Single Flux Quantum Circuit Technology and CAD overview

Single Flux Quantum (SFQ) electronic circuits originated with the advent of Rapid Single Flux Quantum (RSFQ) logic in 1985 and have since evolved to include more energy-efficient technologies such as ERSFQ and eSFQ. SFQ logic circuits, based on the manipulation of quantized flux pulses, have been demonstrated to run at clock speeds in excess of 120 GHz, and with bit-switch energy below 1 aJ. Small SFQ microprocessors have been developed, but characteristics inherent to SFQ circuits and the lack of circuit design tools have hampered the development of large SFQ systems. SFQ circuit characteristics include fan-out of one and the subsequent demand for pulse splitters, gate-level clocking, susceptibility to magnetic fields and sensitivity to intra-gate and inter-gate inductance. Superconducting interconnects propagate data pulses at the speed of light, but suffer from reflections at vias that attenuate transmitted pulses. The recently started IARPA SuperTools program aims to deliver SFQ Computer-Aided Design (CAD) tools that can enable the successful design of 64 bit RISC processors given the characteristics of SFQ circuits. A discussion on the technology of SFQ circuits and the most modern SFQ fabrication processes is presented, with a focus on the unique electronic design automation CAD requirements for the design, layout and verification of SFQ circuits.