Crack detection device

Abstract

To provide a crack detector with which it is possible to non-destructively and accurately detect the depth of a crack that is formed in the inside of a workpiece.SOLUTION: A crack detector (10) comprises: a light source unit (100); an objective lens (504); interface detection means (300, 500) for irradiating a workpiece with first detection light and detecting first reflected light from the workpiece, and detecting on the basis of a detection signal that corresponds to the first reflected light an interface position that indicates the surface or reverse side of the workpiece; and crack detection means (400, 500) for illuminating, by eccentric irradiation, the workpiece with second detection light which is eccentric from the main optical axis and the effective refractive index of which is adjusted in accordance with effective refractive index of first detection light to the workpiece, detecting second reflected light from the workpiece, and detecting on the basis of a detection signal corresponding to second reflected light the crack depth, as referenced to the interface position, of a crack formed in the inside of the workpiece.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a crack detection device for detecting the crack depth of a crack formed inside a workpiece. [Background technology]

[0002] Conventionally, laser processing equipment is known that focuses a laser beam onto the inside of a workpiece such as a silicon wafer, irradiating it along the planned cutting line to form a laser processing region inside the workpiece along the processing line, which serves as the starting point for cutting. The workpiece, with the laser processing region formed, is then divided into individual chips by a cleavage process such as expansion or breaking along the planned cutting line. With such laser processing equipment, a laser processing region is formed inside the workpiece, and the workpiece is divided along the planned cutting line starting from that laser processing region. This offers advantages such as lower dust generation and a reduced possibility of dicing scratches, chipping, or cracks on the material surface compared to general dicing equipment that cuts and divides the workpiece using a blade.

[0003] Incidentally, when a laser processing device forms a laser-processed area on a workpiece, cracks extend from that laser-processed area in the thickness direction of the workpiece. In order to properly separate the workpiece into chips during the cleavage process, it is important to properly form the laser-processed area that serves as the starting point for separating the workpiece, and to accurately detect the crack depth of the cracks formed inside the workpiece.

[0004] Therefore, after forming the laser processing area with the laser processing device, before the splitting process, it is possible to accurately predict whether the laser processing area, which will serve as the starting point for splitting the workpiece, has been properly formed, that is, by detecting the crack depth of the cracks formed inside the workpiece. If there are areas inside the workpiece where the laser processing area has not been properly formed, it is possible to reprocess only those areas with the laser processing device or to change the splitting method in the splitting process. This eliminates the loss of chips in the subsequent splitting process. In addition, the processing conditions in the laser processing device can be modified based on the occurrence of defective areas, thereby reducing the occurrence of defective areas in the laser processing area of ​​workpieces processed thereafter. When reprocessing the laser processing area of ​​a defective area, the time loss required for reprocessing can also be reduced by reducing the number of defective areas.

[0005] On the other hand, evaluating cracks that occur inside a workpiece has traditionally involved cutting and polishing the sample or observing it under limited conditions. Therefore, it has been difficult to apply this method to processing processes using laser processing equipment.

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

[0007] In the technology disclosed in Patent Document 1, light is incident on a crack from one side, the light transmitted through the region containing the crack is detected, and the crack is inspected by utilizing the decrease in the amount of detected light due to scattering by the crack. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2008-222517 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] However, in the technology disclosed in Patent Document 1, the signal level of the photodetector does not directly indicate the depth of the crack, but is treated as a threshold. Therefore, it was necessary to conduct experiments in advance to set the threshold. Furthermore, in order to measure the crack length (crack depth), it was necessary to set multiple thresholds in advance according to the crack length through experiments. In addition, since the crack leading edge position is confirmed by using only the decrease in transmitted light (light that has passed through the region containing the crack), there are limitations to improving the accuracy of crack depth detection.

[0010] This invention has been made in view of these circumstances, and aims to provide a crack detection device that can non-destructively and accurately detect the crack depth of a crack formed inside a workpiece. [Means for solving the problem]

[0011] To solve the above problems, a crack detection device according to a first aspect of the present invention includes a light source unit that emits detection light along a principal optical axis, an objective lens having a lens optical axis coaxial with the principal optical axis and focusing the detection light emitted from the light source unit onto a workpiece, an interface detection means that irradiates the workpiece with a first detection light emitted along the principal optical axis to detect a first reflected light from the workpiece and detects an interface position indicating the front or back surface of the workpiece based on a detection signal corresponding to the first reflected light, and a crack detection means that eccentrically illuminates the workpiece with a second detection light eccentric from the principal optical axis, the amount of eccentricity from the lens optical axis adjusted according to the effective refractive index of the workpiece determined by the first detection light, detects a second reflected light from the workpiece, and detects the crack depth of a crack formed inside the workpiece with respect to the interface position based on a detection signal corresponding to the second reflected light.

[0012] A crack detection device according to a second aspect of the present invention, in the first aspect, has a refractive index of the workpiece n and an effective refractive index of the workpiece determined by the first detection light n eff When the numerical aperture of the second detection light is denoted as NA,

[0013] [Number]

[0014] The numerical aperture NA of the second detection light is adjusted so as to satisfy [Advantages of the Invention]

[0015] According to the present invention, it is possible to nondestructively and accurately detect the crack depth of cracks formed inside a workpiece. [Brief Description of the Drawings]

[0016] [Figure 1] FIG. 1 is a block diagram showing a crack detection device according to an embodiment of the present invention. [Figure 2] FIG. 2 is an explanatory diagram showing a state when deflection illumination of detection light is performed on a workpiece. [Figure 3] FIG. 3 is an explanatory diagram showing a state when deflection illumination of detection light is performed on a workpiece. [Figure 4] FIG. 4 is an explanatory diagram showing a state when deflection illumination of detection light is performed on a workpiece. [Figure 5] FIG. 5 is a diagram showing the state of reflected light received by a photodetector (corresponding to FIG. 2). [Figure 6] FIG. 6 is a diagram showing the state of reflected light received by a photodetector (corresponding to FIG. 3). [Figure 7] FIG. 7 is a diagram showing the state of reflected light received by a photodetector (corresponding to FIG. 4). [Figure 8] FIG. 8 is a diagram for explaining the path through which reflected light from a workpiece reaches an objective lens pupil. [Figure 9] FIG. 9 is a diagram showing an example in which an aperture for crack detection and an aperture for interface detection are used in common. [Figure 10] FIG. 10 is a diagram showing the optical path of detection light incident on a workpiece. [Figure 11]Figure 11 shows the relationship between the pupil of the objective lens and the light beam of the detection light used for crack detection. [Modes for carrying out the invention]

[0017] Hereinafter, embodiments of the crack detection device according to the present invention will be described with reference to the attached drawings.

[0018] [Crack detection device] Figure 1 is a block diagram showing a crack detection device according to one embodiment of the present invention.

[0019] The crack detection device 10 according to this embodiment detects the crack depth of a crack K formed inside a workpiece W, such as a silicon wafer, by irradiating the workpiece W with detection light L1 and detecting the reflected light L2 from the workpiece W. The crack detection device 10 is used in combination with a laser processing device (not shown) that forms a laser processing area inside the workpiece W, but in the following description, the components of the crack detection device 10 will be described, and the configuration of the laser processing device will be omitted.

[0020] In the following explanation, we use a three-dimensional Cartesian coordinate system in which the stage 510 on which the workpiece W is placed is a plane parallel to the XY plane, and the Z direction is the depth direction of the workpiece W.

[0021] As shown in Figure 1, the crack detection device 10 according to this embodiment includes a light source unit 100, an illumination optical system 200, an interface detection optical system 300, a crack detection optical system 400, a control unit 500, a focus adjustment mechanism 502, an objective lens 504, an operation unit 506, and a display unit 508.

[0022] The light source unit 100 emits detection light L1, which is used to detect the interface of the workpiece W and to detect cracks K formed inside the workpiece W. Here, if the workpiece W is a silicon workpiece, it is desirable to use infrared light with a wavelength of 1,000 nm or more as the detection light L1.

[0023] The light source unit 100 includes light sources 102A, 102B, and 102C, and a half mirror 104. The light sources 102A, 102B, and 102C, and the half mirror 104 are arranged along the principal optical axis AX, which is coaxial with the lens optical axis of the objective lens 504.

[0024] Light sources 102A, 102B, and 102C emit detection light L1 along the principal optical axis AX. Light sources 102A, 102B, and 102C can be, for example, laser light sources (infrared laser light sources, laser diodes) or LED (Light Emitting Diode) light sources.

[0025] The light source 102A has a laser aperture capable of illuminating almost the entire surface of the pupil 504a of the objective lens 504. The light source 102A is used for interface detection, which will be described later.

[0026] Light sources 102B and 102C each have a laser aperture capable of illuminating only a portion of the objective lens pupil 504a of the objective lens 504 that is eccentric from the principal optical axis AX (lens optical axis). Light sources 102B and 102C are used for crack detection, as described later.

[0027] The half-mirror 104 reflects the detection light L1 emitted from the interface detection light source 102A and transmits the detection light L1 emitted from the crack detection light sources 102B and 102C. Although not shown in the diagram below, the detection light L1 emitted from the light sources 102A, 102B, and 102C are denoted as L1(A), L1(B), and L1(C), respectively.

[0028] In this embodiment, a half-mirror 104 was used, but instead of the half-mirror 104, a mirror may be inserted into the optical path when the interface is detected, and the mirror may be moved out of the optical path when a crack is detected.

[0029] Light sources 102A, 102B, and 102C are each connected to a control unit 500, and the control unit 500 controls the emission of light sources 102A, 102B, and 102C.

[0030] The control unit 500 includes a CPU (Central Processing Unit) that controls the operation of each part of the crack detection device 10, a ROM (Read Only Memory) that stores the control program, and an SDRAM (Synchronous Dynamic Random Access Memory) that can be used as a working area for the CPU. The control unit 500 receives operation input from the operator via the operation unit 506 and transmits control signals corresponding to the operation input to each part of the crack detection device 10 to control the operation of each part.

[0031] The operation unit 506 is a means for receiving operation input from the operator, and is, for example, a keyboard, and pointing devices such as a mouse and a touch panel.

[0032] The display unit 508 is a device that displays the operation GUI (Graphical User Interface) and images (for example, crack detection results). For example, a liquid crystal display can be used as the display unit 508.

[0033] The illumination optical system 200 guides the detection light L1 emitted from the light source 100 to the objective lens 504. The illumination optical system 200 includes relay lenses 202 and 206 and a mirror 204. The detection light L1 emitted from the light source 100 passes through the relay lens 202 and is reflected by the mirror (e.g., a total internal reflection mirror) 204, causing the optical path to be bent. The detection light L1 reflected by the mirror 204 passes through the relay lens 206, and is then sequentially reflected by the half mirror 304 and the half mirror 302 before being emitted towards the objective lens 504. Here, for example, a dichroic mirror that selectively reflects infrared light with a wavelength of 1,000 nm or more and transmits light in other wavelength bands (observation light) can be used as the half mirror 302. The observation light that has passed through the half mirror 302 can be observed using the observation optical system 600.

[0034] The objective lens 504 focuses (concentrates) the detection light L1 emitted from the illumination optical system 200 onto the workpiece W. The objective lens 504 is positioned opposite the workpiece W and is coaxial with the principal optical axis AX.

[0035] The focus adjustment mechanism 502 adjusts the focusing position of the detection light L1 on the workpiece W. The focus adjustment mechanism 502 includes a drive unit that moves at least one of the objective lens 504 and the stage 510 on which the workpiece W is placed. The focus adjustment mechanism 502 can move the focusing position of the detection light L1 in the XYZ direction by adjusting the relative position in the XYZ direction between the objective lens 504 and the stage 510.

[0036] The reflected light L2, focused by the objective lens 504 and reflected by the workpiece W, is guided to the interface detection optical system 300 and the crack detection optical system 400, and used for interface detection and crack detection of the workpiece W, respectively.

[0037] [Procedure for crack detection] In this embodiment, we will describe an example in which the interface on the back surface of the workpiece W (the surface in contact with the stage 510) is detected, and then the crack depth is detected based on the interface position on the back surface of the workpiece W.

[0038] In this embodiment, the crack depth is detected based on the back surface, but the present invention is not limited thereto. For example, the crack depth may be detected based on the surface of the workpiece W, or the average value of the crack depths detected based on the interface between the front and back surfaces of the workpiece W may be taken.

[0039] [Optical system for interface detection] First, let's explain the detection of the interface of the workpiece W.

[0040] The interface detection optical system 300 is an optical system for detecting the interface (front or back surface) of the workpiece W, and includes a mirror 302, a half mirror 304, a relay lens 306, a half mirror 308, and a photodetector 310.

[0041] When detecting the interface of the workpiece W, the control unit 500 emits light from the light source 102A and irradiates the workpiece W with detection light L1(A). Here, the control unit 500 and the interface detection optical system 300 each function as part of the interface detection means.

[0042] The detection light L1(A) (first detection light) from the light source 102A is a laser beam with an aperture approximately the same size as the pupil 504a of the objective lens 504, and is sequentially reflected by the half mirror 304 and the mirror 302 and guided to the objective lens 504. The detection light L1(A) illuminates approximately the entire surface of the pupil 504a of the objective lens 504.

[0043] Let L2(A) (the first reflected light) be the reflected light from the workpiece W that is reflected from the detection light L1(A). The reflected light L2(A) is reflected by the mirror 302, passes through the half mirror 304, and is then guided to the relay lens 306. The reflected light L2(A) that has passed through the relay lens 306 is reflected by the half mirror 308 and guided to the photodetector 310.

[0044] The photodetector 310 is a device for detecting the interface of the workpiece W by receiving reflected light L2(A) from the workpiece W, and includes a detector body 310A and a pinhole panel 310B.

[0045] The detector body 310A can be a photodetector (for example, a photodiode) that converts the received light into an electrical signal and outputs it to the control unit 500.

[0046] The pinhole panel 310B has a pinhole formed therein to allow a portion of the incident light to pass through. The pinhole panel 310B is positioned upstream of the light-receiving surface of the detector body 310A, and the pinhole of the pinhole panel 310B is positioned on the optical axis of the reflected light L2(A). The position of the pinhole of the pinhole panel 310B is optically confocal to the position of the focal point of the objective lens 504 (confocal pinhole). In addition, the size of the pinhole of the pinhole panel 310B is adjusted to approximately the diffraction limit of the objective lens 504.

[0047] The reflected light L2(A) reflected by the workpiece W is focused at the pinhole position of the pinhole panel 310B, which is optically conjugate to the front focal position of the objective lens 504. When the front focal position of the objective lens 504 coincides with the interface (back surface) of the workpiece W, which is the reflective surface, the light beam of the detected light L1(A) is reflected by the interface (back surface) of the workpiece W, becomes a parallel light beam, and passes through the objective lens 504 and returns. Therefore, the signal output from the detector body 310A has a sharp peak when the front focal position of the objective lens 504 coincides with the position of the interface (back surface) of the workpiece W, which is the reflective surface.

[0048] The control unit 500 irradiates the workpiece W with detected light L1(A) from the light source 102A, and moves the objective lens 504 and the stage 510 relative to each other in the Z direction, thereby moving the front focal position of the objective lens 504 in the Z direction relative to the workpiece W. The control unit 500 detects the reflected light L2(A) from the workpiece W using the photodetector 310, and detects the interface position Z(0) on the back surface of the workpiece W by detecting the peak of the signal from the photodetector 310.

[0049] In this embodiment, the interface of the workpiece W is detected using the confocal method, but the present invention is not limited thereto. For example, the knife-edge method may be used. When using the knife-edge method, it is also possible to apply the aperture for crack detection (photodetector 404 or 406) described later, without providing the half-mirror 308 and the photodetector 310. Furthermore, other focus detection methods such as astigmatism and white light interferometry can also be applied.

[0050] [Optical system for crack detection] Next, we will explain how to detect cracks K formed inside the workpiece W.

[0051] The crack detection optical system 400 includes a relay lens 402, and photodetectors 404 and 406.

[0052] When detecting a crack K formed inside a workpiece W, the control unit 500 emits light from light sources 102B and 102C to irradiate the workpiece W with detection light L1(B) and L1(C) (second detection light). Here, the control unit 500 and the crack detection optical system 400 each function as part of the crack detection means. Light sources 102B and 102C each have laser apertures positioned offset from the principal optical axis AX. As a result, detection light L1(B) and L1(C), which are eccentric with respect to the principal optical axis AX, are irradiated onto the workpiece W.

[0053] The reflected light L2(B) and L2(C) (second reflected light), obtained by the workpiece W from the detected light L1(B) and L1(C), are sequentially reflected by the half mirror 304 and the mirror 302, and then sequentially pass through the relay lens 306 and the half mirror 308 before entering the relay lens 402. The reflected light L2(B) and L2(C) that have passed through the relay lens 402 are received by the photodetectors 404 and 406.

[0054] The photodetectors 404 and 406 are devices for detecting cracks K inside the workpiece W by receiving reflected light L2(B) and L2(C) from the workpiece W. As the photodetectors 404 and 406, photodetectors (e.g., photodiodes) that convert the received light into an electrical signal and output it to the control unit 500 can be used.

[0055] The photodetectors 404 and 406 are positioned conjugate to the pupil 504a of the objective lens, and are also positioned offset from the optical axis to receive the detected light L1(B) and L1(C).

[0056] Figures 2 to 4 are explanatory diagrams showing the situation when the workpiece W is illuminated by the detected light L1. Figure 2 shows the case when a crack K exists at the focal point of the objective lens 504, Figure 3 shows the case when there is no crack K at the focal point of the objective lens 504, and Figure 4 shows the case when the focal point of the objective lens 504 coincides with the crack depth (lower end position of the crack). Figures 5 to 7 show the reflected light L2 received by the photodetectors 404 and 406, respectively, corresponding to the cases shown in Figures 2 to 4. Figure 8 is a diagram to explain the path of the reflected light L2 from the workpiece W to the pupil 504a of the objective lens. Here, we will explain the case in which the detected light L1 passes through the first region G1 on one side of the pupil 504a of the objective lens (right side in Figure 8) and illuminates the workpiece W by the detected light L1.

[0057] As shown in Figure 2, if a crack K exists at the focal point of the objective lens 504, the detected light L1 undergoes total internal reflection at the crack K, and the reflected light L2 follows the same path as the optical path of the detected light L1 with respect to the principal optical axis AX, becoming a component that reaches the same region of the objective lens pupil 504a as the detected light L1. That is, as shown in Figure 8, when the detected light L1 from the light source 100 is irradiated onto the workpiece W via the objective lens 504, the path of the detected light L1 is R1, and the reflected light L2 that undergoes total internal reflection at the crack K inside the workpiece W follows a path R2 that is on the same side (right side in Figure 8) as the path R1 of the detected light L1 with respect to the principal optical axis AX, passing through the first region G1 of the objective lens pupil 504a.

[0058] As shown in Figure 3, if there is no crack K at the focal point of the objective lens 504, the detected light L1 is reflected from the back surface of the workpiece W, and the reflected light L2 becomes a component that reaches the region of the objective lens pupil 504a opposite to the detected light L1. That is, as shown in Figure 8, the reflected light L2 reflected from the back surface of the workpiece W follows a path R3 that is opposite to the path R1 of the detected light L1 with respect to the principal optical axis AX (left side in Figure 8) and passes through the second region G2 of the objective lens pupil 504a.

[0059] As shown in Figure 4, when the focal point of the objective lens 504 coincides with the lower end of the crack K, the detected light L1 is divided into a reflected light component L2a that undergoes total internal reflection at the crack K and reaches the same region of the objective lens pupil 504a as the detected light L1, and a non-reflected light component L2b that is not totally reflected at the crack K, is reflected from the back surface of the workpiece W, and reaches the region of the objective lens pupil 504a opposite to the detected light L1. In other words, as shown in Figure 8, of the reflected light L2, the reflected light component L2a that is totally internalized at the crack K inside the workpiece W follows path R2, which is on the same side as the detection light L1 with respect to the principal optical axis AX (right side in Figure 8), and passes through the first region G1 of the objective lens pupil 504a. The unreflected light component L2b that is not totally internalized at the crack K and is reflected from the back surface of the workpiece W follows path R3, which is on the opposite side of the detection light L1 with respect to the principal optical axis AX (left side in Figure 8), and passes through the second region G2 of the objective lens pupil 504a.

[0060] The photodetectors 404 and 406 are positioned so as to be optically conjugate to the first region G1 and the second region G2 of the objective lens pupil 504a, respectively. This allows the photodetectors 404 and 406 to selectively receive light that has passed through the first region G1 and the second region G2 of the objective lens pupil 504a, respectively.

[0061] In the example shown in Figure 2 (where a crack K exists at the focal point of the objective lens 504), as shown in Figure 5, reflected light L2 is incident on the light-receiving surface 404C of photodetector 404, of the two photodetectors 404 and 406. As a result, the level of the detection signal output from the light-receiving surface 404C of photodetector 404 becomes higher than the level of the detection signal output from the light-receiving surface 406C of photodetector 406.

[0062] On the other hand, in the example shown in Figure 3 (where there is no crack K at the focal point of the objective lens 504), as shown in Figure 6, reflected light is incident on the light-receiving surface 406C of photodetector 406, of the two photodetectors 404 and 406. As a result, the level of the detection signal output from the light-receiving surface 406C of photodetector 406 becomes higher than the level of the detection signal output from the light-receiving surface 404C of photodetector 404.

[0063] Furthermore, in the example shown in Figure 4 (where the focal point of the objective lens 504 coincides with the lower end of the crack K), as shown in Figure 8, the respective components L2a and L2b of the reflected light L2 are incident on the light-receiving surfaces 404C and 406C of the photodetectors 404 and 406, respectively. As a result, the levels of the detection signals output from the light-receiving surfaces 404C and 406C of the photodetectors 404 and 406 become approximately equal.

[0064] Thus, the amount of light received by the light-receiving surfaces 404C and 406C of the photodetectors 404 and 406 changes depending on whether or not a crack K exists at the focal point of the objective lens 504. In this embodiment, this property can be used to detect the crack depth (lower end position of the crack or upper end position of the crack) of a crack K formed inside the workpiece W.

[0065] Specifically, when the outputs of the detection signals from the light-receiving surfaces 404C and 406C of the photodetectors 404 and 406 are denoted as D1 and D2, respectively, the evaluation value S for determining the presence of a crack K at the focal point of the objective lens 504 can be expressed by the following equation.

[0066] S = (D1 - D2) / (D1 + D2) …(1) In equation (1), when the condition S=0 is met, that is, when the amount of light received by the light-receiving surfaces 404C and 406C of the photodetectors 404 and 406 is the same, it indicates that the focal point of the objective lens 504 coincides with the lower end position of the crack (or the upper end position of the crack).

[0067] The control unit 500 controls the focus adjustment mechanism 502 (light-gathering point changing means) to sequentially change the light-gathering point of the objective lens 504 from the interface position Z(0) on the back surface to the thickness direction (Z direction) of the workpiece W, while sequentially acquiring detection signals output from the light-receiving surfaces 404C and 406C of the photodetectors 404 and 406, calculates an evaluation value S shown in equation (1) based on these detection signals, and can detect the crack depth of the crack K (crack lower end position or crack upper end position) by evaluating this evaluation value S and the light-gathering point position information.

[0068] In this embodiment, photodetectors were used as photodetectors 310, 404, and 406, but the present invention is not limited thereto. For example, an infrared camera or the like may be used instead of a photodetector.

[0069] Furthermore, when detecting cracks K, the half-mirror 308 of the interface detection optical system 300 may be moved out of the optical path of the reflected light L2. In this case, a total reflection mirror or a dichroic mirror may be used as the half-mirror 308 of the interface detection optical system 300.

[0070] In this embodiment, separate openings are provided for interface detection (light source 102A) and crack detection (light sources 102B and 102C), but a single opening may be used for both purposes.

[0071] Figure 9 shows an example where the opening for crack detection and the opening for interface detection are used together. Note that in Figure 9, components other than the light source unit 100 are omitted.

[0072] In the example shown in Figure 9, the light source unit 100-1 is provided with one light source 102. The light source 102 has a laser aperture capable of illuminating substantially the entire surface of the pupil 504a of the objective lens 504. When performing interface detection, as shown in Figure 9(a), the detection light L1(A) from the light source 102 is irradiated onto the workpiece W via the illumination optical system 200 and the objective lens 504. Therefore, in the example shown in Figure 9(a), interface detection can be performed in the same manner as in the above embodiment (Figure 1).

[0073] When performing crack detection, as shown in Figures 9(b) and 9(c), a limiting member 106 is inserted between the light source 102 and the illumination optical system 200 downstream of the light source 102 to block a portion of the detection light L1(A). Insertion of the limiting member 106 into and retraction from the optical path may be performed manually, or it may be performed automatically using an actuator controllable by the control unit 500.

[0074] The limiting member 106 is, for example, a disc-shaped member, with a substantially circular opening 106a formed at a position eccentric from the principal optical axis AX (lens optical axis). As shown in Figures 9(b) and 9(c), when the limiting member 106 is rotated 180° around the principal optical axis AX, detection light L1(B) and L1(C) can be obtained that illuminates only a portion of the objective lens pupil 504a of the objective lens 504 that is eccentric from the principal optical axis AX. Therefore, even in the example shown in Figures 9(b) and 9(c), crack detection can be performed in the same manner as in the above embodiment (Figure 1).

[0075] [About the detection light used for crack detection] Next, we will explain the relationship between the detection light L1 (L1(B) and L1(C)) used for crack detection.

[0076] Figure 10 shows the optical path of the detection light incident on the workpiece. In the example shown in Figure 10, the detection light L1 is focused by the objective lens 504 at a depth d of the workpiece W.

[0077] The distance d between the objective lens 504 and the workpiece W airBy changing , the position of the focal point Pc of the detected light L1 (focusing position) is changed. If n is the refractive index of the medium constituting the workpiece W, then d air and the change in d Δd air The relationship between and Δd is expressed by equation (2) below. The refractive index (absolute refractive index) of air is assumed to be 1.

[0078]

number

[0079] According to equation (2), the refractive index n and Δd of the medium constituting the workpiece W are air From this, the change Δd in the light-gathering position within the medium inside the workpiece W can be calculated.

[0080] As shown in equation (2), the amount of change Δd in the focusing position is determined by the refractive index n of the medium constituting the workpiece W, and therefore the refractive index n affects the crack detection result. However, as will be described later, the effective refractive index (hereinafter referred to as effective refractive index n) inside the workpiece W is affected by the angle (angle of incidence) at which the focused beam of detection light L1 enters the interior of the workpiece W. eff The effective refractive index n changes. eff If the value changes, it becomes impossible to accurately calculate the change in the focusing position Δd. Therefore, it is necessary to ensure that there is no inconsistency in positional information between the detection optical system (objective lens 504) for determining the focusing position and the crack detection optical system 400. To achieve this, it is necessary to appropriately determine the position of the pupil 504a of the objective lens 504 for the detection lights L1(B) and L2(C) used for crack detection. By appropriately determining the position of the pupil 504a of the objective lens 504 for the detection lights L1(B) and L2(C) used for crack detection in this way, the positional information indicated by the confocal optical system and the positional information indicated by the crack detection optical system 400 will match.

[0081] Optical path length d in a medium with refractive index n and optical path length d in air air The relationship is expressed by equation (3) below.

[0082] [Number]

[0083] Equation (3) is an equation for a light beam perpendicularly incident on the interface between air and a medium. The crack detection device 10 according to the present embodiment performs deflected illumination on the surface (interface) of the workpiece W. In this case, Equation (3) is modified as Equation (4).

[0084] [Number]

[0085] Here, NA is the numerical aperture of the detection lights L1(B) and L2(C) for crack detection, and when the inclination angle of the light beam of the detection light L1 in air is θ and the refractive index (absolute refractive index) of air is 1, it is expressed as NA = sinθ. From Equation (4), the effective refractive index n eff of the medium is expressed by the following Equation (5).

[0086] [Number]

[0087] When performing crack detection, the crack position is calculated using the value obtained by correcting the optical path length d air in air by the effective refractive index n eff .

[0088] Here, the detection light L1(A) for interface detection has an aperture approximately the same size as the objective lens pupil 504a of the objective lens 504. On the other hand, the detection lights L1(B) and L2(C) for crack detection use a thinner light beam than the detection light L1(A) for interface detection. It is necessary to determine the numerical aperture NA so that the effective refractive index n eff is the same between these two.

[0089] Similarly, the detection light L1(A) used for interface detection is also affected by the inclination of the angle of incidence relative to the workpiece W. In order to accurately detect the interface (back surface), the beam of the detection light L1(A) must be correctly focused on the position detected as the back surface. To focus the beam of the detection light L1(A) on the back surface, it is desirable to determine the numerical aperture (NA) of the detection light L1(A) using the method described below.

[0090] A detection light L1(A) for interface detection is shone onto a sample with a known thickness d to detect the positions of the back and front surfaces of the sample. The difference between the detected positions of the back and front surfaces is then calculated as d. air Let's assume that the effective refractive index n is given by the light beam used for detecting the focal position. eff This is expressed by the following equation (6).

[0091]

number

[0092] For the detection lights L1(B) and L2(C) used for crack detection, the effective refractive index n eff NA is calculated using the following equation (7) so that it is the same as in equation (6).

[0093]

number

[0094] Note that the numerical aperture (NA) of the objective lens 504 is... obj Therefore, the NA values ​​of the detection light L1(B) and L2(C) in equations (4), (5), and (7) are different. <NA obj And, as an example, NA / NA obj = 0.56.

[0095] Figure 11 shows the relationship between the pupil of the objective lens and the light beam of the detection light used for crack detection. In Figures 11(a) and 11(b), the diameter of the objective lens pupil 504a is R. obj The positions of the detection lights L1(B) and L2(C) for crack detection are R mLet's assume it's NA / NA. obj When = 0.56, the effective refractive index n of the medium for the interface detection optical system 300 and the crack detection optical system 400 satisfies equation (8) below. eff They match.

[0096]

number

[0097] In this embodiment, first, the positions of the back and front surfaces of a sample with a known thickness d are detected by an interface detection method, and the difference d between the detected back and front surfaces is air Next, detect the known thickness d and the detected thickness d. air By substituting into equation (4) and solving for NA, the numerical aperture NA of the luminous beams of the detection light L1(B) and L1(C) for crack detection is calculated. Then, the apertures of light sources 102B and 102C are adjusted so that the numerical aperture NA of the luminous beams of the detection light L1(B) and L1(C) for crack detection matches the numerical aperture NA calculated from equation (4). That is, the eccentricity of the laser apertures of light sources 102B and 102C, or the eccentricity of the aperture 106a in the limiting member 106 shown in Figure 9 is adjusted. This makes it possible to match the effective refractive index of the detection light L1(A) for interface detection and the detection light L1(B) and L1(C) for crack detection with respect to the workpiece W. Note that the size of the laser apertures of light sources 102B and 102C, and the size of the aperture 106a in the limiting member 106 shown in Figure 9 are adjusted considering the signal intensity of the reflected light L2, etc.

[0098] According to this embodiment, the effective refractive index of the detection light L1(A) for interface detection and the detection lights L1(B) and L1(C) for crack detection can be matched with respect to the workpiece W. This makes it possible to match the effective refractive index n which changes depending on the angle of incidence of the detection light L1 with respect to the workpiece W. eff This suppresses the influence of [unspecified factor] and allows for accurate calculation of the change in the focusing position Δd due to the change in the relative position between the objective lens 504 and the workpiece W. Therefore, according to this embodiment, the crack depth of a crack K formed inside the workpiece W can be detected non-destructively and accurately.

[0099] In this embodiment, the effective refractive index of the detection light L1(A) for interface detection and the detection lights L1(B) and L1(C) for crack detection were matched with respect to the workpiece W, but the present invention is not limited thereto. For example, the numerical aperture NA may be set so that the difference in the effective refractive index of the detection light L1(A) for interface detection and the detection lights L1(B) and L1(C) for crack detection is approximately ±20%.

[0100] Furthermore, in this embodiment, the back surface of the workpiece W is detected as the interface, and Δd is calculated based on this back surface. air We calculated Δd air The surface used as the basis for calculation is not limited to the back surface of the workpiece W, but may also be the front surface. When the front surface of the workpiece W is used as the basis, it is assumed that the aberration is small when the focus is on the front surface. When the back surface of the workpiece W is used as the basis, it is assumed that the aberration is small on the back surface using a correction ring or the like. [Explanation of Symbols]

[0101] 10...Crack detection device, 100...Light source unit, 102A, 102B, 102C...Light source, 104...Half mirror, 106...Restricting member, 200...Illumination optical system, 202...Relay lens, 204...Mirror, 206...Relay lens, 300...Optical system for interface detection, 302...Mirror, 304...Half mirror, 306...Relay lens, 308...Half mirror, 310...Photodetector, 400...Optical system for crack detection, 402...Relay lens, 404, 406...Photodetector, 500...Control unit, 502...Focus adjustment mechanism, 504...Objective lens, 506...Operation unit, 508...Display unit

Claims

1. A light source unit that emits detection light along the main optical axis, An objective lens having a lens optical axis coaxial with the principal optical axis, which focuses the detection light emitted from the light source onto the workpiece, An interface detection means that irradiates the workpiece with a first detection light emitted coaxially with the principal optical axis and capable of illuminating substantially the entire surface of the pupil of the objective lens, detects the first reflected light that has passed through the interior of the workpiece and been reflected at a second interface opposite to the first interface from which the first detection light was incident, and detects the position of the second interface of the workpiece based on a detection signal corresponding to the first reflected light, A crack detection means provides a second detection light capable of illuminating only a portion of the objective lens pupil that is eccentric from the lens optical axis, wherein the amount of eccentricity from the lens optical axis is adjusted so that the effective refractive index of the first detection light with respect to the workpiece matches the effective refractive index of the second detection light with respect to the workpiece, and the workpiece is selectively illuminated by the second detection light, the second reflected light that has passed through the inside of the workpiece and reflected at the second interface is detected, and the crack depth of a crack formed inside the workpiece is detected based on the position of the second interface, based on the detection signal corresponding to the second reflected light. A crack detection device equipped with the following features.

2. The refractive index of the workpiece is n, and the effective refractive index of the workpiece determined by the first detection light is n. ｅｆｆ When the numerical aperture of the second detection light is denoted as NA, [Math 1] The crack detection device according to claim 1, wherein the eccentricity of the aperture of the second detection light is adjusted to satisfy the condition.

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