Material removal mechanism of Cu-CMP studied by nano-scratching under various environmental conditions

The material removal mechanism of chemical mechanical polishing is simplified as a single nanoparticle scratching the surface of a Cu film based on contact mechanics that relates the size of scratches to the applied force. Scratch experiments were carried out using an atomic force microscope with a SiO2 spherical tip with a radius of 400 nm at low normal forces under three environmental conditions, namely air, deionized (DI) water, and slurry. The experimental results show that for a given force, the scratch depth in a slurry environment is significantly larger than that in a DI water environment, which is due to the slurry chemically interacting with the Cu film surface to generate a soft passivation layer. Consequently, the wear rate in the slurry environment was significantly larger than wear rates in the air environment and in the DI water environment, due to the soft passivation layer generation in the slurry environment. The coefficient of friction (COF) in an air environment is ∼205% and ∼356% larger than those in DI water and slurry environments, respectively, at a given scratch depth. The difference in the COF is due to the effect of buoyancy force in the liquids.