Development of superconducting connection by flip-chip bonding for a multilayer superconducting quantum annealing machine*

To realize practical-scale quantum annealing machines, a large number of qubits are required for the quantum processor. However, implementation of high-density qubits-array on a chip is a difficult problem because the size of qubits is limited by the size of wiring layer, Josephson junction and SQUID. To solve this problem, we have proposed an “QUIP” (Qubit-chip, Interposer and Package-substrate) a 2.5-dimensional (2.5 D) packaging structure. Therefore, development of the packaging technology of qubits is one of the most important issue for QUIP. In this presentation, we focus on flip-chip bonding (FCB) connection as a 2.5 D mounting method. We design and fabricate circular lead/indium alloy solder bumps with a 10 um diameter and 5 um height on the top chip and Nb/Ti/Au-opposing-contact pads on the base chip to form a daisy chain of over 10000 chip-to-chip interconnects. The electrical transport measurements are performed in a cryocooler using a standard dc four-probe technique. We observe a critical current for the daisy chain devices with 15000 bump array, Ic ～ 4 mA.