Inspection method

Abstract

To suppress erroneous determination in internal observation of a wafer after laser processing.SOLUTION: An inspection method performed by a laser processing device 1 according to the present embodiment includes: a first step of attaching a holding member 300 on a rear face 21a of a wafer 20 with a modified region formed therein, and placing a mounting surface 300x of the holding member 300 on a suction table 2; a second step of, to the wafer 20 placed on the suction table 2, outputting transparent light l1 by an imaging unit 4 and detecting the transparent light l1 propagating through the wafer 20; and a third step of determining a state related to the modified region in the wafer 20 based on a captured image output from the imaging unit 4 that detects the transparent light l1. In the first step, the holding member 300 having a thickness set according to a modified region distance is attached to the rear face 21a.SELECTED DRAWING: Figure 24

Description

Technical Field

[0001] One aspect of the present invention relates to an inspection method.

Background Art

[0002] In order to cut a wafer having a semiconductor substrate and having a functional element layer formed on the semiconductor substrate as a back surface along each of a plurality of lines, a laser beam is irradiated onto the wafer from the surface side of the semiconductor substrate, so that a plurality of inspection apparatuses are known that form a plurality of rows of modified regions inside the semiconductor substrate along each of the lines. The inspection apparatus described in Patent Document 1 includes an infrared camera and can observe a modified region formed inside the semiconductor substrate, processing damage formed in the functional element layer, etc. from the surface side of the semiconductor substrate. In this inspection apparatus, for example, based on such internal observation results, the crack state of the wafer after processing is estimated, and based on the estimated result of the crack state, it is determined whether the processing is acceptable (whether the desired processing can be performed under the set processing conditions).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the internal observation as described above, in addition to the direct observation in which imaging is performed by moving the focus from the front surface side toward the back surface side, back surface reflection observation may be performed in which the light reflected from the back surface is imaged by focusing from the front surface side on the region on the side opposite to the front surface with respect to the back surface. During internal observation, usually, a holding member is attached to the back surface side of the wafer, and the above-described direct observation and back surface reflection observation are performed with the holding member placed on the adsorption table. Here, the holding member may have an embossed surface that contacts the adsorption table. Further, with respect to the adsorption table, the surface on which the holding member is placed may have a porous structure, and minute irregularities may be formed by a porous material. When back surface reflection observation is performed in such a case where such a holding member and / or adsorption table is used, the pattern of the embossed surface of the holding member or the pattern of the porous structure of the adsorption table may be reflected in the captured image. In this case, there is a risk that the estimation accuracy of the dents and crack states related to the modified region deteriorates (false determination of the state related to the modified region occurs).

[0005] One aspect of the present invention has been made in view of the above circumstances, and relates to an inspection method capable of suppressing false determination in the internal observation of a wafer after laser processing.

Means for Solving the Problems

[0006] An inspection method according to one aspect of the present invention includes: a first step of attaching a holding member to a second surface opposite to a first surface irradiated with laser light to a wafer in which a modified region is formed inside by irradiation with the laser light, and placing a surface opposite to the surface of the holding member attached to the wafer on an adsorption table; a second step of detecting, by an imaging unit, light having permeability that is output from the imaging unit and propagated through the wafer with respect to the wafer placed on the adsorption table via the holding member; and a third step of specifying a state related to the modified region of the wafer based on a captured image output from the imaging unit that has detected the light having permeability, wherein in the first step, a holding member having a thickness set according to a modified region distance, which is the distance from the second surface of the wafer to the modified region farthest from the second surface, is attached to the second surface.

[0007] In an inspection method according to one aspect of the present invention, a holding member is attached to a second surface, which is the back surface of a wafer having a modified region formed therein, and the holding member is installed on a suction table. Then, light having translucency is output to the wafer placed on the suction table via the holding member, so that the inside of the wafer is observed, and the state related to the modified region of the wafer is specified based on the captured image. Here, when the internal observation is performed with such a configuration, when a back surface reflection observation is performed in which light reflected from the first surface side is focused on a region on the opposite side of the first surface, which is the front surface, with respect to the second surface, which is the back surface, and the captured image is taken, the pattern of the surface of the holding member on which the embossing process is performed and the pattern of the porous structure of the suction table may be reflected in the captured image. In this case, the estimation accuracy of the state related to the modified region of the wafer based on the captured image may deteriorate. In this regard, in the inspection method according to one aspect of the present invention, a holding member having a thickness set according to the modified region distance, which is the distance from the second surface to the modified region farthest from the second surface, is attached to the second surface of the wafer. In this way, by setting the thickness of the holding member in consideration of the modified region distance, it becomes possible to set the thickness of the holding member so that the pattern of the holding member or the like is prevented from being reflected in the captured image in the back surface reflection observation related to the modified region. This can suppress misjudgment in the internal observation of the wafer after laser processing.

[0008] In the first step, a holding member having a thickness set to be larger than the modified region distance may be attached to the second surface. Thereby, it is possible to suppress the pattern of the holding member or the like from being reflected in the captured image in the back surface reflection observation related to the modified region. As a result, it is possible to suppress misjudgment in the internal observation of the wafer after laser processing.

[0009] In the first step, a holding member having a thickness set to be greater than the value obtained by multiplying the modified region distance by the refractive index ratio of the holding member with respect to the wafer may be attached to the second surface. By multiplying the modified region distance by the refractive index ratio, the modified region distance is converted into an approximate distance in the holding member considering the refractive index, so that when a holding member having a thickness greater than the converted value is attached to the second surface, patterns of the holding member and the like are suppressed from being reflected in the captured image in the back surface reflection observation related to the modified region. As a result, false determination in the internal observation of the wafer after laser processing can be suppressed. Further, by setting the thickness of the holding member according to the value obtained by multiplying the modified region distance by the refractive index ratio, for example, compared with the case where the thickness of the holding member is simply set to be greater than the modified region distance, or the case where the thickness of the holding member is set considering only the refractive index ratio, the thickness of the holding member can be reduced. That is, false determination in the internal observation of the wafer after laser processing can be suppressed while avoiding excessive increase in the thickness of the holding member.

[0010] In the first step, a holding member having a thickness set to be greater than the value obtained by multiplying the modified region distance by the dz rate ratio of the holding member with respect to the wafer may be attached to the second surface. By multiplying the modified region distance by the dz rate ratio, the modified region distance is converted into an approximate distance in the holding member considering the refractive index and the numerical aperture (NA) of the objective lens of the imaging unit, so that when a holding member having a thickness greater than the converted value is attached to the second surface, patterns of the holding member and the like are suppressed from being reflected in the captured image in the back surface reflection observation related to the modified region. As a result, false determination in the internal observation of the wafer after laser processing can be suppressed. Further, by setting the thickness of the holding member according to the value obtained by multiplying the modified region distance by the dz rate ratio, for example, compared with the case where the thickness of the holding member is simply set to be greater than the modified region distance, or the case where the thickness of the holding member is set considering only the refractive index ratio, the thickness of the holding member can be reduced. That is, false determination in the internal observation of the wafer after laser processing can be suppressed while avoiding excessive increase in the thickness of the holding member.

[0011] In an inspection method according to an aspect of the present invention, a holding member is attached to a second surface opposite to a first surface irradiated with laser light on a wafer in which a modified region is formed inside by irradiation with the laser light, and the holding member is attached to the second surface. A first step of installing, on a suction table, a surface opposite to the surface of the holding member that is attached to the wafer; a second step of detecting, by an imaging unit, light having transparency that is output and propagated through the wafer with respect to the wafer installed on the suction table via the holding member; and a third step of specifying a state related to the modified region of the wafer based on an imaging image output from the imaging unit that has detected the light having transparency. In the first step, a holding member having a light transmittance of 50% or less for the light having transparency is attached to the second surface.

[0012] In the inspection method according to an aspect of the present invention, since a holding member (a light-shielding holding member) having a light transmittance of 50% or less for the light having transparency is attached to the second surface, in the back surface reflection observation, an image of an object disposed below (on the suction table side) the light-shielding holding member is less likely to be captured in the imaging image. Specifically, it becomes difficult for the pattern on the surface of the holding member where the embossing process is performed and the pattern of the porous structure of the suction table to be captured in the imaging image. As a result, in the back surface reflection observation, it is possible to suppress the pattern of the holding member or the like from being captured in the imaging image, and it is possible to suppress an erroneous determination in the internal observation of the wafer after laser processing.

[0013] In the first step, a holding member having a light transmittance of 30% or less for the light having transparency may be attached to the second surface. By setting the light transmittance of the light having transparency in the holding member to 30% or less, it is possible to more appropriately suppress the pattern of the holding member or the like from being captured in the imaging image in the back surface reflection observation, and it is possible to suppress an erroneous determination in the internal observation of the wafer after laser processing.

[0014] In the first step, a holding member containing a material that absorbs the light having transparency may be attached to the second surface. According to such a configuration, it is possible to appropriately suppress the light transmittance of the light having transparency in the holding member.

[0015] In the first step, a holding member including a material that reflects light having transparency may be attached to the second surface. According to such a configuration, the transmittance of the light having transparency in the holding member can be appropriately suppressed.

[0016] In the second step, while changing the imaging region along the Z direction which is the vertical direction, light having transparency is detected for each imaging region. In the third step, for the imaging images related to each imaging region along the Z direction, feature points and feature amounts of the feature points are detected, and the position of the feature point having a relatively large feature amount may be specified as the formation position of the modification region. In this way, by specifying the formation position of the modification region from the feature amounts of the feature points in the imaging image, the accuracy and efficiency of the internal observation of the wafer after laser processing can be improved.

Advantages of the Invention

[0017] According to the inspection method according to one aspect of the present invention, false determination in the internal observation of the wafer after laser processing can be suppressed.

Brief Description of the Drawings

[0018]

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BEST MODE FOR CARRYING OUT THE INVENTION

[0019] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In each figure, the same or corresponding parts are denoted by the same reference numerals, and redundant descriptions are omitted. [Configuration of Laser Processing Apparatus]

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

[0021] The suction table 2 supports the object 11 by, for example, sucking a film attached to the object 11. Although not shown in FIG. 1, as shown in FIG. 18 and the like, a holding member 300 for holding the wafer 20, which is the object 11, is provided between the wafer 20 and the suction table 2 (details will be described later). The suction table 2 is movable along each of the X direction and the Y direction, and is rotatable about an axis parallel to the Z direction as a center line. The X direction and the Y direction are a first horizontal direction and a second horizontal direction perpendicular to each other, and the Z direction is a vertical direction.

[0022] The laser irradiation unit 3 condenses the laser light L having transparency with respect to the object 11 and irradiates the object 11. When the laser light L is condensed inside the object 11 supported by the suction table 2, the laser light L is particularly absorbed at a portion corresponding to the condensing point C of the laser light L, and a modified region 12 is formed inside the object 11.

[0023] The modified region 12 is a region where the density, refractive index, mechanical strength, and other physical properties are different from those of the surrounding non-modified regions. Examples of the modified region 12 include a melting treatment region, a crack region, a dielectric breakdown region, a refractive index change region, and the like. The modified region 12 has the property that cracks tend to extend from the modified region 12 to the incident side and the opposite side of the laser beam L. Such properties of the modified region 12 are utilized for cutting the object 11.

[0024] As an example, when the adsorption table 2 is moved along the X direction and the condensing point C is relatively moved along the X direction with respect to the object 11, a plurality of modified spots 12s are formed so as to be arranged in a row along the X direction. One modified spot 12s is formed by irradiation with one pulse of the laser beam L. A row of modified regions 12 is a collection of a plurality of modified spots 12s arranged in a row. Adjacent modified spots 12s may be connected to each other or separated from each other depending on the relative moving speed of the condensing point C with respect to the object 11 and the repetition frequency of the laser beam L.

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

[0026] The imaging unit 5 and the imaging unit 6 image the object 11 supported by the adsorption table 2 with light transmitted through the object 11 under the control of the control unit 8. The image obtained by imaging with the imaging units 5 and 6 is used, for example, for alignment of the irradiation position of the laser beam L.

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

[0028] The control unit 8 controls the operations of the adsorption table 2, the laser irradiation unit 3, the plurality of imaging units 4, 5, 6, and the drive unit 7. The control unit 8 is configured as a computer device including a processor, a memory, a storage, a communication device, and the like. In the control unit 8, the processor executes software (program) read into the memory or the like, controls the reading and writing of data in the memory and the storage, and the communication by the communication device.

[0029] The display 150 has a function as an input unit that receives input of information from the user and a function as a display unit that displays information to the user.

[0030] [Configuration of the Object] As shown in FIGS. 2 and 3, the object 11 of the present embodiment is a wafer 20. The wafer 20 includes a semiconductor substrate 21 and a functional element layer 22. In the present embodiment, the wafer 20 is described as having the functional element layer 22, but the wafer 20 may or may not have the functional element layer 22 and may be a bare wafer. The semiconductor substrate 21 has a back surface 21a and a front 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 two-dimensionally 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, circuit elements such as memories, and the like. The functional elements 22a may be three-dimensionally configured with a plurality of layers stacked. Note that the semiconductor substrate 21 is provided with a notch 21c indicating the crystal orientation, but an orientation flat may be provided instead of the notch 21c.

[0031] The wafer 20 is cut for each functional element 22a along each of the plurality of lines 15. The plurality of lines 15 pass between each of the plurality of functional elements 22a when viewed in the thickness direction of the wafer 20. More specifically, the line 15 passes through the center (center in the width direction) of the street region 23 when viewed in the thickness direction of the wafer 20. The street region 23 extends through between adjacent functional elements 22a in the functional element layer 22. In the present embodiment, the plurality of functional elements 22a are arranged in a matrix along the back surface 21a, and the plurality of lines 15 are set in a grid pattern. Note that the line 15 is a virtual line, but may be an actually drawn line.

[0032] [Configuration of Laser Irradiation Unit] As shown in FIG. 4, the laser irradiation unit 3 includes a light source 31, a spatial light modulator 32, and a condenser lens 33. The light source 31 outputs laser light L, for example, by a pulse oscillation method. 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) of a reflective liquid crystal (LCOS: Liquid Crystal on Silicon). The condenser lens 33 condenses the laser light L modulated by the spatial light modulator 32. Note that the condenser lens 33 may be a correction ring lens.

[0033] In the present embodiment, the laser irradiation unit 3 irradiates the wafer 20 with the laser light L from the surface 21b side of the semiconductor substrate 21 along each of the plurality of lines 15, thereby forming two rows of modified regions 12a and 12b inside the semiconductor substrate 21 along each of the plurality of lines 15. The modified region 12a is the modified region closest to the back surface 21a among the two rows of modified regions 12a and 12b. The modified region 12b is the modified region closest to the modified region 12a among the two rows of modified regions 12a and 12b and is the modified region closest to the surface 21b.

[0034] The two modification regions 12a and 12b are adjacent to each other in the thickness direction (Z direction) of the wafer 20. The two modification regions 12a and 12b are formed by relatively moving two condensing points C1 and C2 along the line 15 with respect to the semiconductor substrate 21. The laser beam L is modulated by the spatial light modulator 32 such that, for example, the condensing point C2 is located on the rear side in the traveling direction and on the incident side of the laser beam L with respect to the condensing point C1. Note that, regarding the formation of the modification region, it may be a single focus or a multi-focus, and it may be a single pass or a multi-pass.

[0035] The laser irradiation unit 3 irradiates the wafer 20 with the laser beam L from the surface 21b side of the semiconductor substrate 21 along each of the plurality of lines 15. As an example, for a semiconductor substrate 21 which is a single crystal silicon <100> substrate with a thickness of 400 μm, two condensing points C1 and C2 are respectively aligned with positions 54 μm and 128 μm from the back surface 21a, and the wafer 20 is irradiated with the laser beam L from the surface 21b side of the semiconductor substrate 21 along each of the plurality of lines 15. At this time, for example, when the condition is such that the crack 14 extending across the two modification regions 12a and 12b reaches the back surface 21a of the semiconductor substrate 21, the wavelength of the laser beam L is 1099 nm, the pulse width is 700 nsec, and the repetition frequency is 120 kHz. Also, the output of the laser beam L at the condensing point C1 is 2.7 W, the output of the laser beam L at the condensing point C2 is 2.7 W, and the relative moving speed of the two condensing points C1 and C2 with respect to the semiconductor substrate 21 is 800 mm / sec. Note that, for example, when the number of processing passes is 5, for the wafer 20 described above, for example, ZH80 (position 328 μm from the back surface 21a), ZH69 (position 283 μm from the back surface 21a), ZH57 (position 234 μm from the back surface 21a), ZH26 (position 107 μm from the back surface 21a), ZH12 (position 49.2 μm from the back surface 21a) may be the processing positions. In this case, for example, the wavelength of the laser beam L may be 1080 nm, the pulse width may be 400 nsec, the repetition frequency may be 100 kHz, and the moving speed may be 490 mm / sec.

[0036] [Configuration of Inspection Imaging Unit] As shown in FIG. 5, the imaging unit 4 (imaging section) includes a light source 41, a mirror 42, an objective lens 43, and a light detection unit 44. The imaging unit 4 outputs light that is transmissive to the wafer 20, and images the inside of the wafer 20 by detecting the light that has propagated through the wafer 20. The light source 41 outputs light l1 that is transmissive to the semiconductor substrate 21. The light source 41 is constituted by, for example, 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 mirror 42, passes through the objective lens 43, and irradiates the wafer 20 from the surface 21b side of the semiconductor substrate 21. At this time, the adsorption table 2 supports the wafer 20 in which two rows of modification regions 12a and 12b are formed as described above.

[0037] The objective lens 43 allows the light l1 reflected by the back surface 21a of the semiconductor substrate 21 to pass through. That is, the objective lens 43 allows the light l1 that has propagated through 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 corrects the aberration generated in the light l1 within the semiconductor substrate 21, for example, by adjusting the distance between a plurality of lenses constituting the objective lens 43. Note that the means for correcting the aberration is not limited to the correction ring 43a, and other correction means such as a spatial light modulator may be used. The light detection unit 44 detects the light l1 that has passed through the objective lens 43 and the mirror 42. The light detection unit 44 is constituted by, for example, an InGaAs camera, and detects the light l1 in the near-infrared region. Note that the means for detecting (imaging) the light l1 in the near-infrared region is not limited to the InGaAs camera, and other imaging means such as a transmission-type confocal microscope may be used as long as it performs transmission-type imaging.

[0038] The imaging unit 4 can image each of the two rows of reformed regions 12a and 12b, and the tip of each of the plurality of cracks 14a, 14b, 14c, and 14d (details will be described later). The crack 14a is a crack extending from the reformed region 12a toward the back surface 21a side. The crack 14b is a crack extending from the reformed region 12a toward the front surface 21b side. The crack 14c is a crack extending from the reformed region 12b toward the back surface 21a side. The crack 14d is a crack extending from the reformed region 12b toward the front surface 21b side.

[0039] [Configuration of Imaging Unit for Alignment Correction] As shown in FIG. 6, the imaging unit 5 includes a light source 51, a mirror 52, a lens 53, and a light detection unit 54. The light source 51 outputs light I2 that is transmissive with respect to the semiconductor substrate 21. The light source 51 is composed of, for example, a halogen lamp and a filter, and outputs light I2 in the near-infrared region. The light source 51 may 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 mirror 52, passes through the lens 53, and irradiates the wafer 20 from the front surface 21b side of the semiconductor substrate 21.

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

[0041] The imaging unit 5 irradiates the wafer 20 with light I2 from the surface 21b side under the control of the control unit 8, and images the functional element layer 22 by detecting the light I2 returning from the back surface 21a (functional element layer 22). Similarly, the imaging unit 5 irradiates the wafer 20 with light I2 from the surface 21b side under the control of the control unit 8, and acquires an image of the region including the modified regions 12a and 12b by detecting the light I2 returning from the formation positions of the modified regions 12a and 12b in the semiconductor substrate 21. These images are used for the alignment of the irradiation position of the laser beam L. The imaging unit 6 has the same configuration as the imaging unit 5 except that the lens 53 has a lower magnification (for example, 6 times in the imaging unit 5 and 1.5 times in the imaging unit 6), and is used for alignment in the same manner as the imaging unit 5.

[0042] [Imaging principle by inspection imaging unit] Using the imaging unit 4 shown in FIG. 5, as shown in FIG. 7, for the semiconductor substrate 21 in which the crack 14 extending across the two rows of modified regions 12a and 12b reaches the back surface 21a, the focus F (the focus of the objective lens 43) is moved from the surface 21b side toward the back surface 21a side. In this case, when the focus F is aligned with the tip 14e of the crack 14 extending from the modified region 12b toward the surface 21b side from the surface 21b side, the tip 14e can be confirmed (the right image in FIG. 7). However, even when the focus F is aligned with the crack 14 itself and the tip 14e of the crack 14 reaching the back surface 21a from the surface 21b side, they cannot be confirmed (the left image in FIG. 7). When the focus F is aligned with the back surface 21a of the semiconductor substrate 21 from the surface 21b side, the functional element layer 22 can be confirmed.

[0043] Also, using the imaging unit 4 shown in FIG. 5, as shown in FIG. 8, for the semiconductor substrate 21 in which a crack 14 extending across the two rows of reforming regions 12a and 12b has not reached the back surface 21a, the focus F is moved from the front surface 21b side toward the back surface 21a side. In this case, even when the focus F is aligned with the tip 14e of the crack 14 extending from the reforming region 12a toward the back surface 21a side from the front surface 21b side, the tip 14e cannot be confirmed (the left image in FIG. 8). However, when the focus F is aligned from the front surface 21b side with a region on the side opposite to the front surface 21b with respect to the back surface 21a (that is, the region on the functional element layer 22 side with respect to the back surface 21a), and a virtual focus Fv symmetric to the focus F with respect to the back surface 21a is positioned at the tip 14e, the tip 14e can be confirmed (the right image in FIG. 8). Note that the virtual focus Fv is a point symmetric to the focus F considering the refractive index of the semiconductor substrate 21 with respect to the back surface 21a.

[0044] As described above, it is assumed that the crack 14 itself cannot be confirmed because the width of the crack 14 is smaller than the wavelength of the light l1 which is the illumination light. FIGS. 9 and 10 are SEM (Scanning Electron Microscope) images of the reforming region 12 and the crack 14 formed inside the semiconductor substrate 21 which is a silicon substrate. FIG. 9(b) is an enlarged image of the region A1 shown in FIG. 9(a), FIG. 10(a) is an enlarged image of the region A2 shown in FIG. 9(b), and FIG. 10(b) is an enlarged image of the region A3 shown in FIG. 10(a). Thus, the width of the crack 14 is about 120 nm, which is smaller than the wavelength of the light l1 in the near-infrared region (for example, 1.1 to 1.2 μm).

[0045] Based on the above, the assumed imaging principle is as follows. As shown in Fig. 11(a), when the focus F is positioned in the air, since the light l1 does not return, a dark - looking image is obtained (the right - hand image in Fig. 11(a)). As shown in Fig. 11(b), when the focus F is positioned inside the semiconductor substrate 21, since the light l1 reflected by the back surface 21a returns, a white - looking image is obtained (the right - hand image in Fig. 11(b)). As shown in Fig. 11(c), when the focus F is set on the surface 21b side to the modified region 12, absorption, scattering, etc. occur for a part of the light l1 reflected by the back surface 21a and returning due to the modified region 12. Therefore, an image in which the modified region 12 appears dark against a white - looking background is obtained (the right - hand image in Fig. 11(c)).

[0046] As shown in Figs. 12(a) and 12(b), when the focus F is set on the surface 21b side to the tip 14e of the crack 14, for example, due to optical singularities (stress concentration, strain, discontinuity of atomic density, etc.) occurring near the tip 14e, light confinement occurring near the tip 14e, etc., scattering, reflection, interference, absorption, etc. occur for a part of the light l1 reflected by the back surface 21a and returning. Therefore, an image in which the tip 14e appears dark against a white - looking background is obtained (the right - hand images in Figs. 12(a) and 12(b)). As shown in Fig. 12(c), when the focus F is set on the surface 21b side to a portion other than the vicinity of the tip 14e of the crack 14, at least a part of the light l1 reflected by the back surface 21a returns, so a white - looking image is obtained (the right - hand image in Fig. 12(c)).

[0047] [Detection Algorithm in Internal Observation] Regarding the internal observation of the wafer 20 described above, the algorithms for detecting (identifying) the crack 14 and the algorithms for detecting (identifying) the traces related to the modified region will be described in detail. Such detection of the crack 14 and detection of the traces related to the modified region may be determined and implemented by AI.

[0048] FIG. 13 and FIG. 14 are diagrams for explaining crack detection. In FIG. 13, an internal observation result (an image inside the wafer 20) is shown. The control unit 8 first detects a straight line group 140 for an image inside the wafer 20 as shown in FIG. 13(a). For the detection of the straight line group 140, an algorithm such as Hough transform or LSD (Line Segment Detector) is used, for example. The Hough transform is a method of detecting all straight lines passing through a point on an image and detecting a straight line while weighting a straight line passing through more feature points. LSD is a method of estimating a region that becomes a line segment by calculating the gradient and angle of the luminance value in an image, and detecting a straight line by approximating the region to a rectangle.

[0049] Subsequently, the control unit 8 detects a crack 14 from the straight line group 140 by calculating the similarity with a crack line for the straight line group 140 as shown in FIG. 14. As shown in the upper figure of FIG. 14, the crack line has a feature that the front and back are very bright in the Y direction with respect to the luminance value on the line. Therefore, the control unit 8 compares, for example, the luminance values of all pixels of the detected straight line group 140 with the front and back in the Y direction, and sets the number of pixels whose difference is equal to or greater than the threshold value in both the front and back as a similarity score. Then, the one with the highest similarity score with the crack line among the detected multiple straight line groups 140 is set as a representative value in that image. The higher the representative value, the higher the possibility that the crack 14 exists. The control unit 8 compares the representative values in a plurality of images, and sets the one with a relatively high score as a crack image candidate.

[0050] FIGS. 15 to 17 are diagrams for explaining scratch detection. In FIG. 15, an internal observation result (an image inside the wafer 20) is shown. The control unit 8 detects corners (concentration of edges) in the image as key points for an image inside the wafer 20 as shown in FIG. 15(a), and detects the position, size, and direction thereof to detect feature points 250. As methods for detecting feature points in this way, Eigen, Harris, Fast, SIFT, SURF, STAR, MSER, ORB, AKAZE, etc. are known.

[0051] Here, as shown in FIG. 16, since the dents 280 are arranged at regular intervals in a shape such as a circle or a rectangle, the characteristics as corners are strong. Therefore, by aggregating the feature amounts of the feature points 250 in the image, it becomes possible to detect the dents 280 with high precision. As shown in FIG. 17, when comparing the total feature amounts for each image captured with a shift in the depth direction, a mountain-like change indicating the amount of crack rows for each modified layer can be confirmed. The control unit 8 estimates the peak of the change as the position of the dents 280. By aggregating the feature amounts in this way, it becomes possible to estimate not only the dent position but also the pulse pitch.

[0052] [Details of the internal observation method (inspection method)] Internal observation is performed on the wafer 20 (wafer 20 after laser processing) in which a modified region is formed for the purpose of cutting the wafer 20 or the like. The internal observation method (inspection method) according to the present embodiment includes a first step, a second step, and a third step.

[0053] In the first step, as shown in FIG. 18(a), for the wafer 20 in which the internal modified regions SD1 and SD2 are formed by irradiating laser light, a holding member 300 is attached to the back surface 21a (second surface) opposite to the surface 21b (first surface) irradiated with the laser light, and the mounting surface 300x, which is the surface opposite to the surface of the holding member 300 attached to the wafer 20, is installed on the support surface 2x of the suction table 2.

[0054] The holding member 300 is a tape such as a dicing tape or a back grinding tape. As shown in Fig. 18(b), the mounting surface 300x of the holding member 300 is embossed. Further, the support surface 2x of the suction table 2 has a porous structure, and minute irregularities are formed by a porous material. When such a holding member 300 and / or the suction table 2 are used for internal observation, in the case of rear surface reflection observation, the pattern of the mounting surface 300x of the holding member 300 and the pattern of the support surface 2x of the suction table 2 may be reflected in the captured image (details will be described later). Therefore, as will be described later, in the internal observation method according to the present embodiment, the holding member 300 is optimized so that the patterns of the holding member 300 and the suction table 2 are not reflected in the captured image during rear surface reflection observation, and the optimized holding member 300 is attached to the wafer 20 (details will be described later).

[0055] The second step is an imaging step in which the imaging unit 4 outputs light l1 having permeability (see Fig. 18(a)) to the wafer 20 placed on the suction table 2 via the holding member 300 and detects the light l1 having permeability that has propagated through the wafer 20. In the second step, detection (i.e., imaging) of the light l1 having permeability is performed for each imaging region while changing the imaging region along the Z direction, which is a vertical method. Specifically, in the second step, the control unit 8 controls the drive unit 7 so that the imaging unit 4 sequentially moves to positions where each imaging region along the Z direction in a predetermined imaging range of the wafer 20 can be imaged, and controls the imaging unit 4 so that each imaging region is imaged while changing the imaging region along the Z direction. The imaging range here includes a direct observation region that is imaged by moving the focus from the surface 21b side toward the back surface 21a side, and a back surface reflection region that is imaged by focusing from the surface 21b side on a region on the side opposite to the surface 21b with respect to the back surface 21a and reflecting the light at the back surface 21a. That is, in the internal observation according to the present embodiment, in addition to the direct observation in which imaging is performed by moving the focus from the surface 21b side toward the back surface 21a side, back surface reflection observation is performed in which the light l1 reflected at the back surface is imaged by focusing from the surface side on a region on the side opposite to the surface 21b with respect to the back surface 21a.

[0056] The third step is a step of specifying the state of the modified region of the wafer 20 based on the captured image output from the imaging unit 4 that has detected the transmissive light l1. In the third step, the control unit 8 detects dents, cracks, etc. related to the modified region based on the feature amounts of the feature points shown in the captured images related to the respective imaging regions, and determines whether the state related to the modified region is appropriate based on the detected information. The control unit 8 detects the feature points and the feature amounts of the feature points for the captured images related to the respective imaging regions along the Z direction, and specifies, for example, the position of the feature point with a relatively large feature amount as the formation position of the modified region. As such a detection algorithm in internal observation, for example, the above-described detection algorithm described with reference to FIGS. 13 to 17 is used.

[0057] Here, as described above, in the case of back surface reflection observation in internal observation, there is a problem that the pattern of the mounting surface 300x of the holding member 300 and the pattern of the support surface 2x of the suction table 2 are reflected in the captured image. FIG. 19 is a diagram for explaining the problem in back surface reflection observation in internal observation. As shown in FIG. 19(a), in back surface reflection observation, the focus of the transmissive light l1 is adjusted to a region on the side opposite to the front surface 21b with respect to the back surface 21a, that is, the region on the side of the holding member 300 and the suction table 2. Therefore, when the light l1 reflected by the back surface 21a is imaged by the imaging unit 4, the pattern of the mounting surface 300x of the holding member 300 (see FIG. 19(b)) and the pattern of the support surface 2x of the suction table 2 (see FIG. 19(c)) may be reflected in the captured image. In this case, the accuracy of the determination of the state related to the modified region by the control unit 8, specifically, the estimation accuracy of the dent and crack states related to the modified region may deteriorate.

[0058] FIG. 19(d) is a diagram showing an example of the internal observation result when the pattern of the holding member 300 or the like is reflected in the captured image. In FIG. 19(d), the horizontal axis represents the feature amount, and the vertical axis represents the imaging depth. In the example of FIG. 19(d), in the direct observation region, the feature amount data 901 of the modified region SD1 on the back surface 21a side, the feature amount data 902 of the modified region SD2 on the front surface 21b side, and the feature amount data 910 of the tip of the upper crack are detected. In the back surface reflection region, the feature amount data 903 of the modified region SD1 on the back surface 21a side is detected. Further, in the back surface reflection region, the pattern of the mounting surface 300x of the holding member 300 and the pattern of the support surface 2x of the suction table 2 are detected as the feature amount data 1900. Due to the influence of the feature amount data 1900, the feature amount data 1904 of the modified region SD2 on the front surface 21b side is detected in a state where the feature amount is smaller than when the feature amount data 1900 is not detected. Thus, when the feature amount data 1900 related to the pattern of the holding member 300 or the like is detected, at least the detection of the modified region SD2 on the front surface 21b side is affected, and thereby the accuracy of the determination of the state related to the modified region may deteriorate.

[0059] In order to solve the above-described problems, in the internal observation method according to the present embodiment, in the first step, the holding member 300 is optimized so that the patterns of the holding member 300 and the suction table 2 do not appear in the captured image during the back surface reflection observation, and the optimized holding member 300 is attached to the wafer 20. Specifically, the thickness of the holding member 300 is optimized and / or the material of the holding member 300 is 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 described respectively.

[0060] The optimization of the thickness of the holding member 300 will be described. In the first step, a holding member 300A (300) whose thickness is set according to the distance from the back surface 21a of the wafer 20 to the most distant modified region (hereinafter sometimes referred to as "modified region distance") is attached to the back surface 21a of the wafer 20. Specifically, in the first step, the holding member 300A (300) whose thickness is set to be larger than the value obtained by multiplying the modified region distance by the refractive index ratio of the holding member 300 with respect to the wafer 20 may be attached to the back surface 21a of the wafer 20. For example, in the example shown in FIG. 20(a), the modified region distance is the distance from the back surface 21a to the end portion (upper end portion) on the surface 21b side of the modified region SD2. When the thickness of the holding member 300A is T1, the modified region distance is D, the refractive index of the holding member 300A is R1, and the refractive index of the wafer 20 is R2, the thickness T1 of the holding member 300A is set so as to satisfy the following formula (1). For example, when D = 350 μm, R1 = 1.5, and R2 = 3.5, based on the following formula (1), the thickness T1 of the holding member 300A is calculated as T1 > 150 μm. T1 > D × R1 / R2 ··· (1)

[0061] When such a holding member 300A is used, since the thickness of the holding member 300A is thicker than the distance obtained by converting the modified region distance into an approximate distance in the holding member 300A, as shown in Fig. 20(a), when observing the back surface of the modified regions SD1 and SD2, the light l1 does not reach the mounting surface 300x of the holding member 300A and the support surface 2x of the adsorption table 2. Therefore, when observing the back surface, the pattern of the mounting surface 300x of the holding member 300 and the pattern of the support surface 2x of the adsorption table 2 are not reflected in the captured image. Fig. 20(b) is a diagram showing an example of the internal observation result when the holding member 300A is used. In Fig. 20(b), the horizontal axis represents the feature amount, and the vertical axis represents the imaging depth. In the example of Fig. 20(b), in the back surface reflection region, in addition to the feature amount data 903 of the modified region SD1 on the back surface 21a side, the feature amount data 904 of the modified region SD2 on the front surface 21b side is appropriately detected. And in the back surface reflection region, the feature amount data related to the pattern of the mounting surface 300x of the holding member 300A and the pattern of the support surface 2x of the adsorption table 2 is not detected. As a result, without being affected by the feature amount data related to the pattern of the holding member 300A or the like, the position of the dents and the like of each modified region SD1 and SD2 can be detected with high precision.

[0062] Regarding the thickness of the holding member 300A, it may be determined in consideration of the dz rate calculated from the refractive index for each observation wavelength and the NA (Numerical Aperture) of the objective lens 43. That is, in the first step, the holding member 300 having a thickness set to be larger than the value obtained by multiplying the modified region distance by the dz rate ratio of the holding member 300 with respect to the wafer 20 may be attached to the back surface 21a of the wafer 20. The dz rate is the rate between the Z-axis movement amount of the objective lens in air and the condensing position in the object to be observed. For example, when the wavelength of the light l1 is 1100 nm and the objective lens 43 is moved by 1 μm, and the silicon constituting the wafer 20 moves by 4 μm, the dz rate of the wafer 20 is set to 4.0. In the first step, the holding member 300A having a thickness set to be larger than the value obtained by multiplying the modified region distance by the dz rate ratio of the holding member 300A with respect to the wafer 20 may be attached to the back surface 21a of the wafer 20. Now, for example, when the modified region distance D = 350 μm, the dz rate of the holding member 300A = 1.6, and the dz rate of the wafer 20 = 4.0, the thickness T1 of the holding member 300A is calculated as T1>350×1.6 / 4.0 = 140 μm. Note that the thickness of the holding member 300 may simply be made larger than the modified region distance. That is, in the first step, the holding member 300 having a thickness set to be larger than the modified region distance may simply be attached to the back surface 21a of the wafer 20. However, in this case, since the holding member 300 becomes too thick, it is preferable that the thickness of the holding member 300 is calculated from the above-described formula (1) or the calculation formula considering the dz rate ratio. That is, it is preferable that the thickness of the holding member 300 is calculated in consideration of the wafer thickness, laser processing conditions, wavelength of the observation light, refractive indices of the wafer 20 and the holding member 300, and the NA of the objective lens 43, etc.

[0063] Next, the optimization of the material of the holding member 300 will be described. In the first step, as shown in Fig. 21(a), a holding member 300B with a light 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, a holding member 300B with a light transmittance of 30% or less of the transmissive light l1 may be attached to the back surface 21a of the wafer 20. The holding member 300B may have a transmittance of 50% or less (preferably 30% or less) when the wafer 20 is a silicon wafer and the wavelength band of the transmissive light l1 is 950 nm to 1700 nm, for example.

[0064] When such a holding member 300B having a certain light-shielding property with respect to the transmissive light l1 is used, as shown in Fig. 21(a), the light l1 does not reach the mounting surface 300x of the holding member 300B and the support surface 2x of the suction table 2 when observing the back surface of the modified regions SD1 and SD2. Therefore, when observing the back surface, the pattern of the mounting surface 300x of the holding member 300 and the pattern of the support surface 2x of the suction table 2 are not reflected in the captured image. Fig. 21(b) is a diagram showing an example of the internal observation result when the holding member 300B is used. In Fig. 21(b), the horizontal axis represents the feature amount, and the vertical axis represents the imaging depth. In the example of Fig. 21(b), in the back surface reflection region, in addition to the feature amount data 903 of the modified region SD1 on the back surface 21a side, the feature amount data 904 of the modified region SD2 on the front surface 21b side is appropriately detected. And in the back surface reflection region, the feature amount data related to the pattern of the mounting surface 300x of the holding member 300B and the pattern of the support surface 2x of the suction table 2 is not detected. Thus, the position of the dents and the like of each modified region SD1 and SD2 can be detected with high accuracy without being affected by the feature amount data related to the pattern of the holding member 300B or the like.

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

[0066] The optimization of the thickness of the holding member 300 and the optimization of the material of the holding member 300 described above may be carried out in combination. By making the thickness of the holding member 300 sufficiently thick and including a material with high light-shielding properties in the holding member 300, it is possible to more effectively suppress the pattern of the mounting surface 300x of the holding member 300 and the pattern of the support surface 2x of the suction table 2 from being reflected in the captured image.

[0067] FIG. 22 is a diagram showing an example of the determination result according to the combination of the thickness and transmittance of the holding member 300. Here, the determination result "〇" indicates that the pattern of the holding member 300 or the like is not reflected and false determination is suppressed. "△" indicates that the pattern of the holding member 300 or the like is generally not reflected and false determination is generally suppressed. "×" indicates that the pattern of the holding member 300 or the like is reflected and false determination has occurred. FIG. 23 is a diagram for explaining an example of the distance to the modified region for each processing type. As shown in FIG. 23(a), when SDBG (Stealth Dicing Before Grinding) processing is performed as laser processing, if the thickness of the wafer 20 is 775 μm, the modified region distance is, for example, 230 μm. Further, as shown in FIG. 23(b), when FC (Full Cut) processing in which cracks reach the surface 21b after laser processing is performed as laser processing, if the thickness of the wafer 20 is 775 μm, the modified region distance is, for example, 730 μm. Thus, the modified region distance varies depending on the processing type.

[0068] FIG. 22(a) shows the determination results according to the combination of the thickness and transmittance of the holding member 300 in the SDBG process. In the example shown in FIG. 22(a), when the thickness of the holding member 300 was made greater than 101 μm under the conditions that the thickness of the wafer 20 was 775 μm and the modified region distance was 230 μm, the determination result was "〇" regardless of the transmittance of the holding member 300. Further, when the transmittance of the holding member 300 was 10% or less, the determination result was "〇" regardless of the thickness of the holding member 300. Also, when the thickness of the holding member 300 was 51 to 100 μm and when the thickness of the holding member 300 was 50 μm or less, the determination result was "×" when the transmittance of the holding member 300 was greater than 85%, and the determination result was "△" when the transmittance of the holding member 300 was 30%. Thus, by making the thickness of the holding member 300 sufficiently thick or by making the transmittance of the holding member 300 sufficiently small, the determination result could be made "〇" regardless of the combination of the thickness and transmittance of the holding member 300. Also, even when the determination result did not become "〇" with only the thickness of the holding member 300 or only the transmittance of the holding member 300, the determination result could be made "△" by combining the thickness and transmittance.

[0069] Figure 22(b) shows the determination results according to the combination of the thickness and transmittance of the holding member 300 in the FC process. In the example shown in Figure 22(b), when the thickness of the holding member 300 was greater than 301 μm under the conditions that the thickness of the wafer 20 was 775 μm and the modified region distance was 730 μm, the determination result was "〇" regardless of the transmittance of the holding member 300. Also, when the transmittance of the holding member 300 was 10% or less, the determination result was "〇" regardless of the thickness of the holding member 300. Further, when the thickness of the holding member 300 was 201 - 300 μm, 51 - 200 μm, and 50 μm or less, and the transmittance of the holding member 300 was greater than 85%, the determination result was "×", and when the transmittance of the holding member 300 was 30%, the determination result was "△". Thus, by making the thickness of the holding member 300 sufficiently thick or making the transmittance of the holding member 300 sufficiently small, the determination result could be made "〇" regardless of the combination of the thickness and transmittance of the holding member 300. Also, even when the determination result did not become "〇" with only the thickness of the holding member 300 or only the transmittance of the holding member 300, the determination result could be made "△" by combining the thickness and transmittance.

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

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

[0072] Subsequently, with respect to the wafer 20 placed on the suction table 2 via the holding member 300, the imaging unit 4 outputs light l1 having permeability, the light l1 having permeability that has propagated through the wafer 20 is detected, and the wafer 20 is imaged (step S2, second step).

[0073] Finally, based on the captured image output from the imaging unit 4 that has detected the light l1 having permeability, the state related to the modified region of the wafer 20 is specified (step S3, third step).

[0074] Next, the operation and effect of the internal observation method (inspection method) implemented by the laser processing apparatus 1 according to the present embodiment will be described.

[0075] The internal observation method (inspection method) implemented by the laser processing apparatus 1 according to the present embodiment includes: a first step of attaching the holding member 300 to the back surface 21a of the wafer 20 in which a modified region is formed inside by irradiating laser light, and installing the placement surface 300x of the holding member 300 on the suction table 2; a second step of outputting, by the imaging unit 4, light l1 having permeability with respect to the wafer 20 installed on the suction table 2 via the holding member 300, and detecting the light l1 having permeability that has propagated through the wafer 20; and a third step of specifying the state related to the modified region of the wafer 20 based on the captured image output from the imaging unit 4 that has detected the light l1 having permeability. In the first step, the holding member 300 having a thickness set according to the modified region distance is attached to the back surface 21a.

[0076] In the inspection method according to this embodiment, a holding member 300 is attached to the back surface 21a of a wafer 20 in which a modified region is formed inside, and the holding member 300 is installed on the adsorption table 2. Then, light l1 having permeability is output to the wafer 20 placed on the adsorption table 2 via the holding member 300, so that the inside of the wafer 20 is observed, and the state related to the modified region of the wafer 20 is specified based on the captured image. Here, when internal observation is performed with such a configuration, when back surface reflection observation is performed in which light reflected from the back surface 21a is imaged by focusing from the surface 21b side on a region on the opposite side of the surface 21b from the back surface 21a, the pattern of the surface of the holding member 300 where embossing is performed and the pattern of the porous structure of the adsorption table 2 may be reflected in the captured image. In this case, the estimation accuracy of the state related to the modified region of the wafer 20 based on the captured image may deteriorate. In this regard, in the inspection method according to this embodiment, a holding member 300 having a thickness set according to the modified region distance, which is the distance from the back surface 21a of the wafer 20 to the modified region farthest from the back surface 21a, is attached to the back surface 21a of the wafer 20. In this way, by setting the thickness of the holding member 300 in consideration of the modified region distance, it becomes possible to set the thickness of the holding member 300 so that patterns such as the holding member 300 are prevented from being reflected in the captured image in the back surface reflection observation related to the modified region. This can suppress misjudgment in the internal observation of the wafer 20 after laser processing.

[0077] As a method for avoiding misjudgment in internal observation due to the influence of patterns such as the holding member 300, for example, all imaging images in the Z direction (depth direction) scheduled to be imaged before laser processing are acquired in advance. When internal observation is performed after laser processing, the imaging image captured before laser processing is subtracted from the imaging image after laser processing, and a method of including only the information related to the modified region formed after laser processing within the field of view (pattern avoidance method by filtering) is conceivable. However, in such a method, since it is necessary to acquire all the images in the Z direction scheduled to be imaged before laser processing in advance, the tact deteriorates. Also, if the imaging state such as the amount of light or the luminance value changes even slightly between the imaging images before and after laser processing, the filtering function (subtraction function) does not function properly, and the accuracy of internal observation decreases. In this regard, the internal observation method (inspection method) implemented by the laser processing apparatus 1 according to the present embodiment does not require imaging before laser processing, so the tact can be improved, and since filtering is not performed, the above-described decrease in the accuracy of internal observation does not become a problem. That is, the internal observation method (inspection method) implemented by the laser processing apparatus 1 according to the present embodiment can suppress misjudgment in the internal observation of the wafer 20 after laser processing while improving the tact.

[0078] In the first step, the holding member 300 having a thickness set to be larger than the modified region distance may be attached to the back surface 21a. Thereby, it is possible to suppress the pattern of the holding member 300 or the like from being reflected in the imaging image in the back surface reflection observation related to the modified region. Thereby, it is possible to suppress misjudgment in the internal observation of the wafer 20 after laser processing.

[0079] In the first step, the holding member 300 having a thickness set to be greater than the value obtained by multiplying the refractive index ratio of the holding member 300 with respect to the wafer 20 by the modified region distance may be attached to the back surface 21a. By multiplying the refractive index ratio with the modified region distance, the modified region distance is converted into an approximate distance in the holding member 300 considering the refractive index. Thus, when the holding member 300 having a thickness greater than the converted value is attached to the back surface 21a, it is possible to suppress the pattern of the holding member 300 and the like from being reflected in the captured image in the back surface reflection observation related to the modified region. As a result, it is possible to suppress misjudgment in the internal observation of the wafer 20 after laser processing. Further, by setting the thickness of the holding member 300 according to the value obtained by multiplying the refractive index ratio with the modified region distance, for example, compared with the case where the thickness of the holding member 300 is simply set to be greater than the modified region distance, the thickness of the holding member 300 can be reduced. That is, it is possible to suppress misjudgment in the internal observation of the wafer 20 after laser processing while avoiding excessive increase in the thickness of the holding member 300.

[0080] In the first step, the holding member 300 having a thickness set to be greater than the value obtained by multiplying the distance of the modification region by the dz rate ratio of the holding member 300 with respect to the wafer 20 may be attached to the back surface 21a. By multiplying the distance of the modification region by the dz rate ratio, the distance of the modification region is converted into an approximate distance in the holding member 300 considering the refractive index and the numerical aperture (NA) of the objective lens 43 of the imaging unit 4. Therefore, when the holding member 300 having a thickness greater than the converted value is attached to the back surface 21a, it is possible to suppress the pattern of the holding member 300 or the like from being reflected in the captured image in the back surface reflection observation related to the modification region. As a result, it is possible to suppress misjudgment in the internal observation of the wafer 20 after laser processing. Further, by setting the thickness of the holding member 300 according to the value obtained by multiplying the distance of the modification region by the dz rate ratio, for example, when the thickness of the holding member 300 is simply set to be greater than the distance of the modification region, or when the thickness of the holding member 300 is set considering only the refractive index ratio, the thickness of the holding member 300 can be made smaller. That is, it is possible to suppress misjudgment in the internal observation of the wafer 20 after laser processing while avoiding excessive increase in the thickness of the holding member 300.

[0081] In the internal observation method (inspection method) implemented by the laser processing apparatus 1 according to the present embodiment, in the first step, the holding member 300 having a light transmittance of 50% or less for the transmissive light l1 may be attached to the back surface 21a. In this way, when the light-shielding holding member 300 is attached to the back surface 21a, it becomes difficult for the image of an object disposed below (on the side of the suction table 2) the light-shielding holding member 300 to be reflected in the captured image in the back surface reflection observation. Specifically, it becomes difficult for the pattern of the surface of the holding member 300 where the embossing process is performed and the pattern of the porous structure of the suction table 2 to be reflected in the captured image. As a result, it is possible to suppress the pattern of the holding member 300 or the like from being reflected in the captured image in the back surface reflection observation, and it is possible to suppress misjudgment in the internal observation of the wafer 20 after laser processing.

[0082] In the first step, a holding member 300 having a light transmittance of 30% or less for the transmissive light l1 may be attached to the back surface 21a. By setting the transmittance of the transmissive light l1 in the holding member 300 to 30% or less, it is possible to more appropriately suppress the pattern of the holding member 300 or the like from being reflected in the captured image in the back surface reflection observation, and it is possible to suppress misjudgment in the internal observation of the wafer 20 after laser processing.

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

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

[0085] In the second step, while changing the imaging region along the Z direction, which is the vertical direction, the transmissive light l1 is detected for each imaging region. In the third step, for the captured images related to each imaging region along the Z direction, the feature points and the feature amounts of the feature points are detected, and the position of the feature point with a relatively large feature amount may be specified as the formation position of the modified region. In this way, by specifying the formation position of the modified region from the feature amounts of the feature points in the captured image, the accuracy and efficiency of the internal observation of the wafer 20 after laser processing can be improved.

Explanation of Reference Numerals

[0086] 2... Adsorption table, 4... Imaging unit, 20... Wafer, 21a... Back surface (second surface), 21b... Front surface (first surface), 300... Holding member, L... Laser beam, l1... Transmissive light.

Claims

1. A first step of attaching a holding member to a second surface opposite to a first surface irradiated with a laser beam to a wafer in which a modified region is formed inside by irradiation with the laser beam, and installing a surface opposite to the surface of the holding member attached to the wafer on a suction table; A second step of detecting, by an imaging unit, light having permeability that is output and propagated through the wafer with respect to the wafer installed on the suction table via the holding member; A third step of specifying a state related to the modified region of the wafer based on an imaging image output from the imaging unit that has detected the light having permeability; and The inspection method includes: In the first step, the holding member having a thickness set according to a modified region distance, which is a distance from the second surface of the wafer to the modified region that is most distant from the second surface, is attached to the second surface.

2. The inspection method according to claim 1, wherein in the first step, the holding member having a thickness set to be larger than the modified region distance is attached to the second surface.

3. The inspection method according to claim 1, wherein in the first step, the holding member having a thickness set to be larger than a value obtained by multiplying the modified region distance by a refractive index ratio of the holding member with respect to the wafer is attached to the second surface.

4. The inspection method according to claim 1, wherein in the first step, the holding member having a thickness set to be larger than a value obtained by multiplying the modified region distance by a dz rate ratio of the holding member with respect to the wafer is attached to the second surface.

5. A first step of attaching a holding member to a second surface opposite to a first surface irradiated with a laser beam to a wafer in which a modified region is formed inside by irradiation with the laser beam, and installing a surface opposite to the surface of the holding member attached to the wafer on a suction table; A second step of detecting, by an imaging unit, light having permeability that is output and propagated through the wafer with respect to the wafer installed on the suction table via the holding member; A third step of specifying a state related to the modified region of the wafer based on an imaging image output from the imaging unit that has detected the light having permeability; and The inspection method includes: In the first step, the holding member having a light transmittance of 50% or less is attached to the second surface.

6. The inspection method according to claim 5, wherein in the first step, the holding member having a light transmittance of 30% or less of the light having permeability is attached to the second surface.

7. The inspection method according to claim 5 or 6, wherein in the first step, the holding member containing a material that absorbs the light having permeability is attached to the second surface.

8. The inspection method according to claim 5 or 6, wherein in the first step, the holding member containing a material that reflects the light having permeability is attached to the second surface.

9. In the second step, while changing the imaging region along the Z direction which is the vertical direction, the light having permeability is detected for each imaging region. In the third step, for the imaging images related to each imaging region along the Z direction, the feature points and the feature amounts of the feature points are detected, and the position of the feature point having a relatively large feature amount is specified as the formation position of the modified region. The inspection method according to any one of claims 1 to 8.

Images