JPH0797680B2 - Narrow band laser device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a narrow band laser device which is a light source for exposure used during ultrafine processing of a semiconductor integrated circuit.

2. Description of the Related Art Excimer lasers have been attracting attention as a light source for exposing fine patterns of semiconductor integrated circuits. The excimer laser can obtain an oscillation line having a sufficient output for pattern exposure at several wavelengths between 353 nm and 193 nm by combining a rare gas such as krypton or xenon with a halogen gas such as fluorine or chlorine as a laser medium. it can.

The gain bandwidth of these excimer lasers is as wide as about 1 nm,
When oscillated in combination with an optical resonator, the oscillation line is 0.5n
Has a bandwidth of about m (full width at half maximum). When laser light having a relatively wide bandwidth is used as a light source for exposure as described above, it is necessary to employ an imaging optical system in which chromatic aberration is corrected as an exposure optical system. However, in the ultraviolet region having a wavelength of 350 nm or less, the range of selection of the optical material of the lens used in the imaging optical system is obtained, and it becomes difficult to correct chromatic aberration. When using an excimer laser in an exposure system, the bandwidth of the laser oscillation line is 0.005n.
If monochromatic up to about m, it becomes possible to use an imaging optical system that does not correct chromatic aberration, and it is possible to simplify the optical system of the exposure apparatus, further downsize the exposure apparatus, and reduce the cost.

To make a laser beam having a wide bandwidth monochromatic, a wavelength selection filter having a narrow transmission band may be used. However, with this method, the laser output is significantly attenuated and cannot be put to practical use as a light source for exposure. Therefore, a method is generally adopted in which a wavelength selection element is installed in a resonator and the output is monochromatic without being attenuated. As an example of this, for example, the configuration described in Japanese Patent Laid-Open No. 63-160287 is known.

The structure will be briefly described below. As shown in FIG. 7, the discharge tube 101 is placed in an optical resonator including a total reflection mirror 102 and a semi-transmission mirror 103, and the discharge tube 101 is A medium gas containing a rare gas and a halogen gas is enclosed, and laser oscillation occurs by discharge excitation. A Fabry-Perot etalon 104, which is a wavelength selection element, is installed in the optical resonator. In the excimer laser device having such a configuration, the light 10 having a specific wavelength selected by the Fabry-Perot etalon 104 is used.
Since only 6, 107, 108 and 109 are amplified and oscillated, the output light 105 having a very narrow bandwidth and high output can be obtained.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, in the conventional band-narrowing laser device, since the light of high energy which is standing in the optical resonator passes through the wavelength selection element, the wavelength selection element is deformed or deteriorated to cause the selection wavelength. And a decrease in output occur, and as a result, when used as a light source of an exposure apparatus, there is a problem that a product is defective. The present invention has been made to solve such a problem, and an object of the present invention is to provide a narrow band laser device in which there is no wavelength fluctuation or output reduction due to deformation or deterioration of the wavelength selection element.

Means for Solving the Problems In order to achieve the above object, the present invention provides an optical resonator including first and second reflectors, and a laser medium portion provided in a resonant optical path of the optical resonator. Excitation means for exciting the laser medium portion to generate light in the direction of the resonance optical path; wavelength selection means provided in the resonance optical path for selecting light of a predetermined wavelength component of the light; and the resonance optical path. Polarization conversion means provided inside for converting the polarization state of the light,
A polarization separation unit that is provided in the resonance optical path and separates a first polarization component of the light and a second polarization component of the light, and the wavelength selection unit is in the resonance optical path. A predetermined wavelength component light which is provided other than between the laser medium and the polarization separation means, selects one of the first polarization component and the predetermined polarization component of the second polarization component, and selects the predetermined wavelength component light by the wavelength selection means. The wavelength component light is converted into a polarization component having the other polarization state of the first polarization component and the second polarization component by the polarization conversion means, and then separated by the polarization separation means to have a narrow wavelength band. Narrow band laser device which is emitted as a converted laser output light Action With the above configuration of the present invention, the light emitted from the laser medium is oscillated by the optical resonator, and the light of one polarization component of the light is the laser medium. And the polarization separation means After narrowing the band by the wavelength selecting means placed in the optical resonance optical path other than the above, it is converted into light having the other polarization component by the polarization converting means, and is output as the narrowband output light amplified from the polarization separating means. To be done. Therefore, the optical 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 that the deformation and deterioration of the wavelength selection element are 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 narrow band laser device according to the present invention.
In FIG. 1, laser oscillation is performed in the ultraviolet region by an optical resonator including a discharge tube 1 using a mixed gas of a rare gas and a halogen as a laser medium and total reflection mirrors 2 and 3. A 1/4 wavelength phase shifter 4 and a polarization separator 5 are placed in the resonator optical path formed 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.

The Fabry-Perot etalon 6 which is a wavelength selection element is placed in the resonator optical path other than the output optical paths through which the output lights 7 and 8 pass, and only the wavelength in a specific narrow band is selectively oscillated by the optical resonator. , Only light of a specific narrow wavelength lases.

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

The light 8 amplified by the laser medium in the discharge tube 1 has its propagation direction divided by the polarization separator 5 depending on the polarization component, and one polarization component is output as the output light 7. The light of the other polarization component passes through the polarization separator 5 and becomes light 9 whose wavelength is selected by the Fabry-Perot ethane 6 and the total reflection mirror 2 to become reflected light 10 which again passes through the polarization separator 5. The light 11 amplified by the laser medium enters the 1/4 wavelength phase shifter 4. In the 1/4 wavelength phase shifter 4 and the total reflection mirror 3, the light 11 passes through the 1/4 wavelength phase shifter 4 twice to become the reflected light 12. The light 11 which is equivalent and changed in one direction becomes the reflected light 12 including both variable components. Generally 1/4 wavelength phase shifter 4
It is possible to arbitrarily set both polarization component intensity ratios of the reflecting mirror 12 by rotating the optical axis about the axis through which light passes. Next, the reflected light 12 is amplified by the laser medium of the discharge tube 1, becomes output light 8, and one polarization component is output as output light 7 by the polarization separator 5. Further, the other polarized component passes and becomes light 9 to continue oscillation. here,
The 1/4 wavelength phase shifter 4 is rotated to change the polarization component ratio, thereby arbitrarily changing the ratio of the output light 7 and the passing light 9 to change the laser oscillation coupling ratio of the output light. With such a configuration, as compared with the light 9 that continues to oscillate into the Fabry-Perot etalon 6, the light whose polarization component is converted by the 1/4 wavelength phase shifter 4 is amplified by the laser medium and then amplified. Since the output light 7 passes through the polarization separator 5, the amplification factor of the laser medium is larger than that of the light 9, and the deformation and deterioration of the Fabry-Perot etalon 6 which is the wavelength selection element is significantly reduced. Becomes

As described above, it is clear that the optical load of the wavelength selection element can be significantly reduced with the configuration of the present invention, but a more quantitative description will be given using mathematical expressions.

In FIG. 1, the polarization separator 5 passes, for example, P polarized light and passes S
Assuming that the polarized wave is reflected and the polarized wave is separated, the P wave component intensity of the light 8 is I _8p and the S wave component intensity is I _8s . The intensity of the output light 7 is I _7s with only the S wave component, and the intensity of light 9, light 10 and light 11 is I _9p , I _10p , I _11p with only the P wave component. Light 12 is a 1/4 wavelength phaser
Since SP waves are mixed, both component intensities are set to I _12s and I _12p , respectively. The total reflection mirror has a reflectance of 100%, the Fabry-Perot etalon 6 has a loss of A _E , and (1-A _E ) has a transmittance. Further, since a portion where light is emitted from the discharge tube usually has a window, light loss also occurs in this portion, and therefore the loss is set to A. That is, (1-A) is the transmittance of the window portion.
Also, when the P-wave light 11 becomes the reflected light 12 in the 1/4 wavelength phase shifter, the ratio of the light entering the P-wave to the reflected light 12 as the P-wave is R _pp, and is converted to the S-wave. Is the remaining (1-R _pp ). The amplification factor per unit length of the laser medium with a small amount of light is g _0, and the length of the discharge tube is L.

Here, as an analysis method that is often applied when analyzing a laser having a high amplification factor such as an excimer laser, a saturation effect in high gain laser by Rigrod, Journal of Applied Physics, Volume 36, No. 8, 2487-2490, 1965, (WWR
IGROD, "Saturation Effects in High-Gain Lasers", J
ournal of Applied Physics, Vol.36, No 8, P2487 ~ P249
0, August 1965).

Next, the analysis will be described with reference to the formulas in the above documents. If the present embodiment is adapted from the laser analysis formula of the above document, the following formula is obtained.

Here, β ₂ , γ ₁ , and γ ₂ are values represented in the literature and are given by the following equations in this embodiment. However, I _s is the saturated light intensity.

β ₂ = (I _8p + I _8s ) / {(1-A) I _S } ... (2) γ ₁ = (1-A) ² (I _12p + I _12s ) / I _11p …… (3) γ ₂ = (1-A) ² I _10p / (I _8p + I _8s ) (4) Next, since the amplification rates of S-polarized light and P-polarized light are equal, I _12p and I
The ratio of _12s is equal to the ratio of I _8p and I _8s .

I _12p / I _12s = I _8p / I _8s (5) Also, considering the ratio R _pp and the etalon loss A _E (6),
Expression (7) is obtained.

I _12p = R _pp I _11p , I _12s = (1-R _pp ) I _11p …… (6) I _10p ＝ (1-A _E ) ² I _9p ＝ (1-A _E ) ² I _8p …… (7 ) Since the output light I _out is I _7s and the etalon load light I _E is I _9p , the formula (8) is established.

I _out = I _7s = I _8s , _IE = I _9p = I _8p (8) When the output light intensity I _out and the etalon load light intensity I _E are calculated from the above equations (1) to (8), (9 ) And (10) are obtained.

FIG. 2 specifically shows the calculation results of equations (9) and (10).
Output light intensity I _out / R _pp normalized by the saturated light intensity I _s on the horizontal axis
It is the result of calculating I _s and the etalon load light intensity I _E / I _S.

Next, for comparison, a similar equation is made for the conventional configuration shown in FIG. 7 and examined. Assuming that the reflectance of the semi-transmissive mirror 103 is R and the other conditions are the same as those of the embodiment of FIG. 1, the following formula is obtained by the above-described method. However, since the conventional configuration shown in FIG. 7 does not polarize, the light intensity of each part is I ₁₀₅ , I ₁₀₆ , I against the light 105,106,107,108,109.
₁₀₇ and I ₁₀₈ , the optical loss of the etalon is A _E, and the loss of the window is A, which is the same as the embodiment.

β ₂ = I ₁₀₆ / {(1-A) I _S } ... (11) γ ₁ = (1-A) ² I ₁₀₉ / I ₁₀₈ …… (12) γ ₂ = (1-A) ² I ₁₀₇ / I ₁₀₆ …… (13) I ₁₀₇ ＝ RI ₁₀₆ …… (14) I ₁₀₉ ＝ (1-A _E ) ₂ I ₁₀₈ …… (15) I _out ＝ I ₁₀₅ ＝ (1-R) ₁₀₆ …… ( 16) When the output I _out is obtained from these equations (1) ′ and (11) to (16), equation (17) is obtained.

Next, the load light intensity of the etalon can be obtained from the above document as follows.

Next, the relationship between the etalon load light intensity I _E and β ₄ is given by equation (19).

I _E = I ₁₀₈ = (1-A) β ₄ I _S (19) From formulas (11) to (19), I _E is calculated as formula (20).

FIG. 8 shows concretely the calculation results of the equations (17) and (20).
Output light intensity I _out / I normalized by saturated light intensity I _S on the horizontal axis of R
It is the calculation result of _S and the etalon load light intensity I _E / I _S.

Here, comparing the embodiment of the present invention shown in FIG. 2 with the conventional example shown in FIG. 8, it is apparent that I _E at the same I _out is smaller in the embodiment of the present invention. That is, in FIG. 2, I _out / I _S
When I = 0.3, I _E / I _S = 0.012, whereas in Fig. 8, I _E / I _S = 0.41, there is a difference of more than 30 times, and the incident light intensity of the Fabry-Perot etalon is greatly reduced. Has been done. Also, as a noteworthy feature, in FIG.
Is 0.15, the maximum value of the output is I _out / I _s = 0.31, and only a low output is obtained, but in the embodiment of the present invention, R _pp = 0.25.
The maximum value I _out / I _s = 0.70 was obtained, and the output was twice as high as the output, indicating that the efficiency as a laser device is also excellent.

FIG. 3 shows the actual experimental result of the configuration of the present invention, which is KrF.
It is the measured value of the excimer laser. F ₂ is used as a gas.
22%, Kr is 4.4%, the rest is He gas, and the applied voltage at a total pressure of 1800mb
It is a graph of the output intensity I _out per pulse and the etalon load I _E when the pulse laser oscillation is performed by discharging under the condition of 28 KV.

Further, FIG. 9 shows measured values of the output intensity I _out and the etalon load I _E in the conventional configuration (FIG. 7) under the same conditions.

All the results show that the tendency almost agrees with the logical value, the output of the configuration of the present invention is more than double, and the etalon load is significantly overwhelmingly slightly superior in performance.

As described above, according to the present invention, it is possible to obtain a band-narrowing laser device which significantly reduces light energy passing through the wavelength selection element and exhibits excellent characteristics in efficiency.

Although the Fabry-Perot etalon is used as the wavelength selection element in the embodiments, the present invention can be applied to other wavelength selection elements. These configurations will be described.

First, a second embodiment of the present invention will be described with reference to FIG. In FIG. 4, reference numeral 20 is a grating, and the other parts are the same as in the first embodiment. Grating 20 whose wavelength is selected by light reflection as a wavelength selection element
Is installed in the resonator optical path formed by the total reflection mirrors 2 and 3, and the diffracted light of the grating 20 forms the resonator optical path. The functions of the other parts are the same as those in the first embodiment and will not be described.

Next, a third embodiment of the present invention will be described with reference to FIG. In FIG. 5, reference numeral 30 denotes a prism, and the other parts are the same as in the first embodiment. As a wavelength selection element, a prism 30 whose wavelength is selected by optical refraction is a total reflection mirror 2, 3
The resonator optical path is formed by the light refracted by the prism 30. The functions of the other parts are the same as those in the first embodiment and will not be described.

Above, as the wavelength selection element, the Fabry-Perot etalon,
Although the configuration using the grating and the prism has been explained, when using the Fabry-Perot etalon, this optical element makes two reflecting surfaces face each other, and the wavelength is selected by the interference effect between the two reflecting surfaces. It can be estimated that the reflective surface in which high energy is confined by multiple reflections is easily damaged. On the other hand, according to the method using the grating or the prism described in the second and third embodiments, the wavelength is selected by the light reflection or the light refraction, and therefore the damage of the wavelength selection element is caused by the above-mentioned Fabry-Perot. The threshold value is several times higher than when an etalon is used, and a laser output of 20 W or more can be obtained.

Next, a fourth embodiment of the present invention will be described with reference to FIG. FIG. 6A is a block diagram of a narrow band excimer laser device. In FIG. 6A, laser oscillation is performed in the ultraviolet region by an optical resonator including a discharge tube 1 using a rare gas and a halogen mixed gas as a laser medium and total reflection mirrors 2 and 3. A phase retarder mirror 40 and a polarization separator 5 are placed in the optical path of the resonator formed by the optical resonator, and the output light 7 amplified by the laser medium of the discharge tube 1 passes through the polarization separator 7 and becomes output light 7. Is output. The Fabry-Perot 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 light 7 passes, and only the wavelength in a specific narrow band is selectively oscillated by the optical resonator to cause a specific Only light with a narrow wavelength lases. In the fourth embodiment, 1 / used in the first embodiment
A phase retarder mirror is used instead of the 4-wavelength phase shifter. This part will be described in detail, and the other parts are omitted since they are the same as in the first embodiment.

The phase retarder mirror 40 is provided with a dielectric thin film layer on the surface of the reflecting mirror, and has a difference of 90 ° in the phase between the S polarized wave and the P polarized wave of the reflected light of the obliquely incident light and is a 1/4 wavelength phase. It operates as a resonator and is installed in the optical path of the resonator formed by the total reflection mirrors 2 and 3, and converts the plane of polarization. Due to the angle difference θ (see FIG. 6B) between the direction of the polarization planes separated by the polarization plane separator 5 and the direction created by the phase retarder mirror 40 and the total reflection mirror 3, the above-mentioned ratio R _pp differs, and the output light The binding rate of can be changed. The phase retarder mirror is a quarter-wave phase shifter suitable for a narrow band laser used as a light source for exposure because it has a large aperture that can be easily made, has a strong laser power, and has a small amount of unnecessary multiple reflected light. The wavelength selection element may use the grating or the prism described in the second and third embodiments.

The present invention has been described above with reference to the embodiment. However, the 1/4 wavelength phase shifter of the above embodiment is not limited to the Fresnel rhombus prism, 3
There are various types such as total internal reflection super-achromatic 1/4 wavelength plate, but for obtaining a large diameter beam for exposure, a 1/4 wavelength plate of fourth order or multiple order using a quartz plate is good, 1/4 wavelength phase The device does not have to be a 1/4 wavelength phase shifter as long as it can change the polarization component ratio.

In addition, there are various types of polarization separators such as a multilayer cube polarizer, a transparent plate with a Brewster angle, and a Wollaston prism as the polarization separator of the above-mentioned embodiment.However, in order to obtain a large-diameter beam for exposure, polarization of a dielectric multilayer film is used. Separation mirror is good.

Further, in the above embodiment, the wavelength selection element is provided between the polarization separator and the total reflection mirror, but other than this location, the output light path from the laser medium through which the output light, which is the strongest light, passes to the polarization separator is In the resonator optical path, the light intensity is weak and a wavelength selection element can be provided.

Further, in the above-mentioned embodiment, the 1/4 wavelength phase shifter and the polarization separator are provided in the resonator optical path on the opposite side of the laser medium, but they can be installed in the resonator optical path on the same side.

Further, the wavelength selection element used in the above embodiments may use a plurality of Fabry-Perot etalons, gratings, prisms, or the like, or may combine them. In addition, an element in which a wavelength selection element and a total reflection mirror are integrated, for example, the wavelength selectivity of reflected light of the grating is directly used by a grating with an Echelle grating or an Echelon grating, or one side of a prism is used as a total reflection mirror. Good, the 1/4 wavelength phase shifter and the total reflection mirror are integrated, and one side of the crystal phase plate is made into a total reflection mirror,
That is, the wavelength selection element, the total reflection mirror, the 1/4 wavelength phase shifter, the polarization separator, etc. may be reduced in the number of elements by using elements having a combination of these functions.

Further, the phase retarder mirror used in the above embodiment may be a phase retarder prism having a similar anti-slope surface having a phase retard function.

Further, the total reflection mirror used in the above embodiment does not need to have a reflectance of 100%, and may be any reflectance as long as it constitutes a resonator.

As described above, the present invention provides a wavelength selection element, a 1/4 wavelength phase shifter, and a polarization splitter in the resonator, and polarizes weak oscillation light oscillated in a narrow band with the 1/4 wavelength shifter. Is converted into a narrow band laser light which is amplified by a laser medium and amplified by a polarization separator to reduce the light energy passing through the wavelength selection element. It is possible to provide a narrow-band laser device that is optimal for an exposure light source without any decrease.

[Brief description of drawings]

FIG. 1 is a block diagram of a narrow band laser device according to the first embodiment of the present invention, and FIG. 2 is a diagram showing the output light intensity I _out and the Fabry-Perot etalon incident light intensity I _E with respect to the polarization ratio R _pp of the first embodiment. FIG. 3 is a characteristic diagram showing the calculation result, FIG. 3 is a characteristic diagram showing the relation between I _out and I _E with respect to the actually measured R _pp in the first embodiment, and FIG. 4 is a narrow diagram in the second embodiment of the present invention. FIG. 5 is a block diagram of a band-band laser device, FIG. 5 is a block diagram of a narrow-band laser device in a third embodiment of the present invention, and FIG. 6 is a block diagram of a narrow-band laser device in a fourth embodiment of the present invention. Is a configuration diagram of a conventional narrow band laser device, and FIG. 8 is I _{out with} respect to a semitransparent mirror reflectance R in the conventional narrow band laser device
And FIG. 9 is a characteristic diagram showing calculation results of I _E and I _E , and I _out with respect to R actually measured in the conventional narrow-band laser device, and
It is a characteristic view which shows the relationship of _IE . 1,101 …… Discharge tube, 2,3,102 …… Total reflection mirror, 4 …… 1/4 wavelength phase shifter, 5 …… Polarization separator, 6,20,30 …… Wavelength selector, 7,105 …… Output light.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Keiichiro Yamanaka 3-10-1 Higashisanda, Tama-ku, Kawasaki City, Kanagawa Prefecture Matsushita Giken Co., Ltd. (72) Takeo Miyata 3 Higashisanda, Tama-ku, Kawasaki City, Kanagawa Matsushita Giken Co., Ltd. (56) References JP-A-62-293791 (JP, A) JP-A-64-27773 (JP, A) JP-B-57-40671 (JP, B2)

Claims (7)

[Claims] 1. An optical resonance means including first and second reflectors, a laser medium portion provided in a resonance optical path of the optical resonance means, and a laser medium portion which is excited in a direction of the resonance optical path. Excitation means for generating light, wavelength selection means provided in the resonant optical path for selecting light of a predetermined wavelength component of the light, and provided in the resonant optical path for converting the polarization state of the light. Polarization conversion means and polarization separation means provided in the resonant optical path for separating a first polarization component of the light and a second polarization component of the light, the wavelength selection means, the wavelength selection means The wavelength selecting means is provided in the optical path other than between the laser medium and the polarization separating means, and selects one predetermined wavelength component light of the first polarization component and the second polarization component. The predetermined wavelength component light selected in Is converted into a polarized light component having the other polarization state of the first polarized light component and the second polarized light component, and then separated by the polarized light separating means and emitted as laser output light having a narrowed wavelength band. Narrow band laser device. 2. The band narrowing laser device according to claim 1, wherein at least some of the functions of the first and second reflectors, the wavelength selecting means, the polarization converting means, and the polarization separating means are combined with each other. 3. The band narrowing laser device according to claim 1, wherein the wavelength selecting means is at least one Fabry-Perot etalon, grating, or prism. 4. The band-narrowing laser device according to claim 1, wherein the polarization conversion means is a wave plate, a phase retarder mirror, or a phase retarder prism. 5. The band-narrowing laser device according to claim 1, wherein the polarization splitting means is a polarization splitting mirror. 6. The band-narrowing laser device according to claim 1, wherein the laser medium uses an excimer obtained by combining a rare gas and a halogen gas. 7. An exposure light source device using the narrow band laser device according to claim 1 as an exposure light source.