Micromachined force sensors using thin film nickel-chromium piezoresistors

Micromachined force/tactile sensors using nickel-chromium piezoresistors have been investigated experimentally and through finite-element analysis. The force sensors were designed with a suspended aluminum oxide (Al2O3) membrane and optimally placed piezoresistors to measure the strain in the membrane when deflected with an applied force. Different devices, each with varying size and shape of both the membrane and the piezoresistors, were designed, fabricated and characterized. The piezoresistors were placed into a half-Wheatstone bridge configuration with two active and two passive nickel-chromium resistors to provide temperature drift compensation. The force sensors were characterized using a load cell and a nanopositioner to measure the sensor response with applied load. Piezoresistive gauge factors in the range of 1-5.2 have been calculated for the thin film nichrome (NiCr 80/20 wt%) from the measured results. The force sensors were calculated to have a noise equivalent force of 65-245 nN.