JPH10233542A - Window for gas laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve utility efficiency of discharge excitation energy and precision of wavelength selection by making an optical axis during actual laser oscillation coincide with an optical axis during alignment adjustment even if alignment adjustment of an optical system is performed in the absence of a laser chamber. SOLUTION: One boundary surface of first boundary surface 10 wherein a window is in contact with an outside of a laser chamber 3, and a second boundary surface 11 wherein the window is in contact with an inside of the laser chamber 3, is tilted by a specified angle αto the other boundary surface. When a specific refractive index of outside atmosphere of the laser chamber 3 is (na), a specific refractive index of a material constituting a window is (nw), a specific refractive index of laser gas inside a laser chamber is (nb), an optical incident angle from an outside of a laser chamber to the first boundary surface is θ1, and refraction angle in the first boundary surface is θ2, the tilt angle α is set to allow a relation of tan α=(nw.sinθ2-nb.sinθ1)/(nw.cosθ2-nb.cosθ1) to be satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a window provided in a laser chamber of a gas laser device and functioning as a laser light passing window between the inside and the outside of the laser chamber.

[0002]

2. Description of the Related Art An excimer laser has been attracting attention as a light source of a reduction projection exposure apparatus for manufacturing a semiconductor. In this case, the emitted excimer laser light is required to have a narrow band of 3 pm or less, and Large output power is required.

As a promising technique for narrowing the band of an excimer laser for a reduction projection exposure apparatus, there is a technique using an etalon or a grating (diffraction grating) as a wavelength selection element.

FIGS. 3A and 3B show a configuration of an optical system of a conventional narrow-band excimer laser using a grating as a wavelength selection element. FIG. 3A is a plan view, and FIG. 3B is a front view.

The excimer laser apparatus shown in FIG. 3 has a laser chamber 3 and two prisms functioning as a beam expander between a half mirror 1 functioning as a front mirror and a grating 2 functioning as a rear mirror and a wavelength selecting element. 4a and 4b are provided.

A halogen gas (F 2, Cl 2, etc.), a rare gas and a buffer gas (K
r, Ne, He, Xe, etc.) and a laser gas containing a gas appropriately selected (for example, in a KrF excimer laser, F
2, Kr, Ne, He) are filled so as to be able to circulate,
Also, discharge electrodes 5a and 5b for exciting the laser gas are provided.

Further, windows 6 (front window 6a, rear window 6b) functioning as laser light passing windows between the inside and outside of the laser chamber are provided at the front and rear portions of the laser chamber 3. These windows 6
Is made of glass such as CaF2 or quartz, and is usually inclined so as to have a Brewster angle with respect to the laser optical axis.

In this configuration, an optical resonator is constituted by the half mirror 1 and the grating 2, and the wavelength is selected by the prisms 4a, 4b and the grating. However, the angle of incidence of the laser beam on the prisms 4a, 4b Needs to be maintained at a predetermined angle in order to properly maintain the beam expansion rate and the reflectivity of P-polarized light, and the angle of incidence of the laser beam on the grating 2, which is an angle-dispersion type wavelength selection element, also corresponds to the wavelength to be selected. You need to keep it at an angle. Even when an etalon is used in place of the grating 2, the laser beam must be incident at a predetermined angle on the reflector forming the resonator in order to reduce the side peak.

As described above, in the narrow band laser apparatus, the alignment work for adjusting the optical axis and the arrangement angle of each optical element (in this case, the wavelength selection elements 4 and 2 and the half mirror 1) reduces the quality of the laser light. It is important for improving and stabilizing.

Here, in the alignment work, usually, after the laser chamber 3 is removed from the apparatus, alignment of the optical system is performed using another laser light source (for example, He-Ne laser). Is to be re-installed. In other words, in this alignment work, the alignment of the optical system is performed in a state where the laser chamber 3 does not exist, in other words, in a state where the laser gas in the laser chamber does not exist on the laser optical axis. When laser oscillation is performed, a laser gas exists on the optical axis.

For this reason, according to the prior art, the following problem occurs during actual laser oscillation.

[0012]

That is, when aligning the optical system, the half mirror 1 and the prism 4 are required.
As a medium between a and a, the laser gas (laser chamber 3) existing during the actual laser operation is adjusted by external air instead of the air, but the relative refractive index of the laser gas is different from that of air. Therefore, no matter how accurate the alignment work is, during the actual laser operation, the optical axis of the light entering the laser chamber 3 through the window 6a is the same as the optical axis of the light before entering the window 6a. There is a problem that they do not match and are not parallel.

That is, the optical axis of the laser beam entering the laser chamber is not perpendicular to the discharge direction of the discharge electrode, but is slightly inclined, and as a result, the efficiency of using the discharge excitation energy is reduced. descend.

Similarly, the optical axis of the light exiting the laser chamber through the window 6b is deviated from the laser optical axis when the alignment is adjusted. Is not incident on the wavelength selection elements 4 and 2 at the angle at which the alignment is adjusted, and as a result, the wavelength actually selected deviates from the target wavelength.

FIG. 4 shows the optical axis of the laser beam passing through the laser chamber 3 through the windows 6a and 6b, where the relative refractive index of the atmosphere outside the laser chamber 3 is na,
The relative refractive index of the material forming the windows 6a and 6b is n
w, the relative refractive index of the laser gas in the laser chamber 3 is nb,
Assuming that the light incident angle from the outside of the laser chamber to the window 6a is θ1, the refraction angle at the window 6a is θ2, and the emission angle to the laser chamber 3 is θ3, the following equation is established from Snell's law.

Sin θ2 / sin θ1 = na / nw (1) sin θ3 / sin θ2 = nw / nb (2) When the above (1) and (2) are arranged, the following equation (3) is established.

Sin θ3 = (na / nb) sin θ1 (3) According to the above equation (3), if naｎnb, then θ3 ≠ θ
It turns out that it becomes 1.

Assuming that the incident angle from the inside of the laser chamber 3 to the window 6b is θ4, the refraction angle is θ5, and the emission angle out of the laser chamber is θ6, sin θ5 / sin θ4 = nb / nw in the same manner as described above. (4) sin θ6 / sin θ5 = nw / na (5) sin θ6 = (nb / na) sin θ4 (6) It is understood that θ6 = θ4 does not hold if na = nb does not hold.

As described above, according to the prior art, the alignment adjustment of the optical system is performed in a state where the laser gas does not exist on the laser optical axis. When laser oscillation is performed, the laser optical axis deviates and tilts from the optical axis at the time of alignment adjustment, the selected wavelength in narrowing the band deviates from the target wavelength, and the utilization efficiency of discharge excitation energy decreases. Problems occur.

The present invention has been made in view of such circumstances, and even if the alignment of the optical system is performed in the absence of the laser chamber, the optical axis of the optical system during the actual laser oscillation is not adjusted. An object of the present invention is to provide a window for a gas laser device that is aligned with an axis and solves the above-mentioned problem.

[0021]

According to the present invention, there is provided a window for a gas laser apparatus provided in a laser chamber filled with a laser gas and passing oscillated laser light, wherein the window is in contact with the outside of the laser chamber. One of the first boundary surface and the second boundary surface where the window is in contact with the inside of the laser chamber is inclined at a predetermined angle α with respect to the other boundary surface, and the outside atmosphere of the laser chamber is The relative refractive index is na, the relative refractive index of the material forming the window is nw, the relative refractive index of the laser gas in the laser chamber is nb, the light incident angle from the outside of the laser chamber to the first interface is θ1, Assuming that the refraction angle at the first boundary surface is θ2, the inclination angle α is tanα = (nw · sinθ2−nb · sinθ1) / (nw · cosθ2
−nb · cos θ1).

If one boundary surface of the window is inclined by an angle α with respect to the other boundary surface so as to satisfy the above expression, the laser light emitted into or out of the laser chamber via the window can be obtained. The optical axis is always parallel to the optical axis of the laser beam before entering the window,
Thus, after the alignment of the optical system in the absence of the laser chamber is completed, the problem that the laser optical axis shifts and becomes oblique when the laser chamber 3 is loaded and actual laser oscillation is performed is solved. As a result, the use efficiency of the discharge excitation energy is improved, and if the window of the present invention is applied to a gas laser having a narrow band, a desired wavelength can be accurately and accurately selected.

[0023]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 shows an embodiment of the present invention.

In FIG. 1, a laser chamber 3 is provided with a front window 6a and a rear window 6b, and a laser beam passes through these windows 6a and 6b.

Here, these windows 6a, 6b
Are a first boundary surface 10 in contact with the atmosphere outside the laser chamber 3 and a second boundary surface
Each of these boundary surfaces is constituted by a plane, and one of these boundary surfaces is inclined at a predetermined angle α with respect to the other boundary surface. ing. These windows 6a and 6b are made of glass, such as CaF2 or quartz, like the ordinary windows.

Here, the relative refractive index of the atmosphere outside the laser chamber 3 is na, the relative refractive index of the material forming the windows 6a and 6b is nw, and the relative refractive index of the laser gas in the laser chamber 3 is nb. As shown in (1), a case is considered in which laser light enters the laser chamber from outside the laser chamber via the window 6a.

In this case, the light incident angle from the outside of the laser chamber to the first boundary surface 10 of the window 6a is θ
1. Assuming that the angle of refraction at the first interface 10 is θ2, the angle of incidence on the second interface 11 is φ1, and the angle of refraction at the second interface 11 (the angle of emission into the laser chamber) is φ2. , As before,
The following equation is established from Snell's law.

Sinθ2 / sinθ1 = na / nw (7) sinφ2 / sinφ1 = nw / nb (8) Here, in order to make the incident angle θ1 to the window 6a parallel to the outgoing angle φ2 from the window 6a. Needs to satisfy the following two equations.

Θ2 = α + φ1 (9) θ1 = α + φ2 (10) Then, by substituting the above equations (9) and (10) into the equations (7) and (8) and rearranging, the following equation (11) is obtained. be able to.

Tan α = (nw · sin θ2−nb · sin θ1) / (nw · cos θ2−nb · cos θ1) (11) That is, the two boundary surfaces 10, 1 of the window 6
One of the two boundary surfaces is inclined with respect to the other boundary surface by a predetermined angle α, and the inclination angle α is set to the above (11).
If the window 6 is set in the laser chamber 3 by setting so that the equation is established, the laser chamber 3 is loaded after the alignment of the optical system without the laser chamber 3 is completed. When the laser oscillation is performed, the optical axis of the laser light emitted into or out of the laser chamber via the window is always parallel to the optical axis of the laser light before the window enters,
The above-mentioned conventional problems are solved.

The rear window 6b shown in FIG. 1 is provided with the same inclination angle α as that of the front window 6a.

In the above equation (11), θ1 may be adjusted to an arbitrary angle, but is usually set to the Brewster angle θB. Also, the relative refractive index of air na
Since the relative refractive index nw of the glass constituting the window 6 is also known to be approximately 1.0 and 1.5, respectively, the value of θ2 can be calculated from the above equation (7).

The relative refractive index nb of the laser gas can be calculated using the relative refractive index and the gas composition ratio of each of the gases constituting the laser gas, or can be measured.

Therefore, according to the above equation (11), all parameters required for setting the inclination angle α can be obtained in advance.

Since the relative refractive index nb of the laser gas fluctuates depending on parameters such as the pressure of the laser gas, the composition of the mixed gas, and the temperature, it is important to determine when to determine the relative refractive index nb of the laser gas. It is.

In the case of a device in which the relative refractive index nb immediately after filling the laser chamber with a new gas is small even by subsequent laser oscillation, the relative refractive index nb immediately after filling the laser chamber with a new gas is used. What is necessary is just to determine the inclination angle (alpha).

Further, in the case of a laser which causes a change in relative refractive index nb such that deterioration, reduction, and replenishment of gas due to laser oscillation cannot be ignored, the average value of relative refractive index nb during a predetermined period is used. What is necessary is just to determine the said inclination angle (alpha).

Since the deviation of the optical axis becomes a problem during laser oscillation, the relative refractive index nb during laser oscillation is measured, and the inclination angle α is determined based on the measured value. It may be. In this case, since the relative refractive index nb may fluctuate in a short cycle due to the laser oscillation operation (particularly, discharge of the pulsed laser), the relative refractive index nb may be changed using the average value of the relative refractive index nb during a predetermined period. The inclination angle α
Is preferably determined.

In the above embodiment, the incident angle θ1 at the first boundary surface 10 where the window 6 contacts the atmosphere is set to the Brewster angle θB, but the second boundary surface where the window contacts the laser gas is set. The emission angle φ2 at 11 may be set to the Brewster angle θB. When the relative refractive index na of air and the relative refractive index nb of the laser gas are close to each other, θ is set so that (θ1 + φ2) / 2 = θB holds.
Both 1 and φ2 may be set so as to approach the Brewster angle θB.

If the difference between the relative refractive index na of air and the relative refractive index nb of the laser gas is large, it is not possible to set both θ1 and φ2 so as to approach the Brewster angle θB. The second in contact with
Is set to the Brewster angle θB at the boundary surface 11 of the above, and the other first boundary surface 10 is coated with an anti-reflection film (a P-polarization anti-reflection film when a grating is used). In addition, the present invention is not limited to a narrow-band gas laser using a wavelength selection element, and a total reflection mirror can be used instead of a wavelength selection element. It can also be applied to gas lasers that are not narrowed using.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of the present invention.

FIG. 2 is a diagram for explaining a tilt setting angle of a window.

FIG. 3 is a diagram showing a conventional technique.

FIG. 4 is a diagram showing a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Half mirror 2 ... Grating 3 ... Laser chamber 4 ... Prism 5 ... Discharge electrode 6 ... Window 10, 11 ... Boundary surface

Claims (5)

[Claims] A window for a gas laser device provided in a laser chamber filled with a laser gas and passing oscillated laser light, wherein the window has a first boundary surface in contact with the outside of the laser chamber and the window is inside the laser chamber. And one of the second boundary surfaces in contact with the second boundary surface is inclined by a predetermined angle α with respect to the other boundary surface, the relative refractive index of the atmosphere outside the laser chamber is na, and the material constituting the window is Is the relative refractive index of nw, the relative refractive index of the laser gas in the laser chamber is nb, the light incident angle from the outside of the laser chamber to the first interface is θ1, and the refraction angle at the first interface is θ2. Tan α = (nw · sin θ2−nb · sin θ1) / (nw · cos θ2
−nb · cos θ1) A window for a gas laser device, which is set to a value satisfying the following relationship: 2. The gas laser apparatus window according to claim 1, wherein, at the time of setting the inclination angle α, the relative refractive index of the laser gas is a value before the start of laser oscillation after the laser chamber is filled with the laser gas. . 3. The window for a gas laser device according to claim 1, wherein, when setting the inclination angle α, the relative refractive index nb of the laser gas is a value during laser oscillation. 4. A window for a gas laser device according to claim 1, wherein an anti-reflection film is coated on a first boundary surface of said window. 5. The window for a gas laser device according to claim 1, wherein the window is set such that a second boundary surface of the window and a laser optical axis form a Brewster angle.