Reducing materials- and processing-induced decoherence is critical to the development of utility-scale quantum processors based on superconducting qubits. Here, we report on the impact of two fluorine-based wet etches, which we use to treat the silicon surface underneath the Josephson junctions (JJs) of fixed-frequency transmon qubits made with aluminum base metallization. Using several materials analysis techniques, we demonstrate that these surface treatments can remove germanium residue introduced by our JJ fabrication, with no other changes to the overall process flow. When combined with argon ion milling before JJ fabrication, these surface treatments result in significantly improved energy relaxation times for the highest-performing process, with median $T_1=334\mathrm{\mu }\mathrm{s}$, corresponding to quality factor $Q=6.6\times {10}^6$. This result suggests that the metal-substrate interface directly underneath the JJs was a major contributor to microwave loss in these transmon qubits prior to the integration of these surface treatments. Furthermore, this work illustrates how quantum device performance metrics, in conjunction with materials analysis targeted at identifying the impacts of specific processing steps, can be used to improve the performance of superconducting qubits.

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• Quantum information with solid state qubits
• Superconducting devices
• Superconducting qubits
• Surfaces
• Thin films
• Etching
• Film deposition
• Focused ion beam