Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1998-11-12
2003-06-03
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S493000, C356S498000
Reexamination Certificate
active
06573996
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to interferometric distance measurements using optical signals, and more particularly to a method and apparatus for enhancing precision of such measurement by virtually eliminating polarization leakage problems in an interferometer and by compensating for undesirable variations in the system, including variations caused by air turbulence.
BACKGROUND OF THE INVENTION
Interferometers are currently utilized for distance and position measurement in a variety of applications including lithographic integrated-circuit wafer production, measuring satellite position in global position sensing (GPS) systems, and measuring distance to detectors in earthquake detection systems. In all of these systems, undesirable perturbations of the interferometer optical signals due to natural phenomena or other variations in system parameters can result in small errors in the distances being measured.
One particular source of errors is air turbulence in the measurement space through which an interferometer optical signal, such as a light beam, passes. Turbulence is defined for purposes of this invention as variation in local density of the air in the measurement space. Such air density variations can result from a number of factors, including local temperature variations and air movement. Since the refractive index of the air through which the optical signal passes varies slightly with the density of the air, such turbulence can cause small errors in the distance measurements, as the distance measurement is a function of the wavelength of the optical signal and the refractive index of the air.
Existing high quality single-wavelength interferometers can measure an optical path-length, for example a path-length used in lithography as a measure of stage position, with a theoretical precision on the order of 1 nm or better. However, turbulence of the air in the interferometer optical signal path typically contributes variations of 10-30 nm to the measured path-length during the typical time period in which an integrated-circuit wafer is exposed.
Since such single-wavelength interferometers cannot distinguish between path-length changes due to this air turbulence and those due to stage motion, air turbulence has the effect of degrading the precision of these interferometers to a point where they are marginally capable of supporting 0.25 &mgr;m-design-rule lithography. Hence, 0.1 &mgr;m-design-rule lithography and below, which are becoming increasingly important in the industry, present significant challenges to the accuracy and precision of single-wavelength interferometers. As a result, under typical wafer production conditions, the overlay precision of single-wavelength interferometers is limited by air turbulence to approximately 10-30 nm, which is an unacceptably large imprecision for 0.1 &mgr;m-design-rule lithography.
One solution which has been proposed to overcome the air turbulence problem is for two interferometers employing light beams having significantly different wavelengths (or frequencies) to share a common measurement path. The optical path-length of the measurement path “seen” by each light beam will differ because the refractive index of air is a function of wavelength. This small but significant difference can be used to directly determine the optical “thickness” of the air path, allowing a correction for turbulence to be made.
While conventional interferometer systems utilizing two light beams have purportedly improved measurement precision by correcting for air turbulence, in general such systems are not readily integrated with existing single-wavelength interferometers to improve the precision of single-wavelength interferometers. Additionally, the precision of conventional multi-wavelength interferometers is limited by factors other than air turbulence, which render such interferometers marginally capable of meeting the stringent requirements of very high precision applications, for example, 0.1 &mgr;m-design-rule lithography, as discussed above.
SUMMARY OF THE INVENTION
In view of the foregoing, there is a need for an improved technique to eliminate or compensate for various potential overlay error sources for position lithography applications, particularly in 0.1 &mgr;m-design-rule lithography. In general, an improved precision interferometric distance measurement technique is desirable in order to eliminate or compensate for various sources of measurement error, including air turbulence, in these and other interferometer applications.
For example, an additional problem with conventional single or multi-wavelength interferometers, unrelated to the problem of air turbulence, is polarization leakage or optical nonlinearity. This problem arises because the optical signal splitters typically employed in conventional interferometers to separate the optical signal into two polarized components are imperfect, and therefore some percentage of the optical signal polarized to pass through one of two optical paths is, in fact, in the other of the two optical paths. The nonlinearity of measurements resulting from this “crosstalk” error presents a problematic limit to precision in conventional interferometers, and a solution to this problem would provide an advantage in further enhancing the precision of interferometric distance or position measurements.
It is additionally desirable that an improved interferometer system be achromatic (function equivalently for a wide range of wavelengths), that any precision enhancement and error correction mechanism be compatible with conventional interferometers and require minimal additional space (for purposes of “retrofitting” an existing interferometer with the improved interferometer system), and that the technique utilized for error correction be easily adaptable to perform a “baseline” interferometer distance measuring function, in addition to the error correction function, to provide an enhanced precision distance measurement.
It is also desirable, particularly with respect to lithographic integrated-circuit wafer production applications, that all electrical signals and potential heat generating components be mounted remotely from an interferometer measurement head and from any lithographic components so that the measurement head for the system, which may be mounted to the lithographic stage, is completely passive and contains no thermal sources, thereby eliminating a potential limitation on measurement precision. Further, it is desirable that the passive measurement head itself be extremely insensitive to ambient temperature variations, thereby overcoming temperature drift problems that have also been a source of error in conventional interferometers. Finally, the measurement head should be rugged and inherently insensitive to motion and vibration.
In accordance with the above, the present invention provides a method and apparatus for enhancing the precision of interferometric distance and position measurements.
One example of the present invention is an inexpensive compact two-wavelength interferometer using an analog radio-frequency (RF) heterodyne-mixing signal processing technique, alone or in combination with digital signal processing techniques, and a novel measurement head design, which can significantly reduce measurement errors, such as those due to air turbulence, to less than 1 nm by simultaneously measuring an optical path-length at two different wavelengths. The two different wavelengths may be harmonically related, or may have an arbitrary wavelength relationship.
One aspect of the present invention is an achromatic interferometer design that allows complete integration of an interferometer according to the present invention with a conventional baseline measurement system. The design permits a non-invasive retrofit requiring no modification of the baseline system and allows all optical signals to share a single, compact measurement head.
Another aspect of the present invention is an interferometer system that virtually eliminates the optical nonlinearity “crosstalk” problem due to polariz
Deliwala Shrenik
Flusberg Allen
Swartz Stephen D.
Font Frank G.
Lee Andrew H.
Science Research Laboratory, Inc.
Wolf Greenfield & Sacks P.C.
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