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

The loss of amorphous hydrogenated silicon nitride (a-SiNx: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-SiNx:H film was deposited with different concentrations of hydrogen impurities. We find that quantum-regime dielectric loss tangent tan delta(0) in a-SiNx:H is strongly correlated with N-H impurities, including NH2. By slightly reducing x we are able to reduce tan delta(0) by approximately a factor of 50, where the best films show tan delta(0)similar or equal to 3x10(-5).