Fabrication of Sn-filled superconducting through-silicon vias (SC-TSV) for large-scale superconducting quantum circuits*

Quantum annealing (QA) is one of quantum techniques to solve combinatorial optimization and sampling problems. Recently, a commercial QA machine was developed by D-Wave Systems and has been used to solve various problems. We propose a new three-dimensional packaging structure, which realize a scalability and a flexibility of the circuit size according to the needs for solving problems by stacking qubit-chip, interposer, and package-substrate as much as necessary. However, in the three-dimensional packaging, generating heat in through-silicon vias (TSVs) used to connect the qubit and the package-substrate is a serious issue. In this work, we have developed Sn-filled superconducting TSV (SC-TSV) to solve the above issue. In the process of the SC-TSV fabrication, the Sn was completely filled in holes with a high-aspect-ratio by electroplating process. A diameter and depth of the SC-TSV were 50 μm and 400 μm, respectively. The SC-TSV exhibited a superconducting transition temperature of 3.3 K and a critical current value at 0.3 K was 100 μA. We have plan to improve the critical current for reliable large-scale QA machines.