Metal capping for quality improvement of resonator – the art of choosing the right metal

Aluminum (Al) thin films have been widely used in superconducting circuits and for forming Josephson junctions due to their low kinetic inductance, stable oxide formation, and metal softness. However, they are prone to pinholes, allowing oxygen penetration through grain boundaries and impacting the metal-substrate interface. Increasing film thickness reduces oxidation by promoting larger grains.[1–3] Various pre-processing methods have been utilized to improve the interface characteristics, as the metal-substrate interface plays a critical role in the quality and performance of superconducting quantum devices. [4] To address these issues, we investigated a self-limiting oxidation metal as a capping layer to reduce oxidation at the aluminum-substrate interface. Capped aluminum films showed a crystal orientation shift from [111] to [002], indicating that the capping layer affects nucleation and growth, promoting a different orientation. We are examining how capping layer thickness impacts transition temperature and resonator quality factors and its role as an adhesive layer in altering aluminum's crystal orientation. We will present detailed interface characterizations and the performance of the resonator quality factor at the meeting. Detailed interface characterizations and resonator performance data will be presented.