Horizontal ribbon growth (HRG) promises the growth of crystalline silicon at rates that are orders of magnitude greater than vertical ribbon growth technologies. Unlike vertical meniscus-defined crystal growth processes, such as edge-defined film-fed growth (EFG), which are inherently stable, there are many failure modes that must be avoided in the HRG process. Thus, its successful operation will rely on a thorough understanding of system design and control.

We present a comprehensive thermal-capillary model based on finite-element methods to study the coupled phenomena of heat transfer, melt flow, segregation, and interfacial phenomena (solidification and capillarity) in the HRG process. Bifurcation analysis coupled with transient computations using this model reveals process limitations consistent with known failure modes and suggests operating windows that may allow for stable process operation.  In particular, model results presented here identify failure mechanisms, including the bridging of crystal onto crucible, the spilling of melt from the crucible, and the undercooling of melt at the ribbon tip, that are consistent with prior experimental observations.

Model results also reveal interesting and potentially beneficial redistribution of impurities at the interface with inherent purging characteristics. Impurities have a tendency to accumulate towards a narrow bottom portion of the crystal leaving a majority of the crystal relatively pure. The existence of such a redistribution pattern is explained on the basis of convective flow patterns in the system, such as rotating vortices and solidification flow. High operating pull rates, impurities with low partition coefficients and low diffusivities are shown to further enhance such redistribution effects.

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Supported in part by NSF CBET-075503, the content of which does not necessarily reflect the position or policy of the United States Government, and no official endorsement should be inferred.

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