JPH03250544A - Automatic focusing system for electron microscope - Google Patents

Abstract

PURPOSE:To simply determine correct focal point without misperception when auto-focusing motion is made by the image wobbler system, by varying the inclination angle of an electron beam according to the width of variation in the exciting current of an objective lens. CONSTITUTION:A control device 11 varies the inclination angle theta of an electron beam according to the width of variation in the exciting current instructed by an input device 18, and performs processing to make approx. constant the amount of dislocation of the projected image regardless of the width of variation. That is, a table containing the angles theta corresponding to the widths of variation in the exciting current is stored in a memory 15, and the angle thetadetermined by reference to this table is fed to a beam deflection coil driving circuit 12. Thereby the graph of effective difference for the exciting current assumes a simple figure having one minium to ensure that the focal point is perceived without fault as likely according to the conventional arrangement. The subsequent processes are the same as conventional, and determination of the focal point is completed.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an autofocus system for an electron microscope,
In particular, it relates to autofocus using the image wobbler method.

[Prior Art] In an apparatus such as a transmission electron microscope that irradiates a sample with an electron beam to observe the shape of the sample, autofocus is employed to obtain a clear projected image on a screen. Various autofocus methods have been proposed, one of which is the image wobbler, which irradiates the sample surface with an electron beam from two tilted directions and uses the point where the resulting images match as the focus point. An autofocus method is known. The structure and operation of the image wobbler method are as follows.

Fig. 2 is a diagram showing an example of a configuration when performing autofocus using the conventional image wobbler method, Fig. 3 is a diagram showing the timing of the operation, and Fig. 4 is a diagram for explaining how to find the focus point. It is.

In Fig. 2, 1 is a beam deflection coil, 2 is a sample, and 3 is a beam deflection coil.
is an objective lens, 4 is an imaging system lens, 5 is a screen, 1
1 is a control device, 12 is a beam deflection coil drive circuit, 13
1 is a D/A conversion circuit, 14 is a current stabilization circuit, 15 is a memory, 16 is a synchronization signal generation circuit, 17 is a television camera, 1
8 indicates an input device.

In FIG. 2, the beam deflection coil 1 is used to irradiate the sample 2 at a predetermined angle as indicated by the solid line 6 or the broken line [7].

In the figure, the inclination angles of the solid line 6 and the broken line 7 are each θ with respect to the optical axis O, and the solid line 6 and the broken line 7 are set axially symmetrically with respect to the optical axis O. Note that, hereinafter, being inclined as shown by the solid line 6 will be referred to as a (+) inclination, and being inclined as shown in the broken line 7 will be referred to as a (-) inclination. The sample 2 is placed between the pole pieces (not shown) of the objective lens 3.

The imaging system lens 4 is shown as a block, but it is composed of about three or four stages of intermediate lenses and a projection lens, and forms a projected image of the sample 2 at a predetermined magnification onto a screen 5 made of fluorescent material. It is something that makes you imagine. In addition, in the figure, a solid line 8 shows an enlarged image in the case of a (+) inclination, and a broken line 9 shows an enlarged image in the case of a (-) inclination.

The control device 11 is composed of a microprocessor or the like, and executes processing for autofocus.

That is, the direction of inclination of the electron beam, that is, whether the electron beam should be inclined (+) or (-), and the direction of the inclination angle are output to the beam deflection coil drive circuit 12, and the D/A The excitation current value of the objective lens 3 is output to the conversion circuit 13, the television camera 17 is instructed to capture a projected image, and the focus is determined from the obtained projected image. Perform calculations to find points.

The beam deflection coil drive circuit 12 tilts the electron beam (+) or (-) based on instructions from the control device 11.
) A predetermined excitation current is generated and supplied to the beam deflection coil 1 for tilting the beam.

The D/A conversion circuit 13 converts the digitized excitation current value for the objective lens 3 sent from the control device 11 into an analog value and outputs it to the current stabilization circuit 14 . Thereby, the current stabilizing circuit 14 generates the excitation current instructed by the control device 11 and supplies it to the objective lens 3.

The memory 15 is composed of a RAM that functions as a file for storing image data of the projection image captured by the work area and the TV left camera 7, and a ROM that stores various data necessary for performing autofocus. be done. The synchronization signal generation circuit 16 is necessary for supplying a synchronization signal to the TV left camera 7 and for the operation of the control device 11, and generates a lock. The TV left camera 7 photographs and captures the image of the sample 2 projected on the screen 5 according to instructions from the control device 1 engineer, and the image data of the projected image captured by the TV left camera 7 is sent to an A/D (not shown). The data is digitized by a conversion circuit and stored in a file in the memory 15. The input device 18 instructs the control device 11 to start an autofocus operation, and controls the sweep range and change width of the excitation current value of the objective lens 3 during the autofocus operation, that is, the excitation current value from a certain value to the next certain value. This is used to indicate the increase/decrease in the current value when changing the current value.

Now, when the input device 18 instructs the control device 11 to specify the sweep range and change width of the excitation current value, and further instructs the start of autofocus operation, the control device 11 causes the objective lens 3 to set a predetermined first
In order to supply the excitation current, for example, the lowest current value in the designated sweep range, a digital value having a level shown at 20 in FIG. 3(b) is output at time to. The digital value is converted into an analog value by the D/A conversion circuit 13, and a current corresponding to the level 20 shown in FIG. 3(b) is supplied by the current stabilization circuit 14, and the magnetic field of the objective lens 3 is figure(
It changes as shown in 21 of a). Next, control equipment engineer 1
sends a control signal shown at 22 in FIG. 3(C) to the beam deflection coil drive circuit 12 at time 1 when a predetermined time TO has elapsed since t. Based on the control signal 22, the beam deflection coil drive circuit 12, for example, deflects the electron beam (+
) Generate an excitation current for generating a gradient magnetic field and supply it to the beam deflection coil 1. As a result, the electron beam is tilted by θ (+) as shown at 23 in FIG. 3(d). When the magnetic field of the objective lens 3 and the inclination of the electron beam are stabilized, the control device 11 outputs a control signal shown at 24 in FIG. 3(e) to the TV camera 17, and at time t.
The projected image on the screen 5 is captured during a predetermined time T1 from time t2 to time t3, and the captured image data is stored in the memory 15. When the image data acquisition for the (+) slope is completed in this way, the control device 11 outputs a control signal shown at 25 in FIG. 3(C) to the beam deflection coil drive circuit 12 at time t4. do. As a result, the electron beam becomes (-) as shown at 26 in FIG.
) is tilted. When the inclination of the electron beam is stabilized, the control device 11 controls the TV camera at 27 in FIG. 3(e).
A control signal shown by is output, a projected image on the screen 5 is captured during a predetermined time T1 from time t5 to time t6, and the captured image data is stored in the memory 15. When the image data for the (-) slope has been captured in this way, the control device 11 calculates the difference between the two captured image data and compares the difference with a predetermined threshold value Vn. If the difference exceeds the threshold VIll, it is determined that there is actually a difference between the image data and it is considered an effective difference, and if it is less than the threshold Vn, the difference is determined to be due to noise components contained in the image data. The value of the effective difference is ignored and stored in the memory 15. The difference in image data may be found two-dimensionally or -dimensionally.

Specifically, for example, when the excitation current of the objective lens 3 is at a certain value, the image in the case of (+) slope is 30 in FIG.
If the image in the case of (-) inclination is as shown in 31, then only the difference in the direction shown by 32, 33, 34 or 35 in the figure is calculated. Alternatively, differences may be obtained in all directions indicated by 32 to 35 and these differences may be combined. In the following explanation, in order to facilitate discussion, the difference in direction indicated by 33 in FIG.
It is assumed that the difference is found for each corresponding pixel along the scanning line of the TV camera 17.

When the processing for one excitation current of the objective lens 3 is completed in the above manner, the control device 11 then starts the excitation current at time t7 in order to supply a predetermined second excitation current to the objective lens 3. A digital value having a level indicated by 28 in FIG. 3(b) is output. The level indicated by 28 is the level obtained by adding the specified change width ΔI to the level indicated by 20. The subsequent processing is the same as described above. First, the electron beam is tilted (+) to capture image data, and then (-
) Image data is captured with the camera tilted, and if there is an effective difference between these two image data, the effective difference is stored in the memory 15.

As described above, the control device 11 changes the excitation current of the objective lens 3 in steps over the specified sweep range in a stepwise manner with the specified change width, and for each case of the excitation current, Image data obtained when the electron beam is tilted (+) and image data obtained when the electron beam is tilted (-) are captured, and the process of determining the effective difference between these two image data is repeated, and the effective difference data is collected. When finished,
The point at which the effective difference is minimum is determined and set as the focus point, and the excitation current at that point is supplied to the objective lens 3. As a result, a focused projection image is projected onto the screen 5.

[Problems to be Solved by the Invention] However, according to the conventional method of determining the focus point, the graph plotting the effective difference of the image against the excitation current value of the objective lens 3 is as shown in FIG. 5(a). When the minimum value is a simple single peak characteristic, the point A where the graph has the minimum value is considered to be the focus point, and the objective lens 3
The excitation current value to be supplied to C can be determined, but if the graph becomes a complicated graph with unevenness having multiple minimum values as shown in FIG. 5(b), the correct focus point C However, there is a problem in that point B is mistakenly recognized as the focus point, and the focus point cannot be determined correctly. Possible reasons for this are as follows.

First, in autofocus operations when the position of the focus point cannot be predicted at all, the excitation of the objective lens 3 is used to search for the approximate focus point in a short time. This instructs the current to be swept over a wide range and with a large change width, but a transmission electron microscope has the property that the projected image rotates as the excitation current of the objective lens 3 changes. Therefore, by greatly changing the excitation current of the objective lens 3, the projected image rotates greatly, and the location where the difference between the two images is determined differs depending on the excitation current, and the location where the difference is compared differs. , the effective difference will be different.

Second, even if the image for the (+) tilt and the image for the c-) tilt are actually significantly different from each other,
The apparent dynamic difference may be reduced. In other words, the amount of deviation between the image in the case of a (+) tilt and the image in the case of a (-) tilt is proportional to the tilt angle of the electron beam, and the larger the tilt angle of the electron beam, the greater the shift between the two images. Therefore, there is a large difference in dynamic difference between when the focus is in focus and when it is out of focus, so there is an advantage in being able to clearly identify the focus point.
If the projected image of the sample has a regular pattern, the effective difference may be small even though it is out of focus. Specifically, it is as follows. For example, assume that the projected image of the sample has three patterns 40, 41, and 42 as shown in FIG.
Consider the case where the focus point is determined based on the level difference of the waveforms of the scanning lines shown by . and in case of (+) slope, the sixth
If the waveform shown in Figure 6(b) is obtained, and in the case of a (-) slope, a waveform with the period of the waveform shifted by exactly one cycle is obtained as shown in Figure 6(C). ) and the waveform shown in
The absolute value of the difference from the waveform shown in FIG. 5C is as shown in FIG. 1D, and the effective difference is relatively small despite the large image shift.

For at least two reasons as described above, the graph of the effective difference with respect to the excitation current of the objective lens 3 is as shown in FIG.
), the image becomes uneven with a plurality of minimum values, and the control device 11 sometimes misidentifies the focus point.

Of course, as a method of determining whether there is a difference between two images,
Although it is possible to perform pattern recognition by processing two-dimensional information, the device becomes large and very expensive, which is not practical.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an autofocus method for an electron microscope that can correctly identify a focus point without misrecognizing it.

[Means for Solving the Problems] In order to achieve the above object, the autofocus method of the electron microscope of the present invention tilts the electron beam in two different directions for each of the excitation current values of various objective lenses. The electron microscope autofocus method calculates the difference between the image data obtained by the above-mentioned image data, and when the difference exceeds a predetermined threshold value, the numerical difference is set as the effective difference, and the excitation current when the effective difference is the minimum is set as the focus point. The method is characterized in that the amount of inclination of the electron beam is changed in accordance with the range of change in the excitation current value of the objective lens.

C Effects and Effects of the Invention In the present invention, when performing an autofocus operation using the image wobbler method, the inclination angle of the electron beam is changed according to the width of change in the excitation current of the objective lens, so that the deviation of the projected image is reduced. The amount of is approximately constant regardless of the width of change in the excitation current, and as a result, the graph of the effective difference against the excitation current has a simple single-peak characteristic, so a simple method is to detect the effective difference by comparing the levels of the image signals. Accordingly, the correct focus point can be determined without misrecognizing the focus point, and the processing time for this can also be shortened.

[Example code] Examples will be described below with reference to the drawings.

The hardware configuration of the autofocus system of the electron microscope according to the present invention is similar to that shown in FIG. 2, but the processing performed by the control device 11 is different.

Now, it is known that the amount of deviation δ of the projected image is proportional to the magnification M1, the inclination angle θ of the electron beam, and the amount of focus deviation Δf. That is, if k is a proportionality constant, the following equation holds true.

δ=−M・θ・△f Here, when performing autofocus operation, magnification M
is usually kept constant. Therefore, when performing autofocus using the image wobbler method, if it is predicted that the focus will shift significantly, the tilt angle θ should be set to a small value, so that it is predicted that the focus will not shift too much. It can be seen that by increasing the inclination angle θ, the amount of deviation δ of the projected image can be made substantially constant no matter what autofocus operation is performed.

By the way, as mentioned above, in cases where the position of the focus point cannot be predicted at all, in order to search for the approximate focus point in a short time, the objective lens 3
The sweep range of the excitation current is greatly indicated, and the
As shown in a), the variation width △II is also specified to be large, but
When the approximate focus point is found in this way, the next step is to limit the sweep range of the excitation current to the vicinity of the approximate focus point, and to narrow the range to ΔI2 (ΔI2) in FIG. 1(b).
The focus point is searched by sweeping with a small change width as shown by △I+).

As shown in FIG. 1(a), when the excitation current changes widely, the focus shift becomes large.
Furthermore, as shown in FIG. 1(b), when the variation width of the excitation current is made small, the focus shift is small.

Therefore, by decreasing the tilt angle θ when the specified variation width of the excitation current is large, and by increasing the tilt angle θ when the instructed variation width is small, the projected image can be The amount of deviation δ will be approximately constant.

The above is the feature of the present invention, and therefore, the control device 11:11 performs a process of changing the inclination angle θ of the electron beam according to the change range instructed by the input device 18. In this process, for example, a table that defines the electron beam inclination angle θ with respect to the variation width of the excitation current is stored in the memory 15, the electron beam inclination angle θ is determined by referring to the table, and the obtained inclination angle is This can be done by outputting θ to the beam deflection fill drive circuit 12.

A process of changing the excitation current of the objective lens 3 in steps over a specified range with a specified change width, a process of tilting the electron beam (+) and (-), or capturing image data. Other processes such as processing and focus point determination processing are performed in the same manner as before.

As described above, by changing the electron beam inclination angle θ according to the variation width of the excitation current, the graph of the effective difference with respect to the excitation current can be changed to a simple graph with one minimum value as shown in Figure 5 (a). Therefore, there is no possibility of misidentifying the focus point as in the conventional case.

Although one embodiment of the present invention has been described above, it is clear to those skilled in the art that the present invention is not limited to the above embodiment, and that various modifications can be made.

[Brief explanation of drawings]

Fig. 1 is a diagram for explaining the processing in the autofocus method of the electron microscope according to the present invention, Fig. 2 is a diagram showing an example of the configuration in the case of autofocus using the image wobbler method, and Fig. 3 is a diagram for explaining the operation. Figure 4 is a diagram showing the timing, Figure 4 is a diagram to explain how to find the focus point, Figure 5 is a diagram showing an example of a graph of effective difference with respect to excitation current, and Figure 6 is a diagram to explain conventional problems. This is a diagram. DESCRIPTION OF SYMBOLS 1... Beam deflection filter, 2... Sample, 3... Objective lens, 4... Imaging system lens, 5... Screen, 11... Control device, 12... Beam deflection coil Drive circuit, 13...D/A conversion circuit, 14...Current stabilization circuit, 15...Memory, 16...Synchronization signal generation circuit, 17...TV camera, 18...Input device . Applicant: JEOL Ltd.

Claims (2)

[Claims] (1) For the excitation current values of various objective lenses, calculate the difference between image data obtained by tilting the electron beam in two different directions, and use the difference when the difference exceeds a predetermined threshold. In an autofocus method of an electron microscope in which the excitation current value at which the effective difference is a minimum is set as the focus point, the tilt amount of the electron beam is changed in accordance with the width of change in the excitation current value of the objective lens. An autofocus system for electron microscopes featuring (2) The manner in which the amount of inclination of the electron beam changes is small when the range of change in the excitation current value of the objective lens is large, and large when the range of change in the excitation current value of the objective lens is small. An autofocus system for an electron microscope according to claim 1, characterized in that: