JPH08233747A - Method and apparatus for inspecting defect - Google Patents

Abstract

PURPOSE: To obtain method and apparatus for detecting an abnormality or defect in a pattern formed on a wafer in which correct information of defect can be obtained using a plurality of means. CONSTITUTION: Light projected from a projector 1 is captured by a multifunction mirror 10 having a first function for passing an incident light through the multifunction mirror 10 and projecting the light perpendicularly to an object. The multifunction mirror 10 has a second function for reflecting the incident light toward a predetermined position and the object is irradiated obliquely with the reflected light through a reflector 11 and the like. When the object is irradiated from two different directions, regular reflection image and scattering image can be detected, respectively, by means of identical apparatus. The information of the regular reflection image and scattering image is synthesized thus performing highly accurate defect inspection.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a defect inspection apparatus for detecting a defect in a wafer substrate or a semiconductor device formed on the wafer substrate.

[0002]

2. Description of the Related Art When a semiconductor device is formed, the presence of various foreign substances adhering to a wafer substrate (silicon substrate), which is a semiconductor substrate, causes the formation of patterns such as wiring on the wafer substrate. This may cause pattern defects, short circuits between wires, disconnection, etc. Therefore, it is necessary to detect and remove various foreign substances at an early stage, or to specify a defect occurrence process and manage the countermeasures in order to suppress the defect occurrence. Therefore, there is a defect inspection device as a means for finding various foreign substances.

For example, FIG. 6 is a conceptual diagram of a commonly used defect inspection apparatus (KLA-2310). In the figure, 1 is a projector for irradiating light perpendicularly to a plane portion of an object to be inspected. 2a and 2b are lenses for changing incident light into parallel light, and 3 is a kind of mirror, which is made of a flat plate, one surface of which transmits incident light and the other surface of which reflects incident light. have. 4a, 4
Reference numerals b and 4c are lenses for changing incident light from parallel light to focused light, 5 is a stage on which a wafer substrate 6 as an inspection object is set, and 7 is a reflection image obtained by irradiating the inspection object with light. An image sensor for detecting the information, 8 is a computer unit for calculating the information detected by the image sensor 7,
Reference numerals 9 denote memory units for storing the calculation results obtained by the computer unit.

Next, a method for obtaining one reflection image by this conventional defect inspection apparatus will be described. First, the wafer substrate 6, which is the object to be inspected, is set on the stage 5 of the defect inspection apparatus whose position can be adjusted. Next, light is emitted from the projector 1, the transmitted light is incident on the lens 4a through the mirror 3, and the incident light is focused to irradiate the inspection position 6a of the wafer substrate 6 with vertical light. Next, the reflected image obtained by irradiating the inspection position 6a with light is reflected on the back surface of the mirror 3, and the reflected image is reflected by the lenses 4b, 2b, and 4.
The reflected image is finally taken into the image sensor 7 by sequentially passing through c. The image sensor 7 reads the reflected image as an image and stores this information in the memory unit 9 via the computer unit 8. The above is the steps necessary to obtain one reflection image, and hereinafter, this series of steps will be abbreviated as the reflection image receiving step.

When the inspection object is irradiated with vertical light using the defect inspection apparatus shown in the prior art, an ideal pattern is formed, and when there is no defect in the inspection object, the vertical pattern is generated. Most of the light is specularly reflected, and the reflected image becomes bright. On the other hand, when there is a defect such as a foreign substance or a disconnection, vertical light is scattered at the location where the foreign substance or the disconnection exists, and the image of this reflected image becomes dark. Therefore, in the step of receiving the reflected image, the light emitted from the light projector 1 is scattered or interfered by various foreign substances existing on the inspection position of the object to be inspected, and the detected reflected image becomes bright and dark. Therefore, it becomes possible to inspect whether or not there is a defect on the basis of the reflection image including the light and dark information.

Next, an example of a defect inspection method which can be performed by using the above-described defect inspection apparatus will be described. First, the reflection image receiving step is performed at an arbitrary inspection position formed on the same wafer substrate 6, the brightness of the reflection image is binarized, and this information is stored. Next, another position where the same pattern as the arbitrary inspection position exists is similarly inspected by the image receiving process, binarized image information is obtained based on the brightness of another reflection image, and this information is stored. Next, the two images based on the obtained reflection image are compared. If a difference is found in this image, it means that a defect has been extracted. In addition, the inspection is also performed in the image receiving process at another inspection position where the same pattern is formed on the same wafer substrate 6, and comparison is performed with an arbitrary inspection position that has been previously inspected or an image obtained from another position. , There is a non-matching part,
If there is a difference, it is considered that the defect is extracted, and if there is no difference, it is considered that there is no defect. By repeating this, the defect inspection of the inspection region is performed.

Further, in order to obtain the binarized information from the lightness and darkness of the image of the above defect inspection, for example, the image sensor 7 is CC
There is a method of using a D (CHARGE COUPLED DEVICE) camera. The density of light and dark of the corresponding pattern of each area to be compared for each pixel of this CCD camera is checked, the lightness and darkness at this time are discriminated in 256 steps, and it is determined whether the gradation difference exceeds a certain threshold value. There is a method of determining that a defect has occurred when there is a difference between the two images by obtaining the digitized information and comparing the information (this defect inspection method is described in JP-A-3-48141). Exist). When this method is used, when a minute defect occurs in each pixel,
The shading is averaged within one pixel. Therefore, it may be difficult to detect a defect smaller than the area of one pixel.
Further, the sensitivity may be deteriorated due to the influence of the color tone of the background, and accurate detection may not be possible. Further, in the step of forming the wafer substrate 6 by slicing it from a silicon ingot, if the wafer substrate is not already of uniform thickness, the wafer substrate warps, or a difference in the thickness of the laminated film occurs in pattern formation. In this case, when there is color unevenness on the surface of the wafer substrate, there is a problem that it is erroneously detected as a defect even if there is no defect.

[0008]

Since the conventional defect inspection apparatus and defect inspection method are configured as described above, the presence or absence of a defect is erroneously detected, or detailed information about the shape of the defect cannot be obtained. There was a problem.

The present invention has been made in order to solve the above problems, and it is an object of the present invention to provide a defect inspection apparatus capable of accurately detecting a foreign substance and also detecting a minute defect which could not be found in the past. The object is to further provide a defect inspection method suitable for this apparatus.

[0010]

A defect inspection apparatus according to the present invention transmits the light projected by a light projector, irradiates a predetermined position of the inspection object with the transmitted light, and irradiates the inspection object with the light. The first function is to reflect the reflected light in a certain direction, to reflect the projected light, to irradiate the predetermined position of the inspection object with the reflected light, to receive the reflection image reflected on the inspection object, and to receive the constant direction. It is equipped with a multi-functional mirror that has a second function of reflecting light to the.

In the defect inspection apparatus according to the present invention, the light transmitted by the first function of the multi-function mirror is set so as to irradiate the inspection object vertically, and the second function of the multi-function mirror is set. The light reflected by the function is set so as to irradiate the inspection object from an oblique direction.

In the defect inspection method according to the present invention, the light projected by the light projector is transmitted through the multifunction mirror to irradiate the object to be inspected at a predetermined position to obtain a reflected image, and the light projected by the light projector. Reflects the light in a multifunctional mirror,
The step of irradiating a predetermined position of the object to be inspected with this reflected light through a predetermined path to obtain another reflected image is included.

In the defect inspection method according to the present invention, the specular reflection image obtained by irradiating the inspection object with light perpendicularly and the oblique inspection with the light are obtained. The step of obtaining the scattered image thus obtained is included.

[0014]

In the defect inspection apparatus according to the present invention, the multi-functional mirror having the first function and the second function is used as a constituent element to irradiate the inspection object with light from two different paths, respectively. Is obtained, and the defect inspection result is obtained by calculating the information of these reflection images.

Further, the defect inspection apparatus according to the present invention obtains a specular reflection image by irradiating light to the inspection object perpendicularly, and scatters by irradiating the inspection object with the light obliquely. An image is obtained, and a defect inspection result is obtained by calculating the information of these two reflection images.

In the defect inspection method according to the present invention, the multi-functional mirror having the first function and the second function is used as a constituent element, and the inspection object is irradiated with light from two different paths, respectively. Is obtained, and the defect inspection result is obtained by calculating the information of these reflection images.

Further, in the defect inspection method according to the present invention, a specular reflection image is obtained by irradiating light to the inspection object vertically, and scattered by irradiating the inspection object with the light obliquely. The image is obtained, and the defect inspection result is obtained by calculating the information of these images.

[0018]

【Example】

Example 1. An embodiment of the present invention will be described below with reference to the drawings. In FIG. 1, reference numeral 10 denotes a multifunctional mirror. The multifunctional mirror 10 is capable of transmitting light incident on one surface of a planar body and reflecting incident light on the back surface thereof. Mirror, a second mirror capable of reflecting incident light on both surfaces of the plane body, and the projector 1 on the plane portions of these first and second mirrors.
It has a mirror control unit that adjusts the position of the mirror so that the light emitted from the. Reference numeral 11 denotes a reflector plate that further reflects the light reflected on one surface of the multifunction mirror 10 and irradiates the inspected position with the reflected light, and 12 is provided on a straight line connecting the reflector plate 11 and the inspected position. It is a lens that changes the light from parallel light to focused light. In addition, the same reference numerals as those used in the description of the related art indicate the same or corresponding portions.

Next, the features of the defect inspection apparatus according to the present invention will be described. In this defect inspection apparatus, vertical light can be emitted from the projector 1 to the wafer substrate 6 which is the object to be inspected. Further, as described above, the mirror 3 in the conventional defect inspection apparatus (FIG. 6) is Irradiating the wafer substrate 6 with light from an oblique direction by replacing the multifunctional mirror 10 having two different functions (first and second mirrors) and adding a reflector 11 and a lens 12. The structure is possible. By adopting such a structure, not only the specular reflection image by vertical light but also the scattered image by light from an oblique direction can be detected, and more accurate defect inspection can be performed.

First, the first image receiving process by vertical light of the above defect inspection apparatus will be described with reference to FIG.
The light emitted from the light projector (light source) 1 including a halogen lamp or the like is changed into parallel light by passing through the lens 2a. This parallel light is taken into the multifunctional mirror 10 and is incident on one surface of the first mirror whose position is adjusted by the mirror controller (optical path 13). Since one surface of this first mirror has a characteristic of transmitting the incident light and reflecting the light incident on the back surface thereof, the light incident on the one surface passes through the first mirror and these lenses 2a Similarly to the multifunctional mirror 10, the parallel light is converged by the lens 4a installed on the straight line connecting the projector 1 and the inspection position on the wafer substrate 6 to irradiate the inspection position. As a result, a specular reflection image of the inspection object by vertical light is obtained.

This specular reflection image is reflected on the back surface of the first mirror in the multifunction mirror 10 (optical path 14), as in the case of the prior art, and the lenses 4b, 2b,
The image is captured as an image by the image sensor 7 via 4c in sequence (optical path 15). Further, this image is discriminated on the basis of lightness and darkness in the computer unit 8, or is binarized by discriminating it based on the gradation difference of light and shade, and this information is stored in the memory unit 9. By performing these processes, information due to vertical light can be detected. Or later,
This is the first image receiving step. In the first image receiving step, the first mirror, which is a component of the multifunctional mirror 10, has been described as having a characteristic that one surface thereof transmits incident light and the other surface thereof reflects incident light. If a half mirror is used for this first mirror, part of the light incident on one surface is transmitted and part is reflected, so it is necessary to block the light reflected here. It is conceivable to interpose a light shielding plate between the multifunction mirror 10 and the reflection plate 11.

Next, the second image receiving process by the oblique projection light of the defect inspection apparatus will be described with reference to FIG. As in the case of performing the first image receiving step, the light emitted from the projector (halogen lamp) 1 is changed into parallel light by passing through the lens 2a. This parallel light is a multifunctional mirror 1
0, and the position of the mirror is adjusted by the mirror control unit in the multi-function mirror 10, and both planes are incident on one surface of the second mirror composed of a reflection plate (mirror) (optical path 13 ). The parallel light on one surface of the second mirror is reflected at a predetermined angle, and the reflected light is reflected again by the reflection plate 11 to obliquely reach the position that was the inspection position in the first image receiving step. Irradiate with light. This obliquely projected light is scattered due to the unevenness of the object to be inspected, and a part of the scattered light is applied to the back surface of the multifunctional mirror 10, and the scattered light is reflected by the image sensor 7 on the back surface of the multifunctional mirror 10. Further, it is reflected in the direction (optical path 16). This reflected light is captured as an image by the image sensor 7 via the lenses 4b, 2b, 4c (optical path 15). Subsequent processing is performed in the same manner as in the first image receiving step, and the binarized information obtained by this is stored in the memory unit 9. Hereinafter, the step of obtaining information by the oblique projection light in this manner will be abbreviated as the second image receiving step.

From the information obtained by this oblique projection light, the edge portion of the pattern formed on the wafer substrate 6 and the like where the irregularities are conspicuous, that is, the contour of the pattern and the flat portion are identified. It is possible. Further, if there is a foreign matter such as a protrusion in the inspection region, or if the formed pattern is swollen or thinned, it can be accurately detected by observing the contour of the pattern by the oblique projection light. On the other hand, in the inspection by the second image receiving process, color unevenness due to a subtle film thickness difference of the underlying thin film existing on the surface of the wafer substrate 6 which can be recognized by the first image receiving process by the vertical light described above. Are not recognized as defects such as foreign matter.

Next, how to repeat the first and second image receiving steps as described above and a method for comparing and examining the obtained information will be described with reference to FIG. In FIG. 4, 17 to 19 are specular reflection images obtained by the first image receiving step, and 20 to 22 are scatter images obtained by the second image receiving step. . These images are 17 and 20, 18 and 2
Reference numerals 1, 19 and 22 are images obtained as a result of inspecting exactly the same inspected position by different inspection methods. Further, the patterns (linear patterns such as wirings) 24a and 24b formed at all the inspected positions are patterns of the same shape (the protruding foreign matters 25a and 25b are not patterns intentionally formed). is there. Further, reference numeral 23 is a wafer substrate surface, 24c, 24d, 24e and 24f are images showing pattern end portions detected when a scattered image is detected, and 25a and 25.
b and 25 are protrusion-like foreign substances formed by protruding from the pattern 24a, and 26 is a stain (an impurity such as an oxide film generated on the wafer substrate surface 23 when moisture remains after the wet process, which is herein referred to as 27 is represented by using the expression "spots", and 27 is a minute protrusion existing on the pattern 24b.
The drawings shown at 28 to 34 are the images received at 17 to 22
Is an image obtained by performing a calculation process on the image of.

Next, the order of image reception and the calculation processing method will be described. First, the specular reflection image 17 is obtained by the first image receiving process.
And then switch between the first mirror and the second mirror by adjusting in the mirror control section of the multifunction mirror 10 in the defect inspection apparatus at the same inspection position,
The scattered image 20 is obtained by the second image receiving step. This first,
The second image receiving step is repeated by moving the position to be inspected to sequentially obtain images of 18 and 21 (here, it is assumed that the images 18 and 21 have no defect). When the images 17 and 18 are obtained, the computer unit 8 reads the information of the images 17 and 18 from the memory unit 9, compares the two images, and detects a portion where the binarized information does not match, A specular reflection image difference image 28 is obtained, and in this case, the protruding foreign matter 25a and the spots 26 which are non-matching portions seen in the specular reflection image 17 are recognized as defects, and this information is stored in the memory unit 9. Similarly, the images of the scattered images 20 and 21 are also compared with each other, the protruding foreign matter 25b and the minute protrusions 27, which are the matched portions, are recognized as defects, and this information (scattering difference image 30) is stored in the memory unit 9. Let

Next, a specular reflection image 19 is obtained in the first image receiving step at still another inspected position, and a scattered image 22 is obtained in the second image receiving step at the same inspected position. Here, the computer unit 8 reads the information of the images 18 and 19 from the memory unit 9, compares the two images, detects a portion where the binarized information does not match, and detects the specular reflection image difference image 2
9 is obtained, and in this case, since the images 18 and 19 are exactly matched, a specular reflection difference image 29 having no non-matching portion is obtained, and this information is stored in the memory unit 9. Similarly, images of the scattered images 21 and 22 are compared to obtain a scattered difference image 31, and this information is stored in the memory unit 9.

After that, the information of the images 28 and 29 is read from the memory unit 9, the two images are compared, the non-matching portion is detected, and the final difference image 32 of the specular reflection image is obtained. Next, for the scattered image, a final difference image 33 of the scattered image is obtained from the images 30 and 31 in the same manner. The final difference images 32 and 33 of the regular reflection image and the scattered image are combined, and the foreign matter and defect information obtained from the regular reflection image and the scattered image are combined into one image information 3
4 (composite image). Thus, information 34
By obtaining the (composite image), a minute defect that was overlooked by only the information obtained by vertically irradiating the inspection object with light was accurately detected, and a pattern was formed. Erroneous detection due to color unevenness on the surface of the wafer substrate 6 can be suppressed. Further, in this embodiment, the defect inspection method is shown with three inspection positions, but by performing the inspection with two or more positions as the inspected positions, it is possible to perform the defect inspection utilizing the performance of the defect inspection apparatus. It is believed that

Embodiment 2 FIG. In the first embodiment, the thin silicon oxide film formed because the protrusions and the moisture remain on the pattern formed on the wafer substrate 6 is detected as a defect. In the present embodiment, the protrusions of the pattern (the portions formed by protruding from the pattern in the horizontal direction) are detected as defects, and are detected when the specular reflection image is obtained, and are seen when the pattern of the wiring or the like is visually observed. , A change in color tone due to a slight difference in film thickness, a slight difference in thickness of the wafer substrate 6, warpage, or the like, and a slight bend in the end of the wiring which is detected when a scattered image is obtained. A defect inspection method that does not detect the distortion of the pattern due to the defect is shown.

This embodiment will be described with reference to FIG. Figure 5
In, 36, 37 and 38 are specular reflection images. 9, 40, 4
1 is the scattered image, 42 is the color unevenness seen inside the patterns 24a and 24b, 43a, 43b, 43c and 43d are the end portions of the pattern obtained by the scattered image, 44 and 45 are specular reflection image difference images, and 46 and 47 are Scattering difference image, 48 is the non-common part of the end of the pattern, 49 is the final specular reflection difference image, 5
0 indicates a final scattering difference image, and 51 indicates a composite image.

In this embodiment, the method of obtaining the specular reflection image and the scattered image is exactly the same as the first image receiving step and the second image receiving step of the first embodiment. First, the specular reflection image 36 is obtained at one inspected position by using the first image receiving process, and then the scattered image 39 is obtained by using the second image receiving process. By repeating this, the specular reflection image 37 and the scattered image 40 are sequentially obtained.
At this time, the image information of the specular reflection images 36 and 37 is read out from the memory section, and the portion where the binarized image information of both does not match is set as the difference image 44, and this information is stored in the memory section 9. Similarly, the scattered images 39, 40
Of the image information is read out from the memory unit, and a portion where the two binarized image information do not match is set as the difference image 46, and this image information is stored in the memory unit 9. Next, the specular reflection image 38 and the scattered image 41 are obtained, the difference images 45 and 48 are obtained respectively as in the case of the first embodiment, and the image information is stored in the memory unit 9.

Thereafter, the difference image obtained by reading out the non-common portion of the difference images 44 and 45 of the regular reflection image (final regular reflection difference image).
49, and similarly, a difference image (final scattering difference image) 50 obtained by reading out the non-common part of the difference images 46 and 47 of the scattering image in the same manner.
Get. Next, the image information of the final specular reflection difference image 49 and the final scattering difference image 50 is read from the memory unit 9, and the computer unit 8 extracts the common part of the two images to obtain a composite image 51. I do. This composite image 5
1, 25a and 25b, which are commonly shown in the final specular reflection difference image 49 and the final scattering difference image 50, can be detected as the protruding defects 25, and the colors shown in the difference images 49 and 50 can be detected. The unevenness 42 and the non-common portion 48 at the end of the pattern
Defects that do not affect the quality of the product, such as the above, are not detected, and the defect inspection result is obtained in which it is considered that there are no defects caused by the color unevenness 42 and the non-common portion 48 at the end of the pattern.

In the prior art, even a difference image detected only from the specular reflection image or the scattered image may detect a defect that cannot actually become a defect. Using the defect inspection method, only substantial defects that can be regarded as apparent defects commonly detected in both specular reflection images and scattered images can be finally detected. In this way, it is possible to obtain desired information (composite image) by changing the image information calculation processing method according to the type of defect to be detected. Further, in this embodiment, the defect inspection method is shown with three inspection positions, but by performing the inspection with two or more positions as the inspected positions, it is possible to perform the defect inspection utilizing the performance of the defect inspection apparatus. It is believed that

[0033]

As described above, according to the present invention, since the defect inspection apparatus has a structure in which a multifunctional mirror is added, highly accurate defect inspection can be performed without substantially increasing the cost of the apparatus. There is an effect that a possible device can be obtained.

Further, according to the present invention, it is suitable for a defect inspection apparatus having a structure in which a multifunctional mirror is added, and it is possible to obtain a defect inspection method according to necessary defect information.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a defect inspection apparatus according to an embodiment of the present invention.

FIG. 2 is a diagram showing a method of using the defect inspection apparatus according to the embodiment of the present invention.

FIG. 3 is a diagram showing a method of using the defect inspection apparatus according to the embodiment of the present invention.

FIG. 4 is a diagram necessary for explaining a defect inspection method according to an embodiment of the present invention.

FIG. 5 is a diagram necessary for explaining a defect inspection method according to an embodiment of the present invention.

FIG. 6 is a schematic diagram showing a defect inspection apparatus according to a conventional technique.

[Explanation of symbols]

1. Floodlights, 2a, 2b, 4a, 4b, 4c, 1
2. Lens 3. Mirror, 5. Stage, 6. Wafer substrate,
7. Image sensor 8. Computer Department, 9. Memory section, 10. Multi-function mirror, 11. Reflector 13, 14, 15, 16. Optical path 17, 18, 19, 36, 37, 38. Specular reflection image 20, 21, 22, 39, 40, 41. Scatter image, 2
3. Wafer substrate surface 24a, 24b. Patterns 24c, 24d, 24e, 24f. Pattern end portions 25a, 25b, 25c. Protruding foreign matter, 26. Stains,
27. Microprojections 28, 29, 44, 45. Specular reflection difference image 30, 31, 46, 47. Scatter difference image 43a, 43b, 43c, 43d. Pattern end 32, 49. Final specular reflection difference image, 33, 50. Final scattering difference image 34, 51. Composite image

Claims (4)

[Claims] 1. A projector unit for projecting light to an object to be inspected, light transmitted by the light projector unit being transmitted, and the transmitted light is irradiated to a predetermined position of the object to be inspected. The first function of receiving the reflected image reflected and reflecting it in a certain direction, and reflecting the light projected by the projector unit, irradiating the reflected light to a predetermined position of the inspection object and reflecting it on the inspection object A multifunctional mirror having a second function of receiving the reflected image reflected in a certain direction, an image sensor detecting the reflected image reflected in the certain direction by the multifunctional mirror, and the reflected image detected by the image sensor A defect inspection apparatus comprising: a computer unit for performing a calculation process by taking in as a memory; and a memory unit for storing information of the computer unit. 2. The light projector is installed on a vertical line with respect to the object to be inspected, the light transmitted through the multifunctional mirror is radiated vertically to the object to be inspected, and the light reflected by the multifunctional mirror is reflected onto the object to be inspected. The defect inspection apparatus according to claim 1, wherein the irradiation is performed in an oblique direction. 3. A first step of projecting light from a projector, wherein the light projected in the first step is transmitted through a multifunction mirror to irradiate a predetermined position of an object to be inspected to obtain a reflected image. In the second step, the light emitted in the first step is reflected by the multifunction mirror, and the reflected light is applied to a predetermined position of the object to be inspected through a fixed path to obtain another reflected image. , A fourth step of detecting the reflection image obtained in the second step, a fifth step of detecting the reflection image obtained in the third step, and a comparison of a plurality of reflection images detected by the fourth step Then, the sixth step of detecting the first image information indicating the difference, the plurality of reflected images detected by the fifth step are compared, and the seventh step of detecting the second image information indicating the difference ,
Eighth step of synthesizing the first and second image information obtained by the sixth and seventh steps to obtain the third image information, and storing the third image information obtained by the eighth step A defect inspection method including a ninth step. 4. The reflection image obtained by the second step is
It is a specular reflection image obtained by vertically irradiating the inspection object with light, and the reflection image obtained by the third step is obtained by irradiating the inspection object with light from an oblique direction. The defect inspection method according to claim 3, wherein the defect inspection method is a scattered image.