Fully Superconducting Multi-Chip Module Process with Controlled Interchip Spacing for Quantum Integrated Circuits

We present a fabrication process for a fully superconducting multichip module (MCM), compatible with superconducting qubit technology, to allow a 3D integration of quantum circuits while preserving qubit coherence. In particular, this MCM can be used to couple a Nb-based Single Flux Quantum classical coprocessor for the integrated control and readout of large-scale superconducting multiqubit integrated circuits. We report on the development of superconducting indium-bump MCM technology for the connection of a multi-layer Nb circuit to a qubit chip by integrating indium into the HYPRES fabrication process. The MCM assembly becomes fully superconducting at the transition temperature of 3.4 K, as determined by indium. For the dense MCM bumps, soft malleability of indium may result in poor control of the interchip spacing, which can lead to shorts between bumps. To address this problem, we devised a mechanism in where a more rigid metal supports the indium bumps. This limits the deformation while allowing cold welding and providing a controlled separation between flip-chip and MCM carrier. We characterize the performance of the MCM by measuring the bump array critical current, critical current dependence vs temperature and its mechanical stability over multiple thermal cycles.