Through-Silicon Vias in Quantum Devices
Through-silicon vias are vertical conductive channels that carry signals and ground connections through a chip, enabling dense 3D wiring for large qubit arrays.
Going Vertical
A through-silicon via, or TSV, is a hole etched through a silicon chip and filled with a conductor so that a signal or ground connection can pass from one face to the other. In conventional microelectronics TSVs are a standard tool for stacking chips. In quantum hardware they solve two problems at once: routing control signals to interior qubits from the back of the chip, and stitching ground planes together to suppress unwanted electromagnetic modes.
Two Roles
- Signal delivery: bring control and readout lines to interior qubits vertically, bypassing the crowded qubit plane.
- Ground stitching: connect ground planes on both faces to eliminate slotline and chip modes that would couple to qubits.
Mode Suppression
A large chip with ground metal only on one side supports parasitic electromagnetic modes, chip modes and slotline modes, that can resonate near qubit frequencies and cause loss and crosstalk. Placing TSVs around and between qubits electrically ties the front and back grounds together at many points, raising the frequencies of these parasitic modes above the operating band or removing them. This makes larger chips usable without spurious resonances.
Fabrication Demands
For quantum use, the via must be superconducting or extremely low-loss, and the etching and filling must not introduce lossy interfaces or two-level systems near the qubits. Deep, high-aspect-ratio holes through the wafer must be etched cleanly and lined or filled with superconducting metal without voids. The thermal and mechanical stresses of many vias must not crack the chip or shift qubit parameters.
Role in Scaling
TSVs, together with flip-chip bonding, are the building blocks of the vertical integration that large processors require. They let a wiring interposer feed thousands of qubits from below while keeping the electromagnetic environment clean. Adapting a mature semiconductor technology to the superconducting, low-loss, millikelvin regime is a recurring theme in scaling quantum hardware, and TSVs are one of its clearest examples.