Predication of ablation depth and crater shape in femtosecond laser micromachining of wide bandgap materials

Although the use of the femtosecond laser for micromachining has recently become more popular, the selection of process parameters has been relying mainly on the trial-and-error procedure. This is because at the femtosecond time scale and nanometer length scale the complicated physical phenomena occurring during the laser-material interactions are not well understood. Apparently, fundamental studies to understand the governing laws are required to identify and control key process parameters in order to achieve high quality ablations. This study proposes a quantum model to study the femtosecond laser ablation of wide bandgap materials. The Fokker-Planck equation is employed to calculate the free electron generation through the impact ionization and photoionization processes. The quantum theories are used to calculate the free electron heating, free electron relaxation time, and the spatial and temporal dependent optical properties for the dense plasma generated by the femtosecond pulse. The predicted ablation threshold and ablation depth of barium aluminum borosilicate and fused silica are in agreement with published experimental data. The model greatly improves the prediction precision of ablation depth and can predict the crater shape. The flat-bottom crater, observed experimentally by a femtosecond Gaussian beam, can be well explained by the proposed model.