Development of stable, non-cyanide solutions for electroplating Au-Sn alloy films

Abstract

Au-Sn alloys, with a range of compositions, can be successfully pulse plated onto blank wafers and patterned wafers using a slightly acidic, chloride based solution. However, the plating solution has limited stability (about two to three days), after which deposit compositions are not controllable. The aim of this work was to develop a more stable Au-Sn electroplating solution. An alternative, to a single Au-Sn electroplating solution, is to prepare separate Au and Sn solutions and then mix them as needed to generate the Au-Sn solution. Using this approach, Au and Sn solutions were produced, which were stable for at least 12 months. The stable Au solution consisted of KAuCl4, ammonium citrate, and sodium sulfite; the stable Sn solution consisted of SnCl2 • 2H2O and ammonium citrate. Ascorbic acid is an important bridge component between the 2 solutions. Electroplating results showed that deposit composition, morphology and plating rate from the newly developed mixture were very similar to those obtained from the conventional Au-Sn solution.