JPH11274609A - Gas laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas laser device which can prevent an incident angle deviation of laser to an optical element. SOLUTION: Outer light transmission faces 12b and 14b and internal light transmission faces 12a and 14a of windows 12 and 14 are made to incline to the axis direction of a laser chamber 10, the outer light transmission faces 12b and 14b of each window 12 and 14 are mutually provided in parallel and the internal light transmission faces 12a and 14a are mutually provided in parallel. By the constitution each light axis of incident laser to the front window 12 and emitting laser from the rear window 14 is offset at a slight distance α+β. However, it becomes in parallel and possibility of an angle deviation can not be generated at all.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas laser apparatus having windows at both axial ends of a laser chamber for passing a laser beam, and more particularly to an improvement in the arrangement of these windows.

[0002]

2. Description of the Related Art In recent years, an excimer laser has been attracting attention as a light source of a reduction projection exposure apparatus for manufacturing semiconductors. In this excimer laser device for reduction projection exposure, a narrow band of 1 pm or less in line width and a large output power are required for the laser beam, and an etalon or a grating (diffraction grating) is used as a technique for realizing this. Devices used as wavelength selection elements are promising.

FIG. 8 illustrates an optical system of a general narrow band excimer laser device using a grating as a wavelength selection element.

In this excimer laser device, a half mirror 1 functioning as a front mirror, a grating 2 functioning as a rear mirror and a wavelength selection element, and two prisms 3 functioning as a beam expander,
A laser chamber 6 is arranged between the laser chamber 6 and the band narrowing module 5 having the module 4.

The laser chamber 6 contains a laser gas (eg, a gas selected from a halogen gas (eg, F ₂ or Cl ₂ ), a rare gas, and a buffer gas (eg, Kr, Ne, He, Xe)). In the KrF excimer laser, F ₂ , Kr, Ne, and He are filled so as to be able to circulate, and a pair of main discharge electrodes 7 are provided to excite the laser gas.

[0006] The laser chamber 6 has a front window 8 and a rear window 9 at both ends in the axial direction. The front window 8 and the rear window 9 function as passing windows for laser light between the inside and the outside of the laser chamber 6, and are formed in a plate shape by glass such as CaF ₂ , MgF ₂ or quartz. , Are each inclined so as to have a Brewster angle with respect to the laser optical axis.

In this excimer laser device, an optical resonator is formed by the half mirror 1 and the grating 2, and the wavelength is selected by the prisms 3, 4 and the grating 2.

In this type of narrow band excimer laser apparatus, the angle of incidence of the laser light on the prisms 3 and 4 is set to a predetermined angle in order to appropriately maintain the magnification of the laser light and the reflectivity of the P-polarized light. It is necessary to maintain the angle at which the laser beam is incident on the grating 2, which is an angular dispersion type wavelength selection element, corresponding to the wavelength to be selected. It is extremely important to make the laser light incident on the half mirror 1, the grating 2, and the prisms 3 and 4) at an accurate angle in order to improve the quality of the laser light and to stabilize it. It should be noted that even in a gas laser device using an etalon instead of the grating 2, it is necessary to make the laser light incident on the reflecting mirror constituting the optical resonator at a predetermined angle in order to reduce the side peak.

For this reason, in the conventional gas laser apparatus, the laser beam that has passed through the rear window 9 and entered the laser chamber 6 with the optical axis being offset is returned to the original optical axis again by the front window 8. The above-mentioned rear window 9 and front window 8 are arranged so as to form a letter "C".

[0010]

By the way, the gas laser device constructed as described above has the internal light transmitting surfaces 8a, 9a facing the inside of the laser chamber 6 in the windows 8, 9.
And external light transmitting surfaces 8 b and 9 facing the outside of the laser chamber 6.
When the gas having the same relative refractive index is in contact with b, the above-described function can be surely performed.

However, in an actual gas laser apparatus, as described above, the inside of the laser chamber 6 is filled with a laser gas having a relative refractive index different from that of the external atmosphere. Is slightly inclined with respect to the axial direction of the laser chamber 6,
Further, the tilted laser light is tilted when passing through the rear window 9, and eventually, the laser light with an angular deviation is incident on the band narrowing module 5.

For example, as shown in FIG. 9, the relative refractive index of the external atmosphere is na, the relative refractive index of the material forming the windows 8 and 9 is nw, the relative refractive index of the laser gas is nb, and the relative refractive index of the laser gas is nb. When the incident angle of the laser beam incident on the front window 8 from the outside is θ1, the front window 8
Assuming that the angle of refraction of the laser beam at the time is θ2, and the angle of emission of the laser beam from the front window 8 to the laser chamber 6 is θ3, from Snell's law, sin θ2 / sin θ1 = na / nw sin θ3 / sin θ2 = nw / nb Holds, and from these equations, sin θ3 = (na / nb) sin θ1 holds. Therefore, if na ≠ nb, θ3
≠ θ1.

Similarly, the angle of incidence of the laser beam from the inside of the laser chamber 6 to the rear window 9 is θ 4, the angle of refraction of the laser beam at the rear window 9 is θ 5, and the laser beam from the rear window 9 to the outside of the laser chamber 6. If the light emission angle is θ6, then sinθ5 / sinθ4 = nb / nw sinθ6 / sinθ5 = nw / na sinθ6 = (nb / na) sinθ4 holds, and if na ≠ nb, then θ6 ≠ θ4.

Further, since θ3 ≠ θ4, θ1 ≠ θ6
Holds, and the optical axis of the laser light emitted to the outside of the laser chamber 6, that is, the optical axis of the laser light incident on the band narrowing module 5 is incident on the front window 8 from outside the laser chamber 6. It can be seen that the laser light is inclined with respect to the optical axis of the laser light.

When a laser beam with an angular deviation is incident on the band narrowing module 5 as described above, even if the deviation angle is as small as several milliradians, the gas laser device, particularly the reduced projection Excimer laser equipment for exposure has a great effect on its performance, causing a significant decrease in laser output, causing the selected wavelength to deviate from the target wavelength when narrowing the band, and the light intensity of the laser light itself. Are asymmetric on the left and right, and it is difficult to achieve alignment as an optical resonator.

In addition, in a normal gas laser apparatus, the internal pressure of the laser chamber 6 fluctuates during operation, and the relative refractive index nb of the laser gas fluctuates accordingly. Therefore, it is difficult to keep the incident angle of the laser beam on the band narrowing module 5 constant.

SUMMARY OF THE INVENTION In view of the above circumstances, an object of the present invention is to provide a gas laser device which can prevent a deviation of an incident angle of a laser beam on an optical element.

[0018]

Means for Solving the Problems and Functions and Effects
According to the invention described in the above, in the gas laser device provided with windows for passing the oscillated laser light at both ends in the axial center direction of the laser chamber filled with the laser gas, an external transparent portion facing the outside of the laser chamber in the window The light surface and the internal light transmitting surface facing the inside of the laser chamber in the window are respectively inclined with respect to the axial direction of the laser chamber, and the external light transmitting surfaces of the window are parallel to each other at both ends of the laser chamber. The internal light transmitting surfaces are arranged in parallel with each other.

According to the first aspect of the present invention, since the outer light transmitting surface and the inner light transmitting surface of the window are parallel to each other, the optical element is independent of the deviation of the laser optical axis inside the laser chamber. The laser optical axis incident on the laser chamber is always kept parallel to the laser optical axis of the laser chamber incident light, which reduces the laser output and narrows the band due to the deviation of the incident angle of the laser light to the optical element. In this case, it is possible to reliably prevent problems such as a shift of the selected wavelength from the target wavelength and an asymmetrical light intensity of the laser light on the left and right.

In this case, as in the second aspect of the present invention,
By arranging at least one of the outer light transmitting surface and the inner light transmitting surface of the window at a Brewster angle with respect to the laser optical axis, the laser output can be further improved.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings showing one embodiment. FIG. 1 shows a first embodiment of the present invention, in which only a laser chamber portion is schematically shown to explain the operation principle.

In the first embodiment, both ends in the axial direction of the laser chamber 10 in which the laser gas is circulated therein function as windows for passing a laser beam between the inside and the outside of the laser chamber 10, respectively. The front window 12 and the rear window 14 are respectively arranged obliquely with respect to the axis 16 of the laser chamber 10 so as to be parallel to each other.

The front window 12 and the rear window 14 are made of glass such as CaF ₂ , MgF ₂ or quartz in a plate shape.
The inner light transmitting surfaces 12a and 14a facing the inside of the laser chamber 10 and the outer light transmitting surfaces 12b and 14b facing the outside of the laser chamber 10 are both flat and extend parallel to each other. About the right angle axis)
The light transmitting surfaces 12a, 12b, 14a, and 14b are arranged so as to have a Brewster angle with respect to the laser optical axis.

In the first embodiment, a pair of main discharge electrodes 18 are vertically arranged inside the laser chamber 10, and the discharge direction between the main discharge electrodes 18 is emitted from the front window 12 and the rear window 14. At right angles to the P-polarized plane of the laser beam.

In the first embodiment having the above structure, the relative refractive index of the external atmosphere is na, the relative refractive index of the material forming the windows 12 and 14 is nw, and the relative refractive index of the laser gas is nb. Where the incident angle of the laser light entering the front window 12 from outside the laser chamber 10 is θ11, the refraction angle of the laser light at the front window 12 is θ12, and the emission angle of the laser light from the front window 12 to the laser chamber 10 Is θ13, sin θ12 / sinθ11 = na / nw sinθ13 / sinθ12 = nw / nb sinθ13 = (na / nb) sinθ11 is established from Snell's law, as in the prior art described above. ≠ nb
, Θ13 θ11.

Similarly, the incident angle of the laser beam from the inside of the laser chamber 10 to the rear window 14 is θ 14, the refraction angle of the laser beam at the rear window 14 is θ 15, and the laser beam from the rear window 14 to the outside of the laser chamber 10 Assuming that the light emission angle is θ16, sin θ15 / sin θ14 = nb / nw sin θ16 / sin θ15 = nw / na sin θ16 = (nb / na) sin θ14 holds. If na ≠ nb, then θ16 ≠ θ14.

However, in the first embodiment, since the internal light transmitting surface 12a of the front window 12 and the internal light transmitting surface 14a of the rear window 14 are parallel to each other, θ13 = θ14 is satisfied. Since the external light-transmitting surface 12b and the external light-transmitting surface 14b of the rear window 14 are parallel to each other, θ11 = θ16 holds, and the optical axis of the laser light emitted to the outside of the laser chamber 10 is It can be seen that the light is parallel to the optical axis of the laser light incident on the front window 12 from the outside.

That is, according to the first embodiment, the laser light incident on the front window 12 and the laser light emitted from the rear window 14 have their optical axes offset by a slight distance (α + β). However, they are parallel to each other, and there is no possibility of causing an angle shift.

Moreover, during operation, the laser chamber 1
Even when the internal pressure of 0 fluctuates and the relative refractive index nb of the laser gas changes accordingly, the relationship of θ13 = θ14 always holds, so that the offset distance between the two optical axes slightly changes. Although seen, it has no effect on the parallel relationship between the incoming and outgoing laser light.

For this reason, if the laser chamber 10 shown in the first embodiment is applied to a gas laser device,
Various optical elements disposed on the extension of the front window 12 and the rear window 14, for example, the front mirror, the prism, and the incidence angle of the laser beam to the grating can always be maintained at a desired angle.
The quality of laser light can be improved and stabilized.

FIGS. 2 and 3 show a gas laser apparatus to which the above-described first embodiment is applied, and exemplify a narrow-band excimer laser apparatus used as a light source of a reduction projection exposure apparatus for manufacturing a semiconductor. .

As shown in the figure, in this excimer laser device, a pair of main discharge electrodes 18 are arranged vertically (in FIG. 2 from the front to the depth), and the discharge direction between the main discharge electrodes 18 is the vertical direction. In this state, the above-described laser chamber 10 is positioned and installed on the upper surface of a base (not shown), and a front cavity plate 20 is disposed at a position of the laser chamber 10 located on an extension of the front window 12. A rear cavity plate 22 is arranged at a position located on the extension of the rear window 14. The front cavity plate 20 and the rear cavity plate 22 are formed in a flat plate shape having emission ports 20a and 22a, respectively, and the only upper invar 24 interposed between the upper ends.
And a pair of lower invars 26 interposed between the lower ends in a state where they are connected to each other in a state of being parallel to each other. It is fixedly installed on the upper surface of a base (not shown) via a part.

Further, in this excimer laser device, the front mirror 28 is held at a portion of the outer surface of the front cavity plate 20 which faces the emission port 20a, while the emission port 22 on the outer surface of the rear cavity plate 22 is held.
The band narrowing module 30 is held at a portion facing a.

The front mirror 28 is formed of a half mirror or a mirror having an aperture formed thereon. The optical axis 32 of the front mirror 28 is orthogonal to the front cavity plate 20 and extends in the horizontal direction.
It is attached to 0.

The band narrowing module 30 includes a slit 34
A grating 38 functioning as a rear mirror and a wavelength selection element, and two prisms 40 and 42 functioning as a beam expander are provided inside a housing 36 having
The optical axis 44 is attached to the rear cavity plate 22 in a state where the optical axis 44 is orthogonal to the rear cavity plate 22 and extends in the horizontal direction.

As is apparent from FIG. 2, in this excimer laser device, the exit port 2 of the front cavity plate 20 is used.
The optical axis 32 of the front mirror 28 facing 0a and the optical axis 44 of the band-narrowing module 30 facing the exit 22a of the rear cavity plate 22 are offset by a distance (α + β) in the horizontal direction. The optical axis 3 of these front mirrors 28
The offset distance (α + β) between the laser beam 2 and the optical axis 44 of the band-narrowing module 30 is the laser beam incident on the front window 12 described in the first embodiment,
This is equivalent to the offset distance (α + β) from the laser light emitted from the rear window 14, and is offset horizontally from the axis 16 of the laser chamber 10 by one distance α toward the optical axis 32 of the front mirror 28. On the other hand, the optical axis 44 of the band-narrowing module 30 is horizontally offset from the axis 16 of the laser chamber 10 by the distance β toward the other side.
The laser beam is made to pass between the pair of main discharge electrodes 18 as much as possible inside the laser chamber 10.

Here, in this excimer laser device, the offset of the optical axes 32 and 44 of the front mirror 28 and the band-narrowing module 30 with respect to the axis 16 of the laser chamber 10 from the equation shown in the description of the first embodiment. The distances α and β can be calculated.

Therefore, as shown in FIGS. 4 and 5, for example, the band narrowing module 30 is disposed so as to be movable in the horizontal direction with respect to the rear cavity plate 22, and is positioned at a predetermined position on the rear cavity plate 22. A pin 46 is protruded, and the band narrowing module 30 is connected to the positioning pin 4.
6, the distance β between the optical axis 44 of the band-narrowing module 30 and the axis 16 of the laser chamber 10
However, if it is configured to be equal to the calculated offset distance, when the band narrowing module 30 is attached to the rear cavity plate 22, the band narrowing module 30
Since the alignment between the laser chamber 10 and the laser chamber 10 is accurately determined, subsequent adjustment work is unnecessary.

Although not shown in the drawing, by applying the same configuration between the front mirror 28 and the front cavity plate 20, the front mirror 28
And the alignment with the laser chamber 10 can be easily determined.

As shown in FIG. 6, with the band-narrowing module 30 removed, a laser beam of the alignment adjusting device 48 is irradiated from the emission port 22a of the rear cavity plate 22, and the irradiated laser beam is aligned. After adjusting the position of the laser chamber 10 so that the laser beam enters the adjusting device 48 again, the optical axis 5 of the alignment adjusting device 48 is adjusted.
Even when the band-narrowing module 30 is mounted on the rear cavity plate 22 so as to coincide with the zero, the alignment of the front mirror 28, the laser chamber 10, and the band-narrowing module 30 is easily and accurately determined. It becomes possible.

According to the excimer laser device configured as described above, the front mirror 28 disposed on the extension of the front window 12 and the prisms 40 and 42 and the grating 38 disposed on the extension of the rear window 14 are arranged. As a result, the laser beam is always incident at a desired incident angle to improve the quality of the laser beam and to stabilize the laser beam, so that the laser beam can be applied as a light source of a reduced projection exposure apparatus for semiconductor manufacturing.

In the excimer laser device, the discharge direction of the pair of main discharge electrodes 18 is perpendicular to the wavefront of the laser light emitted from the front window 12 and the rear window 14, that is, the discharge direction is narrow. Since the laser beam expansion directions in the banding module 30 are arranged at right angles to each other, it is extremely advantageous in terms of band-narrowing efficiency.
The arrangement of the front window 12 and the rear window 14 is not necessarily limited to this. For example, even when the direction of discharge by the main discharge electrode 18 and the direction of laser beam enlargement are parallel, the same applies. Needless to say, an effect can be expected.

Further, although a gas band narrowing gas laser device using a wavelength selection element is illustrated, the invention can be applied to a gas laser device using a total reflection mirror instead of the wavelength selection element.

FIG. 7 shows a second embodiment of the present invention, in which only a laser chamber portion is schematically shown to explain the operation principle.

In the second embodiment, as the front window 102 and the rear window 104 disposed at both ends in the axial direction of the laser chamber 100, the internal light transmitting surfaces 102a and 104 facing the inside of the laser chamber 100 are used.
a is the external light transmitting surface 10 facing the outside of the laser chamber 100
2b, 104b, the inner light transmitting surfaces 102a, 104a of the windows 102, 104 are arranged in parallel with each other, and the outer light transmitting surfaces 102b, 104b are And any one of the inner light transmitting surfaces 102a, 104a and the outer light transmitting surfaces 102b, 104b, for example, the outer light transmitting surfaces 102b, 104b, respectively, has a Brewster angle with respect to the laser optical axis. Has been placed.

Here, the relative refractive index of the atmosphere outside the laser chamber 100 is na, the relative refractive index of the material forming the front window 102 is nw, and the relative refractive index of the laser gas is n.
b, and the incident angle of the laser beam incident on the front window 102 from outside the laser chamber 100 is θ21,
The angle of refraction of the laser light on the outer light transmitting surface 102b is θ22, the angle of incidence of the laser light on the inner light transmitting surface 102a is θ22 ′, and the angle of refraction on the inner light transmitting surface 102a, ie, the laser chamber 10
Assuming that the emission angle of the laser light to 0 is θ23, from Snell's law, sinθ22 / sinθ21 = na / nw sinθ23 / sinθ22 '= nw / nb. Further, if the inclination angle φ of the inner light transmitting surface 102a with respect to the outer light transmitting surface 102b satisfies θ22 = θ22 ′ + φθ21 = θ23 + φ, the optical axis of the laser light emitted into the laser chamber 100 becomes The laser beam is parallel to the optical axis of the laser beam incident on the front window 102 from outside the laser chamber 100.

Therefore, in the front window 102, the inner light transmitting surface 102a is opposed to the outer light transmitting surface 102b.
If the inclination angle φ is set so as to satisfy the following expression (1),
The optical axis of the laser light passing through the inside of the laser chamber 100 can be parallel to the optical axis of the laser light incident on the front window 102. The same applies to the rear window 104.

[Equation (1)] tanφ = (nw · sin θ22−nb · sin θ21) / (nw · cos θ22)
−nb · cos θ21) Moreover, the front window 102 and the rear window 104 have internal light transmitting surfaces 102a and 104a parallel to each other, and also have external light transmitting surfaces 102b and 104 mutually.
Since b is parallel, similar to the first embodiment described above,
Although the optical axis of the laser light incident on the front window 102 and the optical axis of the laser light emitted from the rear window 104 are slightly offset, they are parallel to each other, which may cause an angle shift. There is no such thing.

Therefore, if the laser chamber 100 shown in the second embodiment is applied to a gas laser device, the front window 102 and the rear window 1
It becomes possible to always maintain the incident angle of the laser beam to various optical elements arranged on the extension of 04 at a desired angle, to improve the quality of the laser beam and to stabilize it, The axis 106 of the laser chamber 100 and the optical axis of the laser beam can be matched with each other, and the efficiency of using the discharge excitation energy by the main discharge electrode 108 can be improved, and the laser output can be improved. .

In the above equation (1), all parameters required for setting the inclination angle φ are values that can be obtained. That is, θ21 is set to an arbitrary angle, but is usually set to the Brewster angle as described above, and is set to the external atmosphere, that is, the relative refractive index na of air and the glass constituting the window. Since the relative refractive index nw is known to be approximately 1.0 and 1.5, respectively, θ22 can be calculated. further,
The relative refractive index nb of the laser gas can be calculated using the relative refractive index and the gas composition ratio of each gas constituting the laser gas, or can be measured.

In this case, the relative refractive index nb of the laser gas is
Since the pressure of the laser gas, the composition of the mixed gas, and the temperature vary depending on conditions such as the temperature, it is an important choice to determine at which point, for example, immediately after filling the laser chamber 100 with a new gas. In the case of a gas laser device whose relative refractive index does not change much by subsequent laser oscillation, the above-described tilt angle φ may be determined using this value.

Further, in the case of a gas laser apparatus in which deterioration, reduction, and replenishment of gas due to laser oscillation cause a change in relative refractive index that cannot be ignored, an average value of relative refractive indexes during a predetermined period is used. The above-described inclination angle φ may be determined.

Furthermore, the relative refractive index during the laser oscillation may be measured, and the above-described inclination angle φ may be determined based on the measured value. In this case, since the relative refractive index may fluctuate in a short cycle due to the laser oscillation operation, it is also possible to determine the above-described inclination angle φ using the average value of the relative refractive index during a predetermined period. preferable.

[Brief description of the drawings]

FIG. 1 is a schematic view of a main part showing a first embodiment of the present invention.

FIG. 2 is a plan view of a gas laser device to which the first embodiment is applied.

FIG. 3 is a front view of a main part of the gas laser device shown in FIG. 2;

FIG. 4 is a diagram showing a mounting structure of a band narrowing module in the gas laser device shown in FIG. 2;

FIG. 5 is a view taken in the direction of arrow V in FIG. 4;

FIG. 6 is a plan view showing a method for adjusting alignment in the gas laser device shown in FIG. 2;

FIG. 7 is a schematic diagram of a main part showing a second embodiment of the present invention.

FIG. 8 is a plan view showing a conventional gas laser device.

FIG. 9 is a schematic diagram of a main part of the gas laser device shown in FIG.

[Explanation of symbols]

10, 100 ... laser chamber, 12, 102 ... front window, 12b, 14b, 102b, 140b ...
Outer light-transmitting surfaces, 12a, 14a, 102a, 104a ... inner light-transmitting surfaces, 14, 104 ... rear windows, 16, 10
6 ... axis of the laser chamber.

Claims (2)

[Claims] 1. A gas laser device comprising windows at respective axial ends of a laser chamber filled with a laser gas for allowing oscillated laser light to pass therethrough. The surface and the internal light-transmitting surface of the window facing the inside of the laser chamber are respectively inclined with respect to the axial direction of the laser chamber, and the external light-transmitting surfaces of the window are arranged parallel to each other at both ends of the laser chamber. A gas laser device, wherein the internal light transmitting surfaces are arranged in parallel with each other. 2. The gas laser device according to claim 1, wherein at least one of the outer light transmitting surface and the inner light transmitting surface of the window is arranged at a Brewster angle with respect to a laser optical axis.