Showing papers on "Fluence published in 1992"

Laser‐cleaning techniques for removal of surface particulates

Abstract: Flash laser heating using short‐pulsed laser irradiation of a surface is demonstrated to be a promising new approach for effective removal of particulate contaminations of sizes as small as 0.1 μm. This is very useful because micron‐ and submicron‐sized particulates adhere tenaciously onto a solid surface, and conventional cleaning techniques are inadequate for removal. Several varieties of the new laser‐cleaning techniques have been developed by us as well as by others. For example, the pulsed laser irradiation can be used with or without the simultaneous deposition of a thin liquid film on the surface to be laser cleaned. The laser wavelength can also be chosen so that absorption occurs mainly at the sample surface, or in the liquid, or in the particulate, or in a combination of these. In this paper, we discuss and compare examples of these different approaches. We find that laser cleaning with highest efficiency is achieved by choosing a laser wavelength that is strongly absorbed by the surface together with pulse depositing a water film of thickness on the order of microns on the surface momentarily before the pulsed laserirradiation. This permits the effective removal of particles smaller than ∼20 μm, down to as small as 0.1 μm, from a solid surface using a modest ultraviolet laser fluence of ∼0.1 J/cm2.

Nonlinear optical properties of carbon-black suspensions (ink)

Abstract: We performed a series of experiments on suspensions of carbon particles in liquids (ink) and carbon particles deposited on glass to determine the mechanisms for the observed optical-limiting behavior. Both materials show reduced transmittance for increasing fluence (energy per unit area). We found that nonlinear scattering dominates the transmissive losses and that the limiting is fluence dependent, so that limiters based on black ink are effective for nanosecond pulses but not for picosecond pulses. Additionally, the nonlinear scattering and the limiting behavior cease after repeated irradiation. For the liquid, flowing eliminates this effect. All the data obtained are consistent with a model of direct heating of the microscopic-sized carbon particles by linear absorption with subsequent optical breakdown initiated by thermally ionized carriers. A simple calculation gives temperatures higher than the sublimation temperature at the onset of limiting. Emission spectra measurements show singly ionized carbon emission lines with a hot blackbody background emission consistent with temperatures of ≃4000 K. A rapid expansion of the microscopic plasmas generated by the breakdown will effectively scatter further input light. Indeed, in time-resolved experiments the trailing portion of the pulse is most heavily scattered. The time-resolved transmittance of a weak cw probe beam also follows the temporal dependence of the singly ionized carbon emission (≃102 ns). We directly monitored the expansion of the scattering centers by angularly resolving the scattered light for different input fluences and fitting to Mie scattering theory. Since the carbon is black and the microplasmas are initiated by linear absorption, the limiting is extremely broadband. Within the context of this model we discuss the limitations and optimization of ink-based optical limiters.

Type inversion in silicon detectors

Abstract: Silicon strip detectors and photodiodes were irradiated in an 800 MeV proton beam. The change of the effective doping concentration was monitored by measuring diode C-V curves. Type inversion is observed at a fluence Φ = 1.5 × 1013 cm−2. Further evidence for type inversion is obtained from a study of pulses generated by an infrared LED in silicon strip detectors. A two-level parametrization is used to describe donor removal and acceptor state creation during proton irradiation: Neff = N0 exp(−cφ)−βφ. We measure values of c = (5.5 ± 1.1) × 1014 cm2 and β = (0.031 ± 0.006) cm−1. After type inversion the depletion voltage increases with proton fluence. This may set the limit for the lifetime of silicon detectors at future colliders. However, the occurence of type inversion does not degrade the performance of silicon strip detectors. The effective doping concentration showed a complex post irradiation behaviour. After a short term annealing period the doping concentration increased beyond the value that had been reached immediately after the exposure.

Submicron periodic structures produced on polymer surfaces with polarized excimer laser ultraviolet radiation

Abstract: For the first time, pure laser induced periodic structures (without any ablation or any larger structure) of submicron size (spacing and amplitude of 0.2 μm) are developed on polymer surfaces [poly(ethylene terephthalate), poly(butylene terephthalate) and polystyrene] by irradiation with one thousand pulses of the polarized beam of the excimer laser (193 and 248 nm). Fluence is below the ablation threshold and must be chosen in a narrow window which depends on the polymer and the wavelength. The obtained relief is observed by SEM, TEM and characterized by ellipsometry. Structures are obtained also by irradiation of thin films (2000 A) coated on silicon wafers.

Dynamic optical property changes: Implications for reflectance feedback control of photocoagulation

Abstract: During laser treatment, coagulation affects the optical properties of the tissue. In particular, the formation of a white lesion significantly increases the scattering coefficient. This change in the optical properties in turn affects the laser light distribution in the tissue. The white lesion formed during photocoagulation of the retina has a dynamic effect upon reflection and fluence rate. This problem has been simulated on a model medium consisting of a thin absorbing layer covered with a 1 cm thick layer of albumin. The albumin layer is subdivided into coagulated (white) and uncoagulated (clear) layers. The optical properties of each layer have been determined and these values have been used to model light distribution in the medium. One-dimensional adding—doubling and three-dimensional Monte Carlo methods have provided light distributions in the medium for varying thicknesses of the coagulated albumin. Computed fluence reaching the absorbing layer decreased in the presence of a 275 μm or thicker coagulated layer. The coagulated layer attenuates light because it is highly scattering; however, this scattering also leads to a sub-surface peak in fluence rate at a level higher than the incident fluence. The latter effect outweighed the former for coagulated layer thicknesses less than 275 μm. Computed reflectance of argon laser light from a semi-infinite coagulated region initially increased linearly as a function of thickness. As the coagulation thickness increased beyond 4–5 optical depths, the reflectance approached a constant value, R∞, at 9 optical depths (2 mm). Experimentally measured total reflectance is shown to be an inadequate indicator of the thickness of a lesion (finite coagulated volume); however, central reflectance from a lesion measured with a CCD camera confirmed the computed trends. These results provide a theoretical foundation for control of lesion thickness using reflectance images.

Characterization of laser vaporization plasmas generated for the deposition of diamond-like carbon

Abstract: Pulsed laser vaporization of graphite is rapidly emerging as an effective technique for the preparation of high quality diamond‐like carbon films. However, the dynamics of the process and mechanisms by which diamond‐like properties are obtained have not been well understood. The characteristics of the vapor plume generated by 248 nm KrF excimer laser irradiation of a graphite target are investigated using laser induced fluorescence and a Langmuir probe. It is found that the kinetic energy of the C2 molecule increases with laser fluence, reaching a value in excess of 12 eV in the moderate fluence range (3–5 J/cm2) employed for deposition. The Cn+ ions are 5–10 times more energetic and comprise ∼10% of the vapor flux. A notable finding is that irradiation of the surface at an angle of 70° with respect to the target normal increases the ion velocity when compared with 0° laser incidence at the same surface fluence. Analysis of the films prepared under such conditions supports the theory that diamond‐like fil...

Application of the convolution method for calculation of output factors for therapy photon beams.

Abstract: The output factor for a therapy photon beam is defined as the dose per monitor unit relative to the dose per monitor unit in a reference field. Convolution models for photon dose calculations yield the dose in units normalized to the incident energy fluence with phantom scatter intrinsically modeled. Output factors calculated with the convolution method as the dose per unit energy fluence relative to the calculated dose per unit energy fluence in a reference field could deviate as much as 5% if corrections are not made for perturbations due to treatment head scatter. Significant perturbations are particles backscattered from the collimators to the monitor and photons forward scattered from the filter and collimators in the treatment head. The forward scatter adds an "unmonitored" contribution to the total energy fluence of the beam. A model is developed that describes the field size dependence of these perturbations for conversion of output factors, calculated with the convolution method, to machine output factors as an integrated part in treatment planning. The necessary machine characteristics are derived from measurements of the output in air for a limited set of field sizes. The method has been tested using five different multileaf collimated irregular fields at 6 MV and for a large set of rectangular fields at 5, 6, and 18 MV and found to predict output factors with an accuracy better than 1%.

On single‐photon ultraviolet ablation of polymeric materials

Abstract: The nature of uv ablation of organic polymers is discussed in terms of a pseudo‐zeroth‐order rate law of the form dx/dt = k0e−(Eact/kT), where Eact is assumed to be the strength of the weakest bonds in the polymer and T is the local temperature increase from the incident laser pulse. Equations derived from previous models that assumed nonthermal photodecomposition were duplicated from this photothermal model. Even for the simple case of single‐photon absorption, nonideal behavior is affected by radiationless decay, pulse length, and thermal diffusion. These effects were probed. Results indicated that thermal diffusion may have a significant effect on the threshold fluence and to some degree on the shape of the etch depth versus fluence curve. Absorption dynamics (saturation and radiationless decay) appear to be the dominant factor in determining the functional dependence of etch depth on fluence. As a result of competition between absorption saturation and radiationless decay, the penetration depth is int...

Laser induced forward transfer: The effect of support-film interface and film-to-substrate distance on transfer

Abstract: A comparative study on metal pattern deposition of mm2-area by ablating chromium and titanium thin films from an optically transparent support and transferring the ablated material onto another substrate in close proximity with a single laser pulse (LIFT) is reported. The role of support-film interface and film-to-substrate distance in determining both ablation and transfer is discussed. The sequence of events as a function of processing fluence is interpreted by comparing experimental data with calculated temperature distributions. In the case of poorly adhering films the transfer yield is independent of film-to-substrate distance between 0 and 60 μm throughout the fluence range studied. The transmittance of the ablated areas of well adhering films decreases and that of the corresponding prints increases with increasing distance as evaporation becomes dominant.

Composition variations in pulsed-laser-deposited Y-Ba-Cu-O thin films as a function of deposition parameters

Abstract: The composition of pulsed-ultraviolet-laser-deposited Y-Ba-Cu-O films was examined as a function of position across the substrate, laser fluence, laser spot size, substrate temperature, target conditioning, oxygen pressure and target-substrate distance. Laser fluence, laser spot size, and substrate temperature were found to have little effect on composition within the range investigated. Ablation from a fresh target surface results in films enriched in copper and barium, both of which decrease in concentration until a steady state condition is achieved. Oxygen pressure and target-substrate distance have a significant effect on film composition. In vacuum, copper and barium are slightly concentrated at the center of deposition. With the introduction of an oxygen background pressure, scattering results in copper and barium depletion in the deposition center, an effect which increases with increasing target-substrate distance. A balancing of these two effects results in stoichiometric deposition.

Structural and electrical damage induced by high‐energy heavy ions in SiO2/Si structures

Abstract: The structural and electrical properties of SiO2/Si structures irradiated by high‐energy (≳0.5 GeV) Xe and Ni ions have been investigated. Structural analysis of the irradiated SiO2 films, performed with infrared spectroscopy, points to atomic displacements and broken and strained Si—O bonds induced by the irradiation. Using ir data, the damage cross section of the Xe and Ni ions has been deduced. The values are of about 8×10−13 and 6×10−14 cm2 for, respectively, 762 MeV Xe and 551 MeV Ni ions. Electrical measurements of irradiated SiO2/Si structures show an increase of the interface‐state density Dit and of the oxide trapped‐charge density N0t with the ion fluence. These results are compared with defects induced by heavy‐ion irradiation in bulk silica and by light particle radiation in silicon dioxide. Electrically active point defects have been detected in irradiated silicon and are associated with vacancy complexes. The structural and electrical properties of SiO2/Si structures irradiated by high‐energy (≳0.5 GeV) Xe and Ni ions have been investigated. Structural analysis of the irradiated SiO2 films, performed with infrared spectroscopy, points to atomic displacements and broken and strained Si—O bonds induced by the irradiation. Using ir data, the damage cross section of the Xe and Ni ions has been deduced. The values are of about 8×10−13 and 6×10−14 cm2 for, respectively, 762 MeV Xe and 551 MeV Ni ions. Electrical measurements of irradiated SiO2/Si structures show an increase of the interface‐state density Dit and of the oxide trapped‐charge density N0t with the ion fluence. These results are compared with defects induced by heavy‐ion irradiation in bulk silica and by light particle radiation in silicon dioxide. Electrically active point defects have been detected in irradiated silicon and are associated with vacancy complexes.

Damage and aluminum distributions in sic during ion implantation and annealing

Abstract: Damage buildup in SiC during ion implantation of Al + with an energy of 90 keV and flucnces of 10 13 –10 16 ions/cm 2 at room temperature was studied. It was observed that the surface concentration of displaced host atoms and χ min increase rapidly in the fluence range of 3 × 10 13 –10 15 cm −2 and change weakly outside this range. The amorphization of the surface layer occurred at a fluence of φ c ≈ 8 × 10 14 cm −2 . The thickness of the amorphous layer as well as the depth and the width of the Al-atoms profile exceed the values calculated lor an amorphous target. The possibility of recrystallizing the damaged layer depended on the ion fluence. The precipitation of Al-atoms at structural defects was observed.

Phenomenological models for matrix‐assisted laser desorption ion yields near the threshold fluence

Abstract: Phenomenological models were proposed to explain the experimentally observed dependence of protein ion yields with laser fluence in matrix-assisted laser desorption. Assuming that the illuminating laser had a Gaussian intensity profile at the sample being examined, it was possible to fit the experimental points with a model that only assumes a fluence threshold for ion production. No additional dependence of protein ion yield on illuminating fluence above the threshold value was necessary to explain the data.

Study of different ablation models by use of high-speed-sampling photography

Abstract: In our study we investigated the ablation characteristics of an aqueous dye solution with a defined absorption coefficient, irradiated by short (8 ns) and long (100 microsecond(s) ) pulses from a Nd:YAG laser (wavelength: 1064 nm). The experimental technique was schlieren photography with a second Nd:YAG laser at 532 nm as a light source and with a variable delay between the two laser pulses. With a special arrangement of the laser beams and the sample effects below and above the surface of the liquid could be simultaneously observed. We could distinguish three ablation mechanisms, depending on the pulse duration and the incident fluence. With short pulses and a fluence below the vaporization threshold the tensile pulse from the bipolar thermoelastic wave, propagating from the liquid-air interface into the sample, caused rupture and spallation of the liquid. At fluences generating a surface temperature in excess of 100 degree(s)C the short pulses caused explosive vaporization, characterized by shock wave emission both in air and in liquid. At the same fluence the long pulses caused slow vaporization, meaning that vapor and liquid ejection started during the laser pulse and was less violent than with the 8 ns pulses.

Diffusion of iodine into polyimide films modified by ion bombardment

Abstract: Polyimide films have been bombarded with a beam of Ar + ions at different ion fluences and energies ranging from 150 keV to 1.5 MeV. We have studied the diffusion of iodine into these films by Rutherford backscattering spectrometry using a 2 MeV beam of He + particles and we discuss the huge modifications of the diffusion profile in relation to the irradiation beam conditions. For films bombarded with a fluence of 2 × 10 14 Ar + /cm 2 and different irradiation energies increasing from 150 keV to 1.5 MeV, the in-depth distribution of iodine is shown to vary in accordance with the damage profile and to be confined in the irradiated thickness. However the RBS spectra of films irradiated at a constant energy of 150 keV indicate that the diffusion is rapidly blocked when incresing the ion fluence. Films irradiated with the same energy and ion fluence, but with different beams of rare gases, show that the diffusion is blocked at a lower fluence when increasing the mass of the projectiles. These evolutions are interpreted by a competition between two processes: the first results from the trapping of the diffusing iodine by the radiation damage and is dominant at low fluence whereas the second one is attributed to the compaction of the irradiated layer which reduces the diffusion at high irradiation fluences. Application to the formation of diffusion barriers is proposed.

Electron microscopy observations of titanium irradiated with GeV heavy ions

Abstract: GeV heavy ions induce the creation of damage in some metallic targets via electronic excitation. We report here on room temperature electron microscopy observations of titanium irradiated at 15 or 90 K by xenon, tantalum and lead ions. For sufficiently high electronic energy losses ( ⪆ 2.5 keV A −1 ), black dots aligned along the incident ion beam direction are observed. The ratio of the number of such alignments to that of impinging ions depends strongly on the irradiation conditions, namely the irradiation temperature and fluence. A tentative explanation of these observations is proposed. It involves the mechanism of point defect clustering resulting either from thermally activated migration or from athermal processes occuring in the wake of the incident ions.

Investigations of behavior of particles generated from laser‐ablated YBa2Cu3O7−x target using laser‐induced fluorescence

Abstract: YBa2Cu3O7−x target was laser ablated by an ArF excimer laser, and the time of flight (TOF) distributions of Ba atoms were investigated by laser induced fluorescence (LIF) as a function of the irradiation laser fluence (0.2–3 J/cm2) and the surrounding oxygen gas pressure (0–5.32 Pa). At higher irradiation fluence (≳2 J/cm2) without oxygen gas, the TOF distribution showed a twin‐peak distribution with fast and slow velocities, while only the slow velocity component was observed at lower fluence. When oxygen gas was introduced, the low velocity component was quenched at lower oxygen pressure than the high one, indicating that the TOF distribution can be controlled by the oxygen pressure.

Dopant-induced excimer laser ablation of Poly(tetrafluoroethylene)

Abstract: Poly(tetrafluoroethylene) (PTFE) does not exhibit excimer laser etching behavior at conventional, e.g., single photon absorption, emissions of 193, 248, and 308 nm, due to the lack of polymer/photon interaction. This is not surprising since the electronic transitions available to the PTFE molecule are high energy and thus require short wavelength the radiation However, by incorporating a small quantity of material into the non-absorbing fluoropolymer matrix that interacts strongly with the emitted laser energy, e.g., a dopant, successful ablation, both in terms of etch rate and structuring quality occurs. Specifically, excimer laser ablation of PTFE films containing 5, 10, and 15% polyimide (wt/wt) as a dopant was achieved at 308 nm in a fluence range of 1 to 12 J/cm2. Ablation rates for the materials increased with increasing fluence and, at the polyimide levels investigated, varied inversely with dopant concentration. All compositions exhibited excellent structuring quality.

Pulsed excimer laser deposition and characterization of ferroelectric Pb(Zr0.52Ti0.48)O3 thin films

Abstract: Lead zirconate titanate (PZT) thin films were prepared by excimer laser ablation on platinum coated silicon substrates. The composition of the films showed dependence on the fluence at low energy densities (<2 J/cm2), and less dependence on the ablation fluence was observed beyond a fluence of 2 J/cm2. A correlation among the fluence, ablation pressure, and substrate temperature has been established. Crystalline perovskite PZT films showed a dielectric constant of 800–1000, a remnant polarization of 32 μC/cm2, and a coercive field of 130 kV/cm. Films showed fatigue behavior that may be used in a device, and a close comparison of fatigue behavior between the films deposited at different energy densities indicated a better fatigue behavior for a fluence of 4 J/cm2.

High energy ion irradiation of germanium

Abstract: Irradiations of n- and p-doped germanium samples have been performed at room temperature at GANIL (Caen) using Ar (1415 MeV), Xe (5510 MeV and 751 MeV), Ca (272 MeV), Zn (691 MeV), O (195 MeV) and Kr (427 MeV) ions. The sample resistance and Hall mobility were measured in situ as a function of the ion fluence. In p-Ge samples the conductivity and the majority carrier density steadily increase up to the maximum fluences used (about 1012 cm−2). The conductivity σ of n-Ge samples first decreases, reaches a minimum at a fluence depending on the initial doping concentration and then increases. At the minimum value of σ, Hall measurements show a transition from n- to p-type conductivity. These results show that acceptor energy levels are created by irradiation induced defects. At low electronic stopping power values, the creation rate of acceptor levels is approximately a linear function of the calculated number of displacements per atom. This linear dependence becomes invalid at high electronic stopping power values. This last result could be explained by assuming a partial annealing of the defects for the largest electronic stopping power.

The effect of laser fluence on the ablation and deposition of YBa2Cu3O7

Abstract: The effects of laser fluence and shot number on the deposition of YBA2Cu3O7 are investigated. It is found that the evolution of target morphology during ablation is strongly dependent on fluence. Three ablation regimes are identified. In the low fluence region (fluence, less than 0.3 J cm−2, deposition is non-stoichiometric; at intermediate fluences ( 0.3 J cm −2 fluence J cm −2 ), both the stoichiometry and the deposition rate vary strongly with shot number; in the high fluence region (fluence, above 3 J cm−2), deposition is largely independent of shot number but particular contamination of films is a problem.

The polarity, field and fluence dependence of interface trap generation in thin silicon oxide

Abstract: The interface trap generation in thin silicon oxide (SiO) during high-voltage stressing has been measured for oxides fabricated on p- and n-type substrates as a function of stress polarity, field and fluence. The generation of midgap interface traps was independent of stress polarity and substrate type and was dependent on the fluence of electrons through the oxide modulated by the stress voltage. The interface trap generation rate varied as (fluence) − 1 2 . Generation rates of midgap interface traps as high as 1016 states eV−1C−1 were measured at low fluences with the generation rate dropping to 1012 states eV−1C−1 at high fluences as the oxide approached breakdown. The large variation in the interface trap generation rate suggested that high-voltage stress measurements may not be easily extrapolated to lower operating voltages. It has been proposed that the decrease in the trap generation rates as the fluence increased was due to the variable energies required for the breaking of bonds at the silicon-oxide interface. The polarity independence of the interface trap generation was in contrast to the polarity dependence observed in breakdown of SiO. Thus, breakdown in SiO was not correlated with interface trap generation.

Ion-beam effects on optical and rheological properties of polystyrene.

Abstract: High-energy (100-300 keV) ion irradiations in the fluence range 10 11 -5×10 14 ions/cm 2 produces optical defects in polystyrene films (MW=9000 amu). A linear dependence on ion fluence has been detected up to a defect density of 2×10 20 def/cm 3 . Defect production rate depends on the energy loss of incoming ions and the chemical yield (bonds/100 eV) increases from 0.07 to 0.28 going from an energy loss of 10 eV/A to 20 eV/A, being the difference related to the spatial energy distribution inside the ion track. Solubility and molecular weight distribution are modified after irradiation becase of the formation of cross-linkd between the main chains

Effects of light beam size on fluence distribution and depth of necrosis in superficially applied photodynamic therapy of normal rat brain.

Abstract: — The light fluence distributions of 632.8 nm light incident on the exposed surface of normal rat brain in vivo have been measured using an interstitial, stereotactically-mounted optical fiber detector with isotropic response. The dependence of the relative fluence rate on depth and the spatial distribution of fluence were compared for incident beam diameters of 3 and 5 mm. The fluence rate at depth of 1–6 mm along the optical axis within the brain tissue was approximately 70% greater for a 5 mm diameter beam than for a 3 mm beam, at the same incident fluence rate, although the plots of the relative fluence rate vs depth were parallel over the depth range 1–6 mm. The depths of necrosis resulting from photodynamic treatment of brain tissue using the photosensitizer Photofrin and irradiation by 632 nm light with 3 and 5 mm incident beams were also measured. The observed difference in necrosis depths was consistent with the measured difference in fluence. The importance of beam size in photodynamic treatment with small diameter incident light fields is discussed.

Photon field quantities and units for kernel based radiation therapy planning and treatment optimization.

Abstract: The problem of choosing radiation quantities and units for energy deposition kernels and their associated kernel densities is treated with the aim of making them consistent with related classical radiation quantities and units such as restricted mass stopping powers and mass attenuation coefficients It is shown that it is very useful to define the kernels h(r), in terms of the quotient of the mean specific energy imparted to the medium by the radiant energy incident on a volume element centred at the origin of the kernel The basic building block used to generate these kernels is the point energy deposition kernel, h(p), describing the spatial distribution of the energy imparted by a photon interacting at a point in a medium This will allow the kernels to be regarded as generalizations of the traditional mass stopping and attenuation coefficients, which in detail describe the spatial distribution of the mean energy deposition around an interaction site As a consequence, the irradiation or kernel density, f(r) should be expressed in terms of the radiant energy incident per unit volume of the medium It is shown that the kernel density is equal to minus the divergence of the incident unattenuated vectorial energy fluence, and it therefore acts as an irradiation density for the incident vectorial energy fluence The microscopic kernels or the irradiation density may thus be viewed as a perfect 'sink' distribution to the required incident photon energy fluence which is totally absorbed at f(r), and instead replaced by the kernels which describe the detailed energy deposition in the medium in coordinates centred at the sinks From these definitions the required incident energy fluence from an external radiation source used for treatment realization can be determined directly by projecting the irradiation density on the relevant positions of the radiation source This procedure has the valuable property that maximal calculational accuracy is achieved in the tumour because the irradiation density has non-zero values only in the tumour, and the accuracy of the kernel is highest at its origin

Structure, mechanical, and tribological properties of titanium implanted alumina

Abstract: A study of the effects of titanium ion implantation on the structural, mechanical, and tribological properties of single crystal and polycrystalline α-alumina has been carried out. Rutherford Backscattering Spectrometry (RBS) in channeling geometry shows that a great proportion of implanted titanium ions are substitutional at low fluence. This fraction falls to near zero as an amorphous layer is formed. The chemical states for implanted titanium are determined by X-ray Photoelectron Spectroscopy (XPS). Titanium is present in the Ti3+ state near the surface and as metallic Ti0 and as Ti3+ at depths corresponding to higher local concentration of titanium. The same behavior is observed for polycrystalline and single crystal α–alumina. Nanoindentation experiments show that low fluence implantation of titanium results in an increase of mechanical properties whereas high fluence implanted samples exhibit reduced hardness and Young's modulus compared to unimplanted samples. The friction coefficient is not changed by titanium ion implantation. Likewise, the wear characteristics were not changed by low fluence implantation, but amorphization at high fluence leads to a greater disk wear rate.

Incubation/ablation patterning of polymer surfaces with sub-μm edge definition for optical storage devices

Abstract: By exposure to low fluence UV laser radiation, the optical absorption coefficient of subsurface polymer material can be increased (“incubation”) with spatial control, using a suitable contact mask, proper imaging of the mask, or laser direct writing. Spatially selective ablation of polymethylmethacrylate (PMMA) is then achieved with large area XeCl excimer laser pulses at 308 nm. In this way, the transfer of spatial information to the material can be decoupled from the high laser fluence removal (“ablation”) step. The advantages are: The mask is exposed to only low fluence laser radiation — damage is avoided. Since the mask can be removed before the ablation step, mask contamination by the ablated plume cannot occur. Using this incubation/ablation method, PMMA surfaces can be patterned (248 nm/308 nm) with submicrometer spatial control and edge contrasts better than 0.2 μm. This has impact on optical storage technology and laser surface processing techniques in general. The smallest single structure obtained was somewhat smaller than 0.5 μm in diameter up to now, given by the mask.

Effects of post-desorption collisions on the energy distribution of SiCl molecules pulsed-laser desorbed from Cl-covered Si surfaces: Monte-Carlo simulations compared to experiments

Abstract: Si surfaces covered with up to a monolayer of chlorine by exposure to a low chlorine pressure have been irradiated with nanosecond excimer-laser pulses at a fluence just large enough to melt the surface. Angle-resolved time-of-flight (TOF) distributions and surface temperatures have been measured as a function of chlorine dose between laser pulses. The TOF distributions can be fitted well by Maxwell-Boltzmann (MB) distributions for all coverages and at all desorption angles. With increasing coverage, the intensity and kinetic energy distributions become increasingly peaked along the surface normal. Monte-Carlo simulations of the effect of post-desorption collisions, occurring when many molecules are desorbed within a very short time, reproduce the experimental results quite well. It is shown that just a few collisions per molecule are sufficient to convert any initial desorption distribution into a MB one.

Composition and structure of zirconium implanted with nitrogen at high fluence

Abstract: High-fluence implantations of 100 keV N2+ ion into a zirconium layer deposited by an electron beam were carried out at room temperature and at liquid-nitrogen temperature with and without nitrogen gas of a pressure of 1 × 10−4 Torr. Depth profiles of atom concentrations were determined by Rutherford backscattering spectrometry, Auger electron spectroscopy, and resonance nuclear reaction analysis. The experimental results were compared with the theoretical results obtained by Monte Carlo simulations of the dynamic-SASAMAL code. The depth profiles measured by RBS agreed well with the calculated profiles for all fluences from 1 × 1017 to 5 × 1017 ions/cm2, but differed from those obtained by AES indicating the existence of easily released nitrogen in the Zr specimen implanted with nitrogen at high fluence. Glancing angle X-ray diffraction was used to confirm zirconium nitride formation.