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.     

Holographic interferometry for the structural diagnostics of UV laser ablation of polymer substrates

Holographic interferometry is examined for its potential as a diagnostic tool of the structural modifications effected in laser-processing applications. The interferometric ‘comparison’ of the holographic images of the sample recorded before and after irradiation enables the full-field spatially resolved detection of the induced structural modifications. The potential of the method is illustrated in the ablation of polymer (polymethylmethacrylate and polystyrene) films with nanosecond pulses at 193 and 248 nm. The detailed characterization and quantitative monitoring of the growth of the induced modifications as a function of laser-material parameters is attained. A most novel result is the observation of delocalised structural modifications at distances relatively far away (d≈2 cm) from the irradiated area. Received: 15 August 2001 / Accepted: 16 August 2001 / Published online: 2 October 2001     

Femtosecond laser ablation of carbon reinforced polymers

Interaction of intense ultrashort laser pulses (120 fs at 795 nm) with polymer based composites has been investigated. We have found that carbon filled polymers exhibit different ultrafast ablation behaviour depending on whether the filling material is carbon black or carbon fiber and on the polymer matrix itself. The shape and dimensions of the filling material are responsible for some geometrical bad quality effects in the entrance and inner surfaces of drilled microholes. We give an explanation for these non-quality effects in terms of fundamentals of ultrafast ablation process, specifically threshold laser fluences and material removal paths. Since carbon fiber reinforced polymers seemed particularly concerned, this could prevent the use of ultrafast ablation for microprocessing purposes of some of these materials.     

Hydrogenation of carbon fragments after femtosecond laser ablation of solid C60

We have observed several kinds of hydrocarbon cations after the nanosecond and the femtosecond laser ablation (nsLA and fsLA) of solid C₆₀. The observation indicates that the carbon fragments produced just after laser ablation of the C₆₀ molecule react with the hydrogen atoms and ions coexisting in the ablation plume. In the case of fsLA, clear dependence of the product hydrocarbon species on the ablation laser power has been observed although the dependence is not clearly observed in nsLA. The production of C_nH₅⁺ (n = 8, 10, and 12) is only observed in fsLA suggesting the unique nature of the transient carbon fragments produced by fsLA.     

Phase transitions in femtosecond laser ablation

In this study we simulate an interaction of femtosecond laser pulses (100 fs, 800 nm, 0.1-10 J/cm²) with metal targets of Al, Au, Cu, and Ni. For analysis of laser-induced phase transitions, melting and shock waves propagation as well as material decomposition we use an Eulerian hydrocode in conjunction with a thermodynamically complete two-temperature equation of state with stable and metastable phases. Isochoric heating, material evaporation from the free surface of the target and fast propagation of the melting and shock waves are observed. On rarefaction the liquid phase becomes metastable and its lifetime is estimated using the theory of homogeneous nucleation. Mechanical spallation of the target material at high strain rates is also possible as a result of void growth and confluence. In our simulation several ablation mechanisms are taken into account but the main issue of the material is found to originate from the metastable liquid state. It can be decomposed either into a liquid-gas mixture in the vicinity of the critical point, or into droplets at high strain rates and negative pressure. The simulation results are in agreement with available experimental findings.     

Direct observation of the temperature field during ablation of materials by multiple femtosecond laser pulses

We report a direct observation of the temperature field on a steel specimen during ablation by multiple femtosecond laser pulses using an infrared thermography technique. From the experimental results and simulation study of the temperature field, we quantified the deposited thermal power into the specimen during the ablation process. We found that more than two thirds of the incident laser power was deposited in the steel specimen when ablated by multiple femtosecond laser pulses. This result provides further understanding of the heating effect in materials processing by ultrashort laser pulses.     

Femtosecond laser induced submicrometer structures on the ablation crater walls of II-VI semiconductors in water

Femtosecond laser ablations (100 fs, 800 nm, 0.2 mJ/pulse) were performed to produce craters on CdS, ZnS:Cu and ZnSe wafers in water. On the surface of the crater walls, a variety of submicrostructural formations were presented, such as the ripples and network structures for CdS, the subwavelength ripples and columnar structures for ZnS:Cu, even the regular cubic-shaped submicron rods for ZnSe. Based on the field-emission scanning electron microscope (FE-SEM) study of the different characteristic surface morphologies, the possible formation mechanisms were discussed correspondingly. For example, two distinct mechanisms are contributing to the different styles of ripples formed on CdS and ZnS:Cu. The former is the interference effects between the incoming laser beam and scattered surface wave, while the latter is the self-organization structure formation. In addition, the re-crystallization of the water-confined hot plasma would play an important role in the formation of ZnS:Cu column structures and ZnSe rods.     

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)].     

The influence of thermal diffusion on laser ablation of metal films

Single-shot ablation thresholds of nickel and gold films in the thickness range from 50 nm to 7 m have been measured for 14 ns laser pulses at 248 nm, using photoacoustic shock wave detection in air. The metal films were deposited on fused silica substrates. The ablation threshold was found to increase linearly with film thickness up to the thermal diffusion length of the film. Beyond this point it remains independent of film thickness. The proportionality between threshold fluence and thickness allows the prediction of ablation thresholds of metal films from the knowledge of their optical properties, evaporation enthalpies and thermal diffusivities. Physically it proves that ablation is driven by the energy density determined by the thermal diffusion length. A simple thermodynamic model describes the data well. Thermal diffusivities, an essential input for this model, were measured using the technique of transient thermal gratings. In addition, the substrate dependence of the ablation threshold was investigated for 150 nm Ni films.     

Photochemical effects in the UV laser ablation of polymers: Implications for laser restoration of painted artworks

0 ∝t^1/2. The best results are expected for a circular top-hat beam shape. Received: 15 January 1999 / Accepted: 18 January 1999     

Dynamics of plume and crater formation after action of femtosecond laser pulse

Using microinterferometric method, a transition in laser plume from the regime with spallation to the regime without spallation is experimentally studied for the first time. The transition occurs when the fluence F_inc of incident radiation exceeds a threshold of “evaporation” (F_inc)_ev. It has been shown previously that the spallation layer is formed at fluence above the ablation threshold (F_inc)_abl. Thus the spallation exists within the limits (F_inc)_abl<F_inc<(F_inc)_ev. A laser beam has a maximum fluence (F_inc)_c on the axis of the beam. The threshold F_ev separates two cases with qualitatively different morphology: (1) with unbroken shell covering the crater entirely if F_abl<F_c<F_ev, and (2) with the shell having an aperture in the center (like the volcano muzzle) if F_c>F_ev.     

Fundamentals and applications of polymers designed for laser ablation

The ablation characteristics of various polymers were studied at low and high fluences for an irradiation wavelength of 308 nm. The polymers can be divided into three groups, i.e. polymers containing triazene groups, designed ester groups, and reference polymers, such as polyimide. The polymers containing the photochemically most active group (triazene) exhibit the lowest thresholds of ablation (as low as 25 mJ cm^-2) and the highest etch rates (e.g. 250 nm/pulse at 100 mJ cm^-2), followed by the designed polyesters and then polyimide. Neither the linear nor the effective absorption coefficients have a clear influence on the ablation characteristics. The different behavior of polyimide might be explained by a pronounced thermal part in the ablation mechanism. The laser-induced decomposition of the designed polymers was studied by nanosecond interferometry and shadowgraphy. The etching of the triazene polymer starts and ends with the laser pulse, indicating photochemical ablation. Shadowgraphy reveals mainly gaseous products and a pronounced shockwave in air. The designed polymers were tested for an application as the polymer fuel in laser plasma thrusters. Received: 21 October 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +41-56/3104-412, E-mail: thomas.lippert@psi.ch     

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.     

Influence of adsorbates on UV-pulsed laser melting of Si in ultra-high vacuum

The dynamics of laser melting of atomically clean Si is investigated in ultra-high-vacuum (UHV) by transient reflectivity with single-pulse sensitivity in the presence of monitored amounts of chlorine, oxygen or propene. Adsorption of one monolayer (1 ML) leads to measurable variations of the melting dynamics, which are strongly adsorbate-dependent. The variations differ qualitatively and quantitatively from those observed with heavy exposures to gases. The melting dynamics returns to that of clean Si upon subsequent irradiation by laser pulses without readsorption. The required number of pulses for return to clean Si dynamics depends strongly on the type of adsorbate. Adsorbate-induced changes of absorption and reflectivity, and/or incorporation of adsorbates into the substrate, do not explain the results. By contrast, the variations of the melting dynamics are correlated to the photoemitted electron yield, suggesting that laser melting is sensitive to the presence of electrons in the conduction band. These results show that accurate modelling of laser melting of Si interacting with gases should take into account the presence of the gases. Received: 12 September 2000 / Accepted: 9 January 2001 / Published online: 27 June 2001     

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.     

Detection of neutral products formed during excimer laser ablation of polyimide by UV and VUV laser photoionization/mass spectrometry

Some of the neutral species which are produced in the laser ablation of polyimide have been characterized using multiphoton ionization/time of flight mass spectrometry. Three different wavelengths (193 nm, 157 nm, and 118 nm) have been used in an attempt to effect soft ionization of the products formed during or after the initial laser ablation of the polymer. Neutral photo-ablation products detected using this scheme range from atomic to high molecular weight species which, depending on the probe wavelength, include pure carbon clusters as well as a broad distribution of heteroatom containing clusters. However, there is virtually no overlap in the mass spectra recorded at each probe wavelength. When probing with 193 nm, marked changes are observed in the mass spectra as a function of the probe flux used. At moderate fluxes, pure carbon clusters (fullerenes) are observed. The identification of a large distribution of species other than pure carbon clusters is in dramatic contrast to the recent observation [W.R. Creasy, J.T. Brenna: Chem. Phys. 126, 453 (1988)] of the positively charged ionic species produced, which are solely carbon clusters. These results suggest that the neutral and ionic products observed after ablation of the polymer are due to both condensation of the atomic and molecular fragments which form during the ablation laser pulse and nascent polymer fragments. Various implications of this result for the unambiguous determination of the true ablation product distribution are discussed.     

Practical uses of femtosecond laser micro-materials processing

We describe several approaches to basic femtosecond machining and materials processing that should lead to practical applications. Included are results on high-throughput deep hole drilling in glasses in ambient air, and precision high-speed micron-scale surface modification of composite materials and chalcogenide glasses. Ablation of soda-lime silicate glass and PbO lead-silicate is studied under three different sets of exposure conditions, for which both the wavelength and pulse duration are varied. Ablation rates are measured below and above the air ionization threshold. The differences observed are explained in terms of self-channeling in the ablated hole. Fabrication of practical devices such as waveguides and gratings is demonstrated in chalcogenide glass. Received: 11 December 2002 / Accepted: 20 January 2003 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-407/8233-570, E-mail: mrichard@mail.ucf.edu     

Fabrication of Mg-doped ZnO thin films by laser ablation of Zn:Mg target

Mg-doped ZnO thin films were fabricated by laser ablation of Zn:Mg targets consisting of Mg metallic strips and Zn disk in oxygen atmosphere with a goal to facilitate convenient control of Mg contents in the films. The characteristics of the deposited films were examined by analyzing their photoluminescence (PL), X-ray diffraction and X-ray photoelectron spectroscopy (XPS) spectra. Mg contents as analyzed by XPS indicate that the target composition is fairly transferred to the deposited films. The wurtzite structure of ZnO was conserved even for the highly doped ZnO films and there was no Mg- or MgO-related XRD peaks. With increase in the Mg content, the bandgap and PL peak energy shifted to blue and the Stokes shift became larger.     

Molecular dynamics simulation of femtosecond ablation and spallation with different interatomic potentials

Fast heating of target material by femtosecond laser pulse (fsLP) with duration τ_L∼40-100 fs results in the formation of thermomechanically stressed state. Its unloading may cause frontal cavitation of subsurface layer at a depth of 50 nm for Al and 100 nm for Au. The compression wave propagating deep into material hits the rear-side of the target with the formation of rarefaction wave. The last may produce cracks and rear-side spallation. Results of MD simulations of ablation and spallation of Al and Au metals under action fsLP are presented. It is shown that the used EAM potentials (Mishin et al. and our new one) predict the different ablation and spallation thresholds on absorbed fluence in Al: ablation F_a=60{65} mJ/cm²and spallation F_s=120{190} mJ/cm², where numbers in brackets { } show the corresponding values for Mishin potential. The strain rate in spallation zone was 4.3×10⁹ 1/s at spallation threshold. Simulated spall strength of Al is 7.4{8.7} GPa, that is noticeably less than 10.3{14} GPa obtained from acoustic approximation with the use of velocity pullback on velocity profile of free rear surface. The ablation threshold F_a≈120 mJ/cm² and crater depth of 110 nm are obtained in MD simulations of gold with the new EAM potential. They agree well with experiment.     

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.