JP7171131B2 - Workpiece grinding method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method of grinding a workpiece used when grinding the workpiece with a grinding wheel.

In the process of manufacturing optical device chips, an optical device wafer is used in which optical devices such as LEDs (Light Emitting Diodes) are formed in regions defined by dividing lines (streets). By dividing this optical device wafer along the planned division lines, a plurality of optical device chips each having an optical device can be obtained. Similarly, semiconductor device chips are manufactured by dividing a semiconductor wafer on which semiconductor devices such as ICs (Integrated Circuits) and LSIs (Large Scale Integration) are formed.

Various chips represented by the above-mentioned optical device chips and semiconductor device chips are built into various electronic devices. ing. Therefore, a method of thinning the above optical device wafer or semiconductor wafer by grinding with a grinding wheel is used.

For grinding a workpiece such as an optical device wafer or a semiconductor wafer, a grinding apparatus is used that includes a chuck table that holds the workpiece and a grinding means (grinding unit) that grinds the workpiece held by the chuck table. Used. The grinding means of the grinding device has a mount fixed to the tip of a spindle that serves as a rotation axis, and a grinding wheel equipped with a grinding wheel for grinding the workpiece is attached to the mount (for example, See Patent Documents 1 and 2). The workpiece is ground by bringing the grinding wheel into contact with the workpiece while rotating the grinding wheel.

JP 2011-29331 A JP 2011-40631 A

When grinding a workpiece using a grinding apparatus, a grinding wheel attached to a grinding means is pressed against the workpiece held by a chuck table. At this time, a load (processing load) is applied to the grinding means, and if the processing load exceeds a certain value, the grinding means may be damaged. Therefore, the grinding apparatus is designed to stop the operation of the grinding means to avoid damage to the grinding means when the processing load applied to the grinding means exceeds a preset allowable value.

However, depending on the material of the workpiece, grinding may be required under conditions where the machining load exceeds the allowable value. For example, when grinding a workpiece made of a material with relatively high hardness (hard material) such as a sapphire wafer or a SiC wafer, the grinding wheel is strongly pressed against the workpiece in order to properly grind the workpiece. There is a need. When attempting to properly grind such a workpiece, the processing load may exceed the allowable value and the operation of the grinding means may stop, making it difficult to grind using the grinding apparatus.

The present invention has been made in view of such problems, and an object of the present invention is to provide a method of grinding a workpiece that can reduce the load applied to the grinding means during grinding.

According to one aspect of the present invention, grinding means is equipped with a chuck table that holds a workpiece, and a ring-shaped grinding wheel that includes a plurality of grinding wheels for grinding the workpiece held on the chuck table. and a grinding method for grinding the workpiece using a grinding apparatus comprising: an outer periphery in which an outer peripheral region including an outer peripheral edge of the workpiece is ground to a predetermined thickness by the grinding wheel and a central region grinding step of grinding a central region surrounded by the outer peripheral region of the workpiece to a predetermined thickness with the grinding wheel, wherein the outer peripheral region comprises a plurality of rings having different radii. The width of the plurality of regions is set so that the width of the regions closer to the central region becomes wider, and in the outer peripheral region grinding step, the plurality of regions are cut into the workpiece. A method of grinding a workpiece in which grinding is performed sequentially from the outer peripheral edge side toward the central area side, and the workpiece and the grinding wheel are separated between the outer peripheral area grinding step and the central area grinding step. provided.

In a method for grinding a workpiece according to an aspect of the present invention, an outer peripheral area grinding step of grinding an outer peripheral area including an outer peripheral edge of the workpiece to a predetermined thickness with a grinding wheel; and a central region grinding step of grinding the central region to the predetermined thickness with a grinding wheel, thereby grinding the workpiece. In this way, by performing the grinding of the work piece in multiple steps, the contact area between the grinding wheel and the work piece during the grinding process can be increased as compared with the case where the entire work piece is ground at once. This reduces the load on the grinding means on which the grinding wheel is mounted.

It is a perspective view which shows a grinding apparatus. FIG. 4 is a perspective view showing how a workpiece is held by a chuck table; FIG. 4 is a cross-sectional view showing a chuck table; FIG. 4A is a plan view showing the state of the first grinding process, and FIG. 4B is a side view showing the state of the first grinding process. FIG. 5A is a plan view showing the state of the second grinding process, and FIG. 5B is a side view showing the state of the second grinding process. FIG. 6A is a plan view showing the state of the third grinding process, and FIG. 6B is a side view showing the state of the third grinding process. FIG. 7A is a plan view showing the state of the central region grinding step, and FIG. 7B is a side view showing the state of the central region grinding step.

Embodiments of the present invention will be described below with reference to the accompanying drawings. First, a configuration example of a grinding apparatus that can be used for implementing the method for grinding a workpiece according to the present embodiment will be described. FIG. 1 is a perspective view showing a grinding device 2. FIG.

The grinding device 2 includes a base 4 on which each component of the grinding device 2 is mounted, and a rectangular parallelepiped support structure 6 is provided at the rear end of the base 4 . A rectangular opening 4a is provided on the upper surface side of the base 4 when viewed from above. The opening 4a is formed such that its longitudinal direction is along the X-axis direction (front-rear direction).

A ball-screw type X-axis moving mechanism 8 and a dust/splash proof cover 10 covering a part of the X-axis moving mechanism 8 are arranged inside the opening 4a. The X-axis moving mechanism 8 has an X-axis moving table 8a, and moves this X-axis moving table 8a along the X-axis direction. Further, in front of the opening 4a, an operation panel 12 for inputting grinding processing conditions and the like is installed.

A chuck table 14 is provided on the X-axis moving table 8a. The chuck table 14 moves along the X-axis direction together with the X-axis moving table 8 a , and the position of the chuck table 14 in the X-axis direction is controlled by the X-axis moving mechanism 8 . The chuck table 14 holds a workpiece to be ground.

FIG. 2 is a perspective view showing how the workpiece 11 is held by the chuck table 14. As shown in FIG. The workpiece 11 is, for example, a disk-shaped wafer or the like having devices (not shown) such as LEDs (Light Emitting Diodes), ICs (Integrated Circuits), LSIs (Large Scale Integration) formed on the front surface 11a side. .

The material, shape, structure, size, etc. of the workpiece 11 are not limited. A wafer of material can be used. In particular, when forming an optical device such as an LED as a device, it is preferable to use a sapphire wafer which is excellent in mechanical properties, thermal properties and chemical stability.

The workpiece 11 is partitioned into a plurality of regions by a plurality of dividing lines (streets) arranged in a lattice so as to intersect each other, and devices are formed in the plurality of regions. Then, by dividing the workpiece 11 along the dividing lines, a plurality of chips each including a device can be obtained. There are no restrictions on the type, quantity, shape, structure, size, arrangement, etc. of the devices formed on the workpiece 11 .

For the purpose of thinning the chips obtained by dividing the workpiece 11, the workpiece 11 before being divided is subjected to a grinding process. Specifically, the workpiece 11 is thinned by grinding the back surface 11b side of the workpiece 11 with a grinding wheel. When grinding the workpiece 11, the workpiece 11 is placed on the chuck table 14 as shown in FIG.

The workpiece 11 is held by the chuck table 14 so that the back surface 11b side is exposed upward. A protective tape may be attached to the surface 11a of the workpiece 11 to protect the device. In this case, the workpiece 11 is held by the chuck table 14 via the protective tape.

FIG. 3 is a sectional view showing the chuck table 14. As shown in FIG. The chuck table 14 has a disk-shaped suction part 16 made of porous ceramics or the like, and the upper surface of the suction part 16 forms a circular suction surface 16a for sucking the workpiece 11 in plan view. The suction surface 16a is connected to a suction source (not shown) through a suction path 14a formed inside the chuck table 14. As shown in FIG.

The workpiece 11 is sucked and held by the chuck table 14 by applying the negative pressure of the suction source to the suction surface 16a while the workpiece 11 is placed so as to cover the suction surface 16a. Here, an example in which the suction surface 16a is formed in a circular shape in plan view corresponding to the disk-shaped workpiece 11 will be described. It can be changed as appropriate according to the shape of the object 11 or the like.

The suction portion 16 is formed so that its thickness increases from the outer peripheral edge toward the center, and the suction surface 16a is formed in a mountain shape with the center as the apex. That is, the suction surface 16 a has an inclined surface that is slightly inclined with respect to the radial direction of the suction portion 16 . For convenience of explanation, FIG. 3 exaggerates the inclination of the suction surface 16a. For example, when the diameter of the suction portion 16 is about 200 mm or more and 210 mm or less, the height position difference between the center and the outer peripheral edge of the suction surface 16a in the thickness direction of the suction portion 16 is set to about 10 μm or more and 20 μm or less. .

The chuck table 14 is arranged on the X-axis moving table 8a (see FIG. 1) in a slightly inclined state so that part of the inclined surface of the suction surface 16a is parallel to the horizontal direction (XY plane). Also, the chuck table 14 is connected to a rotational drive source (not shown) such as a motor, and rotates around a rotation axis substantially parallel to the thickness direction of the suction portion 16 . The suction surface 16 a is provided so that the center position coincides with the position of the rotation shaft of the chuck table 14 .

A Z-axis movement mechanism 18 is provided on the front side of the support structure 6 shown in FIG. The Z-axis moving mechanism 18 includes a pair of Z-axis guide rails 20 which are rod-shaped and arranged so that their length direction is along the Z-axis direction (vertical direction). A Z-axis moving plate 22 is attached to the pair of Z-axis guide rails 20 so as to be slidable along the Z-axis direction. A nut portion (not shown) is provided on the rear surface side (rear surface side) of the Z-axis moving plate 22, and the Z-axis ball screw is arranged along a direction substantially parallel to the Z-axis guide rail 20 in this nut portion. 24 are screwed together.

A Z-axis pulse motor 26 is connected to one end of the Z-axis ball screw 24 . When the Z-axis ball screw 24 is rotated by the Z-axis pulse motor 26, the Z-axis moving plate 22 moves along the Z-axis guide rail 20 in the Z-axis direction.

A support 28 projecting forward is provided on the front side (surface side) of the Z-axis moving plate 22 . The support 28 supports grinding means (grinding unit) 30 for grinding the workpiece. The grinding means 30 includes a spindle housing 32 fixed to the support 28. The spindle housing 32 rotatably accommodates a spindle 34 serving as a rotating shaft.

A tip (lower end) of the spindle 34 is exposed to the outside of the spindle housing 32 , and a disk-shaped mount 36 is fixed to the tip of the spindle 34 . A ring-shaped grinding wheel 38 having approximately the same diameter as the mount 36 is attached to the lower surface of the mount 36 . The grinding wheel 38 has a ring-shaped base 40, and a plurality of rectangular parallelepiped grinding wheels 42 are fixed along the outer peripheral edge of the base 40 on the side of the base 40 facing the chuck table 14. (See FIGS. 4 to 7).

When grinding the workpiece 11 , first, the workpiece 11 is suction-held by the chuck table 14 . Then, the chuck table 14 is moved by the X-axis moving mechanism 8 to position the chuck table 14 under the grinding wheel 38 .

As shown in FIG. 3, the suction surface 16a of the chuck table 14 is formed in the shape of a mountain with its center as the apex. It becomes a mountain shape reflecting the shape of . Further, as described above, the chuck table 14 is arranged in a slightly inclined state so that a part of the inclined surface of the suction surface 16a is parallel to the horizontal direction. to the outer peripheral edge are held so as to be parallel to the horizontal direction.

Then, while rotating the chuck table 14 and the spindle 34 in predetermined directions at a predetermined number of revolutions, the grinding means 30 is lowered at a predetermined speed, and the grinding wheel 38 attached to the grinding means 30 is moved to the workpiece 11 . press against. As a result, the grinding wheel 38 comes into contact with a region of the back surface 11 b of the workpiece 11 that is parallel to the horizontal direction, and the workpiece 11 is scraped off by the grinding wheel 38 . Thus, the grinding of the workpiece 11 is performed.

When the grinding wheel 38 is pressed against the workpiece 11 when grinding the workpiece 11, a load (processing load) is applied to the grinding means 30, and if the processing load exceeds a certain value, the grinding means 30 is damaged. there is a risk of it happening. Therefore, when the processing load applied to the grinding means 30 exceeds a preset allowable value (processing load allowable value), the grinding device 2 stops the operation of the grinding means 30 to avoid damage to the grinding means 30. is designed to

However, depending on the material of the workpiece 11, grinding may be required under conditions where the processing load exceeds the allowable value. For example, when grinding the workpiece 11 made of a material having relatively high hardness (hard material) such as a sapphire wafer or a SiC wafer, the grinding wheel 38 is used to grind the workpiece 11 appropriately. It is necessary to press firmly against the object 11. If an attempt is made to grind such a workpiece 11, the processing load may exceed the allowable value and the operation of the grinding means 30 may stop, making grinding using the grinding apparatus 2 difficult.

In the grinding method of the workpiece 11 according to the present embodiment, an outer peripheral area grinding step of grinding the outer peripheral area including the outer peripheral edge of the workpiece 11 to a predetermined thickness with a grinding wheel, and an outer peripheral area surrounded by the outer peripheral area of the workpiece 11. and a central region grinding step of grinding the central region to the predetermined thickness with a grinding wheel, thereby grinding the workpiece 11 . In this way, by performing the grinding of the workpiece 11 in multiple steps, the contact area between the grinding wheel and the workpiece 11 during grinding is reduced, and the load applied to the grinding means 30 is reduced. .

When the load applied to the grinding means 30 is reduced, the load applied to the grinding means 30 can be suppressed to the processing load allowable value or less even when the workpiece 11 made of a hard material such as a sapphire wafer or a SiC wafer is ground. The workpiece 11 can be appropriately ground by the grinding device 2 . A specific example of the method for grinding a workpiece according to the present embodiment will be described below.

In the method for grinding a workpiece according to the embodiment, first, an outer peripheral area grinding step is performed in which the outer peripheral area including the outer peripheral edge of the workpiece 11 is ground to a predetermined thickness by the grinding wheel 42 . FIGS. 4 to 6 show the state of the peripheral region grinding process. In the following, as an example, a case where the peripheral region grinding process is further divided into a first grinding process, a second grinding process, and a third grinding process will be described.

FIG. 4A is a plan view showing the state of the first grinding process, and FIG. 4B is a side view showing the state of the first grinding process. 4(A) and 4(B) respectively show the workpiece 11 sucked and held by the chuck table 14 and the grinding wheel 38 mounted on the mount 36 (see FIG. 1) of the grinding means 30. showing. FIG. 4A shows the rotation center O of the workpiece 11 when the chuck table 14 is rotated.

The grinding wheel 38 has a ring-shaped base 40 made of a metal material such as stainless steel or aluminum. A plurality of rectangular parallelepiped grinding wheels 42 are fixed to the base. For convenience of explanation, FIG. 4B shows only the grinding wheel 42a arranged closest to the rotation center O of the workpiece 11 among the plurality of grinding wheels 42 provided in the grinding wheel 38. .

The grinding wheel 42 is formed, for example, by fixing abrasive grains made of diamond, CBN (Cubic Boron Nitride) or the like with a bonding material such as metal bond, resin bond or vitrified bond. However, the material, shape, structure, size, etc. of the grinding wheel 42 are not limited. Also, the number of grinding wheels 42 provided in the grinding wheel 38 can be set arbitrarily.

The workpiece 11 includes a ring-shaped outer peripheral region 13 having a width smaller than the radius of the workpiece 11, and a rotation center O of the workpiece 11 surrounded by the outer peripheral region 13. and a circular central region 15 containing Further, the outer peripheral region 13 has a plurality of ring-shaped regions with different radii.

Specifically, the outer peripheral region 13 includes a first ring-shaped region 13a including the outer peripheral edge of the workpiece 11, a second ring-shaped region 13b surrounded by the first region 13a, and a second region 13b. It is divided into an enclosed ring-shaped third region 13c. The radius of the second region 13b is smaller than the radius of the first region 13a, and the radius of the third region 13c is smaller than the radius of the second region 13b. A region surrounded by the third region 13 c corresponds to the central region 15 .

In FIG. 4A, the width of the first region 13a, the width of the _second region 13b, and the width of the _third region 13c are represented by W1, W2, and W3, _respectively , and the radius of the central region 15 is represented by R. show. The sum of W ₁ , W ₂ and W ₃ corresponds to the width W of the outer peripheral region 13 , and the sum of W and R corresponds to the radius of the workpiece 11 .

In the first grinding step, the grinding wheel 38 and the grinding wheel 38 are arranged so that the grinding wheel 42 overlaps the first region 13a of the workpiece 11 and does not overlap the second region 13b, the third region 13c, and the central region 15. The chuck table 14 is positioned. At this time, the end portion 42b located on the rotation center O side of the grinding wheel 42a is positioned at the boundary between the first region 13a and the second region 13b as shown in FIG. 4(B).

Then, while rotating the chuck table 14 and the grinding wheel 38 in predetermined directions at a predetermined number of revolutions, the grinding wheel 38 is lowered at a predetermined speed to move the grinding wheel 42 to the first region 13a of the workpiece 11. make contact. Thereby, the first region 13 a is ground by the grinding wheel 42 . This grinding is continued until the first region 13a reaches a predetermined thickness (see FIG. 5(B)).

Next, a second grinding step of grinding the second region 13b is performed. FIG. 5A is a plan view showing the state of the second grinding process, and FIG. 5B is a side view showing the state of the second grinding process.

In the second grinding step, the grinding wheel 38 and the chuck table 14 are positioned so that the grinding wheel 42 overlaps the second region 13b of the workpiece 11 and does not overlap the third region 13c and the central region 15. Position. At this time, the end portion 42b located on the rotation center O side of the grinding wheel 42a is positioned at the boundary between the second region 13b and the third region 13c as shown in FIG. 5(B).

Then, while rotating the chuck table 14 and the grinding wheel 38 in predetermined directions at a predetermined number of revolutions, the grinding wheel 38 is lowered at a predetermined speed, and the grinding wheel 42 is moved to the second region 13b of the workpiece 11. make contact. Thereby, the second region 13 b is ground by the grinding wheel 42 . This grinding is continued until the second region 13b reaches a predetermined thickness (about the same thickness as the first region 13a) (see FIG. 6B).

Note that the thickness of the first region 13a is reduced through the first grinding step. Therefore, the grinding wheel 42 and the first region 13a hardly come into contact in the second grinding step.

Next, the 3rd grinding process which grinds the 3rd field 13c is carried out. FIG. 6A is a plan view showing the state of the third grinding process, and FIG. 6B is a side view showing the state of the third grinding process.

In the third grinding step, the grinding wheel 38 and the chuck table 14 are positioned so that the grinding wheel 42 overlaps the third region 13c of the workpiece 11 and does not overlap the central region 15 . At this time, the end portion 42b located on the rotation center O side of the grinding wheel 42a is positioned at the boundary between the third region 13c and the central region 15 as shown in FIG. 6(B).

Then, while rotating the chuck table 14 and the grinding wheel 38 in predetermined directions at a predetermined number of revolutions, the grinding wheel 38 is lowered at a predetermined speed to move the grinding wheel 42 to the third region 13c of the workpiece 11. make contact. Thereby, the third region 13 c is ground by the grinding wheel 42 . This grinding is continued until the third region 13c reaches a predetermined thickness (about the same thickness as the first region 13a and the second region 13b) (see FIG. 7B).

Note that the thickness of the first region 13a and the second region 13b are reduced through the first grinding process and the second grinding process. Therefore, the grinding wheel 42 hardly contacts the first region 13a and the second region 13b in the third grinding step.

As described above, by grinding the first region 13a, the second region 13b, and the third region 13c in order from the outer peripheral edge side of the workpiece 11 toward the central region 15 side, the entire outer peripheral region 13 is ground to a predetermined level. It is processed so as to be thick (peripheral region grinding step). As a result, the workpiece 11 has a shape in which the central region 15 protrudes from the outer peripheral region 13 toward the rear surface 11b.

Next, a central region grinding step is performed in which the central region 15 of the workpiece 11 is ground to a predetermined thickness using a grinding wheel. FIG. 7A is a plan view showing the state of the central region grinding step, and FIG. 7B is a side view showing the state of the central region grinding step.

In the central area grinding process, the grinding wheel 38 and the chuck table 14 are positioned so that the grinding wheel 42 overlaps the central area 15 . For example, as shown in FIG. 7B, the position of the chuck table 14 is adjusted so that the grinding wheel 42a overlaps the rotation center O of the workpiece 11. As shown in FIG.

Then, while rotating the chuck table 14 and the grinding wheel 38 in predetermined directions at a predetermined number of revolutions, the grinding wheel 38 is lowered at a predetermined speed to bring the grinding wheel 42 into contact with the central region 15 of the workpiece 11 . Let Thereby, the central region 15 is ground by the grinding wheel 42 . This grinding is continued until the central region 15 reaches a predetermined thickness (about the same thickness as the outer peripheral region 13).

Note that the thickness of the outer peripheral region 13 is reduced through the outer peripheral region grinding process. Therefore, the grinding wheel 42 and the outer peripheral region 13 hardly come into contact with each other in the central region grinding process. When the central region grinding step is completed, the workpiece 11 is entirely thinned.

As described above, in the grinding method of the workpiece 11 according to the present embodiment, the workpiece 11 is ground by the outer peripheral region grinding step of grinding only the outer peripheral region 13 and the central region grinding step of grinding only the central region 15. be implemented separately. As a result, the contact area between the grinding wheel 42 and the workpiece 11 during grinding is reduced, and the load applied to the grinding means 30 is reduced. Therefore, it is possible to suppress the load applied to the grinding means 30 to the processing load allowable value or less, and to appropriately grind the workpiece 11 by the grinding device 2 .

Further, as shown in FIGS. 4 to 6, by performing the grinding of the outer peripheral region 13 in a plurality of times, the contact area between the workpiece 11 and the grinding wheel 42 in the outer peripheral region grinding process is further reduced, The load applied to the grinding means 30 is further reduced.

Note that the widths (W ₁ , W ₂ , W ₃ ) of the first region 13a, the second region 13b, and the third region 13c that are ground in the outer peripheral region grinding step depend on the dimensions and material of the workpiece 11, respectively. can be set as appropriate. Also, the values of W ₁ , W ₂ and W ₃ may be the same or different.

For example, the widths of the first region 13a, the second region 13b, and the third region 13c can be set such that the widths of the regions closer to the central region 15 are wider (W ₁ <W ₂ <W ₃ ). More specifically, W ₁ , W ₂ and W ₃ can be set so that the areas of the first region 13a, the second region 13b and the third region 13c are substantially equal. As a result, the unevenness in the area of the workpiece 11 to be ground in each of the first grinding process, the second grinding process, and the third grinding process can be suppressed, and the grinding conditions can be uniformed.

Also, in the present embodiment, an example in which the outer peripheral region grinding process is performed by dividing it into three grinding steps has been described, but the aspect of the outer peripheral region grinding process is not limited to this. That is, the grinding of the outer peripheral region 13 may be performed in one grinding process, or may be performed in two or four or more grinding processes.

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

REFERENCE SIGNS LIST 11 Workpiece 11a Front surface 11b Back surface 13 Peripheral area 13a First area 13b Second area 13c Third area 15 Central area 2 Grinding device 4 Base 4a Opening 6 Support structure 8 X-axis moving mechanism 8a X-axis moving table 10 Dust-proof Droplet cover 12 Operation panel 14 Chuck table 14a Suction path 16 Suction unit 16a Suction surface 18 Z-axis movement mechanism 20 Z-axis guide rail 22 Z-axis movement plate 24 Z-axis ball screw 26 Z-axis pulse motor 28 Support tool 30 Grinding means (grinding unit )
32 spindle housing 34 spindle 36 mount 38 grinding wheel 40 base 42 grinding wheel 42a grinding wheel 42b end

Claims (1)

Using a grinding device equipped with a chuck table that holds a workpiece, and a grinding means to which a ring-shaped grinding wheel having a plurality of grinding wheels for grinding the workpiece held on the chuck table is mounted A method of grinding a workpiece by grinding the workpiece by
an outer peripheral region grinding step of grinding the outer peripheral region including the outer peripheral edge of the workpiece to a predetermined thickness with the grinding wheel;
a central region grinding step of grinding the central region surrounded by the outer peripheral region of the workpiece to a predetermined thickness with the grinding wheel ;
The outer peripheral region has a plurality of ring-shaped regions with different radii,
The widths of the plurality of regions are set so that the regions closer to the central region have wider widths,
In the outer peripheral region grinding step, the plurality of regions are sequentially ground from the outer peripheral edge side of the workpiece toward the central region side,
A method of grinding an object to be processed, wherein the object to be processed and the grinding wheel are separated from each other between the step of grinding the outer peripheral region and the step of grinding the central region .

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