Laser focused deposition

Laser assisted chemical vapor deposition is an evolutionary extension of the metal particle catalyzed chemical vapor deposition, wherein a hot laser focus takes the place of a hot solid or liquid metal particle catalyst (Figure 1). (Conventional chemical vapor deposition has no "hot spot" capable of preferentially focusing the vapor phase deposition. 

Figure Metal particle catalyzed and laser assisted chemical vapor deposition. Left Chemical vapor deposition causes the formation of a film or coating on a hot surface. Center and right Metal catalyzed and laser assisted chemical vapor deposition causes the formation of a potentially continuous fiber with a diameter corresponding to the hot metal catalyst particle or laser focus respectively. Redrawn from F. T. Wallenberger, P. C. Nordine and M. Boman, Inorganic fibers and microstructures directly from the vapor phase, Composites Science Technology, 5,193-222 (1994).

Short (or discontinuous) fibers are best prepared in a batch process, e.g., in a small cylindrical reaction chamber. The value of the technology, however, lies in its capability to facilitate the growth of continuous (potentially endless) fibers with a recently discovered automatic self-regulating growth mechanism [2], Finally, the diameter of the laser focus determines the diameter of fibers grown by laser assisted chemical vapor deposition, just as the diameter of the metal particles determines the diameter of the whiskers grown by metal catalyzed chemical vapor deposition. 

Mass transfer in metal catalyzed and in laser assisted CVD processes is driven by highly localized temperature gradients. The relatively small area of either a hot molten metal particle or of a hot laser focus affords whiskers [4] or continuous fibers, respectively [2] [18-19]. The transfer of an equal mass from the vapor to the solid phase in a conventional chemical vapor deposition results in a thin coating over the relatively large area of a hot surface, i.e., that of a flat complex shaped composites part. 

Chemical vapor deposition Deposition of solid material when gaseous reactants encounter a hot surface. Laser assisted CVD employs a hot laser-focus rather than a hot surface. 

Figure 17 Stresses measured from the TGO as a fimetion of distance from laser focus to the surface of the TGO by PLPS teehnique (A) without and (B) with a TBC, The TGO was formed on Fecralloy for 20 hours at 1150 °C, and then a layer of YSZ coating ( 80 jm) was deposited by the EBPVD method.

Fig. 17. Photomicrograph of a UV-lithographic T feature showing 1 pm-wide spaces and 2 pra wide bars in the center of a 300 pm wide square (sample 10 monolayers of polymerized Sa, deposited on an oxidized silicon wafer by the LB-technique). (Reproduced with permission from Ref. >, Copyright 1982, Laser Focus)...

A new discovery exploits a set of multiplexed pulsed lasers applied for the first time to solids in general, and specifically to film deposition. The process utilizes excimer, YAG Nd and CO2 lasers focused on WC/Co substrate in ambient with CO2 and N2 as shrouding gases... 

CVD processing can be used to provide selective deposition on certain areas of a surface. Selective tungsten CVD is used to fill vias or holes selectively through siUcon oxide layers in siUcon-device technology. In this case, the siUcon from the substrate catalyzes the reduction of tungsten hexafluoride, whereas the siUcon oxide does not. Selective CVD deposition can also be accompHshed using lasers or focused electron beams for local heating. 

Electrical and Electronic Applications. Silver neodecanoate [62804-19-7] has been used in the preparation of a capacitor-end termination composition (110), lead and stannous neodecanoate have been used in circuit-board fabrication (111), and stannous neodecanoate has been used to form patterned semiconductive tin oxide films (112). The silver salt has also been used in the preparation of ceramic superconductors (113). Neodecanoate salts of barium, copper, yttrium, and europium have been used to prepare superconducting films and patterned thin-fHm superconductors. To prepare these materials, the metal salts are deposited on a substrate, then decomposed by heat to give the thin film (114—116) or by a focused beam (electron, ion, or laser) to give the patterned thin film (117,118). The resulting films exhibit superconductivity above Hquid nitrogen temperatures. 

Because of the possibility of focusing laser beams, tlrin films can be produced at precisely defined locations. Using a microscope train of lenses to focus a laser beam makes possible tire production of microregions suitable for application in computer chip production. The photolytic process produces islands of product nuclei, which act as preferential nucleation sites for further deposition, and tlrus to some unevenness in tire product film. This is because the subsuate is relatively cool, and therefore tire surface mobility of the deposited atoms is low. In pyrolytic decomposition, the region over which deposition occurs depends on the drermal conductivity of the substrate, being wider the lower the thermal conductivity. For example, the surface area of a deposit of silicon on silicon is nanower dran the deposition of silicon on silica, or on a surface-oxidized silicon sample, using the same beam geomeU y. 

Of special Interest as O2 reduction electrocatalysts are the transition metal macrocycles In the form of layers adsorptlvely attached, chemically bonded or simply physically deposited on an electrode substrate Some of these complexes catalyze the 4-electron reduction of O2 to H2O or 0H while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have Included (a) visible reflectance spectroscopy (b) laser Raman spectroscopy, utilizing surface enhanced Raman scattering and resonant Raman and (c) Mossbauer spectroscopy. This paper will focus on principally the cobalt and Iron phthalocyanlnes and porphyrins. 

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