The sapphire crystals currently available in the marketplace today are produced (grown) by one of the following three methods.
Edge-Defined Film-Fed Growth (EFG)
This method is used when a near net shape product is required. The crystal is formed by dipping a “seed’ crystal into molten alumina oxide and then slowing pulling the re-crystallizing material through a shaped capillary. For window applications the resulting shape would be a flat ribbon or sheet. Although the crystal quality produced by this method is good, it is not as good as the following methods.
Like the growth method described above, the crystal is formed by dipping a “seed” crystal into molten alumina oxide. The seed crystal is slowly pulled upwards and may be rotated simultaneously. By precisely controlling the temperature gradients, rate of pulling and rate of temperature decrease, it is possible to produce a large, single-crystal, roughly cylindrical ingot from the melt. This technique crystallizes the entire feedstock volume into a boule. The size of the crucible holding the molten alumina oxide is close to that of the final crystal, and the crystal grows downward into the crucible, rather than being pulled up and out of the crucible. The upward pulling of the seed is at a much slower rate than the downward growth of the crystal.
Heat Exchanger Method (HEM)
Unlike the other two methods described above, the crystal is formed in-situ. The “seed” crystal is place at the bottom of the crucible covered with the starting “feed” stock. This is placed inside the vacuum furnace which has a cold finger cooled by helium in contact with the base of the crucible where the seed is located. The alumina oxide feed stock is then heated above its melting point, while the seed is maintained in a solid state. The crystallization takes place as the furnace temperature is slowly decreased, and the helium flow to the cold finger is increased. This process produces a large single crystal sapphire boule free of residual stress.