Spectral Analysis of Frictional Forces in ILD CMP
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Spectral Analysis of Frictional Forces in ILD CMP Ara Philipossian 1, Daniel Rosales-Yeomans 1, Leslie Charns 1, Chris Rogers 2, Toshiroh Doy 3 and Masaharu Kinoshita 4 1
Department of Chemical and Environmental Engineering, The University of Arizona, Tucson, AZ 85721 USA 2 Department of Mechanical Engineering, Tufts University, Medford, MA 02155 USA 3 Department of Mechanical Engineering, Saitama University, Urawa, Saitama, 338-8570 JAPAN 4 Rodel-Nitta Company, Tokyo, 104-0061 JAPAN ABSTRACT Spectral analysis of real-time friction data obtained during ILD CMP is used to elucidate the tribology of the process in terms of stick-slip phenomena. Fourier transform analysis is employed to quantify the total amount of mechanical interaction in the pad-slurry-wafer interface as a function of various IC-1000 pad surface textures, PL-4217 fumed silica concentrations, relative pad-wafer velocities and applied wafer pressures. A new parameter termed the ‘Interfacial Interaction Index’ (γ) is defined and determined empirically by integrating the amplitude of the force spectra over a wide range of frequencies. Values of γ extracted from individual force spectra are in qualitative agreement with the tribological information obtained in previous studies using Stribeck curve analysis. This new method is remarkable since it has the potential to eliminate having to perform a multitude of experiments needed for constructing and interpreting Stribeck curves. For a given tribological mechanism, analysis of the spectra for various types of pad textures indicates significant differences between the K-Grooved pad and other types of pads. A qualitative model relating the observed spectra to pad storage modulus is presented as a potential explanation for the above observation. INTRODUCTION In ILD CMP, Stribeck curves are useful in determining process tribology. Regardless of pad texture, results have shown a strong correlation between system tribology and ILD removal rate at silica solids concentrations at or above a certain threshold value [1]. Furthermore, system tribology, especially when considered in conjunction with average coefficient of friction (COF), has been shown to dramatically affect pad life [2]. However, such tests have several drawbacks mainly associated with the laborious nature of experimentation and the analysis involved in constructing the curves. These include the: • •
Need for having to perform a large number of polishing experiments (typically in the order of 20 or more) at various pressures and pad-wafer velocities Uncertainty in estimating the constituents of the Sommerfeld Number [3]. These include approximations of the slurry film thickness between the wafer and the pad [1], the localized pressure applied by the wafer [1], and the potential variability of the viscosity of the slurry at high shear rates which is typical of most CMP processes.
This study strives to determine whether, for a given run, the real-time raw friction data obtained during ILD CMP can be used to quantify the total amount of mecha
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