Study of anodic layers and their effects on electropolishing of bulk and electroplated films of copper

Abstract

Copper anodic layers in solutions of hydroxyethylidenediphosphonic acid (HEDP), phosphoric acid, and phosphoric acid with organic and inorganic additives were analysed in situ using electrochemical impedance spectroscopy (EIS). Salt films formed in HEDP solutions were detected. No salt film was found in solutions of phosphoric acid or phosphoric acid with copper oxide, ethylene glycol and sodium tripolyphosphate as additives. Analysis of EIS data suggests that H2O molecules are the mass transport controlling species in solutions of phosphoric acid and of phosphoric acid with copper oxide, whereas Cu2+ ions are the mass transport controlling species in solutions of HEDP and of phosphoric acid with additives ethylene glycol and sodium tripolyphosphate. Very good results for electropolishing of bulk copper discs were obtained in all the above solutions, but the electropolishing of electroplated copper films on patterned silicon wafers is more challenging. In this case, good planarization was obtained only from HEDP solutions. Experimental results and theoretical analysis indicate that surface profile of anodic layers is an important factor influencing planarization of electroplated copper films on patterned silicon wafers. An electrically resistive anode film with macro surface profile can lead to planarization of the gentle surface undulations of electroplated copper films on trenched silicon wafers due to migration smoothing, diffusion smoothing, and ohmic levelling effects.