Design of a polysilicon-on-insulator pressure sensor with original polysilicon layout for harsh environment

For conventional architectures of polysilicon-on-insulator (PSOI) piezoresistive pressure sensors, metal-on-polysilicon contacts located on the membrane are submitted to repetitive strains and a possible deterioration may occur after a long use period. The metallisation may also be damaged at high temperature or when a corrosive atmosphere is in contact with the membrane. The basic idea developed in this paper consists in reporting the metal-on-polysilicon contacts outside the membrane. A new sensor design based on this idea is presented. The resistance of the polysilicon patterns have been modelled and optimised by finite element analysis. After thermal oxidation, a 0.46 mum thick polysilicon film has been obtained by LPCVD (620 degreesC, 450 mtorr) then boron implanted (30 keV, 2(15) cm(-1)). A crystallisation RTA has been performed at 1100 degreesC during 20 s. The silicon etching of a 20 mum thick membrane has been achieved by means of an aqueous KOH solution. A 8.3 mOmega cm resistivity value has been measured on polysilicon Hall patterns. The measured resistances have been found in a very good agreement with the simulations. The low pressure sensor has been tested in a full Wheatstone-bridge configuration showing a good linearity in the [0-200] mbar range with a 50 mV bar(-1) mA(-1) sensitivity. These results demonstrate the feasibility of a pressure sensor without metal/ polysilicon contact running over the membrane. (C) 2002 Elsevier Science B.V. All rights reserved.