Structuring materials with nanosecond laser pulses (original) (raw)
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Computational and experimental study of nanosecond laser ablation of crystalline silicon
International Communications in Heat and Mass Transfer, 2011
In this paper, a numerical model of nanosecond laser ablation of crystalline silicon has been established. Based on the highly nonlinear model of heat transfer and phase change in crystalline silicon after absorbing laser light, heat transfer equation is solved by using finite element method implemented in ANSYS. The simulation of ablation depth of crystalline silicon is obtained under different conditions of laser fluence and pulse overlap. Comparing with the ablation morphology obtained from SEM observations, the computational results and experimental data show good agreement.
In laser micromachining, the ablation threshold (minimum fluence required to cause ablation) is a key performance parameter and overall indicator of the efficiency of material removal. For pulsed laser microma-chining, this important observable depends upon material properties, pulse properties and the number of pulses applied in a complex manner that is not yet well understood. The incubation effect is one example. It manifests as a change in the ablation threshold as a function of number of laser pulses applied and is driven by photoinduced defect accumulation in the material. Here, we study fem-tosecond (800 nm, 110 fs, 0.1–1 mJ/pulse) micromachin-ing of a material with well-defined initial defect concentrations: doped Si across a range of dopant types and concentrations. The single-pulse ablation threshold (F th,1) was observed to decrease with increasing dopant concentration , from a maximum of 0.70 J/cm 2 (±0.02) for undoped Si to 0.51 J/cm 2 (±0.01) for highly N-type doped Si. The effect was greater for N-type doped Si than for P-type, consistent with the higher carrier mobility of electrons compared to holes. In contrast, the infinite-pulse ablation threshold (F th,?) was the same for all doping levels and types. We attribute this asymptotic behaviour to a maximum defect concentration that is independent of the initial defect concentration and type. These results lend insight into the mechanism of multipulse, femtosecond laser ablation.
THERMOPHYSICAL EFFECTS IN LASER PROCESSING OF MATERIALS WITH PICOSECOND AND FEMTOSECOND PULSES
Journal of Applied Physics, 1995
Application of picosecond and femtosecond laser pulses to the controlled ablation of materials represents a relatively unexplored yet important topic in laser processing. Such ultrashort pulses are of potential value in areas of thin-film deposition, micromachining, and surgical procedures. We report here some early results of systematic studies being done from the femtosecond to the nanosecond regime, as an assessment of the problems and benefits associated with various laser pulse durations and their use in processing optically absorbing media. Experimental data and theoretical results of computer simulations are presented and compared for the threshold energies of ablation in gold as a function of pulse width from 10 ns to 100 fs. This work is then extended to include further numerically computed results for gold and silicon on ablation rates, threshold surface temperatures, liquid thicknesses, and vaporization rates as a function of pulse duration throughout the ultrafast regime from tens of femtoseconds to a few hundred picoseconds.
Nanosecond pulsed laser ablation of silicon in liquids
Applied Physics A, 2009
Laser fluence and laser shot number are important parameters for pulse laser based micromachining of silicon in liquids. This paper presents laser-induced ablation of silicon in liquids of the dimethyl sulfoxide (DMSO) and the water at different applied laser fluence levels and laser shot numbers. The experimental results are conducted using 15 ns pulsed laser irradiation at 532 nm. The silicon surface morphology of the irradiated spots has an appearance as one can see in porous formation. The surface morphology exhibits a large number of cavities which indicates as bubble nucleation sites. The observed surface morphology shows that the explosive melt expulsion could be a dominant process for the laser ablation of silicon in liquids using nanosecond pulsed laser irradiation at 532 nm. Silicon surface's ablated diameter growth was measured at different applied laser fluences and shot numbers in both liquid interfaces. A theoretical analysis suggested investigating silicon surface etching in liquid by intense multiple nanosecond laser pulses. It has been assumed that the nanosecond pulsed laser-induced silicon surface modification is due to the process of explosive melt expulsion under the action of the confined plasma-induced pressure or shock wave trapped between the silicon target and the overlying liquid. This analysis allows us to determine the effective lateral interaction zone of ablated solid target related to nanosecond pulsed laser illumination. The theoretical analysis is found in excellent agreement with the experimental measurements of silicon ablated diameter growth in the DMSO and the water interfaces. Multiple-shot laser ablation threshold of sil-icon is determined. Pulsed energy accumulation model is used to obtain the single-shot ablation threshold of silicon. The smaller ablation threshold value is found in the DMSO, and the incubation effect is also found to be absent.
Spectrochimica Acta Part B: Atomic Spectroscopy, 2017
Multi-parametric theoretical studies to analyze the effect of both the matter properties (absorption coefficient, thermal conductivity and diffusivity) and the heating field parameters (spatial distribution and pulse duration) on the resulted temperature distribution are presented. For heating in sub-micrometric range (< 1 µm), a low dependence of heating temperature distribution on the sample thermal properties and heating source duration was observed. Nano-ablation thresholds are found to be increasing inversely with heating source dimensions. The simulation results demonstrated a good agreement with the nanometer-size craters (100 nm diameters, 10 nm depth) obtained experimentally with a tip-enhanced nearfield ablation (4 ns laser pulse duration, 266 nm wavelength) of Si-and Au-samples. Highlights Modelling of near-field heating. Low dependence of heating temperature distribution on the sample thermal properties and heating source duration. Nano-ablation threshold is a function of heating source dimensions. Good agreement with the craters of 100 nm diameters obtained with a tip-enhanced near-field ablation (4 ns laser pulse duration, 266 nm wavelength) of Siand Au-samples.
2007
This dissertation reports the results of femtosecond (fs, 10-15 seconds) laser ablation studies performed on single crystal silicon with oxide thin films and the single crystal Ni-based superalloy, CMSX-4. Emphasis is placed on near threshold ablation (or material removal) phenomena where fs pulsed lasers show significant promise for industrial machining, characterization, materials processing, fabrication of structures, and other applications. Three specific topics are addressed: fs laser ablation thresholds, ablation morphologies, and ablation dynamics. These investigations demonstrate both fundamental aspects of the Chapter 1
Nanostructure formation upon femtosecond ablation from silicon: Effect of double pulses
Applied Surface Science, 2012
To study the dynamics of laser-ablation induced structure formation (LIPPS), silicon was irradiated by (above-threshold) pulse pairs with a variable time-lag between 100 fs and a few picoseconds. With increasing pulse-to-pulse delay we find a significant change in ablated-area morphology: the central range of the irradiated spot becomes less and less depressed whereas a surrounding ring structure exhibits increasingly coarser modulation, typical for strong irradiation, where the ripples are characterized by an alternation between elevation above and depression below the unaffected surface level. At the spot center the ablation depth decreases with increasing pulse separation, showing only structures usually observed for weak irradiation. Micro-Raman spectroscopy of the modified areas indicates an unexpectedly high, almost mono-dispersed, abundance of confined nanostructures. The results clearly seem to rule out structure formation by any interference-induced modulated ablation. Instead, they support the model of self-organized structure formation upon the creation of a thermally unstable, "soft" state of the target after laser impact.
Laser ablation efficiency of pure metals with femtosecond, picosecond, and nanosecond pulses
High-Power Laser Ablation, 1998
Laser ablation efficiency of metals in terms of ablated volume per unit of energy was studied experimentally in air at atmospheric pressure with sharply focused ns, ps and fs laser pulses. The best efficiency was obtained for fs laser. Crater widening at ps and ns pulses and correlation between the ablation efficiency and melting point temperature were observed. Ablation efficiency dependence of pulse duration and wavelength is discussed.
Metal Ablation with Short and Ultrashort Laser Pulses
Physics Procedia, 2011
In laser microstructuring there is a general conflict between precision and efficiency. Short pulsed micro-and nanosecond systems generally allow high ablation rates. Yet, thermal damage of the workpiece cannot be avoided completely. Ultrafast picoand femtosecond systems allow a higher precision, yet at lower ablation efficiency. This on the one hand can be attributed to the generally lower medium laser power of the ultrafast laser systems, on the other hand to the changed ablation mechanisms. In this contribution a comparative study of the ablation of metal with micro-, nano-, pico-and femtosecond laser pulses shall be presented.