Monolithically Integrated RF MEMS-Based Variable Attenuator for Millimeter-Wave Applications

Abstract

This paper reports a millimeter-wave radio frequency (RF) microelectromechanical systems (MEMS)-based variable attenuator implemented by monolithically integrating CPW-based hybrid couplers with lateral MEMS varactors on a silicon-on-insulator (SOI) substrate. The MEMS varactor features a Chevron-type electrothermal actuator that controls the lateral movement of a thick plate allowing precise change of capacitive loading on a CPW line leading to a change in isolation between input and output. The proposed variable attenuator is successfully fabricated on an SOI substrate with a device footprint of 3.8 $\text {mm}\times 3.1$ mm. The fabrication process provides flexibility to extend this module and implement more complex RF signal conditioning functions, thus making it more appealing to realize a wide range of reconfigurable RF devices. The measured RF performance shows that the device exhibits attenuation levels ( $S_{{21}}$ ) ranging from 10 to 25 dB, at the center frequency of 60 GHz with a bandwidth of 4 GHz and a return loss of better than 20 dB. The device involves laterally moving 20- $\mu \text{m}$ -thick structures, hence offers reliable operation and eliminates MEMS problems like stiction, dielectric charging, and microwelding observed in surface micromachined thin membranes.