JPH06334253A - Narrow band excimer laser - Google Patents

Abstract

PURPOSE:To stabilize the oscillation wavelength by branching a part of output light from a narrow band excimer laser and introducing to a multiphoton reaction cell, detecting the reaction in the form of fluorescent signal or ionization signal, detecting the shift of oscillation wavelength from the detected signal, and then controlling the oscillation wavelength in the correcting direction. CONSTITUTION:A part of output light from a narrow band ArF excimer laser is split through a beam splitter 2 and then further split through another beam splitter 3 into two optical paths so that the laser light impinges on a multiphoton reaction cell 4 encapsulating hydrogen gas from the opposite directions. The laser light entering from the opposite directions are condensed appropriately in order to increase the probability of simultaneous collision of photons entering from different directions. In this regard, the relative speeds for respective photons are offset in the thermal motion of hydrogen molecules and thereby a fluorescent pumping spectrum having no Doppler broadening is obtained. A CPU 6 operates the signal of fluorescent pumping spectrum from a photomultiplier tube 5 and controls a narrow band unit 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a narrow band excimer laser device used for exposure of semiconductors and the like.

[0002]

2. Description of the Related Art As a conventional narrow-band excimer laser device, a document "CLEO '89, Technical Digest, THU4, p.36.
6. ”. FIG. 4 shows a block diagram of the narrow band excimer laser device described in the above document. The oscillation wavelength of the narrow band excimer laser fluctuates due to the effects of thermal expansion of the resonator and changes in the external pressure. In order to prevent this, a part of the output light 1 is branched via the beam splitter 2 and guided to the etalon 15, and the deviation of the oscillation wavelength is calculated by the CPU 6 from the interference fringes of the etalon 15, and the deviation is narrowed in the direction to correct the deviation. By controlling the banding unit 7, the oscillation wavelength is stabilized. Strictly speaking, however, the etalon 1 does not expand due to thermal expansion of the spacers that fix the surface spacing of the etalon 15.
Since the interference fringes of No. 5 also change, the absolute wavelength cannot be precisely stabilized only with the above configuration.

Therefore, as the wavelength reference, the light of the spectrum light source lamp 16 is made incident on the same etalon 15 to observe the change of the interference fringes, and the fluctuation of the measuring system itself is canceled to precisely stabilize the fluctuation of the oscillation wavelength. ing. In addition, the document "The 52nd Annual Meeting of the Applied Physics Society of Japan, Lecture Proceedings 3, 9
Page 42, Lecture No. 11p-L-4, Japan Society of Applied Physics, 1991 "
Since the spectrum spreads due to the effect of isotopes existing in a certain ratio in nature, there is an example in which a lamp with a specially selected isotope is used for high-precision stabilization.

[0004]

As described above, the stability of the oscillation wavelength of the narrow band excimer laser device is limited by the accuracy of the wavelength reference. Even if an isotope lamp is used to improve accuracy, the spectrum width is widened by the Doppler effect because it is heated to a high temperature by discharge or high frequency in order to emit light as a lamp. For example, in the above literature, the mass number
A 198 Hg isotope lamp is lit by high frequency heating to obtain a reference light with a spectral width of about 1 pm. Laser light with an oscillation spectrum width of 1.5 pm is incident on this, and the center wavelength is calculated from the 2.5 pm spread, which is the combined spectral spread of the lamp and laser, for stabilization. However, the spread of the data used as the basis of the calculation is 2.5 pm, so there was a problem of fluctuations of about ± 0.15 pm. Therefore,
The present invention has been made in view of the above-mentioned conventional problems, and an object thereof is to provide a narrow band excimer laser device having a more stable oscillation wavelength.

[0005]

In order to achieve this object, in the narrow band excimer laser device according to the present invention, a part of the output light is branched and enters a multiphoton reaction cell from two opposite directions. An optical system and a control mechanism that stabilizes the oscillation wavelength by detecting the deviation of the oscillation wavelength from the output of the multiphoton reaction cell are provided. Further, the narrow band excimer laser device according to the present invention splits a part of the output light to enter the wavelength conversion cell,
An optical system in which the output light of the wavelength conversion cell is incident on the multiphoton reaction cell from two opposite directions, and a control mechanism for detecting the deviation of the oscillation wavelength from the output of the multiphoton reaction cell and stabilizing the oscillation wavelength. Prepare

[0006]

Function A multi-photon reaction such as two-photon induced fluorescence or two-photon resonance absorption is a reaction that occurs when two or more photons collide with a substance at the same time. In the invention according to claim 1, when a part of the output light of the narrow band excimer laser is branched and guided to the multiphoton reaction cell, the multiphoton reaction occurs only at the wavelength peculiar to the substance in the cell, and a fluorescence signal or By detecting the reaction in the form of an ionization signal or the like, it is possible to detect the wavelength shift of the laser light with respect to the wavelength peculiar to the substance. Here, the material inside the cell also has a Doppler spread due to thermal motion, but when laser light is simultaneously incident from two opposite directions, the thermal motion velocity of the material inside the cell for both photons is canceled out, and the effect of Doppler spread It is possible to detect a signal that is not subject to the noise. By detecting the deviation of the oscillation wavelength from this signal and controlling the oscillation wavelength in the direction of correction, it is possible to provide a narrow band excimer laser device having a more stable oscillation wavelength than before. Further, in the invention described in claim 2, since the laser light is incident on the wavelength conversion cell and the converted light is guided to the multiphoton reaction cell, it can be used even when there is no substance compatible with the wavelength of the laser light. it can.

[0007]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a block diagram of a banded excimer laser device according to the present invention. In the figure, a part of the output light 1 of the narrow band ArF excimer laser is split by a beam splitter 2, and further divided by another beam splitter 3 into two optical paths, and a multiphoton reaction cell is provided from two opposite directions. It is incident on 4. Hydrogen gas is enclosed in the multiphoton reaction cell 4.

FIG. 2 shows the document "IEEE Journal of Quantum".
Electronics, Vol.QE-15, 1979, p.380. ”Is shown. As shown in the figure, A
Two 193 nm photons and ground state 10 of rF excimer laser
When two hydrogen molecules in the same collision, the hydrogen molecules are in the ground state 1
The transition from 0 to the E, F level 11 via the virtual level is made.
Molecules excited to the E and F levels 11 further relax to the B level 12 with a lifetime of about 90 ns. E, F levels 11 to B levels 12
When it is relaxed, it emits fluorescence with a wavelength of 750 nm. B level 1
Since the lifetime of 2 is short at about 1.6 ns, there is almost no self-absorption of fluorescence. By observing this fluorescence intensity with the photomultiplier tube 5, the wavelength between the oscillation wavelength of the ArF excimer laser and the two-photon transition wavelength of the hydrogen molecule can be seen. The difference can be detected.

Here, if the laser beams incident from the two opposite directions are properly condensed, it is possible to increase the probability that photons from different directions collide with hydrogen molecules at the same time. In this case, even if the hydrogen molecule makes a thermal motion, the relative velocities for the respective photons are canceled out, so that a fluorescence excitation spectrum with almost no Doppler spread can be obtained. For example, when narrowing the bandwidth of the ArF excimer laser to a spectral width of 1.5 pm, 2
Since the transition probability of photon excitation is proportional to the square of the spectrum intensity, a fluorescence excitation spectrum with a spectral width of about 1.1 pm can be obtained.

In this embodiment, the signal of the photomultiplier tube is calculated by the CPU 6 so that the fluorescence excitation spectrum with the spread of 1.1 pm has the maximum intensity, and the band narrowing unit 7 is controlled. In the conventional example, it is stabilized at ± 0.15pm based on the spread of 2.5pm, whereas this configuration is based on the spread of 1.1pm, so the fluctuation of the oscillation wavelength can be suppressed to about ± 0.07pm.

Further, it is possible to use the two-photon resonance absorption instead of the two-photon induced fluorescence with the same constitution as the above-mentioned embodiment. In this case, after absorbing two photons and being excited to the E, F level 11, another photon collides with the ionization energy within a lifetime of 90 ns of the E, F11 level to generate an electron-ion pair. To do. If a secondary electron multiplier is used instead of the photomultiplier tube 5 for the generation of electron-ion pairs, it can be detected with high sensitivity. Even with this configuration, an ionization signal with a spectral width of about 1.1 pm can be obtained in the same manner as described above, and similarly, fluctuations in the oscillation wavelength can be suppressed to about ± 0.07 pm. Further, in the above embodiment, A
Although the multiphoton reaction of hydrogen molecules with respect to the rF excimer laser is used, it goes without saying that a multiphoton reaction by another excimer laser or another substance may be used.

FIG. 3 shows the configuration of a narrow band excimer laser device according to the second embodiment of the present invention. A part of the output light 1 of the narrow band ArF excimer laser is split by a beam splitter 2, and a Raman scattering cell 8 which is one type of wavelength conversion cell.
Incident on. Deuterium is enclosed in the Raman scattering cell 8 and a part of the laser light of 193 nm is generated by stimulated Raman scattering.
Converted to 205 nm primary Stokes light. The light is incident on the multiphoton reaction cell 4 from two opposite directions divided into two optical paths.
Hydrogen gas is enclosed in the multiphoton reaction cell 4,
Hydrogen atoms are generated in the cell by high frequency heating.

As is well known in astronomical observation and the like, a hydrogen atom has a Lyman β-ray with a wavelength of 102.6 nm when transitioning from a 3d orbit to a 1s orbit, and a Ballmer with a wavelength of 656 nm when transitioning from a 3d orbit to a 2p orbit. Emit alpha rays. This is because when a light with a wavelength of 205 nm from the Raman scattering cell 8 enters a hydrogen atom in the 1s orbit, it transitions to a 3d orbit by two photons and emits a fluorescence with a wavelength of 656 nm when relaxing from the 3d orbit to the 2p orbit. Is shown. Therefore, as in the case of FIG.
By observing with the photomultiplier tube 5, the wavelength difference between the oscillation wavelength of the ArF excimer laser and the two-photon transition wavelength of hydrogen atoms can be detected, and similarly, the oscillation fluctuation of a laser with a spectral width of 1.5 pm can be suppressed to about ± 0.07 pm. You can

[0014]

As described above, according to the present invention, since an optical system for branching a part of the output light and making it incident on the multiphoton reaction cell from two opposite directions, it is made incident from two directions at the same time. The thermal velocities of the substances in the cell with respect to the photons of the laser light are canceled out, and a signal that is not affected by the Doppler spread can be detected, which stabilizes the oscillation wavelength. Further, since a part of the output light is branched and made incident on the wavelength conversion cell, it can be used even when there is no substance compatible with the wavelength of the laser light.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a narrow band excimer laser device according to the present invention.

FIG. 2 is an energy level diagram of hydrogen molecules.

FIG. 3 is a configuration diagram of a second embodiment of a narrow band excimer laser device according to the present invention.

FIG. 4 is a configuration diagram of a conventional narrow band excimer laser device.

[Explanation of symbols]

 1 Output Light 2 Beam Splitter 4 Multiphoton Reaction Cell 5 Photomultiplier Tube 6 CPU 7 Narrowing Band Unit 8 Raman Scattering Cell 10 Ground State 11 E, F Level 12 B Level

Claims (2)

[Claims] 1. In a narrow band excimer laser device for narrowing an oscillation frequency band and stabilizing an oscillation wavelength, an optical system in which a part of output light is branched and enters a multiphoton reaction cell from two opposite directions. And a control mechanism for stabilizing the oscillation wavelength by detecting the deviation of the oscillation wavelength from the output of the multiphoton reaction cell. 2. In a narrow band excimer laser device for narrowing the oscillation frequency band and stabilizing the oscillation wavelength, a part of the output light is branched and made incident on the wavelength conversion cell, and the output light of this wavelength conversion cell And an optical system for injecting light into the multiphoton reaction cell from two opposing directions, and a control mechanism for detecting the deviation of the oscillation wavelength from the output of the multiphoton reaction cell and stabilizing the oscillation wavelength. Narrow band excimer laser device.