JP2535037B2 - Multi-layer film mirror for X-ray / VUV - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is directed to an optical device, in particular, to light having a wavelength of 200 nm or less, which is called vacuum ultraviolet light from X-rays. It relates to a reflector.

[Conventional technology]

Conventionally, there is no reflector that has a high reflectance for light with a shorter wavelength than the region called vacuum ultraviolet when it is incident at an angle perpendicular to or close to the surface. However, only a reflectance of 1% or less was obtained.

On the other hand, even with an oblique-incidence reflecting mirror having a relatively high reflectance, it was necessary to adjust the incident angle within 1 ° or less or 2 to 3 ° from the mirror surface. Even in this case, since the light beam is incident on the surface at a small angle, a very large reflecting surface is required even for a thin light beam, and the use of the device is difficult and limited.

In addition, the oblique incidence mirror has a small degree of freedom in the optical system configuration, and even in the production of the reflecting mirror, highly accurate flatness is required over a large area, and there are many restrictions in actual use. In the wavelength range from the visible region to 2000 Å, a multi-layered film reflecting mirror utilizing interference of multi-layered thin films has been proposed. For example, as shown in the reflectance characteristics in Fig. 2, Mg is visible in the visible range of 2000 Å or higher.
By forming a multilayer alternating layer with a combination of two dielectrics such as F ₂ and ZnS, a reflectance of approximately 80 to 100% is obtained at a normal incidence.

However, in the case of a dielectric alternating layer, the absorption increases sharply in the wavelength range of 2000 Å or less, and at 1000 Å or less, there is almost no combination of materials that can be used as a reflecting mirror. Even in the case of a metal single layer film such as Al, Au, Pt, etc., whose characteristics are shown in Fig. ² , the reflectance decreases sharply in proportion to λ ² at wavelengths shorter than 700 Å, and becomes 1 at wavelengths shorter than 500 Å and 200 Å. Only a reflectance of less than% can be obtained with normal incidence.

On the other hand, a metal multi-layered film reflecting mirror in which two metal materials having different complex refractive indexes are alternately laminated has been tried, but the reflectivity of most substances is absorbed in the X-ray and VUV region. Is represented by a complex refractive index (n + ik, hereinafter referred to as refractive index) having an imaginary part K representing
Real part n is approximately 1.0 (n = 1-δ, δ = 10 ^-1 to 10 ^-3 ).
Therefore, the reflectance of Fresnel at the boundary between the vacuum and the material thin film is very small, on the order of 0.1% or less. Further, even at the boundary between laminated thin films of different materials, the reflectance does not exceed several% per single boundary surface.

However, high reflectivity is achieved by using a multilayer structure in which different materials are alternately stacked and the reflected light from each layer boundary is strengthened by interference to maximize the reflectivity of the entire multilayer film. Can be obtained. Further, it is known that it is possible to realize a reflecting mirror having a high reflectance by selecting a combination of different kinds of materials such that the difference in refractive index between adjacent layers becomes large and combining with the above film thickness constitution.

As a combination of materials known to date for the structure of a multilayer film, there is a transition metal as a low refractive index material,
Most of the high refractive index materials have used semiconductor elements such as carbon and silicon. Typical examples are a combination of tungsten (W) and carbon (C), a combination of molybdenum (Mo) and silicon (Si), and the like.

In order to obtain a high reflectance in this type of multilayer reflector, in order to obtain a high reflectance, the surface of the substrate, the interface between each layer of the multilayer reflector and the layer adjacent to it, and the surface roughness of the surface of the multilayer reflector are created. Is important to be small. As the evaluation method, for example, the surface roughness of the substrate surface and the surface of the multilayer film reflecting mirror produced is measured by a surface roughness meter of a method of averaging the roughness of a wide area such as a heterodyne interferometric surface roughness meter, that is, an area of several square μm. The method of doing was used conventionally.
However, in this method, not only the averaged information in a wide area can be obtained, but also the surface roughness of the interface between each layer of the multilayer mirror and the layer adjacent thereto cannot be directly known at all. For this reason, unevenness with a size of several tens of Å or more and several thousand Å or less in the direction parallel to the interface between each layer and the layer adjacent to it, which is an important cause for reducing the reflectance of X-rays and VUV Was not taken into account, and no effort was made to control such irregularities.

[Problems to be solved by the invention]

In the above conventional example, although the surface roughness measured by a method of averaging a wide area, that is, an area of several square μm is sufficiently controlled, there is a large difference between the expected reflectance and the measured reflectance. However, there is a drawback in that only a multilayer reflective mirror having a low reflectance can be obtained. For example, the surface of the substrate, the interface between each layer of the produced multilayer reflector and its adjacent layers, and the surface roughness of the produced multilayer reflector with a surface roughness of 10 Å in terms of rms value and its reflectance of 20 Å Comparing the two, for example, soft X-ray with a wavelength of 124 Å
Surface roughness 2 when vertically incident on a single-layer multilayer mirror
The reflectance of a 0 Å multilayer mirror is only about 1/10 of the reflectance of a 10 Å multilayer reflector.

The surface roughness of the substrate surface and the interface of the multi-layered structure can be reflected in the optical path difference of the reflected light by up to twice. Therefore, the X-rays or the vacuum ultraviolet rays reflected by the reflecting mirror having a surface roughness of 1/8 of the used wavelength have a shift of the optical path difference of 1/4 of the wavelength at the maximum and reduce the reflectance. In a region where tens to thousands of X-rays or vacuum ultraviolet rays have coherence, the surface roughness causes phase shifts and causes a coherent decrease in reflectance. For example, the multilayer film consisting of 41 layers of Ru and Si mentioned above has a soft X of 124 Å
When a line is incident, when a cross section is observed with a transmission electron microscope, the reflectance of a multilayer film mirror with a surface roughness of 15Å, which is approximately 1/8 of a wavelength of 124Å, is about that of a multilayer film mirror with almost no surface roughness.
You can only get about 1/4. Generally, surface roughness is 1/15 of wavelength used
When the surface roughness becomes 1/8 of the wavelength used, the reflectance becomes 1/4 when the surface roughness becomes 1/8.

It is an object of the present invention to provide a mirror surface having a high reflectance with respect to X-rays and vacuum ultraviolet rays, and to provide an optical device having an optical characteristic which is not available in the past.

[Means for solving problems]

The problem of the prior art described in detail above is that in a multilayer film reflecting mirror for X-ray / vacuum UV having a reflecting mirror having a multilayer structure composed of alternating layers of substances having different refractive indexes on a substrate, an interface in the multilayer structure is provided. And the surface roughness of the reflecting mirror is a multilayer film reflecting mirror for X-ray / vacuum UV whose rms value is 1/8 or less of the operating wavelength when observing a cross section of a sample with a thickness of 100 Å or more and 700 Å or less by a transmission electron microscope. Solved by. Here, in the case of observing the above sample with a transmission electron microscope, the interfaces between the layers of the multilayer-film reflective mirror and the layers adjacent thereto are as shown in FIG. A negative photographic film of a transmission electron microscope image of a reflecting mirror was measured by a microdensitometer in a direction perpendicular to the surface, and the density was smoothed as a function from the substrate surface, and the density was determined as a point between the peaks and valleys of the density. It is decided.

 Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic view of a cross section of a multilayer film reflecting mirror for X-ray / VUV rays according to the present invention. Here, on the substrate 1, first material layers 2, 4 ...
3,5 ... alternate thicknesses d ₂ , d ₄ ... and d ₃ , d ₅ ... respectively.
Are stacked as. Then, in the reflecting mirror of the present invention, the layers are laminated so that the roughness of the interface and the finally produced surface is 1/8 or less of the used wavelength in rms value when observed by a transmission electron microscope.

The surface of the substrate 1 is processed into a flat surface, a convex surface, a concave surface, or an aspherical surface according to the use of the optical instrument in which the manufactured mirror surface is incorporated, and after polishing with a diamond paste,
Floating polishing, chemical polishing, long-time heating at a temperature slightly lower than the melting point of the substrate material in ultra-high vacuum, flash heating, or the like is repeated until the surface roughness is sufficiently reduced. In this process, impurities attached to the substrate surface are also removed.

In order to keep the surface roughness of the interface within the required range, it depends on the combination of the composition of the first substance and the second substance and the film forming method, but the back pressure should be low and the purity of the film forming material should be low. There are methods such as increasing the temperature, cooling the substrate to suppress the growth of fine crystals to form a uniform amorphous film, and conversely, heating the substrate to form a crystalline film.

In order to prevent the roughness of the substrate surface from affecting the multilayer film, a buffer layer is provided between the substrate surface and the multilayer film, and in the layer thereabove, the interface between each layer and the adjacent layer and the surface roughness of the surface are predetermined. Be smaller than the value of. In addition, in the case of layers near the surface (at least layers within 0.1 μm from the surface) that increase the total number of layers and have a large effect on reflectance, the interface between each layer and the layers adjacent to it and the surface roughness of the surface should be smaller than the specified value. There is also a method such as.

The reflectance developed by the multilayer mirror for X-ray / vacuum ultraviolet ray of the present invention is the difference in the refractive index between two types of substances having different refractive indexes forming the mutual layers, the absorptance of each layer, and the number of layers to be laminated. , It depends on the wavelength of the light to be irradiated, but the difference in the refractive index is practically at least 0.01 when the number of layers is 100 pairs.
It is preferable to have the above.

In order to give each of the alternating layers a difference in refractive index, the alternating layers are formed by using a substance having a high refractive index and a substance having a low refractive index with respect to light in the X-ray / vacuum ultraviolet region, which is the object of the present invention. It may be formed.

The averaged film thicknesses d ₂ , d _3, ... Of the respective layers are approximately 1/4 of the target wavelength, and they are alternately laminated films made of the same material. Meet the condition that all reflected light interferes constructively, or
When the absorption loss in each layer and the reflectance decrease due to the phase shift are compared, it is determined by either or both of the conditions that the reflectance decrease of the entire multilayer film is smaller. At that time, the film thicknesses may be the same for all the same material layers, or may be different for each layer so that the reflectance is maximized, and the thicknesses are not necessarily equal.

As a laminated structure, it is desirable to select a material that has a large difference between the refractive index of the final layer, which is a layer in contact with gas or vacuum, and the refractive index of gas or vacuum. It is also preferable that the difference in refractive index between the substrate and the layer in contact with the substrate be large.

Further, it is preferable that the number of layers is 5 pairs or more because the reflectance increases as the number of alternating layers increases, but if the number is too large, the effect of the absorption layer becomes remarkable, and therefore, considering the ease of production, too. About 200 layers pair is good. Further, a protective layer made of a stable material with little absorption may be provided on the final layer.

As a film forming method for producing the multilayer mirror for X-ray / vacuum ultraviolet ray of the present invention, an electron beam method in an ultra-high vacuum is preferably used. In particular, when a compound material is used, residual oxygen is used. A sputtering method in a vacuum with a sufficiently small amount is an effective method. Furthermore, as a film forming method with high film strength, an ion plating method and a metal organic chemical vapor deposition method (MO
Needless to say, the multilayer film may be formed using CVD or the like.

〔Example〕

Example 1 A silicon substrate (2 ″ φ, 10 mmt) processed to have a surface accuracy of λ / 20 (λ = 6328Å) was polished with diamond paste. When the surface roughness was measured with a heterodyne interferometric surface roughness meter, the rms value was 7.09. It was Å.

Back pressure of the same lot as the substrate whose surface roughness was measured 1 x
Set it on the heater surface of the substrate holder of the device shown in Fig. 4 at 10 ^-10 torr, and set it to 1400 in advance before vapor deposition.
Heated to ° C (2 hours). After that, the substrate was left to cool to room temperature, then the substrate was kept in contact with a copper claw protruding from the liquid nitrogen shroud for 3 hours, and the substrate temperature was measured with a thermocouple type thermometer.

In the apparatus shown in FIG. 4, three electron beam evaporation sources are arranged symmetrically with respect to the substrate (in FIG. 4, one is on the other side). At 9
9.9% ruthenium (hereinafter Ru), 99.999% silicon (hereinafter Si), and 99.999% carbon (hereinafter C) were set in advance. After waiting until the degree of vacuum was restored to 3 × 10 ^-10 torr, vapor deposition was started.

Ru, Si, and C were preheated with an electron beam in advance (shutters on each hearth and the main shutter were closed), and after preheating for 30 minutes to 1 hour, each deposition rate was 3Å / min, A crystal oscillator (not shown) installed near each hearth at 5 Å / min and 3 Å / min was fed back so that the above deposition rate was maintained. Then, vapor deposition was performed while maintaining a vacuum degree of 8 × 10 ^-10 torr. As shown in FIG. 5, first, 200 Å of C was formed as a buffer layer, and then 41 layers of Ru and Si were alternately formed with the respective film thicknesses of 27.2 Å and 36.2 Å. The upper layer and the uppermost layer of C were Ru, and a reflecting mirror A was obtained.

When a soft X-ray having a wavelength of 124 Å was made incident on the manufactured multilayer film reflecting mirror at an inclination of 10 ° from an axis perpendicular to the plane, a reflectance of 59.7% was obtained. There is no buffer layer of C, and the reflectance of the soft X-ray is about 6 times as high as the reflectance (10.2%) of the soft X-ray of the sample B obtained by vapor-depositing the same film thickness as in the above embodiment without cooling the substrate. Became a rate. Although there is no buffer layer for C, the soft X-ray reflectance (23.7%) of the sample C obtained by cooling the substrate and depositing the same film thickness as in the above embodiment was about
The reflectance was 2.5 times. This result indicates that the roughening of the substrate did not affect the multilayer film by inserting the carbon buffer layer between the substrate and the multilayer film structure, and that the substrate was cooled during the evaporation, so that This is because the movement of atoms was suppressed and the formation of island-shaped structures was suppressed, so that the roughness of the interface between each layer in the multilayer portion and the layer adjacent thereto was reduced.

When the surface of the multilayer film prepared by the heterodyne interferometric surface roughness meter was measured for sample C, the rms value was 3.1Å.
When a sample for cross-section observation with a thickness of about 500Å was made and a picture of the image was taken with a transmission electron microscope and the surface roughness was measured,
Roughness of the interface with the buffer layer of 15.9 Å in rms value, wavelength 124
Although it was larger than 1/8 of Å, the roughness of the interface was small in the multilayer film above the buffer layer, and the rms value was 7.2 Å at the interface between the uppermost layer and the layer below it. The interfaces of the other layers in the multilayer film portion were almost the same.

〔The invention's effect〕

As described above, the multilayer mirror for X-ray / vacuum ultraviolet ray of the present invention has several Å or more and several thousand Å or less at the interface between each layer of the multilayer film and the layer adjacent thereto and the surface of the formed multilayer film. By paying attention to even unevenness having a size and setting them to 1/8 or less of the used wavelength as specified in the claims, there is an effect that the reflectance can be increased from several times to 10 times. At the same time, since the scattered light is reduced, there is also an effect of improving the optical characteristics as a reflecting mirror.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a cross section of a multilayer film reflecting mirror for X-ray / vacuum ultraviolet ray according to the present invention, FIG. 2 is a diagram showing the reflectance with respect to the irradiation light wavelength of a dielectric multilayer or a metal single layer, and FIG. About the sample of the example, the principle diagram for determining the interface from the result of the density measurement of the negative film of the image of the multilayer film mirror by the transmission electron microscope by the microdensitometer, and FIG. The figure which shows a vacuum electron beam vapor deposition apparatus, 5th
The figure is a schematic view of a cross section of a multilayer film reflecting mirror for X-ray / vacuum ultraviolet rays having a buffer layer of a third substance. 1 is the substrate, 2 and 4 are the first substance, 3 and 5 are the second substance, 6 is the reflectance curve of the dielectric multilayer, 7 is the reflectance curve of the metal single layer, and 8 is measured by a microdensitometer. Concentration, 9 is smoothed concentration, 10 is interface, 11 is liquid nitrogen shroud, 12
Is a substrate holder provided with a heater, 13 is a substrate, 14 is a claw for cooling the substrate, 15 is a main shutter, 16 is a shutter, 17,1
8 and 19 are electron gun hearths, 17 to Ru, 18
C is set in and C is set in 19. 20 is a crystal oscillator, 21
Represents C and 22 represents a third substance used as a buffer layer.

Claims (1)

(57) [Claims] 1. A multilayer film reflection mirror for X-ray / vacuum ultraviolet light, comprising a substrate having a reflection mirror having a multi-layer structure composed of alternating layers of substances having different refractive indexes, the interface of the multi-layer structure and the surface of the surface of the reflection mirror. Roughness value by transmission electron microscope
Rm when observing a cross section of a sample with a thickness of 100Å or more and 700Å or less
Multi-layer film mirror for X-rays and VUV rays with an s value that is 1/8 or less of the wavelength used.