How Silicon Chips Are Made

Friday, May 29th, 2009

If you want to make a computer, you need silicon chips, but how are silicon chips made? Even getting the “raw” material — pure silicon — is remarkably complicated:

Sand is composed of silica (also known as silicon dioxide), and is the starting point for making a processor. Sand used in the building industry is often yellow, orange or red due to impurities, but the type chosen in the manufacture of silicon is a purer form known as silica sand, which is usually recovered by quarrying. To extract the element silicon from the silica, it must be reduced (in other words, have the oxygen removed from it). This is accomplished by heating a mixture of silica and carbon in an electric arc furnace to a temperature in excess of 2,000°C. The carbon reacts with the oxygen in the molten silica to produce carbon dioxide (a by-product) and silicon, which settles in the bottom of the furnace. The remaining silicon is then treated with oxygen to reduce any calcium and aluminium impurities. The end result of this process is a substance referred to as metallurgical-grade silicon, which is up to 99 per cent pure.

This is not nearly pure enough for semiconductor manufacture, however, so the next job is to refine the metallurgical-grade silicon further. The silicon is ground to a fine powder and reacted with gaseous hydrogen chloride in a fluidised bed reactor at 300°C to give a liquid compound of silicon called trichlorosilane. Impurities such as iron, aluminium, boron and phosphorous also react to give their chlorides, which are then removed by fractional distillation. The purified trichlorosilane is vaporised and reacted with hydrogen gas at 1,100°C so that the elemental silicon is retrieved.

During the reaction, silicon is deposited on the surface of an electrically heated ultra-pure silicon rod to produce a silicon ingot. The end result is referred to as electronic-grade silicon, and has a purity of 99.999999 per cent.

But that’s not good enough:

Although pure to a very high degree, raw electronic-grade silicon has a polycrystalline structure. In other words, it’s made up of lots of small silicon crystals, with defects called grain boundaries between them. Because these anomalies affect local electronic behaviour, polycrystalline silicon is unsuitable for semiconductor manufacturing. To turn it into a usable material, the silicon must be turned into single crystals that have a regular atomic structure. This transformation is achieved through the Czochralski Process.

Electronic-grade silicon is melted in a rotating quartz crucible and held at just above its melting point of 1,414°C. A tiny crystal of silicon is then dipped into the molten silicon and slowly withdrawn while being continuously rotated in the opposite direction to the rotation of the crucible. The crystal acts as a seed, causing silicon from the crucible to crystallise around it. This builds up a rod — called a boule *mdash; that comprises a single silicon crystal. The diameter of the boule depends on the temperature in the crucible, the rate at which the crystal is ‘pulled’ (which is measured in millimetres per hour) and the speed of rotation. A typical boule measures 300mm in diameter.

From there it gets really complicated. Let’s hope we never have to rebuild civilization from scratch.

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