Inscribing diffraction gratings in bulk silicon with nanosecond laser pulses

Femtosecond Laser Percussion Drilling of Silicon Using Repetitive Single Pulse, MHz-, and GHz-Burst Regimes

In this contribution, we present novel results on top-down drilling in silicon, the most important semiconductor material, focusing specifically on the influence of the laser parameters. We compare the holes obtained with repetitive single pulses, as well as in different MHz- and GHz-burst regimes. The deepest holes were obtained in GHz-burst mode, where we achieved holes of almost 1 mm depth and 35 µm diameter, which corresponds to an aspect ratio of 27, which is higher than the ones reported so far in the literature, to the best of our knowledge. In addition, we study the influence of the energy repartition within the burst in GHz-burst mode.

Impact of film thickness in laser-induced periodic structures on amorphous Si films

We report self-organized periodic nanostructures on amorphous silicon thin films by femtosecond laser-induced oxidation. The dependence of structural periodicity on the thickness of silicon films and the substrate materials is investigated. The results reveal that when silicon film is 200 nm, the period of self-organized nanostructures is close to the laser wavelength and is insensitive to the substrates. In contrast, when the silicon film is 50 nm, the period of nanostructures is much shorter than the laser wavelength, and is dependent on the substrates. Furthermore, we demonstrate that, for the thick silicon films, quasi-cylindrical waves dominate the formation of periodic nanostructures, while for the thin silicon films, the formation originates from slab waveguide modes. Finite-difference time-domain method-based numerical simulations support the experimental discoveries.

Internal structuring of gallium arsenide using short laser pulses

Laser writing inside semiconductors attracts attention as a possible route for three-dimensional integration in advanced micro technologies. In this context, gallium arsenide (GaAs) is a material for which the best conditions for laser internal modification (LIM) have not been established yet. We address this question by using laser pulses at a fixed wavelength of 1550-nm. A large parameter space is investigated including the response to the applied pulse energy, pulse duration (from femtosecond to nanosecond) and the focusing conditions. We report that well-defined and reproducible internal modifications are achievable with tightly focused nanosecond pulses. The measured writing thresholds are systematically compared to those obtained in silicon (Si), a more extensively studied material. In comparison to Si, we also observe that GaAs is more prone to filamentation effects affecting the modification responses. The reported specific observations for LIM of GaAs should facilitate the future process developments for applications in electronics or photonics.

Flexible, fast, and benchmarked vectorial model for focused laser beams

In-bulk processing of materials by laser radiation has largely evolved over the last decades and still opens up new scientific and industrial potentials. The development of any in-bulk processing application relies on the knowledge of laser propagation and especially the volumetric field distribution near the focus. Many commercial programs can simulate this, but, to adapt them, or to develop new methods, one usually must create a specific software. Besides, most of the time people also need to measure the actual field distribution near the focus to evaluate their assumptions in the simulation. To easily get access to this knowledge, we present our high-precision field distribution measuring method and release our in-house software InFocus [https://github.com/QF06/InFocus], under the Creative Commons 4.0 license. Our measurements provide 300 nm longitudinal resolution and diffraction limited lateral resolution. The in-house software allows fast vectorial analysis of the focused volumetric field distribution in bulk. Simulations of the linear propagation of light under different conditions (focusing optics, wavelength, spatial shape, and propagation medium) are in excellent agreement with propagation imaging experiments. The aberrations provoked by the refractive index mismatch as well as those induced by the focusing optics are both taken into account. The results indicate that our proposed model is suitable for the precise evaluation of energy deposition.

Laser-induced layers peeling of sputtering coatings at 1064 nm wavelength

Large-scale layers peeling after the laser irradiation of dual ion beam sputtering coatings is discovered and a model is established to explain it. The laser damage morphologies relate to the laser fluence, showing thermomechanical coupling failure at low energy and coating layers separation at high energy. High-pressure gradients appear in the interaction between laser and coatings, resulting in large-scale layer separation. A two-step laser damage model including defect-induced damage process and ionized air wave damage process is proposed to explain the two phenomena at different energy. At relatively high energies (higher than 20 J/cm²), ionization of the air can be initiated, leading to a peeling off effect. The peeling effect is related to the thermomechanical properties of the coating materials.

Inscribing diffraction grating inside silicon substrate using a subnanosecond laser in one photon absorption wavelength

Using focused subnanosecond laser pulses at [Formula: see text] wavelength, modification of silicon into opaque state was induced. While silicon exhibits one-photon absorption at this wavelength, the modification was induced inside [Formula: see text]-thick silicon substrate without damaging top or bottom surfaces. The depth range of the focus position was investigated where inside of the substrate can be modified without damaging the surfaces. Using this technique, diffraction gratings were inscribed inside silicon substrate. Diffraction from the gratings were observed, and the diffraction angle well agreed with the theoretical value. These results demonstrate that this technique could be used for fabricating infrared optical elements in silicon.

Pulse-duration dependence of laser-induced modifications inside silicon

The advent of ultrafast infrared lasers provides a unique opportunity for direct fabrication of three-dimensional silicon microdevices. However, strong nonlinearities prevent access to modification regimes in narrow gap materials with the shortest laser pulses. In contrary to surface experiments for which one can always define an energy threshold to initiate modifications, we establish that some other threshold conditions inevitably apply on the pulse duration and the numerical aperture for focusing. In an experiment where we can vary continuously the pulse duration from 4 to 21 ps, we show that a minimum duration of 5.4 ps and a focusing numerical aperture of 0.85 are required to successfully initiate modifications. Below and above thresholds, we investigate the pulse duration dependence of the conditions applied in matter. Despite a modest pulse duration dependence of the energy threshold in the tested range, we found that all pulse durations are not equally performing to achieve highly reproducible modifications. Taken together with previous reports in the femtosecond and nanosecond regimes, this provides important guidelines on the appropriate conditions for internal structuring of silicon.

Positive- and negative-tone structuring of crystalline silicon by laser-assisted chemical etching

We demonstrate a structuring method for crystalline silicon using nanosecond laser internal irradiation followed by chemical etching. We show a dramatic dependence of the etch rate on the laser-writing speed. Enhanced isotropic etch rates of silicon by laser-induced internal damage were recently demonstrated with strong acids, but our results add the possibility to obtain reduced etch rates leading to different topographies. Material analyses indicate the possibility to efficiently produce high-aspect ratio channels, thanks to laser-induced porosities, as well as silicon micro-bumps due to highly stressed regions. This holds promises for fabricating microfluidic, photovoltaic, and micro-electromechanical systems.