Design and simulation of a high tuning range MEMS digital varactor using SU-8

Abstract

This paper presents the design and simulation of a 5-bit RF MEMS digital variable capacitor with high capacitance ratio (Cr) of 43 using SU-8 material. The proposed design implements five capacitive shunt switches over a coplanar waveguide (CPW) line. A novel horizontal truss fixed-fixed beam design is proposed to achieve RF power handling up to 2.2W. The simulated pull in and lift off voltages are 24.3V and 16V 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 84.5fF to 3.68pF (Cr=43). 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 (er=4) hence reducing the overall minimum capacitance (Cmin). 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 proposed variable capacitor design, a new Aluminium (Al) stopper design is employed to prevent contact between the beams and pulldown electrodes. A 2µm thick Aluminium layer is implemented as a CPW. Aluminum is also used to implement a 2µm beam structure to improve the Q-factor. A 0.25µm of silicon nitride (Si3N4) is used as the dielectric layer. The overall size of the varactor is 740µm × 653µm.