JP2723955B2 - Multilayer reflector for soft X-ray and vacuum ultraviolet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is directed to optical devices, in particular, soft X-rays to light having a wavelength of 2000 ° or less, which is called vacuum ultraviolet light, and whose incident angle is close to perpendicular to a mirror surface. The present invention relates to a multilayer reflector for soft X-rays and vacuum ultraviolet rays which can be suitably used for incidence.

[Conventional technology]

Conventionally, with respect to light on the shorter wavelength side than the region called vacuum ultraviolet, the refractive index of all the materials is close to 1 and the absorption coefficient is so large that it cannot be ignored. A reflector having a high reflectivity at an incident angle close to that angle could not be obtained, and the reflectivity was less than 1%.

However, in recent years, a technique of controlling the thickness of a thin film material on the order of angstroms and stacking a large number of layers has been developed. Multi-layer reflector for X-ray and vacuum ultraviolet has been proposed (E. Spiller, Appl. Phys. Lett, vol 20, 36
5 (1972)), and its research and development has been actively pursued.

The multilayer reflector has a structure in which tens to hundreds of layers of A and B2 materials are alternately laminated, but in order to obtain a high reflectance, the difference in the refractive index between adjacent A and B2 layers is required. It is necessary to select a combination of dissimilar materials that increases. Until now, the combination was tungsten W, nickel N
i, metal such as platinum Pt and carbon C, or molybdenum Mo
Alternating layers of combinations of heavy and light elements such as silicon and silicon Si are known. In addition, in order to adjust the phase condition of the light reflected from the interface between the layers, it is necessary to control the thickness of each layer on the order of several angstroms and manufacture the layers.

[Problems to be solved by the invention]

The above-mentioned multilayer film reflecting mirror is mainly formed by a vacuum film forming method such as vapor deposition or sputtering. However, there is a cause for lowering the reflectance even in a multilayer film which is ideally manufactured by controlling the film thickness.

One of them is that irregularities occur on the surface of the thin film at the time of film formation, and the layer interface is not smooth but rough. Especially at a film thickness of 100 mm or less, since the thin film forms an island-like structure,
This irregularity appears inevitably. As a result, for example, in a multilayer reflector having an interface roughness of 20% with respect to the periodic thickness of the A and B1 layer pairs at the layer interface, the reflectivity is 80 times higher than the theoretical design value.
% Has also dropped.

Another cause of the decrease in reflectivity was that the A and B2 materials mutually diffused at the atomic level during film formation, and the difference in refractive index between adjacent materials was reduced. . Regarding the latter, some proposals have been made to form a buffer for preventing diffusion between the AB layers (JP-A-60-74).
(00,61-128199) A specific invention which solves the former has not been proposed until now except for the method of film formation and the method of selecting the A and B materials.

[Means for solving the problem]

The present invention has been made in view of the above problems, and has as its object to provide a multilayer mirror having high reflectivity, high irradiation durability, and stability while alleviating and eliminating the drawbacks of the conventional example. Is to do.

According to the present invention, in a multilayer reflector for soft X-rays and vacuum ultraviolet rays having a multilayer thin film structure composed of alternating layers of A and B two types of main materials, the sub-material thin film is formed between the AB layer and / or the B-layer. By forming a periodic structure in which at least one or more layers are stacked between the A layers, it is possible to reduce the roughness of the multilayer film interface, thereby achieving the object of the present invention.

FIG. 1 is a schematic view of an embodiment of the multilayer reflector for soft X-rays and vacuum ultraviolet rays according to the present invention.

The multilayer mirror of the present invention shown in FIG. 1 has a flat or curved substrate 1 polished sufficiently smoothly compared to the wavelength used.
The first layers A1, A2, A3, which are low refractive index layers, and the second layers B1, B2, B3, which are high refractive index layers, and the first layer A and the A third layer C1, C2, C3 between the second layer B
Are formed to form a periodic structure of ACCBACCB-. The low refractive index layer is generally tungsten,
It is formed of a high melting point metal material such as molybdenum or a compound containing them as a main component. On the other hand, the high refractive index layer is generally formed of a light element such as carbon, silicon, boron, beryllium, or a compound containing these as main components.

In the present invention, the auxiliary material forming the third layer needs to have an effect of reducing the roughness of the layer interface by lamination. In general, a material and a method of forming a crystal layer by vacuum film formation tend to have an island-like structure with a film thickness of 100 mm or less, so that there is a possibility that unevenness at the interface may be formed or increased.
On the other hand, a material and a method in which an amorphous layer is formed at the time of vacuum film formation hardly form an island structure and have a function of reducing unevenness of an interface by lamination. For example, it has been reported that in a multilayer film system containing vacuum-deposited carbon, which tends to become amorphous, the defects and roughness of the lower layer are reduced by lamination (Y. Le.
petre et al. Proceedings of SPIE, vol 563 p258-26
3). For this reason, a material that forms an amorphous layer by vacuum film formation is desirable as a sub-material.

It is preferable that the auxiliary material forming the third layer has a small soft X-ray absorption coefficient in order to obtain a high reflectance, and it is generally preferable to use a light element having a small soft X-ray absorption coefficient.

For the above reasons, carbon C,
Boron B, beryllium Be, silicon carbide SiC silicon nitride Si ₃
N ₄ , silicon oxide SiO ₂ , boron nitride BN, boron carbide B ₄ C,
Examples include a conductor of a light element having an atomic number of 13 or less such as aluminum nitride AlN or a compound thereof.

In the present invention, it is necessary to set the thickness of the sub-material forming the third layer to such an extent that the absorption coefficient of the main materials A and B is not adversely affected.
A thickness of 5 or less is desirable.

For the production of the multilayer reflector for soft X-rays and vacuum ultraviolet rays of the present invention, vacuum deposition in an ultra-high vacuum is preferably used, but when a high melting point material is used, sputtering is also an effective means, Other methods for forming various thin films such as resistance heating, CVD, and reactive sputtering can be used.

In the multilayer film having the above-described structure according to the present invention, the third layer has the following function.

First, the high-melting-point material forming the low-refractive-index layer forms an island-like structure and crystal grows when vacuum-deposited, thereby causing irregularities on the layer surface and roughening the layer surface. Further, it is known that the lamination of each layer reflects the roughness of the lower layer to the upper layer, and the interface layer becomes rougher toward the upper layer. On the other hand, rough
By laminating so as to insert a third layer made of an amorphous material between the first layer A and the second layer B, the roughness of the surface of the first layer A is reduced, and the roughness is reduced from the lower layer to the upper layer. To act as a buffer layer that prevents propagation to the surface. as a result,
The amount of interface roughness of the multilayer film can be reduced as a whole, and a multilayer mirror having a high reflectance can be obtained.

Second, the light-element amorphous material is uniformly and densely stacked, and acts to prevent mutual diffusion between the low-refractive material A and the high-refractive material B. In particular, when materials A and B are a combination that easily forms a compound (for example, silicide is formed by a small amount of heating with high melting point metal and silicon).
By forming the third layer thin between the A layer and the B layer, the interdiffusion of AB material and the formation of a compound can be prevented.
Interdiffusion often occurs through cracks at the granular boundaries of the layers. For this reason, the buffer layer inserted to prevent the roughness of the interface also functions as a mutual diffusion preventing layer.

In the above embodiment, a third layer as a sub-material is formed between the low-refractive material A and the high-refractive material B so that ACB
Was formed, but if necessary, BCA
, Or a four-layer periodic structure (FIG. 2) such as ACBC or ACBD (D is a sub-material different from C). Thereby, the above-mentioned operation can be further improved.

〔Example〕

Hereinafter, the present invention will be described in more detail with reference to examples of the present invention.

Example 1 A substrate made of fused quartz polished so that the surface roughness becomes 3 ° or less in rms value, ruthenium Ru as a main material forming a first layer, and silicon Si as a main material forming a second layer Used, and carbon C is selected as a sub-material forming the third layer,
The film thicknesses are 23 °, 37 ° and 7 °, respectively, and Ru-
Twenty cycles are formed in the order of C-Si, and finally Ru23Å and C30Å are laminated, and the soft X-ray of the present invention as shown in FIG.
A multilayer reflector for vacuum ultraviolet light was manufactured.

The manufacturing method is to evaporate Ru, C, and Si in order by an electron beam heating method in an ultra-high vacuum of 1 × 10 ⁻⁶ Pa or less so that the thickness of each layer becomes a predetermined value by a quartz crystal film thickness meter. The shutter was opened and closed and controlled.

When this reflecting mirror was evaluated by the small-angle X-ray diffraction method (wavelength: 1.54 °), it was found that the roughness of the layer interface was 5.5 ° rms or less. When light of wavelength 130.0Å was incident on the mirror surface of this reflector at 10 ゜ from the normal, 57.0 法
% Reflectance was obtained.

For comparison, a Ru / Si 20-period multilayer film having a thickness of Ru27Å and Si40Å with no carbon C interposed therebetween was prepared by the same fabrication method to make the layer period equal to that of the multilayer film, and evaluated. Was 12.5Årms, and the reflectance for a wavelength of 130.0Å was 20.0%. From this, it was found that by laminating the C layer thinly between the Ru layer and the Si layer, the roughness of the interface of the multilayer film was reduced, and as a result, the reflectance of the reflecting mirror was increased.

Example 2 A silicon single crystal polished so that the surface roughness was 4 ° or less in rms value was used for a substrate, and molybdenum (Mo) was used as a main material of the first layer, and silicon (Si) was used as a main material of the second layer.
And silicon oxide SiO ₂ was selected as a sub-material for forming the third and fourth layers, and the film thickness was set to Mo22Å and Si33, respectively.
SiO, SiO ₂ 5Å is formed from the lower layer in the order of Mo—SiO ₂ —Si—SiO ₂ in four-layer units for 20 periods, and finally Mo22Å is laminated, and the softness of the present invention as shown in FIG. A multilayer reflector for X-ray and vacuum ultraviolet light was manufactured.

The fabrication method was such that Mo and Si having a diameter of 5 inches were used as targets by RF magnetron sputtering, and Mo was first sputtered at an Ar pressure of 3 × 10 ⁻¹ Pa and laminated to a predetermined thickness. Next, O ₂ is introduced into the vacuum device, and Ar + O ₂ gas pressure is 5 × 10 ^-1 Pa.
After sputtering a Si target to form a SiO ₂ thin film of a predetermined thickness, the introduction of O ₂ gas was stopped, and the Si pressure was reduced to 3 × 10 ^-1 Pa at Ar pressure.
The thin films were laminated to a predetermined thickness. After that, sputtering is performed again by introducing O ₂ gas, and the four layers of Mo—SiO ₂ —Si—SiO ₂ are formed.
A periodic structure was formed. By repeating the above method, 20 cycles were formed, and finally Mo was laminated by 22 mm. When this reflecting mirror was evaluated by the small-angle X-ray diffraction method, the roughness of the layer interface was 6.5 界面 r
It turned out to be less than ms. When X-rays having a wavelength of 130.0 ° were incident on the mirror surface of the reflecting mirror at 10 ° from the normal line, a reflectance of 46.5% was obtained.

The reflecting mirror was heated under vacuum at 650 ° C. for 2 hours, and then the structural change of the multilayer film was observed by small-angle X-ray diffraction.
As a result, almost no change was observed in the periodic structure before and after the heat treatment.

On the other hand, for comparison, a Mo / Si 20-period multilayer film was prepared by sputtering without forming a SiO ₂ layer between Mo 25 ° and Si 42 °, and subjected to the same heat treatment as described above. As a result, in the sample after heating, the intensity of the primary peak among the Bragg diffraction peaks corresponding to the multilayer structure was reduced to about 1/5, and the higher-order peak of the second or higher order was not observed. This is presumably because the heat treatment caused the inter-diffusion of the multilayer film layers to disturb the periodic structure.

From the above results, Mo / Si laminated with SiO ₂ as the intermediate layer
In the multilayer film, it was found that interdiffusion between Mo and Si due to heating was unlikely to occur, and the thermal stability increased.

〔The invention's effect〕

As described above, in the multilayer mirror of the present invention, by forming a periodic structure in which sub-material thin films are stacked as an intermediate layer, the surface and interface roughness due to the island-like structure occurring in the growth process of the thin film is prevented from increasing. Thus, a multilayer mirror for soft X-rays and vacuum ultraviolet rays having a high reflectance can be provided.

In addition, the intermediate layer prevented interdiffusion even in a multi-layer film made of a combination of materials that easily form a compound, thereby achieving thermal stabilization of the multi-layer structure. In particular, since the soft X-ray / vacuum ultraviolet reflecting mirror has many applications in an environment where it is exposed to high-brightness light such as radiation light, the present invention is effective against the aging of the reflecting mirror.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a multilayer reflector embodying the present invention,
FIG. 2 is a schematic sectional view of a multilayer mirror according to another embodiment of the present invention. 1: Substrate A1, A2 ... _{An + 1} : First layer made of main material A (low refractive index) B1, B2 ... _Bn : Second layer made of main material B (high refractive index) C1, C2 ... C _{n + 1:} the third layer (middle layer 1) D1, D2 ... D _n consisting secondary material C: a fourth layer made of secondary material D (intermediate layer _{2) d a, d B,} d C, d _D : Thickness of thin films of layers A, B, C, D

Continuation of the front page (72) Inventor Yoshiaki Fukuda 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (56) References JP-A-60-7400 (JP, A)

Claims (3)

(57) [Claims] 1. A soft X-ray / vacuum ultraviolet multilayer film reflecting mirror having a multilayer thin film structure composed of alternating layers of A, B and two kinds of main materials having different refractive indices from each other. A multilayer film reflecting mirror for soft X-rays and vacuum ultraviolet rays, characterized in that at least one sub-material thin film having the following structure is laminated between the AB layers and / or the BA layers to form a periodic structure. . 2. The multi-layer reflection film for soft X-rays and vacuum ultraviolet rays according to claim 1, wherein the constituent material of the sub-material thin film is an amorphous substance composed of a simple element of a light element or a compound thereof. mirror. 3. The multi-layer reflecting mirror for soft X-rays and vacuum ultraviolet rays according to claim 1, wherein the constituent material of said sub-material thin film is carbon C or silicon oxide SiO ₂ .