Liquid Nitrogen Temperature DC SQUID’s Using Single-Target-Sputtered Films
For the reliable fabrication of thin-film devices with high temperature superconductors, we have developed a simple, single-target rf-magnetron sputtering process involving an unconventional sputtering geometry. The process allows reproducible formation of high transition temperature YBa2Cu3O7-x films from nearly stoichiometric targets and lends itself both to film growth with high temperature post anneals and to low temperature in situ film growth. The post-anneal process has been optimized to routinely yield epitaxial films on lattice matched substrates that are fully superconducting at 86~91 K with current densities at 77 K up to 8x105 A/cm2. Single-level dc SQUID’s have been made by patterning loops in lower current density films with conventional photolithography and ion milling. Noise measurements at 77 K indicate a flux noise level of 3 × 10-4Φo/√Hz at 20 Hz, dominated by low frequency (l/f) noise. The origin of the SQUID noise is elucidated by studying current-biased thin film stripes, which also show low-frequency noise. Both the current-voltage characteristics and the noise voltage spectrum of the stripes are consistent with expectations from flux-flow but not from Josephson coupling between grains. Further improvements in SQUID noise performance will probably require the use of higher current density films with localized Josephson regions.
KeywordsCritical Current Density Dynamic Resistance Flux Flow Flux Noise Stoichiometric Target
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