Laser processing apparatus, laser processing method, and bonded wafer

Abstract

To appropriately detect light reflected on a rear face of a bonded wafer, and to derive an inner processing state with high accuracy.SOLUTION: A laser processing device 1 comprises: a laser irradiation unit 3; an imaging unit 4; and a controller 8 configured to execute a first control of controlling the laser irradiation unit 3 so that one or more reforming regions 12 are formed in a wafer 20A, a second control of controlling the imaging unit 4 to detect light reflected on a rear face 21a and then propagated in the wafer 20A, and a third control of deriving an inner processing state of the wafer 20A on the basis of a signal output from the imaging unit 4. A wavelength of transmissive light with which the bonded wafer 20B is irradiated, a type of a film 70, and a thickness of the film 70 are adjusted so that a light reflectance on the rear face 21a is equal to or more than a reference reflectance that is a light reflectance on the rear face 21a of the wafer 20A used alone.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] One aspect of the present invention relates to a laser processing apparatus, a laser processing method, and a bonded wafer. [Background technology]

[0002] A laser processing device is known that irradiates a wafer having a semiconductor substrate and a functional element layer formed on the rear surface of the semiconductor substrate with laser light from the front surface side of the semiconductor substrate to cut the wafer along each of a plurality of lines, thereby forming a plurality of rows of modified regions inside the semiconductor substrate along each of the plurality of lines. The laser processing device described in Patent Document 1 is equipped with an infrared camera, and makes it possible to observe the modified regions formed inside the semiconductor substrate and processing damage formed in the functional element layer from the front surface side of the semiconductor substrate. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2017-64746 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the laser processing device as described above, for example, when observing the tip of a crack extending from the modified region to the back side of the wafer, the light reflected on the back side of the wafer is detected by the infrared camera or the like as described above. Here, when the processing object of the laser processing device is not a single wafer made of one wafer but a bonded wafer made of two wafers bonded with a film, the reflectance of light on the back side of the wafer described above is likely to be low. That is, in a bonded wafer in which a second wafer is bonded to the back side of a first wafer via a bonding film, the reflectance of light on the back side of the first wafer is likely to be low compared to a single wafer made of only the first wafer. For this reason, when a bonded wafer is processed, the light reflected on the back side of the first wafer cannot be sufficiently detected, and there is a risk that the internal observation of the wafer cannot be properly performed.

[0005] One aspect of the present invention has been made in consideration of the above-mentioned situation, and aims to provide a laser processing apparatus, a laser processing method, and a bonded wafer that are capable of properly detecting light reflected from the back surface of a bonded wafer and deriving the internal processing state with high precision. [Means for solving the problem]

[0006] A laser processing apparatus according to one embodiment of the present invention includes an irradiation unit that irradiates a bonded wafer, in which a second wafer is bonded to the second surface of a first wafer including a first surface and a second surface via a bonding film, with laser light from the first surface side; an imaging unit that irradiates the bonded wafer with light having transparency from the first surface side and detects the light that has propagated through the bonded wafer; and a control unit configured to execute a first control that controls the irradiation unit so that one or more modified regions are formed inside the first wafer by irradiating the bonded wafer with the laser light; a second control that controls the imaging unit so that light reflected at the second surface and propagated through the first wafer is detected; and a third control that derives an internal processing state of the first wafer based on a signal output from the imaging unit in response to the detected light. The wavelength of the light having transparency irradiated to the bonded wafer, the type of film, and the thickness of the film are adjusted so that the reflectance of the light at the second surface is equal to or greater than a reference reflectance, which is the reflectance of the light at the second surface of the first wafer when the second wafer is used alone without being bonded to the first wafer.

[0007] In the laser processing device according to one aspect of the present invention, a laser processing is performed on a bonded wafer to form a modified region on the first wafer, and an internal processing state of the first wafer in which the modified region is formed is derived based on a signal corresponding to a detection result of light reflected on the second surface of the first wafer. In this way, by deriving the internal processing state from the detection result of light reflected on the second surface, for example, it is possible to detect the tip of a crack extending from the modified region to the second surface side, and the internal processing state can be more appropriately estimated (derived). Here, in a bonded wafer in which the first wafer and the second wafer are bonded together via a bonding film, the reflectance of light having transparency on the second surface (the surface on which the film is formed) of the first wafer is generally lower than the reflectance (reference reflectance) on the second surface in the case of the first wafer alone. As a result, there is a risk that the internal processing state of the bonded wafer cannot be appropriately derived from the detection result of light reflected on the second surface. In this regard, in the laser processing device according to one aspect of the present invention, the wavelength of light having transparency, the type of film, and the thickness of the film are adjusted so that the reflectance on the second surface is equal to or greater than the reference reflectance. According to this configuration, even when the processing object is a bonded wafer, the reflectance at the second surface is equal to or greater than that of the first wafer alone, so that the internal processing state can be appropriately derived from the detection result of the light reflected at the second surface. As described above, according to the laser processing device according to one aspect of the present invention, even for bonded wafers, the light reflected at the second surface (rear surface) can be appropriately detected, and the internal processing state can be accurately derived.

[0008] The control unit may further execute a fourth control, which acquires information indicating the wavelength of the light having transparency to be irradiated to the bonded wafer and information indicating the type of the film before the first control, determines the thickness of the film so that the reflectance of the light on the second surface is equal to or greater than the reference reflectance based on the information indicating the wavelength of the light having transparency and the information indicating the type of the film, and outputs information indicating the determined thickness of the film. According to this configuration, before the laser processing, the thickness of the film is determined according to the wavelength of the light to be irradiated to the bonded wafer and the type of the film so that the reflectance of the light on the second surface is equal to or greater than the reference reflectance. This reliably increases the reflectance of the light on the second surface, and reliably detects the light reflected on the second surface.

[0009] The control unit may further execute a fifth control, which acquires information indicating the type of film and information indicating the thickness of the film before the first control, determines the wavelength of light having transparency to be irradiated to the bonded wafer so that the reflectance of light on the second surface is equal to or greater than the reference reflectance based on the information indicating the type of film and the information indicating the thickness of the film, and outputs information indicating the determined wavelength of light having transparency. According to this configuration, before laser processing, the wavelength of light having transparency is determined according to the type of film and the thickness of the film so that the reflectance of light on the second surface is equal to or greater than the reference reflectance. This reliably increases the reflectance of light on the second surface, and reliably detects the light reflected on the second surface.

[0010] The imaging unit has a light source that outputs light having transparency, and a plurality of bandpass filters having different passbands that are switchably provided between the light source and the bonded wafer, and the control unit may further execute a sixth control before the first control, in which one bandpass filter is selected from the plurality of bandpass filters according to the wavelength of light determined in the fifth control, and the selected bandpass filter is set between the light source and the bonded wafer. According to this configuration, the bandpass filter according to the determined wavelength of light is set, so that the reflectance of light on the second surface can be reliably increased by a simple process.

[0011] The imaging unit has a light source that outputs light having transparency, and the control unit may further execute a seventh control prior to the first control in which a wavelength band including the wavelength of the light determined in the fifth control is set in the light source as the wavelength band of the light output from the light source. According to this configuration, the wavelength band including the determined wavelength of the light is set as the wavelength band of the light output from the light source, so that the reflectance of the light on the second surface can be reliably increased by a simple process.

[0012] A bonded wafer according to one aspect of the present invention includes a first wafer, a bonding film formed on one side of the first wafer, and a second wafer bonded to one side of the first wafer by the film, and the type and thickness of the film are adjusted according to the wavelength of the light having transparency so that the reflectance of the light having transparency on one side is equal to or greater than the reference reflectance, which is the reflectance of the light having transparency on one side of the first wafer when the second wafer is used alone without being bonded. According to the bonded wafer according to one aspect of the present invention, the reflectance on one side of the first wafer is equal to or greater than that of the first wafer alone, so that the internal processing state can be appropriately derived from the detection result of the light reflected on the one side. As described above, according to the bonded wafer according to one aspect of the present invention, the light reflected on one side of the bonded wafer can also be appropriately detected, and the internal processing state can be accurately derived.

[0013] The laser processing method according to one aspect of the present invention includes a processing step of irradiating a laser beam from the first surface side of the bonded wafer to form one or more modified regions inside the first wafer in a state in which the wavelength of the light having transparency, the type of film, and the thickness of the film are adjusted so that the reflectance of the light having transparency on the second surface of the bonded wafer, in which the second wafer is bonded to the second surface of the first wafer including the first surface and the second surface through a bonding film, is equal to or greater than the reference reflectance, which is the reflectance of the light having transparency on the second surface of the first wafer when the second wafer is used alone without being bonded to the second wafer; a detection step of irradiating the bonded wafer with light having transparency from the first surface side after the processing step, detecting the light reflected on the second surface and propagating through the first wafer; and a derivation step of deriving the internal processing state of the first wafer based on the light detected in the detection step. According to the laser processing method according to one aspect of the present invention, even when the processing target is a bonded wafer, the reflectance on the second surface is equal to or greater than that of the first wafer alone, so that the internal processing state can be appropriately derived from the detection result of the light reflected on the second surface. As described above, according to the laser processing method of one embodiment of the present invention, even in the case of a bonded wafer, the light reflected on the second surface (rear surface) can be appropriately detected, and the internal processing state can be derived with high accuracy.

[0014] The above-mentioned laser processing method may further include a thickness determination step of determining the thickness of the film based on information indicating the wavelength of the light having transparency to be irradiated to the bonded wafer and information indicating the type of the film before the processing step so that the reflectance of the light on the second surface is equal to or greater than the reference reflectance, and a wafer setting step of setting the bonded wafer having the film thickness determined in the thickness determination step at the irradiation position of the laser light in the processing step. According to such a configuration, before the laser processing, the thickness of the film is determined according to the wavelength of the light to be irradiated to the bonded wafer and the type of the film so that the reflectance of the light on the second surface is equal to or greater than the reference reflectance, and the bonded wafer having the determined film thickness is set at the irradiation position of the laser light. This reliably increases the reflectance of the light on the second surface and reliably detects the light reflected on the second surface.

[0015] The above-mentioned laser processing method may further include a wavelength determination step of determining the wavelength of light having transparency to be irradiated to the bonded wafer based on the information indicating the type of the film and the information indicating the thickness of the film before the processing step so that the reflectance of light on the second surface is equal to or greater than the reference reflectance, and a filter setting step of selecting one bandpass filter from a plurality of bandpass filters having different passbands switchably provided in front of the bonded wafer according to the wavelength of light determined in the wavelength determination step, and setting the selected bandpass filter in front of the bonded wafer. According to this configuration, the bandpass filter according to the determined wavelength of light is set, so that the reflectance of light on the second surface can be reliably increased by a simple process.

[0016] The above-mentioned laser processing method may further include a wavelength determination step of determining the wavelength of light having transparency to be irradiated to the bonded wafer based on the information indicating the type of film and the information indicating the thickness of the film before the processing step so that the reflectance of light on the second surface is equal to or greater than the reference reflectance, and a light source setting step of setting the wavelength band including the wavelength of the light determined in the wavelength determination step as the wavelength band of light output from the light source that outputs the light having transparency to the light source. According to this configuration, the wavelength band including the determined wavelength of the light is set as the wavelength band of light output from the light source, so that the reflectance of light on the second surface can be reliably increased by a simple process.

[0017] The above-mentioned laser processing method may further include a bandpass filter specifying step of sequentially setting a plurality of bandpass filters having different passbands switchably provided in front of the bonded wafer in front of the bonded wafer before the processing step, detecting light reflected on the second surface and propagating through the first wafer in a state in which each bandpass filter is set, and specifying a bandpass filter whose reflectance of light on the second surface is equal to or greater than a reference reflectance based on the detection result, and a filter setting step of setting the bandpass filter specified in the bandpass filter specifying step in front of the bonded wafer before the processing step. According to this configuration, a bandpass filter that can ensure the reflectance of light on the second surface is specified by detecting light reflected on the second surface in a state in which the bandpass filter is actually set, and the specified bandpass filter is set, so that the reflectance of light on the second surface can be reliably increased based on the actual measurement value. Effect of the Invention

[0018] According to one aspect of the present invention, it is possible to appropriately detect light reflected by the back surface of a bonded wafer, and to derive the internal processing state with high accuracy. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a configuration diagram of a laser processing apparatus according to one embodiment. [Diagram 2] FIG. 2 is a plan view of a wafer of one embodiment. [Diagram 3] 3 is a cross-sectional view of a portion of the wafer shown in FIG. 2. [Figure 4] FIG. 2 is a configuration diagram of a laser irradiation unit shown in FIG. [Diagram 5] FIG. 2 is a configuration diagram of an inspection imaging unit shown in FIG. [Figure 6] FIG. 2 is a configuration diagram of an alignment correction imaging unit shown in FIG. [Figure 7] 6A to 6C are cross-sectional views of a wafer for explaining the imaging principle by the inspection imaging unit shown in FIG. 5, and images at various locations by the inspection imaging unit. [Figure 8] 6A to 6C are cross-sectional views of a wafer for explaining the imaging principle by the inspection imaging unit shown in FIG. 5, and images at various locations by the inspection imaging unit. [Figure 9] 1 is a SEM image of modified regions and cracks formed within a semiconductor substrate. [Figure 10] 1 is a SEM image of modified regions and cracks formed within a semiconductor substrate. [Figure 11] 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. [Figure 12] 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. [Figure 13] FIG. 2 is a cross-sectional view of a portion of a bonded wafer. [Figure 14] 1A and 1B are diagrams illustrating rear surface reflections of a single wafer and a bonded wafer. [Figure 15] 11 is a table showing the relationship between film thickness, reflectance, and detectability. [Figure 16] FIG. 13 is a diagram illustrating the reflectance depending on the film thickness. [Figure 17] 1 is a flowchart showing an example of a laser processing method for a bonded wafer. [Figure 18] FIG. 1 is a diagram illustrating reflectance according to wavelength. [Figure 19] FIG. 2 is a configuration diagram of an inspection imaging unit. [Figure 20] 1 is a flowchart showing an example of a laser processing method for a bonded wafer. [Figure 21] FIG. 2 is a cross-sectional view of a portion of a device wafer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and duplicated explanations will be omitted. [Laser processing equipment configuration]

[0021] 1, the laser processing apparatus 1 includes a stage 2, a laser irradiation unit 3 (irradiation section), a plurality of imaging units 4, 5, 6, a drive unit 7, a control section 8, and a display 150 (input section, display section). The laser processing apparatus 1 is an apparatus that forms a modified region 12 in an object 11 by irradiating the object 11 with laser light L.

[0022] The stage 2 supports the object 11, for example, by adsorbing a film attached to the object 11. The stage 2 is movable along both the X direction and the Y direction, and is rotatable about an axis parallel to the Z direction. The X direction and the Y direction are a first horizontal direction and a second horizontal direction that are perpendicular to each other, and the Z direction is a vertical direction.

[0023] 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 by the stage 2, the laser light L is particularly absorbed in a portion corresponding to a focusing point C of the laser light L, and a modified region 12 is formed inside the object 11.

[0024] The modified region 12 is a region that differs from the surrounding non-modified region in terms of density, refractive index, mechanical strength, and other physical properties. Examples of the modified region 12 include a melting treatment region, a crack region, a dielectric breakdown region, and a refractive index change region. The modified region 12 has a 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. Such a property of the modified region 12 is utilized for cutting the target object 11.

[0025] As an example, when the stage 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. One row of modified regions 12 is a collection of multiple modified spots 12s lined up 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 focal point C with respect to the object 11 and the repetition frequency of the laser light L.

[0026] The imaging unit 4 captures an image of the modified region 12 formed in the object 11 and the tip of the crack extending from the modified region 12 .

[0027] Under the control of the control unit 8, the imaging units 5 and 6 capture an image of the object 11 supported on the stage 2 by light transmitted through the object 11. The images obtained by the imaging units 5 and 6 are used, for example, to align the irradiation position of the laser light L.

[0028] 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.

[0029] The control unit 8 controls the operations of the stage 2, the laser irradiation unit 3, the multiple imaging units 4, 5, and 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, etc. In the control unit 8, the processor executes software (programs) loaded into the memory, etc., and controls the reading and writing of data in the memory and storage, as well as communication by the communication device.

[0030] 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.

[0031] [Object composition] The object 11 in this embodiment is a wafer 20, as shown in FIG. 2 and FIG. 3. Here, for convenience of explanation, a single wafer 20 (details will be described later) will be explained, but the wafer mainly explained in this embodiment is a bonded wafer in which two wafers are bonded together via a bonding film. Details of the bonded wafer will be described later. The wafer 20 includes a semiconductor substrate 21 and a functional device layer 22. In this embodiment, the wafer 20 is explained as having the functional device layer 22, but the wafer 20 may or may not have the functional device 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 device layer 22 is formed on the back surface 21a of the semiconductor substrate 21. The functional device layer 22 includes a plurality of functional devices 22a arranged two-dimensionally along the back surface 21a. The functional device 22a is, for example, a light receiving device such as a photodiode, a light emitting device such as a laser diode, a circuit device such as a memory, or the like. The functional element 22a may be configured three-dimensionally by stacking a plurality of layers. Although the semiconductor substrate 21 has a notch 21c indicating the crystal orientation, an orientation flat may be provided instead of the notch 21c.

[0032] The wafer 20 is cut into functional elements 22a along each of the lines 15. The lines 15 pass between the functional elements 22a when viewed from the thickness direction of the wafer 20. More specifically, the lines 15 pass through the center of the street region 23 (the center in the width direction) when viewed from the thickness direction of the wafer 20. The street region 23 extends in the functional element layer 22 so as to pass between adjacent functional elements 22a. In this embodiment, the functional elements 22a are arranged in a matrix along the rear surface 21a, and the lines 15 are set in a lattice pattern. The lines 15 are virtual lines, but may be lines that are actually drawn.

[0033] [Laser irradiation unit configuration] As shown in Fig. 4, the laser irradiation unit 3 has 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 reflective liquid crystal (LCOS: Liquid Crystal on Silicon) spatial light modulator (SLM: Spatial Light Modulator). The condenser lens 33 condenses the laser light L modulated by the spatial light modulator 32. The condenser lens 33 may be a correction ring lens.

[0034] In this embodiment, the laser irradiation unit 3 irradiates the wafer 20 with laser light L from the front surface 21b side 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. The modified region 12a is the modified region closest to the back surface 21a among the two rows of modified regions 12a, 12b. The modified region 12b is the modified region closest to the modified region 12a among the two rows of modified regions 12a, 12b, and is the modified region closest to the front surface 21b.

[0035] 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 the two focusing points C1, C2 along the line 15 relative to the semiconductor substrate 21. The laser light L is modulated by the spatial light modulator 32, for example, so that the focusing point C2 is located on the rear side of the focusing point C1 in the traveling direction and on the incident side of the laser light L. The modified regions may be formed by a single focus or multiple focuses, and may be formed by one pass or multiple passes.

[0036] The laser irradiation unit 3 irradiates the wafer 20 with laser light L from the front surface 21b side of the semiconductor substrate 21 along each of the multiple lines 15. As an example, <100> The two focal points C1 and C2 are aligned at positions 54 μm and 128 μm from the back surface 21a of the semiconductor substrate 21, 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. At this time, for example, when the condition is that the cracks 14 across the 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 light L at the focal point C1 is 2.7 W, the output of the laser light L at the focal point C2 is 2.7 W, and the relative moving speed of the two focal points C1 and C2 with respect to the semiconductor substrate 21 is 800 mm / sec. In addition, for example, when the number of processing passes is 5, 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), and ZH12 (position 49.2 μm from the back surface 21a) may be set as processing positions for the above-mentioned wafer 20. In this case, for example, the wavelength of the laser light 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.

[0037] The formation of the two rows of modified regions 12a, 12b and the cracks 14 is performed in the following case: that is, in a later process, for example, by grinding the surface 21b of the semiconductor substrate 21 to thin the semiconductor substrate 21 and expose the cracks 14 on the surface 21b, and cut the wafer 20 into a plurality of semiconductor devices along each of the plurality of lines 15.

[0038] [Configuration of the inspection imaging unit] As shown in FIG. 5, the imaging unit 4 (imaging section) has a light source 41, a mirror 42, an objective lens 43, and a light detection section 44. The imaging unit 4 captures an image of the wafer 20. The light source 41 outputs light I1 that is transparent to the semiconductor substrate 21. The light source 41 is composed of, for example, a halogen lamp and a filter, and outputs light I1 in the near-infrared region. The light I1 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 side of the semiconductor substrate 21. At this time, the stage 2 supports the wafer 20 on which the two rows of modified regions 12a and 12b are formed as described above.

[0039] The objective lens 43 passes the light I1 reflected by the back surface 21a of the semiconductor substrate 21. That is, the objective lens 43 passes the light I1 propagated through the semiconductor substrate 21. The numerical aperture (NA) of the objective lens 43 is, for example, 0.45 or more. The objective lens 43 has a correction collar 43a. The correction collar 43a corrects aberrations occurring in the light I1 in the semiconductor substrate 21, for example, by adjusting the distances between the multiple lenses constituting the objective lens 43. Note that the means for correcting the aberrations is not limited to the correction collar 43a, and may be other correction means such as a spatial light modulator. The light detection unit 44 detects the light I1 transmitted through the objective lens 43 and the mirror 42. The light detection unit 44 is, for example, configured by an InGaAs camera, and detects the light I1 in the near-infrared region. The means for detecting (capturing) the light I1 in the near-infrared region is not limited to an InGaAs camera, and may be any other imaging means that captures images in a transmission type, such as a transmission confocal microscope.

[0040] The imaging unit 4 can capture images of each of the two rows of modified regions 12a, 12b and the tips of each of the multiple cracks 14a, 14b, 14c, and 14d (details will be described later). The crack 14a is a crack that extends from the modified region 12a to the back surface 21a. The crack 14b is a crack that extends from the modified region 12a to the front surface 21b. The crack 14c is a crack that extends from the modified region 12b to the back surface 21a. The crack 14d is a crack that extends from the modified region 12b to the front surface 21b.

[0041] [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 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 and passes through the lens 53, and is irradiated onto the wafer 20 from the surface 21b side of the semiconductor substrate 21.

[0042] The lens 53 passes the light I2 reflected by the rear surface 21a of the semiconductor substrate 21. That is, the lens 53 passes the light I2 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 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 configured by, for example, an InGaAs camera, and detects the light I2 in the near-infrared region.

[0043] The imaging unit 5, under the control of the control unit 8, irradiates the wafer 20 with light I2 from the front surface 21b side and detects the light I2 returning from the rear surface 21a (functional element layer 22), thereby capturing an image of the functional element layer 22. Similarly, under the control of the control unit 8, the imaging unit 5, under the control of the control unit 8, irradiates the wafer 20 with light I2 from the front surface 21b side and detects the light I2 returning from the positions where the modified regions 12a and 12b are formed in the semiconductor substrate 21, thereby acquiring an image of the region including the modified regions 12a and 12b. These images are used for alignment of 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 (for example, 6x in the imaging unit 5 and 1.5x in the imaging unit 6), and is used for alignment in the same manner as the imaging unit 5.

[0044] [Principle of imaging by inspection imaging unit] Using the imaging unit 4 shown in FIG. 5, the focal point F (the focal point of the objective lens 43) is moved from the front surface 21b side to the back surface 21a side with respect to the semiconductor substrate 21 in which the cracks 14 extending across the two rows of modified regions 12a, 12b reach the back surface 21a as shown in FIG. 7. In this case, when the focal point F is adjusted from the front surface 21b side to the tip 14e of the crack 14 extending from the modified region 12b to the front surface 21b side, the tip 14e can be confirmed (the image on the right side in FIG. 7). However, even if the focal point F is adjusted from the front surface 21b side to the cracks 14 themselves and the tip 14e of the cracks 14 reaching the back surface 21a, they cannot be confirmed (the image on the left side in FIG. 7). It should be noted that when the focal point F is adjusted from the front surface 21b side to the back surface 21a of the semiconductor substrate 21, the functional element layer 22 can be confirmed.

[0045] 5, the focal point F is moved from the front surface 21b side to the back surface 21a side of the semiconductor substrate 21 in which the cracks 14 extending across the 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 side to the tip 14e of the crack 14 extending from the modified region 12a to the back surface 21a side, the tip 14e cannot be confirmed (the image on the left side in FIG. 8). However, when the focal point F is aligned from the front surface 21b side to the region on the opposite side of the back surface 21a from the front surface 21b (i.e., the region on the functional element layer 22 side of the back surface 21a) with respect to the back surface 21a, and a virtual focal point Fv symmetrical to the focal point F with respect to the back surface 21a is positioned at the tip 14e, the tip 14e can be confirmed (the image on the right side in FIG. 8). The virtual focal point Fv is a point symmetrical to the focal point F with respect to the back surface 21a, taking into account the refractive index of the semiconductor substrate 21.

[0046] As described above, the reason why the crack 14 itself cannot be confirmed is assumed to be because the width of the crack 14 is smaller than the wavelength of the light I1, which is the illumination light. Figures 9 and 10 are SEM (Scanning Electron Microscope) images of the modified region 12 and the crack 14 formed inside the semiconductor substrate 21, which is a silicon substrate. (b) of Figure 9 is an enlarged image of the region A1 shown in (a) of Figure 9, (a) of Figure 10 is an enlarged image of the region A2 shown in (b) of Figure 9, and (b) of Figure 10 is an enlarged image of the region A3 shown in (a) of Figure 10. Thus, the width of the crack 14 is about 120 nm, which is smaller than the wavelength (e.g., 1.1 to 1.2 μm) of the light I1 in the near-infrared region.

[0047] Based on the above, the imaging principle is assumed to be as follows. As shown in FIG. 11(a), when the focal point F is located in air, the light I1 does not return, and a dark image is obtained (the image on the right side of FIG. 11(a)). As shown in FIG. 11(b), when the focal point F is located inside the semiconductor substrate 21, the light I1 reflected by the back surface 21a returns, and a whitish image is obtained (the image on the right side of FIG. 11(b)). As shown in FIG. 11(c), when the focal point F is aligned with the modified region 12 from the front surface 21b side, the modified region 12 absorbs, scatters, or the like a part of the light I1 reflected by the back surface 21a and returned, and an image is obtained in which the modified region 12 appears dark against a whitish background (the image on the right side of FIG. 11(c)).

[0048] As shown in (a) and (b) of FIG. 12, when the focal point F is focused on the tip 14e of the crack 14 from the surface 21b side, for example, due to optical singularities (stress concentration, distortion, discontinuity of atomic density, etc.) occurring near the tip 14e, light confinement occurring near the tip 14e, etc., a part of the light I1 reflected by the back surface 21a and returned is scattered, reflected, interfered, absorbed, etc., and an image in which the tip 14e appears black on a whitish background is obtained (the right image in (a) and (b) of FIG. 12). As shown in (c) of FIG. 12, when the focal point F is focused on a part other than the vicinity of the tip 14e of the crack 14 from the surface 21b side, at least a part of the light I1 reflected by the back surface 21a is returned, and a whitish image is obtained (the right image in (c) of FIG. 12).

[0049] [Structure of bonded wafer] The configuration of the bonded wafer will be described below with reference to FIG.

[0050] 13 is a cross-sectional view of a portion of the bonded wafer 20B. The bonded wafer 20B is a wafer in which two wafers are bonded to each other via a bonding film 70. Such a bonded wafer 20B is used, for example, in a form in which a lower wafer is used as a support substrate (a substrate that holds an upper wafer) or in a wafer-to-wafer process for three-dimensional stacked wafers.

[0051] As shown in FIG. 13, the bonded wafer 20B includes a wafer 20A (first wafer), a wafer 80 (second wafer), and a film 70 (lamination film). The wafer 20A is, for example, a wafer similar to the above-mentioned wafer 20, and the semiconductor substrate is, for example, a silicon substrate. The wafer 80 is, for example, a support substrate that supports the wafer 20A, and the semiconductor substrate is, for example, a silicon substrate, similar to the wafer 20A. The film 70 is a lamination film that bonds the wafer 20A and the wafer 80 together, and is, for example, a TEOS (Tetraethyl orthosilicate) film, an oxide film, a nitride film, or the like.

[0052] The wafer 20A has a front surface 21b (first surface) and a back surface 21a (second surface, one surface) like the wafer 20 described above. The front surface 21b is an irradiation surface that is irradiated with laser light from the laser irradiation unit 3. The back surface 21a is a surface on which a film 70 is formed. The film 70 is a bonding film formed on the back surface 21a of the wafer 20A. The wafer 80 is bonded to the back surface 21a of the wafer 20A by the film 70. The wafer 80 has a bonding surface 81b and a back surface 81a. The bonding surface 81b is a surface to be bonded to the back surface 21a of the wafer 20A via the film 70. In the bonded wafer 20B, the type and thickness of the film 70 are adjusted according to the wavelength of the transparent light so that the reflectance of the transparent light on the back surface 21a is equal to or greater than the reflectance of the transparent light on the back surface 21a of the wafer 20A when the wafer 80 is used alone (used as a single plate) without being bonded together (details will be described later).

[0053] [Processing of bonded wafers] Next, the processing of the bonded wafer 20B will be described. The processing of the bonded wafer 20B here may be any processing that detects light propagated through the bonded wafer 20B after forming a modified region on the bonded wafer 20B and derives a processing state (internal processing state) related to the modified region. It may be an inspection processing performed as a pre-processing of a processing to form a modified region for the purpose of cutting the bonded wafer 20B, a processing condition derivation processing to derive processing conditions, or an actual processing processing to form a modified region for the purpose of cutting the bonded wafer 20B. The above-mentioned bonded wafer 20B is, for example, a wafer for inspection used in the above-mentioned inspection processing and processing condition derivation processing. Details of the bonded wafer, which is an actual device used in the actual processing processing, will be described later with reference to FIG. 21. Hereinafter, the processing of the bonded wafer will be described as an inspection processing performed as a pre-processing of a processing to form a modified region for the purpose of cutting the bonded wafer 20B. In the inspection process (pre-processing), a modified area for inspection is formed on the bonded wafer 20B, and, for example, the formation position of the modified area and the tip position of a crack extending from the modified area are detected, and the appropriateness of the processing conditions is judged based on the detected information. By carrying out such an inspection process, for example, if the processing conditions are judged to be inappropriate in the inspection process, the processing conditions can be corrected, and desired processing can be performed in the actual processing.

[0054] In the above-mentioned inspection process, first, the control unit 8 controls the laser irradiation unit 3 (see FIG. 4) so ​​that the bonded wafer 20B is irradiated with laser light to form a plurality of modified regions 12a, 12b on the wafer 20A (first control). The laser irradiation unit 3 irradiates the bonded wafer 20B with laser light from the front surface 21b side of the wafer 20A. Now, in the inspection process, it is desired to detect the tip of the crack 14 extending from the modified region 12a on the back surface 21a side of the wafer 20A to the back surface 21a side. In this case, the imaging unit 4 (see FIG. 5) irradiates the bonded wafer 20B with light having transparency from the front surface 21b side, and detects the light reflected by the back surface 21a and propagated through the wafer 20A, thereby making it possible to detect the tip of the crack extending from the modified region 12a to the back surface 21a side. Therefore, the control unit 8 controls the imaging unit 4 so that the light reflected by the back surface 21a of the wafer 20A and propagated through the wafer 20A is detected (second control). Then, the control unit 8 derives the internal processed state of the wafer 20A based on a signal output in response to the light detected by the imaging unit 4. Specifically, the control unit 8 derives (detects) the position of the tip of the crack 14 extending from the modified region 12a to the back surface 21a based on the signal, and judges whether the processing conditions are appropriate based on the position of the tip of the crack 14.

[0055] Here, when the wafer to be inspected is not a single wafer made of one wafer but a bonded wafer 20B made of two wafers (wafer 20A and wafer 80) bonded together, the reflectance of light (transmissive light output from the imaging unit 4) on the back surface 21a is likely to be low. Fig. 14 is a diagram for explaining the back surface reflection of a single wafer made of only the wafer 20A and the bonded wafer 20B. Fig. 14(a) shows a back surface reflection image of the wafer 20A, which is a single wafer, and Fig. 14(b) shows a back surface reflection image of the bonded wafer 20B.

[0056] In the example shown in FIG. 14(a), when light of a specific wavelength is irradiated onto the wafer 20A, which is a single-plate wafer, the reflectance of light on the back surface 21a is 31%. In contrast, when light of the same specific wavelength is irradiated onto the bonded wafer 20B, the reflectance of light on the back surface 21a of the wafer 20A is 10% as shown in FIG. 14(b). In this way, in the bonded wafer 20B in which the wafer 80 is bonded to the back surface 21a of the wafer 20A via the bonding film 70, the reflectance of light on the back surface 21a of the wafer 20A is lower than that of a single-plate wafer consisting of only the wafer 20A. Such a decrease in reflectance is due to the occurrence of multiple reflections between the film 70 and the adhesive layer present on the bonding surface for light of a specific wavelength. Increasing the amount of light from the imaging unit 4 is considered as a method for increasing the detection rate of light under conditions in which the reflectance is decreased, but this method increases background noise, and as a result, the detection rate of light cannot be increased.

[0057] Therefore, in this embodiment, the wavelength of the light having transparency irradiated from the imaging unit 4 to the bonded wafer 20B, the type of the film 70 for bonding, and the thickness of the film 70 are adjusted as processing conditions for the bonded wafer 20B so that the reflectance of light on the back surface 21a of the bonded wafer 20B is equal to or greater than the reference reflectance, which is the reflectance of light on the back surface 21a of the wafer 20A, which is a single-plate wafer. Below, an example in which the thickness of the film 70 is adjusted with the wavelength of the light having transparency and the type of the film 70 set to fixed values ​​(aspect of film thickness control), and an example in which the wavelength of the light having transparency is adjusted with the type of the film 70 and the thickness of the film 70 set to fixed values ​​(aspect of wavelength control) will be described in order.

[0058] (Film thickness control) Prior to the first control for forming the modified region 12 on the wafer 20A in the inspection process, the control unit 8 may determine the thickness of the film 70 based on information indicating the wavelength of the transparent light (light irradiated from the imaging unit 4) irradiated to the bonded wafer 20B and information indicating the type of film 70, so that the reflectance of light on the back surface 21a of the wafer 20A is equal to or greater than a reference reflectance, and output information indicating the determined thickness of the film 70 (fourth control).

[0059] According to Fresnel's formula, the reflectance of the light is derived based on the wavelength of the irradiated light, the type of polarization of the light (s-wave, p-wave, or mixed), the angle of incidence of the light, the type of film 70 (refractive index of film 70), and the thickness of the film 70. That is, according to Fresnel's formula, if the wavelength of the irradiated light and the type of film 70 are determined, the thickness of the film 70 that realizes the desired reflectance (reflectance equal to or greater than the reference reflectance) can be specified. Now, suppose that the wavelength of the light is set to 1200 nm by a bandpass filter or the like (the type of polarization and the angle of incidence are set to predetermined values), and when a TEOS film with a refractive index of about 1.5 is used as the film 70, it is specified that the reflectance at each thickness of the film 70 is as shown in FIG. 15. Under such conditions, when the reflectance is greater than the reference reflectance (31% as an example here), the detection rate (detection rate of light reflected at the back surface 21a) can be set to 100%, as shown in FIG. 15. The control unit 8 can determine the thickness of the film 70 so that the reflectance is equal to or greater than the reference reflectance based on the information indicating the wavelength of light and the information indicating the type of film 70, by storing information indicating the reflectance relative to the film thickness for each type of film 70 and the wavelength of light, as shown in FIG. 15, in advance. The control unit 8 displays the determined thickness of the film 70 on the display 150 (see FIG. 1). Based on the information displayed on the display 150, the user can grasp the film thickness suitable for the inspection process (allowing the reflected light on the back surface 21a of the bonded wafer 20B to be appropriately detected), and set the bonded wafer 20B with the film thickness at the irradiation position of the laser light on the stage 2. Note that, although the thickness of the film 70 is determined by the control unit 8 in the above description, the process of determining the thickness of the film 70 may be performed by the user. That is, the control unit 8 stores information indicating the reflectance relative to the film thickness, and the user may determine the thickness of the film 70 based on the information.

[0060] FIG. 16 is a diagram for explaining the reflectance according to the film thickness. In the right diagram of FIG. 16, the horizontal axis indicates the film thickness, and the vertical axis indicates the reflectance on the back surface 21a. In the right diagram of FIG. 16, examples of the reflectance according to the film thickness when the wavelength of light is 1100 nm, 1200 nm, and 1300 nm are shown. Now, suppose that the wavelength of light is 1200 nm, and the film thickness of the bonded wafer 20B that was initially intended to be used in the inspection process is 800 nm as shown in the upper left diagram of FIG. 16. In this case, as shown in the right diagram of FIG. 16, the reflectance on the back surface 21a becomes 10% or less, which is smaller than the reference reflectance (for example, 31%) indicated by the dashed line. In such a case, the control unit 8 determines a change in the film thickness of the bonded wafer 20B to be used in the inspection process. Specifically, the control unit 8 determines the bonded wafer 20B to be used in the inspection process to have a film thickness of, for example, 1000 nm (lower left diagram in FIG. 16) so that the reflectance is equal to or greater than the reference reflectance. Note that "outputting information indicating the determined thickness of the film 70" in the above-mentioned fourth control includes outputting various information related to the determination of the thickness of the film 70, and includes outputting information (such as a graph) indicating the reflectance according to the film thickness for each wavelength as shown in FIG. 16.

[0061] FIG. 17 is a flowchart showing an example of a laser processing method for the bonded wafer 20B, and more specifically, a flowchart showing a process for performing the above-mentioned film thickness control in the inspection process.

[0062] 17, when controlling the film thickness, first, the control unit 8 determines the thickness of the film 70 so that the reflectance of light on the back surface 21a is equal to or greater than a reference reflectance based on information indicating the wavelength of light having transparency irradiated to the bonded wafer 20B and information indicating the type of the film 70 (step S1: thickness determination step). Note that step S1 may be performed by a user based on information indicating the reflectance for the film thickness for each wavelength of light stored in the control unit 8.

[0063] Next, the bonded wafer 20B having the thickness determined in step S1 is set by the user at the laser light irradiation position on the stage 2 (step S2: wafer setting process). Note that step S2 may be performed automatically (without the user) under the control of the control unit 8.

[0064] Next, the control unit 8 controls the laser irradiation unit 3, so that laser light is irradiated from the surface 21b side of the bonded wafer 20B, whose film thickness has been adjusted so that the reflectance is equal to or greater than the reference reflectance, to form one or more modified regions 12 inside the wafer 20A (step S3: processing process).

[0065] Next, the control unit 8 controls the imaging unit 4 to irradiate the bonded wafer 20B with transparent light from the front surface 21b side, and the light reflected from the back surface 21a and propagated through the wafer 20A is detected (step S4: detection process).

[0066] Then, the control unit 8 derives the internal processing state of the wafer 20A in the bonded wafer 20B based on the light detected in step S4 (step S5: deriving step). Specifically, the control unit 8 derives (detects) the position of the tip of the crack 14 extending from the modified region 12 to the back surface 21a based on a signal corresponding to the detected light, and judges whether the processing conditions are appropriate based on the position of the tip of the crack 14. The above is the process when performing film thickness control.

[0067] An example of performing film thickness control has been described above, in which "once the wavelength of the irradiated light and the type of film 70, etc. are determined, the thickness of film 70 that will achieve the desired reflectance (reflectance equal to or greater than the reference reflectance) can be specified." Similarly, since "once the wavelength of the irradiated light and the thickness of film 70, etc. are determined, the type of film 70 that will achieve the desired reflectance (reflectance equal to or greater than the reference reflectance) can be specified," the control unit 8 (or the user) may determine the type of film 70 based on information indicating the wavelength of the transparent light (light irradiated from the imaging unit 4) irradiated to the bonded wafer 20B and information indicating the thickness of the film 70, so that the reflectance of the light on the back surface 21a of the wafer 20A is equal to or greater than the reference reflectance.

[0068] (Wavelength Control) The control unit 8 may acquire information indicating the type of film 70 and information indicating the thickness of the film 70 before the first control for forming the modified region 12 on the wafer 20A in the inspection process, and based on the information indicating the type of film 70 and the information indicating the thickness of the film 70, determine the wavelength of the transparent light to be irradiated to the bonded wafer 20B so that the reflectance of light on the back surface 21a of the wafer 20A is equal to or greater than a reference reflectance, and output information indicating the determined wavelength of the transparent light (fifth control).

[0069] As described above, based on Fresnel's formula, the reflectance of the light is derived based on the wavelength of the irradiated light, the type of polarization of the light (s-wave, p-wave, or mixed), the angle of incidence of the light, the type of film 70 (refractive index of film 70), and the thickness of film 70. Therefore, based on Fresnel's formula, once the type of film 70, the thickness of film 70, etc. are determined, the wavelength of light that realizes the desired reflectance (reflectance equal to or higher than the reference reflectance) can be specified.

[0070] FIG. 18 is a diagram for explaining the reflectance according to the wavelength. In the right diagram of FIG. 18, the horizontal axis indicates the film thickness, and the vertical axis indicates the reflectance on the back surface 21a. In the right diagram of FIG. 18, examples of the reflectance according to the film thickness when the wavelength of light is 1100 nm, 1200 nm, and 1300 nm are shown. Now, suppose that the film thickness of the bonded wafer 20B used in the inspection process is 800 nm, and the wavelength of light in the initial inspection process is 1200 nm, as shown in the upper left diagram of FIG. 18. In this case, as shown in the right diagram of FIG. 18, the reflectance on the back surface 21a becomes 10% or less, which is smaller than the reference reflectance (e.g., 31%) indicated by the dashed line. In such a case, the control unit 8 determines a change in the wavelength of the light in the inspection process. Specifically, the control unit 8 changes the wavelength of the light to be irradiated to 1300 nm (lower left diagram of FIG. 18), for example, so that the reflectance becomes equal to or greater than the reference reflectance.

[0071] The control unit 8 may perform a sixth control for switching the bandpass filter so as to obtain the determined wavelength of light, or a seventh control for setting the wavelength band of light output from the light source. When performing the sixth control, the imaging unit 4 may be configured as shown in FIG. 19. FIG. 19 is a configuration diagram of the imaging unit 4. The imaging unit 4 shown in FIG. 19 has a configuration similar to that of the imaging unit 4 shown in FIG. 5, but is different in that it includes a plurality of (here, two) bandpass filters 91 and 92. That is, the imaging unit 4 includes a light source 41, for example an infrared light source, which outputs light having transparency, a mirror 42, an objective lens 43, for example a correction ring lens, a light detection unit 44, for example an infrared detector, and a plurality of bandpass filters 91 and 92. The plurality of bandpass filters 91 and 92 are switchably provided between the light source 41 and the bonded wafer 20B, and are filters having different passbands. Here, it is assumed that bandpass filter 91 is a filter that passes light of 1200 nm, and bandpass filter 92 is a filter that passes light of 1300 nm.

[0072] In the sixth control, the control unit 8 selects one bandpass filter 91 (or bandpass filter 92) from the plurality of bandpass filters 91, 92 according to the wavelength of light determined in the fifth control (control for determining the wavelength) described above, before the first control for forming the modified region 12 on the wafer 20A in the inspection process, and sets the selected bandpass filter 91 (or bandpass filter 92) between the light source 41 and the bonded wafer 20B (more specifically, between the light source 41 and the mirror 42). For example, as shown in FIG. 18, when it is desired to change the wavelength of light from 1200 nm to 1300 nm, the control unit 8 switches the bandpass filter 91 to the bandpass filter 92. In this way, according to the sixth control, the wavelength of light can be appropriately changed and the reflectance on the back surface 21a can be increased by a simple process of switching the bandpass filter.

[0073] In the seventh control, before the first control for forming the modified region 12 on the wafer 20A in the inspection process, the control unit 8 sets a wavelength band including the wavelength of the light determined in the above-mentioned fifth control (control for determining the wavelength) to the light source 41 as the wavelength band of the light output from the light source 41. For example, as shown in Fig. 18, in a case where it is desired to change the wavelength of the light from 1200 nm to 1300 nm, the control unit 8 sets 1300 nm to the light source 41 as the wavelength band of the light output from the light source 41.

[0074] FIG. 20 is a flowchart showing an example of a laser processing method for the bonded wafer 20B, and more specifically, a flowchart showing a process in the case where the above-mentioned wavelength control is performed in the inspection process.

[0075] 20, when wavelength control is performed, first, the control unit 8 determines the wavelength of light having transparency to be irradiated onto the bonded wafer 20B so that the reflectance of light on the back surface 21a of the wafer 20A is equal to or greater than a reference reflectance based on information indicating the type of film 70 and information indicating the thickness of the film 70 (step S11: wavelength determination step). Note that step S11 may be performed by a user based on information indicating the reflectance of light relative to the film thickness for each wavelength stored in the control unit 8.

[0076] Next, the control unit 8 switches between the bandpass filters 91 and 92 or adjusts the wavelength band of the light output from the light source 41 to set the wavelength determined in step S11 (step S12: filter setting step or light source setting step). Note that step S12 may be performed by a user without being controlled by the control unit 8.

[0077] Next, the bonded wafer 20B is set at a laser light irradiation position on the stage 2 by the user (step S13: wafer setting process). Note that step S13 may be performed automatically (without the user) under the control of the control unit 8.

[0078] Next, the control unit 8 controls the laser irradiation unit 3 to irradiate the bonded wafer 20B with laser light from the front surface 21b side, thereby forming one or a plurality of modified regions 12 inside the wafer 20A (step S14: processing step).

[0079] Next, the control unit 8 controls the imaging unit 4 to irradiate the bonded wafer 20B with transparent light from the front surface 21b side, and the light reflected from the back surface 21a and propagated through the wafer 20A is detected (step S15: detection process).

[0080] Then, the control unit 8 derives the internal processing state of the wafer 20A in the bonded wafer 20B based on the light detected in step S15 (step S16: deriving step). Specifically, the control unit 8 derives (detects) the position of the tip of the crack 14 extending from the modified region 12 to the back surface 21a based on a signal corresponding to the detected light, and judges whether the processing conditions are appropriate based on the position of the tip of the crack 14. The above is the process when wavelength control is performed.

[0081] In the aspect of wavelength control, the wavelength of the light having transparency is determined based on information indicating the type and thickness of the film 70 (step S11) as in step S11 above, but this is not limited to this. For example, the reflected light (e.g., the luminance value of the reflected light) from the back surface 21a of the wafer 20A when a plurality of bandpass filters 91, 92 are actually set is detected in the imaging unit 4, and the bandpass filter whose reflectance is equal to or greater than the reference reflectance is identified, thereby determining the wavelength of the light having transparency.

[0082] That is, the laser processing method may further include a bandpass filter specifying step of sequentially setting a plurality of bandpass filters 91, 92 having different passbands switchably provided in front of the bonded wafer 20B in front of the bonded wafer 20B before the processing step, detecting light (e.g., luminance value of light) reflected on the back surface 21a and propagated through the wafer 20A with the bandpass filters 91, 92 set, and specifying a bandpass filter (bandpass filter 91 or bandpass filter 92) with a reflectance of light on the back surface 21a equal to or greater than a reference reflectance based on the detection result, and a filter setting step of setting the bandpass filter specified in the bandpass filter specifying step in front of the bonded wafer 20B before the processing step. According to this configuration, a bandpass filter that can ensure the reflectance of light on the back surface 21a is specified by detecting light reflected on the back surface 21a in a state where the bandpass filter is actually set, and the specified bandpass filter is set, so that the reflectance of light on the back surface 21a can be reliably increased based on the actual measurement value.

[0083] [Effects] Next, the effects of the laser processing apparatus 1 and the laser processing method according to this embodiment will be described.

[0084] The laser processing apparatus 1 according to the present embodiment includes a laser irradiation unit 3 that irradiates a laser beam from the front surface 21b side to a bonded wafer 20B in which a wafer 80 is bonded to the back surface 21a of a wafer 20A including a front surface 21b and a back surface 21a via a bonding film 70, an imaging unit 4 that irradiates the bonded wafer 20B with light having transparency from the front surface 21b side and detects the light that has propagated through the bonded wafer 20B, a first control that controls the laser irradiation unit 3 so that one or more modified regions 12 are formed inside the wafer 20A by irradiating the bonded wafer 20B with the laser beam, and a second control that controls the back surface 21a to detect the laser beam. a control unit 8 configured to execute a second control for controlling the imaging unit 4 so that the light reflected at the back surface 21 a and propagated through the wafer 20A is detected, and a third control for deriving the internal processing state of the wafer 20A based on a signal output from the imaging unit 4 in response to the detected light, and the wavelength of the transparent light irradiated to the bonded wafer 20B, the type of film 70, and the thickness of the film 70 are adjusted so that the reflectance of light at the back surface 21 a is equal to or greater than a reference reflectance, which is the reflectance of light at the back surface 21 a of the wafer 20A when the wafer 80 is used alone without being bonded.

[0085] In the laser processing device 1 according to the present embodiment, the bonded wafer 20B is subjected to laser processing to form the modified region 12 on the wafer 20A, and the internal processing state of the wafer 20A in which the modified region 12 is formed is derived based on a signal corresponding to the detection result of the light reflected on the back surface 21a of the wafer 20A. In this way, the internal processing state is derived from the detection result of the light reflected on the back surface 21a, so that, for example, the tip of the crack 14 extending from the modified region 12 to the back surface 21a side can be detected, and the internal processing state can be more appropriately estimated (derived). Here, in the bonded wafer 20B in which the wafers 20A and 80 are bonded together via the bonding film 70, the reflectance of light having transparency on the back surface 21a (the surface on which the film 70 is formed) of the wafer 20A is generally lower than the reflectance (reference reflectance) on the back surface 21a in the case of the wafer 20A alone. For this reason, there is a risk that the internal processing state of the bonded wafer 20B cannot be appropriately derived from the detection result of the light reflected on the back surface 21a. In this regard, in the laser processing apparatus 1 according to the present embodiment, the wavelength of the light having transparency, the type of the film 70, and the thickness of the film 70 are adjusted so that the reflectance at the rear surface 21a is equal to or greater than the reference reflectance. With this configuration, even when the processing target is the bonded wafer 20B, the reflectance at the rear surface 21a is equal to or greater than that of the wafer 20A alone, so that the internal processing state can be properly derived from the detection result of the light reflected at the rear surface 21a. As described above, with the laser processing apparatus 1 according to the present embodiment, the light reflected at the rear surface 21a of the bonded wafer 20B can also be properly detected, and the internal processing state can be properly derived with high accuracy.

[0086] The control unit 8 may further execute a fourth control, which acquires information indicating the wavelength of the light having transparency to be irradiated to the bonded wafer 20B and information indicating the type of the film 70 before the first control, determines the thickness of the film 70 so that the reflectance of the light on the back surface 21a is equal to or greater than the reference reflectance based on the information indicating the wavelength of the light having transparency and the information indicating the type of the film 70, and outputs information indicating the determined thickness of the film 70. According to such a configuration, before the laser processing, the thickness of the film 70 is determined so that the reflectance of the light on the back surface 21a is equal to or greater than the reference reflectance according to the wavelength of the light to be irradiated to the bonded wafer 20B and the type of the film 70. This reliably increases the reflectance of the light on the back surface 21a, and reliably detects the light reflected on the back surface 21a.

[0087] The control unit 8 may further execute a fifth control, which acquires information indicating the type of the film 70 and information indicating the thickness of the film 70 before the first control, determines the wavelength of light having transparency to be irradiated to the bonded wafer 20B so that the reflectance of light on the back surface 21a is equal to or greater than the reference reflectance based on the information indicating the type of the film 70 and the information indicating the thickness of the film 70, and outputs information indicating the determined wavelength of light having transparency. According to this configuration, before the laser processing, the wavelength of light having transparency is determined according to the type of the film 70 and the thickness of the film 70 so that the reflectance of light on the back surface 21a is equal to or greater than the reference reflectance. This reliably increases the reflectance of light on the back surface 21a, and reliably detects the light reflected on the back surface 21a.

[0088] The imaging unit 4 has a light source 41 that outputs light having transparency, and a plurality of bandpass filters 91, 92 switchably provided between the light source 41 and the bonded wafer 20B, each having a different passband, and the control unit 8 may further execute a sixth control before the first control, in which the control unit 8 selects one bandpass filter 91 (or bandpass filter 92) from the plurality of bandpass filters 91, 92 according to the wavelength of light determined in the fifth control, and sets the selected bandpass filter 91 (or bandpass filter 92) between the light source 41 and the bonded wafer 20B. According to this configuration, a bandpass filter according to the determined wavelength of light is set, so that the reflectance of light on the back surface 21a can be reliably increased by a simple process.

[0089] The imaging unit 4 has a light source 41 that outputs light having transparency, and the control unit 8 may further execute a seventh control before the first control in which a wavelength band including the wavelength of the light determined in the fifth control is set to the light source 41 as the wavelength band of the light output from the light source 41. According to such a configuration, a wavelength band including the determined wavelength of the light is set as the wavelength band of the light output from the light source 41, so that the reflectance of the light on the back surface 21a can be reliably increased by a simple process.

[0090] The bonded wafer 20B according to the present embodiment includes a wafer 20A, a bonding film 70 formed on the back surface 21a, which is one surface of the wafer 20A, and a wafer 80 bonded to the back surface 21a of the wafer 20A by the film 70. The type and thickness of the film 70 are adjusted according to the wavelength of the light having transparency so that the reflectance of light having transparency on the back surface 21a is equal to or greater than the reference reflectance of light having transparency on the back surface 21a of the wafer 20A when the wafer 80 is used alone without being bonded. According to the bonded wafer 20B according to the present embodiment, the reflectance on the back surface 21a of the wafer 20A is equal to or greater than that of the wafer 20A alone, so that the internal processing state can be appropriately derived from the detection result of the light reflected on the back surface 21a. As described above, according to the bonded wafer 20B according to the present embodiment, the light reflected on the back surface 21a of the bonded wafer can also be appropriately detected, and the internal processing state can be accurately derived.

[0091] The laser processing method according to the present embodiment includes a processing step of irradiating the bonded wafer 20B with laser light from the front surface 21b side to form one or more modified regions 12 inside the wafer 20A, a detection step of irradiating the bonded wafer 20B with light having transparency from the front surface 21b after the processing step, detecting the light reflected at the back surface 21a and propagating through the wafer 20A, and a derivation step of deriving the internal processing state of the wafer 20A based on the light detected in the detection step. According to the laser processing method according to the present embodiment, even when the processing target is the bonded wafer 20B, the reflectance at the back surface 21a is equal to or higher than that of the wafer 20A alone, so that the internal processing state can be appropriately derived from the detection result of the light reflected at the back surface 21a. As described above, according to the laser processing method according to the present embodiment, the light reflected at the back surface 21a can be appropriately detected for the bonded wafer 20B as well, and the internal processing state can be accurately derived.

[0092] The above-mentioned laser processing method may further include a thickness determination step of determining the thickness of the film 70 based on information indicating the wavelength of the light having transparency to be irradiated to the bonded wafer 20B and information indicating the type of the film 70 before the processing step so that the reflectance of light on the back surface 21a is equal to or greater than the reference reflectance, and a wafer setting step of setting the bonded wafer 20B having the thickness of the film 70 determined in the thickness determination step at the irradiation position of the laser light in the processing step. According to such a configuration, before the laser processing, the thickness of the film 70 is determined according to the wavelength of the light to be irradiated to the bonded wafer 20B and the type of the film 70 so that the reflectance of light on the back surface 21a is equal to or greater than the reference reflectance, and the bonded wafer 20B having the determined thickness of the film 70 is set at the irradiation position of the laser light. This reliably increases the reflectance of light on the back surface 21a and reliably detects the light reflected on the back surface 21a.

[0093] The above-mentioned laser processing method may further include a wavelength determination step of determining the wavelength of light having transparency to be irradiated to the bonded wafer 20B based on information indicating the type of the film 70 and information indicating the thickness of the film 70 before the processing step so that the reflectance of light on the back surface 21a is equal to or greater than a reference reflectance, and a filter setting step of selecting one bandpass filter from among a plurality of bandpass filters 91, 92 switchably provided in front of the bonded wafer 20B and having different passbands in accordance with the wavelength of light determined in the wavelength determination step, and setting the selected bandpass filter in front of the bonded wafer 20B. According to this configuration, a bandpass filter corresponding to the determined wavelength of light is set, so that the reflectance of light on the back surface 21a can be reliably increased by a simple process.

[0094] The above-mentioned laser processing method may further include a wavelength determination step of determining the wavelength of light having transparency to be irradiated to the bonded wafer 20B based on information indicating the type of the film 70 and information indicating the thickness of the film 70 before the processing step so that the reflectance of light on the back surface 21a is equal to or greater than a reference reflectance, and a light source setting step of setting a wavelength band including the wavelength of the light determined in the wavelength determination step to the light source 41 as the wavelength band of light output from the light source 41 that outputs light having transparency. According to this configuration, the wavelength band including the determined wavelength of the light is set as the wavelength band of light output from the light source 41, so that the reflectance of light on the back surface 21a can be reliably increased by a simple process.

[0095] Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment. For example, as a processing process for a bonded wafer, an inspection process performed as a pre-processing of a process for forming a modified region for the purpose of cutting the bonded wafer, etc., has been mainly described, but the processing process may be a processing condition derivation process for deriving processing conditions, or an actual processing process for forming a modified region for the purpose of cutting the bonded wafer, etc. As a bonded wafer (i.e., a bonded wafer that is a device wafer) when the actual processing process is performed, for example, a bonded wafer 120 shown in FIG. 21 may be used.

[0096] FIG. 21 is a cross-sectional view of a part of a bonded wafer 120 which is a device wafer. In the example shown in FIG. 21, the bonded wafer 120 includes a wafer 121 (first wafer), a wafer 122 (second wafer), a SiO2 film 123 (film for bonding), a SiO2 film 124, a SiN film 125, a wiring layer 126, a SiN film 127, and a SiO2 film 128. The wafers 121 and 122 are, for example, silicon substrates. The SiO2 film 124 and the SiN film 125 which constitute the SNSN structure are formed on the back surface of the wafer 121. The SiO2 film 128 and the SiN film 127 which constitute the SNSN structure are formed on the front surface of the wafer 122. The wiring layer 126 which affects the characteristics of the device is formed on the front surface of the SIN film 127. The wiring layer 126 may have a Cu / Low-k structure in which a low dielectric constant material (Low-k film) is used as an interlayer insulating film and low resistance copper (Cu) is used as a wiring material. A SiO2 film 123 is formed between the SIN film 125 and the wiring layer 126. The SiO2 film 123 is a layer whose thickness can be changed, that is, a layer whose thickness can be controlled to obtain a desired reflectance. In this configuration, for example, the bonded wafer 120, which is a memory wafer, can have a high back surface reflectance and be suitable for internal observation by controlling the thickness of the SiO2 film 123 between the SNSN structure and the wiring layer 126. [Explanation of symbols]

[0097] 1...laser processing apparatus, 3...laser irradiation unit, 4...imaging unit, 8...control unit, 12...modified region, 20B, 120...bonded wafer, 20A...wafer, 21a...rear surface, 21b...front surface, 41...light source, 70...film, 80...wafer, 91, 92...bandpass filter.

Claims

1. an irradiation unit that irradiates a laser beam from a first surface side to a bonded wafer in which a second wafer is bonded to a first wafer having a first surface and a second surface via a bonding film; An imaging unit that irradiates the bonded wafer with light having transparency from the first surface side and detects the light propagated through the bonded wafer; a control unit configured to execute a first control for controlling the irradiation unit so that one or more modified regions are formed inside the first wafer by irradiating the bonded wafer with the laser light, a second control for controlling the imaging unit so that light reflected at the second surface and propagated through the first wafer is detected, and a third control for deriving an internal processing state of the first wafer based on a signal output from the imaging unit in response to the detected light, a laser processing apparatus in which the wavelength of the transparent light irradiated to the bonded wafer, the type of the film, and the thickness of the film are adjusted so that the reflectance of light on the second surface is equal to or greater than a reference reflectance which is the reflectance of light on the second surface of the first wafer when the second wafer is used alone without being bonded.

2. The control unit is 2. The laser processing apparatus of claim 1, further comprising: a fourth control, prior to the first control, for acquiring information indicating a wavelength of the transparent light to be irradiated to the bonded wafer and information indicating a type of the film, determining a thickness of the film based on the information indicating the wavelength of the transparent light and the information indicating the type of the film so that the reflectance of light on the second surface is equal to or greater than the reference reflectance, and outputting information indicating the determined thickness of the film.

3. The control unit is 2. The laser processing apparatus of claim 1, further comprising: a fifth control that acquires, before the first control, information indicating the type of the film and information indicating the thickness of the film, determines a wavelength of the transparent light to be irradiated to the bonded wafer based on the information indicating the type of the film and the information indicating the thickness of the film so that the reflectance of light on the second surface is equal to or greater than the reference reflectance, and outputs information indicating the determined wavelength of the transparent light.

4. The imaging unit includes a light source that outputs light having the transmittance, and a plurality of bandpass filters having different passbands that are switchably provided between the light source and the bonded wafer, The control unit is 4. The laser processing apparatus according to claim 3, further comprising: a sixth control, before the first control, for selecting one bandpass filter from the plurality of bandpass filters in accordance with the wavelength of light determined in the fifth control, and setting the selected bandpass filter between the light source and the bonded wafer.

5. the imaging unit has a light source that outputs light having the transmittance; The control unit is The laser processing apparatus according to claim 3, further comprising a seventh control, prior to the first control, for setting a wavelength band including the wavelength of the light determined in the fifth control as the wavelength band of the light output from the light source to the light source.

6. A first wafer; A bonding film formed on one surface of the first wafer; a second wafer bonded to the one surface of the first wafer by the film; A bonded wafer in which the type and thickness of the film are adjusted according to the wavelength of the transmissive light so that the reflectance of the transmissive light on the one surface is equal to or greater than a reference reflectance, which is the reflectance of the transmissive light on the one surface of the first wafer when the second wafer is used alone without being bonded to the first wafer.

7. a processing step of irradiating a laser beam from the first surface side of the bonded wafer to the second surface of a first wafer including a first surface and a second surface, to form one or more modified regions inside the first wafer, in a state in which the wavelength of the transmissive light, the type of the film, and the thickness of the film to be irradiated to the bonded wafer are adjusted so that the reflectance of the transmissive light on the second surface of the bonded wafer, in which a second wafer is bonded to the second surface of the first wafer via a bonding film, is equal to or greater than a reference reflectance which is the reflectance of the transmissive light on the second surface of the first wafer when the second wafer is used alone without being bonded to the first wafer; a detection step of irradiating the bonded wafer with light having the transmittance from the first surface side after the processing step, and detecting light reflected by the second surface and propagating through the first wafer; and a deriving step of deriving an internal processed state of the first wafer based on the light detected in the detecting step.

8. a thickness determination step of determining a thickness of the film based on information indicating a wavelength of the light having the transmittance to be irradiated to the bonded wafer and information indicating a type of the film before the processing step, so that the reflectance of light on the second surface is equal to or greater than the reference reflectance; 8. The laser processing method according to claim 7, further comprising a wafer setting step of setting the bonded wafer having the film thickness determined in the thickness determination step at a position to be irradiated with laser light in the processing step.

9. A wavelength determination process for determining a wavelength of the light having transparency to be irradiated to the bonded wafer, based on information indicating the type of the film and information indicating the thickness of the film, before the processing process, so that the reflectance of light on the second surface is equal to or greater than the reference reflectance; 8. The laser processing method according to claim 7, further comprising a filter setting step of selecting one bandpass filter from a plurality of bandpass filters having different passbands switchably provided in front of the bonded wafer according to the wavelength of light determined in the wavelength determination step, and setting the selected bandpass filter in front of the bonded wafer.

10. A wavelength determination process for determining a wavelength of the light having transparency to be irradiated to the bonded wafer, based on information indicating the type of the film and information indicating the thickness of the film, before the processing process, so that the reflectance of light on the second surface is equal to or greater than the reference reflectance; 8. The laser processing method according to claim 7, further comprising a light source setting step of setting a wavelength band including the wavelength of the light determined in the wavelength determination step as the wavelength band of the light output from the light source that outputs the light having the transparency to the light source.

11. a bandpass filter identification process for sequentially setting a plurality of bandpass filters having different passbands in front of the bonded wafer, which are switchably provided in front of the bonded wafer before the processing process, detecting light reflected by the second surface and propagating through the first wafer in a state in which each of the bandpass filters is set, and identifying the bandpass filter whose reflectance of light in the second surface is equal to or greater than the reference reflectance based on the detection result; 8. The laser processing method according to claim 7, further comprising a filter setting step of setting the bandpass filter specified in the bandpass filter specifying step in a stage preceding the bonded wafer before the processing step.

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