Coherent light generators – Particular active media – Gas
Reexamination Certificate
1997-03-06
2001-04-10
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular active media
Gas
C372S058000, C372S034000
Reexamination Certificate
active
06215806
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an excimer laser generator used to process various materials such as metal, resin, glass, ceramics, or semiconductor, or used to assist a chemical reaction. More particularly, the present invention relates to an excimer laser generator capable of generating a laser beam which is stable in its energy and shape and having a long life. The present invention also relates to a blower and a heat exchanger for use in such an excimer laser generator. Furthermore, the present invention relates to a step-and-repeat exposure apparatus using such an excimer laser generator.
2. Description of the Related Art
An excimer laser generating system is the only laser generator that can generate a high output laser beam with a wavelength in an ultraviolet region, and is expected to be used in a wide variety of applications such as electronic and chemical industries and energy industries.
Herein the “excimer laser generator” or “excimer laser generating system” refers to a system for generating an excimer laser beam. In an excimer laser generating system, a laser gas such as Ar, Kr, Xe, KrF, or ArF filled in a manifold is excited by means of discharging or electron beam radiation. Excited atoms are combined with atoms in the ground state, thus creating molecules which can exist only in excited states. These molecules are called excimers. Since the excimers are unstable, they immediately emit ultraviolet light and fall to the ground state. This transition is called bond-free transition. In excimer lasers, the ultraviolet light generated by the bond-free transition is amplified by an optical resonator composed of a pair of mirrors. The amplified light is emitted to the outside as a laser beam.
With reference to
FIG. 1
, an excimer laser generating system disclosed in U.S. Pat. No. 5,029,177 is described below.
As shown in
FIG. 1
, the excimer laser generating system includes: a laser chamber
1
; an oscillation spectrum narrowing module
2
for controlling the oscillation line width; a high-voltage pulse power module
3
; an optical monitor module
4
for detecting the energy intensity and the wavelength of the laser output; a high-voltage power supply
5
; and a controller
6
for controlling the entire parts of the excimer laser generating system.
When the excimer laser generating system is used as an exposure light source of a step-and-repeat exposure apparatus for producing an electron device, the computer controller
6
is connected via an interface
7
to a control unit
8
and a computer
9
of the step-and-repeat exposure apparatus.
The laser chamber
1
is filled with a gas such as F
2
, Kr, or Ne. A high-voltage pulse generated by the pulse power module
3
is applied to the cathode electrode in the laser chamber so that a discharge occurs in the gas, which causes light emission. The light is repeatedly reflected between an output mirror
10
and an optical system in the spectrum narrowing module
2
, and is amplified during the travel. The amplified light is emitted to the outside.
To obtain a laser beam having a desired wavelength, a part of the emitted laser beam is reflected by a half-mirror
11
and directed to the optical monitor module
4
. The optical monitor module
4
measures the wavelength and other characteristics of the light. In response to the measurement result provided by the optical monitor module
4
, a stepping motor
12
is driven so that the optical system of the spectrum narrowing module
2
is optimized. The optical power of the laser beam is also monitored by the optical monitor module
4
.
FIG. 2
is a cross-sectional view of the laser chamber
1
, taken in a direction perpendicular to the direction of laser oscillation.
As shown in
FIG. 2
, the laser chamber
1
has two aluminum housing members
13
and
14
which are combined together via an O-ring
15
so as to form a sealed chamber. A cathode electrode
18
is fixed to the chamber
1
via a cathode electrode supporting member
17
and an insulator
16
. Anode electrode
19
is fixed to the housing member
13
via an anode electrode supporting member
20
. The laser chamber
1
further includes a connector
21
for connecting the cathode electrode
18
to the pulse generator
3
. There is also provided a sealing member such as an O-ring
22
.
Inside the laser chamber
1
, there is a blower
23
for circulating gas within the chamber
1
. A heat exchanger including a heat-transfer pipe is also provided in the chamber
1
to cool the gas heated by discharging. The laser chamber
1
, the blower
23
, and the heat exchanger
24
are made of stainless steel or an aluminum alloy wherein the outer surfaces of these elements are usually electro-polished or bright-annealed. An air filter
25
is fixed to the housing so that gas is introduced into the filter via a gas inlet
26
thereby removing particles such as metal fluoride particles produced by the reaction of the electrode metal with fluorine gas.
Referring to
FIG. 3
, a gas supplying system which has been proposed previously by the inventor of the present invention will be described below. This gas supplying system has been designed to properly supply gas into the laser chamber
1
.
In the specific example shown in
FIG. 3
, there are three gas supply lines: a 1% Kr/Ne gas supply line
31
, a 1% F
2
/1% Kr/Ne gas supply line
32
; and a He gas supplying line
33
. The respective gas supply lines have orifices
34
,
35
,
36
for controlling the flow rates and also have valves
37
,
38
, and
39
connected to a manifold
40
. He gas is used as a purging gas when window plates of the laser chamber are replaced with new ones.
A plurality of gas supply pipes are connected to the manifold
40
. Of these gas supply pipes, a gas supplying connection pipe
41
has a chamber valve
42
and a valve
43
. The manifold
40
is also connected to a F
2
gas supply pipe
44
which is connected to the connection pipe
41
via an injection valve
45
and a flow rate control orifice
46
.
The gas supplying system further includes: a spring valve
47
which is opened when the pressure in the manifold
40
increases to an abnormally high level greater than an allowable level; an exhaust line
48
; a valve
49
which is operated by hand as the valve
43
to exhaust the gas when a trouble occurs inside the laser chamber; F
2
gas safening treatment equipment
50
; a vacuum pump
51
; and a pressure gauge
52
.
When it is desirable to fill the laser chamber with a laser discharging gas containing 0.1% F
2
, the laser discharging gas can be introduced into the chamber as follows. First, 1% F
2
/1% Kr/Ne gas is introduced, while monitoring its pressure, into the chamber
53
via the manifold
40
and the pipe
41
until the pressure reaches 30 kPa. Then 1% Kr/Ne gas is introduced until the pressure reaches 300 kPa so that the chamber is filled with the mixed gas containing 0.1% F
2
.
The F
2
concentration decreases with the repetition of laser oscillation. This causes a reduction in the optical output power of the laser. Therefore, each time the optical output power of the laser drops to a level lower than a predetermined value, it is required to supply a proper amount of 1% F
2
/1% Kr/Ne gas into the chamber
53
via the manifold
40
and the pipe
44
.
The conventional excimer laser generating system has the following problems which were not known in the art before the inventor of the present invention found them.
A first problem in the conventional excimer laser generating system is that the F
2
gas has to be supplied as frequently as every 5×10
6
pulses to obtain required optical output power.
A second problem is that the reaction produces fluoride which absorbs laser light. This makes it impossible to obtain stable optical power. For example, the formation of fluoride can cause a variation in the optical output power as large as 5% to 10%. To remove the above fluoride, the entire gas in the chamber
1
must be replaced with a new gas every 5×10
7
to 1×10
8
pulses.
A th
Ohmi Tadahiro
Sano Naoto
Shirai Yasuyuki
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Jr. Leon Scott
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