Laser processing method and laser processing device

Abstract

Provided are a laser processing method and a laser processing device that make it possible to divide a wafer along a division line with high precision while reducing splash damage in laser processing for forming a modified layer inside a wafer by irradiating the division line with a laser. This laser processing method comprises: a first laser processing step for forming a modified layer along a first division line; and a second laser processing step for forming a modified layer along a second division line that intersects the first division line, the second laser processing step being performed after the first laser processing step. In the second laser processing step, a first modified layer 15A and a second modified layer 15B having different positions in the thickness direction of a wafer 10 are formed, and, at the intersection of the first division line 11 and the second division line, the distance to the first division line 11 from the second modified layer 15B is made greater than that from the first modified layer 15A.

Description

Laser processing method and laser processing device

[0001] The present invention relates to a laser processing method and a laser processing apparatus for irradiating a wafer with a laser to form a modified layer inside the wafer.

[0002] Wafers on which various elements such as semiconductor devices and electronic components are formed are divided into individual chips by dividing them along grid-like dividing lines. In relation to such wafer division, for example, Patent Document 1 discloses a technology for forming a first modified layer and a second modified layer at different positions in the thickness direction of the wafer inside the wafer by laser processing in which a laser is irradiated along the dividing lines.

[0003] JP 2018-63987 A

[0004] However, splash damage, which is heat damage caused by laser processing, may occur in wafers subjected to laser processing. Such splash damage is likely to occur at intersections of dividing lines. In Patent Document 1, at the intersections of dividing lines, one of the dividing lines is designated as a non-processing area, but there is still room for improvement in reducing splash damage and accurately dividing the wafer along the dividing lines.

[0005] A laser processing method that solves the above problem includes irradiating a laser along a dividing line to form a modified layer inside a wafer. The laser processing method includes a first laser processing step of forming a modified layer along a first dividing line extending in a first direction, and a second laser processing step, which is performed after the first laser processing step, of forming a modified layer along a second dividing line extending in a second direction intersecting the first dividing line. In the second laser processing step, a first modified layer and a second modified layer are formed at different positions in the thickness direction of the wafer, and the distance between the first dividing line in the second direction at the intersection of the first dividing line and the second dividing line is made different between the first modified layer and the second modified layer.

[0006] A laser processing apparatus that solves the above problem includes a work table that supports a wafer, a laser irradiation unit that irradiates the wafer with a laser, a relative movement mechanism that moves the work table and the laser irradiation unit relative to one another, and a control device that controls the laser irradiation unit and the relative movement mechanism so that a modified layer is formed inside the wafer along a division line, wherein the control device performs a first laser processing process that forms a modified layer along a first division line that extends in a first direction, and after the first laser processing process, a second laser processing process that forms a modified layer along a second division line that extends in a second direction that intersects the first direction and intersects the first division line, and in the second laser processing process, a first modified layer and a second modified layer that are located at different positions in the thickness direction of the wafer are formed, and at the intersection of the first division line and the second division line, the distance between the first division line in the second direction is made different between the first modified layer and the second modified layer.

[0007] According to the present invention, it is possible to accurately divide a wafer along a dividing line while reducing splash damage to the wafer caused by laser processing.

[0008] FIG. 1 is a plan view schematically showing a wafer on which laser processing is performed using an embodiment of a laser processing method in the first embodiment. FIG. 2 is a diagram schematically showing a wafer on which a first modified layer and a second modified layer are formed along a first division line, together with a schematic configuration of an embodiment of a laser processing apparatus in the first embodiment. FIG. 3 is a functional block diagram showing the electrical configuration of the laser processing apparatus in the first embodiment. FIG. 4 is a flowchart showing a procedure of an embodiment of a laser processing method in the first embodiment. FIG. 5 is a flowchart showing an example of a procedure of a first laser processing process in the first embodiment. FIG. 6 is a flowchart showing an example of a procedure of a second laser processing process in the first embodiment. In the first embodiment, FIG. 7(a) is a diagram schematically showing an OFF position for the first modified layer in the second laser processing, and FIG. 7(b) is a diagram schematically showing an ON position for the first modified layer in the second laser processing. In the first embodiment, FIG. 8(a) is a diagram schematically showing an OFF position for the second modified layer in the second laser processing, and FIG. 8(b) is a diagram schematically showing an ON position for the second modified layer in the second laser processing. FIG. 9 is a functional block diagram showing the electrical configuration of the laser processing apparatus in the second embodiment. FIG. 10 is a functional block diagram showing the electrical configuration of the laser processing apparatus in the second embodiment. FIG. 11 is a diagram schematically showing the processing direction in the second laser processing step in the third embodiment. In the third embodiment, FIG. 12(a) is a diagram schematically showing how the first modified layer is formed while moving the work table in the +X direction, and FIG. 12(b) is a diagram schematically showing how the first modified layer is formed while moving the work table in the -X direction. In the third embodiment, Fig. 13(a) is a diagram schematically showing how a second modified layer is formed while moving the work table in the +X direction, and Fig. 13(b) is a diagram schematically showing how the second modified layer is formed while moving the work table in the -X direction. Fig. 14 is a cross-sectional view showing an example of a wafer on which a third modified layer and a fourth modified layer have been formed in a modified example. Fig. 15 is a cross-sectional view showing an example of a wafer on which a fifth modified layer and a sixth modified layer have been formed in a modified example.

[0009] First Embodiment A first embodiment of a laser processing method will be described with reference to Figures 1 to 8. As shown in Figure 1, a wafer 10 is divided into individual chips by being divided along a grid of dividing lines formed by first dividing lines 11 extending in a first direction D1 and second dividing lines 12 extending in a second direction D2. The surface of the wafer 10 on which various elements such as semiconductor devices and electronic components are formed is called the device surface 10a, and the surface opposite the device surface 10a is called the back surface 10b (see Figure 2). The intersections of the first dividing lines 11 and the second dividing lines 12 are called intersections 13.

[0010] In the following description, the planar directions in which the device surface 10a and back surface 10b of the wafer 10 extend are referred to as the XY directions, one direction in the XY directions is referred to as the X direction, and the direction perpendicular to the X direction in the XY directions is referred to as the Y direction. The X direction is the direction in which the parting lines to be processed in each process extend. Regarding the X direction, the direction indicated by the arrow in the figure may be referred to as the +X direction, and the direction opposite to the +X direction as the -X direction. The direction perpendicular to the XY directions is referred to as the Z direction. The Z direction is the thickness direction of the wafer 10.

[0011] 2, the wafer 10 is subjected to laser processing using a laser processing device 20. The laser processing device 20 has a work table 21, a laser irradiation unit 22, a relative movement mechanism 23, an imaging device 24, and a control device 25.

[0012] The work table 21 supports the wafer 10 via the dicing tape 16. The work table 21 is configured to be movable in the X direction and rotatable around the Z direction as a rotation axis by a relative movement mechanism 23. The movement speed of the work table 21 in the X direction is called the laser processing speed.

[0013] The laser irradiation unit 22 irradiates the wafer 10 supported on the work table 21 with a laser LB. The laser irradiation unit 22 is configured to be movable in the Y direction and the Z direction by a relative movement mechanism 23. The laser irradiation unit 22 is configured to be able to temporarily interrupt irradiation of the wafer 10 with the laser LB by shutter control or the like.

[0014] The laser irradiation unit 22 includes a laser emission unit 26 and a condenser lens 27. The laser emission unit 26 and the condenser lens 27 are configured to be relatively movable in the Z direction by a movement mechanism (not shown). The laser emission unit 26 emits a laser beam LB of a predetermined wavelength toward the condenser lens 27. The laser beam LB is a pulsed laser. The condenser lens 27 focuses the laser beam LB emitted by the laser emission unit 26 at a focal point P. The focal point P is the irradiation position of the laser beam LB. A modified layer, in which the crystalline structure is destroyed, is formed near the focal point P inside the wafer 10 irradiated with the laser beam LB. The modified layer is a region that is more fragile than a portion not subjected to laser processing. It is preferable that the laser irradiation unit 22 use an optical element, such as an acousto-optic modulator, that can quickly turn the laser beam LB on and off.

[0015] The irradiation position of the laser LB in the X and Y directions is adjusted by the relative movement between the work table 21 and the laser irradiation unit 22. The irradiation position of the laser LB in the Z direction is adjusted by the movement of the laser irradiation unit 22 in the Z direction or the relative movement between the laser emission unit 26 and the condenser lens 27 in the Z direction.

[0016] The imaging device 24 is attached to the laser irradiation unit 22. The imaging device 24 captures an image of the alignment mark provided on the wafer 10. The imaging device 24 outputs image data indicating the captured image to the control device 25.

[0017] The control device 25 controls the laser irradiation unit 22, the relative movement mechanism 23, and the imaging device 24 to comprehensively control the operation of the laser processing device 20. The laser processing device 20 performs laser processing on a wafer 10 whose device surface 10a is supported on a work table 21 via a dicing tape 16. In this case, the back surface 10b of the wafer 10 is the incident surface of the laser LB. The laser processing device 20 performs laser processing on each first division line 11 and then performs laser processing on each second division line 12. At each division line 11, 12, the laser processing device 20 forms first modified layers 15A aligned near the device surface 10a (farther from the incident surface of the laser LB) and then forms second modified layers 15B aligned near the back surface 10b. Note that FIG. 2 illustrates the wafer 10 in a state in which modified layers 15A and 15B have been formed along the first division line 11. That is, the X direction is the first direction D1, and the Y direction is the second direction D2.

[0018] (Controller) The controller 25 can be realized, for example, by circuitry, i.e., one or more dedicated hardware circuits such as an ASIC, one or more processing circuits that operate according to a computer program (software), or a combination of both. The processing circuit has a CPU and memory (such as ROM and RAM) that stores programs executed by the CPU. Memory, i.e., computer-readable media, includes any available media that can be accessed by a general-purpose or dedicated computer.

[0019] 3, the control device 25 is electrically connected to the laser irradiation unit 22, the relative movement mechanism 23, the imaging device 24, and also to an input unit 29. The input unit 29 receives various information input by an operator to the laser processing device 20. The input unit 29 is configured with, for example, a mouse, a keyboard, a touch panel, etc.

[0020] The operator inputs laser processing conditions through the input unit 29. Specifically, the operator inputs, as the laser processing conditions, the output power and repetition frequency of the laser LB, the position of the first modified layer 15A in the Z direction, the position of the second modified layer 15B in the Z direction, the laser processing speed, etc. The operator also inputs, as the laser processing conditions, the first distance L1 and the second distance L2 to be used in the second laser processing process described below.

[0021] The control device 25 has, as functional units that function by executing various programs, an alignment detection unit 31 and a laser processing control unit 32. The alignment detection unit 31 executes an alignment detection process to detect the positions of the first division lines 11 and the second division lines 12 on the wafer 10.

[0022] For example, in the alignment detection process, the alignment detection unit 31 controls the relative movement mechanism 23 to adjust the position of the image capture device 24 relative to the alignment marks provided on the wafer 10. Next, the alignment detection unit 31 photographs the alignment marks using the image capture device 24 and obtains image data representing the photographed image. Then, the alignment detection unit 31 performs image processing on the image data to detect the positions of each of the first division lines 11 and each of the second division lines 12.

[0023] The laser processing control unit 32 executes a first laser processing process to form modified layers 15A, 15B along each of the first dividing lines 11, and a second laser processing process to form modified layers 15A, 15B along each of the second dividing lines 12. The laser processing control unit 32 holds the laser processing conditions and the positions of each dividing line 11, 12 based on the alignment detection process. The laser processing control unit 32 executes each laser processing process by controlling the relative movement mechanism 23 and the laser irradiation unit 22 based on the laser processing conditions and the positions of each dividing line 11, 12.

[0024] (Laser Processing Method) As shown in FIG. 4, the laser processing method includes an alignment detection step (step S101), a first laser processing step (step S102), and a second laser processing step (step S103).

[0025] In the alignment detection step, the alignment detection unit 31 of the control device 25 executes the above-described alignment detection process, thereby detecting the positions of the first dividing lines 11 and the second dividing lines 12 on the wafer 10.

[0026] In the first laser processing step, the laser processing control unit 32 of the control device 25 executes a first laser processing process. By the first laser processing process, a first modified layer 15A and a second modified layer 15B are formed on the wafer 10 along each of the first dividing lines 11.

[0027] In the second laser processing step, the laser processing control unit 32 of the control device 25 executes the second laser processing. By the second laser processing, a first modified layer 15A and a second modified layer 15B are formed on the wafer 10 along each of the second dividing lines 12.

[0028] 5, in the first laser processing, the laser processing control unit 32 first aligns the first division line 11 to be processed with the laser irradiation unit 22 (step S201). Specifically, the laser processing control unit 32 rotates the work table 21 about the Z direction as the rotation axis so that the first division line 11 to be processed extends in the X direction. The laser processing control unit 32 also moves the work table 21 in the X and Y directions so that the laser irradiation unit 22 is positioned above the end of the first division line 11 in the +X direction.

[0029] Next, the laser processing control unit 32 forms a first modified layer 15A along the first parting line 11 of the workpiece (step S202). Specifically, the laser processing control unit 32 adjusts the irradiation position of the laser beam LB so that the first modified layer 15A is formed. The laser processing control unit 32 then starts irradiating the laser beam LB and moving the worktable 21 along the −X direction. Then, when the worktable 21 moves to a position where the laser irradiation unit 22 is positioned above the end of the first parting line 11 in the −X direction, the laser processing control unit 32 ends irradiating the laser beam LB and moving the worktable 21. As a result, the first modified layers 15A are continuously formed along the first parting line 11 of the workpiece (see FIG. 2).

[0030] Next, the laser processing control unit 32 forms a second modified layer 15B on the first dividing line 11 of the processing target. Specifically, the laser processing control unit 32 adjusts the irradiation position of the laser LB so that the second modified layer 15B is formed. Thereafter, the laser processing control unit 32 starts irradiating the laser LB and moving the work table 21 in the +X direction. Then, when the work table 21 moves to a position where the laser irradiation unit 22 is positioned above the end of the first dividing line 11 in the +X direction, the laser processing control unit 32 ends irradiating the laser LB and moving the work table 21. As a result, second modified layers 15B are continuously formed on the back surface 10b side of the first modified layer 15A on the first dividing line 11 of the processing target, aligned in the X direction (see FIG. 2 ).

[0031] Next, the laser processing control unit 32 determines whether or not the formation of the modified layers 15A, 15B has been completed on all of the first dividing lines 11 (step S204). If the formation of the modified layers 15A, 15B has not been completed on all of the first dividing lines 11 (step S204: NO), the laser processing control unit 32 repeats steps S201 to S203 until the modified layers 15A, 15B have been formed on all of the first dividing lines 11. On the other hand, if the formation of the modified layers 15A, 15B has been completed on all of the first dividing lines 11 (step S204: YES), the laser processing control unit 32 ends the first laser processing process.

[0032] 6, in the second laser processing, the laser processing control unit 32 first aligns the second division line 12 to be processed with the laser irradiation unit 22 (step S301). Specifically, the laser processing control unit 32 rotates the work table 21 about the Z direction as the rotation axis so that the second division line 12 to be processed extends in the X direction. The laser processing control unit 32 also moves the work table 21 in the X and Y directions so that the laser irradiation unit 22 is positioned above the end of the second division line 12 in the +X direction.

[0033] Next, the laser processing control unit 32 starts forming the first modified layer 15A on the second parting line 12 of the processing target (step S302). Specifically, the laser processing control unit 32 adjusts the irradiation position of the laser LB so as to form the first modified layer 15A. Thereafter, the laser processing control unit 32 starts irradiating the laser LB and moving the work table 21 in the −X direction.

[0034] Next, the laser processing control unit 32 determines whether the moving work table 21 has reached the OFF position (step S303). As shown in Fig. 7(a), the OFF position is a position where the first modified layer 15A is formed at a position a first distance L1 away from the first dividing line 11 in the +X direction. In Fig. 7(a), the arrows overlapping the first modified layer 15A indicate the order in which the first modified layer 15A is formed. The first distance L1 is equal to or greater than the interval at which the modified layer is formed based on the laser processing conditions.

[0035] When the work table 21 reaches the OFF position (step S303: YES), the laser processing control unit 32 interrupts the irradiation of the laser LB (step S304). Next, the laser processing control unit 32 determines whether the moving work table 21 has reached the ON position (step S305).

[0036] 7(b), the ON position is a position where the first modified layer 15A is formed at a position a first distance L1 away from the first parting line 11 in the −X direction. The arrows overlapping the first modified layers 15A in FIG. 7(b) indicate the order in which the first modified layers 15A are formed. When the work table 21 reaches the ON position (step S305: YES), the laser processing control unit 32 resumes irradiation of the laser LB (step S306).

[0037] Next, the laser processing control unit 32 determines whether the formation of the first modified layer 15A along the second parting line 12 of the processing target is complete (step S307). For example, the laser processing control unit 32 determines that the formation of the first modified layer 15A is complete when the work table 21 moves to a position where the laser irradiation unit 22 is positioned above the other end of the second parting line 12 in the +X direction. If the formation of the first modified layer 15A is not complete (step S307: NO), the laser processing control unit 32 repeats steps S303 to S307.

[0038] If the formation of the first modified layer 15A is complete (step S307: YES), the laser processing control unit 32 ends the irradiation of the laser LB and the movement of the work table 21, and then starts the formation of the second modified layer 15B on the second parting line 12 of the processing target (step S308). Specifically, the laser processing control unit 32 adjusts the irradiation position of the laser LB so that the second modified layer 15B is formed, and then starts the irradiation of the laser LB and the movement of the work table 21 in the −X direction.

[0039] Next, the laser processing control unit 32 determines whether the work table 21 has reached the OFF position (step S309). As shown in FIG. 8A, the OFF position in this case is the position of the work table 21 where the second modified layer 15B is formed at a position spaced a second distance L2 from the first parting line 11 in the −X direction. In FIG. 8A, the arrows overlapping the second modified layer 15B indicate the order in which the first modified layer 15A is formed. When the work table 21 reaches the OFF position (step S309: YES), the laser processing control unit 32 interrupts the irradiation of the laser LB (step S310).

[0040] Next, the laser processing control unit 32 determines whether the work table 21 has reached the ON position (step S311). As shown in FIG. 8B, the ON position is a position where the second modified layer 15B is formed, a second distance L2 away from the first parting line 11 in the +X direction. In FIG. 8B, the arrows overlapping the second modified layer 15B indicate the order in which the second modified layer 15B is formed. When the work table 21 has reached the ON position (step S311: YES), the laser processing control unit 32 resumes irradiation of the laser LB (step S312).

[0041] Next, the laser processing control unit 32 determines whether the formation of the second modified layer 15B is complete at the second parting line 12 of the processing target (step S313). For example, the laser processing control unit 32 determines that the formation of the second modified layer 15B is complete when the work table 21 moves to a position where the laser irradiation unit 22 is positioned above the end of the second parting line 12 in the +X direction. If the formation of the second modified layer 15B is not complete (step S313: NO), the laser processing control unit 32 repeats steps S309 to S313.

[0042] On the other hand, if the formation of the second modified layer 15B has been completed (step S313: YES), the laser processing control unit 32 determines whether the formation of the modified layers 15A, 15B has been completed at all second division lines 12 after completing the irradiation of the laser LB and the movement of the work table 21 (step S314).

[0043] If the formation of the modified layers 15A, 15B has not been completed on all of the second dividing lines 12 (step S314: NO), the laser processing control unit 32 repeats steps S301 to S314 until the formation of the modified layers 15A, 15B on all of the second dividing lines 12. On the other hand, if the formation of the modified layers 15A, 15B on all of the second dividing lines 12 has been completed (step S314: YES), the laser processing control unit 32 ends the second laser processing process.

[0044] The wafer 10 on which the modified layers 15A and 15B have been formed by the above-described laser processing method is divided into individual chips along the division lines 11 and 12 by a division process such as expanding or breaking.

[0045] The operation and effects of the first embodiment will be described. (1-1) The energy of the laser LB irradiated onto the wafer 10 can be broadly divided into modification energy that modifies the area near the irradiation position, dissipation energy that dissipates as it moves toward the laser incident back surface without reaching the laser incident back surface, and excess energy that is diffused around the irradiation position and reaches the laser incident back surface, causing splash damage. Since the energy dissipated when passing through the modified area is small, the proportion of modification energy that is occupied by the pre-modified portion in the area surrounding the irradiation position increases. On the other hand, the proportion of excess energy that is occupied by the post-modified portion in the area surrounding the irradiation position increases.

[0046] Furthermore, when forming a modified layer along the second dividing line 12, the proportion of the modified portion near the intersection 13 is greater when forming the second modified layer 15B than when forming the first modified layer 15A.

[0047] Based on this, in the second laser processing step (step S103) of the above-described laser processing method, the distance between the second modified layer 15B and the first dividing line 11 is made larger than that between the first modified layer 15A and the second modified layer 15B. This reduces excess energy generated near the intersection 13 when forming the second modified layer 15B. As a result, splash damage to the wafer 10 can be reduced. Furthermore, since the reduced excess energy makes it easier to obtain the desired brittleness near the intersection 13, the wafer 10 can be divided along the dividing lines 11, 12 with high precision.

[0048] (1-2) The first modified layer 15A is formed at a position closer to the device surface 10a of the wafer 10 in the Z direction than the second modified layer 15B. A first distance L1 between the first modified layer 15A and the first dividing line 11 is smaller than a second distance L2 between the first modified layer 15A and the first dividing line 11.

[0049] This configuration increases the brittleness of the device surface 10a of the wafer 10. This allows the wafer 10 to be divided along the first division lines 11 and the second division lines 12 with high precision on the device surface 10a side. As a result, the quality of the device surface 10a of the divided chips can be ensured.

[0050] Second Embodiment A second embodiment of the laser processing method will be described with reference to Figures 9 and 10. The laser processing method of the second embodiment has the same main configuration as the first embodiment. Therefore, in the second embodiment, only the parts that are different from the first embodiment will be described in detail, and the same parts as the first embodiment will be denoted by the same reference numerals and will not be described in detail again.

[0051] It is known that the wafer 10 expands due to laser processing. In the laser processing method of the second embodiment, the OFF position and ON position in the second laser processing step are adjusted based on the degree of expansion of the wafer 10 after the first laser processing.

[0052] 9, the control device 25 has a position correction unit 33 as a functional unit that functions by executing various programs. The position correction unit 33 corrects the position of the second dividing line 12 held by the laser processing control unit 32 based on the degree of expansion of the wafer 10 after the first laser processing.

[0053] Specifically, as shown in FIG. 10 , the position correction unit 33 acquires the size W of the wafer 10 in the second direction D2 near the center in the first direction D1 before and after the first laser processing. For example, the position correction unit 33 acquires the size W of the wafer 10 by performing image processing on image data obtained by capturing an image of a reference mark provided on the wafer 10. The size of the wafer 10 before the first laser processing is referred to as the reference size W1, and the size of the wafer 10 after the first laser processing is referred to as the post-processing size W2. The reference marks may be alignment marks. The position correction unit 33 acquires an average expansion size Wa (=(W1-W2) / n) as the degree of expansion, obtained by subtracting the post-processing size W2 from the reference size W1 and dividing the result by the number n of the second division lines 12. The position correction unit 33 then corrects the position of each second division line 12 stored in the laser processing control unit 32 based on the average expansion size Wa.

[0054] In the second laser processing step, the laser processing control unit 32 determines whether the moving work table 21 has reached the OFF position or the ON position based on the position of the second dividing line 12 after correction.

[0055] The operation and effects of the second embodiment will be described. (2-1) In the second laser processing step, the degree of expansion of the wafer 10 in the second direction D2 is acquired, and the irradiation position of the laser LB in the second direction D2 is adjusted according to the acquired degree of expansion. This improves the accuracy of the OFF and ON positions of the laser LB according to the degree of expansion of the wafer 10 due to the first laser processing step. As a result, the effects described in (1-1) can be more reliably achieved.

[0056] Third Embodiment A third embodiment of the laser processing method will be described with reference to Figures 11 to 13. The laser processing method of the third embodiment has the same main configuration as the first embodiment. Therefore, in the third embodiment, only the parts that differ from the first embodiment will be described in detail, and parts that are the same as those in the first embodiment will be denoted by the same reference numerals and will not be described in detail again.

[0057] In the second laser processing step of the laser processing method of the third embodiment, the laser processing control unit 32 performs laser processing in a processing direction in which a modified layer is formed on both sides of the intersection 13 in the second direction D2 so as to approach the intersection 13.

[0058] Specifically, as shown by arrows 41 and 42 in Figure 11, the laser processing control unit 32 performs laser processing by setting the central area between the intersections 13 as the ON position and positions away from the intersections 13 by distances L1 and L2 toward the ON position as the OFF position.

[0059] More specifically, as shown in Fig. 12(a), when forming the first modified layer 15A, the laser processing control unit 32 moves the work table 21 in the +X direction to form the first modified layer 15A corresponding to the arrow 41. Then, as shown in Fig. 12(b), the laser processing control unit 32 moves the work table 21 in the -X direction to form the first modified layer 15A corresponding to the arrow 42. The ON position corresponding to the arrow 41 and the ON position corresponding to the arrow 42 are different positions in the X direction. The ON position corresponding to the arrow 42 is adjacent to the ON position corresponding to the arrow 41 on the side of the modified layer unformed region in the X direction (the +X direction side in this embodiment).

[0060] 13(a), when forming the second modified layer 15B, the laser processing control unit 32 moves the work table 21 in the +X direction to form the second modified layer 15B corresponding to the arrow 41. Thereafter, as shown in FIG. 13(b), the laser processing control unit 32 moves the work table 21 in the −X direction to form the second modified layer 15B corresponding to the arrow 42.

[0061] The operation and effects of the third embodiment will be described. (3-1) According to the laser processing method described above, the energy of the laser LB irradiated onto the wafer 10 can be stabilized compared to when the ON position is provided near the intersection 13. This improves the accuracy of the formation position of the modified layers 15A, 15B near the intersection 13 and makes it easier to obtain the desired brittleness near the intersection 13. As a result, the effect described in (1-1) can be more reliably obtained, and the wafer 10 can be divided along the second dividing line 12 with higher accuracy.

[0062] (3-2) By making the ON position corresponding to the arrow 41 different from the ON position corresponding to the arrow 42, splash damage near the ON position corresponding to the arrow 42 can be more reliably reduced.

[0063] The first to third embodiments can be modified as follows: The first to third embodiments and the following modifications can be combined with each other to the extent that no technical contradiction occurs.

[0064] The laser processing device 20 may be configured so that the work table 21 and the laser irradiation unit 22 can be relatively moved in the X and Y directions by the relative movement mechanism 23. Therefore, the laser processing device 20 may move the laser irradiation unit 22 during laser processing.

[0065] The laser LB is not limited to a pulsed laser, and may be a CW (Continuous Wave) laser. The back surface 10b of the wafer 10 may be supported on the work table 21 via a dicing tape 16.

[0066] The first distance L1 of the first modified layer 15A may be greater than the second distance L2 of the second modified layer 15B. For example, as shown in FIG. 14 , in addition to the first modified layer 15A and the second modified layer 15B, the wafer 10 may have a third modified layer 15C and a fourth modified layer 15D formed on the back surface 10b opposite the second modified layer 15B as modified layers at different positions in the thickness direction. The third modified layer 15C is formed on the opposite side of the second modified layer 15B from the first modified layer 15A. The fourth modified layer 15D is formed on the opposite side of the third modified layer 15C from the second modified layer 15B. In this case, it is preferable that the laser processing apparatus 20 performs laser processing on the third modified layer 15C with a position that is a third distance L3 (≧L2) away from the first parting line 11 as its OFF position. Furthermore, the laser processing apparatus 20 preferably performs laser processing on the fourth modified layer 15D, with the OFF position set at a position a fourth distance L4 (≧L3) away from the first dividing line 11. That is, in addition to the first modified layer 15A and the second modified layer 15B, a third modified layer, ..., an nth modified layer (where n is an integer greater than or equal to 3) may be formed on the wafer 10. In this case, the laser processing apparatus 20 preferably performs laser processing on the nth modified layer 15n, with the OFF position set at a position an nth distance Ln (≧L2) away from the first dividing line 11. This configuration reduces splash damage to the device surface 10a while increasing the proportion of the modified layer in the thickness direction. As a result, the wafer 10 can be divided along the second dividing line 12 with greater accuracy.

[0067] 15 , in addition to the third modified layer 15C and the fourth modified layer 15D described above, a fifth modified layer 15E and a sixth modified layer 15F may be formed on the back surface 10b side of the fourth modified layer 15D as modified layers at different positions in the thickness direction. The fifth modified layer 15E is formed on the back surface 10b side of the fourth modified layer 15D. The sixth modified layer 15F is formed on the back surface 10b side of the fifth modified layer 15E. The fifth modified layer 15E and the sixth modified layer 15F are back surface modified layers. The back surface modified layer is a modified layer that is sufficiently separated from the device surface 10a and does not affect the device surface 10a due to splash damage caused by the formation of the back surface modified layer. When forming the fifth modified layer 15E and the sixth modified layer 15F, the laser processing apparatus 20 may continuously irradiate the fifth modified layer 15E and the sixth modified layer 15F along the second dividing line 12 without turning the laser on and off. That is, the laser processing apparatus 20 may form the fifth modified layer 15E and the sixth modified layer 15F by irradiating the laser LB along the second dividing line 12 so that the modified layers are formed at predetermined intervals based on the laser processing conditions. This configuration allows for more accurate division of the wafer 10 along the second dividing line 12 while reducing splash damage to the device surface 10a.

[0068] The technical ideas that can be understood from the above embodiments and modifications are described below. [Appendix 1] A laser processing method for forming a modified layer inside a wafer by irradiating a laser along a parting line, the method comprising: a first laser processing step for forming the modified layer along a first parting line extending in a first direction; and a second laser processing step, which is performed after the first laser processing step, for forming the modified layer along a second parting line extending in a second direction intersecting the first direction and intersecting the first parting line, wherein in the second laser processing step, a first modified layer and a second modified layer are formed at different positions in a thickness direction of the wafer, and the first modified layer and the second modified layer are made to have different distances from the first parting line in the second direction at the intersections of the first and second parting lines.

[0069] [Appendix 2] The first modified layer is a modified layer formed at a position closer to the device surface of the wafer in the thickness direction than the second modified layer, and the distance between the first dividing line in the second direction is made smaller for the first modified layer than for the second modified layer. [Appendix 1] A laser processing method as described in

[0070] [Appendix 3] When n is an integer greater than or equal to 3, in the second laser processing step, an n-th modified layer is further formed on the back surface side opposite the device surface to the (n-1)-th modified layer as a modified layer positioned at a different position in the thickness direction of the wafer, and at the intersection, when the distance between the second modified layer and the first dividing line in the second direction is distance L2 and the distance between the n-th modified layer and the first dividing line in the second direction is distance Ln, distance Ln ≧ distance L(n-1). The laser processing method described in [Appendix 2].

[0071] [Appendix 4] In the second laser processing step, a back-side modified layer is formed on the back side of the nth modified layer as a modified layer at a different position in the thickness direction of the wafer, and the back-side modified layer is formed by continuously irradiating the laser along the second dividing line. [Appendix 3] The laser processing method described in

[0072] [Appendix 5] In the second laser processing step, the laser irradiation position in the second direction is adjusted according to the degree of expansion of the wafer due to the first laser processing step. A laser processing method described in any one of [Appendix 1] to [Appendix 4].

[0073] [Appendix 6] In the second laser processing step, laser processing is performed on both sides of the intersection in the second direction, with the direction in which a modified layer is formed so as to approach the first dividing line as the processing direction. A laser processing method described in any one of [Appendix 1] to [Appendix 5].

[0074] 10...wafer, 10a...device surface, 10b...back surface, 11...first division line, 12...second division line, 13...intersection, 15A...first modified layer, 15B...second modified layer, 15C...third modified layer, 15D...fourth modified layer, 15E...fifth modified layer, 15F...sixth modified layer, 16...dicing tape, 20...laser processing device, 21...work table, 22...laser irradiation unit, 23...relative movement mechanism, 24...image capture device, 25...control device, 26...laser emission unit, 27...condensing lens, 29...input unit, 31...alignment detection unit, 32...laser processing control unit, 33...position correction unit.

Claims

1. A laser processing method for forming a modified layer inside a wafer by irradiating a laser along a dividing line, comprising: a first laser processing step for forming a modified layer along a first dividing line extending in a first direction; and a second laser processing step, which is carried out after the first laser processing step, for forming a modified layer along a second dividing line extending in a second direction intersecting the first direction and intersecting the first dividing line, wherein in the second laser processing step, a first modified layer and a second modified layer are formed that are at different positions in the thickness direction of the wafer, and the distance between the first dividing line in the second direction at the intersection of the first dividing line and the second dividing line is made different between the first modified layer and the second modified layer.

2. The laser processing method according to claim 1, wherein the first modified layer is a modified layer formed at a position closer to the device surface of the wafer in the thickness direction than the second modified layer, and the distance between the first dividing line in the second direction is smaller for the first modified layer than for the second modified layer.

3. The laser processing method according to claim 2, wherein, when n is an integer equal to or greater than 3, in the second laser processing step, an nth modified layer is further formed on the back surface side opposite the device surface to the (n-1)th modified layer as a modified layer positioned at a different position in the thickness direction of the wafer, and when, at the intersection, the distance between the second modified layer and the first division line in the second direction is distance L2 and the distance between the nth modified layer and the first division line in the second direction is distance Ln, distance Ln≧distance L(n-1).

4. A laser processing method as described in claim 3, wherein in the second laser processing step, a backside modified layer is formed on the backside of the nth modified layer as a modified layer at a different position in the thickness direction of the wafer, and the backside modified layer is formed by continuously irradiating the laser along the second dividing line.

5. A laser processing method according to claim 1, wherein in the second laser processing step, the laser irradiation position in the second direction is adjusted according to the degree of expansion of the wafer due to the first laser processing step.

6. A laser processing method as described in claim 1, wherein in the second laser processing step, laser processing is performed with the processing direction being a direction in which a modified layer is formed on both sides of the intersection in the second direction so as to approach the first dividing line.

7. A laser processing apparatus comprising: a work table that supports a wafer; a laser irradiation unit that irradiates the wafer with a laser; a relative movement mechanism that relatively moves the work table and the laser irradiation unit; and a control device that controls the laser irradiation unit and the relative movement mechanism so that a modified layer is formed inside the wafer along a division line, wherein the control device executes: a first laser processing process that forms a modified layer along a first division line that extends in a first direction; and, after the first laser processing process, a second laser processing process that forms a modified layer along a second division line that extends in a second direction that intersects with the first division line, and intersects with the first division line; wherein the second laser processing process forms a first modified layer and a second modified layer that are located at different positions in the thickness direction of the wafer, and makes the distance between the first division line in the second direction different between the first modified layer and the second modified layer at the intersection of the first division line and the second division line.

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