In-Depth Parametric Study of Ar or N2 Plasma Activated Cu Surfaces for Cu-Cu Direct Bonding

In this article, we report in-depth parametric study of argon/nitrogen plasma-activated copper surfaces for copper-copper die-to-die direct bonding carried out at room temperature in cleanroom ambient condition. Surface analyses (e.g. water contact angle and X-ray photoelectron spectroscopy) are performed on the control, argon or nitrogen plasma-activated copper surfaces. The results reveal that a thin layer of copper nitride is formed on the copper surface with the nitrogen plasma treatment, which is a potentially effective passivation layer to control the surface oxidation. By fine-tuning the argon or nitrogen plasma (exposure time, plasma power and plasma species), a bonding strength of ∼6 MPa is achieved, and the bonded interface has a specific contact resistivity of $\sim 6.0\times 10^{-4}\ \Omega \cdot \text{cm}^{2}$. Therefore, an optimal plasma recipe is obtained for argon/nitrogen plasma-activated copper-copper direct bonding. This bonding technique is suitable for high-throughput three-dimensional wafer bonding and advanced packaging.