Towards Piezoresistive CMOS Sensors for Out-of-Plane Stress

This paper presents a novel piezoresistive sensor detecting a temperature compensated sum of the three mechanical normal stress components including the Out-of-plane stress iquest_zz. The sensor is based on CMOS-compatible diffusions designed to exploit vertical currents. We show that the temperature compensated stress sum can be extracted via two different sensor designs and even with a single device exploiting the strong influence of the junction field effect in the sensor. For this purpose, an experimental setup that induces homogenous vertical normal stress sigma_zz by applying a vertical force via a gold bump to the sensor is presented. For the characterization, the resulting mechanical stress at the sensor location was evaluated using finite element simulations. Sensitivities, depending on gold bump shape and bias current, were experimentally determined and vertical forces were successfully extracted between 10degC and 60degC, independent of temperature.