A boule is a

single-crystal In materials science, a single crystal (or single-crystal solid or monocrystalline solid) is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries. The absen ...

ingot An ingot is a piece of relatively pure material, usually metal, that is Casting, cast into a shape suitable for further processing. In steelmaking, it is the first step among semi-finished casting products. Ingots usually require a second procedu ...

produced by synthetic means. A boule of

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

is the starting material for most of the

integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

s used today. In the

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

industry synthetic boules can be made by a number of methods, such as the Bridgman technique and the

Czochralski process The Czochralski method, also Czochralski technique or Czochralski process, is a method of crystal growth used to obtain single crystals (monocrystals) of semiconductors (e.g. silicon, germanium and gallium arsenide), metals (e.g. palladium, plati ...

, which result in a cylindrical rod of material. In the Czochralski process a

seed crystal A seed crystal is a small piece of single crystal or polycrystal material from which a large crystal of typically the same material is grown in a laboratory. Used to replicate material, the use of seed crystal to promote growth avoids the otherwi ...

is required to create a larger crystal, or

. This seed crystal is dipped into the pure molten silicon and slowly extracted. The molten silicon grows on the seed crystal in a crystalline fashion. As the seed is extracted the silicon solidifies and eventually a large, cylindrical boule is produced. A semiconductor crystal boule is normally cut into circular

wafers A wafer is a crisp, often sweet, very thin, flat, light biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. They frequently have a waffle surface pattern but may also be patterned with insignia of the foo ...

using an inside hole

diamond saw A diamond blade is a saw blade which has diamonds fixed on its edge for cutting hard or abrasive materials. There are many types of diamond blade, and they have many uses, including cutting stone, concrete, asphalt, bricks, coal balls, glass, a ...

or diamond

wire saw A wire saw is a saw that uses a metal wire or Wire rope, cable for mechanical cutting of bulk solid material such as stone, wood, glass, ferrites, concrete, metals, crystals etc.. Industrial wire saws are usually powered. There are also hand-power ...

, and each wafer is

lapped Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine. Lapping often follows other subtractive processes with more aggressive material removal as a first step ...

and polished to provide substrates suitable for the fabrication of

semiconductor device A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivit ...

s on its surface. The process is also used to create

sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, cobalt, lead, chromium, vanadium, magnesium, boron, and silicon. The name ''sapphire ...

s, which are used for substrates in the production of blue and white

LEDs A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (correspo ...

, optical windows in special applications and as the protective covers for

watches A watch is a Clock, timepiece carried or worn by a person. It is designed to maintain a consistent movement despite the motions caused by the person's activities. A wristwatch is worn around the wrist, attached by a watch strap or another typ ...

. A popular method used to create sapphire boules is the Heat Exchanger Method (HEM), in which aluminum oxide is placed in a molybdenum crucible and heated until melting at 2200°C. It allows for very large crystals over 30 cm wide to be produced. The process takes place in a vacuum. A sapphire seed crystal sits at the bottom of the crucible and is kept from melting by heat exchange (cooling) with helium gas or liquid helium in which is shielded from the vacuum by a cold finger heat exchanger. The furnace is kept at a temperature just above melting, but the heat exchanger is at a temperature just below melting. Then the heat exchanger temperature is lowered to start crystalization, and then the aluminum oxide is cooled over a period of at least 72 hours to 17 days to crystalize it into sapphire. The crucibles are single use, the process is similar to the Bridgman technique and the Stöber methods for crystal growth, and was used for iPhone screens. The crystal grows upward from the bottom of the crucible. Another method is the Edge-defined Film-fed Growth (EFG) method, very similar to the Czochralski method but the material passes through a die before cooling, which shapes the crystal. The crystal does not rotate. Chemical Vapor Deposition (CVD), gradient furnace or vertical bridgman processes can be used for sapphire crystal growth. The temperature gradient method uses a furnace in which a crucible containing material is located. On the bottom of the crucible, a seed crystal is placed. The material is melted and then kept molten for hours until the temperature stabilizes. The furnace is operated in a vacuum and when the temperature reaches 1400°c at the bottom of the crucible, argon gas is injected. Crystallization then starts by cooling the molten material at 1.3 to 3 kelvin per hour, with a linear temperature gradient across the height of the furnace making the material crystallize from the bottom up. The gradient is produced by cooling parts of the furnace with water. The temperature gradient can also be created by having several heaters along the height of the furnace, dividing the heaters into temperature zones and changing the temperature of the zones. Large crystals 50cm across, of water soluble material such as

Monopotassium phosphate Monopotassium phosphate (MKP) (also, potassium dihydrogen phosphate, KDP, or monobasic potassium phosphate) is the inorganic compound with the formula KH2PO4. Together with dipotassium phosphate (K2HPO4.(H2O)x) it is often used as a fertilizer, ...

(KDP) can be made by dissolving the KDP in hot water and salt, creating a growth solution, placing a seed crystal in the solution and then cooling the solution, done in a holden-type crystallizer, in what is known as solution growth.

References

{{reflist Crystals Semiconductor growth