Stepper

A stepper is a device used in the manufacture of integrated circuits (ICs) that is similar in operation to a slide projector or a photographic enlarger. Steppers are an essential part of the complex process, called photolithography, that creates millions of microscopic circuit elements on the surface of tiny chips of silicon. These chips form the heart of ICs such as computer processors, memory chips, and many other devices.

Elements of the circuit to be created on the IC are reproduced in a pattern of transparent and opaque areas on the surface of a quartz plate called a photomask or reticle. The stepper passes light through the reticle, forming an image of the reticle pattern. The image is focused and reduced by a lens, and projected onto the surface of a silicon wafer that is coated with a photosensitive material called photoresist.

After exposure in the stepper, the coated wafer is developed like photographic film, causing the photoresist to dissolve in certain areas according to the amount of light the areas received during exposure. These areas of photoresist and no photoresist reproduce the pattern on the reticle. The developed wafer is then exposed to acids or other chemicals. The acid etches away the silicon in the parts of the wafer that are no longer protected by the photoresist coating. The other chemicals are used to change the electrical characteristics of the silicon in the bare areas. The wafer is then cleaned, recoated with photoresist, then passed through the stepper again in a process that creates the circuit on the silicon, layer by layer. The entire process is called photolithography or photo engineering.

When the wafer is processed in the stepper, the pattern on the reticle (which may contain a number of individual chip patterns) is exposed repeatedly across the surface of the wafer in a grid. The stepper gets its name from the fact that it moves or "steps" the wafer from one shot location to another. This is accomplished by moving the wafer back and forth and left and right under the lens of the stepper. Previous generations of photolithographic equipment would expose the entire wafer, all at once; a stepper, working on a limited area, is capable of higher resolution.

As of 2008, the most detailed patterns in semiconductor device fabrication are transferred using a type of stepper called a scanner, which moves the wafer and reticle with respect to each other during the exposure, as a way of increasing the size of the exposed area and increasing the imaging performance of the lens.

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