Investigation of Aluminum and Gold Flip-Chip Bonding for Quantum Device Integration

The majority of qubit chip integration is realized in a two-dimensional (2D) architecture. Whereas 3D architecture enables more advantages like efficient interconnect routing, allowing for more compact qubit coupling geometries, reducing form factor, and increased connectivity beyond nearest-neighbor interactions. Flip-chip (FC) assembly has been demonstrated to enable 3D architecture connecting qubit chip, interposer, and readout in a sandwich-like structure. Moreover, 3D integration allows the fabrication of hosting chip circuitry without degrading the qubit performance [1]–[4]. Different material considerations have to be taken into account since qubit operational frequency is in the gigahertz range and operating temperature is in the mK range to avoid thermal excitation. Materials that possess superconducting characteristics like Indium (In), Titanium Nitride (TiN), Tantalum Nitride (TaN), and Niobium (Nb) have been discussed as potential interconnect between building blocks [3], [5]. The In bumping on Aluminum (Al) redistribution layers require under-bump metallization (UBM) layers thus introducing multiple fabrication steps before the bonding process. An alternative approach would be to use the existing Al surface to electrochemically grow Al bumps and bond the chip using thermosonic bonding (TSB) at below 150°C to form a homogeneous metal-metal interface [6], [7].