JPH0617875B2 - Pattern inspection method and apparatus - Google Patents

Description

Description: TECHNICAL FIELD The present invention detects a defect in an object to be inspected having a transparent pattern on an opaque substrate such as a photoresist pattern formed on a semiconductor wafer. The present invention relates to a pattern inspection method and an apparatus therefor.

[Conventional technology]

When forming a circuit pattern of an LSI, a stepper (reduction projection exposure apparatus) repeatedly prints a pattern on a reticle, which is an original of the pattern, onto a wafer. Therefore, if foreign matter adheres to the reticle or the stepper optical system, All chips will be defective. Therefore, the reticle pattern is inspected by inspecting the resist pattern printed on the semiconductor surface using the reticle.

Conventionally, in order to inspect this reticle pattern on the exposed and developed photoresist pattern, visual observation using a microscope has been performed. However, visual inspection requires enormous labor, time, and a high degree of skill, and there are individual differences among workers, variations in judgment criteria, and missing of defects. As a result, it has become almost impossible to implement. Therefore, various photoresist pattern appearance inspection techniques have been proposed.

As a method of this appearance inspection, it is conventionally used that a plurality of identical chip patterns are drawn on one reticle, and after transferring this to a photoresist on the wafer, two reticle patterns are printed on the wafer. A method of detecting a pattern defect by comparing chip patterns has been proposed. However, as semiconductors have become highly integrated and have a large area, only one chip pattern is being written on one reticle. For such a chip pattern, the above method is used. Is not applicable.

Therefore, as a method of inspecting such a chip pattern,
For example, a method of comparing and inspecting a chip pattern on a wafer by using, for example, a design pattern at the time of manufacturing a reticle as a model pattern can be considered. This method is widely used in visual inspection of photomasks and reticle patterns for the following reasons. In other words, when a photomask or reticle is illuminated with transmitted light and an image is picked up by an image sensor, the circuit pattern can be picked up with an extremely high contrast, so that the comparison pattern can be easily compared with a design pattern represented by binary data by thresholding. Because you can. However, since the photoresist pattern is a transparent pattern, the contrast between the photoresist portion and the underlying wafer surface is very low, and it is difficult to compare it with the design pattern as compared with the case of a photomask or reticle pattern.

Considering the above points, as a method for comparing and inspecting a photoresist pattern and a design pattern, for example, SPII, Volume 538, Optical Microlithography IV (1985), Pages 122 to 129 (SPIE , Vo
l.538, Optical Microlithography IV (1985) pp122-12
There is a method described in 9). This method limits the wavelength of the reflected light from the wafer by using an optical filter when the photoresist pattern is imaged by the image sensor, and improves the contrast with the circuit pattern. Since the photoresist is a transparent thin film, when light is incident on it, the light reflected from the photoresist surface interferes with the light that has passed through the photoresist and reflected at the boundary surface with the underlying layer. Depending on the thickness, light of one wavelength strengthens each other and light of another wavelength weakens each other.
Therefore, by selecting the transmission wavelength band of the optical filter, the photoresist portion can be detected darkly and an image with high contrast can be obtained, and a design pattern can be easily obtained by the same method as in the case of a photomask or reticle pattern. It is possible to perform a comparative inspection with.

[Problems to be solved by the invention]

The above-mentioned prior art does not consider unevenness in the film thickness of the photoresist pattern, and has a problem that the contrast is lowered in some places due to the unevenness of the film thickness and a comparison inspection with the design pattern cannot be performed. .

In the above conventional technique, the photoresist pattern to be inspected must have a uniform film thickness. That is, the characteristics of the optical filter for detecting the photoresist darkly depend on the thickness of the photoresist, and if the thickness of the photoresist is uneven within one chip, the contrast of the photoresist may vary depending on the location. It becomes difficult to completely binarize only the photoresist portion by thresholding.

On the other hand, in the actual LSI manufacturing process, film thickness unevenness during photoresist coating and film thickness unevenness during development occur. In addition, recently, there is a circuit pattern having a line width smaller than the film thickness, but such a pattern has a smaller film thickness than a pattern having a wide width.

For this reason, in the above-mentioned conventional technique, if the photoresist pattern has unevenness in film thickness within one chip, the pattern cannot be binarized, and the comparison inspection with the design pattern cannot be performed.

It is an object of the present invention to solve the above-mentioned problems of the prior art by making a transparent film on an object to be inspected having a transparent pattern on an opaque substrate, without being affected by film thickness unevenness and film thickness variation of the transparent pattern. A grayscale image signal that corresponds to the brightness according to the change in the film thickness of the pattern and changes darkly from the step of the boundary of the transparent pattern to show an extreme value, and a model pattern signal that serves as a model for the transparent pattern It is an object of the present invention to provide a pattern inspection method and apparatus capable of detecting a defect in a transparent pattern with high reliability based on

[Means for solving problems]

In order to achieve the above-mentioned object, the present invention corresponds to the brightness according to the change of the film thickness of the transparent pattern from the inspection object having the transparent pattern on the opaque substrate and the boundary of the transparent pattern. Detecting a grayscale image signal that changes darkly from the stepped portion to show an extreme value and generate a model pattern signal that serves as a model for the transparent pattern, and the generated model for the detected grayscale image signal. The misalignment of the pattern signal is corrected for alignment, and the grayscale image signal of the normal transparent pattern among the detected grayscale image signals is masked by the aligned model pattern signal and the grayscale that is not masked. A pattern inspection method characterized by detecting a defect of a transparent pattern in the inspection target by an edge signal indicating an extreme value of a change in an image signal. That. Further, the present invention corresponds to the brightness corresponding to the change in the film thickness of the transparent pattern from the inspection object having the transparent pattern on the opaque substrate, and changes from the step difference at the boundary of the transparent pattern to dark. A grayscale image signal showing an extreme value to generate a model pattern signal that serves as a model for the transparent pattern, and corrects the positional deviation of the generated model pattern signal with respect to the detected grayscale image signal. Then, the signal indicating the extreme value of the change is extracted from the detected gray-scale image signal, and the normal signal is extracted from the signals indicating the extreme value of the change extracted by the aligned model pattern signal. Transparent pattern in the object to be inspected by an edge signal showing the extreme value of the change of the grayscale image signal which is not masked by masking the signal showing the extreme value of change due to the transparent pattern A pattern inspection method characterized by detecting defects. Further, the present invention corresponds to the brightness corresponding to the change in the film thickness of the transparent pattern from the inspection object having the transparent pattern on the opaque substrate, and changes from the step difference at the boundary of the transparent pattern to dark. And a grayscale image signal detecting means for detecting a grayscale image signal indicating an extreme value, a model pattern signal generating means for generating a model pattern signal serving as a model for the transparent pattern, and a grayscale image signal detecting means for detecting Position shift correcting means for correcting the position shift of the generated model pattern signal with respect to the generated grayscale image signal to perform positioning, and the grayscale image signal by the model pattern signal aligned by the position shift correcting means. A grayscale image signal not masked by masking the grayscale image signal by a normal transparent pattern among the grayscale image signals detected by the detection means Is a pattern inspection apparatus characterized by comprising a masking defect detecting means for detecting defects of a transparent pattern in the object to be inspected by the edge signal which indicates the extremes of definitive change. Further, the present invention corresponds to the brightness corresponding to the change in the film thickness of the transparent pattern from the inspection object having the transparent pattern on the opaque substrate, and changes from the step difference at the boundary of the transparent pattern to dark. And a grayscale image signal detecting means for detecting a grayscale image signal indicating an extreme value, a model pattern signal generating means for generating a model pattern signal serving as a model for the transparent pattern, and a grayscale image signal detecting means for detecting Position deviation correction means for correcting the position deviation of the generated model pattern signal with respect to the generated grayscale image signal to perform alignment, and an extreme value of change from the grayscale image signal detected by the grayscale image signal detection means. Signal extracting means for extracting the signal indicating the extreme value of the change, and signal for indicating the extreme value of the change by the model pattern signal aligned by the position shift correcting means. The signal indicating the extreme value of the change due to the normal transparent pattern of the signal indicating the extreme value of the change extracted by the extracting means is masked to clear the inspected object by the edge signal indicating the extreme value of the change not masked. A pattern inspection apparatus comprising: masking defect detecting means for detecting a pattern defect. Further, the present invention is the pattern inspection apparatus, wherein the masking defect detecting means comprises:
A masking means for masking the grayscale image signal with the model pattern signal and a binary value for binarizing the grayscale image signal masked by the masking means with a predetermined threshold value to obtain an edge signal showing the extreme value of the change. It is characterized in that it is configured with a conversion means. In the pattern inspection apparatus according to the present invention, the signal extracting means for indicating the extreme value of change is constituted by differentiating means for performing a differentiating process on the grayscale image signal to extract a signal indicating the extreme value of change. It is characterized by According to the present invention, in the pattern inspection apparatus, the masking defect detecting means masks the signal showing the extreme value of the change by the masking defect detecting means and the masking means. And a binarizing means for binarizing the signal indicating the extreme value of the change with a predetermined threshold to obtain an edge signal indicating the extreme value of the change.

[Action]

The present invention focuses on a dark and light image signal that shows a dark and extreme value detected from a step portion at the boundary of a transparent pattern, and focuses on the uneven thickness of a transparent pattern such as a photoresist on an opaque substrate. Even if the brightness of the grayscale image signal detected according to the film thickness fluctuation fluctuates, the fluctuation of the grayscale image signal is not erroneously recognized, and the true defect such as a protrusion defect, an isolated defect, or a short circuit defect of the transparent pattern is generated. The purpose is to make it possible to detect a defect by distinguishing it from the detected grayscale image signal. That is, according to the present invention, according to the above configuration, the projection defect, the isolated defect, the short circuit defect, etc. of the transparent pattern are not affected by the film thickness unevenness and the film thickness variation of the transparent pattern such as the photoresist pattern on the opaque substrate. This is to make it possible to detect the true defect of the with high reliability. By the way, as described above, a defect in a transparent pattern such as a photoresist pattern is caused by a foreign substance attached to the reticle or the stepper optical system. Therefore, the defects existing in the photoresist pattern are, as shown in FIG.
This is a defect in which extra photoresist remains in places where the photoresist should not exist, such as isolated points 2 and short circuits 3. FIG. 11 shows a model pattern for the inspection pattern including the defect shown in FIG. FIG. 12 shows the inspection pattern shown in FIG. 10 and the model pattern shown in FIG. 11 superimposed on each other. The hatched portion is the inspection prohibited area 11, and the other portions are the inspection area 12. As can be seen from FIG. 12, in the inspection area 12, since only defective portions of the photoresist pattern exist on the wafer surface having a flat and substantially uniform reflectance, these defects can be easily detected. It will be possible.

〔Example〕

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

FIG. 1 is a block diagram showing the configuration of a defect detection circuit used in an embodiment of the present invention. In the figure, the inspection pattern detected by the pattern detector 20 is converted into an inspection pattern 27 of a digital signal of several bits by an A / D converter 21. The model pattern generator 22 generates a model pattern 28 synchronized with the pattern generator 20. In order to align the inspection pattern 27 and the model pattern 28,
After the positional deviation detector 23 determines the positional deviation amount between the model pattern 28 and the inspection pattern 27, the positional deviation correction circuit 24 shifts the model pattern 28 to align it with the inspection pattern 27. The masking circuit 25 converts the inspection pattern 27 into a model pattern 2 that has been aligned.
After masking by 9, the output of the masking circuit 25 is binarized by the binarizing circuit 26 to be a defect.

Here, the video signal of the photoresist pattern obtained by the pattern detector 20 will be described. Fig. 2
(a) shows a partial cross section of the inspection pattern, and a photoresist pattern 31 is formed on the wafer 32. The light 33 perpendicularly incident on the surface of the flat photoresist portion is reflected by the photoresist surface or the boundary surface between the photoresist and the wafer and returns in the incident direction. The same applies to the light 35 incident on the wafer surface. However, the light 34 incident on the edge portion of the photoresist pattern is reflected by the photoresist surface and then reflected again by the wafer surface, and the light 36 is refracted from the photoresist surface and enters the inside, and is reflected by the boundary surface with the wafer. There is no light returning to the incident direction. Therefore, the edge portion of the pattern is detected darker than the other portions, as shown in FIG. 7B showing the video signal of the reflected light.

Next, operations of the masking circuit 25 and the binarization circuit 26 shown in FIG. 1 will be described. Figure 3 shows protrusion defect 1
FIG. 4 (a) is a waveform diagram of the video signal of the portion AA ′ in FIG. 3, and FIG. It is detected darkly. In FIG. 2 (a), the video signal levels corresponding to the photoresist patterns 10a and 10b are different, which means that there is a film thickness difference between the patterns 10a and 10b. FIG. 2B is a waveform diagram of the video signal of the model pattern, in which the photoresist portion is "1" and the photoresist-free portion is "0". In the masking circuit 25 shown in FIG. 1, when the model pattern is "1", the maximum value that the inspection pattern can take ("255" in the case of an 8-bit signal) is output, and the model pattern is "0". In case of ", the value of the input inspection pattern is output as it is. FIG. 4 (c) is a waveform diagram of the output of the masking circuit 25. As can be seen from the figure, the normal pattern portion has the maximum value irrespective of the photoresist film thickness. This signal is 2 with an appropriate threshold value V _TH
The value is shown in FIG. 6D, and the portion marked with “0” in the drawing indicates a defect.

FIG. 5 is a block diagram showing a concrete example of the masking circuit 25 of this embodiment. The figure shows the case where the inspection pattern is an 8-bit signal, and the masking circuit is composed of eight OR circuits 401 to 408. When the model pattern 29 is "1", the output is "255",
When it is "0", the value of the input inspection pattern is output as it is. Thus, by setting the output to the maximum value when the model pattern 29 is "1", the normal photoresist pattern portion is set as the inspection prohibited area, and the defect is erroneously detected by the binarization circuit 26 in the next stage. Can be prevented. According to the present embodiment, since the main part of the defect detection circuit can be constructed by only a few OR circuits and a binarization circuit, the circuit can be miniaturized.

FIG. 6 is a block diagram showing the structure of a defect detection circuit used in another embodiment of the present invention. In this embodiment, A /
The D-converted inspection pattern 27 is applied to the edge detection circuit 50.
It is input to the masking circuit 25 via.

The operation of the edge detection circuit 50 will be described below with reference to FIG. FIG. 7 is the same as FIG. 3, and FIG. 7 (a) shows a photoresist pattern 10 including protrusion defects 1.
(b) is a waveform diagram of the video signal of the portion AA 'in FIG.
(c) shows the corresponding model pattern. Now, the video signal of the inspection pattern is, for example, 2 by the edge detection circuit 50.
The second differentiation is performed and the pattern edge signal 51 is converted.
FIG. 7 (d) is a waveform diagram of the pattern edge signal 51. The masking circuit 25 uses the pattern edge signal 5
When 1 is input and the example pattern 29 is "1", the output is "0", and when the example pattern 29 is "0", the input pattern edge signal 51 is output as it is. FIG. 7 (e) is a waveform diagram of the output of the masking circuit 25. This output signal is converted to an appropriate threshold value V by the binarization circuit 26.
The waveform binarized by _TH is the waveform diagram of FIG. 6 (f), and the portion where the output is “1” indicates the defect. In addition,
It is obvious that the same result (shown in FIG. 7 (f)) can be obtained by performing masking by the masking circuit 25 after binarizing by the binarizing circuit 26.

FIG. 8 is a block diagram showing a concrete example of the masking circuit 26 of the present embodiment. As in the case of FIG. 5, the case where the inspection pattern (pattern edge signal) is an 8-bit signal is shown as an example. . This circuit corresponds to the OR circuits 401 to 408 in FIG.
The NOT circuit 61 is additionally inserted in the input circuit on the model pattern side instead of the AND circuits 601 to 608. When the model pattern is "1", the output is "0", and the pattern pattern is When it is "0", the input is output as it is.

Note that, in the above-described embodiment, an example in which the inspection pattern is directly masked with the model pattern generated from the design pattern has been described, but the actual photoresist pattern is
As shown in FIG. 9 (a), the corners are more rounded or the pattern width is narrower than the design pattern shown in FIG. 9 (b). Therefore, the defect detection performance can be further improved by using, as a model pattern, a design pattern that is previously corrected so as to match the shape of the inspection pattern, as is often done when inspecting a reticle or mask pattern. You can

〔The invention's effect〕

According to the present invention, the defect of the transparent pattern is detected by paying attention to the grayscale image signal that changes darkly and shows the extreme value which is detected from the stepped portion of the boundary of the transparent pattern. Even if the brightness of the grayscale image signal detected varies depending on the thickness unevenness of the transparent pattern of the photoresist or the like and the variation of the thickness, the transparent pattern does not erroneously recognize the variation of the grayscale image signal. True defects such as protrusion defects, isolated defects, short-circuit defects, etc. can be detected with high reliability, and remarkable effects can be obtained in improving the yield of semiconductor integrated circuits, improving the reliability of products, and reducing costs.

[Brief description of drawings]

FIG. 1 is a block diagram showing the structure of a defect detection circuit used in an embodiment of the present invention, FIG. 2 is an explanatory diagram of a video signal of a photoresist pattern, and FIGS. 3 and 4 are implementations shown in FIG. 5 is an explanatory view of a signal processing system in an example, FIG. 5 is a block diagram showing a concrete example of a masking circuit in FIG. 1, and FIG. 6 is a block diagram showing a configuration of a defect detection circuit used in another embodiment of the present invention. FIG. 7 is an explanatory view of a signal processing system in the embodiment shown in FIG. 6, FIG. 8 is a configuration diagram showing a concrete example of the masking circuit in FIG. 6, and FIG. 9 is an actual photoresist pattern designed. Explanatory drawing of being transformed from the pattern,
10 to 12 are explanatory views of the principle of the pattern defect inspection method according to the present invention. 20 ... Pattern detector, 21 ... A / D converter, 22 ... Sample pattern generator, 23 ... Positional deviation detector, 24 ... Positional deviation correction circuit, 25 ... Masking circuit, 26 ... Binary circuit, 401 to 408 ... OR circuit, 50 ... Edge detection circuit, 601 to 608 ... AND circuit, 61 ... NOT circuit.

Claims (7)

[Claims] 1. An object to be inspected having a transparent pattern on an opaque substrate, which corresponds to the brightness according to a change in the film thickness of the transparent pattern, and changes from a step portion at the boundary of the transparent pattern to a dark one. A grayscale image signal showing an extreme value to generate a model pattern signal that serves as a model for the transparent pattern, and corrects the positional deviation of the generated model pattern signal with respect to the detected grayscale image signal. Then, the extreme value of the change in the grayscale image signal not masked by masking the grayscale image signal by the normal transparent pattern of the grayscale image signal detected by the aligned model pattern signal A pattern inspection method, wherein a defect of the transparent pattern in the inspection object is detected by an edge signal indicating 2. An object to be inspected having a transparent pattern on an opaque substrate, which corresponds to the brightness according to the change in the film thickness of the transparent pattern, and which changes dark from the step of the boundary of the transparent pattern. A grayscale image signal showing an extreme value to generate a model pattern signal that serves as a model for the transparent pattern, and corrects the positional deviation of the generated model pattern signal with respect to the detected grayscale image signal. Then, the signal indicating the extreme value of the change is extracted from the detected gray-scale image signal, and the normal signal is extracted from the signals indicating the extreme value of the change extracted by the aligned model pattern signal. Of the transparent pattern in the object to be inspected by masking the signal showing the extreme value of the change due to the transparent pattern Pattern inspection method characterized by detecting a Recessed. 3. An object to be inspected having a transparent pattern on an opaque substrate, which corresponds to a brightness corresponding to a change in the film thickness of the transparent pattern, and which changes dark from a step portion at a boundary of the transparent pattern. And a grayscale image signal detecting means for detecting a grayscale image signal indicating an extreme value, a model pattern signal generating means for generating a model pattern signal serving as a model for the transparent pattern, and a grayscale image signal detecting means for detecting Position shift correcting means for correcting the position shift of the generated model pattern signal with respect to the generated grayscale image signal to perform positioning, and the grayscale image signal by the model pattern signal aligned by the position shift correcting means. Of the grayscale image signals detected by the detection means, the grayscale image signal with a normal transparent pattern is masked to obtain a grayscale image signal not masked. Pattern inspection apparatus by the edge signal which indicates the extremes of kicking change characterized by comprising a masking defect detecting means for detecting defects of a transparent pattern in the object to be inspected. 4. The masking defect detection means binarizes a grayscale image signal masked by the model pattern signal and the grayscale image signal masked by the masking means, and binarizes the grayscale image signal with a predetermined threshold value. 4. The pattern inspection apparatus according to claim 3, further comprising a binarizing unit that obtains an edge signal indicating an extreme value of change. 5. An object to be inspected having a transparent pattern on an opaque substrate, which corresponds to the brightness according to the change of the film thickness of the transparent pattern, and which changes darkly from the step portion at the boundary of the transparent pattern. And a grayscale image signal detecting means for detecting a grayscale image signal indicating an extreme value, a model pattern signal generating means for generating a model pattern signal serving as a model for the transparent pattern, and a grayscale image signal detecting means for detecting Position deviation correction means for correcting the position deviation of the generated model pattern signal with respect to the generated grayscale image signal to perform alignment, and an extreme value of change from the grayscale image signal detected by the grayscale image signal detection means. Signal extracting means for extracting the signal indicating the extreme value of the change, and a signal indicating the extreme value for the change by the model pattern signal aligned by the positional deviation correcting means The signal indicative of the extreme value of the change extracted by the output means is masked to the signal indicative of the extreme value of the change due to the normal transparent pattern, and the edge signal indicating the extreme value of the change that is not masked is transparent to the object to be inspected. A pattern inspection apparatus comprising: a masking defect detecting means for detecting a pattern defect. 6. A signal extracting means for indicating the extreme value of the change is constituted by a differentiating means for applying a differentiation process to the grayscale image signal to extract a signal indicating the extreme value of the change. The pattern inspection apparatus according to claim 5. 7. The masking defect detecting means sets a predetermined value for masking means for masking a signal indicating the extreme value of change by the model pattern signal and a signal for indicating the extreme value of change masked by the masking means. With a threshold of 2
A binarizing means for binarizing and obtaining an edge signal indicating the extreme value of the change.
The pattern inspection device according to the item.