The Effect of Pad-asperity Curvature on Material Removal Rate in Chemical-mechanical Polishing

In chemical-mechanical polishing (CMP), surface asperities of the polishing pad play a key role, for they transmit normal force and impart tangential motion to the hard, nano-scale abrasive particles in the slurry. It has been shown recently, however, that the soft pad asperities themselves often generate micro-scale scratches on the surfaces being polished. To mitigate scratching by pad asperities, therefore, topography control by flattening pad asperities has been proposed and experimentally validated. In this study, the effects of asperity-flattening on pad topography and the material removal rate are investigated. It is found both theoretically and experimentally that even at a relatively high pressures only the tallest of the asperities are flattened and the ratio of asperity radius-to-standard deviation of heights is increased, but the average roughness itself is little affected. Specifically, surface profiles of new and asperity-flattened pads indeed show that the average roughness of about 5 μm is changed less than ten percent. Concurrently, the material removal rate is increased by about 30 percent due in part to the increased real area of contact––the result of increased asperity radius of curvature and decreased standard deviation of asperity heights.