High-temperature SOI CMOS compatible MEMS pressure sensors

A MEMS (Micro-Electro-Mechanical System) option for pressure sensors has been integrated into the Fraunhofer IMS 1µm High-Temperature SOI CMOS process. The pressure sensors built using this option consists of polysilicon diaphragm over active areas, together forming a capacitor whose capacitance depends on the deformation of the diaphragm by ambient pressure. The change in capacitance, which is on the order of a few hundred femtofarads over the full pressure range of the sensor, must be converted into an electrical output signal. The monolithic integration of the sensor with the signal conditioning circuits on a single chip eliminates the influence of external noise sources on the sensor output signal and allows a variety of output options. Two chips were designed as test cases for the process option: a basic design consisting of the pressure sensor, a capacitance/voltage (C/V) converter and an additional temperature sensor to provide data for temperature compensation. The other chip is a more elaborate design comprising the sensor array., C/V converter, programmable linearization circuit, output amplifiers, temperature sensor and digital control logic. Both chips were fabricated in several variants targeted at different pressure ranges, with full-scale pressures from 3 bar to 70 bar. The sensors were tested using a custom-built pressure chamber which can be used for temperatures between room temperature and 250°C. They were operational at all temperatures and pressures and showed the expected behavior. The accuracy after linearization and temperature compensation is 1% to 2 % of full scale.