A major difficulty in this growth concept arises from the necessity to balance the large capillary forces which are generated at the meniscus edges, because of the small radius of curvature in the radial direction {RI in Fig. 5), and tend to split the liquid column. This problem has been overcome by different means which determine the overall characteristics and performances of ribbon growth methods. Four of them at least, the WEB, RTR, ESP and RAD processes have proved their ability to deliver ribbon thicknesses below or equal to 100 «m with an acceptable layer quality. The ESP [12] and WEB [13] processes are based on a stabilization of only the edges of the meniscus, its lateral part rising freely from the melt surface, Fig. 6. This is achieved by the use of capillary attachment on wetted filaments in the ESP process and by means of dendrites which grow with the ribbon in the WEB process. The role of the shaping agent is dramatically illustrated in the case of the WEB process which, unlike its competitors, can deliver single crystalline sheets but requires a drastic control of the temperature field in the melt. The RTR process is based on the recrystallization of a ribbon feed-stock by displacement of a narrow molten zone, Fig. 7 [14]. Finally, as opposed to the above methods, the RAD process uses a temporary carbon substrate to shape the freezing meniscus. The RAD Process 1. Basic principle. The RAD process is based on the crystallization of a silicon film drawn along by a carbon ribbon which is pulled upwards through a silicon melt [15]. The carbon ribbon is passed through a slot in the bottom of a silica crucible which contains the melt; as it emerges from the melt surface, the carbon ribbon is coated on
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