Developments toward a 250-nm, Fully Planarized Fabrication Process with Ten Superconducting Layers and Self-Shunted Josephson Junctions

Abstract

We are developing a superconductor electronics fabrication process with up to nine planarized superconducting layers, stackable stud vias, self-shunted Nb/AlO x -Al/Nb Josephson junctions, and one layer of MoN x kinetic inductors. The minimum feature size of resistors and inductors in the process is 250 nm. We present data on the mutual inductance of Nb stripline and microstrip inductors with linewidth and spacing from 250 nm to 1 μm made on the same or adjacent Nb layers, as well as the data on the linewidth and resistance uniformity.

Superconductor Electronics Fabrication Process with MoNx Kinetic Inductors and Self-Shunted Josephson Junctions

“…The evidence for Josephson network behavior in granular MoNx films deposited by thermal evaporation was presented in [28]. However, critical currents in MoNx films formed by reactive sputtering, see below and [29], and our SEM data partially presented in [9] do not indicate granular structure in our MoNx films, although do show it in NbNx films. However, further discussion of these issues goes beyond the scope of this paper.…”

“…This process utilizes resistively shunted Nb/AlOx-Al/Nb Josephson junctions with critical current density Jc = 0.1 mA/μm 2 and geometrical inductors formed on multiple superconducting wiring layers of niobium. Currently it is perhaps the most advanced fabrication process for superconductor electronics, with a theoretical maximum circuit density of about 4•10 6 JJs per cm 2 [8], [9]. However, further increase of the integration scale of SFQ circuits is challenging because of the large area of individual SFQ cells, mainly determined by the area of JJ shunt resistors and geometrical inductors [8].…”

Beyond Moore’s technologies: operation principles of a superconductor alternative

“…Possible solutions here are miniaturization of existing elements and increase of their functionality. The former can be obtained by scaling down the SIS Josephson junction [ 104 ], or search for other high-accuracy technological processes providing nanosized junctions with high critical current density and normal-state resistance [ 104 – 106 ]. Another direction of the research is the substitution of the conventional loop inductance with a kinetic inductance or the inductance of the Josephson junction [ 19 ].…”

Magnetic calorimeter option for the Lynx x-ray microcalorimeter

“…The mutual inductance crosstalk between two superconducting Nb striplines, which are 1 μm apart and 0.25 μm wide, has been reduced to −41 dB using upper and lower ground planes, with data being in agreement with simulations. 17 A third benefit of the multilayer buried wiring approach is that there is a planarized surface on top of the wiring on which to fabricate the main microcalorimeter pixels, with generally more space for the pixel designs and heat-sinking. This makes the subsequent fabrication process dramatically easier and even opens up pixel geometries.…”