Increasing integration scale of superconductor electronics beyond one million Josephson junctions

We review the existing fabrication processes for superconductor digital electronics and describe approaches to increasing the scale of integration of superconducting digital circuits from the current level of about one million Josephson junctions (JJs) on a 1-cm² chip toward ten million JJs per chip. We present designs of ac-clocked Single Flux Quantum (SFQ) shift registers, convenient benchmarking circuits, in a 250-nm-linewidth superconductor electronics fabrication process developed recently at MIT Lincoln Laboratory (MIT LL). For shift registers using resistively shunted JJs with Josephson critical current density, J _cof 100 μA/μm², we achieved a record-high circuit density of 4.2•10⁶ JJs per cm², a factor of three higher than the previous record obtained in the MIT LL 350-nm-linewidth SFQ5ee process. Using self-shunted JJs with J _cof 600 μA/μm², we increased this record circuit density to 7.4•10⁶ JJs per cm².