Optics: measuring and testing – By polarized light examination – Of surface reflection
Reexamination Certificate
2001-05-02
2003-07-29
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
By polarized light examination
Of surface reflection
Reexamination Certificate
active
06600560
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This invention claims priority of a German patent application DE 100 21 378.2 which is incorporated by reference herein.
FIELD OF THE INVENTION
The invention concerns an optical measurement arrangement having an ellipsometer, in which an incident beam of polarized light is directed at an angle of incidence a &agr;≠0° onto a measurement location on the surface of a specimen, and information concerning properties of the specimen, preferably concerning layer thicknesses, optical material properties such as refractive index n, extinction coefficient k, and the like, is obtained from an analysis of a return beam reflected from the specimen.
BACKGROUND OF THE INVENTION
Optical measurement arrangements that are based on the principle of ellipsometry or spectrophotometry, and their use for layer thickness measurement, are known in many varieties from the existing art. They have been successfully utilized in particular in the measurement of thin layers on the patterns of wafer surfaces. Whereas an oblique incidence of the measurement light onto the specimen is required in ellipsometry, a perpendicular light incidence is preferable in spectrophotometry in order to rule out polarization effects as much as possible. A measurement arrangement operating on the principle of spectrophotometry is known, for example, from U.S. Pat. No. 5,120,966.
Since increasingly fine patterns and thinner layers are desirable in particular in wafer manufacture, requirements are also increasing in terms of the accuracy of the optical measurement arrangements with which the dimensional consistency of the patterns and layers can be verified.
To allow even complex patterns and layer systems to be measured, for reliable results it is usually necessary to apply several measurement principles; the measurement operations should be performed as rapidly in succession as possible at a single point, since positioning (given that measurement location sizes are on the order of micrometers wide) is very laborious.
Existing measurement arrangements require different optical assemblies for different measurement principles. Arrangement and coordination of the assemblies with respect to one another must be accomplished in such a way that the pertinent beam paths do not, if possible, substantially influence each other. In the case of a measurement arrangement for the inspection of wafer surfaces, for example, a measurement objective of a spectrophotometer must be arranged over the measurement location. It is also necessary to guide the laser beam of a focusing device onto the measurement location so that the region of a specimen to be examined can be correctly positioned with respect to the measurement objective. An additional ellipsometer must then be arranged alongside the measurement objective of the spectrophotometer, and the incident beams of the ellipsometer must also strike the measurement location. A corresponding device of the ellipsometer for collecting and analyzing an output beam of light reflected from the specimen must furthermore be arranged alongside the spectrophotometer measurement objective. The configuration of a measurement arrangement of this kind is, however, relatively complex.
U.S. Pat. No. 5,042,951 describes a measurement arrangement in which ellipsometric measurement can be performed with only one objective. Many different angles of incidence can be analyzed simultaneously, and even a small measurement spot (approximately 1 &mgr;m or less) can be used. With the arrangement described therein it is not possible, however, simultaneously to analyze several wavelengths separately and spectroscopically.
U.S. Pat. No. 5,596,406 has made improvements over this; it recommends, inter alia, the simultaneous measurement of several wavelengths using a halogen lamp as the illumination source.
The arrangements proposed in U.S. Pat. No. 5,042,951 and in U.S. Pat. No. 5,596,406 consistently use normal dispersive lens optics and glass-plate beam splitters, however, which are suitable for the VIS-IR region but not for the entire UV-VIS-IR region. The reason is the large chromatic aberration of the specimen image, and the decreasing transmission in the deep UV of broadband antireflection coatings and broadband reflection coatings.
SUMMARY OF THE INVENTION
In this context, it is the object of the invention to create an optical measurement arrangement, operating on the principle of ellipsometry, which has a simple, compact configuration and allows a high measurement accuracy, down to the sub-nanometer range, over the entire UV-VIS-IR spectral region.
This object is achieved by an optical measurement arrangement of the kind cited initially in which the incident beam is directed by a mirror objective onto the measurement location on the surface of the specimen, and the return beam is also captured by the mirror objective.
The use of a mirror objective for ellipsometry makes it possible to eliminate the separate optical assemblies hitherto used for the purpose. It is furthermore possible to use the mirror objective simultaneously for spectrography, thus reducing the equipment requirement of the optical measurement arrangement to a single measurement instrument, and allowing a particularly space-saving design for the entire measurement arrangement to be realized.
The mirror objective moreover has the advantage, as compared to optics conventionally used in ellipsometry, of being UV-transparent, so that a measurement with light wavelengths in the spectral region from 190 nm to 800 nm can be performed. In the measurement of small layer thicknesses in particular, measurement with short wavelengths in the UV region results in a high measurement accuracy.
The mirror objective further makes it possible to apply an incident beam onto the measurement location on the specimen within an angular range of 18° to 41° from the optical axis of the mirror objective. The relatively high numerical aperture of the objective, i.e. its large angle of incidence range, allows both thin and thick layers to be measured with high accuracy. Because of the relatively high aperture of the mirror objective, microspot sizes of approximately 400 nm to 2 &mgr;m are possible.
In an advantageous embodiment of the invention, the light reflected from the specimen is introduced via a light-guiding device into an analysis device, the light-guiding device comprising a plurality of individual light-guiding fibers. A further light-guiding device having a plurality of individual light-guiding fibers is provided in order to convey to the analysis device measurement light that is uninfluenced by the specimen. The use of light-guiding fibers permits the analysis device to be arranged very flexibly with respect to the mirror objective and to an illumination source that is necessarily also present. Connecting the light-guiding devices in parallel makes possible a reduction in the occurrence of noise signals upon analysis, since although the measurement signal arrives in noisy fashion at the receiver, that noise is nevertheless correlated with the noise of the reference light channel, so that it can be effectively compensated for.
A polarizing beam splitter is preferably arranged after the mirror objective, in such a way that the return beam coming from the mirror objective is divided, in the polarizing beam splitter, into two s- and p-polarized output beams which are conveyed separately to the analysis device. It is thereby possible to analyze the polarization state of the light reflected from the specimen. The beam splitter can be a Wollaston analyzer or a Rochon analyzer. The Wollaston prism has the advantage over the Rochon prism that the separation angle between the respectively s- and p-polarized output beams is greater.
A focusing lens is preferably arranged between the exit of the polarizing beam splitter and the light-guiding device that is configured in two-channel fashion, in order to focus the s- and p-polarized output light beams obtained from the polarizing beam splitter onto the respective entrances of the channels of
Danner Lambert
Engel Horst
Mikkelsen Hakon
Stey Christof
Leica Microsystems Jena GmbH
Nguyen Tu T.
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