JP2964255B2 - Narrow-band oscillation excimer laser - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a narrow band oscillation excimer laser, and more particularly to an improvement in a narrow band oscillation excimer laser used as a light source of a stepper used in a semiconductor manufacturing apparatus.

[Conventional technology]

Conventionally, an excimer laser has a large beam divergence angle due to multi-mode oscillation.To narrow the band of the excimer laser light by using a grating, in order to reduce the beam divergence angle in the grating,
A pinhole is arranged in the resonator, or the beam is expanded by a beam expander and made incident on the grating to narrow the wavelength line width. For example, in the excimer laser shown in FIG. 7, the grating is arranged in a Littrow arrangement, the band is narrowed by using the grating 1 as a rear mirror, and pinholes 2 are arranged in the optical path before and after the chamber. In this case, a structure using a prism as a beam expander is employed. Also, the eighth
The excimer laser shown in the figure has a grating with a grazing incidence (blaze type) arrangement structure.
And the grating 4 to narrow the band. Further, similarly to the Littrow type, pinholes 5 are arranged in the optical path before and after the chamber. Usually, in the case of the oblique incidence arrangement, a holographic grating is used as the grating, and the angle of incidence is set to be large. Although not shown, an injection-locked excimer laser is also known.

[Problems to be solved by the invention]

By the way, in a narrow-band oscillation excimer laser used for a stepper for a semiconductor manufacturing apparatus, a band is narrowed to a line width of 3 pm or less and a large output is required. When the pinholes 2 and 5 were arranged in the resonator, the output was very small. In addition, in the case of the injection locking method, although a large output is obtained, there are misshots, it is difficult to set the locking efficiency to 100%, and the spectral purity has deteriorated.

Further, when the prism beam expander is used in the Littrow arrangement, it is necessary to arrange a slit (not shown) in order to reduce the line width, increase the beam expansion rate, and further increase the grating. However, it is difficult to produce a large grating or to match the lasing angle and the oscillation wavelength in order to achieve high efficiency.In general, the angle of incidence on the grating does not match the blaze angle. The oscillation efficiency could not be maximized.

Furthermore, in order to oscillate with high efficiency, it is necessary to oscillate in a state where the energy density of the discharge is high, and the life of the discharge electrode has been very short.

The present invention focuses on the above-mentioned conventional problems, and in an excimer laser using a grating as a band-narrowing element, a structure that can always obtain a large output by reflecting diffracted light with the highest efficiency and further reduce the line width. It is an object of the present invention to provide a narrow-band oscillation excimer laser.

[Means for solving the problem]

In order to achieve the above object, a narrow-band oscillation excimer laser according to the present invention includes a grating and a rear mirror as a band-narrowing element outside one of a pair of windows provided on a wall on an optical axis of a laser chamber. In an excimer laser provided with a beam expander between the grating side window and the grating,
The half of the sum of the incident angle and the diffraction angle of the laser light that exits the grating side window and passes through the beam expander and enters the grating is made to substantially match the blaze angle of the grating, and the diffracted light is The grating is arranged so as to transmit through the panda, and a rear mirror that reflects the diffracted light in the optical axis direction of the diffracted light transmitted through the beam expander is arranged between the laser chamber and the beam expander.

In an excimer laser provided with a grating and a rear mirror as a band-narrowing element outside one of a pair of windows provided on a wall on an optical axis of a laser chamber, a gap between the grating-side window and the grating is provided. A beam expander is arranged, and a half of the sum of the incident angle and the diffraction angle of the laser beam that exits the grating side window and passes through the beam expander and enters the grating is matched with the blaze angle of the grating. The grating is arranged so that the diffracted light passes through the beam expander, and the rear mirror that reflects the diffracted light in the optical axis direction of the diffracted light transmitted through the beam expander is opposite to the beam expander with respect to the laser chamber. It is arranged on the side.

In an excimer laser provided with a grating and a rear mirror as a band-narrowing element outside one of a pair of windows provided on a wall on an optical axis of a laser chamber, a gap between the grating-side window and the grating is provided. The beam expander is arranged so that the beam expansion direction of the beam expander is almost orthogonal to the discharge direction of the discharge electrode in the laser chamber, exits the window on the grating side, passes through the beam expander, and enters the grating The half of the sum of the incident angle and the diffraction angle of the laser beam to be made is made to substantially match the blaze angle of the grating, and the grating is arranged so that the diffracted light passes through the beam expander, and passes through the beam expander. Rear mirror to reflect the diffracted light in the optical axis direction of the diffracted light It has a configuration that was placed. still,
In this case, the laser beam incident surface of the beam expander may be coated with an antireflection film for preventing reflection of a polarization component parallel to the beam expansion direction.

In each of the above, it is preferable that the grating is an shale grating, and that the difference between the incident angle and the diffraction angle is set such that the rate of reduction of the diffraction efficiency is 50% or less.

[Action]

In general, when a white parallel ray is incident parallel to the main cross section of the plane grating having the groove interval d, the main maximum of the intensity of the diffracted light having the wavelength λ is: d (sinα + sinβ) = mλ (1) where m is an integer In the direction of the diffraction angle β that satisfies Here, the incident angle α and the diffraction angle β are angles formed by a normal line on the surface of the grating.

 The reflection angle φ is represented by φ = β−α (2).

On the other hand, when a parallel light beam is incident at the same angle as the blaze angle γ, the light intensity at the wavelength of the diffraction angle equal to the blaze angle γ becomes maximum. That is, α = β = γ (3) 2d sin α = mλ (4) where d is a case where both of the lattice constants are satisfied.

However, for example, in the case of a Krf excimer laser, the center wavelength at which the oscillation efficiency becomes maximum is 248.39 nm and the line width is 0.35 nm, and the blaze angle is such that the above equations (3) and (4) hold. It is extremely difficult to manufacture a grating of γ.

On the other hand, in the present invention, the grating is arranged so that a half of the sum of the incident angle and the diffraction angle of the laser beam with respect to the grating is substantially equal to the blaze angle of the grating, and the rear mirror is arranged perpendicular to the diffraction optical axis. To reflect the diffracted light. That is, in the present invention, the relationship between the incident angle α, the diffraction angle β, and the blaze angle γ in the grating is (α + β) / 2 = γ (5). Then, under these conditions, the expressions (1) and (5)
That is, the grating and the rear mirror are arranged so that the following holds.

Thereby, the diffraction intensity can be maximized. Therefore, in such a case, even if the blaze angle slightly fluctuates due to a manufacturing error, by arranging as in the present invention, the diffracted light can always be reflected with the highest efficiency, and a large output can be obtained.

In addition, when the oblique incidence type arrangement structure is used by using an eshale grating as the grating, the dispersion becomes about twice as compared with the case of the Littrow arrangement, so that the line width can be further reduced. The shale grating has a groove whose apex angle is substantially vertical and can be manufactured with a large blaze angle, so that a higher output can be obtained as compared with a normal grating.

In particular, in the case of an shale grating, since the number of grooves is small, the free spectral range is smaller than that of a normal grating, and the above expressions (1) and (5) are satisfied with the reflection angle φ as small as possible. The grating and the rear mirror can be arranged as described above. In this type of device, it is generally known that the efficiency decreases as the reflection angle φ increases, and the efficiency reduction rate A in the case of an shale grating can be expressed by the following equation (6).

A = tan γ · tan (φ / 2) (6) The largest blaze angle γ that can be produced practically is about 76 degrees in an shale grating. Normally, if the rate of decrease A of the diffraction efficiency is set to 0.5 or less, sufficiently efficient laser oscillation becomes possible. However, when the blaze angle γ is 76 degrees, the rate of decrease A of the efficiency of 0.5 or more is caused by the reflection. This is the case where the angle φ is 14.2 degrees or more. Therefore, in the present invention, the reduction rate A of the diffraction efficiency is set to 0.5 or less, and preferably, the reflection angle φ is set to 14.2 degrees or less.

Furthermore, since the beam expander is arranged between the rear window and the grating and the rear mirror is arranged between the rear window and the beam expander, it is easy to arrange the rear mirror and the grating so as to reduce the reflection angle φ. Therefore, as described above, the grating can always reflect the diffracted light with the highest efficiency, and the reduction rate A of the diffraction efficiency can be reduced. Therefore, a large laser oscillation output can be obtained efficiently. In addition, the beam expander expands the beam and makes it incident on the grating, and the area of incidence on the grating can be widened, so the line width can be reduced without using slits and the oscillation efficiency can be improved as described above. As described above, there is no need to increase the energy density of the discharge to increase the laser output, so that the life of the discharge electrode can be prolonged.

Further, since the rear mirror is arranged on the side opposite to the beam expander with respect to the laser chamber, that is, between the front mirror and the laser chamber, the arrangement of the rear mirror and the grating so as to reduce the reflection angle φ is changed. It is even easier than the above arrangement, which is located on the expander side. Therefore, the diffraction light can always be reflected by the grating with the highest efficiency, and the reduction rate A of the diffraction efficiency can be reduced.

In addition, since the beam expansion direction of the beam expander arranged between the rear window and the grating is arranged so as to be substantially perpendicular to the discharge direction of the discharge electrode in the laser chamber, the line width of the oscillating laser light is reduced. It can be made even thinner. The beam expander is coated with an anti-reflection film on the incident side from the laser chamber and the rear mirror in order to prevent reflection of the polarization component parallel to the beam expansion direction. Even when the number of transmissions is large, the efficiency does not decrease, and laser oscillation with higher efficiency can be maintained.

〔Example〕

Hereinafter, specific examples of the narrow-band oscillation excimer laser according to the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram of a narrow-band oscillation excimer laser according to a first embodiment, and FIG. 2 is an explanatory diagram of diffracted light in a grating for explaining the principle of the present invention. The grating has a configuration of oblique incidence arrangement. First, the oscillator body consists of a laser chamber 10 with a built-in discharge electrode,
Windows 12 are provided on both end surfaces in the axial direction of the laser chamber 10 to serve as laser light transmitting portions. Further, a front mirror 14 is disposed on the front end side of the laser chamber 10, and a grating 16 having a fixed inclination angle is disposed on the rear end side. Since the grating 16 has a sawtooth waveform cross section, the grating 16 is constituted by an eshale grating in which the incident angle and the diffraction angle are set to be on the same side with respect to the normal G of the grating 16. In this shale grating, the vertical angle of the groove is vertical. Further, a rear mirror 18 is arranged so as to face the grating 16 so as to reflect the diffracted light from the grating 16 and return it to the grating 16. With this configuration, by adjusting the inclination angles of the grating 16 and the rear mirror 18, diffracted light of a specific wavelength can be selectively reflected, and laser light of the selected wavelength can be efficiently oscillated. You can do it.

By the way, in this device, the grating 16 and the rear mirror 18 have a specific arrangement. That is,
As shown in FIGS. 1 and 2, in the grating 16, the incident angle of the diffracted light reflected via the rear mirror 18 and the diffraction angle returned from the grating 16 to the laser chamber 10 are different from the normal G to the rear mirror. The inclination angles α and β of the 18 and the laser chamber 10 with respect to the respective optical axes are given. The blaze angle γ of the grating 16 is uniquely given at the time of its production. And the reflection angle φ at this time
Becomes φ = β−α.

In such a case, in the present embodiment, the grating 16 is arranged such that a half of the sum of the incident angle α and the diffraction angle β of the laser beam with respect to the grating 16 substantially matches the blaze angle γ of the grating 16, and the rear mirror 18 is arranged perpendicular to the diffraction optical axis to reflect the diffracted light. That is, the previous equation (5); (α + β) / 2 =
Laser chamber of grating 16 so that γ holds
The rear mirror 18 is arranged so as to set an inclination angle θ with respect to the optical axis of 10 and to be orthogonal to the diffraction optical axis from the grating 16 to the rear mirror 18. At this time, the laser chamber is
The reflection angle φ reaching 10 is set to be equal to or less than a predetermined value (for example, 14.2 degrees), and the efficiency reduction rate A is set to be 50% or less. Therefore, for example, when φ = 14 degrees and γ = 76 degrees, α = 69 degrees and β = 83 from the above equations (2) and (5).
Place grating 16 as a degree and at the same time rear mirror
18 is set to be orthogonal to the diffraction optical axis.

According to the narrow-band oscillation excimer laser configured as described above, since the arrangement relationship between the grating 16 and the rear mirror 18 with respect to the laser chamber 10 is set as described above, the blaze angle γ of the grating 16 due to a manufacturing error.
Even if the value fluctuates slightly, diffracted light can always be reflected with the highest efficiency, and a large laser oscillation output can be obtained. In this embodiment, since the oblique incidence arrangement type is employed, the dispersion is about twice as large as in the Littrow arrangement, and the line width can be further reduced. Since the grating 16 is an shale grating, one having a large blaze angle can be manufactured. Therefore, the expressions (1) and (5) are satisfied,
In addition, since the arrangement can be easily set so that the reflection angle φ is 14.2 degrees or less, an excimer laser having a higher efficiency output than a normal grating can be obtained. In addition, the use of the shale grating reduces the number of grooves and reduces the free spectral range as compared with a normal grating, and the above expressions (1) and (5) are satisfied with the reflection angle φ as small as possible. The grating 16 and the rear mirror 18 can be arranged as described above.
Therefore, it is possible to suppress a decrease in efficiency due to a variation in the blaze angle due to a manufacturing error when using the shale grating in the Littrow arrangement.

Next, FIG. 3 shows a narrow-band oscillation excimer laser according to a second embodiment. In this embodiment, a prism 20 is arranged between a rear window 12 and a grating 16 as a beam expander. In order to reduce the reflection angle, the rear mirror 18 is arranged between the laser chamber 10 and the prism 20. According to such a configuration, there is an advantage that the line width can be further reduced and the laser output can be performed with high efficiency as compared with the case of the first embodiment by the beam expanding action of the prism 20.

FIG. 4 shows a narrow-band oscillation excimer laser according to the third embodiment. This is a rear mirror 18 front mirror
It is arranged between 14 and the laser chamber 10. With such a configuration, the reflection angle can be further reduced as compared with the first and second embodiments, and higher efficiency can be achieved.

FIGS. 5 and 6 show a narrow-band oscillation excimer laser according to a fourth embodiment. This narrow-band oscillation excimer laser uses the prism 20 as a beam expander in the same manner as in the second embodiment. In particular, the expanding direction of the beam is substantially perpendicular to the discharge direction of the discharge electrode 24 in the laser chamber 10. Has been placed. The prism 20 includes a laser chamber 10 and a rear mirror 18 in order to prevent reflection of a polarization component parallel to the beam expansion direction.
An anti-reflection film 22 is coated on the side of the light incident surface. (Dashed line in FIG. 5). In the excimer laser configured as described above, the efficiency does not decrease even when the laser light is transmitted through the prism 20 many times, and laser oscillation with high efficiency can be maintained. Further, in this embodiment, since the beam expansion direction by the prism 20 and the discharge direction by the discharge electrode 24 are arranged so as to be substantially perpendicular, the line width of the oscillation laser light can be further reduced. In this embodiment, the slits 26 are arranged before and after the laser chamber 10, and the through holes are particularly long in the discharge direction. By providing such a slit 26,
As described above, laser oscillation with high efficiency can be maintained, and the line width can be further narrowed.

〔The invention's effect〕

As described above, according to the narrow-band oscillation excimer laser according to the present invention, the grating is arranged such that a half of the sum of the incident angle and the diffraction angle of the laser light with respect to the grating is substantially equal to the blaze angle of the grating. In addition, since the rear mirror is arranged perpendicular to the axis of the diffracted light to reflect the diffracted light, the number of times that the diffracted light is diffracted is doubled compared to a normal Littrow arrangement, and the rear mirror As a result, the diffracted light completely returns to the laser chamber, so that the line width can be halved. In particular, according to the present invention, since the half of the sum of the incident angle and the diffraction angle of the laser beam with respect to the grating is approximately equal to the blaze angle, an excellent effect that laser oscillation can be performed with maximum efficiency at all times is achieved. can get. In addition, since the line width can be reduced without reducing the slit width, the discharge energy density can be reduced and the life of the discharge electrode can be extended.

[Brief description of the drawings]

FIG. 1 is a diagram showing the main configuration of a narrow-band oscillation excimer laser according to a first embodiment, FIG. 2 is an explanatory diagram of diffracted light in a grating explaining the principle of the present invention, and FIG. FIG. 4 is a main configuration diagram of a narrow-band oscillation excimer laser according to the third embodiment, and FIG. 5 is a plan view showing a main portion of the narrow-band oscillation excimer laser according to the fourth embodiment. FIG. 6 is a side view of FIG. 5, FIG. 7 is a diagram of a conventional excimer laser using a Littrow grating, and FIG. 8 is an excimer laser using a conventional grazing incidence grating. FIG. 10: laser chamber, 12: window, 14: front mirror, 16: grating, 18: rear mirror, 20: prism (beam expander), 22: anti-reflection film, 24: discharge electrode.

Continuation of the front page (56) References JP-A-56-126994 (JP, A) Laser Research Vol. 16, No. 4, p. 31-39 "Optical Handbook" (Showa 61-2-20) Asakura Publishing Co., Ltd. p. 529-530 "Science and Engineering Basic Course 11 Optics Overview II-Wave Optics-" (Showa 54-2-25) Asakura Shoten Co., Ltd. p. 86 (58) Field surveyed (Int.Cl. ⁶ , DB name) H01S 3/1055 H01S 3/13-3/139

Claims (6)

(57) [Claims] An excimer laser having a grating and a rear mirror as a band-narrowing element outside one of a pair of windows provided on a wall on an optical axis of a laser chamber. A beam expander is placed between the two, and a half of the sum of the incident angle and the diffraction angle of the laser beam that exits the window on the grating side and passes through the beam expander and enters the grating is the blaze angle of the grating. And a grating is arranged so that the diffracted light passes through the beam expander, and a rear mirror that reflects the diffracted light in the optical axis direction of the diffracted light transmitted through the beam expander is connected to the laser chamber and the beam expander. And a narrow-band oscillation excimer laser. 2. An excimer laser in which a grating and a rear mirror are provided as a band narrowing element outside one of a pair of windows provided on a wall on an optical axis of a laser chamber. A beam expander is placed between the two, and the half of the sum of the incident angle and the diffraction angle of the laser beam that exits the window on the grating side and passes through the beam expander and enters the grating is defined as the blaze angle of the grating. The grating is arranged so that the diffracted light passes through the beam expander, and the rear mirror that reflects the diffracted light in the direction of the optical axis of the diffracted light transmitted through the beam expander is positioned on the beam expander. Narrow band oscillation excimer The. 3. An excimer laser in which a grating and a rear mirror are provided as narrow band elements outside one of a pair of windows provided on a wall on an optical axis of a laser chamber. A beam expander is interposed between the beam expanders so that the beam expansion direction is almost perpendicular to the discharge direction of the discharge electrode in the laser chamber. A half of the sum of the incident angle and the diffraction angle of the laser light incident on the grating is made to substantially match the blaze angle of the grating, and the grating is arranged so that the diffracted light passes through the beam expander, and passes through the beam expander. Rear so as to reflect the diffracted light in the optical axis direction of the incoming diffracted light. Narrowband emission excimer laser, characterized in that a color. 4. A narrow-band oscillation excimer laser according to claim 3, wherein an anti-reflection film for preventing reflection of a polarization component parallel to the beam expansion direction is coated on a laser beam incident surface of the beam expander. 5. A narrow-band oscillation excimer laser according to claim 1, wherein the grating is an shale grating. 6. The grating has a reduction rate of diffraction efficiency of 50%.
5. The narrow band oscillation excimer laser according to claim 1, wherein a difference between an incident angle and a diffraction angle is set as follows.