PRINCIPLES OF PRODUCTION OF NEW DEVICES FOR MICRO- AND NANOELECTRONICS ON THE BASE OF MATERIALS WITH TRACKS

Dissertation zur Erlangung des akademischen Grades DOKTOR-INGENIEUR

des Fachbereichs Elektrotechnik und Informationstechnik der FernUniversität in Hagen

von ALEXANDER PETROV Minsk/Weißrussland

Hagen 2004

ii

Eingereicht: 18.05.2004 Mündliche Prüfung: 13.10.2004 1. Berichterstatter: Univ. Prof. Dr. W.R.Fahrner 2. Berichterstatter: Univ. Prof. Dr. G.Gerlach i

CONTENTS

ABSTRACT ...... iii

INTRODUCTION ...... 1

Part 1. GENERAL PROPERTIES OF MATERIALS WITH ION TRACKS

AND METHODS OF THEIR STUDY

1.1. WORKING MATERIALS

1.1.1. ...... 1.1.2. Semiconductors ...... 4 1.1.3. Carbon-based materials ...... 6 14 1.2. TECHNICAL MEANS AND METHODS FOR CHARACTERIZATION OF THE OBTAINED STRUCTURES

1.2.1. Optical microscopy ...... 1.2.2. Scanning microscopy ...... 17 1.2.3. X-ray diffraction spectroscopy ...... 23 1.2.4. Ion transmission spectrometry ...... 29 1.2.5. Electrical measurements ...... 31 47 1.3. ION

1.3.1. General principles ...... 1.3.1.1. Electrostatic accelerators ...... 52 1.3.1.2. High frequency accelerators – cyclotron s ...... 53 1.3.2. Technologies used in the present research ...... 56 57 1.4. LATENT ION TRACKS: FORMATION AND PROPERTIES

1.4.1. Physics of ion-solid interaction and track formati on ...... 1.4.2. Track properties ...... 61 1.4.3. Some applications of latent ion tracks ...... 69 72 1.5. ETCHED ION TRACKS: FORMATION AND PROPERTIES

1.5.1. Wetting and swelling processes ...... 1.5.2. Etching of ion tracks ...... 76 1.5.3. Properties of the etched tracks ...... 81 88 ii

DEPOSITION TECHNIQUES, APPLIED FOR THIS STUDY

1.5.4. Tubules formation by evaporation ...... 95 1.5.5. Galvanic deposition of matter along etched tracks ...... 98 1.5.6. Deposition from a saturated solution ...... 99 1.5.7. Electrodeless or chemical deposition ...... 100

PART 2. ION-TRACK-BASED DEVICES FOR MICRO- AND NANOELECTRONICS

2.1. DEVICES ON THE BASE OF POLYMERS

2.1.1. SPECIFIC FEATURES OF THE ION TRACK MANIPULATIONS

2.1.1.1. tubules formation in tracks ...... 107 2.1.1.2. Thermal behavior of manipulated tracks ...... 127

2.1.2. ION - TRACK – BASED MICRODEVICES

2.1.2.1. Magnets and transformers ...... 135 2.1.2.1.1. Micromagnets ...... 142 2.1.2.1.2. Microtransformers ...... 147 2.1.2.2. Capacitors...... 151

2.1.3. ION - TRACK – BASED NANODEVICES

2.1.3.1. Temperature sensors on the base of PET foils ...... 161 2.1.3.2. Pressure sensors on the base of PI foils ...... 166

2.2. DEVICES ON THE BASE OF SEMICONDUCTORS (TEMPOS)

2.2.1. Basic principles of the TEMPOS ...... 171 2.2.2. TEMPOS with continuous deposited layers in the trac ks ...... 176 2.2.3. TEMPOS with nanoparticles in the tracks ...... 181

SUMMARY AND CONCLUSIONS ...... 189

REFERENCES ...... 192

PUBLICATION LIST ...... 212

ACKNOWLEDGEMENTS ...... 218

CURRICULUM VITAE ...... 220 iii

ABSTRACT

The goal of this work was the investigation of the possibilities of creation of micro- and nanoelectronic devices on the base of th e swift heavy ion tracks, for the sake of the future technology. During the implement ation of the work, the ways of realization of that goal were studied, and some fir st prototypes of ion-track-based micro- and nanoelectronic devices on the base of po lymers and semiconductors were created. Thus, the most important results of t his work are as follows: 1. First prototypes of micromagnets and microtransform ers on the base of PI foils with etched ion tracks have been created. They have been shown to exhibit good quality factors up to ~7 at working frequencie s of about ½ GHz, and reasonable coupling factor of ~90%, respectively; 2. A first prototype of the microcapacitor on the base of PI foil with etched ion tracks has been created. It has shown practically f requency-independent capacity of 0.5 pF - 0.6 pF up to 1 GHz. This resul t was confirmed by a theoretical estimation; 3. According to the studies of temperature and pressur e dependences of resistance of PI and PET foils with fullerite tubules in the ion tracks, it was shown, that temperature and pressure sensors can be created in this way; 4. A family of new electronic devices, denoted by the acronym "TEMPOS" (Tunable Electronic Material with Pores in Oxide on Silicon) has been

fabricated for SiO 2/Si and SiON/Si structures with etched ion tracks i n the

SiO 2 and SiON layers. The functioning of the TEMPOS structures is determined not only by the material and the thickne ss of the dielectric layer, but also by the type of silicon substrate, the diam eter, the length, the shape and areal distribution of the etched tracks, and by the type and the distribution of the (semi)conducting matter deposited within these tracks and on the dielectric surface. Two main classes of the TEMPOS devices hav e been studied: (1) the structures with contunuous deposited layers in the tracks, and (2) the structures with nanoparticles in the tracks. It was found that , depending on the TEMPOS iv

preparation recipe and electrical working point, th e devices may resemble gatable resistors, capacitors, diodes, transistors, photocells or sensors, and therefore they will be are rather universally appli cable in electronics. TEMPOS structures are often sensitive to temperature, ligh t, humidity and organic gases. Light-emitting TEMPOS structures have been produced as well. About 35 TEMPOS-based circuits such as thermosensors, photos ensors, humidity and alcohol sensors, amplifiers, frequency multipliers, amplitude modulators, oscillators, flip-flops, and many others have been already designed and successfully tested. Relative simplicity and low co st of production together with compact size can be attributed to the advantag es of the TEMPOS structures. In general, it can be concluded from the present re search, that etched ion tracks in polymer foils and silicon-based dielectric layer s can be useful for production of micro- and nanoelectronic devices. A large range of possible applications for these structures can be determined. The present scientifi c and technological state-of-the art makes it possible to realize the creation of th e new ion-track-based devices.

v

KURZFASSUNG

Das Ziel dieser Arbeit war die Untersuchung der Mög lichkeiten der Schöpfung mikro- und nanoeletronischer Strukturen auf der Gru ndlage von Spuren hochenergetischer schneller Ionen für die Technolog ie der Zukunft. Während der Fertigstellung dieser Arbeit wurden die Wege studie rt, wie dieses Ziel realisiert werden kann, und einige allererste Prototypen von i onenspur-basierenden mikro- und naoelektronischen Strukturen wurden auf der Gru ndlage von Polymeren und Halbleitern geschaffen. Die wichtigsten Resultate d ieser Arbeit sind: 1. Es wurden erste Prototypen von Mikromagneten und Mi krotransformatoren in PI-Folien mit geätzten Ionenspuren hergestellt. Es konnte gezeigt werden, daß sie gute Qualitätsfaktoren biz zu ~7 bei Arbeitsfrequenzen in der Größenordnung von etwa ½ GHz haben, bzw. daß sie se lbst bei GHz- Frequenzen noch zufriedenstellende Kopplungsfaktore n von etwa 90% aufweisen. 2. Die Studien über Temperatur-und Druckabhängigkeit der Widerstände von Fullerit-Röhrchen in PI-und PET-Folien zeigten, daß auf diese Weise Temperatur-und Drucksensoren hergestellt werden kön nen. 3. Eine Familie neuartiger Bauelemente, durch das Akro nym: „TEMPOS“ (Tunable Electronic Material with Pores in Oxide on Silicon = einstellbares elektronisches Material mit Poren in Oxyd auf Siliz ium) bezeichnet, wurde für

SiO 2/Si- und SiON/Si-Strukturen mit geätzten Ionenspuren in den betr. dielektrischen Schichten geschaffen. Die Funktionsweise der TEMPOS-Strukturen wird nicht nur durch das Material und die Dicke der dielektrischen Schicht sowie den Typ des Silizium-Substrates bestimmt, sondern auch durch den Durchmesser, die Länge, die Form und die Verteilung der geätzten Spuren, und durch den Typ und die Verteilung der halbleitenden Schicht innerhalb dieser Spuren und a uf der Oberfläche des Dielektrikums. vi

Es wurden zwei Hauptklassen von TEMPOS-Struktur en untersucht: (1) Strukturen, bei denen kontinuierliche Schichten auf den Spurenwänden und an der Oberfläche abgelagert wurden, und (2) Strukturen mit Nanopartikeln in Spuren und an der Oberfläche. Es wurde gefunden, daß abhängig von der Herstellungsart und dem elektrischen Arbeitspunkt d er TEMPOS-Strukturen, jene steuerbaren Widerständen, Kondensatoren, Dioden, Transistoren, Photozellen oder diversen Sensoren entsprechen könn en und daß sie deshalb relativ universell in der Elektronik einsetzbar sei n sollten. TEMPOS-Strukturen reagieren oft empfindlich auf Temperatur, Licht, Fe uchte oder organische Gase. Auch lichtemittierende TEMPOS-Strukturen wurden her gestellt. Insgesamt wurden mit TEMPOS-Strukturen bereits etwa 35 Schalt kreise entwickelt, hergestellt und getestet, wie z.B. Thermosensoren, Phtotozellen, Feuchte- und Alkoholsensoren, Verstärker, Frequenzvervielfacher, Amplitudenmodulatoren, Oszillatoren, Flip-Flops, sowie viele andere. Struk turelle Einfachheit, relativ niedrige Produktionskosten und Kompaktheit zählen zu den Vorteilen der TEMPOS-Strukturen. Zusammenfassend kann aus der gegenwärtigen Forschung der Schluß gezogen werden, daß geätzte Ionenspuren in Polymerfolien und dielektrischen Schichten auf Silizium-Grundlage nützlich für die Produktion mikro-und nanoelektronischer Bauelemente eingesetzt werden können. Diese Struktu ren zeichnen sich durch eine große Reichweite möglicher Anwendungen aus. Der geg enwärtige wissenschaftliche und technologische Stand der Fors chung macht es möglich, diese neuartigen Ionenspur-gestützten Strukturen pr oblemlos herzustellen.

INTRODUCTION

The miniaturization of electronic devices belongs t o one of the greatest success stories of the past century. It has been enabled by the consequent development of suitable semiconductor materials, digitalization, l arge-scale integration of switching elements, and their continuous reduction to smaller dimensions, which have now reached typically the order of 100 nm. Pos sibilities of further miniaturization of electronic devices require the u se of new efficient technologies. In this concern the present research, which is conn ected with ion irradiation of various materials, appears to be quite prospective. The impact of energetic into matter results in the formation of narrow trails of damage (with diameters ~ 5-10 nm), being at the same time quite long (ranges ~ 30 to 300 m); these are often called as the "ion tracks" [i1] . In insulating targets these tracks are frequently easily etchable, giving rise to well-defined nanopores [i2, i3]. This has been commercially applied since several decades for creation, for instance, the polymeric filter foils [i4]. During t he past 15 years attempts have been made to fill such tracks with suitable conducting, semiconducting, or insulating materials to form new nanometric structures for app lications in (electro)chemistry, electronics and optics. Several techniques can be a pplied here, such as drying of an embedded solution, pressure injection, galvanic dep osition, chemical deposition, evaporation, decomposition reactions in the gaseous or liquid phase, etc. [i2, i5]. The embedded matter can be put into the shapes of c ompact or nanotubules. One can also deposit sponge-like matte r or dispersed individual nanoparticles on the inner pore walls. Then, the em bedded matter in its peculiar nanoscaled geometrical arrangement gives rise to th e desired electronic properties of the ion-track-based devices. After having studied the basic possibilities of “Si ngle (Swift heavy) Ion Track Electronics“ („SITE“) in the past years [i6, i7], w e have concentrated on the one hand on the production of polymer-based electronic microstructures, such as transformers, capacitors and sensors [i2, i8], and on the other hand we created 2 electronic devices by making use of etched tracks i n silicon oxide and silicon oxynitride on silicon [i9]. These new devices appea r to be capable to cover the bridge between the "classical" silicon-based, and t he new polymer-based electronics. In the first part of the work, a general characteri stic of the materials and samples used in the research, namely polymers, semiconducto rs and fullerenes, is given. The salient features of materials preparation are a lso considered. The main methods of sample characterization, used in the pre sent research, are described, namely optical microscopy, scanning electron micros copy, X-ray diffraction microscopy, ion transmission spectroscopy and elect rical measurements. In the next chapters the technologies of ion irradiation o f studied materials and ion track formation are considered. In this concern it was im portant to give a review about the formation of latent ion tracks in the materials and their possible applications in electronics. Further, the etching procedures are co nsidered, which make it possible to create etched ion tracks of various shapes and d imensions in the investigated samples. Etching recepies used in the preparation o f the samples studied in the present work, prior to some properties of the etche d tracks, are also provided. Thereafter, the various possibilities of deposition of matter into the etched ion tracks, namely evaporation, galvanic deposition and electrodeless deposition, are considered. Main emphasis here is given to the elec trodeless deposition, as it was the main method of material deposition used in this work. In the second part of the work, its main results co ncerning the possibilities of creation of micro- and nanoelectronic devices in p olymers and semiconductors are considered. First, the characteristic features of m etallic tubules formation in the etched tracks in polymer foils are discussed, toget her with the analysis of electrical properties and temperature dependences of the obtai ned structures. In the next chapters the creation and properties of the first prototypes of the micro- and nanodevices on the base of polymer foils with ion tracks, and devices made on the base of SiO 2 and SiON layers containing etched tracks are descr ibed. Two principle strategies can be used for making of these devices. On one hand, it 3 is possible to build up structures, consisting of s everal tracks, being connected with each other by miniaturized electrical wires in a pr e-planned way. On the other hand, one can create the whole structure within one individual etched track each in a polymer foil. The obtained devices can be classif ied as follows: (a) Applications in electrotechnics. The first prototypes of ion-track-based micromagnets, microtransformers and microcapacitors were created by a combined use of electrodeless deposition of through s pecial masks with subsequent evaporation of contacts. (b) Applications in sensorics. The second strategy was used for the creation of ion-track-based nanodevices, namely sensors for tem perature and pressure on the base of polymer foils with fullerite tubules in the tracks. The geometry of these structures enables a rapid access of a gas or liqui d to be probed to the sensor material, so that the resulting device has short re sponse times. (c) Applications in electronics. During investigations of structures, consisting of : (1) a n- or p-doped silicon wafer, (2) a SiO 2 or SiON dielectric layer on top of the silicon wafer, (3) an electrically highly resis tive layer on top of the dielectric layer, (4) very narrow conducting connections (etch ed ion tracks) traversing the whole dielectric layer thus connecting top and bott om, and (5) three electrical contacts, two of them being on the top of the sampl e and one being on its back side, we developed a novel family of electronic dev ices which has been denoted by the acronym "TEMPOS" ( Tunable Electronic Material with Pores in Oxide on Silicon). The last chapter of part 2 is devoted to the comple x characterization of these new structures. Depending on the preparation recipe and electrical working point of the TEMPOS devices, they may resemble gatable re sistors, capacitors, diodes, transistors, photocells or sensors, and therefore t hey will be rather universally applicable in electronics [i9]. Relative simplicity and low cost of production together with compact size can be attributed to the advantages of the TEMPOS structures.

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PART 1. GENERAL PROPERTIES OF MATERIALS WITH ION TRACKS AND METHODS OF THEIR STUDY

1.1. WORKING MATERIALS

1.1.1. POLYMERS

Polymers are macromolecules, which are built up by a large number of repeated molecular units, being linked together by covalent bonds. Different molecules, and separate segments of the same molecule are attracte d to each other by intermolecular van der Waals forces. The involved c ovalent bonds are characterized by high energies (146 kJ - mole -l to 628 kJ - mole -l), short inter- atomic distances (0.11 nm to 0.16 nm) and relativel y constant angles between successive bonds. Covalent bonds govern the thermal and the photochemical stability of the polymers. Polymers have a high str ength to weight ratio, they are easy of moulding and lightness, and their productio n is very cheap. In this research, we have used two polymer material s: (PI, Kapton, produced in Russia) and polyethylene terephthalate (PET, Mylar, produced in Russia). (PI’s) form a class of thermally stable polymers that can withstand higher operational temperatures than most of the po lymeric materials in use today [1.1]. The chemical formula of PI is given in Fig. 1.1. Aromatic PI’s belong to the environmentally most resistant heterochain polymers . The presence of imide and aromatic rings in the structure of the polymer is t he chemical reason for the improved heat resistance and stability to ionizing . The backbone of these aromatized polyethers is quite rigid. The chain can rotate around the phenyl ether linkage. The high density reflects a compact struct ure. The average molecular chain separation distance is about 0.5 nm and the i ntramolecular periodicity is 1.55 nm. PI’s forms ordered aggregates of 2 to 2.5 nm thickness that cannot, however, be considered crystalline as they do not yield well-defined intermolecular

5

Fig. 1.1. Chemical formula of PI [1.5]

reflections. The local alignment of chains is order ed in a smectic fashion along the chain axis for several monomeric units. In addition to the segmental ordering a molecular orientation parallel to the surface of sp in coated films has been observed, being most pronounced for films in the mi crometer range [1.2]. Morphologies ranging from semicrystalline to struct ureless have been reported that apparently depend critically on the i nitial imidization temperature. The molecular order of oriented films improves slig htly above 350 °C, and unoriented films show increasing crystallinity abov e 400 °C. PI can withstand temperatures from –269 °C to at least 430 ° C without any marked degradation. Therefore it finds, application s e.g. in surface coating of supersonic aircrafts. PI does not have a melting po int, and the glass transition temperature is expected to be between 360 ° C and 410 ° C [1.3]. Polyimides are widely used not only for electrical insulation but also for passivation and multilayer interconnection in microelectronics, bec ause of their good electrical insulation and dielectric properties, excellent the rmostability and mechanical strength. Photo-sensitive PI is used in high-energy implantation for IC fabrication as a marking material [1.4]. In the present study, we have used PI foils, which were prepared in JNRI, Dubna, Russia, and had a thicknes s d = 25 m . The PET is formed by repetitive addition of 1,2 eth an diol (´glycol´) and terephthalic acid, and therefore contains two hydro xyl groups (–OH) and two carboxyl groups (-COOH). As this reaction is denote d as “estering”, PET belongs to the category of polyesters [1.1]. Its chemical f ormula is given in Fig. 1.2. PET films are biaxially stretched during their produc tion, so that its crystallites orient

6

Fig. 1.2. Chemical formula of PET [1.5]

parallel to the foil surface. The crystalline fract ion can be quite high - in the order of 50% - and therefore its permeability, for inst ance, is quite low for gases. The crystalline lamella surrounded by the amorphous reg ion makes the whole structure mechanically intact. It possesses a very good mecha nical strength from –196°C to +175°C [1.6]. PET foils, used in the present study, were prepared in JNRI, Dubna, Russia, and had the thickness d = 10 m .

1.1.2. SEMICONDUCTORS

As it is well known from the basic physics, materia ls can be categorised into conductors, semiconductors and insulators by their ability to conduct electricity.

The band theory of materials explains qualitatively the difference between these types of materials [1.7]. occupy energy l evels from the lowest energies upwards. However, some energy levels are forbidden because of the wavelike properties of atoms in the material. The allowed en ergy levels tend to form bands. The highest filled level at T = 0 K is known as the valence band. Electrons in the valence band do not participate in the conduction p rocess. The first unfilled level above the valence band at T = 0 K is known as the conduction band.

Electrons are fermions, and thus follow the Fermi – Dirac distribution function: 1 )E(f = (1.1) Exp []E( - m k/) b )T +1 where kb is the Boltzmann's constant, T is a temperature, E is an energy, and mm mm is the Fermi energy, often denoted as EF , or chemical potential. In semiconductor 7 physics the Fermi energy is the energy, at which th ere would be a fifty percent chance of finding an electron, if all energy levels were allowed. Energy level, corresponding to the Fermi energy is named the Fermi level. In order to apply the statistics, we need the density of states in the c onduction and valence bands. These are derived from the basic principle that the densi ty of states is constant in k-space. In the conduction band, the density of states is gi ven by:

V 2/3 2/1 )E(g = 2( me ) E( - Eg ) (1.2) 2p 2h3 where V is a potential energy, h is the Plank's constant, m e is the electron mass, and E g is the energy gap value.

Fig. 1.3. shows the differences in metals, semicond uctors and insulators in terms of how the energy bands are separated.

Insulators do not conduct electricity, because their valence electrons are not free to migrate throughout the material. In princip le the electrons are free to move around, but, in an insulator there are as many elec trons as there are energy levels for them to occupy. If an electron swaps place with another electron no change is made since electrons are indistinguishable. There a re higher energy levels, but to promote the electrons to these energy levels requir es more energy than is usually practical in insulators, because an energy gap that is far greater than the thermal energy of the electron.

In metals there is no energy gap. The conduction band and th e valence band overlap, allowing free electrons to participate in the conduction process. Therefore, in metals there are more energy levels available th an there are electrons to fill them so very little energy is required to find new energ ies for electrons to occupy, and metals conduct electricity easily.

Semiconductors have the energy gap, the width of which is typically ar ound 1eV. Their conductivity is worse than that of m etals, the resistivity being in the 8

Fig. 1.3. Determination of metals, semiconductors and insulators from the point of view of the band theory [1.7]

9 range between 10 -4 Ohm-m and 10 8 Ohm-m. Specific features of conductive processes in semiconductors will be considered belo w.

It should be mentioned, that polymers are in genera l insulators, but some polymers, especially the ones having undergone a sp ecial treatment or development (irradiation, chemical treatment, etc.) can reveal semiconductor properties and even conductor properties (though, o f course, not as good as those of a metal).

Elementary semiconductors are the ones, where each atom is of the same type, such as Ge, Si. These atoms are bound together by c ovalent bonds, so that each atom shares an electron with its nearest neighbour, forming strong bonds.

Compound semiconductors are made of two or more elements. Common examples are GaAs or InP. These compound semiconduc tors belong to the III-V semiconductors so called because first and second e lements can be found in group III and group V of the periodic table, respectively . In compound semiconductors, the difference in electro-negativity leads to a com bination of covalent and ionic bonding. Ternary semiconductors are formed by the addition of a small quantity of a third element to the mixture, e.g. Al xGa 1-xAs. The subscript x refers to the alloy content of the material, which fraction of th e material is added and which fraction is replaced by the alloy material. Alloying semiconductors in this way allows the energy gap and lattice spacing of the cr ystal to be chosen to suit the application.

Intrinsic semiconductors are pure semiconductors. Both elementary and compound semiconductors can be intrinsic semiconduc tors. At room temperature, the thermal energy of the atoms may allow a small n umber of the electrons to participate in the conduction process. Unlike metal s, where the resistance of semiconductor material decreases with temperature, for semiconductors, as the temperature increases, the thermal energy of the va lence electrons increases, allowing more of them to breach the energy gap into the conduction band. When 10 an electron gains enough energy to escape from the electrostatic attraction of its parent atom, it leaves behind a vacancy which may b e filled be another electron. The vacancy produced can be thought of as a carrier of positive charge. It is known as a hole . As electrons flow through the semiconductor, hole s flow in the opposite direction. If there are n free electrons in an intr insic semiconductor, then there must also be n holes in the equilibrium state. Holes and electrons created in this way are known as intrinsic charge carriers. The carrier con centration or charge density defines the number of charge carriers per unit volu me. This relationship can be expressed as n = p = n i where n i is the intrinsic density; p and n are the numbers of holes and electrons per unit volume, respectively. The variation in the energy gap between different semiconductor materials means tha t the intrinsic carrier concentration at a given temperature varies, too.

An extrinsic semiconductor can be formed from an intrinsic semiconductor by added impurity atoms to the crystal in a process kn own as doping. To take the most simple example, one can consider silicon. Since silicon belongs to group IV of the periodic table, it has four valence electron s. In the crystal form, each atom shares an electron with a neighbouring atom. In thi s state it is an intrinsic semiconductor. B, Al, In and Ga have three electron s in the valence band. When a small fraction of these atoms (less than 1 ´ 10 6) is incorporated into the crystal the dopant atom has an insufficient number of bonds to share bonds with the surrounding silicon atoms. One of the silicon atoms has a vacancy for an electron. It creates a hole, that contributes to the conducti on process at all temperatures. Dopants, that create holes in this manner, are know n as acceptors. This type of extrinsic semiconductor is known as the p-type, as it creates positive charge carriers. Elements that belong to group V of the pe riodic table, such as As, P and Sb, have an extra electron in the valence band. Whe n added as a dopant to intrinsic silicon, the dopant atom contributes an additional electron to the crystal.

Dopants that add electrons to the crystal are known as donors and the semiconductor material is said to be the n-type (see Fig. 1.4). 11

intrinsic p-type n-type

Fig. 1.4. A schematic diagram, showing only the valence electron shell to illustrate intrinsic, p-type and n-type semiconductors [1.7] Doping of compound semiconductors is slightly more complicated. The effect of the dopant atom depends the site occupied by the at om on the lattice. In III-V semiconductors, atoms from group II act as a accept ors when occupying the site of a group III atom, while atoms in group VI act as do nors when they replace atoms from group V. Dopant atoms from group IV have the p roperty that they can act as acceptors or donor depending on whether they occupy the site of group III or group V atoms respectively. Such impurities are known as amphoteric impurities.

Quite often one is interested in transitions, that occur near the bottom of the conduction band minimum to valence band maximum . In this case, it is useful to

draw the band structure energy as a function of pos ition, setting the wave vector k=0. In this representation of the energy bands, the donors and acceptors form levels in the energy gap region. At T = 0 K, any fr ee carriers from donors and acceptors are bound to their atoms, so there is no conduction. For non-zero temperatures, the sites can be thermally ionized, r eleasing carriers in the bands so that conduction can occur (see Fig. 1.5).

These shallow level impurities are known as hydroge nic impurities. For donor atoms, an electron orbits around a lattice site, wh ile for acceptors a hole orbits

around a lattice site with residual negative charge . The energy E n required to ionize this carrier is much less than the binding e nergy of the hydrogen atom since 12

EF

Fig. 1.5. Position of the donor and acceptor levels in the band diagram [1.7] the effective mass is smaller and the radius of the carrier orbit larger than that of the hydrogen atom:

m e4 E = - e (1.3) n 2h2 2 2 2e n 4( pe 0 ) where e is the charge of the electron, h is the Plank's constant, m e is the electron mass, e 0 is the permittivity in vacuum, e is the dielectric constant of a semiconductor material, and n is the number of elec tron energy level.

1 A rough estimate for the temperature of ionization is: k bT ~ E d , k bT = eV at 40 room temperature (E d is the donor level energy, and V is electron poten tial). When the temperature is low, excitation from donors and acceptors can be the only source of carriers: in this range the conductivity is intrinsic. In this regime, the doping of the semiconductor determines whether the semiconductor is n-type or p- type. At higher temperatures, direct thermal excita tion from the valence band to the conduction band swaps the extrinsic density. Then, there is an equal number of electrons and holes; the conductivity is extrinsic with distinction between n and p. 13

In our research we have used n- and p-type silicon wafers (in that case, Si was doped with As, P and B, correspondingly) from 256 m m to 380 m m thickness and resistivities from 0.005 to 35 Ohm-cm, being cut in [111] or [100] direction, as a carrier material. The wafers were cleaned as usual (5 min acetone in an ultrasonic bath, rinsing in distilled (DI) water of 17 M W , 5 min methanol in ultrasonic bath, rinsing in DI water, exposure to a 5H 2O/1H 2O2/1HCL mixture for 15 min at 80 °C , rinsing in DI water, 1 min HF of 1 wt.% at ambi ent temperature, rinsing in

DI water, drying with N 2 gun) and then oxidized. The oxidation took place i n dry atmosphere at 1100 °C for 95 min, with 2 liters of O 2 passing the furnace per minute. In this way, oxide layers between 86 to 135 nm were obtained. Alternatively, silicon oxynitride (SiON) layers of ~ 500 nm to 1 m m thickness were deposited onto the Si wafers by means of plasma che mical vapor deposition (PCVD). For this process the following mixture of g ases was used: 4 sccm (standard cm 3, being the unit which is usually used for the meas uremet of gas flow) of SiH 4 , 4 sccm of NH 3 , 100 sccm of H 2 and 120 sccm of N 2O2. Frequency of plasma was 13.56 MHz, power was 10 Wt, pressure was 200 mTorr, temperature was 390°C, and the deposition process took 10 min. Then the top layers of the wafers were protected by depositing a spin-coated (3000 rpm, 40 s) and baked (110°C, 90 s) photoresist (XAR-P5300/1 from Allresist Ltd.) film, and the oxide of the rear sid e was etched away with the commercial ammoniacal fluoride etchant solution AF8 7.5-12.5 from Merck Ltd. After that, a 2 m m thick Al layer was electron-gun evaporated onto t he rear side at 2 nm/s evaporation speed. All these semiconductor materials were prepared in the Chair of Electronic Devices of the University of Hagen, Germany.

14

1.1.3. CARBON-BASED MATERIALS

In general, carbon-based electronic materials compr ise: fullerenes, which are zero-dimensional quantum dots, carbon nanotubes, be ing one-dimensional quantum wires, graphite which is two-dimensional la yered anisotropic material, and diamond, being a three-dimensional wide gap sem iconductor [1.8 – 1.10]. Within this row of carbon materials, fullerenes and carbon nanotubes occupy a particular place, as the most modern carbon allotro pic modifications.

Each molecule of the fullerene family (C m) appears as a sphere or a spheroid containing carbon atoms [1.8]. The thickness of suc h carbon sphere is about 1Å, which corresponds to size of carbon atom. There are two types of the bonds between carbon atoms on C 60 structure: (a) C–C bond, being a common side of t he pentagon and the hexagon; (b) C =C bond, being the common side of the hexagon couple. Each carbon atom belongs to one pentagon an d two hexagons, and its valence requirements are satisfied completely. The structure of C 60 molecule posesses a truncated icosahedron form, Fig.1.6. Fullerenes and their films are attractive for the c reation of electronic devices, because the spherical shape and the large size o f the fullerene molecule provide a substantial volume of empty intermolecular space in a face-centred cubic lattice of solid fullerite. As a consequence, this material is easily intercalated by different impurities changing its properties. Usually, a pure fullerene possesses dielectric properties, but it can reveal a semiconductor behav ior as a result of intercalation [1.8]. Besides, fullerene in solid fullerite under definite action, for example high pressure, UV irradiation and chemical interaction w ith intercalating substances, can form structures with linear, flat or volumetric polymerization processes.

A solid composed of C 60 molecules forms the van der Waals - bonded crystal , named fullerite, which can be an insulator or a sem iconductor, due to fully occupied valence bonds of each carbon atom. Fulleri te is a molecular crystal, where interaction between carbon atoms in the C 60 molecule is much stronger than the interaction between carbon atoms of neighboring molecules [1.10, 1.11].

15

Fig. 1.6. An example of the closed-shell fullerene C 60 configuration [1.8]

The crystalline structure of the fullerite exists i n two forms, namely the face- cubic centered (fcc) one and the hexagonal one. The form of crystallization depends on interaction of carbon atoms and technolo gy of fullerite preparation.

Some physical constants for the crystalline C 60 are given in the Table 1.1. It is important to note, that during the present re search, we mostly worked with thin layers of polycrystalline fullerite, which res istivity was correspondingly anisotropic. Partial oxidation of fullerene in solid fullerite i s accompanied by the polymerization of fullerene molecules, which is confirmed by the appearance of broad bands in IR spectra in the range of stretc hing vibrations at 800 cm -1 and 1000 cm -1 , being characteristic of fullerite polymerized at high pressures [1.8, 1.11]. Partial polymerization proceeds with the dec rease of the distance between the polymerized fullerene molecules and thus with t he increase of the distance between fullerene molecules bound by van der Waals interaction, which causes the strain in crystallites, as well as between sepa rate fullerite crystallites comprising the film (their size being about 200 nm – 300 n m). As a result of this process,

16

Table 1.1. The main physical constants for С60 [1.8]

Quantity Value fcc lattice constant 1.417 nm C60 - C 60 distance 1.002 nm C60 - C 60 cohesive energy 1.6 eV Mass density 1.72 g/cm 3 Molecular density 1.44 × 10 21 cm -3 Temperature of phase transition 261 K Vol. coeff. of thermal expansion 6.1 × 10 -5 K -1 Band gap 1.7 eV Work function 4.7 ±0.1 eV Thermal conductivity (300 K) 0.4 W/mK Electrical conductivity (300 K) 1.7 ×10 -7 S ×cm -1 Static dielectric constant 4.0 ¸ 4.5 Melting temperature 1180 °C Sublimation temperature 434 °C Heat of sublimation 40.1 kcal/mol Latent heat 1.65 eV/C 60

the film becomes stressed and possesses a large num ber of defects in intermolecular bonds and in the interactions betwee n the crystallites comprising the film. The number of conducting electrical chain s and their conductivity in these films depend on the isotropic pressure. It is typic al, that the dependence of resistance on pressure is observed also at pressure s above the atmospheric pressure.

17

1.2. TECHNICAL MEANS AND METHODS FOR CHARACTERIZATION OF THE OBTAINED STRUCTURES

1.2.1. OPTICAL MICROSCOPY

Optical microscopy can be considered as one of the most versatile and useful methods of the characterization of materials struct ures, since quite a lot of the structure features are sufficiently large to be see n by means of the compound optical microscope. However, the optical microscopy loses ist usefulness as the feature sizes shrink to the submicron regime. In th is case, one would need a help of an electron microscope. The basic optical microscop e can be enhanced by adding phase and differential interference contrast, as we ll as polarizing filters. In general, the optical microscopy is most useful for structure s, larger than one micrometer, and the analysis depends on matching the unknown st ructure with the data on the known one. The essential elements of a compound optical micros cope are illustrated in Fig. 1.30 [1.12]. Its optical elements, the objective and the ocular or eyepiece , are shown as simple lenses; in modern microscopes they consist of six or more highly corrected compound lenses. The object O is placed j ust beyond the first focal point fobj of the objective lens that forms a real and enlarg ed image I. This image lies just within the first focal point foc of the ocular, forming a virtual image of I at I ' . The position of I ' may lie anywhere between near and f ar points of the eye. The objective merely forms an enlarged real image which is examined by the eye, looking through the ocular. The overall magnificati on M is a product of the lateral magnification of the objective and the angular magn ification of the ocular. The simplest microscope is the monocular microscope, wh ich has only one eyepiece. The binocular instrument has two eyepieces essentia lly to make viewing of the sample more convenient. When one objective is used with a binocular microscope, the observed image is generally not stereoscopic.

18

Fig. 1.7. Simplified representation of a compound microscope's optical path [1.12].

Light waves interfere with each other, placing cert ain limits on the performance of microscopes. Airy first computed the diffracted image and showed in 1834 that for diffraction at a circular aperture of diameter d, the angular position of the first minimum (measured from the center) is given by (see Fig. 1.8, a ) [1.13] :

sin ( α) = 1.22 λ / d (1.4) where λ is the wavelength of light in a free space. The ce ntral spot, that contains most of the light, is called the Airy or dif fraction disc. One can do one's own experiment by looking at a bright point source at a distance of several meters, for example, at a lamp, through a small pinhole in a cardboard sheet. The same kind of pattern is formed when a point object is imaged by a microscope. There is no lower limit to the size of an object that can be detected in isolation

19

Fig. 1.8. (a) Diffraction at the aperture of a lens showing the Airy disc and (b) the Raleigh criterion for resolution. I represents the intensity [1.13].

under adequate illumination conditions. Generally, one is not interested in detecting a poi nt object, but a two- or three- dimensional object. Two point objects, a distance s apart, produce overlapping images, as shown in Fig. 1.8, b. If they are too cl ose together, it is impossible to resolve them. As they are separated, it becomes pos sible to tell that there are two objects. The definition of this spacing is somewhat arbitrary. Raleigh suggested that two objects can be distinguished when the cent ral maximum of one coincides with the first minimum of the other. Then, the inte nsity between the two peaks decreases to 80% of the peak height. This is shown in Fig. 1.9 for a microscope. The equ ation

s = 0.61 λ / n sin (Θ) = 0.61 λ / NA (1.5) gives the resolution (the minimum distance between points or parts of an object) and satisfies Raileigh's criterion [1.13]. In the equation, n is the refractive index of the medium separating the object from the objective, and Θ is the half angle 20

Fig. 1.9. The resolution limit of an optical microscope [1.12]. subtended by the lens at the object. The numerical aperture (NA), usually engraved on the objective mount, is a number that expresses the resolving power of the lens and the brightness of the image it forms. The highe r the NA of a lens, the higher is the quality of the lens. For high resolution, that is, small s, NA should be made as large as possible. However, high NA does not only correspond to high resolution, but also to shallow depth of field and shallow wor king distance – the distance from the working point of the object plane to the f ront surface of the objective. The depth of focus Dfocus – the thickness of the image space that is simulta neously in focus – is given by

l Dfocus = (1.6) (4 NA )2

Dfocus is insufficient for both the top and bottom surfac es of an to be in focus simultaneously at 200 ´ magnification. The depth of field Dfield – the thickness of the object space, that is sim ultaneously in focus – is given by

21

n 2 - (NA )2 Dfield = l (1.7) (NA )2

Both Dfocus and Dfield decrease with increasing NA, but the resolution i ncreases. It is possible to purchase long working distance ob jectives that allow greater clearance between the objective lens and the sample . Whereas normal objectives may have working distances of 3 mm ( 20 ´ magnification, 0.46 NA) or 0.5 mm (50 ´ , 0.8 NA), long working objectives have 11 mm (20 ´ , 0.4 NA) or 8 mm (50 ´ , 0.55 NA). However, they generally have lower NA and require higher illumination. The magnification M is related to the resolving power of the microscope objective and the eye. However, the image must be m agnified sufficiently for details to become visible to the eye. The resolving power is the ability to reveal details in an object by means of the eye, microscop e, camera or photograph. An approximate relationship for the magnification is [ 1.14]:

M = (maximum NA (microscope)) / (minimum NA (eye)) = =1.4 / 0.002 = 700 ´ (1.8)

The magnification is sometimes expressed as the rat io of the resolution limits

M = (limit of resolution (eye)) / (limit of resolutio n (microscope)) = = 0.15 mm / 0.0002 mm = 750 ´ (1.9) where the eye resolution is set by the distance bet ween the rod and cone receptors on the retina of the eye. The maximum magnification of a microscope when the image is viewed by the eye is around 750 ´ . Magnification above this is empty magnification; it does not give any additional info rmation. M ≈ 400 ´ - 500 ´ with air as the immersion medium. 22

The eye fatigues easily if used at its limits of re solving power, and it is desirable to supply more magnification than the minimum requi red for convenience. A reasonable rule is to make the magnification about 700 NA, but one should always use the lowest magnification that permits comfortab le viewing. Excessive magnification produces images of lower brilliance a nd poorer definition, with the result that the amount of object details that can b e seen is frequently reduced. Recent objectives have been made with magnification s up to 150 ´ with a "dry" NA of 0.95. The contrast – the ability to distinguish between parts of an o bject – depends on many factors [1.14]. Dirty eyepieces or objectives degrade the image quality. Glare will reduce contrast, especially if the sample is h ighly reflecting. It is most serious when viewing samples with little contrast, and it c an be controlled to some extent by controlling the field diaphragm, the opening tha t controls the area of the lighted region. The diaphragm should never be open more tha n just enough to illuminate the complete field of the microscope. For critical cases it may be reduced to illuminate only a small portion of the normal field . The contrast of the surface-height variation can be improved considerably by the dark-field microscopy. Instead of the light impingi ng vertically onto the sample, it is deflected by appropriate condenser lens and impi nges on the sample at an angle. Only the scattered light reaches the objective, all owing small height variations to be seen. Flat portions of the sample do not reflect the light and appear black. The effect is not unlike the observation of dust partic les in air that appear dramatically when illuminated by sunlight streaming through a wi ndow. The eye sees the light scattered from the dust particles.

23

1.2.2. SCANNING ELECTRON MICROSCOPY

An electron microscope utilizes an electron beam (e -beam) to produce a magnified image of the sample [1.12]. There are thr ee types of electron microscopes: scanning, transmission and emission ones. In the scanning and transmission electron microscopes an electron beam incident on the sample produces an image, whereas in the field-emission mi croscope the specimen itself is the source of electrons. Scanning electron microsco py (SEM) is similar to the light microscopy, with the exception that electrons are u sed instead of . This has two main advantages: much larger magnifications are possible, since electron wavelengths are much smaller than wavelength s, and the depth of field is much larger. De Broglie proposed in 1923, that particles can als o behave as waves [1.15]. The electron wavelength l e depends on the electron velocity v or the accelera ting voltage V as

h h 12 2. l e = = = (Å) (1.10) mv 2qmV V

The wavelength is 0.12 Å for V = 10.000 V – a wavelength significantly below the 4000 Å to 7000Å - wavelength range of visible l ight. Hence the resolution of an SEM can be much higher than that of an optical m icroscope. The image in an SEM is produced by scanning the sam ple with a focused electron beam and detecting the secondary and / or backscattered electrons. Electrons and phonons are emitted at each beam loca tion and subsequently detected. Secondary electrons form the conventional SEM image, and backscattered electrons can also form an image. Fur ther, X-rays are used in the electron microprobe for the information about the m aterial composition. All these signals can be detected and amplified to control th e brightness of a cathode ray display tube (CRT) synchronously with the sample be am scan in the SEM. A one- to-one correspondence is thus established between e ach point on the display and 24 each point on the sample. The magnification M of the mapping process results from the ratio of the dimension scanned on the CRT to the dimension of the scanned sample.

M = (length of CRT display) / (length of sample scan) (1.11)

For a 10-cm-wide CRT displaying a sample scanned ov er a 100 m length, the magnification is 1000 ´ . Modifications of 100,000 ´ or slightly higher are possible in SEMs. It is obvious, that high magnifications ar e easily achieved with SEMs, but low magnifications are more difficult to obtain . For a magnification of 10 ´ the scanned length on the sample is one centimeter, onl y 10 ´ smaller than the CRT scan. Typically, SEM has a large viewing CRT and a high-resolution CRT with typically 2500 lines resolution for photography. The contrast in SEM depends on a number of factors. For a flat, uniform sample the image does not show any contrast. If, however, the sample consists of materials with different atomic numbers, a contrast is observ ed if the signal is obtained from backscattered electrons, because the backscattering coefficient increases with the atomic number Z . The secondary electron emission c oefficient, however, is not a strong function of Z, and atomic number variations do not give appreciable contrast. Contrast is also influenced by surface co nditions and by local electric fields. But the main contrast-enhancing feature is the sample topography. Secondary electrons are emitted from the top 100 Å or so of the sample surface. When the sample surface is tilted from normal beam incidence, the electron beam path depth, lying within 100 Å is decreased by the factor 1 / cos Q , where Q is the angle from normal incidence ( Q = 0 for normal incidence). The interaction of the incident beam with the sample increases with path l ength and the secondary electron emission coefficient increases. The contra st C depends on the angle as [1.16]:

C = tan Q d Q (1.12) 25

For Q = 45 ° a change in angle of d Q = 1 ° produces a contrast of 1.75% , and at 60 ° the contrast increases to 3% for d Q = 1 ° . The sample stage is an important component in SEMs. It must allow precise movement i n tilt and rotation to be able to view the sample at the appropriate angle. The angular effect is responsible for the three-dim ensional of SEM images. But this effect also results from the signa l collection. Secondary electrons are attracted and collected by the detector, even i f they leave the sample in a direction away from the detector. This does not hap pen in optical microscopes, where light reflected away from the detector (the e ye) is not observed. SEM forms its picture in an entirely different manner than a n optical microscope, where light reflected from a sample passes through a lens and i s formed into an image. In an SEM no true image exists. The secondary electrons t hat make up the conventional SEM image are collected, and their density is ampli fied and displayed on a CRT. Image formation is produced by the mapping which tr ansforms information from space to CRT space. A schematic representation of SEM is shown in Fig. 1.10. Electrons emitted from an electron gun pass through a series of lense s to be focused and scanned across the sample. The electron beam should be brig ht with small energy spread. The most common electron gun is a tungsten “hairpin ” filament, emitting electrons thermionically with an energy spread of around 2 eV . Tungsten sources have been replaced to some extent by lanthanum hexaboride (La B 6) sources with higher brightness, lower energy spread ( ≈ 1 eV) and longer life, and field emission guns with an energy spread of about 0.2 eV to 0.3 eV. F ield emission guns are about

100 ´ brighter than LaB 6 sources and 1000 ´ brighter than tungsten sources. The incident or primary electron beam causes second ary electrons to be emitted from the sample and these are ultimately accelerate d to 10 kV to 12 kV. They are most commonly detected with an Everhart-Thornl ey (ET) detector [1.18]. The basic component of this detector is a scintillation material that emits light when struck by energetic secondary electrons accelerated from the sample to detector.

26

Fig. 1.10. Schematic view of a scanning electron microscope [1.17].

The light from the scintillator is channeled throug h a light pipe to a photomultiplier, where the light incident on a phot ocathode produces electrons that are multiplied creating the very high gains, necess ary to drive the CRT. High potentials of 10 kV to 12 kV are necessary for effi cient light emission by the scintillator. For the electron beam not to be influ enced by the high ET detector potential, the scintillator is surrounded by a Fara day cage at a few hundred volt potential. The beam diameter in SEMs is in the range of 50 Å t o 100 Å. Yet the resolution of e-beam measurements is slightly inferior due to the finite shape of the electron- hole cloud, generated in the semiconductor. When el ectrons impinge on a solid, they lose energy by elastic scattering (change of d irection with negligible energy loss) and inelastic scattering (energy loss with ne gligible change in direction). Elastic scattering is caused mainly by interactions of electrons with nuclei and is 27 more probable in high atomic number materials and a t low beam energies. Inelastic scattering is caused mainly by scattering from valence and core electrons. The result of these scattering events is a broadeni ng of the original nearly collimated, well-focused electron beam within the s ample. The generation volume is a function of the e-beam energy and the atomic n umber Z of the sample. Secondary electrons, backscattered electrons, chara cteristic and continuum X-rays, Auger electrons, phonons of various energies, and e lectron-hole pairs are produced. For low-Z samples most electrons penetrat e deeply into the sample and are absorbed. For high-Z samples there is considera ble scattering near the surface and a large fraction of the incident electrons is b ackscattered. The shape of the electron distribution within the sample depends on the atomic number. For low-Z material ( Z £ 15) the distribution is “teardrop” – shaped, as sh own in Fig. 1.12. As Z increases ( 15 < Z < 40), the shape becomes more spherical, and for Z ³ 40, it becomes hemispherical. Teardrop shapes have actuall y been observed by Everhart et al. by exposing polymethylmethacrylate to an ele ctron beam and etching the exposed portion of the material [1.16]. Electron trajectories, calculated with Monte Carlo techniques, also agree with these shape s [1.19].

The depth of electron penetration is the electron r ange Re , defined as the average total distance from the sample surface that an electron travels in the sample along a trajectory. A number of empirical ex pressions have been derived for Re . One such expression is [1.20]:

-6 1.75 Re = ( 4.28 ´´ ´´ 10 E ) / r (cm) (1.13) where r is the sample density (g/cm 3) and E is the electron energy (keV). Thus, the

3 -6 1.75 electron range for Si ( r = 2.33 g/cm ), is: Re (Si) = 1.84 ´ 10 E . Equation (1.13) is sufficiently accurate for 20 keV < E < 200 keV, but underestimates the range slightly, as compared to a more accurate expression , provided by Everhart and Hoff [1.21].

28

Fig. 1.11. Summary of the range and spatial resolut ion of backscattered electrons, secondary electrons, X-rays, and Auger electrons fo r electrons incident on a solid [1.16]

29

1.2.3. X-RAY DIFFRACTION SPECTROSCOPY

Scattering of X-rays with wavelength λ from a crystal with lattice constant d by an angle Θ is described by the Bragg´s rule [1.1] :

n λ = 2 d sin Θ (1.14)

where n is the order of the X-ray reflex (see also Fig.1.12).

Fig. 1.12. Bragg's explanation of specular reflection of definite angles [1.12].

Thus, aiming the beam of X-rays on a sample and reg istering the reflex rays, one can get information about the structure of the mate rial, using the reflection peaks on the X-ray diffraction (XRD) spectrum. An example of such a spectrum is presented in Fig. 1.13.

Fig. 1.13. The example of an X-ray diffraction spectrum [1.12]. 30

The width of the X-ray reflection peaks yields an i nformation about the size of the crystals under examination. According to Debeye and Scherrer [1.22], the full width half maximum of the peak (FWHM D) is correlated with the average crystal or crystallite size , the X ray wavelength λ , and the scattering angle Θ according to: 9.0 l FWHM = (1.15) D < L ×> cos( Q )

It is important to note, that FWHM D signifies the half width corrected by the

2 2 instrumental width FWHM instr : FWHM D = FWHM meas + FWHM instr . Such examinations are of use, for example, during the s tudying of the changes in polymeric crystallinity under irradiation. In our i nvestigations, we have used the XRD method for evaluation of the size of the metal crystallites in ion tracks in polymer foils. These results will be described in t he Part 2 of this work. In the XRD diffractometer (Fig. 1.14), a Geiger cou nter or other electronic detector is used to detect both the presence and in tensity of each of the reflections. In our study, a computer-controlled diffractometer was used, which required only that the sample should be aligned correctly in the X-ray beam and then the instrument in turn records all the reflections up t o a chosen maximum angle Θ, and punches out the values of intensity and estimated error for each reflection [1.12].

Fig. 1.14. The schematic view of the X-ray diffractometer [1.12]. 31

1.2.4. ION TRANSMISSION SPECTROMETRY

A principle idea of the Ion Transmission Spectromet ry (ITS) is illustrated in Fig.

1.15 [1.1]. One can assume, that a monoenergetic io n beam with energy E 0 (a dashed spectrum after inclusion of the detector resolution function in the Fig. 1.15, a) penetrates perpendicularly through an unirradiated thin polymer foil having thickness d and density ρprist . Then the ions will undergo an energy loss dE = S ×d (where S is a stopping power), so that they arrive in a detector placed behind the foil with the energy:

Eprist = E 0 – Sd (1.16)

(smooth spectrum in Fig. 1.15, a, with an account o f the detector resolution). For simplicity, we will restrict ourselves to a penetra tion of light ions through thin target polymer foils (a few m thickness), which is the main field of applicatio n of the special ITS case, being introduced here. In the frame of these restrictions, one can neglect the energy dependence of the stopping p ower and the influence of both energy loss straggling and angular scattering.

a b c Fig. 1.15. Principle sketch of ion transmission spectrometry [1.1]: a) through a pristine polymer foil; b) through a pristine foil with a pore (e.g. an etched track); c) through a foil with a latent track.

32

Let us suppose, that we have n circular pores (e.g . etched ion tracks) with average radii r 0 in our polymer foil of unit area, their axes coinc iding with the surface normal. Then the majority of all ions, impi nging perpendicularly onto the foil, will still suffer the energy loss E prist = E 0 - Sd as before, however particles which happen to pass right through the pores will a rrive at the detector with their full energy E 0 (see the double-peaked spectrum in Figure 1.15, b) . The fraction of these particles is given by the expression: 2 f = n πr0 (1. 17)

This means, that by measuring the ratio of particle s detected with the energies E 0 and E det , one can calculate the pore radius r 0:

1/2 r 0 = (N 0/(n πNtot )) (1.18) were N tot is a total number of particles transmitted per un it area and N 0 is a number of particles, transmitted through all pinholes in t he unit area at full energy E 0. The assumptions that have been made in this direct evaluation procedure, or which are required later for the ITS application, a re: 1. The ion tracks are embedded at random positions wit hin a homogeneous polymeric medium. This always holds for the statistical ion impact. 2. The trajectories of the probing ions have exact par allelity to each other and to the pores to be examined. This is given within the limits of the precision of the analyzing ion beam and of the available goniometer.

3. The lateral deviations δz = L ×δα scat of the trajectories of the probing ions during

their flight paths along the tracks with length L, due to angular scattering δα scat ,

are much less than the average width r 0 of the radial density distribution to be

determined: δz << r 0. Here, δα scat can be approximated by:

1/2 δα scat » 2.36 ×(< >×L) (1.19)

with 33

<> = (M 2/M 1)×(S n/E 0) (1.20)

M1 and M 2 are the atomic masses of the analyzing particle an d the target, respectively, and S n is the average nuclear stopping power, which the p article suffers along the ion track. The latter criterion c an always be fulfilled for suffi- ciently short ion track lengths L (for instance, su fficiently thin polymer foils), or for sufficiently large ion track diameters r 0 (i.e. tracks of energetic heavy ions, and/or tracks in highly sensitive, for example, str ongly degassing polymeric target matter such as self-developing resists). To avoid confusion, let us make clear that here we have to distinguish between the two different types of ions: on the one hand, we de al with the ion, which has produced the track under consideration (the object to be analyzed is the ion track), and on the other hand, we deal with the probing ion , which travels along a specific path through the track-containing foil. One-dimensional tomographic reconstructions can be applied to both latent and etched tracks. In the latter case, the task is to c onvert a measured energy loss spectrum C(E) into another one-dimensional data set r(z), which describes the variation of the etched track radius along the trac k axis z, i.e. the etch track shape [1.23 - 1.25]. One can use for this purpose either a low flux particle beam from an accelerator, or, more conveniently, a radioactive s ource (e.g. 40 kBq Am) emitting α particles. In Fig.1.16 a principle experimental ar rangement is shown for the latter case, and in Fig. 1.17 is presented a photograph of the experimental ITS setup, used in the present research by our group. In order to obtain the etch track shape from the sp ectra, one uses Monte Carlo simulations [1.26], where the input track shapes ar e varied by trial and error until the simulated spectra fit the measured ones. In thi s way, for instance, one can follow the etched track evolution during etchi ng from the first tiny etch pits towards the final extended open pore. In this case, the ion transmission spectrum of

Fig. 1.15 a changes insofar, as a tail emerg es between the peaks E prist and E o

34

Fig. 1.16. Principle arrangement of an ITS experiment [1.1]

Fig. 1.17. The ITS experimental setup used in the present research. 35

(Fig.1.18). The slope of this tail is proportional to (rd/s) 2 [1.23].

If only the track radius r T is of interest, it is sufficient to determine the transmission T of those particles, which do not suf fer any energy loss. When a highly parallel ion beam, aligned exactly along the track axis, is used for this sake, the transmission through the microporous s ample foil is equivalent to the normalized counting rate of the particles (being eq ual to the ratio: counting rate with foil / counting rate without foil) arriving in the detector and given by:

a

b

Fig. 1.18. Examples of ITS, as applied to: (a) a cylindrical etched track, and (b) a track with double conical shape [1.1].

36

2 T par = F t/F 0 = πrT Φ (1.21) with F 0 is a total transmitted sample area, F t is a total etch track area in F 0, Φ is the track density in the sample (or the ion beam fluenc e), and r T is the track radius. One should note that, in case of non-cylindrical (e.g. conical) tracks, r T describes the smallest radius of the track. As ITS was one of the main tools, which was used in the present investigations for evaluation of ion track radii, a simple formula for the correlation between the ITS yield and the ion track radius was derived fo r cylindrical tracks under the impact of both parallel or divergent probing ion beams [1.27]. In the process of the formula derivation, two configurations of the p article beam have been considered: a) Measurement in the parallel beam geometry

Let us assume, that a parallel particle beam (for i nstance, that from an accelerator) hits a target foil (for example, a po lymer) of a thickness L, where Φ 2 capillaries (e.g. etched tracks) per cm with radii r T are embedded, and with the capillary axis coinciding with the beam direction ( Fig. 1.18, a). Then the flux of the transmitted particles (being equal to the count ing rate C F as measured with a detector behind the foil) is given by:

J F ½½ = J 0 F T (1.22) with J 0 being the flux of the incoming particles (i.e. the counting rate of particles without foil, C 0 ), and F T being the total cross-sectional area of all capill aries of the 2 sample. With F T = πrT Φ it follows for r T:

1/2 1/2 r T = [J F ½½ / (J 0 πΦ )] = [T/( πΦ )] (1.23) with T defined as the transmission of the foil. Exp erimentally it can be determined by the ratio C F / C 0 . 37

If the beam direction is tilted against the capilla ry axis by an angle φ, then the directly transmitted beam (i.e. without energy loss in the foil) is limited by the overlap σc of the two circles of diameter r T displaced by δ = L tg φ (Fig. 1.19, b). In the Fig.1.20, the two circles represent the entr ance opening and the exit one of the capillary, as seen by an observer along the beam axis; then:

J F ½½ = J 0 F T σc (1.24) and hence:

1/2 rT = [T/( π Φ σc)] (1.25)

~ 2 ~ For the overlap case (Fig.1.20), σc = s c π r T (with s c = effective opening, normalized to the incident area) and one can derive :

~ 2 ~ ~  d  s~ = 1- d 1- d 2 + arctan   (1.26) c   ~  p   1- d 2 

a

b

Fig. 1.19. Scheme of particle beams, hitting a pol ymer target foil: (a) The case of a parallel particle beam; (b) The case of a tilted particle beam; 38

d

d 2 d 0 x 2

Fig. 1.20. Scheme of the overlapping track geometry.

~ where d = d /( r2 T ) . One can observe, that the separation of the ci rcles d = L ´ ~ tan j depends on the incident angle j and, hence, d =tg j / tg φT , with φT = arctg(2r T/L) being the maximum opening angle of a capillary (the so-called aspect angle, corresponding to the so-called aspect ratio 2L/r T). Thus, for small φ we ~ may replace d by the normalized angle j = j / j T . In Fig. 1.21 the effective ~ ~ opening s c is shown as a function of d » j . For convenient representation, ~ ~ ~ ~ 35.1 the s c (d ) function is replaced by an empirical expression s c » 1( - d ) which fits the data very well (one can note, that Fig. 1. 21 contains 2 curves).

Capillary Aspect Angle

Fig. 1.21. The effective opening as a function of the normalized angle, exact calculation (thick line) and empirical approximation (thin line). 39

As expected, the effective opening is equal to unit y for ion impact along the capillary axis and tends to zero when the incident angle approaches the capillary divergence angle j T = arctg (2r T/L) ( Fig.1.22).

Fig. 1.22. Geometry of the capillary divergence angle. The diagonal lines symbolize the maximum possible beam divergence.

b) Measurement in the divergent beam geometry

Now let us regard an incident particle beam with a divergence, described by a characteristic divergence angle j d , so that part of the incident beam is absorbed at the capillary walls.

Consider first a beam originating from a small area D s at a macroscopic distance d from the capillary, as shown in Fig. 1.23. This a rea may be, for example, an infinitely small segment of a planar radioactive α emitter source. The beam from the area D s enters the capillary under the incident angle j . Then, the current density of ions ejected from the source point is ex pressed as:

r j j d L0 d D s d 0 LT

Fig. 1.23. Incident particle beam with a divergence, originating from a small area. 40

Jd Jˆ = s (1.27 ) s dW ds

~ i. e., D J s = Js D sDW is the current leaving from the source area D s into the solid angle DW . Let us recall the definition of the solid angle DW = s d/ 2 , where s is a small area, limiting the incident or transmitted be am (Fig.1.24). One can note, that the a solid angle is defined in the beam direction.

As in the above case, the transmitted beam D J F is limited by the overlap s c of the two circles of diameter 2r T displaced by d = LT tg j T (Fig. 1.23), as given by expression (1.26). The two circles represent the en trance and exit opening of the capillary seen by an observer at source poin t D s . Similarly, the beam D J 0

2 entering into the capillary is proportional to the incident area p rT . Thus, we obtain:

p r 2 s ˆ T ˆ c D J 0 = J s D s 2 and D J F = J s D s 2 (1.28) L T L T In the next step of the analysis, we introduce a fi nite source area S, as shown in Fig. 1.25. The source area is assumed to have a cir cular symmetry governed by the distance r from the origin located in the center. T he divergence angles j c and j d define circles with radii r T and r d, respectively, as indicated in the figure. Let rs be the radius limiting the source area. We assum e rd < rs , i.e. the source area is

Fig. 1.24. Definition of a solid angle for the present case.

41

f c j c rs rT f d rd j d d 0

LT

L S 0

Fig. 1.25. Geometry of a finite source area. chosen to be relatively large. To treat the case rd > rs , the divergence distance rd is to be replaced by rs , i.e., in general, one should set r d = min(r d, r s).

To evaluate the incident and transmitted beam, we i ntegrate the terms in the expression (1.28). The source area is composed of s mall ring elements D s = 2p r D r , so that:

2p 2 ˆ 2p ˆ J 0 = 2 p rT ∫ J s rdr and J F = ∫ Js s c rdr (1.29) d d 2 yielding for the transmission

ˆ ~ ∫ Js s c rdr T = (1.30) ˆ ∫ Js rdr which correspond to the effective opening averaged over the differential emission ˆ current Js . At this point, some information is needed about the current density function given in the expression (1.27). In the simplest app roach (as given for an α emitter source), step function dependencies for the locatio n and the divergence angle are assumed:

ˆ0 ˆ Js for r < rs and j < j d Js ,r( f ) =  (1.31)  0 otherwise 42

ˆ0 where Js is a constant. The quantities rs and j d , being already defined, refer to source size and the divergence angle respectively.

Hence, the transmission is obtained as:

~ 1 rd ~ 1 rd T = s~ d drr = s~ ~r ~dr ~r (1.32) 2 ∫ c () ~ 2 ∫ c () rd 0 rd 0

~ ~ where r = r/r T and rd = rd r/ T are normalized radii. We have used rd = dtg j d ~ (Fig. 1.24), so that ~r = d . It is seen, that the evaluation reduces to a weig hted ~ integration over the effective opening s c . The results of the transmission calculation, prese nted in Fig.1.26, can be also very well fitted analytically. By expressions

~ 15.1 ~ T= 75.0 1( - rd ) + 25.0 for rd £ 1 and (1.33) ~ 2 ~ T= 25.0 / rd for rd > 1

Capillary Aspect Angle

Fig. 1.26. Transmission data dependence on the divergence angle, obtained from expression (1.32). Exact calculation (thick line) and empirical approximation (thin line).

43

Fig. 1.27. Diagram of the ITS measuring unit. a good agreement is obtained with the integrated va lues (Fig. 1.26 shows two curves, as well). Now let us consider an ITS measuring setup, accordi ng to References [1.28, 1.29]. It includes a 40 kBq source of alpha -particles ( 241 Am) with the TM radius r s, a semiconductor detector (ORTEC ULTRA ) with, for example, a polymeric foil sample in the holder (see Fig. 1.27) . The source and the holder are separated by a distance d.

Then, as r s /d = 2r T/L (see also Fig. 1.24), one can rewrite expression (1.33), so that one obtains the track radius r T from the normalized counting rate T, the ITS system parameters r s and d, and the foil parameter (thickness) L accordi ng to:

rsL ~ rT = for rd £ 1   4T - 1 87.0  2d1-      3   and (1.34) r L T r = s for ~r > 1 T d d

44

Let us now estimate the region of validity of the o btained expression for r T . For this sake we have plotted the values of track radii , as calculated for parallel particle beams versus those ones, which are calculated by in clusion of the beam divergence (Fig. 1.28). In the figure we have plotted theoretical curves fo r non-zero (dashed and dotted lines) and for zero beam divergence (solid line; di agonal) as obtained by the comparison of formulae (1.23) and (1.34). Experimental ITS results (symbols) for the PI foils and the PET foils were also included in the Fig.1.28. The more these data devia te from the diagonal, the larger is the necessity to take into account the beam dive rgence of the experimental setup.

Fig. 1.28. Influence of ITS beam divergence onto track radius calculation (cylindrical track geometry). Symbols: experimental results; dashed and dotted lines: estimations for various cases. Solid line: diagonal (corresponding to a zero beam divergence), to guide the eye. 45

Let us now turn to the case of conical tracks wit h small opening angle a »

2 ( r 0 – r i ) / L, with r 0 being the outer track radius, and r i being the inner one (see Fig.1.29). According to the SEM data, this is mainly the cas e for the experimental results for the PI foil samples, so we need to introduce so me corrections in the expression (1.34). First of all, we have to replace in the integratio n limits of the formula (1.32) by the value of ( r 0 + r i ) / 2. Consequently, after the integration, the s ame formula for

T holds, if the radius r T for the case of a cylindrical track is now re placed by

( r 0 + r i ) / 2 . Therefore, if the expression (1.34) for cylindrical tracks is applied to the conical ones, as a first approximation, then r T has the meaning of an average radius, and the inner and outer radii can be calculated by r i » r T - a L / 4, and r 0 » r T + a L / 4, in the case if the opening angle of the cone, a , is known. Fig. 1.30 compares SEM results with those ones as d etermined by ITS, taking into account the angular divergence of the measurin g setup. In the case of conical tracks, the thus derived ITS radii denote ave rage values between the real inner and outer track radii. This explains the observed d eviations. Taking into account the above mentioned corrections for conical tracks, the data points shift much closer to the diagonal line, and thus indicate that ITS is not only a fast, but also a precise technique for determining track radii.

Fig. 1.29. Geometry of a conical track with small opening angle. 46

Fig. 1.30. Comparison of track radii as determined from the measured SEM data, with those ones as determined by ITS, taking into account the angular divergence of the measuring setup.

Therefore, we can conclude, that comparison with th e SEM data verifies the feasibility of the above derived ITS theory, taking into account the angular divergence of the probing particle. The use of the formula (1.34) should make it possible also to obtain the values of track radii i n polymer foils with material deposition on the track walls, in accordance with t he investigations, which have been carried out during the last years [1.30, 1.31 ].

47

1.2.5. ELECTRICAL MEASUREMENTS

Electrical measurements usualy deal with the d etermination of a conductivity σ = J/E, i.e. the proportionality factor between the current density J and the electric field E, or resistivity r = 1 / σ. Correspondingly, electrical resistance can be determined as R = r × L / S, where L is a distance between contacts, a nd S is a cross-section of the sample. For the samples, used in this research, resistance was measured by the two-probe method and the four-probe method. Creation of reliable ohmic contacts on a sample is very important for electrical mesurements. There are two ways of creating the con tacts [1.5] : (1) Selection of metals with low values of the work fun ction with the goal of creation of an electron injection regime, or select ion of metals with high values of the work function, which is needed for th e hole injection. Due to this selection of contact pairs, the potential barr ier for an effective thermoelectron emission is lower, and corresponding ly, the free carriers density value on the contact is larger than that in the sample bulk; (2) Strong doping of a sample's surface near the contac t, with a result of the potential barrier becoming sufficiently thin for an effective quantum tunelling of carriers; the resistance of organic se miconductors, which are mostly polymers, is usually large, and this makes i t possible to neglect the resistance of an electrical contact, as being compa red with the complete resistance of a sample. A simple possibility for making ohmic test contacts is the application of a conducting paste on a sample. Most frequently the s ilver paste is used [1.32], and more rarely the mixture of a graphitic powder, name d "aquadag" is applied. These contacts enable a satisfactory electrical connectio n of the samples with a measuring circuit, by means of gluing thin wires to the conta cts or the use of needle-like pressing electrodes [1.33]. As the electrodes, one can also use copper plates in the form of stripes, which are tightly pressed to the sample. Another method to

48

Fig. 1.31. The two-probe measuring unit of the needle type [1.5]. 1 – sample; 2 – dielectrical base; 3 – contact needles; 4 – the contact needles pressing holders; apply metallic contacts to a sample is a metal evap oration through special masks. The two-probe method is a simple method of the resi stance measurement in a DC current regime. A diagram of the corresponding measuring unit with a needle- type contacts is given in Fig. 1.31. The two-probe method can be used for samples, which have a correct geometrical form and a constant cross-section. In t hat case, the surface resistivity can be calculated using the measured bulk resistanc e R, by the formula:

r s = R b / L (1.35)

where b is the length (width) of the contacts, and L is the distance between the contacts [1.34]. For the calculation of the bulk resistivity accordi ng to (1.44), one needs to take into account the sample thickness d:

r v = R b d / L (1.36)

It is important to note, that for the electrical me asurements of our samples, one needs devices (e.g. voltmeters) with high input res istances ( > 10 10 Ohm). In the four-probe method (see Fig. 1.32), a small c urrent I from a constant- current source passes between two outer contacts 1 and 4 (with distance d outer ) on the surface of the sample to be probed. The voltage V is measured via two other inner contacts 2 and 3 in between the outer contact s (with a distance d inner between 49 inner and outer contacts). For a thin conduc ting layer with thickness d (d << lateral sample extensions) the sheet resistance R s is then given by [1.1] :

R s = V F c / I (1.37)

where F c is a correction factor depending on the ratio d outer / d inner . Values for F c can be found, for instance, in Ref. [1.35]. One needs to note, that for this case it is worthy to use an AC source to avoid charging-up effects, but one should use low frequen cies to avoid the capacitance effects. This approach does not make much sense, however, in the case of low fluence irradiation of thin polymeric foils with energetic ions, with range >> foil thickness. Here the conductivity along the ion tracks is o f interest, rather than that one along the surface (i.e. normally to the tracks). Th erefore it is desirable to measure the conductivity here via two contacts on the foil´ s front and back sides. Electrical characteristics of the prototypes of the electronic ion-track-based microdevices, produced during this research (see al so the Chapter 2.1.2) were measured by means of vector impedance analys ers HP 4191 and HP 4192 (Fig. 1.33).

Fig. 1.32. The four-probe measuring unit [1.36]: 1 – dielectrical plate; 2 –spring; 3 – contact probes; 4 – sample; 5 - dielectrical base; 50

4

3

2

1

ION

Fig. 1.33. Scheme of the measuring unit for the characterization of the prototypes of the electronic ion-track-based microdevices: 1 – microdevice sample; 2 – contacts; 3 – test fixture (HP 16047A for up to 10 MHz, or HP16092A for up to 1000 MHz); 4 – vector impedance analyser (HP 4191 or HP 4192);

Electrical characteristics of the TEMPOS ( Tunable Electronic Material with Pores in Oxide on Silicon) structures (see the Chapter 2.2) were measu red by means of the experimental setup in Fig. 1.34. As seen from the figure, three connections were nor mally applied to a TEMPOS structure, two on the top side and one on the back side. To avoid any confusion with existing electronic structures, we denoted the surface contacts arbitrarily as “ o” and “ w”, and the base contact as “ v”. Usually, the current-voltage measurements were performed between o and v contacts, whereas the contact w was kept at ground potential . The current-voltage characteristics were observed o n the oscilloscope, and if needed, the setup can be s witched on either to a recorder device, or to the computer.

51

Fig. 1.34. Experimental setup unit for the electrical characterization of the TEMPOS structures [1.37]: 1 – a TEMPOS structure; 2 – bipolar control voltage source of NGMD type; 3 – the oscilloscope of Tektronix TAS-475 type; 4 - measuring generator of MG-47 type.

52

1.3. ION IRRADIATION

1.3.1. GENERAL PRINCIPLES

Energetic ions for materials irradiation can be pro duced in several ways. At present, the most common approach is the use of ion implanters or ion accelerators. Other possibilities include cyclotrons, synchrocycl otrons, radioactive samples or nuclear reactors. In principle, one can produce pure beams of any io n at any energy with ion implanters or ion accelerators [1.1]. The basic dif ferences between both devices are on the one hand the available energy range, and on the other hand, the ion beam currents delivered for each machine. Ion implanters usually work in the energy interval of 10 keV to several 100 keV, delivering high currents, which can vary from tens of m A up to tens of mA. Instead the ion accelerators wo rk in the MeV energy region but with lower currents that in extre me cases can reach the 100 m A region. It should be pointed out, that some high energy nuc lear physics accelerators even reach energies between about 0.5 and 5 GeV (as an example, see Fig. 1.35, a). However only few of them are also used for polymer irradiation and research. For that field, the energy range between 1.5 and 5 MeV per nucleon is most important, as that range covers the maximum of the projectiles ' electronic stopping power, hence yields the highest possible energy densities per ion track in the targets (Fig.1.35, b). On the other hand, that energy range is still below the Coulomb barrier for nuclear reactions, so that one needs no t to care that the irradiated samples might become radioactive. The maximum beam current that is available for a given projectile mass is less important for p olymer applications, due to the great sensitivity of these materials. Let us now consider a general description of some accelerator types, prior to the goals of the present research.

53

a b

Fig. 1.35. Example of a high energy accelerator system, assigned for the materials science investigations: the Centre for Ion Beam Techniques "IonenStrahlLabor" ("ISL") Berlin. Left: available energy range as a function of projectile mass, for the two available accelerator combinations. VdG: combination of van de Graaff injector and cyclotron, RFQ: combination of radio frequency injector and cyclotron. Right: energy per atomic mass as a function of the projectile atomic number. The different colors indicate the corresponding energy transfer per pathlength (calculated here for amorphous Si targets). The range within which the RFQ-cyclotron combination works (dashed rectangle), coincides well with the regime of maximum linear energy transfer (LET) that is most useful for materials science. (Courtesy of the Hahn-Meitner- Institute Berlin and the ISL)

1. 3.1.1. Electrostatic Accelerators

Electrostatic accelerators are operated still today in basically the same way as their prototypes constructed at the beginning of th e thirties. The most successful ones of these were the Cockroft-Walton and van de G raaff accelerators [1.1]. In 1932, J. Cockroft and E. Walton built an acceler ator in Cambridge based on a voltage multiplication principle, which was able to accelerate protons with the energy of 380 keV. The Cockroft-Walton accelerato rs were applied for decades mostly as neutron generators by using (d, n) reacti ons until the recent revival of a similar principle used in a new generation of the m egavolt tandem accelerators, built without moving parts [1.38]. 54

In 1931 R.J. van de Graaff has demonstrated a princ iple in Princeton how to produce a 1 MV potential difference between the ter minals of two belt charged generators. Still, at present the van de Graaff acc elerators are the most known electrostatic accelerator types. The main part of the van de Graaff accelerator is t he high voltage generator. It consists of a continuous conveyor belt that carries electrostatic charges (sprayed by a DC generator) up to a hollow terminal. The high D C potential is maintained by a charge continuously flowing back to the ground thro ugh a voltage divider of very high resistance. A beam of positive ions is extract ed from a radio-frequency source installed at the high voltage terminal. Then, the p ositive ions are accelerated through the acceleration tube to the ground potenti al, thus forming a MeV ion beam. This kind of electrostatic accelerator is cal led single-end accelerator and it is mainly used for gases like H, He, Ne, etc. In Fig. 1.36 is shown the van de Graaff accelerator with 5.5 MV voltage, which is a part of the Centre for Ion Beam Techniques "IonenStrahlLabor" ("ISL") of the Hahn-M eitner-Institut Berlin.

Fig. 1.36. The Van de Graaff accelerator of the ISL (Courtesy of the Hahn Meitner-Institute Berlin and the ISL)

55

More versatile van de Graaff accelerators are the t andems. They have negative ion sources outside the accelerator, an analyzing m agnet, a two-stage acceleration system with a pair of accelerator tubes, a stripper system, a switching magnet and the corresponding beam lines. Usually this kind of accelerator has two negative i on sources, a first one for H ions and He ions that can be of RF or duoplasmatron type, and a second one, which is good for all the other elements - usually a sput ter type source. A detailed description of this kind of negative sources can be found in Reference [1.39]. The analyzing magnet fulfills the same function as the one described for the ion implanters. It is very desirable that it should be of the 90 º type, because under this condition a merit figure of 1/200 is easily achieve d. The stripper channel at the high voltage terminal is used to strip the electron s from the accelerated beam of negative ions. Either a thin carbon film foil (2-5 m g/cm 2), or a low-pressure gas in a narrow channel is used as a stripper. Already acc elerated ions are accelerated again over the same potential difference. The total energy (in keV or MeV) which an ion acquires is (n+1)qV where n is the charge s tate of the ion emerging from the stripper, q is the charge of the electron and V is the terminal voltage. In this calculation we did not take into account the extrac tion voltage, applied to the ion sources, the value of which is of the order of few tens of kV. In the present case the switching magnet fulfills t wo missions, first to select among the ions with different charge states that em erge from the stripper the desired one, and second to direct the ion beam to t he right beam line. Compared with van de Graaff accelerators, a larger range of energies and most of the ion species can be produced in tandem accele rators. Voltages as high as 35 MV have been attained in tandems. However, tandems that accelerate ions in the low MeV energy region are today the most common one s, due to their increasing use in material science research. Instead of the co nventional belt most of the new accelerators today have either a chain of insulatin g and metallic links (NEC pelletrons) [1.39] or a solid state power supply as mentioned before (High Voltage Europe) [1.38]. 56

1. 3.1. 2. High Frequency Accelerators – Cyclotrons

In high frequency accelerators the particles traver se the same accelerating potential difference several times. This is accompl ished in the linear accelerator (LINAC) concepts by a high frequency wave travellin g with the same velocity as a given group of particles. This philosophy implies p hase stability of the particle group, as particles preceeding the wave are deceler ated, whereas particles following the high frequency wave are accelerated. By constructing a consecutive number of gaps with the applied high frequency fiel d in between, so that the wave velocity increases smoothly with traversed distance , the particle group is hence accelerated [1.1]. This concept allows the acceleration of particles w ith different masses to the same velocity by only adjusting the gap voltage acc ordingly. The particle beam is accelerated in form of single pulses. Great lengths of their drift tubes and high frequencies of the accelerating waves characterize the LINAC. In a cyclotron a particle beam travels in circular paths inside hollow electrodes (“D”s), with a gap in between each semi-cycle. The circular particle path is brought about by a constant uniform magnetic field. The accelerating high frequency field is applied to the gaps only, wherea s the D´s remain field free. As the particles traverse each semicycle in the same t ime, independent of their velocity, the high frequency field at the gaps can be constant unless relativistic effects start playing a role. In this case suitable modeling of the magnetic field has to be done to maintain the cycling phase. As the cy clic particle path radius increases with the square root of the particle ener gy, the total particle trajectory becomes a spiral. This means, that the beam has to be injected near the center of the D´s, and it is fed out of the cyclotron at its maximum radius. The resulting ion beam is pulsed with a frequency comparable to LINAC s. In some cases, the cyclotrons can be rather small compact machines, wh ich is an advantage as compared with the electrostatic accelerators. In t he Fig. 1.37 the cyclotron of the ISL is shown.

57

Fig. 1.37. Cyclotron of the ISL [1.40] (Courtesy of the Hahn-Meitner-Institute Berlin and the ISL)

1.3.2. TECHNOLOGIES USED IN THE PRESENT RESEARCH

It is suitable to consider the ion track production technologies, used in the present research on the example of the Centre for I on Beam Techniques "ISL", situated in the Hahn-Meitner-Institut-Berlin (HMI-B erlin). It is exclusively dedicated to the ion beam application technologies, providing the ion beams from various accelarators and combinations of accelerato rs, with energies ranging from 10 eV to 100 MeV. Researchers at the ISL study the basics of the ion interaction with solids, investigate the local properties with a short-life and accelerator- produced radioactive isotopes, analyse the composit ion of thin-layered materials, and perform modification of materials [1.40].

58

Fig. 1.38. The principal diagram of the ISL structure [1.40] (Courtesy of the HMI-Berlin and the ISL)

The most important practical applications of the IS L include the therapy of ocular melanoma with fast protons, and the irradiat ion of polymeric foils with high energy heavy ions for the production of microfilter s, used in various high-tech branches. A principal diagram of the structure of the ISL i s presented in the Fig. 1.8. The cyclotron is the most important installatio n of the ISL, and it produces the highest ion energies (see Fig.1.37). It amplifies t he energy of the injected beams by a fixed factor of 17. With the variable high-freque ncy and magnetic field configuration of the cyclotron, its energy variatio n ranges from 1 MeV to 70 MeV per ion mass unit. The ISL cyclotron produces a quasi-continuous beam with a microstructure, having a pulse width of less than 0.5 ns and a freq uency of 10 MHz to 20 MHz. This time – structure combines the advantages of a quasi-continuous beam for 59 with the possibility of using the puls ed beams. This variety of ion beams, ion energies and beam structures is unique i n Germany. The high ion energies, delivered by the cyclotron provide a wide range of applications. These extend all the way from the eye tumour therapy, with very fast and deeply penetrating protons, to the produc tion of short-life isotopes, using relatively light – to intermediate - mass ions, and to deformation of materials with extremely heavy ions. Depending on the experimental needs, either the Van -de-Graaff accelerator or the RFQ structure is employed as an injector. They are complementary in their specifications and can be used independently. The Van-de-Graaff accelerator with voltage 5.5 MV ( see Fig. 1.36), which on the one hand delivers ions with extremely high ener gy stability and energy resolution, from the other hand permits the rapid e nergy changes. The terminal of the Van-de-Graaff accelerator is equipped with a 5 GHz – Electron Cyclotron Resonance (ECR) source, producing a wide range of c harge states, thus providing ions across a broad energy spectrum. Owing to these features, the accelerator is an excellent tool for materials analysis. The Radio-Frequency-Quadrupole (RFQ) structure, bei ng developed specially for the ISL, delivers ions with the very high beam currents over virtually the entire periodic table, and this device is ideal when high dose rates are required. Ions with the lowest energy are produced by means o f a special decelerator: highly charged ions are decelerated to a few tens o f electron-volts immediately in front of the target. These exotic ions having high potential energy and low kinetic energy are used for specific applications in the su rface analysis and treatment. In order to produce as highly charged ions as possi ble, the ISL relies exclusively on the ECR ion sources. A plasma, confined by a mag netic field is heated by microwaves. The microwave frequency is chosen to fu lfil the cyclotron resonance conditions for electrons in the central region of t he plasma. ECR sources are characterised by the strength of the magnetic field , and both, the power and frequency of the microwaves. 60

The ECRIS4, the most powerful source, is used to pr oduce very highly–charged ions. The other two sources, the Supernanogan and B ECRIS, are designed to operate on high-voltage platforms. Their magnetic f ields are generated entirely by permanent magnets, in order to reduce the needed el ectrical power. Information concerning irradiation of the materials , used in the present research, (see also the Chapter 1.1) is given in the Table 1. 2.

Table 1.2. Irradiation conditions of the materials, used in the present research.

Material Irradiation Ion Fluence of Place of energy type irradiation irradiation

PI 350 MeV Kr 1 ´ 10 6 cm -2 JINR Dubna Russia PET 1 GeV Ar 2 ´ 10 8 cm -2 JINR Dubna Russia 129 20+ 8 -2 SiO 2 / n- and p-Si 390 MeV Xe 2 ´ 10 cm ISL Berlin SiO / n- and p-Si 350 MeV 197 Au 26+ 8 -2 2 1...2.5 ´ 10 cm Germany SiON / n-Si 300 MeV 86 Kr 14+ 1 ´ 10 9 cm -2 ISL Berlin SiON / p-Si 350 MeV 197 Au 26+ 8 -2 1...2.5 ´ 10 cm Germany

61

1.4. LATENT ION TRACKS: FORMATION AND PROPERTIES

Latent ion tracks emerge from the primary depositio n of high energy (~MeV to ~GeV) inside a tiny target volume ("ion track co re", ~10 -15 - 10 -14 cm 3) , and within an extremely short time (~10 -17 - 10 -15 s), by swift heavy ions [1.40]. These extraordinary transient conditions lead to dr amatic modifications of the materials via chemical and structural changes, bein g accompanied by heat and pressure pulses. In spite of repair of much of the primary damage (e.g. the transient breaking of all bonds) during the annealing phase a fter the ion impact ("thermal spike" model, ~10 -12 - 10 -11 s), quite a number of irreversible changes remain. The cylindrical zones with altered properties, where th ese changes prevail, are characterized by a high density of radicals, carbon aceous clusters, new phases (e.g. SiC in case of irradiated polysilanes), amorphizati on, and an altered free volume. These changes influence many macroscopic properties of the irradiated target material such as its permeability, its refraction i ndex, its conductivity, and others.

1.4.1. PHYSICS OF ION-SOLID INTERACTION AND TRACK FORMATION

Ion tracks were discovered by Young in 1958, during the examination of etch pits in a fission-fragment-irradiated LiF, by means of scanning electron microscopy. In 1959, Silk and Barnes [1.41] have ob served latent fission-fragment tracks in in the transmission electron microsc ope. These discoveries have initiated the development of research on ion tracks , including track etching techniques for particle detection and identificatio n, and their use in several areas of science and technology [1.42, 1.43]. Ion tracks are the stable remnants of swift heavy i on (SHI) impact onto solids. An ion is considered as swift (or fast) if its velo city is much higher than the Bohr 62 velocity (0.22 cm/ns ). In spite of the existence o f quite a number of models on the formation mechanism of tracks, there does not yet e xist a general agreement on the details of that process. However, it is generally a ccepted, that the electronic energy loss S e = (dE/dx) e plays a dominant role, that the ionization density must be very high, so that the electronic excitation energy is d ispersed sufficiently slowly, and that the radial extension of the track stems from e nergetic secondary electrons ( d rays) created by the passage of the ions. Therefore , the study of properties of the ejected species from the surface and of the induced damage trail in the bulk can provide information on the track structure [1.1].

The interaction between projectile and target depen ds on the velocity v p of the projectiles, which is the decisive parameter for th e stopping of energetic particles in matter (see Fig. 1.39). A fast ion loses a part or all of its electrons within the first few nanometers upon impact onto matter, and i t maintains only those electrons whose orbital velocity exceeds the projec tile velocity. Depending on the ion speed, the projectile gets an effective charge Z eff which is described by the empirical relation:

2/3 1/2 2/3 1/2 Z eff = Z p [1-exp(-vp/(v 0Zp )] = Z p [1-exp(-125 ß /Z p )] (1.38) with Z p being the original ion charge, v p is the projectile velocity, v 0 is Bohr velocity, ß = v p/c, and c is the speed of light. This formula is va lid for the relative velocity v p/v 0 in the range 3 £ (v p/v 0) £ 100. Below a relative velocity of 3 the Thomas-Fermi atom no longer represents the almost n eutral heavy ion, and the simple physics of the Bohr's stripping model become s inadequate. For very high velocities relativistic effects become important. It is important to note also, that the projectile energy per nucleon [MeV/amu] is deno ted as the specific energy . Ions of equal specific energy have the same speed [ 1.45]. Usually the projectile has a charge lower than the specific charge (i.e. equilibrium charge) upon the ion impact onto the ta rget. This results in a lower projectile stopping during the first few nanometers of the ion trajectory, and hence

63

Fig. 1.39. Interaction of a fast ion with matter [1.44]

the surface-near target damage is reduced. I ndeed, this effect has been found quite frequently, see, e.g. Ref. [1.46]. However, it is not yet clear why the observed regio ns of reduced damage are often much broader than expected. Possibly, this po ints at the self- within damaged polymers. Heavy ions that possess kinetic energies higher than ~1 MeV/amu are slowed down in matter primarily by ioni zation of the target atoms ( ≈ 90%), by the emission of secondary electrons with h igh kinetic energy, and by excitation ( ≈10%) of the electronic system of the target (“elect ronic stopping”) 64

[1.42, 1.47]. The electronic stopping powers of GeV Zn to U ions range from a few keV/nm to a few tens of keV/nm, and vary only sligh tly along the ion path. The 2 energy loss increases with the square of Z eff and decreases with 1/v p . As a major fraction of that enormously high energy loss is tra nsferred to thermal energy after some intermediate steps, corresponding models have been developed to describe the ion tracks as “thermal spikes”. Depending upon the local ion velocity, the projectile interaction time with a single target at om is ~10 -17 s to 10 -16 s. The interaction time of a primary ejected electron or a secondary recoil electron (a δ- ray) with nearby target atoms is typically ~10 -15 s to 10 -14 s. Slowing down of the ion by momentum transfer to substrate atoms ("nucle ar stopping", [1.42]) is important only near the very end of the projectile path, where the ion velocity is smaller than the Bohr velocity. Direct ionization is produced within the so-called infratrack or track core, delimited by the Bohr adiabatic radius, r j. The energetic secondary electrons transport a fraction of the deposited energy away f rom the core up to a maximum radius r u (the "ultratrack" or "penumbra"), limited by the p rojected range of these 3 electrons. In organic solids ( ρ ≈ l g/cm ), r u = 40 nm for a 0.6 MeV/amu primary ion. Different primary ions having the same velocit y produce electrons of the same maximum energy and therefore tracks of the same dia meter, independent of the ion atomic number. However the energy density inside th e track increases with increasing stopping power for the ions with higher atomic numb er. Typically, half of the total projectile energy of a swift heavy i on of 1 MeV/amu to 10 MeV/amu is stored in the core at the stopping power maximum . There have been a number of attempts to explain tra cks in terms of fluctuations of the electronic energy loss. For example, the pro bability of multiple ionizations in a given track region that substantially exceeds the average degree of ionization was estimated in Ref. [1.48]. The energy of the secondary electrons lies in the k eV range; e.g. for a 0.6 MeV/amu ion, the maximum electron energy is about 1 .3 keV. This corresponds to temperatures in the order of some 10 5 K [1.48]. Some of the electrons, the so- 65 called " convoy electrons" , start travelling in the wake of the ion´s electr ic field along with the ion, and others are emitted in backw ard direction from the region of ion impact. The energy spectrum of the electrons co nsists of a broad continuum superposed by various line structures. The broad co ntinuum stems from the δ electrons and exhibits a strong angular dependence. The maximum possible energy, which can be transferred to the electrons i s given by the projectile speed. These electrons determine the radial extension of i on tracks. Many of the discrete lines of energy spectra of emitted electrons stem f rom the so-called “loss electrons”, that have nearly the same velocity as the projecti le ions, and from Auger processes in the target atoms. The width of t he Auger peaks enables one to derive the electron temperature, and the shift of t he measured Auger peaks from their originally expected position by some 50 V ena bles one to derive a corresponding ion track potential V track [1.49]. This means, excited electrons are preferentially removed from the central track area, thus rendering the latter positive. That leads to the building-up of a transi ent track potential. Some electrons, the so-called δ-electrons, are emitted with energies as high as some keV perpendicularly from the track so that the y can migrate up to ~ 100 nm to 1 m. However, the fraction of these electrons is smal l, so that they do not contribute significantly to the overall energy budg et of the ion tracks. Such findings have been made upon the basis of computer simulations, taking into account the electronic structure of the targets inc luding the core levels and the densities of states in the valence band and the ban d gap, and the emission and transport of each excited electron, with the use of the appropriate double- differential cross-sections [1.50]. However, these cross-sections are in general not known. Therefore, in track structure models (" δ-ray model"), the average deposited energy density ρE(r) is approximated by a continuous and radially sy mmetric (cylindrical) distribution of energy deposited arou nd the ion path, which decays roughly with the square of the radial distance from the track centre. This dependence has been confirmed in experiments perfor med on noble gases. lt is problematic to measure ρE(r) in solids, because of difficulties in resolving and 66 detecting radial distributions of charge within nan ometer distances of a track. However, the study of the emission characteristics of electrons ejected from tracks allows one to get first experimental hints for ρE(r). Older concepts speculated with a slow return of the electrons after more than 10 -15 s, due to screening of the central charge. This ti me would be sufficient to initiate the movement of the positive atoms along t he tracks due to their repulsive forces ( “Coulomb Explosion”) [1.42, 1.51]). In this case, the core of the latent ion track would be defined by the range of the atomic c ollision cascade to be less than 10 nm in diameter. However, most of the electrons r eceive relatively low energies (< 100 eV), so that they do not travel more far awa y than a few tens of nanometers from the primary ion track. These electrons produce transient electric fields with strengths of some V track /r u ≈ 50 V / 40 nm ≈ 13 MV/cm, i.e. roughly twenty times the breakthrough voltage of a well-insulating polym er. Thus they will not remain trapped at the end point of their trajectory but wi ll be immediately driven back, to neutralize the track within much less than 10 -15 s. Recombination processes, due to the high density of the target material (mainly rec ombination in three-body collisions), accordingly get major importance. Ther efore, as in any dense plasma, no local positive charge density can arise for a ti me sufficiently long to trigger Coulomb explosions. However, in the surface-near re gion, where the released electrons are permanently lost, no charge recombina tion can occur and Coulomb explosions will occur. For a long time Thermal Spike and Coulomb Explosion models were regarded as competing and contradicting models to describe i on track effects. In spite of many discussions, a final decision could not be bro ught about by experimental evidence, as both models predict a square dependenc e on dE/dx. Furthermore, both models have their disadvantages. For example, the p ossibility of quenching of this effect in its very initial stage by the rapidly ret urning electrons was never treated in a convincing way in the Coulomb Explosion model, an d the Thermal Spike model neglected both surface pressure effects and phase c hanges, which made doubtful their unconditional application. 67

The δ-ray model attempts to describe at least one kind o f energy transport (namely, that mediated by the secondary electrons) out of the immediate path of an impacting ion. Besides the "diffusion" of δ-electrons, there are other mechanisms of energy transport in solid matter. For instance, energy propagation can involve shock-waves (or pressure pulses), arising from the sudden perturbation of the solid by the MeV ions. Diffusion of reactive species (e.g . free radicals and excitons) is another important means of energy transport. The re lative contributions of the various transport mechanisms to the deposited energ y at different radial distances from the ion path are very difficult to estimate. These processes of energy transport are related to the more general problem of the conversion of electronic energy into kinetic en ergy of atomic or molecular motion. Several pathways for this conversion have b een suggested: repulsive dissociation of electronically excited molecules, r epulsion of ionized neighbours (Coulomb Explosion), and direct vibrational excitat ion of molecules by low energy electrons. The efficiency of this conversion is gov erned by the lifetime of the localized electronic excitations and is particularl y high in dielectric materials, which have relatively long-living excitations. Thus , the deposited electronic energy is transformed partially into vibrations and phonon s, i.e. expansion energy, via one of the mechanisms mentioned above and via chemical recombination (reaction heats); also the transformation can take place part ially in bond rearrangements (fast, "hot chemistry” ), which may continue on a longer time scale. A sma ll fraction of the deposited energy is lost to the emi tted electrons, photons and heavier particles () from the near-surfac e layers. A typical time scale of events initiated by the pas sage of a fast ion through a solid is shown in Fig. 1.40. Energy is transferred from the radiation to matter with the creation of excitations and ionizations within the first 10 -14 s. This is sometimes referred to as the physical stage of radiation action. In the time scale of 10 -14 s to 10 -10 s, a major fraction of the energy exists as vibrati onal energy. During the physical-chemical stage, fast (prethermal) chemis try and dissipation of energy to 68

Fig. 1.40. Typical time scale of events occurring in response to the passage of a fast ion through a thin dielectric solid [1.48]. the surroundings occurs. Within typically 10 -14 s to 10 -8 s, unimolecular dissociation takes place, and the subsequent t ime t >10 -12 s is largely devoted to bimolecular chemical reactions. After about 10 -10 s thermal equilibrium is attained, but not yet chemical equilibrium. Diffusion-control led processes, involving radiogenic species with a nonhomogeneous spatial di stribution, dominate. This marks the beginning of the chemical stage, which may alter the pattern of the physical (initial) track structure. After recombination of the energetic electrons with the ionized target nuclei at rest, the track will consist of a multitude of high ly excited but neutral atoms. At that plasma stage of target heating, since heat tra nsfer is carried by electrons, this process is non-linear in respect to the electron te mperature. Thus the heat wave emitted from the track core has a step-like front s tructure, and the temperature at any spatial point has an instantaneous maximum at t he time that the heat wave arrives. The temperature at that spatial point then falls with time as the heat wave propagates. In the region where linear heat conduct ion dominates, the temperature at an arbitrary space point increases with a finite rate, reaches its maximum, and then decreases. The electrons will equilibrate their energy with th e nuclei by electron-phonon coupling. Recent calculations show, that this leads to transient track atom temperatures in the order of about 10 3 K for about 10 -15 s [1.52]. This is what is known since long as “thermal spikes” . Based on statistical thermodynamical 69 calculations in the Thermal Spike model, a detailed mathematical derivation was made to understand the swift heavy ion induced dama ge creation, amorphisation etc. of different solids due to the electron-phonon coupling considering both the electron gas and the atomic lattice as continuous m edia. According to this model, the electronic subsystem is excited by the energy t ransfer from the projectile ion, thus reaching a high temperature (typically some 10 4 K) within a short time (typically 10 -15 s). The electronic system relaxes via the so-calle d electron-phonon coupling. The energy transfer process from excited electrons to the lattice depends on the so-called electron-phonon coupling factor g, which is inversely proportional to the square of the mean free path length λ of the electron/phonon interaction by the relation:

2 (1.39) g = D e(T e)C e(T e)/ l where D e(T e) is the spatial energy distribution and C e(T e) is the electron specific heat. Due to this coupling, an atomic thermal spike (with several thousands of Kelvin) develops in the lattice in the picosecond t ime scale. The spike temperature depends on the electron - phonon coupling strength.

1.4.2. TRACK PROPERTIES

The damage induced by fast ions is not homogeneousl y distributed around and along the track. It is stronger and more concentrat ed in the track core, while becoming more diluted at the limits of the ultratra ck. Besides that, fluctuations in the energy loss events are involved. This can arise , for instance, from the localization of secondary electrons, producing mult iple ionization events in a small volume, or from Auger decay of created inner shell vacancies. Therefore, the region, where damage can occur within an ion track, does not have sharp boundaries (Fig. 1.41). A quantity called the damage cross section , σ, is introduced in order to quantify the effective damag ed zone around the path of the impacting ion. It can be expressed by the formula:

σ = D / (t F N) (1.40) 70

Fig. 1.41. Track segment of a 4He 2+ ion with energy of 2 MeV in water vapour. The arrow at the left indicates the MeV lines [1.47]. where D is a number of damage centers formed during the time interval t, N is a number of target atoms, and F is the ion beam fluence. Quantification of the modifications (or damage) ind uced by the ion beams can be obtained by direct measurements of the impact fe atures or latent tracks by e.g. microscopy. Otherwise it can be done indirectly, by monitoring the changes in the physico-chemical properties of the target with vari ous techniques. In the case of indirect measurements, the size of t he effective modified zones (“effective damage cross sections”) can be extracte d from the evolution of the characteristics of the signals representative for t he damage in pristine material, Y, as a function of fluence Φ. The modifications introduced by different impacti ng ions are a series of statistically independent events, and the probability of creating further damage after a certain number of i mpacts is proportional to the undamaged area of the target. Thus, if a single dam age process operates, this results in an exponential decrease of Y with the fl uence:

Y( Φ)=Y 0 exp (–σ Φ) (1.41)

The quantity σ is identified as the damage cross section. The mag nitude of the cross sections will depend on the "detector" used t o probe the damage (e.g. the degree of crystallinity, molecular vibrations, seco ndary ion yields, optical properties, etc.), and on the structure of the targ et compound. One should differentiate the physical ion track dimensions, gi ven by the infra- and ultratrack, 71 which are fixed for a specific primary ion, from ef fective modification areas with dimensions that depend on the particular structure which is under investigation.

The latent track consists of a core of higher trans ferred energy density D e (eV/atom) surrounded by matter under strain and wea kly affected by the few δ electrons. The damage morphology along the track ev olves with dE/dx. An effective track radius r eff can be defined which can be linked to the damage c ross section σ by the relation r eff = (s / p ) [1.53]. There exists a direct correlation between r eff (and hence σ, as well) and the damage morphology which was firs t derived by Toulemonde and coworkers for high T C superconductors. For σ ≈ -17 -14 2 10 ... 10 cm , corresponding to r eff ≤ ca. 0.5 nm, isolated cylindrical defects of -14 -13 2 some 3 nm diameter appear. For σ ≈ 10 ... 10 cm (i.e. 0.5 nm < r eff ≤ ca. 2 nm), these isolated defects elongate along the trac k direction and start touching; -13 2 -13 2 chemical etchability sets in. For σ between 1 ´ 10 cm and 3 ´ 10 cm , or r eff ≈ 2...3 nm, cylindrical defects of ca. 3 nm radius ap pear. And finally, if σ >3 ´ 10 -13 2 cm or r eff > 3 nm, the cylindrical defects percolate to give r ise to continuously damaged zones along the tracks [1.1]. The transient breaking of all bonds of a polymer al ong the projectile paths upon their impact is a favorite precondition for the pha se changes. Free excited atoms may recombine in all possible new ways, concerning both the chemical composition of material and its structure. For non- polymeric solids, such as silicon, this has been described [1.54] by an amorphization (or “demorphization”) front running behind the projectile and away from the ion trajectory, as a consequence of the heat pulse emerging from the track. Upon subseq uent cooling, recrystallization sets in which were described by a recrystallization front running behind the amorphization front and backwards towards the traje ctory within ~10 -11 s. Depending on the system under consideration, this projectile assisted prompt annealing (PAPA) can be decisive for the track survival. For exampl e, in Si or

CaF 2, complete epitaxial recovery takes place, which ex plains the absence of observable ion tracks in these cases, and also the reduction of in Si. 72

It is assumed, that PAPA is also the underlying rea son to explain the large stability of polymers against ion beam attack. The survival of the vast majority of all polymer molecules and the in general negligible formation of new compounds

(e.g. of aromatic structures and C 60 ) after SHI impact signifies, that PAPA is of great importance also for polymeric targets. There exist only very few examples where major ion-induced phase transitions in polyme rs emerge, such as the formation of SiC along tracks of swift heavy ions p olysilanes (see below).

1.4.3. SOME APPLICATIONS OF LATENT TRACKS

In general, one can determine four major strategies for latent track applications: (1) exploitation of the modified transport properti es along ion tracks, (2) letting metallic atoms or clusters precipitate along the tr acks, (3) exploitation of the material´s chemical changes, and (4) making use of ion-induced phase transitions [1.55]. Concerning the first possibility, swift hea vy ion irradiated polymer foils are already used now as seals to protect sensitive volu mes (of e.g. electromotors) against penetration of ambient dust and moisture wh ile maintaining pressure equilibrium between inside and outside. It had also been suggested [1.56] to dope latent tracks for electronic purpose, which is an e xpansion of the old concept of polymer doping for electronics after low energy ion impact [1.57]. The idea is to insert suitable materials preferentially into the f ree volume along ion tracks to establish there the desired electronic properties. This approach might be applicable not only for polymers but also for diamond [1.58] a s the confined sp 2-rich phase along tracks in diamond is expected to have a more open structure than diamond itself. However, at present the concept of latent t rack doping has hardly left the stage of experimental data acquisition. The second possibility – the formation of conducting nanowires by precipitation of metal atom s or clusters along latent tracks - has not yet been realized, as well. Realizing the third possibility of exploitation of the material´s chemical changes, one can make use of the long-known effect of destru ction of the carbonaceous sp 3 73

structure and its transference to sp 2 by energetic radiation. By SHI irradiation of carbonaceous allotropes such as fullerene or diamon d it is possible to restrict the zone of modification to a cylinder of only a few 10 nm in width. These zones exhibit a conductivity in the order of ~10 -1 Ohm-cm, no matter whether fullerite or diamond was the precursor material (for comparison, one can consider the data for graphite: 8.1 ´ 10 -4 Ohm-cm, amorphous carbon: ~ 1 Ohm-cm, ion-irradiat ed amorphous carbon: 10 -1 Ohm-cm). This can be recorded by means of atomic f orce microscopy (AFM) with a conducting tip, Fig.1.42. With increas ing distance from the track center, the resistivity drops strongly. These AFM i mages reveal, that the modified zones in both fullerite and diamond show up as hill ocks on the surface, due to the lower density of the transformed sp 2-enriched material. The current-voltage characteristics show a non-symmetric and nonlinear behaviour. One can recognize some peaks in the I/V characteristics, which indica tes the occurrence of quantum effects in the carbonaceous nanowires.

Fig. 1.42. AFM image of a SHI ion track (~ 2.2 GeV Au) in fullerite [1.59]. Left: height relief, right: current / voltage characteristics. The ion track, marked by a dashed circle, shows up as a hillock of a few nm height. Whereas the track center is conducting (a, b), the conductivity rapidly decreases with decreasing distance from the track center (c, d).

74

As the conductivity along these carbonaceous nanowi res is higher than that of their environment by many orders of magnitude, and as they protrude somewhat from their neighborhood, it has been proposed to use them as tips for field emission displays. In fact, field emission from irr adiated diamond could already be verified [1.59]. Apart from this, the nonlinear tra ck characteristics suggest possible future applications in electronics. As an example of the fourth possibility (making use of ion-induced phase transitions), one can consider the experimental res ults for polysilanes, irradiated by SHI. They were transformed to a silicon carbide-lik e matter, roughly characterized by SiC x, with x ≈ 1 [1.60]. This residue was insoluble in convention al solvents so that it could be easily separated from the original host matrix by dissolving the latter. It turned out that this residue had a soft spaghetti-like appearance, consisting of many nanowires with some fine structure, Fig.1.4 3, a. Upon annealing of the irradiated target for a prolonged time at a tempera ture around the polysilane´s melting temperature, the SiC x apparently crystallized and gained mechanical stability, Fig.1.43, b. Therefore, it has been prop osed to use such crystalline nanorods as construction material in future microme chanics and microanalytics, e.g. as AFM tips.

75

100 nm a

b

Fig. 1.43. AFM images of SHI-irradiated (2.2 GeV Au) poly(metylphenyl silane), PMPySi [1.60]. (a) appearance of the insoluble residue SiC x (x ~1) after leaching the soluble matrix (left: height image, right: friction image); (b) image of a crystallized swift heavy ion track, after annealing the sample at 400°C for 6 hours in high vacuum;

76

1.5. ETCHED ION TRACKS: FORMATION AND PROPERTIES

Chemical etching of the ion-irradiated polymers is a process of transformation of every single latent track into a hole (pore), so that, depending on the etching conditions, one may obtain a wide variety of etched track shapes - long cylinders, short cones, craters, hemispheres - and many others . The use of ion track etching processes is aiming either at getting information o n the particles, which have created the tracks or at modifying the structure of the pristine monolithic polymer. In the first case the polymer serves as a detector and, sometimes, as a spectrometer. From the number, shapes and sizes of the tracks, on e can determine the particle fluence, the composition of the particle flux, the angle of incidence and the energy of the particles. Numerous applications of polymeri c track detectors in nuclear physics, radiography, cosmic ray studies, applied r adiochemistry, dosimetry, and other fields are based on this principle.

1.5.1. WETTING AND SWELLING PROCESSES

These processes are usually concerned with the wate r uptake conditions for polymers, and some phenomena in this connection. As it is known, most etchants are the dissociating into ions. The so lubility of ions in hydrophobic polymers is extremely small [1.62], as a result of the polarization of the medium around the electrical charges. The "self-energy" of a monovalent ion in a medium with the dielectric constant ee ee can be estimated using the Born formula [1.1]:

2 E = e / 2 e r i (1.42) where e is the elementary electrical charge, and ri is the radius of the ion. Calculations for hydrophobic polymers ( ee ee » 3) and the ion radius of 0.2 nm show that the concentration of the ions in the polymer p hase should be approximately 10 20 times smaller than that in the aqueous phase [1.63 ]. The adsorption of ions in aqueous solutions by hydrophobic surfaces is normal ly negative. In the aqueous phase very close to the polymer surface, the concen tration of ions is lower than in 77 the bulk solution - due to the proximity of the med ium with low ee ee . If the polymer surface is neutral, the negative adsorption is poor and can be neglected in considering the etching kinetics. The thermodynamic parameters of the etchant in the bulk and at the polymer/solution interface can be assumed to be identical. However, that is not the case when the surface has electrical charge due to dissociating chemical groups. An example is the PET treated with an alkaline agent. Its surface acquires negatively charged carb oxylic groups rigidly bound to the surface. The electric field results in increasi ng the local concentration of positive ions (counter-ions) in the adjacent layer of the aqueous phase. Accordingly, the local concentration of negative io ns (co-ions) decreases. This phenomenon produces a considerable effect on the et ching kinetics, especially in diluted etchants [1.64]. In contrast to hydrophobic matrices, diffusion of e lectrolytes in hydrophilic polymers is appreciable. Hydrophilic polymers (poly vinyl alcohol, cellulose, etc.) are characterized by a high content of polar groups and readily take up water, i.e. swell. The presence of water in the polymer signifi cantly reduces the ion "self- energy" [1.65]. Water molecules play the role of ca rriers promoting the transport of ions through the polymer. Other hydrophilic polymer s are of ion-exchanging nature (such as polyamides). In such systems functi onal groups resting positions for diffusing ions are preferable, with energetical ly unfavorable regions in between. The ions hop from one preferred site to an other. Protonated amino-groups can serve as active sites for the diffusion of anio ns, whereas carboxylic groups can provide the route for the transport of protons or o ther cations [1.1]. For these reasons, degrading agents not only etch hydrophilic polymers at the surface, but also diffuse into the depth and initia te overall degradation leading to a decrease in molecular mass, an increase in the number of end gr oups and to further swelling. Consequently, the etchant/polymer contact angle decreases rapidly upon the onset of etching until a new stationary value i s obtained. (e.g. for PET from 79 ° to 67 ° within 50 min). For contrast, a hydrophobic polymer being in contact with a degrading agent retains its average molecular mass. In principle, the etching is 78 solely removing material in this case, as it change s the size and shape of a polymer sample without affecting its interior physical and chemical structure .

The pristine PET is known to be a rather hydrophobi c material with a water contact angle of about 72 °, and hence it hardly uptakes water , so that both solid- to-gel transition and swelling are hindered. Howeve r, after etching of irradiated PET, at least a layer at the surface of unknown thi ckness appears to be hydrophilised due to its much higher concentration of carboxylic (-COOH) and hydroxylic (-OH) end groups in comparison with the pristine polymer. These types of end groups are hydrophilic and can be responsibl e for the water uptake. Fig. 1.44 reveals that, whereas during the onset of NaOH etchant penetration into PET, any change in surface topography could be hardly detected, considerable alterations in surface structure show up after abou t 10 min exposure time at ambient temperature, or ~3 min at 45 ° C. Even when continuing these scanning

Fig. 1.44. Scanning force microscopy images of ion-irradiated PET: (a) without etching; (b) after 10 s etching; (c) after 10 min etching at ambient temperature, as measured on wet samples; (d) only samples, which were dried after etching, reveal pores. Irradiation by 3 GeV U ions at a fluence of 2 ´´ ´´ 10 8 cm -2 [1.1]. The dust particle on the right side of (a) (see also the height profile) was incorporated for the sake of better illustration of the smoothness of the pristine foil.

79 force microscopy (SFM) examinations up that point when the samples disintegrate due to over-etching, SFM observations do not yet re veal any visible hole, Fig. 1.44, c. Only when releasing the uptaken water afte r drying, the shrinking polymer reveals its mass loss in the irradiated zones due t o etching, Fig. 1.44, d [1.1]. For a contrast, recent scanning tunnel microscopy (STM) o bservations on tracks in (PC) [1.66] have shown the existence of open pores during etching, however with diameters much smaller than in the dri ed state. The studies confirmed the existence of a highly conductive surf ace gel. The transition from the latent tracks to etched tra cks by the etching process can also nicely be followed by Ion Transmission Spectro metry (ITS), Fig.1.45. Irradiated unetched PET foils show the same transmi ssion spectra as pristine ones (not shown here), indicating that degassing after i on impact is a process of overall minor importance (i.e. less than a percent loss in density) than is commonly assumed. Incorporation of etchant leads to a shift of the transmission spectra by as much as » 50% towards lower energies, if the foils are measur ed in wet state (with only their surface dried by a cloth to remove drops of the etchant), Fig.1.45, a. Subsequent drying after the etching reverses this p rocess, leading finally to an ion- transparent foil, Fig. 1.45, b [1.1]. These observations indicate that a prerequisite for dissolution of radiochemical destruction products by etchants appears to be thei r transformation from the solid to the gel state by solvent uptake, associated with the material´s swelling [1.1]. This transformation, followed by new free volume fo rmation due to gel dissolution, sets in already at the beginning, but becomes more important when sufficient etching has penetrated into the polymer due to sufficient newly created free polymeric volume.

80

a

b

Fig. 1.45. ITS spectra of different stages of ion track etching of a PET foil. (a) Solvent uptake during the etching leads to a larger energy loss of the transmitted aa aa -particles, (b) solvent loss by drying leads to energy loss reduction and penetration through etched tracks with full projectile energy [1.1]. The final spectrum reveals the porosity of the etched foil (arrow). As the measurements of the transmission spectra were made in air instead of vacuum, scattering of aa aa -particles took place, which has lead to a smearing out of the peak E prist toward the pronounced tail in the ITS spectra, worse energy straggling and a slight shift of peak E o (see Figs. 1.15 and 1.16, Chapter 1.2.4). The foil thickness was 10 m; final etched track diameter is 200 nm.

81

1.5.2. ETCHING OF ION TRACKS

Some general principles of the selection of good et ching conditions were recommended in the Refs. [1.43, 1.67, 1.68]. Summar izing the accumulated experience, one can present these guidelines as fol lows:

1. The etchant should be capable of "distinguishing" b etween regions of different chemical properties. Chemical polishes which tend t o round corners are not suitable since they will obscure etched tracks. 2. The etchant should degrade the molecules at the liq uid/solid interface slowly and carefully. The degradation should occur due to bond breaking and without swelling and dissolving of the matrix. 3. The reaction between the polymer and the etchant sh ould not lead to the formation of insoluble products. Such products are accumulated on the surface and impede further transport of reagents. 4. One should avoid using of highly concentrated etcha nts since they, on the one hand, are often highly viscous, and on the other ha nd, etch very fast. This fast etching leads to the external diffusion-limited kin etics which strongly depends of the efficiency of convection in the liquid phase. 5. The etchant should be stable in time. Its spontaneo us degradation at elevated temperature or under light exposure is undesirable. 6. The etchant should have a low surface tension and, thus, it should wet the polymer surface readily. Bad wettability may cause inhomogeneous etching because the process does not start simultaneously a ll over the sample. In the real practice, the development of an "ideal" etchant is sometimes a hard problem. In many cases it turned out to be impossib le to find a recipe which satisfies all the stated above requirements. So far the method of trial and error plays a major role in the selection of etching cond itions. Though the general rules are really helpful here, still only experimental te sts may give the ultimate answer if the given etchant is suitable or not. 82

The knowledge of the fundamental chemistry, involve d in the etching process enables one to understand and unify results obtaine d under variety of conditions and assists in the search for more satisfactory and application-specific etches [1.69]. Equations of the relevant chemical reaction s can benefit in the case, that one uses stoichiometric relations to estimate consu mption of etchant or determine the etch rate from the amount of etching products. As a matter of fact, all recipes used for track etching in polymers are either degra dation catalyzed by base or oxidation promoted by base or acid. In the former c ase ether or ester bonds in the polymer backbone are destroyed. In the latter case the oxidative degradation provides cleavage of more stable carbon-carbon bond s in the main chain. Carboxylic end groups readily react with alkali and form carboxylates. Hydroxylic ones are less acidic but may also react at high alk ali concentrations. Apart from COOH and OH groups, unsaturated bonds are important for oxidative etching. The alkaline etching of PET is the sequence of clea vages of ester bonds: - ~ O-CH 2-CH 2-O-CO-C6H4-CO ~ + OH  - ~ O-CH 2-CH 2-OH + O-CO-C6H4-CO ~ (1.43)

Final products are the ethylene glycol and the anio n of terephthalic acid. Sodium or potassium salts of terephthalic acid have a low in alkaline etchants. The solubility decreases rapidly with increasing th e alkali concentration. The terephthalates precipitate when a sufficient concen tration of the anion is accumulated in the etching solution [1.1]. In the Table 1.3. are given the recommended etchin g conditions for the polymers, which were used in the present research.

An ion track can be revealed if the etch rate along the track, VT, is larger than the bulk etch rate VB (see Fig. 1.46). The bulk etch rate is the rate of dissolution of the non-disturbed material outside the track. The t rack to bulk etch ratio, V =

VT/V B, depends on the damage density in a narrow zone aro und the ion path. In turn, the damage density is roughly proportional to the deposited energy density along the track. As a first approximation, the stopping power can be taken as a 83

Table 1.3. Recommended etching conditions for the i nvestigated polymers [1.1].

Polymer Etchant, temperature, time Ref. Polyethylene 6.25 mole/l NaOH, 70 oC, 10 min [1.43] terephthalate (PET) o Polyimide (PI) 25% KMnO 4, 100 C, 8 h [1.43] NaClO (10% Cl), 70 oC, hours [1.70] o 30% H 2O2, 100 C, hours [1.71] [1.72]

Fig. 1.46. Initial stage of track etching in an isotropic medium (after Ref. [1.1]). The shape of etch pit is determined by the etch rate along the track, V T, and the etch rate of bulk material, V B. Selective etching is possible if the dissolution rate of material within the track core (diameter d T) is larger than V B.

84 measure of the damage density. However, numerous ex periments have shown that the V vs. dE/dx curve ( dE/dx is the energy loss), measured for a certain ion, i s not a single-valued function. Faster ions produce track s of lower etchability than that for slow ions of the same dE/dx. The maximum dam age density is observed at a lower energy than would be inferred from the dE/d x(E) function. This is due to the fact that the fast ions yield a higher fraction of fast electrons that leave the track core and do not contribute to the damage. The temperature, etchant composition and concentrat ion of chemicals are variables that allow one to control the etch rate a nd the change the ratio between

VB and V T. The reaction rate constant, ks , characterizes a complex interaction between etchant and various species formed in the i on track. The interaction depends on specific properties of particles partici pating in the reaction, e.g. ion size, ion mobilities, etc. This results in differen t values of V T/V B for different etchants used for the same particular substance. The temperature dependence of the reaction ra te constant typically obeys the Arrhenius law:

0 - E k/ b T (1.44) k s = k s e

0 Here k s includes a factor controlled by frequency and a fa ctor controlled by geometry; E is the activation energy, kb is the Boltzmann constant, and T is temperature.

Accordingly, experimentally measured V T and V B are normally exponential functions of the inverse temperature, with the acti vation energies E T and E B, respectively. If E T = E B, the sensitivity does not change with increasing temperature, and vice versa, if E T ¹ E B, the track to bulk etch rate ratio can be varied by changing the etchant temperature. For PET an increase in the etching temperature typi cally improves the track to bulk etch ratio [1.73]. In this polymer, the differ ence between E T and E B depends on the stopping power. The heavier the ion, the hig her is the activation energy for track etching. At high etchant temperature and conc entration, local build-up of etch 85 products in the tracks sets a limit on the etch rat e. The solubility of the etch products (sodium terephthalate) decreases drastical ly at high NaOH concentrations. Precipitation of the etch products within an etch p it results in the change of the apparent activation energy of track etching, as it is seen from changing the slope of the logV T vs. 1/T curve. Radiolytic products in heavy ion tracks are chemica lly active species, undergoing post-irradiation reactions, such as reco mbination, disproportionation, oxidation, photo-oxidation, etc. [1.74]. Some of th e reactions are very fast; for this reason the environmental conditions play a crucial role not only after irradiation, but also during the irradiation. Temperature and am bient atmosphere are the most important factors for the latent track formation i n polymers. The presence or absence of oxygen affects the radiation effects in organic materials and formation of etchable tracks as well. Apart from oxygen, othe r gaseous agents, such as humidity or NO may increase the sensitivity [1.75, 1.76]. Undesirable desensitization of tracks can arise from irradiatio n in vacuum. Depending on the duration of stay of polymeric samples in a pumped i rradiation chamber, the track etch rate can be altered to a greater or lesser deg ree. Thus, individual tracks in the samples, exposed to different ion track densities ( at a fixed ion flux), may have different etching characteristics. The track-etch response sensitivity of polymers (e. g. PC, CR-39, PET, PP) generally decreases with increasing temperature dur ing irradiation [1.77]. However, the track etch rate vs. registration tempe rature dependence can be substantially non-monotone [1.78]. The most pronoun ced changes in the track-etch response occur at temperatures around relaxation tr ansitions [1.79]. A considerable effect can be observed even at temperatures close t o room temperature. The correlation between the relaxation transitions and the alterations of V T is the evidence for the strong influence of mobilities of molecular and radiolysis intermediates on the track formation in polymers. T he balance between chain scissions, repair of cleaved bonds, cross linking a nd oxidation processes changes with changing temperature during irradiation and af ter irradiation. 86

Heating of the ion-irradiated polymers leads to fad ing of the latent tracks. At a certain temperature the tracks disappear completely and cannot be developed any longer by etching. However there are some polymers showing an “anomalous” behavior. An example is , in which a m oderate heating enhances the particle tracks, presumably due to thermo-oxidation [1.80]. The PET foils, used in the present research, were e tched by the solution of 3 mole/l NaOH at the temperature 45 °C, with subsequent rinsing in distilled water. The original PET is a rather hydrophobic material w ith a water contact angle of about 72º and hence hardly uptakes water, so that b oth a sol-to-gel transition and swelling do not occur [1.81]. The PI foils, used in the present research, were e tched by a NaOCl solution of pH = 10 at the temperature 70 °C with subsequent rinsing in distilled water [1.82]. As well as PET, PI is a hydrophobic material, so t hat swelling processes were also not observed here. Besides etching of polymer foils, during the presen t investigation we had a task of etching ion tracks in thin layers of SiO 2 and SiON on a silicon substrates. In general, a very efficient chemical for this purpose is hydrofluoric acid (HF). Still, if used directly such etchant may have a too fast a nd aggresive action on the oxide and oxynitride layers. For such reason, HF is unive rsally used as a "buffered" solution, which can keep the etch rate low and cons tant, by moderating the PH level of the bath. This allows the etching time to be reliably correlated to the etching depth. The industry standard buffered hydrofluoric acid so lution (BHF) has the following formulation:

6 volumes of ammonium floride (NH 4F, 40% solution) + 1 volume of HF

This can be prepared, for example, by mixing 113 g of NH 4F in 170 ml of H 2O, and adding 28 ml of HF. The etch rate at room tempe rature can range from 1000 Å/min to 2500 Å/min. This depends on the actual den sity of the oxide and oxynitride. The following etching reaction holds:

87

SiO 2 + 6HF --> H 2SiF 6 + H 2O (1.45) where H 2SiF 6 is water soluble. Sometimes the BHF is prepared and stored according to the formulation given above, but it is 7:1 diluted in water just before b eing used. This allows an even better control of the etching rate. It is good prac tice to use the diluited BHF only once and then discard it, in order to provide the p rocess repeatability. Another popular etching formulation is the so-calle d "P-etch":

60 volumes of H 2O + 3 vol. of HF + 2 vol. of HNO 3 , that is:

300 ml of H 2O + 15 ml of HF + 10 ml of HNO 3 . The "P-etch" action is strongly dependent on oxide density, as it results from the growth technique. During the present investigations, we have observed , that the etching process preferentially attacks the ion-irradiated zones, bu t leaves both the unirradiated oxide (and SiON, respectively) and also the silicon rather unaffected. This means, that in our experiments the water solutions of HF h ave behaved as quite effective etchants for our purposes, so at this stage of rese arch we did not need to prepare special etchant solutions.

Ion tracks in the SiO 2/Si samples were etched by 4 at% water solution of the hydrofluoric acid (HF) up to 10 min, and subsequent ly rinsed in acetone, alcohol and distilled water [1.83]. For the etching of the tracks in the SiON layers, an etching time of 7 min and a concentration of 40% HF were used to obtain the desired ion track diameter.

88

1.5.3. PROPERTIES OF THE ETCHED TRACKS

Ion track etching process can be effectively contro lled by optical microscopy, scanning electron microscopy (SEM) and ion transmis sion spectrometry (ITS). For example, Fig. 1.47a shows some ITS spectra of 11.4 MeV/amu Pb ion tracks in PET, as measured at different stages of track etchi ng, and Fig. 1.47b illustrates the track shapes as reconstructed from the spectra by m eans of a Monte Carlo program (more details on this concern are given in the Chap ter 1.2.4). From the figure it is seen, that the general etched track shape is usually hyperbolic, which has

a

b

Fig. 1.47. ITS spectra during different stages of track etching (a); reconstructed track shapes (b). In the reconstruction the track has been approximated by a superposition of cones of different shapes. One needs to note also the thinning of the foil due to bulk etching [1.84].

89 been approximated here by the superposition of a nu mber of conical structures. Usually etched tracks can be described by two param eters: the outer track diameter (at the foil surface) and the inner track diameter (at the position where the track is narrowest). The evolution of these paramet ers with etching time is illustrated in Fig.1.48. The longer the etching tim e, the more does the inner track diameter aproach to the outer one, i.e. the more cy lindrical do the tracks become. In this connection one needs to note, that differen t analytical techniques are sensitive to different track parameters. Whereas op tical microscopy and SEM records one of the outer pore radius, ITS gives inf ormation about the changes of the whole track shape. All approaches neglect the s cattering δr of the pore sizes that increase with increasing pore radius r, due to statistical variations of the etching process and the experimental conditions.

In the micrometer scale, the two etch rates, V B and V T, are sufficient to describe the shape of an etch “pit”. The formation of an etc h cone around the ion path can be easily understood when the Huygens’ principle is applied to the track etching process. The process can be considered as a superposition of elementary time- delayed waves emitted from the moving end poi nt of the etched track as it proceeds into the depth of the polymer bulk. The wa ves merge to an envelope that corresponds to the actual shape of the etched track . A satisfactorily quantitative description of the track shape can be accomplished only with the knowledge of the complete form of the V T vs. ion range function. For increasing, constant, and decreasing track etch rates, the shape of the etch pit is concave, conical, or convex, respectively. Situations when V T may be approximated as a constant are generally restricted to small layer removals and to track por tions of higher energy ions where the damage density is only a slowly varying functio n of the range. Directly measurable quantities – the track diameter on the surface D and the visible track length l – are the result of the competition between the tw o etch rates and can be represented in terms of these parameters :

l = (V T – V B) t (1.46)

90

Fig. 1.48. The development of inner and outer track diameters during the etching process. This figure refers to the tracks characterized in Fig. 1.47 [1.84].

1/2 D = 2 V B t [(V –1)/(V + 1)] (1.47)

As the characterization of etched tracks in the pol ymers studied in this research were already considered quite in detail in the Chap ter 1.2.4, we will present here just some results of PI etching, as this material i s most important for this research. Etching kinetics on example of the PI foil (its etc hing conditions were given in the Chapter 1.5.2) with concern to the ion track radius change can be observed in Fig. 1.49. One can see that the form of the "tail" on th e ITS spectra changes with the increase of etching time, until it obtains a peak at 5480 keV, which means that the track is etched through, so that a pore is create d. Dependences of corresponding ion track radii values, as measured by ITS and opti cal microscopy are presented in Fig. 1.50. One can see here some correlation with t he dependences in Fig. 1.48 for the case of PET etching. A good correlation between ITS data and optical microscopy data also can be observed in the Fig. 1. 50. But in the case of PI etching, given in this figure, the increase of the track radii value, which is quite fast on the first stage of etching, then (after approx imately 165 minutes) slows down, showing some saturation, which is different from th e data in Fig.1.48. 91

1

150 min

0.1 Counts 105 min

0.01 45 min 30 min 60 min 120 min

3000 3500 4000 4500 5000 5500 Energy (KeV)

Fig. 1.49. Ion track opening with etching of the irradiated PI foil.

4000

Optical measurements

3000 ITS measurements

2000

1000 Track radius (nm) radius Track

0 120 130 140 150 160 170 180 190 200 Etching time (min)

Fig. 1.50. The development of the inner track radii (ITS measurements) and the outer track radii (optical measurements) with etching in the irradiated PI foil.

92

Some examples of etched tracks in PET and PI, obtai ned in our research are presented in Fig. 1.51 and Fig. 1.52. Irradiation c onditions of the foils are given in the Chapter 1.3.2. Though metals were deposited in the tracks, their form is clearly seen: etched tracks in PET have cylindrical form, a nd etched tracks in PI have hyperbolic form, as the foils were etched simultane ously from both sides.

a b

Fig. 1.51. An example of cylindrical tracks in a polymer foil: tracks in irradiated and etched PET foil, with subsequent chemical (electrodeless) deposition of Ag (SEM images): (a) top view; (b) side view;

a b

Fig. 1.52. An example of hyperbolic tracks in a polymer foil: tracks in irradiated and etched PI foil, with subsequent chemical (electrodeless) deposition of Cu (SEM images): (a) top view; (b) side view; 93

Examples of etched tracks in irradiated SiO 2 layer (conical tracks) and irradiated SiON layer (cylindrical tracks) on Si are presented in Fig. 1.53. Irradiation conditions of these materials are given in the Chap ter 1.3.2.

a

a b

Fig. 1.53. Examples of etched tracks in silicon-based structures (SEM images) : (a) conical tracks in irradiated and etched SiO 2 layer (top view); (b) cylindrical tracks in irradiated and etched SiON layer (side view);

94

1.6. DEPOSITION TECHNIQUES APPLIED FOR THIS STUDY

In principle, etched tracks can be filled with any material for obtaining new nanostructures. The tracks can either be filled com pletely or partly (see Fig.1.54). In the first case, one obtains cylindrical rods (wi res, fibres) of the embedded matter. Partial filling can either signify that a r od is not embedded along the whole track length, or that a tubule is embedded, so that the central track zone is still free of matter. The filling of the tracks can either be accomplished from the gaseous phase or from the liquid phase. In the first case a solid residue can be deposited from a gas decomposing within the tracks (chemical vapor deposition - CVD), one can evaporate matter onto the inner track walls, or it is possible to use plasma deposition processes [1.85]. In the second case, the penetrating liquid can be s ubsequently solidified (fully or partly), e.g. by evaporation of a solvent, or solid matter can precipitate from that liquid. This precipitation inside the etched tracks can be initiated chemically or galvanically. In the case when one needs to create tubules of the penetrant, the inner etched track walls have to be wetted to enabl e a good adsorption. That adsorption can be improved considerably by addition al physical or chemical nucleation centers to initiate bonding reactions (c hemisorption). Before working with very narrow etched tracks, one should verify whether these objects really still exhibit open space after their preparation. Capillary contraction is a well-known effect of “healing” of narrow pores during drying of microporous objects impregnated with a well-wetting liquid in t he usual conditions. The drying process reduces the etched track radius r by about 5 nm, for whatever r. This means that pores below 5 nm to 7 nm radius virtually vani sh during drying, and a second etching of a dried microporous sample virtually beg ins from zero track radius. This track compression is not a surface effect but a bul k one [1.86, 1.87]. Hence, in order to avoid that effect, one should not dry the tracks before their manipulation starts, but store them, for example, in distilled w ater.

95

Fig.1.54. Possibilities of the ion track filling

Now, some deposition techniques, which are importan t for this research, will be considered here.

1.6.1. TUBULES FORMATION BY EVAPORATION

Usually an evaporation source emits atoms or cluste rs isotropically within a given solid angle. Particles originating from the e vaporation source and being emitted from a point along the track axis with traj ectories within a solid angle trans will be transmitted through the tracks, particles e mitted outside the solid angle dep will be deposited on the microporous foil surface, and particles within dep and

trans will be deposited on the inner track walls, thus f orming tubules (Fig. 1.55, a). As the mathematical formulation of this problem is exactly complementary to that one of ITS, including the emission from non-a xial points of origin, we refer to the equations derived there (see the Chapter 1.2.4) . 96

b

Fig. 1.55. Illustration of tubule production by evaporation: (a) evaporation point source aligned with cylindrical tracks, (b) tilted angle geometry [1.85]. In (a) the dashed arrows correspond to trans and the solid arrows correspond to dep . The arrows in (b) illustrate, up to which depth layers can be deposited within etched tracks. (c) In case of conical tracks with sufficiently large opening angle – leading to large angles ζ of impact of the atomic/molecular beams - thick layers can be evaporated up to great depths.

The tubule layer thickness is determined by the cos ine of the angle of incidence ζ between the atomic/molecular beam and the surface normal of the inner track wall. As for narrow cylindrical tracks, the maximum angle ζmax of incidence that is -4 reached for dep , is usually very small (e.g. ~3 ´ 10 degrees in case of a track with 1 m diameter in a 10 m - thick foil at a distance of 10 cm from the evaporation source), only a negligible tubule thick nesses can be obtained in this case. In the given example, a tubule wall thickness of only ~ 1 nm would be obtained when at the same time already ~ 3 m would have been deposited on the 97 foil surface. Therefore, this evaporation geometry has not gained any importance for cylindrical tracks. The situation improves when evaporating under tilte d angles (Fig. 1.55, b), or when evaporating onto conical tracks (Fig. 1.55, c) . For example, evaporation under 45° enables one to obtain tubule thicknesses equal to the layer thickness on the foil surface, however only down to depths corre sponding to the track diameter, and only in one direction. This approach is useful for producing electrical contacts of e.g. sensor material embedded within the etched tracks. In order to obtain circular contacts, either the evaporation source or the sample have to be rotated against each other during evaporation. The eventual disadvantage of evaporating matter int o etched tracks due to angular obstacles is compensated by the great advan tage that nearly any material (such as B, Al, Si, Cr, GaAs, high temperature supe rconductors, etc.) can be deposited in this way - in contrast to the restrict ions of galvanic or chemical deposition. Also, evaporation can be considered as being a more contamination- free process than liquid deposition techniques. However, one should keep in mind, that metal deposi tion from the vapor phase onto polymers is usually accompanied by some metal diffusion into the plastics with subsequent metal precipitation [1.88], i.e. on e might expect the build-up of a “halo” of metallic nanoparticles within the polymer ic environment of the formed tubule. To avoid this effect, it is possible to emp loy: (a) the target cooling, and (b) the fast evaporation techniques, so that metal clus tering dominates the deposition process, and in the case of slow evaporation, the i ndividual atoms rather diffuse into the matrix. One needs to be also aware, that deposition of matt er by evaporation leads to deposition of heat on the sample. In case of excess ive evaporation speed this may lead to intolerable target heating especially for s ensitive polymers such as PET or PC, eventually resulting in track deformations. The refore PI foils are recommended as track carrier materials [1.85]. 98

If the track back side is covered by e.g. a foil or a Si wafer, the transmitted particles will be deposited thereupon, thus forming nanodots, with their diameters equal to the track dimensions. This may be useful f or electronic applications.

1.6.2. GALVANIC DEPOSITION OF MATTER ALONG ETCHED TRACKS

Under the influence of a potential applied to an el ectrode immersed in an , ions in the vicinity of that electrode surface are rearranged, which leads to the formation of an electrical double layer, th e so-called Helmholtz layer, that is further joined by a diffusion layer. Cations migrat e through these layers towards the cathode that is covered by the microporous memb rane. In that way, the galvanic deposition of conducting or semiconducting matter (metals, conducting polymers, semiconductors) inside insulating ion tra cks or other porous matter leads to the formation of conducting rods (wires, fibrill es). The formation of massive templates (e.g. of conducting polymers) as thin as 3 nm in diameter has been reported [1.89]. In this way, nanofibrilles of PPy , PMT (poly(3-methylthiophene), PAni, Cu, Ag, Au, Ni, Pt [1.90], Fe [1.91], Co [1.9 2], and FeCo [1.93] have been produced – in the latter case with oxygen-free nitr ogen as a protective gas. Careful performance of electrodeposition results in produci ng of crystalline structures. The structure of metallic deposits is usually columnar or fibrous along the growth direction. If their reduction potentials are suffic iently far apart from each other, one can electrodeposit separately several atomic specie s from one bath within an etched track. In this way, Cu/Co multilayer wires w ere produced [1.94]. Some examples of the structures, which can be obtai ned by the galvanic deposition, are given in Fig. 1.56.

99

Fig. 1.56. Some examples of the structures, which c an be obtained by galvanic deposition (courtesy of the SDK-Technik Gm bH).

1.6.3. DEPOSITION FROM A SATURATED SOLUTION

Formation of the tubules of fullerite (i.e. the sol id state of fullerene, C 60 – see Chapter 1.1.3) in the etched tracks was carried ou t by a heterogeneous precipitation of fullerite from a saturated solutio n in benzene or toluene onto a microporous foil, where it was deposited in th e form of very smooth thin layers, see Fig. 1.57 [1.95 - 1.97]. These s tructures were used in the present

a b

Fig. 1.57. Fullerite-covered etched tracks in a PI foil (5 layers deposited) [1.96].

(a): view from top, (b): side view onto a fractured edge. The C 60 is seen to be deposited as a smooth layer onto inner and outer polymer walls, with occasional larger-size precipitates. 100 research for creation of sensors (this will be cons idered more in detail in the Part 2 of this work).

Some additional larger fullerite precipitates are found within the etched tracks as well. This deposition process can be repeated multi ple times, with the C 60 tubule resistivity gradually decreasing. However, for more than about 10 deposition cycles, the layer homogeneity deteriorates, resulti ng in less efficient resistivity improvement per deposition step.

1.6.4. ELECTRODELESS OR CHEMICAL DEPOSITION

Gases and liquids can be incorporated in etched ion tracks to initiate there chemical reactions for practical applications. The corresponding reactions can be chemical deposition processes, electrochemical reac tions, polymerization reactions, or just any other chemical reaction [1.8 5]. Chemical deposition comprises deposition from the vapor phase (CVD) and deposition from the liquid phase. The latter is subdivided into ElectrodeLess Deposition (ELD) and Chemical Bath Deposition (CBD), which is also denoted as Che mical Solution Deposition (CSD). In order to form continuous layers of precip itates on the track walls via reduction of the penetrant, it is additionally nece ssary to provide a sufficient areal density of nucleation centers. The ELD technique is widely used for metal depositi on in the sense that it is a chemical process, which does not involve electron e xchange with a conducting substrate as in the electrodeposition technique, an d therefore the ELD does not require conducting substrates. The ELD is an oxydo- reduction process, the presence of the reducting element in the film being a side effect. In the CBD there is no change in the oxidation state of the elements in solution and in the film. They are deposited together as in precipitation reaction s. Basic requirement in the CBD is that the material of interest precipitates from a solution. The nucleation mechanisms in solution belong to two categories [1.98]: 101

(1) Homogeneous nucleation . Here a new phase is formed in a solution which originally did not contain any foreign phase. (2) Heterogeneous nucleation. Here a foreign interface is present on which the growth proceeds selectively. For a homogeneous nucleation, the change in Gibb´s free energy G accompanying a phase transition between n molecul es in a solution and in a cluster is composed of a term, describing the chemi cal potential in the solution and in the solid, and of a term, which describes the va riation of the surface energy. G is also the activation energy of that homogeneous n ucleation process. At equilibrium the chemical potential in the solid equ als that in solution, from which a critical cluster size can be determined [1.98]. The nucleation rate depends sensitively on both the supersaturation ratio S * of the solution and the surface tension σ at the solution/solid interface. Below a critical value of S * (around 12) the nucleation rate drops to very low values. Also an increasing σ leads to markedly decreasing S *. The surface tension can be tailored through modif ication of the pH value, the ion nature and by additives, a s it is related to the adsorption of species at the surface. When a foreign surface is additionally present, the surface tension at the interface between substrate and the precipitating n ucleus has to be considered additionally. This additional term reduces the over all surface energy, needed for the nucleation process. This means, that in presenc e of a foreign substance, which matches with the substrate material, the nucleation energy barrier tends to be suppressed. As a consequence, heterogeneous nucleat ion will, in principle, take place once S * exceeds unity, hence at values of S * far below those ones required for homogeneous precipitation. In the case of heter ogeneous growth from the vapor phase the growth mode of the deposit has to b e related to the interface tension between the two materials. For very low ten sion there is no nucleation barrier and the growth takes place via a two-dimens ional layer by layer mode [1.98]. 102

The more stable is the precipitate, the more negati ve is G and the smaller the solubility product. As many of the materials of int erest in chemical deposition (metals, chalcogenides, etc.) are highly insoluble, very low concentrations of their ions will lead to high supersaturation and hence to homogeneous precipitation. Free ions generally form complex species with ligan ds present in solution – e.g. in aqueous solution OH - for metal cations, or H + for chalcogenide ions. By selected complexing agents, such as NH 3 , additional ligands can be introduced. As a consequence, additional chemical equilibria between ligands and the ions to be precipitated have to be considered, which leads to their increased solubility after complexation [1.85]. The ELD allows for more uniform metal deposition th an electrochemical techniques [1.99]. Such metals as Cu, Ag, Au, Pt, a nd Ni can be chemically deposited. As this procedure is quite common in tod ay´s technology, even commercial solutions for ELD are available [1.100]. Recently the first ELD of metallic alloys such as Co-W-P, Ni-W-P, and Ag-W ha ve been reported [1.101].

If polymers with etched ion tracks are taken as sub strate material, the ELD processes of e.g. metals or chalcogenide compounds lead to their deposition on the etched track walls, which signifies the production of tubules of that matter. For a chemical deposition process, the ratio of the deposition speed of matter onto the inner track walls to the deposition speed onto the outer surface of the foil in which the tracks are embedded will depend on the ratio of the speed v penetr of penetration of the precursor liquid into the tracks to the speed v surf of arrival of the metal ions at the foil surface. For v penetr » v surf , the tubule thickness will be comparable to the layer thickness on the foil surfa ce; for v penetr << v track the tubule growth will be negligible, and the deposition of ma tter will block the nanopore entrance. Therefore, high penetration speeds are im portant for the ELD process [1.85]. Depending on the amount of material deposited on th e etched track walls, one can either form a continuous film or discontinuous islands. In the first case, which occurs after prolonged exposure to the depositing c hemical reaction, massive self- 103 supported tubules emerge, which can be isolated by dissolving the polymeric host matrix in a suitable solvent. In the latter case, w hich emerges after short-time exposure, the dispersed deposited islands of matter on the ion track walls may allow e.g. to tailor devices with specific electron ic transport properties such as given by two-dimensional thermionic electron emissi on, electron tunneling or percolation. This might be exploited e.g. for the u se as pressure or hydrogen sensors. The production of nanotubules is still at its begin ning. Though Cu, Ag, Au, Pt, Ni nanotubules [1.90] and tubules of conducting pol ymers have already been prepared, tubules of other type, e.g. the ones base d on semiconducting compounds, have not yet been formed. Here a great application potential is still abundant. Table 1.4 gives an overview of some of the recipes for de position of metals and conducting polymers for tubule production. Some examples of Ag and Cu nanotubule templates, cr eated by the ELD during the present research, are given in Fig. 1.58 and F ig. 1.59.

Basic requirement for any CBD or ELD technique is a good wetting of the polymer surface, followed by sensitization and acti vation, i.e. the formation of suitable nucleation centers on the etched track wal ls. The most common activators are Sn or Pd metal atoms, attached chemically to th e polymer surface, or dangling bonds introduced via ion or laser irradiation. For the case of Pd activator, let us 2+ - consider as an example the sensitizer complex [Pd ( NH 3)4] (with e.g. Cl as counterion) which, if reduced by a suitable agent, can bind the Pd atom to the polymer via complexation with surface amine, carbon yl, and/or hydroxyl, thus forming groups, such as COOPd.

The sensitized membrane is then activated by exposu re to Ag + [1.102], Sn 2+ , or Pd + [1.31], resulting in the formation of discrete meta llic particles on the polymer surface. Thereafter, the sample is immersed into a Cu, Ag, or Ni plating bath, containing the metallic ions as a reducing agent wh ich results in metal plating on all foil surfaces, including the etched track walls , according to:

104

Table 1.4. Some recipes for electrodeless deposition of metals within ion tracks.

Depos. Recipe PH Temp. Exposure time Ref. Matter value [°°°C] [min] Sensitization 0.2% SnCl 2 in 0.02 M < 7 RT - [1.103], + HCl [1.31] activation step 0.025 mole/l SnCl 2 in - RT 45 [1.104] 0.07 mole/l trifluoroacetic acid. SnCl 2 + 1,3 dihydroxy 5.5 43 ± 7 10 [1.100] benzene + HCl +

[Pd (NH 4)4]Cl 2

Cu Cu 2SO 4 + >7 24±4 - [1.100] formaldehyde + NaOH Deposition speed: 2 m/h Ag Aqueous ammonical >7 ≈20 5-15 [1.100] AgNO 3 solution + reducting agent (glucose, formaldehyde, or sodium hypophosphite) Au K-Au cyanide 6±0.2 85±5 10-20 [1.100]

Commercial Au plating 12, and 5 [1.104] solution + 0.127 mole/l step- Na 2SO 3 + 0.625 mole/l wise formaldehyde + 0.025 de- mole/l NaHCO 3 crease to 10

Ni Ni 2SO 4 + Na acetate 5±0.1 88±1 [1.100] + various agents Deposition speed: 15-20 m/h 105

Fig.1.58. Ag templates in etched tracks of 1 GeV Ar ions in PET

Fig.1.59. Cu templates in etched tracks of 250 MeV Kr ions in PI

II I IV 0 Sn surface + 2 Cu solution  Sn surface + 2 Cu solution (1.48) for the case of Sn activation. Instead of producing the nucleation centers chemica lly, one also can prepare them by laser or ion irradiation [1.105]. For examp le, an energetic laser beam impinging onto a PI surface embedded in an oxidizin g wet medium can activate a ring structure so that oxygen is attached, the latt er enabling the metal addition [1.106] according to the scheme: Cu O O

Ion irradiation is known to modify the polymeric su rfaces chemically and physically [1.107]. Therefore, systematic ion irrad iation experiments have been performed both with very low energy ions at high fl uence from plasma [1.108] and 106 with ions of slightly higher energies (30 keV) [1.8 5]. The experiments indicate that, as a result of nucleation center formation, a n enhanced metal/polymer adhesion takes place. Whereas chemical activation t echniques and laser irradiation activate the etched tracks over their whole length (~10 m to ~100 m), ~50 keV - 500 keV ion irradiation activates the etched track only up to the range of these ions (typically ~ 0.1 m to ~1 m). This enables axial tailoring of track structure s, so that e.g. circular contacts for sensor materials em bedded in the tracks emerge, which guarantees full accessibility of the sensors to the ambient. If no nucleation centers are offered, chemical depo sition will take place only at intrinsic polymeric surface defects, which signifie s the growth of only a few dispersed (semi)conducting nanoclusters on the ion track walls. Though in general regarded as being adverse, this effect can also be beneficial. For example, layers of dispersed Ag nanoclusters (nanocrystallites) on inn er track walls show a high resistivity, which can be tailored to any value by the choice of the Ag precipitation conditions which determine crystallite size and dis tance [1.109]. It is important to note, that a sequential galvanic deposition of different materials leads to axially structured elements, and a sequential chemical deposition leads to radially structured elements [1.41]. For e xample, sequential galvanic deposition of differently semiconducting materials, or of metallic and semiconducting wires (e.g. Cu and Se [1.110, 1.111] , or Ni and CdSe/CdTe [1.112] ) give rise to nanometric diodes. Sequenti al galvanic deposition of different ferromagnetic materials may lead to the g iant magnetoresistance [1.113]. Sequential chemical deposition of different semiconductors, or of metal s and semiconductors should lead to the formation of conc entric diodes, and metal/insulator/metal structures should give concen tric condensors.

107

PART 2. ION-TRACK-BASED DEVICES FOR MICRO- AND NANOELECTRONICS

2.1. DEVICES ON THE BASE OF POLYMERS

2.1.1. SPECIFIC FEATURES OF THE ION TRACK MANIPULATIONS

2.1.1.1. METAL TUBULES FORMATION IN TRACKS

The formation of durable metal layers on polymeric substrates requires the existence of nucleation centers on the polymer surf ace where the metal atoms can precipitate. As for the highly advanced commercial polymers used in the present work, the areal density of the naturally abundant i ntrinsic surface defects which allow metal precipitation and nucleation is far too low to enable the formation of a continuous metallic layer, we introduce nucleation centers by chemical attaching of suitable metal atoms, e.g. Sn, Pd, onto the polymer ic surface (see Chapter 1.6). It has been shown [2.1], that already a few seconds ex posure to a corresponding commercial activation solution gives maximum covera ge of the inner etched track walls with nucleation centers, whereby the metal tu bule quality (as determined by its electrical conductivity) increased by several o rders of magnitude. First experimental series of this kind have been ma de by our group on the silver deposition in a PET matrix with ion tracks [2.2]. R esults of these investigations are summarized in the Table 2.1. For comparison with li terature, silver was chosen as the tubule material. As the previously used [2.3 - 2.8] commercial plating baths contain additives which make a detailed understandi ng of the results difficult, we follow here the classical recipe of silver mirror p roduction [2.9] which was introduced by the French company St. Gobain at the end of the 19 th century [2.10].

Here AgOH is formed by the reaction of AgNO 3 with NaOH. Then just as much concentrated NH 4OH solution is added as is necessary to diss olve all the insoluble hydroxide . In the process of the Ag deposition [2.2] (see also Chapter 1.6), the same amount of concentrated glucose solut ion was added to the 0.2 mole/l 108

Table 2.1. The variation range for the most basic production parameters of Ag tubules in this work (time t SnCl2 of SnCl 2 pretreatment, temperature T depos and time t depos of the Ag complex salt solution exposure). Comparison of SEM and ITS results of this work is made with XRD and AFM results, within the track interior and on the micropore foil surface. d SEM , d ITS : thickness of Ag tubule wall according to SEM and ITS determinations, respectively. Elongated precipitates are characterized by their extensions along the radial (E r) direction and the track (E t) direction [2.2].

SnCl 2 Ag Thickness of Average Ag crystallite size pretreat deposition Ag layer ment time Tdepos tdepos dSEM dITS Inside tracks On foil surface tSnCl2 XRD SEM SEM AFM [°C] [nm] [nm] [nm] [nm] [nm] [nm] [min] 0 5°± 2° 3 days 50 - 30.4 ±3 - - -

0 20°±2° 30 min 40 100 - ~40 ~50 ~50

100 40 100 - ~50 - - min 50 120 24.3±3 Er:50-100 ~40 ~50 3h Et:150- 200 1 1h 40-50 - - 40-50 ~100 -

5 1h 40-50 - - 40-50 ~60 -

10 1 min - 0±20 - 20-30 - - 2 min - 55 - 40-60 - - 5 min - 80 - 40-60 50 - 10 min 40-60 95 - 40-60 200 - 1 h ~150 190±10 - 40-200 ~60 - 0 40°±5° 1 h ~150 - 26.0±3 ~200 ~200-300 ~140 ~200 - - - -

to 0.3 mole/l silver complex salt solutions at the beginning of each deposition experiment. The Ag-processed samples were rinsed in water and dried. As long as none-derivatized Ag-processed samples were still we t, the silver deposited on the unirradiated surface regions of micropore foils cou ld be easily removed by just wiping over them with a soft cloth, due to a low st icking efficiency. For a contrast, 109

silver deposited within the etched tracks, or silve r deposited onto SnCl 2-pretreated micropore foil surfaces cannot be removed that easi ly. In this investigation, we have characterized the fo rmation of silver tubules by ITS, SEM, and by XRD examinations (see the Table 2. 1). Some atomic force microscopy (AFM) results, which are included in the table, just reconfirm the SEM data. We have studied the evolution of silver mirror form ation in both untreated and

SnCl 2-pretreated etch tracks. In both cases silver cryst allites nucleate on the track walls, thus giving rise to Ag nanotubule formation. The difference results from the areal density of nucleation centers for silver crys tallite growth, which is lower in the untreated case than in the SnCl 2 - derivatized one. This signifies in the latt er case a rapid merging of the individual silve r crystallites to a continuous thin Ag layer already after short exposure time to the A g agent solution. For contrast, the few isolated Ag crystallites in untreated track s have to grow up to sizes comparable with the track diameter, or even larger than that, before they eventually can touch each other, and merge. Therefore we find in our measurements on pre- treated tracks a rapid onset of metallic conductanc e after exposure to the Ag salt solution, whereas processed but untreated tracks ne ver obtain good conductivity. Let us first consider the SEM results of non-deriv atized foils at ambient temperature (Table 2.1). After 30 min exposure, onl y a few Ag crystallites (typically, 3 ´ 10 10 cm -2) have been formed on the surfaces of untreated mic ropore foils at room temperature (R.T.), illustrating the low areal density of nucleation centres in none-derivatized foils. The growth of th ese silver precipitates reduces the pore transparency. All crystallites inside the tracks grow only on the track walls, so that silver nanotubules instead of massiv e Ag wires emerge. Even after 3 hours exposure at R.T. the tracks are not completel y filled and individual tracks are still visible in the SEM images. Etched tracks with originally 300 nm diameter now show up as 50 nm large holes, due to coverage with silver precipitates of 50 nm to 100 nm radius. This signifies considerable inner su rface roughness and irregular pore shapes. 110

Variation of the Ag deposition temperature between 5º C and 40º C does not markedly influence the sizes of the observed silver precipitates but only their deposition rate. A pronounced difference between SE M and XRD results (see Table 2.1) indicates, that the small Ag precipitate s identified in the SEM images are polycrystalline, i.e. are composed of several A g crystallites each. This explains the absence of any major cubic shape of the precipi tates. In order to see what the surface-far regions of the silver nanotubules look like, we tore the Ag-treated foils (3 hours Ag deposition at R.T.) into two pieces, and subsequently dissolved the polymer host matrix comp letely, by immersing the samples in etchant for 3 days at R.T., and then we examined their fractured sides. In the case of non-derivatized samples, it was foun d out, that the remaining Ag skeletons consist of silver deposits on front and b ack foil surfaces, which are connected by Ag tubules wherever the etched tracks were. These tubules consist of individual silver domains of typically 150 nm to 20 0 nm length and about 50 nm to 100 nm radial extension. Most of these domains are only loosely connected to each other and often still separated by narrow gaps. Con sequently, they have been partly lost in the fractured regions. This may explain the relatively low conductivity observed on such foils. The elongation of these dom ains along the track direction indicates that the crystallite growth process in th ese pores is not isotropic, possibly due to the reduced potential energy on the track su rface that favours that growth mode. Within the compact 200 nm to 500 nm thick Ag surface layers of these samples, the tubule openings are clearly seen. In f act, these silver surface layers are responsible for the stability of the silver skeleto ns. If the Ag surface layers are removed before etching away the polymer, then the f ragile Ag skeleton collapses to individual, barely interconnected grains. Next let us consider the Ag nanotubule evolution fo r the case when the nanoporous foils have been derivatized before Ag de position with SnCl 2 solution. From Fig. 2.1 a, it follows, that the density o f detectable Ag precipitates is in the 111

8 2 Fig. 2.1. Ag crystallite growth in etched tracks (2x10 tracks/cm ), SnCl 2 pre- treated for 10 min at RT, 6.8 mole/l. Deposition of Ag from solution at R.T. for : (a) 1 min, (b) 2 min, (c) 5 min, (d) 10 min. SEM view of edge of fractured foil, polymer matrix is not dissolved. The first SEM image is darker than the others, due to still negligible secondary electron emission from the insulating PET [2.2]. order of 1 ´ 10 11 cm -2, i.e. that the nucleation centre density is at lea st 3 times higher than the one without SnCl 2 pre-treatment. When gradually increasing the Ag deposition time the isolated silver precipitates grow in size (Fig. 2.1, b) until they touch each other and form a continuous film (Fig. 2.1, c). Only afte r 10 min deposition time these films have got sufficie nt strength to survive the sample 112 fracturing as tubules. Their inner surface rough ness is now in the order of ~40 nm to 100 nm (Fig. 2.1, d). Longer exposure times lead to continuous growth of the tubule walls, but their complete closure has be en not reached in our examinations as yet. During the ion transmission through micropore foils [2.11], three distinct features usually show up in the ITS energy spectra: a sharp peak of the particles transmitted through the etched tracks with their fu ll energy E 0, a broader peak stemming from the particles transmitted through uni rradiated foil regions, and a spectrum part connecting these two peaks, stemm ing from particles, which have been partially stopped in the polymer, and partiall y have travelled through the etched tracks without any energy loss (see the Cha pter 1.2.4).

A pre-treatment by SnCl 2 does not change the ITS spectra of etched tracks. T his indicates, that SnCl 2 activation affects only a polymer layer of marginal thickness on the inner track walls. Deposition of silver on b oth pretreated and untreated track walls leads to reduction of the direct transmission peak - indicating the narrowing of the remaining pore, and to a shift of the polymer foil transmission peak to lower energies – indicating the silver deposition onto th e polymer foil surfaces. Typical changes of the growth velocities, derived from the ITS spectrum are 80 nm/h inside the tracks and 1.3 m/h on the foil surface, respectively, which reflec ts the reduced accessibility of the silver salt solution t o the tracks, in comparison with the freely exposed surfaces. Deducing from the direct transmission peak areas of the ITS spectra the mean inner pore radius, and plotting the latter as a fun ction of deposition time, we obtain Fig. 2.2. As appears for both untreated and pre-tre ated micropore foils, there exists a threshold of typically a minute, below which no c hange in transmission is recorded, as during this time the Ag crystallite si ze is still comparable with the surface roughness of the etched inner pore walls, t hus preventing any identification by ITS. Subsequently, in both cases the Ag precip itate growth proceeds at about the same speed into the radial pore direction. This is understandable, as the growth of each individual precipitate should not de pend on the areal density of all 113

Fig. 2.2. Correlation between inner diameter of Ag nanotubules and exposure time to silver complex salt solution, for untreated (dashed line) and SnCl 2-pretreated foils (solid line). ITS results. The initial inner track diameter values for both experimental series slightly differ from each other [2.2].

precipitates. The inner tubule radius r track correlates with the deposition time t -0.4 according to an empirical power law: r track » t , which corresponds to the dashed line in the Fig. 2.2. This means, the long er the exposure time, the less silver is additionally deposited. This tendency bas ically describes the depletion of the finite supply of silver salt solution by the ch emical reduction process, but also indicates that the inner tubule diam eter becomes narrower with increasing deposition time, which of course reduces their accessibility. Only at larger deposition times pronounced differen ces show up. Whereas untreated foils continue decreasing gradually in th eir transmission, derivatized foils finally close quite abruptly, see Fig. 2.2. The rea son for this difference must be sought in the differences of Ag crystallite growth. It was found, that Ag crystallites on untreated foil walls extend preferentially along the track direction, with the ratio of longitudinal to lateral extent ranging from appr oximately 1.5 to 3. For contrast, 114 the crystallite growth of Ag on derivatized foils a ppears to be rather isotropic, which would explain the differences in pore closure speed. Here it is worth noting, that this well-controllable pore closure process is applied in technology to obtain metal pores of molecular dimensions. The inner tub ule diameter as derived by ITS is somewhat larger than the one deduced from SEM im ages (Table 2.1), as the protrusion of a few crystallites into the tubule ce ntre leads only to marginal stopping of the probing α particles, but to a clear reduction of tubule visi bility in SEM. One can conclude from these investigations [2.1], t hat without foil pre-treatment with acidic SnCl 2 (“derivatization” or “activation”) for the product ion of an excess of nucleation centres (“molecular anchors” [2.3] ) on the inner track walls, one obtains Ag nanotubules of coarse crystallinity and poor coherence between their individual elongated domains (Fig. 2.3, a). The in crease of nucleation center density by activation enables one to obtain a highe r density of more spherical precipitates, with consequently well-coherent tubul e walls (Fig. 2.3, b).

a b

Fig. 2.3. Comparison between ELD-deposited tubules of Ag in PET: intermittent ones (a) and continuous ones (b) [2.1]. Preparation: (a) without addition of chemical activation centers, (b) with their addition. Polymer was etched away after sample preparation for the better visibility of the tubules.

115

During the further research of the tubule formation process in a polymer matrix, a new series of PI and PET foil samples with Cu and Ni tubules has been prepared [2.12]. Formation of the tubules was carried out b y the ElectrodeLess deposition (ELD), as well as Ag tubules formation in the previ ous series of investigations (see the Chapter 1.6.3). For the deposition of Cu and Ni we have used commercial solutions obtained from the Doduco Ltd. company [2.13]. Cu was deposited at ~24 °C, and Ni deposition took place at ~88°C, following the company´s recommendations (see also the Chapter 1. 6.3). Our ITS studies neither revealed any strong surface-near reduction of the i nner tubule diameter nor a complete tubule closure during these processes, in contrast to earlier observations by Martin [2.3]. Still, a decrease of activated tra cks radii with Cu deposition was observed, see Fig. 2.4. In the process of this research, we have used both the information from SEM investigations and the resistance measurements of t he etched track along their axes to describe the state of the emerging tubules. In t he beginning of tubule formation, when the resistances were still very high, this was done by contacting both front and back sides of the tubule-containing polymer foi ls with golden electrodes under a gentle pressure. To determine very low resista nces, special foils were irradiated

Fig. 2.4. Decrease of the radius of activated (derivatized) etched tracks with deposition time for the ELD of Cu [2.14].

116 through tiny pinholes to reduce considerably the to tal number of examined tubules (which were counted). For instance, Fig.2.5 shows the dependence of the C u tubule quality (as determined by the tubule resistivity) on the activa tion time necessary to produce a sufficient number of nucleation centers. The resist ivity of non-activated samples (not shown here), closely resembling to that one af ter 1 s activation time (see Fig.2.5), is determined exclusively by the metal de posited at intrinsic defects. As can be seen, already a few seconds exposure to the activation solution improve the tubule quality by several orders of magnitude. The much higher conductivity of metal tubules formed in PI as compared with those o nes in PET under otherwise identical conditions indicates that metal layers gr own on PI have better quality, for yet unknown reason. Once the nucleation center dens ity has come to saturation, further exposure to the activation solution is inef ficient. The ELD deposition of metals in tubules sets in aft er a system - dependent incubation time (Ni: ~5 s, Ag: ~1 min, Cu: ~1½ min ). At elevated temperature the

Fig. 2.5. Dependence of resistivity of microporous PET and PI foils with embedded Cu tubules along the surface normal of the foil (i.e. along the tubule axis) on the activation time [2.1]. Conditions for chemical activation and ELD deposition in all cases are given by the DODUCO Ltd. company standard recipes [2.13]; 5 min ELD solution exposure.

117 tubule formation speed is faster [2.1]. Fig. 2.6 co mpares the ELD deposition speed at ambient temperature within the etched tracks wit h that one on unirradiated polymer foil surfaces. The metal layers start growi ng ~2 times earlier within the tubules than on the pristine surfaces. This is attr ibuted to the higher density of activation centers in the surfaces, as radiation -damaged inner track walls are expected to bond the sensitizers better than rather inert pristine polymer surfaces. The metal deposition both in the tubules and on uni rradiated surfaces continues up to hours with steadily etched tracks, as compared with the pristine decreasing deposition speed – the latter one either stemming from the exhaust of the given solution, or in the case of narrow tracks, from the ir newly complete filling. There is no remarkable difference in the deposition speed on pristine surfaces and tubule walls, for whatever tubule radius, which demonstr ates, that the diffusion speed of the ELD solution even within very narrow tracks is not a decisive parameter. Both the current/voltage characteristics and the te mperature dependence for a number of intermediate stages of tubule formation w ere measured later on, as well.

Fig. 2.6. The growth of metallic layers on PI by ELD [2.6]. Comparison between growth on surfaces of pristine polymer foils and growth in the interior of etched tracks, for both Cu and Ni.

118

In general, we can conclude, that by the SEM, condu ctivity measurements, and the ITS, the continuous and hollow nature of the me tallic tubules can be proved throughout the whole thickness of the polymer foil. In the process of examination of the time dependen ce of tubule formation via their appearance in SEM images and their electrical resistance, one can clearly distinguish the several stages of tubule formation [2.12]: I. As-etched tracks. The resistance of microporous foils as measured al ong the foil surface normal from one side to the other is s maller by up to several orders of magnitude, as compared with that one of pristine (o r of irradiated but unetched) foils. This has been attributed to the contribution of some residual etchant remaining in the etched tracks even after their car eful leaching in distilled water. These etchant ions (Na +, OH +) have diffused into the polymer bulk through the etched track walls by distances of at least some 10 0 nm, until they are immobilized by sorption or chemical bonding to the polymer chai ns. Applied external fields can release these ions from their shallow traps so that they can make the tracks very weakly conducting [2.15]. Possible contaminant ions from the etchant solution and ambient moisture contribute to this ionic conductan ce, as well. II. Incubation time. The formation of nucleation centers initiates the precipitation of metals on the etched track walls w hich leads to a dramatic and continuous decrease in resistance. This decrease se ts on, however, only after some system and temperature dependent incubation time (f or the above-described cases: Ni: ~5 s, Cu: ~1½ min [2.1]). Interestingly, the in cubation time for metal layer growth within the etched tracks is about half of th at one for metal deposition on unirradiated polymer foil surfaces. This has been a ttributed to a higher density of activation centers in the etched tracks as compared with the pristine surfaces, as radiation-damaged inner track walls are expected to bond the Sn or Pd sensitizers better than inert pristine polymer surfaces [2.1]. The SEM studies show that in this stage the polymer surfaces become covered with high ly dispersed and very small (~5…10 nm) metal crystallites (nanoclusters) at rel atively large distances from each other, with their areal density being given by the nucleation center density. 119

III. Nanocluster growth. The continuing exposure of the etched track walls t o the supersaturated metal complex salt solutions ini tiates the growth of the metallic nanoclusters on the etched track walls in both radi al and lateral directions until they touch each other. At the areal nanoclusters de nsities given here, this happens when the precipitates have grown to sizes of typica lly ~ 50 to 100 nm. IV. Tubule formation. Once the nanoclusters touch and form layers (cylind rical layers on the track walls, planar layers on the uni rradiated foil surfaces), the precipitate coherence will improve, and the layers will grow in thickness. The metal deposition continues up to hours with steadil y decreasing deposition speed – the latter one either resulting from the exhaust of the given ELD solution, or in the case of sufficiently narrow tracks, from their comp lete filling (i.e. wire formation). There is no remarkable difference in the deposition speed on pristine surfaces and tubule walls, for whatever the value of tubule radi us is, which demonstrates that the precipitation speed of the metal is no decisive parameter [2.1]. Fig. 2.7 illustrates SEM images for the transition of stage III to stage IV. One can note, that the nanotubules are by no means perf ectly cylindrical but show a considerable inner wall suface roughness and a grea t variation in their pore shapes.

a b

Fig. 2.7. Metal deposition on and into microporous PET film [2.12]. a) stage III, b) stage IV. Tracks are marked by circles. In the case (a) the observation took place under slightly tilted angle in order to allow for a glimpse into the tubule interior. 120

Besides chemical activation, the ion irradiation is known to modify the polymeric surfaces [2.16]. As several investigatio ns of this kind have shown, the surface nucleation center formation is already obse rved at ion fluences as low as ~10 12 cm -2. The formation of nucleation centers appears to be the result of electronic rather than of nuclear energy transfer, consistent with the laser irradiation results, where electronic excitation pr ocesses are triggered by the energetic photons (see Chapter 1.6.3). As the subse quent chemical deposition is most efficient immediately after the irradiation an d less efficient after a longer delay time, one may conclude that some of the elect ronic defects anneal out already at ambient temperature. The higher nucleation center density obtained by io n irradiation as compared with chemical activiation leads to smoother deposit ed layers, as illustrated in Fig. 2.8. This is important for obtaining multilayer str uctures of good quality. As ion induced activation ranges only up to the pro jectiles´ range, this approach enables one to tailor the metal deposition along a track. In this way it is possible to deposit cylindrical metal contacts on both sides of a track, which are electrically insulated against each other. Fig. 2.9. shows the r esult of a first test experiment undertaken for that purpose. Recent experiments with swift heavy ion irradiated PI (energy of 300 MeV, Xe 17+ ions, fluence of 1 ´ 10 11 cm -2) have shown, that the efficiency of Pd nucleation center formation is also improved by the ion irradiation, probably due to easier Pd-polymer bonding at radiation defects. If, however, the pristine or irradiated PI surfaces are pretreated with HNO 3, the picture reverses. Now metal is chemically deposited on the unirradiated surfaces r ather than on the irradiated regions. This is understood as the result of som e passivation of radiation- induced radicals against subsequent chemical d eposition. Therefore, it appears that one can tailor the tubule formation by specifi c treatment with energetic ions before the activation and/or chemical deposition pr ocesses. Considering the electrical-physical measurements of the polymer foils with metal covering, it can be observed, that a dir ect consequence of the increasing 121

Fig. 2.8. Comparison of Ag layers deposited onto etched tracks in PET (irradiated by 1 GeV Ar ions) after chemical activation (left) and ion-induced activation (right). In the first case, acidic SnCl 2 solution was used, in the second case the sample was ion-irradiated with 300 keV Ne + ions at a fluence of 1 ´´ ´´ 10 15 cm -2, after the ion track etching and before the Ag deposition [2.14].

Fig. 2.9. Cross-sectional view of double-cone shaped etched tracks in PI that were activated with 300 keV Ne + ions at a fluence of 1 ´´ ´´ 10 15 cm -2 before the chemical deposition of Ag. The thereupon formed continuous Ag layer (top) ranges only from the sample surface up to a depth of ~700 nm inside the etched tracks. Below that depth, some Ag is seen to precipitate at intrinsic defects on the track wall as isolated nanoclusters. The polymer is seen to fracture quite irregularly between the perforations [2.14].

122 metallization of the microporous foils is a dramati c change in their conductivity. In the initial stage of metal deposition the resistivi ty falls by many orders of magnitude from the insulating via semiconducting to conducting state, until a continuous metal layer has been formed. The reafter, the further decrease in resistivity is determined by the increase in layer thickness and possibly the decrease of carriers scattering on intergranular ba rriers. The resistivity decreases faster inside the tracks than on the foil surfaces, being consistent with the enhanced metal layer growth within the tracks. Clearly, during the transition from insulating trac ks towards metallic tubules, several different conduction mechanisms that depend on the tubule fine structure (in particular, the density of metallic clusters), will take place. From the Table 2.2, which gives an overview about various mechanisms of this kind, as known from the literature [2.17, 2.18], one can see that the d ifferent mechanisms have different voltage and temperature dependencies of the resisti vity. Eventually, the time dependence of the polymeric co nductance may give hints for the conduction mechanism in a polymer, as well. For example, in the case of ionic conductivity through a polymer, the current g enerally decreases with time, as the ions cannot be injected into the insulator, or be extracted from it fast enough. As a consequence, space charge regions may be built up, being accompanied by large internal fields. The conductance by localized states is of a special importance for irradiated polymers, covered with metallic clus ters. Several models have been developed here, describing tunneling by localized s tates, such as the hopping process (constant or variable range hopping [2.18] ) for highly disordered metals. The other possible conduction mechanisms according to Frenkel-Poole model can be excluded in our case, as in this study we deal w ith tubules of metals and not those of semiconductors.

Therefore, one should be able to derive the conduct ivity mechanisms of the tubules from the given parametric dependencies in their various stages. This is indeed partly possible, see the Fig.2.10 and Fig.2. 11. Fig. 2.10, which shows as an

123

Table 2.2. Basic conduction mechanisms, according to the Refs. [ 2.17] and [ 2.18].

The c i are coefficients, containing information about the barrier height and/or the activation energy of the charge carriers, insulator dynamic permittivity, effective mass of charge carrier, insulator thickness, etc., respectively.

Process Cause of process Voltage and temperature dependences of the current 2 1/2 Schottky Thermionic emission over barrier ~T exp(c 1 V /T - c 2/T) emission 1/2 Frenkel-Poole Field-enhanced thermal excitation of ~V exp [(2c 1V /T) - (c 2/T)] emission trapped electrons into the conduction band 2 Tunnel or field Field emission of trapped electrons ~V exp(-c3/V) emission into the conduction band, or by Temperature independent electron tunneling from the Fermi level in metal into the insulator conduction band Space-charge Carrier injection into the insulator, ~V 2 region limited where no compensating charge is Temperature independent current present Poole-Frenkel, Current is carried by thermally ~exp(V 1/2 / T) Gill excited electrons hopping from one isolated state to the next n Mott or Constant or variable range hopping ~exp(c 4 /T) Shklovski- (tunneling) of electrons by localized (n = 0.25 or 0.5 or 1.0) z Efros states ~exp(c 5V ) (z » 0.5) Ohmic Scattering of carriers by atomic ~V exp(c 6/T) conduction vibrations and defects

Ionic Diffusion of ionic charge carriers ~V/T exp(c 6/T) conduction example the current/voltage characteristics of Ni t ubules in various production stages indicates, that after very short Ni depositi on times (~10 s) the Schottky mechanism is that one which fits best to the measur ed results. For 30 s to 1000 s Ni deposition times, the curves become steeper at high er voltages, which indicates that the Schottky mechanism is overtaken by the reg ular ohmic conduction [2.12]. 124

Fig. 2.10. The current / voltage Fig. 2.11. The temperature depen- characteristics of Ni-filled micro- dence of the resistance of thick Ni porous PET foils, as compared with tubules, as measured and compared several conduction theories [2.12]. with some conduction theories Conductivity values were measured [2.12]. along the ion track directions (i.e. through the foils).

Unfortunately, all the systems, studied up to now, are restricted to the above- listed Stages III and IV, as in the earlier stages extreme ly small currents are involved. The ITS measurements show [2.1], that aft er deposition time of 10 s, average tubule wall thickness of ~0.1 m have emerged, which coincides with the SEM images. For 30 s, 100 s and 300 s deposition ti mes, the corresponding tubule walls have the average thickness of 0.6 m, 1.5 m and 2.0 m, respectively. Thus, we can conclude, that the Schottky model, whi ch is based on thermionic electron emission, is applicable just before the on set of continuous metal tubule formation from the liquid phase, as long as the met allic crystallites still preserve their individuality. Possibly, the presence of adsorbed water and hydrated ions between the metal nanoclusters helps preserving thi s state longer than it would exist during evaporation in vacuum. Only later, whe n the nanoclusters are closely interconnected to form a continuous metallic film, the expected ohmic conduction 125 overtakes. Nevertheless, the overall resistivity of the tubules turns out to be higher than that one of the corresponding bulk metal by up to several orders of magnitude. This indicates either a high density of atomic defe cts in the formed thin layers, at which the charge carriers scatter, or a formation o f intergrain barriers due to the presence of the rest of chemicals between grains af ter deposition. The same findings reported above for Ni also hold for Cu tub ules. This can not, however, signify that in the investig ated systems the electron tunneling through intergranular barriers is superpo sed, as in this case the tubule resistivity would not depend on the temperature [2. 19]. We did not observe that on our samples. Nevertheless, additional experiments i n a broad temperature range will be needed to check this possibility. Based on these findings, it is assumed that anneal ing of the as-formed tubules will remove the remaining intergranular adsorbant l ayers so that better contact between the metallic nanoclusters is achieved, ther eby decreasing the overall tubule resistivity. However, before such study can be performed, it is necessary to examine the thermal stability of the etched tracks and the tubules embedded therein. This has been done [2.20], and the results of that study indicate, that PET is an inadequate host matrix for tubule annealing ( see the Chapter 2.1.1.2). Instead, future experiments of that kind should concentrate on etched tracks in PI.

The ratio of the coefficients c 1 / c 2 (see the Table 2.2) is in the case of the 1/2 Schottky model fit equal to 5:1, with c 1 = (e / (4 π εi d)) and c 2 = e ΦB/k b where e is the elementary charge, εi is the insulator dynamic permittivity, d is the ins ulator thickness or intergranular distance, ΦB is the Schottky barrier height, and k b is the Boltzmann's constant [2.17]. That opens a way to qu antify this conductivity mechanism further, by deducing values for the produ ct of barrier height ΦB and intergranular distance d (which can be obtained fro m the SEM data). After some not complicated transformations, one can get:

1/2 2 ΦB = ( k b (1 / 4 π εi e d ) ) / (c 1 / c 2) (2.1) 126

Therefore, the consequence of the investigation is that the variation of d, by application of e.g. external pressure, sensitively influences the tubule conductivity; i.e. such tubules with disconnected but closely nei ghbored crystallites can be applied as pressure sensors. This really has been f ound in the present research.

127

2.1.1.2. THERMAL BEHAVIOR OF MANIPULATED TRACKS

Some of the proposed future applications suggest th e deposition of matter within the tracks or tubules, with that matter to be subse quently thermally processed to obtain its final desired properties. Metallic tubul es in the etched ion tracks in polymers, forming specific nanostructures, are expe cted to find great applicability in the emerging nanoscience. For this case it is im portant to know the thermal behavior of both the tracks and tubules. Only one s uch work has been published until now, which deals with the thermal stability o f etched tracks in PET foils [2.21]. These measurements are compared in this res earch with new ones on the same type of polymer but from different provenience , and additionally they are compared with the results for PI, as this is anothe r favorite polymeric material for the ion track applications. For this investigation we have selected 10 m thick PET foils from Russian provenience and ~20 m thick PI foils from British provenience (Goodfell ows Ltd.) [2.20]. These foils were irradiated with ~ 30 0 MeV to 500 MeV Kr ions up to fluences of 1 ´ 10 7 cm -2 to 2 ´ 10 8 cm -2 at the heavy ion accelerators of the JINR Dubna and the HMI Berlin, respectively. Then the fo ils were etched according to the usual recepies, as described in the Chapter 1.5 .2. The etching process was constantly monitored by ITS [2.21] (see also the Ch apter 1.2.4). We have etched the tracks up to outer diameters of 75 nm in the ca se of PET and of 1.4 m in the case of PI. From both SEM images and the shape of I TS spectra the exact track shape can be derived. In the case of ITS of non-cyl indrical tracks, a Monte Carlo simulation [2.21] is required to derive the exact t rack parameters (such as inner and outer diameter and cone opening angle) from the spe ctra. The PET and PI microporous foils were cut into two pieces of equal size. Whereas one half was left untreated, the other one was subject to Ag deposition. We decided to use the ELD technique [2.22] (see als o the Chapter 1.6.3) for this purpose. Specifically we applied the historical rec ipe of St. Gobain [2.9, 2.10] for 128 silver mirror production. The thicknesses of the de posited Ag layers on the etched track walls were ~ 20 nm to 30 nm each. In order to obtain good homogeneity of the nanoscal e silver mirrors, the microporous foils had been initially treated with a cidic SnCl 2 solution. It can be proven by SEM, conductivity measurements, and ITS, that the metallic tubules are continuous and hollow throughout the whole thicknes s of the polymer foil, see, e.g. Fig. 2.12, a. The limitations of this technology a re given by the surface roughness of the etched tracks (not shown here) and by the fi nite size of the embedded silver nanoclusters, see Fig. 2.12, b. Our ITS studies, performed on the created Ag nanotu bules neither revealed any strong surface-near reduction of the inner tubule d iameter nor a complete tubule closure during these processes, in contrast to earl ier observations by Martin [2.3]. On the contrary, even a crater-like surface-near wi dening of the tubules was observed occasionally, Fig. 2.12, b. This results f rom different deposition techniques, used by Martin's group and ourselves.

a b

Fig. 2.12. SEM images of Ag tubules in etched tracks in PET foils [2.20]. (a) Side view of a broken foil. It is clearly seen, that the tubules are continuous throughout the whole foil. (b) View into the interior of a narrow Ag tubule (~ 100 nm inner radius), that widens crater-like in the surface-near region. The detailed pore shape is determined by the arrangement and size of the individual tubule-forming Ag nanoclusters . 129

Thereafter, both the microporous foils with and wit hout embedded silver nanotubules were cut into pieces of typically 1 ´ 1 cm 2 size, and annealed for 1 h each up to various temperatures ranging up to 300° C in the case of PET and 700° C in the case of PI. For each sample the corresponding ITS spectra were taken and compared with the Monte Carlo simulations . Fig. 2.13 shows a typical ITS spectrum. Several fea tures can be distinguished. The sharp peak I stems from the particles transmitt ed through the etched tracks with their full energy E 0. A relatively broad peak II stems from the particl es transmitted through unirradiated foil regions with the energy E 0 – S d (where d is the foil thickness and S is the ion stopping power in the polymer foil), and a spectrum part III connecting these two peaks stems from particles, which partly have been stopped in the polymer, and partly have p enetrated through the etched tracks without any energy loss [2.20 ] . If Ag is deposited inside the etched tracks additio nally, the peak I becomes smaller due to reduction of the pore size. Silver d eposited on the polymer foil

surfaces (if not wiped off after deposition) shift s the peak II to lower energies, due

a b

Fig. 2.13. ITS spectra of particles transmitted through as-etched microporous PI foils and microporous PI foils with embedded Ag tubules (a); detail of the ITS spectra (b) [2.20]. As in this case the silver precipitation on the foil surface was wiped off after the tubule preparation, the peak II remains unchanged. A decrease of the peak I height, describing the 130

reduction of the pore size by the Ag deposition, is however quite pronounced, as seen on the part (b). to the additional stopping in that layers. Zone III of the spectrum changes as well, due to altered inner pore geometry. In other words, any variation in the micropore foil topology will show up sensitively by variation s in the ITS spectra. From the analysis of the ITS spectra of all anneale d PI samples without the embedded Ag tubules and with them (not shown here) it became clear, that the spectra generally arrange in two different groups, one ranging from room temperature up to 500° C, and another one ranging from 550° C to 700 °C. This coincides with what is well-known concerning the th ermal stability of PI from, e.g. thermal gravimetry experiments: at ~430° C to 450 °C some mass loss gradually starts, which becomes more pronounced when the temp erature approaches to ~500 °C. Thereafter, a rapid carbonization sets in, which is completed at about 550 °C to 600 °C. The overall mass loss during carbonization leads to a reduction in stopping power, which explains the shift of peak II to higher energies [2.20 ]. The height of peak I reflecting the etched track (o r tubule) diameter is apparently less pronouncedly affected by the anneal ing. This becomes more clear when considering Fig. 2.14a , where are presented t he effective track radii, which were deduced from the ITS spectra. Whereas the trac k radii remain virtually constant during annealing up to ~450 °C, there is a general tendency of track opening at higher temperatures, which comes to satu ration at about 650 °C. It appears, that the embedded silver tubules lead t o some stabilization of the pore size at elevated temperatures, as the inner tubule diameters increase less rapidly with temperature than the etched tracks diameters themse lves - in fact, it was the motivation of that experiment to find the ways to p revent (or at least diminish) the temperature influence on these structures. The trend of pore modification with temperature for PET microporous foils without the embedded silver tubules and with them, that can be derived from the corresponding ITS spectra (not shown here), differ s from the PI results (see Fig. 2.14 b, c). For PET of Russian provenience (solid c urve in Fig.2.14 c), the etched 131 track radii are constant up to 200 °C, and thereafter gradually decrease. With silver embedded, the pores are stable only up to 100° C, and thereafter close stepwise.

Annealing temperature [°C] a

b c

Fig. 2.14. Annealing temperature dependences of average etched track radii and of Ag tubule inner radii for: (a) PI microporous foil and (b,c) PET microporous foil [2.20]. In part (b) the evolution of the track radii is sho wn for two different types of PET foils, one from the British provenienc e (solid curve), and the other one from the Russian provenience (dashed curve). In part (c) is considered the comparison of the change of etched t rack radii and the 132

change of tracks with Ag tubules for the PET foil o f Russian provenience.

The shrinking of the nanoporous etched tracks i s ascribed to the onset of the polymer´s plastic flow, and the decrease in transmi ssion of the Ag tubules is thought to result from their increasing misalignmen t when the host material softens. This effect is not observed for PI, as thi s polymer neither melts nor softens, even at high temperatures. Interestingly, this trend differs, however, dramati cally from the measurements performed with PET microporous foils from the Briti sh provenience – Goodfellows Ltd. (solid curve in Fig. 2.14 b). In that case, th e etched tracks widening has started already at the lowest annealing temperature, 50° C. The track widening had its maximum at 75° C, then shrunk up to 150° C, and thereafter widened again continuously. These processes were attributed to th e loss of solvent, the onset of plastic flow above the glass transition temperature , and to the onset of the polymer´s carbonization [2.20]. It is thought, that this grea t difference in the findings, using the same measuring technique, the same type of polymer and the same etched track radii must be attributed to different additives, which co ntain the two used PET foils. Apparently the product of Russian provenience conta ins less plasticizers (thus making the foil and hence the pores more rigid) and more antioxidants (thus preventing too rapid carbonization). Apart from the rough analytical evaluation as seen in Fig. 2.14, nearly all experimental spectra were also fitted by Monte-Carl o simulations, see Fig. 2.15, which yields much more reliable and accurate result s. Thus, parameters describing the tubule shapes, such as the inner and the outer track radii in the case of the double - cone (hyperbolic) tracks (as given in PI), and the opening angles of the corresponding cones can be derived, as well (Fi g. 2.16). The Monte Carlo results indicate, that both the inner and the outer radii of the double cones slightly decrease above ~300° C, whereas at about the same temperature the cones start widening. Above ~400° C to 450° C the track radii increase again, with further 133

widening of the opening angle. Above 600 °C, when the polymer carbonization has come to an end, the track parameters have been stab ilized, too.

Annealing temperature [°C]

Fig. 2.15. Monte Carlo fit of a typical Fig. 2.16. Parameters of double-cone- shaped etched tracks in PI, ITS spectrum [2.20]. after thermal annealing for As an example, a micro- 1h each [2.20]. Results of the best fit of porous PI foil annealed at Monte Carlo simulation to 450 °C for 1 h is chosen. experimental results.

Therefore, as a result of this research, it was sho wn, that it is really possible to use etched tracks in suitable polymers, such as PI, for the purpose of the production of nanowires or nanotubules, which operation in a r eal devices will demand the high temperature conditions, for processing steps o f up to 450° C and 500° C. This signifies that quite a number of ceramic nanodevice s can also be produced inside polymeric (PI) etched tracks as templates, by conve rting thermally the corresponding organometallic or sol structures prec ursor inside the tracks towards 134 the required sintered ceramic structures. This is o f interest, for instance, for the production of advanced sensors [2.20]. In comparison with e.g. alumina templates, polymer foils as the host of nanopore templates have the advantage of high flexi bility, smaller thickness for self-supporting samples, and lower cost of producti on. Furthermore, the pore shapes and distributions can be tailored by the cho ice of the swift heavy ion species, their energies and fluences. Shape of the irradiated area can be determined by irradiation through masks, and the etching proce ss can be selected so as to obtain pores of any desired diameter and shape. It appears, that polymers of different proveniences may have different thermal behavior, as shown for etched tracks in PET foils o f the Russian and the British proveniences. Therefore, a careful study of the add itives of a given polymeric foil is important, before uncritically applying the latt er. The insertion of metallic tubules into etched track s, which was thought to guarantee the mechanical stability of the nanopores , was only partly successful. Whereas their presence made sense in the case of hi gh - temperature treatment of PI, they failed to be beneficial in the case of PET , due to its easy softening already at moderate temperatures.

135

2.1.2. ION TRACK – BASED MICRODEVICES

2.1.2.1. MAGNETS AND TRANSFORMERS

The research on creation of miniaturized magnets an d transformers is very important, as such elements are vital for applicati ons in communications, aerospace/military applications, the biomedical fie ld and for computer technology. Among the devices, which can be improved in frames of this research, are proximity sensors, filters, resonators, transformer s, power converters, motors, valves, pumps, etc. Regretfully, this miniaturizati on was developing with some delay. Only in the past 15 years first acknowledgab le attempts have been made to improve that situation (see the Table 2.3 and Fig.2 .17). However, for superconducting magnets this development is apparen tly starting now [2.23]. Development of the miniaturized magnets and transfo rmers has started very unspectacularly, by simply reducing the dimensions of conventional spiraling coils and magnetic cores. These first devices of 1 cm to 2 cm size were already sufficient to fulfil quite a number of advanced tas ks, such as the use in SEM miniaturized lens systems [2.24], blood pumps [2.25 ], and high-frequency relays [2.26]. Magnetic microcores, using small-size spira l coils, cloth inductors, and zig- zag coils have been proposed [2.27, 2.28]. When the metal deposition techniques, such as evapo ration, , and printing became well-known, new approaches appeared for the production of smaller-sized inductors (inductance coils) by these ways. Two possibilities were revealed. On the one hand, inductors were deposited in the form of spiralling metallic coils, with in principle only one deposite d layer required [2.29 – 2.32]. In this case, the magnetic core was eventually added a s just another layer on top of the spiral [2.33]. On the other hand, more sophisti cated approaches have been developed, where the wires (and eventually, even a magnetic core) of the inductors were deposited layer-by-layer, with co rresponding insulating layers in between. In this way, planar coils with diam eters of typically a few mm and 136

Table 2.3. Literature scan results, concerning the inductance microcoils preparation, in the sequence of publication dates. The units used here are: Ø is the diameter of the coil [ m]; d is the total length of coil windings [ m]; N is the number of the coil windings; R is the ohmic resistance [ ]; L is the inductance [mH]; Q is the quality factor of the coil.

Purpose, Production, Products, Who? Ref., Materials Technique Parameters Where? When? For Planar Micro-coils G.Y.Tjan, [2.30] displacement conducting Ø = 6000 Z.X. Zhao, 1999 transducers spirals with d ≈ 1000 R.W.Baines, ferrite core R =3.2 to 4.2 P.Corcoran Circuit board attached by L = 0.025 to Derby Univ., glue; 0.041 without UK printing the magnetic core and wires onto L =55 to 77 polymer foil with magnetic core N = 29 – 45, Q = 155 to 170 3 parallel layers Feasibility Scanning the Micro-tubes, S.Kawata, [2.37] demonstration Micro-coils K.Okamoto, 1998 focus in 3 only Ø ≈ 10, d ≈ 20, S.Shoji, dimensions of N=3 Osaka Univ., Coils and Japan a IR laser, tubes of polymer, some nonconducting 10 kW

Photo- polymerisable material NMR Printing of Micro-coils J.R.Spadea, [2.29] microscopic conducting Ø= 400 S.M.Wright, 1997 imaging spirals d = 25 (for Texas Univ., USA wire+substrate) Circuit board, ca. 25 m N = 4 thick

137

Purpose, Production, Products, Who? Ref., Materials Technique Parameters Where? When? For Multilayer Inductors, J.Y.Park, [2.32] transformers, structure Ø min = 150 M.G.Allen, 1997 power production by Ø max =600 Atlanta, USA converters, multilevel dtotal = 15000 etc. (156 layers) R = 4 to 35 electroplating L = 0.00014 to Glass of Cu coils, 0.0004 substrate, each 40 m Q ≤ 3 polyimide wide and 40 Total volume films for m thick, and of structure: insulation Fe/Ni core, 35 4 mm x 4 mm m ´ 500 m x 0.145 mm; cross section up to 3 A (DC) For Toroidal Ø max =4000 J.Y.Park [2.33] transformers structure Ø min =1000 M.G.Allen 1996 40 m thick d= 130 Atlanta, USA Cu wires N = 30 1.5 m thick L=0.7...0.3 Fe core (for 0.1KHz ... 300 kHz) Q = 0.01...1.6 6 regularly Multilayer Ø=10000 D.Taghezout, [2.34] spaced coils structure, d= ca. 50 Grenchen, 1995 for stator of produced by R=8020 Switzerland a disk type metal sputter L=0.12 stepping motor deposition of N=125 Al wires (0.6 Thinned Si and 1 m wafer thickness, separated by 4 m insulat- ing oxide)

138

Purpose, Production, Products, Who? Ref., Materials Technique parameters Where? When? Microsensors, Multilayered Ø = 1000 C.H.Ahn, [2.31] max Y.J.Kim, microactuators Ø min = 139 M.G.Allen, 1994 metallizing d = 4000 Atlanta, micromagnetic USA 40 m thick, N = 30 power devices 100 m wide L = 0.4...0.1 Cu wires, (for 1 kHz...

30 m thick NiFe 1 MHz) permalloy core R = 0.3 in closed Q = 1.5...4.6 toroidal design Microtrans- Zigzag coil Ø = 31500 K.Yamasawa, [2.39] min K.Maruyama, former for 2 coils 35 m Ø max = 41500 I.Hirohama, 1990 micropower thick, insulated d = 210 P.Biringer total Nagano, converter by 20 m foil N=8 Japan 50 m 2 meanders polyimide foil At 1 MHz: L=0.0085 R=25 At 6.7 MHz: L= 0.005 R=400 High current Meander Ø max = 2500 M.Yamaguchi, [2.35] density inductors Ø min = 1 M.Matsumoto, 1990 applications Cu wires 30 m d = 3500 H.Ohzeki, Si substrate wide, N = 10 K.I.Arai, spacing 7-50 m L = 0.013... Sendai, Permalloy core 0.033 Japan R = 4...5 Relay Winding of Coil around SIEMENS, [2.26] Devices wires to hollow continuous Germany 1989 coil axial cavity Ø = 12800 d = 6800 to 7500 15 mWatt 12 V

139

Purpose, Production, Products, Who? Ref., Materials Technique parameters Where? When? High frequency 2 planar parallel Ø=9200 O.Oshiro, [2.38] coils in the d =200 H.Tsujimoto, 1987 spirals insulated integral N =8 K.Shirae, circuits by 10 m thick L=0.0013 Osaka, without mag- Japan foil; netic layer, and optimum L at L=0.0026 with 20 MHz; that maximum pos- sible frequency: 100 MHz Moving coil Wound wires Ø=10000-13000 R.Neuhaus, [2.25] for pressure R=1.3...16 Aachen, 1977 servovalve as Germany a blood pump

Coils for Winding of wires Ø outside =31000 T.Mulvey, [2.24] miniaturized to hollow coils; Ø inside =15000 Birmingham, 1974 lens systems For e-beam D = 5000 UK for SEM; tansmission N = 7500 2 Mylar Imax =10000A/cm insulation thicknesses from 100 m m to a few mm were obtained [2.32 – 2.35]. The meandring coils for linear pulse motors have been r ealized in that way, as well [2.35, 2.36]. Thus, inductance coils of up to 40 m H without magnetic cores, and inductors of 80 mH with those have been obtained, w ith resistivities of a few Ohms and quality factors of up to about 170 [2.30]. One disadvantage of the present design of the induc tors is their size, which often exceeds one millimeter at least in two dimensions b y far. Another disadvantage is that, due to the relatively large aspect ratios (co il diameter/ coil length) which range from about 6 [2.30, 2.31] to 200 [2.29, 2.32 – 2.34], and in the case of non- circular coils, maximum to minimum coil diameter ra tio is about 15 [2.33], the flux leakage is very high and hence the loss is s trong and the quality factor is poor. In this connection, one should point at a new promising approach, which 140

Evolution of Inductance Microcoils Sizes

Proposed ion-track- based coils

Fig. 2.17. Historical evolution of inductance microcoil sizes, for both the full coil volume and the fractional volume per winding. The size range for our proposed ion-track-based microcoils is added here, too. might avoid these disadvantages in future. Here spi ralling coil structures of optimized shape are inscribed by a highly focused l aser beam into a photo- polymerizable material, thus leading to circular co ils, for instance, of about 10 m m diameter and 20 m m length. However, as at present the thus obtained structures are not yet conducting, a well-conducting photopolymer has to be found, befor e any technical application of this approach can be envisaged. The minimum requirement for the use of an inductanc e microcoil in the GHz frequency range oscillators in information technolo gy, is that its quality factor exceeds the value of 10 at its working frequency. F or smaller quality factors, the damping in the wire is too high, so that the select ivity of the coil becomes too small. Inductance coils used at 2 GHz have the typical siz es of 70 mm ´ 35 mm. Of course, due to decreasing wavelengths with increasi ng frequency, devices have to 141 be made increasingly smaller, which increases speci fically the problems of technical reproducibility. 300 GHz are presently re garded as the technical limit of conventional circuits, as in that case the devices become too small. Higher frequencies, corresponding to the far infrared spec trum range, do not have any importance in contemporary information technology. Only beyond, for wavelengths of about 1400 nm to 900 nm, the applica tions arise using optical fibres. Another big problem when going to high frequencies is the dramatic change in material parameters. For example, ferrite cores are good for use only up to about 10 MHz. At higher frequencies (from 10 GHz to 20 GH z) yttrium granate is recommendable, due to its ferroelectrical propertie s. Concerning the insulators, polystyrol has been found out to have the best high frequency properties. In contrast, water and hence the water-like polymers t oo, show high damping above 10 GHz, which makes them unsuitable in the GHz rang e. The increasing resistivity of metals with increasing frequency enhances the pr oblems of their application, as well. For example, a golden wire of 10 m diameter and 700 m length exhibits already a damping of 3 db. At present high frequency coils are embedded in int egrated circuits, but they are not yet available as individual elements. These co ils are usually produced by hand, by winding the wires onto a carrier of plastics (s ilver containing Araldite) which hardens after baking at about 50 o C. In order to insert these coils into the device s by soldering, they must withstand temperatures of a t least 250 o C. Therefore, according to the results given in the Chapter 2.1.1 .2, PI appears to be an adequate base material for these devices. Antennas, operating at 30 GHz to 40 GHz, will have great applications in the next generation of wireless telephone communication (the so-called “widest local loop”), which envisages the mobile telephone connec tion for everybody. At distances of typically 30 m each, radio-modems oper ating at 34 GHz are planned, to enable local reception of a multitude of differe nt participants simultaneously. For this sake, one envisages the development of an integrated circuit with an array 142 of small planar antennae of typically 10 ´ 10 cm 2 size. One intends to make these arrays directional sensitive, by giving different p hase shifts to the subsequent transfer elements of the different antennae. These transfer elements are the candidates for our microdevices. A major limitation of all kinds of miniaturized ele ctrical or electronic devices is the aibility to dissipate the Joule heat, that is g enerated as a result of the electrical current passing through them. This problem is more severe for the polymer-based electronics than for the silicon-based devices, due to the much lower thermal conductivity of polymers (for instance, 0.10 W/mK - 0.35 W/mK for PI, being compared to 148 W/mK for Si). The answer to this pr oblem is efficient cooling. For this purpose metallic replica of conical etched tracks have been developed [2.40], which enable an efficient cooling due to th eir large surface area, if put into a close contact with the electronically active foil s. Another possibility is to exploit the high porosity of the microporous foils themselv es, by including the electronically active devices as tubules embedded t herein, to enable convective flow of air or oil through their pores. Apart from these possibilities, one should take a polymer with high thermal stability such as PI as the microporous support membrane. This material survives temperatures up to ~ 430 °C without any chemical changes, up to ~ 600 °C with limited carbonization, and up to ~800 °C after full carbonization, while maintaining the tra ck shapes. It is important to note, that PI does not become conducting after such a tre atment.

2.1.2.1.1. MICROMAGNETS

Already since some time, our group is working on th e development of micromagnets inside thin PI foils by a sequenti al ELD deposition of Cu within selected ion tracks (see also the Chapter 1.6.3) , and through appropriate masks. The principle concept of this approach is shown in Fig. 2.18. The first step here concerns preparing of ordered groups of metal fille d ion tracks by ELD deposition 143 through one set of appropriate masks from both side s of the polymer foil, and the second step deals with evaporation of Au connection s and contacts through another set of appropriate masks on the top and back sides of the foil. In the Fig. 2.19 is presented the first ion-track-b ased micromagnet prototype [2.41], which was created according to the concept in Fig. 2.18. On the upper row one can see the masks, which were used, and on th e lower row the micromagnet production steps are presented. It has been show n [2.42], that the many parallel metallic nanowires, that form the vertic al connections, behave just as one individual wire with correspondingly larger dia meter. Macroscopic ion track- based test devices, created by our group, have demo nstrated the principle feasibility of this approach. The main problem here is to prevent shortcuts through etched tracks between surface wires crossing each o ther on front and back sides, which is accomplished by closing those tracks with a polymer solution before the deposition of the surface wires. As it is easily po ssible to produce in this way coils with quadratic cross sections, the quality factors of such magnets are supposed to be superior to thin-film magnets obtained by evapor ation, as in the latter case cross-sectional ratios of 1:50 between height and w idth are common, which results in considerable magnetic stray fields. By usi ng conical instead of cylindrical

Fig. 2.18. Principle sketch of the possible concept of creation of the micromagnets [2.14]. 144

Fig. 2.19. The ion - track - based micromagnet prototype with dimensions 6 ´´ ´´ 7 ´´ ´´ 0.02 mm 3; masks (above) and production steps (below) [2.41]. etched tracks, the quadratic cross-section can be r ounded somewhat, which should reduce magnetic stray fields even more. For low-frequency applications, micromagnet coils s hould contain a core of a ferromagnetic, and for higher frequencies a core of ferritic material is needed. As for frequencies above ~10 8 Hz neither ferromagnetic nor ferritic cores are re quired, we decided to use for our research miniaturized ion -track-based magnets embedded in a polymeric matrix. As our measurements have shown [2.41], the micromag net has revealed good operation up to ~ 0.5 GHz , with inductance per win ding of 8 ´ 10 -5 H at 10 Hz and about 2.5 ´ 10 -7 H at frequencies between 10 4 Hz and 5 ´ 10 8 Hz (Fig. 2.20, a). The experimentally defined inductance value coincid es with the theoretical expectation according to Ref. [2.43]:

m N2A L = (2.2 ) 2p rc Here mm mm is magnetic permeability, which in this case we ha ve taken as 1 H/m, N is number of windings, which is 2.5, A is the cross section of the coil, being equal -7 2 -3 to about 1.34 ´ 10 m , and r c is the coil radius, which equals to about 1.6 ´ 10 m. 145

100

Commercial Micromagnet 10

1

Ion-Track-Based Micromagnet, X 10 5

Inductance ( H ) H ( Inductance 0.1

0.01 10 1 10 2 10 3 10 4 10 5 10 6 10 7 Frequency, Hz a

8

7

6

5

4

3 Quality Factor Quality 2

1

0 10 7 10 8 Frequency, Hz b Fig. 2.20. Electrotechnical characterization of the micromagnet prototype, made by means of vector impedance analysers of HP 4191 and HP 4192 type (see Chapter 1.2.5): (a) frequency dependence of inductance (also given for a commercial micromagnet for comparison); (b) frequency dependence of the Q-factor; 146

The estimated inductance is about 8 ´ 10 -5 H. As it is seen from the Fig. 2.20a, the micromagnet inductance is strongly frequency-dependent. Its value first drops down to approximately 2.5 orders of magnitude, and thereafter, after about 5 ´ 10 4 Hz it becomes practically constant. The calculated quality factor of micromagnet is as big as 7 at about 2 ´ 10 8 Hz, and then it drops down to 0 at about 1 GHz, when the m agnet loses its inductive properties (Fig. 2.20, b).

As an attempt to understand this behavior of freque ncy-dependent inductance Lww ww versus frequency, we have tried to simulate it acco rding to the equivalent circuit (Fig.2.21) with the corresponding expression [2.43] :

1(L - w 2LC ) - R 2C L w = (2.3) 1( - w 2LC )2 + (w RC )2

Here L is the inductance, C is the capacity and R is the resistance of the circuit, given in Fig. 2.21, and ww ww is operating frequency. Regretfully, we did not succeed in fitting the expe rimental dependence by this approach as this simulation either yields a frequen cy-independent inductance and the increase of inductance, or its complete breakdo wn, depending on the chosen values of L, C and R.

Fig. 2.21. The equivalent measuring circuit for evaluation of frequency dependence of inductance of the micromagnet [2.43].

147

Therefore we must attribute the observed behavior o f the frequency dependence of inductance (Fig. 2.20, a) to the influence of re sistive and capacitive losses, and also magnetic stray fields taking place in the micr omagnet with the increasing frequency. In our view, the first ion-track-based micromagnet prototype could form a part of future oscillators for the advanced telecommunic ation systems.

2.1. 2.1.2. MICROTRANSFORMERS

The ion-track-based microtransformers have been pro duced according to the same concept as that for the micromagnets (Fig. 2.1 8 and Fig. 2.19), but in this case two windings (a primary one and a secondary on e) have been formed (Fig. 2.22). In the Fig. 2.23 the results of the oscilloscope m easurements of input and output signals of the microtransformer prototype for recta ngular and sinusoidal input at different frequencies are shown. One can see from t he figure, that a good function is recorded in the MHz range (upper graph), but signal transformation is possible

Fig.2.22. Principle sketch of the possible concept of creation of the microtransformers [2.14].

148

Fig. 2.23. Comparison of input (bottom) and output (top) signals of the microtransformer prototype, for rectangular and sinusoidal input at different frequencies [2.44].

Fig. 2.24. The set of stainless-steel masks for creation of the miniaturized ion- track-based transformer. 149 up to the near-GHz range, when gradually a phase la g emerges between input and output signals. We can estimate of the coupling factor K of the mic rotransformer according to [2.43]: M K = (2.4) LpLs

Here, L p is the inductance of primary coil and L s is the inductance of the secondary coil. In the ideal case, without any sca ttering of the magnetic flux, the mutual inductance of the transformer M could be equ al to LpLs ; in reality, however, the mutual inductance is always smaller th an this value. As the difference between L p and L s , being evaluated from measurements results (Fig. 2.23), is about 10%, K value is about 90%. Further development of micromagnets and microtransf ormers production technology with the use of special laser-cut stainl ess steel masks (Fig.2.24) has lead us to creation of a more miniaturized ion-trac k-based transformer (Fig. 2.25). As seen in the Fig. 2.25 b, the individual etched t racks of the PI microporous foil can be clearly recognized in the optical transmissi on microscope image as small dark spots; the evaporated metallic zones show up a s non-transparent areas. This figure indicates, that a further reduction in size by one more order of magnitude is still possible, which could make the microtransform ers interesting for such technical applications, as the use in the anticolli sion radar systems [2.44]. In principle, it should be possible to proceed with the miniaturization so far that coils with less than ~10 -6 cm 3 volume emerge, thus decreasing in size even below the smallest magnets ever produced by conventional lithographic techniques [2.14].

150

a

b

Fig. 2.25. The first miniaturized ion-track-based transformer [2.44 ]: (a) overall view; (b) fragment of the winding; Grey area: microporous PI foil, dark area: evaporated Au wires.

151

2.1.2.2. CAPACITORS

The first prototype of the ion-track-based microcap acitor was created according to the concept, explained in the previous chapter. It was formed first by the sequential ELD depositions of Cu through special st ainless-steel masks on the PI foil with etched tracks, and then Au contacts were evaporated on the top and back sides of the foil (Fig. 2.27, a). An optical microp hotograph of the microcapacitor prototype is provided in the Fig. 2.26, a. First of all, it was important for us to estimate t he capacitance of the microcapacitor at various frequencies, namely in th e range from 10 MHz to 1 GHz. For this sake we have used two evaluation mode ls. The concept of the first one, which we have named "plate model" is shown in the Fig. 2.26. The standard formula for the capacitor with plate contacts was u sed for these calculations:

e 0e S C pm = ( 2.5) d -12 Here, ee ee 0 = 8.85 ´ 10 F/m, is the permittivity of vacuum. ee ee is the dielectric constant of the isolator material, which in our cas e was PI. No data for ee ee were available for our region of interest, but as recent ly a very thorough and reliable measurements of dielectric constant in the region f rom 100 Hz to 10 MHz has been done [2.45] on the same type of PI foil, which is u sed in our research (see the Fig.2.28), we were able to interpolate the needed d ata for our region of interest (shown as a dotted line in the Fig. 2.28). Still, f or the sake of comparability, in this figure we have also plotted the data for other PI f oil types from Refs. [2.46, 2.47]. Thus, we have taken the following ee ee values for the evaluations: 3.01 for 10 MHz, 2.96 for 100 MHz and 2.91 for 1 GHz. S is the contact area, which we have calculated from the microcapacitor parameters (see Fig. 2.26, b): l (the length of the contact plate) equals to 2100 m m, h (the width) equals to 25 m m. Therefore S equals to 5.3 ´ 10 -8 m2. d is the distance between contacts, which equals to 200 m m. 152

Fig. 2.26. (a) The ion-track-based microcapacitor prototype (optical microphotograph) ; (b) Concept for the "plate model" of the microcapacitor capacity evaluation (detail of the two opposing contact strips, which are marked with white colour, as shown in the part (a)); 153

a

b

Fig. 2.27. (a) The concept of creation of the ion-track-based microcapacitor [2.14]; (b) Concept for the "wire model" of the microcapacitor capacity evaluation, view from the top (after Ref. [2.43]); detail of two tracks with opposing polarities, according to the part (a). 154

3.6

3.4

3.2

3.0 Dielectric Dielectric Constant

2.8 Region of Interest

2 3 4 5 6 7 8 9 10 11 10 10 10 10 10 10 10 10 10 10 Frequency, Hz

Fig. 2.28. Frequency dependence of dielectric constant for PI foil : data for the PI of the PMDA-ODA H-type ( ●)[2.45]; data for the PI of HN type () [2.46]; data for the following PI types [2.47]: 6FDA+3,3'-ODA (□); 6FDA+4,4'-ODA ( ▲); PMDA+3,3'-ODA ( ■); HQDEA + 4,4'- ODA ( ►); BDSDA+4,4'-ODA( ▼); BDSDA+4,4'-ODA( ∆∆∆); HQDEA+4,4'-ODA( ◄); ODPA+4,4'-ODA ( ◊); BTDA+4,4'-ODA ( ♦♦♦); PMDA+4,4'-ODA ( ○○○)

According to Fig. 2.26, our special microcapacitor structure consists of 8 pairs of contact plates, so that the total contact area b ecomes: 8 ´ S = 4.24 ´ 10 -7 m2 . Results of the capacity evaluations according to th e "plate model" are given in the Fig. 2.29. One can see, that this is a linear depen dence, which lies in the range of about 0.056 pF – 0.054 pF. Using a more realistic approach, which we have call ed the "wire model", we have considered the capacity between two cylindrica l ion tracks from different contacts of the microcapacitor, thus taking into ac count the stray fields (see Fig. 2.27, b). For the evaluation we have used the expre ssion for the capacity from Ref. [2.43], according to Fig.2.27 : 155

pe 0e h C wm = (2.6)  2  d  d  ln  +   - 1  r2  r2    0  0  

Here h is the ion track length (25 m m), r0 is the ion track radius, which was taken as 1 m m, and d is the distance between ion tracks, opposing each other at different potentials, which we have taken as 200 m m. It is important to note, that the evaluation according to the expression (2.6) ta kes into account only two opposing ion tracks. However, in the actual device, according to Fig. 2.27 a, we have a set of ion tracks, opposing each other. For this case we need to use the expression: l N = (2.7) F where l is the length of the capacitor contact area (see F ig. 2.26), and FF FF is the ion track density, which in this case was equal to 1 ´ 10 6 cm -2 (see the Chapters 1.2.4 and 1.3.2). Hence, the Cwm from expression (2.6) should be multiplied by N to obtain the total capacity of the structure (neglect ing the superposition of the electric field lines). Results of the capacity evaluations according to th e "wire model" are given in the Fig. 2.29. This is also a linear dependence for the capacity between 0.67 pF and 0.65 pF. The capacity of the microcapacitor was measured as a function of frequency, in the range of 10 MHz to 1 GHz. The measurements wer e done by the vector impedance analyser of HP 4192 type (see the Cha pter 1.2.5). These measurements were compared with the measurements on a commercial capacitor or with a nominal capacity of 4.7 pF (see Fig. 2.29). One can see from the 156

10

Commercial Capacitor

"Wire Model" Evaluation 1

Ion-Track-Based Microcapacitor

Capacity, pF Capacity, 0.1

"Plate Model" Evaluation

0.01 7 8 9 10 10 10 Frequency, Hz

Fig. 2.29. Frequency dependences of the capacities of the ion-track based microcapacitor and commercial capacitor with evaluations by different models. figure, that the curves are quite similar by their behavior, though the capacity values of a commercial capacitor were changing from 5.8 pF to 16.4 pF, and the capacity values of our microcapacitor were changing in the range from 0.5 pF to 0.6 pF. One can note, that the "plate model" leads to a str ong disagreement with experimental data, whereas the "wire model" values are rather close to the experimental curve, but still some disagreement tak es place (see Fig. 2.29). The reason of the last disagreement is seen by us in th e insufficiencies of the instrumental calibration, the neglecting of stray f ield overlapping between neighbouring ion tracks, and also in stochastic dis tribution of the track positions. We can conclude, that the classical electrotechnics remains valid, when scaling down from the millimeter size to the micrometer siz e.

157

Our results indicate, that the ion-track-based micr ocapacitors are useful as data storage devices. The replacement of the opposing pl anar capacitor plates by the opposing sets of ion tracks of different polarities leads to the considerable enhancement of the capacity, due to the increased c harge stored per unit volume.

158

2.1.3. ION TRACK – BASED NANODEVICES

As the ion track nanodevices we will consider here sensors on the base of etched ion tracks. In general, it is clear at present, tha t sensors cover a major field of future potential ion track applications. On the one hand, sensors can probe physical, chemical, and biological parameters, such as temperature, pressure, liquid flow-through, magnetic field strength, humidity, th e presence of photons or more energetic particles, etc. On the other hand, sensor s can determine the concentration of components or trace elements in gases or liquids , or biological activities. The sensor market is rapidly growing with 5.3% per year, covering worldwide 27.8 ´ 10 9 Euro in 1998, and probably covering a volume of 46 ´ 10 9 Euro in 2008. At present, the car industry has the greatest use f or sensors, followed by the product processing industry, household and office e quipment production, construction, machine building and aerospace indust ry [2.44]. There is a number of possible approaches for the re alization of ion-track-based sensors. One can either use the micro- and nanoporo us foils just as porous windows of other sensoring devices, or one can inco rporate the sensor material into the microporous foil itself. Whereas the first appr oach has already found some technological applications, the latter one is in it s developing stage. The most simple concept of creating the sensors on the base of polymer foils with etched tracks is to fill the tracks with the c orresponding sensoring matter and to contact the tracks from both sides (Fig. 2.30, a ). In this case the chemicals to be detected have to diffuse through the polymeric bulk to the sensor. If, for contrast, the sensor is deposited in the shape of tubules, an d if it is connected with concentric contacts (Fig. 2.30, b), the matter to b e probed has a direct access to the sensor. Though being slightly more difficult to rea lize, this construction has a much faster response and greater sensitivity [2.14] . One possibility to enable the concentric contact fo rmation is to irradiate the foil with low energy ions (with range << foil thickn ess), so that the sample sur- faces become saturated with dangling bonds within that range, and they can 159

a b

Fig. 2.30. Principle possibilities of ion track sensor arrangement in etched tracks (after Ref. [2.14]): (a) full coverage of the track with the sensor material, top contacts; (b) tubular track coverage with the sensor material, concentric tubular contacting. The chemicals to be probed (arrows) have to diffuse through the polymer in the first configuration, but they have a direct access to the sensor in the latter arrangement.

subsequently bond the desired metals chemically (Fi g. 2.31, a). Another approach is the evaporation of the contact material under ti lted angles (Fig. 2.31, b). Sensor material is introduced in the tracks afterwards. As it was observed earlier, the conductivity of ful lerite (i.e. the solid state of fullerene, C 60 – see the Chapter 1.1.3) has a strong depend ence on humidity, temperature and pressure, and therefore it has been suggested to use it as a sensor material to be incorporated in ion tracks in PI and PET [2.48]. The tracks in the polymer foils have been etched ac cording to the standard recepy , described in the Chapter 1.5.2. The only differenc e was that the PET foils were etched by KOH solution instead of NaOH solutio n, but the effect was the same.

The C 60 has been incorporated in etched tracks in the form of very smooth thin layers by a heterogeneous precipitation from a satu rated solution in benzene or toluene onto the PET and PI foils with etched t racks, which was repeated several

160

Fig. 2.31. Possibilities of the concentric contacts formation in a polymer foil with etched tracks: (a) irradiation with low energy ions; (b) evaporation under tilted angles (rotation of the foil is used for the effective concentric contact formation). times (see the Chapter 1.6.3). Characteristic SEM p ictures of PI foils with fullerite tubules in the tracks are also given in the Chapter 1.6.3. Besides SEM, control of the etching process and ful lerite deposition has been done by ITS (see the Chapter 1.2.4). One of the rep resentative ITS spectra of this kind for PI with fullerite tubules is presented in Fig. 2.32. The existence of a high energy peak of the transmitted α particles at 5.7 MeV indicates the presence of hollow C 60 structures (i.e. tubules) within the tracks. From the decrease of that peak, a C 60 tubule wall thickness of about 200 nm is derive d, and the shift of the peak at 3.9 MeV to lower energies indicates additio nally the deposition of approximately 250 nm - thick C 60 layer on the both foil surfaces. The samples were contacted by depositing silver paste on both sides of a foil. As it was found in the present research, two types of sensors can be created on the base of polymer foils with fullerite tubules in the tracks. These possibilities will be considered below. 161

Fig. 2.32. ITS spectra of etched tracks in a 25 m thick PI foil: (a) empty tracks (squares), and (b) tracks with C 60 tubules (circles).

2.1.3.1. TEMPERATURE SENSORS ON THE BASE OF PET FOILS

The main result of this research was that practical ly all examined samples of

PET foils with tubules of C 60 in tracks have shown a complex semiconducting behaviour (as it is known, fullerite can display se miconducting properties as a result of intercalation, see Chapter 1.1.3), with t wo mechanisms existing at different temperatures, which can bee seen in the F ig. 2.33. For each temperature interval, the electrical resistance in the figure f ollows an individual Arrhenius correlation (Fig.2.34). From the slope of each corr elation, one can derive the activation energy E according to:

lnd R )T( E = 2 k 10 3 (2.8) 10(d 3 / )T where k = 8.625 ´ 10 -5 eV/K, R is electrical resistance and T is temperat ure in Kelvin units [2.49]. 162

1E11

1E10 R, Ohm heating cooling 1E9

10 20 30 40 50 60 70 80 T, 0C

Fig. 2.33. Temperature dependence of electrical resistance of a representative sample of PET foil with the tubules of C 60 [2.49].

3 Activation energy: E = 3,28 eV A1 2 E = 0,94 eV E A2 A1 )] tr 1

E A2 0 ln[R(T)/R(T -1 R(T ) = 2 x 10 10 Ohm tr

-2 T = 30 0C transition -3 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 10 3/T, 1/K

Fig. 2.34. The Arrhenius plots of a representative sample of PET foil with the tubules of C 60 [2.49]. The resistivities are normalized to the transition temperature between the two branches. Indications on the figures are as follows: R(T tr ) is the resistance at transition temperature between the two conduction regions, T transition is the transition temperature, and E A is the activation energy.

163

The temperature dependence of a resistance of a rep resentative PET sample with fullerite tubules of about 200 nm thickness is give n the Fig.2.33. From this dependence one can see that the sample displays the features of a resistive temperature sensor. Clearly pronounced low-temper ature and high-temperature branches on the R(T) dependence, the border of whic h lies in the area of 25 °C - 35 °C can be concerned with the fact, that after the fullerite incorporation and silver contact deposition, a part of the Ag contac t diffuses into the ion tracks and mixes with fullerite. We can estimate the bulk resistivity per fullerite tubule according to the formula:

Scont r bulk = R (2.9 ) Lcont F where R is the resistance of the sample (which was derived from the R(T) dependences in Fig.2.33); Scont is the cross-section of the fullerite tubule (S cont = 2p rh, where r is the ion track radius, taken as 1 m m, h is the tubule thickness, taken as 0.2 m m), Lcont is the distance between contacts, which correspond s to the foil thickness of 10 m m, and FF FF is the ion track density, being taken as 10 8 cm -2 (see the Chapters 1.2.4 and 1.3.2). Therefore, the estimated resistivity was about 10 12 Ohm- cm. Analyzing the Arrhenius plots for several samples, we have found that activation energy of the low-temperature branches w as in the average about 1.77 ± 0.2 eV, which corresponded nicely to the value deri ved earlier for pure fullerite films deposited by chemical vapour deposition withi n protective Ar gas at low pressure [2.48], and hence it could be regarded as another reconfirmation of the existence of highly resistive tubules of fullerite. The absolute resistance value of the low temperature branch of the C 60 tubules was lower than that of evaporated pure fullerite films by several orders of magnitude . The high temperature branches reveal activation energies values in the average ar ound 0.68±0.2 eV, with noticeable variations in the absolute conductivity values from sample to sample. 164

The first observation of this kind indicates a lowe ring of the energy band gap due to diffusion of the silver from the contacts into t he fullerite rod and formation of

C60 - Ag compounds, which should facilitate the curren t transport. In the previous research [2.48] quite a number of hints have been obtained for an efficient thermal fullerite / metal intermixture . For example, when preparing

C60 /M sandwich structures with M = Pd, Ag, or Cu by ev aporation within protective Ar gas onto substrates held at low tempe rature, dramatic differences have been found in the visible and infrared spectra , the refractive index, the layer thickness, the crystallinity, and the conductivity between pure fullerite layers and the prepared fullerite / metal sandwich structures [2.50]. These examinations have indicated, that each fullerene molecule can bond at least 12.4 Ag atoms, thus giving rise to C 60 Ag 12.4 compounds which appear to have much lower specific resistivities than pure fullerite. Therefore, as the result of these first test experi ments, we have revealed temperature-sensor properties of PET samples with t ubules of C 60 . On the base of these results, we could suggest a creation of submi cron-sized resistors with high resistivity, or thermoresistors with negative tempe rature coefficients of resistance (TCR) [2.49]. Still the main drawback of our results is a bad rep roducibility of the values of electrical characteristics of different samples, wh ich could be observed for instance from the Arrhenius plots, so that the values someti mes were different by a factor 100. One reason here could be that one needs to use the better defined contacts not of Ag, but of other metals, for instance, Cr. This was already taken into account in the further research (see below). A second reason c ould be concerned with different contact areas for different samples, whic h was caused by a simple technology of the silver-paste contact application in these first test experiments. For the future proper studies, we would recommend u sing metallic (e.g. Ag, Al or Cr) contacts being evaporated through masks, as it is done in the electronic industry. 165

The third possible reason is concerned with the str ucture of the fullerite tubules. As it was mentioned above, the fullerite tubule lay ers consist of polycrystallites, mainly oriented perpendicular to the tubule axis (s ee Fig. 2.35 a) [2.51]. As given above estimations show, that the resistivity along the tubule walls (which was actually measured) could be by about a factor of 10 5 higher, than the resistivity along the crystallite axis, so that the resis tivity is anisotropic [2.51]. From the other side, as it was mentioned above, during the s ample preparation, several layers of fullerite were deposited in the tracks, wh ich could lead to the situation, shown in the Fig. 2.35 b. The goal of these procedures was increasing of the layer conductivity, proportionally to the incr ease in the cross-section of C 60 . However it appears that the increase was over-propo rtional, due to additional current paths from one layer to the other, as it is illustrated in the Fig. 2.35 b. For the further proper investigations of this effec t, it will be necessary to prepare samples with fullerite layers of equal width, and t o make the systematic electrical measurements as a function of the number of the lay ers [2.51].

Axial: Axial:

Radial : Radial :

a b Fig. 2.35. Principal sketch of the arrangement of fullerite crystallites in the tubule layers with corresponding resistivity values: (a) one-layer arrangement; (b) multilayer arrangement [2.51].

166

2.1.3.2. PRESSURE SENSORS ON THE BASE OF PI FOILS

A PI foil sample with fullerite tubules of ~ 500 nm wall thickness in the tracks, which corresponded to 15 deposition steps, was used for this research. For the measurements the sample was embedded within 2 glass plates, onto which Cr contacts had been evaporated before [2.52]. Afterwards, the PI sample had undergone compression treatments of unidirectional and isotropic character (see Fig. 2.36), with corresponding regular electrical r esistance measurements. Conditions and some results of the measurements are presented in the Table 2.4. The factor of pressure sensitivity, given in the ta ble, was found by the expression:

R P( ) - R P( ) S = 0 1 (2.10) R P( 0 )( P1 - P0 )

Here R(P 0) and R(P 1) are resistances, which are dependent on initial p ressure values P 0 and final pressure values P 1 , measured during the experiments.

a b Fig. 2.36. Principal diagrams of the pressure treatments and electrical measurements of PI foil sample with C 60 tubules: (a) unidirectional compression; (b) isotropic compression; (after Ref. [2.52])

167

Table 2.4. Parameters and results of the measurements of the pressure dependence of electrical parameters of the PI foil sample with fullerite tubules [2.52].

Structural Dimensions Pressure range Resistance, arrangement Resistivity, Pressure sensitivity Fullerite tubules in 5 m m ((0 ¸0.5 kg) ·9.8 m/s2)/ R = 4.3·10 10 ¸ 5·109 W -6 2 11 etched tracks in track diameter, 25·10 m r bulk = 8·10 ¸ 5 10 polyimide foils, Lcont » 25 m m length 0 ¸ 1.96·10 Pa 9.3·10 W ·cm 3 4 -5 -1 unidirectional ~ 0.5 m m wall 3.92·10 ¸ 2.74·10 Pa S1=2.2·10 Pa 4 5 -6 -1 compression thickness 3.92·10 ¸ 1.96·10 Pa S2=4.6·10 Pa -6 2 Scont =25·10 m

Fullerite tubules in -6 2 5 9 9 Scont » 3.14 ·10 m 0 ¸1.13 ·10 Pa R = 8.2·10 ¸ 2.17 ·10 W etched tracks in 4 10 0 ¸1.33 ·10 Pa r bulk = 1.85·10 ¸ polyimide foils, 10 1.33 ·10 4 ¸1.13 ·10 5 Pa 0.5·10 W ·cm isotropic -5 -1 S1 = 3.6·10 Pa compression -6 -1 S2 = 4.9·10 Pa

Thus, the corresponding values of the low-pressure- branch sensitivity, S 1, and the high-pressure-branch sensitivity, S 2, which are provided in the Table 2.4, were calculated.

The values of the bulk resistivity, rr rr bulk have been estimated from the expression (2.9). In this case the values of Scont are given in the Table 2.4, Lcont was taken as 25 m m, and FF FF was taken as 10 6 cm -2 (see the Chapters 1.2.4 and 1.3.2). Results of the measurements are also presented in t he Fig. 2.37, as pressure dependences of resistance. One can see, that the R(P) dependences consist of two different branches for low pressure (S 1) and high pressure (S 2), respectively. We consider, that the low pressure branch ha s to be attributed largely to the reversible rearrangement of the fullerite crystal lites towards the geometrically most favorable distribution (i.e. reduction of the pore volume between them), whereas the high - pressure branch appears pro bably due to the reduction of the distance of the fullerene molecules [2.53]. 168

4x10 10

3x10 10 S 2

R, Ohm R, 2x10 10

S 1 1x10 10

0.0 0.2 0.4 0.6 Mass of load, kg a

1.0 9 R0= 8.2 x 10 Ohm

at P 0= 1.3 Pa 0.8

0 S 2 R/R 0.6

S 0.4 1

0.2 0 40000 80000 120000 Pressure, Pa b

Fig. 2.37. Pressure dependence of electrical resistance of the Pi foil sample with C 60 tubules: (a) unidirectional compression; (b) isotropic compression [2.52].

169

Therefore, in these first test experiments it was s hown, that thin PI films with fullerite tubules can be used as a sensitive materi al to probe both unidirectional mechanical stress and isotropically applied pressur e by the change in their resistance. This means, that fullerite can be used as a new working material in both pressure and mechanical sensors. The sensitivity fa ctor of the possible fullerite- based pressure sensors (FPS) is about 10 -5 Pa -1 - 10 -8 Pa -1, and thus it is comparable with the sensitivity factor of well-known silicon p ressure sensors [2.52]. However the small sizes and simple production technology of FPS´s opens challenging perspectives for creating a new family of cheap er sensors, and probably it would be possible to construct the not expensive two-dime nsional high-resolution sensor fields. As a good pressure sensitivity was found fr om prevacuum to at least atmospheric pressure, FPS´s could also be applied a s vacuum manometers.

170

2.2. DEVICES ON THE BASE OF SEMICONDUCTORS (TEMPOS)

Though the attempts to improve the characteristics of electronic structures, based on silicon, by swift heavy ion irradiation ha ve been undertaken since long, in the vast majority of all cases this has led only to a deterioration of the electronic properties, degrading in extreme cases the sophisti cated silicon-based structures to simple ohmic resistances. Therefore, today swift he avy ion irradiation of electronic structures is essentially used to control their rad iation hardness.

During the present research it was found out, that after etching ion tracks in SiO 2 and SiON thin layers on a silicon substrates and fi lling them up with highly resistive matter, these nanometric pores can be use d as charge extraction or injection paths towards the conducting channel in t he underlying silicon. In this way, a novel family of electronic structures has be en realized, which has been denoted by the acronym "TEMPOS" ( Tunable Electronic Material with Pores in Oxide on Silicon). Functionality of the TEMPOS structures is determined not only by the material and the thickness of the dielectric layer, but also by the type of silicon substrate, the diameter, the length, the sh ape and areal distribution of the etched tracks, and by the type and the distribution of the (semi)conducting matter deposited within these tracks and on the dielectric surface [2.54]. Depending on the TEMPOS preparation recipe and electrical working po int, the structures may resemble gatable resistors, condensors, diodes, tra nsistors, photocells, or sensors, and therefore they will be rather universally appli cable in electronics. TEMPOS structures are often sensitive to temperature, ligh t, humidity and organic gases. Light-emitting TEMPOS structures have been produced as well. About 35 TEMPOS-based circuits such as thermosensors, photos ensors, humidity and alcohol sensors, amplifiers, frequency multipliers, amplitude modulators, oscillators, flip-flops, and many others have been already designed and successfully tested. Sometimes TEMPOS-based circuit s are more compact than those for conventional electronics. As they are bas ed on matter-filled etched ion 171

tracks in SiO 2 and SiON, their minimum sizes can approach to that one of a single ion track with diameters in the order of a few nm t o a few m. This means that we deal here, in principle, with nanoelectronic device s. In a TEMPOS device of 1 cm 2 area, typically ~ 10 6 to 10 8 of such nanodevices operate simultaneously and parallel to each other.

2.2.1. BASIC PRINCIPLES OF THE TEMPOS

Generally the three basic ideas are used for the TE MPOS concept (Fig. 2.38 [2.54]: a) Replacing the metallic gate electrode on top of the dielectric layer of conventional MOS-FET (metal-oxide-semiconductor fie ld-effect transistor) structures [2.55] by a highly resistive layer L , t hat connects both surface contacts. In that way, the potential distribution o f the device (and hence also the overall charge carrier distribution) is altered , especially in the interface- near silicon channel C. b) Inserting etched ion tracks T within the oxide laye r of MOS devices that are filled with highly resistive matter. In this way, a dditional paths for charge extraction or injection between the conducting chan nel C in the silicon and the top resistive layer L on the oxide surface are form ed that are absent in the classical MOS-FET concept. This leads partly to dra matic deviations of the charge carrier distribution from the classical case . c) Adding a base electrode for controlling of function ality of the device. That concept dates back to the beginning of MOS-FET rese arch [2.55] but was often abandoned later. In FET´s, the effect of such a bas e electrode is similar as that one of a gate electrode.

Procedure of the preparation of the SiO 2/Si and SiON/Si structures was described in the Chapter 1.1.2, and etching of i on tracks in the silicon oxide and

172

Ag nano- particles

Si

a b

Fig. 2.38. (a) Principle sketch of a TEMPOS element (shown here with filling of the tracks and on the surface with silver nanoparticles); (b) schematized wiring of the TEMPOS [2.54]. silicon oxynitride layers was described the Chapter 1.5.2. SEM images of etched tracks, which have conical shapes in the SiO 2 layer and cylindrical shapes in the SiON layer, are given in the Chapter 1.5.3. In the next preparation step, a thin homogeneous la yer of a material with high electrical resistivity, such as fullerite and zinc- phthalocyanine, was chemically deposited from a saturated solution onto the surfac e of the dielectric layer and in the interior of the etched tracks (see the Chapter 1.6.3). A thin layer of indium tin oxide (ITO) was deposited by plasma chemical vapor deposition (PCVD). The material and its thickness were chosen so that to o btain some M to G resistance between the surface contacts “ o” and “ w” (Fig. 2.38 b, see below), and usually some k to M resistance along the tracks. Alternatively, we hav e prepared layers of dispersed nanoparticles of metals or semicon ductors, such as Ag, Pd and

TiO 2. These nanoparticles have been directly deposited by the ELD onto the dielectric surfaces (see the Chapter 1.6.4) [2.54]. In a last step of the TEMPOS structure preparation, the electrical connection of the device was made (Fig. 2.38 b). Sample surface l ayer L was contacted at two positions, “ o” and “ w”, spaced by a few millimeters from each other. Al layer, which was electron-gun evaporated onto the rear sid e of the Si layer, served as the 173 base contact, denoted by “ v”. For simplicity, we used commercial conducting silver paste in these test experiments. The principle scheme of experimental setup unit for the electrical characterization of the TEMPOS structures is given in the Chapter 1. 2.5. The current-voltage measurements of the TEMPOS structur es were often performed between o and v contacts, whereas the contact w was kept at ground potential. Fig. 2.39 shows the principle DC characteristics of TEMPOS structures. Depending on the track resistivity, two nearly comp lementary I/V characteristics emerge. In the given example, these different track resistivities were obtained by filling nanoparticles of the same size distribution and same average nanoparticle- to-nanoparticle distances into tracks that were etched for different times ( i.e. up to narrow and wide diameters, respectively) . It appears that, whereas the Type 1 devices (etching from 3 min to 5 min – Fig. 2.39a) are only little influenced by charge extraction from the silicon channels, the Type 2 devices (etching from

a b Fig. 2.39. The two typical DC current/voltage characteristics of a TEMPOS device: (a) Type 1, (b) Type 2 [2.54]. The I/V curves are smoothed to outline the principle behavior better. The corresponding wiring is shown, too. 174

7 min to 10 min – Fig. 2.39b) are dominated by that effect [2.54]. Whereas the characteristics of Type 1 structures resembles some what that one of the p-n-p transistors, Type 2 structures are rather of the n- p-n type ones. This signifies that track conductivity variations (i.e. control of char ge extraction from the channel, or injection into the channel) enables one to obtain t he same effect (the inversion of the I/V characteristics), that in conventional elec tronics requires adequate alteration of the semiconductor doping. Due to their symmetric construction, the contacts o and w are, in principle, interchangeable. Furthermore, if the overall electr ic resistance along the tracks is chosen to be similar in its order of magnitude as that one on the surface between the contacts o and w, then there is also an astonishingly close similarit y obtained between the electrical characteristics when interch anging the top contacts o or w with the base contact v in electronic circuits, Fig. 2.40. TEMPOS structures are found to be rather stable eve n under some electrical load. For example, devices consuming some 10 mW (operating at, e.g., V vw =

60 V and V ow = 15 V at a temperature of ~ 40 °C) are stable in their characteristics for days to weeks. As at more elevated temperatures corrosion and/or oxidation of the conducting layer might modify the device respon se, such possibly destructive experiments were not performed as yet. If, under otherwise identical conditions, the track s in the TEMPOS structures are missing or existing latent tracks are not etched, t hen no such characteristics are recorded. Instead, one gets a linear I/V relation d escribing only the highly resistive surface coverage. If etched tracks, existing in the dielectric layer, are not filled with weakly conductive matter, no current is record ed for whatever applied voltage, and if the tracks and surfaces are covered with highly conductive matter, then the whole structure is just shortcut. All the se findings indicate that TEMPOS structures are not just modified MOS-FETs, but they are virtually new electronic devices that differ distinctly from the existing on es.

175

a b Fig. 2.40. The DC current/voltage characteristics of a Type 2 TEMPOS device, shown for (a) I o(V ow ) with parameter V vw , and (b) I o(V ov ) with parameter V vw [2.53]. For the third possibility, I v(V vw ) with parameter V ow , see Fig. 2.39b. The I/V curves are smoothed to outline the principle behavior better.

The theory of TEMPOS structures is still at its beg inning, but some parts of their I/V characteristics are already understood [2.56]. In contrast to conventional transistors, TEMPOS structures have additional free parameters, such as the etched track density, position, length, diameter, shape, a nd the type of matter embedded in these tracks and on the surface, which gives a larg e variation of possibilities resulting in partly unconventional new properties. This means that TEMPOS is not just one single new electronic structure but a whol e new electronic family. As a general rule, we may state that the higher int ernal complexity of TEMPOS structures as compared with conventional electronic elements allows one to design TEMPOS-containing circuits with greater simplicity. The reduction in the number of peripheric elements for wiring a given circuit l eads to a gain in operation speed, and hence to the inherent feasibility of TEM POS circuits for high frequency operation. Though we restricted our first TEMPOS ex aminations essentially to DC or very low frequencies, we have got already first hints for the feasibility to operate TEMPOS devices also in the MHz regime. By reducing the TEMPOS structure size from at present ~5 mm ´ 5 mm to a few m2 or less (corresponding to the area 176 occupied by two ion tracks), we expect further cons iderable increases in operation frequency [2.54]. TEMPOS structures are also expected to be inherentl y radiation-hard electronic elements, as the transient addition of a few radiat ion-induced discharge channels to some ~10 8 already existing charge transfer channels (i.e. th e etched tracks) should not alter markedly the overall device characteristi cs. Especially the highly resistive top and track connections should damp the effect of any sudden transient shortcut. Now we will consider more in details some types of the TEMPOS devices, which were obtained by our group.

2.2.2. TEMPOS WITH CONTINUOUS DEPOSITED LAYERS IN THE TRACKS

To the TEMPOS with continuous deposited layers in t he tracks, we can attribute the structures filled with homogeneous highly resis tant materials, such as fullerite

(C 60 ), zinc-phthalocyanine (Zn-Phth), and thin indium t in oxide (ITO) layers, see

Fig. 2.41. In the case of C 60 -TEMPOS, the basic current/voltage characteristics favored is that of a diode, Fig. 2.42. Gating the s tructure with an appropriate negative voltage at electrode o leads to a shift of the diode characteristics to t he left. Then considerable currents can flow even at t he applied voltage V vw , being equal to zero [2.57]. The well-known light and humidity dependences of th e fullerite resistivity influence the C 60 -TEMPOS characteristics, as well. With increasing l ight intensity the current inhibition at negative voltage V ov vanishes, and a negative current I v is observed, Fig. 2.43. This is understood by the form ation of new charge carriers in

C60 by the photoeffect.

Humidity changes the I v/V ov characteristics in a similar way, at highly negati ve

Vov . However, whereas at low V ov light impact also leads only to the negative I v, increasing humidity leads to the emergence of posi tive currents I v . The shape of 177

Fullerite

Si

Fig. 2.41. A principle sketch of a TEMPOS element, as shown for the example of the fullerite covering top of the structure and filling in the tracks [2.57].

Fig. 2.42. The DC current / voltage characteristics of a C 60 -TEMPOS- structure is seen to resemble that one of a tunable diode [2.57].

178

Fig. 2.43. The change of the DC dark current of a C60 -TEMPOS-structure (thick curve) upon visible light impact (day light; dashed curves) or humidity increase (solid thin curves) [2.57]. The presence of alcohol vapor shifts the curve in the same direction as light impact. The measurements are performed for zero gating voltage V vw . the current/voltage characteristics turns somewhat towards that one of antiparallel double diodes. It is also remarkable, that even at V ov = 0 the measured currents I v are non-zero, being positive upon enhanced humidity and negative upon irradiation with visible light. This means that in the latter c ase, C 60 -TEMPOS structures act as photocells, and in the first case they act as hygro cells. The complex behaviour of the I v/V ov curves upon humidity increase may indicate that tw o mechanisms might + - play a role – the mobility of H and OH ions within the C 60 latttice at high voltage on the one hand, and electrochemical corrosion reac tions of C 60 in the presence of moisture on the other hand, which makes the C 60 -TEMPOS behave like a galvanic battery upon humidity [2.57].

It should be noted, that the presence of alcohol va por influences the I v/V ov characteristics at low V ov just in the opposite way as humidity does, i.e. gi ves rise to a negative I v. This means, the change from H 2O to C 2H5OH vapor reverses the polarity of the C 60 -TEMPOS battery. As alcohol is known to quench hole s, it is 179 understood that the presence of that vapor rather p romotes electron migration whereas the presumed C 60 corrosion reactions in the presence of moisture ap pear to promote protonic transport. Finally, it should be n oted that toluene vapor does not influence the current/voltage characteristics of C 60 -TEMPOS at all, as it neither dissociates nor traps charge carriers. We have denoted the moisture sensitive TEMPOS devic es as "MOSBIT" which stands for " MO isture S ensitive structures with Buckminsterfullerene in Ion Tracks". A number of electronic circuits has been de veloped to make use of these novel MOSBIT devices, Fig. 2.44. It has proven to b e especially useful to transform the capacitive changes of a MOSBIT device upon humidity to frequency changes as the latter ones can be measured with ext remely high precision. In our test experiments humidity changes of some estimated 20 % to 30 % initiated frequency changes of typically 22.7 kHz, for a base frequency of 2967.50 MHz.

When replacing C 60 by a thin ITO film, one obtains I v/V ov curves, being quite similar to those ones shown in Fig. 2.43. However, the characteristics are somewhat flatter, i.e. the currents I v are lower for the same voltage V ov by factors of up to one order of magnitude. Also ITO-TEMPOS be haves somewhat similar as a gatable battery, insofar as there are non-zero cu rrents I v for zero applied voltage

Vvw and slightly non-zero V ow . Finally, when depositing Zn-phthalocyanine on a TEM POS base structure, one obtains characteristics, such as shown in Fig. 2.45 . In contrast to the diode-like

characteristics of C 60 -TEMPOS and ITO-TEMPOS, the characteristics of Zn- Phth-TEMPOS resembles more that one of a double dio de structure, for yet unknown reason. Upon application of a negative gate voltage, the positive current branch shifts in the same way as observed above for the other structures. It is interesting, that gate voltage tends to block the n egative currents to some extent. Irradiation of Zn-Phth-TEMPOS with visible light le ads to the exactly opposite behaviour as gating by an applied voltage, in bo th quadrants. Zn-Phth-

TEMPOS devices do not act as batteries as do C 60 - TEMPOS structures and ITO-TEMPOS structures, 180

Fig. 2.44. Some circuits made with MOSBIT elements for sensor and power supply applications [2.57].

Fig. 2.45. The DC current / voltage characteristics of a Zn-Phthalocyanine- TEMPOS-structure, in dependence on the gating voltage and visible light impact [2.57]. 181

as there is always I v=0 for V vw =0, for whatever V ow . It is important to note, that the Zn-Phth-TEMPOS curves show some noise. This eff ect has been observed also for the TEMPOS with Ag nanoparticles, and it w as attributed to spontaneous current transitions that are accompanied by light s parks in all colors (see the next chapter). However, visible light emission was not y et verified for Zn-Phth- TEMPOS [2.57].

2.2.3. TEMPOS WITH NANOPARTICLES IN THE TRACKS

As it was already mentioned above, we have found it quite useful to deposit dispersed (semi)conducting nanoparticles (NPs) into the TEMPOS structures, as the variation of the NP distance and NP areal densi ty allows one to adjust the layer resistance to any value between conducting and insu lating ones. Depending on these latter parameters, as well as on the sample t emperature, the applied voltage, and the purity of sample preparation, the physical conduction mechanism is based on either ohmic conductivity, percolation, tunnelin g, Schottky emission, field emission, or ionic conductivity (the latter one ste mming from intrinsic contaminants of the oxide layer, and eventually als o from etchant residues or adsorbed aerosols) [2.17]. In our previous tests t he Schottky emission mechanism was, in general, predominant. Such layers with NPs can be easily tailored to obtain any resistivity value between insulating and conduc ting state, by varying the NP- to-NP distance. We have already used NPs of Ag, Pd, TiO 2, and of NP / polymer composites, such as CdS-NP / polyethylene oxide and Au-NP / teflon. Here we restrict ourselves mostly to the TEMPOS str uctures with thermally grown SiO 2 as the dielectric layer, and with dispersed silver NPs on the oxide layer and in the tracks (Fig. 2.46). The production proce dure of these Ag NPs by chemical or ELD deposition from an oversaturated am oniacaline Ag complex salt solution has been described in the Chapter 1.6.3. Fig. 2.47 shows the SEM image 182 of the surface of such a TEMPOS structure. It was determined from this image, as well as from the other similar images, that the silver NPs have diameters in the ~10 nm to ~100 nm range, and distances be tween the NPs are in the order of ~ 10 nm to ~200 nm. Fig. 2.48 gives the corresponding current/voltage c haracteristics of the surface layer as contacted by o and w. It is important to note, that in this case two currents are flowing parallel to each other: (a) from contac t o through the tracks towards the conducting Si channel, and back from that channel t hrough other tracks towards contact w, and (b) from contact o through the Ag NP surface layer towards contact w. In the example presented here, the latter current appears to be inferior than the first one [2.57]. As expected, the characteristic differs strongly fr om ohmic resistivity (Fig. 2.48c). It is double-diode-like in its shape, due t o the two counteracting Schottky diode transitions between the metallized track s and the semiconducting channel. One can simulate such a layer by an equivalent circ uit consisting of two antiparallel diodes, or even better by two pairs (D o and D w) of antiparallel diodes as shown in the insert of Fig. 2.49a. The currents drop considerably with increasing temperature (Fig. 2.49 b).

Ag NPs

Fig. 2.46. Principle sketch of a TEMPOS structure with silver NPs [2.58].

183

Fig. 2.47. SEM image of Ag NPs deposited onto the oxide layer surface and into the etched tracks (circular structures) [2.58].

Fig. 2.48. I w/V ow DC characteristics of the surface of a TEMPOS structure with p-Silicon and Ag NPs [2.58]: (a) at ambient temperature; (b) at 60 °C; (c) for comparison, current/voltage characteristics of a thin Fe foil showing perfect ohmic behaviour (scale here: current ´´ ´´ 200, voltage /1000).

184

The TEMPOS structures, made of such Ag NPs are also found to be light sensitive (Fig. 2.49 a,b), their resistance decreas ing with increasing light intensity immediately (in the I v(V vw ) diagram, Fig. 2.49 b) or above a threshold of a few volts (in the I o(V ow ) diagram, Fig. 2.49 a) - this may indicate the exi stence of two different mechanisms. Fig. 2.49c shows that both th e TEMPOS capacity and conductivity increase with growing light intensity Φ. This enables one to use TEMPOS elements for the construction of highly sens itive photometers, as capacitive changes can be easily transferred to changes in frequency by inserting the TEMPOS device into an oscillating circuit, and as frequency changes belong to those ones, that can be measured most precisely in physics. It is also important to note in Fig. 2.49 b, that a current I v flows even at zero voltage V vw , the magnitude of the extracted current I v(Φ) depending on the gating voltage V ov - i.e. the TEMPOS device acts here as a gatable photocell. Fro m both the TEMPOS capacity C and resistance R one can derive the diel ectric relaxation time τ = RC to be in the order of 180 s for a TEMPOS device of about 5 mm ´ 10 mm size. Hence, scaling down the TEMPOS elements to m sizes, one might expect relaxation times in the order of ps to ns. The curves shown in Fig. 2.50a can be regarded as b eing characteristic for standard TEMPOS structures. A voltage applied to th e third contact enables a gating of the current between the other two contact s, similarly as it is given for transistor structures. Permutation of the TEMPOS co ntacts used as source, drain and gate hardly alters the overall appearance of th e characteristics (see the Figs. 2.39 and 2.40 in the Chapter 2.2.1). The insert in Fig. 2.50b shows a possibile equivalent circuit of such a TEMPOS structure. It i s seen that such equivalent circuits are capable to explain many of the basic T EMPOS features, such as the diode-like behaviour expressed by the blockade of c urrent I v at negative V vw and the strong current throughput at positive V vw , for whatever gating voltage V ow [2.58]. However, some points cannot be explained by this si mple equivalent circuit. On the one hand, the current/voltage curves are non-z ero for a negative gating voltage 185

a

b

c

Fig. 2.49. DC current/voltage characteristics of a TEMPOS structure, consisting of n-Si / SiO 2 with Ag NPs [2.58]: (a) for the I o(V ow ) relation with V ow = 0, (b) for the I v(V vw ) relation with V ov = -17 V. Both curves show a pronounced dependence on light irradiation. (c) Dependence of the TEMPOS internal capacity Cvw and conductivity G vw on the power Φ of the incident light. The insert in (b) shows the measuring arrangement, and the one in (a) gives the used equivalent circuit.

186

a

b

Fig. 2.50. DC TEMPOS characteristics, as seen in the I v(V vw ) diagram with parameter V ow [2.58]. Measurement with: (a) the TEMPOS structure; (b) the equivalent circuit, as given in the insert;

Vow , especially at negative V vw . On the other hand, the saturation region of near- constant currents I v at excessively applied voltage V vw is much less pronounced in TEMPOS structures than in equivalent circuits. Thes e features may be attributed to 187 leakage currents between the individual diodes of t he equivalent circuit that are not reproduced by the simple equivalent circuit accordi ng to Fig. 2.50 b. Furthermore, noise shows up at elevated drain and g ating voltages (see, e.g. Figs. 2.49a and 2.49b). This finding might indica te some spontaneous discharges of the NPs upon sufficiently high applied field str engths. It is worthy to note that in these cases the local differential resistance occ asionally becomes negative, which means that one obtains amplification at such workin g points. Up to now, the optimum voltage amplification of our test structure s was found to be ~ 5.8, with concomittent current amplification of 3.7 and power amplification of 1.1. In another working point, we even obtained a power amp lification of 24, but here the voltage amplification was only as large as 2.1 [2.5 8]. The above-mentioned current fluctuations are often accompanied by spontaneous light emission in all visible colors fr om sub-micrometric regions [2.53]. For a typical measuring arrangement in this case see Fig. 2.51. Yet we could not clarify, whether all the tracks, all the NPs, or only those NPs, which are situated in tracks, are responsible for this effect . Anyhow, this special type of light emission is most pronounced in the very vicinity of the top electrodes. What concerns the TEMPOS structures with SiON laye rs and Ag nanoparticles, prepared in the similar way as the structures with SiO 2 layers, it is important to note that these ones are capable to emit light in t he vicinity of the contact electrodes. The light emission is not homogeneously distributed over the whole sample surface, but it concentrates only on individual sma ll spots. The light is not emitted continuously but intermittently with different colo rs so that the impression of a sky with sparkling colored stars is obtained under the optical microscope. The intensity of this light emission was larger being compared wi th the similar effect from the

SiO 2-coated samples, however with much more reduced int ensity. Due to the low intensity of the emitted light we di d not yet succeed in taking a picture of this effect. The light emitted fr om SiON-layered structures increases 188

Fig. 2.51. Measuring arrangement for the studies of light emission from TEMPOS structures [2.58]. with increasing current through them. Whereas for I = 5 mA, the light intensity emitted from ~200 miniature light sources was as lo w as 190 pW, it increased via 779 pW at 10 mA to 1.36 nW at 15 mA current (the me asurements were performed with a photocell connected with a high frequency os cillator with a sensitivity of 212 pW/kHz). At present we do not know, which mechanism of light emission works in this case. As only tiny spots are light-emitting, we ass ume that metallic NPs are responsible for that effect. The light emission con centrates in the vicinity of the electrodes made of conducting silver paste. This mi ght be explained by the fact, that the current density increases exponentially wi th decreasing distance to an electrode.

As light emission from Si/SiO 2/Ag or Si/SiON/Ag structures without any etched tracks was not observed, we conclude that conductin g tracks are indeed vital components of the TEMPOS structures, being like med iators for currents between the highly resistant top layer on the dielectric su rface and the conducting channel beneath.

189

SUMMARY AND CONCLUSIONS

The goal of this work was the investigation of the possibilities of creation of micro- and nanoelectronic devices on the base of th e swift heavy ion tracks, for the sake of the future new technology. During the imple mentation of the work, the ways of realization of that goal were studied, and some first prototypes of ion- track-based micro- and nanoelectronic devices on th e base of polymers and semiconductors were created. The following main results of this work are obtaine d: 1. Based on study of the irradiation and etching condi tions of ion tracks in

PI and PET polymer foils and also in SiO 2 and SiON dielectric layers on a Si substrate the properties of the etched tracks were characterized. The main techniques of this study were Ion Transmission Spectrometry (ITS), optical microscopy and scanning electron microscopy (SEM); 2. A new formula for the determination of the radius o f the etched ion tracks in transparent foils has been derived for th e ITS technique, which takes into account the angular divergence of the pr obing particle beam. The formula was successfully tested for the experim ental samples. Also the comparison with SEM data has verified its feasi bility; 3. The conditions of the ElectrodeLess Deposition (ELD ) technique of silver, copper and nickel into the ion tracks have been investigated and determined; 4. The formation of Ag, Cu and Ni nanotubules in etche d ion tracks in the PET and PI polymer foils with ion tracks by means o f the ELD was carried out. The important role of an activation st ep before the ELD for the formation of nanotubules in ion track has been proven. Investigations of electrical properties of the nanotubules were pe rformed with the analysis of the possible conduction mechanisms. The y showed that the tubules reveal Ohmic conduction only after continuo us film on the inner surface of the track is formed, and that in the ini tial stage the Shottky 190

mechanism dominates. Investigations of the temperat ure dependence of the nanotubule stability have shown, that the inser tion of metallic tubules into etched tracks guaranteed their mechanical stab ility within the range of stability of the polymers. This means that in th e case of high temperature - stable PI (up to » 500 °C), the tracks showed good performance up to that temperature. On the other ha nd, they failed to be beneficial in the case of PET, due to its easy soft ening already at moderate temperatures (such as » 100 - 150 °C), the track stability was worse. 5. First prototypes of micromagnets and microtransfor mers on the base of PI foils with etched ion tracks have been created. They have been shown to exhibit good quality factors up to ~7 at working frequencies of about 0.5 GHz, and reasonable coupling factor of ~90%, re spectively; 6. A first prototype of the microcondensor on the base of PI foil with etched ion tracks has been created. It has shown practical ly frequency- independent capacity of 0.5 pF - 0.6 pF up to 1 GHz . This result was confirmed by a theoretical estimation; 7. According to the studies of temperature and pressur e dependences of resistance of PI and PET polymer foils with fulleri te tubules in the ion tracks, it was shown, that temperature and pressure sensors can be created in this way; 8. A family of new electronic devices, denoted by the acronym "TEMPOS" (Tunable Electronic Material with Pores in Oxide on Silicon) has been

realized for SiO 2/Si and SiON/Si structures with etched ion tracks i n the

SiO 2 and SiON dielectric layers. The functioning of the TEMPOS structures is determined not only by the material a nd the thickness of the dielectric layer, but also by the type of silicon s ubstrate, the diameter, the length, the shape and areal distribution of the etc hed tracks, and by the type and the distribution of the (semi)conducting m atter deposited within these tracks and on the dielectric surface. Two mai n classes of the 191

TEMPOS devices have been studied: (1) the structure s with continuous deposited layers in the tracks, and (2) the structu res with nanoparticles in the tracks. It was found that, depending on the TEM POS preparation recipe and electrical working point, the devices ma y resemble gatable resistors, condensors, diodes, transistors, photoce lls, or sensors, and therefore they will be are rather universally appli cable in electronics. TEMPOS structures are often sensitive to temperatur e, light, humidity and organic gases. Light-emitting TEMPOS structures have been produced as well. About 35 TEMPOS-based circuits su ch as thermosensors, photosensors, humidity and alcohol s ensors, amplifiers, frequency multipliers, amplitude modulators, oscill ators, flip-flops, and many others have been already designed and successf ully tested. Relative simplicity and low cost of production together with compact size can be attributed to the advantages of the TEMPOS structur es. In general, it can be concluded from the present re search, that etched ion tracks in polymer foils and silicon-based dielectric layer s can be useful for production of micro- and nanoelectronic devices. A large range of possible applications for these structures can be determined. The present scientifi c and technological state-of-the art makes it possible to realize the creation of th e new ion-track-based devices.

192

REFERENCES

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PART 2

2.1. D.Fink, A.V.Petrov, V.Rao, M.Wilhelm, S.E.Demyanov, P.Szimkowiak, M.Behar, P.S.Alegaonkar, L.T.Chadderton, Productio n Parameters for the Formation of Metallic Nanotubules in Etched Tracks, Radiation Measurements, 2003, V.36, p. 751 – 755 2.2. A.V. Petrov, D. Fink, M.Müller, I.Sieber, M. Wilhelm, N.Stolterfoht, R. Papaleo, A. Schulz, J. Vacik, J. Cervena, V. Hnatow icz, L. T. Chadderton, A. S. Berdinsky, W. Fahrner, Nanotubule Formation by Heterogeneous Nucleation of Silver Along Etched Nuclear Tracks in Polyethylene Terephthalate (PET), Proceedings of the 3 rd Siberian Russian Workshop and Tutorials EDM'2002, 1-5 July 2002, Erlagol, Russia, p. 31-35 2.3. C.R.Martin, Nanomaterials: a Membrane-Based Synthet ic Approach, Science, 1994, V.266, p. 1961-1966 2.4. J.C.Hulteen, C.R.Martin, Template Synthesis of Nano particles in Nanoporous Membranes. In: "Nanoparticles and Nanost ructured Films", Ed. by J.H.Fendler, Wiley-VCH, Weinheim, 1998, p. 235-2 62 2.5. K.B.Jirage, J.C.Hulteen, C.R.Martin, Effect of Thio l Chemisorption on the Transport Properties of Gold Nanotubule Membranes, Anal. Chem., 1999, V.71, p. 4913 – 4918 2.6. C.J.Brumlik, V.P.Menon, C.R.Martin, Template Synthe sis of Microtubule Ensembles Utilizing Chemical, Electrochemical, and Vacuum Deposition Techniques, J. Mater. Res., 1994, V.9, p. 1174-11 82 2.7. V.M.Cepak, J.C.Hulteen, G.Che, K.B.Jirage, B.B.Laks hmi, E.R.Fisher, C.R.Martin, Chemical Strategies for Template Synthe ses of Composite in Micro-and Nanostructures, Chem. Mater., 1997, V.9, p. 1065-1067 2.8. C.R.Martin, Template Synthesis of Polymeric and Met al Microtubules, Adv. Mater., 1991, V.3 , p.457-459 2.9. "Brockhaus´ Konversations-Lexikon", Brockhaus Leipz ig, Berlin und Wien, 14 th ed., 1895, V.15, p. 149 (in German) 206

2.10. K.Thöne, "Chemie im täglichen Leben", Hallwag, Bern, 1955, p. 14-16 (in German) 2.11. J.Vacik, J.Cervena, V.Hnatowicz, D.Fink, R.Klett, N ew Technique for Nondestructive Examination of Latent Track Etching, Radiation Effects and Defects in Solids, 1997, V.140, No. 3-4, p. 307 - 3 11 2.12. A.V. Petrov, D. Fink, S.E.Demyanov, M.Müller, I.Si eber, A.S.Berdinsky, H.-G.Chun, L.T.Chadderton, The Conduction Mechanism s of Cu and Ni Nanotubules at Their Different Formation Stages, Pr oceedings of the 7 th Korean-Russian International Symposium on Science a nd Technology (KORUS-2003), Ulsan, Korea, June 28 – July 6, 2003 (in press) 2.13. AMI DODUCO GmbH & Co., Geschäftsfeld Oberflächentechnik, Im Altgefäll 12, D-75181 Pforzheim, Technical Information Materials, 2001 (in German) 2.14. "Ion Irradiation of Polymers: Fundamentals and Appl ications", Vol.2, Ed. by D.Fink, Springer, Berlin/Heidelberg, 2004 (in press ) 2.15. D.Fink, R.Klett, Conductivity of Single Ion Tracks in Polymers, Radiation Effects and Defects in Solids, 1994, V.132, p. 27-3 0 2.16. M.Gheorgiu, G.Popa, M.Pascu, C.Vasile, Chemical and Physical Surface Modifications of Polymers by Ion Beam Treatments, I n: "Metallized plastics", Ed. by K.L.Mittal, Marcel Dekker Inc., N ew York, 1998, p. 269- 279 2.17. S.M.Sze, "Physics of semiconductor devices", John W iley & Sons, 1981 2.18. "Hopping Transport in Solids", Ed. by M. Pollak and B.I. Shklovski, North- Holland, Amsterdam, 1991. 2.19. A.K.Fedotov, Private communication, Belarussian Sta te University, Minsk, Belarus, 2003

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2.20. D.Fink, P.S.Alegaonkar, A.V.Petrov, J.Cervena, V.H natovicz, J.Vacik, P.Szimkoviak, I.Sieber, M.Wilhelm, A.S.Berdinsky, H .-G.Chun, L.T.Chadderton, Thermal Behavior of Etched Tracks a nd Embedded Metallic Nanotubules, Proceedings of the 7 th Korean-Russian International Symposium on Science and Technology (KORUS-2003), U lsan, Korea, June 28 – July 6, 2003 (in press) 2.21. J.Vacik, J.Cervena, V.Hnatowicz, S.Posta, D.Fink, R .Klett, P.Strauss, Simple Technique for Characterization of Ion-Modifi ed Polymeric Foils, Surf. Coat. Technol., 2000, V.123, p. 97-100. 2.22. D.Lincot, M.Froment, H.Cachet, Chemical Deposition of Chacogenide Thin Films From Solution, Adv. Electrochem. Sci. Eng., 1 999, V.6, p. 165-235. 2.23. H.Yamaguchi, Y.Sato, T.Kataoka, Loss Characteristic s of Air-Core Superconducting Transformer, IEEE Transact. on Magn ., 1992, V.28 p. 2232-2234 2.24. T.Mulvey, Proc. “Scanning Electron Microscopy 1974” , Ed. by O.Johari and I.Corvin, Ill. Technol. Res. Inst., Chicago, 1994, p. 43-50 2.25. R.Neuhaus, Tauchspulensystem zur Ansteuerung eines miniaturisierten Druck-Servoventils, Industrie-Anzeiger, 1977, V.60, p. 1156-1160 (in German) 2.26. Siemens Ltd., Toute l´Electronique, 1989, V.548, p . 24-26 2.27. K.Kawabe, H.Koyama, K.Shirae, Planar Inductor, IEEE Trans. Magn., 1984, Vol. MAG-20, p. 1804-1806 2.28. H.Matsuki, K.Murakami, A New Cloth Inductor Using A morphous Fiber, IEEE Trans. Magn., 1985, Vol. MAG-21, p. 1738 – 174 0 2.29. J.R.Spadea, S.M.Wright, Proc. 19 th Intern. Conf. Of IEEE Engineering in Medicine and Biology Soc., 1997, V.1, p. 464-466 2.30. G.Y.Tjan, Z.X.Zhao, R.W.Baines, P.Corcoran, Mechat ronics, 1999, V.9 p. 317-327 208

2.31. C.H.Ahn, Y.J.Kim, M.G.Allen, A Fully Integrated Pla nar Toroidal Inductor with a Micromachined Ni-Fe Magnetic Bar, IEEE Trans act. on Components, Packaging and Manufactoring Technology, 1994, V.A17 , p.463-469 2.32. J.Y.Park, M.G.Allen, High Current Integrated Microi nductors and Microtransformers Using Low Temperature Fabrication Processes, Microelectronics International, 1997, V.14, p.8 - 11 2.33. J.Y.Park, M.G.Allen, A Comparison of Micromachined Inductors with Different Magnetic Core Materials, Proc. of the IEE E 46 th Electronic Components & Technology Conference, 1996, p.375 - 3 81 2.34. D.Taghezout, A New Micromachined Disc Type Motor, “ Sensor 95”, ACS Organisations GmbH des Fachverbandes für Sensorik, 1995, p. 715-720 2.35. M.Yamaguchi, M.Matsumoto, H.Ohzeki, K.I.Arai, Fabri cation and Basic Characteristics of Dry-Etched Micro Inductors, IEEE Transact. on Magnetics, 1990, V.26, p. 2014-2016 2.36. K.Wakiwaka, S.Suyama, T.Mizuno, S.Yamamoto, Trans. of the Inst. of Electrical Eng. of Japan, 1994, V.114-D, p.325-330 2.37. S.Kawata, K.Okamoto, S.Shoji, Mol. Cryst. Liq.Crys t., 1998, V.314, p.173- 178 2.38. O.Oshiro, H.Tsujimoto, K.Shirae, A Novel Miniature Planar Inductor, IEEE Trans. Magn., 1987, V. MAG-23, p. 3759-3761 2.39. K.Yamasawa, K.Maruyama, I.Hirohama, P.Biringer, Hig h Frequency Operation of a Planar-Type Microtransformer and Its Application to Multilayered Switching Regulators, IEEE Trans. Magn ., 1990, V. MAG-26 p.1204-1209 2.40. A.Schulz, SDK-Technik GmbH Product information, Que dlinburg, Germany, 2000-2002. 2.41. D. Fink, P.S.Alegaonkar, A.V. Petrov, A.S.Berdinsky , V.Rao, M.Müller, K.K.Dwivedi, L.T.Chadderton, The Emergence of New I on Track Applications, Radiation Measurements, 2003, V.36, p.605 – 609 209

2.42. M.Lindeberg, K.Hjort, Statistics on Flexible Circui try Based on Ion Track Defined Nanowire Cluster Links, Proc. of the Worksh op on the European Network on Ion Track Technology, Caen, France, Febr uary 24-26, 2002, p.23, p.5 2.43. K.Küpfmüller, "Einführung in die theoretische Elekt rotechnik", Springer- Verlag, Berlin/Göttingen/Heidelberg, 1962 (in Germa n) 2.44. A.V.Petrov, D.Fink, G.Richter, P.Szimkoviak, A.Chem seddine, P.S.Alegaonkar, A.S.Berdinsky, L.T.Chadderton, W.R. Fahrner, "Creation of Nanoscale Electronic Devices by the Swift Heavy Ion Technology", Proceedings of the 4 th Siberian Russian Workshop and Tutorials EDM'2003, Erlagol, Russia, July 1-4, 2003, p.40 – 45 2.45. P.S.Alegaonkar, "Studies on Radiation Induced damag e and elemental diffusion in polyimide", Ph.D.Thesis, Pune Univers ity, India, 2004 2.46. "Du Pont – Kapton® : Summary of Properties", Inform ation Materials, 2003 2.47. J.O.Simpson, A.K.St.Clair, Fundamental Insight on D eveloping Low Dielectric Constant Polyimides, NASA Technical Repo rts, 1997, p.1-27 2.48. A.S.Berdinsky, Yu.V.Shevtsov, A.V.Okotrub, S.V.Tomb in, L.T. Chadderton, D.Fink, J.M.Lee, Sensor Properties of Fullerene Films and Fullerene Compounds with Iodine, Chemistry for Sust ainable Development, 2000, V.8 , p.141-146 2.49. A.S.Berdinsky, D.Fink, A.V.Petrov, M.Müller, L.T.Ch adderton, J.F.Chubachi, M.H.Tabacniks, Formation and Conducti ve Properties of Miniaturized Fullerite Sensors, Proc. MRS Fall Bos ton, Boston, USA, November 27 –December 1, 2001, Contr. No. Y4.7 2.50. A.S.Berdinsky, Yu.V. Shevtsov, Yu.A.Saranchin, S.V. Trubin, Yu.V. Shubin, B.M. Ayupov, D.Fink, L.T.Chadderton, J.H.Lee, The S tudy on Electro- physical Properties of Sandwich Structures Based on Fullerite Films, Proceedings of the 4 th Korean-Russian International Symposium on Science and Technology (KORUS-2000), Ulsan, Korea, June 27 - July 1, 2000, p.181-186 210

2.51. A.S.Berdinsky, Private communication, Novosibirsk S tate Technical University, Russia, 2004 2.52. A.S.Berdinsky, D.Fink, A.V.Petrov, H.-G.Chun, L.T.C hadderton, V.A.Gridchin, P.S.Alegaonkar, Pressure Dependence o f Conductivity of Fullerite Structures, Proceedings of the 7 th Korean-Russian International Symposium on Science and Technology (KORUS-2003), U lsan, Korea, June 28 – July 6, 2003 (in press) 2.53. A.S.Berdinsky, D.Fink, A.Petrov, L.T.Chadderton, S. M.Krasnoshtanov, E.S.Rylova, The Effect of External Mechanical Stres s On the Fullerite Conductivity, Proc. of the 4 th Siberian Russian Workshop and Tutorials EDM'2003, Erlagol, Russia, July 1-5, 2002, Vol. 1, p.40-44 2.54. D.Fink, A.Petrov, K.Hoppe, W.R.Fahrner, R.M.Papaleo , A.Berdinsky, A.Chandra, A.Chemseddine, A.Zrineh, A.Biswas, L.T.C hadderton, Etched Ion Tracks in Silicon Oxide and Silicon Oxynitride as Charge Injection Channels for Novel Electronic Structures, Nuclear I nstr. and Methods in Physics Research, 2004 (in press) 2.55. P.Richman, "Characteristics and Operation of MOS Fi eld Effect Devices", Mc Graw-Hill, Inc., New York, 1967 2.56. D.Fink, A.Petrov, W.R.Fahrner, K.Hoppe, A.G.Ulyashi n, R.M.Papaleo, TEMPOS - A Novel Electronic Structure Based on Etch ed Ion Tracks in Silicon Oxide and Oxynitride as Charge Extraction C hannels, Proc. of the ISEC, Peking, China, 2003 2.57. D.Fink, A.V.Petrov, W.R.Fahrner, K.Hoppe, R.M.Papal eo, A.S.Berdinsky, A.Chandra, A.Zrineh, L.T.Chadderton, Ion Track Base d Nanoelectronics, Proceedings of the International Conference on Nano -Science and Technology (INCONSAT), Kolkata, India, December 17 – 20, 2003 (in press)

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2.58. D.Fink, A.Petrov, K.Hoppe, W.R.Fahrner, Characteriz ation of "TEMPOS": A New Tunable Electronic Material with Pores in Oxi de on Silicon, Proceedings of the MRS Fall Meeting, Boston, USA, D ecember 1-5, 2003 (in press)

212

PUBLICATION LIST

Results of the research were presented in the follo wing publications:

ARTICLES IN JOURNALS AND PERIODIC ISSUES

1. A.S.Berdinsky, D.Fink, A.V.Petrov, M.Müller, L.T.Ch adderton, J.F.Chubachi, M.H.Tabacniks, Formation and Conductive Properties of Miniaturized Fullerite Sensors, Proc. MRS Fall Boston, Boston, USA, Nove mber 27 – December 1, 2001, Contr. No. Y4.7 2. N.Stolterfoht, J.-H.Bremer, V.Hoffmann, R.Hellhamme r, D.Fink, A.Petrov and B.Sulik, Transmission of 3 keV Ne 7+ Ions through Nanocapillaries Etched in Polymer Foils: Evidence for Capillary Guiding, Phys .Rev.Letters, 2002, VOL.88, NO.13, P.133201-1 – 133201-4. 3. D.Fink, M.Müller, A.Petrov, R.Klett, L.Palmetshofer , V.Hnatowicz, J.Vacik , J.Cervena, L.T.Chadderton, Aqueous Marker Penetrati on into Ion Irradiated Polyimide, Nuclear Instr. and Methods in Physics Re search, 2002, Vol. B191, p.662 – 668. 4. D.Fink, A.Petrov, M.Müller, T.Asmus, V.Hnatowicz, J .Vacik, J.Cervena, Electrolyte Penetration into High Energy Ion Irradi ated Polymers, Surface and Coatings Technology, 2002, Vol. 158-159, p. 228-233 5. A.V. Petrov, D. Fink, M.Müller, I.Sieber, M. Wilhelm, N.Stolterfoht, R. Papaleo, A. Schulz, J. Vacik, J. Cervena, V. Hnatow icz, L. T. Chadderton, A.S. Berdinsky, W. Fahrner, Nanotubule Formation b y Heterogeneous Nucleation of Silver Along Etched Nuclear Tracks in Polyethylene Terephthalate (PET), Proceedings of the 3 rd Siberian Russian Workshop and Tutorials EDM'2002, 1-5 July 2002, Erlagol, Russia, V.1, p. 31-35 6. N. Stolterfoht, D. Fink, A.V. Petrov, M. Müller, J. Vacik, J. Cervena, V. Hnatowicz, L.T. Chadderton, A.S. Berdinsky, Charac terization of Etched Tracks and Nanotubules by Ion Transmission Spectrom etry, Proceedings of the 213

3rd Siberian Russian Workshop and Tutorials EDM'2002, Erlagol, Russia, 2002, V.1, p. 36-39 7. A.S.Berdinsky, D.Fink, A.Petrov, L.T.Chadderton, S. M.Krasnoshtanov, E.S.Rylova, The Effect of External Mechanical Stres s On the Fullerite Conductivity, Proc. of the 4 th Siberian Russian Workshop and Tutorials EDM'2003, Erlagol, Russia, July 1-5, 2002, Vol. 1, p.40-44 8. D. Fink, A. Petrov, N. Stolterfoht, M. Wilhelm, V. Hoffmann, A. Richter, M. Behar, L. Farenzena, R. Papaleo, L. T. Chad derton, A. Schulz, W. R. Fahrner, Creation of Nanoscale Objects by Swift Heavy Ion Tr ack Manipulations, Proceedings of the International Symposium on Mater ial Chemistry in Nuclear Environment, Tsukuba, Japan, March 13 – 15, 2002, p . 171 - 178 9. D. Fink, P.S.Alegaonkar, A.V. Petrov, A.S.Berdinsky , V.Rao, M.Müller, K.K.Dwivedi, L.T.Chadderton, The Emergence of New I on Track Applications, Radiation Measurements, 2003, V.36, p.605 – 609 10. D.Fink, A.V.Petrov, V.Rao, M.Wilhelm, S.E.Demyanov, P.Szimkowiak, M.Behar, P.S.Alegaonkar, L.T.Chadderton, Productio n Parameters for the Formation of Metallic Nanotubules in Etched Tracks, Radiation Measurements, 2003, V.36, p. 751 – 755 11. A.V.Petrov, D.Fink, G.Richter, P.Szimkoviak, A.Chem seddine, P.S.Alegaonkar, A.S.Berdinsky, L.T.Chadderton, W.R. Fahrner, Creation of Nanoscale Electronic Devices by the Swift Heavy Ion Technology, Proceedings of the 4 th Siberian Russian Workshop and Tutorials EDM'2003, Erlagol, Russia, July 1-4, 2003, p.40 – 45

214

CONFERENCE PRESENTATIONS

1. A.S.Berdinsky, D.Fink, A.V.Petrov, M.Müller, L.T.Ch adderton, J.F.Chubachi, M.H.Tabacniks, Formation and Conductive Properties of Miniaturized Fullerite Sensors, Proc. MRS Fall Boston, Boston, USA, Nove mber 27 – December 1, 2001 2. D. Fink, A. Petrov, N. Stolterfoht, M. Wilhelm, V. Hoffmann, A. Richter, M. Behar, L. Farenzena, R. Papaleo, L. T. Cha dderton, A. Schulz, W. R. Fahrner, Creation of Nanoscale Objects by Swift Heavy Ion Track Manipulations, International Symposium on Material Chemistry in Nuclear Environment, Tsukuba, Japan, March 13 – 15, 2002 3. D.Fink, A.Petrov, M.Müller, W.Fahrner, A.Schulz, J. Rochas, L.T.Chadderton, A.S.Berdinsky, V.Hnatowicz, J.Vacik, Perspectives of Swift Heavy Ion Track Technology, Materials Congress, London, UK, April 9 – 11, 2002 4. A.V. Petrov, D. Fink, M.Müller, I.Sieber, M.Wilhelm , N.Stolterfoht, R. Papaleo, A. Schulz, J. Vacik, J. Cervena, V. Hnatow icz L. T. Chadderton, A. S. Berdinsky, W. Fahrner, Nanotubule Formation by Heterogeneous Nucleation of Silver Along Etched Nuclear Tracks in Polyethylene Terephthalate (PET), 3 rd Siberian Russian Workshop and Tutorials EDM'2002, E rlagol, Russia, 1-5 July 2002 5. N. Stolterfoht, D. Fink, A.V. Petrov, M. Müller, J. Vacik, J. Cervena, V. Hnatowicz, L.T. Chadderton, A.S. Berdinsky, Charac terization of Etched Tracks and Nanotubules by Ion Transmission Spectrom etry, 3 rd Siberian Russian Workshop and Tutorials EDM'2002, 1-5 July 2 002, Erlagol, Russia 6. A. S. Berdinsky, D. Fink, A. V. Petrov, L. T. Chadd erton, S M. Krasnoshtanov, E.S. Rylova, The Effect of External Mechanical Stress on the Ful lerite Conductivity, 3 rd Siberian Russian Workshop and Tutorials EDM'2002, Erlagol, Russia, 1-5 July 2002

215

7. N.Stolterfoht, V.Hoffmann, R.Hellhammer, Z.Pesis, D .Fink, A.Petrov, B.Sulik, Guided transmission of 3 keV Ne 7+ ions through nanocapillaries etched in a PET polymer, 14 th International Workshop on Inelastic Ion Surface Co llisions, Ameland, the Netherlands, September 8 – 13, 2002 8. A.Petrov, D.Fink, M.Müller, M.Wilhelm, W.Fahrner, A .Berdinsky, Formation of Nanoscale Structures by Swift Heavy Ion Track Te chnology, International Workshop on Nanostructures for Electronics and Opti cs, Dresden, Germany, October 6 - 9, 2002 9. D.Fink, P.S.Alegaonkar, A.V.Petrov, A.S.Berdinsky, V. Rao, M.Mueller, K.Dwivedi, L.T.Chadderton The Emergence of New Ion Track Applications, 21 International Conference on Nuclear Tracks in Solids, New Dehli, India, October 21-25, 2002 10. D.Fink, A.V.Petrov, V. Rao, M.Wilhelm, S.Demyanov, P.Szimkowiak, M.Behar, P.S.Alegaonkar, L.T.Chadderton, Product ion parameters for the formation of metallic nanotubules in etched tracks , 21 International Conference on Nuclear Tracks in Solids, New Dehli, India, October 21-25, 2002 11. A.V. Petrov, D. Fink, S.E.Demyanov, M.Müller, I.Sieber, A.S.Berdinsky, H.-G.Chun, L.T.Chadderton, The Conduction Mechanism s of Cu and Ni Nanotubules at Their Different Formation Stages, 7 th Korean-Russian International Symposium on Science and Technology ( KORUS-2003), Ulsan, Korea, June 28 – July 6, 2003 12. D.Fink, P.S.Alegaonkar, A.V.Petrov, J.Cervena, V.Hn atovicz, J.Vacik, P.Szimkoviak, I.Sieber, M.Wilhelm, A.S.Berdinsky, H .-G.Chun, L.T.Chadderton, Thermal Behavior of Etched Tracks a nd Embedded Metallic Nanotubules, 7 th Korean-Russian International Symposium on Science and Technology (KORUS-2003), Ulsan, Korea, June 28 – Ju ly 6, 2003

216

13. A.S.Berdinsky, D. Fink, A.V.Petrov, H.-G.Chun, L.T .Chadderton, V.A.Gridchin, P.S.Alegaonkar, Pressure Dependence o f Conductivity of Fullerite Structures, 7 th Korean-Russian International Symposium on Science and Technology (KORUS-2003), Ulsan, Korea, June 28 – July 6, 2003 14. D.Fink, M.Müller, A.Petrov, L.Farenzena, M.Behar, R .M.Papaleo, A.Chandra, Etching Kinetics of Swift Heavy Ion Irradiated Poly mers with Insoluble Additives or Reaction Products, BENSC User's Meetin g, Berlin, Germany, May 16, 2003 15. A.V. Petrov, D. Fink, G.Richter, P.Szimkoviak, A.Ch emseddine, P.S.Alegaonkar, A.S.Berdinsky, L.T.Chadderton, W.R. Fahrner, Creation of Nanoscale Electronic Devices by the Swift Heavy Ion Technology, The 4 th Siberian Russian Workshop and Tutorials EDM'2003, E rlagol, Russia, July 1-4, 2003 16. D.Fink, A.Petrov, W.Fahrner, A.G.Ulyashin, K.Hoppe, R.M.Papaleo, Novel Ion - Track Based MOS-type Structures, Summer scho ol on the evolution of ion tracks in matter "TRACKS'03", Mühlhausen, Germa ny, September 8 – 15, 2003 17. A.V.Petrov, D.Fink, K.Hoppe, A.G.Ulyashin, R.M.Papa leo, A.S.Berdinsky, W.R.Fahrner, Etched Ion Tracks in Silicon Oxide and Silicon Oxynitride as Charge Injection Channels for Novel Electronic Stru ctures, 12 th International Conference on Radiation Effects in Insulators (REI- 12), Gramado, Brazil, August 31 – September 5, 2003 18. A.Petrov, D.Fink, W.Fahrner, K.Hoppe, A.Ulyashin, S.Demyanov, A.Fedotov,

A.Berdinsky, Novel Electronic Structures, Based on Ion Tracks in SiO 2 and SiON, International Conference "Actual Problems of Solid State Physics", Minsk, Belarus, November 4 – 6, 2003 19. D.Fink, A.Petrov, W.Fahrner, K.Hoppe, A.Berdinsky, A.Chandra, New Electronic Devices Based on Ion Tracks in Insulator s, MRS Fall Meeting, Boston, USA, December 1-5, 2003

217

20. D.Fink, A.Petrov, K.Hoppe, W.Fahrner, R.M.Papaleo, A.Berdinsky, A.Chandra, A.Zrineh, L.T.Chadderton, Ion Track Base d Nanoelectronics, International Conference on Nano-Science and Techno logy (INCONSAT) Kolkata, India, December 17 –20, 2003 21. W. R. Fahrner, D. Fink, A. Petrov, K. Hoppe , R. M. Papaleo, TEMPOS – Ein neues nanoelektronisches Bauelement, Workshop of th e Nanotechnologie- Verbund NRW, Paderborn, Germany, February 5, 2004

218

ACKNOWLEDGEMENTS

The author is grateful to Prof. Dr. W.R.Fahrner for his permanent attention to the work, fruitful discussions, important comments and help in organizing of experimental work in the Chair of Electronic Device s of the University of Hagen. The author would like to express his gratitude to D r. D.Fink for his deep attention and help during preparation of the work, many inter esting and efficient discussions, valuable comments and suggestions conc erning form of the work and presentation of the results. The author gratefully acknowledges Mr. K.Hoppe from the University of Applied Sciences of South Westfal ia for his kind help in measuring the TEMPOS structures and presentation of the results. The author gratefully acknowledges the support from Dr. A.Berd insky from the Novosibirsk State Technical University for interesting and impo rtant discussions and help in measurements of experimental samples. The author wo uld like to thank Prof. Dr. S.Demyanov from the Institute of Solid State and Se miconductor Physics of the National Academy of Sciences of Belarus (Minsk, Bel rus) and Prof. Dr. A.Fedotov from the Belarussian State University (Minsk, Belar us) for fruitful discussions, important comments and help in measuring of some sa mples. The author gratefully acknowledges the friendly help and support from the colleagues in the Hahn- Meitner-Institut Berlin: Dr. M.Müller, Dr. S.Klaum ünzer, Dr. N.Stolterfoht, Dr. A.Chemseddine, Mr. P.Szimkoviak, Mr. M.Wilhelm , and Mr. G.Richter. The author would like to thank for kind help and suppor t in making experimental work and discussions the colleagues from the Chair of El ectronic Devices of the University of Hagen: Dr. R.Job, Dr.H.Gabor, Mr. M.S cherff, Mr. B.Wdowiak, Ms. K.Meisinger, and Ms. B.Kleine. The author is grateful to Mr. P.Alegaonkar from the University of Pune, India, for his help in the creation and measurement of the ion -track-based micromagnet and microtransformer prototypes, and several calculatio ns. 219

The author is very grateful to his wife, his and he r parents, and also for his sister and her family for their love, understanding, atten tion, support and inspiration for making the work. Большое вам спасибо , дорогие мои !

220

CURRICULUM VITAE

16.04.1967 born in Minsk, Belarussian SSR (former Soviet Union , now Republic of Belarus)

June 1984 graduated from a secondary school (level of gymnasi um), Minsk

June 1991 MS degree and Diploma in Physics (specialization – solid state physics) from the Physics Faculty of the Belarusian State University, Minsk (MS Thesis “Quantum Effects in Pu re Metals and Alloys on Their Base in the Conditions of Magne tic Breakdown”)

Aug. 1991 – probation researcher at the Cryogenic Research Divi sion of the Sept. 1994 Institute of Solid State and Semiconductor Physics of the National Academy of Sciences of Belarus (Minsk)

Oct. 1994- postgraduate studies at the Institute of Solid Stat e and Oct. 1998 Semiconductor Physics of the National Academy of Sc iences of Belarus (Minsk)

Oct. 1994- junior researcher at the Cryogenic Research Divisi on of the Feb. 2001 Institute of Solid State and Semiconductor Physics of the National Academy of Sciences of Belarus (Minsk)

March 2001 – Ph.D. student in the University of Hagen and the Ha hn-Meitner- Oct. 2004 Insitut-Berlin (Supervisors: Prof. Dr. W.R.Fahrner and Dr. D.Fink)