AC-Biased Shift Registers as Fabrication Process Benchmark Circuits and Flux Trapping Diagnostic Tool

Abstract: We develop an ac-biased shift register introduced in our previous work (V.K. Semenov et al., IEEE Trans. Appl. Supercond., vol. 25, no. 3, 1301507, June 2015) into a benchmark circuit for evaluation of superconductor electronics fabrication technology. The developed testing technique allows for extracting margins of all individual cells in the shift register, which in turn makes it possible to estimate statistical distribution of Josephson junctions in the circuit. We applied this approach to successfully test registers having 8, 16, 36, and 202 thousand cells and, respectively, about 33000, 65000, 144000, and 809000 Josephson junctions. The circuits were fabricated at MIT Lincoln Laboratory, using a fully planarized process, 0.4 {\mu}m inductor linewidth, and 1.33x10^6 cm^-2 junction density. They are presently the largest operational superconducting SFQ circuits ever made. The developed technique distinguishes between hard defects (fabrication-related) and soft defects (measurement-related) and locates them in the circuit. The soft defects are specific to superconducting circuits and caused by magnetic flux trapping either inside the active cells or in the dedicated flux-trapping moats near the cells. The number and distribution of soft defects depend on the ambient magnetic field and vary with thermal cycling even if done in the same magnetic environment.