Chemical processes in the chemical mechanical polishing of copper

Abstract

The mechanisms by which removal and planarization occur during the chemical mechanical polishing (CMP) of copper, used for pattern delineation in a multilevel metallization scheme, are investigated in this paper. We propose that removal occurs as mechanical abrasion of the surface followed by chemical dissolution of the abraded species. Planarization is achieved by the use of a rigid polishing pad that provides mechanical abrasion only to the high areas on the copper surface and by the formation of a surface layer on the copper during polishing to prevent dissolution of copper in the low areas. Fundamentals of electrochemistry are used to explain and predict both the dissolution of copper and the formation of a surface layer in the CMP slurry. Examples of polishing slurries are presented to demonstrate our hypotheses, including a complexing agent (ammonia) plus oxidizer (ferricyanide ion or nitrate ion) slurry and an oxidizing acid (nitric acid) plus corrosion inhibitor (benzotriazole) slurry. Finally, the mechanisms used to explain the CMP of copper are used to explain anomalous behavior during the CMP of titanium, in which the presence of copper ions in the polish slurry accelerates the polish rate of titanium. Titanium is used as a diffusion barrier and adhesion promoter for copper.