The Art of Chip Making

No other manufacturing process is quite like it. Only a single speck of dust can ruin a chip, so work must be done in "clean rooms," where the air is constantly filtered and workers are swathed in surgical-type garb.

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 an n (negative) zone. If two n zones, say, are separated by a p zone, they act as a transistor, which is an electronic switch: a small voltage in the p zone controls the fluctuations in a current flowing between the n zones. In this manner, thousands of transistors can be built into a single chip.

As in silk-screening, a chip's complex circuitry is created a layer at a time. It is a slow, painstaking and error-prone procedure.

First, racks of wafers are placed in long cylindrical ovens filled with extremely hot (about 2,000DEG 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 of the chip's circuitry, is placed over the wafer. Exposed to UV, the resist's shielded areas remain soft and are readily washed away in an acid bath. On the other hand, the unshielded areas harden, forming an outline of the circuit.

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 by a diamond cutter, the bad chips are discarded and the good ones externally wired, sealed in plastic or metal and shipped off to the user.

This file is automatically generated by a robot program, so viewer discretion is required.