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

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Abstract
We describe an improved interconnect technology for the fabrication of multiturn flux transformers from YBa2Cu3O7-x-SrTiO3-YBa2Cu3O7-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 YBa2Cu3O7-x film at 77 K is (2-3)*106 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.

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