JP4343895B2 - Multilayer mirror for soft X-ray - Google Patents

Description

The present invention relates to a multilayer mirror for soft X-rays.
Specifically, multilayer reflectors used for light having a wavelength in the range of 11 to 14 nm, that is, high-level optical elements used for X-ray lasers, X-ray telescopes, X-ray lithography, X-ray microscopes, and the like. The present invention relates to a soft X-ray multilayer mirror having reflectivity.

In recent years, with the progress of miniaturization of semiconductor integrated circuit elements, extreme ultraviolet (hereinafter referred to as EUV) lithography technology, which is lithography using soft X-rays (11 to 14 nm) in addition to conventional ultraviolet rays, has been developed. With respect to light having such a wavelength in the X-ray region, the refractive index of the substance is very close to 1, and the absorption thereof is large, so that a lens utilizing refraction cannot be used in principle.
That is, the refractive index of a substance with respect to light in the X-ray wavelength region is expressed as n = 1−δ−iβ (δ, β: positive real number), and both δ and β are very small compared to 1 (with a refractive index of The imaginary part β represents X-ray absorption). Therefore, the refractive index is close to 1, and X-rays are hardly refracted, and X-rays are always absorbed. Therefore, a lens using refraction like light in the visible light region cannot be used for light in the X-ray wavelength region.
Therefore, an optical system using a reflecting mirror is generally used for light in such a region. However, even in an optical system using a reflecting mirror, since the refractive index is close to 1, the reflectance is very low, and most X-rays are transmitted or absorbed.

  In order to solve such problems, the following reflecting mirrors have been developed. That is, it is a multilayer reflector in which a material having a large difference between the refractive index in the wavelength region of the X-ray used and a refractive index of vacuum (= 1) and a material having a small difference are alternately laminated. In this multilayer-film reflective mirror, the entire interface of each layer is a reflecting surface. And it is comprised by the multilayer film which adjusted the thickness of each layer based on the optical interference theory so that the phase of the reflected wave from each interface may correspond.

  As a representative example of such a multilayer film reflecting mirror, a combination of W (tungsten) / C (carbon), Mo (molybdenum) / Si (silicon), or the like is known. And these multilayer films are produced by thin film formation techniques, such as sputtering, vacuum evaporation, and CVD.

As the development of X-ray multilayer mirrors progresses, multilayer films are being evaluated. Such evaluation has revealed the practicality of some combinations of materials.
For example, the multilayer film of the Mo / Si combination exhibits a high reflectance on the long wavelength side of the silicon absorption edge of 123 mm, and thus is excellent as a multilayer film reflector used in the reflection optical system of a soft X-ray reduction projection exposure apparatus. I have found out.
On the other hand, considering the application of such a multilayer film reflector to the beam line of synchrotron radiation and the optical system of an X-ray laser, the multilayer film mirror is subjected to an extremely strong light load. Destruction or temperature rise of the element due to light absorption occurs.
Therefore, there is a demand for a stable multilayer mirror that can withstand use under severe conditions.

The Mo / Si multilayer film is thermally unstable especially at 300 ° C. or higher, although it exhibits a high reflectance in a normal incidence region near a wavelength of 13 nm.
As a result, it is known that the Si compound is generated at the interface between the Si layer and the Mo layer, and the reflectance decreases due to the blurring of the interface.
For these, S.M. Yulin, T .; Kuhlmaaa, et. al SPIE Vol. 4343 (2001) and T.W. Feigl, S .; Yulin, et. al SPIE Vol. 4506 (2001).

Conventionally, in order to improve the durability of such a Mo / Si multilayer film, Mo / B ₄ C (molybdenum / boron carbide), Mo / SiC (molybdenum / silicon carbide), W / BN (tungsten / boron nitride) For example, heat-resistant materials have been used for light element layers. Alternatively, durability has been improved by using Mo silicide instead of Mo.
Furthermore, in order to improve these durability, for example, Patent Document 1 proposes to use B ₄ C as an intermediate layer between the Mo layer and the Si layer.
Patent Document 2 proposes the use of borides, carbides, silicides, and Patent Document 3 proposes the use of SiO ₂ .
JP 60-7400 A JP-A-63-088503 JP 2002-277589 A

However, according to the conventional examples of Patent Documents 1 to 3 described above, there are the following problems when improving electric field strength resistance and heat resistance.
That is, in these, when a stable substance such as B ₄ C or SiO _{2 is} used as an intermediate layer to form a film between Mo and Si, the conventional Mo / Si can be improved in improving the strength of electric field strength or heat resistance. As a result, the reflectance is greatly reduced as compared with the multilayer mirror. For example, compared with the conventional Mo / Si multilayer mirror, the reflectance is reduced by 1.8% for the multilayer film with 0.5 nm of B ₄ C inserted and by 7.5% for the multilayer film with 0.5 nm SiO ₂ inserted. Will be invited.

  In view of the above problems, an object of the present invention is to provide a soft X-ray multilayer mirror that has electric field strength resistance and heat resistance and can suppress a decrease in reflectance.

In order to solve the above-mentioned problems, the present invention provides a soft X-ray multilayer mirror configured as follows.
First, the soft X-ray multilayer mirror of the present invention includes a high refractive index layer made of a material having a relatively small difference between the refractive index in the X-ray region and the refractive index in a vacuum, and a relative difference in refractive index. A low refractive index layer made of a large material is constituted by a multilayer film layer formed by alternately and repeatedly laminating from the substrate side.
In the multilayer film layer, an intermediate layer is inserted only between the high refractive index layer and the low refractive index layer laminated on the surface side portion of the multilayer film layer.
Furthermore, the multilayer film layer is composed of 30 cycles or more and 60 cycles or less when one combination of the high refractive index layer and the low refractive index layer is one cycle. Further, the intermediate layer is inserted only in a portion of 1 cycle or more and 20 cycles or less when viewed from the surface side of the multilayer film layer.
In the soft X-ray multilayer mirror of the present invention, the high refractive index layer is made of Si, the low refractive index layer is made of Mo, and the intermediate layer is made of B ₄ C. Alternatively, it is characterized by being formed of SiO ₂ .

  ADVANTAGE OF THE INVENTION According to this invention, while having electric field strength tolerance and heat resistance, the multilayer film mirror for soft X-rays which can suppress the fall of a reflectance is realizable.

Next, a high refractive index layer made of a material having a relatively small difference between the refractive index in the X-ray region and the refractive index of vacuum in the embodiment of the present invention ,
A multi-layer mirror for soft X-rays constituted by a multi-layer film layer in which low refractive index layers made of a material having a relatively large difference in refractive index are alternately laminated from the substrate side will be described.
Figure 1 shows a cross-sectional view of a multilayer film Mirror for soft X-ray in this embodiment. In FIG. 1, 1 is a mirror substrate, 2 is a high-refractive index layer made of an object having a relatively small difference between the refractive index at the X-ray wavelength to be used and the refractive index of vacuum (= 1), and 3 is relatively different. A low refractive index layer 4 made of a large material is an intermediate layer.
In the configuration example of the present embodiment, Si is used as the material of the high refractive index layer having a small difference from the refractive index (= 1) of vacuum, and Mo is used as the material of the low refractive index layer having a large difference. For the intermediate layer, a material that is relatively stable and has little influence on the reflectance is desirable, and B ₄ C and SiO ₂ can be used.

In order to explain the present invention, first, FIG. 2 shows the electric field intensity in the multilayer film when the Mo / Si multilayer mirror according to the prior art is irradiated with soft X-rays having a wavelength of 13.5 nm.
In FIG. 2, the horizontal axis represents the film thickness (the right side is the multilayer film surface), and the vertical axis represents the electric field strength. Then, as shown in FIG. 2, the electric field strength distribution in the multilayer film due to incident light increases exponentially as the surface of the multilayer film is approached.

Therefore, in the present invention, B ₄ C or SiO ₂ is inserted as the intermediate layer 4 only between the Mo layer and the Si layer laminated on the surface portion of these multilayer films. As a result, the decrease in reflectance due to the insertion of the intermediate layer was minimized, and the light resistance was improved.
Specifically, when one set of two Mo / Si layers formed by stacking Mo and Si is defined as one cycle, it is composed of 30 cycles or more and 60 cycles or less,
In particular, it has been found that by inserting an intermediate layer only in a portion of 1 cycle or more and 20 cycles or less as viewed from the surface of the multilayer film having a strong electric field strength distribution, the decrease in reflectance can be minimized and the light resistance can be improved. It was.
That is, if the insertion of the intermediate layer is 20 cycles or more, the durability is slightly increased, but the reflectance is greatly reduced, and the difference from the case where the insertion is performed over the entire cycle is eliminated. Therefore, it is effective to insert the intermediate layer 4 only in a portion of 1 cycle or more and 20 cycles or less as viewed from the multilayer film surface side.
Note that the number of layers into which the intermediate layer is inserted at this time depends on the number of Mo / Si layers. That is, if the number of cycles for inserting the intermediate layer is 1/3 or less of the number of cycles of Mo / Si, a multilayer mirror having light durability and good reflectance can be formed. .

Examples to which the present invention is applied will be described below.
[Example 1]
FIG. 3 shows a cross-sectional view of the soft X-ray Mo / Si multilayer mirror of Example 1. FIG.
In FIG. 3, 1 is a mirror substrate, 5 is a layer made of Si which is a substance having a small difference between the refractive index at the X-ray wavelength to be used and the refractive index of vacuum (= 1), and 6 is the refractive index of vacuum (= 1). A layer made of Mo, which is a material having a large difference, and B ₄ C, which is an intermediate layer. In this embodiment, an intermediate layer of B ₄ C is inserted between Mo and Si only up to five periods from the surface of the multilayer film.
In the multilayer structure including the intermediate layer, the thickness of the Mo layer is 2.22 nm, the thickness of the Si layer is 3.68 nm, and the thickness of the intermediate layer of B ₄ C is 0.5 nm.
Further, in the multilayer structure portion not including the intermediate layer, the film thickness of the layer made of Mo is 2.67 nm, and the film thickness of the layer made of Si is 4.23 nm.

[Example 2]
The Mo / Si multilayer mirror for soft X-rays in Example 2 has the same film configuration as that of Example 1 described above, but B ₄ between Mo and Si up to 10 periods from the surface of the multilayer film. An intermediate layer by C is inserted.
In this embodiment, the film thickness of the layer made of Mo is 2.22 nm, the film thickness of the intermediate layer made of B ₄ C is 0.5 nm, and the film thickness of the layer made of Si is 3.68 nm up to 10 cycles on the surface side of the multilayer film. In this order, each layer is alternately stacked for 10 cycles.

[Example 3]
The Mo / Si multilayer mirror for soft X-rays in Example 3 has the same film configuration as that of Example 1 described above, but B ₄ between Mo and Si up to 20 periods from the surface of the multilayer film. An intermediate layer by C is inserted.
In this embodiment, the film thickness of the layer made of Mo is 2.22 nm, the film thickness of the intermediate layer made of B ₄ C is 0.5 nm, and the film thickness of the layer made of Si is 3.68 nm up to 20 cycles on the surface side of the multilayer film. In this order, each layer is alternately stacked for 20 cycles.

(Comparative Example 1)
The Mo / Si multilayer mirror for soft X-rays in Comparative Example 1 is alternately laminated with 60 periods, with no intermediate layer inserted, a film thickness of Mo of 2.67 nm, and a film thickness of Si of 4.23 nm. Is formed.

(Comparative Example 2)
In the Mo / Si multilayer mirror for soft X-rays in Comparative Example 2, the film thickness of the layer made of Mo is 2.22 nm, the film thickness of the intermediate layer made of B ₄ C is 0.5 nm, and the film is made of Si. The film thickness of the layers is 3.68 nm, and each layer is alternately stacked up to 60 cycles in this order.
FIG. 4 shows reflection spectral characteristics in the present example and the comparative example when 60 Mo / Si multilayer mirrors for soft X-rays are stacked and soft X-rays are incident at an incident angle of 0 degrees.
4, 9, 10, and 11 show the reflection spectral characteristics of the soft X-ray Mo / Si multilayer mirrors in Examples 1, 2, and 3, respectively. 8 and 12 show the reflection spectral characteristics of the Mo / Si multilayer mirror for soft X-rays in Comparative Examples 1 and 2.

As can be seen from FIG. 4, the reflectivity for soft X-rays in the vicinity of the wavelength of 13.5 nm of the multilayer mirror of Example 1 shown in 9 is 73.9%.
Further, the reflectance for soft X-rays in the vicinity of the wavelength of 13.5 nm in Example 2 shown in 10 is 73.5%. In addition, the reflectance of Example 2 shown in 11 for soft X-rays having a wavelength of 13.5 nm is 73.2%.
On the other hand, the reflectance for soft X-rays in the vicinity of the wavelength of 13.5 nm in Comparative Example 1 shown in 8 of FIG. 4 is 74.8%. Further, the reflectivity for soft X-rays in the vicinity of the wavelength of 13.5 nm in Comparative Example 2 is 73.0%.
As is apparent from the above, by inserting the intermediate layer of B ₄ C only on the surface side of the multilayer film, it becomes possible to obtain a higher reflectance than when the intermediate layer is inserted over the entire period. Moreover, since the intermediate layer is inserted on the surface side of the multilayer film having a high electric field strength, the light resistance can be improved as compared with the case where there is no intermediate layer.
That is, high light resistance can be obtained while maintaining a good reflectance.

[Example 4]
Next, another embodiment to which the present invention is applied will be described.
FIG. 5 shows a cross-sectional view of a soft X-ray Mo / Si multilayer mirror of Example 4.
In FIG. 5, as in FIG. 3, 1 is a mirror substrate, 5 is a layer made of Si which is a substance having a small difference between the refractive index at the X-ray wavelength to be used and the refractive index (= 1) of vacuum, and 6 is a refractive of vacuum. A layer made of Mo which is a substance having a large difference from the rate (= 1), and 13 is SiO ₂ which is an intermediate layer.
In this embodiment, an intermediate layer made of SiO ₂ is inserted between Mo and Si only up to five periods from the surface of the multilayer film.
In the multilayer structure including the intermediate layer, the thickness of the Mo layer is 2.41 nm, the thickness of the Si layer is 3.5 nm, and the thickness of the SiO ₂ intermediate layer is 0.5 nm.
Further, in the multilayer structure portion not including the intermediate layer, the film thickness of the layer made of Mo is 2.67 nm, and the film thickness of the layer made of Si is 4.23 nm.

[Example 5]
The Mo / Si multilayer mirror for soft X-rays in Example 5 has the same film configuration as that of Example 4 described above, but it is SiO ₂ between Mo and Si up to 10 periods from the surface of the multilayer film. An intermediate layer is inserted.
In this embodiment, up to 10 cycles on the surface side of the multilayer film, the layer thickness of Mo is 2.41 nm, the thickness of the intermediate layer of SiO ₂ is 0.5 nm, the thickness of the layer of Si is 3.5 nm, Each layer is alternately stacked for 10 cycles in this order.

[Example 6]
The Mo / Si multilayer mirror for soft X-rays in Example 6 has the same film configuration as that of Example 4 described above, but it is SiO ₂ between Mo and Si up to 20 periods from the surface of the multilayer film. An intermediate layer is inserted.
In this embodiment, up to 20 cycles on the surface side of the multilayer film, the film thickness of Mo is 2.41 nm, the film thickness of the intermediate layer of SiO ₂ is 0.5 nm, and the film thickness of the layer of Si is 3.5 nm. Each layer is alternately laminated for 20 cycles in this order.

(Comparative Example 3)
The Mo / Si multilayer mirror for soft X-rays in Comparative Example 3 is alternately laminated with 60 periods, with no intermediate layer inserted, the film thickness of Mo being 2.67 nm, and the film thickness of Si being 4.23 nm. Is formed.

(Comparative Example 4)
In the Mo / Si multilayer mirror for soft X-rays in Comparative Example 4, the layer thickness of Mo is 2.41 nm, the thickness of the intermediate layer of SiO ₂ is 0.5 nm, and the layer of Si is 60 cycles. The thickness of each layer is set to 3.5 nm, and each layer is alternately stacked up to 60 cycles in this order.
FIG. 6 shows reflection spectral characteristics in the present example and the comparative example when 60 Mo / Si multilayer mirrors for soft X-rays are stacked and soft X-rays are incident at an incident angle of 0 degrees.
In FIG. 6, 15, 16, and 17 indicate reflection spectral characteristics of the soft X-ray Mo / Si multilayer mirrors in Examples 4, 5, and 6. Reference numerals 14 and 18 show the reflection spectral characteristics of the soft X-ray Mo / Si multilayer mirrors in Comparative Examples 3 and 4, respectively.

As can be seen from FIG. 6, the reflectivity for soft X-rays in the vicinity of the wavelength of 13.5 nm of the multilayer mirror of Example 4 shown in 15 is 71.0%. Further, the reflectivity for soft X-rays in the vicinity of the wavelength of 13.5 nm in Example 5 shown in 16 is 69.2%. Moreover, the reflectance with respect to the soft X-ray of the wavelength of 13.5 nm of Example 6 shown by 17 is 67.8%.
On the other hand, the reflectance with respect to soft X-rays having a wavelength of 13.5 nm in Comparative Example 3 shown in 14 of FIG. 6 is 74.8%.
Moreover, the reflectance with respect to the soft X-ray of wavelength 13.5nm in the comparative example 4 is 67.3%.
As is clear from the above, by inserting an intermediate layer made of SiO ₂ only on the surface side of the multilayer film, it becomes possible to obtain a higher reflectance than when the intermediate layer is inserted over the entire period.
Moreover, since the intermediate layer is inserted on the surface side of the multilayer film having a high electric field strength, the light resistance can be improved as compared with the case where there is no intermediate layer.
That is, high light resistance can be obtained while maintaining a good reflectance.

Sectional drawing of the soft X-ray multilayer mirror in embodiment of this invention. The figure which shows the electric field strength in a multilayer film at the time of irradiating a soft X ray with a wavelength of 13.5 nm to the Mo / Si multilayer film mirror for describing embodiment of this invention. Sectional drawing of the Mo / Si multilayer mirror for soft X rays in the Example of this invention. The figure which shows the spectral characteristic in a present Example when a Mo / Si multilayer film mirror for soft X-rays is laminated | stacked 60 periods, and the soft X-ray near 13.5 nm wavelength injects 0 degree | times. Sectional drawing of the Mo / Si multilayer mirror for soft X rays in the Example of this invention. The figure which shows the spectral characteristic in a present Example when a Mo / Si multilayer film mirror for soft X-rays is laminated | stacked 60 periods, and the soft X-ray near 13.5 nm wavelength injects 0 degree | times.

Explanation of symbols

1: Mirror substrate 2: Layer made of a material having a small difference from the refractive index of vacuum (= 1) 3: Layer made of a material having a large difference from the refractive index of vacuum (= 1) 4: Intermediate layer 5: Layer 6 made of Si : Mo layer 7: B ₄ C intermediate layer 13: SiO ₂ intermediate layer

Claims (4)

A high refractive index layer made of a material having a relatively small difference between the refractive index in the X-ray region and a vacuum refractive index and a low refractive index layer made of a material having a relatively large refractive index difference are arranged on the substrate side. In the multilayer mirror for soft X-rays composed of multilayer layers that are alternately and repeatedly laminated,
In the multilayer film layer, an intermediate layer is inserted only between the high refractive index layer and the low refractive index layer laminated on the surface side portion of the multilayer film layer. Multilayer mirror. The multilayer film layer is composed of 30 cycles or more and 60 cycles or less when a combination of one pair of the high refractive index layer and the low refractive index layer is one cycle,
2. The multilayer film mirror for soft X-rays according to claim 1, wherein the intermediate layer is inserted only in a portion of 1 cycle or more and 20 cycles or less as viewed from the surface side of the multilayer film layer. The multilayer film mirror for soft X-rays according to any one of claims 1 to 3, wherein the high refractive index layer is made of Si and the low refractive index layer is made of Mo. . The multilayer film mirror for soft X-rays according to claim 1, wherein the intermediate layer is made of B ₄ C or SiO ₂ .

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