Fabrication and Characterization of High-T c YBCO DC-SQUID Magnetometers with Recycled SrTiO3 Bi-Crystal Substrates Using an Off-Axis RF Magnetron Sputtering Technique

This study fabricated high- T _c dc-SQUID magnetometers using sputtered YBa ₂ Cu ₃ O _y (YBCO) films grown on recycled 22.6°-tilted bi-crystal SrTiO ₃ (STO) substrates for applications on magnetic sensors. High quality recycled STO bi-crystal substrates were obtained via an optimum polishing process with a surface roughness of ∼0.12 nm, which is comparable with that of a new STO substrate. During sputtering deposition, the optimum growth temperature of the YBCO thin film was controlled to make a smooth surface on the artificial boundary, where the critical temperature and surface roughness of the YBCO thin film were 89 K and 2 nm, respectively. The surface morphology of the Josephson junctions in the fabricated high- T _c SQUIDs was probed by AFM and SEM images. The dc-SQUID magnetometers were designed with a directly coupled pickup coil, and the I − V curves, field modulation pattern, and flux noise spectrum were used to characterize the SQUID magnetometers. With optimum processes for thin-film growth and device fabrication, the SQUIDs revealed a modulation voltage of V _PP > 20 μV and flux noise spectrum of 15–25 μΦ ₀ /Hz ^1/2 at 10 Hz at the temperature of 77 K. The properties of the temperature-dependent critical current I _c and peak-to-peak voltage V _pp ( T ) in the magnetically modulated voltage oscillations for the bi-crystal dc-SQUID magnetometers were discussed.