Residual stress induced deformation of bilayer plate with patterned additional layer

Bilayer material systems find wide application in Micro ElectroMechanical systems (MEMS), especially serving the reflective surface roles in optical MEMS. Unlike the bilayer with fully covered additional layer, the stress related deformation of bilayer with patterned additional layer has been rarely studied. In this paper, we present the out-of-plane deformation response of polysilicon plate with patterned gold layer subjected to residual stress. Stress gradient through the thickness of the polysilicon plate is taken into account to introduce a initially dome shaped deformation into the polysilicon plate. Because this kind of bilayer is mainly used in flat micromirror, the commercially available microlens determines the size of the mirror plate and the fabrication foundry determines the thickness of the additional layer, only the relationship between the deformation and the gold pad size was studied. In order to understand the influence of the residual stress of the gold layer, different residual stresses were tried. The average curvature approach proved not suitable to characterize the deformation of this kind of bilayer plate, and the peak-to-valley (PV) flatness was proposed. The result of numerical analysis shows under given residual stress of gold layer, there is one and only one minimum PV point. Increasing of additional layer residual stress will decrease the minimum PV related gold pad size, but increase the minimum PV.