JPS58176858A - Magnifying lens system of electron microscope - Google Patents

Abstract

PURPOSE:To allow the image to rotate with no change of magnifying power by setting an objective lens and a projection lens at a stationary state, setting two- stage intermediate lenses arranged between them at energizing states opposite to each other, and controlling the energizing strength at a specific level. CONSTITUTION:An objective lens 2 and a projection lens 3 are provided between a sample 1 and a screen 11, and two intermediate lenses 4, 5 are arranged between the lenses 2, 3 to form a four-stage lens system. Power supplies 6-9 of individual lenses are controlled by means of a control unit 10 such as a computer or the like, so that the lenses 2, 3 are fixed at a strong energizing state, the intermediate lenses 4, 5 are set at energizing states opposite to each other and the sum of their energizing strengthes is in proportion to the rotating angle of the image and the difference has a relation approximate to a quadratic curve relative to the rotating angle of the image. Therefore, the image can be rotated at low aberration with no change of magnifying power by using a conventional lens system.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnifying lens system for an electron microscope that can rotate an image without adding a special image rotation lens, without changing magnification, and with low aberrations.

Several lens systems have been devised to perform image rotation, but most of them add special lenses for image rotation, which is a fairly labor-intensive process that involves devising magnetic pole structures and excitation methods. It is something to do. However, although performing image rotation is a requirement in image observation using an electron microscope, it does not lead to new knowledge, and therefore it is necessary to add a particularly complex lens to perform image rotation. There is no point in forcing them to do so. Rather, it is better to use the existing lens as is without adding an additional lens and rotate the image by controlling the electric circuit, which will not have a negative impact on the electron optical system and is economically advantageous, and has a high possibility of realization. That's why.

Now, the rotation angle θ in an electron microscope is θ= (e /8m Vr ) V2f:,' Bz
It is given in dz. Here, e is the electric charge of the electron, m is the rest mass of the electron, Vr is the acceleration voltage of the electron after relative correction, and Bl is the axial magnetic field strength. In this formula, (e
/8RI Vr ) is constant, therefore θ is B
It can be seen that it is determined by the integral value from z20 (sample surface) to zi (image surface). Here, the integral value of 8Z from ZO to 21 is equal to the total excitation (ampere turns (Nl)) of the magnetic field distribution from the sample to the image plane below, and therefore, if this Nl is changed, image rotation will occur. become. However, normally, changing Nl naturally changes the magnification, so if the sole purpose is image rotation, a measure to prevent this change in magnification is required. Now, considering two lenses, if the direction of excitation of these lenses is reversed (therefore, the directions of image rotation are opposite to each other), the excitation NI+ of the first lens will be increased (or decreased), and the excitation of the second lens will be increased (or decreased). If the excitation NI2 of the lens is decreased (or increased), the difference between NI+ and NI2 increases or decreases, but there should be a condition that the magnification does not change.

Therefore, as shown in Fig. 1, the excitation intensity of the first lens is shown as a straight line A, and the excitation intensity of the second lens is shown as a straight line B. ) is changed under conditions such as C that is constant, image rotation can be applied without any problem, but it has been found that a change in magnification is accompanied.

Therefore, an object of the present invention is to provide a novel magnifying lens system that can impart image rotation to existing lens systems without changing the magnification and without increasing aberrations. A lens system having a four-stage or more structure including an objective lens, a projection lens, and at least two intermediate lenses disposed between the two, the objective lens and the projection lens are fixed, and the intermediate lens between the two stages is The lenses are mutually reverse excited, and the sum of the excitation intensities (difference in absolute values) of both lenses has a proportional relationship with respect to the rotation angle of the image, and the difference (sum of absolute values) or the average value is A magnifying lens system for an electron microscope is characterized by a relationship that approximates a quadratic curve with respect to rotation angle.

The inventor conducted detailed computer experiments using a four-stage lens system as shown in FIG. 2 to determine under what conditions image rotation can be performed without a change in magnification. In FIG. 2, 1 is a sample, 2 is an objective lens, and 3 is a projection lens. Two intermediate lenses 4 and 5 are placed between the objective lens and the projection lens. 6.7 and 8.9 are power supplies for each lens, and a control device 1o such as a computer.
Each supply current value is controlled by 11 is a screen for displaying images.

In such a lens system, the lenses are divided into three groups: the objective lens 2, two intermediate lenses 4 and 5, and one projection lens 3, and the objective lens 2 and the projection lens 3 are fixed in a strongly excited state ( image rotation angle when the two intermediate lenses are mutually reverse excited (including slight variations due to focus adjustment, etc.) and the excitation currents IL+ and IL2 are minutely changed (therefore, in high-magnification observation mode). When we calculated the magnification, we obtained the curve shown in Figure 3. In the figure, the horizontal axis is the image rotation angle θ,
The vertical axis is the excitation intensity J (ampere turns) of the intermediate lens 4.5, the curve A group represents the excitation change of the lens 4, and the curve B
The group shows the excitation change of the second lens 5, A1 and 81 are 5
000 times, A2 and 82 are 10,000 times, A3 and B3 are 2
0000 times, and A4 and B4 are 50000 times.

The intersection points a, b, c, and d of the multiple pairs of curves are the cases where the excitation intensities of both lenses are equal. Incidentally, the projection lens excitation intensity Jpl when obtaining this data was 3500 ampere turns. When this Jl)l is changed, the intersection points a, b, c, d of each curve, that is, the rotation angle θ when the excitation strength of the first lens and the second lens become equal, are different, and the curve groups A and B The form is similar to that shown in Figure 3.

A careful study of Figure 3 revealed the following.

<1) Many pairs of curves (A1 and Bl, A2 and 82...
・The slope of the curve is larger on the excitation side than the intersections a, b, c, and d of ), and the slope is small on the weak excitation side, and the sum of the excitation intensity JIL+ of the first lens and the excitation intensity JTL2 of the second lens, In other words, the difference in absolute values lJ[L+ l 1JIL2
1 changes proportionally to the image rotation angle θ as shown in FIG.
Moreover, as a matter of course, the straight line is constant regardless of the magnification.

(2> The difference between the excitation intensity JIL+ of the first lens and the excitation intensity JIL2 of the second lens, that is, the sum of absolute values, or its average value (lJIL+ l+1JIL2+)/2, is the image rotation angle as shown in Figure 5. Changes approximate to a quadratic curve with respect to θ. In the same figure, 5000 times, D2 is 1ooo
o times, DS is 20,000 times, and D4 is 50,000 times.

Taking all the above into account, in order to obtain the desired image rotation angle at a given magnification, the difference in the absolute value of the excitation strength between the first lens and the second lens must be along the straight line shown in Figure 4, and the excitation of both lenses must be It is sufficient if the sum or average value of absolute values of intensity is given along the curve shown in FIG.

These conditions hold true regardless of what shape the intermediate lens is used.

However, when the image is rotated as described above, there is a problem in that various aberrations increase accordingly. Figure 6 shows the changes in the lateral chromatic aberration Cm (-) and distortion aberration Ds (%) with respect to image rotation at 1ooooo times the excitation intensity Jl)l of the projection lens.
This is shown using parameters. Cut 1 in the diagram
．． Ds 1 is C12, D when Jpl=3200A
S2 is the case where Jl)l=23008T. As can be seen from the figure, the change in lateral chromatic aberration Cl11 is small, and Jl
) The higher l tends to be, the smaller it becomes, and it can be seen that when the projection lens 3 is used with strong excitation as in the present invention, no problem occurs over a wide rotation angle range. On the other hand, regarding distortion aberration DS as well, the higher Jl)l is, the better the results are. However, this [)S increases rapidly at θL; 120°. Therefore, the possible image rotation angle θ is this D
It will be determined by S. However, on the large side of θ, D
Since there is no increase in s, there is no restriction in this direction. but,
The direction in which θ increases is the direction in which the excitation increases, so
It cannot be made extremely large. After all, θ can be changed from about 120 degrees to about 220 degrees without any problem.

As explained above, in the present invention, in a four-stage or more-stage imaging lens system, the objective lens and the projection lens are fixed, the two intermediate lenses are reversely excited, and the excitation intensity (absolute value) is The difference between the two lenses is changed linearly to give image rotation, and in order to prevent a change in magnification at that time, the average value of the excitation intensity (absolute value) of both lenses is quadratic with respect to the image rotation angle θ. Since the image is not changed so as to approximate a curve, it is possible to arbitrarily rotate the image using an existing lens system without adding any lenses and without changing the magnification.

Furthermore, unless the rotation angle is made that large, aberrations, especially chromatic aberration of magnification and distortion aberration, do not increase, thus ensuring high-resolution image observation.

Note that the above is an example of the present invention, and it goes without saying that the data may vary depending on the dimensions and shape of the lens used, how other lenses are excited, etc. but,
Even in this case, the trends shown in FIGS. 3 to 5 remain unchanged. Further, the present invention only needs to have at least two stages of intermediate lenses (irrespective of the name) between the objective lens, the projection lens, and the present invention, and therefore it is of course applicable to a lens system having five stages or more as a whole.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the generally considered variable excitation of a lens.
FIG. 2 is a diagram showing an example of a lens system used in the present invention, FIGS. 3 to 5 are diagrams for explaining the present invention, and FIG. 6 is a diagram showing aberrations in the lens system of the present invention. 1: Sample, 2: Objective lens, 3: Projection lens, 4, 5: Intermediate lens, 6.7.8.9: Power supply,
10: Control device, 11 Niscreen. Patent applicant JEOL Ltd. Representative Tadao Kase Figure 1 Section □ 1゜1゛ method l. r one

Claims (1)

[Claims] A lens system having a four-stage or more structure including an objective lens, a projection lens, and at least two intermediate lenses arranged between the two, the objective lens and the projection lens being in a fixed state,
The two stages of intermediate lenses are mutually reversely excited, and the sum of the excitation intensities (difference in absolute values) of both lenses has a proportional relationship with respect to the rotation angle of the image, and the difference (sum of absolute values) or its A magnifying lens system for an electron microscope, characterized in that the average value has a relationship approximating a quadratic curve with respect to the rotation angle of an image.