Comparison of Glycine and Citric Acid as Complexing Agents in Copper Chemical-Mechanical Polishing Slurries
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Comparison of Glycine and Citric Acid as Complexing Agents in Copper ChemicalMechanical Polishing Slurries Venkata Gorantla and S.V.Babu Center for Advanced Materials Processing, and Department of Chemical Engineering, Clarkson University, Potsdam, NY 13699. ABSTRACT Two complexing agents, glycine and citric acid, in hydrogen peroxide based slurries for planarizing copper have been compared. Copper dissolution and polish rates and in situ electrochemical experimental results at various slurry pH values and hydroxyl radical concentrations at pH=8.4 are presented. It was observed that the pH of the slurry has a strong influence on copper dissolution and polish rates. While high copper removal rates were observed with citric acid-peroxide solutions at low pH values, glycineperoxide system yielded high Cu removal rates at alkaline pH values. Copper dissolution rates in both the systems at pH 4 and 8 were consistent with the electrochemical measurements. The concentration of hydroxyl radicals generated in citric acid-peroxide system was less than that of those generated in glycine peroxide system at pH=8.4 indicating low copper removal rates at alkaline conditions in the former system. INTRODUCTION Chemical-mechanical planarization (CMP) involves polishing of wafers against a polyurethane pad under pressure mediated by slurry consisting of abrasives and various chemicals. The material removal and planarization of a patterned or uneven surface during the CMP process occur primarily because of the synergistic effect of the chemical reactions between the surface being polished and chemicals in the slurry and the mechanical action due to abrasives. Though considerable progress has been made in exploring different chemicals as additives in Cu CMP slurries [1-9], significant gaps still exist in obtaining a comprehensive view of their interactions. For example, there is very little discussion of the influence or effectiveness of different complexing agents that can be used for Cu CMP. A comparative study of various reagents will help not only in identifying better CMP slurries but also in determining factors contributing to material removal rate. In this work, the role of two complexing agents, glycine and citric acid, in H2O2 based slurries during CMP of copper was investigated with an emphasis on understanding the difference in dissolution and polish rates as function of slurry pH. In situ Tafel polarization experiments and hydroxyl radical trapping experiments performed to understand the effect of these complexing agents on copper CMP will also be discussed. EXPERIMENTAL DETAILS In situ Electrochemical Experiments The experimental set up and parameters for measuring in situ Tafel polarization curves for a copper disk polished with various slurries against an IC 1400 k-groove pad
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