Nanosecond and femtosecond ablation of La0.6Ca0.4CoO3: a comparison between plume dynamics and composition of the films

Thin films of La_0.6Ca_0.4CoO₃ were grown by pulsed laser ablation with nanosecond and femtosecond pulses. The films deposited with femtosecond pulses (248 nm, 500 fs pulse duration) exhibit a higher surface roughness and deficiency in the cobalt content compared to the films deposited with nanosecond pulses (248 nm, 20 ns pulse duration). The origin of these pronounced differences between the films grown by ns and fs ablation has been studied in detail by time-resolved optical emission spectroscopy and imaging. The plumes generated by nanosecond and femtosecond ablation were analyzed in vacuum and in a background pressure of 60 Pa of oxygen. The ns-induced plume in vacuum exhibits a spherical shape, while for femtosecond ablation the plume is more elongated along the expansion direction, but with similar velocities for ns and fs laser ablation. In the case of ablation in the background gas similar velocities of the plume species are observed for fs and ns laser ablation. The different film compositions are therefore not related to different kinetic energies and different distributions of various species in the plasma plume which has been identified as the origin of the deficiency of species for other materials.     

XPS studies of short pulse laser interaction with copper

The effect of laser ablation on copper foil irradiated by a short 30 ns laser pulse was investigated by X-ray photoelectron spectroscopy. The laser fluence was varied from 8 to 16.5 J/cm² and the velocity of the laser beam from 10 to 100 mm/s. This range of laser fluence is characterized by a different intensity of laser ablation. The experiments were done in two kinds of ambient atmosphere: air and argon jet gas.The chemical state and composition of the irradiated copper surface were determined using the modified Auger parameter (α′) and O/Cu intensity ratio. The ablation atmosphere was found to influence the size and chemical state of the copper particles deposited from the vapor plume. During irradiation in air atmosphere the copper nanoparticles react with oxygen and water vapor from the air and are deposited in the form of a CuO and Cu(OH)₂ thin film. In argon atmosphere the processed copper surface is oxidized after exposure to air.     

Diagnostics of laser-ablated carbon plumes by photoionization using a tunable laser

Tunable laser pulses at wavelengths from 250 to 880 nm were fired at plumes produced by laser ablation of a carbon target in He atmosphere. We observed an electrical current due to photoionization, which roughly reflected the behavior of carbon clusters in the plume. The photoionization current had dynamic temporal variations, comprising a rapid increase at the beginning (T_D<0.2 ms, where T_D is the time after a YAG laser pulse for laser ablation irradiates the target), a gradual increase for 0.2D<3 ms, and a slow decrease for T_D>7 ms. The increasing phase of the photoionization current was synchronized with the decreasing phase of C₂ radical density. For He gas pressures lower than 0.8 Torr no photoionization current was detected. The growth rate of the photoionization current was higher for a higher He gas pressure.     

Plume temperature in the laser ablation of polyimide films measured by infrared emission spectroscopy

The infrared emission spectrum of the plume produced by KrF excimer laser ablation of polyimide films in air and in He was measured in the 680 to 1580 cm^–1 wavenumber range. Using 400 mJ/cm² laser pulses of 248 nm and 35 ns duration yielded a strong emission band characteristic of thev ₂ transitions of hot HCN molecules. Band counters calculations were carried out of thev ₂ emission expected from HCN in thermal equilibrium at various temperatures. They indicate that except for a slight deviation of the measured data from thermal equilibrium, the best fit of the observed results is obtained at a plume temperature of 2250±150K.     

Transmission electron microscopy investigation of crystalline silicon surface irradiated by femtosecond laser pulses in different background atmospheres

This article aims to obtain structural and compositional characteristics of a crystalline silicon surface irradiated by femtosecond laser pulses in SF₆, N₂, air, and vacuum background atmospheres by performing transmission electron microscopy observation of ??110?? cross-sectional specimens. Conical microstructures covered with defective outer layers were formed in SF₆ gas. The elemental sulfur dopants in the surface microstructure, which located in close proximity to defects, were mainly concentrated at the tip region of the microcones, and about several hundred nanometers thick. In N₂ atmosphere, the defects produced regularly on the silicon surface were of the same types with those formed in SF₆ gas and confirmed to be stacking faults and overlapped twins. Furthermore, silicon crystalline grains with different orientations were observed on the silicon surface irradiated in N₂, air, and vacuum atmospheres. Especially, ??-Si₃N₄ crystalline grains were found to be formed in N₂ and air as chemical products when elemental nitrogen exists, and the SiO₂ amorphous phase was formed in air by the oxidation effect. Based on these experimental results, the relevant interaction mechanisms between pulsed laser and crystalline silicon were suggested to be mainly attributed to laser-assisted chemical etching and laser ablation, i.e., if volatile silicon compounds can be produced in a reactive gas atmosphere (e.g., SF₆), the strong laser-assisted chemical etching dominates over the laser irradiation process. Otherwise, laser ablation is the dominant mechanism such as in N₂, air, and vacuum.     

Mass and kinetic energy distribution of the species generated by laser ablation of La<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript>

The mass distributions of the species generated by laser ablation from a La_0.6Ca_0.4MnO₃ target using laser irradiation wavelengths of 193 nm, 266 nm and 308 nm have been investigated with and without a synchronized gas pulse of N₂O. The kinetic energies of the species are measured using an electrostatic deflection energy analyzer, while the mass distributions of the species were analyzed with a quadrupole mass filter. In vacuum (pressure 10⁻⁷ mbar), the ablation plume consists of metal atoms and ions such as La, Ca, Mn, O, LaO, as well as multiatomic species, e.g. LaMnO⁺. The LaO⁺ diatomic species are by far the most intense diatomic species in the plume, while CaO and MnO are only detected in small amounts. The interaction of a reactive N₂O gas pulse with the ablation plume leads to an increase in plume reactivity, which is desired when thin manganite films are grown, in order to incorporate the necessary amount of oxygen into the film. The N₂O gas pulse appears to have a significant influence on the oxidation of the Mn species in the plume, and on the creation of negative ions, such as LaO⁻,O⁻ and O₂⁻.     

Periodic variations of plasma optical emission during repetitive pulsed-laser irradiation of aluminum in ambient air

Optical emission from plasma produced in ambient air by focusing a Nd:YAG laser beam on an aluminum surface was spectrally analysed. A periodic behaviour was observed in spectral line intensities associated with series of laser pulses. Based on simultaneous measurements of diffuse surface reflectivity and complementary measurements in other gases (He, O₂, N₂), this behaviour is ascribed to a competition between thermally-assisted surface oxidation and nitridation, and laser ablation. PACS 52.38.Mf; 52.50.Jm; 81.65; 78.68.+m; 52.70.Kz     

Pulsed laser deposition of hard coatings in atmospheric air

A new laser plasma technique for non-vacuum deposition of thin films has been proposed and experimentally realized. It is based on the fact that the plasma plume, which occurs under ablation of a target in air by high-intensity short laser pulses, can penetrate through a dense gas environment without significant cooling at the distance of about 1 mm. The technique has been applied to deposit diamond-like carbon (DLC) coatings on stainless steel substrates using four different values of pulse duration: 10 ns, 300 ps, 5 ps and 130 fs. Optimization of different experimental parameters including distance between the target and the substrate, laser intensity and gases (He, Ar, N₂, compressed air) blown in the deposition zone, has been performed. The deposition rate in the experiments was estimated as 2–5×10^-4 nm/(cm²pulse) for the pulse energy of 1–4 mJ. The deposited amorphous carbon films with thickness of several hundred nanometers have shown high average nanohardness (10–25 GPa depending on the irradiation conditions) and good adhesion to substrates (60 MPa). According to X-ray electron spectroscopy analysis the films consist of both sp²- and sp³-bonded carbon and contain 3–7% of free oxygen in bulk. The mechanisms of DLC non-vacuum laser deposition are discussed. To demonstrate the large potential of this technique, the first results on deposition of titanium nitride using ablation of titanium in air with nitrogen jet assistance are also presented. PACS 52.38.Mf; 81.15.Fg; 81.05.Uw     

CEMS study of stainless steel films deposited by pulsed laser ablation of AISI316

Thin stainless steel films deposited on SiO₂/Si wafer were prepared by a pulsed laser ablation of austenite stainless steel (AISI316), and characterized by conversion electron Mössbauer spectrometry (CEMS) using a He gas proportional counter. As-deposited films were composed of a magnetic phase. When deposited films were heated in air at various temperatures, the hyperfine field of the magnetic phase increased. Hematite was produced on the surface, and the magnetic orientation changed from parallel to in-plane at random with the increase of heating temperatures. The metallic iron and magnetite were produced at 400°C in dry Ar + 5%H₂ atmosphere. When the film was heated in wet Ar + 5%H₂ atmosphere at 600°C, maghemite was produced on the surface, and austenite phase was produced in the inner film.     

Surface modification of a WTi thin film on Si substrate by nanosecond laser pulses

Interaction of a nanosecond transversely excited atmospheric (TEA) CO₂ laser, operating at 10.6 μm, with tungsten-titanium thin film (190 nm) deposited on silicon of n-type (1 0 0) orientation, was studied. Multi-pulse irradiation was performed in air atmosphere with laser energy densities in the range 24-49 J/cm². The energy absorbed from the laser beam was mainly converted to thermal energy, which generated a series of effects. The following morphological changes were observed: (i) partial ablation/exfoliation of the WTi thin film, (ii) partial modification of the silicon substrate with formation of polygonal grains, (iii) appearance of hydrodynamic features including nano-globules. Torch-like plumes started appearing in front of the target after several laser pulses.     

Monitoring nanoparticle formation during laser ablation of graphite in an atmospheric-pressure ambient

Excimer laser ablation of highly oriented pyrolytic graphite (HOPG) was performed at atmospheric pressure in an N₂ and in an air ambient. During the ablation, nanoparticles condensed from the material ejecta, and their size distribution was monitored in the gas phase by a Differential Mobility Analyzer (DMA) in combination with a Condensation Particle Counter (CPC). Size distributions obtained at different laser repetition rates revealed that the interaction between subsequent laser pulses and formed particles became significant above 15 Hz. This interaction resulted in laser heating, leading to considerable evaporation and a decrease in the size of the particles. X-ray photoelectron spectroscopy revealed that approximately 8% nitrogen was incorporated into the CN_x particles generated in the N₂ ambient, and that the nitrogen was mostly bonded to sp³-hybridized carbon. Monodisperse particles were also deposited and were analyzed by means of Raman spectroscopy to monitor size-induced effects. PACS 81.07.-b; 61.46.+w; 79.70.+q     

Carbon-plasma produced in vacuum by 532 nm-3 ns laser pulses ablation

A study of VIS laser ablation of graphite, in vacuum, by using 3 ns Nd:YAG laser radiation is reported. Nanosecond pulsed ablation gives an emission mass spectrum attributable to C_n neutral and charged particles. Mass quadrupole spectroscopy, associated to electrostatic ion deflection, allows estimation of the velocity distributions of several of these emitting species within the plume as a function of the incident laser fluence. Time gated plume imaging and microscopy measurements have been used to study the plasma composition and the deposition of thin carbon films. The multi-component structure of the plume emission is rationalized in terms of charge state, ions temperature and neutrals temperature. A special regard is given to the ion acceleration process occurring inside the plasma due to the high electrical field generated in the non-equilibrium plasma conditions. The use of nanosecond laser pulses, at fluences below 10 J/cm², produces interesting C-atomic emission effects, as a high ablation yield, a high fractional ionization of the plasma and presence of nanostructures deposited on near substrates.     

Optical emission spectroscopy of thin film fabrication by pulsed laser ablation under high-gravity

We carried out the thin film deposition of iron silicide by pulsed laser ablation (PLA) on a sapphire substrate, which was placed on a high-speed rotating titanium disk. The deposited thin film exhibited a continuous composition gradient. We investigated how the continuous composition gradient was attained, because the strength of the gravity field in our experiment was far below that in the experiment on bulk crystalline compounds. In the present study, we obtain the spatial distribution of several species in the PLA plume of FeSi₂ by using an intensified charge-coupled device (ICCD) camera.     

193-nm laser ablation of CVD diamond and graphite in vacuum: plume analysis and film properties

The pulsed laser ablation of chemical vapor deposition (CVD) diamond and graphite samples in vacuum has been investigated by the use of an ArF excimer laser operating at 5=193 nm. The composition and propagation of both ablation plumes has been probed via wavelength and spatially and temporally resolved measurements of the plume emission and found to be very similar. Electronically excited C atoms and C⁺ and C²⁺ ions are identified among the ablated material. Plume expansion velocities are estimated from time-gated imaging of specific C and C⁺ emissions. Langmuir probe measurements provide further insight into the propagation of the charged components in both ablation plumes. Diamond-like carbon (DLC) films grown by 193-nm laser ablation of both target materials on Si substrates maintained at room temperature have been investigated by laser Raman spectroscopy (325 nm and 488 nm excitation) and by both optical and scanning electron microscopy, and their field emission characteristics investigated. Again, similarities outweigh the differences, but DLC films grown from ablation of the diamond target appear to show steeper I/V dependencies once above the threshold voltage for field emission.     

Impact of open de-ionized water thin film laminar immersion on the liquid-immersed ablation threshold and ablation rate of features machined by KrF excimer laser ablation of bisphenol A polycarbonate

Debris control and surface quality are potential major benefits of sample liquid immersion when laser micromachining; however, the use of an immersion technique potentially modifies the ablation mechanism when compared to an ambient air interaction. To investigate the machining characteristics, bisphenol A polycarbonate has been laser machined in air and under a controllable open liquid film. To provide quantitative analysis, ablation threshold, ablation rate and the attenuation coefficient of the immersing de-ionized (DI) water fluid were measured. In ambient air the threshold fluence was measured to be 37 mJ cm⁻². Thin film immersion displayed two trends: threshold fluences of 58.6 and 83.9 mJ cm⁻². The attenuation of DI water was found to be negligible; thus, the change in ablation rate resulted from increased confinement of the vapour plume by the liquid medium, generating higher Bremsstrahlung attenuation of the beam, lowering the laser etch rate. Simultaneously, splashing motivated by the confined ablation plume allowed release of plume pressure before plume etching commenced. This contributed to the loss of total etching efficiency. Two interaction scenarios were obsereved as a result of splashing: (i) intermediate threshold fluence, where splashing occured after every pulse in a mode that interrupted the flow entirely, leaving an ambient air interaction for the following pulse; (ii) high threshold fluence, where splashing occured for every pulse in a mode that allowed the flow to recommence over the image before the next pulse causing every pulse to experience Bremsstrahlung attenuation. Since attenuation of the immersion liquid was negligible, it is the action of the constrained ablation plume within a thin flowing immersion liquid, the resultant Bremsstrahlung attenuation and splashing events that are the critical mechanisms that modify the primary ablation characteristics.     

Dynamics and spectroscopy of the laser plume from solid targets

The dynamics and the spectral kinetic characteristics of the plume emerging in the vicinity of graphite targets, pressed pellets consisting of zirconium oxide powder stabilized with yttrium (YSZ) and yttrium-aluminum oxides with neodymium (YAO:Nd), and single-crystal YAG:Cr are studied. The targets are irradiated in air at room temperature using a repetitively pulsed CO₂ laser with a wavelength of 10.6 μm, a peak power of up to 9 kW, a pulse energy of 1.69 J, and a pulse duration of 330 μs at a level of 0.1. The plume propagates normally to the target surface at an angle of 45° relative to the laser radiation. The spectral kinetic characteristics of the plume luminescence are discretely measured along the entire length. It is demonstrated that the plumes of all targets (except for the single-crystal YAG:Cr) represent the flows of a weakly nonequilibrium gas plasma with a temperature of 10 kK (graphite) and 3.1–4.7 kK (YSZ and YAO:Nd pressed pellets). The plume size is determined by the peak power of the laser pulse. The luminescence of the two-atom radicals (C₂ in graphite; ZrO and YO in YSZ; and YO, AlO, and NdO in YAO:Nd) dominates in all of the plumes. A single radical (YO) and the spectral lines of atoms and atomic ions are observed in the YAG:Cr plume. A relatively high temperature of the graphite plume is maintained owing to the energy of the exothermic reaction involving the association of carbon atoms and the energy of the vibrationally excited molecules resulting from this reaction. Original Text ? Astro, Ltd., 2006.     

Non-contact acoustic tests based on nanosecond laser ablation: Generation of a pulse sound source with a small amplitude

A method to generate a pulse sound source for acoustic tests based on nanosecond laser ablation with a plasma plume is discussed. Irradiating a solid surface with a laser beam expands a high-temperature plasma plume composed of free electrons, ionized atoms, etc. at a high velocity throughout ambient air. The shockwave generated by the plasma plume becomes the pulse sound source. A laser ablation sound source has two features. Because laser ablation is induced when the laser fluence reaches 10¹²–10¹⁴ W/m², which is less than that for laser-induced breakdown (10¹⁵ W/m²), laser ablation can generate a lower sound pressure, and the sound source has a hemispherical radiation pattern on the surface where laser ablation is generated. Additionally, another feature is that laser-induced breakdown sound sources can fluctuate, whereas laser ablation sound sources do not because laser ablation is produced at a laser beam–irradiation point. We validate this laser ablation method for acoustic tests by comparing the measured and theoretical resonant frequencies of an impedance tube.     

Time-integrated photography of laser-induced plasma plumes

The shape of the plasma plume induced by KrF-laser irradiation of Y?Ba?Cu?O, Cu?O, and Cu targets in O₂ and N₂ atmosphere was investigated by time-integrated photography. The dependence on laser fluence, spot size, and pressure of the ambient gas was studied. Special emphasis was put on the ablation of YBCO in O₂ atmosphere under experimental conditions that are typical for thin-film deposition.     

Double-pulse laser ablation applied to reactive PLD method for synthesis of carbon nitride film: A second laser shot delay

Carbon nitride films were deposited using ablation of graphite target by second harmonic radiation of Nd:YAG laser in nitrogen atmosphere. To produce high hardness films, the deposited particles should have sufficient kinetic energy to provide their efficient diffusion on a substrate surface for formation of crystal structure. However, a shock wave is arisen in ambient gas as a consequence of laser plasma explosive formation. This shock wave reflected from the substrate interacts with plume particles produced by the first laser pulse and decreases their kinetic energy. This results in decrease of film crystallinity. To improve film quality, two successive laser pulses was proposed to be used. At adjusting time delay, the particles induced by the second pulse wilt serve as a piston, which will push forward both stopped particles ablated by the first pulse and arisen from chemical reactions in ambient gas. An X-ray photoelectron spectroscopy (XPS) analysis of deposited films has shown an increase of content of sp ³ carbon atoms corresponding to crystalline phase, if double-pulse configuration is employed. The luminescence of excited C₂ and CN molecules in laser plume at different distances from the target was studied to optimize the delay between laser pulses.     

Growth of preferentially-oriented AlN films on amorphous substrate by pulsed laser deposition

Preferentially-oriented aluminum nitride (AlN) films are grown directly on natively-oxidized Si (100) substrate by pulsed laser deposition (PLD) in nitrogen (N₂) environment. The AlN preferential orientation changes from (002) to (100) with increasing N₂ pressure. Such different behaviors are discussed in terms of deposition-rate-dependent preferential orientation, kinetic energy of depositing species and confinement of laser plume. Finally, sample deposited at 0.9 Pa is proved to have the highest (002) peak intensity, the lowest FWHM value, the highest deposition rate and a relatively low RMS roughness (1.138 nm), showing the optimal growth condition for c-axis-oriented AlN growth at this N₂ pressure.