A Study of Photo Resist Compatibility to Copper Bath during Applied Current
Authors: Lee Ui-Hyoung , Park Ho-Nyun , Morishima Yuji , Hatsukade Tomoko , Takahashi Takuya , Choi Jinho , Won Jaihyung 
Published: February 2020

The copper pillar bump for flip chip application is wafer level processed via through-hole electrodepostion technique. This is differing from the conventional dual damascene plating or TSV (through silicon via) filling which is blanket plating. In this study we found out the mechanism of solder skip plating defects which were originated in leached PR (photo resist) masking seed layer. Constructing WLP (wafer level packaging) or MEMS (micro electro mechanical systems) mostly are masked with PR and their PR patterns’ surface areas ratio are up to 99.8% which means these lipophilic large areas are exposed to the bath. Therefore before adopting these chemistries into production for maintaining bath fresh, PR and electroplating bath compatibility tests are carried out. Most well known compatibility tests are taking FE-SEM image of PR patterns’ shape change before and after dipping or analyzing organic components change of plating bath. Pointing out the specific solder skip plating defect in Fig.1(c), this was observed as a function of applied current and flow. In this study, we investigate a mechanism of skip plating defect and introduce compatibility test method.
Fig.1(a) and (b) shows PR surface visual change with and without applied current respectively. PR was thought to be swelling due to the fact of wafer weight gain. Bump electrodepositing process has to deliver electrons through conductive seed layer for reducing metallic ions in the electroplating bath. Recent needs of increasing throughput, higher depositing speed with larger overpotential surrounding which will lead stronger migration to the wafer surface. When electrolysis with exposed PR pattern directly, protons from bulk supporting electrolyte increase PR solubility inside the patterns while lithographic affected PAG's (photo acid generator) protons (Fig.2(a) and (b)). Change of PR surface following the pattern layout can be seen in Fig.1(b). Defect site was determined with EDS (energy dispersive x-ray spectroscopy) mapping and SIMS (secondary ion mass spectrometry) depth profiling that copper surface has excess amount of carbon, oxygen and sulfur contents which is PR structure material as shown in Fig.1(c) and (d). It is seems that mainly leaching material is a photo sensitizer. Due to large molecular weight and lipophil, we assume that protected polymer isn’t soluble.
And also suppressor with lipophilic components may forms micelle strongly to leach out PR. Triblock copolymers of poly(ethylene oxide) (PEO) (C2H4O) and poly(propylene oxide) (PPO) (C3H7O) are commercially available nonionic surfactants as based block copolymers. Since PPO group have low polarity, they tend to avoid contact with high-polar solvents (such as water), resulting in micelle formation, with a core dominated by PPO and a corona (like shell) dominated by PEO. While the number of PO units increases, it turns out that CMC (critical micelle concentration) decreases and required temperature also decreases. This means micelle forms easily to surround lipoids such as PR (Fig.2(c)). Therefore EO/PO ratio can be mediated CMC to avoid micelle formation for the bath compatibility. Due to these facts we proposed guidelines for chemical developments and its compatibility test method.