The role of interfaces in c-axis microbridges

The c-axis microbridge (CAM) junction is a c-axis interconnect between two superconducting layers. This is an attractive geometry, with low parasitic inductance suitable for single flux quantum logic circuits. Such junctions can be made by the planarisation of a mesa or growth into a window in the insulator. Typical critical current density (J_c) values of the mesa type junctions are ~10⁵ A/cm² at 70 K. However, it has been found that the J_c in the window junctions can be as high as ~10⁶ A/cm ² at 80 K if the window is milled deep into the superconducting base layer. The I-V curves obtained have been modelled using conventional microbridge theory and reasonable agreement with experiment obtained. These results show that the properties of the previous CAM junctions have been determined by a barrier at the grown interface. This places the mesa junctions in the class of `interface engineered´ junctions. Secondary ion mass spectrometry has shown that the ion milled YBCO surface is Y rich, and deficient in Ba and Cu. This is due to the different milling rates of the cations. The effect of the milling and annealing conditions on this interface have been investigated