Word: wafer
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Dates: during 1970-1979
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Monolithic Memories Corp. has a standing order for wafer sorters and probe-card repair technicians; meanwhile, the company is turning down orders for lack of workers. Synerteck needs ion implanters, who manipulate control panels to change the electrical properties of silicon. Fairchild Camera and Hewlett-Packard both have hundreds of positions available that cut across every level of skill. Avantek has an eye out for janitors interested in profit sharing...
Some 250 chips are made from one razor-thin wafer of precisely polished silicon about 3 in. in diameter. These wafers, in turn, are sliced from cylinders of extremely pure (99.9%) crystalline silicon, grown somewhat like rock candy. Why silicon? Because it can be either electrically conducting or nonconducting, depending on the impurities added to it. Thus one small area of a chip can be "doped" (as scientists say) with impurities that give it a deficiency of electrons-making it a so-called p (or electrically positive) zone, while an adjacent area gets a surplus of electrons to create...
First, racks of wafers are placed in long cylindrical ovens filled with extremely hot (about 2,000° F.) oxygen-containing gas or steam. In effect, the wafers are rusted-covered by a thin, electrically insulating layer of silicon dioxide that prevents short-circuiting. Then the wafers are coated with still another substance: the resist, a photographic-type emulsion sensitive only to ultraviolet (UV) light. (To prevent accidental exposure, clean rooms are generally bathed in UV-less yellow light.) Next, a tiny mask, scaled down photographically from a large drawing and imprinted with hundreds of identical patterns of one layer...
Back in the ovens, the wafers are baked again in an atmosphere of gases loaded with "dopants." Like oil stains in a concrete driveway, these impurities soak into the underlying silicon. Since chips usually contain as many as ten layers, all these steps-"rusting," photomasking, etching, baking, etc.-must be repeated for each layer. Then the entire wafer is coated with an aluminum conductor, which also must be masked, etched and bathed in acid. Finally, an eagle-eyed computerized probe scans the wafer for defective circuitry and marks the bad chips in red. The wafer is then separated...
High points on the ride are few. Central Square is worth checking out just to discover that Cambridge is not all Ivy and academia. Kendall is nowhere, halfway between the Necco candy wafer factory and MIT. But after Kendall the train crosses over the Charles, offering one of the best views of downtown Boston (and the river itself) to be found. The Charles-MGH stop is not worth debarking at, although conversation-wise you should remember that it stands between the largest hospital in New England--Mass General--and one of the few neighborhoods in America still using gas street...