Chemical Mechanical Polishing of Tantalum Aqueous Interfacial Reactivity of Tantalum and Tantalum Oxide

The surface reactivity of tantalum metal and its associated oxide in aqueous environments was explored using electrochemical impedance spectroscopy (EIS) and potentiodynamic electrochemical dc tests in combination with tantalum blanket wafer removal rates produced by chemical mechanical polishing. The EIS data indicated that the transient tantalum oxide films formed during polishing are thinner and have a lower charge-transfer resistance at pH 2 compared to pH 10. The thinner film at pH 2 is attributed to easier mechanical removal of the tantalum oxide at low pH compared to high pH. Polishing experiments showed tantalum blanket wafer removal rates 6 to 20 times greater at pH 2 compared to pH 10 for three different oxidizing agents. In addition, there was a measurable induction time before material removal via polishing at pH 10 where pH 2 showed no significant induction time. The mechanical removal of the surface oxide, not charge-transfer, is postulated to be the rate limiting step in tantalum polishing conditions utilizing low downforces, low abrasive concentrations, and a soft pad within the pH range 2-10. The ease of mechanical removal at low pH may result from enhanced interactions of tantalum surface functionalities with abrasive or solution species.