Optics: measuring and testing – By light interference – For refractive indexing
Reexamination Certificate
2002-03-19
2003-02-25
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For refractive indexing
C356S484000
Reexamination Certificate
active
06525826
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to optical instruments for measuring distance and refractive index. The invention relates in particular to interferometric distance measurement independent of the optical path length effects of refractive index of gas in a measurement path including the effects of refractive index fluctuations.
BACKGROUND AND PRIOR ART
A frequently-encountered problem in metrology is the measurement of the refractive index of a column of air. Several techniques exist for measuring the index under highly controlled circumstances, such as when the air column is contained in a sample cell and is monitored for temperature, pressure, and physical dimension. See for example, an article entitled “An air refractometer for interference length metrology,” by J. Terrien,
Metrologia
1(3), 80-83 (1965).
Perhaps the most difficult measurement related to the refractive index of air is the measurement of refractive index fluctuations over a measurement path of unknown or variable length, with uncontrolled temperature and pressure. Such circumstances arise frequently in geophysical and meteorological surveying, for which the atmosphere is obviously uncontrolled and the refractive index is changing dramatically because of variations in air density and composition. The problem is described in an article entitled “Effects of the atmospheric phase fluctuation on long-distance measurement,” by H. Matsumoto and K. Tsukahara,
Appl. Opt.
23(19), 3388-3394 (1984), and in an article entitled “Optical path length fluctuation in the atmosphere,” by G. N. Gibson et al.,
Appl. Opt.
23(23), 4383-4389 (1984).
Another example situation is high-precision distance measuring interferometry, such as is employed in micro-lithographic fabrication of integrated circuits. See for example an article entitled “Residual errors in laser interferometry from air turbulence and nonlinearity,” by N. Bobroff,
Appl. Opt.
26(13), 2676-2682 (1987), and an article entitled “Recent advances in displacement measuring interferometry,” also by N. Bobroff,
Measurement Science
&
Tech.
4(9), 907-926 (1993). As noted in the aforementioned cited references, interferometric displacement measurements in air are subject to environmental uncertainties, particularly to changes in air pressure and temperature; to uncertainties in air composition such as resulting from changes in humidity; and to the effects of turbulence in the air. Such factors alter the wavelength of the light used to measure the displacement. Under normal conditions the refractive index of air is approximately 1.0003 with a variation of the order of 1×10
−5
to 1×10
−4
. In many applications the refractive index of air must be known with a relative precision of less than 0.1 ppm (parts per million) to 0.003 ppm, these two relative precisions corresponding to a displacement measurement accuracy of 100 nm and 3 nm, respectively, for a one meter interferometric displacement measurement.
There are frequent references in the art to heterodyne methods of phase estimation, in which the phase varies with time in a controlled way. For example, in a known form of prior-art heterodyne distance-measuring interferometer, the source emits two orthogonally polarized beams having slightly different optical frequencies (e.g. 2 MHz). The interferometric receiver in this case is typically comprised of a linear polarizer and a photodetector to measure a time-varying interference signal. The signal oscillates at the beat frequency and the phase of the signal corresponds to the relative phase difference. A further representative example of the prior art in heterodyne distance-measuring interferometry is taught in commonly-owned U.S. Pat. No. 4,688,940 issued to G. E. Sommargren and M. Schaham (1987). However, these known forms of interferometric metrology are limited by fluctuations in refractive index, and by themselves are unsuited to the next generation of microlithography instruments.
Another known form of interferometer for distance measurement is disclosed in U.S. Pat. No. 4,005,936 entitled “Interferometric Methods And Apparatus For Measuring Distance To A Surface” issued to J. D. Redman and M. R. Wall (1977). The method taught by Redman and Wall consists of employing laser beams of two different wavelengths, each of which is split into two parts. Frequency shifts are introduced into one part of the respective beams. One part of each beam reflects from an object and recombines with the other part on a photodetector. From the interference signal at the detector is derived a phase, at a difference frequency, that is a measure of the distance to the surface. The equivalent wavelength of the phase associated with the difference frequency is equal to the product of the two laser wavelengths divided by the difference of the two wavelengths. This two-wavelength technique of Redman and Wall reduces measurement ambiguities, but is at least as sensitive to the deleterious effects of refractive index fluctuations of the air as single-wavelength techniques.
Another example of a two-wavelength interferometer similar to that of Redman and Wall is disclosed in U.S. Pat. No. 4,907,886 entitled “Method And Apparatus For Two-Wavelength Interferometry With Optical Heterodyne Processes And Use For Position Or Range Finding,” issued to R. Dändliker and W. Heerburgg (1990). This system is also described in an article entitled “Two-Wavelength Laser Interferometry Using Superheterodyne Detection,” by R. Dändliker, R. Thalmann, and D. Prongué,
Opt. Let.
13(5), 339-341 (1988), and in an article entitled “High-Accuracy Distance Measurements With Multiple-Wavelength Interferometry,” by R. Dändliker, K. Hug, J. Politch, and E. Zimmermann. The system of Dändliker et al., as taught in U.S. Pat. No. 4,907,886, employs laser beams of two wavelengths, each of the beams comprising two polarization components separated in frequency by means of acousto-optic modulation. After passing these beams collinearly through a Michelson interferometer, the polarization components are mixed, resulting in an interference signal, i.e. a heterodyne signal. In that the heterodyne signal has a different frequency for each of the two wavelengths, a so-called superheterodyne signal results therefrom having a frequency equal to the difference in the heterodyne frequencies and a phase associated with an equivalent wavelength equal to the product of the two laser wavelengths divided by the difference of the two wavelengths. According to U.S. Pat. No. 4,907,886 (cited above), the phase of the superheterodyne signal is assumed to be dependent only on the position of a measurement object and the equivalent wavelength. Therefore, this system is also not designed to measure or compensate for the fluctuations in the refractive index of air.
Further examples of the two-wavelength superheterodyne technique developed by Redman and Wall and by Dändliker and Heerburgg (cited above) are found in an article entitled “Two-wavelength double heterodyne interferometry using a matched grating technique,” by Z. Sodnik, E. Fischer, T. Ittner, and H. J. Tiziani,
Appl. Opt.
30(22), 3139-3144 (1991), and in an article entitled “Diode laser and fiber optics for dual-wavelength heterodyne interferometry,” by S. Manhart and R. Maurer,
SPIE
1319, 214-216 (1990). However, neither one of these examples addresses the problem of refractive index fluctuations.
It may be concluded from the foregoing that the prior art in heterodyne and superheterodyne interferometry does not provide a high speed method and corresponding means for measuring and compensating the optical path length effects of air in a measuring path, particularly effects due to fluctuations in the refractive index of air. This deficiency in the prior art results in significant measurement uncertainty, thus seriously affecting the precision of systems employing such interferometers as found for example in micro-lithographic fabrication of integrated circuits. Future interferometers will necessarily incorporate an inventive, new method and
De Groot Peter
Demarest Frank C.
Hill Henry A.
Caufield Francis J.
Font Frank G.
Merlino Amanda
Zygo Corporation
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