Deposition reactor

The epitaxy reactor is a specialized variant of the tubular reactor in which gas-phase precursors are produced and transported to a heated surface where thin crystalline films and gaseous by-products are produced by further reaction on the surface. Similar to this chemical vapor deposition (CVE)) are physical vapor depositions (PVE)) and molecular beam generated deposits. Reactor details are critical to assuring uniform, impurity-free deposits and numerous designs have evolved (Fig. 22) (89). 

Microwave-Plasma Deposition. The operating microwave frequency is 2.45 GHz. A typical microwave plasma for diamond deposition has an electron density of approximately 10 electrons/m, and sufficient energy to dissociate hydrogen. A microwave-deposition reactor is shown schematically in Fig. 5.18 of Ch. 5.P ]P°]... 

Many semibatch reactions involve more than one phase and are thus classified as heterogeneous. Examples are aerobic fermentations, where oxygen is supplied continuously to a liquid substrate, and chemical vapor deposition reactors, where gaseous reactants are supplied continuously to a solid substrate. Typically, the overall reaction rate wiU be limited by the rate of interphase mass transfer. Such systems are treated using the methods of Chapters 10 and 11. Occasionally, the reaction will be kinetically limited so that the transferred component saturates the reaction phase. The system can then be treated as a batch reaction, with the concentration of the transferred component being dictated by its solubility. The early stages of a batch fermentation will behave in this fashion, but will shift to a mass transfer limitation as the cell mass and thus the oxygen demand increase. 

Manufacturing economics require that many devices be fabricated simultaneously in large reactors. Uniformity of treatment from point to point is extremely important, and the possibility of concentration gradients in the gas phase must be considered. For some reactor designs, standard models such as axial dispersion may be suitable for describing mixing in the gas phase. More typically, many vapor deposition reactors have such low L/R ratios that two-dimensional dispersion must be considered. A pseudo-steady model is... 

Steady performance data from the second reactor are shown in Figure 11.10, where the pressure drop did not rise exponentially and the conversion and selectivity remained at 75 and 83%, respectively. The reactor was further analyzed after operation, shown in Figure 11.11, to confirm the lack of carbon deposition. Reactor models were pivotal to developing a robust design for this high-temperature and... 

Application of Supercomputers To Model Fluid Itansport and Chemical Kinetics in Chemical Vapor Deposition Reactors... 

Figure 1. Schematic of the rotating>disk chemical vapor deposition reactor.

Consider Equations (6-10) that represent the CVD reactor problem. This is a boundary value problem in which the dependent variables are velocities (u,V,W), temperature T, and mass fractions Y. The mathematical software is a stand-alone boundary value solver whose first application was to compute the structure of premixed flames.Subsequently, we have applied it to the simulation of well stirred reactors,and now chemical vapor deposition reactors. The user interface to the mathematical software requires that, given an estimate of the dependent variable vector, the user can return the residuals of the governing equations. That is, for arbitrary values of velocity, temperature, and mass fraction, by how much do the left hand sides of Equations (6-10) differ from zero ... 

The ID fluid discharge model has been applied to the ASTER deposition system (see Section 1.2.4). The deposition reactor has an inner volume of 10 1 and an inner diameter of 20 cm. The upper electrode is grounded (see Fig. 4a), and the powered electrode is located 2.7 cm lower. Other typical silane-hydrogen discharge parameters are summarized in Table IV. 

FIG. 68. Cross-sectional view of a HWCVD deposition reactor. (From K. F. Feenstra. Ph D. Thesis, Universiteit Utrecht. Utrecht, the Netherlands. 1998. with permission.)... 

For illustrative purposes, the process of deposition of Si onto graphite is being used as an example. The 15 pm natural graphite precursors were introduced into the industrial size chemical vapor deposition reactor, where a thermal decomposition of silane (SiH4) into the silicon and hydrogen was taking place under inert gas in accordance with the equation (1) ... 

Optimization of Low-Pressure Chemical Vapor Deposition Reactor for the... 

EXAMPLE 14.5 OPTIMIZATION OF LOW-PRESSURE CHEMICAL VAPOR DEPOSITION REACTOR FOR THE DEPOSITION OF THIN FILMS... 

FIGURE 3.10 (a) Chemical-vapour deposition reactor (b) cross section of a 100 pm-thick CVD diamond film grown by DC arc jet. The columnar nature of the growth is evident, as is the increase in film quality and grain size with growth time. (Courtesy of Dr. Paul May and Prof. Mike Ashfold, Bristol University.)... 

Fig. 1.3 Computational simulation of the flow in a chemical-vapor-deposition reactor designed to grow high-temperature superconducting thin films.

There are many important situations where the velocities are low (compared to sound speed), yet the density variations are large, owing to temperature or species variations. For example, in low-speed flames or chemical-vapor-deposition reactors, where the pressure is essentially uniform, large density variations are the result of temperature or species variations. Even though the density may vary by a factor of five, the pressure variations remain small since they are associated primarily with the velocity field. 

Stagnation flows represent a very important class of flow configurations wherein the steady-state Navier-Stokes equations, together with thermal-energy and species-continuity equations, reduce to systems of ordinary-differential-equation boundary-value problems. Some of these flows have great practical value in applications, such as chemical-vapor-deposition reactors for electronic thin-film growth. They are also widely used in combustion research to study the effects of fluid-mechanical strain on flame behavior. 

For many applications, like chemical-vapor-deposition reactors, the semi-infinite outer flow is not an appropriate model. Reactors are often designed so that the incoming flow issues through a physical manifold that is parallel to the stagnation surface and separated by a fixed distance. Typically the manifolds (also called showerheads) are designed so that the axial velocity u is uniform, that is, independent of the radial position. Moreover, since the manifold is a solid material, the radial velocity at the manifold face is zero, due to the no-slip condition. One way to fabricate a showerhead manifold is to drill many small holes in a plate, thus causing a large pressure drop across the manifold relative to the pressure variations in the plenum upstream of the manifold and the reactor downstream of the manifold. A porous metal or ceramic plate would provide another way to fabricate the manifold. 

The theoretical implications of finite-gap stagnation were first recognized by Chapman and Bauer [61] and later extended to combustion environments [214,359]. Evans and coworkers further extended the analysis to support chemical vapor deposition reactors [70, 71,118],... 

There are some good chemical-vapor-deposition reactors that deliberately starve the rotating disk. However, the similarity is broken by the recirculation, and the one-dimensional analysis techniques described herein lose their validity. If the chemical reaction on the surface is sufficiently slow, compared to mass transfer through the boundary layer, then the deposition uniformity will not be much affected by the boundary-layer similarity. In these... 

There are many chemically reacting flow situations in which a reactive stream flows interior to a channel or duct. Two such examples are illustrated in Figs. 1.4 and 1.6, which consider flow in a catalytic-combustion monolith [28,156,168,259,322] and in the channels of a solid-oxide fuel cell. Other examples include the catalytic converters in automobiles. Certainly there are many industrial chemical processes that involve reactive flow tubular reactors. Innovative new short-contact-time processes use flow in catalytic monoliths to convert raw hydrocarbons to higher-value chemical feedstocks [37,99,100,173,184,436, 447]. Certain types of chemical-vapor-deposition reactors use a channel to direct flow over a wafer where a thin film is grown or deposited [219]. Flow reactors used in the laboratory to study gas-phase chemical kinetics usually strive to achieve plug-flow conditions and to minimize wall-chemistry effects. Nevertheless, boundary-layer simulations can be used to verify the flow condition or to account for non-ideal behavior [147]. 

The details of the transitions and the vortex behavior depend on the actual channel dimensions and wall-temperature distributions. In general, however, for an application like a horizontal-channel chemical-vapor-deposition reactor, the system is designed to avoid these complex flows. Thus the ideal boundary-layer analysis discussed here is applicable. Nevertheless, one must exercise caution to be sure that the underlying assumptions of one s model are valid. 

A further consequence of the upstream diffusion to the burner face could be heterogeneous reaction at the burner. Such reaction is likely on metal faces that may have catalytic activity. In this case the mass balance as stated in Eq. 16.99 must be altered by the incorporation of the surface reaction rate. In addition to the burner face in a flame configuration, an analogous situation is encountered in a stagnation-flow chemical-vapor-deposition reactor (as illustrated in Fig. 17.1). Here again, as flow rates are decreased or pressure is lowered, the enhanced diffusion tends to promote species to diffuse upstream toward the inlet manifold. 

While our primary interest in this text is internal flow, there are certain similarities with the classic aerodynamics-motivated external flows. Broadly speaking, the stagnation flows discussed in Chapter 6 are classified as boundary layers where the outer flow that establishes the stagnation flow has a principal flow direction that is normal to the solid surface. Outside the boundary layer, there is typically an outer region in which viscous effects are negligible. Even in confined flows (e.g., a stagnation-flow chemical-vapor-deposition reactor), it is the existence of an inviscid outer region that is responsible for some of the relatively simple correlations of diffusive behavior in the boundary layer, like heat and mass transfer to the deposition surface. 

W.G. Breiland and G. Evans. Design and Verification of Nearly Ideal Flow and Heat Transfer in a Rotating Disk Chemical Vapor Deposition Reactor. J. Electrochem. Soc., 138(6) 1807—1816,1991. 

M.E. Coltrin, RJ. Kee, G.H. Evans, E. Meeks, FM. Rupley, and J.F. Grcar. Spin A Fortran Program for Modeling One-Dimensional Rotating-Disk/Stagnation-Flow Chemical Vapor Deposition Reactors. Technical Report SAND91-8003, Sandia National Laboratories, 1991. 

S. Joh and G.H. Evans. Heat Transfer and Flow Stability in a Rotating Disk Stagnation Flow Chemical Vapor Deposition Reactor. Numer. Heat Transf. Part A— Applications, 31 (8) 867—879,1997. 

C.R. Kleijn and C J. Hoogendoom. A Study of 2- and 3-D Transport Phenomena in Horizontal Chemical Vapor Deposition Reactors. Chem. Engr. Sci., 46(1) 321—334, 1991. 

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