Optics: measuring and testing – By light interference
Reexamination Certificate
2002-07-05
2004-11-09
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Reexamination Certificate
active
06816263
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority of the German patent application 101 31 898.7 which is incorporated by reference herein.
FIELD OF THE INVENTION
The present invention concerns an interferometric measurement apparatus for wavelength calibration, having a laser light source, a detector, and an interferometer, the laser light source emitting light of at least one wavelength, the interferometer separating the light of the laser light source into two sub-beams—a reference beam and a measurement beam—and combining the sub-beams again after at least one reflection at one reflection means each, and the path length difference between the reference beam and measurement beam defining a constant wavelength calibration distance.
BACKGROUND OF THE INVENTION
Interferometric measurement apparatuses are used in many ways for highly accurate spacing and position measurements. In a high-accuracy interferometric measurement, what is usually measured is the relative path length difference between a measurement mirror on a movable measured specimen in the measurement beam path and a reference mirror in the reference beam path. The way in which the phase of the light changes as the measured specimen moves is determined during measurement, the wavelength of the light beam being the measurement criterion. The relative path length is thus indicated using “wavelength” as the unit. The instantaneous value of the wavelength of a light beam depends on the refractive index of the medium traversed by the light beam. The refractive index varies, for example, as a result of slow changes or rapid fluctuations in temperature, air pressure, and atmospheric humidity, or changes in atmospheric composition. In typical measurements on wafers and masks with, for example a coordinate measuring instrument, the fluctuations in the measurement results due to wavelength changes are approximately ±0.1 &mgr;m, i.e. no longer negligible as compared to the features being measured, and therefore not tolerable in terms of the required measurement accuracy.
In order to increase the measurement accuracy, a consideration of the change in the wavelength of the light beam, in the form of a continuous wavelength correction, is necessary.
For high-accuracy distance measurements the coordinate measuring instrument could, as is known e.g. from DE 198 19 492, be operated in a climate chamber. In this at least the temperature, and in some climate chambers the atmospheric moisture as well, is held constant. There are technical limits to the accuracy with which temperature and atmospheric moisture can be controlled. It is also impossible, with acceptable outlay, to manufacture hermetically sealed chambers to maintain a constant air pressure, especially since, taking the example of the coordinating measuring instrument, easy and quick interchange of measured specimens is a necessity. For example, merely actuating the loading opening causes rapid fluctuations in air pressure.
The interferometer wavelength must therefore be continuously acquired in a separate measurement. This can be done by measuring a wavelength calibration distance of constant length (so-called “wavelength tracker”) or by measuring the influencing factors such as temperature, atmospheric moisture, etc. and continuously calculating the instantaneous wavelength. This wavelength correction is inherently subject to error, however, for example because the precision of the measurements underlying it is only finite or because a high-precision measurement is not necessarily fast enough to reproduce rapid changes in the measured variable. The wavelength correction thus also contributes an error to the corrected wavelength.
Especially in interferometric measurement apparatuses in which interferometric measurements are performed with light of two different wavelengths, one considerable error source is constituted by the finite resolution upon measurement of the phase difference at the interferometer output, and by the so-called interpolator error. The latter occurs in heterodyne lasers because the polarization separation of the two laser wavelengths is not ideal. Small amounts of laser light of the wrong wavelength are therefore present in the two polarization components. One example of an interferometric measurement apparatus is the commercially available HP 10702 laser interferometer of the Hewlett-Packard company.
The two aforementioned error components have a fixed maximum magnitude that is independent of the length of the measurement distance. The measurement error &dgr;L upon measurement of a wavelength calibration distance of length L is incorporated into the distance measurement as a relative error &dgr;L/L. It could therefore be suppressed or at least reduced by making the wavelength calibration distance long enough. The reference measurement error will be small compared to the measurement if the reference distance is correspondingly small compared to the wavelength calibration distance. Such wavelength calibration distances do exist and are commercially available—again from the Hewlett-Packard company, as model 10717 A—but they do not meet this length condition for distance measurements in the range exceeding 50 mm.
Very long wavelength calibration distances require a very considerable amount of physical space, especially when built into coordinate measuring instruments for measuring masks and wafers. An obvious solution is therefore to fold the laser beam, i.e. to reflect the laser beam several times within the wavelength calibration distance. Arrangements are known in which the light beam is slightly tilted, so that the laser beam travels several times along the beam path in “zigzag” fashion. This arrangement offers maximum extension of the measurement distance, but the manufacture, assembly, and alignment of this type of mirror arrangement at small tilt angles is very difficult.
SUMMARY OF THE INVENTION
It is therefore the object of the present invention to describe and develop an interferometric measurement apparatus for wavelength calibration of the species, such that in order to increase the measurement accuracy and to decrease the measurement error, the measurement beam distance can be extended without, however, causing problems in manufacture, assembly, and/or alignment.
The interferometric measurement apparatus of the species according to the present invention achieves the aforesaid object by way of the features of claim
1
. According to the latter, an interferometric measurement apparatus of this kind is characterized in that at least one additional reflection means, which reflects the measurement beam at least largely in the opposite direction, is provided in the beam path of the measurement beam.
What has been recognized firstly is that the length of a sub-beam can be extended by way of an additional reflection—as is already known from the existing art. In accordance with the present invention, however, the measurement beam is reflected at least largely in the opposite direction, i.e. the measurement beam striking the additional reflection means and reflected therefrom does not enclose an acute angle of, for example, 10 or 20 degrees, which is problematic to handle especially in terms of manufacture and assembly. What is provided instead is an additional reflection that, for example, reflects the light beam back into itself or parallel to the incident beam. With this action, the measurement error of the wavelength calibration distance can be reduced; for example, if the wavelength calibration distance is doubled, the measurement error can thus be halved. In particularly advantageous fashion, the additional reflection of the measurement beam can be implemented economically with little additional design effort by insertion of an additional reflection means, modest requirements being applied in terms of the alignment and manufacturing tolerances necessary in this context.
In a very particularly preferred embodiment, the additional reflection means reflects back the measurement beam that strikes it in the opposite direc
Kaczynski Ulrich
Rinn Klaus
Connolly Patrick
Crowell & Moring LLP
Leica Microsystems Semiconductor GmbH
Turner Samuel A.
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