Laser-assisted CVD

Chemical Vapor Deposition. In chemical vapor deposition (CVD), often referred to as vapor transport, the desired constituent(s) to be deposited are ia the form of a compound existing as a vapor at an appropriate temperature. This vapor decomposes with or without a reducing or oxidizing agent at the substrate— vapor interface for film growth. CVD has been used successfully for preparing garnet and ortho ferrite films (24,25). Laser-assisted CVD is also practiced. 

There are numerous materials, both metallic and ceramic, that are produced via CVD processes, including some exciting new applications such as CVD diamond, but they all involve deposition on some substrate, making them fundamentally composite materials. There are equally numerous modifications to the basic CVD processes, leading to such exotic-sounding processes as vapor-phase epitaxy (VPE), atomic-layer epitaxy (ALE), chemical-beam epitaxy (CBE), plasma-enhanced CVD (PECVD), laser-assisted CVD (LACVD), and metal-organic compound CVD (MOCVD). We will discuss the specifics of CVD processing equipment and more CVD materials in Chapter 7. 

Donor adducts of aluminum and gallium trihydride were the subject of considerable interest in the late 1960s and early 1970s.1 Thin-film deposition and microelectronic device fabrication has been the driving force for the recent resurgence of synthetic and theoretical interest in these adducts of alane and gallane.24 This is directly attributable to their utility as low-temperature, relatively stable precursors for both conventional and laser-assisted CVD,59 and has resulted in the commercial availability of at least one adduct of alane. The absence of direct metal-carbon bonds in adducts of metal hydrides can minimize the formation of deleterious carbonaceous material during applications of CVD techniques, in contrast to some metal alkyl species.10, 11... 

Pyrolytic-laser-assisted CVD is analogous to thermally driven CVD, but instead of a diffuse heating source, a focused laser beam is used to define deposition areas spatially (32, 38, 39) or to heat the gas phase selectively (228). The use of laser has the added advantages of increased energy flux and rapid heating. To avoid photochemistry, the gas phase must be transparent to the radiation. 

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...

In the case of hot-filament CVD, refractory metal filaments (e.g., W, Ta, Re, etc.) are electrically heated to very high temperatures (between 2000 and 2700°C) to produce the necessary amount of atomic hydrogen that is necessary for the reasons mentioned above for the synthesis of diamond. Except for combustion flame CVD, hot-filament CVD is considered the simplest of all of the methods and also the most inexpensive. Plasma-jet and laser-assisted CVD methods rely on a plasma torch or laser to attain the very high temperatures that are needed to... 

In the first chapter of this book, an overview of CVD techniques has been given, and more detailed descriptions can be found in several textbooks [9, 10]. Many different CVD reactors have been used for the deposition of conducting films, i.e., thermal, UV-enhanced CVD (UVCVD), laser-assisted CVD (LACVD), plasma-enhanced CVD (PECVD) and metal-organic CVD (MOCVD). In addition, two techniques were included, which are not typically part of CVD, chemical transport and spray pyrolysis. 

Laser-assisted CVD (LACVD) is a technique for localized processing. In pyrolytic LACVD the laser beam is used as a heating source. The chemical reactions take place only in the heated spot allowing for direct writing of TiN lines on substrates. Silvestre et al. [57, 58] have studied LACVD of TiN using TiCU, N2 and FI2 and a CO2 laser. They were able to deposit golden colored lines of TiN close to stoichiometry with laser powers of 400 to 700 W. 

Nanometric boron carbide particles can also be prepared by CVD. The reaction of boron trihalides with carbon or gaseous carbon-containing precursors using radiofrequency (RF) plasma [142, 143] or laser-assisted CVD [144] has been appUed. 

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. 

Figure 8. Boron fibers made by hot filament and by laser assisted CVD. This illustration compares the fiber diameter and surface character of a sheath/core boron/tungsten fiber (A,C) with that of a pure boron fiber (B,D). Reproduced from F. T. Wallenberger and P. C. Nordine, Strong, Small Diameter Boron Fibers by Laser Assisted Chemical Vapor Deposition, Materials Letters, 14 [4] 198-202 (1992). With permission from Elsevier Publishers (1992).

F. T. Wallenberger, Inorganic fibers and microfabricated parts by laser assisted CVD synthesis and phase transformations, in Novel Techniques in Synthesis and Processing of Advanced Materials, J. Singh and S. M. Copley, eds., The Metals and Materials Society, pages 251-260 (1995). 

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. 

Laser-assisted CVD of BN can originate from a gaseous reactant [58], or the plasma can be formed by irradiating a target consisting mainly of BN [59 to 63, 139]. The cluster distribution of boron nitride in a laser plasma and the structure of the BN phases in the case of laser-induced plasma deposition have been studied [64]. It is also reported that a combination of an electron cyclotron plasma with laser irradiation produces a coating which consists of p-BN and y-BN [65]. 

There are no obstacles in searching for novel approaches to diamond synthesis by studying new growth processes electrolysis, hydrothermal (15) and laser assisted. Graphite has been transformed to diamond by laser process (16,17) and some laser assisted CVD processes were... 

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