Nb-Nb direct bonding at room temperature for superconducting interconnects

Superconducting devices with high-density integration are required for applications, such as high-performance detectors and quantum computing. Here, the direct bonding of Nb electrodes at room temperature using a surface activated bonding method was investigated. We directly bonded Nb bonding pads and then performed current-voltage measurements in a He-3 cryostat. A superconducting current of 1-5 mA was confirmed for the bonded interconnect. The transition temperature was 7.3-9.3 K. These values are affected by the quality of the bonding interface. According to cross-sectional transmission electron microscopy observations and energy-dispersive x-ray spectroscopy analysis of the bonding interface, the Nb layers were bonded with a 3-nm-thick intermediate layer made of Si and Nb between them. The Si sources were the bonding substrate and the bonding jig in the bonding apparatus.