Laser ablation of copper and aluminium in air

The ablation behavior of copper alloy and aluminium irradiated in air by 1.06 m, 10 ns pulsed laser with power density of 6.4×10⁹W/cm² was studied using scanning electron microscopy (SEM), MCS-RBS and X-ray microanalysis. Evidence of bulk vaporization via bubble formation was observed for the copper alloy under the laser irradiation. Silver-enrichment microregions were found in the ablation crater created by the laser shots on the copper alloy sample. Material removal rates of these materials were determined by crater shape-profile measurement. Using self-similar solutions of the gas-dynamic equations, gas-dynamic parameters of the vaporization waves are obtained. These parameters are used to calculate material removal rates and impulse coupling coefficients of these materials under the pulsed laser irradiation. The calculated mass removal rates and the coupling coefficients are compared with the corresponding experimentally determined values. The surface kinetic energy of the irradiated area on the Al sample is estimated. Possible mechanisms for laser ablation of the materials under study are discussed.     

Femtosecond laser ablation of aluminium

We investigate femtosecond laser ablation of aluminium using a hybrid simulation scheme. Two equations are solved simultaneously: one for the electronic system, which accounts for laser energy absorption and heat conduction, the other for the dynamics of the lattice where the ablation process takes place. For the electron-temperature a generalized heat-conduction equation is solved by applying a finite difference scheme. For the lattice properties, e.g. pressure, density or temperature, we use common molecular dynamics. Energy transfer between the subsystems is allowed by introducing an electron-phonon coupling term. This combined treatment of the electronic and atomic systems is an extension of the well known two-temperature model [Anisimov, Kapeliovich, Perel’man, Electron emission from metal surfaces exposed to ultra short laser pulses, JETP Lett. 39 (2)].     

Modeling of residual thermal effect in femtosecond laser ablation of metals: role of a gas environment

To describe the effect of significant enhancement in thermal energy retained in metal targets following femtosecond laser ablation in a gas environment, we develop a combined model based on both 2D thermal modeling of laser-induced target heating and dynamics of the ambient gas perturbed by multiphoton absorption of laser energy in close proximity to the target. Using our model, we find that thermal energy coupling to the sample is significantly enhanced due to laser-induced gas-dynamic motion in plasma. Another finding is that total thermal energy coupled to the sample due to gas-dynamic energy transfer and thermal energy conduction is close to that measured in our experiment.     

High Coupling Efficiency Generation in Water Confined Laser Plasma Propulsion

High coupling efficiency generation in water confined laser plasma propulsion is investigated. It is found that the coupling efficiency is enhanced over thirty times in water confined ablation compared to that of direct ablation. From calculation of the ablation pressure induced by the plasma on the target surface, it is realized that high coupling efficiency is attributed to the confinement of the water layer on the plasma expansion.     

Effects of laser operating parameters on metals micromachining with ultrafast lasers

180 femtoseconds (1 kHz) and 10 picoseconds (1-50 kHz) ultrafast laser micro-structuring of the metals Ti alloy, Al and Cu have been studied for the purpose of industrial application. The effects of some key laser operating parameters were investigated. The evolution of surface morphology revealed that laser pulses overlap in a range around the spatial FWHM can help to achieve optimal residual surface roughness. While observed ablation rate (unit: μm³ per pulse) changed dramatically with repetition rate due to the combined effects of plasma absorption, residual thermal energy and phase transition, higher throughput can be achieved with higher repetition rate. This study also indicated that residual surface roughness is almost independent of repetition rate at 10 ps temporal pulse length. The ablation depth is approximately proportional to the number of overscan; however, machining accuracy deteriorates, especially for femtosecond laser processing and metals with low thermal conductivity and short electron-phonon coupling time.     

Vaporization and Plasma Shielding during High Power Nanosecond Laser Ablation of Silicon and Nickel

A thermal model to describe the high-power nanosecond pulsed laser ablation is presented. It involves the vaporization and the following plasma shielding effect on the whole ablation process. As an example of Si target, we obtainthe time evolution of the calculated surface temperature, ablation rate and ablation depth. It can be seen that plasma shielding plays a more important role in the ablation process with time. At the same time, the ablation depth with laser fluence based on different models is shown. Moreover, we simulate the pulsed laser irradiation Ni target. The evolution of the transmitted intensity and the variation of ablation depth per pulse with laser fluence are performed. Under the same experimental conditions, the numerical results calculated with our thermal model are more in agreement with the experimental data.     

Dynamic temporal pulse shaping in advanced ultrafast laser material processing

Phase-manipulated ultrafast laser pulses and temporally tailored pulse trains with THz repetition rates are promising new tools for quality micromachining of brittle dielectrics, allowing to adapt the laser energy delivery rate to the material properties for optimal processing. Different materials respond with specific reaction pathways to the sudden energy input depending on the efficiency of electron generation and on the ability to release the energy into the lattice. The sequential energy delivery with judiciously chosen pulse trains may induce softening of the material during the initial steps of excitation and change the energy coupling for the subsequent steps. We show that this can result in lower stress, cleaner structures, and allow for a material-dependent optimization process. Received: 7 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +49-30/6392-1229, E-mail: stoian@mbi-berlin.de RID="**" ID="**"Now at Katana Technologies GmbH, Albert-Einstein-Ring 7, 14532 Kleinmachnow, Germany     

Influence of photoexcitation pathways on the initiation of ablation in poly (methyl methacrylate)

Experimental and theoretical studies of laser ablation of polymers, under various processing conditions, have identified many possible photoexcitation pathways and consequently many likely processes responsible for the onset of ablation. We investigate the role of these processes—namely the thermal, mechanical and chemical processes—occurring in a polymeric substrate during UV irradiation. Molecular dynamics simulations with an embedded Monte Carlo-based reaction scheme were used to study ablation of Poly (methyl methacrylate) at 157 nm. Laser-induced heating and chemical decomposition of the polymeric substrate are considered as ablation pathways. For the heating case, the mechanism of ejection is thermally driven limited by the critical number of bonds broken. This fragmentation process is well reproduced by the existing bulk photothermal ablation model. Alternatively, if the photon energy goes toward direct bond breaking, it initiates chemical reactions, polymer unzipping, and formation of gaseous products leading to near complete decomposition, loss of strength and cohesiveness of the top layers of the polymeric substrate. The ejection of small gaseous molecules weakens and hollows out the substrate, facilitating liftoff of larger fragments of material. These larger clusters are thermally ejected and the photochemical ablation process can be described by the two-step model proposed by Kalontarov.     

Silicon surface morphology study after exposure to tailored femtosecond pulses

Temporal pulse shaping of ultrashort laser pulses has been used for laser ablation of semiconductors. Even the simplest double pulse sequence with a delay of several picoseconds shows remarkable differences in the interaction process, compared to a single pulse of the same total energy. We discuss the interaction of double pulses with single crystal silicon sample in the context of crater morphology for multiple pulses on the same spot. The growth of the typical columnar structures in helium at atmospheric pressure is suppressed and the crater bottom is flat despite the Gaussian beam profile. The influence of the temporal pulse shape has to be treated in conjunction with the influence of the other ablation parameters.     

Analysis of the formation and evolution of oriented microstructures on laser ablated silicon

A theoretical approach and qualitative analysis of the changes induced on the surface morphology and the formation of microstructures on silicon targets irradiated by excimer laser are presented. This study is based on theoretical principles of the laser ablation process, in particular, on the analysis of the contribution of the laser energy density, which involves the laser beam parameters and also the physical properties of the target material. For different laser incident angles, the formation of micro-columns oriented towards the laser incident direction is explained. Moreover, numerical simulations and ablation experiments carried out with an excimer laser corroborate the theoretical analysis.     

纳秒激光烧蚀冲量耦合数值模拟

为研究激光烧蚀靶产生冲量过程和机理, 建立了一个复杂的一维热传导和流体动力学模型. 以空间碎片常见材料Al为例, 用建立的模型数值计算了纳秒脉宽激光烧蚀靶产生的冲量及冲量耦合系数随时间变化情况. 数值结果和已有的实验数据符合的较好. 数值计算表明: 激光脉冲时间内, 靶获得的冲量随时间迅速增加, 在脉冲时间结束后, 冲量变化随时间趋于稳定; 在冲量耦合过程中, 烧蚀等离子体向真空膨胀, 羽流尺度逐渐增大, 同时吸收入射激光能量, 导致激光与靶耦合的能量降低. 关键词： 激光烧蚀 冲量耦合 等离子体     

Femtosecond pulsed laser ablation deposition of tantalum carbide

In this work a frequency-doubled Nd:glass laser with a pulse duration of 250 fs has been used to ablate a TaC target and to deposit thin films on silicon. The results have been compared with those previously obtained by nanosecond pulsed laser deposition and evidence of large differences in the plasma characteristics has been revealed. In particular, in the femtosecond and nanosecond plumes the energy and the velocity of neutral and ionized particles are very different. The features of femtosecond ablation include the delayed emission from the target of large and slow particles. The characteristics of the femtosecond plasma are clearly related to the morphology and composition of the deposited films and the results show a nanostructure consisting of a large number of spherical particles, with a mean diameter of about 50 nm, with a stoichiometry corresponding to Ta₂C. To explain these features, an ablation-deposition mechanism, related to the ejection of hot particles from the target, is proposed.     

Minimization of cavitation effects in pulsed laser ablation illustrated on laser angioplasty

Cavitation effects in pulsed laser ablation can cause severe deformation of tissue near the ablation site. In angioplasty, they result in a harmful dilatation and invagination of the vessel walls. We suggest to reduce cavitation effects by dividing the laser pulse energy into a pre-pulse with low and an ablation pulse with high energy. The pre-pulse creates a small cavitation bubble which can be filled by the ablation products of the main pulse. For suitable energy ratios between the pulses, this bubble will not be enlarged by the ablation products, and the maximal bubble size remains much smaller than after a single ablation pulse. The concept was analyzed by numerical calculations based on the Gilmore model of cavitation dynamics and by high-speed photography of the effects of single and double pulses performed with a silicone tube as vessel model. The use of double pulses prevents the deformation of the vessel walls. The concept works with an energy ratio of up to about 1:30 between the pulses. For the calculated optimal ratio of 1:14.6, the bubble volume is reduced by a factor of 17.7. The ablation pulse is best applied when the pre-pulse bubble is maximally expanded, but the timing is not very critical.     

Laser ablation with short and ultrashort laser pulses: Basic mechanisms from molecular-dynamics simulations

Laser ablation is a technology widely used in many applications. Understanding in detail the mechanisms that lead to ablation remains a formidable challenge because of the complexity of the processes taking place, the variety of species involved, and the range of length and time scales covered. Atomic-level experimental information is difficult to obtain and must be augmented by theory. In this article, we briefly review the progresses that we have accomplished using a simple two-dimensional molecular-dynamics model, insisting on the importance of considering the thermodynamics of the evolution of the systems in order to understand ablation. Through the identification of the thermodynamic pathways followed by the material after irradiation, our model has provided significant insights on the physical mechanisms leading to ablation. It has been demonstrated in particular that these depend strongly on the fluence, and are actually determined by the effective amount of energy received within different regions of the target. Further, internal or external factors, such as inertial confinement, play a key role in determining the route to ablation - and thus the types and sizes of particles ejected - by constraining the thermodynamical evolution of the system. We have established that, for ultrashort pulses in strongly absorbing materials, ablation proceeds by either spallation, phase explosion or fragmentation; the latter, we demonstrate, is the most important mechanism. For longer pulses, ablation may also proceed by trivial fragmentation.     

Modelling the formation of nanostructures on metal surface induced by femtosecond laser ablation

We employ the particle-in-cell method to simulate the mechanisms of femtosecond (fs) laser interactions with a metallic target. The theoretical approach considers the solid as a gas of free electrons in a lattice of immobile ions and the laser fluences close to the ablation threshold. At first moments of the interaction, our simulations mapped out different nanostructures. We carefully characterized the rippling phase and found that its morphology is dependent on the distribution of the electron density and the period of the ripples depends on the laser intensity. The simulation method provides new insights into the mechanisms that are responsible for surface grating formation.     

Molecular dynamics simulation of ultrafast laser ablation of fused silica film

Ultrafast laser ablation of fused silica is studied using molecular dynamics simulations. Ionization and generation of free electrons, absorption of the laser energy by free electrons and energy coupling between free electrons and ions are considered. The BKS potential is applied and modified to describe molecular interactions and the effect of free electrons. Smooth particle mesh of the Ewald method (SPME) is adopted to calculate the Coulomb force. It is found that the electrostatic Coulomb force, which is caused by the ionization, plays an important role in the laser ablation process.     

Residual thermal effects in Al following single ns- and fs-laser pulse ablation

A comparative study of residual thermal effects in aluminum following ns- and fs-laser ablation shows a surprisingly similar trend in their behavior, despite many differences between ns and fs laser-matter interactions. At laser fluences above the ablation threshold where plasmas are produced and at a sufficiently high ambient gas pressure, an enhanced coupling of pulsed laser energy to the sample occurs. This effect appears to be a universal phenomenon for both ns- and fs-laser ablation in gas media. Furthermore, in contrast to the common belief that residual thermal energy is negligible in fs-laser ablation, our study shows that up to 70% of the incident pulse energy can be retained in the sample following single-pulse fs-laserablation in 1-atm air. In both ns- and fs-laser ablation, the major factors governing thermal energy coupling to the sample are the laser fluence and ambient gas pressure. Residual thermal energy deposition decreases with reducing ambient gas pressure. PACS 78.20; 81.05.Bx     

Nanosecond and femtosecond UV laser ablation of polymers: Influence of molecular weight

We examine the nanosecond and femtosecond UV laser ablation of poly(methyl methacrylate) (PMMA) as a function of molecular weight (M_w). For laser ablation with nanosecond laser pulses, at the excimer wavelengths 248 nm and 193 nm, we show that high temperatures develop; yet the dynamics of material ejection differs depending on polymer M_w. The results on the nanosecond ablation of polymers are accounted within the framework of bulk photothermal model and the results of molecular dynamics simulations. Turning next to the 248 nm ablation with 500 fs laser pulses, the ablation threshold and etching rates are also found to be dependent on polymer M_w. In addition, ablation results in morphological changes of the remaining substrate. Plausible mechanisms are advanced.     

Selective ablation of thin films with short and ultrashort laser pulses

Micromachining of CuInSe₂ (CIS)-based photovoltaic devices with short and ultrashort laser pulses has been investigated. Therefore, ablation thresholds and ablation rates of ZnO, Mo and CuInSe₂ thin films have been measured for irradiation with nanosecond laser pulses of ultraviolet and visible light and subpicosecond laser pulses of a Ti:sapphire laser. The experimental results were compared to the theoretical evaluation of the samples heat regime. In addition, the cells photo-electrical properties were measured before and after laser machining. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were employed to characterise the laser-induced ablation channels. Using nanosecond laser pulses, two phenomena were found to limit the laser-machining process. Residues of Mo that were projected onto the walls of the ablation channel and the metallization of the CuInSe₂ semiconductor close to the channel lead to a shunt. The latter causes the decrease of the photovoltaic efficiency. As a consequence of these limiting effects, only subpicosecond laser pulses allowed the selective or complete ablation of the thin layers without a relevant change of the photo-electrical properties.     

Single-pulse drilling study on Au,Al and Ti alloy by using a picosecond laser

Picosecond laser single pulse ablation of Au, Al and Ti alloy (Ti6Al4V) was experimentally investigated with a laser pulse width of 10 ps at a wavelength of 1064 nm for potential industrial micromachining applications. The diameters, depths and morphologies of the drilled craters were studied. Two novel phenomena were found: as hole diameters decreased with fluence, a change of slope of the trend line indicated a change in ablation mechanism for Al and Ti alloy, metallic materials with short electron-phonon coupling times (<10 ps), while Au showed no such transition: an isolated island structure was also observed on Au due to significant melt expulsion. A one-dimensional two-temperature model has been used to discriminate different ablation phenomena. It is shown that metallic materials with different electron–phonon coupling constant have different ablation characteristics in the ps regime. This study could be very helpful for metallic material micromachining with high repetition rate ps lasers pulses which indicates that high throughput may be achieved as well as good machining quality.