JPS6025060Y2 - Scanning electron microscope - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a scanning electron microscope that can always take images of a sample with accurate exposure.

When photographing a sample image in a scanning electron microscope, a cathode ray tube (CR) is used to match the degree of blackening of the film used for photographing.
It is necessary to adjust the brightness of the T), and conventionally, the brightness signal is adjusted in advance according to the film by operating the contrast knob and brightness knob while watching two meters that indicate the amplitude and center level of the brightness signal supplied to the CRT. Adjustments are made to fit within the specified shooting range.

However, since such operations must rely on human power, they are not necessarily accurate.

In addition, conventional methods have focused only on the brightness signal and not on the amount of light actually generated from the CRT screen, so if the brightness of the screen decreases due to changes in the fluorescent surface of the CRT over time, for example,
Even when a CRT transmits a luminance signal of the same intensity, the amount of light generated from the screen decreases, and there is a tendency for underexposure during photographing.

The present invention has been made to improve such conventional disadvantages, and the present invention will be explained in detail below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention, in which numeral 1 represents an electron beam generated from an electron gun (not shown).

The electron beam 1 is narrowly focused onto a sample 3 by a focusing lens, and is two-dimensionally scanned over the sample 3 by a deflection coil 5 to which a scanning signal from a scanning circuit 4 is supplied.

Information such as secondary electrons and backscattered electrons generated from the sample 3 by electron beam scanning is detected by a detector 6, and the obtained detection signal is amplified by an amplifier 7 and then sent to an adjustment circuit 8.

The adjustment circuit 8 includes a contrast adjustment knob 9. Contrast meter 10, brightness adjustment knob 11. Equipped with a brightness meter 12, it measures the amplitude and DC component of the detection signal (
center level) can be adjusted and displayed.

The detection signal to which the desired amplitude and center level have been given by the adjustment circuit 8 is output to the CR directly or via the correction circuit 13.
It is supplied to T14 as a luminance signal.

15 is a switching circuit for switching it.

The deflection coil 16 of the CRT 14 is supplied via a shift circuit 17 with a scanning signal synchronized with the electron beam generated from the scanning circuit 4 on the sample.

The sample image displayed on the screen of CRT14 is
8. Photographs are taken by a camera 20 equipped with an aperture 19.

21 is a photodetector provided to detect light generated from a small area of the screen of the CRT 14;
The average luminance detection circuit 23 via the detector 22 obtained from
sent to.

The average brightness detection circuit 23 is composed of a peak hold circuit 24, 25 that holds the maximum value and minimum value of the detection signal for one screen (one frame), and an average circuit 26 that calculates the average value of the maximum value and the minimum value. There is.

27 is a control circuit for controlling the aperture 19 based on the output signal of the average brightness detection circuit 23.

In such a configuration, the operator first switches the switching circuit 15 to A.
Tilt it to the side, and the sample image and meter 1 displayed on the CRT 14
By adjusting the knobs 9 and 11 while observing 0.12, the amplitude and level of the detection signal sent to the CRT can be kept approximately within a predetermined photographing range.

However, this type of conventional operation causes the disadvantages mentioned above.

Therefore, in this study, the brightness of the image displayed on the CRT screen is detected by the photodetector 21, the average brightness of the image is determined based on the obtained detection signal, and the amount of light falling on the photographic film is determined based on the determined brightness. is under control.

That is, as shown in FIG. 2, a sample image display area a and a brightness detection area are preset on the screen of the CRT 14, and when the shift circuit 17 is not operated, the electron beam in the CRT1J is set in the area a. The device is configured so that the area can be scanned when the image is moved.

Therefore, when the shift circuit 17 is operated, the sample image is displayed in a reduced size.

Therefore, the output signal of the photodetector 21 obtained by detecting the light generated from the area has approximately the same waveform as the luminance signal supplied to the CRT 14.

The peak hold circuits 24 and 25 hold the maximum and minimum values of the output signal of the photodetector 21 for each screen (one frame), and the average circuit 26 adds the maximum and minimum values and divides the sum by 2. The center level (average brightness) of the output signal is detected by calculating the average value.

Next, the control circuit 27 compares the obtained average brightness with a specified brightness set in advance according to the film, and if the actually calculated average brightness is smaller than the specified brightness, controls the aperture 19 to open. The amount of light reaching the photographic film in the camera 20 is increased, and on the other hand, when the actually determined average brightness is greater than the designated brightness, the aperture 19 is controlled in the direction of closing to decrease the amount of light reaching the photographic film in the camera 20. That's what I do.

Therefore, after that, the operation of the shift circuit 17 is stopped and the CRT
By displaying a sample image in area a of the screen and photographing the image with the camera 20, even if there is some error in the result of adjustment by the operator using the adjustment circuit 8, or even if there is insufficient brightness due to aging of the CRT, Based on the signal obtained by detecting the light actually generated from the CRT screen, the aperture 22 operates in a direction to cancel it, so the average brightness of the image photographed on the film is always set in advance. The specified brightness will be reached,
Sample images can always be taken under good exposure conditions.

Furthermore, if we were simply to detect the light from the CRT screen, it would be sufficient to detect the light from the sample image display area, but that would require installing a photodetector inside the camera or placing a semi-transparent mirror in front of the camera. It is necessary to install a camera to extract light and modify a conventional camera, resulting in an extremely complicated structure.

In this regard, in the present invention, the brightness detection area is replaced by the sample image display area a.
Since the photodetector 21 is set at a different position from the camera, it is possible to install the photodetector 21 completely independently of the camera, and this can be done extremely easily without making any equivalent modifications to the conventional camera. .

Although the area set on the CRT screen is different from area a, the scanning time is different, so the brightness is higher in the area with the smaller area.

Therefore, in this embodiment, by setting the switching circuit 15 to B, the correction circuit 13 corrects the brightness signal according to the area ratio of areas a and b, and changes the brightness of the reduced sample image displayed in area a to can be made equal to the brightness of the image displayed on the screen.

Further, in the embodiment described above, the aperture 1 is adjusted by the output of the control circuit 27.
Although the control circuit 9 is not limited to this, any means that can change the amount of light reaching the film may be controlled, for example, the control circuit 8.
It may also be possible to control the

Furthermore, in the above-mentioned embodiment, two-dimensional scanning was also performed in the area of the CRT screen, so a reduced sample image was displayed in that area. However, the scanning in both the X and Y directions may be stopped to create a spot with the focus slightly shifted. In either case, the light from the photodetector 21
An output signal having substantially the same waveform as the luminance signal supplied to the luminance signal is obtained.

[Brief explanation of drawings]

FIG. 1 shows the constituent countries of an embodiment of the present invention, and FIG. 2 is a diagram for explaining its operation. 4...Scanning circuit, 6...Detector, Death...
...Adjustment circuit, 13...Correction circuit...
14...CRT, 15...Switching circuit, 16...Deflection coil, 17...Shift circuit, 19...Aperture, 20...Camera, 21...Photodetector, 23...Average brightness detection circuit, 24.25...Peak hold circuit, 26... ...Average circuit, 27...
control circuit.

Claims (4)

[Scope of utility model registration request] (1) An electron beam is scanned two-dimensionally over a sample, information generated from the sample is detected by the scanning, and the obtained detection signal is transmitted to a cathode ray tube display device synchronized with the electrons on the sample and the beam scanning. In a scanning electron microscope which is adapted to obtain an image of a sample by introducing a camera, a means for controlling the amount of light incident on the camera, a means for controlling the amount of light incident on the camera, and a means for controlling the electron beam of the cathode ray tube so that the image of the sample is displayed. means for deflecting the electron beam so as to irradiate it to a brightness detection area different from the area where the electron beam is applied; Comprising a photodetector, means for determining an average luminance signal based on the maximum and minimum values of the output signal of the photodetector within a predetermined period, and a comparison circuit that compares the obtained average luminance signal with a reference signal. , a scanning electron microscope characterized in that the output signal of the comparison circuit is configured to be supplied to means for controlling the amount of light incident on the camera. (2) The scanning electron microscope according to claim 1, wherein the electron beam of the cathode ray tube scans the brightness detection area two-dimensionally. (3) The scanning electron microscope according to claim 1, wherein the electron beam of the cathode ray tube is line-scanned in the brightness detection area. (4) The scanning electron microscope according to claim 1, wherein the electron beam of the cathode ray tube is stopped in a spot shape with a slightly shifted focus in the brightness detection area.