Fabrication of Sn-Cu alloy superconducting films for filled superconducting through-silicon vias*

Quantum annealing (QA) is one of quantum computers to solve combinatorial optimization and sampling problems. We have developed new three-dimensional (3D) packaging structure, which obtain a scalability and a flexibility in the circuit size to realize customized circuits for solved problems by stacking qubit-chip, interposer, and package-substrate as much as necessary. In the 3D packaging, superconducting TSVs are necessary because generating heat in through-silicon vias (TSVs) used to connect the qubit and the package-substrate is a big problem. So far, we have fabricated superconducting TSVs with pure Sn, however, fabrication yield of the TSVs was low because it was difficult to fill the Sn in deep holes by electroplating process (EP). In this work, we have changed the material from the Sn to Sn-Cu alloy to solve the above problem. A Sn₉-Cu alloy film fabricated by the EP exhibited transition temperature (Tc) of 2.4 K. Although the Tc of the Sn₉-Cu alloy is about 30 % lower than that of the pure Sn, it is sufficient for our QA machine system.