JP2535038B2 - 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 reflecting mirror that has a high reflectance when the light has 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 low degree of freedom in the optical system configuration, and it is required to have high-precision flatness over a large area when manufacturing the reflecting mirror, 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.
A multilayer alternating layer is formed by combining two dielectrics such as F ₂ and ZnS, and 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, a high reflectance can be obtained by using different materials in a multilayer film structure and reflecting light from each layer mirror surface is strengthened by interference to maximize the reflectance of the multilayer film as a whole. Can be obtained. Further, it is known that a reflective mirror having a high reflectance can be realized 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 configuration.

As a material combination that has been known so far for the structure of a multilayer film, there is a transition metal as a low refractive index material, and most of the high refractive index materials use 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.

Conventionally, it has been recognized that it is important to reduce the surface roughness in order to improve the reflectance in this type of multilayer film reflecting mirror. However, the evaluation method is a wide area such as a heterodyne interferometric surface roughness meter,
In other words, it was only evaluated with a surface roughness meter that averages the area of several square μm (Precision Engineering Vo1.52
(1986), No.11, pp7-10), which is an important cause for reducing the reflectance in the X-ray / vacuum-UV region, with a size of several + Å or more and several thousand Å or less in the direction parallel to the surface of the substrate. No consideration was given to the unevenness having a ridge, and therefore no attempt was made to control such an unevenness.

[Problems to be solved by the invention]

Therefore, in the above-mentioned conventional example, although the surface roughness measured by the method of averaging a wide area, that is, an area of several square μm is sufficiently controlled, the expected reflectance and the measured reflectance are large. There is a drawback that there is a gap and only a mirror having a low reflectance as a multilayer-film reflective mirror can be obtained. For example, the surface of the substrate, the interface between each layer of the manufactured multilayer reflective mirror and its adjacent layers, and the surface roughness of the surface of the multilayer reflective mirror manufactured is 10 Å in terms of rms value and its reflectance of 20 Å Comparing the two, when a soft X-ray with a wavelength of 124 Å is vertically incident on a multilayer film mirror consisting of 41 layers of ruthenium (hereinafter Ru) and Si, the reflectance of a multilayer film mirror with a surface roughness of 20 Å has a surface roughness of 10 Å. Only about 1/10 of the reflectance of the multilayer film mirror can be obtained.

The surface roughness of the substrate surface, the interface of the multilayer structure, and the surface of the reflecting mirror is reflected in the optical path difference of the reflected light at a maximum of 2 times.
Therefore, the X-ray or the vacuum ultraviolet ray reflected by the reflecting mirror having the surface roughness of 1/8 of the used wavelength has a deviation of the optical path difference of 1/4 of the wavelength at the maximum, and reduces the reflectance. In a region where a few + Å to a few thousand Å X-rays or vacuum ultraviolet rays have coherence,
The phase shift caused by the surface roughness causes the interference to decrease the reflectance. For example, when 124 Å soft X-rays are incident on the reflectance of 41 layers of Ru and Si described above, when the cross section is observed with a transmission electron microscope, it has a surface roughness of 15 Å which is about 1/8 of the wavelength 124 Å. The reflectance of the multi-layered film mirror is about 1/4 that of the multi-layered film mirror with almost no surface roughness. Generally, when the surface roughness becomes larger than 1/15 of the used wavelength, the reflectance starts to decrease sharply, and when the surface roughness becomes about 1/8 of the used wavelength, the reflectance becomes 1/4.
It will be about.

Further, the surface roughness measured by a method in which the heterodyne interferometric surface roughness is averaged over a wide area of several square μm, such as a meter, non-interferingly reduces the reflectance as irregular reflection. The decrease in reflectance due to this is a little larger than the interference-induced decrease in reflectance due to the roughness of several + Å to several thousand Å. When the surface roughness of the substrate surface and the multilayer film surface is about 1/16 of the wavelength,
The reflectance is about 1 / 2.5 compared to the reflectance with almost no surface roughness.
become.

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 / VUV light having a reflecting mirror having a multi-layer structure composed of alternating layers of substances having different refractive indexes on the substrate, the substrate surface and reflection The roughness value of each surface of the mirror surface is the rms value of the heterodyne interferometric surface roughness meter.
/ 16 or less, and the surface roughness values of the substrate surface, each interface of the multi-layer structure and the mirror surface are 100Å by transmission electron microscope.
The problem was solved by the multilayer mirror for X-ray / vacuum ultraviolet rays, in which the rms value when observing the cross section of the sample having a thickness of 700 Å or less was set to 1/8 or less of the working wavelength. Here, in the case of observing the above sample with a transmission electron microscope, the interface between each layer of the multilayer-film reflective mirror and the layer adjacent thereto is the multi-layer film obtained by the transmission-electron microscope observation as shown in FIG. A negative photographic film of a transmission electron microscope image of a reflecting mirror is measured by a microdensitometer in a direction perpendicular to the surface, and the density is smoothed as a function from the substrate surface, and a density point between the peaks and valleys of the density is measured. Is decided as.

 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, with a heterodyne interferometer type surface roughness meter, the surface roughness was reduced to 1/16 or less of the operating wavelength in terms of rms value by observation with a transmission electron microscope.
Layers 1 and 2 of the first substance and layers 3 and 5 of the second substance, which have different refractive indexes, are alternately transmitted on the substrate 1 that has been polished to 1/8 or less of the wavelength used at the rms value. When observing with an electron microscope, the roughness of the interface is laminated so that the rms value is 1/8 or less of the used wavelength, and the finally manufactured surface is 1 / of the used wavelength at the rms value with a heterodyne interferometric surface roughness meter. The surface roughness is set to 16 or less by observing with a transmission electron microscope so that the surface roughness becomes 1/8 or less of the used wavelength in rms value.

The surface of the substrate 1 to be used is processed into a flat surface, a convex surface, a concave surface, or an aspherical surface according to the use of the manufactured optical instrument having a mirror surface, and after polishing with a diamond paste, a floating polishing method, a chemical Polishing is repeated, such as heating for a long time at a temperature slightly lower than the melting point of the substrate material in ultra-high vacuum, or flash heating until the surface roughness is reduced to the required level. This process also includes removing impurities adhering to the substrate surface.

In order to keep the surface roughness of the interface within a predetermined 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.

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 kinds of substances having different refractive indexes forming the alternating 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 film thickness d ₂ , d ₃ , ... Of each layer is approximately 1 / of the target wavelength.
4 is a laminated film made of the same material alternately,
The film thickness satisfies the condition that all reflected lights at the boundaries between the layers interfere with each other so as to constructively strengthen each other, or the reflectance of the entire multilayer film is compared 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 reduce the decrease of 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.

Also, the reflectivity increases as the number of alternating layers increases, so it is preferable that the number of layers is 5 or more. Good up to a pair. 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) coated with 100 μm thick silicon carbide (hereinafter SiC) processed to have a surface accuracy of λ / 20 (λ = 6328Å) was polished with diamond paste, and then floated. When the surface roughness was measured with a heterodyne interferometric surface roughness meter, the rms value was 2.3Å, which was smaller than 1/16 of the wavelength used 124Å. A sample for a transmission electron microscope with a thickness of about 500Å was made. When the surface roughness was measured by taking a photograph of an image with a transmission electron microscope, almost no surface roughness was observed.

This was set on the heater surface of the substrate holder of the apparatus shown in FIG. 4, and the degree of vacuum was set to 1 × 10 ^-10 torr. Prior to vapor deposition, it was heated to 1400 ° C. (2 hours).
After that, the substrate was left to cool to room temperature, and then the substrate was kept in contact with the copper claw protruding from the liquid nitrogen shroud for 3 hours, and the substrate temperature was measured with a thermocouple type thermometer. .

In the device shown in FIG. 4, the electron beam evaporation sources are arranged symmetrically with respect to the two substrates, and each electron gun hearth has 99.9% ruthenium (Ru) and 99.999% silicon ( (Si) has been set. Vacuum degree is 3 × 10
After waiting until it recovered to ^-10 torr, vapor deposition was started.

Both Ru and Si are preheated by electron beam in advance (shutters on each hearth and main shutter are closed), and after preheating for 30 minutes to 1 hour, each deposition rate is 3Å
Feedback was made so that the above-mentioned vapor deposition rate could be maintained with a crystal oscillator (not shown) installed near each hearth so that the flow rate would be / min, 5Å / min. After this, the shutters of each hearth are opened alternately, and the first substance shown in FIG.
The material is Si and the film thickness is 27.2Å, 36.2Å,
And 41 layers were formed in total. Vacuum degree during film formation is 8
Holds × 10 ^-10 torr.

The multilayer reflector obtained in this way has a wavelength of 124Å
When the soft X-ray of was incident at an inclination of 10 ° from the axis perpendicular to the plane, a reflectance of 61.2% was obtained.

This value is about 5 times as high as the reflectance (11.9%) of the soft X-ray when a commercially available silicon wafer is used as a substrate and the substrate is vapor-deposited to the same film thickness as the above-mentioned embodiment without cooling. Became a rate. The result is that the surface roughness of the substrate was small, and at the same time, the substrate was cooled during vapor deposition, so the mechanism in which the vapor-deposited atoms move around in the vapor-deposition plane to form an island-like structure as a result, and the interface is roughened. Due to the suppression.

Multilayer film mirror produced by the method of the present invention, the surface was measured with a heterodyne interferometric surface roughness meter 2.6 rms value
Å, which was sufficiently smaller than 1/16 of the wavelength used. A sample of a transmission electron microscope with a thickness of about 500 Å was made, and a photograph of the image was taken with a transmission electron microscope to measure the surface roughness. The surface roughness was higher toward the upper layer, but the interface between the uppermost layer and the layer below it. But the rms value was 9.8Å. The surface roughness was 9.8 Å in rms value, both of which were smaller than 1/8 of the wavelength used.

Example 2 The same substrate as that used in Example 1 was set on the heater surface of the substrate holder of the apparatus shown in FIG.

99.99% Mo and 99.999% Si were set in the electron gun hearth. Also, the copper nails protruding from the liquid nitrogen shroud were removed.

After drawing a vacuum to a back pressure of 1 × 10 ^-10 torr, the material was heated to 1400 ° C. (2 hours) in advance before vapor deposition. After waiting until the degree of vacuum was recovered to 8 × 10 ^-10 torr, the substrate temperature was set to 800 ° C. and this temperature was maintained during vapor deposition. Both Mo and Si are preheated by electron beam in advance (shutter on each hearth and main shutter are closed), and after 30 minutes to 1 hour preheating, each evaporation rate is 3
A crystal oscillator (not shown) arranged near each hearth at Å / min and 5 Å / min was fed back so that the above vapor deposition rate could be maintained. After that, the shutters of each hearth are opened alternately, and the first substance in FIG.
The material was Si, the respective film thicknesses were 26, 9Å and 36.0Å, and a total of 41 layers were formed. The degree of vacuum during film formation was maintained at 1 × 10 ^-10 torr.

When electron diffraction was performed, spot-like alternating layers of Mo and Si single crystals were observed, and transmission electron micrographs confirmed that the grain boundaries were about 350Å for Mo and 200Å for Si. It was The roughness of the interface was larger toward the upper layer, but the rms value at the interface between the uppermost layer and the layer below it was 11.3Å. The surface roughness was 11.5 Å in rms value, and both were less than 1/8 of the wavelength used. When measuring the surface of the multilayer film with a heterodyne interferometric surface roughness meter, rm
The s value was 2.7Å, which was smaller than 1/16 of the wavelength used.

Synchrotron radiation is separated into soft X with a wavelength of 124Å
When the line was incident at an angle of 10 ° from the axis perpendicular to the plane, a reflectance of 57.3% was obtained.

〔The invention's effect〕

As described above, in the multilayer mirror for X-ray / vacuum ultraviolet ray according to the present invention, the substrate, the interface of the multilayer film, and the surface average the area of several square μm to the value specified in the claims. The surface roughness measured by the method and the surface roughness caused by irregularities with a size of several tens of Å or more and several thousand Å or more in the direction parallel to the surface of the multilayer film are 1/16 to 1/8 of the operating wavelength in rms value. Because of the following, the effect that the number of mirror reflectors can be increased from several times to 10 times was obtained. At the same time, since the confusion light is reduced, the optical characteristics of the reflecting mirror are also improved.

[Brief description of drawings]

FIG. 1 is a schematic view of a cross section of a multilayer film reflecting mirror for X-ray / vacuum ultraviolet ray of the present invention in FIG. 1, FIG. 2 is a diagram showing the reflectance with respect to the irradiation light wavelength of a dielectric multilayer, a metal single layer, and FIG. 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 reflection mirror by the transmission electron microscope of the silicon substrate used in the example, FIG. 4 is used for the implementation of the present invention. It is a schematic diagram of an ultra-high vacuum electron beam vapor deposition apparatus. 1 is the substrate, 2 and 4 are the first substance, 3 and 5 are the second substance, 6 is the reflectivity curve of the dielectric multilayer, 7 is the reflectivity curve of the single metal 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
And 19 are electron gun hearths 17 to Ru or molybdenum 19 to Si
Set. 20 represents a crystal oscillator.

Claims (1)

(57) [Claims] 1. A multilayer film reflecting mirror for X-ray / vacuum ultraviolet ray, comprising a substrate having a multi-structured reflecting mirror comprising alternating layers of substances having different refractive indexes, on each of the substrate surface and the reflecting mirror surface. Rms value of heterodyne interferometric surface roughness meter is 1/16 or less of the operating wavelength, and the surface roughness values of the substrate surface, the interface of the multilayer structure and the surface of the reflector are 100 Å or more and 700 Å or less by the transmission electron microscope. A multilayer film mirror for X-ray and vacuum ultraviolet rays, whose rms value when observing the cross section of a sample is 1/8 or less of the wavelength used.