Subsurface damage and phase transformation in laser-assisted nanometric cutting of single crystal silicon

Abstract

Single crystal Si subsurface damage and phase transformation caused by laser-assisted nanometric cutting were investigated in this paper through the ultraprecision cutting experiments and molecular dynamics simulation. Post-cutting examination of a crystal's subsurface revealed a distorted SiI layer and an amorphous Si with embedded nanocrystalline Si-III and Si-XII. As a result of insufficient contact pressure during laser-assisted cutting, the amorphous Si was directly generated from the SiI through the collapse of the crystal lattice rather than from the intermediate high-pressure phase Si-II. The newly-formed amorphous Si crystallized partially during the laser-assisted cutting and transformed into metastable Si-III and Si-XII phases caused by the laser annealing effect. In comparison to machining without laser assistance, it was found that dislocation activity was increased by a factor of ~8 × 1014 when laser assistance was applied. This gave rise to enhancement of plastic deformability of the material, with the critical ductile-brittle transition depth of cut increasing from 150 nm to 395 nm and the thickness and extent of stress in the distorted SiI subsurface layer being reduced.