Mechanics of the pad-abrasive-wafer contact in chemical mechanical polishing

Abstract

In chemical mechanical polishing (CMP), a rigid wafer is forced on a rough, elastomeric polishing pad, while a slurry containing abrasive particles flows through the interface. The applied pressure on the wafer is carried partially by the 2-body pad-wafer contact (direct contact) and partially by the 3-body contact of pad, wafer and abrasive particles (particle contact). The fraction of the applied pressure carried by particle contacts is an important factor affecting the material removal rate (MRR) as the majority of the material is removed by the abrasive particles trapped between the pad asperities and the wafer. In this thesis, the contact of a rough, deformable pad and a smooth, rigid wafer in the presence of rigid abrasive particles at the contact interface is investigated by using contact mechanics and finite element (FE) modeling. The interactions between the pad, the wafer and the abrasive particles are modeled at different scales of contact, starting from particle level interactions, and gradually expanding the contact scale to the multiasperity contact of pad and wafer. The effect of surface forces consisting of van der Waals and electrical double layer forces acting between the wafer and the abrasive particles are also investigated in this work. The wear rate due to each abrasive particle is calculated based on the wafer-abrasive particle contact force, and by considering adhesive and abrasive wear mechanisms. A passivated layer on the wafer surface with a hardness and thickness determined by the chemical effects is modeled, in order to characterize the effect of chemical reactions between slurry and wafer on the MRR. The model provides accurate predictions for the MRR as a function of pad related parameters; pad elastic modulus, pad porosity and pad topography, particle related iv parameters; particle size and concentration, and slurry related parameters; slurry pH, thickness and hardness of the passivated surface layer of wafer. A good qualitative agreement between the model and the experiments is found for the variation of the MRR with respect to these parameters. Furthermore, closed form equations are derived in order to optimize the CMP parameters for maximizing the material removal efficiency, which is a measure of the ability of pad to transmit the applied pressure on the abrasive particles. The optimization of the CMP parameters described in this thesis may be particularly important for the low-pressure CMP of ultra-low-k (ULK) dielectric materials, where it is difficult to achieve acceptable MRR without compromising the porous structure of ULK materials.