JPH03233846A - Electron microscope - Google Patents

Abstract

PURPOSE:To search a coma free axis and a spherical aberration axis easily and accurately by giving zero to the output signal value of a circular scanning means applied to a signal adder through depressing a switching means and at this time holding the output signal value of a spiral scan signaling means to supply to the signal adding means. CONSTITUTION:An alignment signaling means 10, circular scan signaling means 13X, 13Y, spiral scan signaling means 14X, 14Y and a signal adding means 15 are provided. 17X, 17Y are also provided for giving zero to the output signal value of the circular scanning means 13X, 13Y supplied to the signal adder 15 through depressing a switching means and at this time holding the output signal value of the spiral scan signaling means 14X, 14Y to supply to the signal adding means 15. It is thus possible to search a coma free axis and a spherical aberration easily and accurately.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electron microscope that can easily search for the coma-free axis and 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 the electron beam emitted from the electron gun onto a sample 3, and 4a and 4b are deflectors consisting of a pair of deflection coils for the X and Y directions. , 5 is an objective lens, 6 is an intermediate lens, 7
8 is a projection lens, 8 is a fluorescent screen, and 9 is an imaging device. 10
11 is an alignment signal generator that generates a deflection signal for making the electron beam incident on a point P on the sample 3 at an arbitrary inclination angle θ; This is a balance circuit for adjusting the intensity ratio of the signals supplied to both deflection coils 4a and 4b so that the signals are incident on the same point P of the sample.

When taking a high-resolution photograph of a sample using a transmission electron microscope configured as described above, it is necessary to align the axis of the incident electron beam with the lens in order to minimize the effects of aberrations of the objective lens. Although considered essential, such axis alignment has been achieved by aligning the voltage centers of the objective lens 5.

In addition, this voltage centering is achieved by slightly varying the high voltage (accelerating voltage) supplied from the electron gun power source 12 to the electron gun 1 to slightly fluctuate the energy of the incident electron beam. An imaged electron microscope? Taking advantage of the fact that the position of the mirror image is varied, the angle of incidence is adjusted by the deflection coil 4b so that the electron microscope image moves symmetrically about the center point of the fluorescent screen, and the axis with the minimum chromatic aberration is found. It is something that is given.

[Problems to be Solved by the Invention] In the electron microscope configured as described above, when the magnetic pole piece spacing and hole diameter of the objective lens 5 are relatively large, the magnetic field distribution ( magnetic field gradient)
is slow, so high-resolution photographs of the sample could be obtained by voltage centering as described above to minimize the effects of chromatic aberration.

However, with the ultra-high resolution of transmission electron microscopes, the spacing between the magnetic pole pieces and the aperture diameter of the objective lens have become smaller, which has sharpened the magnetic field distribution (magnetic field gradient) in the plane perpendicular to the optical axis, making it difficult to center the voltage. If you do this, it will not be possible to align the incident electron beam with the coma-free axis and the spherical aberration axis, so even a slight axis deviation will not allow sufficient resolution to be obtained at the periphery of the ultra-high resolution electron microscope image. The effects of aberrations and spherical aberrations have come to the fore. Therefore, it is necessary to accurately determine the coma-free axis and the spherical aberration axis and to align the incident electron beam with these axes to minimize the influence of aberrations.

The present invention takes the above-mentioned problems into consideration and aims to provide an electron microscope that can easily search for the coma-free axis and spherical aberration axis of the electron microscope.

[r! Means for Solving the 4 Problems] The present invention includes a two-stage deflector for deflecting an electron beam focused by a focusing lens system in front of an objective lens, and a two-stage deflector for deflecting an electron beam focused by a focusing lens system in front of an objective lens, and an alignment signal generating means for generating a deflection signal for making the electron beam incident; and a signal for circularly scanning the electron beam around the pongee of the electron beam tilted by the deflection signal generated by the alignment means. a circular scanning signal generating means; a spiral scanning signal generating means for generating a signal for spirally scanning the tilt axis of the electron beam with the incident point of the electron beam as the center; and a circular scanning signal and a spiral scanning signal for the alignment signal. a signal adding means for superimposing signals, and an output signal value of the circular scanning means supplied to the adder by depression of a switch means is set to zero, and an output signal value of the spiral scanning signal generating means at the time is held. The present invention is characterized in that a means for supplying the signal to the signal addition means is provided.

[Example] Hereinafter, an example of the present invention will be described based on the drawings. 1st
The figures are an apparatus configuration diagram for explaining one embodiment of an electron microscope according to the present invention, FIG. 2, and a diagram for explaining the third operation.

In FIG. 1, the same components as in FIG. 4 are given the same numbers and their explanations are omitted.

The embodiment shown in FIG. 1 differs from the conventional example in that it includes an alignment signal generator 10 that generates a deflection signal for making the electron beam incident on a point P on the sample 3 at an arbitrary inclination angle θ; circular scanning signal generators 13 A helical scanning signal generator 14x generates a signal for scanning the tilt axis in a spiral manner with the incident point P of the electron beam as the axis (fulcrum).

14y, a signal addition circuit 15 for superimposing a circular scanning signal and a spiral scanning signal on the alignment signal, the alignment signal generator 10, circular scanning signal generating means 13x,
13y and the spiral scanning signal generators 14x, 14y, and the circular scanning signal generators 13x, 1 by pressing a switch provided on the operation terminal 18.
The point is that signal holding circuits 17x and 17y are provided to set the output signal value of 3y to zero and to hold the output signals of the spiral scanning signal generators 14x and 14y at that point in time.

Now, the electron beam emitted from the electron gun is focused by a focusing lens system and then incident on the sample 3.

At this time, the electron beam is deflected by the deflection coil 4a, and is also swung back by the deflection coil 4b to the sample 3.
The alignment signal (X OrYo) generated in the alignment signal generation circuit 10 is supplied to the deflectors 4a and 4b via the addition circuit 14 and the balance circuit 1o.

In the state where the electron beam is incident on the point P on the sample 3 at an arbitrary inclination angle as described above, the axis alignment to suppress the influence of spherical aberration has not yet been achieved in any way, and the alignment signal generator 1o The generated alignment signal (X
o, Yo), the irradiation point P and the inclination angle θ are determined.

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

First, the electron beam is directed to a point P on the sample 3 at an arbitrary inclination angle by deflectors 4a and 4b to which alignment signals (xo, Yo) generated by the alignment signal generator 13 are supplied.
is being incident on. Here, the circular scanning signal generator 13X is generated by the $1go means 15.

13y is controlled to generate a signal for circularly scanning the electron beam (alSln ωl t, at Cos ωi
t) 75< is generated, and the signal is supplied to the adder circuit 15 and superimposed on the alignment signal (xO, Yo).

Thereby, the electron beam irradiated onto the sample 3 has a deflection signal (Xo + a + Sin ωlt) outputted from the adding circuit 15 and supplied to the deflectors 4a and 4b.

By Y(1+a 1Cos ωl t ), the second
As shown in Figure (a), the beam is irradiated so as to rotate in a cone shape (hollow cone shape) while maintaining the incident point P onto the sample.

At this time, the image j of the target part of the sample image projected onto the fluorescent screen 8 is influenced by aberrations and moves in an elliptical shape as shown in FIG. 2(b).

In particular, in objective lenses where the magnetic pole piece spacing and aperture diameter have become smaller with the advancement of ultra-high resolution, the magnetic field distribution (magnetic field gradient) in the plane perpendicular to the optical axis has become a key factor, minimizing the influence of spherical aberration. In a state where axis alignment is not achieved, even a slight inclination of the incident angle of the electron beam causes a phenomenon in which the image projected on the fluorescent screen shifts significantly, but this image movement is caused by spherical aberration. Since the image moves in proportion to the cube of the angle θ at which the electron beam is tilted with respect to the axis of , the image moves largely in an elliptical shape, as described above.

However, as shown in Figure 3(a), the axis k of spherical aberration
When an electron beam that rotates in a cone shape with . Become.

Therefore, before the axis alignment for suppressing the spherical aberration is achieved, the operator adjusts the alignment signal while looking at the image moving in an elliptical shape, so that the sample image moves in a substantially perfect circle. By aligning the axes like this, the axis of spherical aberration can be searched.

However, it is extremely troublesome and troublesome for the operator to manually observe the movement of the image while variously changing the incident angle and direction of the electron beam to search for the axis of spherical aberration.

Therefore, a signal (az tsln ω
1 t, a2tCos (JJI t) is supplied to the addition circuit 15, where the helical operation signal is further added to the alignment signal on which the circular scanning signal is superimposed! It is folded i.

Then, the output signal of the adder circuit 15 (Xo +a1Sl
n ω1 to 10a2 tsln (Lll t, Y6
+fi,C.

s ω, t+a2tcos ωl t) is supplied to the two-stage deflectors 4a and 4b via the balance circuit 11, so that the electron beam maintains the incident point P on the sample 3 as shown in FIG. 4(a). However, since the center of rotation of the cone-shaped electron beam is scanned spirally as shown by A-E in the figure, the incident angle and direction of the electron beam can be easily changed to various values. Here, the constant a of the helical scanning signal is
2 is set so that the speed of the spiral scan is about 10 times slower than the scan period of the circular scan (a+<82).

At this time, while observing the sample images A' to E' projected on the fluorescent screen as shown in FIG. When a switch provided on the terminal 18 is pressed, the output signal values of the circular scanning signal generators 13x and 13y are respectively set to zero by the control means 16, and the circular scanning of the electron beam is stopped. The output signal of the helical scanning signal generator (a2tosfr
lωjo + az to Cosω, to) are held in the signal holding circuits 17x, i, 7y, respectively, and the held signals are supplied to the adder circuit 15 to generate the alignment signal (
Xo, Yo). Then, the output signal of the adder circuit 15 (Xo 10a2to Sin ω1to
.

By supplying 'yO+a2t, Cos (ljl to) to the deflectors 4a and 4b via the balance circuit 11, the spherical aberration axis is maintained.

Note that the above-described embodiment is only one embodiment of the present invention, and the present invention can be implemented with various modifications. For example, in the above embodiment, the period of the spiral scan was set to be about 10 times slower than the period of the circular scan, but the period of the circular scan was set to be about 10 times slower than the period of the circular scan. It is sufficient if the speed is such that a circumference is formed by the circular scanning (light), which is sufficiently slower than the period of the circular scanning of the spiral scanning on the - side, and such that the movement of the image formed by the circular scanning can be seen silently. Any speed is fine.

Furthermore, in the above-described embodiment, the roundness of the image was determined while the operator observed the shape of the image projected on the fluorescent screen. The roundness of the image taken by the image sensor may be automatically determined by pattern recognition or the like, and the circular scanning signal generator may be controlled by the control means 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 an electron beam focused by a focusing lens system in the front stage of an objective lens, and alignment signal generating means for generating a deflection signal for making the electron beam incident at an arbitrary inclination angle; and circular scanning of the electron beam around an axis of the electron beam tilted by the deflection signal generated by the alignment means. circular scanning signal generating means for generating a signal for scanning the tilted axis of the electron beam in a spiral shape with the incident point of the electron beam as the center; a signal adding means for superimposing a circular scanning signal and a spiral scanning signal on a signal, and a spiral scanning signal generating means for reducing the output signal value of the circular scanning means supplied to the adder to zero by pressing a switch means, and generating a spiral scanning signal at the time. By providing means for holding the output signal value of and supplying it to the signal addition means, it becomes possible to easily and accurately search for the coma-free axis and the spherical aberration axis in a short time. As a result, it has become possible to take ultra-high resolution photographs with less influence of aberrations.

[Brief explanation of drawings]

FIG. 1 is an apparatus configuration diagram for explaining an embodiment of the present invention, FIGS. 2 to 4 are diagrams for explaining the operation, and FIG. 5 is a diagram for explaining a conventional example. 1: W gun 2: Focusing lens 3: Samples 4a, 4b: Deflector 5: Objective lens 6: Intermediate lens 7: Projection lens 8: Fluorescent screen 9: Imaging device 10: Alignment signal generator 11: Balance circuit 3x, 13)': 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)

[Claims] 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 to make the electron beam incident on a point on the sample at an arbitrary tilt angle. a signal generating means; a circular scanning signal generating means for generating a signal for circularly scanning the electron beam around an axis of the electron beam tilted by a deflection signal generated by the alignment means; and a tilt axis of the electron beam. spiral scanning signal generating means for generating a signal for spirally scanning the electron beam with the incident point of the electron beam as the center; signal adding means for superimposing a circular scanning signal and a spiral scanning signal on the alignment signal; and pressing of a switch means. Accordingly, the output signal value of the circular scanning means supplied to the adder is set to zero, and means is provided for holding the output signal value of the spiral scanning signal generating means at the time and supplying it to the signal adding means. Characteristic electron microscope.