Coherent light generators – Particular pumping means – Electrical
Reexamination Certificate
2000-12-08
2002-11-12
Paladini, Albert W. (Department: 2827)
Coherent light generators
Particular pumping means
Electrical
C372S055000, C372S086000
Reexamination Certificate
active
06480519
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns a gas laser device that emits ultraviolet rays, especially a gas laser device that emits ultraviolet rays, such as an excimer laser device having high oscillation efficiency.
2. Description of Related Art
Higher resolution is demanded of projection exposure equipment as the miniaturization and integration of semiconductor integrated circuits rise. Consequently, the wavelength of exposure light emitted from exposure light sources is becoming shorter, and gas laser devices that emit ultraviolet rays, such as ArF excimer laser devices or fluorine laser devices, would be viable candidates for the next generation of semiconductor exposure light sources.
Mixed gas comprising fluorine (F
2
) gas, argon (Ar) and noble gases, such as neon (Ne), as a buffer gas in an ArF excimer laser device, or mixed gas comprising fluorine (F
2
) gas and noble gases, such as helium (He) as a buffer gas, in a fluorine laser device would be sealed within a laser chamber at a pressure of several 100 kPa, and a pair of main discharge electrodes would be mounted facing each other with a prescribed separation. Laser gas, the laser medium, would be excited within the laser chamber by generating discharge at the main discharge electrodes.
Uniform discharge must be generated between the main discharge electrodes to efficiently generate laser light; but, the laser gas that is present in the discharge space between the main discharge electrodes is commonly subjected to preionization before the main discharge commences in order to generate a uniform discharge in a high-pressure gas atmosphere of several 100 kPa.
One means of generating the preionization would be the preionization method in which two electrodes are disposed facing each other with a dielectric interposed between them. Examples of such preionization units are presented in Japanese Kokai Publication Hei-5-327070, U.S. Pat. No. 2,794,792, Japanese Kokai Publication Hei-10-242553, Japanese patent publication No. 8-502145 and U.S. Pat. No. 5,337,330. All of the preionization units noted are structured with a first electrode (hereinafter abbreviated outer electrode) in contact with the outer surface of a tube formed from a dielectric and a second electrode (hereinafter abbreviated inner electrode) that is inserted within the tube. Corona discharge is created between the outer electrode and the dielectric tube by generating a potential difference between the outer electrode and the inner electrode, and laser gas that is present in the discharge space between the main discharge electrodes is subjected to preionization by ultraviolet light that is generated at this time. There are also cases, in addition to the preionization units in which the dielectric tube and the outer electrode are in proximity without making contact, as well as cases in which the outer electrode is covered by a dielectric substance.
FIG. 5
is a block diagram of an excitation circuit of a gas laser device that emits ultraviolet rays (hereinafter abbreviated gas laser device) using the preionization method. This excitation circuit has a circuit structure termed a charge transfer circuit that uses a solid state switch SW such as a JGBT. In a simple explanation of the operation following this circuit diagram, charge from a high voltage power source HV is held in capacitor C
1
when switch SW is opened. When switch SW is closed while the charge is held in capacitor C
1
, the charge of capacitor C
1
transfers to capacitor C
2
. The charge that had transferred to capacitor C
2
is then transferred to peaking capacitor C
3
via non-linear inductance L
m
termed a saturable inductance or a magnetic switch. The pulse amplitude of the voltage that is applied through the action of magnetic switch L
m
is compressed. The operation of magnetic switch L
m
is that the inductance increases while the charge of capacitor C
1
is transferred to capacitor C
2
, and the inductance rapidly decreases upon saturation when the magnetic flux density has increased, thereby efficiently transferring the charge of capacitor C
2
to peaking capacitor C
3
. Pulse discharge develops between the facing main discharge electrodes
3
,
4
within laser chamber
1
when the voltage of peaking capacitor C
3
has risen and has reached the discharge breakdown voltage. Laser gas is then excited. Specifically, current flows through the discharge circuit loop shown by the thick lines in
FIG. 5
as a result of this discharge.
A differential voltage circuit comprising capacitors C
11
, C
12
and the inductances L
0
are connected in parallel to charge electrodes
3
,
4
. The pulse voltage applied between the main discharge electrodes
3
,
4
is divided, as shown in
FIG. 6
, and is lowered to a range of 25% to 75% thereof, after which voltage is applied in order to attain corona discharge between the outer electrodes
9
and inner electrodes
7
within corona preionization units
15
that are disposed near the upstream side and downstream side of the main discharge space between main discharge electrodes
3
,
4
. The optimum values of the differential voltage ratio, the capacitance of capacitors C
11
, C
12
, and inductance L
0
are selected, the time constant is set to the desired value, and the timing of corona preliminary discharge versus the main discharge is adjusted. The composite capacitance of this differential voltage circuit is adjusted to a level under 10% of peaking capacitor C
3
.
Incidentally, the laser oscillation efficiency is known to be enhanced as the inductance created by the discharge circuit loop falls (Mitsuo Maeda ed. “Excimer laser” pp. 64-65, Gakkai Shuppan Center Inc. first edition Aug. 20, 1983)
FIG. 6
shows a block diagram of an actual discharge circuit loop mentioned above.
FIG. 6
is a cross-sectional view of the principal parts of a gas laser device perpendicular to the direction of laser oscillation. Those constituent elements given the same notation in
FIG. 6
as in
FIG. 5
correspond to the constituent elements shown in FIG.
5
.
In a simple explanation, insulation base
21
is inserted in an airtight manner on the upper wall of laser chamber
1
so as to lie along the longitudinal direction of the discharge space. The other main discharge electrode
3
(for example, a cathode) is attached to the insulation base
21
centrally inside of laser chamber
1
and is connected to high voltage power source
10
via current induction unit
23
penetrating the insulation base
21
. Here, high voltage power source
10
corresponds to the circuit section containing non-linear inductance L
m
on the left side of the peaking capacitor C
3
in
FIG. 5. A
pair of conduction units
25
are attached roughly parallel to insulation base
21
so as to lie on both sides of main discharge electrode
3
within laser chamber
1
. Electroconductive base
26
is extended across the ends of conduction unit
25
and one of the main discharge electrodes
4
(for example, the anode) is attached at the center opposite main discharge electrode
3
at the top in the center. Peaking capacitors C
3
, comprising a plurality of capacitors connected in parallel, are connected to both sides of current induction unit
23
outside of laser chamber
1
. Peaking capacitors C
3
are connected to conduction unit
25
via the current induction unit
24
that pierces insulation base
21
. Furthermore, preionization unit
15
, in which outer electrode
9
and inner electrode
7
are disposed facing each other with interposed dielectric tube
8
, is disposed at the view position of the main discharge space, between the main discharge electrodes
3
,
4
, upstream and downstream of the laser gas stream
2
(denoted by arrows above electroconductive base
26
). Outer electrode
9
is connected directly to electroconductive base
26
while inner electrode
7
is connected between capacitor C
11
and C
12
of high voltage power source
10
via a terminal that is not illustrated.
The section enclosed by broken lines in the structure shown in
FIG. 6
is
Arai Motohiro
Kakizaki Koji
Nixon & Peabody LLP
Paladini Albert W.
Safran David S.
Ushiodenki Kabushiki Kaisha
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