WAFER MANUFACTURING METHOD AND LASER PROCESSING APPARATUS

Abstract

A wafer manufacturing method for manufacturing a SiC wafer (96) from a SiC ingot (72) having a top surface (74) and a bottom surface (76) opposite the top surface, the wafer manufacturing method comprising: a flat surface forming step comprising grinding the upper surface (74) of the SiC ingot (72) to thereby form a flat surface; after performing the flat surface forming step, a coordinate setting step of detecting a facet region (86) of the upper surface (74) of the SiC ingot (72) and further setting the X and Y coordinates of a plurality of points located on a boundary between the facet region (86) and a non-facet region (88) in a state in which an X axis extends in a direction perpendicular to a forming direction of a deviation angle (α) defined as an inclination angle of a c-plane with respect to the upper surface (74) of the SiC ingot (72), and a Y axis extends in a direction perpendicular to the X axis; after carrying out the coordinate setting step, a feeding step comprising setting a focal point (FP) of a laser beam having a transmission wavelength for SiC to a predetermined depth from the upper surface (74) of the SiC ingot (72) inside the SiC ingot (72), wherein the predetermined depth corresponds to a thickness of the SiC wafer (96) to be produced, next applying the laser beam from a focusing means (10) belonging to a laser processing device (2) to the SiC ingot (72) and relatively moving the SiC ingot (72) and the focal point (FP) in an X-direction parallel to the X-axis, thereby forming a band-shaped separation layer extending inside the SiC ingot (72) in the X-direction, wherein the separation layer consists of a modified portion in which SiC is decomposed into Si and C, and of a plurality of cracks,extending from the modified section along the c-plane; , after performing a feeding step, a dividing step comprising relatively moving the SiC ingot (72) and the focal point (FP) in a Y direction parallel to the Y axis, thereby forming a plurality of band-shaped separation layers arranged side by side in the Y direction; and after carrying out the feeding step and the dividing step, a separating step comprising separating the SiC wafer (96) from the SiC ingot (72) along a planar separating layer constructed with the plurality of band-shaped separating layers, wherein in the supplying step, during application of the laser beam to the facet region (86) in accordance with the X and Y coordinates set in the coordinate setting step, an energy of the laser beam is increased compared to an energy of the laser beam and a position of the focusing means (10) when applying the laser beam to the non-facet region (88), and a position of the focusing means (10) is raised.

Description

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

[0001] The present invention relates to a wafer manufacturing method for producing a silicon carbide wafer (SiC wafer) from a SiC ingot and also relates to a laser processing apparatus for forming a separation layer inside a SiC ingot. DESCRIPTION OF THE RELATED PRIOR ART

[0002] A variety of components, such as integrated circuits (ICs), large-scale integrated circuits (LSIs) and light-emitting diodes (LEDs), are manufactured by forming a functional layer on the front side of a wafer made of silicon (Si) or sapphire (Al 2 O 3) and dividing this functional layer into a plurality of separate regions along a plurality of division lines. Furthermore, power devices or optical devices such as LEDs are formed by forming a functional layer on the front side of a wafer formed of hexagonal SiC single crystal and dividing this functional layer into a plurality of separate regions along a plurality of division lines. The division lines of such a wafer having these devices are processed by a processing device such as a dicing device and a laser processing device, thereby separating the wafer into a plurality of individual device chips corresponding to the respective devices. The device chips thus obtained are used in a variety of electrical equipment such as mobile phones and personal computers.

[0003] Generally, a wafer on which components are to be formed is manufactured by cutting a cylindrical ingot with a wire saw. Both sides of the wafer cut from the ingot are polished to a mirror finish (see, for example, Japanese Unexamined Patent Application Laid-Open No. 2000-94221A). However, when the ingot is cut by the wire saw and both sides of each wafer are polished to obtain the product, a large portion of the ingot (70% to 80%) becomes scrap, which is problematic from an economic point of view. In particular, a SiC ingot has high hardness, and it is therefore difficult to cut this ingot with a wire saw. Accordingly, a considerable amount of time is required to cut the ingot, resulting in a reduction in productivity. Since this ingot has a high unit price, this prior art also has the problem of efficiently producing a wafer.

[0004] To solve this problem, a technique has been proposed (see, for example, Japanese Unexamined Patent Application Laid-Open No. 2016-111143 A). This technique includes the steps of setting a focal point of a laser beam having a transmission wavelength for hexagonal SiC single crystal inside a SiC ingot, then applying the laser beam to the SiC ingot while scanning the laser beam along the SiC ingot, thereby forming separation layers inside the SiC ingot on a preset separation plane, and then breaking the SiC ingot along the separation plane where the separation layers have been formed, thereby separating a SiC wafer from the SiC ingot.

[0005] Laser processing methods and devices intended for creating weak points within a semiconductor crystal, but without taking into account a facet region or non-facet region, can be found, for example, in the published patent applications DE 10 2016 205 917 A1 and EP 2 221 138 A1. SUMMARY OF THE INVENTION

[0006] However, there is a case where a facet region different in crystal structure from a non-facet region exists in the SiC ingot. The refractive index in the facet region is larger than that in the non-facet region, and the energy absorption capacity in the facet region is also larger than that in the non-facet region. Accordingly, the depth and state of the separation layer formed in the facet region may become different from those of the separation layer formed in the non-facet region when the laser beam is applied to the SiC ingot. As a result, there is a problem that a step may be formed between the separation layer in the facet region and the separation layer in the non-facet region.In order to grind the wafer produced from the SiC ingot to obtain the desired wafer thickness, the thickness of the wafer to be produced must be increased, taking into account the step between the facet region and the non-facet region. Accordingly, the manufacturing efficiency is insufficient.

[0007] It is therefore an object of the present invention to provide a wafer manufacturing method that can manufacture a SiC wafer from a SiC ingot in the state where there is no step between the facet region and the non-facet region at the separation layer.

[0008] It is a further object of the present invention to provide a laser processing apparatus for use in carrying out this wafer manufacturing method.

[0009] In accordance with one aspect of the present invention, there is provided a wafer manufacturing method for manufacturing a SiC wafer from a SiC ingot having an upper surface and a lower surface opposite to the upper surface, the wafer manufacturing method comprising: a flat surface forming step of grinding the upper surface of the SiC ingot to thereby form a flat surface;after performing the flat surface forming step, a coordinate setting step of detecting a facet region from the top surface of the SiC ingot and further setting the X and Y coordinates of a plurality of points located on a boundary between the facet region and a non-facet region in a state where an X axis extends in a direction perpendicular to a forming direction of a deviation angle defined as an inclination angle of a c-plane with respect to the top surface of the SiC ingot and a Y axis extends in a direction perpendicular to the X axis;After performing the coordinate adjustment step, a supply step of setting a focal point of a laser beam having a transmission wavelength for SiC to a predetermined depth inside the SiC ingot from the top surface of the SiC ingot, the predetermined depth corresponding to a thickness of the SiC wafer to be manufactured, next, applying the laser beam to the SiC ingot from a focusing means belonging to a laser processing apparatus, and relatively moving the SiC ingot and the focal point in an X-direction parallel to the X-axis, thereby forming a band-shaped separation layer extending inside the SiC ingot in the X-direction, the separation layer being composed of a modified portion where SiC is decomposed into Si and carbon (C), and a plurality of cracks extending from the modified portion along the c-plane;after performing the feeding step, a dividing step of relatively moving the SiC ingot and the focal point in a Y direction parallel to the Y axis, thereby forming a plurality of band-shaped separation layers arranged side by side in the Y direction; and after performing the feeding step and the dividing step, a separating step of separating the SiC wafer from the SiC ingot along a planar separation layer constructed with the plurality of band-shaped separation layers; wherein, during the feeding step, an energy of the laser beam is increased and a position of the focusing means when applying the laser beam to the facet region is raised compared to an energy of the laser beam and a position of the focusing means when applying the laser beam to the facet region in accordance with the X and Y coordinates set in the coordinate setting step.

[0010] In accordance with another aspect of the present invention, there is provided a laser processing apparatus for forming a separation layer inside a SiC ingot according to the above wafer manufacturing method, which has an upper surface and a lower surface opposite to the upper surface, the laser processing apparatus comprising a holding table for holding the SiC ingot in a state where the upper surface of the SiC ingot is directed upward; facet region detecting means for detecting a facet region from the upper surface of the SiC ingot held on the holding table;a coordinate setting means that sets and records X and Y coordinates of a plurality of points located on a boundary between the facet region and a non-facet region in a state where an X axis extends in a direction perpendicular to a formation direction of a deviation angle defined as an inclination angle of a c-plane with respect to the top surface of the SiC ingot, and a Y axis extends in a direction perpendicular to the X axis;a laser beam application unit having a focusing means that applies a laser beam to the SiC ingot in a state where a focal point of the laser beam is set from the upper surface of the SiC ingot to a predetermined depth inside the SiC ingot, the predetermined depth corresponding to a thickness of a SiC wafer to be manufactured from the SiC ingot, the laser beam having a transmission wavelength for SiC, thereby forming a separation layer inside the SiC ingot, the separation layer being constructed with a modified portion where SiC is decomposed into Si and carbon (C), and a plurality of cracks extending from the modified portion along the c-plane; an X-movement mechanism for relatively moving the holding table and the focusing means in an X-direction parallel to the X-axis;a Y-movement mechanism for relatively moving the holding table and the focusing means in a Y-direction parallel to the Y-axis; and a control unit that, compared with an energy of the laser beam and a position of the focusing means when applying the laser beam to the non-facet region, increases an energy of the laser beam and raises a position of the focusing means when applying the laser beam to the facet region in accordance with the X and Y coordinates set by the coordinate setting means.

[0011] According to the wafer manufacturing method of the present invention, the depth and state of the planar separation layer formed in the facet region can be made similar to those of the planar separation layer formed in the non-facet region. Accordingly, the SiC wafer can be manufactured in the state where there is no step in the planar separation layer between the facet region and the non-facet region.

[0012] According to the laser processing apparatus of the present invention, the depth and state of the planar separation layer to be formed in the facet region can be made similar to those of the planar separation layer formed in the non-facet region when the laser beam is applied to the SiC ingot. Accordingly, the SiC wafer can be manufactured in the state where there is no step in the planar separation layer between the facet region and the non-facet region.

[0013] The above and other objects, features and advantages of the present invention and the manner of carrying them into effect will become more apparent and the invention itself will be best understood by studying the following description and 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 a preferred embodiment of the present invention; Fig. 2A is a side view of a SiC ingot; Fig. 2B is a top view of the Fig. SiC ingots shown in Figure 2A; Fig. 3 is a perspective view showing a flat surface forming step in a wafer manufacturing method using the Fig. 1; Fig. 4A is a schematic view illustrating an image of a SiC ingot as acquired during a coordinate adjustment step in the wafer manufacturing process; Fig. 4B is a table showing the X and Y coordinates of a plurality of points located on the boundary between a facet area and a non-facet area, set in the coordinate setting step; Fig. 5A is a perspective view illustrating a feeding step in the wafer manufacturing method; Fig. 5B is a sectional view illustrating the feeding step; Fig. 6 is a sectional view illustrating a dividing step in the wafer manufacturing method; and Fig. 7 is a perspective view illustrating a separation step in the wafer manufacturing method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] A preferred embodiment of the wafer manufacturing method and laser processing apparatus in accordance with the present invention will now be described with reference to the drawings. Fig. Figure 1 generally illustrates a laser processing apparatus 2 in accordance with this preferred embodiment. As shown in Fig. 1, the laser processing apparatus 2 includes a holding unit 4 for holding a SiC ingot 72, a facet region detecting means 6 for detecting a facet region from the upper surface of the SiC ingot 72, and a coordinate setting means 8 for setting and receiving the X and Y coordinates of a plurality of points located on the boundary between the facet region and a non-facet region.The laser processing apparatus 2 further includes a laser beam application unit 12 having a focusing means 10 for applying a laser beam to the SiC ingot 72 in the state where the focal point of the laser beam is set to a predetermined depth inside the SiC ingot 72 from the upper surface of the SiC ingot 72, the depth corresponding to the thickness of a wafer to be manufactured, the laser beam has a transmission wavelength for SiC, whereby a separation layer is formed inside the SiC ingot 72, and the separation layer is configured with a modified portion where SiC is decomposed into Si and C and a plurality of cracks extending from the modified portion along a c-plane. The laser processing apparatus 2 further includes an X-movement mechanism 14 for relatively moving the holding unit 4 and the focusing means 10 in the X-direction (feed direction) shown in FIG. Fig. 1 by an arrow X, a Y-movement mechanism 16 for relatively moving the holding unit 4 and the focusing means 10 in the Y-direction (division direction) shown in Fig. 1 is represented by an arrow Y, and a control unit 18 for controlling the operation of the laser processing device 2. The X-direction and the Y-direction are perpendicular to each other in an XY plane. The XY plane defined by the X-direction and the Y-direction is a substantially horizontal plane.

[0015] The laser processing apparatus 2 further includes a base 20, and the holding unit 4 includes an X-movable plate 22 mounted on the base 20 so as to be movable in the X direction, a Y-movable plate 24 mounted on the X-movable plate 22 so as to be movable in the Y direction, a holding table 26 rotatably mounted on the upper surface of the Y-movable plate 24, and an unillustrated motor for rotating the holding table 26.

[0016] The facet area detection means 6 has an imaging unit 28 for imaging the upper surface of the SiC ingot 72 held on the holding table 26. As shown in Fig. 1, an inverted L-shaped support member 30 is mounted on the upper surface of the base 20. The support member 30 is configured with a vertical portion 30a extending upward from the upper surface of the base 20 and a horizontal portion 30b extending horizontally from the upper end of the vertical portion 30a. The imaging unit 28 is mounted with its front end portion on the lower surface of the horizontal portion 30b. Further, a display unit 32 for displaying an image obtained by the imaging unit 28 is provided on the upper surface of the horizontal portion 30b. Preferably, the facet region detecting means 6 includes image processing means for performing image processing, such as binarization processing, on the image of the SiC ingot obtained by the imaging unit 28.Preferably, the imaging unit 28 of the facet area detecting means 6 also serves as an imaging unit for use in performing alignment before applying a laser beam to the SiC ingot 72.

[0017] The coordinate setting means 8 is electrically connected to the imaging unit 28. According to the image of the SiC ingot 72 obtained by the imaging unit 28, the coordinate setting means 8 operates to set and record the X and Y coordinates of a plurality of points located on the boundary between the facet region and the non-facet region in the state where an X axis extends in the direction perpendicular to a formation direction of a deviation angle defined as an inclination angle of the c-plane with respect to the upper surface of the SiC ingot 72, and a Y axis extends in the direction perpendicular to the X axis. The extension direction of the X axis is substantially the same as that shown in Fig. 1, and the extending direction of the Y-axis is essentially the same as that shown in Fig. 1 shown Y-direction.

[0018] The focusing means or condenser 10 of the laser beam application unit 12 is mounted at its front end portion on the lower surface of the horizontal portion 30b so as to be spaced apart in the X direction from the imaging unit 28. The laser beam application unit 12 further includes an unillustrated laser oscillator for generating a pulsed laser beam having a transmission wavelength for SiC, an unillustrated attenuator for adjusting the power of the pulsed laser beam generated by the laser oscillator, and unillustrated focal point position adjusting means for moving the focusing means 10 in the vertical direction to thereby adjust the vertical position of the focal point of the pulsed laser beam.The focal point position adjustment means may be configured by a ball screw connected to the focusing means 10 so as to extend in the vertical direction, and a motor for rotating this ball screw. During operation of the laser beam application unit 12, a pulsed laser beam is generated by the laser oscillator and then adjusted in power by the damper. The pulsed laser beam is then focused by the focusing means 10 and applied to the SiC ingot 72 held on the holding table 26 of the holding unit 4.

[0019] The X-movement mechanism 14 includes a ball screw 34 connected to the X-movable plate 22 so as to extend in the X-direction, and a motor 36 connected to one end of the ball screw 34 for rotating the ball screw 34. Accordingly, a rotational motion of the motor 36 is converted into a linear motion by the ball screw 34, and this linear motion is transmitted to the X-movable plate 22, so that the X-movable plate 22 can be moved in the X-direction relative to the focusing means 10 along a pair of parallel guide rails 20a provided on the base 20. That is, the X-movable plate 22 is slidably mounted on the guide rails 20a extending in the X-direction.

[0020] Similarly, the Y-movement mechanism 16 includes a ball screw 38 connected to the Y-movable plate 24 so as to extend in the Y direction, and a motor 40 connected to one end of the ball screw 38 for rotating the ball screw 38. Accordingly, a rotational motion of the motor 40 is converted into a linear motion by the ball screw 38, and this linear motion is transmitted to the Y-movable plate 24, so that the Y-movable plate 24 can be moved in the Y direction relative to the focusing means 10 along a pair of parallel guide rails 22a provided on the X-movable plate 22. That is, the Y-movable plate 24 is slidably mounted on the guide rails 22a extending in the Y direction.

[0021] The control unit 18 is electrically connected to the coordinate setting means 8. According to the X and Y coordinates of a plurality of points located on the boundary between the facet region and the non-facet region, set by the coordinate setting means 8, the control unit 18 operates to increase the energy of the laser beam compared to the energy of the laser beam and the position of the focusing means 10 when applying the laser beam to the non-facet region, and to raise the position of the focusing means 10 when applying the laser beam to the facet region. The control unit 18, the image processing means of the facet region detection means 6, and the coordinate setting means 8 can be implemented by separate computers or by a common computer.

[0022] The laser processing apparatus 2 further includes a grinding unit 42 for grinding the upper surface of the SiC ingot 72 held on the holding table 26 and a separating mechanism 44 for separating the wafer from the SiC ingot 72 held on the holding table 26 after forming the separation layer inside the SiC ingot 72.

[0023] The grinding unit 42 includes a housing 46 mounted on the side surface of the horizontal portion 30b of the support member 30 so as to be movable in the Y direction, a housing moving means 48 for moving the housing 46 in the Y direction, an arm 50 having a base end vertically movably supported on the housing 46 and extending from the base end in the Y direction, an unillustrated arm lifting means for vertically moving the arm 50, and a spindle housing 52 mounted on the front end of the arm 50.

[0024] A vertically extending spindle 54 is rotatably supported on the spindle housing 52, and a motor (not shown) for rotating the spindle 54 is incorporated in the spindle housing 52. Referring to Fig. 3, a disc-shaped wheel holder 56 is fixed to the lower end of the spindle 54, and an annular grinding wheel 60 is fixed to the lower surface of the wheel holder 56 by bolts 58. A plurality of grinding elements 62 are fixed to the lower surface of the grinding wheel 60 so as to be arranged at predetermined intervals along the outer circumference of the grinding wheel 60.

[0025] Again referring to Fig. 1, the separating mechanism 44 includes a housing 64 which, as in Fig. 1, near the left ends of the guide rails 20a, an arm 66 having a base end supported vertically movably on the housing 64 and extending from the base end in the X direction is provided on the base 20, and an unillustrated arm lifting means for vertically moving the arm 66. A motor 68 is connected to the front end of the arm 66, and a suction member 70 is connected to the lower surface of the motor 68 so as to be rotatable about a vertical axis thereof. A plurality of unillustrated suction holes are formed on the lower surface of the suction member 70. These suction holes of the suction member 70 are connected to an unillustrated suction means for providing negative pressure. Furthermore, the suction member 70 includes an unillustrated ultrasonic vibration applying means for applying ultrasonic vibration to the lower surface of the suction member 70.

[0026] The Fig. 2A and Fig. 2B illustrate the SiC ingot 72 formed from SiC. The SiC ingot 72 has a substantially cylindrical shape. That is, the SiC ingot 72 has a substantially circular first end surface 74, a substantially circular second end surface 76 opposite the first end surface 74, a substantially cylindrical surface 78 formed to connect the first end surface 74 and the second end surface 76, a c-axis ( <0001> -direction) extending from the first end face 74 to the second end face 76 and having a c-plane ({0001} plane) perpendicular to the c-axis.

[0027] In the SiC ingot 72, the c-axis is inclined with respect to a normal 80 to the first end surface 74 by a deviation angle α (for example, α equal to 1°, 3°, or 6°). The deviation angle α is formed between the c-plane and the first end surface 74. The formation direction of the deviation angle α (i.e., the inclination direction of the c-axis) is indicated in the Fig. 2A and Fig. 2B by an arrow A. Furthermore, the cylindrical surface 78 of the SiC ingot 72 is formed with a first orientation plane 82 and a second orientation plane 84, which are perpendicular when viewed from the side and serve to indicate a crystal orientation. The first orientation plane 82 is parallel to the formation direction A of the deviation angle α, and the second orientation plane 84 is perpendicular to the formation direction A of the deviation angle α. As shown in Fig. 2B, which is a plan view in the direction of extension of the perpendicular 80, the length L2 of the second alignment plane 84 is chosen to be shorter than the length L1 of the first alignment plane 82 (L2 <L1).

[0028] The SiC ingot 72 is mainly formed of a hexagonal SiC single crystal, and a facet region 86 which differs in its crystal structure is formed as shown in Fig. 2B, are present locally in the SiC ingot 72. A non-facet region different from the facet region 86 is designated by reference numeral 88. This means that the crystal structure in the facet region 86 differs from that in the non-facet region 88.

[0029] The preferred embodiment of the wafer manufacturing method according to the present invention will now be described. In this preferred embodiment, the above-mentioned laser processing apparatus 2 is used to carry out the wafer manufacturing method. First, the SiC ingot 72 is fixed to the upper surface of the holding table 26 by using a suitable adhesive (e.g., an epoxy resin adhesive) in the state where the first end surface 74 of the SiC ingot 72 is directed upward. That is, the adhesive is interposed between the second end surface 76 of the SiC ingot 72 and the upper surface of the holding table 26. Alternatively, a plurality of suction holes may be formed on the upper surface of the holding table 26, and suction force may be applied to the upper surface of the holding table 26 through these suction holes, thereby holding the SiC ingot 72 on the upper surface of the holding table 26.

[0030] After the above-mentioned fixing of the SiC ingot 72 to the upper surface of the holding table 26, a flat surface forming step is performed to flatten the upper surface of the SiC ingot 72 by grinding except for the case that the upper surface of the SiC ingot 72 has already been flattened.

[0031] When performing the flat surface forming step, the holding table 26 holding the SiC ingot 72 is moved to the position below the grinding unit 42. Thereafter, the holding table 26 is moved as shown in Fig. 3, the spindle 54 is rotated counterclockwise in plan view at a predetermined speed (e.g., 300 revolutions per minute). Furthermore, the spindle 54 is also rotated counterclockwise in plan view at a predetermined speed (e.g., 6000 revolutions per minute). Thereafter, the arm 50 is lowered by operating the arm lifting means, thereby bringing the grinding elements 62 into contact with the upper surface of the SiC ingot 72, that is, the first end surface 74 of the SiC ingot 72 in this preferred embodiment. Thereafter, the arm 50 is further lowered at a predetermined feed rate (e.g., 0.1 µm per second), thereby grinding the upper surface of the SiC ingot 72. Accordingly, the upper surface of the SiC ingot 72 is flattened to such an extent that the incidence of a laser beam on the upper surface of the SiC ingot 72 is not hindered during a later separation layer formation step.Consequently, the upper surface of the SiC ingot 72 is ground to become a flat surface.

[0032] After performing the flat surface forming step, a coordinate setting step is performed to detect the facet region 86 from the top surface of the SiC ingot 72 and set the X and Y coordinates of a plurality of points located on the boundary between the facet region 86 and the non-facet region 88 in the state where an X axis extends in the direction perpendicular to a forming direction of a deviation angle α defined as an inclination angle of the c-plane with respect to the top surface of the SiC ingot 72 and a Y axis extends in the direction perpendicular to the X axis.

[0033] When performing the coordinate adjustment step, the holding table 26 holding the SiC ingot 72 is first moved to the position below the imaging unit 28. Thereafter, the imaging unit 28 is operated to image the upper surface of the SiC ingot 72. According to an image of the SiC ingot 72 as captured by the imaging unit 28 or according to an image obtained by performing image processing, such as binarization processing, on the above image of the SiC ingot 72, the facet region 86 is detected. Thereafter, the coordinate adjustment means 8, as shown in FIGS. Fig. 4A and Fig. 4B, is operated to set and record the X and Y coordinates of a plurality of points (e.g., 24 points) from a point a to a point x located on the boundary between the facet region 86 and the non-facet region 88 in the state where the X axis extends in the direction perpendicular to the formation direction of the deviation angle α and the Y axis extends in the direction perpendicular to the X axis. In addition, the X and Y coordinates of a plurality of points located on the outer edge of the SiC ingot 72 are also set and recorded.Thereafter, the X and Y coordinates in the facet region 86 and the X and Y coordinates in the non-facet region 88 are set and recorded according to the X and Y coordinates of the plurality of points located on the boundary between the facet region 86 and the non-facet region 88 and according to the X and Y coordinates of the plurality of points located on the outer edge of the SiC ingot 72.

[0034] After performing the coordinate adjustment step, a feeding step of setting the focal point of a laser beam having a transmission wavelength for SiC to a predetermined depth from the upper surface of the SiC ingot 72, the predetermined depth corresponding to the thickness of a wafer to be manufactured, next applying the laser beam from the focusing means 10 of the laser processing apparatus 2 to the SiC ingot 72 and relatively moving the SiC ingot 72 and the focal point in the X direction is performed, to thereby form a band-shaped separation layer extending inside the SiC ingot 72 in the X direction, the separation layer being composed of a modified portion where SiC is decomposed into Si and C and a plurality of cracks extending from the modified portion along the c-plane.

[0035] When performing the feeding step, the holding table 26 holding the SiC ingot 72 is first moved in the X direction according to the image of the SiC ingot 72 obtained in the coordinate adjusting step by the imaging unit 28, to thereby adjust the positional relationship between the SiC ingot 72 and the focusing means 10 in the XY plane.

[0036] Thereafter, the focusing means 10 is moved in the vertical direction by operating the focal point position adjusting means, thereby setting a focal point FP (see Fig. 5B) inside the SiC ingot 72 in the non-facet region 88, at a predetermined depth from the upper surface of the SiC ingot 72, the predetermined depth corresponding to the thickness of a wafer to be produced. Next, a pulsed laser beam LB having a transmission wavelength for SiC is applied to the SiC ingot 72 by the focusing means 10 while moving the holding table 26 at a predetermined speed in the X direction, which coincides with the direction perpendicular to the formation direction A of the deviation angle α. Specifically, the pulsed laser beam LB is initially applied to the SiC ingot 72 to thereby decompose SiC into Si and C. Next, the pulsed laser beam LB is applied to the SiC ingot 72 and absorbed by the previously generated C.Consequently, SiC is decomposed into Si and C via a chain reaction with the movement of the holding table 26 in the X direction to thereby linearly form a modified portion 90 extending as shown in FIG. Fig. 6, extends in the X-direction. At the same time, a large number of cracks 92 are formed, so that they extend, as shown in Fig. 6, extend isotropically from the modified portion 90. As a result, a band-shaped separation layer 94 composed of the modified portion 90 and the cracks 92 is formed inside the SiC ingot 72 so that it extends, as shown in Fig. 6, extend in the X direction.

[0037] In the feeding step, the control unit 18 controls the laser beam application unit 12 according to the X and Y coordinates set during the coordinate setting step to increase the energy of the pulsed laser beam LB and raise the position of the focusing means 10 when applying the pulsed laser beam LB to the facet region 86, compared to the energy of the pulsed laser beam LB and the position of the focusing means 10 when applying the pulsed laser beam LB to the non-facet region 88. The refractive index in the facet region 86 is higher than that of the non-facet region 88. Accordingly, the depth of the focal point FP in the facet region 86 can be adjusted by the above-mentioned control of the laser beam application unit 12, as shown in Fig. 5B, can be made substantially the same depth as that of the focal point FP in the non-facet region 88. As a result, the depth of the separation layer 94 to be formed in the facet region 86 can be made substantially the same as the depth of the separation layer 94 to be formed in the non-facet region 88. Moreover, the energy absorption capacity in the facet region 86 is also higher than that of the non-facet region 88. Accordingly, the state of the separation layer 94 to be formed in the facet region 86 can be made similar to the state of the separation layer 94 to be formed in the non-facet region 88 by increasing the pulsed laser beam LB to be applied to the facet region 86 compared to the pulsed laser beam LB to be applied to the non-facet region 88.

[0038] This feeding step can be carried out under the following processing conditions, in which the word “defocusing” corresponds to the amount of movement of the focusing means 10 toward the lower surface 76 of the SiC ingot 72 from the state where the focal point FP of the pulsed laser beam LB is set on the upper surface 74 of the SiC ingot 72. (Non-facet area: refractive index = 2.65) Wavelength of the pulsed laser beam: 1064 nm Average power: 7 watts Repetition frequency: 30 kHz Pulse width: 3 ns Feed speed: 165 mm / s Defocus: 188 µm Depth of the separation layer from the top surface of the SiC ingot: 500 µm (Facet area: refractive index = 2.79)

[0039] Wavelength of the pulsed laser beam: 1064 nm Average power: 9.1 watts Repetition frequency: 30 kHz Pulse width: 3 ns Feed speed: 165 mm / s Defocus: 179 µm Depth of the separation layer from the top surface of the SiC ingot: 500 µm

[0040] After performing the feeding step, a dividing step is performed to move the SiC ingot 72 and the focal point FP relative to each other in the Y direction, thereby forming a plurality of band-shaped separation layers 94 arranged side by side in the Y direction. In this preferred embodiment, the SiC ingot 72 is moved relative to the Y direction by a predetermined dividing amount Li (see Fig. 5A and Fig. 6), and the above feeding step is repeated. As a result, another band-shaped separation layer 94 extending in the X direction is formed adjacent to the previous band-shaped separation layer 94 in the Y direction. By repeating the feeding step and the dividing step, a plurality of similar band-shaped separation layers 94 can be formed at the predetermined depth inside the SiC ingot 72 over the entire upper surface of the SiC ingot 72. The predetermined dividing amount Li is set to a value smaller than twice the length of each crack 92 so that the cracks 92 of any adjacent ones of the plurality of band-shaped separation layers 94 arranged in the Y direction can be overlapped with each other as viewed from above. Accordingly, the wafer can be easily separated from the SiC ingot 72 in the separating step to be performed later.

[0041] After performing the feeding step and the dividing step, which constitute a separation layer forming step, to thereby form the plurality of band-shaped separation layers 94 at the predetermined depth inside the SiC ingot 72, a separation step is performed to separate a wafer from the SiC ingot 72 along a planar separation layer formed with the plurality of band-shaped separation layers 94. In performing the separation step, the holding table 26 holding the SiC ingot 72 is moved to the position below the suction member 70 of the separation mechanism 44. Thereafter, the arm 66 is lowered by operating the arm lifting means, thereby contacting the lower surface of the suction member 70 with the first end surface 74 of the SiC ingot 72, as shown in Fig.7, into close contact. Thereafter, the suction means is actuated to attract the first end surface 74 of the SiC ingot 72 to the lower surface of the suction member 70 via negative pressure. Thereafter, the ultrasonic vibration applying means is actuated to apply ultrasonic vibration to the lower surface of the suction member 70. Simultaneously, the motor 68 is actuated to rotate the suction member 70. As a result, a SiC wafer 96 can be separated from the SiC ingot 72 along the planar separation layer constructed with the plurality of band-shaped separation layers 94. Thus, the SiC wafer 96 is a wafer to be manufactured from the SiC ingot 72.

[0042] After separating the SiC wafer 96 from the SiC ingot 72, the flat surface forming step may be performed on the separation surface (top surface) of the SiC ingot 72, that is, the remaining SiC ingot. Thereafter, the feeding step, the dividing step, and the separating step may be repeated in a similar manner, thereby producing a plurality of similar SiC wafers 96 from the SiC ingot 72. The facet region 86 is formed to extend from the top surface of the SiC ingot 72 to the bottom surface thereof and to have a shape similar to a Japanese Kintaro bar along the thickness of the SiC ingot 72, so that the same Kintaro side appears at any axial position where the bar is cut.Accordingly, the coordinate adjustment step only needs to be performed when manufacturing the first SiC wafer 96 from the SiC ingot 72, and it is not necessary to perform the coordinate adjustment step when manufacturing the second and subsequent SiC wafers 96 from the remaining SiC ingot 72.

[0043] According to the above preferred embodiment, the depth and state of the planar separation layer composed of the plurality of band-shaped separation layers 94 to be formed in the facet region 86 can be made the same as that of the separation layer to be formed in the non-facet region 88. Accordingly, the SiC wafer 96 can be manufactured in a state where there is no step in the planar separation layer between the facet region 86 and the non-facet region 88. That is, it is not necessary to separate the SiC wafer 96 with a step increased in thickness between the facet region 86 and the non-facet region 88, so that manufacturing efficiency can be improved.

[0044] The present invention is not limited to the details of the preferred embodiment described above. The scope of the invention is defined by the appended claims, and all changes and modifications that fall within the equivalent scope of the claims are therefore intended to be embraced by the invention.

Claims

[1] A wafer manufacturing method for manufacturing a SiC wafer (96) from a SiC ingot (72) having an upper surface (74) and a lower surface (76) opposite the upper surface, the wafer manufacturing method comprising: a flat surface forming step comprising grinding the upper surface (74) of the SiC ingot (72) to thereby form a flat surface; after performing the flat surface forming step, a coordinate setting step of detecting a facet region (86) of the upper surface (74) of the SiC ingot (72) and further setting the X and Y coordinates of a plurality of points located on a boundary between the facet region (86) and a non-facet region (88) in a state in which an X axis extends in a direction perpendicular to a forming direction of a deviation angle (α) defined as an inclination angle of a c-plane with respect to the upper surface (74) of the SiC ingot (72), and a Y axis extends in a direction perpendicular to the X axis; after carrying out the coordinate setting step, a feeding step comprising setting a focal point (FP) of a laser beam having a transmission wavelength for SiC to a predetermined depth from the upper surface (74) of the SiC ingot (72) inside the SiC ingot (72), wherein the predetermined depth corresponds to a thickness of the SiC wafer (96) to be produced, next applying the laser beam from a focusing means (10) belonging to a laser processing device (2) to the SiC ingot (72) and relatively moving the SiC ingot (72) and the focal point (FP) in an X-direction parallel to the X-axis, thereby forming a band-shaped separation layer extending inside the SiC ingot (72) in the X-direction, wherein the separation layer consists of a modified portion in which SiC is decomposed into Si and C, and of a plurality of cracks,extending from the modified section along the c-plane;, after performing a feeding step, a dividing step comprising relatively moving the SiC ingot (72) and the focal point (FP) in a Y direction parallel to the Y axis, thereby forming a plurality of band-shaped separation layers arranged side by side in the Y direction; and after carrying out the feeding step and the dividing step, a separating step comprising separating the SiC wafer (96) from the SiC ingot (72) along a planar separating layer constructed with the plurality of band-shaped separating layers, wherein in the supplying step, during application of the laser beam to the facet region (86) in accordance with the X and Y coordinates set in the coordinate setting step, an energy of the laser beam is increased compared to an energy of the laser beam and a position of the focusing means (10) when applying the laser beam to the non-facet region (88), and a position of the focusing means (10) is raised. [2] A laser processing apparatus (2) adapted to form a separation layer inside a SiC ingot (72) having an upper surface (74) and a lower surface (76) opposite to the upper surface, in accordance with the wafer manufacturing method of claim 1, the laser processing apparatus (2) comprising: a holding table (26) configured to hold the SiC ingot (72) in a state in which the upper surface (74) of the SiC ingot (72) is directed upward; a facet area detecting means (6) configured to detect a facet area (86) from the upper surface (74) of the SiC ingot (72) held on the holding table (26); a coordinate setting means (8) configured to set and record X and Y coordinates of a plurality of points located on a boundary between the facet region (86) and a non-facet region (88) in a state in which an X axis extends in a direction perpendicular to a formation direction of a deviation angle (α) defined as an inclination angle of a c-plane with respect to an upper surface of the SiC ingot (72), and a Y axis extends in a direction perpendicular to the X axis; a laser beam application unit (12) equipped with a focusing means (10) that applies a laser beam to the SiC ingot (72) in a state in which a focal point (FP) of the laser beam is set at a predetermined depth inside the SiC ingot from the upper surface (74) of the SiC ingot (72), the predetermined depth corresponding to a thickness of a SiC wafer (96) to be produced from the SiC ingot (72), the laser beam having a transmission wavelength for SiC, whereby a separation layer is formed inside the SiC ingot (72), and the separation layer is constructed with a modified portion in which SiC is decomposed into Si and C and a plurality of cracks extending from the modified portion along the c-plane; an X-movement mechanism (14) configured to move the holding table (26) and the focusing means (10) relatively in an X-direction parallel to the X-axis; a Y-movement mechanism (16) adapted for relative movement of the holding table (26) and the focusing means (10) in a Y-direction parallel to the Y-axis; and a control unit (18) configured to increase an energy of the laser beam when applying the laser beam to the facet region (86) in accordance with the X and Y coordinates set by the coordinate setting means (8), compared with an energy of the laser beam and a position of the focusing means (10) when applying the laser beam to the non-facet region (88), and to raise a position of the focusing means (10).

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