Cold-walled reactors

The CVD process is accomplished using either a hot-wall or a cold-wall reactor (Fig. 13). In the former, the whole chamber is heated and thus a large volume of processing gases is heated as well as the substrate. In the latter, the substrate or substrate fixture is heated, often by inductive heating. This heats the gas locally. 

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). 

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... 

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. 

Figure 5.9. Continuous operation cold-wall reactor for atmospheric pressure deposition of Si02-...

Figure 5.10. Cold-wall reactor with radiant heating.

In two independent studies, InP was grown from the precursor complex [(CH3)2In /i-P(But)2 ]2.255 256 262 First, Cowley et al. employed the use of a cold-wall reactor to deposit InP using H2 or He as the carrier gas, with substrate temperatures between 450 °C and 700 °C. Using an MBE reactor, Bradley and co-workers found that stoichiometric growth was only possible at 480 °C and only when a simultaneous secondary incident flux of dissociated phosphine was added. Lower growth temperatures resulted in indium-rich deposits. 

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).

Even though all three reactors share the same precursor delivery system, each tool offers specific advantages. For example, a cold-wall reactor (reactor B) helps prevent decomposition of the precursor before it reaches the substrate. A pulsed aerosol injection system at low pressure (reactor C) allows the film to grow under better-defined conditions than in a continuous process (reactor A) because of the minimization of undesirable transient effects caused by the high volatility of the solvents used.46 A more detailed description of each of the conditions for film growth, including reactor type, precursor type, delivery method, deposition temperature, growth time, and other parameters are summarized in Table 6.2. Depositions were done on bare and Mo-coated... 

Aixtron has manufactured both a cold-wall planetary reactor and a hot-wall planetary reactor for 7 x 2-inch wafer capacity or 5 x 3-inch. The cold-wall reactor has provided very good uniformities and its performance has been described in... 

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...

Adachi and Mizuhashi investigated the oxidation of dimethyltin dichloride (DMTC) in a cold wall reactor [30]. They reported that the order of reaction is about one half in both oxygen and DMTC, similar to what was found by Ghoshtagore for the oxidation of SnCU [32,33]. Like Ghoshtagore, they also proposed that the reaction was diffusion limited at higher temperatures and surface reaction-limited at lower temperatures. They found an activation energy of 34 kcal moC in the kinetically limited region. 

The apparatus used for the CVD processes [19-21] described below is a cold wall reactor, the principal features of it are described in a former publication [22]. It consists of a vacuum line to which a cyhndrical glass tube is connected, the tube being enrolled by a copper wire, which is used as... 

Abstract The review summarizes recent studies on the synthesis of M - Sb compounds and their potential application to serve as single-source precursor in MOCVD processes. General reaction pathways for the synthesis of simple Lewis acid-base adducts R3M - ER 3 and heterocycles of the type [R2MSbR 2]x (M = Al, Ga, In) are described. As-formed compounds were studied in detail in MOCVD processes using hot-wall and cold-wall reactors. Advantages as well as problems using single-source precursors are described. 

The CVD process is accomplished using either a hot-wall or a cold-wall reactor (Fig. 13). In the former, the whole chamber is heated and thus a... 

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. 

Numerous modeling studies of CVD reactors have been made and are summarized in recent review papers (I, 212). Table 3 in reference 212 lists major examples of CVD models up to mid-1986. Therefore, rather than giving an exhaustive list of previous work, Table V presents a summary of the major modeling approaches and forms the basis for the ensuing discussion, which is most appropriately handled in terms of two groups (1) hot-wall LPCVD systems and (2) cold-wall, near-atmospheric-pressure reactors. In LPCVD reactors, diffusion and surface reaction effects dominate, whereas in cold-wall reactors operated at near-atmospheric pressures, fluid flow and gas-phase reactions are important in predicting performance, as discussed earlier in relation to transport phenomena. 

When we speak of a cold wall CVD reactor, we refer to a continuous flow system where the wafer is kept at the required high temperature, but all other surfaces bounding on the reacting gases are cold. The objective here is to cause the desired reaction only on the hot wafer and keep all other surfaces free of deposits. In practice this is a goal that can only be partially attained. Although reactions will proceed more slowly on colder surfaces, they will proceed-and films will build up. At the same time the films that form on the colder surfaces may be more porous than the normal film and may spall off more easily. All of which says that in spite of our best efforts, cold walled reactors may have their cold walls an undesirable source of particulates which may end up on the hot substrate. The occurence of such particulates can be minimized by frequent cleaning of the chamber walls to remove deposits. 

The two reactors just described are parallel plate reactors. However, they are also cold wall reactors. In other words, the electrode holding the wafers is hot, but all other surfaces exposed to the plasma are cold, or at least not heated. This is done to minimize the deposition on other surfaces so that down time for cleaning can be kept as short as possible. 

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. 

The lowest resistivity silicide film of the four we are considering is the TiSi2 film, so such films have always been of interest. A recent study14 has shown that these films can also be deposited by low-pressure CVD. For these experiments, a cold-wall reactor similar to the parallel-flow tube reactor sketched in Figure 17 of Chapter 1 was used. The wafer was heated by heating the susceptor from below by optical radiation. 

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