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Photolithography: Making the Chips
Once the design of the processor has been completed (or at least a current version of it) and the low-level transistor maps have been defined, it is necessary to actually make the chips. The chips are manufactured using a process called photolithography. In simple terms, transistors and data pathways (internal wires) are created in semiconductors by applying different layers of various materials at exact places, one after the other. Where two layers intersect in the appropriate way, a transistor is formed. A modern processor can be made of a dozen or more of these layers.
A special (very expensive) computer system takes the transistor design of the processor and separates it into these different layers, each of which is called a mask. This name comes from the way the chip is actually made. A wafer is chemically treated with a photo-sensitive substance, and then light is shined through the first layer mask onto the wafer. Where the mask allows the light to shine onto the wafer, it is chemically altered, creating the first layer of the chip. Then another chemical is used to wash off the first one, and the process is repeated with the next mask. In this way, the features of the chip are slowly built onto the silicon, layer by layer.
In order to achieve the miniaturization that gives the processor its great power in a small size, the features (transistors and internal wires) of the processor are shrunk to a microscopic size--less than a micron in width, or one millionth of a meter. Each mask must be created with drawn features that are this size, and the device that does the photolithography must be able to focus light at this level of accuracy as well. Obviously, the equipment to perform this work is amazingly precise, and incredibly expensive.
As you recall, the wafers used in this process are cut from a large crystal, typically eight inches in diameter. Each chip is very small, usually a third of a square inch or less, so each wafer contains many hundreds of chips. The wafer is essentially "chopped up" into little rectangles, each one of which has the potential to become a chip. Some of the surface area of the wafer is also used for test circuits. When the wafer is complete, another special machine breaks it into its individual chips.