JPH10256326A - Method and equipment for inspecting pattern - Google Patents

Abstract

PROBLEM TO BE SOLVED: To detect a defect and to prevent erroneous detection of a pseudo defect even when the depth of focus is small with respect to focal-plane measuring accuracy and positioning accuracy. SOLUTION: An image storing means 14, which stores an image, whose focus agrees with the normal pattern to be inspected (called a reference pattern image), and a means, which automatically selects an image closest to the reference pattern image and sets the image as a comparison pattern image, are provided. The means for selecting the comparison pattern image selects the image closest to the reference pattern image among the images, whose focuses agree with the different patterns, by changing the distance from an objective lens 6 of a microscope to the pattern of the surface of a body under inspection 5. The selected comparison pattern image is stored in a comparison memory 11 and compared by the processing in defect-detection processing parts 16 and 17.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a method and an apparatus for detecting a defect of a fine pattern or a deposit on a surface of a semiconductor such as an LSI or a liquid crystal flat display.

[0002]

2. Description of the Related Art A conventional inspection is performed by comparing a reference pattern image with a pattern image to be inspected. Hereinafter, an inspection of a wafer on which a pattern such as an LSI is formed will be described as an example, but the same applies to an inspection of a liquid crystal flat display and a mask and a reticle used for manufacturing the same.

In these manufacturing processes, a plurality of identical products are formed on one substrate. For example, in the case of an LSI, a plurality of chips are formed on one wafer. The chip includes a portion configured with a regular pattern such as a memory cell and a portion configured with a non-regular pattern such as a peripheral circuit portion. 4 and 5 show examples of regular patterns, and FIG. 6 shows examples of regular patterns. However, these examples are for explanation and are different from actual patterns.

FIGS. 4 and 5 show examples in which the pattern repetition intervals are W1 and W2, respectively. In the inspection of such a regular pattern, a defective portion is detected by performing a comparison by shifting the repetition intervals. In the case of FIG.
By comparing the shifted pattern with the pattern before shifting, for example, a protruding portion (defect) can be detected. In the case of FIG. 5, for example, a portion (defect) missing inside can be detected by comparing the pattern shifted W2 in the horizontal direction with the pattern before shifting.
Such an inspection method is disclosed in, for example, Japanese Patent Application Laid-Open No. 4-27904.
No. 1 publication. Hereinafter, this method is referred to as a defect detection method a.

On the other hand, in the case of a pattern having no regularity as shown in FIG. 6, comparison between adjacent patterns cannot be performed. Therefore, a reference pattern image having no defect is prepared and compared with an image to be inspected. A method of detecting a different portion (defect) of the pattern is used. There are several methods for selecting a pattern image to be a reference, (1) a method of creating from pattern design data, (2) images of chips at different positions are simultaneously taken in by two microscopes, and one of them is used as a reference. (3) There is a method of determining a reference chip position with one microscope, taking an image of the chip, storing the image in a memory, and using this as a reference pattern. These methods are described in, for example, JP-A-1-287449 and JP-A-2-3774.
No. 0 publication.

When the object is a mask or a reticle, the method (1) can be adopted. However, in the inspection of the formed pattern, light reflected from the surface is converted into an image. A method is also adopted in which an image serving as a reference is taken from the formed pattern. In addition, the method (3) has an advantage that the image input unit is simple, for example, only one microscope is required. The method (3) is hereinafter referred to as a defect detection method b.

Inspection by the defect detection method b is possible even for a regular pattern. In general, adjacent patterns have the advantage that the pattern formation conditions in the semiconductor process are close, and the obtained image has the advantage that the difference between the patterns is reduced and the defect can be easily distinguished from a normal portion. In the case of a certain pattern, a method of comparing between adjacent patterns is often used. However, if the repetition interval between adjacent patterns is larger than the size of the imaged area, the repeated pattern will not appear in the imaged area, and the defect detection method a cannot be used. Therefore, in a regular pattern, the defect detection method a and the defect detection method b may be used together depending on the situation.

[0008]

The semiconductor pattern is formed by a two-dimensional pattern as described above, and finally, a three-dimensional pattern in which the two-dimensional pattern is layered. Is formed as FIG. 7 is a diagram illustrating an example of a semiconductor cross section. In the figure, the hatching pattern is changed for each layer. Note that the cross-sectional shape in the figure is a schematic diagram for explanation and is different from the actual formed state.

An image of a pattern to be inspected is optically enlarged and imaged using a microscope, and captured using an image pickup means such as a camera. When the focal plane 70 is determined at a position as shown in FIG. 7 by changing the distance between the magnification optical system of the microscope and the semiconductor surface, pattern defects and deposits within the focal depth 71 are imaged on the imaging plane of the imaging means. However, although it is obtained as an image, a layer or a defect out of the range of the depth of focus 71 has a reduced contrast or does not form an image, so that the defect cannot be detected. For example, in the case of the pattern as shown in FIG. 6, when the focal plane is shifted, as shown in FIG. 8, the contrast of the pattern 80 which has been seen is reduced, and at the same time, the pattern 81 of another layer becomes visible.

On the other hand, as the integration density of semiconductors has increased rapidly in recent years, patterns have been miniaturized, and it has been required to detect finer defects. For this reason, it is necessary to input an image using a microscope with a higher magnification. However, as the magnification becomes higher, the depth of focus becomes shallower, and the heights of the patterns and defects of all the layers fall within the range of the depth of focus of one focal plane. Will not enter.
Therefore, there is a problem in that images of patterns and defects of all layers cannot be obtained by one image input.

To cope with this, for example, Japanese Patent Laid-Open No.
As described in Japanese Patent Publication No. 42055, after a focal plane is determined by measurement or the like, an offset up to a height at which a layer of a pattern to be inspected is seen is obtained, and an image captured under the same offset amount from the focal plane is obtained. There is a method of comparing pattern images between the two.

However, as shown in FIG. 7, there is an effect of the measurement accuracy of the focal plane and the positioning accuracy of the distance between the magnifying optical system and the semiconductor surface. To fluctuate. In addition, the height position of the offset surface fluctuates similarly due to the influence of the positioning accuracy in the height direction on the offset surface. As the magnification becomes higher, the depth of focus becomes smaller. Therefore, a state in which the positioning accuracy is not sufficient occurs, and a difference occurs in an obtained image every time an image is captured.

When a pattern portion having regularity is inspected by the defect detection method a, a difference occurs between images, but there is no difference between patterns in the image. Defects can be detected. Therefore, a defect can be detected in all layers by setting a plurality of offset amounts and combining the detection results when each offset amount is given as disclosed in Japanese Patent Application Laid-Open No. 4-142055.

On the other hand, when a defect is detected by the defect detection method b at a regular pattern portion or a regular pattern portion,
For example, a difference as shown in FIG. 6 and FIG. 8 occurs between images, which causes a problem that the contrast of a detected layer defect is reduced or a pattern of another layer is erroneously detected as a pseudo defect. .

Accordingly, an object of the present invention is to provide means for detecting a defect even when the depth of focus is small with respect to the focal plane measurement accuracy and the positioning accuracy, and preventing erroneous detection of a pseudo defect.

[0016]

In order to solve the above-mentioned problems,
According to the pattern inspection apparatus of the present invention, in a pattern image comparison method, an image storage means for storing an image (hereinafter, referred to as a reference pattern image) focused on a normal pattern to be inspected, Means are provided for automatically selecting a close image and using it as a reference pattern image. The means for selecting the reference pattern image is to change the distance between the microscope objective lens and the pattern surface,
An image closest to the reference pattern image is selected from images focused on different patterns. The selected reference pattern image is stored in the reference memory, and is referred to in the defect detection processing.

That is, according to the present invention, in a pattern inspection method for performing a defect inspection of a pattern by comparing an inspection pattern image with a reference pattern image, a pattern which is closest to a reference pattern image which is accurately focused on a normal pattern is provided. The inspection pattern image is selected and used as a reference pattern image.

The number of reference pattern images is not limited to one.
The inspection pattern image closest to each of the plurality of reference pattern images may be simultaneously selected to be a plurality of reference pattern images.

The present invention also relates to a pattern inspection apparatus for inspecting a defect of a pattern by comparing an inspection pattern image picked up by an image pickup means with a reference pattern image stored in a reference image storage means. Reference image storage means for pre-storing an image which has been accurately focused as a reference pattern image, and image comparison means for comparing the reference pattern image stored in the reference image storage means with the inspection pattern image captured by the imaging means The inspection pattern closest to the reference pattern image is selected and stored as a reference pattern image in the reference image storage means.

The reference image storage means stores a plurality of reference pattern images, and the reference image storage means stores a plurality of inspection pattern images respectively corresponding to the plurality of reference pattern images as a plurality of reference pattern images. Is also good.

When storing the reference pattern, an image having a normal pattern is stored. At this time, an image focused on the pattern on the pattern layer to be inspected is selected and stored as a reference pattern image. Next, after moving to a position of a normal pattern whose formation conditions are close to the pattern to be inspected, the image is captured while changing the height near the selected height when storing the image of the reference pattern, and An image closest to the pattern image is selected and temporarily stored as a reference pattern image.

In the case of inspection, after moving to the position of the pattern to be inspected, the image is taken in while changing the height near the selected height when storing the image of the reference pattern,
An image having the least difference by comparing with the image of the temporarily stored reference pattern is determined as a defect or attached matter detection image.

[0023]

Embodiments of the present invention will be described below with reference to the drawings. Here, a description will be given of a wafer pattern inspection as an example.

FIG. 1 is a block diagram showing a first embodiment of the inspection apparatus according to the present invention. The inspection apparatus includes an image input unit 21 for obtaining a pattern image on a wafer, a reference pattern image selection unit 22, an image processing unit 23, a control unit 4, and the like.

The wafer 5 is placed on a Z-axis stage 2, and the Z-axis stage 2 is mounted on an XY stage 1. The XY stage 1 is a mechanism for moving the inspection pattern position on the wafer 5 in the horizontal direction with respect to the optical axis of the lens 6, and the Z-axis stage 2 is a mechanism for moving the lens 6 and the wafer 5
And a mechanism for moving the wafer in the vertical direction to focus on the inspection pattern on the wafer. The focus measuring unit 3 measures the distance so that the distance from the lens 6 to the surface of the wafer 5 becomes the focal length, and controls the Z-axis stage 2. The light from the light source 8 is reflected on the axis of the lens 6 by the half mirror 7 and is projected on the surface of the wafer 5 via the lens 6. The reflected light on the surface of the wafer 5 is imaged on the imaging surface of the CCD camera 9 by the lens 6, and a pattern image on the wafer surface can be obtained. The above is the configuration of the image input unit 21.

The pattern image formed on the imaging surface of the CCD camera 9 is converted into a digital image by the A / D converter 10 and input to the reference pattern image selector 22 and the image processor 23. The overall control unit 4 includes the image input unit 2
1. Control the operations of the reference pattern image selection unit 22 and the image processing unit 23. The reference memory 11 temporarily stores a reference pattern image.

The reference pattern image selecting section 22 is for selecting a reference pattern image close to a preset reference pattern image. The image storage unit 14 stores a reference pattern image which is a normal pattern and has a focus exactly on a pattern to be inspected, from the image input unit 21. The reference pattern image memory 12 temporarily retrieves an image from the image storage unit 14 and stores the image. The image comparison unit 13 stores the reference pattern image memory 12.
Is compared with the image from the A / D conversion means 10. The control unit 15 drives the Z-axis stage 2 and selects an image closest to the reference pattern in the reference pattern image memory 12 by comparison in the image comparison unit 13. The selected image is sent to the reference memory 11 and stored.

The image processing section 23 detects a defect or an adhering matter from an image obtained by imaging the surface of a wafer to be inspected. The first defect detection processing unit 16 stores the image of the reference memory 11 selected by the reference pattern image
The difference from the image from the / D conversion means 10 is compared, and the difference is generated as a defect image. That is, the comparison is performed between different chips, and is mainly used for inspection of a pattern having no periodicity such as a peripheral circuit portion. Here, the processing by the above-described defect detection method b is performed.

The second defect detection processing section 17 is used in the inspection of a repeated pattern portion in which the same pattern such as a memory cell has a periodicity, and compares adjacent patterns in the same chip. Here, the processing by the above-described defect inspection method a is performed.

Due to the positioning error of the Z-axis stage, the focus position measurement error, etc., the image output from the A / D conversion means 10 is not focused on the pattern or defect of the layer to be inspected, or the reference memory 11 A difference may occur between the image and the image output from the A / D conversion means 10. Therefore, the Z-axis stage 2 is moved up and down, images are taken in from the image input unit 21 at various positions therebetween, and the first defect detection processing unit 16 and the second defect detection processing unit 17 create defect images.

In the defect image from the first defect detection processing section 16, true defects and pseudo defects are mixed. A pseudo defect is a pattern that is normal, but is caused by a positioning error of the Z-axis stage, a focus position measurement error, and the like.
The pattern output from the A / D conversion means 10 is not focused on the pattern or defect of the layer to be inspected in the image output from the A / D conversion means 10, or a difference occurs between the image in the reference memory 11 and the image output from the A / D conversion means 10. This is a location determined as a defect.
Therefore, the first defect image synthesizing unit 18 is provided with the Z-axis stage 2
Are selected from among defect images having different positions.

On the other hand, in the second defect detection processing section 17, since the comparison is made between the adjacent pattern images, the difference as an image is small. Therefore, the second defect image synthesizing unit 19
In the case of synthesizing the accumulated images of the defect images having different positions of the Z-axis stage 2, it means that patterns having different heights have been inspected. Further, by selecting only the defect image having the smallest difference as compared with the image in the reference memory 11, an inspection result of only the pattern having the height stored in the reference pattern image selection unit 22 can be obtained. The images created by the first defect image synthesizing unit 18 and the second defect image synthesizing unit 19 are:
The defect determination unit 20 determines a defect.

The operation of each section will be described below with reference to the flowchart of FIG. First, the reference pattern image memory 1
2, an image of the pattern to be inspected is taken out of the image storage unit 14 and stored (S1). The image storage unit 14
First, a wafer serving as a reference having no defect in the pattern is prepared in advance, mounted on the Z stage 2, and a pattern image is captured and stored in a state where a target normal pattern is accurately focused. At this time, the offset amount Z1 from the Z-axis position set as the focal plane by the focus measurement unit 3 to the Z-axis position where the target pattern is accurately focused is stored.

Next, the wafer 5 to be inspected is mounted on the Z stage 2, the wafer 5 is moved to the reference image acquisition position having the same pattern as the inspection pattern on the XY stage 1, and focused by the focus measuring unit 3 ( S2). The same chip is repeatedly formed on one wafer, and the same pattern to be inspected exists for each chip. For example,
A reference chip position is set in the wafer, and a position in the chip having the same pattern as the inspection pattern is set as a reference image acquisition position.

At the reference image acquisition position, an image is fetched from the image input unit 21 and the A / D conversion means 10 at each position from the Z-axis position offset amount (Z1-a) to (Z1 + a).
The image comparison unit 13 compares the image data with the reference pattern image memory 12. Then, the image having the smallest difference is selected and stored in the reference memory 11 (S3). Here, a is an appropriate value close to the depth of focus, and the movement of each position is controlled by the control unit 15.
Do.

By the processing of steps 2 and 3,
An image focused on the inspection pattern stored in the image storage unit 14 is selected. Also, the pattern formation conditions and the illumination conditions are different between the reference wafer used for storing the image in the image storage unit 14 in advance and the wafer to be inspected, and a difference occurs in the image even though the pattern is a normal pattern. By storing, it is possible to update the inspection pattern image without this difference.

Next, the wafer is moved to a pattern position to be inspected on the XY stage 1, for example, the same inspection pattern position with another chip, and the focus is measured by the focus measuring unit 3 (S4). Then, from the Z-axis offset amount (Z1-a), (Z1 + a)
At each position up to this point, an image is fetched from the image input unit 21 and the A / D conversion means 10, and a defect is detected by the image processing unit 23 (S5). The processing of the image processing 23 is the defect detection methods a and b as described above.

When inspecting each chip having the same inspection pattern on the wafer, the inspection position of each chip is sequentially moved, and steps 2 to 5 in FIG. 3 are repeated.
When the inspection is performed for the XY position and the Z position having different inspection patterns, Steps 1 to 5 are repeated for each XYZ position.

FIG. 2 is a block diagram showing a second embodiment of the inspection apparatus according to the present invention. For the sake of simplicity, parts performing the same functions as in FIG. 1 are denoted by the same reference numerals as in FIG. The reference pattern image selecting section 22
It has a reference pattern memory 32, a second reference pattern memory 33, a first image comparison unit 34, and a second image comparison unit 35. The images selected by the first image comparison unit 34 and the second image comparison unit 35 are stored in the first reference memory 30 and the second reference memory 31, respectively. The image processing unit 23
Has a pair of first defect detection processing units 16 and second defect processing units 17. The pair of first defect processing units 16 and 17 respectively convert the image from the A / D conversion unit 10 into a first image.
The images are compared with the images in the reference memory 30 and the second reference memory.

The inspection apparatus according to the first embodiment has one image storage unit for storing a reference pattern image and one reference memory for temporary storage. One inspection pattern is used for inspection patterns having different Z-axis positions. Each time the inspection is completed, the data is taken out of the image storage unit and stored. The inspection apparatus according to the second embodiment has a configuration in which two reference pattern images are stored at the same time, and the image processing unit 23 also performs an inspection based on a comparison method between the two reference pattern images and the captured image at the same time. Can be.

Hereinafter, a case will be described in which inspection is performed on two patterns having different layers on which patterns are formed, that is, two patterns having different offset positions on the Z axis. Two types of images, which are normal patterns and are accurately focused on a target pattern, are extracted from the image storage unit 14 and stored in the first reference pattern image memory 32 and the second reference pattern image memory 33, respectively.

Next, the XY stage 1 is driven to move the wafer 5 to the pattern position to be inspected, and the Z axis is adjusted to the focal position. An image is fetched from the image input unit 21 and the A / D converter 10 at the Z-axis offset position near the focal position, and the first
In the image comparison unit 34, the first reference pattern image memory 32
The second image comparison unit 35 compares the image with the image in the memory 33 for the second reference pattern image. The image having the smallest difference among the Z-axis offset positions near the focal position is selected and stored in the first reference memory 30 and the second reference memory 31, respectively. Through the above operation, the image closest to the two types of pattern images to be inspected set in advance can be stored again for the inspection wafer.

Next, the wafer 5 is moved to another pattern position to be inspected on the XY stage 1, for example, a position where the inspection pattern is the same on another chip, and the focus is measured by the focus measuring unit 3.
The Z-axis offset position is changed near the focal position, and the first reference memory 30 and the second reference memory 31 are changed at each offset position.
, The pair of first defect detection units 16 and the pair of second defect detection units 17 simultaneously perform defect detection processing.

As described above, since the two reference pattern images are simultaneously processed, the inspection processing can be performed at a high speed. In addition, the second embodiment can be similarly extended to a case where there are three or more reference pattern images. As described above, a wafer such as an LSI has been described as an example, but the present invention can be similarly applied to a semiconductor such as a liquid crystal flat display.

[0045]

According to the present invention, in an inspection by comparing an image with a stored normal reference pattern, a defect can be easily distinguished from a normal pattern even if there is a focus positioning error. Further, it is possible to select and detect only the defect on the focal plane of the reference pattern. Therefore, it is possible to improve the accuracy of defect detection in a high-magnification defect inspection with a small depth of focus.

[Brief description of the drawings]

FIG. 1 is a block diagram of a first embodiment of the present invention.

FIG. 2 is a block diagram of a second embodiment of the present invention.

FIG. 3 is an operation flowchart.

FIG. 4 is an explanatory diagram of a regular pattern.

FIG. 5 is an explanatory diagram of a regular pattern.

FIG. 6 is an explanatory diagram of a pattern having no regularity.

FIG. 7 is a diagram illustrating a cross section and a focal plane of a pattern formed on a wafer.

FIG. 8 is an explanatory diagram of a pattern observation image when a focus shift has occurred.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... XY stage, 2 ... Z-axis stage, 3 ... Focus measuring part, 4 ... Overall control part, 5 ... Wafer, 6 ... Lens, 7 ... Half mirror, 8 ... Light source, 9 ... CCD camera, 10 ... A /
D conversion means, 11: reference memory, 12: reference pattern image memory, 13: image comparison unit, 14: image storage unit,
15: control unit, 16: first defect detection processing unit, 17: second
Defect detection processing unit, 18: first defect image synthesis unit, 19: second defect image synthesis unit, 20: defect determination unit, 21: image input unit, 22: reference pattern image selection unit, 23: image processing unit, 30 ... First reference memory, 31 ... Second reference memory, 32 ... First reference pattern image memory, 33 ... Second
Reference pattern image memory, 34 first image comparison unit, 3
5: second image comparison unit

Claims (4)

[Claims] In a pattern inspection method for performing a defect inspection of a pattern by comparing an inspection pattern image with a reference pattern image, an inspection pattern image that is closest to a reference pattern image that is accurately focused on a normal pattern is determined. A pattern inspection method characterized by selecting the reference pattern image. 2. The pattern inspection method according to claim 1, wherein an inspection pattern image closest to each of the plurality of reference pattern images is simultaneously selected to be a plurality of reference pattern images. Method. 3. A pattern inspection apparatus for performing a defect inspection of a pattern by comparing an inspection pattern image picked up by an imaging unit with a reference pattern image stored in a reference image storage unit. A reference image storage unit that stores an image that matches with the reference pattern image in advance, and an image comparison unit that compares the reference pattern image stored in the reference image storage unit with the inspection pattern image captured by the imaging unit. A pattern inspection apparatus, wherein an inspection pattern closest to the reference pattern image is selected and stored in the reference image storage means as the reference pattern image. 4. The pattern inspection apparatus according to claim 3, wherein said reference image storage means stores a plurality of reference pattern images, and said reference image storage means stores a plurality of inspection patterns respectively corresponding to said plurality of reference pattern images. A pattern inspection apparatus storing a pattern image as a plurality of reference pattern images.