measurement, which in this example are the X and Y axes. These beams are steered to the interferometer optics and plane mirrors prior to their measurement at a remote detector. The detector electronics are located in the same housing as the interferometer optics, providing a compact solution. Some existing laser interferometer solutions require a signal processing board that interfaces directly to the motion controller. In many cases this is done so as to provide a parallel word directly to the motion controller, which allows for high data rates. While this may be required in high speed, high resolution applications, this solution has the distinct disadvantage of making the laser interferometer a proprietary, closed-architecture solution. Interfacing to both the interferometer board and motion controller requires an in-depth knowledge of both devices that is often impractical for most users.

Advances in motion controller technology have nearly made this approach obsolete. Aerotech LZR series laser interferometer output signals are standard A-quad-B, and are electrically identical to the output of a traditional incremental encoder. To the motion controller, the interferometer appears to be a standard feedback device, simplifying system implementation. Aerotech's UNIDEX^® family of stand-alone and PC-bus-based controllers employ high-speed devices, resulting in serial data rates as high as 32 MHz. For a system with a resolution of 6 nm, that results in a speed of nearly 200 mm/s. While Aerotech also manufactures a laser interferometer signal processing board for high-speed applications, the need for this approach has been greatly minimized and often the much simpler serial approach proves to be the optimal solution.

While the position feedback may be straightforward to process, there are other important considerations that must be made when implementing a laser interferometer-based system. Issues such as home-marker implementation, losses of feedback signal, and error-source reduction require unique solutions in an interferometer-based system.

Since the interferometer is strictly an incremental device, there is no way to establish an accurate home reference. Traditional home devices such as LVDTs and optical

proximity switches are only adequate in establishing an approximate home. For accurate wafer measurements, it is often necessary to acquire a fiducial directly from the wafer to establish a sufficiently accurate and repeatable home. Once the mark is acquired, the motion controller counters can be reset to zero (software homed) and the processing continues.

When implementing a laser interferometer as a feedback device it is absolutely necessary for the interferometer to provide a "beam blocked" signal. Unlike a linear encoder that places the read head in close proximity to the encoder glass, it is easy to block the feedback signal (in this case the laser beam) in an interferometer system. This condition requires the motion controller to immediately generate a fault condition and disable the axes. Aerotech's UNIDEX controller and LZR series interferometer implement all fault handling automatically, greatly simplyfying system operation.

Minimize Potential Error Sources

The same requirements that necessitate the use of a laser interferometer _ high resolution and high accuracy _ require that system-wide error sources be minimized. While it is inherently more accurate than alternate feedback schemes, without proper understanding of the error sources, it will be no more effective than a low-cost linear encoder. Environmental conditions, mechanical design and optical alignment must be considered in the design/implementation of any high-accuracy laser interferometer based motion system.

Environmental Errors

The wavelength of light emitted by a He-Ne laser is by definition equal to 632.99072 nm in a vacuum. Interferometer accuracy in a vacuum is accurate to ±0.1 ppm. However, most applications require operation of the system in atmospheric conditions, so this accuracy degrades. The index of refraction of air effectively changes the frequency of the laser light which appears as a path length difference. Fortunately, the effects of temperature, pressure and humidity as they affect the wavelength of light are well known and are related by Edlen's equation. As a result, the LZR series interferometer systems incorporate a "weather station" that samples the environmental conditions. These signals are digitized and processed to create a wavelength scale number that is used to generate a correction factor. An environmentally corrected system will have an accuracy ±1.5 ppm or better. The final accuracy is largely a function of the stability of the environmental conditions.

The most effective, and incidentally also the most expensive, means of compensating for changes in the refractive index of

Environmental Effects on Accuracy

Temperature: 1 ppm / 1°C
Pressure: 1 ppm / 2.5 mm Hg
Humidity: 1 ppm / 85% change

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Aerotech is a motion controller and motion control systems manufacturer.