| Fused silica (amorphous silicon dioxide, a-SiO 2) is a choice optical material in semiconductor-fabrication lithography due to its many excellent optical, mechanical, physical, and chemical properties. UV-laser-induced densification in fused silica has been observed and intensive experimental work has focused on this phenomenon. However, the mechanism of UV-laser-induced densification of fused silica still remains unclear and poorly understood.; In the present thesis, we have performed classical molecular dynamics simulations to study the relationships between 193-nm UV-laser-induced densification and absorbed laser fluence (J/cm2) under different laser pulse durations (from 50 ps to 12500 ps), pressures (-1 GPa, 0 GPa, 2 GPa, and 4 GPa), and temperatures (200 K, 300 K, and 500 K) and how UV-laser irradiation affects the microstructural properties (radial distribution functions, coordination number, neutron static structure factor, total correlation function, bond-angle distributions, and ring-size distribution) and the elastic constants of fused silica. The fused silica simulation cell contains 502 silicon atoms and 1004 oxygen atoms and periodic boundary conditions are applied in all three directions. The effect of laser irradiation is modeled as energy transfer from the absorbed laser photons to bonded silicon and oxygen atoms in silica. The absorbed energy is equivalent to a laser wavelength of 193 nm and two-photon absorption is also applied.; The induced densification is found to, at first, increase with laser fluence, but tends to reach saturation or even decrease at higher fluence. Longer laser pulse duration produces larger densification under the same fluence. Higher pressure or higher temperature also causes more densification. The Si-O bond length increases slightly with fused-silica density, whereas the O-O distance decreases notably. Over-coordinated Si and O atoms appear upon irradiation and increase in number with densification. The medium-range ordering of fused silica decreases with increasing density. New peaks appear in the bond-angle distributions of ∠Si-Si-Si, ∠Si-Si-O, ∠O-Si-O, and ∠Si-O-Si due to the emerging 2- and 3-member rings. The number of dominant 6-member rings decreases in most cases, while that of the others increases and both 6-member and 7-member rings become dominant. The Young's modulus, bulk modulus, shear modulus, and Lame constant are found to increase rapidly with increasing density of fused silica upon irradiation. |
