Cryogenic performance of micromachined silicon rocking actuator for tuning a superconducting resonator

Abstract

A micromachined silicon rocking actuator is used to tune a high temperature superconducting resonator. Its performance at cryogenic temperatures from 77 to 30 K is studied. A significant increase in the quality factor of the resonator, from 376 to 13876, is observed. It is confirmed that the change in resistance of the silicon tuning probe with temperature is the main factor responsible for the improvement. At low temperature, the silicon becomes an excellent dielectric, which makes it a good material for low-loss tuners.Introduction: High temperature superconductor (HTS) resonators exhibit very high quality-factors (Q-factor) at cryogenic temperatures owing to the extremely low surface resistance. Fixed frequency planar HTS resonator filters have been extensively studied [1]. Tunable filters capable of changing the resonator operation frequencies over a wide frequency range are useful for applications in flexible and reconfigurable systems. In the case of tunable HTS filters, several enabling techniques have been demonstrated, such as MEMS switched capacitors [2], nanomotor driven HTS coated tuners [3], and ferroelectric materials [4]. One common challenge is to minimise the degradation of the high Q-factors of the HTS circuits caused by introducing the tuning elements. Various successes have been achieved. Previously we investigated a silicon dielectric-tuner by integrating silicon MEMS actuators with HTS resonators [5,6]. A comb-drive actuated horizontal tuner was first demonstrated in [5]. Later a rocking vertical actuator was reported showing improved tunability and reliability [6]. In both cases, at 77 K, the Q-factors of the HTS resonators were significantly reduced by the resistance of the silicon used (about 20 V . cm based on four-point-probe measurement at room temperature). In this Letter, temperature-dependent cryogenic behaviour of the rocking actuator is studied, this is of interest because temperature affects the carrier density [7] in the silicon, and therefore alters the level of power dissipation and Q-factor of the resonator.

In situ tuning of coupled superconducting microwave resonators

“…There have been many reports of systems to adjust the frequency of a single CMR in situ, utilising magnetic fields 12 , ferroelectric layers 13 , ferromagnetic layers 14 16 and those based on moving a superconducting pin towards and away from the CMR using piezoelectric motors 17 are expensive and require considerable electromagnetic shielding if they are to be operated at very low temperatures. We report here a mechanical system that is simple, easy to construct and is effective in tuning two coupled resonators through the avoided crossing without observable degradation of the high Q, and find very good agreement with simulations.…”