Diffusion furnace operators

The problem of assuring uniform depositions on many wafers closely spaced in a long uniform tube was solved when operation of the reactor at low pressure was considered.22 Normally, in an atmospheric pressure cold wall CVD system, the reactant gas is heavily diluted in N2 in order to reduce gas phase nucleation. At the pressures used for low pressure CVD (0.5-1.0 Torr), this is less of a problem so less diluent is needed. The net effect then is that deposition rates only fall by a factor of five. However, as many as 100 wafers can be processed in such a reactor at one time (see Figure 26), and this more than compensates for the lower deposition rate. In addition, due to the low pressure, diffusion occurs at high rates and the deposition tends to be controlled by the surface temperature. Given the very uniform temperatures available in a diffusion furnace, the deposition uniformity tends to be excellent in such a system. 

Routine chemical exposures to operators of both diffusion furnaces and ion implanters are low— typically less that the detection limit of NIOSH sampling procedures. Chemical concerns with the process center on the possibility of toxic gas releases (see Ch. 11, Toxic Gas Monitoring ) and exposures to residues during maintenance activities. 

During surface eleaning operations where physieal residues ofnonhazardous processes create particulate levels—such as cleaning the packing at the load end of diffusion furnaces (assiuning the fiimace is newer and no asbestos is present)... 

Validation and Application. VaUdated CFD examples are emerging (30) as are examples of limitations and misappHcations (31). ReaUsm depends on the adequacy of the physical and chemical representations, the scale of resolution for the appHcation, numerical accuracy of the solution algorithms, and skills appHed in execution. Data are available on performance characteristics of industrial furnaces and gas turbines systems operating with turbulent diffusion flames have been studied for simple two-dimensional geometries and selected conditions (32). Turbulent diffusion flames are produced when fuel and air are injected separately into the reactor. Second-order and infinitely fast reactions coupled with mixing have been analyzed with the k—Z model to describe the macromixing process. 

The last point is worth considering in more detail. Most hydrocarbon diffusion flames are luminous, and this luminosity is due to carbon particulates that radiate strongly at the high combustion gas temperatures. As discussed in Chapter 6, most flames appear yellow when there is particulate formation. The solid-phase particulate cloud has a very high emissivity compared to a pure gaseous system thus, soot-laden flames appreciably increase the radiant heat transfer. In fact, some systems can approach black-body conditions. Thus, when the rate of heat transfer from the combustion gases to some surface, such as a melt, is important—as is the case in certain industrial furnaces—it is beneficial to operate the system in a particular diffusion flame mode to ensure formation of carbon particles. Such particles can later be burned off with additional air to meet emission standards. But some flames are not as luminous as others. Under certain conditions the very small particles that form are oxidized in the flame front and do not create a particulate cloud. 

SEM microphotogaphs and EDAX scans of the cross section and outer surface of the slag deposit, illustrated in Figure 10, indicate the chemistry of the deposit is not uniform. The bulk of the fused material is rich in silica, low in iron, and virtually depleted of potassium. The outermost layers, no more than 2 to 3p thick, are very rich in iron and frequently also rich in calcium. On occasion, the outer surface is covered with small particulate, several microns in diameter, or undissolved cubic or octahedral crystals whose origin is pyrites. Similar formations have been observed in full-scale operation. The evidence indicates deposits form under axial symmetric flow conditions in the furnace by the fluxing action at the heat transfer surface of small particles, <8p in diameter, of decidedly different chemical composition and mineral source. Migration of the fly ash to the surface is by means of eddy diffusion, thermophoresis, or Brownian motion. 

Combustion processes can create pollutant emissions other than carbon monoxide and oxides of nifrogen. Unbumed hydrocarbons (UHC) is a term describing any fuel or partially oxidized hydrocarbon species that exit the stack of a furnace. The cause for these emissions is typically due to incomplete combustion of the fuel from poor mixing or low furnace temperature. A low temperature environment can be created by operating the furnace at a reduced firing rate or turndown. Particulate matter (commonly called soot) is often produced from fuel rich regions in diffusion flames. Soot becomes smoke if the rate of formation of soot exceeds the rate of oxidation of soot. Oxides of sulfur are formed when sulfur is present in the fuel. 

Typically, a carbon preform is placed within an induction furnace susceptor and heated uniformly to about 1100°C, passing the reactant gas over the surface at such a rate as to maintain the surface reaction rate below the diffusion rate. A number of cycles are undertaken, machining away the surface crust prior to the next densification cycle. Dunlop Aviation operate such a system to manufacture carbon-carbon brake material at their Coventry plant (Figure 14.10) and a typical HT furnace used by Hitco is shown in Figure 14.11. 

Two types of techniques are commonly used for the determination of the internal void fraction and the tortuosity. The first uses a column packed with the catalyst and having a djdp ratio such that the flow approximates the ideal plug flow pattern, it is conveniently inserted in the furnace of a gas chromatograph that has all the parts for detecting the feed and response signals, besides temperature- and flow controls and six way valves. A narrow tracer pulse is injected in the carrier gas flow and the response is measured at the exit of the column. The pulse widens as a consequence of the dispersion in the bed, adsorption on the catalyst surface and effective diffusion inside the catalyst particle. The tracer does not have to be the component A itself The injected pulse should have an adequate residence time in the column and sufficient widening. Preference is given to transient measurements because steady state operation would not measure the effect of the dead end pores. 

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