Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2000-02-25
2001-02-27
Kim, Robert (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S511000, C356S513000
Reexamination Certificate
active
06195168
ABSTRACT:
BACKGROUND OF INVENTION
The invention relates interferometry apparatus and methods, more particularly, to interferometry apparatus and methods for measuring the profile of a surface on an industrial part.
Interferometric techniques are commonly used to measure the profile of a surface of an object. To do so, an interferometer combines a measurement wavefront reflected from the surface of interest with a reference wavefront reflected from a reference surface to produce an interferogram. Fringes in the interferogram indicate spatial variations between the surface of interest and the reference surface. For example, a single-wavelength visible interferometer using phase-shifting techniques can measure surface variations on the order of Angstroms. Such accuracy is important in applications such as characterizing of glass substrates used in lithographic photomasks.
Another type of interferometer is a white-light scanning interferometer, which uses a white-light source and scans the relative path length of the reference and measurement legs of the interferometer. Because the white-light source has a limited coherence length, interference fringes in the interferogram are only present where the optical path difference (OPD) between the measurement and reference paths for corresponding points on the surface of interest and the reference flat is less than the coherence length. Thus, the scanning interferometer can resolve a step, or an otherwise large and/or discontinuous, variation in the surface of interest by scanning the relative path length of the reference and measurement legs, recording multiple interferograms, and determining where in each interferogram interference fringes are present. See, e.g., N. Balasubramanian in U.S. Pat. No. 4,340,306 for additional information regarding scanning interferometers. Furthermore, phase-shifting techniques can be combined with scanning interferometry to interpolate the phase of individual fringes and thereby more accurately determine surface variations.
Complications with interferometric techniques arise, however, when the surface of interest is rough on the scale of the illumination wavelength. The rough surface can scatter the measurement wavefront and corrupt the phase of whatever interference fringes are present in the interferogram. Nonetheless, recent studies have shown that white-light scanning interferometry can be used to measure the profile of a rough surface. In particular, when the surface of interest is sufficiently rough relative to the illumination wavelength, the interferogram includes speckle. Although the speckle corrupts the phase of any interference fringes, they nonetheless exhibit intensity oscillations as a function of OPD when the OPD is within the coherence length of the white-light source. Thus, white-light scanning interferometry can determine the surface profile of a rough surface to within about the coherence length. See, e.g., T. Dresel, G. Häusler, and H. Venzke, in
Applied Optics
31:919-925 (1992).
SUMMARY OF THE INVENTION
The invention features a scanning interferometer employing a broadband infrared source. The broadband infrared source provides the advantages of scanning interferometry and also provides additional advantages relative to interferometers that operate at smaller, visible wavelengths. Notably, there can be less scattering at infrared wavelengths from surfaces that appear rough at visible wavelengths than there would be for visible wavelengths. Therefore, the scanning interferometer can produce broadband interferograms having interference fringes free of speckle. Furthermore, as explained in greater detail below, the larger wavelengths provide a larger field of view (FOV) and lessen the constraints of finite camera pixel size on measuring complex surface profiles. Thus, the scanning interferometer is especially suitable for the measurement of surfaces of precision-engineered components used in industrial manufacturing, such as fuel system components, bearings, brake components, seals, rotors, pumps, turbine blades and disk, etc..
The scanning interferometer can also be used to profile objects that may be substantially opaque at visible wavelengths, but are more transparent in the infrared. For example, the scanning interferometer can be used to measure the profiles of the front and back surfaces of a silicon wafer, as well as its thickness.
In general, in one aspect, the invention features an interferometry system for a measuring a surface profile of a measurement object. The interferometry system includes: a broadband infrared source which during operation generates broadband infrared radiation including central wavelengths greater than about 1 micron; a scanning interferometer which during operation directs a first infrared wavefront along a reference path and a second infrared wavefront along a measurement path contacting the measurement object, and, after the second wavefront contacts the measurement object, combines the wavefronts to produce an optical interference pattern, the first and second infrared wavefronts being derived from the broadband infrared radiation; a detector producing data in response to the optical interference pattern; and a controller which during operation causes the scanning interferometer to vary the optical path difference between the reference and measurement paths over a range larger than the coherence length of the broadband source and analyzes the data as a function of the varying optical path difference to determine the surface profile.
Embodiments of the interferometry system can include any of the following features. The broadband infrared radiation can include central wavelengths within the range of about 3 micron to 11 microns. The coherence length of the broadband infrared radiation can be in the range of about 10 to 100 microns. The system can further include a mount configured to support the measurement object at substantially normal incidence to the measurement path. The scanning interferometer can be of a type that can produce a zero optical path difference between the measurement and reference paths. The interferometer can include at least one imaging optic, which during operation directs the combined wavefronts onto the detector to form the optical interference pattern. The imaging optics can be configured to demagnify the surface of the measurement object with respect to the optical interference pattern formed on the detector. For example, the demagnification can be in the range of about 1× to about 0.1×. The interferometer can produce a field of view on the detector greater than about 5 mm, e.g., in the range of about 10 mm to 100 mm. The interferometer can include a reference mirror positioned along the reference path and a translation stage supporting the reference mirror, the translation stage being connected to the controller for varying the optical path difference between the reference and measurement paths. The interferometer can include a reference mount configured to support a reference object having a surface contacted by the reference path. The interferometer can further include the reference object, which can have flat or shaped reflective surface. The interferometer can include a beam splitter positioned to separate the broadband infrared radiation into the first and second infrared wavefronts and at least one compensating optic positioned along the measurement path to cause curvature in the second infrared wavefront. During operation, the controller can determine the thickness profile of the measurement object based on optical interference pattern data corresponding to reflections of the second infrared wavefront from front and back surfaces of the measurement object.
In general, in another aspect, the invention features an interferometry system for a measuring a surface profile of a measurement object. The interferometry system includes: a broadband infrared source which during operation generates broadband infrared radiation including central wavelengths greater than about 1 micron, the broadband infrared radiation havi
Colonna de Lega Xavier
de Groot Peter
Deck Leslie L.
Fish & Richardson P.C.
Kim Robert
Zygo Corporation
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