Implementation of an Ultra Deep X-Ray Lithography (UDXRL)

Implementation of an Ultra Deep X-ray Lithography

(UDXRL) System at CAMD

Dissertation zur Erlangung des Doktorgrades der Fakultät für Angewandte Wissenschaften der Albert-Ludwigs- Universität Freiburg im Breisgau

vorgelegt von

Georg Aigeldinger

Hauptreferent: Prof. Dr. W. Menz

Korreferent: Prof. Dr. V. Saile

Dekan: Prof. Dr. G.Urban

Juni 2001

1 Zusammenfassung:

In dieser Arbeit wurde die Herstellung von ultratiefen, mehrere Millimeter hohen PMMA Mikrostrukturen mit Hilfe der Röntgentiefenlithographie am Wiggler- Strahlrohr des Center for Advanced Microstructures and Devices (CAMD) realisiert und erstmals experimentell das Konzept der Stapelbelichtung (“Stacked Exposure”) ueberprüft und charakterisiert.

Die Arbeit besteht aus zwei Hauptaufgabengebieten, der Bereitstellung der benötigten Infrastruktur sowie der Durchführung erster Bestrahlungsversuche und dem Vergleich der erzielten Resultate mit bisherigen Arbeiten.

Im Bereich der Infrastruktur musste beginnend vom Quellpunkt, in diesem Fall ein 7T supraleitender Wellenlängenschieber (Wiggler), ein komplettes Strahlrohr inklusive einer Bestrahlungsanlage (Scanner) entworfen und aufgebaut werden. Die Hauptkomponenten der Anlage erlauben den sicheren Betrieb der Anlage sowohl bezüglich der Vakuumanforderungen als auch des Strahlenschutzes. Als Scanner wurde ein einfacher, an Atmosphäre betriebener Linearantrieb realisiert, der eine wassergekühlte Kassette mit integriertem Blendensystem mit der erforderlichen Genauigkeit durch den Wiggler-Strahl bewegt. Der gemessene Taumelfehler der Scannerbewegung liegt unterhalb von 0.2 mrad über eine Wegstrecke von 15 cm und entspricht damit den typischen Anforderungen. Die Steuerung der Anlage wurde über ein PC-Programm realisiert. Die Inbetriebnahme der Anlage hat eine sehr gute Übereinstimmung der theoretischen Daten für die Wigglerquelle als auch der Scannergenauigkeit ergeben. Bei typischen Bestrahlungsbedingungen von 1.5 GeV Elektronenenergie, 6 Tesla Magnetfeldstärke und 100 mA Elektronenstrom wurde eine mittlere Leistung von 6 W/mrad gemessen. Diese Leistung kann zu einem Temperaturanstieg auf bis zu 50ºC führen, welcher zu erheblichen thermo-mechanischen Verzügen sowohl der Maske als auch des Subtrats führt. Die Herstellung einer wassergekühlten Kassette mit integrierten Blenden, welche die Probe in den Umkehrpunkten der Scannerbewegung vor der Strahlung schützt, konnte die Temperaturerhöhung auf ca. 2º reduzieren und damit Abbildungsfehler, die durch thermo-mechanische Verzüge induziert werden, minimieren.

Mit diesen experimentellen Voraussetzungen konnten erfolgreich Proben bis zu einer Resistdicke von 3mm bestrahlt werden. Um den hohen Anforderungen an Maskenkontrast und thermo-mechanischer Stabilität zu genügen, wurde eine Beryllium-Maske mit Teststrukturen hergestellt, die einen direkten Vergleich mit anderen Arbeiten ermöglicht. Die durchgeführten Bestrahlungsversuche gliedern sich in vier Fragestellungen: Zuerst wurde ein unmittelbarer Vergleich von Bestrahlungen am CAMD Ablenkmagneten mit Bestrahlungen am Wiggler anhand von 200µm dicken PMMA Proben durchgeführt. Diese Versuche zeigen, dass die Seitenwandsteilheit am

2 Ablenkmagneten um ca. einen Faktor 4 kleiner ist. Dies deutet auf einen Einfluss des härteren Bestrahlungsspektrums hin, bei dem Photonenenergien bis 45 keV auftreten. Eine zweite Serie von Bestrahlungsversuchen diente dazu, die Entwicklung der Kantensteilheit mit wachsender Strukturhöhe zu bestimmen. Hier konnte gezeigt werden, dass im Bereich von 200µm bis 2800µm die Kantensteilheit einen Wert von ca. 0.25µm pro 100µm Strukturhöhe hat. Dieser konstante Wert deutet daraufhin, dass trotz der zum Teil sehr langen Entwicklungszeiten von 75 Stunden der Kontrast des Resist-Entwicklersystems – es wurde hochmolekularer PMMA-Resist (Mw=2.2 Mil.) und GG-Entwickler bei Raumtemperatur eingesetzt - ausgezeichnet ist. Eine dritte Serie von Strukturuntersuchungen befasste sich mit der Seitenwandrauhigkeit. Hier wurde mit Hilfe eines Weisslichtinterferometers die Rauhigkeit an verschiedenen Stellen entlang der Strukturhöhe bestimmt. Es ergaben sich Werte im Bereich von 10 – 50nm, die typisch sind für LIGA-Strukturen. Eine vierte Serie befasste sich mit der Herstellung von gleichzeitig bestrahlten Proben. Hierzu wurden Graphitsubstrate von 200µm Dicke mit einer 500µm dicken PMMA-Schicht beschichtet und eine Stapelanordnung von 4 Substraten erstmals in einem Bestrahlungsschritt strukturiert. Die eingehende Analyse der Strukturgenauigkeit hat gezeigt, dass diese Methode zu vergleichbaren Ergebnissen bzgl. der Kantensteilheit und der Seitenwandrauhigkeit führt und damit prinzipiell die gleichzeitige Herstellung von mehreren Proben mit guter Genauigkeit möglich ist.

3 Abstract

In this work, ultra-deep, several millimeter high microstructures in PMMA were fabricated, using deep X-ray lithography at the wiggler radiation source at the Center for Advanced Microstructures and Devices (CAMD). For the first time the concept of stacked exposures has been experimentally verified and tested.

The work consists of two main tasks: The preparation of the necessary infrastructure and the conducting of the exposure experiments and their interpretation and comparison to results presented in earlier work.

For the infrastructure a complete ultra high vacuum beamline, starting from the source point of a 7T super conducting wavelength-shifter, had to be designed and built. The beamline was terminated by an in-air exposure station (‘scanner’) that also was implemented. The major components of the built facilities allow the secure operation of the experiments with respect to radiation safety and the vacuum specifications of a synchrotron radiation source. As an in-air scanner, a basic linear driven sled was built. The high precision linear stage with a minimal wobble error was furbished with a fully water-cooled substrate and mask holder. The measured wobble error of the scanner movement is less than 0.2mrad on a distance of 15cm, which matches the typical specifications for X-ray scanners very well. Apertures ensure shielding of the not to be exposed areas of the mask and substrate. The exposure station was controlled by a PC based program and interface.

The commissioning of the beamline and exposure station showed a very good match of the wiggler radiation source and the scanner performance to theoretical data and calculations. At a typical exposure at 1.5 GeV electron energy, 6 T magnetic field in the wiggler and 100mA electron current, a total power of 6W/mrad was measured. This high total incident power can lead to temperature increase from room temperature to up to 50°C, which will lead to significant thermo-mechanical stress and movement on the mask as well as on the substrate. The design and build of a fully water cooled mask – substrate holder that included apertures in the turnaround points to protect mask and substrate from unnecessary exposure was able to reduce the temperature increase to about 2°C. Reducing the temperature increase ensured minimizing the thermo-mechanical stress and movements to guarantee acceptable patterning accuracy in the X-ray lithography process step. With the above experimental infrastructure, samples with a resist thickness of up to 3mm could be exposed. To suffice the high quality requirements at mask contrast and thermo-mechanical stability, a beryllium mask was fabricated, implementing test structures that allow the direct comparison to previous work. The conducted experiments can be divided in four subcategories: First a direct comparison of exposures from the wiggler source to the CAMD bending magnets was made. This comparison was realized by using substrates with 200µm high PMMA. These experiments show that the sidewall slope at the bending magnet source is about a factor of 4 smaller than at the wiggler. This points to a direct

4 influence of the harder exposure spectrum at the wiggler source, which reaches energies of up to 45keV. A second series of exposure experiments determined the sidewall slope at the wiggler source, depending on the structure height. Here it could be shown that in the range of 200-2800µm, the slope remained practically constant at about 0.25µm per 100µm structure height. This constant value shows that regardless of some development times reaching 75 hours, the contrast of the resist/development system is outstanding. GG developer at room temperature was used, along with a high molecular weight PMMA (Mw=2.2Mil). A third exposure series aimed at examining the sidewall roughness of the structures. Using a white light interferometric microscope, roughness at several levels of the structure height was measured. Values between 10 and 50nm average roughness (Ra) were examined which are typical for LIGA structures. The fourth group of experiment evaluated the possibility of exposing several substrates at once, in a stack. Graphite substrates with a thickness of 200µm were furbished with a 500µm thick PMMA resist. The substrates were assembled in stacks of four and exposed in one exposure step. The detailed analysis of the resulting structure accuracy has shown, that this method leads to comparable results regarding sidewall slope and surface roughness compared to the single substrate exposures. In principal the time and resource saving exposure of several samples in one stack is possible without compromising structure accuracy.

1 INTRODUCTION...... 7 2 FUNDAMENTALS OF DEEP X-RAY LITHOGRAPHY ...... 9 2.1 THE LIGA PROCESS ...... 9 2.2 DXRL: EXPOSURE AND DEVELOPMENT ...... 11 2.3 STRUCTURE QUALITY IN DXRL ...... 13 2.4 ULTRA DEEP X-RAY LITHOGRAPHY: STATE OF THE ART...... 16 Exposure and Development ...... 17 3 EXPERIMENTAL SETUP...... 20 3.1 BASIC PROPERTIES OF SYNCHROTRON RADIATION ...... 20 3.2 THE CAMD FACILITY...... 22 3.3 THE CAMD WIGGLER...... 23 3.4 THE WIGGLER BEAMLINE ...... 26 3.4.1 Introduction to Beamlines...... 26 3.4.2 Special Requirements for a Hard X-ray Source Beamline ...... 27 3.4.3 Ray Tracing Simulation ...... 28 3.4.4 Setup of the Wiggler Beamline...... 30 3.4.5 Radiation Shielding: ...... 32 3.5 CHARACTERIZATION OF THE SOURCE...... 35 3.5.1 Beam Width and Vertical Distribution:...... 35 3.5.2 Total Power...... 37 3.6 IN AIR X-RAY SCANNER ...... 40 3.6.1 Mechanical Accuracy...... 41 3.6.2 Mask Fixtures: ...... 43 3.6.3 Further Improvements for the UDXRL Scanner...... 47 4 EXPERIMENTAL RESULTS...... 49 4.1 MASKS FOR UDXRL ...... 49 4.1.1 Graphite Masks...... 52 4.1.2 Beryllium Masks...... 56 4.2 SAMPLE AND EXPOSURE PARAMETERS ...... 58 4.3 MEASUREMENT OF THE UDXRL MICROSTRUCTURE PRECISION ...... 62 4.3.1 Structure Width over Structure Height ...... 62 4.3.2 Sidewall Surface Roughness ...... 70 4.4 STACKED EXPOSURE...... 72 Motivation:...... 72 4.4.1 Dose Calculation ...... 73 4.4.2 Structure Quality...... 74 4.4.3 Sidewall Surface Roughness ...... 75 4.4.4 Structure Width over Structure Height ...... 77 5 SUMMARY ...... 80 BIBLIOGRAPHY...... 82 ACKNOWLEDGEMENTS ...... 86

6 1 Introduction

The field of deep X-ray Lithography (DXRL) is well researched and the technique is routinely performed at the Center for Advanced Microstructures and Devices (CAMD) and at many other laboratories around the world. Maximum heights of 500µm are achievable at reasonable exposure times of less than 2 hours using membrane masks and silicon or ceramic substrates. Thicker resist layers dramatically increase exposure times at a soft synchrotron source like CAMD. X-ray lithography in resist heights of over 1mm is generally denoted Ultra Deep X- ray Lithography (UDXRL). Thicker structures produced in UDXRL are more stable and modular microstructures can profit from this stability, especially in the assembly process of hybrid microsystems. Micro heat exchangers and vibrating shell micro- gyroscopes are only one example of applications pursued at LSU that demand resist heights of 1mm and above [1, 2]. For example in micro motors the height of the motor itself is directly proportional to the amount of torque it can deliver [3] and micro optical components can profit from large height sidewalls for better yield. While these different applications require different levels of precision they all require heights over 1mm in feasible exposure times. CAMD has a well-established X-ray Lithography infrastructure; therefore extending DXRL into UDXRL capabilities at CAMD will require limited effort except for the required exposure tool. A way to achieve higher structures at reasonable exposure times at CAMD is the use of wiggler light [4]. The wiggler insertion device utilizes a set of super conducting magnets, which shift the synchrotron radiation spectrum to higher energy and therefore penetrates the thicker resist. Figure 1 illustrates typical exposure times in hours (calculated) at the CAMD bending magnet beamlines versus the wiggler line.

Comparison Exposure Times Bending Magnet vs. Wiggler

70 Bending Magnet Beamline 1.5GeV 60 Wiggler Source Beamline 1.5GeV 6T Parameters used are typical for the 50 respective beamline. Scan lenth is 5cm. 40

Exposure time[hours @100mA] 0

-10 0 500 1000 1500 2000 2500 3000 Resist Height of PMMA [µm]

Figure 1: Calculated typical exposure times for PMMA at the CAMD bending magnet beamlines compared to the wiggler beamline. Shown is the resist height versus the exposure time in minutes.

7 For CAMD exposure times at the bending magnet increase exponentially and reach the order of 3 days for 3000µm resist height at 100mA average electron current in the synchrotron. The wiggler on the other hand allows exposure of 3000µm in less than 2 hours at 100mA. With this motivation the first goal of this work was to implement an UDXRL beamline and exposure station at the CAMD wiggler. An exposure system consisting of an ultra high vacuum beamline and an in-air scanner end station was built. Besides the state of the art design and construction of the system, radiation safe operation was a major issue because CAMD was originally not designed as a hard X-ray source. The scanner was built from high precision mechanical components comparable to commercially built X-ray scanners. The specifications were verified with measurements including precision mechanics and a temperature check of the mask- substrate cooling system under operating conditions. After successfully commissioning the exposure system, the properties of the newly installed wiggler source were analyzed and compared to calculations. The second goal of this work was to investigate the basic UDXRL properties of the system by running a set of first exposures scaling from small (200µm) to large heights (2800µm). The precision of the fabricated microstructures was measured. The results from these metrology studies were compared to the state of the art in research results in DXRL and UDXRL. For the UDXRL exposures at the wiggler special UDXRL masks were fabricated. Sheet masks were realized using graphite and beryllium as a mask sheet. Finally avenues for high throughput exposures by stacking resist-substrate layers were explored. To the best knowledge of the author this idea has only been briefly discussed in theory [5] but never been studied in detail in an actual UDXRL experiment using feasible materials while examining achievable structure accuracy.

8 2 Fundamentals of Deep X-ray Lithography

2.1 The LIGA Process

Deep X-ray Lithography is part of the LIGA process. LIGA has been developed in the early eighties at the Forschungszentrum Karlsruhe, Germany. Sparking the process development was the search for an inexpensive and ultimately precise method to produce microscopic sized nozzles for the separation of uranium isotopes. The term LIGA is an acronym for the process steps of X-Ray Lithography, electroplating (German: Galvanik) and molding (German: Abformung). Figure 2 shows the basic steps of the LIGA process. The X-ray lithography mask can be fabricated using photolithography methods [6]. Using an UV pattern generator or an electron writer on a chromium mask transfers the design pattern. After gold plating the photolithography pattern, the X-ray mask is used to fabricate the primary microstructure with synchrotron radiation. (The X-ray lithography mask can also be fabricated by direct electron beam writing into a few micrometer thick resist producing a thin intermediate mask. The intermediate mask is then copied in an X-ray lithography step to create the working mask.) The 5 to 20µm high gold structures on the X-ray mask are transferred to the resist by “shadow- printing” of the X-rays. At CAMD the critical energy of the radiation spectrum at 1.3/1.5 GeV Synchrotron ring current is 1.66keV/2.55keV respectively. The commonly used X-ray resist is PMMA (Polymethyl metacrylate) with very high molecular weight. The PMMA is spin coated on a substrate to form a thin layer of up to 15µm. If thicker layers are desired, the PMMA can be directly polymerized on the substrate by using a casting method [7]. For resist layers above 250µm a prefabricated PMMA sheet is solvent bonded [8] or glued to a metal-coated silicon wafer or other rigid conducting substrate. In exposed areas the PMMA reduces its mean molecular weight by main chain scission by two orders of magnitude and can be selectively developed in a bath of organic developer [9]. With this method very precise sidewalls with only minor deviation from the perfect vertical can be produced up to several hundred microns, while the surface roughness of the structure is of optical quality with mean average roughness in the range of 20-50nm or less [10][11]. Starting from the conducting substrate the areas that have been dissolved by the developer can now be filled by means of electroplating with metal, typically nickel or nickel alloys. The structure can be purposely over-plated to ensure filling of different sized structures and so a whole massive unit gets formed that can be released from the substrate. After stripping the resist and substrate from the metal this structure can be used or become a mold insert for injection molding or hot embossing. The molded plastic structures can be the final product or can be used as casts for ceramic microstructure fabrication. The molding step is what makes the LIGA process commercially very interesting because the molding cycle time is faster than the lithography step and a wide variety of materials become available [6][12].

LI: Shadow-printing, using X-rays: Mask membrane with mask structures define which parts of the resist are irradiated. The irradiated parts change their molecular weight and become solvable in a special developer.

Dissolution of exposed resist: Microstructure in PMMA after developing.

G: Form filling using electroplating: Electroplating the metal into the PMMA structures. Over-plating is done on purpose in this example to make an interrelated one-piece mold. A: Replication by Molding: After separating the mold from the resist structure the microstructure is replicated using injection and / or hot embossing tools. At this point other materials can be used such as metals, metal alloys and ceramics.

Figure 2: Basic steps of the LIGA process on an example of a honeycomb structure.

10 2.2 DXRL: Exposure and Development

Patterning of high aspect ratio micro (HARM) structures in DXRL requires the careful control of materials and process parameters. In the following chapter, the basic properties relevant for the DXRL process are described. The average molecular weight of the PMMA used at CAMD is in the range of 2 ⋅106 g/mol and higher [13], [14]. The X-ray exposure of PMMA induces main molecular chains scission in the material. The mean molecular weight is reduced due to the dose absorbed in the resist. The dose is defined as the energy absorbed per unit volume of resist, usually in the units Joule per cubic centimeter [J/cm³]. The exposed PMMA can be developed selectively against the unexposed PMMA with an organic developer named GG, which is a combination of Diethyl- glycolmonobuthylether, Morpholine, Ehanolamine and DI water [15]. The solubility of PMMA in the developer increases with temperature. The temperature of the GG cannot be increased arbitrarily because the higher temperatures can lead to tension cracks and swelling of unexposed areas. 37°C is a temperature that has been used with success in structures up to 500µm height [9]. For thicker structures typically room temperature is used. Agitation supports the development process and is attained by stirring at CAMD. Mega-sonic has been used successfully but bears the risk of damaging very small and high microstructures [16]. GG allows developing PMMA with 10000 g/mol or less [9]. To reach such a low molecular weight a minimum dose of 2kJ/cm³ has to be absorbed in the PMMA. To ensure total development of the exposed resist a minimum dose of 3.5kJ/cm³ is used at CAMD. Further dose deposition in the PMMA does not change the molecular weight significantly. At 20kJ/cm³ the molecular weight of PMMA reaches the range of 1000g/mol. In the range of 20kJ/cm³ the PMMA shows foaming due to the formation of gaseous products relieved from the resist. At CAMD a maximum dose limit of 18kJ/cm³ is usually set for thin resists with heights below 200µm. The top surface of the resist gets the highest dose deposition when exposed, while the bottom gets the lowest dose. The ratio of top to bottom dose is often referred to in X-ray lithography and with CAMD this ratio is generally 5 or less to insure sufficient dose on the bottom of the resist while avoiding an overdose on the top. For more information on structural changes in PMMA exposed to X-rays refer to [17, 18] Synchrotron radiation is highly parallel and has a wide spectrum of energies/wavelengths ranging from the infrared up to hard X-rays. (See chapter 3.1 for detailed information about synchrotron radiation.) Figure 3 illustrates the 1.5 GeV spectrum of CAMD used for a typical exposure and the impact of various materials on the spectrum.

11 Example of an Exposure using the CAMD Bending Magnet Spectrum

CAMD 1.5 GeV AFTER Be WINDOW AFTER Ti MASK AREAS BEHIND Au 1E-3

1E-4 Power [mW/(mrad mA)]

1E-5

0.1 1 10 Photon Energy [keV]

Figure 3: Spectra of a typical exposure at the CAMD bending magnet beamline. Most low energy radiation is absorbed in the 100m thick beryllium window and some more in the 3µm thick Ti membrane. Strong absorption at the Ti-K-edge at 4.96 keV is reflected in the transmitted spectrum. Most of the lower energy photons are absorbed in the 100µm thick Be-vacuum window. Some more low energy photons are absorbed in the mask, which is a 3µm thick Ti membrane in this example. This spectrum is what the PMMA resist is exposed to. The gold absorber is 12µm thick and nearly fully blocks the radiation so that only a negligible dose is deposited underneath the gold pattern of the mask in a 150µm thick PMMA resist. The dose deposition in the resist over height is illustrated in Figure 4. The dose deposition is nearly exponential. It has to be considered, which is especially true for thicker resists that the spectrum in the resist shifts to harder energies with increasing depth because the top layers of the resist act as a filter to the lower regions. To avoid an overdose in the top area of the resist, the spectrum is tailored with additional filters that act as high pass filters to the radiation. To minimize exposure time the spectrum is tailored just hard enough so the minimum dose is reached on the bottom, while the top of the resist does not get over exposed. Also shown in Figure 4 is the dose deposition in PMMA in the shadow of the gold absorber. In this area a ‘dark dose’ of less than 100J/cm³ is required for sufficient contrast.

12 Dose Deposition over Height in PMMA (Bending magnet exposure) 100

] 1 3 Dose Deposit Exposed Areas Dose Deposit Unexposed (Gold Shadow) 0.1 Dose [kJ/cm Dose

0.01

1E-3 -20 0 20 40 60 80 100 120 140 160 Resist Depth From Top [µm]

Figure 4: Typical dose deposition in 150µm of PMMA at the bending magnet beamline. The minimum dose is 3.5 kJ/cm³ at the bottom (at 150µm) and 16kJ/cm³ at the top of the resist (0µm). The area in the shadow of the gold experiences a minimal dose of less than 10J/cm³, which is far from the 100J/cm³, the allowed ‘dark dose’ The ratio of the minimum dose in the exposed area versus the maximum dose deposition in the shadow of the gold absorber is called the contrast of the X-ray mask. The contrast of the mask has to be sufficient and usually better (=larger) than 200. In this example the mask has a contrast of ca. 405. As mentioned above, when using thicker resist, the spectrum has to be ‘hardened’ with filters. Not only the PMMA but also the absorber becomes more transparent to the harder radiation. The mask in this example has a high contrast in the application shown and is can be used for exposures using thicker resist layers.

2.3 Structure Accuracy in DXRL

Many applications call for very tight specifications on the structure accuracy produced with DXRL. The quality and precision can be pinpointed by looking at the following critical points: • Defects in the microscopic structures • Control of absolute dimensions and relative dimensions to mask • The microstructure pattern over the height

Examples for structure defects are adhesion problems between resist and substrate, tension cracks and undeveloped ‘residue’ left on the bottom, especially in small areas of the structures. The defects often result from hard to monitor process parameters like tension in the resist before, during and after exposure and development. Cracks mostly occur in micrometer dimensions in width but can be in the length of millimeters.

13 The sidewall roughness is an important parameter. If not sufficiently smooth, the structure sidewalls cannot be used for optical applications due to scattering of light at the surface. In addition to this, if the microstructure produced with DXRL is basis for a molding tool, de-molding can be complicated due to increased adhesion of the molded plastic to the metal mold insert. Although there are many process parameters affecting the structure and sidewall quality of LIGA microstructures, it is possible by optimally choosing these parameters to achieve good structure quality. Nevertheless, because we drastically change the exposure spectrum at the wiggler compared to bending magnet spectrum, secondary effects caused by radiation interacting with matter need to be considered and will therefore be briefly discussed in the following paragraph. Figure 5 gives an overview of the situation.

Synchrotron radiation

Membrane

Mask Absorber fluorescence

Thermally induced motion

Figure 5: Secondary exposure effects leading to limitations in structure accuracy. Shown are from top to bottom: mask and absorber fluorescence, thermally induced motion of the mask, diffraction, divergence of the synchrotron radiation, secondary electrons, thermal motion in resist and substrate, fluorescence of the substrate.

• Divergence of the synchrotron radiation [19]. o Vertical: typical range 0.1-0.2µm per 500µm structure height with an angle of 0.2-0.4mrad. (For calculation of the angle see chapter 3.1). Scales with X-ray energy: becomes smaller with higher energies.

14 o Horizontal: typical value maximum of 2µm per 500µm at 8mrad beam width at the outer edge of the beam fan (not including source size).

• Diffraction is proportional to the wavelength [19]. At wavelengths in the Angstrom range (2Angstrom=6.15keV), which is typically used for DXRL the structure deviation from perfect over height is in the range of 0.1-0.2µm only. This can be neglected.

• Thermal expansion of the mask versus the resist and thermal expansion in the resist/substrate. This unwanted effect is proportional to the power absorbed in the mask and resist and can be limited for typical DXRL exposures by carefully cooling mask/substrate and by helium atmospheres in the chamber. At sources with even higher integral power than CAMD’s bending magnets (e.g. DCI Lure)1 the effect is in the range of 0.2µm in the center region of the structures when using a beryllium sheet mask [19]. The thermal effects can lead to structure-edge precision loss due to the relative motion induced. Thermal expansion also causes defects such as cracks and rough sidewalls.

• Secondary electron penetration depth: Photo absorption in the resist leads to the emission of secondary electrons. The range of these electrons is determined by the power of the radiation and is limited to about ± 0.1µm for structures of about 500µm on a typical DXRL source [20]. See [19] for more details on the calculated effects of secondary electrons. The penetration depth increases tremendously when using hard X-ray sources.

• Fluorescence occurs from the mask, the absorber and the resist/substrate. It is important to use materials in DXRL that do not have an absorption edge (mostly the K edge) near the critical energy of the source. Titanium for example with its K edge at 4.96 keV has led to problems at sources with the critical energy in this range. The fluorescence radiation corresponding to the Ti K edge has a wavelength of λ = 2.6Å which has the absorption length of 300µm in PMMA. This can severely damage the PMMA especially in the substrate-resist boding area which often has less mean molecular weight than the PMMA sheet, leading to adhesion problems [21]. Other materials that are problematic are silicon and copper.

DXRL dose simulations taking the radiation effects introduced above into account have been performed elsewhere [22] and results are illustrated in Figure 6. The graph shows the absorber of the mask in the top right as a horizontal bold bar and its geometrical shadow cast as a vertical dashed line.

1 DCI: P = 6.1W/(mrad·100mA) versus CAMD 1.5GeV: P = 2.44 W/mrad/100mA. For detailed information about LURE and DCI see http://www.lure.u-psud.fr/

Figure 6: Comparison of measurements (square points) with dose calculations for a 300µm DXRL exposure in [22]. The bold line is the iso-dose line of 1.8kJ/cm³, which is sufficient dose for full development. A recess of the measured developed structure of about 0.2µm is visible while the shape of the sidewall fits the calculated iso-dose profile well.

In the 300µm high resist, calculated iso-dose lines from simulations are entered. An iso-dose line is a line through the cross section of the resist where the deposited dose is equal. The bold line is the iso-dose line of 1.8J/cm³, which is the minimum dose for full development. The calculations are compared to a measurement of an exposed and developed edge. While the basic shape of the measured edge fits the calculated iso- dose lines well, the sidewall is clearly recessed by ca. 0.2µm from the 1.8kJ/cm³ iso- dose line. The sloped shape, narrowing the structure width from bottom to top is typical for DXRL and results from a combination of the effects mentioned above. The bottom recess in the measurement may mainly results from the fluorescence from the substrate.

2.4 Ultra Deep X-ray Lithography: State of the Art

Considering a PMMA resist, the penetration depth of X-ray photons into thick resist layers mainly determines the parameters of the X-ray source. As a measure for this the 1/e absorption length in PMMA as a function of X-ray energy is shown in Figure 7.

Figure 7: Absorption length in PMMA in the hard X-ray region [23]. Concluding from this graph, resist layers of up to 500µm thickness can be patterned within reasonable time, using the X-ray spectrum provided from a CAMD bending magnet. However, for patterning several mm thick resist layers, X-ray photon energies above 10keV are required. The 7T wiggler at CAMD provides such a spectrum and is explained in detail in chapter 3.3. In the next paragraphs a similar process parameter description as for DXRL is presented for UDXRL.

Exposure and Development Emphasis is put to these parameters, which will be different and may have an effect to the overall structure quality. The absorption of higher energy photons occurs in principle identical to the lower energy examples discussed. However, exposure of thick resist layers require high-energy photons, which have in return larger penetration depth resulting in a more uniform dose deposition. Typically the top to bottom (t/b) dose ratio is almost half of that in DXRL [24]. If the t/b ratio is too large the top layers of the resist get dose entries of 15kJ/cm≥ and more in less than an hour time. Foaming will occur faster in this situation than at a slower dose deposition rate since the gaseous byproducts of the PMMA radiation damage cannot diffuse quickly enough. As in DXRL GG is also the developer of choice in UDXRL. It has been shown that for structures over 500µm of height room temperature is preferred [25]. If 37°C is used for these high structures the GG developer acts too aggressive and can start to dissolve parts of the non-exposed structures. Taller structures are also more prone to stress in the resist and therefore will crack more easily when developed aggressively [25]. For the effective exposure spectrum the situation at the CAMD wiggler source is shown in Figure 8: The spectrum is shifted to harder X-rays due to absorption in beamline filters and a 600µm beryllium mask sheet. The low energy cutoff resulting from filters and mask is about 3.5keV photon energy. The spectrum after the beryllium mask is what the resist is exposed to. A layer of 45µm gold absorber ensures that the resist in the protected area is exposed to a negligible remainder of radiation above 25keV.

17 Typical Exposure at the CAMD Wiggler Beamline

CAMD WIGGLER 1.5GeV 6.5T AFTER 250um BE WINDOW AFTER HE/AIR + KAPTON AFTER 600um BE MASK AFTER 45um AU ABSORBER

1E-3

1E-4 Power [mW/(mradPower mA)]

1E-5

110100 Photon Energy[keV]

Figure 8: Spectra of a typical exposure at the CAMD wiggler beamline. Most low energy photons are absorbed in the 250µm thick beryllium window and then some more in the air/helium gap, Kapton window and beryllium mask resulting in a cutoff at ca. 3.5keV The dose deposition in the PMMA is illustrated in Figure 9: A smaller top dose than in DXRL is desired to avoid foaming of the thicker layers under high power exposure. The top dose is ca. 10kJ/cm≥ in this example with the bottom dose still remaining the minimum 3.5kJ/cm≥ known from DXRL. The mask contrast has to be sufficient and is ca. 80 in this scenario using a 45µm gold absorber structure. The dose deposition in the shadow region underneath the gold absorber is 45 J/cm≥

100 Dose Deposition over Height in PMMA (Wiggler exposure)

10 ] 3 Dose Deposit Exposed Areas 1 Dose Deposit Unexposed (Gold Shadow) Dose [kJ/cm Dose

0.1

0.01 0 500 1000 1500 2000 Resist Depth From Top [µm]

Figure 9: Typical dose deposition in 2000µm of PMMA at the wiggler beamline.

18 The main difference between DXRL and UDXRL is the harder spectrum. High energy X-rays above 4-5KeV will induce the radiation damage of the resist. From the discussion of the secondary effects it is expected that this will have an impact on the sidewall quality, mainly verticality and roughness. Furthermore, these effects may lead to processing problems like cracks and lack of adhesion. In addition to the harder spectrum, the UDXRL sources typically have much higher power density, which may cause heat load problems. Going to higher photon energies reduces vertical divergence of the radiation while the horizontal divergence is an energy-independent geometrical effect. See chapter 3.1 for details about divergence of synchrotron radiation. Fluorescence from mask and absorber and from the substrate has been shown to be potentially problematic [26]. Fluorescence from the mask can lead to rounding of the top edges in the PMMA structures. However, going to a low Z mask material like beryllium will significantly reduce this problem [22]. Fluorescence from the substrate will lead to adhesion problems of the PMMA after development. While a dose deposition through fluorescence from the substrate will not lead to full development, it still can damage the PMMA, especially in a bottom spincoat / bonding layer with slightly lower molecular weight than the sheet material [22]. The energy of secondary photoelectrons scales with the energy of the X-ray radiation. While 1keV photoelectrons have a range of only ca. 0.05µm, 10keV electrons have a range of over 1µm in PMMA. This can lead to significant damage of the primary unexposed areas. This will shift the iso-dose lines even further inward from the geometrical shadow cast as it was presented in Figure 6. With a given high power - high-energy spectrum the only effect in the exposure itself that can be actively prevented is the thermal effects. Thermal are minimized by full cooling of the mask and substrate and by using materials that are good heat conductors. Compare [19, 21, 22, 27] and references within for more information on the secondary effects at hard X-ray sources.

19 3 Experimental Setup

3.1 Basic Properties of Synchrotron Radiation

It is known from classical electrodynamics that charged particles emit electromagnetic radiation when accelerated. E.g. electrons in an X-ray tube get decelerated and produce Bremsstrahlung [28]. In circular high-energy electron accelerators relativistic electrons with energies up to several GeV experience a centripetal acceleration from the dipole magnets forcing them onto the orbit. This acceleration results in electromagnetic radiation called ‘synchrotron radiation’ covering a wide spectral range from the infrared up to several 10keV. Figure 10 and Figure 11 show the basic properties and the ‘shape’ of the synchrotron radiation distribution.

Figure 10: To the left: The angular distribution of the emitted intensity from a slow electron (v<

Figure 11: Geometry of the emission of synchrotron radiation. The plane of the accelerator ring is 'filled' with radiation while the intensity in the off-plane direction decreases rapidly with increasing angle ψ [29]. For applications in X-ray lithography the following properties of synchrotron radiation are of particular importance. (They are derived and described along with

20 properties of synchrotron radiation that are less important for lithography more closely in [29] and citations within.)

• Wide spectral range and intense continuum from the infrared to the X-ray region. As an example see Figure 17 for the synchrotron spectra at CAMD. ε ()ε Spectra often are characterized by the characteristic energy c eV . c is the energy that divides the integrated power of the total spectrum in half. It can be calculated using the basic formula (Equation 5) given below. • Highly collimated, excellent directional properties in forward direction. For the CAMD ring at 1.5GeV energy γ = 2935 and the vertical opening angle is in the range Θ = 0.34 mrad. See Figure 11. • Very intense X-ray source with integrated power in the range of several W/cm² • High stability of the delivered beam. The expression ‘storage ring’ already describes the desire to store the electron beam for several hours maintaining only a small decay in current over time. A stable orbit and good vacuum (UHV) in the ring is mandatory to reach this goal.

These specific properties in addition to the ability to calculate the emitted radiation characteristics make synchrotron radiation a precise and intense light source not only for X-ray lithography. The following equations summarize important equations describing the properties of synchrotron radiation relevant for X-ray lithography. Further details and general information can be found in [29],[30].

2 4 = 2 e c E P 2 2 4 3 r (m0c ) Equation 1:Total power radiated by a relativistic electron E 4 [GeV ] δE()keV = 88.5 r[m]

Equation 2:Energy loss per turn per electron ()= ⋅ 3 ⋅ ⋅ Ptot kW 26,6 E[GeV ] B[T ] I[A]

Equation 3:Total power radiated r[m] 18.64 λ ()Å = 5.59 = c E[GeV ]3 B[T ]⋅ E[GeV ]2

Equation 4:Critical wavelength ε ()= 2 c KeV 0.665B[T ]E [GeV ]

Equation 5:Characteristic energy

0.43 1  λ  σ = 0.57 ⋅   nat γ  λ   c  Equation 6: Vertical width of the radiation approximated with a Gaussian distribution and for λ λ / c = 02-100. 1 1 Θ ≈ = γ 1957 ⋅ E λ Equation 7:Vertical emission angle of energies near c 12.398 ε[keV ] = λ()Å

Equation 8:Energy of a photon E[GeV ] ≈ 0.3B[T ]r[m]

Equation 9: Relation magnetic field and radius or a bending magnet

E [GeV] Energy of I [A] Electron current of Synchrotron accelerator B [T] Magnetic field in r [m] Radius of bending bending magnet magnet E ≈ ⋅ 8 γ = c 3 10 m / s mc2 0 Speed of Light

m = 9,1⋅10−31 kg 0 Electron mass

3.2 The CAMD facility

The CAMD facility is owned and operated by Louisiana State University. It provides synchrotron light for microfabrication and analytical sciences [31]. Figure 12 shows a schematic drawing of the ring. The orbit is built with a total of 8 bending magnets providing light to 16 beam ports. Also shown in Figure 12 is a list of the basic characteristics of the machine.

22 16.3 m CAMD CHARACTERISTICS: Energies: 1.3/1.5 GeV Current: 300/150 mA Radius Dipole magnets: 2.928 m Dipole Magnetic Field: 1.46/1.68 T Characteristic Energy: 7 / 2.6 keV Total Power: 1.0W/mrad/0.1A Half-Life Time of Beam: > 8 h Natural Emmitance 2 ⋅10−7 mrad E-beam width / height: 0.6 mm/0.15 mm Figure 12: Schematic view of the CAMD synchrotron with its 8 bending magnets. The basic characteristics of the CAMD synchrotron are listed to the right

CAMD operates at 1.3 and 1.5 GeV electron energies. The characteristic energy of the spectrum from a bending magnet (radius of curvature = 2.928m) is 1.66 keV and 2.55 keV, respectively. In support of its microfabrication efforts a total of 3 beamlines and scanners installed at bending magnet sources is available for research and industry projects [31]. Two of the three beamlines are jointly operated by CAMD and the Institute for Micromanufacturing (IfM), Louisiana Tech. University [32]. One is using an in-house designed scanner; the two jointly used beamlines employ commercial scanners (DEX01 and DEX02) from Jenoptik Mikrotechnik, Jena [33]. In addition, a 200m2 clean room with a variety of processing, metrology and characterization equipment (e.g. optical pattern generator, thin metal film deposition, UV exposure station, reactive ion etching and ion milling, electroplating station and optical, electron and atomic force microscopes) allows pre- and post- processing capability at CAMD and completes CAMD’s microfabrication infrastructure.2

3.3 The CAMD Wiggler

CAMD provides a relatively soft X-ray spectrum (see Figure 17 on page 26) that limits the maximal structure height achievable within a ‘reasonable’ exposure time. Assuming an exposure time of 3 hours or less is ‘reasonable’ the maximum height achievable for a typical scan length of 5cm is less than 500µm. This takes into account typical parameters: t/b ratio < 4, bottom dose 3.5kJ/cm≥, 100mA average beam current, 1.5GeV operation and any of the three lithography beamlines parameters. In order to pattern taller structures in excess of 1mm a “harder” and more intense X- ray source is required. Considering Equation 3 and Equation 9 only two machine parameters enable a higher total power output: The increase of the energy of the electrons (E) or the decrease of the bending radius of the magnets (r) which equals to increasing the magnetic field (B). The same approach would be necessary to increase the critical energy of the

2 Compare the CAMD web page for details on the cleanroom and equipment: http://www.camd.lsu.edu/microfabrication/index.htm

23 ε radiation c . Increasing the energy of the ring is not an option. The bending magnets at CAMD are designed for a maximum of 1.5 GeV operation and cannot accept higher currents (Equation 9). A solution without modification of the already installed bending magnets can be found using a so-called insertion device. In a straight section of the ring the device can be inserted to accomplish a smaller radius curvature of the electron beam while not disrupting the normal ring operation [29]. A super conducting magnet assembly, a ‘wiggler’ consisting of five magnets (of which 3 are super-conducting) with the center pole designed to produce 7T has been purchased in 1998 [4]. The device was operational in solitary mode in fall 1999. Figure 15 shows the straight section of the ring in a top view with the wiggler insertion device installed in the center.

Figure 13: Top view of the straight section of the CAMD ring in which the Wiggler has been inserted. Radiation gets extracted through the special zero degree dipole chamber on the right.

Figure 14 illustrates the operation principle of the 5-pole wiggler. The center pole is the one producing the maximum magnetic field. The other magnets are necessary to pre-bend and to bring the electrons back on the orbit in the straight section. The specialty of the CAMD wiggler is that it is tunable. The magnetic fields in the wiggler are infinitely variable and the center magnetic field can be set to any magnetic field between 2 and 7 T.

Figure 14: Principle of a 5 pole wiggler [34]. At the CAMD wiggler the center three poles are super conducting while the outer two poles are normal conducting.

24 Tuneability is possible while leaving the electron beam trajectory at the center pole in the same location. This results in a fixed source point at any given magnetic field and no adjustment of the beamline extracting the radiation from the ring is required. Figure 15 shows the actual trajectory of the beam in the CAMD wiggler at 7T. Note that the scales X (position in straight section) versus Y (orbit deviation) are compressed by a factor of 100 to better illustrate the orbit deviation.

Figure 15: Trajectory X-coordinate distortion of an electron inside the wiggler at 7T versus the longitudinal coordinate at electron energy of 1.5 GeV[4]. Figure 16 shows the angular variation of the critical energies of the wiggler photon spectra. This figure illustrates that the center pole submits a high-energy photon spectrum of larger than 10keV critical energy into a horizontal angle of over ±25 mrad. The side pole magnets deliver spectra with critical energy close to 2.5keV over merely ±25 mrad horizontal width. This wide horizontal opening angle allows the construction of several beamlines at the wiggler.

Figure 16: Critical energy of photons versus horizontal angle at electron energy of 1.5 GeV [4]. With a maximum magnetic field of 7 T at the center pole, the wiggler defines a radius of curvature of ca. 0.7 m. Figure 17 shows the corresponding power spectrum (center magnet) compared to the spectra of the bending magnets at 1.3 and 1.5 GeV.

CAMD bending magnet 1.3 GeV CAMD bending magnet 1.5 GeV CAMD 0.6 CAMD wiggler 7T 1.5 GeV Bending Magnet at 1.3 GeV: 0.5 Ec = 1.66 keV, Ptot = 1.37 W/mrad/100mA 0.4

0.3 Bending Magnet at 1.5 GeV: Ec = 2.55 keV, Ptot = 2.44 0.2 W/mrad/100mA

0.1 Wiggler Magnet at 1.5 GeV, Power[mW/mrad at 100mA] 7T:

0.0 Ec = 10.5 keV, Ptot = 9.92 W/mrad/100mA 1 10 100 Photon Energy [keV]

Figure 17: Spectral power output from CAMD lithography beamlines for 100 mA electron current. A significant power output of photons with energies up to 40 keV is generated at the CAMD wiggler. The new wiggler source provides CAMD with a powerful tool to effectively expose tall microstructures up to several millimeters. The 10 fold higher integral power also offers the potential to improve the sample throughput and reduces the exposure times dramatically. With this addition, together with the bending magnet beamlines, CAMD’s microfabrication capabilities in DXRL and UDXRL are comparable to other synchrotron sources operating similar beamlines. A few examples are the following light sources: APS [23], NSLS [35] [36] and ANKA [37].

It should be noted that up to this date the wiggler is operational in solitary mode only. Solitary mode means that it is used with an orbit that did not assure proper light at the bending magnet beamlines. While the experiments for this work were conducted on the wiggler, the development state of the operations was at the point where stable operations could only be guaranteed at 6T magnetic field. The orbit of the synchrotron ring and the position of a dipole magnet in the ring were changed during the timeframe of the studies presented. After preliminary exposures in fall 1999 [38] with a basic beamline and scanner setup the beamline and wiggler have been operational again only for 2 months in fall of 2000, also in solitary mode, during nights and weekends only.

3.4 The Wiggler Beamline

3.4.1 Introduction to Beamlines

A beamline on a synchrotron extracts the X-rays from the accelerator ring. The beamline provides internal ultra high vacuum compatible to the vacuum in the synchrotron ring (UHV, pressure <1⋅10−8 mbar). The beamline is pre pumped with a

26 removable combination of roughing and turbo pump and the vacuum is then maintained with ion pumps. Besides extracting the radiation the beamline has to ensure radiation safe operation. The front section of the beamline is located inside the shieldwall and directly connects to the ring vacuum via a vacuum port mounted on a ring vacuum chamber. For sample exchange and general access to the exposure station during synchrotron operation, the beamline is operated in ‘close’ mode. In this state all vacuum valves, and shutters must be closed. The photon shutter in the very front of the beamline stops the high-energy photons and protects further components from overheating. Bremsstrahlung has to be absorbed in a Bremsstrahlungsshutter that consists of a large mass of lead. Both shutters are vacuum compatible and basic components of the beamline. The beamline is disconnected from the ring vacuum by vacuum valves. The beamline is separated by several vacuum gate valves into sections for easier maintenance and modification capability without breaking the vacuum on the entire setup. Each section contains an ion pump, ion gauge and a pump out port for the venting and the pre-pumping with a turbo/roughing pump. Valves and shutters are all pneumatically actuated and controlled electronically. The valve and shutter system is tied into an interlock system with the experimental station and hutch to ensure a safe operation at all times. In the ‘open’ mode the radiation passes directly to the exposure station. A copper aperture in the front section of the beamline determines the beam size. The aperture ensures that no direct radiation hits the stainless steel tubes and bellows. The radiation passes the shieldwall and leaves the UHV section of the beamline shortly before entering the exposure station. The shieldwall penetrating opening is kept minimal for radiation safety. Exposure station and end of the beamline are located inside a radiation safe hutch that is closed during the open mode of the beamline. The beamline has to be in the height of the radiation plane. This height is 48’’ (121.92cm) off the experimental floor at CAMD. The beamline has to be fixed to steel frame stands to achieve this height. Stands do support ion pumps at the same time and have to be fine adjustable in all 3 dimensions. There are several stands holding the various beamline components in place.

3.4.2 Requirements for a Hard X-ray Source Beamline

When building a beamline at the wiggler port, these basic requirements must be considered: • Radiation Safety: o Water Cooled Photon Shutter o Large Bremsstrahlungsshutter o Minimized cross section of vacuum tube and shield wall opening o Radiation hutch • Vacuum: Higher rate of out-gassing demands sufficient vacuum pumping speed • Power deposition in beamline: o Water-cooled beam defining apertures

27 o Ensure no uncooled parts of beamline get exposed to beam, especially thin walled bellows o Rigid, water-cooled beryllium window terminating the UHV section. • Safe remote Operation: Control all vacuum and radiation shutters from a safe area outside the experimental hutch (Beamline control rack). • Scanner o Fully adjustable normal to beam o Accurate movement with wobble error less than 0.2mrad over 15cm scan length (similar specification than Jenoptik DEX02 [33]) o Fast scan speed of up to several cm/s o Fully water-cooled substrate / mask assembly o Blades adjustable to the patterned area of the mask to minimize incident power and heat load. o Computer controlled remote exposure.

3.4.3 Ray Tracing Simulation

Beamline design using ray trace simulation tools allow exact planning of critical components. Ray tracing also provides key information on the properties of the power spectrum. The beamline at the wiggler has no optics except one beam defining aperture and filters acting as high passes to the high energy X-rays. The basic setup of the beamline simulated is shown in top view in Figure 18. Here the beam travels from left to right. The aperture and the beryllium window are shown schematically and with a magnification of 10 towards the dimension of the beam direction. The SHADOW ray-tracing tool (CXrL Wisconsin) has been utilized for this simulation [39]. Simulated source was the wiggler center pole at 6T and 1.5GeV. The source distribution used was Gaussian with a horizontal (x) and a vertical (y) size (standard σ = σ = deviation) of 2 x 3.2mm and 2 y 0.4mm . The horizontal size of the copper aperture at 5.9m distance from the source is 49.02mm, which defines a beam width of ca. 8.5mrad. The 120mm wide beryllium window in 9.85m distance is wide enough to accept the full 8.5mrad safely. Between the beryllium window and the mask plane 28cm of helium separated by 50µm of Kapton and 10cm of air were simulated.

28 Ray Tracing Horizontal Plane

CU APERTURE BE WINDOW 49.02mm x 25.4mm 120mm x 20 mm

SOURCE

A BEAM FAN O

B P

5900mm 3950mm 380mm

Figure 18: Basic setup of the wiggler beamline used for the ray tracing simulation. The vertical distribution is less critical. As shown in chapter 3.1 the synchrotron light is very concentrated in forward direction and the natural vertical opening angle to expect is only 0.34mrad. The vertical beam size therefore does not have to be restricted with the aperture. The aperture vertical size is 1’’=25.4mm and the beryllium window vertical height 20mm. Both do not restrict the natural emission angle of the beam. (The beryllium window is the minimal aperture and reflects ca. 2mrad restriction. This is almost an order of magnitude more than the natural opening angle of 0.34mrad calculated.) The results of the ray tracing simulations are shown in Figure 19 below. To illustrate the vertical (left graph) and horizontal (right graph) distribution a total of 4 simulation runs interpolated using 25000 rays each has been performed. The noise visible in the horizontal distribution is statistical noise.

Vertical Power Distribution Horizontal Power Distribution (horizontally integrated) 0.8 (vertically integrated)

12 0.7

0.6 10

0.5 8 0.4

6 0.3

4 0.2 W/(100mA*mm)

W/(100mA*100um) 0.1 2 Power (vertically integrated) (vertically Power Power (horizontally intergrated) 0.0 0 -0.1 -10 -5 0 5 10 -60 -40 -20 0 20 40 60 Vertical Mask Plane [mm] Horizontal Mask Plane [mm]

Figure 19: Calculated Beam Distribution (Power) in the mask plane. Vertical distribution (horizontally integrated) to the left. Horizontal distribution (vertically integrated) to the right.

29 The vertical beam distribution (horizontally integrated) is not exactly Gaussian because it is the sum of the power of all energies impinging the mask plane. The higher energies get filtered less in the beamline filters and contribute more in the distribution. The high-energy radiation is emitted in a narrower angle than the lower energy radiation (Equation 6, This is shown in detail in [28] Page 677.) A Gaussian fit of the power distribution gives a standard deviation of 2σ=3.6mm. The horizontal distribution (vertically integrated) shows to be 85.2mm wide for Power > 0.3W/(100mA·mm). Integrating the distribution results in a total power of 51.3 W, which is 6.0W/cm vertically, integrated for the 85.2mm beam width.

3.4.4 Setup of the Wiggler Beamline

Based on the simulations and the requirements from chapter 3.4.2 the following beamline and exposure station shown in Figure 20 has been designed and built.

CU APERTURE

GATE VALVE 1 PHOTON BREMS- BERYLLIUM WINDOW (MANUAL) WATER-COOLED SHUTTER STRAHLUNGS- SHIELDWALL EXPOSURE SHUTTER STATION GATE GATE VALVE 2 (SCANNER) VALVE 3 WITH LEAD SHIELDING

1 m

Figure 20: Layout of the CAMD wiggler beamline (side view). The beam direction is from left to right. The wiggler radiation is passing through the UHV beamline shown from left to right. The radiation is leaving the storage ring through a special 0 degree port dipole chamber (not shown in this picture but in the right of Figure 13).

• The first vacuum valve is a manual gate valve 1. This valve allows disconnection of the entire beamline from the ring vacuum for service / maintenance work. Manual action of this valve ensures it being only actuated when the synchrotron is off. This needs to be ensured because there is no other water-cooled shutter placed in front of this valve. • A water-cooled copper aperture is defining a maximum horizontal acceptance angle of 8.5mrad with no vertical limitation.

30 • The photon shutter is water-cooled and can absorb the full power of the beam. The full 8.5mrad of beam at 150mA and 7T produce ca. 130W of power on the photon shutter. • The vacuum gate valve 2 is defining the first vacuum section between gate valve 2 and gate valve 1. One ion pump (Varian star cell, nominal pumping speed 240l/s) is maintaining vacuum better than 5·10−8 mbar under full beam in this section. This section has been designed and built in a manner to be compatible with future plans to attach two beamlines to the wiggler port. • The Bremsstrahlungsshutter ensures a radiation safe operation of the beamline, absorbing all Bremsstrahlung produced when the photon shutter is closed and the synchrotron is in operation.3 The shutter consists of a ca. 20cm diameter, 60kg block of lead that is pneumatically actuated. The Bremsstrahlungsshutter is large enough to fully cover the opening in the shieldwall. • The radiation is extracted from inside the storage ring via a shieldwall through tube (25 mm high x 100 mm wide). Additional lead brick shielding surrounds both sides of the tube ends. • Gate valve 3 defines the second vacuum section that again is maintained by a Varian star cell pump. The vacuum section terminated by this valve is outside the shieldwall and is accessible during synchrotron operation. • A beryllium window terminates the UHV section of the beamline. The window is 250 µm thick and has a 120 mm x 15 mm open aperture. The beryllium window is built to common standards and can withstand a radiation power of more than 10 W/cm vertically integrated [40]. The beryllium foil of the window is vacuum brazed to a copper block that is cooled to 22.5°C by a temperature constant water circuit. The Be-window is located at about 10 m distance of the source point. The atmospheric side of the Be window has been coated with a boron nitride layer to protect the sensitive beryllium from moisture and from ozone produced by radiation interacting with the air [41] This third vacuum section again has a Varian star cell ion pump to maintain UHV. In addition, a helium filled chamber has been attached to the atmospheric side to further protect the beryllium window. This procedure is standard e.g. at NSLS [42]. The chamber is purged with He during beamline operations. It has a 50 µm Kapton window that can be easily exchanged when damaged by radiation. After passing through the Kapton window a ca. 10 cm wide air gap follows before the X-rays hit the mask plane. The air gap allows both, optional filter placement to adapt top to bottom dose ratios to resist thickness and setting defined apertures to minimize the heat load on the mask. • At the end of the beamline, the exposure station is located. It consists of an in-air scanner which is described in detail in chapter 3.6.

3For reference about Bremsstrahlung see 28. Jackson, J.D., Classical Electrodynamics. 3 ed. 1998. pages 702f.

31 The construction of the beamline has been done in two phases. In the first phase the beamline has been terminated inside the shield wall with the beryllium window. See Figure 21 for details of the beamline setup. A fluorescent screen placed behind the window allowed a remote video camera to measure beam position. This permitted the exact measurement of the beam height and provided the opportunity for the machine group to identify the location of a closed orbit when the wiggler was on. Beam height was measured in two positions to establish an absolute vertical beam direction.

Figure 21: First section of the beamline. The beryllium window was installed inside the shieldwall. The beam position was surveyed with a 45-degree fluorescent screen before installing the full beamline. Figure 22 illustrates the beam on the fluorescent screen monitored with the video camera. In air pre-filter of 1mm copper was used. The magnetic field in this particular measurement was 3.75T at 58mA beam current.

Figure 22: Fluorescent screen measurement for beam position survey prior to full installation of the beamline. The scale to the right is in mm. Note that the x scale is compressed due to the angle at which the diagnostic screen was installed.

3.4.5 Radiation Shielding

As shown in Figure 20 most of the beamline is located inside the shieldwall and therefore most access radiation is kept inside this radiation protection. Outside the

32 shieldwall the remaining beamline and the experimental station / scanner are shielded by a roofless steel hutch with 1/16” (1.6mm) thick lead lining.

The radiation levels were measured for experimental conditions (100 mA, 6T, photon shutter open with scanner stage in place). The measurements showed levels of radiation above the allowed 5rem/year (=2500µrem/h). Maximum levels measured were more than 10 mrem/h [43]. Figure 23 shows the radiation seen before and after shielding. After the shielding, described in detail in the next paragraph, the radiation level was measured again and showed to be an order of magnitude less than the allowed limit.

Effects of Shielding the Scanner

Before Shielding Scanner After Shielding Scanner 10000

Radiation Dose Limit 1000

100 urem/(hour 100mA) 10

1 024681012 Measurement Point Number

Figure 23: Radiation from the wiggler beamline exposure station and hutch in 12 points outside the hutch in 6’ (182.88cm) height. Shown are values exceeding the state dose limit before shielding and the values after shielding the scanner with a lead box. Values after shielding are less than one order of magnitude of the dose limit.

Main reason for the high radiation level is scattered radiation produced by the beryllium window and the air gap. This radiation is escaping through the top of the roofless hutch. This effect was enhanced by the utility supply construction running in about 10-foot height above the experimental station. In order to reduce radiation to a tolerable level a lead enclosure on rails was designed and fabricated for the scanner. It slides over the entire scanner stage and the end of the beamline. This lead box can be slid on ball bearings to access the scanner when exchanging samples. To make sure the box is closed during operation the setup is tied into the hutch safety interlock. Two switches checked the box to be closed before allowing the hutch to be searched and secured. Figure 24 shows the scanner at the beamline unshielded, Figure 25 shows the scanner shielded, with the shield pulled open for access. The lead used was 1/8” thick. Figure 26 shows the entire shielding setup in top view.

Figure 24: Scanner at the Wiggler beamline Figure 25: Scanner at the wiggler beamline, (unshielded). shielded, with shield box pulled open.

Figure 26: Top view of the back end of the beamline and the exposure station. The beam is coming in through the shieldwall (bottom of picture). The beryllium window and the entire scanner are shielded with 1/8” thick lead sheets and can be seen in the center of the picture. The entire experiment is surrounded by the hutch wall, which is made out of steel with another 1/16” lead lining.

34 3.5 Experimental characterization of the wiggler-source

The wiggler source is characterized as a pre-requisite for stable and controllable exposures and allows a comparison with the raytrace calculations (chapter 3.4.3).

3.5.1 Beam Width and Vertical Distribution

In a very basic test the beam footprint was taken with radiation sensitive paper.4 The dimensions were then measured with a caliper to be approximately 85mm × 4mm. The print appears to be uniform across the full width. See Figure 27 for an image of the beam footprint done with an additional 1mm Al filter at 6T magnetic field. With this filtration the paper gets exposed only to a hard radiation spectrum with relevant energies above 10 keV.

100 Beam Profile Print in PMMA 90 4mm measurement 80 _____ Gaussian fit: 2σ=2.84mm

m] 70 µ

~ 85mm 40 depth of profile [ profile depth of 30

0 0 2000 4000 6000 8000 10000 Vertical Scan [mm]

Figure 27: Footprint of the beam with 1mm Figure 28: Vertical footprint of the beam (no Al filtration, 6T, and 1.5GeV. filters) in PMMA developed and scanned with profiler.

For another estimate a standing exposure in PMMA at 18 mA beam current and a total dose of 100mA·min was conducted. The sample was developed in GG for 36 hours and scanned with a mechanical surface profilometer (Tencor Alpha Step 50) in the center region. It must be understood that this is not a direct beam profile power measurement because the development characteristics of the PMMA in GG has to be taken into account [44]. After these two preliminary tests a photodiode was used to scan through the beam: The principle setup of the experiment is shown in Figure 29. The photodiode used was a Siemens (Infineon) BPX065. The glass window was lathed off and replaced by a 50 µm Kapton window to increase sensitivity to X-rays while protecting the diode

4 Product information for “Green Detex” rad. sensitive paper used: Sessions of York www.sessionsofyork.co.uk

35 from electron scattering. The photodiode was mounted in the center of a 4” diameter stainless steel plate (thickness=1/2”=12.7mm). In front of the diode a horizontal slit was installed with 100 µm width. An aluminium filter of 1mm thickness was inserted in front of the assembly during this measurement. The assembly was mounted in the mask holder and was scanned through the synchrotron beam with the X-ray scanner at a speed of 0.85mm/s. The diode current was amplified and measured with a voltmeter. The voltage was scaled to an arbitrary number.

SCAN 100 µm slit

Photodiode BEAM

Figure 29: Setup to measure vertical beam profile with a photodiode. (Side view)

20 Photodiode Scan Of Vertical Beamprofile 18

16 Data: Scan with Photodiode Model: Gauss 14 σ = 3.55 ±0.03

intensity 6

[proportional photodiode current] [proportional photodiode 0

-2 0 2 4 6 8 10 12 14 16 vertical scan [mm]

Figure 30: Vertical beam profile measured with the photodiode and slit on scanner.

Figure 30 shows the beam profile measured with this method at 6 T, 1 mm Al filter, 50 mA beam current. Standard deviation (2σ) of the fitted Gaussian is: 3.55 mm,

36 which compares only very roughly to the calculated value of 2.37 mm from a ray tracing simulation including the 1mm filter used. The data in Table 1 summarize the vertical height and horizontal width of the beam, measured and calculated.

Measurement Method Height: 2 σ [mm] Width [mm]

Footprint in Radiation N/A 85 Sensitive Paper Footprint in PMMA 2.84 85 Photodiode Scan 3.55 N/A Calculated Value using Litop [44] 2.56 N/A (Source-point size = 0) Calculated Value using SHADOW 2.37 With 1mm filter 85.2 without add. RAY TRACING [39] 3.63 Without add. filters filter (Source point size 2 σ=0.4mm) Table 1: Beam profile vertical heights5

3.5.2 Total Power

Knowing the actual total power of the wiggler is important to calculate the dose of an exposure. The total power of the wiggler beam has been measured at two times. To measure the total power output of the wiggler, a copper calorimeter was built: It consists of a 0.5” (12.7 mm) thick oxygen free copper plate that is mounted to a horizontal rod in front of the beamline window by two thin nylon strings. (Total measurements: 12.7 x 53.2 x 124.6 mm, two 4mm diameter holes for mounting). A K type thermocouple was attached to the back center of the plate to monitor its temperature [44, 45]. The first measurement was at 6T magnetic field, 74 mA beam current and no forced air-cooling. The photon shutter was opened. The temperature raise was measured with the thermocouple and recorded over time. After 10 minutes the photon shutter was closed. The temperature decrease was recorded in order to estimate the thermal losses of the copper to the environment. The graph in Figure 31 shows the temperature measured over time.

5 Note that during the entire experimental phase of this work a stable beam position and orbit was not provided. It is not possible to quantify the effects the changes in orbits had on these measurements.

37 Temperature of Cu Calorimeter 80

Photonshutter closed 70

Calorimeter 50

40 Temperature [Deg. C] 30

Photonshutter opened 20 -10 0 10 20 30 40 50 60 70 80 Time [min]

Figure 31: Temperature rise of Cu Calorimeter during exposure to the wiggler beam and temperature fall after closing the photon shutter.

7 Temperature Derivatives

3 dT/dt temperature rise 2 dT/dt temperature fall

dT/dt [K/min] dT/dt (dT/dt rise) - (dT/dt fall) 1

-1

-2 30 40 50 60 70 80 T [degr. Celsius]

Figure 32: Temperature Rise dT/dt corrected by the temperature fall. The first ten minutes of the measurement has been differentiated in order to get the temperature increase per time in dependence of the temperature value. The same procedure has been applied to the data of the falling temperature. To correct the rising curve, the falling dT/dt curve had to be subtracted from the rising data. With this correction a dT/dt = 5.6 ± 0.2 K/min was found. With Equation 10, the specific heat of

38 oxygen free copper of 385 J/(kg·K) and the weight of the copper block of 731.2g the total power deposit Equation 10 [46] ∆ ∆ []=  J  ⋅ ∆ [] = T W J C  T K , P C , kg ⋅ K  ∆t in the calorimeter comes to 26.7 ± 0.9 W. The measured width of the beam is 85 mm in 10 m distance, therefore the power/cm and 100 mA beam current is 4.2 ± 0.3W/cm. See Table 2 for a summary of the calculated and measured values. This is only 2/3 of the theoretical value. With all absorbers (250 µm BE, 11”He, 4”Air, 50 µm Kapton), 6T and 100% deposition of the beam energy in the copper a theoretical value of 5.5 W/cm has been calculated for the center pole. In addition to the center pole the two super-conducting side poles contribute 0.3 W/cm each. In comparing measurements the calorimeter has shown accuracy within 2% of the calculated value at the bending magnet beamlines.

Total vertically integrated SOURCE power [W/cm] Measurement #1 4.2 ± 0.3W/cm Measurement #2 (after new orbit) 5.9 ± 0.4 W/cm Calculation (Litop[44]) 6.1 W/cm incl. side poles Calculated (SHADOW) 6.0 side poles not included Table 2: Overview of measurements and theoretical value of the total power output of the wiggler source (after filters). Obviously the orbit has not been optimized when measurement #1 was performed. After moving a dipole magnet in the lattice for improvement of the overall ring performance and after setting a new orbit for wiggler operations the same measurement was performed again. The measured value now was: 5.9 ± 0.4 W/cm. This increase in total power was also seen in the samples exposed. This is a very large discrepancy that shows that an online photon monitor together with a better understanding of the source is mandatory for future UXRL work at the wiggler beamline. It seems that the source point ‘seen’ at the beamline must have changed from an off center peak position to a very precise peak position. The radiation from the secondary (outer) wiggler poles must have been contributing to this effect. The calculated extra power from the secondary magnets is 0.6W/cm total. The electron current in the ring is measured via DCCT 6. While this method is believed to be precise it is not clear how much error it introduces to the measurement process over a longer period of time (months).

6 DCCT: Direct Current Current Transformer = Automatic DC ampere-turn balancing of primary and secondary currents through zero-flux detection in a transformer magnetic core.

39 An online photon monitor using the clipped edges of the wiggler beam is proposed for the future. The CAMD rings re-surveying and adjusting in April 2001 will help to improve and better understand the source.

3.6 In Air X-ray Scanner

The beamline is terminated with an in-air exposure station. It consists of a vertical scanning stage (Aerotech Inc., US) designed with a sled driven by a stepping motor and spindle [47]. The scanner is mounted to a custom made optical table placed on a kinematic mount for precise adjustment on the floor. The bottom part of the kinematic table is designed in a way that the whole upper part of the table is interchangeable with other experiments sharing the bottom platform. Figure 33 shows a 3D CAD view of the scanner optical table with the scanner installed on the top mount. Scan direction is indicated with an arrow.

Figure 33: CAD view of the scanner optical table with the scanning stage in place.

The maximum scan speed is 10 cm/s and the vertical scan length can be set to a maximum of 30 cm. This scan speed helps prevent local heating of the mask [22] and the long scan length allows for long scans and can also be used for multi sample/mask exposures when samples are mounted lined up vertically. The scanner is controlled by a personal computer. A basic user interface was implemented in Visual Basic that allows for changing parameters like scan length, scan range and scan speed.

Figure 34: User interface implemented for the computer-controlled scanner.

3.6.1 Mechanical Accuracy

The accuracy of the scanner has been measured to make sure it is within the specified range of less than 0.2mrad wobble to ensure structure accuracy of the X-ray lithography process. The scanner has been aligned normal to the incoming beam. From surveys with the diagnostic screen it was derived that the wiggler beam is horizontally level. Therefore the mask plane has been surveyed normal to level. Normalizing the other axis of the mask plane to the beam is only possible using the X- ray beam itself. Only a coarse adjustment has been made using a 0.5’’(12.7mm) thick aluminum plate that is cut to a 4” (101.6mm) disk to fit the mask holder of the scanner. This aluminum plate has five 200µm diameter through-holes drilled normal to the surface in a square array. An exposure in a very thin layer of 10µm PMMA resist with this mask shows the 200µm diameter pattern distorted if the mask plane is not normal to the incoming X-ray beam. Visible distortion is about 10µm or above from ideal ‘round’ shape of the through hole pattern. This corresponds to an accuracy of better than 1mrad assuming the mask used has been fabricated to this precision. The fact that 5 holes are used allows minimizing systematic errors introduced by the mask. Another method that can be used is a slit mask with a photodiode measuring the incoming X-rays. (See 3.5.1). This method is not more accurate than to one mrad either [[19] p. 75].

Measurements of scanner wobble: Three tests were performed. In the used coordinate system, the vertical 0 position of the scanner is at the beam height, 4’ (1219 mm) above the experimental floor.

1. Basic check with low-resolution digital level (resolution 0.01 degrees) to check level of mask plane in 0.5” distances across the full scan length. Result: Max. deviation was ± 0.02° = ± 0.35 mrad

41 2. High precision flat mirror attached to the mask plane, Laser autocollimator (Moeller Wedel “elcomat”, 2 axis electronic autocollimator, resolution 0.01 arc sec.) measurement of both the x (tilt around vertical axis) and y tilt (tilt around horizontal axis) of the plane across the full scan length. Figure 12 shows the measurement on the full scan length. The area of ± 2” around the beam height has measurements every 0.1”; the rest of the scan range was measured in 0.5” steps.

60 Scanner Wobble Error

40 Wobble X axis Wobble Y axis

0 Wobble [arc Wobble [arc sec.]

-20

-40 -8-6-4-20246 Scan length [inches]

Figure 35: Wobble of scanner mask plane around vertical (X) and horizontal (X) axis.

Result: Max. deviation of tilt around x axis was ± 40 arc seconds = ± 0.2mrad Max. deviation of tilt around y-axis was ± 8 arc seconds = 0.04mrad.

3. Dynamic measurement: point laser beam on high precision flat mirror. Measure reflected beam position in 30.30 m distance with the scanner in motion across the full scan length. Movement of laser spot in distance resulting from x axis wobble: ± 3.8 mm, movement resulting from y-axis wobble: ± 0.8 mm. These values correspond to the following dynamic errors in mrad:

Result: Max. deviation of tilt around x-axis was ± 0.06 mrad Max. deviation of tilt around y-axis was ± 0.01 mrad

Summary: The basic check with the digital level was a coarse estimate since the accuracy of the instrument is much lower compared to the other tests performed. Measurements two and three show that the allowed wobble error of the scanning stage easily meets the required specification. It is interesting that the dynamic measurement has better results than the static. For this work only the area of ± 2” around the beam

42 height have been used, therefore the error is reduced by a factor of two. This ranks the scanner well within the usually specified range of 0.02-0.05 mrad deviations from perfect movement [22],[33].

3.6.2 Mask Fixtures:

A mask-substrate assembly used for first tests on the wiggler scanner was a copy of the fixture of the CAMD bending magnet XRLM3 beamline exposure station [48]. Figure 36 to Figure 38 show pictures of this basic, uncooled setup.

Figure 36: Mask holder Figure 37: Substrate placed on mask Figure 38: Fully with mask (uncooled). with proximity shims (uncooled). assembled cassette back view (uncooled).

With this uncooled setup temperature rise on the mask and the whole assembly was high. Figure 39 shows the measured temperature of the uncooled assembly, when exposed to the beam at ca. 50 mA (6T, 3cm/s 7cm scan length). The temperature was measured on the mask ring. Temperature reached levels beyond 40° C. A preliminary water-cooled brass back plate was installed behind the substrate using the existing mask cassette. With the water-cooling set to 22.5°C the maximum temperature reached on the mask ring was below 29°C at the same point of the cassette assembly with similar exposure conditions.

43 42

] 38

36 deg C deg

34 Uncooled Fixture Cooled Backplate 32

26 Temperature Mask Ring [ Ring Mask Temperature 24

22 0 10203040 Exposure Time [min]

Figure 39: Temperature of uncooled Mask ring (black), Temperature of mask ring using a preliminary cooled back plate in the mask cassette. A fully water cooled setup (mask and substrate) was added after the described preliminary temperature studies showed cooling to be crucial. The water-cooled setup increased structure quality by reducing the thermal load on the mask-substrate assembly [49][21]. Figure 40 to Figure 42 show pictures of the fully cooled assembly. The assembly is built fully compatible to the CAMD / Jenoptik substrate/mask standard. The brass mask plane that holds the 4” mask mounted on a 304 stainless steel ring is cooled to 22.5°C with two horizontal cooling channels. It features two stainless steel blades for easy vertical aperture setting to avoid turning point exposures of the mask. The brass back plane of the assembly holds the substrate in a 4” diameter depression and it is cooled to 22.5°C by two horizontal channels. The assembly is built in a special sandwich construction where the mask holder and mask can stay in place, hard bolted to the scanner, while the substrate can be changed sliding the substrate cassette only.

Figure 40: Mask holder with Figure 41: Substrate cassette Figure 42: Fully assembled mask place. Note the way the with brass proximity shim and Cassette front view. Adjustable mask is clamped to the flat on substrate installed. apertures are visible. the bottom in the picture.

Figure 43 to Figure 48 are demonstrating the effects of uncooled versus cooled exposures. Figure 43 and Figure 44 show two patterns (the 20 and the 5 µm step structures) on the C04 graphite mask that were used to qualitatively analyze the effects. The uncooled exposure in 1mm PMMA shown in Figure 45 and Figure 46 demonstrates how destructive temperature induced mask versus substrate motion is for patterning accuracy. The structures get transferred only blurry and the 5µm steps are hardly reproduced. Assuming a local temperature increase of 30°K for a beryllium mask (thermal expansion coefficient = 11·10-6 / K) a shifting of 3.3µm on a length of 10cm (4”) is calculated. This value compares to what has been calculated in a finite element analysis of thermal motion on beryllium masks where the shifts were in the 2µm range for 200µm thick Be foil [[22] p. 43].

Figure 43: Graphite Mask (C04) used to Figure 44: Graphite Mask (C04) used to compare cooled and uncooled exposures. Step compare cooled and uncooled exposures. Step height of pattern is 20µm. height of pattern is 5µm.

Figure 45: Exposure in 1mm PMMA uncooled Figure 46: Exposure in 1mm PMMA uncooled using mask C04. Step height 20µm. Note the using mask C04. Step height 5µm. Note the rounding of the concave corners and the upper washed away pattern. The ‘doubling’ of edge due to temperature-induced movements pattern shows well that the movements are in of mask and substrate. the range of several micrometers.

Figure 47: Exposure in 1 mm PMMA cooled Figure 48: Exposure in 1 mm PMMA cooled using mask C04. Step height 20µm. Note the using mask C04. Step height 5µm. Note the much improved pattern accuracy compared to much improved pattern accuracy compared to the uncooled exposure in Figure 45. the uncooled exposure in Figure 46.

46 3.6.3 Further Improvements for the UDXRL Scanner

Building a basic UDXRL scanner is simplified and much less expensive when it is put in air instead of a vacuum chamber. Attenuation radiation in the air gap is not a concern because of the hard radiation used in UDXRL being able to penetrate air easily. Figure shows an example calculated for the wiggler beamline, 6T magnetic field at 1.5GeV electron energy. The spectrum does not get attenuated in the 100mm air gap used for this calculation.

Influence on spectrum when adding 100mm air gap

1E-3

1E-4 Wiggler Beamline Spectrum (250µm Be, 50µm Kapton) Same Wiggler Beamline Spectrum plus 100mm of air Power[mW/(mrad mA)] Power[mW/(mrad

1E-5

1 10 100 Photon Energy [keV]

Figure 49: Influence of adding an air gap in the exposure setup. The spectrum after adding 100mm air to the experimental setup does not change significantly in the energies above 5keV.

Other Synchrotron groups experimenting with UDXRL have built in air scanners as well [50] [23] [36] and reported no problems. In the setup tested at CAMD, however, several problems due to the exposure taking place in air have been recorded. The beryllium window that has been passivated with boron nitride was surrounded by a helium-purged chamber to protect it from moisture and ozone production. Figure 50 shows the window if the sequence works ideally and sufficient helium is delivered to the chamber at all times. Figure 51 shows a window that was installed when the helium supply was cut off, most likely by a nicked supply line. It oxidized its copper part and the Boron Nitride protective coating started to peel. This window would eventually have created a leak if the problem had not been diagnosed early.

Figure 50: Atmospheric side of the Figure 51: Atmospheric side of the beryllium beryllium window. Helium purge was window. The Helium Purge in the helium sufficient on this window and only some chamber must have been interrupted. discoloration of the protective coating Oxidation and heat destroyed the protective occurred which is normal [42]. Boron Nitride coating and oxidized the copper.

In addition, the ozone production had a very harsh impact on the scanner components themselves. The scanner was only exposed to full beam for a maximum total of approximately150 hours. A large air fan to avoid heat buildup and accumulation of ozone in the air gap also vented the Scanner. Most of the scanner components are stainless steel. Still there was a considerable amount of corrosion on critical components that would impact functionality of the unit after only a few hundred operating hours. See Figure 52 and Figure 53 for illustration of the corrosiveness of the environment produced in air.

Figure 52: Corrosion on Scanner Spindle Hub Figure 53: Corrosion on the alloy housing of and Guide plates from excessive Ozone the position readout of the scanner due to production and humidity. excessive ozone production. Derived from these experiences it is highly recommended to build an UDXRL scanner in a vacuum chamber that can be pumped down to 1mbar and then be flooded with 100mbar Helium during exposure.

48 4 Experimental Results

This section will describe in detail requirements for masks suitable for UDXRL and partly fabricated within this work. The sample exposure parameters are summarized and a detailed structure analysis of the samples fabricated in this work is discussed.

4.1 Masks for UDXRL

Membrane masks are not suitable for use at UDXRL sources because of too high sensitivity for temperature gradients and insufficient mechanical stability when plating gold absorber structures above 30µm height [26]. Sheet materials of 100µm or thicker are commonly used for UDXRL. A variety of solutions for UDXRL mask materials are possible and are under development. Relatively thick beryllium sheets (300-600 µm) [10], silicon substrates (50-400 µm) [51] and graphite substrates (125- 250 µm) [52] are all suitable for the application.

Comparison of mask substrates

Figure 54 shows the X-ray transmission of the typical UDXRL mask substrates beryllium, graphite and silicon. The spectrum of the wiggler beamline is used. It is obvious that silicon is not transparent enough for use at the wiggler beamline while beryllium and graphite are equally good candidates.

Comparison of Mask Sheet Materials

3.0x10-3

Spectrum at exposure station -3 2.5x10 Beryllium Mask (600µm) Graphite Mask (200µm) Silicon Mask (100µm) 2.0x10-3

1.5x10-3

1.0x10-3 Power [mW/(mrad mA]

5.0x10-4

345678910 2030 Photon Energy [keV]

Figure 54: Comparison of spectra behind the different mask sheets used. Shown are 600µm Be compared to 200µm graphite and 100µm silicon. The silicon absorbs too many photons in the range of 5-20keV to maintain reasonable exposure times. The scenario calculated is at the wiggler beamline (6T, 1.5GeV, 250µm Be window, Filters: 100µm Kapton, 15’’ (381mm) helium/air.)

49 Besides good X-ray transparency in the energy range of interest (2-50keV) beryllium has good thermal conductivity and the thermal expansion coefficient is small as shown in Table 3.

Potential Mask Flexural Thermal Expansion Thermal Material Strength Coefficient Conductivity [GPa] [10−6 /K] [W/mK] Si (B-dote) 240 2.33 157 Beryllium 318 11.30 205 Graphite 0.085 8.10 95 (PocoDFP1)[53] Table 3: Characteristics for potential sheet mask materials. Disadvantages of Be are its hazardous characteristics [54] and price [55]. While a US$1000 mask sheet price is bearable, taking into account the total cost of making an UDXRL mask, it is not acceptable for fast prototyping and basic research tests that demand high material use and change of designs. Due to the low X-ray absorption, sheets with several hundred microns thickness can be used. The tensile strength of beryllium (Table 3) is outstanding. The high thickness assures ruggedness, a precaution against defects. The high cross section also improves thermal conductance.

Silicon has acceptable material properties (Table 3) but a rather high absorption coefficient in the relevant X-ray range. Typical thickness commonly used is 100µm [51]. Silicon of this thickness as a mask sheet will increase the exposure time significantly, limiting their applications to very intense and hard sources. Figure 56 shows a calculation for a 100µm thick silicon mask sheet in a typical exposure at the CAMD wiggler beamline. The silicon absorbs much of the radiation below 15keV photon energy. This leads to the fact that the gold absorber has to be thicker than on more transparent mask materials. With silicon higher energy photons are used for the exposure that also penetrate the gold absorber easier. Figure 55 shows the minimum gold absorber heights needed for sufficient contrast and a dark dose of less than 100J/cm². Compared to a 600µm thick beryllium sheet, the silicon needs significantly more gold absorber. This is especially true when applied to up to 3000µm PMMA resist thickness. Figure 56 shows calculated exposure times at the wiggler beamline for different PMMA resist heights. Silicon mask sheets of 100µm are compared to 600µm thick beryllium mask sheets with the same beamline and scanner properties. (1.5GeV, 6T, and 250µm Be window, Filters: 100µm Kapton, 15” (381mm) helium/air, scan length 5cm). The exposure times for the silicon sheet masks are about 2 times more compared to beryllium sheet masks for PMMA heights of 3000µm or less. Above 5000µm PMMA thickness, the silicon mask is compatible with the beryllium mask. In this extreme resist heights, the additional filters that have to be used with the beryllium sheet to harden the spectrum sufficiently are equivalent to the amount absorbed by the silicon mask. Nevertheless, the heat load absorbed in a silicon mask remains significantly higher.

50 50 Mask Absorber Gold Height Required for Sufficient Contrast 45

25 100µm Si mask sheet 600µm Be mask sheet Required Height of Gold [um] of Gold Height Required 20

15 0 500 1000 1500 2000 2500 3000 3500 4000 4500 Maximal PMMA resist height [µm]

Figure 55: Minimum gold absorber height for exposures in PMMA of different height. The source used is the wiggler beamline with typical filters (15’’ air / helium, 100 Kapton, t/b ratio < 3). Sheets compared are 600µm thick beryllium vs. 100µm thick silicon. Even though the silicon used is 1/6 as thick, significantly thicker gold absorber is needed for sufficient contrast.

Comparison Exposure Times Silicon / Beryllium Mask 3.5

3.0 Silicon Sheet Mask 100µm Beryllium Sheet Mask 600µm 2.5

2.0

1.5 Exposure Time [h] 1.0

0.5

0 1000 2000 3000 4000 5000 PMMA Resist Height [µm]

Figure 56: Comparison of exposure times using silicon sheet masks and beryllium sheet masks for different PMMA heights. In the entire regime of PMMA resists with heights smaller than 3000µm the silicon masks would lead to significant longer exposure times because. Silicon masks absorb too many photons in the lower energy part of the synchrotron spectrum, which is used for this thickness of PMMA. (Scenario: 6T, 1.5GeV, 250µm Be window, Filters: 100µm Kapton, 15’’ (381mm) helium/air, 5cm scan length, top to bottom dose ratio < 3)

Graphite has been found to be a promising candidate as alternative mask sheet material. It has the advantage of low cost, and is readily available off the shelf [13, 53]. It is electrically conductive and has acceptable mechanical properties (Table 3). It has good transparency to X-rays. It absorbs approximately 4 times higher than beryllium. Figure 54 shows the absorption spectrum of a 200µm thick graphite sheet mask compared to beryllium and silicon. Its absorption characteristic is comparable to the 600µm beryllium sheet. However, there are several problems experienced in the graphite mask technology: • In the mask making itself bad adhesion and underplating has been experienced due to the high porosity of the material. The graphite masks introduced in the following chapter will illustrate the problems in detail. A solution recently found for easier patterning of the mask is direct photolithography using SU8 resists [56]. • In the pattern transfer: First experiments in this work and other studies at CAMD have shown problems with striations on the sidewalls of the patterned resist. X-ray microprobe testing revealed that contamination with heavy metals might lead to this effect and even in ultra purified materials there is also the possibility of small angle scattering at the crystalline structures of the graphite [38, 52, 57]. This issue has not been solved yet but small angle scattering experiments are being prepared for while the search for suitable off the shelf graphite has extended. A promising candidate is pyrolytic graphite.

4.1.1 Graphite Masks

Within this thesis several UDXRL compatible graphite masks using different approaches have been fabricated. They will be briefly described in the following section.

C01 Graphite Mask CAMD CAMD and the Institute for Micromanufacturing, Louisiana Tech University (IfM) jointly fabricated this mask. Off-the-shelf rigid graphite 4“ wafers 125 µm thick and of 99.95% purity from Goodfellow Corp. were used [13]. The mask pattern was generated from a Ti mask used as an intermediate mask. The Ti mask pattern was transferred into a 10µm thick spin-coated PMMA resist using soft X-rays. After development of the irradiated resist, electroplating of the Au absorber pattern up to 5µm followed. Prior to electroplating, the backside was sealed with an optical resist to prevent plating. This 5µm thick Au pattern was used as an intermediate mask. Before stripping the resist, a thick PMMA sheet (up to 100µm) was solvent bonded to the backside of the wafer. X-ray lithography was repeated to transfer the intermediate mask pattern into the thick PMMA resist using the same graphite sheet. The exposed areas were then dissolved in the GG developer at room temperature and the working mask was plated with Au absorber approximately 40µm high. The final mask was achieved by stripping the PMMA resist and by etching off the intermediate mask, using aqua regia. In the copy step through the graphite sheet, striations were already

52 introduced to the working masks gold absorber pattern. The process sequence is described in detail in [57]. This mask is overall presentable but has striations in the absorber sidewalls and significant under-plating in the 1µm range due to porosity of the substrate. Figure 57 and Figure 58 show SEM pictures of a sample gear structure on the mask. The mask is a plain wafer and was not mounted to a ring. The gold surface has a measured minimal radius of 220 mm from tension, even if the mask membrane is placed in the mask / substrate assembly. This radius of curvature leads to insufficient contrast and inaccurate feature reproduction. With this mask first test exposures at the wiggler beamline have been made in October 1999. Details about these first exposures are described in [38]. The exposures suffered in their quality from the striations and the insufficient contrast on the mask due to underplating and radius of the surface. Also, the standard DXRL uncooled mask / substrate cassette introduced in chapter 3.6.2 was used which leads to major problems in patterning accuracy. (See chapter 3.6 for details about sample cooling.)

Figure 57: CO1 Mask sample gear structure, Figure 58: Zoomed view of C01 Mask bottom. note the striations on the gold sidewalls caused See the under plated Gold due to bad resist by the copying through the C foil. adhesion and sealing of C surface.

C03 Graphite mask fabricated within this work Substrate material used is Poco DFP-3 [53]. The graphite has been mounted with AZ P4620 resist to a 3mm thick glass wafer, then it has been flycut on one side. It has been released and mounted on the other side. Both sides have been cut to achieve a thickness of 125 ± 5 µm. The graphite has been “primed” with a 2 µm thick layer of PMMA 9% (spincoat) that has been annealed fast on a hot plate at 150° C to avoid soaking. This step has been repeated once. This step was implemented to help seal the porous graphite surface. A 1mm sheet of high molecular weight PMMA (30x75mm) has been solvent bonded to the pretreated graphite wafer and then was flycut to a height of 75 ± 5 µm. The pattern of this mask was available from a working mask, courtesy of IMT, Forschungszentrum Karlsruhe [49]. Figure 59 shows an overview of the entire mask pattern.

Figure 59: IMT mask pattern used for the fabrication of UDXRL graphite masks for the wiggler exposure system. Lateral dimesion of the structured area is 23mm x 63mm.

This mask pattern is well described in [49] and was specially designed for UDXRL testing. The PMMA was patterned together with IMT scientists at ELSA synchrotron, Bonn. The working mask was a 2.3 µm thick Ti membrane with 8 µm of gold absorber. The gold electroplating was performed at the LSU µSet group facility where 36µm high gold absorber was deposited. The commercial sulfide gold bath was directly used without a strike layer. Current density was 2 mA/cm². SEM pictures of the C03 mask are shown in Figure 62 and Figure 63. They show that the plating looks basically acceptable except for a ridge on top of the structure and worse, for a large underplating foot in the first 5µm of the structures. Insufficient adhesion of the PMMA to the graphite material might have caused this effect. Interesting is, that the adhesion problem is not directly visible in the developed PMMA structures of the same mask as illustrated in Figure 60 and Figure 61. The 5µm foot could correspond to an approximate 5µm thick bottom layer consisting from the spin coated PMMA and the solvent affected zone on the PMMA sheet. Solvent bonding could be partly responsible for this adhesion problem. This mask was mounted to a 4” stainless steel ring to fit Jenoptik scanners.

C04 Graphite mask fabricated within this work The C04 mask was fabricated with identical steps like C03 in the same batch except for the electroplating. A strike layer of ca. 3µm of Nickel was used. Figure 64 and Figure 65 show SEM pictures of a comparable mask feature. The absorber is 30µm high. The foot-like defect due to insufficient adhesion is still produced while the top surface of the gold is smoother than on C03.

Figure 60: 5 µm step structure in PMMA (C04 Figure 61: Zoom in on bottom of 5 µm step mask fabrication step) before electroplating. structure in PMMA (C04 mask fabrication step) before electroplating.

Figure 62: Mask C03, electroplated structure, Figure 63: Mask C03, electroplated structure, 5µm step height. 5µm step height. Zoom on bottom of structure.

Figure 64: Mask C04, electroplated structure, Figure 65: Mask C04, electroplated structure, 5µm step height. 5µm step height.

Conclusion: None of the fabricated graphite masks had vertical sidewalls, which are essential for sufficient contrast of an UDXRL mask. Therefore a more expensive beryllium mask was fabricated using the same intermediate mask.

55 4.1.2 Beryllium Masks

B01 Beryllium Mask fabricated within this work: Beryllium sheets are commercially available from Brush Wellman [55]. Used was a disk with 100mm diameter that was 600µm thick. The surface roughness of the unpolished material (specification PF60) is in the micrometer range, which increases mechanical adhesion of the gold absorber. Processing was done at IMT Karlsruhe. After a flush in ethyl acetate to clean the material, 20nm of chromium for adhesion enhancement and 50nm of gold as an electroplating seed layer are evaporated onto the beryllium. Resist deposition is done by direct polymerization of about 60µm PMMA (Plexidon M727) on the gold. Exposure was performed using the same intermediate mask described with C03 and C04 graphite masks. IMT staff conducted this exposure and also the development in room temperature GG. The mask was transferred to LSU where 45-50µm of gold were electroplated in a sulfite gold bath analog to mask C03. The PMMA resist was stripped by flood exposure and GG development. The beryllium sheet was then glued to a stainless steel ring with 101.6mm diameter (4”). The adhesive used was thermally conductive epoxy7. The slightly smaller diameter of the beryllium sheet prevents the beryllium edge from damage when being handled. The beryllium mask showed to be of good quality and was used as the main mask for this work. Figure 66 and Figure 67 show examples of structures on the mask.

Figure 66: Beryllium mask pattern 20µm step height. The mask has no visible defects and no underplating.

7 OMEGA OB-200 highly conductive epoxy. http://www.omega.com/

Figure 67: Beryllium mask structure. Panorama view on bottom, zoomed views on 5µm wide detail on top.

Both, Figure 66 and Figure 67 show the sidewalls of the beryllium mask to be smooth and without underplating. The close up view on a wall of 5µm nominal width in Figure 67, top with 6k magnification, shows striations on the sidewall that are later to be found to create striations in the resist in the nm range. (See chapter 4.3.2 for details).

IMM beryllium mask A second high contrast beryllium mask for test exposures was borrowed from the Institute of Microtechnology Mainz (IMM), Germany. This mask also consists of a 600µm thick sheet but has 80-120µm thick gold absorber. It is passivated with 0.5µm of Si-Nitride on both sides and an additional 0.1µm of Ti on the processed side. This mask is mounted on a 100mm diameter steel ring with two flats, comparable to the 4” (101.6mm) standard at CAMD. The mask therefore fit in the mask-substrate assemblies without further modifications. Structured process area was in an area of 50mm×50mm diameter. Figure 68 shows a gear test structure on the IMM mask. (SEM picture courtesy of IMM).

Figure 68: Gear structure on the IMM beryllium mask. The mask has smooth sidewalls and 80-120µm of gold absorber thickness [58].

Table 4 summarizes the basic properties of all masks used in this work. For the now following exposure tests, masks B01 and IMM were used.

MASK Sheet Sheet Gold Mounting Comments, Usage name Material Thickness absorber [µm] thickness [µm] C01 Graphite 125 40 None Preliminary exposures C03 Graphite 125 36 4” SS ring Preliminary and Temp. study exposures C04 Graphite 125 30 4” SS ring Temperature study exposures B01 Beryllium 600 45-50 4” SS ring “Work horse”, used for most exposures IMM Beryllium 600 80-120 100 mm Used for qualitative study SS ring exposures Table 4: Summary of the used masks and their basic properties.

4.2 Sample and Exposure Parameters

A total of over 30 samples have been exposed at the wiggler beamline. All used high molecular weight PMMA as resist. (Goodfellow / Plexidon M727) [13, 14]. Heights ranged from 200µm to 3mm. Samples were prepared in three different ways: • PMMA sheet solvent bonded with MMA (9%) to metal (Cu-Ti-Cu) sputtered silicon wafer (500µm thick various crystal orientations) [8]. • PMMA sheet glued to ceramic. Ceramic with 5µm chemically oxidized Ti adhesion layer [24].

58 • Free standing PMMA sheets fixed on mask / substrate back plate with adhesive tape for preliminary tests.

The exposures were all done with identical parameters summarized in Table 5.

Exposure property Value Magnetic field center pole 6T Electron energy 1.5GeV Average current 65mA (max.: 120mA, min.: 20mA) Fixed filters / windows 250µm Be, 15” (381mm) air/he, 100µm Kapton Optional filters 2 x 36µm Al Scan speed 3cm/s Scan length 5 or 7 cm Top to bottom dose ratio <3 Proximity 1mm (up to 2mm on very thick samples for safety reasons in case of overdose foaming) Table 5: Basic exposure properties used for the UDXRL exposures at the wiggler.

Development was performed in the cleanroom fume hood. The full development was insured by individually checking the samples with the optical microscope during the development process. Table 6 summarizes the development properties chosen. The parameters were chosen according to the state of the art for ultra deep structures and in house experience [21], [10].

Developing property Value Developer used GG Temperature Room Temperature (20°C) Agitation Magnetic stir Rinse DBG (80%Diethylglycolmonobuthylether, 20% DI water.) Rinse agitation Stir Average development time 0.2mm 4 h Average development time 0.5mm 10 h Average development time 1.8mm 40 h Average development time 2.8mm 65 h Table 6: Developing Properties of the UDXRL samples.

With the set parameters, samples of all thickness could be successfully fabricated. Figure 69 shows an example of a fabricated gear structure in 1780µm PMMA.

Figure 69: PMMA gear structure fabricated. Panorama view to the right, close up views on the left side demonstrate that the structure details are very good on the top and the bottom of the structure.

It should be mentioned at this point that the cracking and adhesion problems known from literature were observed as well [26] [21] [49]. However, there were sufficient amounts of useable structures on each sample allowing further, more detailed structure measurement and analysis. Table 7 shows the samples selected for detailed examination in chapter 4.3 along with the test protocol and exposure details. Table 8 shows the development details of these samples. The samples can be divided into 4 groups according to the main aspects that were studied:

• Qualitative study (SEM): W09, 1780µm • Comparison to DXRL: W15, W16, 200µm • Structure accuracy as a function of height: o Intermediate height: W14, W20, 1780µm o Maximum height: W18, W19, 2800µm • Stacked Exposure study: W25-1 to W25-4

Based upon preliminary studies with low dose (100J/cm³) samples a maximum limit for the development times in room temperature GG is 3 days. For longer development times, attack of the PMMA is observed. It is for this reason as well as the limited time available for wiggler exposures that the maximal sample height was limited to 2.8mm.

Name Resist Mask Prox Exp. Scan Approx. mA· Bott. Filters Used for Height Gap Time length Average min Dose; Comments [µm] [mm] [min] [cm] Current t/b Sub- Speed [mA] ratio strate [cm/s] date W09 1780 IMM 1.3 165 7.0 88 10470 2700 50µm KA Qualita- Si 3.0 10/12/00 2.8 tive study W14 1780 B01 0.8 112 7.0 92 10700 2700 50µm KA Intermed. Si 3.0 10/23/00 2.7 Height W15 200 B01 1.3 66 7.0 70 5000 2850. 50µm KA Compare Si 3.0 10/23/00 1.2 to DXRL W16 200 B01 1.3 47 7.0 110 5500 3150 50µm KA Compare Ceram. 3.0 1.2 to DXRL c W18 2800 B01 1.2 173 5.0 80 16000 3500 50µm KA Maximum Si 3.0 120-107 2.7 36µm AL heigt -glitch- Power glitch 91-72 av.=80 10/26/00 W19 2700 B01 1.2 143 5.0 131-91 16000 3500 50µm KA Maximum Ceram. 3.0 av=105 2.7 36 µm AL Height 10/29/00 W20 1780 B01 1.3 106 5.0 80 9170 3200 50µm KA Intermed. Cerami 3.0 10/29/00 2.7 Height c W25-1 500 B01 1.5 131 5.0 65 10300 7000 58µm KA Stack Graph. 3.0 11/19/00 1.3 36 µm AL Layer 1 W25-2 500 B01 N/A 131 5.0 65 10300 5270 58µm KA Stack Graph 3.0 11/19/00 1.2 36 µm AL Layer 2 200 µm C 500µm PM W25-3 500 B01 N/A 131 5.0 65 10300 4200 58µm KA Stack Graph 3.0 11/19/00 1.2 36 µm AL Layer 3 400 µm C 1000µm PM W25-4 500 B01 N/A 131 5.0 65 10300 3500 58µm KA Stack Graph 3.0 11/19/00 1.1 36 µm AL Layer 4 600 µm C 1500µm PM Table 7: Parameters of samples selected for the detailed measurements. The use of the sample is indicated below the name. Filters were in addition to the beamline immanent filters (windows, air gap, helium chamber).

Name Development Development Rinse Comments: Height date and times time (all GG Time 20°C )[h] [h] W14 10/23 08:20 – 2 stripes undeveloped 1.7mm 10/24 23:00 38.6 0.75 W15 10/23 09:45 – Hard to tell if developed through 200µm 10/23 16:30 6.75 0.5 Left in dev. long to ensure compatibility with deeper samples W16 10/25 08:00 – Hard to tell if developed through 200µm 10/25 16:00 8.0 4.5 Left in dev. long to ensure compatibility with deeper samples W18 10/26 09:30 – Had problems to develop in some 2.8mm 10/29 13:20 75.7 1.0 sections W19 10/29 16:45 – 2.7mm 11/01 12:00 67.25 2.3 W20 10/29 18:40 – 1.7mm 10/31 15:45 45.75 1.75 W25-1 11/19 12:45 – Looked good a at 6.5 hours 500µm 11/19 20:45 8.0 0.75 development time W25-2 11/19 12:45 – Looked good a at 6.5 hours 500µm 11/19 20:45 8.0 1.0 development time W25-3 11/19 12:45 – Looked good a at 6.5 hours 500µm 11/19 20:45 8.0 1.25 development time W25-4 11/19 12:45 – Looked good a at 6.5 hours 500µm 11/19 20:45 8.0 1.5 development time Table 8: Development Times and Rinse times of the samples chosen for detailed measurements.

4.3 Measurement of the UDXRL Microstructure Patterning Accuracy

Two of the attractive and important features of LIGA microstructures are high precision sidewalls and low surface roughness. It is the goal of this chapter to analyze the structure accuracy and compare it to documented values: This includes a comparison of DXRL and UDXRL studies considering different exposure parameters such as power density and exposure spectrum as well as scaling from small to large heights. In the following, samples patterned at the CAMD wiggler are examined by measuring the structure width over structure height as well as surface roughness at different locations along the sidewall.

4.3.1 Structure Width over Structure Height

Measuring tools In UDXRL the structures are microscopic in their lateral dimensions while possibly being of macroscopic heights of up to several mm. It is important to verify that the accuracy of the microstructure is maintained over height. Measuring such microscopic lateral dimensions with high resolution over tall heights is a challenge. One way to measure the structure width as a function of height is by using the edge of a cuboid and accurately map it.

62 While DXRL structures up to 500µm height have been studied widely for their accuracy over height [7],[22] only one systematic study in UDXRL measured the deviation of the sidewalls from perfectly normal of structures having heights of 1mm and above [21]. For orientation the results of this study are shown in Figure 70. The total deviation of the sidewalls from perfectly normal is 0.9µm for 1500µm structure height (left) and 8µm for the structure of 3000µm height.

Figure 70: Width variation of UDXRL structures over height from [21]. Measurements were performed in steps of 500µm and not related to the mask width. The graph to the left shows a 1500µm high structure, the one to the right shows a measurement of a 3000µm high structure. These measurements translate to a slope of 0.06µm per 100µm structure height in the 1.5mm structure and 0.27µm per 100µm in the 3mm high structure. The 3mm structure has ca. 5 times more slope than the DXRL literature value (0.055µm per 100µm height) reported from a 400µm high sample [7]. In [21] it is speculated that part of the large slope in the 3mm structure is due to the long development time. The measurements of the width in Figure 70 have been taken using an SEM tool. A similar approach is used in this work 8. Measuring the width over height of a PMMA cuboid structure edge, mounted to a substrate without destroying it can be achieved with a scanning electron microscope (SEM). A principle sketch of the setup is illustrated in Figure 71.

8 Used was a Hitachi S-4500II Field emission SEM. Spatial resolution up to 1.5 nm at 15 kV and 4.0 nm at 1 kV.

Figure 71: Principle sketch of the measurement of a very tall microstructure using an SEM. The dimensions of the microstructure are not to scale for demonstration purposes. The width of the microstructure is measured along the hatched edge. In the drawing, the dimensions of the structure itself are not to scale versus the substrate and typically there are many microstructures located nearby. The structure width shown hatched in Figure 71 could ideally be measured by turning the hatched edge normal to the electron microscopes optical axis. However, this is not practical because other structures adjacent to the structure to be measured would obstruct the direct view of the edge. A way to achieve a good view of the edge is by turning the edge to an angle of 45 degrees relative to the electron optical axis (e¯) while aligning the sidewalls parallel to the optical axis. The angle is indicated in Figure 71. Neighboring structures, assuming they are in sufficient distance, will not disrupt the direct view of the edge to be measured. The measurements are performed using the built in measurement tool of the SEM. It allows placing pointers on the edges of the structure and calculates the length of the marked distance directly in µm. In order to achieve comparable measurements, the focus must always be on the plane measured while keeping the working (focal) distance constant together with the accelerating voltage. The estimated accuracy to measure the width, including all systematical and experimental errors, is in the range of ± 0.5µm for a 50-100µm wide edge at 1.5kV accelerating voltage and a magnification of 1000. The height in which the width is measured is determined by using the same measurement option in the vertical (y) direction. However, accounting for the 45- degree tilt by multiplying with a factor of 1/√2 is required. This measurement is less precise because it includes shifting the structure on the sample manipulator away from the focal point to achieve movement to the next measurement height. The error is approximately 10%.

Description of test structures The design width of the examined structure edges is either 100µm or 50µm. Analog to [22] it was differentiated between vertical and horizontal edges. Figure 72 illustrates the definition of horizontal edges and vertical edges relative to the synchrotron beam. The horizontal edge is parallel to the beam fan and the global horizontal. The vertical is normal to the beam fan and parallel to the scan direction.

Figure 72: Illustration of the definition of vertical oriented and horizontal oriented edges. Shown is a part of the CAD drawing of the mask pattern introduced in chapter 4.1.2. The structure is scanned in normal direction through the beam fan, which is horizontal. The edge parallel to the beam fan is called horizontal; the one parallel to the scan direction is called vertical. Due to sample defects, edges corresponding to several different mask structures have been examined. A relation between the mask and PMMA structure could only have been established in single structure cases. However, different mask structures of the same design width were varying by up to ±3µm. The absolute precision from mask to structure could therefore not be examined in a statistical manner. The edge width of all structures have been set to 100µm at the very bottom of the structure directing the focus on the precision of the structure width over height and the sidewall slope.

Comparison of UDXRL wiggler with DXRL bending magnet exposures Using 200µm high resist, exposures have been done at the wiggler scanner. Due to the small sample height, good comparison with typical exposures at a bending magnet is possible and allows a qualitative evaluation of the experimental setup as well as a comparison to measurements reported elsewhere. Figure 73 shows measurements of vertical and horizontal edges of two samples, W15 and W16. (See Table 7 and Table 8 for fabrication details.)

65 Wiggler: Structure Width over Structure Height 200µm

200 W15 vertical edge W15 horizontal edge W16 vertical edge W16 horizontal edge 150

100

50 Structure Height [um]

0 98 99 100 101 102 Structure Width [µm]

Figure 73: 200µm high PMMA structures fabricated at the wiggler: Width over height measurements. The corresponding mask structure is 100µm wide. The structure width of all samples and edges varies only by a maximum of 1µm and the structures are sloped towards the top. There is a tendency of the vertical edges being more precise, in the 0.5µm range. It must be noted that the full structure width is plotted; therefore the per-sidewall deviation from perfectly normal will be 0.5/0.25µm respectively. This translates to 0.25µm per 100µm structure height or better. Compared to the literature value of 0.055µm deviation per100µm height [7] the slope of the sample fabricated at the wiggler is approximately 4.5 times higher. However, the basic shape, narrowing towards the top of the structure, matches the literature [7]. In order to eliminate mask effects and to allow a comparison with typical DXRL exposure parameters, a 1000µm high resist using the same mask (B01) has been exposed at CAMD’s XRLM1 beamline using the DEX02 scanner. The graph in Figure 74 illustrates the results of this measurement. The deviation of the sidewalls in this exposure is maximal 0.075µm per 100µm when excluding measurement points indicated by brackets in the graph. This compares well to the literature value of 0.055µm, which was only measured on a 400µm high structure.

66 Bending Magnet Exposure: Structure Width over 1000µm Structure Height

1000 bend1 (si) horizontal edge bend1 (si) vertical edge bend2 horizontal edge 800 bend2 vertical edge

600

400 ( )

( ) ( )

Structure Height[um] 200 ( )

99 100 101 102 Structure Width [µm]

Figure 74: Reference exposure in 1mm PMMA at the XRLM1 beamline using bending magnet radiation at 1.5GeV, the UDXRL B01 mask and the DEX02 scanner.

The literature value and the reference exposure indicate that even at a very shallow height of 200µm resist there is a factor of at least two less precision over height in the high energy radiation wiggler exposure. Thermal effects, along with secondary radiation effects could be responsible for this significant loss in precision [19]. It can only be speculated at this point that a combination of thermal and secondary radiation effects is causing this loss of precision. As shown in chapter 2.4 and [19] photoelectrons produced by X-rays in the 10keV range can penetrate up to 1µm in PMMA. This may likely be the reason for a shift of the iso-dose-line to which the developer could penetrate and dissolve the resist. Development effects can widely be ruled out because the development time is relatively short (see Table 8).

1780µm high structures The goal was to examine scaling effects in sidewall precision going from small to large height. The first set of structures in the UDXRL range was 1780µm high. This height was chosen as an intermediate height. Again, two samples were fabricated: Samples W14 and W20. Fabrication details are shown in Table 7 and Table 8. Vertical and horizontal edges were examined and the results are illustrated in Figure 75.

67 Structure Width over 1780µm Structure Height

2000 W14 horizontal edge W14 vertical edge W20 horizontal edge 1500 W20 vertical edge

1000

500 Structure Height [um]

90 95 100 105 110 Structure Width [µm]

Figure 75: 1780µm high PMMA structures fabricated at the wiggler: Width over height measurements. The corresponding mask structure is 100µm wide. The width of the horizontal edge of sample W20 is very distorted, especially in the bottom part. It is not clear why and this result has therefore been excluded from further analysis. The variation in width of the remaining edges is ca. 6µm over the full 1780µm height, which translates to a sidewall slope of 0.17µm per 100µm structure height. This structure slope over height is slightly better than the 200µm wiggler samples but it can be stated that the observed error in precision remains constant in the range from 200µm to 1780µm. Compared to the 1500µm high structure results from IMT in Figure 70 these structures have about a factor two higher slope error. This can be explained by the fact that the IMT structure has been patterned with a low photon energy spectrum at ELSA. ELSA is a bending magnet machine. At 2.7GeV electron energy it has 4keV critical energy while the wiggler delivers 9keV at 6T and 1.5 GeV (both without filters).

2800µm high structures A set of 2800µm high samples, W18 and W20, has been patterned and the results of the edge analysis are presented in Figure 76.

68 Structure Width over Structure height 2800µm

3000 W18 horizontal edge (100) W18 vertical edge (50) 2500 W20 horizontal edge (50) W20 vertical edge (50) 2000

1500

1000

Structure Height [um] 500

80 85 90 95 100 105 110 115 120 Structure Width [µm]

Figure 76: 2800µm high PMMA structures fabricated at the wiggler: Width over height measurements. The corresponding mask structure is 100µm wide. Sample W18 has better patterning accuracy than sample W20 which shows severe rounding on the top of the structures and a ‘belly’-shaped sidewall profile of the vertical edge. Considering both samples in the calculation a mean slope error of 0.23µm per 100µm has been derived. Although the absolute value is a little higher, than in 1780µm structures, it still is in the same range. The difference between sample 18 and 20 could result from the different substrates used (W18: Si, W20: ceramic) along with different resist materials. (W18: Goodfellow material, W20: Plexidon material). Although the samples have similar molecular weight and have not shown any differences in all other heights it is possible that differences start showing at the extreme conditions of ultra high resist, long exposure times and development times. Both samples have slope errors that are comparable to the IMT 3mm measurement (0.27µm slope per 100µm height, Figure 70). Note that the IMT uses Plexidon PMMA material, the same material used in sample W20. The increased slope at the top of the structure can be observed in the IMT 3mm measurement also.

Conclusion: The measurements and comparisons above clearly show that the slope per height on the structures stays nearly constant in the range from 200µm over 1780µm to 2800µm. This slope is a factor 4-5 more than in DXRL. Temperature and secondary radiation effects (mostly photo electrons) are believed to be responsible for this effect.

69 The 2800µm samples start to show an influence of the long development times in the 3-day range. While these effects are not significant, for longer development times and thicker resists, alternative strategies might be required. No significant differences between vertical oriented edges versus horizontally oriented edges can be observed.

4.3.2 Sidewall Surface Roughness

As explained in chapter 2.1, the surface roughness of structures fabricated with deep X-ray lithography is in the range of 20-50nm. It has been shown that surface roughness is a crucial quality of LIGA. Because of extreme long development times and a smeared dose deposition from secondary effects it is to be expected that sidewall roughness is increased in UDXRL structures. The sidewall roughness of the structures is measured. The instrument used to measure surface roughness is a white light interferometric microscope from WYKO.9 The RST (Roughness Step Tester) used was the model RSTplus with a resolution of better than 3nm. The PMMA structures to be measured were physically detached from the substrate and glued to a sample holder, the measured sidewall facing the optics. The areas used for roughness testing were larger than 200µm by 200µm and carefully aligned normal to the optical axis. Figure 77 shows an example of a typical WYKO RST measurement of the surface roughness Ra. The striations seen in the picture are in the nanometer range and are explained below.

Figure 77: RST measurement example on the WYKO RSTplus. The area used for surface roughness measurements was always larger than 200µm × 200µm. In this example of the center area on a vertical edge of sample W20 the area used was 312µm × 231µm.

9 http://www.wyko.com/ The IfM, Louisiana Tech. University generously provided measurement time and access to their RSTplus instrument.

70 The four high UDXRL samples introduced in the last chapter have been examined. Top, middle and bottom of the structure have been measured in zones without defects (cracks). Table 9 shows the compiled results.

Sample # Roughness Roughness Roughness Average of bottom area center area top area all areas W14 horizontal 10 19 13 14 W14 vertical 11 18 24 18 W20 horizontal 46 14 83 48 W20 vertical 14 10 13 12 W18 horizontal (14) (129) (119) (87) W18 vertical (46) (141) (125) (104) W19 horizontal 16 29 24 20 W19 vertical 7 7 8 7 All Samples except 17 16 28 20 W18 Table 9: Summary of the roughness measurements of the very high samples 1780 and 3000. All samples show very acceptable average surface roughness with an average below 50nm. There is a clear tendency that the structures are rougher on the top where they are exposed to the developer for longer times. The roughness on sample W18 is not acceptable for LIGA. The surface showed micro-pockmarks on the surface. It is not clear what the origin of this structure defect is. As noted in the last chapter W18 is Goodfellow material while W19 is Plexidon material. It is possible that the Goodfellow material suffers significantly more from long development times than Plexidon with respect to surface roughness. (Note that W18, consistent of Goodfellow material on the other hand performed better with respect to the sidewall precision.) There is no significant difference in surface roughness of the other samples. The 1mm high DXRL structure fabricated with the same B01 mask but at the DXRL bending magnet beamline has been examined as well. The average roughness of this sample was 39nm, which compares well to the UDXRL samples fabricated.

Conclusion: An increase in surface roughness of the UDXRL samples could not be examined except in one sample of 2800µm height. All other samples are comparable to surface roughness examined in DXRL. This illustrates that there is a sharp iso-dose line in the UDXRL samples fabricated.

Nano striations observed When measuring surface roughness, striations with heights in the nanometer scale were observed (Figure 77). These striations were present in every sample exposed with the B01 mask. In order to identify the origin of this effect, a mask feature of a 20µm wide edge is examined in the SEM. Figure 78 shows the SEM picture of the feature at 10kV acceleration voltage (Vacc). To the upper left of the picture is the mask feature with lower Vacc =1.5 kV. Striations are clearly visible. Below the SEM

71 the WYKO RST measurement of the corresponding PMMA structure was inserted for ease of comparison.

Figure 78: Origin of nano striations in the PMMA structures are striations on the gold mask itself. Compared is an SEM picture of the mask at 1.5kV accelerating voltage to the Wyko measurement of the same area in the PMMA strucutre. Roughness of the PMMA sidewall is in the 20nm range.

This test illustrates very clearly that the striations in the PMMA can be traced back to the mask. The mask already carries striations. The mask cannot be measured with the RST instrument without being destroyed to determine if the striations on the mask are also in the nanometer scale. As explained in 4.1.2 the B01 mask has been fabricated using an intermediate mask. It is not clear if the striations were produced in that copy step or if they origin from the intermediate mask. Similar striation effects have been reported in earlier work [59].

4.4 Stacked Exposure

Motivation: It was shown in the previous chapters that hard X-rays have high penetration depths of several millimeters in PMMA, allowing fabrication of very high structures. If a highly transparent substrate material is selected, an entire stack of substrate-mounted resist can be exposed at the same time. This method can reduce total exposure time and enhance efficiency of the DXRL process [5]. A first experiment was conducted with a stack of 4 layers. Figure 79 shows a principle sketch of the setup. The wiggler radiation and the scanner are used together with the

72 B01 beryllium mask. Graphite was chosen as a highly X-ray transparent substrate. The graphite used is the same sheet material used for preparation of graphite masks shown in chapter 4.1.1. The graphite is 200µm thick and the PMMA resist layers are of 500µm thickness. The four resist sheets are glued to four plain graphite substrates. The samples are then stacked directly on top of each other and mounted in the mask- substrate cassette. Exposure is conducted in the same manner as a single substrate.

Figure 79: Principal of stacked exposures. Using the wiggler beam and the UXRLR mask B01 a stack of 4 individual samples was assembled. The stack consists of 4 layers of graphite substrates with PMMA glued to them.

4.4.1 Dose Calculation

The dose calculation for this exposure, just like for a regular exposure, must assure sufficient dose deposition in the bottom of the bottom layer while ensuring that no overdose is deposited in the top layer of the top sample. The top layer has been defined to be Layer 1 and the bottom Layer 4. Figure 80 shows the absorbed dose in all layers of PMMA resist over height. The stack is ‘assembled’ in this graph. The height is put down in the Y-axis while the dose deposition is shown in X-direction.

73 2500 LAYER 1 E =17.0 KeV c 2000

LAYER 2 1500 E =18.1 KeV c

1000 LAYER 3

Stack height [um] height Stack Ec=18.8 KeV

500 LAYER 4 E =19.5 KeV c 0 345678910 Dose [kJ/cm3]

Figure 80: Dose deposition in the different layers of the stack. The highest dose is below 10KJ/cm³ at the top of layer 1; the minimum dose is 3.5 KJ/cm³ on the bottom of layer 4. The bottom of layer 4 receives a sufficient minimum dose of 3.5kJ/cm³ while the top of layer 1 is in safe distance of an overdose at less than 10kJ/ cm³. The top to bottom ratio meets the criteria being smaller than 4. This dose distribution was achieved by adding a 36µm aluminum filter to harden the spectrum sufficiently. The scan length was 5cm. Exposure time was 2 hours and 9 minutes, total dose 10300mA·min. The beam current was 97mA at the start of the exposure and 50mA at the end. These numbers are summarized in the sample overview in Table 7 and Table 8 Included in Figure 80 are the critical energies the different layers are exposed to. From being filtered by the graphite and PMMA layers the power spectrum is shifted to harder X-rays with each layer. The critical energy for the top layer is 17.0keV versus 19.5keV for the bottom layer.

4.4.2 Structure Quality

After exposure the samples were separated and numbered. All were developed simultaneously in GG. Development time for all four layers was 8 hours. See Table 9 on page 62 for a summary of development parameters. Figure 81 through Figure 84 show SEM pictures of the same grating pattern in the four layers after development.

Figure 81: 20µm step structure in layer 1 of the Figure 82: 20µm step structure in layer 2 of stacked exposure. the stacked exposure.

Figure 83: 20µm step structure in layer 3 of the Figure 84: 20µm step structure in layer 4 of stacked exposure. the stacked exposure.

The quality of the top three layers is acceptable though minor cracking is visible in the bottom of all layers. Layer 4, the bottom layer, shows severe cracking. The cracks on the bottom of the other structures could result from processing problems (gluing) in substrate preparation. The severe cracks to layer 4 are believed to have two reasons. First it is possible that the harder radiation of 19.5keV critical energy has an effect on the bottom layer and second, which is more likely, the cooled back plate of the substrate holder keeps the substrate of layer 4 at room temperature while the PMMA can warm up. The temperature gradient between the cooled substrate and the PMMA could be sufficient to induce enough tension in the resist to produce cracks.

4.4.3 Sidewall Surface Roughness

The surface roughness of the structures has been measured at the top and the bottom of the 500µm high PMMA structures using the Wyko RST (Table 10) and it has been qualitatively observed with the SEM. Results are shown in Figure 85 through Figure 88.

Figure 85: Upper edge Figure 86: Upper edge Figure 87: Upper edge Figure 88: Upper edge of layer 1. of layer 2. of layer 3. of layer 4. As expected from graphite properties described in chapter 4.1.1, the graphite sheets increase the surface roughness in the three lower layers. In front of the top layer in Figure 85 no graphite substrate was inserted: no striations are visible. The surface appears to be smooth in the 3k magnification shown. All other layers show fine striations typically experienced using graphite masks. This is another indication that graphite does induce striations leading to increased sidewall roughness. RST measurements were performed on these samples using the WYKO RSTplus described in chapter 4.3.2. An area at the bottom of the 500µm PMMA structures and an area at the top have been examined. It has also been distinguished between vertical and horizontal edges according to the definition in chapter 4.3.2. Table 10 shows a summary of the roughness measurements.

Sample Roughness Roughness Average of bottom area top area both areas Layer 1 horizontal 21 15 18 Layer 1 vertical 13 18 16 Layer 2 horizontal 20 24 22 Layer 2 vertical 17 16 17 Layer 3 horizontal 26 30 28 Layer 3 vertical 36 30 33 Layer 4 horizontal 177 191 184 Layer 4 vertical 44 57 51 Table 10: Surface roughness of the PMMA structures that have been exposed in a stack. As expected the roughness is higher due to the graphite substrates used. Roughness is acceptable though in all layers except in Layer 4. The roughness is increasing with the layer number. The average roughness is twice as high than in the single exposures shown in Table 9 in chapter 4.3.2 but it still is much less than the roughness range usually experienced using graphite as mask sheets. Explanation for this lower than expected roughness could be that in the stacked exposure no proximity between substrates and resist exists. In an exposure with a graphite mask on the other hand, at least several hundred micrometers proximity gap are used which could be responsible for enhancement of the effect. The other major difference to the graphite striations observed in DXRL exposures is that the X-ray energies are much higher in this wiggler exposure. The increased roughness through striations caused by graphite substrates blur the results but it can be said that these measurements illustrate that the surface roughness does not suffer significantly from exposing in a stack.

4.4.4 Structure Width over Structure Height

Like the high structures examined in chapter 4.3.1, the structure width over height has been measured with the SEM. Not only single structures on the different layers were measured, but also corresponding structures to the same mask feature were examined in order to ‘re-assemble’ the data. A horizontal edge and a vertical edge structure have been examined. Both have a nominal width of 100µm. Figure 89 shows the horizontal edge, Figure 90 the vertical. The layers were put together on top of each other in one graph.

MASK ABSORBER MASK ABSORBER 2500 2500 LAYER 1 LAYER 1 PMMA PMMA 2000 2000 C C

LAYER 2 LAYER 2 1500 PMMA 1500 PMMA C C

LAYER 3 1000 LAYER 3 1000 PMMA PMMA C C Stack Height [um] Stack Height [um] 500 500 LAYER 4 LAYER 4 SHADOW CAST OF MASK SHADOW CAST OF MASK OF CAST SHADOW PMMA PMMA C 0 C 0

80 85 90 95 100 80 85 90 95 100 Width [µm] Width [µm]

Figure 89: Width of 100µm wide structures Figure 90: Width of 100µm wide structures fabricated in the stacked exposure. Layer data fabricated in the stacked exposure. Layer data were ‘re-assembled’ to a stack in the graph. were ‘re-assembled’ to a stack in the graph. The width of the structures is shown versus the The width of the structures is shown versus the stack height. This is a horizontal edge. stack height. This is a vertical edge.

The variation of structure width over the entire stack height is ca. 5µm in both orientations of the edges: The side wall slope is about three times less than in the single layer 2800µm exposure shown in Figure 76 on page 69. This is an indication that the development indeed induces a large part of the slope in the ultra deep single exposed samples that were examined. However, the thermal properties are completely different in a stack than in a single layer and could also be responsible for a better accuracy. The single layers of PMMA are fixed to a substrate every 500µm instead of only one layer in the single 2800µm exposure. The recess from the geometrical absorber shadow cast has a minimum of 5µm and a maximum of 10µm. Similar recess effects have been examined in DXRL [19] as illustrated in Figure 6. In this study it is speculated that temperature induced

77 movements in the entire mask – substrate cassette could be responsible for this recess from the shadow cast edge of the mask. 1°K temperature increase translates to 1µm movement over 10cm lateral dimension for beryllium and 8µm for PMMA according to [19]. A temperature study of the exposure setup would be required in order to determine the complicated thermal processes taking place. It can only be speculated that from basic thermal expansion coefficient estimates, movements in the µm range are well possible with temperature increases in only a few °K. Significant temperature increase was indeed measured on the used setup as shown in chapter 3.6.2. The shapes of the edges change with depth from slope to ‘belly’-like. This effect takes place in the two bottom layers in the horizontal edge measurement and only rudimentary in the vertical edges bottom layer: The scenario illustrated in Figure 91 could be responsible for the profiles observed. The center sections of the lower resist layers heat up more during exposure. This effect is shown in the left of the illustration. “Swelling” of the PMMA in the center of the 500µm layer occurs while it is either fixed to the bottom substrate or kept cool by the top substrate. The PMMA is exposed in the ‘swollen’ state. When cooling down, as illustrated to the right, the swelling from temperature increase regresses and the developed structure shows the ‘belly’ type profile.

Figure 91: Scenario possibly responsible for 'belly' type profiles in the lower resist layers of the stacked exposure. On the left the situation during exposure is illustrated, on the right, the scenario after the exposure is presented.

Conclusion: The stacked exposure has proven to be of acceptable accuracy and can present a path to high throughput exposures on hard X-ray sources. The deviation of the structure sidewall from perfectly normal show effects that allow for speculation not only about the stacked exposure, but also about the UDXRL process in general. For example it

78 can be shown, with the stack being more precise over height than a corresponding 2800µm thick resist, that development time and temperature effects in the thermally insulated resist play a major role in precision loss in UDXRL. Effects by secondary radiation seem to be less pronounced. However, this stacked exposure is only a first set of experiments and further systematic studies with varying parameters are needed.

5 Summary

The goal of this work was to fabricate ultra deep, several millimeter high PMMA microstructures with ultra deep X-ray lithography using a 7T wiggler at the Center for Advanced Microstructures and Devices (CAMD). Two main tasks were successfully accomplished within the scope of this work. The first task was to install the beamline and scanner infrastructure at the wiggler, while the second task included first exposure experiments of several millimeters high microstructures and the comparison with other results [21].

In the past CAMD operated lithography beamlines at bending magnets. Due to the relatively soft X-ray spectrum available from these sources, exposures into few hundred micrometers resist are normally performed. With the installation of a super- conducting wiggler, a hard X-ray source became available at the CAMD storage ring. However, this high intensity, hard X-ray source required a complete adaptation of the common CAMD microfabrication beamline design. Beam defining components needed to be water-cooled and the vacuum tube cross section had to be reduced to minimize radiation problems. The reduction of the beamline cross section required a careful alignment of beamline components. The scanner was built on a kinematic mounted table and consisted of a high precision linear motion stage. It was operated in air. The scanner wobble error was measured to be less than 0.2mrad for a total scan distance of 4 inch (101.6mm). The mask plane of the motion stage was aligned to better than 0.1mrad normal to the incident photon beam using a special aperture mask. A copper calorimeter was built and used for measuring the incident power at the mask plane. The approximately 6W/(mrad·100mA) measured compared well to the calculated value. Furthermore, the calorimeter reached temperatures in the 70ºC range for the fully absorbed beam indicating the necessity of actively cooling the mask- substrate fixture. A fully water-cooled mask-substrate cassette fixture was built. With this fixture, the temperature rise of the mask and substrate can be held within 2ºC of room temperature. With this setup, thermally induced motion in the mask and mask- substrate assembly were minimized and well controlled. Operation of the scanner in air caused two major problems: Radiation scattering from the beryllium window and scanner components exposed to the photon beam as well as production of ozone from air. The first problem could be solved by adding a lead box enclosure around the scanner, allowing radiation safe operation with one order of magnitude below the state radiation limit. The ozone problem is a health hazard but also leads to severe corrosion problems on all metal parts of the scanner. Here the use of a vacuum scanner is recommended for the future. The first task of building a state of the art UDXRL beamline and scanner has successfully been completed. With this, a well-qualified experimental setup was available for first experiments in UDXRL at CAMD.

80 Samples of up to 3mm resist height were exposed and developed. To meet the requirements of a high mask contrast and thermal stability a 600µm beryllium mask with a 45µm thick gold absorber was jointly fabricated with the Insitut für Mikrostrukturtechnik (IMT) at Forschungszentrum Karlsruhe. The mask pattern used is a special UDXRL mask design from IMT and allows direct comparison to the IMT results [21]. With this mask UDXRL microstructures have been fabricated, which allow measurements of the sidewall slope error as well as sidewall roughness.

In order to qualify the experimental setup for lithography and also allow comparison to the bending magnet exposure, a 200µm high microstructure was patterned at the wiggler. From this experiment the sidewall slope error of the wiggler sample is approximately 0.25µm / 100µm structure height and about 4 times larger than in DXRL exposures. This indicates that higher photon energies at the wiggler have an impact on the patterning accuracy. In a series of experiments the height was varied from 200-2800µm. The examination of the sidewall slope error resulted in a constant value of approximately 0.25µm/100µm structure height in this range. These results compare well to the results from Achenbach et. al. [21] and proof the state of the art performance of the exposure tool. It also indicates that extremely long development times of up to 75 hours could be used without compromising the performance of the resist / developer system (high molecular weight PMMA was used with GG at room temperature). Measurements of the sidewall roughness have been performed along the sidewall height using a white light interferometer. The mean average roughness (Ra) of the sidewalls is ranging from 10-50nm, comparable to typical LIGA structures. A first experiment in high throughput stacked exposures has been done using a stack of four 200µm thick graphite substrates. On each substrate, a 500µm thick resist was applied. The stack was exposed in 2 hours. The measurements of the patterning accuracy, as well as the slope error and sidewall roughness, demonstrate comparable results to single layer exposures. In conclusion, these first UDXRL experiments demonstrate the potential of the wiggler in efficiently fabricating several millimeter tall high aspect ratio microstructures. Typical structure defects including severe cracks and lack of adhesion have also been observed and require more fundamental process development in order to make high yield fabrication of this kind of structures.

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85 Acknowledgements

Prof. Dr. Wolfgang Menz, Faculty of Microsystem Technology at the University of Freiburg for his support and advice as the thesis advisor for this work.

Prof. Dr. Volker Saile from the IMT, Forschungszentrum Karlsruhe and formerly Director of CAMD, LSU for giving me the unique opportunity to come to CAMD form my dissertation and his ongoing support after leaving CAMD as a co-advisor of the dissertation.

Dr. Jost Göttert, Director of Microfabrication at CAMD, for the fruitful discussions and the valuable advice, especially throughout the final phase of this dissertation project.

Dr. Franz Josef Pantenburg from the IMT Karlsruhe for valuable input, discussions and advice in all aspects of DXRL and UDXRL along with his active support with sample and mask preparation.

Timo Mappes from the Mechanical Engineering Department of the University of Karlsruhe who assisted with sample preparation, exposures and measurements during his internship at CAMD.

Dr. Peter Siddons from the NSLS, Brookhaven National Laboratory for crucial advice concerning synchrotron beamline science and for inviting me to NSLS for preliminary studies early during this dissertation.

Dr. Ben Craft and the CAMD accelerator group for running night and weekend shifts on the wiggler in solitary mode.

The entire staff of CAMD for their continuous support, especially Louis Rupp for his help with assembling the beamline, Yohannes Desta for help with preparing graphite samples and electroplating the masks, Dr. Lorraine Day for the support solving radiation problems at the beamline and exposure station and Dr. John Scott for valuable advice using the SHADOW ray tracing program.

Last but not least I would like to thank my wife Petra, my family and my friends for the unconditional support and encouragement.