Improved Interface of Niobium Superconducting Resonator with Ruthenium as a Capping Layer

The current performance of superconducting circuit-based quantum processors is limited by the poor understanding of interface physics, including native surface oxide formation on the superconducting metal, which causes two-level system (TLS) loss. Niobium (Nb), a superconducting metal with a high energy gap, is an ideal choice for superconducting processors, but unfortunately, it is marred by TLS. Several methods have been proposed to minimize surface oxide on the Nb film, and considerable improvement in TLS loss has been demonstrated. These methods include surface passivation through metal capping, self-assembly of organic molecules, and post-cleaning processes. Among these, metal capping is a suitable choice despite forming a 3-5 nm thick oxide, as self-assembly and post-treatment do not protect the Nb film surface during further fabrication. Here, we have proposed ruthenium (Ru) as a capping layer, forming a self-limiting oxidation with a 0.6 nm oxide thickness and predominantly producing fewer oxide compositions while being chemically resistant for further wafer fabrication processes, thus fulfilling all the criteria of an ideal capping layer. Our investigation suggests that Nb/Ru resonators have great potential as versatile and promising tools for advancing superconducting quantum technologies and integrating quantum interconnects into qubits with minimized TLS loss.