Advanced Electrochemical Plating Processes of Nano-Twinned Copper for Hybrid Bonding

This study introduces an advanced electrochemical plating (ECP) technique specifically designed to enhance the production of high-density nano-twinned copper (nt-Cu) within complete-Cu-seeded features such as lines and vias, which are critical for advanced wafer-level packaging (WLP) applications. As the industry increasingly demands efficient low-temperature hybrid bonding technologies, addressing the limitations of traditional nt-Cu ECP methods has become imperative.Conventional nt-Cu ECP processes are effective for bottom-Cu-seeded features but encounter challenges when applied to complete-Cu-seeded features. These challenges primarily involve balancing the bottom-up fill with maintaining a uniform nt-Cu orientation. The competition between vertical growth from the bottom and horizontal growth from the sidewalls often results in mixed orientations and the formation of undesirable triangle-like structures, which compromise the effectiveness of nt-Cu for low-temperature bonding applications that require a consistent interface orientation.The novel ECP process presented here overcomes these challenges by achieving near-complete nt-Cu density with controlled grain sizes ranging from 0.10 to 1.0 microns, minimal transition layers between the underlying Cu seed and the nt-Cu, reduced occurrence of unwanted triangle-like structures and providing a more reliable and consistent nt-Cu orientation.Our technique utilizes a traditional electrolyte system composed of copper sulfate, sulfuric acid, and chloride ions, which is enhanced by a combination of two or three functional additives tailored to meet the specific needs of different applications and feature dimensions. We have developed two distinct ECP processes based on feature dimension and aspect ratio (A/R):Vertical Growth Orientation Process: This process promotes vertical nt-Cu growth by suppressing conformal deposition and enhancing bottom-up growth. The strategic use of functional additives inhibits sidewall deposition, ensuring that nt-Cu grows predominantly in the vertical direction. This process has demonstrated effectiveness in filling lines with openings as small as 2 m and A/R of approximately 1:1, as well as vias with openings as small as 4 m and similar aspect ratios. Optimization of the plating waveform and additive concentration ratio is crucial for controlling the nt-Cu grain size and columnar structure in the features.Horizontal Growth Orientation Process: This method facilitates horizontal nt-Cu growth in the top half of features. Initially, the process balances conformal and bottom-up filling; however, as feature dimensions decrease with the ECP process, sidewall deposition becomes more dominant, resulting in horizontal growth in the top half of the features. As a result, the top half is filled with horizontal orientation nt-Cu aligned in the direction from the sidewall to the center of the feature. This approach has been validated for vias with openings as small as 1.5 m and A/R of approximately 1.5:1 and is effective for both small, high A/R features and larger, lower A/R features.Both ECP processes ensure that the nt-Cu orientation is consistent on the top part of the features, whether vertical or horizontal. This consistency is crucial for achieving uniform copper crystal orientations on post-CMP (Chemical Mechanical Planarization) wafer surfaces, essential for adequate low-temperature hybrid bonding.In conclusion, the advanced ECP process described in this study represents a significant advancement in the production of nt-Cu for WLP applications. By addressing the limitations of existing methods and achieving high-density, uniformly oriented nt-Cu with minimal defects, this process offers enhanced performance for low-temperature hybrid bonding applications. It marks notable progress in the field of advanced packaging technologies.