Formulation of Sodium Hypochlorite Based Slurry for Copper-Cobalt Chemical Mechanical Planarization Process

Abstract

Chemical mechanical planarization of interconnect metal copper and barrier metal cobalt using NaOCl based slurry is investigated in this study. The slurry consists of 2 wt% silica, 0.5 wt% NaOCl and 5 mM BTA as inhibitor. The formulated slurry gives a combination of low etch rates and comparatively fair removal rates along with selectivity of ∼1:1.006 at pH 9, which are desired to be used in semiconductor industry. A decrease in removal rates for both Cu and Co is observed as the pH regime changes from acidic to alkaline. At acidic pH, the passive layer if any formed on the surface becomes very unstable resulting in higher polish rates. However, at neutral and alkaline medium, the passivation layer comprising of Co (II) oxide/hydroxides is formed which subsequently decreases the polish rate. A highly stable and dense passivation layer of Co (III) oxides comprising of mostly Co3O4 and CoOOH is formed at the highly alkaline region explaining the reason behind lower removal rates. The variation in turntable speed and down force pressure does not have any significant impact on selectivity obtained at optimum conditions. XRD analysis reveals the formation of oxide on Co surface after etching at pH 9.