Title: 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.