Effects of Brighteners on Microstructural Characteristics of Electroplated Copper

Copper electroplating is a fundamental technique for interconnect build-up and surface finish in integrated circuit (IC) and printed circuit board (PCB) industries, taking advantages such as high electric and thermal conductivities of copper and low production and maintenance costs of electroplating process. High-quality electroplating requires good cooperations of baths, tools and parameters, among which electroplating baths play key roles that primarily affect eventual morphology, microstructure, composition and reliability of electroplated copper materials. The active synergies among electroplating additives determine the covering and throwing abilities of electroplating bath to achieve, for example, bottom-up fill or equal-thickness plating. Although more efforts have been devoted to developing novel leveler molecules, a number of research reports indicate that selection and application of an appropriate brightener have chain influences on physicochemical properties of electroplated copper such as appearance, roughness, growth mode, crystal structure, internal stress, ductility, etc. In this work, two typical brighteners, i.e., bis-(sodium sulfopropyl)-disulfide (SPS) and sodium 3-[(dimethylcarbamothioyl)sulfanyl]-1-propanesulfonate (DPS), were formulated with polyethylene glycol (PEG, MW 8000) and comparatively investigated for copper foil electroplating. The results indicate that despite that DPS and SPS show similar depolarizing/accelerating abilities, surface and cross-sectional morphologies, they lead to very distinct residual stress, texture evolution and microstructures.