Coherent light generators – Particular active media – Gas
Reexamination Certificate
1998-08-27
2001-03-27
Davie, James W. (Department: 2881)
Coherent light generators
Particular active media
Gas
C415S111000
Reexamination Certificate
active
06208675
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to laser systems and, more specifically, to a blower assembly for circulating gases within a laser discharge chamber of the laser system.
BACKGROUND OF THE INVENTION
Pulsed laser systems, such as excimer lasers, are well known.
FIG. 11
is an end cross sectional view of a laser chamber, generally illustrated as
10
, used in a conventional pulsed laser system. The laser chamber
10
includes an electrode structure
22
defining an electrical discharge area
28
, a heat exchanger
60
, and a blower assembly
70
. As is well known by those skilled in the art, the pulsed laser system produces energy pulses from a gas mixture that is between the electrode structure
22
. The mixture of gas, which typically includes krypton and fluorine, is maintained at a high pressure (e.g., 3 atm). The electrode structure
22
ionizes the gas mixture to produce a high energy discharge. A life cycle of the gas mixture is measured by the total number of high energy discharges that the gas mixture can produce. Typically the life cycle ranges from about 100 to about 200 million discharges.
The blower assembly
70
plays the important role of circulating the gases in the laser chamber
10
of pulsed laser systems. The circulation of the gases harbors may purposes, among them which include, but are not limited to, maintaining the temperature of the gases at the most efficient level of reaction, maximizing the life cycle of the gases, and facilitating the overall operation of the pulsed laser system. As mentioned above, krypton and fluorine are gases commonly employed by the laser chamber
10
. These gases, however, may adversely affect the mechanical operation of the blower assembly
70
, as well as the performance of the pulsed laser system. To pose the problem more concretely, by way of example, the blower assembly
70
is generally defined by a mechanical structure which includes a motor coupled to a shaft by a lubricated bearing assembly such as a ball bearing. The shaft rotates a fan for the circulation of the gases. The bearing assembly has conventionally been manufactured from ferrous material such as 440C stainless steel. The use of ferrous metals harbors a variety of problems. First, the gases, i.e., krypton and fluorine, are capable of corroding and etching the structure of the bearing assembly, and therefore, diminishing the mechanical integrity of the bearing assembly. Second, fluorine reacts with iron, forming iron (III) fluoride particles (FeF
3
) which contaminate the laser chamber
10
. The iron (III) fluoride particles interfere with the ionization of the gases by the electrode structure
22
for the production of the high energy discharges. Third, the production of iron (III) fluoride also catalyses the degradation of the lubricant used with the bearing assembly. More specifically, perfluoropolyalkylether (PFPE) synthetic oils, such as Krytox 143AB, manufactured by E. I. Du Pont Company, are typically used to lubricate the bearing assembly. The iron (III) fluoride, a Lewis acid catalyst, degrades the PFPE fluid at asperity contact temperatures of up to about 350° C. The scheme for the degradation of PFPE through an autocatalytic pathway is illustrated in
FIG. 12. R
f
and R′
f
in
FIG. 12
are PFPE end groups of an unspecified length. The degradation causes not only the production of volatile acyl fluoride and ketone compounds but also the reduction of the average molecular weight of the lubricant. The degraded product escapes from the bearing assembly, causing mechanical wear and failure of the bearing assembly.
The contamination of the laser chamber
10
with iron (III) fluoride particles diminishes the performance of the pulsed laser system. The requirement to continually replace or re-passivate the ferrous metal bearing, in order to effectively operate the blower assembly
70
, reduces the gas life cycle of the laser chamber
10
. As a result, the overall efficiency and production of the pulsed laser system is vitiated.
SUMMARY OF THE INVENTION
The present invention broadly provides a laser chamber for a compact excimer laser. The laser chamber of the present invention, more specifically, has an electrode structure defining an electrical discharge area for producing a high energy discharge. The high energy discharge ionizes gases, such as krypton and fluorine, and causes the gases to react chemically. A pre-ionizer may be disposed near the electrical discharge area to facilitate the ionization of the gases. A main insulator is also disposed adjacent to the electrical discharge area to insure that a proper electrical discharge is conducted by the electrode structure. The high energy discharge produces a large amount of local heating in the gases, and accordingly, a head exchanger is provided to reduce the temperature of the gases. The laser chamber further includes a blower assembly for the proper circulation and the efficient flow of gases during the operation of the excimer laser.
The blower assembly of the present invention preferably comprises a drive side shaft supported by the laser chamber, a driving assembly operatively engaged to the drive side shaft for rotating the drive side shaft, and a fan assembly engaged to the drive side shaft for circulating the gases. The blower assembly further includes an idle side shaft for supporting the fan assembly. The drive side shaft, the idle side shaft and the fan assembly are manufactured from materials, such as nickel plated aluminum, monel, tin, etc., which are resistant to erosion when exposed to the gases. The blower assembly further includes bearing assemblies circumscribing the drive and idle side shafts for rotatably supporting the drive and idle side shafts to the laser chamber. The bearing assemblies are manufactured from a ceramic compound, such as silicon nitride (Si
3
N
4
). The bearing assemblies, moreover are lubricated with a synthetic lubricant, such as perfluoropolyalkylether (PFPE).
The present invention, moreover, broadly provides a method for circulating gases in a laser chamber comprising:
a) disposing a blower assembly within the laser chamber, the blower assembly comprising a drive side shaft, a fan assembly engaged to the drive side shaft for circulating gases, and a bearing assembly circumscribing the drive side shaft, wherein the bearing assembly comprises a ceramic compound; and
b) rotating the drive side shaft to operatively drive the fan assembly and to circulate gases between the fan assembly and in the laser chamber.
The gases of the laser chamber also circulate against the bearing assembly. The ceramic quality protects the bearing assembly against any essential corrosion. Moreover, the ceramic compound does not react with the gases so as to contaminate the gases or adversely affect the chemical structure of the lubricant.
These, together with the various ancillary advantages and features which will become apparent to those skilled in the art as the following description proceeds, are attained by this novel blower assembly for a pulsed laser system and this novel method, a preferred embodiment thereof shown with reference to the accompanying drawings, by way of example only, wherein:
REFERENCES:
patent: 4959840 (1990-09-01), Akins et al.
patent: 5206873 (1993-04-01), Funakubo et al.
patent: 5291509 (1994-03-01), Mizoguchi et al.
patent: 5771258 (1998-06-01), Morton et al.
patent: 4-109685 (1992-04-01), None
patent: 5-21866 (1993-01-01), None
patent: 10-173259 (1998-06-01), None
Patent Abstracts of Japan, Publication No. 05021866 A, publication date Jan. 29, 1993, 1 page.
Patent Abstracts of Japan, Publication No. 04109685 A, publication date Apr. 10, 1992, 1 page.
Patent Abstracts of Japan, Publication No. 10173259 A, publication date, Jun. 26, 1998, 1 page.
Carre, “The Use of Solid Ceramic and Ceramic Hard-Coated Components to Prolong the Performance of Perfluoropolyalkylether Lubricants,”Surface and Coatings Technology,43/44 (1990) 609-617. (No Month).
Cymer Inc.
Davie James W.
Ogonowsky, E Brian D.
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