LASER PROCESSING METHOD AND MANUFACTURING METHOD OF GaN WAFER

Abstract

To reduce an influence of a laser beam entered into a processed material from an external peripheral side surface in the case where a modification part is formed in an inner part of a processed part by irradiating an external peripheral region of the processed material with a laser beam.SOLUTION: The present invention provides a laser processing method comprising a pulse-like laser beam irradiation step of forming a modification layer into an inner part of a processed material by relatively moving a focal point and the processed material so that the focal point is along an external peripheral region of the processed material in a state where a height position of the focal point is adjusted so that the focal point of a laser beam is positioned in the inner part of the processed material while masking one part of the laser beam with a mask unit so that one part of the laser beam entered into the inner part of the processed material via the external peripheral side surface of the laser beam having a wavelength penetrated to the processed material has a strength that is less than a processing threshold value of the processed material.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a laser processing method for processing a workpiece with a pulsed laser beam having a wavelength that transmits through the workpiece, and a GaN wafer manufacturing method for manufacturing a GaN wafer having a thickness less than that of the workpiece, which is a GaN ingot or a GaN single crystal substrate, by using the laser beam. [Background technology]

[0002] A method has been developed in which a laser beam is used to form multiple peeling layers, each of which includes a modified layer and a crack propagating from the modified layer, at approximately the same depth inside an ingot, and then wafers are peeled off from the ingot using these multiple peeling layers, which have reduced mechanical strength, as starting points (see, for example, Patent Document 1).

[0003] When forming the peeling layer, the laser beam and the ingot are processed and fed relative to each other along a predetermined direction perpendicular to the thickness direction of the ingot, with the focal point of the pulsed laser beam having a wavelength that transmits through the ingot being positioned inside the ingot, thereby forming the peeling layer inside the ingot.

[0004] After one peeling layer is formed, the focal point and the ingot are indexed relative to each other, and then the ingot is processed and fed in the same manner to form additional peeling layers. In this way, multiple peeling layers each along a predetermined direction are formed inside the ingot.

[0005] Also, a method has been developed in which, in a laminated wafer in which a first wafer and a second wafer are stacked, multiple release layers each aligned in a predetermined direction are formed inside the first wafer, and then the first wafer is separated into two disks starting from the multiple release layers (see, for example, Patent Document 2).

[0006] In this method as well, when forming a peeling layer, the focal point of a pulsed laser beam having a wavelength that transmits through the first wafer is positioned inside the first wafer, and the laser beam and the first wafer are moved relative to each other along a predetermined direction perpendicular to the thickness direction of the first wafer, thereby forming a peeling layer inside the first wafer.

[0007] When a laser beam is irradiated onto the outer peripheral region of a workpiece such as an ingot or a laminated wafer, part of the laser beam may be incident on the surface of the workpiece, and another part of the laser beam may be incident on the outer peripheral side surface of the workpiece.

[0008] The focal point of the laser beam incident on the surface of the workpiece and the focal point of the laser beam incident on the outer peripheral side of the workpiece are different in the thickness direction of the workpiece due to the difference in refractive index inside and outside the workpiece. Therefore, the intensity of the laser beam (W / cm2) at the same depth position is different. 2 ) are different.

[0009] In addition, the laser beam is reflected, scattered, etc., on the outer peripheral side surface. Therefore, in the outer peripheral region of the workpiece, the depth position of the modified layer, the formation mode of the modified layer, etc., differ from those in the central part of the workpiece, and in addition, in the outer peripheral region of the workpiece, there is a risk that cracks propagating from the modified layer may propagate in unexpected directions. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2016-111143 A [Patent Document 2] Patent Publication No. 2023-26825 Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention has been made in consideration of these problems, and aims to reduce the influence of a laser beam entering a workpiece from the outer peripheral side when a laser beam is irradiated to the outer peripheral region of a workpiece to form a modified area inside the workpiece. [Means for solving the problem]

[0012] According to one aspect of the present invention, there is provided a laser processing method for a columnar or plate-shaped workpiece having a first surface, a second surface opposite to the first surface, and an outer peripheral side surface intersecting the first surface and the second surface, the method comprising: irradiating an outer peripheral region of the workpiece with a pulsed laser beam having a wavelength that passes through the workpiece in a direction proceeding from the first surface to the second surface; and adjusting a mask unit that masks a portion of the laser beam so that a portion of the laser beam that enters the inside of the workpiece from the outer peripheral side surface has an intensity less than a processing threshold value of the workpiece. and after the mask preparation step, in a state in which the height position of the focal point of the laser beam is adjusted so that the focal point is located inside the workpiece, the focal point and the workpiece are moved relatively so that the focal point is along the outer peripheral region, thereby forming a modified layer inside the workpiece while masking a part of the laser beam with the mask unit so that a part of the laser beam incident on the inside of the workpiece via the outer peripheral side surface has an intensity less than the processing threshold value of the workpiece.

[0013] Preferably, the mask unit has a spatial light phase modulator that at least partially modulates the laser beam, or a metal mask that partially masks the laser beam, and in the laser beam irradiation step, the spatial light phase modulator at least partially modulates the laser beam so that the light intensities are weakened to mask a portion of the laser beam, or the metal mask physically masks a portion of the laser beam.

[0014] Also, preferably, in the laser beam irradiation step, the other portion of the laser beam not masked by the mask unit is branched so that the other portion of the laser beam is focused at a plurality of focusing points arranged along a predetermined direction that intersects, in a planar view, with the direction in which the focusing point and the workpiece move relatively, and then the other portion of the laser beam is irradiated onto the workpiece.

[0015] According to another aspect of the present invention, there is provided a method for producing a GaN wafer having a thickness less than that of a workpiece which is a GaN ingot or a GaN single crystal substrate, the workpiece having a first surface, a second surface located opposite to the first surface, and an outer peripheral side surface intersecting the first surface and the second surface, the method for producing the GaN wafer includes a peripheral region peeling layer forming step of forming a peeling layer in the peripheral region of the workpiece by relatively moving the workpiece and the focal point of a pulsed laser beam having a wavelength which transmits the workpiece so that the focal point is located inside the workpiece while adjusting the height position of the focal point; The method includes a central region peeling layer formation step of forming a peeling layer in a central region of the workpiece located inside the outer circumferential region within the first surface or the second surface by relatively moving the focal point and the workpiece, and a delamination step of peeling the GaN wafer from the workpiece using the peeling layer in the outer circumferential region and the peeling layer in the central region as starting points, wherein in the peripheral region peeling layer formation step, a peeling layer is formed inside the workpiece while masking a portion of the laser beam with a mask unit so that a portion of the laser beam that enters the inside of the workpiece from the outer circumferential side surface has an intensity less than the processing threshold value of the workpiece.

[0016] Preferably, the mask unit has a spatial light phase modulator that at least partially modulates the laser beam, or a metal mask that partially masks the laser beam, and in the peripheral region peeling layer formation process, the spatial light phase modulator at least partially modulates the laser beam so that the light intensities are weakened to mask a portion of the laser beam, or the metal mask physically masks a portion of the laser beam.

[0017] Also, preferably, in the peripheral region peeling layer formation process, the other portion of the laser beam not masked by the mask unit is branched so that the other portion of the laser beam is focused at a plurality of focusing points arranged along a predetermined direction that intersects, in a planar view, with the direction in which the focusing point and the workpiece move relatively, and then the other portion of the laser beam is irradiated onto the workpiece.

[0018] In addition, preferably, the direction in which the focal point and the workpiece are moved relatively in the central region peeling layer formation process forms an angle of 10° or less with the crystal orientation expressed by the following (1) on the (0001) plane of the GaN ingot or the GaN single crystal substrate.

number

[0019] In the laser processing method according to one aspect of the present invention, in the laser beam irradiation step, a modified layer is formed inside the workpiece while a part of the laser beam is masked with a mask unit so that a part of the laser beam incident on the inside of the workpiece through the outer peripheral side surface has an intensity less than the processing threshold value of the workpiece. Therefore, the influence of the laser beam incident on the workpiece from the outer peripheral side surface can be reduced.

[0020] In a method for producing a GaN wafer according to another aspect of the present invention, in the peripheral region peeling layer forming step, a peeling layer is formed inside the workpiece while masking a part of the laser beam with a mask unit so that a part of the laser beam incident on the inside of the workpiece from the peripheral side has an intensity less than the processing threshold of the workpiece. Therefore, the influence of the laser beam incident on the workpiece from the peripheral side can be reduced. [Brief description of the drawings]

[0021] [Figure 1] FIG. 2 is a flow chart showing an example of a laser processing method according to the first embodiment. [Diagram 2] FIG. [Diagram 3] 13 is an example of a modulation pattern displayed on a spatial light phase modulator in a mask preparation process. [Figure 4] FIG. 4(A) is a diagram showing a laser beam irradiation step, and FIG. 4(B) is a partially enlarged view of FIG. 4(A). [Diagram 5] FIG. 5(A) shows a laser beam when forming a focal point closest to the outer peripheral side surface in the Y-axis direction, and FIG. 5(B) shows a laser beam when forming a focal point farthest from the outer peripheral side surface in the Y-axis direction. [Figure 6] FIG. 2 is a plan view of a workpiece in a laser beam irradiation process. [Figure 7] Figure 7(A) is an example of a modulation pattern displayed on the spatial light phase modulator in the first comparative experiment, and Figure 7(B) is a planar photograph of the outer peripheral region of the workpiece after laser processing in the first comparative experiment. [Figure 8] Figure 8(A) is an example of a modulation pattern displayed on the spatial light phase modulator in the second comparative experiment, and Figure 8(B) is a planar photograph of the outer peripheral region of the workpiece after laser processing in the second comparative experiment. [Figure 9] FIG. 9(A) is an example of a modulation pattern displayed on a spatial light phase modulator in the first experiment corresponding to the first embodiment, and FIG. 9(B) is a planar photograph of the outer peripheral region of the workpiece after laser processing. [Figure 10] FIG. 10(A) is a diagram showing a mask preparation step in the modified example, and FIG. 10(B) is a diagram showing a laser beam irradiation step in the modified example. [Figure 11] FIG. 11 is a flow diagram showing an example of a method for producing a GaN wafer according to the second embodiment. [Figure 12] FIG. 1 is a perspective view of a GaN ingot. [Figure 13] FIG. 11 is a top view showing a peripheral region release layer forming step. [Figure 14] FIG. 14A is a diagram showing how approximately half of the laser beam is masked, and FIG. 14B is a diagram showing how the outer periphery of approximately half of the laser beam is masked. [Figure 15] FIG. 11 is a top view showing a central area release layer forming step. [Figure 16] FIG. 16(A) is a diagram showing a separation apparatus, and FIG. 16(B) is a diagram showing a GaN ingot and a GaN wafer after the separation step. [Figure 17] FIG. 2 is a plan view showing a peripheral region separation layer forming step for a rectangular plate-shaped GaN single crystal substrate. [Figure 18] FIG. 1 is a plan view of a GaN ingot showing the angle θ between a predetermined direction and the direction of movement of multiple focusing points in an experiment to evaluate the crystal orientation dependency of the minimum average output at which cracks are formed. [Figure 19] FIG. 19(A) is a graph showing the minimum average output at which a crack is formed versus angle θ, FIG. 19(B) is a plan view of a GaN ingot for θ=30°, and FIG. 19(C) is a plan view of a GaN ingot for θ=90°. [Figure 20] FIG. 13 is a plan view showing a central area release layer forming step according to a second modified example of the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] (First embodiment) An embodiment according to one aspect of the present invention will be described with reference to the attached drawings. Fig. 1 is a flow diagram showing an example of a laser processing method according to the first embodiment. As shown in Fig. 1, in the laser processing method of this embodiment, a holding step S10, a mask preparation step S20, and a laser beam irradiation step S30 are performed in this order.

[0023] First, the laser processing device 2 used in the first embodiment will be described with reference to Fig. 2. In Fig. 2, a plurality of components are shown in functional blocks or simplified shapes. The X-axis direction (processing feed direction), the Y-axis direction (indexing feed direction), and the Z-axis direction (height direction, vertical direction) shown in Fig. 2 are mutually orthogonal.

[0024] The laser processing device 2 has a disk-shaped chuck table 4. The chuck table 4 has a disk-shaped frame made of a metal such as stainless steel. A disk-shaped recess (not shown) having a diameter smaller than that of the frame is formed in the center of the frame. A disk-shaped porous plate (not shown) made of porous ceramics is fixed in this recess.

[0025] A predetermined flow path (not shown) is formed in the frame, and a suction source (not shown) such as a vacuum pump is connected to the predetermined flow path via a pipe (not shown) etc. When negative pressure generated by the suction source is transmitted to the porous plate, negative pressure is generated on the upper surface of the porous plate.

[0026] The annular upper surface of the frame and the circular upper surface of the porous plate are substantially flush and substantially flat, and function as a holding surface 4a for suction-holding the workpiece 11. The holding surface 4a is disposed substantially parallel to the XY plane.

[0027] A rotation drive mechanism 6 that rotates the chuck table 4 is provided below the chuck table 4. The rotation drive mechanism 6 can rotate the rotation axis of the chuck table 4 by any angle. This rotation axis is disposed along the Z-axis direction.

[0028] The chuck table 4 and the rotary drive mechanism 6 are supported by a horizontal movement mechanism 8. The horizontal movement mechanism 8 includes a ball screw type X-axis movement mechanism and a Y-axis movement mechanism, respectively, and can move the chuck table 4 and the rotary drive mechanism 6 along the X-axis and Y-axis directions.

[0029] A laser beam irradiation unit 12 is provided above the holding surface 4a. The laser beam irradiation unit 12 includes a laser oscillator 14. The laser oscillator 14 has a laser medium such as Nd:YAG or Nd:YVO4.

[0030] The laser oscillator 14 further includes an excitation light source (not shown) such as a lamp that irradiates the laser medium with excitation light, and a Q switch (not shown) that controls the timing at which the laser beam L is emitted.

[0031] The laser oscillator 14 generates a pulsed laser beam L having a wavelength (for example, 1064 nm when the workpiece 11 is made of GaN) that is transmitted through the workpiece 11. In addition, in FIG. 2, the traveling path of the laser beam L is indicated by a dashed arrow.

[0032] The laser beam L emitted from the laser oscillator 14 is converted into a burst mode laser beam L in an acousto-optic modulator (AOM) (not shown). The acousto-optic modulator operates according to an electrical signal input to the acousto-optic modulator.

[0033] The acousto-optic modulator has a function of deflecting the laser beam L at predetermined intervals in time according to an input electrical signal. The acousto-optic modulator converts the laser beam L into a burst mode in which a pulse group containing multiple pulses is repeated at a predetermined period by thinning out the pulses.

[0034] In this embodiment, the number of pulses included in one pulse group is referred to as the burst number. The burst number is, for example, 10, but is not necessarily limited to 10. The interval between the pulse groups is, for example, several tens of μs to several hundreds of μs, and the repetition frequency of the pulse groups is, for example, 50 kHz.

[0035] The burst mode laser beam L enters an LCOS-SLM (Liquid Crystal on Silicon - Spatial Light Modulator) (hereinafter referred to as spatial light phase modulator 18) after its output is adjusted in an output adjuster 16 having an attenuator or the like.

[0036] The spatial light phase modulator 18 of this embodiment is a so-called reflective liquid crystal device, but may be a transmissive liquid crystal device. The spatial light phase modulator 18 has a function of changing the phase, polarization plane, amplitude, intensity, propagation direction, etc. of the incident laser beam L (i.e., modulating the laser beam L).

[0037] The spatial light phase modulator 18, for example, changes the phase of the laser beam L when the laser beam L is irradiated to the outer peripheral region 11c (see Figure 6) of the workpiece 11 in a direction proceeding from the upper surface (first surface) 11a to the lower surface (second surface) 11b of the workpiece 11 held by suction on the holding surface 4a.

[0038] Specifically, the spatial light phase modulator 18 masks a portion of the laser beam L by changing the phase of the laser beam L so that the portion of the laser beam L that enters the workpiece 11 from the outer peripheral side surface 11d of the workpiece 11 has an intensity less than the processing threshold of the workpiece 11.

[0039] In this embodiment, when a part of the laser beam L has an intensity less than the processing threshold of the workpiece 11, the part of the laser beam L is expressed as being masked. The part of the laser beam L that is masked is, for example, completely blocked. In this case, the intensity (W / cm 2 ) or energy (J) is approximately zero.

[0040] In one example, the spatial light phase modulator 18 at least partially modulates the laser beam L so that the light intensities weaken each other near the focal point by utilizing an area of ​​the spatial light phase modulator 18 irradiated with the laser beam L excluding a specific semicircular area (see the semicircular area having a striped pattern in Figure 3).

[0041] As a result, approximately half of the luminous flux of the laser beam L (a part of the laser beam L) is masked near the focusing point in a cross section of the laser beam L perpendicular to the traveling direction of the laser beam L. That is, the spatial energy distribution of the laser beam L is adjusted so that approximately half of the luminous flux of the laser beam L has an intensity less than the processing threshold (for example, approximately zero) near the focusing point.

[0042] The light flux of the modulated laser beam L is not limited to approximately half, but is appropriately adjusted according to the processing mode. The spatial light phase modulator 18 adjusts the energy distribution as described above by changing the phase of the laser beam L, and also branches the laser beam L so as to be focused at multiple focusing points P (see FIG. 6).

[0043] In this embodiment, the laser beam L is branched so that ten focal points P are arranged in the Y-axis direction. However, the number of branches of the laser beam L is not limited to 10. The number of branches of the laser beam L is appropriately adjusted depending on the size of the workpiece 11, the average output of the laser beam L, the repetition frequency, etc.

[0044] As described above, the spatial light phase modulator 18 has a function of masking and branching the laser beam L. The operation of the spatial light phase modulator 18 is controlled by a driving circuit 18a. The driving circuit 18a is controlled by a controller 30, which will be described later.

[0045] The spatial light phase modulator 18 has a display area including a plurality of pixels arranged in a matrix. A liquid crystal layer is provided in the display area, and the liquid crystal layer of the spatial light phase modulator 18 forms a predetermined pattern by controlling the tilt of the liquid crystal molecules in each pixel. The phase of the laser beam L, etc. changes according to the pattern of the liquid crystal layer.

[0046] In the mask unit 20 of the present embodiment, the spatial light phase modulator 18 performs the functions of masking and splitting the laser beam L. However, in other examples of the mask unit 20, the function of partially masking the laser beam L may be performed by a metal mask instead of the spatial light phase modulator 18.

[0047] When the mask unit 20 has a metal mask, the spatial light phase modulator 18 has the function of branching the laser beam L, but instead of the spatial light phase modulator 18, a DOE (Diffractive Optical Element) may have the function of branching the laser beam L.

[0048] 2, the laser beam L that has passed through the spatial light phase modulator 18 passes through a pair of lenses 22a and 22b. The pair of lenses 22a and 22b are disposed such that their optical axes are substantially aligned with each other and substantially parallel to the Y-axis direction.

[0049] 2 are approximate, such as the positions and focal distances of the pair of lenses 22a and 22b. The laser beam L, the beam diameter of which has been adjusted by the pair of lenses 22a and 22b, is inverted in the Z-axis direction and the X-axis direction (see FIG. 4(A)), and is reflected by the mirror 24, and then enters the condenser lens 26.

[0050] The mirror 24 and the condenser lens 26 are fixed in a cylindrical condenser (not shown). The laser beam irradiation unit 12 is configured to be movable along the Z-axis direction by a ball screw type Z-axis direction moving mechanism (not shown).

[0051] The position of the focal point P of the laser beam L in the Z-axis direction is adjusted by moving the laser beam irradiation unit 12 along the Z-axis direction. The condenser lens 26 may be configured to be movable along the Z-axis direction within the condenser.

[0052] The laser beam L that has passed through the condenser lens 26 is incident on the workpiece 11 that is sucked and held by the holding surface 4a. The workpiece 11 in this embodiment is a rectangular plate-like (i.e., plate-like) single crystal substrate made of single crystal gallium nitride (GaN) (see FIG. 6).

[0053] The upper surface 11a and the lower surface 11b of the workpiece 11 are located on opposite sides in the thickness direction of the workpiece 11. The outer peripheral side surface 11d of the workpiece 11 is connected so as to be perpendicular (intersect) to the upper surface 11a and the lower surface 11b. In this embodiment in which the workpiece 11 is a rectangular plate, the outer peripheral side surface 11d has four substantially flat planes (see FIG. 6).

[0054] As shown in FIG. 2, a microscope camera unit (not shown) is provided near the condenser in such a manner that it can be moved together with the laser beam irradiation unit 12 by moving in the Z-axis direction.

[0055] The microscope camera unit has a light source including a light emitting element such as an LED (Light Emitting Diode), and a solid-state imaging element such as a CCD (Charge-Coupled Device) image sensor or a CMOS (Complementary Metal-Oxide-Semiconductor) image sensor.

[0056] The operations of the above-mentioned chuck table 4, rotary drive mechanism 6, horizontal movement mechanism 8, laser oscillator 14, drive circuit 18a, etc. are controlled by a controller 30. The controller 30 is configured by a computer including, for example, a processor (processing device) represented by a CPU (Central Processing Unit) and a memory (storage device).

[0057] The memory includes a main storage device such as a dynamic random access memory (DRAM) and an auxiliary storage device such as a flash memory. The auxiliary storage device stores software including a predetermined program. The functions of the controller 30 are realized by operating a processor and the like according to this software.

[0058] Next, a method of performing laser processing on the workpiece 11 will be described according to the procedure shown in Fig. 1. First, as shown in Fig. 2, the workpiece 11 is placed on the chuck table 4 so that the lower surface 11b contacts the holding surface 4a, and the workpiece 11 is suction-held by the holding surface 4a (holding step S10).

[0059] Next, the orientation of chuck table 4 is adjusted using the microscope camera unit so that outer peripheral side surface 11d of workpiece 11, which is a GaN single crystal substrate, is approximately parallel to the X-axis direction (see FIG. 6).

[0060] Next, when the laser beam L is irradiated onto the workpiece 11, a portion of the laser beam L that enters the workpiece 11 from the outer peripheral side surface 11d is masked, and the spatial light phase modulator 18 (i.e., the mask unit 20) is adjusted so that the other portion of the laser beam L that is not masked branches off near the focal point P (mask preparation step S20).

[0061] 2 is a diagram showing the mask preparation step S20. In the mask preparation step S20, the controller 30 sends a predetermined signal conforming to the standard of a video input / output interface such as DVI (Digital Visual Interface) to the drive circuit 18a.

[0062] This controls the tilt of the liquid crystal molecules in the liquid crystal layer, and the liquid crystal layer forms a predetermined pattern. Fig. 3 shows an example of a modulation pattern displayed on the spatial light phase modulator 18 in the mask preparation step S20.

[0063] 3, the phase change (amount of change is 0 rad or more and 2π rad or less) that each pixel of the spatial light phase modulator 18 gives to the laser beam L is expressed by shading. However, the modulation pattern itself is almost invisible to a human even when visually observing the spatial light phase modulator 18.

[0064] By converting the phase change that each pixel gives to the laser beam L into a pixel value (e.g., a luminance value) represented by 8 bits, the modulation pattern when the pixel area of ​​the spatial light phase modulator 18 is viewed in the direction of the arrow 18b in Fig. 2 becomes the image shown in Fig. 3. The images in Fig. 7(A), Fig. 8(A), and Fig. 9(A), which will be described later, are also images of modulation patterns in a similar manner.

[0065] Either the holding step S10 or the mask preparation step S20 may be performed first, or they may be performed simultaneously. After the holding step S10 and the mask preparation step S20, the laser beam L is irradiated from the laser oscillator 14, and the laser beam L passes through the spatial light phase modulator 18, the condenser lens 26, etc., and is irradiated onto the workpiece 11 (laser beam irradiation step S30).

[0066] Fig. 4(A) is a diagram showing the laser beam irradiation step S30, and Fig. 4(B) is a partially enlarged view of Fig. 4(A) in the vicinity of the focal point P. As described above, the workpiece 11 (GaN single crystal substrate) of this embodiment does not have an off-angle, so the upper surface 11a corresponds to (0001) and the thickness direction is parallel to

[0001] . In addition, the outer peripheral side surface 11d is represented by {-1100} or {11-20}.

[0067] In this specification, the Miller-Bravais index is used to identify crystal planes and crystal orientations. Specific crystal planes are expressed using (), but for convenience, they may be written as () planes. Furthermore, crystal planes that are equivalent to each other due to the symmetry of the crystal structure are expressed using {}.

[0068] A specific crystal orientation is expressed using [], but for convenience, it is sometimes written as [] direction. Moreover, crystal orientations that are equivalent to each other are expressed using <>. Incidentally, in the Miller-Bravais index, an index with a minus sign is equal to an index with a bar above it.

[0069] In Figures 4(A) and 4(B), the laser beam L that enters the inside of the workpiece 11 from the top surface 11a is marked with diagonal lines, and the laser beam L that would normally enter the inside of the workpiece 11 from the outer peripheral side surface 11d but is actually masked by the mask unit 20 is marked with dots.

[0070] 4(A) and 4(B), the laser beam L is not branched in the Y-axis direction, but the laser beam L in this embodiment actually branches in the Y-axis direction (see FIG. 6). The Y-axis direction is a predetermined direction that is orthogonal (intersects) in a plan view with the X-axis direction in which the focal point P and the workpiece 11 move relatively.

[0071] 4(A) and 4(B) show a first component L1 of the laser beam L that would normally enter the workpiece 11 from the outer peripheral side surface 11d (but is actually masked), and a second component L2 of the laser beam L that actually enters the workpiece 11 from the top surface 11a. The first component L1 is modulated so as to weaken each other, but the second component L2 is not modulated so as to weaken each other, but is modulated so as to strengthen each other.

[0072] Meanwhile, Figures 5(A) and 5(B) show a comparative example, which is a cross-sectional view of a workpiece 11 when the outer peripheral region 11c of the workpiece 11 is processed with a laser beam L in a state where the laser beam L is not masked and is branched near the focal point P.

[0073] 5(A) and 5(B) exemplarily show a plurality of focal points P branched along the Y-axis direction at a predetermined depth in the workpiece 11. Fig. 5(A) shows the laser beam L when forming the focal point P closest to the outer circumferential side surface 11d in the Y-axis direction.

[0074] In Figure 5 (A), due to the difference in refractive index of the laser beam L incident on the outer peripheral side 11d inside and outside the workpiece 11, and reflection and scattering of the laser beam L at the outer peripheral side 11d, a modified layer 13 (see, for example, Figure 9 (B)) may not be formed near the outer peripheral side 11d, or even if a modified layer 13 is formed, a crack propagating from the modified layer 13 may propagate in an unexpected direction (for example, in the thickness direction of the workpiece 11).

[0075] In contrast, Fig. 5(B) is a diagram showing the laser beam L when forming the focal point P that is the farthest from the outer peripheral side surface 11d in the Y-axis direction. In the case of Fig. 5(B), the laser beam L enters the inside of the workpiece 11 from the upper surface 11a without entering the outer peripheral side surface 11d.

[0076] 5(A) and 5(B), when the laser beam L is irradiated onto the outer peripheral region 11c, the laser beam L may enter the inside of the workpiece 11 from the outer peripheral side surface 11d. Therefore, in this embodiment, the laser beam L is modulated so as to prevent the laser beam L from entering the inside of the workpiece 11 from the outer peripheral side surface 11d.

[0077] In the laser beam irradiation process S30 of this embodiment, as shown in Figures 4(A) and 4(B), the spatial light phase modulator 18 (i.e., the mask unit 20) masks the first component L1 (i.e., a part of the laser beam L) entering the inside of the workpiece 11 from the outer peripheral side surface 11d.

[0078] Furthermore, the spatial light phase modulator 18 branches the second component L2 (i.e., another part of the laser beam L) near the focal point P so that the second component L2, which enters the inside of the workpiece 11 from the upper surface 11a without being masked, is focused at multiple focal points P arranged along the Y-axis direction.

[0079] Then, with the height position of the focal point P of the second component L2 adjusted so that the focal point P is located inside the workpiece 11, the focal point P and the workpiece 11 are moved relative to each other so that the focal point P is along the outer peripheral region 11c of the workpiece 11 (see Figure 6).

[0080] 6 is a plan view of the workpiece 11 in the laser beam irradiation step S30. In this embodiment, the chuck table 4 is fed for processing along the X-axis direction in a state where the ten light focusing points P are arranged in a line along the Y-axis direction, thereby moving the light focusing points P and the workpiece 11 relatively.

[0081] However, the modified layer 13 is formed inside the workpiece 11 while the first component L1 entering the inside of the workpiece 11 through the outer circumferential side surface 11d is masked by the spatial light phase modulator 18.

[0082] An example of the processing conditions in the laser beam irradiation step S30 is shown below. The focal point P located closest to the outer peripheral side surface 11d was set to be a predetermined distance (e.g., 6 μm) away from the outer peripheral side surface 11d that was less than half the distance between the focal points.

[0083] Wavelength: 1064nm Average power: 2.6W Repetition frequency: 50kHz Number of branches: 10 Spacing between focusing points: 12.5μm Depth position of each focal point: 170 μm from the upper surface 11a Spot diameter of each focusing point: 5μm Number of passes: 1 Number of bursts: 10 Machining feed speed: 875mm / s

[0084] In this embodiment, in the laser beam irradiation step S30, the modified layer 13 is formed inside the workpiece 11 while masking the first component L1 with the mask unit 20 so that the first component L1 has an intensity less than the processing threshold of the workpiece 11. Therefore, the influence of the first component L1 incident on the workpiece 11 from the outer circumferential side surface 11d can be reduced.

[0085] (Experiment) Next, the results of an experiment in which laser processing was performed on a rectangular plate-shaped GaN single crystal substrate will be described with reference to Figures 7(A) to 9(B). In the experiment shown in Figures 7(A) to 9(B), the outer peripheral side surface 11d of the workpiece 11 when viewed from above is sometimes referred to as the edge.

[0086] In these experiments, the above-mentioned processing conditions were adopted, except for the average output and the presence or absence of a mask, and laser processing was performed on the outer peripheral region 11c (one side of a rectangle) of the workpiece 11 as shown in Figure 6.

[0087] 7(A) and 7(B) show the first comparative experiment. In the first comparative experiment, the average output was set to 2.6 W, which is the same as the above-mentioned processing conditions, and the laser beam L was not masked by the spatial light phase modulator 18 when laser processing was performed.

[0088] At this time, the laser beam L focused on one or more focusing points P located near the outer peripheral side surface 11d was incident on the inside of the workpiece 11 from the outer peripheral side surface 11d (see, for example, FIG. 5(A)). Note that the intensity of each focusing point P was approximately the same.

[0089] Fig. 7(A) is an example of a modulation pattern displayed on the spatial light phase modulator 18 in the first comparative experiment. Fig. 7(B) is a planar photograph of the outer peripheral region 11c of the workpiece 11 after laser processing in the first comparative experiment.

[0090] The dashed line running vertically through the center of the image shown in Fig. 7(B) corresponds to the edge (i.e., the outer peripheral side surface 11d) of the workpiece 11. In Fig. 7(B), in the outer peripheral region 11c of the workpiece 11 located to the left of this edge, a peeling layer 15 including a modified layer 13 and cracks (note that the reference numerals for the cracks are omitted in Fig. 7(B)) is formed.

[0091] However, as shown in FIG. 7(B), the peeling layer 15 is formed somewhat irregularly in a planar direction parallel to the upper surface 11a, and further, the depth position of the peeling layer 15 in the thickness direction of the workpiece 11 is not constant.

[0092] 8(A) and 8(B) show the second comparative experiment. In the second comparative experiment, as in the first comparative experiment, the laser beam L was not masked by the spatial light phase modulator 18, and laser processing was performed on the outer peripheral region 11c (one side of a rectangle) of the workpiece 11 as shown in FIG. 6, but the average output was set to 1.3 W, half that of the first comparative experiment.

[0093] In the second comparative experiment, the laser beam L focused on one or more focusing points P located near the outer peripheral side surface 11d was also incident on the workpiece 11 from the outer peripheral side surface 11d. The intensity of each focusing point P was approximately the same, and approximately half of the intensity of each focusing point P in the first comparative experiment.

[0094] Fig. 8(A) is an example of a modulation pattern displayed on the spatial light phase modulator 18 in the second comparative experiment, and Fig. 8(B) is a planar photograph of the outer peripheral region 11c of the workpiece 11 after laser processing in the second comparative experiment. The dashed line running vertically through the center of the image shown in Fig. 8(B) corresponds to the edge of the workpiece 11.

[0095] 8(B), a modified layer 13 is regularly formed in a first strip-shaped region along the X-axis direction, which is slightly away from the edge of the workpiece 11. Cracks propagate from the modified layer 13, and a peeled layer 15 is formed.

[0096] However, in a second stripe region of the outer circumferential region 11c that is located relatively closer to the edge than the first stripe region and that is aligned along the X-axis direction, the modified layer 13 is not formed.

[0097] This is thought to be because, due to the difference in refractive index between the inside and outside of the workpiece 11, and the reflection and scattering of the laser beam L incident on the outer peripheral side 11d at the outer peripheral side 11d, the intensity of one or more focal points P located near the outer peripheral side 11d did not reach the processing threshold value capable of forming the modified layer 13.

[0098] Fig. 9(A) is an example of a modulation pattern displayed on the spatial light phase modulator 18 in a first experiment corresponding to the first embodiment, and Fig. 9(B) is a planar photograph of the outer peripheral region 11c of the workpiece 11 after laser processing. In the first experiment, the processing conditions described in the first embodiment were adopted.

[0099] In the first experiment, the laser beam L was incident on the inside of the workpiece 11 from the top surface 11a while a portion of the laser beam L incident from the outer peripheral side surface 11d was masked, among the laser beam L focused at one or more focusing points P located near the outer peripheral side surface 11d.

[0100] The dashed line running vertically through the center of the image shown in Fig. 9(B) corresponds to the edge of the workpiece 11. In the outer peripheral region 11c in Fig. 9(B) , modified layers 13, represented by black dots, are regularly formed in a band-shaped region starting from this edge and having a predetermined width in the Y-axis direction.

[0101] Moreover, the modified layer 13 and the peeling layer 15 including the cracks are formed at approximately the same depth position in the thickness direction of the workpiece 11. In this way, by employing the processing method of the first embodiment, the influence of the first component L1 can be reduced, and the peeling layer 15 can be formed in a band-shaped region reaching the edge of the workpiece 11.

[0102] (Modification) Next, a modification of the first embodiment will be described with reference to Figures 10(A) and 10(B). The mask unit 20 according to the modification has a metal mask 32. The mask 32 is entirely made of aluminum.

[0103] However, as long as the mask 32 is capable of masking the laser beam L, it may be entirely made of other metal materials such as stainless steel or invar material with a low thermal expansion coefficient, or it may have a laminated structure of a glass substrate and a thin chrome film.

[0104] The mask 32 has an opening 32c penetrating from one surface 32a to the other surface 32b. When the one surface 32a is viewed in plan, the opening 32c has, for example, a semicircular shape with the same size and shape as in FIG.

[0105] The mask 32 transmits a part of the laser beam L through the opening 32c, and physically masks the other part of the laser beam L in the area other than the opening 32c. That is, the other part of the laser beam L is substantially blocked.

[0106] In this modification, the mask 32 serves to partially mask the laser beam L, and the spatial light phase modulator 18 serves to split the laser beam L that passes through the opening 32c without being masked. Note that a DOE may be used instead of the spatial light phase modulator 18.

[0107] Fig. 10(A) is a diagram showing a mask preparation step S20 in the modified example, and Fig. 10(B) is a diagram showing a laser beam irradiation step S30 in the modified example. For convenience of explanation, in Fig. 10(A) and Fig. 10(B), the mask 32 is shown in cross section, and some of the components of the laser processing apparatus 2 that are the same as those in the first embodiment are omitted.

[0108] In this modified example, a modified layer 13 can be formed inside the workpiece 11 while masking the first component L1 (i.e., a part of the laser beam L) entering the inside of the workpiece 11 through the outer peripheral side surface 11d with the mask unit 20.

[0109] In the first embodiment and its modified example, the case has been described where the laser beam L is split into a plurality of focusing points P. However, as long as the laser beam L entering the inside of the workpiece 11 from the outer circumferential side surface 11d can be masked, the laser beam L does not have to be split.

[0110] The workpiece 11 is not limited to a rectangular plate shape, and may be a disk-shaped (i.e., plate-shaped) single crystal substrate. The single crystal substrate may be a so-called free-standing substrate, or may be a substrate having an epitaxially grown layer formed on a single crystal substrate serving as a seed crystal.

[0111] The workpiece 11 may be a cylindrical ingot instead of a single crystal substrate. Furthermore, the material of the workpiece 11 is not limited to GaN, and may be a single crystal substrate or an ingot made of a semiconductor material such as silicon (Si), silicon carbide (SiC), or gallium arsenide (GaAs).

[0112] When the workpiece 11 is a cylindrical ingot or a disk-shaped single crystal substrate, the outer peripheral side surface 11d is substantially cylindrical. In this case, however, the outer peripheral side surface 11d may have a flat surface, a notch, or the like that functions as an orientation flat.

[0113] Second Embodiment Next, a second embodiment will be described with reference to Fig. 11 to Fig. 16. The second embodiment relates to a method for producing a GaN wafer 27, in which a delamination layer 15 is formed on a cylindrical GaN ingot (workpiece) 21, and then the GaN wafer 27 is delaminate|separated from the GaN ingot 21 starting from the delamination layer 15.

[0114] 11 is a flow diagram showing an example of a method for manufacturing a GaN wafer 27 according to the second embodiment. In the second embodiment, the holding step S10 to the laser beam irradiation step S30 are performed using the above-mentioned laser processing apparatus 2 (see FIGS. 13 to 15), and then the delamination step S40 is performed using a delamination apparatus 40 (see FIG. 16(A)).

[0115] First, a cylindrical GaN ingot 21 to be processed will be described. Fig. 12 is a perspective view of the GaN ingot 21. The GaN ingot 21 is a single crystal of GaN having a hexagonal crystal structure. However, the conductivity type of the GaN ingot 21 is not particularly limited.

[0116] The GaN ingot 21 has an upper surface (first surface) 21a and a lower surface (second surface) 21b located on opposite sides in a thickness direction 21c. The GaN ingot 21 of this embodiment has no off-angle. The upper surface 21a corresponds to (0001), and the lower surface 21b corresponds to (000-1). The thickness direction 21c is parallel to

[0001] .

[0117] Flat surfaces 23a and 23b are formed on the sides of the GaN ingot 21, and serve as orientation flats for the GaN wafer 27. The flat surface 23a is represented by {-1100}, and the flat surface 23b is represented by {11-20}.

[0118] Between the flat surfaces 23a, 23b, there are a first side surface 23c including a shorter arc connecting the flat surfaces 23a, 23b, and a second side surface 23d including a longer arc connecting the flat surfaces 23a, 23b. The flat surfaces 23a, 23b, the first side surface 23c, and the second side surface 23d are connected to the upper surface 21a and the lower surface 21b so as to be perpendicular (i.e., intersect) with them, respectively.

[0119] In this embodiment, the holding step S10 and the mask preparation step S20 are followed by the laser beam irradiation step S30. The laser beam irradiation step S30 includes an outer peripheral region release layer formation step S32 for forming a release layer 15 in the outer peripheral region 23f as shown in Fig. 13, and a central region release layer formation step S34 for forming a release layer 15 in the central region 23g as shown in Fig. 15.

[0120] In particular, in the laser beam irradiation step S30 of this embodiment, the peripheral region peeling layer forming step S32 is performed before the peripheral region peeling layer forming step S32. Fig. 13 is a top view showing the peripheral region peeling layer forming step S32. In the peripheral region peeling layer forming step S32, the above-mentioned processing conditions are basically adopted.

[0121] However, in the outer peripheral region peeling layer forming process S32, the focal point P and the GaN ingot 21 are not moved linearly relative to each other, but the chuck table 4 is rotated at a predetermined rotation speed by the rotary drive mechanism 6. The rotation speed is set, for example, so that at least one of the focal points P is approximately the same as the above-mentioned processing feed speed.

[0122] As a result, the focal point P and the GaN ingot 21 are moved relatively so that the focal point P is aligned along the outer circumferential region 23f. Note that the annular dashed lines A1, A2, and A3 shown in Fig. 13 indicate the locus of the focal point P located at the innermost position in the radial direction of the GaN ingot 21 among the loci of the movement of the focal point P when the chuck table 4 is rotated.

[0123] At the start of the peripheral region peeling layer formation process S32, the height position of the focal point P of the laser beam L is adjusted so that the focal point P is located at a predetermined depth 25 (see Figure 16 (A)) inside the workpiece 11, and the multiple focal points P are arranged in a row along the Y-axis direction (predetermined direction) that is perpendicular (intersects) with the peripheral side surface 23e in a planar view.

[0124] In the outer periphery region separation layer forming step S32, the Y-axis direction is a direction that is perpendicular (intersecting) in a plan view to the circumferential direction (see dashed lines A1, A2, A3) in which the focal point P and the GaN ingot 21 move relatively.

[0125] In the peripheral region peeling layer formation process S32, the second component L2 (i.e., another part of the laser beam L) is branched near the focal point P so that the second component L2 that is not masked by the mask unit 20 is focused at multiple focal points P arranged along the Y-axis direction near the focal point P, and the second component L2 is irradiated onto the GaN ingot 21.

[0126] Then, the chuck table 4 is rotated once at a predetermined rotation speed in the direction of the arrow 34 shown in Fig. 13. After one rotation, the multiple focusing points P are moved radially inward of the GaN ingot 21. Specifically, the chuck table 4 is indexed and fed by a predetermined index amount (e.g., 106 µm) along the Y-axis direction.

[0127] Thereafter, a second rotation is performed in the same manner, and then a third rotation is performed after indexing in the same manner. In this manner, a plurality of peeling layers 15 arranged concentrically inside the GaN ingot 21 in the outer peripheral region 23f are formed.

[0128] In the peripheral region peeling layer formation process S32, the laser beam L is irradiated while masking the first component L1 with the spatial light phase modulator 18 so that the first component L1 (i.e., a part of the laser beam L) of the laser beam L that enters the inside of the GaN ingot 21 from the peripheral side surface 23e (i.e., the first side surface 23c or the second side surface 23d) has an intensity less than the processing threshold of the GaN ingot 21.

[0129] The range where the laser beam L is masked by the spatial light phase modulator 18 is appropriately adjusted by the controller 30 and the drive circuit 18a according to the irradiation position of the laser beam L. In the AA cross section of FIG. 13, the state where the laser beam L is masked is shown in FIG. 14(A) and FIG. 14(B).

[0130] Fig. 14(A) is a diagram showing a state in which approximately half of the laser beam L incident on the first side surface 23c or the second side surface 23d into the inside of the GaN ingot 21 is masked. For example, Fig. 14(A) corresponds to the situation in the first rotation. Note that Fig. 14(A) omits the branching of the laser beam L and shows one focusing point P, as in Fig. 4(B), but in reality, the laser beam L branches into multiple focusing points P.

[0131] In contrast, Fig. 14(B) is a diagram showing a state in which an even narrower region of the laser beam L is masked compared to the case shown in Fig. 14(A), and shows a state in which the outer peripheral portion of approximately half of the laser beam L is masked. Fig. 14(B) also corresponds to the situation in the first rotation, for example, but the masked region is extremely small.

[0132] 14(A) and 14(B), the laser beam L that enters the inside of the GaN ingot 21 from the top surface 21a is marked with diagonal lines, and the laser beam L that enters the inside of the GaN ingot 21 from the outer peripheral side surface 23e (but is masked) is marked with dots. Also, in FIG. 14(B), similar to FIG. 4(B), the branching of the laser beam L is omitted and one focusing point P is shown, but in reality, the laser beam L branches into multiple focusing points P.

[0133] When the chuck table 4 is rotated, the trajectory of the laser beam L in a plan view crosses the flat surfaces 23a and 23b. During this crossing, the range and time during which the laser beam L enters the interior of the GaN ingot 21 is extremely small.

[0134] Therefore, in the peripheral region separation layer forming step S32 of the second embodiment, the focal point P and the GaN ingot 21 are not moved relatively in a straight line along the flat surfaces 23a and 23b in a plan view.

[0135] In the peripheral region separation layer forming step S32, the focal point P and the GaN ingot 21 are moved relatively to each other so as to move along the first side surface 23c and the second side surface 23d (that is, a part of the peripheral region 23f).

[0136] In addition, although FIG. 13 shows an example in which the chuck table 4 is rotated three times to form three annular peeling layers 15 concentrically, the number of rotations is not limited to three.

[0137] As one guideline for determining the rotation speed, the rotation of the chuck table 4 and the indexing and feeding of the focal point P are alternately repeated until the trajectory of the movement of the focal point P located at the outermost radial position of the GaN ingot 21 when the chuck table 4 rotates is located inside both of the flat surfaces 23a and 23b in the same radial direction.

[0138] When the separation layer 15 is formed in the outer peripheral region 23f in the outer peripheral region separation layer forming step S32, the bond between the Ga atom and the N atom is cut off, and N2 gas (nitrogen gas) is generated.

[0139] If the central region separation layer forming step S34 is performed first, nitrogen gas generated when separation layer 15 is formed in central region 23g may cause an abnormal volume expansion region to be formed radially inward of GaN ingot 21. The same problem occurs when separation layer 15 (see FIG. 8(B)) is formed so as not to reach outer peripheral side surface 11d.

[0140] In contrast, in the present embodiment, by performing the peripheral region peeling layer formation process S32 before the central region peeling layer formation process S34, the peeling layer 15 formed in the peripheral region 23f functions as a passageway for allowing nitrogen gas generated in the central region 23g in the central region peeling layer formation process S34 to escape outside the GaN ingot 21.

[0141] Therefore, by performing the central region separation layer forming step S34 after the peripheral region separation layer forming step S32, abnormal volume expansion of the GaN ingot 21 on the inner side in the radial direction can be suppressed.

[0142] After the peripheral region separation layer forming step S32, the focal point P and the GaN ingot 21 are moved relatively in the central region 23g of the GaN ingot 21 to form a separation layer 15 in the central region 23g at approximately the same depth as that in the peripheral region 23f.

[0143] The central region 23g is located on the inner side of the outer circumferential region 23f in the plane of the upper surface 21a or the lower surface 21b. That is, the central region 23g is located on the inner side of the outer circumferential region 23f in the radial direction of the GaN ingot 21.

[0144] 15 is a top view showing the central region separation layer forming step S34. In the central region separation layer forming step S34, first, the orientation of the chuck table 4 is adjusted so that the [11-20] of the GaN ingot 21 is approximately parallel to the X-axis direction.

[0145] Then, by alternately repeating the relative movement between the focal point P and the chuck table 4 in the X-axis direction (i.e., processing feed) and the relative movement between the focal point P and the chuck table 4 in the Y-axis direction (i.e., indexing feed), multiple linear peeling layers 15 that are approximately parallel to each other are formed over almost the entire specified depth 25 of the GaN ingot 21.

[0146] Regarding the processing feed, for example, the focal point P is moved to the right side in FIG. 15 during odd-numbered relative movements, and the focal point P is moved to the left side in FIG. 15 during even-numbered relative movements, but it is also possible to move the focal point P relatively to the right during odd-numbered movements, and move the focal point P relatively to the left during even-numbered movements.

[0147] Moreover, regarding the indexing feed, the amount of indexing feed in the Y-axis direction is set to a predetermined value (for example, 106 μm). As for the other machining conditions, the machining conditions described in the first embodiment may be adopted.

[0148] At the stage of the central region peeling layer formation process S34, since the peeling layer 15 has already been formed in the peripheral region 23f, the laser beam L incident on the inside of the GaN ingot 21 from the flat surfaces 23a, 23b, the first side surface 23c, and the second side surface 23d does not contribute to the irregular formation of the peeling layer 15.

[0149] After the central region separation layer forming step S34, the GaN wafer 27 is separated from the GaN ingot 21 using a separation device 40, starting from the multiple separation layers 15 in the outer circumferential region 23f and the multiple separation layers 15 in the central region 23g (separation step S40).

[0150] 16(A) is a diagram showing a separation apparatus 40. The separation apparatus 40 has a chuck table 42 having approximately the same diameter as the above-mentioned chuck table 4. The structure of the chuck table 42 is approximately the same as that of the chuck table 4, and the upper surface of the chuck table 42 functions as a holding surface 42a that suction-holds the GaN ingot 21.

[0151] A peeling unit 44 is provided above the chuck table 42. The peeling unit 44 has a cylindrical movable part 46 whose longitudinal side is disposed along the Z-axis direction. A Z-axis direction moving mechanism (not shown) is connected to the movable part 46.

[0152] The Z-axis direction moving mechanism is, for example, a ball screw type moving mechanism, but may be configured with other actuators. The Z-axis direction moving mechanism allows the movable part 46 to move along the Z-axis direction.

[0153] A disk-shaped suction head 48 is provided on the bottom of the movable part 46. The suction head 48 has a frame and a porous plate, similar to the chuck table 42. The lower surfaces of the frame and the porous plate are disposed substantially flush and substantially parallel to the XY plane, and function as a holding surface 48a.

[0154] In the peeling step S40, the lower surface 21b of the GaN ingot 21 having the multiple peeling layers 15 formed thereon is suction-held by the holding surface 42a of the chuck table 42, and the upper surface 21a is suction-held by the holding surface 48a of the suction head 48.

[0155] Next, an external force is applied to the GaN ingot 21. The external force is applied, for example, by driving wedges (not shown) into the side surface of the GaN ingot 21 at the height positions of a plurality of the peeling layers 15. It is preferable to drive the wedges into a plurality of locations along the circumferential direction of the GaN ingot 21, rather than just one location on the side surface of the GaN ingot 21.

[0156] By applying the external force, the cracks further grow at the depth position where the multiple peeling layers 15 are formed. Note that instead of driving in the wedge, the external force may be applied by applying ultrasonic waves (i.e., elastic vibration waves in a frequency band exceeding 20 kHz) to the GaN ingot 21.

[0157] When applying ultrasonic waves, ultrasonic waves are applied to the upper surface 21a side via a liquid such as pure water before the upper surface 21a is sucked and held by the holding surface 48a of the suction head 48. Specifically, the liquid to which ultrasonic waves have been applied is sprayed from a nozzle onto the GaN ingot 21, or ultrasonic waves are applied from a horn to the upper surface 21a side via the liquid.

[0158] When using a nozzle or a horn, an external force is first applied to a local area on the upper surface 21a side having a diameter of about 5 mm to 50 mm by using ultrasonic waves. Next, the nozzle or the horn and the chuck table 42 are moved relatively to apply the external force to other areas on the upper surface 21a side.

[0159] In this way, by gradually moving the area to which the external force is applied toward the upper surface 21a, it is possible to cause the cracks between the modified layers 13 to progress along the upper surface 21a. By applying the external force, the cracks progress further between the adjacent modified layers 13, and the mechanical strength of the multiple peel layers 15 becomes further weakened.

[0160] After the external force is applied, the suction head 48 that suction-holds the GaN ingot 21 is raised. The above-mentioned external force may be applied in parallel with the raising of the suction head 48. As the suction head 48 is raised, the GaN wafer 27 is peeled off from the GaN ingot 21 starting from the multiple peeling layers 15.

[0161] 16(B) is a diagram showing the GaN ingot 21 and the GaN wafer 27 after the delamination step S40. After the delamination step S40, unevenness remaining on the delamination surface of the GaN ingot 21 and the delamination surface of the GaN wafer 27 may be removed by grinding, polishing, or the like.

[0162] In this manner, a GaN wafer 27 having a thickness less than the thickness of the GaN ingot 21 (ie, the distance between the upper surface 21a and the lower surface 21b) is produced.

[0163] Note that a GaN wafer 27 having a thickness less than that of a GaN single crystal substrate may be manufactured from a rectangular plate-shaped or disk-shaped (i.e., plate-shaped) GaN single crystal substrate (workpiece) instead of the GaN ingot 21. The GaN single crystal substrate may be a so-called free-standing substrate, or may be a substrate having an epitaxially grown layer formed on a single crystal substrate serving as a seed crystal.

[0164] In the peripheral region peeling layer formation process S32 of the second embodiment, a spatial light phase modulator 18 is used as the mask unit 20, but instead, the mask 32 described in the modified example of the first embodiment (see Figures 10(A) and 10(B)) may be used.

[0165] Even when the mask 32 is used, it is necessary to appropriately adjust the range over which the laser beam L is masked according to the irradiation position of the laser beam L. However, since the masked range cannot be adjusted by an electrical signal like the spatial light phase modulator 18, the masked range of the laser beam L is adjusted by appropriately selecting a plurality of masks 32 having openings 32c of different sizes.

[0166] Incidentally, the GaN ingot 21 of this embodiment does not have an off-angle, but regardless of the presence or absence of an off-angle, the X-axis direction may be set to within <11-20>±5° of the GaN ingot 21 during processing.

[0167] (First Modification) Next, a first modification of the second embodiment will be described with reference to Fig. 17. In the first modification, a rectangular plate-shaped GaN wafer (not shown) having a thickness less than that of the rectangular plate-shaped GaN single crystal substrate 31 is manufactured from the rectangular plate-shaped GaN single crystal substrate 31.

[0168] In the first modified example, each step is performed according to the flow shown in Fig. 11. However, in the peripheral region peeling layer forming step S32, the locus of the light-concentration point P is not circular but rectangular so as to follow each side of the GaN single crystal substrate 31, but the notation in Fig. 13 is used to describe the peripheral region 23f and the central region 23g.

[0169] Fig. 17 is a plan view showing the peripheral region separation layer forming step S32 for a rectangular plate-shaped GaN single crystal substrate 31. The dashed lines B1, B2, and B3 shown in Fig. 17 show an outline of the trajectory of movement of the light focusing point P closest to the center of the upper surface (first surface) 31a of the GaN single crystal substrate 31 when the chuck table 4 is processed and fed in the peripheral region separation layer forming step S32.

[0170] When processing of the outermost circumference begins, the height position of the focal point P of the laser beam L is adjusted so that the focal point P is located at a predetermined depth inside the GaN single crystal substrate 31, and multiple focal points P are arranged in a line along the Y-axis direction.

[0171] At this time, as described above, a part of the laser beam L is masked so that the laser beam L is not irradiated onto the inside of the GaN single crystal substrate 31 from the outer peripheral side surface located at the outer periphery 31c.

[0172] Then, by feeding the chuck table 4 in a predetermined direction along the X-axis direction (for example, the +X direction) at a predetermined feeding speed, the focal point P and the GaN single crystal substrate 31 are moved relatively along one side of the outer circumferential edge 31c from one end of the side to the other end of the side.

[0173] Next, the chuck table 4 is rotated 90 degrees in a predetermined direction. Then, the chuck table 4 is fed for processing in a direction opposite to the predetermined direction (for example, in the -X direction) at a predetermined processing feed speed, thereby relatively moving the focal point P and the GaN single crystal substrate 31 along the other side of the outer circumferential edge 31c from one end of the other side to the other end of the other side.

[0174] Similarly, after the first round of processing for forming the peeling layer 15 along each side of the outer circumferential edge 31c is performed, the multiple light-converging points P are moved to the inside of the upper surface 31a. Specifically, the chuck table 4 is indexed and fed along the Y-axis direction by a predetermined index amount (e.g., 106 μm). Then, similarly, the second and third rounds of processing are sequentially performed.

[0175] After the peripheral area release layer forming step S32 is completed, the central area release layer forming step S34 and the release step S40 are performed in the same manner as in the second embodiment.

[0176] Incidentally, the relative movement direction between the focal point and GaN ingot 21 or GaN single crystal substrate 31 (that is, the workpiece) is not limited to within <11-20>±5° on the (0001) plane.

[0177] According to the applicant's experiments, it has been found that when forming a peeling layer 15 in which modified layers 13 formed adjacent to each other in a plane are connected by cracks, the average output of the laser beam L can be reduced by setting the relative movement direction between the focal point and the workpiece to within <10-10>±10° on the (0001) plane.

[0178] Referring to Figures 18 to 19(C), we will explain the experimental results that investigated the relationship between the minimum average output of the laser beam L that can form a crack connecting adjacent modified layers 13 and the movement direction of multiple focal points P that move linearly on the (0001) plane.

[0179] 18 is a plan view of a GaN ingot 21 showing the angle θ between [11-20] on the (0001) plane and the moving direction of multiple focusing points P (i.e., the scanning direction of the laser beam L) in an experiment to evaluate the crystal orientation dependency of the minimum average output at which cracks are formed. Note that, also in this experiment, the direction in which the multiple focusing points P are arranged is perpendicular to the moving direction of the multiple focusing points P on the (0001) plane.

[0180] For example, when the movement direction of the multiple focusing points P is the [11-20] direction, θ=0°, and when the movement direction of the multiple focusing points P is the [-1100] direction, θ=90°. The processing conditions in this experiment were as follows.

[0181] Wavelength: 1064nm Machining feed speed: 1000mm / s Repetition frequency: 50kHz Number of bursts: 10 (number of pulses in a pulse group) Number of branches: 10 (number of branches of laser beam L) Number of passes: 1 Spot diameter of each focusing point: approx. 5 μm Distance between adjacent focal points: 12.5μm Depth position of the focal point: Approximately 170 μm from the upper surface 21 a into the GaN ingot 21

[0182] In this experiment, in which the number of branches is 10, the average output of the laser beam L means the average output of the laser beam L that is incident on the GaN ingot 21 after passing through the focusing lens 26 in the focusing device.

[0183] In this experiment, first, the value of θ was fixed to 0° and the average output of the laser beam L was fixed to a predetermined value, and then the upper surface 21a was scanned with the laser beam L. Next, the value of θ was fixed to 0°, and the average output of the laser beam L was increased by 0.05 W.

[0184] In this way, the average output of the laser beam L was gradually increased, and as a result, the minimum average output of the laser beam L capable of forming a crack connecting adjacent modified layers 13 was 1.25 W (θ=0°).

[0185] Next, θ was increased by 10°, and the output of the laser beam L was increased stepwise in the same manner. As a result, the minimum average output of the laser beam L was 1.10 W (θ=10°). In the same manner, θ was increased in increments of 10°, and the minimum average output of each laser beam L that could form a crack connecting adjacent modified layers 13 was searched for at each angle.

[0186] As a result, the minimum average power of the laser beam L was 1.05W (θ=20°), 1.05W (θ=30°), 1.05W (θ=40°), 1.10W (θ=50°), 1.25W (θ=60°), 1.10W (θ=70°), 1.05W (θ=80°), and 1.05W (θ=90°), respectively.

[0187] FIG. 19(A) is a graph showing the minimum average output at which a crack is formed versus angle θ, FIG. 19(B) is a plan view of a GaN ingot 21 when θ=30°, and FIG. 19(C) is a plan view of a GaN ingot 21 when θ=90°.

[0188] 19(A), when θ is in the range of 20° or more and 40° or less (i.e., 30°-10°≦θ≦30°+10°), the average output of the laser beam L is minimum. In other words, the scanning direction of the laser beam L is preferably in the range of [01-10]±10° or less on the (0001) plane.

[0189] Here, experimental results for θ are only available up to 90°, but considering that GaN ingot 21 is a hexagonal crystal and has a 60° rotational symmetry around the c-axis (i.e., the

[0001] direction) in the (0001) plane, it can be reasonably inferred that θ is also in the range of 80° or more and 100° or less (i.e., 90°-10°≦θ≦90°+10°).

[0190] In other words, it can be said that the scanning direction of the laser beam L is similarly preferable even if it is within the range of [-1100]±10° on the (0001) plane. Also, considering the rotational symmetry of 60°, it can be said that the scanning direction of the laser beam L is similarly preferable if it is within the range of <10-10>±10° on the (0001) plane.

[0191] In other words, it can be reasonably assumed that by making the angle formed between the crystal orientation represented by (1) below and the scanning direction of the laser beam L in the (0001) plane 10° or less, cracks connecting adjacent modified layers 13 can be formed without excessively increasing the average output of the laser beam L.

[0192]

number

[0193] (Second Modification) Fig. 20 is a plan view showing a central region peeling layer forming step S34 according to a second modification of the second embodiment based on the above-mentioned experimental results. In Fig. 20, for ease of understanding, two of the multiple light focusing points P are shown as relatively large circles, and some light focusing points located between the two light focusing points P are omitted.

[0194] In the central region separation layer forming step S34 of this embodiment, first, the orientation of the chuck table 4 is adjusted so that the [1-100] of the GaN ingot 21 is approximately parallel to the X-axis direction. Then, the multiple focusing points P are relatively moved in the -X direction, and then the multiple focusing points P are relatively moved in the +X direction. In this manner, the movement in the -X direction and the movement in the +X direction are alternately repeated.

[0195] 20, the movement paths of the multiple focal points P in the GaN ingot 21 are indicated by dashed arrows. Note that instead of moving the multiple focal points P alternately in the -X direction and the +X direction, they may be moved only in the -X direction or only in the +X direction.

[0196] When the relative movement direction between the multiple focal points P and the GaN ingot 21 is along the X-axis direction, the movement direction is parallel to [1-100] and [-1100]. These crystal orientations are two of six equivalent crystal orientations in the GaN ingot 21 having a hexagonal crystal structure, as shown in (2) below.

[0197]

number

[0198] Of course, as is clear from the above experiments, the angle formed between the relative movement direction of the multiple focal points P and the GaN ingot 21 and the crystal orientation represented by (2) on the (0001) plane of the GaN ingot 21 may be 10° or less (i.e., <10-10>±10° or less).

[0199] Similarly, when performing central region peeling layer formation process S34 on a GaN single crystal substrate 31 having the same crystal structure as GaN ingot 21, the relative movement direction between the multiple focusing points P and GaN single crystal substrate 31 may be set to be within <10-10>±10° on the (0001) plane of GaN single crystal substrate 31.

[0200] In the first and second embodiments and their modified examples, the case has been described where the laser beam L is split into a plurality of focusing points P. However, although it is expected that the processing time will be long, in principle, it is not necessary to split the laser beam L, and it is sufficient to be able to mask the laser beam L that enters the inside of the workpiece from the outer circumferential side surface.

[0201] In addition, the structures, methods, and the like according to the above-described embodiments can be modified as appropriate without departing from the scope of the object of the present invention. [Explanation of symbols]

[0202] 2: laser processing device, 4: chuck table, 4a: holding surface 6: Rotation drive mechanism, 8: Horizontal movement mechanism 11: Workpiece, 11a: Top surface (first surface), 11b: Bottom surface (second surface) 11c: Outer area, 11d: Outer side 12: laser beam irradiation unit, 14: laser oscillator, 16: output regulator 13: modified layer, 15: release layer 18: spatial light phase modulator, 18a: driving circuit, 18b: arrow 20: Mask unit 21: GaN ingot (workpiece) 21a: Top surface (first surface), 21b: Bottom surface (second surface), 21c: Thickness direction 22a, 22b: Lenses 23a, 23b: flat surface, 23c: first side, 23d: second side 23e: Outer side, 23f: Outer area, 23g: Center area 24: Mirror, 26: Condenser lens 25: Predetermined depth, 27: GaN wafer 30: Controller 31: GaN single crystal substrate, 31a: upper surface (first surface), 31c: outer periphery 32: mask, 32a: one surface, 32b: other surface, 32c: opening 34: Arrow 40: peeling device, 42: chuck table, 42a: holding surface 44: peeling unit, 46: movable part, 48: suction head, 48a: holding surface A1, A2, A3, B1, B2, B3: dashed lines L: laser beam, L1: first component, L2: second component, P: focal point S10: Holding step, S20: Mask preparation step, S30: Laser beam irradiation step S32: peripheral region peeling layer forming step, S34: central region peeling layer forming step S40: Peeling process

Claims

1. A laser processing method, comprising: a mask preparation step of adjusting a mask unit that masks a portion of a laser beam when a pulsed laser beam having a wavelength that passes through a columnar or plate-shaped workpiece having a first surface, a second surface opposite to the first surface, and an outer peripheral side surface that intersects with the first surface and the second surface is irradiated onto an outer peripheral region of the workpiece in a direction proceeding from the first surface to the second surface, so that a portion of the laser beam that enters the inside of the workpiece from the outer peripheral side surface has an intensity less than a processing threshold value of the workpiece; a laser beam irradiation step of forming a modified layer inside the workpiece while masking a part of the laser beam with the mask unit so that a part of the laser beam incident on the inside of the workpiece via the outer peripheral side surface has an intensity less than the processing threshold value of the workpiece, by relatively moving the focus point and the workpiece so that the focus point is along the outer peripheral region while adjusting the height position of the focus point of the laser beam after the mask preparation step so that the focus point is located inside the workpiece; A laser processing method comprising:

2. the mask unit has a spatial light phase modulator that at least partially modulates the laser beam, or a metal mask that partially masks the laser beam; The laser processing method according to claim 1, characterized in that in the laser beam irradiation step, a portion of the laser beam is masked by at least partially modulating the laser beam by the spatial light phase modulator so that the light intensities are weakened, or a portion of the laser beam is physically masked by the metal mask.

3. The laser processing method according to claim 1 or 2, characterized in that in the laser beam irradiation step, another portion of the laser beam not masked by the mask unit is branched so that the other portion of the laser beam is focused at a plurality of focusing points arranged along a predetermined direction that intersects, in a planar view, with a direction in which the focusing point and the workpiece move relatively, and then the other portion of the laser beam is irradiated onto the workpiece.

4. A method for producing a GaN wafer from a workpiece that is a GaN ingot or a GaN single crystal substrate, the method comprising the steps of: The workpiece has a first surface, a second surface opposite to the first surface, and an outer peripheral side surface intersecting the first surface and the second surface; The method for producing the GaN wafer includes the steps of: a peripheral region peeling layer forming step of relatively moving the workpiece and the focal point of a pulsed laser beam having a wavelength that transmits through the workpiece so that the focal point is located inside the workpiece while adjusting the height position of the focal point so that the focal point is along the peripheral region of the workpiece, thereby forming a peeling layer in the peripheral region; a central region peeling layer forming step of forming a peeling layer in a central region of the workpiece located inside the peripheral region in the first surface or the second surface by relatively moving the focusing point and the workpiece, the central region being located inside the peripheral region; a delamination step of delaminating the GaN wafer from the workpiece, starting from the delamination layer in the outer periphery region and the delamination layer in the central region; Equipped with A method for manufacturing a GaN wafer, characterized in that in the peripheral region peeling layer formation process, a peeling layer is formed inside the workpiece while masking a portion of the laser beam with a mask unit so that a portion of the laser beam that enters the inside of the workpiece from the peripheral side surface has an intensity less than the processing threshold value of the workpiece.

5. the mask unit has a spatial light phase modulator that at least partially modulates the laser beam, or a metal mask that partially masks the laser beam; The method for manufacturing a GaN wafer according to claim 4, characterized in that in the peripheral region peeling layer formation process, a portion of the laser beam is masked by at least partially modulating the laser beam by the spatial light phase modulator so that the light intensities are weakened, or a portion of the laser beam is physically masked by the metal mask.

6. 5. The method for manufacturing a GaN wafer according to claim 4, characterized in that in the peripheral region peeling layer formation process, another portion of the laser beam not masked by the mask unit is branched so that the other portion of the laser beam is focused at a plurality of focusing points arranged along a predetermined direction that intersects in a planar view with a direction in which the focusing point and the workpiece move relatively, and then the other portion of the laser beam is irradiated onto the workpiece.

7. 7. The method for manufacturing a GaN wafer according to claim 4, wherein a direction in which the focal point and the workpiece are moved relatively in the central region peeling layer formation step forms an angle of 10° or less with a crystal orientation expressed by the following (1) on a (0001) plane of the GaN ingot or the GaN single crystal substrate: [0010]

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