Chuck table and wafer grinding method

Abstract

To enable grinding wafers with an off-angle to a uniform thickness while suppressing the need for larger equipment. [Solution] The chuck table (12) holds a wafer (W) with an off-angle when grinding it with a grinding wheel (49) by suction. The chuck table includes a disc-shaped porous member (15) and a trajectory length changing unit (71) that creates a difference in the trajectory length of the grinding wheel such that the holding surface (16), formed by rotating the center (CT) of the porous member around a rotation axis (C2) and grinding its upper surface with a grinding wheel, has a height difference between a first region (R1) and a second region (R2) on either side of a reference line (S) passing through the center of the porous member.

Description

Technical Field

[0001] The present invention relates to a chuck table and a method for grinding a wafer.

Background Art

[0002] As disclosed in Patent Documents 1 and 2, a chuck table used when grinding a wafer with a grinding wheel may perform so-called self-grinding, in which the upper surface of the chuck table is ground with the grinding wheel to form a holding surface for sucking and holding the wafer.

[0003] When grinding a wafer having an orientation flat formed thereon using the chuck tables of Patent Documents 1 and 2, the locus length of the grinding wheel on the orientation flat becomes short. In a portion where such a locus length becomes short, since the grinding load becomes small, it is necessary to prevent the portion from being recessed and not ground compared to other portions. Therefore, a notch similar to the orientation flat is also formed in the chuck table, and the holding surface is formed by self-grinding. That is, by making the grinding load distribution the same when grinding the wafer and when grinding the holding surface by self-grinding, the thickness of the ground wafer is made uniform.

[0004] Here, when grinding a wafer having an off-angle formed thereon, such as a SiC wafer (for example, Patent Document 3), the grinding load within the wafer surface is different due to the off-angle. Further, when grinding while rotating the chuck table holding the wafer having an off-angle and the grinding wheel, the grinding wheel cuts into the surface of the wafer at various angles. For this reason, there is a problem that the grinding resistance changes depending on whether grinding is performed from the downstream side to the upstream side in the off-angle direction (offset direction) of the wafer or from the upstream side to the downstream side, and variations occur in the thickness of the processed wafer.

[0005] Therefore, in the grinding apparatus disclosed in Patent Document 4, grinding is performed by offsetting the rotation center of the chuck table from the center of the wafer held by the chuck table. As a result, Patent Document 4 performs grinding in such a way that the contact area between the grinding wheel and the wafer varies, and the grinding resistance within the wafer surface becomes uniform. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2021-160067 [Patent Document 2] Japanese Patent Publication No. 2023-058925 [Patent Document 3] Japanese Patent Publication No. 2016-111143 [Patent Document 4] Japanese Patent Publication No. 2021-142627 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, Patent Document 4 has a problem in that, because the rotation center of the chuck table and the center of the wafer held by the chuck table are offset, the surface area of ​​the chuck table becomes larger, and furthermore, the entire grinding apparatus also becomes larger.

[0008] The present invention has been made in view of the above, and one of its objectives is to provide a chuck table and a wafer grinding method that can grind wafers with an off-angle to a uniform thickness and suppress the need for larger equipment. [Means for solving the problem]

[0009] A chuck table according to one aspect of the present invention is a chuck table for suction holding a wafer having an off-angle when grinding the wafer with a grinding wheel, comprising: a disc-shaped porous member; and a trajectory length changing unit that creates a difference in the trajectory length of the grinding wheel such that the holding surface formed by grinding the upper surface of the porous member by rotating the center of the porous member on a rotation axis is such that a height difference is formed between one side and the other side of a line passing through the center of the porous member.

[0010] A wafer grinding method according to one aspect of the present invention is a wafer grinding method comprising grinding a wafer having an off-angle, which is held by the holding surface of a chuck table, with a grinding wheel, the method comprising: a holding surface forming step of grinding the upper surface of the chuck table, which rotates on a rotation axis passing through the center of the porous member, with the grinding wheel to form a holding surface that is conical with the center of the porous member as its apex and has a height difference across a line passing through the center of the porous member; a holding step of holding the wafer by attraction with the holding surface, with the direction perpendicular to the line that serves as the reference line for the height difference of the holding surface being the direction of the off-angle of the wafer; and a grinding step of rotating the chuck table and grinding the rotating wafer with the grinding wheel. [Effects of the Invention]

[0011] According to the present invention, the trajectory length changing section creates a difference in trajectory length, which is the portion of the grinding wheel that grinds the chuck table, and a difference in height is created on the holding surface on one side and the other side of a line passing through the center of the porous member. This makes it possible to change the amount of grinding within the wafer surface according to the off-angle of the wafer while suppressing an increase in the flat area of ​​the chuck table. As a result, it is possible to simultaneously achieve uniformity in the thickness of wafers with an off-angle after grinding and suppress an increase in the size of the chuck table and the entire grinding apparatus. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic perspective view of the grinding apparatus according to the embodiment. [Figure 2]FIG. 2A is a schematic view showing a partial longitudinal sectional view of the grinding apparatus according to the embodiment, and FIGS. 2B and 2C are schematic views showing the crystal structure of the wafer. [Figure 3] FIGS. 3A and 3B are a plan view and a front view showing an example of the wafer. [Figure 4] It is a schematic perspective view of the chuck table according to the embodiment. [Figure 5] It is an exploded view of FIG. 4. [Figure 6] FIG. 6A is a plan view for explaining the holding surface forming step, FIG. 6B is a plan view for explaining the holding step, and FIG. 6C is a plan view for explaining the grinding step. [Figure 7] FIGS. 7A to 7D are explanatory views showing how the chuck table is being ground. [Figure 8] FIG. 8A is a schematic perspective view of the chuck table according to the first modification, and FIG. 8B is an exploded view of FIG. 8A. [Figure 9] FIG. 9A is a schematic perspective view of the chuck table according to the second modification, and FIG. 9B is an exploded view of FIG. 9A. [Figure 10] FIG. 10A is a schematic perspective view of the chuck table according to the third modification, and FIG. 10B is an exploded view of FIG. 10A.

MODE FOR CARRYING OUT THE INVENTION

[0013] Hereinafter, a grinding apparatus according to an embodiment will be described with reference to the accompanying drawings. FIG. 1 is a schematic perspective view of the grinding apparatus according to the embodiment.

[0014] As shown in FIG. 1, on the upper surface of the base 10 in the grinding apparatus 1, a rectangular opening extending in the front-rear direction is formed. This opening is covered with a moving plate 13 that is movable in the front-rear direction together with the chuck table 12 and a bellows-shaped waterproof cover 14.

[0015] The chuck table 12 includes a disk-shaped porous member 15. The porous member 15 is made of a porous material such as ceramics, and fine pores are formed throughout. The upper surface of the porous member 15 constitutes a holding surface 16. The holding surface 16 sucks and holds the wafer W by a suction source (not shown).

[0016] In the present embodiment, the wafer W is formed in a substantially disk shape, with the first main surface W1 on the back side facing upward and the second main surface W2 on the front side facing downward. In the grinding process, the second main surface W2 of the wafer W serves as the held surface adsorbed and held by the holding surface 16 of the chuck table 12, and the first main surface W1 of the wafer W serves as the surface to be ground. A protective tape T is adhered to the second main surface W2 of the wafer W. Details of the wafer W and the chuck table 12 will be described later.

[0017] Behind the chuck table 12, a grinding water supply nozzle 17 for supplying grinding water to the holding surface 16 is provided.

[0018] On the base 10, at the position where the chuck table 12 passes, a thickness measuring device 18 for measuring the thickness of the wafer W held by the chuck table 12 is installed.

[0019] The thickness measuring device 18 includes a first height gauge for measuring the height position of the first main surface W1 of the wafer W held on the holding surface 16 of the chuck table 12, and a second height gauge for measuring the height position of the upper surface of the chuck table 12 (see FIG. 2A). Based on the difference between the measured value of the first height gauge and the measured value of the second height gauge by the thickness measuring device 18, the thickness of the wafer W is measured.

[0020] Below the waterproof cover 14, a table movement mechanism 20 is provided for moving the chuck table 12 in the front-rear direction. The table movement mechanism 20 comprises a pair of guide rails 21 extending in the front-rear direction and a ball screw 22, and the mobile table 23 is supported so as to be movable along the guide rails 21. The ball screw 22 is screwed into a moving threaded portion (not shown), and when the motor 25 operates and the ball screw 22 is rotated, the mobile table 23 moves in the front-rear direction.

[0021] Furthermore, below the waterproof cover 14, there is a table rotation mechanism 26 for rotating the chuck table 12 and a tilt adjustment mechanism 27 for adjusting the tilt of the chuck table 12.

[0022] The table rotation mechanism 26 includes a servo motor (not shown) which is a drive source, and an encoder (not shown) which detects the rotation speed, rotation direction, and rotation angle of the servo motor. The tilt adjustment mechanism 27 includes a plurality of support columns provided at predetermined angles apart in the circumferential direction of the chuck table 12 when viewed from above and below, and adjusts the tilt of the chuck table 12 by varying the support height of at least some of the plurality of support columns.

[0023] The grinding device 1 further comprises a lifting unit 30 and a grinding unit 40. The lifting unit 30 is mounted on a column 19 erected at the rear of the base 10 and moves the grinding unit 40 up and down in the vertical direction.

[0024] The lifting unit 30 includes a pair of guide rails 31 positioned on the front side of the column 19 and extending vertically, a lifting table 32 installed to be movable vertically relative to the pair of guide rails 31, and a ball screw 33 that extends vertically and is screwed into a threaded portion (not shown) of the lifting table 32.

[0025] A servo motor 34 is connected to one end of the ball screw 33. In the lifting unit 30, the ball screw 33 is rotated by the driving force of the servo motor 34, causing the lifting table 32 and the grinding unit 40 to move up and down. The rotation speed, direction of rotation, and angle of rotation of the servo motor 34 are detected by the encoder 35.

[0026] The grinding unit 40 comprises a holder 41 mounted on the front of the lifting table 32 and a spindle unit 42 supported by the holder 41. The holder 41 is mounted on the front of the lifting table 32. The spindle unit 42 comprises a spindle housing 44 having a flange portion 43 at its lower end that is supported by the holder 41, and a spindle 45 that rotates by the driving force of a spindle motor (not shown).

[0027] Figure 2A is a schematic diagram showing a partial vertical cross-sectional view of the grinding apparatus according to the embodiment. As shown in Figure 2A, the spindle 45 rotates around a rotation axis C1 that extends in the vertical direction. A mount 47 is connected to the tip (lower end) of the spindle 45, and a grinding wheel 48 is mounted on the mount 47. Multiple grinding wheels 49 are arranged in an annular pattern on the lower surface of the grinding wheel 48, centered on the rotation axis C1. The grinding unit 40 rotates the annular grinding wheels 49 with the spindle 45 and grinds the wafer W, which is held by suction on the holding surface 16 of the rotating chuck table 12, by bringing the lower surface of the grinding wheels 49 into contact with the first main surface W1 of the wafer W.

[0028] The upper surface of the chuck table 12 is formed in a conical shape, with the center CT of the porous member 15 as its apex and sloping downward toward the outer circumference. Therefore, the holding surface 16 formed by the porous member 15 is also formed in a conical shape with the center CT as its apex. Note that in Figure 2A, the slope of the upper surface of the chuck table 12, including the conical holding surface 16, is exaggerated in the illustration, but in reality, this slope is so slight that it cannot be seen with the naked eye.

[0029] The chuck table 12 is driven to rotate in the direction of the arrow by the table rotation mechanism 26, around a rotation axis C2 that passes through the center CT of the porous member 15 (holding surface 16). The center CT is the center of the holding surface 16, the apex of the conical holding surface 16, and the rotation center (rotation axis C2) of the chuck table 12. The rotation axis C2 of the chuck table 12 is tilted by the tilt adjustment mechanism 27 so that a portion of the radial area of ​​the holding surface 16 of the chuck table 12 is parallel to the lower surface of the grinding wheel 49.

[0030] Figures 2B and 2C are schematic diagrams showing the crystal structure of the wafer, with Figure 2B being a partially enlarged view of Figure 2A. As shown in Figure 2B, wafer W is a wafer with an off-angle α, and examples of hexagonal single-crystal wafers include SiC single-crystal wafers and GaN single-crystal wafers. In wafer W, epitaxial growth is performed with an off-angle α of about 1 to 4 degrees to reduce crystal defects. In this embodiment, wafer W is a SiC single-crystal wafer, and the first main surface W1 of wafer W is a C-plane that is tilted by an off-angle α with respect to the {0001} plane (base plane).

[0031] The wafer W has an off-angle direction D. As shown in Figure 2C, the "off-angle direction D" refers to the direction from the leading edge to the base edge of the vector obtained by projecting the normal vector P of the {0001} plane of the wafer W onto a plane. The "upstream side of the off-angle direction D" refers to the side in which the leading edge of the normal vector P obtained by projecting the normal vector P of the {0001} plane of the wafer W onto a plane is pointing. The "downstream side of the off-angle direction D" refers to the side in the opposite direction to the direction in which the leading edge of the normal vector P obtained by projecting the normal vector P of the {0001} plane of the wafer W is pointing.

[0032] Figures 3A and 3B are a plan view and a front view, respectively, showing an example of a wafer. The wafer W in Figures 3A and 3B has a notch W3 on its outer circumference indicating the crystal orientation. The notch W3 of the wafer W in Figure 3A is a notch W31, and the notch W3 of the wafer W in Figure 3B is an orientation flat W32. The notch W31 is cut out in a concave shape toward the center of the first main surface W1, and more specifically, it is formed in a tapered shape (tapered or triangular) toward the center of the first main surface W1. The orientation flat W32 is formed by being cut out in a straight line in a plan view.

[0033] In Figure 3A, the off-angle direction D of the wafer W is parallel to the direction perpendicular to the direction connecting the notch W31 and the center of the first main surface W1, and is parallel to the left-right direction in the figure. In Figure 3B, the off-angle direction D of the wafer W is parallel to the extension direction of the orientation flat W32, and is parallel to the left-right direction in the figure. In both Figure 3A and Figure 3B, the left side is the upstream side of the off-angle direction D, and the right side is the downstream side of the off-angle direction D. In this embodiment, the chuck table 12 is configured to hold the wafer W shown in Figure 3A.

[0034] Figure 4 is a schematic perspective view of the chuck table according to the embodiment. Figure 5 is an exploded view of Figure 4. As shown in Figures 4 and 5, the chuck table 12 includes, in addition to the porous member 15 described above, a ring-shaped (annular) frame 61 arranged around the outer circumference of the porous member 15, and a disc-shaped base portion 62 disposed below the porous member 15 and the frame 61. The frame 61 and the base portion 62 are formed of a dense material.

[0035] The porous member 15 is provided in a shape that corresponds to the wafer W when viewed from above or below, that is, a shape that is approximately circular and has a recessed portion 64 corresponding to the notch W31.

[0036] The frame 61 is provided with an inner circumference shape that has an inner diameter approximately the same as the outer diameter of the porous member 15, and has a bulge 65 whose shape corresponds to the notch W31 of the wafer W and the recess 64 of the porous member 15. Thus, the porous member 15 is fitted snugly into the frame 61 from above. With the porous member 15 housed in the frame 61, the upper surface 66 of the frame 61 and the holding surface 16 which becomes the upper surface of the porous member 15 are positioned on the same conical surface.

[0037] The base portion 62 is provided with a diameter that extends beyond the frame 61 when viewed from above, and has multiple holes formed along its outer circumference for inserting fixing screws. The base portion 62 is connected to the lower end of the frame 61 and together with the frame 61 forms a space for housing the porous member 15. Therefore, the porous member 15 is supported from below by the upper surface of the base portion 62 (see Figure 2A).

[0038] On the upper surface of the base portion 62 inside the frame 61, a circumferential groove 68 and a passage 69 (neither shown in Figure 4) formed on the bottom side of the circumferential groove 68 are formed. The circumferential groove 68 and the passage 69 communicate with a suction source (not shown), and the porous member 15 is used as negative pressure to suction and hold the wafer W at the holding surface 16. When viewed from above, the center position of the base portion 62 coincides with the center CT of the porous member 15 and is provided in a concentric manner.

[0039] Here, when viewed from above, the center of the circle forming the outer circumference of the frame 61 is eccentric to the left with respect to the center CT of the porous member 15 in Figures 4 and 5. As a result, when a virtual line passing through the center CT of the porous member 15 and the recessed portion 64 corresponding to the notch W31 is used as the reference line (hereinafter referred to as "reference line S"), the radial width of the upper surface 66 of the frame 61 differs between one side (right side in the figure) and the other side (left side in the figure) of the reference line S. More specifically, the radial width of the upper surface 66 of the frame 61 is minimum on one side of the reference line S and maximum on the other, resulting in an asymmetrical shape when viewed from above. The trajectory length changing section 71 is formed by the upper surface 66 of the frame 61 having such a shape, and the functions of the trajectory length changing section 71 will be described later.

[0040] The operation of each part of the grinding machine 1 is controlled by the control unit 73 (see Figure 1). The control unit 73 is comprised of a processor that performs various processes, as well as a memory that stores various parameters and programs. The memory stores programs that control the operation of, for example, the table moving mechanism 20, the table rotating mechanism 26, the tilt adjustment mechanism 27, the lifting unit 30, and the grinding unit 40, as part of the control program.

[0041] Next, a method for grinding a wafer W using the grinding apparatus 1 configured as described above will be explained. The wafer grinding method according to this embodiment is a method for grinding a wafer W through a holding surface formation step, a holding step, and a grinding step, and these steps will be explained below with reference to Figures 6A-6C. Figure 6A is a plan view illustrating the holding surface formation step, Figure 6B is a plan view illustrating the holding step, and Figure 6C is a plan view illustrating the grinding step.

[0042] In each process, the rotation axis C1 of the spindle 45 is maintained parallel to the vertical direction, and the rotation axis C2 of the chuck table 12 is tilted relative to the rotation axis C1 of the spindle 45 such that the radial region that becomes part of the holding surface 16 is parallel to the lower surface of the grinding wheel 49.

[0043] [Holding surface forming process] The holding surface formation process is a self-grinding process in which the upper surface of the chuck table 12 is ground in advance, forming the holding surface 16 of the chuck table 12 and the upper surface 66 of the frame 61 so as to be parallel to the lower end surface (grinding surface) of the grinding wheel 49.

[0044] In the holding surface forming process, the chuck table 12 is positioned below the grinding unit 40 by driving the table moving mechanism 20 (see Figure 2A). With this positioning, as shown in Figure 6A, the widthwise centers of the annularly arranged grinding wheels 49 are maintained to pass directly above the center CT of the porous member 15.

[0045] Subsequently, the table rotation mechanism 26 rotates the chuck table 12 around the rotation axis C2, and the spindle 45 rotates the grinding wheel 49 around the rotation axis C1 (see Figure 2A). Then, the lifting unit 30 drives down the grinding unit 40, causing the rotating grinding wheel 49 to contact and press against the upper surface of the rotating chuck table 12. As a result, both the holding surface 16, which is the upper surface of the porous member 15 on the chuck table 12, and the upper surface 66 of the frame 61 are ground, and the holding surface 16 and the upper surface 66 form the same roughly conical surface with the center CT of the porous member 15 as its apex.

[0046] Incidentally, in the grinding process described later, when grinding the wafer W by pressing the grinding wheel 49 against the wafer W while rotating the grinding wheel 49 and the wafer W respectively, the upstream side in the off-angle direction D tends to be thinner than the downstream side. This is because, as shown in Figures 2B and 2C, when grinding from the upstream side to the downstream side in the off-angle direction D, the cutting action is smoother and the grinding resistance becomes relatively smaller, while when grinding from the downstream side to the upstream side, the cutting action is more jarring and the grinding resistance becomes relatively larger. As a result, there was a problem of variations in thickness in the wafer W after grinding.

[0047] To resolve this problem, the chuck table 12 of this embodiment is configured with a trajectory length changing section 71 on the frame 61, and the trajectory length changing section 71 is used in the holding surface forming process to create a difference in the trajectory length of the grinding wheel 49.

[0048] The difference in trajectory length of the grinding wheel 49 will be explained below with reference to Figures 7A to 7D. Figures 7A to 7D are explanatory diagrams showing how the chuck table is being ground. Figures 7A to 7D show the state when the rotation angle of the chuck table 12 is changed, with Figure 7A being θ degrees, Figure 7B being (θ+90) degrees, Figure 7C being (θ+180) degrees, and Figure 7D being (θ+270) degrees. Also, in Figures 7A to 7D, the positions of the outer edges of the multiple grinding wheels 49 arranged in an annular shape are shown by dashed lines. Here, the grinding wheel 49 during grinding comes into contact with the upper surface of the chuck table 12 in the region upstream of the rotation direction of the grinding wheel 49 from the center CT of the porous member 15. In this region, the intersection point between the outer edge of the grinding wheel 49 and the outer edge of the porous member 15 is defined as point A, and the intersection point between the outer edge of the grinding wheel 49 and the outer edge of the frame 61 is defined as point Q.

[0049] During grinding in the holding surface formation process, the trajectory length of the grinding wheel 49 grinding the upper surface 66 of the frame 61 is defined as the length of the outer edge (arc) of the grinding wheel 49 extending between points A and Q, at the rotation angles of the chuck table 12 shown in Figures 7A to 7D. When the rotation angle in Figure 7A is θ degrees and when the rotation angle in Figure 7B is (θ+90) degrees, the trajectory length of the grinding wheel 49 between points A and Q becomes relatively shorter. On the other hand, when the rotation angle in Figure 7C is (θ+180) degrees and when the rotation angle in Figure 7D is (θ+270) degrees, the trajectory length of the grinding wheel 49 between points A and Q becomes relatively longer. Note that in Figures 7A to 7D, the arrows in the off-angle direction D of the wafer W when the wafer W is held in the chuck table 12 are illustrated.

[0050] Since the dense frame 61 has significantly greater grinding resistance than the porous member 15, the influence of the trajectory length on the porous member 15 becomes extremely small in comparative studies of grinding resistance. Therefore, the grinding resistance of the grinding wheel 49 at the rotation angles shown in Figures 7A to 7D is roughly proportional to the trajectory length of the grinding wheel 49 grinding the upper surface 66 of the frame 61. Here, on the holding surface 16 of the chuck table 12, the reference line S, which is a line passing through the center CT of the porous member 15, is used as the boundary, and the right region shown in Figure 5 (the region downstream in the off-angle direction D of the wafer W held on the holding surface 16 (one region)) is defined as the first region R1, and the left region shown in Figure 5 (the region upstream in the off-angle direction D of the wafer W held on the holding surface 16 (the other region)) is defined as the second region R2.

[0051] In the states shown in Figures 7A and 7B, where the trajectory length of the grinding wheel 49 between point A and point Q is relatively short, the upper surface 66 of the frame 61 adjacent to the first region R1 is being ground. In the states shown in Figures 7C and 7D, where the trajectory length of the grinding wheel 49 between point A and point Q is relatively long, the upper surface 66 of the frame 61 adjacent to the second region R2 is being ground. Therefore, in the first region R1 compared to the second region R2, the trajectory length of the grinding wheel 49 between point A and point Q is shorter, the grinding resistance is lower, and the amount of grinding on the upper surface of the chuck table 12 is increased.

[0052] As a result, in the conical holding surface 16 with the center CT of the porous member 15 as its apex, a height difference is formed between one (first region R1) and the other (second region R2) across the reference line S, such that the first region R1 is lower than the second region R2. In other words, the trajectory length changing section 71 forms the shape of the upper surface 66 of the frame 61 described above, and forms a difference in the trajectory length of the grinding wheel 49 between point A and point Q according to the rotation angle of the chuck table 12, so as to form such a height difference. The reference line S, which is the boundary line between the first region R1 and the second region R2, is the same as the reference line for the height difference of the holding surface 16.

[0053] [Holding process] After the holding surface formation process is performed, a holding process is carried out in which the wafer W is held by suction using the holding surface 16, as shown in Figure 6B. In the holding process, for example, the wafer W is placed on the holding surface 16 of the chuck table 12 by a transport device (not shown). At this time, the notch W31 of the wafer W is aligned with the recess 64 of the porous member 15, so that the off-angle direction D of the held wafer W is perpendicular to the reference line S.

[0054] After the wafer W is placed on the holding surface 16, the porous member 15 of the chuck table 12 is connected to a suction source (not shown). As a result, a negative pressure is generated in the porous member 15 by the vacuum drawn by the suction source, and the wafer W is attracted and held in place on the holding surface 16 by this negative pressure. At this time, the wafer W is deformed into an umbrella shape with the center CT of the porous member 15 as its apex, following the shape of the holding surface 16 (see Figure 2A).

[0055] [Grinding process] After the holding process is completed, a grinding process is performed in which the wafer W, which was held by suction on the holding surface 16 of the chuck table 12 during the holding process, is ground with a grinding wheel 49, as shown in Figures 2A and 6C.

[0056] In the grinding process, while measuring the thickness with a thickness measuring instrument 18, the chuck table 12 is rotated by the table rotation mechanism 26 around the rotation axis C2, and the grinding wheel 49 is rotated by the spindle 45 around the rotation axis C1. Then, the grinding unit 40 is lowered by the drive of the lifting unit 30, and the rotating grinding wheel 49 is brought into contact with and pressed against the first main surface W1 of the rotating wafer W, and the first main surface W1 is ground by the rotating grinding wheel 49. During this grinding, the chuck table 12 rotates in the same direction as the spindle 45, causing the wafer W to rotate on its own axis, and the outer edge of the grinding wheel 49 passes through the center of the wafer W as it is ground. When the thickness of the wafer W measured by the thickness measuring instrument 18 reaches a predetermined thickness, the grinding unit 40 is raised and the grinding process ends.

[0057] As described above, when grinding the first main surface W1 (C-surface) of a wafer W having an off-angle α, in Figure 5, the right side tends to be thinner than the left side (downstream side in the off-angle direction D is upstream side), with respect to the reference line S2 passing through the center of the notch W3 of the wafer W and the center of the wafer W. However, by forming a height difference in the holding surface 16 such that the first region R1 is lower than the second region R2, it is possible to grind to a uniform thickness. In other words, by making the distance between the first region R1 of the holding surface 16, which holds the right side of the wafer W that tends to be thinner when grinding with respect to the reference line S2, and the lower surface of the grinding wheel 49 larger than the distance between the second region R2 and the lower surface of the grinding wheel 49, the amount of grinding of the portion of the wafer W held in the first region R1 can be reduced compared to the amount of grinding of the wafer W held and ground on the holding surface 16 where no height difference is formed between the first region R1 and the second region R2. This makes it possible to suppress variations in thickness caused by the off-angle α in the wafer W after the grinding process is completed, and to achieve uniformity in the thickness of the ground wafer W.

[0058] According to the above embodiment, by having a trajectory length changing section 71, as shown in Figures 7A to 7D, the trajectory length of the grinding wheel 49 that grinds the upper surface 66 of the frame 61 during the holding surface formation process can be changed according to the rotation angle of the chuck table 12. This makes it possible to create a height difference on the holding surface 16 between the first region R1 and the second region R2 that straddle the reference line S, and to change the amount of grinding within the wafer W surface according to the off-angle α of the wafer W. Therefore, when grinding a wafer W with an off-angle α, thickness uniformity can be achieved.

[0059] Furthermore, in the above embodiment, since the chuck table 12 has a trajectory length changing section 71, it becomes unnecessary to have a configuration in which the rotation center of the chuck table and the center of the wafer are offset, as in Patent Document 4. This avoids increasing the surface area of ​​the chuck table 12, thereby suppressing the enlargement of the chuck table 12 and the overall enlargement of the grinding apparatus 1.

[0060] Furthermore, since the trajectory length changing section 71 can be constructed by offsetting the inner and outer circumferences of the frame 61, it is possible to prevent the chuck table 12 from becoming complicated. Moreover, the above-mentioned functions and effects can be obtained while easily exchanging the chuck table 12 of this embodiment with an existing chuck table in which the inner and outer circumferences of the frame 61 are centered.

[0061] Furthermore, the present invention is not limited to the embodiments described above, and can be implemented with various modifications. In each of the embodiments described above, the size, shape, etc., shown in the accompanying drawings are not limited thereto, and can be appropriately modified within the scope that allows the present invention to exert its effects. In addition, the present invention can be implemented with appropriate modifications as long as it does not deviate from the scope of its objectives.

[0062] In the above embodiment, the case in which the notch W3 of the wafer W is a notch W31 was described, but as shown in Figure 3B, the notch W3 may be changed to an orientation flat W32. Also, the planar size of the wafer W may be changed to be considerably smaller than the inner circumference shape of the frame 61.

[0063] Furthermore, the trajectory length changing section 71 may be configured by changing the shape of the frame 61 to other shapes, as long as it can perform the same function as in the above embodiment. As an example of making at least one of the above modifications, the first to third modifications shown in Figures 8A to 8B, 9A to 9B, and 10A to 10B will be described below.

[0064] Figure 8A is a schematic perspective view of the chuck table of the first modified example, and Figure 8B is an exploded view of Figure 8A. In the first modified example, the notch W3 of the wafer W is changed to an orientation flat W32 compared to the above embodiment. Also, the wafer W of the first modified example has a smaller planar size than the wafer W of the above embodiment. The porous member 15 has a planar shape corresponding to the wafer W and has a recessed portion 82 that is the same shape as the orientation flat W32. The chuck table 12 of the first modified example is equipped with a holding wall 81 inside the frame 61 that is concentric with the inner circumference of the frame 61. The inner circumference of the holding wall 81 is provided in a shape corresponding to the outer circumference of the porous member 15 having the recessed portion 82. In the first modified example, a passage 83 is formed in the center of the base portion 62 and communicates with a suction source (not shown).

[0065] In the first modified example, the trajectory length changing section 71 is also provided with the same configuration as in the above embodiment. Thus, a space is formed between the frame 61 and the porous member 15, but as with the above embodiment, a height difference is formed on the holding surface 16 between the first region R1 and the second region R2 that straddle the reference line S by the holding surface formation process.

[0066] Figure 9A is a schematic perspective view of the chuck table of the second modified example, and Figure 9B is an exploded view of Figure 9A. In the second modified example, the outer and inner circumferences of the frame 61 are formed concentrically, and the shape of the upper surface 66 of the frame 61 forms the trajectory length changing section 84. The trajectory length changing section 84 of the second modified example is constructed by arranging a plurality of recesses 86 in the portion adjacent to the first region R1. By forming such recesses 86, the grinding trajectory length of the grinding wheel 49 can be shortened when grinding over the recesses 86, and the trajectory length changing section 84 creates a difference in the trajectory length of the grinding wheel 49 according to the rotation angle of the chuck table 12.

[0067] Figure 10A is a schematic perspective view of the chuck table of the third modified example, and Figure 10B is an exploded view of Figure 10A. Similar to the first modified example, the third modified example has the notch W3 of the wafer W changed to an orientation flat W32, and has the same wafer W planar size as the first modified example, with a holding wall 81 inside the frame 61 similar to the first modified example.

[0068] In the third modified example, the trajectory length changing section 84 is also provided with the same configuration as in the second modified example. As a result, a space is formed between the frame 61 and the porous member 15, but the holding surface formation process creates a height difference in the holding surface 16 between the first region R1 and the second region R2 that straddle the reference line S.

[0069] Furthermore, the notch W3 of the wafer W may have a shape other than a notch W31 or an orientation flat W32. Moreover, the notch W3 formed on the wafer W is not limited to one, but may be multiple. Also, the wafer W may not have a notch W3 formed on it.

[0070] As mentioned above, when grinding a wafer W, the upstream side in the off-angle direction D tends to be thinner than the downstream side. This is the case when grinding the C-plane (first main surface W1) of a SiC wafer. In the case of the Si-plane of a SiC wafer or a non-SiC wafer, the upstream side in the off-angle direction D may be thicker than the downstream side. In such cases, the frame 61 is formed so that the trajectory lengths of the grinding wheels 49 on one side and the other side of the reference line S are reversed compared to the above embodiment. [Industrial applicability]

[0071] As described above, the present invention has the effect of being able to grind wafers with an off-angle, such as SiC wafers, to a uniform thickness, and suppressing the increase in size of the grinding apparatus including the chuck table. [Explanation of symbols]

[0072] 1: Grinding device 12: Chuck Table 15: Porous material 16: Holding surface 49: Grinding Wheel 61: Frame 66:Top surface 71: Track length modification section 84: Track length modification section C1: Rotation axis (of the grinding wheel) C2: Rotation axis (of the chuck table) CT: Center (of a porous member) D: Off-angle direction W: wafer

Claims

1. A chuck table for suction holding a wafer with an off-angle when grinding the wafer with a grinding wheel, A disc-shaped porous member, A chuck table comprising: a trajectory length changing section that causes a difference in the trajectory length of the grinding wheel to be formed such that the holding surface, formed by rotating the center of the porous member on a rotation axis and grinding its upper surface with the grinding wheel, has a height difference between one side and the other across a line passing through the center of the porous member.

2. The trajectory length changing section is a ring-shaped frame surrounding the outer circumference of the porous member, The chuck table according to claim 1, wherein the shape of the upper surface of the frame forms the difference in trajectory length.

3. A method for grinding a wafer, wherein the wafer having an off-angle is held by the holding surface of the chuck table according to claim 1 or 2 and ground with the grinding wheel, A holding surface forming step, in which the upper surface of the chuck table, which rotates on a rotation axis passing through the center of the porous member, is ground with the grinding wheel to form a holding surface that is conical with the center of the porous member as its apex and has a difference in height on either side of a line passing through the center of the porous member, A holding step in which the wafer is held by the holding surface by suction, with the direction perpendicular to the line that serves as the reference line for the height difference of the holding surface being the off-angle direction of the wafer, A method for grinding a wafer, comprising: a grinding step of rotating the chuck table and grinding the rotating wafer with the grinding wheel.

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