Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-05-02
2003-09-09
Allen, Stephone (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S369000
Reexamination Certificate
active
06618154
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This invention claims priority of a German patent application DE 100 21 379.0 which is incorporated by reference herein.
FIELD OF THE INVENTION
The invention concerns an optical measurement arrangement, in particular for layer thickness measurement and for ascertaining optical material properties of a specimen, comprising an illumination device for emitting a measurement light beam, a beam splitter for dividing the measurement light beam into a specimen light beam and a reference light beam, a measurement objective for directing the specimen light beam onto a measurement location on the surface of the specimen and for acquiring the light reflected from the measurement location, and an analysis device into which the reference light beam and the specimen light beam reflected from the specimen are coupled in order to obtain information about the specimen, in particular about layer thicknesses present thereon.
BACKGROUND OF THE INVENTION
Optical measurement arrangements that operate on the principle of spectrophotometry, and their particular use for layer thickness measurement, are known in many varieties from the existing art. They have been utilized with particular success in the measurement of thin layers and optical parameters (such as refractive index n and extinction coefficient k) of single-layer and multilayer systems on patterned wafers.
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. In the context of this development, there is an interest in performing measurements both on single-layer systems and on multilayer systems. In addition, it should be possible to make exact measurements, with the same optical measurement arrangement, of thin layers having thicknesses of approximately 1 nm and above and of thicker layers up to approximately 50 &mgr;m.
In addition to the demand for increased accuracy, the desire for greater production volumes must also be taken in account. In the continuous production of wafers, for example, it is necessary to measure them at shorter and shorter time intervals, and whenever possible in-line. On the other hand, space considerations must be also be borne in mind when integrating an optical measurement arrangement into a continuous production line. The arrangement should require little space, and should be flexible in terms of setup. Also desirable is suitability for rapid adaptation to changes in physical conditions. In this context, however, accuracy considerations must never be forgotten.
An optical measurement arrangement of the kind cited initially is known, for example, from U.S. Pat. No. 5,486,701. This is based on the principle of spectral analysis of the light reflected from a measurement location, each thickness having associated with it a characteristic interference-dependent reflection spectrum. The optical measurement arrangement disclosed in U.S. Pat. No. 5,486,701 is, however, specialized for the measurement of extremely thin layers. For that purpose, a separate analysis is made of the UV region and the visible region of the reflected light, so as to obtain therefrom the actual information concerning the measurement location on the specimen. Because of the large number of optical assemblies incorporated into the beam path, the assemblage is complex and sensitive to external influences. The optical deflection devices which transfer a measurement light beam emitted from the illumination device to the specimen and on to the analysis device further limit the design possibilities with regard to a compact assemblage.
The lateral resolution capability for patterns on the specimen depends on the wavelength of the measurement light used. Shorter-wavelength light allows finer patterns to be resolved. In connection with the measurement of very thin layers (in the range of less than one-tenth the wavelength of the measurement light), a further consideration is that the brightness differences that are to be analyzed, from which the information about layer thickness is obtained, become very small. With thin layers in particular, it is important to transfer the light reflected from the specimen to an analysis device with as little loss or interference as possible.
SUMMARY OF THE INVENTION
Against this background, it is the object of the invention to create an optical measurement arrangement that, with a simple and compact configuration, allows accurate information to be obtained concerning properties of a specimen being examined, in particular a patterned wafer.
This object is achieved with an optical measurement arrangement of the kind cited initially in which light-guiding devices having a plurality of light-guiding fibers are provided for coupling the specimen light beam and the reference light beam into the analysis device.
The arrangement at this concrete location of the light-guiding devices, known per se in the field of optical applications, makes possible a good compromise between minimally distorted transfer of the measurement light—i.e. of a component uninfluenced by the specimen (namely a reference light beam) and a component influenced by the specimen (namely a specimen light beam)—and a particularly compact and at the same time flexible configuration of the optical measurement arrangement, which as a result is particularly suitable for being set up in continuous production lines.
Use of the light-guiding devices directly at the entrance of the analysis device allows the latter to be arranged flexibly with respect to the measurement objective. The arrangement is moreover insensitive to interfering influences from the production process, for example vibration.
Preferably a separate light-guiding device is used in each case for the specimen light beam and for the reference light beam, so that beam guidance is mutually independent and thus can be accomplished with as little restriction as possible in accordance with the particular physical conditions that are present.
In a further advantageous embodiment, the exit ends of the light-guiding fibers at the entrance of the analysis device are arranged in correspondence with a receiver provided on the analysis device. Optimum yield of the light reflected from the specimen is thereby obtained. In consideration of analysis devices that are common on the market, for example spectrographs with a downstream CCD detector, the exit ends of the light-guiding fibers are preferably arranged in linear fashion.
To achieve the clearest possible differentiation between maxima and minima in the spectrum of the specimen light beam reflected from the specimen, in a further advantageous embodiment of the invention an aperture stop is arranged in the measurement light beam in front of the beam splitter. The aperture stop preferably has a passthrough opening in the shape of a quarter-circle ring, hereinafter also called a “quarter pupil.” This allows the incident and return light to be guided separately to the measurement objective, and additionally permits the light of a focusing device to be coupled into the measurement objective in order to align the specimen relative to the measurement objective.
For optimum utilization of the light in the aperture, the entrance ends of the light-guiding fibers for the specimen light beam and the reference light beam can be distributed over a surface on which the shape of the opening of the aperture stop is reproduced. The enveloping curve of the light-guiding fibers corresponds substantially to the contour of the opening of the aperture stop. This advantageously also ensures that complete illumination of the entrance ends of the light-guiding devices remains independent of the size of a measurement window on the specimen, and of the settings of the field stop and the pinhole mirror.
It is preferable to use light-guiding devices in which the positional allocation between the entrance ends and exit ends of
Backhaus Kuno
Danner Lambert
Engel Horst
Mikkelsen Hakon
Slodowski Matthias
Allen Stephone
Foley & Lardner
Leica Microsystems Jena GmbH
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