Effect of oxide film on nanoscale mechanical removal of pure iron

In this paper, the properties of an oxide film formed on a pure iron surface after being polished with an H₂O₂-based acidic slurry were investigated using an atomic force microscope (AFM), Auger electron spectroscopy (AES), and angle-resolved X-ray photoelectron spectroscopy (AR-XPS) to partly reveal the material removal mechanism of pure iron during chemical mechanical polishing (CMP). The AFM results show that, when rubbed against a cone-shaped diamond tip in vacuum, the material removal depth of the polished pure iron first slowly increases to 0.45 nm with a relatively small slope of 0.11 nm/μN as the applied load increases from 0 to 4 μN, and then rapidly increases with a large slope of 1.98 nm/μN when the applied load further increases to 10 μN. In combination with the AES and AR-XPS results, a layered oxide film with approximately 2 nm thickness (roughly estimated from the sputtering rate) is formed on the pure iron surface. Moreover, the film can be simply divided into two layers, namely, an outer layer and an inner layer. The outer layer primarily consists of FeOOH (most likely α-FeOOH) and possibly Fe₂O₃ with a film thickness ranging from 0.36 to 0.48 nm (close to the 0.45 nm material removal depth at the 4 μN turning point), while the inner layer primarily consists of Fe₃O₄. The mechanical strength of the outer layer is much higher than that of the inner layer. Moreover, the mechanical strength of the inner layer is quite close to that of the pure iron substrate. However, when a real CMP process is applied to pure iron, pure mechanical wear by silica particles generates almost no material removal due to the extremely high mechanical strength of the oxide film. This indicates that other mechanisms, such as in-situ chemical corrosion-enhanced mechanical wear, dominate the CMP process.