JP3315494B2 - Reflective film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflection film for a laser, which is suitable for laser light in a wavelength range from an ultraviolet region to an infrared region.

[0002]

2. Description of the Related Art In general, a reflection film is composed of a repeating multilayer film in which a λ / 4 film made of a high-refractive-index material and a λ / 4 film made of a low-refractive-index material are alternately laminated. In general, the higher the refractive index, the easier it is to absorb the energy of the illumination light, and therefore, particularly in the case of a reflective film that reflects high-output laser light or the like, the laser proof strength may be insufficient.

Therefore, a high refractive index material having a relatively low refractive index and therefore a small absorption is selected as a material for the high refractive index layer on the side where the laser beam is incident, and a material for the high refractive index layer closer to the substrate is selected. There has been a contrivance such as improving the laser resistance of the reflective film by selecting a high refractive index material having a high refractive index and a large absorption (see Japanese Patent Application Laid-Open No. 2-204702). this is,
Focusing on the point where the laser beam is incident, the intensity of the electric field energy generated inside the reflective film by the standing wave is high, and the absorption of this part is particularly large, so that the laser proof strength is significantly reduced. The laser proof strength is improved by using a material having low absorption as the material of the high refractive index layer.

In addition, an equivalent film is used for each high-refractive-index layer, and the optical film thickness of a layer of a high-refractive-index material having a large absorption among a plurality of layers constituting the equivalent film is designed to be particularly small so that each high-refractive-index layer is designed to have a small optical thickness. It has also been proposed to reduce the absorption of the refractive index layer, thereby improving the laser resistance of the reflective film.

In general, a repeated multilayer film in which high refractive index layers and low refractive index layers are alternately laminated generally has a smaller λ / 4 film as the difference in refractive index between the high refractive index layer and the low refractive index layer is larger. With the number of layers, a high reflectance can be obtained. Also, as mentioned above,
Higher refractive index materials tend to absorb more as the refractive index is higher, and reduce the laser proof strength of the reflective film.

[0006]

However, according to the above-mentioned prior art, as described above, a high refractive index material having relatively low absorption is selected as a material for the high refractive index layer on the laser light incident side. When the proof stress is improved, the difference between the refractive indices of the high refractive index layer and the low refractive index layer becomes small, so that the reflectance of the reflective film is significantly reduced. Accordingly, the number of λ / 4 films constituting the reflection film must be increased in order to prevent a decrease in reflectance, and as a result, the manufacturing cost increases. Also, when an equivalent film in which the optical film thickness of the layer of the high refractive index material is particularly small is used, the total number of layers similarly increases, leading to an increase in manufacturing cost.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and it has been proposed to reduce the absorption without significantly lowering the reflectivity and without significantly increasing the number of layers of the λ / 4 film. It is an object of the present invention to provide a reflective film capable of greatly improving laser proof stress.

[0008]

In order to achieve the above-mentioned object, the reflection film of the present invention is formed by alternately forming at least one film on the surface of a substrate.
It has a multilayer film composed of a high-refractive-index layer and a low-refractive-index layer laminated one by one, and at least one of the high-refractive-index layers is configured such that the refractive index decreases toward the surface of the substrate. Characterized by a heterogeneous film.

It is preferable that the heterogeneous film is a laminated film formed by laminating at least two layers made of materials having different refractive indexes.

Further , a plurality of high refractive index layers are formed on the surface of the substrate.
Having a multilayer film in which a number of low refractive index layers are alternately laminated, and two
The above high refractive index layers each have at least two different refractive indexes.
Is a mixed film composed of two materials.
A material farther from the surface of the substrate has a lower refractive index
Is large, and on the side close to the surface of the substrate.
One high-refractive-index layer is more flexible than the far-side high-refractive-index layer.
Reflection characterized by having a low folding ratio
It may be a membrane .

[0011]

The intensity of the electric field energy generated inside the reflection film by the illumination light incident on the reflection film increases in each high refractive index layer as it approaches the surface of the substrate, and the energy of the illumination light absorbed by the reflection film increases. The larger the intensity of the electric field energy is, the larger the intensity is. Further, the energy of the illumination light absorbed by the reflection film increases as the refractive index of the high refractive index layer increases. If at least one of the high refractive index layers is a non-homogeneous film having a lower refractive index toward the surface of the substrate, the absorption in the portion where the intensity of the electric field energy is high is reduced. Can be greatly reduced. On the other hand, since the portion of the heterogeneous film far from the surface of the substrate has a high refractive index, there is no possibility that the refractive index of the entire heterogeneous film is significantly reduced and the reflectivity of the reflection film is significantly impaired.

[0012]

An embodiment of the present invention will be described.

FIG. 1 shows a reflection film E-1 of the first embodiment, which is formed on a surface 1a of a synthetic quartz substrate 1 by a high refractive index layer 2a of a λ / 4 film of a high refractive index material. And a low-refractive-index layer 2b, which is a λ / 4 film of a low-refractive-index material, is alternately laminated in six layers, and a high-refractive-index layer 2a, which is a λ / 4 film of a high-refractive-index material, is laminated as a final layer. It is provided with a repetitive multilayer film 2 which is a multilayer film composed of a total of 13 layers, wherein SiO ₂ is used as the low refractive index material and each high refractive index layer 2
a is H, which is a high refractive index material having a relatively high refractive index.
fO consists of _two of lambda / 8 film 3 and Al ₂ O ₃ of lambda / 8 film 4 is relatively low refractive index and high refractive index material, the HfO ₂ of lambda / 8 film 3 on the incident side of each of the laser beam It is a laminated film that has been laminated. The reflection film E-1 of this embodiment is designed as a reflection film for a KrF laser, and has a design wavelength λ of 248 nm.
It is. In addition, each high refractive index layer 2a and each low refractive index layer 2
The film forming conditions b are all a film forming temperature of 200 ° C. and a film forming speed of 2
Å5Å / s, oxygen partial pressure was 1-2 × 10 ^-4 torr.

FIG. 2 is a graph in which the intensity of the electric field energy generated by the standing wave in the film when the reflective film E-1 is irradiated with the KrF laser is calculated and plotted from the laser light incident side. As can be seen from this figure, the intensity of the electric field energy due to the standing wave has a tendency to rise from the laser beam incident side in each high refractive index layer 2a and attenuate in the low refractive index layer 2b. In 2a, the intensity of the electric field energy is smaller toward the incident side of the laser beam. As described above, each high-refractive-index layer 2a has a relatively high refractive index on the laser beam incident side, and is thus a high-refractive-index material, HfO ₂ , which has a large absorption.
However, since the intensity of the electric field energy has just risen and is relatively low in this portion, there is no possibility that the absorption is significantly increased. The portion of each high refractive index layer 2a where the intensity of the electric field energy is remarkably large has a relatively low refractive index and therefore a small absorption of Al ₂ O ₃ λ / 8 film 4
, The absorption of each high refractive index layer 2a can be greatly reduced as compared with the case where the entire high refractive index layer 2a is made of HfO ₂ having a high refractive index and therefore high absorption. On the other hand, the high refractive index layer 2a is therefore half of the optical thickness is made of a high refractive index HfO _2, entirely made of high refractive index material such as relatively low refractive index Al ₂ O ₃ In comparison with the case where the number of layers is the same λ / 4 film, it is possible to obtain a reflection film having much better reflection characteristics.

When the laser proof stress of the reflection film E-1 of this embodiment was measured by a known method, it was 4 J / cm ₂ (laser wavelength: 248 nm, pulse width: 15 ns).
The spectral characteristics of the reflectance are as shown in FIG.
The reflectivity at 8 nm was 96.5%, and the bandwidth at a reflectivity of 90% or more was 43 nm. In other words, the reflection film E-1 of this embodiment is a laser reflection film having extremely excellent reflection characteristics and excellent laser proof strength.

FIG. 4 shows a modified example E-2 of the present embodiment, in which a high refractive index layer 21a located closest to the laser beam incident side is formed of a λ / 20 film 31 made of HfO ₂ , Al
A high refractive index layer 21a located on the laser beam incident side is formed of a λ / 5 film 41 made of ₂ O _3.
2 of λ / 14 film 32 and 5λ / 14 of Al ₂ O ₃
Then, the high refractive index layer 21a located on the laser beam incident side is formed of a HfO ₂ λ / 8
The film 33 is composed of a λ / 8 film 43 made of Al ₂ O ₃ , and the remaining high refractive index layer 21a is composed of λ / 4 film made of HfO _2.
It is composed of a film.

That is, as can be understood from FIG. 2, the intensity of the electric field energy generated inside the reflection film is concentrated on the three high refractive index layers located closest to the laser beam incident side.
Each of these three high-refractive-index layers 21a has only a portion where the intensity of the electric field energy is the highest, and A
The laser proof strength is improved by using l ₂ O ₃ , and the device is devised so that the reflection characteristics can be reduced as little as possible. The laser proof stress of this modification E-2 was 5 J / cm ² (laser wavelength 248 nm, pulse width 15 ns), and the intensity of the electric field energy generated inside was calculated, as shown in FIG. ,
The spectral characteristics of the reflectance were as shown in FIG.

As can be seen from FIG. 6, the reflectance at the design wavelength is 97.5%, and the bandwidth of the reflectance of 90% or more is 51%.
nm, which indicates that the laser resistance and the reflection characteristics are superior to those of the reflection film E-1 of the first embodiment. This is probably because three materials of high refractive index layer other than the high refractive index layer positioned on the incident side of the laser light is HfO ₂ is a high-refractive-index material having a refractive index higher than that of Al ₂ O _3.

For comparison, a high refractive index layer, which is a λ / 4 film of HfO ₂ , and a SiO ₂ film were formed on the same substrate as in the first embodiment.
6 low-refractive-index layers, which are λ / 4 films, are alternately laminated,
As a final layer, a high-reflection film composed of a total of 13 repetitive multilayer films formed by laminating a high-refractive-index layer, which is a λ / 4 film of HfO ₂ , was used as a sample A, and the same as in the first embodiment. A high refractive index layer which is a λ / 4 film of Al ₂ O _{3 on} a substrate and Si
A high-reflection film composed of a total of 29 repetitive multilayer films in which 14 low-refractive-index layers, which are λ / 4 films of O ₂ , are alternately laminated, and a λ / 4 film of Al ₂ O ₃ is laminated as a final layer. This was made sample B. The laser proof stress of Samples A and B was measured to be 1 J / cm ² , 5 J / cm ² ,
The spectral characteristics of the reflectance are as shown in FIGS. 7 and 8, and the reflectance at the design wavelength is 97.5% and 94.%, respectively. %,
The bandwidths with a reflectance of 90% or more are 68 nm and 15 nm, respectively.
nm.

Table 1 shows the film structure and laser proof stress of each of the samples A to D, with the reflection film E-1 of this embodiment being Sample C and the modification E-2 being Sample D.

[0021]

[Table 1] FIG. 9 shows a reflection film E-3 of the second embodiment, which is formed on a surface 51a of a synthetic quartz substrate 51 by a high refractive index layer 52a of a λ / 4 film of a high refractive index material and a low refractive index layer. Rate material λ
A low-refractive-index layer 52b, which is a / 4 film, is alternately laminated by four layers, and a high-refractive-index layer 52a, which is a λ / 4 film of a high-refractive-index material, is laminated as a final layer. And a high refractive index layer 52a is formed by using SiO ₂ as a low refractive index material.
Are Ti, which are high refractive index materials each having a relatively high refractive index.
O ₂ and ZrO ₂ is a relatively low refractive index and high refractive index material
Is a mixed film composed of a mixture of

The reflection film E-3 of this embodiment is designed as a reflection film for a fundamental wave of an Nd-YAG laser, and has a design wavelength λ of 1064 nm. Further, each mixed film is set so that the mixing ratio of TiO ₂ on the laser light incident side is the lowest, and becomes higher as approaching the surface 51 a of the substrate 51. The mixing ratio of ZrO ₂ of the high refractive index layer 52a closest to the surface 51a of the substrate 51 is set to zero, and the high refractive index layer 52a as the final layer is formed only of ZrO ₂ .

The film forming conditions for each layer are as follows: a film forming temperature of 200 ° C., an oxygen partial pressure of 1 to 2 × 10 ⁻⁴ torr, a film forming rate of the low refractive index layer 52 b made of SiO _{2 is 2} to 3 ° / s, The high-refractive-index layer 52a, which is a mixed film, is formed by a two-source vapor deposition method. In the high-refractive-index layer 52a closest to the substrate, a TiO ₂ film forming rate of 3 ° / s and a ZrO ₂ film are formed. The speed is controlled to 0 ° / s, the film forming speed of TiO ₂ is gradually reduced and the film forming speed of ZrO ₂ is increased, and the film forming speed of TiO ₂ is set to 0 ° / s and ZrO 2
The film forming speed of No. ₂ was controlled at 3 ° / s.

The intensity of the electric field energy of the reflection film E-3 of this embodiment was calculated as shown in FIG. 10, and the laser proof stress was measured to be 8 J / cm ² (laser wavelength 1).
064 nm and a pulse width of 1 ns). The results of measuring the spectral characteristics of the reflectance are as shown in FIG. 11, and the reflectance at a wavelength of 1064 nm is 95.5% and the reflectance is 9
The band width of 0% or more is 122 nm, which indicates that the reflection film is extremely excellent in both laser proof stress and reflection characteristics.

For the sake of comparison, T
iO high refractive index layer which is a lambda / 4 film ₂ and SiO ₂ of lambda / 4
Samples A and A were obtained by laminating four layers of low refractive index layers alternately and laminating a λ / 4 film of TiO ₂ as a final layer.
Further, a high refractive index layer as a λ / 4 film of ZrO ₂ and a low refractive index layer as a λ / 4 film of SiO ₂ are alternately laminated on the same substrate as the present embodiment, and the final layer is ZrO 2. Λ / 4 of ₂
Sample B was obtained by laminating the films. Samples A and B
The laser proof stress was measured to be ² J / cm ² ,
The spectral characteristics of the reflectance at 10 J / cm ² and the reflectance are as shown in FIGS. 12 and 13. The reflectance at the design wavelength is 95.5% and 83%, and the bandwidths with the reflectance of 90% or more are 228 nm and 0 nm, respectively. Met. Table 2 shows the film configuration and the laser proof stress of each of the samples A to C using the reflection film E-3 of this example as the sample C.

[0026]

[Table 2]

[0027]

Since the present invention is configured as described above, the following effects can be obtained. The absorption is reduced without lowering the reflectivity of the reflective film, and therefore, λ / 4
The laser proof stress can be greatly improved without significantly increasing the number of layers of the film. As a result, an inexpensive reflective film having high reflectivity and laser proof strength can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a first embodiment.

FIG. 2 is a graph showing a distribution of electric field energy intensity according to the first embodiment.

FIG. 3 is a graph showing the spectral characteristics of the reflectance of the first embodiment.

FIG. 4 is a schematic sectional view showing a modification of the first embodiment.

FIG. 5 is a graph showing a distribution of electric field energy intensity of the reflection film of FIG. 4;

FIG. 6 is a graph showing the spectral characteristics of the reflectance of the reflection film of FIG. 4;

FIG. 7 is a graph showing a spectral characteristic of a reflectance of a comparative example of the first embodiment.

FIG. 8 is a graph showing spectral characteristics of reflectance of another comparative example of the first embodiment.

FIG. 9 is a schematic sectional view showing a second embodiment.

FIG. 10 is a graph showing the distribution of the intensity of the electric field energy according to the second embodiment.

FIG. 11 is a graph showing the spectral characteristics of the reflectance of the second embodiment.

FIG. 12 is a graph showing the spectral characteristics of the reflectance of a comparative example of the second embodiment.

FIG. 13 is a graph showing the spectral characteristics of the reflectance of another comparative example of the second embodiment.

[Explanation of symbols]

Reference Signs List 1,51 Substrate 2,52 Repeated multilayer film 2a, 21a, 52a High refractive index layer 2b, 52b Low refractive index layer 3,33 λ / 8 film of HfO ₂ 4,43 λ / 8 film of Al ₂ O ₃ 31 HfO ₂ λ / 20 film 41 λ / 5 film of Al ₂ O ₃ 32 λ / 14 film of HfO ₂ 42 5λ / 28 film of Al ₂ O ₃

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-204702 (JP, A) JP-A-2-192189 (JP, A) JP-A-52-43443 (JP, A) JP-A-55-434 45061 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) G02B 5/08 G02B 1/10 G02B 5/26

Claims (6)

(57) [Claims] 1. A multi-layer film comprising a high refractive index layer and a low refractive index layer alternately laminated at least one by one on a surface of a substrate, wherein at least one of the high refractive index layers is A reflective film, which is a heterogeneous film configured to have a refractive index lower toward a surface of a substrate. 2. The reflection film according to claim 1, wherein said heterogeneous film is a laminated film formed by laminating at least two layers made of materials having different refractive indexes. 3. A high-refractive-index layer comprising the laminated film, comprising a plurality of layers.
The optical film thickness of a layer made of a material having a lower refractive index as the distance from the surface of the substrate is increased among these laminated films. The reflective film as described in the above. 4. The method of claim 2 or 3 reflective film according materials having a lower refractive index, characterized in that a Al ₂ O ₃ among the different refractive index material. 5. A plurality of high refractive index layers and a plurality of high refractive index layers on a surface of a substrate.
It has a multilayer film in which low refractive index layers are alternately laminated, and two or more
At least two high refractive index layers having different refractive indices
It is a mixed film made of materials, and among these mixed films,
A material farther from the surface of the substrate contains a material having a lower refractive index.
Large, and on the side close to the surface of the substrate
High refractive index layer has a higher refractive index than distant high refractive index layer
A reflection film formed so as to be low. 6. A material having a different refractive index among the materials having different refractive indexes.
Wherein the lower material is ZrO _2.
Item 6. The reflective film according to Item 5.