The Making of a Microchip - Ultra-Clean, Robot-Precise
PRECISION is the name of the game in constructing semiconductors. The goal is to make the circuits, which carry electrical signals, as small as possible for maximum speed. By reducing the size of the chip, the chance of contamination is also reduced.
The chips to be made here this fall will be etched with features spaced as close as 1.5 microns (the width of a human hair is 100 microns). Such minute circuitry is formed by projecting light through a ``mask,'' or pattern of the chip design. Prior to this, wafers have been coated with a light-sensitive chemical, called a photoresist, onto which the design was etched as the light hits it. Where the mask blocks the light, the photoresist remains.
The process is repeated with 11 different masks, says David Cunningham, foundry operations manager. Two of the 12 masks etch their patterns on layers of aluminum, which are then topped with photoresist. The aluminum connects the circuits.
On one wafer several hundred chips, and up to 35 different chip designs, will be produced. A particle of dust is enough to impair a chip. All persons in the clean room wear sanitary ``bunny suits''; the air is filtered to remove all but 10 particles per cubic foot.
The building itself is specially designed to be ``vibration isolated.'' The piping, for example, is mechanically isolated from the building to prevent vibrations from disrupting chip fabrication.
Because preventing contamination is so important, the trend is toward robotization, Mr. Cunningham says. ``If you can keep people away from the wafers, you can make better wafers.''