Abstract
Piezoresistors have been used widely for various microsensors, such as accerlerometers and pressure sensors. Conventionally, piezoresistive sensors are fabricated by placing the piezoresistors on the regions where largest strain would occur. In order to satisfy the measurement sensitivity as well as the fabrication processes, the substrate underneath the microstructures are usually removed through the back-sided etching. Hence, the fabrication processes are complex and time consuming. To solve this problem, this article intends to propose an alternative design for piezoresistive sensors. The approach is to form the piezoresistors behind the boundary of the microstructure by diffusion. Therefore this piezoresistive sensor is compatible with front-side etch process and without an additional polysilicon layer deposited. The proposed design also provides the capability of integrating with various microactuators. In this research, the concept is proved by both simulation and experiment. A finite element model was established to analyze the stress at the region 2 to 5 mm away from the microstructure and its boundary. According to the results from the finite element analysis, the stress at the studied region is approximate one order of magnitude smaller than that at the end of the microstructure. In other words, a reasonable signal is still available at this region. During the experiment, silicon dioxide microcantilevers were fabricated, and piezoresistors were formed by diffuse n+ on silicon substrate 3 mm away from the micorcantilever's fixed end. An experimental setup containing micropositioner and piezo actuator was constructed. The output of piezoresistors is then measured when microcatilevers bend or vibrate.