Mechanical characterization of thick polysilicon films: Young's modulus and fracture strength evaluated with microstructures

Abstract

Young's modulus and the fracture strength of thick polysilicon films were evaluated with surface micromachined test structures. The polysilicon films were deposited in an epitaxial reactor and were about 10.5 µm thick. Four different processing schemes of doping and annealing were used and thus four different sets of test structures were micromachined. A micromanipulator system developed at Uppsala University was used to make tensile tests. The micromanipulator is adapted to a scanning electron microscopy environment and all tests were done in situ so that the testing procedure could be monitored at high magnification. From the tensile tests Young's modulus as well as the mean fracture strength and Weibull modulus of each of the films were evaluated. Young's modulus was also evaluated from the collapse voltage of test structures consisting of an electrostatically movable plate suspended by springs. Film texture measurements by x-ray diffraction and cross sectional transmission electron microscopy revealed that the silicon grains had preferential orientations. A mean value of Young's modulus of the film was calculated based on the Young's modulus values of the preferred orientations. It was found that Young's modulus of polysilicon evaluated by the three methods presented agree with each other as well as with other reliable data.