Superconducting Molybdenum Multi-Chip Module Approach for Cryogenic and Quantum Applications

We describe a superconducting multi-chip module (S-MCM) technology using Mo as a robust substrate on which to construct multi-layer superconducting redistribution layers for chip-to-chip signal transmission for densely-integrated cryogenic and quantum electronics. The mechanical robustness and ductile nature of Mo can allow for the integration of chips on a larger scale S-MCM substrate compared to currently available technologies. We demonstrate this integration technology by flip-chip bonding Si chips to Mo substrates using In bumps and epoxy underfill. Superconducting daisy-chain test structures were formed by Mo substrates with polyimide dielectric and superconducting Nb traces connected to Si chips with varying numbers of transitions and bump array densities. Resistance and superconducting transition temperatures of the various daisy-chain configurations were measured from room temperature to 4.2 K. To explore CTE-related challenges, assemblies using Si chips with dimensions up to 27 mm x 22 mm (In bump array size of 20 mm x 20 mm) were found to survive the multiple thermal cycles from room temperature to cryogenic temperatures.