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Claims  |
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We claim:
1. A method of imaging an object illuminated by an X-ray beam, through a
zone plate, comprising the steps of:
disposing a concave mirror with a multilayered film in a path of the X-ray
beam;
reflecting the X-ray beam from the concave mirror with the multilayered
film; and
collecting the reflected X-ray beam onto the object.
2. A method according to claim 1, wherein the disposing of the mirror
includes positioning the mirror to form a reflected beam which is inclined
with respect to an optical axis of the zone plate.
3. A method according to claim 2, wherein a central portion of the X-ray
beam is received by a a first portion of the mirror having a first
reflectivity, and a remaining portion of the X-ray beam is received by a
second portion of the mirror having a second reflectivity, and wherein the
first reflectivity is lower than the second reflectivity.
4. A method according to claim 3, wherein the first portion is not covered
by the multilayered film.
5. A method according to claim 3, wherein at the first portion and
multilayered film structure has been destroyed.
6. A method according to claims 1 or 2, further comprising the steps of
detecting a formed image of the object and preventing a ray emanating from
the object along an optical axis of the zone plate from being superposed
on the formed.
7. A method according to claim 6, further comprising blocking with respect
to a central portion of the zone plate and a plurality of ring zones of
the zone plate around the central portion, the advancement of a ray along
the optical axis of the zone plate.
8. A method according to claim 1, wherein the following condition is
satisfied:
0.1.ltoreq.0.257831.times.[(f.lambda.)/(rn).sup.2
].times.(.lambda./.DELTA..lambda.).ltoreq.10
where .lambda.is the center wavelength of the spectrum of a reflected beam
to be collected on the object, .DELTA..lambda. is a half width of the
spectrum, f is the focal length of the zone plate with respect to the
center wavelength, and rn is the radius of the zone plate.
9. A method according to claim 1, further comprising changing the thickness
of the multilayered film of the mirror with respect to a predetermined
direction along the surface of the mirror to compensate for a change in
wavelength of a reflected beam dependent upon the position on the mirror.
10. An imaging system for imaging an object illuminated by an X-ray source
on a detector, comprising:
a concave mirror with a multilayered film, for reflecting X-rays from the
X-ray source to collect them onto the object; and
a zone plate for forming an image of the object irradiated with the X-rays.
11. An imaging system according to claim 10, wherein said mirror forms a
reflected beam which is inclined with respect to an optical axis of said
zone plate.
12. An imaging system according to claim 11, wherein said mirror has a
first portion having a first reflectivity, for receiving a central portion
of the X-ray beam, and a second portion having a second reflectivity, for
receiving the remaining portion of the X-ray beam, and wherein the first
reflectivity is lower than the second reflectivity.
13. An imaging system according to claim 12, wherein said first portion is
not covered by said multilayered film.
14. An imaging system according to claim 12, wherein at said first portion
said multilayered film structure has been destroyed.
15. An imaging system according to claim 10 or 11, further comprising a
filter for preventing a ray, emanating from the object along an optical
axis of said zone plate, from being superposed on the formed image.
16. An imaging system according to claim 15, further comprising a blocking
member for blocking, with respect to a central portion of said zone plate
and a plurality of ring zones of said zone plate around the central
portion, the advancement of a ray along the optical axis of said zone
plate.
17. An imaging system according to claim 10, wherein the following
condition is satisfied:
0.1.ltoreq.0.257831.times.[(f.lambda.)/(rn).sup.2
].times.(.lambda./.DELTA..lambda.).ltoreq.10
where .lambda. is the center wavelength of the spectrum of a reflected beam
to be collected on the object, .DELTA..lambda. is a half width of the
spectrum, f is the focal length of said zone plate with respect to the
center wavelength, and rn is the radius of said zone plate.
18. An imaging system according to claim 17, wherein the following
condition is satisfied:
0.3.ltoreq.0.257831.times.[(f.lambda.)/(rn).sup.2
].times.(.lambda./.DELTA..lambda.).ltoreq.1
19. An imaging system according to claim 10, wherein, with respect to a
predetermined direction along the surface of said mirror, the thickness of
said multilayered film of said mirror changes so as to compensate a change
in wavelength of a reflected beam dependent upon the position on said
mirror.
20. An X-ray microscope, comprising:
an X-ray source for supplying X-rays;
a concave mirror with a multilayered film, for reflecting the X-rays and
collecting them onto a sample;
a zone plate for forming a magnified image of the sample irradiated with
the X-rays; and
a detector for detecting the formed image.
21. An X-ray microscope according to claim 20, wherein said X-ray source
comprises a laser and a target for receiving a laser beam from said laser
and for producing X-rays.
22. An X-ray microscope according to claim 20, wherein said X-ray source
comprises a synchrotron.
23. An X-ray microscope according to claim 20, wherein said mirror forms a
reflected beam which is inclined with respect to an optical axis of said
zone plate.
24. An X-ray microscope according to claim 23, wherein said mirror has a
first portion having a first reflectivity, for receiving a central portion
of the X-ray beam, and a second portion having a second reflectivity, for
receiving the remaining portion of the X-ray beam, and wherein the first
reflectivity is lower than the second reflectivity.
25. An X-ray microscope according to claim 23, wherein said first portion
is not covered by said multilayered film.
26. An X-ray microscope according to claim 24, wherein at said first
portion the multilayered film structure has been destroyed.
27. An X-ray microscope according to claim 23, further comprising a filter
for preventing a ray, emanating from the object along an optical axis of
said zone plate, from being superposed on the formed image.
28. An X-ray microscope according to claim 27, wherein said filter is an
apertured member.
29. An X-ray microscope according to claim 20, further comprising a
blocking member for blocking, with respect to a central portion of said
zone plate and a plurality of ring zones of said zone plate around the
central portion, the advancement of a ray along the optical axis of said
zone plate.
30. An X-ray microscope according to claim 20, wherein the following
condition is satisfied:
0.1.ltoreq.0.257831.times.[(f.lambda.)/(rn).sup.2
].times.(.lambda./.lambda..DELTA.).ltoreq.10
where .lambda. is the center wavelength of the spectrum of a reflected beam
to be collected on the object, .DELTA..lambda. is a half width of the
spectrum, f is the focal length of said zone plate with respect to the
center wavelength, and rn is the radius of said zone plate.
31. An X-ray microscope according to claim 30, wherein the following
condition is satisfied:
0.3.ltoreq.0.257831.times.[(f.lambda.)/(rn).sup.2
].times.(.lambda./.DELTA..lambda.).ltoreq.1
32. An X-ray microscope according to claim 20, wherein, with respect to a
predetermined direction along the surface of said mirror, the thickness of
said multilayered film of said mirror changes so as to compensate a change
in wavelength of a reflected beam dependent upon the position on said
mirror.
33. A method of imaging an object illuminated by an X-ray beam, through a
zone plate, to detect an image of the object, comprising the steps of:
disposing a concave mirror with a multilayered film in a path of the X-ray
beam;
reflecting the X-ray beam;
reflecting the X-ray beam from the concave mirror with the multilayered
film; and
collecting the reflected X-ray beam onto the object.
34. A method according to claim 33, wherein the disposing of the mirror
includes positioning the mirror to form a reflected beam which is inclined
with respect to an optical axis of the zone plate.
35. A method according to claim 34, wherein a central portion of the X-ray
beam is received by a a first portion of the mirror having a first
reflectivity, and a remaining portion of the X-ray beam is received by a
second portion of the mirror having a second reflectivity, and wherein the
first reflectivity is lower than the second reflectivity.
36. A method according to claim 35, wherein the first portion is not
covered by the multilayered film.
37. A method according to claim 35, wherein at the first portion the
multilayered film structure has been destroyed.
38. A method according to claims 33 or 34, further comprising the steps of
detecting a formed image of the object and preventing a ray emanating from
the object along an optical axis of the zone plate from being superposed
on the formed image.
39. A method according to claim 38, further comprising blocking with
respect to a central portion of the zone plate and a plurality of ring
zones of the zone plate around the central portion, the advancement of a
ray along the optical axis of the zone plate.
40. A method according to claim 33, wherein the following condition is
satisfied:
0.1.ltoreq.0.257831.times.[(f.lambda.)/(rn).sup.2
].times.(.lambda./.DELTA..lambda.).ltoreq.10
where .lambda. is the center wavelength of the spectrum of a reflected beam
to be collected on the object, .DELTA..lambda. is a half width of the
spectrum, f is the focal length of the zone plate with respect to the
center wavelength, and rn is the radius of the zone plate.
41. A method according to claim 40, wherein the following condition is
satisfied:
0.3.ltoreq.0.257831.times.[(f.lambda.)/(rn).sup.2
].times.(.lambda./.DELTA..lambda.).ltoreq.1
42. A method according to claim 33, further comprising changing the
thickness of the multilayered film of the mirror with respect to a
predetermined direction along the surface of the mirror to compensate for
a change in wavelength of a reflected beam dependent upon the position on
the mirror.
43. A method of imaging an object illuminated by an X-ray beam, comprising
the steps of:
reflecting the X-ray beam on a multilayered film mirror; and
collecting the reflected X-ray beam on the object; and
imaging the object through a zone plate.
44. A method of detecting an image of an object illuminated by an X-ray
beam comprising the steps of:
reflecting the X-ray beam on a multilayered film mirror;
collecting the reflected X-ray beam on the object; and
imaging the object through a zone plate.
45. An X-ray microscope according to claim 20, wherein said detector
comprises a photographic film.
46. An apparatus for exposing a radiation sensitive material, comprising:
an X-ray source for supplying X-rays;
a concave mirror with an multilayered film, for reflecting the X-rays and
for collecting them onto an object; and
a zone plate for forming an image of the object, irradiated with the
X-rays, on the radiation sensitive material for exposure thereof.
47. An apparatus according to claim 46, wherein said radiation sensitive
material comprises a photographic film. |
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Claims |
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Description |
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FIELD OF THE INVENTION AND RELATED ART
This invention relates generally to an X-ray microscope.
In X-ray microscopes, there are contact type, scan type and imaging type.
Of these types, the imaging type microscope has highest spatial resolution
and, therefore, many studies have been made to this along developments of
X-ray optical elements.
Generally, an imaging type X-ray microscope comprises a light source, an
illumination optical system (illumination system), a sample (subject of
observation), an imaging optical system (imaging system) and a detector.
The spatial resolution of an X-ray microscope differs with an optical
element used as the imaging optical system. Examples of optical elements
usable as the imaging optical system are a Fresnel zone plate, a grazing
incidence reflector and a multilayered-film reflector. The theoretical
resolution of an imaging optical system that uses a Fresnel zone plate is
of the order of the width of the outermost ring zone. Actually, with this
type of imaging optical system, a resolution of about 50 nm is attainable.
With an imaging system using a grazing incidence reflector, the theoretical
resolution is about a few times the wavelength. However, in this type
imaging system, the roughness or configuration error of the reflection
mirror surface affects the resolution. Therefore, to attain the
theoretical resolution, high precision is required in respect to the
surface roughness and configuration. By way of example, a precision of
nanometer order will be necessary.
Generally, to attain such mirror surface machining precision is very
difficult. As a result, at present, the resolution of a Wolter type mirror
optical system that uses a grazing incidence reflector is as low as about
1 micron.
As for an imaging optical system using a multilayered-film reflector, high
surface precision and configuration precision are required as in the
imaging optical system using a grazing incidence reflector. Additionally,
in respect to the film thickness precision, extraordinarily high precision
of 1/10 nm order is required. Consequently, it is very difficult to attain
high resolution, as with the case of an optical system using a grazing
incidence reflector. For these reasons, currently, many X-ray microscope
studies are being made in respect to ones of the type that the imaging
optical system comprises a Fresnel zone plate.
Generally, a Fresnel zone plate has a low diffraction efficiency, and in
many cases the X-ray microscope should be equipped with a bright light
source or a bright illumination system. Frequently, synchrotron radiation
light is used as a light source. In some cases, as a brighter light
source, an insertion type light source such as undulator or the like is
used. Since, however, there are many limitations in increasing the
luminance of a light source, it-is strongly desired to improve the energy
efficiency of an illumination system to thereby increase the brightness.
A conventional X-ray microscope having a Fresnel zone plate in its imaging
system uses a concave diffraction grating or a Fresnel zone plate in its
illumination system. These elements have a light collecting function for
attaining a bright illumination system as well as a spectroscopic function
for selection of a beam of a particular wavelength or wavelengths. Also,
there is a case wherein a flat diffraction grating and a concave mirror
are used in combination, with the spectroscopic function and the light
collecting function being provided separately.
SUMMARY OF THE INVENTION
Generally, a Fresnel zone plate used in an illumination system has
alternately disposed transparent concentric ring-like zones (transparent
with respect to X-ray, for example) and non-transparent concentric
ring-like zones (non-transparent with respect to X-ray). The total area of
the non-transparent zones is about a half of the whole area, and about 50%
of light is blocked. Also, about a half of the remaining 50% is not
diffracted and goes straight. Thus, if such an amplitude type Fresnel zone
plate is used, there is an inconvenience of a large decrease in light
quantity.
With respect to positive and negative first orders, even an idealistically
fabricated Fresnel zone plate has a low diffraction efficiency which is
not greater than 10%. In an attempt to increasing such a low diffraction
efficiency, a Fresnel zone plate (phase type Fresnel zone plate) wherein a
non-transparent portion is formed by a thin film to provide a phase shift
of .pi. has been proposed. The diffraction efficiency of such phase type
Fresnel zone plate with respect to the positive and negative first orders
is about 10-30%. In order to reduce absorption of the phase type Fresnel
zone plate and to reduce the phase difference, the film thickness should
be maintained thin and constant with good precision. Inconveniently,
therefore, with this method it is not easy to obtain a large area. While a
Fresnel zone plate having a very thin substrate of polyimide or silicon
nitride has been proposed, it depends on the possibility of reducing the
film thickness and attaining high precision. Therefore, inconveniently it
is still difficult to obtain a large area.
If a concave diffraction grating or a flat diffraction grating is used in
combination with a concave mirror to provide an illumination system,
because of low reflectivity of the diffraction grating or the mirror, it
is difficult to attain sufficient brightness. Further, since the
wavelength resolution is higher by one order or more than that of the
Fresnel zone plate, a beam of unnecessarily narrowed bandwidth is
extracted. As a result, it is not easy to obtain a bright illumination
system.
In conventional X-ray microscopes as described above, the efficiency of
energy use is low, and a bright image is not formed and thus is not
detected.
It is accordingly a primary object of the present invention to provide an
imaging method or image detection method having good energy efficiency.
It is another object of the present invention to provide an imaging system
or X-ray microscope having good energy efficiency.
In accordance with one aspect of the present invention, there is provided a
method of imaging an object illuminated by an X-ray beam, through a zone
plate, characterized in that: a concave mirror with a multilayered film is
disposed on a path of the X-ray beam to reflect the same and collect the
same onto the object.
In accordance with another aspect of the present invention, there is
provided a method of imaging an object illuminated by an X-ray beam,
through a zone plate, to detect an image of the object, characterized in
that: a concave mirror with a multilayered film is disposed on a path of
the X-ray beam to reflect the same and collect the same onto the object.
In accordance with a further aspect of the present invention, there is
provided an imaging system, comprising: a concave mirror with a
multilayered film, for reflecting X-rays from an X-ray source to collect
them on an object; and a zone plate for forming an image of the object
irradiated with the X-rays.
In accordance with a yet further aspect of the present invention, there is
provided an X-ray microscope, comprising: an X-ray source for supplying
X-rays; a concave mirror with a multilayered film, for reflecting the
X-rays and collect them on a sample; a zone plate for forming a magnified
image of the sample irradiated with the X-rays; and a detector for
detecting the formed image.
In accordance with yet another aspect of the present invention, there is
provided a method of imaging an object illuminated by an X-ray beam,
characterized in that: the X-ray beam is reflected by a multilayered film
mirror and is collected onto the object thereby; and that the object is
imaged through a zone plate.
In accordance with a further aspect of the present invention, there is
provided a method of detecting an image of an object illuminated by an
X-ray beam, characterized in that: the X-ray beam is-reflected by a
multilayered film mirror and is collected onto the object thereby; and
that the object is imaged through a zone plate.
These and other objects, features and advantages of the present invention
will become more apparent upon a consideration of the following
description of the preferred embodiments of the present invention taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a main portion an X-ray microscope according
to an embodiment of the present invention.
FIGS. 2 and 3 are graphs each showing a relationship between parameters P
and W.
FIG. 4 is a schematic view of a main portion of an X-ray microscope
according to another embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a schematic view of a main portion of an optical arrangement
according to an embodiment of the present invention. Denoted at 1 in FIG.
1 are X-rays emitted by an X-ray source 100, and denoted at 2 is a
multilayered-film concave reflection mirror having a smooth reflection
surface for reflecting the X-rays 1. The reflection mirror 2 has a
substrate S on which multilayered films L1, L2, L3, . . . and Ln of a
number (n) and of materials to be described later, are formed by
deposition. Denoted at 3 is an object (sample) to be observed; at 4 is an
imaging Fresnel zone plate; and at 5 is a detector. An image 6 of the
object 3 is formed on the detector 5 surface. In this embodiment, no
diffraction grating is provided on the path of X-rays and only one
multilayered-film mirror is used.
In the present embodiment, the X-rays are reflected by the
multilayered-film concave reflection mirror 2 and are collected on the
object 3 surface. Simultaneously, wavelength selection is effected to meet
the wavelength to be used. The X-rays transmissively scattered
(diffracted) by the object 3 are incident on the Fresnel zone plate 4. In
response, the zone plate 4 collects the received X-rays and forms an image
6 of the object 3 upon the detector 5 surface.
In the present embodiment, as an example, the light source 100 may comprise
a synchrotron orbital radiation (SOR) source which emanates synchrotron
radiation light. The reflection mirror 2 may comprise a substrate having
its surface polished into a parabolic face on which alternating layers of
Cr (chromium) and C (carbon) are formed.
X-rays emanating from the synchrotron radiation source may be partially
blocked by an aperture means (not shown) and, as a result, a beam of 8 mm
in a horizontal direction (parallel to the sheet of the drawing) and 3 mm
in a perpendicular direction (perpendicular to the sheet of the drawing)
may impinge on the multilayered-film concave mirror 2 surface. The mirror
may preferably comprise a substrate S of silica polished into a paraboloid
of revolution. As regards the configuration precision of the paraboloidal
face of the mirror 2, an error of -.lambda./3 (.lambda.=632.8 nm) from an
idealistic configuration will be adequate. As regards the surface
roughness, a level of 2 .ANG.rms order is adequate. On such a substrate,
alternate layers of Cr and C of a number of 501 (five hundred and one) may
be formed, with the configuration precision and the surface roughness
being substantially unchanged. The mirror 2 may be so set that the
revolution plane of the paraboloid of revolution lies in a perpendicular
plane and that the beam center (central beam of X-rays) is incident on the
mirror at an angle of about 30 deg. as measured with respect to a
perpendicular direction to the mirror surface.
At the opposite end portions A and B of the X-ray beam 1 in a horizontal
plane, the angle of incidence on the reflecting mirror may be 28.2 deg.
and 31.7 deg., respectively. As regards the thickness structure of the
multilayered film of the reflecting mirror 2, the Cr layers L1, L3, L5, .
. . and L501 and the C layers L2, L4, L6, . . . and L500 may have the same
thicknesses, respectively. Additionally, in order to avoid shift in
wavelength of reflected lights from respective portions of the reflection
mirror at the focal point thereof, in a horizontal plane a film thickness
distribution may be provided along the reflection surface of the mirror.
For example, at the end portion A of the X-ray beam, the film thickness
may be 8.4 .ANG. A (Cr layer) and 20.1 521 (C layer) while at the end
portion B of the X-ray beam it may be 8.6 .ANG. (Cr layer) and 21.0 .ANG.
(C layer). Between the end portions A and B, the film thickness may be
continuously changed. The film thickness in the perpendicular direction
may be constant.
In the present embodiment, there may be partial correspondence between the
wavelengths of X-rays as reflected by the multilayered-film reflection
mirror and the wavelengths of the X-rays as diffracted by the Fresnel zone
plate. In order to avoid superposition of the zero-th order light
(transmitted light) (which goes along the optical axis of the Fresnel zone
plate 4) on the image 6 of the object as formed by the Fresnel zone plate
4, at an area on the reflection mirror adjacent to a portion as irradiated
by the center of the X-ray beam, the multilayered film structure may be
destroyed by means of ion beam projection, in a circular range of 100
micron diameter. With this destruction, it may be assured that the object
3 is irradiated with a beam which is inclined with respect to the Fresnel
zone plate 4. In place of destroying the multilayered film structure, such
an area may be covered with by a low-reflectivity material or a material
with an appropriate optical constant in a sufficient thickness to attain
low reflectivity. Alternatively, the multilayered film may originally
formed except for such an area.
In a design example according to this embodiment of the present invention,
the reflectivity of the concave reflection mirror 2 is about 30%
(wavelength=50 .ANG.). With this reflection mirror, an X-ray beam of a
wavelength 50 .ANG. is collected at a distance 66.7 mm (emission angle=30
deg.), and the beam spot size is about 10 microns. If the object (sample)
3 to be observed is placed at this position, the transmitted light passing
the object 3 is collected by the Fresnel zone plate 4 upon the detector 5
surface, by which an image 6 is formed thereon. As regards the Fresnel
zone plate 4, the radius of the innermost ring zone is 5.00 microns and
that of the outermost ring zone is 61.44 microns. The number of the ring
zones is 151 (one hundred and fifty five). The focal length is 5 mm. By
placing an X-ray photographic film at the imaging position as the detector
means 5 and by exposing the film, it is possible to detect the image 6.
The X-ray path may be covered by a vacuum container while the object
(sample) 3 may be placed in an atmosphere by using a sample holder (not
shown) with its X-ray inlet and X-ray outlet being sealed by means of
polyimide films. Where diatom is used as the sample 3 and it is irradiated
with X-rays of 5 nm wavelength, the transmitted X-rays will form a sharp
image 6. According to simulations, the projection magnification of the
image 6 upon the X-ray film is 150 x and the resolution is about 0.3
micron. The necessary exposure time is about 0.05 sec (beam current=150
mA). Such exposure time is very short as about 1/230 of that where a
Fresnel zone plate of a radius 4.2 mm and ring zones of a number of 8001
(eight thousand and one) is used in the illumination system. Thus, with
the present invention, a large increase in energy efficiency is
attainable.
With this embodiment of the present invention as described above, since a
multilayered-film reflection mirror is used in the illumination system, it
is possible to easily fabricate a large area one as compared with a
conventional illumination system using a Fresnel zone plate. Also, it is
possible to provide a bright illumination system with a large
reflectivity, wherein the wavelength resolution is not higher than
required.
In the present embodiment, assuming now that the center wavelength of used
X-rays is .lambda., the spectral half width at the center wavelength
.lambda. is .DELTA..lambda., the focal length and the radius of the
Fresnel zone plate at the center wavelength .lambda. are f and rn,
respectively, and the parameter P is set to be expressed by equation (1)
which reads:
P=0.257831.times.(f.lambda./rn.sup.2)(.lambda./.DELTA..lambda.)(1)
then, the components are so set as to satisfy the following relation:
10.sup.-2 .ltoreq.P.ltoreq.10 (1')
Since .lambda./.DELTA..lambda. in equation (1) is the wavelength resolution
of the X-rays as spectrum-resolved by the multilayered film, the parameter
P represents the wavelength resolution of the illumination system.
On the other hand, it is now assumed that, where a Fresnel zone plate 4 is
used as an imaging element, the spatial resolution of the imaging element
is .delta..infin. if an illumination beam comprises a monochromatic beam
while it is .delta..lambda./.DELTA..lambda. if the wavelength resolution
of the illumination system is .lambda./.DELTA..lambda.. Here, if a
parameter W is set to be expressed by equation (2) which reads:
W=(.delta..lambda./.DELTA..lambda.)/.delta..infin. (2)
then the parameter W can be used as a parameter that represents degradation
of the imaging performance of the imaging element due to chromatic
aberration.
FIGS. 2 and 3 each depicts the relation between the parameters P and W. It
is clear from FIG. 2 that, as the parameter P decreases, the parameter W
increases suddenly. Namely, as the wavelength resolution P of the
illumination system degrades (decreases), the imaging performance degrades
quickly.
FIG. 3 depicts in a log-log graph the relation between the parameters P and
W. It is seen in this graph that, as the parameter P decreases, the
relation of the parameters P and W becomes more rectilinear. Namely, in a
region in which the parameter P is small (P.ltoreq.10.sup.-2), the
following is provided:
W.infin.P.sup.-.varies. (3)
From FIG. 3, .varies. is determined to be about 0.4. In the region in which
the parameter P is small, the parameter W diverges exponentially with a
negative exponent of the parameter P. The limit of starting divergence of
the parameter W with an exponent of the parameter P, is P=10.sup.-2. If
P.gtoreq.10.sup.-2, the degradation of the imaging performance of the
imaging element may be admitted.
On the other hand, as the parameter P increases, the parameter W becomes
close to "1" and the imaging performance improves. If P=1, the imaging
performance becomes substantially equal to that where monochromatic light
is used. If the parameter P increases further, the imaging performance
does not improve any more. To the contrary, the illumination light becomes
dark and, at the wavelength resolution whereat P.gtoreq.10 is provided,
the beam intensity reduces to a fraction or 1/10. Thus, the advantage of
using the multilayered-film reflection mirror is damaged.
For the foregoing reasons, in the present embodiment, it is desirable to
use in an illumination optical system such a multilayered-film reflection
mirror with wavelength resolution .lambda./.DELTA..lambda. by which the
parameter P is maintained in a range of 10.sup.-2 to 10, preferably in a
range of 3.times.10.sup.-2 to 1.
In the present embodiment, the wavelength resolution of the illumination
beam obtainable from the multilayered-film concave reflection mirror may
be so set as to provide a parameter P of 0.222. This may ensure a bright
illumination system with which a sufficiently bright illumination beam may
be obtained and the exposure time may be reduced largely and,
additionally, the resolution of the Fresnel zone plate may not be
decreased.
General features of the multilayered film of the reflection mirror of the
present embodiment will be explained.
The multilayered film of the present embodiment may include alternate
layers of two materials having different optical constants. The
reflectivity of this multilayered film is determined by the wavelength
used and the materials used as well as the film thickness structure.
Generally, such two materials as having small absorption efficiencies with
respect to a used wavelength and having a large difference in refractive
index, may be selected and used. Such a material that has an absorption
limit at a wavelength slightly shorter than a used wavelength may be used.
However, this is not a requirement since, in an observation instrument
such as an X-ray microscope, the wavelength resolution of the multilayered
film affects the resolution. Thus, by selecting two materials having small
absorption efficiencies and having a small difference in refractive index
and by increasing the number of alternate layers, it is possible to obtain
a multilayered film of high reflectivity and high wavelength resolution.
Generally, an X-ray reflector has a reflectivity of a few percentages to
several ten percentages. It has been reported that a plane mirror with
alternate layers of Rh and Si assured a reflectivity of about 80%
(wavelength=130 .ANG.). Therefore, with a multilayered-film reflection
mirror, it is possible to attain an energy efficiency several times or
several ten times higher than the diffraction efficiency of a diffraction
grating or a Fresnel zone plate.
Also, with a multilayered-film reflection mirror, it is easy to obtain a
large area mirror of a size of several ten millimeters and, therefore, it
is possible to uses light from an X-ray source without a large loss. This
is particularly effective where a divergence type source such as a laser
plasma X-ray source or an electron beam X-ray source is used. As described
hereinbefore, when a multilayered-film reflection mirror is used to
provide an imaging element, high precision is required in respect to the
substrate machining, the film thickness control of the multilayered film
and the like. In the present embodiment, however, a multilayered-film
reflection mirror is used only in an illumination system. Only required
for the mirror is that it converges the X-rays. Therefore, as compared
with a case where it is used as an imaging system, not so high precision
is required.
Further, an additional advantage of a multilayered-film reflection mirror
is that the wavelength resolution is not too high. The resolution of a
Fresnel zone plate used in an imaging optical system is of an order of the
width of its outermost ring zone which depends on the number of ring
zones. Also, the wavelength resolution of a Fresnel zone plate is
determined by the number of ring zones. Thus, the resolution and the
wavelength resolution of a Fresnel zone plate are in a proportional
relation. This means that a Fresnel zone plate having high resolution has
high wavelength resolution and, therefore, the illumination system should
have high wavelength resolution. If, however, the wavelength resolution of
the illumination system is too high, the bandwidth of X-rays incident on
the Fresnel zone plate becomes too narrow which renders the optical system
dark. Since the resolution of the device is determined by the width of the
outermost ring zone of the Fresnel zone plate, even if the bandwidth of
X-ray is narrow, the resolution does not improve any more. For this
reason, making the resolution of an illumination system too high is not
preferable.
In a light collecting Fresnel zone plate conventionally used in an
illumination optical system, if the area is made larger while retaining an
appropriate focal length, the number of ring zones increases and the
wavelength resolution increases. In many cases, the wavelength resolution
of a light collecting Fresnel zone plate is higher by one order or more
than that of an imaging Fresnel zone plate. Also, in the case of a
diffraction grating, the wavelength resolution thereof is higher by one
order or more than that of an imaging Fresnel zone plate.
As compared therewith, the wavelength resolution of a multilayered-film
reflection mirror is of the same order of that of an imaging Fresnel zone
plate. This is very effective in a case such as dynamic observation of a
living sample, for example, where a bright optical system is required
although the resolution may not be very high. As compared with a case
where a Fresnel zone plate or a diffraction grating is used in an
illumination optical system, use of a multilayered-film reflection mirror
in an illumination optical system can provide an optical system which is
brighter by several times than the former, even only in respect to the
bandwidth selected.
Further, in respect to the area to be irradiated with X-rays or the
attainable reflectivity, the multilayered-film reflection mirror is
superior. Thus, it can provide a bright optical system. Therefore, the
present embodiment is very effective to an observation device wherein a
bright optical system is required.
A simplest form of light collecting optical system using a multilayered
film may be provided by a multilayered-film reflection mirror having
alternate layers of two different materials formed on a concave surface
substrate. As X-rays are incident on the concave reflection mirror, they
are spectrum-resolved and, at the same time, they are collected to
irradiate a desired position of a sample (object).
The surface configuration of the concave surface substrate may differ,
depending on the type of X-ray to be incident thereon. For X-rays of
substantially parallel light such as synchrotron radiation, a paraboloid
of revolution may be used. For divergent light from a point light source,
an ellipsoid of revolution may be used. If the focal length is long or the
energy width is wide, an approximate sphere may be used.
The thickness of the multilayered film can be determined by the X-ray
wavelength to be used and the convergence conditions such as the angle of
incidence. Generally, the film thickness is defined by alternate provision
of two layers with constant thicknesses. However, in order to obtain a
higher reflectivity, an appropriate film thickness structure may
preferably be adopted. The multilayered film structure is not limited to
alternate layers of two different materials. To avoid roughness of
interface or to prevent diffusion, one or more intermediate layers may be
used. Where a curved surface mirror is used, the angle of incidence
changes with the position of incidence even if a parallel beam is inputted
thereto. In consideration of this, the film thickness of a multilayered
film may preferably be determined so as to provide a thickness
distribution on the curved face, to thereby provide a substantially even
spectral distribution of reflected light as spectrum-resolved by
respective portions of the mirror surface.
Moreover, in consideration of convergence conditions of an optical system
or limitations to device configuration, an optical system including plural
multilayered-film mirrors may be used. By way of example, an optical
system of Wolter type or Schwaltshilt type may be used.
FIG. 4 is a schematic view of a main portion of an optical arrangement of a
second embodiment of the present invention.
In the present embodiment, an X-ray source comprises a laser plasma X-ray
source, and an illumination optical system includes a multilayered-film
reflection mirror having a multilayered film structure formed on an
ellipsoid of revolution. An imaging optical system includes a Fresnel zone
plate.
Denoted in the drawing at 9 is a laser; at 10 is a laser light; at 11 is a
target; at 12 are X-rays; and at 13 is an X-ray aperture cutting aperture
means. Denoted at 14 is a multilayered-film concave reflection mirror of
cylindrical shape having a smooth reflection surface for reflecting the
X-rays. It includes a mirror substrate S. On the inner cylindrical faces
of the substrate, multilayered films L1, L2, L3, . . . and L501 of a
number 501 (five hundred and one) are formed. Denoted at 15 is an object
(sample) to be observed. Denoted at 16 is a scattered light cutting
aperture means, and denoted at 17 is an imaging Fresnel zone plate.
Denoted at 18 is a detector which comprises, for example, an X-ray
photographic film for recording an enlarged image 19 of the sample 15.
Also in this embodiment, no diffraction grating is provided on the X-ray
path and only one multilayered-film reflection mirror is used.
In a design example, the laser plasma X-ray source 100 uses a YAG laser of
1.0 J/pulse and pulse width 10 ns, as the laser 9. The size of the light
point on the target 11 is 10.sup.-2 mm.sup.2. As the target 11, Mo is
used. The target 11 has a cylindrical shape and it is rotatable about an
axis parallel to its generating line, so as to change the pulse
irradiation position continuously.
In this design example, the substrate S of the multilayered-film concave
reflection mirror 14 has a cylindrical (ring) shape of a height 120 mm. It
may be provided by cutting an ellipsoid of revolution having a major
diameter of 424.3 mm and a minor diameter of 300 mm, along its plane of
revolution. As regards the configuration precision, an error of about
.lambda./2 (.lambda.=6328 nm) from an idealistic configuration and surface
roughness of 3 Arms are adequate. In this design example, alternate layers
of Ni and C materials of a total number of 501 (five hundred and one) are
formed on the substrate surface. The angle of incidence of the X-rays is
set to be 45 deg. at the center the concave reflection mirror, and it
changes continuously in the major axis direction of the mirror. At an end
portion F of the region to be irradiated which portion is far from the
X-ray source, the angle of incidence may be 43.1 deg. At another end
portion E which is near to the X-ray source, the angle of incidence may be
47.9 deg. With respect to the minor axis direction, there may be no
distribution of incidence angle. The thickness of the multilayered film
may have a distribution along the major axis of the reflection mirror. At
the end portion E near to the X-ray source, it may be 6.7 .ANG. (Ni layer)
and 24.2 .ANG. (C layer) (constant thickness layers). At the end portion F
far from the X-ray source, the thickness may be 7.1 .ANG. (Ni layer) and
26.6 .ANG. (C layer) (constant thickness layers). Between the end portions
E and F, the film thickness may change continuously. With regard to the
minor axis direction, the film thickness of the multilayered films may be
constant. The angle of emission of the X-rays may be set to be 45 deg.
The X-ray emitting point 11a and the object (sample) to be examined are
placed at the focal point positions of the ellipsoid of revolution of the
multilayered-film concave reflection mirror 14. The X-rays 12 emitted by
the laser plasma X-ray source 100 may be partially blocked by the aperture
means 13, and the transmitted rays may be incident on the reflection
mirror 14. The X-rays may be reflected by the reflection mirror and they
may illuminate the sample 15. Thus, the sample 15 may be irradiated with
beams inclined with respect to the optical axis of the Fresnel zone plate
17. The X-rays may be collected by the reflection mirror so as to form a
beam spot of about 5 micron diameter, upon the sample 15 surface.
Simultaneously, the wavelength selection of the X-rays may be effected,
and the center wavelength may be 4.5 nm and the wavelength resolution
.lambda./.DELTA..lambda. may be about 170.
With regard of regards the Fresnel zone pate 17 for imaging the sample 15,
in this design example, the width of its innermost ring zone is 4.24
microns and the width of its oute | | |
