Signal Power-Insensitive Analog MEMS Tunable Capacitor by Immobilizing the Movable Plates

Abstract

This paper presents an extremely power-insensitive microelectromechanical systems (MEMS) tunable capacitor equipped with immobilization capability in the moving plates when an radio frequency signal is flowing. The proposed tunable capacitor is similar to the conventional metal-insulator-metal capacitor, but the top metal plate is capable of moving laterally as well as vertically; it moves to the left and right to set the capacitance value by modulating the overlap area between the top and bottom plates (analog tuning and high capacitance tuning ratio are the merits), and then the top plate is pulled down to be immobilized resulting in remarkable robustness to the signal power, as well as high capacitance value. The proposed tunable capacitor, which was fabricated by metal surface micromachining, showed the tuning ratio of 181% at 2 MHz (470-852 fF) and 194% at 1 GHz with lateral and vertical actuation voltages under 50 V. It also exhibited small capacitance change against the radio frequency (RF) signal power; the maximum capacitance variation by the signal power of up to 9 W was ˂5.5% over the full tuning range, which is the record-high power insensitivity among the analog MEMS tunable capacitors. This excellent power insensitivity is due to the immobilizing capability of the proposed tunable capacitor. The proposed tunable capacitor maintained the set capacitance value with a variation of ˂8% over 10 million cycles under 1-W signal (cold switching condition). The design, modeling, fabrication, and measurements, including RF characteristics, are all described in this paper.