Fabrication of YBa₂Cu₃O_x thin-film flux transformers using a novel microshadow mask technique for in situ patterning

A novel microshadow mask technique for in situ patterning of multilayers is presented. It is ideally suited to fabricate YBa₂Cu ₃O_x(YBCO) and insulator lines with gently sloping edges, needed for high quality insulated superconducting crossovers. The critical current density j_c (T=77 K) of a YBCO/SrTiO₃/YBCO crossover exceeds 2×10⁶ A/cm² in both the bottom and the top YBCO stripline. The insulating SrTiO₃ layer of 200 nm thickness displays a high resistivity of ρ>10⁸ Ω cm (T=77 K). The extremely smooth morphology of the edges has been revealed by cross sectional transmission electron microscopy, indicating a stepflow mechanism of YBCO growth. Multiturn flux transformers with a 15 μm linewidth input coil spiral have been fabricated by this microshadow mask technique. A transformer with a pickup loop area of 7 mm² has been coupled to a 1 mm² washer dc SQUID in flip chip geometry. In comparison to the bare SQUID a magnetic flux gain factor of 9 has been obtained. The white noise level of this setup was determined to be 8×10^-5 Φ₀/ Hz^1/2 at 77 K. It was entirely due to the intrinsic noise of the employed dc SQUID itself. The 1/f noise level increased a factor of 2.