Coherent light generators – Particular resonant cavity – Specified cavity component
Patent
1997-10-01
2000-08-08
Arroyo, Peresa M.
Coherent light generators
Particular resonant cavity
Specified cavity component
372108, H01S 308
Patent
active
061012114
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a narrow-band laser apparatus employed as a light source for a semiconductor exposure tool for optical microlithography process.
BACKGROUND ART
Owing to the increasingly higher densities of integration in the arrangement of semiconductor elements, semiconductor exposure tools are being required to have higher resolution and deeper depth of focus. To achieve this, there is a trend towards use of shorter wavelengths, specifically, there has been a progressive shift towards shorter wavelengths for the light sources of semiconductor exposure tools, from the g line of high-pressure mercury lamps to the i line, and further, to KrF excimer lasers.
However, when the deep ultraviolet region, such as a KrF excimer laser (248 nm) or ArF excimer laser (193 nm) is employed as the generated laser beam, only a few types of optical material are suitable for use in the projection lens so that it is difficult to correct chromatic aberration. Consequently, in such excimer lasers, a monochromatic lens in which correction for chromatic aberration is not performed is employed as projection lens, and exposure tool light sources are employed in which monochromaticity is raised by narrowing the bandwidth of the excimer laser itself.
However, a narrow band laser apparatus in which all of the output light from the laser generating section 160 is input to a band-narrowing element as shown for example in FIG. 19 suffers from drawbacks such as that the laser beam output becomes small because of large losses in the band-narrowing element and that there are problems of durability due to the large load on the band-narrowing element. This trend is particularly marked in the case of a short-wavelength ArF narrow-band excimer laser. In FIG. 19, reference numeral 160 is the laser generating section, 161 is the band-narrowing element, 162 is a total reflection mirror, and 163 is a half mirror.
Accordingly, in the excimer laser technique illustrated in Japanese patent publication (kokai) No. 3-259583, an attempt to solve these problems is made by dividing the laser beam by a dividing mirror, inputting part of the divided laser beam to a band-narrowing element, and outputting remaining part as laser output: the construction is shown in FIG. 20-FIG. 21.
In FIG. 20, at one end of a laser tube 131 containing laser medium, there are provided a control plate 136 having a window 134 and slit 136a and a high-reflectivity mirror 135, while, at the other end of laser tube 131, there are provided a window 134, dividing mirror 137, first etalon 138, second etalon 139, high reflectivity fold-back mirror 141, beam splitter 142, dispersion plate 143, monitor etalon 144, condensing lens 145, linear line sensor 146, and oscilloscope 147.
In such a construction, some of the laser beam that is output from the left-hand window 134 is divided and reflected by dividing mirror 137 and is subjected to band-narrowing by passing through first etalon 138 and second etalon 139. The laser beam that has been subjected to band narrowing is reflected by high-reflectivity folding-back mirror 141 and dividing mirror 137 so that it is folded back to the laser excitation zone 132 in laser tube 131. After being amplified in laser excitation zone 132, the laser beam again passes through the same optical path as described above, but diverging with an angle .theta., before being emitted through left-hand window 134. Also, part of the laser beam that is emitted is reflected by dividing mirror 137 to be input once more to first etalon 138 and second etalon 139, the remaining part being output as output laser beam L. Part (about 1%) of output laser beam L is reflected at beam splitter 142 and input to linear line sensor 146, so that the intensity distribution of the laser beam can be monitored using the output of the sensor.
With this conventional laser apparatus, an output of high spectral purity containing few ASE (amplified spontaneous emission) components is sought to be obtained by avoiding the use of optical components such as b
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IEE Journal of Quantum Electronics, Direct Bandwidth and Polarization Control of an XEF Unstable Resonator Laser, Nov., 1988, vol. 24, No. 11, pp. 2270-2283.
Komori Hiroshi
Mizoguchi Hakaru
Wakabayashi Osamu
Arroyo Peresa M.
Komatsu Ltd.
Wise Robert E.
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