Plasma enhanced CVD system

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. 

The remaining system is a plasma-enhanced CVD system for the low-temperature deposition of low hydrogen content silicon nitride. The system is shown in Figure 26, and a schematic of the reaction chamber in Figure 27. As can be seen, this reactor is a batch system where the wafers are placed in a square array. 

Several researchers [7] have grown vertically aligned carbon nanotubes using a microwave plasma-enhanced CVD system using a thin-film cobalt catalyst at 825 C. 

Rosier, R. S., The Evolution of Commercial Plasma-Enhanced CVD Systems, Solid State Technology, pp. 67-71 (June 1991)... 

Dielectric Deposition Systems. The most common techniques used for dielectric deposition include chemical vapor deposition (CVD), sputtering, and spin-on films. In a CVD system thermal or plasma energy is used to decompose source molecules on the semiconductor surface (189). In plasma-enhanced CVD (PECVD), typical source gases include silane, SiH, and nitrous oxide, N2O, for deposition of siUcon nitride. The most common CVD films used are siUcon dioxide, siUcon nitride, and siUcon oxynitrides. 

In PECVD, the plasma generation region may be in the deposition chamber or precede the deposition chamber in the gas flow system. The latter configuration is called remote plasma-enhanced CVD (RPECVD). In either case, the purpose of the plasma is to give activation and partial reaction/reduction of the chemical precursor vapors so that the substrate temperature can be lowered and still obtain deposit of the same quaUty. 

A brief review of the literature concerning the several materials employed in the fabrication of both TIR and ARROW structures is given in Table 2. The processes employed are completely different, ranging from molecular beam epitaxy to several chemical vapor deposition (CVD) systems, such as low-pressure CVD (LPCVD) or plasma-enhanced CVD (PECVD). As a rule, all suitable materials for ARROWS (and in general for IOCs) should have homogeneous refractive indexes, high mechanical and chemical stability, few... 

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.)...

In reality, these are only two of many arrangements that could be devised to create and deliver to a substrate large quantities of reactive species using a plasma. Since there are many shortcomings to existing commercial plasma-enhanced CVD reactors, it will be useful to explore other reactor concepts that are under development, but have yet to be widely developed commercially. Whether or not they will lead to practical production systems remains to be seen. 

All of the remaining new CVD reactor systems are cold wall reactors carrying out thermal or plasma-enhanced CVD processes. They are being developed to deposit a variety of films, but each system is initially targeting a particular material. 

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. 

Chemical reactions occurring in the gas-phase can be more or less important in CVD, depending on the system, and can often be analyzed in detail. Gas-phase reactions are more likely to be important with the use of high temperatures and high total reactor pressures, but less likely to be important at low reactor pressures. Many CVD systems are operated in ways that minimize gas-phase reactions in order to avoid particle formation that could interfere with the desired film deposition. Note that the absence of homogeneous nucleation of particles is not synonymous with the absence of gas-phase chemical reactions. In contrast, other CVD systems utilize gas-phase reactions to convert reactant molecules that are relatively unreactive at the surface into more reactive species. Examples where this strategy is used include the combustion CVD processes discussed in Chapter 4 and plasma-enhanced CVD processes. 

A variety of CVD methods and CVD reactors have been developed, depending on the types of precursors used, the deposition conditions applied, and the forms of energy introduced to the system to activate the chemical reactions desired for the deposition of solid fihns on snbstrates. For example, when metalorganic compounds are used as precursors, the process is generally referred to as MOCVD (metalorganic CVD), and when plasma is nsed to promote chemical reactions, this is called plasma-enhanced CVD (PECVD). There are many other modified CVD methods, such as LPCVD (low-pressure CVD), laser-enhanced or assisted CVD, and aerosol-assisted CVD (AACVD). 

Chemical vapor deposition includes various systems, and they are low-pressure CVD (LPCVD), atmospheric pressure CVD (APCVD), plasma enhanced CVD (PECVD), and others. Each type of CVD system has its own advantages and limitations. For instance, in LPCVD, the reactor is usually operated at 1 torr. Under this condition, the diffusivity of the gaseous species increases significantly compared to that under atmospheric pressure. Consequently, this increase in transport of the gaseous species to the reaction sites and the by-products from the reaction sites in LPCVD will not become the rate-limiting steps. This leads to the surface reaction step to be the rate limiting one. 

A wide variety of deposition methods are available, and several systems of each type are produced commercially. A review of typical systems has been published [10]. In regard to the CVD of insulating films, four general reactors are presently used atmospheric pressure CVD (APCVD), low and medium temperature low pressure CVD (LPCVD), and plasma-enhanced CVD (PECVD). 

The two most important CVD technologies in MEMS are the Low Pressure CVD (LPCVD) and Plasma Enhanced CVD (PECVD). The LPCVD process produces layers with excellent uniformity of thickness and material characteristics. The main problems with the process are the high deposition temperature (higher than 600 °C) and the relatively slow deposition rate. The PECVD process can operate at lower temperatures (down to 300 °C) thanks to the extra energy supplied to the gas molecules by the plasma in the reactor. However the quality of the films tends to be inferior to processes running at higher temperatures. Moreover most PECVD deposition systems can only deposit the material on one side of the wafers on 1 to 4 wafers at a time whereas LPCVD systems deposit films on both sides of at least 25 wafers at a time. 

Plasma-enhanced (PECVD) or plasma-assisted (PACVD) CVD, (see chapters in Refs. 5,14, and 15), constitute a smaller category of CVD processes that also involves a variety of reactor designs. In these systems, a plasma is... 

CVD is a well-understood thin film deposition method that uses chemical reactions of vapor-phase precursors. CVD processes have traditionally been initiated and controlled by heat as the source of energy. An elevated deposition temperature is normally required, which restricts the types of substrates that can be used and coating materials that can be deposited, especially thermally sensitive ones (Jones and Hitchman, 2009). However, thermal energy is not the only energy supplied to the system plasmas and photons are widely used in CVD processes. Plasma-enhanced chemical vapor deposition (PECVD), or plasma-assisted CVD, is a CVD technique in which plasma in lieu of thermal energy is used primarily to activate ions and radicals in the chemical reactions leading to layer formation on the substrate. One major advantage of PECVD over... 

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