Laser surface reactions

Though as yet in its infancy, the application of laser Raman spectroscopy to the study of the nature of adsorbed species appears certain to provide unusually detailed information on the structure of oxide surfaces, the adsorptive properties of natural and synthetic zeolites, the nature of adsorbate-adsorbent interaction, and the mechanism of surface reactions. 

Conventional routes to ceramics involve precipitation from solution, drying, size reduction by milling, and fusion. The availability of well-defined mono-dispersed particles in desired sizes is an essential requirement for the formation of advanced ceramics. The relationship between the density of ceramic materials and the sizes and packing of their parent particles has been examined theoretically and modeled experimentally [810]. Colloid and surface chemical methodologies have been developed for the reproducible formation of ceramic particles [809-812]. These methodologies have included (i) controlled precipitation from homogeneous solutions (ii) phase transformation (iii) evaporative deposition and decomposition and (iv) plasma- and laser-induced reactions. 

Figure 16. Examples of deposition shapes in pyrolytic-laser-assisted CVD (a) first-order surface reaction, no mass-transfer effects (b) first-order surface reaction, depletion effects and (c) Langmuir-Hinshelwood surface kinetics, no mass transfer effects. The ratio r/w is the radial position relative to the beam...

From all these XPS informations in order to take into account the fact that after the laser treatments several species like isoimide, C=N-,-CO,0-C-0, and carboxylic groups are present at the surface of polyimide, we propose the following surface reaction under UV laser radiation above the ablation threshold in presence of air moisture ... 

Photo-induced reaction on a metal surface usually consists of several elementary reactions and it is difficult to model the whole reaction process. However, any reactions need to be triggered by electronic excitation. As stated in Section 20.1.4, the major mechanism is indirect excitation thus we focus on modeling the indirect excitation reaction. Since desorption from the surface is one of the simplest processes and can be a prototype for other complex surface reactions, DIET or DIME are clearly the best to study [10, 48, 53, 57, 96]. In photochemistry, continuous wave or nanosecond lasers lead to DIET, where desorption increase linearly with fluence. In contrast, the DIMET process is caused by intense and short laser pulses on the picosecond or femtosecond time scale, with nonlinear dependence on fluence. Since the fluence is proportional to the number of created hot electrons in the bulk, linear... 

During this study, we have found that laser intensity is one of the important factors that control laser surface chemistry. At a small laser intensity, molecules adsorbed on solid surfaces dissociate into atoms and radicals. Some of these atoms or radicals react with atoms of the solid substrates. At a large laser intensity, atoms are photoablated from the solid surfaces to react with the molecules adsorbed or in the gas phase. Hence, we describe in this paragraph a) the dynamical study of UV laser photodissociation of halogen or metal-containing molecules on solid surfaces, b) reactions of atoms generated in the photodissociation of an adsorbate with solid surfaces, and c) reactions of molecules in the gas phase with the photoelectrons or metal atoms generated on intense laser irradiation of solid surfaces. 

Laser-induced reaction has been widely used to stimulate gas-surface interaction. Lasers are also used to probe molecular dynamics in heterogeneous systems as well. In the applied area, the laser photochemical techniques are successfully applied to produce well defined microstructures and new materials for microelectronic devices (1). Enhanced adsorption and chemical reaction on surfaces can be achieved by a photoexcitation of gaseous molecules, adsorbed species as well as solid substrates. The modes of the excitation include vibrational and electronic states of the gaseous species and of the adsorbates surface complexes. Both a single and a multiple photon absorption may be involved in the excitation process. 

Efforts to elucidate the reaction mechanisms of the laser-induced surface reactions have been increasing during these few years. In order to elucidate these reaction mechanisms, features of the reaction on a solid surface should be studied in detail, namely, effect of laser energies, laser fluence, concentration of adsorbed species, the pressure of the gas phase and so on. 

Apart from desorption, surface reaction with adsorbate can be stimulated by the laser irradiation. In this chapter we will demonstrate the formation of new surface species by the CO2 laser induced reaction of CDF3 with the surface of SIO2 (17,18). In order to elucidate the mechanism of the reaction especially to determine the surface species, ir spectroscopy was used. A systematic investigation was performed Including the determination of reaction yields as a function of the laser frequency, the laser intensity and the gas pressure as well as the reaction products, and the determination of the correlation between the excited species and the reaction path. 

The dependence of the laser frequency on the yield of the surface reaction is examined. The decrease in the amount of the IR absorption band of surface OH is shown in Fig. 2, in which the dehydration yield of surface OH exhibited the frequency dependence similar to that of the absorption property of the Si02 substrate rather than that of gaseous CDF3. Dehydration yield was second order to the amount of the surface OH, that is AfOH s A[0H], which indicated that two -OH were involved in this reaction. Furthermore, the effect of laser fluence to the... 

The reaction between COF3 and the Si02 surface under the irradiation of the CO2 laser consists of two reaction paths. Namely, dehydration of the surface OH and the reaction of OH and physisorbed CDF3. For the surface reaction of OH and the CDF3, it is initiated by the excitation of physisorbed CDF3 to the first vibrational state (v=l), followed by an excitation to v=3 by an intermolecular energy transfer. These excited species react with the surface OH to form OD(ad) and CF(ad). The surface species formed by this reaction influence the absorption property of the surface and plays an Important role in the reaction after the initial stage. 

Photochemical surface reactions of polymer systems are an important field not only from the point of view of micro-electronic materials processing, but also from a more general scientific and materials application perspective. We have reviewed our studies in this field, which include investigations of excimer laser ablation, studies of the photo-oxidation of polymer surfaces, and the use of surface cross-linking and surface polymer depositions for microlithographic applications. With the increasing miniaturization of microelectronic devices, the fundamental and the applied aspects of surface photochemistry of polymers becomes increasingly important. 

Finally in this Section on infrared photochemistry, the i.r. laser-induced reaction of SF with a Si surface has been described. Pulsed COj laser radiation in the absence of SF causes momentary heating of a Si target, but no Si removal in the presence of a few Torr of SF, etching of the Si is observed. The mechanism for the process is yet to be established, but it appears that both the reactions of excited species produced by the laser, and the effect of the laser radiation at the gas-surface interface are of importance. ... 

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