Coherent light generators – Particular active media – Gas
Reexamination Certificate
1997-07-11
2001-08-28
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular active media
Gas
C372S087000, C372S038060
Reexamination Certificate
active
06282221
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an excimer laser oscillation apparatus used in processing various works, and its driving method.
2. Related Background Art
An excimer laser has received a lot of attention as the one and only high-power laser that can oscillate in the ultraviolet range, and its applications are expected in the electronics, chemical, and energy industries.
More specifically, the excimer laser is used in processes, chemical reactions, and the like of metals, resins, glass, ceramics, photoresists, semiconductors, and the like.
An apparatus that generates an excimer laser beam is known as an excimer laser oscillation apparatus.
FIG. 15
shows an example of the arrangement of the apparatus.
A pair of electrodes
54
and
55
are arranged in a laser chamber
51
for storing a laser gas. A voltage application circuit for applying a voltage to excite the laser gas is connected to the pair of electrodes
54
and
55
.
A laser gas mixture containing, e.g., F
2
, Ar, Kr, Xe, Cl
2
, and the like filled in the laser chamber is excited by electron beam radiation, discharging, or the like. The excited atoms bind to atoms in the ground or base bottom state to form molecules that can exist in only an excited state. Such molecules are called excimers. Since the excimers are unstable, they immediately emit ultraviolet rays and drop to the ground state. Such phenomenon is called bond-free transition, and the excimer laser oscillation apparatus amplifies ultraviolet rays obtained by that transition in an optical resonator made up of a pair of reflection mirrors and outputs a laser beam.
Since a KrF laser and ArF laser of excimer lasers use highly reactive fluorine gas as a laser gas, the concentration of fluorine in the laser chamber that stores the laser gas and gives discharging energy to the gas drops eventually. In consideration of this, the voltage supplied to the laser chamber is raised so as to obtain a predetermined output. When the predetermined output becomes hard to obtain even by such control, oscillation is interrupted, and fluorine gas is refilled.
When oscillation continues, the predetermined laser output cannot be obtained, even by refilling fluorine, and the laser chamber must be exchanged in such state.
FIG. 16A
shows an example of a conventional laser oscillation circuit. When a thyratron is used as a switch for starting oscillations, after ringing components of a capacitance C
p
shown in
FIG. 16B
are produced. In view of this problem, the thyratron of the laser oscillation circuit is replaced by a thyristor as a semiconductor element to suppress the after ringing components and to prolong the service life of the laser, as shown in FIG.
17
A. Note that no light emission occurs at a positive C
p
voltage of +5 to +6 kV in
FIGS. 16B and 17B
since no discharging takes place and no currents flow at that voltage.
However, even with this apparatus, as shown in
FIG. 18
, fluorine must be refilled at predetermined periods, and oscillations must be done while raising the applied voltage. In other words, fluorine refilling is still required since the fluorine concentration decreases as time passes. For this reason, the service life of the laser chamber is not satisfactory, especially when the laser is used for a long period of time to work articles. The service life of the chamber is an important factor in improving the manufacturing throughput of articles.
SUMMARY OF THE INVENTION
The present invention has been made in consideration of the above-mentioned technical problems and has as its principal object to provide a laser oscillation apparatus which can obtain a desired laser output even after laser oscillations are performed for a longer period of time than the conventional apparatus.
It is another object of the present invention to provide a laser oscillation apparatus which is suitable for the manufacture of semiconductor devices such as photolithography.
According to the present invention, a method of driving an excimer laser oscillation apparatus which has a laser chamber for storing a laser gas, a pair of electrodes arranged in the chamber, and a voltage application circuit for applying a voltage to the pair of electrodes to excite the laser gas, is characterized in that the voltage is applied to change the direction of a current flowing between the pair of electrodes at a predetermined period.
According to the present invention, an excimer laser oscillation apparatus which has a laser chamber for storing a laser gas, a pair of electrodes arranged in the chamber, and a voltage application circuit for applying a voltage to the pair of electrodes to excite the laser gas, is characterized by comprising:
first charge accumulation means for supplying a current in a first direction between the pair of electrodes; and
second charge accumulation means for supplying a current in a second direction different from the first direction.
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Ohmi Tadahiro
Tanaka Nobuyoshi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Jr. Leon Scott
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