The design of multilayered polysilicon for MOEMS applications

A rigorous analysis of a multilayered polysilicon laminated system, constructed by alternating deposition of low-pressure chemical vapor deposition (LPCVD) polysilicon at two different temperatures is presented. Different residual deformation fields are generated in these polysilicon thin films as a function of fabrication temperatures, due to different crystallization behavior at the two temperatures. The combination of the two layers, however, enables precise control of the radius of curvature of released structures, provided the material properties are well defined. We describe a new method, which combines experimental and numerical procedures, to define the material properties, which are responsible for the residual stresses as a function of layer thickness, as well as a procedure to design the desired curvature of a multilayered Micro-optical-electromechanical system (MOEMS) device. A linear deformation field is assumed. It is shown that precise design of the thicknesses of the individual layer is a prerequisite for controlled curvature. The procedure we have developed predicts the curvature of multilayered polysilicon systems with good precision.