A Surface Micromachined Piezoresistive Pressure Sensor with High Sensitivity

This paper presents a surface micromachined piezoresistive absolute pressure sensor with improved performance. A sensitivity of about 11 mV/V/bar in the range 0-500 mbar overpressure has been achieved. The sensor contains four polysilicon piezoresistors arranged in a Wheatstone bridge at the underside of a polysilicon membrane to reduce invironmental influences (e.g. light, moisture). Position and size of the piezoresistors has been optimised by FEA calculations. Three layers of polysilicon deposited by standard CVD techniques are used for membrane, piezoresistors and sacrificial layer of the sensor. Using polysilicon as membrane material thermal stresses generated in the sensor structure could be minimised. With an epitaxial deposition technology in-situ doped membranes with thicknesses significantly higher then 2 ¿m and controlled intrinsic stress could be fabricated. The sacrificial polysilicon was etched with an standard aqueous TMAH solution. Suggested by technological experience and experimental results in a second step a combined sacrificial layer etching technique has been applied. The obtained flat sensor structure provides decidive advantages for fabrication.