ELECTRON-BEAM LITHOGRAPHED STRAIGHT AND WAVY YBA2CU3O7 STEP-EDGE JUNCTIONS

We have developed a process for the fabrication of YBa2Cu3O7 (YBCO) st ep edge junctions. This process greatly improved the uniformity of the step angles, the straightness and sharpness of the step edges on LaAl O3 substrates. We used an electron beam lithography defined amorphous carbon film as an ion milling mask. By studying junctions made across straight and wavy step edges, we showed the importance of the straight ness on the quality of YBCO junctions. Junctions fabricated across the se improved step edges showed less j(c) variations (a factor of 2-3 co mpared to > 10 for Standard step edge junctions) and improved current distribution. We increased the McCumber constant beta(c) by varying th e step height and the YBCO film thickness (h and t). Hysteresis was ob served in the I-V characteristics at 4.2 K for junctions with h = 240 nm and t = 180 mn. The capacitance per unit area was calculated to be 15 fF/mu m(2). The magnetic field (H) dependence of the critical curre nt (I-c) showed well-defined periods with a main peak in the center. T he I-c in the first minimum can be suppressed to 20-40% in most cases. For some junctions, I-c can also be suppressed to zero at higher temp eratures. However, we did not observe a perfect Fraunhofer pattern in the I-c vs. H curve. The field period Delta H was found to have a w(-2 ) dependence on the junction width (w).