JPH03110881A - Wavelength-stabilized laser apparatus - Google Patents

Abstract

PURPOSE:To obtain a wavelength-stabilized laser in which mono-chromaticity of an oscillation wavelength is kept stable and which is optimum for a light source for exposure use by a method wherein a laser power which is incident on an etalon is reduced by using a semitransparent mirror. CONSTITUTION:An optical amplifier 21 and a wavelength selector 22 are coupled so as to be nearly perpendicular by using a semitransparent mirror 4. One part of output light is guided to a wavelength detector 7; a central wavelength of a laser beam is compared with a reference value; an error signal is sent to a pressure control ler 8; a pressure inside an airtight chamber 6 is adjusted so as to set the error signal to zero; the central wavelength of the laser beam is kept always definite. While large light energy is standing inside an optical resonator between a total reflector 3 including a discharge tube 1 and the semitransparent mirror 4, standing light energy is little in a space between the semitransparent mirror 4 and a total reflector 2. Consequently, light energy which passes through etalons 5, 5' is reduced to about 1/5 compared with conventional examples; it is hardly subjected to a thermal deformation and an oscillation line can be kept as one line. Thereby, it is possible to avoid a plurality of oscillation lines and monochromatic light of a definite wavelength can be radiated stably.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a wavelength stabilized laser device used as a light source of a projection exposure apparatus.

(2) (1) Prior Art In recent years, as semiconductor integrated circuits have become more highly integrated, excimer lasers have been attracting attention as a light source for exposing circuit patterns as an alternative to conventional high-pressure mercury lamps. An excimer laser is a type of ultraviolet laser that can produce oscillation lines at a short wavelength between 353 nm and 193 nm by combining a rare gas such as krypton or xenon with a halogen gas such as fluorine or chlorine as a laser medium. It is.

In particular, krypton fluoride excimer laser has a wavelength of 248 nm.
It oscillates at mercury's g-line (436 nm) or i-line (3
It is expected that this technology will pave the way for the production of so-called ultra-LSIs, which have more than twice the degree of integration compared to the case of 65 nm).

The gain bandwidth of these excimer lasers is as wide as approximately 1 nm (
, when oscillating in combination with an optical resonator, the oscillation line is 0.
．． It has a bandwidth (full width at half maximum) of about 5 nm. When a laser beam having such a relatively wide bandwidth is used as an exposure light source, as in the case of a lamp light source, it is necessary to employ an imaging optical system corrected for chromatic aberration in the exposure optical system. However, in the ultraviolet region where the wavelength is 350 nm or less, the range of selection of optical materials for lenses used in the imaging optical system is limited, making it difficult to correct chromatic aberration. The bandwidth of the laser oscillation line is 0.005n.
If it is possible to make the image monochromatic to the order of m and prevent fluctuations in the center wavelength, it will become possible to use an imaging optical system that does not correct chromatic aberration, and a projection exposure apparatus using an excimer laser as a light source will be realized.

In order to make laser light with a wide bandwidth monochromatic, a method has been adopted in which a wavelength selection element with a narrow transmission band is installed inside a laser resonator. FIG. 3 shows a conventional configuration of such a narrow band wavelength stabilized excimer laser. In FIG. 3, a discharge tube 1 is placed within an optical resonator consisting of a total reflection mirror 3 and a semi-transmission mirror 4. The discharge tube 1 is filled with a medium gas containing a rare gas and a halogen gas, and generates laser oscillation by discharge excitation. An airtight chamber 6 is placed between the discharge tube 1 and the total reflection mirror 3, and air space etalons 5, 5', which are wavelength selection elements, are installed in the airtight chamber. Air space etalon (hereinafter simply referred to as etalon) is a wavelength selection element that utilizes the interference of light between opposing parallel plates.
The selected wavelength can be changed depending on the surrounding atmospheric pressure.

A portion of the laser output is guided to a wavelength detector 7 and its center wavelength is measured. The pressure controller 8 controls the airtight chamber 6 so that the center wavelength of the laser beam measured by the wavelength detector 7 is constant.
The internal pressure is adjusted to prevent fluctuations in the center wavelength.

The reason for using two etalons is as follows.

Since the etalon is an interference element, there are always transmission bands of different orders adjacent to a certain transmission band. The ratio of the distance between adjacent transmission bands to the transmission band width is called finesse. The finesse of an etalon that can be manufactured in the excimer laser wavelength range is about 20 at most. Therefore, if one attempts to achieve a band width of 0.005 nm or less with one etalon, the interval between adjacent transmission bands will be 0.1 nm or less, as shown in Figure 4 (a).
), multiple oscillation line forces (appear) within the gain bandwidth of the medium. If multiple oscillation lines occur in this way, the original purpose of imaging a fine pattern cannot be achieved, so the second An etalon is assigned to select only one of the oscillation lines.The situation is shown in FIG. 4(b).

If the transmission band width of the second etalon is set to be approximately the interval between the transmission bands of the first etalon, monochromatic oscillation lines can be realized, and an excimer laser suitable for projection exposure can be obtained. Note that the transmission wavelength of each etalon can be independently selected by changing the inclination of the etalon and the incident angle of the laser beam, and can be easily adjusted to the state shown in FIG. 11(b).

Problems to be Solved by the Invention However, such conventional wavelength-stabilized radars have the drawback that as the laser output is increased, the etalon deforms and a plurality of oscillation lines appear. That is, in order to reach the transmission band width of the two etalons, the gap distances between the parallel plates are designed to be different. For this reason,
When laser light is collected, it exhibits different amounts of variation in the selected wavelength due to thermal deformation. As a result, during laser operation, as shown in FIG. 11(C), a state occurs in which the transmission band of the second etalon straddles the two transmission bands of the etalon in FIG. 1, and multiple oscillation lines appear. Sometimes. If used as an exposure light source under such conditions, not only will it be difficult to detect the center wavelength and it will be impossible to stabilize the wavelength, but the pattern projected by the monochromatic lens will become blurred, leading to product defects.

The present invention was made to solve these problems, and provides a wavelength-stabilized laser device that can always oscillate with one oscillation line.

Means for Solving the Problem In order to solve this problem, the wavelength stabilized laser device of the present invention includes an optical amplifier comprising a laser medium and a first total reflection mirror;
A plurality of wavelength selection elements installed in an airtight chamber and a wavelength selector consisting of a second total reflection mirror are coupled by a semi-transmissive mirror so that their optical axes intersect, forming an optical resonator. Means for detecting the oscillation wavelength and pressure control means are provided to change the pressure within the airtight chamber.

Effect: With this configuration, the energy of the laser light passing through the etalon is reduced to the extent that it is multiplied by the reflectance of the semi-transmissive mirror in the conventional case, so the thermal load on the etalon is significantly reduced, and multiple oscillation lines Deformation of the etalon that would otherwise occur can be prevented.

Embodiment FIG. 1 is a block diagram of an excimer laser which is an embodiment of the present invention. In the laser device according to the embodiment of the present invention shown in FIG. 1, etalons 5 and 5' are installed on the optical axis between the semi-transmitting mirror 4 and the total reflecting mirror 2 to select only wavelengths in a specific narrow band. A wavelength selector 22 is provided. On the other hand, semi-transparent mirror 4
A discharge tube 1 whose laser medium is a mixture of rare gas and halogen gas is installed on the optical axis connecting the total reflection mirror 3 and the total reflection mirror 3, and an optical amplifier 21 is configured. Optical amplifier 21 and wavelength selector 22
are coupled by a semi-transmissive mirror 4 so as to be substantially perpendicular to each other. The monochromatic light selected by the wavelength selector 22 is transmitted to the optical amplifier 21
and is amplified as a laser beam from the semi-transparent mirror 4 to the optical amplifier 2.
1 in the direction of the optical axis. Here, etalon 5,5
It is installed in an airtight room where the internal air pressure can be adjusted.

On the other hand, a beam splitter 10 is placed on the axis connecting the total reflection mirror 2 and the semi-transmission mirror 4, and a part of the output light is guided to the wavelength detector 7. The wavelength detector 7 compares the center wavelength of the laser beam with a reference value and sends an error signal to the pressure controller 8. The pressure controller 8 adjusts the pressure within the airtight chamber 6 so that the error signal becomes zero, so that the center wavelength of the laser beam is always kept constant.

Here, in the optical resonator between the total reflection mirror 3 and the semi-transmission mirror 4 including the discharge tube 1, large optical energy is present, as in the case of the conventional laser device.

On the other hand, in the space between the semi-transmitting mirror 4 and the total reflecting mirror 2, most of the light coming from the direction of the discharge tube is radiated outside the optical resonator by the semi-transmitting mirror, so there is only a small amount of standing optical energy. be. Therefore, assuming that the reflectance of the semi-transmissive mirror 4 is, for example, 20%, the light energy passing through the etalon at 5.5 degrees is about 115 in the conventional example shown in FIG. As a result, the etalon 5.5° is not subject to thermal deformation, and the oscillation line can be kept at one line.

According to experiments conducted by the present inventors, when trying to obtain an output of 2 W or more using a conventional excimer laser as shown in Fig. 3, even if the etalon is initially adjusted so that there is only one oscillation line, After the oscillation started, adjacent oscillation lines appeared within several minutes, resulting in multiple oscillations. However, in the wavelength stabilized laser device according to the present invention, even when the output was 5 W, the number of oscillation lines did not become plural. Furthermore, by simultaneously controlling the air pressure between the gaps between the two etalons in one airtight chamber, the center wavelength could be kept constant.

Since the excimer laser has a high medium gain, the coupling of the optical resonator, that is, the reflectance of the semi-transmissive mirror in the present invention can be as low as 10 to 20%.
It can be said that the present invention is particularly effective in excimer lasers. The inventors conducted experiments and studies regarding the output of the wavelength stabilized laser device according to this example. The experiment was conducted using a KrF excimer laser using krypton and fluorine as the laser medium and helium as the diluent gas. FIG. 7 shows the relationship between the measurement results of the laser output and the reflectance of the semi-transmissive mirror 4, and the etalon absorption power estimated from the laser output.

In the configuration according to the present invention, laser oscillation occurs if the reflectance of the semi-transmissive mirror is set to 4% or more. Also, the reflectance of the semi-transparent mirror is 40
% or less, the absorption power of the etalon could be kept lower than in the conventional example, and multiple oscillations could be avoided.

With the above configuration, the wavelength stabilized laser device of the present invention can avoid multiple oscillation lines due to thermal deformation of the etalon, and can stably emit monochromatic light of a constant wavelength.

As described in detail, the present invention uses a semi-transmissive mirror to reduce the laser power incident on the etalon, so the monochromaticity of the oscillation wavelength is maintained stably, resulting in wavelength stabilization that is optimal for an exposure light source. It can provide lasers.

[Brief explanation of drawings]

Fig. 1 is a diagram illustrating the configuration of a band-narrowing laser device that is an embodiment of the present invention, Fig. 4 is a diagram illustrating the principle of laser wavelength selection, and Fig. 2 is a diagram illustrating the reflectance of the semi-transmissive mirror. A diagram showing the relationship between output and power absorbed by an etalon, and FIG. 3 is a diagram showing the configuration of a conventional wavelength-stabilized laser. 1... Discharge tube, 2, 3... Total reflection mirror,
4...Semi-transparent mirror, 5,5゛...Fabry-Perot etalon, 6...Airtight chamber, 7...
...Wavelength detector, 8...Pressure controller, 10...
... Beam splitter, 21 ... Optical amplifier, 22 ... Wavelength selector.

Claims (2)

[Claims] (1) An optical amplifier consisting of a laser medium and a first total reflection mirror, and a wavelength selector consisting of a plurality of wavelength selection elements and a second total reflection mirror installed in an airtight chamber are arranged so that their optical axes intersect with each other. A wavelength-stabilized laser device, characterized in that the laser beam is coupled with a semi-transparent mirror to form an optical resonator, and is provided with means for detecting the oscillation wavelength of the laser beam and pressure control means to change the pressure within the airtight chamber. . (2) The wavelength stabilized laser device according to claim 1, wherein the reflectance of the semi-transmissive mirror is in the range of 4% to 40% with respect to the incident light in the optical axis direction of the optical resonator.