Processing method and laser processing device for a SiC ingot

Abstract

A method of processing a SiC ingot (86) in which a c-plane is inclined to an end face (88) of the SiC ingot (86) and a direction perpendicular to a direction in which a deviation angle (α) is formed between the end face (88) of the SiC ingot (86) and the c-plane represents an X-axis direction and a direction perpendicular to the X-axis direction represents a Y-axis direction, the method comprising: a peel zone forming step comprising applying a pulsed processing laser beam (LB1) whose wavelength is transmittable through the SiC ingot (86) to the SiC ingot (86), while positioning a focal point (FP1) of the pulsed processing laser beam (LB1) at a depth corresponding to a thickness of a wafer (106) to be peeled from the SiC ingot (86), and processing the SiC ingot (86) and the focal point (FP1) relative to each other in the X-axis direction to form band-shaped peel zones (104) in the SiC ingot (86), each having cracks (102) extending along the c-plane from a region (100) where SiC is separated into Si and C; a feeding step comprising feeding the SiC ingot (86) and the focal point (FP1) relative to each other in the Y-axis direction to line up the peeling zones (104) in the Y-axis direction; a reflected beam detection step comprising applying a verification laser beam (LB2) having a wavelength that is transmissible through the SiC ingot (86) and reflectable by the cracks (102) of the peeling zones (104), and detecting an intensity of a beam reflected by the cracks (102); and a machining laser beam output power adjusting step of adjusting an output power of the pulsed machining laser beam (LB1) to keep the intensity of the reflected beam detected in the reflected beam detecting step within a predetermined range.

Description

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

[0001] The present invention relates to a processing method and a laser processing apparatus for a SiC ingot. DESCRIPTION OF THE RELATED PRIOR ART

[0002] Devices such as integrated circuits (ICs), large-scale integration circuits (LSI circuits), and light-emitting diodes (LEDs) are formed on a wafer made of silicon (Si), sapphire (Al2C3), or the like by depositing a functional layer on the surface of the wafer and forming a plurality of regions on the functional layer with a grid of projected dividing lines thereon. Power devices such as LEDs or the like are formed on a wafer made of single-crystal silicon carbide (SiC) by depositing a functional layer on the surface of the wafer and defining a plurality of regions on the functional layer with a grid of projected dividing lines thereon. The wafer with the devices formed thereon is divided along the projected dividing lines by a dicing device or a laser processing device to produce individual device chips.The component chips are used in electrical devices such as mobile phones and personal computers.

[0003] Wafers for forming devices are generally manufactured by cutting a cylindrical semiconductor ingot with a wire saw. The end and back surfaces of the wafers cut from the ingot are polished to a mirror finish (see, for example, JP 2000-94221 A). However, dividing a semiconductor ingot into wafers with a wire saw and polishing the end and back surfaces of the wafers is uneconomical because a large portion of the semiconductor ingot, such as 70% to 80%, becomes scrap. In particular, SiC ingots have the disadvantage of poor economics because they are hard, difficult and time-consuming to cut with a wire saw, and their unit cost is so high that they do not allow for efficient wafer production.

[0004] The present applicant has proposed a technology in which a laser beam having a wavelength transmissible through single-crystal SiC is applied to a SiC ingot while positioning a focal point of the laser beam inside the SiC ingot to thereby form peeling zones in a projected parting plane in the SiC ingot, and then a wafer is peeled from the SiC ingot along the projected parting plane where the peeling zones are formed (see, for example, JP 2016-111143 A).

[0005] Furthermore, DE 10 2016 008 509 A1 discloses a method for generating control data for post-processing a solid modified by laser beams. DE 10 2018 202 984 A1 proposes semiconductor ingot inspection methods for determining the state of modified layers and cracks. Furthermore, WO 2016 / 081 548 A1 discloses methods for cutting thin, flexible glass substrates using monitoring information regarding a crack tip position relative to an irradiation zone on the glass substrate. SUMMARY OF THE INVENTION

[0006] As the growth of a SiC ingot progresses, its crystal structure becomes uniform. Consequently, it is necessary to apply a laser beam with higher energy to the SiC ingot when peel zones are to be formed in an earlier-grown ingot portion than when peel zones are to be formed in an ingot portion that grew later. Consequently, the proposed technology is problematic in that the laser beam applied to the SiC ingot needs to have different energy levels to form proper peel zones in the SiC ingot at different heights or axial positions within the SiC ingot.

[0007] It is therefore an object of the present invention to provide a machining method and a laser machining apparatus for a SiC ingot capable of forming proper pull-off zones in the SiC ingot at arbitrary heights in the SiC ingot.

[0008] In accordance with one aspect of the present invention, there is provided a method for processing a SiC ingot in which a c-plane is inclined toward an end face of the SiC ingot, and a direction perpendicular to a direction in which a deviation angle is formed between the end face of the SiC ingot and the c-plane is represented as X-axis directions, and a direction perpendicular to the X-axis directions is represented as Y-axis directions, the method comprising a peeling zone forming step of applying a pulsed processing laser beam whose wavelength is transmissible through the SiC ingot to the SiC ingot while positioning a focal point of the pulsed processing laser beam at a depth corresponding to a thickness of a wafer to be peeled from the SiC ingot, and processing the SiC ingot and the focal point relative to each other in the X-axis direction,to form band-shaped peeling zones in the SiC ingot, each having cracks extending from a region where SiC is separated into Si and C along the c-plane, a feeding step of feeding the SiC ingot and the focal point relative to each other in the Y-axis direction to line up the peeling zones in the Y-axis direction, a reflected beam detection step of applying an inspection laser beam having a wavelength transmissible through the SiC ingot and reflectable from the cracks of the peeling zones, and detecting an intensity of a beam reflected by the cracks, and a machining laser beam output power adjustment step of adjusting an output power of the pulsed machining laser beam to maintain the intensity of the reflected beam detected in the reflected beam detection step within a predetermined range.

[0009] Preferably, the method further includes, prior to the peel zone forming step, a flat surface forming step of grinding the end face of the SiC ingot into a flat surface.

[0010] In accordance with another aspect of the present invention, there is provided a laser processing apparatus for forming pull-off zones in a SiC ingot, in which a c-plane is inclined toward one end side of the SiC ingot, and a direction perpendicular to a direction in which a deviation angle is formed between the end side of the SiC ingot and the c-plane is represented as the X-axis direction, and a direction perpendicular to the X-axis direction is represented as the Y-axis direction, the laser processing apparatus comprising a holding table for holding the SiC ingot thereon, a laser beam application unit having a laser oscillator for generating a pulsed processing laser beam, and a beam condenser for applying the pulsed processing laser beam whose wavelength is transmittable to the SiC ingot held on the holding table,while positioning a focal point of the pulsed processing laser beam at a depth corresponding to a thickness of a wafer to be peeled from the SiC ingot to form band-shaped peeling zones in the SiC ingot, each having cracks extending from a region where SiC is separated into Si and C along the c-plane, an X-axis feed mechanism for processing-feeding the holding table and the beam condenser relative to each other in the X-axis direction, a Y-axis feed mechanism for feeding the holding table and the beam condenser relative to each other in the Y-axis direction, a reflected beam detection unit having a light emitter and a light detector for applying an inspection laser beam having a wavelength that is transmissible through the SiC ingot and reflectable from the cracks of the peeling zones, and for detecting an intensity of a beam reflected by the cracks,and a control unit for adjusting an output power of the pulsed machining laser beam to keep the intensity of the reflected beam detected by the reflected beam detection unit within a predetermined range.,

[0011] The method of processing a SiC ingot in accordance with the aspect of the present invention is capable of forming proper pull-off zones in the SiC ingot at arbitrary heights in the SiC ingot.

[0012] The laser processing apparatus in accordance with the other aspect of the present invention is capable of forming proper peel zones in the SiC ingot at arbitrary heights in the SiC ingot, similar to the method for processing a SiC ingot.

[0013] The above and other objects, features and advantages of the present invention and the mode for carrying them out will become more apparent and the invention itself will be best understood by studying the following description and the appended claims with reference to the accompanying drawings which show a preferred embodiment of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of a laser processing apparatus in accordance with an embodiment of the present invention; Fig. 2 is a block diagram of a portion of a structure of the Fig. 1 illustrated laser processing device; Fig. 3A is a front view of a SiC ingot; Fig. 3B is a top view of the SiC ingot; Fig. 4 is a perspective view illustrating a manner in which a flat surface forming step is carried out on the SiC ingot; Fig. 5A is a perspective view illustrating a manner in which a peel zone forming step is performed on the SiC ingot; Fig. 5B is an enlarged partial sectional view illustrating the manner in which the peel zone forming step is carried out on the SiC ingot; Fig. 6 is an enlarged partial sectional view illustrating a manner in which a reflected beam detecting step is performed on the SiC ingot; and Fig. 7 is a perspective view illustrating a manner in which a pulling step is performed on the SiC ingot. DETAILED EXPLANATION OF THE PREFERRED EMBODIMENT

[0014] A processing method and a laser processing apparatus for a SiC ingot in accordance with a preferred embodiment of the present invention will be described below with reference to the drawings.

[0015] First, the laser processing apparatus according to the present embodiment will be described below with reference to Fig. 1. As described in Fig. 1, a laser processing apparatus, generally indicated by 2, comprises a holding unit 4 for holding a SiC ingot 86 thereon, a laser beam application unit 8 having a beam condenser 6 for applying a pulsed processing laser beam having a wavelength transmissible through the SiC ingot 86 to the SiC ingot 86 while positioning a focal point of the laser beam inside the SiC ingot 86 at a depth corresponding to a thickness of a wafer to be peeled from the SiC ingot 86, thereby forming in the SiC ingot 86 band-shaped peeling zones each having cracks extending along a c-plane from a region where SiC is separated into silicon (Si) and carbon (C), an X-axis feeding mechanism 10 for processing feeding the holding unit 4 and the beam condenser 6 relatively to each other in X-axis directions,a Y-axis feeding mechanism 12 for feeding the holding unit 4 and the beam condenser 6 relative to each other in Y-axis directions, a reflected beam detecting unit 14 for applying an inspection laser beam having a wavelength transmissible through the SiC ingot and reflectable by the cracks of the peeling zones, for detecting an intensity of a beam reflected by the cracks, and a control unit 16 (see , Fig. 2) for controlling an output power of the pulsed machining laser beam so that the intensity of the reflected beam detected by the reflected beam detecting unit 14 falls within a predetermined range. The X-axis directions refer to directions defined by a Fig. 1, and the Y-axis directions refer to directions indicated by an arrow in Fig. 1 and are perpendicular to the X-axis directions. The X-axis directions and the Y-axis directions together define an XY plane that is substantially horizontal.

[0016] As in Fig. 1, the holding unit 4 includes an X-axis movable plate 20 movably mounted on a base 18 for movement in the X-axis directions, a Y-axis movable plate 22 movably mounted on the X-axis movable plate 20 for movement in the Y-axis directions, a circular holding table 24 rotatably mounted on an upper surface of the Y-axis movable plate 22, and an unillustrated holding table motor for rotating the holding table 24 about its central axis.

[0017] The laser beam application unit 8 includes a housing 26 having an upwardly directed column extending upwardly from an upper surface of the base 18, and an arm extending substantially horizontally from the upper end of the upwardly directed column. As shown in Fig. 2, the housing 26 accommodates therein a laser oscillator 28 for emitting a pulsed machining laser beam LB1 having a wavelength transmissible through the SiC ingot 86, an attenuator 30 for adjusting the output power of the pulsed machining laser beam LB1 emitted from the laser oscillator 28, and a mirror 32 for reflecting the pulsed machining laser beam LB1 whose output power has been adjusted by the attenuator 30 and for guiding the reflected pulsed machining laser beam LB1 toward the beam condenser 6.

[0018] As in Fig. As illustrated in Figure 1, the beam condenser 6 of the laser beam application unit 8 is mounted on a lower surface of a distal end of the horizontal arm of the housing 26. The laser beam application unit 8 includes a focal position adjusting means (not illustrated). The focal position adjusting means may comprise, for example, a vertical ball screw coupled to the beam condenser 6 and an electric motor for rotating the vertical ball screw about its central axis. The focal position adjusting means adjusts the vertical position of the focal point of the pulsed machining laser beam LB1 emitted from the laser oscillator 28.

[0019] The beam condenser 6 applies the pulsed processing laser beam LB1 to the SiC ingot 86 while positioning the focal point of the pulsed processing laser beam LB1 in the SiC ingot 86 at a depth corresponding to a thickness of a wafer to be peeled from the SiC ingot 86. In the SiC ingot 86 held on the holding unit 4, a c-plane is inclined toward one end side of the SiC ingot 86 by a deviation angle formed along a direction perpendicular to the X-axis directions, which are perpendicular to the Y-axis directions.

[0020] As in Fig. 1, an image pickup unit 34 for capturing an image of the SiC ingot 86 held on the holding unit 4 is mounted on the lower surface of the distal end of the horizontal arm of the housing 26 at a position spaced apart from the beam condenser 6 in one of the X-axis directions. A display unit 36 for displaying the image captured by the image pickup unit 34 is arranged on the upper surface of the horizontal arm of the housing 26.

[0021] The X-axis feed mechanism 10 includes a ball screw 38 extending along the upper surface of the base 18 in the X-axis direction, and an electric motor 40 for rotating the ball screw 38 about its central axis. The ball screw 38 is threadedly engaged with a nut (not shown) coupled to the X-axis movable plate 20. When the electric motor 40 is energized, the ball screw 38 rotates about its central axis, and the nut converts the rotation of the ball screw 38 into linear motion, which is transmitted to the X-axis movable plate 20, which is fed for processing along guide rails 18a on the base 18 in one of the X-axis directions relative to the beam condenser 6.

[0022] The Y-axis feed mechanism 12 includes a ball screw 42 extending along an upper surface of the X-axis movable plate 20 in the Y-axis directions, and an electric motor 44 for rotating the ball screw 42 about its central axis. The ball screw 42 is threadedly engaged with a nut (not shown) coupled to the Y-axis movable plate 22. When the electric motor 44 is energized, the ball screw 42 rotates about its central axis, and the nut converts the rotation of the ball screw 42 into linear motion, which is transmitted to the Y-axis movable plate 22, which is fed in one of the Y-axis directions relative to the beam condenser 6 on the X-axis movable plate 20.

[0023] As in Fig. 1, the reflected beam detection unit 14 includes a light emitter 46 and a light detector 48 mounted on the lower surface of the distal end of the horizontal arm of the housing 26. The light emitter 46 includes a laser oscillator (not illustrated) for emitting a pulsed inspection laser beam LB2 (see Fig. 6) having a wavelength that is transmissible through the SiC ingot 86 and reflectable by the cracks of the peel zones in the SiC ingot 86, and a beam emitter (not shown) for applying the pulsed inspection laser beam LB2 emitted from the laser oscillator to the SiC ingot 86. The light detector 48 may comprise a photodiode or the like.

[0024] Both the light emitter 46 and the light detector 48 are movable in the X-axis directions, the Y-axis directions and vertical directions, and their angles are variable with respect to the end face of the SiC ingot 86. Consequently, the light emitter 46 can have an incident angle θ (see Fig. 6) of the inspection pulsed laser beam LB2 at the end side of the SiC ingot 86, and the light detector 48 can adjust a position of a light detecting surface thereof in the path of the inspection pulsed laser beam LB2 reflected from the cracks of the peeling zones in the SiC ingot 86.

[0025] The control unit 16 is in the form of a computer having an unillustrated central processing unit (CPU) for executing arithmetic processing operations in accordance with control programs, an unillustrated read-only memory (ROM) for storing the control programs, etc., and an unillustrated read / write random access memory (RAM) for storing the results of the arithmetic processing operations.

[0026] As in Fig. 2, the control unit 16 is electrically connected to the light detector 48 so that the light detector 48 can send a signal to the control unit 16 indicative of an intensity of the reflected beam detected by the light detector 48. The control unit 16 is also electrically connected to the attenuator 30 of the laser beam application unit 8. The control unit 16 controls the attenuator 30 based on the intensity of the reflected beam sent from the light detector 48 to adjust the output power of the pulsed machining laser beam LB1 to maintain the intensity of the reflected laser beam detected by the light detector 48 within a predetermined range, for example, to maintain a voltage signal from the light detector 48 in a range of 1 to 1.2 volts.

[0027] The predetermined range referred to above as the range for forming proper peel zones in the SiC ingot 86 and enabling a wafer to be properly peeled from the SiC ingot 86 is determined based on the results of experiments conducted in advance. If the intensity of the reflected beam is less than the lower limit of the above-determined range, there is a possibility that cracks in the peel zones have not grown sufficiently and a wafer cannot be properly peeled from the SiC ingot 86. If the intensity of the reflected beam is less than a lower limit of the above-determined range, the control unit 16 accordingly controls the attenuator 30 to increase the output power of the pulsed machining laser beam LB1 to keep the intensity of the reflected beam within the predetermined range.

[0028] On the other hand, if the intensity of the reflected beam exceeds the upper limit of the above-determined range, cracks of the peeling zones are excessively grown even though a wafer can be properly peeled from the SiC ingot 86. After the wafer is peeled from the SiC ingot 86, and its surfaces that have been separated from each other are planarized by grinding, an amount of SiC ground from the SiC ingot 86 and the wafer is unnecessarily large, which tends to result in an increase in the scrap volume of the SiC ingot. Therefore, if the intensity of the reflected beam exceeds the upper limit of the above-determined range, the control unit 16 controls the attenuator 30 to reduce the output power of the pulsed machining laser beam LB1 to keep the intensity of the reflected beam within the predetermined range.

[0029] As in Fig. 1, the laser processing apparatus 2 according to the present embodiment further includes a peeling unit 50 for peeling a wafer from the SiC ingot 86 with the peeling zones serving as a separation starting point, and a grinding unit 52 for grinding the end face of the SiC ingot 86 into a flat surface.

[0030] The extraction unit 50 includes a housing 54 disposed at terminal ends of the guide rails 18a on the base 18, an arm 56 having a proximal end supported vertically movably on the housing 54 and extending from the proximal end in one of the X-axis directions, and an unillustrated arm raising and lowering means for raising and lowering the arm 56. The arm raising and lowering means may include a ball screw coupled to the arm 56 and extending vertically, and an electric motor for rotating the ball screw about its central axis. An electric motor 58 is mounted at a distal end of the arm 56, and a suction pad 60 is rotatably coupled to a lower surface of the electric motor 58 and connected to the shaft of the electric motor 58 for rotation about its vertical central axis.The suction pad 60 has a plurality of unillustrated suction holes defined in a lower surface thereof and is connected to an unillustrated suction means. An unillustrated ultrasonic vibration applying means for applying ultrasonic vibrations to the lower surface of the suction pad 60 is provided in the suction pad 60.

[0031] The grinding unit 52 includes a mounting wall 62 connected to the housing 26, a vertically movable platen 64 mounted vertically movably on a vertical surface of the mounting wall 62, and a raising and lowering means 66 for raising and lowering the vertically movable platen 64. The raising and lowering means 66 includes a ball screw 68 extending vertically along the vertical surface of the mounting wall 62 and an electric motor 70 for rotating the ball screw 68 about its central axis. The ball screw 68 is threadably engaged with a nut (not shown) coupled to the vertically movable platen 64.When the electric motor 70 is energized, the ball screw 68 rotates about its central axis, and the nut converts the rotation of the ball screw 68 into a linear motion, which is transmitted to the vertically movable plate 64, which is moved in the vertical direction along guide rails 62a attached to the vertical surface of the mounting wall 62.

[0032] A support wall 72, which protrudes in one of the Y-axis directions, is fixed to a vertical surface of the vertically movable plate 64. A spindle 74 is rotatably supported on the support wall 72 for rotation about its vertical central axis, and a spindle motor 76 for rotating the spindle 74 about its central axis is mounted on an upper surface of the support wall 72. As shown in the Fig. 1 and Fig. 4, a disc-shaped wheel holder is fixed to a lower end of the spindle 74, and an annular grinding wheel 82 is fixed to a lower surface of the wheel holders 78 by bolts 80. A plurality of grinding stones 84 arranged in an annular arrangement spaced circumferentially are fixed to an outer peripheral edge portion of a lower surface of the grinding wheel 82.

[0033] The Fig. 3A and Fig. 3B illustrate the SiC ingot 86 made of SiC and having a cylindrical shape. The SiC ingot 86 has a first end face 88 having a circular shape, a second end face 90 having a circular shape opposite the first end face 88, a circumferential surface 92 extending between the first end face 88 and the second end face 90, a c-axis ( <0001> -direction) extending from the first end side 88 to the second end side 90, and a c-plane ({0001}-side) extending perpendicular to the c-axis.

[0034] In the SiC ingot 86, the c-plane is inclined to the first end face 88, i.e., the c-axis is inclined to a line 94 perpendicular to the first end face 88, wherein the first end face 88 and the c-plane form a deviation angle α between them (for example, α is 1°, 3°, or 6°). The deviation angle α is in the direction indicated by an arrow A in the Fig. 3A and Fig. 3B. The peripheral surface 92 of the SiC ingot 86 includes a first orientation plane 96 and a second orientation plane 98, each having a rectangular shape and representing a crystal orientation. The first orientation plane 96 extends parallel to the direction A in which the deviation angle α is formed, and the second orientation plane 98 extends perpendicular to the direction A. As shown in Fig. 3B, a length L2 of the second alignment plane 98 in plan view is smaller than a length L1 of the first alignment plane 96 (L2 <L1).

[0035] The machining method for processing a SiC ingot according to the present embodiment will be described below. The machining method for processing a SiC ingot described below is carried out using the laser machining apparatus 2 described above. In the machining method for processing a SiC ingot according to the present embodiment, the SiC ingot 86 is fixed with the second end side 90 facing downward to an upper surface of the holding table 24 by a suitable adhesive such as an epoxy resin-based adhesive. Alternatively, the upper surface of the holding table 24 may have a plurality of suction holes defined therein, and the SiC ingot 86 may be held on the holding table 24 by suction forces acting via the suction holes on the upper surface of the holding table 24.

[0036] After the SiC ingot 86 is fixed to the holding table 24, a flat surface forming step is performed on the SiC ingot 86 by grinding the first end face 88 into a flat surface, unless the first end face 88 has already been made flat.

[0037] In the flat surface forming step, the holding table 24 is initially positioned under the grinding wheel 82 of the grinding unit 52 by the X-axis feed mechanism 10. Then, the holding table motor is energized to rotate the holding table 24 as shown in Fig. 4 illustrates, at a predetermined rotation speed of, for example, 300 rpm, as viewed counterclockwise from above. The spindle motor 76 is also energized to rotate the spindle 74 at a predetermined rotation speed of, for example, 6000 rpm, as viewed counterclockwise from above. Then, the raising and lowering means 66 lowers the spindle 74 to bring the grindstones 84 into contact with the first end face 88 of the SiC ingot 86. Thereafter, the spindle 74 is lowered at a predetermined grinding feed rate of, for example, 0.1 µm per second. The first end face 88 of the SiC ingot 86 is now ground into a flat surface smooth enough not to hinder the transmission of the pulsed machining laser beam LB1 through the SiC ingot 86.

[0038] After the SiC ingot 86 is held on the upper surface of the holding table 24 and the first end face 88 of the SiC ingot 86 is ground, a peel zone forming step is performed on the SiC ingot 86 in which the c-plane is inclined to the first end face 88 of the SiC ingot 86, and the directions perpendicular to the direction A in which the deviation angle α between the first end face 88 of the SiC ingot 86 and the c-plane is formed are represented as the X-axis directions, and the directions perpendicular to the X-axis directions are represented as the Y-axis directions.The peel zone forming step is performed by applying the pulsed machining laser beam LB1 whose wavelength is transmissible through the SiC ingot 86 to the SiC ingot 86 while positioning a focal point of the pulsed machining laser beam LB1 at a depth corresponding to a thickness of a wafer to be peeled and machining the SiC ingot 86 and the focal point relative to each other in one of the X-axis directions to form band-shaped peel zones each having cracks extending from the region where SiC is separated into Si and C along the c-plane.

[0039] In the peel zone forming step, the image pickup unit 34 initially captures an image of the SiC ingot 86 from above the SiC ingot 86. Then, based on the image of the SiC ingot 86 captured by the image pickup unit 34, the X-axis feed mechanism 10, the Y-axis feed mechanism 12, and the holding table motor are controlled to move and rotate the holding table 24 to set the direction of the SiC ingot 86 to a predetermined direction and also to adjust the positions of the SiC ingot 86 and the beam condenser 6 in the XY plane. To set the direction of the SiC ingot 86 to the predetermined direction, the second alignment plane 98, as shown in Fig. 5A, aligned with the X-axis directions to align the directions perpendicular to the direction A in which the deviation angle α is formed with the X-axis directions, and to align the direction A in which the deviation angle α is formed with the Y-axis directions.

[0040] Then, the focal point position adjusting means raises and lowers the beam condenser 6 to set a focal point FP1 (see Fig. 5B) of the pulsed processing laser beam LB1 from the first end face 88 of the SiC ingot 86 to a depth corresponding to the thickness of a wafer to be peeled from the SiC ingot 86. Then, the X-axis feed mechanism 10 feeds the holding table 24 at a predetermined processing feed speed in one of the X-axis directions for processing aligned with the directions perpendicular to the direction A in which the deviation angle α is formed, while simultaneously the beam condenser 6 applies the pulsed processing laser beam LB1 whose wavelength is transmissible through the SiC ingot 86 to the SiC ingot 86. As shown in Fig. 5B, the pulsed machining laser beam LB1 thus separates SiC into Si and C, and the next applied pulsed machining laser beam LB1 is absorbed by the previously formed C, which separates SiC into Si and C in a chain reaction manner. Pull-off zones 104, each having cracks 102 extending along the c-plane from a region 100 where SiC is separated into Si and C, are now formed in the SiC ingot 86 along the c-plane.

[0041] The peel zone forming step is performed, for example, under the conditions described below. Defocusing referred to in the conditions represents a distance by which the beam condenser 6 is moved toward an upper surface, i.e., the first end face 88, of the SiC ingot 86 after the focal point FP1 of the pulsed machining laser beam LB1 is positioned at the first end face 88 of the SiC ingot 86. Wavelength of the pulsed processing laser beam: 1064 nm Average output power: 7 to 16 watts Repetition frequency: 30 kHz Pulse duration: 3 ns Machining feed speed: 165 mm / s Defocus: 188 µm Position of the pull-off zones from the top surface of the SiC ingot: 500 µm

[0042] While the peel zone forming step is being performed, a reflected beam detecting step is performed to detect the intensity of a beam reflected by the cracks 102 by applying an inspection laser beam having a wavelength that is transmissible through the SiC ingot 86 and is reflectable by the cracks 102 of the peel zones 104.

[0043] In the detection step for a reflected beam, as in Fig. 6 illustrates, a focal point FP2 of the pulsed inspection laser beam, indicated by LB2 and emitted from the light emitter 46, is positioned at the cracks 102 of a peel zone 104 formed by the pulsed processing laser beam LB1. To adjust the position of the light emitter 46, the incident angle θ of the pulsed inspection laser beam LB2 is preferably set to the first end face 88 of the SiC ingot 86 at the Brewster angle. Since the size of a portion of the pulsed inspection laser beam LB2 applied to the SiC ingot 86 and which is not reflected by the first end face 88 but is introduced into the SiC ingot 86 is thus increased, the degree of accuracy with which the beam reflected by the cracks 102 of the peel zone 104 in the SiC ingot 86 is detected is increased.Furthermore, the light detecting surface of the light detector 48 is positioned in the path of the pulsed inspection laser beam LB2 reflected from the cracks 102 of the peel zone 104.

[0044] Then, while the holding table 24 is moved in the X-axis direction for machining, the pulsed machining laser beam LB1 is applied to the SiC ingot 86 to form peeling zones 104 in the SiC ingot 86, and the pulsed inspection laser beam LB2 is applied to the thus-formed cracks 102 of the peeling zones 104. The pulsed inspection laser beam LB2 reflected by the cracks 102 of the peeling zones 104 is detected by the light detector 48, which sends a signal to the control unit 16 indicating the intensity of the reflected beam detected by the light detector 48.

[0045] For example, the detection step for a reflected light beam is carried out under the following conditions. Wavelength of the pulsed inspection laser beam: 1064 nm Average output power: 0.1 watts Repetition frequency: 10 kHz Pulse duration: 10 ns Feed speed: 165 mm / s

[0046] While the peeling zone formation step and the reflected light detection step are being performed, a machining laser beam output adjustment step is further performed to adjust the output power of the pulsed machining laser beam LB1 so that the intensity of the reflected laser beam detected in the reflected light detection step is maintained within a predetermined range. Specifically, while the peeling zones 104 are being formed in the SiC ingot 86 by the pulsed machining laser beam LB1 applied thereto, the inspection pulsed laser beam LB2 is applied to the cracks 102 of the formed peeling zones 104, and the reflected inspection pulsed laser beam LB2 is detected to adjust the output power of the pulsed machining laser beam LB1.

[0047] In the machining laser beam output power adjustment step, the control unit 16 controls the attenuator 30 to adjust the output power of the pulsed machining laser beam LB1 so that the intensity of the reflected beam detected by the light detector 48 remains within a predetermined range. If the intensity of the reflected beam detected by the light detector 48 is less than the lower limit of the predetermined range, the control unit 16 then controls the attenuator 30 to increase the output power of the pulsed machining laser beam LB1 by approximately 1 to 6 watts. On the other hand, if the intensity of the reflected beam detected by the light detector 48 exceeds the upper limit of the predetermined range, the control unit 16 then controls the attenuator 30 to decrease the output power of the pulsed machining laser beam LB1 by approximately 1 to 6 watts.

[0048] Next, a feeding step is performed for feeding the SiC ingot 86 and the focal point FP1 relative to each other in the Y-axis direction to arrange the peeling zones 104 in the Y-axis direction. In the feeding step, the Y-axis feeding mechanism 12 moves the holding table 24 to feed the SiC ingot 86 relative to the focal point FP1 by a predetermined feeding distance Li in the Y-axis direction, which is aligned with the direction A in which the deviation angle α is formed.

[0049] The peel zone forming step and the feeding step are alternately repeated to form the peel zones 104 extending in the X-axis direction as shown in Fig. 5B illustrates aligning the wafers in the Y-axis direction at intervals of the predetermined infeed pitch Li. By maintaining the infeed pitch Li within a range not exceeding the width of the cracks 102 and vertically overlapping the cracks 102 of adjacent ones of the peeling zones 104 in the Y-axis direction, a wafer can be easily peeled from the SiC ingot 86 during a peeling step described below. After the output of the pulsed processing laser beam LB1 is adjusted to keep the reflected beam intensity within the predetermined range, the reflected light detection step and the processing laser beam output adjustment step do not need to be performed at the same depth in the SiC ingot 86.

[0050] After a plurality of peeling zones 104 are formed in the SiC ingot 86 to the depth corresponding to the thickness of a wafer to be peeled from the SiC ingot 86, a peeling step for peeling a wafer from the SiC ingot 86 is performed with the peeling zones 104 acting as a separation starting point.

[0051] In the peeling step, the X-axis feed mechanism 10 positions the holding table 24 under the suction pad 60 of the peeling unit 50. Then, the arm raising and lowering means lowers the arm 56 to contact the lower surface of the suction pad 60, as shown in Fig.7 illustrates bringing the first end face 88 of the SiC ingot 86 into close contact. Next, the suction means is actuated to attract the first end face 88 of the SiC ingot 86 to the lower surface of the suction pad 60 under suction. Then, the ultrasonic vibration applying means is actuated to apply ultrasonic vibrations to the lower surface of the suction pad 60, and the electric motor 58 is energized to rotate the suction pad 60. A wafer 104 is now peeled from the SiC ingot 86, with the peel zones 104 serving as the separation starting point.

[0052] After the peeling step is performed, the flat surface forming step, the peeling zone forming step, the reflected light detecting step, the processing laser beam output power adjusting step, the feeding step, and the peeling step described above are repeated to sequentially peel a plurality of wafers 106 from the SiC ingot 86.

[0053] According to the present embodiment, as described above, since the intensity of the beam reflected from the cracks 102 of the peeling zones 104 to which the inspection pulsed laser beam LB2 is applied is detected and the output power of the machining pulsed laser beam LB1 is adjusted to keep the intensity of the detected reflected light within the predetermined range, proper peeling zones 104 can be formed in the SiC ingot 86 at arbitrary heights in the SiC ingot 86.

[0054] According to the present embodiment, the peeling zone forming step, the reflected light detecting step, and the machining laser beam output adjusting step are illustrated as being performed simultaneously. However, these steps may be performed separately. For example, the peeling zone forming step, followed by the reflected light detecting step, and then the machining laser beam output adjusting step may be performed sequentially.

Claims

[1] A method of processing a SiC ingot (86) in which a c-plane is inclined to an end face (88) of the SiC ingot (86) and a direction perpendicular to a direction in which a deviation angle (α) is formed between the end face (88) of the SiC ingot (86) and the c-plane represents an X-axis direction and a direction perpendicular to the X-axis direction represents a Y-axis direction, the method comprising: a peel zone forming step comprising applying a pulsed processing laser beam (LB1) whose wavelength is transmittable through the SiC ingot (86) to the SiC ingot (86), while positioning a focal point (FP1) of the pulsed processing laser beam (LB1) at a depth corresponding to a thickness of a wafer (106) to be peeled from the SiC ingot (86), and processing the SiC ingot (86) and the focal point (FP1) relative to each other in the X-axis direction to form band-shaped peel zones (104) in the SiC ingot (86), each having cracks (102) extending along the c-plane from a region (100) where SiC is separated into Si and C; a feeding step comprising feeding the SiC ingot (86) and the focal point (FP1) relative to each other in the Y-axis direction to line up the peeling zones (104) in the Y-axis direction; a reflected beam detection step comprising applying a verification laser beam (LB2) having a wavelength that is transmissible through the SiC ingot (86) and reflectable by the cracks (102) of the peeling zones (104), and detecting an intensity of a beam reflected by the cracks (102); and a machining laser beam output power adjusting step of adjusting an output power of the pulsed machining laser beam (LB1) to keep the intensity of the reflected beam detected in the reflected beam detecting step within a predetermined range. [2] A method of processing a SiC ingot (86) according to claim 1, further comprising: before the peel zone forming step, a flat surface forming step comprising grinding the end face (88) of the SiC ingot (86) to a flat surface. [3] A laser processing apparatus for forming peel zones (104) in a SiC ingot (86) in which a c-plane is inclined to an end face (88) of the SiC ingot (86) and a direction perpendicular to a direction in which a deviation angle (α) is formed between the end face (88) of the SiC ingot (86) and the c-plane represents X-axis directions and a direction perpendicular to the X-axis direction represents Y-axis directions, the laser processing apparatus comprising: a holding table (24) for holding the SiC ingot (86); a laser beam application unit (8) having a laser oscillator (28) for generating a pulsed processing laser beam (LB1) and a beam condenser (6) for applying the pulsed processing laser beam (LB1), the wavelength of which can be transmitted to the SiC ingot (86) by the SiC ingot (86) held on the holding table (24), while positioning a focal point (FP1) of the pulsed processing laser beam (LB1) at a depth corresponding to a thickness of a wafer (106) to be peeled from the SiC ingot (86), in order to form band-shaped peeling zones (104) in the SiC ingot (86), each having cracks (102) extending along the c-plane from a region (100) where SiC is separated into Si and C; an X-axis feed mechanism (10) for machining feed the holding table (24) and the beam condenser (6) relative to each other in the X-axis direction; a Y-axis feed mechanism (12) for feeding the holding table (24) and the beam condenser (6) relative to each other in the Y-axis direction; a reflected laser beam detection unit (48) comprising a light emitter (46) and a light detector (48) for applying a checking laser beam (LB2) having a wavelength that is transmissible through the SiC ingot (86) and reflectable by the cracks (102) of the peeling zones (104), and for detecting an intensity of a beam reflected by the cracks (104); and a control unit (16) for adjusting an output power of the pulsed machining laser beam (LB1) to keep the intensity of the reflected beam detected by the reflected beam detection unit (48) within a predetermined range.

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