JP2002116017A - Method and apparatus for measurement of pattern size - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and an apparatus wherein, when a pattern size is measured by using a scanning probe, such as a scanning electron microscope or the like, where dependence on the observation magnification of the pattern size is reduced and the pattern size can be measured precisely, irrespective of the setting of the length-measuring algorithm which finds the pattern size, on the basis of the observation magnification or a line profile. SOLUTION: When the pattern size is observed at a low magnification, a scanning pitch at a step (S4) at which a pattern to be measured is positioned, and a scanning pitch at a step (S7) at which the pattern size is found are changed over. The scanning pitch at the step, at which the pattern size is found, is made fine, to be set to about the diameter of the probe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for measuring a pattern size using a scanning probe such as a scanning electron microscope.

[0002]

2. Description of the Related Art A fine pattern dimension is measured using a probe such as an electron beam, an ion beam, or a light beam, or a mechanical probe. As a typical example, a case will be described in which the dimensions of a pattern processed on a wafer in a semiconductor manufacturing process are measured using a scanning electron microscope (length measuring SEM).

FIG. 1 is a schematic diagram showing the basic principle and configuration of a length measuring SEM. The electron beam 2 emitted from the electron gun 1 is narrowed down by the converging lens 3 and the objective lens 4 and focused on the wafer 5. At the same time, electron beam 2
Is deflected by the deflector 6 to scan the wafer 5 two-dimensionally or one-dimensionally. Secondary electrons 7 are emitted from the wafer portion irradiated with the electron beam 2. After being detected by the secondary electron detector 8 and converted into an electric signal, the secondary electrons 7 undergo processing such as amplification.

[0004] The secondary electron signal is used as a luminance modulation signal or a deflection signal of a display 9 such as a CRT. If the screen of the display 9 is two-dimensionally scanned in synchronization with the two-dimensional scanning of the electron beam 2 on the wafer surface and the luminance signal is modulated with the secondary electron signal, an SEM image is formed on the display 9. On the other hand, the main scanning signal of the electron beam 2 is displayed on the display 9.
If the X-deflection signal is used and the secondary electron detection signal is used as the Y-deflection signal of the display, a line profile is displayed on the display.

An example of a procedure for measuring a pattern dimension on a wafer using a length measuring SEM will be described below. The wafer 5 to be measured taken out of the wafer cassette 10 is pre-aligned on the basis of the orientation flat or the notch of the wafer, and the wafer number written on the wafer is read by a wafer number reader (not shown). The wafer number is unique to each wafer, and the recipe corresponding to the measured wafer 5 is read using the read wafer number as a key. The recipe defines a wafer measurement procedure and measurement conditions, and is registered in advance for each wafer, and all subsequent operations are performed in accordance with the recipe.

After the wafer number is read, the wafer 5
Is transported into the sample chamber 11 held in a vacuum, and is mounted on the XY stage 12. The wafer 5 loaded on the XY stage 12 is aligned using the alignment pattern formed thereon. The alignment pattern is the optical microscope 1 attached to the upper wall of the sample chamber 11.
3 and the image pickup device 14 and the image is magnified to about several hundred times. The captured alignment pattern is compared by the control device 15 with a reference image registered in advance. The controller 15 aligns the wafer 5 by driving the drive motor 16 to correct the position coordinates of the XY stage 12 so that the captured pattern just overlaps the reference image.

After the alignment, the wafer 5 to be measured is moved to a predetermined measuring point on the stage. At the measurement point, the scanning electron beam 2 is irradiated, and a high-magnification SEM image is formed. Using the SEM image, the pattern to be measured formed on the wafer is positioned with high accuracy. Similar to the alignment operation, the positioning is performed by comparing the SEM image of the measurement point with the reference image, and finely adjusting the electron beam scanning region so that the SEM image just overlaps the reference image. In the positioned wafer 5, the pattern to be measured is located substantially at the center of the screen, that is, immediately below the electron beam 2. In this state, the electron beam 2 is one-dimensionally scanned over the pattern to be measured at the same scanning pitch (pixel size) as when forming the SEM image, and the pattern dimensions are obtained from the obtained line profile according to a predetermined length measurement algorithm. If there are a plurality of measurement points on the wafer, the operation after the movement of the measurement points is repeated. If there are a plurality of wafers to be measured in the wafer cassette, the operations after the transfer of the wafers to the sample chamber 11 are repeated.

There are various length measurement algorithms for obtaining a pattern dimension from a line profile.
, The maximum slope value method, the linear approximation method, and the like. In FIG. 2, (a) schematically shows how the pattern to be measured 19 on the wafer 5 is scanned by the electron beam 2, and (b) shows the obtained line profile. (C) is an explanatory diagram explaining a threshold method, (d) is an explanatory diagram explaining a linear approximation method, and (e) is an explanatory diagram explaining a maximum slope value method.

In the threshold method, the position 2 where the slope portions at both ends of the line profile cross a predetermined level (threshold) L
The distance W between the two positions 21 and 22 is determined as the dimension of the pattern 19. The threshold L is
Usually, it is determined as a ratio to the maximum height of the line profile. In the linear approximation method, the slope portions at both ends of the line profile are linearly approximated by the least squares method, and a distance W between intersections 23 and 24 between each straight line and the base line is used as a pattern dimension. In the maximum slope method, a tangent is drawn to a portion where the slope portions at both ends of the line profile have the maximum slope, and an intersection 2 between the tangent and the baseline is drawn.
The distance W between 5 and 26 is used as the pattern dimension.

[0010]

In positioning a pattern to be measured, a relatively low-magnification SEM image is often used in order to easily find a desired pattern from many patterns. Also, when measuring the pattern dimension at a high magnification, the measurement may be performed with the magnification changed.

By the way, even when the same pattern is measured, when the magnification is changed, there is a phenomenon that the measured value and the measurement accuracy change depending on the magnification. For this reason, when performing observation at a low magnification for the above purpose, the measured values greatly deviate from the actual dimensions, and the dispersion of the measured values increases. The linear approximation method and the maximum slope method are
Observation magnification dependence of the measured value is larger than that of the threshold method.

The present invention has been made in view of such problems of the prior art, and accurately determines a pattern dimension regardless of the setting of a length measurement algorithm for determining a pattern dimension from an observation magnification or a line profile. It is intended to provide a method capable of doing so.

[0013]

FIG. 3 shows an example of the measurement of the SEM image magnification dependency of the measured value, and FIG. 4 shows the magnification dependency of the variation (3σ) of the measured value. The horizontal axis is the magnification. The figure shows the measured value by the maximum slope value method and the measured value by the threshold value method. 20%, 50%, 80% by threshold method
Represents a threshold level, for example, a graph shown as 20% is a measurement result when the threshold level is set to 20% of the maximum height of the line profile by the threshold method.

FIGS. 3 and 4 show that when the threshold level is set to 20% by the maximum slope method and the threshold method, the measured value of the pattern width becomes relatively large at a relatively low magnification, and the measurement variation also increases. . If the threshold level is 80
Conversely, in the threshold method with%, the measured value of the pattern width becomes large when measured at a high magnification. As a result of examining the cause of the dependence of the measured pattern size on the observation magnification, it was found that the observation magnification was reduced, the scanning pitch (pixel size) was increased, and the waveform blunting of the line profile was increased. In particular, this tendency becomes remarkable when the scanning pitch is equal to or larger than the electron beam diameter.

FIG. 5 schematically shows the mechanism.
White circles drawn at the scanning pitch indicate sampling points by the electron beam 2, and the pixel size d is equal to the range covered by one sampling point. What is drawn as a line profile is a line profile created by data sampled at the scanning pitch. The figures shown as magnifications are for reference only.
As shown in (a), when the magnification is low, the waveform of the line profile is largely dull, and the size W of the pattern appearing in the line profile is larger than the actual pattern 19. As the magnification is increased in the order of (b), (c) and (d), the size W of the pattern appearing in the line profile changes.

The present invention has been made on the basis of elucidating the cause of the dependence of the measured pattern size on the observation magnification, and scans a sample at a predetermined scanning pitch with a probe and uses a scanning signal obtained from the sample. Forming a sample image, scanning a predetermined portion of the pattern to be measured in the sample image with the probe, and processing the obtained scanning signal in accordance with a predetermined length measurement algorithm to obtain a size of the predetermined portion. The scanning pitch is changed between when the sample image is formed and when the pattern dimension is measured. It is preferable that the scanning pitch at the time of obtaining the pattern size is about the electron beam diameter.

Further, the present invention provides a pattern dimension measuring method for processing a scanning signal obtained by a threshold method to obtain a dimension of a predetermined portion, wherein the threshold is set to a level of about 50% in a low magnification area. I do. Further, the present invention provides a pattern dimension measuring method for obtaining the dimensions of the predetermined portion by processing the obtained scanning signal by a linear approximation method or a maximum inclination value method, and in a low magnification area, by a linear approximation method or a maximum inclination value method. A value obtained by subtracting a predetermined value determined by the observation magnification from the measured dimension value is set as a length measurement value.

Further, the present invention is characterized in that, as a length measurement algorithm, an algorithm for obtaining a dimension by a threshold method after drawing a straight line approximation line or a maximum slope line is used. Further, the present invention is characterized in that the length measurement algorithm is switched to the threshold method in the low magnification area. According to the present invention, since the dependence of the dimension measurement value on the observation magnification is reduced, a measurement value close to the actual dimension can be obtained even when observed at a low magnification, and measurement with high reproducibility is possible.

[0019]

Embodiments of the present invention will be described below with reference to the drawings. Here, a case will be described as an example where a pattern dimension on a wafer is measured using a length measurement SEM. FIG. 6 is an explanatory diagram of the first embodiment. In this embodiment, at the time of observation at a low magnification, the scanning pitch at the time of positioning the pattern to be measured and the scanning pitch at the time of obtaining the pattern size are automatically switched, and the scanning pitch at the time of obtaining the pattern size is about the electron beam diameter. To be fine.

Observation and measurement are performed according to the illustrated operation flow. First, a wafer to be measured is mounted on an XY stage (S1), and wafer alignment is performed using an alignment pattern on the wafer (S2). Thereafter, the XY stage is moved to the measurement point (S3), and the pattern to be measured is positioned using the specific pattern in the SEM image 61 such that the pattern to be measured p is located immediately below the electron beam (S4). In the SEM image 62 at the time when the positioning of the pattern to be measured p is completed, the pattern to be measured p is located at the center of the screen. Subsequently, as shown by the dashed line in the SEM image 63, the part to be measured in the pattern p is designated (S
5).

When step 5 is completed, the scanning pitch is automatically switched so as to be substantially equal to the electron beam diameter (S6), and the pattern dimension is measured at this small scanning pitch (S7). At this time, the electron beam scanning area is as shown by 64. When the relative size of the pixel of the SEM image at the time of the positioning in Step 4 is represented by an area 66, the pixel size of the SEM image at the time of measuring the pattern dimension in Step 7 is represented by an area 67.

As described above, regardless of the observation magnification at the time of positioning the pattern to be measured, the scanning pitch is switched to a predetermined pitch suitable for accurately measuring the actual dimensions of the pattern at the time of pattern dimension measurement, so that the same precision is always obtained. Can be used to measure the size, and it is possible to prevent a change in the measured value due to a difference in magnification. FIG. 7 is an explanatory diagram of the second embodiment. In this embodiment, the threshold is automatically set to a level of approximately 50% during observation with a low magnification and the threshold method.

FIG. 7A is a view for explaining the pattern dimension measurement at a high magnification by the threshold method. The pattern dimension measurement is performed in the area indicated by the broken line in the SEM image shown on the left, and a line profile shown on the right is obtained. In the case of a high magnification, as can be seen from FIG. 3, since the dependence of the length measurement value on the magnification is small, even if the threshold level L1 is arbitrarily set, the fluctuation of the length measurement value by the magnification is small.

However, when the magnification is low, as shown in FIG. 3, the fluctuation of the measured value due to the magnification is large at the threshold of 20% or 80%, but the fluctuation of the measured value due to the magnification is small at the threshold of 50%. Accordingly, as shown in FIG. 7B, when measuring the length at the low magnification by the threshold method, the threshold value L2 is automatically set to about 50%, thereby minimizing the fluctuation of the measured value due to the difference in the magnification. can do.

FIG. 8 is an explanatory diagram of the third embodiment. In this embodiment, at the time of observation by the low magnification and the linear approximation method or the maximum inclination value method, the length measurement value is a value obtained by subtracting a predetermined value determined by the observation magnification from the value obtained by the linear approximation line or the maximum inclination line. To be corrected. FIG. 8 shows an example of the maximum inclination value method. It is assumed that the pattern dimension obtained by the maximum inclination value method from the line profile at the magnification of 50,000 is W1.
In this case, the pattern dimension W is calculated and displayed by the following equation.

W = W1−ΔW Here, as shown in FIG. 3, ΔW is a value measured by the maximum inclination value method at a magnification of 50,000 times and a value measured by the threshold method with a threshold value of about 50%. Is the difference. As described above, when the magnification is low, the variation in the measured value is small according to the threshold method with a threshold of about 50%. By converting to the measured value by
A length measurement value independent of the magnification can be obtained. As is apparent from FIG. 3, since ΔW has a magnification dependency, a value of ΔW is obtained and stored in advance for each magnification, and the value of ΔW corresponding to the magnification is used in the above calculation.

Here, the maximum inclination value method has been described.
Also in the linear approximation method, by converting the measured value into a measured value by a threshold method with a threshold value of about 50% by the same method,
It is possible to remove the dependence of the measured value at low magnification on the magnification. FIG. 9 is an explanatory diagram of the fourth embodiment. In this embodiment, as shown in FIG. 9A, a tangent is drawn at the maximum slope to the slope portion of the line profile, the upper side is approximated by a straight line, and the entire line profile is approximated by a trapezoid. Thereafter, as shown in FIG. 7 (b), the pattern dimension W is determined by a threshold method with a threshold value of about 50% for the tangent line.
Ask for. It is clear from FIG. 5A that the measured value by this method reflects the actual dimensions.

FIG. 10 is an explanatory diagram of the fifth embodiment. In this embodiment, when the length is measured at a low magnification, the maximum inclination value method or the linear approximation method is automatically switched to the threshold method. Now, it is assumed that the measurement method is set to the maximum inclination value method in the length measurement SEM. As shown in FIG. 10A, at the time of length measurement at a high magnification, the pattern dimension W is measured by the maximum inclination value method as set. If the magnification is changed to a low magnification for some reason, as shown in FIG. 10B, the waveform of the line profile becomes blunter, and if the length is measured by the maximum slope method according to the setting, the error increases. Therefore, in this embodiment, when the measurement method is set to the maximum slope method or the linear approximation method, when measuring the length at a low magnification, a threshold value at which the magnification dependence of the measured value is small is about 50.
It should be automatically switched to the threshold method of%.

Here, the length measurement SEM using an electron beam as a probe has been described as an example. However, the probe is a light beam (laser scanning microscope), an ion beam (FI
B) or a mechanical probe (scanning atomic force microscope). Although the case of one probe and one pixel has been described as an example, the present invention is also applicable to a multi-probe and a multi-pixel.

[0030]

According to the present invention, the magnification dependency of the length measurement value can be reduced even in the low magnification range, and the length measurement accuracy can be improved.

[Brief description of the drawings]

FIG. 1 is a view for explaining the principle and configuration of an SEM.

FIG. 2 is an explanatory diagram of a method for obtaining a pattern dimension from a line profile.

FIG. 3 is a diagram showing an example of magnification dependency of a measured value.

FIG. 4 is a diagram showing a relationship between a variation in a measured value and a magnification.

FIG. 5 is a view for explaining a mechanism of a magnification dependency of a dimension measurement value.

FIG. 6 is an explanatory diagram of the first embodiment.

FIG. 7 is an explanatory diagram of the second embodiment.

FIG. 8 is an explanatory diagram of the third embodiment.

FIG. 9 is an explanatory diagram of the fourth embodiment.

FIG. 10 is an explanatory diagram of the fifth embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electron gun, 2 ... Electron beam, 3 ... Convergent lens, 4 ... Objective lens, 5 ... Wafer, 6 ... Deflection coil, 7 ... Secondary electron, 8 ... Secondary electron detector, 9 ... Display, 10 ... Wafer Cassette, 11 sample chamber, 12 XY stage, 1
3 optical microscope, 14 imaging device, 15 control device, 1
6 ... Drive motor, 19 ... Pattern

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2F067 AA21 BB01 BB04 CC17 HH06 JJ05 KK04 NN03 PP12 2F069 AA49 BB15 GG04 HH30 JJ02 MM24 NN16

Claims (3)

[Claims] 1. A sample is scanned at a predetermined scanning pitch by a probe, a sample image is formed using a scanning signal obtained from the sample, and a predetermined portion of a pattern to be measured in the sample image is scanned by the probe. In the pattern dimension measuring method for obtaining the dimensions of the predetermined portion by processing the obtained scanning signal by the linear approximation method or the maximum inclination value method, the magnification is lower than a predetermined magnification, and the linear approximation method or the maximum inclination value method is used. A pattern dimension measuring method, characterized in that at the time of length measurement, a value obtained by subtracting a predetermined value determined by an observation magnification from a determined dimension value is used as a length measurement value. 2. A sample is scanned at a predetermined scanning pitch by a probe, a sample image is formed using a scanning signal obtained from the sample, and a predetermined portion of a pattern to be measured in the sample image is scanned by the probe. In a pattern dimension measuring device for processing the obtained scanning signal by a threshold method to determine the dimension of the predetermined portion, when measuring a length at a magnification lower than a predetermined magnification, automatically regardless of a set threshold level at that time, A pattern dimension measuring apparatus comprising: means for setting a threshold value to approximately 50%. 3. A sample is scanned at a predetermined scanning pitch by a probe, a sample image is formed using a scanning signal obtained from the sample, and a predetermined portion of a pattern to be measured in the sample image is scanned by the probe. In a pattern size measuring apparatus for processing the obtained scanning signal according to a predetermined length measurement algorithm to obtain the size of the predetermined portion, when the magnification is changed to a lower magnification than a predetermined magnification, the maximum length measurement method or the linear approximation method is used. A pattern dimension measuring device comprising means for automatically switching an algorithm to a threshold method.