JPS5848989B2 - Focusing device in electron beam equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focusing device in an electron beam apparatus such as a scanning electron microscope.

In general, in an electron beam apparatus, it is necessary to irradiate a sample with an electron beam bundle converging to a desired narrowness.

For example, in a scanning electron microscope, the resolution is determined by how narrow the electron beam flux can be.

The most important element for converging an electron beam into a narrow beam is focusing the electron lens, and conventionally its operation has relied on the operator's experience.

In other words, in a scanning electron microscope, the operator observes a secondary electron scanning image or a backscattered electron image, and judges how detailed the image is to use as a guide for focusing the electron lens, which makes the operation complicated. It also required skill.

Therefore, research on automating focusing has been actively conducted recently, and examples include a device that differentiates the detection signal over time and controls the excitation current of an electron lens so that the differential value is the best value, and A device has been proposed that integrates the magnitude of the change in the detection signal and controls the excitation current of an electronic lens so that the integrated value becomes the largest value.

However, in such an automatic focusing device, astigmatic aberration exists, and if the direction is close to the direction perpendicular to the scanning direction of the electron beam, the thickness of the electron beam in that direction is the smallest, that is, the electron beam When the focal line is connected, the differential value and the integral value become maximum, and the resulting scanned image contains an astigmatic aberration that flows in one direction.

The present invention has been made in view of the above-mentioned conventional problems, and is a focusing device that can always irradiate a sample with an electron beam in a state of a circle of least confusion even if the electron beam contains astigmatism. The purpose is to provide the following.

In order to achieve such an object, the present invention is characterized in that a focusing operation is performed while circularly scanning the electron beam.

Next, the present invention will be explained in detail using the drawings.

FIG. 1 shows changes in the shape of the electron beam near the focal point when there is astigmatism.

In the figure, 1 is an aperture hole of a magnetic field lens, and the aperture hole 1 is represented by a circle on the XY plane at Z=O.

In this aperture plane, the electron beam emitted from the X-axis focuses at the position Z=Fx, and the electron beam emitted from the Y-axis focuses at the position Z=F.

Therefore, the electron beam has a focal line 2 in the Y direction at Z2Fx, and a focal line 2 in the Y direction at Z2Fx.
A focal line 3 in the X direction is formed.

A circle of least confusion 4 exists between these two focal lines.

At this time, if the sample is located at position 8 in Figure 1, if the excitation current of the magnetic field lens is increased to strengthen the lens strength, the shape of the electron beam on the sample surface will become as shown in Figure 2. ellipse 8
→ focal line 2 → ellipse γ → circle of least confusion 4 → ellipse 6 → focal line 3 → ellipse 5.

Now, let us consider the case where the above-mentioned automatic focusing device is operated while circularly scanning an elliptical electron beam beam on the sample surface as shown in FIG.

Here, a direction t tilted at an arbitrary angle θ with respect to the ellipse axes X and y
Assuming that scanning is performed, the thickness of the electron beam in the l direction corresponds to the distance Xλ'' between the intersections A and A' of the perpendicular line drawn from the ellipse to the t-axis and the t-axis.

And when scanning such an electron beam in a circular manner, θ is 00
This corresponds to a change from 3600 to 3600.

Therefore, the length of λλ1 is θ== o O to θ2360
Considering the integration up to 0, it can be seen that the integral value becomes the minimum value when the length of the major axis and the minor axis of the ellipse match.

Therefore, by repeatedly scanning the electron beam in a circular direction (including an ellipse),
The excitation current of the magnetic field lens is gradually changed step by step for each circular scan, and the integrated value is monitored for each circular scan, and the excitation current value that minimizes the integrated value is determined. is the current value to give the minimum circle of confusion to the electron beam flux.

FIG. 4 is a structural diagram showing an example of a scanning electron microscope based on the principle of the present invention described above, and in the figure, 11 is a sample to be observed.

The sample 11 contains an electron beam E generated from an electron gun (not shown).
B is narrowly focused by the objective lens 12.

Denoted at 13X and 13Y are deflection coils for scanning the sample 11 with the electron beam EB.

Information such as secondary electrons generated from the sample 11 by electron beam irradiation is detected by a detector 14, and the obtained detection signal is sent to an automatic focusing device 16 and a display device 17 via an amplifier 15.

The automatic focusing device 16 operates according to the synchronizing signal a from the timing circuit 18, and conversely sends a stop signal b to the timing circuit 18 to complete the operation.

During operation, the automatic focusing device 16 sends an objective lens current designation signal C to the objective lens drive circuit 19 to gradually change the objective lens current in steps.

20X, 20Y are X, Y scanning circuits that generate X, Y scanning signals having sawtooth waveforms, and the scanning signals are transmitted to the deflection coils 13X, 13Y via switching circuits 21X, 21Y and drive circuits 22X, 22Y, respectively. sent to.

23 is an oscillator that generates a sine signal and a cosine signal for circular scanning;
The ne signal and the cosin signal are sent to switching circuits 21X and 2, respectively.
It is sent to deflection coils 13X and 13Y via 1Y.

Further, the timing circuit 18 is connected to the switching circuits 21X, 21
Y and the above sine signal or cosine
The synchronization signal a is created based on the signal.

In the above configuration, normally the switching circuits 21X, 21Y
The electron beams EB and EB are respectively placed in the state shown in FIG.

When a start is commanded by the operator, the timing circuit 18 sets the switching circuits 21X and 21Y to the state (■) and sends a sine signal and a co to the deflection coils 13X and 13Y, respectively.
A sine signal is sent to cause the electron beam EB to repeatedly scan the sample in a circular or elliptical manner, and a synchronization signal a synchronized with the circular scanning is sent to the automatic focusing device 16 to start the operation.

That is, the automatic focusing device 16 gradually changes the objective lens current step by step each time the electron beam EB is scanned in a circular manner, and also changes the objective lens current in stages each time the electron beam EB is scanned once. The magnitude of the change in the detection signal obtained from the detector 14 during circular scanning is integrated, and when the integrated value reaches the smallest objective lens current value, a stop signal b is issued and the objective lens current is changed to that value, for example, ■ .

Fixed to. This objective lens current value■. As explained earlier, this gives the circle of least confusion, and the sample is irradiated with the electron beam in the state of the circle of least confusion.

Therefore, if the switching circuits 21X and 21Y are returned to the state (2) in synchronization with the stop signal b and the electron beam is scanned in a rask manner in the state of the circle of least confusion, the screen of the display device 17 synchronized with the rask scan will be displayed in one direction. It is possible to obtain clear scanning electron microscopy images without bleeding.

In the above-described embodiments, an automatic focusing device that integrates the magnitude of change in the detection signal is used, but the present invention is not limited to this, and other methods such as a method that differentiates the detection signal over time may also be used. Needless to say, it can be done.

[Brief explanation of the drawing]

Figures 1 and 2 are diagrams for explaining astigmatism, Figure 3 is a diagram for explaining the thickness of the electron beam, and Figure 4 is a configuration showing an example of an apparatus implementing the present invention. It is a diagram. 12: Objective lens, 13X, 13Y: Deflection coil, 14
: Detector, 16: Automatic focusing device, 18: Timing circuit, 20X, 20Y: Scanning circuit, 21X, 21Y: Switching circuit, 23: Oscillator.

Claims (1)

[Claims] 1. An electron beam device that narrowly focuses an electron beam onto a sample using a focusing lens and irradiates the sample, comprising means for repeatedly scanning the electron beam in a circular manner on the sample, and means for changing the current value of the focusing lens for each circular scan. , a means for determining a focusing lens current value that allows the electron beam to be irradiated onto the sample in a state of a circle of least confusion based on a comparison of information generated by the sample during each circular scanning period. Focusing device.