Effects of suppressors on the incorporation of impurities and microstructural evolution of electrodeposited Cu solder joints

Background: Copper (Cu) electrodeposition is a cost-effective and scalable technology commonly used to fabricate interconnects and heat sinks in microelectronic devices. Adding organic functional additives, such as suppressors, to plating solutions is crucial to achieve conformal microvia filling by Cu electroplating. However, codeposition of organic additives is inevitable and leads to reliability concerns regarding impurity residues in the Cu elec-trodeposited layer.Methods: In this study, polyethylene glycol (PEG) at 8,000 g mol-1 and a triblock copolymer EPE (polyethylene oxide-polypropylene oxide-polyethylene oxide) at 2,900 g mol-1 are applied as suppressors and individually added to plating solutions to compare their suppression strength toward Cu deposition using galvanostatic measurements. Their adsorption/desorption behaviors on the Cu surface are theoretically analyzed based on the electronic distribution properties of the two suppressor molecules calculated by quantum chemical calculations. The impurity residues in the Cu layers are measured by a time-of-flight secondary ion mass spectrometer, and their effects on the joining reactions between tin (Sn) solder and Cu are investigated by observing the micro -structural evolution in the Sn/Cu joints subjected to isothermal aging.Significant findings: EPE2900 outperforms PEG8000 in the suppression strength toward Cu deposition. Unlike PEG8000, which causes a high level of impurity incorporation, the impurity residues in the Cu layer constructed by EPE2900 are negligible. As a result, the Sn/Cu solder joint constructed by EPE2900 exhibits high micro -structural stability free of the propagation of detrimental voids during isothermal aging at 200 degrees C. The better suppression efficacy and negligible impurity residues of EPE2900 can be rationalized based on quantum chemical calculations.