Femtosecond laser ablation characteristics of nickel-based superalloy C263

Femtosecond laser (180 fs, 775 nm, 1 kHz) ablation characteristics of the nickel-based superalloy C263 are investigated. The single pulse ablation threshold is measured to be 0.26±0.03 J/cm² and the incubation parameter ξ=0.72±0.03 by also measuring the dependence of ablation threshold on the number of laser pulses. The ablation rate exhibits two logarithmic dependencies on fluence corresponding to ablation determined by the optical penetration depth at fluences below ∼5 J/cm² (for single pulse) and by the electron thermal diffusion length above that fluence. The central surface morphology of ablated craters (dimples) with laser fluence and number of laser pulses shows the development of several kinds of periodic structures (ripples) with different periodicities as well as the formation of resolidified material and holes at the centre of the ablated crater at high fluences. The debris produced during ablation consists of crystalline C263 oxidized nanoparticles with diameters of ∼2–20 nm (for F=9.6 J/cm²). The mechanisms involved in femtosecond laser microprocessing of the superalloy C263 as well as in the synthesis of C263 nanoparticles are elucidated and discussed in terms of the properties of the material.     

Ultraviolet photoablation of a plasma-synthesized fluorocarbon polymer

Plasma polymerized tetrafluoroethylene (PPTFE) is shown to undergo efficient 248 nm excimer laser ablation. The principle difference between this material and the analogous polytetrafluoroethylene (PTFE), which results in only poor quality ablation, is PPTFE's much greater absorption coefficient (7×10⁴ vs. 10² cm^–1). A plot of the ablation depth per pulse versus incident fluence indicates that the threshold for significant ablation occurs near 50 mJ/cm², and that approximately 0.7 m/pulse can be removed at 800 mJ/cm². Near threshold, the ablation rate curve can be fit by a single Arrhenius-type exponential. This suggests that the removal process is at least partially governed by a photothermal process, similar to well-known laser induced thermal desorption experiments. In the very low fluence regime between 10 and 30 mJ/cm², small removal rates are measured in a process likely dominated by non-thermal ablation. The paper concludes with a discussion of the high quality, micron-size features that can be directly patterned into PPTFE surfaces.     

Nanoparticle production by UV irradiation of combustion generated soot particles

Laser ablation of surfaces normally produce high temperature plasmas that are difficult to control. By irradiating small particles in the gas phase, we can better control the size and concentration of the resulting particles when different materials are photofragmented. Here, we irradiate soot with 193 nm light from an ArF excimer laser. Irradiating the original agglomerated particles at fluences ranging from 0.07 to 0.26 J/cm² with repetition rates of 20 and 100 Hz produces a large number of small, unagglomerated particles, and a smaller number of spherical agglomerated particles. Mean particle diameters from 20 to 50 nm are produced from soot originally having a mean electric mobility diameter of 265 nm. We use a non-dimensional parameter, called the photon–atom ratio (PAR), to aid in understanding the photofragmentation process. This parameter is the ratio of the number of photons striking the soot particles to the number of the carbon atoms contained in the soot particles, and is a better metric than the laser fluence for analyzing laser–particle interactions. These results suggest that UV photofragmentation can be effective in controlling particle size and morphology, and can be a useful diagnostic for studying elements of the laser ablation process.     

Sub-picosecond UV-laser ablation of Ni films

Laser ablation of thin Ni films on fused silica by 0.5 ps KrF-excimer-laser pulses at 248 nm is reported. The onset of material removal from different film thicknesses (0.1, 0.3, 0.6 and 1.0 m) was measured in a laser ionization time-of-flight mass spectrometer by the amount of Ni atoms vs laser fluence. Significant amounts of metal atoms are already evaporated at laser fluences around 20 mJ/cm², a threshold up to 100 times smaller compared to the one for 14 ns pulses. In contrast to ns laser pulses, the ablation threshold for 0.5 ps pulses is independent of the film thickness. These results reflect the importance of thermal diffusion in laser ablation of strongly absorbing and thermally good conducting materials and prove that for ablation with short pulses, energy loss to the bulk is minimized.     

Laser induced decomposition of a designed and a commercial polymer studied by ns-interferometry and shadowgraphy

Commercially available polymers often exhibit quite poor laser ablation properties for irradiation wavelengths around 248 nm. At these wavelengths, the absorption is due to photostable aromatic groups. Photolabile triazene polymers were developed to compare the influence of a photolabile group on the laser ablation process. The photochemically active triazene group has a strong absorption band at 332 nm, whereas the second absorption maximum at 220 nm is due to the photostable aromatic group. By irradiating at 308 nm and 193 nm, the influence of the photochemically active group on the ablation process can be studied. The etching of the triazene polymer starts and ends with the laser pulse. No surface swelling, which is assigned to photothermal ablation, is detected for fluences above the threshold of ablation. The expansion of the laser ablation induced shockwave was measured for the photolabile triazene polymer and the photostable polyimide. The speed of the shockwave increases with fluence and is higher for irradiation with 193 nm than with 308 nm. A shockwave with equal or higher velocity is observed for the triazene polymer compared with polyimide. Received: 28 August 2002 / Accepted: 20 September 2002 / Published online: 28 May 2003 RID="*" ID="*"Corresponding author. Fax: +41-56/310-4412, E-mail: thomas.lippert@psi.ch     

Single-shot ablation threshold of chromium using UV femtosecond laser pulses

Single-shot ablation threshold for thin chromium film was studied using 266 nm, femtosecond laser pulses. Chromium is a useful material in the nanotechnology industry and information on ablation threshold using UV femtosecond pulses would help in precise micromachining of the material. The ablation threshold was determined by measuring the ablation crater diameters as a function of incident laser pulse energy. Absorption of 266 nm light on the chromium film was also measured under our experimental conditions, and the absorbed energy single-shot ablation threshold fluence was \(46 \pm 5\)  mJ/cm². The experimental ablation threshold fluence value was compared to time-dependent heat flow calculations based on the two temperature model for ultrafast laser pulses. The model predicts a value of 31.6 mJ/cm² which is qualitatively consistent with the experimentally obtained value, given the simplicity of the model.     

Polymer jacket stripping of optical fibres by laser irradiation

Laser jacket stripping of the two-layer polymer jacket coating of Corning SMF-28 silica fibres has been studied as an alternative approach to chemical and mechanical techniques. These polymer outer layers, although chemically similar, become discernable through laser ablation and depth per pulse experiments. Etch rate measurements using nanosecond UV excimer laser sources (F₂, ArF, KrF and XeCl lasers) reveal that, as expected, the threshold fluence (energy per unit area) for significant material removal drops as the laser wavelength becomes shorter. For some wavelengths and fluences, spontaneous cone formation has been observed, thus providing additional threshold data through apex angle determination. The possible occurrence of deleterious damage to the silica cladding has been assessed using electron microscopy and optical transmission measurements. A wide fluence range over which damage was not observed characterised all the interactions. Irradiation techniques for producing apertures in the polymer coating or complete jacket removal are demonstrated and discussed. Briefly, polymer jacket stripping using a femtosecond laser source (800 nm) has been demonstrated. PACS 42.62.Cf; 52.38.Mf     

Femtosecond pulsed laser ablation of 3CSiC thin film on silicon

A femtosecond pulsed Ti:sapphire laser (pulse width=120 fs, wavelength=800 nm, repetition rate=1 kHz) was employed to perform laser ablation of 1-m-thick silicon carbide (3CSiC) films grown on silicon substrates. The threshold fluence and ablation rate, useful for the micromachining of the 3CSiC films, were experimentally determined. The material removal mechanisms vary depending on the applied energy fluence. At high laser fluence, a thermally dominated process such as melting, boiling and vaporizing of single-crystal SiC occurs. At low laser fluence, the ablation is a defect-activation process via incubation, defect accumulation, formation of nanoparticles and final vaporization of boundaries. The defect-activation process reduces the ablation threshold fluence and enhances lateral and vertical precision as compared to the thermally dominated mechanism. Helium, as an assistant gas, plays a major role in improving the processing quality and ablation rate of SiC thin films due to its inertness and high first ionization energy. PACS 79.20.Ds; 42.62.Cf; 42.70.Qs; 61.72; 61.46     

Dynamics of the ejected material in ultra-short laser ablation of metals

A molecular dynamics model is applied to study the formation and the early stages of ejection of material in ultra-short laser ablation of metals in vacuum. Simulations of the ablation process for iron at a pulse duration of 0.1 ps and at different laser fluences are performed. Different features of the ejection mechanism are observed below, near, and above the ablation threshold. The last is estimated as approximately 0.1 J/cm². The structure of the ablated material is found to depend on the applied laser fluence. The expanded plume consists mainly of large clusters at fluences near to the threshold. With the increase of the laser fluence the presence of the large clusters decreases. Clear spatial segregation of species with different sizes is observed in the direction normal to the surface several tens of picoseconds after the laser pulse onset. The angular distribution of the ejected material is estimated for different regimes of material removal. Above the ablation threshold the distribution is forward peaking. PACS 79.20.Ds; 52.38.Mf; 02.70.Ns; 81.05.Bx     

Femtosecond uv excimer laser ablation

Experiments on the ablation of polymethylmethacrylate (PMMA) with 300 fs uv excimer laser pulses at 248 nm are reported for the first time. With these ultrashort pulses, ablation can be done at fluences up to five times lower than the threshold fluence for 16 ns ablation of PMMA, and the surface morphology is improved, also for several other materials. A model for ablation is proposed, assuming a non-constant absorption coefficient _eff depending on the degree of incubation of the irradiated material and the intensity of the incoming excimer laser pulse. The agreement between our model and our experimental observations is excellent for 16 ns excimer laser pulses, also predicting perfectly the shape of a pulse transmitted through a thin PMMA sample under high fluence irradiation. Qualitative agreement for 300 fs excimer laser pulses is obtained so far.     

Measurements of nanoparticle size distribution produced by laser ablation of tungsten and boron-carbide in N2 ambient

Nanoparticles (NPs) were produced by ablating tungsten and boron-carbide (B₄C) target materials in atmospheric pressure nitrogen ambient using ArF excimer laser pulses. The size distributions of the NPs formed during the ablation were monitored—within a 7-133 nm size window—by a condensation particle counter connected to a differential mobility analyzer. The laser repetition rate was varied between 1-50 Hz, and the fluence was systematically changed in the range of 0.5-15 J/cm², for both materials, allowing a comparative study in an extended laser parameter regime. The multishot ablation threshold (Φ_th) of B₄C was determined to be ∼1.9 J/cm² for the laser used (ArF excimer, λ = 193 nm). Similarly to earlier studies, it was shown that the size distributions consist of mainly small nanoparticles (<∼20 nm) attributed to a non-thermal ablation mechanism below Φ_th. An additional broad peak appears (between 20 and 40 nm) above Φ_th as a consequence of the thermally induced macroscopic ablation. Chemical composition of deposited polydisperse nanoparticles was studied by X-ray photoelectron spectroscopy showing nitrogen incorporation into the boron-carbide.     

Nanoscale laser-induced forward transfer through patterned Cr films

The resolution enhancement of laser-induced forward transfer (LIFT) is investigated through the pre-patterning of Cr on the donor substrate. 85 nm dots are first patterned on a microscope slide, and an 800 nm wavelength and 130 fs pulse laser with a beam waist of ～9 μm is used to transfer the Cr dots to an acceptor substrate. The threshold fluence is found to be ～0.15 the threshold fluence of a similar continuous film, which is thought to be due to the fact that no force is needed to tear away Cr from the film itself, unlike in a continuous film experiment. Since the volume of the material limits the transfer feature sizes instead of the laser parameters, as in a continuous film system, minimum transferable feature diameters are significantly lower compared to the continuous film case. Also, the transferred feature diameters are not dependent on the laser parameters, so the diameters are consistent across a wide range of fluences. The force per unit area generated by the laser at threshold fluence is estimated to be ～3 GPa, which is consistent with previous results in the literature. The simplified model that our pre-patterned Cr LIFT experiment represents would make it an ideal case for benchmarking molecular dynamics simulations of femtosecond laser ablation.     

Features of plasma plume evolution and material removal efficiency during femtosecond laser ablation of nickel in high vacuum

We present an experimental characterization describing the characteristics features of the plasma plume dynamics and material removal efficiency during ultrashort, visible (527 nm, ≈300 fs) laser ablation of nickel in high vacuum. The spatio-temporal structure and expansion dynamics of the laser ablation plasma plume are investigated by using both time-gated fast imaging and optical emission spectroscopy. The spatio-temporal evolution of the ablation plume exhibits a layered structure which changes with the laser pulse fluence F. At low laser fluences (F<0.5 J/cm²) the plume consists of two main populations: fast Ni atoms and slower Ni nanoparticles, with average velocities of ≈10⁴ m/s for the atomic state and ≈10² m/s for the condensed state. At larger fluences (F>0.5 J/cm²), a third component of much faster atoms is observed to precede the main atomic plume component. These atoms can be ascribed to the recombination of faster ions with electrons in the early stages of the plume evolution. A particularly interesting feature of our analysis is that the study of the ablation efficiency as a function of the laser fluence indicates the existence of an optimal fluence range (a maximum) for nanoparticles generation, and an increase of atomization at larger fluences. PACS 52.50.-b; 52.38.Mf; 79.20.Ds; 81.07.-b     

Universal threshold for the steam laser cleaning of submicron spherical particles from silicon

The efficiency of the "steam laser cleaning" process is examined. For the investigation of the physics of particle removal from the particularly interesting surface of silicon we have deposited well-characterized spherical polymer and silica particles of different diameters ranging from several tens to hundreds of nanometers on commercial wafers. As a result of our systematic study we observe a sharp threshold of the steam cleaning process at 110 mJ/cm² (5=532 nm, FWHM=7 ns) which is independent of the size (for particles with diameters as small as 60 nm) and material of the particles. An efficiency above 90% after 20 cleaning steps is reached at a laser fluence of 170 mJ/cm². Experiments with irregularly shaped alumina particles exhibit the same threshold as for spherical particles.     

Nanosecond surface interferometry measurements on designed and commercial polymers

The effect of the ablation mechanism on surface morphology changes during an ablation process was studied by comparing three different polymers: a triazene polymer, a polyimide and poly(methylmethacrylate) (PMMA) with nanosecond surface interferometry. The triazene polymer, for which only indications for a photochemical ablation mechanism had been detected in previous studies, revealed no surface swelling, which could be attributed to a thermal ablation mechanism. For polyimide, a photothermal ablation mechanism is usually used to describe the ablation process at irradiation wavelengths 248 nm. However, the interferometric measurements do not show any surface swelling, which would be a clear indication for a thermal ablation mechanism. A surface swelling was only detected for PMMA with irradiation at 248 nm and fluences below the threshold of permanent surface modification. The detected phase shift, which is proportional to the change of the film thickness and the refractive index, can be explained by the opposite signs of the thermal expansion coefficient and the thermal refractive-index coefficient. PACS 52.38.Mf; 42.87.Bg; 71.20.Rv     

ArF laser surface modification of polyethersulfone film: Effect of laser fluence in improving surface biocompatibility

ArF laser treatment of polyethersulfone (PES) films was performed to improve biocompatibility of surfaces. For this purpose, the threshold fluence for laser ablation of PES was obtained from experimental measurements and then samples were irradiated at 2 separate ranges of fluences, i.e. below and above the ablation threshold. In order to investigate the physico-chemical changes, the modified surfaces were characterized by attenuated total reflectance (ATR) infrared spectroscopy and contact-angle measurements. The biocompatibility of the treated samples in comparison to those untreated was examined in vitro using a platelet adhesion test. The number of adhered platelets was obtained using the lactate dehydrogenase (LDH) method. For surfaces irradiated below the ablation threshold, a high reduction in the number of the adhered platelets was observed; while this number increased in samples treated at the fluence above the ablation threshold. The change in platelet adhesion was attributed to the change in chemistry and roughness of the irradiated surfaces.     

Laser ablation synthesis of indium oxide nanoparticles in water

Colloidal solutions of Indium oxide nanoparticles have been produced by means of laser ablation in liquids (LALs) technique by simply irradiating with a second harmonic (532 nm) Nd:YAG laser beam a metallic indium target immersed in distilled water and varying the laser fluence up to 10 J cm⁻² and the ablation time up to 120 min. At all the investigated fluences the vaporization process of the indium target is the dominant one. It produces a majority (>80%) of small size (<6 nm) nanoparticles, with a very limited content of larger ones (size between 10 and 20 nm). The amount of particles increases regularly with the ablation time, supporting the scalability of the production technique. The deposited nanoparticles stoichiometry has been verified by both X-ray photoelectron spectroscopy (XPS) and Energy Dispersive X-ray (EDX) analysis. Optical bandgap values of 3.70 eV were determined by UV-vis absorption measurements. All these results confirm the complete oxidation of the ablated material.     

A model of laser ablation with temperature-dependent material properties,vaporization, phase explosion and plasma shielding

Laser ablation of metals using nanosecond pulses occurs mainly due to vaporization. However, at high fluences, when the target is heated close to its critical temperature, phase explosion also occurs due to homogeneous nucleation. Due to a wide variation in target temperature, the material properties also show a considerable variation. In this paper, a model of laser ablation is presented that considers vaporization and phase explosion as mechanisms of material removal and also accounts for the variation in material properties up to critical temperature using some general and empirical theories. In addition, plasma shielding due to inverse bremsstrahlung and photo-ionization is considered. The model predicts accurately (within 5 %) the phase explosion threshold fluence of Al. The predictions of ablation depth by the model are in reasonable agreement with experimental measurements at low fluences. Whereas, the degree of error marginally increases at high laser fluences.     

Bimodal Nanoparticle Size Distributions Produced by Laser Ablation of Microparticles in Aerosols

Silver nanoparticles were produced by laser ablation of a continuously flowing aerosol of microparticles in nitrogen at varying laser fluences. Transmission electron micrographs were analyzed to determine the effect of laser fluence on the nanoparticle size distribution. These distributions exhibited bimodality with a large number of particles in a mode at small sizes (3–6-nm) and a second, less populated mode at larger sizes (11–16-nm). Both modes shifted to larger sizes with increasing laser fluence, with the small size mode shifting by 35% and the larger size mode by 25% over a fluence range of 0.3–4.2-J/cm². Size histograms for each mode were found to be well represented by log-normal distributions. The distribution of mass displayed a striking shift from the large to the small size mode with increasing laser fluence. These results are discussed in terms of a model of nanoparticle formation from two distinct laser–solid interactions. Initially, laser vaporization of material from the surface leads to condensation of nanoparticles in the ambient gas. Material evaporation occurs until the plasma breakdown threshold of the microparticles is reached, generating a shock wave that propagates through the remaining material. Rapid condensation of the vapor in the low-pressure region occurs behind the traveling shock wave. Measurement of particle size distributions versus gas pressure in the ablation region, as well as, versus microparticle feedstock size confirmed the assignment of the larger size mode to surface-vaporization and the smaller size mode to shock-formed nanoparticles.