Calibration of MEMS strain sensors fabricated on silicon: theory and experiments

A calibration technique for measuring MEM's strain sensor performance is presented. For resistance based sensors, calibration entails determining a relationship between change in resistance of the sensor and strain (the gauge factor). A modification to the standard calibration method employed for metal foil, resistance strain gauges is presented. The approach entails constructing two nearly identical test specimens: a specimen with the MEM's sensor mounted with adhesive and a specimen with a strain gauge on silicon mounted with adhesive. Data from the strain gauge specimen provide the basis for evaluating the strain at the sensor. Test data are presented which show that strain at the wafer is 52% to 55% of the strain applied to the specimen. A theoretical basis for this strain transfer relationship is presented. Finally, a dimensionless geometry factor, based on shear lag theory, is derived. As the sensor cross section (width and length) and thickness changes, the strain transfer between the specimen and sensor vary linearly with the geometry factor. This result emphasizes the importance in considering the overall sensor geometry when employing semiconductor strain gauges.