Developing a Non-Cyanide Gold Bath Free of Heavy Metals for Via-Fill Applications

Electrochemically filling vias and trenches using gold solutions is increasingly important in developing electronic devices with strict reliability requirements in the compound semiconductor field. The difficulty in developing these solutions is sufficiently suppressing sidewall growth to provide a void-free, bottom-up fill (BUF). Thallium, lead, and bismuth have been used in gold baths to enable BUF. However, new chemistries that exclude these metals are being developed to avoid the high toxicity of thallium and lead, along with the possible effects of bismuth co-deposition. This report describes the preparation and characterization of novel non-cyanide gold baths that do not require additional metal ions to fill vias and trenches. These baths use organic additives to suppress sidewall growth and facilitate acceleration from within the features that enable BUF. Baths were prepared using an accelerator and a variety of suppressors. These baths were tested using chronopotentiometry to predict the fill-type according to Dow’s Model. The baths were then electroplated on coupons with vias, or trenches, and cross-section electron microscopy was used to analyze the fill-type and grain morphology within the plated features. Although typically applied to copper chemistry, Dow’s Model was also found to hold true for gold electroplating baths. This work identified organic additives capable of driving low-impurity, void-free BUF of vias.