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A curl-up-plate microelectromechanical system (MEMS) varactor with an almost linear response and high tuning capacitance ratio is presented. The curl-up in the top plate is realized by the residual stress in the two layers that construct the top plate of the varactor. The linear response is achieved by having the curl-up plate designed to relax on the bottom plate and by having unanchored cantilever beams that prevent the pull-in, while applying a dc bias voltage. The developed varactor exhibits a low parasitic capacitance through etching the lossy substrate underneath the varactor's plates. A thin alumina dielectric layer of 100-nm thickness is deposited using an atomic-layer-deposition technique to provide electrical isolation between the two plates. This MEMS varactor exhibits an almost linear capacitance with a tuning ratio of 5 : 1.