JP2000206300A - X-ray interference microscope and method for testing x-ray reflecting mirror - Google Patents

Abstract

PROBLEM TO BE SOLVED: To dramatically improve the sensitivity of microscope observation in an X-ray range. SOLUTION: A reflecting mirror 4 is located between an X-ray image- formation optical system 1 and an X-ray image pick-up element 3 placed on an image surface S2, a point light source P1 is located in a mirror image position to the image surface S2 with respect to the reflecting mirror 4 and a sample 2 placed on a surface S1 of an object is irradiated and illuminated through a part 1-3 of an opening of the X-ray image-formation optical system 1 through the medium of the reflecting mirror 4 by the point light source P1. The inside of the opening 1-3 of the X-ray image-formation optical system 1 is coherently illuminated by using quasi-monochrome X rays for the point light source 1 and limiting the size of it in terms of space with a pinhole 5. Moreover, the phase change of the X rays due to the sample 2 is allowed to emerge on the X-ray image pick-up element 3 as interference contrast by locating a reflection standard 7 in a symmetrical position to the surface S1 of the object with respect to a beam splitter 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray interference microscope which can be applied to the biological / medical field through microscopic observation of a biological sample and which can be applied in a wide range of fields such as semiconductor manufacturing technology, and an X-ray interference microscope. The present invention relates to a method for inspecting an X-ray reflector used in X-ray lithography or the like using an interference microscope.

[0002]

2. Description of the Related Art X-ray microscopes can be used for microanalysis because they can be expected to have higher resolution because of the shorter wavelength than visible light, and can obtain contrast due to the difference in X-ray absorption of the elements constituting the sample. It has such features as

[0003]

However, in the conventional X-ray microscope, it is difficult to manufacture an X-ray optical element or the like for constituting a microscope optical system in the X-ray region. It was used. Further, the conventional X-ray microscope employs a method of transmitting illumination of the sample (illuminating from the back side of the sample). In order to transmit and illuminate the sample, a very thin thin film ( The sample must be supported by a membrane or the like, which makes it difficult to support the sample. Further, in a conventional X-ray microscope, contrast is obtained by a difference in X-ray absorption by a sample,
It was impossible to obtain information on the phase change.

Further, X-rays such as multilayer reflectors used for X-ray reduction projection exposure, which are expected as a future semiconductor manufacturing technology, will be described.
In a X-ray mirror, there is a problem that a phase difference of X-rays due to minute unevenness of a reflection surface is transferred as a phase-type defect (Reference: KBNguyenet.al., J.Vac.Sci.Techn.
ol.B12 (6), 3833-3840 (1994).). A phase-type defect can become a defect even with an extremely small step of 1/4 (3 to 4 nm) of the wavelength of X-rays, but a conventional defect inspection apparatus using visible light detects such an extremely small step. That was impossible.

The present invention has been made in order to solve such a problem, and an object of the present invention is to use a fragile thin film such as a membrane for supporting a sample, which is not limited to a part for research. An object of the present invention is to provide an X-ray interference microscope which does not need to be provided, and which can obtain information on a phase change and dramatically improve the sensitivity of microscopic observation in an X-ray region. By using this X-ray interference microscope,
An object of the present invention is to provide a method of inspecting an X-ray reflecting mirror capable of detecting a phase-type defect due to a minute step on a reflecting surface of the X-ray reflecting mirror.

[0006]

In order to achieve the above object, an X-ray interference microscope according to the present invention comprises a reflection optical system between an X-ray imaging optical system and an X-ray image pickup device arranged on an image plane. A mirror is disposed, and a point light source is disposed at an image plane and a mirror image position with respect to the reflecting mirror, and the point light source allows the sample placed on the object plane through a part of the opening of the X-ray imaging optical system via the reflecting mirror. Epi-illumination (illumination from the surface side of the sample) is performed.
According to the present invention, the sample placed on the X-ray reflecting mirror is illuminated with incident light by a point light source,
By epi-illuminating the line reflector, there is no need to hold the sample with a fragile thin film such as a membrane. Note that, for example, a quasi-monochromatic X-ray is used as a point light source and the size of the point light source is spatially limited using a pinhole or the like, so as to be within the field of view of the microscope (in the aperture of the X-ray imaging optical system). Is coherently illuminated.

In the X-ray interference microscope according to the present invention, the beam splitter is disposed between the X-ray imaging optical system and the object plane in the above-described configuration, and the beam is reflected at a position symmetrical to the object plane with respect to the beam splitter. A prototype is arranged, and a Mach-Zehnder interferometer is configured by the beam splitter and the reflective prototype so that a phase change of an X-ray due to a sample appears as an interference contrast on an X-ray imaging device.
According to the present invention, since the sample and the reflecting prototype are simultaneously illuminated while maintaining spatial coherence by epi-illumination,
The phase change of the X-ray due to the sample appears as interference contrast on the X-ray imaging device. As a result, the phase change of the X-ray due to the sample is visualized as an interference contrast instead of the contrast due to the difference in the absorption of the X-ray of the sample, and the minute phase change of the X-ray due to the sample is observed as a very large contrast difference. Becomes possible.

Further, the inspection method of the X-ray reflector according to the present invention
An X-ray reflecting mirror is arranged as a sample on the object surface of the X-ray interference microscope described above, and the reflection base or beam splitter of the X-ray interference microscope is slightly moved in the optical axis direction, so that the reflection surface of the X-ray reflecting mirror is This is to detect a defect. According to the present invention, the phase change of the X-ray caused by the sample appears as a contrast due to the change of the interference on the X-ray imaging device while the imaging relationship of the microscope is maintained by slightly moving the reflection prototype or the beam splitter. ,
A defect on the reflection surface of the X-ray reflecting mirror can be detected from the change in contrast or the like. In this case, since the phase change of the X-ray is visualized as interference, the resolution of the X-ray interference microscope is very high, and it is possible to detect an extremely small step below the X-ray wavelength. Further, since the defect detection is performed simultaneously with the microscopic observation, the size of the detectable defect benefits from the resolution of the X-ray interference microscope, and a spatial resolution of 100 nm or less can be expected. Further, it is possible to distinguish and detect a defect (amplitude defect or black defect) due to a partial omission of the X-ray reflecting mirror or adhesion of particles and a phase defect due to a minute step.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments. [X-ray Interference Microscope] FIG. 1 is a view showing a main part of an embodiment of an X-ray interference microscope according to the present invention. In the figure,
1 is an X-ray imaging optical system, 2 is a sample arranged on the object plane S1, and 3 is an X-ray image sensor arranged on the image plane S2.

In this embodiment, a reflective Schwarzschild optical system in which spherical mirrors 1-1 and 1-2 are provided with a reflective coating such as a multilayer film reflective mirror is used as the X-ray imaging optical system 1. . As the multilayer film reflecting mirror, a Mo / Si multilayer film widely used as a combination of multilayer films capable of obtaining a high reflectance for X-rays with a wavelength of 13 nm is used.

Regarding the X-ray imaging optical system 1, a total reflection optical system such as a reflection type optical system using a mirror configuration other than the other Schwarzschild optical system, a Walter type optical system, or the like, or
As long as an optical system such as a Fresnel zone plate lens which can obtain a desired resolution in the X-ray region, the effect is not changed even if any X-ray imaging optical system is used. Further, as for the X-ray imaging element 3, if a two-dimensional image of X-rays can be observed at a desired resolution, a method of observing an X-ray CCD or a fluorescent screen with a visible camera may be used. It is possible.

In FIG. 1, reference numeral 4 denotes a reflecting mirror, which is arranged between the X-ray imaging optical system 1 and an image plane S2. 5 is a point light source or a pseudo point light source (hereinafter simply referred to as a point light source) P
1 and is located at the position of the image plane S2 and the mirror image with respect to the reflecting mirror 4.

The light from the point light source P1 is reflected by the reflecting mirror 4, and irradiates the sample 2 arranged on the object plane S1 through a part 1-3 of the aperture of the X-ray imaging optical system 1. That is, in the present embodiment, the point light source P1 illuminates the sample 2 arranged on the object plane S1 through the reflecting mirror 4 through the part 1-3 of the aperture of the X-ray imaging optical system 1 to perform incident illumination. .

Reference numeral 6 denotes a beam splitter for X-rays, and reference numeral 7 denotes a reflection prototype (reference prototype: reflecting mirror). . The reflection prototype 7 is disposed at a position symmetrical to the object plane S1 with respect to the beam splitter 6. The beam splitter 6 and the reflection standard 7 constitute a Mach-Zehnder interferometer.

The image of the reflection standard 7 and the image of the sample 2 are superposed by the beam splitter 6 and interfere with each other.
The phase change of the line is imaged on the X-ray image sensor 3 as a relative change with respect to the phase of the X-ray by the reflection standard 7, that is, as a contrast due to a difference in interference.

By slightly moving the reflection standard 7 or the beam splitter 6 constituting the Mach-Zehnder type interferometer in the direction of the optical axis, the contrast due to the difference in interference can be changed. The contrast due to the difference in the interference is inverted by a movement amount of about １ ／ of the wavelength of the X-ray used when, for example, the reflection prototype 7 is slightly moved.

The depth of focus of the X-ray imaging optical system 1 is λ / 2NA
^{In the case of} an optical system having a numerical aperture of 0.1 and an NA of 0.1, the depth of focus is 50 times or more the wavelength (0.65 μm at a wavelength of 13 nm). It is possible to change the contrast by the phase difference while maintaining the image state.

As the point light source P1, quasi-monochromatic X-rays are used. Here, the monochromaticity of the point light source P1 limits the alignment accuracy and the fine movement range of the Mach-Zehnder interferometer as the coherence distance. The size of the point light source P1 affects the appearance of interference contrast as spatial coherence.

In order to coherently illuminate the inside of the aperture 1-3 of the X-ray imaging optical system 1, a van Cittert-Zemike is used.
According to the theorem, the diameter d of a circular region that is almost coherently illuminated by an incoherent circular light source (angular diameter α = ρ / R) as shown in FIG. 2 is given by d = 0.16Rλ / ρ, and d Becomes completely incoherent at 0.16 Rλ / ρ.

In the case of the X-ray imaging optical system 1 having an NA of about 0.1, the size of the light source required to illuminate the entire surface almost coherently is NA = d / 2R.
From 0.16 Rλ / d, 2ρ = O. 16λ / NA ~ 21
A pinhole of about nm is required. When the pinhole is placed on the object plane side, the reduction ratio becomes twice, and when the reduction ratio is 1/20, it becomes about 2ρ = 420 nm.

[Inspection of X-ray Reflecting Mirror] When inspecting defects of the X-ray reflecting mirror using the X-ray interference microscope of the present embodiment, an X-ray reflecting mirror such as an X-ray reduction projection exposure is used as the sample 2. Sample (hereinafter, X-ray reflector)
The position of the reflection surface 2 is adjusted so that an image is formed on the X-ray image sensor 3.

At this time, when a reflection type optical system coated with a multilayer reflecting mirror is used as the X-ray imaging optical system 1,
It is necessary to match the wavelength with the X-ray reflecting mirror 2 to be observed. When a Fresnel zone plate lens is used as the X-ray imaging optical system 1, it is necessary to optimize the arrangement of the optical system so that chromatic aberration does not occur at the optimum wavelength of the X-ray reflecting mirror 2 to be observed.

The X-ray interference microscope according to the present embodiment is an epi-illumination type microscope in a state where the reflection surface of the X-ray reflection mirror 2 is adjusted so as to form an image on the X-ray imaging device 3. The second reflective surface can be observed in a bright field.

Next, the position of the reflection prototype 7 is adjusted so that the reflection prototype 7 forms an image on the X-ray imaging device 3.
In the focus adjustment, the X-ray reflecting mirror 2 and the reflecting prototype 7 are used.
It is advisable to perform focus adjustment using a focusing pattern previously arranged above. Further, a shutter 8 that blocks an optical path may be provided to improve the convenience of focus adjustment.

In a state where the images of both the X-ray reflecting mirror 2 and the reflection standard 7 are superimposed on each other by the beam splitter 6 and adjusted so as to be observed by the X-ray imaging device 3, the beam splitter 6 is adjusted.
Is adjusted so that interference fringes can be seen in the field of view. In this state, the X-ray reflecting mirror 2 is used as an interference microscope.
And the relative X-ray phase difference between the reflection standard 7 and the reflection standard 7 can be observed.

If the reflecting prototype 7 (or the beam splitter 6) is finely reciprocated in the optical axis direction, the optical path of the Mach-Zehnder interferometer changes, and the movement of the interference fringes or the change in contrast occurs with the fine reciprocation. In order to inspect the X-ray reflector 2 for defects, the X-ray reflector 2 is inspected in the state of an interference microscope.
Is scanned two-dimensionally.

Defects are detected as differences in contrast. Defects such as adhered particles on the reflecting surface of the X-ray reflecting mirror 2 and partial omissions of the reflecting surface are caused by the reciprocating fine movement of the reflecting prototype 7 (or the beam splitter 6). A phase type defect caused by a minute step or the like embedded under the reflecting surface is observed as a defect whose contrast changes due to the reciprocating fine movement of the reflection prototype 7 (or the beam splitter 6). Is done.

Accordingly, any kind of defect on the reflecting surface of the X-ray reflecting mirror 2 can be detected, and the resolution of the X-ray interference microscope given by λ / 2NA ² is as long as the optical system has NA of 0.1. Depth of focus is 5 times the wavelength (65n at 13nm)
m), and can be observed with a very high spatial resolution of the X-ray interference microscope.

[0029]

As described above, according to the X-ray interference microscope of the present invention, it is not necessary to hold the sample with a fragile thin film such as a membrane by illuminating the sample with a point light source. Since the sample is easily held and the optical system is also simple, it can be spread in various fields without being limited to a part for research. Also,
According to the X-ray interference microscope of the present invention, not the contrast due to the difference in the X-ray absorption of the sample, but the phase change of the X-ray due to the sample appears as an interference contrast on the X-ray imaging device. Can be observed as a very large contrast difference, and the sensitivity of microscope observation in the X-ray region can be dramatically improved. According to the inspection method of the X-ray reflector of the present invention, the X-ray interference microscope described above is used for the defect inspection of the X-ray reflector, so that the phase change of the X-ray due to the minute unevenness of the reflection surface is extremely reduced. Since it is possible to detect with a high resolution, it is possible to detect a phase-type defect, which cannot be detected conventionally, with very high sensitivity.

[Brief description of the drawings]

FIG. 1 is a diagram showing a main part of an embodiment of an X-ray interference microscope according to the present invention.

FIG. 2 is a diagram illustrating the van (Cittert-Zemike) theorem.

[Explanation of symbols]

1. X-ray imaging optical system, 1-1, 1-2 ... spherical mirror, 1-3
... Part of the aperture, 2... Sample (X-ray reflecting mirror), S1... Object plane, S2.
Pinhole, P1: point light source or pseudo point light source, 6: beam splitter, 7: reflective prototype (reference prototype: reflective mirror),
8 ... Shutter.

Claims (3)

[Claims] A reflecting mirror disposed between an X-ray imaging optical system and an X-ray imaging device disposed on an image plane; and a reflecting mirror disposed at a position of the image plane and a mirror image with respect to the reflecting mirror.
An X-ray interference microscope, comprising: a point light source configured to illuminate a sample disposed on an object plane through a part of an opening of the X-ray imaging optical system through the reflecting mirror. 2. The beam splitter according to claim 1, wherein the beam splitter is disposed between the X-ray imaging optical system and the object plane.
A reflection prototype disposed at a position symmetrical with respect to the object plane with respect to the beam splitter; a Mach-Zehnder interferometer is configured by the beam splitter and the reflection prototype; and a phase change of X-rays caused by the sample. Appears as interference contrast on the X-ray image sensor. 3. An X-ray interference microscope according to claim 2, wherein an X-ray reflection mirror is arranged as a sample on the object surface, and the reflection base or beam splitter of said X-ray interference microscope is slightly moved in the optical axis direction. An inspection method of the X-ray reflector, wherein a defect on a reflection surface of the X-ray reflector is detected.