Polysilicon thin film piezoresistive pressure microsensor: design, fabrication and characterization

Abstract

Polysilicon based pressure sensors use a silicon dioxide layer for isolation of piezoresistors from bulk. This helps in reducing the leakage current compared to the p–n junction isolation in silicon piezoresistors. They are also more cost effective than silicon-on-insulator (SOI) based sensors for high temperature applications. This paper reports the design, fabrication process and characterization of a polysilicon piezoresistive pressure sensor with wet bulk micromachined diaphragm. Novel meander shaped polysilicon piezoresistors are placed at optimized locations, found using finite element method (FEM) simulations, to experience high stress. The effect of clamping conditions of the diaphragm on the piezoresistors placement is shown through FEM simulations and the piezoresistor shapes are designed to keep the metal lines outside the diaphragm structure for better reliability. After fabrication and dicing, the mechanical characterization of the sensor is performed using laser doppler vibrometer (LDV) for determining the first mode resonance frequency and transient response of the sensor diaphragm. A first mode resonant frequency of 306.6 kHz and a response time of 0.56 ms are obtained. The sensor is then packaged inside a customized jig and tested with pressure load for determining the static and temperature characteristics of the sensor in the pressure range of 0–30 Bar. The sensor is tested at three different temperatures, viz. −5, 25 and 55 °C. A sensitivity of 3.35–3.73 mV/Bar, non-linearity of less than 0.3 %, and a hysteresis of less than 0.1 % are obtained for all the test temperatures.