Reducing quantum-regime dielectric loss of silicon nitride for superconducting quantum circuits

The loss of amorphous hydrogenated silicon nitride ( a-SiN x : H ) is measured at 30 mK and 5 GHz using a superconducting LC resonator down to energies where a single-photon is stored, and analyzed with an independent two-level system defect model. Each a-SiN x : H film was deposited with different concentrations of hydrogen impurities. We find that quantum-regime dielectric loss tangent tan δ 0 in a-SiN x : H is strongly correlated with N-H impurities, including NH 2 . By slightly reducing x we are able to reduce tan δ 0 by approximately a factor of 50, where the best films show tan δ 0 ≃ 3 × 10 − 5 .