Coherent light generators – Particular beam control device – Optical output stabilization
Patent
1989-09-20
1992-08-25
Scott, Jr., Leon
Coherent light generators
Particular beam control device
Optical output stabilization
372 57, 372 28, 372 19, 372 59, H01S 313
Patent
active
051425432
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to methods and systems for controlling a narrow-band oscillation excimer laser, and more particularly, to a method for controlling an excimer laser that is used as a light source of a reduced projection exposer.
BACKGROUND ART
Much attention has been focused on the utilization of an excimer laser as a light source of a reduced projection exposer for fabrication of semiconductor devices. This is because the utilization of the excimer laser as such a light source can expect many excellent advantages including the limit wavelength of exposure light reduced to below 0.5 .mu.m because of the short wavelength of the excimer laser (the wavelength of a KrF laser being about 248.4 nm), a focus depth larger than that for the g or i rays of a prior art mercury-vapor lamp so long as a resolution power is identical, a small lens numerical) aperture (NA), an enlarged light exposure area, and a high output power.
However, the excimer laser has two big problems when used as the light source of the reduced projection exposer.
One of the problems is that, since the output light wavelength of the excimer laser is as short as 248.35 nm, this limits the materials through which this wavelength of light can pass, to only quartz, CaF.sub.2, and MgF.sub.2, and further among these materials, only quartz can be used as the material of the lens, when taking its homogeneity and machining accuracy into consideration. For this reason, it becomes impossible to design the reduced projection lens subjected to a chromatic aberration correction. Accordingly, this requires the band of the excimer laser to be narrowed to such an extent that the chromatic aberration is negligible.
The other problem is to suppress an undesired speckle pattern developed by narrowing the band of the excimer laser and also to avoid reduction in the laser power caused by the narrowed band.
One of techniques for narrowing the band of an excimer laser is known as the injection lock system. In the injection lock system, wavelength selective elements (such as etalons, diffraction gratings or prisms) are disposed with a cavity at an oscillator stage so that pin holes limits its spatial modes, the laser oscillates in a single mode, and the oscillated laser beam is synchronously injected into its amplification state. For this reason, the output laser beam is highly coherent and thus when such output beam is used as the light source of the reduced projection exposer, an undesired speckle pattern takes place. Generally speaking, the generation of a speckle pattern is considered to depend on the number of spatial transversal modes contained in the laser beam. More specifically, it is known that a smaller number of spatical transversal modes included in the laser beam tends to more develop a speckle pattern, whereas a larger number of spatial transversal modes tends to less develope a speckle pattern. The aforementioned injection lock system, which is a technique for achieving a narrow band essentially by remarkably decreasing the number of spatial transversal modes, cannot be employed in the reduced projection exposer, since it involves the development of a speckle pattern as a big problem.
There is another promising technique for narrowing the band of an excimer laser which uses etalons. As a prior art of such etalon-based techniques, there has been proposed such a technique from the AT & T Bell Laboratory that etalons are disposed between a front mirror and a laser chamber in an excimer laser to realize the narrowed band of the excimer laser. This system, however, has had problems and defects that a spectrum line width cannot be narrowed satisfactorily, a power loss due to the etalon insertion is large, and the number of spatial transversal mode cannot be made considerably large.
To solve the above problems, the inventors of the present application provide such an arrangement that wavelength selective elements having a large effective diameter (about several ten mm) such as etalons are interposed between a rear mirror and a
REFERENCES:
patent: 4823354 (1989-04-01), Znotins et al.
patent: 4829536 (1989-05-01), Kajiyama et al.
patent: 4905243 (1990-02-01), Lokai et al.
Kowaka Masahiko
Wakabayashi Osamu
Jr. Leon Scott
Kabushiki Kaisha Komatsu Seisakusho
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