  Low-loss deposited dielectrics will benefit superconducting devices such as
integrated superconducting spectrometers, superconducting qubits and kinetic
inductance parametric amplifiers. Compared with planar structures, multi-layer
structures such as microstrips are more compact and eliminate radiation loss at
high frequencies. Multi-layer structures are most easily fabricated with
deposited dielectrics, which typically exhibit higher dielectric loss than
crystalline dielectrics. We measured the sub-kelvin and low-power microwave and
mm-submm wave dielectric loss of hydrogenated amorphous silicon carbide
(a-SiC:H), using a superconducting chip with NbTiN/a-SiC:H/NbTiN microstrip
resonators. We deposited the a-SiC:H by plasma-enhanced chemical vapor
deposition at a substrate temperature of 400{\deg}C. The a-SiC:H has a mm-submm
loss tangent ranging from $0.80 \pm 0.01 \times 10^{-4}$ to $1.43 \pm 0.04
\times 10^{-4}$ in the range of 270 to 385 GHz. The microwave loss tangent is
$3.2 \pm 0.2 \times 10^{-5}$. These are the lowest low-power sub-kelvin loss
tangents that have been reported for microstrip resonators at mm-submm and
microwave frequencies. We observe that the loss tangent increases with
frequency. The a-SiC:H films are free of blisters and have low stress: $-$20
MPa compressive at 200 nm thickness to 60 MPa tensile at 1000 nm thickness.
