Fabrication Process and Properties of Fully-Planarized Deep-Submicron Nb/Al-AlOx/Nb Josephson Junctions for VLSI Circuits

TL;DR

This paper presents a new fabrication process for ultra-small Nb/Al-AlOx/Nb Josephson junctions on 200-mm wafers, achieving high uniformity and reproducibility, suitable for high-density superconducting circuits with advanced lithography techniques.

Contribution

Developed a fully-planarized fabrication process for sub-200 nm Josephson junctions compatible with CMOS foundry tools, enabling high-density superconductor integrated circuits.

Findings

Achieved 0.8% to 3% spread in critical current for 1500 nm to 500 nm JJs.

Demonstrated reproducibility and uniformity across 200-mm wafers.

Discussed prospects for circuit densities exceeding 10^6 JJs/cm².

Abstract

A fabrication process for Nb/Al-AlOx/Nb Josephson junctions (JJs) with sizes down to 200 nm has been developed on a 200-mm-wafer tool set typical for CMOS foundry. This process is the core of several nodes of a roadmap for fully-planarized fabrication processes for superconductor integrated circuits with 4, 8, and 10 niobium layers developed at MIT Lincoln Laboratory. The process utilizes 248 nm photolithography, anodization, high-density plasma etching, and chemical mechanical polishing (CMP) for planarization of SiO$_2$ interlayer dielectric. JJ electric properties and statistics such as on-chip and wafer spreads of critical current, $I_c$, normal-state conductance, $G_N$, and run-to-run reproducibility have been measured on 200-mm wafers over a broad range of JJ diameters from 200 nm to 1500 nm and critical current densities, $J_c$, from 10 kA/$cm^2$ to 50 kA/$cm^2$ where the JJs become self-shunted. Diffraction-limited photolithography of JJs is discussed. A relationship between JJ mask size, JJ size on wafer, and the minimum printable size for coherent and partially coherent illumination has been worked out. The $G_N$ and $I_c$ spreads obtained have been found to be mainly caused by variations of the JJ areas and agree with the model accounting for an enhancement of mask errors near the diffraction-limited minimum printable size of JJs. $I_c$ and $G_N$ spreads from 0.8% to 3% have been obtained for JJs with sizes from 1500 nm down to 500 nm. The spreads increase to about 8% for 200-nm JJs. Prospects for circuit densities > $10^6$ JJ/$cm^2$ and 193-nm photolithography for JJ definition are discussed.