Dissolution, corrosion and environmental issues in chemical mechanical planarization of copper

Abstract

Chemical mechanical polishing (CMP) of dielectric and metal films has become a key process in manufacturing devices with ultra large scale integration (ULSI). In a CMP process, planarization is achieved by polishing a wafer with uneven topography using colloidal slurry consisting of sub-micron sized abrasive particles, oxidant and various additives. Hydrogen peroxide and hydroxylamine are commonly used oxidants in copper CMP process. To achieve planarization, the low lying areas have to be protected while the higher areas are polished away. This requires low static dissolution rate of copper in low areas. Another major issue in copper CMP is galvanic corrosion during barrier polishing step where both copper and the barrier metal are exposed to the slurry. The main goal of the research reported in this dissertation is to understand the dissolution and corrosion issues during the removal of copper in hydroxylamine based chemistries. Electrochemical and physical methods such as profilometry were used to obtain copper removal rates. Among the variety of organic compound tested, benzotriazole and salicylhydroxamic acid were identified as potential corrosion inhibitors for copper. The passive film formed on the copper surface by the addition of benzotriazole and salicylhydroxamic acid was characterized by X-ray photoelectron spectroscopy and atomic force microscopy. The passivation and repassivation kinetics were investigated in detail and a passivation mechanism for copper in hydroxylamine in the presence of benzotriazole and salicylhydroxamic acid chemistries is proposed. Copper removal experiments were performed on a specially designed electrochemical abrasion cell (EC-AC) in the presence and absence of inhibitors. The effect of anodic potentials on the dissolution of copper in various chemistries was studied to identify suitable conditions for electro-chemical mechanical planarization process. The extent of galvanic corrosion between copper and tantalum was estimated using electrochemical polarization measurements. A novel setup was designed to make direct measurement of the galvanic current between copper and tantalum and was successfully used to measure galvanic current in various chemical systems. CMP and post CMP cleaning operations account for almost twenty five percent of the total water usage at semiconductor fabrication plants. The waste water has to be treated to remove copper and unused oxidants and organic additives before it can be recycled or disposed. Fundamental studies on the treatment of copper CMP waste water using boron doped diamond electrodes was performed. The feasibility of copper deposition and organic oxidation was established and a design for a novel reactor is proposed.