JPH07301696A - X-ray projection and exposure method and device therefor - Google Patents

Abstract

PURPOSE:To provide an X-ray projection and exposure method high in the transfer accuracy and production efficiency of a pattern and enabling the realization of the long service life of a mask. CONSTITUTION:After diffracting emitted light from an X-ray source by a diffraction grating, only the exposure X-rays of desired wavelength are selectively taken out by a slit, and a mask is irradiated using these X-rays. A synchrotron light source 1 or a laser plasma light source, for instance, is used as the light source. As to the diffraction grating 3, it is desirable that the blaze wavelength is equal to the center wavelength of the exposure X-rays. It is also desirable that the half-width of a filter characteristic obtained by the combination of the diffraction grating 3 and slit 4 is set in a range of one time to twice the half-width of the reflectance wavelength characteristic of the mask 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithographic technique suitable for application to a manufacturing process of semiconductor integrated circuits and the like, and more particularly to a projection exposure method and apparatus using X-rays.

[0002]

2. Description of the Related Art A reduction projection exposure method, which is the mainstream of lithography technology for semiconductor integrated circuits, transfers a pattern drawn on a mask onto a resist on a substrate to be exposed by an imaging optical system. Projection exposure resolution D and depth of focus DO
F is given by the following equation, where NA is the numerical aperture of the imaging optical system and λ is the exposure wavelength.

[0003]

## EQU1 ## D = k ₁ λ / NA (1)

[0004]

DOF = k ₂ λ / NA ² (2) In equations (1) and (2), k ₁ and k ₂ are determined by the characteristics of the light illuminating the mask, the type of resist, or the processing conditions. It is a constant. For example, when both k ₁ and k ₂ are 0.5, a resolution of 0.25 μm and a depth of focus of 0.1 are obtained by using a krypton fluoride laser having an emission wavelength of 248 nm and a projection lens having a numerical aperture of 0.5. It becomes 5 μm and can be applied to a 256-megabit DRAM.

In order to increase the density of semiconductor integrated circuits,
Higher resolution is required, for example 4 Gigabit D
A RAM requires a resolution of about 0.1 μm. formula
As can be seen from (1), the resolution is improved by increasing the numerical aperture or shortening the wavelength. But,
If the numerical aperture is made too large, the depth of focus will decrease according to equation (2), so there is naturally a limit to increasing the numerical aperture. On the other hand, the exposure wavelength is about 3 nm to 20 nm
If it is shortened to, it is possible to achieve a resolution of 0.1 μm or less while securing a depth of focus of 1 μm by an imaging optical system having a small numerical aperture. The wavelength is 3 nm to 20n.
The rays of m include extreme ultraviolet rays in addition to X-rays, but in the present specification, they will be collectively referred to as “X-rays”.

In the X-ray region, since the refractive index of the substance is extremely close to 1, it is practically difficult to use a lens in the imaging optical system, and it is necessary to use a reflecting mirror. In recent years, a multilayer-film reflective mirror has been developed in which a large number of two types of thin films having different refractive indexes are alternately laminated. As a result, a high reflectance can be obtained even in the X-ray region. Attempts have been actively made to perform X-ray projection exposure using a system.

An example of a conventional X-ray projection exposure method is shown in FIG. 7 [Radiation Optics, Vol. 5, pp. 133-142 (1992).
Year)]). Radiant ray 72 of synchrotron light source 71
The X-ray for exposure having a desired wavelength is selected by the thin film filter 73 to illuminate the mask 74. As the mask 74, a mask having a pattern formed of a reflective material on a non-reflective substrate is used. The X-rays reflected by the mask 74 are reflected by the reflecting mirrors 75 and 76 forming the imaging optical system, and then are imaged on the substrate 77 to be exposed. Mask 74 and reflecting mirrors 75, 7
In order to obtain a high reflectance, the reflecting surface 6 is composed of a multilayer film made of molybdenum and silicon (hereinafter referred to as “Mo / Si multilayer film”).

The wavelength of the X-ray for exposure is, for example, 13 nm, but the emitted light of the synchrotron light source 71 is a continuous spectrum light from the X-ray region to the visible region. Therefore, the wavelength component unnecessary for exposure is removed using the thin film filter 73. The thin film filter used is generally made of a carbon film having a thickness of, for example, 0.5 μm, and one example of its characteristics is shown in FIG. The transmittance of the thin film filter is about 10% when the wavelength is 13 nm, and the intensity loss of the exposure X-ray is remarkable. Therefore, the conventional X-ray exposure method is
There is a problem that the exposure requires a long time and the production efficiency is low. Further, as can be seen from FIG. 8, the thin film filter has a short wavelength (about 17 nm) including the exposure wavelength (13 nm).
(Below) has the property of transmitting light rays. As a result, the light rays not required for exposure are absorbed by the mask and all are converted to heat. As a result, the accuracy of the pattern transferred to the substrate to be exposed deteriorates due to the thermal distortion of the mask, or the mask itself is damaged by heat and its life is extended. There was a problem of lowering. Even when a thin-film filter is constructed using beryllium or aluminum,
We were unable to solve these problems.

[0009]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art, to achieve high transfer accuracy and high production efficiency, and to realize a long mask life. It is to provide a projection exposure method and apparatus.

[0010]

The above-mentioned problems of the present invention are as follows.
An effective solution can be achieved by diffracting a radiation beam from a linear light source by a diffraction grating, selectively extracting only an exposure X-ray having a desired wavelength by a slit, and illuminating a mask with the X-ray. It is possible. By adopting such a means, it is possible to almost completely eliminate the rays of unnecessary wavelength contained in the light source, and at the same time, it is possible to significantly reduce the loss of the exposure X-rays as compared with the conventional case.

[0011]

The radiation emitted from the light source is diffracted by the diffraction grating in different directions depending on its wavelength. Therefore, by providing a slit in the direction in which the exposure X-ray of the desired wavelength is diffracted, the exposure X-ray passes through the slit, but all unnecessary light rays having a different wavelength from the X-ray are blocked by the slit. To be done. Further, since the diffraction efficiency of the diffraction grating (the intensity ratio between the incident light beam and the diffracted light beam) can be made larger than the transmittance of the thin film filter, the intensity loss of the exposure X-ray can be greatly reduced. It is desirable to use, for example, a synchrotron light source or a laser plasma light source as the exposure X-ray light source. This is because the emission wavelengths of these light sources cover a wide range from the X-ray region to the visible light region, but a high-output X-ray for exposure can be easily obtained.

As the diffraction grating, a reflection type diffraction grating and a transmission type diffraction grating can be appropriately selected and used. When using a reflection type diffraction grating having a saw-toothed grating groove, it is desirable to make the blaze wavelength determined by the structure equal to the central wavelength of the exposure X-rays, which increases the diffraction efficiency of the diffraction grating. You can Further, it is desirable that the full width at half maximum of the filter characteristic obtained by the combination of the diffraction grating and the slit is set to the range of 1 to 2 times the full width at half maximum of the reflectance wavelength characteristic of the mask. X-rays for exposure can be efficiently extracted.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The X-ray projection exposure method and apparatus according to the present invention will be described below in more detail with reference to some embodiments shown in the drawings. The same symbols in FIGS. 1 to 6 indicate the same or similar items.

<Embodiment 1> In FIG. 1, reference numeral 1 is a synchrotron light source for emitting a light beam including an X-ray for exposure having a desired wavelength, and 3 is a plane reflection used for diffracting a light beam emitted from the light source 1. The type diffraction grating, 4 is a slit for selectively extracting only X-rays for exposure having a desired wavelength from diffracted light rays, 5 is a mask having a predetermined pattern drawn on its surface, 6,
Reference numeral 7 denotes a reflecting mirror which constitutes an image forming optical system, and 8 denotes a substrate to be exposed.

The light beam 2 emitted from the light source 1 is diffracted by the diffraction grating 3 in different directions depending on its wavelength.
The size of the light emitting point of the synchrotron light source is about 0.1 mm and can be regarded as a point light source. The diffraction grating 3 is
A large number of grating grooves are formed on a flat substrate at unequal intervals, and an X-ray for exposure having a desired wavelength converges on a slit 4 and passes through the slit to illuminate a mask 5. on the other hand,
Undesired light rays having a wavelength different from that of the exposure X-rays are reflected by the slit 4
Is blocked by. The mask 5 is formed by drawing a predetermined pattern with a reflective material on a non-reflective substrate. The X-rays reflected by the mask 5 are reflected by the reflecting mirrors 6 and 7 which form the imaging optical system and form an image on the substrate 8 to be exposed. The numerical aperture of the imaging optical system is 0.08, and the numerical aperture on the substrate 8 to be exposed is 0.18.
The pattern could be transferred with a resolution of μm.

The mask 5 and the reflecting surfaces of the reflecting mirrors 6 and 7 are formed of Mo / Si multilayer films in order to obtain high reflectance. The period length of the film is 6.7 nm and the number of layer pairs is 40.
The reflectance is as shown in FIG. The center wavelength of the reflectance wavelength characteristic could be 13 nm, and the half width could be 0.4 nm.

A general description of a plane diffraction grating having sawtooth-shaped grating grooves of unequal intervals is given in, for example, Japanese Patent Laid-Open No. 3-710.
This is described in Japanese Patent No. 24, but the main points are as follows (see FIG. 3). Now, assuming that one lattice groove whose groove interval is d is the origin, a is a constant, and the number of any other lattice groove at the position w in the direction perpendicular to the lattice groove is n, the lattice groove array is expressed by the formula ( It can be defined by 3). Further, if the distances from the position of the above-mentioned lattice groove which is the origin to the object point (light emitting point of the synchrotron light source 1) and the image point (position of the slit 4) are r ₁ and r ₂ , the diffracted light beam is transmitted to the slit 4. The condition for convergence can be expressed by equation (4) (see FIG. 1).

[0018]

N = (w + aw ² ) / d (3)

[0019]

## EQU4 ## cos ² α / r ₁ + cos ² β / r ₂ = −2 mλa / d (4) In the equation (4), α and β are rays that form the centers of the incident ray 32 and the diffracted ray 33 ( The angle formed by the principal ray) with the normal line of the diffraction grating 4, λ is the wavelength, and m is the diffraction order, and the relationship of the following equation is established between them.

[0020]

## EQU5 ## sinα + sinβ = mλ / d (5) In this embodiment, the light emitting point of the synchrotron light source 1 is the object point,
With the position of the slit 4 as the image point, m = 1, λ = 13 nm,
d = 1/300 mm, α = 87 degrees, β = −84.115
Every time, a = -0.002871 / mm, r 1 = 2,000
mm, r ₂ = 500 mm.

The full width at half maximum Δλ of the filter characteristic obtained by the combination of the diffraction grating 3 and the slit 4 is a function of the inverse linear dispersion K (wavelength difference per unit slit width) of the diffraction grating and the slit width Δs. It can be given in (6) and (7).

[0022]

(6) Δλ = K × Δs (6)

[0023]

Equation 7] _{K = dcosβ / mr 2 ... (} 7) In this embodiment, the value of the constant K in equation (6), (7)
Is 0.684 nm / mm. Therefore, the value of the half width Δλ was set to 0.4 nm by setting the value of the slit width Δs to 0.58 mm. The reason for setting in this way is that the X-ray for exposure is efficiently extracted by matching the filter characteristic with the reflectance wavelength characteristic of the mask 5. The full width at half maximum Δλ of the filter characteristic can be widened to twice the full width at half maximum of the reflectance wavelength characteristic of the mask 5, and at this level, the mask 5 does not cause a particular heat load problem. , The entire range of the reflectance can be effectively used.

In order to maximize the diffraction efficiency of the diffraction grating, it is desirable to match the blaze wavelength with the center wavelength of the X-ray for exposure. The blaze wavelength is a wavelength at which the incident light beam 32 and the diffracted light beam 33 form the same angle with the reflecting surface 31. Since the reflecting surface 31 of the grating groove forms an angle θ (blaze angle) with respect to the periodic direction of the grating, the following equation is established between the blaze angle θ and α and β.

[0025]

## EQU00008 ## .theta. = (. Alpha. +. Beta.) / 2 (8) In this example, .alpha. Is +87 degrees and .beta. Is -84.1.
Since it is 15 degrees, the blaze angle θ is set to 1.44 degrees. As a result, the blaze wavelength is the central wavelength λ of the exposure X-ray.
Which is equal to 13 nm, and the diffraction grating using gold as the material of the reflecting surface 31 made it possible to realize a diffraction efficiency of 45%.

The filter characteristic of the diffraction grating according to the present embodiment is as shown in FIG. 4, and a characteristic having a half-value width of 0.4 nm at a center wavelength of 13 nm, which is the same as the reflectance wavelength characteristic of the mask, was obtained. As a result, only the X-rays for exposure having the desired wavelength can be selectively extracted, and the thermal load on the mask due to the light having the undesired wavelength can be avoided. Also, for exposure X
The diffraction efficiency at the center wavelength of the line of 13 nm was 45%, and the intensity of the exposure X-ray illuminating the mask could be significantly increased.

Although the present embodiment has been described with respect to the case where the wavelength of the exposing X-ray is 13 nm, the wavelength of the exposing X-ray is within the range of about 3 nm to 20 nm, and is arbitrarily set according to the required resolution. It is possible to select. As a modified example of the present embodiment, the inventors of the present invention have used an exposure X-ray having a wavelength of 7 nm.
A line was used to perform a trial experiment for obtaining a resolution of 0.05 μm using an imaging optical system having a numerical aperture of 0.08. A Ru / BN multilayer film made of ruthenium and boron nitride was formed on the mask and the reflecting surface of the reflecting mirror (imaging optical system). The cycle length of the multilayer film was 3.6 nm, the number of layer pairs was 200, the center wavelength of the reflectance wavelength characteristics was 7 nm, and the half width was 0.1 nm. However, m = 1, d = 1/600 mm, α = 87
Degree, β = −83.950 degree, a = 0.002808 / m
m, r ₁ = 2,000 mm and r ₂ = 500 mm.
Since the inverse linear dispersion is 0.351 nm / mm, by setting the slit width to 0.29 mm, only the X-ray having the central wavelength of 7 nm and the half-value width of 0.1 nm is selectively taken out from the synchrotron light source and the mask 5 Illuminated. Also, by setting the blaze angle of the diffraction grating to 1.53 degrees,
It was possible to obtain a diffraction efficiency of 40% by setting the blaze wavelength to 7 nm and using a diffraction grating using gold as the material of the reflecting surface.
As a result, also in the case of this modification, it was possible to transfer a fine pattern having a line width of 0.05 μm onto the substrate to be exposed by using an X-ray for exposure having a sufficiently high intensity.

Example 2 An example in which the plane reflection type diffraction grating in Example 1 is changed to a transmission type plane diffraction grating is shown in FIG.
Shown in. In this embodiment, as in the case of the first embodiment, the mask 5 is illuminated by selectively extracting only the exposure X-ray having the center wavelength of 13 nm and the half width of 0.4 nm. Since the transmissive diffraction grating has a rectangular groove cross-section, it can be manufactured more easily than a reflective diffraction grating having a sawtooth grating groove.

<Embodiment 3> FIG. 6 shows another embodiment of the present invention using a concave reflection type diffraction grating. In a concave reflection type diffraction grating in which a large number of grating grooves are arranged at equal intervals, it is usual to adopt a so-called Rowland circle arrangement. Now, assuming that the radius of curvature of the concave surface is R, the conditions for converging the diffracted light rays can be expressed by the equations (9) and (10), and the rays forming the center of the incident light ray and the diffracted light ray (main The relationship shown in Expression (11) is established between the angles α and β formed by the light beam) with the normal line of the concave diffraction grating, the wavelength λ, and the diffraction order m.

[0030]

## EQU00009 ## r ₁ = Rcos α (9)

[0031]

[Equation 10] r ₂ = Rcos β (10)

[0032]

(11) sinα + sinβ = mλ / d (11) In the present embodiment, m = 1, λ = 13 nm, d = 1/300
mm, α = 87 degrees, β = −84.115 degrees, R = 10,
000 mm, r ₁ = 523.360 mm, r ₂ = 1,0
It was set to 25.336 mm. The inverse linear dispersion is dcos β / mr ₂
= 0.333 nm / mm. Therefore, by setting the width of the slit 4 to 1.2 mm, the half-value width of 0.
It was possible to obtain a filter characteristic of 4 nm, and it was possible to selectively extract only the X-rays for exposure having a desired wavelength and illuminate the mask 5. The concave diffraction grating can be easily manufactured as compared with the plane diffraction grating having the non-equidistant grating grooves because the grating grooves are at regular intervals. In the present embodiment, the concave reflecting mirror 9 is provided between the slit 4 and the mask 5 to collect the exposure X-rays diverging from the slit 4 and illuminate the mask 5. As a result, the intensity of the exposure X-rays could be increased as compared with the case where the exposure X-rays were not focused.

Needless to say, the concave reflecting mirror 9 can be used in the same manner as in the first and second embodiments, and the same effect can be obtained.

[0034]

As described above, according to the present invention, it is possible to remove a light beam having an undesired wavelength contained in a light source, and at the same time, to significantly reduce the intensity loss of exposure X-rays, thereby improving the production efficiency. In addition to being able to improve the transfer accuracy of the pattern, the life of the mask can be extended. It should be noted that in the above-described embodiment, the case where the synchrotron light source is used as the light source has been described, but the present invention can be naturally applied to the case where the laser plasma light source is used. The laser plasma light source is a continuous spectrum light source similarly to the synchrotron light source, and the present invention can be applied to remove light rays having a wavelength unnecessary for exposure. Moreover, the laser plasma light source is smaller and cheaper than the synchrotron light source, and has the effect of reducing the cost of the lithography process.

[Brief description of drawings]

FIG. 1 is a first X-ray projection exposure method and apparatus according to the present invention.
FIG. 6 is a layout configuration diagram for explaining the embodiment of FIG.

FIG. 2 is a curve diagram for explaining reflectance wavelength characteristics of a mask.

FIG. 3 is a sectional view for explaining a blaze condition of a diffraction grating.

FIG. 4 is a curve diagram for explaining filter characteristics of a diffraction grating.

FIG. 5 is an arrangement configuration diagram for explaining a second embodiment of the present invention.

FIG. 6 is an arrangement configuration diagram for explaining a third embodiment of the present invention.

FIG. 7 is an arrangement configuration diagram for explaining a conventional X-ray projection exposure method.

FIG. 8 is a curve diagram for explaining the conventional thin film filter characteristics shown in FIG. 7.

[Explanation of symbols]

 1 ... Synchrotron light source 3 ... Diffraction grating 4 ... Slit 5 ... Mask 6 ...

Claims (17)

[Claims] 1. A method for transferring a pattern on a mask to an exposure substrate by illuminating a mask on which a pattern to be transferred is drawn with X-rays and forming an image of the reflected light beam on the exposure substrate.
In the line projection exposure method, after diffracting a radiation beam from a light source by a diffraction grating, only X-rays for exposure having a desired wavelength are selectively taken out using a slit, and the mask is illuminated by the X-rays. X-ray projection exposure method. 2. The X-ray projection exposure method according to claim 1, wherein a plane reflection type diffraction grating is used as the diffraction grating. 3. The X-ray projection exposure method according to claim 1, wherein a concave reflection type diffraction grating is used as the diffraction grating. 4. The X-ray projection exposure method according to claim 1, wherein a transmission diffraction grating is used as the diffraction grating. 5. The X-ray projection according to claim 1, wherein the diffraction grating is a diffraction grating having a blaze wavelength equal to the central wavelength of the X-ray for exposure. Exposure method. 6. The full width at half maximum of the filter characteristic obtained by the combination of the diffraction grating and the slit is set so as to be in the range of 1 to 2 times the full width at half maximum of the reflectance wavelength characteristic of the mask. The X-ray projection exposure method according to any one of claims 1 to 5. 7. The X-ray projection exposure method according to claim 1, wherein a synchrotron light source is used as a light source of X-rays for exposure. 8. The X-ray projection exposure method according to claim 1, wherein a laser plasma light source is used as a light source of X-rays for exposure. 9. A light source for emitting a light beam including an X-ray for exposure having a desired wavelength, a diffraction grating for diffracting a light beam emitted from the light source, and a light beam diffracted by the diffraction grating. The mask is illuminated with a slit for selectively extracting only the exposure X-rays of the wavelength and the exposure X-rays extracted by the slit, and the reflected light beam is imaged on the substrate to be exposed through the imaging optical system. An X-ray projection exposure apparatus comprising at least means for: 10. The X-ray projection exposure apparatus according to claim 9, wherein the diffraction grating is composed of a plane reflection type diffraction grating. 11. The X-ray projection exposure apparatus according to claim 9, wherein the diffraction grating is composed of a concave reflection type diffraction grating. 12. The X-ray projection exposure apparatus according to claim 9, wherein the diffraction grating is composed of a transmission type diffraction grating. 13. The X-ray projection according to claim 9, wherein the diffraction grating is constructed with a diffraction grating having a blaze wavelength equal to the center wavelength of the X-ray for exposure. Exposure equipment. 14. The full width at half maximum of the filter characteristic obtained by the combination of the diffraction grating and the slit is set to be in the range of 1 to 2 times the full width at half maximum of the reflectance wavelength characteristic of the mask. The X-ray projection exposure apparatus according to any one of claims 9 to 13, which is characterized by the above. 15. The X-ray projection exposure apparatus according to claim 9, wherein the light source comprises a synchrotron light source. 16. The X-ray projection exposure apparatus according to claim 9, wherein the light source is constituted by a laser plasma light source. 17. The at least one concave reflecting mirror for condensing X-rays for exposure is arranged between the slit and the mask, according to any one of claims 9 to 16. The X-ray projection exposure apparatus according to.