JP3219197B2 - Laser device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a discharge excitation type laser device used as a light source for material processing, projection exposure, and the like.

[Conventional technology]

Discharge excitation type laser devices are used for material processing such as marking, cutting, welding, etc. In addition to large scale integrated circuits (LSI)
It is used as a light source for optical lithography for the production of the circuit pattern.

For material processing, CO ₂ , excimer laser, or the like is used. In optical lithography, a reduction projection method is mainly used. In this reduction projection method, transmitted light of an original (reticle) pattern illuminated by an illumination light source is transmitted to a photosensitive material on a semiconductor substrate by a reduction projection optical system. Projection forms a circuit pattern. Since the resolution of the projected image is limited by the wavelength of the light source used, the wavelength of the light source for improving the resolution has been gradually shortened from the visible region to the ultraviolet region in recent years. Conventionally, g-rays (wavelength: 463 nm) and i-rays (wavelength: 365 nm) generated from a high-pressure mercury lamp as a light source in the ultraviolet region.
nm) was used. But the minimum pattern line width is 64
When it is less than 0.25 μm, which is required for MB, i-line has already reached its limit for shortening the wavelength. As a solution to this technical limitation, Deep Ultra Violet;
A laser light source (hereinafter referred to as Deep-UV) is promising.
In particular, excimer lasers have high output and high efficiency, and can obtain strong oscillation at short wavelengths such as KrF (wavelength 246 nm) and ArF (wavelength 193 nm) depending on the composition of the medium gas. Deep-UV above
In the region, the glass and crystal material constituting the reduction projection lens system are extremely restricted, so that it becomes difficult to correct the chromatic aberration used in the reduction projection lens system using a mercury lamp. Therefore, instead of correcting the chromatic aberration of the lens system, a wavelength selection element such as an etalon is provided in the laser resonator, and the spectral width of the output light is reduced to such an extent that the chromatic aberration of the lens material can be ignored. With this method, the output which was several nm in the spectral width in the case of natural oscillation can be narrowed to several pm and narrowed. Techniques for narrowing the band include a method in which one or a plurality of Fabry-Perot etalons in which a pair of reflecting films are arranged in parallel are arranged in a resonator, or a method using a diffraction grating (grating) as a total reflection mirror. There is a technology to achieve this. However, the former method using an etalon can narrow the band relatively easily by using a plurality of etalons and superimposing them. Problems such as durability of the film and drift due to loss occur. On the other hand, in the latter method using the diffraction grating, the light intensity is weaker than the method using the etalon because the wavelength can be selected by a single reflection on the diffraction grating surface, so the load on the reflection surface is small, and the number of reflections is only one, so Suitable for high output with low loss. Therefore, a narrow band excimer laser device using a diffraction grating is used as a light source for optical lithography taking advantage of these advantages.

[Problems to be solved by the invention]

Such a narrow band excimer laser is a discharge excitation type laser device, and as shown in FIG.
A laser gas such as KrF is filled, and this laser gas is discharged and excited by electrodes 6 and 7 disposed vertically along the longitudinal direction of the laser chamber 1 to perform laser oscillation. In the drawing, a portion 11 surrounded by a broken line is a discharge excitation region.
The opposite surfaces 6a and 7a of the electrodes 6 and 7 are consumed by the discharge, and the discharge width is reduced.
WA changes. At the same time, the beam width of the output laser light changes. This poses a problem in terms of stabilizing the beam width. In the same figure, the same figure (b) corresponds to C-
FIG. 3C is a vertical cross-sectional view of FIG.
When used for processing, this change in the beam width of the laser degrades the transverse mode, changes the beam condensing property, and causes a change in output, which poses a practical problem. In particular, when used as a light source for optical lithography, this change in the beam width causes the following inconveniences in practical use of a narrow band excimer laser.

That is, as described above, a diffraction grating (grating 5; FIG. 5 (a)) is used as a wavelength selection element in order to narrow the band. Since it is used, the angular dispersion at the operating point is large and the divergence angle of the laser directly affects the spectrum width. That is, when the divergence angle is large, the spectrum width is widened. Therefore, when the discharge width changes and the gain region changes, the beam divergence of the laser light changes, so that the spectrum width also changes greatly. In order to avoid this fluctuation, the conventional aperture 8 (FIG. 5A) is provided to stabilize the gain width. However, in the so-called Chang-type electrode and the like conventionally used, when the output is increased when the electrode is consumed, the discharge width inevitably increases greatly, so that the gain limitation by the aperture 8 makes it difficult to control the output.

Here, a mechanism for increasing the discharge width will be described.
Townsend theory is known as a macroscopic phenomenology for explaining the discharge phenomenon, and is useful for understanding the discharge phenomenon. According to this theory, a gas containing a halogen gas used in an excimer laser is called a negative gas, and captures electrons generated by impact ionization (ionization coefficient: α) of electrons during discharge due to a large electron affinity of halogen. (Electron attachment coefficient:
η) and the apparent ionization coefficient (α-η) are reduced to facilitate discharge concentration. Each of these coefficients and the electric field strength E
Is shown in FIG. P is a constant for normalization.

As apparent from FIG. 6, the ionization coefficient α strongly depends on the electric field strength E, and the electron attachment coefficient η hardly depends on the electric field strength E. Therefore, the apparent ionization coefficient (α−η) has a certain electric field strength E As described above, the value rapidly increases and greatly depends on the electric field intensity E. That is, since the parameter (α-η) for driving the discharge changes according to the electric field intensity distribution on the electrode surface, the discharge width is greatly affected by the electric field intensity distribution on the electrode surface. Therefore, in order to secure the discharge width, it is necessary to increase the uniform electric field intensity region in the electrode width direction as the electrode shape of the discharge excitation type laser device. Conventionally, shapes such as Chang and modified Chang type electrodes have been adopted based on analytical solutions of electric field calculations under ideal conditions.

Here, FIG. 7 (a) shows the potential distribution of the electric field calculation in the case of the modified Chang type electrode. In FIG. 1A, L1... Are equipotential lines. From the figure, the potential distribution between the cathode (upper electrode) 6 and the anode (lower electrode) 7 at the negative high voltage potential is largely bent by the metal plate on which the anode 7 is mounted, the current return wiring 10 and the insulating member 9. You can see that there is. That is, in the actual configuration, the electric field is shifted from the ideal Chang-type electric field due to such wiring and insulation.

FIG. 7B shows the isoelectric field intensity line L2 near the cathode electrode 6.
.., (C) shows how the electric field intensity changes along the surface of the cathode electrode 6. The electric field intensity on the surface of the electrode 6 is clearly about 1/3 of the electric field intensity to the right from the electrode center point A, and the intersection of the electric field intensity line is small in this area, and a uniform electric field portion is formed in this area. You can see that it is done. As shown by the line L3 in FIG. 3C, the electric field intensity rises very slowly at first from the center point of the electrode (point A) to the right end point of the electrode (point B), and rapidly increases as approaching the end point. You can see the rise. FIG. 4D shows the isoelectric field intensity lines L4 in the vicinity of the anode electrode 7, and FIG.
Fig. 7 shows how the electric field intensity changes along the electrode surface. The electric field strength at the center point A is E ₀ . As shown in these figures, the electric field intensity is uniform in the first section of about 4 mm from the electrode center point (point A), and then gradually weakens toward the right end point of the electrode (point B) and then sharply increases after leaving about 12 mm. (See lines L4 and L5). This has a tendency opposite to that of the cathode 6 described above. Although the discharge occurs at the center of the electrode, it is considered that the factor limiting the width of the discharge is on the anode 7 side where the electric field intensity is large at the center.

After 1 × 10 ⁸ shots from the start of discharge, the shape of the electrode at the central part of the depleted electrode was measured, and the electric field intensity distribution at the central part of the depleted electrode was calculated by a simulation analysis method using the shape measurement data. . The simulation results are shown in FIG. 8 with the horizontal axis representing the rightward distance from the center point A and the vertical axis representing the rate of change ΔE / E ₀ (%) of the electric field intensity E with respect to the electric field intensity E ₀ at the central point A. FIG. 2A shows the cathode 6, and FIG. 1B shows the anode 7. The white circles indicate before the start of discharge, and the black circles indicate after 1 × 10 ⁸ shots. As is clear from these figures, 1 × 10
After ⁸ shots, wear has progressed, and the uniform electric field portion has spread near the center point than at the start. The width of this uniform electric field portion substantially corresponds to the observed beam width. This is considered to be due to the fact that the current was concentrated on the uniform electric field portion, which is a strong electric field, and the wear proceeded, and as a result, a wide uniform electric field portion was formed. As described above, in the conventional electrode configuration, when the operation of the laser device is started, the electrodes are worn out with the lapse of time, the uniform electric field portion is enlarged, and the beam width of the output laser light fluctuates accordingly. Become.

The present invention has been made in view of such circumstances,
It is an object of the present invention to provide a laser device capable of reducing a change in a beam width due to electrode consumption.

[Means for solving the problem]

Therefore, according to the present invention, in a laser device which discharges between two electrodes disposed to face each other along the longitudinal direction of a laser chamber and excites a laser gas in the laser chamber to generate laser light, Of the outer lines of a cross section perpendicular to the longitudinal direction of the laser chamber of at least one of the two electrodes, a line facing the other electrode is formed in an arc, and the arc and a line on the opposite side of the arc are formed. Connected by a connection line, the connection line is separated from a portion where the connection line is connected to the arc and a portion where the connection line is connected to a line on the opposite side of the arc. Are formed in an arc shape having a radius equal to or less than the radius of the arc, and a straight line is formed between arcs having a radius equal to or less than the radius of the arc.

[Action]

That is, according to such a configuration, even if electrode wear due to discharge occurs on the opposing surface of the electrode, it is difficult to spread a range in which the electric field intensity is uniform. In addition, the width of the electrode in the discharge width direction does not easily change. For this reason, the fluctuation of the spectrum width of the laser beam due to electrode consumption can be further minimized.

〔Example〕

Hereinafter, embodiments of the laser device according to the present invention will be described with reference to the drawings. In the embodiment, a narrow-band oscillation excimer laser is assumed as the laser device. The apparatus of the embodiment has basically the same configuration as that shown in FIG. 5 previously, and uses electrodes 12 and 13 instead of the electrodes 6 and 7. The laser chamber 1 is filled with a laser gas such as KrF, and the laser gas is supplied to an electrode 12 (cathode electrode), an electrode 13 disposed vertically along the longitudinal direction of the laser chamber 1.
(Anode electrode) discharges and excites to perform laser oscillation. The laser beam is emitted from the laser chamber 1 and the front mirror 2
The laser beam is resonated by an optical resonator composed of a wavelength selection element and a grating 5, and is output as an effective laser beam 4 from the front mirror 2. The grating 5 narrows the oscillation light band and functions as a rear mirror, and has a so-called Littrow arrangement. In the drawing, a portion 11 surrounded by a broken line is a discharge excitation region, 8 is an aperture, 9 is an insulating member, and 10 is an anode current return wiring. WA indicates a discharge width. The cross section of the electrodes 12, 13 perpendicular to the longitudinal direction of the laser chamber 1, that is, the cross section, is symmetrical, and thus the potential distribution during operation of the laser device is symmetrical with respect to the center axis of the cross section. FIG. 1 (a) shows the potential distribution only on the right side of the electrodes 12 and 13.

In FIG. 1A, the upper side is a cathode electrode 12 having an arc-shaped facing surface 12a having a radius R ₁ (13 mm). Side 12b and a bottom surface 12c of the cathode electrode 12 is composed of a straight line, with the opposing face 12a and the side surface 12b are connected by an arc having a radius r _1, and the side surface 12b and the bottom 12c are connected by an arc having a radius r ₂ ing. The size of these radii r ₁ and r ₂ is
It is set to a small predetermined value than the radius R ₁ of a. on the other hand,
The lower side is an anode electrode 13 having an electrode width of 8 mm and an opposing surface 13a formed by an arc having a radius R ₂ (9 mm). This anode electrode
13 is also configured that the side walls 13b and bottom 13c is a straight line, the opposing face 13a and the side 13b is connected by an arc of a radius R _3, are connected by an arc having a radius r ₄ and the side surface 13b and the bottom 13c . The size of these radii r ₃ and r ₄ depends on the facing surface 13a.
It is set to a small predetermined value than the radius R _2. The electrode distance d between the cathode electrode 12 and the anode electrode 13 is 20 mm, and the radii R ₁ , R ₂ , r ₁ , r ₂ , r _3, and r ₄ of the respective arcs are equal to or less than the electrode distance d.

As shown in FIG. 7A, the tendency of the equipotential lines is similar to that of the conventional one shown in FIG. 7A. The potential gradient at is gentle. The potential gradient of the anode 13 becomes gentler as the distance from the center to the right increases.

FIGS. 2B and 2D show the distribution of the electric field intensity near the cathode 12 and the anode 13 in an enlarged manner.
It can be seen that a relatively wide uniform electric field region is formed in the case of, whereas the electric field intensity of the anode 13 decreases rapidly when it is separated rightward from the center. FIG. 4C shows the electric field strength on the surface along the surface of the cathode electrode 12, and from this, 7 to 8 m from the center point A to the right.
It can be seen that a uniform electric field region is formed over the section separated by m. On the other hand, FIG. 7E shows the electric field intensity on the surface along the surface of the anode electrode 13, from which a uniform electric field region is formed in a very narrow part of a section 1 to 2 mm to the right from the center point A. Then, the electric field suddenly weakens as going from there to the periphery. That is, it can be seen that the discharge is concentrated in the narrow area at the center. In any case, the electric field strength of both the electrodes 12 and 13 decreases rapidly toward the periphery of the opposing surface of the electrodes. Therefore, even if the electrodes are consumed by the discharge, the uniform electric field portion is hardly spread, and a narrow-band oscillation excimer laser device in which the fluctuation of the spectrum width due to the consumption of the electrodes is minimized is realized.

In the embodiment, the cross-sectional shape of the electrodes on both sides is formed such that the opposite surface is an arc and the side surface is a straight line, and the straight lines of these arcs are connected by an arc. It is possible. In this case, as shown in FIG. 2, when the process is performed on the anode 13 ', better results can be obtained than when the process is performed on the cathode.

In the embodiment, the line of the cross section of the electrode has a straight line, but the cross section of the electrode may be a perfect circle. In this case, as shown in FIG. 3, a recess 15 corresponding to the size of the circle may be provided in the anode current return wiring 10, and the electrode 14 may be held and fixed by the recess 15. This provides an advantage that the production of the electrode 14 becomes easy.

Also, without the cross-sectional shape of the electrodes to be limited to the embodiments, and a bottom surface which is formed in the opposing surface and the straight line formed by an arc of a radius R ₃ as shown in FIG. 4 (a) radius r _The electrodes 16 connected by an arc of ₅ or the opposed surface formed by an arc of a radius R ₄ and the side surface formed by a straight line are smoothly connected,
A side surface and a bottom surface is a straight line electrode 17 or the like to be connected by an arc of radius r ₆ are conceivable. In short, the present invention is arbitrary as long as at least the opposing surface is formed by an arc and the straight line and the arc, or the straight line are connected smoothly.

〔The invention's effect〕

As described above, according to the present invention, the uniform electric field portion can be suppressed to a constant level even if the facing surface of the electrode is consumed by the discharge. In addition, the width of the electrode in the discharge width direction does not easily change. For this reason, a narrow-band oscillation excimer laser in which fluctuations in the spectral width of laser light due to electrode consumption are further minimized is realized.

[Brief description of the drawings]

FIG. 1 (a) is a diagram showing the distribution of equipotential lines near electrodes used in an embodiment of the laser device according to the present invention, and FIG. 1 (b) is shown in FIG. 1 (a). FIG. 1 (c) showing the distribution of the electric field intensity near the cathode electrode shown in FIG.
FIG. 1D is a graph showing a change in electric field intensity along the surface of the cathode electrode, FIG. 1D is a diagram showing a distribution of electric field intensity near the anode electrode, and FIG. FIG. 2 to FIG. 4 are graphs showing the cross-sectional shape of an electrode used for explaining another embodiment, and FIG. 5 is a graph showing the state of a conventional laser device. FIG. 6 is a graph showing the overall configuration, FIG. 6 is a graph showing the electric field intensity dependence of the ionization coefficient, adhesion coefficient, and apparent ionization coefficient during excimer laser gas discharge, and FIGS. 7 (a), (b), (c),
(D) and (e) respectively show the potential distribution near both electrodes, the electric field intensity distribution near the cathode electrode, the change in electric field intensity at the surface facing the cathode electrode, and the electric field intensity near the anode electrode when the electrodes according to the prior art are used. FIG. 8 is a graph showing the distribution and the change in the electric field strength on the surface facing the anode electrode, and FIG. 8 is a graph showing the rate of change in the electric field strength on the electrode surface before and after the laser operation when using the conventional electrode. 1 ... laser chamber, 2 ... front mirror, 4 ...
Laser light, 5 ... Grating, 12 ... Cathode, 13, 13 ', 16, 17 ... Anode.

Continuation of the front page (56) References JP-A-63-17579 (JP, A) JP-A-57-20488 (JP, A) JP-A-63-227068 (JP, A) , U) Laser Research Vol. 11, No. 5, p. 364-365 (58) Fields investigated (Int. Cl. ⁷ , DB name) H01S 3/03-3/038

Claims (1)

(57) [Claims] 1. A laser device for performing a discharge between two electrodes disposed to face each other along a longitudinal direction of a laser chamber to excite a laser gas in the laser chamber to generate a laser beam, Of the outer lines of a cross section perpendicular to the longitudinal direction of the laser chamber of at least one of the two electrodes, a line facing the other electrode is formed in an arc, and the arc and a line on the opposite side of the arc are formed. A connection is made by a connection line, and a portion of the connection line where the connection line is connected to the arc and a portion where the connection line is connected to a line on the opposite side of the arc are separated from each other. A laser device which is formed in an arc shape having a radius equal to or less than the radius of the circular arc, and a straight line is formed between the arcs having a radius equal to or less than the radius of the circular arc.