(295c) Formulation of Post Cobalt CMP Cleaning Solution Based on Stability Constants of Metal Complexes

Post chemical mechanical planarization (CMP) cleaning has been challenging, with shrinking feature size, especially below 14nm. During polishing, the wafer surface is contaminated with various defects such as residual particles, foreign materials, metallic impurities, etc. One of the main challenges in post CMP cleaning is the removal of the organic contaminants (e.g. metal complexes) formed during polishing. Typical azole derivatives are used as metal passivation agents in the polishing slurries. One such commonly used passivating agent is benzotriazole (BTA), which forms metal complexes during polishing.

In this study, we focus on the formation and the removal of cobalt-benzotriazole (Co-BTA) complexes based on their stability constants. We also discuss how the various components present in the cleaning solution help in the removal of these contaminants from various film surfaces that are used in cobalt interconnects. The stability constant, log K_eq, of Co²⁺-BTA complex was found to lie between 2.7 and 3.9 in the pH range of 6 ∼ 12, much smaller than the Co³⁺-ethylenediaminetetraacetic acid (EDTA) complex (log K_eq = 36.0). The role of potassium persulfate (K₂S₂O₈) and EDTA, that help in the oxidation and ligand exchange of Co²⁺-BTA complexes, respectively, is also established. Additionally, sodium dodecylbenzenesulfonate (SDBS) was able to suppress both the potential difference (∆E_corr) to about 10 mV for the Co/TiN couple, and the corrosion currents of Co film too. Thus, we were able to remove the Co-BTA complexes adsorbed on Co, Titanium Nitride (TiN), Silicon Nitride (SiN), and Silicon dioxide (SiO₂) film surfaces, by exposing them to an aqueous solution of 50 mM EDTA, 1.5 wt% K₂S₂O₈, and 1 mM SDBS at pH 12.