Simulation and fabrication of piezoresistive membrane type MEMS strain sensors

Two piezoresistive (n-polysilicon) strain sensors on a thin Si3N4/SiO2 memb rane with improved sensitivity were successfully fabricated by using MEMS t echnology. The primary difference between the two designs was the number of strips of the polysilicon patterns. For each design, a doped n-polysilicon sensing element was patterned over a thin 3 mu m Si3N4/SiO2 membrane. A 10 00 X 1000 mu m(2) window in the silicon wafer was etched to free the thin m embrane from the silicon wafer. The intent of this design was to fabricate a flexible MEMS strain sensor similar in function to a commercial metal foi l strain gage. A finite element model of this geometry indicates that strai ns in the membrane will be higher than strains in the surrounding silicon. The values of nominal resistance of the single strip sensor and the multi-s trip sensor were 4.6 and 8.6 k Ohm, respectively. To evaluate thermal stabi lity and sensing characteristics, the temperature coefficient of resistance [TCR = (Delta R/R-0)/Delta T] and the gage factor [GF = (Delta R/R-0)/epsi lon] for each design were evaluated. The sensors were heated on a hot plate to measure the TCR. The sensors were embedded in a vinyl ester epoxy plate to determine the sensor sensitivity. The TCR was 7.5 X 10(-4) and 9.5 X 10 (-4)/degrees C for the single strip and the multi-strip pattern sensors. Th e gage factor was as high as 15 (bending) and 13 (tension) for the single s trip sensor, and 4 (bending) and 21 (tension) for the multi-strip sensor. T he sensitivity of these MEMS sensors is much higher than the sensitivity of commercial metal foil strain gages and strain gage alloys. (C) 2000 Publis hed by Elsevier Science S.A. All rights reserved.