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Laser interferometers represent the ultimate feedback device for high-precision motion control application. The combination of high resolution and outstanding accuracy has made it the ideal transducer for wafer steppers, flat panel inspection, and high-accuracy laser micromachining.

A laser interferometer system employs a highly stabilized light source and precision optics to accurately measure distances. Interferometers are superior to glass encoders for several reasons. The most obvious advantage is that interferometers have greater inherent accuracy and better resolution. An additional advantage is that interferometers measure distances directly at the workpiece. Due to mounting considerations linear encoders are often "buried" inside the positioning stage, some distance away from the workpiece introducing an additional source of error. A well-designed interferometer system is able to take measurements directly at wafer height, maximizing accuracy.

Aerotech's LZR3000 series laser interferometer system is based on the Michelson interferometer. It is composed of (refer to Figure 1): (1) a light source, in this case a frequency stabilized He-Ne laser tube; (2) a linear interferometer optic which is made by the combination of a polarizing beam-splitter and retroreflector; (3) a moving linear retroreflector; and (4) detection electronics. When the laser light reaches the interferometer optic, it is separated into two distinct beams (Figure 2). The first beam is reflected back to the detectors and is used as a reference beam. The second beam passes through the optic and is reflected off a moving retroreflector to provide the measurement beam. Due to the motion of the moving retroreflector, the second beam undergoes a shift caused by relative motion of the beam. When the reference beam and measurement beam recombine, they create an interference pattern.

The interference fringe appears as a dark and bright pattern (Figure 3). The intensity of this pattern is a sinusoidal signal that can be treated similar to a standard A-quad-B encoder signal. Aerotech's standard MXH series high-resolution multiplier is capable of multiplication up to x1024. Since the fundamental wavelength (l) of the laser light is 633 nm, and the signal output to the multiplier

There are two basic approaches to the detector electronics. The simplest method is to incorporate the detector in the same housing as the laser. This provides a compact system and is best suited for single-axis applications. For multi-axis applications, use of a remote detector is highly recommended. Aerotech's LZR series remote detection systems embed the detection photodiodes in the same housing as the interferometer optics for optimal beam stability. When coupled with appropriate beam-splitting optics, this allows one laser head to be used as the source for multiple axes. This is useful for XY systems, or systems with active yaw control. Not only does purchasing a single laser source reduce the cost of the laser system, but valuable footprint space is saved as well.

A typical dual-axis implementation is illustrated in Figure 4. To ensure that a beam path is provided at all locations throughout stage travel, two-dimensional implementations require the use of plane mirror optics. The plane mirror implementation has the added benefit of optically doubling the laser signal, resulting in a fundamental resolution of l/4. A single laser source is split to provide a signal to all axes of

Figure 3: Interference fringe patterns created by combination of reference and measurement beam.

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