Self-adjusting microstructures (SAMS)

Composite LPCVD polysilicon/silicon nitride flexures have been fabricated on the sidewalls of previously patterned polysilicon mesas by anisotropic reactive-ion etching. Cantilever beams 450 nm thick (150 nm of silicon nitride and 300 nm of polysilicon) and 2.5 μm wide (the mesa height) were fabricated. Upon release from the sidewall, the cantilever deflects laterally away from the mesa due to a large built-in bending moment arising from the compressive residual stress in the polysilicon layer and the tensile residual stress in the silicon nitride layer. End deflections of about 20 μm are observed for 70 μm-long cantilevers. This self-adjusting microstructure (SAMS) makes use of residual stresses in thin films to reduce intercomponent clearances or to apply preloads in micromechanical systems. The authors present a design theory for SAMS, describe the fabrication process in detail, and discuss the results of initial experiments