Research on Effective Moat Configuration for Nb Multi-Layer Device Structure

One of the most important obstacles to realize high-end single-flux quantum (SFQ) digital circuits is how to eliminate the effects of trapped magnetic flux. In this study, we investigated various moat structures for multi-layer devices with multiple ground planes to determine which moat configuration was most suitable for a cell library. We designed various test circuits with different moat structures and evaluated their effectiveness by measuring the I-V characteristics of superconducting quantum interface device (SQUID) test circuits. Tests were carried out at several milli-gauss to evaluate the moats. We concluded that the most suitable moat configuration for a multi-layer device structure was obtained by surrounding the SFQ circuits with basic via moats in all ground layers of bias ports with ground contacts and by using narrow rectangular moats in only the main ground plane.