Investigation of basic elements and devices in multilayer technology for HTS digital RSFQ circuits

Complex high temperature superconductor digital RSFQ circuits require a multilayer technology that is compatible with freely positionable Josephson junctions. For RSFQ circuits, especially small inductances are required. This can be achieved by using superconducting striplines on a YBCO groundplane. We have produced and examined all basic elements like, e.g., superconducting cross-overs, vias, pinhole free insulation layers and on-chip resistors that are necessary to design and fabricate HTS RSFQ circuits. All structures have been successfully tested and integrated with ramp type junctions with PBCO barriers. For the superconducting lines over the groundplane an inductance of 0.8 pH/square was measured which shows the advantage of a superconducting groundplane. Based on this technology we fabricated a simple multilayer circuit consisting of two magnetically coupled d.c.-SQUIDs. Here we used bicrystal junction stacks due to the smaller spread of junction parameters. The circuit allowed essential operations like generating, erasing and detecting a single flux quantum. One d.c.-SQUID served as storage loop while the other was kept permanently in the voltage state and was used to determine the internal flux state of the storage loop.