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; an observation laser beam irradiating step of positioning a condensing point of a second laser beam, which outputs a power not exceeding a processing threshold value of the wafer, in the inside or the front surface of the wafer and irradiating the second laser beam; 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 irradiated on the wafer in the observation laser beam irradiating step being shaped into a cross-sectional shape in a plane perpendicular to a traveling direction of the second laser beam asymmetrically across the modification layer.

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 region divided by the division predetermined lines, 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, 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 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. If the processing conditions and the like are not appropriate, the crack cannot be properly elongated from the formed modified layer, or the crack is elongated in an unintended direction or the like, so that the wafer cannot be properly divided, and thus the yield of device chips decreases.

[0005] Patent Literature 1: Japanese Patent Application Publication No. 2005-86161

[0006] Patent Literature 2: Japanese Patent Application Publication No. 2010-68009

[0007] Here, in order to confirm whether the processing conditions and the like are appropriate, that is, whether the crack properly 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 the wafer upside down, and carry it into a microscope or the like. Thus, there is a problem that the confirmation of the formation state of the crack takes time. SUMMARY

[0008] The present application has been 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 properly processed.

[0009] 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; a laser beam for observation irradiation step of irradiating 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 from the back surface side of the wafer after the modification layer forming step; a photographing step of photographing reflected light of the second laser beam irradiated in the laser beam for observation 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 laser beam for observation 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 modification layer.

[0010] Preferably, in the determination step, in the image photographed in the photographing step, in a case where the reflected light appears in an area overlapping with an area in which a shape of an irradiated area of the second laser beam on the wafer in the laser beam for observation irradiation step is the same as a shape of the second laser beam, it is determined that a crack extends from the modification layer toward the front surface side of the wafer, and in the image photographed in the photographing step, in a case where the reflected light appears in an area overlapping with an area in which a shape of the irradiated area of the second laser beam on the wafer in the laser beam for observation irradiation step is inverted, it is determined that the crack does not extend from the modification layer toward the front surface side of the wafer.

[0011] Further, according to another aspect of the present application, there is provided a wafer processing method of forming a modification layer inside a wafer having a plurality of division lines provided on a 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 inside the wafer by irradiating a first laser beam having a wavelength that is transmissive to the wafer from a back surface side of the wafer along the division lines while moving a laser beam irradiation unit and the chuck table in directions along the division lines; a laser beam for observation irradiation step of irradiating a second laser beam having a wavelength that is transmissive to the wafer and outputted not more than a processing threshold of the wafer from inside the wafer or the front surface after the modification layer forming step; a photographing step of photographing reflected light of the second laser beam irradiated in the laser beam for observation 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 laser beam for observation irradiation step traveling inside the wafer from a direction other than a direction perpendicular to the back surface of the wafer.

[0012] Preferably, in the laser beam for observation irradiation step, the second laser beam is incident on the wafer in a state where aberration caused by a lens forming a focal point of the second laser beam is corrected.

[0013] Further, preferably, the first laser beam and the second laser beam have the same light source.

[0014] Further, preferably, the laser beam for observation irradiation step is performed by liquid immersion.

[0015] 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 inside a wafer is performed, a laser beam for observation irradiation step, a photographing step, and a determination step are performed. The second laser beam irradiated on the wafer in the laser beam for observation irradiation step and traveling inside the wafer is reflected by the front surface of the wafer and a crack extending from the modification layer to the front surface side. Then, in the photographing step, the reflected light of the second laser beam is photographed.

[0016] Here, in the observation laser beam irradiation step, the second laser beam (reflected light) travels in the region between the modified layer and the front surface. Here, in the case where a crack is not formed from the modified layer to the front surface of the wafer, the second laser beam travels directly in the region. On the other hand, in the case where a crack is formed, an interface is generated between the air layer that enters the crack and the wafer, and the second laser beam is reflected at the interface where the difference in refractive index on both sides is large.

[0017] Therefore, the shape of the reflected light that appears in the image obtained by the photographing step changes depending on the presence or absence of the crack. In other words, the processing state of the wafer, such as the presence or absence, position, and shape of the crack formed in the inside of the wafer, can be determined from the shape of the reflected light that appears in the image. At this time, it is not necessary to move the wafer from the chuck table of the laser processing apparatus, and it is possible to determine whether a crack is formed from the modified layer to the front surface of the wafer without spacing after the modified layer is formed.

[0018] Therefore, according to one embodiment of the present application, a wafer processing method is provided that can easily confirm whether a wafer is processed appropriately. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is a schematic view that schematically shows a wafer.

[0020] Figure 2 is a schematic view that schematically shows a modified layer formation step.

[0021] Figure 3 (A) of FIG. 1 is a schematic view that schematically shows a wafer in which a modified layer is formed in the inside, enlarged, Figure 3 (B) of FIG. 1 is a schematic view that schematically shows a wafer in which a modified layer and a crack are formed in the inside, enlarged.

[0022] Figure 4 is a schematic view that schematically shows an observation laser beam irradiation step.

[0023] Figure 5 (A) of FIG. 4 is a schematic view that schematically shows a second laser beam that is irradiated onto a wafer in which a modified layer is formed in the inside and reflected light thereof, Figure 5 (B) of FIG. 4 is a schematic view that schematically shows a second laser beam that is irradiated onto a wafer in which a modified layer and a crack are formed in the inside and reflected light thereof.

[0024] Figure 6 (A) of FIG. 6 is a schematic view that schematically shows a region in the back surface of a wafer in which a second laser beam is irradiated, viewed from above, Figure 6 (B) of FIG. 6 is a schematic view that schematically shows an example of a region that appears from reflected light in an image in which the reflected light appears, viewed from above, Figure 6of (C) is another example of a plan view of a region appearing by the reflected light in an image appearing by the reflected light.

[0025] Figure 7 of (A) and Figure 7 of (B) is an image appearing by the reflected light in a case where the wafer is formed with a crack, Figure 7 of (C) and Figure 7 of (D) is an image appearing by the reflected light in a case where the wafer is not formed with a crack.

[0026] Figure 8 of (A), Figure 8 of (C), and Figure 8 of (E) is an image appearing by the reflected light, Figure 8 of (B), Figure 8 of (D), and Figure 8 of (F) is an optical microscope photograph appearing by the front surface of the wafer.

[0027] Figure 9 is a cross-sectional view schematically showing a modification example of the modification layer forming step and the observation laser beam irradiation step.

[0028] Figure 10 is a cross-sectional view schematically showing a wafer in which the second laser beam is irradiated from a traveling direction not perpendicular to the back surface of the wafer.

[0029] Figure 11 is a flowchart showing a flow of each step of the wafer processing method.

[0030] Explanation of Reference Signs

[0031] 1: wafer; 1a: front surface; 1b: back surface; 3: division predetermined line; 5: device; 7: modification layer; 9: crack; 2, 48: laser processing apparatus; 4, 50: chuck table; 4a, 50a: holding surface; 6, 52: laser beam irradiation unit; 8, 20, 54: laser oscillator; 10: mirror; 22, 62: dichroic mirror; 12, 24, 64: condenser lens; 14, 66: first laser beam; 16, 30: condensing point; 18: observation laser beam irradiation unit; 26: beam shaping unit; 28, 68: second laser beam; 32, 70: reflected light; 34, 72: photographing unit; 36, 38: image; 40: region; 42a, 42b: region; 56: polarizing plate; 58: spatial light modulator; 60: 4f lens unit. DETAILED DESCRIPTION

[0032] Embodiments of the present application will be described with reference to the accompanying drawings. First, a wafer in which a modification layer is formed by a wafer processing method of the present embodiment will be described. Figure 1 is a perspective view schematically showing a wafer 1.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] The laser oscillator 8 has a function of emitting a first laser beam 14 having a wavelength that is transparent to the wafer 1 (a wavelength that is transmitted through 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 such as Nd:YAG. However, the laser oscillator 8 and the first laser beam 14 are not limited thereto, and can be selected according to the material of the wafer 1 and the like.

[0044] When the modified layer is formed in the inside of the wafer 1, the output of the first laser beam 14 is, for example, 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.

[0045] 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 able to move, for example, in the height direction, and is able to 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.

[0046] 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, a modified layer 7 is formed inside the wafer 1. Here, when the irradiation conditions of the first laser beam 14 and machining conditions such as the machining feed speed are appropriately set, as shown in Figure 3 (B), a 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.

[0047] 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, and thus the yield of device chips decreases. Here, in order to confirm whether the machining conditions and the like are appropriate, that is, whether the crack 9 appropriately advances from the modified layer 7 toward the front surface la of the wafer 1, for example, it is possible to consider observing the front surface la of the wafer 1 using 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, for example, it is necessary to take out the wafer 1 from the laser machining device 2, and to turn the wafer 1 upside down and carry it into a microscope or the like. Thus, there is a problem that it takes time to confirm the machining state of the crack 9 or the like. Therefore, in the wafer machining method of the present embodiment, the time to confirm 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 most simple structure example of the observation laser beam irradiation unit 18 which is able to irradiate 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 includes 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 a second laser beam 28 with an output not exceeding a processing threshold that allows a modified layer to be formed inside the wafer 1.

[0051] The laser oscillator 20 emits, for example, a second laser beam 28 with an output of approximately 0.2W, not exceeding the processing threshold. However, the output of the second laser beam 28 is not limited to this. Since the processing threshold varies depending on the material of the wafer 1, the output of the second laser beam 28 is appropriately determined based on the material of the wafer 1 being processed, so that the output of the second laser beam 28 does not exceed the processing threshold.

[0052] Preferably, the output of the second laser beam 28 is set between one-tenth and one-thousandth of the output of the first laser beam 14. More preferably, the output of the second laser beam 28 is set to approximately one-thirtieth of the output of the first laser beam 14.

[0053] The dichroic mirror 22 has the function of reflecting the second laser beam 28 in a predetermined direction. In addition, as described later, the dichroic mirror 22 has the function of allowing the reflected light 32 to pass through when it reaches the dichroic mirror 22 after the second laser beam 28 is reflected on the front side 1a of the wafer 1.

[0054] The focusing lens 24 has the function of focusing the second laser beam 28 onto the inside or front side 1a of the wafer 1 held by the chuck stage 4. For example, the focusing lens 24 can be moved along the height direction, thereby changing the height position of the focusing point 30.

[0055] Additionally, the observation laser beam irradiation unit 18 can also irradiate the wafer 1 held by the chuck stage 4 with a first laser beam 14 that exceeds the processing threshold of the wafer 1. That is, the observation laser beam irradiation unit 18 can also serve as... Figure 2 The laser beam irradiation unit 6 described herein functions as such. 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 for the first laser beam 14 and the second laser beam 28 can also be the same.

[0056] On the other hand, when the laser processing apparatus 2 has both a laser beam irradiation unit 6 and an observation laser beam irradiation unit 18, and also has a chuck stage, it is possible to process the wafer 1 efficiently. For example, it is possible to irradiate one wafer 1 with a second laser beam 28 while simultaneously irradiating another wafer 1 with a first laser beam 14.

[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 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, with the modification layer 7 interposed. For example, the cross-sectional shape of the second laser beam 28 is a semicircle on one side of two regions separated by the modification layer 7.

[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 to form an image in which the reflected light 32 appears. As will be described later, a determination as to whether the cracks 9 are 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.

[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 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 11 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 is implemented in which the wafer 1 is carried into the laser processing apparatus 2, the front surface la of the wafer 1 is opposed to the chuck table 4, and the wafer 1 is held by the chuck table 4.

[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 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 provisioning step in which a protection member such as an adhesive tape is attached to the front surface la of the wafer 1 in advance can be implemented. In this case, in the holding step S10, the wafer 1 is held by the chuck table 4 with the protection member interposed therebetween.

[0068] Next, a modification layer forming step S20 is implemented in which the first laser beam 14 is irradiated 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. 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 to position one end of one division predetermined line 3 of the wafer 1 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 processing feed direction. Then, the focal point 16 of the first laser beam 14 is positioned at a prescribed height position in the interior of the wafer 1.

[0070] Then, while relatively moving the chuck table 4 and the laser beam irradiation unit 6 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 modified layer 7 along the separation predetermined line 3 is formed in the interior of the wafer 1, and a crack 9 extending from the modified layer 7 to the front surface la of the wafer 1 is formed (refer to (B) of FIG. 6, etc.). Figure 3

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

[0072] After the modified layer 7 is formed along all of the separation predetermined lines 3 in one direction, the chuck table 4 is rotated, and a modified layer 7 is similarly formed along the separation predetermined lines 3 in the other direction. When the first laser beam 14 is irradiated along all of the separation predetermined lines 3 of the wafer 1, the modified layer formation step S20 is completed. Also, in each separation 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 modified layers 7 overlapping each other can be formed.

[0073] When the wafer 1 having the modified layer 7 and the crack 9 extending from the modified layer 7 formed in the interior along the separation predetermined line 3 is ground from the back surface lb side to thin the wafer 1 and remove the modified layer 7, etc., the wafer 1 is separated 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 separated, and there is a case where the quality of the formed device chips does not satisfy the standard requirements or a case where the device chips are damaged, and the yield of the device chips is reduced.

[0074] Figure 3 (A) of FIG. 6 is a cross-sectional view schematically showing the wafer 1 having the modified layer 7 formed in the interior and not having the crack 9 formed. Also, Figure 3 (B) of FIG. 6 is a cross-sectional view schematically showing the wafer 1 having the modified layer 7 formed in the interior and the crack 9 reaching the front surface la from the modified layer 7. 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

[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 onto the microscope. Therefore, in the wafer processing method of the present embodiment, in order to determine the presence or absence of the crack 9 that extends from the modification layer 7 to the front surface la, the observation laser beam irradiation step S30, the photographing step S40, and the determination step S50 are implemented.

[0076] 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 toward 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).

[0077] Figure 4 is a side view that schematically shows the observation laser beam irradiation step S30. When the second laser beam 28 is irradiated from the back surface lb side toward the wafer 1 in which the modification layer 7 is formed inside, the focal point 30 is positioned in advance in 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 that overlaps the modification layer 7.

[0078] 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 in the inside of the wafer 1, thereby converging at the focal point 30.

[0079] The second laser beam 28 that advances in the inside 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 advances in the inside of 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.

[0080] Figure 6 (A) of is a plan view that schematically shows an example of the cross-sectional shape of the second laser beam 28 irradiated to the wafer 1. Specifically, Figure 6 (A) of shows a region 40 in the back surface lb of the wafer 1 in which the second laser beam 28 is irradiated, in which hatching is indicated. Further, for ease of explanation, 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.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] If no crack 9 is formed inside wafer 1, extending from the modified layer 7 to the front surface 1a of wafer 1, then the second laser beam 28 travels through the region beneath the modified layer 7. 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.

[0086] In contrast, if a crack 9 is formed inside the wafer 1, extending from the modified layer 7 to the front surface 1a of the wafer 1, the second laser beam 28 reaches the crack 9 below the modified layer 7 and is affected by the crack 9.

[0087] When crack 9 reaches the front surface 1a of wafer 1, wafer 1 is slightly broken by crack 9, thus forming an interface between the air layer entering crack 9 and wafer 1. Therefore, similar to the case where the second laser beam 28 is reflected by the front surface 1a, the second laser beam 28 reaching crack 9 is reflected by crack 9.

[0088] In this case, such as Figure 5 As shown in (B), the reflected light 32 travels in the same region as the region inside the wafer 1 through which the second laser beam 28 (incident light) is transmitted, and reaches the front side 1a of the wafer 1.

[0089] In the wafer processing method of this embodiment, the following imaging step S40 is then performed: the imaging unit 34 captures the reflected light 32 of the second laser beam 28 that was irradiated onto the wafer 1 in the observation laser beam irradiation step S30. In the imaging step S40, the reflected light 32 is captured to form an image displayed by the reflected light 32.

[0090] Figure 6 (C) is a schematic top view showing the region 42b revealed by reflected light 32 in the image 38 formed by the imaging unit 34 in the absence of a crack 9 forming from the modified layer 7 to the front surface 1a. In the absence of a crack 9, as... 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.

[0091] 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.

[0092] 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 6As shown in (B), the area 42a revealed by the reflected light 32 becomes the same shape as the semicircle.

[0093] 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.

[0094] 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 images 36 and 38 captured in the imaging step S40. Here, the processing state refers to the state of wafer 1 after processing, for example, 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.

[0095] The details of the determination performed in determination step S50 will be explained. In determination step S50, it is determined whether the shape of the reflected light 32 of the second laser beam 28 displayed in the images 36 and 38 captured by the imaging step S40 is a shape that reflects the cross-sectional shape of the second laser beam 28 (incident light) irradiating the wafer 1. In short, it is determined whether the reflected light 32 has the same shape as the cross-sectional shape of the incident light (S51). Here, the cross-sectional shape of the second laser beam 28 refers, for example, to the shape of the irradiated area of ​​the second laser beam 28 on the back surface 1b of the wafer 1.

[0096] The result is, such as Figure 6 As shown in (B), if it is confirmed that the reflected light 32 has the same cross-sectional shape as the incident light, it is determined that the crack 9 extends from the modified layer 7 toward the front side 1a of the wafer 1 (S52). More specifically, in the captured image, the reflected light 32 appears as the area irradiated by the second laser beam 28 in the back side 1b ( Figure 6 If region 40 of (A) overlaps with region 42a of the same shape, it is determined that the crack extends from the modified layer 7 toward the front side 1a of the wafer 1.

[0097] On the other hand, such as Figure 6 As shown in (C), if it is confirmed that the reflected light 32 is the shape after the cross-sectional shape of the incident light is reversed, it is determined that the crack 9 does not extend from the modified layer 7 toward the front side 1a of the wafer 1 (S53). Specifically, in the captured image, the reflected light 32 appears as the area where the second laser beam 28 is irradiated in the back side 1b ( Figure 6 If the area 40) of (A) overlaps with the area 42b of the reversed shape, it is determined that the crack 9 does not extend from the modified layer 7 toward the front side 1a of the wafer 1.

[0098] In the determination step S50, in a case where it is determined that the crack 9 is not extended from the modified layer 7 toward the front surface la, it can be determined that the laser processing performed in the modified layer forming step S20 is not properly performed. In this case, it can be considered that the processing conditions under which the first laser beam 14 is irradiated toward the wafer 1 to process the wafer 1 in the modified layer forming step S20 are inappropriate, or there is some abnormality in the laser processing apparatus 2 including the laser beam irradiation unit 6.

[0099] In the determination step S50, in a case where it is determined that the crack 9 is extended toward the front surface la, then, the wafer 1 is thinned by, for example, grinding from the back surface lb side, and the wafer 1 is divided to manufacture individual device chips. If the crack 9 is properly formed, the wafer 1 is properly divided.

[0100] Figure 7 (A) and Figure 7 (B) of FIG. 10 are photographs showing an example of an image captured by the imaging unit 34 in a case where the crack 9 is formed in the wafer 1. In addition, Figure 7 (C) and Figure 7 (D) of FIG. 11 are photographs showing an example of an image captured by the imaging unit 34 in a case where the crack 9 is not formed in the wafer 1.

[0101] In each photograph, the reflected light 32 reflected by the front surface la of the wafer 1 of the second laser beam 28 is visualized in white. Also, since the shape and position visualized by the reflected light 32 in the image vary depending on whether the crack 9 is formed in the wafer 1 or not, it can be understood that the shape and position of the reflected light 32 visualized in each photograph can serve as a reference for determining the presence or absence of the crack 9.

[0102] In addition, from each photograph, it is known that, in the region visualized by the reflected light 32, the reflected light 32 is not limited to being uniformly visualized. That is, the reflected light 32 is not limited to being uniformly distributed in the entire region of the region 42a shown in (B) of FIG. 10 or in the entire region of the region 42b shown in (C) of FIG. 11. Due to various reasons such as optical phenomena, the reflected light 32 is visualized in a striped or dotted pattern in the image, but even in a case where the reflected light 32 is not uniformly visualized in the image, it is possible to sufficiently determine the presence or absence of the crack 9. Figure 6 Figure 6 In addition, not only determining whether the crack 9 is formed in the inside of the wafer 1 or not, but sometimes it is also desired to evaluate the quality of the crack 9. In the wafer processing method of the present embodiment, it is also possible to evaluate the quality of the crack 9 from the image visualized by the reflected light 32.

[0103] In addition, not only determining whether the crack 9 is formed in the inside of the wafer 1 or not, but sometimes it is also desired to evaluate the quality of the crack 9. In the wafer processing method of the present embodiment, it is also possible to evaluate the quality of the crack 9 from the image visualized by the reflected light 32.

[0104] ​For example, when the crack 9 becomes meandering and there are minute irregularities on the reflecting surface of the second laser beam formed by the crack 9, the image displayed by the reflected light 32 sometimes becomes indistinct. Additionally, sometimes a portion of the reflected light 32 appears outside the area predetermined to be displayed by the reflected light 32 in the image.

[0105] Furthermore, for example, if the crack 9 extending from the modified layer 7 is short and does not reach the front surface 1a, sometimes a non-crack-forming region remains between the modified layer 7 and the front surface 1a. In this case, a portion of the second laser beam 28 passes through this non-crack-forming region, and another portion is reflected by the crack 9. That is, in the image captured by the imaging unit 34, sometimes the reflected light 32 is displayed in two areas: the area where the crack 9 is formed on the wafer 1 and the area where the reflected light 32 is displayed in the absence of the crack 9.

[0106] Another example of the image obtained in step S40 is shown together with an optical microscope photograph showing the front side 1a of the wafer 1. Figure 8 (A) is an image of reflected light 32 taken without the formation of a crack 9 from the modified layer 7 of wafer 1 toward the front side 1a. Figure 8 (B) is an optical microscope photograph showing the front side 1a of the wafer 1.

[0107] Figure 8 (C) is an image of reflected light 32 taken when a quality-deficient crack 9 is formed from the modified layer 7 of wafer 1 toward the front side 1a. Figure 8 (D) is an optical microscope photograph showing the front side 1a of the wafer 1. Furthermore, Figure 8 (E) is an image of reflected light 32 taken when a sufficiently large crack 9 is formed from the modified layer 7 of wafer 1 toward the front side 1a. Figure 8 (F) is an optical microscope photograph showing the front side 1a of the wafer 1.

[0108] exist Figure 8 In the photograph shown in (F), extremely fine lines extending laterally can be identified. When the wafer 1 has a sufficiently large, relatively shallow crack 9, the crack 9 can be seen as such an extremely fine line when the front side 1a is observed using an optical microscope. In this case, Figure 8 The image obtained by performing the observation step S30 of illuminating with a laser beam and the shooting step S40 is shown in (E).

[0109] In contrast, Figure 8 In the photograph shown in (B), the laterally extending line cannot be identified. That is, it can be seen that no crack 9 is formed on wafer 1 that exposes to the front side 1a. In this case,Figure 8 Image (A) is shown obtained by performing the observation step S30 of irradiating with a laser beam and the imaging step S40. Figure 8 The image shown in (A) and Figure 8 In the image shown in (E), it can be seen that the shape and position of the area revealed by the reflected light 32 are reversed.

[0110] Moreover, in Figure 8 In the photograph shown in (D), a horizontally extending line can be identified. It appears in... Figure 8 The lines in the photograph shown in (D) are more prominent than those in the photograph. Figure 8 The extremely fine lines in (F) are thick. When a low-quality crack 9 that produces a meandering effect is formed on wafer 1, the crack 9 can be seen as such a thick line when the front side 1a is observed using an optical microscope.

[0111] In this case, Figure 8 Image (C) is shown obtained by performing the observation step S30 of illuminating with a laser beam and the imaging step S40. Figure 8 In the image shown in (C), Figure 8 The area shown by reflected light 32 in the image shown in (A) and in Figure 8 The areas shown by reflected light 32 in the image (E) are the two regions where reflected light 32 is displayed.

[0112] That is, after taking pictures Figure 7 When viewing the image shown in (C), it can be assumed that a portion of the second laser beam 28 passes through the region between the modified layer 7 and the front surface 1a, and another portion of the second laser beam 28 is reflected by the crack 9. Therefore, it can be determined that the quality of the crack 9 is insufficient. Thus, in the wafer processing method of this embodiment, the quality of the crack 9 formed inside the wafer 1 can be evaluated based on the image obtained by performing the imaging step S40.

[0113] Furthermore, the shape and position of the reflected light 32 displayed in the image obtained in the imaging step S40 are not limited to this. For example, depending on the focusing position of the second laser beam 28 and the arrangement position of the imaging unit 34, it can be considered that if the wafer 1 does not have a crack 9, the reflected light 32 displayed in the image will not have a shape that inverts the cross-sectional shape of the incident light. For example, if a crack 9 is formed, the reflected light 32 displayed in the image may sometimes have a shape that inverts the cross-sectional shape of the incident light.

[0114] The effect of the presence or absence of the crack 9 on the position and shape of the reflected light 32 appearing in the image differs depending on each system. Therefore, in a case where it is intended to determine the presence or absence of the crack 9 extending from the modified layer 7 to the front surface la based on the image taken in the photographing step S40, it is preferable to verify the effect in advance.

[0115] 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. Then, it is preferable to evaluate the effect of the presence or absence of the crack 9 on the image and generate a reference for determining the presence or absence of the crack 9 based on the image.

[0116] Hereinafter, a case where the determination is performed based on whether the shape and position of the reflected light 32 appearing in the image 36, 38 directly reflects the cross-sectional shape of the second laser beam 28 or reflects the cross-sectional shape of the second laser beam 28 after being reversed will be further described. However, the method and reference of the determination and the like are not limited thereto.

[0117] Here, an experiment for investigating the effect of the height position of the focal point 16 inside the wafer 1 at which the first laser beam 14 converges when the modified layer formation step S20 is performed on the formation of the crack 9 or not will be described.

[0118] In this experiment, a Si wafer having a thickness of 775 μm was prepared as the wafer 1, and laser processing was performed using the laser processing apparatus 2. At this time, the wafer 1 was held by the chuck table 4 in a state where the back surface lb side of the wafer 1 was exposed upward, and the first laser beam 14 was irradiated to the wafer 1 to form the modified layer 7 inside the wafer 1. In this experiment, the first laser beam 14 was irradiated to each Si wafer under a plurality of processing conditions in which the distance of the focal point 16 from the front surface la was different, and a plurality of wafers 1 in which the formation depth of the modified layer 7 was different were generated.

[0119] Then, the second laser beam 28 was irradiated to each wafer 1 and the reflected light 32 was photographed, and the presence or absence of the crack 9 was determined based on the formed image. Next, each wafer 1 was transported to a microscope, and the front surface la side of the wafer 1 was observed to confirm the presence or absence of the crack 9. Then, the relationship between the depth D (height) of the modified layer 7 from the front surface la, the determination result of the presence or absence of the crack 9 based on the image, and the confirmation result of the presence or absence of the crack 9 based on the microscope was shown in Table 1.

[0120] [Table 1]

[0121]

[0122] As shown in Table 1, in this experiment, the depth D of the modification layer 7 from the front surface la was set to 12 kinds from 52 μm to 102 μm, and the modification layer 7 was formed in each wafer 1. Then, the second laser beam 28 was irradiated to the wafer 1, the reflected light 32 was captured, and the image was formed.

[0123] For example, Figure 7 (A) is an image formed when the depth D of the modification layer 7 from the front surface la is 81 μm, Figure 7 (B) is an image formed when the depth D is 86 μm. In addition, Figure 7 (C) is an image formed when the depth D is 90 μm, Figure 2 (D) is an image formed when the depth D is 94 μm.

[0124] In the 8 wafers 1 in which the depth D of the modification layer 7 from the front surface la was 86 μm or less, the shape of the reflected light 32 appearing in the image was the same as the cross-sectional shape of the second laser beam 28. On the other hand, in the 4 wafers 1 in which the depth D of the modification layer 7 from the front surface la was 90 μm or more, the shape of the reflected light 32 appearing in the image was the shape in which the cross-sectional shape of the second laser beam 28 was reversed. That is, the following was suggested: when the depth D of the modification layer 7 was 86 μm or less, the crack 9 from the modification layer 7 to the front surface la was formed.

[0125] Then, in this experiment, the wafer 1 was carried out from the laser processing apparatus 2, the front surface la of each wafer 1 was observed with a microscope, and the presence or absence of the crack 9 was confirmed. Then, as shown in Table 1, it was confirmed that the crack 9 was formed in the 8 wafers 1 in which the depth D of the modification layer 7 was 86 μm or less, and it was confirmed that the crack 9 was not formed in the 4 wafers 1 in which the depth D of the modification layer 7 was 90 μm or more.

[0126] In this experiment, it was confirmed that the height of the modification layer 7 had an influence on the presence or absence of the crack 9 from the modification layer 7 to the front surface la. Also, in this experiment, the determination result of the presence or absence of the crack 9 according to the image appearing by the reflected light 32 completely corresponded to the confirmation result of the presence or absence of the crack 9 according to the microscope, and it was confirmed that the processing state of the wafer 1 could be confirmed by the wafer processing method of the present embodiment.

[0127] As described above, in the wafer processing method of the present embodiment, the presence or absence of the crack 9 can be easily determined in place without moving the wafer 1 from the chuck table 4 of the laser processing apparatus 2. That is, the processing state of the wafer 1 can be easily confirmed.

[0128] In addition, the present application is not limited to the description of the above-described embodiments, and various modifications can be made to implement it. For example, in Figure 4 , the simplest structure of the laser beam irradiation unit 6 was described. In addition, inFigure 9 In the above, the simplest structure of the observation laser beam irradiation unit 18 was described. Also, the case where the light source of the first laser beam 14 is different from the light source of the second laser beam 28 was described as the center. However, the wafer processing method of the present embodiment is not limited to this.

[0129] For example, in the modification layer formation step S20 and the observation laser beam irradiation step S30 of the wafer processing method of one embodiment of the present application, other types of laser beam irradiation units can also be used.

[0130] Next, a laser processing device 48 that is a modification example of the laser processing device 2 will be described. Figure 9 is a side view that schematically shows a laser beam irradiation unit 52 that is a modification example mounted on the laser processing device 48. The laser processing device 48 has a chuck table 50 having a holding surface 50a exposed upward and the laser beam irradiation unit 52. The chuck table 50 is configured similarly to the chuck table 4 of the laser processing device 2.

[0131] The laser beam irradiation unit 52 has a function of irradiating the wafer 1 with a laser beam of a wavelength that transmits through the wafer 1. Also, the laser beam irradiation unit 52 is capable of irradiating the wafer 1 with the first laser beam 66 at an output exceeding the processing threshold value and is capable of irradiating the wafer 1 with the second laser beam 68 at an output not exceeding the processing threshold value. That is, the laser beam irradiation unit 52 can be used for the modification layer formation step S20 and the observation laser beam irradiation step S30.

[0132] As shown in Figure 10 , the laser beam irradiation unit 52 has a laser oscillator 54, a polarizing plate 56, a spatial light modulator 58, a 4f lens unit 60, a dichroic mirror 62, and a condenser lens 64. The laser oscillator 54, the dichroic mirror 62, and the condenser lens 64 are the same as the structures corresponding to the above-described laser beam irradiation unit 6 and the observation laser beam irradiation unit 18. In addition, the laser processing device 48 has a photographing unit 72 configured similarly to the photographing unit 34 in the vicinity of the laser beam irradiation unit 52.

[0133] The polarizing plate 56 is used to adjust the polarization direction of the laser beam incident to the spatial light modulator 58. In addition, the spatial light modulator 58 is, for example, an LCOS element. When a laser beam is irradiated to the LCOS element that functions as the spatial light modulator 58, the laser beam is phase-modulated by the liquid crystal and is reflected, so that the wavefront shape can be controlled. That is, when the spatial light modulator 58 is used, the incident laser beam can be shaped into a prescribed cross-sectional shape.

[0134] The 4F lens unit 60 provided between the spatial light modulator 58 and the dichroic mirror 62 has a pair of lenses. Also, the pair of lenses are separated from each other by a prescribed distance. In addition, the distance between each lens and the spatial light modulator 58 or the dichroic mirror 62 is also adjusted. That is, the pair of lenses of the 4F lens unit 60 constitute a telecentric optical system on both sides. Thereby, the image of the laser beam reflected by the reflecting surface of the spatial light modulator 58 is imaged on the entrance surface of the condenser lens 64.

[0135] In the modification layer forming step S20, the condensing point of the first laser beam 66 is positioned at a prescribed height position inside the wafer 1 by the laser beam irradiation unit 52, and the first laser beam 66 is irradiated to the back surface lb side of the wafer 1. Thereby, the modification layer 7 along the division predetermined line 3 is formed inside the wafer 1. At this time, in the spatial light modulator 58, the incident first laser beam 66 is not shaped into a particular shape. Or, it is shaped into a shape suitable for the formation of the modification layer 7.

[0136] In the observation laser beam irradiation step S30, the condensing point of the second laser beam 68 is positioned at the front surface la or inside of the wafer 1 by the laser beam irradiation unit 52, and the second laser beam 68 is irradiated to the back surface lb side of the wafer 1. At this time, the spatial light modulator 58 makes the cross-sectional shape of the second laser beam 68 in the plane perpendicular to the traveling direction when the second laser beam 68 is irradiated to the back surface lb of the wafer 1 asymmetric with respect to the modification layer 7.

[0137] Then, the second laser beam 68 travels inside the wafer 1 and is reflected by the front surface la. Then, the reflected light 70 travels from the back surface lb of the wafer 1 to the outside of the wafer 1, and reaches the photographing unit 72 by passing through the dichroic mirror 62. The photographing unit 72 photographs the reflected light 70 and generates an image.

[0138] Thus, when the laser beam irradiation unit 52 is used, the first laser beam 66 and the second laser beam 68 can be generated using a common light source, and therefore the structure of the laser processing device 48 is simplified.

[0139] Further, in the above embodiment, the case where the cross-sectional shape of the second laser beam 28 is made asymmetric with respect to the modification layer 7 and the second laser beam 28 is irradiated from the normal direction of the back surface lb of the wafer 1 is described, but one embodiment of the present application is not limited thereto.

[0140] Figure 10 is a cross-sectional view schematically showing the case where the traveling direction of the second laser beam 44 is not perpendicular to the back surface lb of the wafer 1 in the observation laser beam irradiation step S30. As shown in FIG. 8, the second laser beam 44 is shaped into a shape that is asymmetric with respect to the modification layer 7 in the plane perpendicular to the traveling direction when the second laser beam 44 is irradiated to the back surface lb of the wafer 1. Figure 4As shown, when no crack 9 is formed from the modified layer 7 to the front surface 1a, the second laser beam 44 (reflected light 46) travels in the region between the modified layer 7 and the front surface 1a. On the other hand, when a crack 9 extending from the modified layer 7 to the front surface 1a is present, the second laser beam 44 (reflected light 46) is reflected by the crack 9.

[0141] Thus, regardless of whether the cross-sectional shape of the second laser beam 44 is asymmetrical, there is a possibility that the presence or absence of crack 9 can be determined based on the image displayed by the reflected light 46. In this case, the second laser beam 44, which irradiates the wafer 1 in the observation laser beam irradiation step S30, travels inside the wafer 1 in a direction not perpendicular to the back surface 1b of the wafer 1. Moreover, in this case, the image displayed by the reflected light 32 changes depending on whether crack 9 is present or not. Therefore, the presence or absence of crack 9 can be determined based on the image displayed by the reflected light 32.

[0142] Furthermore, the second laser beam 28 irradiating the wafer 1 cannot be precisely focused on the focal point 16 due to spherical aberration. As a result, the reflected light 32 is sometimes not clearly visible in the image obtained in the imaging step S40. Therefore, a correction ring to mitigate the effects of spherical aberration can be installed on the condenser lens 24. Moreover, in this case, for example, a correction ring with appropriate performance corresponding to the thickness and material of the wafer 1 can be selected.

[0143] Alternatively, if a spatial light modulator such as an LCOS element is used in the observation laser beam irradiation unit 18, a second laser beam 28 with corrected spherical aberration can be formed and irradiated onto the back surface 1b of the wafer 1.

[0144] Alternatively, the observation step S30, involving laser beam irradiation, can also be performed via liquid immersion. When through... ​ When illustrating the observation step S30 using a laser beam, the space between the condenser lens 24 and the back surface 1b of the wafer 1 is filled with liquid. This liquid can be, for example, a liquid called an oil-based liquid, glycerin, or pure water.

[0145] When the observation laser beam irradiation step S30 is performed by liquid immersion, the numerical aperture of the condenser lens 24, which functions as an objective lens, can be increased. Therefore, the resolution of the image displayed by the reflected light 32 captured by the imaging unit 34 can be improved, thereby enabling more detailed analysis of the crack 9 extending from the modified layer 7.

[0146] Further, in the above-described embodiment, the case where the first laser beam 14 and the second laser beam 28 are mainly irradiated to the wafer 1 on which the device 5 is formed on the front surface 1a side from the back surface 1b side is 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.

[0147] Further, in the above-described embodiment, the case where the cross-sectional shape of the second laser beam 28 is a semicircular shape is 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 other polygonal shape. That is, as long as the distribution of the power is asymmetric across the modified layer 7. For example, when the cross-sectional shape is a shape of half of a semicircle, information related to the elongation direction of the crack 9 is sometimes obtained from the image appearing from the reflected light 32.

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

Claims

1. A method for processing a wafer, wherein a modified layer is formed inside the wafer along the predetermined dividing lines formed on the front side of the wafer, characterized in that, The wafer fabrication method includes the following steps: The holding step involves positioning the front side of the wafer against the chuck stage, using the chuck stage to hold the wafer. In the modified layer formation step, the focus point of a first laser beam with a wavelength that is transparent to the wafer is positioned inside the wafer and the laser beam irradiation unit and the chuck stage are moved relative to each other in the direction along the predetermined dividing line, while the first laser beam is irradiated from the back side of the wafer along the predetermined dividing line to form the modified layer inside the wafer. In the laser beam irradiation step, after the modified layer formation step, the focusing point of a second laser beam with a wavelength that does not exceed the processing threshold of the wafer and is transparent to the wafer is positioned inside the wafer or on the front side, and irradiation is performed from the back side of the wafer. The imaging step involves using an imaging unit to image the reflected light from the front surface of the wafer, which is irradiated by the second laser beam irradiated during the observation laser beam irradiation step; and The determination step involves identifying the shape and position of the reflected light in the image captured during the imaging step to determine the processing status of the wafer. In the observation laser beam irradiation step, the second laser beam irradiating the wafer is shaped such that the cross-sectional shape in the plane perpendicular to the direction of travel of the second laser beam is asymmetrical across the modified layer.

2. The wafer processing method according to claim 1, characterized in that, In this determination step, In the image captured through this imaging step, if the reflected light appears to overlap with an area of ​​the same shape as the irradiated area on the back side of the wafer irradiated by the second laser beam irradiated on the wafer during the observation laser beam irradiation step, it is determined that a crack extends from the modified layer toward the front side of the wafer. In the image captured by this imaging step, if the reflected light appears to overlap with the shape of the irradiated area on the back side of the second laser beam that will irradiate the wafer in the observation laser beam irradiation step, it is determined that the crack does not extend from the modified layer toward the front side of the wafer.

3. A method for processing a wafer, wherein a modified layer is formed inside the wafer along the predetermined dividing lines formed on the front side, characterized in that, The wafer fabrication method includes the following steps: The holding step involves positioning the front side of the wafer against the chuck stage, using the chuck stage to hold the wafer. In the modified layer formation step, the focus point of a first laser beam with a wavelength that is transparent to the wafer is positioned inside the wafer and the laser beam irradiation unit and the chuck stage are moved relative to each other in the direction along the predetermined dividing line, while the first laser beam is irradiated from the back side of the wafer along the predetermined dividing line to form the modified layer inside the wafer. In the laser beam irradiation step, after the modified layer formation step, the focusing point of a second laser beam with a wavelength that does not exceed the processing threshold of the wafer and is transparent to the wafer is positioned inside the wafer or on the front side, and irradiation is performed from the back side of the wafer. The imaging step involves using an imaging unit to image the reflected light from the front surface of the wafer, which is irradiated by the second laser beam irradiated during the observation laser beam irradiation step; and The determination step involves identifying the shape and position of the reflected light in the image captured during the imaging step to determine the processing status of the wafer. In the observation laser beam irradiation step, the second laser beam irradiating the wafer travels inside the wafer from a direction not perpendicular to the back surface of the wafer.

4. The wafer processing method according to any one of claims 1 to 3, characterized in that, In the observation laser beam illumination step, the second laser beam is incident on the wafer in a state where the aberrations caused by the lens forming the focal point have been corrected.

5. The wafer processing method according to any one of claims 1 to 3, characterized in that, The first laser beam and the second laser beam have the same light source.

6. The method for processing a wafer according to any one of claims 1 to 3, characterized in that, The observation was performed by immersion in liquid using a laser beam irradiation step.

7. The method for processing a wafer according to any one of claims 1 to 3, characterized in that, In this determination step, the determination is performed based on a pre-generated benchmark.

Images