Hydrogenated Amorphous Silicon Carbide: A Low-loss Deposited Dielectric for Microwave to Submillimeter Wave Superconducting Circuits

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.