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

This paper presents the design, simulation and analysis of a 5-bit RF MEMS digital variable capacitor using thick SU-8 polymer to achieve both high capacitance ratio (\(\varvec{C}_{\varvec{r}}\)) and Q-factor. The proposed varactor design consists of five capacitive shunt switches loaded over a co-planar waveguide (CPW) line. This results 32 capacitance steps ranging from 102.23 fF to 3.57 pF (\(\varvec{C}_{\varvec{r}}\) = 35). The high capacitance ratio of the varactor is achieved by implementing SU-8 polymer as the base structure of the capacitor due to its low dielectric constant (\(\varvec{\varepsilon}_{\varvec{r}}\) = 4) hence reducing the overall minimum capacitance (\(\varvec{C}_{{\varvec{min}}}\)). The optimization of the SU-8 thickness has been carried out to get the optimum capacitance ratio and Q-factor. 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 and analysis 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 proposed horizontal truss fixed–fixed beam structure. Compared to the conventional doubly clamped solid beam design, the proposed beam has less sensitivity to in-plane residual stress. This makes the pull in voltage to be less affected by the stress due to the fabrication. The simulated pull-in and lift-off voltages are 30 and 25 V respectively while the estimated switching time for the beam is 4.64 μs. 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.