Effect of pH on Chemical-Mechanical Polishing of Copper and Tantalum
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Effect Of pH On Chemical-Mechanical Polishing Of Copper And Tantalum Anurag Jindal1, Ying Li2 and S. V. Babu1 Departments of Chemical1 and Mechanical2 Engineering Center For Advanced Materials Processing Clarkson University, Potsdam, NY 13699 ABSTRACT pH has a strong effect on the polish rates of copper (Cu) and tantalum (Ta) [1]. In this paper, removal rates of Cu and Ta using aqueous slurries containing alumina and silica abrasives in H2O2-glycine solution are studied at varying pH values. It is observed that variation in the Cu and Ta removal rates is a direct result of the change in surface characteristics of the films. Surface characteristics such as presence/absence of a passivating layer and hardness of such layer vary with pH and hence result in removal rate variation. It is also shown that a favorable Cu/Ta polish rate selectivity can be obtained by adjusting the pH of the slurry. INTRODUCTION Cu metallization is achieved by combining the dual damascene process with a suitable liner like Ta or tantalum nitride (TaN), with excess Cu being removed by Chemical-Mechanical Polishing (CMP) in two steps. In the first step bulk of the copper is removed at relatively higher rates, and in the second step the remaining Cu and the liner are removed at lower but equal rates. For an effective application of CMP to sub-0.13 micron device fabrication, a thorough investigation of polishing behavior of Cu and Ta continues to be very important. Significant research on Cu CMP using alkaline and acidic slurries with different oxidants and inhibiting agents has been carried out and reported in the literature [3,4,5]. In contrast, much less is accomplished for Ta polishing. The nature and properties of both Cu and Ta film surfaces vary with changes in the chemistry and pH of the slurry, e.g., Cu forms a variety of oxides and hydroxides at different pH values [6]. Ta however, unlike Cu, is very inert due to its hard, passive oxide (Ta2O5) film that is stable at all pH values. A hard and dense film results in lower polish rates than the one that is soft and porous. In an earlier study [1], the effect of pH on the removal rates of Cu and Ta in DI water were discussed and it was shown that electrostatic interactions between the abrasives and the films play an important role in the polish rate behavior. In this work, the removal rates of Cu and Ta are investigated in the presence of a strong oxidizer system, hydrogen peroxide-glycine as a function of pH. The removal rates are correlated to the hardness and dissolution behavior of Cu and Ta. A comparative study of alumina and fumed silica abrasives is presented with an emphasis on the nature and properties of the film surface at varying slurry pH values. Finally, it is shown that by adjusting the pH of the slurry, Cu/Ta polish rate selectivity can be controlled. EXPERIMENTAL DETAILS Chemical-mechanical polishing Polishing experiments were carried out using a bench-top Struers DAP-V polisher and 3 mm thick copper and tantalum discs (99.99% pure) with a cross sectional area of M6.8.1
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