JPH0522331B2 - - Google Patents

Description

Detailed Description of the Invention The present invention provides an electron beam system that adjusts the path of an electron beam emitted from a sample to obtain a low-magnification wide-field image while minimizing off-axis astigmatism and curvature aberration of a lens system. It is related to the device.

For example, there is an imaging method shown in FIG. 1 as an imaging method for obtaining a low magnification image of about 3000 times or less in various electron beam devices such as an electron microscope. This is an electron microscope that has a lens system consisting of an objective lens 4, an intermediate lens 5, and a projection lens 6. An electron beam 3 emitted from a sample 1 is focused on the object plane of a projection lens 6 as a real image 8 by an objective lens 4 and an intermediate lens 5, and the real image 8 is enlarged and projected onto a screen 9 by the projection lens 6. In the process of obtaining the observed image 10, the magnification is adjusted by adjusting the excitation of the projection lens 6, intermediate lens 5, and objective lens 4.
I am trying to get a low magnification image.

In such an electron beam device that produces low-magnification images, the objective lens 4 is
Since the excitation current of the objective lens 4 does not change significantly, it is possible to suppress thermal sample drift and focus drift caused by changes in the excitation current of the objective lens 4, compared to low-magnification electron beam equipment that almost blocks excitation of the objective lens. , since the objective lens 4 is relatively strongly excited, it is superior in that axial aberrations are reduced. However, due to the fact that the objective lens 4 is relatively strongly excited, the focal length of the objective lens 4 is usually relatively short at several millimeters, and the structural requirements of the electron microscope, the objective lens 4 and the intermediate lens 5 are If the interval is relatively large, e.g. the acceleration voltage is
In a 100 to 200 KV (kilobottle) electron microscope, the above-mentioned interval must be approximately 100 to 200 mm (millimeters), so the spread of the electron beam 3 at the intermediate lens 5 becomes large, and the off-axis beam generated by the intermediate lens 5 increases. It has the disadvantage that aberrations tend to increase and it is difficult to obtain a high-quality low-magnification wide-field image.

Therefore, as shown in FIG. 2, for example, a multi-stage electronic lens 14 and an auxiliary electron lens 15 are provided between the objective lens 4 and the intermediate lens 5, and between the intermediate lens 5 and the projection lens 6, respectively. The electron beam bundle 3 emitted from the sample 1 is formed into a real image 16 by an objective lens 4 and an electron lens 14, and further includes an intermediate lens 5 and an electron lens 15.
A method has also been proposed in which a real image 17 is formed on the object plane of the projection lens 6 and then a low-magnification observation image 10 is formed on the screen 9 by the projection lens 6. In such an electron beam device, since the electron beam bundle 3 takes a path relatively close to the optical axis 2, off-axis aberrations in each lens can be reduced.

However, in the case of the electron beam apparatus shown in FIG. 2 above, the objective lens 4 is strongly excited to reduce the on-axis aberration, and the off-axis aberration generated by each lens is also significantly reduced. However, among off-axis aberrations, off-axis astigmatism and curvature aberration cannot be canceled out by each lens, so in a low-magnification wide-field image of about 1,000 times or less using the above electron beam device, each Depending on the lens arrangement, the pole piece gap, and the hole diameter, there was still a problem that unclear areas tended to remain around the image periphery.

The present invention was made in view of the above-mentioned conventional problems, and its purpose is to maintain the excitation of the objective lens at a strong excitation level during low-magnification observation in the same way as during high-magnification observation in an electron microscope. The objective is to minimize the off-axis astigmatism and curvature aberration of the lens system.

Generally, the size A of an elliptical blur that occurs at a point away from the optical axis on the image recording surface due to off-axis astigmatism and curvature aberration of a lens is A=k∫ ^z1 _z0 [ar ² 〓r ² 〓+br ² 〓r′ ² 〓＋cr〓r〓r′
〓r′〓＋dr′ ² 〓r ² 〓＋er′ ² 〓r′ ² 〓＋】dz……(1)
It is expressed as

Here, k, a, b, c, d, e, f: Relationship among magnetic field distribution, accelerating voltage, etc. r〓, r〓, r'〓, r': Trajectory of paraxial electron beam (on the sample surface r = 0, r' = 1, r = 1, satisfies r = 0 on the objective aperture plane.) Z ₀ : Position of sample surface Z ₁ : Position of image plane (Zworykin, VKet al: “Electron
Optics and the Electron Microscope”1945).

In the case of lenses in ordinary electron microscopes, No. (1)
The terms below the second term in the equation can be ignored compared to the first term, and the size of the blur A is approximately as follows: A=k∫ ^z1 _z0 ar ² 〓r ² 〓dz ……(2) .

Here, r〓r〓 represents, for example, the distance of the electron beams 3a and 3b from the optical axis 2 shown in FIG. 2, respectively.

Here, according to each of the above-mentioned conventional examples, the electron beam 3
a and 3b are optical axes in the intermediate lens 5 and the projection lens 6 in the example shown in FIG. 1, and in the intermediate lens 5, the auxiliary electron lens 15 and the projection lens 6 in the example shown in FIG. Since it is relatively far away from r〓 and
The value of r〓 necessarily becomes relatively large, and the magnitude of the blur A becomes large as well.

The gist of the present invention is that, based on the above equation (2), the size A of the blur caused by the lens of the electron beam device is made as small as possible.

The first type of electron beam device is equipped with an objective lens, an intermediate lens, and a projection lens, and an image of the sample is formed by the objective lens in an excited state, and a high magnification observation image is formed by the intermediate lens and the projection lens. In an electron beam apparatus for imaging, the objective lens is kept in an excitation state with strong excitation during high-magnification observation, and multiple lenses including the objective lens and an intermediate lens following the objective lens are used to generate images that diverge from the back focus of the objective lens. The electron beam bundle is once focused near the main surface of the intermediate lens immediately in front of the projection lens to perform low-magnification observation.

With such a configuration, the electron beam is focused at one end on the main surface of the intermediate lens, so the value of r can be set to 0 at this main surface position, and it can also be made small around it, so that the blur A in each of the above equations can be reduced. can be made smaller.

The second part of the electron beam apparatus is equipped with an objective lens, an intermediate lens, and a projection lens, and an image of the sample is formed by the objective lens in an excited state, and a high-magnification observation image is produced by the intermediate lens and the projection lens. In an electron beam device that forms an image, an electron lens is provided behind the objective lens, and the excitation of the objective lens, which is maintained at a strong excitation level during high-magnification observation, is adjusted so that the image of the sample produced by the objective lens follows the objective lens. Once the image is formed near the main surface of the electron lens,
Furthermore, the excitation state of the electron lens is adjusted so that the electron beam flux emitted from the electron lens is focused and the electron beam flux near the optical axis is incident on the subsequent intermediate lens to perform low magnification observation. It is.

With this configuration, one end of the electron beam is focused on the main surface of the electron lens, so the value of r〓 becomes 0 at this main surface position, and the values around it are also small, making the size of blur A small. be able to.

Furthermore, the excitation state of this electron lens is such that the electron beam flux emitted from this electron lens is incident on the subsequent intermediate lens, and in this lens as well, the electron beam does not leave the optical axis as much as possible, and the above r〓, r〓 Blur A by keeping the value as small as possible
The size of is assumed to be small.

The third electron beam device is equipped with an objective lens, a selected area diaphragm, an intermediate lens, and a projection lens in this order, and forms an image of the sample with the objective lens.
In an electron beam device that forms a high-magnification observation image using an intermediate lens and a projection lens, the objective lens is kept in a highly excited state during high-magnification observation, and electrons are placed between the objective lens and the selected area diaphragm. A lens is arranged, and the objective lens is excited so that an image plane formed only by the objective lens and an image plane formed by the objective lens and the electron lens are formed between the main surface of the electron lens and the selected area aperture surface. This is adjusted to allow low magnification observation.

As for the third electron beam device,
The electron lens may be placed inside the objective lens yoke.

With this configuration, the electron beam is directed near the main surface of the electron lens, that is, the image plane formed only by the objective lens and the image plane formed by the objective lens and the electron lens are connected to the main surface of the electron lens and the limited area diaphragm surface. The value of r〓 becomes 0 at this main surface position, and the value of the integral can be made small, and the size of the blur A can be made small.

With these various electron beam devices, off-axis astigmatism and the like can be significantly reduced, and even clearer observed images can be obtained.

Hereinafter, the present invention will be described in detail based on embodiments shown in the accompanying drawings.

FIG. 3 is a diagram showing an electron beam apparatus according to the first aspect of the present invention. In this electron beam device,
It has a lens configuration similar to that of the conventional electron microscope shown in FIG. 5 and the second intermediate lens 15 to form a real image 17a on the object plane of the projection lens 6, and then the projection lens 6 forms a low magnification observation image 10 on the screen 9. In the excitation adjustment of each lens in this imaging process, the electron beam 3b emitted from the sample 1 parallel to the optical axis 2 is focused on the back focal point 18a of the objective lens 4 and further diverged, and then the second intermediate lens 15 The excitation of the electron lens 14 and the first intermediate lens 5 is mainly adjusted so that the light is focused again near the main surface (position 19).

According to such an electron beam apparatus, after the electron beam 3b emitted from the sample 1 is focused and diverged at the back focal point 18a of the objective lens 4, it is focused again at the second intermediate lens 15.
Since r〓 in the second intermediate lens 15 is approximately zero (see equation (2)) by focusing near the main surface of the lens, off-axis astigmatism in the second intermediate lens 15,
The curvature aberration becomes extremely small, and the blur of the observation image 10 formed on the screen 9 becomes extremely small compared to the electron beam apparatus shown in FIG.

Note that mainly due to the excitation adjustment of the electronic lens 14,
If the electron beam bundle 3b diverging from the back focal point 18a of the objective lens 4 is focused near the main surface of the first intermediate lens 5, the aberration of the first intermediate lens 5 can be reduced as described above.

FIG. 4 is an explanatory diagram showing an electron beam apparatus according to the second invention. In this electron beam device, an electron beam 3a emitted from a sample 1 is focused as a real image 16b near the main surface of an electron lens 14 by adjusting the excitation of an objective lens 4, and is further focused on a first intermediate lens 5.
A real image 17b is formed on the object surface of the projection lens 6 by the second intermediate lens 15, and then a low magnification observation image 1 is formed on the screen 9 by the projection lens 6.
0 is imaged. By adjusting the excitation of the objective lens 4 in this manner, a real image 1 is generated near the main surface of the electron lens 14.
In an electron beam device that obtains the observed image 10 after forming an image of the electron beam 3a, the electron beam 3a emitted from the sample 1 on the optical axis 2
is passed on the optical axis 2 near the main surface of the electron lens 14, and r〓 in the electron lens 14 is made approximately zero (see equation (2)), so the off-axis astigmatism in the electron lens 14 Aberrations and curvature aberrations are extremely small.
Since the objective lens has a relatively short focus in terms of high resolution, in normal use conditions, the electron beam 3 emitted from the sample is subjected to large off-axis astigmatism and curvature aberration at the intermediate lens 5 placed behind the objective lens. However, in the electron beam device of the present invention, since the real image 16b is formed near the main surface of the electron lens 14, the spread of the electron beam bundle 3 in the intermediate lens 5 is reduced, compared to the conventional electron beam device. The blur of the observed image 10 formed on the screen 9 is reduced.

Although the electron lens 14 hardly contributes to image formation, it is installed at a position immediately behind the objective lens 4, and directs the trajectory of the electron beam 3b diverged from the back focal point 18a of the objective lens 4 in the direction of the optical axis 2. Since the aberration in the first intermediate lens 5 is prevented from increasing by refracting it and reducing the r〓 of the equation (2) in the first intermediate lens 5, it is possible to prevent the aberration in the first intermediate lens 5 from becoming large. This is more effective in reducing the blur compared to the case where the image is blurred.

Furthermore, FIG. 5 is an explanatory diagram showing an electron beam apparatus according to the third invention. This electron beam device includes an objective lens 4, a first intermediate lens 5, and a second intermediate lens 1.
5, a projection lens 6, and a selected area diaphragm 11 between the objective lens 4 and the first intermediate lens 5. electronic lens 1
The lens has a structure in which 2 are arranged. As shown in FIG. 6, the actual electronic lens 12 is installed inside the objective lens yoke 4b between the magnetic pole gap 4a of the objective lens 4 and the selected area diaphragm 11 provided at the rear stage of the objective lens 4. For example, in a conventional electron microscope with an accelerating voltage of 100 to 200 KV, the distance between the magnetic pole gap 4a of the objective lens 4 and the selected area diaphragm 11 is approximately 100 to 150 mm. Lens 12 is placed relatively close to objective lens 4.

Here, the electron beam 3 emitted from the sample 1 is formed into a real image 16c by the objective lens 4 and the electron lens 12, and further formed into a real image 17c by the first intermediate lens 5 and the second intermediate lens 15. is formed on the object plane of the projection lens, and then a low magnification observation image 10 is formed on the screen 9 by the projection lens 6. During this imaging process, the excitation of the objective lens 4 is adjusted. An image plane formed only by the objective lens 4 is formed between the main surface of the electron lens 12 and the selected area diaphragm 11. In other words, the electron lens 12 is set to zero excitation, and the sample 1 on the optical axis 2 is
The excitation of the objective lens 4 is adjusted so that the electron beam 3a emitted from the electron beam 3a passes through the optical axis 2 between the electron lens 12 and the selected area diaphragm 11 (indicated by 20 in FIG. 5).

By the way, as mentioned above, when the electron lens 12 is installed near the objective lens 4, the spread of the electron beam 3 at the electron lens 12 is generally reduced, and the electron lens 12 also reduces the spread of the electron beam 3 at the first intermediate lens 5. Since the spread of the electron beam bundle 3 is also reduced, off-axis astigmatism and curvature aberration in the electron lens 12 and the first intermediate lens 5 are reduced.
The state of aberrations in case 2 will be explained.

FIG. 7 shows the major axis d of the elliptical blur caused by off-axis astigmatism and curvature aberration of the electron lens 12 only.
A characteristic graph showing an example of the relationship between the value of (μm) and the position Zi of the image plane when the image is formed only by the objective lens 4 (based on equation (1))
It is. However, the origin of the above image plane position Zi is the center position of the objective lens 4, the observation magnification is 1000x, and the observation point is centered on the optical axis on the image recording plane.
At a point on the circumference of 100 mm, the half-angle of the objective aperture is 1
×10 ^-2 rad. (radian), the focal length of the objective lens 4 is 3 mm, and in the electron lens 12, gap s + hole diameter b = 40 mm.

In FIG. 7, the shaded area indicates the position where the selected area diaphragm 11 is located in a normal electron microscope (a position 100 to 150 mm from the objective lens 4), and the characteristic curve P indicates the position of the electron lens 12 behind the objective lens 4.
500mm (position A), and the characteristic curve Q shows the case where the electron lens 12 is placed behind the objective lens 4.
Each case is shown when placed at 100 mm (position B). For both characteristic curves P and Q, the blur size d is minimized when the image plane position Zi determined only by the objective lens 4 is formed on the principal surface of the electron lens 12 (this indicates the state of the second invention). However, if the image plane position Zi by only the objective lens 4 is formed between the main surface of the electron lens 12 and the selected area aperture 11, the size of the blur caused by aberrations in the electron lens 12 will be at most 10 μm, Considering that the resolution of a film placed on the image recording surface is generally about 20 μm, this is a sufficiently small value.

Figure 8 shows the image plane position Zi using only the objective lens 4.
and the rate of change of excitation in objective lens 4 |ΔJ/
This is a graph showing an example of the relationship between J ₀ |. This characteristic graph assumes that the shape of the objective lens 4 is the gap S + hole diameter b = 14 mm, and J ₀ is the image formed at infinity by the objective lens 4, that is, Zi = When the excitation force is ∞, the relativistically corrected accelerating voltage U〓 is J ₀ /√U
〓 Shows the characteristics obtained with an electron microscope adjusted to a value that satisfies ≒18.

Generally, the magnetomotive force J of the objective lens 4 when the image plane of the objective lens 4 is at the position of Zi is determined as J=J ₀ +ΔJ However, when Zi>0, ΔJ>0 When Zi<0, ΔJ<0 However, the image plane formed only by the objective lens 4 is located between the main surface of the electron lens 12 and the selected area aperture 11 in FIG. 6, for example at position C in FIG.
When forming the objective lens, calculate the excitation change amount ΔJ from this graph as ΔJ = α・J ₀ , then substitute this ΔJ into the first equation, and set the magnetomotive force J such that J = (1 + α) J ₀ . 4 should be adjusted.

This third electron beam device and the first electron beam device are combined, and as shown in FIG. If you focus it near the main surface of
Furthermore, off-axis astigmatism and curvature aberration of the entire lens system can be reduced.

As described above, the first electron beam apparatus of the present invention includes an objective lens, an intermediate lens, and a projection lens, an image of a sample is formed by the objective lens, and the intermediate lens and the projection lens In an electron beam device that forms an observation image, one uses multiple lenses, including an objective lens and an intermediate lens following it, to direct the electron beam diverged from the back focal point of the objective lens to a point immediately in front of the projection lens. The light is once focused near the main surface of the intermediate lens.

Further, according to the second electron beam device, an electron lens is provided behind the objective lens, and the image of the sample by the objective lens is once formed near the main surface of the electron lens following the objective lens, and the electron The electron beam flux emitted from the lens is focused, and the electron beam flux near the optical axis is made to enter the subsequent intermediate lens.

Furthermore, according to the third electron beam device, an objective lens,
In an electron beam apparatus that is equipped with a selected area diaphragm, an intermediate lens, and a projection lens in this order, an image of the sample is formed by the objective lens, and a high-magnification observation image is formed by the intermediate lens and the projection lens. An electronic lens is placed between the aperture and the limited field aperture,
An image plane formed by only the objective lens and an image plane formed by the objective lens and the electron lens are formed between the main surface of the electron lens and the selected area aperture surface.

With the above-described configuration, each invention of the present application maintains the excitation of the objective lens in a strongly excited state during low-magnification observation and during high-magnification observation, and the trajectory of the electron beam in the subsequent lens does not deviate from the optical axis. The off-axis astigmatism and curvature aberration of the lens system can be made smaller to reduce the amount of blur, and a high-quality wide-field image can be obtained at low magnification.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a conventional electron beam apparatus for obtaining a low-magnification wide-field image. FIG. 2 is a diagram showing another example of a conventional electron beam apparatus for obtaining a wide field of view with low magnification. FIG. 3 is a diagram showing an embodiment of the first invention in which the electron beam diverged from the back focal point of the objective lens is focused near the main surface of the intermediate lens. FIG. 4 is a diagram showing an embodiment of the second invention in which the objective lens is imaged near the main surface of the electron lens following the objective lens. FIG. 5 shows a third embodiment of the invention in which the image plane formed only by the objective lens is located between the main surface of the electronic lens provided between the objective lens and the selected area diaphragm surface and the selected area diaphragm surface. It is a figure which shows an example. FIG. 6 is a sectional view showing an example of the electron lens in FIG. 5. Figure 7 shows the major axis d of the elliptical blur caused by off-axis astigmatism and curvature aberration of the electron lens alone in Figure 5, and the image plane position due to the objective lens alone.
It is a graph diagram showing an example of the relationship with Zi. FIG. 8 is a graph diagram showing an example of the relationship between the image plane position Zi due to only the objective lens and the excitation change rate |ΔJ/J ₀ | in the objective lens. 1...Sample, 2...Optical axis (lens axis), 3...
Electron beam flux, 4... Objective lens, 5... Intermediate lens, 6... Projection lens, 9... Screen, 10
... Observation image, 11 ... Selected area aperture, 12, 14
...Electronic lens, 15...Intermediate lens, 18a...
...back focus of the objective lens.

Claims (1)

[Claims] 1. A system comprising an objective lens, an intermediate lens, and a projection lens, in which an image of the sample is formed by the objective lens in an excited state, and a high magnification observation image is formed by the intermediate lens and the projection lens. In an electron beam apparatus, the objective lens is kept in a highly excited state during high-magnification observation, and multiple lenses including the objective lens and an intermediate lens following it are used to capture electrons diverged from the back focus of the objective lens. An electron beam device characterized in that a beam is once focused near the main surface of an intermediate lens immediately in front of a projection lens to perform low magnification observation. 2. In an electron beam device comprising an objective lens, an intermediate lens, and a projection lens, an image of a sample is formed by the objective lens in an excited state, and a high magnification observation image is formed by the intermediate lens and the projection lens, An electron lens is provided behind the objective lens, and the excitation of the objective lens, which is maintained at a strong excitation level during high-magnification observation, is adjusted so that the image of the sample by the objective lens is near the main surface of the electron lens that follows the objective lens. Further, the excitation state of the electron lens is adjusted so that the electron beam flux emitted from the electron lens is focused, and the electron beam flux near the optical axis is incident on the subsequent intermediate lens. An electron beam device characterized by performing magnification observation. 3 An electron beam device that is equipped with an objective lens, a selected area diaphragm, an intermediate lens, and a projection lens in this order, and forms an image of the sample using the objective lens, and forms a high-magnification observation image using the intermediate lens and the projection lens. In this step, the objective lens is kept in a highly excited state during high-magnification observation, an electron lens is placed between the objective lens and the selected area diaphragm, and an image plane formed only by the objective lens and an image plane between the objective lens and the electron lens are arranged. The electron beam is characterized in that the excitation of the objective lens is adjusted so that the image plane formed by the electron lens is formed between the main surface of the electron lens and the selected area diaphragm surface to perform observation at low magnification. Device. 4. The electron beam apparatus according to claim 3, wherein the electron lens is disposed within an objective lens yoke.