Computational and experimental study of a novel L-Cysteine hydrochloride leveler for copper electroplated via fill in redistribution layers

Redistribution layers (RDL) is a key interconnect structure between IC chips and packaging substrate. Copper electroplating is a conventional but important process in RDL manufacture. A copper electroplating bath consists of a virgin-make-up solution of copper sulfate, sulfuric acid and chloride, plus organic additives such as suppressor, accelerator and leveler for control of plating morphologies and material properties. L-Cysteine hydrochloride (CYS) is a natural and non-toxic agent commonly added in foods, medicines and cosmetics. Its chemical structure contains ammonium and thiol groups which are in favor of surface adsorption on copper. In this study, for the first time, CYS is applied and studied as a copper electroplating leveler, together with prototypical suppressor polyethylene glycol (PEG) and sodium bis-(3-sulfopropyl) disulfide (SPS), to electroplate RDL vias. The adsorption and interactions of CYS are characterized with electrochemical measurements and analyzed with numerical simulations. With the PEG-SPS-CYS additive combination, the via fill performances and the crystallographic orientation are measured. The results indicate that CYS levelers exhibit strong adsorption activity and stability on cathode. At a current density of 5 ASD, it exhibits a good RDL via filling rate of 82%. By quantum chemical calculations, it is found that CYS has an appropriate energy gap allowing a high adsorbing activity. This work provides an inexpensive, nontoxic and environmental-friendly leveler for copper electroplating in advanced packaging