Compact and high-accuracy RF MEMS capacitive series devices

摘要

MEMS technology is presented as a promising technology to realize high Q variable capacitors and RF switches with high performance and with high levels of integration. These devices are key elements for systems like phase shifters, tunable filters and matching networks. However, the reliability and the yield of the RF MEMS devices remain the key limiting factors holding the MEMS technology from spreading in the industrial applications. From a RF designer's point of view, reliability and yield are closely related to the accuracy of the definition of the up- and down-state capacitances of the devices. In this paper, we propose a novel compact series capacitive structure with improved predictability and RF performance. The new design mimics a clamped-clamped bridge to lower its sensitivity to the process-induced stress gradient in the up-state. The shape of the device and its consequent parameters, e.g. up- and down-capacitances, are thus more accurately defined even in presence of non-ideal clamping conditions. Unlike the series switchable capacitors with transverse restraining bridge, the novel device does not suffer from high frequency parasitic resonances. Finally, the novel device implements the floating top metal. This allows accurately defining the down-state capacitance of the design at will. Boosted series capacitive switches with inline-restrained cantilever beams have been realized and measured. The isolation is better than 20dB until 1GHz without optimization. The insertion loss in the down-state is better than 0.2dB in the range 1-20GHz. It further slowly and continuously decays to reach 0.4dB at 40GHz without any resonances.