JPS5910530B2 - Automatic brightness adjustment device for scanning electron microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic brightness adjustment device ζζ for a scanning electron microscope.

In a scanning electron microscope, the brightness of the sample image displayed on the cathode ray tube varies greatly depending on the intensity of the electron beam that irradiates the sample, the shape of the sample, the type of material that makes up the sample, the sensitivity of the detector, etc. Change.

Therefore, in order to observe images with appropriate brightness or take photographs, the brightness must be adjusted in some way.

Usually, this is done by changing the sensitivity of the detector and the DC bias voltage of the amplifier.

Of course, this adjustment can be done manually, but there is an extremely high demand for automation due to the complicated operation and the difficulty of visually determining the appropriate brightness.

FIG. 1 shows an embodiment that has been tried in the past in response to such requirements.

A secondary electron beam generated from a sample 2 being scanned by the electron beam 1 is detected by a detector 3, amplified by an amplifier 4, and applied to a cathode ray tube 8.

On the other hand, the output of the amplifier 4 is introduced into a low-pass filter 5 where only the DC component is taken out and applied to the bias setting terminal of the amplifier 4 through a sample and hold circuit 6.

The sample and hold circuit 6 transmits the input to the output only when the switch 7 is pressed, and when the switch 7 is released, the output voltage at that point in time is held.

Furthermore, the amplifier 4, low-pass filter 5, and sample-and-hold circuit 6 constitute a negative feedback loop, and therefore,
Even if the input voltage of the amplifier 4 changes, the DC level of the output voltage of the amplifier 4 is kept constant as long as the switch 7 is pressed, and therefore the brightness of the sample image displayed on the cathode ray tube 8 is Otherwise, it will remain constant.

The problem with this conventional example is the setting of the characteristics of the low-pass filter 5.

That is, since the bias voltage of the amplifier 4 must not fluctuate during one screen scanning time, the time constant of the low-pass filter must be sufficiently longer than one screen scanning time.

However, in a scanning electron microscope, the scanning time for one screen is generally variable from several tens of milliseconds to several tens of seconds, so in order to be usable for all the scanning times for one screen, a low-pass filter 5 is required. The time constant must be set to 100 seconds or more, and if this is done, it will take several tens of seconds for the change in the input signal of the amplifier 4 to be fed back to the bias setting terminal and for the DC level of the output voltage to drop to a constant value. Therefore, it is not practical.

For this purpose, the time constant of the low-pass filter 5 is usually set from a few seconds to several seconds to speed up the scanning time for one screen, and then when the switch 7 or the switch I is pressed, the time constant is automatically set. Either method may be used, such as providing an interlocking mechanism to speed up the screen scanning time.

However, in any case, manual operations must be performed by pressing the switch 7 each time the sample is moved or the sensitivity of the detector 3 is changed to change the contrast of the image, so it is not true automation. .

In FIG. 1, 9 and 10 are X- and Y-axis deflection scanning power supplies.

SUMMARY OF THE INVENTION An object of the present invention is to provide an automatic brightness adjustment device for a scanning electron microscope that can automatically adjust the brightness regardless of whether the scanning time is long or short.

FIG. 2 shows an embodiment of the invention.

The output voltage of the amplifier 4 is integrated by an integrator 11 during one screen scanning time.

When one screen scan is completed, the output voltage of the integrator 11 is applied to the bias voltage generator 12 for a short fixed period during the retrace time.
The output voltage of the bias voltage generator 12 changes by a voltage value proportional to the output voltage of the integrator 11, the bias voltage of the amplifier 4 changes, and the DC component of the output voltage of the amplifier 4 changes. Change.

After the integrator 11 finishes transmitting its output to the bias voltage generator 12 for a certain period of time, it is reset before the next screen scan begins.

The control circuit 13 is a circuit for controlling the transmission of the output of the integrator 11 to the bias voltage generator 12 and the reset operation of the integrator 11.

In FIG. 2, the amplifier 4, the integrator 11, and the bias voltage generator 12 constitute a kind of negative feedback loop, and this negative feedback loop includes a signal transmission device such as a low-pass filter. Since no delay element is included, the negative feedback operation is completed in a short time during the retrace time.

Now, if the gain G of the negative feedback loop is set to G = -1, when the input voltage of the amplifier 4 fluctuates, the time required to settle the DC level of the output voltage to a constant value is one screen scanning time. is equivalent to

Also, the range of gain G in which the negative feedback loop operates normally is O>
Since G>-2, set it to G-1, and even if the value of G fluctuates a little, there is no need to worry about the operation becoming unstable, so set it to G-1, i.e. , can be set to the optimal value.

Figure 3 shows the integrator 11 and bias voltage generator 12 in Figure 2.
, is a more specific embodiment of the control circuit 13.

The integrator 11 consists of an input resistor 18, an amplifier 20, an integrating capacitor 19 and a switch 14.

The bias voltage generator 12 includes a switch 15 and an input resistor 16.
, an amplifier 21, and an integrating capacitor 17.

The control circuit 13 includes a monostable multivibrator 22. 23
The switches 14, 15 each consist of a monostable multi-bi break 22. It is closed by 23.

Assuming that the Y-axis deflection scanning signal of the Y-axis deflection scanning power source 10 is as shown in FIG. 4a, and the output voltage of the amplifier 4 is as shown in FIG. 4e, this voltage is It is time-soaked (see Figure 4f).

When the scanning of one screen is completed, the Y-axis deflection scanning power supply 10
A blanking signal (see FIG. 4b) from the control circuit 13 is input to the control circuit 13.

Thus, monostable multi-by-break 22 switches to switch 1
5 is closed for a certain period of time (see Figure 4, C), and as a result, the output voltage of the bias voltage generator 12, which is composed of an integrating circuit, changes by a voltage value proportional to the output voltage of the integrator 11 (see Figure 4, G). reference).

Next, at the same time as the switch 15 is opened, the switch 14 is closed for a certain period of time by the monostable multi-by-break 23, so that the charge in the integrating capacitor 19 of the integrator 11 is discharged, and the output voltage of the integrator 11 becomes zero.

In the next screen scan, the bias voltage of the amplifier 4 changes so that the average value of the output voltage during one screen scan time becomes zero, so the integral at the end of the next screen scan Vessel 1
The output voltage of the amplifier 4 is zero, so the output voltage of the bias voltage generator 12 does not change, and this state is maintained until the input voltage of the amplifier 4 changes for some reason. The brightness of the sample image displayed on the screen is kept constant.

As described above, in the above embodiment, the average value of the output voltage of the amplifier 4 during one screen scanning time, that is, the average value of the output voltage of the cathode ray tube 8
The deviation of the average brightness of the sample image displayed on the screen from the optimum value is detected, and the average value of the output voltage of the amplifier 4 is set to zero during a short period of time until the next screen scan starts. The bias voltage correction operation is completed.

Even if the gain of the negative feedback loop deviates from -1 to some extent, it will take at most several screen scans to complete the correction of the bias voltage.

Furthermore, since the output voltage of the bias voltage generator 12 does not change during one screen scan, it can be used without any problem even if one screen scan takes several hundred seconds.

As described above, according to the above embodiment, (1) manual operation is not required at all, (2) the time required for the brightness of the sample image to settle to a constant value is equivalent to the time required to scan one screen. (3) It can be applied without any problems even when the scanning time for one screen is long.

Note that the output voltage of the integrator 11 at the end of one screen scan changes depending on the one screen scan time even for the same input voltage. The device 4 must change its integral sensitivity in accordance with the scanning time of one screen.

This can be easily realized by changing the value of the resistor 18 or the integrating capacitor 19 shown in FIG. 4 in conjunction with changing the one-screen scanning time.

As is clear from the above description, according to the present invention, the above-mentioned objects of the present invention are achieved, and the practical effects thereof are enormous.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a conventional brightness adjustment device for a scanning electron microscope, FIG. 2 is a block diagram of an automatic brightness adjustment device for a scanning electron microscope showing an embodiment of the present invention, and FIG.
FIG. 4 is a specific example circuit diagram of a portion in the figure, and is a waveform diagram for explaining the operation of FIGS. 2 and 3. 1... Electron beam, 2... Sample, 3.
...Secondary electron detector, 4...Mampler, 8
... Braun tube, 11 ... Integrator, 1
2...Bias voltage generator, 13...Control circuit. 68

Claims (1)

[Scope of Claims] 1. A scanning electron microscope configured to display an image of a scanning area of a sample on a cathode ray tube based on information signals obtained from the sample by scanning the sample with an electron beam, an integrator that integrates the clear signal over a predetermined scanning time to detect the average brightness of the image; and an integrator that is proportional to the output of the integrator at the end of the predetermined scan. and means for generating a signal that changes during the next predetermined scan period, and maintaining the average brightness constant using the output signal of the means. An automatic brightness adjustment device for a scanning electron microscope characterized by the following features: 2. The automatic brightness adjustment device for a scanning electron microscope according to claim 1, which is configured to switch the sensitivity of the integrator in conjunction with switching of a predetermined scanning time.