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In addition Professor Subramanian is working with doctoral student Qingjun Qin on developing theoretical descriptions of various aspects of polishing in an orbital polishing tool. A SpeedFam/IPEC 676 orbital tool is available at Clarkson for testing predictions from these modeling efforts. Experiments have been performed on the polishing of blanket copper films deposited on 200 mm silicon wafers, using an abrasive-free chemical solution for material removal. In these experiments, the concentration of the chemical and the orbital speed were both varied, and the variation of material removal rate was measured as a function of radial position on the wafer in each case. One of the objectives of the model is to predict these radial variations of removal rate. The pad is constructed with a rectangular grid of grooves, and the slurry is introduced from underneath the pad through a set of 61 holes that are located at the intersections of selected grooves. The model involves describing the flow behavior in the groove structure of the pad, and combining this description with a model of the relative motion of each point on the wafer relative to the pad, along with a convective diffusion model for the transport of the chemical species in the grooves. Clarkson Distinguished Professor Goodarz Ahmadi and his group are developing a model (based on mechanical contact theory) for the chemical-mechanical polishing process. The goal of their research is to provide a fundamental understanding of the parameters that control the effectiveness of CMP for surface planarization. Their current work focuses on the abrasive particle, wafer, and pad contact and the abrasive and adhesive wear mechanisms in the chemical-mechanical polishing process. They are developing a model for interactions of pad asperities with abrasive particles and the wafer. Their analysis includes the influence of abrasive particle adhesion to the surface of the wafer. Also they are looking at the CMP process using hard and soft pads and dilute and concentrated slurries. In addition Professor Ahmadi and his students are studying the effect of abrasive particle shapes, slurry pH, and colloidal forces on the removal rate. Their model predictions are described in detail and compared with the available semi-empirical correlations in the paper " A Model for Mechanical Wear and Abrasive Particle Adhesion During the Chemical-Mechanical Polishing Process," by G. Ahmadi and X. Xia, Journal of the Electrochemical Society , 148 (3) G99-G109 (2001).
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