FUNDAMENTALS OF THERMAL CVD

The several steps that must occur in every CVD reaction are as follows  

In all CVD processes, we are dealing with the change from one state (i.e., the initial, low-temperature reactant gases) to a later one (i.e., the final state with some solid phase and product gases) in time. Since any practical commercial process must be completed quickly, the rate with which one proceeds from the initial to the final state is important. This rate will depend on chemical kinetics (reaction rates) and fluid dynamic transport phenomena. Therefore, in order to clearly understand CVD processes, we will not only examine chemical thermodynamics (Section 1.2), but also kinetics and transport (Section 1.3). 

In this situation, a film is grown on the hot surface (Tw), and its thickness will increase without limit as long as fresh reactants are provided and products can be removed. The gas state will be in quasiequilibrium far from the hot surface and in a strongly nonequilibrium condition close to it. The change from one to the other will occur across a boundary layer where temperature, velocity, and species concentration vary rapidly. The behavior of this boundary layer will be determined by gas transport properties such as viscosity, thermal conductivity, as well as gas-phase kinetics and diffusion coefficients. So, even if the kinetics at the surface are very fast, we must deal with quasiequilibrium phenomena where gas conditions vary rapidly over short distances. 

In the analysis of CVD reactions, it is important to recognize the rates of the various processes. The slowest rate will be controlling, and which one is the slowest or fastest can depend on gas as well as surface conditions. For example, surface reactions may be fast at high surface temperatures. In this case, the CVD process will tend to be limited by the rate at which reactants can get to the surface or products leave it. For this situation, the fluid dynamic boundary layer phenomena will govern the deposition rate. On the other hand, at low pressures diffusion is very rapid and the rate at which surface reactions proceed will tend to govern the deposition rate. Alternatively, low surface temperatures will have low reaction rates, and this will govern no matter how much material diffuses to the surface. 

The essential issues that one is concerned with in all CVD processes are  

---

In addition to thermally-created CVD films, much work has been done using glow discharges to modify the deposition. Therefore, Chapter 2 reviews the fundamentals of plasma-enhanced CVD (PECVD). Initially, the basic character of a plasma is covered. Then we discuss the influence of the reactor configuration on the plasma behavior and PECVD deposition. The two major PECVD reactor systems are reviewed, and then several new concepts are considered. 

Other CVD Processes. CVD also finds extensive use in the production of protective coatings (44,45) and in the manufacture of optical fibers (46-48). Whereas the important question in the deposition of protective coatings is analogous to that in microelectronics (i.e., the deposition of a coherent, uniform film), the fabrication of optical fibers by CVD is fundamentally different. This process involves gas-phase nucleation and transport of the aerosol particles to the fiber surface by thermophoresis (49, 50). Heating the deposited particle layer consolidates it into the fiber structure. Often, a thermal plasma is used to enhance the thermophoretic transport of the particles to the fiber walls (48, 51). The gas-phase nucleation is detrimental to other CVD processes in which thin, uniform solid films are desired. 

In the first chapter, we consider the fundamental nature of the thermally-induced CVD. Initially, we consider the behavior of CVD reactions under the assumption of chemical equilibrium. Much useful information can be derived by this technique, especially for very complex chemical systems where several different solid phases can be deposited. In order to extend our understanding of CVD, it is necessary to consider reacting gas flows where the rates of chemical reactions are finite. Therefore, the next subject considered is the modeling of CVD flows, including chemical kinetics. Depending on processing conditions, the film being deposited may be amorphous, polycrystalline, or epitaxial. 

---