| Abstract [eng] |
Surface roughness is an important parameter that has a great influence on various material properties. It determines the rate of corrosion, wettability, biocompatibility, as well as optical properties of different materials. Low roughness (< 100 nm Ra) surfaces are difficult to achieve through an ablation-based process even with fs pulses, therefore investigation of the theoretical intricacies is of major interest when engraving transparent materials. In our study we present an in-depth investigation of the scanning techniques and how they influence the final surface roughness. We numerically investigate the evolution of the surface roughness when it is scanned with UV-IR femtosecond pulses multiple times (multiple layers) and compare the numerical results to the experimentally acquired values. We found that in the case of a single scan the dominant surface roughness determining factor is the overlap of modifications. The optimal overlap of modifications was found to be somwhere in the range of 10-25 % (minimum achieved surface roughness Ra = 20 +/- 5 nm). The determined optimal overlap of modifications maintained even when different laser sources were used. In the case of a multi-scanned surface we have determined that the resulting surface roughness can be minimized by introducing randomness to prevent the formation of periodic structures that are responsible for increase of surface roughness. It can be achieved by introducing the uncertainty of layer positioning by ± 1 μm , or by rotating every next layer at an angle that is not a repetitive of 90° or similiar (+/- 5°). The minimum achieved surface roughness after 10 pass Ra = 90 nm. The investigated theoretical model is in good relation to the experimentally acquired results and provides valuable information when optimizing the process for minimal-roughness micromachining when performing deep engraving of transparent materials. |
