Low Temperature, Fast Copper–to–Copper Bonding Enabled by Microwave-Induced Interfacial Field Amplification

Abstract

Copper–to–copper (Cu−Cu) bonding has emerged as a promising solution for realizing high-density vertical chip stacking for next-generation 3D integration. However, conventional Cu−Cu bonding generally requires a high thermal budget of ∼300 °C for 1 h, which can cause wafer warpage and compromise device performance. Here, a microwave annealing (MWA)-based Cu−Cu bonding technique is proposed to enable bonding with the substrate maintained at a low temperature of 150 °C and a short bonding time of 3 min. The MWA-bonded samples exhibit electrically and mechanically stable bonding characteristics, with a shear strength of 30–32 MPa and a specific contact resistance of ∼10⁻⁷ Ω·cm². The microwave field is amplified within the nanoscale air gap between the two facing copper surfaces, localizing energy via discharge at the Cu−Cu interface. This localized energy is believed to drive the rapid elimination of the interfacial gap between the Cu−Cu surfaces, allowing rapid bonding while keeping the substrate at a low temperature. Compared to thermocompression (TC) bonding under comparable pressure and an N₂ atmosphere, MWA reduces the bonding temperature by 150 °C while shortening the process time 20-fold. Overall, this study presents a low thermal budget Cu−Cu bonding approach enabled by an MWA strategy, offering a compelling alternative for advanced packaging.