Cold-wall type CVD

YSZ films were prepared a vertical cold-wall type CVD apparatus [II]. Source precursors of Zr(dpm)4 and Y(dpm)3 were vaporized at 483 to 593 and 393 to 473 K, respectively. The source vapors were carried with Ar gas into the CVD reactor. O2 gas was separately introduced by using a double tube nozzle, and mixed with precursor vapors in a mixing chamber placed above a... 

Cold-wall type CVD set-up (1) water-cooled vacuum chamber (2), (7) quartz glass windows (3), (4) gas inlets (5) pressure gauge (6) water-cooled copper electrode (8) graphite heater (substrate) (9) graphite socket (10), (11) gas outlets. 

By combining the cold-wall type CVD set-up, local heating of a graphite substrate and reactive precursors, the deposition rate of CVD SiC plates was significantly enhanced compared to that of conventional thermal CVD. Figure 14.4 presents the temperature dependence of deposition rate of CVD SiC... 

Cold-Wall Reactors. In a cold-wall reactor, the substrate to be coated is heated directly either by induction or by radiant heating whi 1 e th e rest of the reactor remains cool, or at least cooler. Most CVD reactions are endothermic, i.e., they absorb heat and deposition takes place preferentially on the surfaces where the temperature is the highest, in this case the substrate. The walls of the reactor, which are cooler, remain uncoated. A simple laboratory-type reactor is shown... 

Several different types of CVD reactors exist. The cold wall design, which used to be the most common type of reactor, is now less frequently used and the hot-wall reactor has filled its place. Some new and interesting concepts exist as well. These are referred to as chimney-type reactors. The main difference between the hot- or old-wall type reactors and the chimney-style reactor is the transport of materials, which will be explained in the following sections. 

As most organometallic precursors, V(NEt2)4 pyrolysis involves a complicated mechanism highly dependent on the experimental conditions. For this reason, the CVD experiments were conducted at reduced pressure (Table 15.4) in order to improve the diffusivity of the species, reduce their interactions in the gas phase and disfavor subsequent reactions. Two CVD units (hot-wall and cold-wall) of the same geometry were used in this study. Since the reactions in the gas phase are likely to be different in these two types of reactors, we could use them to study the influence of the gas phase chemistry on the growth rate. The composition of the deposits was studied as a function of the substrate temperature under He gas and as a function of the nature of the carrier gas when H2 and NH3 were added in various amounts. 

Dimensionless Quantities and Reactor Types. Transport phenomena in CVD reactors can be described in terms of two broad groups (1) hot-wall, low-pressure reactors and (2) cold-wall, reduced- and atmospheric-pressure reactors. 

In this section we will review the various types of CVD reactors scientists and engineers have used for the development of thermal CVD processes. This will be distinct from the commercial reactors used for production which will be covered in a later chapter. A similar review of reactors for development of plasma-enhanced CVD processes will be made at the end of the next chapter. We will cover the so-called cold wall systems for either single or multiple wafers first, followed by a discussion of continuous belt systems. Finally, we will review the hot wall reactor approach. 

Figure 1 illustrates conventional CVD reactors. These reactors may be classified according to the wall temperature and the deposition pressure. The horizontal, pancake, and barrel reactors are usually cold-wall reactors where the wall temperatures are considerably cooler than the deposition surfaces. This is accomplished by heating the susceptor by external rf induction coils or quartz radiant heaters. The horizontal multiple-wafer-in-tube (or boat) reactor is a hot-wall reactor in which the wall temperature is the same as that of the deposition surface. Therefore, in this type of reactor, the deposition also occurs on the reactor walls which presents a potential problem since flakes from the wall deposit cause defects in the films grown on the wafers. This is avoided in the cold-wall reactors, but the large temperature gradients in those reactors may induce convection cells with associated problems in maintaining uniform film thickness and composition. 

Tlie CVD method is usually used to produce a thin film material which is formed on a heated substrate. However, nanostructured particles of ceria and ceria-yttria have been synthesized by some arrangements of the apparatus. Figure 3.8 shows the schematic CVD reactors for synthesizing ceria-based nanopanicles.Two types of rector has been presented. The nanoparticles are collected either on a cooled quartz susceptor (A) that is in a furnace, or in a cold wall container outside the furnace (B), The precursor cerium chloride set on the container is evaporated and... 

Reactors for conventional thermally activated CVD are of two types cold-wall and hot-wall reactors, respectively internally and externally heated. The disadvantage of a hot-wall reactor is deposition on the wall and partial depletion of reactants leading to nonuniform coatings. A correct reactor geometry and gas inlet manifold can compensate for gas depletion in hot-wall reactors. There is no limit to the form of the objects to be coated, but sizes are restricted. In a cold-wall reactor the substrates to be coated are heated by a graphite susceptor that is inductively heated by an rf generator. Only the hot parts are coated and not the reactor walls, which remain relatively cold. 

Two sample ALD reactor design options are presented schematically in Figure 12.19. Not shown is a conventional MBE environment such as in Figure 11.1. The CVD-type reactor designs may include a conventional CVD reactor tube (hot or cold wall) with pulsed gas sources sending bursts of one reactant and then the other down the tube. This relies on sufficient physical separation of the pulses in the gas phase as the reactant pulse travels down the tube. A second method is to expose a surface sequentially to reactants supplied at fixed locations by rotating the substrate alternately over each. 

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