JPH04104445A - Automatic focusing device in electron microscope - Google Patents

Abstract

PURPOSE:To perform highly accurate automatic focusing by providing a means for removing a stop of an objective from an optical axis during period where an excitation current of the objective is changed. CONSTITUTION:When an excitation current of excitation power supply 3 is appointed by a control circuit 11, a circuit 11 sends a control signal to a stop driving mechanism 5 to return a stop 4 on an optical axis C. As a result, excitation is performed on the basis of the excitation of a coil 2 at an objective optimal excitation current value OUF. When an excitation current value of the objective is variously changed in a state where the stop 4 is arranged on the optical axis C, signal values representing contrast are changed as shown in the attached figure. Namely, the difference between the degrees of contrast at the time of accurate contrast and non-accurate-contrast becomes greater in the case that the stop 4 is removed than in the case that it is arranged. Accordingly, when output signals of an operation circuit 10 are mutually compared by a control circuit 11, an excitation current value having the minimum contrast can be found out with good accuracy so that highly accurate focusing can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION [+Field of Application] The present invention relates to an automatic focusing device in an electron microscope.

[Prior Art] Various types of automatic focusing devices for electron microscopes have been proposed. Among such automatic focusing devices, the excitation current value of the objective lens is changed, and f! 1'1 electron microscope images were obtained, and 1° of each obtained electron microscope image was processed to determine the contrast? 1J1 By finding the excitation current value of the objective lens that becomes the focal point based on this evaluation ('4 ``5
(: There is a device that focuses 4 and 7 times.

[Problems to be Solved by the Invention] In the above-described automatic focusing device in the conventional electron microscope, the difference in the value of the evaluation signal between the in-focus state and the out-of-focus state is small; It was not possible to focus with high precision.

The object of the present invention is to solve these conventional drawbacks and to provide an automatic focusing device for an electron microscope that performs automatic focusing with high precision.

[Means for Solving the Problem] Therefore, the present invention changes the excitation current value of the objective lens, and obtains an image of one electron microscope image under each excitation current value (
In an automatic focusing device for an electron microscope, the automatic focusing device processes the .
The present invention is characterized in that it includes means for moving the aperture of the objective lens off the optical axis during a period when the excitation current of the objective lens is being changed.

[Example] Embodiments of the present invention will be described in detail below based on the drawings.

FIG. 1 is for showing one embodiment of the present invention. In the figure, 1 is a sample, 2 is an excitation coil for the objective lens, and 3 is an objective lens power source. Reference numeral 4 denotes an objective diaphragm, and the objective diaphragm 4 can be inserted and removed from the optical axis C by a diaphragm drive mechanism 5.

The objective aperture 4 is placed near the position where a diffraction spot is formed by the electron beam from the sample, and prevents the electron beam that has been largely diffracted by the sample from contributing to the formation of an electron microscope image. This is to provide contrast. 6 is an intermediate lens, 7
is a projection lens. 8 is an imaging device;
The output signal of is sent to the memory 9.

]0 is the contrast based on the signal sent from the memory 9. Arithmetic circuit] The output signal from 0 is sent to the jli control circuit 11. A control signal from the control circuit 1 is sent to the objective lens power source 3 and also to the aperture drive mechanism 5. The output signal of the control circuit] is also sent to the memory 9. ]2 is a start circuit for instructing the start of automatic focusing.

In this configuration, the start circuit 12 generates a start pulse as shown in FIG. 2(a). Based on this start pulse (j ) and remove it from the optical axis for the period shown in ).

Next, the control circuit] sends a control signal to the objective lens power supply 3, and controls the current supplied to the coil 2 in steps in predetermined increments around the excitation current value at that time, as shown in FIG. 2(d). change to In this way, by avoiding a step change in the excitation state of the objective lens, ・I'-1 allows different electron microscopes (Image 1 1. I 2, I 3, ・I 11
or can be obtained. These electron microscope images are captured by the imaging device 12 as image signals S(11), S(12), S(+3), .
. . , is converted into S(In) and stored in the memory 0 under the control of the control circuit 1. The control circuit 11 is connected to the memory 9
Each image signal S(11), S(+2), S(
+3), . . . , S(In) are sent to the arithmetic circuit 10. Arithmetic circuit] 0 is the sent image signal S(11),
After differentiating S(+2), S(13), . , +3,...
, In contrast CI, C2. C3,... Calculate Cn. By the way, when the objective aperture is removed, the relationship between the contrast level and the excitation current value is as shown by the solid line in FIG. In FIG. 3, the vertical axis represents the contrast level expressed by the signal value obtained by the arithmetic circuit 10, etc., and the horizontal axis represents the deviation Δf from the positive focus or the excitation current value of the objective lens. ing. The control circuit 11 compares the signal values CI, C2, C3, -, Cn with each other, finds the minimum among them, and uses this minimum signal value. The objective lens excitation current value obtained is determined to be the excitation current value corresponding to the positive focal point. Next, the control circuit 11 subtracts a constant value from the signal specifying the excitation current value corresponding to this positive focus, and specifies the excitation current of the objective lens power source 3 based on the subtracted excitation specification signal.

The series of operations from capturing the image signal into the memory 9 to specifying the excitation current of the objective lens power source 3 is shown in Figure 2 (C).
It will be held during the period shown in .

When the excitation current of the excitation power source 3 is specified by the control circuit 1 in this manner, the control circuit 1 sends a control signal to the diaphragm drive mechanism 3 to return the diaphragm 4 to the optical axis C. As a result, based on the excitation of the coil 2, the objective lens is excited with a constant excitation amount and the optimum excitation current value OUF on the underfocus side, and a high-quality image with a uniform contrast and resolution (normally, the two tend to contradict each other) is excited. An image is displayed on the display screen.

Here, when the excitation current value of the objective lens is variously changed with the objective diaphragm 4 placed on the optical axis Cl, the contrast does not show 1 and the total bow value becomes j j Z as shown by the dotted line in Fig. 3. ,
. As is clear from this graph 7, the control circuit 11 changes the output (n, number) of the operation circuit 0 to When comparing with
Automatic focusing of sieve viscosity or +IJ function.

In addition, 1. Furthermore, the embodiments described above can be replaced with, modified from, and implemented as one embodiment of the present invention.

For example, in a series of embodiments, after calculating the contrast, the wave crystal value of the positive signal portion of the differential signal was integrated, or the peak of the signal was simply
Evaluate with l~. Alternatively, the average amplitude of No. 414 may be determined and set as 1,·do value (r; :,).

In addition, in the examples -L Tsuji]~, the evaluation was 4B ``j
For example, we can express the size of the knee trust (, 4 is formed by f'1), or, for example, we can express the size of the resolution, that is, the image (x3 '') Indicates the sharpness of the fall (the knee may be obtained by crushing.

In this way (7), the contrast τF point does not represent the resolution, but the resolution is 1-; This is common to the above embodiment in that the signal representing iI changes more greatly between the focused state and the non-focused state.

Furthermore, in the two consecutive embodiments, the excitation current of the objective object 1 is increased from 1 to 2 in a stepwise manner, images of each excitation state are stored in the memory, and the beliefs of these images are I decided to compare the sizes of the con I and last of the 6 images based on the death, or the excitation current could be changed from the points on both sides of the positive focus to the center side to the intersecting LI, or It may be changed so that it decreases every week.

Alternatively, without sweeping the excitation current over a certain area, when the contrast decreases or stops, the change in the excitation current may be stopped to look at the positive focus (=moon).

[Effects of the invention] As is clear from the above description, the effects of the invention are as follows.
'1L'' According to the l'l dynamic focusing device in the microscope, the evaluation results at both the
1. The value of the evaluation signal in the out-of-focus state is significantly different.
:) Since it is possible to adjust the focus automatically, automatic focusing of ia 'Ri degrees is possible.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention; FIG. 2 is a diagram for explaining the dynamic f+ of the embodiment of the device shown in FIG. 1; negative '5'. Figure 3 is a detailed illustration of the present invention. 1 Sample? Objective L/nose excitation coil 3 Objective L/nose power supply, 4 λ/j objective diaphragm 5 Aperture drive mechanism 6: Intermediate l/nose 71, range IL>
8: Photographing device 0, memory 10 arithmetic circuit 11 control circuit 12/start circuit application no.

Claims (1)

[Claims] The excitation current value of the objective lens is changed, the image signal of the electron microscope image obtained under each excitation current value is processed to obtain an evaluation signal indicating the degree of focus, and focusing is performed based on this evaluation signal. An automatic focusing device for an electron microscope, characterized in that the automatic focusing device for an electron microscope is provided with means for removing the aperture of the objective lens from the optical axis during a period when the excitation current of the objective lens is being changed. Device.