Preparation and piezoresistive properties of reactively sputtered indium tin oxide thin films
Date of Original Version
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:O2 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 Ω, cm after annealing. Room-temperature gage factors (G = ΔR/R01/ε) 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 μin 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 °C-1 were realized in nitrogen ambients at temperatures up to 500°C. In oxygen-bearing ambients, two distinct regions were observed; one having a TCR as low as -429 ppm °C-1 and another, at temperatures up to 1100°C, having a TCR of - 1560 ppm °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 piezo-resistive properties of these ITO films is reviewed and prospects using these films as high-temperature strain sensors is discussed.
Thin Solid Films
Dyer, S. E., O. J. Gregory, P. S. Amons, and A. Bruins Slot. "Preparation and piezoresistive properties of reactively sputtered indium tin oxide thin films." Thin Solid Films 288, 1-2 (1996): 279-286. doi:10.1016/S0040-6090(96)08865-7.