Small-deflection analysis of microelectromechanical varactors

Voltage controlled oscillators (VCO) with high quality (Q) factors require wide tuning ranges and low phase noise performance which depends on the tunability of the capacitors in the LC tank circuit. Microelectromechanical (MEM) varactors are being considered as replacements for VLSI on-chip variable capacitors due to the susceptibility of the latter to phase noise and low Q-factors. This paper presents a theoretical model for the determination of the pull-in voltage in a surface micromachined MEM varactor which is a factor directly affecting the tunability of the device. The associated electrostatic spring softening effects due to the parallel-plate and fringing field capacitances have been taken into account in developing the semi-analytical model with extensive finite element method (FEM) and lumped parameter simulations being carried out.