JP7648902B2 - Crack detection apparatus and method - Google Patents

Description

The present invention relates to a crack detection device and method, and more particularly to a crack detection device and method for detecting cracks formed inside a workpiece.

Conventionally, there is known a laser processing device (also called a laser dicing device) that focuses a laser beam inside a substrate such as a silicon wafer or a glass wafer (hereinafter referred to as the "workpiece") along a planned cutting line, and forms a laser processing area that serves as the starting point for cutting inside the workpiece along the planned cutting line. The workpiece with the laser processing area formed is then cut along the planned cutting line by a cutting process such as expanding or breaking, and separated into individual chips.

When a laser processing area is formed on a workpiece using a laser processing device, a crack propagates from the laser processing area in the thickness direction of the workpiece. If the crack reaches the surface (laser light incident surface) of the workpiece or the back surface on the opposite side, the workpiece can be properly divided into chips in the cutting process. The reason for this is that a crack formed inside the workpiece becomes the starting point when dividing the workpiece, and the degree of propagation of the crack determines the division rate of the workpiece. In addition, in the case of a thick workpiece, the crack may not reach the surface or back surface of the workpiece, so whether or not the laser processing area has been properly formed may not necessarily be determined based on whether or not the crack has reached the surface or back surface of the workpiece.

Therefore, after forming the laser processing area by the laser processing device, before the cutting process, it is possible to accurately predict whether the laser processing area that is the starting point for cutting the workpiece has been properly formed, that is, by detecting the crack depth of the crack formed inside the workpiece. If there is a part inside the workpiece where the laser processing area is not properly formed, it is possible to reprocess only that part again by the laser processing device, or to change the cutting method in the cutting process. This makes it possible to eliminate the loss of chips in the subsequent cutting process. In addition, it is possible to correct the processing conditions in the laser processing device by referring to the occurrence of defective parts, etc., and to reduce the occurrence of defective parts in the laser processing area in the workpiece to be processed thereafter. When the laser processing area of the defective part is reprocessed, the loss of time required for reprocessing can also be reduced by reducing the occurrence of defective parts.

Meanwhile, evaluation of cracks that occur inside workpieces has traditionally been performed by cutting and polishing the sample, or by observation under limited conditions. This has made it difficult to apply this method to processing processes that use laser processing equipment.

In response to this, a technology has been proposed for non-destructively inspecting cracks formed inside a workpiece (see, for example, Patent Document 1).

The technology disclosed in Patent Document 1 uses oblique illumination of cracks inside a workpiece, and in the area where the cracks are formed, detects light that does not hit the crack but is reflected from the back surface of the workpiece, and inspects the cracks by utilizing the reduction in the amount of detected light caused by the crack scattering the incident light.

JP 2017-133997 A

As shown in Patent Document 1, when detecting light that is incident on the inside of a workpiece and is reflected from the back surface of the workpiece without hitting a crack, there is a problem in that the light reflected from the surface of the workpiece becomes disturbance light (noise).

Figure 10 is a cross-sectional view showing the reflected light from the workpiece when the crack inside the workpiece is illuminated with an oblique light. As shown in Figure 10, when light is focused on a crack K inside the workpiece W by a focusing lens, the light reflected on the back surface of the workpiece W without hitting the crack K (hereinafter referred to as back surface reflected light Mt (1) and Mt (2)) is reflected on the back surface side of the workpiece W and returns to the front surface side of the workpiece W and is guided to the detection optical system. Then, when a part of the light that entered the inside of the workpiece W hits the crack K, the detected light amount of the back surface reflected light Mt (1) and Mt (2) decreases. Therefore, the depth position of the crack K can be detected based on the change in the detected light amount of the back surface reflected light Mt (1) and Mt (2).

Here, a portion of the light irradiated to the workpiece W is reflected by the surface of the workpiece W. Hereinafter, the light reflected by the surface of the workpiece W is referred to as surface reflected light N(1) and N(2). If it is not possible to separate the surface reflected light N(1) and N(2) from the back surface reflected light Mt(1) and Mt(2), respectively, the surface reflected light N(1) and N(2) will become disturbance light (noise), and the change in the detected light amount of the back surface reflected light Mt(1) and Mt(2) cannot be accurately measured.

Therefore, in Patent Document 1, a field stop (aperture stop) is placed at a position conjugate with the focal point of the focusing lens to limit the incidence of the surface reflected light N(1) and N(2) on the photodetector.

10A shows a case where the condensing position of the detection light is closer to the back surface (-Z side) of the workpiece W, and FIG. 10B shows a case where the condensing position of the detection light is closer to the front surface (+Z side) of the workpiece W. The dashed dotted line L _D shown in FIG. 10 is a plane indicating the center position in the depth direction (Z direction) of the workpiece W. Also, symbols _FS10 and _FS12 indicate focal planes indicating the condensing positions of the condenser lenses.

10A, when the depth position of the condensing position of the detection light is on the back side (-Z side) of the plane _LD , the ray height hMt(1) of the back-surface reflected light _Mt(1) is lower than the ray height hN(1) of the front-surface reflected light N ₍₁ ) at the focal plane _FS10 . For this reason, it is possible to separate the two by appropriately adjusting the size of the opening of the field stop, which is placed at a position conjugate with the condensing point of the condenser lens.

10B, when the depth position of the condensing position of the detection light is closer to the front surface (+Z side) than the plane _LD , the ray height hMt( ₂ ) of the back surface reflected light _Mt(2) is higher than the ray height hN(2) of the front surface reflected light N ₍₂₎ at the focal plane FS12. For this reason, even if the field stop is narrowed, the back surface reflected light Mt(2) is removed before the front surface reflected light N(2) is removed, making it difficult to separate the two with the field stop.

For this reason, if the depth position of the focused position of the detection light is closer to the surface (+Z side), the surface reflected light N(2) cannot be separated from the back surface reflected light Mt(2), resulting in a problem that the measurement accuracy of the depth position of the crack K decreases or becomes impossible to measure.

The present invention was made in consideration of these circumstances, and aims to provide a crack detection device and method that can accurately measure the depth position of a crack regardless of the crack depth position.

In order to solve the above problem, the crack detection device according to the first aspect of the present invention comprises a light source unit that emits a first detection light and a second detection light whose polarization directions are orthogonal to each other, the light source unit emitting the first detection light along a first light source optical axis that is parallel to the main optical axis and eccentric in one direction relative to the main optical axis, and emitting the second detection light along a second light source optical axis that is parallel to the main optical axis and eccentric in the other direction opposite to the one direction relative to the main optical axis, a focusing lens that has a lens optical axis coaxial with the main optical axis and focuses the first detection light and the second detection light emitted from the light source unit inside the workpiece, and a focusing lens that focuses the first detection light and the second detection light emitted from the light source unit inside the workpiece. The system includes a branching mirror arranged at a position conjugate with the focal point of the optical lens, a first detector arranged downstream along the main optical axis from the branching mirror and detecting a first back-reflected light of the first detection light that is reflected by the back surface of the workpiece without hitting a crack formed inside the workpiece, a second detector arranged downstream of the optical path branched by the branching mirror and detecting a second back-reflected light of the second detection light that is reflected by the back surface of the workpiece without hitting a crack, and a crack detection means that detects the crack depth of the crack based on the detection signals from the first detector and the second detector.

The crack detection device according to the second aspect of the present invention, in the first aspect, includes a first polarizer that is arranged downstream along the main optical axis relative to the branching mirror and has the same polarization direction as the first detection light, and a second polarizer that is arranged downstream of the optical path branched by the branching mirror and has the same polarization direction as the second detection light, and the first detector and the second detector are arranged downstream of the first polarizer and the second polarizer, respectively.

In the third aspect of the present invention, the crack detection device according to the first aspect includes a third detector arranged downstream of the optical path branched by the branching mirror and detecting the first crack reflected light reflected by the back surface of the workpiece after hitting the crack, a fourth detector arranged downstream along the main optical axis relative to the branching mirror and detecting the second crack reflected light reflected by the back surface of the workpiece after hitting the crack, a first polarizing beam splitter arranged upstream of the first detector and the third detector and guiding the first back surface reflected light to the first detector and the second crack reflected light to the third detector, and a second polarizing beam splitter arranged upstream of the second detector and the fourth detector and guiding the second back surface reflected light to the second detector and the first crack reflected light to the fourth detector, and the crack detection means detects the crack depth of the crack based on the detection signals from the first detector, the second detector, the third detector, and the fourth detector.

The crack detection device according to the fourth aspect of the present invention is any of the first to third aspects, and includes a confocal detector capable of detecting the first back surface reflected light and the second back surface reflected light only when the focusing point of the focusing lens coincides with the front surface or back surface of the workpiece, and an interface detection means for detecting the position of the front surface or back surface of the workpiece based on the detection signal from the confocal detector.

A crack detection method according to a fifth aspect of the present invention includes a detection light emission process for emitting a first detection light and a second detection light having mutually orthogonal polarization directions along a first light source optical axis that is parallel to the main optical axis and eccentric in one direction relative to the main optical axis, and emitting a second detection light along a second light source optical axis that is parallel to the main optical axis and eccentric in the other direction opposite to the main optical axis, a focusing process for focusing the first detection light and the second detection light inside the workpiece by a focusing lens having a lens optical axis coaxial with the main optical axis, and a focusing point of the focusing lens arranged at a position conjugate with the focusing point of the focusing lens. The system includes a first optical detection process in which a first detector arranged downstream along the main optical axis from the splitting mirror detects a first back-side reflected light of the first detection light that is reflected by the back side of the workpiece without hitting a crack formed inside the workpiece, a second optical detection process in which a second detector arranged downstream of the optical path split by the splitting mirror detects a second back-side reflected light of the second detection light that is reflected by the back side of the workpiece without hitting a crack, and a crack detection process in which the crack depth is detected based on the detection signals from the first and second detectors.

According to the present invention, the branching mirror can reliably remove surface reflected light that becomes disturbance light, making it possible to accurately detect the depth of a crack even if the crack is close to the surface.

FIG. 1 is a schematic diagram showing a configuration of a crack detection device according to a first embodiment. FIG. 2 is a diagram for explaining the optical system (illumination optical system and detection optical system) in the crack detection device according to the first embodiment of the present invention. FIG. 3 is a diagram for explaining the optical system (illumination optical system and detection optical system) in the crack detection device according to the first embodiment of the present invention. FIG. 4 is a graph showing the simulation results of the outputs from the photodetectors (the first detector and the second detector) and the confocal detector. FIG. 5 is a diagram showing a simulation result by geometrical optics ray tracing of the first rear-surface reflected light obtained from the first detected light and the second detected light. FIG. 6 is a flowchart showing a crack detection method according to the first embodiment of the present invention. FIG. 7 is an enlarged view showing the configuration near a photodetector (first detector) in the detection optical system of a crack detection device according to a second embodiment of the present invention. FIG. 8 is an enlarged view showing the configuration near the photodetector (second detector) in the detection optical system of the crack detection device according to the second embodiment of the present invention. FIG. 9 is a graph showing a simulation result of the output from the photodetectors (the first detector and the third detector). FIG. 10 is a cross-sectional view showing light reflected from a workpiece when a crack inside the workpiece is obliquely illuminated.

Below, an embodiment of the crack detection device according to the present invention will be described with reference to the attached drawings.

[First embodiment]
Fig. 1 is a schematic diagram showing a crack detection device according to the first embodiment. As shown in Fig. 1, the crack detection device 10 according to the first embodiment is a device that detects the crack depth of a crack K formed inside the workpiece W and detects the interface (the position of the front and back surfaces) of the workpiece W by irradiating a detection light L1 onto a workpiece W and detecting a reflected light L2 from the workpiece W.

The crack detection device 10 is combined with a laser dicing device (not shown) that forms a modified region inside the workpiece W, but in order to avoid complicating the drawing, Figure 1 shows only the components related to the crack detection device that are necessary to explain the present invention.

As shown in FIG. 1, the crack detection device 10 includes a light source unit 14, an illumination optical system 16, a dichroic mirror 18, a condenser lens 20, a half mirror 22, a detection optical system 24, a photodetector 26, a focus adjustment mechanism 28, an alignment mechanism 29, and a control unit 30. The workpiece W is placed on a stage (not shown).

The light source unit 14 emits detection light L1 for detecting the depth of a crack K formed inside the workpiece W. In the case where the workpiece W is a silicon wafer, it is preferable to use infrared light having a wavelength of 1000 nm or longer for the detection light L1. The light source unit 14 emits detection light L1 (first detection light L1A and second detection light L1B) along two first light source optical axes _QA and _QB that are parallel to the main optical axis P, which is coaxial with the lens optical axis of the condenser lens 20, and are eccentric with respect to the main optical axis P. The light source unit 14 is connected to a control unit 30, and the control unit 30 controls the emission of the light source unit 14. The configuration of the light source unit 14 will be described later (see FIGS. 2 and 3).

The illumination optical system 16 is composed of a pair of relay lenses 40, 42 and a field stop 44 that limits the range of the detection light L1 irradiated from the light source unit 14 toward the workpiece W. The pair of relay lenses 40, 42 constitute a telecentric afocal optical system, and project the first detection light L1A and the second detection light L1B onto the lens pupil position 20A of the condenser lens 20. The field stop 44 is arranged so as to be in a position conjugate with the condenser lens 20. As a result, the detection light L1 is condensed toward one point on the image plane (condenser surface) of the condenser lens 20 inside the workpiece W to form a light spot, so that unnecessary reflected light and scattered light can be reduced, and it is possible to improve the detection accuracy of the crack depth of the crack K formed inside the workpiece W. Note that if the detection light L1 emitted from the light source unit 14 is collimated light (parallel light), the field stop 44 may be omitted.

The half mirror 22 is disposed between the illumination optical system 16 and the dichroic mirror 18, and transmits a portion of the incident light and reflects a portion of it. That is, the half mirror 22 transmits a portion of the detection light L1 incident from the light source unit 14 via the illumination optical system 16, and guides the transmitted light (detection light L1) to the condenser lens 20 via the dichroic mirror 18, while reflecting a portion of the reflected light L2 of the detection light L1 from the workpiece W, and guides the reflected light (reflected light L2) to the detection optical system 24.

The dichroic mirror 18 is provided for the purpose of separating the processing light and the detection light (first detection light L1A and second detection light L1B) when the condenser lens 20 is shared by the processing optical system of the laser dicing device and the detection optical system 24 for detecting interfaces and cracks. The dichroic mirror 18 bends the main optical axis P by 90 degrees. That is, the dichroic mirror 18 reflects the detection light L1 from the light source unit 14 at a right angle and guides it to the condenser lens 20, and also reflects the reflected light L2 from the workpiece W at a right angle and guides it to the half mirror 22. Note that, if the processing light does not need to be taken into consideration, a total reflection mirror may be placed instead of the dichroic mirror 18.

The focusing lens 20 is disposed at a position facing the workpiece W, and focuses the detection light L1 incident from the light source unit 14 via the illumination optical system 16, the half mirror 22, and the dichroic mirror 18 inside the workpiece W. The lens optical axis of the focusing lens 20 is coaxial with the main optical axis P. When the detection light L1 is focused inside the workpiece W by the focusing lens 20, the reflected light L2 from the workpiece W passes through the focusing lens 20 and the dichroic mirror 18, is reflected by the half mirror 22, and is guided to the detection optical system 24.

The detection optical system 24 guides the reflected light L2 reflected by the half mirror 22 to the photodetector 26, and includes a pair of relay lenses 46, 48 and a branching mirror 70 that limits the range of the reflected light L2 from the workpiece W.

The pair of relay lenses 46, 48 form a telecentric afocal optical system, projecting the pupil of the focusing lens 20 onto the photodetector 26.

The branching mirror 70 is positioned so that it is conjugate with the focal point of the focusing lens 20. The branching mirror 70 functions as a light blocking means that blocks the light reflected from the surface (detection light irradiation surface) of the workpiece W and incident on the photodetector 26 (see FIG. 2).

The light detector 26 is composed of two photodetectors 26A and 26B (omitted in FIG. 1; see FIGS. 2 and 3). The photodetectors 26A and 26B receive the reflected light L2 (first back-surface reflected light Mt(1) and second back-surface reflected light Mt(2)) from the workpiece W when the first detection light L1A and second detection light L1B are irradiated onto the workpiece W, and output a detection signal according to the amount of light received to the control unit 30. The photodetectors 26A and 26B are an example of a light detection means. Note that instead of the photodetectors 26A and 26B, an infrared camera may be used to capture images and perform image processing.

The focus adjustment mechanism 28 is an example of a focal point changing means, and adjusts the focal point of the focusing lens 20 in the thickness direction (Z direction) of the workpiece W. This focus adjustment mechanism 28 has a lens drive unit (e.g., a piezoelectric actuator, not shown) that moves the focusing lens 20 in a direction along the main optical axis P, and the lens drive unit moves the focusing lens 20 in the direction along the main optical axis P to change the distance between the focusing lens 20 and the workpiece W, thereby changing the focal point of the focusing lens 20 in the thickness direction (Z direction) of the workpiece W. The focus adjustment mechanism 28 (lens drive unit) is connected to the control unit 30, and the control unit 30 controls the focal point of the focusing lens 20.

When the focusing lens 20 is configured to be movable in the Z direction together with the processing head (not shown) of the laser dicing device, the focus adjustment mechanism 28 may include a drive mechanism for the processing head. In this case, it becomes possible to combine the position adjustment (coarse adjustment) of the focusing point by the drive mechanism for the processing head and the position adjustment (fine adjustment) of the focusing point by the piezoelectric actuator.

In this specification, the focal point of the focusing lens 20 refers to the position of the focal point of the detection light L1 focused by the focusing lens 20. The depth position (Z direction position) of the focal point of the focusing lens 20 is indicated by the distance from the back surface of the workpiece W.

The alignment mechanism 29 is an example of an alignment means, and performs relative positioning (alignment) between the focusing lens 20 and the workpiece W in the horizontal direction (XY direction). The alignment mechanism 29 has a lens drive unit (not shown) that moves the focusing lens 20 slightly in the horizontal direction perpendicular to the lens optical axis. The lens drive unit is connected to the control unit 30, and the lens drive unit is controlled by the control unit 30 to perform relative positioning between the focusing lens 20 and the workpiece W in the horizontal direction. Note that instead of the alignment mechanism 29, a stage (not shown) on which the workpiece W is placed may be moved relative to the focusing lens 20.

The control unit 30 is composed of a CPU (Central Processing Unit), memory, an input/output circuit unit, etc., and controls the operation of each unit of the crack detection device 10. Specifically, the control unit 30 sequentially acquires detection signals output from the photodetector 26 while changing the focal point of the focusing lens 20 in the thickness direction (Z direction) of the workpiece W by the focus adjustment mechanism 28, and performs a process (crack detection process) to detect the crack depth (crack upper end position or crack lower end position) of the crack K formed inside the workpiece W based on the acquired detection signals, and a process (interface detection process) to detect the interface (front and back surfaces) of the workpiece W. The control unit 30 is an example of a crack detection means and an interface detection means.

(Optical system)
2 and 3 are diagrams for explaining the optical system (illumination optical system and detection optical system) in the crack detection device according to the first embodiment of the present invention. In Fig. 2 and Fig. 3, the optical paths of the first detection light _L1A and the second detection light L1B emitted along the first light source optical axis QA and the second light source optical axis _QB, respectively, are shown. In Fig. 2 and Fig. 3, in order to simplify the optical paths, some optical members such as the dichroic mirror 18 and the focus adjustment mechanism 28 are omitted.

The crack detection device 10 according to this embodiment detects cracks and interfaces formed inside the workpiece W by simultaneously emitting the first detection light L1A and the second detection light L1B and detecting the reflected light (the first back-surface reflected light Mt(1) and the second back-surface reflected light Mt(2) respectively). Here, the first detection light L1A and the second detection light L1B are linearly polarized light whose polarization directions are mutually orthogonal. In addition, in FIG. 2 and FIG. 3, the first detection light L1A and the second detection light L1B and the polarizing filters 60 and 62 are marked with arrows or arrowheads indicating their respective polarization directions.

As shown in Figures 2 and 3, the light source unit 14 includes a light source 32 and a polarizing beam splitter (PBS) 34.

The light source 32 is disposed on a second light source optical axis _QB that is offset downward in the drawing with respect to the main optical axis P, and emits detection light L0 along the second light source optical axis _QB .

The PBS 34 is a combination of a cube-type polarizing beam splitter 34B, which is formed by bonding two right-angle prisms together, and a total reflection prism 34A. The total reflection prism 34A and the polarizing beam splitter 34B are disposed on the first light source optical axis _QA and the second light source optical axis _QB , respectively. The total reflection prism 34A does not need to be bonded to the polarizing beam splitter 34B, and may be separated from the polarizing beam splitter 34B. Also, a total reflection mirror or the like may be used instead of the total reflection prism 34A.

Of the detection light L0 emitted from the light source 32, a parallel component whose polarization direction is parallel to the paper surface (second detection light L1B) passes through the reflecting surface (the bonding surface of the rectangular prism) of the polarizing beam splitter 34B and travels along the second light source optical axis _QB .

On the other hand, of the detection light L0 emitted from the light source 32, a vertical component (first detection light L1A) whose polarization direction is perpendicular to the paper surface is reflected successively by the reflecting surfaces of the polarizing beam splitter 34B and the total reflection prism 34A, and then travels along the first light source optical axis _QA .

The first detection light L1A and the second detection light L1B are guided to the condenser lens 20 via the relay lenses 40 and 42, the half mirror 22, and the dichroic mirror 18 (see FIG. 1 for the dichroic mirror 18; omitted in FIGS. 2 and 3). The first detection light L1A and the second detection light L1B guided to the condenser lens 20 are then focused inside the workpiece W by the condenser lens 20.

As described above, the light source unit 14 according to this embodiment is capable of simultaneously irradiating the workpiece W with the first detection light L1A and the second detection light L1B, which are linearly polarized light whose polarization directions are orthogonal to each other.

The light source unit 14 is not limited to the above example, and for example, the number, structure, and arrangement of the light sources 32 and PBS 34 may be different from those described above. In addition, two light sources 32 may be provided, and a polarizing filter may be provided for each light source.

As shown in Figures 2 and 3, the reflected light from the workpiece W is reflected by the half mirror 22 and guided to the detection optical system 24. The detection optical system 24 includes a 4F optical system composed of relay lenses 46 and 48, and a 4F optical system composed of relay lenses 46 and 50.

A branching mirror 70 is disposed downstream (rear) of the relay lens 46. The branching mirror 70 is disposed at or near a position (focal plane) FS2 optically conjugate with a front focal position (focal plane) _FS1 of the condenser lens 20 in order to separate a light beam passing through the FS(+) region from a light beam passing through the _FS (-) region. Here, the focal plane _FS2 includes the condensing position (focal position) of the relay lens 46 constituting the 4F optical system.

The branching mirror 70 is, for example, a total reflection mirror. The branching mirror 70 is fixed to the FS(-) region on the +y side of the main optical axis P, which is a region slightly upstream of the focal plane _FS2 , and bends the light passing through the FS(-) region. The arrangement of the branching mirror 70 is not limited to the examples shown in Figures 2 and 3. Here, one end of the branching mirror 70 is arranged near the main optical axis P.

A relay lens 48 and a photodetector 26A (first detector) are arranged in this order downstream (rear side) of the branching mirror 70 along the main optical axis P. A relay lens 50 and a photodetector 26B (second detector) are arranged in this order downstream of the optical path branched by the branching mirror 70.

In this embodiment, cracks are detected by detecting reflected light from the workpiece W (first back surface reflected light Mt(1) and second back surface reflected light Mt(2) respectively) using photodetectors 26A and 26B.

A half mirror 80 is disposed downstream of the relay lens 46 (between the relay lens 46 and the focal plane _FS2 (the branching mirror 70)). The half mirror 80 folds a portion of the reflected light from the workpiece W and guides the light to a confocal detector 82. The confocal detector 82 includes, for example, a photodetector or an infrared camera.

A pinhole plate 84 is disposed upstream of the confocal detector 82. The pinhole plate 84 is disposed so that the position of the confocal pinhole is on the main optical axis P and coincides with the rear focal position (focusing position) of the relay lens 46, i.e., a position conjugate with the focal position of the focusing lens 20.

The reflected light from the workpiece W (first back surface reflected light Mt(1) and second back surface reflected light Mt(2)) bent by the half mirror 80 is used to detect the interface of the workpiece W.

(First detection light L1A)
First, the first detection light L1A (vertical component) will be described with reference to FIG.

As shown in FIG. 2, the first detection light L1A is irradiated to the workpiece W via the relay lenses 40 and 42, and is condensed by the condenser lens 20 at the front focal position FS ₁ inside the workpiece W. Of the light condensed at the front focal position FS ₁ , the non-reflected light component of the light that does not hit the crack K and is reflected by the back surface of the workpiece W (first back surface reflected light Mt(1)) passes through the FS(+) region after being reflected by the back surface of the workpiece W, passes through the condenser lens 20 again, and is guided to the detection optical system 24 as a convergent light beam. After passing through the relay lens 46, the first back surface reflected light Mt(1) intersects with the main optical axis P in front of (upstream of) the focal position (focal plane FS ₂ ) of the relay lens 46, and passes through the FS(+) region at the focal plane FS _2. Thereafter, the first back surface reflected light Mt(1) proceeds toward the photodetector 26A via the relay lens 48.

A polarizing filter 60 (first polarizer) that transmits only linearly polarized light whose polarization direction is perpendicular to the paper surface is disposed upstream of the photodetector 26A. The first back surface reflected light Mt(1) is linearly polarized light whose polarization direction is perpendicular to the paper surface, so it transmits through the polarizing filter 60 and reaches the photodetector 26A.

Of the first detection light L1A focused at the front focal position FS ₁ , the light that hits the crack K is totally reflected by the crack K. The reflected light component of the light totally reflected by the crack K (first crack reflected light Mr(1)) passes through the FS(-) region after being reflected by the back surface of the workpiece W, passes through the condenser lens 20 again, and is guided to the detection optical system 24 as a convergent light beam. After passing through the relay lens 46, the first crack reflected light Mr(1) intersects with the main optical axis P in front of (upstream of) the focal position (focal plane FS ₂ ) of the relay lens 46, and proceeds toward the FS(-) region of the focal plane FS _2. The first crack reflected light Mr(1) is then reflected by the branching mirror 70 and proceeds toward the photodetector 26B via the relay lens 50.

A polarizing filter 62 (second polarizer) that transmits only linearly polarized light whose polarization direction is parallel to the paper surface is located upstream of the photodetector 26B. The first crack reflected light Mr(1) is linearly polarized light whose polarization direction is perpendicular to the paper surface, so it is blocked by the polarizing filter 62.

Of the first detection light L1A irradiated to the workpiece W, a virtual extension line of the surface reflected light N(1) reflected on the surface of the workpiece W toward the focal plane _FS2 intersects with the focal plane _FS2 in the FS(-) region, so the surface reflected light N(1) behaves as a light beam emitted from the FS(-) region. Therefore, after intersecting with the main optical axis P, the surface reflected light N(1) passes through the condenser lens 20 again and is guided to the detection optical system 24 as a diverging light beam. After passing through the relay lens 46, the surface reflected light N(1) passes toward the FS(-) region at the focal plane _FS2 , passes through the +y side of the main optical axis P, and becomes a light beam that intersects with the main optical axis P farther (downstream) than the focal position (focal plane _FS2 ) of the relay lens 46.

In the example shown in FIG. 2, the first back surface reflected light Mt(1) and the front surface reflected light N(1) pass through the same side at the lens pupil position 20A of the condenser lens 20, and therefore cannot be distinguished from each other at the lens pupil position 20A. However, at the focal plane _FS2 , the first back surface reflected light Mt(1) and the front surface reflected light N(1) pass through regions on opposite sides of the main optical axis P. At the focal plane _FS2 , a branching mirror 70 is disposed on the side where the front surface reflected light N(1) arrives, and the path of the front surface reflected light N(1) to the photodetector 26A is blocked. For this reason, the front surface reflected light N(1) is reflected by the branching mirror 70 and guided by the relay lens 50 to a region on the opposite side of the photodetector 26B with respect to the main optical axis P.

The surface reflected light N(1) is linearly polarized light whose polarization direction is perpendicular to the paper surface, so it is blocked by the polarizing filter 62, just like the first crack reflected light Mr(1).

(Second detection light L1B)
Next, the second detection light L1B (parallel component) will be described with reference to FIG.

As shown in FIG. 3, the second detection light L1B is irradiated onto the workpiece W and is focused by the focusing lens 20 at the front focal position FS ₁ inside the workpiece W. The non-reflected light component (second back-reflected light Mt(2)) of the light focused at the front focal position FS ₁ that is reflected by the back surface of the workpiece W without hitting the crack K passes through the FS(-) region after being reflected by the back surface of the workpiece W, passes through the focusing lens 20 again, and is guided to the detection optical system 24 as a convergent light beam. After passing through the relay lens 46, the second back-reflected light Mt(2) intersects with the main optical axis P just before (upstream of) the focal position (focal plane FS ₂ ) of the relay lens 46, and heads toward the FS(-) region at the focal plane FS _2. The second back-reflected light Mt(2) is reflected by the branching mirror 70 and travels toward the photodetector 26B via the relay lens 50.

The second back surface reflected light Mt(2) is linearly polarized light whose polarization direction is parallel to the paper surface, so it passes through the polarizing filter 62 and reaches the photodetector 26B.

Of the second detection light L1B focused at the front focal position _FS1 , the light that hits the crack K is totally reflected by the crack K. The reflected light component of the light totally reflected by the crack K (second crack reflected light Mr(2)) passes through the FS(+) region after being reflected by the back surface of the workpiece W, passes through the condenser lens 20 again, and is guided to the detection optical system 24 as a convergent light beam. After passing through the relay lens 46, the second crack reflected light Mr(2) intersects with the main optical axis P in front of (upstream of) the focal position (focal plane _FS2 ) of the relay lens 46, and passes through the FS(+) region at the focal plane _FS2 . The second crack reflected light Mr(2) then travels toward the photodetector 26A via the relay lens 48.

The second crack reflected light Mr(2) is linearly polarized light whose polarization direction is parallel to the paper surface, so it is blocked by the polarizing filter 60.

Of the detection light L1 irradiated on the workpiece W, a virtual extension line of the surface reflected light N(2) reflected on the surface of the workpiece W toward the focal plane _FS2 intersects with the focal plane _FS2 in the FS(+) region, so the surface reflected light N(2) behaves as a light beam emitted from the FS(+) region. Therefore, after intersecting with the main optical axis P, the surface reflected light N(2) passes through the condenser lens 20 again and is guided to the detection optical system 24 as a diverging light beam. After passing through the relay lens 46, the surface reflected light N(2) passes toward the FS(+) region at the focal plane _FS2 , passes through the -y side of the main optical axis P, and becomes a light beam that intersects with the main optical axis P farther (downstream) than the focal position (focal plane _FS2 ) of the relay lens 46.

The second crack reflected light Mr(2) and the surface reflected light N(2) pass through opposite sides of the main optical axis P at the lens pupil position 20A of the focusing lens 20, and therefore can be distinguished from each other at the lens pupil position 20A. Then, on the focal plane _FS2 , the second crack reflected light Mr(2) and the surface reflected light N(2) pass through an area on the same side of the main optical axis P. The surface reflected light N(2) is then guided to an area on the opposite side of the photodetector 26A with respect to the main optical axis P.

The surface reflected light N(2) is linearly polarized light whose polarization direction is parallel to the paper surface, so it is blocked by the polarizing filter 60, just like the second crack reflected light Mr(2).

In addition, the closer the focal plane _FS1 is to the back surface of the workpiece W, the closer the point where the first back-surface reflected light Mt(1) and the second back-surface reflected light Mt(2) intersect with the main optical axis P is to the focal plane _FS2 . Therefore, when the focal plane _FS1 that collects the light irradiated to the workpiece W is close to the back surface of the workpiece W, a part of the convergent light beams of the first back-surface reflected light Mt(1) and the second back-surface reflected light Mt(2) may be blocked. Therefore, the branching mirror 70 may be offset to the +y side so as not to block the convergent light beams of the first back-surface reflected light Mt(1) and the second back-surface reflected light Mt(2).

(Crack detection process)
Below, a procedure for detecting the crack depth of a crack K formed inside the workpiece W using the first back-surface reflected light Mt(1) and the second back-surface reflected light Mt(2) will be described.

When no crack K is present at the focal point of the focusing lens 20, as shown in FIG. 2, the first detection light L1A is reflected by the back surface of the workpiece W without hitting a crack, and is then detected by the photodetector 26A as the first back surface reflected light Mt(1). In this case, the signal level of the detection signal detected by the photodetector 26A is maximized.

On the other hand, if a crack K is present at the focal point of the focusing lens 20, as shown in FIG. 2, the first detection light L1A is totally reflected by the crack, and the signal level of the detection signal detected by the photodetector 26A becomes zero.

In addition, when the focal point of the focusing lens 20 coincides with the crack bottom or top end position of the crack K, the first detection light L1A is split into a non-reflected light component that does not hit the crack and is reflected by the back surface of the workpiece W, and a reflected light component that is totally reflected by the crack and then reflected by the back surface of the workpiece W. The non-reflected light component of the first detection light L1A then enters the photodetector 26A along the path shown in FIG. 2. At this time, the signal level of the detection signal detected by the photodetector 26A is lower than when the crack K is not present at the focal point of the focusing lens 20.

Similarly, in the case of the second detection light L1B, the signal level of the detection signal of the second back surface reflected light Mt(2) detected by the photodetector 26B is maximum when no crack K is present at the focal point of the focusing lens 20, decreases when the focal point of the focusing lens 20 coincides with the lower end position or upper end position of the crack K, and becomes zero when a crack K is present at the focal point of the focusing lens 20.

The control unit 30 sequentially acquires detection signals output from the photodetectors 26A and 26B while controlling the focus adjustment mechanism 28 to change the focal point of the focusing lens 20 in the thickness direction (Z direction) of the workpiece W. This makes it possible to detect the crack depth (bottom end position or top end position) of the crack K.

Figure 4 is a graph showing the simulation results of the output from photodetectors 26A and 26B and confocal detector 82.

Figure 4 shows the output from the photodetectors 26A and 26B and the confocal detector 82 when the back surface position of the workpiece W is set to zero and the thickness is set to 400 μm, and the first detection light L1A and the second detection light L1B are irradiated simultaneously.

In FIG. 4, channel A (Ch-A) and channel B (Ch-B) indicate the outputs of photodetectors 26A and 26B, respectively, and Focus (dashed line) indicates the output of the confocal detector 82. Focus (dashed line) will be described later.

As shown in FIG. 4, the first back surface reflected light Mt(1) and the second back surface reflected light Mt(2) detected by the photodetectors 26A and 26B, respectively, are blocked by the crack K and become zero when the focal point is included in the crack range. This makes it possible to detect the depth position of the crack K.

(Interface detection process)
Next, a procedure for detecting the interface (front and back surfaces) of the workpiece W using the first back-surface reflected light Mt(1) and the second back-surface reflected light Mt(2) will be described.

Figure 5 shows the results of a simulation performed by geometrical optical ray tracing of the first back-reflected light Mt(1) and the second back-reflected light Mt(2) obtained from the first detection light L1A and the second detection light L1B, respectively. Figure 5 shows the detection results of the light beams of the first back-reflected light Mt(1) and the second back-reflected light Mt(2) when a photodetector with a virtual surface shape is placed at the position of the confocal pinhole of the pinhole plate 84.

When the focusing point of the focusing lens 20 does not coincide with the front or back surface of the workpiece W, the first back surface reflected light Mt(1) and the second back surface reflected light Mt(2) are detected as two separate beams, as shown by symbol SP1 in FIG. 5(a). In this case, the first back surface reflected light Mt(1) and the second back surface reflected light Mt(2) are blocked by the pinhole plate 84.

On the other hand, when the focusing point of the focusing lens 20 coincides with the front or back surface of the workpiece W, the light beams of the first back surface reflected light Mt(1) and the second back surface reflected light Mt(2) converge on the main optical axis P, as indicated by reference symbol SP2 in FIG. 5(b). In this case, the light beams of the first back surface reflected light Mt(1) and the second back surface reflected light Mt(2) pass through the confocal pinhole of the pinhole plate 84 and are detected by the confocal detector 82. As a result, peaks are obtained at the positions of the back surface and front surface of the workpiece W, as indicated by the Focus (dashed line) in FIG. 4.

The control unit 30 controls the focus adjustment mechanism 28 to change the focal point of the focusing lens 20 in the thickness direction (Z direction) of the workpiece W, while sequentially acquiring detection signals output from the confocal detector 82. This makes it possible to detect the positions of the front and back surfaces of the workpiece W, and to calculate the distance between the front surface of the workpiece W and the top end position of the crack, and between the back surface of the workpiece W and the bottom end position of the crack.

(Crack detection method)
Next, the crack detection method according to the first embodiment will be described with reference to Fig. 6. Fig. 6 is a flowchart showing the crack detection method according to the first embodiment of the present invention.

In this embodiment, the focusing point of the focusing lens 20 is scanned in the thickness direction (Z direction) of the workpiece W at a preset scanning interval Δz, and each scanning position in the scanning range is designated as Z _i (i=1, 2, ..., n) (where n is a natural number).

The scanning range of the focusing point of the focusing lens 20 can be adjusted depending on the purpose of crack detection. For example, when detecting both the front and back surfaces of the workpiece W, the scanning range of the focusing point of the focusing lens 20 can be set to a wide range that includes the front and back surfaces of the workpiece W. When detecting the distance of the crack K from the front or back surface of the workpiece W, the scanning range of the focusing point of the focusing lens 20 can be set to a narrower range on the front or back surface side.

First, when the operation unit issues an instruction to start the crack detection process, the control unit 30 controls the alignment mechanism 29 to move the focusing lens 20 horizontally to perform relative positioning (alignment) between the workpiece W and the focusing lens 20 (step S10: alignment process).

Next, the control unit 30 controls the lens driving unit of the focus adjustment mechanism 28 to set the focal point of the condenser lens 20 to _Zi . For example, when the first scan is performed (step S12: when i=1), the focal point of the condenser lens 20 is set to the scan start position _Z1 . When the second or subsequent scan is performed (when i≧2), the depth position (position in the Z direction) of the focal point of the condenser lens 20 is set to _Zi =Zi _-1 +Δz (step S14: focal point changing step).

Next, the first detection light L1A and the second detection light L1B are emitted to the workpiece W and are focused inside the workpiece W by the focusing lens 20 (step S16: detection light emission process and focusing process). Then, the first back surface reflected light Mt(1) and the second back surface reflected light Mt(2) are detected by the photodetectors 26A and 26B, respectively, and the outputs of the photodetectors 26A and 26B are obtained. Also, the output of the confocal detector 82 is obtained (step S18: first and second light detection processes).

Next, the control unit 30 determines whether the variable i=n (step S20). If i=n is not true (i.e., the variable i is less than n), the process proceeds to step S22, and the processes from step S14 to step S20 are repeated. On the other hand, if i=n, the process ends scanning of the focal point of the focusing lens 20, and proceeds to step S24.

Next, the control unit 30 detects the peak position (see FIG. 4) from the output of the confocal detector 82 acquired in step S18, and detects the positions of the front and back surfaces of the workpiece W (step S24: interface detection process).

The control unit 30 also detects the crack depth of the crack K based on the outputs of the photodetectors 26A and 26B acquired in step S18 (step S26: crack detection step). In step S26, the control unit 30 calculates the average value _dM (=( _dA + _dB )/2) of the crack depths of the crack K detected based on the outputs of the photodetectors 26A and 26B (referred to as the first crack depth _dA and the second crack depth _dB , respectively) as the crack depth (crack upper end position and crack lower end position). This makes it possible to detect the crack depth with high accuracy, for example, even if the formation position of the crack K is shifted from the main optical axis P.

In addition, only one of the crack depths _dA and _dB of the crack K detected based on the outputs of the photodetectors 26A and 26B may be obtained as the crack depth (crack upper end position and crack lower end position).

The control unit 30 also outputs the positions of the front and back surfaces of the workpiece W, as well as the upper and lower end positions of the crack detected as described above, to a display device (monitor) or the like (not shown) as the detection result of the crack depth of the crack K formed inside the workpiece W.

In this embodiment, a fixed branching mirror 70 is used as a light blocking means for blocking the surface reflected lights N(1) and N(2). Therefore, the surface reflected lights N(1) and N(2), which become disturbance light, can be reliably removed without providing a movable part for blocking. This makes it possible to accurately detect the depth of the crack K even if the crack K is located near the surface.

Furthermore, according to this embodiment, by utilizing the polarization effect, it is possible to simultaneously acquire the first back surface reflected light Mt(1) and the second back surface reflected light Mt(2) as well as the detection signal for interface detection by the confocal detector 82 without switching the light source unit 14. This makes it possible to realize high-speed interface and crack detection.

Second Embodiment
Next, a second embodiment of the present invention will be described. In the following description, the same components as those in the first embodiment are denoted by the same reference numerals and the description thereof will be omitted.

Figures 7 and 8 are enlarged views showing the configuration near the photodetectors 26A and 26B, respectively, of the detection optical system 24 of the crack detection device according to the second embodiment of the present invention.

In this embodiment, instead of the polarizing filters 60 and 62, polarizing beam splitters 90 and 92 are disposed upstream of the photodetectors 26A and 26B, respectively. Then, photodetectors 26A and 26D are disposed downstream of the polarizing beam splitter 90, and photodetectors 26B and 26C are disposed downstream of the polarizing beam splitter 92.

The photodetectors 26A-26D (photodetectors 26) are capable of detecting the reflected light of the first detection light L1A and the second detection light L1B from the workpiece W (first back surface reflected light Mt(1), second back surface reflected light Mt(2), first crack reflected light Mr(1), and second crack reflected light Mr(2)) in both cases where the first detection light L1A and the second detection light L1B are reflected or not reflected by a crack K formed inside the workpiece W.

As shown in FIG. 7, in this embodiment, since there is no polarizing filter 60, the second crack reflected light Mr(2) is not blocked. Therefore, in addition to the first back surface reflected light Mt(1) (see FIG. 2), the second crack reflected light Mr(2) (see FIG. 3) also travels downstream. Similarly, as shown in FIG. 8, in addition to the second back surface reflected light Mt(2) (see FIG. 3), the first crack reflected light Mr(1) (see FIG. 2) also travels downstream.

The polarizing beam splitter 90 (first polarizing beam splitter) transmits the parallel component of the light traveling downstream, the second crack reflected light Mr(2), to the photodetector 26D side, and bends the vertical component, the first back surface reflected light Mt(1), at a right angle and guides it to the photodetector 26A.

The polarizing beam splitter 92 (second polarizing beam splitter) transmits the parallel component of the light traveling downstream, the second back surface reflected light Mt(2), to the photodetector 26B side, and bends the vertical component of the first crack reflected light Mr(1) at a right angle and guides it to the photodetector 26C.

This makes it possible to obtain the output of the first crack reflected light Mr(1) and the second crack reflected light Mr(2) in addition to the first back surface reflected light Mt(1) and the second back surface reflected light Mt(2).

In this embodiment, photodetectors 26A and 26B correspond to the first and second detectors, respectively, and photodetectors 26C and 26D correspond to the third and fourth detectors, respectively.

In this embodiment, for example, crack detection is performed using the first back surface reflected light Mt(1) and the second back surface reflected light Mt(2), as well as the first crack reflected light Mr(1) and the second crack reflected light Mr(2).

The first back surface reflected light Mt(1) and the second back surface reflected light Mt(2) are light reflected by the back surface of the workpiece W without hitting the crack K, whereas the first crack reflected light Mr(1) and the second crack reflected light Mr(2) are light reflected by the crack K and then reflected by the back surface of the workpiece W. Therefore, as shown in FIG. 9, the first crack reflected light Mr(1) is a graph that is approximately line-symmetrical with the graph of the first back surface reflected light Mt(1) inverted upside down. Similarly, the second crack reflected light Mr(2) is a graph that is approximately line-symmetrical with the graph of the second back surface reflected light Mt(2) inverted upside down. For simplicity, FIG. 9 only shows the graphs of the first back surface reflected light Mt(1) and the first crack reflected light Mr(1).

In this embodiment, the intersection points P1 and P2 of the graphs of the first back surface reflected light Mt(1) and the first crack reflected light Mr(1) obtained from the first detection light L1A are detected as the crack bottom end position and the crack top end position, respectively.

Similarly, the intersections of the graphs of the second back surface reflected light Mt(2) and the second back surface reflected light/second crack reflected light Mr(2) obtained from the second detection light L1B are detected as the crack bottom end position and the crack top end position, respectively.

Then, the average values of the crack bottom end position and crack top end position obtained from the graph of the first back surface reflected light Mt(1) and the first crack reflected light Mr(1) and the crack bottom end position and crack top end position obtained from the graph of the second back surface reflected light Mt(2) and the second crack reflected light Mr(2) are detected as the crack bottom end position and crack top end position, respectively.

According to this embodiment, in addition to the first back surface reflected light Mt(1) and the second back surface reflected light Mt(2), the first crack reflected light Mr(1) and the second crack reflected light Mr(2) are detected, and the intersection points of the respective graphs are found, making it easier to detect cracks.

Furthermore, according to this embodiment, by utilizing the polarization effect, it is possible to simultaneously acquire the first crack reflected light Mr(1) and the second crack reflected light Mr(2) in addition to the first back surface reflected light Mt(1) and the second back surface reflected light Mt(2) and the detection signal for interface detection by the confocal detector 82 without switching the light source unit 14. This makes it possible to realize high-speed interface and crack detection.

In this embodiment, cube-type polarizing beam splitters 90 and 92 are used, but plate-type polarizing beam splitters can also be used.

The crack bottom end position and crack top end position may also be detected from either the graph of the first back surface reflected light Mt(1) and the first crack reflected light Mr(1) or the graph of the second back surface reflected light Mt(2) and the second crack reflected light Mr(2).

10...crack detection device, 14...light source unit, 16...illumination optical system, 18...dichroic mirror, 20...condenser lens, 22...half mirror, 24...detection optical system, 26...photodetector, 26A-26D...photodetector, 28...focus adjustment mechanism, 29...alignment mechanism, 30...control unit, 32...light source, 34...polarizing beam splitter, 60, 62...polarizing filter, 70...branching mirror, 80...half mirror, 82...confocal detector, 84...pinhole plate, 90, 92...polarizing beam splitter

Claims (5)

a light source unit that emits first detection light and second detection light whose polarization directions are orthogonal to each other, the light source unit that emits the first detection light along a first light source optical axis that is parallel to a main optical axis and eccentric in one direction with respect to the main optical axis, and emits the second detection light along a second light source optical axis that is parallel to the main optical axis and eccentric in another direction opposite to the one direction with respect to the main optical axis;
a condenser lens having a lens optical axis coaxial with the main optical axis and configured to condense the first detection light and the second detection light emitted from the light source unit inside a workpiece;
A branching mirror disposed at a position conjugate with a focusing point of the focusing lens;
a first detector that is disposed downstream along the main optical axis with respect to the splitting mirror and detects a first back-surface reflected light of the first detection light that is not hit by a crack formed inside the workpiece and is reflected by a back surface of the workpiece;
a second detector that is disposed downstream of the optical path branched by the branching mirror and detects a second back-surface reflected light of the second detection light that is not hit by the crack and is reflected by the back surface of the workpiece;
A crack detection means for detecting a crack depth of the crack based on detection signals from the first detector and the second detector;
A crack detection device comprising: a first polarizer disposed downstream of the splitting mirror along the main optical axis, the first polarizer having the same polarization direction as the first detection light;
a second polarizer disposed downstream of the optical path branched by the branching mirror and having the same polarization direction as the second detection light,
The first detector and the second detector are disposed downstream of the first polarizer and the second polarizer, respectively.
The crack detection device of claim 1 . a third detector that is disposed downstream of the optical path branched by the branching mirror and detects a first crack reflected light of the first detection light that is reflected by the back surface of the workpiece after hitting the crack;
a fourth detector that is disposed downstream along the main optical axis with respect to the splitting mirror and detects a second crack reflected light of the second detection light that is reflected by the back surface of the workpiece after hitting the crack;
a first polarizing beam splitter arranged upstream of the first detector and the third detector, guiding the first back surface reflected light to the first detector and guiding the second crack reflected light to the third detector;
a second polarizing beam splitter arranged upstream of the second detector and the fourth detector, guiding the second back surface reflected light to the second detector and guiding the first crack reflected light to the fourth detector;
The crack detection means detects a crack depth of the crack based on detection signals from the first detector, the second detector, the third detector, and the fourth detector.
The crack detection device of claim 1 . a confocal detector capable of detecting the first back-surface reflected light and the second back-surface reflected light only when a focusing point of the focusing lens coincides with the front or back surface of the workpiece;
an interface detection means for detecting the position of a front surface or a back surface of the workpiece based on a detection signal from the confocal detector;
The crack detection device according to claim 1 , further comprising: a detection light emitting step of emitting first detection light and second detection light having mutually orthogonal polarization directions along a first light source optical axis that is parallel to a main optical axis and eccentric in one direction with respect to the main optical axis, and emitting the second detection light along a second light source optical axis that is parallel to the main optical axis and eccentric in another direction opposite to the one direction with respect to the main optical axis;
a focusing step of focusing the first detection light and the second detection light inside a workpiece by a focusing lens having a lens optical axis coaxial with the main optical axis;
a first light detection step of detecting a first back surface reflected light of the first detection light, the first back surface reflected light being reflected by the back surface of the workpiece without hitting a crack formed inside the workpiece, by a first detector arranged downstream along the main optical axis with respect to a branching mirror arranged at a position conjugate with a focusing point of the focusing lens;
a second light detection step of detecting, by a second detector disposed downstream of the optical path branched by the branching mirror, a second back surface reflected light of the second detection light that is not hit by the crack and is reflected by the back surface of the workpiece;
a crack detection step of detecting a crack depth of the crack based on detection signals from the first detector and the second detector;
A crack detection method comprising:

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