Surface-engineered Nb-Nb Bonding Achieves Oxidation Resistance for Scalable Superconducting Quantum Computing Architectures

The pursuit of scalable quantum computing demands innovative approaches to qubit integration, and researchers are now tackling the limitations of conventional two-dimensional circuits. Harsh Mishra from the Indian Institute of Technology Hyderabad, Yusuke Kozuka from the National Institute for Materials Science (NIMS) and Tohoku University, and Sathish Bonam from Tyndall National Institute, alongside colleagues including Jun Uzuhashi and Praveenkumar Suggisetti, demonstrate a significant advance in three-dimensional superconducting architecture. Their work addresses a critical challenge in niobium-niobium bonding, the formation of resistive oxide layers, by introducing an ultrathin gold capping layer that effectively prevents oxygen incorporation. This surface engineering technique enables robust, low-temperature bonding at significantly reduced pressure, preserving superconductivity and paving the way for denser, more efficient quantum circuits with improved qubit coherence and reduced energy consumption.

D Integration with Through-Silicon Vias

Scalable quantum computing demands the integration of a large number of qubits, but conventional two-dimensional interconnects struggle with wiring congestion, electromagnetic interference, and signal loss. These limitations impede the development of complex quantum circuits and powerful quantum processors. To overcome these challenges, researchers are exploring three-dimensional (3D) integration techniques, which promise high-density, low-latency connections between qubits. This approach involves stacking multiple layers of qubits and interconnects vertically, significantly reducing wiring length and improving signal integrity.

The team investigates a novel 3D integration scheme utilising through-silicon vias (TSVs) to establish vertical connections between qubit layers. TSVs are microscopic channels etched through silicon wafers and filled with conductive materials, enabling high-density, low-resistance connections. The methodology involves fabricating multiple qubit layers on separate silicon wafers, precisely aligning them, and bonding them together using advanced techniques. After bonding, TSVs are etched and filled to create the vertical interconnects. The performance of these 3D integrated qubit interconnects was evaluated through electrical characterisation, including measurements of resistance, capacitance, and signal propagation delay. These measurements confirm the low-resistance, high-bandwidth characteristics of the TSV-based interconnects, demonstrating their potential for high-performance quantum circuits. Furthermore, the team assessed the impact of 3D integration on qubit coherence by analysing signal fidelity, and the results demonstrate minimal degradation to qubit coherence, preserving the quantum information stored in the qubits.

Gold Passivation Improves Niobium Film Bonding

This research details a significant advancement in materials science for superconducting quantum computing. Scientists successfully engineered a method for bonding niobium, a key material in superconducting circuits, at lower temperatures and pressures than previously possible. The team addressed the persistent challenge of native oxide formation on niobium surfaces, which hinders strong, reliable bonding and degrades qubit performance. By applying an ultrathin gold capping layer as a passivation strategy, they effectively suppressed oxidation during the bonding process. Detailed analysis confirms that this gold passivation enables low-temperature thermocompression bonding, resulting in enhanced bonding uniformity and strength while maintaining superconductivity. This achievement represents a crucial step towards fabricating scalable three-dimensional superconducting architectures, essential for building more powerful and complex quantum computers.