electron microscope

Abstract

To obtain an electron microscope with a desired magnification and small off-axis aberrations, especially distortion aberrations, by using a permanent magnet lens whose position can be changed in the optical axis direction. [Solution] The intermediate lens system 11 following the objective lens OL is composed of lens units IL1 and IL2, each of which has two permanent magnets and a yoke supporting the magnets, with the north and south poles of the two permanent magnets reversed to minimize the magnetic field generated in the yoke. The lens units are mechanically adjustable in position, with the lens unit IL1 closer to the objective lens forming a real image that is reduced, equal to, or enlarged from the real image formed by the objective lens, and the lens unit IL2 farther from the objective lens forming an enlarged real image. The subsequent intermediate lens system 12 is configured similarly to the intermediate lens system 11, and both lens units form enlarged real images. The subsequent projection lens is composed of a single lens unit similar to the intermediate lens systems 11 and 12, and forms an enlarged real image from the real image formed by the objective lens.

Description

[Technical Field]

[0001] The present invention relates to an electron microscope that uses a permanent magnet in its imaging system. [Background technology]

[0002] Conventionally, electromagnetic lenses have been mainly used as a means for realizing the lens effect of forming an image in electron microscopes. In contrast, Patent Document 1 is a document relating to a photoelectron microscope that uses a permanent magnet lens instead of an electromagnetic lens. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6954704 Summary of the Invention [Problem to be solved by the invention]

[0004] Patent Document 1 aims to obtain a photoelectron microscope capable of appropriately adjusting the magnification by mechanically driving multiple lens units consisting of permanent magnet lenses, but does not disclose how to arrange and image the multiple lens units to obtain the desired magnification and an image with small off-axis aberrations such as distortion.

[0005] An object of the present invention is to provide an electron microscope that uses a mechanically drivable permanent magnet lens unit in the imaging lens system, thereby enabling a desired magnification to be obtained and minimizing off-axis aberrations such as distortion. [Means for solving the problem]

[0006] In an electron microscope that uses a permanent magnet lens in the imaging system, The intermediate lens system 11 following the objective lens OL is composed of two lens units IL1 and IL2, each having two permanent magnets and a yoke supporting the two permanent magnets, with the north and south poles of the two permanent magnets arranged opposite to each other so as to reduce the magnetic field generated in the yoke. The positions of the lens units IL1 and IL2 can be mechanically changed, and the lens unit IL1 closer to the objective lens OL forms a real image that is reduced, equal in size, or enlarged from the real image formed by the objective lens OL, while the lens unit IL2 farther from the objective lens OL forms an enlarged real image of the real image formed by the objective lens OL.

[0007] Furthermore, the intermediate lens system 12 subsequent to the intermediate lens system 11 is composed of two lens units IL3 and IL4, each having two permanent magnets and a yoke supporting the two permanent magnets, with the north and south poles of the two permanent magnets being arranged opposite to each other so as to reduce the magnetic field generated in the yoke, and the positions of the lens units IL3 and IL4 can be mechanically changed, with the lens unit IL3 closer to the objective lens OL forming a real image that is an enlargement of the real image formed by the lens unit IL2, and the lens unit IL4 farther from the objective lens OL forming a real image that is an enlargement of the real image formed by the lens unit IL2.

[0008] Furthermore, the projection lens PL following the intermediate lens system 12 is composed of a single lens unit PL having two permanent magnets and a yoke supporting the two permanent magnets, with the north and south poles of the two permanent magnets arranged opposite to each other so as to reduce the magnetic field generated in the yoke, and its position is mechanically fixed so as to obtain a real image that is an enlarged version of the real image formed by the lens unit IL4. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic diagram showing the configuration of an imaging lens system of a photoelectron microscope according to an embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing an imaging trajectory of an electron microscope according to one embodiment of the present invention. [Figure 3a]1 is a diagram showing the structure of an objective lens of a photoelectron microscope according to one embodiment of the present invention. [Figure 3b] 10 is a diagram showing the magnetic flux density distribution in the optical axis z direction of the objective lens of the photoemission electron microscope according to one embodiment of the present invention. FIG. [Figure 4a] 1 is a diagram showing the structure of one lens unit that constitutes intermediate lens system 1, intermediate lens system 2, and a projection lens of an electron microscope according to one embodiment of the present invention. [Figure 4b] 1 is a diagram showing the magnetic flux density distribution in the optical axis z direction of one lens unit that constitutes intermediate lens system 1, intermediate lens system 2, and a projection lens of an electron microscope according to one embodiment of the present invention. [Figure 4c] 1 is a diagram showing the object plane position zLo, lens position zL, and image plane position zLi of one lens unit that constitutes intermediate lens system 1, intermediate lens system 2, and a projection lens of an electron microscope according to one embodiment of the present invention. [Figure 5] 1 is a diagram showing an intermediate lens system 1 made up of two lens units of an electron microscope according to one embodiment of the present invention. [Figure 6a] FIG. 10 is a diagram showing the magnification of the lens unit IL1 with respect to the position zL1 when the object plane position of the entire intermediate lens system 11 is zL1o=-50 mm and the image plane position is zL2i=+50 mm in an electron microscope according to one embodiment of the present invention. [Figure 6b] FIG. 6b is a diagram showing the magnitude of off-axis aberration at each point in FIG. 6a. [Figure 7] FIG. 2 is a diagram showing the positions of the lens units IL1, IL2, IL3, IL4 and the projection lens PL of the electron microscope according to one embodiment of the present invention. [Figure 8] FIG. 1 is a diagram showing the magnitude of distortion aberration relative to the magnification of an electron microscope according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] The configuration of a photoemission electron microscope 1 shown in Figure 1 will be described in accordance with one embodiment of the present invention. The photoemission electron microscope 1 is composed of an objective lens OL, an intermediate lens system 11 consisting of two lens units IL1 and IL2, an intermediate lens system 12 consisting of two more lens units IL3 and IL4, and a projection lens consisting of one lens unit PL, all of which are axially symmetrical about the optical axis z. Photoelectrons are emitted from a sample S located at zo on the optical axis and irradiated with excitation light (not shown), and are imaged on an image plane zi by the imaging lens system. Each of the lens units from IL1 to PL is excited by a permanent magnet, has the same magnetomotive force and magnetic pole shape, and therefore has the same focal length.

[0011] Figure 2 shows an example of a paraxial imaging trajectory for the photoelectron microscope 1 shown in Figure 1. Photoelectrons emitted from the sample S, i.e., the object plane zo, are imaged by OL on the OL image plane zoi, then by IL1 on the IL1 image plane z1i, then by IL2 on the IL2 image plane z2i, then by IL3 on the IL3 image plane z3i, then by IL4 on the IL4 image plane z4i, and finally by PL on the image plane zi of the photoelectron microscope 1.

[0012] An example of the objective lens OL10 that constitutes this photoemission electron microscope is shown in Figure 3a. OL is composed of a ring-shaped permanent magnet 101, axially symmetric magnetic poles 102 and 103 made of iron material Fe, and a yoke portion 104. An example of the magnetic flux density distribution Bz in the optical axis z direction is shown in Figure 3b. When excitation light is irradiated onto the sample S, photoelectrons are emitted from the sample surface, and these photoelectrons are focused and imaged at a position zoi = 50 mm in the z direction from the sample position zo (the sample position is the origin: zo = 0) due to the above Bz distribution. Note that OL may also be composed of an electromagnet lens instead of a permanent magnet lens.

[0013] FIG. 4a shows the lens unit 20, which constitutes the intermediate lens systems 1 and 2 and the projection lens following the OL. Similar to the lens unit 20 described in Patent Document 1, the lens unit 20 includes two ring-shaped permanent magnets 201 and 202, axially symmetric magnetic poles 211, 212, and 213 supporting the two permanent magnets, and a yoke 214. The north and south poles of the two permanent magnets are arranged opposite to each other to minimize the magnetic field generated in the yoke. The magnetic poles may have, for example, a diameter of 6 mm, a pole thickness of 4 mm, a pole gap of 4 mm, and a ring-shaped permanent magnet thickness of 4 mm. zL is the midpoint of the central magnetic pole and is the lens position. The dimensions of each magnetic pole do not have to be as described above; IL1, IL2, IL3, IL4, and PL may each be different. Furthermore, each lens unit may have an inclined portion or a tapered tip, as described in Patent Document 1.

[0014] An example of the magnetic flux density Bz in the z-axis direction by this lens unit is shown in Figure 4b. Due to this lens action, an image on object plane zLo is formed on image plane zLi as shown in Figure 4c.

[0015] Figure 5 shows an imaging system using an intermediate lens system 1 made up of two lens units L1 and L2 shown in Figure 4, which have the same focal length f. In Figure 5, the origin of the intermediate lens system 1 is set to 0, with zL1o indicating the object plane position of lens unit L1 located at zL1, zL1i indicating the image plane position of LI, and zL2i indicating the image plane position of lens unit L2 located at zL2. In Figure 5, lens unit L1 operates as a reduction system, and lens unit L2 operates as a magnification system.

[0016] Figure 6a shows the magnification at the image plane zL2i of intermediate lens system 1 relative to the position zL1 of lens unit L1, assuming that the focal length of the two lens units in Figure 5 is constant at f = 7 mm, the object plane position zL1o of intermediate lens system 1 is constant at -50 mm, and the image plane position zL2i of intermediate lens system 1 is constant at +50 mm, while the position of each lens unit is variable. The position of lens unit L2 is not shown. Here, 1R represents the case where L1 operates as a reduction system and L2 operates as a reduction system, 2R represents the case where L1 operates as a reduction system and L2 operates as a magnification system, 3R represents the case where L1 operates as an magnification system and L2 operates as a reduction system, and 4R represents the case where L1 operates as an magnification system and L2 operates as a magnification system.

[0017] Figure 6b shows example calculations of off-axis aberrations at points a to i in Figure 6a, based on a trajectory simulation. The calculation conditions are a parallel incidence electron beam at the object plane position zL1o = -50 mm of the intermediate lens system 1, and a field of view of 4 mmΦ at the image plane position zL2i = +50 mm. Comparing the off-axis aberrations at points a and i, which have the same magnification at the image plane position, reveals a significant difference, particularly in distortion, with the distortion at point a being two orders of magnitude smaller than that at point i. The same is true for points b and h. When the same magnification is obtained using two lens units with the same focal length in the intermediate lens system 1, operating lens unit 1 as a reduction system and lens unit 2 as a magnification system results in smaller distortion than when lens unit 1 is operated as a magnification system and lens unit 2 as a reduction system.

[0018] Figure 7 shows an example of the lens position zL versus magnification (Mag) in the imaging lens system of the present invention shown in Figures 1 and 2. The coordinate origin is the sample position (zo = 0). The position of each lens unit is set based on the findings obtained from Figures 5, 6a, and 6b. By keeping the positions of the intermediate lens system 2, consisting of lens units L3 and L4, and the projection lens PL constant and the position of the intermediate lens system 1, consisting of lens units L1 and L2, variable, magnifications ranging from approximately 300x to 10,000x can be obtained. Fixing the positions of lens units L3, L4, and PL simplifies the configuration and operation. Furthermore, by slightly changing the position of the intermediate lens system 2, magnifications exceeding 10,000x can be obtained.

[0019] Examples of conditions for the photoelectron microscope 1 are: voltage applied to the specimen = -13 kV, OL magnification = 4.4, OL image plane zoi = 50 mm, magnetomotive force of one permanent magnet in lens units L1 to PL = 535 AT, and image plane position zi = 600 mm. When the present invention is applied to a transmission electron microscope, an electron gun and focusing lens system are configured in front of the specimen, which is normally set to earth potential. In this case, the electron gun voltage is set to -13 kV. The objective lens may be configured as an electromagnetic lens instead of a permanent magnet lens.

[0020] Figure 8 shows the distortion ΔR / R × 100 (%) for each magnification in the imaging systems shown in Figures 1, 2, and 7. However, R = 20 mm at the image plane position zi. Figure 8 shows that when observing with an MCP with a diameter of 40 mm, for example, images can be observed with distortion of 3% or less at magnifications of 700x or more. Since distortion is proportional to the cube of R, if the field of view diameter on the image plane is halved to 20 mm, distortion becomes 1 / 8, and images can be observed with distortion of 3% or less at magnifications of 300x or more.

[0021] Although the above has been described with respect to a photoemission electron microscope, the present invention can also be applied to a transmission electron microscope in which the objective lens and sample are configured as transmission electron microscopes, and permanent magnet lenses are used as intermediate lenses following the objective lens. [Explanation of symbols]

[0022] 1. Photoelectron microscope 11 Intermediate lens system 1 12 Intermediate lens system 2 10 Objective Lens 101 Permanent Magnet 102, 103 magnetic pole 104 York 20 Lens unit 201, 202 Permanent magnets 211~213 magnetic pole 214 York

Claims

1. In an electron microscope having a plurality of lens units each made of a permanent magnet and configured to change the distance between the plurality of lens units, An electron microscope characterized in that an intermediate lens system 11 following an objective lens OL is composed of two lens units IL1 and IL2, each having two permanent magnets and a yoke supporting the two permanent magnets, with the north and south poles of the two permanent magnets being arranged opposite to each other so as to reduce the magnetic field generated in the yoke, the positions of the lens units IL1 and IL2 being changeable, the lens unit IL1 closer to the objective lens OL forming a real image that is reduced, equal in size, or enlarged from the real image formed by the objective lens, and the lens unit IL2 farther from the objective lens OL forming an enlarged real image of the real image.

2. The intermediate lens system 12 subsequent to the intermediate lens system 11 is composed of two lens units IL3 and IL4, each having two permanent magnets and a yoke supporting the two permanent magnets, and the north and south poles of the two permanent magnets are arranged opposite to each other so that the magnetic field generated in the yoke is reduced, the positions of the lens units IL3 and IL4 are changeable, and the lens unit IL3 closer to the objective lens OL forms a real image that is an enlargement of the real image formed by the lens unit IL2, and the lens unit IL4 farther from the objective lens OL forms a real image that is an enlargement of the real image formed by the lens unit IL2.

2. The electron microscope according to claim 1, wherein the projection lens PL following the intermediate lens system 12 is composed of a single lens unit having two permanent magnets and a yoke supporting the two permanent magnets, with the north and south poles of the two permanent magnets being arranged inversely to each other so as to reduce the magnetic field generated in the yoke, and the position of the lens unit is fixed so as to obtain a real image that is an enlarged version of the real image formed by the lens unit IL4.

3. 3. An electron microscope according to claim 2, wherein said intermediate lens system is fixed in position.

4. 4. An electron microscope according to claim 1, wherein the object plane position and the image plane position of said intermediate lens system 11 as a whole are kept constant regardless of the magnification.

5. 4. An electron microscope according to claim 1, wherein the object plane position and the image plane position of said intermediate lens system 12 as a whole are kept constant regardless of the magnification.

6. 6. An electron microscope according to claim 1, wherein the object plane position and the image plane position of said projection lens PL are kept constant regardless of the magnification.

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