Surface reaction control chemical vapor deposition

This CVD procedure is somewhat different from that used to deposit semiconductor layers. In the latter process, the primary reaction occurs on the substrate surface, following gas-phase decomposition (if necessary), transport, and adsorption. In the fiber optic process, the reaction takes place in the gas phase. As a result, the process is termed modified chemical vapor deposition (MCVD). The need for gas-phase particle synthesis is necessitated by the slow deposition rates of surface reactions. Early attempts to increase deposition rates of surface-controlled reactions resulted in gas-phase silica particles that acted as scattering centers in the deposited layers, leading to attenuation loss. With the MCVD process, the precursor gas flow rates are increased to nearly 10 times those used in traditional CVD processes, in order to produce Ge02-Si02 particles that collect on the tube wall and are vitrified (densified) by the torch flame. 

Chemical vapor deposition is a key process for the growth of electronic materials for a large variety of devices essential to modern technology. Its flexibility and relatively low deposition temperatures make CVD attractive for future device applications in Si and compound-semiconductor technologies. The process involves gas-phase and surface reactions that must be controlled to achieve desired material and electronic properties. 

Chemical vapor deposition (CVD) is an atomistic surface modification process where a thin solid coating is deposited on an underlying heated substrate via a chemical reaction from the vapor or gas phase. The occurrence of this chemical reaction is an essential characteristic of the CVD method. The chemical reaction is generally activated thermally by resistance heat, RF, plasma and laser. Furthermore, the effects of the process variables such as temperature, pressure, flow rates, and input concentrations on these reactions must be understood. With proper selection of process parameters, the coating structure/properties such as hardness, toughness, elastic modulus, adhesion, thermal shock resistance and corrosion, wear and oxidation resistance can be controlled or tailored for a variety of applications. The optimum experimental parameters and the level to which... 

Generally speaking, PEVD is a modified form of chemical vapor deposition (CVD). A comparison between PEVD and conventional CVD is schematically shown in Figure 1 for a product (D) formed from reactants (A) and (B). In a CVD process, both reactants (A) and (B) are supplied through a vapor phase at the same side of a solid substrate (E). They react chemically at the surface of the solid substrate (E), aided by some type of catalytic effect, to form a desired product (D). In a PEVD process, one reactant (A) is transported from one side (source) of a solid substrate (E) to the other side (sink) under well-controlled thermodynamic and kinetic conditions. At the sink side, reaction with (B) occurs to form (D). Further growth of (D) into a continuous thin film with the desired thickness in a PEVD process also relies on (A) transported in the solid state now through (E) and (D) to react with (B). 

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. 

Chemical vapor deposition (CVD) of tetraethoxysilane on HZSM5 was performed stepwise under well-controlled, mild conditions. Several test reactions were performed over the series of modified samples. Under mild conditions, CVD follows first order kinetics with respect to uncovered external sites on the zeolite crystals. The external surface is homogeneous with regard to both CVD and catalytic activity. Reactions, which are controlled by strong internal mass transfer restrictions, do respond in a way, which indicates that CVD causes pore mouth plugging rather than pore mouth narrowing. 

Chemical vapor deposition is an important technique in the production of all kinds of solid state devices. In the process, the active metal organic vapor is swept into a two-dimensional slit reactor by a carrier gas, and deposition occurs at the hot top and bottom plates. The reaction at the plate surfaces can be written MO —> M - - O. Assuming a laminar operation, develop an expression to compute the rate of loss of MO for diffusion-controlled process. 

Another technique proposed for the inside process is plasma chemical vapor deposition (PCVD). In this version of the process, a low-pressure plasma is generated inside the tube that does not lead to soot generation, but rather allows a heterogeneously nucleated chemical reaction to occur at the inner surface of the tube, so that the glass is built up in molecular-scale layers. This is a true chemical vapor deposition (CVD) process. More layers are required than with soot, but the process can be controlled more precisely. Unfortunately, the process has not yet proved commercially economical. 

---