Surface-initiated vapor deposition

The first is a pyrolytic approach in which the heat dehvered by the laser breaks chemical bonds in vapor-phase reactants above the surface, allowing deposition of the reaction products only in the small heated area. The second is a direct photolytic breakup of a vapor-phase reactant. This approach requires a laser with proper wavelength to initiate the photochemical reaction. Often ultraviolet excimer lasers have been used. One example is the breakup of trimethyl aluminum [75-24-1] gas using an ultraviolet laser to produce free aluminum [7429-90-5], which deposits on the surface. Again, the deposition is only on the localized area which the beam strikes. 

Inside" processes—such as modified chemical vapor deposition (MCVD) and plasma chemical vapor deposition (PCVD)—deposit doped silica on the interior surface of a fused silica tube. In MCVD, the oxidation of the halide reactants is initiated by a flame that heats the outside of the tube (Figure 4.8). In PCVD, the reaction is initiated by a microwave plasma. More than a hundred different layers with different refractive indexes (a function of glass composition) may be deposited by either process before the tube is collapsed to form a glass rod. 

To get around the difficulties involved in preparing for study an interface consisting of a monolayer of polyimide on a metal substrate, vapor-deposited components of polyimide (9-111 and model molecules for parts of the polyimide (11-121 on metals have been studied recently. Due to the limited size of these model molecules, they can be vapor-deposited in monolayers on the surface of clean metallic substrate in an ultra high vacuum system. In this way, useful information concerning the initial interface formation of (parts of) polyimide on a metal surface can be obtained. 

Alternatively PAA can be obtained without solvent by vapor deposition polymerization as described first by Salem et al. [2], In this technique the dianhydride py-romellitic and the dianhydride diamine (4,4 -oxidianiline) are codeposited onto a substrate, where they react to form PAA. Again the transformation to Polyimide is obtained by subsequent heating to temperatures up to 350°C. By comparison to spun dn films, initial interaction of the polymer with the substrate occurs in the uncomplexed PAA state. The chemical interaction between PAA and the metal establishes the adhesion of the final polyimide film. This is discussed in this communication for evaporated gold cluster and bulk silver surfaces. 

Chain initiation occurs when two monomer radicals are coupled to form a dimer biradical and proceeds further." This is an endothermic reaction requiring a heat of formation of 16 kcal/mol. Because of energetic concerns, chain initiation is unlikely to happen in the gas phase at low pressure. When the monomers are adsorbed onto the surface of the substrate, it is believed that, the high local concentration of monomers promotes the formation of biradicals assisted by van der waals forces. Models developed for vapor deposition polymerization of parylene-N indicate that initiation is a third order reaction with an activation energy of 24.8 kcal/mol. 

There is little information regarding the surface chemistry involved in the nucleation of amorphous silicon by photo-induced chemical vapor deposition (photo-CVD). The reason seems to be that effective chemical and physical means of detecting a small amount of silicon are hardly available at present. In our laboratory, the initial process of amorphous silicon (a-Si) formation from silanes or disilanes on Si02 substrate by photo-CVD has been studied by a new technique of chemical... 

Research into light-initiated chemical reactions and processes on solid surfaces is a growing new field which promises to yield a number of useful applications molecular photo-devices for super memory, photo-chemical vapor deposition to produce thin-layered electronic semiconducting materials, sensitive optical media, and the control of photochemical reaction paths, etc. In fact, photochemistry on solid surfaces is now a major field in a national research project on "Frontiers of Highly Efficient Photochemical Processes" sponsored by the Ministry of Education, Science and Culture of Japan. 

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