Cold-Wall Thermal Systems

All of the remaining new CVD reactor systems are cold wall reactors carrying out thermal or plasma-enhanced CVD processes. They are being developed to deposit a variety of films, but each system is initially targeting a particular material. 

The obvious question to ask is why not run this process in a hot tube and treat 50 to 100 wafers at a time rather than eight The explanation is that the 

WSi2 process (WF6 + SiH4 - WSi2 +.) proceeds very rapidly so that 

In comparison to the Genus reactor, this system holds the wafer upside down to minimize any particulate on the wafer. Also, since this is a singlewafer machine, a loadlock is provided to ensure that the reaction chamber is never opened to the atmosphere. Attempts to provide this feature on a batch reactor are difficult and expensive, due to the size of the chamber needed. Heating is done in a way similar to the Genus system. High-intensity lamps shine on the back of a chuck to heat it to processing temperature. 

A final point should be made concerning the single-wafer CVD reactor concept. This approach only makes sense if each wafer can be processed in 1 to 2 minutes, so reasonable throughput can be achieved. In many applications, conducting films can be thin, 2000 A, so deposition rates of 1000 to 1500 A/min would be suitable. Such rates are not unreasonable, for example, for WSi2 films. 

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Thermal CVD requires high temperature, generally from 800 to 2000°C, which can be generated by resistance heating, high-frequency induction, radiant heating, hot plate heating, or any combination of these. Thermal CVD can be divided into two basic systems known as hot-wall reactor and cold-wall reactor (these can be either horizontal or vertical). 

Another example of a cold-wall reactor is shown in Fig. 5.9. It uses a hot plate and a conveyor belt for continuous operation at atmospheric pressure. Preheating and cooling zones reduce the possibility of thermal shock. The system is used extensively for high-volume production of silicon-dioxide coatings for semiconductor passivation and interlayer dielectrics. 

The ThFFF separation system is made up of a flat ribbon-like channel obtained by placing a trimming-spacer between two flat bars kept at different temperatures (at the upper wall) and (at the lower wall), with AT = Tg- The thickness of the spacer defines the channel thickness w. In the channel cross section, the thermal diffusion process pushes the analyte toward the so-called accumulation wall, usually the cold wall (thermophobic substances) the combination of the flow profile and the thermal diffusion produces the fractionation. 

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. 

Once the calibration constants and n have been determined for a given polymer-solvent system, Eq. (4) can be used for all thermal FFF channels, provided the temperature of the cold wall (T ) is held constant. The cold-wall temperature affects the calibration plot because the Soret coefficient DjID) and, therefore, varies with T. For a detailed discussion of temperature effects see the entry Cold-Wall Effects in Thermal FFF. In a thorough study of temperature effects, Myers and co-workers [3] demonstrated that the dependence of the Soret coefficient on can be accurately modeled by... 

In a cold-wall CVD only the substrates are heated either inductively or resistively and the wall of the reactor is colder than that of the substrate. Therefore, the deposition mainly occurs on the heated substrate, and negligible deposition on the walls of the reactor. Cold-wall reactors are mainly used for continuous deposition of fibres and depositions where a thermal gradient is required to facilitate CVI. Hot-wall CVD reactors represent one of the major categories of CVD reactors. In such systems, the chamber containing the parts is heated by a furnace from outside. In general, hot-wall reactors have the advantages of being... 

Monospher Colloidal Silica. Superimposed thermal FFF fracto-grams (system Therm I) of various sizes of Monospher colloidal silicas (E. Merck) run in acetonitrile are shown in Figure 7. A AT of 53 K (cold wall temperature of 290 K) was used to obtain these data. The broadness and excessive tailing of the peaks suggests a high polydispersity for the particle populations. In addition, the proximity of elution of the Monospher 100 and 150 samples indicates that their mean sizes are not as different as... 

Thermal-expansion and -contraction loads occur when a piping system is prevented from free thermal expansion or contraction as a result of anchors and restraints or undergoes large, rapid temperature changes or unequal temperature distribution because of an injection of cold liquid striking the wall of a pipe cariying hot gas. 

When a fluid is heated, the hot less-dense fluid rises and is replaced by cold material, thus setting up a natural convection current. When the fluid is agitated by some external means, then forced convection takes place. It is normally considered that there is a stationary film of fluid adjacent to the wall and that heat transfer takes place through this film by conduction. Because the thermal conductivity of most liquids is low, the main resistance to the flow of heat is in the film. Conduction through this film is given by the usual relation (74), but the value of h is not simply a property of the fluid but depends on many factors such as the geometry of the system and the flow dynamics for example, with tubes there are significant differences between the inside and outside film coefficients. 

Convective flow is used in both the thermogravitational (Clusius-Dickel) column and in electrodecantation. In the thermogravitational system, one wall of the channel is heated or, alternatively, a hot wire is placed along the axis of the channel. The fluid at the cold surface then tends to sink relative to that at the hot surface. Simultaneously, thermal diffusion (Section 8.8) causes different levels of enrichment in the hot and cold regions of the channel. The enriched solutes then move up and down the channel at a rate depending upon their distribution between hot and cold regions. In binary... 

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