Hot-cold reactor

B5H11 can be prepared in 70% yield by the reaction of B2H6 and B4H10 in a carefully dimensioned hot/cold reactor at -1-120°/—30° ... 

B4H10 forms slowly by decomposition of diborane, B2H6, in a slightly exothermic reaction (AH = -14 kj mol 1) and it is produced in a hot-cold-reactor (120 °C/ -78 °C). At present the best method is to react Bu4N[B3H8] with BC13 or A1C13 in the presence of toluene as shown in Eq. (26) ... 

Disproportionation of P—P bond-containing phosphines can produce new P—H bonds . Gaseous P2H4 in a hot-cold reactor or neat liq PjH at 30- 40 C reacts to form PH P,H, ... 

An improved synthesis of nido-dicarbaoctaborane( 10) has been reported.68 This was carried out in a concentric cylindrical hot-cold reactor of capacity 750 ml. If a 1 2 molar mixture of C2B3HS and B2H6 was kept in the reactor for 4 hours, a 35% yield of C2B6Hi0 resulted. 

B4H8CO may be prepared in a convenient one-step synthesis from B2H6 and CO in a hot-cold reactor.136 The carbonyl derivative may then react with C2H4 to give (CH2)2B4H8. The structure of this is (22), as shown by 3H... 

Chlorine, under reduced temperature and in an inert solvent, reacts with closo-1,5-C2B3H5 to produce 2-Cl-l,5-C2B3H4- In a hot/cold reactor, BMc3 and 1,5-C2B3HS form a mixture of B-mono-, -di-, and -tri-methyl derivatives of the c/oso-carbaborane. The dimer 2,2 -C2B3H4—C2B3H4 and B-methyl derivatives are also produced ... 

The relative proportions of the products are markedly dependent on the conditions. Up to 90% conversion to Bi H o may be realized in a hot-cold reactor in which an inner cylindrical surface is heated to 100°C while the outer surface is maintained at -80 C to trap out the tetraborane as it is formed (179). Stock and Mathing had earlier shown that pyrolysis at higher temperatures (180°C) leads to B5H11 as a major product, whereas at still higher temperatures (250-300°C) B5H9 predominates (306). More recently Owen... 

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-waH reactor is most often used in small-size systems. The hot-waH reactor, by contrast, is most often used in large-volume production 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). 

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. 

Hot reactor walls are sometimes used as a means to increase the density of the films that are deposited on the walls. This reduces the amount of adsorbed contaminants on the walls, and leads to lower outgassing rates. A hot wall is particularly of interest for single-chamber systems without a load-lock chamber. Material quality is similar to the quality obtained with a cold reactor wall [145],... 

This method was similar to that used by Hiteshue et al (3). In this method sand (50 g, mesh 0.42 - 0.15 mm) was mixed with the coal (25 g, mesh 0.5 - 0.25 mm). The addition of sand to the coal helped to prevent agglomeration (4). All the experiments used an aqueous solution of stannous chloride impregnated on the coal as a catalyst. The amount of catalyst added on a tin basis was 1% of the mass of the coal. These mixtures were placed in a hot-rod reactor and heated to 500°C at a heating rate of 200°C per minute. Residence time at temperature was 15 minutes. Hydrogen at a flow rate of 22 liters/minute and a pressure of 25 MPa was continously passed through the fixed bed of coal/sand/catalyst. The volatile products were collected in high-pressure cold traps. A schematic of the apparatus used is shown in Figure 2. 

In the preparation of 2-(tricarbonylferra)hexaborane(10) by co-pyrolysis of the reactants in a hot-cold Pyrex tube reactor, the latter was severely etched and weakened, sometimes splintering. At 230° a maximum cumulative service life of 4 months was observed, and at 260°C the reactor was replaced at the first signs of etching, usually after 6 runs. 

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).

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. 

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...

If the system consists of a series of adiabatic reactors, there are two basic configurations. The first has heat exchangers or furnaces between each of the reactors to cool or heat the reactor effluent before it enters the next reactor. The second configuration uses cold shot cooling. Some of the cold reactor feed is bypassed around the upstream reac-tor(s) and mixed with the hot effluent from the reactor to lower the inlet temperature to the downstream catalyst bed. 

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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