A typical boule will be around 300mm across. A boule is a rod made from a single silicon crystal, and its size depends on the temperature, the rate of spin and the rate at which the crystal is pulled from the liquid. This attracts silicon from the crucible, creating what's known as a boule. A tiny silicon crystal is then dipped into the molten silicon, and it's drawn out while rotating constantly in the opposite direction to the rotation of the crucible. The process of doing that is called the Czochralski Process, and it involves melting the silicon crystal in a quartz crucible at just over the melting point of 1,414 degrees C. Those boundaries can cause chaos with electronic signals, so the structure of the silicon has to be changed. That means it's composed of lots of small silicon crystals, and the joins between those crystals can suffer from defects known as grain boundaries. Its purity: 99.999999%.Д®lectronic-grade silicon still isn't perfect, because it has a polycrystalline structure. An electrically heated, ultra-pure silicon rod collects the silicon, and the result is electronic-grade silicon. These are removed by fractional distillation and the trichlorosilane is vaporised in hydrogen at 1,000 degrees C. This creates a liquid silicon compound called tricholrosilane, and it also creates chlorides of unwanted elements such as iron, aluminium, boron and phosphorus. So the metallurgical grade silicon is refined further, this time by grinding it into a fine powder, adding hydrogen chloride and heating it in a fluidised bed reactor at 300 degrees C. Unfortunately for the chipmakers, that still isn't pure enough to meet the requirements of microscopic microprocessors. That silicon is then treated with oxygen to remove impurities such as calcium or aluminium, leaving what's known as metallurgical grade silicon. At those temperatures the carbon reacts with the oxygen, becoming carbon dioxide and leaving pure silicon in the bottom of the furnace. To get the silicon, the oxygen is removed by mixing it with carbon and heating it in an electric arc furnace to temperatures beyond 2,000 degrees C. Silica sand is also known as silicon dioxide, and as you've no doubt guessed from the name it's a compound of silicon and oxygen. What chipmakers need is silica sand, which you usually get from quarrying. Normal sand and the sand used in the building industry is usually coloured red, yellow or orange due to the presence of impurities. As you've probably guessed, chipmakers don't head for the nearest beach with JCBs or place bulk orders with the local builders' merchant.

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