Novel YBa₂Cu₃O_7-x and YBa ₂Cu₃O_7-x/Y₄Ba₃O ₉ multilayer films by bias-masked "on-axis" magnetron sputtering

In situ YBa₂Cu₃O_7-x (YBCO) films have been fabricated on SrTiO₃ (001) and LaAlO₃ (001) substrates by on-axis biased-radio-frequency magnetron sputtering in Ar-10% O₂ at total pressures as low as 3 Pa (3×10^-2 mbar) and a deposition rate 210 nm/h. Negative oxygen ion-resputtering has been considerably reduced by introducing a biased copper mask between the substrate and target. The surface morphology and physical properties of the films are greatly improved on applying a positive dc substrate bias with respect to the grounded deposition chamber. We have obtained superconducting YBCO films with transport critical current as high as 10⁶ A/cm² at 77 K and low normal-state resistivity by this approach. Scanning tunneling microscopy analyses of the films with the best superconducting properties reveal a spiral growth mechanism. However, films deposited by negative dc bias under identical sputtering conditions are insulating. From x-ray θ-2θ and rocking curve measurements, we identify the insulating films to be c-axis oriented Y₄Ba ₃O₉ (YBO) films. Furthermore, YBCO films could be grown on the YBO layers without any degradation of T_C and c-axis orientation. This novel bias sputtering feature gives us a unique opportunity to produce superconductor/insulator, YBCO/YBO, multilayers from a single YBCO target.