Influence of the Superconducting Ground Plane on the Performance of RSFQ Cells

Single-flux-quantum (SFQ) digital circuits are mostly based on cells that rely on reliable foundry processes that make use of a superconducting ground plane as a reference for the active elements and the microstrip line interconnects. The quantum of magnetic flux h/2e, associated with the binary information, corresponds to a magnetic field energy density that needs to be localized in space to limit interactions between adjacent cells. In other words, mutual inductances can harm the proper behaviour of circuits unless they are taken carefully into account during the design phase. We studied extensively the Josephson transmission line cell with different geometrical configurations of the ground plane and bias pads. We found with the use of InductEx that the return current sometimes follows paths that are far from what intuition tells, which can lead to nonoptimized designs. In this paper, we emphasize the limitations due to the presence of external or internal magnetic fields. Then, we compare obtained performances with the ones with optimized geometries for which the presence of the magnetic field is taken into account from the design phase.