Grinding Method

Abstract

To provide a new grinding method capable of suppressing a difference between a surface roughness of a central part of a plate-like workpiece and a surface roughness of an outer peripheral part small compared to a conventional grinding method.SOLUTION: A grinding method includes: a first grinding step of adjusting a relative inclination of a chuck table and a grinding wheel to a first state, then bringing a grind stone into contact with a workpiece, and grinding the workpiece; and a second grinding step of adjusting the relative inclination of the chuck table and the grinding wheel to a second state different from the first state, then bringing the grind stone into contact with the workpiece, and grinding the workpiece. In the second grinding step, the workpiece is ground under a condition that a surface roughness can be made small compared to that in the first grinding step.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to a grinding method that is applied when grinding a plate-shaped workpiece such as a wafer. [Background technology]

[0002] In order to realize small and lightweight device chips, there are increasing opportunities to thin wafers on which devices such as integrated circuits are mounted on the front side. The front side of the wafer is held by a chuck table, and the chuck table and a grinding wheel on which multiple grinding stones containing abrasive grains are fixed are rotated together, and the grinding stones are pressed against the back side of the wafer while supplying liquid such as pure water, thereby grinding and thinning the wafer (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2014-124690 A Summary of the Invention [Problem to be solved by the invention]

[0004] In a grinding method called in-feed grinding, in which the grinding wheel and the chuck table are rotated relative to each other so that the grinding wheel passes through the rotation axis of the chuck table, a difference in surface roughness between the center and outer periphery of the wafer is likely to occur. Since the difference in surface roughness leads to a difference in the mechanical strength of the device chip, there has been a demand to minimize the difference in surface roughness between the center and outer periphery of the wafer after grinding.

[0005] The present invention has been made in consideration of these problems, and its purpose is to provide a new grinding method that can reduce the difference in surface roughness between the central part and the outer periphery of a plate-shaped workpiece, compared to conventional grinding methods. [Means for solving the problem]

[0006] According to one aspect of the present invention, there is provided a grinding method for grinding a plate-shaped workpiece held by the holding surface of the chuck table with a grinding wheel having a plurality of grinding wheels arranged in a ring shape, while rotating both the chuck table having a holding surface and the grinding wheel so that the plurality of grinding wheels pass through a rotation axis of the chuck table, the method comprising adjusting a relative inclination between the chuck table and the grinding wheel to a first state so that the distance between the grinding wheel and the holding surface is greater at other positions than at a position overlapping with the rotation axis of the chuck table, and then bringing the holding surface and the grinding wheel relatively close to each other to bring the grinding wheel into contact with the workpiece to grind the workpiece. a grinding stopping step of, after the first grinding step, moving the grindstone away from the workpiece to stop grinding of the workpiece; and a second grinding step of, after the grinding stopping step, adjusting the relative inclination between the chuck table and the grinding wheel to a second state different from the first state so that the grindstone does not come into contact with the workpiece at a position overlapping with the rotation axis of the chuck table, and then bringing the holding surface and the grindstone relatively close to each other to bring the grindstone into contact with the workpiece to grind the workpiece, wherein in the second grinding step, the workpiece is ground under conditions that result in a smaller surface roughness than in the first grinding step. At the same time, grinding is terminated before the grindstone comes into contact with the workpiece at a position overlapping the rotation axis of the chuck table. A method of grinding is provided.

[0007] Preferably, in the second grinding step, the chuck table is rotated slower than in the first grinding step. Also, preferably, in the second grinding step, the grinding wheel is rotated faster than in the first grinding step. Also, preferably, in the second grinding step, the holding surface and the grindstone are brought closer to each other slower than in the first grinding step. Effect of the Invention

[0008] In a grinding method called in-feed grinding, in which the chuck table and grinding wheel are rotated together so that the grinding wheel passes through the rotation axis of the chuck table, the volume of the workpiece removed per unit time is greater at the periphery than at the center, and as a result, the surface roughness of the periphery of the workpiece after grinding tends to be greater than that of the center.

[0009] Therefore, in a grinding method according to one aspect of the present invention, the relative inclination between the chuck table and the grinding wheel is adjusted to a first state, thereby grinding the workpiece so that the central portion is thinned (first grinding step), and then the relative inclination between the chuck table and the grinding wheel is adjusted to a second state different from the first state, thereby grinding the workpiece so that the central portion is not removed (second grinding step).

[0010] Furthermore, in the second grinding step, the workpiece is ground under conditions that result in a smaller surface roughness than in the first grinding step. This allows the surface roughness of the outer periphery of the workpiece to be adjusted separately from the surface roughness of the central portion, making it smaller. In other words, the difference between the surface roughness of the central portion and the surface roughness of the outer periphery of the workpiece can be kept smaller than in conventional grinding methods. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view showing an example of a grinding device. [Diagram 2] FIG. 2 is a perspective view showing how a workpiece is ground. [Diagram 3] FIG. 3 is a cross-sectional view showing a state in which a workpiece is ground under a first condition. [Figure 4] FIG. 4 is a cross-sectional view showing the workpiece after being ground under the first condition. [Diagram 5] FIG. 5 is a cross-sectional view showing the state where the grinding wheel has been separated from the workpiece. [Figure 6] FIG. 6 is a cross-sectional view showing a state in which the workpiece is ground under the second condition. [Figure 7]FIG. 7 is a cross-sectional view showing the workpiece after being ground under the second condition. [Figure 8] FIG. 8 is a cross-sectional view showing a workpiece after being ground under a first condition in the modified example. [Figure 9] FIG. 9 is a cross-sectional view showing a workpiece after being ground under the second condition in the modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1 is a cross-sectional view showing a grinding device 2 used in this embodiment, and Fig. 2 is a perspective view showing a state in which a plate-shaped workpiece 11 is ground by this grinding device 2. As shown in Fig. 1, the grinding device 2 has a rectangular parallelepiped base 4 that supports multiple components.

[0013] A chuck table 10 capable of holding a workpiece 11 is disposed on the upper portion of the base 4. The chuck table 10 includes a cylindrical (disk-shaped) frame body 10a formed from a metal such as stainless steel. A recess having a circular opening at the upper end is formed on the upper surface side of the frame body 10a.

[0014] A porous disk-shaped holding plate 10b made of ceramics or the like is fixed in this recess. An upper surface 10c of the holding plate 10b is configured in a shape corresponding to the side surface of a cone, for example. A flow path (not shown) is provided inside the frame 10a, one end of which opens at the bottom of the recess, and the other end of this flow path is connected to a suction source (not shown) such as an ejector via a valve (not shown) or the like.

[0015] That is, the lower surface of the holding plate 10b is connected to a suction source via a flow path, a valve, etc. provided inside the frame 10a. Therefore, when the workpiece 11 is placed on the upper surface 10c of the holding plate 10b, the valve is opened, and negative pressure from the suction source is applied, the lower surface of the workpiece 11 is sucked by the chuck table 10. In this way, the upper surface 10c of the holding plate 10b functions as a holding surface that sucks and holds the workpiece 11.

[0016] A disk-shaped table base 12 is fixed to the lower surface side of the frame body 10a. A rotation drive source 14 such as a motor is connected to the lower part of the table base 12. In other words, the chuck table 10 is linked to the rotation drive source 14 via the table base 12. By operating the rotation drive source 14, the chuck table 10 rotates around a predetermined rotation axis 10d that passes through the center of the holding surface (upper surface 10c).

[0017] A tilt adjustment mechanism 16 capable of adjusting the tilt of the table base 12 is disposed below the table base 12. The tilt adjustment mechanism 16 includes one fixed support part 16a and two movable support parts 16b. The one fixed support part 16a and the two movable support parts 16b are disposed at equal angular intervals (i.e., intervals of 120°) in the circumferential direction of the table base 12, and the upper end parts of each are connected to the lower surface of the table base 12. Note that in FIG. 1, one of the two movable support parts 16b is shown.

[0018] The height of the upper end of the fixed support part 16a is constant. On the other hand, the height of the upper end of the movable support part 16b is configured to be changeable. By changing the height of the upper end of the movable support part 16b, the direction (angle) of the rotation axis 10d of the chuck table 10 can be adjusted within a predetermined range.

[0019] A thickness measuring unit 18 is provided near the chuck table 10. The thickness measuring unit 18 includes a first height measuring device 18a disposed above the frame 10a and a second height measuring device 18b disposed above the holding plate 10b. For example, the height of the upper surface of the frame 10a is measured by the first height measuring device 18a, and the height of the upper surface of the workpiece 11 held on the chuck table 10 is measured by the second height measuring device 18b, and the difference between these measured values ​​is calculated as the thickness of the workpiece 11.

[0020] Further, a grinding fluid supply unit 19 is provided near the chuck table 10. The grinding fluid supply unit 19 includes a pipe portion 19a extending upward from the base 4, and a nozzle portion 19b extending in a substantially horizontal direction from the upper end of the pipe portion 19a toward the rotation axis 10d of the chuck table 10. The lower end side of the pipe portion 19a is connected to a grinding fluid supply source (not shown) that stores a grinding fluid such as pure water.

[0021] A rectangular pillar-shaped support structure 6 extending upward is provided at a side end of the base 4. A grinding feed unit 20 is provided on the side surface of the support structure 6 facing the chuck table 10. The grinding feed unit 20 includes a pair of guide rails 20a that are long in the vertical direction and are fixed to the side surface of the support structure 6 facing the chuck table 10. A moving plate 20b is attached to each guide rail 20a in a slidable manner.

[0022] A nut portion 20c constituting a ball screw is provided on the surface of the movable plate 20b facing the support structure 6, and a vertically long screw shaft 20d is rotatably connected to the nut portion 20c. A motor 20e is connected to one end of the screw shaft 20d. By rotating the screw shaft 20d by the motor 20e, the movable plate 20b moves up and down (along the Z-axis in FIG. 1) along the guide rail 20a.

[0023] A grinding unit 22 is provided on the surface of the moving plate 20b opposite to the support structure 6. The grinding unit 22 includes a cylindrical holder 22a with a bottom. The side of the holder 22a is fixed to the moving plate 20b, and a cylindrical spindle housing 22b is housed inside the holder 22a. The spindle housing 22b is supported on the bottom of the holder 22a via an optional spacer 22c or the like.

[0024] A part of a cylindrical spindle 22d is accommodated inside the spindle housing 22b. A rotation drive source (not shown) such as a motor is connected to the upper end of the spindle 22d. When the rotation drive source is operated, the spindle 22d rotates around a rotation axis 22e that is substantially parallel to the axis of the spindle 22d.

[0025] The lower end of the spindle 22d is exposed to the outside from the lower end of the spindle housing 22b. The upper surface side of a disk-shaped mount 22f having circular upper and lower surfaces is fixed to the lower end of the spindle 22d. A grinding wheel 24 is attached to the lower surface of the mount 22f.

[0026] The grinding wheel 24 is provided with an annular wheel base 26 made of a metal such as stainless steel or aluminum and formed to have approximately the same diameter as the mount 22f. When the grinding wheel 24 is attached to the mount 22f, the annular upper surface of the wheel base 26 is brought into contact with the circular lower surface of the mount 22f. On the annular lower surface of the wheel base 26, a plurality of grinding stones 28 are arranged in an annular shape, each of which is made of abrasive grains such as diamond or cBN (cubic boron nitride) fixed with a binder such as vitrified or resinoid.

[0027] When grinding the workpiece 11, the positional relationship between the chuck table 10 and the grinding unit 22 is adjusted so that a part of the rotation axis 10d of the chuck table 10 (a part above the upper surface 10c) overlaps with the grinding wheels 28 of the grinding wheel 24. Therefore, when the chuck table 10 and the grinding wheel 24 are rotated together to grind the workpiece 11, the multiple grinding wheels 28 pass through the rotation axis 10d in order. Note that when the grinding unit 22 is lowered when grinding the workpiece 11, the nozzle portion 19b is disposed inside the grinding wheel 24.

[0028] The workpiece 11 is typically a disk-shaped wafer made of a semiconductor such as silicon (Si). The front surface 11a (see FIG. 3, etc.) of the workpiece 11 is partitioned into a plurality of small regions by a plurality of mutually intersecting division lines (streets), and devices such as ICs (Integrated Circuits) are formed in each small region. In this embodiment, the workpiece 11 is thinned by grinding from the back surface 11b (FIG. 2) side located opposite the front surface 11a.

[0029] In this embodiment, the workpiece 11 is a disk-shaped wafer made of a semiconductor such as silicon, but there are no limitations on the material, shape, structure, size, etc. of the workpiece 11. For example, a substrate made of other materials such as semiconductors, ceramics, resins, metals, etc. can also be used as the workpiece 11. Similarly, there are no limitations on the type, number, shape, structure, size, arrangement, etc. of devices. The workpiece 11 may not have any devices formed thereon.

[0030] In the grinding method according to this embodiment, first, the above-mentioned workpiece 11 is held by the chuck table 10 of the grinding device 2 (holding step). Specifically, for example, the front surface 11a side of the workpiece 11 is brought into contact with the upper surface 10c of the holding plate 10b, a valve is opened, and negative pressure from the suction source is applied. As a result, the front surface 11a side of the workpiece 11 is held by the chuck table 10, and the back surface 11b side is exposed upward. Note that a protective member suitable for protecting the device may be attached in advance to the front surface 11a side of the workpiece 11.

[0031] After the workpiece 11 is held by the chuck table 10, the workpiece 11 is ground under a first condition (first grinding step). Fig. 3 is a cross-sectional view showing the state in which the workpiece 11 is ground under the first condition. In Fig. 3, the back surface 11c of the workpiece 11 after being ground under the first condition is shown by a dashed line. Specifically, for example, the positional relationship between the chuck table 10 and the grinding unit 22 is adjusted so that the grindstone 28 is located above the center of the chuck table 10.

[0032] Also, the inclination of the chuck table 10 is adjusted by the inclination adjustment mechanism 16 so that the distance between the grindstone 28 and the holding plate 10b at the center of the chuck table 10 is closer than the distance between the grindstone 28 and the holding plate 10b in the outer peripheral region of the chuck table 10. That is, the relative inclination between the chuck table 10 and the grinding wheel 24 is adjusted to a first state so that the distance between the upper surface 10c of the holding plate 10b and the lower surface of the grindstone 28 is larger at other positions than at the position where the grindstone 28 overlaps with the rotation axis 10d of the chuck table 10.

[0033] The relative inclination between the chuck table 10 and the grinding wheel 24 is set arbitrarily depending on the total thickness variation (TTV) required for the workpiece 11 after grinding, etc. Here, the relative inclination between the chuck table 10 and the grinding wheel 24 is adjusted so that the angle α between the rotation axis 10d of the chuck table 10 and the rotation axis 22e of the spindle 22d is about 0° to 0.004°, preferably about 0.0034°.

[0034] Thereafter, while rotating both the chuck table 10 and the grinding wheel 24, the grinding feed unit 20 lowers the grinding wheel 24 to bring the upper surface 10c of the holding plate 10b relatively close to the grinding wheel 28. Then, the grinding wheel 28 is brought into contact with the workpiece 11 on the holding plate 10b, and the workpiece 11 is ground from the back surface 11b side. When grinding the workpiece 11, the multiple grinding wheels 28 pass through the rotation axis 10d of the chuck table 10 in sequence.

[0035] Conditions such as the speed at which the chuck table 10 is rotated, the speed at which the grinding wheel 24 is rotated, and the speed at which the grinding wheel 24 is lowered (the speed at which the upper surface 10c of the holding plate 10b is brought closer to the grindstone 28) are set arbitrarily according to the surface roughness required for the workpiece 11 after grinding (the surface roughness of the central portion of the workpiece 11). Here, the chuck table 10 is rotated at a speed of 100 rpm to 900 rpm, the grinding wheel 24 is rotated at a speed of 1000 rpm to 3000 rpm, and the grinding wheel 24 is lowered at a speed of 1.5 μm / s to 5.0 μm / s.

[0036] The relative approach between the upper surface 10c of the holding plate 10b and the grinding wheel 28 continues until the workpiece 11 has a desired thickness. FIG. 4 is a cross-sectional view showing the workpiece 11 after it has been ground under the first condition. As described above, the relative inclination between the chuck table 10 and the grinding wheel 24 is adjusted to the first state (i.e., the state in which the rotation axis 10d and the rotation axis 22e form an angle α). Therefore, the central portion of the workpiece 11 ground under the first condition including this first state is thinner than the outer periphery.

[0037] After the workpiece 11 is ground under the first conditions, the grinding wheel 28 is moved away from the workpiece 11 to stop grinding of the workpiece 11 (grinding stop step). Fig. 5 is a cross-sectional view showing a state in which the grinding wheel 28 has been moved away from the workpiece 11. Specifically, for example, the grinding wheel 24 is raised by the grinding feed unit 20 to relatively separate the upper surface 10c of the holding plate 10b from the grinding wheel 28. The distance by which the grinding wheel 24 is raised may be about 10 µm to 100 µm.

[0038] After grinding of the workpiece 11 is stopped, the workpiece 11 is ground under second conditions different from the first conditions (second grinding step). Fig. 6 is a cross-sectional view showing the state in which the workpiece 11 is ground under the second conditions. In Fig. 6, the back surface 11d of the workpiece 11 after grinding under the second conditions is shown by a dashed line.

[0039] Specifically, the inclination of the chuck table 10 is adjusted by the inclination adjustment mechanism 16 so that the distance between the grindstone 28 and the workpiece 11 at the center of the chuck table 10 is greater than the distance between the grindstone 28 and the workpiece 11 in an area on the outer periphery of the chuck table 10. In other words, the relative inclination between the chuck table 10 and the grinding wheel 24 is adjusted to a second state different from the first state so that the grindstone 28 does not come into contact with the workpiece 11 at a position overlapping with the rotation axis 10d of the chuck table 10.

[0040] The relative inclination between the chuck table 10 and the grinding wheel 24 is set arbitrarily depending on the TTV required for the workpiece 11 after grinding, etc. Here, the relative inclination between the chuck table 10 and the grinding wheel 24 is adjusted so that the angle β between the rotation axis 10d of the chuck table 10 and the rotation axis 22e of the spindle 22d is about 0.007° to 0.012°, preferably about 0.008°.

[0041] Thereafter, while rotating both the chuck table 10 and the grinding wheel 24, the grinding feed unit 20 lowers the grinding wheel 24 to bring the upper surface 10c of the holding plate 10b relatively close to the grinding wheel 28. The grinding wheel 28 is then brought into contact with the workpiece 11 on the holding plate 10b, and the workpiece 11 is ground from the back surface 11b side. Even when grinding the workpiece 11, the multiple grinding wheels 28 pass through the rotation axis 10d of the chuck table 10 in sequence.

[0042] In the so-called in-feed grinding applied to the grinding method according to the present embodiment, the outer periphery of the workpiece 11 moves faster than the central portion due to the rotation of the chuck table 10. Therefore, the volume of the outer periphery of the workpiece 11 removed by grinding is larger than the volume of the central portion. Therefore, in grinding under the above-mentioned first condition, the surface roughness of the outer periphery of the workpiece 11 becomes larger than the surface roughness of the central portion.

[0043] Therefore, in this embodiment, the outer periphery of the workpiece 11 is ground under second conditions different from the first conditions so as to reduce the difference between the surface roughness of the central portion and the surface roughness of the outer periphery of the workpiece 11. For example, any one of the speed at which the chuck table 10 is rotated, the speed at which the grinding wheel 24 is rotated, and the speed at which the grinding wheel 24 is lowered (the speed at which the upper surface 10c of the holding plate 10b is brought closer to the grindstone 28) is adjusted so as to reduce the surface roughness of the outer periphery of the workpiece 11 compared to grinding under the first conditions.

[0044] When only the rotation speed of the chuck table 10 is adjusted, the second condition is set so that the chuck table 10 is rotated slower than that in the grinding under the first condition. Specifically, the chuck table 10 is rotated at a speed that is 1 / 3 or less of the first condition, typically at a speed of 30 rpm to 300 rpm. The rotation speed of the grinding wheel 24 and the speed at which the grinding wheel 24 is lowered may be the same as those in the first condition.

[0045] When only the rotation speed of the grinding wheel 24 is adjusted, the second condition is set so that the grinding wheel 24 is rotated faster than the grinding under the first condition. Specifically, the grinding wheel 24 is rotated at a speed at least twice as fast as that under the first condition, typically at a speed of 2000 rpm to 6000 rpm. The rotation speed of the chuck table 10 and the speed at which the grinding wheel 24 is lowered may be the same as those under the first condition.

[0046] When only the speed at which the grinding wheel 24 is lowered is adjusted, the second condition is set so that the grinding wheel 24 is lowered slowly, i.e., so that the upper surface 10c of the holding plate 10b and the grindstone 28 are brought closer together slowly. Specifically, the grinding wheel 24 is lowered at a speed that is 1 / 5 or less of the first condition, typically at a speed of 0.3 μm / s to 1.0 μm / s. The speed at which the chuck table 10 is rotated and the speed at which the grinding wheel 24 is rotated may be the same as those in the first condition.

[0047] By grinding under such second conditions, for example, the surface roughness of the outer periphery of the workpiece 11 after grinding can be kept within a range of ±20% of the surface roughness of the central portion, thereby reducing the difference between the surface roughness of the central portion and the surface roughness of the outer periphery. Note that in this embodiment, only one of the speed at which the chuck table 10 is rotated, the speed at which the grinding wheel 24 is rotated, and the speed at which the grinding wheel 24 is lowered is changed from the first conditions, but multiple items can also be changed.

[0048] The upper surface 10c of the holding plate 10b and the grinding wheel 28 continue to move relatively close to each other until the workpiece 11 has a desired thickness. FIG. 7 is a cross-sectional view showing the workpiece 11 after it has been ground under the second conditions. As described above, the relative inclination between the chuck table 10 and the grinding wheel 24 is adjusted to the second state (i.e., the state in which the rotation axis 10d and the rotation axis 22e form an angle β). Therefore, the outer periphery of the workpiece 11 ground under the second conditions including this second state becomes thinner than the outer periphery of the workpiece 11 before it is ground under the second conditions.

[0049] On the other hand, the thickness of the central portion of the workpiece 11 after grinding under the second condition is 1 / 1000 of that of the workpiece 11 before grinding under the second condition. center That is, the central portion of the workpiece 11 is not ground under the second condition. This allows the surface roughness of the outer periphery of the workpiece 11 to be adjusted separately from the surface roughness of the central portion, and made smaller.

[0050] If too wide an area of ​​the workpiece 11 is machined by grinding under the second conditions, it becomes impossible to properly adjust the surface roughness of the outer periphery and the surface roughness of the central portion of the workpiece 11. Similarly, if too narrow an area of ​​the workpiece 11 is machined by grinding under the second conditions, it becomes impossible to properly adjust the surface roughness of the outer periphery and the surface roughness of the central portion of the workpiece 11.

[0051] Therefore, in grinding under the second condition, it is preferable to machine an area where the distance from the outer peripheral edge of the workpiece 11 is 1 / 3 to 2 / 3 of the radius of the workpiece 11, and it is more preferable to machine an area where the distance from the outer peripheral edge is 2 / 5 to 3 / 5 of the radius. Note that in this embodiment, by grinding under the second condition, an area where the distance from the outer peripheral edge of the workpiece 11 is about 1 / 2 of the radius of the workpiece 11 is machined.

[0052] As described above, in a grinding method called in-feed grinding in which the chuck table 10 and the grinding wheel 24 are rotated together so that the grindstone 28 passes through the rotation axis 10d of the chuck table 10, the volume of the workpiece 11 removed per unit time is larger at the outer periphery than at the center. Therefore, the surface roughness of the outer periphery of the workpiece 11 after grinding tends to be larger than the surface roughness of the center.

[0053] Therefore, in the grinding method of this embodiment, the relative inclination between the chuck table 10 and the grinding wheel 24 is adjusted to a first state, thereby grinding the workpiece 11 so that the central portion is thinned (first grinding step), and then the relative inclination between the chuck table 10 and the grinding wheel 24 is adjusted to a second state different from the first state, thereby grinding the workpiece 11 so that the central portion is not removed (second grinding step).

[0054] Furthermore, in the second grinding step, the workpiece 11 is ground under conditions (second conditions) that result in a smaller surface roughness than in the first grinding step. This allows the surface roughness of the outer periphery of the workpiece 11 to be adjusted separately from the surface roughness of the central portion, making it smaller. In other words, compared to conventional grinding methods, the difference between the surface roughness of the central portion and the surface roughness of the outer periphery of the workpiece 11 can be kept small.

[0055] The present invention is not limited to the above-described embodiment, and can be modified in various ways. For example, in the above-described embodiment, the workpiece 11 is ground under conditions that the back surface 11b of the workpiece 11 is machined into a concave shape in the first grinding step, and the workpiece 11 is ground under conditions that the back surface 11b of the workpiece 11 is machined into a convex shape in the second grinding step. However, the workpiece 11 can also be ground under conditions that the back surface 11b of the workpiece 11 is machined into a flat shape in the second grinding step.

[0056] Fig. 8 is a cross-sectional view showing the workpiece 11 after being ground under the first condition in the modified example, and Fig. 9 is a cross-sectional view showing the workpiece 11 after being ground under the second condition in the modified example. Fig. 8 shows the back surface 11e of the workpiece 11 after being ground under the first condition in the modified example, and Fig. 9 shows the back surface 11f of the workpiece 11 after being ground under the second condition in the modified example.

[0057] In the second condition according to the modified example, the angle β between the rotation axis 10d of the chuck table 10 and the rotation axis 22e of the spindle 22d is set to be substantially zero. On the other hand, in the first condition according to the modified example, the angle α between the rotation axis 10d and the rotation axis 22e is set to be larger than the angle α in the above-mentioned embodiment. Other specific conditions (first condition and second condition) may be the same as those in the above-mentioned embodiment. This modified example can also achieve a TTV and surface roughness equivalent to those of the grinding method in the above-mentioned embodiment.

[0058] It is also possible to grind the workpiece 11 under conditions in which the back surface 11b of the workpiece 11 is machined flat in the first grinding step, and grind the workpiece 11 under conditions in which the back surface 11b of the workpiece 11 is machined convexly in the second grinding step. While this condition makes it difficult to increase the TTV, it is still possible to sufficiently obtain the effect of minimizing the difference in surface roughness between the central portion and the outer periphery of the workpiece 11. Therefore, such conditions may be applied if permitted by the required quality, etc.

[0059] In addition, the structures, methods, and the like according to the above-described embodiments and modifications can be implemented with appropriate modifications without departing from the scope of the present invention. [Explanation of symbols]

[0060] 11: Workpiece 11a: Surface 11b: Back side 11c: Back side 11d: Back 11e: Back side 11f: Back 2: Grinding equipment 4: Base 6:Support structure 10: Chuck table 10a: Frame 10b: Holding plate 10c:Top surface 10d: Rotation axis 12: Table base 14: Rotation drive source 16: Tilt adjustment mechanism 16a: Fixed support part 16b: Movable support part 18: Thickness measurement unit 18a: First height measuring device 18b: Second height measuring device 19: Grinding fluid supply unit 19a: Pipe section 19b: Nozzle section 20: Grinding feed unit 20a: Guide rail 20b: Moving plate 20c: Nut part 20d: Screw shaft 20e: Motor 22: Grinding unit 22a: Holder 22b: Spindle housing 22c: Spacer 22d: Spindle 22e: Rotation axis 22f: Mount 24: Grinding wheel 26: Wheel base 28: Grindstone α :Angle β :Angle

Claims

1. A grinding method for grinding a plate-shaped workpiece held by the holding surface of the chuck table with a grinding wheel having a plurality of grinding wheels arranged in a ring shape, while rotating the chuck table and the grinding wheel together so that the plurality of grinding wheels pass through a rotation axis of the chuck table, comprising: a first grinding step of adjusting a relative inclination between the chuck table and the grinding wheel to a first state so that a distance between the grinding wheel and the holding surface is larger at other positions than at a position overlapping with the rotation axis of the chuck table, and then bringing the holding surface and the grinding wheel relatively close to each other, thereby bringing the grinding wheel into contact with the workpiece to grind the workpiece; a grinding stop step of moving the grinding wheel away from the workpiece after the first grinding step to stop grinding the workpiece; a second grinding step of adjusting, after the grinding stopping step, a relative inclination between the chuck table and the grinding wheel to a second state different from the first state so that the grinding wheel does not contact the workpiece at a position overlapping with the rotation axis of the chuck table, and then bringing the holding surface and the grinding wheel relatively close to each other to bring the grinding wheel into contact with the workpiece to grind the workpiece, In the second grinding step, the workpiece is ground under conditions that result in a smaller surface roughness than in the first grinding step, and grinding is terminated before the grinding wheel comes into contact with the workpiece at a position overlapping with the rotation axis of the chuck table.

2. 2. The grinding method according to claim 1, wherein in the second grinding step, the chuck table is rotated slower than in the first grinding step.

3. 3. The grinding method according to claim 1, wherein the grinding wheel is rotated faster in the second grinding step than in the first grinding step.

4. 4. The grinding method according to claim 1, wherein in the second grinding step, the holding surface and the grindstone are brought closer to each other slower than in the first grinding step.

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