JP7717596B2 - Inspection method - Google Patents

Description

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

An inspection device is known that includes a semiconductor substrate and cuts a wafer having a functional device layer formed on the back surface along multiple lines by irradiating the wafer from the front side of the semiconductor substrate with laser light to form multiple rows of modified regions within the semiconductor substrate along each of the multiple lines. The inspection device described in Patent Document 1 is equipped with an infrared camera, which makes it possible to observe modified regions formed within the semiconductor substrate and processing damage formed in the functional device layer from the front side of the semiconductor substrate. For example, the inspection device estimates the state of cracks in the wafer after processing based on the results of this internal observation, and determines whether the processing is successful (whether the desired processing was performed under the set processing conditions) based on the estimated crack state.

JP 2017-64746 A

In the internal observation described above, in addition to direct observation, in which an image is captured by moving the focal point from the front side to the back side, backside reflection observation may also be performed, in which the focal point is adjusted from the front side to the area opposite the back side and the light reflected from the back side is captured. During internal observation, a holding member is typically attached to the backside of the wafer, and the above-mentioned direct observation and backside reflection observation are performed with the holding member placed on a suction table. Here, the holding member may have an embossed surface that contacts the suction table. Furthermore, the suction table may have a porous surface on which the holding member is placed, resulting in minute irregularities created by the porous material. When backside reflection observation is performed using such a holding member and/or suction table, the pattern on the embossed surface of the holding member or the pattern of the porous structure of the suction table may appear in the captured image. This may result in a decrease in the accuracy of estimating the state of scratches and cracks in the modified region (leading to incorrect determination of the state of the modified region).

One aspect of the present invention was conceived in consideration of the above-mentioned circumstances, and relates to an inspection method that can reduce erroneous judgments when observing the inside of a wafer after laser processing.

An inspection method according to one aspect of the present invention comprises the following steps: a first step of attaching a holding member to a second surface of a wafer, the second surface being opposite the first surface irradiated with laser light, and placing the surface of the holding member opposite the surface attached to the wafer on a suction table; a second step of using an imaging unit to output transparent light to the wafer placed on the suction table via the holding member and detect the transparent light that has propagated through the wafer; and a third step of identifying the state of the modified region of the wafer based on the captured image output from the imaging unit that detected the transparent light. In the first step, a holding member is attached to the second surface of the wafer, the thickness of which is set according to the modified region distance, which is the distance from the second surface of the wafer to the modified region farthest from the second surface.

In one aspect of the present invention, an inspection method involves attaching a holding member to the second surface (rear surface) of a wafer having a modified region formed therein, and placing the holding member on a suction table. Transmissive light is emitted through the holding member to the wafer placed on the suction table, thereby performing internal observation of the wafer, and identifying the state of the modified region of the wafer based on the captured image. When performing internal observation using this configuration, if backside reflection observation is performed by focusing on an area on the opposite side of the second surface (rear surface) from the first surface (front surface) and capturing an image of light reflected by the second surface, the embossed surface pattern of the holding member or the porous structure of the suction table may appear in the captured image. This could result in a decrease in the accuracy of estimating the state of the modified region of the wafer based on the captured image. In this regard, in an inspection method according to one aspect of the present invention, a holding member whose thickness is set according to the modified region distance, which is the distance from the second surface to the farthest modified region from the second surface, is attached to the second surface of the wafer. In this way, by setting the thickness of the holding member taking the modified region distance into consideration, it is possible to set the thickness of the holding member so that the pattern of the holding member, etc., is prevented from appearing in the captured image when observing the back surface reflection of the modified region. This makes it possible to reduce erroneous judgments when observing the inside of a wafer after laser processing.

In the first step, a holding member with a thickness set to be greater than the modified region distance may be attached to the second surface. This prevents the pattern of the holding member, etc. from appearing in the captured image when observing the back surface reflection of the modified region. This makes it possible to reduce erroneous determinations when observing the inside of the wafer after laser processing.

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 to the wafer may be affixed to the second surface. Multiplying the modified region distance by the refractive index ratio converts the modified region distance to an approximate distance within the holding member that takes the refractive index into account. Therefore, by affixing a holding member having a thickness greater than the converted value to the second surface, the pattern of the holding member or the like is prevented from appearing in the captured image during backside reflection observation of the modified region. This reduces erroneous determinations in internal observation of the wafer after laser processing. Furthermore, by setting the thickness of the holding member according to the value obtained by multiplying the modified region distance by the refractive index ratio, the thickness of the holding member can be made smaller than, for example, when the thickness of the holding member is simply set to be greater than the modified region distance. In other words, it is possible to prevent the holding member from becoming too thick while also reducing erroneous determinations in internal observation of the wafer after laser processing.

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 relative to the wafer may be attached to the second surface. Multiplying the modified region distance by the dz rate ratio converts the modified region distance into an approximate distance within the holding member that takes into account the refractive index and the NA (numerical aperture) of the objective lens of the imaging unit. Therefore, by attaching a holding member having a thickness greater than the converted value to the second surface, the pattern of the holding member or the like is prevented from appearing in the captured image during backside reflection observation of the modified region. This reduces erroneous determinations in internal observation of the wafer after laser processing. Furthermore, by setting the thickness of the holding member according to the value obtained by multiplying the modified region distance by the dz rate ratio, the thickness of the holding member can be made smaller than, for example, when the thickness is simply set to be greater than the modified region distance or when the thickness of the holding member is set taking only the refractive index ratio into consideration. In other words, it is possible to prevent the holding member from becoming too thick while reducing erroneous determinations in internal observation of the wafer after laser processing.

An inspection method according to one aspect of the present invention comprises the following steps: a first step of attaching a holding member to a second surface of a wafer, the second surface being opposite the first surface irradiated with laser light, and placing the surface of the holding member opposite the surface attached to the wafer on a suction table; a second step of using an imaging unit to output transparent light to the wafer placed on the suction table via the holding member and detect the transparent light that has propagated through the wafer; and a third step of identifying the state of the modified region of the wafer based on the captured image output from the imaging unit that detected the transparent light. In the first step, a holding member with a transmittance of 50% or less for transparent light is attached to the second surface.

In an inspection method according to one aspect of the present invention, a holding member (light-blocking holding member) with a transmittance of 50% or less for transmitted light is affixed to the second surface. This makes it less likely that an image of an object located below the light-blocking holding member (toward the suction table) will appear in the captured image during backside reflection observation. Specifically, the pattern of the embossed surface of the holding member and the pattern of the porous structure of the suction table will not appear in the captured image. This prevents the pattern of the holding member, etc. from appearing in the captured image during backside reflection observation, thereby reducing erroneous determinations when observing the interior of a wafer after laser processing.

In the first step, a holding member having a transmittance of 30% or less for transparent light may be attached to the second surface. By ensuring that the transmittance of the holding member is 30% or less for transparent light, it is possible to more appropriately prevent patterns from appearing in captured images during backside reflection observation, thereby reducing erroneous determinations during internal observation of the wafer after laser processing.

In the first step, a holding member containing a material that absorbs light with transparency may be attached to the second surface. With this configuration, the transmittance of light with transparency through the holding member can be appropriately reduced.

In the first step, a holding member containing a material that reflects light with transparency may be attached to the second surface. With this configuration, the transmittance of light with transparency through the holding member can be appropriately reduced.

In the second step, the imaging area is changed along the vertical Z direction, and light that transmits through each imaging area is detected. In the third step, feature points and the feature amounts of the feature points are detected for the captured image of each imaging area along the Z direction, and the position of the feature point with a relatively large feature amount may be identified as the formation position of the modified area. In this way, by identifying the formation position of the modified area from the feature amounts of the feature points in the captured image, the accuracy and efficiency of internal observation of the wafer after laser processing can be improved.

An inspection method according to one aspect of the present invention can reduce erroneous determinations when observing the inside of a wafer after laser processing.

1 is a configuration diagram of a laser processing apparatus according to an embodiment; FIG. 2 is a plan view of a wafer according to an embodiment. 3 is a cross-sectional view of a portion of the wafer shown in FIG. 2. FIG. 2 is a configuration diagram of a laser irradiation unit shown in FIG. 1 . FIG. 2 is a configuration diagram of an inspection imaging unit shown in FIG. 1 . FIG. 2 is a configuration diagram of an imaging unit for alignment correction shown in FIG. 1 . 6A and 6B are cross-sectional views of a wafer for explaining the imaging principle of the inspection imaging unit shown in FIG. 5, and images at various locations taken by the inspection imaging unit. 6A and 6B are cross-sectional views of a wafer for explaining the imaging principle of the inspection imaging unit shown in FIG. 5, and images at various locations taken by the inspection imaging unit. 1 is a SEM image of modified regions and cracks formed within a semiconductor substrate. 1 is a SEM image of modified regions and cracks formed within a semiconductor substrate. 6 is a light path diagram for explaining the imaging principle of the inspection imaging unit shown in FIG. 5, and a schematic diagram showing an image at a focal point of the inspection imaging unit. FIG. 6 is a light path diagram for explaining the imaging principle of the inspection imaging unit shown in FIG. 5, and a schematic diagram showing an image at a focal point of the inspection imaging unit. FIG. FIG. 10 is a diagram illustrating crack detection. FIG. 10 is a diagram illustrating crack detection. FIG. 10 is a diagram illustrating the detection of a scratch. FIG. 10 is a diagram illustrating the detection of a scratch. FIG. 10 is a diagram illustrating the detection of a scratch. 3A and 3B are diagrams illustrating detailed configurations of a holding member and an adsorption table. 1A and 1B are diagrams illustrating problems with back surface reflection observation for internal observation. 10A and 10B are diagrams illustrating an example of a holding member suitable for rear surface reflection observation. 10A and 10B are diagrams illustrating an example of a holding member suitable for rear surface reflection observation. 10A and 10B are diagrams showing determination results according to combinations of thickness and transmittance of a holding member. 10A and 10B are diagrams illustrating an example of the distance to the modified region for each processing type. 10 is a flowchart illustrating an example of an inspection method.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are designated by the same reference numerals, and redundant explanations will be omitted.
[Configuration of laser processing device]

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

The suction table 2 supports the object 11, for example, by adsorbing a film attached to the object 11. Although not shown in Figure 1, as shown in Figure 18 and other figures, a holding member 300 that holds the wafer 20 is provided between the object 11 (wafer 20) and the suction table 2 (details will be described later). The suction table 2 is movable along both the X and Y directions, and is rotatable around an axis parallel to the Z direction. The X and Y directions are first and second horizontal directions that are perpendicular to each other, and the Z direction is the vertical direction.

The laser irradiation unit 3 focuses laser light L, which is transparent to the object 11, and irradiates the object 11. When the laser light L is focused inside the object 11 supported on the suction table 2, the laser light L is particularly absorbed in the area corresponding to the focusing point C of the laser light L, forming a modified region 12 inside the object 11.

The modified region 12 is a region whose density, refractive index, mechanical strength, and other physical properties differ from those of the surrounding unmodified region. Examples of modified region 12 include a melt-processed region, a crack region, an insulation breakdown region, and a refractive index change region. The modified region 12 has the property that cracks tend to extend from the modified region 12 to the incident side of the laser light L and to the opposite side. These properties of the modified region 12 are utilized to cut the object 11.

As an example, when the suction table 2 is moved along the X direction and the focal point C is moved along the X direction relative to the object 11, multiple modified spots 12s are formed in a row along the X direction. One modified spot 12s is formed by irradiating one pulse of laser light L. A row of modified regions 12 is a collection of multiple modified spots 12s lined up in a row. Adjacent modified spots 12s may connect with each other or may separate from each other depending on the relative movement speed of the focal point C with respect to the object 11 and the repetition frequency of the laser light L.

The imaging unit 4 captures images of the modified area 12 formed in the object 11 and the tip of the crack extending from the modified area 12.

Under the control of the control unit 8, the imaging units 5 and 6 capture an image of the object 11 supported on the suction table 2 using light that passes through the object 11. The images captured by the imaging units 5 and 6 are used, for example, to align the irradiation position of the laser light L.

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

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

The display 150 functions as an input unit that accepts information input from the user and as a display unit that displays information to the user.

[Configuration of the object]
As shown in FIGS. 2 and 3 , the object 11 in this embodiment is a wafer 20. The wafer 20 includes a semiconductor substrate 21 and a functional device layer 22. While the present embodiment describes the wafer 20 as having the functional device layer 22, the wafer 20 may or may not include the functional device layer 22, and may be a bare wafer. The semiconductor substrate 21 has a back surface 21 a and a front surface 21 b. The semiconductor substrate 21 is, for example, a silicon substrate. The functional device layer 22 is formed on the back surface 21 a of the semiconductor substrate 21. The functional device layer 22 includes a plurality of functional devices 22 a arranged two-dimensionally along the back surface 21 a. The functional devices 22 a may be, for example, light-receiving devices such as photodiodes, light-emitting devices such as laser diodes, or circuit devices such as memories. The functional devices 22 a may be three-dimensionally configured by stacking multiple layers. The semiconductor substrate 21 has a notch 21 c indicating the crystal orientation, but an orientation flat may be provided instead of the notch 21 c.

The wafer 20 is cut into individual functional elements 22a along each of the multiple lines 15. When viewed in the thickness direction of the wafer 20, the multiple lines 15 pass between each of the multiple functional elements 22a. More specifically, when viewed in the thickness direction of the wafer 20, the lines 15 pass through the center of the street region 23 (the center in the width direction). The street region 23 extends in the functional element layer 22 so as to pass 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 lattice pattern. Note that although the lines 15 are virtual lines, they may also be actually drawn lines.

[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 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 reflective liquid crystal (LCOS: Liquid Crystal on Silicon) spatial light modulator (SLM). 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.

In this embodiment, the laser irradiation unit 3 irradiates the wafer 20 with laser light L from the front surface 21b of the semiconductor substrate 21 along each of the multiple lines 15, thereby forming two rows of modified regions 12a, 12b inside the semiconductor substrate 21 along each of the multiple lines 15. Of the two rows of modified regions 12a, 12b, modified region 12a is the modified region closest to the back surface 21a. Of the two rows of modified regions 12a, 12b, modified region 12b is the modified region closest to modified region 12a and closest to the front surface 21b.

The two rows of modified regions 12a, 12b are adjacent to each other in the thickness direction (Z direction) of the wafer 20. The two rows of modified regions 12a, 12b are formed by moving two focal points C1, C2 along a line 15 relative to the semiconductor substrate 21. The laser light L is modulated by the spatial light modulator 32 so that, for example, focal point C2 is located behind focal point C1 in the traveling direction and on the incident side of the laser light L. The modified regions may be formed using a single focus or multiple focuses, and in one pass or multiple passes.

The laser irradiation unit 3 irradiates the wafer 20 with laser light L from the front surface 21b of the semiconductor substrate 21 along each of the multiple lines 15. As an example, for a semiconductor substrate 21 that is a 400 μm-thick single-crystal silicon <100> substrate, two focusing points C1 and C2 are aligned at positions 54 μm and 128 μm from the back surface 21a, respectively, and the laser light L is irradiated from the front surface 21b of the semiconductor substrate 21 along each of the multiple lines 15 to the wafer 20. In this case, for example, if the conditions are such that the cracks 14 spanning two rows of modified regions 12a and 12b reach the back surface 21a of the semiconductor substrate 21, the wavelength of the laser light L is 1099 nm, the pulse width is 700 ns, and the repetition frequency is 120 kHz. The output of the laser beam L at focal point C1 is 2.7 W, the output of the laser beam L at focal point C2 is 2.7 W, and the relative movement speed of the two focal points C1 and C2 relative to the semiconductor substrate 21 is 800 mm/sec. If the number of processing passes is 5, for example, the processing positions for the wafer 20 described above may be ZH80 (328 μm from the back surface 21 a), ZH69 (283 μm from the back surface 21 a), ZH57 (234 μm from the back surface 21 a), ZH26 (107 μm from the back surface 21 a), or ZH12 (49.2 μm from the back surface 21 a). 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 movement speed may be 490 mm/sec.

[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 transparent to the wafer 20 and detects the light that has propagated through the wafer 20, thereby capturing an image of the interior of the wafer 20. The light source 41 outputs light 11 that is transparent to the semiconductor substrate 21. The light source 41 is configured, for example, with a halogen lamp and a filter, and outputs light 11 in the near-infrared region. The light 11 output from the light source 41 is reflected by the mirror 42, passes through the objective lens 43, and is irradiated onto the wafer 20 from the front surface 21b of the semiconductor substrate 21. At this time, the suction table 2 supports the wafer 20 on which the two rows of modified regions 12a and 12b have been formed, as described above.

The objective lens 43 passes light l1 reflected by the back surface 21a of the semiconductor substrate 21. In other words, the objective lens 43 passes light l1 that has propagated through the semiconductor substrate 21. The numerical aperture (NA) of the objective lens 43 is, for example, 0.45 or greater. The objective lens 43 has a correction collar 43a. The correction collar 43a corrects aberrations that occur in the light l1 within the semiconductor substrate 21, for example, by adjusting the distance between the multiple lenses that make up the objective lens 43. Note that the means for correcting aberrations is not limited to the correction collar 43a, and other correction means such as a spatial light modulator may also be used. The light detection unit 44 detects light l1 that has passed through the objective lens 43 and the mirror 42. The light detection unit 44 is, for example, composed of an InGaAs camera, and detects light l1 in the near-infrared region. Note that the means for detecting (imaging) light l1 in the near-infrared region is not limited to an InGaAs camera, and other imaging means that perform transmission imaging, such as a transmission confocal microscope, may also be used.

The imaging unit 4 can capture images of each of the two rows of modified regions 12a and 12b, and the tips of each of the multiple cracks 14a, 14b, 14c, and 14d (details will be described later). Crack 14a is a crack that extends from modified region 12a toward the back surface 21a. Crack 14b is a crack that extends from modified region 12a toward the front surface 21b. Crack 14c is a crack that extends from modified region 12b toward the back surface 21a. Crack 14d is a crack that extends from modified region 12b toward the front surface 21b.

[Configuration of the imaging unit for alignment correction]
6 , the imaging unit 5 has 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 transparent to the semiconductor substrate 21. The light source 51 is configured, for example, with 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 and passes through the lens 53, and is irradiated onto the wafer 20 from the front surface 21b side of the semiconductor substrate 21.

The lens 53 passes light I2 reflected by the back surface 21a of the semiconductor substrate 21. In other words, the lens 53 passes light I2 that has propagated through the semiconductor substrate 21. 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 greater than the numerical aperture of the lens 53. The light detection unit 54 detects light I2 that has passed through the lens 53 and mirror 52. The light detection unit 54 is composed of, for example, an InGaAs camera, and detects light I2 in the near-infrared region.

Under the control of the control unit 8, the imaging unit 5 irradiates the wafer 20 with light I2 from the front surface 21b and detects light I2 returning from the back surface 21a (functional device layer 22), thereby capturing an image of the functional device layer 22. Similarly, under the control of the control unit 8, the imaging unit 5 irradiates the wafer 20 with light I2 from the front surface 21b and detects light I2 returning from the positions where the modified regions 12a and 12b are formed on the semiconductor substrate 21, thereby capturing an image of the region including the modified regions 12a and 12b. These images are used to align the irradiation position of the laser light L. The imaging unit 6 has a similar configuration to the imaging unit 5, except that the lens 53 has a lower magnification (e.g., 6x in the imaging unit 5 and 1.5x in the imaging unit 6), and is used for alignment in the same way as the imaging unit 5.

[Principle of imaging by inspection imaging unit]
Using the imaging unit 4 shown in Figure 5, the focal point F (the focal point of the objective lens 43) was moved from the front surface 21b toward the back surface 21a of a semiconductor substrate 21 in which cracks 14 spanning two rows of modified regions 12a and 12b reached the back surface 21a, as shown in Figure 7. In this case, when the focal point F was focused on the tip 14e of the crack 14 extending from the modified region 12b toward the front surface 21b from the front surface 21b, the tip 14e could be seen (image on the right in Figure 7). However, when the focal point F was focused on the front surface 21b from the front surface 21b, the crack 14 itself and the tip 14e of the crack 14 reaching the back surface 21a could not be seen (image on the left in Figure 7). Furthermore, when the focal point F was focused on the back surface 21a of the semiconductor substrate 21 from the front surface 21b, the functional element layer 22 could be seen.

Using the imaging unit 4 shown in FIG. 5, the focal point F is moved from the front surface 21b toward the back surface 21a of a semiconductor substrate 21 in which cracks 14 spanning two rows of modified regions 12a, 12b have not yet reached the back surface 21a, as shown in FIG. 8. In this case, even if the focal point F is aligned from the front surface 21b to the tip 14e of the crack 14 extending from the modified region 12a to the back surface 21a, the tip 14e cannot be seen (the image on the left in FIG. 8). However, by aligning the focal point F from the front surface 21b to the region on the opposite side of the back surface 21a from the front surface 21b (i.e., the region on the functional device layer 22 side of the back surface 21a) and positioning a virtual focus Fv symmetrical to the focus F with respect to the back surface 21a at the tip 14e, the tip 14e can be seen (the image on the right in FIG. 8). The virtual focus Fv is a point symmetrical with the focus F, which takes into account the refractive index of the semiconductor substrate 21, with respect to the back surface 21a.

As described above, the reason why the crack 14 itself cannot be seen is presumably because the width of the crack 14 is smaller than the wavelength of the illumination light l1. Figures 9 and 10 are SEM (Scanning Electron Microscope) images of the modified region 12 and crack 14 formed inside a semiconductor substrate 21, which is a silicon substrate. Figure 9(b) is an enlarged image of region A1 shown in Figure 9(a), Figure 10(a) is an enlarged image of region A2 shown in Figure 9(b), and Figure 10(b) is an enlarged image of region A3 shown in Figure 10(a). As such, the width of the crack 14 is approximately 120 nm, which is smaller than the wavelength of light l1 in the near-infrared region (e.g., 1.1 to 1.2 μm).

Based on the above, the imaging principle is assumed to be as follows. As shown in Figure 11(a), when the focus F is positioned in air, light l1 does not return, resulting in a dark image (the image on the right in Figure 11(a)). As shown in Figure 11(b), when the focus F is positioned inside the semiconductor substrate 21, light l1 reflected from the back surface 21a returns, resulting in a whitish image (the image on the right in Figure 11(b)). As shown in Figure 11(c), when the focus F is aligned with the modified region 12 from the front surface 21b side, the modified region 12 absorbs, scatters, etc., a portion of the light l1 reflected from the back surface 21a and returns, resulting in an image in which the modified region 12 appears dark against a whitish background (the image on the right in Figure 11(c)).

As shown in (a) and (b) of Figure 12, when the focal point F is focused on the tip 14e of the crack 14 from the surface 21b side, for example, optical peculiarities (stress concentration, strain, discontinuity in atomic density, etc.) occurring near the tip 14e and light confinement occurring near the tip 14e cause scattering, reflection, interference, absorption, etc. of a portion of the light l1 reflected from the back surface 21a and returned, resulting in an image in which the tip 14e appears dark against a whitish background (the images on the right in (a) and (b) of Figure 12). As shown in (c) of Figure 12, when the focal point F is focused on a portion of the crack 14 other than the vicinity of the tip 14e from the surface 21b side, at least a portion of the light l1 reflected from the back surface 21a returns, resulting in a whitish image (the image on the right in (c) of Figure 12).

[Detection algorithm for internal observation]
The following describes in detail an algorithm for detecting (identifying) cracks 14 and an algorithm for detecting (identifying) traces associated with modified regions in relation to the above-described internal observation of wafer 20. The detection of cracks 14 and traces associated with modified regions may be determined and performed by AI.

Figures 13 and 14 are diagrams illustrating crack detection. Figure 13 shows the results of internal observation (an image of the inside of the wafer 20). The control unit 8 first detects a group of straight lines 140 from the image of the inside of the wafer 20 as shown in Figure 13(a). To detect the group of straight lines 140, an algorithm such as the Hough transform or LSD (Line Segment Detector) is used. The Hough transform is a method of detecting straight lines by detecting all straight lines passing through a point on an image and weighting the lines that pass through more characteristic points. The LSD is a method of detecting straight lines by calculating the gradient and angle of brightness values in the image to estimate the area that will become a line segment, and then approximating this area to a rectangle.

Next, the control unit 8 detects a crack 14 from the group of lines 140 by calculating the similarity between the group of lines 140 and the crack line, as shown in Figure 14. As shown in the upper diagram of Figure 14, a crack line is characterized by being very bright in the Y direction both before and after the crack line compared to the brightness value on the line. For this reason, the control unit 8, for example, compares the brightness values of all pixels in the detected group of lines 140 with those before and after in the Y direction, and determines the number of pixels where the difference is greater than or equal to a threshold value both before and after as the similarity score. Then, the group of lines 140 detected with the highest similarity score to the crack line is determined to be the representative value for that image. A higher representative value is an indicator that the possibility of the presence of a crack 14 is higher. By comparing the representative values in multiple images, the control unit 8 determines those with relatively high scores as crack image candidates.

Figures 15 to 17 are diagrams explaining scratch detection. Figure 15 shows the results of internal observation (image of the inside of wafer 20). For the image of the inside of wafer 20 as shown in Figure 15(a), the control unit 8 detects corners (concentrations of edges) within the image as key points, and detects their position, size, and direction to detect feature points 250. Known methods for detecting feature points in this way include Eigen, Harris, Fast, SIFT, SURF, STAR, MSER, ORB, and AKAZE.

As shown in Figure 16, the dent 280 has a strong corner characteristic, as it is made up of circles, rectangles, and other shapes arranged at regular intervals. Therefore, by aggregating the feature amounts of the feature points 250 in the image, it is possible to detect the dent 280 with high accuracy. As shown in Figure 17, by comparing the total feature amounts for each image captured by shifting in the depth direction, a change in the peaks can be confirmed, indicating the amount of crack rows for each modified layer. The control unit 8 estimates the peak of this change as the position of the dent 280. By aggregating the feature amounts in this way, it is possible to estimate not only the dent position but also the pulse pitch.

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

As shown in Figure 18(a), the first step is to attach a holding member 300 to the back surface 21a (second surface) of a wafer 20, which has been irradiated with laser light to form modified regions SD1 and SD2 therein, on the opposite side of the front surface 21b (first surface) to which the laser light is irradiated, and to place the mounting surface 300x of the holding member 300 opposite the surface attached to the wafer 20 on the support surface 2x of the suction table 2.

The holding member 300 is, for example, a tape such as dicing tape or backgrind tape. As shown in FIG. 18(b), the mounting surface 300x of the holding member 300 is embossed. Furthermore, the support surface 2x of the suction table 2 has a porous structure, with minute irregularities formed by the porous material. When such a holding member 300 and/or suction table 2 is used for internal observation, the pattern on the mounting surface 300x of the holding member 300 or the pattern on the support surface 2x of the suction table 2 may appear in the captured image during backside reflection observation (details will be described later). Therefore, as will be described later, in the internal observation method according to this embodiment, 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 backside reflection observation, and the optimized holding member 300 is attached to the wafer 20 (details will be described later).

The second step is an imaging step in which the imaging unit 4 outputs transparent light l1 (see FIG. 18(a)) to the wafer 20 placed on the suction table 2 via the holding member 300 and detects the transparent light l1 propagating through the wafer 20. In the second step, the imaging area is changed along the Z direction, which is the vertical direction, and the transparent light l1 is detected (i.e., imaged) for each imaging area. Specifically, in the second step, the control unit 8 controls the drive unit 7 to sequentially move the imaging unit 4 to positions where each imaging area along the Z direction in a predetermined imaging range of the wafer 20 can be imaged, and also controls the imaging unit 4 to image each imaging area while changing the imaging area along the Z direction. The imaging range here includes a direct observation area, which is imaged by moving the focus from the front surface 21b toward the back surface 21a, and a back surface reflection area, which is imaged by focusing from the front surface 21b on an area on the opposite side of the back surface 21a to the front surface 21b and capturing light reflected by the back surface 21a. That is, in internal observation according to this embodiment, in addition to direct observation, in which an image is captured by moving the focal point from the front surface 21b toward the back surface 21a, back surface reflection observation is also performed, in which the focal point is set from the front surface side to the area on the opposite side of the back surface 21a from the front surface 21b, and light l1 reflected by the back surface is captured.

The third step is a step of identifying the state of the modified regions of the wafer 20 based on the captured images output from the imaging unit 4 that detected the transparent light 11. In the third step, the control unit 8 detects scratches, cracks, etc., related to the modified regions based on the feature amounts of the feature points shown in the captured images of each captured region, and determines whether the state of the modified regions is appropriate based on the detected information. The control unit 8 detects the feature points and their feature amounts for the captured images of each captured region along the Z direction, and identifies, for example, the positions of feature points with relatively large feature amounts as the formation positions of the modified regions. The detection algorithm used for such internal observation is, for example, the detection algorithm described above using Figures 13 to 17.

As described above, when using back-surface reflection observation for internal observation, there is a problem in that the pattern on the mounting surface 300x of the holding member 300 and the pattern on the support surface 2x of the suction table 2 are captured in the captured image. Figure 19 is a diagram illustrating the problem with back-surface reflection observation for internal observation. As shown in Figure 19(a), in back-surface reflection observation, transparent light l1 is focused on the area on the opposite side of the back surface 21a from the front surface 21b, i.e., the area on the holding member 300 and suction table 2 side. Therefore, when light l1 reflected by the back surface 21a is captured by the imaging unit 4, the pattern on the mounting surface 300x of the holding member 300 (see Figure 19(b)) and the pattern on the support surface 2x of the suction table 2 (see Figure 19(c)) may be captured in the captured image. This may reduce the accuracy of the control unit 8's determination of the state of the modified region, specifically, the accuracy of estimating the state of scratches and cracks in the modified region.

Figure 19(d) shows an example of an internal observation result when a pattern such as that of the holding member 300 is captured in the captured image. In Figure 19(d), the horizontal axis represents feature amount, and the vertical axis represents imaging depth. In the example of Figure 19(d), feature amount data 901 of the modified region SD1 on the back surface 21a, feature amount data 902 of the modified region SD2 on the front surface 21b, and feature amount data 910 of the tip of the upper crack are detected in the direct observation region. Feature amount data 903 of the modified region SD1 on the back surface 21a are detected in the back surface reflection region. Furthermore, 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 feature amount data 1900. Due to the influence of the feature amount data 1900, feature amount data 1904 of the modified region SD2 on the front surface 21b are detected with a smaller feature amount than when the feature amount data 1900 is not detected. In this way, detection of feature data 1900 related to the pattern of the holding member 300, etc., affects at least the detection of the modified region SD2 on the surface 21b side, which may result in a decrease in the accuracy of determining the state of the modified region.

To solve the above-mentioned problems, in the first step of the internal observation method according to this embodiment, 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 backside reflection observation, and the optimized holding member 300 is affixed to the wafer 20. Specifically, the thickness of the holding member 300 and/or the material of the holding member 300 are optimized. Below, the optimization of the thickness of the holding member 300 and the optimization of the material of the holding member 300 are each described.

The optimization of the thickness of the holding member 300 will be described. In the first step, a holding member 300A (300) having a thickness set according to the distance to the farthest modified region from the back surface 21a of the wafer 20 (hereinafter sometimes referred to as the "modified region distance") is attached to the back surface 21a of the wafer 20. Specifically, in the first step, the holding member 300A (300) 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 300 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 (upper end) of the modified region SD2 on the front surface 21b side. 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 to satisfy the following formula (1): For example, when D=350 μm, R1=1.5, and R2=3.5, the thickness T1 of the holding member 300A is calculated to be greater than 150 μm based on the lower-level formula (1).
T1>D×R1/R2...(1)

When such a holding member 300A is used, the thickness of the holding member 300A is greater than the approximate distance within the holding member 300A obtained by converting the modified region distance. Therefore, as shown in FIG. 20(a), when observing the backside of the modified regions SD1 and SD2, light 11 does not reach the mounting surface 300x of the holding member 300A or the support surface 2x of the suction table 2. Therefore, when observing the backside, the patterns on the mounting surface 300x of the holding member 300A and the support surface 2x of the suction table 2 are not captured in the captured image. FIG. 20(b) shows an example of an internal observation result when the holding member 300A is used. In FIG. 20(b), the horizontal axis represents feature amount, and the vertical axis represents imaging depth. In the example of FIG. 20(b), in the backside reflection region, in addition to feature amount data 903 of the modified region SD1 on the backside 21a, feature amount data 904 of the modified region SD2 on the frontside 21b are properly detected. Furthermore, in the rear surface reflection area, feature data related to the pattern on the mounting surface 300x of the holding member 300A and the pattern on the support surface 2x of the suction table 2 is not detected. This allows the positions of the dents in each modified area SD1, SD2 to be detected with high accuracy without being affected by feature data related to the patterns of the holding member 300A, etc.

The thickness of the holding member 300A may be determined taking into account the refractive index for each observation wavelength and the dz rate calculated from the NA (Numerical Aperture) of the objective lens 43. That is, in the first step, a holding member 300 having a thickness set to be greater than the modified region distance multiplied by the dz rate ratio of the holding member 300 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 of the objective lens in air and the focal position within the observed object. For example, if the wavelength of light l1 is 1100 nm and the objective lens 43 is moved 1 μm, the dz rate of the wafer 20 is set to 4.0 if the objective lens 43 moves 1 μm, resulting in a 4 μm movement within the silicon that constitutes the wafer 20. In the first step, a holding member 300A having a thickness set to be greater than the modified region distance multiplied by the dz rate ratio of the holding member 300A to the wafer 20 may be attached to the back surface 21a of the wafer 20. For example, if the modified region distance D is 350 μm, the dz rate of the holding member 300A is 1.6, and the dz rate of the wafer 20 is 4.0, the thickness T1 of the holding member 300A is calculated as T1 > 350 × 1.6 / 4.0 = 140 μm. The thickness of the holding member 300 may simply be greater than the modified region distance. That is, in the first step, a holding member 300 having a thickness greater than the modified region distance may simply be attached to the back surface 21a of the wafer 20. However, in this case, the holding member 300 would be too thick. Therefore, it is preferable to calculate the thickness of the holding member 300 using the above-mentioned formula (1) or a calculation formula that takes the dz rate ratio into account. That is, it is preferable to calculate the thickness of the holding member 300 taking into account the wafer thickness, laser processing conditions, the wavelength of the observation light, the refractive index of the wafer 20 and holding member 300, the NA of the objective lens 43, etc.

Next, we will explain how to optimize the material of the holding member 300. In the first step, as shown in FIG. 21(a), a holding member 300B having a transmittance of 50% or less for the transparent 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 having a transmittance of 30% or less for the transparent 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 transparent light l1 is 950 nm to 1700 nm, the holding member 300B may have a transmittance of 50% or less (preferably 30% or less).

When a holding member 300B having a certain degree of light blocking ability against the transparent light l1 is used, as shown in FIG. 21(a), light l1 does not reach the mounting surface 300x of the holding member 300B or the support surface 2x of the suction table 2 during rear surface observation of the modified regions SD1 and SD2. Therefore, during rear surface observation, the patterns on the mounting surface 300x of the holding member 300 and the support surface 2x of the suction table 2 are not captured in the captured image. FIG. 21(b) shows an example of an internal observation result when the holding member 300B is used. In FIG. 21(b), the horizontal axis represents feature amount, and the vertical axis represents imaging depth. In the example of FIG. 21(b), in the rear surface reflection region, in addition to feature amount data 903 of the modified region SD1 on the rear surface 21a, feature amount data 904 of the modified region SD2 on the front surface 21b is properly detected. Furthermore, in the rear surface reflection area, feature data related to the pattern on the mounting surface 300x of the holding member 300B and the pattern on the support surface 2x of the suction table 2 is not detected. This allows the positions of the dents in each modified area SD1, SD2 to be detected with high accuracy without being affected by feature data related to the patterns of the holding member 300B, etc.

In the first step, a holding member 300B containing a material that absorbs the transparent light 11 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 transparent light 11 may be attached to the back surface 21a of the wafer 20.

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 implemented in combination. By making the holding member 300 sufficiently thick and including a material with high light-blocking properties, it is possible to more effectively prevent the pattern on the mounting surface 300x of the holding member 300 and the pattern on the support surface 2x of the suction table 2 from appearing in the captured image.

Figure 22 shows an example of a judgment result according 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., was not reflected and erroneous judgment was suppressed, "△" indicates that the pattern of the holding member 300, etc., was largely not reflected and erroneous judgment was largely suppressed, and "×" indicates that the pattern of the holding member 300, etc., was reflected and erroneous judgment occurred. Figure 23 is a diagram illustrating an example of the distance to the modified region for each processing type. As shown in Figure 23(a), when SDBG (Stealth Dicing Before Grinding) processing is performed as the laser processing, assuming that the thickness of the wafer 20 is 775 μm, the modified region distance is, for example, 230 μm. Also, as shown in Figure 23(b), when FC (Full Cut) processing is performed as the laser processing, in which the crack reaches the surface 21b after laser processing, assuming that the thickness of the wafer 20 is 775 μm, the modified region distance is, for example, 730 μm. As such, the modified area distance varies depending on the processing type.

Figure 22(a) shows the evaluation results for different combinations of the thickness and transmittance of the holding member 300 during SDBG processing. In the example shown in Figure 22(a), under the conditions of a wafer 20 thickness of 775 μm and a modified region distance of 230 μm, when the thickness of the holding member 300 was greater than 101 μm, the evaluation result was "Good" regardless of the transmittance of the holding member 300. Furthermore, when the transmittance of the holding member 300 was 10% or less, the evaluation result was "Good" regardless of the thickness of the holding member 300. Furthermore, when the thickness of the holding member 300 was 51 to 100 μm or when the thickness of the holding member 300 was 50 μm or less, the evaluation result was "Poor" if the transmittance of the holding member 300 was greater than 85%, and the evaluation result was "Good" if the transmittance of the holding member 300 was 30%. In this way, by making the thickness of the holding member 300 sufficiently thick or by making the transmittance of the holding member 300 sufficiently small, the judgment result could be "Good" regardless of the combination of the thickness and transmittance of the holding member 300. Furthermore, even in cases where the judgment result would not be "Good" based on the thickness or transmittance of the holding member 300 alone, the judgment result could be "Poor" by combining the thickness and transmittance.

Figure 22(b) shows the evaluation results for different combinations of the thickness and transmittance of the holding member 300 during FC processing. In the example shown in Figure 22(b), under the conditions of a wafer 20 thickness of 775 μm and a modified region distance of 730 μm, when the thickness of the holding member 300 was greater than 301 μm, the evaluation result was "Good" regardless of the transmittance of the holding member 300. Furthermore, when the transmittance of the holding member 300 was 10% or less, the evaluation result was "Good" regardless of the thickness of the holding member 300. Furthermore, when the thickness of the holding member 300 was 201-300 μm, 51-200 μm, or 50 μm or less, the evaluation result was "Poor" if the transmittance of the holding member 300 was greater than 85%, and the evaluation result was "Good" if the transmittance of the holding member 300 was 30%. In this way, by making the thickness of the holding member 300 sufficiently thick or by making the transmittance of the holding member 300 sufficiently small, the judgment result could be "Good" regardless of the combination of the thickness and transmittance of the holding member 300. Furthermore, even in cases where the judgment result would not be "Good" based on the thickness or transmittance of the holding member 300 alone, the judgment result could be "Poor" by combining the thickness and transmittance.

Figure 24 is a flowchart of one example of the internal observation method (inspection method) described above. As shown in Figure 23, in this inspection method, first, a holding member 300 is attached to the back surface 21a of a wafer 20, which has been irradiated with laser light to form a modified region therein, opposite the front surface 21b irradiated with laser light, and the mounting surface 300x of the holding member 300, opposite the surface attached to the wafer 20, is placed on the suction table 2 (step S1, first process).

In step S1, for example, a holding member 300 having a thickness set to be greater than the modified region distance multiplied by the refractive index ratio of the holding member 300 to the wafer 20 is attached to the surface 21b of the wafer 20. Alternatively, in step S1, a holding member 300 having a transmittance of 50% (preferably 30%) or less for the transparent light 11 may be attached to the surface 21b of the wafer 20.

Next, the imaging unit 4 outputs transparent light l1 to the wafer 20 placed on the suction table 2 via the holding member 300, and detects the transparent light l1 that propagates through the wafer 20, thereby capturing an image of the wafer 20 (step S2, second process).

Finally, the state of the modified region of the wafer 20 is identified based on the captured image output from the imaging unit 4 that detected the transparent light l1 (step S3, third process).

Next, we will explain the effects of the internal observation method (inspection method) performed by the laser processing device 1 according to this embodiment.

The internal observation method (inspection method) performed by the laser processing apparatus 1 according to this embodiment includes the following steps: a first step of attaching a holding member 300 to the back surface 21a of a wafer 20, which has a modified region formed therein by irradiation with laser light, and placing the mounting surface 300x of the holding member 300 on the suction table 2; a second step of using the imaging unit 4 to output transparent light l1 to the wafer 20 placed on the suction table 2 via the holding member 300 and detect the transparent light l1 that has propagated through the wafer 20; and a third step of identifying the state of the modified region of the wafer 20 based on the captured image output from the imaging unit 4 that detected the transparent light l1. In the first step, the holding member 300, whose thickness is set according to the modified region distance, is attached to the back surface 21a.

In the inspection method according to this embodiment, a holding member 300 is affixed to the back surface 21a of a wafer 20 having a modified region formed therein, and the holding member 300 is placed on the suction table 2. Then, transparent light 11 is output to the wafer 20 placed on the suction table 2 via the holding member 300, thereby performing internal observation of the wafer 20, and the state of the modified region of the wafer 20 is identified based on the captured image. When performing internal observation with this configuration, if back surface reflection observation is performed by focusing on an area on the opposite side of the back surface 21a from the front surface 21b and capturing an image of light reflected by the back surface 21a, the pattern of the embossed surface of the holding member 300 or the pattern of the porous structure of the suction table 2 may appear in the captured image. This may result in a deterioration in the accuracy of estimating the state of the modified region of the wafer 20 based on the captured image. In this regard, in the inspection method according to this embodiment, a holding member 300 whose thickness is 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 affixed to the back surface 21a of the wafer 20. In this way, by setting the thickness of the holding member 300 taking the modified region distance into consideration, it is possible to set the thickness of the holding member 300 so that patterns such as the holding member 300 are prevented from appearing in captured images during back surface reflection observation of the modified region. This makes it possible to reduce erroneous determinations during internal observation of the wafer 20 after laser processing.

One possible method for avoiding erroneous internal observation due to the influence of patterns on the holding member 300, etc., is to capture all images in the Z direction (depth direction) to be captured before laser processing. Then, when performing internal observation after laser processing, subtract the images captured before laser processing from the images captured after laser processing, thereby capturing only information related to the modified region formed after laser processing within the field of view (a pattern avoidance method using filtering). However, this method requires capturing all images in the Z direction to be captured before laser processing, which reduces takt time. Furthermore, even slight changes in the imaging conditions, such as light intensity or brightness, between the images captured before and after laser processing prevent the filtering function (subtraction function) from functioning properly, resulting in reduced accuracy in internal observation. In this regard, the internal observation method (inspection method) performed by the laser processing device 1 according to this embodiment eliminates the need for pre-laser processing imaging, thereby reducing takt time. Furthermore, because no filtering is performed, the aforementioned reduction in accuracy in internal observation is not an issue. In other words, the internal observation method (inspection method) performed by the laser processing device 1 according to this embodiment can reduce tact time while suppressing erroneous determinations in the internal observation of the wafer 20 after laser processing.

In the first step, a holding member 300 with a thickness set to be greater than the modified region distance may be attached to the back surface 21a. This prevents the pattern of the holding member 300, etc., from appearing in the captured image during back surface reflection observation of the modified region. This makes it possible to reduce erroneous determinations during internal observation of the wafer 20 after laser processing.

In the first step, a holding member 300 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 300 to the wafer 20 may be attached to the back surface 21a. Multiplying the modified region distance by the refractive index ratio converts the modified region distance to an approximate distance within the holding member 300 that takes the refractive index into account. Therefore, by attaching a holding member 300 having a thickness greater than the converted value to the back surface 21a, the pattern of the holding member 300 or the like is prevented from appearing in the captured image during back surface reflection observation of the modified region. This reduces erroneous determinations in internal observation of the wafer 20 after laser processing. Furthermore, by setting the thickness of the holding member 300 according to the value obtained by multiplying the modified region distance by the refractive index ratio, the thickness of the holding member 300 can be made smaller than when, for example, the thickness of the holding member 300 is simply set to be greater than the modified region distance. In other words, it is possible to prevent the holding member 300 from becoming too thick while reducing erroneous determinations in internal observation of the wafer 20 after laser processing.

In the first step, a holding member 300 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 300 relative to the wafer 20 may be attached to the back surface 21a. By multiplying the modified region distance by the above dz rate ratio, the modified region distance is converted into an approximate distance within 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. Therefore, by attaching a holding member 300 having a thickness greater than the converted value to the back surface 21a, patterns such as those of the holding member 300 are prevented from appearing in the captured image during back surface reflection observation of the modified region. This makes it possible to prevent erroneous determinations during internal observation of the wafer 20 after laser processing. Furthermore, by setting the thickness of the holding member 300 according to the value obtained by multiplying the modified region distance by the dz rate ratio, the thickness of the holding member 300 can be made smaller than when, for example, the thickness of the holding member 300 is set simply to be larger than the modified region distance, or when the thickness of the holding member 300 is set taking only the refractive index ratio into consideration. In other words, it is possible to prevent the thickness of the holding member 300 from becoming too large, while suppressing erroneous determinations during internal observation of the wafer 20 after laser processing.

In the internal observation method (inspection method) performed by the laser processing apparatus 1 according to this embodiment, in the first step, a holding member 300 having a transmittance of 50% or less for the transparent light 11 may be affixed to the back surface 21a. By affixing the light-blocking holding member 300 to the back surface 21a in this manner, images of objects located below the light-blocking holding member 300 (toward the suction table 2) are less likely to appear in the captured image during back surface reflection observation. Specifically, the pattern of the embossed surface of the holding member 300 and the pattern of the porous structure of the suction table 2 are less likely to appear in the captured image. This prevents the pattern of the holding member 300, etc., from appearing in the captured image during back surface reflection observation, thereby reducing erroneous determinations during internal observation of the wafer 20 after laser processing.

In the first step, a holding member 300 having a transmittance of 30% or less for the transparent light 11 may be attached to the back surface 21a. By ensuring that the transmittance of the holding member 300 for the transparent light 11 is 30% or less, it is possible to more appropriately prevent the pattern of the holding member 300, etc. from appearing in the captured image during back surface reflection observation, thereby reducing erroneous determinations when observing the inside of the wafer 20 after laser processing.

In the first step, a holding member 300 containing a material that absorbs the transparent light 11 may be attached to the back surface 21a. With this configuration, the transmittance of the transparent light 11 through the holding member 300 can be appropriately reduced.

In the first step, a holding member 300 containing a material that reflects the transparent light 11 may be attached to the back surface 21a. With this configuration, the transmittance of the transparent light 11 through the holding member 300 can be appropriately reduced.

In the second step, the imaging area is changed along the vertical Z direction, and light 11 that transmits light from each imaging area is detected. In the third step, feature points and the feature amounts of the feature points are detected for the captured image of each imaging area along the Z direction, and the position of the feature point with a relatively large feature amount may be identified as the formation position of the modified area. In this way, by identifying the formation position of the modified area from the feature amounts of the feature points in the captured image, the accuracy and efficiency of internal observation of the wafer 20 after laser processing can be improved.

2...suction table, 4...imaging unit, 20...wafer, 21a...rear surface (second surface), 21b...front surface (first surface), 300...holding member, L...laser light, l1...transparent light.

Claims (9)

a first step of attaching a holding member to a second surface of a wafer having a modified region formed therein by being irradiated with laser light, the second surface being opposite to a first surface irradiated with the laser light, and placing the surface of the holding member opposite to the surface attached to the wafer on a suction table;
a second step of outputting light having transparency to the wafer placed on the suction table via the holding member and detecting the light having transparency propagated through the wafer by an imaging unit;
a third step of identifying a state of the modified region of the wafer based on an image output from the imaging unit that detects the light having transmittance;
In the first step, the holding member, whose thickness is set according to the modified area distance, which is the distance from the second surface of the wafer to the modified area that is farthest from the second surface, is attached to the second surface. The inspection method described in claim 1, wherein in the first step, the holding member, whose thickness is set to be greater than the modified region distance, is attached to the second surface. The inspection method described in claim 1, wherein in the first step, the holding member is attached to the second surface, the thickness of the holding member being set to be greater than the modified region distance multiplied by the refractive index ratio of the holding member to the wafer. The inspection method described in claim 1, wherein in the first step, the holding member is attached to the second surface, the thickness of the holding member being set to be greater than the value obtained by multiplying the modified region distance by the dz rate ratio of the holding member to the wafer. a first step of attaching a holding member to a second surface of a wafer having a modified region formed therein by being irradiated with laser light, the second surface being opposite to a first surface irradiated with the laser light, and placing the surface of the holding member opposite to the surface attached to the wafer on a suction table;
a second step of outputting light having transparency to the wafer placed on the suction table via the holding member and detecting the light having transparency propagated through the wafer by an imaging unit;
a third step of identifying a state of the modified region of the wafer based on an image output from the imaging unit that detects the light having transmittance;
In the first step, the holding member having a transmittance of the transparent light of 50% or less is attached to the second surface. The inspection method described in claim 5, wherein in the first step, the holding member having a transmittance of 30% or less for the transparent light is attached to the second surface. The inspection method according to claim 5 or 6, wherein in the first step, the holding member containing the light-absorbing material having the transmittance is attached to the second surface. 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 the transmittance is attached to the second surface. In the second step, the light having the transmittance is detected for each imaging region while changing the imaging region along a Z direction which is a vertical direction;
The inspection method according to any one of claims 1 to 8, wherein in the third step, feature points and feature amounts of the feature points are detected for the captured images of each imaging area along the Z direction, and the position of the feature point with a relatively large feature amount is identified as the formation position of the modified area.