Showing papers on "Fluence published in 1983"

Kinetics of the ablative photodecomposition of organic polymers in the far ultraviolet (193 nm)

Abstract: The efficiency of ablative photodecomposition of poly(methyl methacrylate) caused by pulsed radiation at 193 nm from an excimer laser (argon–fluorine fill) has been studied as a function of fluence. A threshold for the onset of the process lies at 10 mJ/cm2 per pulse of 14 ns. The efficiency of the process increases rapidly with fluence up to 250 mJ/cm2 per pulse after which it levels off sharply. The etch depth of ∼3100 A/pulse at 250 mJ/cm2 can be compared to a depth of 4800 A/pulse at a fluence of >20 J/cm2/pulse. A model is suggested for the ablative photodecomposition of organic materials and the photochemistry of the process at different fluences is discussed. The variation in the efficiency with the composition of the polymer is briefly considered.

Self‐developing UV photoresist using excimer laser exposure

Abstract: Nitrocellulose functions as a self‐developing photoresist which can be patterned using pulsed excimer laser radiation. The material exhibits a threshold fluence for ablation of 20 mJ/cm2 at a wavelength of 193 nm; this threshold results in higher contrast than can be obtained with most conventional photoresists. The effect of varying the laser wavelength has been examined. A simple model of ablative development has been used to predict the etch rate. The processing stability of the resist has been increased without changing the optical development rate by the addition of a dopant. The resolution of the resist is better than 0.3 μm.

Dye stability under excimer-laser pumping

Abstract: The stability of polymethine dyes under XeCl laser excitation has been investigated (For these dyes emitting in the ir the difference between absorbed and emitted photoenergy is 2 eV) The stability was measured in an amplifier device operating in the saturated regime While the influence on dye concentration is negligible, the fluence dependence shows the importance of two photon absorptions A meausure for the stability is the number of photons which can be emitted per dye molecule before the dye solution has degraded to 50% of the initial value This value is in the range of 150–280 The stability is clearly related to the formation of photoproducts absorbing at the pump and the laser wavelength A model starting from the change in the dye solution absorption spectra is in very good agreement with the observed energy decays

Electrical properties of ion‐implanted poly(p‐phenylene sulfide)

Abstract: Ion implantation of impurities into thin films of poly(p-phenylene sulfide) (PPS) is found to increase the conductivity of the material by up to 12 orders of magnitude. The increase is stable under exposure to ambient conditions, in contrast to the instability of the conductivity increases in PPS produced by chemical doping with AsF5. PPS films 0.1–0.2 μm thick are spin cast from solution onto interdigitated electrodes patterned on an oxidized silicon substrate. The room-temperature interelectrode resistance is measured as a function of implantation fluence. An estimate of film conductivity is obtained from this resistance with a simple model for the electrode and film geometry. A first experiment yielded similar conductivity increases for implantation of either arsenic or krypton. At a fluence of 1 × 1016cm−;2, which corresponds to an average impurity concentration of 2.5 × 1021cm−3, the conductivity reaches an apparently saturated value of 1.5 × 10−5 (Ω cm)−1. Infrared spectra of the films before and after implantation suggest that crosslinking may be present in the implanted films, and Auger studies show stoichiometric changes throughout the implanted layer. These results suggest that the observed conductivity changes are the result of molecular rearrangements produced by the implantation rather than the result of specific chemical doping. Specific chemical doping may, however, explain the results of a second experiment in which implantation of bromine resulted in substantially larger conductivities found to increase at an approximate linear rate from a value of 1.0 × 10−4 (Ω cm)−1 at a fluence of 1 × 1016 cm−2 to a value of 4.0 × 10−4 (Ω cm)−1 at a fluence of 3.16 × 1016 cm−2.

Distribution of nitrogen implanted in iron as a function of fluence

Abstract: The effect of the fluence on the evolution of the distributions of 40 keV nitrogen-ions implanted into iron has been investigated. The distributions were determined by using the 15N(p, αγ)12C nuclear resonance reaction at 429 keV. The study shows a considerable reduction in the saturation areal density of ions retained, in relation to the simplest ion collection model. The mean sputtering yield for a 5–1017 ions-cm−2 fluence has been determined by 2 MeV-4He backscattering. Some experiments of isotopic marking by 14N and 15N implantations have brought some interesting features concerning the nitrogen distribution and the mixing. Two simple models were designed in order to justify our experimental results, but only one is presented here. This model is based upon a nitrogen outdiffusion linked to chemical reactions under beam.

High-fluence he-implantation in Ni trapping, re-emission, and surface modification

Abstract: Simultaneous measurements have been made of blister formation and gas re-emission during bombardment of Ni with 8–40 keV 3He+ at room temperature. It is shown that re-emission and blister formation start at the same critical fluence. An initial peak in the re-emission rate is correlated to a step in the total number of blisters. A more detailed analysis of the experimental data shows that the area contributing to gas re-emission is much larger than the blister area. This suggests the formation of an interconnected layer below the surface in which cracks at blister sites provide gas emission channels to the surface.

Laser-induced nonlinear absorption in silicon: Free-carrier absorption versus thermal effects

Abstract: The optical-absorption coefficient of silicon has been determined under moderate ($\ensuremath{\lesssim}0.15$ J/${\mathrm{cm}}^{2}$) nanosecond laser excitation, between 2.0 and 2.6 eV. With respect to the cw low-power value, an enhancement under irradiation is observed on the whole range of fluence and wavelength. This enhancement builds up with the pump pulse and persists a long time after its end. It is shown that the features of the effect, i.e., dependences on time, fluence, and wavelength, can be interpreted by a thermally induced absorption enhancement. This mechanism is operative above 2.0 eV, whereas, under this value, the free-carrier absorption process is dominant.

Acoustic signals from laser-annealed amorphous silicon

Abstract: Acoustic signals generated by Q‐switch laser irradiation of amorphous Si have been investigated as a function of incident laser fluence. Marked discontinuities in the signal at the recrystallization threshold and at the damage threshold are found. The results indicate that the first effect is due to melting and the second is due to evaporation.

Thermal desorption spectroscopy of he from Ni at and below saturation

Abstract: Trapping of helium after implantation at energies of 8 to 150 keV and fluences up to 1019 He-ions/cm2 in nickel at room temperature is studied by measuring the thermal desorption spectra during linear heating up to 1000°C. At several annealing stages the trapped helium is measured by means of the nuclear reaction 3He(d, α)H and the target surface is observed by laser scattering and with the scanning electron microscope. The thermal desorption spectra depend strongly on the implantation fluence but only slightly on the implantation energy, indicating a similar trapping of He in the lattice for the implantation energies used here, The temperature at which desorption starts decreases with increasing fluence. Above the critical fluence for blistering an additional low temperature (150°C) desorption maximum is found. The desorption peak at 150°C can be approximated theoretically with a single jump desorption process of first order and a Gaussian distribution of activation energies around 1 eV. The mea...

Magnetic Properties of Iron Implanted Polymers and Graphite

Abstract: We have measured the magnetic properties of highly oriented pyrolytic graphite (HOPG), polyethylene (PE), and polyvinylidine fluoride (PVF2), implanted with fluences of 25 keV iron atoms ranging from 1016 to 1017 atoms/cm2 . The lowest fluence specimens were paramagnetic down to 2 K, with evidence for clusters of only a few spins, while the highest fluence specimens were clearly ferromagnetic, with magnetization curves resembling those of a set of randomly oriented soft magnetic planes. The critical fluence for formation of a ferromagnetic state appears to be between 1 and 3 × 1016 atoms/cm2 at 25 keV. Theseresults can be qualitatively understood based on the critical density for percolation of near neighbor exchange interactions.

Fluence and absorbed dose in high energy electron beams.

Abstract: The depth dependence of fluence, vectorial and planar fluence, energy fluence, vectorial and planar energy fluence and absorbed dose has been calculated in 5 and 20 MeV electron beams in water. The shape of these distributions has been compared with analytic expressions for fluence and absorbed dose and the general mechanisms governing the shape of the depth dose curve have been explained. In particular it is demonstrated that the depth dependence of planar energy fluence and mean energy is very similar, and so is the case for fluence and absorbed dose and primary fluence and absorbed dose from primaries with inclusion of fast secondaries. The results are useful for dosimetry and dose planning in high energy electron beams.

Dependence of laser‐induced damage of surface layers of GaP on pulse width and wavelength

Abstract: Disorder induced by irradiation with a laser pulse on surface layers of GaP has been studied by means of the Rutherford backscattering channeling technique. It is found that a 0.6‐μs laser pulse near the direct band gap and a 15‐ns laser pulse near the indirect band gap induce damages more pronounced than a 15‐ns laser pulse near the direct band gap of the same fluence. Moreover, decomposition of the surface layers into Ga‐rich and P layers is found to be induced by irradiation with a 0.6‐μs laser pulse. It is suggested that such a decomposition occurs when heating is not substantial.

Directional dependence of radiation fluence from 192Ir and 198Au sources.

Abstract: Radiation fluence distribution around 192Ir and 198Au seeds of the type used for brachytherapy application was measured using a NaI crystal connected to a multi-channel analyzer The radiation fluence, determined as a function of the angle between the seed axis and the direction of detection, is anisotropic with the fluence along the seed axis amounting to 78% of that perpendicular to the seed axis for seeds of both isotopes The region of fluence reduction is broader for 198Au seeds, extending from 0-45 degrees; for 192Ir the decrease in fluence is evident for angles of 0-20 degrees Integration of the angular distribution over 4 pi geometry indicated the effective activity of the implants to be 4% and 1% less than the nominal activity for the 198Au and 192Ir seeds, respectively, based on calibration measurements perpendicular to the seed axis

Ion Mixing Processes

Abstract: We consider ion mixing in the low temperature regime, where it is insensitive to temperature. Mixing of most thin markers is Gaussian, independent of irradiation flux, and varies linearly with fluence. However, the mixing in some media varies widely between markers of similar mass and appears to correlate with thermal diffusion constants. In bilayer systems, the profile of long-range mixing is exponential, and the number of mixed atoms scales linearly with fluence. This can be modeled successfully with simple collisional theory. Short range mixing scales with the square root of the fluence, but again shows strong correlations with known bulk chemical properties. We conclude that chemical driving forces and low energy transport mechanisms such as interstitial migration play major roles in ion mixing, even at low temperatures.

Model for calculating depth dose distributions for broad electron beams

Abstract: Central axis electron beam depth dose distributions can be transformed by replacing dose by fluence and depth by a measure of angular dispersion. This transformation was applied to a set of broad beam central axis depth dose distributions calculated by Monte Carlo code for beams with initial energies ranging from 1 to 60 MeV in homogeneous media of water, aluminum, and copper. The resulting fluence distributions belong to a family of curves that can be parametrized by a single-valued function of initial beam energy and medium and can be used to calculate fluence distributions for accelerators. Fluence curves can be easily transformed to depth dose curves.

Rapid Thermal Annealing of Ion Implanted GaAs and InP

Abstract: This paper discusses the properties of high intensity lamp-annealed silicon or beryllium-implanted GaAs and InP samples. We find this annealing process can result in efficient activation of dopants. Conventional furnace annealing at the same temperature does not result in increased electrical activation of the dopants. High fluence silicon implants can be activated in anneal times as short as 2 seconds, while low fluence silicon implants require more extended annealing. Activation of low fluence implants in GaAs depends strongly on the properties of the bulk semi-insulating material.

Ion irradiation smoothing and film bonding for laser mirrors.

Abstract: Irradiation with high energy heavy ion beams has been investigated as a technique for improving the quality of highly reflecting metallic surfaces to be used as laser mirrors. Properties such as reflectivity, corrosion resistance, film bonding, and threshold to laser surface damage have been examined. Modifications of composition and microstructure of the material associated with the heavy ion irradiation have been measured with RBS, TEM, SEM, Auger, and ESCA. Reflectivity and extinction coefficient measurements were made using ellipsometry techniques. Observations indicate that keV heavy ion irradiations in the fluence range of 1015 to 1016 cm−2 produce significant surface smoothing. Additionally, MeV implants of heavy ions into films of Cu, Ag, Au and Al deposited on molybdenum substrates resulted in improvements to both tarnish resistance and structural bonding integrity.

Simple model of space radiation damage in GaAs solar cells

Abstract: A simple model is derived for the radiation damage of shallow junction gallium arsenide (GaAs) solar cells. Reasonable agreement is found between the model and specific experimental studies of radiation effects with electron and proton beams. In particular, the extreme sensitivity of the cell to protons stopping near the cell junction is predicted by the model. The equivalent fluence concept is of questionable validity for monoenergetic proton beams. Angular factors are quite important in establishing the cell sensitivity to incident particle types and energies. A fluence of isotropic incidence 1 MeV electrons (assuming infinite backing) is equivalent to four times the fluence of normal incidence 1 MeV electrons. Spectral factors common to the space radiations are considered, and cover glass thickness required to minimize the initial damage for a typical cell configuration is calculated. Rough equivalence between the geosynchronous environment and an equivalent 1 MeV electron fluence (normal incidence) is established.

Photoelectric Emission Studies from Crystalline Silicon at 266 NM

Abstract: Three different photoelectric regimes are observed in the interaction of 15 ps, 266 nm laser pulses with crystalline silicon samples versus light fluence. A superposition of linear and quadratic photoionization is followed by a space charge limited regime up to the critical fluence $$F_{th}^{4w}$$ for the surface amorphization where highly nonlinear ion emission is observed. Ion and electron emissions become equal in magnitude at a fluence $$\sim\;F_{th}^{4w}$$ . The absence of observable thermionic effects indicates that thermal equilibrium of the electron-hole plasma and the lattice is reached during the laser pulse duration.

Ultra-Violet Induced Insulator Flashover as a Function of Material Properties

Abstract: : Previous work has shown that a small fluence of UV photons is sufficient to initiate flashover at the insulator/vacuum interface of a vacuum insulated transmission line. 1 In this paper we examine the behavior of polyethylene, polystyrene, and teflon insulators in which the dielectric/vacuum interface was oriented at a 45-degree angle to the applied electric field. Field stress was varied over the range 10-70 kV/cm, both positive and negative polarity. The insulators were exposed to various UV spectra with photon energy up to 12 eV. All insulator materials were unaffected by radiation with photon energy 3 eV or less regardless of power, The geometry which withstood the greatest photon fluence was, for every material, the inverse of the geometry suggested as preferable by applying conventional DC breakdown criteria. A model explaining the polarity phenomena as well as the insulators' behavior under various spectra is proposed.