Laser Pulse Energy Density Research Articles

Characterization of a-Si:H resists for a vacuum-compatible photolithography process

We report the characterization of amorphous hydrogenated silicon (a-Si:H) films for use as a dry photoresist, in a vacuum-compatible photolithography process. a-Si:H films are deposited, patterned via excimer laser exposure, and developed in a hydrogen plasma. Experiments on the effects of exposure environment and laser pulse energy density were carried out to determine the optimum conditions for the technique. Transmission electron microscopy (TEM) shows the excimer-exposed a-Si:H surfaces to be polycrystalline in nature. The polycrystalline surfaces serve very well as an etching mask during plasma development, exhibiting etch selectivities over 600:1. Preliminary methods for reducing undesirable surface roughness at the exposed surfaces are also discussed.

Compaction versus expansion behavior related to the OH-content of synthetic fused silica under prolonged UV-laser irradiation

The behavior of different types of synthetic fused silica under prolonged 193 nm excimer laser irradiation at very low energy densities was investigated. Irradiation experiments at energy densities below 50 μJ/cm 2 for tens of billions of laser pulses were performed. Effects of changes in the density in the irradiated region of the samples were measured at 633 nm, both with an interferometer as an optical path difference and as the distribution of stress induced birefringence. Samples of synthetic fused silica of type III and IIIa with different levels of hydroxyl-content and different levels of molecular hydrogen content were manufactured. Depending on the composition of the fused silica samples, compaction as well as rarefaction was observed. The OH-content in the sample is found to be the dominating factor in the expansion-versus-compaction behavior. A qualitative model is postulated in order to describe the dependence on the OH-content. The induced optical path difference as a function of energy density and number of laser pulses is described with a phenomenological model function.

Surface state of CdTe crystals irradiated by KrF excimer laser pulses near the melting threshold

We have studied pulsed-laser-induced melting and following crystallization of the surface of CdTe crystals subjected to irradiation with nanosecond KrF ( λ=248 nm) excimer laser pulses. The surface state, morphology and reconstruction of the surface layer of (1 1 1) B oriented CdTe wafers were investigated by measurements of time-resolved optical reflectivity, atomic force microscopy and reflection of high-energy electron diffraction. It was established that a jump in the reflection coefficient of a probe He–Ne beam, increase in the surface smoothness and change in the crystallinity of the surface layer occurred at the certain threshold value of laser pulse energy density W m≈50 mJ/cm 2. The peculiarities of the laser-induced melting and transformation of the structure of CdTe surface as dependence of KrF excimer laser energy density were discussed.

Hydrodynamic effects on the molten surface of a laser-irradiated aluminum sample

The role of excimer laser pulse energy density was investigated in connection to the turbulence of the molten bath induced by laser irradiation. Gentle hydrodynamic effects are observed on the surface of a laser-irradiated aluminum surface, when the laser energy density substantially exceeds the melting threshold. Chaotic fluid motion inside the molten bath is observed above a laser energy threshold value which, in the present case of KrF excimer laser irradiating aluminum, is between 7.5 and 8.5 J/cm 2. Liquid wave motion, which results in typical wavelike frozen structure, can be induced in a reproducible way thus permitting a controlled tailoring of the surface morphology.

Magnetic field assisted increase of growth rate and reduction of particulate incorporation in pulsed laser deposited boron nitride films

The deflection of the ions ablated from an h-BN target in inhomogeneous magnetic fields was investigated, aimed finally at the reduction of the number of particulates in pulsed laser deposited c-BN films. As a large fraction of the ablated species are ions at the high laser pulse energy densities used, the film growth rates can be significantly increased on substrates that are mounted above and sideways of the ablation area by using appropriate magnetic fields. Consequently, fewer laser pulses are needed for the deposition of films with the same thickness and the area density of particulate can be decreased.

Laser-induced changes in the photoelectric properties of Mn xHg 1−xTe crystals

The application of pulsed laser irradiation to photoconductive n-Mn x Hg 1− x Te crystals was investigated as a possible procedure for modifying their photoelectric characteristics and increasing the photosensitivity. The photoconductivity spectra and the temperature dependences of the excess carrier lifetime of Mn x Hg 1− x Te crystals, subjected to laser irradiation with nanosecond ruby laser pulses, were studied. It was found that a range of energy density of laser pulses can be chosen to decrease the surface recombination rate and to increase the photosensitivity of the crystals. Changes in the photoelectric properties of the irradiated samples are attributed to the laser-stimulated cleaning of the surface as well as to modification of the defect structure of Mn x Hg 1− x Te crystals.

The influence of coatings on subsurface mechanical properties of laser peened 2011-T3 aluminum

High power Q-switched laser systems are currently being developed for use in a process known as laser shock processing or “laser peening” which results in significantly improved fatigue properties in aluminum components. An ablative, sacrificial coating such as paint or metal foil is used to protect the aluminum component from surface melting by the laser pulse, which adversely affects fatigue life. This paper, using nano-indentation, analyzes the effect of the paint and foil coatings on the shock wave propagation into the aluminum specimen and the resulting change in mechanical properties versus depth. Near the surface, hardness was found to be increased by the laser peening, however this process decreased the measured elastic modulus. The laser pulse energy density and properties of the foil including its adhesion to the aluminum alloy were found to influence the change in surface mechanical properties.

Characterization of ion-assisted pulsed laser deposited cubic boron nitride films

Cubic boron nitride films were deposited by pulsed laser deposition from a boron nitride and a boron target using a KrF-excimer laser, where the growing films were bombarded by a nitrogen or a nitrogen/argon ion beam. The variation of the film properties with laser and ion beam parameters and substrate temperature has been investigated by infrared spectroscopy, cross-section and plan-view high-resolution transmission electron microscopy, electron-energy-loss spectroscopy and in situ ellipsometry. It will be shown that c-BN films with high phase purity can be prepared at sufficiently strong ion bombardment as well as substrate temperatures above 160°C. The c-BN phase was found to grow exclusively on top of the well-known hexagonal interlayer with c-axis orientation parallel to the substrate surface. Two types of nucleation were observed, the first characterized by c-BN (111) and the second by c-BN (001) lattice planes growing parallel to the (0002) lattice planes of the initially formed h-BN layer. c-BN films were prepared at maximum growth rates of 16 nm/min. Additional UV-photon irradiation of the growing films results in distinct modifications of the microstructure of the BN films. Using laser pulse energy densities on the substrate surface above 200 mJ/cm2 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. In contrast, films irradiated at 100 to 160 mJ/cm2 are cubic. Electron microscopic observations show that in this range the mean diameter of crystallite in the excimer laser irradiated regions increased by a factor of two in comparison with unirradiated regions of the same sample. The experimental results will be discussed in context with the results of temperature field calculations.

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.

Triplet quantum yield determination by picosecond laser double-pulse fluorescence excitation

The quantum yields of S 1 → T 1 triplet formation of some organic dyes are determined by picosecond double-pulse laser excitation and time-resolved fluorescence detection. The second pulse fluorescence signal is reduced due to first pulse induced singlet-triplet intersystem crossing. Disturbances due to higher excited state triplet-singlet back transfer are minimized by proper laser pulse duration and pulse energy density adjustment. The saturation energy density of triplet ground state depopulation has to be higher than the saturation energy density of singlet ground state depopulation, and the energy density of the second excitation pulse should be comparable to or less than the saturation energy density of ground state depopulation.

Excimer laser interactions with an aluminum alloy

An AlCuMg alloy was irradiated using a KrF pulsed excimer laser. A microstructural study showed the presence of a laser treated zone having a uniform depth of approximately 20 μm. The surface layer of the laser treated aluminum alloy exhibited a wavy topography and its surface roughness was found to depend on the number of laser pulses per step. A X-ray diffraction analysis revealed the presence of aluminum oxides and nitrides on the surface of the laser irradiated specimens. Corrosion measurements showed that the laser treated aluminum alloy exhibited a higher corrosion resistance. The corrosion behavior of the irradiated specimens is related to the energy density of the incident laser pulses.

Microstructures of Luminescent nc-Si by Excimer Laser Annealing of a-Si:H

AbstractWe have reported for the first time on visible photoluminescence (PL) in crystallized a-Si:H/a-SiNx:H multilayer structures by CW Ar ion laser annealing treatments. In this paper we present new results on visible PL from crystallized a-Si:H by using KrF excimer pulse laser (wavelength 248 nm) irradiating treatments. The transmission electron microscopy and Raman scattering studies reveal the microstructures of crystallized Si films, which depend on the pulse number and the pulse energy density of KrF laser. When the laser pulse energy density is higher than 520 mJ/cm2, the nanosized Si crystallites (nc-Si) can be formed from a-Si:H layers with a thickness of 100 nm and strong PL with a peak wavelength of 610 nm has been observed at room temperature.

Comparison of one-photon and two-photon effects in the photosensitivity of germanium-doped silica optical fibers exposed to intense ArF excimer laser pulses

The growth rate of fiber Bragg gratings written using 193 nm light from an ArF excimer laser is linearly proportional to the laser pulse energy density for fibers with high germanium doping but proportional to the square of the pulse energy density for standard telecommunications fibers with low germanium concentration. The two-photon process in standard fibers yields refractive index increases that saturate around 10−3, an order of magnitude improvement over previous results in this type of fiber without sensitization treatment. The two types of photoinduced refractive index gratings have comparable thermal stability and preserve about 50% of their initial magnitude after 30 min at 600 °C.

Excimer laser sputtering deposition of TiO 2 optical coating for solar cells

We have investigated the ArF laser sputtering deposition of TiO 2 anti-reflection coatings, aiming at large scale production of II–VI and III–V solar cells, the main theme of our research group. As a first step, layers were deposited on silicon wafers and glass. A reasonable growth rate for production, considering possible pulse frequencies of a few tens Hertz, of 0.1 Å per 1 J/cm −2 laser pulse energy density was obtained. The structure of the oxide layer, interface and interdiffusion with the substrate are characterized using cross sectional high resolution transmission electron microscopy and Rutherford backscattering, as a function of substrate temperature on the one hand and argon, nitrogen and oxygen pressure during deposition on the other hand. The surface optical quality of the deposited films versus thickness and growth conditions is presented. We will apply this technique to a solar cell prototype and a GaSb based photodetector prototype fabricated in our group.

Study of PbTe(Ga) evaporation using a nanosecond pulsed laser

Vapor composition and temperature, and time-of-flight (TOF) distributions of the vaporized particles have been studied for vacuum evaporation of PbTe(Ga) by pulses of a Nd 3+:YAG laser. A quadrupole mass spectrometer and the optical multichannel analyser (emission spectroscopy) have been used for this purpose. It has been observed that the composition of the evaporated substance, ion-neutral particle ratio and TOF distributions depended strongly on the laser pulse energy density w in the region of w = 0.47–4.8 J/cm 2. Estimations of the kinetic energies of the ions have been carried out. The laser plasma temperature changed from 8000 to 4500 K at w = 4.8 J/cm 2 for a space of 25 mm from the target surface along the normal.

Morphology of Si 1−xGe x thin crystalline films obtained by pulsed-excimer-laser annealing of heavily Ge-implanted Si

Thin crystalline Si 1− x Ge x films have been obtained by irradiating heavily (∼ 1.5 × 10 17 ions/cm 2) Ge-implanted <100> Si with a pulsed ArF excimer laser. The crystallization of the layer amorphized by implantation occurs by a melting-solidification process during and after the laser pulse. The influences of implantation conditions of laser energy density or number of laser pulses on the layer configuration (crystallinity and grain size) are observed by transmission electron microscopy on cross-sections and by grazing X-ray diffraction. The beam energy for implantations ranged from 50 to 150 keV and the laser energy density from 0.9 to 1.2 J cm 2. Structures obtained after complete as well as incomplete melting of the whole amorphized layer have been investigated. For example: a monocrystalline layer can be observed (implantation: 100 keV; laser: 0.9 J/cm 2, 20 pulses) or successive polycrystalline layers: large grains of Si 1− x Ge x near the surface on poly-Si (implantation: 80 keV up to 8 × 10 16 atoms/cm 2 then 50 keV up to 10 17 atoms/cm 2; laser: 1.2 J/cm 2, 10 pulses). The observations also show the grain growth induced by increasing number of pulses.

Polymerisation of acrylic oligomers photoinitiated by XeCl laser radiation

Polymerisation of OKM-2 and TGM-3 films was photoinitiated by pulse-periodic XeCl laser radiation (λ = 308 nm). The radiation-dose dependences of the mass and thickness of hardened films, and of the degree of conversion of double bonds were determined for polymerisation both in air and in an inert gas. A study was made of the influence of the energy density of laser pulses on the polymer yield. The changes in the transmission spectra of the irradiated films were studied. The energy expenditure in the hardening of the films depended strongly on the energy density of the laser pulses. The parameters of the polymerisation kinetics were estimated.

Preparation of Si 1−xGe x thin crystalline films by pulsed excimer laser annealing of heavily Ge implanted Si

Thin crystalline Si 1−xGe x layers were obtained by irradiating heavily Ge implanted (10 17 atoms cm −2) 〈100〉 oriented Si substrates with a pulsed excimer (ArF) laser. The respective influences of the implantation conditions, laser energy density or number of successive laser pulses were investigated using Rutherford backscattering channeling analysis and Raman spectroscopy. In particular, it is shown that layers of good crystalline quality can be readily obtained, the width almost constant Ge concentration being controlled by the irradiation conditions. Optimizing these conditions leads to 200 nm thick alloy layers with a constant Ge content x = 0.14. These results were interpreted using a computer simulation based on the melting-resolidification process which occur during the pulsed laser irradiations.

Self-emission and enhancement of laser-induced emission of electrons from ferroelectrics

We report on laser-induced electron emission (LIEE) from ferroelectrics (FE) at 266, 355 and 532 nm wavelength. The self-emission of charges up to 20 nC/cm 2 with kinetic energies up to several keV was observed with PLZT ceramics at laser-pulse energy densities of 13 mJ/cm 2 and a pulse width of 5 ns FWHM after high-voltage-induced polarization switching. The driving electric field is generated by the laser-induced change of the spontaneous polarization in a time scale of 1 ns. The dependence of the emission on the laser-pulse energy density is shown and the relation between the enhancement of LIEE and the laser-induced self-emission is discussed.

Kinetics of defect creation by pulsed laser illumination in hydrogenated amorphous silicon films deposited from pure silane and from silane-helium mixtures

The pulsed laser-soaking technique was employed to study the kinetics of defect creation in two types of samples. The laser pulse energy density U was varied in the range (2.5 − 33) 10 −3 J cm −2. After a rise time nearly independent of U, a steady-state defect density N SS is observed which varies linearly with U. A model including light-induced annealing of defects fits the present data. N SS as well as the light-induced creation and light-induced annealing terms are found to be higher in standard samples than in He-diluted samples and confirm the better stability of the latter.