Method of processing a wafer

Abstract

The present application provides a wafer processing method that easily confirms the length of a crack formed in the interior of a wafer. A wafer processing method that forms a modified layer in the interior of a wafer along a division intended line has the following steps: a modified layer forming step that forms a modified layer in the interior of the wafer by irradiating a first laser beam from the back surface side of the wafer; a laser beam for observation irradiating step that positions a focal point of a second laser beam that outputs not more than a processing threshold value of the wafer in the interior or the front surface of the wafer and irradiates the second laser beam while moving the focal point in the thickness direction of the wafer; a photographing step that photographs the reflected light of the second laser beam using a photographing unit; and a determination step that determines the processing state of the wafer based on the image photographed in the photographing step, the second laser beam being shaped so that the cross-sectional shape in the plane perpendicular to the traveling direction of the second laser beam is asymmetric across the modified layer.

Description

TECHNICAL FIELD

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

[0002] In a manufacturing process of a device chip, a plurality of separation predetermined lines crossing each other are set on a front surface of a wafer, a device is formed in each region divided by the separation predetermined lines, and the wafer is separated along the separation 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 a back surface side of the wafer to the wafer, and the laser beam is condensed in the inside of the wafer along the separation predetermined lines. At this time, a modified layer as a separation starting point is formed in the vicinity of a 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 separated along the separation predetermined lines (for example, refer to Patent Literature 1, Patent Literature 2).

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

[0005] If the processing condition or the like is not appropriate, the wafer cannot be appropriately separated because the crack is elongated from the formed modified layer in an unintended direction or the like, and thus the yield of the device chip is reduced. The same is true in a case where the crack is not elongated from the modified layer with a sufficient length, and the crack does not reach the front surface of the wafer.

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

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

[0008] Here, in order to confirm whether the processing condition or the like is appropriate, that is, whether the crack appropriately travels from the modified layer formed in the wafer 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 out the wafer from a laser processing device, turn upside down the wafer, and carry in the wafer to the microscope or the like. Thus, there is a problem that confirmation of the formation condition of the crack takes time.

[0009] In addition, even if a crack extends from the modified layer toward the front surface, in a case where the length of the crack is not sufficient and the crack does not reach the front surface of the wafer, the crack cannot be visually confirmed even if the front surface of the wafer is observed with a microscope, and the length of the crack cannot be evaluated even if a microscope is used. In this case, if the wafer is not divided to observe the cross section, the length of the crack cannot be evaluated. SUMMARY

[0010] 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 confirming the processing state of a wafer even in a case where a crack extending from a modified layer formed in the inside does not reach the front surface of the wafer.

[0011] According to one embodiment of the present application, a wafer processing method is provided, in which a modified layer is formed in the inside of a wafer along a plurality of division predetermined lines provided on a front surface of the wafer, and the wafer processing method is 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 modified layer forming step of forming the modified layer in the inside of the wafer by positioning a focal point of a first laser beam having a wavelength that is transmissive to the wafer in the inside of the wafer and moving a laser beam irradiation unit and the chuck table relatively in a direction along the division predetermined line while irradiating the first laser beam from a back surface side of the wafer along the division predetermined line; 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 value of the wafer in the inside of the wafer or the front surface after the modified layer forming step, moving the focal point in a thickness direction of the wafer, 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, 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 asymmetrically across the modified layer.

[0012] It is preferable that, in the determination step, in the image captured by the capturing step, in a case where the reflected light appears in an area overlapping with an area in which the second laser beam irradiated on the wafer in the observation laser beam irradiation step is the same shape in the back surface, it is determined that a crack is formed in the height position of the focal point of the second laser beam, and in the image captured by the capturing step, in a case where the reflected light appears in an area overlapping with an area in which the shape of the second laser beam irradiated on the wafer in the observation laser beam irradiation step is reversed in the back surface, it is determined that the crack is not formed in the height position of the focal point of the second laser beam.

[0013] Further, it is preferable that the observation laser beam irradiation step be performed by liquid immersion.

[0014] In the wafer processing method of one embodiment of the present application, after the modification layer formation step of forming a modification layer by condensing a first laser beam inside a wafer is performed, an observation laser beam irradiation step, a capturing step, and a determination step are performed. A second laser beam that is irradiated to the back surface side of the wafer and travels inside the wafer is reflected by the front surface of the wafer in the observation laser beam irradiation step. Then, the reflected light of the second laser beam is captured in the capturing step.

[0015] Further, in the observation laser beam irradiation step, the second laser beam is irradiated to the wafer while moving the focal point of the second laser beam in the thickness direction of the wafer. Then, the second laser beam enters a region between the modification layer and the front surface. Here, when a crack extends from the modification layer, an interface is generated between the air layer entering the crack and the wafer. The interface is large in difference in refractive index on both sides. Thus, when the second laser beam reaches the crack, the second laser beam is reflected at the interface.

[0016] The shape of the reflected light in the image in which the reflected light appears greatly changes depending on whether the second laser beam is reflected by the crack. For example, when the second laser beam is repeatedly irradiated to the wafer while moving the focal point in the thickness direction of the wafer, the shape of the reflected light appearing in the image changes. Depending on the height of the focal point at which the change occurs, the processing state of the wafer such as the length of the crack can be determined. At this time, the wafer does not need to be moved from the chuck table of the laser processing apparatus, and the processing state of the wafer can be determined without spacing after the modification layer is formed.

[0017] Thus, according to one embodiment of the present application, a wafer processing method in which the processing state of a wafer can be confirmed even in a case where a crack extending from a modification layer formed inside does not reach the front surface of the wafer is provided. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a perspective view schematically showing a wafer.

[0019] Figure 2 is a cross-sectional view schematically showing the modification layer forming step.

[0020] Figure 3 (A) of FIG. 1 is a cross-sectional view schematically showing a wafer in which a modification layer is formed inside, enlarged, Figure 3 (B) of FIG. 1 is a cross-sectional view schematically showing a wafer in which a modification layer and a crack are formed inside, enlarged.

[0021] Figure 4 is a cross-sectional view schematically showing the observation laser beam irradiation step.

[0022] Figure 5 (A) of FIG. 2 is a cross-sectional view schematically showing a second laser beam and its reflected light which are irradiated onto a wafer in which a modification layer is formed inside, Figure 5 (B) of FIG. 2 is a cross-sectional view schematically showing a second laser beam and its reflected light which are irradiated onto a wafer in which a modification layer and a crack are formed inside.

[0023] Figure 6 (A) of FIG. 3 is a plan view schematically showing a region in the back surface of a wafer which is irradiated with a second laser beam, Figure 6 (B) of FIG. 3 is a plan view schematically showing an example of a region which is visualized by reflected light in an image visualized by the reflected light, Figure 6 (C) of FIG. 3 is another example of a region which is visualized by reflected light in an image visualized by the reflected light.

[0024] Figure 7 (A) of FIG. 4 and Figure 7 (B) of FIG. 4 is an image visualized by reflected light in a case where a wafer is formed with a crack, Figure 7 (C) of FIG. 4 and Figure 7 (D) of FIG. 4 is an image visualized by reflected light in a case where a wafer is not formed with a crack.

[0025] Figure 8 is a cross-sectional view schematically showing a wafer in which a crack which does not reach the length of the front surface of the wafer is elongated from a modification layer.

[0026] Figure 9 (A) of FIG. 5 is a cross-sectional view schematically showing a forward route of a second laser beam and its reflected light when the second laser beam is reflected by a crack, Figure 9 (B) of FIG. 5 is a cross-sectional view schematically showing a forward route of a second laser beam and its reflected light when the second laser beam is not reflected by a crack.

[0027] Figure 10 is a flowchart showing a flow of each step of a processing method of a wafer.

[0028] BRIEF DESCRIPTION OF DRAWINGS

[0029] 1: wafer; 1a: front surface; 1b: back surface; 3: division intended line; 5: device; 7: modified layer; 9: crack; 2: laser processing apparatus; 4: chuck table; 4a: holding surface; 6: laser beam irradiation unit; 8, 20: laser oscillator; 10: mirror; 22: dichroic mirror; 12, 24: condensing lens; 14: first laser beam; 16, 30: condensing point; 18: observation laser beam irradiation unit; 26: beam shaping unit; 28: second laser beam; 32: reflected light; 34: imaging unit; 36, 38: image; 40: region; 42a, 42b: region; 44a, 44b: height position. DETAILED DESCRIPTION

[0030] Embodiments of the present application are described with reference to the accompanying drawings. First, a wafer in which a modified layer is formed by a wafer processing method of the present embodiment is described. Figure 1 is a perspective view schematically showing a wafer 1.

[0031] The wafer 1 is, for example, a substantially circular plate-shaped substrate or the like made of a material such as Si (silicon), SiC (silicon carbide), GaN (gallium nitride), GaAs (gallium arsenide), or another semiconductor, or a material such as sapphire, glass, quartz, or the like. The glass is, for example, alkali glass, non-alkali glass, soda lime glass, lead glass, borosilicate glass, quartz glass, or the like.

[0032] A plurality of division intended lines 3 that cross each other are provided on the front surface 1a of the wafer 1. The division intended lines 3 are also referred to as streets. A device 5 is formed in each region divided by the division intended lines 3 on the front surface 1a of the wafer 1. The device 5 is, for example, an IC (Integrated Circuit) or an LSI (Large-Scale Integrated circuit) or the like. However, the wafer 1 is not limited thereto. The material, shape, structure, size, or the like of the wafer 1 is not limited, and the device 5 can not be formed on the wafer 1.

[0033] When the wafer 1 is divided along the division intended lines 3, individual device chips each mounting the device 5 are formed. When the wafer 1 is divided, for example, a laser beam is condensed inside the wafer 1 along the division intended lines 3, a modified layer is formed inside the wafer 1, and a crack extending in the thickness direction from the modified layer toward the front surface 1a of the wafer 1 is formed.

[0034] Next, a laser processing apparatus 2 that implements the wafer 1 processing method of the present embodiment is described using, for example, Figure 2 Figure 2 ​is a sectional view schematically showing a case where a modification layer is formed in a wafer 1 using a laser processing apparatus 2. The laser processing apparatus 2 has a chuck table 4 which holds the wafer 1, and a laser beam irradiation unit 6 which irradiates a laser beam to the wafer 1 held by the chuck table 4.

[0035] The chuck table 4 has a porous member (not shown) on the upper surface side. The upper surface of the porous member becomes a holding surface 4a which holds the wafer 1. The chuck table 4 is rotatable about an axis perpendicular to the holding surface 4a. The chuck table 4 has a suction source (not shown) connected to the porous member.

[0036] When the wafer 1 is processed by the laser processing apparatus 2, the wafer 1 is placed on the holding surface 4a with the front surface la facing the holding surface 4a, and then a negative pressure generated by 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 where the back surface lb side is exposed upward. The wafer 1 is processed by the laser by irradiating the laser beam from the exposed back surface lb side.

[0037] When the wafer 1 is held by the chuck table 4, a frame unit which is formed by integrating a ring-shaped frame, an adhesive tape which is attached to the ring-shaped frame on the outer periphery, and the wafer 1 can be formed in advance. When the frame unit is formed, the front surface la side of the wafer 1 is attached to an adhesive surface of the adhesive tape which is exposed at the opening of the ring-shaped 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 frame unit interposed therebetween.

[0038] 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) which is 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) which is parallel to the holding surface 4a and perpendicular to the processing feed direction.

[0039] Figure 2 A simplest structure example of the laser beam irradiation unit 6 which is capable of irradiating a 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 which oscillates a laser, a mirror 10, and a condenser lens 12.

[0040] The laser oscillator 8 has a function of emitting a first laser beam 14 having a wavelength which is transparent to the wafer 1 (a wavelength which is transmitted through the wafer 1). For example, the first laser beam 14 uses a laser having a wavelength of 1099 nm which is oscillated in a medium such as Nd:YAG. However, the laser oscillator 8 and the first laser beam 14 are not limited thereto, and can be selected in accordance with the material of the wafer 1 and the like.

[0041] When the modification layer is formed inside the wafer 1, the output of the first laser beam 14 is about 2 W to 3 W, for example. However, the output of the first laser beam 14 is not limited to this, and can be any output that enables the formation of the modification layer inside the wafer 1. The first laser beam 14 emitted from the laser oscillator 8 is reflected by the mirror 10 in a prescribed direction, and is irradiated onto the wafer 1 held by the chuck table 4 via the condenser lens 12.

[0042] 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 movable in the height direction, for example, and can change the height position of the condensing point 16. The condensing point 16 of the first laser beam 14 is positioned at a prescribed height position inside the wafer 1.

[0043] As shown in Figure 2 , when the laser beam irradiation unit 6 and the chuck table 4 are relatively moved in the machining feed direction and the first laser beam 14 is condensed inside the wafer 1, the modification 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), a crack 9 extending from the modification layer 7 toward the front surface la of the wafer 1 is formed, and thus the wafer 1 can be easily and appropriately divided.

[0044] 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 modification layer 7, or the crack 9 extends in an unintended direction. Or, even if the crack 9 is formed, the length is not sufficient, and the crack 9 does not reach the front surface la. In these cases, since the wafer 1 cannot be appropriately divided, the yield of the device chips decreases.

[0045] Here, in order to confirm whether the machining conditions and the like are appropriate, that is, whether the crack 9 appropriately advances from the modification layer 7 toward the front surface la of the wafer 1, it is possible to consider, for example, observing the front surface la of the wafer 1 with a microscope or the like.

[0046] However, in order to observe the front surface la side of the wafer 1 on 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 the microscope or the like. Thus, there is a problem that it takes time to confirm the machining state such as the presence or absence of the crack 9.

[0047] Moreover, even if the crack 9 extending from the modification layer 7 is formed inside the wafer 1, in the case where the length of the crack 9 is not sufficient and the crack 9 does not reach the front surface la, even if the front surface la of the wafer 1 is observed with the microscope or the like, the crack 9 cannot be visually confirmed. Thus, in the case where it is desired to detect the length of the crack 9, it is necessary to divide the wafer 1 to observe the cut surface.

[0048] Therefore, in the wafer processing method of the present embodiment, the processing time for the confirmation is reduced by confirming the processing state of the wafer 1 in the laser processing apparatus 2. Next, the structure for the confirmation of the processing state will be described.

[0049] As shown in FIG. 1, the laser processing apparatus 2 has an observation laser beam irradiation unit 18. The observation laser beam irradiation unit 18 has a function of irradiating the wafer 1 on which the modified layer 7 is formed with the second laser beam 28 as an observation laser beam. Figure 4 Figure 4 A simplest structure example of the observation laser beam irradiation unit 18 capable of irradiating the wafer 1 held by the chuck table 4 with the second laser beam 28 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 not exceeding the processing threshold value capable of forming the modified layer 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 not exceeding 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 the reflected light reaches the dichroic mirror 22 after being reflected on the front face la side of the wafer 1.

[0054] The condenser lens 24 has a function of condensing the second laser beam 28 inside or on the front face 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 wafer 1 held by the chuck table 4 with the first laser beam 14 with an output exceeding the processing threshold value of the wafer 1. That is, the observation laser beam irradiation unit 18 can also function as a processing laser beam irradiation unit. Figure 2 ​The laser beam irradiation unit 6 described above functions as the observation laser beam irradiation unit 18. 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 also be the same.

[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 wafers 1 can be processed efficiently. For example, the first laser beam 14 can be irradiated to one wafer 1 while the second laser beam 28 is irradiated to another wafer 1.

[0057] The beam shaping unit 26 of 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) that shields the second laser beam 28 around the through window. The through window is formed 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 able to shape 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 in the cross-sectional shape in the plane 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 irradiated region in the back surface lb) across the modification layer 7. For example, the cross-sectional shape of the second laser beam 28 is a semicircle on one side of the two regions separated by the modification layer 7.

[0061] The second laser beam 28 is irradiated to the wafer 1 from the back surface 1b side and travels inside the wafer 1. Then, the second laser beam 28 that has reached the front surface 1a of the wafer 1 is reflected by the front surface 1a 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 1b.

[0062] The reflected light 32 of the second laser beam 28 is converted to parallel light by the condenser lens 24 and passes through the dichroic mirror 22. Further, a photographing unit 34 that photographs the reflected light 32 is disposed on the advancing route of the reflected light 32 that has passed 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 photographing unit 34 photographs the reflected light 32 and forms an image that appears from the reflected light 32. As described later, a determination as to whether or not the crack 9 is appropriately elongated from the modified layer 7 formed inside the wafer 1 to the front surface 1a is made based on the image formed by the photographing of the reflected light 32 by the photographing unit 34.

[0064] Next, a wafer processing method according to the present embodiment will be described. The wafer processing method is implemented, for example, in the laser processing apparatus 2. In the wafer processing method, the modified layer 7 is formed inside the wafer 1 along the plurality of division predetermined lines 3 provided on the front surface 1a of the wafer 1. Figure 10 A flowchart showing a flow of the steps of the wafer processing method will be described. The steps will be described in detail below.

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

[0066] In the holding step S10, the wafer 1 is placed on the chuck table 4 with the front surface 1a side of the wafer 1 opposed to the holding surface 4a of the chuck table 4 in a manner that the back surface 1b side of the wafer 1 is exposed upward. Then, when the suction source of the chuck table 4 is activated to apply 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 implemented, a protection member disposing step of previously attaching a protection member such as an adhesive tape to the front surface 1a of the wafer 1 can be implemented. 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 is performed as follows: a first laser beam 14 is irradiated from the back surface lb side of the wafer 1 along the division predetermined line 3, and a modification layer 7 is formed in the inside of the wafer 1. The first laser beam 14 is a laser beam of a wavelength that is transmissive to the wafer 1 (a wavelength that can transmit the wafer 1). Figure 2 is a cross-sectional view schematically showing the modification layer forming step S20.

[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 division predetermined line 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 division predetermined 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 in the inside of the wafer 1.

[0070] Then, the chuck table 4 and the laser beam irradiation unit 6 are relatively moved along the machining feed direction, and 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 division predetermined line 3 is formed in the inside of the wafer 1, and a crack 9 extending from the modification layer 7 to the front surface la of the wafer 1 is formed (see (B) of FIG. 6, etc.). Figure 3

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

[0072] After the modification layer 7 is formed along all of the division predetermined lines 3 in one direction, the chuck table 4 is rotated, and the modification layer 7 is similarly formed along the division predetermined lines 3 in the other direction. When the first laser beam 14 is irradiated along all of the division predetermined lines 3 of the wafer 1, the modification layer forming step S20 is completed. In addition, in each division predetermined line 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 that overlap each other can be formed.

[0073] When the wafer 1 in which the modification layer 7 and the crack 9 extending from the modification layer 7 are formed in the inside along the division predetermined line 3 is ground from the back surface lb side to thin the wafer 1 and remove the modification layer 7, etc., the wafer 1 is divided to obtain individual device chips. However, if the crack 9 does not properly extend to the front surface la of the wafer 1, the wafer 1 cannot be properly divided, and there is a case where the quality of the formed device chips does not satisfy a reference requirement or a case where the device chips are damaged, and the yield of the device chips decreases.

[0074] Figure 3 ​(A) is a cross-sectional view schematically showing a wafer 1 in which a modification layer 7 is formed inside and no crack 9 is formed. In addition, Figure 3 (B) is a cross-sectional view schematically showing a wafer 1 in which a modification layer 7 is formed inside and a crack 9 reaches the front surface la from the modification layer 7. As shown in (B), 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 no crack 9 is formed, the crack 9 cannot be visually confirmed on the front surface la. Figure 3

[0075] 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. In addition, in the case where the crack 9 does not reach the front surface la, the crack 9 cannot be confirmed even with a microscope.

[0076] 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 having an output that does not exceed the processing threshold of the wafer 1, and is a laser beam having a wavelength that is transmissive to the wafer 1 (a wavelength capable of transmitting the wafer 1).

[0078] Figure 4 is a side view schematically showing the observation laser beam irradiation step S30. When the second laser beam 28 is irradiated to the wafer 1 in which the modification layer 7 is formed inside from the back surface lb side, the focal point 30 is positioned in advance to the inside of the wafer 1 or the front surface la. It is preferable that the focal point 30 be positioned at a position of the front surface la of the wafer 1 overlapping the modification layer 7.

[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, after transmitting through the condenser lens 24, the second laser beam 28 is irradiated to the back surface lb of the wafer 1 and advances inside the wafer 1, thereby being condensed at the focal point 30.

[0080] ​The second laser beam 28, traveling inside the wafer 1, is reflected by the front surface 1a of the wafer 1. Then, the reflected light 32 of the second laser beam 28 travels inside the wafer 1, passes through the back surface 1b of the wafer 1, and travels outward. Then, the reflected light 32 passes through the condenser lens 24 and the dichroic mirror 22 and reaches the imaging 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 side 1b of wafer 1) perpendicular to the direction of travel, which is asymmetrical across the modified layer 7.

[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 5As shown in (B) of FIG. 6, the second laser beam 28 that is irradiated to the back surface 1b side of the wafer 1 is condensed to a condensing point 30. Then, the second laser beam 28 is reflected by the front surface 1a of the wafer 1, and the reflected light 32 travels inside the wafer 1 to reach the back surface 1b of the wafer 1.

[0086] If the crack 9 that reaches the front surface 1a 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 (B) of FIG. 6, 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 that reaches the front surface 1a 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 1a of the wafer 1, since the wafer 1 is slightly broken by the crack 9, an interface is formed between the air layer that enters 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 that reaches 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 1a.

[0089] In this case, as shown in (B) of FIG. 6, the reflected light 32 travels in reverse in the same region inside the wafer 1 as the region through which the second laser beam 28 (incident light) transmits to reach the front surface 1a of the wafer 1. Figure 5

[0090] Further, in the observation laser beam irradiation step S30, the height of the condensing lens 24 or the like is changed so that the condensing point 30 moves in the thickness direction (Z-axis direction) of the wafer. Then, the second laser beam 28 is sequentially irradiated to the back surface 1b side of the wafer 1 while the condensing point 30 is moved.

[0091] In the wafer processing method of the present embodiment, then, a photographing step S40 is performed in which the reflected light 32 of the second laser beam 28 that is irradiated on 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 appears.

[0092] Figure 6 (C) of FIG. 6 is a plan view that schematically shows the region 42b in which the reflected light 32 appears in the image 38 that is formed by the photographing unit 34 in a case where the crack 9 that reaches the front surface 1a from the modified layer 7 is not formed. In a case where the crack 9 is not formed, as shown in (C) of FIG. 6, the reflected light 32 that is reflected by the front surface 1a of the wafer 1 and the reflected light 32 that is reflected by the back surface 1b of the wafer 1 are present in the same region inside the wafer 1. Figure 5 ​​As shown in (A), the second laser beam 28 (incident light) and the reflected light 32 are in a reversed state separated by the modified layer 7.

[0093] Therefore, the shape of the reflected light 32 of the second laser beam 28 displayed in image 38 is the same as the shape after reversing the cross-sectional shape of the second laser beam 28. In the case where the cross-sectional shape of the second laser beam 28 is semi-circular, as... Figure 6 As shown in (C), the area 42b revealed by the reflected light 32 becomes a shape that reverses the semi-circular shape.

[0094] in addition, Figure 6 (B) is a schematic top view showing the region 42a revealed by reflected light 32 in the image 36 formed by the imaging unit 34 when the crack 9 extends from the modified layer 7 towards the front side 1a. When the crack 9 extends from the modified layer 7 towards the front side 1a, as... Figure 5 As shown in (B), the paths of the second laser beam 28 (incident light) and the reflected light 32 overlap. Therefore, when the cross-sectional shape of the second laser beam 28 is semi-circular, as... Figure 6 As shown in (B), the area 42a revealed by the reflected light 32 becomes the same shape as the semicircle.

[0095] Thus, the shape of the reflected light 32, as shown in images 36 and 38 formed by photographing the reflected light 32 in step S40, changes depending on the presence or absence of the crack 9. Therefore, it is possible to determine whether a crack 9, extending from the modified layer 7 to the front side 1a, is formed inside the wafer 1 based on images 36 and 38.

[0096] Figure 7 (A) and Figure 7 (B) is a photograph showing an example of an image captured by imaging unit 34 when a crack 9 is formed on wafer 1. Additionally, Figure 7 (C) and Figure 7 (D) is a photograph showing an example of an image captured by the imaging unit 34 when the wafer 1 has not formed a crack 9.

[0097] In each photograph, the reflected light 32 of the second laser beam 28 reflected by the front surface 1a of the wafer 1 is shown in white. Moreover, since the shape and position of the reflected light 32 shown in the image change depending on whether a crack 9 is formed on the wafer 1, it can be understood that the shape and position of the reflected light 32 shown in each photograph can serve as a reference for determining the processing state of the wafer 1, represented by the presence or absence of crack 9 or the length of crack 9.

[0098] Furthermore, as can be seen from the photographs, the area displayed by reflected light 32 is not limited to the reflected light 32 displaying with uniform intensity. That is, it is not limited to the area where reflected light 32 is displayed. Figure 6the entire region of the region 42a shown in (B) or in the entire region of the region 42b shown in (C). The reflected light 32 appears in the image in a striped or dotted pattern due to various reasons such as optical phenomena, but even in the case where the reflected light 32 does not appear uniformly in the image, the processing state of the wafer 1 can be sufficiently determined. Figure 6

[0099] In the processing method of the wafer of the present embodiment, a determination step S50 is implemented in which the processing state of the wafer 1 is determined from the image taken by the photographing step S40. Here, the processing state refers to the state of the wafer 1 processed by irradiation of the first laser beam 14, including the result of the processing. For example, the result of the processing refers to the presence or absence of the crack 9 from the modification layer 7 to the front surface la, the height position formed by the modification layer 7, and the presence or absence of meandering of the modification layer 7, and the like.

[0100] In addition, in the processing state of the wafer 1, the length of the crack 9 extending from the modification layer 7 in the case where the crack 9 does not reach the front surface la is also included. Figure 8 is a cross-sectional view schematically showing the wafer 1 in which the modification layer 7 is formed inside, implemented by the modification layer forming step S20. Figure 8 is a cross-sectional view showing the wafer 1 in which the crack 9 extending from the modification layer 7 toward the front surface la is formed together with the modification layer 7 in the modification layer forming step S20, but the crack 9 does not reach the front surface la.

[0101] In the determination step S50 of the processing method of the wafer of the present embodiment, the processing state of the wafer 1 including the length of the crack 9 is determined. Hereinafter, the details of the determination implemented in the determination step S50 will be described. In the determination step S50, the processing state of the wafer 1 is determined from the position and shape of the reflected light 32 appearing in the image taken by the photographing step S40.

[0102] Figure 8 is a cross-sectional view showing the height positions 44a, 44b at which the focal point 30 is located when the second laser beam 28 is irradiated on the back surface lb of the wafer 1 in the observation laser beam irradiation step S30. The description is divided into the case where the focal point 30 is positioned at the height position 44a higher than the lowest point of the crack 9 and the case where the focal point 30 is positioned at the height position 44b lower than the lowest point.

[0103] Figure 9 ​(A) is a sectional view schematically showing a travel path of the second laser beam 28 in a case where the focal point is positioned at the height position 44a in the observation laser beam irradiation step S30. The second laser beam 28, which is irradiated to the back surface Ib of the wafer 1, converges toward the focal point positioned at the height position 44a. The crack 9 extends to this height position 44a, the second laser beam 28 reaches the crack 9, and is reflected by the crack 9.

[0104] The second laser beam 28 reflected by the crack 9 further travels downward inside the wafer 1 to reach the front surface la, and is reflected by the front surface la. Then, the reflected light 32 of the second laser beam 28 travels upward in the wafer 1 to travel outside from the back surface Ib. At this time, when viewed from the back surface Ib side of the wafer 1, the reflected light 32 belongs to the side opposite to the side to which the second laser beam 28 belongs among the two regions separated by the modified layer 7.

[0105] Therefore, when the reflected light 32 is photographed by the photographing unit 34 in the photographing step S40, the reflected light 32 appears as an area 42a overlapping the cross-sectional shape of the second laser beam 28 in the image 36 formed as in (B) of FIG. 4. Figure 6

[0106] On the contrary to this, Figure 9 (B) is a sectional view schematically showing a travel path of the second laser beam 28 in a case where the focal point is positioned at the height position 44b in the observation laser beam irradiation step S30. The second laser beam 28, which is irradiated to the back surface Ib of the wafer 1, converges toward the focal point positioned at the height position 44b. The crack 9 does not extend to this height position 44b, and the second laser beam 28 is not reflected by the crack 9.

[0107] Therefore, the second laser beam 28 that has passed through the focal point positioned at the height position 44b further travels downward inside the wafer 1 to reach the front surface la, and is reflected by the front surface la. Then, the reflected light 32 of the second laser beam 28 travels upward in the wafer 1 to travel outside from the back surface Ib. At this time, when viewed from the back surface Ib side of the wafer 1, the reflected light 32 belongs to the side opposite to the side to which the second laser beam 28 belongs among the two regions separated by the modified layer 7.

[0108] Therefore, when the reflected light 32 is photographed by the photographing unit 34 in the photographing step S40, the reflected light 32 appears as an area 42b overlapping the shape of the cross-sectional shape of the second laser beam 28 reversed in (C) of FIG. 5 in the image 38 formed as in (C) of FIG. 5. Figure 6

[0109] ​​Thus, the shape and position of the reflected light 32 appearing in the image captured by the imaging unit 34 greatly change depending on whether or not the second laser beam 28 is reflected by the crack 9. Therefore, it is possible to determine whether or not the second laser beam 28 is reflected by the crack 9 depending on the shape and position of the reflected light 32 appearing in the image.

[0110] Then, in the image appearing in the reflected light 32, it is determined whether or not the reflected light 32 appears on the same side as the second laser beam 28 (incident light) with respect to the modified layer 7 (S51). Then, in the case where the reflected light 32 appears on the same side as the incident light, it is determined that the crack 9 extends to the depth at which the focal point 30 is located (S52). On the other hand, in the case where the reflected light 32 does not appear on the same side as the incident light (i.e., in the case where it appears on the opposite side), it is determined that the crack 9 does not extend to the depth at which the focal point 30 is located (S53).

[0111] In other words, in the image captured by the imaging step S40, in the case where the reflected light 32 appears to overlap with a region having the same shape as the irradiated region of the second laser beam 28 in the back surface lb, it is determined that the crack 9 is formed at the height position of the focal point 30. On the other hand, in the image, in the case where the reflected light 32 appears to overlap with a region having a shape obtained by inverting the shape of the irradiated region of the second laser beam 28 in the back surface lb, it is determined that the crack 9 is not formed at the height position of the focal point 30 of the second laser beam 28.

[0112] Further, when the second laser beam 28 is repeatedly irradiated toward the back surface lb of the wafer 1 while the focal point 30 is moved in the thickness direction of the wafer 1, the shape and position of the reflected light 32 appearing in the image captured by the imaging unit 34 change when the focal point 30 becomes a particular height. Also, at the timing at which the shape of the reflected light 32 appearing in the image changes, the particular height at which the focal point 30 is located becomes the height position of the lower end of the crack 9. When the height position of the lower end of the crack 9 is ascertained, the length of the crack 9 can be calculated.

[0113] Here, in the case where the second laser beam 28 is condensed in the vicinity of the lower end of the crack 9, in the image appearing in the reflected light 32, sometimes both the light reflected by the crack 9 and the light traveling without being reflected by the crack 9 appear. Or, sometimes the light is diffusely reflected and no particular light appears in the image. In any case, it is possible to determine the height position of the lower end of the crack 9 depending on the change in the shape of the reflected light 32 appearing in the image.

[0114] Further, the crack 9 formed inside the wafer 1 is sometimes interrupted halfway. In the wafer processing method of the present embodiment, even if the second laser beam 28 is condensed at the interrupted position of the crack 9, a change can be recognized in the image appearing from the reflected light 32. That is, the height position of the wafer 1 at the portion where the crack 9 is interrupted can be discriminated.

[0115] If the height position of the lower end of the crack 9 or the like can be derived, the processing conditions of the first laser beam 14 to the wafer 1 can be adjusted so that the crack 9 formed together with the modified layer 7 reaches the front surface la.

[0116] Further, the shape and position of the reflected light 32 appearing in the image obtained by the photographing step S40 are not limited to this. For example, depending on the optical system possessed by the laser beam irradiation unit 18 and the arrangement position of the photographing unit 34, it can be considered that the reflected light 32 appearing in the image does not become the shape after the cross-sectional shape of the incident light is reversed in the case where the second laser beam 28 is not reflected by the crack 9. For example, it can be considered that the reflected light 32 appearing in the image becomes the shape after the cross-sectional shape of the incident light is reversed in the case where the second laser beam 28 is reflected by the crack 9.

[0117] The influence of the presence or absence of the reflection of the crack 9 to the second laser beam 28 on the position and shape of the reflected light 32 appearing in the image differs depending on each system. Therefore, in the case where it is intended to determine the length of the crack 9 extending from the modified layer 7 from the image taken in the photographing step S40, it is preferable to verify this influence in advance.

[0118] For example, a wafer 1 formed with a crack 9 of a known length is prepared, the second laser beam 28 is irradiated to the wafer 1 while changing the height position of the condensing point 30, and the reflected light 32 is photographed to obtain an image. Then, it is preferable to evaluate the relationship between the length of the crack 9 and the shape and position of the reflected light 32 appearing in the image, and to generate a reference for determining the length of the crack 9 from the image.

[0119] As explained above, in the wafer processing method of the present embodiment, even in the case where the crack 9 extending from the modified layer 7 formed inside does not reach the front surface la of the wafer 1, the processing state of the wafer 1 can be confirmed. At this time, it is not necessary to move the wafer 1 from the chuck table 4 of the laser processing apparatus 2, and the length of the crack 9 can be easily determined in situ. That is, the processing state of the wafer 1 can be easily confirmed.

[0120] Further, the present application is not limited to the above-described embodiments, and various modifications can be made to implement the present application. For example, in the above-described embodiments, the case where the first laser beam 14 and the second laser beam 28 are irradiated to the wafer 1 from the back surface 1b side has been mainly described, but one embodiment of the present application is not limited thereto. For example, the first laser beam 14 and the second laser beam 28 can be irradiated to the front surface 1a side of the wafer 1. Further, 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.

[0121] Further, in the above-described embodiments, the case where the cross-sectional shape of the second laser beam 28 is a semicircular shape has been described as an example, but the cross-sectional shape is not limited thereto. For example, the cross-sectional shape can be a triangular shape, a quadrangular shape, or another polygonal shape. That is, as long as the distribution of the power is asymmetric across the axis along the division predetermined line 3 (for example, the modified layer 7), it is acceptable.

[0122] Further, the second laser beam 28 irradiated to the wafer 1 cannot be precisely condensed to the condensing point 16 due to the influence of spherical aberration, and as a result, the reflected light 32 can not be clearly visualized in the image obtained in the photographing step S40. Therefore, a correction ring that mitigates the influence of spherical aberration can be attached to the condensing lens 24. Also in this case, for example, a correction ring having appropriate performance corresponding to the thickness and the material of the wafer 1 is selected.

[0123] 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 that is corrected for spherical aberration can be formed and irradiated to the back surface 1b of the wafer 1.

[0124] Further, the observation laser beam irradiation step S30 can be implemented by liquid immersion. When this case is described with reference to FIG. 8, 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. Figure 4

[0125] In the case where the observation laser beam irradiation step S30 is implemented by liquid immersion, the numerical aperture of the condensing lens 24 that functions as an objective lens can be increased. Therefore, the resolution of the image visualized by the reflected light 32 photographed by the photographing unit 34 can be improved, and thus the processing state of the wafer 1 can be analyzed in more detail.

[0126] The configuration, method, and the like of the above-described embodiments can be appropriately changed and implemented without departing from the scope 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 predetermined lines provided on the front surface of the wafer, characterized by comprising the steps of: 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 of a wavelength that is transmissive to the wafer from the back surface side of the wafer along the division predetermined line 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 predetermined line; an observation laser beam irradiation step of, after the modification layer forming step, positioning a focal point of a second laser beam of a wavelength that is transmissive to the wafer and that outputs not more than a processing threshold value of the wafer in the inside of the wafer or on the front surface, irradiating the second laser beam from the back surface side of the wafer while moving the focal point in a thickness direction of the wafer; a photographing step of photographing, by a photographing unit, reflected light of the second laser beam irradiated in the observation laser beam irradiation step that is reflected by the front surface of the wafer; and a determination step of determining a processing state of the wafer based on a shape and a position of the reflected light in an image in which the reflected light appears 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 asymmetrically across the modification layer.

2. The wafer processing method according to claim 1, wherein, in the determination step, in the image photographed by the photographing step, in a case where the reflected light appears in an area that is the same shape as an irradiated area in the back surface of the second laser beam irradiated on the wafer in the observation laser beam irradiation step, it is determined that a crack is formed at a height position of the focal point of the second laser beam, and in the image photographed by the photographing step, in a case where the reflected light appears in an area that is a shape obtained by inverting the shape of the irradiated area in the back surface of the second laser beam irradiated on the wafer in the observation laser beam irradiation step, it is determined that the crack is not formed at the height position of the focal point of the second laser beam.

3. The wafer processing method according to claim 1 or 2, wherein the observation laser beam irradiation step is performed by liquid immersion.

4. The wafer processing method according to claim 1 or 2, wherein, in the determination step, determination is performed based on a reference for determination generated in advance. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Images