Laser-induced Reactions Research Articles

Excimer Laser-Induced Surface Reaction of Fluoropolymers with Liquid Water

Excimer Laser-Induced Surface Reaction of Fluoropolymers with Liquid Water

Unconventional Laser Chemistry. Laser-Induced Chemical Modification of the Surface of Fluoropolymers.

Excimer laser-induced photochemical reactions were applied to the surface modification of fluoropolymers. Irradiation of excimer laser pulses onto fluoropolymer films in contact with various solutions of reactants provided the surface with chemical functionalities of chemical affinity to water, solvents, and adhesives. The reactions proceed through electron transfer from excited states of the reactants, produced in high concentrations by the intense laser irradiation, to the polymer surfaces.

Computational modeling of solid-state reactions in the NiSi systems induced by pulsed lasers

A mathematical model describing pulsed laser-induced solid-state chemical reactions in thin metal film-semiconductor systems is developed in this paper. Following experimental evidence, the chemical reactions are supposed to be driven by diffusion effects. In accord with the usual experimental conditions in pulsed-laser irradiation of thin films, the problem is treated in one space dimension. The mass, momentum, and energy balances are formulated under the assumption that the convective transfer and the cross effects are negligible. This leads to a system of partial differential equations for heat conduction and species diffusion coupled through the heat-source/sink term due to chemical reactions and the temperature dependence of the diffusion coefficients. Numerical solution of the mathematical model is performed using the finite element method. Computational implementation of the model is discussed and typical results of numerical experiments are presented.

Time-Resolved REMPI Detection of Methyl Radicals Generated in Laser-Induced Unimolecular Reactions

Time-Resolved REMPI Detection of Methyl Radicals Generated in Laser-Induced Unimolecular Reactions

Deposition and analysis of silicon clusters generated by laser-induced gas phase reaction

Silicon clusters have been generated by CO2-laser-induced decomposition of SiH4 in a flow reactor. By introducing a conical nozzle into the reaction zone, they are extracted into a molecular beam apparatus and analyzed with a time-of-flight mass spectrometer. The mass spectra show that the cluster source emits, besides small clusters, also nanosized species containing around 103 atoms. These clusters were deposited on silicon and sapphire targets at room temperature. The deposited films have been analyzed with a Raman spectrometer and with a field emission scanning electron microscope (FE-SEM). The Raman spectra reveal a broad amorphouslike band and a relatively sharp peak at 518.1 cm−1. Interpretation of the sharp Raman feature, based on the phonon confinement model, suggests the presence of silicon nanocrystallites in the deposited films with a particle size of about 3–3.6 nm in diameter. The FE-SEM micrographs show an agglomerate of spherical particles of 3–12 nm in diameter, with a pronounced maximum in the size distribution at around 3.5 nm. The various methods of characterization allow us to conclude that the size of the nanoclusters is largely preserved if they are deposited on the substrate. Therefore, the technique presented here might be an efficient means to produce silicon quantum dots of about 3 nm in diameter.

Numerical simulation of the formation of Ni silicides induced by pulsed lasers

Pulsed laser-induced solid-state chemical reactions in an Ni-Si system are studied in the paper. Physico-chemical and mathematical models of the Ni-silicides formation are formulated, a numerical solution using the Galerkin finite-element method is described, and a computational algorithm is developed. Further, the XeCl excimer laser irradiation of a thin nickel film (1 nm to 2000 nm)-silicon system is simulated, with energy densities of the laser pulse which are insufficient to melt the surface of the sample. Typical results of the numerical experiments are presented and the influence of the diffusion coefficients of Ni in Si and Si in Ni recommended by various authors on the calculated temperature and concentration fields is studied. A comparison of the results of numerical simulations with experimental findings previously published in the literature reveals that the lattice diffusion cannot explain the experimental data of laser synthesis of silicides and that grain-boundary diffusion is probably the dominant mechanism of mass transfer at the laser irradiation of Ni-Si systems.

Production of boron carbides by IR laser-induced gas phase reactions

Abstract The irradiation of gaseous mixtures of nido-decaborane (B10H14) and ethylene by a TEA CO2 laser results in the formation of solid deposits which can be identified, on the basis of X-ray photoelectron spectroscopy (XPS) and Fourier transform IR (FTIR), to consist of boron-carbon compounds. Deposits with a wide range of B C stoichiometries can be prepared by varying the ratio of decaborane to ethylene partial pressures.

Collinear Quantum Calculation of Transition-State Spectrum for the K + NaCl Reaction

Abstract The laser-induced reaction, K + NaCl + hν → KCl + Na*, has been studied quantum-mechanically, with consideration of only collinear geometries. Laser-dressed equations are numerically solved by the R-matrix propagation method. Quantum reaction probabilities are dominated by many resonances corresponding to the vibrational states of the reaction complex formed in the reaction. The results obtained are compared with surface hopping trajectory calculations. The trajectory surface hopping model fails in predicting the translational energy dependence of the reaction probabilities but a Landau–Zener model is qualitatively correct in the calculation of the laser wavelength dependence of the reaction probabilities. These results suggest that the dynamics of the laser-induced reaction can be divided into two independent processes; the ground state reaction K + NaCl → KCl + Na and the non-adiabatic transition from the laser-dressed ground state to the excited state.

TEA CO 2 laser-induced telomerization chemistry in conventional and waveguide photoreactors

Experimental approaches to the chemical synthesis of gas phase radicals by IR multiple photon excitation of polyatomic molecules are considered. The transverselyxcited atmospheric (TEA) CO 2 laser-induced telomerization reaction of C 2F 4 and CF 3I was studied in conventional and waveguide photolysis reactors. The difference in the product distribution in the two cases can be explained in terms of the various competing rate constants and the concentration of the perfluoroalkyl radicals generated in the irradiated volume.

Laser chemical vapour deposition for microelectronics

The laser microchemical direct-write process is a powerful method to fabricate microstructures required for drawing interconnection networks in customised circuits and remodelling prototype circuits. The laser microchemical direct-write technique is presented by reviewing some laser-induced chemical reactions involved in the deposition of materials used in the microelectronics field. Investigation of the deposition of metals and silicon micrometer-size lines or dots using a CW argon-ion laser operating at wavelengths around 0.5 μm, is described. The growth kinetics, morphology and electrical resistivity of the deposits are studied in detail at various scanning speeds of the laser spot, laser-beam powers and reactant gas pressures. Several photon fluxes have been used to emphasise any photolytic effect. Investigations of the kinetics of the chemical reactions allow us to propose reaction mechanisms for the decomposition of the reactive molecules used. On the basis of reported results, the use of the laser direct-write process as a tool for restructuring integrated circuits is presented.

Laser ablation of refractory material, cluster formation and deposition

The mass spectrum of titanium-carbon clusters produced in the gas phase by laser-induced plasma reaction shows that Ti 8C 12 (met-car) is a stable species, which coexists with Ti n C 2 n clusters. But, the transmission electron microscopy measurements indicate that only TiC nanometer crystals with rocksalt-like structure are in the laser-deposited TiC films, and have a mean size of about 50 Å. The scanning electron microscopy observation and the measurement of the film conductivity show that the films are continuous and uniform above a thickness of 40 Å.

Synthesis of iron(III) oxide ultrafine powders by laser-induced vapour-phase reaction

Iron(III) oxide ultrafine powders have been synthesized by laser-induced vapour-phase reaction using an IR pulsed CO2 laser as light source and a mixture of Fe(CO)5–O2 as reactant without adding any sensitizes. The optimum experimental conditions were presented. A single laser pulse can initiate an explosive reaction provided that the total Fe(CO)5–O2 pressure exceeds 5 kPa and the O2–Fe(CO)5 partial pressure ratio is 1. The reaction results in the complete disappearance of iron carbonyl and the energy consumption is very low compared with the conventional chemical process. The maximum quantum efficiency is ca. 102, suggesting that the explosive reaction may proceed according to a chain reaction mechanism. The relationships between the total pressure and the quantum efficiency and laser fluence threshold were both derived. IR, XRD, TEM and XPS were used to characterize the powders. The experimental results showed that the powders consisted of both crystalline γ-Fe2O3 and amorphous Fe2O3. The former phase is 50–100 nm in diameter while the latter is 5–12 nm in diameter. This is the first time that crystalline γ-Fe2O3 prepared in this way has been found to be present as regular polyhedra and not spherical particles.

Laser-induced Reactions in a Deep UV Resist System: Studied with Picosecond Infrared Spectroscopy

ABSTRACTOne of the most technologically important uses of organic photochemistry is in the imaging industry where radiation-sensitive organic monomers and polymers are used in photoresists. A widely-used class of compounds for imaging applications are diazoketones; these compounds undergo a photoinduced Wolff rearrangement to form a ketene intermediate which subsequently hydrolyses to a base-soluble, carboxylic acid. Another use of organic molecules in polymer matrices is for dopant induced ablation of polymers. As part of a program to develop diagnostics for laser-driven reactions in polymer matrices, we have investigated the photo-induced decomposition of 5-diazo-2,2-dimethyl-l,3-dioxane-4,6-dione (5-diazo Meldrum's acid, DM) in a PMMA matrix using picosecond infrared spectroscopy. In particular, irradiation of DM with a 60 ps 266 nm laser pulse results in immediate bleaching of the diazo infrared band (v = 2172 cm-1). Similarly, a new band appears within our instrument response at 2161 cm-1 (FWHM = 29 cm-1) and is stable to greater than 6 ns.; we assign this band to the ketene photoproduct of the Wolff rearrangement. Using deconvolution techniques we estimate a limit for its rate of formation of τ < 20 ps. The linear dependence of the absorbance change with the pump power (266 nm) even above the threshold of ablation suggest that material ejection take place after 6ns.

Laser-Assisted Dry Etching Ablation for Microstructuring of III-V Semiconductors

ABSTRACTLaser-assisted dry etching ablation (LADEA) has been reviewed with an emphasis on its applicability for the microstructuring of III-V semiconductor compounds. The method is based on the application of an excimer laser ( λ= 308 nm) for pulsed heating of a wafer which is placed in a stream of Cl2/He gas. Both the products of chemical reaction and the depth to which a laser-induced reaction takes place depend on laser fluence. This makes possible the ablation of a well defined volume of the material. Little or no structural damage to the surface is observed because ablation is carried out with laser fluences below those required to melt the matrix material. The laser fluence dependence of the etch rate indicates that the process is primarily temperature driven with a characteristic energy for desorption. We have investigated LADEA as a method for in-situ processing of III-V semiconductors and the fabrication of nanostructures. An atomic force microscopy study has shown that atomically smooth surfaces can be obtained if the etch rate is near 1/2 atomic layer per laser pulse. The lateral resolution of LADEA has been found to be at least 20 nm. This, as well as the results of in-situ photoluminescence and Auger electron spectroscopy measurements, indicate that LADEA can be used for the direct (photoresist-free) fabrication of high quality microstructures and, ultimately, for the nanostructuring of III-V semiconductor compounds.

Laser-induced photochemical reaction of bis(ethynylstyryl) benzene prepared by molecular beam deposition.

We have investigated the photochemical reaction in the external photon field induced with KrF excimer laser (λ=248nm) for the bis(ethynylstyryl)benzene (BESB) films deposited by a molecular beam deposition technique. The cis-cis isomer crystalline films have grown on the KBr (001) substrate without laser irradiation. The laser irradiation to the film surface after and during deposition at the substrate temperature of 25°C caused the cis-to-trans photoisomerization and gave amorphous films due to disordering of the molecular arrangement. On the contrary, the photoisomerized trans-trans isomer crystals were found to be grown upon the irradiation during deposition at the substrate temperature of 60°C. These results indicate that the applied photon field to the molecular beam and the substrate temperatures during deposition play a significant role in thefilm structures.

Raman characterization of amorphous carbon produced by fullerite laser-induced transformation

MicroRaman spectroscopy has been used to investigate the products of laser-induced cage opening reactions in fullerite films. Laser irradiation of fullerite under oxidative conditions produces a conversion to amorphous carbon. The analysis of the Raman spectra suggests the formation of a partially ordered a-C phase. The relation between its structure and the fullerene fragmentation process is discussed.

Carbon Dioxide laser Synthesis of Ultrafine Silicon Carbide Powders from Diethoxydimethylsilane

Ultrafine SiC powders have been synthesized from diethoxydimethylsilane by laser‐induced vapor‐phase reactions. The powders produced under different flow rates of the reactant vapor have been characterized by chemical analysis, transmission electron microscopy, infrared spectros‐copy, and X‐ray diffractometry. The results show that the powders are less than 30 nm in average particle diameter, uniformly distributed, spherical in shape, and amorphous. The oxygen and free carbon of the powders vary with the vapor flow rate of the reactant. Annealing of the powders at 1873 K in nitrogen for 1 h results in a complete transformation from amorphous SiC to β‐SiC and a small amount of α‐SiC.

Laser-induced oxidation reactions in supercritical ethaneO2 mixtures

Abstract The laser-induced oxidation of ethane by O 2 at 318 K was investigated with varying the pressure between 12–91 atm. The reaction condition was regarded as the supercritical phase above 50–60 atm, depending on the O 2 fraction. Ethanol, acetaldehyde, and CO 2 were mainly produced at any reaction conditions, together with small amounts of C 1 compounds and formic esters. The kinetic discussion for the time dependence indicated that the consecutive photolysis of primary products takes place during the subsequent laser irradiation period. The branching ratio to CO 2 formation in the primary process in the supercritical phase is much smaller than that in the gas phase, and the selectivities for ethanol and acetaldehyde show a discontinuous change near the critical point. These facts show that the supercritical phase affects this complex radical reaction system. The primary photoabsorption process is also discussed.

Laser chemical processing of magnetic materials for recording-head application

A laser-induced thermochemical reaction has been used to etch single crystalline magnetic ferrite for industrial application to recording-head processing. Scanning focused Ar-ion laser irradiation to ferrite in flowing aqueous solutions of KOH or H3PO4 resulted in etch rates of 350–400 μm/s with aspect ratios of up to 40. Metal-In-Gap (MIG) head structures with Sendust (FeAlSi) have been successfully fabricated using laser chemical processing. The MIG head fabricated by laser chemical processing showed better performance than that by conventional mechanical machining.

Laser synthesis of ultrafine Si 3N 4SiC powders from hexamethyldisilazane

Ultrafine Si 3Ni 4SiC powders have been successfully prepared from hexamethyldisilazane ((Me 3Si) 2NH) (Me  CH 3) and NH 3 by a laser-induced gas phase reaction. An optimization experiment aimed at increasing the Si 3N 4 content of the powders has been done with an orthogonal design of the main processing parameters, including the laser power and the gas flow rates of (Me 3Si) 2NH and NH 3. The Si 3N 4 content of the powders has been raised to 70 wt.% and the corresponding processing parameters determined. The powders produced have been characterized by chemical analysis, transmission electron microscopy, IR spectroscopy and X-ray diffraction. They are white-grey in colour, free of hard agglomerates, of average particle size 20 nm and their chemical composition changed with the processing parameters.