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ST426 - Fabrication Process and Properties of Fully-Planarized Deep-Submicron Nb/Al-AlOx/Nb Josephson Junctions for VLSI Circuits (Contest winner)
Fabrication Process and Properties of Fully-Planarized Deep-Submicron Nb/Al-AlOx/Nb Josephson Junctions for VLSI Circuits
Sergey K. Tolpygo, Vladimir Bolkhovsky, Terence J. Weir, William D. Oliver, Member, IEEE, and Mark A. Gouker, Senior Member, IEEE
Lincoln Laboratory, Massachusetts Institute of Technology,
Lexington, MA 02420, USA
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 a CMOS foundry. This process is a core of several nodes of fully-planarized fabrication processes developed at MIT Lincoln Laboratory for superconductor integrated circuits with 4, 8, and 10 niobium layers. 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, Ic, normal-state conductance, GN, and run-to-run reproducibility have been measured on 200-mm wafers in a broad range of JJ diameters from 200 nm to 1500 nm and critical current densities, Jc, from 10 kA/cm2 to 50 kA/cm2 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 GN and Ic 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. Ic and GN spreads from 0.8% to 3% have been obtained for JJs with sizes from 1500 nm down to 500 nm to be utilized in Single-Flux-Quantum circuits with Jc from 10 kA/cm2 to 50 kA/cm2. The spreads increase to about 8% for 200-nm JJs. Prospects for circuit densities > 106 JJ/cm2 and 193-nm photolithography for JJ definition are discussed.
Keywords (Index Terms) — Nb/Al-AlOx/Nb Josephson junctions, RSFQ, RQL, superconducting integrated circuit, superconductor electronics, self-shunted junction, 248-nm photolithography, minimum printable size, mask error enhancementIEEE/CSC & ESAS SUPERCONDUCTIVITY NEWS FORUM (global edition), January 2015.
Received August 21, 2014; Selected October 8, 2014. Reference ST426; Category 4.
ASC 2014 manuscript 2EPo1A-02 published online in IEEE Trans. Appl. Supercond. (IEEE XPLORE) DOI: 10.1109/TASC.2014.2374836, on November 26, 2014.