Optics: measuring and testing – By light interference – Spectroscopy
Reexamination Certificate
2000-06-21
2003-01-21
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Spectroscopy
C356S519000
Reexamination Certificate
active
06509970
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a wavelength monitoring apparatus for laser light for semiconductor exposure. More particularly, the present invention relates to a wavelength monitoring apparatus for laser light for semiconductor exposure that measures the wavelength of laser light for semiconductor exposure relative to reference light, whose wavelength is known, simultaneously with the measurement of the reference light.
With the achievement of small and fine semiconductor integrated circuits, the wavelength of semiconductor exposure light is becoming shorter. Accordingly, ArF excimer laser of wavelength 193.4 nm or fluorine laser of wavelength 157 nm is promising as a next-generation light source for semiconductor lithography. Regarding ArF excimer laser, its spectral line width is generally wide, i.e. of the order of 400 pm. However, as optical materials usable for exposure apparatus in the vacuum ultraviolet region, there are only synthetic quartz and fluorite, and it is extremely difficult to achromatize the projection optical system of the exposure apparatus. To avoid the problem of chromatic aberration in the projection optical system of the exposure apparatus, it is necessary to narrow down the spectral line width to 1 pm or less, and it is essential to stabilize the wavelength so that variations in the center wavelength are within ±0.1 pm.
Fluorine laser also needs to narrow the spectral line width when it is used in an exposure apparatus having a projection optical system.
Narrowing of the spectral line width is realized, for example, by a spectral width-narrowing optical system including a beam diameter-enlarging prism and a diffraction grating. Wavelength selection is performed by controlling the angle of incidence of light on the diffraction grating.
Realization of the above-described wavelength stabilization requires a wavelength monitoring apparatus for measuring the wavelength and spectral line width of the narrowed laser light for semiconductor exposure during exposure and for feedback-controlling the laser on the basis of data obtained by the measurement.
FIG. 4
shows the arrangement of a wavelength monitoring apparatus available from Burleigh Instruments, Inc. as a conventional wavelength monitoring apparatus of the type described above. In this apparatus, reference light from a He—Ne laser of wavelength 632.8 nm used as a standard light source is incident on a beam splitter via a reflecting mirror and a shutter A. The reference light is reflected by the beam splitter to reach an etalon via a reflecting mirror and a concave mirror. In the etalon, the reference light is subjected to multiple beam interference and then passes through a focusing lens to form interference fringes of concentric circles or parallel lines on the back focal plane of the focusing lens. A linear array sensor (CCD), which is placed in the back focal plane, corrects variations in the refractive index of air in the etalon and variations in the mirror spacing on the basis of data concerning the position of each fringe on the CCD. Next, the shutter A is closed, and light under wavelength measurement from, for example, an ArF excimer laser, for semiconductor exposure, which has a wavelength in the vicinity of 193.4 nm is input through an entrance aperture and a shutter B from the right-hand side as viewed in the figure. The laser light passes through the beam splitter to reach the etalon via the reflecting mirror and the concave mirror. The laser light is subjected to multiple beam interference in the etalon and passes through the focusing lens to form interference fringes of concentric circles or parallel lines on the CCD placed in the back focal plane of the focusing lens. The wavelength of the light under measurement is calculated from data concerning the positions on the CCD of the fringes of the light under measurement.
In the conventional wavelength monitoring apparatus, the fringes produced by the standard light source (He—Ne laser) and the fringes by the excimer laser for semiconductor exposure are formed in the same area on the CCD. If it is intended to observe these fringes simultaneously (both the shutters A and B are opened to allow the reference light and the light under wavelength measurement to enter the system), the fringes of the two light beams undesirably overlap each other on the linear array sensor. Accordingly, it becomes difficult to measure the positions of the fringes accurately. For this reason, each fringe pattern has to be measured individually in the conventional apparatus. Consequently, there is an interval of time between the calibration of the etalon using the fringes produced by the standard light source (i.e. the measurement of the fringe positions) and the wavelength measurement using the fringes of the excimer laser for semiconductor exposure (i.e. the measurement of the fringe positions) . Accordingly, there is a possibility that an error will occur in the measurement.
In the wavelength monitoring apparatus shown in
FIG. 4
, a He—Ne laser of high frequency stability, for example, is used as a standard light source. Because such a laser needs an unfavorably long time to become stabilized from start-up, it is necessary to make the laser oscillate continuously from the start-up to the completion of measurement. Therefore, it is necessary to provide a shutter mechanism (shutter A) for blocking light between the He—Ne laser and the etalon so that the fringes produced by the He—Ne laser are not formed on the CCD during the wavelength measurement of the excimer laser for semiconductor exposure. As the repetition frequency of the excimer laser increases, it becomes impossible for the open-close operation of the shutter A to follow up the repetition frequency if it is intended to perform wavelength measurement for each shot of the excimer laser.
Regarding the etalon coating, He—Ne laser light and excimer laser light for semiconductor exposure pass through a single etalon. Therefore, reflecting mirrors that constitute the etalon require a dielectric multilayer coating for two wavelengths: the wavelength of He—Ne laser light, i.e. 632.8 nm, and the wavelength of ArF excimer laser light, i.e. about 193 nm. It is difficult and costs a great deal to make a coating that provides satisfactory reflectance and low-loss characteristics for both the wavelengths.
These problems are associated with not only the ArF excimer laser but also the fluorine laser.
SUMMARY OF THE INVENTION
In view of the above-described problems with the prior art, an object of the present invention is to provide a wavelength monitoring apparatus which is capable of measuring both standard light and laser light for semiconductor exposure simultaneously and highly accurately, without a time lag, and which does not require a shutter mechanism for switching between the standard light and the laser light for semiconductor exposure but only needs to provide an etalon with coatings exhibiting satisfactory reflectance and low-loss characteristics for the standard light and the laser light for semiconductor exposure, respectively.
To attain the above-described object, the present invention provides a wavelength monitoring apparatus for laser light for semiconductor exposure. The wavelength monitoring apparatus includes an entrance-side optical system for making the laser light for semiconductor exposure and reference light incident on different areas of a single etalon in the form of diverging light, converging light or diffused light in such a manner that the respective center axes of the laser light and the reference light are displaced relative to each other. Two focusing optical systems are provided in approximately coaxial relation to the respective center axes of the laser light and reference light passing through the etalon. A one-dimensional array optical sensor is placed in a plane coincident with the back focal planes of the focusing optical systems to receive interference fringes produced by the laser light and the reference light. The positions of the interf
Hotta Kazuaki
Kobayashi Takao
Seki Kyohei
Connolly Patrick
Dellett & Walters
Turner Samuel A.
Ushio Denki Kabushiki Kaisya
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