Continuously tunable MEMS capacitor with high tuning ratio and large power handling capability

Abstract

An innovative method is proposed to achieve high power handling capability and high tuning ratio in a tunable MEMS capacitor with two-axis movement. With the lateral movement, the proposed tunable capacitor can tune its capacitance by changing an over-lap area. In case of the vertical movement, it can handle high signal power without the variation of the pre-set capacitance due to less movable space of an air-gap. To predict the characteristics of the proposed tunable capacitor, the proper mechanical and electrostatic models were developed. Also, the structure design for realizing the proposed tunable capacitor was proposed. In addition, to control the stress gradient of the fabricated structure, the dual-step current electroplating (DuCE) method has been newly proposed and successfully demonstrated. The proposed tunable MEMS capacitor, which was fabricated via metal surface micromachining, showed a capacitance tuning ratio of 1.81 in lateral actuation with 50 V and vertical actuation with 50 V, and the variation of less than 5.5 % of the pre-set capacitance under 9W of the signal power. The total response on and off time of the fabricated tunable capacitor were 1.7 ms and 0.65 ms. In reliability tests, the device was well actuated and covered almost full moving range after $1 x 10^7$ cycles in lateral actuation and until $3 x 10^7$ cycles in vertical actuation with the bipolar actuation.