Studies of interfacial adhesion and copper plating chemistry for metallization of through-package-vias in glass interposers
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High quality copper plating in blind-vias and through-vias is crucial to achieve high electrical performance and reliability from the integrated circuit (IC) to the package substrate and printed wiring board (PWB). Cu has been deposited and patterned by several chemical and vapor deposition methods, but the most common approach for high volume manufacturing is electroless and electrolytic Cu plating. In efforts to meet the ever-increasing demand for improved system performance, recent trends in microelectronic packaging require finer Cu lines, micro vias, and fine-pitch Cu-filled through-package-vias (TPVs). Additionally, non-traditional package substrate materials such as glass are being investigated for their electrically insulating properties. These trends pose various material and process-related challenges in achieving high quality Cu metallization. Poor adhesion between copper and glass can result in electrical failures caused by delamination. Low-quality Cu filling in high-aspect ratio TPVs can lead to void formation and mechanical reliability failures. These challenges associated with poor-quality Cu metallization can be addressed through a better understanding of copper to glass adhesion and Cu deposition mechanisms. Therefore, there is a strong need to investigate the fundamental aspects related to Cu adhesion and Cu plating chemistry. The objective of this research was to develop a fundamental understanding of various aspects related to copper deposition on glass for advanced package and interposer applications. This included investigating the following topics: 1) metallization and adhesion of Cu to ultra-thin glass substrates with small diameter, fine-pitch TPVs, 2) electrochemical characteristics of organic additives in electrolytic Cu plating, and 3) the mechanism of how organic additives in electrolytic Cu plating enable void-free Cu filling in vias. Various Cu to glass adhesion mechanisms were investigated. Glass surface roughness was first studied as an approach to enable direct electroless Cu deposition. The feasibility was evaluated by determining the minimum required surface roughness and the interfacial failure mechanism by x-ray photoelectron spectroscopy (XPS). Then, a via-first process using a thin epoxy-based adhesion layer was developed. The process enabled electroless Cu deposition to ultra-thin glass substrates (100 µm) with small diameter (20 µm), fine pitch (40 µm) TPVs. Cu metallized glass substrates with TPVs passed thermomechanical reliability with strong adhesion after thermal cycle testing and highly accelerated stress testing. The impact of organic additives on electrolytic copper plating behavior was studied by electrochemical characterization. Using cyclic voltammetry and injection chronopotentiometry, the potential dependence and polarization kinetics of individual additives and combinations of additives were characterized. Their impact on Cu filling performance in blind-vias was investigated, and a mechanism was proposed to describe the additive interactions with the Cu via during plating. Finally, surface enhanced Raman spectroscopy (SERS) was used for in-situ observation of the chemical interactions between organic additives and the Cu surface during electroplating.