Fine pitch superconducting interconnects obtained with Nb–Nb direct bonding

For cryogenic systems, the development of superconducting interconnects is nowadays essential to reach large-scale integration schemes while addressing the associated challenges of signal dispersion and cross talk as well as thermal management. To achieve interconnects with micrometer and potentially sub-micrometer pitches, we investigate wafer-to-wafer direct bonding of Nb pads, using 200 mm processes developed for Cu/SiO _2 hybrid bonding and for Nb routing levels. Contrarily to standard hybrid bonding, the Nb pads of these interconnects are surrounded by air instead of dielectric to reduce signal and thermal losses in between the wafers. We report on the fabrication and characterization of this technology using 20–7 μ m-pitch direct bonded Nb pads. Transmission electron microscopy of the bonded interface and wafer-level parametric tests at 300 K of various daisy chains assess the bonding quality with a yield exceeding 90%. Low temperature electrical measurements in a cryostat demonstrate the promise of these interconnects with a critical temperature of 4.6 K, a critical magnetic field of 3.2 T and a critical current density of 1.25 kA/cm ^2 at 2 K and 0 T for 10 µ m × 10 μ m bonded Nb pads.