About the fluorine chemistry in MCVD: The mechanism of fluorine incorporation into SiO₂ layers

Abstract

Fluorine incorporation during t h e deposition of silica layers is studied under MCVD conditions using C,F,Cl, as fluorine source. The experimental results are compared with known models of the incorporation mechanism. It is found that a gaspolid equilibrium during the consolidation step determines the final fluorine content of the layers.Der Fluoreinbau wahrend der Abscheidung von Quarzglasschichten wird unter MCVDBedingungen studiert, wobei C,F,Cl, als Fluorquelle Verwendung findet. Die experimentellen Ergebnisse werden mit bekannten Modellen Zuni Einbaumechanisnius verglichen. Es zeigt sich, dalj ein Gas-Festkorper-Gleichgewicht wahrend der Konsolidierungsphase den endgultigen Fluorgehalt der Schichten bestimmt.

Thermodynamics of fluorine incorporation into silica glass

“…Moreover, a small amount of POCl 3 was applied, leading to a slight phosphorus content (<0.5 mol% P 2 O 5 ) of the fluorinedoped layers. This improves their optical quality, however without influence on the fluorine concentration, as shown in [21]. It can therefore be neglected if considering the fluorine incorporation mechanism.…”

“…This has been demonstrated for the reaction of SiCl 4 , which is practically fully converted to SiO 2 and Cl 2 (partially dissociated to Cl), for the incorporation of other dopants such as germanium and phosphorus [27] and also for fluorine, using the starting compounds CF 2 Cl 2 , SiF 4 [16] and C 2 F 3 Cl 3 [21]. The chemistry in the system Si/F/O/Cl/C or S is quite complex and a lot of gaseous species can be formed in principle.…”

“…Here, we present results of glass preparation with several fluorine sources both for the direct deposition method and the incorporation during sintering, and interpret them on a thermodynamic basis following throughout the approach already described in [16,20,21]. The special goal of our investigation is to describe the experimental results by a simple but consistent equilibrium model where only the minimum of reactions really needed is included to describe the results within the limits of experimental evidence.…”

“…The deposition experiments have been carried out according to two different doping regimes, compare [13,21] and Fig. 1.…”

“…Several authors have already investigated thermally activated fluorine incorporation via gas phase [11][12][13][14] and have discussed the mechanisms of fluorine incorporation and the influence of fluorine on the deposition efficiency of silica (it is reduced in general) with different fluorine source compounds [15][16][17][18][19][20][21][22][23][24][25][26]. Most authors agree that the mechanism of the fluorine chemistry is based on the existence of chemical equilibria in the gas phase and between the gas and condensed phase, based on the original ideas and investigations of Walker et al [16].…”