JPH0243787A - Laser device - Google Patents

Abstract

PURPOSE:To obtain a laser device capable of retaining the quality of a monochromized laser beam for a long period by a method an optical resonator and a wavelength selecting element, installed inside the resonator and in an inert atmosphere, are provided. CONSTITUTION:A laser device of this design is provided with an optical resonator and a single or two or more wavelength selecting elements 4 installed inside the resonator and in an inert gas. For instance, laser rays in an ultraviolet region are oscillated through a discharge tube 1, which employs a mixed gas of rare gas and halogen gas as a laser medium, together with the optical resonator composed of a total reflection mirror 2 and an output mirror 3. An air space etalon 4 serving as a wavelength selection element is provided to an optical axis of the optical resonator. The air space etalon 4 is a kind of a Fabry-Perot etalon, which is provided with two parallel plane quartz plates which have an adequate refractive index toward the laser wavelength and face each other at a very small gap, and installed in a hermetically sealed vessel 5. And, the hermetically sealed vessel 5 is charged with an inert gas such as nitrogen, helium, or argon.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a laser device used for microfabrication and the like.

BACKGROUND OF THE INVENTION In recent years, laser devices that oscillate in the ultraviolet region have attracted attention as light sources for microfabrication. Among these, excimer lasers use a combination of rare gases such as krypton and xenon as laser media and halogen gases such as fluorine and chlorine.
Strong oscillation lines can be obtained at several wavelengths between 363 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 about 5 nm.

When a laser device having such a relatively wide line width is used as a light source, it is necessary to employ an optical system that corrects chromatic aberration for the light transmission 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 optical systems is limited, making it difficult to correct chromatic aberration. Therefore, when using an excimer laser with a wavelength of 350 nm or less, the line width of the oscillation line should be set to 0. Monochromatization to about O05 nm is being carried out. As a result, it is possible to employ an optical system that does not correct chromatic aberration, which makes it possible to simplify the apparatus, further downsize it, and reduce its cost. Furthermore, the pattern of the focused laser beam can be made as small as possible, opening up applications as a light source for ultrafine processing.

Further, when the oscillation wavelength of the excimer laser is made monochromatic in this way, the center wavelength can be set to any value within the gain bandwidth. Therefore, if the oscillation wavelength can be precisely set and a mechanism is built into the excimer laser body, the wavelength-tunable ultraviolet laser device can be used not only for microfabrication but also for controlling photochemical reactions.
It is expected to have a wide range of applications such as isotope separation.

A wavelength selection element such as a Lunija grating or an etalon that narrows the oscillation line width of the laser, that is, makes it monochromatic, may be placed in the resonator. FIG. 4 shows an example of a laser device in which the oscillation line width is narrowed using an air space etalon 4. The air space etalon is a type of Fabry-Berrot etalon, which consists of two parallel plane plates facing each other and configured to transmit only specific wavelengths due to the interference effect between them. Air space etalon has an accuracy of 1 of the opposing reflective surface.
Since the transmission band width changes depending on the reflectance, it is only necessary to design and manufacture the device so that the desired oscillation line width can be obtained. To adjust the reflectance of opposing surfaces, a coating is applied that has a specific reflectance at the laser wavelength used. Multilayer metal oxide coatings are common in the excimer laser wavelength range.

Problems to be Solved by the Invention However, in a laser resonator, a powerful laser beam passes through a wavelength selection element, which damages the coating on the reflective surface, causing changes in the shape of the laser beam, a decrease in output, and even damage to the oscillation line. There was a problem that it was easy to cause the width to expand. The present invention was made to solve such problems,
The present invention provides a laser device that can maintain the quality of a monochromatic laser beam for a long period of time.

Means for Solving the Problem In order to solve this problem, the present invention includes an optical resonator and a single or plural wavelength selection elements installed in the optical resonator and placed in an inert gas atmosphere. It is equipped with

Function: With this configuration, it is possible to suppress the deterioration reaction that occurs through active gases such as oxygen in the air when the coating surface of the wavelength selection element is irradiated with laser light, dramatically extending the life of the wavelength selection element. It can be stretched.

Embodiment FIG. 1 is a block diagram of an excimer laser which is an embodiment of the present invention. In FIG. 1, the laser device according to the embodiment of the present invention emits a laser beam in the ultraviolet region by an optical resonator consisting of a discharge tube 1 using a mixed gas of rare gas and halogen gas as a laser medium, a total reflection mirror 2, and an output mirror 3. oscillate. An air space etalon 4, which is a wavelength selection element, is placed on the optical axis of the optical resonator. The air space etalon 4 is a type of Fabry-Bello etalon, in which two parallel plane quartz plates having an appropriate reflectance at the laser wavelength face each other with a small gap, and is installed in an airtight container 5. There is. The airtight container 5 is filled with an inert gas such as nitrogen, helium, or argon.

Next, the principle of making the excimer laser monochromatic with the above configuration will be explained. Generally, the gain bandwidth of an excimer laser is about 1 nm, and when oscillated without monochromatic, the gain bandwidth is 0.5 nm.
It has an oscillation linewidth of about nm. When the air space etalon 4 is inserted into the resonator, the oscillation line width can be made monochromatic to, for example, the order of 0.001 nm according to its transmission characteristics.

FIG. 2 is a diagram showing the principle by which a specific wavelength is selected by an air space etalon. The opposing surfaces of the air space etalon have a reflectance of several tens of percent, and the incident laser beam undergoes multiple reflections before exiting in its original direction of travel. At this time, between the light that is emitted after being reflected N times and the light that is emitted after being reflected N+1 times, Δ=2ndcosθ −・−・−(1
) occurs, so mλ=2ndcosθ...
Only light with a wavelength λ that satisfies the relationship @) can pass through the air space etalon. Here, n is the refractive index between the opposing surfaces, d is the distance between the opposing surfaces, θ is the inclination angle of the incident light with respect to the optical axis, and m is an integer.

As can be seen from the above considerations, as a result of the laser light making many round trips between the opposing surfaces of the air space etalon, high energy ultraviolet light is constantly present. For this reason, it is inevitable that the reflective film coating will gradually deteriorate. In particular, when the air space etalon is used in air without being placed in an airtight container, the wavelength selection characteristics change after operating the excimer laser device with about 106 pulses, causing the oscillation linewidth to become wider than its original value, or the center wavelength A phenomenon occurred in which the In addition, as a result of local or overall unevenness on the surface of the coating, changes in the beam shape and declination of the laser output were observed.

One of the reasons why the coating deteriorates is that oxygen that absorbs ultraviolet light becomes an active species and combines with the coating film, which also absorbs ultraviolet light and becomes reactive.

Therefore, in the laser device of this embodiment, this problem was solved by placing the air space etalon in an inert gas atmosphere. That is, the air space etalon is housed in an airtight container, and the gas in the airtight container is replaced with nitrogen, a rare gas, etc., thereby suppressing the reaction that occurs due to the presence of oxygen. According to experiments conducted by the present authors, the life of the coating was extended by more than two orders of magnitude by using an inert gas atmosphere around the air space etalon. The effect was particularly great when rare gases such as helium and argon were used, which do not produce excited species at the wavelength of the excimer laser.

FIG. 3 is a diagram showing a different embodiment of the present invention.

In the embodiment shown in FIG. 3, the center wavelength of laser oscillation is made variable by changing the gas pressure between the gaps in the air space etalon. That is, when the pressure between the opposing surfaces of the air space etalon changes, the refractive index n changes, so the optical path difference Δmo changes. As a result, the wavelength selected by the airspace etalon will also change. In this embodiment, in order to control the center wavelength of the laser beam to a desired value, a part of the laser beam is guided to a wavelength detector 7 by a beam splitter 6, and a high pressure gas source 8 is activated in accordance with the signal from the wavelength detector 7. Pulp 9 or Pump 10
The pressure inside the airtight container 6 is adjusted by opening the pulp 11. Needless to say, in this case as well, the gas supplied from the high-pressure gas source 8 to the airtight container 5 is nitrogen, rare gas, or the like.

With the above configuration, in the laser device of the present invention, the lifetime of the wavelength selection element can be dramatically extended.

In this example, an example in which an air space etalon is used as a wavelength selection element is shown. Similar effects can be expected even when other wavelength selection elements such as gratings are used.

As described in detail, the laser device according to the present invention extends the life of the wavelength selection element by placing the wavelength selection element in an inert gas atmosphere, and maintains the quality of a monochromatic laser beam stably over a long period of time. A laser device having such excellent effects can be provided.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a laser device which is an embodiment of the present invention,
FIG. 2 is a diagram illustrating the entire principle of the air space etalon, FIG. 3 is a diagram showing another embodiment of the present invention, and FIG. 4 is a diagram illustrating a conventional method of monochromating laser light. 1...discharge tube, 2...total reflection mirror, 3.
... Output mirror, 4 ... Air space etalon, 6 ... 'A-tight container, 6 ... Beam splitter, 7 ... Wavelength detection Vessel, 8...
・High pressure gas source, 9...Pulp, 10...
・Pump, 11...Pulp. Name of agent: Patent attorney Shigetaka Awano and 1 other person1-
3--1 714t +...-Works s'-7 Eta a So 5-Kijo disturbance No. 4

Claims (1)

[Claims] A laser device comprising: an optical resonator; and one or more wavelength selection elements installed in the optical resonator and placed in an inert gas atmosphere.