The effect of substrate bias on the growth of vanadium oxide thin films

Open Access
Author:
Kozlowski, Scott M
Graduate Program:
Engineering Science and Mechanics
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 10, 2014
Committee Members:
  • Mark William Horn, Thesis Advisor
Keywords:
  • sputtering
  • magnetron
  • reactive
  • vanadium oxide
  • vanadium
  • VOx
  • pulsed DC
  • bias
  • microbolometer
  • thermal imaging
Abstract:
Current portable high performance thermal imaging devices are possible as a result of uncooled focal plane arrays of microbolometers. In order to increase efficiency, portability and performance over their cooled counterparts, uncooled thermal imaging utilizes materials able to operate at room temperature. These thin film detectors absorb incoming thermal radiation, resulting in a temperature change and a corresponding resistivity change of the thin film imaging layer. This resistivity change is measured by the CMOS read-out circuitry the microbolometers are fabricated on, and converted to an image. One of the most widely used materials is a vanadium oxide thin film due to its low resistivity, high temperature coefficient of resistance, and low noise characteristics. While these films have been manufactured commercially for years using ion beam deposition, this study concentrates on thin film vanadium oxide films deposited using magnetron sputtering. In this study, vanadium oxide thin films are deposited using pulsed DC reactive magnetron sputtering of a metallic vanadium target. Various properties of these films were studied in order to get a better understanding of the structural and electrical characteristics of the films. In particular, this study looked at the effect on the film properties of a substrate bias applied to the thin film during deposition. The vanadium oxide thin films in this study were deposited in an argon atmosphere at various oxygen partial pressures with total flow rates between 15 and 45 sccm at pressures of 2.5 and 5 mTorr. RF as well as both positive and negative DC substrate biases were applied to the films during deposition. Structural characteristics of the films were studied using spectroscopic ellipsometry, grazing incident x-ray diffraction and transmission electron microscopy. Electrical properties such as resistivity and the temperature coefficient of resistivity were also studied. As seen in previous works, the cathode current at the target, as well as various electrical properties, exhibited a hysteresis effect between films made at increasing %O2 and decreasing %O2. These films have been shown to have resistivites on the order of 1 ohm-cm with TCR values as large as -4%/K. GIXRD has been used to show that these films consist of FCC rock salt nanocrystallites in an amorphous matrix. The application of a substrate bias during deposition has been shown to increase the TCR to resistivity ratio of the films, reduce their process hysteresis, densify the films, and, through the use of TEM images, form a nanocrystalline columnar structure containing micro-twins embedded within an amorphous vanadium oxide matrix. The substrate bias has been shown to dramatically change the structure of the films as well as to improve numerous film properties critical to their application as imaging layers in uncooled IR microbolometer arrays.