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

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 SiO2 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.