Current status and future prospect of the Nb-based fabrication process for single flux quantum circuits

The Superconductivity Research Laboratory has successfully fabricated large quantities of single flux quantum (SFQ) large scale integrated circuits, including several thousands of Josephson junctions (JJs). Using a J(c) = 2.5 kA cm(-2) process in which the number of Nb layers was four and the minimum JJ size was 2 mu m square. We developed a new advanced fabrication process that produced a J(c) = 10 kA cm(-2), nine Nb layers and a minimum JJ size of I Am square. The increase in the number of Nb layers was achieved by using a planarization technique. The target of our next generation process is a J(c) = 40 kA cm(-2) with a 0.5 mu m square for the minimum junction size. This specification will be achieved by using advanced semiconductor technologies. This process will enable SFQ circuits to be produced with one million JJs on a chip and achieve a clock frequency greater than 100 GHz.