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Sub-Nano Depth Scratches on Various Crystal Surfaces During Chemical Mechanical Polishing

Surface defects deteriorate the performance of optical crystals and semiconductor crystals. Chemical mechanical polishing technology is one of the most effective methods to smooth surfaces and meanwhile to reduce surface defects like cracks, pits and scratches. However, it was found that scratches still remain on some crystal surfaces even after chemical mechanical polishing. Various crystals such as Si, YAG, Lu2O3 and Al2O3 were polished with silicasol with the particle diameter of 80 nm, and the surfaces morphologies were measured by AFM. The surfaces of Si and YAG crystals are covered with sub-nanometer depth scratches, while there are atomic step structures formed on the surfaces of Lu2O3 and Al2O3 crystals. To reveal the underlying mechanism of the formation of sub-nanometer depth scratches, the interactions between particles and crystal surfaces during CMP are studied through Hertz theory and First-principle calculation. It is found that the appearance of such scratches is closely related to the properties of atomic structure.