Fabrication of functional structures on thin silicon nitride membranes

A process to fabricate functional polysilicon structures above large (4 x 4 mm(2)) thin (200 nm), very flat LPCVD silicon rich nitride membranes was developed. Key features of this fabrication process are the use of low-stress LPCVD silicon nitride, sacrificial layer etching, and minimization of membrane stress and deformation after fabrication, which was done by complete removal of the sacrificial layer, avoidance of sharp corners in the design of the functional polysilicon structures, and annealing of the polysilicon after doping. A polysilicon layer that was directly attached to the silicon nitride membrane was used. The polysilicon was doped locally to separate the rotors from the stators electrically, preventing short-circuiting. The layer was patterned with a hexagonal supporting structure that serves three main functions: membrane strengthening, electrical wiring and microactuation. As a demonstration of the technology a pseudo hexagonal polysilicon framework with embedded comb-drive actuated microshutter suspended above a similar to 150-nm thick membrane, was successfully fabricated.