Novel Wafer-Level Ta-Ta Direct Thermocompression Bonding for 3D Integration of Superconducting Interconnects for Scalable Quantum Computing System

As quantum computing evolves from small-scale qubit systems to more complex large-scale processors, the demand for individual qubit control and scalability will lead to the use of 3D integration and packaging technologies. Although advances in 3D integration in traditional CMOS technology are notable at room temperature, its potential in cryogenic environments, especially in quantum computing, remains largely unexplored. Superconducting qubit technology demands improvements in qubit relaxation and coherence time. Tantalum (Ta), renowned for its minimal loss, remarkable coherence time and superior stability, emerges as a promising candidate for superconducting materials. This study presents a novel approach by utilizing Ta-Ta thermo-compression bonding to create superconducting joints between wafers for vertical integration for the first time. A successful bonding temperature (500 °C) and pressure (0.3 MPa) results in the emergence of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula>-tantalum, which improves the coherence time significantly as reported earlier. The shear strength test shows a bond strength of 200 MPa which is a clear indication of a good bond between the two layers. Hence, we demonstrate the feasibility of achieving 3D integration of superconducting chips using this approach, thus opening doors for inventive quantum computing architectures.