Electron microscope

Abstract

The present invention provides an electron microscope capable of suppressing light scattering due to a hole for electron passage in a reflector. The electron microscope comprises an electron source that emits an electron beam with which to irradiate a sample, a light source that emits excitation light with which to irradiate the sample, a reflector which has a hole that the electron beam passes through and which reflects the excitation light toward the sample, a detector that detects electrons emitted from the sample irradiated with the electron beam and outputs a detection signal, and a control unit that generates an observation image of the sample on the basis of the detection signal. The electron microscope is characterized by further comprising: a condensing lens disposed between the light source and the reflector to condense the excitation light and also form a dark area covering the hole in the center of the excitation light.

Description

electron microscope

[0001] The present invention relates to an electron microscope that generates an observation image of a sample by irradiating the sample with an electron beam and light and detecting electrons emitted from the photoexcited sample.

[0002] An electron microscope is a device that irradiates a sample with an electron beam and generates an observation image of the sample by detecting secondary electrons and backscattered electrons emitted from the sample. If the sample becomes charged by the electron beam irradiation, the brightness of the observation image may fluctuate or become distorted.

[0003] Patent Document 1 discloses that in order to prevent the sample from being charged, light is irradiated onto the area to be irradiated with the electron beam, and the light irradiated onto the electron beam irradiation area is irradiated onto the sample via a reflecting mirror having a hole through which the electron beam passes.

[0004] International Publication No. 2020 / 115876

[0005] However, as in Patent Document 1, the light irradiated onto the sample through a reflecting mirror having a hole for passing the electron beam is scattered by the hole. Therefore, when observing a photoexcited sample, the scattered light may generate noise, which may degrade the quality of the observed image.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an electron microscope that can suppress scattering of light at holes in a reflecting mirror for passing electron beams.

[0007] In order to achieve the above object, the present invention provides an electron microscope comprising: an electron source that emits an electron beam with which a sample is irradiated; a light source that emits excitation light with which the sample is irradiated; a reflecting mirror that has a hole through which the electron beam passes and that reflects the excitation light toward the sample; a detector that detects electrons emitted from the sample irradiated with the electron beam and outputs a detection signal; and a control unit that generates an observation image of the sample based on the detection signal, wherein the electron microscope further comprises a focusing lens that is disposed between the light source and the reflecting mirror and focuses the excitation light and forms a dark area at the center of the excitation light to cover the hole.

[0008] According to the present invention, it is possible to provide an electron microscope capable of suppressing scattering of light at holes in a reflecting mirror for passing electron beams.

[0009] FIG. 1 is a diagram illustrating an example of the overall configuration of an electron microscope according to the first embodiment; FIG. 2 is a diagram illustrating a dark portion formed at the center of excitation light; FIG. 3 is a diagram illustrating another example of the overall configuration of an electron microscope according to the first embodiment; FIG. 4 is a diagram illustrating another example of the configuration for forming a dark portion; FIG. 5 is a diagram illustrating another example of the configuration for forming a dark portion; FIG. 6 is a diagram illustrating another example of the configuration for forming a dark portion;

[0010] An embodiment of an electron microscope according to the present invention will be described below with reference to the accompanying drawings. An electron microscope is a device that irradiates a sample with electrons and detects secondary particles such as secondary electrons, backscattered electrons, and transmitted electrons emitted from the sample to generate an observation image of the sample. As an example of an electron microscope, a scanning electron microscope (SEM) that detects secondary electrons and backscattered electrons emitted from a sample will be described below. Note that the present invention can also be applied to transmission electron microscopes (TEM) and scanning transmission electron microscopes (STEM) that detect transmitted electrons.

[0011] An example of the overall configuration of the electron microscope of Example 1 will be described with reference to Figure 1. The electron microscope includes a vacuum chamber 101 and a control unit 124. The vacuum chamber 101 is evacuated by a vacuum pump (not shown), and includes a sample stage 122, a photocathode 107, an electron beam deflector 125, an objective lens 109, and an electron detector 111 therein.

[0012] The sample stage 122 holds the sample 110 to be observed, and the position thereof is adjusted by the sample stage adjustment unit 123 .

[0013] The photocathode 107 is an electron source that emits an electron beam 108 when irradiated with laser light from the laser light source 102 via the optical fiber 103, lens 104, entrance window 105, and lens 106. The photocathode 107 is, for example, a highly doped p-type semiconductor formed on a transparent substrate. The photocathode 107 has a negative electron affinity (NEA) surface due to the adsorption of cesium (Cs) and oxygen onto the surface of p-type GaAs. The NEA surface of the photocathode 107 provides an electron source with brightness characteristics equivalent to a Schottky electron source and a more monochromatic electron source with a narrower energy spread than a cold cathode electron source. Pulsed irradiation of laser light from the laser light source 102 onto the photocathode 107 causes the photocathode 107 to emit pulsed electron beams 108, generating pulsed electron beams with a width as short as picoseconds. The photocathode 107 is not limited to one having an NEA surface, and may be a Schottky type electron source or a lanthanum hexaboride (LaB ６ ) may be used as a thermionic emission electron source.

[0014] An electron beam 108 emitted from the photocathode 107 is deflected by an electron beam deflector 125 and focused by an objective lens 109. Deflection by the electron beam deflector 125 causes the electron beam 108 to two-dimensionally scan the surface of the sample 110. The region scanned by the electron beam 108 becomes the observation region of the sample 110.

[0015] The electron detector 111 detects secondary electrons and reflected electrons emitted from the surface of the sample 110 scanned by the electron beam 108 , and outputs a detection signal to the control unit 124 .

[0016] The control unit 124 is a device that controls each unit, and is, for example, a general-purpose computer. The computer includes a processor such as a CPU (Central Processing Unit) and memories such as RAM (Random Access Memory) and ROM (Read Only Memory). The control unit 124 also generates an observation image of the sample 110 based on the detection signal output from the electron detector 111.

[0017] When generating an observation image of the sample 110, the sample 110 may be irradiated with light for exciting the sample 110. The electron microscope illustrated in Fig. 1 includes a laser light source 113, an optical fiber 114, a collimating lens 115, an entrance window 117, a condenser lens 116, and a perforated plane mirror 118 in order to irradiate the sample 110 with excitation light 112.

[0018] A laser light source 113 emits excitation light 112, which is light for exciting the sample 110. The excitation light 112 emitted from the laser light source 113 travels through an optical fiber 114 to a collimating lens 115, where it is collimated into a parallel beam. The parallel beam of excitation light 112 passes through an entrance window 117 to a condenser lens 116 provided in the vacuum chamber 101, where it is condensed by the condenser lens 116. The condensed excitation light 112 is reflected by a perforated plane mirror 118 and irradiated onto the sample 110. The condensing by the condenser lens 116 causes the excitation light 112 to irradiate the sample 110 so as to cover the observation region of the sample 110.

[0019] When examining the transient response of the sample 110 due to irradiation with the excitation light 112, the excitation light 112 may be emitted in pulses from the laser light source 113, and the control unit 124 may control the irradiation timing of the laser light source 102 and the laser light source 113.

[0020] An angle adjustment unit 120 and a position adjustment unit 121 may also be provided to adjust the irradiation area of ​​the excitation light 112. The angle adjustment unit 120 is a mechanism for adjusting the angle of the excitation light 112 incident on the perforated plane mirror 118, and is configured, for example, by a goniostage. The position adjustment unit 121 is a mechanism for adjusting the position of the excitation light 112 incident on the perforated plane mirror 118, and is configured, for example, by a fine adjustment stage.

[0021] The perforated plane mirror 118 is provided with a hole through which the electron beam 108 passes. Of the excitation light 112 incident on the perforated plane mirror 118, light that enters the hole through which the electron beam 108 passes is scattered by the hole and irradiates an area other than the observation area of ​​the sample 110, which may generate noise and degrade the image quality of the observation image. Therefore, in Example 1, a dark area that covers the hole through which the electron beam 108 passes is formed in the center of the excitation light 112, thereby suppressing scattering of the excitation light 112 at the hole.

[0022] The dark area 200 formed at the center of the excitation light 112 will be described using FIG. 2 . FIG. 2 shows a perforated plane mirror 118 having a hole 118a through which the electron beam 108 passes and reflecting the excitation light 112 toward the sample 110, and a condenser lens 116 that condenses the excitation light 112 and forms a dark area 200 covering the hole 118a at the center of the excitation light 112. Because the dark area 200 is a portion that does not contain light, covering the hole 118a with the dark area 200 can suppress light scattering due to the hole 118a. Furthermore, because distortion may occur around the hole 118a due to the processing of the hole 118a, it is preferable that the dark area 200 be large enough to cover the periphery of the hole 118a. Since the excitation light 112 reflected by the perforated plane mirror 118 is condensed by the condenser lens 116, the dark area 200 disappears on the surface of the sample 110. The formation of the dark portion 200 will be described later with reference to FIGS.

[0023] Another example of the overall configuration of the electron microscope of Example 1 will be described using FIG. 3 . The difference between FIG. 3 and FIG. 1 is the position of the condenser lens 116, so a description of only the condenser lens 116 and its periphery will be omitted. The condenser lens 116 provided in the electron microscope illustrated in FIG. 3 is disposed outside the vacuum chamber 101 and integrated with the collimator lens 115, and its angle and position are adjusted together with the collimator lens 115 by an angle adjustment unit 120 and a position adjustment unit 121. This configuration makes it easy to adjust the irradiation position of the excitation light 112 on the sample 110. Furthermore, it is possible to replace the module composed of the collimator lens 115, the condenser lens 116, the angle adjustment unit 120, and the position adjustment unit 121 with another module, for example, an optical module for observing the surface of the sample 110.

[0024] An example of a configuration for forming the dark portion 200 will be described with reference to Fig. 4. Fig. 4(a) is a front view of a shielding plate 401 that forms the dark portion 200, and Fig. 4(b) is a side view of the shielding plate 401. Fig. 4(b) also shows incident light rays 402 to the shielding plate 401 and emitted light rays 403 from the shielding plate 401. Fig. 4(c) shows the intensity distribution of the incident light rays 402, and Fig. 4(d) shows the intensity distribution of the emitted light rays 403.

[0025] The shielding plate 401 is a transparent flat plate 401a having a shielding region 401b in the center thereof that shields the excitation light 112. The transparent flat plate 401a is, for example, a glass plate or an acrylic plate, and the shielding region 401b is formed by applying a light absorbing material that absorbs the excitation light 112 in a circular shape.

[0026] 4A and 4B, the center of the excitation light 112 incident on the shielding plate 401 is absorbed by the shielding region 401b, and a dark portion 200 is formed at the center of the excitation light 112. As a result, the circular excitation light 112 shown in FIG. 4C is converted into annular excitation light having a dark portion 200 at its center, as shown in FIG. 4D. The dark portion 200 covers the hole 118a of the perforated plane mirror 118, thereby suppressing scattering of the excitation light 112.

[0027] The size of the shielding region 401b is determined so that the hole 118a of the perforated plane mirror 118 is covered by the dark portion 200 formed by the shielding plate 401. The shielding plate that forms the dark portion 200 is not limited to the one exemplified in FIG.

[0028] 5A and 5B, a shielding plate 501 will be described as another example of a configuration for forming a dark portion 200. Note that (a) in Fig. 5 is a front view of the shielding plate 501, (b) is a side view of the shielding plate 501, and incident light rays 502 to the shielding plate 501 and emitted light rays 503 from the shielding plate 501 are also shown. Also, (c) in Fig. 5 shows the intensity distribution of the incident light rays 502, and (d) shows the intensity distribution of the emitted light rays 503.

[0029] The shielding plate 501 illustrated in FIG. 5 has four openings formed in a light-absorbing material that absorbs the excitation light 112. The four openings are arranged in rotational symmetry with the optical axis of the excitation light 112 as the four-fold axis. The number of openings is not limited to four, and N openings may be arranged in rotational symmetry, where N is an integer greater than or equal to 2. Even with this configuration, a dark area 200 is formed at the center of the excitation light 112. When the number of openings N is infinite, the shielding plate 401 illustrated in FIG. 4 is obtained, and therefore the smaller the number of N, the more the number of manufacturing steps can be reduced. The configuration for forming the dark area 200 is not limited to the shielding plates illustrated in FIGS. 4 and 5.

[0030] Another example of the configuration for forming the dark space 200 will be described using FIG. 6 . FIG. 6 shows a front view and an A-A′ cross-sectional view of a meniscus axicon mirror 601 that converts circular light into annular light. The meniscus axicon mirror 601 is a transparent member, such as a glass member, in which a conical space having the same central axis and height as the truncated cone is provided within a truncated cone, and the conical surfaces of the truncated cone and the cone are parallel. The angle between the conical surface and the central axis is preferably 45°. The light entrance and exit surfaces are surfaces that are perpendicular to the central axis, with the end surface on the apex side of the cone being the entrance surface and the end surface on the base side of the cone being the exit surface.

[0031] Circular parallel light rays incident on the meniscus axicon mirror 601 shown in FIG. 6 pass through the entrance surface, are totally reflected by the conical surface of the cone, and then radially spread in a direction perpendicular to the central axis. The radially spread light is then totally reflected by the conical surface of the truncated cone, becoming circular parallel light rays and emerging from the exit surface. In other words, the meniscus axicon mirror 601 can form a dark space 200 at the center of the excitation light 112. Furthermore, while a portion of the light incident on the shielding plate is absorbed, the light incident on the meniscus axicon mirror 601 is not absorbed, thereby suppressing light loss. Light loss can be minimized particularly when the angle between the conical surface and the central axis is 45°.

[0032] The size of the dark space 200 is determined by the size of the base of the cone. The meniscus axicon mirror that forms the dark space 200 is not limited to the one exemplified in FIG.

[0033] Referring to Figure 7, a meniscus axicon mirror 701 will be described as another example of a configuration for forming a dark space 200. Figure 7 shows a front view and an A-A' cross-sectional view of the meniscus axicon mirror 701. The meniscus axicon mirror 701 is configured by providing a space shaped like a combination of a second cylinder and a second truncated cone within a glass member shaped like a combination of a first truncated cone and a first cylinder, with the two conical surfaces parallel to each other. The glass member and the space therein have the same central axis and height. The bottom surface of the first truncated cone, which is a glass member, and the top surface of the first cylinder are the same size, and the bottom surface of the second cylinder, which forms the space, and the top surface of the second truncated cone are the same size. A light-absorbing material is disposed in the hole formed by the second cylinder. Furthermore, the angle between the conical surface and the central axis is preferably 45°. The light incident surface and light exit surface are surfaces that are perpendicular to the central axis, with the upper end surface of the first truncated cone being the incident surface and the lower end surface of the second truncated cone being the exit surface.

[0034] 7, a circular parallel light beam incident on the meniscus axicon mirror 701 passes through the entrance surface, undergoes total reflection at the conical surface of the second truncated cone, and then undergoes total reflection at the conical surface of the first truncated cone, and then becomes a circular parallel light beam and exits from the exit surface. In other words, the meniscus axicon mirror 701 can form a dark space 200 at the center of the excitation light 112.

[0035] Furthermore, the meniscus axicon mirror 701 is easier to manufacture and its optical axis is easier to adjust than the meniscus axicon mirror 601 in Figure 6. That is, when forming a conical space in a glass cylindrical base by lathe machining of the meniscus axicon mirror 601, it is difficult to form the apex of the cone because the radius of curvature of the tip of a diamond turning tool is finite. Furthermore, in order to totally reflect light on the conical surface of the meniscus axicon mirror 601, it is not possible to bring other members into contact with the conical surface, and therefore the meniscus axicon mirror 601 must be held at the outer periphery of the exit surface, making optical axis adjustment difficult.

[0036] In contrast, the meniscus axicon mirror 701 is easier to manufacture because it does not require the formation of a cone apex. Also, the meniscus axicon mirror 701 can be held by the side surface of the first cylinder, making it easier to adjust the optical axis.

[0037] Another example of the configuration for forming the dark space 200 will be described using Figure 8. Figure 8 shows a front view and an A-A' cross-sectional view of a hybrid axicon mirror 801. The hybrid axicon mirror 801 has a transparent substrate 802, a convex axicon mirror 803, a concave axicon mirror 804, a lens barrel 805, and a ring screw 806. Each part will be described below.

[0038] The transparent substrate 802 is a circular glass plate, but may also be an acrylic plate.

[0039] The convex axicon mirror 803 is a conical member with a mirror formed on the conical surface of the cone. The mirror formed on the conical surface is made of a vapor-deposited film of metal such as aluminum or silver, or a dielectric multilayer film. The convex axicon mirror 803 is bonded onto the transparent substrate 802 so that it has the same central axis as the transparent substrate 802.

[0040] The concave axicon mirror 804 is a component that has a truncated cone-shaped space inside a cylinder, with the space having the same central axis and height as the cylinder, and a mirror formed on the conical surface of the truncated cone. The convex axicon mirror 803 is placed in the truncated cone-shaped space so that they share the same central axis. The conical surface of the truncated cone of the concave axicon mirror 804 is parallel to the conical surface of the cone of the convex axicon mirror 803, and the angle between the central axis and the conical surface is, for example, 45°. The outer diameter of the concave axicon mirror 804 is the same as the outer diameter of the transparent substrate 802, and the inner diameter of the hole in the concave axicon mirror 804, i.e., the diameter of the top surface of the truncated cone that forms the space, is the same as the outer diameter of the bottom surface of the cone of the convex axicon mirror 803.

[0041] The lens barrel 805 is a cylindrical member that holds the transparent substrate 802 to which the convex axicon mirror 803 is adhered, and the concave axicon mirror 804. One end of the lens barrel 805 may be provided with a protrusion 805a that supports the concave axicon mirror 804, and the other end may be provided with a spiral groove into which the ring screw 806 is fitted. The inner surface of the lens barrel 805 is preferably blackened. By blackening the inner surface of the lens barrel 805, light that reaches the inner surface of the lens barrel 805 is absorbed, thereby reducing noise generation.

[0042] The ring screw 806 is an annular screw that is fitted into a groove provided in the lens barrel 805. The ring screw 806 is fitted into the transparent substrate 802 to which the convex axicon mirror 803 is adhered, and the lens barrel 805 in which the concave axicon mirror 804 is incorporated, thereby fixing each component and assembling the hybrid axicon mirror 801. Note that the method of fixing the transparent substrate 802 and the concave axicon mirror 804 is not limited to using the protrusion 805a of the lens barrel 805 and the ring screw 806, and adhesive may also be used, for example. The light incident surface and light exit surface are surfaces that are perpendicular to the central axis, and the end surface on which the protrusion 805a is provided is the incident surface, and the end surface on which the ring screw 806 is fitted is the exit surface.

[0043] 8, a circular parallel light beam incident on the hybrid axicon mirror 801 is reflected by the conical mirror of the convex axicon mirror 803 and by the conical mirror of the concave axicon mirror 804, and then becomes a circular parallel light beam and exits from the exit surface. In other words, the hybrid axicon mirror 801 can form a dark space 200 at the center of the excitation light 112.

[0044] Furthermore, the hybrid axicon mirror 801 is easier to manufacture and to adjust its optical axis than the meniscus axicon mirror 601 in Figure 6 and the meniscus axicon mirror 701 in Figure 7. Specifically, the hybrid axicon mirror 801 is assembled using the transparent substrate 802, convex axicon mirror 803, concave axicon mirror 804, lens barrel 805, and ring screw 806, which are manufactured separately, making it easier to manufacture. Furthermore, because none of these components has a conical space like the meniscus axicon mirror 601, they can be easily manufactured by lathe machining. Furthermore, the hybrid axicon mirror 801 can be held by the side of the lens barrel 805, which is longer in the central axial direction than the side of the first cylinder of the meniscus axicon mirror 701, making optical axis adjustment easier.

[0045] Another example of the configuration for forming the dark space 200 will be described using Figure 9. Figure 9 shows a front view and an A-A' cross-sectional view of a meniscus axicon mirror 901 manufactured by resin molding. The meniscus axicon mirror 901 is a transparent resin member that has a shape in which the meniscus axicon mirror 601 shown in Figure 6, a disk-shaped member, and an annular member with a flange portion 901a attached to its outer periphery are integrated together. The disk-shaped member is provided on the entrance surface of the meniscus axicon mirror 601, and the annular member with the flange portion 901a is provided on the exit surface.

[0046] 9, a circular parallel light beam incident on the meniscus axicon mirror 901 passes through the entrance surface, undergoes total reflection at the conical surface of the cone and at the conical surface of the truncated cone, and then emerges from the exit surface as a circular parallel light beam, similar to the case of FIG. 6. In other words, the meniscus axicon mirror 901 can form a dark space 200 at the center of the excitation light 112.

[0047] Furthermore, the meniscus axicon mirror 901 is easier to manufacture and to adjust the optical axis than the meniscus axicon mirror 601 in Figure 6. That is, although the meniscus axicon mirror 901 has a conical space, it is manufactured by resin molding, so the apex of the cone can be easily formed. Note that although the mold used for resin molding has a conical shape, the apex of the cone is convex, so it can be easily manufactured by lathe processing. Furthermore, the meniscus axicon mirror 901 has a flange portion 901a, which makes optical axis adjustment easy. Note that it is preferable that the flange portion 901a be formed longer in the central axis direction, for example, equal to or longer than that of the meniscus axicon mirror 601. Furthermore, the components manufactured by resin molding are not limited to those exemplified in Figure 9.

[0048] Using Figure 10, a combination of an incident-side diffractive optical element 1001 and an exit-side diffractive optical element 1002 will be described as another example of a component manufactured by resin molding. Figure 10(a) is a front view of the incident-side diffractive optical element 1001, (b) is a side view, and (c) is a front view of the exit-side diffractive optical element 1002. Note that Figure 10(b) also shows incident light ray 1003 to the incident-side diffractive optical element 1001 and exit light ray 1004 from the exit-side diffractive optical element 1002. The incident-side diffractive optical element 1001 and the exit-side diffractive optical element 1002 are manufactured by resin molding.

[0049] The incident-side diffractive optical element 1001 is composed of circular transparent members of different diameters arranged concentrically with no gaps between them. The cross section of the circular transparent members is a right-angled triangle with its inclined surface facing the central axis, and the exit surface is flat. Circular parallel light rays incident on the incident-side diffractive optical element 1001 having this shape are refracted in a direction away from the central axis, as shown in Figure 10(b).

[0050] The exit-side diffractive optical element 1002 is composed of annular transparent members of different diameters arranged concentrically with no gaps between them. However, the annular transparent members are not arranged in the area where the dark region 200 is formed. The cross section of the annular transparent member is a right-angled triangle with its inclined surface facing the outer periphery, and the incident surface side is flat. By arranging the exit-side diffractive optical element 1002 having such a shape so as to have the same central axis as the entrance-side diffractive optical element 1001, light refracted by the entrance-side diffractive optical element 1001 is refracted as shown in FIG. 10B and becomes annular parallel light rays with the dark region 200 formed at its center. Note that the oblique angles of the right-angled triangles of the entrance-side diffractive optical element 1001 and the exit-side diffractive optical element 1002 are preferably set appropriately to satisfy the blaze condition according to the wavelength of the incident light.

[0051] The above describes the embodiments of the present invention. The present invention is not limited to the above embodiments, and the components can be modified and embodied without departing from the spirit of the invention. Furthermore, multiple components disclosed in the above embodiments may be combined as appropriate. Furthermore, some components may be omitted from all the components shown in the above embodiments.

[0052] 101: vacuum chamber, 102: laser light source, 103: optical fiber, 104: lens, 105: entrance window, 106: lens, 107: photocathode, 108: electron beam, 109: objective lens, 110: sample, 111: electron detector, 112: excitation light, 113: laser light source, 114: optical fiber, 115: collimating lens, 116: condensing lens, 117: entrance window, 118: perforated plane mirror, 118a: hole, 120: angle adjustment unit, 121: position adjustment unit, 122: sample stage, 123: sample stage adjustment unit, 124: control unit, 125: electron beam deflector, 200: dark area, 401: shielding plate, 401a: transparent Bright flat plate, 401b: shielding region, 402: incident light beam, 403: exit light beam, 501: shielding plate, 502: incident light beam, 503: exit light beam, 601: meniscus axicon mirror, 701: meniscus axicon mirror, 801: hybrid axicon mirror, 802: transparent substrate, 803: convex axicon mirror, 804: concave axicon mirror, 805: lens barrel, 805a: protrusion, 806: ring screw, 901: meniscus axicon mirror, 901a: flange, 1001: incident-side diffractive optical element, 1002: exit-side diffractive optical element, 1003: incident light beam, 1004: exit light beam.

Claims

1. An electron microscope comprising: an electron source that emits an electron beam with which a sample is irradiated; a light source that emits excitation light with which the sample is irradiated; a reflecting mirror that has a hole through which the electron beam passes and that reflects the excitation light toward the sample; a detector that detects electrons emitted from the sample irradiated with the electron beam and outputs a detection signal; and a control unit that generates an observation image of the sample based on the detection signal and controls each unit, characterized in that the electron microscope further comprises a focusing lens that is positioned between the light source and the reflecting mirror and focuses the excitation light and forms a dark area at the center of the excitation light to cover the hole.

2. An electron microscope according to claim 1, wherein the condenser lens has a mirror that converts the excitation light from a circular shape to a circular shape.

3. An electron microscope as claimed in claim 2, characterized in that the mirror has a transparent member in which a conical space having the same central axis and height as the truncated cone is provided within the truncated cone, and the two conical surfaces are parallel.

4. An electron microscope according to claim 3, characterized in that the angle between the conical surface and the central axis is 45°.

5. An electron microscope as claimed in claim 3, wherein the mirror is a resin member in which the transparent member, a transparent disk, and a transparent ring with a flange portion attached to the outer periphery are integrated, the transparent disk being provided on one end face of the transparent member, and the transparent ring being provided on the other end face of the transparent member.

6. An electron microscope according to claim 5, wherein the flange portion has a length in the central axis direction equal to or greater than that of the transparent member.

7. An electron microscope according to claim 2, wherein the mirrors comprise a concave axicon mirror and a convex axicon mirror, the concave axicon mirror being a member having a truncated cone-shaped space within a cylinder, the space having the same central axis and height as the cylinder, and a mirror formed on the conical surface of the truncated cone, and the convex axicon mirror being a member having a conical shape and a mirror formed on the conical surface of the cone, and being arranged so as to have the same central axis within the truncated cone-shaped space of the concave axicon mirror.

8. An electron microscope according to claim 7, characterized in that the mirror further comprises a transparent disk to which the convex axicon mirror is bonded so that they have the same central axis, and a cylindrical lens barrel that holds the concave axicon mirror and the transparent disk to which the convex axicon mirror is bonded.

9. An electron microscope according to claim 8, wherein the inner surface of the lens barrel is blackened.

10. An electron microscope according to claim 8, wherein one end of the lens barrel is provided with a protrusion for supporting the concave axicon mirror, and the other end is fitted with a ring screw.

11. An electron microscope as claimed in claim 2, characterized in that the mirror is a glass member shaped as a combination of a first truncated cone and a first cylinder, with a space shaped as a combination of a second cylinder and a second truncated cone provided within the glass member, and the two conical surfaces are parallel.

12. An electron microscope as described in claim 2, wherein the mirror has an incident-side diffractive optical element and an exit-side diffractive optical element, the incident-side diffractive optical element is made up of first transparent members in the shape of annular rings of different diameters arranged concentrically with no gaps between them, the cross section of the first transparent member being a right-angled triangle with its inclined surface facing the central axis, the exit-side diffractive optical element is made up of second transparent members in the shape of annular rings of different diameters arranged concentrically with no gaps between them, the cross section of the second transparent member being a right-angled triangle with its inclined surface facing the outer periphery, and the planes of the incident-side diffractive optical element and the exit-side diffractive optical element are arranged facing each other and have the same central axis.

13. An electron microscope according to claim 1, wherein the condenser lens has a shielding plate for shielding the center of the excitation light.

14. An electron microscope according to claim 13, characterized in that the shielding plate has a plurality of openings arranged in rotational symmetry.

15. An electron microscope according to claim 1, characterized in that the condenser lens is disposed outside the vacuum chamber.

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