Light detection device and method

Abstract

To provide a photodetection device and method capable of detecting back-reflected light reflected from a back surface of a workpiece out of detection light emitted to the workpiece.SOLUTION: A photodetection device (10) includes: a light source section (32) that emits detection light along a light source optical axis that is parallel to a main optical axis and eccentric from the main optical axis; a condenser lens (20) that has a lens optical axis coaxial with the main optical axis and focuses the detection light emitted from the light source into the inside of a workpiece; shading means (60, 70) that is placed at a position conjugate to the focused point of the condenser lens and shades surface-reflected light against the main optical axis by blocking an area on the side where the surface-reflected light reflected from the surface of the workpiece among the detection light reaches; and light detection means (26) that detects back-reflected light reflected from the back surface of the workpiece out of the detection light and generates a detection signal corresponding to the detected light thereof.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a crack detection device and method, and more particularly to a crack detection device and method for detecting a crack formed inside a workpiece. [Background technology]

[0002] Conventionally, there is known a laser processing device (also called a laser dicing device) that irradiates a laser beam along a line to be cut by focusing the beam inside a substrate such as a silicon wafer or a glass wafer (hereinafter referred to as a "workpiece"), and forms a laser processing area that serves as a starting point for cutting inside the workpiece along the line to be cut. The workpiece with the laser processing area formed therein is then cut along the line to be cut by a cutting process such as expanding or breaking, and divided into individual chips.

[0003] Incidentally, when a laser processing area is formed on a workpiece by 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 breaking 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.

[0004] 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 is 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 also possible to correct the processing conditions in the laser processing device by referring to the occurrence status 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.

[0005] On the other hand, evaluation of cracks that have occurred inside a workpiece has traditionally been performed by cutting and polishing the sample, or by observation under limited conditions, making it difficult to apply this method to processing processes using laser processing equipment.

[0006] In response to this, a technique has been proposed for non-destructively inspecting cracks formed inside a workpiece (see, for example, Patent Document 1).

[0007] The technology disclosed in Patent Document 1 involves applying oblique illumination to cracks inside a workpiece, detecting light that is reflected from the back surface of the workpiece without hitting the crack in the area where the crack is formed, and utilizing the reduction in the amount of detected light caused by the incident light being scattered by the crack to inspect the crack. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2017-133997 A Summary of the Invention [Problem to be solved by the invention]

[0009] As shown in Patent Document 1, when detecting light that enters the inside of a workpiece and is reflected from the back surface of the workpiece without hitting a crack, there is a problem that the light reflected from the surface of the workpiece becomes disturbance light (noise).

[0010] Fig. 13 is a cross-sectional view showing the reflected light from the workpiece when the crack inside the workpiece is obliquely illuminated. As shown in Fig. 13, when light is focused on the 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 (detection light) Mt(1) and Mt(2)) is reflected on the back surface side of the workpiece W, 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 incident on 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).

[0011] 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 will be 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), making it impossible to accurately measure the change in the detected light amount of the back surface reflected light Mt(1) and Mt(2).

[0012] Therefore, in Patent Document 1, a field stop (aperture stop) is placed at a position conjugate with the focusing point of the focusing lens to limit the incidence of the surface reflected lights N(1) and N(2) on the photodetector.

[0013] FIG. 13(a) 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. 13(b) shows a case where the condensing position of the detection light is closer to the front surface (+Z side) of the workpiece W. Note that the dashed line L D is a plane showing the center position in the depth direction (Z direction) of the workpiece W. Also, symbol FS 10 and F.S. 12 indicates a focal plane showing the focusing position of the focusing lens.

[0014] As shown in FIG. 13(a), the depth position of the focused position of the detection light is on the plane L D If it is on the back side (-Z side) of the focal plane FS 10 The height of the light beam Mt(1) reflected from the back surface is h Mt(1) is the height of the surface reflected light N(1) h N(1) Therefore, it is possible to separate the two by appropriately adjusting the size of the aperture of the field stop, which is placed at a position conjugate with the focal point of the condenser lens.

[0015] On the other hand, as shown in FIG. 13(b), the depth position of the focused position of the detection light is on the plane L D If it is closer to the surface (+Z side), the focal plane FS 12 In this case, the height of the light beam Mt(2) reflected from the back surface is h Mt(2) is the height of the surface reflected light N(2) h N(2) Therefore, it is difficult to separate the two using a field stop.

[0016] Therefore, when 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.

[0017] The present invention has been made in consideration of the above circumstances, and aims to provide a crack detection device and method that can accurately measure the depth position of a crack regardless of the depth position of the crack. [Means for solving the problem]

[0018] In order to solve the above problems, a crack detection device according to a first aspect of the present invention comprises a light source unit that emits detection light along a light source optical axis that is parallel to a main optical axis and decentered from the main optical axis, a focusing lens having a lens optical axis coaxial with the main optical axis and focusing the detection light emitted from the light source unit inside a workpiece, a shading means that is arranged at a position conjugate with the focusing point of the focusing lens and blocks the surface reflected light by shielding an area on the side where the surface reflected light reflected by the surface of the workpiece reaches, relative to the main optical axis, a light detection means that detects back side reflected light reflected by the back side of the workpiece among the detection light and generates a detection signal corresponding to the detected light, and a crack detection means that detects the crack depth of a crack formed inside the workpiece based on the detection signal.

[0019] A crack detection device according to a second aspect of the present invention is the first aspect, wherein the light blocking means blocks an area at the focusing point position of the focusing lens that is conjugate with the area on the side where the detection light is incident with respect to the main optical axis.

[0020] A crack detection device according to a third aspect of the present invention, in the first or second aspect, includes a light source unit comprising a first light source that emits a first detection light along a first light source optical axis that is parallel to the main optical axis and arranged at a position eccentric in one direction relative to the main optical axis, and a second light source that emits a second detection light along a second light source optical axis that is parallel to the main optical axis and arranged at a position eccentric in the other direction opposite to the main optical axis, the light blocking means is a mirror, and the light detection means comprises a first detector that is arranged rearward along the main optical axis relative to the mirror, and a second detector that is arranged downstream of the optical path branched by the mirror, and the first detector detects back-reflected light of the first detection light that is reflected by the back surface of the workpiece without hitting a crack, and the second detector detects back-reflected light of the second detection light that is reflected by the back surface of the workpiece without hitting a crack.

[0021] A crack detection device according to a fourth aspect of the present invention is similar to the third aspect in that the first detection light and the second detection light emitted from the first light source and the second light source, respectively, are linearly polarized light that are perpendicular to each other, and further includes a first polarizer arranged upstream of the first detector and having the same polarization direction as the first detection light, and a second polarizer arranged upstream of the second detector and having the same polarization direction as the second detection light.

[0022] A crack detection device according to a fifth aspect of the present invention is configured in such a manner that, in the fourth aspect, the first light source and the second light source simultaneously emit first detection light and second detection light, and the first detector and the second detector detect back surface reflected light that is reflected by the back surface of the workpiece without hitting a crack, from the first detection light and the second detection light emitted simultaneously.

[0023] A crack detection method according to a sixth aspect of the present invention includes a detection light emitting step of emitting detection light along the direction of a light source optical axis that is parallel to a main optical axis and decentered from the main optical axis; a focusing step of focusing the detection light inside a workpiece using a focusing lens having a lens optical axis coaxial with the main optical axis; a light detection step of blocking an area on the side where surface reflected light reflected from the surface of the workpiece reaches, relative to the main optical axis, of the detection light, which is positioned conjugate to the focusing point of the focusing lens, thereby blocking the surface reflected light, detecting back surface reflected light reflected from the back surface of the workpiece of the detection light, and generating a detection signal corresponding to the detected light; and a crack detection step of detecting a crack depth of a crack formed inside the workpiece based on the detection signal. Effect of the Invention

[0024] According to the present invention, the shading means can reliably remove surface reflected light that becomes disturbing light, making it possible to accurately detect the depth of a crack even if the crack is located near the surface. [Brief description of the drawings]

[0025] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a crack detection device according to a first embodiment. [Diagram 2]FIG. 2 is a diagram for explaining the light-shielding means (light-shielding plate) according to the first embodiment of the present invention. [Diagram 3] FIG. 3 is a diagram for explaining the light-shielding means (light-shielding plate) according to the first embodiment of the present invention. [Figure 4] FIG. 4 is a flowchart showing a crack detection method according to the first embodiment of the present invention. [Diagram 5] FIG. 5 is a flow chart showing a light detection process according to the first embodiment of the present invention. [Figure 6] FIG. 6 is a diagram for explaining a light blocking means (branching mirror) according to the second embodiment of the present invention. [Figure 7] FIG. 7 is a diagram for explaining a light blocking means (branching mirror) according to the second embodiment of the present invention. [Figure 8] FIG. 8 is a flow chart showing a light detection process according to the second embodiment of the present invention. [Figure 9] FIG. 9 is a diagram showing an optical system of a crack detection device according to the third embodiment of the present invention. [Figure 10] FIG. 10 is a diagram showing an optical system of a crack detection device according to the third embodiment of the present invention. [Figure 11] FIG. 11 is a flowchart showing a crack detection method according to the third embodiment of the present invention. [Figure 12] FIG. 12 is a flow chart showing a light detection process according to the third embodiment of the present invention. [Figure 13] FIG. 13 is a cross-sectional view showing light reflected from a workpiece when a crack inside the workpiece is obliquely illuminated. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of a crack detection device according to the present invention will be described with reference to the accompanying drawings.

[0027] [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 measures the crack depth of a crack K formed inside the workpiece W by irradiating a detection light L1 onto the 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 FIG. 1, in order to avoid complicating the drawing, only components related to the crack detection device necessary for explaining the present invention are shown. In addition, in this embodiment, the crack depth of the crack K is described as indicating the distance from the back surface of the workpiece W to the lower end position or the upper end position of the crack K, but of course, it is not limited to this, and may be the distance from the surface (detection light irradiation surface) of the workpiece W.

[0028] 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).

[0029] The light source unit 14 emits detection light L1 for detecting the depth of a crack K formed inside the workpiece W. When the workpiece W is a silicon wafer, it is preferable to use infrared light having a wavelength of 1000 nm or longer as the detection light L1. The light source unit 14 has a first light source optical axis Q parallel to a main optical axis P, which is coaxial with the lens optical axis of the condenser lens 20, and eccentric in one direction (the lower side in FIG. 1) from the main optical axis P. A and the second light source optical axis Q B That is, the first light source 32A and the second light source 32B emit detection light L1 along the main optical axis P from positions eccentric from the main optical axis P. The first light source 32A and the second light source 32B are connected to the control unit 30, and the control unit 30 controls the emission of the first light source 32A and the second light source 32B.

[0030] 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 first light source 32A and the second light source 32B toward the workpiece W. The pair of relay lenses 40, 42 constitute a telecentric afocal optical system, and project the light source 32 onto the lens pupil position 20A of the condenser lens 20. The field stop 44 is disposed 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, when the detection light L1 emitted from the first light source 32A and the second light source 32B is collimated light (parallel light), the field stop 44 may be omitted.

[0031] The half mirror 22 is disposed between the illumination optical system 16 and the dichroic mirror 18, and transmits a part of the incident light and reflects a part of it. That is, the half mirror 22 transmits a part of the detection light L1 incident from the first light source 32A and the second light source 32B via the illumination optical system 16, and guides the transmitted light (detection light L1) to the condenser lens 20 via the dichroic mirror 18, and also reflects a part 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.

[0032] 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 first light source 32A and the second light source 32B 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 a total reflection mirror may be disposed in place of the dichroic mirror 18.

[0033] The condenser lens 20 is disposed at a position facing the workpiece W, and condenses the detection light L1 incident from the first light source 32A and the second light source 32B via the illumination optical system 16, the half mirror 22, and the dichroic mirror 18 into the workpiece W. The lens optical axis of the condenser lens 20 is coaxial with the main optical axis P. When the detection light L1 is condensed inside the workpiece W by the condenser lens 20, the reflected light L2 from the workpiece W passes through the condenser lens 20 and the dichroic mirror 18, is reflected by the half mirror 22, and is guided to the detection optical system 24.

[0034] The detection optical system 24 is intended to guide the reflected light L2 reflected by the half mirror 22 to the photodetector 26, and is composed of a pair of relay lenses 46, 48 and a light shielding plate 60 that limits the range of the reflected light L2 from the workpiece W.

[0035] The pair of relay lenses 46 and 48 constitute a telecentric afocal optical system, and project the pupil of the condenser lens 20 onto the photodetector 26 .

[0036] The light shielding plate 60 is disposed so as to be at a position conjugate with the focusing point of the focusing lens 20. The light shielding plate 60 functions as a light shielding means for blocking light reflected from the surface (detection light irradiation surface) of the workpiece W and entering the photodetector 26 (see FIG. 2).

[0037] The photodetector 26 is disposed at a position conjugate with the condenser lens pupil 20a of the condenser lens 20, and is provided to detect the reflected light L2 that changes depending on the presence or absence of a crack K formed inside the workpiece W. The photodetector 26 is composed of two photodetectors 26A and 26B. The photodetectors 26A and 26B each receive the reflected light L2 from the workpiece W, generate a detection signal according to the amount of light received, and output the detection signal to the control unit 30. The photodetector 26 may be a split type photodetector having a light receiving surface split into multiple parts, and may be, for example, a two-split photodetector or a four-split photodetector having a light receiving surface (light receiving element) split into two. The photodetector 26 is an example of a light detection means. Instead of the photodetector 26, an infrared camera may be used to capture an image and perform image processing.

[0038] The focus adjustment mechanism 28 is an example of a focal point changing means, and adjusts the focal point of the condenser lens 20 in the thickness direction (Z direction) of the workpiece W. The focus adjustment mechanism 28 includes a lens drive unit (not shown) that moves the condenser lens 20 in a direction along the main optical axis P, and the lens drive unit moves the condenser lens 20 in the direction along the main optical axis P to change the distance between the condenser lens 20 and the workpiece W, thereby changing the focal point of the condenser 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 condenser lens 20.

[0039] In this specification, the focal point of the condenser lens 20 refers to the position of the focal point of the detection light L1 focused by the condenser lens 20. The depth position (Z direction position) of the focal point of the condenser lens 20 is represented by the distance from the back surface of the workpiece W.

[0040] The alignment mechanism 29 is an example of an alignment means, and performs relative positioning (alignment) between the condenser lens 20 and the workpiece W in the horizontal direction (XY direction). The alignment mechanism 29 has a lens drive unit (not shown) that slightly moves the condenser lens 20 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 condenser lens 20 and the workpiece W in the horizontal direction. Instead of the alignment mechanism 29, a stage (not shown) on which the workpiece W is placed may be moved relatively to the condenser lens 20.

[0041] The control unit 30 is composed of a CPU (Central Processing Unit), a 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 condenser 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) of detecting 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. The control unit 30 is an example of a crack detection means.

[0042] (Example of light blocking means) 2 and 3 are diagrams for explaining a light blocking means (light blocking plate) according to the first embodiment of the present invention. In the following explanation, an xyz Cartesian coordinate system is used in which the light traveling direction is the z direction. In addition, in order to simplify the optical path, some optical members such as the dichroic mirror 18 and the focus adjustment mechanism 28 are omitted in Fig. 2 and Fig. 3.

[0043] 2 and 3, the light blocking plate 60 according to this embodiment is disposed at or near a position (focal plane) FS2 that is optically conjugate with a front focal position (focal plane) FS1 of the focusing lens 20. Here, the focal plane FS2 includes a focusing position (focal position) by the relay lens 46 that constitutes the 4F optical system.

[0044] Taking an example where the first light source 32A is used as a light source for crack depth detection, the light blocking plate 60 is disposed so as to block one of the areas (FS(-) area) obtained by dividing the optical path on the focal plane FS2 with respect to the main optical axis P. Here, the FS(-) area is an area conjugate with the area (FS(-) area) on the side where the first detection light L1A from the first light source 32A is incident, at the front focal position (focus point position) FS1 of the focusing lens 20.

[0045] The light blocking plate 60 is movable by an actuator 62. The control unit 30 controls the light emission of the first light source 32A and the second light source 32B, and also controls the actuator 62 to control the position of the light blocking plate 60 in accordance with the detection light of the detection target.

[0046] Although the actuator 62 is used in this embodiment, the position of the light blocking plate 60 may be controlled by using, for example, a magnetic force or an electromagnet.

[0047] Hereinafter, the position control of the detection light and the light shielding plate 60 will be described with reference to Fig. 2 and Fig. 3. Fig. 2 shows a case where, of the first detection light L1A irradiated to the workpiece W, a component reflected by the back surface of the workpiece W without hitting the crack K is detected.

[0048] As shown in FIG. 2, the first detection light L1A emitted from the first light source 32A is irradiated onto the workpiece W and is focused by the focusing lens 20 at the front focal position FS1 inside the workpiece W. Of the light focused at the front focal position FS1, the light reflected by the back surface of the workpiece W without hitting the crack K (back surface reflected light (first back surface reflected light) Mt(1)) is reflected by the back surface (FS(+) area side) 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 first back surface reflected light Mt(1) 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(+) area (the area on the -y side with respect to the main optical axis P) on the focal plane FS2. Thereafter, the first back surface reflected light Mt(1) reaches the photodetector 26A via the relay lens 48.

[0049] On the other hand, of the first detection light L1A irradiated to the workpiece W, surface reflected light N(1) reflected by the surface of the workpiece W (FS(-) area side) intersects with the main optical axis P, then passes through the condenser lens 20 again, and is guided as a diverging light beam to the detection optical system 24. After passing through the relay lens 46, the surface reflected light N(1) travels toward the FS(-) area (area on the +y side with respect to the main optical axis P) on the focal plane FS2, 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.

[0050] As described above, 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 collecting lens 20, and therefore cannot be distinguished from each other at the lens pupil position 20A. However, at the focal planes FS1 and 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. Therefore, by arranging a light shielding plate 60 that shields the side on the focal plane FS2 where the front-surface reflected light N(1) reaches, it is possible to prevent the front-surface reflected light N(1) from reaching the photodetector 26A.

[0051] It should be noted that 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) intersects with the main optical axis P is to the focal plane FS2. For this reason, 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 flux of the first back-surface reflected light Mt(1) may be blocked. Therefore, the light blocking plate 60 may be positioned offset to the +y side so as not to block the convergent light flux of the first back-surface reflected light Mt(1).

[0052] FIG. 3 shows a case where a component of the first detection light L1A irradiated to the workpiece W and totally reflected by a crack K is detected.

[0053] As shown in Fig. 3, the first detection light L1A focused at the front focal position FS1 and the light that hits the crack K is totally reflected by the crack K. The light totally reflected by the crack K (back surface reflected light (second back surface reflected light) Mr(1)) is reflected by the back surface (FS(-) area side) 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. The second back surface reflected light Mr(1) passes through the relay lens 46, 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(-) area on the focal plane FS2. When the second back surface reflected light Mr(1) is detected, the light shielding plate 60 is removed from the optical path (FS(-) area), and the second back surface reflected light Mr(1) reaches the photodetector 26B via the relay lens 48.

[0054] On the other hand, of the first detection light L1A irradiated to the workpiece W, as described with reference to Figure 2, the surface reflected light N(1) 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.

[0055] The second back-surface reflected light Mr(1) and the front-surface reflected light N(1) pass on opposite sides of the main optical axis P in the lens pupil of the collecting lens 20, and can be distinguished from each other at the lens pupil. Then, on the focal planes FS1 and FS2, the second back-surface reflected light Mr(1) and the front-surface reflected light N(1) pass through an area (FS(-) area) on the same side of the main optical axis P to reach the photodetectors 26B and 26A, respectively. Therefore, the second back-surface reflected light Mr(1) and the front-surface reflected light N(1) do not enter the same photodetector 26B.

[0056] As a result, the workpiece W reaches a predetermined depth position Z i By focusing the light at the focusing point, it is possible to detect the first back-surface reflected light Mt(1) that is reflected by the back surface of the workpiece W without hitting the crack K, and the second back-surface reflected light Mr(1) that is reflected by the back surface of the workpiece W after being totally reflected by the crack K.

[0057] When no crack K is present at the focal point of the focusing lens 20, the first detection light L1A does not hit a crack, is reflected by the back surface of the workpiece W, and is then detected by the photodetector 26A, as shown in Fig. 2. On the other hand, when a crack K is present at the focal point of the focusing lens 20, the first detection light L1A is totally reflected by the crack, is reflected by the back surface of the workpiece W, and is then detected by the photodetector 26B, as shown in Fig. 3.

[0058] When the focusing point of the focusing lens 20 coincides with the crack lower end position or crack upper end position of the crack K, the first detection light L1A is divided 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 and the reflected light component are incident on the photodetectors 26A and 26B along the paths shown in FIG. 2 and FIG. 3, respectively. At this time, the levels (detected light amounts) of the detection signals of the non-reflected light component and the reflected light component detected by the photodetectors 26A and 26B are approximately equal. In this embodiment, the crack depth (crack lower end position or crack upper end position) of the crack K formed inside the workpiece W is detected by utilizing the above-mentioned property. In the following description, the non-reflected light component and the reflected light component are also referred to as the first back surface reflected light Mt(1) and the second back surface reflected light Mr(1), respectively.

[0059] Specifically, when the detection signal outputs of the first back surface reflected light Mt(1) and the second back surface reflected light Mr(1) output from the photodetectors 26A and 26B, respectively, are D1 and D2, an evaluation value indicating the relationship between the focusing point of the focusing lens 20 and the crack depth of the crack K can be expressed by equation (1).

[0060] S = (D1 - D2) / (D1 + D2) … (1) In equation (1), when the condition S=0 is satisfied, i.e., when the amount of light received by photodetectors 26A and 26B is the same, this indicates that the focal point of the focusing lens 20 and the bottom end position of the crack (or the top end position of the crack) coincide.

[0061] Therefore, 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 photodetectors 26A and 26B, calculates the evaluation value S shown in equation (1) based on the acquired detection signals, and evaluates this evaluation value S to detect the crack depth (crack lower end position or crack upper end position) of the crack K.

[0062] Furthermore, in this embodiment, the second light source 32B is used as a light source for crack depth detection to detect the back surface reflected light (the first back surface reflected light Mt(2) and the second back surface reflected light Mr(2)). In this case, when detecting the first back surface reflected light Mt(2) reflected by the back surface of the workpiece W without hitting the crack K, the light shielding plate 60 is arranged so as to shield the area (FS(+) area on the opposite side (-y side) from the example shown in FIG. 2) where the front surface reflected light L reaches on the focal plane FS2. On the other hand, when detecting the second back surface reflected light Mr(2) reflected by the back surface of the workpiece W after being totally reflected by the crack K, the light shielding plate 60 is removed from the optical path (FS(+) area). Here, the FS(+) area is an area conjugate with the area (FS(+) area) on the side where the second detection light L1B (not shown) from the second light source 32B is incident at the front focal position (focus point position) FS1 of the focusing lens 20.

[0063] Then, an average value (eg, arithmetic mean) of the crack depths of the cracks K obtained using the first light source 32A and the second light source 32B is calculated (see step S32 in FIG. 4).

[0064] (Crack detection method) Next, a crack detection method according to a first embodiment of the present invention will be described with reference to Figs. 4 and 5. Fig. 4 is a flowchart showing the crack detection method according to the first embodiment of the present invention. In this embodiment, the focus of the condenser 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 referred to 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 is set according to the crack depth of the crack K to be detected, and may be set to the entire range in the thickness direction of the workpiece W, for example, or may be set to a partial range thereof. For example, when detecting only the crack bottom position as the crack depth of the crack K, the detection efficiency can be improved by limiting the scanning range to a partial range in a deep position of the workpiece W (closer to the back surface).

[0065] (Step S10) First, when the operation unit instructs the start of 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 (alignment process).

[0066] (Steps S12 to S14) Next, the control unit 30 sets a variable k to 1 (k=1) and sets a variable i to 1 (i=1).

[0067] (Step S16) Next, the control unit 30 controls the lens driving unit of the focus adjustment mechanism 28 to move the focusing point of the focusing lens 20 to Z i For example, when the first scan is performed (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 (Z direction position) of the focal point of the condenser lens 20 is set to Z i =Z i-1 Set to +Δz (focus point changing step).

[0068] (Step S18) Next, the first back-surface reflected light and the second back-surface reflected light are detected (light detection step). Fig. 5 is a flow chart showing the light detection step according to the first embodiment of the present invention.

[0069] First, the control unit 30 controls the first light source 32A to emit the first detection light L1A from the first light source 32A (step S50: detection light emission process). The first detection light L1A from the first light source 32A is guided to the condenser lens 20 via the illumination optical system 16, the half mirror 22, and the dichroic mirror 18. Then, the first detection light L1A guided to the condenser lens 20 is focused inside the workpiece W by the condenser lens 20 (light focusing process).

[0070] 2, the control unit 30 controls the actuator 62 to insert the light blocking plate 60 into the FS(-) region on the optical path (step S52). This blocks the front surface reflected light N(1), and only the first back surface reflected light Mt(1) is guided to the photodetector 26A via the condenser lens 20, the dichroic mirror 18, the half mirror 22, and the detection optical system 24. The first back surface reflected light Mt(1) is then received by the photodetector 26A, and a detection signal according to the amount of received light is output to the control unit 30 (step S54: first detection step).

[0071] 3, the control unit 30 controls the actuator 62 to move the light blocking plate 60 away from the optical path (FS(-) area) (step S56). Then, the photodetector 26B detects the second rear-surface reflected light Mr(1) (step S58: second detection step).

[0072] In addition, when k=2, using the second light source 32B, the light shielding plate 60 is inserted into the FS(+) area on the optical path when the first back-surface reflected light Mt(2) is detected, and is removed from the optical path (FS(+) area) when the second back-surface reflected light Mr(2) is detected.

[0073] 5, the light shielding plate 60 is inserted and retracted during the execution of the oblique illumination, but the present invention is not limited to this. For example, the light shielding plate 60 may be inserted and retracted each time the oblique illumination is executed.

[0074] As a result, the control unit 30 obtains detection signals D1 and D2 of the first back-surface reflected light Mt(1) and the second back-surface reflected light Mr(1) output from the photodetectors 26A and 26B, respectively.

[0075] (Step S20) Next, the control unit 30 determines the current scanning position (the focal point of the condenser lens 20) Z based on the detection signals D1 and D2 acquired in step S18. i The evaluation value S corresponding to i The evaluation value S i is calculated by the formula (2). The control unit 30 calculates the calculated evaluation value S i is stored in a storage unit (not shown).

[0076] S i =(D1-D2) / (D1+D2) …(2) (Step S22) Next, the control unit 30 judges whether or not the variable i = n. If i = n is not true (i.e., the variable i is less than n), the process proceeds to step S24, and if i = n, the process proceeds to step S26.

[0077] (Step S24) If i=n is not satisfied in step S22, the control unit 30 increments i by 1 (i=i+1), returns to step S14, and repeats the processes from step S14 to step S22.

[0078] (Step S26) If i=n in step S22, the control unit 30 determines the scanning position Z i The evaluation value S calculated for each i Based on this, the crack depth of the crack K formed inside the workpiece W is detected (crack detection process).

[0079] Specifically, the control unit 30 controls each scanning position Z i The evaluation value S corresponding to iAmong these, the scanning position (i.e., the depth position of the focusing point of the focusing lens 20) at which the evaluation value is 0 (zero) is detected as the crack depth (crack bottom end position or crack top end position) of the crack K. i The evaluation value S corresponding to i If the evaluation value that is 0 is not included in the evaluation value, the scanning position corresponding to the evaluation value closest to 0 may be detected as the crack depth of crack K.

[0080] In addition, each scanning position Z i The evaluation value S corresponding to i If there is no evaluation value of 0 in i The evaluation value S corresponding to i The scanning position where the evaluation value becomes 0 may be calculated by performing an interpolation process, and the scanning position may be detected as the crack depth of the crack K.

[0081] Also, at each scanning position Z i The evaluation value S corresponding to i are of the same polarity (positive or negative), it can be determined that no crack K is present in the scanning range of the focusing point of the focusing lens 20, or that crack K is present over the entire range.

[0082] (Step S28) Next, the control unit 30 judges whether or not k = 2. If k = 2 is not true, the process proceeds to step S30, and if k = 2, the process proceeds to step S32.

[0083] (Step S30) If k=2 is not satisfied in step S28, k is incremented by one (k=k+1), the process returns to step S14, and the processes from step S14 to step S26 are repeated using the second light source 32B.

[0084] (Step S32) Next, the control unit 30 calculates the crack depth (first crack depth) d of the crack K detected using the first light source 32A. Aand the crack depth (second crack depth) d of the crack K detected using the second light source 32B. B Average value of d M (=(d A +d B ) / 2) as the true crack depth (bottom end position of the crack). Note that the control unit 30 calculates the true crack depth d M is displayed on a display device (monitor) not shown.

[0085] In this embodiment, a movable light shielding plate 60 is used as a light shielding means for shielding the surface reflected light N(1). This makes it possible to reliably remove the surface reflected light N(1), which becomes disturbance light, so that even if the crack K is located near the surface, it becomes possible to accurately detect the depth of the crack K.

[0086] [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.

[0087] In the first embodiment, the movable light blocking plate 60 is used as the light blocking means, but in the second embodiment, a branching mirror 70 is used instead.

[0088] 6 and 7 are diagrams for explaining a light blocking means (branching mirror) according to a second embodiment of the present invention. Fig. 6 and Fig. 7 show the optical paths of the first detection light L1A and the second detection light L1B emitted from the first light source 32A and the second light source 32B, respectively. In Fig. 6 and Fig. 7, some optical members such as the dichroic mirror 18 and the focus adjustment mechanism 28 are omitted in order to simplify the optical paths.

[0089] 6 and 7, the branching mirror 70 according to this embodiment is disposed at or near a position (focal plane) FS2 that is optically conjugate with a front focal position (focal plane) FS1 of the focusing lens 20. Here, the focal plane FS2 includes a focusing position (focal position) by the relay lens 46 that constitutes the 4F optical system.

[0090] 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 example shown in Figs. 6 and 7. The branching mirror 70 may be arranged, for example, in the FS(+) region on the -y side of the main optical axis P, which is a region slightly downstream of the focal plane FS2. Here, one end of the branching mirror 70 is arranged near the main optical axis P.

[0091] A photodetector 26A (first detector) is disposed on the rear side along the main optical axis P of the branching mirror 70. A relay lens 50 and a photodetector 26C (second detector) are disposed in this order on the downstream side of the optical path branched by the branching mirror 70. The relay lens 50, together with the relay lens 46 and the like, constitute a detection optical system 24 (4F optical system).

[0092] First, the first detection light L1A from the first light source 32A will be described. As shown in Fig. 6, the first detection light L1A emitted from the first light source 32A is irradiated to the workpiece W and is collected by the collecting lens 20 at the front focal position FS1 inside the workpiece W. Of the light collected at the front focal position FS1, 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 (back surface reflected light (first back surface reflected light) Mt(1)) is reflected by the back surface (FS(+) area side) of the workpiece W, passes through the collecting 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 FS2) of the relay lens 46, and passes through the FS(+) area on the focal plane FS2. Thereafter, the first back-surface reflected light Mt(1) reaches the photodetector 26A via the relay lens 48.

[0093] Of the first detection light L1A 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 (rear surface reflected light (second rear surface reflected light) Mr(1)) is reflected by the rear surface (FS(-) area side) 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. The second rear surface reflected light Mr(1) passes through the relay lens 46, intersects with the main optical axis P just before (upstream of) the focal position (focal plane FS2) of the relay lens 46, and heads toward the FS(-) area of ​​the focal plane FS2. The second rear surface reflected light Mr(1) is then reflected by the branching mirror 70 and reaches the photodetector 26C via the relay lens 50.

[0094] Of the first detection light L1A irradiated to the workpiece W, surface reflected light N(1) reflected by the surface of the workpiece W (FS(-) area side) intersects with the main optical axis P, then passes through the condenser lens 20 again, and is guided as a diverging light beam to the detection optical system 24. After passing through the relay lens 46, the surface reflected light N(1) travels toward the FS(-) area 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 of the relay lens 46 (focal plane FS2).

[0095] 6, 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 planes FS1 and 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. Therefore, at the focal plane FS2, the 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. The front-surface reflected light N(1) is reflected by the branching mirror 70 and guided by the relay lens 50 to the opposite side of the photodetector 26C with respect to the main optical axis P.

[0096] It should be noted that 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) intersects with the main optical axis P is to the focal plane FS2. For this reason, 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 flux of the first back-surface reflected light Mt(1) may be blocked. Therefore, the branching mirror 70 may be offset to the +y side so as not to block the convergent light flux of the first back-surface reflected light Mt(1).

[0097] Next, the second detection light L1B from the second light source 32B will be described. As shown in Fig. 7, the second detection light L1B emitted from the second light source 32B is irradiated onto the workpiece W and is focused by the focusing lens 20 at the front focal position FS1 inside the workpiece W. Of the light focused at the front focal position FS1, 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 (back surface reflected light (first back surface reflected light) Mt(2)) is reflected by the back surface (FS(-) area side) 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 first back surface reflected light Mt(2) intersects with the main optical axis P just before (upstream) the focal position (focal plane FS2) of the relay lens 46, and heads toward the FS(-) area on the focal plane FS2. Then, the first back-surface reflected light Mt(2) is reflected by the branching mirror 70, passes through the relay lens 50, and reaches the photodetector 26C.

[0098] 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 (rear surface reflected light (second rear surface reflected light) Mr(2)) is reflected by the rear surface (FS(+) area side) 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 rear surface reflected light Mr(2) intersects with the main optical axis P just before (upstream of) the focal position (focal plane FS2) of the relay lens 46, and passes through the FS(+) area on the focal plane FS2. The second rear surface reflected light Mr(2) then reaches the photodetector 26A via the relay lens 48.

[0099] Of the detection light L1 irradiated onto the workpiece W, surface reflected light N(2) reflected by the surface of the workpiece W (FS(+) area side) intersects with the main optical axis P, then passes through the condenser lens 20 again, and is guided as a diverging light beam to the detection optical system 24. After passing through the relay lens 46, the surface reflected light N(2) travels toward the FS(+) area 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 of the relay lens 46 (focal plane FS2).

[0100] The second back-surface reflected light Mr(2) and the front-surface reflected light N(2) pass through opposite sides of the main optical axis P at the lens pupil position 20A of the collecting lens 20, and therefore can be distinguished from each other at the lens pupil position 20A. Then, at the focal planes FS1 and FS2, the second back-surface reflected light Mr(2) and the front-surface reflected light N(2) pass through areas on the same side of the main optical axis P. Thereafter, the front-surface reflected light N(2) reaches an area on the opposite side of the photodetector 26A with respect to the main optical axis P. Therefore, the second back-surface reflected light Mr(2) and the front-surface reflected light N(2) do not enter the same photodetector 26A.

[0101] Next, a crack detection method according to this embodiment will be described. The crack detection method according to this embodiment differs from the first embodiment in the light detection step (step S18) shown in Fig. 4. Therefore, only the light detection step according to this embodiment will be described, and descriptions of the other steps will be omitted.

[0102] Fig. 8 is a flowchart showing a light detection process according to the second embodiment of the present invention. As shown in Fig. 8, the control unit 30 controls the first light source 32A (when k = 1; see Fig. 4) to emit the detection light L1 from the first light source 32A (step S60: detection light emission process). Then, as shown in Fig. 6, the photodetectors 26A and 26C detect the first back surface reflected light Mt(1) and the second back surface reflected light Mr(1), respectively (step S62: detection process). On the other hand, when k = 2, as shown in Fig. 7, the photodetectors 26C and 26A detect the first back surface reflected light Mt(2) and the second back surface reflected light Mr(2), respectively.

[0103] In this embodiment, a fixed branching mirror 70 is used as a shading means for shading the surface reflected light N(1), so that the surface reflected light N(1), which becomes disturbance light, can be reliably removed without providing a means for moving the shading means.

[0104] In the first and second embodiments, the crack depth of the crack K is detected using detection light from two light sources (the first light source 32A and the second light source 32B) and the average value of the depth is calculated, but the present invention is not limited to this. For example, the crack depth of the crack K may be detected using detection light from only one light source.

[0105] [Variations] In the first and second embodiments, the evaluation value S (see equations (1) and (2)) is calculated using the non-reflected light components (first back-surface reflected lights Mt(1) and Mt(2)) that do not hit the crack K and are reflected on the back surface of the workpiece W, and the reflected light components (second back-surface reflected lights Mr(1) and Mr(2)) that are totally reflected on the crack K, but the present invention is not limited to this. For example, if the surface of the crack K is rough, the measurement accuracy of the reflected light components may decrease. For this reason, it is considered to perform crack detection using only the non-reflected light components.

[0106] In this case, the scanning position (focus point) Z of the condenser lens 20 i For each, the output D of the detection signal of the non-reflected light component (the first rear surface reflected light Mt(1) and Mt(2)) i The scanning position Z of the condenser lens 20 is stored. i When the detection light L1 (L1A and L1B) is moved from the back side or the front side, the output of the non-reflected light component that passes through the area without the crack K becomes maximum, and when the light beam of the detection light L1 (L1A and L1B) hits the crack K, the output of the non-reflected light component gradually decreases. Therefore, for example, the maximum value D of the detection signal output max Depth Z is half of i is detected as the bottom end position or the top end position of the crack.

[0107] In addition, each scanning position Z i Output D corresponding to i D in i =D max If the output for Z / 2 is not included, i Output D corresponding to i Z i By performing an interpolation process as a function of D i =D max Scan position Z = / 2 i may be calculated and the scanning position may be detected as the crack depth of the crack K.

[0108] In the first embodiment, when crack detection is performed using only the non-reflected light component, there is no need to retract the light shielding plate 60 in order to detect the reflected light component (the second rear surface reflected light Mr(1)), and steps S56 and S58 in Fig. 5 can be omitted (see Fig. 3). Furthermore, in this case, when crack detection is performed using only the first light source 32A without the second light source 32B, the light shielding plate 60 can be fixed, and the actuator 62 and photodetector 26B are not required. Furthermore, when the first light source 32A is omitted and only the second light source 32B is used, the photodetector 26A can be omitted.

[0109] In the second embodiment, when crack detection is performed using only the non-reflected light components, detection of the reflected light components (second rear surface reflected light Mr(1) and Mr(2)) can be omitted. Furthermore, in this case, when the first light source 32A is omitted and only the second light source 32B is used, the photodetector 26A can be omitted.

[0110] [Third embodiment] Next, a third embodiment of the present invention will be described. In the following description, the same components as those in the first and second embodiments are denoted by the same reference numerals and the description thereof will be omitted.

[0111] In the first and second embodiments, the first light source 32A and the second light source 32B perform oblique illumination separately, but in this embodiment, by using polarizers 80 and 82, the first light source 32A and the second light source 32B perform oblique illumination simultaneously.

[0112] 9 and 10 are diagrams showing the optical system of a crack detection device according to a third embodiment of the present invention. FIG. 9 and FIG. 10 show the optical paths of the first detection light L1A and the second detection light L1B emitted from the first light source 32A and the second light source 32B, respectively. In FIG. 9 and FIG. 10, in order to simplify the optical paths, some optical members such as the first light source 32A, the second light source 32B, the half mirror 22, the relay lenses 40 and 42, the dichroic mirror 18, and the focus adjustment mechanism 28 are omitted.

[0113] In this embodiment, the first detection light L1A and the second detection light L1B emitted from the first light source 32A and the second light source 32B, respectively, are linearly polarized light that is orthogonal to each other. In the following description, the polarization direction of the first detection light L1A emitted from the first light source 32A is perpendicular to the paper surface, and the polarization direction of the second detection light L1B emitted from the second light source 32B is parallel to the paper surface.

[0114] 9 and 10, in the crack detection device of this embodiment, a first polarizer 80 and a second polarizer 82 are disposed upstream of the photodetectors 26A and 26C, respectively. The polarization direction of the first polarizer 80 is perpendicular to the plane of the paper, and the polarization direction of the polarizer 82 is parallel to the plane of the paper.

[0115] In addition, in Figs. 9 and 10, the first detection light L1A, the second detection light L1B, the first polarizer 80, and the second polarizer 82 are marked with arrows or arrowheads indicating their respective polarization directions.

[0116] First, the first detection light L1A from the first light source 32A will be described. As shown in Fig. 9, the first detection light L1A emitted from the first light source 32A is irradiated to the workpiece W and is collected by the collecting lens 20 at the front focal position FS1 inside the workpiece W. Of the light collected at the front focal position FS1, 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 (back surface reflected light (first back surface reflected light) Mt(1)) is reflected by the back surface (FS(+) area side) of the workpiece W, passes through the collecting 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 FS2) of the relay lens 46, and passes through the FS(+) area on the focal plane FS2. Thereafter, the first back surface reflected light Mt(1) travels via the relay lens 48 toward the photodetector 26A.

[0117] A first polarizer 80 whose polarization direction is perpendicular to the plane of the paper is disposed between the relay lens 48 and the photodetector 26A (upstream of the photodetector 26A). Since the polarization direction of the first back surface reflected light Mt(1) is also perpendicular to the plane of the paper, the first back surface reflected light Mt(1) passes through the first polarizer 80 and reaches the photodetector 26A.

[0118] Of the first detection light L1A 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 (rear surface reflected light (second rear surface reflected light) Mr(1)) is reflected by the rear surface (FS(-) area side) 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. The second rear surface reflected light Mr(1) passes through the relay lens 46, intersects with the main optical axis P just before (upstream of) the focal position (focal plane FS2) of the relay lens 46, and heads toward the FS(-) area of ​​the focal plane FS2. The second rear surface reflected light Mr(1) is then reflected by the branching mirror 70 and heads toward the photodetector 26C via the relay lens 50.

[0119] A polarizer 82, whose polarization direction is parallel to the plane of the paper, is disposed between the relay lens 50 and the photodetector 26C (upstream of the photodetector 26C). Since the polarization direction of the second back-surface reflected light Mr(1) is perpendicular to the plane of the paper, the second back-surface reflected light Mr(1) is blocked by the polarizer 82.

[0120] Of the first detection light L1A irradiated to the workpiece W, surface reflected light N(1) reflected by the surface of the workpiece W (FS(-) area side) intersects with the main optical axis P, then passes through the condenser lens 20 again, and is guided as a diverging light beam to the detection optical system 24. After passing through the relay lens 46, the surface reflected light N(1) travels toward the FS(-) area 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 of the relay lens 46 (focal plane FS2).

[0121] 9, a branching mirror 70 is disposed on the side of the focal plane FS2 where the surface reflected light N(1) reaches, and the surface reflected light N(1) is reflected by the branching mirror 70 and guided by the relay lens 50 to the opposite side of the photodetector 26C with respect to the main optical axis P. The surface reflected light N(1) is also blocked by the polarizer 82.

[0122] Next, the second detection light L1B from the second light source 32B will be described. As shown in Fig. 10, the second detection light L1B emitted from the second light source 32B is irradiated onto the workpiece W and is focused by the focusing lens 20 at the front focal position FS1 inside the workpiece W. Of the light focused at the front focal position FS1, 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 (back surface reflected light (first back surface reflected light) Mt(2)) is reflected by the back surface (FS(-) area side) 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 first back surface reflected light Mt(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 heads toward the FS(-) area on the focal plane FS2. Then, the first back-surface reflected light Mt(2) is reflected by the branching mirror 70 and travels via the relay lens 50 toward the photodetector 26C.

[0123] The polarization direction of the first back-surface reflected light Mt(2) is parallel to the plane of the paper, just like the polarizer 82, so the first back-surface reflected light Mt(2) passes through the polarizer 82 and reaches the photodetector 26C.

[0124] 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 (rear surface reflected light (second rear surface reflected light) Mr(2)) is reflected by the rear surface (FS(+) area side) 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 rear surface reflected light Mr(2) intersects with the main optical axis P just before (upstream of) the focal position (focal plane FS2) of the relay lens 46, and passes through the FS(+) area on the focal plane FS2. The second rear surface reflected light Mr(2) then travels toward the photodetector 26A via the relay lens 48.

[0125] The polarization direction of the second back-surface reflected light Mr(2) is parallel to the plane of the paper, whereas the polarization direction of the first polarizer 80 is perpendicular to the plane of the paper, so the second back-surface reflected light Mr(2) is blocked by the first polarizer 80.

[0126] Of the detection light L1 irradiated to the workpiece W, the surface reflected light N(2) reflected by the surface (FS(+) area side) of the workpiece W crosses the main optical axis P, then passes through the condenser lens 20 again, and is guided to the detection optical system 24 as a diverging light beam. The surface reflected light N(2) passes through the relay lens 46, then travels toward the FS(+) area on the focal plane FS2, and becomes a light beam that crosses the main optical axis P farther (downstream) than the focal position (focal plane FS2) of the relay lens 46. The surface reflected light N(2) then reaches 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 also blocked by the polarizer 80. Therefore, the second back surface reflected light Mr(2) and the surface reflected light N(2) do not enter the same photodetector 26A.

[0127] As described above, in this embodiment, the first detection light L1A and the second detection light L1B are linearly polarized light that is orthogonal to each other, and the first polarizer 80 and the second polarizer 82, which have the same polarization direction as the first detection light L1A and the second detection light L1B, are arranged upstream of the photodetectors 26A and 26C. Therefore, as shown in FIG. 9, the non-reflected light component (first back-surface reflected light Mt(1)) of the first detection light L1A that is reflected by the back surface of the workpiece W without hitting the crack K passes through the first polarizer 80 and reaches the photodetector 26A. On the other hand, as shown in FIG. 10, the reflected light component (second back-surface reflected light Mr(2)) of the second detection light L1B that is totally reflected by the crack K is blocked by the first polarizer 80. 10, the non-reflected light component (first back-surface reflected light Mt(2)) of the second detection light L1B that does not hit the crack K and is reflected by the back surface of the workpiece W passes through the second polarizer 82 and reaches the photodetector 26C. On the other hand, the reflected light component (second back-surface reflected light Mr(1)) of the first detection light L1A that is totally reflected by the crack K is blocked by the second polarizer 82.

[0128] That is, in this embodiment, it is possible to block the reflected light components (second back-surface reflected lights Mr(2) and Mr(1)) heading toward the photodetectors 26A and 26C. Therefore, according to this embodiment, even if the first detection light L1A and the second detection light L1B are emitted simultaneously from the first light source 32A and the second light source 32B, respectively, it is possible to simultaneously detect the non-reflected light components (first back-surface reflected lights Mt(1) and Mt(2)).

[0129] The control unit 30 determines the scanning position (focus point) Z of the focusing lens 20. i For each, a detection signal D of the non-reflected light component (first rear surface reflected light Mt(1) and Mt(2)) is i (1) and D i (2) is stored. Then, as in the modified example, the detection signal D i (1) and D i (2) is the maximum value of the detection signal output D max Depth Z is half of i is detected as the bottom end position or the top end position of the crack.

[0130] Next, a crack detection method according to a third embodiment will be described with reference to Fig. 11 and Fig. 12. Fig. 11 is a flowchart showing the crack detection method according to the third embodiment of the present invention.

[0131] (Step S70) First, when the operation unit instructs the start of 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 (alignment process).

[0132] (Step S72) Next, the control unit 30 sets a variable i to 1 (i=1).

[0133] (Step S74) Next, the control unit 30 controls the lens driving unit of the focus adjustment mechanism 28 to move the focusing point of the focusing lens 20 to Z i For example, when the first scan is performed (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 (Z direction position) of the focal point of the condenser lens 20 is set to Z i =Z i-1 Set to +Δz (focus point changing step).

[0134] (Step S76) Next, the first back-surface reflected light and the second back-surface reflected light are detected (light detection step). Fig. 12 is a flow chart showing the light detection step according to the third embodiment of the present invention.

[0135] First, the control unit 30 controls the first light source 32A and the second light source 32B to emit the first detection light L1A and the second detection light L1B, respectively (step S70: detection light emission step). Then, as shown in Fig. 9 and Fig. 10, the control unit 30 detects the first back-surface reflected light Mt(1) and Mt(2) by the photodetectors 26A and 26C, respectively (step S92).

[0136] (Step S78) Next, the control unit 30 judges whether or not the variable i = n. If i = n is not true (i.e., the variable i is less than n), the process proceeds to step S80, and if i = n, the process proceeds to step S82.

[0137] (Step S80) If i=n is not satisfied in step S78, the control unit 30 increments i by 1 (i=i+1), returns to step S74, and repeats the processes from step S74 to step S80.

[0138] (Step S82) If i=n in step S78, the control unit 30 determines the scanning position Z i The detection signal D calculated every i (1) and D i Based on (2), the depth of the crack K formed inside the workpiece W is detected (crack detection process). i (1) and D i (2) is the maximum value of the detection signal output D max Depth d is half of A and d B is detected as the bottom end position or the top end position of the crack.

[0139] (Step S84) Next, the control unit 30 calculates the crack depth (first crack depth) d of the crack K detected using the first light source 32A. A and the crack depth (second crack depth) d of the crack K detected using the second light source 32B. B Average value of d M (=(d A +d B ) / 2) as the true crack depth (bottom end position of the crack). Note that the control unit 30 calculates the true crack depth d M is displayed on a display device (monitor) not shown.

[0140] According to this embodiment, the first detection light L1A and the second detection light L1B can be simultaneously emitted from the first light source 32A and the second light source 32B, respectively, and there is no need to switch between the first light source 32A and the second light source 32B. This makes it possible to shorten the time required for crack detection.

[0141] In this embodiment, by reversing the polarization directions of the first polarizer 80 and the second polarizer 82, it is also possible to detect the crack depth using the reflected light components (second back-surface reflected lights Mr(1) and Mr(2)) that are totally reflected by the crack K. However, as described above, the second back-surface reflected lights Mr(1) and Mr(2) are affected by the roughness of the surface of the crack K, so it is preferable to use the first back-surface reflected lights Mt(1) and Mt(2), which are non-reflected light components.

[0142] Also, a polarizing beam splitter may be used instead of the first polarizer 80 and the second polarizer 82. In this case, in addition to the photodetectors 26A and 26C, a photodetector for detecting the second back surface reflected light Mr(1) and Mr(2) is provided. This makes it possible to simultaneously detect the second back surface reflected light Mr(1) and Mr(2) in addition to the first back surface reflected light Mt(1) and Mt(2). In this case, the crack depth of the crack K can be detected using the detection signals of the second back surface reflected light Mr(1) and Mr(2) in addition to the first back surface reflected light Mt(1) and Mt(2). As specific methods for calculating the crack depth, for example, the following [A] and [B] are considered.

[0143] [A] As in steps S82 and S84 in Fig. 11, the depth at which the detection signals of the second back-surface reflected lights Mr(1) and Mr(2) are half the maximum detection signal outputs is detected as the crack depth. Then, the average value of the four crack depths detected from the first back-surface reflected lights Mt(1) and Mt(2) and the second back-surface reflected lights Mr(1) and Mr(2) is calculated as the true crack depth.

[0144] [B] The depth at which the first back surface reflected light Mt(1) and the second back surface reflected light Mr(1) obtained using the first light source 32A are equal is detected as the first crack depth. Also, the depth at which the first back surface reflected light Mt(2) and the second back surface reflected light Mr(2) obtained using the second light source 32B are equal is detected as the second crack depth. Then, the average value of the first crack depth and the second crack depth is calculated as the true crack depth. [Explanation of symbols]

[0145] 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, 28...focus adjustment mechanism, 29...alignment mechanism, 30...control unit, 32A...first light source, 32B...second light source, 60...light shielding plate, 70...branching mirror, 80...first polarizer, 82...second polarizer

Claims

1. a light source unit that emits detection light along a light source optical axis that is parallel to a main optical axis and decentered from the main optical axis; a condenser lens having a lens optical axis coaxial with the main optical axis and configured to condense the detection light emitted from the light source unit inside the workpiece; a light shielding means arranged at a position conjugate with a focusing point of the focusing lens, and shielding an area on a side of the main optical axis where surface reflected light reflected by the surface of the workpiece reaches, thereby shielding the surface reflected light; a light detection means for detecting back-reflected light reflected by the back surface of the workpiece out of the detection light, and generating a detection signal corresponding to the detected light; The light source unit includes: a first light source that emits a first detection light along a first light source optical axis that is parallel to the main optical axis and is disposed at a position that is eccentric in one direction with respect to the main optical axis; a second light source that emits a second detection light along a second light source optical axis that is parallel to the main optical axis and is disposed at a position that is eccentric in another direction opposite to the one direction with respect to the main optical axis, the light blocking means is a mirror, the light detection means includes a first detector disposed on the rear side of the mirror along the main optical axis, and a second detector disposed on the downstream side of the optical path branched by the mirror, The first detector detects back-reflected light of the first detection light that is not reflected inside the workpiece and is reflected by the back surface of the workpiece, The second detector is a light detection device that detects back-reflected light of the second detection light that is not reflected inside the workpiece and is reflected by the back surface of the workpiece.

2. the light blocking means blocks an area conjugate with an area on a side where the detection light is incident with respect to the main optical axis at a light focusing point position of the focusing lens, 2. The optical detection device according to claim 1.

3. the first detection light and the second detection light emitted from the first light source and the second light source, respectively, are linearly polarized lights orthogonal to each other; a first polarizer disposed upstream of the first detector and having the same polarization direction as the first detection light; a second polarizer disposed upstream of the second detector and having the same polarization direction as the second detection light; The optical detection device according to claim 1 or 2, further comprising:

4. the first light source and the second light source simultaneously emit the first detection light and the second detection light, The first detector and the second detector detect back-surface reflected light that is not reflected inside the workpiece and is reflected by the back surface of the workpiece, out of the first detection light and the second detection light that are simultaneously emitted.

4. The optical detection device according to claim 3.

5. a detection light emitting step of emitting a first detection light along a direction of a first light source optical axis that is parallel to a main optical axis and disposed at a position eccentric in one direction from 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 disposed at a position eccentric in another direction opposite to the one direction from 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 mirror that is arranged at a position conjugate with a focusing point of the focusing lens and that blocks an area on a side where surface reflected light of the first detection light and the second detection light reflected by the surface of the workpiece reaches with respect to the main optical axis, and blocks the surface reflected light; a first detector disposed on the rear side of the mirror along the main optical axis detects back-reflected light of the first detection light that is not reflected inside the workpiece but is reflected by the back surface of the workpiece; a second detector disposed downstream of the optical path branched by the mirror detects back-surface reflected light of the second detection light that is not reflected inside the workpiece but is reflected by the back surface of the workpiece; a light detection step of generating a detection signal corresponding to the detected light; A light detection method comprising:

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