Grinding method of workpiece

Abstract

To provide a grinding method for a workpiece capable of suppressing to a minimum reduction of productivity with setting of a grinding stone.SOLUTION: The present invention relates to a grinding method for a workpiece including: a grinding step of grinding a front face side of the workpiece, of which the modified layer is exposed on a front face, by selectively using an annular first grinding wheel comprising a first grinding stone in which abrasive grains are dispersed and an annular second grinding wheel comprising a second grinding stone in which abrasive grains having an average grain size smaller than that of the abrasive grains of the first grinding stone are dispersed; and a determination step of determining whether or not setting for aligning the second grinding stone into a state suitable for grinding is required before the grinding step. In the grinding step, in a case where it is determined in the determination step that the setting is required, the front face side of the workpiece is ground using the second grinding wheel without grinding the front face side of the workpiece using the first grinding wheel, thereby grinding the front face side of the workpiece while performing the setting of the second grinding stone in accordance with front face ruggedness caused by the modified layer.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a method for grinding a workpiece in the form of a plate having a modified layer exposed on its surface.

Background Art

[0002] In the production of device chips incorporated in various electronic devices, for example, a disk-shaped semiconductor wafer made of a semiconductor material such as single-crystal silicon carbide (SiC) is used. This semiconductor wafer can be obtained, for example, by cutting a columnar semiconductor ingot with a wire saw to form a disk-shaped as-cut wafer and then improving the flatness of its front and back surfaces by lapping or the like.

[0003] On the other hand, silicon carbide is very hard and cannot necessarily be easily cut by a method using a wire saw as described above. Therefore, the production of semiconductor wafers made of silicon carbide has required a great deal of cost. In response to this problem, in recent years, a technique has been proposed in which a region having a predetermined depth is modified from the surface of a semiconductor ingot by a laser beam, and a semiconductor wafer is separated from the semiconductor ingot with this modified region (hereinafter, modified layer) as a boundary (see, for example, Patent Document 1).

[0004] The modified layer formed by irradiation with a laser beam is brittle compared to other regions, and cracks are generated in the vicinity thereof as the modified layer is formed. Therefore, by using this technique, a semiconductor wafer can be easily separated from a semiconductor ingot. The modified layer remaining on the surface of the obtained semiconductor wafer is later removed by a method such as grinding using a grinding wheel in which abrasive grains are dispersed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, as described above, since silicon carbide is very hard, when a grinding wheel in which abrasive grains with a small particle size are dispersed is used for grinding, the performance of the grinding wheel is likely to deteriorate due to clogging or the like caused by wear of the abrasive grains. When the performance of the grinding wheel deteriorates, dressing to adjust the state of the grinding wheel is performed using a dedicated dressing board. At that time, the grinding apparatus is stopped to install the dressing board. Therefore, there has been a problem that productivity decreases when the frequency of dressing increases.

[0007] Therefore, an object of the present invention is to provide a method for grinding a workpiece that can minimize a decrease in productivity associated with dressing of a grinding wheel.

Means for Solving the Problems

[0008] According to one aspect of the present invention, there is provided a method for grinding a workpiece used when grinding a plate-shaped workpiece having a modified layer modified by a laser beam exposed on the surface, including: an annular first grinding wheel having a first grinding wheel in which abrasive grains are dispersed; and an annular second grinding wheel having a second grinding wheel in which abrasive grains having an average particle size smaller than that of the abrasive grains of the first grinding wheel are dispersed, and selectively using the first grinding wheel and the second grinding wheel to grind the surface side of the workpiece. A grinding step of grinding, and a determination step of determining whether or not dressing for adjusting the second grinding wheel to a state suitable for grinding is necessary before the grinding step. In the grinding step, when it is determined in the determination step that dressing of the second grinding wheel is unnecessary, the surface side of the workpiece is ground using the first grinding wheel, and then the surface side of the workpiece is ground using the second grinding wheel. When it is determined in the determination step that dressing is necessary, the surface side of the workpiece is ground using the second grinding wheel without grinding the surface side of the workpiece using the first grinding wheel, thereby performing dressing of the second grinding wheel due to unevenness of the surface caused by the modified layer. A method for grinding a workpiece is provided in which the surface side of the workpiece is ground while performing the dressing.

[0009] Preferably, in the grinding step, when it is determined in the determination step that dressing of the second grinding wheel is necessary, the surface side of the workpiece is ground while dressing the second grinding wheel by the unevenness of the surface caused by the modified layer, and then, after grinding the surface side of the workpiece using the first grinding wheel, the surface side of the workpiece is ground using the second grinding wheel. Further, preferably, the workpiece is a semiconductor wafer separated from a semiconductor ingot starting from the modified layer.

Advantages of the Invention

[0010] In the method for grinding a workpiece according to one aspect of the present invention, when it is determined that dressing of the second grinding wheel in which abrasive grains having an average particle size smaller than that of the abrasive grains of the first grinding wheel are dispersed is necessary, the surface side of the workpiece with the modified layer remaining is ground using the second grinding wheel, so that the surface side of the workpiece is ground while dressing the second grinding wheel by the unevenness of the surface caused by the modified layer. Therefore, it is not necessary to use a dedicated dressing board for dressing the second grinding wheel.

[0011] That is, since it is not necessary to stop the grinding apparatus for installing the dressing board, even if the dressing frequency increases, it is difficult for productivity to decrease. Thus, according to the method for grinding a workpiece according to one aspect of the present invention, it is possible to minimize the decrease in productivity associated with dressing of the grinding wheel.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

[0013] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. First, an example of a grinding apparatus used in the method for grinding a workpiece according to the present embodiment will be described. FIG. 1 is a perspective view schematically showing a grinding apparatus 2, and FIGS. 2 and 3 are cross-sectional views schematically showing a state in which a workpiece 11 is being ground. In FIG. 1, some elements constituting the grinding apparatus 2 are represented by functional blocks. In the following description, the X-axis direction (front-rear direction), the Y-axis direction (left-right direction), and the Z-axis direction (vertical direction) are perpendicular to each other.

[0014] As shown in FIG. 1, the grinding apparatus 2 includes a base 4 that supports various elements constituting the grinding apparatus 2. On the front end side of the upper surface of the base 4, a concave accommodating portion 4a having an open upper end is provided, and a first conveying mechanism 6 used for conveying the plate-shaped workpiece 11 is accommodated in the accommodating portion 4a. The first conveying mechanism 6 is typically a robot arm having a plurality of joints, and can not only convey the workpiece 11 but also invert the workpiece 11 upside down.

[0015] The workpiece 11 is, for example, a disk-shaped semiconductor wafer separated from a columnar semiconductor ingot made of a semiconductor material such as single-crystalline silicon carbide (SiC). That is, the workpiece 11 has a circular front surface 11a and a circular back surface 11b opposite to the front surface 11a. As shown in FIG. 2, a modified layer 11c that was the starting point of separation remains on the front surface 11a side of the workpiece 11 when the workpiece 11 is separated from the semiconductor ingot.

[0016] The modified layer 11c is formed, for example, by condensing a laser beam having a wavelength that is difficult to be absorbed by the semiconductor ingot (a wavelength that is easy to transmit) onto a region having a predetermined depth from the surface of the semiconductor ingot to cause multi-photon absorption. The modified layer 11c is brittle compared to other regions of the semiconductor ingot, and cracks are generated in the vicinity thereof as the modified layer 11c is formed. Therefore, the semiconductor wafer can be easily separated from the semiconductor ingot with a small force using the modified layer 11c as a starting point for separation.

[0017] As described above, the modified layer 11c remains on the surface 11a side of the workpiece 11 thus obtained, and unevenness due to separation from the semiconductor ingot is formed on the surface 11a. In other words, unevenness due to the modified layer 11c exists on the surface 11a of the workpiece 11. In the grinding method of the workpiece according to the present embodiment, the modified layer 11c together with the unevenness is removed from the workpiece 11 by grinding the surface 11a side of the workpiece 11 with the grinding device 2.

[0018] In the present embodiment, a disk-shaped semiconductor wafer made of a semiconductor material such as single-crystalline silicon carbide is used as the workpiece 11, but the material, shape, structure, size, etc. of the workpiece 11 are not limited. For example, a diamond wafer made of diamond or the like can also be used as the workpiece 11. Further, the modified layer 11c does not necessarily remain on the surface 11a side of the workpiece 11. However, for the conspicuous implementation described later, it is necessary that unevenness of a certain size is formed on the surface 11a of the workpiece 11.

[0019] As shown in FIG. 1, in front of the housing portion 4a, cassette tables 10a and 10b on which cassettes 8a and 8b capable of housing a plurality of workpieces 11 are placed are provided. A position adjustment mechanism 12 for adjusting the position of the workpiece 11 is provided diagonally rearward of the housing portion 4a. The position adjustment mechanism 12 includes a disk-shaped position adjustment table and a plurality of pins arranged around the position adjustment table.

[0020] By moving a plurality of pins along the radial direction of the position adjustment table, for example, the center of the workpiece 11 carried out from the cassette 8a by the first transfer mechanism 6 and placed on the position adjustment table is aligned to a predetermined position in the X-axis direction and the Y-axis direction. The workpiece 11 is placed on the position adjustment table so that its surface to be ground (in this embodiment, the surface 11a) is exposed upward.

[0021] On the side of the position adjustment mechanism 12, a second transfer mechanism 14 for holding and transferring the workpiece 11 rearward is provided. The second transfer mechanism 14 includes, for example, an arm and a holding pad connected to the tip of the arm and capable of sucking and holding the upper surface (surface to be ground) side of the workpiece 11. By rotating the holding pad by the arm, the workpiece 11 whose position has been adjusted by the position adjustment mechanism 12 is transferred rearward.

[0022] Behind the second transfer mechanism 14, a turntable 16 is provided. The turntable 16 is connected to a rotational drive source (not shown) such as a motor and rotates around a rotation axis substantially parallel to the Z-axis direction. On the upper surface of the turntable 16, three chuck tables 18 used for holding the workpiece 11 are provided at substantially equal angular intervals. There is no limitation on the number of chuck tables 18 provided on the turntable 16.

[0023] The turntable 16 rotates in the direction of the arrow in FIG. 1 and in the direction opposite to the arrow, and moves each chuck table 18 in order to the loading / unloading area A adjacent to the second transfer mechanism 14, the rough grinding area B behind the loading / unloading area A, and the finish grinding area C on the side of the rough grinding area B. The second transfer mechanism 14 transfers the workpiece 11 held by the holding pad from the position adjustment table of the position adjustment mechanism 12 to the chuck table 18 arranged in the loading / unloading area A.

[0024] As shown in FIGS. 2 and 3, each chuck table 18 includes a disk-shaped frame body 20 made of, for example, ceramics or the like. On the upper surface side of the frame body 20, a concave portion 20a having a circular opening at the upper end is formed. A holding plate 22 made of ceramics or the like and having a porous disk shape is fixed to the concave portion 20a of the frame body 20.

[0025] When the workpiece 11 is held by the chuck table 18, the lower surface (in this embodiment, the back surface 11b) of the workpiece 11 contacts the upper surface 22a of the holding plate 22. A protective member made of resin or the like may be affixed to the lower surface of the workpiece 11 in advance. In that case, the protective member contacts the upper surface 22a of the holding plate 22.

[0026] The lower surface side of the holding plate 22 is connected to a suction source (not shown) such as an ejector via a flow path 20b provided inside the frame body 20 and a valve (not shown). Therefore, by bringing the lower surface of the workpiece 11 or the like into contact with the upper surface 22a of the holding plate 22, opening the valve, and applying the negative pressure of the suction source, the workpiece 11 is held by the chuck table 18 by the suction force resulting from the negative pressure.

[0027] A rotary drive source (not shown) such as a motor is connected to the lower part of the frame body 20. The chuck table 18 rotates around a rotation axis substantially parallel to the Z-axis direction or a rotation axis slightly inclined with respect to the Z-axis direction such that the center of the upper surface 22a becomes the center of rotation by the force generated by this rotary drive source.

[0028] As shown in FIG. 1, columnar support structures 24 are provided behind (behind the turntable 16) the rough grinding region B and the finish grinding region C, respectively. On the front side of each support structure 24, a Z-axis movement mechanism 26 is provided. Each Z-axis movement mechanism 26 includes a pair of guide rails 28 substantially parallel to the Z-axis direction, and a moving plate 30 is attached to the guide rails 28 in a slidable manner.

[0029] On the rear side (back side) of each moving plate 30, a nut (not shown) constituting a ball screw is fixed, and a screw shaft 32 substantially parallel to the guide rail 28 is connected to this nut in a rotatable manner. A motor 34 is connected to one end of the screw shaft 32. By rotating the screw shaft 32 by the motor 34, the moving plate 30 moves along the guide rail 28 (Z-axis direction). On the front surface (front side) of each moving plate 30, a fixture 36 is provided.

[0030] Each fixture 36 supports a grinding unit 38 capable of grinding the workpiece 11. Each grinding unit 38 includes a spindle housing 40 fixed to the fixture 36. Each spindle housing 40 houses a spindle 42 that serves as a rotation axis parallel to the Z-axis direction or a rotation axis slightly inclined with respect to the Z-axis direction in a rotatable manner.

[0031] The lower end of each spindle 42 is exposed from the lower end surface of the spindle housing 40, and a disk-shaped mount 44 is fixed to this lower end. A plurality of holes (not shown) penetrating the mount 44 in the thickness direction are provided on the outer peripheral portion of the mount 44, and bolts 46 etc. are inserted into each hole.

[0032] On the lower surface of the mount 44 of the grinding unit 38 on the rough grinding region B side, a first grinding wheel 48a for rough grinding is attached with bolts 46. Further, a motor (not shown) connected to the upper end side of the spindle 42 is housed in the spindle housing 40 of the grinding unit 38 on the rough grinding region B side. By the power of this motor, the first grinding wheel 48a rotates together with the spindle 42.

[0033] As shown in FIG. 2, the first grinding wheel 48a includes an annular first wheel base 50a formed using a metal such as stainless steel or aluminum. On the lower surface of the first wheel base 50a, a plurality of first grinding wheels 52a in which abrasive grains such as diamond are dispersed in a binder such as vitrified or resinoid are fixed along the circumferential direction of the first wheel base 50a.

[0034] Beside the first grinding wheel 48a, a liquid supply nozzle (not shown) capable of supplying a liquid (processing fluid) such as water to the portion (processing point) where the workpiece 11 and the first grinding stone 52a come into contact is provided. However, instead of this liquid supply nozzle, or together with the liquid supply nozzle, a liquid supply port used for supplying the liquid may be provided on the first grinding wheel 48a or the like.

[0035] On the other hand, a second grinding wheel 48b for finish grinding is attached to the lower surface of the mount 44 of the grinding unit 38 on the finish grinding region C side by bolts 46. Further, a motor (not shown) connected to the upper end side of the spindle 42 is housed in the spindle housing 40 of the grinding unit 38 on the finish grinding region C side. By the power of this motor, the second grinding wheel 48b rotates together with the spindle 42.

[0036] As shown in FIG. 3, the second grinding wheel 48b includes an annular second wheel base 50b formed of a metal such as stainless steel or aluminum. On the lower surface of the second wheel base 50b, a plurality of second grinding stones 52b in which abrasive grains such as diamond are dispersed in a binder such as vitrified or resinoid are fixed along the circumferential direction of the second wheel base 50b.

[0037] Beside the second grinding wheel 48b, a liquid supply nozzle (not shown) capable of supplying a liquid (processing fluid) such as water to the portion (processing point) where the workpiece 11 and the second grinding stone 52b come into contact is provided. However, instead of this liquid supply nozzle, or together with the liquid supply nozzle, a liquid supply port used for supplying the liquid may be provided on the second grinding wheel 48b or the like.

[0038] The average grain size of the abrasive grains contained in the second grinding wheel 52b for finish grinding is smaller than the average grain size of the abrasive grains contained in the first grinding wheel 52a for rough grinding. In other words, the average grain size of the abrasive grains contained in the first grinding wheel 52a for rough grinding is larger than the average grain size of the abrasive grains contained in the second grinding wheel 52b for finish grinding. Thereby, the first grinding wheel 48a suitable for rough grinding and the second grinding wheel 48b suitable for finish grinding are realized.

[0039] Although there is no limitation on the specific size of the abrasive grains, for example, by setting the average grain size of the abrasive grains contained in the second grinding wheel 52b to 1 μm or less, the flatness of the workpiece 11 can be sufficiently improved by the finish grinding using this second grinding wheel 52b. In this case, the average grain size of the abrasive grains contained in the first grinding wheel 52a becomes larger than 1 μm. For example, by making the average grain size of the abrasive grains contained in the first grinding wheel 52a larger than 3 μm, the workpiece 11 can be efficiently machined by the rough grinding using the first grinding wheel 52a.

[0040] The workpiece 11 held by the chuck table 18 in the rough grinding area B is ground on the upper surface side by the grinding unit 38 on the rough grinding area B side described above. Also, the workpiece 11 held by the chuck table 18 in the finish grinding area C is ground on the upper surface side by the grinding unit 38 on the finish grinding area C side described above.

[0041] Therefore, by moving the chuck table 18 holding the workpiece 11 in the order of the loading / unloading area A, the rough grinding area B, and the finish grinding area C, the rough grinding of the workpiece 11 and the finish grinding after the rough grinding can be continuously performed. The chuck table 18 in the finish grinding area C is positioned again in the loading / unloading area A when the finish grinding of the workpiece 11 is completed.

[0042] As shown in FIG. 1, a third transfer mechanism 54 for holding and transferring the workpiece 11 after grinding forward is provided at a position in front of the loading / unloading area A and on the side of the second transfer mechanism 14. The third transfer mechanism 54 includes a holding pad that sucks and holds the upper surface side of the workpiece 11, and an arm connected to the holding pad. By rotating the holding pad with the arm, the workpiece 11 after grinding is transferred forward from the chuck table 18 in the loading / unloading area A.

[0043] A cleaning unit 56 for cleaning the workpiece 11 unloaded by the third transfer mechanism 54 is provided on the side of the third transfer mechanism 54. The cleaning unit 56 includes, for example, a spinner table that rotates while holding the lower surface side of the workpiece 11, and a cleaning nozzle that injects a cleaning fluid onto the upper surface side of the workpiece 11 held by the spinner table.

[0044] The cleaning fluid used in this cleaning unit 56 is typically a mixed fluid (two-fluid) in which air and water are mixed. Of course, water or the like without air mixed in may be used as the cleaning fluid. The workpiece 11 cleaned by the cleaning unit 56 is transferred by the first transfer mechanism 6 and is, for example, housed in the cassette 8b.

[0045] A control unit 58 is connected to each element of the grinding device 2. This control unit 58 is constituted by, for example, a computer including a processing device, a storage device, and an input device, and controls the operations of each element of the grinding device 2 described above so that the workpiece 11 is properly ground.

[0046] The processing device is typically a CPU (Central Processing Unit), and performs various processes necessary for controlling the above-described elements. The storage device includes, for example, a main storage device such as a DRAM (Dynamic Random Access Memory) and an auxiliary storage device such as a hard disk drive or a flash memory.

[0047] The input device is, for example, a touch panel and also serves as an output device (display device). Note that a keyboard, a mouse, or the like may be adopted as the input device. The function of this control unit 58 is realized, for example, by the processing device operating according to software stored in the storage device.

[0048] Figure 4 is a flowchart showing a method for grinding a workpiece. For example, when grinding a hard workpiece 11 such as a semiconductor wafer made of a semiconductor material such as silicon carbide (SiC) using a second grinding wheel 52b containing abrasive grains with a small average particle size, phenomena such as clogging where the exposed abrasive grains are lost due to wear are likely to occur. Therefore, in this embodiment, first, it is determined whether dressing is necessary to prepare the second grinding wheel 52b in a state suitable for grinding (determination step ST1).

[0049] Specifically, it is determined based on an arbitrary criterion whether the second grinding wheel 52b is in a state suitable for grinding. For example, when the performance of the second grinding wheel 52b deteriorates due to clogging or the like described above, the workpiece 11 cannot be properly ground, and the load associated with the rotation of the second grinding wheel 48b increases. Therefore, by utilizing this phenomenon, it is possible to determine whether the second grinding wheel 52b is in a state suitable for grinding.

[0050] For example, if the load current value of the motor that rotates the second grinding wheel 48b (hereinafter, the previous load current value) measured during the finish grinding of the previous workpiece 11 exceeds a predetermined threshold value, it is determined that the second grinding wheel 52b is not in a state suitable for grinding and dressing is necessary (YES in determination step ST1). On the other hand, if the previous load current value does not exceed the predetermined threshold value, it is determined that the second grinding wheel 52b is in a state suitable for grinding and dressing is unnecessary (NO in determination step ST1).

[0051] Instead of the load current value of the motor, it may be determined whether the second grinding wheel 52b is in a state suitable for grinding based on the load applied from the second grinding wheel 48b (second grinding wheel 52b) to the workpiece 11.

[0052] In this case, for example, if the load measured during the finish grinding of the previous workpiece 11 exceeded a predetermined threshold value, the second grinding wheel 52b was not in a suitable state for grinding, and it was determined that dressing was necessary (YES in determination step ST1). If the previous load did not exceed the predetermined threshold value, the second grinding wheel 52b was in a suitable state for grinding, and it was determined that dressing was unnecessary (NO in determination step ST1).

[0053] Incidentally, immediately after the new second grinding wheel 48b is replaced, the abrasive grains are not properly exposed on the surface of the second grinding stone 52b, and the performance of the second grinding stone 52b is not sufficient. Therefore, even if the second grinding wheel 48b was replaced immediately before, it is determined that the second grinding stone 52b is not in a suitable state for grinding and that dressing is necessary (YES in determination step ST1).

[0054] These determinations are made, for example, by the control unit 58 of the grinding apparatus 2. However, an operator or the like of the grinding apparatus 2 may also make these determinations. After determining whether or not dressing of the second grinding stone 52b is necessary, the first grinding wheel 48a and the second grinding wheel 48b are selectively used to grind the surface 11a side of the workpiece 11 (grinding step).

[0055] For example, if it is determined that dressing of the second grinding stone 52b is unnecessary (NO in determination step ST1), the control unit 58 moves the chuck table 18 holding the workpiece 11 from the loading / unloading area A to the rough grinding area B, and then uses the first grinding wheel 48a (the first grinding stone 52a) to rough grind the workpiece 11 (rough grinding step ST2).

[0056] Specifically, as shown in FIG. 2, the chuck table 18 and the first grinding wheel 48a are rotated together, and while supplying liquid from the liquid supply nozzle, the first grinding wheel 48a is gradually lowered. Thereby, the first grinding stone 52a is brought into contact with the surface 11a of the workpiece 11, and the surface 11a side of the workpiece 11 can be ground with high efficiency. For example, when the modified layer 11c on the surface 11a side is completely removed, the rough grinding of the workpiece 11 using the first grinding wheel 48a is completed.

[0057] When the grinding of the workpiece 11 by the first grinding stone 52a is completed, the control unit 58 moves the chuck table 18 holding the workpiece 11 from the rough grinding area B to the finish grinding area C, and then uses the second grinding wheel 48b (second grinding stone 52b) to finish grind the workpiece 11 (finish grinding step ST3).

[0058] Specifically, as shown in FIG. 3, the chuck table 18 and the second grinding wheel 48b are rotated together, and while supplying liquid from the liquid supply nozzle, the second grinding wheel 48b is gradually lowered. Thereby, the second grinding stone 52b is brought into contact with the new surface 11d of the workpiece 11 from which the modified layer 11c has been removed, and the surface 11d side of the workpiece 11 can be ground with high precision. For example, when the flatness of the surface 11d side is enhanced to a certain extent, the finish grinding of the workpiece 11 using the second grinding wheel 48b is completed.

[0059] On the other hand, when it is determined that the dressing of the second grinding stone 52b is necessary (YES in the determination step ST1), the control unit 58 moves the chuck table 18 holding the workpiece 11 from the loading / unloading area A to the finish grinding area C, and then uses the second grinding wheel 48b to grind the workpiece 11. That is, the workpiece 11 is ground using the second grinding wheel 48b without grinding the workpiece 11 using the first grinding wheel 48a (dressing grinding step ST4).

[0060] On the surface 11a side of the workpiece 11, as described above, the modified layer 11c remains, and there are irregularities on the surface 11a due to the modified layer 11c. Therefore, when grinding the surface 11a side of the workpiece 11 where the modified layer 11c remains using the second grinding wheel 48b, the second grinding wheel 52b can be sufficiently consumed to expose the abrasive grains on the surface. That is, the dressing of the second grinding wheel 52b can be performed. FIG. 5 is a cross-sectional view schematically showing the state in which the dressing of the second grinding wheel 52b is performed.

[0061] Specifically, as shown in FIG. 5, the chuck table 18 and the second grinding wheel 48b are rotated together, and while supplying liquid from the liquid supply nozzle, the second grinding wheel 48b is gradually lowered. As a result, the second grinding wheel 52b is brought into contact with the surface 11a of the workpiece 11 where the modified layer 11c remains, and the surface 11a side of the workpiece 11 can be ground while dressing the second grinding wheel 52b due to the irregularities of the surface 11a caused by the modified layer 11c.

[0062] Note that the grinding conditions (the rotation speed of the chuck table 18, the rotation speed of the spindle 42, the lowering speed of the second grinding wheel 48b, etc.) may be the same as the finishing grinding conditions using the second grinding wheel 48b. Of course, other conditions suitable for dressing the second grinding wheel 52b can also be applied.

[0063] Also, the average value of the height difference of the irregularities on the surface of the workpiece 11 is typically 10 μm or more, preferably 20 μm or more and 30 μm or less. Due to the irregularities of the surface 11a having such a height difference, appropriate dressing of the second grinding wheel 52b becomes possible. For example, when the surface 11a side of the workpiece 11 is ground to a certain extent, the grinding of the workpiece 11 accompanied by the dressing of the second grinding wheel 52b is completed. In this grinding, it is not necessary to completely remove the modified layer 11c from the workpiece 11.

[0064] After the grinding of the workpiece 11 with the prominent second grinding wheel 52b is completed, the workpiece 11 is roughly ground using the first grinding wheel 48a (first grinding stone 52a) (rough grinding step ST2), and then the workpiece 11 is finish-ground using the second grinding wheel 48b (second grinding stone 52b) (finish grinding step ST3).

[0065] Thereby, the modified layer 11c remaining on the workpiece 11 can be removed, and the flatness of the workpiece 11 can be enhanced. In addition, when the modified layer 11c is sufficiently thin and the modified layer 11c can be removed in a short time by grinding the workpiece 11 with the prominent second grinding stone 52b, the subsequent rough grinding and finish grinding may be omitted.

[0066] As described above, in the method for grinding a workpiece according to the present embodiment, when it is determined that the prominent second grinding stone 52b in which abrasive grains having an average particle size smaller than that of the abrasive grains of the first grinding stone 52a are dispersed is required, the surface 11a side of the workpiece 11 in which the modified layer 11c remains is ground using the second grinding wheel 48b, so that while making the prominent second grinding stone 52b prominent due to the unevenness of the surface caused by the modified layer 11c, the surface 11a side of the workpiece 11 is ground. Therefore, it is not necessary to use a dedicated dressing board to make the prominent second grinding stone 52b prominent.

[0067] That is, since it is not necessary to stop the grinding apparatus 2 to install the dressing board, even if the frequency of making prominent is increased, it is difficult for the productivity to decrease. Thus, according to the method for grinding a workpiece according to the present embodiment, the decrease in productivity associated with making the grinding stone prominent can be minimized.

[0068] Note that the present invention is not limited to the description of the above-described embodiments and can be implemented with various modifications. For example, in the above-described embodiment, the case where the workpiece 11 is ground using two types of grinding wheels has been described as an example. However, the present invention can also be applied to the case where the workpiece 11 is ground using three or more types of grinding wheels. Typically, before rough grinding, rougher grinding can be performed using a grinding wheel (grinding wheel) containing abrasive grains having a larger average particle size than the abrasive grains of the first grinding wheel 52a.

[0069] In addition, the structures, methods, etc. according to the above-described embodiments and modified examples can be appropriately modified and implemented without departing from the scope of the object of the present invention.

Description of Reference Numerals

[0070] 11: Workpiece 11a: Surface 11b: Back surface 11c: Modified layer 11d: Surface ST1: Judgment step ST2: Rough grinding step ST3: Finish grinding step ST4: Glazing grinding step 2: Grinding device 4: Base 4a: Accommodating portion 6: First conveying mechanism 8a: Cassette 8b: Cassette 10a: Cassette table 10b: Cassette table 12: Position adjusting mechanism 14: Second conveying mechanism 16: Turntable 18: Chuck table 20: Frame body 20a: Recess 20b: Flow path 22: Holding plate 22a: Upper surface 24: Support structure 26: Z-axis moving mechanism 28: Guide rail 30: Moving plate 32: Screw shaft 34: Motor 36: Fixture 38: Grinding unit 40: Spindle housing 42: Spindle 44: Mount 46: Bolt 48a: First grinding wheel 48b: Second grinding wheel 50a: First wheel base 50b: Second wheel base 52a: First grinding stone 52b: Second grinding stone 54: Third conveying mechanism 56: Cleaning unit 58: Control unit

Claims

1. A method for grinding a work piece in the form of a plate, having a modified layer modified by a laser beam exposed on its surface, comprising: a grinding step of selectively using an annular first grinding wheel having a first grinding stone with abrasive grains dispersed therein and an annular second grinding wheel having a second grinding stone with abrasive grains dispersed therein, the average particle size of which is smaller than that of the abrasive grains of the first grinding stone, to grind the surface side of the work piece; a determination step of determining whether dressing of the second grinding stone is necessary to prepare it for grinding before the grinding step; In the grinding step, when it is determined in the determination step that dressing of the second grinding stone is unnecessary, the surface side of the work piece is ground using the first grinding wheel and then the surface side of the work piece is ground using the second grinding wheel; when it is determined in the determination step that dressing is necessary, the surface side of the work piece is ground using the second grinding wheel without grinding the surface side of the work piece using the first grinding wheel, and while dressing the second grinding stone due to the unevenness of the surface caused by the modified layer, the surface side of the work piece is ground. A method for grinding a work piece.

2. In the grinding step, when it is determined in the determination step that dressing is necessary, the surface side of the work piece is ground while dressing the second grinding stone due to the unevenness of the surface caused by the modified layer, and then the surface side of the work piece is ground using the first grinding wheel and then the surface side of the work piece is ground using the second grinding wheel. The method for grinding a work piece according to Claim 1.

3. The method for grinding a work piece according to Claim 1 or Claim 2, wherein the work piece is a semiconductor wafer separated from a semiconductor ingot starting from the modified layer as a separation starting point.

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