Nb-based superconducting silicon interconnect fabric for cryogenic electronics

Abstract

Quantum computing for real world applications requires large arrays of qubits. This requires advanced integration technologies. In this work, we propose a simple integration method with fine interconnect pitch (10 μm) and close spacing that can overcome the large input and output (I/O) and high wiring level requirements of very large scale of quantum circuits. A system-on-wafer (SoW) packaging concept called superconducting silicon interconnect fabric (superconducting-IF), based on silicon interconnect fabric (Si-IF), is proposed, with the help of a Au interlayer technology onto the superconducting-IF. The fine-pitch and die-to-wafer-scale Au interlayer is the first demonstration of direct metal–metal thermocompression bonding (TCB) that is optimized for superconducting applications without the use of solders. The developed Au interlayer integration technology is demonstrated to be Josephson-junction-compatible (<150 °C), mechanically robust (>30 MPa), and electrically reliable down to 2 K. The mechanical strength of the Au interlayer integration method is optimized through shear tests with a shear force around 150 N on 2 × 2 mm2 dies. The transition temperature (Tc) of Nb, which is at 9 K, is experimentally verified to be unchanged after each fabrication process. Electrical and temperature cycling measurements on over 20 bonded dies of large-pitch Kelvin structures as well as fine-pitch daisy chain structures reveal reliable connections in the low-temperature regime. This work pushes quantum computing a step closer to realize its potential through 3D integration of a very large scale of quantum circuits with a high density of I/O (>10 000 per mm2) as well as high wiring capability and without introducing lossy amorphous dielectrics.