Electrical Properties Of Reactive Magnetron Sputtered Vanadium Oxide Thin Films (2006)

It is well known that vanadium oxide can take many different forms. However for this study, only the amorphous phase was investigated. Amorphous vanadium oxide (VOx) thin films were deposited on thermally grown silicon dioxide by DC magnetron sputtering using a vanadium metal target in an argon / oxygen atmosphere. The driving force of this study was to investigate the temperature coefficient of resistance (TCR) and low resistivity in the amorphous films. Sheet resistance is very sensitive to small changes in temperature, making amorphous VOx very attractive to thermal sensor applications such as infrared detectors. To form the vanadium oxide, physical vapor deposition of vanadium metal at 200 Watts of DC power was used with varied amounts of oxygen in a primary argon atmosphere. During deposition, the concentration of oxygen was controlled by using a 20:80 mixture of O2 and Ar in conjunction with high purity Ar supply. Flow control techniques were derived and calculated to predict the percentage of oxygen before and during deposition to understand the reaction between the vanadium metal and oxygen. Concentrations of O2 in the deposition chamber were varied from 0.025% to 3.000% with the purpose of gaining an understanding of the affects of O2 concentration in amorphous VOx films. TCR and resistivity measurements were performed to characterize the films. The results showed a resistivity decrement with decreasing oxygen concentration. The films with lower concentrations of oxygen were found to have better TCR values then those with higher percentages of oxygen. To further reduce the resistivity of the VOx and maintain the TCR value, co-sputtering of noble metals (gold and platinum) with VOx was studied. The metals were co-sputtered at various power settings with the vanadium oxide reactive process at a fixed percentage of oxygen. The TCR and resistivity results showed that the additions of Au and Pt into VOx reduced the resistivity. However, only Au was found to improve TCR value. The results of these experiments showed that by reducing the amount of oxygen in the film, the ratio between TCR and resistivity further improved. Mechanical limits of the gas delivery system and the relatively low sensitivity to oxygen detection, gas flow control is limited when sputtering with only a single target. Several targets were therefore used during sputtering to allow for higher gas flows thereby increasing the effective sensitivity of the oxygen control. To increase the amount of available vanadium and still have a sufficient amount of detectible oxygen present, four vanadium targets were sputtered simultaneously. The measurements appeared to have a trend of increase in TCR values with a decrease in resistivity. For an ideal case, thermal sensor material should incorporate high TCR and low resistivity for better sensitivity. The amorphous vanadium oxide deposited by 4 vanadium targets seems to satisfy that requirement. In conclusion, a novel method has been established to fabricate amorphous vanadium oxide thin films with high TCR and low resistivity for infrared detectors.