Our goal is to develop inter-substrate connection technology to increase the density of quantum circuits integrated with peripherals. We are working on inter-substrate connection structures using superconductive TSVs to achieve high-density inter-substrate signal transmission.

In a previous study, we successfully demonstrated microwave signal transmission in the 1–12 GHz range using an inter-substrate vertical coaxial structure inside stacked substrates[1]. Our inter-substrate vertical coaxial structure consists of two through-silicon holes facing each other on the stacked substrates. The inner walls of the holes are coated with aluminum, and the holes are vertically connected by indium bumps.

In this study, we implement a microwave resonator on the inner layer of stacked substrates and connect it with the vertical coaxial structure. We confirmed the resonance of the inner-layer resonator at around 11 GHz. However, we did not find signal transmission at frequencies below 1 GHz.

In this poster, we present the structure of the resonator and its transmission spectrum measured through the inter-substrate vertical coaxial structure. We also investigated the reason for the poor transmission below 1 GHz. We show the results of conductivity measurements of the aluminum-indium interface under different plasma ashing conditions applied to resist residues on the aluminum surface before indium evaporation.