A Study of Photo Resist Compatibility to Copper Bath during Applied Current
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.