JP2967623B2 - Narrow band laser oscillator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser oscillation device such as an excimer laser used as a light source of a reduction projection exposure apparatus having a refraction type optical system, and more particularly to a laser oscillation light having a narrow band. And a laser oscillation device for the same.

[0002]

2. Description of the Related Art In order to obtain high resolution in reduced projection exposure for semiconductor exposure, it is necessary to increase the numerical aperture of an optical system or shorten the exposure wavelength. In general, since the focal depth of the optical system decreases with an increase in the numerical aperture, it can be said that shortening the wavelength of the light source is a more desirable means. In particular, an excimer laser oscillation device has an oscillation wavelength in a region from ultraviolet to vacuum ultraviolet, and thus many applications as an exposure light source have been attempted. However, since synthetic glass is the only suitable glass material in such a wavelength region, it is generally not easy to achieve achromatism with a refractive optical system. Therefore, a light source such as a refraction-type reduction projection exposure apparatus needs to have a narrow wavelength band.

FIG. 5 shows a magazine (Can. J. Phys., 6).
3, 214 ('85)) is a conceptual diagram of the configuration of a conventional narrow-band excimer laser oscillation device. In FIG. 5, a discharge tube 51 is arranged in an optical resonator constituted by a total reflection mirror 52 and a semi-transmission mirror 53. Discharge tube 51
Is filled with a medium gas containing a rare gas and a halogen gas, and oscillates laser by discharge excitation. A wavelength selecting element 54 such as an etalon or a prism is provided in the optical resonator, and slits 55 and 56 are provided so as to sandwich the laser medium. In the excimer laser oscillation device having such a configuration, the wavelength selection element 54
Since oscillation can be obtained only at the specific wavelength selected in the above, a narrow band light with high output can be obtained.

[0004]

However, in such a conventional narrow-band excimer laser oscillation apparatus, in order to reduce the loss in the wavelength selection element and to function effectively, the transverse mode order of the laser must be reduced. Need to reduce. Therefore, it is necessary to set the slit width according to the mode volume of the low-order transverse mode. However, in a general resonator configuration, the low-order transverse mode has a small cross-sectional area and a very narrow slit width, so that only a small part of the emitted laser light is extracted. High output cannot be obtained due to low utilization efficiency of the excitation volume. Also, although it is easier to narrow the band as the number of transverse modes decreases, the reduction of the number of transverse modes results in high spatial coherence, and when used in a semiconductor exposure apparatus, speckles occur on the exposed surface.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and a high output is obtained by effectively utilizing an excitation volume and improving a laser light extraction efficiency, and further, a narrow space coherence is reduced. It is an object to provide a banded excimer laser oscillation device.

[0006]

Means for Solving the Problems The present invention includes a laser oscillating means including a total reflection mirror and a semi-transparent mirror optical resonator constituted by the opposed sides of the laser medium, Ru oscillated from the oscillation means Having an optical dispersion element for narrowing the spectrum of the oscillation wavelength of the laser light, and having a slit shape
Narrowband laser, characterized by installing a variable magnification afocal optical system composed of a concave cylindrical mirror and the size of the limited convex cylindrical mirror in the optical resonator so as to sandwich the laser medium having an opening An oscillation device.

[0007]

By using a variable power afocal optical system, a laser beam can be extracted from a wide volume according to the excitation volume of the laser, and a high output can be obtained due to a high laser extraction efficiency. In addition, since the size of the opening provided in the concave cylindrical mirror and the size of the convex cylindrical mirror can be designed according to a desired mode, oscillation light in a mode necessary for narrowing the band can be obtained without using a slit. further,
In addition to the large excitation volume, the variable-magnification afocal optical system is composed of a cylindrical mirror, so that the laser beam has a slit-shaped cross section, so that higher-order transverse mode light can be generated in directions not limited by the slit. You. That is, since oscillation of a multi-transverse mode single-wavelength laser beam becomes possible, spatial coherence can be reduced, and speckle, which is a problem particularly in semiconductor exposure, can be avoided.

[0008]

FIG. 1 shows a first embodiment of the present invention. FIG.
The narrow-band laser oscillation device shown in FIG. 1 has an optical resonator composed of a diffraction grating 12 and a semi-transmissive mirror 13 disposed opposite to each other with a discharge tube 11 interposed therebetween, and a prism 14 for narrowing the spectrum of the oscillation wavelength. And The laser medium is discharge-excited by the electrodes 18 and 19. Further, a variable-magnification afocal optical system comprising a concave cylindrical mirror 15 having an opening 16 and a concave cylindrical mirror 17 sandwiching the discharge tube 11 in the optical resonator is provided.

In narrowing the band using the prism in the arrangement of FIG. 1, it is necessary that the number of transverse modes in the x direction in the figure be small. FIG. 2 is a sectional view of the excitation volume of the excimer laser on the xy plane. The excitation volume 20 generally has an elongated shape along the discharge direction. Therefore, in the case of the electrode position shown in FIG. 1, the opening 16 can be formed in a slit shape by disposing a cylindrical mirror having a curvature only with respect to the y-axis. At this time, the number of transverse modes in the x-axis direction can be controlled by setting the size of the opening 16 and the convex mirror 17 to desired sizes and shapes. In this case, the number of transverse modes in the y-axis direction is not limited, so that multi-mode single-wavelength oscillation is realized, and generation of speckle can be avoided. Further, since the entire area of the excitation volume 20 of the discharge tube 11 can be used, the efficiency of extracting the laser beam is improved.

FIG. 3 shows a second embodiment of the present invention. The second embodiment is characterized in that a partially total reflection mirror 33 is used as a semi-transmission mirror on the output side. FIG. 4 shows the configuration of the partial total reflection mirror 33. This has a structure in which a convex cylindrical mirror 41 and a semi-transmissive mirror 42 are integrated.
When viewed from the direction, the reflectance of the convex cylindrical total reflection portion 41 is set to 100%, and the reflectance of the plane semi-transmission portion 42 is set to be approximately equal to the output mirror of a conventional laser.

A convex aspherical optical system is constituted by the convex cylindrical total reflection portion 41 and the convex cylindrical mirror 15 of the partial total reflection mirror 33, and the plane semi-transmission portion 42 and the total reflection mirror 52 of the partial total reflection mirror 33 Constitutes an optical resonator.

In the excimer laser oscillation device, since the lifetime of the upper level of the laser is short, the duration of the population inversion is short, and the round trip of the laser beam is not sufficient. Therefore, in FIG. 3, by using the partial total reflection mirror 33, an optical resonator length shorter than that of the laser oscillation device shown in FIG. 1 is realized.

[0013]

According to the present invention, the size of the aperture provided in the concave cylindrical mirror and the size of the convex cylindrical mirror can be designed according to a desired mode. Therefore, it is possible to obtain oscillation in a mode necessary for narrowing the band without using a slit. it can. In addition, by using the variable power afocal optical system, laser light can be extracted from the entire excitation volume of the laser, so that the laser extraction efficiency is high and high output can be obtained. Further, in addition to a large excitation volume, by forming the variable power afocal optical system with a cylindrical mirror, the laser beam has a slit-shaped cross section. Therefore, a multi-transverse mode single-wavelength laser beam can be oscillated and spatial coherence can be reduced, so that speckle, which is a problem particularly in semiconductor exposure, can be avoided.

[Brief description of the drawings]

FIG. 1 is a sectional view of a first embodiment of the present invention.

FIG. 2 is a diagram showing shapes of a discharge electrode and an excitation volume.

FIG. 3 is a sectional view in a second embodiment of the present invention.

FIG. 4 is a diagram showing a configuration of a partial total reflection mirror.

FIG. 5 is a diagram showing a conventional example of a narrow-band laser device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Discharge tube 12 Diffraction grating 13 Semi-transmission mirror 14 Prism 15 Concave cylindrical mirror 16 Opening 17 Convex cylindrical mirror 18, 19 Discharge electrode 20 Excitation volume 32 Total reflection mirror 33 Partial total reflection mirror 41 Convex cylindrical total reflection part 42 Flat semi-transmission part 51 Discharge Tube 52 Total Reflection Mirror 53 Semi-Transmission Mirror 54 Wavelength Selector 55, 56 Slit

Claims (1)

(57) [Claims] 1. A laser oscillating means including a total reflection mirror or the optical resonator constituted by the diffraction grating and the semitransparent mirror across the laser medium arranged opposite, oscillation of the laser a laser beam that will be oscillated by the oscillator means An optical dispersion element for narrowing the spectrum of the wavelength, and a variable magnification lens comprising a concave cylindrical mirror having a slit-shaped opening and a convex cylindrical mirror having a limited size. A narrow-band laser oscillation device wherein a focal optical system is arranged in an optical resonator so as to sandwich a laser medium.