Superconductor digital electronics using Josephson junctions as ultrafast switches and magnetic-flux encoding of information was proposed over 30 years ago as a sub-terahertz clock frequency alternative to semiconductor electronics based on complementary metal-oxide-semiconductor (CMOS) transistors. Recently, interest in developing superconductor electronics has been renewed due to a search for energy saving solutions in applications related to high-performance computing. The current state of superconductor electronics and fabrication processes are reviewed in order to evaluate whether this electronics is scalable to a very large scale integration (VLSI) required to achieve computation complexities comparable to CMOS processors. A fully planarized process at MIT Lincoln Laboratory, perhaps the most advanced process developed so far for superconductor electronics, is used as an example. The process has nine superconducting layers: eight Nb wiring layers with the minimum feature size of 350 nm, and a thin superconducting layer for making compact high-kinetic-inductance bias inductors. All circuit layers are fully planarized using chemical mechanical planarization (CMP) of SiO2 interlayer dielectric. The physical limitations imposed on the circuit density by Josephson junctions, circuit inductors, shunt and bias resistors, etc., are discussed. Energy dissipation in superconducting circuits is also reviewed in order to estimate whether this technology, which requires cryogenic refrigeration, can be energy efficient. Fabrication process development required for increasing the density of superconductor digital circuits by a factor of ten and achieving densities above 10^7 Josephson junctions per cm^2 is described.
Universal logic circuit for gate-controlled superconductor-based switches operating at liquid-helium temperatures
Budapest University of Technology and Economics（布达佩斯技术与经济大学）；MTA-BME Superconducting Nanoelectronics Momentum Research Group（MTA-BME超导纳米电子动量研究组）；Aalto University School of Science（阿尔托大学理学院）；Institute of Technical Physics and Materials Science, HUN-REN Centre for Energy Research（匈牙利研究与创新网络能源研究中心技术物理与材料科学研究所）；MTA-BME Correlated van der Waals Structures Momentum Research Group（MTA-BME关联范德华结构动量研究组）
