Studies of microvia filling mechanism and a novel Cu plating formula

Microvia filling by copper electroplating has been a key process for fabrication of high density interconnection (HDI) of advanced PCBs or IC substrates. Currently, conventional copper plating formulas used for the microvia filling typically contain at least two or three components, such as suppressor, accelerator, and leveler. A specific synergy between these additives leads to a bottom-up deposition of copper in the microvia, so that the microvia can be fully filled by the copper deposit without a void [1-6]. However, copper is also deposited simultaneously on the board surface during plating, when the conventional copper plating formula is employed. Consequently, a pattern with fine lines formed by an etching process is not easily obtained due to the thick copper film plated on the board surface. Therefore, we have developed a novel copper plating formula herein in order to achieve microvia filling without an increase in copper thickness on the board surface after the copper electroplating. In other words, this novel copper plating formula can offer a highly selective filling for microvia metallization, that is, the whole copper was exclusively deposited in the microvia. A comparison of cross-sections of copper filled microvias using two different plating formulas, one conventional and the other novel, is shown in this work. A basic cyclic voltammetry (CV) analysis was carried out to explain the filling results. A simple mathematical simulation of current density distribution is also presented to explain the filling mechanism.