National Geographic : 1968 Aug
Slices of silicon ingot house the circuits Plastic mask, when vastly reduced, will plot "wiring" Furnace heat diffuses circuit elements into wafers SILICON DIOXIDE PROTECTIVE LAYER- Simplified diagram shows a transistor in cross-section 286 SIIC IIr The miraculous "IC" AKE A TINY CHIP of synthetic crystal, cram it with scores of minute electronic devices, and you have the integrated circuit, or IC, a little giant that has revolutionized electronics technology. Suc cessor to the bulky vacuum tube and the transistor, the IC goes through an ingenious manufacturing process, described below. The end product is an engineer's delight: rugged, long-lasting, low in cost, fantastically speedy. Magnified 12 times (right), the IC with its gold leads and parallel prongs for soldering also reveals unsuspected beauty. SLICING OFF AWAFER 12/1000 of an inch thick, IC technicians polish it even thinner. Silicon provides a good material for the circuits because its electri cal properties can be precisely altered by adding controlled amounts of impurities, called dopants. A wafer will eventually carry a grid of hundreds of IC's, not yet cut apart in the sample at top. TO PATTERN THE COMPONENTS and wiring for each chip, a computer-controlled machine cuts a series of large plastic stencils, called masks. Here an engineer inspects a mask at Texas Instruments, the firm that pioneered in IC's a decade ago. A mask is reduced photographically at least 500 times to exact chip size, then reproduced over and over on a glass plate so there is one for each chip in the wafer. Complex photographic and chemical steps transfer the images to the wafer surface. Now, guided by the masks' openings, or "windows," technicians can implant the circuit elements. LIKE BAKED COOKIES, wafers emerge from a fur nace at North American Rockwell Autonetics in Anaheim, California. Temperatures up to 2,5000 F. diffuse impurities into the wafers. To illustrate this miniaturized manufacturing, a GEOGRAPHIC artist drew two cross-sections of an IC transistor, lower left. To form its negative col lector, technicians cover the chip with a mask. Then they slide the wafer into a furnace containing a dopant of vaporized phosphorus. Bombarding the wafer through the mask's window, the phos phorus atoms penetrate and form a negative pock et-the collector area. A similar baking with a different mask and a different dopant-vaporized boron-creates a positive base area. A third such process creates the emitter area. To interconnect these areas and link them with the rest of the circuit, the wafer is coated with a conducting material such as aluminum. Then, with circuit routes plotted by a mask, the unwanted conducting material is etched away, leaving a microscopic raised web-the IC "wiring." FINAL PRODUCT: The black IC chip and its mounting are smaller than a postage stamp, right. En larged (opposite), the chip shows its aluminum "wiring." This IC has already been surpassed by others still smaller (page 289). KODACHROMES BYJAMESP. BLAIR© N.G.S.