The Role of Fluid Flow Phenomena in the Czochralski Growth of Oxides
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THE ROLE OF FLUID FLOW PHENOMENA IN THE CZOCHRALSKI GROWTH OF OXIDES.
D.C. MILLER Airtron Division Litton Systems, Inc., 200 East Hanover Avenue, Morris Plains, New Jersey 07950.
ABSTRACT In the Czochralski growth of single crystals from large melts, fluid flow phenomena have a major effect on interface shape, growth striations, defect density and the length of crystals which can be grown from a melt of given volume and thermal geometry. Because of the technical difficulties encountered in making direct measurements in molten oxides, simulation experiments have been extensively utilized to gain insight into melt behavior. Both temperature profile and flow geometry results from simulation experiments are discussed. This data is supported by direct melt observations and results from the characterization of grown crystals. When reviewed together, this information offers new insights into the complex behavior of Czochralski growth processes, including the role of thermal gradients, crystal rotation, and surface tension driven (Marangoni) convection. INTRODUCTION As the size of melts in the Czochralski process are increased in order to grow larger crystals, melt flow phenomena become increasingly important in controlling many aspects of the growth process as well as the quality of the grown In terms of the growth process this includes how often a single cryscrystal. tal is obtained each time a seed is dipped, the external shape of the crystal and the amount of the melt remaining in the crucible when the run is terminated (growth yield). Dislocation density, dopant or nonstoichiometry striations, and voids or inclusions can all be included in defining the quality of the crystals (defect yield). All of these factors contribute to the overall yield of a crystal growth operation. In this work many of these factors will be illustrated with examples from the growth of gadolinium gallium garnet, (GGG), a material used primarily as substrates in the manufacturing of magnetic bubble memory devices,[l-3] and fluid flow simulation experiments.[4-61 DEFECT PROBLEM Perhaps the most famous defect problem shown to be related to fluid flow is illustrated in Fig. 1., a 2 inch diameter GGG boule with both ends polished, between crossed polars. A cluster of many dislocations is apparent in the center. The particular dislocation generation mechanism operating here is believed to be related to an instability in the direction of fluid flow in the melt arising from the opposing forces of natural convection and that induced by the rotating crystal. The direction of convective flow is upwards next to the hot crucible wall and down in the center. The crystal driven flow is in just the opposite direction. The driving force for this flow varies directly with the square of the diameter of the crystal and directly with the rotation rate.[7]
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Fig. 1. End on view of a 2" diameter GGG boule with a dislocation cluster in the center, between crossed polars.
Fig. 2. A sectioned top of a 2" diameter GGG boule between crossed polars. [3]
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