JP7776271B2 - Grinding wheels - Google Patents

Description

The present invention relates to a grinding wheel for grinding a workpiece using creep feed grinding.

The device chip manufacturing process uses a wafer on which devices are formed in multiple areas defined by multiple intersecting streets (planned division lines). By dividing this wafer along the streets, multiple device chips, each equipped with a device, are obtained. The device chips are incorporated into various electronic devices such as mobile phones and personal computers.

In recent years, with the miniaturization of electronic devices, there has been a demand for thinner device chips. To address this, a process of thinning wafers by grinding them using a grinding device is sometimes carried out before they are divided. The grinding device is equipped with a chuck table with a holding surface for holding the workpiece, and a grinding unit for grinding the workpiece. A grinding wheel containing a grinding stone is attached to the grinding unit. The grinding device grinds the workpiece by rotating the grinding wheel and bringing the grinding stone into contact with the workpiece.

When using a grinding device to grind a workpiece such as a wafer, the relative positions of the chuck table and grinding unit are adjusted so that the center of the workpiece held by the chuck table is aligned with the trajectory of the grinding wheel. Then, while the chuck table and grinding wheel are both rotated, the grinding wheel is lowered in the processing feed direction (vertical direction) perpendicular to the holding surface, causing the bottom surface of the grinding wheel to come into contact with the top surface of the workpiece, grinding it. This type of grinding method is called in-feed grinding.

On the other hand, a grinding method known as creep feed grinding is sometimes used to grind workpieces. In creep feed grinding, the grinding wheel is positioned outside the workpiece, and the relative positions of the chuck table and grinding unit are adjusted so that the bottom surface of the grinding wheel is positioned below the top surface of the workpiece. Then, while rotating the grinding wheel, the chuck table is moved along the processing feed direction (horizontal direction) parallel to the holding surface. As a result, the top surface of the workpiece is ground by the grinding wheel, cutting away an arc from the side of the workpiece (see Patent Documents 1 and 2).

Japanese Patent Application Laid-Open No. 2017-56522 Japanese Patent Application Laid-Open No. 2020-93318

When grinding a workpiece with a grinding device, the grinding conditions are selected so that the workpiece is ground efficiently using a grinding wheel with high grinding capacity, and so that the surface of the ground workpiece (ground surface) is highly flat. For example, the particle size of the abrasive grains contained in the grinding wheel is determined depending on the material of the workpiece, etc.

Grinding wheels containing large-grain abrasive grains have the advantage of high grinding ability and the ability to grind workpieces efficiently in a short amount of time, but the disadvantage of easily leaving roughness on the ground surface of the workpiece. On the other hand, grinding wheels containing small-grain abrasive grains have the advantage of reducing the surface roughness of the ground surface of the workpiece, but the disadvantage is that the debris (grinding debris) generated by grinding the workpiece is likely to adhere to the grinding wheel, causing the abrasive grains to protrude insufficiently (clogging), and reducing grinding ability. Therefore, it is difficult to achieve both grinding efficiency and the quality of the workpiece after grinding.

The present invention was made in consideration of these problems, and aims to provide a grinding wheel that can improve the efficiency and quality of grinding processes.

According to one aspect of the present invention, there is provided a grinding wheel for grinding a workpiece by creep feed grinding, the grinding wheel comprising: a disk-shaped wheel base; a plurality of first grinding stones containing first abrasive grains and arranged in a ring shape on one end face side of the wheel base; and a plurality of second grinding stones containing second abrasive grains and arranged in a ring shape on one end face side of the wheel base, the second grinding stones being disposed closer to the center of the wheel base than the first grinding stones and the second grinding stones being disposed so as not to overlap with two adjacent first grinding stones in the radial direction of the wheel base; the hardness of the first grinding stones is lower than the hardness of the second grinding stones; the average grain size of the first abrasive grains is less than three times the average grain size of the second abrasive grains; and the flexural strength of the first grinding stones is 30% to 60% of the flexural strength of the second grinding stones.

Preferably , the concentration of the first abrasive grains in the first grinding wheel is less than 60% of the concentration of the second abrasive grains in the second grinding wheel. Also, preferably, the number of the first grinding wheels is less than the number of the second grinding wheels. Also, preferably, the width of the first grinding wheel in the radial direction of the wheel base is smaller than the width of the second grinding wheel in the radial direction of the wheel base.

When using a grinding wheel according to one aspect of the present invention, the workpiece is ground efficiently by the first grinding wheel, and then the workpiece is ground flat by the second grinding wheel. This improves the efficiency and quality of grinding the workpiece.

FIG. FIG. 2A is a perspective view showing the chuck table and the grinding unit, and FIG. 2B is an enlarged cross-sectional view showing a part of the grinding wheel. FIG. 2 is a bottom view showing the grinding wheel. FIG. 4A is a side view showing the chuck table and the grinding unit in the preparation step, and FIG. 4B is a side view showing the chuck table and the grinding unit in the grinding step. Figure 5(A) is a cross-sectional view showing a workpiece contacting a first grinding wheel, Figure 5(B) is a cross-sectional view showing a workpiece contacting a second grinding wheel, and Figure 5(C) is a cross-sectional view showing a workpiece being ground by the second grinding wheel.

An embodiment of the present invention will now be described with reference to the accompanying drawings. First, an example configuration of a grinding device capable of grinding a workpiece using a grinding wheel according to this embodiment will be described. Figure 1 is a perspective view showing a grinding device 2. Note that in Figure 1, the X-axis direction (processing feed direction, front-to-back direction, first horizontal direction) and the Y-axis direction (left-to-right direction, second horizontal direction) are mutually perpendicular. Furthermore, the Z-axis direction (vertical direction, up-down direction, height direction) is a direction perpendicular to the X-axis and Y-axis directions.

The grinding device 2 includes a base 4 that supports and houses each of the components that make up the grinding device 2. A rectangular opening 4a is formed on the top surface of the base 4, with its longitudinal direction aligned with the X-axis direction. Furthermore, a rectangular support structure 6 that protrudes upward from the top surface of the base 4 and is provided at the rear end of the base 4 along the Z-axis direction.

A first movement mechanism (first movement unit) 8 is provided inside the opening 4a. For example, the first movement mechanism 8 is a ball screw-type movement mechanism that includes a pair of guide rails (not shown) arranged along the X-axis direction and a flat movement table (not shown) slidably mounted on the pair of guide rails. A nut portion is provided on the rear (lower) side of the movement table, and a ball screw (not shown) arranged between the pair of guide rails along the X-axis direction is threadedly engaged with this nut portion. A pulse motor (not shown) is also connected to the end of the ball screw. When the ball screw is rotated by the pulse motor, the movement table moves in the X-axis direction along the pair of guide rails.

A chuck table (holding table) 10 that holds the workpiece 11 is provided on the surface (top) side of the moving table of the first moving mechanism 8. The first moving mechanism 8 also has a table cover 8a that surrounds the chuck table 10. Furthermore, accordion-shaped dust-proof and drip-proof covers 12 that are extendable and contractible along the X-axis direction are provided in front and behind the table cover 8a. The table cover 8a and dust-proof and drip-proof covers 12 cover the components of the first moving mechanism 8 (guide rails, moving table, ball screw, pulse motor, etc.) that are located inside the opening 4a.

The upper surface of the chuck table 10 is a flat surface roughly parallel to the horizontal plane (XY plane) and constitutes a holding surface 10a that holds the workpiece 11. The holding surface 10a is made of a porous material such as porous ceramics, and is connected to a suction source (not shown) such as an ejector via a flow path (not shown) and a valve (not shown) formed inside the chuck table 10. Note that Figure 1 shows an example in which the holding surface 10a is circular, assuming that it will hold a disk-shaped workpiece 11, but the shape of the holding surface 10a can be modified as appropriate depending on the shape of the workpiece 11.

The chuck table 10 is moved along the X-axis direction together with the table cover 8a by the first movement mechanism 8. A rotary drive source (not shown), such as a motor, is connected to the chuck table 10, and this rotary drive source rotates the chuck table 10 around a rotation axis that is roughly parallel to the Z-axis direction. In other words, the rotation axis of the chuck table 10 is set along a direction perpendicular to the holding surface 10a.

A second movement mechanism (second movement unit) 14 is provided on the front side of the support structure 6. The second movement mechanism 14 has a pair of guide rails 16 arranged along the Z-axis direction. A flat movement table 18 is attached to the pair of guide rails 16 so that it can slide along the pair of guide rails 16.

A nut portion (not shown) is provided on the rear side (back side) of the moving table 18. A ball screw 20, which is arranged along the Z-axis direction between a pair of guide rails 16, is threadedly engaged with this nut portion. A pulse motor 22 is also connected to the end of the ball screw 20. When the ball screw 20 is rotated by the pulse motor 22, the moving table 18 moves in the Z-axis direction along the pair of guide rails 16.

A support member 24 that protrudes forward from the front surface of the moving table 18 is fixed to the front side (surface side) of the moving table 18. The support member 24 supports a grinding unit 26 that grinds the workpiece 11. The movement (lifting and lowering) of the grinding unit 26 in the Z-axis direction is controlled by the second moving mechanism 14.

The grinding unit 26 includes a hollow, cylindrical housing 28 supported by the support member 24. The housing 28 accommodates a cylindrical spindle 30 arranged along the Z-axis direction. The tip (lower end) of the spindle 30 protrudes downward from the underside of the housing 28. A rotational drive source (not shown), such as a motor, is connected to the base (upper end) of the spindle.

A disk-shaped wheel mount 32 made of metal or the like is fixed to the tip of the spindle 30. A grinding wheel 34 for grinding the workpiece 11 is attached to the underside of the wheel mount 32. For example, the grinding wheel 34 is fixed to the wheel mount 32 with a fastener such as a bolt.

The grinding wheel 34 rotates around a rotation axis roughly parallel to the Z-axis direction by power transmitted from the rotation drive source via the spindle 30 and wheel mount 32. In other words, the rotation axis of the grinding wheel 34 is set along a direction perpendicular to the holding surface 10a of the chuck table 10. In addition, a grinding fluid supply passage (not shown), such as a nozzle, is provided inside or near the grinding unit 26 to supply a liquid (grinding fluid) such as pure water to the workpiece 11 held by the chuck table 10 and the grinding wheel 34.

A control unit (control unit, control device) 36 that controls the grinding device 2 is provided inside or outside the grinding device 2. The control unit 36 is connected to each component of the grinding device 2 (first movement mechanism 8, chuck table 10, second movement mechanism 14, grinding unit 26, etc.) and generates control signals to control the operation of each component.

For example, the control unit 36 is configured by a computer and includes an arithmetic unit that performs calculations necessary to control the grinding device 2, and a storage unit that stores various information (data, programs, etc.) used to control the grinding device 2. The arithmetic unit includes a processor such as a CPU (Central Processing Unit). The storage unit includes memory such as a ROM (Read Only Memory) and RAM (Random Access Memory).

The grinding device 2 described above grinds the workpiece 11. For example, the workpiece 11 is a disk-shaped wafer made of a semiconductor material such as silicon, and has a front surface 11a and a back surface 11b. The workpiece 11 is divided into multiple rectangular regions by multiple streets (planned division lines) arranged in a grid pattern so as to intersect with each other. Furthermore, devices (not shown), such as ICs (Integrated Circuits), LSIs (Large Scale Integration), and LEDs (Light Emitting Diodes), are formed on the front surface 11a side of each of the multiple regions divided by the streets.

By dividing the workpiece 11 along the streets using cutting, laser processing, or the like, multiple device chips, each equipped with a device, are manufactured. Furthermore, if the back surface 11b of the workpiece 11 is ground using a grinding device 2 to thin the workpiece 11 before dividing it, a thinned device chip can be obtained.

There are no restrictions on the material, shape, structure, size, etc. of the workpiece 11. For example, the workpiece 11 may be a substrate made of a semiconductor other than silicon (GaAs, InP, GaN, SiC, etc.), sapphire, glass, ceramics, resin, metal, etc. Furthermore, there are no restrictions on the type, number, shape, structure, size, arrangement, etc. of devices formed on the workpiece 11, and the workpiece 11 does not necessarily have to have any devices formed on it.

Figure 2(A) is a perspective view showing the chuck table 10 and grinding unit 26. When the workpiece 11 is placed on the chuck table 10 and suction force (negative pressure) from the suction source is applied to the holding surface 10a, the workpiece 11 is sucked and held by the chuck table 10. Note that a protective tape made of resin or the like may be attached to the surface 11a of the workpiece 11 to protect the surface 11a side (device side) of the workpiece 11. In this case, the workpiece 11 is held by the holding surface 10a of the chuck table 10 via the protective tape.

A grinding wheel 34 is attached to the grinding unit 26. The grinding wheel 34 has a disk-shaped (annular) wheel base 40 made of metal or the like and formed with approximately the same diameter as the wheel mount 32. The wheel base 40 includes a first surface (upper surface) 40a and a second surface (lower surface) 40b that are generally parallel to each other, and an outer circumferential edge (side surface) 40c connected to the first surface 40a and the second surface 40b.

The grinding wheel 34 also includes multiple grinding stones 42. The grinding stones 42 are fixed to one end face (the second surface 40b side) of the wheel base 40 via adhesive or the like. For example, the grinding stones 42 are formed in a rectangular parallelepiped shape and arranged along the outer periphery of the wheel base 40.

Figure 2(B) is an enlarged cross-sectional view showing a portion of the grinding wheel 42. The grinding wheel 42 includes abrasive grains 44 and a bonding material 46 that secures the abrasive grains 44. Diamond, cBN (Cubic Boron Nitride), etc. are used as the abrasive grains 44. Furthermore, metal bond, resin bond, vitrified bond, etc. are used as the bonding material 46.

Figure 3 is a bottom view showing the grinding wheel 34. A circular opening 40d is provided in the center of the wheel base 40 provided on the grinding wheel 34, penetrating the wheel base 40 in the thickness direction. The center position of the opening 40d roughly coincides with the center O of the wheel base 40. In other words, the wheel base 40 and the opening 40d are arranged concentrically.

Two types of grinding wheels 42 (first grinding wheel 42A and second grinding wheel 42B) are fixed to one end face (second surface 40b) of the wheel base 40. The first grinding wheel 42A is arranged in an annular shape so as to overlap the circumference of a first circle 48A that is concentric with the wheel base 40. The second grinding wheel 42B is also arranged in an annular shape so as to overlap the circumference of a second circle 48B that is concentric with the wheel base 40 and has a smaller diameter than the first circle 48A. For example, the first grinding wheel 42A is arranged so that its length (longitudinal direction) follows the circumference of the first circle 48A. Similarly, the second grinding wheel 42B is arranged so that its length (longitudinal direction) follows the circumference of the second circle 48B.

Each second grinding wheel 42B is provided closer to the center O of the wheel base 40 than all of the first grinding wheels 42A. Specifically, the distance d _A from the center O of the wheel base 40 to the end of the first grinding wheel 42A located on the outer peripheral edge 40c side of the wheel base 40 is longer than the distance d _B from the center O of the wheel base 40 to the end of the second grinding wheel 42B located on the outer peripheral edge 40c side of the wheel base 40. Furthermore, for example, the second grinding wheel 42B is arranged so as not to overlap with two adjacent first grinding wheels 42A in the radial direction of the wheel base 40, as shown in FIG.

There are no restrictions on the dimensions (length, width, height), number, or arrangement of the first grinding wheel 42A and the second grinding wheel 42B. Furthermore, the dimensions and number of the first grinding wheel 42A may be the same as or different from the dimensions and number of the second grinding wheel 42B.

When the grinding wheel 34 is rotated, the first grinding wheel 42A and the second grinding wheel 42B each move along annular movement paths (rotation paths) that are generally parallel to the horizontal plane. At this time, the outer diameter of the trajectory (movement path) of the first grinding wheel 42A is larger than the outer diameter of the trajectory (movement path) of the second grinding wheel 42B.

Here, the hardness of the first grinding wheel 42A is lower than the hardness of the second grinding wheel 42B. In other words, the second grinding wheel 42B is harder than the first grinding wheel 42A. Therefore, when grinding the workpiece 11 with the grinding wheel 34 (see Figure 2(A)), the first grinding wheel 42A is more likely to wear than the second grinding wheel 42B.

There are no limitations on the method for adjusting the hardness of the first grinding wheel 42A and the second grinding wheel 42B. For example, by using a binder 46 (see Figure 2(B)) with a different material or density (porosity) for the first grinding wheel 42A and the second grinding wheel 42B, the magnitude relationship between the hardness of the first grinding wheel 42A and the hardness of the second grinding wheel 42B can be adjusted.

The size of the abrasive grains 44 (first abrasive grains) contained in the first grinding wheel 42A and the size of the abrasive grains 44 (second abrasive grains) contained in the second grinding wheel 42B are set appropriately depending on the material of the workpiece 11, etc. However, the first abrasive grains and second abrasive grains are selected so that the average grain size of the first abrasive grains is less than three times the average grain size of the second abrasive grains.

The grinding wheel 34 is attached to the grinding unit 26 (see Figures 1 and 2(A)), and the workpiece 11 is ground by the grinding wheel 34. In this embodiment, the workpiece 11 is thinned by performing creep feed grinding, in which the chuck table 10 and the grinding wheel 34 are moved relatively in a direction parallel to the holding surface 10a to process the workpiece 11. Below, a specific example of a method for grinding the workpiece 11 using the grinding device 2 is described.

First, as shown in FIG. 2(A), the workpiece 11 is held by the chuck table 10 (holding step). For example, the workpiece 11 is placed on the chuck table 10 so that the front surface 11a faces the holding surface 10a and the back surface 11b is exposed upward. In this state, when the suction force (negative pressure) of the suction source is applied to the holding surface 10a, the workpiece 11 is suction-held by the chuck table 10. As mentioned above, protective tape may be attached to the front surface 11a of the workpiece 11.

Next, the positional relationship between the chuck table 10 and the grinding unit 26 is adjusted (preparation step). Figure 4(A) is a side view showing the chuck table 10 and grinding unit 26 in the preparation step. In the preparation step, the positional relationship between the chuck table 10 and the grinding unit 26 is adjusted so that the workpiece 11 and the grinding wheel 42 are spaced apart from each other in the processing feed direction (X-axis direction) parallel to the holding surface 10a, and the lower surface of the grinding wheel 42 is positioned a predetermined distance below the upper surface (rear surface 11b) of the workpiece 11.

Specifically, first, the position of the chuck table 10 in the X-axis direction is adjusted by the first movement mechanism 8 (see FIG. 1) so that the workpiece 11 is positioned in front of the grinding wheel 34 (to the left of the page in FIG. 4A) without overlapping with the grinding wheel 34. The position of the grinding unit 26 in the Z-axis direction is also adjusted by the second movement mechanism 14 (see FIG. 1) so that the bottom surface of the grinding wheel 42 is positioned lower than the top surface of the workpiece 11. The difference ΔH in height (position in the Z-axis direction) between the top surface of the workpiece 11 and the bottom surface of the grinding wheel 42 at this time corresponds to the target value for the amount of workpiece 11 ground (the difference in thickness of the workpiece 11 before and after grinding) in the grinding step described below.

Next, while rotating the grinding wheel 34, the chuck table 10 and the grinding unit 26 are moved relative to each other in the processing feed direction (X-axis direction), and the workpiece 11 is ground from one end to the other end by the grinding wheel 42 (grinding step). Figure 4(B) is a side view showing the chuck table 10 and grinding unit 26 during the grinding step.

In the grinding step, the workpiece 11 is ground by creep feed grinding. Specifically, the spindle 30 is first rotated to rotate the grinding wheel 34 around a rotation axis that is approximately perpendicular to the holding surface 10a of the chuck table 10. The rotation speed of the grinding wheel 34 is set to, for example, between 1000 rpm and 3000 rpm.

Then, with the grinding wheel 34 rotating and the chuck table 10 not rotating, the chuck table 10 is moved along the X-axis direction at a predetermined speed by the first movement mechanism 8 (see Figure 1). As a result, the chuck table 10 and the grinding wheel 34 move relatively toward each other along the processing feed direction at a predetermined processing feed speed. The movement speed of the chuck table 10 (processing feed speed) is set, for example, to a range of 1 mm/s to 20 mm/s.

When the chuck table 10 moves and one end of the workpiece 11 (the front end in the direction of movement of the workpiece 11, the right end on the paper in Figure 4(B)) reaches the trajectory of the grinding wheel 42, that end is ground away by the grinding wheel 42. The chuck table 10 then moves along the X-axis direction until the other end of the workpiece 11 (the rear end in the direction of movement of the workpiece 11, the left end on the paper in Figure 4(B)) is positioned so that it overlaps with the trajectory of the grinding wheel 42. As a result, the workpiece 11 is ground from one end to the other by the grinding wheel 42, thinning the entire workpiece 11.

When the workpiece 11 is ground with the grinding wheel 42, the bonding material 46 (see Figure 2(B)) of the grinding wheel 42 gradually wears away, causing the exposed abrasive grains 44 (see Figure 2(B)) to fall off and newly exposing the abrasive grains 44 embedded within the bonding material 46 (a phenomenon known as self-sharpening). This prevents the sharpness of the grinding wheel 42 from decreasing due to wear of the abrasive grains 44. It also prevents the grinding wheel 42 from clogging, maintaining the abrasive grains 44 in a protruding state.

In addition, when the workpiece 11 is ground by the grinding wheel 42, a grinding fluid such as pure water is supplied to the workpiece 11 and the grinding wheel 42. This cools the workpiece 11 and the grinding wheel 42 and washes away any debris (grinding debris) generated by the grinding process.

Next, details of grinding the workpiece 11 with the grinding wheel 34 in the grinding step will be described with reference to Figures 5(A) to 5(C). Figure 5(A) is a cross-sectional view showing the workpiece 11 in contact with the first grinding wheel 42A. Note that the difference in height between the lower surfaces of the first grinding wheel 42A and the second grinding wheel 42B and the lower surface (surface 11a) of the workpiece 11 corresponds to the finished thickness T, which is the target value for the thickness of the workpiece 11 after grinding.

When processing feed begins, one end of the workpiece 11 first comes into contact with the rotating first grinding wheel 42A and is ground by the first grinding wheel 42A. The first grinding wheel 42A has low hardness and is easily worn away by contact with the workpiece 11. This makes it easy for the first grinding wheel 42A to self-sharpen while grinding the workpiece 11. This allows the first grinding wheel 42A to grind the workpiece 11 while maintaining high grinding ability, ensuring that the back surface 11b of the workpiece 11 is reliably ground away by the first grinding wheel 42A.

Figure 5(B) is a cross-sectional view showing the workpiece 11 in contact with the second grinding wheel 42B. Note that Figure 5(B) exaggerates the wear on the first grinding wheel 42A.

While the workpiece 11 is being ground with the first grinding wheel 42A, the lower-hardness first grinding wheel 42A wears, gradually reducing its height and causing the height position of the underside of the first grinding wheel 42A to fluctuate. As a result, the area of the workpiece 11 ground by the first grinding wheel 42A becomes slightly thicker than the finishing thickness T. The area not ground by the first grinding wheel 42A then comes into contact with the second grinding wheel 42B, which rotates inside the first grinding wheel 42A.

Figure 5(C) is a cross-sectional view showing the workpiece 11 being ground by the second grinding wheel 42B. After the workpiece 11 comes into contact with the second grinding wheel 42B, as the processing feed progresses further, the area that was not ground by the first grinding wheel 42A is removed by the second grinding wheel 42B.

The second grinding wheel 42B has high hardness and is resistant to wear even when it comes into contact with the workpiece 11. Therefore, even when the workpiece 11 is ground with the second grinding wheel 42B, the height position of the lower surface of the second grinding wheel 42B is unlikely to fluctuate. As a result, the thickness of the area of the workpiece 11 ground by the second grinding wheel 42B is roughly equal to the finishing thickness T.

Furthermore, the hardness of the second grinding wheel 42B is higher than that of the first grinding wheel 42A, and the second grinding wheel 42B is less likely to develop self-sharpening edges than the first grinding wheel 42A. As a result, the second grinding wheel 42B is less likely to have the abrasive grains 44 (see Figure 2(B)) excessively protruding from the binder 46 (see Figure 2(B)), reducing the surface roughness of the back surface 11b of the workpiece 11 after grinding.

The second grinding wheel 42B is less likely to self-sharpen than the first grinding wheel 42A, and is therefore more prone to clogging. However, by the time the workpiece 11 reaches the second grinding wheel 42B, most of the area to be ground on the workpiece 11 has already been removed by the first grinding wheel 42A, and the amount of grinding allocated to the second grinding wheel 42B is small. As a result, the amount of grinding waste generated when the workpiece 11 is ground with the second grinding wheel 42B is reduced, and in reality, the grinding ability of the second grinding wheel 42B is not significantly reduced due to clogging.

Here, if the average particle size of the abrasive grains 44 (first abrasive grains) contained in the first grinding wheel 42A were three times the average particle size of the abrasive grains 44 (second abrasive grains) contained in the second grinding wheel 42B, rough irregularities would be formed in the area of the workpiece 11 ground by the first grinding wheel 42A, and this roughness would likely remain in the workpiece 11 even after subsequent grinding by the second grinding wheel 42B.

Furthermore, when the workpiece 11 is ground with the first grinding wheel 42A, abrasive grains 44 that fall off from the first grinding wheel 42A may get caught between the workpiece 11 and the second grinding wheel 42B when the workpiece 11 is ground with the second grinding wheel 42B. In this case, if the average grain size of the first abrasive grains is three times the average grain size of the second abrasive grains, the large first abrasive grains that have gotten between the workpiece 11 and the second grinding wheel 42B may scrape off the workpiece 11, potentially leaving unexpected roughness on the workpiece 11 after grinding.

However, as mentioned above, the average particle size of the first abrasive grains is set to less than three times the average particle size of the second abrasive grains. Therefore, rough irregularities are unlikely to remain in the area of the workpiece 11 ground by the first grinding wheel 42A. Furthermore, even if the first abrasive grains get between the workpiece 11 and the second grinding wheel 42B, unexpected roughness will not remain in the workpiece 11.

The grinding of the workpiece 11 by the grinding device 2 is achieved by the control unit 36 (see Figure 1) controlling the operation of each component of the grinding device 2. Specifically, the memory of the control unit 36 stores a program that describes the series of operations of each component of the grinding device 2 required to perform the holding step, preparation step, and grinding step in that order. When grinding the workpiece 11, the control unit 36 reads and executes the program, sequentially outputting control signals to each component of the grinding device 2. This controls the operation of the grinding device 2, and the workpiece grinding method according to this embodiment is automatically performed.

In order to increase the cutting ability of the first grinding wheel 42A compared to the second grinding wheel 42B, it is preferable that the size of the first abrasive grains contained in the first grinding wheel 42A be larger than the size of the second abrasive grains contained in the second grinding wheel 42B. Specifically, the average grain size of the first abrasive grains is larger than the average grain size of the second abrasive grains. Alternatively, the grain size of the first abrasive grains is smaller than the grain size of the second abrasive grains. However, as mentioned above, the average grain size of the first abrasive grains is kept to less than three times the average grain size of the second abrasive grains. For example, diamonds with a grain size of #3000 can be used as the first abrasive grains, and diamonds with a grain size of #5000 can be used as the second abrasive grains.

Furthermore, to promote wear of the first grinding wheel 42A, the first grinding wheel 42A and the second grinding wheel 42B may be different in factors other than hardness. For example, the flexural strength (bending strength) of the first grinding wheel 42A may be lower than that of the second grinding wheel 42B, as long as it does not interfere with the grinding of the workpiece 11 by the first grinding wheel 42A. Specifically, the flexural strength of the first grinding wheel 42A is preferably 30% to 60% of the flexural strength of the second grinding wheel 42B. This makes the first grinding wheel 42A more susceptible to wear than the second grinding wheel 42B, promoting self-sharpening of the first grinding wheel 42A. The flexural strengths of the first grinding wheel 42A and the second grinding wheel 42B can be measured using a three-point bending test.

Furthermore, the concentration of the first abrasive grains in the first grinding wheel 42A may be smaller than the concentration of the second abrasive grains in the second grinding wheel 42B. Specifically, the concentration of the first abrasive grains in the first grinding wheel 42A is preferably less than 60% of the concentration of the second abrasive grains in the second grinding wheel 42B. This makes the first grinding wheel 42A more susceptible to wear than the second grinding wheel 42B, promoting self-sharpening of the first grinding wheel 42A.

The number of first grinding wheels 42A may be smaller than the number of second grinding wheels 42B. Furthermore, the width W _A (see FIG. 3) of the first grinding wheels 42A in the radial direction of the wheel base 40 may be smaller than the width W _B (see FIG. 3) of the second grinding wheels 42B in the radial direction of the wheel base 40. This makes the first grinding wheels 42A more susceptible to wear than the second grinding wheels 42B, promoting self-sharpening of the first grinding wheels 42A.

As described above, when using the grinding wheel 34 according to this embodiment, the workpiece 11 is ground efficiently by the first grinding wheel 42A, and then the workpiece 11 is ground flat by the second grinding wheel 42B. This improves the efficiency and quality of grinding the workpiece 11.

In addition, because the average particle size of the first abrasive grains contained in the first grinding wheel 42A is kept to less than three times the average particle size of the second abrasive grains contained in the second grinding wheel 42B, rough irregularities are less likely to remain in the area of the workpiece 11 ground by the first grinding wheel 42A, and first abrasive grains that fall off the first grinding wheel 42A are less likely to adversely affect the grinding of the workpiece 11 by the second grinding wheel 42B. As a result, deterioration in the quality of the workpiece 11 after grinding is prevented.

The number of times creep feed grinding is performed (the number of times the preparation step and grinding step are performed) can be set appropriately depending on the material of the workpiece 11, the amount of grinding, etc. In other words, the preparation step and grinding step may be performed two or more times to thin the workpiece 11 to its finishing thickness.

In addition, the structures, methods, etc. related to the above embodiments can be modified as appropriate without departing from the scope of the present invention.

11 Workpiece 11a Front surface 11b Back surface 2 Grinding device 4 Base 4a Opening 6 Support structure 8 First moving mechanism (first moving unit)
8a Table cover 10 Chuck table (holding table)
10a Holding surface 12 Dustproof/waterproof cover 14 Second moving mechanism (second moving unit)
16 Guide rail 18 Moving table 20 Ball screw 22 Pulse motor 24 Support member 26 Grinding unit 28 Housing 30 Spindle 32 Wheel mount 34 Grinding wheel 36 Control unit (control unit, control device)
40 Wheel base 40a First surface (upper surface)
40b 2nd side (bottom side)
40c Outer edge (side)
40d Opening 42 Grinding wheel 42A First grinding wheel 42B Second grinding wheel 44 Abrasive grains 46 Bonding material (bond material)
48A 1st Circle 48B 2nd Circle

Claims (4)

1. A grinding wheel for grinding a workpiece by creep feed grinding, comprising:
A disc-shaped wheel base,
a plurality of first grinding wheels including first abrasive grains and arranged in an annular shape on one end surface side of the wheel base;
a plurality of second grinding wheels including second abrasive grains and arranged in an annular shape on one end surface side of the wheel base;
the second grinding wheel is provided closer to the center of the wheel base than the first grinding wheel,
the second grinding wheels are arranged so as not to overlap with two adjacent first grinding wheels in the radial direction of the wheel base;
The hardness of the first grinding wheel is lower than the hardness of the second grinding wheel;
the average particle size of the first abrasive grains is less than three times the average particle size of the second abrasive grains;
A grinding wheel characterized in that the flexural strength of the first grinding wheel is 30% or more and 60% or less of the flexural strength of the second grinding wheel. 2. The grinding wheel of claim 1 , wherein the concentration of the first abrasive grains in the first grinding wheel is less than 60% of the concentration of the second abrasive grains in the second grinding wheel. 3. The grinding wheel according to claim 1, wherein the number of the first grinding wheels is less than the number of the second grinding wheels. 4. The grinding wheel according to claim 1, wherein the width of the first grinding wheel in the radial direction of the wheel base is smaller than the width of the second grinding wheel in the radial direction of the wheel base.