| With the rapid development of semiconductor industry towards miniaturization and integration,the research of advanced precision machining technology for micro devices has attracted the attention of the world.It is of great value to fabricate precise microscale structures on the micro device surface.In recent years,due to its excellent thermal and chemical stability,single crystal silicon carbide has been rapidly developed and applied.Micromachining technology with high efficiency and high quality for hard and brittle silicon carbide need to be developed urgently.Ultrafast laser processing technology has become an essential tool for micromachining due to its wide available range of materials,high flexibility and high accuracy,and it is also one of the best methods for machining silicon carbide.In the air environment,the problem of ablation debris shielding still exists when ultrafast laser processing silicon carbide.Although the liquid-assisted ultra-fast laser processing technology can avoid the above problems,the laser-induced cavitation bubbles will interfere with the laser beam,failing to accurately predict and control the processed microstructure shape.Owing to the instantly removing of ablation debris by laser-induced spontaneous microjet,the ultrafast laser-induced microjet-assisted ablation technology is developed in this dissertation,which realizes the high-efficient and highquality machining of silicon carbide.Moreover,according to the law that the polarization effect and the secondary ablation effect jointly affect the microstructure shape,an ultrafast laser ablation prediction model is established.Combined with the beam polarization shaping technology,the processing of controllable microstructure is realized.This dissertation mainly includes the following aspects:The dynamic evolution behavior of laser-induced cavitation bubbles is observed with a high-speed camera,and theoretical analysis is carried out to reveal the mechanism of continuous microjets formed by the accumulation and acceleration of pulsed laserinduced microjets.The laser and liquid parameter conditions for generating stable and continuous microjets are obtained.Through the high-speed camera observation experiment,the reflection effect of the solid wall on the microjet and the formation mechanism of continuous microjet opposite to the laser scanning direction are explained.The role of laser-induced microjet in removing debris and bubbles is explored.Based on the role of laser-induced microjet in removing ablation debris and bubbles,an efficient laser-induced microjet-assisted ablation processing technology is explored.According to the coaxial on-line observation experiment and theoretical analysis,the mechanism of material removal in laser-induced microjet-assisted ablation process is revealed.Through single factor experiment,the influence of laser processing parameters on processing efficiency and surface quality is explored.According to the high-speed camera observation experiment of laser-induced cavitation bubble evolution,the influence of laser repetition frequency on the processing stability is analyzed.Through theoretical calculation,the effect of flowing liquid film on increasing the maximum laser repetition frequency for stable processing is revealed.The influence of flowing liquid film thickness and velocity on processing stability and processing efficiency is studied.Based on the accurate description of laser polarization effect and secondary ablation effect,the prediction model of ultrafast laser ablation is established,and the accuracy of the model is verified.Through the experiment of linear polarization laser ablation with changing the polarization direction,the influence of the angle between the polarization direction and the laser scanning direction on the ablation shape is explored.And the reason for the formation of asymmetric microstructure by linear polarization laser ablation is explored.According to the simulation calculations and experimental results,the reason for the formation of symmetrical V-shaped structure is revealed to be secondary ablation of reflected laser beam.Combining laser-induced microjet-assisted ablation technology and beam polarization shaping technology,the preparation methods of two special microstructures(W-type and U-type)are proposed.Owing to the symmetrically distributed of polarization component along the scanning direction,the cylindrical vector beam can process symmetrical microstructure that does not change with the scanning path direction.Based on the ablation prediction model and the laser parameter optimization experiment,the mapping relationship between the microstructure shape and the laser parameter is established.Then the process parameter optimization matching strategy is proposed to control the microstructure profile,and is verified by experiments. |
