JPH03209887A - Narrow band laser device - Google Patents

Abstract

PURPOSE:To prevent a laser device of this design from varying in wavelength and deteriorating in output by a method wherein a wavelength selection element is provided into an optical path other than an output optical path which extends from a laser medium passing through a polarized light separator, and a wavelength phase shifter and a third reflective mirror are provided into an optical path other than a resonator optical path which branches off from a partial polarized light separator. CONSTITUTION:An optical resonator composed of a first reflective mirror 2 and a second reflective mirror 3, a partial polarized light separator 4 and a polarized light separator 5 provided into a resonator optical path, a wave length selection element provided into the resonator optical path other than an output optical path passing through the polarization light separator 5, and a wavelength phase shifter 7 and a third reflector mirror 8 provided into an optical light path other than the resonator optical path branching off from the partial polarized light separator 4 are provided. A part of light, which is specified in wavelength and formed in the optical resonator passing through the laser medium and the wavelength selection element 6, is extracted from the partial polarized light separator 4, converted in plane of polarization through a wavelength phase shifter 7, amplified again through the laser medium, and then outputted from the polarized light separator 5. By this setup, optical energy is made decrease to such an extent that an output light is divided by the amplification factor of the laser medium, and a wavelength selection element is sharply decreased in deformation and deterioration.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a narrowband laser device used as a light source for exposure light in ultrafine processing of semiconductor integrated circuits, etc.

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

An excimer laser uses a combination of a rare gas such as krypton or xenon and a halogen gas such as fluorine or chlorine as a laser medium to obtain an oscillation line with sufficient output for pattern exposure at several wavelengths between 353 nm and 193 nm. I can do it.

The gain bandwidth of these excimer lasers is as wide as about 1 nm, and when oscillated in combination with an optical resonator, the oscillation line has a bandwidth (full width at half maximum) of about 0.5 nm. When laser light 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 in the exposure optical system. However, in the ultraviolet region where the wavelength is 350 nm or less, the range of selection of optical materials for lenses used in the imaging optical system is limited, making it difficult to correct chromatic 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, it becomes possible to use an imaging optical system that does not correct chromatic aberration, which simplifies the optical system of the exposure device. Furthermore, it is possible to realize miniaturization and cost reduction of the entire exposure apparatus.

To make laser light with a wide bandwidth monochromatic, it can be passed through a wavelength selection filter with a narrow transmission band. but,
In this method, the laser output is significantly attenuated and cannot be used practically as an exposure light source. Therefore, a method is generally adopted in which a wavelength selection element is installed in a resonator to monochromate the output without attenuating it. As an example of this, the configuration described in Japanese Patent Application Laid-open No. 160287/1987 is known.

The configuration will be briefly described below. As shown in FIG. 8, a discharge tube 101 is installed within an optical resonator consisting of a total reflection mirror 102 and a semi-transmission mirror 103. The discharge tube 101 is filled with a medium gas containing rare gas and nitrogen gas, and generates laser oscillation by discharge excitation.

In the optical resonator, a 51\-7 wavelength selection element, ie, a van Riverot ethane 104, is installed.

An excimer laser device having such a configuration emits light 106 .of a specific wavelength selected by the Fabry-Perot etalon 104 .
Since only 107, 108, and 109 are amplified and oscillated, it is possible to obtain high output light 105 with a very narrow bandwidth.

Problems to be Solved by the Invention However, in the above-mentioned conventional narrowband laser device, the high-energy light that resides within the optical resonator passes through the wavelength selection element, leading to deformation and deterioration of the wavelength selection element. Variations in the selected wavelength and reduction in output occur, resulting in problems such as product defects when used as a light source for an exposure device.

The present invention solves the problems of the prior art, and aims to provide a narrowband laser device that is free from wavelength fluctuations and output drops due to deformation and deterioration of wavelength selection elements.

A means to solve problems The technical solution of the present invention is 6 bases to achieve the above object.

The means includes an optical resonator comprising at least a laser medium, first and second reflecting mirrors forming a resonator optical path passing through the laser medium, and a partial polarization separator and a polarization separator provided in the resonator optical path. and a wavelength selection element provided in the resonator optical path other than the output optical path through which output light is output from the laser medium through the polarization separator, and an optical path other than the resonator optical path separated from the partial polarization separator. It is equipped with a wavelength phase shifter and a third reflecting mirror provided therein.

The reflecting mirror, wavelength phase shifter, partial polarization separator, polarization separator, and wavelength selection element can combine at least some of the functions of these elements. Further, the wavelength phaser can be selected from a first-order or multiple-order wave plate, a phase retarder mirror, or a phenyl retarder prism. , a dielectric multilayer polarization splitting mirror can be used as the partial polarization splitter.

In addition, the above laser medium is a rare gas.

Excimer combined with gas can be used.

Further, the partial polarization separator can be constructed by combining a polarization separator and a semi-transparent mirror, or can be constructed by forming a polarization separation film and a semi-transmission film on both sides of a transparent substrate.

According to the present invention, a part of light of a specific wavelength plane created by an optical resonator that passes through a laser medium and passes through a wavelength selection element is extracted by a partial polarization splitter, and the polarization plane is changed by a wavelength phase shifter. After being converted and amplified again by a laser medium, it is output as output light by a polarization splitter. 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, and deformation and deterioration of the wavelength selection element are significantly reduced.

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings.

First, a first embodiment of the present invention will be described.

FIG. 1 is a configuration diagram showing a band narrowing laser device in a first embodiment of the present invention. As shown in Fig. 1, a discharge tube '1 whose laser medium is a mixture of rare gas and halogen gas.
and the first -J, - which creates a resonator optical path that penetrates the laser medium.
The laser oscillates in the ultraviolet region by an optical resonator consisting of the second total reflection mirror 2 and the second total reflection mirror 3. In the resonator optical path created by the optical resonator, a partial polarization separator 4 is installed between the discharge tube 1 and the first total reflection mirror 2, and a polarization separator 4 is installed between the discharge tube 1 and the second total reflection mirror 3. A separator 5 is installed. Partial polarization separator 4
Although it separates the propagation directions of light with different polarizations, it does not completely separate them, and only partially transmits or reflects one polarization component. For example, it has the function of reflecting 100% of S-polarized light, but transmitting P-polarized light by a ratio T and reflecting a ratio (L−T). Although this ratio T will be explained in detail later, it is an exposure value of about 0.05 to 0.8. The polarization separator 5 separates the propagation directions of light of different polarization, for example, 10 for P-polarized light.
It has the function of transmitting 0% of light and reflecting 100% of S-polarized light. The light oscillated by discharge excitation in the discharge tube 1 passes through the polarization separator 5 and is output as output light 19 in the resonator optical path other than the output optical path, that is,
A Fabry-Perot etalloy 6, which is a wavelength selection element, is installed between the partial polarization separator 4 and the first total reflection mirror 2.
A quarter wavelength phase shifter 7 for converting polarization and a third total reflection mirror 8 are installed in an optical path other than the resonator optical path separated from the partial polarization separator 4.

The operation of the above configuration will be described below.

A portion of the light oscillated by the discharge tube 1 is P-polarized by the partial polarization separator 4 and transmitted therethrough. Only light of a specific wavelength is selected from this light 9 by a Fabry-Perot etalon 6, which is a wavelength selection element, and is reflected by the first total reflection mirror 2 to return as light 10. Since the light 10 is P-polarized as described above, a portion thereof passes through the partial polarization separator 4 to become light 11, which is amplified by the discharge tube 1 and becomes light 12. Since the light 12 is P-polarized light, it passes through the polarization separator 5, is reflected by the second total reflection mirror 3, passes through the polarization separator 5 again, and becomes light 1310 -7. The light 13 is amplified by the discharge tube 1 and becomes the light 14,
A part of it passes through the partial polarization separator 4 and becomes p1 light 9, which continues to oscillate in the same manner as above. The other part of the light 14 is reflected by the partial polarization separator 4, becomes light 15, passes through a 1/4 wavelength phaser, is reflected by the third total reflection mirror 8, and becomes 1/4 light again.
The wavelength phase shifter 7 is converted to aD, and the p polarized light is converted to S polarized light and becomes light 1.
It becomes 6.

Generally, when light passes through the 1/4 wavelength phase shifter 7 twice, the 1/4 wavelength phase shifter 7 has the same function as a 1/2 wavelength phase shifter, and the optical axis direction of the phase shifter 7 is used to polarize the incident light. Wave front and 45°
It is known that when set at an angle, all incident P-polarized light is converted to S-polarized light. The S-polarized light 16 is 100% reflected by the partial polarization separator 4 to become S-polarized light 17, amplified by the discharge tube 1 to become S-polarized light 18, and 100% reflected by the polarization separator 5. It is output as S-polarized output light 19.

With such a configuration, compared to the intensity of the light 9 incident on the Fabry-Perot etalon 6, the output light 19 is extracted after being amplified by the discharge tube 1, so that the intensity of the laser medium is The amplification factor is large, and deformation and deterioration of the Fabry-Perot etalon 6, which is a wavelength selection element, is relatively significantly reduced.

From the above description, it is clear that according to the configuration of this embodiment, it is possible to significantly reduce the optical load on the Fabry-Perot etalon 6, which is a wavelength selection element.
Furthermore, it will be explained quantitatively using mathematical formulas.

In Figure 1, the intensity of lights 10 to 18 is rlO to 118
shall be. Since the light 9 is particularly a load light for the Fabry-Perot etalon 6, its intensity is designated as IE, and since the light 19 is an output light, its intensity is designated as I out.

However, lights 9 to 15 are P-polarized lights, and lights 16 to 19
is S-polarized light. As described above, the partial polarization separator 4 transmits the P-polarized light by a ratio T, reflects the ratio (1-T'), and reflects 100% of the S-polarized light. First,
The second and third total reflection mirrors 2.3.8 have 100% reflectance, and the Fabry-Perot etalon 6 has a loss of AE ('
t-AE) is the transmittance. In addition, the portion from which υ light exits from the discharge tube 1 usually has a window, so light loss occurs in this portion as well, and this loss is designated as A. That is, (1-A
) is the transmittance of the window. Assuming that the 1/4 wavelength phase shifter 7 converts P-polarized light into 100% S-polarized light, the polarization separator 5
reflects 100% of S-polarized light and transmits 100% of P-polarized light. Go to the amplification factor per unit of minute light in the laser medium.
Let the length of the discharge tube 1 be L.

Here, as an analysis method that is considered to be well suited when analyzing a laser with a high amplification factor such as an excimer laser, the method described by Rigrod, Saturation Effect in High-Cain Laser, Journal of Applied Physics, Vol. 36, No. 8 .2487-2490 pages,
1965, (W, W, RI GROD, ”
5aturationEffects in High
-Gain La5ers, Journalof
Applied Physics, Vol. 36
．． No8゜P2487~P2490. August
There is a method described in (1965).

Next, the analysis will be explained by quoting the formula in the above document131,
-7.

If the laser analysis formula in the above literature is adapted to this example, the following formula is obtained ((force, (Japanese formula used) in the literature.

Here, β4, γ1, γ2 are the values expressed in the literature,
The configuration of this embodiment is given by the following equation.

β4 = I out/((17A) (1-R) I
s ) ++++++++・++(21γ1=β1/β
4-(1-A)2R・・・・・・(5)γ2-β
3/β2-(1-A)2T2(1-AE)2/R...
(4) Here, the constant R is the ratio of the P polarization component intensity to the line intensity of light propagating leftward in the discharge tube 1, and is given by the following equation. Moreover, Is is the saturation light intensity.

・・・・・・・・・・・・・・・(5) Above (11~(
From equation 5), the output light intensity I out is determined by the following equation.

14 pages.

・・・・・・・・・・・・(6) Etalon load light IE is given by the following equation.

IE=T114=T(I A)β2Is ・
・・・・・・・・・・・・(7) Adding the above (1) to the above (71 formula)
), (2), (5), (4), and (5), we get
The etalon load light IE is determined by the following equation.

FIG. 2 specifically shows the total results of equations (6) and (8) above. These are the results of calculating the output light intensity I out/Is normalized by the saturated light intensity Is with respect to the transmittance T of P-polarized light of the partial polarization separator 4 and the etalon load light intensity IE/Is.

Next, for comparison, a similar formula will be created and studied for the configuration of the conventional example shown in FIG. Assuming that the reflectance of the semi-transmissive mirror 103 is R and the other conditions are the same as in the embodiment of the present invention shown in FIG. 1, the following equation can be obtained by the above method. However, in the conventional configuration shown in FIG. 8, the light is not polarized, so the light intensity at each part is 15 l<-.

The degree is 105, 106, 107, 108, 109 for light 105.

β2=I+o6/((1-A)Is) γ1=(I A)211o9/11osγ2-(1-
A) 2I+o7/I 1a61107=RI 106 1109= (1-AE)21108 Iout=11os-(I R)
1' and equations (9) to (141), the following equation is obtained.

・・・・・・・・・・・・(9 ・・・・・・・・・・・・(10) ・・・・・・・・・・・・(11) ・・・・・・・・・・・・(12) ・・・・・・・・・・・・(15) ・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・・(15) Next, the load light intensity of the etalon can be determined from the above-mentioned literature as shown in the following equation.

β2/β4-675 units (11') Next, the relationship between the load light intensity IE of the etalon and β4 is expressed by the following equation.

I” = 11oa = (1-A)β4 Is
−・−・−・−・(%1above(1bi, (9)～(
From formula 161, engineering E can be found as the following formula.

Figure 9 specifically shows the calculation results of formulas (15) and (171) above. Output light intensity I out / I s normalized by saturation light intensity Is with respect to transmittance T of P-polarized light and etalon load light intensity I This is the result of calculating E/Is.

Here, when comparing the embodiment of the present invention shown in FIG. 2 and the conventional example shown in FIG. 9, it is clear that the embodiment of the present invention can obtain a smaller value for IE at the same -Iout. . That is, in FIG. 2, I out/Is =
0.3, IE/l5=0.004, but in Fig. 8, there is a difference of more than 100 times from IE/Is-〇, 41, which is a large incidence of Fabry-Perot etalon 6. Light intensity is being reduced. Also, as a noteworthy feature, in the conventional example shown in FIG. 9, the value of R is 0.15,
The maximum output value is 17-, -L value Iout/Is = 0.31, and only a low output can be obtained, but in the embodiment of the present invention, T = 0.58,
The maximum value I out/I s = 0.83 was obtained, and the output was 2.7 times as high, indicating that the laser device was also excellent in effect.

As described above, in this embodiment, it is possible to obtain a narrowband laser device that significantly reduces the optical energy passing through the Fabry-Perot etalon 6, which is a wavelength selection element, and exhibits excellent characteristics in terms of efficiency.

Although the above embodiment has been described using the Fabry-Perot etalon 6 as the wavelength selection element, the present invention can also be applied using other wavelength selection elements. These configurations will be explained below.

First, while referring to the configuration diagram shown in FIG.
An example will be described. In FIG. 3, 20 is a grating, and other parts are the same as in the first embodiment. A grating 20 that selects a wavelength by light reflection as a wavelength selection element is connected to first and second total reflection mirrors 2.
3 creates a resonance of 18 bases.

It is installed in four optical paths, and a resonator optical path is formed by the diffracted light of the grating 20. Since the functions of other parts are the same as those of the first embodiment, their explanation will be omitted.

Next, referring to the configuration diagram shown in FIG.
An example will be described. In FIG. 4, 30 is a prism, and the 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 first and second total reflection mirrors 2.3, and the refracted light of the prism 30 forms the resonator optical path. Since the functions of other parts are the same as those of the first embodiment, their explanation will be omitted.

Next, a fourth embodiment of the present invention using another wavelength phase shifter and a third reflecting mirror will be described.

5 and 6 show a fourth embodiment of the present invention, FIG. 5 (at is a configuration diagram, FIG. 5 (bl is a bottom view of the phase retarder prism, and FIG. This is a detailed configuration diagram of the fifth factor (at, fbl
, 40 is a phase retarderp 19.\7 rhythm, and the other parts are the same as in the first embodiment. As shown in FIG. 6, the phase retarder prism 40 has the function of converting the incident P-polarized light into 15'eS-polarized light and returning it as light 16 in the opposite direction. It has a function of integrating the functions of the long plate 7 and the third total reflection mirror 8. This phase leader prism 40 is installed at an angle of 45 degrees, as shown in FIGS.
It is converted into 0% S polarized light and emitted as light 16 in the opposite direction. Generally, by changing the angle of inclination from 0 to 45 degrees, the ratio of conversion to S-polarized light can be freely set from 0 to 100%. The phase retarder prism 40 is made of a high transmittance material such as synthetic quartz CaF2, and as shown in FIG. The surface is tilted approximately 2 degrees so that the direct reflection of the light (light 15) from the surface does not mix with the emitted light (light 16). For this reason, unlike a normal 45° prism, 45°, 47°, and 88° prisms are used. The surface on which the light 15 enters the prism and is reflected is formed by a 90° phase reflecting surface 42. This surface 42 is a dielectric multilayer film that has a 90° phase difference in the reflection of P-polarized light waves and S-polarized light waves by designing the structure and thickness of the dielectric film, and is used as a 1/4 wavelength plate and an optical has equivalent functionality. 90
The light reflected by the phase reflection surface 42 is specularly reflected by the total reflection surface 41, and becomes outgoing light (light 16) on a reverse course. The total reflection surface 41 can also be easily formed using a dielectric multilayer film.

In this way, this embodiment has a difference that is 1/1 of that of the first embodiment.
Since the phase retarder prism 40 having the function of integrating the four-wavelength phase shifter 7 and the third total reflection@8 is used,
The configuration can be simplified and adjustments can be made easily. Note that the Fabry-Perot etalon 6 may be used as the wavelength selection element, and the grating 20 or prism 30 used in the second or third embodiment may also be used.

In each of the above embodiments, the configuration was explained in which P-polarized light is oscillated, converted to S-polarized light, amplified, and output. It may be configured to oscillate with light, convert it into P-polarized light, amplify it, and output it. In this case, the ratio of the reflectance transmittance of the partial polarization separator 4 and polarization separator 5 to that of S-polarized light may be used. It is sufficient to use P-polarized light and reverse the above embodiment, and it is possible to select a polarized light that is easy to implement.

The present invention has been described above using examples, but there are various other types of quarter-wave phase shifter 7 in the above-mentioned embodiments, such as a Fresnel rhombic prism, a three-time total reflection super achromatic quarter-wave plate, etc. In order to obtain a large diameter beam for exposure, a first-order or multiple-order quarter-wave plate using MgF2 is better than a quartz plate, and the quarter-wave phase shifter 7 is a phase shifter with exactly 174 wavelengths. It is not necessary that the polarized light be used as long as it can change the component ratio of polarized light.

Further, the partial polarization separator 4 of the above embodiment may be constructed by combining a complete polarization separator 50 and an anti-transmitting mirror 51 to have the same function as shown in FIG. 7(a), or as shown in FIG. 7(b). As shown in the figure, one side of a transparent substrate made of quartz or CaF2 is formed with a dielectric multilayer polarization separation film 22. A partial polarization separator may also be configured.

There are various types of polarization separator 5 in the above embodiment, such as a multilayer Cupp polarizer, a Brewster angle transparent plate, and a Wollaston prism. The polarization splitting mirror is good.

Further, in the above embodiment, the wavelength selection element is used as the partial polarization separator 4.
and the first total reflection mirror 2, however, the light can be transmitted in any optical path of the resonator other than the output optical path from the laser medium to the polarization splitter 5 through which the output light, which is the strongest light, passes. Since the intensity is weak, a wavelength selection element can be provided.

Furthermore, as the wavelength selection element, a plurality of the Fabry-Perot etalons 6, gratings 20, prisms 30, etc. used in the above embodiments may be used, or they may be combined.

In addition, elements that integrate a wavelength selection element and a total reflection mirror, such as a grating that uses the wavelength selectivity of the reflected light directly from the Echare grating or Echelon grating, or a grating that uses one side of a prism for total reflection. It may be used as a cast material, or by integrating the 1/4 wavelength phase shifter and the total reflection mirror, one side of the MgF2 or quartz phase plate may be made into a total reflection mirror, that is, the wavelength selection element, the total reflection mirror, and the 1/4 wavelength phase plate may be used as a total reflection mirror. For the four-wavelength phase shifter, polarization separator, etc., the number of elements may be reduced by using elements that combine these functions.

Furthermore, the phase retarder prism 40 used in the above embodiments may be a phase retarder mirror whose reflective surface has a phase retard function.

Further, the total reflection mirror 2.3 used in the above embodiment does not need to have a reflectance of 100%, but may have a reflectance that constitutes a resonator.

Effects of the Invention As described above, according to the present invention, only a portion of the light of a specific wavelength plane created by the optical resonator that passes through the laser medium and passes through the wavelength selection element is extracted by the partial polarization splitter, and the wavelength The plane of polarization is converted by a phase shifter, amplified again by a laser medium, and then output as output light by a polarization splitter. 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, and deformation and deterioration of the wavelength selection element are significantly reduced. It is possible to provide a narrowband laser device that does not deteriorate and is optimal for an exposure light source.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram showing a band narrowing laser device in the first embodiment of the present invention, and FIG. 2 shows the relationship between the output light intensity and the Fabry-Perot etalon incident light intensity with respect to the transmittance of the partial polarization separator of the above embodiment. A characteristic diagram showing calculation results, FIG. 3 is a configuration diagram showing a band narrowing laser device in a second embodiment of the present invention, and FIG. 4 shows a band narrowing laser device in a third embodiment of the present invention. Please refer to the configuration diagram, Figure 5.
Or a detailed configuration diagram of the phase retarder prism, FIG. 7 (al, (
b125B-7 is a block diagram showing another example of a partial polarization separator used in the present invention, FIG. 8 is a block diagram showing a conventional narrowband laser device, and the ninth factor is the above conventional narrowband laser device. FIG. 2 is a characteristic diagram showing calculation results of output light intensity and etalon incident intensity with respect to semi-transmissive mirror reflectance in the device. DESCRIPTION OF SYMBOLS 1... Discharge tube, 2, 3... Total reflection mirror, 4... Partial polarization separator, 5... Polarization separator, 6... Fabry-Perot etalon (wavelength selection element), 7...・1/4 wavelength phase shifter, 8... Total reflection mirror, 19... Output light, 20...
- Grating (wavelength selection element), 30... prism (wavelength selection element).

Claims (1)

[Scope of Claims] (1) An optical resonator comprising at least a laser medium, first and second reflecting mirrors forming a resonator optical path passing through the laser medium, and a partially polarized light provided in the resonator optical path. a separator, a polarization separator, a wavelength selection element provided in the optical path of the resonator other than the output optical path through which the output light passes from the laser medium through the polarization separator, and is separated from the partial polarization separator. A band narrowing laser device including a wavelength phase shifter and a third reflecting mirror provided in an optical path other than a resonator optical path. (2) The band narrowing laser device according to claim 1, wherein the reflecting mirror, the wavelength phase shifter, the partial polarization separator, the polarization separator, and the wavelength selection element have at least a part of their functions combined. (5) Claim 1 or 2 in which the wavelength phaser is selected from a first order or multiple order wave plate, a phase retarder mirror, or a phase retarder prism.
The band-narrowing laser device described. (4) 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. (5) The band narrowing laser device according to claim 1 or 2, wherein the partial 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 rare gas and halogen gas. (7) The band narrowing laser device according to claim 1 or 2, wherein the partial polarization separator is constituted by a combination of a polarization separator and a semi-transmissive mirror. (8) The band narrowing laser device according to claim 1 or 2, wherein the partial polarization separator comprises a transparent substrate, and a polarization separation film and a semi-transparent film formed on both sides of the transparent substrate.