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A stepper is a device, similar to a slide projector or a photographic enlarger, that is used in photolithography to transfer patterns that will become part of an integrated circuit (IC) onto the surface of a silicon wafer. This is done by exposing photosensitive material on the wafer to light passing through a pattern on a transparent plate called a reticle.

The wafer has patterns for many ICs arranged across its surface in a grid. The stepper is named after the fact that it moves or "steps" from one location to another on 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

A silicon wafer is coated with a material, called photoresist, that changes chemically when struck by light of a particular frequency. The wafer is then placed in a cassette or "boat" that holds a number of wafers, which is then placed in 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 to be printed on the wafer is contained in a pattern etched in chrome on a photomask (also called a "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 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 alignement 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 unload by the wafer loader robot, and another wafer takes its place on the stage. The exposed wafer eventually is 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 abberations can be ignored outside of this area, because they will not affect the exposed area on the wafer.

Successful scanning requires extremely precise syncronization between the moving reticle and wafer stages during the exposure. Accomplishing this presents many technological challenges.

See also

Photolithography

Semiconductor

Integrated Circuit

External links

Stepper makers

• ASML
• Canon
• Nikon

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