Research On Precise Micro Processing Technology By Shaped Femtosecond Laser

Femtosecond laser micromachining is an important application of femtosecond laser in the field of material processing, which is closely related to the advanced device manufacturing technology, and it plays a direct role in promoting the development of national defense, medical and industrial manufacturing. The mechanisms of femtosecond laser micromachining processes for different materials involve atomic and molecular physics, plasma physics and material science, etc. Thus, the study of micromachining process of different materials not only is significant in understanding their physical mechanism, but also has instructive guidance for micromachining applications. The development of new laser control methods(femtosecond laser shaping technology), and the study of shaped laser processing of different materials, can not only promote the understanding of the mechanisms of material processing and developing new micromachining methods, but also improve the accuracy and speed of material micromachining. Based on the demands for laser fusion target micromachining, the femtosecond laser pulse shaping techniques are used in precision micromachining of different materials in this work. The ablation mechanism and micromachining accuracy for metal materials, hydrocarbon polymer material films and microspheres under the irradiation of femtosecond laser double pulse and shaped laser pulse have been studied. The main contents of this thesis include:The study on drilling processes of aluminum and copper is performed by using femtosecond laser double pulses. The ablation mechanisms for aluminum and copper have been discussed by analyzing the signal of transmitted light and morphology of the drilled holes. According to the experimental results, the diameter of the holes decreases, and the drilling speed slows down, with increasing double pulse delay time. Larger granules are observed around the holes at short delay times and only finer ash is observed at long delay times. It is found that, plasma shielding effect plays an important role both on hole formation and on nanoparticles generation, and the oxidation of aluminum during the drilling process will affect the ablation processes.The study of drilling copper film has been performed by changing the laser pulse shape, repetition rate, laser polarization, laser fluence and relative position between laser focus and target. The transmitted laser light signal and the ablation morphologies for copper film are measured under different experimental conditions. From which we study the ablation mechanisms for metallic materials, and the micromachining technology for producing high-quality round micro-holes is found experimentally. It’s found that, by using a laser pulse sequence of five pulses with the interval time of td=0.6 ps, micro-holes with less splashing can be obtained; by using high repetition rate laser pulses, splashed particle size can be reduced; a good round hole can be produced by using the circularly polarized laser pulse. By repeatedly optimizing experimental parameters, a micro-hole with small diameter, good roundness, smallest diameter difference between front and back apertures and no splashes, can be produced.In the study of the laser ablation of glow discharge polymers, it is found that the femtosecond laser pulse shape has important effect on the ablation efficiency and accuracy of ablation results. We also find that a micro-hole with no obvious splashing can be obtained when the laser ablation of the target is done in vacuum environment. The focal position of the laser is another important factor to affect processing efficiency and accuracy. When the laser reaches its focus before reaching the surface, the micro-holes obtained will be "small" and "round". By selecting appropriate focus position in experiments, ablation efficiency can be highly optimized and high precision micro-holes can be obtained.We developed a method that a film and a glass baseplate is used together to simulate the front and inner surface of polymer microsphere; by characterizing the baseplate after ablation, the splashing condition inside the microsphere is simulated, and the no damage processing conditions for inner wall of microsphere is obtained. It is found that by using the multiple pulse shaped femtosecond laser, with an energy of 6.0 μJ and a vacuum of 2.5 Pa, not only the splashed materials into the microsphere during processing but also the heat affected zone generated when the laser irradiated on the inner surface can be reduced, and high quality micro-holes are obtained. There is no ablation marks on the baseplate when the laser energy is reduced to 4.0 μJ. Using laser pulses with this energy, higher quality micro-holes can be obtained, which can be used for further application in laser fusion.In the study of femtosecond laser shaped pulse processing of the polymer microsphere, we developed a technique to find the spatial position and orientation of microsphere, to ensure the laser direction is strictly perpendicular to the surface and through the center of microsphere. We perform studies on the shaped femtosecond laser drilling of microspheres with different wall thickness. It is found that the minimum energies required for drilling though samples with different wall thicknesses are different. Higher laser energies are required for drilling through the thicker microspheres, because the reflection, refraction and scattering occur during laser propagation inside the hole drilled, and the thicker the wall the lesser energy of laser can be delivered to the bottom. The high quality micro-holes on microsphere is successfully produced in this work, and our study also proves that the accuracy of microsphere processing can be improved by using shaped laser pulses, and the obtained microsphere with high quality micro-holes can be used in the further practical applications.The above studies show that femtosecond laser shaping technology used in micromachining increases the controllability of material processing, the best laser shapes can be designed according to the actual needs to obtain the best processing condition, hence improve the efficiency and accuracy of micromachining. Thus, the femtosecond laser shaping technology can be widely used in applications and researches of micromachining for different materials.