JPH03227584A - Narrow-band oscillation excimer laser - Google Patents

Abstract

PURPOSE:To make is possible to improve efficiency and to reduce the width of a line by making 1/2 of the sum of the incident angle of laser light and a diffraction angle approximately agree with a blaze angle, arranging a grating, arranging a rear mirror orthogonally with respect to the axis of diffracted light, and reflecting the diffracted light. CONSTITUTION:The laser is an excimer laser using a grating 16 as a narrow- band element. An angle which is 1/2 of the sum of an incident angle alpha of laser light into the grating 16 and a diffraction angle beta is made to agree approximately with a braze angle gamma of the grating 16. The grating 16 is arranged in this way. A rear mirror 18 is arranged orthogonally with the axis of diffracted light so as to reflect the diffracted light. In this way, the diffracted light is reflected at the maximum efficiency all the time, and the large output can be obtained. The width of the line can be made also narrow.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application 1] The present invention relates to a narrowband oscillation excimer laser, and more particularly to improvement of a narrowband oscillation excimer laser used as a light source for a stepper used in semiconductor manufacturing equipment.

[Conventional technology] Conventionally, in order for excimer lasers to oscillate in multiple modes,
When the beam divergence angle is large and the excimer laser beam is to be made into a narrow band using a grating, in order to reduce the beam divergence angle in the grating, it is necessary to place a pinhole in the resonator or to narrow the beam using a beam expander. The wavelength line width was narrowed by enlarging the beam and making it incident on the grating. For example, in the excimer laser beam draw arrangement shown in FIG. 7, the grating 1 is used as a rear mirror to narrow the band, and pinholes 2 are arranged in the optical path before and after the chamber. In this case, the structure uses a prism as a beam expander.

The excimer laser shown in FIG. 8 has an oblique incidence (glaze type) arrangement of gratings, and a total reflection mirror 3 and a grating 4 are used to narrow the band. Also, like the Littrow type, pinholes 5 are placed in the optical path before and after the chamber. In the case of this 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] Incidentally, in a narrow band oscillation excimer laser such as that used in a stepper for semiconductor manufacturing equipment, a narrow band with a line width of 3p or less and a large output are required. When a pinhole was placed inside the resonator, the output was extremely small. In addition, in the case of the injection locking method, although a large output can be obtained, there are miss shots, it is difficult to achieve a locking efficiency of 100%, and the spectral purity is poor.

In addition, when using a prism beam expander in a retro arrangement, a slit is placed to narrow the line width.
It was necessary to increase the beam expansion ratio and to make the grating even larger.

However, because it is difficult to manufacture large gratings or to match the phrase angle and oscillation wavelength to achieve high efficiency, the incident angle to the grating generally does not match the phrase angle, and as a result, oscillation always occurs. It was not possible to maximize efficiency.

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 is extremely short.

The present invention has focused on the above-mentioned conventional problems, and in an excimer laser that uses a grating as a band narrowing element, it is possible to always reflect diffracted light with the highest efficiency to obtain a large output, and to further narrow the line width. (The purpose is to provide a narrowband oscillation excimer laser with a structure that can be used.

[Means for Solving the Problem] In order to achieve the above object, the narrowband oscillation excimer laser according to the present invention is an excimer laser using a grating as a band narrowing element, in which the incident angle of laser light with respect to the grating and The gratings were arranged so that one-half of the sum of the diffraction angles substantially coincided with the phrase angle of the grating, and the rear mirror was arranged perpendicular to the diffraction optical axis to reflect the diffracted light.

In this case, the grating is an echère grating, and the grating is configured such that the difference between the angle of incidence and the angle of reflection with respect to the grating is set to 14.2 degrees or less.

[Operation] In general, when a white parallel light beam is incident parallel to the main cross section of a planar grating with a groove spacing of d, the main maximum of the intensity of the diffracted light of wavelength λ is as follows: d (sina + cos β) = theory λ・(1) However, , ■ occur in the direction of a diffraction angle β that satisfies an integer. Here, the angle of incidence α and the angle of diffraction β are the angles formed by the normal line to the surface of the grating.

Also, the reflection angle φ is φ=β−α...(2) It can be expressed as

On the other hand, when parallel light rays are incident at the same angle as the phrase angle γ, the light intensity of the wavelength of the diffracted light at the same angle as the phrase angle γ becomes maximum. That is, α=β=γ (3) 2dsinα=m7 (4) where d is the case where both equations of the lattice constant hold true.

However, for example, in the case of a KrF excimer laser, the center wavelength at which the oscillation efficiency is maximum is 248°39nm.
, the line width is 0.35 nm, and it is extremely difficult to manufacture a grating with a phrase angle γ that satisfies the above equations (3) and (4).

In contrast, in the present invention, the grating is arranged so that one-half of the sum of the incident angle of the laser beam to the grating and the diffraction angle approximately coincides with the phrase angle of the grating, and the rear mirror is arranged perpendicular to the diffraction optical axis. It is configured to reflect the diffracted light. That is,
In the present invention, the relationship between the incident angle α, the diffraction angle β, and the phrase angle γ in the grating is (α+β)/2=γ (5). Under this condition, the above (1), (5
) The rear mirrors are arranged so that the formula holds true.

This allows the diffraction intensity to be maximized.

Therefore, in such a case, even if the phrase angle varies slightly due to manufacturing errors, by arranging it as in the present invention,
Diffracted light can always be reflected with maximum efficiency, resulting in large output.

In addition, by using an echère grating as a grating and creating an oblique incidence arrangement structure, it is possible to
Since the dispersion is approximately twice that in the case of arrangement, the line width can be further narrowed.

Since the apex angle of the grooves of the Echère grating is almost vertical,1 and it is possible to manufacture a grating with a large phrase angle, higher output can be obtained than in the case of a normal grating.

In particular, in the case of an echère grating, since the number of grooves is small, the fleece vector range is smaller than that of a normal grating, and the above equations (1) and (5) hold true when the reflection angle φ is made as small as possible. You can position the rear mirror as you like. It is known that in this type of device, the efficiency usually decreases as the reflection angle φ increases, and the rate of decrease in efficiency A in the case of an Echère grating is expressed by the following equation.

A=jan7・jan(φ/2)・(6) Okawakami: The largest phrase angle that can be manufactured is about 76 degrees for the Echère grating. At this phrase angle, the efficiency decrease rate A is 05 or more because the reflection angle φ is 1
This is a case where the temperature is 4.2 degrees or more.

Therefore, in the present invention, the reflection angle φ is set to be 14.2 degrees or less.

[Embodiments] Specific embodiments of the narrowband oscillation excimer laser according to the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a configuration diagram of a narrowband oscillation excimer laser according to a first embodiment. This device has an oblique incidence configuration. First, the oscillator main body consists of a laser chamber 10 containing an excitation electrode, and windows 12 are provided on both axial end faces of the chamber 10 to serve as a transmitting portion for laser light. A front mirror 14 is disposed on the front end side of the laser chamber 10, and a grating 16 set at a constant inclination angle is disposed on the rear end side. This grating 16 has a sawtooth-shaped wave-shaped cross section, and is constructed of an echère grating in which the incident angle and the diffraction angle are set on the same side with respect to the normal line of the grating 16. In this echère grating, the apex angle of the grooves is vertical. Further, a rear mirror 18 is arranged facing the grating 16 to reflect the diffracted light from the grating 16 and return it to the grating 16. With this configuration, diffracted light of a specific wavelength can be selectively reflected by adjusting the inclination angle of the rear mirror 18.
Laser light of this selected wavelength can be oscillated.

Incidentally, in this device, the grating 16 and the mirror 18 are arranged in a specific configuration. That is, as shown in FIG. 1, 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 each rear mirror 18. and the tilt angles α and β of the laser chamber 10 to the optical axis. Also,
The phrase angle γ of the grating 16 is uniquely given at the time of its manufacture. And the reflection angle φ at this time is φ=
Becomes β−α.

In such a case, in this embodiment, the grating is arranged so that one-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 phrase angle γ of the grating, and the rear mirror is It is arranged perpendicular to the diffraction optical axis to reflect the diffracted light. That is, the laser chamber 1 of the grating 16 is
0 with respect to the optical axis, and the rear mirror 18 is arranged so as to be perpendicular to the diffraction optical axis from the grating 16 to the rear mirror 18. At this time,
The reflection angle φ from the rear mirror 18 to the laser chamber 10 across the grating 16 is set to be 14.2 degrees or less, so that the efficiency decrease rate is 50% or less. Therefore, for example, when φ = 14 degrees and γ = 76 degrees, from equations (2) and (5), α = 69 degrees and β
= 83 degrees, and at the same time, the rear mirror 18 is set to be perpendicular to the diffraction optical axis.

According to the narrow-band oscillation excimer laser configured in this way, since the arrangement relationship between the grating 16 and the mirror 18 with respect to the laser chamber 10 is set as described above, the phrase angle γ of the grating 16 may be slightly changed due to manufacturing errors. Even when the light fluctuates, it is possible to always reflect the diffracted light with the highest efficiency, and a large output can be obtained.

In this example, since the oblique incidence arrangement type is used, the dispersion is approximately twice that of the Littrow arrangement, and the line width can be further narrowed. Since the grating 16 is an echère grating, it can have a large phrase angle and can have a high output compared to a normal grating. In addition, by using an Echère grating, the number of grooves is small and the free spectral range is smaller than that of a normal grating, so that (1) and (5) above are satisfied with the reflection angle φ as small as possible. By arranging the rear mirror 18, it is possible to suppress a decrease in efficiency due to the use of the echère grating.

Next, FIG. 3 shows a narrow band oscillation excimer laser according to a second embodiment. In this embodiment, a prism 2 is used as a beam expander between the rear window 12 and the grating 16.
0 is placed.

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, the effect of the prism 20 has the advantage that the line width can be further narrowed and the output can be increased compared to the case of the first embodiment.

FIG. 4 shows a narrowband oscillation excimer laser according to a third embodiment. This mirror 18 is arranged between the front mirror 14 and the laser chamber 10.

By adopting such a configuration, the reflection angle can be further reduced than in the case of the previous embodiment, and higher efficiency can be achieved.

FIGS. 5 and 6 show a narrow band oscillation excimer laser according to the fourth embodiment. This narrow band oscillation excimer L/-zer uses the prism 20 as an expander as in the second embodiment, but in particular, the beam expansion direction is almost perpendicular to the discharge direction in the laser chamber 10. It is arranged so that This prism 20 is coated with an antireflection film 22 on the side of the incident surface from the laser chamber 10 and the rear mirror 18 in order to prevent reflection of polarized light components parallel to the beam expansion direction (dashed line in the figure). ). In the excimer laser configured in this way, even if the laser beam passes through the prism 20 many times, the efficiency does not decrease and 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 to be substantially perpendicular, the line width of the oscillated laser beam can be further narrowed. In this embodiment, the slits 26 are arranged before and after the laser chamber 10, and are formed into long through holes particularly in the discharge direction. Providing such a slit 26 not only increases efficiency but also enables a narrow band with a thin line width.

[Effects of the Invention] As explained above, according to the narrowband oscillation excimer laser according to the present invention, one-half of the sum of the incident angle and the diffraction angle of the laser beam with respect to the grating is abbreviated as the phrase angle of the grating. In addition to arranging the gratings so that they match, the rear mirror is also arranged perpendicular to the diffraction optical axis to reflect the diffracted light, so the diffracted light is diffracted twice as many times as compared to a normal glue-draw arrangement. Since the diffracted light is doubled and the rear mirror completely returns the diffracted light to the laser chamber, the line width can be halved. In particular, in this invention, since the incident angle and the diffraction angle of the laser beam on the grating are adapted to the phrase angle, an excellent effect can be obtained in that laser oscillation can be performed with the efficiency always maximized. In addition, since the line width can be made thinner without reducing the slit width, the discharge energy density may be small and the life of the discharge electrode can be extended.

[Brief explanation of drawings]

FIG. 1 is a main configuration diagram of a narrow band oscillation excimer laser according to the first embodiment, FIG. 2 is an explanatory diagram of diffracted light in a grating to explain the present invention in detail, and FIG. A main configuration diagram of a band oscillation excimer laser, 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 the main parts 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 with a glue-draw arrangement, and FIG. 8 is a diagram of a conventional excimer laser with an oblique incidence arrangement. 10... Laser chamber, 14... Front mirror 16... Grating, 18
・・・・・・Rear mirror

Claims (1)

[Claims] 1) In an excimer laser using a grating as a band-narrowing element, half of the sum of the incident angle and the diffraction angle of the laser beam with respect to the grating is made to approximately match the phrase angle of the grating. A narrow-band oscillation excimer laser characterized by arranging a grating and by arranging a rear mirror perpendicular to the diffraction optical axis to reflect diffracted light. 2) The narrowband oscillation excimer laser according to claim 1, wherein the grating is an echère grating. 3), the grating has a difference between the angle of incidence and the angle of reflection of 14
．． 2. The narrowband oscillation excimer laser according to claim 1, wherein the narrowband oscillation excimer laser is set to 2 degrees or less.