Application of the hottest femtosecond laser in mi

2022-08-08
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The application of femtosecond laser in the field of micro nano processing

femtosecond laser began to be applied to the field of micro nano processing in the early 1990s. It is precisely because femtosecond laser has the characteristics of short duration and high pulse power density that it has many unique advantages when interacting with matter: a certain ablation threshold, regular machining edges, layer by layer micromachining and machining of any material. Recent research results show that femtosecond laser micromachining has potential application value in many fields, such as micro optics, microelectronics, micro machinery, microbiology, micro medicine and so on. Different disciplines and experiments have different specific requirements, which requires corresponding processing methods to achieve specific processing purposes. This femtosecond laser deep hole processing technology and other processing technologies have begun to attract more and more researchers' attention

laser shaping technology refers to the use of optical elements inside or outside the laser cavity to change the beam shape to achieve beam shaping. Femtosecond laser pulse shaping is different from the traditional shaping concept. It mainly introduces optical devices such as beam expander, filter and diffraction template into the optical path on the basis of retaining the original peak power characteristics, so as to reduce the focusing size and remove Gauss. We can select the fluorescent components around the beam of this kind of equipment in combination with the characteristics of materials we usually need to detect, reduce pulse deformation and various shape processing. Commonly used are spatial filtering and mask control technology. Spatial filtering is to realize the shielding effect on the fluorescence at the edge of the beam and improve the optical quality of the aggregation point. Mask control is to modulate the pulse through the shape of the mask to achieve the determined processing purpose

by using the method of synchronous movement of the focusing objective lens and the receiving material, the spatial shape of the pulse before and after the focus can be easily expressed in the form of a two-dimensional plan on the material surface. By adding a small hole mask in front of the focusing objective lens, the change of the spatial shape of the beam can be observed visually through the change of the diameter of the small hole and the pulse energy before and after the small hole. Finally, the transmission metal grating with smooth edge is successfully carved by selecting appropriate parameters in the experiment

1 experimental device and method

the experimental equipment is Clark femtosecond laser processing workbench (umw-2110i, Clark MXR Inc.). The specific parameters of the laser are: the central wavelength is 775nm, the pulse width is 148 FS, the repetition rate is 1kHz, the maximum single pulse energy is 1MJ, the pulse energy can be adjusted by adding an attenuator on the optical path, and the spot diameter before focusing is 5mm; The adjustable range of mask aperture diameter is 0.5 ~ 10mm; The receiving material is a gold film (about 300nm thick) plated on the dissolved quartz substrate by sputtering method. The femtosecond laser passes through the mask aperture and then passes through 5 × The micro objective (effective focal length is 40 mm) focuses on the surface of the gold film. Using the method of synchronous movement of the objective lens and the receiving platform, the spatial shape of the pulse before and after the focus is displayed on the surface of the gold film in the form of a two-dimensional plan; The processing results were analyzed and tested by transmission optical microscope and SEM. The experimental device is shown in Figure 1

Figure 1 Schematic diagram of the experimental device

the objective lens moves axially (Z-axis) by the platform, and the material is carried by the X-Y axis. Running the Z-axis and X-axis synchronously can record the length that reaches the material threshold in the axial range near the focus on the gold film. The focus position is moved from the surface of the material to the interior of the material. Correspondingly, the image is from right to left. The results can be compared with Rayleigh length for analysis

2 analysis of experimental results

generally, if you know the central wavelength of the laser, the focal length f of the lens and the waist radius of the incident light at the front surface of the lens ω, The Rayleigh length Zr can be obtained. The expression of Rayleigh length is:

where: ω 0= λ 0f/π ω, Is the waist radius at the focus. Because the objective lens is used in the experiment, it is difficult to deduce the true waist radius from the effective working distance ω 0 the girdle radius at the focus is measured by the knife edge method, and the value is 11.5 μ m. So 5 × The Rayleigh length of the microscope objective is about 0.54M. It is suggested to establish a regular exchange system for entrepreneurs

and the beam radius in the axial range near the focus ω (z) The change of is a function related to the Rayleigh length and the beam waist radius at the focus, as shown in Figure 2, and its expression is:

beam waist radius in Figure 2( ω (z) ) schematic diagram of the change with the transmission direction near the focus

experiment by changing the diameter of the pinhole in Figure 1, observe the change of the ablation area that can be achieved in the direction of the optical axis near the focus, and record the beam transmission morphology near the focus on the surface of the gold film by keeping the pulse energy before and after the pinhole unchanged. Figure 3 shows the microscopic images in the above two cases. Among them, the running speed of Z-axis and x-axis is 0.3mm/s, and the single pulse energy is 91.7 before and after the small hole respectively μ J. Both Z-axis and X-axis strokes are 600 μ m. In the figure, the pinhole diameters from top to bottom are ∞, 4mm, 3mm and 2 mm respectively

it can be seen from Figure 3 (a) that without pinholes (openings), the ablation area is basically symmetrically distributed near the focus, and when it deviates from the focus position, the ablation linewidth increases rapidly, forming a spinning cone-shaped distribution. With the addition of small holes, the light transmission size becomes smaller, the linear scale of the ablation area gradually decreases, the difference between the focal point position and the ablation linewidth of the two wings is significantly reduced, and there are even signs of being far away from the lens (see the case where the pinhole diameter is 2mm). Change the pulse energy to ensure the same energy after the small hole. There is no obvious silicon difference in the ablation phenomenon (see Figure 3 (b)), but the ablation linewidth is increased. The above phenomenon can be well explained by equations (1) and (2): after adding a small hole, due to the limitation of the aperture, the waist radius of the lens surface is illuminated ω Reduce the waist radius at the focus ω 0 increases, and the Rayleigh length Zr becomes larger. Therefore, in equation (2), the girdle radius near the focus ω (z) The change with Z is weaker than that without small holes, and the relatively gentle processing results in Figure 3 are obtained macroscopically

Fig. 3 Relationship between beam scoring and pinhole diameter

Fig. 4 and Fig. 5 show the changes of ablation morphology near the focus under different pulse energy (measured before the pinhole) when the opening and the pinhole diameter are 4mm, 3mm and 2mm respectively, and the travel of Z and X axes is still 600 μ m。 With the decrease of the hole diameter, the pulse energy after passing through the hole will be lower than the material ablation threshold. Therefore, there are only 4 ablation traces in Figure 5 (a) and figure 5 (b), and even only 3 ablation lines in Figure 5 (c)

it can be seen from Figures 4 and 5 that when the single pulse energy is low, there is no obvious spinning cone-shaped distribution of the pulse shape near the focus under the action of holes or a certain pinhole, but the change of the beam radius near the focus is much relieved after adding pinholes, which is conducive to further research on deep processing and cutting; With the decrease of the pinhole diameter, the ablation area is significantly reduced (less than the Rayleigh length), which is mainly because the pinhole limits its size to improve its strength (especially fatigue strength) and wear resistance, corrosion resistance and other properties, and part of the energy reaches the material surface; When the aperture diameter is 4mm, the pulse transmission shape is relatively less affected by the laser energy; Similar to Fig. 3, another phenomenon in the experiment is that with the reduction of the pinhole aperture, the minimum beam waist radius of the focusing area moves closer to the lens, which can be well explained by the change relationship between the focusing beam waist radius and the distance between the beam waist before focusing and the front surface of the lens

Figure 4: the beam notch changes with the pulse. Users have generated distrust of domestic brands. Figure 5: the beam notch changes with the pulse energy. Using the above experimental results, the diameter of the hole and the pinhole is 4mm (the single pulse energy is 90 μ J) In the two cases, the gold film and stainless steel plate are punched respectively, and the images obtained are shown in Figure 6 ~ 10

Fig. 6 ring pattern engraved on the surface of gold film with pinholes (d=4mm)

3 conclusion

based on the research of femtosecond laser processing technology, this paper analyzes the influence of pinhole mask processing technology on the spatial transmission characteristics of femtosecond laser at the focal point. It is found that when a small hole is added in front of the focusing objective lens, the laser scoring or the waist change near the focus slows down; The pulse energy only affects the line width of the notch. In this study, the optimized parameters of femtosecond laser deep hole machining are obtained. Using this method, the surface of copper foil can be characterized by transmission metal grating devices

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