JPH03250776A - Narrowband oscillation excimer laser - Google Patents

Abstract

PURPOSE:To contrive narrowing the band of a laser and the prolongation of a device life as well as to improve the controllability of the wavelength of a laser beam and moreover, to make it possible to obtain a high output of the laser where the wavelength is not effected by the discharge width by a method wherein a space filter is arranged between a grating and a laser chamber. CONSTITUTION:In a narrow-band oscillation excimer laser, a laser chamber 3 and a space filter 20 to function as a wavelength selecting element are arranged between a front mirror 1 and a grating 6 having the functions of a rear mirror and the wavelength selecting element. Since a laser beam incident on the grating 6 is restricted from spreading by the filter 20, the line width of a laser wavelength is not changed even if a discharge width is changed. Thereby, it becomes possible that the discharge width is made large and an energy density can be made small. Accordingly, the life of an optical element and a discharge electrode can be drastically prolonged and at the same time, the control of the output of the laser becomes possible at a high efficiency.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a narrowband oscillation excimer laser used as a light source for a stepper.

[Prior Art and Problems to be Solved by the Invention] The use of an excimer laser as a light source for a reduction projection exposure apparatus (hereinafter referred to as a stepper) for manufacturing semiconductor devices is attracting attention. This is because the wavelength of excimer laser is short (the wavelength of KrF is approximately 248.4 nm), so the limit of optical exposure is 0.5μ.
m or less, the depth of focus is deeper than the g-line and i-line of conventional mercury lamps for the same resolution, and the numerical aperture (NA) of the lens can be small, making it possible to expand the exposure area. This is because it can be expected to have many excellent advantages such as being able to be made larger and having 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 places a wavelength selection element (etalon, diffraction grating, prism, etc.) inside the cavity of the oscillator stage, limits the spatial mode with a pinhole, causes single mode oscillation, and amplifies this laser light. Injection locking by stage. Although relatively large output power can be obtained with this method, there are drawbacks such as miss shots, difficulty in achieving 100% locking 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.

That is, it is known that when the number of spatial transverse modes included in laser light is small, speckle patterns are more likely to occur, and conversely, when the number of spatial modes is large, speckle patterns are less likely to occur.

The above-mentioned injection rotary method is essentially a technology that narrows the band by significantly reducing the number of spatial transverse modes, and because the generation of speckle patterns is a major problem, it is not used in reduction projection exposure equipment. Can not.

Another promising technique for narrowing the band of excimer lasers is one that uses an air gear tube etalon, which is a wavelength selection element. In the conventional technology using this air gear tube etalon, a plurality of air gap etalons are arranged within the cavity of the excimer laser in an attempt to narrow the band of the excimer laser.

However, when an etalon is placed inside a laser cavity, the energy density transmitted through the etalon is extremely high in narrow-band oscillation excimer lasers for single-stage steppers, so the etalon's reflective film deteriorates with a small number of shots. , the spectral purity and power of the output laser beam were significantly reduced due to a significant decrease in the transmittance of the etalon and a decrease in finesse. Furthermore, according to this conventional technology, since the energy density transmitted through the etalon is high, the etalon gap changes due to heat, resulting in a large shift in the transmission wavelength of the etalon, which significantly reduces wavelength controllability. There was a problem.

In addition, other conventional technologies use prism beam expanders and gratings as band narrowing elements, but in this case, it is necessary to insert an aperture to narrow the line width, which results in high efficiency. In order to oscillate, the energy density of the discharge was increased to cause oscillation.

Therefore, this conventional device has a problem in that the life of the discharge electrode is extremely short.

Furthermore, these conventional techniques generally have a large beam spread, so when the discharge voltage is changed to control the output, the discharge width changes, causing a change in the wavelength line width and oscillation efficiency.

This invention was made in view of these circumstances,
The purpose of the present invention is to provide a narrowband oscillation excimer laser that can narrow the band and extend the life of the device, improve wavelength controllability, and obtain high output without having the wavelength line width influenced by the discharge width. shall be.

[Means to solve the problem]

In order to achieve the above object, in the present invention, in a narrow band oscillation excimer laser including a grating as a band narrowing element, a spatial filter is disposed between the grating and the laser chamber.

[Effect]

According to this configuration, the beam spread of the laser light incident on the grating is reduced by the spatial filter.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a side view showing an embodiment of a narrow band oscillation excimer laser according to the present invention. In the narrowband oscillation excimer laser of this embodiment, a laser chamber 3 and a spatial filter 20 that functions as a wavelength selection element are arranged between a front mirror 1 and a rear mirror 6 that functions as a wavelength selection element. will be established. In this case, the spatial filter 20 is composed of a light collecting lens 4,4' and an aperture 5. That is, in this case, an optical resonator is configured between the front mirror 1 and the grating 6.

This grating arrangement is a so-called loose draw arrangement.

The laser chamber 3 is filled with a laser gas containing KrF or the like in a circulatory manner, and windows 2a and 2b are provided at both ends of the laser chamber 3 at a predetermined angle.

The grating 6 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 6 selects light of a specific wavelength by varying the angle θ of the grating 6 with respect to the incident light within a plane including the drawing direction. That is, the grating 6 reflects only a specific light corresponding to the angle 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.

By arranging the spatial filter 20 between the laser chamber 3 and the grating 6 in this manner, the spread of the beam incident on the grating 6 is reduced, and the band is efficiently narrowed.

Note that in this embodiment, a slit may be arranged instead of the aperture 5. However, when slits are arranged, they are arranged so that the longitudinal direction of the slits and the drawing direction of the grating 6 are perpendicular to minimize beam distortion in the grating 6 and efficiently narrow the band.

FIGS. 2(a) and 2(b) show a plan view and a side view of another embodiment of the present invention in which the spatial filter 20 of the embodiment shown in FIG. This is what is shown. In this embodiment, the cylindrical lens 8.8' focuses the laser beam as a linear beam, so the energy density is lower than in the embodiment shown in FIG. 1, and the life of the slit 7 can be further extended. can.

FIGS. 3(a) and 3(b) show a plan view and a side view of another embodiment of the present invention in which the relationship between the discharge direction and the arrangement direction of the slits 7 in the embodiment shown in FIG. 1 is specified. It is something.

In this embodiment, by making the longitudinal direction of the slit 7 substantially coincide with the discharge direction of the laser, the amount of laser light blocked by the slit 7 is reduced and the transmittance of the spatial filter 20 is improved. In other words, the spread angle of the laser beam output from the window 2b of the laser chamber 3 is generally smaller in the direction perpendicular to the discharge direction than in the direction of discharge by the electrodes 9.9, that is, the direction in which the electrodes 9.9 are arranged. , its shape is a vertically long rectangle. Therefore, by arranging the slit 7 so as to correspond to this laser cross-sectional shape, the transmittance of the spatial filter 20 is improved and efficient band narrowing can be achieved.

FIGS. 4(a) and 4(b) show that in the embodiment shown in FIG. 2, the discharge direction, the longitudinal direction of the slit 7, and the mechanical axis of the cylindrical lens 8.8' are substantially aligned, and furthermore, these directions are aligned with the grating. 6 drawing direction (Fig. 4(b)
) shows a plan view and a side view of another embodiment of the present invention, which is arranged substantially perpendicular to the plane of the drawing.

With this configuration, the amount of laser light that is rejected by the slit 7 is reduced, and the transmittance of the spatial filter 20 is improved.Moreover, since the laser light is focused by the cylindrical lens 8.8', the energy density is reduced, and the slit 7 The lifespan of can be further extended. Furthermore, since the grating 6 is arranged so that the drawing direction of the grating 6 is also perpendicular to the discharge direction by the electrodes 9.9, the beam spread in the grating 6 can be minimized. Bandwidth can be narrowed efficiently.

5(a) and 5(b) show a plan view and a side view of another embodiment of the present invention in which a prism beam expander 10 is added to the embodiment shown in FIG. 4. The panda 1o is arranged between the spatial filter 20 and the grating 6.

Here, the direction of beam expansion by the beam expander 10 coincides with the direction perpendicular to the direction of discharge by the electrodes 9 in the laser chamber 3.

According to such a configuration, since the beam is expanded by the beam expander 10, the energy density of the laser beam is further reduced, and the life of the slit 7 and the grating 6 can be extended. Furthermore, since the direction of expansion is only perpendicular to the direction of discharge by the electrodes 9.9, the beam divergence angle entering the grating can be further reduced, thereby making it possible to narrow the band more efficiently.

6(a) and 6(b) show that the cylindrical lens 8' of the embodiment shown in FIG. 4 is arranged in a different position so that the functions of the spatial filter 20 and the beam expander are replaced by two cylindrical lenses 8.8'. A side view and a plan view of another embodiment of the present invention realized using only the slit 7 and the slit 7 are shown.

By doing so, the beam expander can be omitted.

Figure 7 shows the spatial filter 20 as a concave mirror (or parabolic mirror).
]], 11' and a slit 7. Note that if the concave mirror (or parabolic mirror) 11, 11' is configured in a cylindrical shape, the durability of the slit 7 can be increased because the linear beam is focused on the slit 7.

In the embodiments, only the retro arrangement is illustrated, but the present invention can also be applied to an incident arrangement. Further, in the embodiment, a single lens is used as an example of the spatial filter 20, but even better band narrowing can be achieved by using a lens set with aberration correction, an aspherical lens, or the like. Furthermore, if chromatic aberration correction is performed, even if the color of the lens changes, the focal position will not change, so that even better band narrowing can be achieved.

〔Effect of the invention〕

As explained above, according to the present invention, (1) the spatial filter reduces the beam spread, so it is possible to efficiently narrow the band without inserting an aperture inside the cavity; It is also possible to obtain high output.

(2) Since the beam spread is shifted to a small value using a spatial filter, the wavelength line width does not change even if the discharge width changes, which makes it possible to increase the discharge width and reduce the energy density. As a result, the lifespan of the optical element and the discharge electrode can be dramatically extended, and output control can be performed with high efficiency.

(3) When a cylindrical lens and a slit are used as a spatial filter, a ray with low energy density is incident on the slit, and the life of the slit can be dramatically extended.

It has excellent effects such as

[Brief explanation of drawings]

FIG. 1 is a plan view showing an embodiment of the present invention, and FIG.
)(b) to 6(a) and 6(b) are respectively a plan view and a side view showing other embodiments of the present invention, and FIG. 7 is a plan view showing still another embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... Front mirror 2a, 2b... Window, 3... Laser chamber, 4... Condensing lens, 5...
・Aperture, 6... Grating, 7... Slit, 8... Cylindrical lens, 9... Discharge electrode, 10... Prism beam extractor, 20.
...Spatial filter 1 Figure 0 (b) Figure 2 Figure 3 Figure (0) 0 Figure 4 (b) Figure 5 (CI> To) Figure 6

Claims (9)

[Claims] (1) A narrowband oscillation excimer laser including a grating as a band narrowing element, characterized in that a spatial filter is disposed between the grating and the laser chamber. (2) The narrowband oscillation excimer laser according to claim (1), wherein the spatial filter comprises a condenser lens and a slit. (3) The narrowband oscillation excimer laser according to claim (1), wherein the spatial filter comprises a cylindrical lens and a slit. (4) The narrowband oscillation excimer laser according to claim (1), wherein the spatial filter comprises a concave mirror and a slit. (5) The narrowband oscillation excimer laser according to claim (1), wherein the spatial filter comprises a parabolic mirror and a slit. (6) The narrowband oscillation excimer laser according to claim 2, wherein the longitudinal direction of the slits and the drawing direction of the grating are substantially perpendicular to each other. (7) The cylindrical lens according to claim (3), characterized in that the direction of the mechanical axis of the cylindrical lens and the longitudinal direction of the slit are substantially aligned, and these directions are arranged to be substantially perpendicular to the drawing direction of the grating. Narrowband oscillation excimer laser. (8) Claim (8) characterized in that the cylindrical lens is arranged so that the direction of the mechanical axis substantially coincides with the discharge direction.
7) The narrowband oscillation excimer laser described above. (9) The narrowband oscillation excimer laser according to claim (1), further comprising a beam expander disposed between the spatial filter and the grating.