CN108015673B

CN108015673B - Grinding device

Info

Publication number: CN108015673B
Application number: CN201710990740.8A
Authority: CN
Inventors: 吉田真司
Original assignee: Disco Corp
Current assignee: Disco Corp
Priority date: 2016-11-01
Filing date: 2017-10-23
Publication date: 2021-06-04
Anticipated expiration: 2037-10-23
Also published as: TWI720254B; TW201817543A; CN108015673A; KR102305383B1; JP2018069411A; KR20180048374A; JP6791551B2

Abstract

Provided is a grinding device capable of detecting scratches appropriately with a simple configuration without requiring a complicated optical system configuration. The grinding device (1) is provided with a scratch detection unit (50) which detects scratches formed on a ground wafer (W). The scratch detection unit includes: a line sensor (53) that photographs a radius portion of the wafer; a light emitter (54) extending in the same direction as the line sensor; and a determination unit (55) that determines the presence or absence of a scratch on the basis of the image captured by the line sensor. The entire surface of the wafer is imaged by rotating the wafer one turn around the center as the axis while imaging the radius portion of the wafer with the line sensor. The judgment unit performs coordinate conversion on the shot image to edit the shot image into a strip-shaped image, and judges whether the scratch exists according to the strip-shaped image.

Description

Grinding device

Technical Field

The present invention relates to a grinding apparatus.

Background

When the wafer is subjected to the plunge grinding by the grinding apparatus, saw cuts as grinding cuts are formed on the ground surface of the wafer. The saw cuts are formed radially from the center toward the outer periphery of the wafer. Among the saw cuts, abrasive grains detached from the grinding wheel during machining may contact the surface of the wafer to be ground, and so-called scratches may be generated as scratches. Since the scratches affect devices formed on the wafer, it is necessary to check whether the scratches are present or absent at the end of grinding.

Therefore, a grinding apparatus has been proposed which detects scratches on a wafer after grinding (see, for example, patent document 1). In patent document 1, a polished surface of a processed wafer is irradiated with a light beam, and the presence or absence of a scratch is determined based on the amount of light reflected by the light beam.

Patent document 1: japanese laid-open patent publication No. 2009-95903

However, in the grinding apparatus described in patent document 1, a complicated configuration of an optical system is required to detect the scratches of the wafer, and as a result, the configuration of the entire apparatus may become complicated.

Disclosure of Invention

The present invention has been made in view of the above problems, and an object of the present invention is to provide a grinding apparatus capable of appropriately detecting a scratch with a simple configuration without requiring a complicated configuration of an optical system.

A grinding device according to one embodiment of the present invention includes: a grinding unit having a mounting base on which a grinding wheel having a grinding wheel in a ring shape is mounted, and a spindle unit rotating about a center of the grinding wheel; a holding unit having a table rotating unit for rotating the chuck table about the center of the wafer held by the holding surface of the chuck table; and a scratch detection unit, wherein the holding surface of the chuck table is formed as an inclined surface with a center as a vertex and a lower periphery, the grinding unit enables a grinding wheel which rotates by the spindle unit to pass through the center of the wafer held by the chuck table and grind a part to be ground of an arc in a radius area between the center and the periphery of the wafer, the scratch detection unit comprises: a line sensor of a radius length for photographing a radius of the wafer; a light body extending in the same length as the line sensor; and a determination unit having: an editing unit that edits a band image with a radial direction of a captured image captured by the line sensor as a vertical axis and a circumferential direction as a horizontal axis; and a determination unit that determines that there is a scratch when the width of the straight line having regularity is larger than a preset width or when a line other than the straight line having regularity exists in the band-shaped image edited by the editing unit, and that determines that there is no scratch when the width of the straight line having regularity is equal to or smaller than the preset width.

According to this configuration, the chuck table is rotated while the line sensor is caused to capture an image of a radius portion of the wafer, thereby obtaining a captured image of the entire surface of the wafer. In the imaging, since the light emitter illuminates the radial portion of the wafer, the presence or absence of the scratch can be determined from the brightness of the captured image. In particular, by editing the captured image into a band image, the scratch can be expressed by a straight line having regularity. Therefore, the width of the straight line can be easily compared with the width of the preset straight line. In addition, lines other than straight lines having regularity can be easily found. As a result, the presence or absence of the scratch can be easily determined. Thus, the scratch can be appropriately detected with a simple configuration without requiring a complicated optical system configuration.

According to the present invention, the scratch can be appropriately detected with a simple configuration without requiring a complicated configuration of an optical system.

Drawings

Fig. 1 is a perspective view of a grinding apparatus according to the present embodiment.

Fig. 2 is a schematic view of a wafer ground by the grinding apparatus of the present embodiment (grinding step).

Fig. 3 (a) and (B) are schematic top surface views showing an example of imaging the top surface of the wafer after grinding.

Fig. 4 (a) and (B) are schematic diagrams illustrating an example of the imaging process according to the present embodiment.

Fig. 5 (a) and (B) are schematic diagrams showing an example of the editing process according to the present embodiment.

Fig. 6 (a) and (B) are schematic diagrams showing a specific example of the scratch detection.

Description of the reference symbols

1: a grinding device; 20: a chuck table; 21 a: a holding surface; 24: a table rotating unit; 40: a grinding unit; 42: a spindle unit; 45: a mounting seat; 46: grinding the grinding wheel; 47: grinding the grinding tool; 50: a scratch detection unit; 53: a line sensor; 54: a light emitter; 55: a judgment unit; 56: an editing unit; 57: a judgment section; w: a wafer; s, S1, S2, S_A、S_B、S_C: scratching; d: width.

Detailed Description

Hereinafter, a grinding apparatus according to the present embodiment will be described with reference to the drawings. Fig. 1 is a perspective view of a grinding apparatus according to the present embodiment. Fig. 2 is a schematic view of a wafer ground by the grinding apparatus of the present embodiment (grinding step). The grinding apparatus is not limited to the structure of the apparatus dedicated for grinding as shown in fig. 1, and may be incorporated into a fully automatic type processing apparatus that performs a series of processing such as grinding, polishing, and cleaning, for example, in a fully automatic manner.

As shown in fig. 1 and 2, the grinding apparatus 1 is configured to grind the wafer W held by the chuck table 20 using a grinding wheel 46 in which a plurality of grinding stones 47 are arranged in an annular shape. The wafer W is carried into the grinding apparatus 1 with the protective tape T bonded thereto and held on the chuck table 20. The wafer W may be a semiconductor wafer such As silicon or gallium arsenide, an optical device wafer such As ceramic, glass, or sapphire, or an As-cut (As-Slice) wafer before device pattern formation, As long As it is a plate-like member to be ground.

A rectangular opening extending in the X-axis direction is formed in the upper surface of the base 10 of the grinding apparatus 1, and the opening is covered with a moving plate 11 movable together with the chuck table 20 and a corrugated waterproof cover 12. A ball screw type advancing and retreating unit (not shown) for moving the chuck table 20 in the X-axis direction is provided below the waterproof cover 12. The chuck table 20 is coupled to a table rotating unit 24, and can be rotated around the center of the wafer W by driving the table rotating unit 24. The chuck table 20 and the table rotating unit 24 together serve as a holding unit.

A holding surface 21a is formed on the upper surface of the chuck table 20, and the holding surface 21a sucks and holds the wafer W by a porous material. Specifically, the chuck table 20 is a porous chuck for sucking and holding the wafer W, and is configured by attaching a disk-shaped porous plate 21 to a frame 22 as a main body.

The porous plate 21 is a porous material such as ceramic, and fine pores for adsorption are formed in the entire body. The frame 22 has a circular shape having a diameter larger than that of the porous plate 21, and a circular recess 23 for accommodating the porous plate 21 is formed in the center. The inner side surface of the circular recess 23 is formed to have the same inner diameter as the outer diameter of the porous plate 21. The depth of the circular recess 23 is formed to be substantially the same as the thickness of the porous plate 21.

A communication path (not shown) communicating with a suction source (not shown) is formed in the housing 22. The porous plate 21 is fitted into the circular recess 23, and the communication path communicates with the porous plate 21. Thus, a holding surface 21a capable of holding the wafer W by suction by the negative pressure of the suction source is formed on the upper surface of the porous plate 21. As shown in fig. 2 in particular, the holding surface 21a has an inclined surface whose outer periphery is slightly inclined with the rotation center of the chuck table 20 (the center of the holding surface 21a) as a vertex. When the wafer W is sucked and held on the holding surface 21a inclined in a conical shape, the wafer W is deformed into a conical shape inclined gradually along the shape of the holding surface 21 a.

A grinding feed unit 30 that performs grinding feed of the grinding unit 40 in a direction (Z-axis direction) approaching and separating from the chuck table 20 is provided on the column 15 on the base 10. The grinding feed unit 30 has: a pair of guide rails 31 disposed on the column 15 in parallel with the Z-axis direction; and a Z-axis table 32 provided slidably on the pair of guide rails 31 and driven by a motor. A nut portion, not shown, is formed on the back surface side of the Z-axis table 32, and the ball screw 33 is screwed into these nut portions. The grinding unit 40 is moved in the Z-axis direction along the guide rail 31 by rotationally driving the ball screw 33 by the drive motor 34 coupled to one end portion of the ball screw 33.

The grinding unit 40 is attached to the front surface of the Z-axis table 32 via a housing 41, and a grinding wheel 46 is rotated around the central axis by a spindle unit 42. The spindle unit 42 is a so-called air spindle, and rotatably supports a spindle 44 inside the housing by high-pressure air.

A mount 45 is coupled to a tip end of the main shaft 44, and a grinding wheel 46 having a grinding wheel 47 in an annular shape is attached to the mount 45. The grinding wheel 47 is configured by bonding diamond abrasive grains having a predetermined abrasive grain diameter with a ceramic bond, for example. The grinding wheel 47 is not limited to this, and may be formed by fixing diamond abrasive grains with a binder such as a metal binder or a resin binder.

The grinding apparatus 1 is provided with a control unit 90 that centrally controls the respective units of the apparatus. The control unit 90 is constituted by a processor, a memory, or the like that executes various processes. The Memory is configured by one or more storage media such as a ROM (Read Only Memory), a RAM (Random Access Memory), and the like according to the use. The control unit 90 controls, for example, the grinding feed amount, the grinding feed speed, and the like (and, for example, the rotation speed of the grinding wheel) of the grinding unit 40. The control unit 90 controls various operations of the scratch detection unit 50 described later.

A scratch detection unit 50 for detecting a scratch formed on the wafer W after grinding is provided on a side of the chuck table 20. The scratch detection unit 50 includes: an erected portion 51 erected from the upper surface of the base 10; and a scanning unit 52 extending from the standing unit 51 in the Y-axis direction. The scanner unit 52 includes: a line sensor 53 that photographs the upper surface of the wafer W; and a light emitter 54 disposed along the line sensor 53 (both see fig. 4).

The line sensor 53 is formed of, for example, an image sensor, and extends in a length corresponding to a radius portion of the wafer W. The line sensor 53 can take an image of a region corresponding to a radius portion of the wafer W. The light emitter 54 extends in the same direction and the same length as the line sensor 53, and irradiates the upper surface of the wafer W with light. Specifically, the light emitter 54 irradiates the wafer W with light so that the imaging range of the line sensor 53 is bright. As will be described in detail later, the upper surface of the wafer W can be imaged by rotating the chuck table 20 once while the scanner unit 52 images a radius portion of the upper surface of the wafer W.

The scratch detection unit 50 further includes a determination unit 55, and the determination unit 55 determines whether or not there is a scratch based on the captured image captured by the scanner unit 52. The determination unit 55 is constituted by a part of the control unit 90. The determination unit 55 has: an editing unit 56 that edits the captured image; and a determination unit 57 for determining the presence or absence of a scratch on the basis of the edited captured image. The details of the determination unit 55 will be described later.

In the grinding apparatus 1 configured as described above, so-called infeed grinding is performed in which the grinding whetstone 47 is brought into rotational contact with the front surface of the wafer W in a state in which the rotational axis of the grinding whetstone 46 is offset from the rotational axis of the chuck table 20. Here, a grinding process of the wafer W will be described with reference to fig. 2.

As shown in fig. 2, the wafer W is placed on the holding surface 21a of the chuck table 20. Specifically, the wafer W is placed on the holding surface 21a such that the surface to which the protective tape T is bonded is positioned on the lower side. The wafer W is sucked and held by the negative pressure generated on the holding surface 21a, and is formed into a gently inclined conical shape following the shape of the holding surface 21 a.

The chuck table 20 is positioned below the grinding unit 40. At this time, the rotation axis of the chuck table 20 is positioned at a position eccentric with respect to the rotation axis of the grinding stone 47. Further, the chuck table 20 adjusts the inclination of the rotation axis by an inclination adjustment mechanism, not shown, so that the grinding surface 47a of the grinding stone 47 is parallel to the holding surface 21 a.

Then, while the chuck table 20 is rotated, the grinding unit 40 is lowered (grinding and feeding) toward the holding surface 21a by the grinding and feeding unit 30 while rotating the grinding wheel 47 by the spindle unit 42. The grinding surface 47a of the grinding stone 47 is in contact with a radius portion from the center to the outer periphery of the wafer W in an arc shape.

In this way, the grinding stone 47 of the grinding unit 40 grinds the portion to be ground of the circular arc of the wafer W in the radial region between the center and the outer periphery of the wafer W through the center of the wafer W. The wafer W is thinned by gradually grinding and feeding the wafer W in the Z-axis direction while the grinding wheel 47 is brought into rotational contact with the wafer W. When the wafer W is thinned to a desired thickness, the grinding process is ended.

However, when the wafer is ground by the lateral feed grinding apparatus, grinding marks (saw marks) including scratches are sometimes formed on the ground surface of the wafer. Examples of the scratches include arc-shaped patterns (grinding marks) regularly formed from the center toward the outer periphery of the wafer. In addition, abrasive grains detached from the grinding wheel during processing may contact the surface of the wafer to be ground, thereby causing scratches. The scratches have an influence on devices formed on the wafer, and thus the formation of the scratches is less desirable.

For example, it is considered that a large amount of grinding water is supplied to the upper surface of the wafer to perform grinding processing, and the peeled abrasive grains are removed from the surface to be ground of the wafer, so that scratches are not easily formed. However, increasing the amount of grinding water supplied is uneconomical, and a large amount of grinding water is a factor to reduce the force with which the grinding wheel contacts the wafer, i.e., to reduce the engagement of abrasive grains. As a result, grinding efficiency may deteriorate. As described above, the generation of scratches can be suppressed by increasing the grinding water, but it is difficult to achieve a balance between grinding efficiency. Therefore, it is necessary to confirm the presence or absence of the scratch at the end of grinding.

For example, a grinding apparatus has been proposed which irradiates a polished surface of a processed wafer with a light beam and determines the presence or absence of a scratch based on the amount of light reflected by the light beam. However, in this grinding apparatus, a complicated configuration of an optical system is required to detect the scratches of the wafer, and as a result, the configuration of the entire apparatus may become complicated.

Therefore, the present inventors conceived to appropriately detect a scratch with a simple structure without requiring a complicated optical system structure. Specifically, in the present embodiment, the entire surface of the wafer W is imaged by rotating the chuck table 20 once while imaging the radius portion of the wafer W by the scanner unit 52, and the presence or absence of a scratch is determined from the obtained image.

Here, the scratch detection unit of the present embodiment will be described with reference to fig. 3. Fig. 3 is a top surface view showing an example of imaging the top surface of the wafer after grinding. Specifically, fig. 3 (a) shows a wafer shot of a comparative example, and fig. 3 (B) shows an example of the wafer shot of the present embodiment.

As shown in fig. 3, numerous regular arc-shaped scratches are formed on the upper surface of the wafer W after grinding, from the center toward the outer periphery of the wafer W. In the example shown in fig. 3 (a), the scanner unit 60 corresponding to the diameter length of the wafer W is positioned above the wafer W. The scanning unit 60 extends in the Y-axis direction. The scanner unit 60 scans the entire surface of the wafer W by moving (scanning) the wafer W relative to the wafer W in the X-axis direction while irradiating the wafer W with light.

In this case, among the scratches formed in the left side region of the wafer W with respect to the center line C along the X-axis direction (for example, the scratch S1) and the scratches formed in the right side region of the wafer W (for example, the scratch S2), the orientations of the scratches irradiated with light in the scanning direction of the scanner unit 60 are different.

Thus, the following is assumed: since the irradiation of the scratch with light becomes uneven depending on the position of the scratch, a suitable captured image cannot be obtained. For example, in fig. 3 (a), the center position of the wafer W may be shifted by changing the light irradiation direction on the left and right sides of the paper surface.

In contrast, in the present embodiment shown in fig. 3 (B), the scanner unit 52 is disposed on the upper surface of the wafer W at a length and position corresponding to the radius of the wafer W. The entire surface of the wafer W is imaged by rotating the wafer W one turn while irradiating the wafer W with light from the scanner unit 52. In this case, the light can be uniformly applied to the scratch S at all times. As a result, an appropriate captured image of the wafer W can be obtained without shifting the center of the wafer W.

As will be described in detail later, the image captured by the scanner unit 52 is coordinate-converted to facilitate observation of the scratch, and the scratch can be easily and appropriately detected.

Next, the scratch detection method of the present embodiment will be described with reference to fig. 4 to 6. Fig. 4 is a schematic diagram illustrating an example of the imaging process according to the present embodiment. Fig. 4 (a) is a view seen from arrow a of fig. 1, and fig. 4 (B) is a view seen from arrow B of fig. 1. Fig. 5 is a schematic diagram showing an example of the editing process according to the present embodiment. Fig. 5 (a) is a captured image before editing, and fig. 5 (B) is a band image after editing. Fig. 6 is a schematic diagram showing a specific example of the scratch detection.

The scratch detection method of the present embodiment is implemented by the following steps: an imaging step of imaging a ground surface of the ground wafer W (see fig. 4); an editing step of performing coordinate conversion on the captured image of the wafer W to edit the captured image into a band-shaped image (see fig. 5); and a determination step of determining the presence or absence of a scratch on the basis of the edited band image (see fig. 6).

First, an imaging process will be described. As shown in fig. 4 (a), the ground wafer W is positioned below the scanning unit 52 while being held by suction on the chuck table 20. Further, the chuck table 20 adjusts the inclination of the rotation axis by an inclination adjustment mechanism (not shown) so that the extending direction of the scanning unit 52 (line sensor 53) is parallel to the upper surface (holding surface 21a) of the wafer W.

As shown in fig. 4 (a) and (B), the imaging area of the line sensor 53 corresponds to a radius portion of the wafer W directly below the line sensor 53. The light emitter 54 irradiates light to the imaging area of the line sensor 53. The scanner 52 captures an image of a radius portion of the wafer W irradiated with the light from the light emitter 54 by the line sensor 53, and rotates the wafer W on the chuck table 20 once, thereby acquiring a captured image of the entire surface of the wafer W. The light emitted from the light emitter 54 has a wavelength that reflects on the front surface of the wafer W, and a wavelength that is transparent to the wafer W is not used.

Next, the editing process will be described. As shown in fig. 5 (a), in the captured image obtained in the capturing step, the reflected light of the captured light is reduced (scattered) due to the fine irregularities formed by the scratch, and therefore the scratch can be recognized from the contrast of the light and the shade. In the editing step, the captured image shown in fig. 5 a is subjected to coordinate conversion, and is edited into an image (a band-shaped image shown in fig. 5B) in which the presence or absence of a scratch is easily determined by the determination unit 57 (see fig. 1).

Specifically, the editing unit 56 (see fig. 1) performs coordinate conversion with the radial direction (from the wafer center to the wafer periphery) of the captured image of fig. 5 a as the vertical axis and the circumferential direction (0 ° to 360 °) of the captured image as the horizontal axis. As shown in (B) of FIG. 5, byThe edited image resulting from the coordinate conversion is represented as a rectangular image (band-shaped image) long in the circumferential direction. For example, the thick line scratch S shown in fig. 5 (a) is a thick line scratch S in the band image of fig. 5 (B)_AAnd (4) showing.

In this way, in the actual captured image, the scratch is represented by an arc-shaped curve, whereas in the edited band image, the scratch is represented by a substantially straight line having regularity. Thus, the determination section 57 can easily determine the presence or absence of the scratch in the subsequent determination step.

Next, the determination step will be described. In the judging step, the presence or absence of a scratch is judged from the band image obtained in the editing step. Specifically, the determination unit 57 determines that there is a scratch when the width of the straight line having regularity in the band image is larger than a predetermined width, and determines that there is no scratch when the width of the straight line having regularity is equal to or smaller than the predetermined width. Further, the determination unit 57 determines that there is a scratch even when there is a line other than a straight line having regularity in the band image.

For example, as shown in fig. 6 (a), it is considered that a thicker straight line S is displayed along a straight line having regularity in a band-shaped image_BThe case (1). The determination unit 57 determines the straight line S from the strip image pair_BThe width D of (2) is detected and compared with a linear width which is a preset criterion for judging the presence or absence of a scratch. As a result, at the straight line S_BIf the width of (2) is large, the straight line S is set_BIs identified as a scratch S_B. That is, the determination unit 57 determines that there is a scratch.

As another example, as shown in fig. 6 (B), a straight line S formed so as to intersect with a straight line having regularity is displayed in a band image_CIn the case of (1), the determination unit 57 determines the straight line S_CIs identified as a scratch S_C. That is, in this case, the determination unit 57 also determines that there is a scratch.

As described above, in the present embodiment, even when a regular scratch such as a grinding scratch is displayed in the captured image, a relatively large scratch and a random scratch can be detected from the edited band image. That is, scratches that may affect subsequent steps or devices formed on the wafer W can be selected and determined.

In addition, as described above, when it is determined that there is a scratch, the grinding process is performed again, and the scratch which may become a problem later can be removed. Thus, a series of steps of grinding, scratch detection, and scratch removal can be performed by a single grinding apparatus 1. This eliminates the need to transport the wafer W to another apparatus for detecting or removing the scratches.

As described above, according to the present embodiment, the line sensor 53 can capture an image of the entire surface of the wafer W while rotating the chuck table 20. Since the light emitter 54 illuminates the radial portion of the wafer W during imaging, the presence or absence of scratches can be determined based on the brightness of the captured image. In particular, by editing the captured image into a band image, the scratch can be expressed as a straight line having regularity. Therefore, the width of the straight line can be easily compared with the width of the preset straight line. In addition, lines other than straight lines having regularity can be easily found. As a result, the presence or absence of the scratch can be easily determined. Thus, the scratch can be appropriately detected with a simple configuration without requiring a complicated optical system configuration.

In the present embodiment, the entire surface of the wafer W is imaged by rotating the chuck table 20 once, but the present invention is not limited to this configuration. For example, the scanner 52 may be rotated around the center of the wafer W.

In the present embodiment, the scanner unit 52 is configured to extend along a length corresponding to a radius portion of the wafer W, but is not limited to this configuration. The scanner 52 (line sensor 53 and light emitter 54) may be shorter than the radius of the wafer W. When the scanner 52 is shorter than the radius of the wafer W, the scanner 52 is moved away from the wafer W to capture an image and detect a scratch, or the scanner 52 is moved closer to the wafer W in the radial direction of the wafer W to capture an image of the entire surface of the wafer W and detect a scratch. Thus, the distance between the scanner unit 52 and the wafer W can be adjusted. Further, if the scanner unit 52 is short, an inexpensive scanner unit 52 can be selected.

Further, although the present embodiment and the modification example have been described, the above embodiment and the modification example may be combined in whole or in part as another embodiment of the present invention.

The embodiments of the present invention are not limited to the above-described embodiments, and various changes, substitutions, and alterations can be made without departing from the spirit and scope of the technical idea of the present invention. In addition, if the technical idea of the present invention can be realized in other ways by other technologies derived or advanced from the technology, the method can also be used for implementation. Therefore, the claims cover all the embodiments that can be included in the scope of the technical idea of the present invention.

As described above, the present invention has the following effects: the present invention can appropriately detect scratches with a simple configuration without requiring a complicated optical system configuration, and is particularly useful in a grinding apparatus that grinds the front surface of a wafer.

Claims

1. A grinding apparatus, comprising:

a grinding unit having a mounting base on which a grinding wheel having a grinding wheel in an annular shape is mounted, and a spindle unit that rotates about a center of the grinding wheel;

a holding unit having a table rotating unit for rotating the chuck table about the center of the wafer held by the holding surface of the chuck table; and

a scratch detection unit for detecting a scratch on the substrate,

wherein the content of the first and second substances,

the holding surface of the chuck table is formed as an inclined surface having a center as a vertex and a lower periphery,

the grinding unit grinds a portion to be ground of a circular arc in a radius region between the center and the outer periphery of the wafer by passing the grinding stone rotated by the spindle unit through the center of the wafer held by the chuck table,

the scratch detection unit has:

a line sensor for photographing the radius of the wafer for the radius length;

a light body extending in the same length as the line sensor; and

a judging unit for judging whether the received signal is a signal,

the judgment unit has:

an editing unit that edits a band image with a radial direction of a captured image captured by the line sensor as a vertical axis and a circumferential direction as a horizontal axis; and

and a determination unit that determines that there is a scratch when the width of the straight line having regularity is larger than a preset width or when a line other than the straight line having regularity exists in the band-shaped image edited by the editing unit, and that determines that there is no scratch when the width of the straight line having regularity is equal to or smaller than the preset width.