As a matter of fact, the optical characteristics of thin films are substantially different from those of bulk materials. In the case of the film thickness comparable or smaller than phonon mean free paths (MFPs), it appears that thermal conductivities ...
As a matter of fact, the optical characteristics of thin films are substantially different from those of bulk materials. In the case of the film thickness comparable or smaller than phonon mean free paths (MFPs), it appears that thermal conductivities are reduced than that of bulk materials because of phonon scattering at the interfaces between thin film and substrate. Electric and optical characteristics of materials are significantly changed by ultrashort laser pulses which are shorter than a few picoseconds, typically 10 ps. In spite of wide applications of thin film structures in semiconductor industry, the energy transport mechanism and optical characteristics in thin film structures irradiated by ultra-short pulsed lasers have been poorly understood. Thus, the ultimate goal of this study lies in investigating the energy transfer mechanism in dielectric and semiconductor thin film structures irradiated by nanosecond to femtosecond pulsed lasers, in examining optical characteristics changing with laser pulses and film thicknesses, and in studying fundamental interactions among photons, energy carriers, and phonons.
First, this study investigates numerically the ultrafast laser-induced optical breakdown in fused silica (SiO2) with the non-local type of Fokker-Planck equation. This equation derived from Boltzmann transport equation (BTE) can account for electron distributions in energy space, avalanche ionization, multiphoton ionization, and three-body recombination. The optical response of electron plasma and reflectivity at the surface boundary is given by the complex Drude dielectric function and Fresnel formulas, respectively. With very intense laser irradiation, the strong absorption of laser energy takes place and an initially transparent solid is converted to a metallic state. Full ionization is achieved at intensities above threshold and all further laser energy is deposited within a thin skin depth. It is also shown that further increase in fluence does not make the ablation depth to increase any more because of the substantial increase in reflectivity and absorption of laser energy in skin depth.
Second, the extensive numerical simulations are rigorously carried out for conductive and radiative heat transfer characteristics in thin silicon layers irradiated by pulsed lasers. The electromagnetic theory is used to predict energy absorption in nanoscale thin films, and the wave interference effects are considered through thin film optics. The optical properties of silicon are constituted from the function of lattice temperature and carrier density and the reduced thermal conductivity in thin film and thermal boundary resistance (TBR) at interfaces are estimated by equation of phonon radiative transport (EPRT). In femtosecond laser irradiation, carrier-lattice energy transfer, mostly taking place after the laser pulse is over, will then rapidly heat the ions to much higher temperatures compared to the long pulse cases. The reflectivity of silicon thin film is strong function of film thickness unlike bulk material due to the wave interference effect. The maximum reflectivity of silicon thin film is about seven times larger than the minimum reflectivity. For nanosecond pulse lasers, the spatial distributions of energy absorption appear periodic with depth, whereas the carrier and lattice temperature distributions do not show wavy patterns. On the other hand, for picosecond pulse lasers, the spatial carrier and lattice temperature distributions appear periodic with depth because of shorter pulse duration than diffusion time. It is found that thin film optics should be used in predicting temperature distribution in the semiconductor material as well as in developing optical diagnostic methods.