Showing papers on "Fluence published in 2000"

Double layer effects in laser-ablation plasma plumes

Abstract: Charge-collector probe measurements have been performed to elucidate ion acceleration in laser-induced plasma plumes over a range of laser fluences important for pulsed laser deposition. The fundamental (1064 nm) or second (532 nm) harmonics of a Nd:YAG laser were used for ablation. The evolution of the time-of-flight ion signal from single-peaked to double-peaked and again to single-peaked with increasing laser fluence in the range of $2--25 {\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}$ has been followed. The analysis of the ion velocity distributions shows that increasing laser fluence results in the appearance of a portion of accelerated ions that can be recognized as an additional fast peak in the time-of-flight distribution. The dependencies of the ion signal on the target-to-collector distance, the background pressure, and the wavelength of laser radiation have been studied. The results are discussed from the viewpoint of the generation of a self-consistent ambipolar electric field (so-called double layer). The observed ion acceleration suggests that formation in the plume of a high-energetic electron tail due to absorption of laser radiation is responsible for the development of a double layer.

Charge storage and interface states effects in Si-nanocrystal memory obtained using low-energy Si + implantation and annealing

Abstract: Thin SiO2 oxides implanted by very-low-energy (1 keV) Si ions and subsequently annealed are explored with regards to their potential as active elements of memory devices. Charge storage effects as a function of Si fluence are investigated through capacitance and channel current measurements. Capacitance–voltage and source–drain current versus gate voltage characteristics of devices implanted with a dose of 1×1016 cm−2 or lower exhibit clear hysteresis characteristics at low electric field. The observed fluence dependence of the device electrical properties is interpreted in terms of the implanted oxide structure.

Methods for producing uniform large-grained and grain boundary location manipulated polycrystalline thin film semiconductors using sequential lateral solidification

Abstract: Methods for processing an amorphous silicon thin film sample into a polycrystalline silicon thin film are disclosed. In one preferred arrangement, a method includes the steps of generating a sequence of excimer laser pulses, controllably modulating each excimer laser pulse in the sequence to a predetermined fluence, homoginizing each modulated laser pulse in the sequence in a predetermined plane, masking portions of each homoginized fluence controlled laser pulse in the sequence with a two dimensional pattern of slits to generate a sequence of fluence controlled pulses of line patterned beamlets, each slit in the pattern of slits being sufficiently narrow to prevent inducement of significant nucleation in region of a silicon thin film sample irradiated by a beamlet corresponding to the slit, irradiating an amorphous silicon thin film sample with the sequence of fluence controlled slit patterned beamlets to effect melting of portions thereof corresponding to each fluence controlled patterned beamlet pulse in the sequence of pulses of patterned beamlets, and controllably sequentially translating a relative position of the sample with respect to each of the fluence controlled pulse of slit patterned beamlets to thereby process the amorphous silicon thin film sample into a single or polycrystalline silicon thin film.

Machining of transparent materials using an IR and UV nanosecond pulsed laser

Abstract: Channels are traditionally machined in materials by drilling from the front side into the bulk. The processing rate can be increased by two orders of magnitude for transparent materials by growing the channel from the rear side. The process is demonstrated using nanosecond laser pulses to drill millimeter-sized channels through thick silica windows. Absorbing defects are introduced onto the rear surface to initiate the coupling of energy into the material. Laser drilling then takes place when the fluence exceeds a threshold. The drilling rate increases linearly with fluence above this threshold. While UV light drills about four times faster than IR light, the pulse length (in the nanosecond regime) and the pulse repetition rate (in the 0.1–10 Hz range) do not greatly influence the drilling rate per pulse.

Surface planarization of thin silicon films during and after processing by the sequential lateral solidification method

Abstract: Systems and methods for reducing a surface roughness of a polycrystalline or single crystal thin film produced by the sequential lateral solidification process are disclosed. In one arrangement, the system includes an excimer laser (110) for generating a plurality of excimer laser pulses of a predetermined fluence, an energy density modulator (120) for controllably modulating the fluence of the excimer laser pulses such that the fluence is below that which is required to completely melt the thin film, a beam homoginizer (144) for homoginizing modulated laser pulses in a predetermined plane, a sample stage (170) for receiving homoginized laser pulses to effect melting of portions of the polycrystalline or single crystal thin film corresponding to the laser pulses, translating means for controllably translating a relative position of the sample stage (170) with respect to the laser pulses, and a computer (110) for coordinating the excimer pulse generation and fluence modulation with the relative positions of the sample stage (170) to thereby process the polycrystalline or single crystal thin film by sequential translation of the sample stage (170) relative to the laser pulses.

Systems and methods using sequential lateral solidification for producing single or polycrystalline silicon thin films at low temperatures

Abstract: System and methods for processing an amorphous silicon thin film sample into a single or polycrystalline silicon thin film are disclosed. The system includes an excimer laser (110) for generating a plurality of excimer laser pulses (111) of a predetermined fluence, an energy density modulator (120) for controllably modulating fluence of the excimer laser pulses, a beam homogenizer (144) for homogenizing modulated laser pulses (146) in a predetermined plane, a mask (150) for masking portions of the homogenized modulated laser pulses into patterned beamlets, a sample stage (180) for receiving the patterned beamlets to effect melting of portions of any amorphous silicon thin film sample (170) placed thereon corresponding to the beamlets, translating means for controllably translating a relative position of the sample stage with respect to a position of the mask and a computer (100) for controlling the controllable fluence modulation of the excimer laser pulses and the controllable relative positions of the sample stage and mask, and for coordinating excimer pulse generation and fluence modulation with the relative positions of the sample stage and mask, to thereby process amorphous silicon thin film sample into a single or polycrystalline silicon thin film by sequential translation of the sample stage relative to the mask and irradiation of the sample by patterned beamlets of varying fluence at corresponding sequential locations thereon.

Relationship between fluence gradient and lateral grain growth in spatially controlled excimer laser crystallization of amorphous silicon films

Abstract: In order to clarify the relationship between excimer laser fluence gradient and the length of lateral grain growth, the excimer laser fluence used for crystallization is modulated by a beam mask. The crystallized 50-nm-thick a-Si films are secco etched in order to observe the lateral grain growth by scanning electron microscope. The fluence distribution across the pattern is measured by a negative photoresist that has a linear relationship between laser fluence and resist thickness after development. This mapped fluence distribution is utilized to deduce the fluence gradient for each laser energy output. It is shown that lateral growth length increases and the directionality of the grains improves as the fluence gradient increases. Lateral growth length as long as 1.5 μm can be driven in a 50-nm-thick a-Si film by a single excimer laser pulse without any substrate heating. Electrical conductance measurement is used to probe the solidification dynamics. The lateral solidification velocity is estimated to be 7 m/s. A model is proposed to explain the formation of grain microstructure.

Formation mechanism of silver nanocrystals made by ion irradiation of ion-exchanged sodalime silicate glass

Abstract: Sodalime silicate glass surface layers were doped with up to 7.0 at.% Ag+ ions by ion-exchange in a AgNO3/NaNO3 solution at 330–355°C. Ion irradiation using either 400 and 500 keV He, 1 MeV Ne or 2 MeV Xe was then used to induce the growth of metallic nanocrystals in the ion-exchanged region. The ion fluences ranged from 1.3×10 14 ions/cm 2 to 1.1×10 17 ions/cm 2 . X-ray and electron diffraction show small Ag nanocrystals with a broad size distribution, up to a diameter of 10–15 nm, after irradiation. Optical transmission measurements show the characteristic surface plasmon resonance of metallic Ag around 420 nm. The absorption resonance sharpens and increases in strength with increasing ion irradiation fluence, indicating that both nanocrystal size and volume fraction increase with irradiation fluence. Depending on ion fluence, up to ∼15% of the ion-exchanged Ag+ ions is incorporated in nanocrystals. From a systematic comparison of the degree of nanocrystal formation as a function of ion species, fluence and energy, it is concluded that nanocrystal formation is mainly caused by the atomic displacement energy loss component of the incoming ion beam; the electronic energy deposition component is less efficient.

Thermal and nonthermal ion emission during high-fluence femtosecond laser ablation of metallic targets

Abstract: We have investigated the emission of positive ions from metallic targets irradiated with intense, ultrashort laser pulses (≈120 fs) at 780 nm, in both S and P polarized states. The measured energy spectra show the presence of a nonthermal, high-energy (several keV) ion component accompanying low-energy ions (tens of eV) produced by a thermal mechanism. The yield of the high-energy component shows a strong dependence on both laser fluence and light polarization. For the low-energy component a higher ablation efficiency was observed for P polarization, and ascribed to a more effective absorption mechanism active during the laser–target interaction.

Influence of the laser fluence in infrared matrix-assisted laser desorption/ionization with a 2.94 m Er : YAG laser and a flat-top beam profile

Abstract: The dependence of the signal intensity of analyte and matrix ions on laser fluence was investigated for infrared matrix-assisted laser desorption/ionization (IR-MALDI) mass spectrometry using a flat-top laser beam profile. The beam of an Er : YAG laser (wavelength, 2.94 microm; pulse width, 90 ns) was coupled into a sapphire fiber and the homogeneously illuminated end surface of the fiber imaged on to the sample by a telescope. Three different laser spot sizes of 175, 350 and 700 microm diameter were realized. Threshold fluences of common IR matrices were determined to range from about 1000 to a few thousand J m(-2), depending on the matrix and the size of the irradiated area. In the MALDI-typical fluence range, above the detection threshold ion signals increase strongly with fluence for all matrices, with a dependence similar to that for UV-MALDI. Despite the strongly different absorption coefficients of the tested matrices, varying by more than an order of magnitude at the excitation laser wavelength, threshold fluences for equal spot sizes were found to be comparable within a factor of two. With the additional dependence of fluence on spot size, the deposited energy per volume of matrix at threshold fluence ranged from about 1 kJ mol(-1) for succinic acid to about 100 kJ mol(-1) for glycerol.

Laser induced removal of spherical particles from silicon wafers

Abstract: Laser-induced removal of spherical silica particles from silicon wafers was investigated. The cleaning efficiency and laser cleaning thresholds for particles with diameters of 0.5, 1.0, 2.5, and 5.0 μm were carefully measured. It is found that the cleaning efficiency is more sensitive to laser fluence than laser pulse number and repetition frequency. The particle ejecting energies were found to increase with laser fluence. The threshold laser fluences for removing particles with sizes of 0.5 and 1.0 μm are 225 and 100 mJ/cm2, respectively, when KrF excimer laser is used. The threshold laser fluences are only a value below 5.0 mJ/cm2 for particles with a size of 2.5 and 5 μm. A model including both linear expansion and elastic deformation model was proposed to explain the experimental results. With this model, the particle movement and deformation in laser cleaning process were calculated. The expressions for threshold laser fluences were derived. The theoretical predictions are found to be greater than the experimental results. The difference can be explained by the enhancement of light intensity near the contacting area, due to the focusing and scattering by spherical particles. This model is useful to the study of laser cleaning as well as particle adhesion and deformation on solid surfaces.

Electronic excitation induced mass transport on 200 MeV 107Ag+14 ion irradiated Si surface

Abstract: Modification on Si (111) surface resulting from 200 MeV 107Ag+14 ion irradiation at an incidence angle of 15° with respect to the surface normal has been investigated by atomic force microscope (AFM) and x-ray diffraction techniques. Root mean square roughness measured using AFM was found to increase from 0.96 nm in the pristine sample to 18.33 nm in the sample irradiated with an ion fluence of 5×1013 ions/cm2. This spectacular increase in surface roughness is ascribed to the displacement of Si atoms driven by the creation of unsaturated dangling bonds on the Si surface during swift heavy ion irradiation. At a fluence of 5×1013 ions/cm2, a new type of ditch and dike structure was observed. These structures were distributed over the irradiated surface. Each ditch is followed by a dike. This is attributed to the cumulative effect of ion irradiation arising due to overlapping of ion-induced damaged zones and electronic excitation-induced shear motion of the atoms towards the surface.

Rapid long-period grating formation in hydrogen-loaded fibre with 157 nm F2-laser radiation

Abstract: Long-period gratings were fabricated in hydrogen-loaded telecommunication fibre using 157 nm F/sub 2/-laser radiation. A >20 dB attenuation peak at 1590 nm was induced with only 5.0 J/cm/sup 2/ accumulated fluence, a value >250-fold smaller in comparison with fluence requirements for 248 nm KrF-laser irradiation.

Chemical modification of polypropylene induced by high energy carbon ions

Abstract: Polypropylene was irradiated with 12 C + ions of 3.6 and 5.4 MeV energy using 3 MV Pelletron. The spectral changes owing to ion bombardment were investigated by UV–VIS and Fourier-transform infrared (FTIR) spectroscopy. A gradual increase in absorbance was observed around visible and near visible region with increase in fluence of bombarding ions. The difference absorption spectra show formation of chromophoric groups with wavelength maximum near 380 nm at lower fluence, but at high fluence a shift in peak is observed. The chromophoric groups are likely to be the extended conjugated polyene system and the red shift in peak postion at high fluence may be attributed to the greater degree of conjugation. The formation of unsaturated linkage is confirmed by the FTIR spectra with observed stretching band around 1650 cm −1 and its intensity was found to increase with increase in ion fluence studied. The gases (in the range 2–80 amu) which were evolved due to interaction of polypropylene with 12 C + ions were measured with Residual Gas Analyzer (RGA). A large number of gaseous components were detected. This shows that polymer chains break into some smaller fragments which concomitantly leads to extended conjugation.

Raman scattering characterization of Si(100) implanted with mega-electron-volt Sb

Abstract: Dose dependent structural modifications in Si(100) due to 1.5 MeV implantation of Sb have been characterized using Raman spectroscopy and Rutherford backscattering spectrometry/channeling (RBS/C) techniques. With increasing fluence, an intensity reduction of the first order Raman peak, characteristic of crystalline Si, is observed. The amorphicity in Si lattice appears at a dose of 1×1013 ions/cm2 and it increases with each dose. For a dose of 5×1014 ions/cm2 the Raman spectrum resembles that of amorphous Si. RBS/C studies also support a fully amorphized lattice at this dose though for smaller doses it suggests lower disorder. For the fluences of 1×1013 and 1×1014 ions/cm2 a coexistence of undamaged crystalline Si regions and amorphous zones is observed. Consequently, phonon confinement is observed. Lattice recovery achieved by subsequent annealing has also been investigated using Raman spectroscopy. By annealing at 600 °C, sample crystallinity is fully recovered in all the cases up to the fluence of 5×10...

Behavior of fused silica irradiated by low level 193 nm excimer laser for tens of billions of pulses

Abstract: Fused silica samples from seven suppliers have been irradiated with low-level (0.1 mJ/cm2) 193 nm irradiation using an ArF excimer laser. At high pulse counts of tens of billions of pulses, a decrease of the optical path length is observed in the irradiated area. Two of the samples were irradiated with a higher fluence source of 3 mJ/cm2 for 100 million pulses in a separate area. That higher level exposure produced an increase in the optical path length as expected due to compaction. Birefringence measurements were also made which show that the character of birefringence is different for the high fluence and low fluence irradiated areas. A description of the test and measurements is presented along with data covering pulse counts to 40 billion pulses.

Raman spectroscopy of phenylcarbyne polymer films under pulsed green laser irradiation

Abstract: The polycarbyne polymer films were coated on silicon substrates and then irradiated by a pulsed Nd:yttrium–aluminum–garnet laser (λ=532 nm) with various fluences in argon gas atmosphere. Significant changes in microstructures and chemical bonding (sp3/sp2) during laser treatment were investigated by Raman spectroscopy. At a laser fluence below 50 mJ/cm2, the Raman spectrum of the film was similar to that of the original polymer film. In a fluence range from 100 to 650 mJ/cm2, the thermal decomposition of the polymer occurred, resulting in upshift of G peak and downshift of the D peak both with narrower peak width. With increasing laser fluence from 650 to 950 mJ/cm2, both the G and D peaks downshifted simultaneously due to the thermal decomposition. The simultaneous downshift of both peaks indicated the increased sp3/sp2 ratio in the carbon film converted from the polymer. With increasing laser fluence in this range, the thermal decomposition also induced the narrowing of both peaks. The narrowing of both...

Reduced threshold ultraviolet laser ablation of glass substrates with surface particle coverage: A mechanism for systematic surface laser damage

Abstract: A recent study of ultraviolet laser cleaning of silica glass surfaces contaminated with medium density alumina particles has shown a systematic type of laser-induced surface damage. This is characterized as a pit which increases in diameter and depth with increasing irradiating fluence. The damage pit occurs at lower fluence levels when smaller contaminant particles are used. The pits are due to laser ablation of the particle coated glass surface. The threshold for this laser ablation is well below that for a sample of the glass in its uncontaminated condition and the efficiency of ablation is estimated as being about 1000 times greater than that for silica. This laser damage is difficult to visualize by standard optical microscopy and could easily go undetected in laser cleaning studies. It may have general implications in the application of the new laser cleaning technologies to a variety of surfaces, including dielectrics and semiconductors, where medium to high densities of contaminant particles are p...

Effects of laser fluence, size, and shape of the laser focal spot in pulsed laser deposition using a multielemental target

Abstract: Two-dimensional distributions of thickness and of composition of the deposit produced by the room temperature pulsed laserablation of lead zirconate titanate in vacuum were studied experimentally as a function of laser fluence, of size, and of elongation of the rectangular laser focal spot. The flip over and the elliptical shape of the deposit were observed. Increase in laser fluence, increase in elongation, and decrease in size of the spot resulted in a stronger broadening of the thickness profiles. The deposit was lead deficient, with the lead profiles “inverse” to the thickness profiles. Excess and/or nominal content of zirconium and of titanium were observed with the profiles resembling those of the thickness. The thickness profiles were in general formal agreement with the model of adiabatic expansion of the monoelemental plume. An additional broadening of the profiles was ascribed to the spatial distribution of the composition in the deposit. The behavior of the composition was qualitatively analyzed in terms of sorption of ablated species at the substrate. Good agreement between experimental observations and the conclusions of the sorption analysis suggests a determining role of the plume-substrate interaction in the deposition process.

Effect of vacuum on the occurrence of UV-induced surface photoluminescence, transmission loss, and catastrophic surface damage

Abstract: Vacuum degrades the transmittance and catastrophic damage performance of fused-silica surfaces, both bare and silica-sol anti-reflective coated. These effects may be important in certain space application of photonics devices. When exposed to hundreds of 355-nm, 10-ns laser pulses with fluences in the 2 - 15 J/cm2 range, transmittance loss is due to both increased reflectance and absorption at the surface. Spectroscopic measurements show that the absorbed light induces broadband fluorescence from the visible to infrared and that the peak photoluminescence wavelength depends cumulative fluence. The effect appears to be consistent with the formation of surface SiOx4/ (x < 2 with progressively lower x as cumulative fluence increases. Conversely, low fluence CW UV irradiation of fluorescent sites in air reduces the fluorescence signal, which suggests a photochemical oxidation reaction back to SiO2. The occurrence of catastrophic damage (craters that grow on each subsequent pulse) also increases in a vacuum relative to air for both coated and uncoated samples.

Transport of oxygen atoms mediated by electronic excitation

Abstract: Diffusion of oxygen from substrate to the film was observed under the influence of large electronic excitation. CuO thin film ∼210 nm on float glass was irradiated with 210 MeV I ions. We noticed the transport of oxygen from the substrate to the CuO film through the interface. The amount of oxygen transport from glass to the film was found to be fluence dependent. The loss of oxygen from the films was also observed. The erosion of Cu atoms was also observed beyond a fluence of 9.6×10 13 ions/cm2. The measurements were performed by on-line elastic recoil detection using a large area position sensitive detector. Since the electronic energy loss dominates in the present case of 210 MeV I ions, the observed changes at the interface and surface are attributed to inelastic collisions of the incident ions with the atomic electrons in the sample.

Damage evolution in Xe-ion irradiated rutile (TiO2) single crystals

Abstract: Rutile (TiO 2 ) single crystals with (1 1 0) and (1 0 0) orientations were irradiated with 360 keV Xe 2+ ions at 300 K to fluences ranging from 1×10 17 to 5×10 20 Xe/m 2 . Irradiated samples were analyzed using Rutherford backscattering spectroscopy combined with ion channeling analysis (RBS/C) and transmission electron microscopy (TEM). RBS/C results showed that much of the instantaneous displacement damage produced under ion irradiation is recovered under ambient temperature irradiation conditions. Upon irradiation to a fluence of 2×10 19 Xe/m 2 , the radiation damage-induced microstructure was observed by TEM to consist of three distinct layers: (1) a layer near surface (thickness about 12 nm) exhibiting relatively homogeneous TEM contrast; (2) a second layer with a low density of relatively large-sized defects; and (3) a third layer consisting of a high concentration of small defects. After the fluence was increased to 5×10 19 Xe/m 2 , a buried amorphous layer was observed by TEM. The thickness of the amorphous layer was found to increase with increasing Xe ion fluence. The uppermost damage layer, which accounts for the surface subpeak in RBS/C spectra, was found to be polygonized by ion irradiation.

Excimer laser annealing of glasses containing implanted metal nanoparticles

Abstract: Silver and copper nanoparticles have been synthesized by ion implantation in silica and soda-lime silicate glass at 60 keV to a dose of 4×1016 ion/cm2 and at 50 keV to a dose of 8×1016 ion/cm2, respectively. The glasses were annealed using pulses of a high-power KrF excimer laser (248 nm) in ambient atmosphere. This employed a single (25 ns) pulse fluence of 0.25 J/cm2 for Ag-implanted samples or of 0.21 J/cm2 for Cu-implanted ones. Several pulses from 1 to 250 of the same energy density at a frequency of 1 Hz were accumulated in the same area on the surface. The formation and modification of metal nanoparticle was assessed via optical reflectance, combined with Rutherford backscattering analysis. Generally, changes induced by laser pulses suggest there are both reductions of the nanoparticles and some longer-range diffusion of metal atoms into the glass. However, before the total dissolution of metal nanoparticles is accrued, the substrate temperature, increasing during many-pulse treatments, initiates the regrowth of new metal nanoparticles that leads to a rise of the reflectance. These results are discussed on the basis of a surface substrate melting. This work is part of an attempt to gain control over the size and depth distribution of such metal nanoparticles.

Surface defect generation in optical materials under high fluence laser irradiation in vacuum

Abstract: It is shown that irradiation of optical materials with high fluence ultraviolet laser pulses at low pressure leads to a rapid growth in the surface defect population and degradation of their performance. Experiments have been performed in nonlinear KH/sub 2/PO/sub 4/ crystal and in fused silica, and surface defect populations have been observed following a few tens of high fluence 351 nm pulses.

Phase change phenomena during high power laser-materials interaction

Abstract: This work investigates phase change phenomena due to high power pulsed laser irradiation. During high power laser heating, the intense radiation flux from the laser is transformed to the target material and raises the temperature of the target surface rapidly. When the laser fluence is high enough, melting and superheating of liquid are possible. At even higher laser fluences, the superheated liquid undergoes a phase explosion that turns the liquid into a mixture of liquid and vapor. In this paper, we describe theoretical, numerical and experimental studies of the materials’ response under nanosecond pulsed excimer laser irradiation. Experiments are performed in a laser fluence range between 2.5 and 10 J/cm 2 (between 100 and 400 MW/cm 2 ) on nickel specimens. The velocity of the laser-evaporated vapor, absorption of the laser energy by the laser-evaporated vapor, the threshold laser fluence for phase explosion, and the pressure and temperature at the target surface are determined. Results of these studies reveal phase change mechanisms during high power laser interaction with metal.

Excimer laser-induced microstructural changes of alumina and silicon carbide

Abstract: Surface treatments with a KrF excimer laser were applied on alumina and silicon carbide ceramic materials. Results on the surface modifications induced by laser were related to the processing parameters: laser fluence (1.8 and 7.5 J/cm2), number of laser pulses (1 to 500), frequency (1 to 120 Hz), pulse duration (25 ns), sample speed under the laser beam and working atmosphere. It was ascertained that alumina can be laser treated under air, while silicon carbide needs an inert atmosphere to avoid surface oxidation. Microstructural analyses of surface and cross section of the laser processed samples evidenced that at low fluence (1.8 J/cm2) the surface of both ceramics is covered by a scale due to melting/resolidification. At high fluence (7.5 J/cm2) there are no continuous scales on the surfaces; material is removed by decomposition/vaporisation and the depth of material removal is linearly dependent on the number of pulses. On alumina surface, a network of microcracks formed, while on silicon carbide different morphologies (flat and rugged areas, deposits of debris and discontinuous thin remelted scales) were detected. The evolution of surface morphology and roughness is discussed with reference to composition, microstructure and physical and optical properties of the two tested ceramics and to laser processing parameters.