Time-Dependent Evolution Study of Ar/N₂ Plasma-Activated Cu Surface for Enabling Two-Step Cu-Cu Direct Bonding in a Non-Vacuum Environment

Abstract

In this paper, a two-step copper-copper direct bonding process in a non-vacuum environment is reported. Time-dependent evolution of argon/nitrogen plasma-activated copper surface is carefully studied. A multitude of surface characterizations are performed to investigate the evolution of the copper surface, with and without argon/nitrogen plasma treatment, when it is exposed to the cleanroom ambient for a period of time. The results reveal that a thin layer of copper nitride is formed upon argon/nitrogen plasma activation on copper surface. It is hypothesized that the nitride layer could dampen surface oxidation. This allows the surface to remain in an “activated” state for up to 6 hours. Afterwards, the activated dies are physically bonded at room temperature in cleanroom ambient. Thereafter, the bonded dies are annealed at 300ºC for varying duration, which results in an improvement of the bond strength by a factor of 70 ~ 140 times. A sample bonded after plasma activation and 2-hour cleanroom ambient exposure demonstrates the largest shear strength (~5 MPa). The degradation of copper nitride layer at elevated temperature could aid in maintaining a localized inert environment for the initial diffusion of copper atoms across the interface. This novel bonding technique would be useful for high-throughput three-dimensional wafer bonding and heterogeneous packaging in semiconductor manufacturing.