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IEICE Technical Report; IEICE Tech. Rep.

Operations of Single Flux Quantum Circuits Based on 40-kA/cm^2 Process

Kouketsu Tomohito, Akaike Hiroyuki, Fujimaki Akira, T. Satoh, K. Hinode, Nagasawa Shuichi, Mutsuo Hidaka

Published Jul 15, 2010• 0 citations
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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 Jc = 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 Jc = 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 Jc = 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.

Superconductor Science and Technology•Feb 15, 2006•62 citations
Designs and test results for a single-flux-quantum (SFQ) eight-bit shift register circuit operating at frequencies above 100 GHz are described. The high-speed performance was realized by introducing a planarized 10 kA cm −2 Nb fabrication process as an advanced process and by adopting middle-damped junctions with McCumber parameters β c of 1.8–2.9 in the circuit. The middle-damped junctions were used to reduce the repulsion between SFQ pulses and to adjust the timing. The circuit was designed using a cell-based design method and was tested by constructing an on-chip test system with a ladder-type four-bit high-frequency clock generator. We confirmed its correct operations up to 120 GHz.
Abstract We have demonstrated digital-to-analog (D/A) operations using single-flux-quantum (SFQ) pulse-frequency modulation (PFM) D/A converters for future AC voltage standards. In this paper, for the improvement of SFQ-PFM D/A converters, we investigated a double-flux-quantum amplifier (DFQA) and a magnetically-coupling SFQ driver/receiver circuit (MC-SFQ-DR) fabricated using a 10 kA cm −2 Nb/AlO x /Nb integration process. The critical current density J c of 10 kA cm −2 was four times larger than that of the integration process we had used. A DFQA and an MC-SFQ-DR included unshunted Josephson junctions, and therefore, it was unclear if the high- J c process improved their performances. We measured test chips cooled in a liquid helium bath. The maximum input voltages for a +20 fold and a −20 fold DFQA were 147 and 126 μ V, for which the corresponding Josephson frequencies were 70.9 and 61.1 GHz. It was confirmed that the operation frequencies of the DFQAs and MC-SFQ-DR were improved approximately two-fold.
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 Jc = 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 Jc = 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 Jc = 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.