JPH04314374A - Narrow band laser device - Google Patents

Abstract

PURPOSE:To reduce fluctuation of beam profile and to acquire laser light of stable output by providing an air flow generating means which makes gas flow to a reflection surface of a diffraction grating. CONSTITUTION:Air flow is made to occur in a reflection surface of a diffraction grating 30 by blowing gas against the diffraction grating 30 by a fan 40. Thereby, a surface of the diffraction grating 30 is cooled and gas in an area near a surface of the diffraction grating 30 is forcibly moved to prevent gas in an area near a surface of the diffraction grating 30 from being heated by the diffraction grating 30. Since air flow is generated forcibly in a surface of the diffraction grating 30, gas is not heated by heat of the diffraction grating 30 and does not generate natural convection. Thereby, it is possible to prevent beam profile from fluctuating.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser device that outputs laser light with a narrow spectral width, and more particularly to a narrow band laser device suitable as a light source for a reduction projection exposure apparatus.

0002

[Background Art] With the increasing density of semiconductor integrated circuits, patterns have become significantly finer, and in order to form fine patterns, a laser light source that outputs so-called narrowband laser light with a narrow spectral width is required. There is. Currently, excimer laser devices are attracting attention as light sources for reduction projection exposure devices because they have high energy conversion efficiency and can provide large output. However, the excimer laser device has a wide spectrum width, and the oscillated light cannot be used as it is as a light source for a reduction projection exposure device. Therefore, the spectral width of the oscillated laser beam is narrowed using a wavelength selection element (band narrowing element) such as an etalon or a diffraction grating.

FIG. 7 shows an example of a conventional narrow band laser device. This laser device has windows 1 at both ends of a laser chamber 10 in which a laser medium such as KrF is sealed.
2 and 14 are provided so that the laser beam 16 generated within the laser chamber 10 can be emitted to the outside of the laser chamber 10. A front mirror 18 constituting a resonator is provided in front of the window 12 on the output side.

Further, etalons 20 and 22 as wavelength selection elements are arranged behind the other window 14 (on the right side in the figure), and a rear mirror 24 forming a resonator is arranged behind the etalon 22. There is.

The narrow band laser device shown in FIG.
Since it aims to narrow the band, there is no need to limit the discharge width, and there is an advantage that the band narrowing efficiency is high. However, since the laser light 16 emitted from the laser chamber 10 directly enters the etalons 20 and 22, the energy density of the incident light is high, and the etalons 20 and 22 are heated by the laser light 16, resulting in thermal changes in wavelength selection characteristics. Stability is extremely poor,
Moreover, the life of the etalons 20 and 22 is shortened. Also, 2
Complex wavelength control is required to match the selected wavelengths of the two etalons 20 and 22, which slows down the wavelength and output control speed.

Therefore, a thermally stable diffraction grating is used, and prisms 26 and 28 are placed behind the window 14, as shown in FIG.
and the diffraction grating 30 are arranged in Littrow, and the window 12
, 14 are provided with apertures 32, 34 adjacent to each other. In this narrow band laser device, the diffraction grating 30 is thermally stable, and the wavelength can be controlled using the diffraction grating 30.
Since only the angle of the wavelength needs to be adjusted, wavelength control and output control can be performed very quickly.

Furthermore, as shown in FIG. 9, an etalon 36 is provided between the prism 28 arranged in Littrow and the diffraction grating 30.
There are also devices equipped with This device consists of prisms 26, 2
8 and the etalon 36, the wavelength spectrum width can be narrowed.
2.34 is not required. In the apparatus shown in FIG. 9, the laser beam 16 is dispersed (expanded) by the prisms 26 and 28, and then enters the etalon 36 and the diffraction grating 30. Since the energy density of the laser beam is small and the amount of wavelength shift is small, only the etalon 36 needs to be controlled for wavelength control, so the wavelength and output can be controlled quickly.

Furthermore, as a narrowband laser device using an etalon and a diffraction grating as a wavelength selection element, the diffraction grating is arranged at an oblique incidence, and the etalon and the diffraction grating are arranged in an airtight chamber, so that the pressure in the airtight chamber is reduced. There is also a method for absorbing fluctuations in the selected wavelength by controlling the gap of the etalon through the method (Japanese Patent Application Laid-open No. 84766/1983).

[0009]

However, in a narrowband laser device using the above-mentioned diffraction grating 30, the diffraction grating 30 is heated to a high temperature due to the incident laser beam 16, and this heat causes the diffraction grating 30 to become heated. The gas near the reflecting surface (surface) is warmed and fluctuates, causing the beam profile of the laser beam 16 to fluctuate.

That is, the inventors measured the beam profile of a device in which an etalon and a diffraction grating are arranged in a resonator as shown in FIG. 9 by making laser light 16 incident on a one-dimensional photodiode array sensor. The results shown in FIG. 10 were obtained. As is clear from the figure, the light intensity at each point from the center of the beam is different, resulting in a wavy beam profile. Moreover,
The waves in the beam profile change over time due to the fluctuation of the gas near the surface of the diffraction grating 30 (A).
) to (C) were observed.

[0011] Therefore, in order to measure the period of the wave,
Center part of laser beam 16 (0.025 x 0.5 mm2)
When the temporal change in the light intensity was measured, the results shown in FIG. 11 were obtained, and the period of fluctuation was about 1 second. This beam profile fluctuation is a big problem when a narrow band laser device is used as a light source for a reduction projection exposure device because it affects the imaging of a fine pattern.

The present invention has been made in order to eliminate the drawbacks of the prior art, and aims to provide a narrow band laser device that can reduce fluctuations in the beam profile and obtain a laser beam with stable output. The purpose is

[0013]

[Means for Solving the Problems] The inventors conducted various experiments in order to investigate the cause of the above-mentioned beam profile fluctuation. When gas was blown onto each optical element, there was no change in the beam profile fluctuations even when the gas was blown onto the prism and etalon, but when the gas was blown onto the surface of the diffraction grating, the beam profile The fluctuation has become smaller.

[0014] This is because the surface of the diffraction grating absorbs the laser beam and generates heat, increasing the temperature of the gas near the surface and changing the refractive index of the gas. It is thought that the beam profile changes only at high points and no longer matches the selected wavelength of the etalon, causing the beam profile to wave. It is thought that the heat generated in the diffraction grating causes natural convection in the gas near the diffraction grating, causing the beam profile to fluctuate.

The present invention has been made based on the above findings, and provides a narrowband laser device having a diffraction grating as a wavelength selection element, including providing an airflow generating means for causing gas to flow on the reflective surface of the diffraction grating. It is a feature.

The airflow generating means may be a blower, or may be composed of a gas source of purge gas supplied into the apparatus and a gas discharge port connected to this gas source and disposed near the diffraction grating. good.

[0017]

[Operation] In the present invention constructed as described above, a gas flow is forcibly generated on the reflecting surface (surface) of a diffraction grating by means of an air flow generating means. As a result, the diffraction grating is cooled, and the gas near the surface of the diffraction grating is forcibly moved to prevent the temperature of the gas near the diffraction grating from rising. Therefore,
The generation of heat distribution in the gas near the reflection surface of the diffraction grating and the influence of natural convection are eliminated, and the fluctuations in the beam profile are reduced, making it possible to obtain a stable beam profile.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the narrowband laser device according to the present invention will be described in detail in accordance with the attached figures. Note that the same reference numerals are given to the parts corresponding to the parts explained in the prior art, and the explanation thereof will be omitted. FIG. 1 is an explanatory diagram of a narrowband laser device according to an embodiment of the present invention.

In FIG. 1, the narrowband laser device has prisms 26, 28, an etalon 36, and a diffraction grating 30 arranged in a Littrow arrangement, and a fan 40, which is a blower, is installed near the diffraction grating 30 as an airflow generating means. It has been placed. The fan 40 blows gas (for example, nitrogen) 4 inside the device from diagonally above onto the reflective surface of the diffraction grating 30 in which fine grooves are formed.
2 to generate an airflow on the surface of the diffraction grating 30.

In this way, by blowing gas onto the diffraction grating 30 using the fan 40 and creating an air flow on the reflective surface of the diffraction grating 30, the surface of the diffraction grating 30 is cooled and the vicinity of the surface of the diffraction grating 30 is cooled. The gas near the surface of the diffraction grating 30 is forced to move, and the gas near the surface of the diffraction grating 30
0 to prevent heating. Therefore, in the embodiment, the temperature of the gas near the surface of the diffraction grating 30 rises partially, the refractive index of that part changes, the selected wavelength of the diffraction grating 30 changes, and the beam profile appears to be wavy. This situation can be avoided.

Furthermore, since a forced airflow is generated on the surface of the diffraction grating 30, the gas is not heated by the heat of the diffraction grating 30 and natural convection is not generated, and the beam profile is prevented from fluctuating. Can be done. In particular, when the wind speed of the airflow near the surface of the diffraction grating 30 is set to 2 m/s or more, as shown in FIG. It will be done.

FIG. 3 shows another embodiment, in which a sirocco fan 44 as an airflow generating means is arranged on one side in the pitch direction of the grooves of the diffraction grating 30, and is connected to the reflecting surface of the diffraction grating 30. Gas 42 is blown out in parallel. In this embodiment as well, the same effects as in the previous embodiment can be obtained.

FIGS. 4 and 5 show still other embodiments, in which a cross flow fan 46 and an axial fan 48 are used as airflow generating means to blow gas 42 along the grooves of the diffraction grating 30. It was designed to be attached.

In FIG. 6, a purge gas such as nitrogen gas is used to generate an air flow on the surface of the diffraction grating. That is, in FIG. 6, a nitrogen gas cylinder 50 serving as a nitrogen gas source constitutes an airflow generating means, and a valve 52 is provided to which a flowmeter 54 is connected.
A tube 56 having a diameter is connected thereto. The distal end discharge port 58 of the tube 56 is arranged near the reflective surface of the diffraction grating 30, and discharges the nitrogen gas 60 along the pitch direction of the grooves formed in the diffraction grating 30, almost parallel to the surface of the diffraction grating 30. It seems to be gushing out.

[0025] In this embodiment as well, the same effects as in each of the above embodiments can be obtained. Note that the nitrogen gas 60 is
You can also make it blow out along the groove of the diffraction grating 3.
The light may be blown out obliquely to the 0 reflecting surface.

In the above embodiments, a narrowband laser device using prisms 26, 28, an etalon 30, and a diffraction grating 30 as wavelength selection elements has been described. It may also be a laser device. In addition, in the embodiment described above, the apparatus in which the diffraction grating 30 is arranged in Littrow was explained.
The diffraction grating 30 may be of a Grace type. In the above embodiments, an excimer laser device has been described, but other laser devices such as a gas laser such as a He-Ne laser or a dye laser may also be used.

[0027]

As explained above, according to the present invention, by blowing gas onto the reflective surface of the diffraction grating to generate an airflow, the diffraction grating is cooled and It is possible to prevent the gas near the surface from being heated, and it is possible to stabilize the output of the laser light and the beam profile.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of an embodiment according to the present invention.

FIG. 2 is a diagram showing changes in light intensity over time in an example.

FIG. 3 is a partial perspective view of an embodiment in which a sirocco fan is disposed as an airflow generating means at one end in the pitch direction of the grooves of the diffraction grating.

FIG. 4 is a partial perspective view of an embodiment using a crossflow fan as an airflow generating means.

FIG. 5 is a partial perspective view of an embodiment using an axial fan as the airflow generating means.

FIG. 6 is an explanatory diagram of an embodiment in which airflow is generated by purge gas.

FIG. 7 is an explanatory diagram of a conventional narrow band laser device in which the wavelength selection element is an etalon.

FIG. 8 is an explanatory diagram of a conventional narrowband laser device using a prism and an etalon as a wavelength selection element.

FIG. 9 is an explanatory diagram of a conventional narrowband laser device using a prism, an etalon, and a diffraction grating as wavelength selection elements.

FIG. 10 is an explanatory diagram of temporal variations in the beam profile of a conventional narrowband laser device.

FIG. 11 is an explanatory diagram of temporal fluctuations in light intensity at the beam center of a conventional narrowband laser device.

[Explanation of symbols]

10 Laser chamber 16 Laser beams 26, 28 Prism 36 Etalon 30 Diffraction grating 40 Air flow generating means (fan) 42
gas

Claims (3)

[Claims] 1. A narrowband laser device having a diffraction grating as a wavelength selection element, characterized in that the narrowband laser device is provided with airflow generating means for causing gas to flow through a reflective surface of the diffraction grating. 2. The narrowband laser device according to claim 1, wherein the airflow generating means is a blower. 3. The airflow generating means comprises a gas source of purge gas supplied into the apparatus, and a gas discharge port connected to the gas source and disposed near the diffraction grating. 1. The narrowband laser device according to 1.