High quality YBCO thin films - laser deposition, co-evaporation, and device fabrication

High quality superconducting YBa 2 Cu 3 O 7-δ (YBCO) thin films were fabricated with ex situ co-evaporation on SrTiO 3 and in situ laser deposition on (001) MgO substrates. The co-evaporated films gave a superconducting transition temperature, T c ∼ 90 K, and a critical current density, j c ∼ 5 × 10 5 A/cm 2 at 77 K after an ex situ post anneal at 890°C. For laser deposited films, T c was above 90 K, and j c , about 10 6 A/cm 2 at 77 K. Two kinds of Josephson weak links were developed. The first kind consisted of a 0.8 μm or wider constriction in a co-evaporated YBCO film. A lift-off stencil was used to pattern the film before the post anneal. The second kind of weak link utilized the inherent grain boundaries in the laser deposited YBCO film. A 70-130 nm wide strip in A1 was deposited across a 4-6 μm wide YBCO bridge. After a 10-20 min long anneal at 300-450°C, the A1 had diffused into the film. We believe that the diffusion occurs mainly along the grain boundaries. By the proximity effect, the normal metal weakened the superconductivity at the grain boundary, and a weak link was formed with I c R n ∼ 0.9 mV at 77 K. Dry an wet etching were also used to pattern some structures.