Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2001-03-28
2004-02-03
Pyo, Kevin (Department: 2878)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S496000
Reexamination Certificate
active
06687013
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to displacement measurement techniques, instrumentation techniques, evaluation techniques, precision patterning techniques, fine patterning techniques, semiconductor patterning techniques, and master mask patterning techniques. More particularly, the present invention relates to a displacement measurement technique which requires accuracy of the order of nanometer.
2. Description of the Related Art
For example, a laser interferometer displacement measuring system is often used as high accuracy displacement measurement means for controlling such as a stepper, employed in the photolithography process for fabricating semiconductor devices, and for controlling X-Y stages for use such as in precision machining equipment. A nominal value of resolution is 0.3 nm for the displacement measurement system which provides the most accurate displacement measurement and has been developed particularly for stepping control.
Concerning the provision of increased accuracy, even a general displacement measurement technique that does not employ the laser interferometer displacement measuring technique but employs noise processing by averaging is disclosed in Japanese Patent Laid-Open Publication No. Hei 7-306034 in relation to the non-contact displacement measurement. Optical measurement employing an optical interferometer is disclosed in Japanese Patent Laid-Open Publication No. Hei 9-178567 in relation not to position but to wavelength measurement.
However, in many cases, even the current laser interferometer displacement measuring system having a nominal value of resolution of 0.3 nm actually provides only an absolute accuracy of the order of ±2 nm for displacement measurement. The resolution and the absolute accuracy are essentially different from each other. An interferometer displacement measuring system may apparently have an accuracy of 0.3 nm in the range of about 10 nm, but in some cases, a gradual undulation may be generally found with the magnitude reaching 3 nm or more in the range of 100 to 300 nm. These problems were not made clear until a high-speed real-time displacement measurement approach was developed to thereby make it possible to measure the displacement of a moving object with high accuracy at a frequency greater than that of mechanical vibrations.
In general, to provide an increased accuracy, the noise processing by averaging over time is performed as mentioned above in order to improve the accuracy (i.e., relative accuracy) of stability of measurement values under a standstill condition of the object. However, with a recent increasing demand for increased accuracy, the absolute accuracy of measurement values has become necessary. In the course of study to the present invention, such a problem has become clear that the prior-art noise processing by averaging cannot provide a sufficient absolute accuracy.
In view of the aforementioned problems, it is therefore the object of the present invention to provide a high-accuracy interferometer displacement measuring system which provides an absolute accuracy in the range of ±2 nm to ±1 nm or less for a displacement measurement value using interference of laser light.
SUMMARY OF THE INVENTION
In consideration of the fact that the interference of light itself, which is the principle of laser interferometry, causes an error, the present invention is adapted to eliminate errors, concerning absolute accuracy, which cannot be eliminated only by averaging over time. Approaches to increased accuracy that focus attention on such a cause of error have never discussed before. More specifically, a correction value corresponding to the laser wave cycle of a displacement is added to a displacement output of the laser interferometer displacement measuring system, thereby correcting the distortion error in the interferometer displacement measuring system.
Upon measurement of a continuously moving object as a measurement target, the laser interferometer displacement measuring system according to the present invention stores and corrects, as a measurement error caused by the interference effect, an oscillatory component that appears in the cycle consistent with the frequency of laser light, thereby implementing an increased accuracy. Even such a correction method for allowing a relatively simple sinusoidal wave to be added to or subtracted from a measurement value can reduce the range of error of absolute position about ±2 nm to within ±1 nm, thereby making it possible to provide an increased accuracy.
That is, a laser interferometer displacement measuring system according to the present invention is characterized by comprising a displacement measurement mechanism making use of laser interference, and corrector means for adding a correction value to or subtracting the correction value from a measurement value of the displacement measurement mechanism. The corrector means uses a cyclic correction value having a cycle corresponding to a wave cycle of laser light.
Furthermore, a laser interferometer displacement measuring system according to the present invention is characterized by comprising a displacement measuring mechanism making use of laser interference, and corrector means for adding a correction value to or subtracting the correction value from a measurement value of the displacement measurement mechanism. The corrector means has storage means for storing a cyclic correction value having a cycle corresponding to a wave cycle of laser light, and the correction value is read out of the storage means in accordance with the measurement value and is added to or subtracted from the measurement value. It is possible to employ a rewritable memory as the storage means.
Furthermore, a laser interferometer displacement measuring system according to the present invention comprises a laser light source, an interferometer for dividing laser light of wavelength &lgr; emitted from the laser light source into a reference path beam and a measurement path beam to interfere the reference path beam with the measurement path beam having been reflected from a subject body, a light detector for detecting the light subjected to the interference in the interferometer, and measurement value output means for converting a detection signal of the light detector into a measurement value to output the resulting value. In the system, a displacement of the subject body causes an n-fold variation in length of an optical path between the interferometer and the subject body. The laser interferometer displacement measuring system is characterized by further comprising corrector means for adding a correction value to or subtracting the correction value from the measurement value of the measurement value output means. The system is also characterized in that, with the measurement value being employed as a variable, the corrector means uses, as the correction value, a cyclic function having a cycle of &lgr;
or a sum of a plurality of cyclic functions having the cycle of &lgr;
as a fundamental cycle. The plurality of cyclic functions having the cycle of &lgr;
as a fundamental cycle can be the cyclic function having a cycle of &lgr;
and harmonic cyclic functions thereof.
The aforementioned laser interferometer displacement measuring system according to the present invention can comprise means for performing feedback control so as to carry out tracking adjustment of a phase and amplitude of the correction value.
A laser interferometer displacement measuring system according to the present invention is characterized by comprising a displacement measurement mechanism making use of laser interference, and corrector means for adding a correction value to or subtracting the correction value from a measurement value of the displacement measurement mechanism. The corrector means prepares a plurality of types of cyclic functions having a cycle corresponding to a wave cycle of laser light, and assigns weights to each of the cyclic functions to allow the resulting cycl
Hosaka Sumio
Isshiki Fumio
Sugaya Masakazu
Suzuki Tatsundo
Yamaoka Masahiro
Antonelli Terry Stout & Kraus LLP
Hitachi , Ltd.
Pyo Kevin
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