Design considerations in MEMS parallel plate variable capacitors

This paper presents some important design considerations usually ignored that need to be taken into account when designing MEMS parallel plate variable capacitors. Explicitly, it introduces an accurate method that incorporates parasitic and fringing capacitances in calculating the tuning range. Secondly, it shows, by the finite element method, that the etching hole effects for two-plate and three- plate varactors on the capacitance are low and can be ignored in the initial design stages. Finally a novel closed form expression, which calculates the capacitance of a varactor with a warped top suspended plate due to residual stress, is introduced; the expression is based on a second order model.