Optics: measuring and testing – By polarized light examination – Of surface reflection
Reexamination Certificate
2001-04-23
2003-07-08
Epps, Georgia (Department: 2873)
Optics: measuring and testing
By polarized light examination
Of surface reflection
C356S073000, C356S326000, C356S445000
Reexamination Certificate
active
06590655
ABSTRACT:
TECHNICAL FIELD
The present invention relates to polarimetry, ellipsometry, reflectometry, spectrophotometry and the like, and more particulalry to systems for, and methods of controlling radial energy density profiles in, and/or cross-sectional dimensioning of electromagnetic beams.
BACKGROUND
It is well known that polarimeter, ellipsometer and spectrophotometer, (often reflectometers as well), and the like systems generally involve causing an electromagnetic beam to non-destructively impinge upon the surface of a sample system along a locus removed from a normal to said sample system surface. Such oblique angles of incidence, it is noted, cause a beam of electromagnetic radiation which is initially essentially circular in cross-section, to form an essentially elliptically shaped “spot” on a sample system surface whereat impingement occurs. Further, it is known that electromagnetic beams provided by typically available sources thereof, generally do not present with radially uniform energy density, but instead have a peak energy density centrally located, which decreases radially. In addition, it has been determined that often said energy density does not decrease in a simple manner such as, for instance, linearly or as a square of increasing radius all the way to zero, but rather upon so decreasing to approximately ten percent (10%) of maximum, “oscilations” as a function of increasing radius occur, much like those seen in the Fourier Transform of a Square Wave. It is noted that the presence of said “oscillations” cause trouble in analysis of ellipsometric data obtained via use of electromagnetic beams which contain them.
While it is also known to apply optical elements, (eg. lenses), which have spherical and aspherical surfaces, (but characterized by radial symmetry), in reflectometer, spectrophotometer, polarimeter, ellipsometer and the like systems, said optical elements generally do not serve specifically to change a spot shape, where an electromagnetic beam impinges on a sample system surface, to substantially circular. It is also known to apply optical elements with other than radial symetry in ellipsometer systems, however, typical application thereof is to enter compensation for various aberational effect. An example of this is discussed in U.S. Pat. No. 5,608,526 to Piwonka-Corle et al.
Additional known Patents include U.S. Pat. No. 5,859,424 to Norton et al. which discloses use of an apodizing filter used to reeuce spot size in optical measurements. U.S. Pat. No. 5,910,842 to Piwonka-Corle et al. is disclosed as it describes application of an elliptically shaped mirror in an ellipsometer system, which elliptically shaped mirror is applied to reduce off-axis aberations. U.S. Pat. No. 5,917,594 to Norton describes application of spherical mirrors in spectroscopic measurement systems. U.S. Pat. No. 6,184,984 to Lee et al. describes application of an off-axis paraboloid mirror in a system for measuring properties of a sample.
A Patent to Burghardt et al., U.S. Pat. No. 5,414,559 is also disclosed as it describes a device for homogenizing a light beam. In addition, U.S. Pat. No. 5,796,521 to Kahlert et al. is disclosed as it describes optical apparatus for homogenizing electromagentic beams which comprises cylindrical lenses.
Utility would be inherrant in a system, and method of its application which would tailor electromagnetic beams so that they present with substantially radially uniform energy density, and/or which would cause a spot at the surface of a sample system whereupon impingement thereof occurs, to be essentially circular.
A need is identified for systems and methods which provide electromagnetic beams in polarimeter, ellipsometer, reflectometer, spectrophotometer and the like systems that present with essentially radially uniform energy density content and/or other than essentially circular cross-sectional shape before they impinge on a sample system surface.
DISCLOSURE OF THE INVENTION
The present invention can be considered to, in the context of application in reflectometer, spectrophotometer, polarimeter and ellipsometer and the like systems, variously apply or combine and apply selections from the group:
means for effecting cross-sectional, substantially radially uniform energy density in electromagnetic beams; and
means for effecting desired electromagnetic beam spot shape whereat impingement upon a surface of a sample system occurs.
The means for effecting cross-sectional, substantially radially uniform energy density in electromagnetic beams comprises a sequential combination of a beam expander, a first beam collimator, at least one multi-faceted optical element, a beam condenser and a second beam collimator. Multi-faceted elements can be considered to be comprised of a plurality of small lenses or, for instance, can be constructed from a plurality of half circular, (in cross-section), longitudinal elements aligned parallel to one another; directly adjacent to a second plurality of half circular, (in cross-section), longitudinal elements aligned parallel to one another, wherein the longitudinal orientations of the first and second plurality of half circular, (in cross-section), longitudinal elements are oriented other than parallel, (eg. at an angle of 90 degrees), to one another.
In use an electromagnetic beam with an arbitrary cross-sectional energy density profile is caused to enter said beam expander which serves to expand the beam diameter, then said expanded diameter beam is collimated and caused pass through said at least one multi-faceted optical element which can be thought of as serving to form a multiplicity of spatially separated images of portions of said expanded beam. The beam condensor next serves to focus said spatially separated images atop one another, with the second beam collimator then serving to provide an electromagnetic beam of more uniform radial energy density than was demonstrated by the input beam. A plurality of such multi-faceted optical elements can be sequentially present adjacent to one another to provide further improved uniform radial energy density. And, it is to be understood, where a plurality of multi-faceted optical elements are present, to reduce dispersion effects, they can be constructed of different materials, which different materials which have different wavelength transmission characteristics, and/or specularly dependent indicies of refraction.
The means for effecting desired electromagnetic beam spot shape, whereat impingement on a surface of a sample system occurs, can comprise selection from the group:
lenses which present with non-radial symmetry, (eg. astigmatic/toroidal lenses);
focusing mirrors with non-radial symetry, (eg. parabolic mirrors); and
apertrues with non-radial symetry:
where the terminology “non-radial symetry” is to be interpreted to mean, for instance, that a “width-wise” dimension is different from an orthogonally oriented “length-wise” dimension. That is there is an aspect ratio offset from 1.0. Application of non-radial symetry optical elements can result in a circular cross-section electromagnetic beam becomming shaped into an elliptical beam of electromagnetic radiation. The purpose of this is to, in combination with a “length-wise” elongating effect on the beam, which results from its non-normal oblique angle of incidence on the surface of a sample system, result in an essentially circular beam spot shape at the location where it impinges on the surface of said sample system.
Of course, it is within the scope of the present invention to combine the effects of non-radial symetry aperture and/or non-radial symetry lenses and/or non-radial symetry focusing mirrors to effect essentially circular beam spot shape at the location whereat impingement on the surface of a sample system occurs. Further, any of the identified beam shaping optical elements can be combined with the means for effecting cross-sectional, essentially radially uniform energy density in electromagnetic beams, and such combinations provide a preferred embodiment.
More precisely the present invention is then a re
He Ping
Johs Blaine D.
Liphardt Martin M.
Welch James D.
Epps Georgia
J.A. Woollam Co. Inc.
O'Neill Gary
Welch James D.
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