JP3226338B2 - Koehler illumination optical system for soft X-ray microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an illumination optical system of an imaging optical system such as a microscope, and more particularly to an illumination optical system in which a light source having a small light emitting area is used as a light source. For example, the present invention relates to a case where a laser plasma light source having a small light emission area or a radiation light source is used as in a soft X-ray microscope.

[0002]

2. Description of the Related Art Conventionally, critical illumination and Koehler illumination are known as illumination methods used for microscopes. Critical illumination, as shown in Figure 6, as it is projected onto the sample T and the image of the light source S ₁ by the condenser lens L. On the other hand, as shown in FIG. 7, the Koehler illumination forms an image of the light source S ₁ on the focal plane of the object space of the condenser lens L ₂ using light emitted from one point of the light source S ₁ using a lens L _1. The light beam illuminates the sample T, and the Koehler illumination requires a more complicated optical system.
However, in the case of the Koehler illumination, uniform illumination can be performed on the surface of the sample T without being affected by the light emission shape of the light source. Therefore, at present, Koehler illumination is employed in almost all microscopes.

In a conventional optical microscope using visible light, infrared light, ultraviolet light, or the like, a light source such as a halogen lamp or a mercury lamp having a light emitting area larger than an illumination visual field is used. I have. In such a case, Koehler illumination can be easily realized. However, in a soft X-ray microscope using soft X-rays, a light source having a large light emitting area cannot be used as compared with a required size of an illumination visual field.
In this case, it is difficult to easily realize the Koehler illumination, and the inability to use a refractive optical system such as an optical system in the visible region increases the difficulty of using the Koehler illumination.
Under such circumstances, in a soft X-ray microscope,
Critical lighting has been used exclusively.

FIGS. 8 and 9 show a conventional example of critical illumination.
And in FIG. 8, the Walter type objective lens L _w, an example using a Wolter type condenser C. The light source S ₁ is projected by the Walter condenser C, and the luminous flux excluding the central part blocked by the light shielding plate 1 is the sample T.
Is irradiated on the image of the sample T is further enlarge the image A in position 3 by Wolter objective lens L _w.

FIG. 9 shows an objective lens Z ₂ of a zone plate.
In an example using a capacitor Z ₁ of zone plate. The light source S ₁ is projected at the same magnification by the zone plate condenser Z ₁ to irradiate the sample T at the position of the pinhole 2 provided on the light shielding plate 1, and the image of the sample T is moved to the position 3 by the objective lens Z ₂ of the zone plate. Is enlarged as an image A.

FIG. 10 shows an example in which a spheroidal condenser C is used for a Schwarzschild type objective lens L _S. Light source S ₁ is sample T by spheroidal condenser C
And the image of the sample T is magnified as an image A at a position 3 by the Schwarzschild type objective lens L _S.

[0007]

In the conventional soft X-ray microscope, since critical illumination is used for the illumination optical system, illumination unevenness occurs and a microscope image of good image quality cannot be obtained. This is because, in the case of critical illumination, since the image of the light source is directly projected onto the sample, the information of the light source overlaps the image of the sample to be observed. In a microscope using visible light or the like, a device is provided in which a diffuser plate is provided immediately after the light source so that an image of the light source can be blurred and uniform illumination can be performed. However, in the case of light that hardly passes through a substance, such as soft X-rays, it is not easy to use a transmission type diffusion plate. As a countermeasure, a large loss of light quantity is involved, but the inventor of the present application has previously proposed the use of a reflection type diffusion plate in Japanese Patent Application No. 03-129467. Under such circumstances, Koehler illumination is suitable for effectively using light from the light source and obtaining uniform illumination. This is evident from the fact that almost all optical microscopes now employ Koehler illumination. However, with a soft X-ray microscope,
As mentioned earlier, it is difficult to realize Koehler lighting.

The present invention has been made in view of the above circumstances, and realizes a Koehler illumination in a soft X-ray microscope in which light from a light source can be used effectively and uniform illumination can be obtained. It is intended to be.

[0009]

The Koehler illumination optical system of the microscope according to the present invention is characterized in that first, light from a secondary light source is irradiated onto a sample by a zone plate. Further, in addition to the first feature, the principal ray forming the secondary light source is a convergent ray. Specifically, a secondary light source of a light source is formed by a grazing incidence type total reflection mirror, a zone plate, a Schwarzschild optical system, and the like, and a sample is illuminated by a zone plate provided immediately after the secondary light source. A soft X-ray microscope capable of uniform illumination and obtaining a high-quality image is realized.

[0010]

First, the basic configuration of the Koehler illumination optical system will be described with reference to FIG. 1 showing a simplified illumination optical system. The following analysis is performed with a thin-walled optical system, and a lens includes not only a so-called refractive optical system made of glass but also generally all optical elements having a lens function. In the figure, S ₁ is a light source having a size (object height) of S ₁ (hereinafter, referred to as a light source S ₁ ), S ₂ is a secondary light source having a size of S ₂ (hereinafter, a secondary light source S ₂ ), L ₁ is a lens, L ₂ having a focal length f ₁ is a lens having a focal length f _2. The lens L ₁ to form a secondary light source S ₂ of the light source S _1. The magnification at this time is m. Then, irradiating the sample with light of a secondary light source S ₂ with the lens L _2. Here, to simplify the focal position f ₂ the secondary light source S ₂ of the lens L ₂ is are to be positioned. Accordingly, the position of the sample T is the focal position of the opposite side of the lens L _2.

When the illumination optical system is configured as described above, if the numerical aperture of the lens L ₁ is ψ ₁ , ψ ₂ , the numerical aperture on the exit side of the lens L ₂ is θ, and the size of the illumination field is φ. Tan θ = S ₂ / f ₂ = mS ₁ / f ₂ (1) φ = 2 f ₂ ψ ₂ = 2 f ₂ ψ ₁ / m (2)

Let us consider a little what the above expressions (1) and (2) mean. First, in order to obtain a condenser having a large numerical aperture θ on the emission side, the light source S ₁ must be large or the lens L ₁ must be large.
Or magnification m by a large, or the focal length f ₂ of the lens L ₂ is small is a condition. Also, if large in order to obtain the illumination field φ is the magnification m by a small lens L _1, the lens L ₂
Focal length f or ₂ larger, or a numerical aperture of [psi ₁ of the lens L ₁ is large is a condition of. In the case of a normal optical microscope,
Since a light source having a large light emitting area such as a halogen lamp can be used as the light source, m, f ₂ satisfying the expression (2) can be used.
, A sufficient numerical aperture θ is obtained. When using an extremely low magnification objective lens, increasing the value of m
A not so large numerical aperture θ can be easily satisfied. At this time, in order to satisfy the large illumination field of several times the light source, m although it is necessary to increase the [psi ₁ more than it increases, which is by the lamp side of the condenser lens in an aspheric Has been resolved.

Next, consider a condenser in the soft X-ray region where almost only a point light source having a small light emitting area can be used. In this case, in order to obtain a large numerical aperture θ, since S ₁ is small in the equation (1), the magnification m is increased, and
It is necessary to reduce the focal length f ₂ of the lens L _2. Further, equation (2), by reducing the focal length f ₂ of the lens L ₂ by increasing the magnification m, the size of the illumination field φ decreases, it is necessary to significantly increase the [psi _1. Magnification m
, By moving the position of the light source S _1, is susceptible to large, in order to reduce the focal length f ₂ of the lens L ₂ it is necessary to reduce the focal length in the design itself of the lens L ₂ is there.

By the way, soft X-rays have poor material transmittance,
Since the refractive index is almost 1, a refractive optical system cannot be used. In addition, since the reflectance at direct incidence is low, the imaging optical system for soft X-rays includes a zone plate using diffraction, a Walter-type reflecting mirror using total reflection at oblique incidence, and reflection at normal incidence with a multilayer film. A Schwarzschild type optical system or the like having an improved efficiency is used. Among them, the Walter type and the Schwarzschild type have a ring-shaped opening as shown in FIGS. 8 and 10, and the illumination is also ring-shaped.
It is difficult to use the lens L ₂ shown in. Moreover, not be much smaller focal length f _2.

On the other hand, the zone plate is not suitable for the lens L ₂ because it can have a small focal length instead of an annular opening. The use of zone plate lens L _2,
By the lens L ₁ to form a secondary light source S _2, by illuminating on the sample T plane in a short zone plate the light of the focal length, it is possible to Koehler illumination with soft X-ray region. However, with the current technology, the numerical aperture of the zone plate itself cannot be increased so much, and further contrivance is required to make the condenser compatible with an objective lens having a large numerical aperture.

If the numerical aperture of the zone plate is NA _Z , the width of the outermost zone of the zone plate is d, and the wavelength is λ, there is a relationship of NA _Z = λ / (2d) (3). For example, the wavelength is set to a wavelength range of 40 ° which is important for observing a biological sample and is called a “water window”. The width d of the outermost zone that can be made with current technology, since it is about 30 nm, (3) the numerical aperture NA _Z 0.07 is limited from the equation, 1
Cannot cope with an objective lens having a numerical aperture larger than 0.07. As a countermeasure, a device as shown in FIG. 2 is required.

[0017] In Figure 2, the secondary light source S ₂ principal ray 4 of the light rays forming the can so that the convergence beam, and use two lenses. Projecting an image of the light source S ₁ by the lens L ₁ having a focal length f _1. A real image or a virtual image of the light source S ₁ projected by the lens L ₁ is converted into a lens L having a focal length f ₂ .
₂ by the secondary light source S _2. At this time, the lens L ₁
A lens the distance p between L _2, the focal length f ₂ of the lens L ₂
By taking larger (p> f ₂ ), the secondary light source S
The principal ray 4 of the rays forming ₂ can be a convergent ray. With such a configuration, a large numerical aperture as a capacitor as compared to the numerical aperture NA _Z zone plate Z NA
Can be obtained. In summary, a condenser suitable for a large numerical aperture can be formed by making the principal ray 4 forming the secondary light source a convergent ray.

[0018]

First Embodiment FIG. 3 shows a first embodiment of a Koehler illumination optical system according to the present invention, and shows a condenser configured for a Walter type objective lens. To correspond to the above description, substantially the same members are denoted by the same reference numerals. A light source (laser plasma light source) S ₁ having a size of about 50 μm is projected at the same magnification by a zone plate Z ₁ to form a secondary light source S ₂ at a position of a pinhole 2 provided in a light shielding plate 1. Wavelength selection is performed by the light shielding plate 1. Further, the secondary light source S ₂
The zone plate Z ₂ light from irradiating onto the surface of the sample T. Image of the illuminated sample T is expanded to the position 3 by Wolter type objective lens L _W as an image A.

As described above, the wavelength of soft X-rays is 40 ° in a wavelength range that is important for observing biological samples and is called a “water window”. The numerical aperture NA of the Walter type objective lens is 0.05, and therefore, the necessary numerical aperture on the exit side of the condenser sin θ = 0.
05, The visual field to be illuminated is about 50 μm. If the light source is a laser plasma light source, the size of the light source is about 50μ as described above.
m. And the numerical aperture NA _Z of the zone plate, annular width d of the outermost shell, the relationship between the wavelength lambda, of the above (3), NA _z = lambda /
Is a (2 d), the relationship between the radius r and the focal length f of the zone plate, is obtained relationship because represented by _{f = r / NA Z (4} ) f = 2 dr / λ (5) . Here, a condition of r ≧ fθ + φ / 2 (6) must be satisfied between the numerical aperture θ of the condenser and r in order to effectively use the light beam.

[0020] As described above, since the zone width d of outermost get made with current technology is 30 nm, the numerical aperture NA _Z 0.07 (3) from equation zone plate. If the radius r of the outermost shell of the zone plate is 0.2 mm, f =
3 mm. As a result, fθ + φ / 2 = 0.175 ≦ 0.2, which satisfies the condition of Expression (6). Thus, the illumination optical system of FIG. 3 having the numerical values in the present embodiment constitutes a Koehler illumination optical system.

Second Embodiment FIG. 4 shows a second embodiment of the Koehler illumination optical system according to the present invention, which shows a condenser configured for a Schwarzschild objective lens. A light source S ₁ having a size of about 50 μm is enlarged and projected by a zone plate Z ₁ to form a real image, and a secondary light source S ₂ enlarged by the zone plate Z ₂ is formed using the real image. In zone plate Z ₃ is irradiated with light from the secondary light source S ₂ on the surface of the sample T. The image of the illuminated sample T is a Schwarzschild objective lens L
_The image is enlarged to the position 3 by _S as the image A.

As in the first embodiment, the soft X-ray has a wavelength of 40 °.
It is. Numerical aperture NA of Schwarzschild objective lens L _S
Is 0.25, so the required condenser exit side numerical aperture sin θ = 0.25, and the illuminated field is about 50 μm. If the light source is a laser plasma light source, the size of the light source is about 50μ
m. Consider the transmission of the secondary light source S _2. Zone plate Z ₁ (focal length f ₁₎ Position p ₀ of the real image by _{'is, p 0' = f 1 p} 0 / (p 0 -f 1) (7) magnification m ₁ of zone plate Z ₁ is, m ₁ = −f ₁ / (p ₀ −f ₁ ) (8) The position b of the secondary light source S ₂ by the zone plate Z ₂ is a = p ₀ ′ −p b = f ₂ a / ( _{f 2 + a) = f 2} (p 0 '-p) / {f 2 + (p 0' -p)} (9) magnification m ₂ of zone plate Z _{2 is, m 2 = -f 2 / {} f 2 + (P ₀ '-p)｝ (10)

Consider the projection of the pupil. The projection position p ′ by the zone plate Z ₂ (focal length f ₂ ) is: p ′ = f ₂ p / (p−f ₂ ) (11) The relationship between the angles ψ ₁ and ψ ₂ of the principal ray 4 is 、 ₂ / ψ ₁ = (p / f ₂ ) −1 (12) The position c of the illumination visual field by the zone plate Z ₃ (focal length f ₃ ), that is, the distance c between the zone plate Z ₃ and the surface of the sample T is c < is _{f 3, c = f 3 {} p '- (b + f 3)} / [f 3 + {p' - (b + f 3)}] (13) Therefore, the relationship between the numerical aperture NA, NA / [psi ₂ = 1 + {p ′ − (b + f ₃ )} / f ₃ (14)

The focal length f ₁ of the zone plate Z ₁ is set to 1.98.
mm, the distance p ₀ between the light source S ₁ and the zone plate Z ₁ is 2 mm
From the equations (7) and (8), the real image position p ₀ ′ is 20
0 mm and magnification m ₁ are 100. Therefore, the size 50μm
Light source is expanded to 5mm. Also, [psi ₁ becomes 0.0125. Then, at a distance p = 180 mm from the zone plate Z _1, the focal length f ₂ to form a secondary light source by disposing a zone plate Z ₂ of 10 mm, (9) and (10), a = P ₀ '-p = 200-180 = 20 b = f ₂ a / (f ₂ + a) = (10 × 20) / (10 + 20) = 6.67, and a = 20 mm and b = 6z67 mm are obtained. Magnification m ₂ =
b / a = 1/3, and the size of the secondary light source is S ₂ =
5/3 = 1.67mm. The projection position p 'of the pupil in this case is
From equation _{(11), p '= f 2 p /} (p-f 2) = (10 × 180) / (180-10) = 10.59 next, p' is = 10.59Mm obtained. From the equation (12), the angle change of the principal ray 4 is as follows: ψ ₂ = ψ ₁ × ｛(p / f ₂ ) -1｝ = 0.0125 × ｛(180/10) -1｝ = 0.2125, and ψ ₂ = 0.2125 Is obtained.

[0025] In addition, the focal length f ₃ of the zone plate Z ₃
Assuming that = 2 mm, from equation (13), c = f ₃ {p ′ − (b + f ₃ )} / [f ₃ + {p ′ − (b + f ₃ )}] = 2 × {10.59− (6.67 + 2) ｝ / [2 + {10.59−8.67}] = 0.98, and c = 0.98 mm is obtained. The numerical aperture NA is (1
4) from the _{equation, NA = ψ 2 × [1+} {p '- (b + f 3)} / f 3] = 0.2125 × [1+ {10.59 - (6.67 + 2)} / 2] = 0.4165 next, NA = 0.4165 is obtained Can be At this time, when the numerical aperture of the zone plate Z ₁ and 0.07 pupil diameter, from approximately 0.28 mm, the diameter of the illuminated field becomes 8.4 [mu] m. That is, Koehler illumination corresponding to a Schwarzschild objective lens having a numerical aperture of 0.41 is possible.

Third Embodiment A different numerical value is adopted for the illumination optical system of FIG. The focal length f of the zone plate Z _1
1 = _3.92mm, the distance between the light source S ₁ and the zone plate Z ₁ p
Assuming that ₀ is 4 mm, the real image position p is obtained from Equations (7) and (8).
₀ 'is 200 mm and the magnification m ₁ is 50. Therefore, a light source with a size of 50 μm is enlarged to 2.5 mm. Also, ψ ₁ is 0.
00625. Then, p = 180m from the zone plate Z ₁
m to a position away, to form a secondary light source focal length f ₂ Place the zone plate Z ₂ of 10 mm, (9) and _{(10), a = p 0 '-p =} 200-180 = 20 b = f ₂ a / (f ₂ + a) = (10 × 20) / (10 + 20) = 6.67, magnification m ₂ = b / a = 1/3, and the size of the secondary light source is S ₂ = 2.5 / 3 = 0.83 mm. The projection position of the pupil when p 'is from _{(11), p' = f 2 p /} (p-f 2) = (10 × 180) / (180-10) = 10.59 next, p '= 10.59 mm. From equation (12), the angle change of the principal ray 4 is as follows: ψ ₂ = ψ ₁ × ｛(p / f ₂ ) -1｝ = 0.00625 × ｛(180/10) -1｝ = 0.00625 × 17 = 0.10625 ψ ₂ = 0.10625 is obtained.

[0027] The focus of the zone plate Z ₃ distance f ₃ = 2 mm
From equation (13), c = 0.98 mm, and from equation (14), NA
= 0.20825 is obtained. At this time, the zone plate Z ₁
If the numerical aperture is 0.07, the pupil diameter is about 0.28 mm, and the diameter of the illumination field is 16.8 μm. Therefore, also in this case, Koehler illumination corresponding to a Schwarzschild objective lens having a numerical aperture of 0.21 is possible. In addition, by appropriately selecting the specifications of each zone plate, it is possible to correspond to an arbitrary numerical aperture and the size of the illumination visual field.

[0028] A fourth embodiment using the radiation source to the fourth embodiment the light source in FIG. The light from the radiation light source S ₁ is reflected by the grazing incidence reflection mirror 5,
In zone plate Z ₁ to form a secondary light source S _2. The zone plate Z ₂ irradiates the sample T with light from the secondary light source S _2, and the image of the illuminated sample T is enlarged to the position 3 by the objective lens L as an image A.

[0029]

As described above, the Koehler illumination optical system of the soft X-ray microscope according to the present invention firstly forms a secondary light source as a light source, and a zone provided immediately after the secondary light source. with the configuration of the light from the secondary light source Te <br/> through the plate is irradiated onto the sample surface,
Light from a light source can be used effectively, and Koehler illumination capable of obtaining uniform illumination can be realized in a soft X-ray microscope.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a basic configuration of a Koehler illumination optical system.

FIG. 2 is a configuration diagram of a Koehler illumination optical system having a zone plate condenser corresponding to an objective lens having a large numerical aperture.

FIG. 3 is a configuration diagram of a first embodiment of a Koehler illumination optical system according to the present invention, including a zone plate condenser configured for a Walter objective lens.

FIG. 4 is a configuration diagram including a zone plate condenser configured for a Schwarzschild type objective lens in a second embodiment of the Koehler illumination optical system according to the present invention.

FIG. 5 is a configuration diagram of a fourth embodiment of the Koehler illumination optical system according to the present invention, in which a radiation light source is used as a light source.

FIG. 6 is an explanatory diagram of critical illumination.

FIG. 7 is an explanatory diagram of Koehler illumination.

FIG. 8 is a configuration diagram of a conventional critical illumination optical system using a Walter condenser for a Walter objective lens.

FIG. 9 is a configuration diagram of a conventional critical illumination optical system using a zone plate condenser for a zone plate objective lens.

FIG. 10 is a configuration diagram of a conventional critical illumination optical system using a spheroidal condenser for a Schwarzschild type objective lens.

[Explanation of symbols]

Position of the light-shielding plate 2 pinhole 3 image 4 principal ray 5 grazing incidence reflection mirror A image L ₁ lens L ₂ lens L ₃ lens S ₁ source S ₂ the secondary light source T samples Z ₁ zone plate Z ₂ zone plate Z ₃ Zone plate

Claims (2)

(57) [Claims] 1. A lighting optical system of the soft X-ray microscope, cable
Light from the secondary light source of the illumination optics through the zone plate
Soft X, characterized in that so as to irradiate onto the sample and
Koehler illumination optics for a line microscope. 2. A Koehler illumination optical system for a soft X-ray microscope according to claim 1, wherein a chief ray forming said secondary light source is a convergent ray.