Wafer grinding method

Abstract

To provide a wafer grinding method that can reduce the likelihood of processing defects such as chipping or cracks occurring when forming a recess having a circular bottom on the back side of a wafer.SOLUTION: After forming a first recess having a bottom shaped to correspond to the side of an inverted cone on the back side of a wafer and before forming a second recess having a circular bottom on the back side of the wafer, a chuck table and a spindle are separated to separate the multiple grinding wheels from the back side of the wafer, and then the inclination of at least one of the chuck table or spindle is adjusted to change the angle between the rotation axis of the chuck table and the rotation axis of the spindle.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a wafer grinding method for grinding a wafer. [Background technology]

[0002] Device chips such as ICs (Integrated Circuits) are essential components in various electronic devices such as mobile phones, personal computers, etc. Such chips are manufactured, for example, by dividing a wafer on which multiple devices are formed on the front surface into regions each including an individual device.

[0003] The wafer may be thinned prior to division in order to produce smaller chips. The wafer may be thinned, for example, by grinding the back side of the wafer in a grinding device. The grinding device generally includes a chuck table having a holding surface and rotatable about a rotation axis that is a straight line passing through the center of the holding surface, and a spindle provided above the chuck table and extending in the vertical direction.

[0004] A grinding wheel having a plurality of grinding stones arranged in a circular pattern is attached to the lower end (tip) of the spindle. The grinding wheel is designed so that the outer diameter of the locus of the lower surface (grinding surface) of each of the plurality of grinding stones when the spindle is rotated is longer than the radius of the wafer.

[0005] In this grinding device, the wafer is thinned, for example, in the following order: First, the wafer is held on the holding surface of the chuck table so that the back side of the wafer is exposed, and then the relative positions of the chuck table and the spindle are adjusted so that the center of the wafer is positioned directly below the trajectory of the grinding surfaces of the multiple grinding wheels when the spindle is rotated.

[0006] Next, while rotating both the chuck table and the spindle, the spindle is lowered so that the grinding wheels come into contact with the backside of the wafer. This starts grinding the wafer. Furthermore, while still rotating both the chuck table and the spindle, the spindle is lowered until the wafer has a desired thickness. This causes the entire backside of the wafer to be ground, thinning the wafer.

[0007] Here, if the entire wafer is thinned, the rigidity of the wafer decreases, which may make it difficult to handle the wafer in the subsequent steps. Therefore, a method for grinding the back side of the wafer by a method also called TAIKO grinding has been proposed (for example, see Patent Document 1).

[0008] In short, in this method, a grinding wheel, in which the outer diameter of the grinding surface of each of the grinding wheels when the spindle is rotated is shorter than the radius of the wafer, is attached to the tip of the spindle, and the back side of the wafer is ground as described above. In this case, the back side (upper surface) of the central region of the wafer is ground, and the region surrounding this central region (peripheral excess region) remains as a ring-shaped reinforcing portion.

[0009] As a result, a recess having a circular bottom surface is formed on the back side of the wafer. However, in this case, the outer surfaces of the multiple grinding wheels contact the side surface of the recess (i.e., the inner surface of the ring-shaped reinforcement part) from when grinding of the wafer starts until a recess having a predetermined shape is formed on the back side of the wafer. Therefore, in this case, chipping may occur in the area near the inner surface of the ring-shaped reinforcement part due to contact with the multiple grinding wheels.

[0010] In consideration of this, a method of performing TAIKO grinding has been proposed that shortens the time that the outer surfaces of the grinding wheels are in contact with the inner surface of the ring-shaped reinforcing part (see, for example, Patent Document 2). Specifically, in this method, first, the spindle is tilted slightly from the vertical direction so that the part of the trajectory of the grinding surfaces of the grinding wheels that is located directly above the center of the workpiece when the spindle is rotated is positioned at the lowest point.

[0011] Next, TAIKO grinding (first grinding step) is performed with the spindle tilted in this manner, whereby a first recess having a bottom shaped to correspond to the side of the inverted cone is formed on the back side of the wafer without the outer side of each of the multiple grinding wheels coming into contact with the inner side of the ring-shaped reinforcing part.

[0012] Next, the spindle is returned to its original state, that is, the state extending along the vertical direction, so as to continue from the first grinding step, and TAIKO grinding (second grinding step) is performed, whereby a second recess having a circular bottom surface is formed on the back side of the wafer.

[0013] As a result, this method makes it possible to produce wafers with a uniform thickness in their central region while suppressing the occurrence of chipping in the region near the inner surface of the ring-shaped reinforcement portion due to contact with multiple grinding wheels. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] JP 2007-19461 A [Patent Document 2] JP 2021-150347 A Summary of the Invention [Problem to be solved by the invention]

[0015] When the second grinding step is performed consecutively from the first grinding step, the corners of the first recess, i.e., the vicinity of the outer edge of the bottom, tend to be subjected to a large load, which may cause cracks to form in the corners of the first recess, resulting in damage to the wafer.

[0016] In view of this, an object of the present invention is to provide a wafer grinding method capable of reducing the likelihood of processing defects such as chipping or cracks occurring when forming a recess having a circular bottom on the back side of a wafer. [Means for solving the problem]

[0017] According to the present invention, in a grinding apparatus including a chuck table having a holding surface and rotatable around a first rotation axis passing through the center of the holding surface, and a spindle having a grinding wheel attached to its tip, on which a plurality of grinding wheels are provided in a circularly spaced arrangement, and rotatable around a second rotation axis, there is provided a method for grinding a wafer having a plurality of devices formed on its front surface side, the radius of which is longer than the outer diameter of a path of a grinding surface of each of the plurality of grinding wheels when the spindle is rotated, the method comprising: a holding step of holding the wafer on the holding surface of the chuck table so that the back surface side of the wafer is exposed; a first grinding step of grinding the back surface side of the wafer by rotating both the chuck table and the spindle after the holding step, and bringing the chuck table and the spindle closer to each other in a predetermined direction so that the plurality of grinding wheels come into contact with the back surface side of the wafer while the center of the wafer and the path overlap in the predetermined direction; a separating step of separating the chuck table and the spindle in the predetermined direction after the step of separating the plurality of grinding wheels from the back side of the wafer; an adjusting step of adjusting an inclination of at least one of the chuck table or the spindle to change an angle between the first rotation axis and the second rotation axis after the separating step; and a second grinding step of grinding the back side of the wafer after the adjusting step by rotating both the chuck table and the spindle and bringing the chuck table and the spindle closer to each other in the predetermined direction so that the plurality of grinding wheels come into contact with the back side of the wafer with the center and the trajectory of the wafer overlapping in the predetermined direction, wherein in the first grinding step, a first recess having a bottom shaped corresponding to a side surface of an inverted cone is formed on the back side of the wafer, and in the second grinding step, a second recess having a circular bottom is formed on the back side of the wafer.

[0018] The wafer grinding method of the present invention preferably further comprises a cleaning step of cleaning the bottom of the first recess after the separation step and before the second grinding step. Also, the wafer grinding method of the present invention preferably further comprises a position adjustment step of moving the chuck table and the spindle closer to or farther away from each other in a direction perpendicular to the predetermined direction after the separation step and before the second grinding step. Effect of the Invention

[0019] In the present invention, after the first grinding step and before the second grinding step, the chuck table and the spindle are separated so as to separate the multiple grinding wheels from the back side of the wafer, and then the inclination of at least one of the chuck table or the spindle is adjusted so as to change the angle between the rotation axis of the chuck table (first rotation axis) and the rotation axis of the spindle (second rotation axis).

[0020] That is, in the present invention, the inclination of at least one of the chuck table and the spindle is not adjusted while the grinding wheels are in contact with the back side of the wafer. Therefore, in the present invention, the angle between the rotation axis of the chuck table and the rotation axis of the spindle is changed without applying a load to the vicinity of the outer edge of the bottom of the first recess. As a result, in the present invention, it is possible to reduce the probability of processing defects such as chipping or cracks occurring when forming the second recess having a circular bottom on the back side of the wafer. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 is a partially sectional side view that illustrates an example of a grinding device. [Diagram 2] FIG. 2 is a partially sectional side view that typically shows a spindle, a wheel mount, and a grinding wheel included in the grinding device. [Diagram 3] FIG. 3 is a perspective view that illustrates an example of a wafer to be ground in the grinding apparatus. [Figure 4]FIG. 4 is a flow chart that illustrates a schematic example of a wafer grinding method for grinding a wafer in a grinding apparatus. [Diagram 5] FIG. 5(A) is a cross-sectional view that typically shows the state of the holding step, and FIG. 5(B) is a side view that typically shows the state of the first adjusting step. [Figure 6] FIG. 6(A) is a partially sectional side view showing a schematic diagram of the first grinding step, and FIG. 6(B) is a partially sectional side view showing a schematic diagram of the separation step. [Figure 7] FIG. 7(A) is a side view that shows a schematic view of the second adjusting step, and FIG. 7(B) is a partial cross-sectional side view that shows a schematic view of the second grinding step. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] An embodiment of the present invention will be described with reference to the accompanying drawings. Fig. 1 is a partially sectional side view showing a schematic diagram of an example of a grinding device. Note that the X-axis direction and the Y-axis direction shown in Fig. 1 are directions perpendicular to each other on a horizontal plane, and the Z-axis direction is a direction (vertical direction) perpendicular to the X-axis direction and the Y-axis direction.

[0023] The grinding device 2 shown in Fig. 1 has a base 4 that supports each of the components. An X-axis direction moving mechanism 6 is provided on the upper surface of the base 4. This X-axis direction moving mechanism 6 has a pair of guide rails 8 each extending along the X-axis direction. A rectangular parallelepiped X-axis direction moving plate 10 is attached to the top of the pair of guide rails 8 in a manner that allows it to slide along the X-axis direction.

[0024] A screw shaft 12 extending along the X-axis direction is disposed between the pair of guide rails 8. A stepping motor 14 for rotating the screw shaft 12 is connected to one end of the screw shaft 12. A nut 16 that houses a large number of balls that circulate in response to the rotation of the screw shaft 12 is provided on the outer circumferential surface of the screw shaft 12 on which the threads are formed, thereby forming a ball screw.

[0025] Furthermore, the nut 16 is fixed to the underside of the X-axis direction moving plate 10. Therefore, when the screw shaft 12 is rotated by the stepping motor 14, the X-axis direction moving plate 10 moves along the X-axis direction together with the nut 16. Furthermore, a disk-shaped chuck table 18 that moves along the X-axis direction together with the X-axis direction moving plate 10 is provided on the X-axis direction moving plate 10.

[0026] The chuck table 18 has a disk-shaped frame 20 made of ceramics. A disk-shaped recess having a predetermined depth is formed on the upper surface of the frame 20. A flow path (not shown) that opens at the bottom surface of the recess is formed inside the frame 20. This flow path is also connected to a suction source (not shown) such as an ejector.

[0027] Furthermore, a circular porous plate 22 having a diameter roughly equal to that of the recess formed on the upper surface of the frame 20 is fixed to the recess. The porous plate 22 is made of, for example, porous ceramics. The porous plate 22 also has an upper surface that is shaped to correspond to the side surface of a cone (the center of the porous plate protrudes beyond the outer periphery).

[0028] When a suction source communicating with a flow path formed inside the frame 20 is operated, a suction force acts on the space near the upper surface of the porous plate 22. Therefore, the upper surface of the porous plate 22 and the upper surface of the frame 20 which is flush with the upper surface of the porous plate 22 function as a holding surface of the chuck table 18 for holding a wafer.

[0029] Further, a rotation mechanism (not shown) for rotating the chuck table 18 is provided below the chuck table 18. This rotation mechanism includes, for example, a pulley and a servo motor. When this rotation mechanism is operated, the chuck table 18 rotates about a straight line passing through the center of the holding surface as a rotation axis (first rotation axis).

[0030] In addition, a cylindrical bearing 24 and a cylindrical support plate 26 that rotatably support the chuck table 18 are provided below the chuck table 18. The chuck table 18 is rotatably connected to a chuck table side adjustment mechanism 28 via the bearing 24 and the support plate 26.

[0031] The chuck table side adjustment mechanism 28 has two movable shafts 28a, 28b and one fixed shaft 28c that are arranged at approximately equal angular intervals along the circumferential direction of the chuck table 18 and have their lower ends fixed to the upper surface side of the X-axis direction moving plate 10. When at least one of the two movable shafts 28a, 28b partially raises and lowers the chuck table 18, the inclination of the rotation axis of the chuck table 18 is adjusted.

[0032] In addition, for example, in the initial state, the chuck table side adjustment mechanism 28 is connected to the chuck table 18 in a state in which the line segment connecting a point on the outer periphery of the holding surface of the chuck table 18 that is located in the X-axis direction when viewed from the center in a plan view and the center of the holding surface is parallel to the X-axis direction.

[0033] A wall 30 extending along the Z-axis direction is provided on an end of the base 4. A Z-axis direction moving mechanism 32 is provided on the surface of the wall 30 facing the chuck table 18. The Z-axis direction moving mechanism 32 has a pair of guide rails 34 each extending along the Z-axis direction. A rectangular parallelepiped Z-axis direction moving plate 36 is attached to the surface of the pair of guide rails 34 in a manner that allows it to slide along the Z-axis direction.

[0034] A screw shaft 38 extending along the Z-axis direction is disposed between the pair of guide rails 34. A stepping motor 40 for rotating the screw shaft 38 is coupled to one end (upper end) of the screw shaft 38. A nut 42 that houses a large number of balls that circulate in response to the rotation of the screw shaft 38 is provided on the outer circumferential surface of the screw shaft 38 on which the threads are formed, thereby forming a ball screw.

[0035] The nut 42 is fixed to the back surface side of the Z-axis direction moving plate 36. Therefore, when the stepping motor 40 rotates the screw shaft 38, the Z-axis direction moving plate 36 moves along the Z-axis together with the nut 42. A cylindrical cover 44 with a bottom is fixed to the front surface side of the Z-axis direction moving plate 36. A through hole 44a is formed in the center of the bottom of the cover 44, through which a spindle 52, which will be described later, passes.

[0036] A housing 46 is provided inside the cover 44. The housing 46 is supported on the bottom of the cover 44 via two spacers 48a and 48b. The spacer 48a is located in the X-axis direction as viewed from the spindle 52, and the spacer 48b is located in the opposite direction to the X-axis direction as viewed from the spindle 52.

[0037] The housing 46 is also connected to a spindle-side adjustment mechanism 50. The spindle-side adjustment mechanism 50 has a bolt 50a including a shaft portion screwed into a female thread formed to penetrate a portion of the bottom of the cover 44 located below the spacer 48a and a screw hole formed in the underside of the spacer 48a, and a head portion located below the bottom of the cover 44.

[0038] A stepping motor (not shown) for rotating the bolt 50a is connected to the bolt 50a. The spacer 48a can be raised by rotating the bolt 50a, for example, clockwise, with the stepping motor. Specifically, in this case, the spacer 48a can be raised until the head of the bolt 50a comes into contact with the bottom of the cover 44 after the spacer 48a is separated from the bottom of the cover 44.

[0039] Also, by rotating the bolt 50a, for example, counterclockwise, using the stepping motor, it is possible to lower the spacer 48a. Specifically, in this case, it is possible to lower the spacer 48a until the spacer 48a comes into contact with the bottom of the cover 44 after the head of the bolt 50a has been separated from the bottom of the cover 44. Note that, for example, in the initial state, the spindle-side adjustment mechanism 50 is connected to the housing 46 with the spacer 48a in contact with the bottom of the cover 44 and the head of the bolt 50a separated from the bottom of the cover 44.

[0040] Furthermore, in the initial state, for example, a part of a spindle 52 extending along the Z-axis direction is rotatably accommodated in the housing 46. The housing 46 also accommodates a servo motor (not shown) connected to a base end (upper end) of the spindle 52. The spindle 52 protrudes downward through a through hole 44a formed in the bottom of the cover 44, and a disk-shaped wheel mount 54 is provided at a tip end (lower end) of the spindle 52.

[0041] The wheel mount 54 is made of a metal material such as stainless steel and has a generally flat lower surface. A plurality of through holes are formed in the outer periphery of the wheel mount 54 at generally equal angular intervals along the circumferential direction of the wheel mount 54. A grinding wheel 56 is attached to the wheel mount 54 by using fasteners (not shown) such as bolts that are inserted into each of the through holes.

[0042] The grinding wheel 56 includes an annular wheel base 58 made of a metal material such as stainless steel. A plurality of grinding stones 60 are arranged in a circular pattern on the lower surface of the wheel base 58. Each of the grinding stones 60 includes a binder such as vitrified or resinoid, and abrasive grains such as diamond dispersed in the binder. The abrasive grains are exposed on the lower surface of each of the grinding stones 60, and the lower surface functions as a grinding surface for grinding the wafer.

[0043] When the servo motor connected to the base end of the spindle 52 is operated, the grinding wheel 56 rotates together with the spindle 52 around a straight line along the extension direction of the spindle 52 as a rotation axis (second rotation axis). Note that the outer diameter of the trajectory of the grinding surface of each of the multiple grinding wheels 60 when the spindle 52 is rotated is, for example, slightly shorter than the diameter of the porous plate 22 of the chuck table 18.

[0044] Furthermore, when the spindle-side adjustment mechanism 50 is operated to raise the spacer 48a, the inclinations of the spindle 52, the wheel mount 54, and the grinding wheel 56 are adjusted so that the grinding wheels 60 farther from the wall 30 are positioned lower than the grinding wheels 60 closer to the wall 30. That is, when the spindle-side adjustment mechanism 50 is operated in this manner, the inclination of the rotation axis of the spindle 52 is adjusted. Note that the rotation axis of the spindle 52 is parallel to the Z-axis direction in the initial state, for example, that is, when the spacer 48a is in contact with the bottom of the cover 44.

[0045] 2 is a partial cross-sectional side view showing the spindle 52, the wheel mount 54, and the grinding wheel 56. The wheel mount 54 has a base portion 62 having a flow passage 62a formed in the center thereof, the flow passage 62a communicating with the flow passage 52a formed in the spindle 52, and a bottomed cylindrical hanging portion 64 provided to hang down and surround the flow passage 62a. The hanging portion 64 has a plurality of through holes 64a formed therein, which extend in a direction perpendicular to the Z-axis direction and communicate with the flow passage 62a formed in the base portion 62.

[0046] Furthermore, the multiple through holes 64a formed in the hanging portion 64 communicate with a liquid supply source (not shown), such as a pump, that supplies liquid such as water, via a flow path 62a formed in the base portion 62 and a flow path 52a formed in the spindle 52. When this liquid supply source is operated, liquid is supplied radially toward the space below and inside the grinding wheel 56 via the flow paths 52a, 52a, and the multiple through holes 64a.

[0047] Fig. 3 is a perspective view showing a schematic example of a wafer ground by the grinding apparatus 2. The wafer 11 shown in Fig. 3 is made of a semiconductor material such as silicon. The radius of the wafer 11 is longer than the outer diameter of the trajectory of the grinding surfaces of the grinding wheels 60 when the spindle 52 is rotated, and is shorter than the radius of the holding surface of the chuck table 18.

[0048] The front surface 11a of the wafer 11 is divided into a plurality of regions by a plurality of mutually intersecting division lines (streets) 13, and a plurality of devices 15 such as ICs are formed in each of the regions. The plurality of devices 15 are arranged, for example, in a matrix in a circular central region 17a of the wafer 11, and are not arranged in an annular peripheral excess region 17b surrounding the central region 17a.

[0049] The radius of the central region 17a of the wafer 11 is approximately equal to the outer diameter of the trajectory of the grinding surface of each of the multiple grinding wheels 60 when the spindle 52 is rotated. In addition, the boundary between the central region 17a and the outer peripheral surplus region 17b is shown by a dashed line in Fig. 3, but this dashed line is an imaginary line and does not exist on the actual wafer 11. In addition, for the sake of convenience, the wafer 11 is shown with a squared outer edge in Fig. 3, but the outer edge of the actual wafer 11 is often chamfered.

[0050] Furthermore, a disk-shaped protective tape 19 is attached to the front surface 11a of the wafer 11 to prevent damage to the multiple devices 15 when the back surface 11b is ground. This protective tape 19 has a diameter roughly equal to that of the wafer 11, and is composed of, for example, a flexible film-like base layer and an adhesive layer (glue layer) provided on the front surface (surface on the wafer 11 side) of the base layer.

[0051] Specifically, the base layer is made of polyolefin (PO), polypropylene (PP), polyethylene terephthalate (PET), polyvinyl chloride (PVC), polystyrene (PS), etc. The adhesive layer is made of ultraviolet-curable silicone rubber, acrylic material, epoxy material, etc.

[0052] Fig. 4 is a flow chart that shows a schematic example of a method for grinding the wafer 11 in the grinding apparatus 2. In this method, the wafer 11 is first held on the holding surface of the chuck table 18 so that the back surface 11b of the wafer 11 is exposed (holding step S1). Fig. 5(A) is a cross-sectional view that shows a schematic view of the holding step S1.

[0053] In this holding step S1, the wafer 11 is placed on the holding surface of the chuck table 18 via the protective tape 19 so that the center of the holding surface of the chuck table 18 coincides with the center of the front surface of the protective tape 19. Then, a suction source communicating with a flow path formed inside the frame 20 of the chuck table 18 is operated. As a result, the wafer 11 is held on the holding surface of the chuck table 18 with the back surface 11b side of the wafer 11 exposed.

[0054] After the holding step S1, the inclination of the spindle 52 is adjusted (first adjustment step S2) to form a first recess having a bottom shaped to correspond to the side of an inverted cone on the back surface 11b of the wafer 11 in a first grinding step S3 described later. Fig. 5(B) is a side view showing the first adjustment step S2.

[0055] In this first adjustment step S2, the spindle-side adjustment mechanism 50 raises the spacer 48a so that the angle θ between the rotation axis A of the spindle 52 and a straight line L parallel to the Z-axis direction becomes, for example, more than 0° and less than 2°. As a result, the grinding surface of the grinding wheel 60 farthest from the wall 30 is positioned lower than the grinding surface of the grinding wheel 60 closer to the wall 30.

[0056] After the first adjustment step S2, the back surface 11b of the wafer 11 is ground so as to form a first recess on the back surface 11b of the wafer 11 (first grinding step S3). Fig. 6(A) is a partially sectional side view showing the first grinding step S3.

[0057] In this first grinding step S3, first, the chuck table 18 is moved along the X-axis direction so that the center of the holding surface of the chuck table 18 is positioned directly below the lowest portion of the trajectory of the grinding surfaces of each of the multiple grinding wheels 60 when the spindle 52 is rotated.

[0058] Next, both the chuck table 18 and the spindle 52 are rotated, and liquid (not shown) is supplied to the space below the inner side of the grinding wheel 56 via the flow passage 52a, the flow passage 52a, and the plurality of through holes 64a, while the spindle 52 is lowered so that the plurality of grinding stones 60 contact the back surface 11b side of the wafer 11. This starts grinding of the wafer 11.

[0059] Furthermore, both the chuck table 18 and the spindle 52 are rotated, and the spindle 52 is lowered while liquid is being supplied to the back surface 11b side of the wafer 11 until the thickness of the center of the wafer 11 reaches a predetermined thickness. Note that this predetermined thickness is set, for example, to be equal to or slightly thicker than the thickness of the central region 17a of the wafer 11 after a second grinding step S6 described later.

[0060] As a result, a first recess 21 having a bottom 21a shaped to correspond to the side of the inverted cone is formed on the back surface 11b of the wafer 11. Note that grinding debris is generated on the back surface 11b of the wafer 11 as the wafer 11 is ground. Most of the grinding debris is washed away by the liquid supplied to the back surface 11b of the wafer 11.

[0061] In addition, in the first grinding step S3, since the rotation axis A of the spindle 52 is inclined as described above, the outer side surfaces of the grinding wheels 60 do not contact the side surface 21b of the first recess 21. Therefore, in the second grinding step S6, the occurrence of chipping due to contact with the grinding wheels 60 is suppressed.

[0062] Next, the spindle 52 is raised so as to separate the grinding wheels 60 from the back surface 11b of the wafer 11 (separation step S4). Fig. 6(B) is a partially sectional side view that illustrates the separation step S4.

[0063] In this separation step S4, the spindle 52 is raised until the grinding wheels 60 are positioned at least above the outer peripheral surplus region 17b of the wafer 11. After the spindle 52 is raised in this manner, the rotation of both the chuck table 18 and the spindle 52 may or may not be stopped.

[0064] After the separation step S4, the inclination of the spindle 52 is adjusted (second adjustment step S5) to form a second recess having a circular bottom surface on the back surface 11b of the wafer 11 in a second grinding step S6 described later. Fig. 7(A) is a side view showing the second adjustment step S5.

[0065] In this second adjustment step S5, the spindle 52 is returned to its original state so as to change the angle between the rotation axis of the chuck table 18 and the rotation axis of the spindle 52. Specifically, the spindle-side adjustment mechanism 50 lowers the spacer 48a so that the spacer 48a contacts the bottom of the cover 44 so that the rotation axis of the spindle 52 becomes parallel to the Z-axis direction.

[0066] After the second adjustment step S5, the back surface 11b of the wafer 11 is ground so as to form a second recess on the back surface 11b of the wafer 11 (second grinding step S6). Fig. 7(B) is a partially sectional side view that illustrates the second grinding step S6.

[0067] In this second grinding step S6, first, the components of the grinding device 2 are operated in the same manner as in the first grinding step S3 to start grinding of the wafer 11. Then, while both the chuck table 18 and the spindle 52 are rotated, the spindle 52 is lowered until the thickness of the central region 17a of the wafer 11 reaches a predetermined thickness, for example. As a result, a second recess 23 having a circular bottom surface 23a is formed on the back surface 11b side of the wafer 11.

[0068] In the wafer grinding method shown in FIG. 4, after the first grinding step S3 and before the second grinding step S6, the chuck table 18 and the spindle 52 are separated so as to separate the multiple grinding wheels 60 from the back surface 11b of the wafer 11, and then the inclination of the spindle 52 is adjusted so as to change the angle between the rotation axis of the chuck table 18 and the rotation axis of the spindle 52.

[0069] That is, in this method, the inclination of at least one of the chuck table 18 and the spindle 52 is not adjusted in a state where the grinding wheels 60 are in contact with the back surface 11b of the wafer 11. Therefore, in this method, the angle between the rotation axis of the chuck table 18 and the rotation axis of the spindle 52 is changed without applying a load to the vicinity of the outer edge of the bottom 21a of the first recess 21. As a result, in this method, it is possible to reduce the probability of processing defects such as chipping or cracks occurring when the second recess 23 having a circular bottom surface 23a is formed on the back surface 11b of the wafer 11.

[0070] The above is one aspect of the present invention, and the present invention is not limited to the above. For example, in the present invention, the rotation axis of the spindle 52 may be inclined in advance with respect to the Z-axis direction. That is, the spindle-side adjustment mechanism 50 may be connected to the housing 46 in an initial state with the spacer 48a separated from the bottom of the cover 44. In this case, the first adjustment step S2 may be omitted.

[0071] Also, in the present invention, in at least one of the first adjustment step S2 and the second adjustment step S5, only the inclination of the chuck table 18 may be adjusted without adjusting the inclination of the spindle 52. Alternatively, in the present invention, the inclinations of both the chuck table 18 and the spindle 52 may be adjusted in at least one of the first adjustment step S2 and the second adjustment step S5.

[0072] The present invention may also be a wafer grinding method including steps other than the multiple steps S1 to S6 shown in Fig. 2. For example, in the present invention, a cleaning step of cleaning the bottom 21a of the first recess 21 may be performed after the separation step S4 and before the second grinding step S6. This cleaning step is performed, for example, by rotating the chuck table 18 while supplying liquid radially below the inner side of the grinding wheel 56 via the flow paths 52a, 62a, and the multiple through holes 64a.

[0073] Furthermore, if the position in the X-axis direction of the trajectory of each of the grinding surfaces of the multiple grinding wheels 60 changes when the spindle 52 is rotated before and after the second adjustment step S5, a position adjustment step may be performed to move the chuck table 18 along the X-axis direction after the separation step S4 and before the second grinding step S6.

[0074] For example, when the entire trajectory is positioned inside the first recess 21 in a planar view after the second adjustment step S5, the chuck table 18 and the spindle 52 may be separated in the X-axis direction so that a point on the side 21b of the first recess 21 that is located in the X-axis direction when viewed from the center of the wafer 11 overlaps with one end of the trajectory in the X-axis direction in a planar view.

[0075] On the other hand, if a portion of the locus is positioned outside the first recess 21 in a planar view after the second adjustment step S5, the chuck table 18 and the spindle 52 may be brought closer together in the X-axis direction so that a point on the side 21b of the first recess 21 that is located in the X-axis direction when viewed from the center of the wafer 11 overlaps with one end of the locus in the X-axis direction in a planar view.

[0076] Also, when intentionally shifting the diameter of the second recess 23 formed on the back surface 11b of the wafer 11 from the diameter of the first recess 21 in the second grinding step S6, a position adjustment step of moving the chuck table 18 along the X-axis direction may be carried out after the separation step S4 and before the second grinding step S6.

[0077] For example, when forming a second recess 23 having a diameter shorter than the diameter of the first recess 21, the chuck table 18 and the spindle 52 may be brought closer together in the X-axis direction so that the entire trajectory is positioned inside the side surface 21b of the first recess 21 in a planar view.

[0078] On the other hand, when forming a second recess 23 having a diameter longer than the diameter of the first recess 21, the chuck table 18 and the spindle 52 may be separated in the X-axis direction so that a portion of the trajectory is positioned outside the side surface 21b of the first recess 21 in a planar view.

[0079] 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]

[0080] 2: Grinding equipment 4: Base 6:X-axis direction movement mechanism 8: Guide rail 10: X-axis moving plate 11: Wafer (11a: front surface, 11b: back surface) 12: Screw shaft 13: Planned division line (street) 14: Stepping motor 15: Device 16: Nut 17a: Central area 17b: Surplus outer area 18: Chuck table 19: Protective tape 20:Frame body 21: First recess (21a: bottom, 21b: side) 22: Porous plate 23: second recess (23a: bottom surface) 24: Bearing 26: Support plate 28: Chuck table side adjustment mechanism (28a, 28b: movable shaft, 28c: fixed shaft) 30: Wall 32:Z-axis direction movement mechanism 34: Guide rail 36: Z-axis moving plate 38: Screw shaft 40: Stepping motor 42: Nut 44: Cover (44a: Through hole) 46: Housing 48a, 48b: Spacer 50: Spindle side adjustment mechanism (50a: bolt) 52: spindle (52a: flow path) 54: Wheel mount 56: Grinding wheel 58: Wheel base 60: Grinding wheel 62: Base portion (62a: flow path) 64: Drooping part (64a: Through hole)

Claims

1. A method for grinding a wafer, comprising: a grinding apparatus including a chuck table having a holding surface and rotatable around a first rotation axis passing through the center of the holding surface; and a spindle having a grinding wheel, the grinding wheel having a plurality of grinding stones provided in a circularly spaced arrangement, attached to a tip thereof and rotatable around a second rotation axis, the method comprising the steps of: grinding a wafer having a plurality of devices formed on a front surface side thereof, the radius of the devices being longer than an outer diameter of a path of a grinding surface of each of the plurality of grinding stones when the spindle is rotated, the method comprising the steps of: a holding step of holding the wafer on the holding surface of the chuck table so that a backside of the wafer is exposed; a first grinding step of grinding the back side of the wafer by rotating both the chuck table and the spindle in a predetermined direction while bringing the grinding wheels into contact with the back side of the wafer in a state in which the center of the wafer and the locus overlap in the predetermined direction, while rotating the chuck table and the spindle in the predetermined direction; a separation step of separating the chuck table and the spindle in the predetermined direction so as to separate the plurality of grinding wheels and the back side of the wafer after the first grinding step; an adjustment step of adjusting an inclination of at least one of the chuck table or the spindle so as to change an angle between the first rotation axis and the second rotation axis after the separation step; a second grinding step of grinding the back side of the wafer by rotating both the chuck table and the spindle in the predetermined direction while bringing the chuck table and the spindle closer to each other in the predetermined direction so that the grinding wheels come into contact with the back side of the wafer in a state in which the center of the wafer and the trajectory overlap in the predetermined direction, after the adjusting step; In the first grinding step, a first recess having a bottom shaped to correspond to a side surface of an inverted cone is formed on the back side of the wafer; In the second grinding step, a second recess having a circular bottom surface is formed on the back surface side of the wafer.

2. 2. The method for grinding a wafer according to claim 1, further comprising a cleaning step of cleaning the bottom of the first recess after the separating step and before the second grinding step.

3. 3. The method for grinding a wafer as described in claim 1 or 2, further comprising a position adjustment step of moving the chuck table and the spindle closer to or farther apart in a direction perpendicular to the predetermined direction after the separation step and before the second grinding step.

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