Hydrogenated amorphous silicon (a-Si:H) grown by PECVD has a lower loss tangent (tan δ) among conventional dielectrics (such as SiO 2 and SiN x ) and hence is considered as the best amorphous dielectric material for superconducting qubit application. The incorporation of PECVD a-Si:H into the Nb technology requires attention due to the possible degradation of the superconductivity of the Nb films. Superconducting transition temperature (T c ) and residual resistivity (ρ 0 ) of 20 nm, 50 nm and

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... 00 nm thick Nb films were measured before and after a-Si:H deposition. The penetration of oxygen and hydrogen inside the Nb films was evaluated from the variation of the lattice parameter obtained by X-ray diffraction. The high process temperature (250 • C) and the presence of energetic hydrogen ions during the a-Si:H layer growth caused a decrease of T c and increase of ρ 0 through two physical processes: 1) oxygen diffusion from the surface Nb oxides and 2) hydrogen diffusion inside the Nb films. The degradation of T c was reduced with the increase of the film thickness. Nitridation of Nb films and deposition of a sputtered thin amorphous silicon layer (a-Si) on the Nb films (in both cases made in situ after the Nb film deposition) were investigated as surface treatments to protect the Nb films during PECVD. It was demonstrated that both methods markedly reduce oxygen and hydrogen diffusion into Nb films during a-Si:H deposition, but the a-Si layer was more effective to protect the Nb films.