Effects of Surface Forces on Material Removal Rate in Chemical Mechanical Planarization

In chemical mechanical planarization, abrasive particles are pushed onto a wafer by a deformable pad. In addition to the pad-particle contact force, surface forces also act between the wafer and the particles. Experimental studies indicate the signifi- cance of slurry pH and particle size on the material removal rate MRR. In this work, a model for MRR, including the contact mechanics of multiple particles caught in the interface of a rough, porous, and deformable pad and a rigid wafer, including the influences of van der Waals and double-layer forces, is introduced. The effects of surface forces on MRR were investigated for different applied pressures, pad elastic moduli, particle sizes, molar concentrations of ions, and zeta potentials of the wafer and particles. The attractive van der Waals forces increase the MRR, while the double-layer forces, calculated to be repulsive, lower the MRR. The relative magnitude of surface forces compared to the pad-particle force increases with a smaller particle size and a pad elastic modulus. The experimental trends for the variation in MRR with slurry pH were predicted well by the model when the variation in the zeta potential of the wafer and particles with respect to slurry pH is considered.