JP7794601B2 - Grinding method for workpiece - Google Patents

Description

The present invention relates to a method for grinding a workpiece using a grinding device that includes a chuck table having a holding surface on its upper surface and a grinding unit equipped with a grinding wheel having grinding stones arranged in a ring shape.

In the manufacturing process for device chips used in electronic devices such as mobile phones and personal computers, first, multiple intersecting division lines (streets) are set on the surface of a wafer made of a material such as a semiconductor. Then, devices such as ICs (Integrated Circuits) and LSIs (Large-scale Integration) are formed in each area defined by the division lines. The wafer is then divided along the division lines to form individual device chips.

In recent years, SiC wafers have been used to manufacture device chips equipped with high-voltage power devices capable of high-temperature operation. SiC wafers are manufactured by cutting cylindrical SiC ingots. 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. This forms a modified layer inside the SiC ingot, which serves as the starting point for delamination (see Patent Document 1).

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, and further grinding is carried out to flatten the surface. Grinding of workpieces such as SiC wafers is carried out using a grinding device. The grinding device used to grind wafers includes a chuck table that can hold the workpiece, and a grinding unit equipped with a grinding wheel having grinding stones arranged in a ring (see Patent Document 2).

JP 2016-111143 A JP 2009-90389 A

When grinding a damaged layer, the grinding wheel tends to wear out rapidly. Therefore, to reduce the frequency of grinding wheel replacement, it is desirable to use a grinding wheel that does not wear out easily. On the other hand, when grinding after removing the damaged layer, the condition of the grinding wheel tends to deteriorate rapidly due to glazing and other issues. Therefore, it is desirable to use a grinding wheel that wears out easily. When a grinding wheel that wears out easily is used, the grinding wheel wears out at a rate that is sufficient to keep up with the rate of glazing and other issues, so unused abrasive grains are exposed one after another, making it easier to maintain the performance of the grinding wheel.

However, when grinding a workpiece that includes areas with different grinding efficiencies, it is not practical to switch types of grinding wheels mid-grinding. Attempting to grind such a workpiece with a single grinding wheel would result in excessive wear on the grinding wheel, necessitating frequent replacement of the grinding wheel, and the workpiece would not be ground to a satisfactory quality.

The present invention was made in consideration of these problems, and its purpose is to provide a method for grinding workpieces that can grind workpieces with high efficiency and high quality.

According to one aspect of the present invention, there is provided a grinding unit including a chuck table having a holding surface, capable of suction-holding a workpiece placed on the holding surface and rotatable around a table rotation axis passing through the center of the holding surface, a spindle along the table rotation axis and having a grinding wheel with annularly arranged grinding stones attached to its tip, the grinding wheel being rotated by rotating the spindle, and the grinding stone being rotated and moved around a circular orbit while grinding the workpiece held on the chuck table with the grinding stone; a first moving mechanism that relatively moves the chuck table and the grinding unit along a first direction along the table rotation axis, and a second moving mechanism that relatively moves the chuck table and the grinding unit along a second direction perpendicular to the first direction, wherein the chuck table and the grinding unit are relatively moved by the second moving mechanism so that the center of the holding surface and the circular orbit of the grinding wheel do not overlap along the first direction; a first grinding step in which the grinding wheel is rotationally moved on the circular orbit by rotating the chuck table around the table rotation axis and rotating the spindle, and the chuck table and the grinding unit are moved relatively by the first moving mechanism to bring the grinding wheel into contact with the workpiece and grind the outer periphery of the workpiece (excluding the case where only a part of the bottom surface of the grinding wheel comes into contact with the workpiece); and after the first grinding step, the chuck table and the grinding unit are moved relatively by the second moving mechanism. a second grinding step of moving the chuck table around the table rotation axis and rotating the spindle to rotate the grinding wheel on the circular orbit, and moving the chuck table and the grinding unit relatively by the first moving mechanism to bring the grinding wheel into contact with the workpiece and grind the center of the workpiece.

Preferably, the method further includes a third grinding step following the second grinding step, in which the outer periphery and the center of the workpiece are ground with the grinding wheel without operating the second moving mechanism.

More preferably, the workpiece surface to be ground by the grinding wheel has a first layer and a second layer located inside the first layer, and the grinding wheel is less likely to wear when grinding the second layer than when grinding the first layer.

In one aspect of the present invention, a method for grinding a workpiece involves grinding the outer periphery of the workpiece in a first grinding step, followed by grinding the center of the workpiece in a second grinding step, rather than grinding the entire surface of the workpiece all at once. Therefore, the degree of change in the state of the grinding wheel in each of the first and second grinding steps is smaller than the degree of change in the state of the grinding wheel when the entire surface of the workpiece is ground all at once.

In other words, the amount of fluctuation in the state of the grinding wheel in each grinding step in one aspect of the present invention is smaller than the amount of fluctuation in the state of the grinding wheel when the entire surface of the workpiece is ground simultaneously. In other words, changes in the state of the grinding wheel can be averaged out. This prevents excessive wear and excessive progression of glazing of the grinding wheel, allowing for long-term, stable use of the grinding wheel. More specifically, the frequency of grinding wheel replacement can be reduced, improving the grinding efficiency of the workpiece, and preventing a decline in processing quality due to glazing of the grinding wheel, etc.

Therefore, the present invention provides a method for grinding a workpiece that grinds the workpiece with high efficiency and high quality.

FIG. 2 is a perspective view schematically showing a grinding device. FIG. 2 is a cross-sectional view schematically showing a grinding device. Figure 3(A) is a side view schematically showing the positional relationship between the grinding wheel and the chuck table in the first grinding step in the first example, and Figure 3(B) is a side view schematically showing the workpiece being ground in the first grinding step in the first example. Figure 4(A) is a plan view schematically showing the positional relationship between the grinding wheel and the chuck table in the first grinding step in the first example, and Figure 4(B) is a plan view schematically showing the workpiece ground in the first grinding step in the first example. Figure 5(A) is a side view schematically showing the positional relationship between the grinding wheel and the chuck table in the first grinding step in the second example, and Figure 5(B) is a side view schematically showing the workpiece being ground in the first grinding step in the second example. Figure 6(A) is a plan view schematically showing the positional relationship between the grinding wheel and the chuck table in the first grinding step in the second example, and Figure 6(B) is a plan view schematically showing the grinding area of the workpiece ground in the first grinding step in the second example. Figure 7(A) is a side view schematically showing the positional relationship between the grinding wheel and the chuck table in the second grinding step, and Figure 7(B) is a side view schematically showing the workpiece being ground in the second grinding step. FIG. 8(A) is a plan view schematically showing the positional relationship between the grinding wheel and the chuck table in the second grinding step, and FIG. 8(B) is a plan view schematically showing the ground workpiece. Figure 9(A) is a graph showing a schematic representation of the change in the state of a grinding wheel over time when the entire surface of a workpiece is ground simultaneously, and Figure 9(B) is a graph showing a schematic representation of the change in the state of a grinding wheel over time in a method for grinding a workpiece according to an embodiment. 1 is a flowchart illustrating a flow of a method for grinding a workpiece according to an embodiment.

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the method for grinding a workpiece according to this embodiment, the workpiece is ground using a grinding device. First, the workpiece to be ground using the grinding device will be described. Figure 3(A) includes a cross-sectional view that schematically shows the workpiece 1 before grinding, and Figure 4(A) includes a plan view that schematically shows the workpiece 1 before grinding.

The workpiece 1 is a disk-shaped wafer made of, for example, Si (silicon), SiC (silicon carbide), GaN (gallium nitride), GaAs (gallium arsenide), or other semiconductor material.

Alternatively, the workpiece 1 may be a substantially circular substrate made of a material such as sapphire, glass, or quartz. Examples of such glass include alkali glass, alkali-free glass, soda-lime glass, lead glass, borosilicate glass, and quartz glass. The workpiece 1 may also be a package substrate, a ceramic substrate, or the like. However, the workpiece 1 is not limited to these.

The surface of the workpiece 1 is divided by a number of planned division lines (not shown) arranged in a grid pattern called streets. Devices (not shown), such as ICs and LSIs, are formed in each area of the surface of the workpiece 1 divided by the planned division lines. The workpiece 1 is ground from the back side to thin it, and then divided along the planned division lines to obtain individual thin device chips.

In recent years, SiC wafers have come to be used when manufacturing device chips equipped with high-voltage power devices capable of high-temperature operation. SiC wafers are manufactured by cutting cylindrical SiC ingots. 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. This forms a modified layer inside the SiC ingot, which serves as the starting point for delamination.

Damage caused by peeling and the formation of modified layers remains on the surface of a SiC wafer sliced from a SiC ingot. Therefore, the surface of the SiC wafer is ground to remove the damaged layer, and further grinding is carried out to flatten the surface. The surface of the SiC ingot from which the SiC wafer was sliced is also ground and flattened before the next SiC wafer is sliced. These SiC wafers and SiC ingots can also serve as the workpiece 1 in the method for grinding a workpiece according to this embodiment.

Next, we will explain the grinding device used in the workpiece grinding method according to this embodiment. Figure 1 is a schematic perspective view of the grinding device 2, and Figure 2 is a schematic cross-sectional view of the grinding device 2. The grinding device 2 has a roughly rectangular parallelepiped base 4 that supports each component. A recess 4a is formed on the top surface of the base 4, extending along the Y-axis direction. The recess 4a is fitted with a chuck table 6 that can hold the workpiece 1 and is movable in the Y-axis direction, and a dust-proof and drip-proof cover 4b that covers the opening of the recess 4a while exposing the chuck table 6.

In Figure 2, the dustproof and drip-proof cover 4b is omitted. A Y-axis movement mechanism 8 that movably supports the chuck table 6 is disposed inside the recess 4a. The Y-axis movement mechanism 8 includes a pair of guide rails 10 along the Y-axis direction, a moving table 12 slidably supported on the guide rails 10, and a ball screw 16 along the Y-axis direction that is threaded into a nut portion 14 provided on the underside of the moving table 12. A pulse motor 18 that rotates the ball screw 16 is connected to one end of the ball screw 16.

The chuck table 6 is disposed on the moving table 12. When the Y-axis moving mechanism 8 is operated, the chuck table 6 can be moved along the Y-axis direction.
The nozzle 6 has a ceramic frame 6a. A flow path (not shown) is provided inside the frame 6a, and one end of the flow path is connected to a suction source (not shown) such as an ejector.

The frame 6a has a recess on its upper surface, consisting of a disk-shaped space. A roughly disk-shaped porous plate 6b is fixed in this recess. The porous plate 6b has flat, circular lower and upper surfaces. The diameter of the porous plate 6b is roughly the same as the diameter of the workpiece 1.

The other end of the flow path in the frame 6a is connected to the underside of the porous plate 6b. When the suction source is operated, negative pressure is generated on the upper surface of the porous plate 6b, and the workpiece 1 is sucked and held on this upper surface. Therefore, the upper surface of the chuck table 6 functions as the holding surface 6c.

The chuck table 6 is rotatable around a table rotation axis 6e that passes through the center of the holding surface 6c and is generally perpendicular to the holding surface 6c. The tilt of the holding surface 6c of the chuck table 6 is also adjustable. The grinding device 2 is equipped with a support mechanism 20 that supports the chuck table 6 so that the tilt of the table 6 can be adjusted and the table can rotate.

The support mechanism 20 has an inclination adjustment mechanism with one or more fixed support members 20a and two or more movable support members 20b. For example, one fixed support member 20a and two movable support members 20b are connected to the movable table 12 at 120 degrees apart in the circumferential direction of the movable table 12. Note that Figure 2 shows one movable support member 20b.

The height of the upper end of the fixed support member 20a is fixed, but the upper end of the movable support member 20b is movable in the vertical direction. By adjusting the height of the upper end of the movable support member 20b, the holding surface 6c of the chuck table 6 can be tilted at a predetermined angle relative to the vertical direction (Z-axis direction).

A disk-shaped table base 6d is connected to the underside of the frame 6a of the chuck table 6. A drive mechanism 20c, which constitutes part of the support mechanism 20, is provided on the central underside of this table base 6d. The drive mechanism 20c includes, for example, a motor and is connected to the frame 6a via the table base 6d. By operating the drive mechanism 20c, the chuck table 6 rotates around the table rotation axis 6e.

The grinding device 2 includes a grinding unit 22 that grinds the workpiece 1 held by the chuck table 6, and a lifting unit 24 that raises and lowers the grinding unit 22. A support section 26 is erected at the rear of the grinding device 2, and this support section 26 supports the grinding unit 22 via the lifting unit 24. A pair of guide rails 28 are provided on the front of the support section 26 along the Z-axis direction (processing feed direction). A movable plate 30 is slidably attached to each guide rail 28.

A nut portion 32 is provided on the back side (rear side) of the movable plate 30, and a ball screw 34 parallel to the guide rail 28 is threadedly engaged with this nut portion 32. A pulse motor 36 is connected to one end of the ball screw 34. When the pulse motor 36 rotates the ball screw 34, the movable plate 30 moves in the Z-axis direction along the guide rail 28.

A grinding unit 22, which performs grinding on the workpiece, is fixed to the front side of the movable plate 30. By moving the movable plate 30, the grinding unit 22 can move in the Z-axis direction (processing feed direction). The grinding unit 22 has a cut cylindrical holding member 38. The holding member 38 is fixed to the surface on the front side of the movable plate 30.

A spindle housing 40 is provided inside the holding member 38. A circular buffer member 42 made of rubber or the like is provided at the bottom of the spindle housing 40. The spindle housing 40 is supported on the bottom surface of the holding member 38 via the buffer member 42.

A portion of the spindle 44 is rotatably housed in the spindle housing 40. A rotation drive mechanism (not shown), such as a motor, is connected to the upper end (base end) of the spindle 44. The spindle 44 is generally aligned with the table rotation axis 6e, and when the rotation drive mechanism is operated, the spindle 44 rotates around the rotation axis 46.

The lower end (tip) of the spindle 44 is located below the bottom of the holding member 38. The upper surface of a disk-shaped wheel mount 48 is connected to the lower end of the spindle 44. The upper surface of an annular grinding wheel 50 is attached to the lower surface of the wheel mount 48.

The grinding wheel 50 has an annular wheel base 52. The wheel base 52 is made of a metal such as aluminum or stainless steel, and has a diameter corresponding to the diameter of the workpiece 1. The upper surface of this wheel base 52 is connected to the lower surface of the wheel mount 48. In other words, the grinding wheel 50 is attached to the lower end (tip) of the spindle 44 via the wheel mount 48.

A number of grinding wheels 54 are arranged in a ring shape on the underside (one surface) of the wheel base 52. Each grinding wheel 54 is formed by mixing abrasive grains such as diamond or cBN (cubic boron nitride) with a binder such as vitrified or resinoid, and then sintering the mixture. When the spindle 44 is rotated around the rotation axis 46 to rotate the grinding wheel 50, the grinding wheels 54 rotate and move along a circular orbit.

The grinding device 2 is equipped with a thickness gauge 56 located above the movement path of the chuck table 6. The thickness gauge 56 measures the height from the holding surface 6c of the top surface of the workpiece 1 held by the chuck table 6, thereby measuring the thickness of the workpiece 1. The thickness gauge 56 measures the height of the top surface of the workpiece 1 by, for example, irradiating the top surface of the workpiece 1 with a laser beam and detecting the reflected light, thereby measuring the thickness of the workpiece 1. However, the thickness gauge 56 is not limited to this, and may also be a contact-type thickness gauge.

When grinding the workpiece 1 with the grinding device 2, first, the workpiece 1 is placed on the holding surface 6c and then suction-held by the chuck table 6. At this time, the surface of the workpiece 1 to be ground is the upper surface 1a. Next, the Y-axis movement mechanism 8 is operated to move the chuck table 6 below the grinding unit 22. After that, the lifting unit 24 is operated to lower the grinding unit 22, while the grinding wheel 54, which rotates on a circular orbit, comes into contact with the surface to be ground, and the workpiece 1 is ground.

During the grinding process, the circular orbit of the grinding wheel 54 is positioned so that it passes above the center of the chuck table 6. Grinding is then performed while measuring the thickness of the workpiece 1 using a thickness gauge 56 at a position away from the center of the chuck table 6.

Here, the Y-axis movement mechanism 8 may move the grinding unit 22 along the Y-axis direction, and the lifting unit 24 may raise and lower the chuck table 6. That is, the grinding device 2 is equipped with a first movement mechanism (lifting unit 24) that moves the chuck table 6 and the grinding unit 22 relatively along a first direction (Z-axis direction) along the table rotation axis 6e. It is also equipped with a second movement mechanism (Y-axis movement mechanism 8) that moves the two relatively along a second direction (Y-axis direction) perpendicular to the first direction.

When the workpiece 1 is ground with the grinding wheel 54, the abrasive grains contained in the grinding wheel 54 gradually wear away, gradually reducing the grinding ability of the grinding wheel 54. Meanwhile, as the workpiece 1 is ground, the binder that makes up the grinding wheel 54 also gradually wears away, causing new abrasive grains to be exposed one after another on the bottom surface of the grinding wheel 54. In other words, as the grinding wheel 54 wears away from the bottom surface while grinding the workpiece 1, it is possible to grind the workpiece 1 with a grinding ability above a predetermined level.

Here, for example, a SiC wafer sliced from a SiC ingot may be ground as the workpiece 1 by the grinding device 2. The SiC wafer has a damaged layer on its surface due to peeling and the formation of a modified layer when it is sliced from the SiC ingot, and is ground to remove this damaged layer and flatten the surface.

Figure 3(A) shows a schematic cross-sectional view of a workpiece 1 including a first layer 3 and a second layer 5. For example, the first layer 3 is a layer in which damage has occurred, and the second layer 5 is a layer in which no damage has occurred. However, the first layer 3 and the second layer 5 are not limited to this. Furthermore, the boundary between the first layer 3 and the second layer 5 is not necessarily clear.

When grinding a damaged layer, the grinding wheel 54 wears out rapidly. Therefore, it is desirable to use a grinding wheel 54 that does not wear out easily in order to reduce the frequency of replacing the grinding wheel 50. On the other hand, when grinding after removing the damaged layer, glazing and other problems occur, rapidly deteriorating the condition of the grinding wheel 54, so it is desirable to use a grinding wheel 54 that wears out easily. When a grinding wheel 54 that wears out easily is used, the grinding wheel 54 wears out at a rate that is sufficient to keep up with the rate at which glazing and other problems progress, so unused abrasive grains are exposed one after another, making it easier to maintain the performance of the grinding wheel 54.

However, when grinding a workpiece 1 that includes portions with different grindability characteristics, it is not practical to change the grinding wheel 50 and switch the type of grinding stone 54 during grinding. Attempting to grind such a workpiece 1 with a single grinding stone 54 would result in excessive wear on the grinding stone 54, necessitating frequent replacement of the grinding stone 50, and would also result in the workpiece 1 not being ground with sufficient quality.

In order to mitigate these problems, the workpiece grinding method according to this embodiment grinds the workpiece 1 according to the procedure described below. The workpiece grinding method according to this embodiment is described in detail below. Figure 10 is a flowchart showing the flow of the workpiece grinding method according to this embodiment.

In the grinding method for a workpiece according to this embodiment, first, the workpiece 1 is placed on the holding surface 6c of the chuck table 6 so that the surface to be ground is exposed upward, and the workpiece 1 is held by suction on the chuck table 6. Figure 3(A) shows a cross-sectional view of the workpiece 1 held by suction on the chuck table 6. In this case, the upper surface 1a of the workpiece 1 becomes the surface to be ground, and the lower surface 1b faces the holding surface 6c. Note that a tape-like protective member may be attached to the lower surface 1b of the workpiece 1 in advance, and the workpiece 1 may be held on the chuck table 6 via the protective member.

When placing the workpiece 1 on the chuck table 6, the workpiece 1 is precisely positioned so that its center 7 coincides with the center of the holding surface 6c. At this time, the planar position of the center 7 of the workpiece 1 coincides with the planar position of the center of the holding surface 6c, and the table rotation axis 6e passes through the center 7 of the workpiece 1.

Then, a first grinding step is performed to grind the outer periphery 9 (described below) of the workpiece 1. First, the chuck table 6 and the grinding unit 22 are moved relatively by the second movement mechanism (Y-axis movement mechanism 8) so that the center of the holding surface 6c and the circular orbit of the grinding wheel 54 do not overlap in the first direction (Z-axis direction). Figure 4(A) is a plan view that schematically shows the positional relationship between the circular orbit 54a of the grinding wheel 54 and the chuck table 6 in this state. In other words, the center of the holding surface 6c is shifted from the circular orbit 54a (S11).

Next, the chuck table 6 is rotated around the table rotation axis 6e, and the spindle 44 is rotated to rotate the grinding wheel 54 on the circular orbit 54a. The chuck table 6 and the grinding unit 22 are then moved in the first direction (Z-axis direction) by the first movement mechanism (lifting unit 24) so that they approach each other. The grinding wheel 54 then comes into contact with the upper surface 1a, which is the surface to be ground, of the workpiece 1, and grinding of the workpiece 1 begins (S12).

Here, because the circular orbit 54a of the grinding wheel 54 and the center of the chuck table 6 do not overlap, the area including the center 7 of the workpiece 1 is not ground. This area including the center 7 of the workpiece 1 is called the central portion 11, and the annular area surrounding this central portion 11 from the outside is called the outer periphery 9.

In this case, the grinding wheel 54 comes into contact with the workpiece 1, grinding the outer periphery 9 of the workpiece 1. This stage in which the outer periphery 9 of the workpiece 1 is ground is referred to as the first grinding step. In the first grinding step, as shown in Figure 4(A), the innermost periphery of the outer periphery 9 to be ground comes into contact with the outermost periphery of the path of the grinding wheel 54, which rotates and moves on the circular orbit 54a.

Figure 3(B) is a side view showing the first grinding step, including a cross-sectional view showing the workpiece 1 with its outer periphery 9 ground. Figure 4(B) is a plan view showing the workpiece 1 with its outer periphery 9 ground. In the first grinding step, grinding proceeds while the thickness of the outer periphery 9 of the workpiece 1 is monitored by the thickness gauge 56. When it is confirmed that the outer periphery 9 has reached the predetermined thickness, the operation of the first moving mechanism (lifting unit 24) stops, and grinding of the outer periphery 9 is completed.

The first grinding step for grinding the outer periphery 9 of the workpiece 1 is not limited to the case where the innermost periphery of the outer periphery 9 to be ground comes into contact with the outermost periphery of the path of the grinding wheel 54 that rotates and moves on the circular orbit 54a, as shown in Figure 4(A). Next, we will explain the first grinding step of the second example, which is a variation of the first grinding step of the first example described above.

Figure 5(A) is a side view schematically showing the workpiece 1, grinding unit 22, and chuck table 6 at the start of the first grinding step according to the second example, and Figure 6(A) is a plan view schematically showing the first grinding step according to the second example. In the first grinding step according to the second example, as shown in Figure 6(A), the innermost periphery of the outer peripheral portion 9 to be ground comes into contact with the innermost periphery of the path of the grinding wheel 54 that rotates and moves on the circular orbit 54a.

Figure 5(B) is a side view schematically showing the first grinding step according to the second example, and Figure 6(B) is a plan view schematically showing the workpiece 1 whose outer periphery 9 has been ground in the first grinding step according to the second example. In this case, too, the chuck table 6 is rotated around the table rotation axis 6e, and the grinding wheel 54 is rotated and moved on the circular orbit 54a, and the grinding wheel 54 is brought into contact with the upper surface 1a of the workpiece 1, thereby grinding the outer periphery 9 of the workpiece 1.

Whether the first grinding step according to the first example or the first grinding step according to the second example is performed, the outer periphery 9 of the workpiece 1 is ground. Note that in Figures 4(B) and 6(B), grinding marks formed as extremely small irregularities on the upper surface 1a of the outer periphery 9 of the workpiece 1 are shown by dashed lines.

In the workpiece grinding method according to this embodiment, a second grinding step is performed after the first grinding step, in which the central portion 11 of the workpiece 1 is ground. Figure 7(A) is a side view schematically showing the grinding unit 22, chuck table 6, etc. at the start of the second grinding step. Figure 8(A) is a plan view schematically showing the positional relationship between the circular orbit 54a of the grinding wheel 54 and the chuck table 6 during the second grinding step.

In the second grinding step, first, the second movement mechanism (Y-axis movement mechanism 8) is operated to move the chuck table 6 and the grinding unit 22 relative to each other, so that the center of the holding surface 6c and the circular orbit 54a of the grinding wheel 54 are aligned along the first direction (Z-axis direction). That is, the center of the holding surface 6c is aligned with the circular orbit (S21). Figure 8(A) is a plan view schematically showing the positional relationship between the circular orbit 54a of the grinding wheel 54 and the chuck table 6 in this state. At this time, the center 7 of the workpiece 1 and the circular orbit 54a are aligned.

Next, the chuck table 6 is rotated around the table rotation axis 6e, and the spindle 44 is rotated to rotate the grinding wheel 54 on the circular orbit 54a. The chuck table 6 and the grinding unit 22 are then moved in the first direction (Z-axis direction) by the first movement mechanism (lifting unit 24) so that they approach each other. The grinding wheel 54 then comes into contact with the upper surface 1a, which is the surface to be ground, of the workpiece 1, and grinding of the workpiece 1 begins (S22).

Then, in the second grinding step, the central portion 11 of the workpiece 1, which was not ground in the first grinding step, is primarily ground. Figure 7(B) is a side view that schematically shows the second grinding step, and includes a cross-sectional view that schematically shows the workpiece 1 with the central portion 11 ground. Figure 8(B) is a plan view that schematically shows the workpiece 1 with the central portion 11 ground.

In the second grinding step, grinding proceeds while the thickness of the central portion 11 of the workpiece 1 is monitored by the thickness gauge 56. When it is confirmed that the central portion 11 has reached the specified thickness, the operation of the first movement mechanism (lifting unit 24) stops, and grinding of the central portion 11 is completed. When the first grinding step and the second grinding step are performed, the entire upper surface 1a, which is the surface to be ground, of the workpiece 1 is ground, and the entire workpiece 1 reaches the specified thickness.

Here, we will explain the changes in the state of the grinding wheel 54 using the example of the workpiece 1 being a SiC wafer sliced from a SiC ingot. In this example, the first layer 3 exposed on the top surface 1a of the workpiece 1 before it is ground is a damaged layer, while the second layer 5 located inside (below) the first layer 3 is an undamaged layer. Then, by grinding, the first layer 3 is removed, and part of the second layer 5 is also removed.

In conventional grinding methods that do not use the workpiece grinding method according to this embodiment, the grinding surface of the workpiece 1 is simultaneously ground, removing the first layer 3 and grinding away part of the second layer 5. Figure 9(A) is a graph that schematically shows the change in the state of the grinding wheel 54 during this process. In this graph, the vertical axis represents the condition (C) of the grinding wheel 54, which particularly indicates the quality of the grinding ability. This condition (C) is not a number based on a measurement, but rather a virtual index used solely to explain the change in the state of the grinding wheel 54.

When grinding of the workpiece 1 begins, the grinding wheel 54 first grinds the first layer 3, which is the layer in which damage occurred on the SiC wafer. When the damaged layer is the first layer 3, the grinding wheel 54 is severely worn in grinding the first layer 3. However, as the grinding wheel 54 is worn, unused abrasive grains are successively exposed, and the condition (C) of the grinding wheel 54 rises to a predetermined level _C1 . However, once the condition (C) of the grinding wheel 54 exceeds the predetermined level _C1 , it does not improve, and the grinding wheel 54 simply continues to wear severely thereafter.

Thereafter, removal of the first layer 3 is completed, and grinding of the undamaged second layer 5 begins. The time at this point is designated as time _TA . During grinding of the second layer 5, glazing and the like occur, and the condition (C) of the grinding wheel 54 deteriorates as grinding progresses. Then, at time _TB when the workpiece 1 reaches a predetermined thickness, it is conceivable that the condition (C) of the grinding wheel 54 does not meet the required standard and is in an unsuitable state for grinding the workpiece 1.

The boundary between the first layer 3 and the second layer 5 in the workpiece 1 is not always clear, and changes in the condition (C) of the grinding wheel 54 are not necessarily linear. Changes in the condition of the grinding wheel 54 do not necessarily progress in the same manner as the graph shown in Figure 9(A). However, the problems that arise when using a single grinding wheel 54 to grind two layers that differ in their susceptibility to wear and glazing are clearly shown in the graph in Figure 9(A).

In contrast, in the workpiece grinding method according to this embodiment, first, the outer periphery 9 of the workpiece 1 is ground to remove the first layer 3 and a portion of the second layer 5, and then the center portion 11 is ground to remove the first layer 3 and a portion of the second layer 5. In other words, because the outer periphery 9 and center portion 11 of the workpiece 1 are ground separately, the area ground at one time is relatively small compared to when the entire top surface 1a of the workpiece 1 is ground.

9(B) is a graph schematically showing the change in the state of the grinding wheel 54 at this time. Similar to the graph shown in FIG. 9(A), this graph shows the change in the state (C) of the grinding wheel 54. Here, time _T1A is the time when removal of the first layer 3 from the outer periphery 9 of the workpiece 1 is completed in the first grinding step, and time _T1B is the time when the first grinding step ends. Furthermore, time _T2A is the time when removal of the first layer 3 from the center 11 of the workpiece 1 is completed in the second grinding step, and time _T2B is the time when the second grinding step ends.

The amount of change in the condition (C) of the grinding wheel 54 is roughly proportional to the amount of workpiece 1 ground by the grinding wheel 54. Therefore, the change in the condition (C) of the grinding wheel 54 when grinding and removing the first layer 3 of the workpiece 1 in each of the first and second grinding steps is smaller than when the entire upper surface (grinding surface) 1a of the workpiece 1 is ground simultaneously.

Similarly, the change in the state (C) of the grinding wheel 54 when grinding and removing a portion of the second layer 5 of the workpiece 1 in each of the first and second grinding steps is smaller than when the entire upper surface (grinding surface) 1a of the workpiece 1 is ground simultaneously.

For example, in the first grinding step, grinding of the first layer 3 is completed before the grinding wheel 54 is excessively worn, and grinding of the second layer 5 is completed before the condition of the grinding wheel 54 deteriorates excessively. Similarly, in the second grinding step, grinding of the first layer 3 is completed before the grinding wheel 54 is excessively worn, and grinding of the second layer 5 is completed before the condition of the grinding wheel 54 deteriorates excessively. In this way, changes in the condition (C) of the grinding wheel 54 while grinding the workpiece 1 are averaged out, making it less likely that the grinding wheel 54 will wear out excessively or its condition will deteriorate excessively.

Here, the sizes of the outer periphery 9 and the central portion 11 set on the workpiece 1 will be explained using the plan view shown in Fig. 4(A). As shown in Fig. 4(A), the radius of the top surface (surface to be ground) 1a of the workpiece 1 is R, the radius of the central portion 11 is _R1 , and the width of the outer periphery 9 is _R2 . In order to smooth out changes in the state of the grinding wheel 54 as evenly as possible, it is most preferable that the areas of the outer periphery 9 and the central portion 11 are approximately equal.

To make the areas of the outer periphery 9 and the center 11 equal, the radius _R1 of the center 11 should be set to a value obtained by multiplying the radius R of the upper surface (surface to be ground) 1a by 1/√2. In this case, the width _R2 of the outer periphery 9 should be a value obtained by multiplying the radius R of the upper surface (surface to be ground) 1a by 1-1/√2. For example, if the workpiece 1 is a wafer with a diameter of 200 mm, the diameter of the center 11 should be set to approximately 141 mm, and the width of the outer periphery 9 should be set to approximately 30 mm.

However, the areas of the outer peripheral portion 9 and the central portion 11 do not need to be strictly equal. The area of the central portion 11 is preferably 80% to 120% of the area of the outer peripheral portion 9, more preferably 90% to 110%, and even more preferably 95% to 105%. However, the areas and ratios of the outer peripheral portion 9 and the central portion 11 are not limited to these.

As explained above, when the first grinding step and the second grinding step are performed, the outer periphery 9 and the center 11 of the workpiece 1 are ground, respectively, and the first layer 3 and part of the second layer 5 are removed. However, it is not easy to precisely match the height positions of the lower ends of the grinding wheels 54 when the first grinding step and the second grinding step are completed.

For example, the height position of the lower end of the grinding wheel 54 may be slightly higher when the second grinding step is completed. In this case, the central portion 11 of the upper surface 1a of the workpiece 1 after grinding will be higher than the outer periphery 9, forming a step on the upper surface 1a and making it non-flat. Therefore, after grinding the central portion 11 of the workpiece 1 in the second grinding step, the lifting unit 24 may be operated to lower the grinding unit 22, thereby performing a third grinding step in which the outer periphery 9 and central portion 11 of the workpiece 1 are ground with the grinding wheel 54.

If the third grinding step is performed following the second grinding step without operating the second movement mechanism (Y-axis movement mechanism 8) and the outer periphery 9 and center 11 of the workpiece 1 are ground simultaneously, the top surfaces of the outer periphery 9 and center 11 of the workpiece 1 will be at the same height. In other words, the top surface 1a of the workpiece 1 will be flattened.

When the third grinding step is performed following the second grinding step, it is preferable to continue the operation of the first movement mechanism (lifting unit 24) without stopping the operation of the first movement mechanism that was in progress during the second grinding step. In this case, the moment when the second grinding step ends and the moment when the third grinding step begins are not necessarily clear, and the second grinding step and the third grinding step switch seamlessly.

However, in the workpiece grinding method according to this embodiment, the third grinding step does not have to be performed, and a step may ultimately remain between the outer periphery 9 and the center 11 of the workpiece 1. In this case, after the workpiece grinding method according to this embodiment is performed, the top surface 1a of the workpiece 1 removed from the grinding device 2 may be subjected to finish grinding using another grinding device or polished using a polishing device, thereby removing this step.

As explained above, in the method for grinding a workpiece according to this embodiment, the entire surface of the workpiece 1 is not ground all at once. Instead, the outer periphery 9 of the workpiece 1 is ground in the first grinding step, and then the center 11 of the workpiece 1 is ground in the second grinding step. Therefore, the degree of change in the state of the grinding wheel 54 in each of the first and second grinding steps is smaller than the degree of change in the state of the grinding wheel 54 when the entire surface of the workpiece 1 is ground all at once.

In other words, the amount of fluctuation in the state of the grinding wheel 54 in each grinding step is smaller than the amount of fluctuation in the state of the grinding wheel 54 when the entire surface of the workpiece 1 is ground simultaneously. In other words, the changes in the state of the grinding wheel 54 can be averaged out. This prevents excessive wear and excessive progression of glazing of the grinding wheel 54, allowing the grinding wheel 54 to be used stably for a long period of time. More specifically, the frequency of replacement of the grinding wheel 54 can be reduced, improving the grinding efficiency of the workpiece 1 and preventing a decline in processing quality due to glazing of the grinding wheel 54, etc.

The present invention is not limited to the above-described embodiment and can be implemented with various modifications. For example, in the above-described embodiment, the upper surface (surface to be ground) 1a of the workpiece 1 is divided into a central portion 11 and an outer periphery 9, and the workpiece 1 is ground in two stages. However, the grinding method for a workpiece according to one aspect of the present invention is not limited to this. In other words, the upper surface (surface to be ground) 1a of the workpiece 1 may be divided into three or more regions, and the workpiece 1 may be ground in three or more stages.

In this case, to minimize changes in the state of the grinding wheel 54, it is preferable that the areas of each region of the divided upper surface (grinding surface) 1a of the workpiece 1 be roughly equal. The boundaries of each region are preferably set concentrically around the center 7 of the workpiece 1. Furthermore, it is preferable that each region be ground in sequence, starting from the outside of the workpiece 1.

In the above embodiment, the grinding wheel 54 wears out and improves when the first layer 3 on the top surface 1a of the workpiece 1 is ground, and the grinding wheel 54 becomes dull and deteriorates when the second layer 5 is ground. However, the workpiece 1 and grinding wheel 54 are not limited to this. For example, the grinding wheel 54 may deteriorate when the first layer 3 is ground, and improve when the second layer 5 is ground.

The workpiece 1 may also include layers other than the first layer 3 and the second layer 5, and these layers may be ground with the grinding wheel 54. Furthermore, the boundaries between the layers included in the workpiece 1 are not necessarily clear, and the boundary between the first layer 3 and the second layer 5 is not necessarily clear. One important aspect of the present invention is that by dividing the workpiece 1 into multiple regions and grinding them sequentially, the range of variation in the state of the grinding wheel 54 is reduced. Therefore, the workpiece 1 does not necessarily have to include the first layer 3 and the second layer 5.

The structures, methods, etc. described in the above embodiments can be modified as appropriate without departing from the scope of the present invention.

REFERENCE SIGNS LIST 1 Workpiece 1a Upper surface 1b Lower surface 3 First layer 5 Second layer 7 Center 9 Outer periphery 11 Center 2 Grinding device 4 Base 4a Recess 4b Dustproof/waterproof cover 6 Chuck table 6a Frame 6b Porous plate 6c Holding surface 6d Table base 6e Table rotation axis 8 Y-axis movement mechanism (second movement mechanism)
REFERENCE SIGNS LIST 10 guide rail 12 moving table 14 nut portion 16 ball screw 18 pulse motor 20 support mechanism 20a fixed support member 20b movable support member 20c drive mechanism 22 grinding unit 24 lifting unit 26 support portion 28 guide rail 30 moving plate 32 nut portion 34 ball screw 36 pulse motor 38 holding member 40 spindle housing 42 buffer member 44 spindle 46 rotating shaft 48 wheel mount 50 grinding wheel 52 wheel base 54 grinding stone 54a circular track 56 thickness measuring device

Claims (3)

a chuck table having a holding surface, capable of suction-holding a workpiece placed on the holding surface, and rotatable around a table rotation axis passing through the center of the holding surface;
a grinding unit having a spindle along the table rotation axis and a grinding wheel having grinding stones arranged in a circular pattern attached to the tip thereof, the grinding wheel being rotated by rotating the spindle, and the grinding stone being rotated and moved along a circular orbit while grinding the workpiece held on the chuck table with the grinding stone;
a first moving mechanism that moves the chuck table and the grinding unit relatively in a first direction along the table rotation axis;
a second moving mechanism that relatively moves the chuck table and the grinding unit along a second direction perpendicular to the first direction,
a first grinding step in which the chuck table and the grinding unit are moved relatively by the second moving mechanism so that the center of the holding surface and the circular orbit of the grinding wheel do not overlap along the first direction, the chuck table is rotated around the table rotation axis and the spindle is rotated to rotate the grinding wheel on the circular orbit, and the chuck table and the grinding unit are moved relatively by the first moving mechanism to bring the grinding wheel into contact with the workpiece and grind the outer periphery of the workpiece (excluding the case where only a portion of the bottom surface of the grinding wheel comes into contact with the workpiece);
a second grinding step of, after the first grinding step, relatively moving the chuck table and the grinding unit by the second moving mechanism so that the center of the holding surface and the circular orbit of the grinding wheel overlap along the first direction, rotating the chuck table around the table rotation axis and rotating the spindle to rotationally move the grinding wheel on the circular orbit, and moving the chuck table and the grinding unit relatively by the first moving mechanism so that the grinding wheel comes into contact with the workpiece and grinds the center of the workpiece. The method for grinding a workpiece according to claim 1, further comprising a third grinding step following the second grinding step, in which the outer periphery and the center of the workpiece are ground with the grinding wheel without operating the second moving mechanism. a first layer and a second layer located inside the first layer are provided on a grinding surface side of the workpiece to be ground by the grinding wheel;
3. The method for grinding a workpiece according to claim 1, wherein the grinding wheel is less likely to wear when grinding the second layer than when grinding the first layer.