Abstract Body

We investigate an integration method for superconducting quantum computers, where we arrange vertically stacked substrates for qubits and peripheral circuits. To accommodate millions of qubits required for fault-tolerant quantum computing, we need high-density interconnect in the qubit and peripheral circuits for sending and receiving control and readout signals to/from the physical qubits. More demanding signal integrity for interconnect is required than in conventional digital signal processing circuits because of the analog nature of the signals. We demonstrate a vertical coaxial structure as a fundamental element in three-dimensional integrated devices. In the device structure, the control and readout waveforms are perpendicularly guided through the substrates, allowing access to the inner layer qubits and readout circuits from the top and bottom layers of the device.

Here, we design and fabricate the coaxial signal transmission structure and evaluate it through microwave measurement. Figure 1(a) shows a schematic illustration of the device. We can arrange qubits and readout circuitry in the inner layers of the stacked silicon substrates, and we control and observe the qubits through microwaves guided perpendicular to the substrate. We implement the coaxial structure for vertical signal transmission on 300-um silicon substrates using through-silicon vias (TSVs). In addition to a 150-um-diameter TSV as the center conductor, eight TSVs, functioning as the ground electrodes, are placed on the circumference of the center one. Figure 1(b) depicts the coaxial structure. The facing substrates are bonded through 10-um-diameter indium micro-bump electrodes. We use electromagnetic field simulation for impedance control.

We fabricate the vertical coaxial structure, in which we make through holes by deep reactive ion etching after a circuit patterning with 100-nm-thick aluminum and metalize the inner walls of the holes with c.a. 2.8 μm indium film using oblique evaporation. The two substrates are bonded using a flip-chip bonder under normal pressure to finalize the device. The practical difficulties in the fabrication are the angled evaporation of the indium via metalization with an evaporator without the substrate tilting function and the long-time lift-off process for 4-um-height indium micro bumps due to a thick patterning resist.

We have designed a dedicated sample package for evaluating the stacked structure. In this enclosure, we make a galvanic connection of the sample package to the stacked chips by touching spring pins to the electrodes on the substrate and realize the microwave transmission by organizing the spring-pin contacts for the center and outer conductors to form a coaxial waveguide. We obtain a sample package capable of transmitting and receiving signals through the stacked substrates with an arrangement of the coaxial structures on both the top and bottom sides of the substrates. We measure the transmission characteristics of the stacked substrate device up to 20 GHz at room temperature (Fig. 1(c)). We find the flat amplitude- and phase transmission coefficient from 1 GHz to 20 GHz, leading to an expectation to low-distortion interconnect. Furthermore, in the poster, we discuss the vertical coaxial readout of inner-layer resonant circuits and the scalability of the structure.

Acknowledgment

We acknowledge K. Kusuyama, K. Nittoh, B. Alexander, S. Laszlo, and M. Ozawa for their fruitful inputs in the fabrication process. This work is supported by JST Moonshot R&D (Grant No. JPMJMS2067) and ARIM in MEXT (Grant No. JPMXP1223UT1033). A part of the fabrication was conducted in Takeda Cleanroom, Center of U-Tokyo for ARIM and Data Hub.