Strain sensitivity and temperature influence on sputtered thin films for piezoresistive sensors

The use of Ni-Cr thin-film resistors as strain-sensitive gages has been studied. Composition and thermal treatments are optimized for the following parameters: gage factor (FG) and temperature coefficient of resistance (TCR). TCR has been determined using an automatic data acquisition system with implemented software. FG has been determined by the cantilever method. The optimum thin-film composition (Ni 50%-Cr 50%) has been determined. Low stable TCR values (less than 20 ppm in the 30-150 °C temperature range) are obtained. Resistance stability is achieved with a 24 h heat treatment at 150 °C. The desired 40.Ω/square sheet resistance has been attained for 50 nm thick films. A longitudinal gage factor of 2.3 is obtained for this composition (and a lower transversal gage factor of 0.83). With the established process, Ni-Cr thin-film strain gages satisfy the main properties required for their application to piezoresistive pressure transducers: adequate strain sensitivity, high sheet resistance, low thermal sensitivity and good thermal stability.