Femtosecond Laser Ablation Mechanism And Morphology Chracteristics Of Quartz Glass

Femtosecond laser micromachining technology is a kind of micro-manufacturing method which has many unique characteristics, such as ultra-short duration, super-peak power and high repetition rate. The materials could generate locally microburst in a ultra-short time by produce high temperature and pressure plasmas, then the materials can be removed precisely. In addition, femtosecond laser can ablate dielectrics with high machining accuracy, small interaction area, and it can break through the diffraction limit. So the silica glass could be ablated precisely by femtosecond laser. Quartz glass has excellent high temperature performance and a low thermal expansion coefficient. Meanwhile, its transmittance is so high in the ultraviolet region that it can be used for lithography, space optics and laser fusion systems. In the above application fields, the method of precision ablation processing for quartz glass surface is used frequently, and the high surface quality is required. In a word, it has many practical values to investigate the removal mechanisms and processing rules of femtosecond laser. In this paper, the physical models of femtosecond laser ablation dielectric were established, and the ablation mechanism was discussed. Then the pulse, line and surface ablation experiments were conducted respectively by using the femtosecond laser system which has sapphire Ti regenerative amplifier. What’s more, the optical microscope, ESEM, ultra-depth microscope, surface steps tester and3D surface profiler were applied to analyze the ablation morphology, etching morphology, ablation depth and surface roughness. The specific work was presented as follows:(1) The physical models of femtosecond laser ablation quartz glass were built based on electron density evolution equation of dielectric and avalanche, multiphoton ionization theory. The ablation threshold, depth and volume were calculated at certain parameters by MATLAB, and then the results were verified.(2) The line ablation experiments of different scanning velocity and laser power were carried out by linearly and circularly polarized femtosecond laser, respectively. Also, the ablation line morphology of the two laser polarization states was compared. Besides, the ablation line width and depth were tested by optical microscope and ESEM respectively, and the reasons of the differences were analyzed.(3) The surface ablation experiments of different line overlap rate were conducted based on the optimized parameters of line ablation experiments, and the ablation surface morphology of the two laser polarization states was compared. Besides, the surface roughness was detected by3D surface profiler, and the optimized line overlap rate was investigated for a lower surface roughness value. In addition, the ablation depth of removed surface was detected by surface steps tester.(4) The "V" type assumptions of line ablation and surface ablation were proposed based on the experimental results of the cross-sectional profile. The relational expressions of the line and surface ablation scallop height with scanning velocity, line overlap rate were established, respectively. Then the results were calculated and verified by experiments.(5) The influences of HF etching time on the ablated surface morphology were investigated and the differences of the morphology chracteristics at different line overlap rate were analyzed. Also, the variation laws of roughness with etching time were researched, and they have guiding significance for manufacture low roughness ablation surface.