JPH0387084A - Narrow bandwidth laser equipment - Google Patents

Abstract

PURPOSE:To obtain the most suitable laser equipment for an exposing light source free from the fluctuation of selected wavelength and the decrease of output, by installing a wavelength selecting element, a quarter-wavelength phase shifter, and a polarization separator, converting the polarization of a weak oscillation light in a narrow bandwidth with a quarter-wavelength phase shifter, amplifying the light with laser medium, and outputting the light from the polarization separator. CONSTITUTION:The propagation direction of a light 8 amplified by medium in a discharging tube 1 is divided by a polarization separator 5, in response to the component of a polarized light, and one component is radiated as an output light 7. The other component light is made to travel through the polarization separator 5; the wavelength of the above light 9 is selected by a Fabry-Perot etalon 6 and a total reflection mirror 2; the reflected light 10 is again made to travel through the separator 5, and amplified with the medium, this amplified light 11 is made to enter a quarter-wavelength phase shifter 4. After that, the light 11 is again made to travel through the phase shifter 4 by the phase shifter 4 and the total reflection mirror 3, thereby forming a light 12. As the result of two times travelling, the same effect as the travelling of a half-wavelength phase shifter is obtained, thereby transforming the light 11 polarized in one direction into the reflected light 12 having both polarization components. Hence the deformation and the deterioration of the etalon 6 can be remarkably reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a band-narrowing laser device that is an exposure light source used in ultra-fine processing of semiconductor integrated circuits.

2. Description of the Related Art Excimer lasers have been attracting attention as a light source for exposing fine patterns of semiconductor integrated circuits.

By combining a rare gas such as krypton or xenon with a halogen gas such as fluorine or chlorine as an excimer L/-f laser medium, an oscillation line with sufficient output for pattern exposure at several wavelengths between 353 nm and 193 nm is produced. Obtainable.

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 bandwidth (full width at half maximum) of about 5 nm. When a laser beam having such a relatively wide bandwidth is used as an exposure light source, it is necessary to employ an imaging optical system that corrects chromatic aberration as the N-point optical system. However, in the ultraviolet region where the wavelength is 350 nm or less, the range of optical materials for lenses used in the imaging optical system is limited, making it difficult to correct monochromatic aberration. When using an excimer laser in an exposure device, if the bandwidth of the laser oscillation line can be made monochromatic to about 0.005 nm, an imaging optical system that does not correct chromatic aberration can be used, which will simplify the optical system of the exposure device. The overall size and price can be reduced.

To make laser light with a wide bandwidth monochromatic.

Pass it through a wavelength selective filter with a narrow transmission band.

good. However, with this method, the laser output is significantly attenuated,
It cannot be used practically as a light source for exposure. There,
Generally, a method is adopted in which a wavelength selection element is installed inside a resonator to make the output monochromatic without attenuating it. As an example of this, the configuration described in Japanese Patent Laid-Open No. 63-160287 is known.

The structure will be briefly explained below. As shown in FIG. 7, the total reflection mirror 1o2. and semi-transparent mirror 103
A discharge tube 101 is placed in an optical resonator consisting of
01 is filled with a medium gas containing a rare gas and a halogen gas, and generates laser oscillation by discharge excitation. In the optical resonator, there is a Fabry bellow etalon 10 which is a wavelength selection element.
4 is set. In an excimer laser device with such a configuration, only the light 106, 107, 108, 109 of a specific wavelength selected by the fiber bellow etalon (04) is amplified and oscillated. Output light 105 can be obtained.

Problems to be Solved by the Invention However, in conventional narrowband laser devices, high-energy light that is trapped in the optical resonator passes through the wavelength selection element, leading to deformation and deterioration of the wavelength selection element. Fluctuations in the selected wavelength and reduction in output occur, resulting in 1! When used as a light source for an optical device, there were problems such as defects in the product.The present invention was made to solve these problems.

It is an object of the present invention to provide a narrowband laser device that is free from wavelength fluctuations and output decreases due to deformation and deterioration of wavelength selection elements.

Means for Solving the Problems In order to achieve the above object, the technical solution of the present invention is as follows:
a laser medium and a resonator optical path passing through the laser medium;
and a second reflecting mirror, and a wavelength phase shifter and a polarization separator are provided in the optical path of the dedicated resonator, and output light is output from the laser medium through the polarization separator. A wavelength selection element is provided in the optical path of the resonator other than the output optical path.

Effects of the present invention The light of a specific polarization plane created by an optical resonator that passes through a laser medium and passes through a wavelength selection element is converted into a polarization plane by a wavelength phase shifter and amplified by the laser medium. It passes through a polarization separator and is output as output light. Therefore, the light energy passing through the wavelength selection element is reduced to the extent that the output light is divided by the amplification factor of the laser medium, so deformation and deterioration of the wavelength selection element is significantly reduced.

EXAMPLE Hereinafter, a first example of the present invention will be described with reference to FIG.

FIG. 1 is a block diagram of a band narrowing laser device of the present invention.

In FIG. 1, an optical resonator consisting of a discharge tube 1 and a total reflection mirror 2.3 using a mixture of rare gas and halogen as a laser medium oscillates as a laser in the ultraviolet region. A 174-wavelength phase shifter 4 and a polarization separator 5 are placed in the resonator optical path created by the optical resonator, and the output light 8 amplified by the laser medium of the discharge tube 1 passes through the polarization separator 5, and the output light 7 is output.

A Fabry bellows etalon 6, which is a wavelength selection element, is placed in the resonator optical path other than the output optical path through which the output lights 7 and 8 pass, and only wavelengths in a specific narrow band selectively form an optical resonator. As a result, only light with a specific out-of-range wavelength oscillates as a laser.

The operation of the configuration shown in FIG. 1 as described above will be explained below.

The propagation direction of the light 8 amplified by the laser medium of the discharge tube 1 is separated by the polarization separator 5 depending on the polarization component, and one polarization component is output as the output light 7.

The other polarized light component passes through the polarization separator 5, and the light 9
Then, the wavelength is selected by the 77 prepero ethane 6 and the total reflection mirror 2, and the reflected light 10 passes through the polarization separator 5 again, and becomes the light 11 amplified by the laser medium, which is sent to the 174 wavelength phase shifter 4. Light 1 enters the 01/4 wavelength phase shifter 4 and the total reflection mirror 3.
1 passes through the 174-wavelength phaser 4 twice and becomes the reflected light 12, but the two passes are equivalent to passing through the 172-wavelength phaser, and the light 11 which is polarized in one direction 11Fi combines the polarization components of both. In general, by rotating the quarter wavelength phase shifter 4 around the axis through which the light passes, it is possible to arbitrarily set the intensity ratio of both polarized light components of the reflecting mirror 12.

Next, the reflected light 12 is amplified by the laser medium of the discharge tube 1 to become output light 8, and one polarized light component is outputted by the polarization separator 5 as output light 7. The other polarized light component passes through and continues to oscillate as light 9. Here, 1
By rotating the /4 wavelength phase shifter 4 and changing the polarization component ratio, the ratio of the output light 7 and the passing light 90 can be arbitrarily changed, and the laser oscillation coupling rate of the output light can be changed. With this configuration, compared to the light 9 that continues to oscillate that enters the optical fiber bellows etalon 6, the light whose polarized component is converted by the quarter wavelength phase shifter 4 is amplified by the laser medium. After that, it passes through the polarization separator 5 and becomes the output light 7, so the amplification factor of the laser medium is larger than that of the light 9, and the wavelength selection element Fabry Bellows etalon 6 is relatively deformed and deteriorated significantly. This will result in a reduction.

As described above, it is clear that the configuration of the present invention can significantly reduce the optical load on the wavelength selection element, but this will be explained in a more stationary manner using mathematical formulas.

In FIG. 1, assuming that the polarization separator 5 separates the polarization by passing the P-polarized wave and reflecting the S-polarized wave, the P-wave component intensity of the light 8 is Iapt, and the S-wave component intensity is I8°. Output light 7 has only the S wave component and has an intensity of I7s, and light 9. light 1
0. Light 11 has an intensity of only P wave component 19Py 110p
*rltp. Light 12 is S with a 174 wavelength phase shifter.
Since the P wave is mixed, the intensity of both components is I 129e.
I shall be 12p. The total reflection mirror has a reflectance of 100%, and the loss of the Fabry Bellow etalon 6 is set to AE.
(1-AI) is the transmittance. In addition, since there is usually a window in the area from which light exits the discharge tube, light loss occurs in this area as well, and this loss is designated as A. That is, (1-A
) is the transmittance of the window part. Also, when P-wave light 11 becomes reflected light 12 in a 174-wavelength phase shifter, Rpp is the ratio at which light entering as P-wave becomes reflected light 12 as P-wave, and the ratio at which the light is converted into S-wave is the remaining signal. (1-Rpp). The amplification factor per unit length of the laser medium for minute light is g.

Let the length of the discharge tube be L.

Here, an analysis method that is considered to be well suited when analyzing a laser with a high amplification factor such as an excimer laser is the one described by Rigrod, Saturation Effect in High Gain Laser, Journal of Applied Physics, Vol. 36.

No. 8. pp. 2487-2490, 1965, (W, W
.

RIGROD, ” 5aturation Effe
cts in H3gh-Ga1n La5ers
Journal of ApPliedPhysi
ca, Vol, 36. No. 8. P 24
87-P2490, August 1965)
There is a method described in

Next, the analysis will be explained by citing the formula in the above document. If the laser analysis formula in the above-mentioned document is adapted to this embodiment, the following formula is obtained.

Here, β2. γ1. γ2 is the value expressed in the literature,
In this embodiment, it is given by the following equation. However, let d be the saturated light intensity.

β2 γ1 γ2 = (Iap+l5s)/((1-A)I,)=(1-A
)2(112p+112S)/111P=(1-A)2
)1op/(Iap+Ias)...(2)...(3)...(4) Next, since the amplification factors of S-polarized light and P-polarized light are equal, 112p
The ratio of Iap and IB8 is equal to the ratio of Iap and IB8.

112P/I 128 = Iap/I8s...
・・・・・・・・・・・・・・・(5) Also, the ratio Rp
Considering p and etalon loss AE, the following formula (6+, (71) is obtained.

112P=Rpp111p, I+z3=(1-Rp
p) I+1p-(6111op=(1-Ar,)2Is
+p=(1-AE)2Iap-(71 output light ■.

ut is I7g and etalon load light 1. Since is I9p (formula 81 holds true).

Engineering. ut” I7s =: IB3 mIE = l
9p=Iap (8) As shown in equations (11 to (8) above), the output light intensity is calculated as follows.If ut and etalon load light intensity II are determined, equation (9) is obtained.

・・・・・・・・・・・・(?) Figure 2 specifically shows the calculation results of equations (9) and (1o).

The output light intensity is normalized by the saturation light intensity (RpI) on the horizontal axis. ut/Is and etalon load light intensity IE/I,
This is the result of calculating.

Next, for comparison, a similar formula will be created and studied for the conventional configuration shown in FIG. The reflectance of the semi-transparent mirror 103 is R
Assuming that other conditions are the same as in the embodiment shown in FIG. 1, the following equation can be obtained using the method described above. However, in the conventional configuration shown in Figure 7, the light is not polarized, so the intensity of the light at each part is 105, 106, 107, 108, 109.
corresponding to 1105+1106+ 1107+ 1
108, the optical loss of the etalon is AE, and the loss of the window is A, which is the same as in the example. 0 β2 = 1106/ ((1-A) IB) γ1 = (
1-A) 2I 109 / I 1oaγ2=(1-A
)2) 107/11061107 = RI 106 1109 = (1-Ag)2 I 1oaI□ut
=11os=(1-R)1106...6
1) ・・・・・・・・・(12) ・・・・・・・・・(13) ・・・・・・・・・(14) ・・・・・・・・・(151 ・・・・・・・・・・(161 These (1)' and 011-06) formulas. u
When t is determined, equation (17) is obtained.

......07) Next, the load light intensity of the etalon can be obtained from the above literature as follows: 0 β2/β4 ) Next, the relationship between etalon load light intensity Ig and β4 is expressed by equation 69).

IB ” 11os = (1-A)β4
・-”■(19161)～(Example Yos Ig
(g) can be found as the formula.

FIG. 8 specifically shows the calculation results of the formula (171,i1).

Output light intensity normalized by saturation light intensity with R on the horizontal axis.

These are the calculated results of ut/Is and etalon load light intensity IE/I8.

Here, when comparing the embodiment of the present invention shown in FIG. 2 with the conventional example shown in FIG. 8, it is clear that they are the same. A smaller value of rg at ut can be obtained in the embodiment of the present invention. That is, in Figure 2 I
. When ut/I, = 0.3, IE/I, = 0.01
2, whereas in Fig. 8, II/I, =
0.41, which is more than 30 times the difference, and the intensity of the light incident on the Fabry Bellows etalon has been significantly reduced. Also, as a noteworthy feature, in the conventional example shown in FIG. 8, the value of R is 0.
．． 15, the maximum output value Iout/I = 0.31, and only a low output can be obtained, but in the embodiment of the present invention, the maximum value Iout/I = 0.25. ut/I,=0.7
0 is obtained, and even if you press the output button, the output will be more than twice as high.
This shows that the efficiency as a laser device is also excellent.

Figure 3 shows the actual experimental results of the configuration of the present invention.
This is an actual measurement value of F excimer laser. F2 as gas used
'f, 0.22%, Kr 4.4%, remaining He gas, total pressure 1800 mb, applied voltage 28 KV, discharge, pulsed laser oscillation, output intensity per pulse. It is a graph of ut and etalon load IH.

Further, FIG. 9 shows actual measured values of the output intensity Iout and the etalon load IE applied to the conventional configuration (FIG. 7) under the same conditions.

All of the results show that the tendency is almost the same as the theoretical value, and that the configuration of the present invention has more than twice the output, has significantly less etalon load, and has overwhelmingly good performance.

As described above, according to the present invention, it is possible to obtain a narrowband laser device that significantly reduces the optical energy passing through the wavelength selection element and exhibits excellent characteristics in terms of efficiency.

Although the embodiments have been described above using a Fabric bellow etalon as a wavelength selection element, the present invention can also be applied to other wavelength selection elements. These configurations will be explained.

First, a second embodiment of the present invention will be described with reference to FIG. In FIG. 4, 20 is a grating, and the other parts are the same as in the first embodiment. A grating 20, which selects a wavelength by light reflection, as a wavelength selection element is installed in the resonator optical path formed by the total reflection mirror 2.3, and the resonator optical path is formed by the diffracted light of the grating 20. The functions of other parts are the same as those of the first embodiment, and therefore will be omitted.

Next, a third embodiment of the present invention will be described with reference to FIG. In FIG. 5, 30 is a prism, and other parts are the same as in the first embodiment.

A prism 30 that selects a wavelength by optical refraction as a wavelength selection element is installed in the resonator optical path formed by the total reflection mirrors 2 and 3, and the refracted light of the prism 30 forms a resonator optical path. The functions of other parts are the same as those of the first embodiment, and therefore will be omitted.

As mentioned above, as a wavelength selection element, a Fabry bellow etalon,
We have explained the configuration using gratings and prisms, but when using the Fabry Bellows etalon, this optical element has two reflecting surfaces facing each other, and the wavelength is selected by the interference effect between them, so the opposing reflections It can be assumed that the reflective surfaces, where high energy is trapped between the surfaces due to multiple reflections, are likely to be damaged. In comparison, according to the method using a grating or a prism explained in the second and third embodiments, wavelength selection is performed by light reflection or light refraction, so damage to the wavelength selection element is less likely to occur.
Compared to using the Fapriperot etalon mentioned above,
The threshold value is several times higher, making it possible to obtain a laser output of p20W or more.

It is a block diagram of a device. The laser oscillates in the ultraviolet region using an optical resonator consisting of a discharge tube 1 using a rare gas and halogen mixture as a laser medium, and a total reflection mirror 2.3. There is a phase retarder in the resonator optical path created by the optical resonator.
A mirror 40 and a polarization separator 5 are placed, and the output light 7 amplified by the laser medium of the discharge tube 1 passes through the polarization separator 7 and is output as output light 7. A Fabry-Péronie YS Talon 6, which is a wavelength selection element, is placed in the resonator optical path other than the output optical path through which the above output light 7 passes, and the wavelength in a specific narrow band is selectively formed into an optical resonator. By doing this, only light with a specific narrow wavelength is lased. In the fourth embodiment, a phase retarder mirror is used in place of the 1/4 wavelength phase shifter used in the first embodiment.This part will be explained in detail, and the other parts will be explained in detail in the first embodiment. It is omitted because it is the same as .

The phase retarder □G40 has a dielectric thin film layer on the surface of a reflecting mirror, and is used to separate the S polarization and P polarization of the reflected light of obliquely incident light.
It has a 90' difference in polarization phase and operates as a 1/4 wavelength phase shifter, and is installed in the resonator optical path formed by the total reflection mirror 2.3 to convert the plane of polarization. The polarization planes separated by the polarization plane separator 5 have different ratios RPp, and the coupling rate of the output light can be changed. The phase retarder mirror is a 1/4 wavelength phase shifter that can be easily made with a large diameter, is strong against laser power, and has fewer unnecessary multiple reflections, and is used for narrowband lasers used as light sources for exposure. . Incidentally, the wavelength first selection element is the above-mentioned 412. Alternatively, the grating or prism described in the third embodiment may be used.

The present invention has been described above using embodiments, but the 1/4 wavelength phase shifter of the above embodiments may also include a Fresnel rhombic prism,
There are various types of 3-time total reflection super achromatic 174-wave plates, but to obtain a large diameter beam for exposure, a first-order or multiple-order quarter-wave plate using a quartz plate is best.
The 1/4 wavelength phase shifter does not have to be an exact 1/4 wavelength phase shifter as long as it can change the component ratio of polarized light.

There are various types of polarization separators used in the above embodiments, such as a multilayer cube polarizer, a Brewster angle transparent plate, and a Wollaston prism. Separation mirror is good.

In addition, in the above embodiment, the wavelength selection element was provided between the polarization separator and the total reflection mirror, but it is also possible to use a wavelength selection element other than the output optical path from the laser medium to the polarization separator through which the output light, which is the strongest light, passes. If it is in the resonator optical path, the light intensity is weak and a wavelength selection element can be provided.Also, in the above embodiment, the 1/4 wavelength phase shifter and the polarization separator were provided in the resonator optical path on the opposite side of the laser medium. They can be installed on the same side as long as they are in the resonator optical path.

However, the wavelength selection elements used in the above embodiments may include a plurality of optical fibers such as bellows etalons, gratings, prisms, etc., or may be combined. In addition, an element that integrates a wavelength selection element and a total reflection mirror, for example, a grating with a rainbow grating or a long grating, and a grating that uses the wavelength selectivity of the reflected light of a direct grating, totally reflects one side of the prism. It can be used as a mirror, or it can be used by integrating a 1/4 wavelength phase shifter and a total reflection mirror, making one side of the crystal phase plate a total reflection mirror,
That is, the wavelength selection element and the total reflection mirror.

For the 1/4 wavelength phase shifter, polarization separator, etc., the number of elements may be reduced by using elements that combine these functions.

Further, the phase retarder mirror used in the above example may be a phase retarder prism whose similar reflecting surface has a phase retard function.Also, the total reflection mirror used in the above example has a reflectance of 100%. It is not necessary that the reflectance is the same as that of the resonator.

DETAILED DESCRIPTION OF THE INVENTION As described in detail, the present invention includes a wavelength selective element within a resonator,
A 1/4 wavelength phase shifter and a polarization separator are installed, and the weak oscillation light oscillated in a narrow band is converted into polarized light by the 1/4 wavelength phase shifter, amplified by a laser medium, and outputted from the polarization splitter. C,
By reducing the light energy that passes through the wavelength selection element, fluctuations in one selected wavelength and reduction in output can be minimized, making it possible to provide a narrowband laser device that is optimal for an exposure light source.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram of a narrowband laser device according to a first embodiment of the present invention, and FIG. 2 is a calculation of output light intensity Iout and optical intensity IE incident on the Fabry Bellows etalon with respect to the polarization ratio Rpp in the first embodiment. Characteristic diagram showing the results, Figure 3 is the same as Figure 1.
Iout for RpP actually measured in Examples
A characteristic diagram showing the relationship between s and IH, FIG. 4 is the second characteristic diagram of the present invention.
A configuration diagram of a narrowband laser device in an embodiment, FIG. 5 is a configuration diagram of a narrowband laser device according to a third embodiment of the present invention,
FIG. 6 is a configuration diagram of a narrowband laser device according to a fourth embodiment of the present invention, FIG. 7 is a configuration diagram of a conventional narrowband laser device,
FIG. 8 is a characteristic diagram showing the calculation results of Iout and IE with respect to the semi-transmissive mirror reflectance R in a conventional narrow band laser device,
FIG. 9 is a characteristic diagram showing the relationship between Ioutm and IE with respect to R actually measured in a conventional narrow band laser device. 1.101...Discharge tube, 2,3,102...Total reflection mirror, 4...1/4 wavelength phase shifter, 5...Polarization separator,
6.20゜30...Wavelength selection element, 7゜105...Output light.

Claims (7)

[Claims] (1) At least an optical resonator including a laser medium and a first and second reflecting mirror in a resonator optical path passing through the laser medium, and a wavelength phase shifter and a polarization separator in the resonator optical path. . A narrowband laser device, characterized in that a wavelength selection element is provided in the resonator optical path other than the output optical path in which output light is output from the laser medium through the deflection separator. (2) The band narrowing laser device according to claim 1, wherein the reflecting mirror, the wavelength phase shifter, the polarization separator, and the wavelength selection element are elements that combine at least some of the functions of these elements. (3) The band-narrowing laser device according to claim 1 or 2, wherein the wavelength selection element comprises one or more optical bellows etalons, gratings, or prisms. (4) The band-narrowing laser device according to claim 1 or 2, wherein the wavelength phaser comprises a first-order or multiple-order wavelength plate using a quartz single crystal plate, a phase retarder mirror, or a phase retarder prism. (5) The band-narrowing laser device according to claim 1 or 2, wherein the polarization separator comprises a polarization separation mirror made of a dielectric multilayer film. (6) The band narrowing laser device according to claim 1, wherein the laser medium uses an excimer that is a combination of a rare gas and a halogen gas. (7) A light source device for exposure, characterized in that the narrow band laser device according to claim 1 is used as a light source for exposure.