Method for grinding a workpiece

Abstract

To surely grind workpiece with high efficiency.SOLUTION: A grinding method for grinding workpiece includes a first grinding step of grinding the workpiece so that an outer peripheral part of the workpiece is thinner than a central part of the workpiece, a measurement step of measuring the thickness in the outer peripheral part of the workpiece or the height in the outer peripheral part on an upper surface of the workpiece, and a second grinding step of grinding the workpiece, in which the second grinding step starts grinding from the central part of the workpiece, and ends grinding when the thickness of the workpiece reaches a finish thickness determined based on the thickness of the workpiece in the outer peripheral part measured by the measurement step or the height in the outer peripheral part on the upper surface of the workpiece.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a method for grinding a workpiece, which grinds and flattens a workpiece made of a hard material such as SiC (silicon carbide). [Background technology]

[0002] When manufacturing device chips equipped with devices such as power devices capable of high-temperature operation and high-voltage resistance, and LSI (Large Scale Integration), for example, disk-shaped SiC wafers are used. Multiple devices are provided on the surface of the SiC wafer, and the SiC wafer is divided into individual devices to obtain individual device chips.

[0003] A disk-shaped SiC wafer is manufactured by cutting a cylindrical SiC ingot. For example, a laser beam with a wavelength that can penetrate SiC is focused at a depth corresponding to the thickness of the wafer to be manufactured, and the SiC ingot is irradiated with the laser beam. Then, a modified layer that becomes the starting point for peeling is formed inside the SiC ingot (see Patent Document 1).

[0004] Damage caused by peeling and the formation of modified layers remains on the surface of SiC wafers cut from SiC ingots. Therefore, the surface of the SiC wafer is ground to remove the damaged layer. Grinding of workpieces such as SiC wafers is performed using a grinding machine.

[0005] In the grinding machine, the grinding of the workpiece is performed in several stages. First, a first grinding step called rough grinding is performed, in which the workpiece is largely ground at a relatively high speed to remove the damaged layer. Since the rough grinding causes a further damaged layer to form on the ground surface of the workpiece, a second grinding step called finish grinding is performed, in which the workpiece is ground at a relatively slow speed to a high quality to remove the damaged layer.

[0006] The grinding device includes a holding table, the upper surface of which serves as a holding surface for holding the workpiece, and a first grinding unit and a second grinding unit disposed above the holding table. Each grinding unit includes a grinding wheel having grinding stones arranged in an annular shape. The grinding device can rotate the holding table around a table rotation axis passing through the center of the holding surface, and can rotate each grinding wheel to rotate the grinding stones on an annular orbit. When the grinding units are lowered to bring the rotating grinding stones into contact with the workpiece, the workpiece is ground.

[0007] The descent speed of the grinding unit in finish grinding is relatively slow, and at the moment when the grinding wheel comes into contact with the grinding surface of the workpiece when finish grinding is started after rough grinding has been performed, if the increase in the contact area is too steep, it will be difficult for the grinding wheel to bite into the workpiece. Therefore, the grinding surface of the workpiece when rough grinding is completed is made into a conical surface that gently slopes from the center to the outer periphery of the workpiece, and the center of the workpiece is made thicker than the outer periphery.

[0008] This can be achieved by forming the holding surface of the holding table into a gently inclined conical surface, and then tilting the holding table relative to the grinding wheel so that the generatrix constituting the holding surface that is closest to the rotational surface including the annular orbit of the grinding wheel is non-parallel to the rotational surface. Then, when performing finish grinding, the relative inclination of the holding table and the grinding wheel is adjusted so that the generatrix of the holding surface that is closest to the rotational surface is parallel to the rotational surface, and the grinding wheel moving on the annular orbit is brought into contact with the workpiece whose center is thicker than its outer periphery.

[0009] In this case, when finish grinding starts, the grinding wheel first comes into contact with the relatively high central part of the workpiece. At this time, the contact area of ​​the grinding wheel with the workpiece is small, so the grinding wheel easily bites into the workpiece. Then, as finish grinding progresses and the height of the central part decreases, the contact area gradually increases, and finally the grinding wheel comes into contact with the entire surface of the workpiece to be ground. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] JP 2016-111143 A Summary of the Invention [Problem to be solved by the invention]

[0011] When rough grinding hard workpieces such as SiC wafers, even a slight change in the condition of the grinding wheel causes a large variation in the grinding load, resulting in a large variation in the grinding results. For example, the height difference between the center and periphery of the workpiece varies greatly. If this variation is large, the periphery of the workpiece may not be sufficiently ground when the workpiece is finish ground.

[0012] To cope with this, it is possible to set the thickness to be removed from the workpiece in the finish grinding to a large value, but this can result in an unnecessarily large amount of grinding. A large amount of grinding increases the processing time and wears the grinding wheel, requiring frequent replacement, which reduces the processing efficiency.

[0013] The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for grinding a workpiece, which can grind the workpiece highly efficiently and reliably. [Means for solving the problem]

[0014] According to one aspect of the present invention, a grinding method for grinding a workpiece includes a holding step of placing the workpiece on a holding surface of a holding table having a conical holding surface on an upper surface thereof and rotatable around a table rotation axis passing through the center of the holding surface, and holding the workpiece on the holding table; a first grinding step of rotating a first spindle having a first grinding wheel having a first grinding wheel on a bottom surface thereof fixed to a lower end thereof to rotate the first grinding wheel on a first annular orbit, and bringing the holding table and the first grinding wheel close together while making the generatrix of the conical holding surface closest to the first grinding wheel non-parallel to the first annular orbit, thereby grinding the workpiece so that an outer periphery of the workpiece is thinner than a central portion of the workpiece; and a second grinding step of grinding the workpiece so that the outer periphery of the workpiece is thinner than a central portion of the workpiece after the first grinding step. a measuring step of measuring a thickness at an outer periphery or a height of an upper surface of the workpiece at the outer periphery; and a second grinding step of rotating a second spindle having a second grinding wheel having a second grinding wheel on a bottom surface thereof fixed to a lower end thereof to rotate the second grinding wheel on a second annular track, and bringing the holding table and the second grinding wheel close to the second grinding wheel while making the generatrix of the conical holding surface closest to the second grinding wheel parallel to the second annular track, thereby grinding the workpiece, wherein in the second grinding step, grinding is started from the center of the workpiece, and grinding is terminated when the workpiece reaches a finishing thickness determined based on the thickness of the workpiece at the outer periphery measured in the measuring step or the height of the upper surface of the workpiece at the outer periphery. The finishing thickness is determined by subtracting a depth at which a damaged layer formed in the workpiece in the first grinding step can be removed from the thickness of the workpiece at the outer periphery measured in the measuring step or the height of the upper surface of the workpiece at the outer periphery. The present invention provides a method for grinding a workpiece, comprising the steps of:

[0015] Preferably, the second grinding wheel contains abrasive grains having a smaller grain size than the grain size of the abrasive grains contained in the first grinding wheel. Effect of the Invention

[0016] In a method for grinding a workpiece according to one aspect of the present invention, after a first grinding step is performed, a measuring step is performed to measure the thickness of the workpiece at its outer periphery, etc. Then, in a second grinding step, grinding is terminated when the workpiece reaches a finish thickness determined from the measured thickness of the workpiece at its outer periphery, etc.

[0017] That is, the thickness of the workpiece when grinding in the second grinding step is to be ended is determined based on the thickness of the outer periphery, which is thinner than the central part at the time when the first grinding step is ended. Therefore, the workpiece can be ground with a necessary and sufficient grinding amount to remove the layer at the outer periphery of the workpiece, where the layer damaged in the first grinding step reaches its deepest.

[0018] Therefore, the present invention provides a method for grinding a workpiece that can grind the workpiece highly efficiently and reliably. [Brief description of the drawings]

[0019] [Figure 1] FIG. 2 is a perspective view showing a grinding device and a workpiece; [Diagram 2] FIG. 2(A) is a cross-sectional view that typically shows a first grinding step, and FIG. 2(B) is a cross-sectional view that typically shows the workpiece at the end of the first grinding step. [Diagram 3] FIG. 4 is a cross-sectional view illustrating a second grinding step. [Figure 4] FIG. 4(A) is a cross-sectional view showing a schematic of a workpiece during the second grinding step, and FIG. 4(B) is a cross-sectional view showing a schematic of a workpiece that has been sufficiently ground in the second grinding step. [Diagram 5] 3 is a flowchart showing the flow of each step of a method for machining a workpiece. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] An embodiment of the present invention will be described with reference to the drawings. In the method for grinding a workpiece according to this embodiment, the workpiece is ground and thinned by a grinding device. First, the workpiece will be described. FIG. 1 includes a perspective view showing a workpiece 1 in a schematic manner.

[0021] The workpiece 1 is, for example, a substantially disk-shaped wafer made of a material such as Si, SiC, GaN (gallium nitride), GaAs (gallium arsenide), or other semiconductor. In particular, the method for processing a workpiece according to this embodiment can suitably grind the workpiece 1 such as a wafer made of a hard material such as SiC or GaN. However, the workpiece 1 is not limited to this.

[0022] A workpiece such as a disk-shaped wafer is formed, for example, by cutting a cylindrical ingot. A disk-shaped wafer is formed, a plurality of devices are arranged in a matrix on the surface of the wafer, and the wafer is divided into individual device chips.

[0023] In recent years, SiC wafers have been attracting attention as wafers used in manufacturing device chips equipped with power devices capable of high-temperature operation and high-voltage resistance, LSIs, etc. SiC wafers are formed by slicing SiC ingots (e.g., hexagonal single crystal ingots).

[0024] When cutting a SiC ingot, for example, a laser beam with a wavelength that transmits SiC is irradiated onto the SiC ingot. At this time, the focal point of the laser beam is positioned at a predetermined depth position corresponding to the thickness of the SiC wafer to be manufactured, and the laser beam is irradiated onto the SiC ingot while the focal point is relatively moved horizontally. Then, a modified layer that becomes the peeling starting point is formed inside the SiC ingot. Then, the SiC ingot is cut using the modified layer as the starting point to obtain a SiC wafer.

[0025] Since the cut surface of the obtained SiC wafer has layers damaged during the cutting process and minute irregularities, the SiC wafer is ground. A grinding device 2 is used to grind a workpiece 1 such as a SiC wafer. Hereinafter, a case where a SiC wafer is ground as the workpiece 1 by the grinding device 2 will be described, but the workpiece 1 in the grinding method of the workpiece according to this embodiment is not limited to this. A tape-like protective member 3 is attached in advance to a back surface 1b of the workpiece 1 opposite to the grinding surface 1a to be ground by the grinding device 2.

[0026] Next, the grinding device 2 in which the grinding method for the workpiece 1 according to this embodiment is carried out will be described in detail. The grinding device 2 includes a base 4 that supports each of the components. Cassette mounting tables 26a and 26b are fixed to the front end of the base 4. For example, a cassette 28a that contains the workpiece 1 before grinding is placed on the cassette mounting table 26a, and a cassette 28b for containing the workpiece 1 after grinding is placed on the cassette mounting table 26b.

[0027] A wafer transport robot 30 is installed at a position adjacent to the cassette mounting tables 26a, 26b on the base 4. The wafer transport robot 30 takes out the workpiece 1 from the cassette 28a placed on the cassette mounting table 26a, and transports the workpiece 1 to a positioning table 32 provided at a position adjacent to the wafer transport robot 30 on the base 4.

[0028] The positioning table 32 has a plurality of positioning pins arranged in a ring shape. When the workpiece 1 is placed in the central placement area, the positioning table 32 moves each of the positioning pins radially inward in an interlocking manner, thereby positioning the workpiece 1 at a predetermined position.

[0029] A loading arm 34 and an unloading arm 36 are provided at positions adjacent to the positioning table 32 on the upper surface of the base 4. The workpiece 1 positioned at a predetermined position by the positioning table 32 is transported by the loading arm 34.

[0030] A disk-shaped turntable 6 is provided rotatably in a horizontal plane on the central upper surface of the base 4. Three holding tables 8 spaced 120 degrees apart from each other in the circumferential direction are provided on the upper surface of the turntable 6. When the turntable 6 is rotated, each holding table 8 can be moved.

[0031] The holding table 8 has an internal suction path (not shown) one end of which is connected to a suction source (not shown), and the other end of the suction path is connected to a holding surface 8a on the holding table 8. As shown in FIG. 2(A) and other figures, the holding surface 8a is made of a porous member 8b, and the porous member 8b is housed in a frame 8c having a recess on its upper surface. The holding table 8 suction-holds the workpiece 1 placed on the holding surface 8a by applying negative pressure generated by the suction source through the porous member 8b to the workpiece 1.

[0032] 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 can rotate around a table rotation axis 8d that is set to pass through the center 8f of the holding surface 8a. The bottom of the holding table 8 is supported by a number of support shafts (not shown), one or more of which are extendable and retractable. By adjusting the length of these support shafts, the inclination of the holding surface 8a (the inclination of the table rotation axis 8d) can be changed.

[0033] Returning to Fig. 1, the explanation will be continued. The workpiece 1 is loaded onto and unloaded from the holding table 8 in the wafer loading / unloading area of ​​the turntable 6. In the wafer loading / unloading area, the workpiece 1 can be loaded onto the holding table 8 by the loading arm 34, and can be unloaded from the holding table 8 by the unloading arm 36. After the workpiece 1 is loaded onto the holding table 8 positioned in the wafer loading / unloading area by the loading arm 34, the turntable 6 is rotated to move the holding table 8 to the next rough grinding area.

[0034] A first grinding unit 10a that roughly grinds the grinding surface 1a of the workpiece 1 held by a holding table 8 positioned in a rough grinding area is disposed outside the turntable 6 on the rear upper surface of the base 4. After the workpiece 1 has been roughly ground by the first grinding unit 10a, the turntable 6 is rotated to move the holding table 8 to a finish grinding area adjacent to the rough grinding area.

[0035] A second grinding unit 10b is disposed outside the turntable 6 on the upper rear surface of the base 4, for finish-grinding the grinding surface 1a of the workpiece 1 held on the holding table 8 positioned in the finish grinding area. After the second grinding unit 10b has performed finish grinding of the workpiece 1, the turntable 6 is rotated to return the holding table 8 to the wafer load / unload area, and the workpiece 1 is unloaded from the holding table 8 by the unloading arm 36.

[0036] A spinner cleaning device 38 that cleans and spin-dries the ground workpiece 1 is disposed near the unloading arm 36 on the upper surface of the base 4 and the wafer transport robot 30. The workpiece 1 cleaned and dried by the spinner cleaning device 38 is then transported from the spinner cleaning device 38 by the wafer transport robot 30 and stored in a cassette 28b placed on the cassette mounting table 26b.

[0037] Columns 22a and 22b are provided in an upright position at the rear of the base 4. A grinding feed unit 24a that supports the first grinding unit 10a so as to be movable along the vertical direction is provided in front of the column 22a. A grinding feed unit 24b that supports the second grinding unit 10b so as to be movable along the vertical direction is provided in front of the column 22b.

[0038] The first grinding unit 10a supported by the grinding feed unit 24a includes a first spindle 14a extending along the vertical direction and a spindle motor 12a connected to the upper end of the first spindle 14a. The second grinding unit 10b supported by the grinding feed unit 24b includes a second spindle 14b extending along the vertical direction and a spindle motor 12b connected to the upper end of the second spindle 14b. The orientation of each of the spindles 14a and 14b may be adjustable.

[0039] A disk-shaped wheel mount 16a is disposed at the lower end of the first spindle 14a, and a first grinding wheel 18a is fixed to the lower surface of the wheel mount 16a. A plurality of first grinding stones 20a arranged in an annular shape are attached to the lower surface of the first grinding wheel 18a.

[0040] A disk-shaped wheel mount 16b is disposed at the lower end of the second spindle 14b, and a second grinding wheel 18b is fixed to the lower surface of the wheel mount 16b. A plurality of second grinding stones 20b arranged in an annular shape are attached to the lower surface of the second grinding wheel 18b.

[0041] When the spindle motor 12a is operated to rotate the first spindle 14a, the first grinding wheel 18a rotates and the first grinding stone 20a moves on the first annular track. Then, the grinding feed unit 24a is operated to lower the first grinding unit 10a, and the first grinding stone 20a is brought into contact with the grinding surface 1a of the workpiece 1 held on the holding table 8, and the workpiece 1 is ground.

[0042] When the spindle motor 12b is operated to rotate the spindle 14b, the second grinding wheel 18b rotates and the second grinding wheel 20b moves on the second annular track. Then, the grinding feed unit 24b is operated to lower the second grinding unit 10b, and the second grinding wheel 20b is brought into contact with the grinding surface 1a of the workpiece 1 held on the holding table 8, whereby the workpiece 1 is ground.

[0043] In grinding the workpiece 1 using the first grinding unit 10a, the grinding feed unit 24a performs grinding feed at a relatively high speed to roughly grind the workpiece 1. In the rough grinding by the first grinding unit 10a, a damaged layer and minute irregularities formed on the grinding surface 1a of the workpiece 1 are mainly removed. For example, when the workpiece 1 is a SiC wafer cut from a SiC ingot, a damaged layer formed on the grinding surface 1a due to cutting is removed by rough grinding.

[0044] In grinding the workpiece 1 using the second grinding unit 10b, the grinding feed unit 24b performs grinding feed at a relatively low speed to finish grind the workpiece 1. In the finish grinding by the second grinding unit 10b, the damaged layer formed on the grinding surface 1a by the rough grinding is mainly removed. In the grinding surface 1a after the finish grinding, the damaged layer and minute uneven shapes are removed, and devices can be suitably formed.

[0045] The first grinding wheel 20a and the second grinding wheel 20b each contain abrasive grains made of diamond or the like and a binder for dispersing and fixing the abrasive grains. The second grinding wheel 20b used for finish grinding preferably contains abrasive grains having a smaller grain size than the grain size of the abrasive grains contained in the first grinding wheel 20a used for rough grinding. In this case, the first grinding wheel 20a can quickly rough-grind the workpiece 1, while the second grinding wheel 20b can finish-grind the workpiece 1 with high quality.

[0046] A first thickness measuring unit 40 for measuring the thickness of the workpiece 1 to be roughly ground by the first grinding unit 10a is disposed on the upper surface of the base 4 near the first grinding unit 10a. A second thickness measuring unit 42 for measuring the thickness of the workpiece 1 to be finish ground by the second grinding unit 10b is disposed on the upper surface of the base 4 near the second grinding unit 10b.

[0047] The first thickness measuring unit 40 and the second thickness measuring unit 42 are, for example, contact type thickness measuring units that come into contact with the grinding surface 1a of the workpiece 1. The contact type thickness measuring unit includes, for example, two probes extending above the holding table 8.

[0048] Each probe has a contact part extending downward from the tip of an arm extending in the horizontal direction. One probe measures the height of the grinding surface 1a of the workpiece 1 by bringing the lower end of the contact part into contact with the grinding surface 1a 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 part into contact with the holding surface 8a.

[0049] The workpiece 1 is placed and held on the holding surface 8a of the holding table 8 via the protective member 3. Therefore, the contact type thickness measuring unit can calculate the total thickness of the workpiece 1 and the protective member 3 from the difference between the measured height of the grinding surface 1a of the workpiece 1 and the height of the holding surface 8a of the holding table 8.

[0050] Alternatively, the first thickness measuring unit 40 and the second thickness measuring unit 42 are non-contact type thickness measuring units that do not physically come into contact with the grinding surface 1a of the workpiece 1. The non-contact type thickness measuring unit measures the height of the grinding surface 1a of the workpiece 1 by, for example, sending ultrasonic waves or probe light from a measuring unit disposed directly above the grinding surface 1a of the workpiece 1 to the grinding surface 1a, receiving the reflected ultrasonic waves or the like at the measuring unit, and analyzing the ultrasonic waves or the like.

[0051] 2(A) and other figures, the holding surface 8a of the holding table 8 is configured as a gently sloping cone surface with the center 8f as the apex. If the holding surface 8a is a cone surface, when the workpiece 1 is suction-held by the holding table 8, the workpiece 1 will deform slightly in accordance with the holding surface 8a. Note that the shapes of the workpiece 1, holding table 8, and other components shown in the figures have been exaggerated for ease of explanation.

[0052] When grinding the workpiece 1, in this state, the holding table 8 is rotated around the table rotation axis 8d, and the grinding units 10a, 10b are lowered while rotating the spindles 14a, 14b, and the grinding wheels 20a, 20b are brought into contact with the grinding surface 1a of the workpiece 1. Then, the workpiece 1 placed on the holding table 8 rotates while grinding progresses in an arc-shaped region from the outer periphery 1d to the center 1c of the workpiece 1, and the entire area of ​​the workpiece 1 is ground.

[0053] Here, since the descending speed of the second grinding unit 10b in finish grinding is relatively low, if the contact area rises too steeply at the moment when the second grinding wheel 20b contacts the grinding surface 1a of the workpiece 1, the second grinding wheel 20b will have difficulty biting into the workpiece 1. Therefore, rough grinding is performed by the first grinding unit 10a so that the grinding surface 1a of the workpiece 1 becomes a conical surface that gently slopes from the central portion 1c to the outer periphery 1d of the workpiece 1, and the central portion 1c of the workpiece 1 is made thicker than the outer periphery 1d.

[0054] This can be achieved by tilting the holding table 8 relative to the first grinding wheel 18a so that the generatrix constituting the holding surface 8a, which is composed of a conical surface, that is closest to the rotational plane including the first annular orbit of the first grinding wheel 20a is non-parallel to the rotational plane.

[0055] Thereafter, when performing finish grinding, the relative inclination between the holding table 8 and the second grinding wheel 18b is adjusted so that the rotation plane including the second annular orbit of the second grinding wheel 20b is parallel to the generatrix of the holding surface 8a closest to the rotation plane. Then, the second grinding wheel 20b moving on the second annular orbit is brought into contact with the workpiece 1 whose central portion 1c is thicker than its outer periphery 1d.

[0056] In this case, when finish grinding is started, the second grinding wheel 20b first comes into contact with the relatively high central portion 1c of the workpiece 1. At this time, the contact area of ​​the second grinding wheel 20b with the workpiece 1 is small, so the second grinding wheel 20b easily bites into the workpiece 1. Then, as finish grinding progresses and the height of the central portion 1c decreases, the contact area gradually increases, and finally the second grinding wheel 20b comes into contact with the entire grinding surface 1a of the workpiece 1.

[0057] For example, when the workpiece 1 is a 4-inch SiC wafer, rough grinding by the first grinding unit 10a is performed so that the height of the central portion 1c of the workpiece 1 is about 5 μm higher than the height of the outer peripheral portion 1d. Furthermore, the layer damaged by rough grinding can be sufficiently removed by finish grinding the workpiece 1 to a depth of about 5 μm from the grinding surface 1a. Therefore, in the finish grinding, the workpiece 1 is ground so that the entire upper surface is exposed at a height position about 5 μm lower than the height position of the outer peripheral portion 1d of the grinding surface 1a of the workpiece 1 after rough grinding.

[0058] When performing finish grinding using the second grinding unit 10b, the height of the ground surface 1a of the workpiece 1 is conventionally monitored using the second thickness measuring unit 42 in the intermediate region between the center 1c and the outer periphery 1d of the ground surface 1a. For example, in this example, when rough grinding is completed, the height position of this intermediate region of the ground surface 1a is about 2.5 μm higher than the outer periphery 1d. Therefore, in order to remove the layer in which damage has occurred in the entire area including the outer periphery 1d, it is considered to perform finish grinding so that the ground surface 1a is about 7.5 μm lower in the intermediate region.

[0059] However, when rough grinding a hard workpiece 1 such as a SiC wafer, the grinding results vary greatly due to reasons such as the grinding load fluctuating greatly due to a slight change in the state of the first grinding wheel 20a. For example, the height difference between the center 1c and the outer periphery 1d of the workpiece 1 varies greatly. If this height difference becomes larger than expected and the grinding surface 1a is inclined more than expected, the outer periphery 1d of the workpiece 1 may not be sufficiently ground when the workpiece 1 is removed by a predetermined thickness in finish grinding.

[0060] In the above example, when the height of the central portion 1c is about 10 μm higher than the height of the outer peripheral portion 1d in the rough grinding, the height of the intermediate region between the central portion 1c and the outer peripheral portion 1d of the grinding surface 1a is 5 μm higher than the height of the outer peripheral portion 1d. In this case, if the finish grinding is not performed so that this intermediate region is about 10 μm lower, the layer in which damage has occurred over the entire grinding surface 1a cannot be removed. Therefore, if the finish grinding is performed so that the intermediate region is about 7.5 μm lower as originally expected, the outer peripheral portion 1d of the workpiece 1 will not be sufficiently ground, and the layer in which damage has occurred due to rough grinding will remain on the workpiece 1.

[0061] Therefore, it is possible to set the thickness to be removed from the workpiece 1 in the finish grinding to be large, but in this case, the amount of grinding increases. If the amount of grinding increases, the processing time increases, and the second grinding wheel 20b is worn out more, requiring frequent replacement, which reduces the processing efficiency. Furthermore, if the height difference between the center 1c and the outer periphery 1d of the grinding surface 1a becomes smaller than expected when rough grinding is performed, the workpiece 1 will be excessively ground in the finish grinding.

[0062] Therefore, in the grinding method of the workpiece according to the present embodiment described below, the thickness of the outer periphery 1d of the workpiece 1 is measured after rough grinding by the first grinding unit 10a is completed, and the finishing thickness of the workpiece 1 is determined based on the measured value. Next, the grinding method of the workpiece according to the present embodiment will be described. FIG. 5 is a flowchart showing the flow of each step of the grinding method of the workpiece according to the present embodiment. Each step of the grinding method of the workpiece according to the present embodiment will be described in detail below.

[0063] In the grinding method of the workpiece according to this embodiment, first, a holding step S10 is performed in which the workpiece 1 is placed on the holding surface 8a of the holding table 8 and the workpiece 1 is held by the holding table 8. The holding step S10 will be described in detail with reference to FIG.

[0064] In the holding step S10, first, the wafer transfer robot 30 transfers the workpiece 1 from the cassettes 28a, 28b placed on the cassette placement stages 26a, 26b to the positioning table 32, and the position of the workpiece 1 is adjusted. Then, the loading arm 34 transfers the workpiece 1 onto the holding surface 8a of the holding table 8 positioned in the wafer loading / unloading area. At this time, the grinding surface 1a of the workpiece 1 is exposed upward, and the back surface 1b faces the holding surface 8a. Then, the holding table 8 holds the workpiece 1 by suction.

[0065] Thereafter, in order to carry out the first grinding step S20 described next, the turntable 6 is rotated to move the holding table 8 holding the workpiece 1 to a rough grinding area below the first grinding unit 10a.

[0066] Next, a first grinding step S20 is performed. Fig. 2(A) is a cross-sectional view that typically shows the first grinding step S20. Fig. 2(A) includes a side view of a portion of the first grinding unit 10a and a cross-sectional view of the holding table 8 that suction-holds the workpiece 1. In the first grinding step S20, the workpiece 1 is roughly ground by the first grinding unit 10a.

[0067] In the first grinding step S20, first, the relative orientation of the first grinding unit 10a and the holding table 8 is adjusted. More specifically, the generatrix of the conical holding surface 8a that is closest to the first grinding wheel 20a is made non-parallel to the first annular orbit. For convenience of explanation, in FIG. 2(A), an extension 8e of the generatrix and a surface 20c including the first annular orbit are shown by dashed lines.

[0068] In this state, the first spindle 14a is rotated around the spindle rotation axis 14c to rotate the first grinding wheel 20a on the first annular orbit, and the holding table 8 is rotated around the table rotation axis 8d. Then, the grinding feed unit 24a is operated to lower the first grinding unit 10a, etc., to bring the holding table 8 and the first grinding wheel 18a closer to each other. Then, the bottom surface of the first grinding wheel 20a comes into contact with the grinding surface 1a of the workpiece 1, and the workpiece 1 is ground.

[0069] The lowering speed of the first grinding unit 10a at this time is set to be 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 roughly ground. At this time, the thickness of the workpiece 1 is monitored using the first thickness measuring unit 40. Then, when the workpiece 1 has been roughly ground to a predetermined thickness, the lowering of the first grinding unit 10a is stopped, and the rough grinding is completed.

[0070] Here, when the workpiece 1 is a SiC wafer cut from an ingot, this predetermined thickness is set so that a damaged layer formed on the grinding surface 1a by cutting is removed. Fig. 2(B) is a cross-sectional view showing the workpiece 1 after the first grinding step S20 is completed and the workpiece 1 is roughly ground. As shown in Fig. 2(B), when the workpiece 1 is roughly ground by carrying out the first grinding step S20, the outer periphery 1d of the workpiece 1 becomes thinner than the central portion 1c.

[0071] After rough grinding, damage such as minute irregularities and fissures called cracks remains on the grinding surface 1a of the workpiece 1. The workpiece 1 is then finish-ground to remove the layer containing the damage and flatten the grinding surface 1a of the workpiece 1. In the grinding method for the workpiece according to this embodiment, a measurement step S30 is performed after the first grinding step S20 and before the second grinding step S40.

[0072] In the measuring step S30, the thickness of the outer periphery 1d of the workpiece 1 or the height of the outer periphery 1d of the upper surface (ground surface 1a) of the workpiece 1 is measured. This measurement is performed, for example, immediately after the first grinding step S20 is completed by a first thickness measuring unit 40 provided near the first grinding unit 10a. Alternatively, in the measuring step S30, the turntable 6 is rotated to send the holding table 8 to a finish grinding area below the second grinding unit 10b, and the measurement is performed by the second thickness measuring unit 42.

[0073] After the measurement step S30, a second grinding step S40 is performed. If the holding table 8 that suction-holds the workpiece 1 is not moved in the measurement step S30, the turntable 6 is rotated to send the holding table 8 to the finish grinding area in the second grinding step S40. FIG. 3 is a cross-sectional view that typically shows the second grinding step S40. FIG. 3 includes a side view that typically shows the second grinding unit 10b that grinds the workpiece 1 in the second grinding step S40.

[0074] 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 wheel 20b is made parallel to the second annular orbit of the second grinding wheel 20b. For ease of explanation, an extension 8e of the generatrix and a surface 20d including the second annular orbit are shown by dashed lines in Fig. 3.

[0075] In this state, the second spindle 14b is rotated around the spindle rotation axis 14d to rotate the second grinding wheel 20b on the second annular orbit, and the holding table 8 is rotated around the table rotation axis 8d. Then, the grinding feed unit 24b is operated to lower the second grinding unit 10b, etc., to bring the holding table 8 and the second grinding wheel 18b closer to each other. Then, the bottom surface of the second grinding wheel 20b comes into contact with the grinding surface 1a of the workpiece 1, and the workpiece 1 is ground.

[0076] Here, the second grinding wheel 20b moving on the second annular track first comes into contact with the highest central portion 1c of the grinding surface 1a of the workpiece 1. FIG. 4(A) is a cross-sectional view showing the workpiece 1 during the second grinding step S40. Then, as the height of the workpiece 1 becomes lower at the central portion 1c, the contact area of ​​the second grinding wheel 20b with the grinding surface 1a gradually increases. Therefore, the second grinding wheel 20b properly bites into the grinding surface 1a, and the grinding surface 1a is properly ground.

[0077] In the second grinding step S40, grinding is terminated when the workpiece 1 reaches a finishing thickness determined based on the thickness of the workpiece 1 at the outer periphery 1d measured in the measuring step S30, or the height of the outer periphery 1d of the upper surface (ground surface 1a) of the workpiece 1. This finishing thickness can be derived, for example, by subtracting a depth that can sufficiently remove a damaged layer formed on the ground surface 1a of the workpiece 1 by rough grinding from the thickness of the workpiece 1 at the outer periphery 1d measured in the measuring step S30.

[0078] For example, if finish grinding to a depth of 5 μm or more is required to fully remove the layer where this damage occurred, a height position that is 5 μm lower than the height of the ground surface 1a at the outer periphery 1d before finish grinding is defined as the finish height position for the entire area of ​​the workpiece 1. The thickness of the workpiece 1 when the entire area of ​​the ground surface 1a of the workpiece 1 reaches this finish height is defined as the finish thickness of the workpiece 1.

[0079] 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 reaches this finish thickness, the lowering of the second grinding unit 10b is stopped. Fig. 4(B) is a cross-sectional view showing the workpiece 1 when grinding by the second grinding unit 10b is completed. For convenience of explanation, the second grinding unit 10b and the like are omitted in Fig. 4(B).

[0080] According to the grinding method of the workpiece according to the present embodiment, even in the outer peripheral portion 1d of the workpiece 1 where the layer damaged in the rough grinding in the first grinding step S20 progresses to the deepest extent, the finish grinding of the workpiece 1 can be performed so as to remove the layer in the second grinding step S40. Therefore, the finish grinding will not end in a state where the finish grinding of the outer peripheral portion 1d of the workpiece 1 has not been performed as shown in FIG. 4(A).

[0081] That is, even if a large variation occurs in the height difference between the outer periphery 1d and the central portion 1c of the workpiece 1 during rough grinding, the necessary finish grinding can be reliably performed on the workpiece 1. On the other hand, even if the height difference between the outer periphery 1d and the central portion 1c of the workpiece 1 during rough grinding becomes smaller than expected, the workpiece 1 is not excessively finish ground.

[0082] As described above, according to the method for grinding a workpiece according to this embodiment, even if the workpiece 1 to be ground is a hard member and the rough grinding results vary widely, the workpiece 1 can be ground efficiently and reliably. However, there is no restriction on the workpiece 1, and efficient and reliable grinding can be performed even if the workpiece 1 is not hard.

[0083] The present invention is not limited to the above embodiment, and can be modified in various ways. For example, the above embodiment describes a case where the grinding surface 1a is inclined so that the outer periphery 1d of the workpiece 1 is lower than the center 1c in the rough grinding performed in the first grinding step S20, but one aspect of the present invention is not limited to this.

[0084] In order to facilitate the second grinding wheel 20b biting into the grinding surface 1a of the workpiece 1 when starting the second grinding step S40, it is also possible to roughly grind the workpiece 1 so that the central portion 1c is thinner than the outer periphery 1d by performing the first grinding step S20. In this case, in the measuring step S30, the thickness of the central portion 1c of the workpiece 1 or the height position of the central portion 1c of the upper surface (grinding surface 1a) of the workpiece 1 is measured by the first thickness measuring unit 40 or the second thickness measuring unit 42.

[0085] Then, in the second grinding step S40, grinding is terminated when the workpiece 1 reaches a finishing thickness that is determined based on the thickness of the workpiece 1 at the central portion 1c measured in the measuring step S30, or the height position of the central portion 1c of the upper surface of the workpiece 1. In this way, even if the central portion 1c of the workpiece 1 becomes thin in rough grinding, the workpiece can be ground efficiently and reliably according to the grinding method for a workpiece according to one aspect of the present invention.

[0086] In addition, if the variation in the processing results of the rough grinding performed in the first grinding step S20 is extremely large, it may be unclear whether the workpiece 1 will be thinner at the central portion 1c or at the outer periphery 1d. In this case, in the measuring step S30, it is preferable to measure the thickness, etc., of both the central portion 1c and the outer periphery 1d of the workpiece 1. Then, in the second grinding step S40, it is preferable to end grinding when the workpiece 1 reaches a finishing thickness determined based on the thickness of the workpiece 1 measured at the thinner of the central portion 1c and the outer periphery 1d.

[0087] The structures, methods, and the like according to the above-described embodiments can be modified as appropriate without departing from the scope of the object of the present invention. [Explanation of symbols]

[0088] 1 Workpiece 1a Surface to be ground 1b back side 1c central part 1d outer periphery 3 Protective materials 2 Grinding equipment 4 Foundation 6. Turntable 8 Holding Table 8a Holding surface 8b Porous material 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 rotation axis 16a,16b Wheel mount 18a, 18b Grinding wheels 20a, 20b Grinding wheel 20c,20d plane 22a, 22b Column 24a, 24b Grinding feed unit 26a, 26b Cassette placement table 28a, 28b Cassette 30 Wafer transport robot 32 Positioning table 34 Loading Arm 36 Unloading Arm 38 Spinner cleaning device 40,42 Thickness measurement unit

Claims

1. A grinding method for grinding a workpiece, comprising the steps of: a holding step of placing the workpiece on a holding surface of a holding table having a conical holding surface on an upper surface thereof and rotatable around a table rotation axis passing through a center of the holding surface, and holding the workpiece with the holding table; a first grinding step of rotating a first spindle having a first grinding wheel fixed to its lower end, the first grinding wheel having a first grinding stone on its bottom surface, rotating the first grinding stone on a first annular track, and bringing the holding table and the first grinding wheel close to each other while making the generatrix of the conical holding surface closest to the first grinding stone non-parallel to the first annular track, thereby bringing the first grinding stone into contact with the workpiece, thereby grinding the workpiece so that the outer periphery of the workpiece is thinner than the center of the workpiece; a measuring step of measuring a thickness of the workpiece at the outer periphery or a height of a top surface of the workpiece at the outer periphery after the first grinding step; a second grinding step of rotating a second spindle having a second grinding wheel having a second grinding wheel on a bottom surface thereof fixed to a lower end thereof to rotate the second grinding wheel on a second annular track, and bringing the holding table and the second grinding wheel close together while aligning a generatrix of the conical holding surface closest to the second grinding wheel parallel to the second annular track, thereby bringing the second grinding wheel into contact with the workpiece, thereby grinding the workpiece; In the second grinding step, grinding is started from the center of the workpiece, and grinding is terminated when the workpiece reaches a finish thickness determined based on the thickness of the workpiece at the outer periphery measured in the measuring step or the height of the upper surface of the workpiece at the outer periphery; A method for grinding a workpiece, characterized in that the finishing thickness is determined by subtracting a depth that can remove a damaged layer formed in the workpiece in the first grinding step from the thickness of the workpiece at the outer periphery measured in the measurement step, or from the height of the upper surface of the workpiece at the outer periphery.

2. 2. The method of claim 1, wherein the second grinding wheel contains abrasive grains having a grain size smaller than that of the first grinding wheel.

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