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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.
The stepper's role in photolithography
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 each area received during exposure. These areas of photoresist and no photoresist reproduce the pattern on the reticle. The developed wafer is then exposed to acids that etch away the silicon in the bare areas or to chemicals that change the electrical characteristics of the silicon in those areas. The entire process is called photolithography.
Each IC chip is rather small (22x22 mm for example) and the patterns for many individual chips (called dies) are arranged across the surface of the wafer in a grid. The stepper gets its name from the fact that it moves or "steps" from one die to another across the wafer, exposing each in turn.
Currently (as of 2005) the most detailed patterns in semiconductor 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.
Basic operation
The silicon wafers are coated with photoresist, and placed in a cassette or "boat" that holds a number of wafers. This is then placed in a part of the stepper called the wafer loader, usually located at the lower front of the stepper.
A robot in the wafer loader picks up one of the wafers from the cassette and loads it onto the wafer stage where it is aligned to enable another, finer alignment process that will occur later on.
The pattern of the circuitry for each chip is contained in a pattern etched in chrome on the reticle, which is a plate of transparent quartz. A typical reticle used in steppers is approximately 114 by 134 mm square, and about one centimeter thick.
A variety of reticles, each appropriate for one stage in the process, are contained in a rack in the reticle loader, usually located at the upper front of the stepper. Before the wafer is exposed a reticle is loaded onto the reticle stage by a robot, where it is also very precisely aligned. Since the same reticle can be used to expose many wafers, it is loaded once before a series of wafers is exposed, and is realigned periodically.
Once the wafer and reticle are in place and aligned, the wafer stage, which is moved very precisely in the X and Y directions (front to back and left to right) by worm screws or linear motors, carries the wafer so that the first of the many patterns (or "shots") to be exposed on it is located below the lens, directly under the reticle.
Although the wafer is aligned after it is placed on the wafer stage, this alignment is not sufficient to insure that the layer of circuitry to be printed onto the wafer exactly overlays previous layers already there. Therefore each shot is aligned using special alignment marks that are located in the pattern for each final IC chip. Once this fine alignment is completed, the shot is exposed by light from the illumination system of the wafer that passes through the reticle, through a reduction lens, and on to the surface of the wafer. A process program or "recipe" determines the length of the exposure, the reticle used, as well as other factors that affect the exposure.
Each shot located in a grid pattern on the wafer is exposed in turn as the wafer is stepped back and forth under the lens. When all shots on the wafer are exposed, the wafer is unloaded by the wafer loader robot, and another wafer takes its place on the stage. The exposed wafer is eventually moved to a developer where the photoresist on its surface is exposed to developing chemicals that wash away areas of the photoresist, based on whether or not they were exposed to the light passing through the reticle. The developed surface is then subjected to other processes of photolithography.
Major subassemblies
A typical stepper has the following subassemblies: wafer loader, wafer stage, wafer alignment system, reticle loader, reticle stage, reticle alignment system, reduction lens, and illumination system. Process programs for each layer printed on the wafer are executed by a control system centering on a computer that stores the process program, reads it, and communicates which the various subassemblies of the stepper in carrying out the program's instructions. The components of the stepper are contained in a sealed chamber that is maintained at a precise temperature to prevent distortions in the printed patterns that might be caused by expansion or contraction of the wafer due to temperature variations. The chamber also contains other systems that support the process, such as air conditioning, power supplies, control boards for the various electrical components, and others.
Scanners
Scanners are steppers that increase the length of the area exposed in each shot (the exposure field) by moving the reticle stage and wafer stage in opposite directions to each other during the exposure. Instead of exposing the entire field at once, the exposure is made through an "exposure slit" that is as wide as the exposure field, but only a fraction of its length (such as a 9x25 mm slit for a 35x25 mm field). The image from the exposure slit is scanned across the exposure area.
There are several benefits to this technique. The field can be exposed with a lesser reduction of size from the reticle to the wafer (such as 4x reduction on a scanner, compared with 5x reduction on a stepper), while allowing a field size much larger than that which can be exposed with a typical stepper. Also the optical properties of the projection lens can be optimized in the area through which the image of the projection slit passes, while optical aberrations can be ignored outside of this area, because they will not affect the exposed area on the wafer.
Successful scanning requires extremely precise synchronization between the moving reticle and wafer stages during the exposure. Accomplishing this presents many technological challenges.
See also
External links
Stepper makers
"Stepper"