JP2586656B2 - Wavelength stabilized laser device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. 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. Description of the Related Art In recent years, as semiconductor integrated circuits have become more highly integrated, an excimer laser has attracted attention as a light source for circuit pattern exposure as an alternative to a conventional high-pressure mercury lamp. An excimer laser is an ultraviolet laser that can obtain oscillation lines at several wavelengths 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. is there. In particular, a krypton fluoride excimer laser oscillates at a wavelength of 248 nm,
It is expected to pave the way for so-called VLSI manufacturing, which has more than twice the integration density of mercury g-line (436 nm) or i-line (365 nm).

These excimer lasers have a wide gain bandwidth of about 1 nm, and when oscillated in combination with an optical resonator,
It has a bandwidth of about 0.5 nm (full width at half maximum). When a laser beam having a relatively wide bandwidth is used as a light source for exposure as in the case of a lamp light source, it is necessary to employ an imaging optical system in which chromatic aberration is corrected for the exposure optical system. However, in the ultraviolet region having a wavelength of 350 nm or less, the selection range of the optical material of the lens used in the imaging optical system is limited, and it becomes difficult to correct chromatic aberration. If the bandwidth of the laser oscillation line is made monochromatic to about 0.005 nm and the fluctuation of the center wavelength can be prevented, an imaging optical system without chromatic aberration correction can be used, and a projection exposure apparatus using an excimer laser as a light source can be realized.

In order to monochromaticize a laser beam having a wide bandwidth, a method of installing a wavelength selection element having a narrow transmission band in a laser resonator has been adopted. FIG. 3 shows a conventional configuration of such a narrow band wavelength stabilized excimer laser. In FIG. 3, a discharge tube 1 is placed in an optical resonator including a total reflection mirror 2 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 oscillates laser by discharge excitation. An airtight chamber 6 is placed between the discharge tube 1 and the total reflection mirror 2, and airspace etalons 5, 5 'as wavelength selection elements are installed in the airtight chamber. An air space etalon (hereinafter simply abbreviated as an etalon) is a wavelength selection element utilizing interference of light between opposed parallel flat plates,
The selected wavelength can be changed depending on the ambient pressure.

A part of the laser output is guided to the wavelength detector 7 and its center wavelength is measured. The pressure controller 8 adjusts the pressure in the hermetic chamber 6 so that the center wavelength of the laser beam measured by the wavelength detector 7 becomes constant, thereby preventing the center wavelength from fluctuating.

The reason for using two etalons is as follows. Since the etalon is an interference element, a transmission band of a different order always exists adjacent to a certain transmission band. The ratio of the distance between adjacent transmission bands to the transmission bandwidth is called finesse. The finesse of an etalon that can be manufactured in the wavelength range of an excimer laser is at most about 20. Therefore, 0.005n per etalon
In order to realize a bandwidth of less than m, the interval between adjacent transmission bands is less than 0.1 nm, and a plurality of oscillation lines appear within the gain bandwidth of the medium as shown in FIG. 4 (a). If a plurality of oscillation lines are generated in this way, the initial purpose of imaging a fine pattern cannot be achieved. Therefore, a second etalon is arranged and only one oscillation line is selected.
This is shown in FIG. 4 (b). If the transmission bandwidth of the second etalon is approximately equal to 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. The transmission wavelength of each etalon can be independently selected by tilting the etalon and changing the incident angle of the laser beam, and can be easily adjusted to the state shown in FIG. 4 (b).

Problems to be Solved by the Invention However, such a conventional wavelength-stabilized laser has a disadvantage that the etalon is deformed as the laser output is increased, and a plurality of oscillation lines appear. That is, since the two etalons have different transmission bandwidths, the two etalons are designed to have different gap intervals between the parallel plates. Therefore, when the laser beam is absorbed, different amounts of the selected wavelength change due to thermal deformation. As a result, during the laser operation, as shown in FIG. 4 (c), a state occurs in which the transmission band of the second etalon straddles the two transmissions of the first etalon, and a plurality of oscillation lines appear. There is. When used as an exposure light source in such a state, not only is it difficult to detect the center wavelength and it becomes impossible to stabilize the wavelength, but also the pattern projected by the monochromatic lens is blurred, leading to defective products. The present invention has been made to solve such a problem, and an object of the present invention is to provide a wavelength-stabilized laser device that can always oscillate with one oscillation line.

Means for Solving the Problems To solve this problem, a wavelength stabilized laser device according to the present invention comprises 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 composed of two total reflection mirrors and a semi-transmission mirror are arranged on a straight line to constitute an optical resonator, and a means for detecting an oscillation wavelength of laser light and a pressure control means are provided. The pressure in the airtight chamber is changed.

Operation With this configuration, the energy of the laser beam passing through the etalon is reduced to a level obtained by multiplying the energy of the conventional example by the reflectivity of the semi-transmissive mirror. This can prevent the etalon from being deformed.

Embodiment FIG. 1 is a configuration diagram of an excimer laser according to an embodiment of the present invention. In FIG. 1, in the laser apparatus according to the embodiment of the present invention, etalons 5, 5 'are installed on the optical axis between the semi-transmissive mirror 4 and the total reflection mirror 2, and only a wavelength in a specific narrow band is selected. A wavelength selector 22 is provided. On the other hand, a discharge tube 1 using a mixed gas of a rare gas and a halogen gas as a laser medium is installed on an optical axis connecting the semi-transmissive mirror 4 and the total reflection mirror 3, and an optical amplifier is provided.
Make up 21. The monochromatic light selected by the wavelength selector 22 is amplified by the optical amplifier 21 and extracted from the semi-transmissive mirror 4 in a direction substantially perpendicular to the optical axis as laser light. Where the etalon
5, 5 'are installed in an airtight room where the internal pressure can be adjusted.

A beam splitter 10 is placed on the output beam, and a part of the output light is guided to a wavelength detector 7. The wavelength detector 7 compares the center wavelength of the laser light with a reference value, and sends an error signal to the pressure controller 8. As a result of the pressure controller 8 adjusting the pressure in the hermetic chamber 6 so that the error signal becomes zero, the center wavelength of the laser beam is always kept constant.

Here, in the optical resonator between the total reflection mirror 3 including the discharge tube 1 and the semi-transmission mirror 4, a large amount of light energy is present as in the case of the conventional laser device. On the other hand, in the space between the semi-transmissive mirror 4 and the total reflection mirror 2, most of the light coming from the discharge tube direction is radiated out of the optical resonator by the semi-transmissive mirror, so that the standing light energy is small. Therefore, the transflective mirror 4
Is 20%, for example, the etalon 5,
The light energy passing through 5 'is about 1/5 that of the conventional example shown in FIG. As a result, the etalons 5, 5 'are less susceptible to thermal deformation and can maintain a single oscillation line.

According to the experiments of the present inventors, when an output of 2 W or more is to be obtained with a conventional excimer laser as shown in FIG. 3, even if the etalon is initially adjusted so that the number of oscillation lines becomes one, In some cases, adjacent oscillation lines appear within a few minutes after the start of oscillation, resulting in multiple oscillations. However, in the wavelength stabilized laser device according to the present invention, even when the output was set to 5 W, the oscillation line did not become plural. Further, the center wavelength could be kept constant by simultaneously controlling the pressure between the gaps of the two etalons in one hermetic chamber.

Since the excimer laser has a high medium gain, the coupling of the optical resonator, that is, the transmittance of the transflective mirror in the present invention can be as low as 10 to 20%. Therefore, it can be said that the present invention exerts its effect particularly in an excimer laser. The inventors conducted experiments and studies on the output of the wavelength stabilized laser device according to the present embodiment. The experiment was performed on a KrF excimer laser using krypton and fluorine as a laser device and helium as a diluent gas. FIG. 2 shows the relationship between the transmittance of the semi-transmissive mirror 4 and the measurement results of the laser output, and the relationship between the etalon absorption power estimated from the laser output. In the configuration according to the present invention, laser oscillation occurs when the transmittance of the semi-transmissive mirror is 4% or more. It was also found that if the transmissivity of the semi-transmissive mirror is 40% or less at the maximum, the absorption power of the etalon can be reduced as compared with the conventional example, and multiple oscillations can be avoided.

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

Effect of the Invention As described above, the present invention aims at reducing the laser power incident on the etalon by the semi-transmissive mirror,
An object of the present invention is to provide a wavelength-stabilized laser that is stable in monochromaticity of the oscillation wavelength and is optimal as a light source for exposure.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the configuration of a narrow-band laser device according to one embodiment of the present invention, FIG. 2 is a diagram showing the relationship between the laser output and the power absorbed by an etalon with respect to the reflectance of a semi-transmissive mirror, FIG. 3 is a diagram showing a configuration of a conventional wavelength stabilizing laser, and FIG. 4 is a diagram for explaining the principle of laser wavelength selection. 1 ... discharge tube, 2,3 ... total reflection mirror, 4 ... semi-transmission mirror, 5,
5 '... Fabry-Perot etalon, 6 ... Airtight room, 7 ...
… Wavelength detector, 8… signal processing circuit, 10… beam splitter, 21… optical amplifier, 22… wavelength selector.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Mutsumi Sansho 1006 Kazuma Kadoma, Osaka Pref.Matsushita Electric Industrial Co., Ltd. (56) References JP-A-64-84767 (JP, A) JP-A-64-84682 (JP, A) JP-A-1-113377 (JP, U)

Claims (2)

(57) [Claims] An optical amplifier comprising a laser medium and a first total reflection mirror; a plurality of wavelength selection elements installed in an airtight chamber;
A wavelength selector consisting of a total reflection mirror is arranged on a straight line with a semi-transmissive mirror interposed therebetween to form an optical resonator, and a means for detecting an oscillation wavelength of laser light and a pressure control means are provided, and the airtightness is provided. A wavelength stabilizing laser device characterized by changing a pressure in a room. 2. The wavelength stabilized laser device according to claim 1, wherein the transmittance 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.