Modified chemical-vapor deposition MCVD process

Fig. 21. Schematic illustration of the four primary vapor-phase deposition processes used in optical-fiber fabrication outside vapor deposition (OVD), modified chemical vapor deposition (MCVD), plasma vapor deposition (PVD), and vapor axial deposition (VAD) (115).

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. 

Modified carbon fibers, 13 383-385 Modified cellulosic membranes, in hemodialysis, 26 825, 826-828t Modified chemical vapor deposition (MCVD), in fiber optic fabrication, 11 136-137, 138, 139 Modified-Claus sulfur recovery process, 23 601, 602... 

This CVD procedure is somewhat different from that used to deposit semiconductor layers. In the latter process, the primary reaction occurs on the substrate surface, following gas-phase decomposition (if necessary), transport, and adsorption. In the fiber optic process, the reaction takes place in the gas phase. As a result, the process is termed modified chemical vapor deposition (MCVD). The need for gas-phase particle synthesis is necessitated by the slow deposition rates of surface reactions. Early attempts to increase deposition rates of surface-controlled reactions resulted in gas-phase silica particles that acted as scattering centers in the deposited layers, leading to attenuation loss. With the MCVD process, the precursor gas flow rates are increased to nearly 10 times those used in traditional CVD processes, in order to produce Ge02-Si02 particles that collect on the tube wall and are vitrified (densified) by the torch flame. 

Inside processes such as modified chemical vapor deposition (MCVD) had a different origin. Chemical vapor deposition (CVD) had long been used In the electronics industry for fabrication of silicon devices and was adapted to produce silica layers inside substrate tubes [18j. In CVD, the concentration of reactants is very low to inhibit gas phase reaction in favor of a heterogeneous wall reaction which produced a vitreous, particle-free deposit on the substrate. This is fine for the 1000 A films required for semiconductor processing, but fails to produce thick deposits required for fiber. CVD was therefore reversed, the reactant concentration was increased and large volumes of particles were produced inside the silica substrate tube. They deposited on the tube wall and were sintered to glass. 

The inside process is somethnes called inside vapor phase oxidation (IVPO) or inside vapor deposition (IVD). However, it is perhaps most commonly referred to as modified chemical vapor deposition (MCVD). In one variation, perhaps not currently in commercial use, an argon/oxygen plasma is generated within the tube by microwave radiation to provide the thermal energy for the chemical reaction, instead of relying on heat from a burner outside the tube. 

The MCVD modified chemical vapor deposition) method also relies on the production of glass from halide vapors. The deposition process occurs inside a vitreous silica tube, which is heated from the outside and which serves as the cladding for the fiber. The reaction of the vapors now occurs without contamination by gases from the flames, which never contact the deposited material. Consolidation of the soot occurs simultaneously with deposition. The process continues until the desired layer thickness is reached, after which the entire tube is collapsed by increasing the external temperature to complete the preform. 

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. 

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