How Does Silica Sand Turn Into Silicon Crystals?


July 2, 2021

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Did you know that a miracle of engineering happens every day at a silicon (Si) wafer fabrication plant? Silica sand goes through an amazing journey before it turns into crystals used to make an Si wafer.

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How Silica Sand Turns Into Silicon Crystals

Silica Sand Is Quarried

Chipmakers get silica sand or silicon dioxide from quarrying. Next, they mix it with carbon and heat it to temperatures beyond 2,000°C to remove the oxygen. While silica sand is inside an electric arc furnace, the carbon reacts with oxygen and turns into carbon dioxide.Once the heating process is complete, the pure silicon that’s left at the bottom of the furnace is treated with oxygen to eliminate calcium or aluminum. The removal of these impurities produces metallurgical grade silicon that can be up to 99% pure. Since this isn’t pure enough to meet the necessary requirements of microscopic microprocessors, the metallurgical grade silicon is ground into a fine powder.Next, hydrogen chloride is added before it’s heated at 300°C to create a liquid silicon compound which is also known as trichlorosilane. Unwanted elements that include iron, aluminum, phosphorus, and boron are further removed via fractional distillation before the trichlorosilane is vaporized in hydrogen. Lastly, the electronic-grade silicon is collected by an electrically heated, ultra-pure silicon rod.

Crystals Are Made

Since electronic-grade silicon has a polycrystalline structure, it still isn’t perfect. To change its structure, it has to go through the Czochralski process which involves melting the silicon crystal in a quartz crucible.Next, a small silicon crystal is dipped into the molten silicon and drawn out while it’s being constantly rotated in the opposite direction to the crucible’s rotation. This process creates a rod made from one silicon crystal called a boule. The boule is cut into wafers.

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