A Practical Electrolyte for the Electrodeposition of Eutectic Gold/Tin Alloy

Gold-Tin alloys are useful in many microelectronic applications, including chip bonding and wafer bump plating. The 80-20 wt% (70-30 at%) Gold-Tin eutectic alloy is particularly desirable as a solder. Electro-deposition is a low cost alternative to currently employed techniques such as sputtering, plasma jet or the use of solid alloy preforms. Prior art electroplating baths for the deposition of Gold-Tin alloy have been found to be incapable of depositing the eutectic alloy over a usable current density range and have suffered from poor solution stability. As a result, these processes have limited commercial application. An electroplating bath has now been developed which can deposit the eutectic Gold-Tin alloy over a practical current density range while maintaining very tight alloy control. Additionally, stability problems have been addressed, allowing extended electrolyte operation. The current work reports on the electrolyte operating parameters and their influence on the deposit composition and morphology. Results for typical applications, such as backside wafer plating and bump plating, are presented.