Shaping processes in crystal growth
Shaping processes in crystal growth are a collection of techniques for growing bulk crystals of a defined shape from a melt, usually by constraining the shape of the liquid meniscus by means of a mechanical shaper. Crystals are commonly grown as fibers, solid cylinders, hollow cylinders (or tubes), and sheets (or plates). More complex shapes such as tubes with a complex cross section, and domes have also been produced. Using a shaping process can produce a near net shape crystal and reduce the manufacturing cost for crystals which are composed of expensive or difficult to machine materials.
Edge-defined film-fed growth or EFG was developed for sapphire growth in the late 1960s by Harold LaBelle and A. Mlavsky at Tyco Industries. A shaper (also referred to as a die) having dimensions approximately equal to the crystal to be grown rests above the surface of the melt which is contained in a crucible. Capillary action feeds liquid material to a slit at the center of the shaper. When a seed crystal is touched to the liquid film and raised upwards, a single crystal forms at the interface between the solid seed and the liquid film. By continuing to pull the seed upwards, the crystal expands as a liquid film forms between the crystal and the top surface of the shaper. When the film reaches the edges of the shaper, the final crystal shape matches that of the shaper.
The exact dimensions of the crystal will deviate from the dimensions of the shaper because every material has a characteristic growth angle, the angle formed at the triple interface between the solid crystal, liquid film, and the atmosphere. Because of the growth angle, varying the height of the meniscus (i.e. the thickness of the liquid film) will change the dimensions of the crystal. The meniscus height is affected by pulling speed and crystallization rate. The crystallization rate depends on the temperature gradient above the shaper, which is determined by the configuration of the hot-zone of the crystal growth furnace, and the power applied to the heating elements during growth. The difference in thermal expansion coefficients between the shaper material and the crystal material can also cause appreciable size differences between the shaper and the crystal at room temperature for crystals grown at high temperatures.
The shaper material should be non-reactive with both the melt and growth atmosphere, and should be wet by the melt.
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