Application of Multipurpose Femtosecond Laser Interferometry in High Precision Silicon Nanostructures

Researchers from the Laser Processing Group of the IO-CSIC Institute of Optics in Spain report on the application of multi-purpose femtosecond laser interference in high-precision silicon nanostructures. The related research was published in Optics&Laser Technology with the title "Versatile femtosecond laser interference pattern applied to high precision nanostructured of silicon".

Highlights:
1. A novel fs laser nanostructure configuration based on commercial Ti: Sa fs lasers.
2. A grating and spot array with a manufacturing cycle adjustable to 650 nanometers.
3. Produce amorphous stripes and spots with characteristic sizes as small as 120 nm in silicon.
4. High stripe contrast and clear local flux.
5. Suitable for various materials and applications.

Researchers have introduced a high-precision nanostructure technology based on commercial Ti: Sa femtosecond amplified laser (800 nm, 120 fs, 1 kHz, 1 mJ) beam interference. By applying this technology to the nanostructure of silicon, its potential and versatility have been demonstrated. By utilizing commercial and specialized laser manufactured diffractive optical elements as well as standard optical elements, periodic line gratings and spot arrays with adjustable periods (as low as 650 nm) can be easily manufactured. In addition, the process of manufacturing millimeter level diffraction gratings at a processing speed of up to 0.5 mm/s through multi pulse irradiation was also demonstrated. The various structures written in crystalline silicon have amorphous stripes and spots with a width as low as 300 nm. The corresponding complex morphology profiles include surface protrusions and depressions, with a minimum size of up to 120 nm, which can be controlled by laser flux and stripe width, indicating the existence of multiple competing material recombination processes in the molten phase. The special high contrast of stripe intensity and the near Gaussian intensity envelope of laser spot enable it to be patterned with well-defined local flux, paving the way for the study of single pulse flux dependence. This technology can obtain high-quality experimental data, which is crucial for modeling attempts aimed at revealing the basic formation mechanisms of complex surface morphologies of various materials. In addition, the technology introduced in this article has outstanding potential in the rapid and versatile manufacturing of high-precision metasurfaces.

Figure 1. Experimental setup for femtosecond direct laser interference patterning (DLIP) of nanostructures and intensity distribution on the sample plane recorded in the experiment.

Figure 2. A series of single pulse irradiation with increasing pulse energy were conducted in Si (111), ranging from below the amorphization threshold to 2-3 times the ablation threshold.

Figure 3: Optical micrograph and large-area AFM scan, covering the entire local flux window from below the amorphization threshold to above the ablation threshold.

Figure 4. Imprinted pattern with two cycles of 50 μ m rotated 90 ° and magnification of 20x.

Figure 5. Figure 7 (a) shows an optical micrograph of an area of the pattern recorded through transmission. The diameter of the ablation (transmission) area d ≈ 30 µ m can be easily adjusted through pulse flux and objective lens. b) Optical micrographs of single pulse laser imprinted areas in silicon (111) using laser-generated masks and Mag=20x. The image contrast is set to [0.91, 1.17]. (c) Same as (b), but after the sample moved halfway between the nanodots in both directions, a second irradiation pulse appeared on the pattern.

Figure 6. (a) shows the result of writing a 5 mmx5 mm grating at a speed of v=0.5 mm/s, where the light diffracts into rainbow colors under white light irradiation. Observing under an optical microscope (Figure 8 (b)), it can be observed that the entire grating area extends with highly periodic uniform lines.

Figure 7. Researchers wrote several 5 x 5 millimeter gratings under ablation conditions to explore the effects of laser flux, scanning speed, and grating period. The figure shows the results of gratings A1, A2, and A3.

The fs-DLIP system introduced in this article has strong versatility and can manufacture user selectable nanogrids with periods as low as 650 nm and nanodots with diameters less than 300 nm, and can be extended to large areas through sample scanning. Using the same device to manufacture diffractive optical elements and generate user designed interference patterns provides greater flexibility. In addition, using Ti: Sa fs amplification laser as the light source for fs-DLIP device has several advantages compared to more complex systems used in other works. Firstly, its commercial availability allows for wider deployment. Secondly, it has a higher repetition rate and can achieve higher processing throughput. Thirdly, due to the fewer stages of amplification/nonlinearity, the stability between pulses is relatively high. Fourthly, its Gaussian beam envelope can perform high-resolution imprinting on nanostructures with a clearly defined local flux, paving the way for the study of single pulse flux dependence.

Regarding the specific results of silicon patterning, high-precision amorphous and ablative nanostructures with very sharp boundaries have been obtained. In the amorphous state, the lateral morphology of the stripes is closely related to their width, indicating the existence of different non ablative material recombination processes in the molten phase, including Marangoni convection and surface capillary convection. The versatility of this technology in adjustable stripe period and width is expected to provide high-precision experimental data for studying the underlying mechanisms of complex surface morphology, which is not only applicable to silicon but also to various other materials. In addition, advanced and reliable control of this technology brings broad prospects for manufacturing metasurface based devices.

Source: Yangtze River Delta Laser Alliance