Design and Evaluation of Magnetic Field Tolerant Single Flux Quantum Circuits for Superconductive Sensing Systems

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  • YAMANASHI Yuki
    Department of Electrical and Computer Engineering, Yokohama National University
  • YOSHIKAWA Nobuyuki
    Department of Electrical and Computer Engineering, Yokohama National University

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A promising application of a single-flux quantum (SFQ) circuit is read-out circuitry for a multi-channel superconductive sensor array. In such applications, the SFQ read-out circuit is expected to operate outside a magnetic shield. We investigated an SFQ circuit structure, which is tolerant to an external magnetic field, using the AIST 2.5kA/cm2 Nb standard 2 process, which has four Nb wiring layers including the ground plane. By covering the entire circuit using an upper Nb wiring layer called the control (CTL) layer, the influences of the external magnetic field on the SFQ circuit operation can be avoided. We experimentally evaluated the sheet inductance of the wiring layer underneath the CTL shielding layer to design a magnetic-field-tolerant SFQ circuit. We implemented and measured test circuits comprising toggle flip-flops (TFFs) to evaluate their magnetic field tolerances. The operating margin and maximum operating frequency of the designed TFF did not deteriorate with increases in the magnetic field applied to the test circuit, whereas the operating margin of the conventional TFF was reduced by applying the magnetic field. We have also demonstrated the high-speed operation of the designed TFF operated in an unshielded environment at a frequency of up to 120GHz with a wide operating margin.

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