Method of processing wafer

Abstract

To provide a method of processing a wafer, in which the residue of a chamfered portion does not remain in the corner between the side wall of one wafer of a bonded wafer and a bonding surface, in a case of removing the chamfered portion from the one wafer of the bonded wafer.SOLUTION: A method of processing a wafer includes: a first modified layer forming step of applying a laser beam LB to an inner side adjacent to a chamfered portion 18 formed on an outer periphery of one wafer 4 by focusing the laser beam LB and transmitting the laser beam LB through the one wafer 4 to form a ring-shaped modified layer 28; and a second modified layer forming step of applying the laser beam LB to a bonded surface of the one wafer 4 and the other wafer 6, by focusing the laser beam LB and transmitting the lased beam LB through the one wafer 4 to form a bonding-force reducing modified layer 30 to reduce the bonding force. In the second modified layer forming step, the bonding-force reducing modified layer 30 is formed to be connected with a bottom portion of the ring-shaped modified layer 28 which is already formed in the first modified layer forming step, or with a bottom portion of the ring-shaped modified layer 28 which is formed after the second modified layer forming step.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a wafer processing method for processing a bonded wafer in which one wafer and another wafer are bonded together. [Background technology]

[0002] A wafer has multiple devices such as ICs and LSIs formed on its surface, separated by planned dividing lines. The back surface is ground to a specified thickness, and then the wafer is divided into individual device chips using a dicing machine and laser processing machine. Each of these device chips is then used in electrical equipment such as mobile phones and personal computers.

[0003] Because a chamfer is formed on the outer periphery of a wafer, when the back surface of the wafer is ground to thin it, the chamfer becomes a sharp knife-edge. This requires operators to exercise extra caution when handling the wafer. Furthermore, when the chamfer becomes a sharp knife-edge, cracks tend to form from the outer periphery of the wafer to the inside, increasing the risk of device damage. To address this issue, the present applicant has proposed a technology in which a laser beam is irradiated onto the chamfer of the wafer to form a ring-shaped modified layer and then remove the chamfer (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-88187 Summary of the Invention [Problem to be solved by the invention]

[0005] There is a technology in which one wafer is bonded to another wafer to improve device functionality, and then the backside of one wafer is ground to a desired thickness. In this technology, when the chamfered portion is removed from one wafer, a residue of the chamfer remains at the corner between the sidewall and the bonding surface of the other wafer, which becomes a source of contamination.

[0006] An object of the present invention is to provide a wafer processing method in which, when a chamfered portion is removed from one of the wafers of bonded wafers, no residue of the chamfered portion is left at the corner between the sidewall and the bonding surface of one of the wafers. [Means for solving the problem]

[0007] According to the present invention, there is provided the following wafer processing method that solves the above-mentioned problems. "A wafer processing method for processing a bonded wafer in which one wafer and another wafer are bonded, a first modified layer forming step of irradiating one of the wafers with a laser beam by positioning a focal point of the laser beam inside adjacent to a chamfered portion formed on the outer periphery of the wafer and forming a ring-shaped modified layer; a second modified layer forming step of irradiating a bonding surface between one wafer and the other wafer with a laser beam by positioning a focal point of the laser beam passing through one wafer to form a bonding strength reducing modified layer that reduces the bonding strength, A wafer processing method is provided in which, in the second modified layer formation step, the bonding strength reducing modified layer is formed so as to be connected to the bottom of the ring-shaped modified layer already formed in the first modified layer formation step or the ring-shaped modified layer formed after the second modified layer formation step.

[0008] Preferably, the method includes a fluid supplying step of supplying a fluid that weakens the bonding strength to the bonding surface between one wafer and the other wafer from the outer periphery of the bonded wafers, allowing the fluid to penetrate into the region leading to the bonding strength reducing modified layer. The same laser beam may be used in the first modified layer forming step and the second modified layer forming step. The fluid supplying step is desirably carried out before the second modified layer forming step, after the second modified layer forming step, or simultaneously with the second modified layer forming step.

[0009] It is preferable to include a chamfer removing step of removing a chamfer from one of the wafers after the first modified layer forming step and the second modified layer forming step.When the top surface of one of the wafers is ground to thin it after the first modified layer forming step and the second modified layer forming step, a grinding step of removing a chamfer from one of the wafers may be included.

[0010] One wafer and the other wafer are bonded together by a Si-O-Si siloxane bond, and the fluid that weakens the bonding force contains any one of water, water vapor, and mist, and the Si-O-Si bond is converted into a Si-OH-OH-Si bond, thereby weakening the bonding force in the fluid supplying step. [Effects of the Invention]

[0011] The wafer processing method of the present invention includes: A wafer processing method for processing a bonded wafer in which one wafer and another wafer are bonded together, comprising: a first modified layer forming step of irradiating one of the wafers with a laser beam by positioning a focal point of the laser beam inside adjacent to a chamfered portion formed on the outer periphery of the wafer and forming a ring-shaped modified layer; a second modified layer forming step of irradiating a bonding surface between one wafer and the other wafer with a laser beam by positioning a focal point of the laser beam passing through one wafer to form a bonding strength reducing modified layer that reduces the bonding strength, In the second modified layer formation process, the bonding strength reducing modified layer is formed so as to connect to the bottom of the ring-shaped modified layer already formed in the first modified layer formation process or the ring-shaped modified layer formed after the second modified layer formation process, so that when the chamfered portion is removed from one of the wafers of the bonded wafers, no residue of the chamfered portion remains at the corner between the sidewall and the bonding surface of one of the wafers. [Brief explanation of the drawings]

[0012] [Figure 1] (a) A perspective view of the bonded wafer, (b) A partial cross-sectional view of the wafer shown in (a). [Figure 2] (a) An oblique view showing the first modified layer formation process, (b) a partial cross-sectional view of a bonded wafer in which a ring-shaped modified layer is formed on one of the wafers, and (c) a partial cross-sectional view of a bonded wafer in which multiple ring-shaped modified layers are formed at different radial positions. [Figure 3] FIG. 10 is a plan view of a wafer on which radially modified layers extending radially outward from a ring-shaped modified layer are formed. [Figure 4] (a) An oblique view showing the second modified layer formation process, (b) a partial cross-sectional view of a bonded wafer in which a bonding strength reducing modified layer is formed on the bonding surface between one wafer and the other wafer, and (c) a partial cross-sectional view of a bonded wafer in which the second modified layer formation process has been carried out before the first modified layer formation process. [Figure 5] FIG. 10A is a perspective view showing a fluid supplying step, FIG. 10B is a side view showing the fluid supplying step, and FIG. 10C is a perspective view showing a state in which the fluid supplying step is being carried out simultaneously with the second modified layer forming step. [Figure 6] FIG. [Figure 7] (a) A perspective view showing the start of the grinding process, (b) A perspective view showing the end of the grinding process, and (c) A perspective view of the bonded wafers in which the chamfered portion of one wafer has been removed by the grinding process. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, preferred embodiments of the wafer processing method according to the present invention will be described with reference to the drawings.

[0014] (Bonded wafer 2) 1(a) and 1(b) show a disk-shaped bonded wafer 2 that can be processed by the method of the present invention. The bonded wafer 2 is made by stacking two wafers, one 4 and the other 6, into one.

[0015] The first wafer 4 and the second wafer 6 are made of silicon (Si) and may have a diameter of approximately 200 mm and a thickness of approximately 700 μm. As shown in FIG. 1(a), the surface 6a of the second wafer 6 has a device region 12 in which a plurality of devices 8, such as ICs and LSIs, are partitioned by lattice-shaped division lines 10, and a peripheral surplus region 14 surrounding the device region 12. For convenience, FIG. 1(a) shows a ring-shaped boundary 16 between the device region 12 and the peripheral surplus region 14 with a two-dot chain line, but in reality, there is no line indicating the boundary 16. Although not shown, the surface 4a of the first wafer 4 has the same configuration as the surface 6a of the second wafer 6. Furthermore, a curved chamfered portion 18 is formed on the outer periphery of each of the first wafer 4 and the second wafer 6, and a notch 20 indicating the crystal orientation is formed.

[0016] When forming the bonded wafer 2, the device region 12 of one wafer 4 is bonded to the device region 12 of the other wafer 6. At this time, the notch 20 of one wafer 4 is aligned with the notch 20 of the other wafer 6, and the one wafer 4 and the other wafer 6 are bonded together in a state where the crystal orientation of the one wafer 4 and the crystal orientation of the other wafer 6 are aligned. After bonding the one wafer 4 and the other wafer 6, it is preferable to perform a heat treatment to bond the one wafer 4 and the other wafer 6 together through siloxane bonding. The siloxane bond is a Si-O-Si bond in which silicon (Si) and oxygen (O) are alternately bonded, and a strong bond can be maintained even at high temperatures.

[0017] (First modified layer forming step) In this embodiment, first, a first modified layer formation process is carried out in which a laser beam is transmitted through one of the wafers 4, the focal point of the laser beam being positioned inside adjacent to the chamfered portion 18 formed on the outer periphery of the wafer 4, and the laser beam is irradiated to form a ring-shaped modified layer.

[0018] The first modified layer forming step can be performed using, for example, a laser processing device 22 shown in Fig. 2(a) The laser processing device 22 includes a chuck table 24 that suction-holds the wafer, an oscillator (not shown) that oscillates a pulsed laser beam LB having a wavelength that is transparent to the bonded wafer 2, and a condenser 26 that condenses the laser beam LB oscillated by the oscillator and irradiates one of the wafers 4 with the condensed laser beam LB.

[0019] In the first modified layer forming step, first, the bonded wafer 2 is suction-held on the upper surface of the chuck table 24. At this time, the bonded wafer 2 is placed on the upper surface of the chuck table 24 with the rotation center of the chuck table 24 and the center of the bonded wafer 2 aligned. At this time, one of the wafers 4 from which the chamfered portion 18 is to be removed is positioned on top. Then, a suction force is generated on the upper surface of the chuck table 24 by a suction means (not shown), and the back surface 6b of the other wafer 6 is suction-held on the upper surface of the chuck table 24.

[0020] Once the bonded wafer 2 is suction-held on the chuck table 24, a processing line along which the laser beam LB should be irradiated is set. To do this, an image of one of the wafers 4 is captured from above using an imaging means (not shown) of the laser processing device 22, and the outer periphery and center position of the one of the wafers 4 are detected based on the image of the one of the wafers 4 captured by the imaging means. Next, based on the detected outer periphery and center position of the one of the wafers 4, a ring-shaped line located inside and adjacent to the chamfered portion 18 formed on the outer periphery of the one of the wafers 4 is set as the processing line. For example, if the chamfered portion 18 is formed in a ring-shaped region about 2 mm wide from the outer periphery of the one of the wafers 4, a ring-shaped line located 2.5 mm radially inward from the outer periphery of the one of the wafers 4 can be set as the processing line.

[0021] After setting the line to be processed along which the laser beam LB is to be irradiated, the focal point of the laser beam LB is positioned at a required position on the line to be processed. At this time, the height of the back surface 4b of one of the wafers 4 (i.e., the height of the upper surface of the bonded wafer 2) is detected by a height detection means (not shown) of the laser processing device 22. Then, using the detected height of the back surface 4b as a reference, the focal point of the laser beam LB is positioned at a required position on the line to be processed inside the one of the wafers 4 (inside the outer peripheral excess region 14) after passing through the one of the wafers 4.

[0022] Once the focal point of the laser beam LB is positioned at the required position, one of the wafers 4 is irradiated with the laser beam LB having a wavelength that is transparent to the bonded wafers 2, thereby forming a ring-shaped modified layer 28 along the chamfered portion 18. That is, by irradiating the laser beam LB onto one of the wafers 4 while rotating the chuck table 24 in the direction indicated by the arrow R1 in Figure 2(a), a ring-shaped modified layer 28 is formed along the entire circumference of the ring-shaped intended processing line.

[0023] After one ring-shaped modified layer 28 (one circumference) has been formed, the height position of the focal point of the laser beam LB is shifted to a shallower position, and the laser beam LB is irradiated onto one of the wafers 4 in the same manner as above. By repeating this process of shifting the height position of the focal point and irradiating the laser beam LB, multiple ring-shaped modified layers 28 are formed at intervals in the vertical direction, as shown in Figure 2(b). Note that vertically adjacent modified layers 28 may be connected by cracks (not shown) extending from the modified layers 28.

[0024] Alternatively, after forming one ring-shaped modified layer 28 (one circumference), multiple ring-shaped modified layers 28 with different depths may be formed at different radial positions, as shown in Fig. 2(c). That is, multiple ring-shaped modified layers 28 may be formed while changing the radial position so that the multiple ring-shaped modified layers 28 as a whole are inclined radially outward from the back surface 4b (upper surface) toward the front surface 4a (lower surface) of one wafer 4. In this case, the previously formed modified layer 28 does not interfere with the focusing of the laser beam LB for forming the next modified layer 28, so that the formation of the ring-shaped modified layer 28 can be started from a shallow position (a position close to the back surface 4b of one wafer 4).

[0025] The first modified layer forming step can be performed, for example, under the following processing conditions: The defocus described below is the amount of movement of the condenser 26 when the condenser 26 is moved toward the bonded wafer 2 from a state in which the focal point of the laser beam LB is positioned on the back surface 4b (exposed surface) of one wafer 4. Laser beam wavelength: 1342nm Repetition frequency: 80kHz Focus point feed speed: 60 rpm Average output: 2W Defocus: 700μm, 500μm, 300μm, 150μm

[0026] In the first modified layer forming step, modified layers 29 may further be formed radially from the ring-shaped modified layer 28 (see FIG. 3). That is, a plurality of linear modified layers 29 (three in this embodiment) extending radially outward from the ring-shaped modified layer 28 to the outer periphery of one wafer 4 may be formed radially at equal intervals in the circumferential direction inside one wafer 4. It is preferable that the radial modified layers 29 are formed in a plurality of layers spaced apart in the vertical direction, similar to the ring-shaped modified layer 28. By forming such radial modified layers 29, the chamfered portion 18 can be finely divided and effectively removed in the chamfered portion removing step described later.

[0027] (Second modified layer forming step) After the first modified layer formation process is carried out, a second modified layer formation process is carried out in which a laser beam is irradiated onto the bonding surface between one wafer 4 and the other wafer 6 by positioning the focal point of the laser beam through one wafer 4 and forming a bonding strength reducing modified layer that reduces the bonding strength.

[0028] The second modified layer forming process can be performed using the above-mentioned laser processing device 22. In other words, the same laser beam LB can be used in the first and second modified layer forming processes. Specifically, the wavelength, repetition frequency, feed speed of the focal point, and average output of the laser beam LB can be the same in the first and second modified layer forming processes.

[0029] In the second modified layer forming step, the focal point of the laser beam LB is positioned at a required position while continuing to suction and hold the bonded wafer 2 by the chuck table 24. Specifically, the focal point of the laser beam LB is positioned on the outer periphery of the bonding surface between one wafer 4 and the other wafer 6, passing through one wafer 4.

[0030] After the focal point of the laser beam LB is positioned at the required position, the bonding surface is irradiated with the laser beam LB having a wavelength that is transparent to the bonded wafers 2, thereby forming a ring-shaped bonding strength reducing modified layer 30 that reduces the bonding strength of the bonded wafers 2. That is, the laser beam LB is irradiated onto the bonding surfaces of the bonded wafers 2 while rotating the chuck table 24 in the direction shown by the arrow R1 in Figure 4(a), thereby forming the ring-shaped bonding strength reducing modified layer 30.

[0031] After forming one bonding strength reducing modified layer 30 (one circumference), the radial position of the focal point of the laser beam LB is changed, and the laser beam LB is irradiated onto the bonding surface in the same manner as described above. By repeatedly changing the radial position of the focal point and irradiating the laser beam LB in this manner, multiple bonding strength reducing modified layers 30 are formed in a ring shape within a range of a predetermined distance (e.g., approximately 30 μm to 300 μm) radially outward from the lower end of the ring-shaped modified layer 28 (see FIG. 4( b)). That is, in the second modified layer forming process, the bonding strength reducing modified layer 30 is formed so as to connect to the bottom of the ring-shaped modified layer 28 already formed in the first modified layer forming process. Note that adjacent bonding strength reducing modified layers 30 in the radial direction may be connected by cracks (not shown) extending from the bonding strength reducing modified layers 30. Therefore, the bonding strength reducing modified layers 30 can be formed at intervals in the radial direction.

[0032] The second modified layer forming step can be carried out, for example, under the following processing conditions. Laser beam wavelength: 1342nm Repetition frequency: 80kHz Focus point feed speed: 60 rpm Average output: 2W Defocus: 700μm±30μm

[0033] In this embodiment, the second modified layer forming step is performed after the first modified layer forming step, but the second modified layer forming step may be performed before the first modified layer forming step. That is, the bonding strength reducing modified layer 30 may be formed before forming the ring-shaped modified layer 28 (see FIG. 4(c)). In this case, the bonding strength reducing modified layer 30 is formed so as to be connected to the bottom of the ring-shaped modified layer 28 formed after the second modified layer forming step.

[0034] (Fluid supply process) After the second modified layer formation process is performed, a fluid supply process is performed in which a fluid that weakens the bonding strength is supplied to the bonding surface where one wafer 4 and the other wafer 6 are bonded from the outer periphery of the bonded wafer 2, and the fluid penetrates into the area leading to the bonding strength reducing modified layer 30.

[0035] The fluid supplying step can be performed using, for example, a fluid supplying device 34 shown in Fig. 5(a). The fluid supplying device 34 includes a movable nozzle 36 and a fluid supply source (not shown) that supplies fluid F to the nozzle 36. The fluid supplying device 34 can be attached to the laser processing device 22. In this embodiment, an example in which the fluid supplying device 34 is attached to the laser processing device 22 will be described.

[0036] In the fluid supplying step, first, the chuck table 24 is moved near the nozzle 36 of the fluid supply device 34. Next, the tip of the nozzle 36 is positioned to the side of the bonded wafer 2. Next, the chuck table 24 is rotated in the direction indicated by the arrow R1 in FIG. 5(a). Next, a fluid F is supplied from a fluid supply source to the nozzle 36, and as shown in FIGS. 5(a) and 5(b), the fluid F (e.g., pure water) that weakens the bonding strength of the bonding surfaces of the bonded wafers 2 is supplied from the nozzle 36. This causes the fluid F to permeate the region from the outer periphery of the bonded wafer 2 to the bonding strength reducing modified layer 30. Note that the fluid F is not limited to a liquid form and may be in the form of vapor or mist.

[0037] As described above, the bonding surfaces of the bonded wafer 2 are bonded by siloxane bonds (Si-O-Si bonds). Therefore, by supplying the fluid F from the side of the bonded wafer 2 toward the bonding surfaces of the bonded wafer 2, the fluid F gradually permeates the bonding surfaces from the outer periphery of the bonded wafer 2, and the region into which the fluid F has permeated changes to an Si-OH-OH-Si bond. As a result, the bonding strength in the region into which the fluid F has permeated is weakened, and a ring-shaped bond-weakened region 37 is formed on the outer periphery of the bonding surfaces.

[0038] In this embodiment, the fluid supplying step is performed after the second modified layer forming step, but the fluid supplying step may be performed before the second modified layer forming step, or alternatively, as shown in FIG. 5(c), the second modified layer forming step and the fluid supplying step may be performed simultaneously.

[0039] (chamfered portion removal process) After the fluid supplying step is performed, a chamfer removing step is performed to remove the chamfer 18 from one of the wafers 4 (see FIG. 6). However, if the fluid supplying step is omitted, the chamfer removing step is performed after the first modified layer forming step and the second modified layer forming step are performed.

[0040] In the chamfered portion removing step, the chamfered portion 18 can be removed from one of the wafers 4 by applying an external force to the chamfered portion 18 of the one of the wafers 4 using an appropriate external force applying means. For example, the external force can be applied to the chamfered portion 18 by spraying a high-pressure fluid such as water or air onto the chamfered portion 18 from the outer periphery of the bonded wafers 2. Note that while Fig. 6 shows an example in which the chamfered portion 18 is removed from one of the wafers 4 while maintaining its ring-shaped shape, the chamfered portion 18 may be removed in the form of multiple broken pieces.

[0041] As described above, in this embodiment, the ring-shaped modified layer 28 and the bonding strength reducing modified layer 30 are formed in the first and second modified layer forming steps, so that the chamfered portion 18 can be completely removed from one wafer 4, using the ring-shaped modified layer 28 and the bonding strength reducing modified layer 30 as interfaces. Therefore, when the chamfered portion 18 is removed from one wafer 4, no residue of the chamfered portion 18 remains at the corner C between the sidewall and the bonding surface of one wafer 4. Furthermore, when the fluid supplying step is performed to form the bonding strength weakened region 37 on the outer periphery of the bonding surface, the chamfered portion 18 can be removed even more effectively.

[0042] (Grinding process) Instead of the above-mentioned chamfered portion removal process, a grinding process may be carried out to remove the chamfered portion 18 from one of the wafers 4 when grinding and thinning the top surface of one of the wafers 4 after carrying out the first modified layer formation process and the second modified layer formation process or after carrying out the fluid supply process.

[0043] The grinding step can be performed using, for example, a grinding device 38 shown in Figures 7(a) and 7(b). The grinding device 38 includes a chuck table 40 that holds the bonded wafer 2 by suction, and grinding means 42 that grinds the bonded wafer 2 held by suction on the chuck table 40. The grinding means 42 includes a spindle 44 that extends vertically and a disk-shaped wheel mount 46 fixed to the lower end of the spindle 44. An annular grinding wheel 50 is fastened to the lower surface of the wheel mount 46 by bolts 48. A plurality of grinding stones 52 are fixed to the outer periphery of the lower surface of the grinding wheel 50, and are arranged in an annular shape at intervals in the circumferential direction.

[0044] In the grinding process, first, the back surface 4b of one wafer 4 is placed facing upward, and the back surface 6b of the other wafer 6 is suction-held on the upper surface of the chuck table 40. Next, the chuck table 40 is rotated in the direction indicated by arrow R2 at a predetermined rotational speed (e.g., 300 rpm). The spindle 44 is also rotated in the direction indicated by arrow R3 at a predetermined rotational speed (e.g., 6000 rpm). Next, the spindle 44 is lowered to bring the grinding wheel 52 into contact with the back surface 4b of the one wafer 4, and grinding water is supplied to the portion of the back surface 4b of the one wafer 4 where the grinding wheel 52 is in contact. Thereafter, the spindle 44 is lowered at a predetermined grinding feed rate (e.g., 0.1 μm / s). As a result, the back surface 4b of the one wafer 4 is ground to form the one wafer 4 to a desired thickness, as shown in FIGS. 7(b) and 7(c). Furthermore, during grinding, the downward force acting on the bonded wafers 2 from the grinding means 42 and the outward force of the grinding stone can be used to completely remove the chamfered portion 18 from one of the wafers 4, with the ring-shaped modified layer 28 and the bonding strength reducing modified layer 30 as interfaces.

[0045] As described above, in this embodiment, the ring-shaped modified layer 28 and the bonding strength reducing modified layer 30 are formed in the first and second modified layer forming steps, so that the chamfered portion 18 can be completely removed from one wafer 4, with the ring-shaped modified layer 28 and the bonding strength reducing modified layer 30 serving as interfaces. Therefore, when the chamfered portion 18 is removed from one wafer 4, no residue of the chamfered portion 18 remains at the corner C between the sidewall and the bonding surface of one wafer 4. [Explanation of symbols]

[0046] 2: Bonded wafer 4: One wafer 6: The other wafer 18: Chamfered part 28: Ring-shaped modified layer 30: Bond strength reduction modified layer F:Fluid

Claims

1. A wafer processing method for processing a bonded wafer in which one wafer and another wafer are bonded together, comprising: a first modified layer forming step of irradiating one of the wafers with a laser beam by positioning a focal point of the laser beam inside adjacent to a chamfered portion formed on the outer periphery of the wafer and forming a ring-shaped modified layer; a second modified layer forming step of irradiating a bonding surface between one wafer and the other wafer with a laser beam by positioning a focal point of the laser beam passing through one wafer to form a bonding strength reducing modified layer that reduces the bonding strength, A wafer processing method in which, in the second modified layer formation process, the bonding strength reducing modified layer is formed so as to connect to the bottom of the ring-shaped modified layer already formed in the first modified layer formation process or the ring-shaped modified layer formed after the second modified layer formation process.

2. 2. The wafer processing method according to claim 1, further comprising a fluid supply step of supplying a fluid that weakens the bonding strength to the bonding surface where one wafer and the other wafer are bonded from the outer periphery of the bonded wafers, allowing the fluid to penetrate into the region that reaches the bonding strength reducing modified layer.

3. 2. The wafer processing method according to claim 1, wherein the same laser beam is used in the first modified layer forming step and the second modified layer forming step.

4. 3. The wafer processing method according to claim 2, wherein the fluid supplying step is carried out before the second modified layer forming step, after the second modified layer forming step, or simultaneously with the second modified layer forming step.

5. 2. The wafer processing method according to claim 1, further comprising a chamfer removing step of removing a chamfer from one of the wafers after the first modified layer forming step and the second modified layer forming step.

6. 2. The wafer processing method according to claim 1, further comprising a grinding step of removing a chamfered portion from one of the wafers when grinding the top surface of the one of the wafers to thin it after the first modified layer forming step and the second modified layer forming step.

7. One wafer and the other wafer are bonded together by a Si—O—Si siloxane bond, the fluid that weakens the bonding force includes any one of water, water vapor, and mist; 3. The wafer processing method according to claim 2, wherein the bonding strength is weakened in the fluid supplying step by converting Si--O--Si bonds into Si--OH--OH--Si bonds.

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