JPH01302884A - Wavelength-stabilized laser apparatus - Google Patents

Abstract

PURPOSE:To obtain an apparatus which can stably restrict a center wavelength to a definite value by a method wherein a wavelength detection means is constituted in such a way that it detects the center wavelength of a laser beam by using a Fabry-Perot etalon installed inside an airtight chamber. CONSTITUTION:Means 13, 7 to 10 used to control a selection wavelength of a wavelength selection element 4 and a means 12 used to detect an oscillation wavelength of a laser beam are built in a laser apparatus which is provided with a laser medium and an optical resonator composed of a total reflector 2 and an output mirror 3 and which can select a specific oscillation wavelength within a gain bandwidth of the laser medium by installing the single or two or more wavelength selection elements 4. The wavelength detection means 12 detects a center wavelength of the laser beam by using a Fabry-Perot etalon 14 installed inside an airtight chamber 15. By this setup, a refractive index between gaps of the air-space etalon 14 inside the airtight chamber 15 can be kept definite; as a result, a wavelength detector which is not affected by an ambient temperature and an atmospheric pressure can be constituted; accordingly, the center wavelength can be restricted to a definite value.

Description

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

2. Description of the Related Art In recent years, with the miniaturization of LSI patterns, there has been a tendency for light sources for pattern exposure to have shorter wavelengths. One such light source is a high-pressure mercury lamp, whose g-line (436 nm) or i-line (3 esnm) has been used in the LSI manufacturing process. So-called ultra-LSI with even finer patterns
, a light source with a much shorter wavelength is required, and laser light sources such as excimer lasers are attracting attention as a device that meets this requirement. Excimer lasers use rare gases such as krypton and xenon and fluorine as laser media.

35 by combining halogen gas such as chlorine.
It is possible to obtain oscillation lines with sufficient output for exposure of LSI patterns at several wavelengths between 3 nm and 193 nm.

The gain bandwidth of these excimer lasers is as wide as approximately 1 nm, and when oscillated in combination with an optical resonator, the oscillation line is 0.
．． It has a line width (full width at half maximum) of approximately 51 m.

When a laser beam having such a relatively wide line width 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 choices for optical materials for lenses used in the imaging optical system is limited.
It becomes difficult to correct chromatic aberration. Therefore, the wavelength is 350 n
When using a laser light source with a wavelength of less than m in an exposure device, an imaging optical system that does not correct chromatic aberration can be used by making the line width of the laser oscillation line monochromatic to about αoosnm and restricting its center wavelength to a constant value. Therefore, the optical system of the exposure apparatus can be simplified, and the entire exposure apparatus can be made smaller and its price can be reduced.

The present inventors have already proposed a configuration of a laser device that achieves the above object. In other words, it is a wavelength tunable laser device in which means for stabilizing the center wavelength of the laser is built into the main body, and the oscillation wavelength can be set and constrained to an arbitrary value. In order to realize such a laser device, a method for precisely detecting and controlling the laser oscillation wavelength is required. There are also several proposals regarding methods of detecting the oscillation wavelength of a laser. For example, Japanese Patent Application No. 62-242378 describes a method in which a laser beam is incident on a Fabry-Perot interferometer and the oscillation wavelength of the laser is read from the position of interference fringes formed.

The Fapry-Perot interferometer can achieve the same resolution as a large grating spectrometer with an extremely small-scale device, so it can be used as a laser device with a built-in mechanism to stabilize the center wavelength, as proposed by the present inventors. It is one of the best wavelength detectors.

Problems to be Solved by the Invention However, the Fapry-Perot interferometer has the disadvantage that even if the wavelength of the incident laser light is kept constant, interference fringes are created at different positions depending on changes in the surrounding air pressure and temperature. . FIG. 3 is a diagram showing the principle of wavelength detection using a Fabry-Perot interferometer.

Here, the Fabry-Perot interferometer employs a so-called air space etalon 14 in which two parallel flat plates of quartz glass are opposed with a small gap maintained therebetween. When a laser beam is incident on the air space etalon 14, an interference pattern called a fringe 17 is projected by the lens 16. By reading the position of the fringe 17 using, for example, a linear image sensor 18&, the relative wavelength of the laser beam can be determined.

In a wavelength detector having such a configuration, when the refractive index between the gaps of the air space etalon 14 changes due to a change in atmospheric pressure or temperature, the diameter of the fringe changes as shown in 172L. As a result, even though the wavelength of the laser beam has not changed, an erroneous signal is output as if there had been a change, resulting in a problem that the center wavelength of the laser beam fluctuates.

An alternative to the air space etalon is a solid etalon, which is made of a single quartz plate with parallel planes. However, solid etalons also suffer from the same drawbacks as air space etalons because the refractive index of the material changes with temperature. Although it is possible to eliminate fluctuations in the refractive index by providing a mechanism to keep the temperature or pressure constant, the device would be large and the compactness of the Schiff App-Perot interferometer would not be utilized.

In the end, the conventional wavelength control method using a 7-application-Perot interferometer has a problem in that it is difficult to constrain the center wavelength of the laser beam to a constant value. The present invention was made to solve these problems, and provides a wavelength-stabilized laser that can stably constrain the center wavelength to a constant value.

Means for Solving the Problem Equipped with an optical resonator consisting of a laser medium, a total reflection mirror and an output mirror, and specifying a single or multiple wavelength selection elements within the gain bandwidth of the laser medium in the optical resonator. A laser device capable of selecting the oscillation wavelength of the wavelength selection element has a built-in means for controlling the selected wavelength of the wavelength selection element and a means for detecting the oscillation wavelength of the laser beam, and the wavelength detection means is installed in an airtight chamber. It is configured to detect the center wavelength of the laser beam using a Fabry-Perot etalon installed at the center.

Effect: With this configuration, the center wavelength of the laser beam can be stably constrained to a constant value.

Embodiment FIG. 1 is a block diagram of a wavelength stabilized laser device which is an embodiment of the present invention. In FIG. 1, the wavelength stabilized laser device according to the embodiment of the present invention uses a discharge tube 1 which uses a mixture of rare gas and halogen gas as a laser medium, an optical resonator consisting of a total reflection mirror 2 and an output mirror 3 to generate ultraviolet light. Laser oscillation occurs in the area.

On the optical axis of the optical resonator, there is an air space etalon 4, which is a wavelength selection element. Grating 6 is placed. The air space etalon 4 is a type of Fabry-Berrot etalon in which two parallel plane quartz plates with appropriate reflectance are faced with a small gap at the laser wavelength.
It is installed in an airtight container θ. A high pressure air source 7 and a low pressure air source 8 are connected to the airtight container 6 via pulps 9 and 1o, respectively, and the air pressure between the gaps of the air space etalo/4 and around the grating 5 can be changed. Further, the sample beam extracted by the semi-transmissive mirror 11 is guided to the wavelength detector 12. The wavelength detector 12 outputs a signal corresponding to the center wavelength of the sample beam to the comparator 13. The comparator 13 compares the signal sent from the wavelength detector 12 with a reference signal, and outputs a signal to open or close the pulp 9 or the pulp 10 depending on the amount of deviation.

Next, the operation of the wavelength stabilized laser device of the present invention having the above configuration will be explained. Generally, the gain bandwidth of an excimer laser is 11 m, as shown in Figure 2B. If this is to be made monochromatic to about 0.005 nm as shown in FIG. 2, it is necessary to use a wavelength selection element with a transmission band width of about the same level. Such a wavelength selection element can be realized, for example, by combining a Fabry-Perot etalon and a grating as shown in the embodiment.

The center wavelength of the laser beam made monochromatic in this way may vary during operation of the laser. This is because the transmission center wavelength of the wavelength selection element changes depending on temperature, atmospheric pressure, and the like. Fluctuations in the center wavelength of the laser beam due to such causes can be suppressed by keeping the temperature and air pressure around the wavelength selection element constant, but the temperature increase of the wavelength selection element itself due to absorption of a portion of the laser beam Yes, there is a limit to this too. Therefore, in the laser device according to the present invention, a wavelength detector 12 is provided to obtain a wavelength signal corresponding to the center wavelength of the laser beam to keep the selected wavelength of the wavelength selection element at a constant value.
Fix the center wavelength of the laser beam. In order to keep the selected wavelength of the wavelength selection element constant, for example, as shown in the embodiment shown in FIG. Accordingly, by opening and closing the pulp 9 between the high-pressure air source 7 or the pulp 1o between the low-pressure air source 8 and changing the air pressure inside the airtight chamber 6, the refractive index between the gaps of the air space etalon 4 can be changed. Just adjust it.

In the wavelength detector 12, the spectrum of laser oscillation is developed and imaged on a concentric fringe 17 by an air space etalo/14, and a photodiode 18 is placed at the position of the fringe.
is placed. When the center wavelength of the laser beam changes, the diameter of the fringe 17 changes and the output of the photodiode 18 decreases, so the comparator 13 controls the center wavelength of the laser beam to return to its original value and the output of the photodiode 18 to recover. generates a signal.

In order to maintain the center wavelength of the laser beam at a constant value in a wavelength detector having such a configuration, a fringe pattern 17 must always be formed at the same position when a laser beam of a constant wavelength is incident. However, if the refractive index between the gaps in the air space etalon 14 changes due to changes in ambient temperature or atmospheric pressure, the diameter of the fringe will change even if a laser beam of a certain wavelength is incident, causing a signal that appears to have changed in wavelength. This is the same as mentioned above.

Therefore, a material whose refractive index changes due to temperature is negligible is needed, but such a material cannot be found in the wavelength range excavated by excimer lasers. However, the present inventors have come up with the idea that by installing the air space etalon in an airtight chamber, it is possible to configure a wavelength detector that is not affected by temperature changes.

FIG. 1 shows an embodiment of a wavelength detector according to the present invention. In this embodiment, the air space etalon 14 is
It is housed in an airtight chamber 15 whose interior is completely shut off from outside air. Since the refractive index of a gas is determined only by its density, the refractive index between the gaps of the air space etalon 14 in the hermetic chamber 16 is kept constant, resulting in a wavelength detector that is not affected by the ambient temperature or atmospheric pressure. Can be configured.

Since the wavelength detector according to the present invention has the characteristic that the detection wavelength is not affected by external factors, it is possible to provide a wavelength-stabilized laser that can be incorporated into the laser main body to stably constrain the wavelength. Additionally, this wavelength detector does not require temperature or atmospheric pressure control means, making the laser device more compact.

Lower prices can also be achieved.

Note that in the wavelength detector shown in Figure 1, an example was shown in which a photodiode was used to determine only whether the fringe was in a predetermined position, but it could also be configured to read the shape of the fringe using a linear image sensor or the like. You may. In addition, the gas sealed in the airtight chamber 16 may be other than air.
For example, noble gases.

Nitrogen gas or the like may also be used.

Further, since the property that the refractive index of a gas is determined only by the density holds true in all wavelength ranges, it goes without saying that the wavelength detector shown in this embodiment can be used to detect the wavelength of general light sources other than excimer lasers.

DETAILED DESCRIPTION OF THE INVENTION As described in detail, the wavelength stabilized laser device according to the present invention can provide a laser device having the excellent effect of stably constraining the center wavelength of laser light to a constant value.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the configuration of a wavelength stabilized laser device that is an embodiment of the present invention, Fig. 2 is a diagram illustrating monochromatic laser light, and Fig. 3 is a diagram showing the configuration of a conventional wavelength detector. It is. 1...discharge tube, 2...total reflection mirror, 3.
... Output mirror, 4 ... Air space etalon, 5 ... Grating, 6 ... Airtight container, 7 ... High pressure air source, 8 ...Low pressure air source, 9.10...Pulp, 11...
...Semi-transparent mirror, 12...Wavelength detector, 13...
... Comparator, 14 ... Air space etalon, 16 ... Airtight chamber, 16 ... Lens, 17 ... Fringe, 171L・River・・Changed fringe, 18...Photodiode, 18
2L...Linear image sensor. Name of agent: Patent attorney Toshio Nakao and 1 other person1-
- air 4c 萱朱 14...- air χ χ L r J 5 5-- g b- quining +, 9- = J long r jinsu

Claims (1)

[Claims] A laser that is equipped with an optical resonator consisting of a laser medium, a total reflection mirror, and an output mirror, and that can select a specific oscillation wavelength within the gain bandwidth of the laser medium with a single or multiple wavelength selection elements in the optical resonator. In the apparatus, means for controlling the selected wavelength of the wavelength selection element and means for detecting the oscillation wavelength of the laser beam are built into the laser apparatus, and the wavelength detecting means is installed in an airtight chamber. A wavelength-stabilized laser device characterized by detecting the center wavelength of laser light using an etalon.