JPS6091540A - Alignment unit for transmission electron microscope - Google Patents

Abstract

PURPOSE:To accurately and easily align the direction of the electron rays that are incident on a lens to the optical axis of the lens by providing a deflection means that freely varies the incidence angle of the electron rays while the irradiation area is being fixed and a means that supplies step-type deflection signals to a deflection means. CONSTITUTION:Deflection coils 6x and 7x are kept so that the deflection direction to electron rays can be inverse reciprocally and the ratio of a deflection angle can be fixed and are adjusted so that the electron ray irradiation point on a sample 1 cannot be moved. The output of a pulse rectangular wave generation circuit 11 that outputs a rectangular wave with the wave height value + or -DELTAl is applied to an addition circuit 8 through a switch 10. The electron microscope picture of a square hole can be focused almost at the center of a phosphor plate and the switch 10 is turned on by slightly defocusing an objective lens 3. When the output I0 value of a deflection power supply 9 is adjusted so that the oscillation lengths d1 and d2 of a picture on the phosphor plate can be equaled and the switch 10 is turned off, the incident direction of the electron rays and the optical axis of a lens can accurately be aligned for the X-axis direction. The same way applies also to the Y-axis direction.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an alignment device for accurately and easily aligning an electron beam incident on an objective lens of an electron microscope with an optical axis of the objective lens.

In an electron microscope, it is necessary to align the lens axes of a plurality of imaging lenses, and various adjustments such as so-called voltage axis alignment are performed. However, even if the axis alignment between the imaging lenses is perfect, if the central axis of the electron beam is incident even slightly tilted to the optical axis of the objective lens, the focus position will shift depending on the direction in which the electron beam passes through the sample. This phenomenon occurs, and when pursuing the ultimate resolution, it becomes impossible to obtain an image with good symmetry, making image interpretation extremely difficult.

Therefore, especially when observing a crystalline sample, it is important to accurately align the incident direction of the electron beam with the optical axis of the objective lens.

In FIG. 1, an electron beam 2 enters a crystalline sample 1 at an angle α to the optical axis Z of an objective lens 3, and forms an electron microscope image on a fluorescent plate 5 through an imaging lens system 4. This shows the state of image formation. At this time, an electron microscope image (bright field image) of the electron beam that passed through sample 1 without being scattered.
The amount of deviation Uo from the original imaging position on the fluorescent plate 5
And, the amount of deviation of the electron microscope image (dark-field image) created by the Bragg-reflected electron beam at the Bragg angle ±α0 from the original image-forming position on the fluorescent plate 5 is 0, which is the image-forming position including the objective lens. The image magnification of the lens system is M1, the spherical aberration coefficient of the objective lens is O81, and the defocus amount of the objective lens is 8f.
Then, l-1e1-1eine is expressed by the following equation.

vo=ト'I・(Cslo'bl"+△+)°α7・
... (1) U = H-(C5, lc/l-1゜1'''
+JMc(u:C4), <2> This (1) and (
From equation 2), the deviation ΔU of the dark-field image with respect to the bright-field image is expressed as follows.

bU soil = Hv(Cs(Wtknu l' (u, ±'-)-
1o&l'c/l) Sat t-'r-u, E ,,, (3)
As can be seen from equation (3), if αt is not zero, the imaging positions of the two dark-field images, which should be formed at symmetrical positions with respect to the bright-field image, will no longer be symmetrical with respect to the bright-field image. . In this way, if the symmetry is broken, it will be a major hindrance to the analysis of electron microscope images and diffraction patterns, so it is necessary to adjust the tilt angle by the deflection device so that it becomes -αt so as to cancel the angle α1. However, it is extremely difficult to know whether the electron beam is tilted with respect to the optical axis of the lens and to what extent the angle α [of that tilt is Therefore, it was not possible to accurately correct the inclination of the electron beam with respect to the lens optical axis.

In order to solve this problem, the present invention makes it possible to monitor the degree of inclination between the central axis of the incident electron beam and the lens optical axis of the objective lens, thereby making it possible to easily control the accurate incident angle of the electron beam. The purpose of this device is to provide a deflection means that arbitrarily varies the incident angle of the electron beam while fixing the sample irradiation area with the electron beam. A fixed DC signal is expressed as ΔI
The present invention is characterized in that means is provided for supplying a step-like deflection signal to the deflection means which switches in three stages: Io-ΔI, Io, and To+Δ■.

Now, an electron beam is applied to a gold foil sample with a square hole with a side of about 10 nm to 1100 nm at an angle α with the axis Z of the lens.
When the objective lens is defocused from a state in which the sample with the (001) plane as the surface is irradiated with the electron beam tilted, the electron microscope image on the fluorescent plate becomes as shown in FIG. In Figure 2, A is a bright-field image (000) of a square hole in gold foil, and B and C on both sides are dark images formed by the (020) and 3-(020>) diffracted electron beams from the sample, respectively. This is a field image.In this way, the aperture F placed on the diffraction pattern forming surface
When a plurality of diffraction points are made incident on the imaging lens system below the intermediate lens, an image A is obtained.

The reason why the image formation positions of B and C are shifted is because they are out of focus, the center of the electron beam is not aligned with the optical axis Z of the lens, etc. If the electron beam is tilted in the (020) direction from this state (tilt angle -αt), the image shift will be as shown in FIG. 3. That is, the dark-field images B and C, which should be formed at symmetrical positions with respect to the bright-field image A, are formed at asymmetrical positions due to the influence of the angle αt.

From the state shown in FIG. 3, if the electron beam is tilted by an angle (±α1) equal to the symmetrical direction with respect to the tilt angle αt,
The image on the fluorescent screen is shown in Figure 4. Figure 4 shows the angle ±
How images vibrate according to the added inclination centering on images A+, B+, and C+ when α1 is not added (C2→C1→
C3, A2→A1→A3, B2→B1→B3). The deviation of each dark-field image in FIG. 4 is calculated using the following formula.

Δku = M, [cs・(1 line ±α.
f・evening] ... (4) Here, α1=0, and Δf is Δf=cs, (1 地的士CItl〜tezu.しσI'l−
1 spear - σ1) in. ... If defocusing is performed so that (5) is obtained, Δ♂ becomes zero, and the images of A, B, and C match. In other words, these images cannot be made to coincide unless αt is made zero. Therefore, by changing the standard tilt angle αt using a deflection device while changing the defocus amount of the objective lens and adjusting it so that images A, B, and C overlap, the tilt angle α°C can be set to zero. can do.

FIG. 5 is a schematic diagram showing the main parts of an embodiment of the present invention based on the above principle, and the same reference numerals as in FIG. 1 represent the same components. Deflection coils 5x and 7x in FIG. 4 are respectively X-direction deflection coils constituting two stages of deflection means, and are supplied with the output Io of the deflection power supply 9 via the adder circuit 8. The deflection directions of the electron beams of the respective deflection coils are kept opposite to each other and the ratio of the deflection angles is kept constant, and the electron beam irradiation point on the sample 1 is adjusted so as not to move. One input of the adder circuit 8 has
Through the switch 10, the peak value ±ΔI as shown in FIG.
The output of a pulse rectangular wave generation circuit 11 which outputs a rectangular wave of is applied, and a deflection signal of Io±Δ■ is supplied to the deflection coil.

The procedure for accurately aligning the inclination angle of the electron beam entering the sample with the lens optical axis Z using the apparatus shown in FIG. 5 is as follows.

First, we used the aforementioned gold foil sample with a square hole as an observation sample, and after adjusting the voltage axis of the imaging lens system, we focused the electron microscope image of the square hole on a roughly medium flame on the fluorescent plate. Adjust the power output of the imaging lens system. At this time, the output of the deflection power source 9 is kept at zero, and the switch 10 is kept in an off state. Next, the output of the lens power supply (not shown) of the objective lens 3 is slightly defocused from the just focus state (at this time, an image as shown in FIG. 2 is displayed), and then the switch 10 is turned on. turn on. Then, a deflection signal having the signal waveform shown in FIG. 6 is supplied to the deflection coil, and an image as shown in FIG. 4 is displayed on the fluorescent plate.

At this time, if the angle αt is not zero, the distance @ (range) dl over which the two images vibrate as shown in FIG.

d2 are not equal. Next, when the value of the output Io of the deflection power source 9 is slightly changed, the center level value of the deflection signal waveform shown in FIG. 6 changes from 0 to 10, and the vibration distances 111td1 and d2 of the image on the fluorescent screen change. Therefore, this distance 1
1d1. Adjust so that d2 is equal. With this adjustment, the distance was 11dl.

The state where d2 becomes equal means that the deflection angle of the electron beam by the deflection signal 1o becomes equal to -αt, that is, the angle α[ in equation (3) becomes zero. Therefore, by turning off the switch 10 and readjusting the objective lens 3 to just focus, it is possible to display an image in which the incident direction of the electron beam and the optical axis of the lens accurately match in the X-axis direction.

Furthermore, although not shown in Fig. 5, the same deflection means and deflection power source as in the X direction are provided for the Y direction, and by performing the same axis alignment adjustment as for the X direction, electronic It becomes possible to completely match the direction of incidence of the line with the optical axis of the objective lens.

As explained in detail above, according to the present invention, the direction of the electron beam incident on the lens can be accurately and easily aligned with the optical axis of the lens by simply incorporating a simple means into a normal deflection device. , a remarkable effect is exhibited in image observation of crystalline samples using a transmission electron microscope.

[Brief explanation of the drawing]

1 to 4 are schematic diagrams for explaining the imaging state of a crystalline sample image by a transmission electron microscope, FIG. 5 is a schematic diagram showing an apparatus according to an embodiment of the present invention, and FIG. It is a schematic diagram for explaining the operation of the device. 1: Sample, 2: Electron beam, 3: Objective lens, 4: Imaging lens system, 5: Fluorescent screen, 6X, 7y: Deflection coil, 8
: addition circuit, 9: deflection power supply, 10: switch, 11: pulse square wave generation circuit. 8-

Claims (1)

[Claims] A deflection means is provided to arbitrarily vary the incident angle of the electron beam with the sample irradiation area fixed by the electron beam, and when a minute variable DC signal is lo and a minute fixed DC signal is ΔI, Io -Δ1. An axis alignment device for a transmission electron microscope, characterized in that: means is provided for supplying a step-like deflection signal switching in three stages of Io and Io+Δ■ to the deflection means.