Pulsed KrF Excimer Laser Research Articles

Microstructure and tribological performance of CN x–TiN x composite films prepared by pulsed laser deposition

CN x –TiN x composite films were prepared on high-speed steel (HSS) substrate by pulsed KrF excimer laser co-deposition process with graphite/Ti combined targets and a substrate temperature of 200 °C. The composition, morphology and microstructure of the films were characterized by energy dispersive X-ray spectrum (EDS), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The adhesion and tribological performance of the films were investigated using a conventional scratch tester and a ball-on-disk tribometer, respectively. In the graphite/Ti range of 0.5–2.0 of the target, TiN x , a-CN x and metallic Ti phase were found in the composite films. The TiN x disappeared in the films at a high graphite/Ti ratio of the target. With increasing the graphite/Ti ratio of the target, the adhesion to substrate of the composite films deteriorated from 46 N to 26 N, and the friction coefficient decreased from 0.23 to 0.17. The composite film deposited at the graphite/Ti ratio of 1.0 showed a low friction coefficient, good adhesion and wear rate of 3.2 × 10 −7 mm 3/Nm in humid air.

Effects of Laser Sources on Damage Mechanisms and Reverse-Bias Leakages of Laser Lift-Off GaN-Based LEDs

The frequency-tripled neodymium-doped yttrium aluminum garnet laser and the KrF pulsed excimer laser were employed to separate GaN thin films from sapphire substrates and to transfer the films to bond with other substrates. The different laser lift-off processes would generate the dislocation density on different regions. In this study, the effects of these two laser sources on structural damage mechanisms and reverse-bias leakages of InGaN–GaN light-emitting diodes (LEDs) were studied.

Periodic structures in GeO2-B2O3-SiO2 glass films fabricated using ultraviolet laser pulses

We firstly observed the surfaces of periodic structures consisting of Ge nanoparticles in channel waveguide cores of GeO2-B2O3-SiO2 glasses. Such periodic structures were formed only by direct laser writing and subsequent annealing. Refractive index patterns were written in the glass films by irradiation with KrF excimer laser pulses. Then, the patterns were inverted by thermo-induced precipitation of Ge nanoparticles at unirradiated areas. Brown channel cores were obtained due to predominant precipitation of the nanoparticles in the cores. The periodic structures of 530 nm pitch and microchannels were directly observed by using wet-etched samples. The periodic structures were confirmed only inside the cores.

Time-resolved measurements during backside dry etching of fused silica

The laser-induced backside dry etching (LIBDE) investigated in this study makes use of a thin metal film deposited at the backside of a transparent sample to achieve etching of the sample surface. For the time-resolved measurements at LIBDE fused silica samples coated with 125 nm tin were used and the reflected and the transmitted laser intensities were recorded with a temporal resolution of about 1 ns during the etching with a ∼30 ns KrF excimer laser pulse. The laser beam absorption as well as characteristic changes of the reflection of the target surface was calculated in dependence on the laser fluence in the range of 250–2500 mJ/cm 2 and the pulse number from the temporal variations of the reflection and the transmission. The decrease of the time of a characteristic drop in the reflectivity, which can be explained by the ablation of the metal film, correlates with the developed thermal model. However, the very high absorption after the film ablation probably results in very high temperatures near the surface and presumably in the formation of an absorbing plasma. This plasma may contribute to the etching and the surface modification of the substrate. After the first pulse a remaining absorption of the sample was measured that can be discussed by the redeposition of portions of the ablated metal film or can come from the surface modification in the fused silica sample. These near-surface modifications permit laser etching with the second laser pulse, too.

Short soft x‐ray sources

AbstractIn this work we report on the characterization of pulsed soft x‐rays emitted from laser‐produced plasma. The plasma was generated by a pulsed KrF excimer laser focused on pure Si, Cu and Ta targets by 40, 80 and 120 mJ laser energies. The utilized detector was a very sensitive Faraday cup which opportunely biased was able to record time‐resolved signals of x‐rays and to estimate their energy. The found x‐rays energy values were compared with the ion temperature of the plasma obtained by fitting the time‐resolved ion current signals with a shifted Maxwell‐Boltzmann velocity distribution. The results showed that the laser‐produced Ta plasma induced bunches of x‐rays having in average the highest energy values and it was also characterized by ion temperature higher than the ones of the laser‐produced Si and Cu plasmas. Copyright © 2009 John Wiley & Sons, Ltd.

Pulsed KrF excimer laser induced degradation of cellulose based insulating paper

We report on the accelerated ageing of cellulose based insulating paper by means of pulsed UV laser irradiation (λ = 248 nm) under various experimental conditions including paper composition, background gas (He, N2 and air) and moisture content of the paper. The temperature reached by the paper samples during their laser irradiation was monitored by means of real-time IR imaging. It is shown that the equilibrium temperature (Teq) reached by the paper increases from ~30 to ~270 °C when the laser energy density was raised from 15 to 550 mJ cm−2. The laser irradiated samples were systematically characterized by means of scanning electron microscopy (SEM) observations and degree of polymerization (DPv) measurements. Interestingly, it is found that, for a given moisture content, the degradation level of the cellulose is mainly triggered by the Teq value reached during the laser irradiation. Moreover, their moisture content was found to influence significantly the number of laser produced bond scissions (it doubles when the moisture content is increased from 0.5 to 6%); the paper degradation is apparently not affected by the presence of oxygen as the background gas. These results suggest that the laser induced cellulose degradation occurs through a direct photolysis (i.e. direct breakage of C–C, C–O and C–H bonds), leading to radicals formation, which, in turn, are believed to induce the acid hydrolysis degradation mechanism, the latter being moisture dependent. The activation energy (Ea) of each gaseous species collected after the laser degradation was estimated. Their Ea values were found to be in good agreement with the one associated to the laser depolymerisation of cellulose (i.e. ~56 kJ mol−1), suggesting thereby a direct correlation between the cellulose degradation and the formation of the detected gaseous species. Finally, the pulsed laser irradiation can be seen as an attractive tool to identify primarily generated molecules, on a very short time scale, that can be used as relevant chemical markers for the monitoring of the ageing of transformers materials with cellulose.

Properties of Amorphous Transparent Conductive In-Ga-Zn Oxide Films Deposited on Fused Quartz by the PLD Method

Transparent conductive oxide (TCO) films of In-Ga-Zn-O were deposited by sputtering, for applications involving shorter wavelengths than visible light wavelengths. The films were deposited on fused quartz at a substrate temperature of 250° by pulsed laser deposition (PLD) using a pulsed KrF excimer laser (λ=248 nm). The targets were synthesized by mixing In2O3-Ga2O3-6ZnO powders and Ga2O3+3ZnO powders. Weight % of the mixed Ga2O3+3ZnO powders was estimated by x, where x was changed from 0 to 0.2 in order to increase the fraction of Ga atoms in the film. Near the stoichiometric composition of In, Ga and Zn, we obtained low resistivity films and high carrier mobility. All the films deposited were amorphous. The optical bandgap energy was increased with increasing Ga content. With appropriate incorporation of extra Ga in the target, we could deposit stoichiometric In-Ga-Zn-O amorphous films. We obtained the lowest resistivity of 3 × 10-3 Ωcm and a high Hall mobility of 20-25 cm2/Vs. The carrier concentration was 1 × 1020 cm-3. The optical band gap energy was changed from 3.91 to 4.06 eV with increasing x value from 0 to 20 wt%. It was confirmed that the slight deviation from the stoichiometric composition did not influence the electrical properties. Even for a thickness of 50-80 nm, the film properties were the same as those of much thicker films. The present films are very homogeneous and the surface is very flat, due to the fact that the films are amorphous. [DOI: 10.1380/ejssnt.2009.273]

Pulsed laser ablated off-stoichiometric thin films of the Heusler alloy Co 2TiSn on Si (100) substrate

We report the growth of thin films of ferromagnetic Heusler alloy Co 2TiSn on Si (100) substrate using KrF excimer pulsed laser ablation. Films of thicknesses ranging from 8 to 220 nm were deposited on Si (100) substrate heated up to 200 ± 10 °C, with an aim to study the structural, morphological and magnetic properties. The grown films are off-stoichiometric, polycrystalline, having single-phase with high degree of (220) texturing. Angle dependent fluorescence measurements suggest no segregation of alloying elements as a function of depth. X-ray reflectivity measurements indicate that all the films are having low density layer at the top as well as at the film-substrate interface. Magneto optical Kerr effect measurements at room temperature reveal clear hysteresis loops suggesting ferromagnetic behavior of the films. Thermal annealing at temperature ≥ 220 °C suggest transformation of Co 2TiSn phase into cobalt silicide phase, which confirms the necessity of low substrate temperature (< 220 °C) to produce such single-phase Co 2TiSn films.

Altering the catalytic activity of thin metal catalyst films for controlled growth of chemical vapor deposited vertically aligned carbon nanotube arrays

The growth rate and terminal length of vertically aligned carbon nanotube arrays (VANTAs) grown by chemical vapor deposition have been dramatically improved through pulsed KrF excimer laser pretreatments of multilayer metal catalyst films. Silicon wafers coated with Al, Mo, and Fe layers were laser processed in air with single laser shots of varying fluence through circular apertures, then heated to ∼750°C and exposed to acetylene and ferrocene containing gas mixtures typically used to grow vertically aligned nanotube arrays. In situ videography was used to record the growth kinetics of the nanotube arrays in both patterned and unpatterned regions to understand the changes in catalytic activity, growth rates, and termination of growth. The height of the patterned regions varied with fluence, with the most successful treatment resulting in 1.4 cm tall posts of nanotubes embedded in a 0.4 cm tall nanotube carpet. High-resolution transmission electron microscopy images from the nanotubes in the posts revealed fewer walls, smaller diameters, and a much narrower distribution of diameters compared to the nanotubes grown in the carpet. This information, along with data obtained from weighing the material from each region, suggests that pulsed laser processing can also significantly increase the areal density of VANTAs.

Effect of Laser Sources on the Structural Damage Mechanisms and Reverse-bias Leakages of Laser Lift-off GaN-based LEDs

The frequency-tripled Nd YAG laser (355nm) and the KrF pulsed excimer laser (248 nm) were used to separate GaN from sapphire substrates. The laser damage mechanism of these two laser sources and effects on the reverse-bias leakages of GaN-based light-emitting diodes (LEDs) were studied.

Pulsed plasma ion source to create Si nanocrystals in SiO 2 substrates

A pulsed KrF excimer laser of irradiance of about 10 8 W/cm 2 was utilized to synthesize Si nanocrystals on SiO 2/Si substrates. The results were compared with that ones obtained by applying low bias voltage to Si(1 0 0) target in order to control the kinetic energy of plasma ions. Glancing incidence X-ray diffraction spectra indicate the presence of silicon crystalline phases, i.e. (1 1 1) and (2 2 0), on SiO 2/Si substrates. The average Si nanocrystal size was estimated to be about 45 nm by using the Debye–Scherrer formula. Scanning electron microscopy and atomic force microscopy images showed the presence of nanoparticles of different size and shape. Their distribution exhibits a maximum concentration at 49 nm and a fraction of 14% at 15 nm.

Time-resolved transmission study of fused silica during laser-induced backside dry etching

Laser-induced backside dry etching (LIBDE) is a promising technique for micro- and nanomachining of transparent materials. Although several experiments have already proved the suitability and effectiveness of the technique, there are several open questions concerning the etching mechanism and the concomitant processes. In this paper time-resolved light transmission investigations of etching process of fused silica are presented. 125 nm thick silver coating was irradiated through the carrying 1 mm thick fused silica plate by single pulses of a nanosecond KrF excimer laser. The applied fluences were 0.38, 0.71 and 1 J/cm 2. During the etching process the irradiated spots were illuminated by an electronically delayed nitrogen laser pumped dye laser. The delay between the pump and probe pulses was varied in the range of 0 ns and 20 μs. It was found that the transmitted probe beam intensity strongly depends on the applied delays and fluences. Scanning electron microscopy and energy dispersive X-ray spectrometry of the etched surface showed the existence of silver droplets and fragments on the illuminated surfaces and silver atoms built into the treated surface layer influencing the transmission behavior of the studied samples.

CRACKING AND EXFOLIATION OF TiO2 FILM IRRADIATED WITH EXCIMER LASER

TiO 2 film deposited on glass was irradiated in air with single-shot KrF excimer laser pulse. The surface roughened as the result of the laser ablation. It is further noted that single-pulse irradiation with fluence ranging from 400 to 1200 mJ/cm2 gave rise to protrusion of the irradiated surface above the original surface, which is in contrast to usual expectation that irradiated surface is below the unirradiated surface. The surface protrusion is mainly attributed to the effect of surface tension. At the laser fluence of 1000 mJ/cm2, cracks were formed in the irradiated area and severe film exfoliation was observed at the periphery of the irradiated area due to the release of internal stress. With higher laser fluence above 1000 mJ/cm2, patches of film were observed to peel off within the irradiated areas. Hydrodynamic ablation is proposed to account for film exfoliation. The observed phenomenon is useful for further understanding how TiO 2 film reacts to strong UV laser irradiation.

Synthesis and control of size and structure of porous carbon films

Nanoporous carbon films are directly synthesized using a pulsed KrF excimer laser ablation deposition technique. Variations of the laser power density, substrate temperature, angle and distance between the substrate and target yield different sizes (0.2–2 μm) of porous structures of carbon films. Optical microscope was used to examine the surfaces of the samples. Raman scattering was used to characterize the samples, and the typical G- and D-bands have been identified. Intensities and profiles of the G- and D-bands strongly depend on the angle of deposition. The relationship between the deposition rate and the power density was also studied.

Carbon nanotube formation by laser direct writing

This letter presents carbon nanotube (CNT) formation by laser direct writing using 248nm KrF excimer pulsed laser in air at room temperature, which was applied to irradiate amorphous carbon (a-C) assisted by Ni catalysts underneath for the transformation of carbon species into CNTs. The CNTs were synthesized under appropriate combination of laser energy density and a-C thickness. The growth mechanism and key parameters to determine the success of CNT formation were also discussed. The demonstration of the CNT growth by laser direct writing in air at room temperature opens an opportunity of in-position CNT formation at low temperatures.

Preparation of TiO2 thin films by laser ablation for photocatalytic applications

Thin titanium dioxide (TiO2) films were deposited by pulsed KrF excimer laser ablation deposition system using a titanium oxide target. The effects of substrate temperature and oxygen partial pressure on the phase formations of various microstructures were investigated by x-ray diffraction (XRD), scanning electron microscopy (SEM), Raman scattering spectroscopy, and UV-Vis spectrophotometer. The film structures range from amorphous to mixed phase of anatase and rutile to pure anatase. Increasing the substrate temperature improves the quality of anatase phase of crystalline TiO2 films. The oxygen pressure in the range between 15 and 60 mTorr shows a single anatase phase in TiO2 films while the rutile and anatase mixed phases were observed elsewhere. The band gap of TiO2 films varied from 2.72 to 3.27 eV with increasing oxygen partial pressure while the film structures changed from rutile phase to anatase phase. The surface area of TiO2 films increased as oxygen partial pressure in film deposition. The photocatalytic performance evaluated by degradation of methylene blue in UV light was distinguishable in the TiO2 films with high anatase phase and surface area.

Effect of KrF excimer laser irradiation on the properties of ZnO thin films

At room temperature, the effect of KrF pulsed excimer laser irradiation on the structural, photoluminescence, and electrical properties and on the surface morphology of ZnO thin films under different laser energy densities was investigated. Compared to the as grown sample, at an irradiation energy density of about 450 mJ/cm2, the ZnO thin film exhibits a series of desirable properties: UV emission is distinctly higher, resistivity is decreased by three orders of magnitude, and the surface is flat and smooth and, at the same time, it maintains a good epitaxial orientation and a wurtzite crystal lattice structure. UV emission enhancement after laser irradiation is discussed in detail. It is suggested that the mechanism responsible for the UV enhancement is the increase in donors and not the improvement in the crystalline quality.

Effects of Temperature on Microstructure and Tribological Performance of a-CN&lt;sub&gt;x&lt;/sub&gt; Films Prepared by Pulsed Laser Deposition

a-CNx films were deposited onto silicon wafers at temperatures from RT up to 600 °C by using pulsed KrF excimer laser deposition. The composition, morphology and microstructure of the CNx films were characterized by X-ray photoelectron spectrum (XPS), scanning electron microscopy (SEM) and Raman spectrum. The tribological performance of the films was investigated using a ball-on-disk tribometer. With increasing the deposition temperature ranging from RT to 400 °C, the N content of films dropped from 36 at.% to 22 at.%, the ratio of N-sp3 C bonds, hardness and friction coefficient of the film decreased. Further increase of deposition temperature led to the lack of nitrogen and the increasing degree of order in ringed sp2 C=C bonds of the amorphous carbon film. The mechanical and tribological performances became worse. The film deposited at 300°C showed a low friction coefficient of 0.11 and a preferable wear resistance of 1.65×10–7 mm3 Nm–1 in humid air.

Study of soft X-ray induced by plasma laser

X-ray radiation is generated by laser-induced plasma also at very low laser fluence. In this work, the diagnostic of a pulsed X-ray source utilizing as detector a very sensitive fast polarized Faraday cup (FC) is described. The plasma was generated by a pulsed KrF excimer laser focused on a pure copper target. The X-ray energy was studied at 30 and 102 mJ laser energy focused by a convergent lens of 15 cm focal length which provided an intensity of 1.3×108 and 4.5×108 Wcm−2, respectively. The maximum X-ray energy was approximately 1 keV for the both laser energies, whereas the energy peak value was of about 140 eV for the lower laser energy and about 160 eV for the higher. This behaviour was compared with the calculation of plasma temperature performed by the FC at long time. The plasma temperature was determined by the best fit of the modified Maxwell–Boltzmann equation which resulted of about 600 kK (57 eV) and 970 kK (84 eV) for the lower and higher applied laser energy. Such a source is useful for studying on living cells and on sequencing of DNA.

Proton conducting BaZr 0.8Y 0.2O 3−x thin films by pulsed laser deposition technique

Here we report the synthesis of high-quality single-phase thin films of BaZr 0.8Y 0.2O 3− x (BZY20) by pulsed laser deposition (PLD) technique. BZY20 films were deposited on various substrates including single-crystal sapphire (0 0 0 1), LaAlO 3 (0 0 1), LaAlO 3/LaNiO 3 (0 0 1), and polycrystalline Cu and Fe foils. Depositions were performed using a high-power pulsed KrF excimer laser (wavelength 248 nm, 25 ns pulse duration) under controlled oxygen ambience. Substrate temperature and laser energy density during the depositions were maintained in the range 500–600 °C and 2–3 J/cm 2, respectively. The microstructural, electrical, and gas-sensing properties of BZY20 films were studied, with the goal of understanding growth characteristics and optimizing their sensory response. X-ray diffraction (XRD) and orientation imaging microscopy (OIM) results showed that for BZY20 films grown on a sapphire (0 0 0 1) substrate, films tend to orient in the 〈1 1 1〉 and 〈2 1 1〉 directions, while for those grown on LaAlO 3 (0 0 1) and LaAlO 3/LaNiO 3(0 0 1) substrates, films tend to orient preferably in the 〈2 1 1〉 direction. Possible atomic positions of the constituent atoms at the film/substrate interface were determined to understand the growth characteristics and to estimate the epitaxial strains at the film–substrate interface. Typical conductivity of BZY20 thin films grown on single-crystal substrates was ∼1×10 −3 and ∼10×10 −3 S/cm at 500 and 800 °C, respectively. BZY20 films grown on single-crystal substrates were found to be very sensitive to H 2O vapor pressures with film resistance changing by >18% at temperatures of 400 °C when water vapor partial pressure changed from 2 to 50 kPa.