WAFER MANUFACTURING PROCESS

Abstract

A wafer manufacturing method for producing a wafer having a deviation angle α from a hexagonal single-crystal ingot (11) having a top surface (11a), a c-plane exposed to the top surface (11a), and a c-axis perpendicular to the c-plane, the wafer manufacturing method comprising: a supporting step comprising supporting the ingot (11) by a wedge member (13) having a wedge angle α on a support table (26) having a horizontal support surface, the wedge angle α being equal to the deviation angle α, whereby the upper surface of the ingot (11) is inclined by the deviation angle α with respect to a horizontal plane; a first modified layer forming step comprising setting a focal point of a laser beam (LB) having a transmission wavelength for the ingot (11) to a predetermined vertical position inside the ingot (11), and next applying the laser beam (LB) to the upper surface (11a) while relatively moving the focal point and the ingot (11) in a first direction perpendicular to a second direction where the deviation angle α is formed, whereby a first modified layer (19) and first cracks (21) extending from the first modified layer (19) along the c-plane are linearly formed inside the ingot (11); a first dividing step comprising a relative movement of the focal point in the second direction to move the focal point by a predetermined amount; and an initial wafer manufacturing step comprising separating an initial wafer (23) from the ingot (11) at a separation plane including the first modified layers (19) and the first cracks (21) formed by repeating the first modified layer (19) forming step and the first dividing step inside the ingot (11); wherein in the first dividing step, a dividing amount L as a distance between any adjacent ones of the plurality of first modified layers (19) is set less than or equal to a distance defined such that the front ends of the adjacent first cracks (21) extending from the adjacent first modified layers (19) along the c-plane overlap each other.

Description

BACKGROUND OF THE INVENTIONField of the InventionThe present invention relates to a wafer manufacturing method for slicing a hexagonal single crystal ingot to manufacture a wafer.Description of the Related ArtVarious devices such as ICs and LSIs are formed by forming a functional layer on the front side of a wafer formed of silicon or the like and dividing this functional layer into a plurality of regions along a plurality of crossing division lines. The division lines of the wafer are processed by a processing apparatus such as a cutting apparatus and a laser processing apparatus, to thereby divide the wafer into a plurality of individual device chips corresponding to the devices, respectively. The device chips thus obtained are widely used in various electronic devices such as mobile phones and personal computers. Further, power devices or optical devices such as LEDs and LDs are formed by forming a functional layer on the front side of a wafer formed of a hexagonal single crystal such as SiC and GaN and dividing this functional layer into a plurality of regions along a plurality of intersecting division lines.Generally, the wafer on which the devices are to be formed is manufactured by cutting an ingot with a wire saw. Both sides of the wafer obtained above are polished to mirror finish (see, for example, Japanese Patent Application Laid-Open No. 2000-94,221 A). This wire saw is configured in such a manner that a single wire, such as a wire rope having a diameter of approximately 100 μm to 300 μm, is wound around many grooves formed on usually two to four guide rollers to form a plurality of cutting portions spaced in parallel at a given pitch. The wire is operated to travel in one direction or in opposite directions, thereby cutting the ingot into a plurality of wafers.An ingot such as a hexagonal single crystal ingot includes numerous c-planes in which atoms grow in a planar manner and a c-axis extending in a direction perpendicular to each c-plane, the atoms growing along this direction so as to be stacked. Generally, a hexagonal single crystal ingot is manufactured such that the c-plane is exposed to the upper surface of the ingot and the c-axis extends vertically. A functional layer is formed on the upper surface of a wafer as a substrate to be manufactured from the ingot. In order to improve the affinity between the functional layer and the substrate, the upper surface of the wafer has an off angle of 3.5°, 4.0° or 8.0° relative to the c-plane according to the type of the functional layer. Up to this time, the hexagonal single crystal ingot is usually manufactured in the direction inclined by 3.5°, 4.0°, or 8.0° with respect to the c-axis, and this ingot is next formed into a cylindrical shape. Thereafter, the cylindrical ingot is cut to produce a hexagonal single crystal wafer.In addition, Laid-Open Patent Publication CN 1 01 461 039 A discloses a laser processing method for cutting a plate-shaped processing object along a predetermined line.SUMMARY OF THE INVENTIONHowever, when the ingot is cut by the wire saw and both sides of each wafer are polished to obtain the product, 70% to 80% of the ingot becomes scrap, causing low economics. In particular, for example, a hexagonal single crystal ingot of SiC or GaN has a high Mohs hardness, and it is therefore difficult to saw this ingot with a wire saw, causing a decrease in productivity.Further, when a hexagonal single crystal ingot having a c-plane produced by crystal growth on the upper surface thereof is processed to obtain a cylindrical hexagonal single crystal ingot whose upper surface is inclined by 3.5°, 4.0° or 8.0° with respect to the c-plane, and this cylindrical ingot is next cut to obtain a hexagonal single crystal wafer, the following problem arises. That is, in processing the original hexagonal single crystal ingot manufactured by crystal growth to obtain the cylindrical hexagonal single crystal ingot whose upper surface is inclined by a predetermined angle with respect to the c-plane, many undesired wastes from the expensive ingot are discarded, which is uneconomical.It is therefore an object of the present invention to provide a wafer manufacturing method that can manufacture a hexagonal single crystal wafer economically from a hexagonal single crystal ingot having a c-plane on its upper surface, whose upper surface has a predetermined off angle.According to an aspect of the present invention, there is provided a wafer manufacturing method for manufacturing a wafer having an off angle α from a hexagonal single crystal ingot having an upper surface, a c-plane exposed to the upper surface, and a c-axis perpendicular to the c-plane, the wafer manufacturing method comprising a supporting step of supporting the ingot by a wedge member having a wedge angle α on a supporting table having a horizontal supporting surface, the wedge angle α being equal to the off angle α, whereby the upper surface of the ingot is inclined by the off angle α with respect to a horizontal plane; a first modified layer forming step of setting a focal point of a laser beam having a transmission wavelength for the ingot to a predetermined vertical position inside the ingot, and next applying the laser beam to the upper surface while relatively moving the focal point and the ingot in a first direction perpendicular to a second direction where the off angle α is formed, so as to linearly form a first modified layer inside the ingot and first cracks extending from the first modified layer along the c-plane; a first dividing step of relatively moving the focal point in the second direction to further move the focal point by a predetermined amount; and an initial wafer manufacturing step of separating an initial wafer from the ingot at a separation plane including the first modified layers and the first cracks formed inside the ingot by repeating the first modified layer forming step and the first scheduling step; wherein in the first scheduling step, a scheduling amount L as a distance between any two adjacent ones of the plurality of first modified layers is set to be less than or equal to a distance defined such that or in which the leading ends of the adjacent first cracks extending from the adjacent first modified layers along the c-plane overlap each other.Preferably, the wafer manufacturing method further includes a separation start point forming step of setting the focus of the laser beam having a transmission wavelength for the ingot to a predetermined depth inside the ingot from a new upper surface of the ingot as a newly exposed surface obtained by separating the initial wafer in the initial wafer manufacturing step, the depth corresponding to a thickness of the wafer to be manufactured, and next applying the laser beam to the new upper surface upon relatively moving the focus and the ingot, to thereby form a second modified layer parallel to the new upper surface and second cracks extending from the second modified layer along the c-plane, thereby forming a separation start point; and after performing the separation start point forming step, a wafer includes a separation step of separating a plate-shaped member having a thickness corresponding to the thickness of the wafer to be manufactured from the ingot at the separation start point, thereby manufacturing the wafer from the ingot; wherein the separation start point forming step includes a second modified layer forming step of relatively moving the focus of the laser beam in the first direction perpendicular to the second direction, thereby linearly forming the second modified layer extending in the first direction; and a second dividing step of relatively moving the focus in the second direction, thereby further moving the focus by the predetermined amount.Preferably, after performing the initial wafer manufacturing step and before performing the separation start point forming step, the wafer manufacturing method further includes a flattening step of grinding the new upper surface of the ingot on which the first modified layers and the first cracks are exposed, thereby flattening the new upper surface of the ingot.According to another aspect of the present invention, there is provided a wafer manufacturing method for manufacturing a wafer having an off angle α from a hexagonal single crystal ingot having an upper surface, a c-plane exposed to the upper surface, and a c-axis perpendicular to the c-plane, the wafer manufacturing method including a first modified layer forming step of setting a focal point of a laser beam having a transmission wavelength for the ingot to a first depth from the upper surface inside the ingot, and next applying the laser beam to the upper surface while relatively moving the focal point and the ingot to linearly form a first modified layer inside the ingot and first cracks, extending from the first modified layer along the c-plane; a second modified layer forming step of relatively moving the focal point and the ingot in a direction perpendicular to the first modified layer to thereby advance the focal point by a predetermined amount after performing the first modified layer forming step, next setting the focal point to a second depth larger than the first depth, and next applying the laser beam to the upper surface upon relatively moving the focal point and the ingot in a direction parallel to the first modified layer to form a second modified layer parallel to the first modified layer inside the ingot and second cracks extending from the second modified layer along the c-plane; wherein an indexing amount L as a distance between the first modified layer and the second modified layer in the second modified layer forming step is set to be less than or equal to a distance defined when leading ends of the first cracks and the second cracks overlap each other; wherein a difference h between the first depth and the second depth in a c-axis direction is set to be h=L×tanα; a second modified layer forming repeating step of repeating the second modified layer forming step upon sequentially lowering the focal point of the laser beam to an intersection of a line defined by the inclination tanα and a line defined by the indexing amount L, whereby the second modified layers and the second cracks are formed in the entire separation plane in the ingot; and an initial wafer manufacturing step of separating an initial wafer from the ingot at the separation plane including the first and second modified layers and the first and second cracks formed inside the ingot.Preferably, the wafer manufacturing method further includes a separation start point forming step of setting the focus of the laser beam having a transmission wavelength for the ingot to a predetermined depth inside the ingot from a new upper surface of the ingot as a newly exposed surface obtained by separating the initial wafer in the initial wafer manufacturing step, the depth corresponding to a thickness of the wafer to be manufactured, and next applying the laser beam to the new upper surface upon relatively moving the focus and the ingot, to thereby form a third modified layer parallel to the new upper surface and third cracks extending from the third modified layer along the c-plane, thus forming a separation start point; and a wafer separating step of separating a plate-shaped member having a thickness corresponding to the thickness of the wafer to be manufactured from the ingot at the separation start point after performing the separation start point forming step, thereby manufacturing the wafer from the ingot; wherein the separation start point forming step includes a third modified layer forming step of relatively moving the focus of the laser beam in a first direction perpendicular to a second direction where the off angle α is formed, thereby linearly forming the third modified layer extending in the first direction; and a classifying step of relatively moving the focal point in the second direction to thereby move the focal point further by the predetermined amount, and next, setting the focal point at the intersection of the line defined by the slope tanα and the line defined by the classifying amount L.According to the wafer manufacturing method of the present invention, it is not necessary to process a hexagonal single crystal ingot obtained by crystal growth into a cylindrical shape so that the side surface of the processed ingot is inclined by 3.5°, 4.0°, or 8.0° with respect to the c-axis. Accordingly, no scrap is generated from the hexagonal single crystal ingot, thereby achieving good economicalness. Further, the laser beam is applied to the ingot to thereby form the modified layers inside the ingot and the cracks extending from each modified layer along the c-plane, the modified layers and the cracks forming the separation start point. Thereafter, the wafer is separated from the ingot at this separation start point, thereby producing the wafer from the ingot. Accordingly, it is not necessary to cut the ingot by using a wire saw in producing the wafer from the ingot, so that the problem of disposing 70% to 80% of the ingot can be prevented to thereby further improve the economical efficiency.The above and other objects, features and advantages of the present invention and the manner of realizing them will thereby become more apparent, and the invention itself will best be understood by studying the following description and appended claims with reference to the appended drawings which show some preferred embodiments of the invention.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a perspective view of a laser processing apparatus suitable for use in carrying out the wafer manufacturing method of the present invention; FIG. 2 is a block diagram of a laser beam generation unit; FIG. 3 is an exploded perspective view showing a supporting step according to a first preferred embodiment of the present invention; FIG. 4A is a perspective view showing the supporting step; FIG. 4B is a side view showing the supporting step; FIG. 5 is a perspective view for illustrating a first modified layer forming step according to the first preferred embodiment; FIG. 6 is a schematic sectional view for illustrating the first modified layer forming step and a first classifying step according to the first preferred embodiment; FIG. 7A is a schematic plan view for illustrating the first scheduling step; FIG. 7B is a schematic sectional view for illustrating an amount of division in the first division step;FIGS. 8A and 8B are perspective views for illustrating an initial wafer manufacturing step according to the first preferred embodiment; FIG. 9 is a schematic sectional view for illustrating the initial wafer manufacturing step; FIG. 10 is a perspective view showing a newly exposed upper surface of the ingot obtained by performing the initial wafer manufacturing step; FIG. 11 is a perspective view showing a flattening step according to the first preferred embodiment; FIG. 12 is a schematic sectional view for illustrating a separation start point forming step according to the first preferred embodiment; FIG. 13 is a schematic sectional view showing a wafer separating step according to the first preferred embodiment;FIGS. 14A and 14B are perspective views showing a supporting step according to a second preferred embodiment of the present invention; FIG. 15 is a schematic sectional view for illustrating a first modified layer forming step, a second modified layer forming step, and a second modified layer forming repeating step in accordance with the second preferred embodiment; FIG. 16 is a schematic sectional view showing a separation start point forming step according to the second preferred embodiment; and FIG. 17 is a schematic sectional view showing a wafer separating step according to the second preferred embodiment.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTSPreferred embodiments of the present invention will now be described in detail with reference to the drawings. Referring to FIG. 1, there is shown a perspective view of a laser processing apparatus 2 suitable for use in carrying out the wafer manufacturing method of the present invention. The laser machining apparatus 2 includes a stationary base 4 and a first slide block 6 mounted on the stationary base 4 so as to be movable in the X direction. The first link block 6 is moved in a feeding direction or in the X direction along a pair of slide rails 14 by a feeding mechanism 12 composed of a ball screw 8 and a stepping motor 10.A second link block 16 is mounted on the first link block 6 so as to be movable in the Y direction. The second link block 16 is moved in an indexing direction or in the Y direction along a pair of guide rails 24 by an indexing mechanism 22 constructed of a ball screw 18 and a stepping motor 20. A support table 26 is mounted on the second slide block 16. The support table 26 is movable in the X direction and the Y direction by the feeding mechanism 12 and the indexing mechanism 22, and is also rotatable by a motor accommodated in the second link block 16.A column 28 is provided on the stationary base 4 so as to project upward therefrom. A laser beam applying mechanism (laser beam applying means) 30 is mounted on the column 28. The laser beam applying mechanism 30 is configured of a housing 32, a laser beam generating unit 34 (see FIG. 2 ) accommodated in the housing 32, and a focusing means (laser head) 36 mounted on the front end of the housing 32. An imaging unit 38 including a microscope and a camera is also mounted on the front end of the housing 32 so as to be aligned with the focusing means 36 in the X direction.As shown in FIG. 2, the laser beam generating unit 34 includes a laser oscillator 40 such as a YAG laser and a YVO4laser for generating a pulsed laser beam, repetition frequency setting means 42 for setting the repetition frequency of the pulsed laser beam to be generated by the laser oscillator 40, pulse width setting means 44 for setting the pulse width of the pulsed laser beam to be generated by the laser oscillator 40, and power setting means 46 for setting the pulsed laser beam to be generated by the laser oscillator 40. Although not particularly shown, the laser oscillator 40 has a Brookfield window, so that the laser beam generated by the laser oscillator 40 is a laser beam with linearly polarized light. After the power of the pulsed laser beam is adjusted to a predetermined power by the laser adjusting means 46 of the laser beam generating unit 34, the pulsed laser beam is reflected by a mirror 48 included in the focusing means 36, and next focused by a focusing lens 50 included in the focusing means 36. The focusing lens 50 is positioned so that the pulsed laser beam is focused inside a hexagonal single crystal ingot 11 as a workpiece fixed to the support table 26. The focusing means 36 is mounted on the housing 32 so as to be finely movable in a vertical direction (Z direction).Referring to FIG. 3, there is shown an exploded perspective view showing a supporting step in a wafer manufacturing method according to a first preferred embodiment of the present invention. FIG. 4A is a perspective view showing the supporting step, and FIG. 4B is a side view showing the supporting step. Reference numeral 11 denotes the hexagonal single crystal ingot as a workpiece, hereinafter referred to simply as an ingot. The ingot 11 has an upper surface 11 a, and a c-plane is exposed to the upper surface 11 aof the ingot 11. Accordingly, a c-axis 17 perpendicular to the c-plane of the ingot 11 extends in a direction perpendicular to the upper surface 11 aof the ingot 11. Reference numeral 15 denotes a vertical axis. The hexagonal single crystal ingot 11 is selected from a SiC single crystal ingot and a GaN single crystal ingot.The wafer manufacturing method according to the first preferred embodiment is a method for manufacturing a wafer having an off angle α from the ingot 11 with a c-plane exposed to the upper surface 11 a. First, a supporting step is performed in such a manner that the ingot 11 is supported by a wedge member 13 having a wedge angle α on the support table 26 having a horizontal support surface (upper surface), the wedge angle α being equal to the deviation angle α. By performing the supporting step, the ingot 11 is supported on the support table 26 in the state where the upper surface 11 aof the ingot 11 is inclined with respect to a horizontal surface (horizontal plane) by the off angle α. In this preferred embodiment, the off angle α is set to 4°.However, the off angle α is not limited to 4°, but may be freely set in a range of 1° to 8°, for example. In this case, the ingot 11 may be supported on the support table 26 via a wedge member having a predetermined wedge angle corresponding to the above freely set off angle. The wedge member 13 is fixed to the support table 26 using a wax or adhesive. Similarly, the ingot 11 is fixed to the wedge member 13 by using a wax or adhesive.Referring again to FIG. 1, a column 52 is fixed to the left side of the stationary base 4. The column 52 is formed with a vertically elongated opening 53, and a pressing member 54 is vertically movably mounted on the column 52 so as to protrude from the opening 53.After performing the support step to support the ingot 11 by the wedge member 13 on the support table 26 in the state where the off angle α is formed in the Y direction as shown in FIG. 5, a first modified layer forming step is performed in such a manner that a laser beam LB is applied to the ingot 11 upon relative movement of the focusing means 36 in the X direction, thereby linearly forming a first modified layer 19 inside the ingot 11 and first cracks 21 extending from the first modified layer 19 along the c plane. Thereafter, as shown in FIG. 6, a first classifying step is performed in such a manner that the focal point of the laser beam LB is relatively moved to be further moved in the Y direction by a predetermined amount where the off angle α is formed. Thereafter, the first modified layer forming step is performed again without changing the height (vertical position) of the focal point of the laser beam LB.Thereafter, the first dividing step and the first modified layer forming step are repeated to form a plurality of first modified layers 19 and first cracks 21 in the entire separation plane in the ingot 11 as shown in FIG. 6. As shown in FIGS. 7A and 7B, the division amount L as a distance between any two adjacent ones of the plurality of first modified layers 19 is set to be less than or equal to a distance defined such that the front ends of the adjacent first cracks 21 extending from the adjacent first modified layers 19 along the c-plane overlap each other. Preferably, the distance h between the adjacent first cracks 21 in the c-axis direction is set to 50 μm or less.In the wafer manufacturing method according to the first preferred embodiment, it is important that the scanning direction of the laser beam to be applied from the focusing means 36 is set to the X direction perpendicular to the Y direction where the off angle α of the ingot 11 is formed in the state where the ingot 11 is supported by the wedge member 13 on the support table 26. By the above-mentioned adjustment of the scanning direction of the laser beam, the first cracks 21 propagating from each first modified layer 19 formed inside the ingot 11 can be formed to extend very long along the c-plane.For example, the first modified layer forming step and the first classifying step are performed under the following laser processing conditions.Light source: pulsed Nd:YAG laserWavelength: 1064 nmRepetition frequency: 80 kHzaverage power: 3.2 WPulse width: 4 nsSpot diameter: 10 μmnumerical aperture (NA) of the focusing lens: 0,45Scheduling amount: 250 μmIn this way, the focal point of the laser beam is successively advanced to form the plurality of first modified layers 19 in the entire separation plane in the ingot 11 and the first cracks 21 extending from each first modified layer 19 along the c-plane. Thereafter, an initial wafer manufacturing step is performed in such a manner that an external force is applied to the ingot 11 to thereby separate an initial wafer from the ingot 11 at the separation plane including the first modified layers 19 and the first cracks 21. This initial wafer manufacturing step is performed using the printing mechanism 54 shown in FIG. 1. The construction of the print mechanism 54 is shown in Figs. 8A and 8B. The pressing mechanism 54 includes a head 56 vertically movable by a not-shown moving mechanism installed in the column 52 shown in FIG. 1, and a pressing member 58 rotatable with respect to the head 56 in the direction shown by an arrow R in FIG. 8B. The pusher 58 is tiltable with respect to the head 56.As shown in FIG. 8A, the pressing mechanism 54 is relatively positioned above the ingot 11 fixed to the support table 26 by the wedge member 13. Thereafter, the pressing member 58 is inclined and the head 56 is lowered until the pressing member 58 comes into pressure contact with the upper surface 11 aof the ingot 11. In the state where the pressing member 58 is in pressure contact with the upper surface 11 aof the ingot 11, the pressing member 58 is rotated in the direction of the arrow R to thereby generate a torsional stress in the ingot 11. As a result, the ingot 11 is broken at the separation plane where the first modified layers 19 and the first cracks 21 are formed. Accordingly, as shown in FIG. 9, an initial wafer 23 is separated from the ingot 11 (initial wafer manufacturing step). The initial wafer 23 is disposed of.In FIGS. 9 and 10, reference numeral 11 sindicates a newly exposed upper surface of the ingot 11 obtained by performing the above-mentioned initial wafer manufacturing step. As shown in FIG. 10, the newly exposed upper surface 11 sof the ingot 11 is a slightly rough surface where the first modified layers 19 and the first cracks 21 are partially left. Accordingly, it is preferable to perform a flattening step of flattening this slight rough surface by grinding. When performing the flattening step, the ingot 11 having the wedge member 13 is removed from the support table 26 of the laser processing apparatus 2. Thereafter, as shown in FIG. 11, the ingot 11 is held under suction via the wedge member 13 on a chuck table 74 incorporated in a grinding apparatus not shown. The ingot 11 is held under suction on the chuck table 74 by the wedge member 13 having the off angle α such that the upper surface 11 sof the ingot 11 is a horizontal surface.In FIG. 11, reference numeral 60 denotes a grinding unit incorporated in the grinding apparatus. The grinding unit 60 includes a spindle 62 adapted to be driven in a rotational direction by a motor not shown, a wheel mount 64 fixed to the lower end of the spindle 62, and a grinding wheel 68 detachably mounted to the lower surface of the wheel mount 64 by a plurality of bolts 66. The grinding wheel 68 is configured of an annular wheel base 70 and a plurality of grinding members 72 fixed to the lower surface of the wheel base 70 so as to be arranged along the outer periphery thereof.In the flattening step, the chuck table 74 is rotated at 300 U / min, for example, in the direction shown by an arrow a in FIG. 11. At the same time, the grinding wheel 68 is rotated at 1000 U / min, for example, in the direction shown by an arrow b in FIG. 11. Further, a stepping motor in a grinding unit feeding mechanism not shown is normally operated to lower the grinding unit 60 until the grinding members 72 of the grinding wheel 68 come into contact with the upper surface 11 sof the ingot 11 held on the chuck table 74. Then, the grinding unit 60 is supplied to press the grinding members 72 against the upper surface 11 sof the ingot 11 under a predetermined load, and thereby grind the upper surface 11 sof the ingot 11 to flatten the upper surface 11 s. After performing the flattening step, the upper surface 11 sis preferably polished to become a mirror surface 11 sshown in FIG. 12.As described above, the initial wafer 23 is separated from the ingot 11 by performing the initial wafer manufacturing step. Thereafter, the flattening step is preferably performed to flatten the upper surface 11 sof the ingot 11. Thereafter, more preferably, the upper surface 11 sof the ingot 11 is polished to obtain the mirror surface 11 fas a new upper surface of the ingot 11. Thereafter, as shown in FIG. 12, a separation start point forming step is performed in such a manner that the focal point of the laser beam LB having a transmission wavelength for the ingot 11 fixed to the support table 26 via the wedge member 13 is adjusted to a predetermined depth inside the ingot 11 from the upper surface 11 f, the depth corresponding to the thickness of a wafer to be manufactured, and the laser beam LB is next applied to the upper surface 11 fwhen the focal point and the ingot 11 relatively move, to thereby form a modified layer (second modified layer) 19 parallel to the upper surface 11 fand cracks (second cracks) 21 extending from the modified layer 19 along the c-plane, thereby forming a separation start point.This separation start point forming step includes a second modified layer forming step of relatively moving the focal point of the laser beam LB in a first direction (X direction) perpendicular to a second direction (Y direction) where the off angle is formed, thereby linearly forming the modified layer (second modified layer) 19 extending in the first direction, and also includes a second dividing step of relatively moving the focal point in the second direction, thereby further moving the focal point by the predetermined amount. This separation start point forming step is repeated to form a plurality of second modified layers 19 and second cracks 21 in an entire separation plane at the predetermined depth in the ingot 11. Thereafter, a wafer separating step is performed in such a manner that a plate-shaped member having a thickness corresponding to the thickness of the wafer to be manufactured is separated from the ingot 11 at the separation start point, thereby manufacturing a hexagonal single crystal wafer 25 shown in FIG. 13.This wafer separating step can be carried out by using the pressing mechanism 54 used in the initial wafer manufacturing step mentioned above with reference to FIGS. 8A and 8B. That is, the pressing member 58 is brought into pressure contact with the upper surface 11 fof the ingot 11. In this state, the pressing member 58 is rotated to generate a torsional stress in the ingot 11. As a result, the ingot 11 is broken at the separation start point where the modified layers 19 and the cracks 21 are formed. Accordingly, as shown in FIG. 13, the hexagonal single crystal wafer 25 can be separated from the ingot 11. After separating the wafer 25 from the ingot 11, the separation surface of the wafer 25 and the separation surface of the ingot 11 are preferably ground and polished to mirror finish. Thereafter, the separation start point forming step and the wafer separating step are repeatedly applied to the ingot 11 to thereby produce a plurality of wafers 25 from the ingot 11.A wafer manufacturing method according to a second preferred embodiment of the present invention will now be described with reference to Figs. 14A to 17. In the wafer manufacturing method according to the second preferred embodiment, the hexagonal single crystal ingot 11 having a c-plane exposed to the upper surface 11 ais directly fixed to the support surface (upper surface) of the support table 26 by using a wax or adhesive as shown in FIGS. 14A and 14B (support step).Thereafter, a first modified layer forming step as shown in FIG. 15 is performed in such a manner that the focal point of a laser beam LB having a transmission wavelength (for example, 1064 nm) for the ingot 11 is adjusted to a first depth inside the ingot from the upper surface 11 aof the ingot 11 and the laser beam LB is next applied to the upper surface 11 awhen the focal point and the ingot 11 are relatively moved, thereby forming a first modified layer 19 inside the ingot 11 and first cracks 21 extending from the first modified layer 19 along the c-plane.After executing the first modified layer forming step, a second modified layer forming step is executed in such a manner that the ingot 11 and the focal point are moved in the direction (Y direction) perpendicular to the first modified layer 19 to thereby advance the focal point by a predetermined amount, and that the focal point is next set to a second depth larger than the first depth, and the laser beam LB is next applied to the upper surface 11A upon relatively moving the ingot 11 and the focal point in the X direction parallel to the first modified layer 19, thereby forming a second modified layer 19 parallel to the first modified layer 19 inside the ingot 11 and second cracks 21, extending from the second modified layer 19 along the c-plane.In the second modified layer forming step, the division amount L as the distance between the first modified layer 19 and the second modified layer 19 is set to be less than or equal to a distance defined such that the front ends of the first cracks 21 and the second cracks 21 overlap each other as shown in FIG. 7B.If h is made to represent the difference between the first depth and the second depth of the c-axis direction, the difference h is set as h=L×tanα. Thereafter, the second modified layer forming step is repeated with sequentially lowering the focal point of the laser beam LB to the intersection of a line defined by the inclination tanα and a line defined by the indexing amount L, thereby forming a plurality of second modified layers 19 and second cracks 21 in the entire separation plane in the ingot 11 (second modified layer forming repeating step).The Y coordinate and Z coordinate of the intersection of the line defined by the slope tanα and the line defined by the division amount L are preliminarily stored in a memory of a controller included in the laser processing apparatus 2. Thereafter, the focusing means 36 is sequentially lowered in repeating the second modified layer forming step corresponding to the Y coordinate and Z coordinate stored in the memory. The laser processing conditions in the second preferred embodiment are similar to those in the above-mentioned first preferred embodiment.Thus, the first modified layer forming step, the second modified layer forming step, and the second modified layer forming repeating step are performed to form the separation plane including the modified layers 19 and the cracks 21 formed inside the ingot 11. Thereafter, an initial wafer manufacturing step is performed in such a manner that an initial wafer is separated from the ingot 11 at this separation plane. This initial wafer manufacturing step is similar to that in the first preferred embodiment described above with reference to FIGS. 8A, 8B and 9, and therefore the description thereof is omitted.After performing the initial wafer manufacturing step, a flattening step is preferably performed by using a grinding device to flatten the upper surface of the ingot 11 in the first preferred embodiment. Further, a polishing step is preferably performed after performing the flattening step to obtain a mirror surface 11 fas a new top surface as in the first embodiment. In the second preferred embodiment, the upper surface 11 fof the ingot 11 is inclined with respect to a horizontal surface (horizontal plane) by the off angle α.As mentioned above, after performing the initial wafer manufacturing step, and preferably performing the flattening step and the polishing step, a separation start point forming step as shown in FIG. 16 is performed in such a manner that the focal point of the laser beam LB having a transmission wavelength to the ingot 11 is adjusted to a predetermined depth inside the ingot 11 from the upper surface 11 f, the depth corresponding to the thickness of the wafer to be manufactured, and the laser beam LB is next applied to the upper surface 11 fwhen the focal point and the ingot 11 are relatively moved, to thereby form a modified layer (third modified layer) 19 parallel to the upper surface 11 fand horizontal cracks (third cracks) 21, extending from the modified layer 19 along the c-plane, thereby forming a separation start point.This separation start point forming step includes a third modified layer forming step of relatively moving the focal point of the laser beam LB in the X direction (first direction) perpendicular to the Y direction (second direction) where the off angle is formed, thereby linearly forming the third modified layer 19 extending in the X direction, and also includes a dividing step of relatively moving the focal point in the Y direction to thereby further move the focal point by a predetermined amount, and also lowering the focal point to the intersection of the line defined by the inclination tanα and the line defined by the dividing amount L.In the separation start point forming step of the second preferred embodiment, the focal point of the laser beam LB is set to the intersection of the line defined by the inclination tanα and the line defined by the indexing amount L, and the laser beam LB is scanned in the X direction to thereby form the separation start point inside the ingot 11, the separation start point being composed of the modified layers 19 parallel to the upper surface 11 fof the ingot 11 and the horizontal cracks 21 extending from each modified layer 19 along the c-plane. After performing the separation start point forming step, a wafer separating step is performed in such a manner that a plate-shaped member having a thickness corresponding to the thickness of the wafer to be manufactured is separated from the ingot 11 composed of the modified layers 19 and the cracks 21 at the separation start point, thereby manufacturing a hexagonal single crystal wafer 25 as shown in FIG. 17.As described above, the wafer manufacturing method according to the second preferred embodiment is characterized in that the dividing step includes not only the step of advancing the focal point in the Y direction by the predetermined amount but also the step of setting the focal point at the intersection of the line defined by the inclination tanα and the line defined by the dividing amount L. After separating the wafer 25 from the ingot 11, the separation surface of the wafer 25 and the separation surface of the ingot 11 are preferably ground and polished to mirror finish. Thereafter, the separation start point forming step and the wafer separating step are repeatedly applied to the ingot 11 to thereby produce a plurality of wafers 25 from the ingot 11.

Claims

A wafer manufacturing method for manufacturing a wafer having an off angle α from a hexagonal single crystal ingot (11) having an upper surface (11a), a c-plane exposed to the upper surface (11a), and a c-axis perpendicular to the c-plane, the wafer manufacturing method comprising: a supporting step of supporting the ingot (11) by a wedge member (13) having a wedge angle α on a supporting table (26) having a horizontal supporting surface, wherein the wedge angle α is equal to the off angle α, thereby inclining the upper surface of the ingot (11) with respect to a horizontal plane by the off angle α; a first modified layer forming step of setting a focal point of a laser beam (LB) having a transmission wavelength to the ingot (11) to a predetermined vertical position inside the ingot (11), and next applying the laser beam (LB) to the upper surface (11a) upon relatively moving the focal point and the ingot (11) in a first direction perpendicular to a second direction where the off angle α is formed, thereby linearly forming, inside the ingot (11), a first modified layer (19) and first cracks (21) extending from the first modified layer (19) along the c-plane; a first dividing step of relatively moving the focal point in the second direction to further move the focal point by a predetermined amount; and an initial wafer manufacturing step of separating an initial wafer (23) from the ingot (11) at a separation plane including the first modified layers (19) and the first cracks (21) formed inside the ingot (11) by repeating the first modified layer forming step (19) and the first slicing step; wherein in the first slicing step, a slicing amount L as a distance between any adjacent ones of the plurality of first modified layers (19) is set to be less than or equal to a distance defined such that the leading ends of the adjacent first cracks (21) extending from the adjacent first modified layers (19) along the c-plane overlap each other.The wafer manufacturing method according to claim 1, further comprising: a separation start point forming step of setting the focal point of the laser beam (LB) having a transmission wavelength for the ingot (11) to a predetermined depth inside the ingot (11) from a new upper surface (11s) of the ingot (11) as a newly exposed surface obtained by separating the initial wafer (23) in the initial wafer manufacturing step, the depth corresponding to a thickness of the wafer to be manufactured, and next applying the laser beam (LB) to the new upper surface (11s) upon relatively moving the focal point and the ingot (11), to thereby form a second modified layer (19) parallel to the new upper surface (11s) and second cracks (21), extending from the second modified layer (19) along the c-plane, thereby forming a separation start point; and a wafer separating step, after performing the separation start point forming step, of separating a plate-shaped member having a thickness corresponding to the thickness of the wafer to be manufactured from the ingot (11) at the separation start point, thereby manufacturing the wafer (25) from the ingot (11); wherein the separation start point forming step includes: a second modified layer forming step (19) of relatively moving the focal point of the laser beam (LB) in the first direction perpendicular to the second direction, thereby linearly forming the second modified layer (19) extending in the first direction; and a second dividing step of relatively moving the focal point in the second direction, thereby advancing the focal point by the predetermined amount.The wafer manufacturing method according to claim 2, further comprising: a flattening step of grinding the new upper surface (11s) of the ingot (11) on which the first modified layers (19) and the first cracks (21) are exposed, after performing the initial wafer manufacturing step and before performing the separation start point forming step, thereby flattening the new upper surface (11s) of the ingot (11).A wafer manufacturing method for manufacturing a wafer having an off angle α from a hexagonal single crystal ingot (11) having a top surface (11a), a c-plane exposed to the top surface (11a), and a c-axis perpendicular to the c-plane, the wafer manufacturing method comprising: a first modified layer forming step (19) of setting a focal point of a laser beam (LB) having a transmission wavelength for the ingot (11) to a first depth from the top surface (11a) inside the ingot (11), and next applying the laser beam (LB) to the top surface (11a) while relatively moving the focal point and the ingot (11), whereby a first modified layer (19) is linearly formed inside the ingot (11) and first cracks (21) extending from the first modified layer (19) along the c-plane; a second modified layer forming step (19) of relatively moving the focal point and the ingot (11) in a direction perpendicular to the first modified layer (19) to thereby, after performing the first modified layer forming step, advance the focal point by a predetermined amount, next, adjust the focal point to a second depth larger than the first depth, and next, apply the laser beam (LB) upon relatively moving the focal point and the ingot (11) in a direction parallel to the first modified layer (19) to the upper surface (11s), to thereby linearly form a second modified layer (19) parallel to the first modified layer (19) inside the ingot (11) and second cracks (21) extending from the second modified layer (19) along the c-plane; wherein an indexing amount L as a distance between the first modified layer (19) and the second modified layer (19) is set to be less than or equal to a distance defined such that leading ends of the first cracks (21) and the second cracks (21) overlap each other in the second modified layer forming step (19); wherein a difference (h) between the first depth and the second depth in a c-axis direction is set as h=L×tanα; a second modified layer formation repeating step (19) including repeating the second modified layer formation step (19) with sequentially lowering the focal point of the laser beam (LB) to an intersection of a line defined by the inclination tanα and a line defined by the indexing amount L, thereby forming the second modified layers (19) and the second cracks (21) in the entire separation plane in the ingot (11); and an initial wafer manufacturing step (23) including separating an initial wafer (23) at the separation plane from the ingot (11) including the first and second modified layers (19) and the first and second cracks (21) formed inside the ingot (11).The wafer manufacturing method according to claim 4, wherein the division amount L is set such that the difference h is less than 50 μm or less.The wafer manufacturing method according to claim 4 or 5, further comprising: a separation start point forming step of setting the focal point of the laser beam (LB) having a transmission wavelength for the ingot (11) to a predetermined depth inside the ingot (11) from a new upper surface (11s) of the ingot (11) as a newly exposed surface obtained by separating the initial wafer (23) in the initial wafer manufacturing step, the depth corresponding to a thickness of the wafer to be manufactured, and next applying the laser beam (LB) to the new upper surface (11s) upon relatively moving the focal point and the ingot (11), to thereby form a third modified layer (19) parallel to the new upper surface (11s) and third cracks (21), extending from the third modified layer (19) along the c-plane, thereby forming a separation start point; and a wafer separation step, after performing the separation start point forming step, of separating a plate-shaped member from the ingot at the separation start point with a thickness corresponding to the thickness of the wafer to be manufactured, thereby manufacturing the wafer (25) from the ingot (11); wherein the separation start point forming step includes: a third modified layer forming step (19) of relatively moving the focus of the laser beam (LB) in a first direction perpendicular to a second direction where the off angle α is formed, thereby linearly forming the third modified layer (19) extending in the first direction; and a dividing step of relatively moving the focus in the second direction, thereby advancing the focus by the predetermined amount, and next setting the focus at the intersection of the line defined by the inclination tanα and the line defined by the dividing amount L.The wafer manufacturing method according to any one of the preceding claims, wherein the hexagonal single crystal ingot (11) is selected from a SiC single crystal ingot and a GaN single crystal ingot.

Images