Showing papers on "Fluence published in 1988"

Optical limiting with semiconductors

Abstract: We present a detailed characterization of the output of passive semiconductor-based optical limiters. These devices utilize two-photon absorption along with photogenerated carrier defocusing within the material to limit the output fluence and irradiance. In addition to protecting downstream optical components, the focusing geometry combined with these nonlinearities makes the devices self-protecting. Such devices have a broad working wavelength range since both the initial two-photon absorption and the subsequent carrier refraction are slowly varying funtions of wavelength. For example, ZnSe should have a useful range of from 0.5 to 0.85 μm. In this material we have observed the onset of limiting at input powers as low as 80 W when using 10-nsec, 0.53 μm input pulses. At the same wavelength, when 30 psec pulses into a monolithic ZnSe limiter are used, limiting begins at ≃300 W or 10 nJ. We also monitored the output spatial energy distribution along with the temporal response at each position, using a 2-psec-resolution streak camera. We found that the output fluence along with the output irradiance is effectively limited below detector damage thresholds over an input range of 4 orders of magnitude. Additionally, since both two-photon absorption and the associated self-defocusing increase with decreasing band-gap energy, similar devices using narrow-gap semiconductors should have considerably lower limiting thresholds.

Pulsed CO2 laser tissue ablation: measurement of the ablation rate.

Abstract: Ablation of guinea pig skin using a CO2 laser emitting 2-mu sec-long pulses has been quantified by measuring the mass of tissue removed as a function of incident fluence per pulse. The mass-loss curves show three distinct regimes in which water evaporation, explosive tissue removal, and laser-induced plasma formation dominate. The data are fit to two models that predict that the mass removed depends either linearly or logarithmically on fluence. Although the data are best fit by a linear dependence upon fluence, plasma formation at high fluences prohibited obtaining data over a wide enough fluence range to differentiate unambiguously between the two models. Ablation efficiency, ablation thresholds, and the optical penetration depth at 10.6 micron were obtained from the measurements.

Excimer laser light induced ablation and reactions at polymer surfaces as measured with a quartz-crystal microbalance

Abstract: We have developed a new technique to measure the etch rate of polymer films under the ablative photodecomposition condition obtained by absorption of the far‐UV radiation of the excimer laser. The technique is based on the use of a quartz‐crystal microbalance interfaced with a microcomputer. It has high accuracy (17 ng/cm2). Etch rates of poly(ethylene terephthalate), polycarbonate, and polystyrene were measured as a function of the laser intensity, at 193 and 248 nm. The etch rate in the region of the ablation threshold intensity is obtained precisely. A slow ablation regime between the threshold and the linear regime is evidenced. For polycarbonate and polystyrene, a reaction of the irradiated surface with molecular oxygen accelerates the ablation process at low fluence of the 248 nm radiation. The degradation reaction of poly(ethylene therephthalate) is monitored as a function of fluence by selective dissolution in acetone. A sudden increase of the degraded depth is seen at the transition between the t...

Theory of polymer ablation

Abstract: A new formula is presented for the etch depth l per pulse of an excimer laser of fluence F. Incremental ablation is defined as the etch depth per pulse after many pulses. We show that l is proportional to F, rather than ln(F).

Systematics of thin films formed by excimer laser ablation: Results on SmBa2Cu3O7

Abstract: Excimer laser ablation was used to prepare thin films from a bulk target of SmBa2Cu3O7. The systematic variation in film thickness, stoichiometry, and microstructure, as a function of laser fluence and angle from the target surface normal, was determined. We show that films which are stoichiometric over wide solid angles can be rapidly prepared from this material by use of high laser fluence. Films prepared from various Bi‐Sr‐Ca‐Cu‐O targets at high laser fluence also reproduce bulk stoichiometry. In contrast, films prepared by this technique from a YBa2Cu3O7 target are deficient in Ba and Cu for laser fluences required to produce compositional homogeneity over wide areas.

Studies of Raman scattering from overdense targets irradiated by several kilojoules of 0.53 μm laser light

Abstract: In this paper a study of stimulated Raman scattering (SRS) from relatively planar plasmas irradiated with short‐wavelength (0.53 μm) laser light is reported. The Novette Laser Facility [Laser Part. Beams 3, 173 (1985)] produced several kilojoules of light in 1 nsec, which allowed it to irradiate a large spot with enough intensity to produce significant Raman scattering. These experiments measured the fluence, angular distribution, spectrum, and timing of the Raman light, as a function of the average laser intensity. Reductions in the Raman fluence at low laser intensity are attributed to collisional damping. The measured SRS fluence was larger than that predicted from convective amplification of bremsstrahlung noise, as calculated using the average properties of the laser beam and the plasma. Possible contributions to the observed scattering from enhanced noise, Raman scattering within filaments, and the absolute Raman instability at density extrema are discussed.

Controlled etching of silicate glasses by pulsed ultraviolet laser radiation

Abstract: Fused glass which is used as an insulation layer in the electrical packaging of integrated circuits can be precisely patterned by etching with pulsed, ultraviolet laser radiation. The glass absorbs weakly at both 193 and 248 nm lines of the excimer laser. Etching by 193 nm pulses (∼20 ns FWHM) begins at a threshold fluence of 0.5 J/cm2 and rises to 1200 A/pulse at ∼2.5 J/cm2. The etch depth increases linearly with the number of pulses and the tolerance achieved is better than 5 μm. The etched holes are well‐defined in outline. Etching by 248 nm pulses has a threshold fluence of ∼1.2 J/cm2. The etched pattern is irregular and shows damage due to shattering that is well outside the irradiated area. The etch depth is difficult to control since the surface that is produced is uneven. The maximum etch depth per pulse that was realized was only 150 A. The etching by 193 nm laser pulses is believed to depend on the creation of color centers which strongly increases the absorption cross section.

Radiation damage test of silicon multistrip detectors

Abstract: Radiation damage test of silicon multistrip detectors were performed using an 800 GeV proton beam. The local proton fluence was up to 1014/cm2. The observed prominent changes were the proportional increase of the leakage current with the integrated beam intensity and the change of the effective impurity density. The effective impurity density decreases with fluence up to ≈4×1013/cm2 but for greater fluences, it increases. This may indicate the type conversion of the bulk silicon. We have also observed the change of the carrier collection properties, which may be caused by the synergistic effect of the charge-up of surface SiO2 and the decrease of the effective impurity density in bulk silicon.

Amorphization of Silicon by Ion Irradiation: The Role of the Divacancy

Abstract: Fixed fluence ion irradiation of silicon is shown to produce either defected crystal or amorphous silicon depending on the ion flux employed. The amorphous threshold flux, defined as the minimum flux required to generate a continuous amorphous layer for a fixed fluence irradiation, is measured as a function of irradiation temperature. This critical flux for amorphization is shown to satisfy an Arrhenius expression with a unique activation energy of ∼1.2eV, which corresponds to the migration/dissociation energy of the silicon divacancy. These observations lead to the conclusion that the stability of the silicon divacancy controls the competition between defect production and dynamic defect annealing, and hence the crystalline to amorphous phase transformation.

Production of 0.1–3 eV reactive molecules by laser vaporization of condensed molecular films: A potential source for beam-surface interactions

Abstract: A versatile, repetitively pulsed source of translationally fast, reactive molecules is described that is suitable for materials processing experiments. The pulsed beams are generated by excimer laser vaporization of cryogenic molecular films that are continuously condensed on transparent substrates. The generation of fast, energy variable pulsed molecular sources of Cl2 and NO is demonstrated. The most probable translational energies of Cl2 and NO molecules can be reproducibly varied monotonically by adjusting the laser fluence or film thickness. Here, the most probable translational energy is quoted as the energy corresponding to the maximum of the time-of-flight trace. Using laser fluences of 2–25 mJ cm−2 from a 193 nm excimer laser, the most probable translational energies of Cl2 are 0.4–2 eV. Significant fractions of molecules with translational energies greater than 3 eV are observed at the leading edges of the distributions. Very similar results are obtained by vaporizing Cl2 with 248 and 351 nm radiation. Pulses of translationally fast NO molecules are generated in a similar manner; most probable energies from 0.1–0.4 eV, with the fastest molecules up to 0.8 eV, are obtained using laser fluences of 1–11 mJ cm−2 at 193 nm. Approximately 1013−1014 molecules per cm2 of the film are vaporized per laser pulse, depending on film thickness and laser fluence.

The effects of high‐fluence neutron irradiation on the superconducting properties of magnetron sputtered NbN films

Abstract: Neutron irradiation experiments on sputtered NbN films up to a fluence of 1023 m−2 (E>0.1 MeV) are reported. Both the transition temperatures Tc and the normal state resistivities change only slightly with neutron fluence. While the critical current densities degrade (up to 30%) in an intermediate field range (6–14 T), the high field data show virtually no change of Jc due to the radiation‐induced increase of Bc2 and the occurrence of peak effects near the upper critical field. It is therefore proven experimentally that NbN is an extremely radiation‐hard high field superconductor.

Effects of varying argon ion laser intensity and exposure time on the ablation of atherosclerotic plaque.

Abstract: Using continuous wave (CW) argon ion laser light, a total of 253 laser exposures of varying power (1.5, 3, 5, 8 or 10 W) and duration (20-1,333 ms) were delivered to four segments of human atheromatous aorta obtained at autopsy. Exposure conditions were controlled by using an optically shielded laser catheter that provided a 500 micron spot of light of known power. Two thresholds for consistently reproducible ablation could be defined-an intensity threshold at 25.5 W/mm2 and a fluence threshold at 3.2 J/mm2. Above threshold, a fluence of 5.1 J/mm2 was found to produce the most efficient ablation, ie, removed the greatest volume (mm3) per energy delivered (J) compared to other fluence levels employed (p less than 0.0001). Between aortic segments, however, considerable variability in efficiency (mm3/J) was observed, possibly owing to different optical properties and/or plaque composition. Low-intensity laser radiation produced inconsistent ablation and extensive coagulation effects to surrounding tissue. When a fluence of 5.1 J/mm2 was constructed with a high-intensity laser beam and a short exposure time, consistent and efficient tissue removal resulted without histologic evidence of coagulation necrosis.

The anomalous thermal radiation from metals produced by ultrashort laser pulses. I

Abstract: The spectral-time characteristics of the secondary radiation induced in silver and tungsten by picosecond laser pulses of varying duration and fluence have been studied theoretically and experimentally. It is established that the secondary radiation is due to heating, but in silver, nevertheless, it is not usual and does not correspond, for example, to grey-body radiation. This radiation—the anomalous thermal radiation proceeds by the following mechanism: When the electron and phonon subsystems in a metal are heated (including also the nonisothermal process), there appears a glow with a continuous spectral distribution and an intensity exceeding the radiation intensity of the grey body whose temperature is equal to the temperature of the ion or electron subsystem of a metal. This anomaly is either due to overheating of the electron subsystem with respect to the ion subsystem or due to recombination of electrons and thermions in a microlayer above the metal surface.

Ultraviolet laser‐induced ion emission from silicon

Abstract: Si and Ge ion ejection is observed to occur from clean silicon and germanium surfaces upon excimer laser irradiation at fluences (0–150 mJ/cm2) well below that necessary to cause thermionic emission or melting of the substrate. Quadrupole mass spectrometric techniques were employed to for the detection of emitted positive ions. Laser intensity dependent measurements were performed at 193, 248, and 351 nm to elucidate the laser fluence and photon energy threshold behavior of each of the charged species.

Effect of excimer laser alloying of Ti on the sliding friction of AISI 304 stainless steel

Abstract: Dry sliding friction measurements made on titanium layers evaporated on AISI 304 stainless steel in the as-deposited and excimer laser mixed form show a dependence on the film thickness and the amount of mixing. The effect of laser mixing is dependent on the incident fluence with high fluences and/or large numbers of pulses producing surfaces with poor frictional properties. The optimum total fluence depends on the thickness of the surface layer, a result consistent with thorough mixing of the alloyed layer without the surface damage that results from large numbers of pulses.

A time-of-flight study on the nanosecond laser induced etching of Cu with Cl 2 at 308 nm

Abstract: Chemical etching of Cu is studied using Cl2 and a ns pulsed UV laser at 308 nm. At Cl2 pressures in the range of 10−6–10−4mbar and a laser fluence up to 0.82 J/cm2 the velocity distributions of the ejected species are determined. CuCl and Cu3Cl3 are the main products. The time-of-flight spectra of these particles can be fitted with Maxwell-Boltzmann distributions at high temperatures viz. 1750

Effects of irradiation — fast neutrons and implantation on sintered YBaCuO superconductors

Abstract: Effects of irradiation damage by fast neutrons, and nitrogen implantation on magnetic and electrical properties of Y Ba Cu O materials have been studied. The samples were subjected to a fluence of 2 × 10 17 and 1 × 10 17 neutrons / cm 2 . In the implantation studies dosages of 10 15 and 10 16 ions/ cm 2 of N + were used. Here we will report on the detailed changes in the magnetic hysteresis loop due to irradiation. It is found that the transport inter-grain critical current decreases with irradiation, while the critical current inside the grains is enhanced.

Phase transformation in He + and H + ion irradiated type 304 stainless steel

Abstract: γ (fcc)→α (bcc) phase transformation in type 304 stainless steel has been observed after irradiation of He+ and H+ ions up to fluence levels of 1017 and 1019 ions/cm2, respectively. Depth selective conversion Mossbauer spectroscopy and surface-sensitive X-ray diffractometry were employed to study the effect of irradiation. It is shown that the amount of the ion induced phase is highly sensitive to the fluence, the ion species and depth from the surface. It is worth noting that H+ ion irradiation is rather ineffective in inducing the transformation.

Trapping of hydrogen implanted into titanium

Abstract: Depth profiles and annealing behaviors of hydrogen implanted into titanium samples at an energy of 10 keV per nucleon were measured in the implantation temperature and fluence ranges of 77–400 K and of 6.3×1016–3.3×1018 H/cm2, respectively. Trapping enthalpies are deduced from the annealing curves. It was found that the trapping mechanism of hydrogen implanted into Ti depends on both the temperature and the fluence at the implantation. When implanted at a temperature sufficiently higher than 273 K, hydrogen moves from the surface layer to an underlying bulk and precipitates as hydride irrespective of fluence. On the other hand, at a temperature around or below 273 K, the implanted hydrogen atoms are trapped by the vacancies at a low concentration of hydrogen. However, hydride formation is realized where the local atomic ratio of H/Ti becomes close to the stoichiometric value of TiH2.

Excimer laser direct etching of GaAs

Abstract: The direct etching of GaAs substrates by excimer laser light has been studied at wavelengths of 193 nm (ArF). 248 nm (KrF) and 351 nm (XeF). For absorbed laser fluences greater than the direct etching fluence thresholds of 23 mJ cm-2 for ArF, 33 mJ cm-2 for KrF and 80 mJ cm-2 for XeF, the authors observe direct etching in air to depths of order one nanometre per pulse. For low-fluence exposures in either vacuum (10-2 Torr) or N2 (1 atm) environments the etch depth per pulse is less than a quarter of that observed when exposure takes place in air. For higher fluence exposures ( approximately 250 mJ cm-2) the etched regions show signs of melting. Using a line-narrowed KrF excimer laser and holographic interference techniques gratings of period 1.5 mu m were directly etched in GaAs substrates.

Boron Doping of Silicon Using Excimer Lasers

Abstract: The use of lasers in the doping of semiconductors has been investigated extensively these last years both for photovoltaic and microelectronic applications. In this work, doping of single-crystal silicon in BCl3 ambients using a pulsed UV laser has been studied as a function of laser wavelength and fluence in order to investigate the effects of photochemical decomposition of the BCl3 gas and the effects of thermal decomposition of adsorbed layers on the doping process. Different parameters involved in the process (laser energy density, number of pulses per frame, BCl3 gas pressure) were investigated. The electrical characteristics of the doped layers were discussed.

Molecular beam study of laser-induced chemical etching of Si(111) by chlorine molecules

Abstract: Chemical etching of Si(111) surface by chlorine molecules under 355 and 560 nm irradiation has been studied using a continuous wave supersonic molecular beam. Only two products, SiCl and SiCl2, were observed. The translational energy distributions of the gaseous products have been measured as a function of laser fluence, and can be fitted with Maxwell–Boltzmann distributions. Study on the effect of translational energy of incident chlorine molecules on the reaction rate is also presented for the first time.

Projection patterned Si doping of GaAs in ambient SiH4 gas by a KrF excimer laser

Abstract: Potential application of a projection system with reduction optics to the patterned doping process has been demonstrated. A KrF excimer laser beam is projected on GaAs substrates enclosed in a SiH4 gas ambient cell to achieve the Si doping with a field size of 5×5 mm2. At a laser fluence of 380 mJ/cm2, the n‐type conduction layer with a surface carrier density of 2.27×1014 cm−2 in the semi‐insulating GaAs substrate and with an activation efficiency of ∼81% can be obtained. The minimum linewidth of 2.5 μm is discussed together with temperature profiles calculated by transient heat conduction in the substrate.

ESR and track-etch studies of irradiated polymers

Abstract: Radiation damage of the polymer CR-39 (allyl diglycol carbonate), due to neutrons and alpha-particles, and of Kapton due to He + , Ar + , and Xe + ions is examined by electron spin resonance (ESR) and track-etch (chemical) methods. Characteristic electron spin resonance signals depend upon fluence, and are subject to annealing. Particle track-etch rates vary with annealing temperature and time. It is shown for the first time that there is a clear correlation between the ESR signal and the corresponding track registration and retention properties of a polymer, regarded as a detector. Strong evidence is also adduced for the separating-out of nuclear and electronic stopping. Finally, the dependence of radiation damage as a function of fluence (dose) and annealing temperature is studied by using scanning electron microscopy (SEM) and interactive image analysis (INTIMAN).

Improvement of Tc in the Grain Surface Superconductor La2CuO4 by GeV Heavy-Ion Irradiation

Abstract: The critical temperature of La2CuO4 specimens showing a grain surface superconductivity after annealing under normal O2 pressure can be improved by 2.9 GeV krypton ion bombardment. The midpoint temperature has thus been raised from 32.5 to 35.0 K by a fluence of 4 · 1012 cm-2. Such a result probably enhanced by the high value of the electronic stopping power might be related to the pressure effect of the defects induced by irradiation and/or the existence of a spin density wave phase.

Temperature Dependence of Ion Assisted Epitaxial Growth of Chemical Vapor Deposited Si Layers

Abstract: Thin layers of Si were chemical vapor deposited onto as - received p-type Si wafers and implanted with 80 KeV of As or Ge to a fluence of 1 × 1015 /cm2. Irradiation at 450°C with 600 KeV Kr++ ions causes the epitaxial growth of the entire deposited and amorphized Si layer. At lower irradiation temperatures the regrowth rate of the deposited layers is substantially reduced with respect that of the implanted amorphous layers. The presence of As enhances the regrowth rate of a factor 2.5. The results are explained qualitatively in terms of a dynamical bond breaking of SiO2, and of a dopant influence on the migration energy of the defects responsible for the growth.