  Superconducting resonators used in millimeter-submillimeter astronomy would
greatly benefit from deposited dielectrics with a small dielectric loss. We
deposited hydrogenated amorphous silicon films using plasma-enhanced chemical
vapor deposition, at substrate temperatures of 100\deg C, 250\deg C and 350\deg
C. The measured void volume fraction, hydrogen content, microstructure
parameter, and bond-angle disorder are negatively correlated with the substrate
temperature. All three films have a loss tangent below $10^{-5}$ for a
resonator energy of $10^5$ photons, at 120 mK and 4-7 GHz. This makes these
films promising for microwave kinetic inductance detectors and on-chip
millimeter-submilimeter filters.
