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National Geographic : 1982 Oct
Contents
enlarging it slightly and making it as com plex as a city nearly 1,000 miles square. How do you build a megalopolis almost twice the size of Alaska? Manufacturing any chip is a painstaking, protracted process. Just south of San Fran cisco Bay, at the Intel Corporation in Silicon Valley, I found that it can take as long as three months to make a microprocessor (see the article about Silicon Valley beginning on page 459). "Some magic's involved," engineer Ralph Leftwich said as I pulled on a baggy white nylon jump suit, cap, and bootees. Voila! I was a conjurer's illusion in my bunny suit, required fashion in the "clean rooms" where Intel pioneered the microprocessor in 1971 and where filtered air holds fewer than 100 particles of dust or other contaminants per cubic foot. To a microscopic chip circuit, motes are as menacing as boulders. In one clean room, trays held razor-thin silicon wafers, polished mirror smooth and racked like diminutive records. They were slices of a sausagelike crystal grown from molten silicon so pure that if contaminants were redheads, there would be but 15 of them on earth. Such crystals yield wafers as large as five inches across; each wafer be comes the base of hundreds of chips. Two things make silicon, a semiconduc tor, the favored material for chips. Its ability to carry electricity can be precisely altered by ingraining its crystal structure with mea sured amounts of chemical impurities, or dopants. And silicon surfaces can be conve niently oxidized-rusted, in effect-into an electrically insulating glaze. "Chips are sandwiches," Ralph said as I peered at a silvery oxidized wafer. He ex plained that techniques reminiscent of silk screening would stack and stencil the wafer with layers of insulation and crystal, the crystal doped with infinitesimal pockets of impurities laid out in some 300 identical chip-scale circuit patterns (pages 426-7). "The impurities form conducting areas that overlap from top to bottom of a wafer. By etching 'windows' between them, we cre ate transistors." At the end, with as many as 12 detailed levels demanding interconnec tion, a wafer receives an aluminum coating and a final etch that leaves conducting fila ments invisible to the naked eye. 434 A new chip's ultrafine "wiring" offers so little entree to its transistors that they defy individual quality testing. But their collec tive performance is judged as needlelike probes jab at metal pads on the rim of each chip on a wafer, running 10,000 electrical checks a second. Sound chips are diced from wafers by a diamond saw, then bonded and wired to gold frames and sealed in small ceramic cases propped on stubby plug-in prongs. Packaged, a wafer's worth of chips looks like a swarm of black caterpillars. ters by the dozens in a personal com puter, and in their cocoon they might metamorphose into a journalist's tool as useful as pen or notebook. Cows and computers are a profitable combinationfor Missouri dairyfarmer Tom Kessler. For 100 pounds of milk daily, his cows should eat exactly 35 National Geographic,October 1982
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