FABRICATION ISSUES IN OPTIMIZING YBA2CU3O7-X FLUX TRANSFORMERS FOR LOW 1/F NOISE

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) X 10(6) cm-2. In situ trilayers exhibited 1/f flux noise levels at 1 Hz below the measurement sensitivity of 15 muPHI0 Hz-1/2, where PHI0 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 muPHI0 Hz-1/2 and 75 muPHI0 Hz-1/2.