JPH03250777A - Narrow-band oscillation excimer laser - Google Patents

Abstract

PURPOSE:To improve the controllability of power and a wavelength and the efficiency by a method wherein light emitted from the front mirror of an optical resonator is turned back toward the interior of a discharge chamber, passes through the interior of the chamber, is made to output and at the same time, a grating is arranged in such a way that the direction of the wire drawing of the grating becomes almost parallel to the direction of discharge for causing discharge excitation. CONSTITUTION:Prisms 41 and 42 and a grating 30 are used as band narrowing elements for narrowing the band of a laser beam and these are arranged on the side of a window 21 of a discharge chamber 20. On the other hand, a front mirror 10 and a rectangular prism 50 installed at 45 deg. to the mirror 10 are arranged on the side of a window 22 on the opposite side to the window 21 of the chamber 20. The arrangement of the grating 30 to electrodes 23 and 24 in the chamber 20 is selected in such a way that the direction of the wire drawing of the grating 30 becomes parallel to the direction of discharge using the electrodes 23 and 24 in the chamber 20. Accordingly, even if the discharge width in the chamber 20 is changed, a spectral line width is not changed. Thereby, it becomes possible to hold reliably a stable spectral purity and the controllability of the power of a laser and the wavelength of the laser beam are significantly improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a narrow band oscillation excimer laser, and is particularly suitable for use as a light source for a reduction projection exposure apparatus.

[Conventional technology]

Excimer lasers have been used as light sources for reduction projection exposure apparatuses (hereinafter referred to as steppers) for manufacturing semiconductor devices. This is because the wavelength of excimer laser is short (the wavelength of KrF laser is about 248.4 nm), so it is possible to extend the limit of light exposure to 0.5 μm or less, and if the resolution is the same, the g-line of the mercury lamp used conventionally. This is because compared to I-line and I-line, many excellent advantages can be expected, such as a deeper depth of focus, a smaller numerical aperture (NA) of the lens, the ability to enlarge the exposure area, and the ability to obtain greater power. .

Incidentally, an excimer laser used as a light source for a stepper is required to have a narrow line width of 3 pm or less, and is required to have a large output power.

As a technique for narrowing the band of an excimer laser, there is a conventional technique called an injection lock method. This injection locking method uses a wavelength selection element (etalon, diffraction grating, prism, etc.) inside the cavity of the oscillator stage.
is placed, the spatial mode is restricted by a pinhole to produce single mode oscillation, and this laser light is injection-locked by an amplification stage. Although relatively large output power can be obtained with this method, there are drawbacks such as miss shots, difficulty in achieving 100% mouth/sock efficiency, and poor spectral purity. Furthermore, in the case of this method, the output light has high coherence, and if this is used as a light source for a reduction exposure device, a speckle pattern will occur. It is generally believed that the generation of speckle patterns depends on the number of spatial transverse modes included in laser light. In other words, it is known that when the number of spatial transverse modes included in laser light is small, speckle patterns occur, and when the number of spatial modes increases, speckle patterns become less likely to occur. .

The above-mentioned injection lock method is a technique for narrowing the band by significantly reducing the number of spatial transverse modes in terms of wood quality, and cannot be used in a reduction projection exposure apparatus because the occurrence of speckle patterns is a major problem.

Another promising technique for narrowing the band of excimer lasers is the use of an air gear knob etalon, which is a wavelength selection element. A conventional technology using this air gear/knob etalon is a technology developed by AT&T Bell Laboratories that attempts to narrow the band of the excimer laser by placing an air gear/knob etalon between the front mirror of the excimer laser and the laser (discharge) chamber. Proposed. However, this method has the problem that the spectral linewidth cannot be narrowed very much, the power loss due to the insertion of an air gap etalon is large, and the number of spatial motes cannot be increased very much. Furthermore, since the change in the air gap due to the heat absorption of the etalon immediately after oscillation is very large, the oscillation wavelength changes greatly, and the wavelength controllability is not very good. Air gap etalons also have durability issues.

Therefore, an excimer laser has been proposed in which a relatively durable grating is used as a wavelength selection element.

In addition, in conventional excimer lasers, discharge-excited light is reciprocated within the discharge chamber by an optical resonator and oscillated laser light is output from the front mirror. The optical axis of the light directed toward the rear mirror in the discharge chamber coincided with the optical axis of the light reflected by the rear mirror and directed toward the front mirror, that is, the emission optical axis.

[Problem to be solved by the invention]

When a grating is selected as a wavelength selection element, especially when combined with a beam expander, the wavelength controllability is very good and the durability of the optical element used is also improved, but the linewidth is narrowed. Therefore, it is necessary to limit the oscillation region using a slit, resulting in poor efficiency and a narrow power control range.

In addition, the spectral linewidth changed due to the change in discharge width.

The present invention has been made in view of these circumstances, and is designed to provide a narrow band oscillation excimer laser that has an optical resonator composed of a grating and a front mirror that are effective as wavelength selection elements, regardless of changes in discharge width. Our objective is to provide a narrowband oscillation excimer laser that can ensure stable spectral purity, significantly improve power and wavelength controllability, and significantly improve efficiency. It has been the target.

[Means to solve the problem]

Therefore, in the present invention, in a narrow band oscillation excimer laser that has an optical resonator including a grating and outputs oscillation laser light by reciprocating discharge-excited light within a discharge chamber by the optical resonator, the optical resonator A folding means is provided for folding the light emitted from the front mirror into the discharge chamber and outputting the light through the discharge chamber. They are arranged so that they are approximately parallel.

[Effect]

According to this configuration, since the drawing direction of the grating is made substantially parallel to the discharge direction, the spectral linewidth does not change even if the discharge width of the discharge chamber changes. Additionally, since the laser beam oscillated from the front mirror is folded back and amplified, efficiency is extremely high.

〔Example〕

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

FIG. 1 shows an embodiment of a narrow band oscillation excimer laser according to the present invention, in which prisms 4, 42 and a grating 30 are used as band narrowing elements to narrow the band of laser light. These are arranged on the window 21 side of the discharge chamber 20. On the other hand, a front mirror 1 is placed on the opposite side of the discharge chamber 20 and on the window 22 side.
0 and a 45 degree right angle prism 50 are arranged. These front mirror 10 and grating 30 constitute an optical resonator.

KrF or the like is sealed in the discharge chamber 20 as a laser gas, and an electrode 23 is used to discharge and excite this laser gas.
．． 24 is provided, and the discharge chamber 20 is further provided with windows 21 and 22 through which the oscillated laser light passes.

The grating 30 uses light diffraction to select light of a specific wavelength, and has a large number of grooves arranged in a fixed direction. In this specification, the direction perpendicular to the many grooves is referred to as the drawing direction.

The grating 30 can select light of a specific wavelength by varying the angle θ of the grating 30 with respect to the incident light within a plane including the drawing direction. That is, the grating 30 reflects only a specific light corresponding to the angle θ of the grating with respect to the incident light in a predetermined direction (in this case, the direction of the incident light), thereby performing a selection operation for light of a specific wavelength.

In this embodiment, the grating 30 is arranged with respect to the electrode 23.24 in the laser chamber 20 so that the drawing direction of the grating 30 is parallel to (coinciding with) the direction of discharge by the electrode 23.24 in the discharge chamber 20. It is characterized by selected things. Further, as described above, the grating 30 is arranged on the window 21 side of the discharge chamber 20, and the grating 30 is placed on the opposite side of the discharge chamber 20.
On the window 22 side, a front mirror 10 and a 45-degree right angle prism 50 are arranged to shift and return the light emitted from the front mirror 10 upward in the discharge direction. and,
between the prism 41 and the window 21, and the window 2
There is a slit 91.92 between 2 and the front mirror 10.
are placed respectively. Here, the slits 91 and 92 have a long shape in a direction perpendicular to the direction of discharge by the electrodes 23 and 24 in the hk electric chamber 20.

In this configuration, the grating 30 functions as a so-called rear mirror. That is, the discharge chamber 20
The light that has passed through is expanded into a light beam by a beam expander formed by prisms 42 and 41, and is irradiated onto the grating 30.

Here, since the discharge direction and the drawing direction of the grating 30 match, the light beam is expanded by the beam expander formed by the prisms 42 and 41, and the light beam is expanded to the grating 3.
The beam divergence angle of the light irradiated to 0 is small. The light reflected by the grating 30 passes through the discharge chamber 20 and is output from the front mirror 10 as oscillated laser light.
The light is incident on a 5 degree right angle prism 50. The incident light is bent by the 45-degree right angle prism 50 so that the optical axis of the reflected light is above the optical axis of the incident light in the discharge direction, and enters the discharge chamber 20 again, where it is amplified and sent to the window 2.
1 as an oscillation laser beam L1.

Here, the prisms 41 and 42 constitute a beam expander that expands the laser beam, but the beam expansion direction (that is, the direction perpendicular to the ridge line direction of the prism)
are coincident with (parallel to) the direction of the discharge by the electrodes 23,24 in the discharge chamber 20. Note that the beam expansion direction of the beam expander by the prisms 41 and 42 does not need to exactly match the discharge direction. It is sufficient that the beam expander beam expansion direction substantially coincides with the discharge direction.

FIG. 2 shows another embodiment of the narrow band oscillation excimer laser according to the present invention, and the embodiment shown in the same figure is different from the embodiment shown in FIG.
Prisms 43 and 44 constituting a beam expander are arranged between the 5-degree right angle prism 50 and the 5-degree right angle prism 50.

The other parts are the same as those shown in FIG. In the embodiment shown in FIG. 2 as well, the front mirror 10 and the grating 30 constitute an optical resonator.

Also in this embodiment, the arrangement of the grating 30 with respect to the electrodes 23.24 in the laser chamber 20 is selected so that the drawing direction of the grating 3o is parallel (coinciding) with the discharge direction by the electrodes 23.24 in the discharge chamber 20. are doing. Further, a grating 30 is disposed on the window 21 side of the discharge chamber 20, and a front mirror 10 is provided on the opposite side of the discharge chamber 2o, on the window 22 side, and the light emitted from the front mirror 10 is shifted upward in the discharge direction and reflected by 45 degrees. A right angle prism 50 is arranged. Slits 91 and 92 are arranged between the prism 41 and the window 21 and between the window 22 and the front mirror 10, respectively. Here, the slits 91 and 92 have a long shape in a direction perpendicular to the direction of discharge by the electrodes 23 and 24 in the discharge chamber 20.

In this configuration, the grating 30 functions as a so-called rear mirror. That is, the discharge chamber 20
The light that has passed through is expanded into a light beam by a beam expander formed by prisms 42 and 41, and is irradiated onto the grating 30.

Here, since the discharge direction and the drawing direction of the grating 30 match, the light beam is expanded by the beam expander formed by the prisms 42 and 41, and the light beam is expanded to the grating 3.
The spread angle of the beam of light irradiated to 0 is small. The light reflected by the grating 30 passes through the discharge chamber 20 and is output from the front mirror 10 as an oscillated laser beam, but this light is expanded by a beam expander formed by prisms 43 and 44. and enters the 45 degree right angle prism 50. In the 45-degree right angle prism 5o, the incident light is bent so that the optical axis of the reflected light is above the optical axis of the incident light in the discharge direction, enters the discharge chamber 20 again, is amplified, and exits from the window 21. It is emitted as oscillation laser light L2. In this embodiment, a higher output can be obtained than in the embodiment shown in FIG. 1 by expanding the light beam by the prisms 43 and 44.

Here, the prisms 41, 42 and 43, 44 constitute a beam expander that expands the laser beam, but the beam expansion direction (that is, the direction perpendicular to the ridge line direction of the prism) is within the discharge chamber 20. electrode 23.2
It coincides with (parallel to) the discharge direction according to 4. Note that the beam expansion direction of the beam expander by the prisms 41.42 and 43.44 does not need to exactly match the discharge direction. It is sufficient that the beam expansion direction by the beam expander substantially coincides with the discharge direction.

FIG. 3 shows another embodiment of the narrowband oscillation excimer laser according to the present invention, and the embodiment shown in the same figure is different from the embodiment shown in FIG.
A polarizing element 80 is arranged between the 5 degree right angle prism 50 and the 5 degree right angle prism 50. Other parts are the same as those shown in FIG. Also in the embodiment shown in FIG. 3, the front mirror 10
and grating 30 constitute an optical resonator.

Also in this embodiment, a bulleting is applied to the electrodes 23.24 in the laser chamber 20 so that the drawing direction of the grating 30 is parallel to (coinciding with) the discharge direction by the electrodes 23.24 in the discharge chamber 20. 30 placements are selected. Further, a grating 30 is arranged on the window 21 side of the discharge chamber 20, and a front mirror 10 is arranged on the opposite side of the discharge chamber 2o and on the window 22 side.
A 45 degree right angle prism 50 is arranged to shift the light emitted from the front mirror 10 upward in the discharge direction and turn it back. A slit 92 is arranged between the window 22 and the front mirror 10, respectively. here,
The slit 92 has an elongated shape in a direction perpendicular to the direction of discharge by the electrodes 23, 24 in the discharge chamber 20.

In such a configuration, the portion of the grating 30 functions as a so-called rear mirror. That is, discharge chamber 2
The light that has passed through 0 is expanded into a light beam by a beam expander using a prism 42.4, and is irradiated onto the grating 30.

Here, since the discharge direction and the drawing direction of the grating 30 match, the light beam is expanded by the beam expander formed by the prisms 42 and 41, and the light beam is expanded to the grating 3.
The beam divergence angle of the light irradiated to 0 is small. The light reflected by the grating 30 passes through the discharge chamber 20 and is output from the front mirror 10 as an oscillated laser beam, but this light is expanded by a beam expander formed by prisms 43 and 44. be done. Furthermore, the light is incident on the 45-degree right angle prism 50 via the polarizing element 80.

In the 45-degree right angle prism 50, the incident light is bent so that the optical axis of the reflected light is above the optical axis of the incident light in the discharge direction, enters the discharge chamber 20 again, is amplified, and exits from the window 21. It is emitted as oscillation laser light L3. In this embodiment, a higher output can be obtained than the embodiment shown in FIG. 1 by expanding the light beam by the prisms 43 and 44, and the polarization direction of the light is controlled by the polarizing element 80, so that the light beam is expanded from the front mirror 10. Oscillating components can be reduced, improving efficiency.

Here, the prisms 41, 42 and 43, 44 constitute a beam expander that expands the laser beam, but the beam expansion direction (i.e., the direction perpendicular to the ridge line direction of the prism) is directed toward the electrode in the discharge chamber 20. 23.2
It coincides with (parallel to) the discharge direction according to 4. Note that the beam expansion direction of the beam expander by the prisms 41.42 and 43.44 does not need to exactly match the discharge direction. It is sufficient that the beam expansion direction of the beam expander substantially coincides with the discharge direction. In the embodiment shown in FIG. 3, the polarizing element 80 is arranged between the prisms 43 and 44 and the 45-degree right angle prism 50, but the invention is not limited to this. It can be placed at any location as long as it is on the optical path from when the light is turned back and enters the discharge chamber 20 again.

FIG. 4 shows another embodiment of the narrowband oscillation excimer laser according to the present invention, and the embodiment shown in the same figure is different from the embodiment shown in FIG. As shown by the broken line, AR cocoing is applied to allow only the P-polarized component of light to pass through. Note that the slit 91 is omitted. Other parts are the same as those shown in FIG. In the case of FIG. 4 as well, the front mirror 10 and the grating 30 constitute an optical resonator.

Also in this embodiment, the arrangement of the grating 30 with respect to the electrodes 23.24 in the laser chamber 20 is selected so that the drawing direction of the grating 30 is parallel to (coinciding with) the direction of discharge by the electrodes 23.24 in the discharge chamber 20. are doing. Further, a grating 30 is arranged on the window 21 side of the discharge chamber 20, and a front mirror 10 is arranged on the opposite side of the discharge chamber 20 and on the window 22 side.
A 45-degree right angle prism 50 is arranged to shift the light emitted from the front mirror 10 upward in the discharge direction and turn it back. A slit 92 is arranged between the window 22 and the front mirror 10, respectively. here,
The slit 92 has an elongated shape in a direction perpendicular to the direction of discharge by the electrodes 23, 24 in the discharge chamber 20.

In such a configuration, the portion of the grating 30 functions as a so-called rear mirror. That is, discharge chamber 2
The light beam passing through the grating 30 is expanded by a beam expander formed by prisms 42 and 41, and is irradiated onto the grating 30.

Since the prisms 42 and 41 are AR-coated as described above, only P-polarized light passes therethrough, and the prisms 42 and 41 are oscillated as P-polarized light. Since the discharge direction and the drawing direction of the grating 30 match, the light beam is expanded by the beam expander formed by the prisms 42 and 41, and the beam spread angle of the light irradiated onto the grating 30 is small. Then, the light reflected by the grating 30 passes through the radiation MF Yamba 2o, and the light is reflected by the front mirror 10.
This light will be output as an oscillation laser light, but this light will be expanded into a light beam by a beam expander using prisms 43 and 44. Like the prisms 42 and 41, these prisms 43 and 44 are also AR-coated, allowing only P-polarized light to pass through, and the 45-degree right-angle prism 5o
is incident on the In the 45 degree right angle prism 5o, the incident light is
The reflected light is bent so that its optical axis is upward in the discharge direction with respect to the optical axis of the incident light, and then enters the discharge chamber 20 again, where it is amplified and oscillated laser light L4 through the window 21.
is ejected as. In this example, the prism 43.44
By expanding the light beam by , it is possible to obtain a higher output than the embodiment shown in FIG. Furthermore, prism 41.
The polarization direction of the laser beam oscillated by the grating 3o and the front mirror 10 becomes P-polarized light due to the AR coating on the prisms 43 and 42, while the laser beam is emitted from the front mirror 10 and output via the discharge chamber 20 due to the AR coating on the prisms 43 and 44. The polarization direction of the oscillation laser light L4 becomes P-polarized light, so that both light directions can be matched. For this reason, the front mirror 10
This makes it possible to further reduce the amount of spontaneous oscillation.

The prisms 41.42 and 43.44 constitute a beam expander that expands the laser beam, and the beam expansion direction (that is, the direction perpendicular to the ridge line direction of the prism) is directed toward the electrodes 23.24 in the discharge chamber 20. coincides with (parallel to) the direction of discharge due to In addition, the prism 41
．． The beam expansion direction of the beam expander according to 42 and 43.44 does not have to coincide exactly with the discharge direction.

The beam expansion direction of the beam expander is approximately the discharge direction.
Just do it.

In any of the above embodiments, the drawing direction of the grating and the discharge direction are made to match, so that: 1) Even if the discharge width changes, the spectral linewidth does not change.

2) The power control rotation becomes wider.

3) - Because of the degree of freedom, the wavelength control speed is fast.

4) Since the energy density of the light incident on the grating and the beam expander is small, the amount of wavelength shift is small, and the life of the band narrowing element is extended.

Furthermore, since a polarizing element is placed in the optical path of the laser beam emitted from the front mirror until it returns and enters the discharge chamber again, the amount of spontaneous oscillation caused by the front mirror can be reduced, improving efficiency. I can do it. Then, the efficiency can be further improved by matching the polarization direction of the light that enters the discharge chamber again from the front mirror with the polarization direction of the laser light oscillated by the grating and the front mirror.

In the embodiments shown in FIGS. 1 and 2, slits (apertures) are inserted in the optical resonator at two locations, but the slits (apertures) are not limited to two locations, but may be inserted at one location, or may be inserted at three or more locations. Similarly, in the embodiments shown in FIGS. 3 and 4, the slit (aperture) is not limited to being inserted at one location, but may be inserted at two or more locations.

In addition, in the embodiment described above, the direction of discharge by the electrodes 23 and 24 in the laser channel and the direction of drawing of the grating 30 do not necessarily have to be exactly parallel. It is sufficient that the drawing direction of the grating 30 is approximately parallel to the direction of discharge by the electrodes 23,24.

Furthermore, in the embodiment, a 45 degree right angle prism is used as a return mirror for returning the light emitted from the front mirror 10 to the discharge chamber 20 side, but the present invention is not limited to this, and a single mirror or The two mirrors may be aligned 11,'>.

Further, in the embodiment, the laser beam is folded back only once, but the laser beam is not limited to this, and it is also possible to fold the laser beam two or more times.

In addition, although a prism is used as the beam expander in the embodiment, the beam expander is not limited to this, and the beam expander may be configured by a combination of a concave lens and a convex lens, or a combination of convex lenses. . Alternatively, the beam expander may be configured using a cylindrical lens.

Furthermore, in the embodiments shown in FIGS. 1 to 4, the grating is arranged as a rear mirror, but an oblique incidence arrangement is also possible. In the oblique incidence arrangement, for example, a total reflection mirror is integrally attached to the grating, and this total reflection mirror functions as a rear mirror. That is, the present invention may have any configuration as long as the grating is included within the optical resonator.

〔Effect of the invention〕

As explained above, according to the present invention, stable spectral purity can be ensured. Therefore, controllability of power and wavelength is greatly improved. Furthermore, since the oscillated laser beam emitted from the front mirror is turned back to enter the discharge chamber and amplified, efficiency is greatly improved.

[Brief explanation of drawings]

FIG. 1 shows an embodiment of the narrowband oscillation excimer laser according to the present invention, and FIG. 2 shows a prism as a beam expander between the front mirror shown in FIG. 1 and the 45-degree right-angle prism. FIG. 3 is a diagram showing another embodiment of the present invention in which the front mirror shown in FIG.
FIG. 4 is a diagram showing another embodiment of the present invention in which a polarizing element is inserted between a 5-degree right-angle prism and a beam expander shown in FIG. It is a figure which shows the other Example of this invention which carried out. 10... Front mirror 20... Discharge chamber,
21.22... Window, 23.24... Electrode, 3
0... grating, 41. ．． 42.43.44・
... Prism, 50 ... 45 degree right angle prism, 80 ... Polarizing element rear mirror Fig. 1 3 Fig. 2

Claims (1)

[Scope of Claims] A narrow band oscillation excimer laser that has an optical resonator including a grating, and outputs oscillated laser light by reciprocating discharge-excited light within a discharge chamber by the optical resonator, wherein the optical resonator A folding means is provided for folding the light emitted from the front mirror of the device into the discharge chamber and outputting the light through the discharge chamber, and the drawing direction of the grating is set in the discharge direction for the discharge excitation. A narrow band oscillation excimer laser characterized by being arranged so as to be substantially parallel to.