Characteristics of a Reactively Sputtered Indium Tin Oxide Thin Film Strain Gage for Use at Elevated Temperatures

Abstract

AbstractStrain sensors based on thin films of indium tin oxide (ITO) have been developed for a variety of applications, where the measurement of both static and dynamic strain are required at elevated temperatures. ITO thin films were prepared by rf reactive sputtering in Ar:02 mixtures from high density, electrically conductive targets having a nominal composition of 90% In203 and 10% Sn02. The resulting ITO films exhibited room temperature resistivities between 2x10−2 and 2x102 ω cm, an optical bandgap of 3.5 ev and tested “n” type by hot probe. These same films exhibited large negative gage factors (G=δρ/ ρδε) when tested at room temperature and a relatively low temperature coefficient of resistance when tested at elevated temperature in air. Specifically, gage factors approaching -100 with little hysteresis were observed for strains up to 700 μin/in and TCR's as low as 195 ppm/°C have been measured for the sputtered ITO films. In addition, these films were electrically stable and readily formed ohmic contacts with platinum at temperatures up to 1180°C. In this paper, we report on the electrical properties and piezoresistive properties of ITO based strain gages at temperatures up to 1180°C. Prospects of using ITO thin films as the active strain elements in high temperature strain gages and the characteristics of strain sensors based on ITO are discussed.