Design and Finite Element Analysis of Micromachined Piezoresistive Polyimide Nanocantilevers for Surface Stress Sensing Applications

The rapid progress in the field of micro- and nano- electromechanical systems (MEMS and NEMS) coupled with the advent of nanotechnology have led to the development of a very promising field of bioMEMS. A typical example of this class of devices are MEMS based sensors for biomolecular detection which use knowledge of various domains like biology, nanotechnology, microfabrication, material science, optics, electrical and mechanical engineering. This thesis explores the suitability of polyimide based nanocantilevers for surface stress sensing applications. Polyimide is a mechanically flexible biocompatible polymer. In our study, the cantilevers are modeled and analyzed using the commercial Finite Element Analysis tool oventorWare. The von Mises stress and the relative percentage change in resistance of the integrated polysilicon piezoresistor are analyzed by changing the geometric parameters of the cantilever. The effect of partial and complete etching of the cantilever structure near the clamped end is also studied. It is shown that these etched parts act as stress raisers and result in approximately 50 85% increase in surface stress sensitivity of the device. It is also shown that the effect of Joule heating on the thermal sensitivity is critical for overall improvement of signal to noise ratio of the device.