Demonstration of Tantalum-Tantalum Thermocompression Bonding for Vertical Integration of Superconducting Qubit

A quantum processor based on superconducting quantum bits can be scaled up by stacking multiple chips that individually perform different computational functions. While advancements in 3D integration within conventional CMOS technology have been notable at room temperature, its potential in cryogenic temperatures, particularly in quantum computing, remains largely unexplored. Superconducting qubit technology demands improvements in both qubit relaxation and coherence time. Tantalum, renowned for its minimal loss and remarkable coherence time of 0.3 milliseconds, emerges as a promising candidate for superconducting materials. This study introduces a novel method that employs tantalum (Ta) to bond superconducting joints between wafers using the thermocompression technique and subsequently converts β-tantalum to α-tantalum after bonding at 500°C, achieving a bond strength of 700 N in a shear test. additionally, we will present the use of passivation techniques to mitigate the oxide, resulting improvement in TLS Losses and reducing the bonding thermal budget. Hence, we demonstrate the feasibility of achieving 3D integration of superconducting chips using this approach, thereby opening the door to inventive quantum computing architectures.