Low Temperature Superconducting Tunable Bandstop Resonator and Filter Using Superconducting RF MEMS Varactors

A novel niobium-based superconducting RF microelectromechanical system (MEMS) varactor is presented, and its mechanical performance is characterized at both room and cryogenic temperatures. The device is amenable to integration with superconducting microelectronics (SME) technology. The RF performance of the varactor at cryogenic (4 K) temperatures indicates capacitance variations from 40 fF to 0.46 pF. Hence, the varactor is used to design a monolithically integrated tunable resonator, whose measured results at cryogenic temperature show a sweep of frequency from 2.62 to 2.54 GHz and a discrete shift from 2.54 to 1.95 GHz, when the biasing voltage varies from 0 to 58 V. Using the same fabrication process, a fixed niobium-based three-pole bandstop filter is designed with a center frequency of 2 GHz and a size of 5 mm × 0.85 mm. Tunable versions of the three-pole filter using semiconductor varactors and monolithically integrated MEMS varactors are also designed and characterized at 4 K. The results of the tunable bandstop filters are analyzed both theoretically and experimentally.