Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-09-14
2002-02-05
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S521000
Reexamination Certificate
active
06344898
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to apparatus and methods used for measuring surface shapes (surface topography) with high accuracy and precision. More specifically, the invention concerns such apparatus and methods that employ interferometry for measuring topographical characteristics of a surface of a sample such as an optical element.
BACKGROUND OF THE INVENTION
A conventional use of a Fizeau interferometer or a Twyman-Green interferometer is the measurement of the shape (i.e., surface topography) of a spherical surface, such as the surface of a spherical lens. To perform such measurements using either of these types of interferometers a reference surface is conventionally required. I.e., conventional measurements of the surface topography of a spherical surface of a sample are determined by comparison with an actual corresponding “ideal” reference surface. As a result, the accuracy of the measurements cannot exceed the accuracy of the reference surface.
A conventional way in which to solve such a problem is disclosed in Japanese Laid-Open (Kokai) Patent Application No. 2-228505 disclosing an interferometer not requiring a reference surface. Specifically, this reference discloses a so-called point-diffraction interferometer, abbreviated “PDI.” A PDI employs an ideal spherical-surface wave, generated by diffraction of light passing through a pinhole, for use as a reference wavefront. Such a device allows high-accuracy and high-precision measurements of the topography of aspherical surface.
Unfortunately, the conventional PDI technique summarized above is not usable for measuring the surface topography of an aspherical surface. This is because, when measuring a spherical surface, very few interference fringes (i.e., a “sparse” pattern of fringes) are generated by regions of the surface where the curvature radius of the spherical wave produced by the pinhole coincides with the curvature radius of the sample. But, when measuring an aspherical surface, the curvature radius varies according to the location on the surface; as a result, the spacing of interference fringes is sparse only in regions that are too dense for measuring.
SUMMARY OF THE INVENTION
In view of the shortcomings of the conventional art as summarized above, an object of the invention is to provide apparatus and methods for performing high-accuracy measurements of the surface topography of aspherical surfaces as well as spherical surfaces.
To such end, and according to a first aspect of the invention, various representative embodiments of apparatus for measuring the surface topography of a test surface of a sample are provided. A first representative embodiment of such an apparatus comprises a point light source configured and situated relative to a detector and the sample so as to produce, from an input light, a beam of light divergently propagating as a prescribed wavefront from a point on the point light source. The beam comprises a measurement-beam portion, directed toward the test surface so as to reflect from the test surface, and a reference-beam portion. The light detector is configured to produce an output signal encoding data corresponding to an interference characteristic of light received by the detector. The point light source also comprises a reflective surface oriented so as to receive the measurement-beam portion reflected from the test surface and to cause the measurement-beam portion returning to the point light source to reflect from the reflective surface toward the detector. The measurement-beam portion reflected from the test surface and the reference-beam portion interfere with each other so as to produce an interference fringe received by the detector. The interference fringe has a characteristic corresponding to a surface topography of the test surface relative to the prescribed wavefront. An actuator is configured and situated so as to move at least one of the sample and the point light source relative to each other so as to change the distance between the test surface and the point light source as required. A processor is situated so as to receive the output signal from the detector. The processor is configured to provide a measurement of the surface topography from the interference fringe received by the detector.
The point light source desirably comprises a reflective mirror defining a pinhole, wherein the prescribed wavefront is produced by diffraction of the input light as the input light passes through the pinhole. The reflective mirror can be oriented so as to define a plane that is perpendicular to a propagation axis of the input light incident on the point light source. Alternatively, the reflective mirror can be oriented so as to define a plane that is at an angle of less than 90° to a propagation axis of the input light incident on the point light source.
In an alternative configuration, the point light source can comprise an optical fiber configured to conduct the input light. Such an optical fiber desirably comprises an end face serving as the reflective surface of the point light source. The end face also serves as the point from which the prescribed wavefront divergently propagates due to diffraction of the input light.
The prescribed wavefront can be a spherical wavefront. Alternatively, the prescribed wavefront can be any of various suitable aspherical wavefronts.
The point light source can be configured and oriented such that the measurement beam interferes with the reference beam as the measurement beam, reflected from the test surface, propagates to the reflective surface of the point light source. Alternatively, the point light source can be configured and oriented such that the measurement beam interferes with the reference beam as the measurement beam, reflected from the reflective surface of the point light source, propagates to the detector.
The apparatus can further comprise a light source configured to produce the input light. The input light can comprise a single or multiple wavelengths of light as required. If the input light comprises multiple wavelengths, a wavelength selector can be included to permit selection, from the multiple wavelengths of input light, a particular wavelength for input to the point light source. In such a configuration, the point light source can be configured to produce, from the particular wavelength, the measurement beam portion and the reference beam portion.
The apparatus can further comprise a light-path adjuster situated between the input light source and the point light source. The light-path adjuster is desirably configured to cause a path length of the reference-beam portion to coincide with a path length of the measurement-beam portion so as to permit adjustment of a contrast parameter of the interference fringe.
According to another aspect of the invention, methods are provided for measuring a profile of a test surface of a sample. According to a representative embodiment of such a method, a point light source is provided having a reflective surface. The point light source is situated so as to receive an input light and to produce from the input light a measurement-beam portion and a reference-beam portion divergently propagating as a prescribed wavefront from a point. The test surface is irradiated with the measurement-beam portion so as to cause the measurement-beam portion to reflect from the test surface and then reflect from the reflective surface. At a first distance of the test surface from the point light source, the measurement-beam portion reflected from the test surface interferes with the reference-beam portion so as to produce a first pattern of interference fringes. A pattern characteristic of the first pattern of interference fringes is detected. The first distance of the test surface from the point light source is changed to a second distance. At the second distance, the measurement-beam portion reflected from the test surface interferes with the reference-beam portion so as to produce a second pattern of interference fringes. A pattern characteristic of the second pattern of interfere
Gemma Takashi
Ichihara Hiroshi
Ichikawa Hajime
Jacobsen Bruce
Nakayama Shigeru
Klarquist & Sparkman, LLP
Nikon Corporation
Turner Samuel A.
LandOfFree
Interferometric apparatus and methods for measuring surface... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Interferometric apparatus and methods for measuring surface..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Interferometric apparatus and methods for measuring surface... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2983919