liquid column. Second, it provides mechanical strength for the thin layers thus obtainable. 2. Experimental aspects. Growth is currently made with opposite silicon layers of equal thickness. This condition of symmetrical growth can only be obtained if the ribbon is pulled in a vertical plane without lateral excursions in that plane [15]. The puller has been conceived in order to fulfill the related mechanical constraints; it is also equipped with a melt replenishment line for continuous operation (Fig. 9). The puller consists of three segments, i.e., from bottom to top, the carbon shaper case, the growth chamber and the pulling system. The ribbon is extracted from the growth chamber, protected against air contamination by an argon stream, via an opening in its upper part. The control of the thickness of the silicon layers boils down to that of the melt level in the crucible and the reference temperature, currently taken on the susceptor wall and measured by means of an optical pyrometer. Under typical growth conditions, the layer thickness can be controlled by the operator within a few micrometres at a pull rate of about 10 cm/min. 3. Carbon substrate aspects. The ribbon substrate is a composite structure which consists of a low-density, soft graphite ribbon 300 pm thick, coated with pyrocarbon layers 2 to 5 pm thick. Its geometry and stiffness are given by the graphite ribbon whereas its wetting properties and chemical resistance to molten silicon are provided by the pyrocarbon coating [16]. The potential difficulties raised by the use of an ‘internal shaper' during growth are basically due to its compositional impurities, geometrical imperfections and thermo-mechanical properties which result in melt contamination, curved layers and built-in stresses respectively. They have been
RkJQdWJsaXNoZXIy MTU5NjU0Mg==