Coherent light generators – Particular pumping means – Electrical
Reexamination Certificate
2002-08-22
2004-10-26
Wong, Don (Department: 2828)
Coherent light generators
Particular pumping means
Electrical
Reexamination Certificate
active
06810061
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a discharge electrode used in a laser apparatus in which a laser gas containing a halogen gas is excited by a discharge performed between electrodes, and a discharge electrode manufacturing method.
2. Description of the Related Art
FIGS. 27A and 27B
are sectional views of an excimer laser apparatus or fluorine molecular laser apparatus.
In a gas laser apparatus such as an excimer laser apparatus or fluorine molecular laser apparatus, a laser gas is sealed inside a laser chamber
10
. A mixed gas comprised of a rare gas (krypton Kr in the case of a KrF excimer laser, argon Ar in the case of an ArF excimer laser, and the like), a halogen gas (fluorine F
2
or the like) and a buffer gas (neon Ne) is used as the laser gas. In order to cause laser oscillation by exciting this laser gas, main discharge electrodes (cathode and anode)
2
and
3
which are disposed facing each other across the optical axis L of the laser light, and preparatory dissociation electrodes
41
and
42
which cause preparatory dissociation of the laser gas between the main discharge electrodes
3
and
3
in order to facilitate the generation of a discharge between the main discharge electrodes
2
and
3
, are disposed inside the laser chamber
10
.
In the case of such a gas laser apparatus, as a result of the preparatory dissociation of the laser gas between the main discharge electrodes
2
and
3
by the preparatory dissociation electrodes
41
and
42
, a discharge is generated between these main discharge electrodes
2
and
3
, and the laser gas is thus excited so that a laser oscillation is generated. Furthermore, as is universally known, the generation of a stable discharged between the main discharge electrodes
2
and
3
results in a stable laser oscillation, so that a stable laser output is obtained.
The main discharge electrodes used in such a gas laser apparatus include the electrodes described in Japanese Utility Model Application Laid-Open No. 61-174764 (hereafter referred to as “Reference 1”), Japanese Patent Application Laid-Open No. 62-199078 (hereafter referred to as “Reference 2”), and Japanese Patent Application Laid-Open No. 63-227069 (hereafter referred to as “Reference 3”).
In the electrodes described in the abovementioned Reference 1, the side surface portions of the main discharge electrodes other than the portions of the surfaces of the electrodes where a discharge is performed (hereafter referred to as the “discharge portions”) are closely covered with an insulating material in order to prevent a discharge from occurring between the main discharge electrodes and the arc discharge electrodes (corresponding to the preparatory dissociation electrodes) even in cases where the gap between the main discharge electrodes and the arc discharge electrodes is short.
Furthermore, in the electrodes described in the abovementioned Reference 2, at least portions of the laser tube or discharge members are coated with a halogen corrosion-resistant resin layer in order to suppress problems such as corrosion of the inside walls of the laser tube or discharge members, deterioration of the sealed gas and the like caused by the generation of strong ultraviolet radiation, ions, electrons and the like in large quantities in the vicinity of the main discharge electrodes.
Furthermore, in the electrodes described in the abovementioned Reference 3, insulators are mounted on the end portions, e.g., the local surface portions of the end portions, of the main discharge electrodes so that a stable glow discharge is obtained at the flat surface portions located in the centers of the main discharge electrodes, and so that insulation breakdown and arcing in the end portions of the main discharge electrodes are prevented.
Furthermore, main discharge electrodes other than the electrodes described in these references include main discharge electrodes in which the cathode surfaces in the main discharge electrodes constructed from an anode and cathode are coated with a dielectric thin film. In such electrodes, deterioration of the anode (deformation of the electrode) caused by discharge bombardment is alleviated by utilizing the drop in the discharge initiation voltage that occurs as a result of the cathode surfaces being coated with a dielectric thin film.
However, in cases where the main discharge electrodes of the abovementioned References 1 through 3 are actually installed inside the laser chamber of an excimer laser apparatus such as a krypton-fluorine (KrF) excimer laser, argon-fluorine (ArF) excimer laser or the like, or the laser chamber of a fluorine molecular (F
2
) laser apparatus, a reaction occurs between the halogen gas (fluorine or the like) contained in the laser gas and the discharge part of the anode as a result of the repetition of the laser oscillation operation, so that the anode is halogenated (fluorinated in the case of fluorine). Furthermore, as a result of discharge bombardment and heat, the discharge part of the anode is deformed from a flat state into a shape with indentations and projections. Moreover, the discharge part of the cathode is also halogenated, although to a slight degree compared to the anode, and the discharge part of the cathode also deteriorates as a result of sputtering.
As a result, the discharge between the main discharge electrodes becomes unstable, so that the output energy of the laser drops, thus making it impossible to obtain the desired laser output characteristics.
In order to counter this problem, it is necessary to take measures such as raising the gas pressure inside the laser chamber, raising the voltage that is applied across the main discharge electrodes, or the like. In some cases, it may be necessary to replace the deteriorated main discharge electrodes. Furthermore, even in cases where the deteriorated electrodes are replaced with fresh electrodes, problems similar to those described above still occur; as a result, there is a frequent electrode replacement cycle. When the electrodes are replaced, the entire laser chamber must be re-assembled, so that the working characteristics are extremely poor; this results in the problem of increased maintenance costs.
Even in the case of main discharge electrodes in which the cathode surface is coated with a dielectric thin film, the discharge part of the anode where a discharge is performed between the anode and cathode is not coated with such a thin film. Accordingly, as was described above, stable laser output characteristics cannot be obtained because of degeneration of the electrodes (anode) caused by halogenation (e.g., fluorination) of the electrode material, and deterioration of the anode (deformation of the electrode) caused by discharge bombardment.
Thus, if the abovementioned conventional techniques are used, a metal fluoride film is formed on the surfaces of the main discharge electrodes in proportion to the number of laser shots, so that there is an effect on the stability of the main discharge. If the stability of the main discharge reaches a range that is not permitted from the standpoint of laser performance, the electrodes must be replaced.
In regard to this problem, W/O 01/97344, U.S. Patent Application Laid-Open No. 2001/50939 (hereafter referred to as “Reference 4”) indicates the following:
The metal fluoride film (copper fluoride in the case of copper electrodes) that is formed on the electrode discharge parts increases in proportion to the number of laser shots, and when this film reaches approximately 50 to 80% of the electrode discharge part regions, this is viewed as being the useful life of the electrodes. In cases where the main discharge is performed in regions that are covered by a metal fluoride film, the current flows through small holes with a diameter of 50 to 100 &mgr;m. In regions where a metal fluoride film is formed, there is no further progression of erosion caused by fluorine. However, as the regions that are covered by a metal fluoride film in the discharge parts become smaller, the rate of
Chiba Teiichirou
Fujimoto Junichi
Hori Tsukasa
Mizoguchi Hakaru
Sumitani Akira
Komatsu Ltd.
Menefee James
Varndell & Varndell PLLC
Wong Don
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