Nanoscratching of copper surface by CeO₂

CeO₂ nanoparticles based slurries are widely used for chemical mechanical polishing in integrated circuit manufacturing. However, the fundamental processes of material removal and planarization remain elusive. By combining a nanoindenter system with a homemade CeO₂ tip, we investigated the nanoscratching behavior of copper film quantitatively under both constant load and ramp load modes. Based on the evolution of the coefficient of friction, the nanoscratching behavior can be divided into three regimes. For regime I, the coefficient of friction decreases sharply along with the increasing normal load and the copper undergoes mainly elastic deformation. The friction wear begins to enter regime II once the normal load reaches a critical value from where both the coefficient of friction and scratch damage begin to exhibit a changing elastic-plastic characteristic with the increasing of normal load. In regime III, the coefficient of friction reaches a steady value and becomes independent of the normal load and the deformation of copper film enters a steady elastic-plastic state. The coefficient of friction in regime I and II can be well modeled by Hertz contact theory and the classical friction models, respectively. Detailed analysis demonstrates the transition between the two models occurs when the stress concentration approaches the yield strength of copper and the material removal rate can be predicted by adjusting the parameter of the normal force and the abrasive particle size.