As semiconductor technology advances toward the 5 nm node and beyond, cobalt has attracted increasing attention as a potential replacement for copper in interconnect structures for its good resistance to electromigration and stable thermal performance. The chemical mechanical polishing process of cobalt strongly depends on the formation and removal of surface reaction layers. As a result, even small changes in the chemical environment of the polishing slurry can lead to noticeable differences in the material removal rate and surface quality. In practical polishing processes, potassium hydroxide is commonly used to adjust the slurry to a mildly alkaline pH, which inevitably introduces potassium ions into the system. Previous studies show that potassium ions can influence material removal by altering interfacial electrochemical behavior. However, when hydrogen peroxide is used as an oxidizing agent, the role of potassium ions in cobalt polishing and the related mechanisms are still not fully understood. In this work, the effects of potassium ion concentration on cobalt removal behavior were systematically studied at a pH value of 8 through two polishing slurry systems: one without hydrogen peroxide and the other containing 0.25wt.% hydrogen peroxide. Under the condition containing hydrogen peroxide, the influence of potassium ion concentration on the static corrosion rate of cobalt was also investigated. To better understand the possible interaction between potassium ions and hydrogen peroxide, four different solution environments were prepared at the same pH value, including deionized water, potassium chloride solution, hydrogen peroxide solution, and a mixed solution of potassium chloride and hydrogen peroxide. Electrochemical measurements were carried out on cobalt samples in these solutions. In addition, metallographic microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy were used to analyze the surface morphology, elemental composition, and valence state changes of cobalt after immersion. On this basis, succinic acid was introduced as a carboxyl-containing additive to regulate the performance of polishing systems containing potassium ions. The experimental results showed that when hydrogen peroxide was not present in the polishing slurry, the addition of potassium ions had only a limited effect on the cobalt removal rate and surface roughness, and the surface roughness remained at about 2 nm after polishing. In contrast, when the slurry contained 0.25wt.% hydrogen peroxide, introducing potassium ions significantly increased the cobalt removal rate, reaching a maximum of 305.67 nm/min. Meanwhile, obvious corrosion pits appeared on the polished cobalt surface, and the surface roughness increased from 2.27 nm to 101.5 nm. Electrochemical test results indicated that, when potassium ions and hydrogen peroxide coexisted, the corrosion current density of cobalt increased to 34.774 μA/cm², while the interfacial reaction impedance decreased significantly, indicating that the electrochemical reactivity of the cobalt surface was significantly enhanced. EDS and XPS results further showed that, under the combined action of potassium ions and hydrogen peroxide, the oxidation of the cobalt surface became more severe, with an increase in the content of high-valence oxides and a marked decrease in the content of metallic cobalt. Furthermore, when 40 mmol/L succinic acid was added to the polishing system containing both potassium ions and hydrogen peroxide, the surface quality of cobalt after polishing was significantly improved, and the surface roughness was reduced to 0.7 nm, reaching the atomic-scale level. Overall, potassium ions alone have a limited influence on the chemical mechanical polishing behavior of cobalt in polishing systems without hydrogen peroxide. However, in the presence of hydrogen peroxide, potassium ions enhance interfacial electrochemical reactions, weaken the stability of the surface reaction layer, and accelerate oxidation and corrosion processes. As a result, the material removal rate increases, but surface quality deteriorates. By introducing carboxyl-containing chelating agents such as succinic acid, this negative synergistic effect can be effectively moderated, leading to improved surface integrity. This improvement is attributed to the adsorption and complexation of carboxyl groups on the cobalt surface, which slows the dissolution of the surface reaction layer caused by the combined action of potassium ions and hydrogen peroxide. This study provides a clearer understanding of the role of alkali metal ions in cobalt chemical mechanical polishing and offers practical guidance for the design and optimization of polishing slurries for advanced-node cobalt interconnect applications.
Key words
chemical mechanical polishing /
cobalt /
potassium ion /
hydrogen peroxide /
synergistic effect
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Funding
National Key Research and Development Program of China (2023YFB3405500); National Natural Science Foundation of China (52350411)
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