Overview of Materials and Fabrication Technologies
3.6 MODIFIED CHEMICAL VAPOR DEPOSITION
Inside processes, such as MCVD, had a different origin. Following the tradition of the electronics industry, chemical vapor deposition (CVD) tech-niques were used to produce doped silica layers inside silica substrate tubes [27]. As in CVD, the concentration of reactants was very low to inhibit gas phase reaction in favor of a heterogeneous wall reaction that produced a vitreous particle-free deposit on the tube wall. The tube was collapsed to a rod and relatively low loss fiber obtained. However, deposition rates were impractically low and attempts to increase them always produced silica particles that deposited on the tube wall and resulted in excess loss. The solution was to exactly reverse the CVD practice: intentionally produce a gas phase reaction by increasing the reactant flows by more than 10 times. Submicron particles were, thus, produced that deposited on the tube wall and were fused into clear pore-free glass as the torch traversed along the tube. MCVD was, thus, developed [28] as the process shown in Fig. 3.6. High-purity gas mixtures are injected
I. TUBE SETUP
II. DEPOSITION
HEAT SOURCE
HEAT SOURCE
HEAT SOURCE
GLASS DEPOSIT EXHAUST
FIBER
III. COLLAPSE
IV. FIBER DRAWING
CHEMICALS
Figure 3.6 The modified chemical vapor deposition (MCVD) process consists of deposition of glass layers inside a silica tube, collapse of tube to a solid rod, and drawing of preform into fiber.
into a rotating tube, which is mounted in a glass working lathe and heated by a traversing oxy-hydrogen torch. Homogeneous gas phase reaction occurs in the hot zone created by the torch to produce amorphous particles, which deposit downstream of the hot zone. The heat from the moving torch sinters this deposit to form a pure glass layer. Typical torch temperatures are suffi-ciently high to sinter the deposited material, but not so high as to deform the substrate tube. The torch is traversed repeatedly to build up, layer by layer, the core or cladding. Composition of the individual layers is varied between traversals to build the desired fiber index structure. Typically, 30 to 100 layers are deposited to make either single-mode or graded index multimode fiber.
3.6.1 Chemical Equilibria: Dopant Incorporation
After the initial demonstration of feasibility, fundamental investigations es-tablished the knowledge required to create a commercial process. For instance, it was necessary to better understand the chemistry of the MCVD process in order to control the incorporation of GeO2and limit hydroxyl impurities. In addition, to increase fabrication efficiency, it was necessary to understand the mechanism by which particles deposit on the substrate tube, as well as the manner in which the silica particles are sintered into pore-free glass. Although process develop-ment preceded quantitative understanding, optimization of the commercial process required this knowledge.
The chemistry of SiCl4 and GeCl4 oxidation was investigated by infrared spectroscopy [29]. Samples of effluent gases from typical MCVD reactions demonstrated that as the maximum hot-zone temperature reached 1300K, SiCl4 began to oxidize to Si2OCl6 (Fig. 3.7). Up to 1450K, the amount of oxychloride increases to a maximum, whereas at higher temperatures the SiCl4, Si2OCl6, and POCl3contents decrease until their concentration in the effluent is insignificant above about 1750K. Above this temperature, all reactants are converted to oxides.
The behavior of GeCl4is different. Its concentration in the effluent gas stream decreases between 1500 and 1700K, but above 1700K remains approximately 50% of its original value. It is clear that the majority of the initial germanium is unreacted and escapes in the effluent.
These results indicate that at low temperatures (T < 1600K), the extent of the reaction for SiCl4, GeCl4, and POCl3 is controlled by reaction kinetics, while at higher temperatures thermodynamic equilibria become dominant. It is clear from rate studies that the residence times in the hot zone are sufficient to produce equilibrium above 1700K. The SiCl4and GeCl4concentrations at high temperatures are strongly influenced by the equilibria:
SiCl4 (g)þ O2 (g)! SiO2(s)þ 2Cl2(g) (3:2) and
GeCl4 (g)þ O2 (g)! GeO2 (s)þ 2Cl2(g) (3:3) Equilibrium constants for these reactions may be written
KSiO2¼ (aSiO2)(PCl2)2=(PSiCl4)(PO2) (3:4) KGeO2¼ (aGeO2)(PCl2)2=(PGeCl4)(PO2), (3:5) where Pi are the partial pressures of gaseous species and ai represents the chemical activities of the solid species. The activities can be approximated by gixi, where xiis the mole fraction of the particular species in the solid and giis the activity coefficient. An activity coefficient of unity implies an ideal solution obeying Raoult’s law. The equilibrium constants for these reactions have been determined as a function of temperature and indicate that Eq. (3.2) strongly favors the formation of SiO2at high temperature, as verified by the experiments described earlier. Oxidation of GeCl4by Eq. (3.3), on the other hand, is incom-plete because the equilibrium constant, KGeO2, is less than unity at temperatures higher than 1400K. This means that only a fraction of the germanium starting composition will be present as GeO2. The presence of significant Cl2
10−1
1000 1200 1400 1600 T (K)
SiCl4 GeCl4 Si2OCl6 POCl3
P (atm)
1800 2000 2200 10−2
10−3 10−4 10−5
Figure 3.7 Modified chemical vapor deposition (MCVD) effluent composition as a function of hot-zone temperature. Starting reactants: 0:5 g=min SiCl4, 0:05 g=min GeCl4, 0:016 g=min POCl3, 1540 cm3=min O2.
concentration resulting from the complete oxidation of SiCl4 shifts the equilib-rium further toward GeCl4 by the law of mass action. Low oxygen partial pressure has the same effect.
3.6.2 Purification from Hydroxyl Contamination
A second important aspect of MCVD chemistry is the incorporation of the impurity OH[30] because reduction of OH in optical fibers to ppb levels is essential for realization of low attenuation in the 1.3- to 1:55 mm region. Hydro-gen species originate from three sources: diffusion of OH from the substrate tube during processing, impurities in the starting reagents and carrier oxygen gas, and contamination from leaks in the chemical delivery system.
The OHlevel in the fiber is controlled by the reaction
H2Oþ Cl2! 2HCl þ1=2O2 (3:6) with equilibrium constant
KOH¼ (PHCl)2(PO2)1=2=(PH2O)(PCl2): (3:7) The concentration of OHincorporated into the glass, cSiOH[3], is described by CSiOH¼ (PH2Oinitial)(PCl2)1=2=(PO2)1=4: (3:8) During deposition in MCVD, Cl2 is typically present in the 3–10% range because of oxidation of the chloride reactants. This is sufficient to reduce OH by a factor of about 4000. However, chlorine is typically not present during collapse and significant amounts of OH can be incorporated by diffusion of torch byproducts through the silica tube. Figure 3.8 shows the dependence of the SiOH concentration in the resultant glass as a function of typical PO2 and PCl2
concentrations used during MCVD deposition and collapse with 10 ppm H2O in the starting gas. Figure 3.8 also shows typical consolidation of the VAD and OVD soot processes.
3.6.3 Thermophoresis
Turning now from the reaction equilibria, we consider the mechanism of deposition of particles on the tube walls. The SiO2particles produced by vapor phase reaction have diameters in the range 0:02–0:1 mm and are, thus, entrained in the gas flow. Without the imposition of a temperature gradient, they would remain in the gas stream and exit from the tube end. However, temperature gradients in the gas stream produced by the traveling torch give rise to the phenomenon of thermophoresis [31]. Here, particles residing in a thermal
gradient are bombarded by energetic gas molecules from the hot region and less energetic molecules from the cool region. A net momentum transfer forces the particle toward the cooler region. Within an MCVD substrate tube, because the wall is cooler than the center of the gas downstream of the torch, particles are driven toward the wall where they deposit. The MCVD process is shown sche-matically in Fig. 3.9 in terms of (1) heat transfer in the hot zone, (2) reaction, (3) particle formation, (4) particle deposition beyond the hot zone where the tube wall becomes cool relative to the gas stream, and (5) consolidation of previously deposited particles in the hot zones as the torch traverses to the right.
MCVD COLLAPSE
Figure 3.8 Typical incorporation of OHduring processing stages of modified chemical vapor deposition (MCVD), for 10 ppm H2O in chemical precursors.
REACTION ZONE QUARTZ SUBSTRATE
Figure 3.9 Particle formation and thermophoretic deposition in modified chemical vapor deposition (MCVD).
A mathematical model for thermophoretic deposition [32], experimentally verified, concluded that deposition efficiency (ratio of SiO2equivalent entering tube to that contained in exhaust) may be expressed as e¼ 0:8(1 Te=Trxn), where Trxnis the gas reaction temperature and Teis the temperature downstream of the torch at which the gas and the tube wall equilibrate. Typically, Teis about 400C and Trxn about 2000C, giving an efficiency value on the order of 60%.
Note that the efficiency is not a function of the maximum tube temperature.
Examination of the process of consolidation of the soot layer on the inner surface of the silica tube revealed the mechanism to be viscous sintering [33]. By this mechanism, the rate of consolidation is proportional to the sintering time and surface tension and inversely proportional to the void size, initial soot density, and glass viscosity.