PCVD

Whereas the OVD, PCVD, and MCVD processes build a refractive index profile layer by layer, the VAD process uses gaseous constituents in the flame to control the shape and temperature distribution across the face of the growing soot boule. 

Excimer lamps have opened the possibiHty of cost-effective large-area direct photochemical vapor deposition (PCVD). PCVD of stoichiometric, insulating Si02 onto Si wafer has been reported using SiH and N2O as gas-phase precursors and the 172-nm radiation from a Xe 2 lamp (54). Deposition... 

In plasma chemical vapor deposition (PCVD), the starting materials are typically SiCl, O2, 2 6 GeCl (see Plasma technology). Plasma chemical vapor deposition is similar to MCVD in that the reactants are carried into a hoUow siUca tube, but PCVD uses a moving microwave cavity rather than a torch. The plasma formed inside the microwave cavity results in the deposition of a compact glass layer along the inner wall of the tube. The temperatures involved in PCVD are lower than those in MCVD, and no oxide soots are formed. Also, the PCVD method is not affected by the heat capacities or thermal conductivities of the deposits. 

Inside" processes—such as modified chemical vapor deposition (MCVD) and plasma chemical vapor deposition (PCVD)—deposit doped silica on the interior surface of a fused silica tube. In MCVD, the oxidation of the halide reactants is initiated by a flame that heats the outside of the tube (Figure 4.8). In PCVD, the reaction is initiated by a microwave plasma. More than a hundred different layers with different refractive indexes (a function of glass composition) may be deposited by either process before the tube is collapsed to form a glass rod. 

A schematic of the Philips PCVD unit is shown in Figure 11. 

Plasma chemical synthesis, 1 717 Plasma chemical vapor deposition (PCVD), in fiber optic fabrication, 11 139-140 Plasma coatings, 5 665 Plasma deposition, in vitreous silica manufacture, 22 414, 415 Plasma derivatives, 12 129t... 

Up to now three chemical vapor deposition (CVD) techniques have proved suitable for the preparation of high quality optical fibers the outside vapour phase oxidation (OVPO) process8, the modified CVD (MCVD) process9 and the plasma-activated CVD (PCVD) process10. The last mentioned process will be the main subject of this article. To give a better appreciation of the principles the alternative processes will be described briefly. 

The main conclusion that can be drawn from our discussion is that the formation of macromolecules in the gas phase can be ruled out under the experimental conditions applied. This was not obvious from the beginning in view of the results of MCVD- and OVPO-processes where dust formation is observed in the gas phase. For the PCVD-process a gradual formation of soot in the gas phase can ala) be observed for increasing pressures. In the following only those experimental conditions will be considered where the soot formation can be neglected. 

Fig. 18. Typical optical attenuation curve of a fiber produced by means of PCVD. The upper and lower curve, respectively, indicate the accuracy of the measurement...

The PCVD process has now been in operation under pilot plant conditions for some years. Figures 20 and 21 give the attenuation and full width half maximum pulse broadening of 200 successive fibers selected at random from the production24). The Figures reflect the good control that can be achieved by the PCVD process under production conditions. 

This chapter gives an introduction into the preparation of optical fibers by means of the plasma-activated chemical vapour deposition (PCVD) method. 

Up to now three preparation methods, namely the OVPO, MCVD and PCVD method have been proven suitable for the preparation of high quality optical fibers. 

A short review of the OVPO and the MCVD method is given. A more detailed discussion of the PCVD method is presented. Emphasis has been laid on the description of experiments in which pure and Ge02 -oxide doped silica have been deposited. It turns out that the PCVD method has some unique properties such as deposition without soot formation in the gas phase, moderate substrate temperature, high deposition efficiency and the possibility of rapid reactor movement. Besides germanium, other dopant elements such as boron and fluorine have also been successfully deposited simultaneously with silica. 

High quality optical fibers obtained using the PCVD method demonstrate that the plasma activated deposition fulfills the extreme requirements for optical fibers almost ideally. This is underlined by the optical properties of a large number of fibers prepared under the same PCVD conditions on a pilot plant scale which show a narrow distribution in their attenuation and pulse broadening values. This in turn is directly correlated with the good process control which is achievable. 

Figure 15 TEM of aluminum thin film deposited in L.PCVD hot tube furnace. After Levy.21 (Reprinted by permission of the publisher. The Electrochemical Society, Inc.)...

Figure 1.3 clearly demonstrates the luminous gas phase created under the influence of microwave energy coupled to the acetylene (gas) contained in the bottle. This luminous gas phase has been traditionally described in terms such as low-pressure plasma, low-temperature plasma, nonequilibrium plasma, glow discharge plasma, and so forth. The process that utilizes such a luminous vapor phase has been described as plasma polymerization, plasma-assisted CVD (PACVD), plasma-enhanced CVD (PECVD), plasma CVD (PCVD), and so forth. 

The material deposition that occurs in the low-pressure electrical discharge has been discussed under various terminologies such as plasma polymerization (PP), plasma-enhanced chemical vapor deposition (PECVD), plasma-assisted chemical vapor deposition (PACVD), plasma chemical vapor deposition (PCVD), and so forth [1]. However, none of these terminologies seems to represent the phenomenon adequately. The plasma aspect in the low-pressure discharge is remote, although it plays a key role in creating the environment from which material deposition occurs to the extent that no chemical reaction occurs without the plasma. In this sense, PECVD and PACVD could be out of the context in many cases in which nothing happens without plasma. In such cases, PP or PCVD would describe the phenomenon better. If the substrate was not heated substantially above the ambient temperature, the use of PECVD or PACVD should be avoided. 

In order to find the domain of LCVD, it is necessary to compare various vacuum deposition processes chemical vapor deposition (CVD), physical vapor deposition (PVD), plasma chemical vapor deposition (PCVD), plasma-assisted CVD (PACVD), plasma-enhanced CVD (PECVD), and plasma polymerization (PP). All of these terms refer to methods or processes that yield the deposition of materials in a thin-film form in vacuum. There is no clear definition for these terms that can be used to separate processes that are represented by these terminologies. All involve the starting material in vapor phase and the product in the solid state. 

Plasma polymerization or PCVD Electrical discharge Gas Coupled... 

Ceramic Preparative Methods 17.2.5. Chemical Vapor Deposition 17.2.5.3. Plasma CVD (PCVD)... 

CVD can also be classified using its activation methods. Thermal activated CVD processes are initiated only with the thermal energy of resistance heating, RF heating or by infrared radiation. They are widely used to manufacture the materials for high-temperature and hard-to-wear applications. In some cases enhanced CVD methods are employed, which includes plasma-enhanced CVD (PECVD), laser-induced CVD (LCVD), photo CVD (PCVD), catalysis-assisted CVD and so on. In a plasma-enhanced CVD process the plasma is used to activate the precursor gas, which significantly decreases the deposition temperature. 

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