A compact AQFP logic cell design using an 8-metal layer superconductor process

Adiabatic quantum-flux-parametron (AQFP) is an adiabatic logic based on the quantum-flux-parametron. It has extremely low switching energy, as well as high signal gain and high robustness. In this work, we present the development of an AQFP cell library using the MIT LL 100 μA μm⁻² superconductor electronics fabrication process, SFQ5ee. Four types of basic sub-cells, i.e. buffer, inverter, constant, and branch, are designed as the building blocks for combinational AQFP logic cells. Taking advantage of the 8 metal layers, we place the dc superconducting quantum interference device (SQUID) part of a buffer above the ground plane and the signal transformer part below the ground plane to mitigate unwanted couplings. In addition, by vertically overlapping the signal transformer and the dc SQUID, the dimension of a buffer is reduced to 15 μm × 20 μm, which is only 30% of that of the conventional design. More complex AQFP logic cells, such as the majority gate, AND gate, and OR gate, are designed by arraying the sub-cells together. Correct operations and wide excitation current margins are confirmed for the established AQFP logic cells by both simulation and experiments. (paper)