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

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-μA/μm ² MIT LL fabrication process SFQ5ee with eight niobium layers. The studied two-junction memory cell with a two-junction nondestructive readout occupied 168 μm ² , resulting in an over 0.5 Mbit/cm ² functional density. Then, we reduced the cell area down to 99 μm ² (corresponding to over 0.9 Mbit/cm ² functional density) by utilizing self-shunted Josephson junctions (JJs) with critical current density JC of 600 μA/μm ² and eliminating shunt resistors. The fabricated high-JC 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: 1) miniaturization of superconducting transformers by using soft magnetic materials; and 2) reduction of JJ area by using planar high-JC junctions similar to variable thickness bridges.