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| Silicon wafers are cut from a silicon crystal, such as this one |
Computer chips are made from silicon and this is a very good thing because silicon is the second most common element on the earth. However, the silicon must be made very, very pure and grown as a single crystal. Silicon crystals are made somewhat like candles. A silicon crystal seed is dipped in molten silicon, rotated, and pulled slowly to create a cylinder of silicon. These silicon cylinders are the purest crystals on the planet.
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These are polished silicon wafers in a quartz wafer boat ready to be processed |
Once the silicon cylinder is grown to the desired diameter, it is then sawed into circular wafers. The earliest wafers were about an inch in diameter. Today, wafers are being grown to a massive twelve inches in diameter. These wafers are then polished to achieve a very flat surface. As a side result, polished wafers have a mirror finish. Once the polished wafer is complete the process of building the chip design can begin.
What happens next is the true magic. Transistors, and other electronic parts, are built upon the surface of this polished circular wafer. Transistors are built out of different types of doped silicon. Adding small amounts of other elements like phosphorous and boron creates p-type and n-type silicon. Transistors are built using three alternating layers of these types of silicon.
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Each finished wafer has reflective characteristics, just like a gemstone |
Due to the electrical properties of these layers, an electronic switch can be created. The challenge is to build up thousands, even millions, of these devices and then wire them together into one circuit. Not a very easy task, as you might have guessed.
To create the transistors, a process called etching the chip is used. The chip design is created in layers on the polished wafer. Each layer is created by depositing the silicon types, one at a time, on the wafer surface and then etching away the unwanted material. At least two layers are needed to complete the transistor; the polished wafer counts as the first layer. A last layer is added to electrically interconnect the devices we have created on the chip, and this is done either with polysilicon or aluminum.
The wafer is then cut into the individual chips. A single wafer can have hundreds of chips. The individual chip is then mounted in an electronic package that serves to protect it and connect it to the outside world. People often mistake theses packages as the chip, when the chip is really hidden inside.
It has been said that computer chips are the greatest value added product in the world. We essentially take a pile of sand and change it into thousands of dollars worth of computer chips.