JPH0346284A - Narrowband oscillation excimer laser - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] This 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 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 locking 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 contained 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 injection locking method described above is essentially a technology that narrows the band by significantly reducing the number of spatial transverse modes.
Since the generation of speckle patterns is a serious problem, it cannot be used in a reduction projection exposure apparatus.

Another promising technique for narrowing the band of excimer lasers is the use of an etalon, which is a wavelength selection element. Conventional techniques using this etalon include a technique developed by AT&T Bell Laboratories in which an etalon is placed between the front mirror of an excimer laser and a laser chamber to narrow the band of the excimer laser, and a technique developed by the present inventors in which an etalon is placed between the front mirror of an excimer laser and a laser chamber, and an excimer laser developed by the present inventors. A technique has been proposed in which an etalon is disposed between a laser beam mirror and a laser chamber to narrow the band of the excimer laser.

FIG. 6 shows two etalons 2a between the front mirror 1 of the excimer laser and the laser chamber 4.

2b shows a conventional device configured by inserting the 2b. In addition, in FIG. 5, 5 beam mirror 3a.

3b shows the window of the laser chamber 4.

Further, FIG. 7 shows two etalons 2a between the excimer laser beam mirror 5 and the laser chamber 4.

2b shows another conventional device configured by inserting the 2b.

Furthermore, as shown in FIG. 8, a collimator made of, for example, a combination of a convex lens 6 and a concave lens 7 is inserted between the laser chamber 4 and the etalon 2a so that an etalon with a small effective area can be used. .

[Problem to be solved by the invention]

By the way, narrowband oscillation excimer lasers, which are constructed by arranging an etalon inside a laser cavity as shown in Figs. The reflective film deteriorates with a small number of shots, resulting in a significant decrease in etalon transmittance and finesse.

In this case, the spectral purity or power of the output laser light was significantly reduced. This became a particular problem when an excimer laser was used as a light source for a stepper.

Also, as shown in Figure 8, using a collimator lens,
This tendency became even more pronounced when the laser beam area incident on the etalon was made smaller than the output laser beam area.

Therefore, an object of the present invention is to provide a narrowband oscillation excimer laser in which deterioration of the etalon is suppressed as much as possible.

[Means to solve the problem]

In order to achieve the above object, the present invention is configured such that a beam expander is disposed between the laser chamber and the etalon to expand the laser beam incident on the etalon.

[Effect]

As a result, the energy density of the laser beam that passes through the etalon is significantly reduced, and deterioration of the etalon can be dramatically suppressed.

〔Example〕

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

FIG. 1 shows an embodiment of a narrow band oscillation excimer laser according to the present invention. In FIG. 1, a laser chamber 4, a beam expander 8, and etalons 2a and 2b are arranged between a front mirror 1 and an rear mirror 5. Here, the front mirror 1 and the rear mirror 5 constitute an optical resonator.

The laser chamber 4 is filled with a laser gas containing KrF or the like in a circulable manner, and is provided with a discharge electrode (not shown) for exciting the laser gas. Furthermore, fin windows 3 are arranged at predetermined angles at both ends of the laser chamber 4.
a and 3b are provided.

The beam expander 8 is for expanding the area of the laser beam output from the window 3b of the laser chamber 4, and the laser beam expanded by this beam expander is irradiated onto the etalons 2a and 2b.

The etalons 2a and 2b function as band narrowing elements, and one of the etalons 2a and 2b has a free spectrum with a large tral range, and the other has a small tral range.

Narrowing of the band is achieved by overlapping the transmission bands of 2b. Note that the transmission characteristics of each etalon 2a, 2b are controlled by controlling each etalon 2a, 2b with respect to the optical axis.
This can be done by changing the angle of , the gap length of each etalon 2a, 2b, the pressure in the gap space, etc.

According to such a configuration, the output laser light from the laser chamber 4 is expanded by the beam expander 8 and irradiated onto the etalons 2a, 2b.
The energy density of the laser beam transmitted through the etalons 2a and 2b is significantly reduced, thereby significantly suppressing deterioration of the etalons 2a and 2b.

FIG. 2 shows an embodiment in which a concave lens 6 and a convex lens 7 are used as a beam expander. In this embodiment, the laser light output from the window 3b of the laser chamber 4 is converted into parallel light that is expanded by a concave lens 6 and a convex lens 7, and is irradiated onto the etalons 2a and 2b. This reduces the energy density of the laser beam that passes through the etalons 2a, 2b.

In the embodiment shown in FIG. 2, the laser beam output from the window 3b of the laser chamber 4 is configured to expand at a constant ratio in all directions, but this is expanded only in a specific direction. It may be configured to expand. With this configuration, the life of the etalon can be extended without increasing the effective diameter of the etalon too much.

FIGS. 3(a) and 3(b) show another embodiment of the invention constructed in this manner. FIG. 3(a) shows its plan view, and FIG. 3(b) shows its side view. Figure 3(a)
In the embodiment shown in (b), a beam expander is constituted by two cylindrical lenses 10 and 11. In this case, the laser light output from the window 3b of the laser chamber 4 is expanded only in a direction perpendicular to the direction of discharge by the electrode 9 in the laser chamber 4, and the laser beam is expanded into the etalon 2a.
, 2b.

FIGS. 4(a) and 4(b) show another embodiment in which tRl1ffi is achieved using two prisms 12a and 12b as beam expanders. Here, Fig. 4(a)
shows a plan view, and FIG. 4(b) shows a side view. In this case as well, the direction of beam expansion of the beam expander is configured to be substantially perpendicular to the direction of discharge by the electrodes 9.

In addition, in the configuration of FIG. 4, two prisms 12a, 1
Although the beam expander is constructed using the prism 2b, the beam expander may be constructed using one prism or three or more prisms.

Figures 5(a) and 5(b) show another embodiment in which a beam expander 13 that expands only in one direction is used so that the shape of the laser beam incident on the etalon is approximately square. be. Here, FIG. 5(a) shows its plan view, and FIG. 5(b) shows its side view.

Generally, the laser oscillation area of the laser chamber 4 has a rectangular shape that is long in the direction of discharge by the electrode 9. Therefore, in this embodiment, only the direction perpendicular to the discharge direction by the electric current 1ft9 is expanded so that the shape of the laser beam incident on the etalons 2a and 2b is approximately square as shown in FIG. 5(C). .

Note that in this example, only the direction perpendicular to the discharge direction is expanded, but even if the discharge direction and the direction perpendicular to this are expanded at different ratios, the shape of the beam incident on the etalon will be approximately square. can do.

With this configuration, the band can be efficiently narrowed without increasing the effective diameter of the etalon, and the life of the etalon can be extended.

Although the above embodiment shows the case where the etalon is inserted between the beam mirror and the laser chamber, the present invention can be similarly applied to the case where the etalon is inserted between the front mirror and the laser chamber. Further, the number of etalons to be inserted is not limited to two, but may be one or three or more.

〔Effect of the invention〕

As explained above, according to the present invention, the following effects can be obtained.

(1) A beam expander is placed between the laser chamber and the etalon to expand the laser beam incident on the etalon, thereby reducing the energy density that passes through the etalon, thereby dramatically extending the life of the etalon.

(2) Furthermore, by arranging the beam expansion direction to be perpendicular to the discharge direction, it is possible to effectively narrow the band without increasing the effective diameter of the etalon and dramatically extend the life of the etalon. can.

(3) Furthermore, by making the laser beam incident on the etalon square, it is possible to effectively narrow the band and dramatically extend the life of the etalon without increasing the effective diameter of the etalon.

[Brief explanation of drawings]

FIG. 1 is a plan view showing an embodiment of a narrow band oscillation excimer laser according to the present invention, FIG. 2 is a plan view showing an embodiment using a combination of a concave lens and a convex lens as a beam expander, and FIG. a) and (b) are a plan view and a side view showing another embodiment configured using two cylindrical lenses as a beam expander, and FIG.
b) is a plan view and a side view showing another embodiment configured using two prisms as a beam expander;
Figures (a) and (b) are plan and side views showing still another embodiment in which the shape of the beam incident on the etalon is approximately square, and Figure 5 (C) is the same as Figure 5 (a). (b
), FIG. 6, FIG. 7, and FIG. 8 are diagrams showing conventional examples of the present invention. 1...Front mirror 2a, 2b...Etalon,
3a, 3b...Window, 4...Laser chamber, 5...Rear mirror 6...Concave lens, 7...Convex lens, 8.13...Beam expander, 9...
Electrode, 10° 11... Cylindrical lens, 12a
, 12b...prism. Figure 2 Figure 3 (a) Figure 3 (b) Figure 4 (0) Figure 4 (b) Figure 5 (c)

Claims (5)

[Claims] (1) In a narrowband oscillation excimer laser using an etalon as a band narrowing element, a beam expander is disposed between a laser chamber and the etalon, and the area of the laser beam incident on the etalon is at least larger than the area of the output laser beam. A narrowband oscillation excimer laser. (2) The narrowband oscillation excimer laser according to claim (1), wherein the beam expander is configured using a prism or a cylindrical lens and expands only one direction of the laser beam. (3) The narrow band oscillation excimer laser according to claim 1, wherein the beam expander has a beam expansion direction substantially perpendicular to the discharge direction of the discharge electrode. (4) The narrowband oscillation excimer laser according to claim (1), wherein the beam expander is constructed using a collimator lens. (5) Claim (1) characterized in that the shape of the laser beam incident on the etalon is approximately square.
) Narrowband oscillation excimer laser.