JP3383802B2 - Reflecting mirror and manufacturing method thereof - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflecting mirror for laser light, a reflecting mirror for forming an optical path and a method for manufacturing the same, and more particularly to a large reflecting mirror for short wavelength light and a method for manufacturing the same.

[0002]

2. Description of the Related Art As is well known, lasers are widely used in various fields and their applications are becoming more and more diverse. Particularly, a mirror for generating a vacuum ultraviolet light region and a soft X-ray region having a shorter wavelength and for forming an optical path is required to be developed immediately as an essential tool for scientific research and future industry.

However, conventionally used vapor-deposition film reflecting mirrors of aluminum, gold, platinum, etc. have the drawbacks of low heat resistance and low reflectance for light in such a short wavelength range.

Next, as a material having a relatively high reflectance and a little heat resistance, quartz glass, single crystal silicon, single crystal molybdenum, a reflecting mirror obtained by polishing the surface with high precision, or a dielectric multilayer on the surface thereof. Reflective mirrors with modified membranes are also being considered. But this like insufficient noted in terms of heat resistance and reflectance, in terms of the total energy, 50 m J, in terms of peak power is only 5MW is obtained.

As described above, the reflector for short wavelength light (generally, about 200 nanometers or less) has a high melting point and excellent heat resistance, unlike ordinary reflectors for visible light, infrared rays, and far infrared rays. Properties, high thermal conductivity, high reflectivity, and uniform (isotropic) physical and chemical properties of the entire mirror body are desired. Conventionally, no reflecting mirror having such various required characteristics has been developed.

[0006]

Therefore, the problem to be solved by the present invention is to develop a high-power reflecting mirror for short-wavelength light, which has the above-mentioned required characteristics.

In response to such a request, silicon carbide (S
A reflective mirror (hereinafter referred to as SiC / SiC-based reflective mirror) in which a dense silicon carbide film is formed on the surface of an iC) -based material as a base material by a CVD method and is highly accurately polished has been proposed. With this SiC / SiC-based reflecting mirror, the heat resistance and the reflectivity are remarkably improved as compared with the reflecting mirrors using other conventional materials. However, in the case of this material-based reflecting mirror, there are some drawbacks. That is, the silicon carbide-based material that is the base material is a silicon carbide-based material that is produced by separately molding powdery silicon carbide under pressure using an appropriate binder component and firing at high temperature with or without pressure. A base material is obtained, and a CVD method silicon carbide film is formed on the surface thereof.

Therefore, (a) contamination and deterioration of the mirror surface of the surface due to the leaching and diffusion of the binder component from the deep part, (b) the cost burden of the molding and sintering apparatus, and (c) the hardness of the silicon carbide sintered body. Examples of such problems include difficulty in shape processability and restrictions on shape independence, (d) localized temperature rise due to low thermal diffusion characteristics of the silicon carbide based sintered body, and the resulting phenomenon of damage to the base material.

In order to make up for the drawbacks of the SiC / SiC-based reflecting mirror, the present inventors previously formed a high-density isotropic graphite material (C) as a base material and formed it on the surface by the CVD method. A reflecting mirror (hereinafter C / Si) obtained by polishing a silicon carbide film with high precision.
We have developed and filed an application for a C-type reflector (abbreviated as C-mirror)
2-255192).

As a result, (A) graphite material has a high melting point and high heat resistance, (B) is excellent in thermal conductivity and thermal diffusion characteristics, has a small partial temperature rise, and (C) has a high total ash content of 10 ppm or less. With purity,
By using an isotropic graphite material with an anisotropic ratio of 1.10 or less, the characteristics and uniformity as a reflecting mirror are secured. (D) Since it is an isotropic material, it can be arbitrarily shaped like metal. By making the expansion coefficient of the (E) graphite material which is possible to be the same as that of the CVD silicon carbide film, peeling and cracking of the surface mirror surface can be prevented. The characteristics such as things were confirmed.

However, with the progress of optical technology, the precision of the mirror surface of the reflecting mirror is required to be higher, and more and more large is required.

[0012] Such a large size (for example, the long axis side of the mirror body is 5
0 to 150 cm) and high accuracy (for example, roughness with partial unevenness is 10 angstroms rms or less, surface accuracy showing overall flatness is 1/10 to 1/3 λ or less)
In the production of the mirror surface, it was found that in the above C / SiC-based reflecting mirror, the hardness and rigidity of the isotropic graphite material used as the base material were insufficient.

To further explain, in the case of highly accurate polishing of the CVD silicon carbide layer, rough polishing, medium polishing, spherical polishing with submicron diameter, and other polishing agents are sequentially performed.
In the process of precision polishing and increasing precision, due to insufficient rigidity of the base material, the base material itself cannot withstand the pressing force during polishing, a so-called stiff phenomenon occurs, and the predetermined accuracy and flatness may not be satisfied. there were. This phenomenon was not so large and was negligible in a reflector having a relatively small size, for example, a reflector having a length of 5 cm to 20 cm, but the length of the reflector was 50 cm or more, 1 m to 1.5 m. In the case of a giant reflecting mirror, when it is pressed for polishing, it causes a total bending and a partial depression phenomenon, resulting in a phenomenon of insufficient mirror surface accuracy and flatness.

[0014]

[Means for Solving the Problems] As a means for solving such a phenomenon and enabling the production of a high-precision large-sized mirror, the underlying graphite base material is partially silicon carbide to improve the rigidity and hardness of the base material. When the method of forming a silicon carbide layer by the CVD method on top of it was adopted after it was raised, a remarkable effect was found and it became possible to manufacture an ultra-large reflecting mirror having a highly accurate reflecting surface.

[0015]

[Structure and operation of the invention] Graphite material (reflector base material)
Several methods of partially converting the above into silicon carbide have been known so far, and any of these conventional methods can be applied to the present invention alone or in combination.

For example, the methods described in JP-A-1-264969, JP-A-1-249679, JP-A-1-242408 and the like can be exemplified. These can be roughly classified into the followings based on chemical formulas. (A) CVR method

[0017]

[Chemical 1]

(B) Si impregnation method or reaction method with Si vapor

[0019]

[Chemical 2]

(C) CVR method + Si impregnation method

[0021]

[Chemical 3]

However, in [Chemical formula 1], * is a raw material added separately, and in [Chemical formula 1] to [Chemical formula 3] is a base material. Also, in [Chemical 3], ** is C that remains in the substrate.

In the reflector substrate obtained by these methods, the raw material graphite (C) is partially contained in silicon carbide (Si).
The material is reformed and converted to C), that is, a material of (C + SiC) mixture is reformed.

The thickness of the silicon carbide layer depends on the quality and pore size of the graphite material as the base material, the contact conditions with the SiO gas and the S
i Depending on the conditions such as the amount of impregnation, the difference from the surface layer is 2
It can be controlled by selecting the reaction conditions from the one modified to about 3 mm to the one uniformly modified to the deep part.

The ratio of SiC to C in the modified layer is
It can be controlled as appropriate by selecting the reaction conditions.
In the range of 95%, particularly in the range of 30 to 80% when considering the physical properties of the reflector substrate, it is sufficient to achieve the purpose.

When the ratio is less than 20%, it does not differ from the case of the reflecting mirror base material which is substantially composed of only graphite, and conversely, when the ratio exceeds 95%, substantially only silicon carbide is contained. It becomes close to the base material.

It is particularly preferable that the hardness and rigidity of the graphite base material are enhanced while leaving the good characteristics as the graphite base material as described above, and the folding range with the required characteristics is 30 to 80%.

Also, depending on the reaction conditions selected, there is also a so-called gradient composition in which the [SiC / C] ratio near the surface layer of the reflector substrate is changed to 70% and the ratio of the deep portion of the substrate is changed to 30%. It can be manufactured.

The graphite base material thus modified (C + Si
For C), a more dense silicon carbide film is formed on the surface by CVD, and the silicon carbide film is polished with high precision to obtain a reflecting mirror (hereinafter referred to as (C + SiC) / SiC-based reflecting mirror).

The SiC film is formed by the CVD method by a known method, for example, the method disclosed in Japanese Patent Laid-Open No. 1-265203.

By using a base material of [carbon and silicon carbide] mixture in which the vicinity of the surface layer is partially silicidized as described above, the CVD for forming the outermost surface layer is achieved in addition to the desired effect described above.
It was also confirmed that the bondability between the method silicon carbide film layer and the base material was further strengthened and peeling was eliminated.

The embodiments of the present invention will be exemplarily described below.

[1] Substrate shape processing step High density isotropic graphite (C) material [SI manufactured by Toyo Tanso Co., Ltd.
C-120] length 1000 mm x 100 mm x 60 m
[Substrate C] cut into a rectangular shape of m. This material could be easily processed into an arbitrary shape by an ordinary metal processing machine.

[2] Silicification Step of Base Material The above base material (C) was subjected to a silicidation treatment (SiC conversion) by the method described in JP-A-1-264969. That is, quartz powder (SiO ₂ ) and graphite pieces (C consumed as a raw material) are put into a graphite crucible and reacted at 1900 to 2300 K to generate SiO gas, and the SiO gas reacts with the reflector substrate (C). Then, the vicinity of the surface layer was silicified (SiC). The thickness of the silicified layer was controlled to a depth of about 2 to 8 mm from the surface layer by selecting the reaction conditions, particularly the time. When the surface and the cut surface are observed by the SEM photograph, the surface layer is 9
5% was converted to SiC, and the extent to which it was converted to SiC decreased to the deeper part, resulting in a graded composition. Further, it was found that the portion deeper than 10 mm deeper than the surface layer was not silicified and remained as graphite [hereinafter, referred to as the base material (C + Si
C)].

[3] CVD-SiC Coating Forming Step A dense and homogeneous silicon carbide layer was formed on the reflecting mirror substrate treated by the above method [2] by the CVC method according to a conventional method. As one example, the method described in Japanese Patent Application Laid-Open No. 1-265203, that is, [base material (C + SiC)] is placed in a CVD reaction furnace, and SiH ₄ , SiCl ₄ , Si as silicon-based raw materials are used.
In addition to HCl ₃ , (CH ₃ ) ₃ SiCl, etc., and a carbon material as a raw material can be involved as a carbon-based raw material, CCl ₄ , CH ₄ , C ₃ H ₈ , C as a separate carbon source.
It is also possible to introduce ₃ H ₆ or the like.

In addition to these, if necessary, a carrier gas such as H ₂ or Ar can be appropriately introduced. The reaction temperature is preferably 1300 to 1800K. The CVD silicon carbide layer formed to a thickness of about 300 to 400 μm by such a method was composed of β-type SiC only, and was dense and homogeneous.

Further, the thermal expansion coefficient of the isotropic graphite material used as the base material is almost the same as that of the β-based CVD method silicon carbide film, and the entire vicinity of the surface layer of the base material or In combination with the result of partial silicidation (SiC conversion), even in a large-sized mirror body having a length of 1 meter, there were no phenomena such as cracking or peeling in the CVD-processed silicon carbide layer which becomes a mirror surface.

[4] Mirror-polishing step Polishing is carried out by a conventional method such as ceria powder, chromia powder, etc., and roughening is carried out by a pressing method.
Using μm spherical diamond grains, high-precision polishing is performed by the pressing method and the floating method.

Substrate (C + S) which has been subjected to the step of silicifying the vicinity of the surface layer of the substrate [C] according to the method of the present invention (the above [ 2 ]).
In the case of iC), even if pressed and rubbed with a strong force at the time of polishing, there is almost no deformation due to bending of the base material, and sufficient rigidity is obtained,
The flatness of the entire mirror body was not impaired.

The effect of the present invention was remarkably recognized as the size of the mirror body increased. Regarding the surface accuracy, since the hardness of the base material (C + SiC) is much higher than that of the base material (C), the amount of dents was small against the pressing force during polishing, and good surface accuracy was obtained.

[0041]

[Comparative Example 1] A base material of the same type and shape as the base material (C) obtained in the step (I) of Example 1 was used without performing the step (II) (base material silicidation step). Step (III), Step (I
In the step V), the mirror body obtained by treating in the same manner as in Example 1 is
The rigidity of the substrate as a whole and the hardness of the surface are insufficient, and the pressing force during polishing cannot be resisted. As a result, bending and partial depressions occur as a whole, and the flatness and precision of the mirror surface are insufficient. there were. If the pressing force is weakened to compensate for this, much polishing time and labor are required, and the mirror surface finally obtained is inferior to the mirror surface obtained by carrying out the method of the present invention.

Table 1 shows the results of measuring the flatness and surface accuracy of the mirror surfaces obtained in Example 1 and Comparative Example 1 .

[0043]

[Table 1]

However, (Note 1) to (Note 3) in Table 1 indicate the following.

(Note 1) Measured values and average values at 5 points on the central axis of the elongated mirror body. Unit: Angstrom, rms ZYGO Roughness measurement method

(Note 2) Measurement value by diffraction image method However, λ = 632.8 nm (nanometer) Measurement by ZYGO interferometer (using He-Ne light)

(Note 3) Visual observation of surface by 400 times optical microscope

[0048]

EFFECT OF THE INVENTION Graphite is used as the base material, and the surface is CVD method SiC
In the reflection mirror obtained by coating and polishing, the surface and the vicinity of the surface of the graphite base material is partially siliconized to increase the rigidity and hardness of the base material, and the polishing operation is easy and the obtained mirror surface is flat. It was possible to remarkably improve both the property and the precision of the mirror surface.

─────────────────────────────────────────────────── ─── Continuation of the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G21K 1/06 G02B 5/08 H01S 3/08

Claims (4)

(57) [Claims] 1. An isotropic graphite material as a base material, the surface of which is CV.
In a reflecting mirror having a mirror surface obtained by forming a silicon carbide film by the D method and polishing it with high precision, after the isotropic graphite material as a base material is partially converted to silicon carbide, A reflecting mirror having a mirror surface obtained by forming a silicon carbide film on the surface by a CVD method and polishing this with high precision. 2. An isotropic graphite material as a base material, the surface of which is CV
In a reflecting mirror having a mirror surface obtained by forming a silicon carbide film by the D method and polishing it with high precision, after the isotropic graphite material as a base material is partially converted to silicon carbide, When a silicon carbide film is formed on the surface by a CVD method, and a mirror having a mirror surface obtained by polishing the silicon carbide film with high precision is manufactured, an isotropic graphite material as a base material is coated with silicon monoxide vapor (SiO gas). ) Is contacted to partially convert the isotropic graphite material into silicon carbide, and then CVD is performed on the surface.
A method of manufacturing a reflecting mirror, which comprises forming a silicon carbide film by a method and polishing it with high precision. 3. An isotropic graphite material as a base material, the surface of which is CV
In a reflecting mirror having a mirror surface obtained by forming a silicon carbide film by the D method and polishing it with high precision, after the isotropic graphite material as a base material is partially converted to silicon carbide, When a silicon carbide film is formed on the surface by a CVD method, and a reflecting mirror having a mirror surface obtained by polishing this with high precision is manufactured, an isotropic graphite material as a base material is impregnated with silicon or silicon vapor. And a part of the isotropic graphite material is converted into a silicon carbide material, a silicon carbide film is formed on the surface by a CVD method, and high-precision polishing is performed. 4. The reflecting mirror according to claim 1, which is a reflecting mirror used for light having a short wavelength of 200 nm or less.