JP2003204104A - Laser unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser unit as a short wavelength light source of an exposure apparatus having a refractive projection optical system, whose output light is in a narrow band region while satisfying a required output. <P>SOLUTION: The laser unit includes a band narrowing means using etalon which consists of two substrates having coating on opposing surfaces, and comprises a laser resonator that emits a laser beam whose band is narrowed by the band narrowing means; and a laser amplifier that amplifies the laser beam emitted from the laser oscillator. The etalon is constituted such that the optical reflectivity R of the coating satisfies a predetermined equation supposing a spectrum waveform width of the laser beam emitted from the laser resonator when its band is not narrowed is set to 2A and the number of times that laser beam passes through the band narrowing means is M. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser device which outputs a narrow band short wavelength light, and more particularly to a laser device which is suitable as a light source of an exposure apparatus having a refraction projection optical system.

[0002]

2. Description of the Related Art As the degree of integration of semiconductor devices has improved, 70
As a light source of an exposure apparatus for realizing a circuit having a marking width of nm or less, a light source that emits light having a wavelength of 160 nm or less is required. In particular, F ₂ (fluorine molecule: molecular f) that generates light having a wavelength near 157 nm
A laser device is considered to be a promising light source.

By the way, the projection optical system of the exposure apparatus is roughly classified into two types: a dioptric system and a catadioptric system. The refraction system is a projection optical system that has been widely used in conventional exposure apparatuses, but the correction of chromatic aberration of lenses and the like poses a serious problem. Conventionally, chromatic aberration has been corrected for a plurality of wavelengths included in light generated by a light source by combining a plurality of optical elements such as lenses having different refractive indexes. However, the F ₂ laser device generates 15
As a material having sufficient transmissivity for light in the wavelength region near 7 nm, materials other than CaF ₂ cannot be used. On the other hand, when a catadioptric projection optical system is used, the occurrence of chromatic aberration can be suppressed. However, in the catadioptric system, since many reflecting mirrors are used and they are multiaxial, it is difficult to adjust the optical axis as compared with the uniaxial structure of the refractive system.

Therefore, in order to use the F ₂ laser device as a light source of an exposure device having a refraction type projection optical system, it is desired to narrow the output light of the F ₂ laser device. Full width at half maximum (FWHM) of the spectrum of the output light of the F ₂ laser device in the case of free running
th at half maximum) is about 0.8 pm to 1.3 pm, but in order to use it as a light source of an exposure apparatus having a projection optical system of refraction system, it must be 0.2 pm or less. As an example of narrowing the band, a technique is known in which an etalon is arranged in the optical path of a resonator of a laser device.

FIG. 11 shows an example of a one-stage band narrowing laser device in which an etalon is arranged in the optical path of a resonator. Figure 11
In the laser chamber 1, a pair of discharge electrodes 2 composed of an anode electrode and a cathode electrode arranged in a direction perpendicular to the plane of the drawing, and a fan 3 for circulating a gas.
And a radiator (not shown) for cooling the gas heated to a high temperature by the discharge. Further, windows 4 and 5 are arranged on the rear side wall and the front side wall of the laser chamber 1, respectively, in a V-shape so as to form a predetermined Brewster angle with the optical axis passing through the laser chamber 1. On the rear side of the laser chamber 1, a band narrowing module 50 including an etalon 51 and a total reflection mirror 52 is arranged. The etalon 51 functions as a filter for realizing a narrow band of output light.

The laser chamber 1 is filled with a laser gas as a laser medium, and a high voltage pulse generator 6 is used.
Thus, when a high voltage is applied to the discharge electrode 2, the laser gas is excited to generate seed light which becomes laser light. The generated seed light is transmitted from the rear window 4 to the band narrowing module 5
Etalon 5 in the band narrowing module
After passing through 1, the laser beam is totally reflected by the total reflection mirror 52, passes through the etalon 51 again, passes through the laser chamber 1, and reaches the front mirror 7 installed on the front side. After that, a part of the light is reflected again to the laser chamber 1 according to the reflectance of the front mirror, and the total reflection mirror 52
After reciprocating between the front mirror 7 and the front mirror 7, laser light is output from the front mirror 7.

On the right side of the front mirror 7, there is arranged a beam splitter 8 which splits light into two directions.
The wavelength monitor 9 measures the output, the center wavelength, etc. of a part of the laser light divided for the monitor, and the controller 10 determines the incident angle of the etalon 51 and the etalon gap between them based on the measurement result of the wavelength monitor 9. The oscillation center wavelength is controlled by adjusting the gas pressure and the like.

[0008]

When the above laser device is used as a light source of an exposure device, it is 5 mJ to 10 m.
Although a high output of J is required, in order to obtain such a high output, light of high intensity is passed through the etalon, which causes deterioration or deformation of the coating of the etalon. Therefore, only the etalon without coating can be used, and the finesse cannot be increased, so that the line width cannot be narrowed.

As described above, it is desired to narrow the output light of the F ₂ laser device as the light source of the exposure apparatus having the refraction type projection optical system. When the etalon is used as the narrowing means, the exposure apparatus is used. In order to satisfy the output and durability required as the light source of the above, there was a problem that the requirement of the spectral width of the output light could not be achieved.

In view of the above points, the present invention provides a short wavelength light source for an exposure apparatus having a refractive projection optical system,
An object of the present invention is to provide a laser device in which the output light is narrowed while satisfying the required output.

[0011]

In order to solve the above problems, the laser device according to the first aspect of the present invention is a band narrowing means using an etalon composed of two substrates having coatings on opposite surfaces. Including a laser oscillator that emits a laser beam narrowed by the band narrowing means, and a laser amplifier that amplifies the laser beam emitted from the laser oscillator, and The width of the spectrum when it is not narrowed is 2
Assuming that A is the number of times that the laser light passes through the band narrowing means, and M is the light reflectance R of the coating,

[Equation 4] The etalon is configured to meet.

A laser device according to a second aspect of the present invention is a beam splitter which is arranged on the optical axis of laser light at a predetermined inclination angle and has a coating on either one of the surfaces, and this beam splitter. It includes a band narrowing means using a FOX-SMITH interferometer, which is composed of two total reflection mirrors which are arranged above and below the splitter with their respective reflecting surfaces facing each other, and is narrowed by the band narrowing means. A laser oscillator that emits a laser beam and a laser amplifier that amplifies the laser beam emitted from the laser oscillator, and the FOX-SMITH interferometer is arranged on the front side of the laser oscillator to emit the laser beam. If the width of the spectrum of the laser light to be narrowed is 2A and the number of times the laser light passes through the narrowing means is M, Computing the light transmittance T is of the formula shown below

[Equation 5] The FOX-SMITH interferometer is configured to satisfy

Further, a laser device according to a third aspect of the present invention includes a beam splitter which is arranged on the optical axis of laser light at a predetermined inclination angle and has a coating on the surface opposite to the laser medium side. , A narrowing means using a FOX-SMITH interferometer comprising two total reflection mirrors arranged so that the total reflection surface faces the coating surface of the beam splitter, and the narrowing means narrows the band. A laser oscillator that emits the emitted laser light and a laser amplifier that amplifies the laser light emitted from the laser oscillator, and the FOX-SMITH interferometer is arranged on the rear side of the laser oscillator. Assuming that the width of the spectrum of the emitted laser light when the bandwidth is not narrowed is 2A and the number of times the laser light passes through the narrowing means is M, Computing the optical reflectance R is represented by the formula shown below

[Equation 6] The FOX-SMITH interferometer is configured to satisfy

According to the present invention, since the laser light emitted from the laser oscillator is amplified by the laser amplifier in a two-stage configuration, the intensity of light incident on the etalon or the like arranged in the laser oscillator can be suppressed. Therefore,
Since it is possible to prevent the deterioration and deformation of the coating such as etalon, the high finesse (Fi
It is possible to sufficiently narrow the output light band by using an etalon or the like having a nesse).

[0015]

DETAILED DESCRIPTION OF THE INVENTION Referring to the accompanying drawings,
An embodiment of the present invention will be described. In each of the following embodiments, an F ₂ laser device will be described as an example. In addition, the same reference numerals are given to the same components, and the description thereof will be omitted.

First, a first embodiment of the present invention will be described. FIG. 1 is a diagram showing a configuration of a laser device according to a first embodiment of the present invention. This laser device has a two-stage configuration in which laser light emitted from a laser oscillator is amplified by a laser amplifier. Here, an injection locking type two-stage laser device will be described.
It may be a two-stage laser device of the er oscillator power amplifier) type. Here, the injection lock method is a method in which a laser amplifier constitutes a resonator, that is, reflection mirrors are arranged on both the rear side and the front side, and in the MOPA method, the laser amplifier does not constitute a resonator. That is, this is a system in which no reflecting mirror is arranged on both the rear side and the front side.

As shown in FIG. 1, a laser oscillator 100 using fluorine molecules as a laser medium includes a laser chamber 11 containing a mixed gas of fluorine and a buffer gas. Electrodes (not shown) are arranged inside the laser chamber 11 so as to face each other vertically, and windows 14 and 15 are provided on both sides of the laser chamber 11 with a predetermined optical axis that penetrates the laser chamber 11. They are arranged in a V shape so as to form Brewster's angle. This Brewster angle makes it possible to suppress the loss due to the reflection of light on the surfaces of the windows 14 and 15.

A band narrowing module 150 is arranged on the left side of the laser chamber 11 via an aperture 101 having a through hole on the optical axis of the laser chamber for passing laser light therethrough. The band narrowing module 150 is the etalon 1
51 and a rear mirror 152.

The etalon 151 is installed with a predetermined inclination angle with respect to the optical axis of the laser light. Etalon 1
51 has two substrates 153 and 154 facing each other at a distance d, and the facing surfaces of these substrates are provided with a partial reflection coating. In addition, as the substrate material of etalon,
CaF ₂ can be used. In the case of air gap etalon, contact materials such as Zerodur and ULE
Low expansion glass such as Instead of the air gap etalon, it is also possible to use a solid etalon in which the above-mentioned partial reflection coating is applied to both surfaces of a highly transparent parallel substrate such as CaF ₂ .

An output mirror 103 is arranged on the right side of the laser chamber 11 via an aperture 102 having a through hole on the optical axis passing through the laser chamber 11 for passing laser light therethrough. It should be noted that the apertures 101 and 102 may actually have through holes formed therein, or an aperture having a black coating around the through holes may be used to reduce reflection.

Laser oscillator 100 and laser amplifier 200
A transmission system 300 including two reflection mirrors 301 and 302 arranged so as to transmit the laser light emitted from the laser oscillator to the laser amplifier is provided between and.

The laser light generated in the laser chamber 11 passes through the window 14 and the aperture 101, and the etalon 151 arranged in the band-narrowing module 150.
It emits toward. The laser light that has entered the etalon 151 is totally reflected by the rear mirror 152, then again enters the etalon 151, and is directed in different directions depending on the wavelength. As a result, only the laser light having the predetermined wavelength passes through the aperture 101. The laser light that has passed through the aperture 101 passes through the inside of the laser chamber 11 and is emitted toward the aperture 102. Here, too, only the laser light having the predetermined wavelength passes through the aperture 102, so that the laser light has a narrow band.

A part of the laser beam that has passed through the aperture 102 is reflected by the output mirror 103 and returns to the laser chamber 11. In this way, the resonance of the laser light occurs between the rear mirror 152 and the output mirror 103. The other part of the laser light that has passed through the aperture 102 passes through the output mirror 103 and is emitted toward the reflection mirror 301 arranged in the propagation system 300.

In this embodiment, an unstable resonator having a magnification of 3 times or more is used as the laser amplifier. Figure 1
As shown in FIG. 1, the laser amplifier 200 includes a laser chamber 201 having windows 202 and 203 installed at Brewster's angles on both sides. Laser chamber 201
A concave mirror 205 is arranged on the laser light incident side, and a convex mirror 204 is arranged on the laser light emitting side.

The concave mirror 205 is formed with a through hole 206 for passing the incident laser beam on the optical axis, and a highly reflective HR (high r) is formed around the through hole 206.
eflection) coating is applied. It should be noted that the concave mirror 205 may actually have a through hole formed therein, or a mirror having an AR (anti-reflection) coating for antireflection so that a portion corresponding to the through hole becomes a transmissive portion. A substrate may be used. An HR coating is applied to the central portion of the surface of the convex mirror 204 facing the laser chamber, and an AR coating is applied to the surrounding portion of the convex mirror 204 where the laser light is emitted.

In the present invention, as coating materials for AR coating, partial reflection coating and HR coating, AlF ₃ , MgF ₂ , NdF ₂ , GdF ₃ ,
A fluoride (dielectric multilayer film material) such as LaF ₃ or LiF ₃ can be used. Further, as a coating material for the HR coating, an overcoat of Al, Al + MgF _2, an overcoat of an Al + dielectric multilayer film material, or the like can be used.

In FIG. 1, the laser light emitted from the laser oscillator is reflected by a reflection mirror 30 arranged in the propagation system.
It is reflected by 1, 302 and transmitted to the laser amplifier 200. The laser light enters the laser chamber 201 through the through hole 206 at the center of the concave mirror 205 arranged in the laser amplifier 200. The laser light that has passed through the laser chamber is incident on the convex mirror 204. The laser light reflected by the convex mirror 204 is reflected by the concave mirror 205 installed on the right side of the laser chamber, and is amplified while reciprocating between the convex mirror 204 and the concave mirror 205. The central portion of the convex mirror 204 is provided with a total reflection coating, and the light hitting the central portion is reflected toward the concave mirror 205. Convex mirror 20
The portion other than the central portion of 4 is AR-coated, and when the reflected light from the concave mirror 205 hits this portion, it is transmitted and output from the convex mirror 204.

In the present embodiment, since the laser light emitted from the laser oscillator is amplified by the laser amplifier, the output of the laser light emitted from the laser oscillator is 0.1 mJ / cm ² or less. Therefore, the intensity of the laser light incident on the etalon installed in the laser oscillator can be suppressed to a low level, and the etalon coating is not deteriorated or deformed. Therefore, it is possible to use the etalon with high finesse. it can.

Next, a second embodiment of the present invention will be described. In the second embodiment of the present invention, a FOX-SMITH interferometer is used as the band narrowing means instead of the etalon. FIG. 2 shows the configuration of a laser oscillator included in the laser device according to the second embodiment of the present invention.
In this laser oscillator, a laser chamber 1 having windows 14 and 15 installed on both sides at Brewster's angle is provided.
A rear mirror 111 that totally reflects the laser light generated in the laser chamber is arranged on the rear side of 1, and a narrowing module 160 is arranged on the front side.

A FOX-SMITH interferometer including a beam splitter 117 and two total reflection mirrors (end mirrors) 112 and 114 is arranged in the band-narrowing module 160. The beam splitter 117 is arranged on the optical axis of the laser light at a predetermined inclination angle. The two total reflection mirrors 112 and 114 include the beam splitter 11
7 above and below the respective coated surfaces 113,
115 are arranged to face each other. Here, the distance between the total reflection mirror 112 and the total reflection mirror 114 is indicated by D. Note that CaF ₂ can be used as the substrate material of the beam splitter.

The laser light generated in the laser chamber 11 passes through the window 14 and the aperture 101, is totally reflected by the rear mirror 111, then passes through the laser chamber 11, passes through the aperture 102, and enters the beam splitter 117. To do. The beam splitter 117 is provided with a partially reflective coating, which splits the laser light into two directions. A part of the laser light is reflected toward the total reflection mirror 114, travels back and forth between the total reflection mirror 114 and the total reflection mirror 112, and then is returned to the laser chamber 11 via the aperture 102.
As a result, the rear mirror 111 and the total reflection mirror 112,
Resonance of the laser light occurs with 114 and the laser light is narrowed. The other part of the laser light is emitted toward the reflection mirror 301 arranged in the propagation system 300.

Next, a third embodiment of the present invention will be described. Also in the third embodiment of the present invention, the FOX-SMITH interferometer is used instead of the etalon as the band narrowing means. FIG. 3 shows the configuration of a laser oscillator included in the laser device according to the third embodiment of the present invention.
In this laser oscillator, a band narrowing module 170 is arranged on the rear side of the laser chamber 11 having windows 14 and 15 installed at Brewster's angles on both side walls.

A beam splitter 121 and two total reflection mirrors 122 and 124 are provided in the band narrowing module 170.
A FOX-SMITH interferometer including and is arranged.
The beam splitter 121 is arranged at a predetermined inclination angle on the optical axis of the laser light. The total reflection mirror 122 is arranged on the optical axis of the laser light so that the beam splitter 121 is sandwiched between the total reflection mirror 122 and the laser chamber 11. Further, the total reflection mirror 124 is arranged below the beam splitter 121 so that the coating surface faces the beam splitter. Here, the length of the optical path between the two total reflection mirrors 122 and 124 is indicated by D.

The laser light generated in the laser chamber 11 passes through the window 14 and the aperture 101 and enters the beam splitter 121. Beam splitter 12
1 has a partially reflective coating 123, which splits the laser light in two directions. The laser light transmitted to the total reflection mirror 122 reciprocates between the total reflection mirror 122 and the total reflection mirror 124, and then is returned to the laser chamber 11. The laser light emitted from the laser chamber 11 is emitted from the window 15 and the aperture 102.
And is emitted toward the output mirror 126. Resonance of the laser light occurs between the total reflection mirrors 122 and 124 and the output mirror 126, and the laser light is narrowed. The laser light passing through the output mirror 126 passes through the aperture 102 and is emitted toward the reflecting mirror 301 arranged in the propagation system 300.

The etalon arranged in the band narrowing module of the laser oscillator in FIG. 1 and the FOX-SMI arranged in the band narrowing module in FIGS. 2 and 3.
The FSR in the TH interferometer is set as shown below. Fig. 4 shows the case where laser oscillation is performed in the laser oscillator without using the band narrowing module (free run: Free R
FIG. 5 and FIG. 6 show spectrum waveforms when the band is narrowed by the band narrowing module using the etalon in the laser oscillator.

In FIG. 4, it is assumed that the spectrum waveform at the time of free running has a width of 2A. This spread width is defined by, for example, the spectral purity during free running. “Spectral purity” is one index relating to the degree of concentration of spectral energy, and means a line width drawn so as to have “a certain area ratio” with respect to the total area of the spectrum waveform. For example, the commonly used "95% purity"
Is defined by the line width drawn so as to have an area of 95% from the center of the spectrum waveform in the total area of the spectrum waveform.

As shown in FIGS. 5 and 6, the spectral waveform when the band is narrowed is almost determined by the transmission function waveform of the etalon having a plurality of periodic mountain-shaped shapes. Here, in FIG. 5, the FSR of the etalon is narrow, and a plurality of peak shapes of the transmission function of the etalon are superimposed on the spectrum waveform at the time of free running, so the spectrum waveform at the time of narrowing the band is at the center. It becomes a shape with bumps on both sides of the mountain shape.

In the spectrum waveforms during free running shown in FIGS. 5 and 6, the full width at half maximum (FWHM) of the spectrum
Both are the same, but when used as a light source for exposure, in addition to the full width at half maximum (FWHM) of the spectrum, the spectral purity of the narrowed laser light is also important. The definition of the spectral purity is the same as the above-mentioned spectral purity during free running. The bumps on both sides in FIG. 5 cause deterioration of the spectral purity. In order to prevent the deterioration of the spectral purity, it has been invented that it is desirable to set FSR = 2A as shown in FIG.

FOX-SMIT as shown in FIGS. 2 and 3.
When the band narrowing is performed using the H interferometer, the transmission function of the etalon in FIGS. 5 and 6 becomes the beam splitter 117 in FIG. 2 and the beam splitter 1 in FIG.
The FSR of the FOX-SMITH interferometer is the same as that of the etalon except that it corresponds to the reflection function of the light reflected toward the laser chamber 11 by FSR = 2.
It is desirable to set to A.

In FIG. 1, the finesse (Fi) in the etalon arranged in the band-narrowing module of the laser oscillator is shown.
nesse) and FSR and full width at half maximum (FWH) of spectrum
The relationship between M) and Δλ is obtained by the following equations (1) and (2).

[Equation 7]

In equations (1) and (2), Finesse
_Multi indicates the effective finesse of the etalon when the laser beam passes through the band-narrowing element in the band-narrowing module a plurality of times (M times) (multi-pass). Further, λ is the central wavelength of the laser beam, n is the refractive index of the gap between the etalon substrates, and d is the length of the gap between the etalon substrates.

The Finesse _Multi and the Finesse _Single when light passes through the etalon once have the relationship shown in the equation (3). For formula (3), refer to "The Fab
ry-Perot Interferometer ", by JM Vauhan MA-D.Phil, I
See OP Publishing Ltd. Publishing, 1989, ISBN 0-85274-138-3.

[Equation 8]

Here, Finesse _Single is the reflection finesse (F
inesse _R ), Finesse _Single is
It is obtained by the expression of the reflection finesse of the expression (4).

[Equation 9] In the equation (4), R is the reflectance of the reflection surface of the etalon.

The Finesse _Single may be obtained by combining the reflection finesse and the parallelism finesse. At that time, the reciprocal of Finesse _Single is the sum of the reciprocals of each finesse.

Next, the specifications of the etalon used for the laser oscillator are determined based on the FSR and the effective finesse setting conditions. As described above, when the F ₂ laser device is used as the light source of the exposure apparatus having the refractive projection optical system, the full width at half maximum (FWHM) of the spectrum needs to be narrowed to 0.2 pm or less. Also, FSR = 2A (p
It is desirable to set m). Therefore, the expression (5) is derived from the expression (1).

[Equation 10] In the equation (5), A (pm) is 1/2 of the width 2A of the spectrum waveform at the time of free run, and is, for example, 1/2 of the spectrum purity width at the time of free run.

From Equations (3) and (5), Finesse
_Single is obtained and is shown by the equation (6).

[Equation 11]

Finesse _#Single is a reflection finesse
When approximated to e _R ), the following expression (7) is derived from the expressions (4) and (6).

[Equation 12] Since A> 0 and M ≧ 1, the left side of equation (7) is 0.
Greater than

Here,

[Equation 13] Then, the following equation is derived from the equations (7) and (8).

[Equation 14] Here, since C> 0, the equation (9) is derived.

[Equation 15]

In the expression (9), the value on the right side exceeds 1, but since 0 <R <1, the expression (9) becomes the expression (10).

[Equation 16] That is, the reflective surface of the etalon may be coated so that the reflectance of the reflective surface of the etalon satisfies the expression (10).

When narrowing the band using a FOX-SMITH interferometer as shown in FIG. 2 or 3 instead of the etalon, the finesse and FSR and the full width at half maximum Δλ (FWHM) In relation to
The expressions (1) and (2) are established, and the effective finesse is obtained from these expressions. However, in equation (2),
n is the refractive index of the medium between the total reflection mirrors, d is the distance between the total reflection mirrors through the beam splitter, and corresponds to D in FIGS. 2 and 3.

In the case of the second embodiment shown in FIG. 2, Finesse _Single in equation (3) is approximated by the transmission Finesse _T of the FOX-SMITH interferometer. When the transmittance of the beam splitter 117 in FIG. 2 is T, the transmission finesse (Finesse _T ) is calculated by the equation (11).

[Equation 17]

The Finesse _Single may be obtained by combining the transmission finesse and the parallelism finesse. At that time, the reciprocal of Finesse _Single is the sum of the reciprocals of each finesse.

In the case of the third embodiment shown in FIG. 3,
Finesse _{Single in} equation (3) is similar to the case where an etalon is used as the band narrowing means, and FOX-SMITH
It approximates the reflection finesse of an interferometer. Reflective finesse
sse _R ) is calculated by the equation (4). The reflectance R
Is the reflectance of the reflecting surface of the beam splitter 121 in FIG.

In the third embodiment shown in FIG. 3, the reflectance R of the reflecting surface of the beam splitter 121 may be set so as to satisfy the expression (10). On the other hand, the second shown in FIG.
In the embodiment, the transmittance T of the beam splitter 117 may be set so as to satisfy the expression (12).

[Equation 18]

Next, in order to verify the above set conditions,
In the demonstration example, an actual narrowing band module was designed, and the full width at half maximum of the spectrum of the laser beam emitted from the laser device using the narrowing band module was measured. Demonstration Example 1 In the first embodiment shown in FIG. 1, a total pressure of 400 k is set in the laser chamber 11 of the laser oscillator 100.
Pa, a fluorine (F ₂ ) gas of 10% or less, and a laser gas containing helium (He) as a buffer gas were filled and free-running. The spectral purity (95% purity) of the emitted laser beam was about 4 pm. became. Therefore, the etalon FSR = 4 pm, that is, A = 2 pm.

Further, in the laser resonator actually used as the laser oscillator of the exposure laser device, at least one round trip can be expected due to the resonance of the laser beam, so M = 2.

When A = 2 and M = 2, C =
It becomes 12.87, and if this value is substituted into the equation (10),
0.7839 <R. Therefore, the reflective surface of the etalon was coated so that the reflectance (R) of the reflective surface of the etalon was R = 79%. As the coating material, the above-mentioned fluoride (dielectric multilayer film material) was used. Further, the length d of the gap between the etalon substrates was determined based on the equation (2) so that FSR = 4 pm.

Using the band-narrowing module having the etalon thus designed, the laser oscillator was band-narrowed, and the full width at half maximum (FWHM) of the spectrum of the laser beam emitted from the laser oscillator was measured. .1
It was 8 pm, and FWHM satisfied the condition of 0.2 pm or less.

Demonstration Example 2 In the first embodiment shown in FIG. 1, the total pressure is 400 kPa, the fluorine (F ₂ ) gas is 10% or less, and the buffer gas is neon (Ne) in the laser chamber 11 of the laser oscillator 100. When the above laser gas was filled and free-run was performed, the spectral purity (95% purity) of the emitted laser beam was about 2 pm. So the etalon's FSR = 2pm, that is, A =
It was set to 1 pm. In addition, as in Verification Example 1, M = 2.

When A = 1 and M = 2, from the equation (8), C =
6.436 is obtained, and when this value is substituted into the equation (10),
0.6166 <R. Therefore, the reflective surface of the etalon was coated so that the reflectance R of the reflective surface of the etalon was 62%. As the coating material, the above-described fluoride (dielectric multilayer film material) was used. Also FS
The length d of the gap between the etalon substrates was determined based on the equation (2) so that R = 2 pm.

Using the band-narrowing module having the etalon thus designed, the laser oscillator was band-narrowed, and the full width at half maximum (FWHM) of the spectrum of the laser beam emitted from the laser oscillator was measured. .1
It was 8 pm, and FWHM satisfied the condition of 0.2 pm or less.

As is clear from the demonstration examples 1 and 2, neon (Ne) is used as the buffer gas for the laser gas.
It tends to have a narrower spectrum width 2A in the free run than the case of using helium (He), and the reflectance of the etalon (in the case of the second embodiment, the transmittance of the beam splitter, the third In the case of the above embodiment, the reflectance of the reflecting surface of the beam splitter may be small. That is, the narrower the spectrum width 2A during the free run is, the easier the etalon or beam splitter can be manufactured.

As shown in FIG. 7, when the laser gas pressure in the laser chamber of the F ₂ laser device is lowered, the spectral line width becomes narrow. For example, the laser gas pressure is 1
As the atmospheric pressure decreases, the full width at half maximum (FWHM) of the spectrum during free running becomes narrower by 0.2 to 0.3 pm. Similarly,
The free-run spectrum width 2A also tends to narrow as the laser gas pressure decreases. As described above, the narrower the spectrum width 2A during free running, the easier the production of the etalon and the beam splitter. Therefore, the laser gas pressure is preferably as low as possible. However, when the laser gas pressure decreases, the laser output also decreases, so the laser gas pressure cannot be extremely decreased.

In the demonstration example 1 and the demonstration example 2,
The case where the buffer gas of the laser gas is He or Ne is shown, but He and Ne are used as the buffer gas.
You may use the mixed gas of.

Further, in the above-mentioned demonstration examples 1 and 2, the number of times (M) the laser light passes through the band narrowing means is set to two, but the value of M may be set according to the characteristics of the laser.

In FIG. 8, assuming that M = 2, equation (8) and (1)
The laser pulse waveform and the sidelight waveform in the laser oscillator whose band is narrowed by using the etalon designed by obtaining the reflectance of the coating based on the equation (0) are shown. Figure 8
In, the vertical axis represents intensity and the horizontal axis represents time. However, the time on the horizontal axis is represented by the number of times light passes through the band-narrowing means (the number of round trips of the resonator) M. For example, when the resonator length is 1500 mm, M = 2 shown in FIG. 8 is 2
X (1.5 m x 2 / c) [c (speed of light): about 3 x 10 ⁸ m
/ S] = corresponding to about 20 ns.

Here, the light generated from the laser gas excited by the discharge generated between the pair of electrodes will be referred to as "sidelight". The sidelight is observed from a position not on the laser resonator (for example, an electrode side position in a direction substantially perpendicular to the longitudinal direction of the electrode). The sidelight shows the gain distribution during laser oscillation. That is, at the peak of the sidelight, the laser pulse rises sharply beyond the threshold value.

In FIG. 8, before the occurrence of the first peak of the sidelight, the time point of 5% of the intensity of the first peak is the starting point of the sidelight waveform, and before the occurrence of the first peak of the laser pulse waveform, the first peak of The time point of 5% of the intensity is the starting point of the laser pulse waveform.

Here, if the time from the starting point of the sidelight waveform to the starting point of the laser pulse waveform is Δt, then Δt
Is the time during which the light generated by the discharge reciprocates in the laser resonator until the laser pulse rises. In FIG. 8, Δt = X, the number of times M light passes through the etalon during the time Δt is X, and X> 2. Therefore, with M = X, the reflectance of the coating can be obtained based on the equations (8) and (10).

That is, when M = X> 2, finesse such as reflection finesse can be reduced as compared with when M = 2. This means that the design of the etalon is loosened, and it becomes easy to improve the transmittance of the etalon and to manufacture the etalon (machining of surface accuracy, relaxation of surface parallelism processing accuracy). In the case of the laser device having the characteristics shown in FIG. 8, it is desirable to redesign the etalon with M = X in terms of transmittance and processing accuracy.

The number M of times that the laser beam passes through the band narrowing means is such that L is the laser cavity length, c is the speed of light, and Δt is the time from the sidelight waveform starting point to the laser pulse waveform starting point.

[Formula 19] Required by.

As described above, in the ordinary laser oscillator, the light makes one round trip between the laser resonators at a minimum. Therefore, if the etalon is designed with M = 2, the desired line width can be surely obtained. Therefore, M = 2 is a design guide. However, if it is difficult to design the etalon with M = 2, it is possible to facilitate the design of the etalon by increasing Δt and increasing M.

As a method of lengthening Δt, it is effective to lower the gain of the laser. In the case of the F ₂ laser apparatus, the gain of the laser can be lowered by reducing the discharge injection power or optimizing the conditions of the operating laser gas. As the operating laser gas conditions of the F ₂ laser device, it is desirable that the total pressure is 3500 hPa or less and the F ₂ concentration is 10% or less. This can be applied whether the buffer gas is He only, Ne only, or a mixture of He and Ne. When Ne is included as the buffer gas component, the addition of Xe increases the laser output under high repetition conditions. Therefore, when Ne is contained as a buffer gas component, Xe is added to the buffer gas.
It is preferable to contain

Next, in the first embodiment shown in FIG. 1, the relationship between the transmittance of the etalon and the laser output energy was examined. The result is shown in FIG. Here, the laser output energy during free running without using an etalon is data when the etalon transmittance is 100%.

2 consisting of laser oscillator and laser amplifier
In the stage type F ₂ laser device, the output of the laser beam emitted from the laser oscillator can be suppressed to a low level as described above. However, as is apparent from FIG. 9, when the transmittance of the etalon is less than 20%, almost no laser output energy is obtained, and it is necessary for the laser beam emitted from the laser oscillator in the two-stage F ₂ laser device. Often less than the laser output energy. Therefore, the transmittance of the etalon is preferably 20% or more.

Further, three types of etalons having coatings with reflectances R of the reflecting surfaces of the etalons set to 60%, 70%, and 80% were designed, and for each etalon,
The relationship between the film absorption of the coating (reflection film) and the transmittance of the etalon was investigated. The result is shown in FIG. As is apparent from FIG. 10, in the etalon having the coating (reflection film) having the reflectance (R) of 60%, the transmittance of the etalon becomes less than 20% when the film absorption rate exceeds 20%. . Further, in an etalon having a coating (reflecting film) having a reflectance (R) of 80%, when the film absorptivity exceeds 10%, the etalon transmittance falls below 20%.

In the F ₂ laser apparatus used for exposure, the spectrum width 2A during the free run can be maximized in the vicinity of the conditions of Demonstration Example 1 in which the laser gas pressure is 400 kPa and He is used as the buffer gas. I understand. The reflectance of the etalon in the demonstration example 1 is 79.
%, But if the reflectance of the etalon corresponding to the maximum value of the spectrum width 2A during free running is 80%,
From 0, the film absorption of the coating is preferably 10% or less.

The above-mentioned demonstrative examples and consideration regarding the etalon show the same tendency also in the case of the FOX-SMITH interferometer, and the FOX-S shown in FIGS. 2 and 3.
The film absorptance of the coating applied to the beam splitter of the MITH interferometer is also preferably 10% or less.

[0079]

As described above, according to the present invention,
The intensity of light incident on the etalon or the like arranged in the laser oscillator can be suppressed. Therefore, it is possible to prevent deterioration and deformation of the coating such as etalon.
It is possible to sufficiently narrow the output light band by using an etalon having a high finesse due to the coating.

Further, by determining the condition of the reflectance of the coating of the etalon and the condition of the reflectance or the transmittance of the coating of the beam splitter in the FOX-SMITH interferometer, not only the spectral line width but also the spectral purity can be set to a desired value. It is possible to narrow the band.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a laser device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a laser oscillator included in a laser device according to a second embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of a laser oscillator included in a laser device according to a third embodiment of the present invention.

FIG. 4 is a diagram showing a spectrum waveform during a free run.

FIG. 5 is a diagram showing a first spectral waveform having a narrow band and a transmission function of an etalon.

FIG. 6 is a diagram showing a narrowed second spectrum waveform and a transmission function of an etalon.

FIG. 7 is a diagram showing a relationship between a working gas pressure in a laser chamber and a line width.

FIG. 8 is a diagram showing a sidelight waveform and a laser pulse waveform.

FIG. 9 is a graph showing the relationship between etalon transmittance and laser output energy.

FIG. 10 is a graph showing the relationship between the film absorption rate of etalon and the etalon transmittance.

FIG. 11 is a diagram showing a configuration of a conventional one-stage laser device.

[Explanation of symbols]

1, 11 Laser chamber 2 Electrode 3 Fan 4, 5, 14, 15 Window 6 High-voltage pulse generator 7 Front mirror 8, 117, 121 Beam splitter 9 Wavelength monitor 10 Controller 50, 150, 160, 170 Band narrowing module 51 , 151 Etalon 52, 112, 114, 122, 124, 152 Total reflection mirror 100 Laser oscillator 101, 102 Aperture 103 Output mirror 111 Rear mirror 113, 115 Coating surface 153, 154 Substrate 200 Laser amplifier 201 Laser chamber 202, 203 Window 204 Convex surface Mirror 205 Concave mirror 300 Propagation system 301, 302 Reflecting mirror

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Osamu Wakabayashi             1200 Gigaphoton Co., Ltd. Manda 1200, Hiratsuka City, Kanagawa Prefecture             Inside the company F-term (reference) 2H048 GA01 GA09 GA13 GA25 GA48                 5F046 CA04                 5F072 AA04 FF08 JJ03 JJ05 JJ09                       JJ13 KK06 KK08 KK09 KK15                       KK30 RR05 YY09

Claims (9)

[Claims] 1. A laser oscillator including a band narrowing means using an etalon composed of two substrates having coatings on opposite surfaces, and emitting a laser beam narrowed by the band narrowing means, and the laser. A laser device comprising a laser amplifier for amplifying laser light emitted from an oscillator, wherein the spectrum width of the laser light emitted from the laser oscillator when not narrowed is 2A, and the narrowing means is a laser. Assuming that the number of times light passes through is M, the light reflectance R of the coating is expressed by the following equation: A laser device in which the etalon is configured to satisfy the above condition. 2. A beam splitter, which is arranged on the optical axis of the laser beam at a predetermined inclination angle and has a coating on one of the surfaces, and a reflecting surface facing the upper and lower sides of the beam splitter. A laser oscillator for emitting a laser beam narrowed by the narrowing means, which includes a narrowing means using a FOX-SMITH interferometer composed of two total reflection mirrors arranged in parallel. A laser device comprising a laser amplifier for amplifying laser light emitted from an oscillator, wherein the FOX-SMITH interferometer is arranged on a front side of the laser oscillator, and a narrow band of laser light emitted from the laser oscillator is provided. Assuming that the width of the spectrum when it is not converted is 2 A and the number of times the laser beam passes through the band narrowing means is M, Transmittance T is represented by the formula [number 2] shown below The laser device, wherein the FOX-SMITH interferometer is configured to satisfy the above condition. 3. A beam splitter, which is arranged on the optical axis of the laser beam at a predetermined inclination angle and has a coating on the surface opposite to the laser medium side, and respective reflecting surfaces on the coating surface of the beam splitter. FOX-SMIT consisting of two total reflection mirrors arranged so that
A laser oscillator including a band narrowing unit using an H interferometer, emitting a laser beam narrowed by the band narrowing unit, and a laser amplifier amplifying the laser beam emitted from the laser oscillator. In the laser device described above, the FOX-SMITH interferometer is arranged on the rear side of the laser oscillator, and the width of the spectrum of the laser light emitted from the laser oscillator when the band is not narrowed is 2A, and the band is narrowed. Letting M be the number of times the laser light passes through the means, the light reflectance R of the coating is expressed by the following equation: The laser device, wherein the FOX-SMITH interferometer is configured to satisfy the above condition. 4. The width 2 of the spectrum waveform of the laser beam emitted from the laser oscillator when the band is not narrowed.
4. The laser device according to claim 1, wherein A is equal to spectral purity. 5. The laser device according to claim 1, wherein the number of times M the laser light passes through the band narrowing means is 2. 6. The laser device according to claim 1, wherein a film absorption rate of the coating is 10% or less. 7. The laser oscillator and the laser amplifier are
Injection lock method or MOPA (master o
A laser device according to any one of claims 1 to 6, which constitutes a two-stage laser system of a scillator power amplifier type. 8. The laser oscillator excites a laser gas to generate laser light by applying a voltage to a pair of discharge electrodes arranged in a laser chamber filled with a laser gas as a laser medium, The laser device according to claim 1, wherein F ₂ (fluorine molecule) is contained as a laser medium. 9. The laser device according to claim 8, wherein the laser gas further contains xenon gas.