JPH0963528A - Charged particle microscope image photographing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To determine precisely the exposure time to fit for a desired image part and to provide a satisfactory photographed picture, by providing a means for detecting brightness of an arbitrary part of a charged particle microscope image. SOLUTION: A charged particle beam from a charged particle source 1 irradiates, via an irradiation lens system 2, a specimen held by a specimen holding means 3, and an image formed by charged particles scatteredly transmitted by and emitted from the specimen is, via magnifying/image-forming system lenses 4, 4', converted into an optical image of the specimen on a fluorescent screen 5. A charged particle detector 6 comprizing an avalanche photodiode is installed behind a central part of the fluorescent screen 5 and charged particles thereby detected through a relatively small sized opening 5a for taking out charged particles are counted by a counter 10. An operator operates a deflecting system so that an arbitrary part of a microscope image may agree with a position of the opening 5a and a control device 13, determining appropreate exposure time from data of the counter 10, controls a shutter 11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charged particle microscope image capturing apparatus for a charged particle microscope such as an electron microscope, and more particularly, it automatically sets a proper exposure when capturing a charged particle microscope image. The present invention relates to a charged particle microscope image capturing device capable of taking a photograph or recording a moving image.

[0002]

2. Description of the Related Art FIG. 3 is a block diagram of an example of a conventional charged particle microscope image taking apparatus. In the figure, 17 is a column, 18 is a shutter, 19 is a shutter control mechanism, 20 is a fluorescent screen provided on the image forming surface, 21 is a camera for taking a picture or video, 22 is an ammeter, and 23 is a controller. Show.

The fluorescent screen 20 can take a horizontal position and a vertical position. In the horizontal position, a magnified image of a sample (not shown) is projected, and an observable charged particle microscope image is emitted on the fluorescent screen surface. To be done.

Conventionally, the determination of the exposure when the charged particle microscope image is taken on the fluorescent screen 20 by using the camera 21 has been performed by the following operation. First, as shown in the figure, a fluorescent screen 20 on which an electron microscope image is projected is used as an electrode, and an electron flow incident on this electrode is input to an ammeter 22 by a cable to measure a current value I. Here, the current density J is calculated from the area S of the fluorescent screen 20 and the current value I as follows.

J = I / S (1) Further, assuming that the amount of charge per unit area required to expose the image recording medium in the camera 21 is K, an appropriate exposure time t
Can be calculated as follows.

T = K / J (2) The exposure time calculated in this way is set in the control device 23 to control the opening time of the shutter 18.

However, the sensitivity of the recording medium of a typical electron microscope is 10 ⁻¹¹ to 10 ⁻¹² C / cm ² , and therefore 1
Taking an exposure time of ² seconds, an area of 1 cm ² is 10
Therefore, a current amount of 1 pA is emitted as imaging electrons. Further, when the exposure time is long, the current amount becomes smaller. Since it is quite difficult to measure such a minute amount of current, the area of the fluorescent screen 20 used to capture the electron flow is increased to increase the amount of current, thereby enabling accurate measurement. Therefore, the size of the area used to determine the exposure can be several cm ² . Since the standard size of a medium for recording an electron microscope image is about 10 cm square, this means that the average current density of a considerably wide portion of the image is measured. Therefore, in the case of a sample having a uniform image brightness, that is, a current density, a good result can be obtained by this method, but when the current density is not uniform, such a method cannot provide proper exposure. The case is coming. One example is the case of image capturing by a method called the dark field observation method.

FIG. 4A is an explanatory view of image capturing by the dark-field observation method. In the figure, 24 is a sample, 25 is an enlarged image forming lens system, 26 is a diaphragm, 26a is an aperture, and 27 is dark. The visual field image is shown.

In this method, only the electrons scattered by the sample 24 can pass through the aperture 26a of the diaphragm 26 by the action of the magnifying and imaging lens system 25, and all the remaining electrons are imaged on the surface of the diaphragm 26. Then shut off. For this reason, as shown in FIG. 4B, electrons do not enter the remaining most area of the dark field image 27 except the central portion corresponding to the sample, so that the image of the sample captures electrons. , The average current density of the image area detected with the fluorescent screen as an electrode is smaller than the current density of only the image part of the sample, and as a result, the exposure time determined by the exposure determination method described above. Then, the exposure time becomes excessive.

Another example is the case of capturing an image called an electron diffraction image. This image is a collection of electrons in many small dots as shown in FIG. At this time, the difference in intensity between the brightest part in the center, the peripheral part, and the weak spots is several orders of magnitude or more, so that the dynamic range of the recording characteristics of a single recording medium is often insufficient. In such a case, it is necessary to determine the exposure time in accordance with the brightness of the point of interest, but the brightness of such a point is impossible by the above-mentioned measurement of the average current density. In addition, the exposure time of the diffraction image often exceeds 1 minute, and the current is very weak at this time, so the sensitivity of the ammeter is insufficient in the above method, and an appropriate exposure time can be determined. There wasn't.

[0011]

As described above, in many conventional charged particle microscope image capturing, it is difficult to perform automatic exposure, and the operator takes in a fluorescent screen or a CRT image gradation displayed by a video camera and displayed by the operator. It was often the case that the exposure time was properly judged and the exposure was adjusted manually.

[0012]

DISCLOSURE OF THE INVENTION The present invention has a means for automatically controlling the exposure time for photographing by accurately determining the exposure time that matches the brightness of an arbitrary portion of interest in a charged particle microscope image. An object is to provide a particle microscope image photographing device.

The structure of the present invention for solving the above-mentioned problems is a charged particle source, an irradiation lens system, a sample holding means,
In a charged particle microscope image photographing apparatus for photographing an enlarged sample image on a fluorescent screen in a charged particle microscope in which a magnifying imaging lens system and a fluorescent screen are arranged in series, charged particles incident on a part of the fluorescent screen. The charged particle extracting means for extracting only the charged particle, the charged particle detector for detecting the charged particles extracted by the charged particle extracting means, the counting means for counting the charged particles detected by the charged particle detecting means, and the counting means for counting Exposure control means for controlling the exposure time of photographing according to the number of detected charged particles.

According to the configuration of the charged particle microscope image taking apparatus of the present invention, by providing a means for taking out charged particles which is sufficiently smaller than the entire image, for example, an opening in the fluorescent screen, the charged particles which are incident only on that part. The current density of the wire can be measured.

At that time, the charged particle flow to be measured becomes small, but in the above configuration, it is possible to measure even a minute current by counting the charged particles one by one. If a diode or avalanche photodiode is used as a charged particle detector, when a charged particle with high energy is incident, a plurality of carriers are generated inside the detector to multiply the charge, so that one charged particle can be used in the subsequent stage. A sufficiently large output pulse that can process the output pulse signal can be obtained by the electronic circuit.
For example, a minute current can be measured and the dynamic range of several digits can be easily dealt with by counting the output pulse for a certain period of time after separating the output pulse from noise by separating it from noise.

The current density is calculated from the thus measured current value and the area of the opening in which the charged particle beam is taken in, the exposure time is determined from the sensitivity of the recording medium, and the shutter or the like is automatically controlled to obtain an appropriate exposure. Shooting can be done depending on the time.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a configuration diagram of an embodiment of the present invention, in which 1 is a charged particle source, 2 is an irradiation lens system, and 3
Is a sample holding means to be observed, 4, 4'is a magnifying and imaging lens system, 5 is a fluorescent screen, 5a is an opening for extracting charged particles, 6 is a charged particle detector, 7 is an amplifier, 8 is a wave height discriminator, 9 Is a waveform shaper, 10 is a counter, 11 is a shutter, 12 is a shutter control mechanism, 13 is a control device, 1
Reference numeral 4 denotes an image recording device.

The beam of charged particles emitted from the charged particle source 1 is focused by the irradiation lens system 2 and irradiates the sample held by the sample holding means 3. Transmitted and scattered in the sample,
The sample image of the charged particles emitted from the sample is magnified and projected on the surface of the fluorescent screen 5 by the magnifying and forming lens systems 4 and 4 '. On the fluorescent screen 5, the magnified sample image formed by the charged particles is converted into a light image, and the image recording device 14 makes it possible to optically capture and record. The exposure time of the photographing at this time is controlled by the shutter control mechanism 12 for the opening time of the charged particle flux passage by the shutter 11 provided between the magnifying imaging lens systems 4, 4'and the fluorescent screen 5, that is, the shutter speed. As a result, the length of the light emission time of the fluorescent screen 5 is changed to adjust.

The mechanism for automatically determining this exposure time is
It is as follows. The opening 5a provided in the central portion of the fluorescent screen 5 has a considerably small area with respect to the entire area of the fluorescent screen 5, and has a function of extracting the charged particles at the central portion of the charged particle flux incident on the fluorescent screen 5 in a spot manner. With. Behind the fluorescent screen 5, a charged particle detector 6 using an avalanche photodiode is provided so as to face the opening 5a, and the charged particles passing through the opening 5a are detected. The charged particle detector 6 converts the energy of each incident charged particle into a hole-electron pair and further avalanche multiplies it to generate a relatively large output pulse.

An output pulse from the charged particle detector 6 is amplified by an amplifier 7, a pulse height discriminator 8 extracts only a pulse having a level higher than a noise level, and a waveform shaper 9 shapes the pulse waveform. Is input to the counter 10. The counter 10 is controlled by the control device 13 and counts input pulses within a certain period.

The control device 13 calculates an appropriate exposure time from the count value of the counter 10 and controls the shutter control mechanism 12 based on the calculated exposure time. Operators of charged particle microscopes
The image deflection operation is performed so that the target portion of the sample image comes to the position of the opening 5a of the fluorescent screen 5, and the exposure time is appropriately determined at the target portion of the sample image.

FIG. 2 shows a sectional structure of an example of an avalanche photodiode used as the charged particle detector 6, which is manufactured by epitaxially growing a π layer, a p layer and an n + layer on a p + substrate. It P layer and n during operation
A large reverse voltage is applied between the + layers to form an avalanche multiplication region 15. The π layer having a low impurity concentration functions as the absorption region 16 that converts the energy of the incident charged particles into hole-electron pairs. The detection of charged particles by this avalanche photodiode is based on the following principle.

When the charged particles are incident on the avalanche photodiode, a number of carriers, which is calculated from the energy E of the charged particles and the generation energy e of electron-hole pairs, is generated in the absorption region 16 as follows.

E / e (3) This carrier is accelerated by the depletion layer electric field in the avalanche multiplication region 15 and amplifies the carrier by impact ionization. When the multiplication factor is M, the finally output charge amount is as follows.

QME / e (4) Here, q is the charge of the electron. If this charge is output with a pulse width t, the maximum current i of the pulse can be expressed as follows.

I = qME / et (5) Assuming that the avalanche photodiode is made of silicon, e = 3.61V and the avalanche multiplication factor M = 2.
0, pulse width t = 500 ps, charged particles 100 keV
Assuming the electrons are, the current is about 0.2 mA. This value is an output of a level large enough to be processed by the electronic circuit in the subsequent stage.

As described above, the detection output of the charged particles by the avalanche photodiode is separated from the noise by the wave height discriminator 8 and converted into a pulse which can be input to the counter 10 by the waveform shaper 9. In this way, the counter 10 counts for a certain period of time, thereby opening the opening 5a.
The current density of charged particles incident on can be measured. This value is input to the control device 13, and the first equation, the second equation
Calculate the exposure time from the formula, control the shutter 11,
It is designed to shoot with proper exposure.

The avalanche photodiode may be another charged particle detector, for example, an ordinary diode. In this case, since there is no avalanche multiplication function, the level of the output pulse becomes small, and therefore it is suitable for a charged particle device having high energy.

These semiconductor diode type charged particle detectors have a short dead time after detecting one charged particle until the next charged particle can be detected. Even if the rate goes up, 10 ⁹ per second
Can count more than one. Since each charged particle is counted, it has a very large dynamic range of about 10 digits from 1 to 10 ⁹ or more.

If the current of the charged particles is small, a detector having a long dead time, such as a channeltron or a microchannel plate, may be used. In addition, even if the output from the detector is measured as an analog current, the charge of the charged particles is multiplied by the detector, so even if the current of the charged particles is directly measured using the conventional screen as an electrode, Can measure with high sensitivity. This method has the advantage of simplifying the device configuration.

The number of the openings 5a provided in the fluorescent screen 5 is not limited to one, and a plurality of appropriately arranged openings can be used to detect charged particles respectively, and each detected output can be detected. May be used selectively or in combination.

[0032]

According to the present invention, since the current density at any point of the charged particle microscope image can be measured, it is possible to record the charged particle microscope image on the recording medium with an appropriate exposure time for the object of interest. Become.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a charged particle microscope image capturing apparatus according to an embodiment of the present invention.

FIG. 2 is a sectional structural view of an example of an avalanche photodiode.

FIG. 3 is a configuration diagram of an example of a conventional charged particle microscope image capturing device.

FIG. 4 is an explanatory diagram of image capturing by a dark field observation method.

FIG. 5 is an explanatory diagram showing an electron diffraction image.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Charged particle source 2 Irradiation lens system 3 Sample holding means 4, 4'Magnification imaging lens system 5 Fluorescent screen 5a Aperture for taking out charged particles 6 Charged particle detector 7 Amplifier 8 Wave height discriminator 9 Waveform shaper 10 Counter 11 Shutter 12 shutter control mechanism 13 control device 14 image recording device 15 avalanche multiplication region 16 absorption region 17 column 18 shutter 19 shutter control mechanism 20 fluorescent screen 21 camera 22 ammeter 23 controller 24 sample 25 magnifying imaging lens system 26 diaphragm 26a aperture 27 Darkfield image

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Junji Endo 3-3-33 Tsurumai, Sakado City, Saitama Prefecture (72) Inventor Tetsuji Kodama 2-21-43 Shirotori House, 2-4-43 Daiho, Atsuta-ku, Nagoya-shi, Aichi Prefecture ( 72) Inventor Tsuneyuki Urakami 586-1 Mori-cho, Mori-cho, Suchi-gun, Shizuoka Prefecture (72) Koji Tsuchiya 353 Noguchi-cho, Hamamatsu-shi, Shizuoka Prefecture B-502 (72) Shinji Osuka 675-146, Arimanishi-cho, Hamamatsu-shi, Shizuoka Prefecture

Claims (3)

[Claims] 1. A magnified sample image on a fluorescent screen in a charged particle microscope in which a charged particle source, an irradiation lens system, a sample holding means, a magnifying imaging lens system, and a fluorescent screen are arranged in series. In the charged particle microscope image capturing apparatus for, a charged particle extraction means for extracting only charged particles incident on a part of the fluorescent screen, a charged particle detector for detecting charged particles extracted by the charged particle extraction means, and a charged particle Charged particles characterized by comprising: counting means for counting the charged particles detected by the detecting means; and exposure control means for controlling the exposure time of photographing according to the number of detected charged particles counted by the counting means. Microscope imager. 2. The charged particle extracting means according to claim 1, wherein the charged particle extracting means is an opening provided in the surface of the fluorescent screen, and the charged particle detector is provided so as to face the opening and detects the charged particles through the opening. A charged particle microscope image capturing apparatus, which is configured as described above. 3. The charged particle microscope image capturing apparatus according to claim 1, wherein an avalanche photodiode is used as the charged particle detecting means.