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Abstract
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.
Highlights
Iron is one of the most common elements on earth
Based on the existence of a turning point at 4 μN, one can infer that the polished pure iron surface is layered, probably due to the formation of the oxide film, and the resultant mechanical strength is inhomogeneous along the depth direction
The mechanical properties of the oxide film formed on the pure iron surface after being polished with the slurry containing 0.01 wt% H2O2 at pH 4.0 were investigated using an atomic force microscope (AFM) against the diamond tip in vacuum
Summary
Iron-based materials such as steels and pure iron have been widely used in various high technology applications. Pure iron has been intensively used in high energy density physics [1, 2], as a liner material [3], and in some comparative experiments as a reference material [4]. An ultra-smooth surface with excellent surface integrity is required and is even indispensable for satisfactory device performance [5]. It is known that chemical mechanical polishing (CMP), which has been widely used to manufacture ultra-large scale integrated circuits, can yield an ultra-smooth surface with nano and even sub-nano surface roughness and nearly zero subsurface damage by taking advantage of the synergetic effects of chemical corrosion and mechanical wear [7−9]. Jiang et al [10] reported that an ultra-smooth copper
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