JPS5816746B2 - Focusing method and device in electron beam equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focusing method and apparatus for 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 narrowly converging the electron beam flux;
has an electronic lens focusing system, whose operation traditionally 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 maximized, and A device has been proposed that integrates the magnitude of change in a detection signal and controls the excitation current of an electron lens so that the integrated value is maximized.

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 provides a focusing method that can always irradiate a sample with an electron beam in a state of a circle of least confusion even if the electron beam includes astigmatism. The purpose of the invention is to provide a device for

In order to achieve such an object, in the present invention, focusing operations are performed in at least two different electron beam scanning directions (for example, X and Y directions), and the electron lens determined by focusing in each scanning direction is It is characterized by calculating the average value of the excitation current and sending an excitation type and current corresponding to the average value to the electron lens.

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

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

In the figure, reference numeral 1 denotes the aperture hole of the magnetic field lens, and the aperture hole 1 is a circle on the X-Y plane at Z-0.

In this aperture plane, the electron beam emitted from the X-axis is focused at the position Z=Fx, and the electron beam emitted from the X-axis is focused at the position Z=Fy.

Therefore, the electron beam has focal line 2 in the Y direction at Z=Fx, and Z=Fy
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 an ellipse as shown in Figure 2. 8→
It changes continuously as focal line 2 → ellipse 7 → 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 scanning an elliptical electron beam as shown in Fig. 3 on the sample surface in a direction l tilted at an arbitrary angle θ with respect to the ellipse axes X and y. .

If we consider the tangents of the ellipse that intersect at right angles to the l-axis and let their intersection points be A and A', the length of AA' corresponds to the thickness of the electron beam when scanning in the axial direction.

Then, when the automatic focusing device gradually changes the excitation current and gradually changes the lens strength, the shape of the electron beam changes as shown in Fig. 2, and Kτ is, for example, the focal line 2 and the circle of least confusion 4. It always has a minimum value between.

Moreover, as mentioned above, in the automatic focusing device, when the width of the electron beam in the scanning direction, that is, AA', is at its minimum value, the differential value or the integrated value becomes the maximum, the focusing operation is stopped, and the excitation current of the lens is reduced. The current value I7 at that time is fixed.

That is, the excitation current value at which AA' is minimum can be determined by the automatic focusing device.

Next, the scanning direction of the electron beam flux is set to the m-axis direction perpendicular to the l-axis,
If the automatic focusing device is operated again, the thickness of the electron beam in the m-axis direction, ie, B B', will always have a minimum value between the focal line 3 and the circle of least confusion 4, so The excitation current value Im at that time can be determined in exactly the same manner.

By the way, if the shape of the electron beam on the sample when the excitation current I7 is, for example, 7, and when the excitation current Im is 6, then the circle of least confusion is between the two, and the shape of the electron beam on the sample is The shape of the beam bundle can be made into the state of the circle of least confusion 4.

Therefore, if the electron beam flux is returned to normal two-dimensional scanning in this state, a clear scanning electron microscope image without flowing in one direction can be obtained.

Note that θ-90°Xn (n=0, 1, 2...)
Needless to say, in the case of , IA and Im are given by the excitation current when the electron beam connects the focal lines 2 and 3.

FIG. 4 is a configuration diagram showing an example of a scanning electron microscope that implements the method according to 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.

13X. 13Y is a deflection coil for scanning the electron 7iEB on the sample 11.

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

The automatic focusing device 16 starts its operation in response to a start signal a from a timing circuit 17, and conversely sends a stop signal A to the timing circuit 17 to end its operation.

During operation, the automatic focusing device 16 passes the objective lens current designation signal C to the objective lens drive circuit 19 via the switching circuit 18.
The objective lens current is gradually changed step by step.

20 and 21 are memory circuits, and the memory circuits 20 and 21
The value of the objective lens current designation signal C at the end of the operation is stored in via the switches 22 and 23.

24 is an averaging circuit, which calculates the average value d of the signal values stored in the storage circuits 20 and 21;
The average value d is sent to the first drive circuit 19 via the switching circuit 18.

25X and 25Y are scanning signal generators, and the X and Y scanning signals from the generators are respectively sent to a switching circuit 26X.

The signal is sent to deflection coil drive circuits 27X and 27Y via 26Y.

The driving circuit 27Y is configured so that an X scanning signal can also be sent via a switching circuit 26Y.

Further, the scanning signal generators 25X and 25Y also generate synchronization signals synchronized with the X and Y scanning signals, and the synchronization signals are sent to the timing circuit 17.

In addition, the above switching circuits 18, 26X, 26Y and switch 22
．． 23 is controlled by a timing signal from the timing circuit 17.

When a start is commanded by the operator in the configuration described above, the timing circuit 17 switches the switching circuit 18 to the automatic focusing device side, switches the switching circuits 26X and 26Y to the terminal 2, and opens the other switches 22 and 23. ” state.

Next, the timing circuit 17 sends a start signal a to the automatic focusing device 16 to start the focusing operation.

That is, the automatic focusing device 16 adjusts the electron beam EB to 1 in the X direction.
The objective lens current is gradually changed in stages for each horizontal scan, and the magnitude of the change in the detection signal obtained from the detector 14 is integrated for each horizontal scan, and this integrated value is the largest. When the objective lens current value is , a stop signal is issued to fix the objective lens current to that value, for example, Ix.

The timing circuit 17 closes the switch 22 for a short period in synchronization with the stop signal, thereby setting the objective lens current designation signal value Cx corresponding to the objective lens current value Ix.
is stored in the memory circuit 20.

After Cx is stored in the memory circuit 20, the timing circuit 17 turns the switching circuits 26X and 26Y to the terminal 3, sends an X scanning signal to the Y direction deflection coil 13Y, and changes the scanning direction of the electron beam to the previous one. The Y directions differ by 90 degrees.

Then, with the timing circuit 17 activated, the start signal a is again sent to the automatic focusing device 16 to perform focusing in the Y direction scanning.

When focusing is completed, a stop signal is sent to the timing circuit 17.
, the timing circuit 17 closes the switch 23 for a short period of time and causes the storage circuit 20 to store the objective lens current designation signal cy corresponding to the objective lens current value y at that time.

When Cx and Cy are respectively stored in the memory circuits 20 and 21 as described above, the timing circuit 17 switches to the switching circuit 26X,
26Y is returned to terminal 1, and the switching circuit 18 is turned to the averaging circuit 24 side to send the signal.

Therefore, the objective lens current becomes a small circle of confusion.

In this state, the electron beam is X. Since the sample is scanned two-dimensionally by the Y scanning signal, by supplying the detection signal to a display device (not shown), it is possible to obtain a clear scanning electron microscope image that does not flow in one direction.

[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 in the scanning direction,
FIG. 4 is a configuration diagram showing an example of an apparatus implementing the present invention. 12: Objective lens, 13X, 13Y: Deflection coil, 14
: Detector, 16: Automatic focusing device, 17: Timing circuit, 18, 26X, 26Y: Switching circuit, 20, 21
: Memory circuit, 22, 23: Switch, 24: Average circuit, 25X, 25Y: Scanning signal generator.

Claims (1)

[Claims] 1. In an electron beam device that narrowly focuses and irradiates an electron beam onto a sample using a focusing lens, the electron beam is scanned in at least two different directions on the sample, and the electron beam generated from the sample is scanned in at least two different directions on the sample. Based on the information obtained, find the focusing lens current value that minimizes the thickness of the electron beam in each scanning direction, and
A focusing method characterized in that the lens current of the focusing lens is set to the average value of the plurality of lens current values obtained. 2. A focusing lens for narrowly focusing the electron beam on the sample, a deflection means for scanning the electron beam in at least two directions on the sample, and a system based on information generated from the sample as the electron beam scans. means for detecting a focusing lens current value that minimizes the thickness of the electron beam in the scanning direction; and focusing that minimizes the thickness of the electron beam in each direction determined by the detection means in at least two different scanning directions. 1. A focusing device for an electron beam apparatus, comprising means for determining an average value of lens current values, and configured to supply a lens current corresponding to the average value to the focusing lens.