Trilayer superconducting membranes for high-Q, stress-engineered vacuum-gap mechanics

Chip-integrated mechanical resonators underpin quantum memories and hybrid interfaces, but practical devices need to reconcile strong electromechanical coupling with ultra-low dissipation. On vacuum-gap drumhead capacitors, we previously achieved ultra-high mechanical quality factor with aluminum membranes, but the single-photon coupling was limited by the smallest reliable gap. In this work, we replace the aluminum layer with an aluminum-niobium-aluminum trilayer, whose large tensile stress helps stabilize the fabrication to suspend large-sized membranes at lower gaps and improve mechanical resonators based on dissipation dilution engineering.

Here, we design and fabricate an array of superconducting vacuum-gap drumhead optomechanical resonators with drumhead diameters ranging from 50 to 400 µm. From the dependence of mechanical frequencies, we infer a low-temperature film stress of 1.3 GPa. The high stress supports sub 100 nm gaps and large dissipation dilution: we observe mechanical quality factors above 5 million. Besides, we show the preliminary implementation of soft clamping structures: steering wheel resonators and phononic-crystal membrane resonators.