JPH0619964B2 - electronic microscope - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electron microscope capable of achieving both high-resolution observation at high magnification and observation of a high-quality wide-field image at low magnification.

[Prior Art] Observation of a biological slice sample is performed using an electron microscope. Observation of such biological section samples is generally between 1000 and 50,0
It is carried out in a magnification range of 00 times, but in order to improve the image quality in this case, the following two requirements must be satisfied.

(1) Increase the contrast.

(2) Reduce blurring due to off-axis chromatic aberration.

First, consider the first request.

The contrast in observing a biological section sample is mainly amplitude contrast, and as shown in FIG. 3, the smaller the opening angle α of the electron beam from the sample, the higher the contrast. However, the third
In the figure, 1 is an objective lens, 2 is a sample, 3 is an objective diaphragm, 4 is an image of the sample 2 by the objective lens 1, and 5 is an electron beam. Since it can be considered that the position of the objective diaphragm 3 is substantially equal to the position of the objective lens 1, if the focal length of the objective lens 1 is f ₀ , the following relationship is established between α and the hole diameter Dap of the objective diaphragm 3. To do.

αtan α = Dap / 2f ₀ (1) From the above formula, it is necessary to increase the focal length of the objective lens 1 or reduce the hole diameter of the objective diaphragm 3 in order to increase the contrast. .

However, as will be described later, the focal length f ₀ of the objective lens 1
When is increased, the axial difference, that is, the spherical aberration coefficient Cs and the axial aberration coefficient Cc are increased, so that the focal length f ₀
Can not be so large. Therefore, in order to increase the contrast of the low-magnification image, it is necessary to make the aperture diameter Dap of the objective diaphragm 3 quite small.

Now consider the second request.

Assuming that the off-axis chromatic aberration coefficient is Cmr, U is the acceleration voltage of the electron beam, ΔU is the energy loss of the electron beam due to the sample, and R is the distance from the center on the film surface, the amount of image blur Dcmr on the film surface is It is expressed by the following formula.

Dcmr = Cmr · (ΔU / U) · R (2) On the other hand, as shown in FIG. 4, the distance between the objective lens 1 and the point where the electron beam irradiating the sample 2 intersects the optical axis C is δ, It is known that Cmr is expressed as follows, where Z _{0 is} the distance between the sample 2 and the objective lens 1 and Z ₁ is the distance between the objective lens 1 and the image 4 formed by the objective lens 1.

Cmr (1 / Z ₀ -1 / Z ₁ ) / (1 / δ-1 / Z ₀ ) ... (3) Therefore, in order to minimize the off-axis chromatic aberration,
It suffices that 8 is infinity, which is desirable to irradiate the sample with an electron beam from an infinite point, in other words, an electron beam with good parallelism.

On the other hand, in an electron microscope, in order to increase the resolution when performing high-magnification observation, it is necessary to reduce the spherical aberration coefficient Cs and the axial chromatic aberration coefficient Cc of the objective lens, which requires the focal length f ₀ of the objective lens. It needs to be small. Further, when an appropriate forward magnetic field is used, the effect of reducing the Cs and Cc can be obtained, so that an optical diagram near the objective lens in the conventional electron microscope is as shown in FIG.

In FIG. 5, reference numeral 1 is an objective lens, and 1a and 1b are front and rear magnetic field lenses near the objective lens, respectively. In FIG. 5, the same components as those in FIGS. 3 and 4 are designated by the same reference numerals. Has been done. As is clear from FIG. 5,
For the above-mentioned reason, the focal length f ₀ of the objective lens 1 is extremely short, so high resolution can be obtained at high magnification, but the position P at which the electron beam 5 crosses over is the objective aperture 3.
Since it is considerably ahead of the position of, the electron beam 5 is partially cut. Such cutting of the electron beam 5 does not pose a problem during high-magnification observation, but it narrows the visual field during low-magnification observation, making it impossible to observe a wide visual field.
Of course, if the objective diaphragm 3 can be arranged at the position P, such a problem does not occur.
When the sample 2 is brought closer to, the inclination angle of the sample 2 is limited, so that it is necessary to separate the objective aperture 3 and the sample 2 by at least the distance shown in FIG.

[Problems to be Solved by the Invention] Therefore, conventionally, although high resolution observation can be performed at high magnification, when a high quality image is observed at low magnification, only an image with a narrow visual field can be observed. There wasn't.

It is an object of the present invention to solve such conventional drawbacks and to provide an electron microscope capable of observing a high-resolution image at a high magnification and a high-quality wide-field image at a low magnification.

[Means for Solving Problems] Therefore, the present invention provides an irradiation lens system, an objective lens having a strong front magnetic field lens, and a sample arranged between the front magnetic field lens and the rear magnetic field lens of the objective lens. In an electron microscope equipped with an objective diaphragm and an imaging lens system arranged in the latter stage of the objective lens, an electron beam incident substantially parallel to the front magnetic field lens of the objective lens at the time of high-magnification observation has a rear magnetic field lens of the objective lens. And an objective diaphragm, a crossover is formed between the objective lens and the objective lens, and an electron beam that is once crossed just before entering the front magnetic field lens of the objective lens at the time of low-magnification observation is converted into a substantially parallel electron beam sample by the objective lens. So that a crossover is formed at the position of the objective aperture after entering the objective lens, the excitation intensity of the irradiation lens system changes the excitation state of the objective lens to strong excitation. It is characterized by an electron microscope configured so that it can be changed in conjunction with the switching of the observation magnification while maintaining it.

EXAMPLES Examples of the present invention will be described in detail below with reference to the drawings.

In FIG. 1 showing an embodiment of the present invention, reference numeral 6 in the figure denotes an electron gun, and an electron beam 5 from the electron gun 6 is a first, a second, and a third.
After being focused on the focusing lenses 7, 8 and 9 of
It is incident on the sample 2 through the front magnetic field lens 1a. The electron beam 5 that has passed through the sample 2 passes through the rear magnetic field lens 1b of the objective lens 1 and the aperture 3a of the objective diaphragm 3 and then from the first, second, third intermediate lenses 10, 11, 12 and the projection lens 13. It is incident on the formed imaging lens system 14. as a result,
An electron microscope image of the sample 2 is formed on the fluorescent plate 15.
Reference numeral 16 denotes an exciting power source for the third focusing lens 9,
Is an imaging lens system excitation power supply for supplying an excitation current to each of the first, second, and third intermediate lenses 10, 11, and 12. Reference numeral 18 denotes an arithmetic and control unit, and the arithmetic and control unit 1
A storage device 19 is connected to the storage device 8. The storage device 19 stores, as a table, data for designating the exciting current values sent from the imaging lens system exciting power source 17 to the respective lenses 10, 11 and 12 corresponding to the respective observation magnification values.
Further, the storage device 19 stores, as a table, data for designating an exciting current sent from the focusing lens power source 16 to the third focusing lens 9 in correspondence with each observation magnification value. Reference numeral 20 is an operation console connected to the arithmetic and control unit 18 for instructing an observation magnification.

In such a configuration, when the operator gives an instruction to perform observation at a high magnification, for example, a magnification M1 on the console 20, the console 20
Based on the instruction signal from the above, the arithmetic and control unit 18 reads the exciting current data corresponding to the magnification M1 from the exciting current value data table of the third focusing lens 9 stored in the storage unit 19, DA-converts it, and then focuses it. Send to the lens power supply 16. At the same time, based on a signal from the console 20 that indicates the observation magnification M1, the arithmetic and control unit 18 causes the storage unit 1 to operate.
The first, second, and third intermediate lenses 1 stored in 9
Data corresponding to the magnification M1 is read out from the exciting current value data of 0, 11, and 12, and the imaging lens system exciting power supply 17 is read.
Send to. As a result, the second focusing lens 9 is not or weakly excited as shown in FIG.
At, the electron beam is incident on the front magnetic field lens 1a of the objective lens 1 without being focused. The electron beam 5 that has entered the front magnetic field lens 1a is further focused by the front magnetic field lens 1a, then enters the sample 2, and the electron beam 5 scattered from the sample 2 is further focused by the rear magnetic field lens 1b of the objective lens 1. It After the electron beam passing through the rear magnetic field lens 1b crosses over considerably before the objective aperture 3,
The light passes through the diaphragm 3 and further enters the imaging lens system 14 in the subsequent stage. As is clear from the optical diagram of FIG. 2 (a),
Since the focal length f ₀ of the objective lens 1 is set to a sufficiently short value, the axial chromatic aberration coefficient Cc, the spherical aberration coefficient Cs, etc. have small values, and a high-resolution, high-magnification image can be observed.

Next, using the operation console 20, the observation magnification is low, for example, M2.
Is issued, the arithmetic and control unit 18 causes the storage unit 19 to
The exciting current value data corresponding to the magnification M2 is read out from the exciting current value data of the third focusing lens 9 stored in. The read excitation current value data is AD-converted and then sent to the excitation power supply 16. At the same time, the arithmetic and control unit 18 at the same time, based on the signal from the console 20 for instructing the magnification M2, the arithmetic control unit 18 stores the first, second, and third intermediate lenses 10, 11, Of the 12 exciting current value data, the data corresponding to the magnification M2 is read out and sent to the imaging lens system exciting power supply 17. As a result, the third focusing lens 9 is more strongly excited than during high-magnification observation, so that the electron beam 5 once passes between the third focusing lens 9 and the front magnetic field lens 1a as shown in FIG. 2 (b). After crossing over, the light enters the front magnetic field lens 1a. The electron beam 5 that has entered the front magnetic field lens 1a is further focused by the lens 1a and enters the sample 2 as a substantially parallel electron beam bundle. The electron beam 5 that has passed through the sample 2 enters the rear magnetic field lens 1b and the lens 1b. As a result, it just crosses over at the opening 3a of the objective aperture 3 and passes through the objective aperture 3. As described above, the focal length f ₀ of the objective lens is set to be extremely short to enable observation of a high resolution image at a high magnification, and the diameter Dap of the objective diaphragm 3 is set to a small value to reduce a high contrast low magnification. Although the image is obtained, the electron beam is hardly cut by the objective diaphragm, so that an image with a wide field of view can be observed. Further, at this time, since the sample 2 is irradiated with the electron beam flux with good parallelism, the off-axis chromatic aberration coefficient becomes a small value, and an image with less blur can be observed.

The above-described embodiment is only one embodiment of the present invention, and many variations can be considered.

For example, in the above-described embodiment, the exciting current of the third focusing lens is specified by reading the exciting current value table stored in the storage device, but based on the exciting current value signal obtained by calculation. You may specify by specifying.

[Effects of the Invention] As is apparent from the above description, in the electron microscope based on the present invention, the electron beam incident substantially parallel to the front magnetic field lens of the objective lens at the time of high-magnification observation becomes the rear magnetic field lens of the objective lens. An electron beam is connected to a position between the objective diaphragm and a crossover at the time of low-magnification observation immediately before entering the front magnetic field lens of the objective lens. After the incident light, the excitation intensity of the irradiation lens system can be changed in association with the switching of the observation magnification while maintaining the excitation state of the objective lens in the strong excitation state so that a crossover is formed at the position of the objective diaphragm. Since it is configured, it can be used under the condition that this axial chromatic aberration is small at the time of high magnification observation where the influence of the axial chromatic aberration of the objective lens is dominant. In low-magnification observation in which the effect of off-axis chromatic aberration is dominant, the off-axis chromatic aberration can be used under a small condition. Therefore, according to the electron microscope based on the present invention, not only a high-resolution high-magnification image can be observed, but also the image quality of a low-magnification image can be improved while maintaining a wide field of view.

[Brief description of drawings]

FIG. 1 is a diagram showing one embodiment of the present invention, FIG. 2 is an optical diagram for explaining the operation of the above-mentioned one embodiment device, and FIG. 3 shows the conditions required for the hole diameter of the objective aperture. FIG. 4 is a diagram for explaining the relationship between the off-axis chromatic aberration coefficient and the electron beam irradiating the sample, and FIG. 5 is a diagram for explaining the defects of the conventional apparatus. 1: Objective lens 1a: Front magnetic field lens of objective lens 1b: Rear magnetic field lens of objective lens 2: Sample 3: Objective diaphragm 4: Image of sample by objective lens 5: Electron beam, 6: Electron gun 7, 8, 9 : Focusing lens 10, 11, 12: Intermediate lens 13: Projection lens, 14: Imaging lens system 15: Fluorescent plate, 16: Focusing lens power supply 17: Imaging lens system power supply 18: Arithmetic control device 19: Storage device, 20: Console

Claims (1)

[Claims] 1. An irradiation lens system, an objective lens having a strong front magnetic field lens, a sample arranged between the front magnetic field lens and the rear magnetic field lens of the objective lens, and an objective diaphragm.
In an electron microscope equipped with an imaging lens system arranged at the rear end of the objective lens, an electron beam incident substantially parallel to the front magnetic field lens of the objective lens at the time of high-magnification observation is connected to the rear magnetic field lens of the objective lens and the objective diaphragm. Between the positions of the crossover, and immediately before entering the front magnetic field lens of the objective lens at the time of low-magnification observation, after the electron beam once crossing the beam enters the sample as a substantially parallel electron beam bundle by the objective lens. It is configured such that the excitation intensity of the irradiation lens system can be changed in association with the switching of the observation magnification while maintaining the excitation state of the objective lens in the strong excitation state so that a crossover is formed at the position of the objective diaphragm. An electron microscope.