In situ deposited Al₂O₃ passivated and native AlO_x capped aluminum resonators – comparative studies

Superconducting (SC) devices form a leading platform for quantum computing, where the coherence time is often limited by losses at the surfaces and interfaces. Prior studies have shown that diffusion of ambient species from the surface into the metal-substrate interface along grain boundaries in SC films significantly increases the losses. Approaches such as increasing film thickness or enlarging grain size via higher growth temperatures effectively suppress this diffusion. Recently, replacing native AlO_x with deposited Al₂O₃ has emerged as a promising route for protecting SC films, especially in Josephson junctions with thin electrodes and limited thermal budgets.

We present comparative studies of Al resonators capped with in situ deposited Al₂O₃ and native AlO_x. With Al₂O₃ caps, resonators show internal quality factors (Q_i) exceeding 10⁶, significantly higher than their AlO_x-capped counterparts. Moreover, these resonators demonstrate remarkable aging resilience, maintaining Q_i ≈ 5 × 10⁵ after seven months of air exposure, whereas AlOₓ-capped devices degrade to Q_i ≈ 1 × 10⁵ within two weeks. Even for 10-nm films, Al₂O₃ passivation yields Q_i ≈ 5 × 10⁵, outperforming the AlO_x counterparts and highlighting superior protection of thin SC films. Our findings underscore the crucial role of in situ deposited Al₂O₃ in mitigating surface and interface degradation, providing a robust pathway toward high-coherence and long-term stable SC quantum devices.