JP2726538B2 - electronic microscope - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electron microscope capable of easily searching for a coma-free axis and a spherical aberration axis of the electron microscope.

[Prior Art] Conventionally, a transmission electron microscope having a configuration as shown in FIG. 5 is known. In FIG. 5, 1 is an electron gun, 2 is a focusing lens for focusing an electron beam emitted from the electron gun on a sample 3, and 4a and 4b are deflectors comprising a pair of deflection coils for X and Y directions. Reference numeral 5 denotes an objective lens, 6 denotes an intermediate lens, 7 denotes a projection lens, 8 denotes a fluorescent plate, and 9 denotes an imaging device. Reference numeral 10 denotes an alignment signal generator for generating a deflection signal for causing an electron beam to enter the point P on the sample 3 at an arbitrary tilt angle θ.
Is the electron beam deflected by the deflection coil 4a.
This is a balance circuit for adjusting the intensity ratio of the signals supplied to the two deflection coils 4a and 4b so that the deflection coils 4a and 4b are turned back by the 4b and enter the same point P of the sample.

Using a transmission electron microscope with the configuration described above,
When taking a high-resolution photograph of a sample, it is indispensable to align the axis of the incident electron beam with the lens in order to minimize the effect of the aberration of the objective lens. Has been achieved by matching the voltage centers of the objective lens 5.

This voltage centering is performed by slightly changing the high voltage (acceleration voltage) supplied from the electron gun power supply 12 to the electron gun 1 to slightly change the energy of the incident electron beam, thereby forming a voltage on the fluorescent screen. Utilizing the fact that the position of the electron microscope image to be imaged is changed, the deflection coil 4b adjusts the incident angle so that the electron microscope image moves symmetrically about the center point of the fluorescent screen, and the chromatic aberration is minimized. Find the axis.

[Problems to be Solved by the Invention] In the electron microscope having the above-described configuration, when the distance between the pole pieces and the hole diameter of the objective lens 5 are relatively large, the magnetic field distribution on a plane perpendicular to the optical axis in the lens ( Since the magnetic field gradient is slow, a high-resolution photograph of the sample could be obtained by performing voltage centering as described above to minimize the influence of chromatic aberration.

However, with the ultra-high resolution of the transmission electron microscope, the gap between the magnetic pole pieces and the hole diameter of the objective lens have been further reduced, so that the magnetic field distribution (magnetic field gradient) on the plane orthogonal to the optical axis has become steep, and the voltage center alignment has become difficult. Since the incident electron beam cannot be made to coincide with the coma-free axis and the spherical aberration axis only by performing the method, sufficient resolution cannot be obtained in the periphery of the ultra-high-resolution electron microscope image even with a slight axis shift, that is, The effects of aberrations and spherical aberrations have surfaced. Therefore, it is necessary to accurately determine the coma-free axis and the spherical aberration axis, and to make the incident electron beam coincide with the axes to minimize the influence of the aberration.

An object of the present invention is to provide an electron microscope capable of easily searching for a coma-free axis and a spherical aberration axis of an electron microscope in consideration of the above-described problems.

[Means for Solving the Problems] The present invention provides a two-stage deflector for deflecting an electron beam focused by a focusing lens system in front of an objective lens, and an electron beam at one point on a sample at an arbitrary tilt angle. Alignment signal generating means for generating a deflection signal for causing
Circular scanning signal generating means for generating a signal for circularly scanning the electron beam around the axis of the electron beam tilted by the deflection signal generated by the alignment means; Helical scanning signal generating means for generating a signal for helically scanning around a point, signal adding means for superposing a circular scanning signal and a helical scanning signal on the alignment signal, and the adder by pressing switch means And a means for setting the output signal value of the circular scanning means to be supplied to 0 to zero, holding the output signal value of the spiral scanning signal generating means at that time, and supplying the output signal value to the signal adding means.

[Example] Hereinafter, an example of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing an apparatus configuration for explaining an embodiment of an electron microscope according to the present invention, and FIG. 2 and FIG.

In FIG. 1, the same components as those in FIG. 4 are denoted by the same reference numerals, and description thereof will be omitted.

The embodiment shown in FIG. 1 is different from the conventional example in that an alignment signal generator 10 for generating a deflection signal for causing an electron beam to enter the point P on the sample 3 at an arbitrary inclination angle θ, and the alignment signal Circular scanning signal generators 13x and 13y for generating a signal for circularly scanning the electron beam around an inclination axis O of the electron beam inclined by an alignment signal generated by the generator 10, and an inclination axis of the electron beam. Is the incident point P of the electron beam.
Helical scanning signal generators 14x and 14y for generating signals for helically scanning with the axis as a fulcrum, a signal adding circuit 15 for superimposing a circular scanning signal and a helical scanning signal on the alignment signal, and the alignment signal The generator 10, the circular scanning signal generators 13x, 13y, and the control means 16 for controlling the spiral scanning signal generators 14x, 14y, and the circular scanning signal generators 13x, 13x, by pressing a switch provided on the operation terminal 18. 13y
Signal holding circuit 1 that holds the output signal values of the spiral scanning signal generators 14x and 14y at that time as zero
7x and 17y are provided.

Now, the electron beam emitted from the electron gun is focused by the focusing lens system, and then enters the sample 3. At this time, the electron beam is deflected by the deflection coil 4a, is turned back by the deflection coil 4b and is incident on a desired point P on the sample 3, but the deflectors 4a and 4b have alignment signal generation circuits. The alignment signal (x ₀ ,
Y ₀ ) is supplied through the addition circuit 15 and the balance circuit 11.

In the state where the electron beam is incident on the point P on the sample 3 at an arbitrary tilt angle as described above, no axis alignment for suppressing the influence of the spherical aberration has been performed yet, and the alignment signal generator 10 The irradiation point P and the inclination angle θ are determined only by the generated alignment signal (X ₀ , Y ₀ ).

Therefore, in the present invention, a case will be described in which the axis of the spherical aberration is obtained from a state where the axial alignment for suppressing such spherical aberration is not performed.

First, the electron beam is incident on the point P on the sample 3 at an arbitrary tilt angle by the deflectors 4a and 4b to which the alignment signal (x ₀ , Y ₀ ) generated by the alignment signal generator 13 is supplied. Here, the circular scanning signal generators 13x and 13y are controlled by the control means 16 to generate signals (a ₁ Sin ω ₁ t, a ₁ Cos ω ₁ t) for circularly scanning the electron beam. Is supplied to the addition circuit 15 and is superimposed on the alignment signal (x ₀ , Y ₀ ). As a result, the electron beam irradiated onto the sample 3 is output from the addition circuit 15 and
The deflection signal (x ₀ + a ₁ Sin ω ₁ t, Y ₀ + a ₁
Cos ω ₁ t) is applied so as to rotate in a cone shape (hollow cone shape) while maintaining the point of incidence P on the sample as shown in FIG. 2 (a). At this time, the image j of the target portion of the sample image projected on the fluorescent screen 8 is affected by the aberration and moves in an elliptical shape as shown in FIG. 2 (b).

In particular, in an objective lens in which the pole piece spacing and the hole diameter have been reduced with the increase in ultra-high resolution, the magnetic field distribution (magnetic field gradient) in a plane perpendicular to the optical axis becomes steep, and the influence of spherical aberration is minimized. The axis is not aligned for
The phenomenon that the image projected on the phosphor screen is greatly shifted by the slight inclination of the incident angle of the electron beam,
This movement of the image moves in proportion to the cube of the angle θ at which the electron beam is inclined with respect to the axis of the spherical aberration, and therefore the image moves largely in an elliptical shape as described above.

However, as shown in FIG. 3 (a), when an electron beam that rotates in a cone around the axis k of spherical aberration is incident on the sample 3, the movement of the sample image projected on the fluorescent screen 8 is
As shown in FIG. 3B, the robot moves while drawing a substantially perfect circle.

Therefore, in a state before the axial alignment for suppressing the spherical aberration is performed, the operator adjusts the alignment signal while watching the image moving in an elliptical shape, and the movement of the sample image draws a substantially perfect circle. The axis of spherical aberration can be searched for by performing the axial alignment as described above.

However, the operation of manually moving and observing the image while changing the incident angle and the incident azimuth of the electron beam manually to search for the axis of the spherical aberration is very troublesome and troublesome.

Therefore, signals (a ₂ tSinω ₂ t, a ₂ tCosω) generated by the spiral scanning signal generators 14x and 14y for spirally scanning the tilt axis O of the electron beam with the incident point P of the electron beam as a fulcrum.
₂ t) is supplied to the adder circuit 15, the helical operation signal in the summing circuit 15 is further added superimposed on the superimposed alignment signal of the circular scan signal.

Then, the output signal (x ₀ + a ₁ Sinω ₁ t + a) of the adding circuit 15
₂ tSinω ₂ t, Y ₀ + a ₁ Cosω ₁ t + a ₂ tCosω ₂ t) is a balanced circuit 11
The electron beam is supplied to the two-stage deflectors 4a and 4b through the, so that the electron beam rotates in a cone shape while maintaining the incident point P on the sample 3 as shown in FIG. Since the center is spirally scanned as shown by A to E in the figure, the incident angle and incident direction of the electron beam can be easily changed to various values. Here, the constant ω ₂ of the spiral scanning signal is set so that the speed of the spiral scanning is about 10 times slower than the scanning cycle of the circular scanning.

At this time, while observing the sample images A ′ to E ′ projected on the fluorescent screen as shown in FIG. 4 (b), when the movement of the images becomes a perfect circle (time t ₀ ), the operation terminal When a switch provided on 18 is depressed, the output signal values of the circular scanning signal generators 13x and 13y are each made zero by the control means 16,
The circular scanning of the electron beam is stopped, and the output signal (a ₂ t ₀ Sin ω ₂ t ₀ , a ₂ t ₀ Cos) of the spiral scanning signal generator at that time is stopped.
ω ₂ t ₀ ) are held in the signal holding circuits 17x and 17y, respectively, and the held signals are supplied to the addition circuit 15 and added to the alignment signals (X ₀ , Y ₀ ). Then, the output signal (X ₀ + a ₂ t ₀ Sin ω ₂ t ₀ , Y ₀ + a ₂ t ₀ Cos ω ₂ t ₀ ) of the addition circuit 15 is supplied to the deflectors 4 a and 4 b via the balance circuit 11. , The spherical aberration axis is maintained.

Note that the above-described embodiment is merely an embodiment of the present invention,
The present invention can be implemented with various modifications. For example,
In the above-described embodiment, the period of the spiral scan is set to be about 10 times slower than the period of the circular scan, but the period of the circular scan is an afterimage (afterglow) of an image moving on the fluorescent screen. It is sufficient if the speed is such that the circumference is formed, and if the speed of the one spiral scan is sufficiently slower than the cycle of the circular scan, and the speed is such that the movement of the image formed by the circular scan can be viewed silently. Good.

In the above-described embodiment, the roundness of the image projected on the fluorescent screen is obtained while the operator observes the shape of the image. However, the image projected on the fluorescent screen is captured by an imaging unit 9 such as a television camera. The circularity of the image captured by the method may be automatically determined by pattern recognition or the like, and the control unit may control the circular scanning signal generator based on the determination result.

[Effects of the Invention] As is clear from the above description, according to the present invention,
A two-stage deflector for deflecting the electron beam focused by the focusing lens system in front of the objective lens, and an alignment for generating a deflection signal for causing the electron beam to enter a point on the sample at an arbitrary tilt angle Signal generating means, circular scanning signal generating means for generating a signal for circularly scanning the electron beam around the axis of the electron beam tilted by the deflection signal generated by the alignment means, and tilt axis of the electron beam Scanning signal generating means for generating a signal for spirally scanning around the electron beam incident point, signal adding means for superimposing a circular scanning signal and a spiral scanning signal on the alignment signal, and pressing of a switch means The output signal value of the circular scanning means supplied to the adder is set to zero, and the output signal value of the spiral scanning signal generation means at that time is held and the signal addition means It has become possible to search easily and accurately coma-free axis and spherical aberration axis in a short time by providing the means for supplying. Therefore, it has become possible to take an ultra-high-resolution photograph with little influence of aberration.

[Brief description of the drawings]

FIG. 1 is a diagram showing the configuration of an apparatus for explaining an embodiment of the present invention, FIGS. 2 to 4 are diagrams for explaining the operation, and FIGS.
The figure is a diagram for explaining a conventional example. 1: Electron gun 2: Focusing lens 3: Sample 4a, 4b: Deflector 5: Objective lens 6: Intermediate lens 7: Projection lens 8: Fluorescent plate 9: Imaging device 10: Alignment signal generator 11: Balance circuit 3x, 13y : Circular scanning signal generator 14x, 14y: Spiral scanning signal generator 15: Signal addition circuit 16: Control means 17: Signal holding circuit 18: Operation terminal

Claims (1)

(57) [Claims] 1. A two-stage deflector for deflecting an electron beam focused by a focusing lens system in front of an objective lens, and a deflection signal for causing an electron beam to enter a point on a sample at an arbitrary tilt angle. Alignment signal generating means for generating;
Circular scanning signal generating means for generating a signal for circularly scanning the electron beam around the axis of the electron beam tilted by the deflection signal generated by the alignment means; Helical scanning signal generating means for generating a signal for helically scanning around a point, signal adding means for superposing a circular scanning signal and a helical scanning signal on the alignment signal, and the adder by pressing switch means An electron microscope provided with means for making the output signal value of the circular scanning means supplied to the device zero and for holding the output signal value of the spiral scanning signal generating means at that time and supplying it to the signal adding means. .