Showing papers on "Fluence published in 1985"

Excimer laser etching of polyimide

Abstract: It is reported that thin films of polyimide are efficiently etched in air at pulsed excimer laser wavelengths of 248, 308, and 351 nm. Etch rate versus incident fluence data are found to obey a Beer–Lambert etching relation. Sharp laser fluence thresholds for significant etching are found to correlate with the wavelength‐dependent absorption coefficient. The absorbed energy density required to initiate significant etching is found, within experimental error, to be independent of the wavelengths examined. It is felt that this information demonstrates the predominantly thermal nature of the laser etching mechanism. Additionally, infrared spectroscopy and coupled gas chromatography/mass spectroscopy were used to identify several gases evolved during pulsed laser etching of polyimide in both air and vacuum.

Excimer laser ablation and thermal coupling efficiency to polymer films

Abstract: Thermal coupling and etch rate measurements are reported for polyethylene terephthalate and polyimide films irradiated at excimer laser wavelengths of 193, 248, and 308 nm. Thermal energy balance is observed up to a threshold fluence but above this the energy absorbed remains approximately constant, the excess energy being carried away by the ablated material. The ablated material appears to have a temperature >103 K as determined by calculations based on the thermal energy loading and relaxation time estimated from IR measurements. These results provided useful information on the interaction mechanism.

Damage center formation in SiO2 thin films by fast electron irradiation

Abstract: The concentrations of E’ centers (ESR‐active oxygen vacancies) produced by 30–160 keV electron irradiation have been measured in thermal SiO2 films at doses far exceeding any previously reported. The dependences of these concentrations on electron energy and fluence were measured in both steam‐grown and dry oxides. Results indicated that ionization rather than atomic displacement is the predominant formation mechanism. Significant differences in dose dependence were found between oxide types, reflecting the role of hydrogen in damage annealing.

Excimer laser‐induced deposition of InP: Crystallographic and mechanistic studies

Abstract: InP thin films have been deposited on several types of substrates via 193‐nm excimer laser‐induced photochemical decomposition of (CH3)3In and P(CH3)3 gas‐phase precursors. The characteristics of the deposited films are studied over a wide range of conditions. A photochemical model is proposed which explains the stoichiometry and rate at which the film deposits. Approximate fluences are given for the onset of (in order of increasing fluence) In‐precursor photochemistry, P‐precursor photochemistry, CHx photochemistry, laser‐induced crystallization, and laser damage. Crystallinity of InP films deposited on (100) InP substrates has been studied by scanning electron microscopy, transmission electron microscopy, and Rutherford backscattering spectroscopy. Films range from amorphous to epitaxial, depending upon conditions (most notably fluence incident on the substrate). The best film deposited at ∼0.1 J/cm2 and at a steady‐state temperature of only ∼320 °C had a backscattering spectrum indistinguishable from t...

Laboratory scale-up of two-stage laser chemistry separation of13C from CF2HCl

Abstract: The laboratory scale-up of a two-stage laser enrichment process for carbon isotopes, involving infrared multiphoton dissociation of freon-22, is described. Unmodified commercial equipment and materials were used. An initial study of the effect of fluence, laser frequency, freon-22 pressure and pressure of argon, nitrogen and trifluoromethyl chloride was made in short irradiation cells (constant fluence) in order to define optimum process parameters. The process was then scaled to higher throughput in longer cells (1–5 m) in which compensation for beam-energy depletion by absorption was made by reduction in the beam area by focussing. From the scale-up experiments, measurements of yield and enrichment of the tetrafluoroethylene product gave demonstrated production rates. These, coupled with measurements of the absorption, allowed extrapolation to production rates assuming total utilization of the available output energy. Using a 100 W TEA CO2 laser (10 J, 10 Hz) we have demonstrated production rates of 0.20 g h−1 carbon-12 at 99.99% carbon-12, 11 mg h−1 carbon-13 at 72% carbon-13 and 2 kg per annum carbon-13 at 50%. Energy absorption measurements imply a capability to produce 3 kg per annum carbon-13 at over 95% carbon-13 in a two-stage process. The apparatus was used to produce gram quantities of carbon-13 depleted freon-22 (99.99% carbon-12). A comparison of the infrared multiphoton dissociation of this material with that of natural freon-22 (1.11% carbon-13) showed that under the conditions required to give selective dissociation of13CF2HCl that12CF2HCl was excited as a result of a dominantly radiative interaction and that collisional transfer from13CF2HCl molecules played a minor role.

Action spectra for phototropic balance in Phycomyces blakesleeanus: dependence on reference wavelength and intensity range.

Abstract: — Action spectra for phototropic balance of Phycomyces blakesleeanus sporangiophores were measured for various reference wavelengths and intensity ranges. Balance action spectra were made at fluence rates of 10-4 W m-2 with reference wavelengths of 450 nm, 394 nm, 507 nm, and broadband blue light. For broad-blue light and 450 nm light as references, typical flavin-like action spectra were found with a ma jor peak at 455 nm, a secondary peak at 477 nm, and a minor peak at 383 nm; these peaks are wider for broad blue than for 450 nm light. With the 394 nm reference, there is a major peak at 455 nm, a secondary peak at 477 nm and a minor peak at 394 nm. An action spectrum with 507 nm reference has a major peak at 455 nm and a minor peak at 383 nm, but no peak at 477 nm. A balance action spectrum was made with 450 nm reference light near threshold intensity (2 times 10-8 W m-2); there, the 386 nm peak is greatly reduced, while the 455 nm peak is enhanced. The intensity dependence of the 386 nm peak was studied in detail for reference light of 450 nm. We found that the relative quantum efficiency of the 386 nm light increases with the logarithm of the 450 nm fluence rate; in the high intensity range (0.3 W m-2) the relative quantum efficiency of the 386 nm light is 1.3 and approaches zero at 10-9 W m-2. These findings indicate that P. blakesleeanus phototropism is mediated by multiple interacting pigments or by a photochromic photoreceptor.

Excimer laser etching of Al metal films in chlorine environments

Abstract: The etching of Al metal films on Si substrates in up to 1 Torr of Cl2 gas using the 308‐nm XeCl laser pulses is reported. Etch rates of up to ∼1 μm per pulse were obtained which were laser fluence independent in the range 3≳φ≳0.25 J/cm2. The etch rates decreased, however, at high Cl2 pressures and high laser repetition rate. Processes such as redeposition of the ablated material and slowing down of the ablation at increased Cl2 pressures are considered as possible explanations of the observed results. By subsequently exposing the film to Cl2 and then irradiating it under vacuum, we found that the main etching mechanism is reaction in the absence of light to form an aluminum chloride layer followed by the ablation of this layer with the subsequent laser pulse.

Ion beam processes in Si

Abstract: Observation of the effects of implants of energetic ions at high dose rates into-Si have produced some exciting and interesting results. The mechanism whereby displacement damage produced by ions self-anneals during high dose rate implantation will be discussed. It will be shown that ion beam annealing (IBA) offers in certain situations unique possibilities for damage annealing. Annealing results of the near surface in Si with a buried oxide layer, formed by high dose implantation, will be presented in order to illustrate the advantages offered by IBA. It will be also shown that ion irradiation can stimulate the epitaxial recrystallization of amorphous overlayers in Si. The nonequilibrium alloying which results from such epitaxial processes will be discussed as well as mechanisms which limit the solid solubility during irradiation. Finally, a dose rate dependency for the production of stable damage by ion irradiation at a constant fluence has been observed. For low fluence implants, the amount of damage is substantially greater in the case of high flux rather than low flux implantation.

Nonlinear Photoemission from Picosecond Irradiated Silicon

Abstract: Three distinctly different regimes of photoelectric emission are observed over a wide fluence range of uv-laser pulses irradiating single-crystal silicon samples. The role of the electron-hole plasma in the nonlinear photoemission is demonstrated by temporal correlation measurements. The diffusion properties of hot carriers are analyzed by investigating the influence of energy transport by hot carrier diffusion on the fluence threshold for melting with uv photons.

Experimental and theoretical investigations into the origin of cross-contamination effects observed in a quadrupole-based SIMS instrument

Abstract: The minimum-detection limits achievable in SIMS analyses are often determined by transport of material from surrounding surfaces to the bombarded sample. This cross-contamination (or memory) effect was studied in great detail, both experimentally and theoretically. The measurements were performed using a quadrupole-based ion microprobe operated at a secondary-ion extraction voltage of less than 200 V (primary ions mostly 8keV O 2 + ). It was found that the flux of particles liberated from surrounding surfaces consists of neutrals as well as positive and negative ions. Contaminant species condensing on the bombarded sample could be discriminated from other backsputtered species through differences in their apparent energy spectra and by other means. The apparent concentration due to material deposited on the sample surface was directly proportional to the bombarded area. For an area of 1 mm2 the maximum apparent concentration of Si in GaAs amounted to ∼5 × 1016atoms/cm3. The rate of contamination decreased strongly with increasing spacing between the bombarded sample and the collector. The intensities of backsputtered ions and neutrals increased strongly with increasing mass of the target atoms (factor of 10 to 50 due to a change from carbon to gold). The effect of the primary ion mass (O 2 + , Ne+, and Xe+) and energy (5–10keV) was comparatively small. During prolonged bombardment of one particular target material, the rate of contamination due to species not contained in the sample decreased exponentially with increasing fluence. In order to explain the experimental results a model is presented in which the backsputtering effect is attributed to bombardment of surrounding walls by high-energy particles reflected or sputtered from the analysed sample. The level of sample contamination is described by a formula which contains only measurable quantities. Cross-contamination efficiencies are worked out in detail using calculated energy spectra of sputtered and reflected particles in combination with the energy dependence of the sputtering yield of the assumed wall material. The experimental findings are shown to be good agreement with the essential predictions of the model.

Flaking and wave-like structure on MeV energy high-dose 4He+ bombarded silicon

Abstract: In previous work different types of metallic glasses were irradiated under the same experimental conditions by 1 or 2 MeV 4 He + ions up to a fluence of 10 18 ions/cm 2 . It was found that, at a critical dose, flaking occurred and on the flaked surface a wave pattern consisting of periodical elevations was observed. The appearance of this pattern was independent of the material composition and the manufacturing technology of the target as well as on the applied He + energy provided the current was kept low to avoid beam heating effects. Surprisingly a similar pattern was also observed in the present work on the flaked surface of single crystal silicon after irradiation with MeV energy He + ions up to the same fluence. Consequently it is suggested that this pattern is not related to amorphous structure, as was previously thought to be the most probable, but rather seems to be a more general phenomenon. Possible mechanisms of the formation are discussed.

Face-centered-cubic Nb-Si solid solutions produced by picosecond pulsed laser quenching

Abstract: Face‐centered‐cubic Nb100−xSix solid solutions (10≤x≤27) have been produced by 30‐ps pulsed laser irradiation, using a Nd:YAG laser with a fluence of about 1 J/cm2. The lattice parameters of these solutions suggest that the solute atoms can be interstitial or substitutional, probably as a result of a change in the quenching conditions.

Spectral characteristics of IR-multiphoton excitation in supersonic molecular beams

Abstract: The multiphoton absorption of SF6 was investigated in supersonic molecular beams in dependence on the fluence and the wavelength of the CO2 laser. The temperatures of the SF6 molecules have been reduced using seeded beams of different concentrations. The experimental results are discussed on the basis of the known spectroscopic data of SF6 and provide some novel information about the spectral characteristics of the ir-multiphoton excitation of strongly cooled molecules in the collision-free case.

Alteration Of The Transmission Characteristics Of Fused Silica Optical Fibers By Pulsed Ultraviolet Radiation

Abstract: Several all-silica fibers advertises as transmitting ultraviolet radiation were tested with a rare-gas-fluoride excimer laser Test wavelengths were 193nm (ArF*), 248nm (KrF*), and 351nm (XeF*) Pulse duration was 15ns, repetition rates were from 1 to 25 Hz, and in-put fluences ranged from a few millijoules per square centimeter up to the surface damage threshold Under some conditions the fiber changed its transmission as it was irradiated The effects of wavelength, fluence and previous history of the fiber have been characterized Macroscopic inhomogeneities, such as inclusions in the fiber, contribute to the transient effects along with molecular processes in the drawn fused silica

Laser-induced doping of GaAs

Abstract: Thin layers of GaAs are heavily doped locally by laser induced Se or Zn diffusion. H2Se or diethylzinc gases are used to provide Se or Zn dopant atoms. The surface is locally heated with 3 ns light pulses from a Q-switched frequency doubled Nd-YAG laser. The doping process is described in detail. Doping profiles and sheet carrier concentrations are measured as a function of substrate temperature, laser fluence and processing time. Dopant concentrations of more than 1021 cm−3, with a thickness of the doped layer of less than 20 nm can be achieved.

Ultraviolet (193 nm) laser‐induced oxygen exchange and bulk diffusion in gallium arsenide native oxides: 18O experiments

Abstract: Pulsed laser irradiation (193 nm, t=14 ns) at low fluence (0.045 J/cm2) of GaAs 18O enriched native oxides leads to a very efficient oxygen exchange between 18O in the oxides and 16O coming from the surrounding atmosphere. 1000 laser pulses caused a complete exchange of oxygen from an oxide layer of ∼30‐nm thickness. This showed a very high rate of oxygen self‐diffusion under laser irradiation although the total oxygen content in the oxide was preserved. As this process is believed to have a threshold fluence, a model based on the hole‐pair mechanism of Itoh and Nakayama [Phys. Lett. A 92, 471 (1982)] is suggested. Preferential loss of As and preferential oxidation of Ga are also observed to some extent.

Observations on the spectral dependence andT/D isotope selectivity in the CO2 laser multiple-photon dissociation of trifluoromethane

Abstract: Pulsed CO2 laser multiple-photon dissociation of CTF3 (v2 mode) bathed in argon and CDF3, CHF3 or CCl4 is examined as a function of laser wavelength (9.2–9.6 μm) and fluence. The dependence of the dissociation profile on wavelength is analyzed and comparisons are made to prior work. The single-stepT/D enrichment factor for infrared photolysis of trifluoromethane is measured; potential practical isotope separation is discussed. Pulsed infrared laser photolysis of CTF3 (v5 mode) using a 12 μm NH3 laser is also attempted.

Laser-induced optical reflection and absorption of GaP

Abstract: Changes in the optical absorption and reflection coefficients of GaP induced by irradiation with 600 ns laser pulses of the wavelenghts at the direct and indirect band gaps have been measured. It is found that a persistent increase in the absorption coefficient and a permanent decrease in the reflection coefficient, in addition to transient increases in the absorption and reflection coefficients, are induced, by irradiation at the band gap. The persistent component is found to be evolved with a surprisingly long time constant of about 10 ms. This component is ascribed to laser-induced modification of the surface layers or damage formation. The transient component is ascribed to temperature rise on the basis of time-resolved optical absorption measurements. It is found that the persistent changes are induced by irradiation with the indirect band gap photons at a fluence which induces only a little change in the transient reflectivity. The threshold laser fluence to create the surface modification by the indirect band-gap irradiation is found to be only four times that by direct band-gap irradiation. A delayed reflectivity change is found to be induced by irradiation at the indirect band gap and is ascribed to the modification of surface layers by photons at the surface layer, which enhances the absorption coefficient. We interpret these experimental results in terms of non-thermal laser-induced atomic processes.

Self and Ion Beam Annealing of P,Ar, and Kr in Silicon

Abstract: The damage produced by high current density ∿l0µA/cm 2 implants of 120 keV P + into and silicon wafers, 500 °m thick, has been investigated in the fluence range 1×l0 1 5 /cm 2 -l×l0 16 /cm 2 by ion channeling and by transmission electron microscopy. For both orientations the thickness of the damage layers increases with the fluence up to 2×10 15 /cm 2 and then decreases. The rate of regrowth is a factor two faster for the with respect to the oriented Si crystals. Similar ratios have been found in pre-amorphized samples and irradiated with Kr + ions in the temperature range 350°C-430°C. The TEM analysis reveals the presence of hexagonal silicon and of twins in small amounts for both orientations. The beam induced epitaxial growth depends also on the species present in the amorphous layer. A comparison between self-annealing and beam annealing in Si preamorphized with Ar + or P + shows a noticeable retardation of the growth rate in the presence of Ar + .

Crystalline-to-amorphous phase transition in irradiated silicon

Abstract: The amorphous(a)-to-crystalline (c) phase transition has been studied in electron(e ) and/or ion irradiated silicon (Si). The irradiations were performed in situ in the Argonne High Voltage Microscope-Tandem Facility. The irradiation of Si, at <10K, with 1-MeV e to a fluence of 14 dpa failed to induce the c-to-a transition. Whereas an irradiation, at <10K, with 1.0 or 1.5-MeV Kr+ ions induced the c-to-a transition by a fluence of approx.0.37 dpa. Alternatively a dual irradiation, at 10K, with 1.0-MeV e and 1.0 or 1.5-MeV Kr+ to a Kr+ fluence of 1.5 dpa - where the ratio of the displacement rates for e to ions was approx.0.5 - resulted in the Si specimen retaining a degree of crystallinity. These results are discussed in terms of the degree of dispersion of point defects in the primary state of damage and the mobilities of point defects.

Interdependence of fluence and pressure in the laser-induced isomerization of methyl isocyanide

Abstract: The infrared laser-induced isomerization of methyl isocyanide has previously been shown to exhibit a marked pressure dependence; large-scale isomerization occurs when threshold conditions have been exceeded. In this work the wavelength and fluence dependence of the pressure threshold have been investigated. It has been determined that threshold conditions reflect a balance between the average level of excitation and the sample pressure. The multiphoton spectrum and the threshold as a function of wavelength have been determined and both reflect the structure of the linear absorption spectrum, indicating that the effective anharmonicity is small. In addition, the earlier conclusion that the threshold is not due to a laser-triggered, thermal explosion is corroborated. 24 references, 6 figures, 2 tables.

Pin Photodiodes Irradiated with 40- and 85-MeV Protons

Abstract: PIN photodiodes were bombarded with 40- and 85-MeV protons to a fluence of 1.5 × 1011 p/cm2, and the resulting change in spectral response in the near infrared was determined. The photocurrent, dark current and pulse amplitude were measured as a function of proton fluence. Changes in these three measured properties are discussed in terms of changes in the diode's spectral response, minority carrier diffusion length and depletion width. A simple model of induced radiation effects is presented which is in good agreement with the experimental results. The model assumes that incident protons produce charged defects within the depletion region simulating donor type impurities.