Minimizing stress in large-area surface micromachined perforated membranes with slits

This paper presents the effectiveness of both design and fabrication techniques for avoiding the rupturing or excessive bending of perforated membranes after release in surface micromachining. Special lateral designs of arrays of slits in the membrane were investigated for a maximum yield at a given level of residual stress. Process parameters were investigated and optimized for minimum residual stress in multilayer thin-film membranes. A 2 mu m thick sacrificial TEOS layer and a structural membrane that is composed of silicon nitride and polysilicon layers in the stack is the basis of this study. The effect of sharp corners on the local stress in membranes was investigated, and structures are proposed that reduce these effects, maximizing the yield at a given level of residual stress. The effects of perforation and slits were studied both theoretically and using finite element analysis. While the overall effect of perforation is negligible in typical MEMS structures, an optimum design for the slits reduces the von Mises stress considerably as compared to sharp corners. The fabrication process was also investigated and optimized for the minimum residual stress of both the layers within the stack and the complete layer stack. The main emphasis of this work is on placing a stress-compensating layer on the wafer backside and simultaneously removing it during the surface micromachining, as this has been found to be the most effective method to reduce the overall stress in a stack of layers after sacrificial etching. Implementation of a stress compensating layer reduced the total residual stress from 200 MPa compressive into almost 60 MPa, tensile. Even though a particular structure was studied here, the employed methods are expected to be applicable to similar MEMS design problems.