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The Pennsylvania State University 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
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