An Investigation on the Interface Corrosion Behaviors of Cobalt Interconnects in Chemical Mechanical Polishing Slurry

Abstract

Cobalt is widely regarded as the most promising interconnect material for 10 nm node and beyond. The development of a chemical mechanical planarization (CMP) slurry suitable for cobalt interconnect is a critical component for the application of cobalt interconnect. During CMP process of the interconnect layer, the achievement of high-quality surface after planarization is greatly challenged by the metal corrosion in CMP slurry. In this contribution, the corrosion behavior of cobalt in a slurry with potassium persulfate (KPS) as an oxidizer, glycine as a complexing agent, and benzotriazole (BTA) as an inhibitor was investigated. Static erosion rates (SER) of cobalt in the slurry at various pH values with and without the inhibitor were examined. The result showed that SER of cobalt increased slightly with increasing pH, whereas BTA clearly inhibited the corrosion of cobalt in the slurry. Scanning electronic Microscopic analysis revealed that BTA could improve the morphology of cobalt surface which was deteriorated due to corrosion in planarization slurry of pH = 9. Electrochemical corrosion measurements were conducted to further investigate the effects of BTA. The potentiodynamic polarization curves indicated that as the BTA concentration increased, the corrosion potential increased, while the corrosion current density decreased. The corrosion of cobalt was effectively inhibited by adding 0.4wt% BTA in the slurry, with an inhibition efficiency of 99.02%. The electrochemical impedance data showed that the Nyquist plots of cobalt contained two rings in the slurry without BTA. The high-frequency ring was formed by cobalt oxide, and the low-frequency ring was formed by double layers. While in the BTA-containing slurry, the Nyquist plots contained only one ring at a high frequency formed by double layers, with a significantly larger diameter than that in the slurry without BTA. It can be concluded that BTA is capable of preventing cobalt from forming an oxide layer, and thereby, reducing electrochemical corrosion. Finally, the X-ray photoelectron spectroscopy was implemented to quantitatively analyze the surface's valence composition of cobalt in various solutions. The results showed that when the KPS was added as an oxidizer, a double-layer of passivation was formed on the surface of cobalt, with a Co²⁺ rich inner layer and Co³⁺ rich outer layer. The addition of glycine resulted in the dissolution of the outer layer oxide, reducing the content of Co³⁺ in the passivation layer. The addition of BTA could suppress the oxidation of Co by KPS, and lowered the Co³⁺ content on the cobalt surface. It can be demonstrated that the CMP slurry developed in this work effectively inhibited the corrosion of cobalt in an acid solution, which may solve the problem of galvanic corrosion between the cobalt interconnect and barrier layer in CMP process.

Keywords

cobalt interconnects / chemical mechanical planarization / corrosion / glycine / benzotriazole

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