Manufacturing method for gallium nitride substrate

Abstract

To provide a manufacturing method of a gallium nitride substrate capable of efficiently cutting out a gallium nitride substrate from a gallium nitride ingot.SOLUTION: A manufacturing method of a gallium nitride (GaN) substrate includes: a release layer forming step of forming a release layer at a depth corresponding to a thickness of a GaN substrate to be manufactured by relatively moving a GaN ingot and a light-converging point of a laser beam having a wavelength transmitting through GaN along a direction of a crystal orientation represented by the following formula (1) of the GaN ingot while positioning the light-converging point inside the GaN ingot; and a release step of releasing the GaN substrate from the GaN ingot using the release layer as a starting point. In the release layer forming step, the laser beam is branched to form the plurality of light converging points, and a straight line connecting the branched light converging points is set along a direction parallel to a direction of the crystal orientation represented by the following formula (2).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a gallium nitride substrate. [Background technology]

[0002] Gallium nitride (GaN) has a band gap three times larger than that of silicon (Si), and therefore its use in devices such as power devices and light-emitting diodes (LEDs) is being considered. It is known that gallium nitride substrates (GaN substrates) are cut from gallium nitride ingots (GaN ingots) using an inner peripheral blade, which can have a thinner blade thickness than an outer peripheral blade (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-084469 Summary of the Invention [Problem to be solved by the invention]

[0004] However, even if an inner diameter cutting blade is used to cut out a GaN ingot, the thickness of the inner diameter cutting blade is, for example, about 0.3 mm compared to the thickness of the GaN substrate (for example, 150 μm), so 60 to 70% of the GaN ingot is cut off and discarded during cutting, which is uneconomical.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for manufacturing a gallium nitride substrate that enables gallium nitride substrates to be efficiently cut out from a gallium nitride ingot. [Means for solving the problem]

[0006] In order to solve the above-mentioned problems and achieve the object, the method for manufacturing a gallium nitride substrate of the present invention is a method for manufacturing a gallium nitride substrate from a gallium nitride ingot having a first surface and a second surface opposite to the first surface, and comprises: a holding step for holding the gallium nitride ingot; a delamination layer formation step for positioning a focal point of a laser beam having a wavelength that transmits through gallium nitride from the first surface to inside the gallium nitride ingot and relatively moving the gallium nitride ingot and the focal point along the direction of the crystal orientation of the gallium nitride ingot expressed by the following formula (1), thereby forming a delamination layer to a depth corresponding to the thickness of the gallium nitride substrate to be manufactured; and a delamination step for delaminating the gallium nitride substrate from the gallium nitride ingot using the delamination layer as a starting point, wherein in the delamination layer formation step, the laser beam is branched to form a plurality of focal points, and the straight lines connecting each of the branched focal points are set to be parallel to the direction of the crystal orientation expressed by the following formula (2).

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[0007] In the peeling layer formation step, the moving speed of the gallium nitride ingot and the plurality of focusing points may be set so that a straight line connecting adjacent processing marks formed by moving the gallium nitride ingot and the plurality of focusing points relative to each other is formed along the direction of the crystal orientation expressed by the formula (2). [Effects of the Invention]

[0008] The present invention makes it possible to efficiently cut out and manufacture gallium nitride substrates from gallium nitride ingots. [Brief explanation of the drawings]

[0009] [Figure 1]FIG. 1 is a perspective view showing an example of a gallium nitride ingot used in the method for producing a gallium nitride substrate according to the first embodiment. [Figure 2] FIG. 2 is a top view illustrating the crystal orientation of the gallium nitride ingot of FIG. [Figure 3] FIG. 3 is a flowchart showing the procedure of the method for manufacturing a gallium nitride substrate according to the embodiment. [Figure 4] FIG. 4 is a cross-sectional view illustrating the holding step and the release layer forming step of FIG. [Figure 5] FIG. 5 is a top view illustrating the release layer forming step of FIG. [Figure 6] FIG. 6 is a top view illustrating the branching of the laser beam in the peeling layer forming step of FIG. [Figure 7] FIG. 7 is a top view illustrating processing marks formed by the laser beam in the peeling layer forming step of FIG. [Figure 8] FIG. 8 is a cross-sectional view illustrating the peeling step of FIG. [Figure 9] FIG. 9 is a cross-sectional view illustrating the peeling step of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0010] Modes (embodiments) for carrying out the present invention will be described in detail with reference to the drawings. The present invention is not limited to the contents described in the following embodiments. Furthermore, the components described below include those that can be easily imagined by a person skilled in the art and those that are substantially the same. Furthermore, the configurations described below can be combined as appropriate. Furthermore, various omissions, substitutions, or modifications of the configuration can be made within the scope of the gist of the present invention.

[0011] [Embodiment] A method for manufacturing a gallium nitride substrate according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a perspective view showing a gallium nitride ingot 100, which is an example of a gallium nitride ingot used in the method for manufacturing a gallium nitride substrate according to embodiment 1. FIG. 2 is a top view illustrating the crystal orientation of the gallium nitride ingot 100 of FIG. 1. The method for manufacturing a gallium nitride substrate according to an embodiment is a method for manufacturing a gallium nitride substrate (GaN substrate, GaN wafer) 130 (see FIG. 9) from a gallium nitride ingot (GaN ingot) 100. The GaN ingot 100 is a single crystal of gallium nitride (GaN) having a hexagonal crystal structure. However, the conductivity type of the GaN ingot 100 is not particularly limited. The GaN ingot 100 may be p-type containing p-type impurities such as magnesium (Mg) or beryllium (Be), or may be n-type containing n-type impurities such as silicon (Si) or germanium (Ge).

[0012] 1, the GaN ingot 100 in this embodiment is formed in an overall cylindrical shape, and has a flat circular first surface 101 exposed upward, a flat circular second surface 102 exposed downward on the opposite side of the first surface 101, and a peripheral surface 103 located between the first surface 101 and the second surface 102. The GaN ingot 100 has a diameter of 4 inches (approximately 100 mm) and a thickness of 500 μm, although the diameter and thickness are not limited to these values.

[0013] 1, flat rectangular orientation flats 104 and 105 are formed on peripheral surface 103 of GaN ingot 100. However, the present invention is not limited to this, and instead of orientation flats 104 and 105, notches extending in an axial direction perpendicular to first surface 101 and second surface 102 may be formed at similar positions on peripheral surface 103.

[0014] In this specification, Miller indices are used to identify the crystal orientations and crystal planes of GaN single crystals. In this specification, specific crystal orientations are expressed using [ ], and crystal orientations that are equivalent due to the symmetry of the crystal structure are expressed using < >. In addition, specific crystal planes are expressed using ( ), and crystal planes that are equivalent due to the symmetry of the crystal structure are expressed using {}.

[0015] As shown in FIG. 1 , the first face 101 corresponds to the crystal plane (1-1) below and is perpendicular to the crystal orientation (2-1) below. The second face 102 corresponds to the crystal plane (1-2) below and is perpendicular to the crystal orientation (2-2) below. The orientation flat 104 is planar, corresponds to the crystal plane (1-3) below and is perpendicular to the crystal orientation (2-3) below. The orientation flat 105 is planar, corresponds to the crystal plane (1-4) below and is perpendicular to the crystal orientation (2-4) below. That is, the GaN ingot 100 is manufactured so that the following crystal plane (1-1) is exposed on the first surface 101, the following crystal plane (1-2) is exposed on the second surface 102, the following crystal plane (1-3) is exposed on the orientation flat 104, and the following crystal plane (1-4) is exposed on the orientation flat 105. The GaN ingot 100 may also be manufactured so that the following crystal plane (1-3) is exposed on the orientation flat 105 and the following crystal plane (1-4) is exposed on the orientation flat 104. The crystal orientations (2-1), (2-3), and (2-4) are perpendicular to each other. Therefore, the orientation flat 104 is formed parallel to the crystal orientation (2-4), and the orientation flat 105 is formed parallel to the crystal orientation (2-3).

[0016]

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[0017] The three crystal orientations (2-4), (2-5), and (2-6) shown in Figure 2, which form angles of 120° with each other, all belong to the crystal orientations represented by the following formula (2), which are equivalent to each other, due to the symmetry of the hexagonal crystal structure of GaN ingot 100.

[0018]

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[0019] The GaN ingot 100 has a property that the following crystal plane (3-3) including the crystal plane (1-4) is less likely to become a cleavage plane than the following crystal plane (3-1) including the crystal plane (1-1) and the crystal plane (1-2) and the following crystal plane (3-2) including the crystal plane (1-3). That is, the GaN ingot 100 has a property that the GaN ingot 100 is less likely to cleave along the crystal plane (3-3) than along the crystal plane (3-1) and the crystal plane (3-2).

[0020]

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[0021] Furthermore, the three crystal orientations (2-3), (2-7), and (2-8) that form angles of 120° with each other shown in Figure 2, and the three crystal orientations (2-9), (2-10), and (2-11) that form angles of 120° with each other, all belong to crystal orientations that are equivalent to each other and are expressed by the following formula (1) due to the symmetry of the hexagonal crystal structure of GaN ingot 100.

[0022]

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[0023] Next, this specification will explain a method for manufacturing a gallium nitride substrate according to an embodiment with reference to the drawings. Figure 3 is a flowchart showing the processing steps of the method for manufacturing a gallium nitride substrate according to an embodiment. The method for manufacturing a gallium nitride substrate according to an embodiment is a method for manufacturing a GaN substrate 130 from a GaN ingot 100, and includes a holding step 1001, a separation layer forming step 1002, and a separation step 1003. The manufacturing method of the gallium nitride substrate according to the embodiment is a method of manufacturing a GaN substrate by forming a delamination layer 110 in a GaN ingot 100 and breaking the GaN ingot 100 along the crystal plane (3-1) using the delamination layer 110, thereby delaminating a GaN substrate 130 from the GaN ingot 100. By arranging multiple focal points 19 (see Figure 4) of an irradiated laser beam 18 (see Figure 4) along a crystal orientation expressed by equation (2) that is perpendicular to a crystal plane (3-3) that is relatively difficult to cleave, and arranging multiple processing marks 25 (modified layers) formed by irradiation with the laser beam 18, the method can reduce cleavage along the crystal plane (3-2) more than conventionally.

[0024] Fig. 4 is a cross-sectional view illustrating the holding step 1001 and the peeling layer forming step 1002 in Fig. 3. Fig. 5 is a top view illustrating the peeling layer forming step 1002 in Fig. 3. Fig. 6 is a top view illustrating the branching of the laser beam 18 in the peeling layer forming step 1002 in Fig. 3. Fig. 7 is a top view illustrating the processing marks 25 formed by the laser beam 18 in the peeling layer forming step 1002 in Fig. 3. Fig. 7 is an enlarged view of VII in Fig. 5.

[0025] The holding step 1001 and the peeling layer forming step 1002 are performed by a laser processing apparatus 10 shown in Fig. 4. As shown in Fig. 4, the laser processing apparatus 10 includes a holding table 11 that holds a GaN ingot 100 on a holding surface 12, an oscillator 13, an output adjustment unit 14, a branching unit 15, a mirror 16, a condenser 17, a moving unit (not shown), and a control unit (not shown).

[0026] The holding table 11 is, for example, a chuck table that suction-holds the GaN ingot 100 from the second surface 102 side on a holding surface 12 with the first surface 101 side exposed. The holding table 11 is provided so as to be rotatable about an axis parallel to the Z-axis direction, which is vertical and perpendicular to the holding surface 12, by a rotary drive source (not shown).

[0027] The oscillator 13 oscillates a laser beam 18 having a wavelength that is transparent to GaN (GaN ingot 100). The oscillator 13 has, for example, Nd:YAG, Nd:YVO4, or the like as a laser medium, and emits a pulsed (e.g., several tens of MHz) laser beam 18 having a wavelength (e.g., 1064 nm) that is transparent to GaN (GaN ingot 100).

[0028] The output adjustment unit 14 adjusts the output of the laser beam 18 oscillated by the oscillator 13. The output adjustment unit 14 is, for example, an acousto-optic modulator (AOM), and operates in accordance with an input electrical signal, deflecting the laser beam 18 for a predetermined time in accordance with the signal, and converting the laser beam 18 into a burst mode in which the laser beam 18 is thinned out for a predetermined time. In this embodiment, the laser beam 18 adjusted by the output adjustment unit 14 has a pulse repetition frequency of about several kHz to several tens of kHz (e.g., 50 kHz), and the number of bursts is about several to several tens (e.g., 10).

[0029] The branching unit 15 branches the laser beam 18, whose output has been adjusted by the output adjustment unit 14, into a plurality of beams (several to a dozen or so beams, five beams in the example shown in FIG. 4) at predetermined intervals in a predetermined direction in the XY plane. The branching unit 15 has, for example, an LCOS-SLM (Liquid Crystal on Silicon - Spatial Light Modulator), but a diffraction grating may be used instead of the LCOS-SLM.

[0030] Mirror 16 reflects the multiple laser beams 18 branched by branching unit 15 to change the direction of the optical axis. Concentrator 17 focuses the multiple laser beams 18 reflected by mirror 16 to form multiple focal points 19, which are then irradiated onto GaN ingot 100. In this embodiment, the spot diameter of focal points 19 is set to about several μm (for example, about 5 μm).

[0031] The moving unit moves the holding table 11 and the condenser 17 relatively along the processing feed direction and the indexing feed direction, thereby moving the GaN ingot 100 held on the holding table 11 relatively along the processing feed direction and the indexing feed direction, and the GaN ingot 100 and the focal points 19 of the multiple laser beams 18 formed by the condenser 17 relatively along the processing feed direction and the indexing feed direction. Here, in this embodiment, the processing feed direction is set in the X-axis direction of the laser processing apparatus 10, and the indexing feed direction is set in the Y-axis direction of the laser processing apparatus 10.

[0032] The control unit of the laser processing apparatus 10 controls the operation of each component of the laser processing apparatus 10 to cause the laser processing apparatus 10 to perform the holding step 1001 and the peeling layer forming step 1002. In this embodiment, the control unit of the laser processing apparatus 10 includes a computer system. The computer system included in the control unit of the laser processing apparatus 10 includes an arithmetic processing device having a microprocessor such as a CPU (Central Processing Unit), a storage device having memory such as a ROM (Read Only Memory) or RAM (Random Access Memory), and an input / output interface device. The arithmetic processing device of the control unit of the laser processing apparatus 10 performs arithmetic processing in accordance with a computer program stored in the storage device of the control unit of the laser processing apparatus 10, and outputs control signals for controlling the laser processing apparatus 10 to each component of the laser processing apparatus 10 via the input / output interface device of the control unit of the laser processing apparatus 10.

[0033] The holding step 1001 is a step in which the GaN ingot 100 is held by the holding table 11 of the laser processing apparatus 10, as shown in FIG. Specifically, in the holding step 1001, the GaN ingot 100 is transported onto the holding table 11 by a transport unit or the like (not shown), the GaN ingot 100 is placed on the holding surface 12 with the first surface 101 facing upward, and the second surface 102 of the GaN ingot 100 is held by suction on the holding surface 12 of the holding table 11. After that, the holding table 11 is rotated around the Z axis by a rotation drive source (not shown), so that the orientation flat 104 formed parallel to the crystal orientation (2-4) of the GaN ingot 100 is aligned in a direction rotated 30° from the direction along the processing feed direction (the X-axis direction of the laser processing apparatus 10), and the orientation flat 105 formed parallel to the crystal orientation (2-3) of the GaN ingot 100 is aligned in a direction rotated 30° from the direction along the indexing feed direction (the Y-axis direction of the laser processing apparatus 10). That is, in the holding step 1001, the crystal orientation (2-8) of the GaN ingot 100 held by the holding table 11 is set to be along the X-axis direction of the laser processing device 10. Alternatively, the orientation flat 105 and the X-axis direction of the laser processing device 10 are aligned, and the crystal orientation (2-9) is set to be along the X-axis direction.

[0034] Note that the holding step 1001 is not limited to this in the present invention, and it is sufficient to set a specific crystal orientation included in the crystal orientation expressed by the above formula (1) of the GaN ingot 100 held by the holding table 11 to be aligned with the X-axis direction of the laser processing apparatus 10. Here, in this embodiment, the specific crystal orientation included in the crystal orientation expressed by formula (1) refers to any of crystal orientations (2-3), (2-7), (2-8), (2-9), (2-10), and (2-11). Furthermore, in this embodiment, aligning (setting) to be aligned with a predetermined orientation or direction refers to aligning (setting) such that the angle formed with the predetermined orientation or direction is 10° or less.

[0035] As shown in Figures 4 and 5, the peeling layer formation step 1002 is a step in which a focal point 19 of a laser beam 18 having a wavelength that transmits through GaN (GaN ingot 100) is positioned inside the GaN ingot 100 from the first surface 101, and the GaN ingot 100 and the focal point 19 are moved relatively along the direction of the crystal orientation of the GaN ingot 100 expressed by the above-mentioned equation (1), thereby forming a peeling layer 110 to a depth corresponding to the thickness 120 (see Figure 8) of the GaN substrate 130 to be manufactured.

[0036] In this embodiment, the delamination layer forming step 1002 includes a laser beam irradiation step and an indexing step. After the holding step 1001 is performed, the delamination layer forming step 1002 alternately performs the laser beam irradiation step and the indexing step to form a delamination layer 110 including a plurality of modified layers and cracks extending from the modified layers along a direction parallel to the first surface 101 inside the GaN ingot 100.

[0037] The laser beam irradiation step is a step in which the control unit of the laser processing apparatus 10 uses the moving unit to move (process) the focal point 19 of the laser beam 18 and the GaN ingot 100 relatively in the processing feed direction, i.e., along the direction of a specific crystal orientation (crystal orientation (2-8) in this embodiment) that is parallel to the first face 101 of the GaN ingot 100 and included in the crystal orientation expressed by the above-mentioned formula (1), while irradiating the laser beam 18 using the concentrator 17 to form a modified layer and cracks extending from the modified layer inside the GaN ingot 100 along a direction parallel to the first face 101. When the GaN ingot 100 is irradiated with the laser beam 18 in the laser beam irradiation step, a modified layer is formed in a direction parallel to the first surface 101 near the focal point 19 of the laser beam 18 along a line parallel to the processing feed direction of the laser beam 18, and cracks are formed extending from both sides of the modified layer in a direction parallel to the first surface 101. The modified layer is a region in which, for example, the density, refractive index, mechanical strength, or other physical properties are different from those of the surrounding area.

[0038] Further, in this embodiment, in the laser beam irradiation step of the peeling layer formation step 1002, the branching unit 15 of the laser processing apparatus 10 branches the laser beam 18 to form a plurality of focal points 19, and the straight lines 21 connecting each of the branched focal points 19 are set to be parallel to the direction of a specific crystal orientation included in the crystal orientation expressed by the above formula (2). More specifically, in the laser beam irradiation step of the peeling layer formation step 1002, the straight lines 21 connecting adjacent focal points 19 formed by branching the laser beam 18 by the branching unit 15 are set to be parallel to the direction of a specific crystal orientation included in the crystal orientation expressed by the above formula (2), as shown in FIG.

[0039] In the laser beam irradiation step of the peeling layer formation step 1002, the spacing between adjacent focal points 19 formed by branching the laser beam 18 using the branching unit 15 is set to be 5 μm or more and 20 μm or less. In this embodiment, the set value for the spacing in the X-axis direction (the direction parallel to the crystal orientation (2-8)) is, for example, 14.4 μm, and in this case, the set value for the spacing in the direction parallel to the Y-axis direction is 12.5 μm. Adjacent focal points 19 refer to a pair of focal points 19 whose spacing is within a range obtained by adding a predetermined error to this set value. In this embodiment, the predetermined error is ±10% or less of this set value, and preferably ±5% or less.

[0040] By setting the laser beam irradiation step of the peeling layer formation step 1002 in this manner, each of the multiple focal points 19 formed by branching from the laser beam 18 by the branching unit 15 can be formed in one of six specific crystal orientations included in the crystal orientation expressed by equation (2) (including the three crystal orientations expressed by equation (2) as well as the corresponding crystal orientations in the opposite positive and negative directions), as shown in Figure 6, and can be arranged so that they are located on the intersections of a grid whose side length is the set value of the spacing between adjacent focal points 19.

[0041] Specifically, in the branching pattern of the focal points 19 from the laser beam 18 shown in Fig. 6(A), all four straight lines 21 connecting adjacent focal points 19 extend along a direction parallel to the crystal orientation (2-4) from the bottom to the top of the paper in Fig. 6. In the branching pattern of the focal points 19 from the laser beam 18 shown in Fig. 6(B), the straight lines 21 connecting adjacent focal points 19 extend along a direction parallel to the crystal orientation (2-4), a direction parallel to the crystal orientation (2-5), a direction parallel to the crystal orientation (2-4), and a direction parallel to the crystal orientation (2-5), from the bottom to the top of the paper in Fig. 6, respectively. In the branching pattern of the focal points 19 from the laser beam 18 shown in Figure 6(C), the straight lines 21 connecting the adjacent focal points 19 extend along the direction parallel to the crystal orientation (2-5), the direction parallel to the crystal orientation (2-5), the direction parallel to the crystal orientation (2-4), and the direction parallel to the crystal orientation (2-4), respectively, from the bottom to the top of the page in Figure 6. In the branching pattern of the focal points 19 from the laser beam 18 shown in Figure 6(D), the straight lines 21 connecting the adjacent focal points 19 extend along the direction parallel to the crystal orientation (2-5), the direction parallel to the crystal orientation (2-6), and the direction parallel to the crystal orientation (2-4), respectively, from the bottom to the top of the page in Figure 6.

[0042] Furthermore, in this embodiment, in the laser beam irradiation step of the peeling layer formation step 1002, the control unit of the laser processing apparatus 10 moves (processing feeds) the GaN ingot 100 and the multiple focal points 19 formed by branching the laser beam 18 relatively to each other, and sets the moving speed (processing feed speed) of the moving unit between the GaN ingot 100 and the multiple focal points 19 so that, as shown in Figure 7, the straight line 27 connecting the formed adjacent processing marks 25 is formed along the direction of a specific crystal orientation included in the crystal orientation expressed by the above equation (2).

[0043] In the laser beam irradiation step of the peeling layer formation step 1002, when the GaN ingot 100 is irradiated with a laser beam 18 that forms a group of multiple focal points 19 shown in Figure 6(A), a group of multiple processing marks 25 (processing mark group 26) having the same arrangement as the group of multiple focal points 19 is formed, as shown in Figure 7. Then, in the laser beam irradiation step of the peeling layer formation step 1002, the control unit of the laser processing apparatus 10 sets the processing feed rate based on the time interval between irradiation of the laser beam 18 so that the product of the time interval between irradiation of the laser beam 18 and the processing feed rate is an integer multiple of (2 x cos30° x the interval between adjacent focal points 19 (the length of the straight line 21)) within a predetermined error range. Here, this integer multiple is preferably the smallest value that prevents the focal point 19 of the next laser beam 18 from overlapping the position irradiated with the previous laser beam 18. For example, when irradiating laser beams 18 having the patterns shown in Figures 6(A), 6(B), 6(C), and 6(D), it is preferable to set the processing feed rate with this integer multiple set to 1 (equal). In the laser beam irradiation step of the peeling layer formation step 1002, the processing feed rate is set to, for example, 1000 mm / s.

[0044] In the laser beam irradiation step of the peeling layer formation step 1002, by setting the processing feed rate in this manner, the straight lines 27 connecting the adjacent processing marks 25 between the adjacent processing mark groups 26 formed are formed along the direction of a specific crystal orientation included in the crystal orientation expressed by the above formula (2), as shown in Fig. 7. Then, as shown in Fig. 7, each of the processing marks 25 in the multiple processing mark groups 26 formed by multiple irradiations of the laser beam 18 can be formed in one of six specific crystal orientations included in the crystal orientation expressed by formula (2), and can be arranged so that it lies on the intersection of a grid whose side length is the set value for the spacing between adjacent focusing points 19.

[0045] The indexing step is a step in which the control unit of the laser processing apparatus 10 uses the movement unit to relatively index and feed the GaN ingot 100 and the focal point 19 of the laser beam 18 along an indexing direction, i.e., a direction parallel to the first face 101 of the GaN ingot 100 and perpendicular to the processing feed direction in which the GaN ingot 100 and the focal point 19 of the laser beam 18 were moved relative to each other when forming the modified layer in the laser beam irradiation step. In this embodiment, the length of the indexing feed (indexing feed amount) in the indexing feed step of the peeling layer formation step 1002 is set to about 100 μm (e.g., 106 μm). The indexing feed amount may be set so that the group of processing marks 26 formed in the laser beam irradiation step immediately before the indexing feed and the group of processing marks 26 formed in the laser beam irradiation step immediately after the indexing feed partially overlap.

[0046] By alternately performing a laser beam irradiation step and an indexing step, modified layers are formed on the GaN ingot 100 along multiple lines parallel to the processing feed direction, near the focal point 19 of the laser beam 18, in a direction parallel to the first surface 101, and cracks extending from the modified layers formed along adjacent lines connect to each other. As a result, by applying a predetermined external force, the GaN ingot 100 can be delaminated, starting from the delamination layer 110 containing these modified layers and cracks, into a GaN substrate 130 having a thickness 120 that includes the first surface 101 and corresponds to the depth from the first surface 101 when the focal point 19 of the laser beam 18 is positioned inside the GaN ingot 100.

[0047] 8 and 9 are cross-sectional views illustrating the delamination step 1003 in Fig. 3. As shown in Fig. 8 and 9, the delamination step 1003 is a step of delaminating the GaN substrate 130 from the GaN ingot 100 starting from the delamination layer 110. The delamination step 1003 is performed by a delamination apparatus 30 shown in Fig. 8 and 9. As shown in Fig. 8 and 9, the delamination apparatus 30 includes a holding table 31 that holds the GaN ingot 100 on a holding surface 32, a delamination unit 33, and a control unit (not shown).

[0048] The holding table 31 is similar to the holding table 11 of the laser processing apparatus 10 described above. The delamination unit 33 includes a suction holder 34 and a moving unit 35. The suction holder 34 is formed in a disk shape and suction-holds the first surface 101 of the GaN ingot 100 on its underside. The moving unit 35 moves the holding table 31 and the suction holder 34 relative to each other, for example, along the Z-axis direction. The moving unit 35 applies a force to the suction holder 34, which suction-holds the first surface 101 of the GaN ingot 100 held on the holding table 31, in a direction away from each other along the Z-axis direction relative to the holding table 31, thereby applying a force that pulls the GaN ingot 100 along the Z-axis. The control unit of the delamination apparatus 30 includes a computer system similar to the control unit of the laser processing apparatus 10.

[0049] In the peeling step 1003, as shown in Figures 8 and 9, the GaN ingot 100 is suction-held from the second surface 102 side on the holding surface 32 of the holding table 31 with the first surface 101 side exposed, and after the first surface 101 of the GaN ingot 100 is suction-held by the suction holding portion 34 of the peeling unit 33, a pulling force is applied along the Z-axis direction to the GaN ingot 100 held on the holding table 31 by the moving unit 35 of the peeling unit 33, thereby peeling the GaN substrate 130 from the GaN ingot 100 at the peeling surface 140 starting from the peeling layer 110.

[0050] In the manufacturing method of the gallium nitride substrate according to the embodiment, an external force application step such as inserting a wedge or applying ultrasonic waves may be performed after the separation layer formation step 1002 and before the separation step 1003, or simultaneously with the separation step 1003.

[0051] In the external force applying step, for example, by driving a wedge into the peripheral surface 103 of GaN ingot 100 at a height position of delamination layer 110, it is possible to further extend the crack in delamination layer 110 along a direction parallel to first surface 101. The wedge may be driven into one location, or may be driven into multiple locations along the peripheral direction of GaN ingot 100.

[0052] In the external force applying step, instead of driving in a wedge, ultrasonic waves (elastic vibration waves in a frequency band exceeding 20 kHz) may be applied to the GaN ingot 100, which can further extend the crack in the separation layer 110 along a direction parallel to the first surface 101. In this case, in the external force applying step, ultrasonic waves are applied to the first surface 101 side via a liquid such as pure water before the first surface 101 of the GaN ingot 100 is suction-held by the lower surface of the suction holding unit 34. Specifically, in the external force applying step, the liquid to which ultrasonic waves have been applied may be sprayed toward the first surface 101 of the GaN ingot 100, or ultrasonic waves may be applied from an ultrasonic horn via the liquid to the first surface 101 side of the GaN ingot 100. Furthermore, in the external force application step, ultrasonic waves are first applied to a localized area of ​​approximately 5 mm to 50 mm in diameter on the first surface 101 side of the GaN ingot 100, and then the area to which the ultrasonic waves are applied is gradually expanded, which more preferably allows the crack in the peeling layer 110 to be further extended along a direction parallel to the first surface 101.

[0053] By carrying out the external force applying step, cracks between adjacent modified layers are connected, and the mechanical strength of delamination layer 110 becomes weaker than the regions of GaN ingot 100 other than delamination layer 110. Therefore, GaN substrate 130 can be delaminated from GaN ingot 100 with a smaller force than when the external force applying step is not carried out.

[0054] In the method for manufacturing a gallium nitride substrate according to the embodiment having the above configuration, in the peeling layer formation step 1002, the branching unit 15 of the laser processing apparatus 10 branches the laser beam 18 to form a plurality of focal points 19, and the straight lines 21 connecting each of the branched focal points 19 are set to be parallel to the direction of a specific crystal orientation included in the crystal orientation expressed by the above formula (2). Therefore, in the method for manufacturing a gallium nitride substrate according to the embodiment, a group of multiple processing marks 25 (modified layers) formed by irradiation with the laser beam 18 can be arranged so that, similar to the focal points 19, the straight lines 21 connecting adjacent processing marks 25 (modified layers) are parallel to the direction of the specific crystal orientation included in the crystal orientation expressed by the above formula (2). This reduces the risk of cracks (cleavage) occurring along the crystal plane (3-2), which would increase the unevenness of the peeled surface 140. Therefore, the method for manufacturing a gallium nitride substrate according to the embodiment can preferably extend cracks in the separation layer 110 along a direction parallel to the first surface 101, thereby reducing the unevenness of the separation surface 140, thereby achieving the advantageous effect of efficiently cutting out and manufacturing the GaN substrate 130 from the GaN ingot 100.

[0055] In addition, in the method for manufacturing a gallium nitride substrate according to the embodiment, the control unit of the laser processing apparatus 10 relatively moves (processes and feeds) the GaN ingot 100 and the plurality of focal points 19 formed by branching the laser beam 18, and sets the movement speed (processing feed speed) of the GaN ingot 100 and the plurality of focal points 19 by the movement unit so that the straight line 27 connecting adjacent processed marks 25 formed is formed along the direction of a specific crystal orientation included in the crystal orientation expressed by the above formula (2), as shown in Figure 7. Therefore, in the method for manufacturing a gallium nitride substrate according to the embodiment, the processed marks 25 of the plurality of processed mark groups 26 formed by multiple irradiations of the laser beam 18 can be arranged so that, like the focal points 19, the straight line 21 connecting adjacent processed marks 25 (modified layers) between adjacent processed mark groups 26 is parallel to the direction of the specific crystal orientation included in the crystal orientation expressed by the above formula (2), thereby further reducing the risk of cracks (cleavage) occurring along the crystal plane (3-2), which would increase the unevenness of the peeled surface 140. Therefore, the method for manufacturing a gallium nitride substrate according to the embodiment can more preferably extend the cracks in the separation layer 110 along a direction parallel to the first surface 101, thereby further reducing the unevenness of the separation surface 140, thereby achieving the advantageous effect of more efficiently cutting out and manufacturing the GaN substrate 130 from the GaN ingot 100.

[0056] Furthermore, in the method for manufacturing a gallium nitride substrate according to the embodiment, the processing feed direction is set to be parallel to the direction of a specific crystal orientation included in the crystal orientation expressed by the above formula (1). This reduces the laser output required to form cracks in the peeling layer 110, thereby contributing to a reduction in the damaged layer and an improvement in the processing capacity of the device. That is, by positioning the focal point 19 inside the gallium nitride ingot 100 with the laser beam 18 branched so that the processing marks 25 (modified layer) are parallel to the direction of the specific crystal orientation included in the crystal orientation expressed by the above formula (2), and relatively moving the focal point 19 and the gallium nitride ingot 100 in the direction parallel to the direction of the specific crystal orientation included in the crystal orientation expressed by the above formula (1), it is possible to suppress the formation of defective cracks and excessive cracks, thereby achieving the effect of efficiently and uniformly forming cracks. This significantly reduces the amount of material that is discarded when slicing with a wire saw, allowing gallium nitride substrates 130 to be manufactured from gallium nitride ingots 100 without waste, thereby improving productivity.

[0057] The present invention is not limited to the above-described embodiment, and can be implemented in various modifications without departing from the gist of the present invention. [Explanation of symbols]

[0058] 18 Laser Beam 19 Focus point 21,27 straight line 25 Machining marks 100 Gallium nitride ingot (GaN ingot) 101 First Side 102 Second Side 110 Peeling layer 120 thickness 130 Gallium nitride substrate (GaN substrate)

Claims

1. A method for producing a gallium nitride substrate from a gallium nitride ingot having a first surface and a second surface opposite to the first surface, comprising: a holding step of holding a gallium nitride ingot; a separation layer forming step of positioning a focal point of a laser beam having a wavelength that transmits through gallium nitride inside the gallium nitride ingot from the first surface, and relatively moving the gallium nitride ingot and the focal point along the direction of the crystal orientation of the gallium nitride ingot expressed by the following formula (1), thereby forming a separation layer to a depth corresponding to the thickness of the gallium nitride substrate to be manufactured; a peeling step of peeling the gallium nitride substrate from the gallium nitride ingot starting from the peeling layer, In the peeling layer forming step, the laser beam is branched to form a plurality of focal points, and a straight line connecting each of the branched focal points is set to be parallel to a direction of a crystal orientation expressed by the following formula (2): A method for manufacturing a gallium nitride substrate. [Equation 1] [Equation 2]

2. In the peeling layer forming step, the moving speeds of the gallium nitride substrate and the plurality of focusing points are set so that straight lines connecting adjacent processing marks formed by moving the gallium nitride substrate and the plurality of focusing points relatively are formed along the direction of the crystal orientation expressed by the formula (2). The method for producing a gallium nitride substrate according to claim 1 .

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