RF MEMS for Cryogenic Applications

RF MEMS (Radio Frequency Microelectromechanical systems) has been a rapidly expanding field for the last decade. This expansion is driven by the need for low power, high linearity and high performance devices for wireless, defense, and scientific applications. NASA has a vested interest in RF MEMS switches for signal routing and phase modulation in their next generation of Cosmic Microwave Background (CMB) measurements. Recently detected anisotropies in the polarization of the CMB by the BICEP2 detector have provided insight to the structure and energy of the inflationary period right after the Big Bang and NASA looks to expand on that success with the Beyond Einstein Inflation Probe which will map the polarization of the CMB over the entire sky.
This work investigates the the fabrication and reliability of RF MEMS devices as possible candidates for phase modulators operating on a satellite platform for CMB measurements at temperatures close to absolute zero. RF MEMS DC-contact series switches have been implemented from DC - 30 GHz on superconducting Nb microstrip on 5 um silicon-on-insulator (SOI) substrates. The devices were built using a novel fabrication process that uses the device layer of the SOI as the microstrip dielectric. Tunable resonators (fc = 16GHz) have been designed and fabricated using the RF MEMS switches to tune fc in 1 GHz steps. RF performance of the switches and resonators are presented.
The contact resistance (Rc) when the switch is closed is also of interest because variations in Rc could introduce errors into CMB measurements. A closed-cycle cryostat has been modified to measure shifts in Rc as the devices are actuated. Reliability testing of the devices at 4 K show that the devices are currently unreliable for long term operation at cryogenic temperatures, with device lifetimes less that one million contacts. Device failure is in the form of stiction, or a permanently closed switch that will no longer actuate. Charging of the substrate underneath the MEMS switch has been determined as the main cause of stiction at 4 K. Results and possible solutions to prolong operation lifetime are discussed.