Design of a Low Voltage and High Capacitance Tuning Ratio Electrothermally Actuated RF-MEMS Varactor using Standard Microfabrication Constraints

This paper presents the design and analysis of a symmetric radio frequency microelectromechanical systems (RF-MEMS) varactor mounted on a coplanar waveguide (CPW) for a phased array application at low gigahertz frequencies. This varactor is driven by single crystal silicon (SCS) electrothermal actuator capable of generating large contact forces, paired with a compliant mechanical amplifier (CMA) for an amplified and linear bidirectional in-plane motion. The RF-MEMS varactor, utilizing parallel sixty-four beam actuators and a micro displacement amplifier, having a loaded amplification ratio of 7.5, requires a driving voltage of 1.3 V for a 3 μm displacement. The varactor exhibits a high achievable tuning ratio of 1100 %. The RF characteristics of the device show a return loss of better than -10 dB at 2 GHz and an insertion loss better than -1 dB at 2 GHz. The device is designed according to SOI MUMPS constraints, covering an area of 4×4 mm. The varactor avoids problems related to stiction and dielectric charging, by measuring the nominal capacitance for a 0.5 μm gap.