JPS5824899B2 - Focusing device for electron beam equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focusing device for an electron beam device, and more particularly to an improvement in a focusing device for a transmission electron microscope.

Conventionally, a device commonly called a wobbler has been used as a focusing device for a transmission electron microscope.

This Wappler is a device that changes the incident angle of the electron beam on the sample over time, and observes the movement of the sample image that appears on the fluorescent plate. Focusing is performed by changing the applied current.

In this case, the brightness of the phosphor plate must be good enough to be observed with the naked eye, so a considerably large amount of electron beam will be irradiated onto the sample.

Therefore, there is a drawback that the sample may be damaged by changing its morphology or characteristics.

In order to avoid this, a problem arises in that focusing must be done in dark conditions when photographing images at high magnification, and a solution to this problem has been eagerly awaited.

As a solution to the above problem, there has conventionally been a volatility doubling device capable of photographing and observing an image of a fluorescent plate.

However, this device is difficult to operate and maintain, and is extremely expensive, so it is hardly used at present.

SUMMARY OF THE INVENTION An object of the present invention is to provide a focusing device for an electron beam device that allows easy and reliable focusing even in a dark state that is conventionally impossible to observe with the naked eye.

FIG. 1 is a diagram explaining the principle of the present invention.

The image of the sample 1 is formed at the position of the slit 6 by the objective lens 4 and is in focus, but if it is not adjusted, the focal length of the objective lens is not appropriate, so the image on the object plane 3 will be at the slit 6. It occurs at position 6.

When an electron beam 2 deflected by a deflection coil (not shown) on the top of the sample 1 is made incident on the sample 1 in this state, an image of the sample is created in occurs in

Similarly, when the deflected electron beam 2 is incident on the same point on the sample, the image on the object plane 3 becomes A'' and becomes the image plane 5.
An image of A" is formed at position B".

If the distance between the sample 1 and the object surface 3 is Δf, and the incident angle of the deflected electron beams 2 and 2' on the sample is α, then λ at the object surface 3
, A'/ is 2α・Δf, and the distance of ' in the slit 6 is
The distance B" is equal to the distance A'A" multiplied by the magnification of the objective lens 4.

``Bτ is further magnified by a normal magnifying lens and projected onto the fluorescent plate of the electron microscope, but for convenience of explanation, the slit 6 is located near the fluorescent plate and close to the bottom thereof. It is assumed that a fluorescent surface 7 and a photomultiplier tube 8 closely attached thereto are placed.

When B%Z comes to the right side of the slit 6, the electron beam reaching the fluorescent surface 7 decreases, and when y, the electron beam is released and the electron beam amount increases.

That is, the larger Δf is, the more the amount of electron beam passing through the gap 6 changes depending on the direction of incidence of the electron beam incident on the sample.

When the focal length of the objective lens 4 is set appropriately and the sample 1 and the object surface 3 match, the above I¥B(-!) will match at the center of the slit 6 opening, so the direction of the electron beam incident on the sample will change. However, the amount of electron beam detected by the photomultiplier tube 8 remains constant.

That is, when the detected amount of electron beam (fluorescence amount) remains unchanged even if the direction of incidence of the electron beam incident on the sample 1 changes, it can be said that the state is in focus.

If there is a difference in the amount of electron beam entering the slit 6 depending on the direction of the incident electron beam with the sample 1 removed, the correction value should be given to the detected value in advance (by doing so, the deflection when the sample is inserted is Only changes caused by the electron beam can be detected.

Practical products require such a correction device.

It is important that the width of the slit 6 and its position can be adjusted.

In other words, if the target sample is small and the image size is within the narrow gap, and the amount of displacement on the image plane 5 is smaller than the gap, there will be no change in the electron beam amount due to defocus, so the image will be Being able to change the relative position of the slit or the inside of the slit is an important requirement for improving measurement accuracy.

To adjust the relative position, the slit 6 may be moved relative to the image, or the slit 6 may be fixed and the image may be deviated.

In the case of deflecting the latter image, the sample may be moved using a sample fine movement device, or the electron beam may be deflected using conventional means using a deflection coil in the electron beam path.

The above-mentioned movement of the slit or movement of the sample image may be performed manually or automatically.

FIG. 2 is an explanatory diagram of a transmission electron microscope which is one embodiment of the present invention.

The electron beam generated by the electron source 9 is multiplied by the anode 10, condensed by the condenser lens 11, and periodically deflected by the deflection coil 12 to irradiate the sample 1.

The electron beam that has passed through the sample is passed through the objective lens 4 and the intermediate lens 13.
A sample image magnified by the projection lens 14 is generated on the fluorescent surface of the detector 15.

This detector 15 is equipped with a slit 6, a fluorescent surface 7 and a photomultiplier tube as shown in FIG.

As long as the slit 6 is placed near the final image phosphor plate of the electron microscope, the depth of focus will be deep, so it can be placed either above or below it.

The electrical signal generated by the detector 15 is amplified by an amplifier 16 and the amount of electricity is displayed on a display 17, and the variation is extracted by a variation detector 18 and transmitted to an automatic adjustment device 19. It will be done.

The automatic adjustment device 19 increases or decreases the excitation current applied to the objective lens 4 in a direction to reduce the amount of variation in the amount of electricity, and automatically adjusts so that the amount of variation always becomes the minimum value.

As stated above in the explanation of the principle of the present invention, the fact that the change in the amount of electricity generated by the detector when irradiating electron beams with different incident directions is minimal means that the excitation current flowing through the objective lens 4 is in an optimal state. This indicates that the focal length of the objective lens is in the optimum state.

This situation can be seen on the display 17, which may be an electric meter or a CRT tube, or a display whose light intensity or color changes.

In order to process the quantity of electricity from the detector 15, it is better to detect the variation thereof rather than simply comparing the quantity of electricity due to the deflection of the electron beam.

The difference in electron dose due to deflection direction switching is detected by the variation detector 18.
is detected by the objective lens 4, and the minimum value of the amount of change is detected by the objective lens 4.
The focal length is optimal and the image is in focus.

As mentioned earlier, if the detection signal changes simply by deflecting the electron beam without inserting a sample, it is possible to correct both values in advance, or to calculate the change in the ratio between the two. You can monitor it.

To perform automatic focusing, means for detecting the minimum value of the change in the detected quantity of electricity and an automatic adjustment device 19 that automatically changes the excitation current of the objective lens 4 are used.

While changing the excitation current of the objective lens 4 continuously or minutely intermittently, the change value of the detected quantity of electricity is detected, and the excitation current is stopped at the minimum value.

The minimum value of the change can be obtained by using a storage device and comparing successive changes with previous values.

Furthermore, there is no essential difference whether the incident angle of the electron beam changes intermittently or continuously.

The photomultiplier tube used as the means for detecting the electron beam has a power of about 10-
It has a high sensitivity of 0-15a/crA.

Therefore, when photographing the final sample image at a magnification of 10,000 copies, the density of the electron beam irradiating the sample is 'el O'? amp
This means that it may be lowered to /crtH.

When observing a sample image on a conventional fluorescent plate with the naked eye and performing focusing, the electron beam density with which the sample is irradiated is 1010-3a/
Compared to the case where about crl was required, there is much less damage to the sample.

In addition, the photomultiplier tube is inexpensive, has coordination, and the processing rotation of its detection current can be constructed at a relatively low cost.

Other detectors that can be used include a Faraday cage that directly measures the electron beam, and an electron beam multiplier that directly amplifies the charged particle beam.

FIG. 3 is a plan view showing another row of detectors used in the apparatus of the present invention, and FIG. 4 is a sectional view thereof.

Since the fixed slit plate 20 and the movable slit plate 21 having a plurality of slits are arranged one on top of the other, it is possible to adjust the slits to the optimum size according to the fineness of the sample composition and its enlarged image magnification. There are advantages that can be achieved.

This slit can be adjusted by moving the movable slit plate 21, and the longitudinal direction of the slit is perpendicular to the deflection plane of the electron beam to be deflected.

Arranging the slits in this manner has the effect of increasing the electron beam dose without impairing detection performance.

Although the embodiments of the present invention have been described above with reference to a transmission electron microscope, the present invention is not limited to this; however, it can be used as a focusing device for a device that converges and deflects a wide range of electron beams.

INDUSTRIAL APPLICATION This invention has the remarkable effect that damage to a sample by an electron beam can be prevented and focusing of a sample body can be performed easily and reliably.

[Brief explanation of the drawing]

Figure 1 is a diagram explaining the principle of the present invention, Figure 2 is 1 implementation 1 of the present invention.
FIG. 3 is a plan view showing another embodiment of the detector used in the apparatus of the present invention, and FIG. 4 is a sectional view of the detector shown in FIG. 3. Explanation of symbols, 1... Sample, 2. z... Deflected electron beam, 3... Object surface, 4... Objective lens, 5... Image plane, 6... Slit, 7... Fluorescent surface, 8... Photoelectric detector, 9... Electron source, 10... S pole, i i... Condenser lens, 12... Deflection coil, 13...
Intermediate lens, 14... Projection lens, 15... Detector, 16... Amplifier, 17... Display device, 18... Fluctuation detector, 19... Third dynamic adjustment device, 20 ... Fixed slit plate, 21... Movable slit plate.

Claims (1)

[Scope of Claims] 1. A means for changing the angle of incidence of an electron beam incident on a sample, an electron lens for forming an image of the electron beam transmitted through the sample, and a means for transmitting the electron beam imaged by the electron lens. a slit arranged such that its length direction is substantially perpendicular to the direction of change in the incident angle of the electron beam incident on the sample;
means for detecting the electron beam passing through the slit; means for automatically changing the focal length of the electron lens; and in response to an output signal of the detecting means, when the change in the signal becomes minimum. A focusing device for an electron beam apparatus, comprising means for stopping a change in focal length of the electron lens. 2. A focusing device for an electron beam apparatus according to claim 1, characterized in that the width of the slit through which the electron beam passes can be adjusted.