Investigation of high rate magnetron sputtering of niobium films for Josephson integrated circuits

High rate Dc-magnetron sputtering of 30 nm niobium thin films is shown to be a very good technique to achieve reproducible and stable ground plane, tunnel junctions electrodes, control lines and even resistors for Josepson Integrated Circuits. Pure niobium films have low residual resistivity (ρ
10
k
\simeq 2\mu \Omega
cm), low stresses, and are no longer in the "dirty limit". High quality ratio (
R_{J}/R_{N} \simeq 20
at 4 K). Josephson Junctions were performed with niobium base electrode. Critical temperatures either below 4.2K used for resistors or between 4.2K and 16K can be controlled accurately by reactive magnetron sputtering in an argon-nitrogen plasma. Homogeneous low critical temperature can be also selectively obtained by liquid cathodization or by RF plasma post-treatments on a pure niobium film through a photoresist mask. Nb, Nb(N), Nb(O) and Nb (H) films are studied by TEM and X ray diffraction. The critical temperature change obtained can be explained by lattice BCC structure dilatation or distorsion and by impurities inclusion.