JPH0868898A - Reflection mirror and its production method - Google Patents

Abstract

PURPOSE: To provide a reflection mirror capable of suppressing thermal distortion due to X-ray irradiation and the like low enough. CONSTITUTION: Provided are a base 1 made of Invar having linear thermal expansion coefficient of 1×10<-7> /K or less and a thin film 2 of glass which is formed on the surface of the base 1 and its surface is polished optically smoothly. In the case used for an X-ray optical system, multiple layer film 4 reflecting X-ray of specific wave length is formed on the surface of the thin film 2.

Description

Detailed Description of the Invention

The present invention relates to a reflecting mirror used in an X-ray optical system such as an X-ray reduction projection exposure apparatus or an optical system for light rays in a wavelength range other than X-rays, and a manufacturing method thereof.

[0002]

2. Description of the Related Art In recent years, with the miniaturization of semiconductor integrated circuit devices, X-rays having a shorter wavelength are used in place of conventional ultraviolet rays in order to improve the resolution of an optical system limited by the diffraction limit of light. The projection lithography technology has been developed. The X-ray projection exposure apparatus used in this technique is mainly composed of an X-ray source, an illumination optical system, a mask, an imaging optical system, a wafer stage and the like.

A synchrotron radiation source or a laser plasma X-ray source is used as the X-ray source. The illumination optical system is composed of a grazing incidence mirror, a multilayer film mirror, a filter that reflects or transmits only X-rays of a predetermined wavelength, and illuminates the mask with X-rays of a desired wavelength. The mask includes a transmission type mask and a reflection type mask. The transmissive mask has a pattern formed by providing an X-ray absorbing substance in a predetermined shape on a thin membrane made of a substance that transmits X-rays well. On the other hand, the reflective mask has a pattern formed by providing a portion having a low reflectance in a predetermined shape on a multilayer film that reflects X-rays, for example. The pattern formed on such a mask is imaged on a wafer coated with a photoresist by a projection imaging optical system composed of a plurality of multilayer film mirrors, and is transferred to the resist. Since X-rays are absorbed by the atmosphere and attenuated, all optical paths thereof are maintained at a predetermined vacuum degree.

In the X-ray wavelength range, since no transparent substance exists and the reflectance on the surface of the substance is very low, ordinary optical elements such as lenses and mirrors cannot be used. Therefore, the X-ray optical system includes an oblique-incidence mirror that reflects X-rays obliquely incident on the reflecting surface by using total reflection, and interference by matching the phases of reflected light at each interface of the multilayer film. It is composed of a multi-layer film mirror or the like that obtains a high reflectance by the effect.

Since the grazing incidence optical system has a large aberration, it is impossible to obtain a diffraction limit resolution. On the other hand, the multilayer mirror can reflect X-rays vertically, and can form a diffraction-limited X-ray optical system. Therefore,
The imaging optical system of the X-ray projection exposure apparatus is composed of multilayer mirrors.

Such an X-ray multilayer mirror has the highest reflectance when a multilayer film made of molybdenum and silicon is used on the long wavelength side of the L absorption edge (12.3 nm) of silicon. At -15 nm, it is about 70% regardless of the incident angle. On the shorter wavelength side than the L absorption edge of silicon, a multilayer film that can obtain a reflectance of 30% or more at normal incidence has not been developed. As a substrate material of the multilayer mirror, a glass material such as quartz is used, which has a high shape accuracy and can be processed with a small surface roughness.

[0007]

In the X-ray projection exposure apparatus as described above, a practical throughput (for example, 8
In order to obtain 30 sheets / inch hour with an inch wafer), X-rays of a certain intensity (for example, about 10 mW / cm ² ) are formed on the surface of the multilayer mirror that constitutes the imaging optical system.
Need to be irradiated. On the other hand, the reflectance of the multilayer film mirror is about 70% at the highest, and the rest is absorbed, transmitted, and scattered without being reflected by the multilayer film. The loss due to scattering is slight, and the X-ray transmitted through the multilayer film is completely absorbed by the mirror substrate. That is, most of the X-rays not reflected by the multilayer mirror are absorbed by the multilayer mirror, and the energy is converted into heat. This heat causes the temperature of the multilayer film mirror to rise, causing thermal deformation.

In general, in order to obtain a diffraction-limited resolution in an optical system, the shape error of the mirror or lens forming the optical system must be made sufficiently smaller than the wavelength of the light used. Therefore, in the optical system using X-rays, the allowable error becomes stricter as the wavelength is shorter than that in the optical system using visible light or ultraviolet light. Therefore, the thermal deformation of the multilayer mirror due to the irradiation of X-rays as described above has a great influence on the imaging characteristics, and there is a possibility that the resolution as designed cannot be obtained.

In order to prevent the influence of such thermal deformation, the mirror is cooled from the back surface, but the sufficient effect cannot be obtained. Further, since the X-ray optical system is used in vacuum, there is almost no heat radiation from the surface of the mirror.

Therefore, in order to prevent the influence of thermal deformation, there is no choice but to limit the intensity of X-rays incident on the mirror, which limits the throughput of the X-ray projection exposure apparatus. That is, the conventional technique cannot achieve both high resolution and high throughput of the X-ray projection exposure apparatus.

Although the problems of the X-ray optical system have been described above, the problems associated with the thermal deformation of the reflecting mirror are not so different even in an optical system using a light beam in a wavelength range different from the X-ray wavelength range. It happens even if there is.

An object of the present invention is to provide a reflecting mirror capable of sufficiently suppressing thermal deformation due to irradiation light such as X-rays, and a manufacturing method thereof.

[0013]

SUMMARY OF THE INVENTION First, a reflecting mirror used in an X-ray optical system will be described as an example. The amount of deformation of the mirror surface when the surface of the multilayer mirror is irradiated with X-rays will be described. Since the actual deformation of the mirror differs greatly depending on its size and shape, it is necessary to perform calculations by the finite element method, etc. for accurate estimation, but here we will estimate the approximate value of deformation by the following simplification. To do.

As shown in FIG. 2A, the back surface of the substrate of thickness d is in contact with a constant temperature heat bath (corresponding to cooling the back surface to keep it at a constant temperature), and When a constant heat flux Q (energy of the irradiated X-rays that is absorbed by the substrate without being reflected) is applied to the portion, the extension of the portion in the vertical direction (x direction) ( Or shrinkage) Δ
Consider x. Here, it is assumed that heat conduction in the lateral direction is not considered, and that all X-rays are absorbed by the surface. At this time, since a uniform temperature gradient is generated in the x direction inside the substrate as shown in FIG. 2B, the temperature at the position x (temperature difference from the heat bath) T (x) is

[Equation 1] However, λ is given by the wavelength. The elongation Δ (δx) of a thin layer of thickness δx in the substrate is

[Equation 2] Given in. α is the thermal expansion coefficient (coefficient of linear expansion) of the substrate material
Is. Therefore, the elongation Δx of the entire substrate is

(Equation 3) Becomes

FIG. 3 shows the thermal conductivity, the coefficient of thermal expansion, and the thermal deformation Δx of the substrate calculated by the equation (3) for various materials. Heat flux Q input to the substrate is 10 mW
/ Cm2. In order to secure a practical exposure area size in an X-ray projection exposure apparatus, the diameter of the mirror is 200
The thickness d is required to be about mm, and the thickness d is set to 50 mm because the thickness is generally required to be about a quarter of the diameter in order to maintain the shape of the mirror with high accuracy. As can be seen from FIG. 3, fused silica, which is widely used in the refractive optical system of the projection exposure apparatus using ultraviolet light, has a deformation of 45.3 nm. This is many times larger than the wavelength (for example, 13 nm) of the X-ray used, which is out of the question.

Zero-Jur from Schott and ULE from Corning, which are generally well known as low thermal expansion glasses.
Has a low coefficient of thermal expansion, but since it is glass, it has a low coefficient of thermal conductivity. On the other hand, although silicon carbide (SiC) and silicon (Si) have a higher thermal conductivity than glass, they have a large thermal expansion coefficient. These materials are all 1 to several n
A deformation of m will occur.

Using Rayleigh's condition that the wavefront aberration of the optical system is within a quarter of the wavelength, the shape accuracy per mirror forming the optical system is

[Equation 4] Must be suppressed within. n is the number of mirrors forming the optical system, and is multiplied by 1/2 because it is a reflective system. For example, when an optical system composed of four mirrors is used at a wavelength of 13 nm, the shape error allowed for one mirror is 0.81 nm. Zerojua, UL
When E, SiC, or Si is used for the substrate, the thermal deformation amount becomes larger than this value. Therefore, it is not possible to obtain a diffraction limit resolution with an optical system that is configured by a mirror using these materials as a substrate.

Metal has a large thermal conductivity due to the contribution of free electrons, but generally has a large thermal expansion coefficient. However, the Invar type alloy is known as a material exhibiting a remarkably small coefficient of thermal expansion due to the effect of magnetostriction.
Fe-Ni alloy, Fe-Ni-Co alloy, Fe-Co-Cr alloy, Fe-Pt alloy,
There are Fe-Pd alloy, Zr-Nb-Fe alloy, Cr-Fe-Sn alloy, Mn-Ge-Fe alloy, Fe-B amorphous alloy, Fe-Ni-Zr amorphous alloy and the like. Hereinafter, the Invar type alloy will be referred to as Invar. From FIG. 3, if Invar is used for the substrate, the amount of thermal deformation due to X-ray irradiation is 0.1 nm or less, which can be suppressed sufficiently smaller than the above allowable shape error.

However, since metal has fine crystal grain boundaries, it is difficult to polish its surface to a smooth surface of the order of nanometers. O by Alan G. Michelette
ptical Systems for X Rays (1986 Plenum Press, New
(York), page 74, describes the surface roughness that can be obtained by polishing a substance that is a candidate for a mirror material for X-rays. According to it, the rms value (root mean square value) of the minimum surface roughness obtained for each material is
SiC produced by D (Chemical Vapor Deposition) method
Is the smallest and is 0.4 nm. Since these materials are amorphous substances having no fine structure, a smooth surface can be obtained. However, metal Invar can be processed only to a surface roughness of about 2.8 nm.

The surface roughness required for the substrate of the multilayer mirror for X-rays can be estimated by the following equation.

(Equation 5) Where R ₀ is the reflectance without surface roughness R is the reflectance with scattering loss due to surface roughness σ is the rms value of the surface roughness λ is the wavelength of X-rays θ is the oblique incident angle λ = 13 nm , Θ = 90 ° (normal incidence),
The value of R / R ₀ with respect to the surface roughness σ is shown in FIG. As is clear from this figure, if SiC or fused silica having a surface roughness of 0.4 nm is used for the substrate of the multilayer film mirror, a reflectance of nearly 90% of the ideal case without roughness can be obtained. When Invar having a roughness of 2.8 nm is used for the substrate, X-rays are not reflected at all.

[0021] Therefore, as a result of intensive studies, the present inventors have found that if glass having a small surface roughness is attached to the surface of a metal substrate such as Invar having a small thermal deformation, both the thermal deformation and the surface roughness can be sufficiently achieved. It has been found that a small mirror substrate can be obtained. That is, the reflecting mirror for an X-ray optical system according to the present invention, as shown in FIG. 1, a glass thin plate 2 is bonded onto a metal substrate 1 such as Invar to prepare a substrate 3, and an X-ray is formed thereon. Is formed by forming a multilayer film 4 that reflects light. In this case, in order to suppress the thermal deformation to be small, it is effective that the material on the upper side (the side on which X-rays are incident) has a small thermal expansion coefficient and the material on the lower side has a large thermal conductivity.

FIG. 5 shows the thermal deformation Δx of the substrate in which various kinds of glass are attached to the surface of the Invar. Similar to Figure 3,
The heat flux Q applied to the substrate was 10 mW / cm ² , and the total thickness of the substrate was 50 mm. When ULE with a thickness of 20 mm or less is attached to the invar, and when the ULE has a thickness of 5 mm
When the following zero-jure is attached, the thermal deformation can be suppressed sufficiently smaller than the allowable shape error.
When pasting fused quartz, the thickness must be less than 1 mm, so the glass pasted on the Invar should be UL
A low thermal expansion glass such as E or Zerodur is desirable. In addition,
In order to use it as an X-ray reflecting mirror, a multilayer film for reflecting X-rays is further formed on this surface, and the thickness of this multilayer film is 0. Since it is only a few μm or less, its thermal deformation can be ignored.

When the glass is attached to the metal such as Invar, it is necessary that the thermal contact at the interface is good. When sticking with an adhesive, it is desirable to use an adhesive having a good thermal conductivity. The most preferred method for bonding metal and glass is, for example, J. Appl. Phys., 4
0 (1969) P3946. In this method, as shown in FIG. 6, the metal and the glass are brought into contact with each other, a positive voltage is applied to the metal side, and the two are joined by the electrostatic force generated at the interface. When this method is used, the metal and glass are in close contact with each other at the atomic level, so that the thermal contact is very good and the bonding strength is high.

[0024]

EXAMPLE An example of the reflecting mirror according to the present invention will be described below. FIG. 1 is a diagram of a reflecting mirror that is a first embodiment of the present invention. In this example, the ULE 2 was attached to the Invar 1 and the multilayer film 4 was formed on the surface of the ULE 2 to manufacture an X-ray multilayer mirror having a diameter of 200 mm, a radius of curvature of 500 mm, and a central thickness of 50 mm. The manufacturing process will be described in order.

First, the Invar is ground to a diameter of 200.
mm, the central thickness was 30 mm, the front surface was a concave surface having a radius of curvature of 500 mm, and the back surface was a flat metal substrate 1. The surface (the surface to which the glass is bonded) was finished by electropolishing into a mirror surface having a surface roughness of 10 nm (rms) or less. The glass substrate 2 to be attached thereto was made of ULE as a material, and was processed by grinding and polishing to have a concave surface and a convex surface each having a diameter of 200 mm, a thickness of 20 mm and a radius of curvature of 500 mm. Both surfaces were mirror finished with a surface roughness of 10 nm (rms) or less.

Next, the metal substrate 1 and the glass substrate 2 were anodically bonded using an apparatus as shown in FIG. The metal substrate 1 was placed on the heater 5, and the glass substrate 2 was placed thereon. These are heated to 400 ° C. by the heater 5, and the electrodes 6 are formed on the surface of the glass substrate 2 as shown in the figure.
And a voltage of 1000 V was applied between the metal substrate 1 and the glass substrate 2 by the DC power supply 7. When the voltage was continuously applied for about 5 minutes, the metal substrate 1 and the glass substrate 2 were completely bonded. Then, the surface of the glass substrate 2 was finish-polished to smooth the surface roughness to 0.4 nm (rms). In this way, a substrate 3 for a reflecting mirror was produced by bonding the metal substrate 1 made of Invar and the glass substrate 2 made of ULE.

Finally, an X-ray multilayer mirror is formed by forming a multilayer film 4 composed of molybdenum (Mo) and silicon (Si) with a period length of 6.7 nm and a stacking number of 50 layers on the surface of the substrate 3 by ion beam sputtering. Was completed. This multi-layered film mirror has a back surface that is cooled down and kept at a constant temperature.
Even if a heat flux of 0 mW / cm ² is incident on the whole surface or a part of the heat flux, the thermal deformation is 0.3 nm or less.
An X-ray of nm can be used to construct a diffraction-limited optical system.

FIG. 7 is a diagram of a reflecting mirror which is a second embodiment of the present invention. In this example, the X-ray multilayer mirror having a diameter of 200 mm, a radius of curvature of 1000 mm, and a center thickness of 50 mm was produced by sticking the zero jure 2A to the invar 1 and forming the multilayer film 4 on the surface of the zero joa 2A. . The manufacturing process will be described in order.

First, the Invar is ground to a diameter of 200.
A disc-shaped metal substrate 1 having a center thickness of 45 mm and a center thickness of 45 mm was produced. The surface (the surface to which the glass is bonded) was finished by electropolishing into a mirror surface having a surface roughness of 10 nm (rms) or less. The glass substrate 2 to be attached to this has a diameter of 200 m as a result of grinding and polishing by using Zerodur as a material.
m, the central thickness was 5 mm, the front surface was a concave surface with a curvature radius of 1000 mm, and the back surface was flat. Both surfaces were mirror finished with a surface roughness of 10 nm (rms) or less.

Next, by using an apparatus as shown in FIG. 6, the metal substrate 1 and the glass substrate 2A are anodically bonded in the same manner as described above, and further the surface thereof is subjected to finish polishing to obtain a surface roughness. Was 0.4 nm (rms).
In this way, a substrate 3 for a reflecting mirror was produced in which the metal substrate 1 made of Invar and the glass substrate 2A made of Zerojure were bonded together.

Finally, in the same manner as described above, a period length of 6.7 nm composed of molybdenum (Mo) and silicon carbide (SiC) is obtained by ion beam sputtering.
An X-ray multilayer mirror was completed by forming a multilayer film 4 of 50 layers on the surface of the substrate 3. Similarly to the first embodiment, if the back surface of this multilayer film mirror is cooled and kept at a constant temperature, even if a heat flux of 10 mW / cm ² is incident on the whole surface or a part thereof, thermal deformation is 0. Since it is less than or equal to 0.3 nm, a diffraction-limited optical system can be constructed using X-rays with a wavelength of 13 nm.

In the above embodiments, the anodic bonding was used to bond the metal substrate and the glass substrate together, but the present invention is not limited to this and may be bonded with an adhesive or the like. . The material of the metal substrate 1 is not limited to Invar, but when it is used as a reflection mirror for an X-ray optical system, it needs to have a coefficient of thermal expansion of 1 × 10 ⁻⁷ / K or less. The material of the glass substrate 2 is not limited to ULE and zerojure. Low thermal expansion glass is desirable, but when using a material whose thermal expansion coefficient is not so small, the thickness may be reduced.

In the above description, the reflecting mirror used in the optical system of the X-ray reduction projection exposure apparatus has been described as an example, but it can be applied to other X-ray optical system reflecting mirrors. Also, the same effect can be obtained by using as a reflecting mirror of an optical system used for ultraviolet light or infrared light in a wavelength range different from the X-ray wavelength range. In this case, a multilayer film that reflects X-rays is unnecessary. Further, the coefficient of thermal expansion of the metal substrate used is not required to be as small as that of the X-ray optical system. For example, when used for visible light, it may be about 5 × 10 ⁻⁶ / K.

[0034]

As described above, in the reflecting mirror according to the present invention, a thin glass plate is attached to the surface of a metal substrate having a small coefficient of thermal expansion, and the surface is polished to be optically smooth. It is possible to suppress the thermal deformation of the reflecting mirror due to absorption of incident light such as, for example, remarkably small as compared with the conventional technique, and it is possible to provide a highly accurate optical system. A metal substrate having a linear expansion coefficient of 1 × 10 ⁻⁷ / K or less as in the reflecting mirror of claim 4,
Moreover, a reflecting mirror having an X-ray reflection multilayer film formed on the surface of a thin film of an amorphous material can be used as an X-ray optical system reflecting mirror. Even if the light is irradiated, the image forming characteristic of the optical system does not deteriorate due to thermal deformation. Therefore, the throughput can be improved without impairing the resolution of the X-ray projection exposure apparatus.

[Brief description of drawings]

FIG. 1 is a diagram showing a reflecting mirror according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating thermal deformation of a substrate.

FIG. 3 is a diagram showing various materials, thermal conductivity, thermal expansion coefficient, and deformation amount.

FIG. 4 is a diagram showing a decrease in reflectance of a multilayer film mirror due to surface roughness of a substrate.

FIG. 5 is a diagram illustrating the amount of deformation of a substrate in which various glass thin plates are attached to Invar.

FIG. 6 is a diagram showing a method of anodic bonding used in the present invention.

FIG. 7 is a diagram showing a reflecting mirror according to a second embodiment of the present invention.

[Explanation of symbols]

 1 Metal Substrate 2 Glass Substrate 3 Reflector Substrate 4 Multilayer Film 5 Heater 6 Electrode 7 DC Power Supply

Claims (7)

[Claims] 1. A reflecting mirror comprising: a metal substrate; and a thin glass plate which is attached to the surface of the metal substrate and whose surface is polished to be optically smooth. 2. The linear expansion coefficient of the metal substrate is 1 × 10 ^−7.
/ K or less, a reflecting mirror. 3. The reflecting mirror according to claim 1, wherein the metallic substrate is made of an Invar type alloy. 4. A metal substrate having a coefficient of linear expansion of 1 × 10 ⁻⁷ / K or less, a thin glass plate bonded to the surface of the metal substrate and having its surface polished optically smooth, and this glass. A reflecting mirror comprising: a multilayer film formed on a thin plate and reflecting X-rays of a predetermined wavelength. 5. The method for manufacturing a reflecting mirror according to claim 1,
A method for manufacturing a reflecting mirror, which comprises at least a step of laminating a metal substrate and a glass thin plate, and a step of polishing the surface of the glass thin plate to an optically smooth surface. 6. The method for manufacturing a reflecting mirror according to claim 4,
At least a step of laminating a metal substrate and a glass thin plate, a step of polishing the surface of the glass thin plate to an optically smooth surface, and an X-ray having a predetermined wavelength on the surface of the thin plate after the polishing step. And a step of forming a reflective multilayer film. 7. The method of manufacturing a reflecting mirror according to claim 5, wherein the metal substrate and the glass substrate are bonded by anodic bonding. [Claims] [0001]