Improved Interface\nof Niobium Superconducting Resonator\nwith Ruthenium as a Capping Layer

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