Chengjuan Yang

Shape characteristics of microstructures in femtosecond laser multi-pulse ablation of metals

Abstract In femtosecond laser multi-pulse ablation, the ablated surface and the fluence irradiated on this surface are ever changing. Thus, the simple summarization of single pulse ablation is not an accurate shape forecast method and the influence of beam spot change must be considered. In this paper, a new simulation method is proposed to forecast the microstructure shape, in which fluence distribution, ablation directions, and ablation rates of each pulse on the ablated surface are determined according to the spatial propagation characteristics of laser and two logarithmic ablation models. Further, the microstructure shape is obtained by accumulating the effects of all pulses. Three kinds of ablation — on-focus ablation, off-focus convergent beam ablation, and divergent beam ablation — are investigated using the proposed method. Simulation results show that the influence of beam spot change on microstructure shape is obvious. In convergent beam ablation, the structure has a sharp bottom, and the sidewall changes from concave to convex with the increase of pulses. In contrast, the structure obtained by divergent beam ablation has a much higher draft-angle and flatter bottom. On-focus ablation can be regarded as parallel beam ablation when ablations proceed within the Rayleigh range and the profile keeps to the parabola function. In experiments, microstructures are machined with three kinds of ablation in typical metals, and it is proved that the proposed structure-developing model is a valid shape forecast method when vaporization is the main ablation mechanism.

Comparative experimental study of laser-induced transitions in crystalline silicon by femtosecond, picosecond, and millisecond laser ablation

In order to study laser-induced transitions of the crystalline silicon, comparative ablation experiments by femtosecond-, picosecond-, and millisecond-pulsed laser were carried out on<111>crystalline silicon wafers in this study. For each laser ablating process, final chemical composition and microstructural state of ablated material on sample surface were analyzed by X-ray photoelectron spectroscopy and transmission electron microscopy, respectively. Then the influences of laser pulse duration variation on the composition and microstructure of ablated material were also discussed. Therefore, the experimental results were considered to provide more completed and further understandings of laser-induced transitions of crystalline silicon, which may have some contribution to the development of laser-semiconductors micromachining.

Comparison of femtosecond laser-induced front- and rear-side ablation of films

Femtosecond laser rear-side ablation, in which the laser beam first passes through the substrate, then irradiates on the film at the substrate–film interface, can be used to fabricate films on a transparent substrate. In this method, the material removal mechanism is different from that in front-side ablation, and the ablation occurs in an enclosed space. The different material removal process in rear-side laser ablation will bring about some unique ablation results. In this paper, the material removal mechanisms and ablation results of Cr film in rear-side ablation are investigated by comparing with those in front-side ablation. It is found that rear-side ablation can achieve smaller feature size than front-side ablation, especially when the laser fluence is small. However, owing to its different material breaking and ejecting process, the rear-side ablation tends to have worse bottom morphology and irregular edge morphology. In addition, the microstructures in the two ablations have opposite cross-sectional shapes and the ripple period in rear-side ablation is larger than that in front-side ablation.

High-precision fabrication of Pd film by laser rear-side ablation with thick sacrificial layer

Thermal damage can be a major problem in laser machining thin films. This paper proposes a laser rear-side ablation technique that incorporates a sacrificial layer to absorb excess energy, thereby lessening the level of thermal damage created in the laser processing of metallic films. The sacrificial layer is placed between the quartz substrate and the metallic film. Experiments were performed on the laser machining of holes in a palladium film using phenol formaldehyde resin as the sacrificial layer in which the pulse energy and sacrificial layer thickness were varied. The experimental results validate the proposed approach in that the judicious choice of the values of the pulse energy and the thickness of the sacrificial layer lead to the creation of high-quality holes that do not display the effects of thermal damage. Thus, the proposed technique can be considered to be a promising method for the laser machining of metal films.

Study on the thermal effects of femtosecond laser ablation on Ti-6A1–4V

In order to have a deeper understanding of femtosecond (fs) laser ablation mechanism and its thermal effects on material, comparative experiments of a millisecond laser with high average power and a femtosecond laser with high peak energy were carried out on Ti-6Al-4V. The elemental composition of ablated material and the surrounding region was detected and observed by Oxford INCA energy dispersive x-ray spectroscopy (EDS) and its electronic digital imaging system respectively. The experimental results indicate that the recast layer that is typically produced in millisecond laser ablation and characterized by microcracks, bubbles, impurity, together with content increase of oxygen is not bypassed even when the material is irradiated by femtosecond laser pulses with ultrashort pulse duration and high peak energy. In contrast with the presence of recast layers in both two laser ablations, heat-affected zone (HAZ) was not found in femtosecond laser experiment although it is a representative production in millisecond laser ablation.

Superhydrophobic surfaces with micro-/nano-structures prepared by femtosecond laser

Synthetic superhydrophobic surfaces have attracted considerable attention because they reduce effects such as friction, erosion, and contamination of devices. In this paper, a simple method for fabricating superhydrophobic metallic surfaces with femtosecond laser pulses is presented. The laser irradiation creates a surface morphology that exhibits structures on the micro- and nanoscales. By varying the laser fluence, focus position and scanning mode, we can tune the feature sizes and morphologies of microstructures and nanostructures. The wetting properties of surface change with the micro/nano structure are characterized. The contact angles for water on polished surface and surface with micro- and nano-structures are measured and compared. It is found that the contact angles for water on the micro- and nano-structured surfaces is higher about 40° than that on the polished surface. The results indicate that laser ablation is effective surface texturing method and the composite structures with micro- and nanoscales improve the hydrophobic property of metals effectively.Synthetic superhydrophobic surfaces have attracted considerable attention because they reduce effects such as friction, erosion, and contamination of devices. In this paper, a simple method for fabricating superhydrophobic metallic surfaces with femtosecond laser pulses is presented. The laser irradiation creates a surface morphology that exhibits structures on the micro- and nanoscales. By varying the laser fluence, focus position and scanning mode, we can tune the feature sizes and morphologies of microstructures and nanostructures. The wetting properties of surface change with the micro/nano structure are characterized. The contact angles for water on polished surface and surface with micro- and nano-structures are measured and compared. It is found that the contact angles for water on the micro- and nano-structured surfaces is higher about 40° than that on the polished surface. The results indicate that laser ablation is effective surface texturing method and the composite structures with micro- and n...