Pulsed KrF Excimer Laser Research Articles

KrF Excimer Laser-induced Ozone Formation in Supercritical Carbon Dioxide

Laser-induced reactions by a pulsed KrF excimer laser were studied using UV absorption spectroscopy in sub- and supercritical O2/CO2 mixtures up to the pressure of 15 MPa (corresponding density, 17 mol dm-3). Although the 248 nm excimer laser photon energy is smaller than the energy required for dissociating O2, ozone formation was observed in O2/CO2 mixtures. Under the laser irradiation, O3 concentration increased monotonically with the increase of the irradiation time and then stayed constant, which is satisfactorily expressed by the equation d[O3]/dt = a − b[O3]. a corresponds to O3 formation rate and b to O3 decomposition rate constant. The value of a increased with the increase of CO2 density up to 3 mol dm-3 and was then kept almost constant with further increase. O2 absorbs a photon to yield an oxygen molecule in the Herzberg III state O2(A‘ 3Δu), being augmented along with the increase of CO2 density. In pure O2, the predominant pathway of O3 formation is the reaction between excited O2 in Herzberg states and ground state O2 to yield O3 and atomic oxygen. In high-density O2/CO2 mixtures, O3 is considered to be produced through reaction between the Herzberg states O2 and CO2. Taking account of the quenching effect for the above reaction together with the augmentation of O2 absorption of laser light by the high-density CO2, the behavior of a with respect to CO2 density was satisfactorily explained. The behavior of b suggested a certain inhibition of O3 recovery in high-density CO2 after the photodecomposition of the product O3, which was ascribed to the formation of CO3 from the O(1D) reaction with CO2. A certain cage effect for the O3 photodecomposition was also suggested. No specific pressure effect was observed near the critical point.

Cubic boron nitride deposition on silicon substrates at room temperature by KrF excimer laser ablation of h-BN

A hexagonal-BN target was ablated by high-fluence (6 and 12 J/cm2) KrF excimer laser pulses in low-pressure (5 Pa) N2 atmosphere, without any aid of ion bombardment and heating of the Si substrate. Investigations of the deposited films show that the cubic-BN phase was deposited. The film deposited at 6 J/cm2 appears to contain a higher cubic phase content with respect to the one deposited at 12 J/cm2.

Ablation study on pulsed KrF laser annealed electroluminescent ZnS:Mn/Y 2O 3 multilayers deposited on Si

The ZnS:Mn active layer of a thin film electroluminescent (TFEL) device has been annealed under 1.034 MPa (10.34 bar, 150 psi) of argon pressure using a 20-ns pulsed KrF excimer laser. We investigate the effects of multiple shots at various power densities upon the ablation rates of the ZnS:Mn layer. The results are compared to a thermal simulation of the laser-matter interaction using single pulse irradiation, and it is inferred that the cubic to hexagonal transition and melting of ZnS:Mn decrease the ablation rate.

The effect of target temperature on the deterioration of metal surfaces under pulsed laser irradiation

The effects of pulsed laser irradiation on preheated solid metal surfaces have been studied. The surface of low-melting point metals (In and Bi) is irradiated by KrF excimer laser pulses of fluences below 400 mJ/cm 2. Evaluation of the optical microscopic pictures of the processed metal surfaces reveals that the characteristic lateral dimension of laser-induced surface structures increases as the melting point is approached. The results are explained in terms of the lifetime of the melt pool, obtained by numerical temperature calculations. Close to the melting point, the increased lifetime of the molten phase allows ample time for smoothing of the surface before its resolidification.

Optical Properties Of Tantalum Oxide Films Deposited On BK7 Substrates By Excimer Laser Ablation

AbstractThin amorphous films of tantalum oxide were grown on borosilicate crown glass substrates by KrF excimer pulsed laser ablation of a Ta2O5 target, in an oxygen environment. The deposition was performed at a temperature of 250 or 400 °C, while the oxygen pressure was set in the range 5 to 30 mTorr. The optical properties of the tantalum oxide coatings, as evaluated by reflectance/transmittance spectrophotometry, were found to be dependent on the oxygen gas pressure. At a pressure of 5 mTorr, absorbing films were obtained, with extinction coefficients above 10−2 (at λ=633 nm), along with an optical energy band-gap as low as 0.7 eV. At a pressure of 10 mTorr and above, the coatings had refractive indices up to 2.25 (at λ=633 nm), extinction coefficients below 10−4 (for λ>390 nm), and an optical energy band-gap in the range 3.9 to 4.0 eV.

Properties of the magnetoresistive La0.8Sr0.2MnO3 film and integration with PbZr0.52 Ti0.48O3 ferroelectrics

AbstractThe colossal magnetoresistive La0.8Sr0.2MnO3 (LSMO) thin film was prepared on the MgO (100) single crystal substrate using KrF excimer pulsed laser deposition technique. The LSMO film deposited at the substrate temperature of 850 °C, oxygen pressure of 500 mTorr and laser energy density of 2 J/cm2(5 Hz) showed the resistivity peak temperature (Tp) of 330 K and the magnetoresi stance change of 15 %(H=0.7 T) at the room temperature. The large lattice mismatch with the substrate increased Tp and decreased the resistivity of the LSMO film.The X-ray diffraction measurement for the PbZr0.52Ti0.48O3 (PZT) / LSMO heterostructures indicated both c-axis and in- plane orientation, with the good PZT surface morphology.

Laser Induced Fluorescence Measurement Of Plasma Plume During Pulsed Laser Deposition Of Diamond-Like Carbon

AbstractDynamics of carbon ablation plasma plume during the preparation of diamond-like carbon films by KrF excimer pulsed laser deposition was investigated using laser induced fluorescence (LIF) and optical emission spectroscopy. LIF signal from C2molecule (Swan band, d 3Φg – a3Φu) was detected using a photomultiplier tube and an intensified CCD camera. Temporal evolution and spatial distribution of C2 molecules in the ablated plume were measured as a function of laser energy density and ablation area. LIF intensity is found to be weaker in the central part of the plume than that at the periphery at incident energy greater than 6 J/cm2. It is conjectured that some of C2molecules are dissociated by collision with energetic species in central part of the ablation plume. Dynamics of ablation plasma plume is strongly dependent on the size of ablated area.

Nanocrystalline growth and diagnostics of TiC and TiB 2 hard coatings by pulsed laser deposition

Nanocrystalline coatings of TiC and TiB2 were grown by pulsed laser deposition on Si(100) and on X155 steel at low substrate temperatures ranging from 40 °C to 650 °C. A pulsed KrF excimer laser was used with the deposition chamber at a base pressure of 10-6 mbar. The morphology and structure of the films, studied with SEM, XRD, and TEM, showed that nanocrystalline films with a fine morphology of TiC and TiB2 were deposited with a grain size of 10 nm–70 nm at all substrate temperatures. The growth of the polycrystalline coatings possessed a columnar morphology with a preferred orientation. The hardness of the coatings was determined to be 40 GPa and the elastic modulus, 240 GPa. The composition and the kinetics of the plume produced during the pulsed laser deposition of TiC and TiB2 was studied under film growth conditions. The mass analysis of ions of the ejected material was performed by time-of-flight mass spectroscopy (TOF-MS) and showed the presence of Ti+ and C+ during TiC ablation and B+, B2+, and Ti+ during TiB2 ablation. The kinetic energies (KE) of the ions depended on the laser fluence which was between 0.5 eV and 340 eV. The kinetic energy and the evolution of the plasma was studied with a streak camera. The velocity of the plasma was of the order of 106 cm/sec and was linearly dependent on the energy fluence of the laser. The emission spectroscopy of the plasma plume confirmed the atomic neutral and single excited species of Ti. These results show that coating growth basically occurs by the recombination of the ionic species at the surface of the substrate.

Integration of GaN thin films with dissimilar substrate materials by Pd-In metal bonding and laser lift-off

Gallium nitride (GaN) thin films grown on sapphire substrates were successfully bonded and transferred onto GaAs, Si, and polymer “receptor” substrates using a low-temperature Pd-In bond followed by a laser lift-off (LLO) process to remove the sapphire growth substrate. The GaN/sapphire structures were joined to the receptor substrate by pressure bonding a Pd-In bilayer coated GaN surface onto a Pd coated receptor substrate at a temperature of 200°C. X-ray diffraction showed that the intermetallic compound PdIn3 had formed during the bonding process. LLO, using a single 600 mJ/cm2, 38 ns KrF (248 nm) excimer laser pulse directed through the transparent sapphire substrate, followed by a low-temperature heat treatment, completed the transfer of the GaN onto the “receptor” substrate. Cross-sectional scanning electron microscopy and x-ray rocking curves showed that the film quality did not degrade significantly during the bonding and LLO process.

Pulsed laser deposition of photosensitive a-Si thin films

Si films have been fabricated by pulsed KrF excimer laser deposition (PLD) through the use of various Si targets and deposition conditions. The deposits consisted of droplets and homogeneous films, which were assigned to be crystalline and amorphous silicon, respectively, by the micro Raman scattering measurements. Not only the crystalline but also the amorphous Si part scarcely (<1 atomic %) contained hydrogen regardless of whether or not the films are prepared in the presence of H2 gas. Conditions were explored to reduce the droplet formation and to produce photosensitive films. Amorphous Si films with photosensitivity (σph/σd) exceeding 103 were obtained, and they exhibited high stability against light soaking. Thus, PLD is a promising method to fabricate photosensitive and photostable a-Si films.

Determining the properties of pulsed laser deposited thin films by controlling the kinetic energy of the film-forming particles

The deposition of Al2O3 thin films by pulsed KrF excimer laser radiation (248 nm) on fused silica substrates is investigated as a function of the processing variables: laser fluence, processing gas pressure and target-to-substrate distance. The kinetic energy of the Al species in the laser-generated plasma, as measured by time-of-flight optical emission spectroscopy and time-of-flight quadrupole mass spectrometry, is described as a function of the type and pressure of the processing gas, the distance from target, and the laser fluence. The influence of the kinetic energy of the film-forming particles on the density and the refractive index of the resulting films, determined by ellipsometry, is investigated. The densification of the Al2O3 thin films to 94% of the bulk value is achieved by film-forming Al particles impinging on the growing surface with mean kinetic energies of about 25 eV.

Pulsed laser deposition of metals at target temperatures close to the melting point

Tin and bismuth metals are irradiated by KrF excimer laser pulses of fluences around 5 J/cm2. Supressing the formation of surface inhomogeneities on the target is achieved by increasing the target temperature to close to its melting point. The characteristics of surface structures developed on targets ablated at room temperature and heated to approximately 40 °C below the melting point of the respective metal, are presented. Optical microscopy is used to follow the changes in the surface morphology of the deposited films, and especially in the surface number density of particulates. Approaching the melting point of the tin target resulted in a threefold decrease in surface number density of particulates, while for bismuth only a slight decrease was obtained. In the latter case, the anisotropic growth properties of bismuth precluded the effective smoothing of certain domains on the target surface, even at temperatures close to its melting point.

Formation of Si nanoclusters in amorphous siliconthin films by excimer laser annealing

It is shown that an Si nanocluster is formed in an amorphous silicon (a-Si) thin film following irradiation using a pulsed KrF excimer laser. The photoluminescence spectrum of the irradiated 70 nm thick a-Si film at a power density of 180 mJ/cm2 at one shot shows two luminescence bands centred at ~1.31 and 1.76 eV. The peak emission wavelength depends on the silicon nanocluster size, which is ~3 – 4 nm. A mechanism for the formation of Si nanoclusters is also proposed.

Influence of ion-beam energy and substrate temperature on the synthesis of carbon nitride thin films by nitrogen-ion-assisted pulsed laser deposition

Carbon nitride thin films were deposited on silicon wafers by pulsed KrF excimer laser (wavelength 248 nm, duration 23 ns) ablation of graphite with assistance of nitrogen ion beam bombardment. X-ray photoelectron spectroscopy, Raman spectroscopy, and ellipsometry were used to identify the binding structure, nitrogen content, and optical properties of the deposited thin films. The influence of the nitrogen ion beam energy on the compositional, electronic, and optical properties of the deposited thin films was investigated. The thin films deposited with nitrogen ion bombardment had an N/C ratio of 0.43. Raman spectroscopy measurements indicated the formation of CN triple bonds in the deposited thin films. The optical band gap Eopt was observed to decrease with the nitrogen ion energy. A nitrogen ion energy between 50 and 100 eV was deduced to be the optimal condition for depositing the carbon nitride thin films.

Optical emission study of ablation plasma plume in the preparation of diamond-like carbon films by KrF excimer laser

Optical emission study of the laser ablation plasma plume during the preparation of diamond-like carbon (DLC) films using KrF excimer (248 nm) pulsed laser deposition (PLD) has been carried out by means of a monochromator equipped with an intensified optical multichannel analyzer. In high vacuum (1×10−7 Torr), the emission lines from carbon ions of C+, C2+, and C3+ are observed in addition to atomic carbon emission lines, while no emission from the diatomic carbon molecule (C2) is observed. With increasing background nitrogen pressure up to 500 mTorr, the emission intensities of the C2 Swan band and the carbon nitride (CN) violet band increase. The diamond-like character of deposited DLC film degrades with background nitrogen pressure. The vibrational temperature of C2 and CN molecules decreases with the increasing of nitrogen pressure. The CN vibrational temperature for the first 2 μs after the laser pulse is very high and in agreement with the kinetic energy of monatomic carbon ions. The C2 vibrational temperature is as low as 0.6 eV and is consistent with the electron temperature of about 0.8–3.0 eV. It is conjectured that CN molecules are formed directly in reactions involving energetic ionic monatomic carbon, and that the formation of excited C2 molecules is the result of molecular recombinations of C atoms and ions. From the emission intensity measurements and the estimation of the vibrational temperature, it is suggested that the C2 molecule in the ablated plasma plume is not important, but energetic species, such as C+, are very important for producing high quality DLC films using PLD.

Structural and optical quality of GaN/metal/Si heterostructures fabricated by excimer laser lift-off

Gallium nitride (GaN) thin films grown on sapphire substrates were successfully bonded and transferred onto Si substrates using a Pd–In metallic bond. After bonding, a single 600 mJ/cm2, 38 ns KrF (248 nm) excimer laser pulse was directed through the transparent sapphire followed by a low-temperature heat treatment to remove the substrate. Channeling-Rutherford backscattering measurements revealed the thickness of the defective interfacial region to be approximately 350 nm. The full width at half maximum, low-temperature (4 K), donor-bound exciton photoluminescence (PL) peak was larger by 25% on the exposed interfacial layer compared to the original GaN surface. Ion milling of the exposed interface to a depth of 400 nm was found to remove the interfacial layer and associated defects. The minimum channeling yield and PL linewidths from the exposed interface were found to be comparable to those obtained from the original GaN surface after ion milling.

Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off

Indium–gallium nitride (InGaN) multiple-quantum-well (MQW) light-emitting diode (LED) membranes, prefabricated on sapphire growth substrates, were created using pulsed-excimer laser processing. The thin-film InGaN MQW LED structures, grown on sapphire substrates, were first bonded onto a Si support substrate with an ethyl cyanoacrylate-based adhesive. A single 600 mJ/cm2, 38 ns KrF (248 nm) excimer laser pulse was directed through the transparent sapphire, followed by a low-temperature heat treatment to remove the substrate. Free-standing InGaN LED membranes were then fabricated by immersing the InGaN LED/adhesive/Si structure in acetone to release the device from the supporting Si substrate. The current–voltage characteristics and room-temperature emission spectrum of the LEDs before and after laser lift-off were unchanged.

Pair generation of Ge electron centers and self-trapped hole centers inGeO2−SiO2glasses by KrF excimer-laser irradiation

Formation processes of paramagnetic centers in ${\mathrm{GeO}}_{2}{\ensuremath{-}\mathrm{S}\mathrm{i}\mathrm{O}}_{2}$ glasses by irradiation with KrF excimer-laser pulses (5 eV) were investigated at 77 K using electron-spin resonance (ESR). The essential photochemical reaction was the pair generation of electron-trapped centers associated with fourfold coordinated Ge ion (GEC) and self-trapped hole centers of bridging oxygen (STH). This reaction proceeds by band-to-band excitation via two-photon absorption process. A part of GEC was converted to $\mathrm{Ge}{E}^{\ensuremath{'}}$ center during prolonged irradiation at 77 K. The difference between ESR spectra before and after annealing at room temperature revealed that the majority of STH disappears by recombination with GEC (or $\mathrm{Ge}{E}^{\ensuremath{'}})$ during annealing at room temperature. The dominant paramagnetic centers remaining after annealing were GEC and $\mathrm{Ge}{E}^{\ensuremath{'}},$ and their total concentration was approximately 50% of those before annealing. The intimate relation between the generation of paramagnetic centers and the decay of emission due to ${\mathrm{Ge}}^{2+}$ suggested that the hole transfer from STH to ${\mathrm{Ge}}^{2+}$ is followed by their structural relaxation.

Excimer laser crystallization of amorphous indium–tin oxide thin films and application to fabrication of Bragg gratings

Amorphous (a-) indium–tin oxide (ITO) thin films were crystallized at ambient temperature by irradiation with KrF (248 nm) or ArF (193 nm) excimer laser pulses of ≥40 mJ/cm2 per pulse. Electrical resistivity of the specimen at ∼300 K decreased from 5.9×10−4 Ω cm to 2.7×10−4 Ω cm upon laser crystallization. This decrease was due to the increase in the carrier concentration which arises from the occupation of In3+ sites in the lattice with Sn4+ upon crystallization. Photo-imprinted Bragg gratings of crystalline ITO thin films were successfully fabricated by irradiation of a-ITO thin films with the excimer laser pulses through a silica phase mask and subsequent chemical etching of the irradiated thin films utilizing the large differences in the etching rate between crystallized and amorphous ITOs.

Germanium dioxide whiskers synthesized by laser ablation

We obtained germanium dioxide (GeO2) whiskers in bulk quantity by ablating a germanium target at 820 °C with a pulsed KrF excimer laser in an argon atmosphere. Most of the GeO2 whiskers were smooth and straight with hexagonal or triangular, or quadrilateral cross sections while some of them had a bamboo-shoot-shaped form. Results of scanning electron microscopy, transmission electron microscopy, and x-ray diffraction showed that the whiskers are hexagonal crystalline GeO2.