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Time
resolved Fourier Transform Electrochemical Impedance Spectroscopy (FT-EIS):
Chemical roles of high-pH peroxide-based polishing slurries in chemical
mechanical polishing (CMP) of Ta
Hydrogen peroxide is commonly used as an oxidizer in CMP slurries. Using typical abrasives (like 3 wt% alumina) in such a peroxide-based (~ 5 wt% peroxide) medium, the polish rate of Ta is found to increase sharply as the pH of the slurry is increased between 8 and 12. We have investigated the underlying chemical reactions for this effect by combining cyclic voltammetry (CV) and Fourier Transform Electrochemical Impedance Spectroscopy (FT-EIS). The results indicate that in the present case (with an alkaline solution background), hydrogen peroxide may actually play a role in oxide removal from the Ta surface. Hand-polished Ta contains a thin native oxide film. It includes discontinuous TaO clusters, covered by approximately 3 nm thick porous Ta2O. Electrochemically formed oxide is predominantly Ta pentoxide, regardless of pH and formation-method (potentiostatic or galvanostatic). We apply a triangular voltage ramp from -0.1 V to 0.4 V, and back to -0.1 V (vs. SCE) at a rate of 10 mV/s. This causes electro-oxidation of Ta (on top and below the native oxide film) through a process commonly known as the "high-field mechanism". Due to the relatively small anodic limit used here, the degree of electro-oxidation is "moderate". Nevertheless, electro-oxidation of Ta is essentially irreversible with respect to voltage cycling. Thus, during the anodic voltage scan, the Ta surface oxidizes; at the same time, catalytic breakdown of hydrogen peroxide promotes a set of surface reactions that seem to compete with the oxidation process resulting in partial dissolution of the Ta pentoxide film on the electrode. During the cathodic voltage cycle, the oxide layer is not reduced, but the peroxide-supported reactions are likely to continue to dissolve the oxide layer. The time resolved FT-EIS data (for one full voltage cycle) are presented in the form of animated Nyquist plots as functions of the applied D.C. voltage. Click here to see these animated Nyquist plots for CV of Ta in 0.1 M NaOH + 5 wt% hydrogen peroxide. Note: The time resolved EIS animations presented here are best viewed with any recent version of MS Explorer. |