Manufacturing method of wafer

Abstract

To reduce a cutting margin when manufacturing a wafer of GaN.SOLUTION: A manufacturing method of a wafer, for manufacturing the wafer from a processed material as a single crystal substrate of an ingot of a gallium nitride or the gallium nitride, comprises: a separation layer formation step of forming a separation layer to a processed material; and a separation step of separating the wafer from the processed material at a start point of a separation layer. In a predetermined direction in the separation layer formation step, an angle formed by as space from a crystal orientation expressed by an orientation (1) on a (0001) surface is 10° or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a wafer having a thickness less than the distance between a first surface and a second surface from a workpiece which is an ingot of gallium nitride or a single crystal substrate of gallium nitride each having a first surface and a second surface located on the opposite side of the first surface.

Background Art

[0002] Gallium nitride (GaN) is called a wide bandgap semiconductor and has a bandgap approximately three times that of silicon (Si). Using this relatively large bandgap of GaN, devices such as power devices and LEDs are manufactured.

[0003] A single crystal substrate of GaN (i.e., a wafer) is usually manufactured by slicing an ingot of GaN. For wafer manufacturing, for example, an annular slicer having a cutting blade provided at an inner peripheral portion instead of an outer peripheral portion is used (see Patent Document 1).

[0004] However, since the thickness of the cutting blade of the slicer is relatively large (for example, 0.3 mm) compared to the thickness of the wafer (for example, 0.15 mm), about 60% to 70% per wafer is discarded as cutting waste when cutting waste and wafers are combined. Thus, using a cutting blade is uneconomical because the ratio of cutting waste to the total of cutting waste and wafers (i.e., the waste rate) becomes relatively high.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention has been made in view of such problems, and an object thereof is to reduce switching when manufacturing a GaN wafer from a GaN ingot.

Means for Solving the Problems

[0007] According to one aspect of the present invention, a wafer manufacturing method for manufacturing a wafer having a thickness less than the distance between a first surface and a second surface from a workpiece that is a gallium nitride ingot or a gallium nitride single crystal substrate having the first surface and the second surface located on the opposite side of the first surface, the method comprising: a holding step of sucking and holding the second surface of the workpiece; after the holding step, irradiating a pulsed laser beam having a wavelength that penetrates the workpiece from the side opposite to the second surface to the first surface and positioning the condensing point of the laser beam at a predetermined depth position of the workpiece, and relatively moving the workpiece and the condensing point along a predetermined direction to form a separation layer in the workpiece; and a separation step of separating the wafer from the workpiece starting from the separation layer after the separation layer formation step, wherein the predetermined direction in the separation layer formation step provides a wafer manufacturing method in which the angle formed with the crystal orientation represented by the following (1) on the (0001) plane is 10° or less.

Number

[0008] Preferably, the wafer manufacturing method further comprises an annular machining step of forming a separation layer in the outer peripheral region of the workpiece by irradiating the laser beam in an annular shape along the outer peripheral edge of the workpiece after the holding step and before the separation layer formation step while positioning the condensing point at the predetermined depth position.

[0009] Also, preferably, in the separation layer formation step, after relatively moving the workpiece and the condensing point in a regular hexagonal shape along the predetermined direction, the condensing point is moved to the central side in the radial direction of the workpiece, and then the workpiece and the condensing point are relatively moved in a hexagonal shape along the predetermined direction.

[0010] Also preferably, in the separation layer forming step, the laser beam is branched into a plurality of laser beams, and the condensing points of each of the plurality of laser beams are arranged to be aligned along a first direction, and then a second direction orthogonal to the first direction is set as the predetermined direction.

[0011] Also preferably, in the separation layer forming step, after moving a plurality of condensing points along the second direction, the plurality of condensing points are shifted along the first direction so as to partially overlap a moving region including the locus of the movement of the plurality of condensing points in the second direction as viewed from the first surface, and then the plurality of condensing points are moved along the second direction.

[0012] Also preferably, in the separation layer forming step, the interval between a plurality of condensing points aligned along the first direction is 5 μm or more and 20 μm or less.

[0013] Also preferably, the separation layer formed in the separation layer forming step includes a plurality of modified regions. The interval between the plurality of modified regions formed side by side along the first direction is defined as a (μm), and the interval between the plurality of modified regions formed side by side along the second direction is defined as b (μm) by relatively moving the plurality of condensing points and the workpiece along the second direction. In this case, the aspect ratio represented by (b / a) is 0.5 or more and 3.0 or less.

[0014] Also preferably, in the separation layer forming step, the laser beam irradiated onto the workpiece is irradiated onto the workpiece in burst mode.

Advantages of the Invention

[0015] In the manufacturing method according to one aspect of the present invention, a separation layer is formed on a workpiece by relatively moving the workpiece and a condensing point in a predetermined direction with the condensing point of a pulsed laser beam having a wavelength that penetrates the workpiece positioned at a predetermined depth position in the workpiece (separation layer forming step).

[0016] Then, starting from the separation layer, the wafer is separated from the workpiece (separation step). By using a laser beam, the thickness of the separation layer can be made, for example, about 60 μm (i.e., 0.06 mm), so that the cutting cost in the thickness direction of the workpiece can be reduced compared with the case of using a cutting blade.

Brief Description of the Drawings

[0017]

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Embodiments for Carrying Out the Invention

[0018] With reference to the accompanying drawings, embodiments according to one aspect of the present invention will be described. FIG. 1 is a flowchart of a manufacturing method for manufacturing a single crystal substrate of GaN (i.e., wafer 15) (see FIG. 8(B)) that is thinner than ingot 11 from GaN ingot (workpiece) 11.

[0019] In the first embodiment, wafer 15 is manufactured by sequentially performing the holding step S10, the separation layer formation step S20, and the separation step S30 shown in FIG. 1. First, with reference to FIG. 2, ingot 11 will be described. FIG. 2 is a perspective view of ingot 11.

[0020] Ingot 11 is a single crystal of GaN having a hexagonal crystal structure. However, the conductivity type of ingot 11 is not particularly limited. Ingot 11 may be p-type containing p-type impurities such as magnesium (Mg) and beryllium (Be), or may be n-type containing n-type impurities such as silicon (Si) and germanium (Ge).

[0021] Ingot 11 of the present embodiment has a diameter of 4 inches (about 100 mm) and a thickness of 500 μm, but the diameter and thickness are not limited to these values. Ingot 11 has a first surface 11a and a second surface 11b that is located on the opposite side of the first surface 11a in the thickness direction 11c and is parallel to the first surface 11a. The first surface 11a corresponds to the c-plane shown in (2) below.

[0022]

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[0023] In this specification, crystal planes and crystal orientations are specified using Miller indices. A specific crystal plane is expressed using (), and crystal planes that are equivalent to each other due to the symmetry of the crystal structure are expressed using {}. Similarly, a specific crystal orientation is expressed using [], and crystal orientations that are equivalent to each other are expressed using <>.

[0024] The crystal orientation perpendicular and upward with respect to the first plane 11a (i.e., the c-plane) is represented by the following (3). This crystal orientation is called the c-axis and corresponds to the thickness direction 11c of the ingot 11.

[0025]

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[0026] The ingot 11 of this embodiment has a plurality of flat surfaces on its side surface. More specifically, the ingot 11 has a first side surface 13a and a second side surface 13b that are in a perpendicular positional relationship with each other. The first side surface 13a corresponds to the crystal plane shown in the following (4), and the second side surface 13b corresponds to the crystal plane shown in the following (5).

[0027]

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[0028]

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[0029] The first orientation flat (hereinafter abbreviated as the first OF13a1) where the first plane 11a and the first side surface 13a intersect is parallel to the crystal orientation of the following (6).

[0030]

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[0031] In addition, a second orientation flat (hereinafter abbreviated as the second OF13b1) where the first surface 11a and the second side surface 13b intersect is parallel to the crystal orientation of the following (7).

[0032]

Number

[0033] Next, with reference to FIG. 3, a laser processing apparatus 2 for performing laser processing on the ingot 11 will be described. FIG. 3 is a schematic diagram of the laser processing apparatus 2. In FIG. 3, a plurality of components are shown in the form of functional blocks or simplified shapes.

[0034] The X-axis direction (processing feed direction, second direction, predetermined direction), Y-axis direction (indexing feed direction, first direction), and Z-axis direction (height direction) shown in FIG. 3 are perpendicular to each other.

[0035] In this specification, the X-axis direction is parallel to the +X direction and -X direction which are opposite to each other. Similarly, the Y-axis direction is parallel to the +Y direction and -Y direction which are opposite to each other, and the Z-axis direction is parallel to the +Z direction and -Z direction which are opposite to each other.

[0036] The laser processing apparatus 2 has a disk-shaped chuck table 4. The chuck table 4 has a disk-shaped frame body made of a metal such as stainless steel. A disk-shaped recess (not shown) having a diameter smaller than that of the frame body is formed at the center of the frame body. A disk-shaped porous plate made of porous ceramics is fixed in this recess.

[0037] A predetermined flow path (not shown) is formed in the frame body, and a suction source (not shown) such as a vacuum pump is connected to the predetermined flow path via a pipe portion (not shown) or the like. When the negative pressure generated by the suction source is transmitted to the porous plate, a negative pressure is generated on the upper surface of the porous plate.

[0038] The annular upper surface of the frame body and the circular upper surface of the porous plate are substantially flush and substantially flat, and function as a holding surface 4a for sucking and holding the ingot 11. The holding surface 4a is arranged parallel to the XY plane.

[0039] A rotary drive mechanism (not shown) for rotating the chuck table 4 is provided below the chuck table 4. The rotary drive mechanism can rotate the chuck table 4 by a predetermined angle about a predetermined rotation axis along the Z-axis direction.

[0040] The chuck table 4 and the rotary drive mechanism are supported by a horizontal movement mechanism (not shown). The horizontal movement mechanism includes a ball screw type X-axis direction movement mechanism and a Y-axis direction movement mechanism, respectively, and can move the chuck table 4 and the rotary drive mechanism along the X-axis direction and the Y-axis direction.

[0041] A laser beam irradiation unit 6 is provided above the holding surface 4a. The laser beam irradiation unit 6 has a laser beam generation unit 8. The laser beam generation unit 8 includes a laser oscillator 10.

[0042] The laser oscillator 10 has, for example, Nd:YAG, Nd:YVO4, etc. as a laser medium. From the laser oscillator 10, a pulsed (for example, several tens of MHz) laser beam L having a wavelength (for example, 1064 nm) that penetrates the GaN ingot 11 A is emitted.

[0043] The laser beam L emitted from the laser oscillator 10 A is converted into a burst-mode laser beam L in an acousto-optic modulator (AOM) 12. B is converted.

[0044] The acousto-optic modulator 12 operates according to an electrical signal input to the acousto-optic modulator 12, and deflects the laser beam L A for a predetermined time. Thereby, the laser beam LA The laser beam L in a state thinned out for a predetermined time B is emitted from the acousto-optic modulator 12 to the output adjustment unit 14.

[0045] FIG. 4(A) is a schematic diagram of the pulsed laser beam L incident from the laser oscillator 10 to the acousto-optic modulator 12 A and FIG. 4(B) is a schematic diagram of the pulsed laser beam L incident from the acousto-optic modulator 12 to the output adjustment unit 14 B .

[0046] In FIGS. 4(A) and 4(B), the horizontal axis indicates time and the vertical axis indicates the magnitude of the output. The laser beam L A is, in the acousto-optic modulator 12, as shown in FIG. 4(B), the laser beam L in burst mode in which a pulse group 12a including a plurality of pulses is repeated at a predetermined period T B is converted.

[0047] The time interval t corresponding to the interval between the pulse groups 12a is, for example, from several tens of μs to several hundreds of μs. Note that the reciprocal of the period T (i.e., the repetition frequency) between the pulse groups 12a with the pulse group 12a as the repetition unit is, for example, 50 kHz.

[0048] Returning to FIG. 3, the laser beam L B is then adjusted to an appropriate output by the output adjustment unit 14 including an attenuator (attenuator) or the like, and then spatially branched by the branching unit 16.

[0049] The branching unit 16 of the present embodiment has an LCOS-SLM (Liquid Crystal on Silicon - Spatial Light Modulator) (not shown), but a diffraction grating may be used instead of the LCOS-SLM.

[0050] The laser beam L that has passed through the branching unit 16 CIt is guided to the irradiation head 20 through a collimator lens (not shown), a mirror 18, etc. The irradiation head 20 has a condenser lens (not shown). The condenser lens focuses the laser beam L at a predetermined depth position of the ingot 11 sucked and held on the holding surface 4a. C to focus.

[0051] The laser beam L shown in FIG. 3 C is branched into a plurality of laser beams L C1 , L C2 , L C3 , L C4 and L C5 by the branching unit 16, and the respective condensing points P (P1, P2, P3, P4, P5) of the laser beams L from L C1 to L C5 are arranged to be aligned along the Y-axis direction at a predetermined depth position of the ingot 11.

[0052] The interval between the plurality of condensing points P arranged along the Y-axis direction is set to a predetermined value, for example, 5 μm or more and 20 μm or less. In the example shown in FIG. 3, for convenience of explanation, the number of branches of the laser beam L C is set to 5, but the number of branches is not limited to 5. The number of branches may be 2 or more and 16 or less, and in a preferred example, the number of branches is 10.

[0053] An imaging unit (not shown) for imaging a subject is provided in the housing (not shown) of the laser beam irradiation unit 6. The imaging unit has a light emitting device (not shown) that emits light downward along the Z-axis direction.

[0054] The light emitting device includes a light emitting element such as an LED (Light Emitting Diode) that functions as a light source. The imaging unit further has an imaging element (not shown) that receives the reflected light of the light irradiated from the light emitting device via a lens (not shown). The light from the light emitting device has a wavelength of visible light.

[0055] The imaging device can photoelectrically convert the wavelength of light from the light-emitting device. The imaging device is, for example, a CCD (Charge-Coupled Device) image sensor or a CMOS (Complementary Metal-Oxide-Semiconductor) image sensor. The light-emitting device, the lens, the imaging device, etc. constitute a microscope camera unit that images a subject with visible light.

[0056] Operations of the above-described chuck table 4, the rotational drive mechanism, the horizontal movement mechanism, the laser beam irradiation unit 6, etc. are controlled by a control unit (not shown). The control unit is constituted by a computer including, for example, a processor (processing device) typified by a CPU (Central Processing Unit) and a memory (storage device).

[0057] The memory includes a main storage device such as a DRAM (Dynamic Random Access Memory), an SRAM (Static Random Access Memory), a ROM (Read Only Memory), etc., and an auxiliary storage device such as a flash memory, a hard disk drive, a solid state drive, etc.

[0058] The auxiliary storage device stores software including a predetermined program. By operating the processing device etc. according to this software, the functions of the control unit are realized. Next, a method for manufacturing the wafer 15 according to the first embodiment will be described according to the procedure shown in FIG. 1.

[0059] FIG. 5 is a side view showing a holding step S10 of sucking and holding the second surface 11b of the ingot 11 by the holding surface 4a. In the holding step S10, the ingot 11 is sucked and held by the holding surface 4a in such a manner that the second surface 11b is in contact with the holding surface 4a and the first surface 11a is exposed upward.

[0060] Also, in the holding step S10, after suction holding, the imaging unit images the first surface 11a side to identify the deviation of the second OF13b1 with respect to the X-axis direction of the laser processing apparatus 2. Then, the chuck table 4 is rotated by the rotation drive mechanism so as to cancel this deviation, making the second OF13b1 substantially parallel to the X-axis direction.

[0061] After the holding step S10, a burst-mode laser beam L is irradiated from above the first surface 11a (i.e., on the side opposite to the second surface 11b) toward the first surface 11a. C to form a separation layer 11d at a predetermined depth position from the first surface 11a.

[0062] FIG. 6 is a plan view showing the separation layer forming step S20. In FIG. 6, for ease of understanding, two of the plurality of condensing points P are shown as relatively large circles, and some of the condensing points located between these two condensing points P are omitted.

[0063] In the separation layer forming step S20, after arranging the respective condensing points P so as to be aligned along the Y-axis direction at a predetermined depth position 11e (see FIGS. 3 and 8(A)) of the ingot 11, the plurality of condensing points P and the ingot 11 (i.e., the chuck table 4) are relatively moved along the X-axis direction (predetermined direction).

[0064] In the separation layer forming step S20 of the present embodiment, after relatively moving the plurality of condensing points P in the -X direction, the plurality of condensing points P are relatively moved in the +X direction. In this way, the movement in the -X direction and the movement in the +X direction are alternately repeated.

[0065] In FIG. 6, the movement paths of the plurality of condensing points P in the ingot 11 are indicated by dashed arrows. Instead of alternately moving the plurality of condensing points P in the -X direction and the +X direction, they may be moved only in the -X direction or only in the +X direction.

[0066] When the relative movement direction of the plurality of condensing points P and the ingot 11 is along the X-axis direction, the movement direction is parallel to the crystal orientation shown in the following (8).

[0067] [Number]

[0068] Note that the two crystal orientations shown in (8) are two of the six equivalent crystal orientations in the ingot 11 having a hexagonal crystal structure as shown in (9) below.

[0069] [Number]

[0070] Incidentally, the relative movement direction between the plurality of condensing points P and the ingot 11 does not have to be completely parallel to the crystal orientation specified in (8), and the angle formed with the crystal orientation specified in (8) on the c-plane (see (2) above) may be 10° or less. Even in this case, the applicant has confirmed in experiments that the separation layer 11d is formed.

[0071] An example of the processing conditions used in the separation layer formation step S20 is shown below.

[0072] Wavelength: 1064 nm Processing feed rate: 1000 mm / s Index feed amount: 106 μm (i.e., the index amount) Repetition frequency: 50 kHz Number of bursts: 10 (number of pulses included in the pulse group 12a) Number of branches: 10 (number of branches of the laser beam L C ) Number of passes: 1 Spot diameter of each condensing point: approximately 5 μm Depth position of the condensing point: approximately 170 μm from the first surface 11a into the ingot 11

[0073] In addition, under these processing conditions, the distance between adjacent light condensing points among the 10 light condensing points is set to, for example, 12.5 μm. Further, when the 10 light condensing points P are arranged, the laser beam L is irradiated within a range of 12.5 μm × 9. C Therefore, the irradiation of the plurality of light condensing points P arranged along the Y-axis direction becomes 112.5 μm (see Fig. 7).

[0074] When the plurality of light condensing points P are relatively moved along the X-axis direction, the movement locus of the plurality of light condensing points P is included in the first movement region 22a shown by the solid line in Fig. 7. After moving the plurality of light condensing points P along the X-axis direction, the indexing feed is performed by relatively moving the irradiation head 20 and the chuck table 4 along the Y-axis direction by the above-described predetermined index amount.

[0075] In this state, the plurality of light condensing points P are similarly moved relatively along the X-axis direction. The movement locus of the plurality of light condensing points P after the indexing feed is included in the second movement region 22b shown by the broken line in Fig. 7. As shown in Fig. 7, the first movement region 22a and the second movement region 22b partially overlap in the overlapping region 22c when viewed from the first surface 11a.

[0076] Fig. 7 is a plan view showing the overlap of the first movement region 22a and the second movement region 22b. Under the above-described processing conditions, the width of the overlap in the Y-axis direction is 6.5 μm. Such an overlapping region 22c is formed on both sides in the Y-axis direction of each movement region, excluding the two movement regions located at the Y-axis ends of the first surface 11a.

[0077] By the way, in the vicinity of each of the plurality of light condensing points P, the crystallinity of the ingot 11 changes due to multi-photon absorption. For example, in a region where multi-photon absorption occurs, a modified region with a reduced mechanical strength is formed compared to a region where multi-photon absorption does not occur.

[0078] In addition, cracks extend from the modified region along the XY plane direction. Depending on the processing conditions, the cracks may also extend from the modified region along the Z-axis direction. In the present embodiment, the region in the ingot 11 where the modified region and the cracks are formed is referred to as the separation layer 11d.

[0079] After the separation layer formation step S20, as shown in FIGS. 8(A) and 8(B), the ingot 11 is separated into the wafer 15 and the other ingot 17 using the separation device 32 (separation step S30). With reference to FIG. 8(A), the separation device 32 will be described.

[0080] The separation device 32 has a chuck table 34 having substantially the same diameter as the above-described chuck table 4. The structure of the chuck table 34 is substantially the same as that of the chuck table 4, and the upper surface of the chuck table 34 functions as a holding surface 34a for sucking and holding the ingot 11. Above the chuck table 34, a separation unit 36 is provided.

[0081] The separation unit 36 has a columnar movable part 38 whose long hand part is arranged along the Z-axis direction. A Z-axis direction movement mechanism (not shown) is connected to the movable part 38, and the movable part 38 is movable along the Z-axis direction. The Z-axis direction movement mechanism is, for example, a ball screw type movement mechanism, but may be composed of other actuators.

[0082] A disk-shaped suction head 40 is provided at the bottom of the movable part 38. The suction head 40 has a frame body and a porous plate, similar to the chuck table 34. The lower surfaces of the frame body and the porous plate are arranged substantially flush and substantially parallel to the XY plane, and function as a holding surface 40a.

[0083] FIG. 8(A) is a diagram showing the separation step S30. In the separation step S30, the second surface 11b of the ingot 11 in which the separation layer 11d is formed is sucked and held by the holding surface 34a of the chuck table 34, and the first surface 11a is sucked and held by the holding surface 40a of the suction head 40.

[0084] Next, an external force is applied to the ingot 11. The application of the external force is performed, for example, by driving a wedge (not shown) into the ingot 11 at the height position of the separation layer 11d with respect to the side surface of the ingot 11. It is preferable to drive the wedge not only at one location on the side surface of the ingot 11 but also at a plurality of locations along the circumferential direction of the ingot 11.

[0085] By applying the external force, cracks are further extended in the XY plane direction at the depth position 11e where the separation layer 11d is formed. Instead of driving the wedge, an external force may be applied by applying ultrasonic waves (i.e., elastic vibration waves in a frequency band exceeding 20 kHz) to the ingot 11.

[0086] When applying ultrasonic waves, before sucking and holding the first surface 11a with the holding surface 40a of the suction head 40, ultrasonic waves are applied to the first surface 11a side through a liquid such as pure water. Specifically, the liquid to which ultrasonic waves are applied is jetted from the nozzle onto the ingot 11, or ultrasonic waves are applied from the horn to the first surface 11a side through the liquid.

[0087] The applicant has confirmed in experiments that if an external force is applied to the entire first surface 11a side at once, undesirable cracks will occur. Therefore, when using a nozzle or a horn, first, an external force is applied to a local region on the first surface 11a side with a diameter of about 5 mm to 50 mm using ultrasonic waves.

[0088] Next, by relatively moving the nozzle or the horn and the chuck table 34, an external force is applied to other regions on the first surface 11a side. In this way, by gradually expanding the region to which the external force is applied on the first surface 11a side, the cracks between the modified regions can be extended along the first surface 11a.

[0089] By applying the external force, the cracks connect between adjacent modified regions, and the mechanical strength of the separation layer 11d becomes weaker than that of the regions other than the separation layer 11d of the ingot 11. Therefore, the wafer 15 can be separated from the ingot 11 with a smaller force compared to the case where no external force is applied.

[0090] After applying an external force, the suction head 40 is raised (i.e., moved in the +Z direction). As a result, the wafer 15 is separated from the ingot 11 starting from the separation layer 11d. FIG. 8(B) is a diagram showing the wafer 15 and the like separated from the ingot 11. Note that the application of the above-described external force may be performed in parallel with the raising of the suction head 40.

[0091] The separation layer 11d has a thickness of about 50 μm to 60 μm (for example, 58 μm) in the thickness direction 11c, and the thickness of this separation layer 11d corresponds to the above-described switching. By laser-processing the ingot 11, the switching in the thickness direction 11c of the ingot 11 can be reduced as compared with the case of using a slicer.

[0092] Therefore, the productivity of the wafer 15 when manufacturing the wafer 15 from the ingot 11 is improved. Note that even when using a wire saw, at least about 150 μm of switching is required. Therefore, the manufacturing method of the present embodiment is also superior as compared with the case of using a wire saw.

[0093] In the above example, it has been described that a plurality of condensing points P are arranged at a predetermined depth position 11e of the ingot 11 to form the separation layer 11d. However, instead of the ingot 11, the separation layer 11d can also be formed at a predetermined depth position of a GaN single crystal substrate (workpiece), and the wafer 15 can be separated from this single crystal substrate.

[0094] In this case, a GaN single crystal substrate thicker than the thickness of the separated wafer 15 (i.e., the length in the c-axis direction) may be used. That is, the thickness of the wafer 15 is less than the distance between both surfaces (the first surface and the second surface) in the c-axis direction of the GaN single crystal substrate.

[0095] (Modification Example) Next, a modification example of the separation layer formation step S20 will be described. FIG. 9 is a diagram showing a modification example of the separation layer formation step S20. In the separation layer formation step S20 according to the modification example, the relative movement between the plurality of condensing points P and the ingot 11 at the above-described machining feed rate is not linear along the X-axis direction, but is a regular hexagon. This point is different from the first embodiment, but other points are the same as those in the first embodiment.

[0096] For example, the plurality of condensing points P are relatively moved in the order of the following (10), (11), (12), (13), (14), and (15). Such machining can be realized, for example, by appropriately combining the linear movement of the chuck table 4 by the horizontal movement mechanism and the rotation of the chuck table 4 by the rotation drive mechanism.

[0097]

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[0098]

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[0101]

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[0102]

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[0103] After relatively moving a plurality of condensing points P so as to draw one regular hexagon, the plurality of condensing points P are moved by the above-described predetermined index amount toward the center side in the radial direction of the ingot 11, and then, similarly, the plurality of condensing points P are relatively moved in the order from (10) to (15).

[0104] As a result, the moving regions of the plurality of condensing points P become a plurality of regular hexagon shapes arranged concentrically, as shown in FIG. 9. Note that, as shown in (9), all of (10) to (15) are included in the crystal orientation indicated by (1).

[0105] In this modification, at the start of laser processing, the plurality of condensing points P are moved in the direction indicated by (10). However, as long as the plurality of condensing points P can be relatively moved in a regular hexagon shape, laser processing may be started from any crystal orientation from (10) to (15).

[0106] Further, the relative moving direction between the plurality of condensing points P and the ingot 11 does not have to be completely parallel to the crystal orientation specified by (1), and the angle formed with the crystal orientation specified by (1) on the c-plane may be 10° or less.

[0107] For example, when relatively moving the plurality of condensing points P and the ingot 11 along the crystal orientation specified by (10), this relative moving direction may be such that the angle formed with the crystal orientation specified by (10) on the c-plane is 10° or less.

[0108] The same applies when relatively moving the plurality of condensing points P and the ingot 11 along the crystal orientations specified by (11) to (15). Note that this modification can be similarly applied to a single crystal substrate of GaN instead of the ingot 11.

[0109] (Second Embodiment) Next, referring to FIGS. 10 and 11, a second embodiment will be described. FIG. 10 is a flowchart of a method for manufacturing a wafer 15 according to the second embodiment, and FIG. 11 is a plan view showing an annular processing step S15.

[0110] In the manufacturing method according to the second embodiment, after the holding step S10 and before the separation layer forming step S20, a circular processing step S15 of irradiating a laser beam L in a circular shape along the outer peripheral edge 11f of the ingot 11 is further provided. C is further provided.

[0111] In this embodiment, irradiating the laser beam L in a circular shape along the outer peripheral edge 11f C means that, after considering that the edge of the first surface 11a chipped due to the presence of the first OF 13a1 and the second OF 13b1 is complemented to be circular, the laser beam L is irradiated along the circular edge of the first surface 11a. C is irradiated.

[0112] Also in the circular processing step S15, the plurality of condensing points P are positioned at the same predetermined depth position 11e as when forming the separation layer 11d in the separation layer forming step S20. In the circular processing step S15, first, the plurality of condensing points P are arranged side by side along the Y-axis direction at a predetermined depth position 11e of the ingot 11.

[0113] At this time, one condensing point located on the outermost side is located, for example, inside the ingot 11 in the radial direction by a predetermined distance 24 from the outer peripheral edge 11f. The predetermined distance 24 is, for example, 4 μm or more and 8 μm or less, and a preferred example is 5 μm or more and 6 μm or less.

[0114] In this state, the chuck table 4 is rotated once at a predetermined rotational speed in the direction of the arrow shown in FIG. 11. After one rotation, the plurality of condensing points P are moved inside the ingot 11 in the radial direction. Specifically, the chuck table 4 is indexed and fed by a predetermined index amount 26 along the Y-axis direction. The predetermined index amount 26 is, for example, 106 μm.

[0115] The predetermined rotational speed of the chuck table 4 is appropriately adjusted, for example, so that the peripheral speed at the plurality of condensing points P is substantially equal to the above-described processing feed speed. The predetermined rotational speed of the chuck table 4 may be adjusted to realize a preferred aspect ratio (b / a) described later.

[0116] In this way, in the annular machining step S15, the separation layer 11d is also formed in the outer peripheral region 28 of the ingot 11. In FIG. 11, an example is shown in which the chuck table 4 is rotated three times to form three annular separation layers 11d concentrically, but the number of rotations is not limited to three.

[0117] Other machining conditions (wavelength, repetition frequency, number of bursts, number of branches, number of passes, spot diameter of each focusing point, depth position of the focusing point) are, for example, the same as those in the first embodiment. Thereby, after the annular machining step S15, the separation layer formation step S20 can be executed smoothly.

[0118] In the separation layer formation step S20, when the separation layer 11d is formed, the bond between Ga atoms and N atoms is broken to form N2 (nitrogen molecules), and nitrogen gas is generated.

[0119] If the separation layer 11d is not formed in the outer peripheral region 28 after the annular machining step S15, an abnormal volume expansion region may be formed inside the ingot 11 in the radial direction due to the nitrogen gas formed in the separation layer formation step S20.

[0120] In the second embodiment, the separation layer 11d formed in the outer peripheral region 28 functions as a path for discharging the nitrogen gas generated inside the ingot 11 in the radial direction to the outside of the ingot 11 in the separation layer formation step S20.

[0121] Therefore, abnormal volume expansion inside the ingot 11 in the radial direction can be suppressed. Further, by forming the separation layer 11d in the outer peripheral region 28, the extension of cracks in an undesirable direction (for example, the c-axis direction) can be suppressed, and the extension of cracks on the outside of the c-plane of the ingot 11 can be promoted.

[0122] (Experiment showing the relationship between crystal orientation and crack formation) Next, referring to FIGS. 12 to 13(C), a laser beam L capable of forming a crack connecting adjacent modified regions CThe experimental results of examining the relationship between the minimum output and the moving direction of a plurality of focusing points P that linearly move on the c-plane will be described.

[0123] Fig. 12 is a plan view of the ingot 11 showing the angle θ formed between the predetermined direction (the first OF13a1) on the c-plane and the moving direction of a plurality of focusing points P (i.e., the scanning direction of the laser beam L C in an experiment for evaluating the crystal orientation dependence of the minimum output at which cracks are formed. In this experiment as well, the direction in which the plurality of focusing points P branch is orthogonal to the moving direction of the plurality of focusing points P on the c-plane.

[0124] For example, when the moving direction of the plurality of focusing points P is the crystal orientation shown in (6) above, θ = 0°, and when the moving direction of the plurality of focusing points P is the crystal orientation shown in (7) above, θ = 90°. The processing conditions in this experiment were as follows.

[0125] Wavelength: 1064 nm Processing feed rate: 1000 mm / s Repetition frequency: 50 kHz Number of bursts: 10 (the number of pulses included in the pulse train 12a) Number of branches: 10 (the number of branches of the laser beam L C Number of passes: 1 Spot diameter of each focusing point: approximately 5 μm Interval between adjacent focusing points: 12.5 μm Depth position of the focusing point: approximately 170 μm from the first surface 11a into the ingot 11

[0126] In this experiment where the number of branches is 10, the output of the laser beam L C means the sum of the outputs of the laser beam L C1 from L C10 that enters the ingot 11 after passing through the condenser lens in the irradiation head 20.

[0127] In this experiment, first, the value of θ was set to 0°, and the laser beam L C ​After fixing the output of [[ID=]] at a predetermined value, the first surface 11a (i.e., the c surface) was scanned with the laser beam L C Next, with the value of θ fixed at 0°, the output of the laser beam L C was increased by 0.05 W.

[0128] In this way, as a result of increasing the output of the laser beam L C step by step, the minimum output of the laser beam L C that can form cracks connecting adjacent modified regions was 1.25 W (θ = 0°).

[0129] Next, after increasing θ by 10°, the output of the laser beam L C was similarly increased step by step. As a result, the minimum output of the laser beam L C was 1.10 W (θ = 10°). Similarly, while increasing θ in 10° increments, the minimum output of each laser beam L C that can form cracks connecting adjacent modified regions at each angle was explored.

[0130] As a result, the minimum outputs of the laser beam L C were 1.05 W (θ = 20°), 1.05 W (θ = 30°), 1.05 W (θ = 40°), 1.10 W (θ = 50°), 1.25 W (θ = 60°), 1.10 W (θ = 70°), 1.05 W (θ = 80°), and 1.05 W (θ = 90°), respectively.

[0131] Figure 13(A) is a graph showing the minimum output at which cracks are formed with respect to the angle θ, Figure 13(B) is a plan view of the ingot 11 when θ = 30°, and Figure 13(C) is a plan view of the ingot 11 when θ = 90°.

[0132] As shown in Figure 13(A), θ is in the range of 20° or more and 40° or less (i.e., 30° - 10° ≤ θ ≤ 30° + 10°), and the output of the laser beam L C is the minimum. That is, the laser beam L CThe scanning direction is preferably in the range of ±10° of the crystal orientation shown in the above (15) on the c-plane.

[0133] Here, regarding θ, there are only experimental results up to 90°. However, considering that the ingot 11 is hexagonal and the ingot 11 has a 60° rotational symmetry on the c-plane, it is reasonably speculated that the same applies to the range of θ being 80° or more and 100° or less (i.e., 90° - 10° ≤ θ ≤ 90° + 10°).

[0134] That is, the laser beam L C The scanning direction is similarly preferable in the range of ±10° of the crystal orientation shown in the above (10) on the c-plane. Also, considering the 60° rotational symmetry, the scanning direction of the laser beam L C is similarly preferable if it is in the range of ±10° of the six equivalent crystal orientations shown in the above (9) on the c-plane.

[0135] That is, the laser beam L C By setting the scanning direction of the laser beam L to be in the range of ±10° of the six equivalent crystal orientations shown in the above (9) on the c-plane, it can be reasonably speculated that cracks connecting adjacent modified regions can be formed without making the output of the laser beam L C excessively high.

[0136] (Experiment regarding the arrangement of the modified regions and the formation mode of the separation layer 11d) Next, in the separation layer formation step S20, the experimental results when changing the interval (distance b shown in FIG. 15) between adjacent condensing points in the scanning direction of the laser beam L C with respect to the interval (distance a shown in FIG. 15) between adjacent condensing points among the plurality of branched condensing points P will be described with reference to FIGS. 14 to 17.

[0137] In this experiment, a single crystal substrate of GaN was processed using the above-described laser processing apparatus 2. The wavelength, repetition frequency, burst number, branching number, pass number, spot diameter of each condensing point, depth position of the condensing point, and interval between the condensing points were the same as those in the first embodiment, but the processing feed rate (mm / s) and the laser beam LC The output was appropriately changed according to the aspect ratio (b / a). Also in this experiment, the direction in which the plurality of condensing points P branch is orthogonal to the direction in which the plurality of condensing points P move on the c-plane.

[0138] Also, when processing the single crystal substrate shown in each of FIGS. 14, 16, and 17, the indexing feed amount was set to 112.5 μm, and laser processing was performed on three parallel linear regions. In FIGS. 14, 16, and 17, the three parallel linear regions are collectively indicated by reference numeral 11g. However, laser processing was performed so as not to form the overlapping region 22c (see FIG. 7).

[0139] FIG. 14 is a photograph of a single crystal substrate in which sufficient cracks 11i were formed between some of the modified regions 11h, but sufficient cracks 11i were not formed throughout the entire modified region 11h. This photograph was obtained by photographing the first surface 11a side of the single crystal substrate after laser processing with a visible light camera. Note that the photographs in FIGS. 16 and 17 described later were also obtained by photographing with a visible light camera.

[0140] The straight line that crosses the center of the image shown in FIG. 14 in the horizontal direction is a reference line 30 that is displayed so as to cross the center of the imaging field of view. The belt-shaped linear region 11g is a region where laser processing was performed along the crystal orientation indicated by (1).

[0141] In the image, a modified region 11h is formed in the region indicated by the black circles, and cracks 11i are formed in the bright regions between the modified regions 11h.

[0142] FIG. 15 is a diagram schematically showing a plurality of modified regions 11h. The distance a is the interval between a plurality of modified regions 11h arranged along the Y-axis direction (that is, corresponding to the interval between a plurality of condensing points P arranged along the Y-axis direction), and the unit is μm.

[0143] Further, the distance b is the interval between a plurality of reforming regions 11h arranged along the X-axis direction (i.e., corresponding to the interval between a plurality of condensing points P arranged along the X-axis direction), and the unit is μm. The distance b is determined according to the machining feed rate (i.e., the relative moving speed between the plurality of condensing points P and the ingot 11) and the repetition frequency.

[0144] According to experiments, it has been clarified that the quality of machining is determined according to the aspect ratio represented by (b / a). Specifically, when the aspect ratio (b / a) exceeds 3.0, the reforming regions 11h are separated from each other, so that the crack 11i does not sufficiently extend in the XY plane direction as shown in FIG. 14.

[0145] On the other hand, when the aspect ratio (b / a) is less than 0.5, the reforming regions 11h approach each other, and the crack 11i extends relatively sufficiently in the XY plane direction as shown in FIG. 16, but a relatively large crack 11j is formed in the Z-axis direction.

[0146] FIG. 16 is a photograph of a single crystal substrate in which a relatively large crack 11j is formed in the c-axis direction. Note that the crack 11j extends in the Z-axis direction (depth direction), and in the photograph shown in FIG. 16, the focus is not adjusted and the contour is slightly blurred.

[0147] In contrast, when the aspect ratio (b / a) is 0.5 or more and 3.0 or less, the crack 11i can be extended relatively sufficiently in the XY plane direction, and the formation of a relatively large crack 11j in the Z-axis direction can be prevented.

[0148] FIG. 17 is a photograph of a single crystal substrate in which the crack 11i sufficiently extends between the entire reforming regions 11h, and a relatively large crack 11j is not formed in the Z-axis direction. Note that the aspect ratio (b / a) may be 0.8 or more and 2.5 or less, or may be 1.0 or more and 1.4 or less.

[0149] According to the above-described embodiments, modifications, and experimental results, by forming the separation layer 11d on the workpiece using the laser processing apparatus 2, it is possible to reduce the cutting in the thickness direction of the workpiece compared to the case of using a slicer.

[0150] In addition, the structures, methods, etc. according to the above-described embodiments can be appropriately modified and implemented without departing from the scope of the object of the present invention.

Explanation of Reference Numerals

[0151] 2: Laser processing apparatus, 4: Chuck table, 4a: Holding surface 6: Laser beam irradiation unit, 8: Laser beam generation unit 10: Laser oscillator, 12: Acousto-optic modulator, 12a: Pulse train 11: Ingot (workpiece), 11a: First surface, 11b: Second surface, 11c: Thickness direction 11d: Separation layer, 11e: Depth position, 11f: Outer peripheral edge 11g: Linear region, 11h: Modified region, 11i, 11j: Cracks 13a: First side surface, 13b: Second side surface First orientation flat (First OF): 13a1 Second orientation flat (Second OF): 13b1 14: Output adjustment unit, 16: Branch unit, 18: Mirror, 20: Irradiation head 15: Wafer, 17: Other ingot 22a: First moving region, 22b: Second moving region, 22c: Overlap region 24: Distance, 26: Index amount, 28: Outer peripheral region, 30: Reference line 32: Separation device, 34: Chuck table, 34a: Holding surface 36: Separation unit, 38: Movable part, 40: Suction head, 40a: Holding surface L A 、L B 、L C 、L C1 、L C2 、L C3 、L C4 、LC5 : Laser beam P, P1, P2, P3, P4, P5: Focus points S10: Holding step, S15: Annular machining step S20: Separation layer formation step, S30: Separation step a, b: Distances, t: Time interval, T: Period

Claims

1. A method for manufacturing a wafer having a thickness less than the distance between a first surface and a second surface from a workpiece which is an ingot of gallium nitride or a single crystal substrate of gallium nitride, each having the first surface and the second surface located on the opposite side of the first surface, comprising: a holding step of sucking and holding the second surface of the workpiece; After the holding step, a separation layer forming step of irradiating a pulsed laser beam having a wavelength that penetrates the workpiece from the side opposite to the second surface to the first surface, positioning the condensing point of the laser beam at a predetermined depth position of the workpiece, and relatively moving the workpiece and the condensing point along a predetermined direction to form a separation layer in the workpiece; After the separation layer forming step, a separation step of separating the wafer from the workpiece starting from the separation layer; comprising In the separation layer forming step, the predetermined direction is characterized in that the angle formed with the crystal orientation represented by the following (1) on the (0001) plane is 10° or less. A method for manufacturing a wafer. 【Equation 1】

2. After the holding step and before the separation layer forming step, a circular ring machining step of positioning the condensing point at the predetermined depth position and irradiating the laser beam in a circular ring shape along the outer peripheral edge of the workpiece to form a separation layer in the outer peripheral region of the workpiece is further provided. The method for manufacturing a wafer according to claim 1.

3. In the separation layer forming step, after relatively moving the workpiece and the condensing point in a regular hexagonal shape along the predetermined direction, moving the condensing point to the central side in the radial direction of the workpiece, and then relatively moving the workpiece and the condensing point in a hexagonal shape along the predetermined direction. The method for manufacturing a wafer according to claim 1 or 2.

4. In the separation layer forming step, the laser beam is branched into a plurality of laser beams, and the condensing points of each of the plurality of laser beams are arranged to be aligned along a first direction. Then, a second direction orthogonal to the first direction is set as the predetermined direction. The method for manufacturing a wafer according to claim 1, characterized in that.

5. In the separation layer forming step, after moving a plurality of condensing points along the second direction, the plurality of condensing points are shifted along the first direction so that a moving region including the locus of the movement of the plurality of condensing points in the second direction partially overlaps when viewed from the first surface. Then, the plurality of condensing points are moved along the second direction. The method for manufacturing a wafer according to claim 4, characterized in that.

6. In the separation layer forming step, the interval between the plurality of condensing points arranged along the first direction is 5 μm or more and 20 μm or less. The method for manufacturing a wafer according to claim 4, characterized in that.

7. The separation layer formed in the separation layer forming step includes a plurality of modified regions. Let the interval between the plurality of modified regions formed side by side along the first direction be a (μm), and let the interval between the plurality of modified regions formed side by side along the second direction be b (μm) by relatively moving the plurality of condensing points and the workpiece along the second direction. When the aspect ratio represented by (b / a) is 0.5 or more and 3.0 or less. The method for manufacturing a wafer according to claim 6, characterized in that.

8. In the separation layer forming step, the laser beam irradiated on the workpiece is irradiated on the workpiece in a burst mode. The method for manufacturing a wafer according to claim 1, characterized in that.

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