Sn-Cu codeposition from a non-aqueous solution based on ethylene glycol for wafer-bonding applications: direct and pulse electroplating

Metal wafer bonding is being investigated in industry and research fields due to the relatively low process temperatures (T < 500 °C) and, at the same time, to obtain a metallic junction able to offer greater reliability compared with other materials. In this work, electrodeposition of eutectic Sn-Cu alloy, suitable for wafer bonding, was studied using ethylene glycol (EG) as a solvent. The resulting organic electrolyte was used for its promising properties, such as a wider electrochemical window than traditional aqueous electroplating baths. Ethylene glycol solutions containing copper(II) and tin(II) chloride salts were characterised electrochemically by means of cyclic voltammetry (CV) and linear sweep voltammetry (LSV). Deposition was performed both under direct (DC) and pulsed (PC) current conditions and the advantages of the latter are discussed, also in terms of surface morphology as observed by scanning electron microscope (SEM). Wafer bonding was successfully achieved and the interdiffusion of copper and tin species in the bonding region is discussed.