Effect of Additives on the Microstructure of Electroplated Tin Films

Electroplated tin films are important for several technologies such as microelectronics, Li-ion batteries, and corrosion-resistant coatings. In this paper, we demonstrate that the microstructure of tin films can be precisely controlled by addition of certain combinations of organic additives to a methanesulfonic acid based electroplating bath. Tin films are deposited on a copper substrate by varying the concentrations of two such additives, namely, hydroquinone and gelatin. The plating process is characterized by linear sweep voltammetry and insights into the resulting microstructure are obtained using imaging by scanning electron microscopy. It is found that the addition of hydroquinone alone results in non-uniform tin films having significant dendrites and does not affect the hydrogen gas evolution during electroplating. The addition of gelatin alone is found to suppress the evolution of hydrogen gas and affects the film nucleation process that controls its grain morphology, but also creates pinholes in the film and sludge in the bath. A combination of the two additives, however, yields a highly uniform grain structure with little to no dendrite formation and a delayed onset of hydrogen gas evolution, which is highly desirable during the electroplating process. Experiments are carried out that show that the mechanism of tin film nucleation and growth during electroplating is altered in the presence of the additives, where gelatin suppresses the tin nucleation rate (thereby controlling the grain size) as well as the formation of large dendrites; while hydroquinone, acting as an antioxidant, prevents pinholes, creates a uniform film, and avoids sludge formation in the presence of gelatin. This work demonstrates a path to control electroplated tin microstructures by varying bath chemistries and process parameters which will prove highly useful to industry. (c) 2018 The Electrochemical Society.