JP2008139525A - Multilayer film optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To materialize a multilayer film optical element which is little deformed by reducing stress of an optical multilayer film constituting an antireflection film and a mirror. <P>SOLUTION: A stress relaxation layer 4 comprising an amorphous ScF3 film is inserted in the optical multilayer film comprising a high refractive index film 2 and a low refractive index film 3 layered on a substrate 1. Since ScF3 has larger compressive stress compared with other materials, tensile stress of the optical multilayer film can be canceled even when film thickness is small. When the substrate 1 has a metal film 5 thereon, effect of moisture prevention and rust prevention are expectable by inserting the stress relaxation layer 4 comprising the amorphous ScF3 film between the optical multilayer film and the metal film 5 so that the stress relaxation layer 4 lies on the metal film 5. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a multilayer optical element having optical characteristics such as an antireflection film and a mirror.

  Multilayer film optical elements such as mirrors and antireflection films using optical thin films are used in various products, and these realize desired optical characteristics by combining optical thin films having different refractive indexes. In order to obtain the desired quality of the optical element, the optical multilayer film has low optical loss such as light absorption and light scattering, optical characteristics as designed, beautiful appearance, environmental durability, laser durability, etc. Desired.

  However, some optical multilayer films have strong film stress. When the film stress is large, cracks and peeling occur in the film, resulting in abnormal appearance, deterioration of environmental durability, and distortion of the element shape. Even if the problem of stress is solved, the refractive index and film thickness may not be as expected in some cases, and optical characteristics as designed may not be obtained.

  Furthermore, even if optical characteristics as designed are obtained, there may be problems with subsequent environmental durability and laser durability. In particular, the properties of metal films often deteriorate due to reaction of the surface with oxygen, nitrogen, water, etc. in the atmosphere.

In order to solve these problems, for example, in the case of a stress problem, a method of inserting a film having a reverse stress as disclosed in Patent Document 1 as a stress relaxation layer is known.
JP 2003-29024 A

  However, when the stress relaxation layer absorbs light with respect to a target wavelength or does not have a desired refractive index, a film configuration in which desired optical characteristics cannot be realized by insertion may be obtained.

For example, for light in the ultraviolet region, an oxide film stress relaxation layer cannot be used due to problems such as light absorption and laser durability. Even in the case of a fluoride film having low light absorption with respect to light in the ultraviolet region, for example, in the stress relaxation layer using SrF ₂ described in Patent Document 1, the stress of the SrF ₂ film is sufficiently strong. Therefore, the desired stress is often not obtained depending on external factors such as thermal stress when the substrate is heated. In order to obtain a desired stress, it is necessary to increase the film thickness of the SrF ₂ film. In this case, however, the optical characteristics of the element are limited by SrF ₂ . Thus, in many cases, the stress relaxation layer cannot be freely inserted in optical design.

  The present invention has been made in view of the above-mentioned unsolved problems of the prior art, and has low optical loss, optical characteristics as designed, beautiful appearance, environmental durability, laser durability, appropriate shape, etc. An object of the present invention is to provide a multilayer optical element provided.

The multilayer optical element of the present invention is inserted into the optical multilayer film in order to cancel the stress of the optical multilayer film, and the optical multilayer film having at least two thin films having different refractive indexes laminated on the substrate. And a stress relaxation layer made of at least one layer of ScF ₃ film.

As shown in FIG. 11, the film stress of ScF ₃ is a strong stress in the opposite direction with respect to other fluoride films. Compared to SrF ₂ etc., its size can be seen. For this reason, an optical element in which stress is suppressed can be obtained by using the ScF ₃ film as a stress relaxation layer.

Since the insertion film made of ScF ₃ has a strong stress, the effect of stress relaxation can be obtained even with a relatively thin film thickness. Then, by limiting the film thickness to an optical film thickness of 0.01λ or more and 0.11λ or less, it is possible to obtain optical characteristics equivalent to the case without the stress relaxation layer.

The ScF ₃ film is preferably amorphous. Since an amorphous film does not become rough due to the growth of crystal grains unlike a crystalline film, a relatively smooth surface is easily obtained. Since ScF ₃ is an amorphous film if the substrate temperature is up to about 250 ° C., the insertion film of ScF ₃ makes it easy to form an optical multilayer film as designed, and has optical characteristics as designed. The obtained device is obtained.

  In this way, an optical element with controlled stress and optical characteristics as designed can be obtained.

  The best mode for carrying out the present invention will be described with reference to the drawings.

FIG. 1A shows a film configuration according to the first embodiment. This multilayer optical element is a film in which a high refractive index film 2 and a low refractive index film 3 constituting an optical multilayer film are laminated on a substrate 1, and a stress relaxation layer 4 made of a ScF ₃ film is inserted therebetween. It has a configuration. The optical characteristics of the optical multilayer film constituting the element are substantially realized by the high refractive index film 2 and the low refractive index film 3. The stress relaxation layer 4 has a reverse stress (compressive stress) that cancels out the total stress (tensile stress) generated by the high refractive index film 2 and the low refractive index film 3. As described above, the stress relaxation layer 4 suppresses the stress of the element, and it is possible to suppress abnormal appearance and environmental durability due to cracks and peeling and further distortion of the element shape.

  The stress relaxation layer 4 has an optical film thickness of 0.02λ or more and 0.11λ or less when the target wavelength is λ. For this reason, the optical characteristics of the entire element are hardly affected.

The film material is formed by vacuum deposition or the like using LaF _{3 as} a high refractive index film, MgF ₂ as a low refractive index film, and ScF ₃ as a stress relaxation film.

The substrate preferably has a very small coefficient of linear thermal expansion of 0.6 × 10 ⁻⁶ / K or less. When the linear thermal expansion coefficient of the substrate is very small of 0.6 × 10 ⁻⁶ / K or less, film stress is likely to occur due to the difference in thermal expansion coefficient between the film and the substrate due to temperature change during film formation. However, according to the present embodiment, the stress is sufficiently controlled even when a substrate having a very small linear thermal expansion coefficient of 0.6 × 10 ⁻⁶ / K or less is used. Therefore, it is possible to obtain an optical element having desired optical characteristics that has little thermal expansion due to environmental temperature changes, that is, little shape change.

  In this way, by solving problems such as stress and optical characteristics, it has low optical loss with less light absorption and light scattering, and also has optical characteristics as designed, beautiful appearance, laser durability, etc. A multilayer optical element can be realized.

FIG. 1B shows a second embodiment. This multilayer optical element has a metal film 5 on a substrate 1, and a low refractive index film 3 and a high refractive index film 2 are formed on the metal film 5 via a stress relaxation layer 4 made of amorphous ScF _3. It has a laminated film structure. The optical characteristics of the element are realized by the metal film 5, the low refractive index film 3, and the high refractive index film 2. The stress relaxation layer 4 made of ScF ₃ has a reverse stress (compressive stress) that cancels out the total stress (tensile stress) generated by the metal film 5, the low refractive index film 3, and the high refractive index film 2. The stress relaxation layer 4 controls the stress of the element, and it is possible to suppress the appearance abnormality due to cracks and peeling, the problem of environmental durability, and the distortion of the element shape. Further, since the stress relaxation layer 4 made of ScF ₃ is amorphous, it is possible to effectively prevent the penetration of gas in the element atmosphere such as water and oxygen into the metal film 5.

In a multilayer film including a metal film, a film configuration in which an amorphous ScF ₃ film having a stress opposite to that of the multilayer film is inserted on the incident light side of the metal film is desirable.

The metal film reacts with oxygen, nitrogen, moisture and other gases in the atmosphere, and its characteristics are likely to change. In addition, fluoride dielectric films are used as protective films for metal films for the ultraviolet region, and these fluoride dielectric films often have a structure such as a columnar structure that easily transmits gas. Therefore, by inserting an amorphous ScF ₃ film that does not have a columnar structure, it is preferable to control the stress and simultaneously improve the durability.

The film material is formed by vacuum deposition or the like using Al as the metal film, MgF ₂ as the low refractive index film, LaF ₃ as the high refractive index film, and ScF ₃ as the stress relaxation film.

As in the first embodiment, the substrate preferably has a very small linear thermal expansion coefficient of 0.6 × 10 ⁻⁶ / K or less.

FIG. 2 shows the film configuration of the mirror according to the first embodiment. The substrate 11 of this embodiment is a zerodur substrate (trade name) manufactured by Schott with a very small coefficient of linear thermal expansion of 0.02 × 10 ⁻⁶ / K, and an optical film thickness of 1.38λ is formed thereon. The ScF ₃ film 16 is laminated. Next, an LaF ₃ film (high refractive index film) 12 having an optical film thickness of 0.25λ, an ScF ₃ film (stress relaxation layer) 14 having an optical film thickness of 0.05λ, and MgF ₂ having an optical film thickness of 0.22λ. A total of four layers of the film 13 (low refractive index film) and the ScF ₃ film (stress relaxation layer) 14 having an optical film thickness of 0.05λ are used as one constituent layer 17, and 17 constituent layers 17 are laminated. A surface layer of the LaF ₃ film 18 having an optical film thickness of 0.26λ is provided.

The ScF ₃ films 14 and 16 are amorphous, and the LaF ₃ films 12 and 18 and the MgF ₂ film 13 are crystalline.

(Comparative Example 1)
As a comparative example of Example 1, FIG. 3 shows a film configuration of a mirror according to Comparative Example 1. In this comparative example, 17 layers of an LaF ₃ film 12a with an optical film thickness of 0.28λ and an MgF ₂ film 13a with an optical film thickness of 0.29λ are alternately stacked on a substrate 11a made of a shotd zerodur substrate. As a layer, a LaF ₃ film 18a having an optical film thickness of 0.28λ is laminated.

The LaF ₃ films 12a and 18a and the MgF ₂ film 13a are crystalline.

(Comparative Example 2)
As a comparative example of Example 1, FIG. 4 shows a film configuration of a mirror of Comparative Example 2. The substrate 21 of Comparative Example 2 is a zerodur substrate manufactured by Schott, on which an ScF ₃ film 26 having an optical film thickness of 2.91λ is laminated. Next, 17 layers of LaF ₃ films 22 with an optical film thickness of 0.28λ and MgF ₂ films 23 with an optical film thickness of 0.29λ are alternately stacked, and the final layer is LaF with an optical film thickness of 0.28λ. _Three films 28 are laminated.

The film thickness of the ScF ₃ film 26 is the same as the total film thickness of the ScF ₃ film of Example 1.

The LaF ₃ films 22 and 28 and the MgF ₂ film 23 are crystalline.

  FIG. 5 shows the simulation value and actual measurement value of the reflectance of Example 1, the simulation value and actual measurement value of the reflectance of Comparative Example 1, and the actual measurement value of Comparative Example 2 when λ = 193 nm. The angle of light incident on the element is 45 °.

The reflectance of the simulation value of Example 1 is almost the same as the simulation value of Comparative Example 1 when there is no ScF ₃ film as the stress relaxation layer, and the ScF ₃ film affects the optical characteristics. You can see that it is not. Moreover, the actual measurement value of Example 1 is also a value close to the design simulation value.

  However, it can be seen that the reflectance of the actually measured value in Comparative Example 1 is lower than the simulation value.

  Table 1 shows simulation values of total stress in Example 1 and Comparative Example 1. In Example 1, the total stress value is reduced to about 1/3 that of Comparative Example 1.

  Due to this stress difference, no cracks were seen in the appearance of Example 1, but cracks were seen in the appearance of Comparative Example 1.

  Further, regarding Example 1 and Comparative Example 2, when the presence or absence of appearance abnormality after storage in a high humidity and high temperature environment of 60 ° C. and 90% humidity was examined, as shown in Table 2, abnormality was found in Example 1. However, in Comparative Example 2, cracks and grain were generated in the film.

  Thus, it was found that the film configuration of Example 1 provides an element having a high optical design freedom, good optical characteristics as designed, and a beautiful appearance with controlled stress. .

(Comparative Example 3)
As a comparative example of Example 1, FIG. 6 shows a film configuration of a mirror of Comparative Example 3. The substrate 31 in this comparative example is a zerodur substrate manufactured by Schott, on which the ScF ₃ film 36 is laminated. Next, a total of four layers of the LaF ₃ film 32, the ScF ₃ film 34, the MgF ₂ film 33, and the ScF ₃ film 34 are used as one constituent layer 37, and 17 constituent layers 37 are laminated. The LaF ₃ film 38 is laminated as a surface layer. In this film configuration, the optical film thickness of the ScF ₃ film 34 serving as a stress relaxation layer is changed from 0.01λ to 0.21λ, and the ScF ₃ film 36, LaF ₃ film 32, and MgF ₂ film 33 are adjusted accordingly. The simulation value was obtained by optimizing the optical film thickness of the LaF ₃ film 38 each time.

FIG. 7 shows the reflectance by the simulation at λ = 193 nm. If the film thickness of the ScF ₃ film 34 is 0.11λ or less, it can be seen that the reflectance does not decrease much when there is no ScF ₃ film 34 as a stress relaxation layer.

Further, the smaller the film thickness of the ScF ₃ film 34 is, the more advantageous in optical design, but 0.02λ or more is desirable from the viewpoint of stress relaxation and film thickness control.

FIG. 8 shows the film configuration of the mirror according to the second embodiment. In this embodiment, a circular synthetic quartz substrate having a linear thermal expansion coefficient of 0.55 × 10 ⁻⁶ / K, a thickness of 2 mm, and a diameter of 100 mm is used as the substrate 41, and an Al metal film 45 is formed on the physical film thickness. 180 nm is stacked. Next, the ScF ₃ film 46 is laminated with an optical thickness of 0.52λ. Next, the LaF ₃ film (high refractive index film) 42 has an optical film thickness of 0.29λ, the ScF ₃ film 44 has an optical film thickness of 0.13λ, and the MgF ₂ film (low refractive index film) 43 has an optical film thickness. _Three constituent layers 47 each having a total thickness of 0.03λ and an ScF ₃ film 44 having an optical thickness of 0.13λ are stacked. The LaF ₃ film 48 is laminated as a surface layer with an optical film thickness of 0.24λ.

The ScF ₃ films 44 and 46 are amorphous, and the MgF ₂ film 43 and the LaF ₃ films 42 and 48 are crystalline.

(Comparative Example 4)
As a comparative example of Example 2, Comparative Example 4 is shown in FIG. In this comparative example, a circular synthetic quartz substrate having a thickness of 2 mm and a diameter of 100 mm is used as the substrate 41a, and an Al metal film 45a is laminated thereon with a physical thickness of 180 nm. Next, the LaF ₃ film 42a has an optical film thickness of 0.28λ, and the MgF ₂ film 43a has an optical film thickness of 0.29λ. A total of two constituent layers 47a are stacked, and the LaF ₃ film 48a is stacked. Is a structure in which 0.28λ is laminated as the surface layer with an optical film thickness.

The MgF ₂ film 43a and the LaF ₃ films 42a and 48a are crystalline.

  The measured values of the reflectance of Example 2 and Comparative Example 4 are shown in FIG. These are both mirrors for ArF and KrF lasers with an incident angle of 45 °, but have similar characteristics.

However, when the elements of Example 2 and Comparative Example 4 were irradiated with 1 billion shots of ArF laser with a fluence of 1 mJ / cm ² , the change in reflectance was less than 1% in Example 2, but in Comparative Example 4 The change in reflectivity was about 5%.

  Further, as shown in Table 3, the deformation amount of the substrate due to the lamination of the films is 0.9λ (λ = 193 nm) and 0.7λ (λ = 248 nm) in Example 2, and 9. They were 6λ (λ = 193 nm) and 7.5λ (λ = 248 nm). It can be seen that the deformation amount of Example 2 is 1/10 or less of that of Comparative Example 4.

  Thus, it can be said that the structure of Example 2 is an element that satisfies the optical characteristics and laser durability, and is controlled in stress.

It is a figure which shows the film | membrane structure by 1st and 2nd embodiment. 1 is a diagram illustrating a film configuration according to Example 1. FIG. 6 is a diagram showing a film configuration according to Comparative Example 1. FIG. 6 is a diagram showing a film configuration according to Comparative Example 2. FIG. It is a graph which shows the reflectance of Example 1 and Comparative Examples 1 and 2. FIG. 6 is a diagram showing a film configuration according to Comparative Example 3. FIG. It is a graph which shows the result of having investigated the optical film thickness relationship of the reflectance by the comparative example 3, and the stress relaxation layer. 6 is a diagram showing a film configuration according to Example 2. FIG. 6 is a diagram showing a film configuration according to Comparative Example 3. FIG. 6 is a graph showing the reflectance of Example 2 and Comparative Example 4. It is a graph which shows the stress value of a fluoride film | membrane.

Explanation of symbols

1, 11, 11a, 21, 31, 41, 41a Substrate 2 High refractive index film 3 Low refractive index film 4 Stress relaxation layer 5 Metal film

Claims (5)

An optical multilayer film having at least two thin films having different refractive indexes laminated on a substrate, and at least one ScF ₃ film inserted in the optical multilayer film to cancel the stress of the optical multilayer film A multilayer optical element, comprising: a stress relaxation layer. The multilayer optical element according to claim 1, wherein the tensile stress of the optical multilayer film is canceled by the compressive stress of the ScF ₃ film. _3. The multilayer optical element according to claim 1, wherein an optical film thickness of the ScF ₃ film is 0.02λ or more and 0.11λ or less with respect to a target wavelength λ. The multilayer optical element according to any one of claims 1 to 3, wherein the ScF ₃ film is amorphous.   5. The multilayer optical element according to claim 1, wherein the optical multilayer film includes a metal film and at least two thin films having different refractive indexes.

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