Single crystal Sapphire’s properties as an electrical insulator while providing high heat conductivity makes it an excellent substrate for high heat generating electrical circuits such as Light Emitting Diodes and high power integrated circuits as found in satellite and other high frequency communication technologies. For the production of LED’s, Sapphire wafers are used as a non-conducting substrate for the growth of devices based on Gallium Nitride (GaN). Sapphire crystals also have uses other than as LED substrates, for example sighting windows on missile and missile guidance hardware, furnace sight windows, and watch glass.
There are a number of ways to make Sapphire crystals. For the LED market, the most popular are the Kryopolous and Czochralski techniques. Wafers are cut from an ingot which is produced by melting high-purity Alumina in a Tungsten or Molybdenum crucible within a vacuum furnace using either a Molybdenum or Tungsten hot zone. A precisely oriented rod-mounted seed crystal is dipped into the molten Alumina. The seed crystal’ʹs rod is slowly pulled upwards and rotated simultaneously. By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single-crystal, cylindrical ingot from the melt.
Other methods exist, including edge-defined film-fed growth (EFG), heat-exchanger method (HEM), and vertical‑horizontal gradient freezing (VHFG). The method of growth determines the type and form of refractory metal used.
Sapphire melting furnace hot zones are manufactured from thin‑gauge Molybdenum or Tungsten and formed into a cylindrical heat shield around the crucible. These shields are similar to those used in vacuum-furnace heat shields, except that their operating temperature and environment are more aggressive. The crucibles are either spun from Molybdenum or Tungsten sheet to produce a thin wall crucible or fabricated from either Molybdenum or Tungsten using CIP and sinter technology depending on the furnace design.