Hot wall reactors

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). 

Hot-Wall Reactors. A hot-wall reactor is essentially an isothermal furnace, which is often heated by resistance elements. The parts to be coated are loaded in the reactor, the temperature is raised to the desired level, and the reaction gases are introduced. Figure 5.6 shows such a furnace which is used for the coating of cutting tools with TiC, TiN, and Ti(CN). These materials can be deposited alternatively under precisely controlled conditions. Such reactors are often large and the coating of hundreds of parts in one operation is possible (see Ch. 18). 

Hot wall reactors have the advantage of close temperature control. A disadvantage is that deposition occurs everywhere, on the part as well as on the walls of the reactor, which require periodic cleaning or the use of a disposable liner. 

Typical Reactor Design. Table 5.1 lists typical CVD production reactors which include cold-wall and hot-wall reactors operating at low or atmospheric pressures. The decision to use a given system should be made after giving due consideration to all the factors of cost, efficiency, production rate, ease of operation, and quality. 

Patterson, D. E., et al., Thermochemical Vapor Deposition of Diamond in a Carbon-Halogen-Oxygen and/or Sulfur Atmospheric Hot-Wall Reactor, in Applications of Diamond Films and Related Materials, (Y. Tzeng, et al., eds.), Elsevier Science Publishers, pp. 569-576 (1991)... 

ZnS, CdS, (ZnxCd x)S Hot-wall reactor,under active secondary vacuum Glass and III-V substrates, growth at 400 °C. ZnS poor-quality films. CdS and (ZnxCdi x)S good-quality films. CdS epitaxially growth on (lOO)-oriented GaAs and InP substrates 180... 

ZnSe, CdSe Hot-wall reactor Deposited elemental selenium 181... 

Figure 6.11. Schematic of two of the reactors used (a) atmospheric pressure horizontal hot-wall reactor (Reactor A) and (b) vertical cold-wall reactor (Reactor B).

Three different reactors were used to deposit CuInS2 films via AACVD. Reactor A, shown schematically in Fig. 6.11a, was primarily used in the parametric studies described below. This is a horizontal, atmospheric pressure, hot-wall reactor with a plate-type 2.5-MHz ultrasonic nebulizer from Sonaer Ultrasonics. The precursor (1.5-3.5g) was dissolved into distilled toluene (50-400 ml) and fed into the nebulizer using a syringe pump. The nebulizer... 

ATMOSPHERIC PRESSURE HOT-WALL REACTOR PARAMETRIC STUDY... 

The reactor used for this study was the horizontal atmospheric-pressure hot-wall reactor (reactor A in Fig. 6.11). The susceptor accommodated three substrates side by side at an angle of 15.5 ° above horizontal. The leading edge of the film experiences a more reactant-rich gas stream and is closer to the... 

Atmospheric Pressure Hot-Wall Reactor Parametric Study 181... 

Similar to chemical vapor deposition, reactants or precursors for chemical vapor synthesis are volatile metal-organics, carbonyls, hydrides, chlorides, etc. delivered to the hot-wall reactor as a vapor. A typical laboratory reactor consists of a precursor delivery system, a reaction zone, a particle collector, and a pumping system. Modification of the precursor delivery system and the reaction zone allows synthesis of pure oxide, doped oxide, or multi-component nanoparticles. For example, copper nanoparticles can be prepared from copper acetylacetone complexes [70], while europium doped yttiria can be obtained from their organometallic precursors [71]. 

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. 

The behavior of the hot-wall reactor has been investigated thoroughly. A short but comprehensive study by Wagner and Irmscher captures the main points [50]. 

Wafer rotation, as previously mentioned, has been introduced in the hot-wall reactor (see Figure 1.10), which gives it a capacity of 3 x 2-inch (that is, three 2-inch wafers can be run simultaneously to increase reactor throughput). The uniformity in this reactor is routinely 1-2% in thickness and 5-7% in doping [Rune Berge, Epi-gress, private communication]. 

Figure 1.10 The hot-wall reactor with a 3 x 2-inch rotating platter. The mode of rotation is generally gas-foil rotation.

Fig. 2 Reported growth rate of tin oxide, prepared from (Ctf3)4Sn + O2, as a function of temperature. Borman et al. [39] used a hot wall reactor with various diameters shown in the legend, [TMT] = 99-390 ppm. Ghostagore [32,33] used a horizontal cold wall reactor with [TMT] = 117-310 ppm. Chow et al. [54] used a stagnation-point flow reactor, and Vetrone et al. [55] a horizontal hot-wall reactor with a tilted substrate...

Each heating technique has its advantages and disadvantages, and changing from one technique to another may involve significant changes in the process variables. The cold-wall reactor is most often used in small-size systems. The hot-wall reactor, by contrast, is most often used in large-volume production reactors. 

Nitride Coatings. Carbide tips coated with titanium nitride or titanium carbonitride are usually manufactured by a CVD process using T1CI4, H4, and N2 in a hot-wall reactor. 

Hot-Wall Reactors. Because of the large mass diffusivities and nearly isothermal conditions (except for the entrance zone) in hot-wall, low-pressure reactors (50 Pa), multicomponent diffusion and chemical reactions are critical... 

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. 

Finally, we can comment on the influence of the reactor type on the films that can be deposited. Evidently, the hot-wall reactor tends to deposit very Ta-rich films. Although it may be possible to alter the stoichiometry in this type of reactor, the choices are limited. One must operate under conditions where uniform depositions are achieved both on each wafer and from wafer to wafer, because this is a batch system. In the cold-wall reactor, it was possible to obtain the proper stoichiometry at high deposition rates. Since the higher deposition rates permit development of a single-wafer reactor, there are more choices in the process conditions to be used. 

Plasma oxide can be grown from a number of oxidizers plus SiH4. Among these are N20, 02, C02 and even TEOS (tetraethoxysilane). Generally, 02 is not used as it too often leads to homogeneous nucleation. The preferred reactants have proven to be SiH4 and N20, so we will restrict our discussion to these. Films grown in both cold-wall and hot-wall reactors will be considered. 

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