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 mu m 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 mu m are observed for 70 mu m-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.<>