High Speed Electrochemical Deposition Bath for Sn-Ag Alloy and Evaluation of Micro Solder Bump Plated on Ni-Coated Si Wafer

In flip chip packaging, especially high density interconnection, electrodeposition of Sn-Ag alloy has been attracted increasing attention from semiconductor industry as a replacement of toxic tin-lead solders. With aim of improving plating speed, life-time, and enhancing quality of micro solder bump, various electrochemical characteristics of strong acid electroplating bath composing of methane sulfonic acid (MSA), Ag-MSA, Sn-MSA, and several modification agents were investigated by using linear voltammetry techniques, cyclic voltammetric stripping (CVS) with controlled hydrodynamic condition with RDE. In addition, surface morphology, metallurgical structure and composition of Sn-Ag solder bump formed on Si wafer and deposition properties were characterized by using XRD, WDXRF, and SEM. The CVS measurements showed that suppressing the hydrogen evolution reaction at high cathodic current above 30mA/cm² was critical, and small amount, less than 0.6 g/L, of benzylidene acetone was effective for this troublesome. The fundamental electrochemical responses revealed that the concentration of metallic complex ions, silver (I) and tin (II & IV), and its stability are crucial for better electroplating. In this study, the effects of complexing agent of thiourea, anti-oxidizer of hydroquinone, and several surfactants were also examined in terms of potential shift, current density, solution stability, and deposition rate. The results also showed that most electrochemical reactions in this bath composition are influenced by rotation speed, hence hydrodynamic conditions. With optimized bath solution, electroplating parameters, such as current efficiency, deposition rate, and elemental decay, and bath lifetime were also measured on micro-patterned Si wafer, and plating performance was compared with electrochemical properties. The optimized plating bath showed high deposition rate of 0.8μm/min/ASD, longer lifetime of 95 AH/L, proper eutectic alloy composition of 3.5 w/o and suitable for micro solder bump size of less than 20 μm. In this study, the relation of electrochemical and deposition characteristics with various additives and electrical properties of micro solder bumps will be discussed.