Prediction of Thermal Damage upon Ultrafast Laser Ablation of Metals.

Ultrafast lasers micromachining results depend on both the processing parameters and the material properties. The obtained thermal effects are negligible if a good combination of processing parameters is chosen. However, optimizing the processing parameters leading to the required surface quality on a given material can be quite complex and time consuming. We developed a semi-empirical model to estimate the heat accumulation on a surface as a function of the laser fluence, scanning speed and repetition rate. The simulation results were correlated with experimental ones on different materials, and compared with the transient temperature distributions calculated using an analytical solution to the heat transfer equation. The predictions of the proposed model allow evaluating the heat distribution on the surface, as well as optimizing the ultrafast laser micromachining strategy, yielding negligible thermal damage.

High-quality high-throughput silicon laser milling using a 1 kW sub-picosecond laser.

We report on high-quality high-throughput laser milling of silicon with a sub-ps laser delivering more than 1 kW of average laser power on the workpiece. In order to avoid heat accumulation effects, the processing strategy for high-quality laser milling was adapted to the available average power by using five-pulse bursts, a large beam diameter of 372 µm to limit the peak fluence per pulse to approximately 0.7J/cm2, and a high feed rate of 24 m/s. As a result, smooth surfaces with a low roughness of Sa≤0.6µm were achieved up to the investigated milling depth of 313 µm while maintaining a high material removal rate of 230mm3/min.

Effects of Top-hat Laser Beam Processing and Scanning Strategies in Laser Micro-Structuring.

The uniform energy distribution of top-hat laser beams is a very attractive property that can offer some advantages compared to Gaussian beams. Especially, the desired intensity distribution can be achieved at the laser spot through energy redistribution across the beam spatial profile and, thus, to minimize and even eliminate some inherent shortcomings in laser micro-processing. This paper reports an empirical study that investigates the effects of top-hat beam processing in micro-structuring and compares the results with those obtainable with a conventional Gaussian beam. In particular, a refractive field mapping beam shaper was used to obtain a top-hat profile and the effects of different scanning strategies, pulse energy settings, and accumulated fluence, i.e., hatch and pulse distances, were investigated. In general, the top-hat laser processing led to improvements in surface and structuring quality. Especially, the taper angle was reduced while the surface roughness and edge definition were also improved compared to structures produced with Gaussian beams. A further decrease of the taper angle was achieved by combining hatching with some outlining beam passes. The scanning strategies with only outlining beam passes led to very high ablation rates but in expense of structuring quality. Improvements in surface roughness were obtained with a wide range of pulse energies and pulse and hatch distances when top-hat laser processing was used.