The Pennsylvania State University

The Graduate School

Department of Engineering Science and Mechanics

PROPERTIES OF PULSED DC SPUTTERED VANADIUM OXIDE THIN FILMS

USING A V2O3 TARGET FOR UNCOOLED MICROBOLOMETERS

A Thesis in

Engineering Science

by

Kerry Elizabeth Wells

2008 Kerry Elizabeth Wells

Submitted in Partial Fulfillment of the Requirements for the Degree of

Master of Science

December 2008

ii The thesis of Kerry Elizabeth Wells was reviewed and approved* by the following:

Mark W. Horn Associate Professor Engineering Science and Mechanics Thesis Advisor

Michael Lanagan Associate Professor of Engineering Science and Mechanics, and Materials Science and Engineering

S. Ashok Professor of Engineering Science

S.S.N Bharadwaja Research Associate

Nikolas Podraza Research Associate

Judith A. Todd P. B. Breneman Department Head Head of the Department of Engineering Science and Mechanics

*Signatures are on file in the Graduate School

iii ABSTRACT

Vanadium oxide (VOx) thin films are known as feasible materials for sensing applications in uncooled microbolometers. A great deal remains unknown about the relationship between the films‟ material properties and the deposition parameters. This study involved the deposition and analysis of VOx films made by pulsed DC magnetron sputtering of a V2O3 target with a 200 W power source at 225 kHz at room temperature.

The depositions consisted of thin films made at total pressures varied from 2.5 to 50 mTorr and oxygen partial pressures between 0 and 10%. Electrical, optical and microstructural properties were investigated to determine the effects of varied oxygen partial pressure and total pressure during deposition. Variations of thickness and post deposition annealing and aging were also studied to determine the effects on the film properties.

The results of this study showed the temperature coefficient of resistances and resistivity values of the films were in the range of -3 to -5% (K-1) and 10 - 200 kΩ-cm respectively. Both atomic force microscopy and field emission scanning electron microscopy data established the microstructure of the films followed the structure zone model with an increase in columnar size as total pressure was increased. Spectroscopic ellipsometry analysis indicated significant variations in the dielectric functions in the

VOx films with similar electrical properties due to variation in the oxygen content of the films. Films deposited for various durations with the same deposition parameters showed a trend of decreasing resistivity with increased thickness. Substantial variations in the electrical properties occurred when some films were subjected to post deposition iv annealing. Lastly, the study showed that aging while in desiccators did not result in significant variation of the films‟ properties. v TABLE OF CONTENTS

LIST OF FIGURES ...... vii

LIST OF TABLES ...... x

ACKNOWLEDGEMENTS ...... xi

Chapter 1 INTRODUCTION ...... 1

1.1 Thermal Imaging ...... 1 1.1.1 Microbolometers ...... 5 1.2 Vanadium Oxide Thin Films ...... 8 1.2.1 Fabrication of Thermal Sensing Material ...... 9 1.2.1.1 Ion Beam Sputtering ...... 11 1.2.1.2 RF Sputtering ...... 12 1.2.1.3 Pulsed DC Sputtering ...... 14 1.2.2 Material properties ...... 15 1.2.2.1 Temperature Coefficient of Resistance ...... 15 1.2.2.2 1/f Noise ...... 16 1.2.2.3 Resistivity ...... 18 1.2.2.4 Additional Figures of Merit: Responsivity, Detectivity, Noise Equivalent Power, NETD ...... 18 1.3 Thesis Outline ...... 19

Chapter 2 LITERATURE REVIEW ...... 21

2.1 Infrared Sensing Materials in Microbolometers ...... 21 2.1.1 Commercial Materials ...... 22 2.1.1.1 Amorphous Silicon ...... 25 2.1.1.2 Vanadium Oxide ...... 27 2.1.2 Novel Materials ...... 33 2.2 Structure Zone Model ...... 35

Chapter 3 EXPERIMENTAL METHODS OF MATERIAL CHARACTERIZATION ...... 37

3.1 Experimental Procedure ...... 37 3.1.1 Sample Preparation ...... 37 3.1.2 Sputter Deposition ...... 38 3.2 Characterization Methods ...... 43 3.2.1 Profilometry ...... 43 3.2.2 Temperature Dependant Current-Voltage Measurements ...... 43 3.2.3 Spectroscopic Ellipsometry ...... 47 3.2.4 Atomic Force Microscopy ...... 52 vi 3.2.5 Field Emission Scanning Electron Microscope ...... 53 3.2.6 Annealing ...... 54 3.2.7 Thickness Study ...... 54

Chapter 4 RESULTS AND DISCUSSIONS ...... 56

4.1 Deposition Rate ...... 58 4.2 Surface Structure ...... 59 4.3 Cross-sectional Microstructure ...... 62 4.4 Optical Properties ...... 67 4.5 Resistivity and Temperature Coefficient of Resistance ...... 71 4.5.1 Reproducibility and Aging ...... 80 4.5.2 Annealing ...... 88 4.5.3 Thickness Effects ...... 92

Chapter 5 CONCLUSIONS ...... 94

Chapter 6 FUTURE WORK ...... 97

Bibliography ...... 98

vii LIST OF FIGURES

Figure 1-1: Microbolometer pixel [6] ...... 6

Figure 1-2: Cross-sectional microbolometer diagram [7] ...... 8

Figure 1-3: Magnetron sputtering ...... 11

Figure 1-4: Power waveforms of DC, RF, asymmetric bipolar pulsed DC [13] ...... 14

Figure 2-1: TCR as a function of resistivity for a-Si where resistivity has been varied by incorporation of different doping concentrations [2,26] ...... 23

Figure 2-2: TCR vs. resistivity for VOx thin films created in studies done by (a) Hongchen Wang, et al. and (b) R.A. Wood. [27, 6] ...... 24

Figure 2-3: Vanadium oxide phase diagram [34] ...... 27

Figure 2-4: VO2 metal-insulator phase transition [16] ...... 29

Figure 2-5: Thornton‟s Structure Zone Model [49] ...... 36

Figure 3-1: Internal layout of the deposition chamber...... 39

Figure 3-2: Pressure gauge with partial pressure of argon (a) and a total pressure of 5 mTorr, 10 % oxygen partial pressure (b) after adjusting the oxygen flow rate...... 40

Figure 3-3: Optical micrograph of the sputtered nickel contacts ...... 45

Figure 3-4: Two point measurement system ...... 46

Figure 3-5: Ex situ spectroscopic ellipsometer schematic [50] ...... 48

Figure 3-6: Geometry of the incoming, reflected, and transmitted electromagnetic waves showing the p and s axes in addition to the place of incidence. [52]...... 49

Figure 3-7: Optical model structure consisting of crystalline silicon substrate (c-Si) / native oxide (SiO2) / bulk VOx film / surface roughness / air ambient…………50

Figure 4-1: Thin film deposition rates as a function of oxygen partial pressure with varied total deposition pressures ...... 59

Figure 4-2: Surface roughness images of films made at 2.5% P from atomic O2 force microscopy ...... 60 viii Figure 4-3: Root mean square roughness vs. total pressure ...... 61

Figure 4-4: Cross sectional images of the films made at 0% P for total pressures O2 of (a) 2.5 mTorr, (b) 5 mTorr, (c) 10 mTorr (d) 50 mTorr ...... 63

Figure 4-5: Films do not have apparent grain boundaries deposited at 2.5 mTorr with changes in oxygen partial pressures of or at 5 mTorr and oxygen partial pressures (e) 0%, (f) 2.5, (g) 5% (h) 10% ...... 65

Figure 4-6: Varied apparent columnar size of films at a total pressure of 10 mTorr with changes in oxygen partial pressures of (a) 0%, (b) 2.5, (c) 5% (d) 10% ...... 66

Figure 4-7: Ellipsometric spectra (symbols) in ( , ) measured at three angles of o o o incidence, i = 55 , 70 , and 85 , for a VOx thin film deposited on a native oxide covered c-Si substrate. The VOx film was prepared at a total pressure of 2.5 mTorr, and an oxygen partial pressure of 0%...... 69

Figure 4-8: Dielectric function spectra ( 1, 2) corresponding to VOx films prepared with oxygen partial pressure from 0 to 10 % and a total pressure of 2.5 mTorr...... 70

Figure 4-9: Current vs. voltage for a sample deposited at 5 mTorr 5% ...... 72

Figure 4-10: Example TCR calculation ...... 72

Figure 4-11: 2.5 mTorr films with (a) resistivity vs. temperature and (b) trends in TCR and resistivity with varied oxygen partial pressure...... 75

Figure 4-12: 5 mTorr films with (a) resistivity vs. temperature and (b) trends in TCR and resistivity with varied oxygen partial pressure...... 76

Figure 4-13: 10 mTorr films with (a) resistivity vs. temperature and (b) trends in TCR and resistivity with varied oxygen partial pressure...... 77

Figure 4-14: 50 mTorr films with (a) resistivity vs. temperature and (b) trends in TCR and resistivity with varied oxygen partial pressure...... 78

Figure 4-15: TCR vs. resistivity data from samples deposited at 2.5, 5, 10, 50 mTorr total pressures and 0%, 2.5%, 5% and 10% oxygen partial pressure...... 80

Figure 4-16: Resistivity vs. temperature for the initial and repeated 5 mTorr series .. 81

Figure 4-17: Basic TCR and resistivity trends as a function of oxygen partial pressure are maintained between depositions ...... 82 ix Figure 4-18: Aging effects on resistivity and TCR for (a) 5 mTorr 0% , (b) 50 P mTorr 0% and (c) 5 mTorr 10% films ...... O2 84

Figure 4-19: Aging effects on several 5 mTorr 5% samples ...... 86

Figure 4-20: Effects of annealing 200˚C for 30 minutes in air on TCR and resistivity...... 90

Figure 4-21: Effects of thickness on resistivity vs. temperature ...... 93

Figure 4-22: TCR and resistivity as a function of thickness ...... 93

x LIST OF TABLES

Table 1-1: Types of infrared detectors [2] ...... 2

Table 2-1: Summary of a-Si microbolometer material literature ...... 26

Table 2-2: Summary of VOx microbolometer material literature ...... 33

Table 3-1: Pulsed DC sputtering ramp sequence ...... 42

Table 3-2: RF sputtering ramp sequence ...... 42

Table 4-1: Experimental matrix of deposition parameters, electrical properties and thickness ...... 57

xi ACKNOWLEDGEMENTS

I want to thank several people that have helped me greatly during the past two years I have been working on my master‟s degree. Without their knowledge and support,

I do not believe I could have accomplished what I have.

My lab mates have always taken the time to help me learn how to use the equipment and ensured that I clearly understood how and why everything works the way it does in the lab. For this I would like to thank Guneet Sethi, Ryan Carey, Nick

Fieldhouse and Chandru Venkatasubmanian. Sean Pursel‟s expertise in FESEM imaging provided me with cross sectional images of the majority of my films. Dr. Nik Podraza assisted me in acquiring spectroscopic ellipsometry data. Dr. Srowthi S. N. Bharadwaja

(Raja) helped greatly with data analysis and provided me with insight on subsequent data acquisition steps.

Without the support from my friends and family, the time spent getting my masters degree would not have been so enjoyable. I appreciate everyone who helped me keep my head up when the stress from my work load got me down.

I would also like to thank Dr. Mark Horn for advising me through this process.

His style has allowed me to investigate areas I was interested in while still obtaining useful information for the MURI project. He gave me direction but never to the extent where it turned into commands. I am very thankful for the support and knowledge I have gained while working with Dr. Horn.

Research was sponsored by the U.S. Army Research Office and U.S. Army

Research Laboratory and was accomplished under Cooperative Agreement Number xii W911NF-0-2-0026. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Office, Army Research Laboratory, or the

U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon. 1

Chapter 1

INTRODUCTION

Vanadium oxide thin films have been used as infrared sensing materials in uncooled microbolometers for over two decades. With an improved understanding of vanadium oxide thin film and deposition processes, higher sensitivity, faster response times and better overall infrared imaging devices could be made. This work explores the potential of utilizing the pulsed DC sputtering of a V2O3 target and the effects of variable deposition parameters on the films electrical, structural and optical properties in order to assess the sustainability of these materials for uncooled microbolometer devices.

1.1 Thermal Imaging

Infrared imaging devices provide the ability to detect the temperature differences between objects much as eyes or a CCD camera enables the viewer to see either colors or detect intensity differences in the visible wavelength range. Such devices are of great importance for the military, search and rescue missions, space applications and even heat leakage monitoring. What was previously an exclusive tool for the military has now become available to the general public for such applications as hunting, driving aid and security surveillance [1]. Due to the increasing popularity and more strenuous operational demands of infrared (IR) imaging devices, it has been necessary to improve the sensitivity and functionality of these devices. 2 There are two basic types of IR detectors; photon and thermal (see Table 1-1).

In a photon detector, also known as a photodetector, the incident photons interact with the sensing material‟s electrons [2]. The response measured depends on the specific photodetector varieties, examples of which include intrinsic, extrinsic, photoemissive and quantum well [1]. The response will vary relative to the number of photons absorbed by the material. Thermal detectors, in contrast, work based on a change in temperature of the sensing material. This change can be measured as a consequence of a variation in an electrical, optical, or magnetic property, which also depends on the specific type of detector. Various IR thermal detectors include; bolometer, thermopiles and pyroelectric detectors [3].

Table 1-1: Types of infrared detectors [2] Infrared Detectors Photon Thermal Photoconductivity Bolometer Intrinsic Semiconductor Extrinsic Metal

Photovoltaic Thermopile

Photoemissive Pyroelectric

Thermopiles and pyroelectric detectors provide two different ways to measure the incoming IR radiation. A thermopile consists of a series of thermocouples connected together which create a voltage based on their temperature change. The use of only one 3 thermocouple would not allow for sufficient sensitivity thus it is necessary to connect several thermocouples in series. A thermocouple consists of two different materials that, when heated at their junction, create a specific voltage depending on the materials

Seebeck coefficients s . The detector is designed so that the incoming radiation is focused at the junction of the two metals. Materials with high , low thermal conductivity and low resistivity are essential to produce effective thermal IR detectors.

However, the calibration of these sensors is not straightforward since each pixel output signal requires pre-amplification and individual calibration along with the ambient temperature compensation. In addition, the achievable temperature resolution is approximately 1 K which is 2 orders smaller than the required sensing resolution (10mK).

On the other hand, a pyroelectric detector senses changes in temperature with changes in polarization current. Two types of pyroelectric detectors are being used. One variety is passive and reacts to changes in temperature with the changing polarization while remaining well below the Curie temperature denoted pyroelectric mode. The other goes above the Curie temperature but uses an applied DC bias denoted bolometric mode.

However, current pyroelectric detectors based on ferroelectric thin films require high deposition temperatures and thus are a challenge to integrate onto current complementary metal oxide semiconductor (CMOS) based read out circuitries [3].

Bolometers are another type of thermal IR detector which measure changes in resistance to determine the corresponding changes in temperature due to incumbent IR radiation. In general, the resistance of the sensing material in semiconducting bolometers decreases with increasing temperature. Thus these materials have a negative temperature 4 coefficient of resistance (TCR). On the other hand, if the sensing material is a metal, a temperature increase will cause the materials resistance to increase and thus have a positive TCR [2].

Historically, the predominantly used night vision device incorporated a photon detector that required cryogenic coolers to operate properly [4]. Similar cooling systems are also used in thermal imaging. The cooling systems have a limited life and are rather bulky. These devices also require a sufficient start up time before they work properly making them impractical for many applications. Although the benefit of using a photonic

IR detector with a cooling system is higher resolution, these semiconductor IR detectors have small band gaps at room temperature (approximately 4kT) and exhibit large dark currents during operation. This can cause the detecting signal to be swamped at and above room temperature applications when no cooling unit it used. Moreover, the cooling units are also very expensive to purchase and maintain making the devices uneconomical for most civilian applications [5].

More recent technologies have focused largely on thermal detectors, specifically bolometers. Bolometers and microbolometers operate by measuring changes in resistance that occur due to changes in the materials temperature caused by incoming IR radiation.

The choice of detecting material in a microbolometer is crucial to producing a high resolution IR image. Materials with high TCR, low resistivity, low noise and compatibility with CMOS read out circuitry possess the necessary properties to achieve high sensitivity in IR detection.

Uncooled microbolometer pixel focal plane arrays (FPA) in IR imaging devices possess many desirable advantages over cryogenically cooled devices. An uncooled 5 microbolometer does not require a cool down period before it is capable of functioning.

Also, cryogenically cooled systems often require a great deal of maintenance and are easily prone to damage. Imaging systems containing a microbolometer sensor can be fabricated to comply with military specifications allowing them to withstand the rugged use for both military and civilian applications. The size restraints that accompany a large cooling unit are eliminated using imaging devices with microbolometers. These devices are small enough that they can be incorporated into handheld devices and even into goggles, often used by the military. In addition, uncooled IR detectors provide an advantage with a wide spectral response compared to semiconductor based photon detectors. A cooling system often generates a great deal on noise whereas microbolometers can operate silently. Uncooled microbolometers have several distinct advantages over their cooled counterparts making their technological advancement a necessity for the future. Reduced weight and size decreases the array production cost and simplifies package form factors due to the non-requirement of cooling systems [2, 3].

1.1.1 Microbolometers

A generalized schematic shown in Figure 1-1 provides a basic view of the structure of a microbolometer pixel. An array of these microbolometer pixels makes up the sensor in a thermal imaging device. State of the art devices contain over 300,000 pixels usually expressed as 640×480 to describe their resolution. The sensing material of the device is isolated above the readout circuitry and forms a bridge like structure.

Because bolometers work based on changes in temperature, it is important for the area 6 sensing the incoming IR radiation to be thermally isolated [6]. For the device to have a better sensitivity it must be able to heat up and cool down quickly. Therefore, the area heated is raised up off of the circuit by tiny legs. The legs are a composite of materials but always contain a metal to permit electrical contact to the read-out circuitry. In addition, the whole structure must carefully balance heat conduction and heat capacitance.

Figure 1-1: Microbolometer pixel [6]

Microbolometers are now typically made on silicon wafer substrates 6” or larger.

CMOS fabrication or silicon micromachining processes have been well developed for other devices and this same technology is compatible in manufacturing microbolometers.

Beginning with a silicon wafer, a read out integrated circuit (ROIC) is created. Then, the bridge like structure is formed over the ROIC by the deposition of several layers and the use of lithography and etching processes. A sacrificial layer is deposited and later removed allowing the sensing material to be suspended above the substrate [6]. 7 Figure 1-2 provides a generalized cross sectional view of a microbolometer and includes some common materials used for each component. The ROIC provides a link between the sensing material and the computer that converts the electrical change into an image. Fabrication of ROICs has already been well developed through CMOS technology. The electrodes that connect the ROIC and the bridge structure are made of a metal encapsulated by Si3N4. The metal is also a good heat conductor and thus helps to cool the detecting material. A thin membrane, usually made of Si3N4 coupled with SiO2, is used to help support the sensing material. The most commonly used materials for detecting the incoming IR radiation are amorphous silicon (a-Si) and VOx. The sensing material of a microbolometer can be coated with a passivation layer for protection and to increase the stability of the film. A reflector on top of the ROIC and a quarter-wave cavity design make up the rest of the microbolometer pixel. These are added to the pixel to aid in reflecting IR radiation that is not initially absorbed back towards the detector to be absorbed on the second pass thus maximizing the amount of absorbed energy [3, 6].

8

Detecting Electrode Legs Material (VOx) Thin

(Si3N4) Membrane (Si3N4) Gold Contacts

Reflector ROIC/Silicon Substrate

Figure 1-2: Cross-sectional microbolometer diagram [7]

Honeywell Technology Center was the first company to successfully develop the uncooled microbolometer in the early 1980‟s. They received a government contract and remained the sole supplier of this product until the military declassified the project in

1992. Honeywell then licensed their technology to several companies including Boeing and BAE, which now produce thermal imaging devices containing microbolometer arrays

[6].

1.2 Vanadium Oxide Thin Films

Many materials such as VOx, a-Si, poly-SiGe, YBaCuO and a even a protein based cytochrome C bovine serum albumen have been investigated to determine their usefulness as the sensing material in microbolometers [2, 8-11]. Two of the most commonly used temperature sensitive imaging materials are VOx and a-Si. Both materials possess several desirable properties for a sensing material including high TCR, 9 low 1/f noise and low resistivity. With comparable TCR values, VOx thin films are found to have lower resistivity values than those of a-Si. Industrial VOx and a-Si films for microbolometers have generally been deposited by ion beam sputtering and plasma enhanced chemical vapor deposition respectively. The preferred film used today as a sensing material in microbolometers is ion beam sputtered VOx [3].

1.2.1 Fabrication of Thermal Sensing Material

In order to fabricate cost efficient microbolometers, it is necessary that the thin film fabrication process should be able to integrate easily with silicon micromachining processes. The main compatibility issue with the micromachining process is that temperatures must remain as low as possible throughout the entire fabrication with a maximum temperature below the melting point of typical aluminum wiring levels for the

ROIC. There are several different ways to deposit VOx that allows a smooth incorporation into the CMOS fabrication process. Typically, physical vapor deposition techniques, in which an atomized material condenses on a surface, are used to form a film. Specific methods include reactive ion beam, RF and DC sputtering.

Depending on the desired composition of x in the VOx film and resultant microstructure, different target materials and deposition conditions must be applied.

Targets used to deposit vanadium oxide films include, but are not limited to, V, VO2,

V2O3, and V2O5. Some deposition methods require the source material to be conductive and thus a V target works efficiently. On the other hand, the remaining targets listed above are poor conductors and may not be compatible with some deposition techniques. 10 For example, a DC current applied to a poorly conducting material cannot sustain a plasma. In reactive sputtering even a conductive target can become insulating due to the buildup of an oxide layer on the target which may cause arcs. When arcing takes place macroparticles are released from the target. Films created during plasma arcing incorporate larger clusters of material and may lead to non-uniform film growth on the substrate [12].

In order to create a film with the desired oxygen content, it is convenient to use reactive sputtering. Sputtering does not necessarily produce the same stoichiometry in the film as was initially in the target. Using a pure metal target and allowing oxygen gas to flow through the chamber during deposition will result in the creation of a VOx film.

Similarly, the introduction of oxygen flow using a VOx target will change the resultant films V:O ratio [12].

The use of magnetron sputtering has become vastly popular among all variations in sputtering technology. With a typical magnetron sputtering deposition, a circular target is used with circular ring magnets placed at the back of the target. There are typically three magnetic rings of alternating polarity. The alternating polarity creates a magnetic field which helps to direct electrons towards the target. Figure 1-3 shows a diagram of a sputtering system with the inclusion of the magnets used during magnetron sputtering. The use of magnetron, as compared to non-magnetron, sputtering subsequently increases the thin film deposition rate. If there is a limit on the allowable applied voltage, identical current and deposition rates can be obtained using lower voltages with magnetron sputtering in comparison to conventional, non-magnetron sputtering. There exists a minimum pressure at which sputtering can occur depending on 11 the specifics of an individual chamber, power supply, target, etc. Magnetron sputtering, however, can operate in reduced pressure environments. One drawback to this type of sputtering is that due to the magnetic field, material from the target is sputtered off on a preferential ring also known as a “racetrack”. This limits the useful life of the target, because once the “racetracks” have eroded through to the backing plate, the target can no longer be used even though a great deal of target material remains [12].

Figure 1-3: Magnetron sputtering

1.2.1.1 Ion Beam Sputtering

The majority of VOx films used in commercial microbolometers have been deposited using reactive ion beam sputtering. This method is compatible with CMOS fabrication and was used in the original microbolometers fabricated by Honeywell 12 Technology Center in 1982 [2, 6]. There is no obvious published explanation as to why ion beam sputtering became the acceptable method for producing the vanadium oxide sensing material.

Ion beam sputtering occurs with the use of a beam of Ar+ ions which is formed in a source separate from the target. An ion gun utilizes a strong magnetic field which aids in the creation of ions. The ions created are then directed towards a target by an electric field. Material from any type of target can be sputtered using this method. Deposition rates are lower compared to those of magnetron sputtering, although this slower deposition method allows for greater control over the film composition through deposition parameters. Increased control of composition by such parameters may be one reason for using ion beam sputtering over other methods. Sputtering pressures are generally an order of magnitude lower with ion beam sputtering (10-4 Torr) than for magnetron sputtering (10-2 – 10-3), possibly leading to less impurity incorporation. This may be another reason industry has chosen ion beam sputtering over other deposition methods [12].

1.2.1.2 RF Sputtering

To sputter material from a target a bias must be applied to the target material. In the case of RF sputtering the voltage is varied from positive to negative at a radio frequency of 13.56 MHz. Figure 1-4 includes the waveform of an RF power source.

Because the frequency is very high, ions cannot follow the signal, thus inducing a DC self-bias enables sputtering by the ions. This self-bias is a strong function of reactor 13 geometry, target material and pressure. The major advantage of RF sputtering is that any target material can be sputtered, not just conductive targets.

Some major drawbacks to using an RF power source limit its usefulness to industry. Very low deposition rates make RF sputtering inefficient for use in industry.

Product output rates are very important to a company attempting to produce as much product as possible. Lower deposition rates mean the VOx films will take longer to deposit and productivity becomes limited. Although film stoichiometry can be varied with this technique, films tend to be rather metallic. Additionally, RF power supplies require an impedance matching network to ensure efficient coupling between the power supply and the target. Furthermore, RF power supplies can be rather expensive and this may limit the quantity a company can purchase once again resulting in limited productivity [12]. A final shortcoming associated with using an RF power source is that the energy of the sputter species cannot be easily controlled, which limits control of the microstructure of the resultant film. 14

Figure 1-4: Power waveforms of DC, RF, asymmetric bipolar pulsed DC [13]

1.2.1.3 Pulsed DC Sputtering

Pulsed DC sputtering of VOx is a fairly new deposition technique and consequently there has not been much research done on this topic thus far. It appears to be a very viable technique as deposition rates are higher than ion beam and RF sputtering, 15 and issues with insulation build up on the target are no longer an issue as with straight

DC sputtering. Figure 1-4 includes the difference between RF, DC, and pulsed DC waveforms. The pulse time for pulsed DC sputtering can also be varied and is set to the appropriate duration for each deposition system. These variations in the power source have an effect on the film growth.

Pulsed DC power includes the benefits of both RF and DC power. Using RF power results in low deposition rates because thin film deposition is limited to the ion flux/ion energy product which is only partially related to input power. With pulsed DC the negative bias is applied for a longer duration and thus deposition rates are increased.

A short pulse of positive bias is still applied and controls target poisoning and reduces the chance of arcing [13, 14]. This positive pulse is also very effective at conditioning or stabilizing the surface of the target in reactive sputtering.

1.2.2 Material properties

1.2.2.1 Temperature Coefficient of Resistance

TCR is one of the primary material properties used to determine acceptable materials for sensing IR radiation. Most materials undergo a change in resistance when heated or cooled. Thin film materials that undergo a large change in resistance for a small change in temperature are necessary for IR detectors. TCR (K-1), α, is given by

Eq. 1.1 below, where R is the resistance and T is the temperature. Materials that experience these large changes in resistance with small changes in temperature are said to 16 have a high TCR. A thin film that becomes more resistive when heated, such as a metal, is described as having a positive TCR. On the other hand, films that become more conductive after heating, such as a semiconductor, are denoted as having a negative TCR

[6].

1 dR 1.1 R dT

VOx thin films are semiconductors and therefore have a negative TCR value [2].

The TCR values of VOx thin films have been found to be between near zero, where the film is near metallic, to over -6 %K-1 for more resistive films [16]. The conductivity of a material is directly related to concentration of charge carriers, density of states and charge carrier mobility. When VOx is heated, the mobility of the electrons increases and the film becomes more conductive. The TCR of the film is dependant on deposition conditions, but a practical model to predict TCR has unfortunately not yet been developed. This work, in addition to other research efforts, has begun to provide an understanding of the relationship between the film deposition parameters and the electrical properties.

1.2.2.2 1/f Noise

Noise is one of the most limiting factors in thin film materials for microbolometers. Unfortunately, there does not appear to be a great deal of information relating noise to the materials other electrical and microstructural properties. Films that are highly resistive typically have higher noise, and thus lower resistivity films are 17 desired. If noise is too high the signal created due to the incoming IR radiation will be swamped out by the noise of the system thus compromising the sensitivity of the device.

In electrical systems, there are several different types of noise: 1/f noise, Johnson noise, temperature fluctuation noise and background fluctuation noise [2]. The first, 1/f noise, also known as pink noise or flicker noise, is a fluctuation where the spectral density is proportional to the inverse frequency. 1/f noise is also commonly referred to as low-frequency noise because it is typically the predominant noise at lower frequencies.

On the other hand, Johnson noise exists when the spectral density is equal to a constant and does not depend on frequency [6]. Temperature fluctuation noise is a result of the individual pixels trying to return to equilibrium after being subjected to IR radiation.

These fluctuations occur because of differences between the pixel and substrate temperature. Background noise is related to the temperature difference between the pixel and the ambient conditions it is subjected to, such as, whether the device is in a hot desert or an arctic climate [1].

1/f noise is considered most carefully when choosing an IR detecting material because the imaging devices typically use low frequencies. A typical frame rate of a microbolometer pixel is 30Hz [2]. Although faster frame rates produce better video images of the scene, it is necessary to allow sufficient time for the sensing element to return to equilibrium between measurements. 18 1.2.2.3 Resistivity

Film resistivity is an important material property to consider when choosing a material for the microbolometer sensor. A films resistivity, ρ, is given in Eq. 1.2 where R is the film resistance (Ω), A is the cross-sectional area resistance and is measured across

(cm2) and l is the length between the contacts (cm). A microbolometer works based on changes in resistance so it would only be appropriate that resistance of the film was considered. As mentioned above, a film with large resistance typically has higher TCR, but also higher 1/f noise [6]. However, films with lower resistance could use less power during operation. In order to develop devices that can operate via battery power it is important to limit power consumption to ensure a longer running life of the device.

RA 1.2 l

1.2.2.4 Additional Figures of Merit: Responsivity, Detectivity, Noise Equivalent Power, NETD

There exist several figures of merit for comparing the performance of microbolometer sensor arrays in IR imaging devices. The material‟s responsivity, Rv, provides a value describing how well the power incident upon the device is converted to an output voltage [2]. Responsivity can also be directly related to the films TCR, α;

Films with higher TCR values have higher responsivity as shown in Eq. 1.3.

Ib R RV 1.3 G 1 2 2 19

where Ib is the bias current, R is the resistance, η is the absorbtivity of the sensing film,

G is the thermal conductance, ω is the frequency of the chopper and τ is the thermal time constant. Recent literature shows devices currently have a responsivity of approximately

250kV/W [3]. Detectivity and noise equivalent power (NEP) both provide information about the detector with respect to the incident power, the output voltage and its relation to the noise voltage. In order to compare similar device with varied detector areas normalize detectivity, D*, is often used [2, 3]. Another commonly used figure of merit of infrared detector performance is noise equivalent temperature difference (NETD). This value is the minimum temperature difference necessary to observe a variation [15].

These device properties are dependent on the imaging material‟s electrical properties, in addition to the steps of the device fabrication process.

1.3 Thesis Outline

Uncooled microbolometers have greatly enhanced the capabilities of IR imaging in many applications. The ability to produce images in situations where visible light does not exist has come a long way from the first uncooled microbolometer developed at the

Honeywell Technology Center only 25 years ago. With all the advancements already made, however, there still exists a great deal of improvements for future study. This study continues to look into improving the microbolometer by gaining a better understanding of how the deposition parameters of VOx affect the film properties. 20 First, a review of literature has been completed describing the predetermined relationships between VOx and its properties. Additional relevant detecting materials and the thin film structure zone model are described. A V2O3 target was used to produce thin films in this study as its x value, 1.5, is near what has been stated to be ideal for microbolometers between 1.75 and 2 [9, 35]. A full description of all experimental procedures used to create and analyze the films is included. The results and discussions section includes an evaluation of all experiments and provides a great deal of insight on the trends occurring throughout the various films. The conclusions tie together the acquired results and explain the usefulness of this study. 21

Chapter 2

LITERATURE REVIEW

Although uncooled microbolometers are fairly new technological devices, a great deal of research has already been performed to determine the best materials for sensing

IR radiation. A wide variety of materials have been studied to establish their usefulness in this application. Various deposition techniques and parameters have also been investigated. The resultant properties of these films have been analyzed and compared for device optimization. Generalizations have been made about which materials “appear” to work best in sensing IR radiation, but there still exists a great deal of information to be learned. A review of the literature in the field provides framework for the experiments and analysis presented in this thesis.

2.1 Infrared Sensing Materials in Microbolometers

Industry currently produces FPA consisting of microbolometer pixels with detector elements typically made of VOx or a-Si [2,3]. Films of these materials, made with specific processing parameters, possess desirable TCR, resistivity and 1/f noise properties. When subjected to IR radiation, these films have thus far been deemed the best materials for uncooled bolometer applications. Other materials with acceptable properties have also been investigated for use in uncooled microbolometers.

22 2.1.1 Commercial Materials

Amorphous silicon and vanadium oxide thin films are commonly used in the commercial fabricated of uncooled IR imaging devices [10]. These materials are found to have acceptable film properties and may be processed with methods compatible to the

CMOS fabrication process. Although other material may possess more desirable specific property, such as TCR or resistivity, the best trade off amongst all desired properties has been found with a-Si and VOx.

For both a-Si and VOx, a similar trend in TCR occurs as a function of resistivity.

The films TCR and resistivity increase proportionally, following a linear slope. As mentioned, a-Si films have much higher resistivity values in comparison with VOx.

Although these values are higher, ROIC designs have been developed to accommodate each materials resistance range. Figure 2-1 shows the relationship between TCR and resistivity for a-Si films whereas Figure 2-2 demonstrates the same relationship seen with

VOx films.

23

Figure 2-1: TCR as a function of resistivity for a-Si where resistivity has been varied by incorporation of different doping concentrations [2,26]

24

(a)

(b)

Figure 2-2: TCR vs. resistivity for VOx thin films created in studies done by (a) Hongchen Wang, et al. and (b) R.A. Wood. [27, 6] 25 2.1.1.1 Amorphous Silicon

Silicon is regularly used during CMOS fabrication processes and can also be used to create acceptable detecting materials for microbolometers. a-Si has TCR values ranging from -2.5 %K-1, when doped to lower resistivity, to -8 %K-1 with a higher resistivity [3]. As mentioned, tradeoffs must always be made when choosing the detection material and in the case of a-Si high TCR corresponds to higher resistance and also high 1/f noise. Therefore, a-Si films with the highest TCR and resistivity may not always be used. In general, a-Si typically has much higher resistance values than VOx and therefore devices containing a-Si apply a continuous bias rather than the pulsed bias used with VOx in an imaging device [2].

There has been a great deal of research done to develop a-Si films and determine their effectiveness in microbolometers. Xing-Ming Liu et al. created a structure that possessed a TCR value as high as -2.8 %K-1 [28]. Another study done by A.J. Syllaios et al. at the Raytheon Electronic Systems Company created films with the same TCR, -

2.8 %K-1, and measured the resistivity to be 200 Ω-cm. Higher TCR films (13 %K-1) were also created, although their correspondingly high resistivity values prevented them from being compatible with current ROIC designs [29]. LETI LIR developed an a-Si based microbolometer which has an optimized TCR and resistivity of -2.5 %K-1 and 86

Ω-cm respectively [30]. A micro infrared camera used for unattended ground surveillance contained a-Si films with similar TCR performance values, -2.7 %K-1 [21].

When reported, the value of NETD was less than 100mK [21, 29, 30]. a-Si film performance specifications are summarized in Table 2-1. 26

Table 2-1: Summary of a-Si microbolometer material literature Authors TCR Resistivity NETD Responsivity Resistance (%K-1) X.M. Liu et al. -2.8 3-10 MΩ A.J. Syllaios et al. -2.8 200 Ω-cm <100mK 30 MΩ

C. Vedel et al. -2.5 86 Ω-cm <100mK 10mV/K

T. Schimert et al. -2.7 67mK 1x106 V/W 30 MΩ S. Li et al. -2.0 540 Ω-cm

A recent study observed the difference between a-Si, already found to have acceptable performance properties, and microcrystalline silicon (μc-Si). The group aimed to determine the effects of microcrystallinity on the films relevant properties in regards to IR detection. The group first made a-Si films with values of TCR (-2.0 %K-1) and resistivity (540 Ω-cm) comparable to those previously published. μc-Si films were made by slightly increasing the substrate temperature and the TCR and resistivity were determined to be -3.8 %K-1 and 435 Ω-cm respectively. 1/f noise was measured for both films and the μc-Si film showed 4 times lower noise than that of the a-Si film. The results of this study show that small changes in deposition parameters have a large effect on the films properties [31]. Similar changes to deposition parameters may be applied when investigating the effects in VOx thin films. 27 2.1.1.2 Vanadium Oxide

Vanadium has been mined from various minerals including caronite ore and petroleum ash. Vanadium is typically found as V2O5 in these natural materials [32].

Vanadium ions exist in several valence states including +2, +3, +4 and +5. These correspond to VO, V2O3, VO2, and V2O5 respectively, for vanadium oxide [33]. The phase diagram in Figure 2-3 shows the states listed above in addition to many mixed phases of vanadium oxide. Vanadium oxides also exist in a range of different crystal structures depending on several factors including phase and temperature [34].

Figure 2-3: Vanadium oxide phase diagram [34] 28

VO2, V2O3, V2O5 and other mixed phases of VOx have been studied as potential microbolometer materials. VOx for microbolometers has been stated as near ideal when x is between 1.75 and 2 [9, 35]. As there are several properties that make VOx an attractive material for bolometric sensing applications, a tradeoff between these properties must occur when the ideal material is chosen. Depending on the value of x, the film may undergo phase changes, consequently affecting the performance in a device.

Some VOx materials undergo a phase transition between the semiconductor and metallic phases. VO2 switches from a semiconductor to a metal at approximately 67˚C and has been investigated for microbolometers working within this temperature range.

This material changes structure between a monoclinic symmetric lattice as a semiconductor to a metal with a tetragonal symmetric lattice. Characteristic of this transition is a resistance change of over 4 orders of magnitude. In addition to this large resistance change there exists a hystersis effect on the curve when the material cools below the transition temperature. The switching effect and the hysteretic curve make crystalline VO2 undesirable for microbolometers [16, 36]. Figure 2-4 shows a typical temperature versus resistance curve for VO2. Other phases of vanadium oxide, such as

V2O3, undergo a similar phase change outside the temperature range of interest and therefore would not exhibit this drastic transition during use, leaving many phases of vanadium oxide as viable options for microbolometers [38].

29

Figure 2-4: VO2 metal-insulator phase transition [16]

Additionally, the deposition method and parameters will greatly affect the final film properties of VOx thin films. Post deposition treatments will also have further effects on the final film properties. Ideally, the films must possess strong stability, so as to not change their functional device lifetime. The run-to-run reproducibility of the films is also important for device yield in manufacturing [37]. It has been difficult to determine an optimized fabrication method for VOx thin films because numerous variables must be considered during deposition. Research has shown films have been deposited by ion beam sputtering, DC sputtering, RF sputtering and electron beam evaporation [27, 35, 38-

42]. Limited data has been published on the used of pulsed DC sputtering to create VOx films [45]. The majority of sputtered films have been reactively grown by ion sputtering using pure vanadium targets. The various studies, several to be described below, examined the electrical, optical and structural properties of such films. 30

There exist a small number of publications related to VOx thin films prior to 1998 and limited, if any, publications regarding VOx to be used in microbolometers before that time. Before 1998 the formation, electrical, structural and optical properties of VOx films were studied by several groups. VOx films were also assessed for usefulness in many other applications including gas sensors, microwave switches and electrodes in lithium batteries [46-48]. The initial studies of VOx led the way for the future examination of the material for microbolometers.

An early study published in 1985 by a group at the University of Wisconsin-

Milwaukee examined the crystal structure and optical absorption behavior of films made using reactive RF sputtering to deposit vanadium oxide [40]. Films were grown at 0.1 mTorr with various oxygen partial pressures (0-25%) using a V target and the deposition temperature was estimated to have remained below 100˚C A relationship between the deposition rate and the oxygen partial pressure was determined. As the percentage of oxygen present during deposition was increased the deposition rate was decreased. This was attributed to the formation an oxidation layer on the target, which is also known as target poisoning. The crystal structure of the films with oxygen percentages between 2-8% appeared to be V2O5 according to XRD. Films made with greater amounts of oxygen present did not yield diffraction peaks. Optical data was in agreement as the absorption curves showed the film to be crystalline V2O5 when less oxygen was present during the deposition and an amorphous film at higher oxygen partial pressures [40].

After the declassification of the microbolometer project by the Honeywell

Technology Center in 1992, articles began being published on the characterization of

VOx for microbolometers. One such article by Yong-Hee Han et al. from 2002 tested 31 using several layers of films to create a high TCR, low resistivity film. Using RF sputtering, V2O5 was deposited with a layer of V above it. By annealing this film in an oxygen atmosphere, 3 layers resulted; V2O5: V: V2O5. Films made with an optimized anneal time had TCR values greater than 2 %K-1 and resistivity values below 0.1 Ω-cm, making them suitable for use in microbolometers. These films also showed good results with respect to reproducibility [38].

Another more recent study, performed by the School of Electronics Information

Engineering, China, in 2006, deposited films via facing targets DC reactive sputtering with TCR values as high at -4.5 %K-1. These films were accompanied by high resistance typical of high TCR films. To lower the resistance, the films were annealed in high vacuum. The resultant films maintain high TCR, -4.4 %K-1 and their sheet resistance was about 20 kΩ/sq. The films also exhibit finer grain sizes and therefore were more homogenous after the 400˚C anneal. These films appear to be suitable for use in microbolometers [42].

Although the majority of published articles pertaining to VOx thin films have used

V as the target, a 2002 article used a V2O3 target. In this study the oxygen partial pressure was varied between depositions using a DC sputtering system, as they claimed the targets resistivity was low enough to do so. Power, chamber pressure and Ar flow were held constant throughout the runs. A deposition at room temperature with no additional oxygen present resulted in a film that appeared to be V2O3 according to XPS data. Addition of oxygen gas to the chamber resulted in films more similar to V2O5. The films were amorphous V2O5 when deposited at room temperature and polycrystalline

V2O5 when the substrate was heated [41]. The intention of this study was to create V2O5 32 films, not particularly for use in microbolometers, as the traditional figures of merit were not investigated. To make these films more applicable for IR detection by increasing their homogeneity, the use of a pulsed DC power supply can and has been used in future experiments.

A study comparing the difference between DC and pulsed DC sputtered V2O5 thin films found similar results to those above with respect the amorphous nature of the unheated films grown using DC sputtering. Although the target used in these experiments was pure V metal and not ceramic V2O3, the film‟s structure showed a similar trend. Limited adatom mobility was used to explain the nature of the amorphous structure. In comparison, the films created using pulsed DC were all polycrystalline when grown at varied deposition temperatures. The use of a pulsed DC power source created higher energy ions moving towards the substrate, thus higher adatom mobility on the surface, allowing the growth of more crystalline films [45]. Studying the films crystallinity is only one of the properties that must be determined to fully understand VOx.

TCR and resistivity remain the most highly investigated properties of VOx pertinent for use in microbolometers.

-1 The TCR values of VOx thin films are usually between -1.5 and -5 %K depending on the material deposition parameters [42]. Films suited for microbolometers

-1 have been reported to have TCR values near -2%K . Understanding the exact effect each property has on the films overall performance in a device is the ultimate goal of this area of research. Table 2-2 provides a summary of various processing parameters and the corresponding resistance and TCR values as they have been reported in the literature. 33

Table 2-2: Summary of VOx microbolometer material literature

Deposition Processing Resistivity/ Authors Stoichmetry TCR (%K-1) Method Temp (˚C) Sheet Rho

Yuqiang Reactive DC VO 400 in vacuum 3 hrs <20 kOhm/sq Lv et al. Sputtering x -4.4

Yong-Hee Reactive RF V O /V/V O 300 in O 30 min 0.1 Ohm-cm -2 to -2.5 Hang et al. Sputtering 2 5 2 5 2

S.B. Wang Ion beam sputtering VO 395 in O 1 hr 50 kOhm/sq -2.1 et al. x 2

Hongchen Reactive Ion Beam 200 Wang et VO (B) 32 kOhm/sq -2.5 Sputtering 2 al.

R.T. Rajendra V O + small PLD 2 5 RT 1.7 ohm-cm -2.8 Kumar et VO2 al.

C. Chen et Reactive Ion Beam VO 623 -2.0 al. Sputtering x

2.1.2 Novel Materials

Polycrystalline silicon germanium (poly-SiGe) thin films have been studied to determine their viability as the detecting material in a microbolometers. TCR values have been reported between -0.65 and 4 %K-1 [17-19]. In a specific study done by affiliates of the Tsingshau University in Beijing poly-SiGe films were fabricated by chemical vapor deposition (CVD) and subsequently doped with boron via ion implantation [20]. A resultant film exhibited a TCR of -1.91 %K-1 and a sheet resistance of 0.35 MΩ/sq.

Microbolometer bridge structures were made using this material to study the 1/f noise. 34 Significant noise contribution, when the frequency was greater than 150 Hz, was attributed to Johnson noise. On the other hand, the 1/f noise contribution was the largest when then frequency was less than 100 Hz. None the less, the values of 1/f noise

1/2 reported were less than 2 μV/Hz , as typically measured in amorphous Si microbolometers [21].

Yttrium barium copper oxide (YBCO) is superconducting material that has also been considered for fabrication in microbolometers. Published data has shown that

-1 YBCO thin films possess TCR values ranging from -2.8 to -4 %K [10, 22-24]. In a study done by Sandeep Kumar et al. YBCO was sputtered via RF magnetron sputtering at room temperature. The resultant TCR values ranged from -3 to -3.5 %K-1. The NETD was less than 100mK and noise was similar to that in VOx devices. An activation energy of 0.22 eV was determined using the Arrhenius plot. [10].

Poly-SiGe and YBCO are two of the more commonly studied alternative materials to a-Si and VOx for microbolometers whereas globular proteins are fairly unique. Research done at the U.S. Army Research Lab in collaboration with Boeing has found a material with high TCR and low sheet resistance. Thin films of cytochrome c have been evaluated with Honeywell CMOS electronics and were found to have TCR values as high at 35 %K-1. The lowest values reported, 5-7 %K-1, were still well above

2 that of found with VOx. The low resistivity of these materials, as low as 0.5 Ω/cm , adds to their figures of merit [11]. Unfortunately, the films undergo variations attributed to aging which may lead to the demise of their potential implementation in microbolometer

[25]. 35 2.2 Structure Zone Model

Structure zone models (SZM) have been developed to help predict the structures of films made at varied gas pressures and substrate temperatures. Thorton introduced a

SZM in the early 1970s which depicted the morphologies of sputtered thin films [49].The model, depicted in Figure 2-5, shows that variations in both the gas pressure and the substrate temperature largely affect the films microstructure. At low substrate temperatures the films remain within Zone T and Zone 1. In Zone 1 the films are more openly packed due to limited adatom mobility. In this zone, if the deposition pressure is increased to a point where the mean free path of the sputtered material and the Ar+ is greater than that between the target and substrate the films will typically exhibit a more open structure and shadowing effects may occur. The films in this zone have many defects and the boundaries between the fibrous grains are highly voided. These films characteristically have a dome like surface texture. In Zone T, the transition region between Zone 1 and Zone 2, films have a finer grain structure and the grain boundaries are dense. These films exhibit smooth surfaces in Zone T as can be seen in Figure 2-5

[12, 44].

36

Figure 2-5: Thornton‟s Structure Zone Model [49]

37

Chapter 3

EXPERIMENTAL METHODS OF MATERIAL CHARACTERIZATION

3.1 Experimental Procedure

A great deal of information pertaining to the properties of about VOx thin films remains unknown. This study characterizes films produced by pulsed DC sputtering using a V2O3 target under various deposition conditions. Electrical, structural and optical properties, as well as film stability and thickness dependency have been investigated.

3.1.1 Sample Preparation

Each VOx film was deposited onto both, glass and native oxide coated crystalline silicon substrates. The glass substrates were prepared by cutting 1-mm thick Corning

Micro Slides into rectangles approximately 0.5” by 1”. P-doped Si wafers with <100> orientation, 13-17 mil thickness and a resistivity between 1-10 Ω-cm were also used as substrates. A native oxide remained on the Si wafers. The original 3” circular Si wafers were cut into pieces of approximate dimensions 0.6” by 0.6”

To assure the substrates were clean, they underwent two ultrasonic baths. The substrates were placed in a beaker containing acetone and bathed for 15 minutes. This was followed by the substrates undergoing a similar ultrasonic bath in ethanol. Once the ultrasonic cleanings were complete the substrates were removed from the ethanol and 38 blown dry with nitrogen gas. The substrates were placed into a clean container and kept in a desicator under vacuum before being used.

3.1.2 Sputter Deposition

The vacuum system used to deposit the VOx thin films was fitted with 3” Torrus guns from the Kurt J. Lesker company. The system was developed to simultaneously carry out sputtering from two targets, also known as dual sputtering, although, in these experiments only one target was used. The chamber was a circular system and the sputtering gun was setup to be pointed at 45˚ offset from the stage. Because there was only a single target being used with these experiments, a wedge was placed on the stage in order for the gun and the substrate to be parallel. The substrates were taped to a wedge with kapton vacuum tape to hold them in place. The throw distance between the sputtering gun and the substrates was 3.5”. A shield was used to cover the substrates during the presputter step of the deposition. The stage was equipped with a thermometer and there did not appear to be significant temperature variations of the stage during deposition. Figure 3-1 is a photograph of the interior of the chamber.

39

Sputtering gun Shutter Substrate Holder

Substrate

Figure 3-1: Internal layout of the deposition chamber

To control the pressure within the chamber, two pumps were used. Because the chamber was opened between depositions, the base pressure before a deposition was reduced to at least 6x10-6 torr to ensure minimal impurities remained. A mechanical pump was first used to reduce the initial pressure within the chamber. Switching to a

Pfeiffer 520 1/s turbo pump allowed the chamber to reach base pressures below 6x10-6 torr, reducing the impurity incorporation during deposition.

Both total pressure and oxygen partial pressure were varied throughout the study.

Argon gas (99.99% purity) was present in the chamber during all depositions with a flow rate of 30 sccm measured by a MKS flow controller. To obtain the correct oxygen partial pressure, the valve between the chamber and the turbo pump, the gate valve, was closed 40 until the MKS Signal Controller as shown in the left image of Figure 3-2, attached to a

MKS Baratron, read the appropriate partial pressure of argon. Beginning oxygen gas

(99.994% purity) flow and adjusting the flow rate allowed for the desired total pressure to be obtained as seen in Figure 3-2. Once set, the gate valve and flow rates were left untouched, with the exception of oxygen, which was shut off and only allowed to flow one minute before deposition and turned off immediately afterwards. Also, it was occasionally necessary to increase the pressure within the chamber when using the RF power supply because the plasma would not strike at low pressures (below 5 mTorr).

The gate valve was slowly adjusted back to its exact location prior to turning on the RF power supply once the plasma struck. The gases used were purchased from Messer, MG

Industries.

(a) (b) Figure 3-2: Pressure gauge with partial pressure of argon (a) and a total pressure of 5 mTorr, 10 % oxygen partial pressure (b) after adjusting the oxygen flow rate. 41

A V2O3 target was purchased from the ACI Alloys, Inc. and used within a Kurt J.

Lesker Company sputtering gun. The target had a 3” diameter, .25” thickness and a purity of 99.9%. All samples were made using the same target. Due to the targets insulating nature, it was necessary to perform a sufficient pre-sputtering ramp up. This precaution was taken to ensure the target would not crack due to a shock from the rapid increase in power applied. Details about sputtering times are mentioned below in Table 3-

1 and Table 3-2.

The majority of films produced in this work were created using an asymmetric bipolar pulse power supply. A negative bias was applied to the target which, with the right conditions, lead to plasma formation and sputtering. In order to neutralize the change build up on the target a short, a small reverse bias, also described as a positive bias, is applied. The short pulse can range from 500-8000 ns depending on the equipment used according to the ENI RPG-50 pulsed DC power supply operation manual.

Films produced via pulsed DC sputtering used a ENI RPG-50 pulsed DC power supply. These films were deposited using a power of 200 W, a pulse frequency of 225 kHz and a pulse width of 1616 ns. The RF film production was carried out by a RFX600

RF power supply and a ATN500 impedance matching network, both made by Advanced

Energy.

The ENI RPG-50 pulsed DC power supply allowed control over power ramping, pulse frequency, pulse width and run time. The programmability of this device ensured accurate reproducibility with respect to power and run time. A similar approach was used with the RF power supply, although it was not equipped with the same 42 programming capabilities. Much attention was given to assure that similar ramp up, sputtering and ramp down processes were preformed with the RF depositions. Table 3-1 and Table 3-2 provide details on the ramping sequences for both power supplies.

Table 3-1: Pulsed DC sputtering ramp sequence

Power (W) Time (sec)

0-50 0

50 60

50-200 1300

200 1800 (Deposition Time)

200-50 1300

50-0 0

Table 3-2: RF sputtering ramp sequence

Power (W) Time (sec)

0-20 0 (increased quickly till plasma struck)

20-200 1300

200 1800 (Deposition Time)

200-0 1300

43 3.2 Characterization Methods

3.2.1 Profilometry

The Tencor Alpha-Step 500 Surface Profilometer was used to measure the thickness of the films. During deposition, as part of the mechanism of keeping the substrates on the wedge, a piece of kapton tape was placed over an edge of the substrate.

Once the deposition was complete the tape was removed, leaving a clean step for thickness measurements. This method provided sharp steps required for profilometry measurements.

The profilometer consists of a stylus which touches the surface and scans over the step created by the kapton tape. During these experiments the force of the stylus was maintained at 3.7 mg and the scan length at 500 um. Measurements were taken at three locations along the step and averaged to determine the film thickness. Standard deviation was calculated from these three measurements to determine error for the thickness measurements.

3.2.2 Temperature Dependant Current-Voltage Measurements

The TCR determination involved sputtering nickel contacts on the film and using them to measuring the current while a voltage was applied. The chamber used to deposit the nickel contacts was similar to that used to make the VOx films. As determined by previous experimentation, highly resistant films deposited on silicon could potentially allow the applied voltage to tunnel through the silicon substrate instead of the film itself 44 and lead to innaccurate results [43]. To eliminate the chance of the current tunneling, through the substrate rather than the film, glass substrates, highly insulating materials with resistivity of approximately 1x10-13Ω-m, were used in place of the silicon substrates for these electrical measurements. The samples were taped to a shadow mask and positioned within the chamber to be aligned parallel with the sputtering gun containing a nickel target (99.9% purity) purchased from the Kurt J. Lesker Company.

-6 As with the VOx film deposition, the chamber was evacuated below to 6x10 Torr to ensure purity. After a five minute ramp up, the contacts were deposited for fifteen minutes at 15 mTorr and 150 W using a MDX Magnetron Drive. Contacts deposited by this procedure were approximately 3000 Å thick as determined by profilometery.

Figure 3-3 shows the pattern produced by the shadow mask. Although there are several contacts and various spacing created, only the two contacts with the widest spacing, shown at the far left, were used in these measurements.

45

Figure 3-3: Optical micrograph of the sputtered nickel contacts

In the second step of the procedure, the sample was placed on a temperature stage within an MMR technologies probing station; the set point temperature was maintained to be very precise using the MMR Technologies K-20 Temperature Controller. The sample was soaked at each temperature for at least two minutes before a measurement was taken.

The window in the system was covered and the measurements were taken in the dark.

Measurements were taken in 10˚C increments between 25˚C and 105˚C during both temperature ramp up and ramp down.

With the contacts in place and temperature set, the Keithley 2400 Sourcemeter was used in conjunction with LabTracer 2.0 to create I-V curves at the sequence of 46 temperatures described above. A measurement was taken using a two point system with four probes. A voltage was applied across the nickel contacts using the probes in the upper left hand side and the current measured through the lower right probes as shown in

Figure 3-4. The voltage was swept from 0 to 5 V while 200 measurements were taken at each temperature. Films were also measured from -5 to 0 V to ensure accuracy. The resultant I-V curves allowed for the resistance to be calculated using Eq. 3.1.

Figure 3-4: Two point measurement system

V 3 R I 3.1 47 Once resistance was measured, a simple calculation allowed for the TCR of each film to be determined. To calculate TCR, the natural logarithm of resistance as a function of temperature was plotted and the slope of this curve was derived using Eq. 1.1.

3.2.3 Spectroscopic Ellipsometry

Spectroscopic ellipsometry measurements allow for the extraction of information about the film‟s dielectric function, thickness, and surface roughness. When polarized light is reflected from a sample‟s surface, changes occur to the amplitude and the polarization state at each interface and throughout each layer of material. The polarization state and the amplitude changes can then be analyzed to extract the complex dielectric function and the thickness of each component layer.

The J.A. Woollam Co. M2000-DUV variable-angle rotating-compensator multichannel spectroscopic ellipsometer was used to perform the measurements.

Figure 3-5 provides a diagram of this spectroscopic ellipsometer. Ex situ measurements were taken at three angles of incidents θi = 55˚, 70˚, and 85˚ over a spectral range from

0.75 to 6.5 eV. The data acquisition time of each measurement was less than 2 minutes.

48

Figure 3-5: Ex situ spectroscopic ellipsometer schematic [50]

Ellipsometry measures the change in the polarization of the light when reflected from a sample‟s surface through the complex electric field vector, Eo. This vector is typically depicted in several electric field components. The electromagnetic wave can first be simplified by reducing one direction, that of which the beam propagates which would usually be denoted as the z-axis in the Cartesian coordinate system. By assuming the beam propagates in that direction, the remaining components can be described with two orthogonal components. These components are then described by two orthogonal axes x and y in the Cartesian system or more commonly redefined in ellipsometry as fields that are parallel (p) and perpendicular (s) to the plane of incidence. Figure 3-6 is a schematic view of the incoming, reflected and transmitted light, providing a visual depiction of the wave components.

49

Figure 3-6: Geometry of the incoming, reflected, and transmitted electromagnetic waves showing the p and s axes in addition to the place of incidence. [52]

The two parameters measured during reflectance in ellipsometry are Ψ and Δ, which are the amplitude and the phase shift difference, respectively. The complex amplitude reflection ratio, ρr is the ratio of the complex amplitude reflection coefficients, rp and rs. rp and rs, are the ratio of the reflected electric field amplitude to the incident electric field amplitude in the p-polarized and s-polarized fields, respectively, as shown in

Eq. 3.2 and Eq. 3.3. Ep and Es are the complex electric field parallel and perpendicular to the plane of incidence and φp and φs are the phase shift for the p-polarized and s-polarized fields. ρr is correlated to Ψ and Δ by Eq. 3.4 .

r E p rp i rp exp i p 3.2 E p

50

r Es rs i rs exp i s 3.3 Es

rp rp r exp i( p s tan exp i 3.4 rs rs

Ellipsometry, as previously mentioned, measures Ψ and Δ but does not directly determine the optical properties or microstructure of a thin film on a substrate. A stratified media model is implemented to extract the film‟s dielectric functions, thickness and surface roughness. The layers that make up the model include the substrate, native oxide, VOx film, and a roughness layer as shown in Figure 3-7. Reference dielectric functions are used for the substrate and the native oxide. The surface roughness layer is represented as a Bruggeman effective medium approximation consisting of a 0.50/0.50 mixture of bulk VOx material and void [53].

Surface roughness (ds)

VOx (db)

Native Oxide SiO2 (dox)

c-Si Substrate

Figure 3-7: Optical model structure consisting of crystalline silicon substrate (c-Si) / native oxide (SiO2) / bulk VOx film / surface roughness / air ambient. 51 Once spectra in ( , ) are acquired, WVASE, a commercial data analysis program is used to extract the complex dielectric function of the VOx material and the thickness of the bulk and surface roughness layers. The stratified media model mentioned above was used for fitting the data. Film thickness values were initially set using profilometry data and then allowed to vary to obtain a better fit. Data from all three angles of incidence were fit simultaneously to the stratified media model shown in

Figure 3-7. The VOx dielectric functions used in this model are represented by a parameterization (using a least squares regression analysis with a weighted error function) [54].

Parameterized VOx dielectric functions are represented by two or three Tauc-

Lorenz oscillators sharing a common band gap, a zero broadened high energy pole, and a

Lorenz oscillator [55-57]. The Lorenz oscillator is represented by Eq. 3.5

AE0 22 , 3.5 E0 E i E where A is the amplitude, Γ is broadening and 0 is the resonance energy. Each Tauc-

Lorenz oscillator can be described by Eq. 3.6

2 AE EEg 0 E E 2 22g 2 2 E, 2 EEE0 0 E E g 3.6 and

2 2 1 P22 d , Eg E 52 where again A is the amplitude, Γ is broadening, 0 is the resonance energy and Eg is the band gap. The pole oscillator used is given is Eq. 3.7

A 22, 3.7 EE0 where A is the amplitude and 0 is a resonance wavelength which must be above the spectral limit of 6.5 eV.

3.2.4 Atomic Force Microscopy

To produce surface images and obtain the roughness of the films, a Digital

Instruments Nanoscope III-D was utilized. A small piece of the VOx thin film on the silicon substrate was cleaved off from each sample in order to fit on the holder of the

AFM. Once the sample was mounted it was cleaned with compressed air to remove any dust particles that may have accumulated.

The AFM measurement was performed in tapping mode over a 2μm square area.

A cantilever with a tip is scanned across a surface as it oscillates at a given frequency.

Tip interactions with the surface cause changes in the oscillation frequency. To measure these changes a laser is directed at a mirrored surface on the cantilever. The reflected laser beam is then directed at a set of photodiodes. The tip interactions with the surface can then be converted into an electronic signal and used to create an image. Images displaying the surface topography can be used to determine the films roughness and also the grain size of the nanocrystalline material. 53 3.2.5 Field Emission Scanning Electron Microscope

Scanning electron microscopy (SEM) is typically used to obtain high resolution images of a material‟s surface and cross section [12]. SEM characterization must be performed under vacuum conditions because any gas present during the measurement could interact with the electrons and disrupt the signal. To create the electron beam for field emission scanning electron microscope (FESEM), a field emitter tip is used. The beam is much smaller in diameter than in typical SEM resulting in a highly concentrated, high current density beam. The beam is directed towards the sample and scanned across a small area in a raster fashion. After interacting with the sample, electrons of various energies are emitted. These electrons can then be converted into electronic signals and processed to create an image.

Using the process described above, an FESEM was used to take cross-sectional images of the films to provide insight into the microstructure. A Leo 1530 FESEM, operated within a class 100 cleanroom, was used to perform these measurements. This device operates at low voltage (1kV) so issues that arise due to sample charging are significantly reduced and there is usually no need for an additional conductive layer on non-conductive samples. Under these conditions, the device has a rated resolution of

3nm. The working distance between the lower portion of the lens and the samples surface ranged from 1 to 3 mm. Magnifications of the films imaged varied from 150kX to 5000kX.

VOx samples deposited on a silicon substrate were imaged by the FESEM.

Sample preparation involved cleaving off of a small piece of the original 0.6” x 0.6” 54 samples. Freshly cleaved samples were immediately loaded into the chamber for measurement. The holder allowed several samples to be loaded simultaneous but imaged individually in order to save time during the pump down process.

3.2.6 Annealing

Annealing was performed on several samples to determine how they would withstand elevated temperatures. Several VOx samples deposited on glass were cleaved in half, one half was saved to be used as a control and the other half was annealed. A hot plate, in ambient air, was set to 200˚C and maintained for over an hour to assure the temperature was stable. Samples were then placed on the hot plate and a timer started.

After the 30 minute anneal, samples were removed and allowed to cool to room temperature. Following the annealing process, contacts were deposited on both the annealed samples and the un-annealed samples.

3.2.7 Thickness Study

A thickness study was conducted to determine the effects of thickness on the films TCR and resistivity. The chamber setup was changed slight as to accommodate 4 samples in one deposition in order to save time during the pump down cycle and to ensure the conditions were as similar as possible for all 4 samples during the deposition.

A rotating substrate holder was created so that after the first sample was deposited a quarter rotation occurred and aligned the next sample in front of the target for deposition. 55 The throw distance was shortened to 2.5” so that the deposition time could be reduced to create films of similar thickness as to those prepared with a 3.5” throw distance.

Deposition time was varied from 200 to 2800 seconds. Two separate runs were performed in order to create a sufficient amount of samples to determine the effects of thickness. 56

Chapter 4

RESULTS AND DISCUSSIONS

In order to better understand how growth parameters and post deposition treatments affect film properties several analysis methods were implemented. In order to properly optimize detecting materials for microbolometers, it is necessary to quantify the resultant different material properties with each other in addition to the deposition and modification conditions. Toward that goal, this work has examined and correlated several key electrical, optical and microstructural properties which are described within this chapter.

Due to the belief that the required chemical composition for current IR sensors is

VO1.8, and because pulsed DC sputtering of a V2O3 has not previously been examined, this thesis work reflects an investigation of a VO1.5 target for creating acceptable IR sensitive films [9, 35]. The initial experiments involved a total of 16 films made at 4 different total pressures (2.5, 5, 10 and 50 mTorr) and 4 oxygen partial pressures (0, 2.5,

5 and 10 % P ) by sputtering a V2O3 target at 200 W in room temperature. These films O2 were chosen to be investigated to explore effects occurring due to variable total pressures and oxygen partial pressures. The films deposited for further studies were chosen based on the properties determined by characteristics of the initial 16 samples. Table 4-1 provides a summary of the thin film deposition parameters and their corresponding electrical and physical properties to be described within this chapter.

57

Table 4-1: Experimental matrix of deposition parameters, electrical properties and thickness

58 4.1 Deposition Rate

The thin film deposition rate was determined by dividing the films thickness with the deposition time. A 30 minute total deposition time was maintained for most samples to determine the deposition rates. It can be seen from Figure 4-1 that as the oxygen partial pressure increases the deposition rate decreases. In pure argon background, the sputtering rates are found to be higher whereas the deposition rates decrease when the oxygen atoms interact with the target and poison its surface.

Figure 4-1 shows that films deposited with total pressures of 2.5, 5, and 10 mTorr have fairly similar deposition rates. Hence, in this pressure range one could assert there exists minimal effects on deposition rate due to total pressure variations. However, it can be seen that there are slightly lower deposition rates for the series with a total pressure of

5 mTorr. At 0% P , the deposition rate is the highest at approximately 130 to 150 O2

Å/min. The lowest deposition rates, approximately 70 Å/min, occur when oxygen partial pressure is 10%. In contrast, the films deposited at a total pressure of 50 mTorr and those made with an RF power source exhibited an overall lower deposition rate. The deposition rates for films processed at 50 mTorr are approximately half the values of those processed at low pressure. During deposition of the films made using RF power, target poisoning may have contributed to the resultant lower deposition rates.

Furthermore, the RF sputtered thin films exhibited high resistance values (greater than

109 ohms) and the I-V measurements were not reliable. These high resistance films are simply not ideal for bolometer sensing applications with their current configuration.

59

Figure 4-1: Thin film deposition rates as a function of oxygen partial pressure with varied total deposition pressures

4.2 Surface Structure

In order to gain a better understanding of the films microstructure, the film surface morphologies were examined using atomic force microscopy. All of the films were found to be very smooth with less than 2nm root mean square (rms) roughness and appeared to consist of small uniform grains. Figure 4-2 shows the AFM surface data of films made at 2.5% P . The film deposited with a total pressure of 5 mTorr had the O2 smoothest surface. The low surface roughness exhibited at 5 mTorr total pressure was also seen with other films irrespective of oxygen partial pressures as shown in Figure 4-3.

It can be seen that with low total pressures, 2.5 and 5 mTorr, the roughness values are 60 near 0.5 and 0.2 nm respectively. On the other hand, the measured rms roughness values at 10 and 50 mTorr are found to be slightly higher and are approximately 1 nm.

2.5 mTorr 5 mTorr

10 mTorr 50 mTorr

Figure 4-2: Surface roughness images of films made at 2.5% P from atomic force O2 microscopy

61

Figure 4-3: Root mean square roughness vs. total pressure

The roughness trend of the films may be attributed to the Structure Zone Model

[12, 49]. The films at lower pressures appear to be in Zone T with fine grains and dense grain boundaries corresponding to smooth surfaces. As the total pressure is increased, the roughness values appear to correspond to the rougher domed surface texture characteristic of Zone 1. Although the films exact stoichiometry was not determined, the

T/TM is assumed to be less than 0.04 because of the high melting temperature associated various VOx phases. Because the temperature of the substrates is not varied and T/TM is very small, it would follow that a transition between Zone T and Zone 1 of the SZM is consistent with an increase in pressure as seen in Figure 2-5. 62 4.3 Cross-sectional Microstructure

Further examination of the microstructure of the films was preformed using a field emission scanning electron microscope. Cross sectional images illustrate that some of the films exhibit columnar structure. Figure 4-4 shows a representative series of 0%

P films prepared at different total pressures. It can be seen that at 2.5 mTorr the film O2 exhibit a very fine grained structure. As total pressure is increased, the visibility of a columnar structure becomes apparent. This trend in column size appears to be in agreement with the SZM [12, 49]. With lower total pressure, the plasma particles are more energetic and thus the resultant film can be packed more densely. At higher pressures there were more collisions causing greater scattering of the gasses which lead to more loosely packed materials. Additionally, as the pressure increases the adatom mobility is reduced, making it more likely for the shadowing effect to occur and columns to develop.

63

VOx VOx

Si 2.5 mTorr Si 5 mTorr

(a) (b)

VO x VOx

Si 10 mTorr Si 50 mTorr

200nm

(c) (d)

Figure 4-4: Cross sectional images of the films made at 0% P for total pressures of (a) O2 2.5 mTorr, (b) 5 mTorr, (c) 10 mTorr (d) 50 mTorr

After examining changes that occurred due to varied oxygen percentages in the plasma, several trends became apparent. At both 2.5 and 5 mTorr all films were highly uniform as observed in the images in Figure 4-5. X-ray diffraction data has shown all films in this study to be x-ray amorphous. On the other hand, with both 10 and 50 mTorr series some columnar growth across the film thickness can be noticed as shown in

Figure 4-6. Additionally, as the oxygen partial pressure is increased for films made at these total pressures, the apparent columns become less obvious and the grains instead 64 appear to be equiaxed. These microstructural features were found to show influence on the films electrical and optical properties, as will be discussed in subsequent sections.

65

VOx VOx

Si 0%PO2 Si 2.5%PO2

(a) (b)

VOx VOx O Si 5%PO2 Si 10%P 2

(c) (d)

VO x VOx

Si 0%PO2 Si 2.5%PO2

(e) (f)

VO x VOx

Si 5%PO2 Si 10%PO2 200 nm

(g) (h)

Figure 4-5: Films do not have apparent grain boundaries deposited at 2.5 mTorr with changes in oxygen partial pressures of or at 5 mTorr and oxygen partial pressures (e) 0%, (f) 2.5, (g) 5% (h) 10%

66

VOx VOx

Si 0%P Si 2.5%P O2 O2 (a) (b)

VOx VOx

Si 5%PO2 Si 10%PO2

200nm

(c) (d) Figure 4-6: Varied apparent columnar size of films at a total pressure of 10 mTorr with changes in oxygen partial pressures of (a) 0%, (b) 2.5, (c) 5% (d) 10% 67

4.4 Optical Properties

The optical properties of the films, as represented by the complex dielectric function spectra = 1 + i 2, can be used to identify variations in the phase composition and electrical properties of the material. As described previously in Section 4.2.1, the ellipsometric spectra of the films were fit using a stratified media model as shown in

Figure 3-7. The dielectric functions are represented with a parameterized model and

Figure 4-7 provides an example of the fit of the model to experimental spectra for a film prepared at a total pressure of 2.5 mTorr and a 0% P . The dielectric functions were O2 then compared for three series of films prepared at total pressure of 2.5, 5, and 10 mTorr where the oxygen partial pressure was varied from 0 to 10%. The greatest differences were observed in the 2.5 mTorr series shown in Figure 4-8. As the oxygen partial pressure is increased, critical point features in the dielectric function vary. The amplitude of the low energy feature shown in the ε2 spectrum near approximately 2 eV decreases as the oxygen partial pressure is increased. As will be shown in Section 4.5, the films deposited at 0 and 10% have very similar resistivity and TCR values, but their dielectric functions exhibit drastic differences as shown in Figure 4-8. It has been observed that the 0% film in this series exhibits features similar to nanocrystalline face centered cubic VO phase material deposited using dual vanadium and vanadium dioxide sputter targets [50]. For the fcc VO phase materials, it has been observed that the amplitude of the critical point feature in the 2 spectrum in the vicinity of 2.0 eV 68 increases with decreasing film resistivity. In the series of films prepared with the V2O3 target, however, it has been observed that this feature quickly decreases as the oxygen partial pressure is increased, even though the resistivity may have decreased.

Additionally, the low energy resonance feature disappears completely for the film made at 10% P , which also begins to exhibit critical point features similar to that of single O2 crystal V2O5 phase [50]. The variations observed in the dielectric function spectra indicate that films prepared with a V2O3 target exhibit varying phase composition, and different opto-electrical characteristics, as a function of oxygen partial pressure during deposition. These types of variations are observed across the series of films prepared at 5 and 10 mTorr

69

Figure 4-7: Ellipsometric spectra (symbols) in ( , ) measured at three angles of o o o incidence, i = 55 , 70 , and 85 , for a VOx thin film deposited on a native oxide covered c-Si substrate. The VOx film was prepared at a total pressure of 2.5 mTorr, and an oxygen partial pressure of 0%.

70

Figure 4-8: Dielectric function spectra ( 1, 2) corresponding to VOx films prepared with oxygen partial pressure from 0 to 10 % and a total pressure of 2.5 mTorr. 71 4.5 Resistivity and Temperature Coefficient of Resistance

As mentioned previously, all TCR values were measured from VOx thin film samples deposited on highly resistive glass substrates. The high resistance of glass ensured that the maximum electric field drop occurred through the VOx thin film. A two point, four probe measurement system, applied voltages between -5 to 5 volts in steps and the corresponding current between the electrodes was detected, as shown in a representative example in Figure 4-9. The linear I-V characteristics signify the Ni electrodes were ohmic in nature. The slopes of Ln R vs. temperature curves provided the

TCR values of the films. For example, a 5 mTorr and 5% P representative sample was O2 used to illustrate the temperature dependant resistance of the films calculated using

Eq. 3.1 and as shown in Figure 4-10. Subsequently, the resistivity values of all the films were calculated taking into account the respective geometrical factors for a comparative analysis.

72

Figure 4-9: Current vs. voltage for a sample deposited at 5 mTorr 5% P O2

Figure 4-10: Example TCR calculation 73 Figures 4-11 through 4-14 illustrate the resistivity as a function of temperature for variable oxygen partial pressure films prepared at fixed total pressures and also trends in

TCR and resistivity as functions of oxygen partial pressure. In the case of films deposited at 2.5 and 5 mTorr total pressures as in Figures 4-11 and 4-12, an increase in the oxygen partial pressure results in decreasing resistivity values until a certain point, followed by a subsequent increase. In addition, the TCR values of films deposited at 2.5 mTorr total pressure follow the same trend as that of its resistivity with variation in oxygen partial pressure. However, in the case of 5 mTorr total pressure films, the TCR values were found to decrease monotonically. According to the microstructure data analysis shown in Figure 4-5, the films exhibiting similar resistivity trends both appear to have a fine apparent columnar structure and/or appear to be amorphous in nature. The 10 mTorr total pressure films, shown in Figure 4-13 exhibit variations in TCR that show an increasing and then decreasing trend as the oxygen partial pressures is increased. The microstructure data shown in Figure 4-6 indicates with increase in the oxygen partial pressure the columnar structure becomes less apparent. This variation is coupled with electrical trends as illustrated by an increase in resistivity, until a small decrease in resistivity at 10% P . Also, the structure of the films appears to be equiaxed columns. O2

The TCR values of this 10 mTorr series begin low at -3.5% (K-1) for 0% , stabilizes between 2.5 and 5% to -4.17% (K-1) and then finally decreases to -3.4% when the oxygen partial pressure is increased to 10%. The least variation of resistivity is shown by films made at 50 mTorr depicted in Figure 4-14. The microstructure of films prepared at varying oxygen partial pressure (not shown here) remains similar throughout 74 and may be a reason for the relatively small variations in resistivity. Although the film resistivity for the 50 mTorr series remains constant, the TCR values experience a variable

TCR trend similar to that of the 2.5 mTorr series.

Most films in this series have a room temperature resistivity between 10 and 100 kΩ-cm. TCR values of these films range between -3 and -5% (K-1). All these films exhibited fairly high TCR values irrespective of the total pressure and oxygen partial pressures during deposition. However, as mentioned previously, a close observation of the data in Figures 4-11 through 4-14 indicates that the TCR and resistivity values do not exhibit similar trends as a function of oxygen partial pressures at fixed total pressures.

75

(a)

(b) Figure 4-11: 2.5 mTorr films with (a) resistivity vs. temperature and (b) trends in TCR and resistivity with varied oxygen partial pressure 76

(a)

(b)

Figure 4-12: 5 mTorr films with (a) resistivity vs. temperature and (b) trends in TCR and resistivity with varied oxygen partial pressure 77

(a)

(b) Figure 4-13: 10 mTorr films with (a) resistivity vs. temperature and (b) trends in TCR and resistivity with varied oxygen partial pressure 78

(a)

(b)

Figure 4-14: 50 mTorr films with (a) resistivity vs. temperature and (b) trends in TCR and resistivity with varied oxygen partial pressure 79 The trends seen in the TCR and resistivity values with respect to changes in oxygen partial pressure have been suggested to relate to the microstructure of the films, but the phase, crystallinity and/or exact oxygen content may also have effects on these values. As was mentioned in the introduction chapter, VOx can exist in multiple crystalline phases [34]. Moreover, all films were deposited at room temperature and x- ray diffraction studies have indicated that these films are amorphous in nature.

Therefore, these differences in TCR and resistivity trends with processing conditions could be attributed to major variation in the stoichiometry and/or „x‟ value in VOx thin films.

The correlation between film resistivity and respective TCR values is plotted in

Figure 4-15. It can be observed that the increase in TCR values resulted in an increase in the room temperature resistivity values. This trend followed the previously reported data shown in Figure 2-2. For resistivity values between 15 and 30 kΩ-cm the corresponding

TCR values fall within a wide range from -3 to -4.2 %(K-1). The scatter of these points is somewhat contradictory of the general trend curve. In general, the required resistivity for current microbolometer applications is far below the exhibited as deposited resistivity values of these films, although the TCR is sufficiently high. In order to reduce and control the film resistivity, post deposition annealing experiments will be discussed in a subsequent section.

80

Figure 4-15: TCR vs. resistivity data from samples deposited at 2.5, 5, 10, 50 mTorr total pressures and 0%, 2.5%, 5% and 10% oxygen partial pressure.

4.5.1 Reproducibility and Aging

An important factor to be considered for commercial production of microbolometers is reproducibility. Deposition of identical films between runs creates increased yield and ease throughout the device fabrication process. The series of films made at 5 mTorr with variable oxygen partial pressures were repeated in order to quantity 81 the run to run variability. The series made at 5 mTorr was chosen to be tested for reproducibility because it showed the widest range of TCR and resistivity values.

Figure 4-16 shows that there are noticeable differences between depositions with same processing parameters. These differences may be attributed to the chamber conditions prior to film deposition and post deposition surface conditions of the films due to handling, aging possible surface re-dox reactions, etc. Although there are noticeable variations in TCR and resistivity between films produced under the same deposition conditions, Figure 4-17 shows that the overall trends of these properties appear to be similar with those observed for the initial 5 mTorr series.

Figure 4-16: Resistivity vs. temperature for the initial and repeated 5 mTorr series

82

Figure 4-17: Basic TCR and resistivity trends as a function of oxygen partial pressure are maintained between depositions

In addition, VOx films should ideally possess stable electrical properties throughout their usage time. In order to examine the aging effects on the electrical properties of these films, the resistivity vs. temperature measurements were repeated and compared with the original measurements. These samples were kept in a vacuum desicator and were exposed to ambient condition only during measurements. Figure 4-18 shows the aging studies of the electrical properties for 3 different samples processed with 83 differing conditions. Figure 4-18 (a) shows a sample with high resistivity and high TCR that exhibited minimal changing after approximately 8 months. Similarly the aging response of a lower resistivity film also exhibited minimal changes as shown in Figure 4-

18 (b). Interestingly, both samples were processed with 0% P at two different total O2 pressures. Samples that were processed with a higher oxygen partial pressure, 10% at 5 mTorr total pressure, however, suffer significant aging after 6 months as shown in

Figure 4-18 (c). At present, no structural and/or chemical reasons for such large aging effects are known, thus further studies are of interest.

84

(a)

(b)

Figure 4-18: Aging effects on resistivity and TCR for (a) 5 mTorr 0% P , (b) 50 mTorr O2 0% and (c) 5 mTorr 10% films 85

(c) Figure 4-18 cont. Aging effects on resistivity and TCR for (a) 5 mTorr 0% , (b) 50 mTorr 0% and (c) 5 mTorr 10% films

In order to establish if the aging effects are consistent and reproducible, three films were made with identical deposition parameters, 5 mTorr 5% P and were O2 subsequently analyzed. Even though the initial resistivity values of these films are slightly different from one another, the overall aging studies indicated that these films experienced insignificant changes over time as can be seen from Figure 4-19. Figure 4-

19 (a) shows a sample made specifically to examine aging effects. This sample was measured regularly and as can be seen there were no significant aging effects within the first 3 months. Figure 4-19 (b) and (c) also show limited changes due to aging. A common trend amongst all the graphs is a slight increase in resistivity and equal or 86 greater TCR values as the films age. Accordingly, increased resistivity corresponds to the increase in TCR as was seen in Figure 4-15. The increase in resistivity is most likely a result of a change in the oxidation state of vanadium in VOx film.

Figure 4-19: Aging effects on several 5 mTorr 5% P samples O2

87

Figure 4-19 cont. Aging effects on several 5 mTorr 5% P samples O2

88 4.5.2 Annealing

During manufacturing and packaging processes, as well as during the use of IR microbolometers, the VOx thin films may undergo high temperature exposures. Device insensitivity to such high temperature exposures requires (high temperature) stability of the VOx films. Therefore, in this study, several films were subjected to an annealing process at 200˚C for 30 minutes in air. To determine the effects of annealing, a sample with high resistivity and high TCR (5 mTorr, 0% P ), two samples with the lowest O2 resistivity and moderate TCR values (5 mTorr, 5% ) and finally a sample with the highest TCR value and a fairly low resistivity (50 mTorr, 0% ) were chosen to be annealed. The results reported in Figure 4-20 show the possibility of drastic changes in

TCR and resistivity values under these conditions.

Films made at 0% and pressures of 5 mTorr and 50 mTorr, Figure 4-20 (a) and (b), displayed small changes in resistivity. The sample made at 5 mTorr underwent a drop in resistivity, whereas the film processed at 50 mTorr was the only film in this study to show an increase in resistivity (and was repeated to verify the results). It has been noted (previously in the literature) that films annealed in an oxygen environment have undergone increases in resistivity [35]. The changes in the 0% films were far less extreme then the results shown in Figure 4-20 (c) and (d), both made at 5 mTorr and 5%

. The resistivity changes by more than three orders of magnitude, and the TCR drops to half the as-deposited value. When the films are annealed this may cause the material to reorder and become more structured. This may result in fewer defects in the material 89 and therefore reduce the resistivity of the film. These drastic changes occur only when subjected to extreme temperatures, as the samples did not undergo these changes during aging studies. If the films in device configurations are subjected to such temperatures, it would be necessary to recalibrate the circuitry of the devices to account for these changes.

90

(a)

(b)

Figure 4-20: Effects of annealing 200˚C for 30 minutes in air on TCR and resistivity 91

(c)

(d) Figure 4-20 cont.: Effects of annealing 200˚C for 30 minutes in air on TCR and resistivity 92 4.5.3 Thickness Effects

In order to determine if thickness has an effect on the films properties, several films of differing thickness were made. The films were all deposited with identical deposition parameters, excluding deposition time. The deposition time was varied between 200 and 2800 seconds. Seven samples were made in two deposition runs to minimize any possible drift that may have otherwise occurred from run to run variation.

For this purpose, a specially designed shutter was used to control the thickness of the samples as explained in the experimental procedures section. Figure 4-21 shows the resistivity vs. temperature data for all of the films prepared at 5 mTorr, 5% P with O2 thicknesses between 827 and 7868Å. There exist slight variations in both TCR and resistivity between the various films. Figure 4-22 shows an apparent decrease in TCR and resistivity as the thickness of the films is increased. A study of the surface roughness by AFM (not shown here) showed that the films rms roughness increases with thickness.

The roughness variations with thickness are most likely a result of a microstructural evolution in the films, which could possibly be the source for the decreasing TCR and resistivity trend seen in Figure 4-22.

93

Figure 4-21: Effects of thickness on resistivity vs. temperature

Figure 4-22: TCR and resistivity as a function of thickness 94

Chapter 5

CONCLUSIONS

The purpose of this work has been to better understand the effects growth parameters have on the relevant microstructural, optical and electrical properties of the vanadium oxide films. An initial set of 16 films was created consisting of films produced at 4 total pressures and 4 different oxygen partial pressures. Initial characterization was preformed on these films and further studies were done to evaluate the effects of time, temperature and thickness. After a thorough investigation of all films made with a V2O3 target and variable deposition parameters several key observations have been made.

Films prepared by pulsed DC sputtering of a V2O3 target possessed high TCR values (-3 to -5% K-1) accompanied by resistivities in the range of 10 to 100 kΩ-cm. The resistivities of these films are somewhat high and not presently integratable with the

ROIC of current microbolometers. Several studies were done on the series of films made to determine their reproducibility, stability and effects of thickness variation. Although there were slight variations in the TCR and resistivity values, trends that occurred with variable oxygen partial pressure were consistent. Films that were measured for aging did not undergo drastic changes in TCR and resistivity, but remained stable for the duration of experimentation. When the films were annealed in air several films changed only slightly, whereas other films experienced a dramatic change in both TCR and resistivity.

Thus it has been established that this issue must be addressed if exposure to such harsh 95 conditions is expected. During implementation in a microbolometer, the thickness of a film should be given close attention as there is a general trend of decreased TCR and resistivity as thickness is increased, possibly a result of film surface structure.

In addition to the comprehensive studies on the electrical properties an examination of the optical and microstructural properties were also performed. The films dielectric functions showed that there were noticeable differences across series of fixed total pressure and variable oxygen partial pressure. Films that appeared similar with respect to TCR and resistivity had very different optical properties indicating that the composition and phase vary as a function of variable oxygen partial pressure under these deposition conditions. Surface analysis done with AFM showed all films to be very smooth, having rms roughness below 2 nm. FESEM provided the cross sectional images which have indicated that low oxygen partial pressure, low total pressure films appeared amorphous whereas high oxygen partial pressure, high total pressure films had distinct grains boundaries.

The trends due to variation in total pressure and oxygen partial pressure have yet to be completely explained. The microstructure of the films appears to have some influence on the resistivity and TCR trends. Variations in film stoichmetry and phase may play large roles in determining the electrical properties.

The study of films made using a V2O3 target is just one of many studies in progress and the combination of the various studies will aid in determining how all film deposition parameters affect film properties. Although the films made in this study exhibit resistivities higher than incorporation into current technology permits, alteration 96 of circuitry, or further post-processing of the films may allow for their application in the future generation technology. 97

Chapter 6

FUTURE WORK

The majority of this study focused on electrical data acquisition for the films studied while ignoring the films composition. Future work should include determining the phase and composition of the films to determine their role in controlling the trends seen in the TCR and resistivity. Thus far a concrete explanation for variations in the optical properties of the films prepared using V2O3 targets possessing similar TCR and resistivity values have also yet to be determined. Characterization using Rutherford back scattering or transmission electron microscopy will provide useful information pertaining to the films stoichiometry and phase while providing a viable explanation. In order to make these films more compatible with current microbolometer fabrication, further investigation of the result of annealing by attempting various temperatures, durations and environments may result in lower resistivity films. Another possible method for reducing the films resistivity would be to deposit films using dual target sputtering of V and V2O3 targets. The use of a sandwich structure containing layers of V and V2O3 may help to make the films more compatible with current technologies. A final important property of the sensing material in a microbolometer is the 1/f noise. Noise measurements were not done in this study but should be a major focus of future work. 98

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