Piezoresistive CMOS sensor for out-of-plane normal stress

A piezoresistive sensor for measuring the temperature compensated sum (σxx + σyy)/2 − βσzz of the three mechanical normal stress components, where σxx + σyy, σzz, and β denote the in-plane normal stress sum, the out-of-plane normal stress, and a numerical factor close to 1 is presented. The device is based on CMOS-compatible diffusions and exploits the piezoresistive effect acting on vertical current components. The sensor signal and resulting possibilities to extract mechanical stress components are analyzed. Experimental results illustrate the influence of design parameters and operating conditions on the sensor performance and are compared to a simple analytical sensor model. A second-order cross-sensitivity to in-plane shear stresses is discussed and determined experimentally. The temperature compensation is demonstrated by a force measurement with a deviation between the numerically expected and the experimentally extracted value smaller than 15% over the temperature range from 10 °C to 60 °C.