Excimer Laser Wavelength Research Articles

Studies of carbon nitride thin films synthesized by KrF excimer laser ablation of graphite in a nitrogen atmosphere

Carbon nitride thin films were deposited on silicon wafers by pulsed KrF excimer laser (wavelength 248 nm, duration 23 ns) ablation of graphite in a nitrogen atmosphere. Different excimer laser fluences and pressures of the nitrogen atmosphere were used in order to achieve a nitrogen content as high as possible in the deposited thin films. Fourier transform infrared and x-ray photoelectron spectroscopies were used to identify the binding structure and the content of the nitrogen species in the deposited thin films. The N/C ratio 0.42 was obtained at an excimer laser fluence of 0.8 J cm−2 at a repetition rate of 10 Hz under a nitrogen pressure of PN=100 mTorr. A high content of C=N double bond instead of C≡N triple band was indicated in the deposited thin films. Ellipsometry was used to analyze the optical properties of the deposited thin films. The carbon nitride thin films have amorphous-semiconductorlike characteristics with an optical band gap Eopt of 0.42 eV.

Ultraviolet laser ablation of polymers: spot size, pulse duration, and plume attenuation effects explained

A versatile model for ultraviolet (UV) laser ablation of polymers is presented, which is very successfully applied to the calculation of a variety of different properties of this process, including the influence of plume attenuation dynamics. The polymer is described as a system of chromophores with two possible electronic states. The model is based on the combination of photothermal decomposition and photodissociative bond breaking in the electronically excited state. Laser induced chemical modifications are incorporated via different absorption coefficients for the initial and for the modified polymer after absorption of UV light. Dynamic attenuation of the expanding ablation plume and heat conduction are taken into account. The results of the theoretical calculations are compared with the results of three different series of experiments performed with polyimide (PI) and polymethylmethacrylate at the excimer laser wavelength 248 nm and with PI also at 308 nm: (1) Measurement of the ablation rate as a function of fluence for four different pulse durations between 20 and 250 ns; (2) Measurements of the ablation rate as a function of fluence for five different laser irradiation spot radii between 10 and 150 μm, and (3) Time resolved measurement of the dynamic plume attenuation at the ablating laser wavelength as a function of fluence for four different pulse durations between 20 and 250 ns. The model leads to a prediction of etch rates, ablation thresholds, plume attenuation, and surface temperatures during the ablation process, which is in good agreement with the experimental results. The observed increase of the ablation rate with increasing pulse length and with decreasing laser spot size can be explained by the model as a consequence of laser induced modified absorption in combination with the dynamic shielding of the expanding plume.

Pulsed laser deposition and modification of cubic boron nitride films

Pulsed laser ablation using an excimer laser of 248-nm wavelength was applied to prepare boron nitride films, where the ablation from elemental boron targets as well as boron nitride was studied. The growing films were bombarded by a nitrogen or a nitrogen/argon ion beam to obtain stoichiometric films and to investigate ion induced modifications of structure and properties. Moreover, in order to study the influence of ultraviolet photon bombardment on the structure of the growing boron nitride films, a certain region of each film was irradiated using an excimer laser beam of 248-nm wavelength. The films have been investigated by in-situ ellipsometry, infrared spectroscopy, transmission electron microscopy including diffraction measurements and electron-energy-loss spectroscopy. The results show that the boron nitride films have either predominantly hexagonal or predominantly cubic structure dependent on the laser and ion beam parameters and the substrate temperature. At the optimum laser and ion beam parameters, nearly pure cubic films could be prepared even at a relatively low substrate temperature of 200°C. However, those films prepared on silicon substrates exhibit the typical layered structure with a 5 to 10-nm thick amorphous layer formed initially on the substrate, a 10 to 30-nm thick layer of highly oriented h-BN and the c-BN layer. The c-BN layer consists of nanocrystallites up to 30 nm in diameter. Additional excimer laser irradiation of the growing films resulted in distinct modifications of the microstructure of the BN films. Using laser fluences above 200 mJ/cm 2, the laser irradiation leads to the formation of turbostratic h-BN even though the unirradiated film regions of the same sample show the cubic structure. The most interesting range of laser fluence lies between 100 and 160 mJ/cm 2. Electron microscopic observations show that in this range, the mean diameter of crystallite in the excimer laser irradiated regions increased by a factor of 2 in comparison with unirradiated regions of the same sample.

Deposition of hydroxyapatite thin films by excimer laser ablation

The influence of the pressure of a sole water atmosphere during the pulsed laser deposition of hydroxyapatite thin films on Ti–6Al–4V substrates has been studied. The rest of the technological parameters involved in the process have been fixed near the conditions where the best crystalline coatings are obtained. The pressure of the water atmosphere has been varied between 0.15 and 1.5 mbar. The films properties have been analysed by means of XRD, SEM, FT-IR spectroscopy and SIMS. An optimal region, in order to obtain thin films of highly crystalline hydroxyapatite, has been found near 0.5 mbar for the two excimer laser wavelengths (193 nm and 248 nm) used in this study. These films have a preferential orientation in the (100) direction.

Microstructuring of Si(100) by light induced dry etching in the VUV

Light-induced dry etching of Si(100) in the VUV range using synchrotron radiation (SR) and a halogen-containing gas (XeF2) has been investigated with respect to selectivity, anisotropy, quantum efficiency, optimal wavelength, spatial resolution and quality of the photochemical etching processes. Microstructuring of Si with XeF2can be optimized to achieve etched structures in the sub-micrometre range by increasing the contrast in choosing a wavelength with minimal unselective etching. The strength of unselective etching is strongly wavelength dependent and follows the XeF2gas phase absorption coefficient. Fragments from dissociation of the XeF2reach the Si surface and thus cause unselective etching. Optimal dry etching occurs for wavelengths around 120 nm because the selectivity is high due to an excitation of a surface layer and also the quantum efficiency is very large. An efficiency of 10 removed Si atoms per incoming photon, which exceeds that in the visible spectral range by more than four orders of magnitude, combined with the higher spatial resolution at 120 nm compared to the conventional excimer laser and I-line wavelengths and the availability of optical materials for imaging present a perspective for generating line densities in the Gbit range.

Structure control of pulsed-laser-deposited Pb0.6Sr0.4TiO3/La0.5Sr0.5CoO3 thin films on various substrates

Structure development is an important issue for the growth of ferroelectric thin film. In the present experiment, Pb0.6Sr0.4TiO3(PSrT)/La0.5Sr0.5CoO3 (LSCO) thin films have been grown on various substrates, such as: single crystal Si with (100) and (111) orientations, Au/Si, Pt/Si and glass, using XeCl excimer laser (wavelength = 308 nm) under various processing conditions. X-ray diffraction results show that highly (100)-oriented LSCO films can be deposited successfully under appropriate processing conditions regardless the type of the substrate. PSrT films can be deposited upon LSCO layer with the same structure arrangements if LSCO can develop desired structures. It appears that appropriate processing parameters are more important than lattice parameter consideration for growing LSCO films using pulsed laser deposition. Microstructure investigations indicate that the substrate with an amorphous surface layer may help the development of the desired structure.

Studies of Carbon Nitride Thin Films Synthesized by KrF Excimer Ablation of Graphite in Nitrogen Atmosphere

AbstractCarbon nitride thin films were deposited on silicon wafers by pulsed KrF excimer laser (wavelength 248 nm, duration 23 ns) ablation of graphite in nitrogen atmosphere. Different fluences of the excimer laser and pressures of the nitrogen atmosphere were used in order to achieve a high nitrogen content in the deposited thin films. Fourier Transform Infra-red (FTIR) and X-ray photoelectron spectroscopy (XPS) were used to identify the binding structure and the content of the nitrogen species in the deposited thin films. The highest N/C ratio 0.42 was achieved at an excimer laser fluence of 0.8 Jcm -2with a repetition rate of 10 Hz under the nitrogen pressure of PN=100 mTorr. A high content of C=N double bond instead of C-N triple bond was indicated in the deposited thin films. Ellipsometry was used to analyze the optical properties of the deposited thin films. The carbon nitride thin films have amorphous-semiconductor-like characteristics with the optical band gap Eop, as high as 0.42 eV.

Novel Method for Controlling Nano Thin Coatings on Particulate Materials

AbstractParticulate 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).

Electron impact and multiphoton ionization and fragmentation of molybdenum-cyclopentadienyl-dicarbonyl-nitrosyl at 351, 248 and 193 nm

Electron impact (EI) and multiphoton ionization (MPI) mass spectra of CpMo(CO) 2NO (Cp  C 5H 5, cyclopentadienyl ligand) are reported. The ionization energy (8.20 eV) and appearance energies of major fragment ions are determined. Ion dissociation energies and formation enthalpies are derived from these data. On EI ionization molecular ions and large fragment ions are formed. A primary loss of CO ligands from molecular ions is favoured over NO or Cp elimination. Mo + ions are generated in low abundance only. On multiphoton excitation at the excimer laser wavelengths 351 and 248 nm and low laser intensities MoO + ions and, with lower abundance, Mo + ions are detected. At increasing laser intensities Mo + ions are becoming predominant. At 193 nm and low laser intensities CpMoNO + and CpMo + ions are formed. At higher laser intensities an increasing yield of smaller ions, particularly Mo +, are found. The number of absorbed photons for the formation of prominent ions is determined and compared with EI appearance energies. The MPI results at 351 and 248 nm are interpreted with a dominant dissociation of the neutral parent compound followed by multiphoton ionization of the fragments. At 193 nm, however, a resonant multiphoton absorption leads to intermediate molecular ionization and subsequent fragmentation.

Investigation into excimer laser radiation damage of deep ultraviolet optical phase masking films

A variety of materials based on various oxides, nitrides, fluorides, and composites have been found to be potentially suitable for use as attenuated phase masking materials for use at excimer laser wavelengths. Presented here are results from the investigation into 193 nm excimer laser radiation of zirconium–nitride-based, aluminum–nitride-based, chrome–fluoride, amorphous-carbon, and tantalum–silicon–nitride (TaN/Si3N4) attenuated phase-shift mask materials at fluence levels mask materials experience during wafer exposure. Spectroscopic photometric and ellipsometric methods were utilized to quantify damage through measurement of transmission and reflection properties and extraction of optical constants. Results show that understoichiometric zirconium– and aluminum–nitride materials are susceptible to radiation-induced modification through oxidation effects while tantalum–silicon–nitride and chrome–fluoride films are able to withstand exposure to several thousand J/cm2 without significant degradation.

Optical loss estimation of coatings at the ArF excimer laser wavelength

A measurement system that estimates the absorption loss of coatings at the wavelength of ArF excimer laser (193 nm) has been developed. Utilizing the photoacoustic waves generated by pulsed laser irradiations, the quantitative measurements with simple procedures became possible. The holder that holds the samples in a liquid led to stable acoustic matching condition and high reproducibility (< 2%) of measurements. Effects of scattering noises could be reduced by selecting suited materials for the holder and setting an appropriate time gate for the signal processing. The attained sensitivity was 0.001% in the condition of a low irradiation power (< 3 mJ).

Fast wavelength switching of narrow-band excimer lasers

A novel system was developed, which allows one to switch the wavelength of a narrow-band excimer laser between two successive light pulses at a repetition rate of at least 250 Hz. This is realized by a periodically driven piezo actuator, which is attached to the diffraction grating of the narrow-band KrF excimer laser. The achieved position accuracy of the grating leads to a wavelength reproducibility of ±0.2 pm, which allows one to apply this system to laser spectroscopic investigations like LIF or LIPF of OH in flames. Using the fast wavelength switching system background reduced concentration and temperature fields in flames can be measured within one sequence. Some possible realized and planned applications like the measurement of gas temperature, the diagnostic of turbulent combustion processes, and the investigation of combustion processes under microgravity are discussed.

Transient recrystallization of amorphous silicon films

We have investigated crystallization of amorphous silicon films on SiO 2/Si substrates using conventional furnace annealing, incoherent light-based rapid thermal annealing (RTA), and pulsed excimer laser (wavelength = 248 nm, energy density = 0.1–0.6 J cm −2). The effect of a 50 nm SiO 2 capping layer on the final microstructure was examined for each annealing technique, using transmission electron microscopy (TEM) and X-ray diffraction (XRD). A discussion of the phenomenological effects of the capping layer on resultant microstructure is included. For laser annealing, the effects of varying the thickness of the intermediate SiO 2 layer were examined and the resultant microstructure was characterized for grain size and crystallized orientation using TEM. Grain sizes ranging from 50 to 200 nm were observed. Electrical properties of the crystallized films were presented for the various resultant microstructures. The effect of laser interactions with multi-layered structures were simulated by solving the heat condition equation with the appropriate boundary conditions and the results are included.

Mutagenic effects of the excimer laser using a fibroblast transformation assay

For over 10 years fiber-guided laser systems have been used in arthroscopic surgery. The possibility of a mutagenic risk by the use of lasers has not been sufficiently repudiated, especially for the Excimer laser at a wavelength of 308 nm, which is close to the absorption spectrum of DNA. The aim of this study therefore was to approximate the mutagenic risk of UV-laser wavelengths using an in vitro transformation assay. BALB/3T3 fibroblasts were irradiated with Excimer laser wavelengths of 248 nm and 308 nm. The resulting transformation rates were compared to those of untreated cells and to cells that were subjected to an x-ray dose of 1 Gy. Pulse energy, frequency, and irradiation time were varied over a wide range, from sublethal fluences to total cell death. Furthermore, the effect of repeated irradiation (split-dose) was analyzed. The results for the 248 nm irradiation showed a highly significant transformation rate ( P < 10 −9). In contrast, results for the 308 nm irradiation were not at all significant ( P < .18). The transformation rate of cells that were treated with x-rays was increased exponentially compared with the controls. Although an increased transformation rate was found after 248 nm irradiation, that of the wavelength 308 nm was not enhanced. The study therefore shows that the use of the Excimer wavelength 308 nm in arthroscopic procedures does not imply a specific mutagenic risk.

Excimer laser cleaning of mold-contaminated paper: Sterilization and air quality considerations

The Excimer laser can be used to remove biological debris from underlying substrates. The ability of the krypton fluoride (KrF) Excimer laser to removeAspergillus niger mold, which had been grown on filter paper, was evaluated in the present study. The air in the immediate vicinity of the laser cleaning apparatus was sampled to determine if viable mold fragments were released into the air during treatment. It was found that filter paper could be sterilized with the Excimer laser wavelength with the proper selection of parameters. During this process, viable spores and mold fragments are released into the atmosphere, and pose a relative risk unless proper measures to prevent contamination of the work area are taken.

Novel method for synthesis of engineered particulates with thin coatings

ABSTRACTParticulate coatings have wide ranging applications in several new technologies such as flatpanel 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 (size 1–800μm). The pressure and nature of the background gas (inert or active) controls the cluster size of the particles in the laser plume. Experiments were conducted for laser deposition of silver particles on alumina core particles by excimer laser (wavelength = 248 nm and pulse duration = 25 nanosecond) irradiation of silver targets. The surface coverage and the coating of the film wasf found to depend on the laser parameters (energy density and number of laser pulses) as well as the residence time of the core particles in the plume regime. The films were characterized by wavelength and energy dispersive x-ray spectroscopy and x-ray photoelectron spectroscopy techniques.

Excimer laser ablation of aluminum nitride

Excimer laser wavelengths ablate aluminum nitride at rates up to 0.2 μm/pulse where the rate increases with decreasing background pressure and increasing fluence. The ablation threshold for AlN at 248 nm is approximately 2 J/cm2. Blind vias are produced with flat bottoms, straight walls, and a decomposed metallic layer remaining on the surface. Ablation rate dependence on fluence saturates at high fluences due to absorption by the ablation plume. The influence of processing variables on ablation rate and ablation mechanisms are discussed. Statistical design of experiments is used to compare data sets.

Luminescent oxide thin films grown by pulsed laser deposition

Abstract— Transparent photoluminescent ZnGa2O4 and Tb‐doped Y3Al5O12 (YAG:Tb) thin films were deposited on sapphire substrates at 300°C by laser ablation. The targets were made from ZnGa2O4 and YAG:Tb powders prepared with a novel ceramic processing technique (combustion synthesis). The deposition of the films was performed in ultrahigh vacuum with a KrF excimer laser (wavelength = 248 nm) with a fluence of 3.6 J/cm2 and a repetition rate of 30 Hz. X‐ray diffraction and Auger measurements taken on as‐deposited films indicated that the films were amorphous and showed a reduction in oxygen when compared to the bulk targets, and were not photoluminescent. Post‐annealing treatments in air for at least 1 hour in the range 800–1200°C induced crystallinity and recovered blue photoluminescence on ZnGa2O4 and green photoluminescence on YAG:Tb thin films, respectively.

High efficiency in dry etching of Si for wavelengths around 120 nm

Microstructuring of Si with XeF2 can be optimized by increasing the contrast in choosing a wavelength with minimal nonselective etching. The efficiency of selective etching with optimal quality can be increased by factors of 100 and 500 by using wavelengths around 120 and 110 nm, respectively, in comparison to longer wavelengths around 200 nm. The high efficiency of typically 10 removed atoms per photon, the availability of optical materials for imaging and the potentially high spatial resolution at 120 nm compared to the conventional excimer laser and I-line wavelengths present a perspective for generating line densities required in the Gbit range.

Development of a technique for quantifying in-cylinder A/F ratio distribution using LIF image processing

Abstract Mixture formation in the cylinder greatly influences combustion. In this study, a system was developed for analyzing mixture formation by using LIF to visualize the formation process and image processing to quantify the air/fuel ratio distribution. Iso-octane was used as the fuel, DMA as the fluorescent tracer and a KrF excimer laser (wavelength of 248 nm) as the excitation light source. This system was used to visualize mixture formation in an engine with a tumble-type swirl control valve, and the effect of varying the fuel injection timing on mixture behavior and the A/F ratio distribution was examined.