Showing papers on "Fluence published in 2004"

Femtosecond laser ablation properties of borosilicate glass

Abstract: We study the femtosecond laser ablation properties of borosilicate glass using atomic force microscopy and laser pulses of 200 fs duration, centered at 780 nm wavelength. We show that both single-shot and multishot ablation threshold fluences can be determined by studying the diameter and the depth of single-shot ablated craters. The linear relationship between the square of the crater diameter and the logarithm of the laser fluence in the form of D2=2w02ln(F0∕FthN=1) provides the single-shot ablation threshold, FthN=1, whereas the linear relationship between the ablation depth and the logarithm of laser fluence in the form of ha=αeff−1ln(F0∕FthN>1) provides the multishot ablation threshold, FthN>1. The results depict a multishot ablation threshold of ≈1.7J∕cm2 independent of the atmospheric conditions. The slopes of the linear fits also provide a precise estimate of the beam radius at the surface, w0≈5.9μm, and the “effective optical penetration depth,” αeff−1≈238nm in air. The method is systematic, prov...

Study of hard x-ray emission from intense femtosecond Ti:sapphire laser–solid target interactions

Abstract: Interaction of intense Ti:sapphire laser with solid targets has been studied experimentally by measuring hard x-ray and hot electron generation. Hard x-ray (8–100 keV) emission spectrum and Kα x-ray conversion efficiency (ηK) from plasma have been studied as a function of laser intensity (1017–1019 W/cm2), pulse duration (70–400)fs, and laser pulse fluence. For intensity I>1×1017 W/cm2, the Ag ηK increases to reach a maximum value of 2×10−5 at an intensity I=4×1018 W/cm2. Hot electron temperature (KTh) and ηK scaling laws have been studied as a function of the laser parameters. A stronger dependence of KTh and ηK as a function of the laser fluence than on pulse duration or laser intensity has been observed. The contribution of another nonlinear mechanism, besides resonance absorption, to hard x-ray enhancement has been demonstrated via hot electron angular distribution and particle-in-cell simulations.

Laser etching of fused silica using an adsorbed toluene layer

Abstract: A new method for laser etching of transparent materials with a low etch rate and a very good surface quality is demonstrated. It is based on the pulsed UV-laser backside irradiation of a transparent material that is covered with an adsorbed toluene layer. This layer absorbs the laser radiation causing the etching of the solid. The threshold fluence for etching of fused silica amounts to 0.7 J/cm2. The constant etch rate of about 1.3 nm/pulse that has been observed in a fluence interval from 2 to 5 J/cm2 is evidence of a saturated process. The limited thickness of the adsorbed layer causes the low etch rates and the rate saturation. The etched surface structures have well defined edges and low surface roughness values of down to 0.4 nm rms.

Nanometer-scale measurements of Fe3+/ΣFe by electron energy-loss spectroscopy: A cautionary note

Abstract: The effects of electron-beam damage on the Fe 3+ /ΣFe (total iron) ratio were measured by electron energy-loss spectroscopy (EELS) with a transmission electron microscope (TEM). Spectra were acquired from crushed and ion-beam-thinned cronstedtite. For fluences below 1 × 10 4 e/A 2 , the Fe 3+ /ΣFe values from crushed grains range between 0.43 and 0.49, consistent with undamaged material. These measurements were acquired from flakes 180 to 1000 A thick. With increase in fluence, samples 3+ /ΣFe values >0.5. The critical fluence for radiation damage by 100 kV electrons as defined by Fe 3+ /ΣFe 4 e/A 2 . The absorbed dose to the speciman during acquisition of a typical EELS spectrum is large, with values around 2.2 × 10 10 Gy (J/kg), equivalent to the deposition of 620 eV/A 3 . Cooling to liquid N 2 temperature did not significantly slow the damage process. Ion-beam thinning produces an amorphous layer on crystal surfaces. Spectra from the thinnest regions, which are amorphous, exhibit Fe 3+ /ΣFe >0.7. With increase in sample thickness, the Fe 3+ /ΣFe values decrease to a minimum, consistent with data from the undamaged material. The increase of Fe 3+ /ΣFe with respect to electron-beam irradiation is likely caused by loss of H. At low fluences, the loss of H is negligible, thus allowing consistent Fe 3+ /ΣFe values to be measured. The cronstedtite study illustrates the care required when using EELS to measure Fe 3+ /ΣFe values. Similar damage effects occur for a range of high-valence and mixed-oxidation state metals in minerals. EELS is the only spectroscopic method that can be used routinely to determine mixed-valence ratios at the nanometer scale, but care is required when measuring these data. Consideration needs to be given to the incident beam current, fluence, fluence rate, and sample thickness.

Fabrication of beam homogenizers in quartz by laser micromachining

Abstract: The combination of gray tone phase masks with a laser-based wet etching process can be applied as a one step micromachining process. This technique utilizes a XeCl excimer laser and an absorbing liquid which is in contact with quartz. Microstructures with continuous profiles, such as plano-convex microlenses in quartz, can be created with laser fluence well below the damage threshold of quartz. The roughness of the etched features varies from several micrometers to below 10 nm, depending on the laser fluence and applied absorbing solution. The etch rates of quartz using different organic solutions such as pyrene in acetone or pyrene in tetrahydrofurane reveal a complex behavior, suggesting that several processes dominate at various laser fluences.

Structural and sputtering effects of medium energy ion bombardment of silicon

Abstract: Molecular dynamics simulation is used to study argon ion bombardment of an initially perfect silicon crystal up to its damaged state at a total fluence of 4 × 10 14 impacts/cm 2 . Lower and higher energy processes are considered: one process with ions at 500 eV and another process with ions at 700 eV, which are like those used in a particular microelectromechanical systems (MEMS) fabrication technique. These energies are intermediate relative to most previous silicon ion bombardment studies, higher than those typically used in ion-assisted deposition and lower than in typical ion implantation. In all, up to 118 impacts are simulated in a 5.43 by 5.43 nm periodically replicated cell of a target (0 0 1) surface of silicon. After an impact, the material is cooled slowly to 77 K by a process that models thermal conduction in to the bulk target material. It is assumed that defects are immobile at this temperature and that no further structural relaxation occurs before the next impact. Multiple simulations of more than 100 ion impacts are conducted for both ion energies and averaged to converge statistical descriptions of structural evolution and sputtering. Surface roughening is observed with increase in ion fluence. Damage throughout the sample is studied using a planar radial distribution function. Using a crystallinity measure based on this function, it is found that the damage region extends 2.2 nm into the material for 500 eV ions and 3.0 nm for 700 eV ions after 80 impacts. The damaged region is separated from deeper, undamaged crystal by a nearly flat interface. Sputter yields are in reasonable agreement with experimental data, reaching nearly steady rates of 0.5 and 0.7 sputtered atoms per incident argon atom for the 500 and 700 eV cases, respectively. For the number of ions considered, implanted argons do not significantly affect Si sputtering.

Elongated Co nanoparticles induced by swift heavy ion irradiations

Abstract: Spherical nanoparticles have been formed in SiO 2 layers by 160 keV Co + ion implantation. Different particle sizes have been obtained depending on implantation fluence (2 × 10 16 , 5 × 10 16 and 1 × 10 17 Co + cm −2 ) and substrate temperature (77, 295 and 873 K). These spherical particles have been subsequently irradiated with 200 MeV 127 I ions at fluences from 10 11 to 10 14 cm −2 at 300 K. Modifications of magnetic properties of the irradiated samples, studied by a SQUID magnetometer at 5 and 295 K in a magnetic field applied perpendicular or parallel to the incident beams, have been related to the modifications of the particle size and shape observed by transmission electron microscopy (TEM). The results are in good agreement with calculations based on a model describing two different regimes: a spherical growth at low irradiation fluences and a prolate deformation along the beam direction for the high fluences.

Structural, electrical transport, magnetization, and 1∕f noise studies in 200MeV Ag ion irradiated La0.7Ce0.3MnO3 thin films

Abstract: The effect of 200MeV Ag ion irradiation on structural, electrical transport, magnetization, and low-frequency conduction noise properties of electron-doped La0.7Ce0.3MnO3 thin films have been investigated. The as-grown thin films show c-axis epitaxial structure along with a small amount of unreacted CeO2 phase. After the irradiation, at the lowest fluence both the magnetization and metal-insulator transition temperature increase. Further increase in fluence reduces the metal-insulator transition temperature and leads to larger resistivity; however, the unreacted phase of CeO2 disappears in the x-ray diffraction pattern. On the other hand, the normalized electrical noise is greatly enhanced even at the lowest nonzero fluence. Surprisingly the conducting noise in the irradiated samples is much higher in the metallic state than in the semiconducting one. The observed modifications in structural, electrical, magnetic, and noise properties of 200MeV Ag ion irradiated La0.7Ce0.3MnO3 thin films have been explain...

Step edge sputtering yield at grazing incidence ion bombardment.

Abstract: The surface morphology of Pt(111) was investigated by scanning tunneling microscopy after 5 keV Ar+ ion bombardment at grazing incidence in dependence of the ion fluence and in the temperature range between 625 and 720 K. The average erosion rate was found to be strongly dependent on the ion fluence and the substrate temperature during bombardment. This dependence is traced back to the variation of step concentration with temperature and fluence. We develop a simple model allowing us to determine separately the constant sputtering yields for terraces and for impact area stripes in front of ascending steps. The experimentally determined yield of these stripes--the step-edge sputtering yield--is in excellent agreement with our molecular dynamics simulations performed for the experimental situation.

Study of heavy-ion induced modifications in BaF2 and LaF3 single crystals

Abstract: BaF2 and LaF3 single crystals were irradiated with different ions from Ne to U having energies between 1.4 and 13.3 MeV/u and were subsequently analyzed with scanning force microscopy (SFM), optical spectroscopy and surface profilometry. Similar to numerous other ionic crystals, ion-induced hillocks were observed by SFM, the mean size on the nanometer scale depending on the electronic energy loss. BaF2 shows optical color-center absorption spectra, which consist of broad bands ascribed to F-center aggregates, whereas LaF3 does not exhibit any specific absorption bands. Depending on the beam parameters (energy, fluence, etc.), the irradiation of both crystals leads to more or less pronounced out-of-plane swelling. In order to study the thermal behaviour of ion-induced modifications, isochronal annealing has been performed. Both crystals show a decrease of the relative step height and the areal density of the hillocks with increasing temperature.

Multi-Dot Floating-Gates for Nonvolatile Semiconductor Memories - Their Ion Beam Synthesis and Morphology

Abstract: Scalability and performance of current flash memories can be improved substantially by replacing the floating poly-Si gate by a layer of Si dots. This multi-dot layer can be fabricated CMOS-compatibly in very thin gate oxide by ion beam synthesis (IBS). Here, we present both experimental and theoretical studies on IBS of multi-dot layers consisting of Si nanocrystals (NCs). The NCs are produced by ultra low energy Si ion implantation, which causes a high Si supersaturation in the shallow implantation region. During post-implantation annealing, this supersaturation leads to phase separation of the excess Si from the SiO2. Till now, the study of this phase separation process suffered from the weak Z contrast between Si and SiO2 in Transmission Electron Microscopy (TEM). Here, this imaging problem is resolved by mapping Si plasmon losses with a Scanning Transmission Electron Microscopy equipped with a parallel Electron Energy Loss Spectroscopy system (PEELS-STEM). Additionally, kinetic lattice Monte Carlo simulations of Si phase separation have been performed and compared with the experimental Si plasmon maps. It has been predicted theoretically that the morphology of the multi-dot Si floating-gate changes with increasing ion fluence from isolated, spherical NCs to percolated spinodal Si pattern. These patterns agree remarkably with PEELS-STEM images. However, the predicted fluence for spinodal patterns is lower than the experimental one. Because oxidants of the ambient atmosphere penetrate into the as-implanted SiO2, a substantial fraction of the implanted Si might be lost due to oxidation.

Prediction of solar cell degradation in space from the electron–proton equivalence

Abstract: From a comparison between the degradation rates, induced by proton and electron irradiations, of the performances of GaAs quantum well based vertical cavity surface emitting laser, we deduce a coefficient of equivalence between the two types of irradiations. This coefficient allows us to calculate the fluence of protons of a given energy which produces the same degradation as a fluence of electrons of a standard energy. We apply the result of this analysis to the degradation of GaAs solar cells, demonstrating that this coefficient allows a quantitative prediction of the degradation induced by a proton irradiation, when the degradation induced by electrons at one energy is known.

Sub-surface damage in indium phosphide caused by micromachining of grooves with femtosecond and nanosecond laser pulses

Abstract: Grooves laser-micromachined in InP using 130 fs and 8 ns pulses with fluences ≈2 and 0.7 J/cm2 are investigated by cross-sectional transmission electron microscopy. At the fluence of 2 J/cm2, irradiation with both femtosecond and nanosecond laser pulses yield substantial resolidified layers with a maximum thickness of ≈0.5 μm. In contrast, at the fluence of 0.7 J/cm2, irradiation with nanosecond pulses leads to a layer of similar thickness, while femtosecond irradiation produces laser induced periodic surface structures with minimal resolidified material. For both fluences, femtosecond pulses generate substantial densities of defects extending over a few microns in depth, while nanosecond laser irradiation leads to no observable damage beneath the resolidified layer. The high peak power density and the stress confinement obtained from femtosecond pulses, along with incubation effects, are identified as the major factors leading the observed plastic deformations.

Bistable defect in mega-electron-volt proton implanted 4H silicon carbide

Abstract: Epitaxial 4H-SiC n-type layers implanted at room temperature with a low fluence of mega-electron-volt protons have been measured by deep level transient spectroscopy (DLTS). The proton fluence of 1 ...

Single-pulse KrF laser ablation and nanopatterning in vacuum of β-titanium alloys used in biomedical applications

Abstract: We report the KrF laser (248 nm) nanosecond single-pulse irradiation in vacuum of a mirror-polished surface of a model β-titanium alloy. A series of single-pulse laser-annealing experiments is performed with increasing fluence adjusted to obtain conditions from submelting (a few mJ/cm2) up to intense laser ablation (30 J/cm2). The structural and morphological changes are followed by atomic force microscopy and scanning electron microscopy, as well as with other analytical techniques, and compared with the starting αβ equilibrium mixture of the untreated sample. The results are discussed and related to the theoretical estimation of fluences corresponding to the β-transus, melting, boiling and intense ablation (phase-explosion) thresholds as well as to the estimates of melt depth and lifetime evaluation as a function of fluence. By carrying out laser annealing in vacuum the surface oxidation is avoided while roughening and phase transformations are favoured within the laser-heated depth. The mirror-polished starting surface is composed of nanometric α-precipitates embedded in a β-matrix, whose surface is systematically 5–10 nm above the α-surface. Upon laser annealing with increasing fluence the α-nanocrystals transform progressively into β-phase, resulting in an increasing relief. At higher fluence the conditions for increasing roughness are studied. Submicrometre roughness is expected to improve cell adhesion to β-titanium alloys and hence osteointegration.

Dynamics of femtosecond laser-induced melting and amorphization of indium phosphide

Abstract: Laser-induced melting and resolidification of single-crystalline indium phosphide (InP) upon irradiation with 150fs laser pulses at 800nm has been investigated by means of real-time-reflectivity measurements with subnanosecond time resolution. Melting of the surface is observed to occur very rapidly on a time scale shorter than our experimental resolution while the lifetime of the liquid phase is several tens of nanoseconds. As a result of the subsequent rapid solidification process, a thin layer of amorphous material with a thickness of several tens of nanometers is formed on the surface. The formation of this amorphous layer has been observed for every fluence above the melting and below the ablation threshold. The evolution of the reflectivity has been modeled for several different solidification scenarios and compared to the experimental results. This comparison shows that solidification proceeds interfacially from the solid interface towards the surface. A lower limit for the critical solid-liquid in...

Radiation-induced change of polyimide properties under high-fluence and high ion current density implantation

Abstract: Polyimide (PI) films were implanted with 40-keV Ar+ and 80-keV Ar(2+) ions in a fluence range of 5.0*10^14-1.5*10^17 cm-2 at ion-current densities of 1-16 uA*cm2. It is shown that the conductivity of the samples rises with the ion-current density at a fixed fluence. Electrophysical parameters of the polyimide change stepwise with the implantation fluence when it exceeds a certain value. The change of electrical parameters of the implanted PI correlates with that of the optical and paramagnetic characteristics. The phenomenon of complete volatilisation of argon implanted with an energy of 40 keV due to surface heating and disordering under the high-power beam is found. It is shown that the change of ion charge and energy at constant beam-power density causes only a quantitative change in the polymer characteristics. A model of PI alteration and carbonised phase formation, taking into account the action of the high-power ion beam and the peculiarities of PI chemical structure, is developed to explain the observed effects. Implantation at high ion-current density can be suggested as an efficient practical means to control polymer conductivity and other parameters

Pulsed laser deposition: metal versus oxide ablation

Abstract: We present experimental results of pulsed laser interaction with metal (Ni, Fe, Nb) and oxide (TiO2, SrTiO3, BaTiO3) targets. The influence of the laser fluence and the number of laser pulses on the resulting target morphology are discussed. Although different responses for metal and oxide targets to repetitive laser irradiation could be expected due to the different band structures of metals and oxides, the optical response is quite similar for 248-nm laser irradiation. Therefore, the difference in response is largely caused by differences in thermal properties.Metal targets show periodic structures of the order of micrometers after consecutive pulses of laser radiation, while the SrTiO3 and BaTiO3 targets show a flat surface after ablation for relatively low fluences (1.0 Jcm?2). The observed TiO2 target ablation characteristics fall in between those of the ablated metals and perovskites, because ablation results in the presence of Ti-rich material, which shields the underlying toichiometric target material from ablation. The final target morphology is dependent on fluence, number of pulses, and the movement of the target itself (rotating, scanning, or stationary). It can take between 15 and 75 pulses to reach a steady-state target morphology on a stationary target.