New contact design for the ex situ fabrication of small size, low resistivity normal metal contacts to epitaxial c-axis YBCO films

We report a new method for the ex situ fabrication of low resistance ohmic contacts between planar epitaxial c-axis YBCO films and Ag/Au metallizations. The process involves the etching of slots into the superconductor prior to metal deposition and annealing at 450°C. We confirmed the optimum dimensions of the superconductor line and slot width to be 1 μm. The patterning processes were investigated which permitted us to fabricate these dimensions with no discernible degradation of the high temperature superconductors (HTSC) edges and surface degradations below 30 nm thickness. Specific contact resistivities in the 10^-6...10^-7 Ω·cm ² region have been achieved which is an advantage over planar contacts of up to two orders of magnitude and can be obtained for contact pads of widths above 10 μm. Contacts provided with slots are characterized by a significant increase in homogeneity and reproducibility compared to annealed planar contacts. The temperature and size dependence of the contact resistances of planar and slotted contacts have been compared and studied as a function of film morphology and contact metal. The resulting contact resistance can be related to the interface resistance in band c-directions as well as to the formation of point contacts and a contribution of the spreading resistance of the contact layer