Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-07-09
2001-04-17
Font, Frank G. (Department: 2077)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06219146
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical systems and particularly to an apparatus and method for accurate alignment of optically reflective surfaces relative to a beam of parallel light, usually a laser beam.
BACKGROUND
Many optical and mechanical systems require precise angular alignment. For example, equipment for electron beam and optical lithography includes moving stages in which the stage position is measured using interferometers that require precise alignment. Interferometric metrology is also used in precise machine tools. These and similar devices require mirrors carried on the moving stages to be aligned perpendicularly to a laser beam. Interferometers use other optical surfaces such as those of glass prisms which require accurate alignment. Using conventional methods, it is difficult to know accurately whether an interferometer axis is aligned, since its location in space depends on glass or mirror geometry and since it cannot readily be viewed in a manner that a laser beam is viewed. Misalignment of an interferometer can give rise to error terms of the first order in stage position and in first and second order in yaw and pitch, in accordance with conventional analysis.
A traditional method of aligning a reflective surface employs an autocollimator as an alignment tool. An autocollimator is configured basically as a telescope with a reticle, usually illuminated with a self contained light source. If an autocollimator is pointed at a distant flat mirror that is closely but not exactly perpendicular to the axis of the instrument, then an observer will observe through an eyepiece an image of the reticle that is slightly offset from the directly viewed reticle. When the reticle and its image superpose exactly, in focus, size and position, then light from the instrument is collimated and is perpendicular to the mirror. These devices are usually constructed to allow small angular displacements of the mirror to be measured, sometimes automatically. A disadvantage of an autocollimator is the requirement for an illuminated reticle, that must be precisely aligned with both the laser beam and the target reflector.
Another traditional instrument used for optical alignment is a sextant, which superposes the images of two objects, typically the sun and the horizon, for the purpose of measuring their apparent angular separation. Small motions of the instrument do not change the apparent relative positions of the objects. However, a sextant and similar devices depend on accurately movable mirrors and accurately calibrated circles, and generally cannot view sources located in opposite directions.
A traditional method recommended by metrological interferometer manufacturers to align mirrors and other reflective surfaces in a laser beam path is to place a white card at the laser output aperture with a hole in it just big enough to transmit the beam. The reflected beam from the mirror being aligned returns to the card, illuminating a visible spot on the surface of the card facing away from the laser. When the mirror is adjusted so that the spot appears centered on the hole, the mirror is perpendicular to the beam. This method suffers from several disadvantages: The laser beam may be several millimeters in diameter, making small errors hard to detect. The offset of the spot is twice the angular error of the mirror multiplied by the distance between the card and the mirror, which may be quite short, often less than a meter, and there may be little geometric magnification and no optical magnification of the error. The two spots being superposed may be large and are on opposite sides of the card. The hole in the card prevents the centers of the spots from being seen. The card is often not conveniently visible from the location where the adjustment is being made.
It is desirable in the art to provide a method and apparatus to determine accurately and simply whether optically reflective surfaces are in accurate alignment with a laser beam or similar parallel light beam. A null method is most desirable, which gives an unambiguous indication of accurate alignment independent of calibration, and in which any error of alignment is magnified. It is desirable that the accuracy not be degraded when the laser beam diameter is large. It is desirable that the apparatus not disturb the laser or system optics to be aligned and that removal of the apparatus from the system being aligned not disturb its alignment. These requirements do not preclude the apparatus being a permanent part of the system, nor do they preclude the apparatus providing a calibrated measurement of error, should any exist.
SUMMARY
An optical device, hereinafter called “the apparatus”, and a method are provided for measuring the relative alignment of a laser or similar collimated light source with a mirror or other reflective surface and for providing a null indication if alignment is exact. In the description that follows, the laser-mirror configuration is referred to as “the system,” and word “direction” implies a direction toward a point on the celestial sphere: all light rays parallel to one another have the same direction. “Counterparallel” implies directions toward two points on any great circle on the celestial sphere that are exactly 180 degrees apart.
The apparatus includes a beamsplitter, which divides the laser beam of the system into a sample beam and a reference beam. The sample beam is transmitted substantially undeflected in direction through the beamsplitter and is then reflected back to the beamsplitter from the optically reflective surface of the system to be aligned. The reference beam is reflected from the beamsplitter approximately at a right angle to the laser beam and is then reflected in a counterparallel direction from a retroreflector in the apparatus back to the beamsplitter. The sample beam from the reflective surface is then partially reflected from the beamsplitter toward a telescope. The reference beam from the retroreflector is partially transmitted undeflected in direction through the beamsplitter toward the telescope. The telescope is the third principal optical component of the apparatus, but is not necessarily physically attached to other components. The angle between the sample and reference beams at the telescope is proportional to the orientation angle between the laser beam and the normal to the reflective surface.
The telescope collects both the sample and reference beams and transforms each beam into a point image (or ‘star’). The apparent angular separation between the two point images produced by the telescope is equal to twice the product of the telescope magnification times the angle formed between the mirror normal and the laser beam, independent of the distance to the mirror. If the reflective surface is accurately aligned relative to the laser beam, then the two point images or stars are superposed on one another. If viewed directly by eye, the laser beam is preferably attenuated using optical filters to prevent eye injury. Alternatively, for direct eye viewing, the beamsplitter mirror can be nearly completely reflective, transmitting only a small fraction of light.
The point images can be projected onto a viewing screen or into an suitable image detecting device, thereby providing an output signal in response to the lateral separation between the point images. In some embodiments, the output signal provides feedback to an actuator, which maintains perpendicularity of the laser beam and the reflective surface within a desired tolerance. Ideally, the beamsplitter in the apparatus is inclined at a 45-degree angle relative to the laser beam of the system, but deviations in this angle do not affect the relative positions of the two stars. If the stars superpose, becoming a single star, the system is in alignment.
The apparatus does not require that the laser beam and reflective surface each be aligned with respect to the apparatus to insure their correct alignment relative to one another. If the apparatus is an integral part of the laser-reflector system, for example an
Eckes William A.
Innes Robert
Brooks Ken
Etec Systems, Inc.
Font Frank G.
Natividao Phi
Skjerven Morrill & MacPherson LLP
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