Coherent light generators – Particular active media – Gas
Reexamination Certificate
1999-12-03
2002-10-15
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Gas
C372S057000
Reexamination Certificate
active
06466599
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a discharge unit for an excimer or molecular gas laser, particularly having a narrow discharge width and aerodynamic gas flow.
2. Discussion of the Related Art
Pulsed gas discharge lasers, emitting in the deep ultraviolet region (DUV) and/or vacuum ultraviolet region (VUV), are important tools for a wide range of industrial applications. For example, microlithography applications currently use a line narrowed excimer laser (e.g., ArF, KrF, XeCl, KrCl or XeF) or a molecular fluorine (F
2
) laser having high efficiency and stability at high repetition rates (e.g., 1000 Hz or more).
An electrode chamber design and electrode configuration of a conventional discharge unit are illustrated in FIG.
1
. The electrode chamber of
FIG. 1
houses a pair of elongated main electrodes
2
,
4
. The main electrodes
2
,
4
are separated by a gap or discharge area
6
through which a gas mixture is flowed. A set of high voltage capacitors or “peaking” capacitors Cp is preferably positioned as close as possible to the main discharge electrodes
2
,
4
, and as uniformly as possible over the length of the electrodes
2
,
4
. One or two or more preionization units
10
are used to preionize the gas mixture in the discharge area
6
prior to the main discharge.
One of the main electrodes, in this case electrode
2
, is connected to a pulsed high voltage generator. The high voltage generator typically includes a thyratron or a solid state switch for providing a fast and powerful charge to the peaking capacitors
8
up to the electrical breakdown voltage of the gas discharge gap
6
. The other main electrode
4
is usually connected to ground potential. Fast and powerful discharge of the peaking capacitors Cp, followed by electrical breakdown of the active laser gases in the gas mixture provides the necessary pumping of the gas mixture.
The peaking capacitors in both cases are disposed outside of the electrode chamber (that is not necessary, but common, because it easily avoids exposure of the peaking capacitors to the aggressive halogen gas). One of the main discharge electrodes, the ground electrode, is connected directly to the metal body of the electrode chamber. The other or high voltage electrode is connected to the peaking capacitors and is separated from the grounded metal body of the electrode chamber by means of a dielectric (e.g., ceramic) insulator.
The gas mixture is characterized as being strongly electronegative and maintained at an elevated pressure (e.g., a few bars). The gas mixture for an excimer laser includes an active rare gas such as krypton, argon or xenon, a halogen containing species such as fluorine or HCl, and a buffer gas such as neon or helium. A molecular fluorine laser includes molecular fluorine and a buffer gas such as neon and/or helium.
A typical preionization arrangement includes two preionization units
10
each including a conducting electrode inside a dielectric tube. The preionization units
10
are connected to a pulsed high voltage source and preionize the gas mixture by forming a uniform surface glow discharge. The preionization units
10
are typically positioned in the vicinity of the discharge area
6
between the main electrodes
2
,
4
and provide an initial ionization of the laser gas during the charging of the peaking capacitors Cp by the high voltage pulsed generator. UV-preionizers typically include arrays of electrical sparks, sometimes stabilized by dielectric surfaces, or other configurations of barrier or corona discharge sources. Soft x-ray radiation sources are also sometimes used.
Examples of preionization arrangements which could be used for UV- preionization are shown in
FIGS. 2
a
and
2
b
.
FIG. 2
a
shows a corona preionization arrangement including two corona units
10
a
. Each corona unit
10
a
shown includes an cylindrical electrode
16
surrounded by a dielectric tube
18
. An external electrode
20
a
provides a potential difference for each preionization unit
10
a
. The UV radiation emitted by the preionization units
10
a
preionizes gaseous components within the discharge area
6
.
FIG. 2
b
shows a cross section of a UV-spark preionization arrangement wherein the preionization units
10
b
include separate pins
22
surrounded by dielectrics
24
. These pins
22
are fed-through the chamber and connected to a pulsed power source outside the chamber. A plurality of spark gaps
26
are formed due to a potential difference between an electrode
20
b
in proximity to the pins
22
and produces preionization of the gas in the discharge area
6
.
Besides the discharge unit having a pulser circuit and a laser tube including an electrode chamber such as that illustrated in
FIG. 1
, the laser tube of the discharge unit further includes a gas vessel
11
having a gas flow system or blower
12
and a heat exchanger
14
as illustrated in
FIG. 3. A
vane
15
is also shown extending from the blower
12
generally to the electrode
4
of the discharge chamber. The blower
12
forces the gas to flow generally as indicated by the arrows in FIG.
3
. The gas mixture is naturally heated as it is excited by the electrical discharge in the discharge area
6
. The heat exchanger
14
cools the heated gas after it exits the electrode chamber. The portion of the gas mixture which participates in a laser pulse is replaced by fresh gas before the next laser pulse occurs. Although not shown, a gas supply unit also typically supplies fresh gas to the system from outside gas containers to replenish each of the components of the gas mixture. In particular, halogen containing gas is typically supplied because the halogen concentration in the gas mixture tends to deplete rapidly during operation, while it is desired to maintain a constant or near constant halogen concentration in the gas mixture. Means for releasing some of the gas mixture is also typically provided so that the gas pressure can be controlled and to expel contaminated gases.
Above, various components of a pulsed gas discharge laser such as an excimer or molecular laser have been discussed with respect to their design and arrangement within the electrode chamber. The design and placement of the electrode chamber itself relative to the gas vessel
11
, the placement of the peaking capacitors Cp, and the insulation of the high voltage electrode
2
are further considerations in effective discharge unit design. Examples of laser designs are illustrated in cross-sectional views at the
FIGS. 4
a
and
4
b.
The discharge unit illustrated at
FIG. 4
a
includes a dielectric frame or one or two or more dielectric insulators
28
(see Industrial Excimer Lasers: Fundamentals, Technolgy and Maintenance, Dirk Basting, Ed., 2
nd
edition (1991); Litho laser tube of Lambda Physik, GmbH). Each dielectric insulator
28
is mechanically connected to the gas vessel
11
that is connected to the grounded discharge electrode
4
. The dielectric frame or insulator(s)
28
electrically isolate the high voltage electrode
2
. That is, the roof
31
connected to the high voltage electrode
2
is insulated from the grounded main electrode
4
by the dielectric insulator(s)
28
.
Where the electrode chamber, e.g., as shown in
FIG. 4
a
, meets the gas vessel
11
, an arrangement
30
of conducting ribs are connected electrically to the grounded electrode
4
. The rib arrangement
30
of the discharge unit includes several rectangular ribs
32
separated by openings to permit gas flow from the gas vessel
11
into the electrode chamber and into the discharge area
6
. The relationship between the rectangular ribs
32
and the opening separating them are illustrated at
FIGS. 4
c
-
4
e
. The ribs
32
serve as low inductive current conductors in the discharge circuitry. A lower inductivity of the discharge electrical current loop is advantageous as better matching may be provided between the wave impedance of the electrical discharge loop and the gas discharge impedance.
The discharge unit of
FIG. 4
a
advantageously al
Berger Vadim
Bragin Igor
Rebhan Ulrich
Stamm Uwe
Ip Paul
Lambda Physik AG
Menefee James
Sierra Patent Group Ltd.
Smith Andrew V.
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