Wafer manufacturing method

Abstract

To provide a wafer manufacturing method that reduces cutting allowance when manufacturing a GaN wafer.SOLUTION: A manufacturing method of a wafer having a thickness less than a 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 comprises: a separation layer forming step of forming a separation layer on the workpiece by relatively moving the workpiece and a focal point along a predetermined direction in a state in which a pulsed laser beam with a wavelength that passes through the workpiece is irradiated onto the first surface from the opposite side to the second surface and the convergence point of the laser beam is positioned at a predetermined depth position of the workpiece; and a separation step of separating the wafer from the workpiece starting from the separation layer. The predetermined direction in the separation layer forming step has an angle formed between the (0001) plane and the crystal orientation represented by (1) below of 5° 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 the first and second surfaces, from a workpiece which is a gallium nitride ingot or a gallium nitride single crystal substrate having a first surface and a second surface located opposite the first surface. [Background technology]

[0002] Gallium nitride (GaN) is a wide-bandgap semiconductor, possessing a bandgap approximately three times larger than that of silicon (Si). This relatively large bandgap of GaN is utilized in the manufacture of power devices, LEDs, and other devices.

[0003] GaN single-crystal substrates (i.e., wafers) are typically manufactured by slicing GaN ingots. For wafer manufacturing, for example, an annular slicer with cutting edges on the inner circumference rather than the outer circumference is used (see Patent Document 1).

[0004] However, since the thickness of the slicer's cutting blade is relatively large (e.g., 0.3 mm) compared to the wafer thickness (e.g., 0.15 mm), approximately 60% to 70% of each wafer is discarded as cutting material when combined with the wafer. Thus, using a cutting blade results in a relatively high ratio of cutting material to the total amount of wafer (i.e., a high waste rate), making it uneconomical. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2011-84469 [Overview of the project] [Problems that the invention aims to solve]

[0006] This invention has been made in view of the aforementioned problems, and aims to reduce the cutting allowance when manufacturing GaN wafers from GaN ingots. [Means for solving the problem]

[0007] According to one aspect of the present invention, a wafer manufacturing method is provided for manufacturing a wafer having a thickness less than the distance between the first surface and the second surface from a workpiece which is a gallium nitride ingot or a gallium nitride single crystal substrate having a first surface and a second surface located opposite to the first surface, wherein the second surface of the workpiece is retention A wafer manufacturing method is provided, comprising: a holding step; a separation layer formation step, after the holding step, irradiating the first surface from the opposite side of the second surface with a pulsed laser beam having a wavelength that penetrates the workpiece, positioning the focal point of the laser beam at a predetermined depth position in the workpiece, and then moving the workpiece and the focal point relative to each other along a predetermined direction to form a separation layer in the workpiece; and a separation step, after the separation layer formation step, separating the wafer from the workpiece starting from the separation layer, wherein the predetermined direction in the separation layer formation step has an angle of 5° or less between it and the crystal orientation represented by (1) below in the (0001) plane.

number

[0008] Preferably, the wafer manufacturing method further comprises an annular processing step, after the holding step and before the separation layer formation step, in which the focusing point is positioned at a predetermined depth and the laser beam is irradiated in an annular manner along the outer edge of the workpiece to form a separation layer in the outer region of the workpiece.

[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 formation 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 formation step, after moving a plurality of condensing points along the second direction, the plurality of condensing points are moved 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, and then the plurality of condensing points are moved along the second direction.

[0012] Also, preferably, in the separation layer formation step, the interval between a plurality of condensing points arranged 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 formation step includes a plurality of modified regions. When the interval between the plurality of modified regions formed side by side along the first direction is a (μm), and the interval between the plurality of modified regions formed side by side along the second direction is b (μm) by relatively moving the plurality of condensing points and the workpiece along the second direction, the aspect ratio represented by (b / a) is 0.5 or more and 3.0 or less.

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

Advantages of the Invention

[0015] In a manufacturing method according to one aspect of the present invention, a separation layer is formed in the workpiece by positioning the focal point of a pulsed laser beam having a wavelength that penetrates the workpiece at a predetermined depth position in the workpiece, and then moving the workpiece and the focal point relative to each other in a predetermined direction (separation layer formation step).

[0016] Then, the wafer is separated from the workpiece starting from the separation layer (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), which reduces the cutting allowance in the thickness direction of the workpiece compared to when using a cutting blade. [Brief explanation of the drawing]

[0017] [Figure 1] This is a flowchart of the manufacturing process. [Figure 2] This is a perspective view of an ingot. [Figure 3] This is a schematic diagram of a laser processing machine. [Figure 4] Figure 4(A) is a schematic diagram of the laser beam LA, and Figure 4(B) is a schematic diagram of the laser beam LB. [Figure 5] This is a side view showing the holding step. [Figure 6] This is a plan view showing the separation layer formation step. [Figure 7] This is a plan view showing the overlap of the movement regions of multiple focal points. [Figure 8] Figure 8(A) shows the separation step, and Figure 8(B) shows the wafers etc. separated from the ingot. [Figure 9] This figure shows a modified example of the separation layer formation step. [Figure 10] This is a flowchart of the manufacturing method according to the second embodiment. [Figure 11] This is a plan view showing the annular machining step. [Figure 12] This is a photograph of a single-crystal substrate where sufficient cracks were not formed between the modified regions. [Figure 13]This diagram schematically shows multiple modification regions. [Figure 14] This is a photograph of a single-crystal substrate with a relatively large crack formed in the c-axis direction. [Figure 15] This is a photograph of a single-crystal substrate in which sufficient cracks have formed between the modified regions. [Modes for carrying out the invention]

[0018] An embodiment of one aspect of the present invention will be described with reference to the attached drawings. Figure 1 is a flowchart of a manufacturing method for producing a GaN single-crystal substrate (i.e., a wafer 15) (see Figure 8(B)) that is thinner than the GaN ingot (workpiece) 11.

[0019] In the first embodiment, the wafer 15 is manufactured by sequentially performing the holding step S10, the separation layer formation step S20, and the separation step S30 shown in Figure 1. First, the ingot 11 will be described with reference to Figure 2. Figure 2 is a perspective view of the 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 n-type containing n-type impurities such as silicon (Si) and germanium (Ge).

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

[0022]

number

[0023] In this specification, Miller indices are used to identify crystal planes and crystal orientations. Specific crystal planes are represented using (), and crystal planes that are equivalent to each other due to the symmetry of the crystal structure are represented using {}. Similarly, specific crystal orientations are represented using [], and crystal orientations that are equivalent to each other are represented using <>.

[0024] The crystal orientation perpendicular to the first face 11a (c face) and pointing upward is represented by (3) below. This crystal orientation is called the c axis and corresponds to the thickness direction 11c of the ingot 11.

[0025]

number

[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 positional relationship that is orthogonal to each other. The first side surface 13a corresponds to the crystal plane shown in (4) below, and the second side surface 13b corresponds to the crystal plane shown in (5) below.

[0027]

number

[0028]

number

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

[0030]

number

[0031] Furthermore, the second orientation flat (hereinafter abbreviated as second OF13b1) where the first face 11a and the second side surface 13b intersect is parallel to the crystal orientation described in (7) below.

[0032]

number

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

[0034] As shown in Figure 3, the X-axis direction (machining feed direction, second direction, predetermined direction), the Y-axis direction (indexing feed direction, first direction), and the Z-axis direction (height direction) are orthogonal to each other.

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

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

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

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

[0039] A rotational drive mechanism (not shown) for rotating the chuck table 4 is provided at the bottom of the chuck table 4. The rotational drive mechanism can rotate the chuck table 4 by a predetermined angle around a predetermined axis of rotation 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 movement mechanism and a Y-axis movement mechanism, respectively, which can move the chuck table 4 and the rotary drive mechanism along the X-axis and Y-axis directions.

[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 the laser medium. From the laser oscillator 10, a pulsed laser beam L with a wavelength (e.g., tens of MHz) having a wavelength (e.g., 1064 nm) that penetrates the GaN ingot 11 is emitted. A Launch.

[0043] Laser beam L emitted from laser oscillator 10 A In the Acousto-Optic Modulator (AOM) 12, the burst-mode laser beam L B It will be converted.

[0044] The acoustic-optic modulator 12 operates according to the electrical signal input to the acoustic-optic modulator 12, and the laser beam L operates according to the signal. A The laser beam L is deflected for a predetermined time.A The laser beam L in a state where it is 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 represents time and the vertical axis represents 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. The reciprocal of the period T between the pulse groups 12a (i.e., the repetition frequency) with the pulse group 12a as the repetition unit is, for example, at 50 kHz.

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

[0049] The branch unit 16 of the present embodiment has a 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 passing through the branch unit 16 CThe laser beam L is guided to the irradiation head 20 via a collimator lens (not shown), mirror 18, etc. The irradiation head 20 has a focusing lens (not shown). The focusing lens directs the laser beam L to a predetermined depth position in the ingot 11, which is held by attraction at the holding surface 4a. C To concentrate the light.

[0051] The laser beam L shown in Figure 3 C The branching unit 16 provides multiple laser beams L C1 , L C2 , L C3 , L C4 and L C5 It is branched into, and the laser beam L C1 From L C5 Each of the focusing points P(P1, P2, P3, P4, P5) is arranged to be aligned along the Y-axis at a predetermined depth position in the ingot 11.

[0052] The spacing between multiple focal points P aligned along the Y-axis is set to a predetermined value, for example, 5 μm to 20 μm. In the example shown in Figure 3, for the sake of explanation, the laser beam L C Although the number of branches is set to 5, the number of branches is not limited to 5. The number of branches may be between 2 and 16, and a suitable example of the number of branches is 10.

[0053] The housing (not shown) of the laser beam irradiation unit 6 is equipped with an imaging unit (not shown) for imaging the subject. 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 includes an image sensor (not shown) that receives reflected light from the light-emitting device via a lens (not shown). The light from the light-emitting device has wavelengths of visible light.

[0055] The image sensor is capable of photoelectric conversion of the wavelength of light from a light-emitting device. The image sensor is a CCD (Charge-Coupled Device) image sensor or a CMOS (Complementary Metal-Oxide-Semiconductor) image sensor, etc. The light-emitting device, lens, image sensor, etc., constitute a microscope camera unit that images a subject using visible light.

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

[0057] Memory includes main memory such as DRAM (Dynamic Random Access Memory), SRAM (Static Random Access Memory), and ROM (Read Only Memory), and auxiliary memory such as flash memory, hard disk drives, and solid-state drives.

[0058] The auxiliary storage device stores software, including a predetermined program. The control unit's functions are realized by operating the processing unit and other components according to this software. Next, a method for manufacturing the wafer 15 according to the first embodiment will be described according to the procedure shown in Figure 1.

[0059] Figure 5 is a side view showing the holding step S10 in which the second surface 11b of the ingot 11 is held by the holding surface 4a through suction. In the holding step S10, the ingot 11 is held by the holding surface 4a through suction in such a manner that the second surface 11b is in contact with the holding surface 4a and the first surface 11a is exposed upwards.

[0060] Furthermore, in the holding step S10, after suction holding, the first surface 11a side is imaged with the imaging unit to identify the misalignment of the first OF13a1 with respect to the X-axis direction of the laser processing apparatus 2. Subsequently, the chuck table 4 is rotated with the rotation drive mechanism to cancel out this misalignment, thereby making the first OF13a1 approximately parallel to the X-axis direction.

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

[0062] Figure 6 is a plan view showing the separation layer formation step S20. In Figure 6, for ease of understanding, two of the multiple focal points P are shown as relatively large circles, and several focal points located between these two focal points P are omitted.

[0063] In the separation layer formation step S20, the light-gathering points P are positioned so as to be aligned along the Y-axis at a predetermined depth position 11e of the ingot 11 (see Figures 3 and 8(A)), and then the multiple light-gathering points P and the ingot 11 (i.e., the chuck table 4) are moved relative to each other along the X-axis (a predetermined direction).

[0064] In the separation layer formation step S20 of this embodiment, the multiple focal points P are moved relative to each other in the +X direction, and then the multiple focal points P are moved relative to each other in the -X direction. In this way, movement in the +X direction and movement in the -X direction are repeated alternately.

[0065] In Figure 6, the movement paths of the multiple focal points P within the ingot 11 are shown by dashed arrows. Note that instead of moving the multiple focal points P alternately in the +X and -X directions, they may be moved only in the +X direction or only in the -X direction.

[0066] When the relative movement direction between multiple focal points P and the ingot 11 is along the X-axis, this movement direction is parallel to the crystal orientation shown in (8) below.

[0067]

number

[0068] The two crystal orientations shown in (8) are two of the six equivalent crystal orientations in ingot 11, which has a hexagonal crystal structure, as shown in (9) below.

[0069]

number

[0070] Incidentally, the relative movement direction between the multiple focal points P and the ingot 11 does not have to be perfectly parallel to the crystal orientation specified in (8), and the angle formed between the crystal orientation specified in (8) and the c-plane (see (2) above) may be 5° 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: 1064nm Machining feed rate: 875 mm / s Indexing feed amount: 106 μm (i.e., index amount) Repetition frequency: 50kHz Burst count: 10 (number of pulses included in pulse group 12a) Number of branches: 10 (Laser beam L C (Number of branches) Number of passes: 1 Spot diameter at each focusing point: approximately 5 μm Depth position of the focal point: 170 μm from surface 11a

[0073] In this processing condition, the spacing between adjacent focal points in the 10 focal points is set to, for example, 12.5 μm. Also, when 10 focal points P are arranged, the laser beam L is within a 12.5 μm × 9 area. C Since the light is irradiated, the irradiation distance of the multiple focal points P arranged along the Y-axis is 112.5 μm (see Figure 7).

[0074] When multiple focusing points P are moved relative to each other along the X-axis, the trajectories of the movement of the multiple focusing points P are included in the first movement region 22a shown by the solid line. After moving the multiple focusing points P along the X-axis, the irradiation head 20 and the chuck table 4 are moved relative to each other along the Y-axis to perform indexing feed by the predetermined index amount described above.

[0075] In this state, multiple focal points P are moved similarly along the X-axis relative to each other. The trajectories of the movement of the multiple focal points P after indexing are included in the second movement region 22b (see Figure 7), shown by the dashed line. As shown in Figure 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] Figure 7 is a plan view showing the overlap of the first moving region 22a and the second moving region 22b. Under the processing conditions described above, the width of the overlap in the Y-axis direction is 6.5 μm. Such overlapping regions 22c are formed on both sides in the Y-axis direction of each moving region, except for the two moving regions located at the Y-axis ends of the first surface 11a.

[0077] Incidentally, near each of the multiple focal points P, the crystallinity of the ingot 11 changes due to multiphoton absorption. For example, in the regions where multiphoton absorption occurs, modified regions are formed in which the mechanical strength is lower compared to regions where multiphoton absorption does not occur.

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

[0079] After the separation layer formation step S20, the ingot 11 is separated into wafer 15 and other ingots 17 using the separation apparatus 32, as shown in Figures 8(A) and 8(B) (separation step S30). The separation apparatus 32 will be described with reference to Figure 8(A).

[0080] The separation device 32 has a chuck table 34 which is approximately the same diameter as the chuck table 4 described above. The structure of the chuck table 34 is approximately the same as that of the chuck table 4, and the upper surface of the chuck table 34 functions as a holding surface 34a that sucks and holds the ingot 11. A separation unit 36 ​​is provided above the chuck table 34.

[0081] The separation unit 36 ​​has a cylindrical movable part 38 whose longitudinal portion 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 disc-shaped suction head 40 is provided at the bottom of the movable part 38. The suction head 40, like the chuck table 34, has a frame and a porous plate. The lower surfaces of the frame and the porous plate are arranged to be substantially flush and substantially parallel to the XY plane, and function as a holding surface 40a.

[0083] Figure 8(A) shows the separation step S30. In the separation step S30, the second surface 11b of the ingot 11 on which the separation layer 11d is formed is held by suction using the holding surface 34a of the chuck table 34, while the first surface 11a is held by suction using the holding surface 40a of the suction head 40.

[0084] Next, an external force is applied to the ingot 11. This external force is applied, for example, by driving a wedge (not shown) into the side of the ingot 11 at the height of the separation layer 11d. It is preferable to drive the wedge into multiple locations along the circumferential direction of the ingot 11, rather than just one location on the side of the ingot 11.

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

[0086] When applying ultrasound, before the first surface 11a is sucked and held by the holding surface 40a of the suction head 40, ultrasound is applied to the first surface 11a side via a liquid such as pure water. Specifically, the liquid to which ultrasound has been applied is sprayed from the nozzle onto the ingot 11, or ultrasound is applied to the first surface 11a side from the horn via the liquid.

[0087] The applicant has confirmed in experiments that applying an external force to the entire surface 11a at once causes undesirable cracking. Therefore, when using a nozzle or horn, an external force is first applied using ultrasound to a localized area on the surface 11a with a diameter of approximately 5 mm to 50 mm.

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

[0089] When an external force is applied, cracks connect between adjacent modified regions, and the mechanical strength of the separation layer 11d becomes even weaker than that of the ingot 11 outside the separation layer 11d. Therefore, the wafer 15 can be separated from the ingot 11 with less force than when no external force is applied.

[0090] After applying an external force, the suction head 40 is raised (i.e., moved in the +Z direction). This separates the wafer 15 from the ingot 11, starting from the separation layer 11d. Figure 8(B) shows the wafer 15 separated from the ingot 11. Note that the external force described above may be applied in parallel with the raising of the suction head 40.

[0091] The separation layer 11d has a thickness of approximately 50 μm to 60 μm (for example, 58 μm) in the thickness direction 11c, and this thickness of the separation layer 11d corresponds to the cutting allowance mentioned above. By laser processing the ingot 11, the cutting allowance in the thickness direction 11c of the ingot 11 can be reduced compared to when a slicer is used.

[0092] Therefore, the productivity of wafer 15 when manufacturing wafer 15 from ingot 11 is improved. Even when using a wire saw, a cutting allowance of at least 150 μm is required. Therefore, the manufacturing method of this embodiment is superior to the method using a wire saw.

[0093] In the example described above, a separation layer 11d is formed by arranging multiple focusing points P at a predetermined depth position 11e of the ingot 11. 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) can be used. In other words, the thickness of the wafer 15 will be less than the distance between the two surfaces (first and second surfaces) in the c-axis direction of the GaN single-crystal substrate.

[0095] (Modified Version) Next, a modified version of the separation layer formation step S20 will be described. Figure 9 shows a modified version of the separation layer formation step S20. In the modified version of the separation layer formation step S20, the relative movement between the multiple focusing points P and the ingot 11 at the processing feed rate described above is not in a straight line along the X-axis direction, but in a regular hexagonal shape. This is the only difference from the first embodiment, but all other aspects are the same as the first embodiment.

[0096] For example, multiple focusing points P are moved relative to each other in the order of (10), (11), (12), (13), (14), and (15) below. Such machining can be achieved, for example, by appropriately combining the linear movement of the chuck table 4 by a horizontal movement mechanism and the rotation of the chuck table 4 by a rotational drive mechanism.

[0097]

number

[0098]

number

[0099]

number

[0100]

number

[0101]

number

[0102]

number

[0103] After moving multiple focal points P relative to each other to form a regular hexagon, the multiple focal points P are moved by the predetermined index amount described above toward the radial center of the ingot 11, and then similarly, the multiple focal points P are moved relative to each other in the order of (10) to (15).

[0104] As a result, the movement regions of the multiple focal points P become multiple regular hexagonal shapes arranged concentrically, as shown in Figure 9. Note that, as shown in (9), all of (10) through (15) are included in the crystal orientation shown in (1).

[0105] In this modified example, at the start of laser processing, multiple focal points P are moved in the direction shown in (10). However, as long as the multiple focal points P can be moved relative to each other in a regular hexagonal shape, laser processing may be started from any of the crystal orientations from (10) to (15).

[0106] Furthermore, the relative movement direction between the multiple focal points P and the ingot 11 does not have to be perfectly parallel to the crystal orientation specified in (1), and the angle formed between the crystal orientation specified in (1) and the c-plane may be 5° or less.

[0107] For example, when moving multiple focal points P and the ingot 11 relative to each other along the crystal orientation specified in (10), the direction of this relative movement can be such that the angle formed between the c-plane and the crystal orientation specified in (10) is 5° or less.

[0108] The same applies when multiple focal points P and the ingot 11 are moved relative to each other along the crystal orientation specified in (11) to (15). Furthermore, this modified example can be applied to a GaN single-crystal substrate instead of the ingot 11.

[0109] (Second Embodiment) Next, a second embodiment will be described with reference to Figures 10 and 11. Figure 10 is a flow chart of the wafer manufacturing method of the second embodiment, and Figure 11 is a plan view showing the annular processing step S15.

[0110] In the manufacturing method according to the second embodiment, after the holding step S10 and before the separation layer formation step S20, a laser beam L is applied in an annular manner along the outer peripheral edge 11f of the ingot 11. C The system further includes an annular machining step S15 in which light is irradiated.

[0111] In this embodiment, the laser beam L is formed in an annular shape along the outer edge 11f. C Irradiating means assuming that the edges of the first surface 11a, which were missing due to the presence of the first OF13a1 and second OF13a2, have been filled in so that they become circular, and then irradiating along the circular edges of the first surface 11a with a laser beam L C It means to irradiate with light.

[0112] In the annular machining step S15, the multiple focusing points P are positioned at the same predetermined depth position 11e as when forming the separation layer 11d in the separation layer formation step S20. In the annular machining step S15, first, multiple focusing points P are arranged in a line along the Y-axis direction at the predetermined depth position 11e of the ingot 11.

[0113] In this case, the outermost focusing point is located radially inward from the outer edge 11f by a predetermined distance 24 from the outer edge 11f. The predetermined distance 24 is, for example, 4 μm to 8 μm, and a preferred example is 5 μm to 6 μm.

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

[0115] The predetermined rotational speed of the chuck table 4 is adjusted as appropriate, for example, so that the peripheral speed at multiple focusing points P is approximately equal to the machining feed rate described above. The predetermined rotational speed of the chuck table 4 may also be adjusted to achieve a suitable aspect ratio (b / a) as described later.

[0116] In this way, the annular processing step S15 also forms a separation layer 11d in the outer peripheral region 28 of the ingot 11. In Figure 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 processing 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 the same as in the first embodiment, for example. This allows the separation layer formation step S20 to be executed smoothly after the annular processing step S15.

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

[0119] If the separation layer 11d is not formed in the outer peripheral region 28 after the annular processing step S15, the nitrogen gas formed in the separation layer formation step S20 may cause an abnormal volume expansion region to form on the radially inner side of the ingot 11.

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

[0121] Therefore, abnormal volume expansion in the radial direction of the ingot 11 can be suppressed. Furthermore, by forming a separation layer 11d in the outer peripheral region 28, the propagation of cracks in undesirable directions (e.g., the c-axis direction) can be suppressed, while the propagation of cracks outward on the c-plane of the ingot 11 can be promoted.

[0122] (Experimental Example) Next, in the separation layer formation step S20, the experimental results when the spacing between adjacent focal points and the processing feed rate are changed will be explained using Figures 12 to 15. In the experimental example, a GaN single crystal substrate was processed using the laser processing apparatus 2 described above.

[0123] The wavelength, repetition frequency, burst count, number of passes, spot diameter of each focal point, depth position of the focal points, and spacing of the focal points were the same as in the first embodiment, but the processing feed rate (mm / s) and pulse energy (μJ) were changed as appropriate.

[0124] However, when processing the single crystal substrate shown in Figure 12, the laser beam L C The number of branches is set to 6, and when processing the single crystal substrate shown in Figures 14 and 15, the laser beam L C The number of branches was set to 10.

[0125] Furthermore, when processing the single crystal substrates shown in Figures 12, 14, and 15, the indexing feed rate was set to 112.5 μm, and laser processing was performed on three parallel linear regions. However, the laser processing was performed in a manner that avoided forming the overlapping region 22c (see Figure 7).

[0126] Figure 12 is a photograph of a single-crystal substrate in which sufficient cracks were not formed during the modified region 11h. This photograph was obtained by taking a visible light camera of the first surface 11a of the single-crystal substrate after laser processing. Similarly, the photographs in Figures 14 and 15, described later, were also obtained by taking a visible light camera.

[0127] The straight line that crosses the center of the image in Figure 12 is a reference line 30 that is displayed so as to cross the center of the imaging field of view. The band-shaped linear region 11g is a region that has been laser-processed along the crystal orientation shown in (1).

[0128] In the image, modified regions 11h are formed in the areas indicated by black circles, and cracks 11i are formed in the bright areas between the modified regions 11h.

[0129] Figure 13 is a schematic diagram showing multiple modified regions 11h. Distance a is the interval between multiple modified regions 11h aligned along the Y-axis (corresponding to the interval between multiple focal points P aligned along the Y-axis), and its unit is μm.

[0130] Furthermore, distance b is the interval between multiple modification regions 11h aligned along the X-axis (corresponding to the interval between multiple focusing points P aligned along the X-axis), and its unit is μm. Distance b is determined according to the processing feed rate (i.e., the relative movement speed between the multiple focusing points P and the ingot 11) and the repetition frequency.

[0131] Experiments revealed that the quality of the processing is determined by the aspect ratio, expressed as (b / a). Specifically, when the aspect ratio (b / a) exceeds 3.0, the modified regions 11h separate from each other, and as shown in Figure 12, the cracks 11i do not extend sufficiently in the XY plane.

[0132] Furthermore, when the aspect ratio (b / a) is less than 0.5, the modified regions 11h approach each other, and cracks 11i extend relatively sufficiently in the XY plane direction as shown in Figure 14, but relatively large cracks 11j are formed in the Z axis direction.

[0133] Figure 14 is a photograph of a single-crystal substrate in which a relatively large crack 11i has formed in the c-axis direction. Note that the crack 11i extends in the Z-axis direction (depth direction), and in the photograph shown in Figure 14, it is out of focus and its outline is blurred.

[0134] In contrast, when the aspect ratio (b / a) is between 0.5 and 3.0, it is possible to extend the crack 11i relatively sufficiently in the XY plane direction, while preventing the formation of a relatively large crack 11i in the Z axis direction.

[0135] Figure 15 is a photograph of a single-crystal substrate in which cracks 11i have sufficiently extended during the modified region 11h, and relatively large cracks 11i have not formed in the Z-axis direction. The aspect ratio (b / a) may be between 0.8 and 2.5, or between 1.0 and 1.4.

[0136] According to the embodiments, modifications, and experimental results described above, forming a separation layer 11d on the workpiece using the laser processing apparatus 2 reduces the cutting allowance in the thickness direction of the workpiece compared to using a slicer.

[0137] Furthermore, the structures, methods, etc., related to the embodiments described above can be modified as appropriate without departing from the scope of the present invention. [Explanation of symbols]

[0138] 2: Laser processing device, 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 group 11: Ingot (workpiece), 11a: First surface, 11b: Second surface, 11c: Thickness direction 11d: Separation layer, 11e: Depth position, 11f: Outer edge 11g: linear region, 11h: modified region, 11i, 11j: cracks 13a: 1st side, 13b: 2nd side First Orientation Flat (1F): 13a1 Second Orientation Flat (2OF): 13a2 14: Output adjustment unit, 16: Branching unit, 18: Mirror, 20: Illumination head 15: Wafers, 17: Other ingots 22a: First movement region, 22b: Second movement region, 22c: Overlap region 24: Distance, 26: Index amount, 28: Outer 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 , L C5 : Laser beam P, P1, P2, P3, P4, P5: Focus point S10: Holding step, S15: Annular machining step S20: Separation layer formation step, S30: Separation step a, b: distance, t: time interval, T: period

Claims

1. A method for manufacturing a wafer having a thickness less than the distance between the first and second surfaces, from a workpiece which is a gallium nitride ingot or a gallium nitride single crystal substrate having a first surface and a second surface located opposite the first surface, A holding step of holding the second surface of the workpiece, After the holding step, a separation layer formation step is performed in which a pulsed laser beam having a wavelength that penetrates the workpiece is irradiated onto the first surface from the opposite side of the second surface, and the focal point of the laser beam is positioned at a predetermined depth position in the workpiece, and the workpiece and the focal point are moved relative to each other along a predetermined direction to form a separation layer in the workpiece. After the separation layer formation step, a separation step is performed to separate the wafer from the workpiece starting from the separation layer, Equipped with, In the separation layer formation step, the predetermined direction is such that the angle formed between the (0001) plane and the crystal orientation represented by (1) below is 5° or less. A wafer manufacturing method characterized in that, in the separation layer formation step, the workpiece and the focusing point are moved relative to each other in a regular hexagonal shape along the predetermined direction, the focusing point is moved towards the radial center of the workpiece, and then the workpiece and the focusing point are moved relative to each other in a hexagonal shape along the predetermined direction. [Math 1]

2. A method for manufacturing a wafer having a thickness less than the distance between the first and second surfaces, from a workpiece which is a gallium nitride ingot or a gallium nitride single crystal substrate having a first surface and a second surface located opposite to the first surface, A holding step of holding the second surface of the workpiece, After the holding step, a separation layer formation step is performed in which a pulsed laser beam having a wavelength that penetrates the workpiece is irradiated onto the first surface from the opposite side of the second surface, and the focal point of the laser beam is positioned at a predetermined depth position in the workpiece, and the workpiece and the focal point are moved relative to each other along a predetermined direction to form a separation layer in the workpiece. After the separation layer formation step, a separation step is performed to separate the wafer from the workpiece starting from the separation layer, Equipped with, In the separation layer formation step, the predetermined direction is such that the angle formed between the (0001) plane and the crystal orientation represented by (1) below is 5° or less. In the separation layer formation step, the laser beam is split into multiple laser beams, and the focal points of each of the multiple laser beams are arranged to line up along the first direction, and the second direction perpendicular to the first direction is set to the predetermined direction, and A method for manufacturing a wafer, characterized by moving a plurality of focal points along the second direction, and then moving the plurality of focal points along the second direction while shifting the plurality of focal points along the first direction such that the moving region including the trajectory of the plurality of focal points moving in the second direction partially overlaps with the first surface when viewed from the first surface. [Math 1]

3. A method for manufacturing a wafer having a thickness less than the distance between the first and second surfaces, from a workpiece which is a gallium nitride ingot or a gallium nitride single crystal substrate having a first surface and a second surface located opposite to the first surface, A holding step of holding the second surface of the workpiece, After the holding step, a separation layer formation step is performed in which a pulsed laser beam having a wavelength that penetrates the workpiece is irradiated onto the first surface from the opposite side of the second surface, and the focal point of the laser beam is positioned at a predetermined depth position in the workpiece, and the workpiece and the focal point are moved relative to each other along a predetermined direction to form a separation layer in the workpiece. After the separation layer formation step, a separation step is performed to separate the wafer from the workpiece starting from the separation layer, Equipped with, In the separation layer formation step, the predetermined direction is such that the angle formed between the (0001) plane and the crystal orientation represented by (1) below is 5° or less. In the separation layer formation step, the laser beam is split into multiple laser beams, and the focal points of each of the multiple laser beams are arranged to line up along the first direction, and the second direction perpendicular to the first direction is set to the predetermined direction. In the separation layer formation step, the spacing between the multiple light-gathering points aligned along the first direction is 5 μm or more and 20 μm or less. The separation layer formed in the separation layer formation step includes a plurality of modified regions, A method for manufacturing a wafer, characterized in that when the distance between the plurality of modified regions formed in a line along the first direction is a (μm), and the distance between the plurality of modified regions formed in a line along the second direction by relatively moving the plurality of focal points and the workpiece in the second direction is b (μm), the aspect ratio expressed as (b / a) is 0.5 or more and 3.0 or less. [Math 1]

4. A method for manufacturing a wafer according to any one of claims 1 to 3, further comprising an annular processing step, after the holding step and before the separation layer formation step, in which the focusing point is positioned at a predetermined depth and the laser beam is irradiated in an annular manner along the outer edge of the workpiece to form a separation layer in the outer peripheral region of the workpiece.

5. The method for manufacturing a wafer according to any one of claims 1 to 3, characterized in that, in the separation layer formation step, the laser beam irradiated onto the workpiece is irradiated onto the workpiece in burst mode.

Images