JPH04328234A - Automatic brightness/contrast adjusting system of electron microscope - Google Patents

Abstract

PURPOSE:To make automatic adjustment of brightness and contrast. CONSTITUTION:A reference histogram table 9 contains a reference histogram for various reference images specified previously and parameters for defining the gain and DC level. A control device 8 prepares a histogram by taking in the image data for the specimen written in a frame memory 7 and counting the number of picture elements per brightness. The histogram is compared with the reference histogram registrated in the reference histogram table 9, and a reference histogram the most similar to the prepared histogram shall be selected. The gain to be given to a gain adjusting circuit 3 and the DC level to be given to a DC level adjusting circuit 4 are determined based on parameters, which have been written in correspondence to the selected reference histogram.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic brightness/contrast adjustment method in an electron microscope.

0002

[Prior Art] Electron microscopes, particularly scanning electron microscopes, not only observe the shape of a sample on a CRT;
This is often photographed, and in that case, it is required to appropriately adjust the brightness and contrast of a detection signal obtained by detecting secondary electrons or reflected electrons emitted from the sample. Therefore, in the past, the maximum value V of the detection signal in one screen was
MAX and minimum value VMIN are determined, and the gain adjustment and DC level adjustment of the detection signal is performed based on these values. For example, the gain G is G=(VMAX-VMIN)/G0, where the desired contrast is G0.
...Determined by (1), the DC level L is L=(VMAX+VMIN)/2
...I was looking for it in (2).

0003

[Problem to be Solved by the Invention] However, in the detection signal, there may be locally very high brightness parts or low brightness parts due to dust etc. attached to the sample. Signal components that should not originally be used for brightness/contrast settings are also used, resulting in the brightness and contrast of the part you want to observe not being at appropriate values and having to be manually reset. there were. The present invention solves the above-mentioned problems, and aims to provide an automatic brightness/contrast adjustment method for an electron microscope that can automatically perform appropriate brightness/contrast adjustments.

0004

[Means for Solving the Problems] In order to achieve the above object, the automatic brightness/contrast adjustment method in the electron microscope of the present invention is based on a secondary electron detection signal or a backscattered electron detection signal emitted from a sample. Create a histogram of the number of pixels with respect to brightness, compare the histogram with a plurality of predetermined reference histograms, select the reference histogram that most closely approximates the shape, and emit light from the sample based on the selected reference histogram. be done 2
It is characterized by determining the brightness and contrast of the secondary electron detection signal or the reflected electron detection signal.

[0005]

[Operation] Reference histograms for various predetermined reference images and parameters for determining the gain and DC level are written in the reference histogram table 9. The control device 8 creates a histogram from the image data of the sample, compares the histogram with the reference histogram registered in the reference histogram table 9, selects the reference histogram that is most similar to the created histogram, and calculates the reference histogram based on the parameters. The gain given to the gain adjustment circuit 3 and the DC level given to the DC level adjustment circuit 4 are determined.

[0006]

Embodiments Hereinafter, embodiments will be described with reference to the drawings. FIG. 1 is a diagram showing the configuration of an embodiment of an automatic brightness/contrast adjustment method in an electron microscope according to the present invention, in which 1 is a detector, 2 is an amplifier, 3 is a gain adjustment circuit, and 4
is a DC level adjustment circuit, 5 is a signal processing circuit, and 6 is an A/D
A conversion circuit, 7 a frame memory, 8 a control device, 9 a reference histogram table, and 10 a CRT.

A detector 1 detects secondary electrons or reflected electrons emitted from a sample (not shown), and its output is amplified by an amplifier 2 and input to a gain adjustment circuit 3. The gain adjustment circuit 3 amplifies the detection signal with a gain instructed by the control device 8, and is for example a D/
It is composed of a multiplication circuit using an A conversion circuit. The output of the gain adjustment circuit 3 is input to a DC level adjustment circuit 4, and is given a DC level instructed by a control device 8. This DC level adjustment circuit 4 can also be constructed from a D/A conversion circuit. The output of the DC level adjustment circuit 4 is a CRT.
The signal is supplied to the signal processing circuit 10 and displayed, and is also subjected to predetermined signal processing in the signal processing circuit 5, digitized by the A/D conversion circuit 6, and written into the frame memory 7. The control device 8 includes a reference histogram table 9, performs processing to be described later to obtain a gain to be applied to the gain adjustment circuit 3, and a DC level to be applied to the DC level adjustment circuit 4, and is configured by a microprocessor. The reference histogram table 9 is a table in which histogram patterns for various reference images and parameters used for setting gains and DC levels are written.

An example is shown in FIG. In FIG. 2, it is assumed that pixels are digitized with 8 bits and 256 gradations. FIG. 2A is a reference histogram pattern for a reference image having a fibrous pattern 11 and a background 12 as shown in FIG. 3A, and parameters V0, V1, and V2 for setting the DC level and gain are determined. . In FIG. 2A, V0 is set at the peak position on the high brightness side, and the signal voltage corresponding to this brightness is adopted as the DC level. V1 is set at the position where the number of pixels between the two peaks is minimum, and V2 is set at the position where the number of pixels is a certain percentage of the number of pixels at the peak position on the high brightness side. , V1, V2 are respectively adopted as the above VMIN, VMAX, and the gain given to the gain adjustment circuit 3 is determined by the above equation (1). According to this, the gain and DC level are
Since the peak 13 corresponding to 2 is removed and only the image pattern 11 to be observed is set, the brightness and contrast of the displayed image can be appropriately set.

FIG. 2B is a reference histogram pattern for a reference image in which relatively small patterns are distributed almost evenly over the entire screen as shown in FIG. 3B, and V0, which is a parameter for setting the DC level, is at the peak. The parameters V1 and V2 for setting the gain are set at the position of the luminance having a number of pixels equal to a certain percentage of the number of pixels at the peak position, or at the position of the luminance having a predetermined number of pixels on both sides of the peak. set in position. Similarly, FIG. 2C is a reference histogram pattern for a reference image having a stripe pattern, such as a resist pattern on a silicon substrate, as shown in FIG. 3, and is a parameter for setting the DC level V0.
is set at the position where the number of pixels between the two peaks is minimum, and V1, which is a parameter for setting the gain,
V2 is set at a luminance position having a number of pixels equal to a certain percentage of the number of pixels at the peak position, or at a luminance position having a predetermined number of pixels outside the two peaks. Having explained the three types of reference histograms above,
It goes without saying that more reference histograms can be defined, and the method of describing the positions of the parameters V0, V1, and V2 in the table can be determined as needed. For example, as will be clear from what will be described later, a processing procedure for determining the positions of V0, V1, and V2 as parameters can be described. In addition, noise components due to dust adhesion, for example, appear as relatively low peaks on the high brightness side on the histogram.
It is also clear that the reference histogram can be created taking this pattern into account as well.

Next, the operation of the control device 8 will be explained with reference to the flowchart shown in FIG.

When an instruction for automatic brightness/contrast adjustment is given through an input device such as a button (not shown), the control device 8 instructs the gain adjustment circuit 3 and the DC level adjustment circuit 4 to set a predetermined gain and DC level, respectively. S1). For example, the gain is set to 1, and the DC level is set to 0V. As a result, the gain adjustment circuit 3 and the DC level adjustment circuit 4 are put into a through state. The detection signal that has passed through the gain adjustment circuit 3, DC level adjustment circuit 4 and signal processing circuit 5 is A/
The D conversion circuit 6 digitizes the data into, for example, 8 bits and writes it into the frame memory 7. When the writing of one screen of image data to the frame memory 7 is completed, the control device 8
captures image data pixel by pixel from the frame memory 7 and creates a histogram (S2). When creating this histogram, it is natural to count the number of pixels at the same brightness step as when creating the reference histogram. In other words, if the standard histogram is set for all luminances of 256, then the number of pixels is 25.
6. Count the number of pixels for all luminances, and if the number of pixels is set for each predetermined luminance range as shown in Figure 5, count the number of pixels for each predetermined luminance range. do.

When the creation of the histogram is completed, the control device 8 extracts the features of the created histogram, compares them with the reference histogram stored in the reference histogram table 9 (S3), and determines which features are most similar to the features of the created histogram. A reference histogram is selected as the optimal reference histogram (S4). As criteria for selecting the optimal standard histogram, for example, the number of peaks, the position of each peak in the histogram, the half-width of each peak, etc. may be adopted as features, and comparisons may be made.

Next, the control device 8 reads the parameters V0, V1, V2 for setting the gain and DC level written corresponding to the selected optimal reference histogram from the reference histogram table 9, and reads out the created histogram. The gain and DC level are determined by determining the positions to which these parameters correspond (S5
), the obtained gain and DC level are set in the gain adjustment circuit 3 and the DC level adjustment circuit 4, respectively (S6), and the process ends. For example, if the reference histogram shown in FIG. 2A is selected as the optimal reference histogram in the process of S4, the brightness of the peak on the high brightness side of the histogram created in the process of S2 is determined, and the signal voltage corresponding to that brightness is determined. DC level, and the brightness at which the number of pixels between the two peaks of the created histogram is minimum, and the number of pixels that is higher than the high brightness side peak and is a predetermined ratio of the number of pixels of the high brightness side peak. For example, the brightness having 1/4 the number of pixels of the high brightness peak is determined, and the signal voltages corresponding to these determined brightnesses are determined, and the gain given to the gain adjustment circuit 3 is determined using the above equation (1).

When the above processing is completed, the gain adjustment circuit 3 and the DC level adjustment circuit 4 are respectively given the optimum gain and DC level for the sample image, so that the CRT 10
An image with good brightness/contrast is displayed. Note that if a large number of reference histograms are stored in the reference histogram table 9, two reference histograms may be selected as the optimal reference histograms in the process of S4 in FIG. A coefficient may be given to the histogram created based on the degree of similarity between the two reference histograms, and the gain and DC level may be set by incorporating the characteristics of both reference histograms.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible. For example, in the above embodiment, the digitized image data is once written into the frame memory 7, but the histogram can be created by sequentially inputting the digitized image data to the control device 8 without using the frame memory 7. is also possible.

[0016]

[Effects of the Invention] As is clear from the above description, according to the present invention, since the brightness and contrast that suit the user's preference can be written in advance in the reference histogram, images with the same brightness and contrast can be displayed at the time of measurement. can be observed. In addition, even if there are locally high or low brightness areas in the image, the brightness and contrast are set for the necessary parts of the image based on the reference histogram, so the brightness and contrast are always optimal for the observed image. It is possible to obtain an image with .

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of an embodiment of the present invention.

FIG. 2 is a diagram for explaining a reference histogram.

FIG. 3 is a diagram showing an example of an image corresponding to the reference histogram of FIG. 2;

FIG. 4 is a flowchart showing the processing of the control device.

FIG. 5 is a diagram for explaining creation of a histogram.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1...Detector, 2...Amplifier, 3...Gain adjustment circuit, 4...DC level adjustment circuit, 5...Signal processing circuit, 6...A/D conversion circuit, 7...Frame memory, 8...Control device, 9...Reference histogram Table, 10...CRT.

Claims (1)

[Claims] Claim 1: Create a histogram of the number of pixels with respect to brightness based on the secondary electron detection signal or backscattered electron detection signal emitted from the sample, and compare the histogram with a plurality of predetermined reference histograms to find the most A reference histogram having a similar morphology is selected, and the amount of 2 emitted from the sample is determined based on the selected reference histogram.
An automatic brightness/contrast adjustment method for an electron microscope, characterized by determining the brightness and contrast of a secondary electron detection signal or a backscattered electron detection signal.