In situ test structures for the thermal expansion coefficient and residual stress of polysilicon thin films

Abstract

In this research, micromachined devices consisting of four micro-rotating structures for the in situ determination of the thermal expansion coefficient (TEC), tensile and compressive residual stress of polysilicon thin films are studied. The structures are heated electrically and deflect due to the thermal expansion. The lateral displacements of the devices are related to the thermal stress and residual stress of the test beams. The micro-rotating structures are arranged, so that the lateral displacements are designed to be either a constant value which is used to determine the TEC of the thin film or a variable value that changes with the residual stress of the thin film. An analytical model of the test structure is presented. The finite element software ANSYS is used to verify the analytical model and provide guidelines for the structure design. Experimental results with a surface micromachined polysilicon thin film are used to demonstrate the proposed method. In the experiments, a current–voltage measurement system only is required. The TEC for the polysilicon thin film is obtained to be (2.61 ± 0.04) × 10−6 K−1 from 400 to 420 K and the residual stress is measured as −(10.15 ± 0.70) MPa.