Superconductor Digital Electronics: Scalability and Energy Efficiency Issues

TL;DR

This paper reviews the current state, fabrication processes, and scalability challenges of superconductor digital electronics using Josephson junctions, focusing on energy efficiency and potential for large-scale integration.

Contribution

It provides an in-depth analysis of fabrication processes and physical limitations, proposing advancements to increase circuit density and assess energy efficiency in superconductor electronics.

Findings

Current process achieves 8 Nb wiring layers with 350 nm features.

Physical limitations restrict circuit density and scalability.

Energy dissipation analysis suggests potential energy efficiency despite cryogenic requirements.

Abstract

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.