JP4332371B2 - Appearance inspection device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an appearance inspection apparatus and an appearance inspection method for detecting defects by detecting images of a plurality of patterns formed on an inspection target object such as a semiconductor wafer, a photomask, and a printed board.
[0002]
[Prior art]
In general, in a pattern appearance inspection, a one-dimensional image sensor is used as image detection means for capturing an image of an inspection object. FIG. 3 is a configuration diagram of the image sensor, and FIG. 4 is a plan view of the inspection area of the inspection object. When the image sensor scans the pattern inspection area, the area where the image can be detected by one scan is shown. Show. The inspection object is scanned in a direction orthogonal to the arrangement direction of the pixels 81 of the photodetector of the image sensor 8 to obtain a two-dimensional image. The length in the direction in which the plurality of pixels 81 are arranged is called the height of the image sensor 8. Further, when the inspection object is a semiconductor wafer having a plurality of semiconductor chips being manufactured, the length in the same direction as the height of the image sensor 8 of one chip is called a chip height. Since the chip height is larger than the height of the image sensor 8, only one image of the chip can be detected by one scan of the image sensor 8. The size of the partial image of the chip is determined by the height of the image sensor 8 that performs detection and the imaging magnification of the imaging optical system that projects the chip image onto the image sensor 8. This height is called the effective imaging height of the image sensor 8. In general image sensors, the height of the area that can be detected by one scan of the image sensor is smaller than the chip height, so in order to inspect the entire surface of the chip, scanning is performed by shifting in the height direction multiple times. Method is used.
[0003]
FIG. 5 is a plan view showing a sensor scanning locus 101 when a plurality of chips 21 formed on a wafer are scanned a plurality of times by an image sensor. In general, in an appearance inspection apparatus, an image sensor is fixed, and a wafer to be inspected is moved using a stage. First, the inspection object is moved in the X direction to acquire an image. Information for each pixel of one chip 21 is stored in the memory in the scanning order. After the image detection is completed in one scan, the inspection object is moved in the Y direction by the effective imaging height of the image sensor, and the stage is moved so that the scanning direction is reversed as shown by a broken line in FIG. By repeating this, images are detected sequentially.
[0004]
The inspection of the chip 21 is performed by comparing the images of the same pattern, but when inspecting the region b, the image of the inspection region c is used as a reference image. However, when inspecting the area a which is the first chip in each scanned row, the inspection cannot be performed because there is no reference image of the previous area of the same pattern. For this reason, an area that cannot be inspected occurs on the outer periphery of the wafer. For the above reasons, an area that cannot be inspected by the wafer outer peripheral chip is generated.
[0005]
The above problem can be solved by changing the scanning method (see, for example, Patent Document 1). FIG. 6 is a plan view showing the scanning trajectory of the image sensor, as in FIG. In this case, the effective imaging height of the image sensor is 1/3 of the chip height, and the inspection area is scanned into three areas in a strip shape. The scanning in the first row in the X direction is the same as that shown in FIG. 5, but the second row is scanned by moving the object to be inspected by the height of one chip in the Y direction. This is repeated, and comparative inspection is performed on images of the same pattern corresponding to 1/3 of the chip area. When the last line is completed, as shown by a broken line in FIG. 6, an image of the same pattern in the next 1/3 area of the chip is comparatively inspected. When the first chip is completed, an image of the same pattern in the remaining 1/3 of the chip is comparatively inspected.
[0006]
When inspecting the area b immediately after folding, the pixel information of the area c to be a reference image is read out in an order reverse to the order at the time of storage, thereby comparing the area b and the area c as images of the same pattern. Can do. Note that, by storing an image of another region of the same pattern in advance as a reference image of the region a at the start of the inspection, the region a can be inspected.
[0007]
In this example, the scanning directions of the image sensors in the first and second rows are different. The difference in scanning direction causes the following problems.
[0008]
FIG. 7 is a conceptual diagram illustrating the focusing of the image sensor in the direction perpendicular to the wafer. Focusing is performed while the surface of the chip is scanned with an image sensor by an automatic focusing function of the appearance inspection apparatus. The focal position in the scanning direction 103 becomes the focal positions 106 and 107 with respect to the ideal focal position 105 due to the response delay of the focusing function. On the other hand, the focal position in the scanning direction 104 is the focal positions 108 and 109 with respect to the ideal focal position 105. As a result, there is a difference in the follow-up performance of the focusing, and even though the region b and the region c shown in FIG. 6 have the same pattern, a difference for each pixel occurs. It will be extracted. Such imaginary defect information, that is, illicit information, is a problem that greatly affects the reliability of the inspection apparatus because it is difficult to distinguish it from true defects.
[0009]
[Patent Document 1]
Japanese Patent Laid-Open No. 11-160247
[Problems to be solved by the invention]
An object of the present invention is to provide an appearance inspection apparatus and an appearance inspection method capable of eliminating an image difference caused by a scanning direction and improving pattern defect inspection accuracy.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, in the embodiment of the present invention, the scanning direction of the image sensor with respect to the reference image is the same as the scanning direction of the image sensor when obtaining the detection image.
[0012]
Further, an image storage means for storing an image immediately before returning of the scan of the image sensor acquired by scanning the image sensor in the same direction as the scan direction of the image sensor at the time of acquisition of the detected image, and a scan of the image sensor A defect is detected by comparing the detected image immediately after the return and the reference image stored in the image storage means.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described with reference to the drawings.
[0014]
FIG. 9 is a schematic configuration diagram of an appearance inspection apparatus. A wafer 3 that is an object to be inspected is fixed on a Zθ stage 2 that can move and rotate in the height direction, and the Zθ stage 2 is placed on an XY stage 1 that can move in the X direction and the Y direction, which are horizontal directions. Is installed.
[0015]
Above the wafer 3, there is a half mirror 5 that directs illumination light from the illumination light source 4 toward the wafer 3, and the illumination light reflected on the wafer 3 illuminates the wafer 3 through the objective lens 6. Reflected light from the wafer 3 passes through the objective lens 6 and the half mirror 5 and is received by the image sensor 8 as detection light. The light branched by the half mirror 7 enters the automatic focus detection means 9, calculates the optimum focus position, gives a movement command to the Z stage of the Zθ stage 2, and can detect an image with the optimum focus. The detection light received by the image sensor 8 is converted into a digital image signal through the A / D converter 10, and after the image signal detected in the scanning area of the first chip is recorded as a reference image by the image recording means 11, The image comparison unit 12 compares the detected image detected in the scanning area of the next chip with the reference image to extract a defect, and the defect position determination unit 14 specifies the defect position.
[0016]
FIG. 2 is a plan view of the semiconductor wafer and the enlarged portion. Generally, in defect inspection, an area adjacent to an inspection area is stored as a reference image, the reference image is compared with an image in the inspection area, and a difference between the two is extracted as a defect. As an example of the object to be inspected, a case of comparative inspection between chips on a semiconductor wafer will be described. On a single wafer 3 as shown in FIG. 2, a plurality of chips 21 as individual products are arranged. These chips 21 are continuously arranged at regular intervals in the matrix direction as patterns having the same shape. Further, a memory cell pattern 31 is formed in the chip 21 as a repeated pattern at a constant period. Taking advantage of the characteristics that the memory cell patterns 31 are arranged in the same shape, at regular intervals, and at regular intervals, in defect inspection, pattern defects are detected by the following method using the detection image of the inspection area and the reference image stored in advance. To do.
[0017]
FIG. 1 is a schematic diagram of an image obtained by inspection. As shown in FIG. 1, the difference image 21c is obtained by taking the difference between the reference image 21b having the same shape as the detected image 21a detected in the inspection region. When there is no defect, as shown in FIG. 1A, the pattern 23a of the detected image 21a and the pattern 23b of the reference image 21b are exactly the same, so that there is no density difference in the pixels of the difference image 21c.
[0018]
On the other hand, as shown in FIG. 1B, when a defect 25a such as a foreign substance exists in addition to the pattern 24a of the detection image 22a, the defect 25c appears in the difference image 22c from the reference image 22b. In this way, the presence or absence of a defect can be detected.
[0019]
As a reference image, there are a case where an image of a pattern having no defect is stored in advance, and a case where a detection image immediately before the detection image is used as a reference image. In the example of FIG. 1B, it is not known whether the detected image 22a or the reference image 22b has the defect 25c. However, the detected image 22a is compared with the next detected image as a reference image, and similarly to the difference image 22c. If the defect 25c is detected, the defect exists in the detected image 22a, and if the defect 25c is not detected, it can be understood that the defect exists in the reference image 22b.
[0020]
In general, in a pattern defect inspection, a one-dimensional image sensor is used as an image detection means for capturing an image of an inspection target object. As described above, this one-dimensional image sensor has small photodetectors arranged in a one-dimensional direction as shown in FIG. 3. In a pattern defect inspection apparatus, the one-dimensional image sensor is orthogonal to the arrangement direction of the pixels of this photodetector. A two-dimensional image is acquired by scanning the object to be inspected.
[0021]
In order to inspect all the chips on the wafer, folding is necessary, but the problem described with reference to FIG. 7 occurs. FIG. 8 is a plan view showing the scanning trajectory of the image sensor, as in FIGS. When the area of the chip 21 of the wafer that is the inspection object is divided by the effective imaging height of the image sensor, the height of one chip is divided by an even number. For example, as shown in FIG. 8, the chip 21 is considered divided into two strip-shaped inspection areas. When inspecting the first area b of the turn-up portion of the scanning trajectory 103, the scanning direction of the image sensor is reverse in the previous area a, so that it is not a reference image, and the scanning area of the area b is the same as the scanning direction of the image sensor. Must be used as a reference image. Similarly, when the region d is a detection image, the region b is used as a reference image. In this way, for all the chips, the chips are divided into even numbers so that the same region has the same scanning direction. Other inspection areas can be sequentially compared. The first region e uses a region f scanned from the same direction as a reference image.
[0022]
Therefore, since the reference image is not necessarily the image immediately before the inspection image, it is necessary to temporarily store the inspection image until the reference image corresponding to the inspection image is acquired. For example, a recording unit such as a frame memory 13 is added to the image recording unit 11 shown in FIG. 9, and an inspection image and a reference image can be recorded until necessary. When comparing detected images immediately after folding, the image data recorded in the frame memory is read as a reference image and compared. Based on the result calculated by the image comparison means 12, the defect position determination means 14 specifies the defect position.
[0023]
As described above, in the embodiment of the present invention, not only the image data of the chip adjacent to the image storage means but also the image immediately before the folding detected in advance in the same direction is stored in the frame memory, and the folding is performed. In the case of inspecting the immediately following image, the chip image immediately before the turn-up before one row detected by scanning from the same direction is read out and used as a reference image, resulting from the difference in image due to the difference in the scanning direction in the entire inspection region. It is possible to eliminate false alarms and miss defects and to inspect correctly.
[0024]
In the embodiment of the present invention, the adjacent chip is used as a reference image as usual except that the detection image immediately after the folding is compared. An image detected by scanning from the same direction at the position can be employed.
[0025]
In the embodiment of the present invention, when the inspection start area cannot be inspected, the problem can be solved by capturing a reference image before starting the inspection in order to eliminate the area. At this time, as long as the condition that the reference image is an image at a corresponding position of a chip having the same shape scanned from the same direction is satisfied, the location of the chip for capturing the reference image and the scanning method do not matter.
[0026]
In the embodiments of the present invention, an image sensor is used as the image detecting means. However, the present invention is not limited to this, and the concept of the present invention is applied to those that perform image detection and image comparison. It is possible.
[0027]
As described above, according to the embodiment of the present invention, when inspecting a plurality of patterns formed on an object to be inspected such as a semiconductor wafer, a false alarm or a defect caused by a difference in an image due to a difference in scanning direction. It is possible to eliminate the problem of oversight and to accurately and accurately perform defect inspection of all inspection areas of the inspection object.
[0028]
【The invention's effect】
According to the present invention, it is possible to provide an appearance inspection apparatus and an appearance inspection method capable of eliminating the image difference caused by the scanning direction and improving the inspection accuracy of pattern defects.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of an image obtained by inspection.
FIG. 2 is a plan view of a semiconductor wafer and an enlarged portion.
FIG. 3 is a configuration diagram of an image sensor.
FIG. 4 is a plan view of an inspection area of an inspection object.
FIG. 5 is a plan view showing a sensor scanning trajectory when a plurality of chips are scanned a plurality of times by an image sensor.
FIG. 6 is a plan view showing a scanning locus of the image sensor.
FIG. 7 is a conceptual diagram illustrating focusing of an image sensor in a direction perpendicular to a wafer.
FIG. 8 is a plan view showing a scanning locus of the image sensor.
FIG. 9 is a schematic configuration diagram of an appearance inspection apparatus.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... XY stage, 2 ... Z (theta) stage, 3 ... Wafer, 4 ... Illumination light source, 5, 7 ... Half mirror, 6 ... Objective lens, 8 ... Image sensor, 9 ... Automatic focus detection means, 10 ... A / D converter 11 ... Image recording means, 12 ... Image comparing means, 13 ... Frame memory, 14 ... Defect position determining means, 21 ... Chip, 31 ... Memory cell pattern, 81 ... Pixel.

Claims (1)

Comparison of a detected image obtained by scanning an image sensor with a pattern having the same shape continuously arranged at equal intervals in a matrix direction formed in a chip on an inspection object and a reference image stored in advance In the visual inspection device that detects defects,
Dividing the height of the chip by an even number, and having a recording unit for recording the reference image,
After the image detection is completed in one scan, the scan is turned back to the next scan row, the scan direction is reversed to the scan direction before the turn, and the image is sequentially detected by repeating this,
In the case where the detected image is an image on a chip immediately after the scan is folded , the detected image at a position corresponding to the detected image is a chip immediately after folding in the same scanning direction in a different scan row. Read from the recording unit as a reference image , detect the defect by comparing the detected image and the reference image ,
When the detected image is an image on a chip other than the chip immediately after turning, refer to the detected image at the position corresponding to the detected image, which is the chip immediately before the chip that detected the detected image. A visual inspection apparatus characterized in that a defect is detected by comparing an image of a pattern without a defect stored in advance as a reference image at a position corresponding to a detection position of the detected image as a reference image .

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