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Molecular dynamic simulation is applied in analyzing the deformation of silicon surface under the impact of large silica cluster. The mechanism of such a deformation is largely different from the cases of ion bombardment and indentation. With the impact of large silica cluster, the silicon surface is extruded due to the combinational effects of thermal spread, phase transformation, and crystallographic slip. It is found that thermal spread is the most significant one among these three effects. The extrusions on silicon surface will be in embryo during the impact unloading stage and will grow up during the cluster rebounding stage. Furthermore, the critical impact velocity to induce the formation of extrusions on silicon surface is associated with the incidence angle of the cluster, while it is independent from the size of the cluster. The findings are instructive in optimizing the process parameters for ultraprecision machining of silicon wafer.

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