Compliant piezoresistive MEMS sensors exhibit great promise for improved on-chip sensing. As compliant
sensors may experience complex loads, their design and implementation require a greater understanding of the
piezoresistive effect of polysilicon in bending and combined loads. This paper presents experimental results
showing the piezoresistive effect for these complex loads. Several n-type polysilicon test structures were tested.
Results show that, while tensile stresses cause a linear decrease in resistance, bending stresses induce a nonlinear
rise in resistance contrary to the effect predicted by available models. The experimental data illustrate the
inability of published piezoresistance models to predict the piezoresistive trends of polysilicon in bending and
combined loads, indicating the need for more complete models appropriate for these loading conditions and more
complete understanding of the piezoresistive effect.
This paper presents the design, fabrication, and testing of a force sensor for integrated use with thermomechanical
in-plane microactuators. The force sensor is designed to be integrated with the actuator and fabricated in the
same batch fabrication process. This sensor uses the piezoresistive property of silicon as a sensing signal by
directing the actuation force through two thin legs, producing a tensile stress. This tensile load produces a
resistance change in the thin legs by the piezoresistive effect. The resistance change is linearly correlated with
the applied force. The device presented was designed by considering both its piezoresistive sensitivity and out-of-
plane torsional stability. A design trade-off exists between these two objectives in that longer legs are more
sensitive yet less stable. Fabrication of the sensor design was done using the MUMPs process. This paper presents
experimental results from this device and a basic model for comparison with previously attained piezoresistive
data. The results validate the concept of integral sensing using the piezoresistive property of silicon.
We examine exploiting the inherent piezoresistivity of a polysilicon compliant mechanism to provide feedback sensing of the mechanism displacement. As the piezoresistive compliant mechanism deflects to produce motion its resistance changes producing a usable signal. The goal of this work is to improve the transient response of a thermal actuator through piezoresistive feedback control. Implementing feedback control significantly improves the actuators transient response. The actuator response time to step inputs is reduced from 800μs to 230μs with proportional control alone. The system bandwidth was increased from 500~Hz to 4~kHz with proportional control. The large overshoot in the step response or the resonant peak in the frequency response can be reduce by an appropriately tuned 2~kHz notch prefilter.