JPH0714086B2 - Excimer laser device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an excimer laser device, and more specifically,
The present invention relates to an excimer laser device that can obtain an output beam cross section that is highly effective in a rare gas halide excimer laser, which is expected to be applied and applied in the field of semiconductor processes, etc., particularly when the gas pressure is low.

2. Description of the Related Art A rare gas halide discharge-excited excimer laser (hereinafter referred to as an excimer laser) is capable of performing high-power and high-repetition laser oscillation in the ultraviolet region, and of course it can be used not only as a light source for research, but also as a semiconductor. Applications are expected to be expanded in the fields of process, chemical industry, medicine and energy.

As a typical type of excimer laser, an excimer laser having an oscillation wavelength, ArF of 193 nm, KrF of 248 nm, and XeCl of 308 nm is known. In both cases, a gain medium composed of a rare gas and a halogen gas is sealed in a gas pressure vessel (discharge chamber) and discharge excitation is performed to obtain laser oscillation.

FIG. 3 shows a cross section of a portion of a conventional excimer laser device according to the present invention. The power supply, electric circuit, etc. required for laser oscillation are not shown. In FIG. 3, gain medium 1
Are enclosed in the discharge chamber 2. In the discharge chamber 2, two main electrodes 3 and 4 extending in the direction of the optical axis 7 are arranged. Although not shown, the main electrodes 3 and 4 are electrically connected to each other so that the energy stored in the capacitor can be charged and discharged. Immediately before the high-voltage pulse is applied between the main electrodes 3 and 4, UV light is irradiated from the preionization means 5 and the gap between the main electrodes 3 and 4 is ionized in advance. When the pulse voltage applied between the main electrodes 3, 4 reaches the breakdown voltage of the gain medium 1, the main electrodes 3, 4
A glow discharge (discharge path) 6 is generated between the four, the gain medium 1 is excited, and the partial transmission mirrors are arranged facing each other in the direction of the optical axis 7.
Laser oscillation occurs between 10 and a total reflection mirror (not shown).

At this time, it is well known that the higher the gas pressure of the enclosed gain medium, the larger the laser output energy per single pulse, and the operation at 2 to 4 atmospheric pressure is general.

On the other hand, the optimum value of the distance between the two main electrodes extending in the optical axis direction (height H of the discharge path) depends on the enclosed gas pressure (P),
Empirically, the relationship is as follows.

H (height) / P (pressure) 6kg / cm Therefore, the higher the filling pressure, the narrower the electrode spacing,
It is preferable to set the electrode interval wider as the gas pressure becomes lower. In the chemical industry or in the field of energy, it is desired to increase the output energy per single pulse, so high gas pressure operation of 3 atm or more is suitable. In this case, the general output beam cross section is H ≒ 20mm, W≈15mm.

As a light source for photolithography in semiconductor processes, rather than output energy in a single pulse,
It is desired to increase the average output by increasing the repetition rate. Because exposure of a photosensitive material such as photoresist does not require too much single pulse energy. For example, even in a material such as PMMA, irradiation with a single pulse energy exceeding 300 m ^J / cm ² causes ablation, which is rather a problem. Therefore, to expose one chip,
A method in which irradiation is performed multiple times with a small pulse energy is preferable. What is required as a photolithography light source is to have a high repetition performance of at least 200 Hz or higher.

Problems to be Solved by the Invention In order to realize a high repetition rate, it is a condition that the flow velocity of the gain medium passing between the two main electrodes is high. For example, the width of the discharge path formed between the main electrodes is W (m), the repetition frequency is N (1 / sec), and after the main discharge, the laser gas in the discharge path is completely replaced during the next discharge. Therefore, the required flow velocity U (m / sec) of the laser gas is defined by U = _CR · W · N. The value of C _R is ideally 1, but it is a value that greatly changes depending on the structure of the oscillator, the shape of the electrodes, etc., and generally C _R = 2 to 3. For example, the width W of the discharge path
= 0.02 m, if a repetition number N = 5001 / sec of the laser oscillator of the C _R = 2 and made structure, the laser gas flow rate becomes necessary 20 m / sec.

In order to obtain the above-mentioned flow velocity with a high gas pressure excimer laser, not only a large capacity motor is required to drive the fan for gas circulation provided in the discharge chamber, but The structure of the fan, the support, the bearings, etc. are required to be fairly sturdy, and the device becomes bulky.
High cost and not a good idea.

Therefore, in order to achieve a high repetition rate, it can be said that the excimer laser operating at a low gas pressure is more advantageous. However, in order to obtain efficient oscillation in the low gas pressure excimer laser, it is necessary to widen the distance between the two main electrodes as described above. As a result, the height (H) of the discharge path that can be used for laser emission increases.

This means that in the output beam cross section of the excimer laser, the ratio of the beam width (W ') to the beam height (H') becomes large. Low gas pressure operation For example, when the filling pressure is 2 atm, the optimum electrode interval is about 3 cm,
The cross-sectional shape of the extracted excimer laser beam is a rectangle having a height (H ') to about 3 cm and a width (W') to about 1.5 cm, which is about H '/ W'≈2. Further, when the filling gas pressure is low, a rectangular beam having a larger H '/ W' ratio can be obtained. This is by no means desirable considering the application of excimer laser beams. For example, even when it is used for photolithography in a semiconductor process, the size of the lens optical system becomes unnecessarily large and becomes expensive in view of effective use of laser output. Further, when using an effective lens diameter smaller than the height (H ') of the excimer laser beam, it is necessary to limit the effective laser diameter in accordance with the effective lens diameter by using an aperture or the like. It has the drawback that it cannot be done.

As described above, in the conventional technique, since the folded structure is not adopted, the width (W ') and the height (H') of the discharge path 6 are the same as the sectional shape of the laser beam extracted from the oscillator. Therefore, there is a drawback that the H '/ W' ratio becomes large in the case of low gas pressure operation.

An object of the present invention is to solve the above-mentioned problems of the prior art, and to provide a low gas pressure operation and high repetition rate excimer laser having a simple structure and a high beam utilization rate.

Means for Solving the Problems Technical means of the present invention for solving the above problems include a discharge chamber containing a gain medium, at least two main electrodes extending in the direction of the optical axis, and a main discharge before the main discharge. In a gas laser consisting of a means for pre-ionizing and an oscillator with a total internal reflection optical element and a partially transmissive output element, the height (H) of the discharge path available for laser emission is the width of the discharge path ( W), and the oscillator divides the height (H) into two equal parts in the discharge path.
It has at least one optical element that folds the laser beam so that it folds parallel to each other in the discharge path that is available for laser radiation formed between the main electrodes.

Action The action of the above technical means is as follows.

Since the laser beam is folded back in parallel by at least one optical element so as to divide the height of the discharge path into two equal parts in the discharge path, the H / W ratio of the output beam can be almost 1 . Therefore, it is possible to extract the excimer laser beam with high utilization efficiency, and it is possible to reduce the size of the lens optical system.

Examples Examples of the present invention will be described below with reference to the drawings. First, a first embodiment of the present invention will be described.

1a is a schematic cross-sectional view perpendicular to the optical axis of the excimer laser oscillator according to the first embodiment of the present invention, and FIG.
FIG. 4 is a schematic cross-sectional view in the optical axis direction. In FIG.
A gain medium (laser gas) 1 composed of a rare gas and a halogen gas is enclosed in a closed space of the discharge chamber 2 at a predetermined pressure of 3 atm (2280 torr) or less, and is not shown in the drawing, but is like a cross fan. , Between the gaps formed by the main electrodes 3 and 4 at a high speed in the direction perpendicular to the optical axis 7 (20 to 30 m / se
It is circulated in c). The main electrodes 3 and 4 are formed at least similar to the Rogowski or Chung type cross-sectional shape so as to obtain a uniform glow discharge. Although not shown, the two main electrodes 3 and 4 are adapted and connected to, for example, a high voltage capacity transfer type pulse circuit,
Due to the charge and discharge energy of the capacitor, uniform glow discharge is performed in the gap between the main electrodes for a short time, and a discharge path 6 that can be used for laser radiation is formed. The preionization device 5 is set to automatically emit UV light when a high voltage pulse is applied to the main electrodes 3 and 4, and preliminarily ionizes the discharge path 6 prior to the main discharge. , Helps to make the main discharge uniform glow. In the discharge chamber 2, the height (H) of the discharge path formed between the main electrodes 3 and 4 that can be used for laser radiation.
The optical axes 7 and 7'are arranged so as to be divided into two, for example, divided into two. A partial transmission optical element 10 and a total reflection optical element 11 which are integral with each other are arranged at right angles on one side of the discharge chamber corresponding to the optical axes 7, 7 '. On one side of the discharge chamber 2, an integral folding optical element 9 is fitted to the optical axis 7, 7 ', divides the height (H) of the discharge path 6 into two equal parts, and the other optical element 10, 11 is provided. They are arranged in phase with each other to form an optical resonator.

The integral folding optical element 9 includes a single total reflection surface 91 that is perpendicular to the directions of the optical axes 7 and 7'and a Brewster angle (tan φ = n ₁ / n ₀ n) with respect to the optical axes 7 and 7 '. ₁ is a refractive index at the oscillation wavelength of the constituent materials, n ₀ is a refractive index of the laser medium), and is composed of two transmission planes 92 provided so that the surface reflectance for P-polarized light is as zero as possible.

In the first embodiment of the present invention configured as described above, the discharge path available for laser emission is effectively utilized, and the rectangular ratio of the cross-sectional shape of the output beam ( By dividing the H / W) into two in the height direction, the H / W ratio can be brought close to 1 as in high gas pressure operation, and the effective size of the optical system used externally can be minimized. It will be possible. Further, in the first embodiment of the present invention, the laser is selectively vertically polarized (P-polarized) by the folding optical element 9 which is integrated, so that it is stable when an oblique incidence optical system is used outside the laser oscillator. One of the advantages is that the divided light amount can be divided.

Next, a second embodiment of the present invention will be described. Figure 2 shows
It is a schematic cross section along the optical axis showing a second embodiment of the present invention. The second embodiment is the same as the first embodiment except for the folding optical element 12.

The folding optical elements 12 of the second embodiment form a right angle with each other,
In addition, it has two reflecting mirror surfaces 13 and 13 'forming an angle of 45 ° with respect to the optical axes 7 and 7', respectively, and is formed by a transmission plane 14 perpendicular to the optical axes 7 and 7 '. It consists of a transparent body.

The features of the second embodiment that the first embodiment does not have are the folding optical element.
12 is a reflecting mirror surface so that the optical axis does not shift depending on the wavelength.
If 13 and 13 'have a broadband emissivity in the excimer laser wavelength range, they can oscillate from XeCl (308 nm) to ArF (193 nm) simply by changing the enclosed gain medium. The other effects of the second embodiment are similar to those of the first embodiment.

EFFECTS OF THE INVENTION As described above, according to the present invention, since the height (H) of the discharge path that can be used for laser radiation is divided into two parallel to the main electrode by one optical element, it is excited by discharge. The gain medium is effectively utilized, and at low gas pressure operation, which is important for high repetition,
The cross section of the laser output beam can be made into a square or a shape close to it without impairing the characteristics, the effective size of the optical system used outside can be reduced, and the cost can be reduced.

In addition, since the laser selectively oscillates vertically polarized light by the integrated folding optical element, it becomes possible to easily use the oblique incidence optical system outside the laser oscillator, and the output monitor,
The accuracy of beam splitting is improved.

Furthermore, by using a right-angle prism type folding optical element, XeCl, Kr
It became possible to oscillate with high efficiency and high repetition at low gas pressure without modifying F and ArF to optical elements, and the application was widened.

The above effects obtained in the present invention are not limited to the fact that the partial transmission optical element and the total reflection optical element are necessarily integrated. Furthermore, the effect of the present invention is
The right-angle prism type folding optical element is not necessarily limited to be integrated, and may be separate reflecting mirror surfaces that form a right angle with each other.

The present invention can be applied not only to low gas pressure operation but also to high gas pressure operation.

[Brief description of drawings]

FIG. 1 is a schematic transverse sectional view showing a laser oscillator according to a first embodiment of the present invention, a is a sectional view in a direction perpendicular to the optical axis, b is a sectional view in the optical axis direction, and FIG. FIG. 3 is a schematic cross-sectional view showing a laser oscillator of the second embodiment of FIG. 3, and FIG. 3 is a schematic cross-sectional view showing a laser oscillator of the prior art. 1 ... Gain medium (excimer laser gas), 2 ... Discharge chamber, 3,
4 ... Main electrode, 5 ... Pre-ionization device, 6 ... Discharge path, 7, 7 '... Optical axis, 9 ... Laser beam folding optical element, 10 ... Partial transmission optical element, 11 ... Total reflection optical element, 12 ... Right angle prism Shape-folding optical element, 13, 13 '... Reflective mirror surface, 14 ... Transmission plane.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takeo Miyata 3-10-1 Higashisanda, Tama-ku, Kawasaki City, Kanagawa Prefecture Matsushita Giken Co., Ltd. 3-10-1 Matsushita Giken Co., Ltd.

Claims (6)

[Claims] 1. A discharge chamber containing a gain medium, at least two main electrodes extending in the direction of the optical axis, means for pre-pre-ionization prior to the main discharge, and a total reflection optical element, a partial transmission output. An excimer laser device comprising: an element and at least one optical element that folds a laser beam so as to divide the height into two parts in a discharge path. 2. The laser beam is turned back so as to be substantially parallel to each other in a discharge path that can be used for radiation formed between the main electrodes. Excimer laser device. 3. An optical element for returning a laser beam corresponds to one reflecting surface substantially perpendicular to an optical axis in a discharge path that can be used for laser radiation, and each has a Brewster's angle to the optical axis. The excimer laser device according to claim 1, wherein the excimer laser device comprises two planes formed at an angle. 4. An optical element for folding a laser beam has two reflecting mirror surfaces that are substantially perpendicular to each other and each have an angle of about 45 ° with respect to the optical axis. An excimer laser device according to claim 1. 5. The excimer laser device according to claim 1, wherein the total pressure of the gain medium required for laser oscillation is 3 atm or less. 6. An excimer laser device according to claim 1, wherein the total reflection optical element and the partial transmission output element are integrally formed.