JPH03139893A - Narrow band oscillation excimer laser - Google Patents

Abstract

PURPOSE:To lengthen life and narrow the spectral line width while improving controllability of the wavelength by arranging a beam expander and an etalon between a laser chamber and a grating and making the beam expanding direction of the beam expander almost vertical to the discharge direction and further arranging the grating so that its wire drawing direction may be almost vertical to the discharge direction. CONSTITUTION:An etalon 5, which functions as a laser chamber 3, a beam expander 4 and a wavelength selection element is arranged between a front mirror 1 and a grating 6 having a function of a rear mirror and a wavelength selection element is arranged. The laser chamber 3 is circulatably filled with laser gas and discharge electrodes 7, 7' for exciting gas are arranged in line and the discharge direction is made in the vertical direction. The grating 6 can select light having the specific wavelength by making an angle theta to incident light variable. The beam expanding direction is in accord with the vertical direction to the discharge direction by the electrodes 7, 7' inside a laser chamber 3. Further, the wire drawing direction of the grating 6 also selects its arrangement so as to be vertical to the discharge direction by the electrodes 7, 7'.

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.

[Conventional technology]

The use of excimer lasers as light sources for reduction projection exposure apparatuses (hereinafter referred to as steppers) for manufacturing semiconductor devices is attracting attention. This is due to the short wavelength of excimer laser (KrF
The wavelength of
．． It is possible to extend the length to 5 μm 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 grow in size and obtain a large amount of 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 technology for narrowing the band of excimer lasers, there is a conventional technique called the injection locking method.6゜This injection locking method uses a wavelength selection element (etalon, diffraction grating, prism, etc.) within the cavity of the oscillator stage. The spatial mode is restricted by a pinhole to cause 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% 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, speckles and patterns are more likely to occur, and conversely, when the number of spatial modes is large, speckles and patterns are less likely to occur.

The injection lock method described above is essentially a technique for narrowing the band by significantly reducing the number of spatial transverse modes, and cannot be used in reduction projection exposure apparatuses because the generation of speckle patterns poses a major problem.

Another promising technique for narrowing the band of excimer lasers is the use of an air gap etalon, which is a wavelength selective element. As a conventional technique using this air gear tube etalon, a technique has been proposed in which a plurality of air gap etalons are arranged within a cavity of an excimer laser to narrow the band of the excimer laser.

[Problem to be solved by the invention]

However, when an etalon is placed inside a laser cavity, the energy density transmitted through the etalon is extremely high in single-stage narrowband oscillation excimer lasers for use in stereo/sono systems, so the etalon's reflective film deteriorates with a small number of shots. As a result, the transmittance of the etalon is significantly reduced, the finesse is reduced, and the spectral purity and power of the output laser beam are significantly reduced. 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 etalon transmission wavelength, which significantly reduces wavelength controllability. There was a problem.

This invention was made in view of these circumstances,
It is an object of the present invention to provide a narrowband oscillation excimer laser that has a narrower band and a longer lifespan, as well as a narrower spectral linewidth and improved wavelength controllability.

[Means to solve the problem]

In order to achieve the above object, in the present invention, a beam expander, an etalon, and a grating are used as band narrowing elements, and the beam expander and etalon are arranged in this order between the laser chamber and the grating, and the beam expander The beam expansion direction of the grating is made substantially perpendicular to the discharge direction, and the grating is arranged so that its drawing direction is substantially perpendicular to the discharge direction.

[Effect]

According to this configuration, wavelength selection is performed using the etalon and grating, and beam expansion is performed using the beam expander. The beam expansion direction of the beam expander is made substantially perpendicular to the discharge direction, and the grating is arranged so that its drawing direction is substantially perpendicular to the discharge direction. Beam expansion by the beam expander reduces energy density and extends the lifetime of the etalon and grating.

The beam divergence angle of an excimer laser is larger in the laser discharge direction than in the direction perpendicular to the laser discharge direction.

The cross section of the oscillation region is also larger in the discharge direction than in the direction perpendicular to the discharge direction.

Therefore, by making the beam expansion direction of the beam expander substantially perpendicular to the laser discharge direction, the band can be efficiently narrowed without increasing the effective diameter of the etalon.

Furthermore, by making the drawing direction of the grating substantially perpendicular to the laser discharge direction, the band can be narrowed more efficiently.

Snake Embodiment] 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. The narrowband oscillation excimer laser of this embodiment includes a laser chamber 3, a beam expander 4, and an etalon 5 that functions as a wavelength selection element between a front mirror 1 and a grating 6 that functions as a rear mirror and a wavelength selection element. will be placed. That is, 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 discharge electrodes 7 and 7' for exciting the laser gas are arranged in parallel in a direction perpendicular to the plane of the drawing. That is, the discharge direction is perpendicular to the paper surface. Furthermore, windows 2a and 2b are provided at both ends of the laser chamber 3 and are arranged 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 can select 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.

Here, the direction of beam expansion by the beam expander 4 coincides with the direction perpendicular to the direction of discharge by the electrodes 7, 7' in the laser chamber 3. Further, the arrangement of the grating 6 with respect to the electrode 7.7' in the laser chamber 3 is selected so that the drawing direction of the grating 6 is also perpendicular to the discharge direction by the electrode 7.7'.

According to such a configuration, since the beam is expanded by the beam expander 6, the energy density of the laser beam is reduced, and the life of the etalon 5 and the grating 6 can be extended. Furthermore, since the energy density of the laser beam is reduced, changes in the etalon gap due to heat are suppressed, and wavelength controllability is improved. Furthermore, since the direction of expansion is only perpendicular to the direction of discharge from the electrodes 7, 7', the shape of the laser beam can be approximately square as shown in Figure 2, which makes the etalon effective. The band can be narrowed efficiently without increasing the diameter too much. That is, the divergence 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 discharge direction by the electrodes 7.7-, that is, the arrangement direction of the electrodes 7.7'. , its shape is a horizontally long rectangle. Therefore, by setting the direction of expansion to be perpendicular to the direction of discharge by the electrode 7.7', the shape of the laser beam can be made approximately square.

Further, according to this embodiment, the etalon 5 is inserted and the grating 6 is arranged so that the drawing direction of the grating 6 is also perpendicular to the discharge direction by the electrodes 7.7'. It is possible to minimize the beam spread at , thereby efficiently narrowing the band.

3(a) and 3(b) show another embodiment of the present invention using a beam expander constituted by two cylindrical lenses 8.9 as the beam expander 4 of the embodiment shown in FIG. An example is shown in side view and top view. In this embodiment as well, the drawing direction of the grating 6 is perpendicular to the discharge direction by the electrode 7.7' in the laser chamber 3, and the beam expansion direction of the beam expander by the cylindrical lens 8.9 is the same as that of the electrode 7.7'. 7′
It is configured to be perpendicular to the discharge direction.

FIGS. 4(a) and 4(b) show two prisms 10 and 11 as the beam expander 4 of the embodiment shown in FIG.
FIG. 3 shows a side view and a plan view of another embodiment of the present invention using a beam expander configured as shown in FIG. In this embodiment as well, the drawing direction of the grating 6 is perpendicular to the discharge direction by the electrode 7.7' in the laser chamber 3, and the beam expansion direction of the beam expander by the prism 10.11 is made perpendicular to the discharge direction by the electrode 7.7' in the laser chamber 3. ′ is arranged perpendicular to the discharge direction. In this case, the beam expander was constructed using two prisms, but only one prism was used.
The number may be one or more.

5(a) and 5(b) show that the prism 10 is connected between the prism 10 of the embodiment shown in FIG. 4 and the laser chamber 3.
．． 11 is a side view and a plan view showing another embodiment of the present invention in which a polarizing element 12 is interposed to selectively transmit only polarized waves substantially parallel to the beam expansion direction of a prism beam expander consisting of a prism beam expander 11. . In this embodiment as well, the drawing direction of the grating 6 is perpendicular to the discharge direction by the electrodes 7,7' in the laser chamber 3, and the prism 1
The beam expansion direction of the beam expander according to 0.11 is arranged to be perpendicular to the discharge direction by the electrodes 7.7'.

The polarizing element 12 selectively transmits only polarized waves substantially parallel to the beam expansion direction of the prism beam expander formed by the prism 10.11. As the polarizing element 12, for example, a polarizing prism using a birefringent material (crystal, calcite, etc.), a Brewster dispersion prism, a glass substrate (quartz, CaF2 or MgF2) arranged at Brewster's angle, or a glass substrate It can be constructed from a material coated to transmit a predetermined polarized light beam.

According to such a configuration, the apparatus shown in FIGS. 5(a) and 5(b) selectively oscillates with linearly polarized waves that are approximately parallel to the beam expansion direction of the prism beam expander composed of prisms 1 and 11. Ru.

Here, the prism beam extractor (Linearly polarized light parallel to the beam expansion direction of the beam has a large transmittance even if the incident angle to the prism is large, so the beam expander's expansion ratio is increased to narrow the spectral line width. However, the losses will not be that large.

In other words, a narrow band oscillation excimer laser with low loss can be constructed.

6(a) and 6(b) show another embodiment of the present invention, FIG. 6(a) being a side view thereof, and FIG. 6(b) being a plan view thereof. In this embodiment, the rear window 2b of the laser chamber 3 is configured to select a specific linearly polarized light wave.

In this embodiment, the rear window 2b of the laser chamber 3 has an approximately Brewster angle θa with respect to the laser optical axis in the beam expansion direction of the prism beam expander consisting of the prism 10.11 and in the plane of the laser optical axis. are arranged so that

In addition, in the configuration shown in FIG. 6, the rear window 2b
Although only the rear window 2b and the front window 2a may be arranged at a predetermined Brewster angle, only the front window 2a may be arranged at the Brewster angle. You may.

FIGS. 7(a) and 7(b) show still another embodiment of the present invention. Here, FIG. 7(a) shows its side view, and FIG. 7(b) shows its plan view. In this embodiment, the prism 10.1 constituting the prism beam expander is
1 is coated with an antireflection film that selectively prevents reflection of only polarized light components parallel to the expansion direction of the prism beam expander. The portions 13 and 14 indicated by dotted lines in FIG. 7(a) indicate this coating portion.

In this case, the coating may be applied to one surface of at least one prism that transmits light. In this case, even if the angle of incidence with respect to the prism becomes large, a transmittance of 99% or more can be maintained for polarized light components parallel to the beam expander direction.

In the above embodiment, a cylindrical lens or a prism was used as a beam expander to expand the laser beam in only one direction, but as shown in FIG.
, a convex lens 16 may be used. Furthermore, the beam expander may be configured with a convex lens.

In the embodiment of the above-mentioned retro-arrangement, it is preferable to use a Nizier grating as shown in FIG. 9 as the grating 6. As shown in FIG. 9, Nizier gratings have grooves whose apex angles are almost right angles, and can be manufactured with large blaze angles β, resulting in high efficiency and high resolution. Therefore, in each of the above-mentioned and draw arrangement embodiments, if a Nizier grating as shown in FIG. 9 is used as the grating 6, and the incident angle and the diffraction angle of the laser beam are made to coincide with the blaze angle, it will be more efficient. Narrowing of the band becomes possible, and sufficient narrowing of the band can be achieved even with a single grating stage.

In the above-described embodiment, the discharge direction by the electrodes 7, 7' in the laser chamber 3 and the drawing direction of the grating 6 do not necessarily have to be exactly perpendicular. If the drawing direction of the grating 6 is substantially perpendicular to the direction of discharge by the electrodes 7, 7', sufficiently highly efficient wavelength control becomes possible.

Further, the beam expansion direction of the beam expander by the prism 10.11 or the cylindrical lens 8.9 does not need to be exactly perpendicular to the discharge direction by the electrode 7.7'. The beam expansion direction of the beam expander is the electrode 7.7
If the direction of discharge is approximately perpendicular to the direction of discharge caused by ゛, sufficiently highly efficient wavelength control becomes possible.

〔Effect of the invention〕

As explained above, according to the present invention, (1) a beam expander is arranged between the laser chamber and the etalon;
By enlarging the laser beam incident on the etalon, the energy density □ incident on the etalon and grating is reduced, so the life of the etalon and grating is dramatically extended.

(2) Since the energy density incident on the etalon and grating is low, thermal deformation does not occur in the etalon and grating, thereby reducing the amount of wavelength shift and improving wavelength controllability.

(3) Since the beam expansion direction of the beam expander is made almost perpendicular to the discharge direction, and the grating is arranged so that its line drawing direction is substantially perpendicular to the discharge direction, the effective diameter of the etalon is increased. To efficiently narrow the band without any problems.

(4) The drawing direction of the grating and the beam expansion direction of the prism beam expander are made to almost match, and a selective transmission means is provided to selectively oscillate linearly polarized waves approximately parallel to the beam expansion direction of the prism beam expander. , loss in the prism beam expander can be almost eliminated. Therefore, it becomes possible to narrow the spectral line width and obtain a large output.

[Brief explanation of the drawing]

FIG. 1 is a side view showing an embodiment of the present invention, FIG. 2 is a sectional view showing the shape of a laser beam according to the embodiment of the invention, and FIGS. 3(a), (b) to 7(a) (b) is a side view and a plan view showing another embodiment of the invention, FIG. 8 is a side view showing still another embodiment of the invention, and FIG. 9 is a side view showing a Nisier grating. . DESCRIPTION OF SYMBOLS 1... Front mirror 2a, 2b... Window, 3... Laser chamber, 4... Beam expander, 5... Etalon, 6... Grating, 7.
7'9. . Discharge electrode, 8.9... Cylindrical lens, 10.
11. ... Prism, 12... Polarizing element 13.1
4... Anti-reflection film 15.16... Collimator lens Fig. 3 (Q) Fig. 3 (b) Straight direction Fig. 1 Fig. 2 Fig. 4 Fig. 4 (b) Fig. 5 (( 1) Figure 5 (b) Figure 7 (Q) Figure 7 (b) Figure 6 ((1) Figure 6 (b) Figure 8 Figure 9 Procedural amendment (voluntary) 1゜ Case Display 1999 Patent Application No. 278141 2゜Name of the inventionNarrowband oscillation excimer laser 3゜Relationship with the case Patent applicant (123) Komatsu Ltd. 4゜Representative (〒104) Chuo, Tokyo 2-11-2-5, Ginza-ku, drawing subject to correction.

Claims (5)

[Claims] (1) A beam expander, an etalon, and a grating are used as band narrowing elements, and the beam expander and etalon are arranged in this order between the laser chamber and the grating, and the beam expansion direction of the beam expander is set in the discharge direction. and the grating is arranged so that its drawing direction is substantially perpendicular to the discharge direction. (2) The narrow band oscillation excimer laser according to claim (1), further comprising selective transmission means for selectively transmitting linearly polarized waves substantially parallel to the beam expansion direction of the beam expander. (3) The narrowband oscillation excimer laser according to claim 1, wherein the selective transmission means is a polarizing element disposed within an optical resonator. (4) The selective transmission means is arranged on the rear side of the laser chamber so that the beam expansion direction of the beam expander and the optical axis of the laser form a substantially Brewster angle with respect to the optical axis of the laser. The narrowband oscillation excimer laser according to claim 1, further comprising a window on the front side. (5) The beam expander is a prism, and the selective transmission means selectively prevents reflection of only polarized light components substantially parallel to the beam expansion direction of the prism, and includes an antireflection film coated on at least one surface of the prism. The narrowband oscillation excimer laser according to claim 1, characterized in that the narrowband oscillation excimer laser comprises: