Very Large Scale Integration of Josephson-Junction-Based Superconductor Random Access Memories

Abstract

Arrays of Vortex Transitional (VT) memory cells with functional density up to $1 Mbit/cm^2$ have been designed, fabricated, and successfully demonstrated. This progress is due to recent advances in design optimization and in superconductor electronics fabrication achieved at MIT Lincoln Laboratory. As a starting point, we developed a demo array of VT cells for the 100-${\mu}A/{\mu}m^2$ MIT LL fabrication process SFQ5ee with 8 niobium layers. The studied two-junction memory cell with a two-junction nondestructive readout occupied $168 {\mu}m^2$, resulting in an over $0.5 Mbit/cm^2$ functional density. Then, we reduced the cell area down to $99 {\mu}m^2$ (corresponding to over $0.9 Mbit/cm^2$ functional density) by utilizing self-shunted Josephson Junctions (JJs) with critical current density, $J_c$ of $600 {\mu}A/{\mu}m^2$ and eliminating shunt resistors. The fabricated high-$J_c$ memory cells were fully operational and possessed wide Read/Write current margins, quite close to the theoretically predicted values. We discuss approaches to further increasing the integration scale of superconductor memory and logic circuits: a) miniaturization of superconducting transformers by using soft magnetic materials; b) reduction of JJ area by using planar high-$J_c$ junctions similar to variable thickness bridges.