Grazing incidence interferometer and method

Details Grazing incidence interferometer and method Grazing incidence interferometer and method

1999-06-03

2001-06-19

Font, Frank G. (Department: 2877)

Optics: measuring and testing

By light interference

For dimensional measurement

C356S521000

Reexamination Certificate

active

06249351

ABSTRACT:

FIELD OF THE INVENTION
The invention relates generally to optical metrology instrumentation for imaging, analysis and characterization of surfaces, and more particularly, to grazing-incidence interferometers.
BACKGROUND OF THE INVENTION
Detecting surface defects and form errors with sizes in the micron range has become an important issue for contemporary part manufacturers. Contemporary parts often do not have highly polished surfaces and thus are not amenable to interferometric examination using visible light reflected at normal incidence. For such parts, grazing incidence interferometers may be useful measuring tools. These interferometers use light reflected off a surface at grazing angles to measure the surface's properties.
There are several known types of grazing incidence interferometers, as is described, for example, on pp. 76, 259-260 of the book “Optical shop testing” by D. Malacara, Second Edition (John Wiley & Sons, Inc., New York, 1992). The density of fringes generated by a grazing incidence interferometer having an illumination wavelength &lgr; is characterized by an equivalent wavelength &Lgr;, given by the formula
&Lgr;=&lgr;/cos(&agr;)  (1)
Since the equivalent wavelength &Lgr; is larger than &lgr;, many surface textures that are too rough for interferometry at the illumination wavelength &lgr; appear smoother when illuminated at grazing angles than when illuminated at normal incidence. It is consequently easier to perform interferometry at grazing incidence angles for these surfaces.
Various methods have been employed for separating and recombining measurement and reference wavefronts used for grazing incidence interferometry. Abramson, in Optik, 30, 56-71 (1969), describes a well-known type of grazing incidence interferometer that employs a large right-angle prism placed nearly in contact with a sample surface. In this case, the hypotenuse of the prism serves both to separate and to recombine the measurement and reference wavefronts.
Birch, in J. Phys. E: Sci. Instru. 6, 1045-1048 (1973), describes a grazing incidence interferometer that relies on diffraction gratings for separating and recombining the wavefront rather than the hypotenuse of a right-angle prism. Birch's interferometer has the advantage that the sample surface need not be placed close to optical components of the interferometer.
Although there are several types of grazing incidence interferometer in the prior art, including those cited above, grazing incidence interferometry has enjoyed only a limited success for on-line, production yield enhancement of precision manufactured surfaces. There is accordingly an unmet need for an accurate, efficient and convenient geometry for grazing incidence interferometry.
SUMMARY OF THE INVENTION
In accordance with a first embodiment of the invention, the interferometer includes a source, beam expansion optics that generate an initial illumination wavefront and a diffractive-optic beam splitter for separating the initial wavefront into a reference wavefront and a measurement wavefront by amplitude division. In some embodiments, the reference and measurement wavefronts correspond to the first-order diffracted beams of the diffractive-optic beam splitter.
The measurement wavefront reflects from the sample surface at a grazing angle. The interferometer also includes a diffractive-optic beam combiner to recombine the reference and measurement wavefronts into an output wavefront and an optical imaging system to generate an image of the sample surface containing interference fringe information from the output wavefront. Because the recombined reference and measurement wavefronts undergo the same number of inversions, they have the same relative relationship as they had when initially split from the initial illumination wavefront.
In various embodiments, the reference wavefront reflects from a reference mirror, generally disposed parallel to and facing the sample surface. Then, the reference and measurement wavefronts overlap after reflection.
In various embodiments, grazing incidence and reflection angles are angles of greater than 75° with respect to a normal to the sample surface.
In various embodiments, the splitter and recombiner are configured so that the equivalent wavelength &Lgr;, which characterizes the interference fringes, is substantially independent of the source wavelength &lgr;.
Various embodiments of the interferometer position optical elements so that optical path lengths traversed by the measurement and reference wavefronts are substantially equal. In some of these embodiments, the source includes a spectrally broadband source, such as, a multimode laser diode, a light emitting diode, filtered white light or another source of low-coherence light.
Various embodiments employ auxiliary mirrors, e.g., a reference mirror and a fold mirror. The fold mirror may be located to facilitate the forming the beam splitter and the beam combiner with a single diffractive-optic element.
Various embodiments perform phase shifting interferometry using electronic data acquisition, computer control and mechanical devices that introduce precise phase shifts in the interference pattern.
Various embodiments position optical elements and/or other devices so that the reference and measurement wavefronts traverse unequal path lengths. In these interferometers, the source wavelength may be a tunable laser light source that facilitates phase shifting.
Various embodiments are configured to simultaneously measure the front and back surfaces of a sample. Some such embodiments can determine the thickness profile of a silicon wafer.
Various embodiments are adapted for the analysis of non-planar surfaces, e.g., cylinders.
In various embodiments, the diffractive-optics causes the reference wavefront to converge to a focus and then to diverge while propagating between the beam splitter and beam combiner. The propagation inverts the reference wavefront.
Various embodiments restrict the sample height range over which high-contrast fringes form in the image of the sample surface. Some such interferometers employ an extended source, that is, a source having a large surface area, such as a ground glass screen illuminated by a laser beam or a light-emitting diode. These interferometers further include data acquisition devices, a computer control and a mechanical actuator to scan the interference pattern by moving the sample in a direction substantially perpendicular to the sample surface. Such embodiments can profile the sample surface without the fringe ambiguity by using fringe-contrast scanning.
Various embodiments of the invention employ birefringent optical elements in place of the diffractive-optic elements.


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“On-Line Detection of Surface Defects

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