Characterization of high-Q laterally moving RF MEMS tuneable capacitor

This paper investigates an implementation of RF MEMS tuneable capacitors based on the lateral displacement of the sidewalls of a three-dimensional micromachined coplanar transmission line. The concept achieves high Q since the mechanical actuation elements are completely de-coupled from the RF path, which is achieved by capacitive coupling of the ground signal to the moving sidewall. The fabricated capacitor has a tuning range of 40.53 to 60 fF (C_max/C_min = 1.48) in three discrete steps. This paper reports for the first time on Q-factor measurements, reliability, self-actuation robustness measurements, and linearity analysis. The Q-factor was determined by a transmission-line resonator as 88 at 40 GHz, resulting in a Q×f product of 3.52×10¹² which is higher than of any previously reported tuneable capacitor. Measurement data demonstrates high self-actuation robustness of 41.5 dBm at actuation and release voltages of 30.7 V and 21.15 V, a pull-in time of 60 μs, and a mechanical resonance frequency of 5.3 kHz. The linearity of the device has been determined to an IIP3 of 71.4, 74.4 and 91.0 dBm for the three states of the capacitor. Reliability tests up to 1 billion cycles show no detectable performance degradation of this all-metal tuneable capacitor concept.