Investigation of the Superconducting Properties of Nb Filmscovered by PECVD a-Si:H Layers for Superconducting Qubit Application

Hydrogenated amorphous silicon (a-Si:H) grown by PECVD has a lower loss tangent (tan _) among conventional dielectrics (such as SiO2 and SiNx) 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 (Tc) and residual resistivity (_0) of 20 nm, 50 nm and 100 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 di_raction. The high process temperature (250_ C) and the presence of energetic hydrogen ions during the a-Si:H layer growth caused a decrease of Tc and increase of _0 through two physical processes: 1) oxygen di_usion from the surface Nb oxides and 2) hydrogen di_usion inside the Nb films. The degradation of Tc 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 a s surface treatments to protect the Nb filmsduring PECVD. It was demonstrated that both methods markedly reduce oxygen and hydrogen di_usion into Nb films during a-Si:H deposition, but the a-Si layer was more e_ective to protect the Nb films.