Effect of Pad Surface Texture and Slurry Abrasive Concentration on Tribological and Kinetic Attributes of ILD CMP
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Effect of Pad Surface Texture and Slurry Abrasive Concentration on Tribological and Kinetic Attributes of ILD CMP Ara Philipossian and Scott Olsen Department of Chemical and Environmental Engineering The University of Arizona, Tucson, AZ 85721 USA ABSTRACT Real-time coefficient of friction (COF) analysis is used to determine the extent of normal and shear forces during CMP and identify the lubrication regimes associated with the process. Pads with different surface textures and slurries with varying abrasive concentrations are used to polish ILD films over a wide range of operating parameters. Results show that by varying abrasive concentration and pad surface texture, one can cause the process tribology to change from ‘boundary lubrication’ to ‘partial lubrication’, to ‘hydrodynamic lubrication’. A two-phase model relating average coefficient of friction and Preston’s constant is presented. At abrasive concentrations up to 9 percent, material removal is proportional to the extent of contact between the abrasives and the wafer. At abrasive concentrations between 9 to 25 percent, removal rate is directly influenced by average COF. A new parameter termed the ‘tribological mechanism indicator’ is defined and extracted from the data, which coupled with the information on COF and ILD removal rate, results in a series of ‘universal’ correlations. A qualitative model based on pad storage modulus is used to explain the trends. INTRODUCTION Given the nature of CMP, one area that can potentially impact ILD removal rate is the COF associated with the pad, wafer and slurry abrasive particles. Based on limited reported effects of COF on polishing of silicon dioxide wafers [1,2], it would be of interest to establish relationships between the slurry abrasive concentration, the coefficient of friction, and the amount of material removal during CMP. This work has been based on the premise that during planarization, the COF in the pad-slurry-wafer region affects ILD removal rate, thus necessitating a fundamental understanding and control of the magnitude of forces involved in the process. Given the above postulation, identification of the tribological mechanisms and determination of key factors contributing to the extent of pad-slurry-wafer contact will be critical. THEORY AND EXPERIMENTAL PROCEDURE Tribology and Removal Rate Measurements: A scaled polisher was used for all experiments [3]. To measure the shear force between the pad and the wafer during polish, a sliding table was placed beneath the polisher. The sliding table consisted of a bottom and a top plate that the polisher was set upon. As the wafer and pad were engaged, the top plate would slide with respect to the bottom plate in only one direction due to friction between the pad and wafer. The degree of sliding was quantified by coupling the two plates to a load cell. The load cell was attached to a strain gauge amplifier that would send a voltage to a data acquisition board. The apparatus was calibrated to report the force associated with a particular voltage rea
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