Design, simulation and analysis of a digital RF MEMS varactor using thick SU‑8 polymer

This paper presents the design and simulation of a 5-bit RF MEMS digital variable capacitor using SU-8 material to achieve both high capacitance ratio and Q-factor. The proposed varactor design implements five capacitive shunt switches over a co-planar waveguide (CPW) line. A novel horizontal truss fixed-fixed beam design is employed to achieve RF power handling up to 2.2 W. The simulated pull-in and lift-off voltages are 30V and 16 V respectively. The contact area of the capacitor between the beams and the CPW line are varied in a binary configuration to realise 32 capacitance steps ranging from 102.23 fF to 3.57 pF. In order to increase the capacitance ratio of the varactor, SU-8 polymer has been utilised to form the base structure of the capacitor due to its low dielectric constant hence reducing the overall minimum capacitance. Moreover, the inclusion of semi-elliptical slot at the ground of CPW and variable sizes of CPW line further reduce the parasitic capacitance. To improve the reliability of the variable capacitor design, a new aluminium (Al) stopper design is used to prevent contact between the beams and the pull-down electrodes. Mechanical simulations have been carried out to investigate the effects of in-plane residual stress and stress gradient on the spring constant and the initial displacement of the beam structure. The CPW line and the beam are made of 2μm thick aluminium to improve the Q-factor. The overall size of the varactor is 740 μm × 653 μm. The proposed RF MEMS varactor could be integrated in reconfigurable filters, phase shifters and matching networks targeting future multi-standard wireless communication systems.