JP2993261B2 - X-ray multilayer reflector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an X-ray reduction projection exposure,
The present invention relates to a multilayer film reflecting mirror used in a reflection optical system in the X-ray wavelength range in a X-ray telescope, an X-ray microscope, an X-ray laser, various X-ray analyzers, and the like.

[0002]

2. Description of the Related Art For light in the X-ray wavelength range, the refractive index of a substance is expressed as n = 1-δ-iβ (δ, β: positive real number). Very small (the imaginary part β of the refractive index represents X-ray absorption). Therefore, the refractive index is almost close to 1, and the X-ray is hardly refracted.
Absorb lines. Therefore, a lens utilizing refraction like light in the visible light region cannot be used for light in the X-ray wavelength region.

Therefore, an optical system utilizing reflection is used. However, since the refractive index is close to 1, the reflectance is very low, and most of the X-rays are transmitted or absorbed. In order to solve this problem, a material having a large difference between the refractive index in the wavelength range of the X-ray to be used and a vacuum refractive index (= 1) and a material having a small difference are alternately laminated in layers. Thus, a multi-layer reflecting mirror was developed in which a large number of reflecting surfaces were provided as interfaces, and the thickness of each layer was adjusted based on the theory of optical interference so that the phases of the reflected waves from the respective interfaces matched. A typical example of such a multilayer reflector is W
(Tungsten) / C (carbon), Mo (molybdenum) /
Combinations of Si (silicon) and the like are known. And
These multilayer films are formed by sputtering, vacuum deposition, CVD
(Chemical Vaper Deposition) and the like.

[0004]

The above-mentioned multilayer film has an artificial periodic structure and is inherently unstable. In particular, when such a multilayer reflector is used for high-intensity X-rays, part of the X-rays is absorbed by the multilayer film and its substrate, and the energy increases the temperature of the multilayer film. Therefore, reactions such as interdiffusion and compound formation are promoted in the multilayer film, so that the multilayer film structure is destroyed in a short time and the function as a reflector is lost.

[0005] Recently, as the practical use of the X-ray multilayer film reflecting mirror has progressed, the heat resistance of the multilayer film has been evaluated, and the heat resistance of some combinations of materials has been clarified. 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 °, and is therefore excellent as a multilayer mirror used in a reflection optical system of an X-ray reduction projection exposure apparatus. ing. However, this multilayer film has low heat resistance, and when heated to about 400 ° C. in a vacuum, the multilayer film structure is destroyed. It is known that this destruction phenomenon is caused by diffusion of silicon into the molybdenum layer and formation of molybdenum silicide. (See, for example, DGSterns et.al., J. Appl. Phys. 67 (1990) 2415.) In order to prevent the destruction of the multilayer structure due to such diffusion and to improve heat resistance, it is necessary to prevent diffusion at the interface. What is necessary is just to provide a layer. However, a substance having an anti-diffusion effect is not preferable in terms of optical characteristics, and lowers the reflectance of the multilayer film itself. Therefore, it is necessary to form the anti-diffusion layer as thin as possible in order to minimize the decrease in reflectance.

[0006] By the way, with respect to the formation of a thin film, it is known that most substances grow in the form of islands in the initial stage of thin film growth (for example, see Akira Kanehara thin film P39 Shokabo 1979). That is, a very thin thin film grows in an island shape without forming a continuous film, and becomes a continuous film after the film thickness is increased to some extent. The film thickness that changes from an island-like film to a continuous film varies depending on the substance, but is approximately several to several tens angstroms. On the other hand, the period length d of the X-ray multilayer mirror
(The thickness of one cycle of the multilayer structure) is given by the following Bragg's equation, where λ is the wavelength of the X-ray used and θ is the incident angle (with respect to the normal).

2d cos θ = λ For example, when a multilayer film is used at normal incidence, the cycle length of this multilayer film is 波長 of the wavelength. The X-ray multilayer mirror is used for soft X-rays having a wavelength of several tens to one hundred and several tens of angstroms, and the cycle length of the multilayer film is of the same order. Since the thickness of the diffusion preventing layer needs to be sufficiently smaller than the period length, the thickness is several to several tens of angstroms, which is a limit for a continuous film.

Accordingly, when the thickness of the diffusion preventing layer is reduced to prevent the reflectance from decreasing, the diffusion preventing layer becomes an island-like thin film, and in this case, the diffusion preventing effect is lost. In addition, since the interface roughness increases due to the island-like growth, scattering may occur and the reflectivity may rather decrease. As described above, in the method in which the diffusion preventing layer is simply provided at the interface, the reflectance is reduced because the thickness of the diffusion preventing layer cannot be made sufficiently small. The tendency was remarkable when a diffusion preventing layer was provided on a multilayer film having a small cycle length.

An object of the present invention is to solve such a problem.

[0010]

According to the present invention, there is provided an X-ray multilayer mirror comprising two layers of materials having different refractive indices in an X-ray wavelength range used alternately laminated. In at least one of the layers, the composition of the substance is different between a center portion and an interface portion with the other layer, and the composition is gradually changed between the center portion and the interface portion.

[0011]

According to the present invention, as shown in FIG. 1 (a), at least one of the layers (layer A and layer B) of a multilayer film composed of two substances having different refractive indices (layer A and layer B). Here, the layer B is composed of two types of substances (B-1 and B-1).
-2). And FIG. 1 (b)
As shown in the figure, the composition distribution in the depth direction of the B layer in the multilayer film is as follows:
Consists of During this time, the composition gradually changed. At this time, the substance B-2 is selected from substances having a refractive index that gives high reflectance but easily diffused, and the substance B-1 is a substance that is inferior to the substance B-2 in terms of reflectivity but hardly diffuses. did. Therefore, there is no possibility that the substance B-2 diffuses into the layer A. Note that there is no clear interface between the substance B-1 and the substance B-2, and no interface roughness occurs between them. Therefore, the reflectance of the multilayer film is not reduced by scattering. In addition, even if the cycle length of the multilayer film is reduced, the effect of preventing diffusion can be effectively exhibited.

Further, the substance B-1 for preventing diffusion is A
The composition ratio becomes 100% at the interface with the layer, and the composition ratio decreases toward the inside of the B layer. Therefore, FIG.
As shown in (2), the amount of the substance B-1 required to obtain the same diffusion prevention effect can be reduced as compared with the case where another layer is newly provided as the diffusion prevention layer. Therefore, it is possible to suppress a decrease in the reflectance.

As described above, according to the present invention, it is possible to prevent mutual diffusion between substances of each layer forming a multilayer film without greatly reducing the reflectance. As a result, the heat resistance of the multilayer film is improved. Although the number of layers is omitted in the drawing, several tens to several hundreds of such periodic structures are actually stacked.

[0014]

Embodiment 1 FIG. 3 is a schematic structural view of an ion beam sputtering apparatus used for producing a multilayer film of the present invention. This apparatus is provided with a substrate holder 2 for mounting a substrate 1 in a vacuum chamber 10, two types of ion sources 3a and 3b, and two types of target holders 4a and 4b. One target holder 4a is rotatable in the direction of arrow S in the figure by rotating means (not shown). Targets 5a and 5b made of different materials are attached to both surfaces of the holder 4a, respectively, so that a target of a material to be formed into a film can be opposed to the ion source 3a. The other target holder 4b is fixed, and a target 5c is attached. At the time of film formation, the argon ion beams 6a and 6b are emitted from the ion source 3a or 3b to collide with the target. Then, vapor generated by sputtering the target material is attached to the substrate 1 to form a thin film.

In this embodiment, molybdenum is used for the target 5a, silicon is used for the target 5b, and silicon oxide (SiO ₂ ) is used for the target 5c. Further, a silicon wafer was used as the substrate 1. Here, a manufacturing process of the multilayer film will be described. First, a target 5a made of molybdenum is irradiated with an ion beam 6a to form a molybdenum layer having a thickness of 25 ° on the substrate 1. Next, the target holder 4a is rotated to make the silicon target 5b face the ion source 3a. Then, the ion beam 6 is applied to the target 5b.
a, and the target 5c was also irradiated with the ion beam 6b. At this time, by controlling the outputs (acceleration voltage and beam current of the ion beam) of each of the ion sources 3a and 3b, only silicon oxide is formed at the start and end of the film formation, and the silicon composition is formed in the middle. By gradually increasing the ratio, the silicon composition ratio becomes 100% at the center of the layer,
Thereafter, the composition ratio of silicon was gradually reduced again. In this way, a layer (B layer) composed of silicon oxide and silicon having a thickness of 50 ° and having a modulated composition distribution as shown in FIG. 1 is formed. The above operation was repeated 50 times to form a multilayer film having a period length of 75 °. This multilayer film
In FIG. 1, the substance A forming the A layer is made of molybdenum, and the substances B-1 and B-2 forming the B layer are made of silicon oxide and silicon, respectively.

The multilayer film produced as described above was heated in vacuum to examine the heat resistance. As a result, there was no change in reflectance up to 600 ° C., and at 800 ° C., the multilayer structure was broken and no longer reflected. As a comparative example, a film thickness of 25
モ molybdenum layer and 50Å silicon layer alternately
A multilayer film was prepared by laminating 50 layers each. And the heat resistance was examined similarly. As a result, at 400 ° C., the light was no longer reflected.

Next, the soft X-ray reflectivity of each of the multilayer film of the present embodiment and the conventional multilayer film was measured with S-polarized light using emitted light. The incident angle of synchrotron radiation is
The angle was set to 15 ° so that the peak of the reflectance occurred near the wavelength of 135 °. The result is shown in FIG. In the graph of FIG. 4, a curve P indicates the reflectivity of the conventional multilayer film of the combination of molybdenum / silicon, and a curve Q indicates the reflectivity of the multilayer film of the present embodiment. As can be seen from the graph, the wavelength is 137Å
The reflectivity of the multilayer film of the present example was 66%, whereas the reflectivity of the conventional reflector was 72% with respect to the above light, and no significant decrease in the reflectivity was observed.

In order to obtain the same heat resistance as the multilayer film of this embodiment in the multilayer film having the diffusion preventing layer C at the interface as shown in FIG.
Requires a layer made of silicon oxide. The result of similarly measuring the soft X-ray reflectivity of this multilayer film is shown as a curve R in the graph of FIG. As can be seen from the graph, the reflectance of the multilayer film in this case was 58%, which was lower than that of the multilayer film of the present example. If the thickness of silicon oxide is smaller than 10 °, a sufficient diffusion preventing effect cannot be obtained.

[0019]

[Embodiment 2] In this embodiment, a multilayer film was produced by changing the target material in the same apparatus as in Embodiment 1. Then, molybdenum was used for the target 5a, silicon was used for the target 5b, and silicon carbide (SiC) was used for the target 5c. As the substrate 1, a silicon wafer is used.

First, a target 5a made of molybdenum is irradiated with an ion beam 6a to form a film thickness on the substrate 1.
A 25 mm molybdenum layer was formed. Next, the target holder 4a is rotated to make the silicon target 5b face the ion source 3a. Then, while irradiating the target 5b with the ion beam 6a, the target 5c
Was also irradiated with the ion beam 6b. At this time, by controlling the outputs (acceleration voltage and beam current of the ion beam) of each of the ion sources 3a and 3b, only silicon carbide is formed at the start and end of the film formation, and the silicon composition is formed in the middle. Gradually increase the silicon composition ratio at the center of the layer.
After that, the composition ratio of silicon gradually decreased again. Thus, a layer composed of silicon carbide and silicon having a thickness of 50 ° and having a modulated composition distribution as shown in FIG. 1 was formed. The above operation was repeated 50 times to form a multilayer film having a period length of 75 °. In this multilayer film, the substance A forming the layer A in FIG.
The substance B-1 and the substance B-2 forming the B layer are made of silicon carbide and silicon, respectively.

The multilayer film produced as described above was heated in a vacuum to examine the heat resistance. As a result, there was no change in reflectance up to 800 ° C., and at 950 ° C., the multilayer structure was broken and no longer reflected. As a comparative example, a film thickness of 25
モ molybdenum layer and 50 と silicon layer alternately
A multilayer film was prepared by laminating the layers one by one. And the heat resistance was examined similarly. As a result, at 400 ° C., the light was no longer reflected.

Next, the soft X-ray reflectivity of each of the multilayer film of the present embodiment and the conventional multilayer film was measured with S-polarized light using emitted light. The incident angle of synchrotron radiation is
The angle was set to 15 ° so that the peak of the reflectance occurred near the wavelength of 135 °. The result is shown in FIG. In the graph of FIG. 5, a curve P indicates the reflectance of a conventional multilayer film of a combination of molybdenum / silicon, and a curve Q indicates the reflectance of the multilayer film of the present embodiment. As can be seen from this graph, the wavelength 137
The reflectivity of the multilayer film of the present example was 68%, whereas the reflectivity of the conventional multilayer film was 72% for the light of Å, and no significant decrease in the reflectivity was observed.

In order to obtain the same heat resistance as the multilayer film of the present embodiment in the multilayer film having the diffusion preventing layer C at the interface as shown in FIG. A layer made of silicon is required. The result of similarly measuring the soft X-ray reflectivity of this multilayer film is shown as a curve R in the graph of FIG. As can be seen from the graph, the reflectance of the multilayer film in this case was 63%, which was lower than that of the multilayer film of the present example. When the thickness of silicon oxide is smaller than 15 °, a sufficient diffusion preventing effect cannot be obtained.

[0024]

As described above, according to the present invention, it is possible to prevent the mutual diffusion between the substances forming the multilayer film while keeping the decrease in the reflectivity of X-rays small. Therefore, the heat resistance of the multilayer film can be significantly improved. Further, since it is not necessary to newly provide a diffusion prevention layer at the interface portion as in the related art, the heat resistance of a multilayer film having a small cycle length can be effectively improved.

In the present invention, the use of a multilayer optical system used for a high-intensity X-ray source such as synchrotron radiation, or extremely severe durability such as an X-ray laser resonator is required. It can greatly exert its effect on the intended use and the like.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a structure of a multilayer film according to the present invention and a composition distribution in a depth direction thereof.

FIG. 2 is a schematic view of a conventional multilayer film provided with a diffusion preventing layer at an interface.

FIG. 3 is a schematic configuration diagram of an ion beam sputtering apparatus used for manufacturing a multilayer film of the present invention.

FIG. 4 is a graph showing measured values of soft X-ray reflectivity of the multilayer film of Example 1.

FIG. 5 is a graph showing measured values of soft X-ray reflectivity of the multilayer film of Example 2.

[Description of Signs of Main Parts]

 Reference Signs List 1 substrate 2 substrate holder 3a ion source 3b ion source 4a target holder 4b target holder 5a target 5b target 5c target 6a ion beam 6b ion beam 10 vacuum chamber

Claims (1)

(57) [Claims] 1. An X-ray multilayer mirror comprising two layers made of materials having different refractive indices in an X-ray wavelength range to be used, wherein at least one layer has a central portion and the other layer. An X-ray multilayer mirror, wherein the composition of the substance differs between the interface and the interface, and the composition gradually changes between the center and the interface.