Experimental Verification of Moat Design and Flux Trapping Analysis

Soft defects caused by flux quanta not captured by moats during superconductor integrated circuit cooldown are known to degrade circuit performance. However, haphazard or random moat placement does not necessarily improve circuit performance and may even degrade it. Under the IARPA SuperTools program we have both developed numerical simulation tools to extract simulation models of circuit structures in the presence of trapped fluxons and designed experiments to measure the probability of flux trapping in moats and the coupling of such trapped flux to circuit structures. Depending on moat structure, the coupling from a few moat-captured fluxons is shown to reduce the critical current for a SQUID by 20% or more, which justifies a thorough analysis. We present results for this very important aspect of magnetic rule checking and show how controlled flux trapping experiments in designated moats yield measurement results that fit simulated predictions very well. With the simulation tools validated, we show how flux trapping is incorporated into compact simulation models and we detail the extraction of magnetic coupling and the calculation of the compact models for both branch and loop-type simulations. We also show how phase-based simulation with the circuit simulator JoSIM allows arbitrary fluxon insertion in moats during transient circuit simulation and conclude with some recommended design rules for moat geometry, size, and placement.