Long-term Changes in Parameters for Nb-based Superconducting Digital Circuits

The same process evaluation chips for Nb-based superconducting digital circuits were measured at liquid He temperature (4 K) just after fabrication and after storage at room temperature for up to 16 years. The critical current (I<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub>) of the Josephson junction (JJ), resistance, inductance, and I<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the Nb–Nb contacts before and after storage were compared. I<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> decreased by around 5% and resistance increased about 2% after 10 years. Neither change depended on JJ area or resistor width. Therefore, we concluded they were changes in critical current density (J<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub>) and sheet resistance (R<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sq</sub>). Because J<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> did not change in the chip thermally annealed just after fabrication, we concluded that the long-term J<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> changes were caused by the same mechanism as thermal annealing. Although the long-term change in J<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> is not critical for superconducting digital circuit operation, it could cause circuit malfunctions in some cases. Therefore, some measures are required, such as light annealing after fabrication or cold storage. The effect of the resistance change on superconducting digital circuit operation was small. No inductance changes were observed. I<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of Nb–Nb contacts decreased by several tens of percent over 10 years but was still sufficient for the digital circuits to function.