SUMMARYWe describe the recent progress on a Nb nine-layer fabrication process for large-scale single flux quantum (SFQ) circuits. A device fabricated in this process is composed of an active layer including Josephson junctions (JJ) at the top, passive transmission line (PTL) layers in the middle, and a DC power layer at the bottom. We describe the process conditions and the fabrication equipment. We use both diagnostic chips and shift register (SR) chips to improve the fabrication process. The diagnostic chip was designed to evaluate the characteristics of basic elements such as junctions, contacts, resisters, and wiring, in addition to their defect evaluations. The SR chip was designed to evaluate defects depending on the size of the SFQ circuits. The results of a long-term evaluation of the diagnostic and SR chips showed that there was fairly good correlation between the defects of the diagnostic chips and yields of the SRs. We could obtain a yield of 100% for SRs including 70,000 JJs. These results show that considerable progress has been made in reducing the number of defects and improving reliability.

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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 µm 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 1 µm 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 µm 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.