Plasma-Enhanced Systems

The system operates at about 400 mTorr and with flow rates of about 1 slm, so a roots blower is used to pump the system to base pressure ( 1 mTorr) and then handle the gas flow at these low pressures. 

As noted earlier, wafer throughput is a key concern. The more wafers that can be placed on the platen, the higher the throughput. For 3 wafers, a load of 42 wafers could be handled. As wafer sizes have increased, however, the capacity of this system has been severely impacted. For 5 wafers, only 16 can be loaded at one time. Obviously, the problem is more difficult for 6 and eventually 8 wafers. This difficulty eventually led to the development of the hot tube PECVD system to be discussed next. 

It should also be noted that this reactor has to occupy space in the clean room, and is not operated under computer control. For application to modern fab lines, these are two disadvantages of this system. 

The major advantage of the hot tube approach is in wafer throughput. A typical wafer load would be seventy 3 wafers, as contrasted to the 42-wafer load in the AMP-3300. The advantage is not quite 2 1, as the hot tube wafer boat cools off when it is withdrawn to replace finished wafers. Time to reheat this structure lengthens processing time. 

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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. 

At the end of last century, a near frictionless carbon (NFC) coating was reported, which is practically hydrogen contained DLC film grown on steel and sapphire substrates using a plasma enhanced chemical vapor deposition (PECVD) system [50]. By using a ball on a disk tribo-meter, a super low friction coefficient of 0.001-0.003 between the films coated on both the ball and the disk was achieved [50]. A mechanistic model was proposed that carbon atoms on the surface are partially di-hydrogenated, resulting in the chemical inertness of the surface. Consequently, adhesive interaction becomes weak and super low friction is achieved [22],... 

For the most part, plasma-enhanced etching and deposition are performed in four basic reactor types (Figure 7 2, 46). Each reactor has several basic components a vacuum chamber and pumping system to maintain reduced pressures, a power supply to create the discharge, and gas- or vaporhandling capabilities to meter and control the flow of reactants and products. 

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. 

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. 

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. 

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. 

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. 

The next three chapters review the deposition of thermally-induced dielectric films (Chapter 3) and metallic conducting films (Chapter 4), as well as plasma-enhanced films of either type (Chapter 5). The many chemical systems employed to create these films are considered, and the nature of the resulting films is presented. Films studied are silicon dioxide, silicon nitride, polysilicon, epitaxial silicon, the refractory metal silicides, tungsten and aluminum. 

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... 

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. 

The different PEC VD film/substrate systems are schematically presented in Figure 6a. The substrates correspond to 99.99% pure Al, mechanically polished with a 0.3 pm alumina powder, then finally electrolytically in a 70% methanol-30% nitric acid solution. When exposed to air, a native aluminum oxide of about 3 nm is produced. The substrates were coated with a dielectric film of a passivation material either SijN or Si02 4.5 wt.% P. These systems are, respectively, denoted as system A and system C. The SijN films were produced by plasma enhanced chemical vapor deposition at a temperature of 360°C, while the SiO 4.5 wt.% P films were chemically vapor deposited at a temperature of 420°C. For both passivation materials, the thickness of the films was 0.8 pm. 

Complement is a plasma system comprised of interrelated proteases arranged in a cascade fashion that becomes activated in response to bacterial signals and Ags associated withbacteria. The role of complement is tw ofold 1) to serve as an initial, first line of defense to invading microorganisms and 2) to enhance systemic host defense by activating and orchestrating the... 

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