Workpiece grinding method

Abstract

To provide a workpiece grinding method which enables a reduction in probability that the workpiece breaks when one face side of a workpiece such as a wafer or an ingot having surface roughness on one face side larger than surface roughness on the other face is made flat to be thinned to a desired thickness.SOLUTION: A workpiece grinding method includes: a holding step of holding the other face side of the workpiece on the holding surface of a chuck table such that one face side of the workpiece mis exposed; a first grinding step of grinding, after the holding step, one face side of the workpiece until one face side of the workpiece is flattened; and a second grinding step of grinding one face side of the workpiece while changing an area to be ground of the workpiece, after the first grinding step, by repeating increases and decreases in contact area of the workpiece with a plurality of grinding grindstones.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for grinding a workpiece, in which one side of the workpiece is ground by bringing a plurality of grinding wheels into contact with one side of the workpiece, the one side having a surface roughness greater than the other side. [Background technology]

[0002] Semiconductor device chips are generally manufactured using wafers made of single crystals of silicon (Si), silicon carbide (SiC), gallium nitride (GaN), lithium tantalate (LiTaO3:LT), lithium niobate (LiNbO3:LN), etc. The wafers are manufactured, for example, by separating an ingot so that a portion of the front surface side of the wafer is peeled off.

[0003] As a method for separating an ingot, a method using a laser beam with a wavelength that transmits through the material of the ingot has been proposed (see, for example, Patent Document 1). Specifically, in this method, first, the focal point of the laser beam is irradiated onto the ingot while the focal point and the ingot are moved relatively to each other so that the focal point is positioned inside the ingot.

[0004] As a result, a separation layer is formed inside the ingot, which includes a modified portion and a crack extending from the modified portion. In this method, an external force is applied to the ingot so as to further extend the crack. As a result, the ingot is separated at the separation layer, i.e., a portion of the front surface side is peeled off from the ingot, and a wafer is produced. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2013-49161 A Summary of the Invention [Problem to be solved by the invention]

[0006] When the wafer is manufactured as described above, unevenness reflecting the distribution of modified parts and cracks contained in the separation layer is formed on one side (separation layer side) of each of the ingot and the wafer. That is, the wafer and the ingot are formed such that the surface roughness of one side is greater than the surface roughness of the other side.

[0007] Therefore, the wafers thus manufactured are ground so as to be flattened on one side and thinned to a desired thickness before being used for the manufacture of chips, and the ingots remaining after the wafers are manufactured are ground so as to be flattened on one side and thinned to a desired thickness before being used for the manufacture of other wafers.

[0008] Such grinding is performed by using a grinding wheel including an annular base and a plurality of grinding stones arranged in a circularly spaced fashion on one side of the base, by bringing the plurality of grinding stones into contact with one side of the wafer or ingot while rotating both the wafer or ingot and the grinding wheel.

[0009] However, as described above, if the ingot and the wafer have unevenness on one side, the grinding wheel may be excessively worn during the period during which the grinding is performed until the one side of the ingot or the wafer is flattened. Therefore, the grinding is often performed using a grinding wheel equipped with a grinding wheel that is resistant to wear (e.g., a grinding wheel with high hardness).

[0010] On the other hand, when grinding an ingot or wafer using a grinding wheel having a grinding stone that is resistant to wear, the self-sharpening of the grinding stone may be suppressed during the period after one side of the ingot or wafer is flattened during the grinding process. In this case, a large load is applied to the ingot or wafer for a long period of time, increasing the probability of the ingot or wafer being damaged.

[0011] In view of these points, an object of the present invention is to provide a method for grinding a workpiece, such as a wafer or ingot, which has a surface roughness on one side greater than the surface roughness on the other side, and which can reduce the likelihood of the workpiece being damaged when one side of the workpiece is flattened and then thinned to a desired thickness. [Means for solving the problem]

[0012] According to the present invention, a grinding method for a workpiece is provided, which includes a grinding device including a chuck table rotatable about a rotation axis that is a straight line passing through the center of a holding surface, and a spindle having a grinding wheel attached to its tip, the grinding wheel including an annular base and a plurality of grinding wheels provided in a circularly spaced pattern on one surface of the base, and which rotates both the chuck table and the spindle while bringing the plurality of grinding wheels into contact with one surface of the workpiece, the one surface of which has a surface roughness greater than that of the other surface, thereby grinding the one surface of the workpiece. a holding step of holding the other side of the workpiece on the holding surface of the chuck table so that the one side of the workpiece is exposed; a first grinding step of grinding the one side of the workpiece until the one side of the workpiece is flattened after the holding step; and a second grinding step of grinding the one side of the workpiece while changing the area of ​​the workpiece to be ground by repeatedly increasing and decreasing the contact area between the workpiece and the multiple grinding wheels after the first grinding step.

[0013] In the second grinding step, it is preferable to repeatedly increase and decrease the contact area by changing the distance between the spindle and the center of the holding surface in a direction perpendicular to the direction along the spindle, or to repeatedly increase and decrease the contact area by changing the angle between the direction along the spindle and the direction along the rotation axis of the chuck table.

[0014] Furthermore, the method for grinding a workpiece of the present invention preferably further comprises, after the second grinding step, a final grinding step of grinding the one surface side of the workpiece so as not to change the contact area. Effect of the Invention

[0015] In the present invention, one side of the workpiece is ground until the one side is flattened, and then the one side of the workpiece is ground while changing the area of ​​the workpiece to be ground by repeatedly increasing and decreasing the contact area between the workpiece and the multiple grinding wheels. When the contact area between the workpiece and the multiple grinding wheels decreases, the load applied to the workpiece decreases.

[0016] Furthermore, by repeatedly increasing and decreasing the contact area between the workpiece and the multiple grinding wheels, the self-sharpening of the multiple grinding wheels is promoted, and the load on the workpiece is reduced. Therefore, in the present invention, it is possible to reduce the probability that the workpiece will be damaged due to a large load being applied to the workpiece over a long period of time. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a cross-sectional view that illustrates an example of a workpiece. [Diagram 2] FIG. 2 is a schematic diagram showing an example of a grinding device for grinding one surface side of the workpiece shown in FIG. [Diagram 3] FIG. 3 is a flow chart that illustrates an example of a method for grinding one surface side of the workpiece shown in FIG. 1 using the grinding apparatus shown in FIG. [Figure 4] FIG. 4 is a diagram illustrating an example of the holding step illustrated in FIG. [Diagram 5] FIG. 5 is a diagram illustrating an example of the first grinding step illustrated in FIG. [Figure 6] FIG. 6 is a diagram illustrating a first operation in the example of the second grinding step illustrated in FIG. [Figure 7]FIG. 7 is a diagram illustrating a schematic view of a second operation in the example of the second grinding step illustrated in FIG. [Figure 8] FIG. 8 is a diagram illustrating a third operation in another example of the second grinding step illustrated in FIG. [Figure 9] FIG. 9 is a diagram illustrating a fourth operation in another example of the second grinding step illustrated in FIG. [Figure 10] FIG. 10 is a flow chart that illustrates a schematic diagram of another example of a method for grinding one surface side of the workpiece shown in FIG. 1 using the grinding apparatus shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] An embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1 is a cross-sectional view showing an example of a workpiece. The workpiece 11 shown in Fig. 1 is made of a single crystal such as Si, SiC, GaN, LT, or LN.

[0019] This workpiece 11 is, for example, a wafer produced by using a laser beam to form a separation layer inside a cylindrical ingot and then separating the ingot at the separation layer, or the ingot that remains after the wafer has been produced.

[0020] Moreover, the surface roughness of one surface 11a (e.g., the separation layer side) of the workpiece 11 is greater than the surface roughness of the other surface 11b of the workpiece 11. In other words, the one surface 11a of the workpiece 11 has uneven portions 11c, whereas the other surface 11b of the workpiece 11 is generally flat.

[0021] Fig. 2 is a schematic diagram showing an example of a grinding device for grinding one surface 11a side of a workpiece 11. The grinding device 2 shown in Fig. 2 has a disk-shaped chuck table 4 having a diameter larger than that of the workpiece 11. This chuck table 4 has a frame 6 made of ceramics or the like.

[0022] The frame 6 has a disk-shaped bottom wall 6a and a cylindrical side wall 6b standing on the outer periphery of the bottom wall 6a. That is, a recess is defined by the bottom wall 6a and the side wall 6b on the upper surface side of the frame 6. A disk-shaped porous plate 8 made of porous ceramics or the like is fixed in this recess.

[0023] The diameter of the porous plate 8 is smaller than the diameter of the workpiece 11. The upper surface of the side wall 6b of the frame 6 and the upper surface of the porous plate 8 have a shape corresponding to the side surface of a cone, and function as a holding surface of the chuck table 4 when holding the other surface 11b of the workpiece 11.

[0024] In addition, a flow path 6c is formed in the bottom wall 6a, which opens at the bottom surface of the recess and penetrates the bottom wall 6a. The flow path 6c communicates with the suction source 12a via a valve 10a and with the fluid supply source 12b via a valve 10b.

[0025] The suction source 12a includes, for example, an ejector, etc. The fluid supply source 12b includes, for example, a tank for storing high-pressure gas, a filter for removing foreign matter mixed in the gas supplied from the tank, and a regulator for adjusting the pressure of the gas supplied from the tank.

[0026] Furthermore, the chuck table 4 is connected to a rotation mechanism (not shown). This rotation mechanism includes, for example, a motor and a pulley. When this rotation mechanism operates, the chuck table 4 rotates around a straight line passing through the center of the holding surface of the chuck table 4 as a rotation axis.

[0027] The chuck table 4 is supported by a tilt adjustment mechanism (not shown) via bearings (not shown) or the like. This tilt adjustment mechanism includes, for example, two movable shafts and one fixed shaft that are arranged at approximately equal angular intervals along the circumferential direction of the chuck table 4. When at least one of the two movable shafts partially raises or lowers the chuck table 4, the tilt of the rotation axis of the chuck table 4 is adjusted.

[0028] A spindle 14 extending in the vertical direction is provided above the chuck table 4. The lower end (tip) of the spindle 14 is a disk-shaped mount 16. The diameter of the mount 16 is larger than the radius of the workpiece 11.

[0029] Additionally, a plurality of holes (not shown) are formed on the outer periphery of the mount 16, penetrating the mount 16 in the thickness direction of the mount 16, and a bolt (not shown) is inserted into each hole. Additionally, a grinding wheel 18 is attached to the underside of the mount 16 using a plurality of bolts.

[0030] The grinding wheel 18 has an annular base 20 made of a metal material such as stainless steel or an aluminum alloy. The outer diameter of the base 20 is approximately equal to the diameter of the mount 16, and a plurality of grinding stones 22 are provided on the underside thereof in an annular, discrete manner.

[0031] Each of the grinding wheels 22 includes a bond material such as vitrified or resinoid, and abrasive grains such as diamond dispersed in the bond material. The abrasive grains are exposed on the lower surface of each of the grinding wheels 22, and the lower surface functions as a grinding surface when grinding the one surface 11a of the workpiece 11.

[0032] In addition, when the uneven portion 11c existing on the one surface 11a side of the workpiece 11 is ground by a plurality of grinding wheels 22, there is a risk that each grinding wheel 22 may be excessively worn. Therefore, it is preferable to use a grinding wheel that is less likely to wear as each grinding wheel 22. Specifically, it is preferable that each grinding wheel 22 contains a vitrified bond as its bonding material.

[0033] Furthermore, a motor (not shown) is connected to the base end (upper end) of the spindle 14. When this motor operates, the grinding wheel 18 rotates together with the spindle 14, with a straight line along the direction in which the spindle 14 extends (here, the vertical direction) as the rotation axis. At this time, the grinding surface of each of the multiple grinding wheels 22 describes a circular locus whose outer diameter is larger than the radius of the workpiece 11.

[0034] The spindle 14 is connected to a horizontal movement mechanism (not shown) and a vertical movement mechanism (not shown). Each of these movement mechanisms includes, for example, a motor and a ball screw. When the horizontal movement mechanism operates, the spindle 14 moves in the horizontal direction, and when the vertical movement mechanism operates, the spindle 14 moves in the vertical direction.

[0035] 3 is a flow chart showing a schematic example of a method for grinding one surface 11a of the workpiece 11 in the grinding device 2. In this method, first, the other surface 11b of the workpiece 11 is held on the holding surface of the chuck table 4 so that the one surface 11a of the workpiece 11 is exposed (holding step S1).

[0036] 4 is a diagram showing an example of the holding step S1. In this holding step S1, the workpiece 11 is placed on the holding surface of the chuck table 4 with its one surface 11a facing upward and covering the porous plate 8. Next, the suction source 12a is operated and the valve 10a is opened.

[0037] As a result, a suction force acts on the other surface 11b of the workpiece 11 through the flow passage 6c formed in the bottom wall 6a of the frame 6 of the chuck table 4 and the porous plate 8. As a result, the workpiece 11 is held on the holding surface of the chuck table 4 in a state in which the workpiece 11 is elastically deformed so as to conform to the holding surface of the chuck table 4, specifically, so that the other surface 11b has a shape corresponding to the side surface of a cone. With the above, the holding step S1 is completed.

[0038] After the holding step S1, the one surface 11a of the workpiece 11 is ground until the one surface 11a of the workpiece 11 is flattened (first grinding step S2). Flattening the one surface 11a of the workpiece 11 in the first grinding step S2 means removing the uneven portion 11c from the one surface 11a of the workpiece 11.

[0039] In other words, this means that the thickness of the entire area of ​​the workpiece 11 is set to be equal to or smaller than the distance between the most recessed point of the uneven portion 11c and the other surface 11b. Also, the thickness of the uneven portion 11c, i.e., the distance between the most protruding point and the most recessed point of the uneven portion 11c, is, for example, 10 μm or more and 50 μm or less.

[0040] 5 is a diagram showing a schematic view of an example of the first grinding step S2. In this first grinding step S2, first, the spindle 14 is moved horizontally so that the trajectory of each grinding surface of the multiple grinding wheels 22 when the spindle 14 is rotated is positioned directly above the center of the holding surface of the chuck table 4. Next, the inclination of the chuck table 4 is adjusted so that the part of the holding surface of the chuck table 4 that overlaps with the trajectory is positioned at the top.

[0041] Next, the spindle 14 is lowered while rotating both the chuck table 4 and the spindle 14. The rotation speed of the chuck table 4 in the first grinding step S2 is, for example, 1000 rpm or more and 4000 rpm or less, and the rotation speed of the spindle 14 is, for example, 40 rpm or more and 500 rpm or less. The lowering speed of the spindle 14 in the first grinding step S2 is, for example, 0.2 μm / s or more and 0.8 μm / s or less.

[0042] As a result, the grinding wheels 22 come into contact with the uneven portion 11c located on the one surface 11a side of the workpiece 11, and the most protruding portion of the uneven portion 11c is ground. Then, this grinding, that is, the lowering of the spindle 14 while rotating both the chuck table 4 and the spindle 14, continues until the uneven portion 11c is removed. With the above, the first grinding step S2 is completed.

[0043] After the first grinding step S2, the one surface 11a side of the workpiece 11 is ground while changing the area of ​​the workpiece 11 to be ground by repeatedly increasing and decreasing the contact area between the workpiece 11 and the multiple grinding wheels 22 (second grinding step S3).

[0044] In this second grinding step S3, for example, the spindle 14 is lowered and swung while rotating both the chuck table 4 and the spindle 14. Note that the swinging of the spindle 14 means, for example, the repetition of a first operation for reducing the contact area between the workpiece 11 and the multiple grinding wheels 22 and a second operation for increasing the contact area.

[0045] 6 is a diagram showing a schematic view of the first operation. Specifically, the first operation means the movement of the spindle 14 toward a predetermined direction perpendicular to the direction in which the spindle 14 extends (here, the vertical direction). Then, this first operation is continued, for example, until the distance between the spindle 14 and the center of the holding surface of the chuck table 4 in the predetermined direction increases and the grinding surfaces of the multiple grinding wheels 22 are separated from the center of one surface 11a of the workpiece 11.

[0046] 7 is a diagram showing the second operation. Specifically, the second operation means the movement of the spindle 14 in the direction opposite to the predetermined direction. This second operation is continued until, for example, the distance between the spindle 14 and the center of the holding surface of the chuck table 4 in the predetermined direction decreases and the grinding surfaces of the grinding wheels 22 and the center of the surface 11a of the workpiece 11 come into contact with each other again.

[0047] In the second grinding step S3, the rotation speed of the chuck table 4 is, for example, 1000 rpm or more and 4000 rpm or less, and the rotation speed of the spindle 14 is, for example, 40 rpm or more and 500 rpm or less. In addition, the descending speed of the spindle 14 in the second grinding step S3 is, for example, 0.2 μm / s or more and 0.8 μm / s or less.

[0048] The moving speed of the spindle 14 along the predetermined direction is, for example, 50 μm / s or more and 5000 μm / s or less. The switching between the first operation and the second operation in the second grinding step S3 is performed at a timing when the moving distance of the spindle 14 along the predetermined direction in each of the first operation and the second operation becomes a predetermined distance of 5 mm or more and 50 mm or less.

[0049] This alternately repeats grinding of the area other than near the center of one side 11a of the workpiece 11, i.e., grinding involving a first operation, so as to leave a protrusion 11d having a shape equivalent to the side of a cone in the center of one side 11a of the workpiece 11, and grinding of the entire area of ​​one side 11a of the workpiece 11, i.e., grinding involving a second operation, so as to flatten the one side 11a of the workpiece 11.

[0050] The alternating repetition of grinding with the first operation and grinding with the second operation is continued until the workpiece 11 reaches a predetermined finishing thickness. In this case, in the second grinding step S3, grinding with the second operation is finally performed to flatten the one surface 11a side of the finally obtained workpiece 11.

[0051] 3, the first surface 11a of the workpiece 11 is ground until the first surface 11a of the workpiece 11 is flattened, and then the first surface 11a of the workpiece 11 is ground while changing the area of ​​the workpiece 11 to be ground by repeatedly increasing and decreasing the contact area between the workpiece 11 and the multiple grinding wheels 22. When the contact area between the workpiece 11 and the multiple grinding wheels 22 decreases, the load applied to the workpiece 11 decreases.

[0052] Furthermore, by repeatedly increasing and decreasing the contact area between the workpiece 11 and the multiple grinding wheels 22, the self-sharpening of each of the multiple grinding wheels 22 is promoted, reducing the load applied to the workpiece 11. Therefore, in the method for grinding the workpiece shown in Fig. 3, it is possible to reduce the probability that the workpiece 11 will be damaged due to a large load being applied to the workpiece 11 over a long period of time.

[0053] The above is one embodiment of the present invention, and the present invention is not limited to the above. For example, the present invention may be implemented in a grinding apparatus provided with a horizontal movement mechanism and / or a vertical movement mechanism for moving the chuck table 4 instead of or in addition to the horizontal movement mechanism and / or the vertical movement mechanism for moving the spindle 14.

[0054] Similarly, the present invention may be implemented in a grinding apparatus provided with a tilt adjustment mechanism for adjusting the tilt of the spindle 14 instead of or in addition to the tilt adjustment mechanism for adjusting the tilt of the chuck table 4. That is, in the present invention, it is only necessary that the chuck table 4 and the spindle 14 can be moved relatively to each other, and there is no limitation on the structure for this purpose.

[0055] In the present invention, for example, in the second grinding step S3, grinding involving the above-mentioned first operation and grinding involving the second operation may be alternately repeated by oscillating the chuck table 4 instead of or in addition to oscillating the spindle 14.

[0056] Furthermore, in the second grinding step S3 of the present invention, it is sufficient to grind one surface 11a of the workpiece 11 while changing the area of ​​the workpiece 11 to be ground by repeatedly increasing and decreasing the contact area between the workpiece 11 and the multiple grinding wheels 22, and there is no limitation on the operation for this purpose.

[0057] For example, in the second grinding step S3 of the present invention, the contact area between the workpiece 11 and the multiple grinding wheels 22 may be repeatedly increased and decreased by changing the angle between the direction along the spindle 14 and the direction along the rotation axis of the chuck table 4.

[0058] Specifically, in the second grinding step S3 of the present invention, while both the chuck table 4 and the spindle 14 are rotating, the spindle 14 may be lowered and the chuck table 4 may be rocked. Note that rocking the chuck table 4 means, for example, repetition of a third operation for reducing the contact area between the workpiece 11 and the multiple grinding wheels 22 and a fourth operation for increasing the contact area.

[0059] 8 is a diagram showing a schematic view of the third operation. Specifically, the third operation means adjusting the inclination of the chuck table 4 so as to separate the outer periphery of the workpiece 11 from the grinding wheels 22. This third operation is continued, for example, until at least the region other than the center of the one surface 11a of the workpiece 11 is separated from each grinding surface of the grinding wheels 22.

[0060] 9 is a diagram showing a schematic view of the fourth operation. Specifically, the fourth operation means adjusting the inclination of the chuck table 4 so that the outer periphery of the workpiece 11 comes into contact with the grinding surfaces of the grinding wheels 22. This fourth operation is continued, for example, until the entire area from the center to the outer periphery of the one surface 11a of the workpiece 11 comes into contact with the grinding surfaces of the grinding wheels 22.

[0061] This alternately repeats grinding of the area near the center of one side 11a of the workpiece 11 so as to make the one side 11a of the workpiece 11 into a shape equivalent to an inverted truncated cone, i.e., grinding involving the third operation, and grinding of the entire area of ​​one side 11a of the workpiece 11 so as to flatten the one side 11a of the workpiece 11, i.e., grinding involving the fourth operation.

[0062] The alternating repetition of grinding with the third operation and grinding with the fourth operation is continued until the workpiece 11 reaches a predetermined finishing thickness. In this case, in the second grinding step S3, grinding with the fourth operation is finally performed to flatten the one surface 11a side of the finally obtained workpiece 11.

[0063] In addition, in the present invention, for example, in the second grinding step S3, grinding involving the above-mentioned third operation and grinding involving the fourth operation may be alternately repeated by swinging the spindle 14 instead of or in addition to swinging the chuck table 4.

[0064] In the present invention, the workpiece 11 may be ground to the finish thickness by further grinding the one surface 11a side of the workpiece 11 thereafter, without grinding the workpiece 11 to the finish thickness in the second grinding step S3. Fig. 10 is a flow chart showing a schematic example of a grinding method for a workpiece ground in this manner.

[0065] In this method, after carrying out the holding step S1, the first grinding step S2, and the second grinding step S3 as described above, the one surface 11a side of the workpiece 11 is ground so as not to change the contact area between the workpiece 11 and the multiple grinding wheels 22 (final grinding step S4). This final grinding step S4 is carried out, for example, by operating the components of the grinding device 2 in the same manner as in the first grinding step S2.

[0066] In the final grinding step S4, the rotation speed of the chuck table 4 is, for example, 1000 rpm or more and 4000 rpm or less, and the rotation speed of the spindle 14 is, for example, 40 rpm or more and 500 rpm or less. In addition, the descending speed of the spindle 14 in the final grinding step S4 is, for example, 0.2 μm / s or more and 0.8 μm / s or less.

[0067] In the grinding method of the workpiece shown in FIG. 10, it is possible to more easily flatten one surface 11a of the workpiece 11 having a finishing thickness, compared to the grinding method of the workpiece shown in FIG. 3.

[0068] In addition, the structures and methods according to the above-described embodiments can be modified as appropriate without departing from the scope of the present invention. [Explanation of symbols]

[0069] 2: Grinding equipment 4: Chuck table 6: Frame (6a: bottom wall, 6b: side wall, 6c: flow path) 8: Porous plate 10a: Valve 10b: Valve 11: Workpiece (11a: one side, 11b: other side, 11c: uneven part, 11d: protruding part) 12a: Suction source 12b: Fluid supply source 14: Spindle 16: Mount 18: Grinding wheel 20: Foundation 22: Grinding wheel

Claims

1. A grinding method for a workpiece, comprising: a chuck table rotatable about a rotation axis that is a straight line passing through the center of a holding surface; and a spindle having a grinding wheel attached to a tip thereof, the grinding wheel including an annular base and a plurality of grinding wheels disposed in a circularly spaced pattern on one surface of the base, the method comprising the steps of: rotating both the chuck table and the spindle while bringing the plurality of grinding wheels into contact with one surface of the workpiece, the one surface of the workpiece having a surface roughness greater than that of the other surface of the workpiece, the method comprising the steps of: a holding step of holding the other side of the workpiece on the holding surface of the chuck table so that the one side of the workpiece is exposed; a first grinding step of grinding the one side of the workpiece until the one side of the workpiece is flattened after the holding step; a second grinding step of grinding the one surface side of the workpiece while changing the area to be ground of the workpiece by repeatedly increasing and decreasing the contact area between the workpiece and the plurality of grinding wheels after the first grinding step; A method for grinding a workpiece comprising:

2. 2. The method for grinding a workpiece according to claim 1, wherein in the second grinding step, the contact area is repeatedly increased and decreased by changing the distance between the spindle and the center of the holding surface in a direction perpendicular to the direction along the spindle.

3. 2. The method for grinding a workpiece according to claim 1, wherein in the second grinding step, the contact area is repeatedly increased and decreased by changing the angle between the direction along the spindle and the direction along the rotation axis of the chuck table.

4. 4. The method for grinding a workpiece according to claim 1, further comprising a final grinding step, after the second grinding step, of grinding the one surface side of the workpiece so as not to change the contact area.

Images