The Thermal Effects of CMP as a Particle Augmented Mixed Lubrication Tribosystem
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The Thermal Effects of CMP as a Particle Augmented Mixed Lubrication Tribosystem Gagan Srivastava and C . Fred Higgs III Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh PA 15213 ABSTRACT Most chemical mechanical polishing (CMP) researchers assume that the polishing occurs in the mixed-lubrication regime, where the applied load on the wafer is supported by the hydrodynamic slurry pressure and the contact stress generated during the pad-wafer contact. Consequently, the particle augmented mixed lubrication (PAML) approach has been employed as an extremely high-fidelity asperity-scale mixed-lubrication CMP model in the past. Recently, a more computationally efficient PAML approach, PAML-lite, which considers the slurry’s fluid and particle dynamics, the pad/wafer contact mechanics, and the resulting material removal, was introduced. The current work presents the PAML-lite framework with the isothermal assumption relaxed. As a result, wafer-scale interfacial temperatures during CMP can be predicted by considering asperity heating and dissipation of the heat into the solid and fluid media in the sliding contact. INTRODUCTION There are several factors that affect the material removal procedure such as the chemical and / or mechanical effects of the contact pressure, relative velocity, types of pad and slurry, polishing temperature and other process conditions. Among these factors temperature is one of the most prevalent, still often overlooked factor that affects the material removal. White et al. [1] showed that the energy required to remove oxide and copper on a wafer is strongly correlated with the temperatures measured on the pad. Rentlen and Ninh [2] showed that the activation energy of copper removal is on the order of 0.5 – 0.6 eV. This meant that 100C increase in the pad temperature was sufficient to double the polishing rate. Pad temperature has also been used to monitor metal CMP process. Multiple authors have claimed that pad temperature can be used for endpoint detection. Hocheng and Huang [3] presented a list of patents that used thermal methods to monitor CMP process. There have been multiple, somewhat similar explanations for the heat addition to the wafer-slurry-pad tribosystem. White et al. have claimed that the temperatures measured on the surface of the pad are are a function of frictional force, relative velocity and polishing time. According to Seok et al. [4] the mechanical abrasion by the abrasive particles causes friction that generates frictional heat on the contacting interfacial area, and this heat plays a key role in accelerating the chemical reactions for material removal. In this paper, a novel multiphysics framework has been introduced that can be used to model the thermal interactions between the wafer, pad and the slurry. The model accounts for majority of the physics affecting the CMP process: the hydrodynamic pressure generated by the slurry, stresses generated due to solid-solid contact, material removal by abrasives wearing down the surface, and heat gene
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