| Abstract [eng] |
Nowadays, laser cutting, drilling and micromachining devices use mirror systems that make possible the fast translation of the laser beam across the working space. In such systems, scanning mirror changes its position over million times per second. Therefore, a scanning mirror needs to be capable of handling deformation force without compromising on accuracy and moving speed. Different materials, from glass to ceramics and metals, are used as scanning mirrors. Nevertheless, for high-tech application it is used silicon carbide (SiC). It bolsters variety of useful properties making it superior among other materials. Its non-toxicity, stiffness, high radiation hardness, chemical, mechanical and thermal stability makes it very attractive. It is widely used in LIDAR scanning systems and high-rate laser micromachining. However, due to the high hardness of SiC, it is not easy to obtain high surface smoothness by traditionally mechanical polishing process. Microstructure defects, like pores, grain boundary damages are unavoidable. Thus, further SiC surface modification is required. In this work, the technology of SiC surface quality improve by silicon (Si) layer was developed. Firstly, uncoated sintered SiC substrate surfaces were investigated. From AFM surface topography images, it was seen that microstructure defects, like pores and polishing scratches present on a surface. Such surface defects have a major influence on light scattering at 355 nm wavelength. The SiC substrate with 30.9 nm Root Mean Square (RMS) roughness leads to ~1200 times higher light scattering than bare fused silica substrate with 0.3 nm RMS roughness. From the literature overview, it was found out that one of the best solutions to improve SiC surface quality is to coat thick silicon layer on top and then polish it. Nevertheless, it was not shown detailed coating and polishing parameters. Therefore, evaporated Si layer properties depending on the deposition parameters were studied. It was shown that coated with Si layer SiC substrate's RMS roughness has no significant dependence on substrate temperature or deposition rate. However, chemical-mechanical polishing with 70 nm size cerium oxide particles is capable to reduce coated SiC surface roughness up to 2 times in comparison to initial. Compressive stress up to 200 MPa is observed at low temperature evaporated Si layers. This is caused by the difference in thermal expansion coefficient and intrinsic stress due to layer microstructure. Such compressive stress decreases with substrate temperature increase. With the increase of substrate temperature to 300°C during deposition the compressive stress value was reduced to only 60 MPa. Such stress caused by Si layer has minor impact on SiC substrate flatness. It was found, that a 2000 nm thick Si layer was cracking on a SiC substrate. Layer cracking is caused by high residual tensile stress and SiC surface pores acting as cracking predecessors. The found optimized deposition parameters are: layer thickness of 1300 nm, substrate temperature of 300°C and deposition rate are — 4 Å/s. Finally, modified (coated with Si and then polished) SiC substrate were coated with multilayer dielectric Bragg mirror. It was designed to reflect 355 nm wavelength radiation at zero angle of incidence. In this case, no significant pores or other surface defects were found. Furthermore, relatively low 2.3 nm RMS roughness and high 99.6% reflectance value at 355 nm wavelength were successfully achieved. The same reflectance level was measured for the fused silica substrate coated in the same deposition process. In contrast, unmodified SiC substrate coated with such multilayer mirror showed only 98.5% reflectance at 355 nm wavelength. In this work, it was developed technology for a significant increase of SiC substrate UV reflectivity. This opens further possibility of effective use of such optical elements in laser scanning systems. |
