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

Abstract

Low temperature dielectric loss of amorphous hydrogenated silicon nitride (a-SiN$_{x}$:H) is studied in a quantum-regime and correlated with hydrogen defects in the film. The loss is measured at 30 mK at approximately 5 GHz using a superconducting LC resonator containing a-SiN$_{x}$:H dielectric in a parallel-plate capacitor down to the energies where a single-photon is stored, and analyzed with an independent two-level system (TLS) defect model. Each a-SiN$_{x}$:H film was deposited with inductively-coupled PECVD where the N$_{2}$ to SiH$_{4}$ precursor gas ratio is varied to produce films with different hydrogen defect concentrations. We find that quantum-regime dielectric loss in a-SiN$_{x}$:H is strongly correlated with NH$_{2}$ impurity and are able to reduce the low-temperature low-power loss of a-SiN$_{x}$:H by more than a factor of 30 by reducing $x$. With this method, the lowest loss tangent of a-SiN$_{x}$:H obtained at 5 GHz in the quantum-regime is approximately 3 $\times$ 10$^{-5}$.