Fabrication issues in optimising YBa2Cu3O7-xflux transformers for low 1/f noise

We describe an improved interconnect technology for the fabrication of multiturn flux transformers from YBa 2 Cu 3 O 7-x -SrTiO 3 -YBa 2 Cu 3 O 7-x multilayers. The essential improvements are reductions in the thicknesses of the trilayer films, typically to 100 nm, 250 nm and 250 nm respectively, and in the deposition rate, to 0.07 nm/laser pulse. This process yields crossovers in which the critical current density in the upper YBa 2 Cu 3 O 7-x film at 77 K is (2-3)*10 6 cm -2 In situ trilayers exhibited 1/f flux noise levels at 1 Hz below the measurement sensitivity of 15 mu Phi 0 Hz -1/2 , Where Phi 0 is the flux quantum. However the flux noise of trilayers in which each layer had been patterned was significantly higher. The best flip-chip magnetometer had a white noise of 40 fT Hz -1/2 , increasing to 340 fT Hz -1/2 at 1 Hz; the corresponding flux noise levels were 9 mu Phi 0 Hz -1/2 and 75 mu Phi 0 Hz -1/2 .