Showing papers on "Fluence published in 1987"

Time-resolved observation of electron-phonon relaxation in copper.

Abstract: Amplified 150--300-fs laser pulses are applied to monitor the thermal modulation of the transmissivity of thin copper films. Non- equilibrium electron-lattice temperatures are observed. The process of electron-phonon energy transfer was time resolved and was observed to be 1--4 ps increasing with the laser fluence.

Ultraviolet laser ablation of polyimide films

Abstract: Pulsed laser radiation at 193, 248, or 308 nm can etch films of polyimide (DuPont KaptonTM). The mechanism of this process has been examined by the chemical analysis of the condensible products, by laser‐induced fluorescence analysis of the diatomic products, and by the measurement of the etch depth per pulse over a range of fluences of the laser pulse. The most important product as well as the only one condensible at room temperature is carbon. Laser‐induced fluorescence analysis showed that C2 and CN were present in the ablation plume. At 248 nm, even well below the fluence threshold of 0.08 J/cm2 for significant ablation, these diatomic species are readily detected and are measured to leave the polymer surface with translational energy of ∼5 eV. These results, when combined with the photoacoustic studies of Dyer and Srinivasan [Appl. Phys. Lett. 48, 445 (1986)], show that a simple photochemical mechanism in which one photon or less (on average) is absorbed per monomer is inadequate. The ablation proces...

Quantitative and ultrastructural studies of excimer laser ablation of the cornea at 193 and 248 nanometers.

Abstract: Excimer laser radiation at 193 nm and 248 nm was used to create linear etch perforations of enucleated calf corneas. The etch depth per pulse was determined for various exposures, and specimens were examined by light and transmission electron microscopy. Compared to 248 nm, excimer laser ablation at 193 nm was found to have a lower threshold for onset of ablation, less increase in etch depth per pulse at increasing fluences, and less structural alteration in adjacent cornea. For 193 nm, structural alterations were minimal, confined to an area less than 0.3 micron wide, and did not increase with increasing fluence. These studies suggest that clinical strategies for excimer laser refractive surgery will employ the 193-nm wavelength, with fluence chosen depending on surgical strategy. Ablation exposures above 600 mJ/cm2 at 193 nm may give the most repeatable etch depth.

Fast neutron irradiation of YBa2Cu3O7

Abstract: The influence of fast neutron irradiation on the superconducting transition temperature,T c , the transition width, upper critical field,B c2 , and critical current density,j c , has been investigated in YBa2Cu3O7 up to a fluence of 1019 cm−2 (E>1 MeV). TheT c degradation with fluence is slightly less than in PbMo6S8, but larger than in A 15 compounds. The irradiation induced increase of the normal state resistivity is accompanied by a remarkable decrease of both the intergrainj c and the superconducting volume fraction.

Laser-induced chemical etching of silicon in chlorine atmosphere

Abstract: Chemical etching of single-crystalline (100)Si induced by pulsed laser irradiation at 308, 423, and 583 nm has been investigated as a function of the laser fluence and C12 pressure. Without laser-induced surface melting, etching requires Cl radicals which are produced only at laser wavelengths below 500 nm. With low laser fluences (Φ(308 nm) 440 mJ/cm2) etching is thermally activated. In the intermediate region both thermal and non-thermal mechanisms contribute to the etch rate.

Time resolved transmission studies of poly(methyl methacrylate) films during ultraviolet laser ablative photodecomposition

Abstract: The time resolved transmission of P10, poly(methyl methacrylate), resist films has been studied during ablative photodecomposition (APD) by KrF, KrCl, and ArF excimer lasers. The observed changes in transmission are most likely due to scattering and absorption by the ablated material during and after the laser pulse. Measurements of the energy per unit volume absorbed at the threshold laser fluence for APD as a function of wavelength indicate that APD is not caused solely by a thermal degradation process.

Orientation and doping effects in ion beam annealing of α-silicon

Abstract: The regrowth of amorphous layers in and Si substrates has been studied under irradiation with 600 keV Kr+ at target temperature in the 250–400°C range. The amorphous layers were obtained by P+ or Si+ implants. 2.0 MeV He+ backscattering in combination with channeling effect technique was adopted as the analytical technique. The regrown thickness increases linearly with fluence and with the energy density deposited into nuclear collisions at the amorphous-single crystal interface in all the investigated cases. The rate of regrowth for the orientation is two times that for the orientation. The measured activation energy of 0.22–0.05 eV is equal within the experimental accuracy for both orientations. The influence of impurities has been investigated in Si substrates preamorphized with P+ or Si+ ions. The regrowth is enhanced in the presence of phosphorus by a factor of ≈- 4 with respect to the Si self-ion implantation. The results are discussed in terms of the existing models of thermal and ion assisted epitaxial regrowth.

Energy deposition at insulator surfaces below the ultraviolet photoablation threshold

Abstract: A surface deformation technique measures the energy deposited in sapphire and other surfaces upon irradiation by focused excimer laser light. For sapphire, the thermal deformation indicates a linear dependence of absorbed energy on excimer fluence with about 3% of the incident light absorbed per surface at 193 nm. Just below etching threshold this absorbed energy should raise the surface temperature to about 430 K, an insufficient temperature for ablation of Al2O3 by thermal vaporization. In contrast the absorbed energy creates ≲3 electron–hole pairs/oxygen atom, consistent with a photochemical (electronic type) etching process.

Retention of hydrogen in graphite

Abstract: The retention of hydrogen in POCO AXF‐5Q graphite has been measured at room temperature as a function of fluence and flux for H+2 ions at energies from 250 to 500 eV provided by a glow discharge. More than 2×1018 H/cm2 has been retained, and no indication of saturation has been observed to a fluence of 5×1019 H/cm2. In this experiment retention was found to increase linearly with fluence for constant flux. A flux dependence was observed; i.e., the retention rate was observed to decrease monotonically as the flux increased. A changeover experiment, deuterium to hydrogen, was conducted; the results show that significant changeover occurs (i.e., about 30% changeover for a fluence of 5×1017 D/cm2).

Wavelength dependence of the excimer laser etching characteristics of a polymeric resist

Abstract: The direct etching by excimer laser light of thin films of a polymeric resist, PMTM, has been studied at wavelengths of 157, 193, 222, and 248 nm. The energy absorbed per unit ablated volume near the etching threshold fluence indicates that a wavelength‐dependent reaction mechanism causes ablation of the film. When laser wavelengths shorter than 222 nm are used at fluences close to threshold no evidence of heating of the polymer is observed. At 222 nm we observe thermal effects which cause melting and flow of the polymer while at 248 nm trapped pockets of volatile gas, which can subsequently explode, appear to be formed below the (molten) surface.

Ion beam mixing effects in the Ar+-irradiated Fe/SiO2 system

Abstract: Ion beam mixing effects at the metal/insulator interface have been studied by irradiating at room temperature the Fe film/SiO2 substrate system with 100 keV Ar ions in the fluence range 3 × 1015 – 1.3 × 1017 ions/cm2. The aim of the work was to quantify the ion induced mixing of Fe atoms and clarify the mechanisms involved in the atomic transport processes. SEM analyses of the irradiated surfaces reveal ion beam induced surface morphological modifications as hole formation in the Fe film and Fe island growth. Computer controlled image analyses allowed us to evaluate the Fe coverage at any irradiation fluence, φ , and to introduce a fluence dependent “coverage” factor ƒ(φ) in calculations. The amount of mixed Fe atoms, Qm, has been measured by 1.8 MeV He+ Rutherford backscattering, after a chemical etching procedure to remove the unmixed Fe film. The mixing process is well described by the relation Q = Aφ +B√φ, indicating that both collisional and diffusive mechanisms are effective. The relative influence of these two main mechanisms has been evaluated by determining the coefficients A and B, taking into account the morphological effects expressed by the “coverage” factor ƒ(φ).

Radiation Effects in High Tc Superconductors for Space Applications

Abstract: YBa2Cu3O7 is one of the new class of superconductors having a critical temperature (Tc) above 90 K. Devices using these materials would require minimal refrigeration to operate in space. It is important, therefore, to determine the effect of natural space radiation on these materials. Preliminary experiments have been performed to study displacement damage and ionization effects in 100?m thick films of YBa2Cu3O7. Samples of these films were irradiated at room temperature with 1) 56 MeV electrons up to a total fluence of 1.6×1015 cm-2, 2) 6 MeV protons up to a total fluence of 5.0×1013 cm-2, and 3) 63 MeV protons up to a total fluence of 1.09×1014 cm-2. It is concluded from the electron irradiations that the major effect of ionization is to increase the normal resistivity of the material. Although Tc(onset) was not much affected by either the electron or the proton irradiations, Tc(completion) varied substantially with proton fluence and at the higher fluences there was evidence of a non-superconducting region even at 4 K. It appears from the shape of the resistivity-temperature curve that high energy proton irradiation induces a partially superconducting multi-phase mosaic. Comparing worst case space environments to the radiations described above, it is concluded that this high Tc material could be made to remain superconducting for extended space missions with only moderate shielding.

Amorphous phase formation in ion irradiated Ni3B

Abstract: A monocrystalline Ni3B compound is irradiated at 90 and 300 K with ions of different masses (Kr, Ni, B) and the amorphous phase formation studied via in situ Rutherford backscattering and channeling experiments. For a given ion mass, total amorphization of the irradiated layer occurs at lower fluence at 90 than at 300 K. At both temperatures, the amorphization kinetics varies nearly linearly with the number of atoms displaced by the bombarding ions, the amorphization rate being independent of the mass of the irradiating ion at 90 K.

Plasmon‐phonon‐assisted electron‐hole recombination in silicon at high laser fluence

Abstract: We present both theoretical and experimental results in silicon which clearly demonstrate that at time scales of 20 to 40 ps, after the pump laser pulse and at fluences greater than 100 mJ/cm2, the carrier density of the electron‐hole plasma drops for increasing fluence; this is not explained by Auger recombination. We show that this drop is specific to plasmon‐phonon‐assisted recombination, which naturally explains this behavior.

Precipitate Evolution in Type 316 Stainless Steels Irradiated in EBR-II

Abstract: The irradiation temperature and fluence dependences of precipitates formed in Type 316 stainless steels with and without titanium have been studied for specimens irradiated over a fluence range of 3 x 10 2 6 to 25 x 10 2 6 n/m 2 (E > 0.1 MeV) and a temperature range of 370 to 730°C. A precipitate formation diagram using irradiation temperature and fluence as the two coordinates is constructed. The composition of M 6 C is found to be irradiation temperature dependent. The composition of Laves is found to be both irradiation temperature and fluence dependent. The variable composition behavior of the precipitates, M 6 C and Laves, during irradiation is discussed in terms of nonequilibrium segregation and the radiation-induced off-stoichiometric characteristics of the precipitates.

Damage created by high-current-density implants of phosphorus into and silicon wafers

Abstract: The damage left by high-current-density, ∼9 μA/cm2, implants of 120 keV phosphorus into 〈100〉 and 〈111〉 silicon oriented substrates was investigated as a function of the fluence in the range 4×1015–1.5×1016/cm2. The samples were analyzed by 2 MeV He+ channeling and transmission electron microscopy. Initially a buried amorphous layer forms at low fluences until the wafer temperature saturates at ∼450 °C at a fluence of ∼4.5×1015/cm2. As the fluence is further increased ion-assisted regrowth of this initial buried amorphous layer takes place and is 2 to 2.5 times faster (with respect to ion fluence) for 〈100〉 substrates than for 〈111〉 substrates. At higher fluences, most of the residual damage is located at a depth equal to the sum of the projected range and of the straggling. In the regrown layers twins are found in both orientations, and in some cases a hexagonal silicon phase is present at high fluences. The results are compared with the ion assisted regrowth of amorphous layers at well defined temperatures in the 250°–400 °C range.

Atomic mixing during multiple species implantation of Si

Abstract: Serial implantation of, probably high fluence, different ion species into a common substrate may be a useful method of generating materials standards for surface analytical techniques. Such a method may, however, suffer from atomic relocation processes within and from the substrate resulting from ballistic and diffusion processes and sputtering. In this work we examine theoretically and computationally the influence of these processes on the collection of different mass ions, as a function of ion fluence in Si. Results are presented for single species implantation and for different mass combinations of two sequential species implantations. These results are compared with experimental measurements of range profiles and total collected quantities of 40 keV As+ and Bi+ implanted separately or sequentially up to fluences of the order of 1017 cm−2 into Si. Substantial evidence of atomic redistribution is shown and conclusions about optimal conditions for obtaining composition precision are drawn.

Current effects in BF2 implants in silicon

Abstract: Boron implantion in (100) silicon with BF2 molecular ions was studied as a function of the implant current and fluence, using a medium current, commercial ion implanter equipped with a naturally cooled end station. Sample characterization was performed with Rutherford backscattering spectrometry (RBS), secondary ion mass spectrometry (SIMS), nuclear reaction analysis (NRA) and sheet resistance measurements. The thickness of the amorphous layer created by the impinging beam was found to increase, for fluences larger than the amorphization threshold, as the logarithm of the fluence for beam current densities lower than 50 μ A/CM2. For higher current densities the formerly created amorphous layer was found to recover by solid state epitaxy; the anneal is nearly complete for sample implanted at 500 μA/cm2 beam current density and at a fluence of 1 × 1016 cm−2. In high fluence samples boron and fluorine are snow-plowed by the travelling amorphous/crystalline interface both during the ion implantation step at high beam current and during furnace annealing at 500–600°C of low current implanted samples. For high current implanted samples fluorine is severely desorbed, and only a small fraction of the implanted dose is retained at the end of the implantation step. In addition, boron is redistributed up to the surface. During furnace annealing of high fluence, low current implanted samples, fluorine is not desorbed, and accumulates in two regions, at about half and twice the projected range of fluorine, where a high density of implantation originated defects are expected. In these samples boron does not reach the surface, but accumulates in the external fluorine-rich layer.

N-ion irradiation studies of VN thin films

Abstract: Thin films of VN have been formed by reacting films of vanadium metal, which were previously prepared by e-beam evaporation, with N 2 gas. The superconducting transition temperature T 2 , the electrical resistivity ϱ, the X-ray diffraction spectra, and the temperature dependent upper critical field H c2 of these films were studied as a function of nitrogen ion irradiation. T c is found to decrease exponentially with incident ion fluence to a total change of 42%, while the residual resistivity increases, both saturating at about the same fluence. This exponential behavior is derived from a simple defect production and annealing model. H c2 is found to fit WHH theory at all ion doses, with a maximum H c2 (0) at 4 × 10 14 N-ions/cm 2 . The electronic heat capacity coefficient γ * , calculated from the slope of H c2 at T c , is observed to be correlated with T c ; therefore, γ * also decreases with fluence.