Study on the microscopic removal mechanism of interconnect metal Co using AFM with Si, SiO2, and diamond probes

Abstract

Cobalt is a promising alternative to copper as interconnect metal when semiconductor technology advances to 5 nm nodes and below. Co needs to be polished but the polishing mechanism is not clear enough. This study employs AFM to investigate the microscopic removal mechanism of Co CMP in an air environment, using Si, SiO₂, and diamond probes as abrasives. The microstructure and chemical state of the scribed Co surface were analyzed using Hertz contact theory, AFM, SEM, XPS, EDS, and HRTEM. The results demonstrate that the probe material and applied load significantly affect the wear depth and width. The depth obtained by SiO₂, Si, and diamond probes is about 1, 7, and 70 nm, respectively, under the same load. The Si and diamond probes primarily remove material through mechanical plowing, whereas the SiO₂ probe combines mechanical plowing with chemical reactions. XPS analyses revealed that the SiO₂ probe facilitated the formation of more Co₃O₄ on the Co surface. HRTEM results showed a Co oxide film (∼1.5 nm thickness) and an amorphous Co layer (∼15 nm thickness) in the scribed region. The oxide film enhanced adhesion, forming a protective layer that hindered further material removal and reduced friction. This study provides theoretical support for optimizing Co CMP processes, which is crucial to next-generation integrated circuits.