JPH06347246A - Scanning electron microscope provided with length-measuring function - Google Patents

Abstract

PURPOSE:To easily judge whether the result of an automatic length measurement is correct or not when the result is deviated from a tolerance by a method wherein the differentiated waveform of a line profile signal waveform is displayed on a screen. CONSTITUTION:An electron beam 2 from an electron gun 1 is narrowed by an objective lens 6, and a sample 7 is irradiated with the beam. The lens 6 is excited by an objective-lens power supply 11. A deflecting-signal generator 14 is instructed by a computer 21, it scans the beam 2 on the sample 7 two- dimensionally via a deflecting coil 3. The computer 21 reads the whole or a part of image data in an image memory 16, and it superposes and displays the scanned image of an object to be observed and the differentiated signal waveform expressing the change rate of a line profile signal waveform on the screen of an image-displaying CRT 19. Then, since many extreme values exist in the differentiated waveform and the largest extreme value of peak values does not correspond to a normal edge, a cause that the result of a length measurement is outside a tolerance can be recognized as caused by a length- measuring error.

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 a length measuring function, and more particularly to a length measuring function suitable for automatically measuring dimensions such as width and pitch of a fine pattern on a semiconductor wafer. The present invention relates to a scanning electron microscope having a function.

[0002]

2. Description of the Related Art In the manufacturing process and inspection process of semiconductor devices, the contact hole diameter and the pattern width are accurately measured and the measurement result is acceptable in order to suppress the occurrence rate of defective products and produce highly reliable products. If it is out of the range, the control parameter of the production line is changed based on the measurement result.
Therefore, if the measurement result is inaccurate, great damage may occur.

In recent years, a scanning electron microscope has been widely used for measuring dimensions on the order of submicrons. In dimension measurement using a scanning electron microscope, edge detection is performed based on a line profile signal obtained by scanning one or a plurality of lines in a scanning region, and the length is measured by detecting the distance between the edges.

Here, an outline of conventional automatic measurement by the maximum tilt method using a line profile signal will be described with reference to FIG. Generally, in automatic length measurement using a scanning electron microscope, first, a scan image of the length measurement target is displayed on the screen and
As shown in (a), the length measurement range is designated by, for example, two cursor lines 40a and 40b. Then, a line profile signal is obtained by vertically integrating a plurality of scanning signals in a range sandwiched by the two cursor lines 40a and 40b. FIG. 2B shows a typical waveform diagram of the line profile signal 32 obtained by such line integration.

The pattern width in this signal waveform is represented by the distance between the pattern edges E1 and E2. In the maximum gradient method, the gradient is the steepest in the signal waveform in order to determine the pattern edges E1 and E2. Position A1 (A2) is searched for, and a straight line Ls1 (Ls2) determined by signal data at several points around A1 (A2) and a straight line Lb1 (Lb2) forming the base of the signal waveform are obtained. The intersection is defined as the pattern edge E1 (E2). The position A1 (A2) is generally determined based on the differential signal 33 representing the rate of change of the line profile signal waveform as shown in FIG.

In the differential signal waveform 33, the position of the maximum value indicates the position of the maximum inclination to the right, and the position of the minimum value indicates the position of the maximum inclination to the right. Therefore, if the maximum value is found on the differential signal waveform 33, the position A1 is found, and if the minimum value is found, the position A2 is found.

Regarding the dimension measurement using such a line profile signal, for example, Japanese Patent Laid-Open No. 63-2 is used.
The method disclosed in Japanese Patent Laid-Open No. 3-289507 is disclosed in Japanese Patent Laid-Open No. 3-289507, for example, regarding a method for determining an edge based on the maximum value of a secondary signal.

[0008]

As the process rules of semiconductor wafers become finer, the pattern shapes of the wafers are diversified, and it becomes difficult to secure the reliability of automatic dimension measurement of a sample using a scanning electron microscope. Came.

For example, when the pattern shape of a laminated structure having a sectional structure as shown in FIG.
As shown in (b), the slope of the line profile waveform 32 changes in two steps at the edge portion of the sample. Such a phenomenon occurs because the emission efficiency of secondary electrons differs depending on the materials A and B of the sample. If the automatic measurement by the maximum tilt method is applied to this waveform, the position E1 (E
2) is determined as the pattern edge position, and the original pattern edge position e1 (e2) cannot be detected. Further, the line profile signal waveform does not represent the cross-sectional shape of the sample as it is, not only in such automatic length measurement but also in manual length measurement. It was difficult to determine the cross-sectional shape and the original edge position.

Further, it is difficult to accurately and accurately measure all of the wafer shapes, which will be diversified in the future, without error, using any method other than the maximum tilt method described above. The operator has no choice but to rely on the operator.

In addition to the above, in the fully automatic length measurement using the scanning electron microscope, when the image is out of focus or the foreign matter is judged to be an edge, the length measurement start point or end point is measured. It is known that a large error may occur in the measurement result due to the incorrect determination of the measurement result, and even if the measurement result is out of the allowable range, the dimension and diameter of the pattern are not always abnormal. . However, in the past, since only the length measurement result was respected, there was a problem in that even those that actually fell within the allowable range were judged to be defective.

An object of the present invention is to solve the above-mentioned problems of the prior art so that when the result of the automatic length measurement is out of the allowable range, it can be easily judged whether or not the length measurement result is correct. To do.

[0013]

In order to achieve the above-mentioned object, the present invention is characterized in that the following means are taken. (1) A means for displaying a scan image of a measuring object by scanning a focused electron beam on a sample, a means for detecting a line profile signal of the scan image, and a rate of change indicating the rate of change of the line profile signal. A means for detecting the signal, a means for displaying the waveform of the change rate signal together with at least one of the scanning image and the line profile signal, a means for determining the measurement start point and the end point based on the line profile signal, A means of detecting the distance between a long start point and an end point. (2) In addition to the above configuration (1), means for displaying one set of marks on the screen, means for moving each of the marks independently on the screen, and two on the scan image indicated by each mark. A means to detect the distance between points.

[0014]

With the configuration (1) described above, the differential waveform of the line profile signal waveform is displayed on the screen. Therefore, if the operator refers to the differential waveform, the measurement result deviates from the expected allowable range. Even in such a case, it is possible to easily understand whether it is due to a pattern defect or a length measurement error due to the shape of the length measurement object.

According to the above configuration (2), even if a length measurement error occurs, the normal length measurement start point and the end point can be designated by a set of marks so that the measurement can be performed at the normal position again. The length can be easily measured.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of a scanning electron microscope having a length measuring function to which the present invention is applied. The electron beam 2 emitted from the electron gun 1 is narrowed down by the objective lens 6 and applied to the sample 7. The objective lens 6 is excited by the objective lens power supply 11. Deflection signal generator 1
Reference numeral 4 supplies a deflection signal corresponding to the scanning range and scanning position of the electron beam 2 on the sample, which is instructed by the computer 21, to the deflection coil 5 via the deflection amplifier 10 to excite the deflection signal to emit the electron beam 2. The sample 7 is two-dimensionally scanned.

A secondary signal (secondary electron signal, reflected electron signal, etc.) generated from the sample 7 in response to the irradiation of the electron beam 2 is detected by the detector 12 and converted into an electric signal, and then A The analog signal is converted into a digital signal by the / D converter 15 and stored in the image memory 16.
The contents of the image memory 16 are always converted from digital signals to analog signals by the D / A converter 17,
It is added to the grid as a luminance signal of an image display CRT (cathode ray tube) 19. At this time, the A / D converter 15, the image memory 16, and the D / A converter 17 store a timing signal for A / D conversion and storage, and further D / A conversion for image display. Receive from 14 The deflection coil 20 of the image display CRT 19 is excited by a signal obtained by amplifying the deflection signal generated by the deflection signal generator 14 by the deflection amplifier 18.

On the other hand, the sample stage 8 on which the sample 7 is placed
Are moved by the stage drive circuit 13, which changes the scanning position of the electron beam 2 on the sample 7 and moves the field of view. In the same visual field movement, the image shift coil 3 is excited by the DC amplifier 9, and the sample 7 of the electron beam 2 is moved.
This can also be done by offsetting the scanning position above. The movement amount of these visual field movements is controlled by the computer 21.

The cursor signal generated by the cursor signal generator 22 is the trackball 24 or the computer 21.
Changes the display position of the cursor displayed on the CRT 19 for image display. The computer 21 can acquire the position information on the image display CRT 19 from the state of the cursor signal generator 22. Further, the computer 21 can read all or a part of the image data in the image memory 16, and by combining it with the cursor position information in the previous period, the line integration of the image from a part of the image data centering on the cursor position can be performed. Generate a signal waveform and change the corresponding position in the image memory 16,
The signal waveform (line profile) is displayed on the image display CRT 19. For the display of the signal waveform, a memory dedicated to the signal waveform display is separately provided, the corresponding position on the memory is changed, the exclusive OR of the memory and the image memory 16 is calculated, and the image display CRT 19 is displayed. It can also be done by displaying.

Next, the present invention will be described by taking as an example the case where the length measurement is performed on a sample in which a length measurement error occurs in the automatic length measurement as described with reference to FIGS. 11 (a) and 11 (b). . Figure 2
FIG. 11 is a diagram showing the display contents on the image display CRT 19 according to the first embodiment of the present invention when the pattern of the two-layer structure as described with reference to FIG. 11 (a) is observed. In the example, the scanning image 31 of the observation target and the line profile waveform 32, as shown in FIG.
It is characterized in that the differential signal waveform 33 representing the rate of change of the line profile signal waveform 32 is superimposed and displayed on the screen.

When the pattern of the two-layer structure as described with reference to FIG. 11 (a) is observed, the line profile waveform 32 has a two-step inclination at the edge portion of the sample, as shown in FIG. 11 (b). Therefore, if the automatic measurement by the maximum tilt method is applied, the wrong edge positions E1 and E2 are misjudged as the normal measurement start point and the end point, and a measurement error occurs, and the measurement result is out of the allowable range. It may end up.

In such a case, the pattern defect was uniquely determined in the prior art, but in the present embodiment, the differential waveform 33 of the line profile waveform 32 is displayed.
Since there are many extreme values in the differential waveform 33 and the extreme value having the largest peak value does not correspond to the normal edge, the operator can refer to the differential waveform 33 to obtain the measurement result. It can be recognized that there is a high possibility that the cause of being out of the allowable range is not due to a pattern defect but due to a length measurement error.

That is, since there are many extreme values in the example described with reference to FIG. 11, an operator who has some knowledge of the theory of edge judgment by the maximum gradient method can use another edge different from the normal edge. It becomes possible to easily recognize that there is a high possibility that is erroneously determined to be the measurement start point and end point.

In this way, when the length measurement result deviates from the predetermined allowable range, in this embodiment, the left edge designating mark 50L and the right edge designating mark 50R are automatically or in response to an instruction from the operator. As shown in FIG. 3, it is displayed in a superimposed manner at the planned position on the screen. Here, the operator uses the appropriate input means such as a trackball or a mouse to move the marks 50L and 50R to the regular edge positions as shown in FIG.

Marks 50L and 50 are made by the trackball.
When moving R, in the present embodiment, it is possible to select either the left edge mark or the right edge mark by the changeover switch. The distance between the left and right marks that changes during movement is multiplied by a unit length on the signal waveform and a coefficient for calibrating the unit length on the actual pattern, and converted into the actual dimension of the pattern in real time for display. Therefore, the actual size of the pattern displayed when the mark has been moved to the correct position is the desired pattern width.

The shape of the mark for designating the normal edge position is not limited to the arrow as described above, and for example, as shown in FIG. It may be linear marks 51L and 51R that are perpendicular to the vertical direction and reach the differential waveform 33. The means for moving the mark is not limited to the track ball, and may be a mouse or an arrow key.

According to the present embodiment, since the differential waveform of the line profile waveform is displayed and shown to the operator, even if the length measurement result is outside the predetermined allowable range,
It becomes easy to understand whether it is due to a pattern defect or a length measurement error due to the shape of the length measurement object.

Further, by specifying a regular edge position by an appropriate means such as a mark, it is possible to measure the length at that position. Also, the regular distance between edges can be easily re-measured.

In the above-described embodiment, the positions at which the marks 50L and 50R are first displayed (see FIG. 3) have no special meaning, but as shown in FIG. Alternatively, the position where the end point is determined may be designated.

Further, in the above embodiment, the change rate signal 33
Has been described as being displayed together with the scanning image 31 and the line profile signal 32, the present invention is not limited to this, and the scanning image 31 and the line profile signal 32 are not limited thereto.
It may be displayed together with any one of the above. Even with such a display mode, it is possible to recognize that the measurement start point and the end point have been erroneously determined and to specify the normal edge position at the time of erroneous determination.

Next, a second embodiment of the present invention will be described in which the normal edge position can be more easily and accurately specified when the edge is misjudged and the length measurement result is out of the predetermined allowable range. To do.

FIG. 6 is a diagram showing a screen display example according to the second embodiment of the present invention. The present embodiment is characterized in that the rate of change of the line profile waveform 32 is displayed as a quantized signal 36 that discretely displays the quantization level of the extreme value of the rate-of-change signal. That is, in this embodiment,
The convex peak (upper limit value) and the convex peak (lower limit value) of the differential signal are plotted together with the quantization level at that position.

Here, when the length measurement result deviates from the predetermined allowable range, the operator refers to the quantized signal 36, and the reason why the length measurement result is out of the standard is the pattern defect as described above. It is possible to recognize that there is a high possibility that it is due to a length measurement error rather than a thing. At this time, similarly to the above, marks 51L and 51R are superimposed and displayed on the screen on the left edge and the right edge determined by the maximum tilt method as shown in FIG. 7 automatically or in response to an instruction from the operator. To be done. In this embodiment, it can be seen that the pair of edge positions P1L and P1R having the largest rate of change of the line profile signal waveform 32 are erroneously determined as the length measurement start point and the end point, respectively.

By the way, even if the edge position is erroneously determined in this way, there is a very high possibility that the original edge position is on any of the extreme values. For example, in this embodiment, the regular edge is the fourth quantization level position P4L,
Equivalent to P4R. Therefore, in the present embodiment, when the operator presses the scheduled key switch once, as shown in FIG.
The positions of the pair of marks 51L and 51R are the second edge positions P2L and P2R from the maximum edge positions P1L and P1R.
To move to. Further, if the key switch is pressed once, it moves from the second edge positions P2L, P2R to the third edge positions P3L, P3R. If you press the key switch once more, the third edge position P3L, P3R
To the fourth edge position P4 which is a normal edge position
After moving to L and P4R, the mark interval at this time is displayed as a regular distance between edges.

As described above, in this embodiment, the measuring device successively selects new edge candidates for each simple operation of pressing a button, so that the edge position is designated by using the above-mentioned trackball or mouse. Compared with the case, it becomes possible to specify the regular edge position easily and accurately.

In the above-described embodiment, the priority order for pressing the key switch to select the edge position is determined based on the quantization level, but the present invention is not limited to this. Of the peak signal information, the leftmost one for the left edge and the rightmost one for the right edge may be sequentially selected.

[0037]

As described above, according to the present invention, the following effects can be achieved. (1) Since the differential waveform of the line profile waveform is displayed and shown to the operator, even if the length measurement result is out of the expected tolerance range, whether it is due to a pattern defect, It becomes easy to understand whether or not this is due to a measurement error due to the shape of the long object.

If such a judgment becomes easy, the operator can be given a sense of security and the mental burden can be reduced. You will be able to significantly reduce. (2) Since it is possible to specify the regular edge position with an appropriate means such as a mark and perform the length measurement at that position, even if the edge is misjudged and a length measurement error occurs, The distance between edges can be easily remeasured. As a result, it is possible to prevent the result of the length measurement error from being used for quality control. (3) The rate of change of the line profile waveform is displayed as a quantized signal that discretely displays the quantization level of the extreme value of the rate-of-change signal, and the measurement device is updated one after another with each simple operation of pressing a button. Since a proper edge candidate is selected, the regular position can be specified easily and accurately.

[Brief description of drawings]

FIG. 1 is a block diagram of a scanning electron microscope to which the present invention is applied.

FIG. 2 is a diagram showing an example of a display form of a length measurement object according to the first embodiment of the present invention.

FIG. 3 is a diagram showing an example of a method of measuring a regular distance between edges when a length measurement error occurs.

FIG. 4 is a diagram showing an example of a method for measuring a regular distance between edges when a length measurement error occurs.

FIG. 5 is a diagram showing an example of a method for measuring a regular distance between edges when a length measurement error occurs.

FIG. 6 is a diagram showing an example of a display form of a length measuring object according to a second embodiment of the present invention.

FIG. 7 is a diagram showing an example of a method for measuring a regular distance between edges when a length measurement error occurs.

FIG. 8 is a diagram showing an example of a method for measuring a regular distance between edges when a length measurement error occurs.

FIG. 9 is a diagram showing an example of a method of measuring a regular distance between edges when a measurement error occurs.

FIG. 10 is a diagram showing an example of a method for measuring a regular distance between edges when a measurement error occurs.

FIG. 11 is a diagram for explaining the cause of a length measurement error in automatic length measurement that employs the maximum tilt method.

FIG. 12 is a diagram showing an automatic length measuring method adopting the maximum inclination method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electron gun, 2 ... Electron beam, 3 ... Deflection coil, 5 ... Image shift coil, 6 ... Objective lens, 7 ... Sample, 8 ...
Sample stage, 9 ... DC amplifier, 10 ... Deflection amplifier, 1
DESCRIPTION OF SYMBOLS 1 ... Objective lens power supply, 12 ... Detector, 13 ... Stage drive circuit, 14 ... Deflection signal generator, 15 ... A / D converter,
16 ... Image memory, 17 ... D / A converter, 18 ... Deflection amplifier, 19 ... Image display monitor, 20 ... Deflection coil, 2
1 ... Computer, 22 ... Cursor signal generator, 24 ...
Trackball, 31 ... Scan image, 32 ... Line profile signal waveform, 33 ... Differential signal waveform, 36 ... Quantized signal waveform, 50L, 51L ... Left edge mark, 50R, 5
1R ... Mark for right edge

Claims (7)

[Claims] 1. A means for displaying a scan image of a length-measuring object by scanning a focused electron beam on a sample, a means for detecting a line profile signal at a predetermined position of the scan image, and a change in the line profile signal. Means for detecting a rate-of-change signal, a means for displaying the waveform of the rate-of-change signal as an image together with at least one of a scanning image and a line profile signal, and determining a measurement start point and an end point based on the line profile signal. And a means for detecting a distance between the length measurement start point and the end point, the scanning electron microscope having a length measurement function. 2. The scanning electron microscope with a length measuring function according to claim 1, wherein the change rate signal is a differential signal calculated based on a differential value of the line profile signal. 3. The scanning electron microscope with a length measuring function according to claim 2, wherein the differential signal is a quantized signal that discretely represents a quantized level at each extreme value of the differential value. . 4. A means for displaying a set of marks on the screen, a means for moving each of the marks independently on the screen, and a distance between two points on the scanning image indicated by each mark. A scanning electron microscope having a length measuring function according to any one of claims 1 to 3, further comprising: 5. The mark of one set selectively designates each of the positions at which a pair of extreme values is generated in response to the measurement start point and end point of the differential value in response to an input operation. A scanning electron microscope having a length measuring function according to claim 4. 6. The mark includes a position at which a pair of extreme values corresponding to a measurement start point and an end point of the differential value are generated, a position at which a pair of extreme values has the largest quantization level, and a smallest pair. 6. The scanning electron microscope with a length measuring function according to claim 5, wherein the scanning electron microscope is sequentially designated from any one of the positions where the extreme value is generated. 7. The set of marks sequentially designates a pair of extreme values in response to the measurement start point and end point of the differential value from either the inside or the outside of the quantized signal. A scanning electron microscope having a length measuring function according to claim 5.