Corrosion-Free Copper Pad Recess Enabled by Wet Acid Etching and Functional Water Rinse for Hybrid Bonding Applications

This study presents a novel cyclic wet etching process based on citric acid for the controlled recessing of copper (Cu) pads, aimed at improving wafer-to-wafer (W2W) and die-to-wafer (D2W) hybrid bonding in advanced packaging applications. Conventional chemical mechanical polishing (CMP) struggles to prevent corrosion and excessive recessing of the Cu pads, particularly when they are connected to long metal lines. These issues compromise the quality and reliability of interconnects. To address these challenges, we propose an alternative strategy. The proposed citric acid-based approach enables nanometer-scale precision of Cu pad recess without causing corrosion. The work includes detailed investigations into the formation and aging of native Cu oxide, the kinetics of Cu oxide etching, and the interaction of citric acid with both Cu and silicon carbon nitride (SiCN). A stable etch rate of 24.6 nm/min is achieved using 0.05 wt% citric acid. Furthermore, a cyclic process involving ultraviolet (UV) exposure and wet etching enables controlled Cu pad recessing by approximately 1.5 nm per cycle. Optimized rinse chemistries such as functional water (ultra-dilute NH 4 OH) are also developed to improve surface quality and minimize Cu corrosion in connected pads. This method offers a robust and reliable solution for precise Cu pad height control and reduces Cu corrosion, contributing to enhanced performance in hybrid bonding with high-density interconnects.