Thin Film Polymer Stress Measurement Using Piezoresistive Anisotropically Etched Pressure Sensors

Abstract

ABSTRACTStresses in thin polymer films have been studied for some time by using wafer bowing, bending beams, microstructure release, and laser holographic techniques. An alternative technique for measuring stresses in thin films is discussed in the following paper. Piezoresistive anisotropically etched single crystal silicon pressure sensors are sensitive not only to applied pressure, but also to applied package stress. Deposited passivation materials, like silicone gels and polyimides, have been observed to change the sensitivity of the pressure sensor. In the current work, a thin, conformal polymeric coating (parylene C) is being developed for these pressure sensors. This thin film has been observed to reduce the sensitivity of the device as a function of the film thickness and modulus and the silicon thickness and modulus. The parylene C thin films exhibit a consistent change in film stress during annealing indicating a modification to polymer crystallinity and a corresponding change in material properties. Qualitatively, the electrical output on the pressure sensor compares favorably with measurements taken using wafer bowing. Experimental DMA and TMA work has been performed to determine the modulus (7.84 × 105 psi) and CTE (39 ppm/°C at 25 °C) of the material. However, literature values of modulus (4.1 × 105 psi) have been used with finite element analysis to model the stress effect more accurately for the thin conformal coating on the pressure sensor device. These results indicate that the sensitivity of the pressure sensor will be reduced approximately quadratically as a function of the polymer coating thickness. An empirical function has been derived to estimate sensitivity loss as a function of substrate (i.e., initial diaphragm material) modulus and thickness and coating modulus and thickness.