JP2001148230A - Scanning electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scanning electron microscope by which the optimum image contrast can be obtained. SOLUTION: A measure is equipped in which the peak value of signal is detected at the stage of analog signal that is input into AD converter, or detected from the data after AD conversion, and in which the gain control of secondary electron detector and amplifier is carried out so that this nearly coincides with the full scale of AD converter. Apart from this, a measure is equipped to detect the image contrast that is formed on the image memory, and based on the detected results, to convert the concentration value of image so that the whole range of concentration value that can be displayed by the display device is utilized, and to optimize the contrast of displayed image.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a scanning electron microscope, and more particularly to adjustment of image contrast.

[0002]

2. Description of the Related Art In a scanning electron microscope, weak secondary electrons generated from a sample by irradiating the sample with an electron beam are detected by a detector, the amount thereof is amplified, and an analog signal is converted by an AD converter. The signal is converted into a digital signal, and an image is generated from the signal by a signal processing device. The amount of secondary electrons strongly depends on the shape and material of the surface of the sample, and the amplitude fluctuation range of the scanned image differs. Therefore, the signal level is automatically adjusted so that the optimum dynamic range can be obtained for each field scan. Adjustments are required.

In the prior art, the maximum peak voltage and the bottom voltage of the analog signal detected and amplified by the detector during one field scanning period are detected, and the values are controlled so as to fall within a constant value. . The input signal removes high-frequency noise through a low-pass filter whose set value can be selected, and is input to a detection circuit. The detection circuit includes a peak hold for peak detection and a peak hold for bottom detection. The gain of the input signal level is adjusted so as to fall within a preset range with respect to the data full scale of the AD converter. For example, a setting is made so that the minimum value of the image signal is 20% of the data full scale and the maximum value is 80% of the data full scale. The difference between the peak and bottom voltages is compared with a reference value, and the gain of the detector is set according to the relationship. Further, the bias value of the amplifier is set by comparing the bottom value with the reference value.

In order to observe an insulator such as a semiconductor,
In order to reduce the effect of electrification on the sample surface, the amount of electrons irradiating the sample, that is, the probe current, is made
In addition, it is necessary to make the repetition cycle of electron beam scanning as fast as possible. For example, if the scanning cycle is 30 frames per second,
A technique of setting the probe current to several pA is generally used. However, since a single scan does not provide a sufficient signal amount to form an image, an image is formed by accumulating and adding signals at each pixel of the image by inter-frame addition or inter-frame moving average method. . Under these conditions,
The output signal of the detector becomes a pulse train signal in which each of the secondary electrons is separated and detected, and the contrast of the image is formed not by the peak value of the pulse signal but by the difference in the number of pulse signals per unit time. You.

FIG. 2 is a graph showing an example of a waveform representing a detected secondary electron signal. Each peak corresponds to one secondary electron, and the magnitude of the peak depends on the probability variation of the signal amplification factor of the detection system, not on the intensity of the secondary electrons. The contrast of the image created by this signal is
Higher peaks are brighter and lower peaks are darker.

In the above-mentioned prior art, since the magnitude of the signal is detected, it is not possible to accurately obtain a detection value corresponding to the contrast of the image. Assuming that the probe current is 1 pA and the time of one pixel of an image during scanning is 100 ns, the number of sample irradiation electrons per pixel is only 0.6 on average. This is because even if the maximum value of the secondary electron emission efficiency from the sample is 1 and all the emitted secondary electrons can be detected, even if the signal pulse peak exactly matches the full scale of the AD converter. In the normal addition method, even a portion having the largest signal in an image has only a brightness of 60% of the full scale. Here, the normal addition method is a method in which input signal data is divided by the number of additions and added so that the maximum result of the addition does not exceed the full scale of the image memory.

Further, in the conventional method, since a low-pass filter is passed through the peak / bottom detection circuit, when the peak density is low, the detected peak value is attenuated and smaller than the peak value of the input signal, and the peak value is detected. In order to match the small peak voltage to the reference, the gain becomes excessively large, and a situation is likely to occur in which the full scale of the AD converter greatly exceeds the saturation.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a scanning electron microscope capable of obtaining an optimum image contrast.

[0009]

In order to solve the above-mentioned object, the present invention provides a detector for detecting secondary electrons obtained by scanning an electron beam on a sample and a signal detected by the detector. In a scanning electron microscope including an amplifier for amplifying and a signal processing device for forming an image from a signal amplified by the amplifier, a peak value of a signal for forming an image formed by the signal processing device is detected. A first signal detection unit that performs, based on a peak value of the signal detected by the first signal detection unit, an adjustment unit that generates a signal to be fed back to the gain of the detector or the amplifier, and the signal processing device A second signal detecting unit for detecting a contrast of the formed image; and a density value converting unit for converting a density value of the image detected by the second signal detecting unit.

[0010] Further, an AD converter for converting the signal from an analog signal to a digital signal is provided, and the adjusting means is adapted to substantially match the full scale of the AD converter.
In this configuration, the gain of the detector or the amplifier is set.

[0011] Further, a display device for displaying the image is provided, and the density value conversion means converts the density value of the image so as to use substantially the entire area of the density value that can be displayed by the display device. Things.

Further, the present invention provides a detector for detecting secondary electrons obtained by scanning an electron beam on a sample, an amplifier for amplifying a signal detected by the detector, and a signal amplified by the amplifier. In a scanning electron microscope provided with an AD converter for converting a digital signal into a digital signal and an image forming means for forming an image from the digital signal, a peak of an input signal of the AD converter during one frame scanning by the electron beam is provided. / First signal detecting means for detecting a bottom value or detecting an output signal of the AD converter to calculate a histogram, and adjusting the gain of the detector or the amplifier based on a result of the first signal detecting means. Means for performing inter-frame addition or inter-frame moving average processing on the basis of the value of the signal detected by the first signal detection means, calculating a histogram of the image, and performing AD conversion. A second signal detecting means for detecting a peak / bottom value of the obtained voltage signal, and setting a data of a look-up table based on a result of the second signal detecting means to enhance a contrast of the image to the image forming means. Image adjusting means for displaying the image.

[0013]

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

FIG. 1 is a control block diagram showing a control configuration of the scanning electron microscope. In the figure, an electron beam 3 emitted from an electron gun 2 of a mirror unit 1 of a scanning electron microscope is converged by an electron lens (not shown) and is irradiated on a sample 5. Deflection signal 2 sent from signal controller 24 to deflector 4
The sample 6 is raster-scanned on the sample 5 by the electron beam 3.
Secondary electrons or reflected electrons generated from the surface of the sample 5 by irradiation of the electron beam 3 are detected by the electron detectors 6 and 7, amplified by the amplifiers 8 and 9, and added by the signal adder 10. You. A signal output from the signal adder 10 or the amplifiers 8 and 9 is selected by a selector 11 and input to an AD converter 12 to be converted into a digital signal. Alternatively, a moving average between frames is performed, and the result is taken into the image memory 14 as digital data. The image data in the image memory 14 passes through a look-up table 20 for gradation conversion, is converted into an analog signal by a DA converter 22, and is displayed on a display 23 as an image.

Signal value control is performed so that the peak voltage of the input signal of the AD converter 12 substantially matches the full scale of the AD converter 12. There are two ways to do this.

The first method is a method of detecting a peak voltage and a bottom voltage of an analog video signal as in the related art. However, a frequency band is secured so that accurate peak detection can be performed without passing through a low-pass filter. The input signal of the AD converter 12 is input to the peak / bottom voltage hold circuit 17 via the signal selector 13 to detect the maximum peak voltage and the bottom voltage during one frame scanning period of the image, and the detected signals are detected by the AD converter 18. Is converted to digital data, read by the central processing unit 21, and the gain control DA converter 25 resets the gain of the amplifier 8 and the gain of the amplifier 9 so that the peak voltage and the bottom voltage of the analog video signal become desired values. I do.

The second method is to detect the peak voltage of a video signal using a histogram. Selector 2
7 is set on the side (lower side in the figure) where the output data of the AD converter 12 is input to the histogram arithmetic circuit 16, and the frequency distribution of the output data of the AD converter 12 during one frame scanning period is set by the histogram arithmetic circuit 16. From the calculated and obtained shape of the signal histogram, the peak value distribution of the current video signal is detected, and from this value and the current set value, the maximum of the signal peak voltage value is equivalent to the full scale of the AD converter 12. To calculate the gain required to perform
The A converter 25 is set.

FIG. 3 is a histogram of the peak value distribution of the video signal. The histogram of the peak value distribution of the acquired video signal is as shown in FIG. W indicates the full scale of the AD converter 12. The maximum value A of the peak value of the video signal is obtained, and the gains of the amplifiers 8 and 9 are corrected to W / A times the current value. Then, the histogram of the peak value distribution of the video signal acquired in the next one frame is as shown in FIG. 3B, and the image data can be formed by effectively using the full scale.

The AD converter 1 according to any of the above methods
2 can be automatically adjusted to an optimum level. In particular, if the second method is used, the input signal voltage of the AD converter 12 can be accurately adjusted to a value at which full scale can be used effectively without saturation.

Next, an image control for adjusting the gradation of the image formed on the image memory and displaying the image with an appropriate contrast will be described below.

The selector 27 in FIG. 1 is set on the side for selecting the output signal of the image calculation unit 15, and the data of all the pixels during one frame scanning period of the data subjected to the inter-frame calculation processing by the image calculation unit 15 Is obtained by the histogram calculation circuit 16. The data of the lookup table 20 is calculated and set so as to extend the peak / bottom of the histogram data to a desired range with respect to the full scale (for example, 80/20% of the full scale).

FIG. 4 is a histogram obtained by conversion using a look-up table. In FIG. 4A, the minimum value of the acquired histogram is T, and the width of the distribution is C.
And W indicates the full scale of the AD converter 12 shown in FIG. Then, as shown in FIG. 4B, the output is 0 until the input data is (T-0.2C), and the output is from (T-0.2C).
The output changes linearly from 0 to the full scale in the range of (T + 1.2C), and the lookup table 20 is set so that the output becomes the full scale in the range of (T + 1.2C) or more. Thus, an image having the histogram distribution shown in FIG. 4C is obtained by passing the image having the histogram distribution shown in FIG. 4A through the output look-up table shown in FIG. 4B. be able to.

The image contrast formed on the image memory 14 shown in FIG. 1 is obtained by converting the output signal of the image calculation unit 15 into an analog signal by a DA converter 19 and passing through a signal selector 13 to a peak / bottom voltage hold circuit. 17 in 1
It can also be realized by a method of detecting minimum and maximum peaks during frame scanning.

As described above, the signal value control based on the gain feedback shown in FIG. 3 and the image control shown in FIG.
An optimum image contrast can be obtained for an image finally displayed.

When the signal value control and the image control by the gain feedback are alternately repeated, and when the interlace scanning mode is used, for example, a histogram of the peak value distribution of the video signal is obtained in an even-numbered field. If the gain is set during the line period, the histogram of the image is obtained in the next odd field, and the image is controlled by the look-up table, the image contrast can be automatically controlled with the fastest response. If the change of the visual field is not so fast, the change may be performed alternately every frame or alternately every several frames. Since the peak value of the signal is determined by the energy of the secondary electrons and the amplification factor of the detector, once adjusted, it does not change in a short time, and the image contrast mainly depends on the number of generated secondary electrons. It changes every time. Therefore, the image control may be performed constantly, and the signal value control may be performed intermittently. Also, when the inter-frame calculation processing is integration, only signal value control by gain feedback is performed for each frame,
Once at the end of image integration, the input of the histogram calculation circuit may be switched to the side for selecting the output signal of the image calculation unit 15 in FIG. 1 to obtain a histogram of the image, and the above image control may be performed.

[0026]

As described above, according to the present invention, it is possible to obtain a scanning electron microscope capable of obtaining an optimum image contrast.

[Brief description of the drawings]

FIG. 1 is a control block diagram showing a control configuration of a scanning electron microscope.

FIG. 2 is a graph showing an example of a waveform representing a detected secondary electron signal.

FIG. 3 is a histogram of a peak value distribution of a video signal.

FIG. 4 is a histogram when converted using a look-up table.

[Explanation of symbols]

3 ... Electron beam, 5 ... Sample, 6,7 ... Electron detector, 8,9 ...
Amplifier, 10 ... signal adder, 11, 27 ... selector, 1
2, 18 AD converter, 13 signal selector, 14 image memory, 15 image operation unit, 16 histogram operation circuit, 17 peak / bottom voltage hold circuit, 19,
22: DA converter, 20: Look-up table, 21
... Central processing unit, 23 ... Display, 24 ... Signal controller, 25 ... Gain control DA converter.

Claims (4)

[Claims] 1. A detector for detecting secondary electrons obtained by scanning an electron beam on a sample, an amplifier for amplifying a signal detected by the detector, and forming an image from the signal amplified by the amplifier. A signal processing device, a first signal detecting means for detecting a signal for forming an image formed by the signal processing device, and a signal detected by the first signal detecting device. Adjusting means for generating a signal to be fed back to the gain of the detector or the amplifier, a second signal detecting means for detecting a contrast of an image formed by the signal processing device, and the second signal detecting means And a density value converting means for converting the density value of the image detected in the step (a). 2. An apparatus according to claim 1, further comprising an A / D converter for converting said signal from an analog signal to a digital signal, wherein said adjusting means adjusts said detector so as to substantially match a full scale of said A / D converter. Alternatively, a scanning electron microscope characterized by setting the gain of an amplifier. 3. A display device according to claim 1, further comprising a display device for displaying said image, wherein said density value conversion means uses the density of said image so as to use substantially the entire area of the density value that can be displayed by said display device. A scanning electron microscope characterized by converting a value. 4. A detector for detecting secondary electrons obtained by scanning an electron beam on a sample, an amplifier for amplifying a signal detected by the detector, and converting the signal amplified by the amplifier into a digital signal. In a scanning electron microscope provided with an AD converter for converting and an image forming means for forming an image from the digital signal, the A
First signal detecting means for detecting a peak / bottom value of an input signal of the D converter or detecting an output signal of the AD converter and calculating a histogram; and detecting the detector or the signal based on a result of the first signal detecting means. An adjusting means for setting the gain of the amplifier; and a voltage obtained by performing an inter-frame addition or an inter-frame moving average process on the basis of the value of the signal detected by the first signal detecting means, calculating a histogram of the image, and performing A / D conversion. A second signal detecting means for detecting a peak / bottom value of the signal, and a look-up table data set based on the result of the second signal detecting means to enhance the contrast of the image and to provide the image forming means with an image. A scanning electron microscope, comprising: an image adjusting means for displaying the image.