Study of femtosecond laser pulse drilling of silicon

Femtosecond (fs) laser drilling has been proved to be an efficient tool to drill various materials. In the drilling process, however, there are constant needs to eliminate spatter, control the taper, increase the aspect ratio and material removal rate. In addition, it is necessary to further understand the laser material interaction mechanism. In this thesis, some new features were proposed into the femtosecond laser drilling area in order to overcome these problems. In this study, the effect of six alcohol liquids on the fs laser drilling process was evaluated and compared. The relation between the volatility of the liquids and the material removal rate of laser drilling was found. A more volatile liquid could assist better in carrying away the debris and allow more laser energy to reach the ablation front. The material removal efficiency was increased by 40% when applying methanol as the assist liquid. The spatial wavelength of laser induce period surface structure (LIPSS) as fabricated in ethanol was found to be 455 nm which is about 40% less than that in air (772 nm). This finding could improve the applicability of LIPSS as optical gate and catalyst support. A systematic assessment of geometry evolution of the laser drilled through hole at different substrate temperatures was conducted. The result suggested that the entrance hole diameter was increased by 25% while the exit hole was increased by 30% when the substrate temperature was increased to 900 K. The laser drilling efficiency was also greatly increased by elevating the substrate temperature. This high drilling efficiency was attributed to the enhanced laser energy absorption of silicon wafer and thereafter wave guiding effect. The spatter area was found, however, to be continuously decrease with increasing the substrate temperature. This study provided useful knowledge for better understanding the fs laser energy absorption of silicon wafer at higher temperature. In order to further study the hole wall’s effect on the laser beam propagation inside the micro-hole during the laser drilling, a theoretical analysis was conducted by numerically solving the time harmonic Maxwell’s wave equation. The taper angle of micro-hole was varied to investigate its effect on the laser energy distribution at the bottom of the hole. The ellipse entrance hole shape and zonal structure at the hole bottom which were observed in the previous experiment were reasonably explained by using the laser intensity distribution as obtained in the present model.DOCTOR OF PHILOSOPHY (MAE