JPS5854464B2 - Drift correction method in electron microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a novel method for correcting drift in an electron microscope.

It is well known that sample drift poses a major obstacle to high-resolution observation in electron microscopes.

This drift is caused by the temperature and mechanical imbalance of structures near the sample, and is caused by temperature changes during sample exchange, temperature changes in the objective lens, mechanical changes due to sample movement, etc. It can take several hours to reach this point, which poses a major hindrance to high-resolution observation.

In the past, some research has been conducted to reduce sample drift, but most of this research has focused on improving the sample stage, and active correction of drift has not been put into practice.

The present invention proposes a method for correcting drift that can be put to practical use, and will be explained below with reference to the drawings.

FIG. 1 is a block diagram of an example of an apparatus implementing the method of the present invention.
1 is an objective lens, 2 is an intermediate lens, and 3 is a projection lens.

The electron beam transmitted through the sample 4 is formed into an image and magnified by an objective lens 1, and further magnified by an intermediate lens 2 and a projection lens 3 to form an image 5. A fluorescent screen is usually placed at the position of this image formation. The electronic image can be observed as a visible image.

For example, a solid-state imaging plate γ is placed at an appropriate location near the fluorescent screen, and is capable of converting an electron microscope image into a time-varying electric signal in two orthogonal directions (X and Y directions).

Reference numeral 8 denotes a horizontal (X direction) synchronization signal generation circuit, which generates pulse signals at regular intervals as shown in FIG.
As a result, a video signal (FIG. 2b) having a temporal change scanned in the It will be done.

In this binarization circuit, a constant level E1 (see diagram b)
The above signal is converted into a 1" signal, and the below signal is converted into a "0" signal (Fig. 2C), and the two ant games - N1a and 11
sent to b.

On the other hand, the signal from the synchronizing signal generating circuit 8 is sent to a sawtooth wave signal generating circuit 12, which generates a sawtooth wave as shown in Figure d.

This signal is sent to a comparator circuit 13, where it is compared with a slice signal E2 having a level approximately at the center, and when this level is exceeded, a signal is generated (FIG. 2e).
send to.

A part of the signal from the comparison circuit 13 passes through the inversion circuit 14 to become the signal shown in FIG. 2f, and is sent to the AND gate 11b.

A clock pulse of a constant period is sent to both AND gates 11a and 11b from a clock pulse oscillator 15, and therefore the outputs of both gates are as shown in FIG. 2g and h, respectively.

As shown in FIG. 3, the time τ1 of this clock pulse corresponds to the length 11 on the right side of the slice position of the image portion 5a showing a value of 1'', and τ2 corresponds to the length 12 of the left side portion of the same figure. do.

Since the slice position is set at level E2 in FIG. 2d and is a fixed position, if the image portion 5a moves left or right (
drift), the difference or ratio between 11 and N12 will change,
The values of τ1 and τ2 change accordingly.

This time duration is converted into a number of clock pulses, each pulse being counted by a counter 16a, 16b.

The counted values of both counters are sent to the memory 17 and temporarily stored.

In the present invention, since measurements are made between parts at a certain time, four counts are stored in the X direction.

These stored values are read out by a control circuit 18 (for example, a computer) having an arithmetic function, and the drift speed is determined based on the four values.

This drift speed is displayed by the display device 19.

On the other hand, once the drift speed is measured, a control signal is sent from the control circuit 18 to the power supply 20, which generates a deflection voltage or current in the deflection system 21 that exactly cancels out the drift,
supply

The deflection system 21 does not need to be placed below the projection lens, but may be placed inside the objective lens or inside the intermediate lens, as shown at 22.

Further, this deflection coil 22 is conveniently used for electrically moving the field of view of the sample.

Although the above circuit system is for the X direction, it goes without saying that a similar circuit is constructed in a direction perpendicular to this, that is, in the Y direction, although not shown.

In this case, the slice position and scan position in FIG. 3 are opposite.

In the above configuration, the slice level E of the comparison circuit 13
2 at an appropriate (center) position, and position an image portion 5a of an appropriate shape (for example, circular) approximately at the center of the fixed image pickup plate 7;
The binarization circuit 1
Adjust 0.

In this state, the control circuit 18 sends a signal to the synchronization signal generation circuit 8 and the counters 16a and 16b, and at the same time as resetting the counter, a horizontal synchronization signal is generated and sent to the solid-state image pickup plate γ and the sawtooth wave generation circuit 12.

As a result, a time-varying video signal is obtained from the image pickup plate, which passes through the binarization circuit 10 and becomes the signal shown in FIG. 2C.
It is sent to AND gates 11a and 11b.

Further, the signal shown in FIG. 2d from the sawtooth wave generating circuit 12 passes through the comparison circuit 13 and becomes the signal shown in FIG.
1a, and further passed through the inverting circuit 14 to the ant game N.
Sent to lb.

Since each AND gate is supplied with clock pulses from the oscillator 15, the counter 16a counts the number of clock pulses (g in FIG. 2) corresponding to FIG. 3, 11 during the period τ1.

Also, the signal during the period τ2 from the anime game) 11b is 12
The number of clocks corresponding to 1 is counted by the counter 16b.

The counted values of both counters are temporarily stored in the memory 17.

After a certain period of time (for example, 1 minute) has elapsed, the control circuit again switches between the circuit 8 and the counter 16a.

A control signal and a reset signal are sent to 16b, and the same operation as described above is performed.

As a result, four count values are stored in the memory.

For example, in FIG. 3, the sample image 5 during the first scan
If a drifts as shown by the dotted line from the center during the second scan, counters 16a and 16b have τ1
As shown by ' and 72', clocks with different widths (numbers) are introduced.

The control circuit 18 then reads out the four signals stored in the memory 17 and determines the drift speed.

The speed of this drift is displayed on the display device 19.

On the other hand, a signal is sent from the control circuit to the power supply 20, which generates a deflection voltage or current that reduces the speed of this drift to zero, and sends it to the deflection system 21.

If you do the above, you can use the short time before taking the photo.
Drift speed can be measured and corrected to zero, making it possible to obtain extremely high-resolution photographs.

In the above description, it is convenient to construct the solid-state imaging plate T so that it can be taken in and out of the electron beam path.

Further, this imaging plate is merely an example, and any plate may be used as long as it can convert an electron microscope image into a signal that changes over time.

Moreover, although the position of the image has been measured by dividing the image portion to be specified and measuring the time of each part, the distance (time) from the reference point to the specific image may also be measured.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of an apparatus that can carry out the method of the present invention, and FIGS. 2 and 3 are diagrams illustrating its operation. 1: Objective lens, 2: Intermediate lens, 3: Projection lens, 4
: Sample, 5: Image formation, γ: Solid-state imaging plate, 8: Horizontal synchronization signal generation circuit, 10: Binarization circuit, 11a and 11b: AND gate, 12: Sawtooth wave oscillator, 13: Comparison circuit, 1
4: Inverting circuit, 15: Clock oscillator, 16a and 16
b: Counter, 17: Memory 18: Control circuit, 1
9: Display device, 20: Power supply, 21: Deflection system.

Claims (1)

[Scope of Claims] 1. At a certain point in time, a specified image area of a formed electron microscope image is changed into an electric signal that changes over time in two orthogonal directions, and from this signal, a signal regarding the position of the specified area is determined. After a certain period of time, a similar temporal change signal is obtained for the specific image area, and from this signal, a signal regarding the position of the specific area after a certain period of time is obtained, and the drift of the sample is determined based on the position signals at both times. A method for correcting drift in an electron microscope, characterized in that the velocity is determined and the electron microscope image is deflected and moved so as to make the drift velocity in each of the directions zero. 2. Converting a specified image area of a formed electron microscope image into an electrical signal that changes over time in two orthogonal directions at a certain point in time, and dividing a specific part of the signal with a reference position as a boundary,
Measure the temporal length of the image portion, then obtain a temporal change signal regarding the specific area after a certain period of time has elapsed, and measure the temporal length of each specific portion separated from the reference position. , in an electron microscope characterized in that the drift velocity of the sample is determined based on the measured values of each temporal length at both time points, and the electron microscope image is deflected and moved so as to make the drift velocity in each of the directions zero. Drift correction method.