Silicon Surface Micro-structure Fabrication By Femtosecond Laser

The fabrication of surface micro-structure has many interesting applications, such as the changes of the material surface color, hydrophobic properties and surface roughness. The most important applications, for silicon surface micro-structure, are the change of the optical properties. In this paper, we summarize the ways to form surface micro-structure, detailed introduce the formation process and mechanism of the preparation of silicon surface micro-structure by using the ultra-short pulse laser, and the formation process of the femtosecond laser pulse induce silicon surface micro-structure has been further investigated.When the femtosecond laser pulse irradiate at the silicon surface, and its fluence beyond a certain threshold value, the micro-nano scale structure will form spontaneous at the radiation area. The micro-structure, on one hand, broaden the light-absorb area of the silicon surface, on the other hand, leads to the incident light reflect repeatedly between the side walls of the adjacent structures, which form a so-called "light trapping effect" to promote the absorption and transmission of the incident light. In addition, the formation of the micro-structure results in the refraction index mutation of the interface between air and substrate translate into a gradient change, thereby reducing the reflectivity. We detailed investigate the formation process and mechanism of the silicon surface micro-structure irradiated by the ultra-short pulse laser, and we mainly explore how the parameter of the femtosecond laser influence on the silicon surface micro-structure formation, such as pulse number, scan speed and polarization direction. We also research how the variation of the silicon surface micro-structure morphology relies on the parameter of the femtosecond laser, and the porous silicon structures are fabricated at the certain laser parameters.Aiming at the traditional laser induced silicon surface micro-structure processing has the demerits of irregular distribution and uncontrollable period, in order to form the silicon surface micro-structure of uniform distribution and controllable period, we propose a new method to modulate the field distribution of the radiation laser by using multi-beam interference, so as to realize the control of the characteristics of microstructures. In this paper, we use the pure phase spatial light modulator (SLM) to modulate the spatial phase of femtosecond laser to realize the multi-beam interference, and form the two-dimensional multi-spot with the distribution and period controllable. By using these multi-spots, we fabricated the regular distribution concave structures on silicon surface, the distribution character and period of which was flexible controlled by shift the phase masks on SLM. The morphology and optical character of the microstructure was measured with scanning electron microscopy (SEM) and spectrophotometer. Experimental results show that the close-packed concave structure with the period of about3.3μm was fabricated on the silicon surface under a special parameter (10×focusing lens, laser power is25mW, exposing time is30s) by loading the phase of4-facet pyramid lens with the base angle of2°, and the formed structure shows good effect of anti-reflection, its transmission between1.2μm-2μm enhance11.5%compare with the polished silicon.In the end, we detailed summarize the work, both the achievements and that need to be improved of our research are analyzed, then we show out the problems that need to be further researched.