JPS61176178A - Laser chamber of gas circulation system discharge type - Google Patents

Abstract

PURPOSE:To make a cross section of an output beam square and enlarge its area and to improve a pulse oscillation repeating frequency thereby enabling the uniformity without enlarging a fan by circulating a gas in a manner a gas penetrates through a main discharge electrodes of a mesh structure. CONSTITUTION:The laser gas in the gas circulation path of annular form which is sandwiched by the concentric inner wall 10 and outer wall 9 flows in directions designated by the arrows and B while drawing almost a circle by a fan 13. A pair of main discharge electrodes 2 and 3 having the principal parts of mesh structure are arranged to make a right angle with the gas circulation path. When a spark is caused between the main discharge electrode 3 and a preparatory ionization electrode 8, the generated ultraviolet ray transmits the mesh part of the electrode 3 and ionizes the laser gas between the main discharge electrodes 2 and 3. When a discharge voltage is applied between the electrodes 2 and 3 after the preparatory ionization, the main discharge occurs to cause laser oscillation. The generated impurity gas is exhausted through the mesh-form main discharge electrode 2 and the laser gas of low impurity concentration is introduced through the mesh-form electrode 3.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to an improvement in a gas circulating discharge type laser chamber.

(Background of the invention) Excimer lasers that can emit strong ultraviolet rays are used in lithography, photo-etching, photo-CVD, photo-oxidation,
It is attracting attention as a light source for photoannealing, photodoping, etc.

Figure 2 shows the basic structure of an early excimer laser chamber. Laser gas placed in a chamber housing 1 is discharged by electrolysis applied to a pair of main discharge electrodes 2 and 3, and is discharged from a pair of mirrors. The laser oscillates in an optical resonator (not shown). In FIG. 2, 6 and 7 are insulating materials.

However, in order to obtain a uniform laser oscillation beam with good reproducibility, a pair of main discharge electrodes 2.
It is necessary to pre-ionize the laser gas between 3 and 3. For the pre-ionization method.

There are (i) an X-ray preionization method, (ii) an electron beam preionization method, (iii) a corona discharge preionization method, and (iv) an ultraviolet preionization method. Methods (i) and (ii) can uniformly ionize a large amount of gas, but have the disadvantage that the apparatus becomes large-scale. On the other hand, method 1ii) requires a simple structure such as arranging an electrode that is likely to cause corona discharge, such as a thin wire, near the main discharge electrode, but method (fv) is not practical in terms of reliability. I can say that there is.

In the method (iv), as shown in FIG.
Alternatively, a spark is generated between the main discharge electrode 5 and the main discharge electrode 3, and the ultraviolet rays generated thereby pre-ionize the laser gas. Note that sparks may be caused between the pre-ionization electrode 4 and another pre-ionization electrode placed close to it.

Therefore, when the width of the main discharge electrode is narrow, a pre-ionizing electrode can be placed on the side as shown in Figure 2, but when the width or spacing of the main discharge electrode is large, the pre-ionizing light can pass through. The chamber must be placed in a position that allows uniform gas irradiation. For this reason, when the width of the main discharge electrode is large, it has been devised that the main discharge electrode 3 has a mesh structure as shown in FIG. 3, and a preliminary ionization electrode 8 is provided inside the mesh structure.

However, with the structure shown in FIGS. 2 and 3, even the relatively well-developed XeCl excimer laser
When 10' pulse oscillations are performed.

There is a drawback that the pulse output decreases to the initial value A due to deterioration of the laser gas. Furthermore, the repetition frequency and 1
The higher the energy per pulse, the more rapidly the laser gas deteriorates.

The biggest reason for this is that during the main discharge in the main discharge region sandwiched between main discharge electrodes 2 and 3, the laser gas reacts with the electrode material and the chamber housing material to generate impurity gas, which leads to the 9th main discharge. If this is done, the impurity gas mixed in the laser gas will reduce the discharge excitation efficiency, cause internal absorption of one oscillation light, contaminate the electrode, and increase the transition to arc discharge which is harmful to laser oscillation.

In order to solve this drawback, a laser gas circulation type chamber as shown in FIGS. 4 and 5 (cross-sectional view) was developed.

In the chamber shown in FIG. 4, the charges stored in external capacitors 11 and 12 are discharged between main discharge electrodes 2 and 3, thereby causing laser oscillation. In this case as well, the optical resonator is connected to the main discharge electrode 2 in the direction perpendicular to the plane of the paper.
They are arranged to sandwich 3. Then, the impurity gas generated by the reaction between the laser gas, electrode material, and housing material due to the main discharge is transferred from the region sandwiched between the main discharge electrodes 2 and 3 by the rotation of the fan 13 (referred to as the main discharge region) to the arrow A.
Drive along. Instead, fresh laser gas in the laser gas reservoir 14 sandwiched between the housing outer wall 9 and inner wall 10 is guided along arrow B to the main discharge region. Therefore, when performing the ninth main discharge, the impurity gas concentration in the main discharge region becomes low, so that the above-described drop in pulse output is alleviated.

Of course, it is desirable that the volume ratio between the reservoir and the main discharge region be large. However, in the example shown in FIG. 4, the main discharge region is separated from the laser gas reservoir, so gas circulation cannot be carried out smoothly.

Therefore, in the example shown in FIG. 5, the main discharge electrode 2.3 is incorporated into the IJ reservoir 14. Also.

Since it is advantageous for laser oscillation to arrange the capacitor 11 and the main discharge electrode 2.3 as close as possible, the capacitor 11 is also incorporated into the reservoir 14. Therefore, although the laser gas circulation path itself has a simple circular shape, the condenser 11 now becomes an obstruction to the gas flow.

In order to increase the pulse repetition frequency f of the excimer laser, it is necessary to circulate the laser gas at high speed, so the obstruction of the gas flow by the condenser 11 becomes an increasingly serious problem. Furthermore, when used for the above-mentioned applications (lithography, optical etching...), it is often desirable for the cross-sectional shape of the excimer laser's output beam to be close to a square rather than an elongated rectangle, and for this reason conversion requires a complex optical system. was needed. In order to make the output beam cross section of the excimer laser large and square, the main discharge electrode 2.
3 may be made larger and have the same dimensions, but the pulse repetition frequency f is limited by the capacity of the fan 13.

The reason for this will be explained next. Therefore, first, the relationship among the laser gas circulation speed, pulse oscillation repetition frequency f, and electrode dimensions will be explained using FIG. 6-7.

Assume that the width of the gas circulation path is the width of the main discharge electrode, the width of the main discharge electrode is W9, and the interval is d, and the gas flow rate that flows through the main discharge region when the fan 13 is rotated at a constant rotation speed is V.

The relationship between V/V6 and d/D is as shown in FIG.

However, the length of the main discharge electrodes 2 and 3 in the oscillation axis direction is constant, and the laser gas circulation speed when the fan 13 is rotated at the same rotation speed with the main discharge electrodes 2 and 3 removed is v.
o. From FIG. 7, in the region where d/D is 0.5 to 1,
v= CD/d) There is a relationship of V6, but d/D<0.5
In this case, V/V6 has a maximum value, and in the region of d/D~0, V/V6 rapidly decreases.

On the other hand, to obtain a beam cross-sectional shape that is close to a square.

It is necessary to make it dew, but W can be arbitrarily thick (although it can be done, d is only D at most.

After all, in order to obtain a shape as large as possible and as close to a square as possible, the ratio must be d/D-1. Therefore, if we focus on the gas flow velocity near d/D, 1, we can see that, as mentioned above, the relationship between vs (D/d)V@, where f is the repetition frequency, during 1/f there is an impurity generated by the main discharge. All that is required is to expel the gas from the main discharge area. This condition is v
/ f≧W and r v = CD/d)Vo, so w d faD v .

/f. Therefore, in order to increase the beam cross-sectional shape (area) Wd, it is necessary to decrease f or increase IDIvo. However, D.

Increasing vo requires a larger fan.

In other words, in not only excimalizers but also conventional gas circulation discharge type laser chambers, if the cross-sectional shape of the output beam is squared and the area is increased without increasing the size of the fan, the pulse oscillation repetition frequency f can be increased. Also, uniformity decreases.

(Objective of the Invention) The object of the present invention is to make the cross-sectional shape of the output beam square and enlarge the area without increasing the size of the fan in a gas circulation discharge type laser chamber as shown in FIG. The objective is to improve the pulse oscillation repetition frequency f and make it uniform.

(Summary of the Invention) Therefore, the feature of the present invention is that, as shown in FIG. The object is to have a mesh structure and to circulate gas through the mesh structure.

Hereinafter, the present invention will be specifically explained with reference to Examples.

(Embodiment) FIG. 1 is a schematic vertical sectional view of a chamber of this embodiment, and the laser gas in the donut-shaped gas circulation path sandwiched between the concentric inner wall lO and the outer wall 9 is fed by a fan 13. Draw a circle and arrow A.

Flows in direction B. A pair of main discharge electrodes 2 and 3.

The main parts of both have a mesh structure and are arranged at right angles to the gas circulation path.

When a spark is caused between the main discharge electrode 3 and the pre-ionization electrode 8, which have the main part of a mesh structure.

The ultraviolet rays generated here pass through the mesh portion of the main discharge electrode 3 and ionize the laser gas in the main discharge region between the pair of main discharge electrodes 2 and 3. At this time, the preliminary ionization electrode 8
is preferably arranged inside the main discharge electrode 2. After preionization,
When an appropriate discharge voltage is applied between the mesh-shaped main discharge electrodes 2 and 3, a main discharge occurs and laser oscillation is performed. When the main discharge occurs, impurity gas that is disadvantageous to laser oscillation is also generated in this region, so this is expelled through the mesh-like main discharge electrode 2. And mesh-like electrode 3
A laser gas with a low impurity concentration is sent through the tube.

° Here, the relationship among the laser gas circulation speed, pulse oscillation repetition frequency, and electrode dimensions will be explained. The width of the gas circulation path is determined by the width of the mesh-shaped main discharge electrode 2.3, and the interval W9 is determined by d.
When the fan 13 is rotated at a constant rotation speed, the gas flow velocity in the main discharge region is V, and the aperture ratio of the mesh is α, the relationship between V/V 6 and αw/D is shown in Figure 7. It is expressed as d-αW in . However, since the influence of the pre-ionization electrode 8 is small, it is ignored, and the length of the mesh-like main discharge electrode 2.3 in the oscillation axis direction is kept constant.
Further, let vo be the laser gas circulation speed when the fan 13 is rotated at the same rotation speed with the electrode 2.3 removed. Let f be the pulse oscillation repetition frequency.

Therefore, the beam cross-sectional area (Wd) obtained in this embodiment is St,
The beam cross-sectional area (wd) obtained in the conventional example (Fig. 5) is S
, then +S1 and S! The ratio r is r=S2/5I
=1/α, and the beam cross-sectional area of this embodiment is several times larger. In addition.

The amount of pre-ionizing ultraviolet light passing through the mesh is determined by the aperture ratio α
Therefore, it is necessary to select α from about 0.3 to 0.7.

Next, some operational data of the chamber of this example will be shown.

[Data 1] Laser wavelength λ = 249 nm / Laser gas composition He:Kr:Fz #97:3:0.2 (volume ratio) Total atmospheric pressure 2.5 atm Main discharge electrode length 1m Width 3ca+ Interval 33 Pulses Oscillation repetition frequency 300Hz・Aperture ratio α 0.5・Output 45W・Life span (time to reach 90% of initial value) 2×10′ pulse・Beam cross-sectional shape 2.5×2.5 cIll
Square [Data 2] Laser wavelength λ-308nm Laser gas composition Ne:Xe:Hcl#98.9:
0.95: 0.15 (volume ratio) 〃 Total atmospheric pressure 2.5ata+ ・Main discharge electrode length 1m 〃 Width 3clI 〃 Spacing 3cm ・Pulse oscillation repetition frequency 300Hz ・Opening ratio α
0.5 - Output 25W - Lifespan 2xlO' pulse - Beam cross-sectional shape 2.5 x 2.5 cs square [Data 3] Laser wavelength λ = 222ruw - Laser gas composition
Ne:'Kr:Hcl -98:1.5:0.5 (volume ratio) ・〃 Total atmospheric pressure 2.5 atm ・Main discharge electrode length 1 m 〃 Width 3c11 〃 Spacing 3 cm ・Pulse oscillation repetition frequency 300 Hz ・Aperture ratio α 0.5 ・Output 5W ・Life span 2×10' pulse ・Beam cross-sectional shape 2.5
He:Ar:F, = 97:2.7:0.3 (volume ratio) ・I Total atmospheric pressure 2.5 atm ・Main discharge electrode length 1m, Width 3al ・Spacing 3am ・Pulse oscillation repetition frequency 300 Hz・Aperture Rate α 0.5 - Output 20w - Lifespan 4.5 x 10' pulse - Beam cross-sectional shape 2.3 x 2.3 cm square Note that the pre-ionization electrode 8 is not only inside the main discharge electrode 2 or 3, but also inside the electrode 2 They may be placed slightly apart between .3 and 3.

Furthermore, as shown in FIG. 8, the chamber of the present invention contains a chamber for impurity gas adsorption, which has been conventionally performed.

In that case, a cold trap may be placed.

The pulse oscillation repetition life is improved by 10 to 20%.

When the oscillation output power decreases, it can be restored to the initial oscillation output power by replenishing halogen gas or reinjecting mixed laser gas.

Furthermore, the circulation fan 13 may be located at the center of the reservoir as shown in FIG.

(Effects of the Invention) As described above, according to the present invention, the main discharge electrode has a mesh structure, and the laser gas circulation path is set to penetrate the mesh structure, so that impurity gas is efficiently removed from the main discharge area wear. In addition to being able to circulate gas without increasing the size of the fan, it is also possible to obtain a square beam cross section over a large area with good uniformity while oscillating at a high pulse repetition frequency.

[Brief explanation of the drawing]

FIG. 1 is a schematic vertical sectional view of a chamber according to an embodiment of the present invention. 2-6 are schematic vertical cross-sectional views of conventional chambers. FIG. 7 is a graph showing the relationship between the gas flow rate and the dimensions of the main ionization electrode. 8 and 9 are schematic vertical sectional views of a chamber according to another embodiment of the present invention. [Explanation of symbols of main parts] t −−−−−−−−−−−・−・Housing 2.
3 ・−・・−・ Pair of main discharge electrodes 4.5.8 ・
−−−1111・Preliminary ionization electrode 9−−−−−−−
-----1 Outer wall (housing) 10 -------
−−−−1 Inner wall 13−・−・−Fan

Claims (1)

[Scope of Claims] 1. In a gas circulation discharge type laser chamber in which laser gas circulates in a nearly circular manner, the main parts of a pair of main discharge electrodes have a mesh structure, and the laser passes through the mesh structure. A chamber characterized in that the electrodes are arranged so that gas circulates. 2. The chamber according to claim 1, wherein a preliminary ionization electrode is disposed inside one of the pair of main discharge electrodes.