High quality factor RF MEMS tunable capacitor

This paper presents the design of a tunable capacitor that integrates two vertical comb-drive tunable capacitors and a parallel plate tunable capacitor. The parallel plate tunable capacitor is in the middle of the comb-drive tunable capacitors. The vertical comb-drive part has sets of fixed and moving fingers while the parallel plate has fixed and moving plates. The capacitor is fabricated using the MetalMUMPs microfabrication process, which has only one thick structural layer i.e. metal (20 μm nickel and 0.5 μm gold). The nickel of the metal layer is subject to residual stress gradients along its thickness. After release of the metal layer, the stress gradients bend the curve-up beams to raise the moving fingers and the moving plate of the capacitor above the substrate and the fixed plate. Hence, one structural layer (i.e. the metal) is used to form the moving and the fixed comb fingers without requiring two structural layers. Vertical comb-driven tunable capacitors offer high capacitance density when a large number of fingers with narrow gap are used. Therefore, high capacitance ratio and high quality factors can be achieved by using vertical comb-drives with in a relatively small device area compared to lateral comb-drives. The parallel plate driving part increases the displacement and capacitance ratio while reducing the actuation voltage required for driving the capacitor. The quality factor and tuning ratio of the fabricated tunable capacitor are 118.5 and 101% at 0.8 GHz; respectively at a driving voltage of 100 V. Based on the experimental results of the fabricated tunable capacitor, an optimized design is presented for both high tuning ratio and high quality factor. The tuning ratio of the optimized capacitor is found to be 143.1% at a “pull-in” displacement of 9.5 µm and a “pull-in” voltage of 90 V. The quality factors of the optimized capacitor are 520 at 0 V and 363.5 at 90 V, respectively at 0.8 GHz. The tunable capacitor achieves a displacement of more than one-half of the gap between the parallel plates without “pull-in” effect.

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