Method of processing a wafer

Abstract

The present application provides a wafer processing method, which easily confirms whether the wafer is processed under proper conditions. The wafer processing method of forming a modification layer in the inside of a wafer along a division predetermined line has the following steps: a modification layer forming step of forming a modification layer in the inside of the wafer by irradiating a first laser beam from the back surface side of the wafer; a laser beam for observation irradiating step of positioning a focal point of a second laser beam, which outputs not more than a processing threshold value of the wafer, in the inside or the front surface of the wafer and irradiating the second laser beam while moving the focal point; a photographing step of photographing reflected light of the second laser beam by a photographing unit; and a determining step of determining a processing state of the wafer based on an image photographed in the photographing step, the second laser beam being shaped so as to be not line-symmetrical across an axis along the division predetermined line with respect to a cross-sectional shape in a plane perpendicular to a traveling direction of the second laser beam.

Description

TECHNICAL FIELD

[0001] The present application relates to a wafer processing method in which a laser beam is irradiated from the back surface side of a wafer and condensed in the inside of the wafer to form a modified layer as a starting point for dividing the wafer, and a crack is elongated from the modified layer toward the front surface side of the wafer. BACKGROUND

[0002] In a device chip manufacturing process, a plurality of division predetermined lines intersecting each other are set on the front surface of a wafer, a device is formed in each of the divided regions, and the wafer is divided along the division predetermined lines.

[0003] For example, a laser beam having a wavelength (a wavelength capable of transmitting the wafer) that is transparent to the wafer is irradiated from the back surface side of the wafer to the wafer, and the laser beam is condensed in the inside of the wafer along the division predetermined lines. At this time, a modified layer as a division starting point is formed in the vicinity of the condensing point of the laser beam. When a crack is elongated from the formed modified layer toward the front surface of the wafer, the wafer is divided along the division predetermined lines (for example, refer to Patent Literature 1 and Patent Literature 2).

[0004] In this processing method, in order to form the modified layer and cause the crack to travel from the modified layer toward the front surface of the wafer, it is necessary to appropriately set the formation position of the modified layer in the depth direction of the wafer and the processing conditions such as the irradiation conditions of the laser beam.

[0005] If the processing conditions and the like are not appropriate, the crack cannot be appropriately elongated from the formed modified layer, or the crack is elongated in an unintended direction or the like, and thus the wafer cannot be appropriately divided, and therefore the yield of the device chips decreases. In addition, if an offset occurs in the optical axis of the optical system, the modified layer cannot be formed at a prescribed position, and the wafer still cannot be appropriately divided.

[0006] Patent Literature 1: Japanese Patent Application Publication No. 2005-86161

[0007] Patent Literature 2: Japanese Patent Application Publication No. 2010-68009

[0008] Here, in order to confirm whether the processing conditions and the like are appropriate and the modified layer is formed as intended and whether the crack travels appropriately from the modified layer toward the front surface of the wafer, it is possible to consider, for example, observing the front surface of the wafer with a microscope or the like. However, in order to observe the front surface side of the wafer irradiated with the laser beam from the back surface side, it is necessary to, for example, take the wafer out of the laser processing device, turn the wafer upside down, and carry it into the microscope or the like. Therefore, there is a problem that the confirmation of the processing state of the wafer takes time. SUMMARY

[0009] The present application was achieved in view of the above-described problems, and an object thereof is to provide a wafer processing method capable of easily confirming whether a wafer is appropriately processed.

[0010] According to one embodiment of the present application, there is provided a wafer processing method of forming a modification layer in the inside of a wafer along a plurality of division lines provided on the front surface of the wafer, the wafer processing method comprising: a holding step of opposing the front surface of the wafer to a chuck table and holding the wafer by the chuck table; a modification layer forming step of forming the modification layer in the inside of the wafer by irradiating a first laser beam having a wavelength that is transmissive to the wafer from the back surface side of the wafer along the division lines while positioning a focal point of the first laser beam in the inside of the wafer and relatively moving a laser beam irradiation unit and the chuck table in a direction along the division lines; an observation laser beam irradiation step of positioning a focal point of a second laser beam having a wavelength that is transmissive to the wafer and outputting not more than a processing threshold of the wafer in the inside of the wafer or the front surface after the modification layer forming step, and irradiating the second laser beam from the back surface side of the wafer while relatively moving the wafer and the focal point in a direction along the division lines; a photographing step of photographing reflected light of the second laser beam irradiated in the observation laser beam irradiation step by a photographing unit; and a determination step of determining a processing state of the wafer based on an image photographed in the photographing step, the second laser beam irradiated on the wafer in the observation laser beam irradiation step being shaped into a cross-sectional shape in a plane perpendicular to a traveling direction of the second laser beam, and the second laser beam being asymmetric with respect to an axis along the division lines.

[0011] Further, according to another aspect of the present application, there is provided a wafer processing method of forming a modification layer in the inside of a wafer along a plurality of division lines provided on the front surface of the wafer, the wafer processing method comprising: a holding step of opposing the front surface of the wafer to a chuck table and holding the wafer by the chuck table; a modification layer forming step of forming the modification layer in the inside of the wafer by irradiating a first laser beam having a wavelength that is transmissive to the wafer from the back surface side of the wafer while positioning a focal point of the first laser beam in the inside of the wafer and relatively moving a laser beam irradiation unit and the chuck table in a direction along the division lines; an observation laser beam irradiation step of, after the modification layer forming step, positioning a focal point of a second laser beam having a wavelength that is transmissive to the wafer and outputted so as not to exceed a processing threshold of the wafer in the inside of the wafer or the front surface, relatively moving the wafer and the focal point in a direction perpendicular to the direction along the division lines so that the focal point passes over the modification layer, and irradiating the second laser beam from the back surface side of the wafer; a photographing step of photographing reflected light of the second laser beam irradiated in the observation laser beam irradiation step by a photographing unit; and a determination step of determining a processing state of the wafer based on an image photographed in the photographing step, wherein the second laser beam irradiated on the wafer in the observation laser beam irradiation step is shaped into a cross-sectional shape in a plane perpendicular to a traveling direction of the second laser beam, and is asymmetric with respect to an axis along the division lines.

[0012] Preferably, the observation laser beam irradiation step is performed by liquid immersion.

[0013] In the wafer processing method of one embodiment of the present application, after the modification layer forming step of forming a modification layer by condensing a first laser beam in the inside of a wafer is performed, an observation laser beam irradiation step, a photographing step, and a determination step are performed. In the observation laser beam irradiation step, a second laser beam is irradiated from the back surface side of the wafer while relatively moving the wafer and a focal point.

[0014] The second laser beam irradiated to the back surface side of the wafer and traveling in the inside in the observation laser beam irradiation step is reflected by the front surface of the wafer. Then, in the photographing step, the reflected light of the second laser beam is photographed to form an image. Here, the shape and position of the reflected light appearing in the image are determined depending on the positional relationship between the modification layer and a crack and the second laser beam, and the like.

[0015] Thus, if an image appearing by the reflected light of the second laser beam is formed while relatively moving the wafer and the focal point, the processing state of the wafer, such as the formation position and height of the modification layer, the presence or absence of a crack, and the like can be determined.

[0016] According to one embodiment of the present application, a wafer processing method is provided that allows easy confirmation of whether a wafer has been processed properly. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a schematic cross-sectional view illustrating a wafer.

[0018] Figure 2 is a schematic cross-sectional view illustrating a modification layer forming step.

[0019] Figure 3 (A) of FIG. 1 is a schematic cross-sectional view illustrating a wafer in which a modification layer is formed inside, enlarged, Figure 3 (B) of FIG. 1 is a schematic cross-sectional view illustrating a wafer in which a modification layer and a crack are formed inside, enlarged.

[0020] Figure 4 is a schematic cross-sectional view illustrating an observation laser beam irradiation step.

[0021] Figure 5 (A) of FIG. 2 is a schematic cross-sectional view illustrating a second laser beam and its reflected light irradiated onto a wafer in which a modification layer is formed inside, Figure 5 (B) of FIG. 2 is a schematic cross-sectional view illustrating a second laser beam and its reflected light irradiated onto a wafer in which a modification layer and a crack are formed inside.

[0022] Figure 6 (A) of FIG. 3 is a schematic plan view illustrating a region in a back surface of a wafer in which a second laser beam is irradiated, Figure 6 (B) of FIG. 3 is a schematic plan view illustrating an example of a region appearing by reflected light in an image appearing by the reflected light, Figure 6 (C) of FIG. 3 is a schematic plan view illustrating another example of a region appearing by reflected light in an image appearing by the reflected light.

[0023] Figure 7 (A) of FIG. 4 and Figure 7 (B) of FIG. 4 are images appearing by reflected light in a case where a wafer has a crack, Figure 7 (C) of FIG. 4 and Figure 7 (D) of FIG. 4 are images appearing by reflected light in a case where a wafer does not have a crack.

[0024] Figure 8 (A) of FIG. 5 is a schematic plan view illustrating an irradiated region of a second laser beam, Figure 8 (B) of FIG. 5 is a schematic cross-sectional view illustrating a path of a second laser beam and reflected light.

[0025] Figure 9 (A) of FIG. 6, Figure 9 (B) of FIG. 6, Figure 9 (C) of FIG. 6,Figure 9 (D) and Figure 9 (E) is a schematic top view showing the area revealed by reflected light in an image.

[0026] Figure 10 (A) is a schematic top view showing the area irradiated by the second laser beam. Figure 10 (B) is a cross-sectional view schematically showing the path of the second laser beam and the reflected light.

[0027] Figure 11 (A) Figure 11 (B) Figure 11 (C) Figure 11 (D) and Figure 11 (E) is a schematic top view showing the area revealed by reflected light in an image.

[0028] Figure 12 It is a flowchart illustrating the steps of the wafer fabrication method.

[0029] Label Explanation

[0030] 1: Wafer; 1a: Front side; 1b: Back side; 3: Pre-defined dividing line; 5: Device; 7: Modified layer; 9: Crack; 2: Laser processing device; 4: Chuck stage; 4a: Holding surface; 6: Laser beam irradiation unit; 8, 20: Laser oscillator; 10: Reflector; 22: Dichroic mirror; 12, 24: Focusing lens; 14: First laser beam; 16, 30: Focusing point; 18: Observation laser beam irradiation unit; 26: Beam shaping unit; 28: Second laser beam; 32, 32a, 32b, 32c, 32d: Reflected light; 34: Imaging unit; 3 6, 38: Image; 40: Region; 42a, 42b: Region; 44a, 44b, 44c, 44d, 44e: Illuminated region; 46a, 46b, 46c, 46d, 46e: Image; 48a, 48b, 48c, 48d, 48e: Region; 50b, 50c, 50e: Region; 52a, 52b, 52c, 52d, 52e: Illuminated region; 54a, 54b, 54c, 54d, 54e: Image; 56a, 56b, 56c, 56d, 56e: Region; 58b, 58d: Region. Detailed Implementation

[0031] Embodiments of the present invention will be described with reference to the accompanying drawings. First, a wafer for which a modified layer is formed by the wafer processing method of this embodiment will be described. Figure 1 This is a schematic three-dimensional view of chip 1.

[0032] The wafer 1 is, for example, a generally circular substrate made of materials such as Si (silicon), SiC (silicon carbide), GaN (gallium nitride), GaAs (gallium arsenide), or other semiconductors, or materials such as sapphire, glass, or quartz. The glass is, for example, alkali glass, alkali-free glass, soda lime glass, lead glass, borosilicate glass, or quartz glass.

[0033] Multiple intersecting pre-defined dividing lines 3 are formed on the front side 1a of the wafer 1. These pre-defined dividing lines 3 are also called spacers. Devices 5 are formed in the regions defined by the pre-defined dividing lines 3 on the front side 1a of the wafer 1. These devices 5 may be, for example, integrated circuits (ICs) or large-scale integrated circuits (LSIs). However, the wafer 1 is not limited to these. The material, shape, structure, and size of the wafer 1 are not limited, and devices 5 may not be formed on the wafer 1.

[0034] When wafer 1 is diced along the predetermined dicing line 3, individual device chips each equipped with device 5 are formed. During the dicing of wafer 1, for example, a laser beam is focused along the predetermined dicing line 3 inside wafer 1 to form a modified layer inside wafer 1, and a crack is formed extending from the modified layer toward the front surface 1a of wafer 1 along the thickness direction.

[0035] If the processing conditions of wafer 1 are inappropriate, or the state of the laser processing equipment is not suitable for processing, the crack may not extend from the modified layer, or the crack may extend in an unpredictable direction, making it impossible to properly divide wafer 1. In this case, the yield of the device chip decreases due to the generation of defective products.

[0036] Next, use Figure 2 The laser processing apparatus 2 for implementing the wafer 1 processing method of this embodiment will be described. Figure 2 This is a schematic cross-sectional view illustrating the formation of a modified layer on a wafer 1 using a laser processing apparatus 2. The laser processing apparatus 2 includes: a chuck stage 4 that holds the wafer 1; and a laser beam irradiation unit 6 that irradiates a laser beam onto the wafer 1 held by the chuck stage 4.

[0037] The chuck stage 4 has a porous component (not shown) on its upper surface. The upper surface of the porous component serves as a holding surface 4a for holding the wafer 1. The chuck stage 4 is capable of rotating about an axis perpendicular to the holding surface 4a. The chuck stage 4 has an attraction source (not shown) connected to the porous component.

[0038] When the wafer 1 is processed by the laser processing apparatus 2, the front surface la is opposed to the holding surface 4a, and the wafer 1 is placed on the holding surface 4a, and then, the negative pressure generated from the suction source is applied to the wafer 1 through the porous member. In this case, the wafer 1 is held by the chuck table 4 in a state that the back surface lb side is exposed upward. The wafer 1 is processed by laser by irradiating the laser beam from the exposed back surface lb side.

[0039] When the wafer 1 is held by the chuck table 4, a frame unit in which the annular frame, the adhesive tape attached to the annular frame, and the wafer 1 are integrated can be formed in advance. When the frame unit is formed, the front surface la side of the wafer 1 is attached to the adhesive surface of the adhesive tape exposed at the opening of the annular frame. In this case, when the chuck table 4 holds the frame unit, the wafer 1 is placed on the holding surface 4a with the adhesive tape interposed therebetween.

[0040] The chuck table 4 and the laser beam irradiation unit 6 are relatively movable in a direction parallel to the holding surface 4a. For example, the chuck table 4 is movable in a processing feed direction (X-axis direction) set to be parallel to the holding surface 4a, and the laser beam irradiation unit 6 is movable in an indexing feed direction (Y-axis direction) parallel to the holding surface 4a and perpendicular to the processing feed direction.

[0041] Figure 2 A simplest structure example of the laser beam irradiation unit 6 capable of irradiating the laser beam to the wafer 1 held by the chuck table 4 is schematically shown. The laser beam irradiation unit 6 has a laser oscillator 8 that oscillates a laser, a mirror 10, and a condenser lens 12.

[0042] The laser oscillator 8 has a function of emitting a first laser beam 14 having a wavelength (a wavelength that transmits the wafer 1) that is transmissive to the wafer 1. For example, the first laser beam 14 uses a laser having a wavelength of 1099 nm that is oscillated in a medium of Nd:YAG or the like. However, the laser oscillator 8 and the first laser beam 14 are not limited thereto, and can be selected according to the material or the like of the wafer 1.

[0043] When the modified layer is formed in the inside of the wafer 1, for example, the output of the first laser beam 14 is about 2 W to 3 W. However, the output of the first laser beam 14 is not limited thereto, and can be an output capable of forming the modified layer in the inside of the wafer 1. The first laser beam 14 emitted from the laser oscillator 8 is reflected by the mirror 10 in a predetermined direction, and is irradiated onto the wafer 1 held by the chuck table 4 through the condenser lens 12.

[0044] The condenser lens 12 has a function of condensing the first laser beam 14 at a prescribed height position inside the wafer 1 held by the chuck table 4. The condenser lens 12 is, for example, movable along the height direction, and can change the height position of the condensing point 16. The condensing point 16 of the first laser beam 14 is positioned at the prescribed height position inside the wafer 1.

[0045] As shown in Figure 2 , when the laser beam irradiation unit 6 and the chuck table 4 are relatively moved along the machining feed direction and the first laser beam 14 is condensed inside the wafer 1, the modified layer 7 is formed inside the wafer 1. Here, when the irradiation conditions of the first laser beam 14 and the machining conditions such as the machining feed speed are appropriately set, as shown in Figure 3 (B), the crack 9 extending from the modified layer 7 toward the front surface la of the wafer 1 is formed, and the wafer 1 can be easily and appropriately divided.

[0046] However, if the machining conditions and the like are inappropriate, as shown in Figure 3 (A), the crack 9 does not appropriately extend from the formed modified layer 7, or the crack 9 extends in an unintended direction or the like, and the wafer 1 cannot be appropriately divided. Further, if a shift occurs in the optical axis of the optical system, the modified layer 7 cannot be formed at a prescribed position and the wafer 1 cannot be appropriately divided. Therefore, the yield of the device chips decreases.

[0047] Here, in order to confirm whether the wafer 1 is appropriately machined, it is possible to consider, for example, observing the front surface la of the wafer 1 with a microscope or the like. That is, it is possible to consider confirming whether the crack 9 appropriately advances from the modified layer 7 toward the front surface la of the wafer 1, and further, confirming whether the modified layer 7 is appropriately formed at a prescribed position, by observing the front surface la of the wafer 1 with a microscope or the like.

[0048] However, in order to observe the front surface la side of the wafer 1 from which the first laser beam 14 is irradiated from the back surface lb side, it is necessary to, for example, take out the wafer 1 from the laser machining device 2, turn the wafer 1 upside down, and carry it into a microscope or the like. Therefore, there is a problem that it takes time to confirm the machining state such as the presence or absence of the crack 9. Therefore, in the wafer machining method of the present embodiment, the time for confirmation is reduced by confirming the machining state of the wafer 1 in the laser machining device 2. Next, the structure for the confirmation of the machining state will be described.

[0049] As shown in Figure 4 , the laser machining device 2 has an observation laser beam irradiation unit 18. The observation laser beam irradiation unit 18 has a function of irradiating a second laser beam 28 as an observation laser beam to the wafer 1 on which the modified layer 7 is formed. Figure 4 A simplest structure example of the observation laser beam irradiation unit 18 capable of irradiating the second laser beam 28 to the wafer 1 held by the chuck table 4 is schematically shown.

[0050] The observation laser beam irradiation unit 18 has a laser oscillator 20, a dichroic mirror 22, a condenser lens 24, and a beam shaping unit 26 that shapes the second laser beam 28 into a specific shape. The laser oscillator 20 is capable of emitting the second laser beam 28 with an output that does not exceed the processing threshold value at which a modified layer can be formed inside the wafer 1.

[0051] The laser oscillator 20, for example, emits the second laser beam 28 with an output of about 0.2 W that does not exceed the processing threshold value. However, the output of the second laser beam 28 is not limited to this. Since the processing threshold value differs depending on the material of the wafer 1, the output of the second laser beam 28 is appropriately confirmed depending on the material of the wafer 1 to be processed so that the output of the second laser beam 28 does not exceed the processing threshold value.

[0052] It is preferable that the output of the second laser beam 28 be set to between one-tenth and one-thousandth of the output of the first laser beam 14. It is more preferable that the output of the second laser beam 28 be set to about one-thirtieth of the output of the first laser beam 14.

[0053] The dichroic mirror 22 has a function of reflecting the second laser beam 28 toward a prescribed direction. In addition, as will be described later, the dichroic mirror 22 has a function of making the reflected light 32 of the second laser beam 28 transmitted when it reaches the dichroic mirror 22 after being reflected by the front surface la side of the wafer 1.

[0054] The condenser lens 24 has a function of condensing the second laser beam 28 inside the wafer 1 or on the front surface la of the wafer 1 held by the chuck table 4. The condenser lens 24, for example, is capable of moving along the height direction and is capable of changing the height position of the condensing point 30.

[0055] In addition, the observation laser beam irradiation unit 18 can also irradiate the first laser beam 14 with an output that exceeds the processing threshold value of the wafer 1 toward the wafer 1 held by the chuck table 4. That is, the observation laser beam irradiation unit 18 can also function as the laser beam irradiation unit 6 described in the first embodiment. In this case, the laser beam irradiation unit 6 can be omitted, thereby simplifying the structure of the laser processing apparatus 2. Therefore, the light sources of the first laser beam 14 and the second laser beam 28 can be the same. Figure 2

[0056] On the other hand, in the case where the laser processing apparatus 2 has both the laser beam irradiation unit 6 and the observation laser beam irradiation unit 18, when there is also one chuck table, the wafer 1 can be processed efficiently. For example, the first laser beam 14 can be irradiated toward one wafer 1 while the second laser beam 28 is irradiated toward another wafer 1.

[0057] ​The beam shaping unit 26 provided in the observation laser beam irradiation unit 18 has a function of shaping the second laser beam 28 emitted from the laser oscillator 20 into a specific shape. The beam shaping unit 26 is, for example, a plate-like member having a through window (not shown) having a shape corresponding to the specific shape and a shielding portion (not shown) shielding the second laser beam 28 around the through window. The through window is formed so as to penetrate the beam shaping unit 26.

[0058] The beam shaping unit 26 adjusts the direction so that the penetration direction of the through window coincides with the traveling direction of the second laser beam 28, and is incorporated in the observation laser beam irradiation unit 18. When the second laser beam 28 reaches the beam shaping unit 26, a part passes through the through window, and the remaining part is shielded by the shielding portion, thereby shaping the second laser beam 28 into the specific shape.

[0059] Alternatively, a DOE (Diffractive Optical Element) can also be incorporated in the observation laser beam irradiation unit 18 as the beam shaping unit 26. In this case, the DOE is designed and manufactured so as to be capable of shaping the second laser beam 28 into a prescribed shape. Further, a spatial light modulator including an LCOS (Liquid crystal on silicon) element can also be incorporated in the observation laser beam irradiation unit 18 as the beam shaping unit 26.

[0060] In the wafer processing method of the present embodiment, the second laser beam 28 is shaped so as to be asymmetric with respect to an axis along the division predetermined line 3 in the cross-sectional shape in a plane (for example, the back surface lb) perpendicular to the traveling direction of the second laser beam 28 when irradiated to the back surface lb of the wafer 1. For example, the cross-sectional shape of the second laser beam 28 is a semicircle on one side of two regions separated by the axis.

[0061] The second laser beam 28 is irradiated to the wafer 1 from the back surface lb side, and travels inside the wafer 1. Then, the second laser beam 28 reaching the front surface la of the wafer 1 is reflected by the front surface la of the wafer 1. Then, the reflected light 32 of the second laser beam 28 travels in the opposite direction inside the wafer 1, and travels outside the wafer 1 from the back surface lb.

[0062] The reflected light 32 of the second laser beam 28 is converted into parallel light by the condenser lens 24, and passes through the dichroic mirror 22. Further, a photographing unit 34 which photographs the reflected light 32 is disposed on the advancing route of the reflected light 32 passing through the dichroic mirror 22. The photographing unit 34 has, for example, an image sensor such as a CMOS sensor or a CCD sensor.

[0063] The imaging unit 34 images the reflected light 32, forming an image in which the reflected light 32 appears. As will be described later, a determination of the processing state of the wafer 1, such as whether the crack 9 is properly extended from the modification layer 7 formed in the interior of the wafer 1 to the front surface la, is made based on the image formed by the imaging unit 34 imaging the reflected light 32. In addition, this image is also used when detecting the position at which the modification layer 7 is formed, and the like, to determine whether the modification layer 7 is properly formed at a predetermined position.

[0064] Next, the processing method of the wafer according to the present embodiment will be described. The processing method of the wafer is performed, for example, in the laser processing apparatus 2. In the processing method of the wafer, the modification layer 7 is formed in the interior of the wafer 1 along the plurality of division predetermined lines 3 provided on the front surface la of the wafer 1. Figure 12 A flowchart showing a flow of each step of the processing method of the wafer will be described. Hereinafter, each step will be described in detail.

[0065] First, a holding step S10 of carrying the wafer 1 into the laser processing apparatus 2, opposing the front surface la of the wafer 1 to the chuck table 4, and holding the wafer 1 by the chuck table 4 is performed.

[0066] In the holding step S10, the wafer 1 is placed on the chuck table 4 with the front surface la side of the wafer 1 opposed to the holding surface 4a of the chuck table 4 in such a manner that the back surface lb side of the wafer 1 is exposed upward. Then, when the suction source of the chuck table 4 is activated to apply a negative pressure to the wafer 1, the wafer 1 is held by suction by the chuck table 4. Figure 2 A cross-sectional view of the wafer 1 held by suction by the chuck table 4 is schematically shown.

[0067] In addition, before the holding step S10 is performed, a protection member provisioning step of previously attaching a protection member such as an adhesive tape to the front surface la of the wafer 1 can be performed. In this case, in the holding step S10, the wafer 1 is held to the chuck table 4 with the protection member interposed therebetween.

[0068] Next, a modification layer forming step S20 of irradiating the first laser beam 14 from the back surface lb side of the wafer 1 along the division predetermined line 3 to form the modification layer 7 in the interior of the wafer 1 is performed. The first laser beam 14 is a laser beam having a wavelength that is transmissive to the wafer 1 (a wavelength capable of transmitting the wafer 1). Figure 2 A cross-sectional view of the modification layer forming step S20 is schematically shown.

[0069] In the modification layer forming step S20, first, the chuck table 4 and the laser beam irradiation unit 6 are relatively moved, and one end of one of the dicing pre-planned lines 3 of the wafer 1 is positioned below the laser beam irradiation unit 6. At the same time, the chuck table 4 is rotated to align the dicing pre-planned line 3 of the wafer 1 with the machining feed direction. Then, the focal point 16 of the first laser beam 14 is positioned at a prescribed height position inside the wafer 1.

[0070] Then, while the chuck table 4 and the laser beam irradiation unit 6 are relatively moved in the machining feed direction, the first laser beam 14 is irradiated to the wafer 1. When the first laser beam 14 is irradiated to the wafer 1 under conditions suitable for machining of the wafer 1, a modification layer 7 along the dicing pre-planned line 3 is formed inside the wafer 1, and a crack 9 extending from the modification layer 7 to the front face la of the wafer 1 is formed (see (B) of FIG. 8, etc.). Figure 3

[0071] After the modification layer 7 is formed along one of the dicing pre-planned lines 3 of the wafer 1, the chuck table 4 and the laser beam irradiation unit 6 are moved in the indexing feed direction, and the modification layer 7 is similarly formed inside the wafer 1 along the other dicing pre-planned line 3.

[0072] After the modification layer 7 is formed along all of the dicing pre-planned lines 3 in one direction, the chuck table 4 is rotated, and the modification layer 7 is similarly formed along the dicing pre-planned lines 3 in the other direction. When the first laser beam 14 is irradiated along all of the dicing pre-planned lines 3 of the wafer 1, the modification layer forming step S20 is completed. In addition, in each of the dicing pre-planned lines 3, the first laser beam 14 can be irradiated two or more times with the height of the focal point 16 changed, and a plurality of modification layers 7 overlapping each other can be formed.

[0073] When the wafer 1 having the modification layer 7 formed inside along the dicing pre-planned line 3 and the crack 9 extending from the modification layer 7 is ground from the back face lb side to thin the wafer 1 and remove the modification layer 7, etc., the wafer 1 is divided to obtain individual device chips.

[0074] However, if the crack 9 does not properly extend to the front face la of the wafer 1, the wafer 1 cannot be properly divided. In addition, in a case where the formation position of the modification layer 7 is not at a prescribed position or in a case where the modification layer 7 is not linear but meanders along the dicing pre-planned line 3, the wafer 1 cannot be properly divided. In these cases, sometimes the quality of the formed device chips does not satisfy a criterion. In addition, sometimes damage occurs on the device chips. That is, the yield of the device chips decreases.

[0075] Figure 3 (A) of FIG. 8 is a schematic cross-sectional view showing the wafer 1 having the modification layer 7 formed inside and the crack 9 not formed. In addition, Figure 3 ​(B) is a schematic cross-sectional view that is enlarged to show the wafer 1 in which the modification layer 7 is formed inside and a crack 9 reaches the front surface la from the modification layer 7. As shown in (B) of FIG. 6, the crack 9 reaches the front surface la. As shown in (B) of FIG. 6, when the crack 9 reaches the front surface la, the crack 9 can be visually confirmed when the front surface la of the wafer 1 is observed with a microscope. On the other hand, when the crack 9 is not formed, the crack 9 cannot be visually confirmed on the front surface la. Figure 3

[0076] Therefore, after the modification layer 7 is formed in the wafer 1, in order to confirm the presence or absence of the crack 9, it is conceivable to observe the front surface la side of the wafer 1 with a microscope. However, in order to observe the front surface la with a microscope, the wafer 1 must be carried out from the chuck table 4 and moved to the microscope. Therefore, in the wafer processing method of the present embodiment, in order to determine the processing state of the wafer 1, the observation laser beam irradiation step S30, the photographing step S40, and the determination step S50 are implemented.

[0077] Next, the observation laser beam irradiation step S30 implemented after the modification layer formation step S20 will be described. In the observation laser beam irradiation step S30, the second laser beam 28 is irradiated as an observation laser beam from the observation laser beam irradiation unit 18 to the wafer 1 held by the chuck table 4. The second laser beam 28 is a laser beam of an output that does not exceed the processing threshold of the wafer 1 and is a laser beam of a wavelength that is transmissive to the wafer 1 (a wavelength that can transmit the wafer 1).

[0078] Figure 4 (B) is a schematic cross-sectional view that is enlarged to show the wafer 1 in which the modification layer 7 is formed inside and a crack 9 reaches the front surface la from the modification layer 7. As shown in (B) of FIG. 6, the crack 9 reaches the front surface la. As shown in (B) of FIG. 6, when the crack 9 reaches the front surface la, the crack 9 can be visually confirmed when the front surface la of the wafer 1 is observed with a microscope. On the other hand, when the crack 9 is not formed, the crack 9 cannot be visually confirmed on the front surface la.

[0079] The second laser beam 28 emitted from the laser oscillator 20 reaches the beam shaping unit 26 and is shaped into a prescribed shape by the beam shaping unit 26. Then, the second laser beam 28 is reflected by the dichroic mirror 22 and advances toward the chuck table 4. Then, the second laser beam 28, after transmitting through the condenser lens 24, is irradiated to the back surface lb of the wafer 1 and advances inside the wafer 1, thereby condensing at the condensing point 30.

[0080] The second laser beam 28 advancing inside the wafer 1 is reflected by the front surface la of the wafer 1. Then, the reflected light 32 of the second laser beam 28 advances inside the wafer 1 and advances to the outside through the back surface lb of the wafer 1. Then, the reflected light 32 transmits through the condenser lens 24 and the dichroic mirror 22 and reaches the photographing unit 34.

[0081] Figure 6 ​(A) is a top view schematically showing an example of the cross-sectional shape of the second laser beam 28 irradiating the wafer 1. Specifically, Figure 6 (A) shows a region 40 on the back surface 1b of wafer 1 that was irradiated by the second laser beam 28, with shaded lines marked in region 40. Furthermore, for ease of illustration, in Figure 6 (A) shows a dashed line schematically indicating the planar position of the modified layer 7 formed inside the wafer 1 along the predetermined dividing line 3 and a point schematically indicating the planar position of the focusing point 30.

[0082] like Figure 6 As shown in (A), the cross-sectional shape of the second laser beam 28 is, for example, a semi-circular shape. Figure 6 As shown in (A), the second laser beam 28 is pre-shaped by the beam shaping unit 26 into a cross-sectional shape in a surface (e.g., the back surface 1b of wafer 1) perpendicular to the direction of travel, which is asymmetrical about an axis conceived along the predetermined dividing line 3.

[0083] Here, the path of the reflected light 32 of the second laser beam 28 reflected from the focusing point 30 located on the front side 1a of the wafer 1 is described in detail. Figure 5 (A) is a schematic cross-sectional view showing the travel paths of the second laser beam 28 and the reflected light 32 without the formation of a crack 9 extending from the modified layer 7 to the front side 1a of the wafer 1. Figure 5 (B) is a schematic cross-sectional view showing the paths of the second laser beam 28 and the reflected light 32 in the case where a crack 9 extending from the modified layer 7 is formed.

[0084] in addition, Figure 5 The sectional view shown in (A) and Figure 5 The cross-sectional view shown in (B) is a diagram illustrating the effect of the presence or absence of crack 9 on reflected light 32. Figure 5 The sectional view shown in (A) and Figure 5 In the cross-sectional view shown in (B), for ease of explanation, the relative positional relationships of the wafer 1, the modified layer 7, the pre-defined dividing line 3, and the crack 9, as well as the characteristics such as the travel angle of the second laser beam 28 and the reflected light 32, are emphasized.

[0085] like Figure 5 (A) and Figure 5 As shown in (B), the second laser beam 28, which is irradiated onto the back side 1b of the wafer 1, is focused at the focusing point 30. Then, the second laser beam 28 is reflected by the front side 1a of the wafer 1, and the reflected light 32 travels inside the wafer 1 and reaches the back side 1b of the wafer 1.

[0086] In a case where the focal point 30 is positioned below the modified layer 7 of the front surface la, if a crack 9 reaching the front surface la of the wafer 1 from the modified layer 7 is not formed inside the wafer 1, the second laser beam 28 travels through the region below the modified layer 7. As shown in (A) of Fig. 8, the second laser beam 28 (incident light) and the reflected light 32 become a state of being reversed across the modified layer 7. Figure 5

[0087] On the other hand, in a case where the crack 9 reaching the front surface la of the wafer 1 from the modified layer 7 is formed inside the wafer 1, the second laser beam 28 reaches the crack 9 below the modified layer 7, and is affected by the crack 9.

[0088] In a case where the crack 9 reaches the front surface la of the wafer 1, since the wafer 1 is slightly broken by the crack 9, an interface is formed between the air layer entering the crack 9 and the wafer 1. Since the light is reflected at this interface where the difference in refractive index on both sides is large, the second laser beam 28 reaching the crack 9 is reflected by the crack 9, as in the case where the second laser beam 28 is reflected by the front surface la.

[0089] In this case, as shown in (B) of Fig. 8, the reflected light 32 reaches the front surface la of the wafer 1 in the same region inside the wafer 1 as the region through which the second laser beam 28 (incident light) transmits. Figure 5

[0090] Further, in the observation laser beam irradiation step S30, the chuck table 4 and the observation laser beam irradiation unit 18 are relatively moved along the division predetermined line 3. For example, the chuck table 4 is moved in the machining feed direction (X-axis direction). That is, the wafer 1 and the focal point 30 are relatively moved along the division predetermined line 3, and the second laser beam 28 is sequentially irradiated to the back surface lb of the wafer 1.

[0091] In a case where the modified layer 7 formed by the modified layer formation step S20 is not meandering but formed in a straight line along the division predetermined line 3, the second laser beam 28 is similarly reflected by the front surface la each time the second laser beam 28 is irradiated to the back surface lb of the wafer 1. As a result, the reflected light 32 travels inside the wafer 1 in the same path.

[0092] On the other hand, in a case where the wafer 1 is not machined under appropriate conditions and the modified layer 7 is meandering, when the second laser beam 28 is irradiated to the meandering portion of the modified layer 7, the advancing route of the reflected light 32 changes. Therefore, the machining state of the wafer 1 can be evaluated by repeatedly observing the reflected light 32.

[0093] ​​Alternatively, in the observation laser beam irradiation step S30, for example, the observation laser beam irradiation unit 18 is moved in the indexing feed direction (Y-axis direction). That is, while moving the wafer 1 and the focal point 30 in a direction perpendicular to the direction along the division predetermined line 3 in such a manner that the focal point 30 is across the modified layer 7 (in a through manner), the second laser beam 28 is sequentially irradiated to the back surface 1b of the wafer 1.

[0094] For example, in the modified layer formation step S20, the processing conditions are inappropriate and the modified layer 7 is formed at a position farther from the front surface 1a than intended, as a result of which the crack 9 extending from the modified layer 7 is not formed at times. In this case, in order to obtain information related to the formation depth of the modified layer 7, the focal point 30 is moved in such a manner that the focal point 30 is across the modified layer 7, and the second laser beam 28 is repeatedly irradiated to the wafer 1.

[0095] At this time, the second laser beam 28 or the reflected light 32 is irradiated to the modified layer 7. Also, when the focal point 30 is moved, the irradiation manner of the second laser beam 28 or the like to the modified layer 7 changes, and thus the advancing route of the reflected light 32 changes. The change depends on the formation height of the modified layer 7, and thus the processing state of the wafer 1 can be evaluated by repeatedly observing the reflected light 32.

[0096] In the wafer processing method of the present embodiment, next, a photographing step S40 is performed in which the reflected light 32 of the second laser beam 28 irradiated to the wafer 1 in the observation laser beam irradiation step S30 is photographed by the photographing unit 34. In the photographing step S40, the reflected light 32 is photographed to form an image 38 in which the reflected light 32 is visualized.

[0097] Figure 6 (C) is a plan view schematically showing a region 42b visualized by the reflected light 32 in the image 38 formed by the photographing unit 34 in the case where the crack 9 from the modified layer 7 to the front surface 1a is not formed. In the case where the crack 9 is not formed, as shown in (A) of FIG. 14, the second laser beam 28 (incident light) and the reflected light 32 become a state of being reversed across the modified layer 7. Figure 5

[0098] Therefore, the shape of the reflected light 32 of the second laser beam 28 visualized in the image 38 becomes a shape in which the cross-sectional shape of the second laser beam 28 is reversed. In the case where the cross-sectional shape of the second laser beam 28 is a semicircular shape, as shown in (C) of FIG. 14, the region 42b visualized by the reflected light 32 becomes a shape in which the semicircular shape is reversed. Figure 6

[0099] In addition, in the case where the crack 9 is formed, as shown in (B) of FIG. 14, the second laser beam 28 (incident light) and the reflected light 32 become a state of being reversed across the crack 9. Figure 6 ​​(B) is a plan view schematically showing a region 42a in which the reflected light 32 appears in the image 36 formed by the photographing by the photographing unit 34, in a case where the crack 9 extends from the modified layer 7 toward the front surface la. In the case where the crack 9 extends from the modified layer 7 toward the front surface la, as shown in (B) of FIG. 6, the second laser beam 28 (incident light) overlaps with the path of the reflected light 32. Therefore, in a case where the cross-sectional shape of the second laser beam 28 is a semicircular shape, as shown in (B) of FIG. 6, the region 42a in which the reflected light 32 appears becomes the same shape as the semicircular shape. Figure 5 Figure 6

[0100] Thus, the shape and the like of the reflected light 32 appearing in the images 36, 38 formed by the photographing of the reflected light 32 in the photographing step S40 change depending on the presence or absence of the crack 9. Therefore, it is possible to determine from the images 36, 38 whether or not the crack 9 from the modified layer 7 to the front surface la is formed in the wafer 1.

[0101] Figure 7 (A) and Figure 7 (B) are photographs showing an example of an image photographed by the photographing unit 34 in a case where the crack 9 is formed in the wafer 1. In addition, Figure 7 (C) and Figure 7 (D) are photographs showing an example of an image photographed by the photographing unit 34 in a case where the crack 9 is not formed in the wafer 1.

[0102] In each photograph, the reflected light 32 after the second laser beam 28 is reflected by the front surface la of the wafer 1 is shown in white. Also, since the shape and the position in which the reflected light 32 appears in the image change depending on whether or not the crack 9 is formed in the wafer 1, it is understood that the shape and the position in which the reflected light 32 appears in each photograph can become a determination reference of the processing state of the wafer 1 represented by the presence or absence of the crack 9 and the like.

[0103] In addition, from each photograph, it is known that, in the region in which the reflected light 32 appears, the reflected light 32 does not necessarily appear with a uniform intensity. That is, it is not limited to that the reflected light 32 is uniformly distributed in the entire region of the region 42a shown in (B) of FIG. 6 or in the entire region of the region 42b shown in (C) of FIG. 6. The reflected light 32 appears in a striped or dotted shape in the image due to various reasons such as optical phenomena, but even in a case where the reflected light 32 does not uniformly appear in the image, it is possible to sufficiently determine the processing state of the wafer 1. Figure 6 Figure 6

[0104] ​​​​In the wafer processing method of this embodiment, the following determination step S50 is performed: the processing state of wafer 1 is determined based on the image captured by the imaging step S40. Here, the processing state refers to, for example, the state of wafer 1 processed by irradiating the first laser beam 14, including the processing result. For example, the processing result refers to the presence or absence of cracks 9 from the modified layer 7 to the front side 1a, the height position of the modified layer 7, and the presence or absence of meandering in the modified layer 7, etc.

[0105] The details of the determination performed in determination step S50 will be explained. In determination step S50, the processing state of wafer 1 is determined based on the position and shape of the reflected light 32 displayed in the image captured by the imaging step S40. First, determination step S50 will be explained when the second laser beam 28 is irradiated onto wafer 1 while the focusing point 30 is moved relative to the wafer 1 in the direction (X-axis direction) along the predetermined dividing line during the observation laser beam irradiation step S30.

[0106] Figure 8 (A) is a schematic top view showing the area of ​​the back surface 1b of wafer 1 irradiated by the second laser beam 28 during the observation of the laser beam irradiation step S30. Figure 8 As shown in (A), a modified layer 7 is formed inside the wafer 1 along the predetermined dividing line 3. Figure 8 In (A), the formation location of the modified layer 7 is indicated by a dashed line. Furthermore, in Figure 8 In the wafer 1 shown in (A), a crack is formed from the modified layer 7 to the front side 1a, but a meandering is produced in the modified layer 7.

[0107] exist Figure 8 In the example shown in (A), the focusing point 30 is positioned on the front side 1a at a location overlapping with the modified layer 7. While moving the focusing point 30 relative to the pre-defined dividing line 3, a second laser beam 28 is irradiated five times onto the back side 1b of the wafer 1. Figure 8 In (A), shaded lines are marked in the irradiated areas 44a, 44b, 44c, 44d, and 44e of the second laser beam 28, indicating the positions in the back side 1b. Figure 9 (A) to Figure 9 (E) is a top view schematically showing the image formed by capturing reflected light 32 in the shooting step S40.

[0108] For example, Figure 9 (A) is schematically shown in the direction of Figure 8 A top view of the image of reflected light 32 captured by the imaging unit 34 when the illuminated area 44a in (A) is illuminated by the second laser beam 28. Similarly, Figure 9 (B) to Figure 9Figures (E) are schematic top views showing the images of reflected light 32 captured by the imaging unit 34 when the second laser beam 28 is irradiated into the irradiated areas 44b, 44c, 44d, and 44e.

[0109] When the second laser beam 28 irradiates the irradiated area 44a, the travel paths of the second laser beam 28 and the reflected light 32 traveling inside the wafer 1 are... Figure 5 The travel path shown in (B) is the same. That is, the second laser beam 28 is reflected by the front side 1a and crack 9 of the wafer 1, and the reflected light 32 travels in the opposite direction in the travel path of the second laser beam 28 from the back side 1b to the outside of the wafer 1. Therefore, in Figure 9 In image 46a shown in (A), reflected light 32 is displayed in region 48a, which has the same shape as the cross-sectional shape of the second laser beam 28.

[0110] Figure 8 (B) shows the travel paths of the second laser beam 28 traveling inside the wafer 1 and the reflected light 32a, 32b when the second laser beam 28 irradiates the irradiated area 44b. In this case, a portion of the second laser beam 28 traveling inside the wafer 1 reaches the focusing point 30 and is reflected by the front surface 1a of the wafer 1. The reflected light 32a travels outward from the back surface 1b of the wafer 1 and reaches the imaging unit 34. Then, in Figure 9 In the top view shown in (B), the reflected light 32a is shown in region 48b, which is schematically displayed.

[0111] On the other hand, such as Figure 8 As shown in (B), another portion of the second laser beam 28 traveling inside the wafer 1 reaches the modified layer 7 and the crack 9 and is reflected, and is further reflected by the front side 1a at a position that is not the focal point 30. Then, the reflected light 32b travels outward from the back side 1b of the wafer 1 and reaches the imaging unit 34. Then, in Figure 9 In the top view shown in (B), the reflected light 32b is shown in region 50b in the schematically illustrated image 46b.

[0112] Similarly, in Figure 9 In image 46c, which is schematically shown in (C), reflected light unaffected by the modified layer 7 is displayed in region 48c, while reflected light affected by the modified layer 7 is displayed in region 50c. Figure 9 In image 46d, schematically shown in (D), reflected light unaffected by the modified layer 7 is displayed in region 48d. Figure 10In the image 46e shown schematically in (E), a part of the reflected light of the second laser beam 28 reflected at the condensing point 30 is reflected by the modification layer 7 to appear in the region 50e, and the remaining reflected light does not reach the modification layer 7 to appear in the region 48e.

[0113] In addition, in the image obtained by photographing the reflected light 32 in the photographing step S40, a change appearing in the region where the reflected light 32 appears is not limited to this depending on the processing state of the wafer 1. That is, depending on the thickness of the wafer 1 and the formation depth of the modification layer 7, the position of each structural element of the observation laser beam irradiation unit 18, and the like, a change appears in various ways in the region where the reflected light appears in the image.

[0114] However, in any case, the position and shape of the region where the reflected light appears in the image are determined by the processing state of the wafer 1. Therefore, the processing state of the wafer 1 can be determined from the image obtained by the photographing step S40.

[0115] For example, according to the images 46a, 46d, it is suggested that the modification layer 7 is formed at a predetermined position in the irradiated region 44a, 44d irradiated with the second laser beam 28. On the other hand, a change is found in the reflected light appearing in the images 46b, 46c, 46e, and it is suggested that the modification layer 7 is formed away from the predetermined position in the irradiated regions 44b, 44c, 44e. Therefore, in the determination step S50, as the processing state of the wafer 1, it can be determined that a local meander is generated in the modification layer 7.

[0116] Next, the determination step S50 in the case where the second laser beam 28 is irradiated to the wafer 1 while moving the wafer 1 and the condensing point 30 relatively in a direction (Y-axis direction) perpendicular to the direction along the division predetermined line so that the condensing point 30 traverses the modification layer 7 will be described.

[0117] Figure 10 (A) of FIG. 46 is a plan view schematically showing a region of the back surface 1b of the wafer 1 irradiated with the second laser beam 28 in the observation laser beam irradiation step S30. In Figure 10 The modification layer 7 is formed inside the wafer 1 along the division predetermined line 3 as shown in (A) of FIG. 46, and Figure 10 In (A) of FIG. 46, the formation position of the modification layer 7 is indicated by a broken line. However, in Figure 10 In the wafer 1 shown in (A) of FIG. 46, a crack is not formed from the modification layer 7 to the front surface 1a.

[0118] In Figure 10In the example shown in (A), the focus point 30 is positioned at a position offset from the modified layer 7 along the Y-axis on the front side 1a, and the focus point 30 is moved relative to the modified layer 7 along the Y-axis while the second laser beam 28 is irradiated five times onto the back side 1b of the wafer 1.

[0119] exist Figure 11 In (A), shaded lines are marked in the irradiated areas 52a, 52b, 52c, 52d, and 52e of the second laser beam 28, indicating the positions in the back side 1b. Figure 11 (A) to Figure 11 (E) is a top view schematically showing the image formed by capturing reflected light 32 in the shooting step S40.

[0120] For example, Figure 10 (A) is schematically shown in the direction of Figure 11 A top view of the image displayed by the reflected light 32 generated by the imaging unit 34 when the illuminated area 52a in (A) is illuminated by the second laser beam 28. Similarly, Figure 11 (B) to Figure 11 Figures (E) are schematic top views showing the images projected by the reflected light 32 formed by the imaging unit 34 when the second laser beam 28 is irradiated onto the irradiated areas 52b, 52c, 52d, and 52e.

[0121] Because the irradiated area 52a is significantly farther from the modified layer 7, the second laser beam 28 traveling inside the wafer 1 and its reflected light 32 do not irradiate the modified layer 7. That is, the second laser beam 28 is reflected by the front surface 1a of the wafer 1, and the reflected light 32 travels outwards from the wafer 1. Therefore, in Figure 10 In image 54a shown in (A), the reflected light 32 appears in region 56a, which is the shape of the cross-sectional shape of the second laser beam 28 after it has been inverted.

[0122] Figure 11 (B) schematically illustrates the travel paths of the second laser beam 28 traveling inside the wafer 1 and the reflected light 32c, 32d when the second laser beam 28 irradiates the irradiated area 52b. In this case, the second laser beam 28 traveling inside the wafer 1 reaches the focusing point 30 and is reflected by the front surface 1a. Then, a portion of the reflected light 32c travels outward from the back surface 1b of the wafer 1 and reaches the imaging unit 34. Then, in Figure 10 In the top view shown in (B), the reflected light 32c is shown in region 56b, which is schematically displayed.

[0123] On the other hand, such as Figure 11(B) and reaches the modified layer 7 and is reflected, traveling outside the wafer 1 from the back surface lb, to the imaging unit 34. Then, in the image 54b schematically shown in the plan view of (B), the reflected light 32d appears in the region 58b. Figure 5

[0124] In a case where the second laser beam 28 is irradiated to the irradiated region 52c, the second laser beam 28 and the reflected light 32 travel in the same travel paths as those shown in (A). Therefore, in the image 54c schematically shown in (C), the reflected light 32 appears in the region 56c in a shape reversed from the cross-sectional shape of the second laser beam 28. Figure 11 Figure 11

[0125] In a case where the second laser beam 28 is irradiated to the irradiated region 52d, a part of the second laser beam 28 reaches the modified layer 7 and is reflected, being reflected by the front surface la at a position other than the condensing point 30. In a case where the second laser beam 28 is irradiated to the irradiated region 52e, the second laser beam 28 traveling inside the wafer 1 and its reflected light 32 do not irradiate the modified layer 7. That is, the second laser beam 28 is reflected by the front surface la of the wafer 1, and the reflected light 32 travels outside the wafer 1. Therefore, in the image 54e shown in (E), the reflected light 32 appears in the region 56e in a shape reversed from the cross-sectional shape of the second laser beam 28. Figure 11

[0126] Figure 11

[0127] Thus, it is possible to determine whether the second laser beam 28 or the reflected light 32 irradiates the modified layer 7, from each of the images shown in (A) to (E). Then, during movement of the condensing point 30 in a manner so as to cross the modified layer 7, the presence or absence of reflection of the second laser beam 28 or the like on the modified layer 7 is switched. Figure 11 Figure 5

[0128] ​​​​​​​​Accordingly, the formation height of the modified layer 7 can be derived from the position of the condensing point 30 and the like during detection of the reflection of the second laser beam 28 and the like on the modified layer 7, the shape of the second laser beam 28, and the configuration of the optical system possessed by the observation laser beam irradiation unit 18 and the like. Accordingly, in the determination step S50, the formation height of the modified layer 7 can be determined as the processing state of the wafer 1. Further, the quality of the modified layer 7 can be determined from the distribution of the reflected light 32 in the image in which the reflected light 32 appears, the state of the unevenness of the reflection surface of the modified layer 7 which becomes the reflection surface, and the like.

[0129] Further, the position of the modified layer 7 in the Y-axis direction can also be precisely determined from each image. When the position of the modified layer 7 can be precisely determined, the condensing point 30 can be positioned so as to overlap the modified layer 7 to irradiate the second laser beam 28. For example, when the condensing point 30 is precisely positioned at a position of the front surface la of the wafer 1 which overlaps the modified layer 7 and the second laser beam 28 is irradiated to form the image in which the reflected light 32 appears, it is possible to highly accurately determine the presence or absence of a crack 9 which extends from the modified layer 7 to the front surface la (refer to (A) of FIG. 10 and (B) of FIG. 11). Figure 5 Figure 4

[0130] As described above, in the determination step S50, the processing state of the wafer 1 is determined from the image in which the reflected light 32 of the second laser beam 28 appears.

[0131] Here, the observation laser beam irradiation step S30, the imaging step S40, and the determination step S50 can also be repeatedly performed. For example, first, the observation laser beam irradiation step S30 is performed while the condensing point 30 is moved in the Y-axis direction so as to cross the modified layer 7. Then, in the determination step S50, the position of the modified layer 7 in the Y-axis direction can be determined.

[0132] Then, the observation laser beam irradiation step S30 is performed while the condensing point 30 is moved in the X-axis direction with the condensing point 30 positioned so as to overlap the modified layer 7. Then, in the determination step S50, it is possible to determine whether or not a meander is generated in the modified layer 7.

[0133] Further, the shape and position of the reflected light 32 which appears in the image obtained by the imaging step S40 are not limited thereto. For example, depending on the condensing position of the second laser beam 28 and the arrangement position of the imaging unit 34, it can be considered that the reflected light 32 which appears in the image does not become a shape after the cross-sectional shape of the incident light is reversed in the case where the crack 9 is not formed in the wafer 1.

[0134] ​​That is, the influence of the processing state of the wafer 1 on the position and shape of the reflected light 32 appearing in the image differs for each system. Therefore, in a case where it is intended to determine the processing state of the wafer 1, represented by the presence or absence of the crack 9 extending from the modification layer 7 toward the front surface la, based on the image taken in the photographing step S40, it is preferable to verify the influence in advance.

[0135] For example, a wafer 1 in which the crack 9 is formed in advance and a wafer 1 in which the crack 9 is not formed are prepared, the second laser beam 28 is irradiated to each wafer 1, and the reflected light 32 is similarly photographed to obtain an image. In addition, a wafer 1 in which the position and the depth of formation of the modification layer 7 are known is prepared, the condensing point 30 is moved variously with respect to the modification layer 7, the second laser beam 28 is irradiated to the wafer 1, and the reflected light 32 is similarly photographed to obtain an image.

[0136] Then, the influence of the presence or absence of the crack 9 and the depth and position of formation of the modification layer 7 on the image and the like is evaluated from various angles. That is, it is preferable to evaluate the influence of the processing state of the wafer 1 on the image and generate a reference for determining the processing state of the wafer 1 based on the image.

[0137] In addition, when the wafer 1 is formed with the modification layer 7, even in a case where the modification layer 7 is formed without meandering, a straight-line-shaped modification layer 7 is sometimes formed at a position separated from the center line of the division intended line 3. The distance between the position of formation of this modification layer 7 and the center line of the division intended line 3 is referred to as a kerf offset. In the wafer processing method of the present embodiment, the amount of the kerf offset can also be evaluated by determining the position of formation of the modification layer 7.

[0138] In addition, when the wafer 1 is formed with the modification layer 7, there is a case where the crack 9 not reaching the front surface la is elongated from the modification layer 7. Also, there is a case where a wafer 1 in which the length of the crack 9 elongated from the modification layer 7 is known is prepared, and an image appearing the reflected light 32 is similarly formed, so that the relationship between the length of the crack 9 and the position and shape of the region appearing the reflected light 32 in the image can be obtained. In this case, in the determination step S50, the length of the crack 9 can be calculated based on the image appearing the reflected light 32.

[0139] Further, in the wafer processing method of the present embodiment, the second laser beam 28 is irradiated to the back surface lb side of the wafer 1 while moving the condensing point 30, and a plurality of images appearing the reflected light 32 are obtained, so that information related to the position at which a processing abnormality occurs in the wafer 1 is obtained. Therefore, in the wafer processing method of the present embodiment, the information related to the position at which the processing abnormality occurs can also be stored in the control unit or the like of the laser processing apparatus 2.

[0140] In this case, for example, based on the information about the position where the processing abnormality occurs, for each device chip obtained by dividing the wafer 1, the device chip obtained from the position where the processing abnormality occurs can be identified as a defective product. In addition, the operator of the laser processing apparatus 2 can perform adjustment and repair of each structural element of the laser processing apparatus 2 based on the information about the position where the processing abnormality occurs in the wafer 1.

[0141] As explained above, in the processing method of the wafer of the present embodiment, it is possible to easily determine the presence or absence of the crack 9 in situ without moving the wafer 1 from the chuck table 4 of the laser processing apparatus 2. That is, it is possible to easily confirm the processing state of the wafer 1.

[0142] In addition, the present application is not limited to the description of the above-described embodiment, and various modifications can be made to implement it. For example, in the above-described embodiment, mainly the case where the first laser beam 14 and the second laser beam 28 are irradiated from the back surface 1b side toward the wafer 1 is explained, but one mode of the present application is not limited to this. For example, the first laser beam 14 and the second laser beam 28 can be irradiated toward the front surface 1a side of the wafer 1. In addition, the wafer 1 on which the device 5 is not formed can be subjected to laser processing to form the modified layer 7 inside the wafer 1.

[0143] In addition, in the above-described embodiment, the case where the cross-sectional shape of the second laser beam 28 is a semicircular shape is explained as an example, but the cross-sectional shape is not limited to this. For example, the cross-sectional shape can be a triangular shape, a quadrangular shape, or other polygonal shape. That is, as long as the distribution of the power is asymmetric across the axis along the division intended line 3 (for example, the modified layer 7).

[0144] In addition, the second laser beam 28 irradiated to the wafer 1 cannot be precisely condensed to the condensing point 30 due to the influence of spherical aberration, and as a result, sometimes the reflected light 32 cannot be clearly shown in the image obtained in the photographing step S40. Therefore, a correction ring that mitigates the influence of spherical aberration can be installed on the condensing lens 24. Also in this case, for example, a correction ring with appropriate performance corresponding to the thickness and material of the wafer 1 is selected for use.

[0145] Alternatively, in the case where a spatial light modulator such as an LCOS element is used in the observation laser beam irradiation unit 18, the second laser beam 28 corrected for spherical aberration can be formed and irradiated to the back surface 1b of the wafer 1.

[0146] Furthermore, the observation laser beam irradiation step S30 can be implemented by liquid immersion. When this case is explained with reference to the observation laser beam irradiation step S30 shown in FIG. 4, the space between the condensing lens 24 and the back surface 1b of the wafer 1 is filled with a liquid. The liquid can be, for example, a liquid called immersion oil, glycerin, or pure water. ​ When this case is explained with reference to the observation laser beam irradiation step S30 shown in FIG. 4, the space between the condensing lens 24 and the back surface 1b of the wafer 1 is filled with a liquid. The liquid can be, for example, a liquid called immersion oil, glycerin, or pure water.

[0147] In a case where the laser beam irradiation step S30 for observation is performed by liquid immersion, the numerical aperture of the condenser lens 24 functioning as an objective lens can be increased. Therefore, the resolution of the image appearing from the reflected light 32 captured by the capturing unit 34 can be improved, and thus the processing state of the wafer 1 can be analyzed in more detail.

[0148] The configuration, method, and the like of the above-described embodiments can be appropriately changed and implemented within a range not departing from the object of the present application.

Claims

1. A wafer processing method of forming a modification layer in the inside of a wafer along a plurality of division lines provided on the front surface of the wafer, characterized by comprising: a holding step of opposing the front surface of the wafer to a chuck table and holding the wafer by the chuck table; a modification layer forming step of forming the modification layer in the inside of the wafer by irradiating a first laser beam having a wavelength that is transmissive to the wafer along the division lines from the back surface side of the wafer while positioning a focal point of the first laser beam in the inside of the wafer and relatively moving a laser beam irradiation unit and the chuck table in a direction along the division lines; an observation laser beam irradiation step of, after the modification layer forming step, positioning a focal point of a second laser beam for observation having a wavelength that is transmissive to the wafer and outputting not more than a processing threshold value of the wafer in the inside of the wafer or the front surface, relatively moving the wafer and the focal point of the second laser beam for observation in a direction along the division lines while irradiating the second laser beam from the back surface side of the wafer; a photographing step of photographing reflected light of the second laser beam irradiated in the observation laser beam irradiation step by a photographing unit; and a determination step of determining a processing state of the wafer based on an image photographed in the photographing step, wherein a shape of the second laser beam irradiated on the wafer in the observation laser beam irradiation step satisfies that a cross-sectional shape in a plane perpendicular to a traveling direction of the second laser beam is not linearly symmetrical with respect to an axis along the division lines.

2. The wafer processing method according to claim 1, characterized in that the observation laser beam irradiation step is performed by liquid immersion.

3. A wafer processing method of forming a modification layer in the inside of a wafer along a plurality of division lines provided on the front surface of the wafer, characterized by comprising: a holding step of opposing the front surface of the wafer to a chuck table and holding the wafer by the chuck table; a modification layer forming step of forming the modification layer in the inside of the wafer by irradiating a first laser beam having a wavelength that is transmissive to the wafer along the division lines from the back surface side of the wafer while positioning a focal point of the first laser beam in the inside of the wafer and relatively moving a laser beam irradiation unit and the chuck table in a direction along the division lines; an observation laser beam irradiation step of, after the modification layer forming step, positioning a focal point of a second laser beam for observation having a wavelength that is transmissive to the wafer and outputting not more than a processing threshold value of the wafer in the inside of the wafer or the front surface, relatively moving the wafer and the focal point of the second laser beam for observation in a direction perpendicular to a direction along the division lines so that the focal point straddles the modification layer while irradiating the second laser beam from the back surface side of the wafer; a photographing step of photographing reflected light of the second laser beam irradiated in the observation laser beam irradiation step by a photographing unit; and a determination step of determining a processing state of the wafer based on an image photographed in the photographing step. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ a determination step of determining a processing state of the wafer based on the image captured in the capturing step, the second laser beam irradiated on the wafer in the observation laser beam irradiation step is shaped so as to be non-linearly symmetrical with respect to an axis along the division predetermined line in a plane perpendicular to a traveling direction of the second laser beam.

4. The wafer processing method according to claim 3, wherein the observation laser beam irradiation step is performed by liquid immersion.

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