## Mathematical and Computing Sciences Question #320

Jonathan Weiler, a 14 year old male from the Internet asks on January 21, 1998,

How are the tiny chips and wires that go into a computer made? They seem so tiny and fragile.

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### Barry Shell answered on January 21, 1998

The technique is called photolithography which means "printing with light on stone". The stone is the silicon waver--a big crystal of "sand" that has been sliced very thin. The light can be x-rays or even an electron beam these days.

Engineers are able to make the light very sharp and perfect so that it draws really small lines. There are many ways to do this, but one way it works is like this.

1. First, they coat the silicon with a fine layer of metal, usually aluminum. This is sometimes done just by letting a few layers of aluminum atoms condense on the wafer from aluminum vapour in much the same way that frost will form on a cold window from water vapour in the air. There are special machines for this.
2. After this layer of aluminum is down, they spray a type of paint on it. This paint is resistant to acid.
3. Next, they shine light on it in a pattern that has the wires. In a way it's like a slide projector, but the slide and everything is really really small. Wherever the light hits the paint, the paint becomes harder and more difficult to wash off.
4. Finally, then they put the whole thing in a bath and the paint gets washed off. Then they put it in acid. The acid eats away all the aluminum, but not where the paint is left. Then they use a solvent to wash off the paint and, tada, there are all the little tiny aluminum wires.
Now in real chip factories this process is repeated several times, so there are layers and layers of material. Some is metal, some is plastic and some is semiconductor. In the end, there is a complex circuit with millions of transistors and other electrical components. All of it is done by "printing" the wires and other structures onto the silicon base in a manner similar that described above. Finally at the end, a machine comes and connects very tiny hairlike wires to little pads all around the circuit. These fine wires go out to the "package" that holds the chip and connect to the pins that stick out of the back or sides of the package. These pins connect to the circuit board in your computer or VCR or telephone or whatever. When you see a chip on a circuit board, you are looking at the chip package. The real chip is way smaller and is inside this black package.

Everything is microscopic and everything is very fragile.

The tiniest piece of dust can wreck the whole thing, so all the factories have to be extremely clean. Workers go through special rooms and change their clothes to special dust-free clothes before they can enter the factory. They even have to change shoes and vacuum their feet in special vacuum cleaners.

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