Showing papers on "Fluence published in 1986"

Dynamics of UV laser ablation of organic polymer surfaces

Abstract: A model based on a time‐dependent treatment of the ablation of organic polymer surfaces by UV laser radiation is proposed. It relates the dynamics of the etching process to the experimental parameters such as the fluence, wavelength, and the width of the laser pulse. The model is applied to poly(methyl methacrylate) and to polyimide in order to predict the etch characteristics quantitatively and to compare them systematically to the experimental results. This model accounts for a wide variety of observations such as fluence thresholds, wavelength‐dependent etch rates, ‘‘incubation’’ pulses, filtering by photofragments, fast intrapulse etching, thermal contributions, and even the influence of pulse compression so that a consistent picture of the mechanisms underlying the ablation process becomes apparent.

Mechanism of the ultraviolet laser ablation of polymethyl methacrylate at 193 and 248 nm: laser-induced fluorescence analysis, chemical analysis, and doping studies

Abstract: The formation of C2 in the ablation of polymethyl methacrylate (PMMA) by 193- and 248-nm laser pulses was studied by laser-induced fluorescence. The formation of C2 is observable at a fluence at 248 nm that is well below the fluence threshold where significant etching takes place. The opposite is true at 193 nm. The velocity distribution of the diatomic form was also measured and found to approach but not exactly fit a Maxwell–Boltzmann equation. Average translational energies as high as 6 eV were recorded even at the fluence threshold for this product. The other products of the laser ablation of PMMA at 193 or 248 nm are methyl methacrylate (MMA) and a solid that is a low-molecular-weight fraction of PMMA. Although the products are the same at both wavelengths, the mix is quite different. At 193 nm, 18% of the ablated polymer is MMA, whereas at 248 nm less than 1% of the polymer appeared as MMA. A semilog plot of the mass of material removed versus the fluence shows three distinct regions. The central portion at each wavelength (80–300 mJ/cm2 at 193 nm; 600–2000 mJ/cm2 at 248 nm), which corresponds to rapid etching, is identified as ablative photodecomposition. At lower fluences, the etching efficiency falls off rapidly, indicating that there is a threshold fluence. At fluences above the range for ablative photodecomposition, the etching levels off, probably because of the secondary absorption of the incoming photons by the products. The mass of material ablated per joule of energy absorbed in the ablated volume was remarkably similar at both wavelengths. It is suggested that ablative photodecomposition involves both a one-photon process that produces MMA and low-molecular-weight polymeric fragments and a many-photon process that gives rise to products such as C2 with high translational energy. In order to establish the role of the many-photon process, samples of PMMA doped with acridine were exposed to laser pulses. The presence of as little as 2% of acridine lowers the threshold for significant etching at 248 nm from 400 to 90 mJ/cm2. The absorption characteristics of the doped polymer at this wavelength show that the acridine molecules absorb >10 photons each, whereas the absorption of the polymer changes little. The drop in the threshold energy for etching should therefore be attributed to the many-photon decomposition of the acridine and the consequent ablation of the polymer as a whole.

Pulsed CO2 laser etching of polyimide

Abstract: Etching of thin polyimide films in air was investigated using a line tunable, pulsed CO2 laser. The threshold fluence for etching at a wavelength of 944 cm−1 (10.6 μm) exceeds that at 1087 cm−1 (9.2 μm) by a factor of 4. This is consistent with the infrared absorption spectrum which shows polyimide to be significantly more absorbing at 1087 cm−1. As a result, etching at 1087 cm−1 produces a cleaner, more precisely defined region. Analysis of the vapors generated during laser etching shows the simple gases CO2, H2O, and CO to be present.

Velocity distribution of molecular fragments from polymethylmethacrylate irradiated with UV laser pulses

Abstract: Polymethylmethacrylate samples are irradiated with pulses from an ArF excimer laser at λ=193 nm The velocities of ablated particles are measured with the use of a quadrupole mass spectrometer For laser fluences <120 mJ/cm2 we observe thermal, Maxwell–Boltzmann velocity distributions The photofragments possess a common temperature which increases with fluence to values as high as 3000 K

Fluence perturbation in photon beams under nonequilibrium conditions

Abstract: The perturbation effect in parallel-plate ionization chambers used for buildup measurements has been investigated. The fluence perturbation due to electrons emitted through the side walls are thoroughly investigated by measurements using film and extrapolation chambers and by calculations. The electron fluence varies both with side wall material and chamber geometry. In order to obtain a small perturbation effect, the chamber should have a large guard width compared to the electrode separation and the side walls should have as large an angle as possible with the central axis. The side wall should be of the same material as the rest of the chamber. The perturbation effect is also dependent on the electron contamination of the beam and angular distribution of the electron fluence. It is thus not possible to correct the perturbation effect in one parallel-plate chamber with fixed plate separation with correction factors obtained with extrapolation chambers of other dimensions. In order to make accurate surface dose measurements extrapolation chambers are therefore strongly recommended in favor of fixed parallel-plate chambers.

Excimer laser surgery of the cornea: Qualitative and quantitative aspects of photoablation according to the energy density

Abstract: Using an excimer laser working in the far ultraviolet and cutting fresh corneas of human cadaver eyes, we were able to demonstrate that there is an ideal fluence (joule/cm2/pulse) that allows a maximum tissue ablation depth per shot. Above the threshold of 1 J/cm2/pulse the depth of tissue ablated does not increase and both thermal and mechanical damage appears at the cut edges.

Solid and fluid xenon in Xe implanted aluminium

Abstract: The evolution of xenon precipitates in Xe implanted aluminium as a function of fluence and implantation temperature is investigated using transmission electron microscopy. Implantations at room temperature lead to a high density of overpressurized solid xenon precipitates; with increasing fluence, their number and the inside pressure decrease. After implantation at 200°C with a fluence of 5×1015 Xe2+ cm−2, xenon is in two different phases: solid precipitates and fluid bubbles and the latter are surrounded by membranes of solid xenon. In situ TEM annealing and cooling experiments were performed and lattice parameter changes were observed. The results are discussed and an attempt is made to give a qualitative analysis of the possible mechanisms of xenon precipitation in aluminium.

Laser curing of coatings and inks

Abstract: A method of curing polymer coatings by laser photoche­ mistry in which UV laser irradiation of the coating containing a photoinitiator sensitive to a selected wavelength of the laser light effects a low energy cure of the coatings. The total energy to effect a cure is markedly reduced by applying a series of pulses of the laser light at a low single pulse fluence. A reduction of the single pulse fluence at a constant pulse repetition rate or a reduction of the pulse repetition rate at a constant single pulse fluence tends to further reduce the total energy required to effect a cure of the coating.

Ion beam mixing studies in the Sn-Si system

Abstract: The process of ion beam mixing in the Sn-Si system is investigated by varying both the incident Ar+ ion energy (100–220 keV) at a fixed dose (2 × 1016/cm2) and the fluence (5 × 1015/cm2− 5 × 1016/cm2) at a constant energy (130 keV). The analyses of the spectra. obtained from Rutherford backscattering (RBS) of 3 MeV alpha particles, have shown that, in these samples (i) the width of the intermixed Sn-Si region increases as the Ar+ energy or fluence is increased (ii) the number of Si atoms/cm2 increases linearly with Ar+ dose and (iii) no significant penetrating tail is present in the Sn profile. From conversion electron Mossbauer (CEM) studies of these systems, it is observed that, at the most, about 40% of Sn in the intermixed layer contributes to form the Sn(1−x)Six compound. It is deduced from these results that x is in the range 0.5 ≦ x ≦ 0.8. These observations also suggest a large transport of Si atoms into the Sn layer, possibly due to radiation damage effects caused by Ar+ ion bombardment.

Roughness studies of ion beam processed molybdenum surfaces

Abstract: Mechanically polished molybdenum samples have been irradiated with 150 keV molybdenum ions at fluences from zero to 16×1015 ions/cm2 to study the effects on surface smoothing. Both fluence and substrate temperature during irradiation (25, 250, and 500 °C) had considerable effects on the optical properties. Ellipsometry, profilometry, and Nomarski photography were used to characterize the surfaces. Multiple wavelength, multiple angle of incidence ellipsometry results were analyzed using effective medium approximation models, with molybdenum, molybdenum oxide, and voids as the principal constituents. Generally, we find that the reflectance calculated from the ellipsometric measurements increases continuously as a function of fluence. However, for high substrate temperature and short wavelength the reflectance as a function of fluence reaches a maximum and then decreases. Effective medium calculations indicate that there is a corresponding change in the surface roughness with fluence, and that effects due to...

Sputtering of rare gas solids by keV ions

Abstract: The sputtering of Ar, Kr, and Xe solids by keV ions has been investigated by measuring the absolute sputtering yield and the fluence dependence of atomic ejection. In some instances the ejection rate changes by an order of magnitude or more before a steady state rate is established. It is therefore essential to know the fluence range covered in a yield measurement before it can be compared to other measurements or theory.

Wavelength dependence of laser‐induced sputtering from the (111) surface of BaF2

Abstract: Using blue tunable pulse laser radiation of low fluence, we have investigated laser‐induced sputtering from cleaved BaF2 (111) surfaces under ultrahigh vacuum conditions. Time correlated with the laser pulses the positive ions Ba+, Ba++, (BaF)+ and F+ were observed. Practically no negative ions were found. Neutral atomic fluorine (F 0) was desorbed abundantly. A pulse correlation of F 0 as well as the relative amount of F 0 and F+ could not be established at this stage. The emission yield of all positive ions as well as of F 0 was strongly wavelength dependent and showed a broad resonance around 2.9 eV.

Electron irradiation-induced amorphism of some silicates

Abstract: Irradiation with 200 keV electrons induced amorphism in natural beryl and cordierite, at fluences of ∼ 1026e/m2, a factor of ≈ 10 times that required to produce a similar effect in quartz. After dehydration of the beryl and cordierite by heating at 1050°C, the fluence required to induce amorphism was increased to ∼ 1027e/m2. The electron fluence required to produce amorphism in both materials decreased with decreasing electron energy, as expected if solid-phase radiolysis is responsible. CsAlSi5O12, prepared by dry-firing ceramic techniques, was rendered amorphous at a fluence of 3 × 1024 e/m2 (E = 200 keV). However, there was little apparent dependence of the electron fluence required to produce amorphism on electron energy over the 50–200 keV range, and Cs+ migration may play a critical role in the crystalline ← amorphous transformation. A natural sphene was unaffected structurally by irradiation to a fluence of 2 × 1027 e/m2 (E = 200 keV). The data are consistent with established theories that...

Defects created by self-implantation in Si as a function of temperature and fluence

Abstract: Self-amorphization of silicon by implantation of 150 keV Si + ions has been studied using channeling, step height measurements, and high resolution electron microscopy. It is shown that the value of the implantation temperature must be lower than RT in order to obtain an amorphous layer. Contamination by O, N an C during implantation has been evaluated by measuring the thermal regrowth rate of the amorphous layer and by nuclear reaction analysis. Owing to such contamination, the amorphization fluence should not exceed 3 × 10 15 >Si + /cm 2 or 10 16 Si + /cm 2 for implantation temperatures of 77 K and RT respe fluences, the epitaxial regrowth rate decreases.

Ion beam annealing during high current density implants of phosphorus into silicon

Abstract: The damage left by high current density∼10 μA/cm2 implants of 120‐keV P+ into 4‐in. (500‐μm‐thick) and 5‐in. (600‐μm‐thick) Si wafers of 〈100〉 orientation has been measured by 2.0‐MeV He backscattering in combination with the channeling effect technique. The fluences ranged between 1 and 7.5×1015/cm2. The amount of disorder is highest at 1×1015/cm2 and then decreases with fluence. The annealing of the amorphous layer takes place by the movement of two and one amorphous–single crystal interfaces for the 500‐ and 600‐μm‐thick wafers, respectively. The experimental data are compared with a beam annealing model based on the temperature‐rise profile, the amount of point defects generated by the ion in the collision cascade volume, and the assumption of a regrowth process governed by an activation energy of 0.25 eV.

XeCl laser controlled chemical etching of aluminum in chlorine gas

Abstract: The 308 nm XeCl laser assisted etching process of thin Al metal films on Si substrate in Cl2 gas was investigated. Etch rates were measured versus the laser fluence on the sample, the laser repetition rate, the Cl2 pressure and the sample temperature. Irradiation experiments under vacuum of films which were previously exposed to Cl2, and laser assisted etching in rare gases, nitrogen and air mixtures with Cl2 were also performed to elucidate the mechanism of the etching process. The surface morphology was investigated by scanning electron microscopy. The results show that a) Etch rates of up to about 1.5 μm per pulse are obtained which are strongly dependent on the Cl2 pressure and sample temperature. b) The etching mechanism is essentially a chemical chlorination of the Al in between the laser pulses which is followed by photo-ablation of the reaction products, c) AlCl3 evaporation and redeposition processes can explain the observed results. d) The Al films can be etched fully and cleanly without damage to the smooth Si substrate. e) Etching through adjacent or imaged mask on the Al film yielded relatively smooth and well defined Al walls with structures of the order of 1 μm.

Rapid Thermal Annealing And Solid Phase Epitaxy Of Ion Implanted InP

Abstract: The production and annealing of damage in (100) InP implanted with Si ions at 77K, room temperature and 200°C have been studied by channeling, Secondary Ion Mass Spectrometry (SIMS), Hall measurement and electrochemical profiling An ion energy of 180 keV and a fluence of 10 14 cm -2 produces an amorphous layer which extends to the surface in samples implanted at 77K or room temperature, while samples implanted at 200°C with a fluence as high as 10 15 cm -2 remain crystalline For room temperature or 77K implants, the maximum epitaxial regrowth thickness is about 2000A An amorphous layer less than 2000A thick can be nearly completely recrystallized by epitaxial growth, at a temperature of 250°C The post-anneal residual disorder of thicker layers, following furnace annealing (FA) at 750°C for 15 minutes, is linearly dependent on the initial amorphous layer thickness A high degree of electrical activation is reached for high temperature implants and low annealing temperature (500°-600°C) In contrast, for room temperature implants annealing at higher temperature using furnace or rapid thermal annealing was required to activate the dopants The percentage of electrical activation of both low and high fluence ion doses is nearly the same at the optimal condition for rapid thermal annealing (RTA) or furnace annealing, even for the case of residual disorder layers following room temperature implants and furnace annealing Comparison of the SIMS Si profile with the carrier concentration profile shows compensation at the damage-crystalline interface

Fluence-rate dependence of monophotonic reactions of nucleic acids in vitro and in vivo.

Abstract: The Bunsen-Roscoe law, also known as the reciprocity law (E=f(F) with F=I t) has only limited validity for monophotonic reactions of nucleic acids. Especially at low fluence rates, the extent of in vitro and in vivo photoreactions of nucleic acids in the far-UV and near-UV range is a function of the fluence and of the fluence rate (E=f (F;I)). In vitro experiments with poly(dA)poly(dT) clearly show that the far-UV (254 nm) response, indicated by the changes of the ellipticity at 315 nm, does not obey the Bunsen-Roscoe law at low fluence rates in the range between 1 W m-2 and 20 W m-2. In vivo experiments with Escherichia coli revealed very similar anomalies. Studying the growth delay after irradiation with far-UV light at 280 nm or near-UV light at 334 nm, we have confirmed the lack of reciprocity in both spectral ranges. The failure of the Bunsen-Roscoe law for the 280 nm and 334 nm UV irradiation effect at low fluence rates was in the range O < I < 40 W m-2. In both cases reciprocity occurred at higher fluence rates (40 < I < 100 W m-2).

Effects of ion irradiation on conductivity of CrSi2 thin films

Abstract: Electrical resistivity measurements are used to study damage in CrSi_2 thin films induced by Ne, Ar, or Xe ion irradiation over a fluence range of 10^(10)–10^(15) ions cm^(−2). Irradiation produces a factor of 5–12 increase in film conductivity at the higher fluences. The influence of defect generation and recombination is evident. We speculate that formation of a compound defect is a dominant factor enhancing film conductivity. A temperature dependence at low fluences is reported and tentatively identified.

Dependence of ion-induced Pd-siliclde formation on nuclear energy deposition density

Abstract: Pd 2 Si formation at the Pd-Si interface induced by irradiation with ions having a wide range of nuclear energy deposition density has been investigated. It is found that the thickness of the suicide layer formed by irradiation is proportional to the ion fluence for irradiation with ions having low energy-deposition densities, while it is proportional to the square root of the fluence for irradiation with ions having energy-deposition densities. The results indicate that Pd 2 Si formation is reaction limited when the energy-deposition density at the interface is low and is diffusion limited when it is high. The results are compared with the phenomenological theory developed by Horino et al. and it is shown that such a dependence of the limiting processes on the energy deposition density is induced when the diffusion is thermally activated while the reaction at the interface is radiation-enhanced.

Incorporation of Monte Carlo Electron Interface Studies into Photon General Cavity Theory

Abstract: Electron Monte Carlo calculations using CYLTRAN and a new PHSECE (Photon-Produced Secondary Electrons) technique were carried out to estimate electron fluences and energy deposition profiles near LiF/Al and LiF/Pb material interfaces undergoing /sup 60/Co gamma irradiation. Several interesting and new features emerge: (a) although the buildup of the secondary electron fluences at the interfaces of the irradiated media is approximately exponential, the value of the electron mass fluence buildup coefficient, ..gamma.., is not equal to the electron mass fluence attenuation coefficient, ..beta..; (b) the ..beta.. value of the attenuation of the gamma generated electron fluences at the cavity/medium interfaces is strongly dependent on the Z of the adjacent material; and (c) for LiF/Pb there is a significant ''intrusion'' energy deposition mode arising from sidescattering in the wall material (lead).

Incorporation of Monte Carlo Electron Interface Studies into Photon General Cavity Theory

Abstract: Electron Monte Carlo calculations using CYLTRAN and a new PHSECE (Photon-Produced Secondary Electrons) technique were carried out to estimate electron fluences and energy deposition profiles near LiF/Al and LiF/Pb material interfaces undergoing /sup 60/Co gamma irradiation. Several interesting and new features emerge: (a) although the buildup of the secondary electron fluences at the interfaces of the irradiated media is approximately exponential, the value of the electron mass fluence buildup coefficient, ..gamma.., is not equal to the electron mass fluence attenuation coefficient, ..beta..; (b) the ..beta.. value of the attenuation of the gamma generated electron fluences at the cavity/medium interfaces is strongly dependent on the Z of the adjacent material; and (c) for LiF/Pb there is a significant ''intrusion'' energy deposition mode arising from sidescattering in the wall material (lead).

Calculations concerning voltage ripple of x-ray generators

Abstract: Formulae are given for absorbed dose, fluence and energy fluence of narrow X-ray beams generated by pulsating-potential X-ray generators with any waveform. A computer program has been written for calculating these and some other quantities, including mean energies, for particular waveforms. A new definition is suggested for the ripple correction factor of pulsating-potential X-ray generators for the assessment of their performance. Some numerical results and applications are discussed.

Laser produced percolation structures

Abstract: Pulsed laser irradiation of thin Ti films yields structures consisting of metallic islands connected by oxide filaments. The sheet resistivity of the structure is mostly determined by the degree of oxidation of the filaments and can be varied through the laser fluence. By adding gold to the initial layer, the same structures are obtained but oxidation is inhibited. The material is essentially a two‐dimensional bond percolator. The temperature coefficient of the resistance is drastically reduced compared to that of the previously investigated Ti‐TiOx structures.

Production of Electronically Excited P2 and in from ArF Excimer Laser Irradiation of InP

Abstract: We have investigated the decomposition of single-crystal InP surfaces irradiated by a 193 nm ArF excimer laser. These studies provide insight into mechanisms of thermal decomposition, surface diffusion and epitaxy. Pulsed laser exposure leads to evolution of P2 from the surface which is detected by resonance fluorescence resulting from a fortuitous overlap of the C1Σ u + , v′ = 11 ← X1Σ g + , v″ = 0 with the laser frequency. P2-evolution occurs above a threshold fluence of 0.12 J/cm2 and lags the peak laser intensity by ~20 nsec. These observations are explained by a thermally activated decomposition mechanism, as opposed to any direct, photochemical ejection process. Peak surface temperatures have been calculated and are used to predict P2 yields as a function of fluence and time which are in good agreement with experiments. These findings are also discussed in relation to previous studies of excimer laser stimulated growth of InP.

Study of Ion Implanted Copper Laser Mirrors by Spectroscopic Ellipsometry

Abstract: Implantation with 400 keV Ag or Cu ions improves the near-surface microstructural quality and reflectance of diamond turned and mechanically polished flat copper laser mirrors. Spectroscopic ellipsometry is sensitive to changes in either the microscopic surface roughness, or in the nearsurface substrate void fraction, and both parameters are observed to change upon implantation. Substrate density as a function of ion fluence peaks at about 5 × 1015cm-2. Low energy (300 eV) Ar ion implantation can cause either a reduction or increase in microscopic surface roughness, depending on fluence.

Shortcut Method for the Analysis of Reaction Characteristics in Infrared Multiphoton Dissociation

Abstract: It has been shown that the fluence dependence of the fractional conversion per pulse (which is often called the reaction characteristics) of infrared multiphoton dissociation is very useful in designing and evaluating the laser isotope separation processes. It was reported in our preceding works that it was possible to obtain this reaction characteristics through the procedure we proposed. In this note, this procedure is improved for promptanalysis using both numerical expressions and approximating equations for generalized relations between the dimensionless reaction volume and the dimensionless fluence.