Coherent light generators – Particular active media – Gas
Patent
1990-02-26
1992-01-14
Scott, Jr., Leon
Coherent light generators
Particular active media
Gas
372 31, 372 32, 372 33, 372 98, 372 20, H01S 322
Patent
active
050816357
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates generally to an apparatus for controlling an output from a narrow band excimer laser device. More particularly, the present invention relates to an apparatus for controlling an output from an excimer laser device wherein the apparatus is preferably employable as a light source particularly for an unit for projecting and exposing figures in a reduced size.
BACKGROUND ART
Attention has been paid to an excimer laser device which is usable as a light source for an unit for projecting and exposing figures in a reduced size for the purpose of producing semiconductor devices. This is attributable to many remarkable advantages derived from the facts that it is possible to extend a limit of light exposure to a level of wavelength of excimer laser shorter than 0.5 micron because an excimer laser has a short wavelength (e.g., KrF laser has a wavelength of about 248.4 nm). the excimer laser device has a deep focus depth compared with a g line and an i line emitted from a mercury lamp which has been heretofore used, provided that each device has a same resolution, the number of lens apertures (NA) can be reduced, an exposing range can be widened and large power can be generated by the device.
However, the excimer laser device has two significant problems to be solved when it is used as a light source for an unit for projecting and exposing figures in a reduced size.
One of them is that the excimer laser has a short wavelength of 248.35 nm and only quartz can be used as a starting material for lenses in view of uniformity and machining accuracy due to the fact that material through which it can transmit is limited only to quartz, CaF.sub.2 and MgF.sub.2. This makes it impossible to design a lens for projecting figures in a reduced size with its color aberration corrected. Therefore, there is a need of narrowing an operative range of the excimer laser device to such an extent that any color aberration can be neglected.
The other problem to be solved is how to prevent a speckle pattern from appearing as the operative range of the excimer laser device is narrowed and how to suppress reduction of power caused as the operative range of the excimer laser device is narrowed.
As an prior technique of narrowing the operative range of the excimer laser device, there is known a so-called injection lock process. This injection lock process is such that a wavelength selection element (etalon, diffraction grating, prism or the like) is disposed in a cavity at an oscillator stage, laser light is oscillated in a single mode while a spatial mode is restricted by a pin hole, and the thus oscillated laser light is synchronously introduced into the excimer laser device via an amplifying stage. This arrangement allows laser light outputted from the excimer laser device to have a high property of coherence. Thus, when the excimer laser device operable in accordance with the injection lock process is used as a light source for an unit for projecting and exposing figures in a reduced size, a speckle pattern is generated. In general, it has been considered that generation of a speckle pattern depends on the number of spatial lateral modes involved in laser light. Further, it has been known that when the spatial lateral modes involved in the laser light has a small number, a speckle pattern is liable to appear and that, to the contrary, when the number of spatial lateral modes increase, the speckle pattern hardly appears. The above-described injection lock process is substantially concerned with a technique for narrowing the operative range of the excimer laser device by largely reducing the number of spatial lateral modes. However, since the injection lock process has a significant problem of generation of a speckle pattern, the excimer laser device can not be employed for an unit for projecting and exposing figures in a reduced size.
In addition, as other promising technique for narrowing the operative range of the excimer laser device, there is known a technique of using etalons each serving a
REFERENCES:
patent: 4829536 (1989-05-01), Kajiyama et al.
patent: 4926428 (1990-05-01), Kajiyama et al.
patent: 4947398 (1990-08-01), Yasuda et al.
patent: 4977563 (1990-12-01), Nakatani et al.
patent: 4991178 (1991-02-01), Wani et al.
Ito Noritoshi
Kowaka Masahiko
Wakabayashi Osamu
Jr. Leon Scott
Kabushiki Kaisha Komatsu Seisakusho
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