Laser device having an electrode with auxiliary conductor members

A laser device is capable reducing fluctuation of the beam width of the output laser beam due to abrasion of its discharge electrodes. A part of the contour shape of a section perpendicular to the longitudinal axis of the chamber of the laser device in at least one of the electrodes, which part confronts the other one of the electrodes, has a shape of a circular arc having a predetermined radius. The width of at least one of the two electrode may be substantially equal to the width of electric discharge taken place between the electrodes. Further, a conductor member may be disposed on each side of the electrode along its longitudinal axis to reduce electric field strength therearound.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a discharge pumped laser device used for light 
sources for machining works, projection light, etc. and more particularly 
to discharge electrodes in the laser device which minimize the fluctuation 
of the beam width of the laser beam due to the abrasion of the electrodes. 
2. Description of the Related Art 
A discharge pumped laser device is used for machining of works such as 
marking, cutting and welding, and also used as a light source for optical 
lithography to transcribe circuit patterns for large scale integrated 
circuits. 
For machining works, carbon dioxide lasers, excimer lasers, etc., are used. 
For optical photolithography, a reduction projection process is typically 
used. In the reduction projection process, light irradiated on and passed 
through a reticle pattern is projected through a reduction projection 
aligner onto a photo-sensitive material deposited on a semiconductor 
substrate to thereby form a circuit pattern. The resolution of the 
projected image is inverse proportional to the wavelength of light; 
therefore, the wavelength of light has been shifted from a visible region 
to an ultraviolet region in order to improve the resolution. 
Conventionally, g-line (having a wavelength of 463 nm) and i-line (having a 
wavelength of 365 nm), which are produced by a high pressure mercury lamp, 
are used as an ultraviolet light source. However, for the production of a 
64 megabyte memory, the line width is 0.25 m or less at the minimum 
pattern. Therefore, a light source having a wavelength shorter than those 
of the i-line has been expected. 
A deed ultraviolet (deep-UV) laser light source is now considered promising 
to break the technological limit for shorter wavelength. Especially, the 
excimer laser produces high output with high efficiency. It provides 
strong oscillation at short wavelengths in a certain composition of the 
medium gas such as KrF (wavelength of 246 nm) and ArF (wavelength of 193 
nm). 
However, the selection of glass and crystalline materials which constitute 
the reduction projection lens system is greatly restricted in the deep-UV 
region. Therefore, the correction of the chromatic aberration is difficult 
to perform in the reduction projection lens system which uses a mercury 
lamp. In place of correcting the chromatic aberration of the lens system, 
a wavelength selection device such as an etalon is disposed in the laser 
cavity (resonator) to thereby reduce the spectrum width of the output 
light to such an extent that the chromatic aberration is negligible. 
According to this method, the output having a spectrum width of several 
nanometers in the natural oscillation can be reduced to a narrow band of 
several picometers. To reduce the bandwidth, it has been proposed the 
methods in which one or more Fabry-Perot etalons including a pair of 
parallel reflective films are provided the cavity or a diffraction grating 
is used as a total reflection mirror. 
The former method uses a plurality of etalons which are superposed to each 
other. This method can reduce the bandwidth relatively easily. However, 
since the optical intensity between the reflective films is very high, the 
durability of the reflective films and a drift due to the loss become 
problems when high output is to be produced. 
The latter method using a diffraction grating can achieve wavelength 
selection by a single reflection on the diffraction grating surface so 
that the optical intensity is lower than that of the former method. As a 
result, the load on the reflective surface is small. Further, since the 
reflection takes place only one time, a loss at the reflective surface is 
low and high output is obtained. Because of these advantages in the latter 
method, a narrow band excimer laser device widely employs a diffraction 
grating as a light source for optical lithography. 
Referring to FIG. 23(a), a narrow band excimer laser is of a discharge 
pumped type in which laser oscillation is performed by discharging and 
pumping a laser gas such as KrF filled in a laser chamber 1 through a pair 
of electrodes 6, 7 disposed above and below the laser chamber along the 
longitudinal axis of the chamber. A discharge pumped region 11 is shown 
enclosed by the broken lines. As discharge advances, the opposing surfaces 
6a, 7a of the electrodes 6, 7 are abraded and the discharge width WA 
changes. Along with the change in discharge width, the beam width of the 
output laser light 4 changes, which is a problem in terms of stabilization 
of the beam width. 
When this laser is used for machining works, due to the change in the beam 
width, the transverse mode is deteriorated, the beam condensing 
performance is changed, and the output is changed. This causes a practical 
problem. Especially, when this laser is used as a light source for optical 
photolithography, a change in the beam width would cause the following 
undesirable problems in the practical use of the narrow band excimer 
laser. 
To reduce the bandwidth, the diffraction grating 5 (FIG. 23(a)) is used as 
a wavelength selective device and high order diffraction is used in the 
diffraction grating, so that angular dispersion at the operating point 
becomes large and the divergent angle of the laser beam directly 
influences the spectrum width. When the divergent angle is large, the 
Spectrum width would increase. Therefore, when the discharge width, that 
is the gain region, changes, the divergent angle of the laser beam 
changes, so that the spectrum width greatly changes. In order to avoid 
this change, an aperture 8 (FIG. 23(a)) is conventionally provided to 
stabilize the gain width. In a so-called Chang type electrodes used 
conventionally, the discharge width would greatly increase inevitably in 
the abraded electrodes when the output is increased. As a result, it 
becomes difficult to control the output due to the restriction on the gain 
by the aperture 8. 
The mechanism of enlarging the discharge width will now be described. 
Townsent's theory is known as a macroscopic phenomenological theory which 
explains discharge, and is useful for understanding discharge phenomena. 
In this theory, a gas including a halogen used in the excimer laser is 
called a negative gas. Electrons produced by the collision and ionization 
of the electrons (ionization coefficient: .alpha.) in the discharge 
process by a large electron affinity of halogen are captured (electron 
attachment coefficient: .eta.) to reduce an apparent ionization 
coefficient (.alpha.-.eta.) to thereby facilitate condensation of the 
discharge. The relationship between these coefficients and electric field 
strength E is shown in FIG. 24. in which a reference character P denotes a 
normalization coefficient. 
As seen from FIG. 24, the ionization coefficient is greatly dependent on 
the electric field strength E while the electron attachment coefficient 
.eta. is not substantially dependent on the electric field strength E, so 
that the apparent ionization coefficient (.alpha.-.eta.) rapidly increases 
in excess of a certain electric field strength and greatly depends on the 
electric field strength E. Thus, the parameter (.alpha.-.eta.) which 
drives the discharge changes in accordance with the electric field 
strength distribution on the surface of the electrodes, so that the 
discharge width is greatly influenced by the electric field strength 
distribution on the electrode surface. In order to ensure the discharge 
width, a uniform large electric field strength region transverse to the 
electrodes of the discharge pumped type laser device is required to be 
provided as its electrode shade. Conventionally, the shades of Chang and 
modified Chang electrodes are used on the basis of the analysis of the 
electric field under ideal conditions. 
FIG. 25(a) shows a potential distribution derived from the electric field 
calculation in the case of a modified Chang type electrodes, in which L1 . 
. . denotes equipotential lines. It will be seen from FIG. 25(a) that the 
potential distribution between a cathode (upper electrode) 6 at high 
negative potential and an anode (lower electrode) 7 is greatly bent by a 
metal plate on which the anode 7 is placed, a current return lead 10 and 
an insulating member 9. In the actual configuration, the existence of such 
leads and isolation causes the electric field to deviate from an ideal 
Chang type electric field. 
FIG. 25(b) shows equifield strength line L2 . . . in the vicinity of the 
cathode electrode 6 while FIG. 25(c) shows changes in the electric field 
strength along the surface of the cathode electrode 6. As seen from FIG. 
25(b), the electric field strength on the surface of the electrode 6 does 
not so often intersect the equifield strength lines in the region ranging 
from the electrode central point A to about 1/3 of the right half 
electrode width and a uniform electric field is formed in that region. 
As shown by a line L3 in FIG. 25(c), the electric field intensity initially 
rises very slowly from the electrode central point A toward the right-hand 
electrode end point B and then rises more rapidly closer to the end point. 
FIG. 25(d) shows an equifield strength line L4 . . . in the vicinity of 
the anode electrode 7. FIG. 25(e) shows changes in the electric field 
strength along the electrode surface. The electric field strength at the 
center A is denoted by E.sub.0. As shown by these figures, the electric 
field strength is uniform in the first rightward section of about 4 mm 
starting from the electrode central point A. It then becomes slowly lower 
toward the right-hand end B and becomes rapidly lower after a distance of 
about 12 mm (see lines L4, L5). This is a trend reverse to that in the 
case of the cathode 6. Discharge occurs at the central portion of the 
electrode. The factor of restricting the discharge width is considered to 
be on the anode 7 side where its central portion is at high electric field 
strength. 
After 1.times.10.sup.8 shots, the shape of the abraded electrode central 
portion was measured and the electric field strength distribution in the 
abraded electrode central portion was calculated by using finite element 
method on the basis of the measured shape data. 
FIGS. 26(a) and 26(b) show the result of this simulation in which the axis 
of abscissa expresses a rightward distance from the center A and the 
ordinate axis shows the ratio of a change .DELTA.E in the electric field 
strength E to electric field strength E.sub.0 at the central point A (%). 
FIG. 26(a) concerns the cathode 6 while FIG. 26(b) concerns the anode 7. 
The white dot shows the state of new electrode while the black dot shows 
the state after 1.times.10.sup.8 shots. 
As seen from these figures, abrasion has increased after 1.times.10.sup.8 
shots, so that a uniform electric field has extended in the vicinity of 
the central point compared with that of new electrode. The width of this 
uniform electric field portion corresponds to the observed beam width. In 
this respect, it is considered that an electric current is concentrated at 
the uniform electric field portion which is a strong electric field and 
abrasion has increased to thereby form a wide uniform electric field 
portion. 
As described above, when the operation of the laser device starts, the 
conventional electrode is abraded as time passes to thereby increase the 
area of the uniform electric field portion and to cause the fluctuation of 
the beam width of the output laser beam. 
SUMMARY OF THE INVENTION 
The present invention is made in view of the above-described situations in 
the conventional art, and it is an object of the present invention to 
provide a laser device which reduces fluctuations in the laser beam width 
caused by the electrode abrasion. 
According to the first aspect of the present invention, there is provided a 
laser device of the type in which electric discharge takes places between 
a pair of electrodes to excite a laser gas in the laser chamber thereby 
producing laser beam, wherein the width of at least one of the pair of 
electrodes substantially equal to the width of the electric discharge. 
With this construction, even if electrode has been abraded due to discharge 
across the confronting surfaces of the electrodes, a uniform electric 
field strength range is difficult to extend. Thus, fluctuation in the 
spectrum width of the laser beam due to the electrode abrasion is 
minimized. 
According to the second aspect of the present invention, there is provided 
a laser device of the type in which discharge takes places between a pair 
of electrodes to excite a laser gas in the laser chamber thereby producing 
a laser beam, characterize in that the width of at least one of the pair 
of electrodes substantially coincides with the discharge width. 
With this construction, the discharge width becomes a constant value which 
corresponds to the electrode width. Therefore, the beam width of the 
output laser beam is constant to thereby operate the laser device in a 
stabilized manner. 
According to the third aspect of the present invention, there is provided a 
laser device of the type in which electric discharge takes place between a 
pair of electrodes to excite a laser gas in the laser chamber thereby 
producing laser beam, wherein the width of at least one of the pair of 
electrodes substantially equal to the width of the electric discharge and 
wherein there is further provided with a pair of conductor members 
disposed on respective sides of said at least one of the electrodes along 
the longitudinal axis of said one electrode. 
With this construction, the discharge width becomes a constant value which 
corresponds to the electrode width. Therefore, the width of the output 
laser beam is constant to thereby operate the laser device in a stabilized 
manner. Further, the abrasion rate of the electrode is suppressed to a 
small value because the electric field is eased by the conductors disposed 
on the respective sides of the electrode to thereby reduce contaminations 
generated from the electrodes.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
Referring to the drawings, embodiments of the laser device according to the 
present invention will be described hereinafter. In the embodiments, the 
laser device is assumed to be a narrow band oscillation excimer laser. The 
excimer laser of the embodiments to be described below is basically the 
same structure as the conventional example of FIG. 23(a). Reference 
numerals 1, 2, 4, 8 and 11 denote similar elements of the embodiments of 
the present invention and the example of the conventional device. 
In a first embodiment, as shown in FIG. 1(a), electrodes 12 and 13 are used 
in place of electrodes 6 and 7 in FIG. 23(b), respectively. Referring to 
FIG. 23(a), a laser chamber 1 is filled with a laser gas, for example, of 
KrF, which is discharged and pumped for laser oscillation through the 
electrode 12 (cathode electrode) and the electrode 13 (anode electrode) 
disposed above and below the laser chamber 1 along the longitudinal axis 
of the laser chamber. The laser beam is resonated by a light resonator 
(cavity) composed of the laser chamber 1, a front mirror 2 and a grating 5 
which is a wavelength selective element, and output as an effective laser 
beam 4 from the front mirror 2. The grating 5 reduces the bandwidth of the 
oscillating beam and also functions as a rear mirror, which is disposed in 
so-called Littrow arrangement. 
In FIGS. 23(a)-(c), an area 11 enclosed by the broken line is a discharge 
pumped region; reference numeral 8 denotes an aperture; and WA denotes a 
discharge width. In FIG. 1(a), reference numeral 9 denotes an insulating 
member; 10 denotes an anode current return lead. 
In the second and third embodiments to be described later, the reference 
numerals 9 and 10 denote members same as those in the first embodiment. 
The sections of the electrodes 12, 13 perpendicular to the longitudinal 
axis of the laser chamber 1 are each in lateral symmetrical. Therefore, 
the potential distribution during the operation of the laser device is 
also in lateral symmetry around the central axis of the section. Thus, 
only the potential distribution on the right-half portion of the 
electrodes 12, 13 is shown in FIG. 1(a). 
The upper portion of FIG. 1(a) shows the cathode electrode 12 having a 
confronting surface 12a in the shape of circular arc with a radius R.sub.1 
(of 13 mm). The cathode electrode side 12b and bottom 12c are each 
composed of a straight line while the confronting surface 12a and the side 
surface 12b are connected by an arc with a radius of r.sub.1 while the 
side 12b and the bottom surface 12c are connected with an arc of a radius 
r.sub.2. These radii r.sub.1, r.sub.2 are set to predetermined values 
which are smaller than the radius R.sub.1 of the surface 12a. The lower 
portion of FIG. 1(a) shows an anode electrode 13 which has a width of 8 
mm, and a confronting surface 13a in the shade circular arc with a radius 
R.sub.2 (of 9 mm). The anode electrode 13 has a side surface 13b and a 
bottom surface 13c, each of which is composed of a straight line. The 
confronting surface 13a and side surface 13b are connected by an arc of a 
radius r.sub.3 while the side surface 13b and bottom surface 13c are 
connected by an arc of a radius r.sub.4. The radii r.sub.3, r.sub.4 are 
set to predetermined values which are smaller than the radii R.sub.2 of 
the confronting surface 13a. The inter-electrode distance d between the 
cathode and anode electrodes 12 and 13 is 20 mm. All radii R.sub.1, 
R.sub.2, r.sub.1, r.sub.2, r.sub.3 and r.sub.4 of the arcs are smaller 
than the inter-electrode distance d. 
As shown in FIG. 1(a), the tendency of the equipotential lines is similar 
to that of the conventional one shown in FIG. 25(a). Because the distance 
between the cathode 12 and the insulating member 9 is larger than that in 
FIG. 25(a), the potential gradient at the electrode 12 side is lower than 
that in FIG. 25(a). For the anode 13, the potential gradient decreases 
from its central portion toward the right end. 
FIGS. 1(b) and 1(d) shows a distribution of the electric field strength in 
the vicinity of the cathode 12 and anode 13 on enlarged scale. It will be 
seen that a relatively wide uniform field region is established in the 
cathode 12 while in the anode 13 the field strength rapidly decreases from 
the central portion of the anode toward its right-hand end. FIG. 1(c) 
shows the field strength along the surface of the cathode electrode 12. It 
will be seen that a uniform field region is established through a region 
ranging from the central point A to the right-hand point distant by 7-8 mm 
from the central point. FIG. 1(e) shows the field strength on the surface 
of the anode electrode 13. A uniform field region is established in a very 
narrow area ranging from the central point A to a rightward point distant 
by 1-2 mm from the central point, and the field strength is rapidly 
reduced from that right-hand point toward the periphery of the anode 
electrode. It will be understood that discharge occurs as concentrated in 
the central narrow region. In any event, the field strength rapidly 
decreases with the electrodes 12 and 13 toward the peripheries of their 
confronting surfaces. Therefore, a narrow band oscillation excimer laser 
device is realized where even if electrode abrasion occurs due to 
discharge, a uniform field portion is difficult to extend to thereby 
minimize fluctuations in the spectrum width due to the electrode abrasion. 
Although in the first embodiment both electrodes are shown as having 
arcuate confronting cross-sections and a straight-line side surface, and 
the arc surface and the straight line side surface are connected by an 
arc, only one of those electrodes may take such configuration as shown in 
FIG. 2. If the anode electrode 13' has such configuration, a more 
advantageous result is obtained than the case where the cathode electrode 
has such configuration. 
While in the first embodiment the configuration of the electrode 
cross-section is shown as having a straight line, it may be a complete 
circle as shown in FIG. 3 in which an anode current return lead 10 may 
have a concavity 15 corresponding to the size of the circle so as to hold 
and fix the electrode 14 therein. Thus, the electrode 14 is easy to 
manufacture. 
The sectional configuration of the electrode is not limited to the 
above-described one. As shown in FIG. 4(a), an electrode 16 may have an 
confronting surface formed by an arc with a radius R.sub.3 and a bottom 
surface formed by a straight line, the arc and straight line being 
connected by an arc with a radius r.sub.5. FIG. 4(b) show an electrode 17 
which has an confronting surface formed by an arc with a radius R.sub.4 
and a side surface formed by a straight line which is connected smoothly 
with the arc with a radius R.sub.4, with the side surface and the bottom 
which are both straight lines being connected by an arch with a radius 
r.sub.6. 
In summary, the electrode may take any configuration as long as it has at 
least an arcuate confronting surface where a straight line is connected 
smoothly with an arc or where straight lines are connected smoothly to 
each other. 
FIGS. 5(a)-(c) show a second embodiment in which electrodes 18 and 19 are 
used in place of the conventional electrodes 6 and 7, respectively. The 
electrode 19 is characterized in that a part of the contour shade of the 
section of the electrode 19 perpendicular to the longitudinal axis of the 
laser chamber 1 and confronting the electrode 18 has a shade of an 
elliptical arc. The sections of the electrodes 18 and 19 are in lateral 
symmetry. Therefore, the potential distribution during the operation of 
the laser device is in lateral symmetry around the center axis of the 
sections. In FIGS. 5(a)-(c), the potential distribution for only the 
typical right-hand portions of the electrodes 18 and 19 is shown. 
The upper portion of FIGS. 5(a)-(c) shows the cathode electrode 18 having 
an arcuate confronting surface 18a with a radius of 13 mm while the lower 
portion of FIGS. 5(a)-(c) shows the anode electrode 19 having a width of 8 
mm and an confronting surface 19a formed as an elliptical arc, as 
mentioned above. This second embodiment is constructed such that the width 
of the anode electrode 19 coincides with the length of the longer diameter 
of the ellipse while the center axis of the section coincides with the 
shorter diameter of the ellipse. The distance between the cathode and 
anode electrodes 18 and 19 is assumed to be 25 mm. That is, the width of 
the electrodes are less than the distance between the electrodes. 
FIG. 5(a) shows the result of a numerical analysis performed when the ratio 
r of longer diameter/shorter diameter of an ellipse involving the 
confronting surface 19a of the anode electrode 19 is 1, that is, when the 
confronting surface 19a is an arc. FIG. 5(b) shows the result of a 
numerical analysis performed when r=2. FIG. 5(c) is the result when r=4. 
It is to be noted that (a), (b) and (c) of each of FIGS. 5-9 show that the 
ratio of the longer diameter/shorter diameters r=1, 2 and 4, respectively. 
While the tendency of all the equipotential lines is similar to that of 
those conventional ones shown in FIG. 25(a), the potential gradient is 
increased because the equipotential lines in the vicinity of the anode 19 
are pushed up by the rod-like electrode, so that the potential gradient at 
the electrode upper end portion (the central axis portion) is high. This 
situation will be better understood in FIGS. 6(a)-(c) which show the 
vicinity of the anode 19 on enlarged scale. 
FIGS. 7(a)-(c) show the distribution of field strength. FIGS. 8(a)-(c) show 
the details of the field strength distribution in the vicinity of the 
anode electrode 19. FIGS. 9(a)-(c) show a change in the field strength E 
on the surface of the electrode 19. As will be seen from these figures, 
when the cross section of the electrode is a complete circle in (a) of 
these figures, the field strength is high at the electrode upper end 
portion (central portion) and decreases rapidly toward the right-hand end 
to thereby provide a sharpened distribution. When the cross section of the 
electrode is an ellipse (the ratio of the longer diameter/shorter diameter 
r=2) in (b), a field distribution is provided in which a slight concavity 
is formed at the upper end with a flat uniform field strength width of 
about 6 mm. It will be understood that the absolute values of the field 
strength at the electrode upper end portions increase in the order of (a), 
(b) and (c). As the ellipse becomes oblater, the central field strength 
decreases and the field strength at the shoulder of the uniform field 
strength region conversely increases. 
Now assume that the operator turns on an operation switch to start the 
operation of the laser device and discharge starts thus from the state of 
the ellipse (the ratio of longer diameter/shorter diameter r=2) in (b). 
Since the field is considerably uniform and the discharge extends through 
the overall elliptical arc portion 19a of the electrode 19, the abrasion 
of the electrode 19 increases simultaneously through the overall electrode 
elliptical portion 19a. As will be seen in FIG. 9(b), since a more local 
discharge current flows through the central portion where a stronger field 
exists, that portion is abraded more rapidly. The situation then comes 
close to the state of (c) and the field in the vicinity of the center 
point A becomes lower and abrasion is decreased. Now, the field of the 
periphery D of the central portion increases, a more discharge current 
flows through this periphery D where the increased field exists to thereby 
abrade the electrode more rapidly. The situation then returns to the state 
of (b). In this manner, the discharge portion is settled to take a 
stabilized shape between (b) and (c). Since a self-shape maintenance 
mechanism operates due to a kind of negative feedback, a long time 
stabilized operation is performed while maintaining a uniform field region 
constant without changing the discharge width WA. This is clarified by the 
result of the numerical analysis. 
When the ratio of the longer diameter/shorter diameter r is small, the 
field strength E is high in the vicinity of the electrode central point A 
and rapidly decreases toward the periphery of the central portion, so that 
discharge is concentrated at the narrow central region. If r increases 
excessively, the field strength E increases in the periphery of the 
central portion and the discharge is likely to be concentrated on the 
peripheral portion of the electrode. Thus, there is an optimal value of r 
where discharge is not locally concentrated. In an experiment where r=2, 
the field strength became constant through substantially the overall 
region of the confronting surface 19a of the electrode 19 and a discharge 
width WA was obtained which was substantially the same as the length of 
the electrode width. 
As described above, when the confronting surface 19 a of the electrode 19 
is formed with an optimal ratio of the longer diameter/shorter diameter to 
thereby operate the laser, discharge extends through substantially the 
overall electrode elliptical portion 19a and abrasion increases 
simultaneously through the overall surface of the electrode elliptical 
portion 19a. Even if there is a local strong field, a more discharge 
current flows through that portion, which is abraded more rapidly. A kind 
of negative feedback mechanism operates in which the field at that portion 
and hence abrasion is decreased. Therefore, the uniform field region is 
constant for a long time to thereby provide a stabilized operation without 
changes in the discharge width. Such experimental results coincide 
substantially well with the result of the above numerical analysis, 
advantageously. 
While in this second embodiment a constant field strength is obtained 
through the overall surface of the electrode elliptical portion 19a when 
the ratio of longer/shorter diameters r=2 to thereby obtain a discharge 
width WA which is the same as the electrode width, this optimal value 
changes depending on the overall width, inter-electrode distance, and 
geometrical layout of the electrodes, the permittivity of the insulating 
member, and the nature of the discharge medium gas, so that the optimal 
value should be obtained depending on the situation. 
While in the second embodiment only the anode electrode 19 is shown as 
having a discharge width WA which is the same as the electrode width, the 
electrodes 18' and 19' on both sides may be provided so as to have 
electrode widths which are the same as the discharge width WA, as shown in 
FIG. 10, to thereby produce effects similar those produced by the second 
embodiment. 
While in the second embodiment the confronting surface is illustrated 
having an elliptical form, the present invention is not limited to it. 
Even if the confronting surface has a configuration other than an 
elliptical arc, it may be used as long as the field strength distribution 
is uniform on the confronting surface. As an example, an optical surface 
20a of the electrode 20 may be defined by a combination of an arc and a 
straight line, as shown in FIG. 11(a). Alternatively, as shown in FIG. 
11(b), the electrode confronting surface 21a may be defined by a 
combination of an ellipse and straight lines. Furthermore, the confronting 
surface may be formed of a curved line of a smooth function system. In 
these cases, if straight lines or a curved line and a straight line are 
connected smoothly, field concentration is difficult to occur to thereby 
prevent the occurrence of surface discharge or unnecessary discharge. 
A third embodiment will be described next. In the third embodiment, 
electrodes 22 and 23 are used as shown in FIGS. 12(a)-(c) in place of the 
conventional electrodes 6 and 7, respectively. The electrode 23 is 
characterized in that part of the contour shape of the cross section of 
the electrode 23 perpendicular to the direction of longitudinal axis of 
the laser chamber 1 and confronting the electrode 22 has the shape of an 
elliptical arc. It is further characterized in that a column-like metal 
structure 24 having a circular cross section which reduces the electric 
field strength (hereinafter referred to as a field easing electrode) is 
provided on each of the right and left sides of the electrode 23. In FIGS. 
12(a)-(c), the right-hand side one is denoted by 24R while the left-hand 
side one is denoted by 24L (not shown). The cross sections of the 
electrodes 22 and 23 are in lateral symmetry, so that the potential 
distribution occurring during the operation of the laser device is in 
lateral symmetry around the mid-axis of the cross section. 
FIGS. 12(a)-(c) show the potential distribution for only the right-hand 
side portions of the electrodes 22 and 23. The upper portions of FIGS. 
12(a)-(c) show a cathode electrode 22 having an arcuate confronting 
surface with a radius of 13 mm while the lower portions of FIGS. 12(a)-(c) 
show an anode electrode 23 having an elliptical confronting surface 23a 
with a transverse electrode width of 8 mm. In the third embodiment, the 
transverse width of the electrode 23 is formed such that it coincides with 
the length of the longer diameter of the ellipse and the vertical center 
axis of the electrode 23 coincides with the shorter diameter axis of the 
ellipse. The distance between the cathode and anode electrodes 22 and 23 
is assumed to be 25 mm. Thus, the transverse width of the electrode 23 is 
less than the distance between the electrodes. The transverse surface of 
the field easing electrode 24R on the right-hand side of the anode 
electrode 23 is a circle with a diameter of 8 mm and connected 
electrically with the electrode 23. This applies to the field easing 
electrode 24L on the left side of the anode electrode 23. That is, the 
field easing electrode 24 is such that it is lower than the confronting 
surface of the electrode 23. Since the electrodes 22, 23 perform discharge 
therebetween, the materials of the electrodes are required to be a high 
melting point metal having high resistance to discharge. The field easing 
electrodes 24R and 24L do not perform discharge and are only required to 
be maintained at a static electrical potential, so that they are not 
necessarily required to be made of a high melting point metal having 
resistance to discharge. Thus, they may be made of a metal material 
different from those of the electrodes 22 and 23. 
FIG. 12(a) shows the result of the numerical analysis when the ratio of the 
longer diameter/shorter diameters r of an ellipse involving the anode 
electrode confronting surface 23a is 1 or when the confronting surface 23a 
is an arc. FIG. 12(b) shows the result when the ratio r=2 while FIG. 12(c) 
shows the result when the ratio r=4. It is to be noted that (a), (b) and 
(c) of FIGS. 13-19 show the cases where the ratios of the longer 
diameter/shorter diameter r=1, 2 and 4, respectively. 
While the tendency of the overall equipotential lines is similar to that of 
the conventional ones shown in FIG. 23(a), the equipotential lines in the 
vicinity of the anode 23 are pushed up by the field easing electrode 24R 
and the potential gradient at the electrode upper end portion (center axis 
portion) is low. This will be better understood from FIGS. 13(a)-(c) which 
show the vicinity of the anode 23 on enlarged scale. 
FIGS. 14(a)-(c) show the details of the field strength distribution in the 
vicinity of the anode electrode 23, and FIGS. 15(a)-(c) show a change in 
the field strength E on the surface of the electrode 23. When the cross 
section of the electrode is a true circle in (a), the field strength is 
high at the central point A of the electrode 23 and rapidly decreases 
toward its right-hand end to thereby provide a sharpened distribution. 
When the cross section of the electrode is an ellipse (the ratio of the 
longer diameter/shorter diameter r=2) in (b), the field distribution is 
flatter than that in the vicinity of the central point A in (a). In the 
case of an ellipse (the ratio of the longer diameter/shorter diameter r=4) 
in (c), conversely, the field strength in the vicinity of the central 
point A is lower than that at the periphery D of the central portion. 
The advantages produced by provision of the field easing electrode 24 will 
be described below. FIGS. 16(a-c), 17(a-c) show the details of the field 
strength distribution in the vicinity of the anode electrode 23 where no 
field easing electrode 24 is disposed and a change in the field strength E 
on the surface of the electrode 23, respectively. As will be obvious from 
the comparison between FIGS. 16(a-c), 17(a-c) and FIGS. 14(a-c) and 
15(a-c), the provision of the field easing electrode 24 reduces the field 
in the periphery D of the central portion of the anode electrode 23, that 
is, the difference in field magnitude between the central point A and the 
periphery D is small. 
The absolute values of the field strength at the upper end portions of the 
anode electrode 23 are large in the order of (a), (b) and (c). It will be 
seen that as the ellipse becomes flatter, the field strength at the 
central point A decreases and the field strength at the periphery D 
conversely increases. The field strength at the central point A of the 
anode electrode 23 is uniformly reduced when the field easing electrode 24 
is provided compared to the case where no field easing electrode is 
provided. 
Assume now that the operator turns on the operating switch to start the 
operation of the laser device and discharge starts in the state of (b). 
Since the field is considerably even through a wide range of the central 
portion of the confronting surface 23a and discharge extends over the 
overall elliptical portion 23a of the electrode 23, the abrasion of the 
electrode 23 increases simultaneously through the overall surface of the 
electrode elliptical portion 23a. 
As will be seen in FIG. 15(b), a larger current flows locally through the 
region where the field is strong in the vicinity of the central point A, 
so that this region is abraded more rapidly. Then, the state of (c) 
appears and the field in a region in the vicinity of the central point is 
reduced to thereby decrease the abrasion of the electrode. Now, the field 
strength in the periphery D of the central portion increases and a more 
discharge current flows through the periphery D where the field is strong 
to thereby abrade the electrode more rapidly. In this manner, the state of 
(b) is recovered and the shade of the discharge is thus settled at a 
stabilized one between (b) and (c). Since a self-shape maintaining 
mechanism works by a kind of negative feedback, a uniform field is 
maintained in a predetermined region to thereby operate the laser for a 
long time in a stabilized manner without changing the discharge width WA. 
This was clarified by the result of the numerical analysis. 
Experiments showed that if the ratio of the longer diameter/shorter 
diameter r is smaller, the field strength E is higher at the central point 
A of the electrode and rapidly decreases toward its periphery D, so that 
discharge is concentrated in the central narrow region. If the ratio of 
the longer diameter/shorter diameters r is large excessively, the field 
strength E increases at the periphery D of the central portion and hence 
discharge is likely to be concentrated in the periphery portion. 
The ratio of the longer diameter/shorter diameters r has an optimal value 
where no discharge is concentrated in a single place. In experiments, when 
the ratio of the longer diameter/shorter diameter r=3, the field strength 
became substantially even through the electrode width to thereby obtain a 
discharge width WA substantially equal to the electrode width. If the 
confronting surface 23a of the electrode 23 is formed with such optimal 
ratio of the longer diameter/shorter diameters r and the laser is 
operated, discharge extends through substantially the overall surface of 
the electrode elliptical portion 23a, so that abrasion simultaneously 
advances through the overall surface of the electrode elliptical portion 
23a. 
Even if there is a local intense field, a more discharge current flows 
therethrough to thereby abrade that portion more rapidly. A kind of 
negative feedback mechanism operates where the field strength and hence 
abrasion soon decreases. Therefore, a uniform field is maintained in a 
region determined by the electrode width for a long time to thereby 
operate the laser device with a constant discharge width. 
The advantages produced by the provision of such field easing electrode 24 
will be described below. If no field easing electrode 24 is provided, the 
field strength on the surface of the anode electrode 23 increases and much 
contaminations are produced from the electrode 23, disadvantageously. 
However, the provision of such field easing electrode 24 reduces this 
problem. The electrical intensity distribution on the surface of the anode 
electrode 23 changes greatly depending on the shape of the anode electrode 
23. If no field easing electrode 24 was provided, an unstable discharge 
often occurred at the start up of the laser operation. However, this was 
improved by the provision of the field easing electrode 24. Such 
experiment coincides advantageously with the result of the numerical 
analysis and is proved appropriate. 
While in the third embodiment a constant field strength is obtained through 
the overall surface of the electrode elliptical portion 23a with the ratio 
of the longer diameter/shorter diameters r=3 to provide a discharge width 
WA which is the same as the electrode width, this optimal value changes 
depending on the overall width, inter-electrode distance, and geometrical 
layout of the electrodes, the permittivity of the insulating member, and 
the nature of the discharge medium gas, so that the optimal value is 
required to be obtained depending on the situation. 
While in the third embodiment the cross-section of the field easing 
electrode 24 is shown as a circle, the present invention is not limited to 
this particular configuration, and a field easing electrode 25 may have a 
cross-section of a quadrant as shown in FIGS. 18(a) and 18(b). It will be 
seen that FIGS. 18(a), 18(b) and 19(a), 19(b) correspond to FIGS. 14(a-c) 
and 15(a-c), respectively and produce similar effects. 
The sectional configuration of the field easing electrodes is not limited 
to that of the third embodiment. It may be a combination of an elliptical 
arc and a straight line as shown by a field easing electrode 26 in FIG. 
20(a) or a combination of an arc and a straight line as shown by a field 
easing electrode 27 in FIG. 20(b). 
Alternatively, another possible sectional configuration may be an ellipse 
or a curve belonging to a smooth function system. In any case, a smooth 
connection of straight lines or a curve and a straight line renders field 
concentration difficult to occur to thereby prevent surface discharge and 
unnecessary discharge. In summary, the field easing electrode is only 
required to be disposed on both sides of the discharge electrode. 
Provision of a field easing electrode on each side of the discharge 
electrode allows the cathode electrode and the anode electrode alone to be 
exchanged, if necessary, and no field easing electrodes are required to be 
exchanged. Therefore, the exchanging cost and electrode manufacturing cost 
are reduced advantageously compared to the case of the anode electrode and 
cathode electrodes being each made of a single member. 
While in the third embodiment the anode electrode 23 alone is arranged to 
have the discharge width WA which is the same as the electrode width and 
the field easing electrode 24 is provided, both the electrodes 22' and 23 
may have their respective confronting surface formed so as to obtain the 
discharge width WA which is the same as the electrode width and field 
easing electrodes 24' and 24 may be disposed corresponding to the 
electrodes 22' and 23, respectively to thereby produce advantages similar 
to those produced by the third embodiment. 
The material of the field easing electrodes may be metal, ceramics or 
plastic if it is a conductive material. The field easing electrode 24 is 
only required to maintain a potential in a static manner and may be 
covered all with an insulating tube 28, as shown in FIG. 22(a). 
Alternatively, as shown in FIG. 22(b), the field easing electrode 24 may 
be covered partially with an insulating material 29 and connected 
electrically (capacitatively, inductively or resistively) with the 
electrode 23. 
While in the third embodiment the field easing electrode is separately 
provided in addition to the discharge electrode on each side of the 
discharge electrode, these elements may be molded integrally with each 
other. 
As described above, according to the present invention, the discharge 
pumped type laser device can be operated in a stabilized manner for a long 
time without changing the discharge width. Therefore, the use of the 
present laser device for material working ensures stabilized material 
working in a stabilized manner for a long time without changes in the beam 
width and transverse mode and is very advantageous from a standpoint of 
practical use. The use of a wavelength selective device within the 
resonator of the present laser device or in a part of the laser device 
makes realized a narrow band oscillation laser where fluctuations in the 
spectrum width are very small. Long time stabilization of the oscillation 
spectrum width which is the problem with the conventional laser 
lithography is achieved, which is very useful in practical operations.