Hot Wall Systems

One of the problems with cold wall systems is the difficulty in maintaining a very uniform temperature on the wafers. Such concern can be eliminated if the entire reactor chamber is placed within a furnace maintained at a very uniform temperature. An ideal candidate for such a furnace is the standard diffusion tube furnace already in wide use for integrated circuit fabrication. If in addition, wafers could be loaded vertically as in a diffusion furnace, the reactor throughput could be substantial. 

Zeleznik, F.J., and Gordon, S., Calculation of complex chemical equilibria. 

Eriksson, G., Thermodynamic studies of high temperature equilibria. 

Cruise, D.R., Notes on the rapid computation of chemical equilibria. J. 

Chemical Vapor Deposition (L.F. Donaghey, P. Rai-Choudhury, R.N. Tauber, eds.), p. 47, Electrochemical Society, Princeton, NJ (1977). These figures were originally presented at the Fall 1977 Meeting of the Electrochemical Society, Inc. held in Atlanta, Georgia. 

---

C.H.J., The deposition of silicon from silane in a low-pressure hot-wall system../ Crys. Growth 57 259 (1982). 

The same reactor concept would be valid in a hot wall system, if the entire reactor were placed in a furnace. In this way, temperature uniformity would be excellent by definition. Obviously, this would be awkard and not economically attractive. However, if our electrode geometry were of two parallel, narrow rectangular electrodes, the structure could conveniently fit into a hot tube (much like a diffusion furnace). In this way, a large batch load could be run in a relatively hot furnace tube. Such a hot wall system is shown in Figure 15. 

Multiple reactangular electrodes are arranged so that they fit down the length of a tube and are alternately powered by a 400-kHz power supply. The electrodes are fabricated of graphite. A major attraction of the hot wall system is the large wafer load that can be run (i.e., 84 4-inch wafers) at onetime. This is offset to some extent by the fact that the electrode structure cools off each time wafers are unloaded, and the time needed to reheat upon insertion into the furnace detracts from wafer throughput. 

Another approach to this problem involves heating the wafer at 750 F at very low pressures (<10 10 Torr prior to deposition.28 This has the effect of removing the native oxide by evaporation of SiO. Depositions were achieved in the temperature range of 750° to 850°C in SiH4 + H2. Since the authors were developing a hot-wall system with many wafers stacked close to each other, the deposition was carried out at 2 mTorr. Deposition rates of 20 to 45 A/min were achieved. As expected, dopant transition widths were very narrow, several hundred angstroms. Again, device studies on such a system have not yet been done. 

It is probable that a fundamental difference exists in processes operating in the two types of reactors considered here. In the hot-wall system, the reactant gases have ample time to react before reaching the wafers, so gas phase chemistry probably plays a role. In the cold-wall system, this is probably minimized. 

Chapter 6 is devoted to typical commercially-available CVD reactor systems, including cold-wall and hot-wall systems. Several new commercial reactors are also reviewed. 

Selectivity Many of the early studies were focused on selective tungsten (based on WF6/H2). A clear disadvantage of the tube systems is that the wafers in the rear will see more reaction products than those in the front. As we have seen in chapter III, the reaction products are a major cause for the loss of selectivity. Indeed, poor selective results are normally seen in such furnaces. Another disadvantage of the hot wall system is that as soon as tungsten coating of the wall occurs, there is a tremendous increase of the reaction by-products partial pressures, again leading to poor selectivities. 

Reaction-hmited processes produce good uniformity of coverage and may grow material in batch mode in a hot-wall system but are relatively slow. 

At its simplest, a HW heating cycle is the circulation of HW from a boiler (or heat pump or similar device) through a supply and distribution piping system to various appliances and then back to the boiler. Hot water systems are hydronic systems and, when of any size, are designed to operate via various primary and secondary circuits. These circuits are provided with their own circulating pumps of different capacities to provide proper layout flow, usually to perimeter-wall m-tube convectors, fan coil units, or other space heating equipment. 

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],... 

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]. 

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. 

At low pressures, the electron temperature is much higher than the temperature of the gas. The temperature of an electron with energy of 2 eV will be 23,200 K. Even though the individual electrons are very hot, the system or gas remains at ambient temperature. Because of the very low density and very low heat capacity of the electrons, the amount of heat transferred to the gas and to the walls of the container is very small. Thus the term cold plasma derives its meaning from the small amount of heat transferred to the gas or solids in contact with it. 

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. 

Two types of CVD systems can be considered. One is a closed system into which a finite quantity of reactant gas is introduced, such as shown in Figures 1A and 1 B. Initially, the silane (SiH4> is introduced at a low temperature (Tc). Silane will then diffuse to the hot wall through a concentration gradient layer, adsorb on the walls, dissociate there and leave solid silicon behind while H2 diffuses back into the gas. After a finite time, an equilibrium is reached where no more silicon is deposited. 

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. 

Figure 15 Hot-wall, parallel-plate reactor for plasma-enhanced CVD. (Courtesy of Pacific Western Systems, Inc.)...

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 hot wall approach to the plasma-enhanced CVD system has been described in Chapter 3. A schematic of a typical system is shown in Figure 21. The elements of this system are similar to that of the cold-wall system just described. There is a gas panel, vacuum system, and an RF power supply to create the discharge. The RF frequency typically used is 400 kHz. The reaction chamber of such a system is shown in Figure 22. The electrodes are a set of several long narrow rectangular slabs of graphite with pockets cut into them. The graphite electrodes lead to some problems with particulate contamination, but attempts to use aluminum have not been successful. 

Figure 21 Schematic of hot wall piasma deposition system, PWS-450. Pacific Western Systems, Inc.

One of the difficulties with the traditional LPCVD hot tube reactor is depletion of reactant gases as they flow down the length of the tube. To overcome this problem, designers either ramp up the temperature at the tube back end to increase deposition rates, or introduce the gases in a distributed fashion along the tube length. Neither solution is ideal, so several years ago Anicon introduced a new hot wall configuration. In this system, a hot quartz bell jar is used to provide the uniform temperature ambient for deposition. 

System downtime Low because of hot wall design High because of particle generation... 

---