Method for grinding a workpiece

Abstract

The present invention provides a workpiece grinding method that efficiently and reliably grinds a workpiece. The workpiece grinding method grinds a workpiece, wherein the workpiece grinding method has the following steps: a first grinding step that grinds the workpiece so that an outer peripheral portion of the workpiece is thinner than a central portion of the workpiece; a measurement step that measures a thickness at the outer peripheral portion of the workpiece or a height at the outer peripheral portion of an upper surface of the workpiece; and a second grinding step that grinds the workpiece, wherein in the second grinding step, grinding is started from the central portion of the workpiece, and grinding is ended when the workpiece reaches a finished thickness determined based on the thickness at the outer peripheral portion of the workpiece or the height at the outer peripheral portion of the upper surface of the workpiece measured by the measurement step.

Description

TECHNICAL FIELD

[0001] The present application relates to a grinding method of a workpiece which is ground to be flattened by a hard material such as SiC (silicon carbide). BACKGROUND

[0002] When manufacturing a device chip mounting a device such as a power device or an LSI (Large Scale Integration) capable of high-temperature operation and high withstand voltage, for example, a SiC wafer in a round plate shape is used. When a plurality of devices are provided on the front surface of the SiC wafer and the SiC wafer is divided for each device, each device chip is obtained.

[0003] The SiC wafer in a round plate shape is manufactured by a method of cutting a SiC ing in a cylindrical shape. For example, a condensing point of a laser beam capable of transmitting a wavelength of SiC is positioned at a depth corresponding to the thickness of the wafer to be manufactured, and the laser beam is irradiated to the SiC ing. Then, a modified layer as a peeling starting point is formed in the inside of the SiC ing (refer to Patent Literature 1).

[0004] Damage accompanying peeling or formation of the modified layer remains on the front surface of the SiC wafer cut from the SiC ing. Therefore, the front surface of the SiC wafer is ground to remove the layer in which the damage has been generated. Grinding of the workpiece such as the SiC wafer is implemented by a grinding device.

[0005] In the grinding device, grinding of the workpiece is implemented in a plurality of stages. First, a first grinding step called rough grinding is implemented: the workpiece is roughly ground at a relatively fast speed to remove the layer in which the damage has been generated. A layer in which the damage has been generated due to the rough grinding is further formed on the ground surface of the workpiece, and therefore a second grinding step called finish grinding is implemented: the workpiece is finely ground at a relatively slow speed to remove the layer in which the damage has been generated.

[0006] The grinding device has a holding table of which the upper surface is a holding surface that holds the workpiece, and a first grinding unit and a second grinding unit disposed above the holding table. Each grinding unit has a grinding wheel on which grinding stones arranged in a circular ring are mounted. Also, the grinding device is capable of rotating the holding table around a table rotation axis passing through the center of the holding surface, and is capable of rotating each grinding wheel to rotate the grinding stones on a circular track. When the grinding unit is lowered to bring the rotating grinding stones into contact with the workpiece, the workpiece is ground.

[0007] The lowering speed of the grinding unit in finish grinding is relatively low, and at the instant when the grinding tool contacts the ground surface of the workpiece at the start of finish grinding after rough grinding, if the contact area increases too rapidly, the grinding tool is not easily bitten into the workpiece. Therefore, the ground surface of the workpiece at the end of rough grinding is made into a conical surface that gradually slopes from the central portion to the outer peripheral portion of the workpiece, and the central portion of the workpiece is made thicker than the outer peripheral portion.

[0008] This can be achieved by making the holding surface of the holding table into a conical surface that gradually slopes, and relatively tilting the holding table with respect to the grinding wheel in such a manner that the generatrix of the holding surface that is closest to the rotating surface of the annular track is not parallel to the rotating surface. Then, at the time of finish grinding, the relative tilt of the holding table and the grinding wheel is adjusted in such a manner that the generatrix of the holding surface that is closest to the rotating surface is parallel to the rotating surface, so that the grinding tool that moves on the annular track contacts the workpiece whose central portion is thicker than the outer peripheral portion.

[0009] In this case, at the start of finish grinding, the grinding tool first contacts the relatively high central portion of the workpiece. At this time, the contact area of the grinding tool with the workpiece is small, so the grinding tool is easily bitten into the workpiece. And as finish grinding proceeds, the height of the central portion decreases, and the contact area gradually increases, and eventually the grinding tool contacts the entire ground surface of the workpiece.

[0010] Patent Literature 1: Japanese Patent Application Publication No. 2016-111143

[0011] When rough grinding is performed on a hard workpiece such as a SiC wafer, the grinding load greatly fluctuates when the state of the grinding tool slightly changes, so the deviation of the grinding result increases. For example, the difference in height between the central portion and the outer peripheral portion of the workpiece greatly fluctuates. In this case where the fluctuation is large, when finish grinding is performed on the workpiece, sometimes the outer peripheral portion of the workpiece is not sufficiently ground.

[0012] Therefore, it is considered to set the removal thickness of the workpiece during finish grinding to be large, but in this case, sometimes the grinding amount is unnecessarily increased. When the grinding amount increases, the processing time increases, and the consumption of the grinding tool increases and frequent replacement work and the like are required, which becomes a major cause of reducing the processing efficiency. SUMMARY

[0013] The present application was completed in view of this problem, and aims to provide a workpiece grinding method that can efficiently and reliably grind a workpiece.

[0014] According to one embodiment of the present application, there is provided a method for grinding a workpiece, the method comprising: a holding step of placing the workpiece on a holding surface of a holding table having a conical surface on an upper surface and being rotatable about a table-rotating shaft passing through the center of the holding surface, and holding the workpiece by the holding table; a first grinding step of rotating a first main shaft having a first grinding wheel with a first grinding tool on a bottom surface fixed to a lower end, rotating the first grinding tool on a first annular track, bringing the holding table and the first grinding wheel closer to each other in a state where a generatrix of the conical surface closest to the first grinding tool is not parallel to the first annular track, and bringing the first grinding tool into contact with the workpiece, thereby grinding the workpiece to make the outer peripheral portion of the workpiece thinner than the central portion of the workpiece; a measuring step of measuring the thickness at the outer peripheral portion of the workpiece or the height at the outer peripheral portion of the upper surface of the workpiece after the first grinding step; and a second grinding step of rotating a second main shaft having a second grinding wheel with a second grinding tool on a bottom surface fixed to a lower end, rotating the second grinding tool on a second annular track, bringing the holding table and the second grinding wheel closer to each other in a state where a generatrix of the conical surface closest to the second grinding tool is parallel to the second annular track, and bringing the second grinding tool into contact with the workpiece, thereby grinding the workpiece, wherein in the second grinding step, grinding is started from the central portion of the workpiece, and grinding is ended when the workpiece reaches a finished thickness determined based on the thickness at the outer peripheral portion of the workpiece or the height at the outer peripheral portion of the upper surface of the workpiece measured by the measuring step.

[0015] Preferably, the second grinding tool includes abrasive grains having a smaller particle diameter than the abrasive grains included in the first grinding tool.

[0016] In the method for grinding a workpiece according to one embodiment of the present application, the thickness at the outer peripheral portion of the workpiece or the like is measured by the measuring step after the first grinding step is performed. Also, in the second grinding step, grinding is ended when the workpiece reaches a finished thickness determined based on the measured thickness at the outer peripheral portion of the workpiece or the like.

[0017] That is, at the time when the first grinding step is ended, the thickness of the workpiece at the time when grinding in the second grinding step is ended is determined based on the thickness of the outer peripheral portion which is thinner than the central portion. Therefore, the workpiece can be ground by a sufficient amount of grinding required to remove the layer in which damage is generated by the first grinding step, to the outer peripheral portion of the workpiece which is the deepest.

[0018] Therefore, according to the present invention, a method for grinding a workpiece capable of grinding the workpiece efficiently and reliably is provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a perspective view schematically showing a grinding device and a workpiece.

[0020] Figure 2 (A) is a cross-sectional view schematically showing the first grinding step, Figure 2 (B) is a cross-sectional view schematically showing the workpiece at the end of the first grinding step.

[0021] Figure 3 It is a cross-sectional view schematically showing the second grinding step.

[0022] Figure 4 (A) is a cross-sectional view schematically showing the workpiece in the middle of the second grinding step. Figure 4 (B) is a cross-sectional view schematically showing the workpiece that has been fully ground in the second grinding step.

[0023] Figure 5 This is a flowchart showing the flow of each step of a method for processing a workpiece.

[0024] Description of labels

[0025] 1: Workpiece; 1a: Grinding surface; 1b: Back surface; 1c: Center; 1d: Peripheral; 3: Protective member; 2: Grinding device; 4: Base; 6: Turntable; 8: Holding table; 8a: Holding surface; 8b: Porous member; 8c: Frame; 8d: Table rotation axis; 8e: Extension line; 8f: Center; 10a, 10b: Grinding unit; 12a, 12b: Spindle motor; 14a, 14b: Spindle; 14c, 14d: Spindle Axis rotation axis; 16a, 16b: grinding wheel mounting seat; 18a, 18b: grinding wheel; 20a, 20b: grinding tool; 20c, 20d: surface; 22a, 22b: column; 24a, 24b: grinding feed unit; 26a, 26b: box loading table; 28a, 28b: box; 30: wafer transfer robot; 32: positioning workbench; 34: loading arm; 36: unloading arm; 38: rotary cleaning device; 40, 42: thickness measuring unit. DETAILED DESCRIPTION

[0026] The embodiment of the present invention will be described with reference to the accompanying drawings. In the grinding method of the workpiece of this embodiment, the workpiece is ground and thinned by a grinding device. First, the workpiece will be described. Figure 1 A perspective view schematically showing the workpiece 1 is included.

[0027] The workpiece 1 is, for example, a substantially circular plate-shaped wafer or the like formed of a material such as Si, SiC, GaN (gallium nitride), GaAs (gallium arsenide), or another semiconductor. In particular, in the workpiece processing method of the present embodiment, a wafer or the like formed of a hard material such as SiC or GaN can be appropriately ground. However, the workpiece 1 is not limited thereto.

[0028] The circular plate-shaped wafer or the like workpiece is formed by cutting a cylindrical ingot. When a circular plate-shaped wafer is formed, a plurality of devices are arranged in a matrix on the front surface of the wafer, and the wafer is divided for each device, thereby obtaining individual device chips.

[0029] In recent years, attention has been focused on SiC wafers as wafers used when manufacturing device chips in which a device such as a power device or an LSI capable of high-temperature operation and high withstand voltage is mounted. An SiC wafer is formed by cutting an SiC ingot (for example, a hexagonal single-crystal ingot).

[0030] When cutting the SiC ingot, for example, a laser beam of a wavelength that transmits through SiC is irradiated to the SiC ingot. At this time, a focal point of the laser beam is positioned at a prescribed depth position corresponding to the thickness of the SiC wafer to be manufactured, and the SiC ingot is irradiated with the laser beam while the focal point is moved relatively horizontally. As a result, a modified layer that serves as a peeling starting point is formed inside the SiC ingot. Furthermore, when the SiC ingot is cut with the modified layer as a starting point, an SiC wafer is obtained.

[0031] On the cut surface of the obtained SiC wafer, a layer in which damage has occurred during cutting and a minute uneven shape or the like remain, and thus grinding is performed on the SiC wafer. In the grinding of the workpiece 1 such as an SiC wafer, a grinding device 2 is used. Hereinafter, a case in which the SiC wafer as the workpiece 1 is ground by the grinding device 2 will be described, but the workpiece 1 in the grinding method of the workpiece of the present embodiment is not limited thereto. A band-shaped protective member 3 is preliminarily attached to the back surface 1b of the workpiece 1 on the opposite side from the ground surface 1a.

[0032] Next, the grinding device 2 that implements the grinding method of the workpiece 1 of the present embodiment will be described in detail. The grinding device 2 has a base 4 that supports each component. Box placement stages 26a and 26b are fixed to the front end of the base 4. For example, a box 28a that accommodates the workpiece 1 before grinding is placed on the box placement stage 26a, and a box 28b that accommodates the workpiece 1 after grinding is placed on the box placement stage 26b.

[0033] A wafer transfer robot 30 is installed on the stage 4 at a position adjacent to the cassette placement stages 26a, 26b. The wafer transfer robot 30 takes out the workpiece 1 from the cassette 28a placed on the cassette placement stage 26a and transfers the workpiece 1 to a positioning table 32 provided on the stage 4 at a position adjacent to the wafer transfer robot 30.

[0034] The positioning table 32 has a plurality of positioning pins arranged in a ring shape. With respect to the positioning table 32, when the workpiece 1 is placed on the central placement area, each of the positioning pins is moved in linkage toward the radially inner side, thereby positioning the workpiece 1 at a predetermined position.

[0035] A loading arm 34 and an unloading arm 36 are provided on the upper surface of the stage 4 at positions adjacent to the positioning table 32. The workpiece 1 positioned at the predetermined position by the positioning table 32 is transferred by the loading arm 34.

[0036] A turntable 6 in the shape of a circular plate is provided on the central upper surface of the stage 4 so as to be rotatable in a horizontal plane. Three holding tables 8 are provided on the upper surface of the turntable 6 at positions separated from each other by 120 degrees in the circumferential direction. Each of the holding tables 8 is movable when the turntable 6 is rotated.

[0037] The holding table 8 has, inside, a suction passage (not shown) connected at one end to a suction source (not shown) and connected at the other end to a holding surface 8a on the holding table 8. As shown in (A) and the like of FIG. 8, the holding surface 8a is composed of a porous member 8b housed in a frame 8c having a recess in the upper surface. The holding table 8 applies, through the porous member 8b, a negative pressure generated by the suction source to the workpiece 1 placed on the holding surface 8a, thereby suction-holding the workpiece 1. Figure 2

[0038] In addition, a rotary drive source (not shown) such as a motor is connected to the bottom of the holding table 8, and the holding table 8 is rotatable about a table rotation axis 8d provided so as to pass through the center 8f of the holding surface 8a. In addition, the bottom of the holding table 8 is supported by a plurality of support shafts (not shown), one or more of which are extendable and retractable. Also, the inclination of the holding surface 8a (the inclination of the table rotation axis 8d) can be changed by adjusting the lengths of these support shafts.

[0039] Returning to Figure 1 ​The explanation will be continued. The carrying-in and carrying-out of the workpiece 1 with respect to the holding stage 8 is performed in the wafer carrying-in and carrying-out area of the turn table 6. In the wafer carrying-in and carrying-out area, the workpiece 1 can be carried-in to the holding stage 8 by the loading arm 34, and the workpiece 1 can be carried-out from the holding stage 8 by the unloading arm 36. After the workpiece 1 is carried-in to the holding stage 8 positioned in the wafer carrying-in and carrying-out area by the loading arm 34, the turn table 6 is rotated to move the holding stage 8 to the next rough grinding area.

[0040] On the outer side of the turn table 6 on the rear side upper surface of the base 4, a first grinding unit 10a that performs rough grinding of the ground surface la of the workpiece 1 held by the holding stage 8 positioned in the rough grinding area is arranged. After the rough grinding of the workpiece 1 is performed by the first grinding unit 10a, the turn table 6 is rotated to move the holding stage 8 to the fine grinding area adjacent to the rough grinding area.

[0041] On the outer side of the turn table 6 on the rear side upper surface of the base 4, a second grinding unit 10b that performs fine grinding of the ground surface la of the workpiece 1 held by the holding stage 8 positioned in the fine grinding area is arranged. After the fine grinding of the workpiece 1 is performed by the second grinding unit 10b, the turn table 6 is rotated to move the holding stage 8 to the wafer carrying-in and carrying-out area, and the workpiece 1 is carried-out from the holding stage 8 by the unloading arm 36.

[0042] In the vicinity of the unloading arm 36 and the wafer transfer robot 30 on the upper surface of the base 4, a spin cleaning device 38 that performs cleaning and spin drying of the ground workpiece 1 is arranged. Further, the workpiece 1 cleaned and dried by the spin cleaning device 38 is carried from the spin cleaning device 38 by the wafer transfer robot 30 and is housed in the cassette 28b placed on the cassette placement table 26b.

[0043] On the front surface of the column 22a, a grinding feed unit 24a that supports the first grinding unit 10a so as to be movable in the vertical direction is arranged. On the front surface of the column 22b, a grinding feed unit 24b that supports the second grinding unit 10b so as to be movable in the vertical direction is arranged.

[0044] The first grinding unit 10a supported by the grinding feed unit 24a has a first main shaft 14a extending in the vertical direction and a main shaft motor 12a connected to the upper end of the first main shaft 14a. Further, the second grinding unit 10b supported by the grinding feed unit 24b has a second main shaft 14b extending in the vertical direction and a main shaft motor 12b connected to the upper end of the second main shaft 14b. Further, the orientation of each main shaft 14a, 14b can be adjusted.

[0045] A circular plate-shaped grinding wheel mounting seat 16a is provided at the lower end of the first main shaft 14a, and a first grinding wheel 18a is fixed to the lower surface of the grinding wheel mounting seat 16a. A plurality of first grinding tools 20a arranged in a circular ring shape are mounted to the lower surface of the first grinding wheel 18a.

[0046] A circular plate-shaped grinding wheel mounting seat 16b is provided at the lower end of the second main shaft 14b, and a second grinding wheel 18b is fixed to the lower surface of the grinding wheel mounting seat 16b. A plurality of second grinding tools 20b arranged in a circular ring shape are mounted to the lower surface of the second grinding wheel 18b.

[0047] When the main shaft motor 12a is caused to operate to rotate the first main shaft 14a, the first grinding wheel 18a is rotated, and the first grinding tools 20a move on the first annular track. Also, when the grinding feed unit 24a is caused to operate to lower the first grinding unit 10a and bring the first grinding tools 20a into contact with the ground surface la of the workpiece 1 held by the holding table 8, the workpiece 1 is ground.

[0048] Also, when the main shaft motor 12b is caused to operate to rotate the main shaft 14b, the second grinding wheel 18b is rotated, and the second grinding tools 20b move on the second annular track. Also, when the grinding feed unit 24b is caused to operate to lower the second grinding unit 10b and bring the second grinding tools 20b into contact with the ground surface la of the workpiece 1 held by the holding table 8, the workpiece 1 is ground.

[0049] In the grinding of the workpiece 1 using the first grinding unit 10a, rough grinding of the workpiece 1 is performed at a relatively fast speed based on the grinding feed of the grinding feed unit 24a. In the rough grinding based on the first grinding unit 10a, a damaged layer and a minute uneven shape formed on the ground surface la of the workpiece 1 are mainly removed. For example, in the case where the workpiece 1 is a SiC wafer obtained by cutting a SiC ingot, the damaged layer formed on the ground surface la in association with the cutting is removed by the rough grinding.

[0050] In the grinding of the workpiece 1 using the second grinding unit 10b, finish grinding of the workpiece 1 is performed at a relatively low speed based on the grinding feed of the grinding feed unit 24b. In the finish grinding based on the second grinding unit 10b, the damaged layer formed on the ground surface la due to the rough grinding is mainly removed. The ground surface la after the finish grinding is removed of the damaged layer and the minute uneven shape, and thus a device can be appropriately formed.

[0051] The first grinding abrasive 20a and the second grinding abrasive 20b contain abrasive grains formed of diamond or the like and a binding material that dispersively fixes the abrasive grains. The second grinding abrasive 20b used in finish grinding preferably contains abrasive grains having a smaller particle diameter than the abrasive grains contained in the first grinding abrasive 20a used in rough grinding. In this case, the workpiece 1 can be rapidly rough-ground by the first grinding abrasive 20a, and on the other hand, the workpiece 1 can be finish-ground with high quality by the second grinding abrasive 20b.

[0052] In the vicinity of the first grinding unit 10a on the upper surface of the base 4, a first thickness measurement unit 40 that measures the thickness of the workpiece 1 that has been rough-ground by the first grinding unit 10a is provided. In the vicinity of the second grinding unit 10b on the upper surface of the base 4, a second thickness measurement unit 42 that measures the thickness of the workpiece 1 that has been finish-ground by the second grinding unit 10b is provided.

[0053] The first thickness measurement unit 40 and the second thickness measurement unit 42 are, for example, contact-type thickness measurement units that come into contact with the ground surface la of the workpiece 1. The contact-type thickness measurement unit has, for example, two probes that extend upward from the upper side of the holding table 8.

[0054] Each probe has a contact portion that extends downward from the front end of an arm portion that extends in the horizontal direction. One probe measures the height of the ground surface la of the workpiece 1 by bringing the lower end of the contact portion into contact with the ground surface la of the workpiece 1. The other probe measures the height of the holding surface 8a of the holding table 8 by bringing the lower end of the contact portion into contact with the holding surface 8a.

[0055] The workpiece 1 is held on the holding surface 8a of the holding table 8 with the protective member 3 interposed therebetween. Therefore, the contact-type thickness measurement unit can calculate the total thickness of the workpiece 1 and the protective member 3 from the difference between the height of the ground surface la of the workpiece 1 and the height of the holding surface 8a of the holding table 8 that are measured.

[0056] Alternatively, the first thickness measurement unit 40 and the second thickness measurement unit 42 are non-contact-type thickness measurement units that do not physically come into contact with the ground surface la of the workpiece 1. The non-contact-type thickness measurement unit, for example, transmits ultrasonic waves or probe light toward the ground surface la of the workpiece 1 from a measurement portion provided directly above the ground surface la, and receives the reflected ultrasonic waves or the like with the measurement portion, analyzes the ultrasonic waves or the like, and thereby measures the height of the ground surface la of the workpiece 1.

[0057] Here, as Figure 2As shown in (A) and the like, the holding surface 8a of the holding table 8 is constituted of a gentle conical surface having the center 8f as the apex. When the holding surface 8a is a conical surface, the workpiece 1 is slightly deformed following the holding surface 8a when the workpiece 1 is held by suction by the holding table 8. Further, the shape of the workpiece 1 or the holding table 8 or the like described in each drawing is exaggerated for the sake of explanation.

[0058] When the workpiece 1 is ground, the holding table 8 is rotated in this state about the table rotation axis 8d, and the grinding units 10a, 10b are lowered while the spindles 14a, 14b are rotated, so that the grinding tools 20a, 20b are brought into contact with the ground surface la of the workpiece 1. Then, the workpiece 1 placed on the holding table 8 is rotated while being ground in the circular arc-shaped region from the outer peripheral portion Id to the central portion lc of the workpiece 1, so that the entire region of the workpiece 1 is ground.

[0059] Here, the lowering speed of the second grinding unit 10b in the finish grinding is relatively low, and therefore, when the contact area rises too sharply at the instant when the second grinding tool 20b comes into contact with the ground surface la of the workpiece 1, the second grinding tool 20b does not easily bite into the workpiece 1. Therefore, the first grinding unit 10a is operated to perform rough grinding in such a manner that the ground surface la of the workpiece 1 becomes a conical surface gently inclined from the central portion lc to the outer peripheral portion Id of the workpiece 1, so that the central portion lc of the workpiece 1 is made thicker than the outer peripheral portion Id.

[0060] This can be achieved by relatively inclining the holding table 8 with respect to the first grinding wheel 18a in such a manner that the generatrix of the holding surface 8a (which is constituted of a conical surface) closest to the rotational surface of the first grinding tool 20a including the first annular track is not parallel to the rotational surface.

[0061] Then, when the finish grinding is performed, the relative inclination of the holding table 8 with respect to the second grinding wheel 18b is adjusted in such a manner that the rotational surface of the second grinding tool 20b including the second annular track is parallel to the generatrix of the holding surface 8a closest to the rotational surface. Also, the second grinding tool 20b moving on the second annular track is brought into contact with the workpiece 1 whose central portion lc is thicker than the outer peripheral portion Id.

[0062] In this case, when the finish grinding is started, first, the second grinding tool 20b comes into contact with the relatively high central portion lc of the workpiece 1. At this time, the contact area of the second grinding tool 20b with the workpiece 1 is small, and therefore, the second grinding tool 20b easily bites into the workpiece 1. Also, as the finish grinding proceeds, the height of the central portion lc decreases, and the contact area gradually increases, and finally, the second grinding tool 20b comes into contact with the entire ground surface la of the workpiece 1.

[0063] For example, in the case where the workpiece 1 is a 4-inch diameter SiC wafer, rough grinding based on the first grinding unit 10a is performed in such a manner that the central portion 1c of the workpiece 1 is higher than the outer peripheral portion 1d by about 5 μm. Further, by finish grinding the workpiece 1 to a depth of about 5 μm from the ground surface 1a, the layer in which damage has been caused by rough grinding can be sufficiently removed. Therefore, in finish grinding, the workpiece 1 is ground in such a manner that the entire upper surface is exposed at a height position that is lower than the outer peripheral portion 1d of the ground surface 1a of the workpiece 1 after rough grinding by about 5 μm.

[0064] In the case where finish grinding using the second grinding unit 10b is performed, the height of the ground surface 1a is monitored using the second thickness measuring unit 42 at an intermediate region between the central portion 1c and the outer peripheral portion 1d of the ground surface 1a of the workpiece 1. For example, in this example, at the time when rough grinding is completed, the height position of this intermediate region of the ground surface 1a is higher than the outer peripheral portion 1d by about 2.5 μm. Therefore, in order to remove the layer in which damage has been caused in the entire region including the outer peripheral portion 1d, it is considered that finish grinding is performed in such a manner that the ground surface 1a is lowered by about 7.5 μm at the intermediate region.

[0065] However, in the case where rough grinding is performed on a hard workpiece 1 such as an SiC wafer, the deviation of the grinding result becomes large due to a change in the state of the first grinding tool 20a, a large variation in the grinding load, or the like. For example, the difference in height between the central portion 1c and the outer peripheral portion 1d of the workpiece 1 greatly varies. In the case where this difference in height is larger than expected and the ground surface 1a is inclined more than expected, when the workpiece 1 has been removed by finish grinding to a prescribed thickness, the outer peripheral portion 1d of the workpiece 1 is sometimes not sufficiently ground.

[0066] In the above example, in rough grinding, in the case where the height of the central portion 1c is higher than the height of the outer peripheral portion 1d by about 10 μm, the height of the intermediate region between the central portion 1c and the outer peripheral portion 1d of the ground surface 1a is higher than the height of the outer peripheral portion 1d by about 5 μm. In this case, if finish grinding is performed in such a manner that the intermediate region is lowered by about 10 μm, the layer in which damage has been caused cannot be removed in the entire region of the ground surface 1a. Therefore, when finish grinding is performed in such a manner that the intermediate region is lowered by about 7.5 μm as originally expected, the outer peripheral portion 1d of the workpiece 1 cannot be sufficiently ground, and the layer in which damage has been caused by rough grinding remains in the workpiece 1.

[0067] Therefore, it is considered to set the removal thickness of the workpiece 1 in the finish grinding to be large, but in this case, the grinding amount increases. When the grinding amount increases, the processing time increases, and the consumption of the second grinding tool 20b increases, and frequent replacement work and the like are required, which becomes a main cause of reducing the processing efficiency. In addition, in the case where the difference in height between the central portion 1c and the outer peripheral portion 1d of the ground surface 1a is smaller than expected when the rough grinding is performed, the workpiece 1 is excessively ground in the finish grinding.

[0068] Therefore, in the workpiece grinding method of the present embodiment described below, after the rough grinding based on the first grinding unit 10a is finished, the thickness at the outer peripheral portion 1d of the workpiece 1 is measured, and the finished thickness of the workpiece 1 is determined based on the measured value. Next, the workpiece grinding method of the present embodiment will be described. Figure 5 is a flowchart showing the flow of each step of the workpiece grinding method of the present embodiment. Hereinafter, each step of the workpiece grinding method of the present embodiment will be described in detail.

[0069] In the workpiece grinding method of the present embodiment, first, the holding step S10 of placing the workpiece 1 on the holding surface 8a of the holding table 8 and holding the workpiece 1 by the holding table 8 is performed. The holding table 8 is used Figure 1 The holding step S10 will be described in detail.

[0070] In the holding step S10, first, the workpiece 1 is carried from the cassettes 28a, 28b placed on the cassette placement tables 26a, 26b to the positioning table 32 by the wafer carrying robot 30, and the position of the workpiece 1 is adjusted. Then, the workpiece 1 is carried to the holding surface 8a of the holding table 8 positioned in the wafer carrying-in and carrying-out area by the loading arm 34. At this time, the ground surface 1a side of the workpiece 1 is exposed upward, and the back surface 1b side faces the holding surface 8a. Then, the workpiece 1 is suction-held by the holding table 8.

[0071] Then, in order to perform the first grinding step S20 described below, the turntable 6 is rotated to move the holding table 8 holding the workpiece 1 to the rough grinding area below the first grinding unit 10a.

[0072] Next, the first grinding step S20 is performed. Figure 2 (A) of FIG. 10 is a cross-sectional view schematically showing the first grinding step S20. In Figure 2 (A) of FIG. 10 is a cross-sectional view schematically showing the first grinding step S20. In

[0073] In the first grinding step S20, the relative orientation of the first grinding unit 10a and the holding table 8 is first adjusted. More specifically, the generatrix of the conical holding surface 8a closest to the first grinding tool 20a is made non-parallel to the first annular track. In Figure 2 In (A) of FIG. 8, the extension line 8e of the generatrix and the surface 20c including the first annular track are shown by single-dot chain lines for ease of explanation.

[0074] In this state, the first spindle 14a is rotated around the spindle rotation axis 14c, the first grinding tool 20a is rotated on the first annular track, and the holding table 8 is rotated around the table rotation axis 8d. Also, the grinding feed unit 24a is operated to lower the first grinding unit 10a, and the like, so as to approach the holding table 8 and the first grinding wheel 18a. Then, the bottom surface of the first grinding tool 20a comes into contact with the ground surface la of the workpiece 1, and the workpiece 1 is ground.

[0075] The lowering speed of the first grinding unit 10a at this time is greater than the lowering speed of the second grinding unit 10b in the second grinding step S40 described later. When the first grinding step S20 is performed, the workpiece 1 is rough-ground. At this time, the thickness of the workpiece 1 is monitored using the first thickness measuring unit 40. Also, when the workpiece 1 is rough-ground to a prescribed thickness, the lowering of the first grinding unit 10a is stopped, and the rough grinding is ended.

[0076] Here, in the case where the workpiece 1 is a SiC wafer cut from an ingot, the prescribed thickness is set in a manner that can remove a damage layer formed on the ground surface la due to the cutting. Figure 2 (B) of FIG. 8 is a cross-sectional view schematically showing the workpiece 1 that has been rough-ground when the first grinding step S20 is ended. As shown in Figure 2 (B) of FIG. 8, when the workpiece 1 is rough-ground by the first grinding step S20, the outer peripheral portion Id of the workpiece 1 is thinner than the central portion Ic.

[0077] On the ground surface la of the workpiece 1 that has been rough-ground, there remain minute unevenness or damage such as chipping, which is called a crack, and the like, generated due to the rough grinding. Therefore, the ground surface la of the workpiece 1 is next planarized while removing the layer in which the damage has been generated by performing finish grinding on the workpiece 1. Here, in the workpiece grinding method of the present embodiment, the measuring step S30 is performed after the first grinding step S20 and before the second grinding step S40 is performed.

[0078] In the measurement step S30, the thickness at the outer peripheral portion Id of the workpiece 1 or the height at the outer peripheral portion Id of the upper surface (ground surface la) of the workpiece 1 is measured. This measurement is performed, for example, by the first thickness measurement unit 40 provided in the vicinity of the first grinding unit 10a immediately after the first grinding step S20 is completed. Alternatively, in the measurement step S30, the turntable 6 is rotated to send the holding table 8 to the fine grinding area under the second grinding unit 10b, and the measurement is performed by the second thickness measurement unit 42.

[0079] After the measurement step S30, the second grinding step S40 is performed. In the case where the holding table 8 holding the workpiece 1 is not moved in the measurement step S30, first, the turntable 6 is rotated to send the holding table 8 to the fine grinding area in the second grinding step S40. Figure 3 Fig. 8 is a cross-sectional view schematically showing the second grinding step S40. In Figure 3 Fig. 9 is a side view schematically showing the second grinding unit 10b that grinds the workpiece 1 in the second grinding step S40.

[0080] Here, the relative orientation of the second grinding unit 10b and the holding table 8 is adjusted. More specifically, the generatrix of the conical holding surface 8a closest to the second grinding tool 20b is made parallel to the second annular track of the second grinding tool 20b. In Figure 3 In Fig. 9, for convenience of explanation, the extension line 8e of the generatrix and the surface 20d including the second annular track are shown by single-dot chain lines.

[0081] In this state, the second spindle 14b is rotated around the spindle rotation axis 14d, the second grinding tool 20b is rotated on the second annular track, and the holding table 8 is rotated around the table rotation axis 8d. Also, the grinding feed unit 24b is operated to lower the second grinding unit 10b, and the like, so that the holding table 8 and the second grinding wheel 18b are brought close to each other. Then, the bottom surface of the second grinding tool 20b comes into contact with the ground surface la of the workpiece 1, and the workpiece 1 is ground.

[0082] Here, the second grinding tool 20b moving on the second annular track first comes into contact with the highest central portion lc of the ground surface la of the workpiece 1. Figure 4 (A) of Fig. 10 is a cross-sectional view schematically showing the workpiece 1 at the middle of the second grinding step S40. Also, in the central portion lc, as the height of the workpiece 1 becomes lower, the contact area of the second grinding tool 20b with the ground surface la gradually increases. Therefore, the second grinding tool 20b appropriately bites into the ground surface la, and appropriately grinds the ground surface la.

[0083] In the second grinding step S40, grinding is ended when the workpiece 1 reaches a finished thickness determined from the thickness at the outer peripheral portion Id of the workpiece 1 or the height at the outer peripheral portion Id of the upper surface (ground surface la) of the workpiece 1 measured by the measuring step S30. The finished thickness can be derived, for example, by subtracting the depth of the damaged layer formed on the ground surface la of the workpiece 1 by the rough grinding from the thickness at the outer peripheral portion Id of the workpiece 1 measured by the measuring step S30.

[0084] For example, in a case where finish grinding to a depth of 5 μm or more is required in order to sufficiently remove the damaged layer, a height position that is 5 μm lower than the height of the ground surface la at the outer peripheral portion Id before the finish grinding is set as the finished height position of the entire region of the workpiece 1. Also, the thickness of the workpiece 1 when the entire region of the ground surface la of the workpiece 1 reaches this finished height is set as the finished thickness of the workpiece 1.

[0085] In the second grinding step S40, for example, the workpiece 1 is finish ground while the thickness of the workpiece 1 is monitored by the second thickness measuring unit 42, and when the thickness of the workpiece 1 becomes the finished thickness, the descent of the second grinding unit 10b is stopped. Figure 4 (B) of FIG. 10 is a cross-sectional view schematically showing the workpiece 1 at the completion of grinding by the second grinding unit 10b. In Figure 4 In (B) of FIG. 10, the second grinding unit 10b and the like are omitted for convenience of explanation.

[0086] According to the grinding method of the workpiece of the present embodiment, even when the damaged layer by the rough grinding of the first grinding step S20 advances to the outer peripheral portion Id of the workpiece 1 that is the deepest, finish grinding of the workpiece 1 can be performed in a manner that removes the damaged layer by the second grinding step S40. Therefore, finish grinding is not ended in a state where finish grinding is not performed on the outer peripheral portion Id of the workpiece 1 as shown in (A) of FIG. 10. Figure 4 (B) of FIG. 10.

[0087] That is, even in a case where a large deviation occurs in the difference in height between the outer peripheral portion Id and the central portion Ic of the workpiece 1 in the rough grinding, finish grinding required for the workpiece 1 can be reliably performed. On the other hand, even in a case where the difference in height between the outer peripheral portion Id and the central portion Ic of the workpiece 1 is smaller than expected in the rough grinding, the workpiece 1 is not excessively finish ground.

[0088] As explained above, the workpiece grinding method according to the present embodiment can efficiently and reliably grind the workpiece 1 even in a case where the workpiece 1 as a grinding target is a hard member and the deviation of the processing result of the rough grinding is large. However, the workpiece 1 is not limited, and efficient and reliable grinding can be performed even in a case where the workpiece 1 is not hard.

[0089] In addition, the present application is not limited to the above-described embodiments, and various modifications can be made and implemented. For example, in the above-described embodiments, a case where the ground surface la is inclined so as to be lower at the outer peripheral portion Id than at the central portion Ic in the rough grinding performed by the first grinding step S20 is explained, but one embodiment of the present application is not limited thereto.

[0090] In order to easily bite the second grinding tool 20b into the ground surface la of the workpiece 1 at the start of the second grinding step S40, the first grinding step S20 can also be performed to rough grind the workpiece 1 so as to be thinner at the central portion Ic than at the outer peripheral portion Id. In this case, in the measurement step S30, the thickness at the central portion Ic of the workpiece 1 or the height position at the central portion Ic of the upper surface (ground surface la) of the workpiece 1 is measured by the first thickness measurement unit 40 or the second thickness measurement unit 42.

[0091] Further, in the second grinding step S40, grinding is ended when the workpiece 1 reaches the finished thickness decided from the thickness at the central portion Ic of the workpiece 1 or the height position at the central portion Ic of the upper surface of the workpiece 1 measured by the measurement step S30. Thus, even in a case where the central portion Ic of the workpiece 1 is thinned in the rough grinding, the workpiece grinding method according to one embodiment of the present application can efficiently and reliably grind the workpiece.

[0092] Further, it is also considered that in a case where the deviation of the processing result of the rough grinding performed by the first grinding step S20 is extremely large, it is not determined which of the central portion Ic and the outer peripheral portion Id of the workpiece 1 is thinned. In this case, in the measurement step S30, it is preferable to measure the thickness or the like at an arbitrary position of the central portion Ic and the outer peripheral portion Id of the workpiece 1. Further, in the second grinding step S40, it is preferable to end grinding when the workpiece 1 reaches the finished thickness decided from the thickness of the workpiece 1 measured at the position of the thinner one of the central portion Ic and the outer peripheral portion Id.

[0093] The configuration, method, and the like of the above-described embodiments can be appropriately changed and implemented without departing from the scope of the present application.

Claims

1. A method for grinding a workpiece, wherein the workpiece is ground, The workpiece grinding method comprises the following steps: a holding step of placing the workpiece on a holding surface of a holding table having a conical holding surface on its upper surface and rotatable about a table rotation axis passing through the center of the holding surface, and holding the workpiece by the holding table; The first grinding step comprises rotating a first spindle having a first grinding wheel having a first grinding tool on its bottom surface fixed at its lower end, rotating the first grinding tool on a first annular track, bringing the holding table and the first grinding wheel closer together in a state where a generatrix of the conical holding surface closest to the first grinding tool is not parallel to the first annular track, and bringing the first grinding tool into contact with the workpiece, thereby grinding the workpiece so that the outer periphery of the workpiece is thinner than the central portion of the workpiece. a measuring step of measuring, after the first grinding step, a thickness of the workpiece at the peripheral portion or a height of the upper surface of the workpiece at the peripheral portion; as well as The second grinding step comprises rotating a second spindle having a second grinding wheel having a second grinding tool on its bottom surface fixed at its lower end, rotating the second grinding tool on a second annular track, bringing the holding table and the second grinding wheel closer together in a state where a generatrix of the conical holding surface closest to the second grinding tool is parallel to the second annular track, and bringing the second grinding tool into contact with the workpiece, thereby grinding the workpiece. In the second grinding step, grinding is started from the center portion of the workpiece, and grinding is terminated when the workpiece reaches a finished thickness determined based on the thickness at the peripheral portion of the workpiece or the height at the peripheral portion of the upper surface of the workpiece measured in the measuring step.

2. The method for grinding a workpiece according to claim 1, wherein: The second grinding stone includes abrasive grains having a smaller grain size than the abrasive grains included in the first grinding stone.

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