The Bridgman and the Czochralski Methods are well known methods of growing bulk single crystals from the melts of congruently melting compounds. These methods basically involve "pulling" a single crystal from the melt or liquid reservoir of material. The Czochralski method involves pulling the crystal from a position in the melt which is displaced away from the container or crucible walls. The size and shape of the crystal are determined by the temperature distribution around the growth site. Since the container has some influence over the temperature distribution of the melt, it has an indirect influence on the size and shape of the crystal, but such effect is only secondary. The crystal shape and size is primarily determined by the size and shape of the interface formed between the crystal and the melt as the crystal is continually formed and pulled upward from the melt.
Modifications of the Czochralski method have been developed in which the shape of the crystal during growth is controlled by a shaper or die. Typical shaped crystal growth methods include the Stepanov method, as described by Antonov (Antonov, P. I., and Nikanorov, S. P., Journal of Crystal Growth 50 3 (1980), or the Edge-Defined Film Fed Growth (EDFG)) method, as described by LaBelle (LaBelle H. E., Jr., Journal of Crystal Growth 50, 8 (1980)). These methods are basically modifications of the general Czochralski technique, wherein a shaper or die is provided which has walls that are sufficiently close to the crystal-melt interface to affect and partially control the size and shape of the crystal as it is pulled from the melt as it passes through the shaper.
FIGS. 1 and 2 are schematic representations of the Stepanov method of crystal growth wherein FIG. 1 shows EDFG shaped crystal growth and FIG. 2 depicts a non-wetting melt which does not wet the shaper's surface. As is apparent, the shape of the meniscus of the liquid melt between the crystal and the shaper is determined by the wettability of the shaper surface. Due to the possible variations in meniscus shape, the size of the crystal being pulled through the shape is only partially dependent upon the shaper walls. The actual size of the crystal will depend upon the surface area of the crystal-melt interface, which is, in turn, determined by the wettability of the melt on the shaper as demonstrated in FIGS. 1 and 2. The crystal formed in FIG. 1 will be approximately the same diameter as the outer diameter of the shaper, while the diameter of the crystal formed in FIG. 2 will be approximately the same as the shaper's inside diameter. Thus, the crystal formed in FIG. 1 will be wider than the crystal formed in FIG. 2 even though the shapers are of the same dimensions. This difference is due to the different wetting of the shaper by the melts.
It would be desirable to provide an apparatus in which the size and shape of the crystal are controlled by the shaper only and are not dependent upon the shape of the meniscus of melt material which developes between the crystal and the shaper as is presently the case in shaped crystal growing systems based on the Czochralski method.
A possible solution, to overcome the inherent variability in crystal shape and size for Czochralski-type methods, would be to lower the crystal melt interface into the die as shown by the horizontal dotted line in FIG. 2. However, if the crystal melt interface is lowered into the shaper to accurately control the crystal shape, the resulting surface adhesion and frictional forces between the crystal and the shaper frequently result in breakage of the crystal as it is pulled from the shaper.
In addition to problems involving control of crystal fiber widths, the Czochralski-type systems (where the crystal melt interface is displaced from the shaper walls) also present problems involving alignment of the crystal over the shaper. Continual care must be taken to insure that the crystal is aligned directly over the shaper. Since there is no contact between the crystal and shaper, some external alignment means are required in order to insure that the crystal remains in the desired position over the shaper. As is well known, when pulling crystal fibers of any length, the fibers tend to vibrate. Continual maintenance of crystal alignment over the shaper can be especially difficult when such crystal vibrations occur.
It would be desirable to provide an apparatus for growing shaped crystal fibers in which both the shape and size of the fiber can be accurately controlled over long periods of time, crystal breakage due to adhesion and frictional forces at the shaper wall is minimized, and wherein alignment of the crystal with the shaper is maintained at all times and even during crystal vibrations which may be induced during the crystal pulling process.
USPTO Backgrounds
USPTO Backgrounds