Effect of indentation speed on deformation behaviors of surface modified silicon: A molecular dynamics study

To explore the effect of indentation speed on the deformation behaviors of silicon (Si) surface coated with silica (SiO2) in chemical mechanical polishing process, the nanoindentation test is performed by molecular dynamics (MD) simulation. It is found that the force and indentation depth at which the force drops for high speed are lower than those for low speed, implying the mechanical strength of the bilayer composite increasing with a reducing indention speed. The percentage variations of atom number of coordinated silicon and Si-O bond number consistently indicate that the SiO2 film at higher speed tends to fracture preferentially without sufficient densification and the amount of deformation is also larger. As amorphous SiO2 film tends to fracture during indentation, the original Si-I and newly generated Si-II phases induced by indentation within underlying silicon begin to transform to a Si amorphous structure, which reveals the reason for why the CN5 number for higher indentation speed are larger than those for lower speed at the same indentation depth but with lower CN6 number when indentation depth grows from 4.0 nm to 8.2 nm. Stress analysis indicates the much higher shear stress subjected to silicon at higher speed facilitates the crystalline-to-amorphous transformation.