Preparation and piezoresistive properties of reactively sputtered indium tin oxide thin films

Oxygen deficient thin films of indium tin oxide (ITO) were prepared by r.f. reactive sputtering from a high-density ITO target (90 wt.% In2O3 and 10 wt.% SnO2) in various Ar:O-2 mixtures for the purpose of investigating their use in a variety of strain gage applications. The resulting thin films were transparent in the visible spectrum (optical bandgap of 3.3-3.4 eV), tested n-type by hot probe and exhibited room-temperature resistivities in the range 0.01 to 0.10 Omega cm after annealing. Room-temperature gage factors (G=Delta R/R(0)1/epsilon) as large as -77.71 were measured on patterned ITO films. These gage factors are considerably larger than those reported for refractory metal alloys (G=2). A large, negative piezoresistive response (negative gage factor) was observed for all ITO films similar to the responses observed for n-type silicon. The piezoresistive response was reproducible and linear, with little or no hysteresis observed with strains up to 700 mu(in in)(-1). Additionally, optical gage factors based on changes in the ITO bandgap due to strain were established for these films using UV-Vis spectroscopy. Temperature coefficients of resistance (TCRs) of ITO films as low as + 230 ppm degrees C-1 were realized in nitrogen ambients at temperatures up to 500 degrees C. In oxygen-bearing ambients, two distinct regions were observed; one having a TCR as low as -429 ppm degrees C-1 and another, at temperatures up to 1100 degrees C, having a TCR of -1560 ppm degrees C-1. Large gage factors combined with relatively low TCRs make these ITO films excellent candidates for use as high-temperature strain sensors. The relationship between processing parameters and piezoresistive properties of these ITO films is reviewed and prospects using these films as high-temperature strain sensors is discussed.