PREPARATION, CHARACTERIZATION AND POST-DEPOSITION MODIFICATION OF PULSED-DC MAGNETRON SPUTTERED VANADIUM OXIDE THIN FILMS FOR MICROBOLOMETER APPLICATIONS

Open Access
Author:
Venkatasubramanian, Chandrasekaran
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 29, 2010
Committee Members:
  • S Ashok, Dissertation Advisor/Co-Advisor
  • S Ashok, Committee Chair/Co-Chair
  • Mark William Horn, Committee Chair/Co-Chair
  • Michael T Lanagan, Committee Member
  • Thomas Nelson Jackson, Committee Member
Keywords:
  • Pulsed dc sputtering
  • vanadium oxide
  • annealing
  • ion implantation
  • microbolometer
  • infrared imaging
Abstract:
Vanadium Oxide (VOx) thin films have been at the heart of uncooled microbolometers for several years, however very little is known about their structure and material properties. Also, process control remains an issue because the films are formed under oxygen-starved conditions resulting in films with “x” less than 2.0 in VOx. Hence it is desirable to optimize the film deposition conditions so as to obtain the required properties (high temperature coefficient of resistance [TCR], low resistivity and low noise) for the microbolometer application. In this work, the parameter space for pulsed-dc magnetron sputtering was explored to arrive at optimum deposition conditions. A metallic vanadium target was used in a reactive environment under different Ar/O2 ratios. The gas flow rates and oxygen partial pressures were varied systematically, and the corresponding changes in the cathode (target) current were monitored. The cathode current was found to exhibit a hysteresis behavior between forward and reverse directions for changes in the oxygen percentage as well as the total flow rate. The width and position of the hysteresis curve depended on the relative values of the gas flow rates and the oxygen partial pressures. Films were deposited along various points in the hysteresis curve, and their structural and electrical properties were evaluated. The resistivity and the TCR of the films were also found to exhibit a hysteretic behavior similar to that of the cathode current. The film microstructure changed from columnar at low flow rates to multi-grained features at higher flow rates. Also, the TCR of the films exhibited a linear relation with log of resistivity – the higher the resistivity, the higher the TCR. The current read-out circuitry for VOx microbolometer arrays requires a material with high TCR but low resistivity. Post-deposition modification was investigated to see if the combination of resistivity and TCR could be improved from the as-deposited properties. The sputtered films were annealed in inert (nitrogen) and oxidizing (oxygen) atmospheres at four different temperatures for varying time periods. Depending on the exact annealing conditions, several orders of magnitude change in resistance and significant variations in TCR were observed. Optimal results were obtained for 300 oC anneal in nitrogen atmosphere. Ion implantation was also tried out to further enhance the trade-off between resistivity and TCR. Two species - Hydrogen (active) and Helium (inert) were implanted, which caused both the TCR and resistivity to go up. Subsequent anneal in nitrogen dropped the resistivity significantly, without changing the TCR much. This resulted in up to 40% improvement in TCR, for any given resistivity, as compared to the as-deposited films. In summary, magnetron sputtering, in combination with post-deposition process, can produce VOx thin films comparable to or better than those used by industry. In addition, cathode current during deposition is a helpful processing metric to produce low resistivity, high TCR films.