METHOD FOR GRINDING A WORKPIECE

Abstract

Method for grinding a surface (102; 201) of a workpiece (100; 200) having surface irregularities using grinding stones (77), the method comprising: a holding step to hold the workpiece (100; 200) on a clamping table (31); a first grinding step involving rotating the workpiece (100; 200) held on the clamping table (31) and grinding the surface (102; 201) of the workpiece with the grinding stones (77) by a predetermined amount of material removal, while a measuring element of a height gauge is held outside of contact with the surface (102; 201) of the workpiece (100; 200); a height difference measurement step involving bringing the measuring element of the height gauge into contact with the surface (102; 201) of the workpiece (100; 200) that was ground during the first grinding step, and measuring a height difference of the surface irregularities on the surface (102; 201) of the workpiece (100; 200) with the height gauge; and a second grinding step with a pause in the grinding of the surface (102; 201) of the workpiece (100; 200) when the height difference measured in the height difference measuring step falls within a preset range, and grinding of the surface (102; 201) of the workpiece (100; 200) when the measured height difference is greater than the preset range, while the measuring element of the height gauge is kept in contact with the surface (102; 201) of the workpiece (100; 200) until the measured height difference falls within the preset range.

Description

BACKGROUND OF THE INVENTION AREA OF THE INVENTION

[0001] The present invention relates to a method for grinding a workpiece. DESCRIPTION OF THE RELATED STATE OF THE ART

[0002] To planarize both surfaces of a workpiece cut from a semiconductor ingot, the surfaces of the cut-off workpiece are ground. For example, after one surface of a workpiece has been ground, the other surface of the workpiece is ground while the thickness of the workpiece is measured, as disclosed in JP 2009-72851A. The workpiece can thus be ground to a predetermined thickness.

[0003] DE 10 2018 208 190 A1 discloses a wafer manufacturing device for producing a SiC wafer from a SiC single-crystal ingot. SUMMARY OF THE INVENTION

[0004] In accordance with the technology disclosed in JP 2009-72851A, the thickness of the workpiece is measured while the probe of a height gauge is held in contact with the other surface of the workpiece. However, in cases where at least the other surface of the workpiece exhibits significant surface irregularities, the probe tends to bounce off the other surface and may find it difficult to measure the workpiece thickness.

[0005] It is therefore an object of the present invention to provide a method for grinding a workpiece which has large surface irregularities on its surface to a predetermined thickness.

[0006] In accordance with one aspect of the present invention, a method for grinding a surface of a workpiece having surface irregularities with grinding stones is provided, comprising a holding step involving holding the workpiece on a clamping table, a first grinding step involving rotating the workpiece held on the clamping table and grinding the surface of the workpiece with grinding stones by a predetermined amount of material removal while a measuring element of a height gauge is prevented from contacting the surface of the workpiece, a height difference measuring step involving bringing the measuring element of the height gauge into contact with the surface of the workpiece that has been ground in the first grinding step, and measuring a height difference of the surface irregularities on the surface of the workpiece with the height gauge, andIf the height difference measured in the height difference measurement step falls within a preset range, a second grinding step is performed, including stopping the grinding of the workpiece surface if the measured height difference is greater than the preset range, and grinding the workpiece surface while the measuring element of the height gauge remains in contact with the workpiece surface until the measured height difference falls within the preset range.

[0007] Preferably, the workpiece can include a plate-shaped object having a surface, a second surface opposite the first surface, a c-axis extending from the first surface to the second surface and inclined at an angle of deviation to a line perpendicular to the first surface, and a c-plane perpendicular to the c-axis, wherein the plate-shaped object has been removed from an ingot made of a single crystal of silicon carbide by applying a laser beam with a wavelength that is transmittable to the ingot through its first surface and linearly moving a focal point of the laser beam relative to the ingot in a second direction perpendicular to a first direction in which the angle of deviation is formed, by repeatedly moving the focal point of the laser beam relative to the ingot by a predetermined path in the first direction.Repeatedly modified layers are formed in the ingot by forming separation start points of the modified layers and cracks extending from the modified layers in the first direction along the c-plane, and by applying external forces to the separation start points to peel off the plate-like object along the separation start points as a boundary, wherein the surface of the workpiece exhibiting the surface irregularities includes a surface of the plate-like object to be peeled off that is opposite the first surface.

[0008] Alternatively, the workpiece can include the ingot from which the plate-shaped object is removed, and the surface of the workpiece to be ground, which has the surface irregularities, can include a surface of the ingot from which the plate-shaped object has been removed.

[0009] Preferably, the method may further include a material removal quantity calculation step prior to the start of the first grinding step, in which the predetermined material removal quantity during the first grinding step is calculated in accordance with the equation: material removal quantity = feed distance × tan(angle of deviation).

[0010] In this method, in accordance with the present invention, the surface of the workpiece is planarized, i.e., ground, before the height of the ground surface is measured by the height gauge. Consequently, the measuring element of the height gauge is prevented from bouncing off the ground surface, thus making it difficult to measure the height of the ground surface. In the second grinding step, it is therefore possible to grind the workpiece with the grinding stones while measuring the height of the workpiece surface with the height gauge until the height difference of the surface irregularities falls within the preset range.

[0011] The above and other problems, features and advantages of the present invention, as well as the manner of its implementation, will best become clearer by studying the following description and attached claims, with reference to the attached drawings, which show some preferred embodiments of the invention, and the invention itself will be best understood thereby. BRIEF DESCRIPTION OF THE DRAWINGS Fig. Figure 1 is a perspective view of a grinding device performing a method for grinding a workpiece in accordance with an embodiment of the present invention; Fig. Figure 2 is a graph illustrating an example of the relationship between the height of a first surface of an ingot and a time, where the height is measured at a height difference measurement step and a second grinding step of the process; Fig. Figure 3 is a perspective view of an ingot; Fig. 4 is a front view of the ingot; Fig. Figure 5 is a schematic side view of a section of a laser processing device; Fig. Figure 6 is a top view of the ingot; Fig. Figure 7 is a sectional view illustrating a training step of a modified layer of the process; Fig. Figure 8 is a top view illustrating the formation step of a modified layer; and Fig. Figure 9 is a sectional view illustrating a predetermined material removal rate during a first grinding step of the process. DETAILED EXPLANATION OF THE PREFERRED FORM OF EXECUTION

[0012] As in Fig. Figure 1 illustrates a grinding device 1, which performs a method for grinding a workpiece in accordance with a preferred embodiment of the present invention. The device is for grinding a semiconductor ingot 200 (hereinafter referred to simply as "ingot") or a semiconductor wafer 100 (hereinafter referred to simply as "wafer") as the workpiece. The ingot 200 can, for example, be an ingot made of SiC. The wafer 100 can be a plate-shaped object, which is, for example, peeled from the ingot 200.

[0013] As in Fig. As illustrated in Figure 1, a three-dimensional coordinate system is established with respect to the grinding device 1. This three-dimensional coordinate system includes an X-axis, a Y-axis, and a Z-axis, all of which extend perpendicular to each other. The X-axis extends in Y-directions that include a +Y-direction and a -X-direction, the Y-axis extends in Y-directions that include a +Y-direction and a -Y-direction, and the Z-axis extends in Z-directions that include a +Z-direction and a -Z-direction. The X-axis and the Y-axis extend horizontally, while the Z-axis extends vertically and perpendicular to the X-axis and the Y-axis. In the following description, reference is made to these directions with respect to the grinding device 1.

[0014] The grinding device 1 includes a base 10, which has a rectangular cuboid shape, a column 11 extending upwards from the base 10, and a control unit 7 belonging to the grinding device 1. The base 10 has an opening 13 defined in its upper surface. A holding unit 30 is arranged in the opening 13. The holding unit 30 includes a clamping table 31 with a holding surface 32 for holding a workpiece and a support element 33 that supports the clamping table 31 at its upper end.

[0015] The holding surface 32 of the clamping table 31 is connected to a suction source (not illustrated) to hold the workpiece by suction. In other words, the holding unit 30 holds the workpiece against the holding surface 32 by suction.

[0016] The clamping table 31 is rotatably supported about a table center axis by the support element 33 arranged below it, which extends in the Z-axis directions through the center of the holding surface 32, while the workpiece is held on the holding surface 32. Therefore, the workpiece is held on the holding surface 32 and is rotatable about an axis that extends through the center of the holding surface 32.

[0017] As in Fig. As illustrated in Figure 1, the clamping table 31 is surrounded horizontally by a cover plate 39. A bellows plate 12, which can expand and contract in the Y-axis directions, is coupled to opposite ends of the cover plate 39 in the opening 13. The holding unit 30 is arranged on a Y-axis motion mechanism 40, which is housed in the base 10 beneath the holding unit 30.

[0018] The Y-axis motion mechanism 40 represents an example of a horizontal motion mechanism. The Y-axis motion mechanism 40 moves the holding unit 30 and the grinding unit 70 in the Y-axis directions relative to each other, parallel to the holding surface 32. In accordance with the present embodiment, the Y-axis motion mechanism 40 is arranged to move the holding unit 30 relative to the grinding unit 70 in the Y-axis directions. The horizontal motion mechanism can be a rotary table with several holding units 30 arranged on it.

[0019] The Y-axis motion mechanism 40 includes a pair of Y-axis guide rails 42 that are parallel to the Y-axis directions, a table 45 movable in the Y-axis direction and slidable on and along the Y-axis guide rails 42, a Y-axis ball screw 43 that extends parallel to the Y-axis guide rails 42, a Y-axis servo motor 44 that is connected to the Y-axis ball screw 43, and a holding base 41 that itself holds the Y-axis guide rails 42, the Y-axis ball screw 43, and the Y-axis servo motor 44.

[0020] The table 45, which is movable in the Y-axis direction, is slidably mounted on the Y-axis guide rails 42. A nut (not illustrated) is fixedly mounted on a lower surface of the table 45 and is operatively engaged with the Y-axis ball screw 43 in threaded engagement. The Y-axis servo motor 44 is coupled to one end of the Y-axis ball screw 43.

[0021] When the Y-axis servo motor 44 is energized, it rotates the Y-axis ball screw 43, as shown in Fig. Figure 1 illustrates the movement of the table 45, which is movable in the Y-axis direction, along the Y-axis guide rails 42 in one of the Y-axis directions. The support element 33 of the holding unit 30 is mounted on the table 45, which is movable in the Y-axis direction. Consequently, as the table 45 moves in one of the Y-axis directions, the holding unit 30, including the clamping table 31, also moves in the same Y-axis direction.

[0022] In accordance with the present embodiment, the holding unit 30 is generally moved by the Y-axis movement mechanism 40 between a workpiece placement area as a front area in the -Y direction, where the workpiece is placed on the holding surface 32 of the clamping table 31, and a grinding area as a rear area in the +Y direction, where the workpiece is ground on the holding surface 32, along the Y-axis directions.

[0023] As in Fig. As illustrated in Figure 1, the column 11 is furthermore erected on the base 10 in a rearward area in the +Y direction. The grinding unit 70 for grinding the workpiece on the clamping table 31 and a grinding feed mechanism 50 are mounted on a front surface of the column 11. The grinding feed mechanism 50 operates to move the holding unit 30 and the grinding unit 70 relative to each other in the Z-axis directions, i.e., grinding feed directions, perpendicular to the holding surface 32. In accordance with the present embodiment, the grinding feed mechanism 50 operates to move the grinding unit 70 relative to the holding unit 30 in the Z-axis directions.

[0024] The grinding feed mechanism 50 includes a pair of Z-axis guide rails 51 parallel to the Z-axis directions, a Z-axis movable plate 53 which is displaceable on and along the Z-axis guide rails 51, a Z-axis ball screw 52 which extends parallel to the Z-axis guide rails 51, a Z-axis servo motor 54 and a support housing 56 which is mounted on a front surface, i.e. an end face, of the Z-axis movable plate 53 and supports the grinding unit 70 thereon.

[0025] The plate 53, which is movable in the Z-axis direction, is slidably mounted on the Z-axis guide rails 51. A nut (not illustrated) is fixed to a rear surface, i.e., a back side, of the plate 53 and is operable via the Z-axis ball screw 52 in threaded engagement. The Z-axis servo motor 54 is coupled to one end of the Z-axis ball screw 52.

[0026] When the Z-axis servomotor 54 is energized, it rotates the Z-axis ball screw 52 about its central axis, which extends in a vertical direction. This causes the nut to move the Z-axis-movable plate 53 in one of the Z-axis directions along the Z-axis guide rails 51. Therefore, when the Z-axis-movable plate 53 moves in one of the Z-axis directions, the support housing 56 mounted on the Z-axis-movable plate 53 and the grinding unit 70 supported by the support housing 56 also move with the Z-axis-movable plate 53 in the same Z-axis direction.

[0027] The grinding unit 70 represents an example of a machining unit. As in Fig. As illustrated in Figure 1, the grinding unit 70 includes a spindle housing 71 attached to the support housing 56, a spindle 72 rotatably held by the spindle housing 71, a rotary motor 73 for rotating the spindle 72 about its central axis extending in a vertical direction, a disc holder 74 attached to a lower end of the spindle 72, and a grinding wheel 75 supported on the disc holder 74.

[0028] The spindle housing 71 is held in the support housing 56 and extends in the Z-axis directions. The spindle 72 extends in the Z-axis directions perpendicular to the holding surface 32 of the clamping table 31 and is rotatably supported by the spindle housing 71.

[0029] The rotary motor 73 is coupled to an upper end of the spindle 72. When the rotary motor 73 is energized, it rotates the spindle 72 about an axis 701 (see figure). Fig. 2) as its central axis, which extends in the Z-axis directions. The disc holder 74 has the form of a circular plate and is attached to a lower end, i.e., a distal end, of the spindle 72. The disc holder 74 supports the grinding wheel 75 on its lower surface.

[0030] The grinding wheel 75 has essentially the same diameter as the wheel holder 74. The grinding wheel 75 includes an annular wheel base, that is, a ring-shaped base 76 made of a metallic material, such as an aluminum alloy or similar, and an annular row of grinding stones 77 attached to a lower surface of the wheel base 76 along its entire circumferential edge. When the annular row of grinding stones 77 is held in contact with a surface of a workpiece to be ground, that is, a first surface 201 of the ingot 200 or a surface of the wafer 100, which is arranged in the grinding area and held on the clamping table 31, and is rotated about its central axis by the rotary motor 73 via the spindle 72, the wheel holder 74, and the wheel base 76, the grinding stones 77 grind the surface of the workpiece to be ground.

[0031] As in Fig. As illustrated in Figure 1, a linear scale 65 for measuring the vertical position of the grinding unit 70 is arranged on the column 11. The linear scale 65 includes a reading device 66, which is mounted on the Z-axis movable plate 53 to move with it in the Z-axis directions, and a scale element 67, which is arranged on a front surface of one of the Z-axis guide rails 51. When the linear scale 65 is in operation, the reading device 66 reads subdivisions of the scale element 67 to detect the vertical position of the grinding unit 70 as it is moved by the grinding feed mechanism 50.

[0032] As in Fig. As illustrated in Figure 1, a holding surface height measuring mechanism 8 for measuring the height of the holding surface 32 of the holding unit 30 is arranged on the upper surface of the base 10 along the opening 13. The holding surface height measuring mechanism 8 includes a housing 82, which is arranged in the -X direction on one side of the opening 13 on the upper surface of the base 10, an arm 81 coupled to a side surface of the housing 82, and a probe 80 mounted at the distal end of the arm 81. The holding surface height measuring mechanism 8 can measure the height of the holding surface 32 by bringing the lower end of the probe 80 into contact with the holding surface 32. Alternatively, the holding surface height measuring mechanism 8 can incorporate, for example, a non-contact rangefinder, such as a laser distance measuring device, instead of the probe 80.

[0033] A height measuring mechanism for an upper surface 9, for measuring the height of the upper surface of a workpiece, is arranged on the upper surface of the workpiece in the vicinity of the holding surface height measuring mechanism 8. The height measuring mechanism for an upper surface 9 represents an example of a height gauge and includes a probe 90 as a measuring element and a probe movement mechanism 95 for raising and lowering the probe 90 in the Z-axis directions.

[0034] The probe movement mechanism 95 includes a rear plate 96 erected on the base 10, a pair of guide rails 91 arranged on a front surface of the rear plate 96 and extending parallel to the Z-axis directions, a movable plate 93 slidable on the guide rails 91, a ball screw 98 extending parallel to the guide rails 91, an electric motor 92, and an L-shaped bracket 94 mounted on a front surface, i.e., an end face, of the movable plate 93. The L-shaped bracket 94 supports the probe 90 on a lower surface of the bracket.

[0035] The movable plate 93 is slidably arranged on the guide rails 91. A nut (not illustrated) is fixed to a rear surface, i.e., a back side, of the movable plate 93 and is operatively engaged with the ball screw 98 in threaded engagement. The electric motor 92 is coupled to one end of the ball screw 98.

[0036] The probe movement mechanism 95 is actuated as follows. When the electric motor 92 is energized, it rotates the ball screw 98, which causes the nut to move the movable plate 93 along the guide rails 91 in one of the Z-axis directions. The L-shaped bracket 94, which is mounted on the movable plate 93, and the probe 90, which is supported by the L-shaped bracket 94, are also moved with the movable plate 93 in the same Z-axis direction.

[0037] A linear scale 97 is arranged on a side face of the rear plate 96, which, when positioned adjacent to one of the guide rails 91, faces the +X direction. The linear scale 97 includes a reading device 972, which is mounted on the movable plate 93 for movement with it in the Z-axis directions, and a scale element 971, which is arranged on the end face of the rear plate 96. The scale element 971 extends parallel to the guide rail 91, which is positioned adjacent to the linear scale 97.

[0038] The height measuring mechanism for an upper surface 9 operates as follows. The probe movement mechanism 95 moves the probe 90 into contact with a surface of a workpiece to be ground, such as the first surface 201 of the ingot 200, which is held against the holding surface 32 of the clamping table 31. The reading device 972 then reads the divisions of the scale element 971 to determine the height or vertical position of the movable plate 93. The height of the first surface 201 of the ingot 200 can be measured based on the height of the movable plate 93. If the workpiece to be ground is a wafer 100, the height measuring mechanism for an upper surface 9 measures the height of an upper surface of the wafer 100 in a similar manner.

[0039] In accordance with the present embodiment, the thickness of the workpiece at the holding surface 32 can be calculated by subtracting the height of the holding surface 32, measured by the holding surface height measuring mechanism 8, from the height of the upper surface of the workpiece, measured by the height measuring mechanism for an upper surface 9.

[0040] The controller 7 includes a central processing unit (CPU) for executing arithmetic operations in accordance with control programs and a storage medium, such as a memory or similar device, for storing the control programs and various data. The controller 7 controls the components of the grinding device 1 described above in order to perform a grinding operation on the ingot 200 or the wafer 100.

[0041] The grinding process performed by the grinding device 1 is described below. The grinding process is carried out as a method for grinding a workpiece, also referred to as a workpiece grinding method, in accordance with the present embodiment, in order to grind a surface to be ground which has irregularities of the workpiece held by the holding unit 30.

[0042] The workpiece grinding method in accordance with the present embodiment comprises an ingot planarization step which planars the first surface 201 of the ingot 200 to be ground, a stripping step which strips the wafer 100 from the ingot 200, and a wafer planarization step which planars a surface to be ground of the wafer 100 which has been stripped from the ingot 200. (1) Ingot planarization step

[0043] The first surface 201 of the ingot 200, which has not been planarized, exhibits surface irregularities. In the ingot planarization step, the grinding device 1 grinds the first surface 201 of the ingot 200 to planarize it. Specifically, the operator of the grinding device 1 places the ingot 200 with its first surface 201 to be ground facing upwards on the holding surface 32 of the holding unit 30, which is positioned in the workpiece placement area as the front area in the -Y direction. Then, in response to a command from the operator, the control unit 7 brings the holding surface 32 into fluid contact with the suction source (not illustrated).

[0044] The control unit 7 now holds the ingot 200 under suction force against the holding surface 32 of the holding unit 30 (holding step).

[0045] The controller 7 then controls the Y-axis movement mechanism 40 to move the holding unit 30 into the grinding area, specifically the area furthest back in the +Y direction, in order to position the first surface 201 of the ingot 200, held on the holding surface 32, under the grinding stones 77 of the grinding unit 70. The controller 7 energizes the rotary motor 73 of the grinding unit 70 to rotate the spindle 72 about its central axis. The grinding stones 77 of the grinding wheel 75, which is connected to the spindle 72 via the wheel holder 74, are now rotated about the central axis of the spindle 72. Furthermore, the controller 7 controls the support element 33 of the holding unit 30 to rotate the clamping table 31 about its central axis.

[0046] The controller 7 then controls the grinding feed mechanism 50 to bring the rotating grinding stones 77 into contact with the first surface 201 of the ingot 200, which is held against the rotating holding surface 32. In this way, the controller 7 grinds the first surface 201 of the ingot 200 with the grinding stones 77 by a predetermined amount of material (first grinding step). The predetermined amount of material removal was, for example, previously set by the operator.

[0047] In particular, before the first grinding step, the measuring element of the height gauge, i.e., the probe 90 of the height mechanism for an upper surface 9, is brought into contact with the first surface 201 of the ingot 200 held on the holding surface 32, while the clamping table 31 is not rotated, and then the height measuring mechanism for an upper surface 9 measures the height of the first surface 201 of the ingot 200. Then the control 7 controls the grinding wheels 77 to grind off the first surface 201 of the ingot 200 by the predetermined amount of material from the measured height of the first surface 201 of the ingot 200.

[0048] After the height of the surface to be ground has been measured, the measuring element of the height gauge is moved away from the surface to be ground. The grinding wheels 77 can then grind the workpiece during the first grinding step, while the measuring element of the height gauge is held out of contact with the surface to be ground. In other words, in accordance with the present embodiment, the control unit 7 performs the first grinding step while holding the probe 90 of the height measuring mechanism for an upper surface 9 out of contact with the first surface 201 of the ingot 200.

[0049] After the first grinding step, the controller 7 brings the probe 90 of the height measuring mechanism for an upper surface 9 into contact with the first surface 201 of the ingot 200, which was ground in the first grinding step, at several points. At this time, the controller 7 brings the probe 90 into contact with the first surface 201 of the ingot 200 while controlling the support element 33 to rotate the ingot 200 together with the clamping table 31, or it controls the Y-axis movement mechanism 40 to move the ingot 200 together with the clamping table 31 in one of the Y-axis directions. The controller 7 can thus bring the probe 90 into contact with the first surface 201 of the ingot 200 at several points.

[0050] In this way, the controller 7 measures the height of the first surface 201 of the ingot 200 at several locations using the height measuring mechanism for an upper surface 9. Then, the controller 7 measures a height difference of the surface irregularities on the first surface 201 based on the measured height (height difference measurement step).

[0051] The control unit 7 then accesses the measured height difference of the surface irregularities. In particular, if the height difference of the surface irregularities falls within a predetermined range, i.e., a permissible range, the control unit 7 then determines that the first surface 201 of the ingot 200 has been planarized and stops the grinding of the first surface 201 of the ingot 200.

[0052] If, on the other hand, the height difference of the surface irregularities is greater than the permissible range, i.e., falls outside the permissible range, then the controller 7 grinds the first surface 201 of the ingot 200 with the grinding stones 77 while keeping the probe 90 of the height measuring mechanism for an upper surface 9 in contact with the first surface 201 of the ingot 200, i.e., while the height measuring mechanism for an upper surface 9 continues to measure the height difference of the surface irregularities. The controller 7 continuously grinds the first surface 201 of the ingot 200 with the grinding stones 77 until the height difference of the surface irregularities, as measured by the height measuring mechanism for an upper surface 9, falls within the permissible range. In this way, the first surface 201 of the ingot 200 is planarized (second grinding step).

[0053] Fig. Figure 2 illustrates a graph that shows an example of the relationship between the height (H: µm) of the first surface 201 of the ingot 200, measured by the height measurement mechanism for an upper surface 9, and a time (t) during the height difference measurement step and the second grinding step. In the example shown in Fig. As illustrated in graph 2, the height difference measurement step is performed until time t1, and at time t1 the height difference measurement step is completed and the grinding step begins.

[0054] In the example that is illustrated by the one in the Fig. As illustrated in graph 2, the measured height difference of the surface irregularities of the first surface 201, that is, the height difference of the measured heights (H), during the height difference measurement step up to time t1 falls outside a permissible range R, which is defined as the minimum value of the measured heights (H) between an upper limit H2 and a lower limit. In particular, the maximum value of the measured heights (H) of the first surface 201 exceeds the upper limit H2.

[0055] During the grinding step, which begins at time t1, the maximum value of the measured heights (H) of the first surface 201 gradually approaches the upper limit H2 over time and is smaller than the upper limit H2 at time t2. In this example, the controller 7 determines that the height difference of the measured surface irregularities of the first surface 201, i.e., the height difference of the measured heights (H), falls within the permissible range R at time t2 and terminates the second grinding step. (2) Pulling step

[0056] In the peeling step, the wafer 100 is peeled off the ingot 200, whose first surface 201 was planarized in the ingot planarization step.

[0057] The structural details of the 200 ingot are described below. Fig. 3 and Fig. Figure 4 illustrates Ingot 200. Ingot 200 is made, for example, from a single crystal of silicon carbide (SiC), that is, it is an ingot made from a hexagonal single crystal of silicon carbide. Ingot 200 has the first face 201, which was referenced above, and a second face, that is, a back face 202, which is opposite the first face 201. As described above, the first face 201 of Ingot 200 was planarized during the ingot planarization step and, as described later, is to be irradiated with a laser beam.

[0058] The ingot 200 has a first alignment plane 211 and a second alignment plane 212 on its outer circumferential surface, the latter extending perpendicular to the first alignment plane 211. The first alignment plane 211 is longer than the second alignment plane 212.

[0059] The ingot 200 has a c-axis 216 and a c-plane 217, which is perpendicular to the c-axis 216. The c-axis 216 extends from the first surface 201 to the second surface 202 and is inclined by a deviation angle α to a line 215 that is perpendicular to the first surface 201. The c-plane 217 is inclined by the deviation angle α to the first surface 201. In accordance with the present embodiment, the c-axis 216 is inclined in a direction perpendicular to the direction in which the shorter second alignment plane 212 extends.

[0060] The c-plane 217 is structured at the molecular level of the ingot 200 as countless planes within the ingot 200. In accordance with the present embodiment, the deviation angle α is set to 4°. However, the deviation angle α is not limited to 4°, but can be freely set to any degree value in the range of 1° to 6°.

[0061] In the peeling step according to the present embodiment, a laser beam is applied to the ingot 200 to form modified layers in the ingot 200. According to the present embodiment, a Fig. 5 illustrated laser processing device 300 used to apply a laser beam to the ingot 200.

[0062] The laser processing device 300 includes a laser beam generation unit 301, which generates a laser beam having a wavelength transmissible through the ingot 200, a beam condenser 303, which directs the laser beam towards the ingot 200, and a support table 309, which supports the ingot 200 on it.

[0063] The laser beam generated by the laser beam generation unit 301 is reflected by a reflecting mirror 305 of the beam condenser 303, causing it to travel in the -Z direction towards a condenser lens 307. The condenser lens 307 focuses and directs the laser beam onto the ingot 200 on the support table 309.

[0064] In the laser processing device 300, the laser beam generation unit 301, the beam condenser 303 and the support table 309 are movable relative to each other along the X-axis directions and the Y-axis directions.

[0065] The focal point of the laser beam is fed for processing in one of the X-axis directions when the laser beam generation unit 301, the beam condenser 303, and the support table 309 are moved relative to each other along the X-axis directions. Therefore, the X-axis directions represent the processing feed directions of the laser processing unit 300. The ingot 200 on the support table 309 is processed by the laser beam applied to it when the focal point of the laser beam is moved linearly in one of the X-axis directions relative to the ingot 200.

[0066] Furthermore, the focal point of the laser beam is adjusted in one of the Y-axis directions when the laser beam generation unit 301, the beam condenser 303 and the support table 309 are moved along the Y-axis directions relative to each other.

[0067] In the peeling step in accordance with the present embodiment, the ingot 200, as in Fig. Figure 5 illustrates the ingot 200 being placed on the support table 309 of the laser processing device 300, so that the laser beam from the beam condenser 303 is applied to the first surface 201 of the ingot 200. In particular, the ingot 200 is positioned as shown in Figure 5. Fig. Figure 6 illustrates that the second alignment plane 212 is firmly placed on the support table 309, so that it extends in the X-axis directions.

[0068] In particular, the ingot 200 is held on the support table 309 such that the direction perpendicular to the -Y direction in which the deviation angle α is formed, that is, the direction extending through line 215 perpendicular to the first surface 201 of the ingot 200 and through an intersection point 220 between the c-axis 216 and the first surface 201, or the direction indicated by arrow 401, is parallel to the X-axis directions (holding step during the screeding step). The intersection point 220 refers to an intersection point between the first surface 201 and the c-axis 216, which crosses line 215 perpendicular to the first surface 201 on the c-plane 217 (see Fig. 4).

[0069] The laser beam scans the first surface 201 of the ingot 200 along the direction perpendicular to the direction in which the deviation angle α is formed, i.e., the direction indicated by arrow 401, so that the focal point of the laser beam is moved linearly relative to the first surface 201. In accordance with the present embodiment, the directions perpendicular to the -Y direction in which the deviation angle α is formed, i.e., X-axis directions, represent the processing feed directions of the support table 309.

[0070] The focal point of the laser beam applied to the ingot 200 by the beam condenser 303 is positioned at a depth in the ingot 200 that corresponds to the thickness of the wafer 100 to be removed from the ingot 200. The focal point of the laser beam and the ingot 200 are moved linearly relative to each other along the X-axis perpendicular to the -Y-direction, in which the deviation angle α is formed, in order to form the modified layers in the ingot 200 (formation step of a modified layer) at the depth referred to above.

[0071] Then, the focal point of the laser beam and the ingot 200 are moved relative to each other by a predetermined approach path, i.e., a predetermined distance, in the -Y direction in which the deviation angle α is formed (approach movement step). Afterwards, the formation step of a modified layer and the approach movement step are repeated.

[0072] By repeating the training step of a modified shift and the delivery movement step, as in the Fig. 7 and Fig. Figure 8 illustrates several modified layers 501 linearly along the X-axis directions in the ingot 200 at a depth corresponding to the thickness 500 of the wafer 100 to be peeled from the ingot 200. Each of the modified layers 501 is spaced from an adjacent layer by a feed path W along the Y-axis directions. Simultaneously, cracks 503 extending from the modified layers 501 in the Y-axis directions are formed in the ingot 200 parallel to the c-plane 217. The modified layers 501 and the cracks 503 act as separation points where the wafer 100 is peeled from the ingot 200.

[0073] The laser beam from the beam condenser 303 is applied across the entire first surface 201 of the ingot 200 to form separation start points of the modified layers 501 and the cracks 503 in the ingot 200. Subsequently, external forces are applied to the separation start points, causing the wafer 100 to detach from the ingot 200 as a plate-like object along the separation start points as the boundary. For example, a known pressure mechanism, such as that disclosed in JP 6 355 540 B2, can be used to apply torsional stresses to the ingot 200, causing the ingot 200 to tear along the separation start points of the modified layers 501 and the cracks 503, thus detaching the wafer 100 from the ingot 200. (3) Wafer planarization step

[0074] The wafer 100, which was pulled from the ingot 200 during the pull-off step, has a first wafer surface 101 and a second wafer surface 102, which is a surface opposite the first wafer surface 101, i.e., a back side. The first wafer surface 101 is the first surface 201 of the ingot 200 and was planarized during the ingot planarization step. The second wafer surface 102, on the other hand, is a surface that was pulled from the ingot 200 during the pull-off step, which is also referred to as the pull-off surface and exhibits surface irregularities. During the wafer planarization step, the second wafer surface 102 of the wafer 100 is planarized.

[0075] The wafer planarization step is similar to the ingot planarization step described above. In the wafer planarization step, the operator places the wafer 100 with its second wafer surface 102 to be planed facing upwards on the holding surface 32 of the holding unit 30 (see Fig. 1) In other words, the first wafer surface 101 of the wafer 100, which has been planarized, is held against the holding surface 32. The controller 7 controls the suction source to apply a vacuum to the holding surface 32, so that the wafer 100 is held against it by suction (holding step).

[0076] As in the first grinding step during the ingot planarization step, the controller 7 controls the grinding feed mechanism 50 to bring the rotating grinding stones 77 into contact with the second wafer surface 102 of the wafer 100, which is held against the rotating holding surface 32. In this way, the controller 7 grinds the second wafer surface 102 of the wafer 100 by a predetermined amount of material removal using the grinding stones 77 (first grinding step).

[0077] At this point, the controller 7 calculates the material removal quantity L in accordance with the following equation (material removal quantity calculation step): Material removal rate L = delivery distance W × tan(angle of deviation α) where the feed path W refers to the path over which the focal point of the laser beam is moved in the -Y direction during the feed step of the peel-off step, and the deviation angle α refers to the in Fig. 3 illustrated deviation angles α.

[0078] In particular, as in Fig. Figure 9 illustrates that modified layers 501 are formed with spacing intervals that correspond to the feed path W on the second wafer surface 102, along which the wafer 100 was pulled from the ingot 200 during the peeling step. Cracks 503 are also formed on both sides of each of the modified layers 501, i.e., between the modified layers 501. While the planarized first wafer surface 101 is held in contact with the holding surface 32, the cracks 503 are inclined by the angle of deviation α to the horizontal directions, i.e., the directions parallel to the holding surface 32 on the second wafer surface 102 to be ground. Therefore, the second wafer surface 102 exhibits surface irregularities whose height depends on the amount of material removed L, which is given by equation (1).

[0079] Consequently, the second wafer surface 102 can be planarized by grinding it down during the first grinding step by the material removal rate L, which is given by equation (1), as a predetermined material removal rate. If the infeed distance is 100 µm and the deviation angle α is 4°, then the material removal rate L is approximately 27.97 µm.

[0080] Control unit 7 sets the predetermined material removal rate as described above and then executes the first grinding step. In other words, the predetermined material removal rate is set before the first grinding step is performed.

[0081] Next, as in the ingot planarization step, the controller 7 performs a height difference measurement step to measure the height difference of the surface irregularities on the second wafer surface 102 of wafer 100. If the height difference of the surface irregularities falls within a preset range, i.e., an acceptable range, then the controller 7 determines that the second wafer surface 102 of wafer 100 has been planarized and terminates the grinding process.

[0082] If, however, the height difference of the surface irregularities is greater than the permissible range, i.e., falls outside the permissible range, then the controller 7 grinds the second wafer surface 102 of the wafer 100 with the grinding stones 77, while the height measuring mechanism for an upper surface continues to measure the height difference of the surface irregularities. In this way, the second wafer surface 102 of the wafer 100 is planarized (second grinding step).

[0083] The ingot 200, from which the wafer 100 was peeled, also exhibits surface irregularities on its peeled surface. The operator performs the ingot planarization step described above on the peeled surface and then performs the peeling step and the wafer planarization step. In this way, several wafer 100s, each with both surfaces planarized, can be obtained from the ingot 200.

[0084] In accordance with the present embodiment, in both the ingot planarization step and the wafer planarization step, the first grinding step is performed to grind down the first surface 201 of the ingot 200 or the second wafer surface 102 of the wafer 100 by a predetermined amount of material, after which the height of the ground surface is measured by the height measuring mechanism for a top surface 9. In other words, the surface to be ground is planarized, that is, ground, before the height of the ground surface is measured by the height measuring mechanism for a top surface 9. Consequently, the probe 90 of the height measuring mechanism for a top surface 9 is prevented from bouncing off the ground surface and thus from encountering difficulties in measuring the height of the ground surface.In the second grinding step, it is therefore possible to grind the workpiece with the grinding stones 77 of the grinding unit 70 until the height difference of the surface irregularities, which are measured by the height measuring mechanism for an upper surface 9, falls within the permissible range.

[0085] In accordance with the present embodiment, the ingot planarization step is performed prior to the peeling step in order to planarize the first surface 201 of the ingot 200 onto which the laser beam is to be applied. Therefore, the modified layers 501 and the cracks 503 can be well formed in the ingot 200 to obtain the wafer 100 as a plate-shaped object to be peeled from the ingot 200.

[0086] In accordance with the present embodiment, the ingot 200 is formed from a hexagonal single crystal of silicon carbide with a deviation angle. When the wafer 100 is peeled from the ingot 200 as a plate-shaped workpiece, surface irregularities are formed on the peeled surfaces of the ingot 200 and the wafer 100 due to the interconnected inclined cracks 503.

[0087] In accordance with the present embodiment, the peeling surface of the ingot 200, which exhibits the surface irregularities, is planarized during the ingot planarization step. Consequently, the modified layers 501 and the cracks 503 can be easily formed in the ingot 200 as a plate-shaped object to be peeled from the ingot 200 for the purpose of obtaining a next wafer 100.

[0088] In accordance with the present embodiment, the peeling step and the wafer planarization step are performed after the ingot planarization step has been performed in order to planarize the first face 201 of the ingot 200. However, the ingot planarization step can be omitted.

[0089] In a case where the ingot planarization step is omitted, the laser beam is applied to the ingot 200 through the first surface 201, which has not been planarized, in order to, as in Fig.Figure 7 illustrates the formation of separation start points of the modified layers 501 and cracks 503 in the ingot 200. External forces are then applied to these separation start points, causing a wafer 100 to be pulled away from the ingot 200 as a plate-like object along the separation start points. The resulting wafer 100 exhibits surface irregularities on both the first wafer surface 101 and the second wafer surface 102. The wafer planarization step is then performed on both the first wafer surface 101 and the second wafer surface 102, producing a wafer 100 with both surfaces planarized.

Claims

[1] Method for grinding a surface (102; 201) of a workpiece (100; 200) having surface irregularities using grinding stones (77), the method comprising: a holding step to hold the workpiece (100; 200) on a clamping table (31); a first grinding step involving rotating the workpiece (100; 200) held on the clamping table (31) and grinding the surface (102; 201) of the workpiece with the grinding stones (77) by a predetermined amount of material removal, while a measuring element of a height gauge is held outside of contact with the surface (102; 201) of the workpiece (100; 200); a height difference measurement step involving bringing the measuring element of the height gauge into contact with the surface (102; 201) of the workpiece (100; 200) that was ground during the first grinding step, and measuring a height difference of the surface irregularities on the surface (102; 201) of the workpiece (100; 200) with the height gauge; and a second grinding step with a pause in the grinding of the surface (102; 201) of the workpiece (100; 200) when the height difference measured in the height difference measuring step falls within a preset range, and grinding of the surface (102; 201) of the workpiece (100; 200) when the measured height difference is greater than the preset range, while the measuring element of the height gauge is kept in contact with the surface (102; 201) of the workpiece (100; 200) until the measured height difference falls within the preset range. [2] The method of claim 1, wherein the workpiece (100) comprises a plate-shaped object formed from an ingot (200) made of a single crystal of silicon carbide, comprising a first surface (201), a second surface (202) opposite the first surface, a c-axis (216) extending from the first surface (201) to the second surface (202) and inclined at an angle of deviation (α) to a line (215) perpendicular to the first surface (201), and a c-plane perpendicular to the c-axis (216), by repeatedly depositing modified layers (501) in the ingot (200) by applying a laser beam of a wavelength transmissible through its first surface (201) to the ingot (200) and linearly moving a focal point of the laser beam relative to the ingot (200) in a second direction perpendicular to the first direction in which the angle of deviation (α) is formed.The focal point of the laser beam is repeatedly moved relative to the ingot (200) along a predetermined path in the first direction, separation start points of the modified layers (501) and cracks (503) are formed, extending from the modified layers (501) in the first direction along the c-plane, and external forces are applied to the separation start points to pull off the plate-shaped object (100) along the separation start points as a boundary, and the surface (102) of the workpiece (100) which has the surface irregularities, has a surface of the plate-shaped object to be peeled off which is opposite the first surface (201). [3] The method of claim 1, wherein the workpiece has an ingot (200) made from a single crystal of silicon carbide and a first surface (201), a second surface (202) opposite the first surface, a c-axis (216) extending from the first surface (201) to the second surface (202) and inclined by an angle of deviation (α) to a line (215) perpendicular to the first surface (201), and a c-plane perpendicular to the c-axis (216), in which repeatedly modified layers (501) are formed in the ingot (200) by applying a laser beam with a wavelength that can be transmitted to the ingot (200) through its first surface (201) and linearly moving a focal point of the laser beam relative to the ingot (200) in a second direction perpendicular to a first direction in which the angle of deviation (α) is formed,The focal point of the laser beam is repeatedly moved in the first direction relative to the ingot (200) by a predetermined feed path, forming separation start points of the modified layers (501) and cracks (503) extending from the modified layers (501) in the first direction along the c-plane, and external forces are applied to the separation start points to subtract the plate-shaped object (100) along the separation start points as a boundary, and The surface (201) of the workpiece to be ground, which has surface irregularities, has a surface of the ingot (200) from which the plate-shaped object (100) has been removed. [4] The method of claim 2, further comprising: Before the start of the first grinding step, a material removal quantity calculation step is performed, calculating the predetermined material removal quantity during the first grinding step in accordance with a subsequent equation: Material removal rate = delivery distance × tan(angle of deviation). [5] The method of claim 3, further comprising: Before the start of the first grinding step, a material removal quantity calculation step is performed, calculating the predetermined material removal quantity during the first grinding step in accordance with a subsequent equation: Material removal rate = delivery distance × tan(angle of deviation).

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