Influence of intrinsic film stress on superconducting quantum circuit performance

The reliable fabrication of high coherence superconducting qubits requires the identification and mitigation of many sources of microwave losses. In recent years, much work has gone into reducing the losses of the base metalization layer, including chemical surface treatments, etching optimization, and various capping processes. One unexplored avenue is reducing base layer losses by tuning the intrinsic stress of the superconducting thin film. In this work, we tune the intrinsic stress of DC sputtered Nb thin films by changing the pressure of Ar gas in the chamber during sputter deposition. Using wafer-bow profilometry and asymetric X-ray diffraction, we show that we can produce films in both the compressive and tensile stress regimes. Films are characterized with x-ray diffraction, atomic force microscopy, RRR, superconducting critical temperature, and microwave loss measurements. A two-fold decrease of coplanar waveguide resonator losses are observed for highly tensioned films versus highly compressed films.