JPH05290783A - Scanning electron microscope - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a scanning electron microscope capable of accurately performing contrast correction in a short time. The image signal peak hold circuit 9 obtains the current image contrast, and the obtained image contrast is subtracted by a subtraction circuit 14
Send to. The subtraction circuit 14 calculates the difference between the optimum image contrast and the obtained current image contrast. This difference is sent to the control characteristic storage unit 15. In the control characteristic storage unit 15,
An acceleration voltage signal V1 is sent from a control device (not shown), and in this case, the control characteristic at the acceleration voltage V1 is used. The gain changing means 16 changes the gain of the variable gain amplifier 8 according to the signal from the control characteristic storage unit 15. As a result, an image with optimum contrast is immediately displayed on the screen of the cathode ray tube 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning electron microscope having functions of contrast correction and brightness correction.

[0002]

2. Description of the Related Art A scanning electron microscope uses an electron beam to a sample.
It is a device that dimensionally scans and irradiates, detects a signal emitted from a sample, and displays a sample image on a display screen.

FIG. 3 is a schematic view of a scanning electron microscope. The electron beam emitted from the electron gun 1 is focused by the focusing lens 2 and the objective lens 4 and irradiates the sample 5. Further, since the scanning signal is sent from the scanning signal generating circuit 13 to the deflector 3, the electron beam scans the sample 5 two-dimensionally. Secondary electrons are emitted from the sample 5 by irradiation with an electron beam, and the secondary electrons are converted into light by the scintillator 6,
This light is converted into a video signal by the photomultiplier tube 7. This video signal is sent to the cathode ray tube 12 via the variable gain amplifier 8. Since the scanning signal generating circuit 13 sends a scanning signal synchronized with the scanning signal sent to the deflector 3 to the cathode ray tube 12, an image of the sample scanning area is displayed on the screen of the cathode ray tube 12.

The video signal from the variable gain amplifier 8 is also sent to the image signal peak hold circuit 9. The image signal peak hold circuit 9 holds a video signal for one sample scanning area, obtains a signal representing the current image contrast from these signals, and sends this signal to the subtraction circuit 10. The subtraction circuit 10 holds a signal representing the optimum image contrast, and calculates the difference between the signal representing the optimum image contrast and the obtained signal representing the current image contrast. A signal representing this difference is sent to the control characteristic storage unit 11. Figure 4
Are shown for explaining the gain correction signal stored in the control characteristic storage unit 11. In the figure, the horizontal axis represents the difference between the optimum image contrast and the current image contrast, and the vertical axis represents the gain correction amount of the variable gain amplifier 8. That is, the control characteristic storage unit 11 stores information indicating the amount of gain correction necessary to obtain an image having the optimum contrast when the contrast is not optimum. For example, if the difference sent from the subtraction circuit 10 is ΔC1, the difference is ΔG from the control characteristic storage unit 11.
The output signal of 1 is sent to the variable gain amplifier 8. Then, the gain of the variable gain amplifier 8 changes by an amount corresponding to ΔG1. As a result, an image with optimum contrast is immediately displayed on the screen of the cathode ray tube 12.

[0005]

However, the control characteristics may not be optimum depending on the observation conditions, for example, the acceleration voltage and the type of the sample, and the contrast correction may take a long time, or the accuracy of the contrast correction may decrease. Sometimes. Further, such a problem similarly occurs in the brightness correction.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a scanning electron microscope capable of accurately performing contrast correction or brightness correction in a short time.

[0007]

Means for Solving the Problems A scanning electron microscope of the present invention that achieves this object is provided with a means for amplifying a video signal based on irradiation of a sample with an electron beam, and a video signal from the amplification means. Display means, information detecting means for obtaining the current image information signal based on the video signal from the amplifying means, and means for calculating the difference between the current image information signal obtained by the information detecting means and the optimum image information signal. And a control characteristic storage unit in which the gain correction amount of the amplification means is stored corresponding to the difference in the image information signal and the observation condition, and the control characteristic storage unit by inputting the signal in the difference between the image information signal and the observation condition And a gain varying means for controlling the gain of the amplifying means based on the gain correction signal output from the.

[0008]

The present invention will be described below with reference to the drawings.

FIG. 1 is a schematic view of a scanning electron microscope shown as an embodiment of the present invention. In the figure, the same reference numerals as those in FIG. 3 indicate the same components. Reference numeral 14 is a subtraction circuit, 15 is a control characteristic storage unit, and 16 is a gain varying means. The contrast correction in such an apparatus will be described below.

A feature of the present invention is that the control characteristic for each acceleration voltage or sample is determined and stored.
First, obtaining the control characteristic at the acceleration voltage V1 will be described.

First, the gain varying means 16 sets the gain of the variable gain amplifier 8 to a certain value G2. This value is held in the gain changing means 16. The video signal converted into the video signal by the photomultiplier tube 7 is sent to the cathode ray tube 12 via the variable gain amplifier 8. At the same time, the video signal from the variable gain amplifier 8 is also sent to the image signal peak hold circuit 9. The image signal peak hold circuit 9 holds a video signal for one sample scanning area, obtains a signal representing the current image contrast from these signals, and sends the obtained signal to the subtraction circuit 14. The subtracting circuit 14 holds a signal representing the optimum image contrast, and calculates the difference between the signal representing the optimum image contrast and the signal representing the current image contrast obtained. This difference is sent to the control characteristic storage unit 15. FIG. 2 is shown for explaining the gain correction signal stored in the control characteristic storage unit 15. In the figure,
The horizontal axis represents the difference between the optimum image contrast and the current image contrast, and the vertical axis represents the gain correction amount of the variable gain amplifier 8. By the way, it is assumed that the difference ΔC2 is sent from the subtraction circuit 14 to the control characteristic storage unit 15. Subtraction circuit 1
4 sends a signal indicating that there is a difference in contrast to the gain varying means 16. Receiving this signal, the gain changing means 16 changes the gain of the variable gain amplifier 8 by a predetermined amount. By changing the gain of the variable gain amplifier 8,
The image contrast obtained by the image signal peak hold circuit 9 is different from that before the gain change. The subtraction circuit 14 calculates the difference between the optimum image contrast and the current image contrast, and if there is a difference, sends a signal to the gain varying means 16. The subtraction circuit 14 sends only the result ΔC2 of the first calculation to the control characteristic storage unit 15 and does not send the second and subsequent calculation results to the control characteristic storage unit 15. The gain changing means 16 which has received the signal from the subtraction circuit 14 changes the gain of the variable gain amplifier 8 by a predetermined amount as described above. Such an operation is performed until the difference becomes zero by the calculation of the subtraction circuit 14, and when the difference becomes zero, the subtraction circuit 14 sends an end signal to the gain varying means 16. Upon receiving this signal, the gain varying means 16 obtains the difference between the current gain of the variable gain amplifier 8 and the held gain G2, and this difference ΔG2 is used as the control characteristic storage unit 1
Send to 5. The control characteristic storage unit 15 stores the gain difference ΔG2 corresponding to the image contrast difference ΔC2 sent from the subtraction circuit 14. In FIG. 2, point A shows this relationship.

Next, the gain of the variable gain amplifier 8 is set to a value different from the set value G2, for example, G3. If the result of the first operation of the subtraction circuit 14 is ΔC3, the difference ΔC3 is stored in the control characteristic storage unit 15. Then, the same operation as described above is performed until the calculation result of the subtraction circuit 14 becomes zero. The difference ΔG3 between the gain of the variable gain amplifier 8 and the gain G3 held by the gain changing means 16 at this time is sent to the control characteristic storage unit 15. In FIG. 2, point B shows this relationship.

Thereafter, the same operation is performed, and C, D ...
Point is required. Then, the control characteristic storage unit 15 obtains the gain correction signal at the acceleration voltage V1 by the least square method.

The operation for obtaining the control characteristics as described above is performed by changing each accelerating voltage and sample.

Next, the operation of the apparatus shown in FIG. 1 will be described for the case of observing a sample with the acceleration voltage V1.

The video signal converted into the video signal by the photomultiplier tube 7 is sent to the cathode ray tube 12 through the variable gain amplifier 8. At the same time, the video signal from the variable gain amplifier 8 is also sent to the image signal peak hold circuit 9. The image signal peak hold circuit 9 holds the video signals for one sample scanning area, obtains a signal representing the current image contrast from these signals, and sends this signal to the subtraction circuit 14. The subtracting circuit 14 holds a signal representing the optimum image contrast, and calculates the difference between the signal representing the optimum image contrast and the signal representing the current image contrast obtained. This difference is sent to the control characteristic storage unit 15. At this time, a signal indicating that there is a difference in contrast is controlled so as not to be sent to the gain varying means 16. An acceleration voltage signal V1 is sent to the control characteristic storage unit 15 from a control device (not shown), and in this case, the control characteristic at the acceleration voltage V1 is used. For example, if the difference sent from the subtraction circuit 14 is ΔC2, the output signal of ΔG2 is sent from the control characteristic storage unit 15 to the gain changing means 16. The gain changing means 16 uses this signal to make the variable gain amplifier 8
The gain of is changed by an amount corresponding to ΔG2. As a result, an image with optimum contrast is immediately displayed on the screen of the cathode ray tube 12.

Instead of changing the gain of the variable gain amplifier 8, the gain of the photomultiplier tube 7 may be changed.

The present invention can also be applied to brightness correction.

[0019]

Since the apparatus of the present invention stores the control characteristic for each observation condition, it is possible to accurately perform contrast correction or brightness correction in a short time even when the observation condition is changed.

[Brief description of drawings]

FIG. 1 is a schematic view of a scanning electron microscope shown as an embodiment of the present invention.

FIG. 2 shows control characteristics.

FIG. 3 is a schematic view of a conventional scanning electron microscope.

FIG. 4 shows control characteristics.

[Explanation of symbols]

 1 electron gun 2 focusing lens 3 deflector 4 objective lens 5 sample 6 scintillator 7 photomultiplier tube 8 variable gain amplifier 9 image signal peak hold circuit 10, 14 subtraction circuit 11, 15 control characteristic storage unit 12 cathode ray tube 13 scanning signal generation Circuit 16 Gain varying means

Claims (1)

[Claims] 1. A means for amplifying a video signal based on irradiation of a sample with an electron beam, a display means to which the video signal from the amplification means is supplied, and a current image based on the video signal from the amplification means. Information detecting means for obtaining an information signal, means for calculating a difference between the current image information signal and the optimum image information signal obtained by the information detecting means, and the amplification corresponding to the difference between the image information signals and the observation condition. The gain of the amplification means is controlled on the basis of a control characteristic storage section in which the gain correction amount of the means is stored, and a gain correction signal output from the control characteristic storage section by the input of the signal of the difference between the image information signals and the observation condition signal. And a scanning electron microscope provided with a gain varying means.