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Ultrashort-pulse lasers with fundamental wavelengths ranging from near-infrared to near-ultraviolet are increasingly being used for laser-induced surface modification of non-metallic solids. The relaxation of the initial electronic excitation into vibrational relaxation modes can produce efficient ablation and other desirable surface modifications with little collateral damage because the laser energy is deposited on a time scale much shorter than thermal diffusion times. Little is known, however, about how ultrashort pulses interact with insulators at wavelengths in the vibrational infrared. This paper describes surface modifications achieved by picosecond laser irradiation in the 2-10 lim range. The laser source was a tunable, free-electron laser (FEL) with I-ps micro-pulses spaced 350 ps apart in a macropulse lasting up to 4 μs, with an average power of up to 3 W. This unusual pulse structure makes possible novel tests of the influences vs fluence and intensity, as well as the effects of resonant vibrational excitation. As model materials systems, we studied calcium carbonate, its isoelectronic cousin sodium nitrate, and fused silica. Particularly intriguing are surface modifications achieved by tuning the laser into vibrational resonances and overtones of the target materials, or by tailoring the energy content of the pulse. The mechanisms underlying these effects, and their implications for materials-modification strategies, are discussed.
The existence inside targets during pulsed laser ablation of a sub-surface superheating effect (SSSH) has been predicted by numerical temperature estimations. The experimental evidence has been so far only indirect, based on the modification of the surface morphology caused by the explosive volume boiling induced by the SSSH effect. However, round-shaped micrometer-sized cavities formed by gas release due to volume boiling have been found on several target materials even when the temperature estimations did not predict any SSSH effect. Although the SSSH effect could exist under certain conditions, it seems that it is not a prerequisite for explosive volume boiling which is the actual mechanism responsible for droplets emission. Volume boiling could occur whenever a thick liquid layer, whose temperature is much higher than the equilibrium boiling value is formed and lasts for several tens of nanoseconds on the target surface, a situation usually found when the laser wavelength is poorly absorbed by the target material.
We have developed a mesoscopic model for the study of pulsed laser ablation of transparent granular ceramics. The model enables the understanding of several features that happen at the surface of a transparent target and which play an important role in the evolution of the evaporation process.
The results show that electron emission happens early, much before atom evaporation. High defect concentration regions (including grain boundaries, surface) are crucial for energy absorption. The model also predicts the generation of high intensity electric fields in places with high defect concentration, such as grain boundaries. Low fluences should have selective species removal, but higher fluences should give congruent removal.
The dependence of the density of generated electrons, the evaporated species and energy on the material properties is included.
N-type CdSe films with thicknesses of 470 - 630 nm were grown on (001) and 2°- miscut GaAs wafers by ArF (193 nm) pulsed laser ablation of stoichiometric CdSe targets at platen temperatures (Tp) of 250 - 425°C in vacuum and ambient Ar gas. Film-substrate interdiffusion was studied with Auger depth profiling, as well as energy dispersive x-ray fluorescent spectroscopy (EDS). Both techniques showed that extensive interdiffusion took place at the film-substrate interface for CdSe films grown at Tp≥ 355°C but was greatly reduced at Tp=250°C. Tilting the substrate to be approximately parallel to the ablation plume as well as decreasing the ambient gas pressure also reduced film-substrate interdiffusion.
The initial growth of pulsed laser deposited SrTiO3 on SrTiO3 has been studied using high pressure Reflection High Energy Electron Diffraction (RHEED) and Atomic Force Microscopy (AFM). For this, we developed a Pulsed Laser Deposition (PLD)-RHEED system, with the possibility to study the growth and to monitor the growth rates, in situ, at typical PLD pressures (10-50 Pa). Using perfect single crystal SrTiO3 substrate surfaces, we observe true 2D intensity oscillations at different temperatures. Simultaneously, information on the diffusion of the deposited material on the surface could be extracted from the relaxation of the intensity after each laser pulse. The characteristic times depend on pressure and temperature as well as the 2D coverage during growth.
Pulsed laser ablation of granulated Si target was carried out at 1200 °C in an Ar atmosphere. Multishot ablated target surface forms intensity dependent features, including porous, skeleton, and columnar structures. Very long columnar structures were observed when the angle of the target surface with respect to the direction of the laser beam was small. Evidence on preferable remove of smaller particles has been observed. Formation of the columnar structures started from the biggest particles at the surface and grew deeper, straight in the laser beam direction, by consuming the removed Si species from the deep channel between columns. The Si species ablated off the granulated Si target deposited as Si nanowires or nanoparticles down stream of the Ar flow. Significant decrease in the deposition rate of Si nanostructures has been observed upon the formation of the columnar structures at the target surface.
The dynamics of gas phase nanoparticle formation by pulsed laser ablation into background gases are revealed by imaging photoluminescence and Rayleigh-scattered light from gas-suspended SiOx nanoparticles following ablation of c-Si targets into 1-10 Torr He and Ar. Two sets of dynamic phenomena are presented for times up to 15 s after KrF-laser ablation. Ablation of Si into heavier Ar results in a uniform, stationary plume of nanoparticles while Si ablation into lighter He results in a turbulent ring of particles which propagates forward at 10 m/s. The effects of gas flow on nanoparticle formation, photoluminescence, and collection are described. The first in situ time-resolved photoluminescence spectra from 1-10 nm diameter silicon particles were measured as the nanoparticles were formed and transported. Three spectral bands (1.8, 2.5 and 3.2 eV) similar to photoluminescence from oxidized porous silicon were measured, but with a pronounced vibronic structure. The size and composition of individual gas-condensed nanoparticles were determined by scanning transmission electron microscopy and correlated with the gas-phase photoluminescence. Weblike-aggregate nanoparticle films were collected at room temperature and 77K on c-Si substrates. After standard passivation anneals, the films exhibited strong room temperature photo-luminescence consisting of 3 spectral bands in agreement with the gas-phase measurements, however lacking the vibronic structure. These techniques demonstrate new ways to study and optimize the luminescence of novel optoelectronic nanomaterials during synthesis in the gas phase, prior to deposition.
Laser ablation of metals and reaction with hydrocarbon produces clusters of metal carbides. Metal precursors are from group IVb to VIb of the periodic system. The mass distribution is analysed by TOF in the gas phase. The clusters are deposited on carbon and investigated by TEM and STM. The materials are very hard and very flat and include Ti-C, Zr-C, V-C, Nb-C, and TaC.
Nanoparticles of iron oxide were prepared by pulsed laser ablation on carbon coated mica substrates. An ArF excimer laser was used to irradiate a Fe2O3 target in atmospheres of Ar at room temperature. The effects of ambient pressure on size and morphology of nanoparticles were investigated using transmission electron microscopy. The morphology of the deposited nanoparticles was strongly dependent on the ablation pressure. The formations of nanoparticles and their aggregates were observed at pressures higher than 46.7 Pa and 267 Pa of Ar, respectively. The size of the primary nanoparticles ranged from 2 – 9 nm and their size distribution agreed with a log-normal distribution function. The aggregate size increased with ambient pressure and the primary particle size was independent of ambient pressure.
Quasi one-dimensional materials have attracted considerable attention in recent years because of its potential to both fundamental physics and nanoelectronic applications. More recently, we have achieved large scale synthesis of silicon nanowires (SINW) at a high growth rate by laser ablation of Si target at 1200 °C. The laser source was a pulsed KrF excimer laser and the Si targets were made by pressing Si powder of 5 microns in size. 50 sccm Ar was used as a carrying gas flowing from the side near the Si target towards a water-cooled copper finger. Si nanowires have been grown with diameters ranging from 3 to 43 nm and several hundreds microns in length after 2 hours of laser ablation of Si target. The SLNWs were analyzed by XRD, Raman, EDS, TEM and HRTEM. Successful large scale synthesis of SINW by laser ablation extends the pulsed laser ablation method from depositing thin films to synthesis of nanowires.
Low pressure differential mobility analysis and transmission electron microscopy were used to study the aerosol formed during 1064-nm pulsed Nd3+:YAG laser ablation of a solid cesium iodide (CsI) target. Measurements reveal that the aerosol is comprised of particles in three size regimes: particles with approximately log-normal size distributions and Dpg ~ 10 nm, compact spherical CsI particles with Dpg ~ 80 nm, and fractal-like agglomerates with Dpg ~ 1-3 μm.
Optical emission spectrum of aluminum plasma induced by a 1064 nm Nd:YAG laser is investigated by an Optical Multichannel Analyzer (OMA). Spectroscopic study shows that more number of Al, Al+, and Al++ spectral lines can be observed with increasing the incident laser fluence. Al, Al+, Al++ spectral lines are also observed successively with high fluence. The atomic spontaneous radiation is analyzed to interpret the calibrated plasma spectrum. The laser energy threshold for the appearance of excited Al, Al+, and Al++ spectral lines are about 0.8, 1.0 and 1.5 J/cm2 respectively. Assuming LTE (Local Thermodynamic Equilibrium) conditions, the plasma density is derived to be in the range of 0.7×1017 to 2×1017 cm-3 from the profiles of Al+ (358.7 and 286.1 nm) spectral lines with different gated times and incident laser fluences. The plasma temperature is also estimated to be 4000 ~ 8000 K, from relative intensities of two different Al I spectral lines (309.2 and 396.2 nm) with different fluence.
Audible acoustic wave detection is applied to investigate KrF excimer laser ablation of Indium Tin Oxide (ITO) thin film layer for Liquid Crystal Display (LCD) patterning. It is found that there is no acoustic wave generation if laser fluence is lower than ITO ablation threshold. For laser fluence higher than the threshold, audible acoustic wave will be detected due to shock wave generation during ITO laser ablation. The amplitude of the acoustic wave is closely related to the laser ablation rate. With more laser pulse applied, the amplitude is dropped to zero because the ITO layer is completely removed. However, if laser fluence is increased higher than ablation threshold for glass substrate, the amplitude is also dropped with pulse number but not to zero. It is due to laser ablation of ITO layer and glass substrate at the same time. Since the thickness of ITO layer is in a scale of 100 nm, laser interaction with glass substrate will happen even at the first pulse of high laser fluence irradiation. Laser ablation induced ITO plasma emission spectrum in visible light region is analyzed by an Optical Multi-channel Analyzer (OMA). Specific spectral lines are In I (325.8, 410.2 and 451.1 nm) and In II 591.1 nm. Spectral intensities of 410.2 and 451.1 nm lines are selected to characterize the evolution of ITO plasma intensity with laser fluence and pulse number. It is found that the spectral intensities are reduced to zero with laser pulse number. It is also found that spectral lines other than ITO plasma will appear for laser fluence higher than ablation threshold for glass substrate. Threshold fluences for glass and ITO ablation are estimated for setting up a parameter window to control LCD patterning in real-time.
Inhomogeneous low-k thin films of SiOxNy have been deposited by laser ablation of a Si3N4 sintered target in presence of oxygen gas. The high oxidation rate of silicon nitride has been used to control the stoichiometry of the films by modifying the oxygen partial pressure. The refractive index of the deposited material was able to be tailored at any value between 1.47 (SiO2) to 2.03 (Si3N4) by this approach. In situ optical characterization of the growing films was possible using kinetic and spectro ellipsometry. The refractive index was determined by applying the Effective Medium Approximation (EMA) and considering a mixture of SiO2, Si3N4, and voids. The volumetric composition obtained by ellipsometry was compared to the results determined by AES and XPS characterization. The purpose of this application is to show that reactive PLD can be used to produce high quality optical filters.
Particulate coatings have wide ranging applications in several new technologies such as flat-panel displays, sintering of advanced ceramics, rechargeable batteries, etc. In this paper, we show the feasibility of the pulsed laser ablation technique to make very thin, uniformly distributed and discrete coatings in particulate systems so that the properties of the core particles can be suitably modified. Presently, laser ablation techniques have been primarily applied to deposit thin films on flat substrate materials. To deposit discontinuous particulate coatings, the laser induced plume from the target comes in contact with an agitated bed of core particles. The pressure and nature of the background gas (inert or active) controls the cluster size of the nano particles in the laser plume. Experiments were conducted for laser deposition of Ag nano particles on Al2O3 and SiO2 core particles by pulsed excimer laser (wavelength = 248 nm and pulse duration = 25 nanosecond) irradiation of a Ag sputtering target The surface coverage and coating uniformities of the film were found to depend on the synthesis parameters (energy density, # laser pulses, gas pressure backfill gas, molecular weight) as well as the residence time of the core particles in the plume regime. The films were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), wavelength dispersive x-ray analysis (WDX), scanning transmission electron microscopy (STEM), and x-ray photoelectron spectroscopy (XPS).
A new deposition method, inspired from the crossed fluxes technique, which employs a concave, conic-shaped target is presented here. The rectangular excimer laser beam used for ablation was focused so that the middle of the spot laid exactly on the tip of the concave-shaped target. Each half of the laser spot created a plasma plume on one side of the concave target which was the symmetrical image across the cone axis of that created by the other half of the laser spot. The heavy droplets passed through the plasma interaction region without collisions and, maintaining their direction of motion, moved away from the system axis. The majority of the ablated ions and atoms emitted from one side of the spot collided with those emitted from the other side and, because of the symmetry of the concave-shaped target, acquired a velocity component along the system axis, moving towards the substrate. Scanning electron microscopy investigations showed a significant reduction of droplet density onto the surface of hydroxyapatite layers grown from such concave-shaped targets as compared to films grown from the usual cylindrical targets.
A laser spark atomizer (LINA-SPARK™), LSA, has been used for preparing powder particles from SnO2, Al2O3 and ZrO2 ceramic specimen. It is shown that this technique can be used for preparing thin films by direct deposition on a substrate. The as-prepared powder can also be redispersed and deposited using ultrasonic nebulization (Pyrosol) deposition. The latter approach is especially suited for deposition of controlled-size and multicomponent thin films.
The coupling of the LSA to an induced coupled plasma (ICP) emission spectrometer is also discussed and compared with laser ablation. Generally powder particles produced from LSA present a narrower size distribution as powders prepared by laser ablation. As a result, the quantitative elemental analysis of solids are improved with full benefit of the sensitivity and detection limits of the ICP are lowered.
Chemically clean etching of thermodynamically metastable diamond using a 500 femtosecond (fs), 248 nm KrF excimer laser is reported. The experimental results, characterized by micro-Raman spectroscopy indicate that unlike nanosecond (ns) pulsed laser processing of diamond, fs laser irradiation of the surface does not generate any graphite or amorphous carbon residues. Microstructural analysis of the fs pulsed laser etched surface indicates streaks.
Diamond thin films promise excellent performance in several application fields such as high temperature and high frequency electronics, but practical applications are presently limited by the polycrystalline morphology of deposited films.
A laser treatment was performed to smooth the surface of diamond films, produced by HFCVD, with the aim to allow a suitable patterning and tayloring of diamond films and their use as coatings on specific tools. Different laser wavelengths (193, 532 nm), times of exposure, and energy densities were employed during the treatments.
A SEM characterization has shown a structural modification of the surface morphology and a noticeable weakening of surface roughness. A microRaman analysis indicated the appearance of a glassy carbon component which, together with the surface smoothing occurring at the treated zone, seems to justify: a) the large reduction of the intensity of Raman spectra (the diamond and silicon optical phonon lines and the photoluminescence emission are about 50 times weaker); b) the enhancement of conductivity and reflectivity.
The thickness dependence of ablation rates following 193nm UV-laser irradiation of single HfO2 layers on fused silica (SiO2) is investigated using scanning electron microscopy and stylus profilometry to determine quantitatively substrate roughness and ablation depth. Thin dielectric films of the investigated kind build up dielectric mirrors, which are patterned to prepare masks for excimer-laser micromachining. The single pulse ablation thresholds are found to increase approximately linearly with increasing HfO2 thickness and consequently the threshold fluence for obtaining clean ablation of the total HfO2 coating increases exponentially with its thickness. At elevated fluences both ablation of the coating as well as ablation of the substrate are observed. The results provide important quantitative values for a future treatment of more complicated multilayer systems of HfO2/SiO2 bilayers.