Femtosecond Laser And The Silicon Nitride Crystal Interaction

The silicon nitride crystal film(β- Si₃N₄) be widely used in micro-electronic industry, photo-electronic industry, machinery industry, etc. Studying the interaction between femtosecond laser and Si₃N₄ crystal film can not only extend the theory of interaction between femtosecond pulse and the dielectric, provide reference for other insulating materials, but also extend the use of silicon nitride crystal film in micro-electronic, photo-electronic and nanometer materials device.In this paper, some results of theoretical studies and numerical calculations about the interaction between femtosecond laser andβ-Si₃N₄ crystal film are presented. The main contents are classified as follows:Firstly, the process of the interaction between femtosecond laser andβ- Si₃N₄ crystal film is described according to the theory of the interaction between femtosecond laser and insulating materials. Multiphoton ionization and impact ionization are two primary nonlinear effects in the process. There are two processes in the process of the interaction between femtosecond laser andβ- Si₃N₄ crystal film, the first one is that electron ionized through multiphoton ionization and impact ionization ,and produced plasma in the material. After that plasma absorbed laser energy greatly untilβ-Si₃N₄ is evaporated.Secondly, the interaction between femtosecond laser and silicon nitride crystal film (β-Si₃N₄) is analyzed and numerical studied in detail with Fokker-Planck (F-P) equation. First, we get the multiphoton ionization coefficient and impact ionization coefficient in different laser parameters . Then we evaluate F-P equation in detail with MATLAB programmer, and get the damage threshold ,crater shape, the verse of the electronic density, the verse of laser fluence,the verse of reflectivity of Si₃N₄ film in three different laser parameters, which are 780nm, pulse width greater than 100fs; 800nm, pulse width greater than or equal 100fs; 800nm, 15fs. we also consider different laser intensity and laser radius.Finally, considering laser intensity distributing in time and space, we calculate the breakdown point, the earliest damage time and plasma absorption etc, with the moving breakdown model which improved by Docchio, when long laser pulse ablation silicon nitride crystal film. we calculate the breakdown point and the earliest damage time etc, with the breakdown model which improved by Fan, when femtosecond ablations silicon nitride crystal film. We get the conclusion that the first breakdown point in the medium is mainly determined by pulse width and the laser radius.