| Many thin films with micrometer/nanometer scale are often used in MEMS/NEMS process. The mechanical properties of the thin films have a great influence on the performance of MEMS device, especially the thin film acting as structure layer. The mechanical parameters of a thin film mainly include the Young’s modulus, the poisson’s ratio, the residual stress and the stress gradient.This paper is based on silicon microfabrication technique, by coating positive photoresist BP212on a silicon substrate as the sacrificial layer and negative photoresist SU-8as the structure layer, and then micro-structures such as cantilever beams, double clamped beams and center-anchored circular plates have been made.The micro-structures which are made of SU-8photoresist will bend due to the stress gradient in the direction of thickness after being released.3D surface profile information of the structures after deforming can be measured by using the phase-shifting microscopic interferometry.The Stoilov algorithm is used in this paper. Conventional phase-shifting equations require phase shift amounts to be known, but it is unneeded in this algorithm, only identical phase shift amounts in every step should be guaranteed. The real phase information of the structures can be gotten by means of the phase-unwrapping technique, and then the real height of the object can be obtained.In order to eliminate tilt error in surface profile measurement, a robust approach based on least square method is proposed, which is implemented by finding a reference plane with least squares fit to get the height information of the upper surface of micro-structures relative to the reference plane.Microcantilevers are used as test structures in this paper, using the phase-shifting microscopic interferometry to measure the surface deflection, and then calculate the curvature of the surface. Finally, the stress gradient of the thin film can be obtained. This method is fast and non-contact with high precision. |
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