JP2001027699A - Multi-layer film reflecting mirror and reflecting optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multi-layer film reflecting mirror comprising a high reflectivity for soft X-ray region, especially laser plasma X-ray which uses Xe as a target comprising peak wavelength of 10.9 nm. SOLUTION: An Ru12 where a difference between the refractive index at a soft X-ray wavelength region and that of vacuum is large and an SrB6 layer 13 with small one are alternately laminated on an Si substrate 11 to form a multi-layer film reflecting mirror 1. The multi-layer film reflecting mirror 1 provides a high reflectivity at peak wavelength 10.9 nm of laser plasma X-ray using Xe as a target when a cycle length is 56.3 Å, Γ value which is the ratio between the layer thickness of Ru layer 12 and cycle length is 0.4, and the number of layers is 80 layer pairs.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multilayer mirror and a reflection optical system used in an X-ray apparatus such as an X-ray telescope, an X-ray laser, and an X-ray lithography.

[0002]

2. Description of the Related Art The complex refractive index of a substance is represented by the following equation (1). Since both δ and k are much smaller than 1 in the X-ray region, the optical system of an apparatus using X-rays is used. Uses a reflection optical system.

N = 1−δ−ik (1) In the formula (1), i ² = −1, and the imaginary part k represents X-ray absorption by the substance.

However, in the case of an oblique incidence optical system using total reflection, the reflectivity is very small at an incident angle close to normal which is smaller than the critical angle θc for total reflection. For this reason, in a reflection optical system used in the X-ray region, a multilayer film reflection mirror having a large number of reflection surfaces formed by alternately laminating two types of substances having a large amplitude reflectance at an interface is used. The thickness of each layer is set based on the theory of optical interference so that the phases of the reflected waves reflected at each interface match. At this time, a material having a small difference between the refractive index at the used X-ray wavelength and the refractive index of vacuum (= 1) is used as one of the materials to be laminated, and a material having the large difference is used as the other material. Can be

[0004] Further, since the multilayer film reflecting mirror can also reflect X-rays vertically, in an optical system using vertical reflection,
The aberration can be reduced as compared with the oblique incidence optical system using the total reflection. Further, since the multilayer mirror reflects X-rays strongly only when the Bragg condition represented by the formula (2) is satisfied, it has a characteristic of wavelength selectivity.

2d · sin θ = m · λ (2) In equation (2), d is the period length of the multilayer film, θ is the oblique incident angle, λ is the wavelength of the X-ray, and m is the order.

[0005] Examples of the multilayer film used in the multilayer mirror include a W / C multilayer film in which W (tungsten) and C (carbon) are alternately laminated, and a Mo / Mo layer in which Mo (molybdenum) and C are laminated. / C multilayer films and the like are conventionally known. These multilayer films are formed by sputtering, vacuum evaporation, CV
It is formed by a thin film forming technique such as D (Chmical Vapor Deposition).

Among the multilayer films used in such a multilayer mirror, the Mo / Si multilayer film has an L absorption edge (wavelength) of Si.
A multilayer film having a high reflectance on the long wavelength side (12.6 nm) and having a reflectance of 60% or more (direct incidence) at a wavelength around 13 nm can be relatively easily produced. Also, Mo /
In the Be multilayer film, Be (beryllium) at a wavelength of 11.1 nm is used.
Is known to exhibit a high reflectance on the long wavelength side of the K absorption edge, and also in this case, a multilayer film having a reflectance of 60% or more at a wavelength near 13 nm can be relatively easily produced. .

[0007] Such a Mo / Si multilayer film or Mo / Be
Multi-layer reflecting mirrors are used in research fields such as X-ray telescopes and X-ray laser resonators, and EUVL (Extrem
e Ultraviolet Lithography) is expected to be applied to reduction projection lithography technology using soft X-rays. As the soft X-ray light source for EUVL, laser plasma X-rays and the like are known in addition to the emitted light. Laser plasma X-rays refer to X-rays generated from plasma by irradiating a gaseous or solid target with laser light in a vacuum to convert the gas or solid into a plasma state. The spectrum of the generated X-rays is a spectrum specific to the type of target used.

[0008] In the case of laser plasma X-rays, when a solid is used as a target, scattering particles called debris are generated, and there is a problem that an optical system is damaged. For this reason, laser plasma X-rays using a gas as a target have recently attracted attention. For example, research using a Xe (xenon) gas as a gas target has been conducted (Glenn D. Kubiak, et al., SPIE vol. .3331,199
8). When Xe gas is used as a target for laser plasma X-rays, the X-ray intensity becomes maximum at a wavelength of about 10.9 nm.

[0009]

[Problems to be solved by the invention] However, the wavelength 1
Since 0.9 nm is on the short wavelength side of the absorption edge of each of Si and Be, the X-ray absorption of Si and Be increases, and Mo
High reflectance cannot be obtained in any of the / Si multilayer film and the Mo / Be multilayer film. Usually Mo / Si
In the case of the multilayer film and the Mo / Be multilayer film, respectively, around 13 nm,
Since X-rays with a wavelength near 11 nm are used, the peak wavelength 1
When 0.9 nm laser plasma X-rays are used, light having a wavelength outside the peak wavelength is used. In laser plasma X-rays, the X-ray intensity at wavelengths around 13 nm and 11 nm is the peak intensity (X-ray intensity at the peak wavelength).
However, there is a drawback that the light source intensity cannot be used effectively because it is only a fraction. In addition, the Mo / Be multilayer film has a problem that Be and Be compounds are highly toxic and difficult to handle.

An object of the present invention is to provide a soft X-ray region, in particular, Xe
An object of the present invention is to provide a multilayer mirror and a reflection optical system having a high reflectance for X-rays having a wavelength of 10.9 nm, which is the peak wavelength of laser plasma X-rays using gas.

[0011]

An embodiment of the present invention will be described with reference to FIG. (1) The invention according to claim 1 is characterized in that the first layer 12 formed of a substance having a large difference between the refractive index in the soft X-ray wavelength region and the refractive index of vacuum, and the first layer 12 formed of a substance having a small difference. Layer 1 of 2
The above-mentioned object is achieved by using the strontium boride as a material for forming the second layer 13, which is applied to the multilayer reflector 1 in which a plurality of layer pairs consisting of I do. (2) According to a second aspect of the present invention, in the multilayer film reflecting mirror 1 according to the first aspect, ruthenium is used as a material for forming the first layer 12. (3) The invention according to claim 3 is the multilayer reflector according to claim 1 or 2, wherein the period length is a layer thickness of a layer pair;
In addition, the layer thickness ratio between the first layer 12 and the second layer 13 is adjusted so that the center wavelength of the reflectance of the multilayer film becomes approximately 10.9 nm. (4) The invention according to claim 4 is applied to a reflection optical system of an apparatus using a laser plasma X-ray source using xenon as a target, and has the above object by having a plurality of multilayer film reflection mirrors according to claim 3. To achieve.

In the section of the means for solving the above-mentioned problems, which explains the configuration of the present invention, the drawings of the embodiments of the present invention are used to make the present invention easier to understand. However, the present invention is not limited to the embodiment.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. In order to increase the reflectance of the multilayer mirror, it is most important to select a material to be used for the multilayer film, and it is preferable to use a material that increases the reflectance at each interface. The amplitude reflectance r at the interface of the multilayer film in the case of normal incidence is represented by n1 = 1−δ1−ik1 and n2 = 1−δ2, respectively, of the refractive indices of the two substances constituting the multilayer film.
When −ik2 is given, it is given by Fresnel equation (3).

R = (n2−n1) / (n2 + n1) = {(δ1−δ2 + i (k1−k2)} / {(δ1 + δ2-2) + i (k1−k2)} (3) where i ² = -1, k is a constant representing the absorption of the substance.
Since a substance having a small k (small absorption) is used as the substance used for the multilayer mirror, δ≫k, and the equation (3) can be approximated as the following equation (4).

R = (δ1−δ2) / (δ1 + δ2-2) (4) Therefore, in order to increase the reflectance at the multilayer film interface, a material having a large difference in δ (= δ1−δ2) is required. It is better to use a combination.

The inventor of the present invention has examined the above-mentioned conditions, that is, a combination of many substances for a substance having a small value of k and a large difference in δ between two substances. When SrB ₆ (strontium boride) or Y (yttrium) is used and Ru (ruthenium) or Rh (rhodium) is used as a substance having a large δ, Xe (xenon) is used in a soft X-ray region, particularly, a target.
It has been found that this method is preferable for producing a multilayer mirror having a high reflectance with respect to a laser plasma X-ray peak wavelength of 10.9 nm using the method.

In particular, it has been found that a multilayer film formed of a combination of the following substances has a high reflectance with respect to a peak wavelength of 10.9 nm of laser plasma X-rays using Xe as a target. SrB ₆ multilayer Figure 1 by (strontium boride) and Ru multilayer Y by the (ruthenium) (yttrium) and Ru is a sectional view of a Ru / SrB ₆ multilayer reflector 1 using the SrB ₆ and Ru In this case, a Ru layer 12 and a SrB ₆ layer 13 are alternately laminated on a substrate 11 such as a Si substrate or a glass substrate.

Here, when the above-mentioned multilayer film is formed on a substrate, it is preferable to select a material which is unlikely to undergo chemical changes such as oxidation as a material of the uppermost layer of the multilayer film, and by doing so, the reflection of the multilayer film is preferred. The aging of the mirror can be reduced. In the case of a Ru / SrB ₆ multilayer film, the SrB ₆ layer is preferably the uppermost layer, and in the case of the Ru / Y multilayer film, the Ru layer is preferably the uppermost layer.

FIG. 2 is a diagram showing the spectrum intensity of laser plasma X-rays using Xe as a target, wherein the vertical axis represents the X-ray intensity and the horizontal axis represents the wavelength. The peak wavelength of the laser plasma X-ray is 10.9 nm, and the X-ray intensity (peak intensity) at that wavelength is about 7.9 times the intensity near the wavelength of 13.1 nm used in the Mo / Si multilayer film. Furthermore, it is 1.7 times as high as the X-ray intensity at a wavelength near 11.4 nm used in the Mo / Be multilayer film.
Therefore, by adjusting the reflection peak wavelength of the multilayer film to the peak wavelength of laser plasma X-ray using Xe of 10.9 nm,
Laser plasma X-rays from the light source can be used effectively.

Next, the case where the above-described Ru / SrB ₆ multilayer film reflecting mirror is used in the optical system of the reduction projection exposure apparatus using soft X-rays will be described. FIG. 3 shows a schematic configuration of the soft X-ray exposure apparatus, focusing on an optical system. This exposure apparatus is a projection exposure apparatus that performs an exposure operation by a step-and-scan method, and projects an image of a part of a circuit pattern drawn on a reflective mask 5 onto a wafer 8 via a projection optical system 6. In addition, the mask 5 and the wafer 8 are relatively scanned with respect to the projection optical system 6 in a one-dimensional direction (here, the Y-axis direction), so that the entire circuit pattern of the mask 5 is exposed to a plurality of shot areas ( (One step area) by a step-and-scan method.

In the apparatus shown in FIG. 3, the illumination optical system 3 is constituted by two mirrors 3a and 3b, and the projection optical system 6 is constituted by four mirrors 6a to 6d. Therefore, FIG.
The exposure apparatus has a reflection optical system composed of six mirrors and one reflection type mask 5. Each mirror 3a,
3b, the 6a~6d, Ru / SrB ₆ multilayer film described above is formed. Reference numeral 2 denotes a light source that generates laser plasma X-rays.
The light is reflected by a and 3b, and is irradiated on the reflective mask 5 supported on the reticle stage 4 by Koehler illumination.

FIG. 9 is a plan view of the reflecting mirror 3a. The reflecting mirror 3a is a reflecting mirror having a plurality of concave reflecting surfaces 95 having a shape similar to the illumination area on the reflective mask 5. Each of these reflecting surfaces 95 has the same focal length, forms a secondary light source, becomes parallel light by the reflecting mirror 3a, and is irradiated onto the reflective mask 5.

A reflective film pattern made of a Ru / SrB ₆ multilayer film is provided on the reflective mask 5, and the reflective film pattern has a pattern shape corresponding to the pattern to be transferred onto the wafer 8. . The X-rays reflected by the mask 5 and including the pattern information of the mask 5 enter the projection optical system 6.

The projection optical system 6 includes a concave first mirror 6.
a, a second mirror 6b having a convex shape, a third mirror 6c having a concave shape, and a fourth mirror 6d having a concave shape, and are arranged such that their optical axes are coaxial. Each of the mirrors 6a to 6d is shaped so as not to block a reciprocating optical path formed by the mirrors, and an aperture stop (not shown) is provided at the position of the third mirror 6c.

The X-rays reflected by the mask 5 are sequentially reflected by a first mirror 6a to a fourth mirror 6d, and
A predetermined magnification β (for example, | β | = 1 /
(4, 1/5, 1/6), a reduced image of the pattern of the mask 5 is formed. The mask 5 can be moved at least along the Y direction by the reticle stage 4, and the wafer 8 is supported by the wafer stage 7 that can move along three directions of XYZ. During the exposure operation, the mask 5 and the wafer 8 are moved in opposite directions at a predetermined speed ratio determined by the reduction magnification of the projection optical system 6. As a result, the pattern of the mask 5 is scanned and exposed in a predetermined shot area on the wafer 8.

Next, a laser / plasma X-ray source targeting Xe having a spectrum as shown in FIG. 2 is used as a light source, and the Mo / Si multilayer film ( 50 layer pair) and Ru
A comparison is made with the case where a / SrB ₆ multilayer film (80 layer pairs) is used. Assuming that these multilayer films have a theoretical reflectance (normal incidence), the reflectance of the Mo / Si multilayer film is 7
5.2%, reflectivity of Ru / SrB ₆ multilayer film is 62.4%
It is. The optical systems 3 and 6 and the reflective mask 5 constitute a reflective optical system composed of seven mirrors. Is calculated in consideration of Mo /
Assuming that the strength when using a Si multilayer film is 100, Ru
The strength when the / SrB ₆ multilayer film is used is 114.

FIG. 4 is a diagram showing the reflectivity and X-ray intensity of a reflecting optical system comprising seven mirrors (for example, mirrors 3a, 3b, 6a, 6b, 6c, 6d and mask 5 in FIG. 3). The horizontal axis represents the wavelength of the X-ray, the left scale on the vertical axis represents the reflectance, and the right scale on the vertical axis represents the intensity. In FIG. 4, MS1 and MS2 indicate the reflectance in the case of the Mo / Si multilayer film and the X-ray intensity when passing through a reflection optical system using seven multilayer mirrors. On the other hand, RS1 and RS2 show the X-ray intensity when it passes through the reflectance in the case of Ru / SrB ₆ multilayer film, and the multilayer film reflecting mirror 7 Like using reflective optics. Also, a curve X indicated by a dotted line
e-LPX indicates the spectral intensity of laser plasma X-rays targeting Xe.

As shown in FIG. 4, the reflective optical system using the Mo / Si multilayer film reflects only X-rays having a wavelength of about 13.1 nm, and the reflective optical system using the Ru / SrB ₆ multilayer film has the wavelength of 1 nm.
Only X-rays near 0.9 nm are reflected. Comparing the reflectances MS1 and RS1, the Mo / Si multilayer film is larger, but
Since the spectral intensity of laser plasma X-ray has a peak near the wavelength of 13.1 nm, the X-ray intensity of RS2 is lower than that of RS2.
Becomes larger than S2. The intensity of the X-ray passing through the reflecting optical system is represented by the area of a region surrounded by the curves RS2 and MS2 and the horizontal axis.
As described above, if the Mo / Si multilayer film is 100, the Ru / SrB ₆ multilayer film is 114.

At present, a Mo / Si multilayer film having a high reflectivity of about 67.5% is obtained (C. Montca).
lm, et al., SPIE vol. 3331, 1998). Therefore, an X-ray intensity equivalent to the X-ray intensity obtained by using a Mo / Si multilayer film having a reflectivity of 67.5% in the above-described reflective optical system is set to Ru / SrB _6.
To obtain a multilayer film, the reflectance A of Ru / SrB ₆ multilayer film is calculated by the following equation (5).

A = 0.624 × (0.675 / 0.752) / (1.14 ^1/7 ) (5) From the equation (5), A is 0.55, and the Ru / SrB ₆ multilayer film is obtained. Is 55%, Mo / S with a reflectance of 67.5%
X-ray intensity equivalent to the case of using the i multilayer film can be obtained.

Since this reflectivity of 55% is lower than the ideal reflectivity of the Ru / SrB ₆ multilayer film of 62.4%, if a Ru / SrB ₆ multilayer film having a reflectivity of 55% or more is manufactured, Mo is obtained.
An optical system (optical system of seven mirrors) having higher reflected X-ray intensity than the case where the / Si multilayer film is used can be obtained. Also,
13.1nm wavelength X used in Mo / Si multilayer mirror
Since X-rays having a wavelength of 10.9 nm shorter than the X-rays are used,
It becomes possible to project a finer pattern.

In the above-described comparison, a general multilayer film having a reflectance peak at a wavelength of 13.1 nm has been described as an example of the Mo / Si multilayer film.
A Mo / Si multilayer having a peak at 10.9 nm is also possible. This Mo / Si multilayer film has a period length of 55.63 °, Mo
The value of Γ, which is the ratio between the layer thickness of the layer and the period length, is 0.4,
The number of layers is 50 layer pairs. The reflectance (ideal reflectance) at a wavelength of 10.9 nm is 30.1%. This reflectivity is equal to R
is lower than the ideal reflectance of u / SrB ₆ multilayer film, in terms of X-ray intensity is advantageous for Ru / SrB ₆ multilayer film, the more advantage the number of mirrors constituting the optical system is increased is increased.

By the way, when the theoretical reflectance when the number of stacked Ru / SrB ₆ multilayer films is changed is calculated, it is found that the reflectance is 6 for 50 layer pairs.
0.5%, 62.4% for 80 layer pairs, 62.4% for 100 layer pairs.
5%. From this, it is considered that the reflectance is almost saturated in the 80 layer pairs.

When a laser plasma X-ray source using Xe as a target must be used as a light source,
May be used Y shown in the following examples (yttrium) instead of SrB _6. Since the absorption edge of this substance called yttrium is shorter than 10.9 nm, 10.
The reflectivity at 9 nm is better than that of a mirror made of a Mo / Si multilayer film. Instead of ruthenium, rhodium (Rh) having substantially the same optical constants may be used.

[0032]

(Embodiment 1) The multilayer mirror of this embodiment has the same structure as the Ru / SrB ₆ multilayer film shown in FIG. 1 described above, and has a Ru layer 12 and a SrB ₆ layer on a Si substrate 11. Tier 1
Ru / SrB ₆ multilayer film in which Ru and
It is formed by ion beam sputtering using each target of SrB ₆ (see FIG. 1). Ru /
The period length of the SrB ₆ multilayer film is 56.3 °, the value of Γ which is the ratio of the layer thickness of the Ru layer to the period length is 0.4, and the number of stacked layers is 8
0 layer pairs. FIG. 6 shows the reflectance (calculated value) of the Ru / SrB ₆ multilayer film with respect to wavelength.
The reflectance at 10.9 nm is 62.4%. In addition, when the soft X-ray reflectivity of the manufactured Ru / SrB ₆ multilayer film at normal incidence was measured using synchrotron radiation, a reflectivity of 57% was obtained. This value is sufficient for a multilayer mirror used for soft X-ray reduction lithography.

(Embodiment 2) FIG. 7 is a cross-sectional view of a multilayer mirror 9 of Embodiment 2. This multilayer film reflecting mirror 9 is composed of a Ru substrate in which Y layers 92 and Ru layers 93 are alternately stacked on a Si substrate 91.
/ Y multilayer film is formed by ion beam sputtering using Ru and Y targets. Ru
The period length of the / Y multilayer film was 56.2 °, the value of Γ, which is the ratio between the layer thickness of the Ru layer and the period length, was 0.4, and the number of layers was 80 pairs. FIG. 8 shows the reflectance (calculated value) of the Ru / Y multilayer film with respect to the wavelength. The reflectance at a wavelength of 10.9 nm is 63.8%. Further, when the reflectance of the manufactured Ru / Y multilayer film at normal incidence was measured by using synchrotron radiation, a reflectance of 56% was obtained. This value is sufficient for a multilayer mirror used for soft X-ray reduction lithography.

The stoichiometric ratio of strontium boride used for the multilayer mirror is Sr: B = 1: 6.
It is not limited to only things. In fact, even if the stoichiometric ratio of the film material at the time of film formation varies from this value, there is no significant optical effect.

In the correspondence between the embodiment described above and the elements of the claims, the Ru layers 12 and 93 are the first layers.
, And the SrB ₆ layer 13 and the Y layer 92 each constitute a second layer.

[0036]

As described above, according to the first to third aspects of the present invention, a multilayer mirror having a high reflectance in the soft X-ray region can be obtained. In particular, in the invention of claim 3, the laser plasma X using Xe as a target is used.
Since it has a high reflectance for X-rays having a peak wavelength of 10.9 nm, for example, in an exposure apparatus using a laser plasma X-ray using Xe as a target as a light source, the X-rays from the light source can be effectively used. Can be used. According to the fourth aspect of the present invention, there is provided a reflection optical system capable of effectively utilizing X-rays from a laser plasma X-ray source using Xe as a target.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a multilayer reflector according to the present invention.

FIG. 2 shows a laser plasma X using Xe as a target.
The figure which shows the spectral intensity of a line.

FIG. 3 is a schematic view of an exposure apparatus using a multilayer mirror according to the present invention in an optical system.

FIG. 4 is a diagram showing a reflectance and an X-ray intensity of a reflection optical system including seven mirrors.

FIG. 5 is a view showing an ideal reflectance of a Mo / Si multilayer film having a reflection peak at a wavelength of 10.9 nm.

FIG. 6 is a diagram showing an ideal reflectance of a Ru / SrB ₆ multilayer film.

FIG. 7 is a cross-sectional view of a multilayer reflecting mirror 9 according to the second embodiment.

FIG. 8 is a view showing an ideal reflectance of a Ru / Y multilayer film.

FIG. 9 is a schematic diagram of a reflecting mirror 3a.

[Explanation of symbols]

1, 9 multilayer film reflecting mirror 2 light source 3 illumination optical system 4 reticle stage 5 reflective mask 6 projection optical system 7 wafer stage 8 wafer 11, 91 Si substrate 12, 93 Ru layer 13 SrB ₆ layer 92 Y layer 95 concave reflecting surface

Claims (4)

[Claims] 1. A first layer formed of a material having a large difference between a refractive index in a soft X-ray wavelength region and a refractive index of a vacuum, and a second layer formed of a material having a small difference therebetween. A multilayer mirror comprising a plurality of layer pairs laminated on a substrate, wherein strontium boride or yttrium is used as a material forming the second layer. 2. The multilayer reflector according to claim 1, wherein ruthenium or rhodium is used as a material forming the first layer. 3. The multilayer reflector according to claim 1, wherein a period length, which is a layer thickness of the layer pair, and a layer thickness ratio between the first layer and the second layer. A multilayer reflector, wherein the central wavelength of the reflectance of the multilayer film is adjusted to be approximately 10.9 nm. 4. A reflection optical system of an apparatus using a laser plasma X-ray source using xenon as a target, comprising a plurality of multilayer mirrors according to claim 3.