Annealing reduces silicon nitride microwave-frequency two-level systems loss*

Silicon nitride Si₃N₄ is used in a wide variety of quantum applications at microwave-frequencies for its dielectric, thermal, and mechanical properties. We report measurements of the dielectric loss of stoichiometric Si₃N₄ with superconducting circuits, and explain the material’s behavior with the resonant and relaxation components of the two-level systems (TLS) model. Post-deposition annealing of the Si₃N₄ film drastically reduces the loss compared to as-deposited films. Infrared spectroscopy points to the presence of hydrogen impurities in the as-deposited film as the origin of the TLS loss.