Inspection methods

Abstract

The inspection method implemented by the laser processing apparatus includes: a first step, attaching a holding member to the back side of a wafer with a modified region formed thereon, and placing the mounting surface of the holding member on an adsorption stage; a second step, using an imaging unit to output penetrating light and detecting the penetrating light propagating through the wafer; and a third step, determining the state of the modified region of the wafer based on the image output from the imaging unit that detected the penetrating light. In the first step, a holding member with a thickness set according to the distance of the modified region is attached to the back side.

Description

[Technical Field]

[0001] The present invention relates to an inspection method. [Previous Technology]

[0002] An inspection apparatus is known in which, in order to cut a wafer having a semiconductor substrate and with the side of the semiconductor substrate containing a functional element layer as the back side, along each of a plurality of lines, laser light is irradiated onto the wafer from the surface side of the semiconductor substrate, forming multiple rows of modified regions inside the semiconductor substrate along each of the plurality of lines. The inspection apparatus described in Japanese Patent Application Publication No. 2017-64746 includes an infrared camera, capable of observing modified regions formed inside the semiconductor substrate and processing damage formed in the functional element layer from the surface side of the semiconductor substrate. In this inspection apparatus, for example, the crack state of the processed wafer is estimated based on such internal observation results, and the processing qualification (whether the desired processing was performed under set processing conditions) is determined based on the estimated crack state. [Summary of the Invention]

[0003] [The problem the invention aims to solve]

[0004] In the internal observation described above, besides direct observation by moving the focus from the surface side to the back side, there is also back reflection observation, where the focus is aligned from the surface side to the area opposite to the surface on the back side, and light reflected from the back side is captured. During internal observation, a holding member is typically attached to the back side of the wafer, and the aforementioned direct and back reflection observations are performed with the holding member positioned on the adsorption stage. Here, the holding member may have embossing on the surface in contact with the adsorption stage. Furthermore, the adsorption stage may have a porous structure on the surface where the holding member is located, resulting in minute irregularities based on the porous material. If back reflection observation is performed using such a holding member and / or adsorption stage, the patterns of the embossed surface on the holding member and the porous structure of the adsorption stage may be reflected in the captured image. In this case, there is a concern that the accuracy of estimating the dents or cracks in the modified area may be compromised (leading to incorrect determinations of the condition related to the modified area). [Technical Means for Solving the Problem]

[0005] One aspect of the present invention was developed in view of the above-mentioned actual situation, and relates to an inspection method capable of suppressing erroneous judgments in the internal observation of a wafer after laser processing.

[0006] An inspection method for a sample of the present invention comprises: a first step, for a wafer in which a modified region is formed internally by irradiating laser light, attaching a holding member to a second side opposite to the first side irradiated by laser light, and placing the side of the holding member opposite to the side attached to the wafer on an adsorption stage; a second step, for the wafer placed on the adsorption stage by the holding member, outputting penetrating light by an imaging unit and detecting the penetrating light propagating in the wafer; and a third step, determining the state related to the modified region of the wafer based on the image output from the imaging unit that detected the penetrating light, wherein in the first step, a holding member with a thickness set according to the modified region distance is attached to the second side, the modified region distance being the distance from the second side of the wafer to the modified region furthest from the second side.

[0007] In one aspect of the inspection method of the present invention, a holding member is attached to the back side, i.e., the second side, of a wafer in which a modified region is formed internally, and the holding member is disposed on an adsorption stage. Furthermore, internal observation of the wafer is performed by outputting penetrating light through the holding member onto the adsorption stage, and the state of the modified region of the wafer is determined based on the captured image. Here, when performing internal observation with this configuration, if a back reflection observation is performed, with the focus aligned from the surface (i.e., the first side) to the area opposite to the first side (i.e., the second side) of the back side, the pattern of the embossed surface of the holding member and the pattern of the porous structure of the adsorption stage are reflected in the captured image. In this case, there is a concern that the accuracy of estimating the state of the modified region of the wafer based on the captured image may deteriorate. In this regard, according to one aspect of the inspection method of the present invention, a holding member with a thickness set according to the distance from the second side to the modified region furthest from the second side, i.e., the modified region distance, is attached to the second side. In this way, by taking into account the distance of the modified region when setting the thickness of the retaining member, the thickness of the retaining member can be set in a manner that prevents the patterns of the retaining member, etc., from being reflected in the captured image during back-reflection observation of the modified region. This helps to suppress erroneous judgments during internal observation of the wafer after laser processing.

[0008] Alternatively, in the first process, a retaining member with a thickness set to be greater than the distance to the modified area is attached to the second surface. This allows the patterns of the retaining member, etc., to be suppressed from appearing in the captured image during back-reflection observation of the modified area. Thus, erroneous judgments can be suppressed during internal observation of the wafer after laser processing.

[0009] Alternatively, in the first process, a holding member with a thickness set greater than the distance of the modified region multiplied by the refractive index ratio of the holding member relative to the wafer is attached to the second surface. By multiplying the distance of the modified region by the aforementioned refractive index ratio, the distance of the modified region is converted into an approximate distance within the holding member that takes into account the refractive index. By attaching a holding member with a thickness greater than the converted value to the second surface, patterns of the holding member, etc., can be suppressed from appearing in the captured image during back-reflection observation of the modified region. In this way, erroneous judgments during internal observation of the wafer after laser processing can be suppressed. Furthermore, by setting the thickness of the holding member based on the value obtained by multiplying the distance of the modified region by the refractive index ratio, the thickness of the holding member can be reduced compared to, for example, setting the thickness of the holding member only in a manner greater than the distance of the modified region. That is, excessive thickness of the holding member can be avoided, and erroneous judgments during internal observation of the wafer after laser processing can be suppressed.

[0010] Alternatively, in the first step, a holding member with a thickness set greater than the distance of the modified region multiplied by the dz ratio of the holding member relative to the wafer can be attached to the second surface. By multiplying the distance of the modified region by the aforementioned dz ratio, the distance of the modified region is converted into an approximate distance in the holding member considering the refractive index and the NA (numerical aperture) of the objective lens of the imaging unit. By attaching a holding member with a thickness greater than the converted value to the second surface, the pattern of the holding member, etc., can be suppressed from appearing in the image captured during back-side reflection observation of the modified region. In this way, erroneous judgments in the internal observation of the wafer after laser processing can be suppressed. Furthermore, by setting the thickness of the holding member based on the value obtained by multiplying the distance of the modified region by the dz ratio, the thickness of the holding member can be reduced compared to, for example, setting the thickness of the holding member only to be greater than the distance of the modified region, or setting the thickness of the holding member only considering the refractive index ratio. That is, it is possible to avoid excessive thickness of the holding member and suppress erroneous judgments in the internal observation of the wafer after laser processing.

[0011] The inspection method of one aspect of the present invention comprises: a first step, for a wafer in which a modified region is formed internally by irradiation with laser light, attaching a holding member to a second surface opposite to the first surface irradiated by laser light, and placing the side of the holding member opposite to the surface to which it is attached on an adsorption stage; a second step, for the wafer placed on the adsorption stage by the holding member, outputting penetrating light by an imaging unit and detecting the penetrating light propagating in the wafer; and a third step, determining the state related to the modified region of the wafer based on an image output from the imaging unit that detects the penetrating light; in the first step, attaching a holding member with a transmittance of penetrating light of 50% or less to the second surface.

[0012] In one aspect of the inspection method of the present invention, because a holding member (a light-shielding holding member) with a transmittance of light of 50% or less is attached to the second surface, during back-reflection observation, the image of an object disposed below (on the adsorption stage side) of the holding member with light-shielding properties is difficult to be reflected in the captured image. Specifically, the patterns of the embossed surfaces of the holding member and the patterns of the porous structure of the adsorption stage are difficult to be reflected in the captured image. Thus, by suppressing the reflection of patterns of the holding member, etc., in the captured image during back-reflection observation, erroneous judgments in the internal observation of the wafer after laser processing can be suppressed.

[0013] Alternatively, in the first process, a holding member with a transmittance of 30% or less of transmissible light is attached to the second surface. By setting the transmittance of the holding member to 30% or less of transmissible light, the pattern of the holding member, etc., can be more appropriately suppressed from appearing in the captured image during back-side reflection observation, thereby suppressing erroneous judgments during internal observation of the wafer after laser processing.

[0014] Alternatively, in the first step, a retaining member comprising a material that absorbs transmissive light is attached to the second surface. With this configuration, the transmissivity of transmissive light of the retaining member can be appropriately suppressed.

[0015] Alternatively, in the first step, a retaining member comprising a material that reflects transmissive light is attached to the second surface. With this configuration, the transmissivity of the transmissive light of the retaining member can be appropriately suppressed.

[0016] Alternatively, in the second step, while changing the imaging area along the Z direction (which is the vertical direction), penetrating light is detected in each imaging area; in the third step, feature points and feature values ​​of the imaging images related to each imaging area along the Z direction are detected, and the positions of feature points with relatively large feature values ​​are determined as the formation positions of the modified region. In this way, by determining the formation positions of the modified region based on the feature values ​​of feature points in the imaging images, the accuracy and efficiency of internal observation of the wafer after laser processing can be improved.

[0017] The inspection method of the present invention can suppress erroneous judgments in the internal observation of a wafer after laser processing.

Implementation Method

[0019] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Furthermore, in each drawing, the same or equivalent parts are labeled with the same symbols, and repeated descriptions are omitted.

[0020] [Composition of the laser processing apparatus] As shown in Figure 1, the laser processing apparatus 1 includes an adsorption stage 2, a laser irradiation unit 3, multiple imaging units 4 (imaging sections), 5, 6, a drive unit 7 (drive section), a control unit 8, and a display 150. The laser processing apparatus 1 is a device that forms a modified region 12 on a target object 11 by irradiating the target object 11 with laser light L.

[0021] The adsorption stage 2 supports the object 11, for example, by adsorbing a thin film attached to the object 11. Furthermore, although not shown in FIG1, as shown in FIG18, a holding member 300 for holding the wafer 20 (details described later) is provided between the wafer 20, which is the object 11, and the adsorption stage 2. The adsorption stage 2 is movable along each of the X and Y directions and can rotate about an axis parallel to the Z direction. Furthermore, the X and Y directions are mutually perpendicular first and second horizontal directions, and the Z direction is a vertical direction.

[0022] The laser irradiation unit 3 focuses the laser light L, which is penetrable to the object 11, and irradiates the object 11. When the laser light L is focused inside the object 11 supported by the adsorption stage 2, the portion of the laser light L corresponding to the focusing point C of the laser light L is particularly absorbed, and a modified region 12 is formed inside the object 11.

[0023] The modified region 12 is a region whose density, refractive index, mechanical strength, and other physical properties differ from those of the surrounding unmodified regions. Examples of modified regions 12 include melt-treated regions, cracked regions, insulation-damaged regions, and regions with refractive index changes. The modified region 12 has the characteristic that it facilitates the extension of cracks from the modified region 12 towards the incident side of the laser light L and the opposite side. This characteristic of the modified region 12 is utilized in the cutting of the object 11.

[0024] As an example, if the adsorption stage 2 is moved along the X direction and the focusing point C is moved relative to the object 11 along the X direction, multiple modified particles 12s will be formed into a row along the X direction. A modified particle 12s is formed by irradiation with a pulse of laser light L. A row of modified regions 12 is a collection of multiple modified particles 12s arranged in a row. Adjacent modified particles 12s may be connected or separated depending on the relative movement speed of the focusing point C relative to the object 11 and the repetition frequency of the laser light L.

[0025] The imaging unit 4 captures images of the modified region 12 formed on the object 11 and the front end of the crack extending from the modified region 12.

[0026] The imaging units 5 and 6, under the control of the control unit 8, use light that penetrates the object 11 to photograph the object 11 supported by the adsorption table 2. As an example, the images obtained by the imaging units 5 and 6 are used for aligning the irradiation position of the laser light L.

[0027] The drive unit 7 supports the laser irradiation unit 3 and multiple imaging units 4, 5, 6. The drive unit 7 moves the laser irradiation unit 3 and multiple imaging units 4, 5, 6 along the Z direction.

[0028] The control unit 8 controls the operation of the adsorption stage 2, the laser irradiation unit 3, the multiple imaging units 4, 5, 6, and the drive unit 7. The control unit 8 is configured as a computer device including a processor, memory, storage, and communication equipment. In the control unit 8, the processor executes software (programs) loaded into the memory, etc., and controls the reading and writing of data in the memory and storage, as well as communication based on the communication equipment.

[0029] The display 150 has the functions of an input section for receiving information from a user and a display section for displaying information to a user.

[0030] [Composition of the Object] As shown in Figures 2 and 3, the object 11 of this embodiment is a wafer 20. The wafer 20 includes a semiconductor substrate 21 and a functional element layer 22. Furthermore, although the wafer 20 described in this embodiment has a functional element layer 22, the wafer 20 may or may not have a functional element layer 22, and may also be a bare wafer. The semiconductor substrate 21 has a back surface 21a and a surface 21b. The semiconductor substrate 21 is, for example, a silicon substrate. The functional element layer 22 is formed on the back surface 21a of the semiconductor substrate 21. The functional element layer 22 includes a plurality of functional elements 22a arranged in two dimensions along the back surface 21a. The functional elements 22a are, for example, light-receiving elements such as photodiodes, light-emitting elements such as laser diodes, and circuit elements such as memory. The functional elements 22a may also be formed by stacking multiple layers to create a three-dimensional structure. In addition, a notch 21c indicating the crystal orientation is provided on the semiconductor substrate 21, but an orientation plane may be provided instead of the notch 21c.

[0031] The wafer 20 is cut along each of the multiple lines 15 for each functional element 22a. When viewed from the thickness direction of the wafer 20, the multiple lines 15 pass between each of the multiple functional elements 22a. More specifically, when viewed from the thickness direction of the wafer 20, the lines 15 pass through the center (center in the width direction) of the street region 23. The street region 23 extends in the functional element layer 22 in a manner that passes between adjacent functional elements 22a. In this embodiment, the multiple functional elements 22a are arranged in a matrix along the back surface 21a, and the multiple lines 15 are set in a grid pattern. Furthermore, the lines 15 are imaginary lines, but may also be actually drawn lines. [Configuration of the laser irradiation unit]

[0032] As shown in Figure 4, the laser irradiation unit 3 includes a light source 31, a spatial light modulator 32, and a focusing lens 33. The light source 31 outputs laser light L, for example, via pulse oscillation. The spatial light modulator 32 modulates the laser light L output from the light source 31. The spatial light modulator 32 is, for example, a spatial light modulator (SLM) for reflective liquid crystal (LCOS: Liquid Crystal on Silicon). The focusing lens 33 focuses the laser light L modulated by the spatial light modulator 32. Alternatively, the focusing lens 33 can also be a lens with a correction ring.

[0033] In this embodiment, the laser irradiation unit irradiates laser light L from the surface 21b of the semiconductor substrate 21 toward the wafer 20 along each of the plurality of lines 15, thereby forming two rows of modified regions 12a, 12b inside the semiconductor substrate 21 along each of the plurality of lines 15. Modified region 12a is the modified region closest to the back surface 21a among the two rows of modified regions 12a, 12b. Modified region 12b is the modified region closest to modified region 12a among the two rows of modified regions 12a, 12b, and is also the modified region closest to the surface 21b.

[0034] Two rows of modified regions 12a, 12b are adjacent in the thickness direction (Z direction) of wafer 20. The two rows of modified regions 12a, 12b are formed by moving two focusing points C1, C2 relative to semiconductor substrate 21 along line 15. The laser light L is modulated by spatial light modulator 32, for example, such that focusing point C2 is located behind focusing point C1 in the travel direction and on the incident side of laser light L. Furthermore, the formation of the modified regions can be single-focus or multi-focus, and can be one-pass or multi-pass.

[0035] The laser irradiation unit irradiates laser light L from the surface 21b side of the semiconductor substrate 21 toward the wafer 20 along each of the plurality of lines 15. As an example, for the semiconductor substrate 21, which is a single-crystal silicon <100> substrate with a thickness of 400 μm, two focusing points C1 and C2 are aligned with positions 54 μm and 128 μm away from the back surface 21a, respectively, and laser light L is irradiated from the surface 21b side of the semiconductor substrate 21 toward the wafer 20 along each of the plurality of lines 15. At this time, for example, under the condition that the crack 14 spanning the two columns of modified regions 12a, 12b reaches the back surface 21a of the semiconductor substrate 21, the wavelength of the laser light L is 1099 nm, the pulse width is 700 nsec, and the repetition frequency is 120 kHz. Furthermore, the output of laser light L at focal point C1 is set to 2.7W, the output of laser light L at focal point C2 is set to 2.7W, and the relative moving speed of the two focal points C1 and C2 relative to the semiconductor substrate 21 is set to 800 mm / s. Additionally, for example, when the number of processing passes is set to 5, for the aforementioned wafer 20, for example, ZH80 (position 328 μm away from the back surface 21a), ZH69 (position 283 μm away from the back surface 21a), ZH57 (position 234 μm away from the back surface 21a), ZH26 (position 107 μm away from the back surface 21a), and ZH12 (position 49.2 μm away from the back surface 21a) can be set as processing positions. In this case, for example, the wavelength of laser light L can be 1080 nm, the pulse width 400 nsec, the repetition frequency 100 kHz, and the moving speed 490 mm / s.

[0036] [Composition of the Inspection Imaging Unit] As shown in FIG5, the imaging unit 4 (imaging section) includes a light source 41, a reflector 42, an objective lens 43, and a light detection unit 44. The imaging unit 4 images the interior of the wafer 20 by outputting penetrating light to the wafer 20 and detecting the light propagating within the wafer 20. The light source 41 outputs penetrating light l1 to the semiconductor substrate 21. The light source 41 is, for example, composed of a halogen lamp and a filter, and outputs light l1 in the near-infrared region. The light l1 output from the light source 41 is reflected by the reflector 42 and passes through the objective lens 43 to illuminate the wafer 20 from the surface 21b side of the semiconductor substrate 21. At this time, the adsorption stage 2 supports the wafer 20, on which two rows of modified regions 12a, 12b are formed as described above.

[0037] The objective lens 43 allows light l1 reflected from the back surface 21a of the semiconductor substrate 21 to pass through. That is, the objective lens 43 allows light l1, after propagation in the semiconductor substrate 21, to pass through. The numerical aperture (NA) of the objective lens 43 is, for example, 0.45 or more. The objective lens 43 has a correction ring 43a. The correction ring 43a adjusts the distance between the multiple lenses constituting the objective lens 43, thereby correcting aberrations caused by light l1 within the semiconductor substrate 21. Furthermore, the means of correcting aberrations is not limited to the correction ring 43a, but may also be other correction means such as a spatial light modulator. The light detection unit 44 detects light l1 that has passed through the objective lens 43 and the reflecting mirror 42. The light detection unit 44 is, for example, composed of an InGaAs camera, and detects light l1 in the near-infrared region. Furthermore, the means of detecting (capturing) light l1 in the near-infrared region is not limited to an InGaAs camera, but may be any means of transmission imaging such as a transmission confocal microscope, or other imaging means.

[0038] The imaging unit 4 is capable of imaging the front ends of each of the two modified regions 12a and 12b, and the various cracks 14a, 14b, 14c, and 14d (details to be described later). Crack 14a is a crack extending from the modified region 12a toward the back surface 21a. Crack 14b is a crack extending from the modified region 12a toward the surface 21b. Crack 14c is a crack extending from the modified region 12b toward the back surface 21a. Crack 14d is a crack extending from the modified region 12b toward the surface 21b.

[0039] [Configuration of the Imaging Unit for Alignment Correction] As shown in FIG6, the imaging unit 5 includes a light source 51, a reflector 52, a lens 53, and a light detection unit 54. The light source 51 outputs light I2 that is penetrating to the semiconductor substrate 21. The light source 51 is, for example, composed of a halogen lamp and a filter, and outputs light I2 in the near-infrared region. The light source 51 may also be shared with the light source 41 of the imaging unit 4. The light I2 output from the light source 51 is reflected by the reflector 52 and passes through the lens 53 to illuminate the wafer 20 from the surface 21b side of the semiconductor substrate 21.

[0040] Lens 53 allows light I2 reflected from the back surface 21a of semiconductor substrate 21 to pass through. That is, lens 53 allows light I2, after propagation within semiconductor substrate 21, to pass through. The numerical aperture of lens 53 is 0.3 or less. That is, the numerical aperture of the objective lens 43 of imaging unit 4 is larger than the numerical aperture of lens 53. Light detection unit 54 detects the light I2 that has passed through lens 53 and mirror 52. Light detection unit 54 is, for example, an InGaAs camera, and detects light I2 in the near-infrared region.

[0041] The imaging unit 5, under the control of the control unit 8, illuminates the wafer 20 with light I2 from the surface 21b side and detects the light I2 returning from the back surface 21a (functional element layer 22), thereby capturing an image of the functional element layer 22. Similarly, under the control of the control unit 8, the imaging unit 5 illuminates the wafer 20 with light I2 from the surface 21b side and detects the light I2 returning from the formation locations of the modified regions 12a, 12b in the semiconductor substrate 21, thereby acquiring an image of the region including the modified regions 12a, 12b. These images are used for aligning the irradiation position of the laser light L. The imaging unit 6, except that the lens 53 has a lower magnification (e.g., 6x in imaging unit 5, 1.5x in imaging unit 6), has the same configuration as the imaging unit 5 and is used for alignment in the same way as the imaging unit 5.

[0042] [Based on the imaging principle of the inspection imaging unit] Using the imaging unit 4 shown in FIG5, as shown in FIG7, for the semiconductor substrate 21 where the crack 14 spans the two rows of modified regions 12a, 12b and reaches the back surface 21a, the focal point F (the focal point of the objective lens 43) is moved from the surface 21b side toward the back surface 21a side. In this case, if the focal point F is aligned from the surface 21b side with the tip 14e of the crack 14 extending from the modified region 12b toward the surface 21b side, the tip 14e can be identified (the right image in FIG7). However, even if the focal point F is aligned from the surface 21b side with the crack 14 itself and the tip 14e of the crack 14 reaching the back surface 21a, it is still impossible to identify them (the left image in FIG7). Furthermore, if the focal point F is aligned from the surface 21b side with the back surface 21a of the semiconductor substrate 21, the functional element layer 22 can be identified.

[0043] Furthermore, using the imaging unit 4 shown in FIG5, as shown in FIG8, for the semiconductor substrate 21 where the crack 14 spanning the two columns of modified regions 12a, 12b does not reach the back surface 21a, the focus F is moved from the surface 21b side toward the back surface 21a side. In this case, even if the focus F is aligned from the surface 21b side with the front end 14e of the crack 14 extending from the modified region 12a toward the back surface 21a side, the front end 14e cannot be identified (the left image in FIG8). However, if the focus F is aligned from the surface 21b side with the region on the back surface 21a opposite to the surface 21b (i.e., the region on the side of the functional element layer 22 of the back surface 21a), and an imaginary focus Fv symmetrical about the back surface 21a and the focus F is located at the front end 14e, the front end 14e can be identified (the right image in FIG8). Furthermore, the imaginary focus Fv is a point symmetrical about the back surface 21a to the focus F taking into account the refractive index of the semiconductor substrate 21.

[0044] The reason why the crack 14 itself could not be confirmed as described above is that the width of the crack 14 is smaller than the wavelength of the light 11 used as illumination light. Figures 9 and 10 are SEM (Scanning Electron Microscope) images of the modified region 12 and the crack 14 formed inside the semiconductor substrate 21, which is a silicon substrate. Figure 9(b) is a magnified image of region A1 shown in Figure 9(a), Figure 10(a) is a magnified image of region A2 shown in Figure 9(b), and Figure 10(b) is a magnified image of region A3 shown in Figure 10(a). Thus, the width of the crack 14 is about 120 nm, which is smaller than the wavelength of the light 11 in the near-infrared region (e.g., 1.1~1.2 μm).

[0045] The shooting principle envisioned based on the above explanation is as follows. As shown in Figure 11(a), if the focal point F is located in the air, a black image is obtained because the light l1 will not return (the image on the right in Figure 11(a)). As shown in Figure 11(b), if the focal point F is located inside the semiconductor substrate 21, a white image is obtained because the light l1 reflected from the back surface 21a returns (the image on the right in Figure 11(b)). As shown in Figure 11(c), if the focal point F is aligned with the modified region 12 from the surface 21b side, the modified region 12 absorbs and scatters a portion of the light l1 reflected back from the back surface 21a, thus obtaining an image of the modified region 12 presented in black against a white background (the image on the right in Figure 11(c)).

[0046] As shown in Figures 12(a) and (b), if the focal point F is aligned from the surface 21b side to the front end 14e of the crack 14, due to optical specificities (stress concentration, strain, atomic density discontinuities, etc.) and light confinement near the front end 14e, a portion of the light l1 reflected back from the back surface 21a is scattered, reflected, interfered with, or absorbed, thus obtaining an image of the front end 14e appearing black against a white background (the right-hand image in Figures 12(a) and (b)). As shown in Figure 12(c), if the focal point F is aligned from the surface 21b side to a portion other than the front end 14e of the crack 14, at least a portion of the light l1 reflected from the back surface 21a returns, resulting in a white image (the right-hand image in Figure 12(c)).

[0047] [Detection Algorithm in Internal Observation] Regarding the internal observation of wafer 20 described above, the algorithms for detecting (determining) cracks 14 and detecting (determining) dents in the modified region are explained in detail. The detection of cracks 14 and dents in the modified region can also be determined and implemented by AI.

[0048] Figures 13 and 14 illustrate the crack detection. Figure 13 shows the internal observation results (image inside wafer 20). The control unit 8 first detects the group of straight lines 140 in the image inside wafer 20 shown in Figure 13(a). The detection of the group of straight lines 140 is performed using algorithms such as Hough transform or LSD (Line Segment Detector). Hough transform is an algorithm that detects all straight lines passing through a point in the image, while weighting the lines passing through more feature points. LSD is an algorithm that estimates the region that is a line segment by calculating the gradient and angle of the brightness value in the image, and detects straight lines by approximating the region as a rectangle.

[0049] Next, as shown in FIG14, the control unit 8 calculates the similarity between the line group 140 and the crack line, thereby detecting the crack 14 based on the line group 140. As shown in the upper part of FIG14, the crack line has the characteristic of being very bright before and after the brightness value on the line in the Y direction. Therefore, the control unit 8 compares the brightness value of all pixels of the detected line group 140 with the before and after in the Y direction, and uses the number of pixels whose difference is above the threshold in both directions as the similarity score. Moreover, the one with the highest similarity score to the crack line among the detected line groups 140 is used as the representative value in the image. The higher the representative value, the higher the probability that the crack 14 exists. The control unit 8 compares the representative values ​​in multiple images and uses the one with the relatively high score as a candidate image for crack.

[0050] Figures 15-17 illustrate the dent detection. Figure 15 shows the internal observation results (image of the interior of wafer 20). The control unit 8 detects the corners (concentration of edges) in the image of the interior of wafer 20 as shown in Figure 15(a) as key points, and detects their position, size, and orientation to detect feature points 250. As a method for detecting feature points in this way, Eigen, Harris, Fast, SIFT, SURF, STAR, MSER, ORB, AKAZE, etc. are known.

[0051] Here, as shown in FIG16, the indentations 280 are arranged at regular intervals in shapes such as circles and rectangles, serving as strong corner features. Therefore, by summing the feature quantities of feature points 250 within the image, the indentations 280 can be detected with high precision. As shown in FIG17, by comparing the sum of the feature quantities of each image captured by displacement in the depth direction, the change in the peak representing the amount of cracks in each modified layer can be confirmed. The control unit 8 estimates the peak value of this change as the position of the indentation 280. By summing the feature quantities in this way, not only the position of the indentation can be estimated, but also the pulse interval can be estimated.

[0052] [Details of the Internal Inspection Method (Inspection Method)] Internal inspection is performed on a wafer 20 (laser-processed wafer 20) with modified regions formed for purposes such as wafer dicing. The internal inspection method (inspection method) of this embodiment includes a first process, a second process, and a third process.

[0053] As shown in FIG18(a), in the first process, for the wafer 20 in which modified regions SD1 and SD2 are formed inside by irradiating laser light, a holding member 300 is attached to the back side 21a (second side) opposite to the surface 21b (first side) irradiated by laser light, and the side of the holding member 300 opposite to the surface of the wafer 20, i.e., the placement surface 300x, is provided on the support surface 2x of the adsorption stage 2.

[0054] The retaining member 300 is, for example, a cutting tape or a back-grinding tape. As shown in FIG18(b), the mounting surface 300x of the retaining member 300 is embossed. In addition, the support surface 2x of the adsorption stage 2 has a porous structure, forming tiny bumps and depressions based on a porous material. When using such a retaining member 300 and / or adsorption stage 2 for internal observation, during back-reflection observation, the patterns of the mounting surface 300x of the retaining member 300 and the support surface 2x of the adsorption stage 2 may be reflected in the captured image (details will be described later). Therefore, as described below, in the internal observation method of this embodiment, the retaining member 300 is optimized so that the patterns of the retaining member 300 and the adsorption stage 2 are not reflected in the captured image during back-reflection observation, and the optimized retaining member 300 is attached to the wafer 20 (details will be described later).

[0055] The second process is the imaging process. A wafer 20, held on the adsorption stage 2 by the holding member 300, is subjected to penetrating light l1 (see Figure 18(a)) output by the imaging unit 4, and the penetrating light l1 propagating through the wafer 20 is detected. In this second process, the imaging area is changed along the Z direction (which is considered vertical) while detecting (i.e., imaging) the penetrating light l1 in each imaging area. Specifically, in this second process, the control unit 8 controls the drive unit 7 to move the imaging unit 4 sequentially to each imaging area along the Z direction within a predetermined imaging range of the wafer 20, making them available for imaging. The imaging unit 4 is controlled to image each imaging area while changing the imaging area along the Z direction. The imaging range here includes: a direct observation area where the focus is moved from the surface 21b side towards the back surface 21a side for imaging, and a back reflection area where the focus is aligned from the surface 21b side with the area opposite to the back surface 21a for imaging the light reflected from the back surface 21a. That is, in the internal observation of this embodiment, not only is direct observation performed by moving the focus from the surface 21b side to the back surface 21a side to take pictures, but also back reflection observation is performed by moving the focus from the surface side to the area of ​​the back surface 21a opposite to the surface 21b to take pictures of the light l1 reflected from the back surface.

[0056] The third step is to determine the state of the modified region of the wafer 20 based on the captured image output from the imaging unit 4 that detects the penetrating light 11. In the third step, the control unit 8 detects dents and cracks in the modified region based on the feature values ​​of the feature points displayed in the captured images of each captured region, and determines whether the state of the modified region is appropriate based on the detected information. The control unit 8 detects feature points and feature values ​​of the feature points in the captured images of each captured region along the Z direction, and for example, determines the position of the feature point with a relatively large feature value as the formation position of the modified region. Such a detection algorithm in internal observation uses, for example, the detection algorithm described above as shown in Figures 13 to 17.

[0057] Here, as described above, there is a problem that when performing back-reflection observation for internal observation, the patterns on the mounting surface 300x of the retaining member 300 and the support surface 2x of the adsorption stage 2 are reflected in the captured image. Figure 19 illustrates the problem in back-reflection observation for internal observation. As shown in Figure 19(a), in back-reflection observation, the focal point of the penetrating light l1 is focused on the area opposite to the surface 21b of the back surface 21a, that is, the area on the side of the retaining member 300 and the adsorption stage 2. Therefore, if the light l1 reflected from the back surface 21a is captured by the imaging unit 4, the patterns on the mounting surface 300x of the retaining member 300 (see Figure 19(b)) and the patterns on the support surface 2x of the adsorption stage 2 (see Figure 19(c)) will be reflected in the captured image. In this case, there is concern that the accuracy of the determination of the state related to the modified area of ​​the control unit 8 (specifically, the estimation accuracy of the dent or crack state of the modified area) will deteriorate.

[0058] Figure 19(d) is an example of the internal observation results when the pattern of the retaining member 300, etc., is reflected in the captured image. In Figure 19(d), the horizontal axis represents the feature quantity, and the vertical axis represents the shooting depth. In the example of Figure 19(d), feature quantity data 901 of the modified region SD1 on the back side 21a, feature quantity data 902 of the modified region SD2 on the surface 21b, and feature quantity data 910 of the upper crack tip are detected in the direct observation area. Feature quantity data 903 of the modified region SD1 on the back side 21a is detected in the back reflection area. Moreover, in the back reflection area, the pattern of the mounting surface 300x of the retaining member 300 and the pattern of the support surface 2x of the adsorption stage 2 are detected as feature quantity data 1900. Due to the influence of this feature quantity data 1900, the feature quantity data 1904 of the modified region SD2 on the surface 21b is detected with a smaller feature quantity compared to the case where the feature quantity data 1900 is not detected. Thus, since the characteristic data 1900 of the pattern of the retaining member 300 is detected, it will affect the detection of the modified region SD2 on the surface 21b side, and there is a concern that the accuracy of the determination of the state related to the modified region will deteriorate.

[0059] To solve the above problems, in the internal observation method of this embodiment, the holding member 300 is optimized in the first step so that the pattern of the holding member 300 and the adsorption stage 2 will not be reflected in the captured image during back-reflection observation, and the optimized holding member 300 is attached to the wafer 20. Specifically, the thickness of the holding member 300 and / or the material of the holding member 300 are optimized. Hereinafter, the optimization of the thickness of the holding member 300 and the optimization of the material of the holding member 300 will be explained respectively.

[0060] The optimization of the thickness of the holding member 300 will be explained. In the first process, a holding member 300A (300) with a thickness set according to the distance of the modified region farthest from the back surface 21a in the wafer 20 (hereinafter referred to as "modified region distance") is attached to the back surface 21a of the wafer 20. Specifically, in the first process, a holding member 300A (300) with a thickness set in a manner greater than the modified region distance multiplied by the refractive index ratio of the holding member 300 relative to the wafer 20 may be attached to the back surface 21a of the wafer 20. For example, in the example shown in FIG20(a), the modified region distance is the distance from the back surface 21a to the end (upper end) of the surface 21b side of the modified region SD2. With the thickness of the retaining member 300A set to T1, the distance of the modified region set to D, the refractive index of the retaining member 300A set to R1, and the refractive index of the wafer 20 set to R2, the thickness T1 of the retaining member 300A is set in a manner that satisfies the following equation (1). For example, with D = 350 μm, R1 = 1.5, and R2 = 3.5, the thickness T1 of the retaining member 300A is calculated to be > 150 μm according to the following equation (1): T1 > D × R1 / R2 (1)

[0061] When using such a holding member 300A, the thickness of the holding member 300A is greater than the distance used to convert the distance of the modified region into an approximate distance within the holding member 300A. Therefore, as shown in FIG20(a), when observing the back side of the modified regions SD1 and SD2, light l1 does not reach the mounting surface 300x of the holding member 300A and the support surface 2x of the adsorption stage 2. Consequently, when observing the back side, the patterns of the mounting surface 300x of the holding member 300A and the support surface 2x of the adsorption stage 2 are not reflected in the captured image. FIG20(b) is a diagram showing an example of the internal observation results when using the holding member 300A. In FIG20(b), the horizontal axis represents the characteristic quantity, and the vertical axis represents the shooting depth. In the example shown in Figure 20(b), in the back reflection area, in addition to detecting the characteristic data 903 of the modified region SD1 on the back surface 21a side, the characteristic data 904 of the modified region SD2 on the surface 21b side is also appropriately detected. Furthermore, in the back reflection area, the characteristic data of the pattern on the mounting surface 300x of the holding member 300A and the pattern on the support surface 2x of the adsorption stage 2 are not detected. Therefore, the position of the indentation in each modified region SD1, SD2 can be detected with high accuracy without being affected by the characteristic data of the pattern on the holding member 300A, etc.

[0062] The thickness of the holding member 300A can also be determined based on the dz rate calculated from the refractive index of each observation wavelength and the NA (Numerical Aperture) of the objective lens 43. That is, in the first process, the holding member 300, with a thickness set greater than the distance to the modified region multiplied by the dz rate ratio of the holding member 300 relative to the wafer 20, can be attached to the back surface 21a of the wafer 20. The dz rate is the ratio of the amount of Z-axis movement of the objective lens in air to the focusing position within the object being observed. For example, when the wavelength of light l1 is 1100 nm and the objective lens 43 is moved by 1 μm, and the light moves 4 μm within the silicon constituting the wafer 20, the dz rate of the wafer 20 is 4.0. Alternatively, in the first step, a retaining member 300A with a thickness set greater than the distance of the modified region multiplied by the dz ratio of the retaining member 300A relative to the wafer 20 is attached to the back surface 21a of the wafer 20. Here, for example, when the distance of the modified region D = 350 μm, the dz ratio of the retaining member 300A = 1.6, and the dz ratio of the wafer 20 = 4.0, the thickness T1 of the retaining member 300A is calculated to be 350 × 1.6 / 4.0 = 140 μm. Furthermore, the thickness of the retaining member 300 can be set to be only greater than the distance of the modified region. That is, in the first step, a retaining member 300 with a thickness set greater than the distance of the modified region is attached to the back surface 21a of the wafer 20. However, in this case, the retaining member 300 becomes too thick, so it is preferable to calculate the thickness of the retaining member 300 according to the above formula (1) or a formula that takes into account the dz ratio. That is, the thickness of the holding member 300 is preferably calculated by taking into account the wafer thickness, laser processing conditions, wavelength of the observation light, refractive index of the wafer 20 and the holding member 300, and the NA of the objective lens 43.

[0063] Next, the optimization of the material of the holding member 300 will be explained. In the first step, as shown in FIG21(a), the holding member 300B with a transmittance of 50% or less of the transmissive light l1 output from the imaging unit 4 may be attached to the back surface 21a of the wafer 20. More preferably, in the first step, the holding member 300B with a transmittance of 30% or less of the transmissive light l1 may be attached to the back surface 21a of the wafer 20. For example, when the wafer 20 is a silicon wafer and the wavelength band of the transmissive light l1 is 950nm to 1700nm, the transmittance of the holding member 300B may be 50% or less (preferably 30% or less).

[0064] When using a holding member 300B that provides some light-blocking properties against penetrating light l1, as shown in FIG21(a), when observing the back side of the modified regions SD1 and SD2, light l1 does not reach the mounting surface 300x of the holding member 300B and the support surface 2x of the adsorption stage 2. Therefore, when observing the back side, the patterns on the mounting surface 300x of the holding member 300B and the support surface 2x of the adsorption stage 2 will not be reflected in the captured image. FIG21(b) is a diagram showing an example of the internal observation results when using the holding member 300B. In FIG21(b), the horizontal axis represents the feature quantity, and the vertical axis represents the shooting depth. In the example of FIG21(b), in the back reflection area, in addition to detecting the feature quantity data 903 of the modified region SD1 on the back side 21a, the feature quantity data 904 of the modified region SD2 on the surface 21b side is also appropriately detected. Furthermore, in the back reflection area, no characteristic data of the pattern on the mounting surface 300x of the holding member 300B or the pattern on the support surface 2x of the adsorption stage 2 were detected. Therefore, the positions of indentations in each modified region SD1 and SD2 can be detected with high precision, regardless of the characteristic data of the pattern on the holding member 300B, etc.

[0065] Alternatively, in the first step, a holding member 300B comprising a material that absorbs transmissive light l1 may be attached to the back surface 21a of the wafer 20. Alternatively, in the first step, a holding member 300B comprising a material that reflects transmissive light l1 may be attached to the back surface 21a of the wafer 20.

[0066] The optimization of the thickness of the retaining member 300 and the optimization of the material of the retaining member 300 described above can also be implemented by combining both. By making the thickness of the retaining member 300 sufficiently thick and by including the retaining member 300 in a material with high light-blocking properties, it is possible to more effectively suppress the reflection of the pattern on the mounting surface 300x of the retaining member 300 and the pattern on the support surface 2x of the adsorption stage 2 in the captured image.

[0067] Figure 22 is a diagram showing an example of the judgment result corresponding to the combination of the thickness and transmittance of the holding member 300. Here, the judgment result "○" indicates that the pattern of the holding member 300, etc., is not reflected and the case of erroneous judgment is suppressed, "△" indicates that the pattern of the holding member 300, etc., is approximately not reflected and the case of erroneous judgment is suppressed, and "×" indicates that the pattern of the holding member 300, etc., is reflected and an erroneous judgment occurs. Figure 23 is a diagram illustrating an example of the distance to the modified region for each processing category. As shown in Figure 23(a), when the laser processing is SDBG (Stealth Dicing Before Grinding) processing, if the thickness of the wafer 20 is set to 775 μm, the distance to the modified region is set to, for example, 230 μm. Furthermore, as shown in Figure 23(b), in the case of FC (complete cut-off) processing where cracks reach surface 21b after laser processing, if the thickness of wafer 20 is set to 775 μm, then the distance of the modified region is set to, for example, 730 μm. Thus, the distance of the modified region varies depending on the processing type.

[0068] Figure 22(a) shows the judgment results corresponding to the combination of the thickness and transmittance of the holding member 300 in SDBG processing. In the example shown in Figure 22(a), under the conditions that the thickness of the wafer 20 is 775 μm and the distance of the modified region is 230 μm, when the thickness of the holding member 300 is greater than 101 μm, the judgment result is "○" regardless of the transmittance of the holding member 300. In addition, when the transmittance of the holding member 300 is set to 10% or less, the judgment result is "○" regardless of the thickness of the holding member 300. In addition, when the thickness of the holding member 300 is 51~100 μm and when the thickness of the holding member 300 is 50 μm or less, when the transmittance of the holding member 300 is greater than 85%, the judgment result is "×", and when the transmittance of the holding member 300 is 30%, the judgment result is "△". Thus, by making the thickness of the retaining member 300 sufficiently thick, or by making the transmittance of the retaining member 300 sufficiently low, regardless of the combination of the thickness and transmittance of the retaining member 300, the determination result can be "○". Furthermore, even if the determination result cannot be "○" by the thickness of the retaining member 300 alone, or by the transmittance of the retaining member 300 alone, by combining the thickness and transmittance, the determination result can be "△".

[0069] Figure 22(b) shows the judgment results corresponding to the combination of the thickness and transmittance of the holding member 300 in FC processing. In the example shown in Figure 22(b), with the thickness of wafer 20 being 775 μm and the distance between modified regions being 730 μm, when the thickness of the holding member 300 is greater than 301 μm, the judgment result is "○" regardless of the transmittance of the holding member 300. In addition, when the transmittance of the holding member 300 is set to 10% or less, the judgment result is "○" regardless of the thickness of the holding member 300. Furthermore, when the thickness of the holding member 300 is 201~300 μm, 51~200 μm, or 50 μm or less, the judgment result is "×" when the transmittance of the holding member 300 is greater than 85%, and the judgment result is "△" when the transmittance of the holding member 300 is 30%. Thus, by making the thickness of the retaining member 300 sufficiently thick, or by making the transmittance of the retaining member 300 sufficiently low, regardless of the combination of the thickness and transmittance of the retaining member 300, the determination result can be "○". Furthermore, even if the determination result cannot be "○" by simply making the thickness of the retaining member 300, or by simply making the transmittance of the retaining member 300, the determination result can be "△" by combining the thickness and transmittance.

[0070] Figure 24 is a flowchart of an example of the above-described internal observation method (inspection method). As shown in Figure 23, in this inspection method, firstly, for a wafer 20 in which a modified region is formed internally by irradiating laser light, a holding member 300 is attached to the back surface 21a opposite to the surface 21b irradiated by the laser light. The mounting surface 300x of the holding member 300, which is attached to the opposite side of the surface of the wafer 20, is provided on the adsorption stage 2 (step S1, first process).

[0071] In step S1, for example, a retaining member 300 with a thickness set in a manner greater than the distance of the modified region multiplied by the refractive index ratio of the retaining member 300 relative to the wafer 20 is attached to the surface 21b of the wafer 20. Alternatively, in step S1, a retaining member 300 having a transmittance of light l1 of 50% (preferably 30%) or less may be attached to the surface 21b of the wafer 20.

[0072] Next, the wafer 20, which is placed on the adsorption stage 2 through the holding member 300, is photographed by the imaging unit 4 by outputting penetrating light l1 and detecting the penetrating light l1 after propagation in the wafer 20 (step S2, second process).

[0073] Finally, based on the captured image output from the imaging unit 4 that detects penetrating light l1, the state related to the modified region of the wafer 20 is determined (step S3, third process).

[0074] Next, the effects of the internal observation method (inspection method) implemented by the laser processing apparatus 1 of this embodiment will be explained.

[0075] The internal observation method (inspection method) implemented by the laser processing apparatus 1 of this embodiment includes: a first step, for a wafer 20 in which a modified region is formed internally by irradiating laser light, attaching a holding member 300 to the back surface 21a, and setting the mounting surface 300x of the holding member 300 on the adsorption stage 2; a second step, for the wafer 20 set on the adsorption stage 2 by the holding member 300, outputting penetrating light l1 by the imaging unit 4 and detecting the penetrating light l1 after propagation in the wafer 20; a third step, determining the state related to the modified region of the wafer 20 based on the image output by the imaging unit 4 from the detected penetrating light l1; in the first step, attaching the holding member 300, whose thickness is set according to the distance of the modified region, to the back surface 21a.

[0076] In the inspection method of this embodiment, a holding member 300 is attached to the back surface 21a of a wafer 20 in which a modified region is formed internally, and the holding member 300 is disposed on an adsorption stage 2. Furthermore, by outputting penetrating light 11 through the holding member 300 onto the wafer 20 placed on the adsorption stage 2, internal observation of the wafer 20 is performed, and the state related to the modified region of the wafer 20 is determined based on the captured image. However, when performing internal observation in this manner, if a back reflection observation is performed, focusing from the surface 21b side onto the area opposite to the surface 21a of the back surface 21a, the patterns of the embossed surfaces of the holding member 300 and the porous structure of the adsorption stage 2 may appear in the captured image. In this case, there is a concern that the accuracy of the estimation of the state related to the modified region of the wafer 20 based on the captured image may deteriorate. In this respect, in the inspection method of this embodiment, a retaining member 300 with a thickness set according to the distance from the back surface 21a of the wafer 20 to the modified region furthest from the back surface 21a, i.e., the modified region distance, is attached to the back surface 21a of the wafer 20. By setting the thickness of the retaining member 300 in consideration of the modified region distance, the thickness of the retaining member 300 can be set in a way that prevents patterns such as those on the retaining member 300 from being reflected in the captured image during back-side reflection observation of the modified region. This suppresses erroneous judgments during internal observation of the wafer 20 after laser processing.

[0077] Furthermore, as a method to avoid incorrect judgment in internal observation based on the influence of patterns on the holding member 300, for example, the following method can be considered: before laser processing, all images in the predetermined Z direction (depth direction) are acquired; when performing internal observation after laser processing, the images acquired before laser processing are removed from the images acquired after laser processing, so that only the information of the modified area formed after laser processing is within the viewing angle (a pattern avoidance method based on filtering). However, because such a method requires acquiring all images in the Z direction before laser processing, the production cycle deteriorates. In addition, if the shooting state, such as the amount of light or the brightness value, changes slightly in the images before and after laser processing, the filtering function (subtraction function) cannot function properly, and the accuracy of internal observation will decrease. In this respect, the internal observation method (inspection method) implemented by the laser processing apparatus 1 of this embodiment does not require pre-laser processing photography, thus improving the production cycle. Furthermore, since filtering is not required, the aforementioned decrease in the accuracy of internal observation is not a problem. That is, the internal observation method (inspection method) implemented by the laser processing apparatus 1 in this embodiment can improve the production cycle and suppress erroneous judgments in the internal observation of the laser-processed wafer 20.

[0078] Alternatively, in the first process, a retaining member 300 with a thickness set to be greater than the distance to the modified area is attached to the back surface 21a. This can suppress the reflection of patterns from the retaining member 300, etc., in the captured image during back-reflection observation of the modified area. Thus, erroneous judgments during internal observation of the wafer 20 after laser processing can be suppressed.

[0079] Alternatively, in the first process, a retaining member 300 with a thickness set greater than the distance of the modified region multiplied by the refractive index ratio of the retaining member 300 relative to the wafer 20 is attached to the back surface 21a. By multiplying the distance of the modified region by the aforementioned refractive index ratio, the distance of the modified region is converted into an approximate distance within the retaining member 300 that takes into account the refractive index. By attaching a retaining member 300 with a thickness greater than the converted value to the back surface 21a, the patterns of the retaining member 300, etc., can be suppressed from appearing in the captured image during back-reflection observation of the modified region. In this way, erroneous judgments in the internal observation of the laser-processed wafer 20 can be suppressed. Furthermore, by setting the thickness of the retaining member 300 according to the value of the distance of the modified region multiplied by the refractive index ratio, the thickness of the retaining member 300 can be reduced compared to, for example, setting the thickness of the retaining member 300 only in a manner greater than the distance of the modified region. That is, it is possible to avoid the retaining member 300 being too thick and to suppress erroneous judgments in the internal observation of the laser-processed wafer 20.

[0080] Alternatively, in the first process, a holding member 300 with a thickness set to a value greater than the distance of the modified region multiplied by the dz ratio of the holding member 300 relative to the wafer 20 is attached to the back surface 21a. By multiplying the distance of the modified region by the aforementioned dz ratio, the distance of the modified region is converted into an approximate distance in the holding member 300 that takes into account the refractive index and the NA (numerical aperture) of the objective lens 43 of the imaging unit 4. By attaching a holding member 300 with a thickness greater than the converted value to the back surface 21a, it is possible to suppress the reflection of patterns such as those of the holding member 300 in the image captured during back reflection observation of the modified region. In this way, erroneous judgments can be suppressed during internal observation of the laser-processed wafer 20. Furthermore, by setting the thickness of the holding member 300 based on the value obtained by multiplying the distance of the modified region by the dz ratio, the thickness of the holding member 300 can be reduced compared to cases where the thickness of the holding member 300 is set only to be greater than the distance of the modified region, or where the thickness of the holding member 300 is set only considering the refractive index ratio. That is, it is possible to avoid the holding member 300 being too thick and to suppress erroneous judgments in the internal observation of the wafer 20 after laser processing.

[0081] In the internal observation method (inspection method) implemented in the laser processing apparatus 1 of this embodiment, the first step may involve attaching a holding member 300 with a transmittance of less than 50% for the light 11 that has penetrating power to the back surface 21a. In this way, by attaching the light-shielding holding member 300 to the back surface 21a, during back-reflection observation, the image of an object disposed below the light-shielding holding member 300 (on the adsorption stage 2 side) is difficult to be reflected in the captured image. Specifically, the patterns on the embossed surfaces of the holding member 300 and the patterns of the porous structure of the adsorption stage 2 are difficult to be reflected in the captured image. Thus, by suppressing the reflection of patterns from the holding member 300, etc., in the captured image during back-reflection observation, erroneous judgments in the internal observation of the laser-processed wafer 20 can be suppressed.

[0082] In the first step, a holding member 300 with a transmittance of 30% or less of the transmissive light l1 can be attached to the back surface 21a. By setting the transmittance of the transmissive light l1 of the holding member 300 to 30% or less, the pattern of the holding member 300, etc., can be more appropriately suppressed from appearing in the captured image during back-side reflection observation, thereby suppressing erroneous judgments during internal observation of the laser-processed wafer 20.

[0083] In the first step, a retaining member 300 containing a material that absorbs transmissive light l1 may be attached to the back surface 21a. With this configuration, the transmittance of transmissive light l1 of the retaining member 300 can be appropriately suppressed.

[0084] In the first step, a retaining member 300 containing a material that reflects transmissive light l1 may be attached to the back surface 21a. With this configuration, the transmittance of transmissive light l1 of the retaining member 300 can be appropriately suppressed.

[0085] Alternatively, in the second step, while changing the imaging area along the Z direction (which is the vertical direction), penetrating light l1 is detected in each imaging area; in the third step, feature points and feature values ​​of the imaging images related to each imaging area along the Z direction are detected, and the positions of feature points with relatively large feature values ​​are determined as the formation positions of the modified region. In this way, by determining the formation positions of the modified region based on the feature values ​​of feature points in the imaging images, the accuracy and efficiency of internal observation of the laser-processed wafer 20 can be improved. [Simplified Explanation of the Diagram]

[0018] [Fig. 1] is a structural diagram of a laser processing apparatus according to an embodiment. [Fig. 2] is a top view of a wafer according to an embodiment. [Fig. 3] is a cross-sectional view of a portion of the wafer shown in Fig. 2. [Fig. 4] is a structural diagram of the laser irradiation unit shown in Fig. 1. [Fig. 5] is a structural diagram of the inspection imaging unit shown in Fig. 1. [Fig. 6] is a structural diagram of the alignment and correction imaging unit shown in Fig. 1. [Fig. 7] is a cross-sectional view of a wafer based on the imaging principle of the inspection imaging unit shown in Fig. 5, and images of various parts of the inspection imaging unit. [Fig. 8] is a cross-sectional view of a wafer based on the imaging principle of the inspection imaging unit shown in Fig. 5, and images of various parts of the inspection imaging unit. [Fig. 9(a),(b)] are SEM images of the modified region and cracks formed inside a semiconductor substrate. [Fig. 10(a),(b)] are SEM images of the modified region and cracks formed inside a semiconductor substrate. [Figures 11(a)~(c)] are optical path diagrams illustrating the imaging principle of the inspection imaging unit shown in Figure 5, and schematic diagrams showing the image at the focal point of the inspection imaging unit. [Figures 12(a)~(c)] are optical path diagrams illustrating the imaging principle of the inspection imaging unit shown in Figure 5, and schematic diagrams showing the image at the focal point of the inspection imaging unit. [Figures 13(a),(b)] are diagrams illustrating crack detection. [Figure 14] is a diagram illustrating crack detection. [Figures 15(a),(b)] are diagrams illustrating dent detection. [Figure 16] is a diagram illustrating dent detection. [Figure 17] is a diagram illustrating dent detection. [Figures 18(a),(b)] are diagrams illustrating the detailed structure of the holding member and the adsorption stage. [Figures 19(a)~(d)] are diagrams illustrating problems in back-reflection observation during internal observation. [Figure 20(a),(b)] illustrates an example of a retaining member suitable for back-reflection observation. [Figure 21(a),(b)] illustrates an example of a retaining member suitable for back-reflection observation. [Figure 22(a),(b)] shows the judgment results corresponding to the combination of the thickness and transmittance of the retaining member. [Figure 23(a),(b)] illustrates an example of the distance to the modification area for each processing category. [Figure 24] is a flowchart of an example of an inspection method.

Claims

1. An inspection method comprising: a first step, for a wafer in which a modified region is formed internally by irradiation with laser light, attaching a holding member to a second surface opposite to the first surface irradiated by the laser light, and placing the side of the holding member opposite to the surface attached to the wafer on an adsorption stage; a second step, for the wafer placed on the adsorption stage by the holding member, outputting penetrating light by an imaging unit and detecting the penetrating light propagating in the wafer; and a third step, determining a state related to the modified region of the wafer based on an image output from the imaging unit that detected the penetrating light, wherein in the first step, the holding member, whose thickness is set according to the modified region distance, is attached to the second surface, the modified region distance being the distance from the second surface of the wafer to the modified region furthest from the second surface.

2. The inspection method as described in request item 1, wherein, In the aforementioned first step, the aforementioned retaining member, whose thickness is set to be greater than the aforementioned modified area distance, is attached to the aforementioned second surface.

3. The inspection method as described in request item 1, wherein, In the aforementioned first step, the aforementioned retaining member, whose thickness is set to a value greater than the aforementioned modified region distance multiplied by the refractive index ratio of the aforementioned retaining member relative to the aforementioned wafer, is attached to the aforementioned second surface.

4. The inspection method as described in Request 1, wherein, In the aforementioned first process, the aforementioned retaining member, whose thickness is set to a value greater than the distance of the aforementioned modified region multiplied by the dz ratio of the aforementioned retaining member relative to the aforementioned wafer, is attached to the aforementioned second surface.

5. An inspection method comprising: a first step, for a wafer in which a modified region is formed internally by irradiation with laser light, attaching a holding member to a second surface opposite to the first surface irradiated by the laser light, and placing the side of the holding member opposite to the surface attached to the wafer on an adsorption stage; a second step, for the wafer placed on the adsorption stage by the holding member, outputting penetrating light by an imaging unit and detecting the penetrating light propagating in the wafer; and a third step, determining a state related to the modified region of the wafer based on an image output from the imaging unit that detected the penetrating light, wherein in the first step, the holding member having a transmittance of the penetrating light of 50% or less is attached to the second surface.

6. The inspection method as described in claim 5, wherein, In the aforementioned first step, the aforementioned retaining member with a light transmittance of less than 30% is attached to the aforementioned second surface.

7. The inspection method as described in request item 5 or 6, wherein, In the aforementioned first step, the aforementioned retaining member, which contains a material that absorbs the aforementioned transmissive light, is attached to the aforementioned second surface.

8. The inspection method as described in request item 5 or 6, wherein, In the aforementioned first step, the aforementioned retaining member, which contains a material that reflects the aforementioned translucent light, is attached to the aforementioned second surface.

9. The inspection method as described in any one of requests 1 to 6, wherein, In the aforementioned second step, while changing the shooting area along the Z direction, which is the vertical direction, the aforementioned penetrating light is detected in each shooting area. In the aforementioned third step, feature points and feature values ​​of the aforementioned shooting images related to each shooting area along the aforementioned Z direction are detected, and the position of the feature point with the relatively large feature value is determined as the formation position of the aforementioned modified region.

Images