Reactions between liquid silicon and vitreous silica
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Oxygen incorporation in silicon crystals during Czochralski growth is dependent on many factors, of which the dissolution of the silica crucible is of great importance. In this paper the reactions between vitreous silica and molten silicon have been analyzed, both in sealed ampoules and in Czochralski crucibles. It was found that the vitreous silica crystallizes to cristobalite by lateral growth. For this reaction to occur it is necessary that liquid silicon is present. The vitreous silica dissolves and the cristobalite grows with a thin layer of liquid silicon between them. Different oxygen concentrations in the melt in equilibrium with the amorphous and crystallized silica are necessary for the reaction to proceed. The oxygen flux in the melt is dependent upon the dissolution of both vitreous silica and cristobalite as well as the reaction between these phases.
I. INTRODUCTION
light on the mechanisms of oxygen transport from the crucible to the liquid, the current study was initiated. In this study closed ampoules charged with silicon were used to study the equilibrium reactions. Having a low internal pressure, the silica ampoules collapse on heating, giving direct contact between silica and silicon without evaporation or oxygen loss. Optimum conditions in which to study the interface reactions are thus obtained. As a reference, crucibles from real Czochralski growth were also studied.
It is well known that the oxygen content of silicon wafers is very important for device performance and device yield. The oxygen in the wafers is incorporated during the growth process. The main source of oxygen, during crystal growth from the melt by the Czochralski method, is the silica crucible used as charge container. Most of the work aiming at control of oxygen during Czochralski pulling has concentrated on the influence of the crystal growth parameters, as reviewed by Moody.1 Where the oxygen source is concerned, the interface reaction between liquid silicon and the crucible is, of course, extremely important, but so far, little work has been done on this topic. An important experimental study of the reaction was initiated by Chaney and Varker,2 who concluded that besides the dissolution of vitreous silica, spots are formed at some surface sites in the presence of molten silicon by the devitrification of vitreous silica to cristobalite. The paper gave neither details on the formation of cristobalite nor indications as to the oxygen content. Later, Zulehner and Huber3 reported that spots nucleate at points where the surface is contaminated; the initial shape of the cristobalite spots are spherical crystal clusters; further growth occurs parallel to the silica surface as perpendicular growth is halted by dissolution into the melt. They also speculated that the growth rate of cristobalite into the silica is similar to its dissolution rate into the melt. In spite of the above-mentioned studies, there are still many uncertainties. For example, there are no experimental studies on the initial formation of cristobalite, its grow
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