Abrasive tool

Abstract

To provide a polishing tool that can reliably eliminate the static electricity caused by the polishing of a wafer.SOLUTION: There is provided a polishing tool for polishing a wafer. The polishing tool includes a base and a polishing layer fixed to the base. The polishing layer includes an electrically conductive material dispersed therein to eliminate static electricity generated when the polishing layer comes into contact with the wafer. Preferably, the electrically conductive material is carbon fiber, with the content of the carbon fiber being 3 wt.% or more and 15 wt.% or less.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a polishing tool for polishing a wafer.

Background Art

[0002] In the manufacturing process of device chips, a wafer in which devices are formed in a plurality of regions partitioned by a plurality of streets (division planned lines) arranged in a grid pattern is used. By dividing this wafer along the streets, a plurality of device chips each having a device are obtained. The device chips are incorporated into various electronic devices such as mobile phones and personal computers.

[0003] In recent years, with the miniaturization of electronic devices, thinning of device chips has been demanded. Therefore, a process of thinning the wafer using a grinding device may be performed before dividing the wafer. The grinding device includes a chuck table that holds a workpiece and a grinding unit that grinds the workpiece, and a grinding wheel including a grinding stone is attached to the grinding unit. The wafer is held by the chuck table, and while rotating the chuck table and the grinding wheel, the grinding stone is brought into contact with the wafer, so that the wafer is ground and thinned (see Patent Document 1).

[0004] On the surface of the wafer (the surface to be ground) ground by the grinding stone, fine scratches (grinding marks, saw marks) formed along the path of the grinding stone remain. When the wafer in this state is divided to manufacture device chips, the grinding marks remain on the device chips and the flexural strength (bending strength) of the device chips decreases. Therefore, the ground wafer is subjected to a polishing process. For the polishing process, a disk-shaped polishing tool (polishing pad) having a polishing layer that contacts the workpiece is used. By pressing the polishing layer against the surface to be ground of the wafer while rotating the polishing tool, the surface to be ground is flattened and the grinding marks remaining on the surface to be ground are removed.

[0005] However, when polishing a wafer with a polishing tool, static electricity may be generated between the wafer and the polishing layer that are in contact with each other, and the surface (polished surface) side of the wafer to be polished by the polishing layer may be charged. As a result, the devices formed on the wafer may be damaged or malfunction, and the quality of the device chips may deteriorate.

[0006] In response to the above problems, Patent Document 2 discloses a method of polishing a wafer using a polishing tool including a polishing layer in which a columnar static eliminator is embedded. In this polishing tool, the static eliminator is exposed on the lower surface of the polishing layer, and the static eliminator contacts the polished surface of the wafer during wafer polishing. Thereby, the static electricity generated by the contact between the wafer and the polishing layer is removed through the static eliminator, and it becomes difficult for the device to be damaged or malfunction due to static electricity.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] As described above, by using a polishing tool in which a static eliminator is embedded in the polishing layer, it is possible to remove the static electricity generated during the polishing process. However, since the material of the static eliminator is different from the material of the base material of the polishing layer, the region where the static eliminator of the polishing layer is provided may be more likely to wear than other regions during wafer polishing. In this case, when the wafer is polished with the polishing tool for a certain period of time, the region where the static eliminator of the polishing layer is provided becomes thinner than other regions, and it becomes difficult for the static eliminator to contact the wafer. As a result, the static electricity removing effect may not be sufficiently exerted, and there is a risk that the occurrence of device damage or malfunction is not suppressed.

[0009] The present invention has been made in view of such problems, and an object thereof is to provide a polishing tool capable of surely removing static electricity generated by polishing a wafer.

Means for Solving the Problems

[0010] According to one aspect of the present invention, there is provided a polishing tool for polishing a wafer, including a base and a polishing layer fixed to the base, and the polishing layer is is dispersed with a conductive material for removing static electricity generated when the polishing layer contacts the wafer including a binder the conductive material is carbon fiber, and the content of the carbon fiber is 3 wt% or more and 15 wt% or less and the carbon fiber forms a conductive path from the upper surface to the lower surface of the binder A polishing tool is provided.

[0011] Preferably, The content of the carbon fiber is 5 wt% or more.

[0012] The polishing tool according to one aspect of the present invention includes a polishing layer in which a conductive material is dispersed. Thereby, when polishing a wafer with the polishing tool, the state in which the conductive material is in contact with the wafer is maintained, and static electricity generated between the wafer and the polishing layer is surely removed.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments according to one aspect of the present invention will be described with reference to the accompanying drawings. First, a configuration example of a polishing apparatus capable of polishing a workpiece using the polishing tool according to the present embodiment will be described. FIG. 1 is a perspective view showing a polishing apparatus 2. In FIG. 1, the X-axis direction (first horizontal direction, front-rear direction) and the Y-axis direction (second horizontal direction, left-right direction) are perpendicular to each other. Also, the Z-axis direction (vertical direction, up-down direction, height direction) is perpendicular to the X-axis direction and the Y-axis direction.

[0015] The polishing apparatus 2 includes a rectangular parallelepiped base 4 that supports or houses each component constituting the polishing apparatus 2. At the front end of the base 4, cassette mounting regions (cassette mounting tables) 6a and 6b on which cassettes 8a and 8b are placed are provided. The cassettes 8a and 8b are containers capable of housing a plurality of wafers 11, and are arranged on the cassette mounting regions 6a and 6b. For example, the cassette 8a houses the wafers 11 before polishing, and the cassette 8b houses the wafers 11 after polishing.

[0016] FIG. 2 is a perspective view showing a wafer 11. For example, the wafer 11 is a disk-shaped single crystal wafer made of a semiconductor material such as silicon, and includes surfaces 11a and back surfaces 11b that are substantially parallel to each other.

[0017] Wafer 11 is partitioned into a plurality of rectangular regions by a plurality of streets (lines to be divided) 13 arranged in a grid pattern so as to intersect each other. Further, devices 15 such as IC (Integrated Circuit), LSI (Large Scale Integration), LED (Light Emitting Diode), and MEMS (Micro Electro Mechanical Systems) devices are formed on the surfaces 11a of the plurality of regions partitioned by the streets 13, respectively.

[0018] However, there are no restrictions on the type, material, shape, structure, size, etc. of the wafer 11. For example, the wafer 11 may be a wafer made of a semiconductor other than silicon (such as GaAs, InP, GaN, SiC), sapphire, glass, ceramics, resin, metal, etc. Also, there are no restrictions on the type, quantity, shape, structure, size, arrangement, etc. of the devices 15.

[0019] By dividing the wafer 11 along the streets 13, a plurality of device chips each including the device 15 are manufactured. Also, by thinning the wafer 11 by grinding the back surface 11b side of the wafer 11 with a grinding wheel before dividing the wafer 11, a thinned device chip can be obtained.

[0020] Note that fine scratches (grinding marks, saw marks) formed along the path of the grinding wheel remain on the back surface 11b (the surface to be ground) of the wafer 11 ground by the grinding wheel. When the device chip is manufactured by dividing the wafer 11 in this state, the grinding marks remain on the device chip and the flexural strength (bending strength) of the device chip decreases. Therefore, the back surface 11b side of the ground wafer 11 is polished by a polishing device 2 (see FIG. 1). Thereby, the back surface 11b side of the wafer 11 is flattened, and the grinding marks remaining on the back surface 11b side of the wafer 11 are removed.

[0021] When polishing the back surface 11b side of the wafer 11 with the polishing device 2, a protective member 17 is attached to the front surface 11a side of the wafer 11. For example, as the protective member 17, a tape having substantially the same shape as the wafer 11 is used. The tape includes a flexible film-like base material and an adhesive layer (paste layer) provided on the base material. The base material is made of a resin such as polyolefin, polyvinyl chloride, or polyethylene terephthalate, and the adhesive layer is made of an epoxy-based, acrylic-based, or rubber-based adhesive or the like. Note that the adhesive layer may be an ultraviolet curable resin that cures by irradiation with ultraviolet rays.

[0022] The wafer 11 is accommodated in the cassette 8a shown in FIG. 1 in a state where the protective member 17 is attached. Then, the cassette 8a containing a plurality of wafers 11 is placed on the cassette placement area 6a.

[0023] An opening 4a is provided in a region located between the cassette placement areas 6a and 6b on the upper surface side of the base 4. And inside the opening 4a, a first transfer mechanism 10 for transferring the wafer 11 is provided. Also, an operation panel 12 for inputting various information (processing conditions, etc.) to the polishing device 2 is provided in a region in front of the opening 4a.

[0024] A position adjustment mechanism 14 for adjusting the position of the wafer 11 is provided diagonally rearward of the first transfer mechanism 10. The wafer 11 accommodated in the cassette 8a is transferred onto the position adjustment mechanism 14 by the first transfer mechanism 10. Then, the position adjustment mechanism 14 adjusts the position of the wafer 11 by sandwiching the wafer 11. Also, a second transfer mechanism (loading arm) 16 for holding and turning the wafer 11 is arranged in the vicinity of the position adjustment mechanism 14.

[0025] In a region located behind the second transfer mechanism 16 on the upper surface side of the base 4, a rectangular opening 4b is provided, the longitudinal direction of which is formed along the X-axis direction. And a moving mechanism 18 is provided inside the opening 4b. For example, the moving mechanism 18 is a ball screw type moving mechanism and includes a ball screw (not shown) arranged along the X-axis direction, a pulse motor (not shown) for rotating the ball screw, and the like. Further, the moving mechanism 18 includes a flat moving table 20 and moves the moving table 20 along the X-axis direction. Furthermore, bellows-shaped dust and drip-proof covers 22 that cover the components (such as the ball screw and the pulse motor) of the moving mechanism 18 and expand and contract along the X-axis direction are provided in front of and behind the moving table 20.

[0026] On the moving table 20, a chuck table (holding table) 24 for holding the wafer 11 is provided. The upper surface of the chuck table 24 is a flat surface that is generally parallel to the horizontal direction (XY plane direction) and constitutes a holding surface 24a for holding the wafer 11. The holding surface 24a is connected to a suction source (not shown) such as an ejector via a suction path 24b (see FIG. 6) formed inside the chuck table 24, a valve (not shown), and the like. The wafer 11 that has been aligned by the position adjustment mechanism 14 is transferred onto the holding surface 24a of the chuck table 24 by the second transfer mechanism 16 and is held by the chuck table 24.

[0027] When the moving mechanism 18 moves the moving table 20, the chuck table 24 moves along the X-axis direction together with the moving table 20. Further, a rotational drive source (not shown) such as a motor for rotating the chuck table 24 around a rotation axis that is generally parallel to the Z-axis direction is connected to the chuck table 24.

[0028] At the rear end of the base 4, a rectangular parallelepiped-shaped support structure 26 is provided. A moving mechanism 28 is provided on the front side of the support structure 26. The moving mechanism 28 includes a pair of guide rails 30 arranged along the Z-axis direction on the front side of the support structure 26. A moving plate 32 is slidably mounted on the pair of guide rails 30 along the guide rails 30.

[0029] On the rear surface side (back surface side) of the moving plate 32, a nut portion (not shown) is provided. A ball screw 34 arranged along the Z-axis direction is screwed into this nut portion between a pair of guide rails 30. Further, a pulse motor 36 is connected to an end portion of the ball screw 34. When the ball screw 34 is rotated by the pulse motor 36, the moving plate 32 moves in the Z-axis direction along the guide rails 30.

[0030] On the front surface side (front side) of the moving plate 32, a support member 38 is provided. The support member 38 supports a polishing unit 40 that performs polishing on the wafer 11.

[0031] The polishing unit 40 includes a hollow cylindrical housing 42 supported by the support member 38. A cylindrical spindle 44 arranged along the Z-axis direction is rotatably accommodated in the housing 42. The tip end portion (lower end portion) of the spindle 44 is exposed outside the housing 42, and a rotational drive source (not shown) such as a motor is connected to the base end portion (upper end portion) of the spindle 44.

[0032] A disk-shaped mount 46 is fixed to the tip end portion of the spindle 44. And a disk-shaped polishing tool (polishing pad) 48 for polishing the wafer 11 is mounted on the lower surface side of the mount 46. For example, the polishing tool 48 is fixed to the mount 46 by a fixture such as a bolt 50. The polishing tool 48 rotates around a rotation axis substantially parallel to the Z-axis direction by the power transmitted from the rotational drive source via the spindle 44 and the mount 46.

[0033] The chuck table 24 holding the wafer 11 is positioned below the polishing unit 40 by the moving mechanism 18. Then, while rotating the chuck table 24 and the spindle 44, the polishing unit 40 is lowered at a predetermined speed by the moving mechanism 28. Thereby, the rotating polishing tool 48 contacts the wafer 11, and the wafer 11 is polished.

[0034] At a position adjacent to the second transfer mechanism 16, a third transfer mechanism (unloading arm) 52 that holds and rotates the wafer 11 is disposed. Further, a cleaning mechanism 54 for cleaning the wafer 11 is disposed on the front side of the third transfer mechanism 52. For example, the cleaning mechanism 54 includes a spinner table that holds and rotates the wafer 11, and a nozzle that supplies a cleaning liquid such as pure water to the wafer 11 held by the spinner table.

[0035] The wafer 11 polished by the polishing unit 40 is transported to the cleaning mechanism 54 by the third transfer mechanism 52 and is cleaned by the cleaning mechanism 54. Then, the cleaned wafer 11 is transported by the first transfer mechanism 10 and stored in the cassette 8b.

[0036] When polishing the wafer 11 with the polishing apparatus 2, a polishing tool 48 is attached to the mount 46. FIG. 3(A) is a perspective view showing the upper surface side of the polishing tool 48, and FIG. 3(B) is a perspective view showing the bottom surface side of the polishing tool 48. The polishing tool 48 includes a disk-shaped base 60 and a disk-shaped polishing layer 62 fixed to the base 60.

[0037] The base 60 is made of a metal such as stainless steel or aluminum, and has a plurality of screw holes 60a that open on the upper surface side of the base 60. The screw holes 60a are arranged at substantially equal intervals along the circumferential direction of the base 60. Further, a columnar through hole 60b that penetrates the base 60 in the thickness direction is provided at the center of the base 60.

[0038] The polishing layer 62 is formed in a disk shape having substantially the same diameter as the base 60, and is joined to the lower surface side of the base 60 by an adhesive or the like. The lower surface of the polishing layer 62 constitutes a flat polishing surface 62a that contacts the wafer 11 and polishes the wafer 11. Further, a columnar through hole 62b that penetrates the polishing layer 62 in the thickness direction is provided at the center of the polishing layer 62.

[0039] With the upper surface of the abutment 60 in contact with the lower surface of the mount 46 (see FIG. 1), the bolt 50 (see FIG. 1) is inserted through a through hole (not shown) provided in the mount 46 into the threaded hole 60a and screwed therein, whereby the polishing tool 48 is attached to the mount 46.

[0040] FIG. 4 is an enlarged cross-sectional view showing a part of the polishing layer 62. The polishing layer 62 includes a binder (base material) 64 that is the base material of the polishing layer 62, abrasive grains (fixed abrasive grains) 66 and a conductive material 68 contained in the binder 64. In FIG. 4, for convenience of explanation, the abrasive grains 66 and the conductive material 68 are enlarged and shown with respect to the thickness of the binder 64.

[0041] The binder 64 is a disc-shaped member that functions as a bonding material for fixing the abrasive grains 66, and includes an upper surface 64a and a lower surface 64b that are generally parallel to each other. The lower surface 64b of the binder 64 corresponds to the polishing surface 62a of the polishing layer 62 (see FIGS. 3(A) and 3(B)).

[0042] For example, the binder 64 is made of felt, resin (such as foamed urethane, rubber particles, etc.), and the thickness of the binder 64 is set to be 5 mm or more and 15 mm or less. As the abrasive grains 66, for example, silica (SiO2) having an average particle size of 1 μm or more and 10 μm or less is used. However, the material and thickness of the binder 64 and the material and particle size of the abrasive grains 66 can be appropriately changed according to the material of the wafer 11, which is the object to be polished.

[0043] In addition, the conductive material 68 is generally uniformly dispersed in the polishing layer 62 (binder 64). A part of the conductive material 68 is exposed on the upper surface 64a of the binder 64, and another part of the conductive material 68 is exposed on the lower surface 64b of the binder 64. The conductive material 68 exposed on the upper surface 64a and the conductive material 68 exposed on the lower surface 64b are connected through the conductive material 68 embedded in the binder 64. Thereby, a conductive path from the upper surface 64a to the lower surface 64b of the binder 64 is formed, and the polishing layer 62 has conductivity in the thickness direction of the polishing layer 62 (the thickness direction of the binder 64).

[0044] The conductive material 68 is a conductive substance for removing static electricity generated when the polishing layer 62 comes into contact with the wafer 11. As the conductive material 68, carbon fibers can be used. For example, carbon fibers with an average value of the length (average fiber length) of 1 μm or more and 20 μm or less and an average value of the diameter (average fiber diameter) of 0.1 μm or more and 0.5 μm or less are used. Also, the content of the carbon fibers is adjusted so that a conductive path from the upper surface 64a to the lower surface 64b of the binder 64 is appropriately formed. Specifically, the content rate of the carbon fibers is preferably 3 wt% or more and 15 wt% or less. This content rate corresponds to the ratio of the mass of the carbon fibers to the mass of the polishing layer 62 including the abrasive grains 66 (the total of the mass of the binder 64, the mass of the abrasive grains 66, and the mass of the carbon fibers).

[0045] For example, a felt impregnated with a liquid mixed with the abrasive grains 66 and carbon fibers can be obtained by a method of impregnating a felt with a liquid in which the abrasive grains 66 and carbon fibers are mixed, or a method of mixing the abrasive grains 66 and carbon fibers into the raw material of the felt in the manufacturing process of the felt. Then, by impregnating this felt with a liquid adhesive (such as an epoxy resin-based adhesive or a phenolic resin-based adhesive), a polishing layer 62 in which the abrasive grains 66 and carbon fibers are dispersed is formed on the binder 64 made of felt. Also, by performing compression molding and firing after mixing or kneading a resin material, the abrasive grains 66, and the carbon fibers, a polishing layer 62 in which the abrasive grains 66 and carbon fibers are dispersed is formed on the binder 64 made of resin.

[0046] Note that there are no restrictions on the shape, number, and size of the polishing layer 62 fixed to the base 60. FIG. 5(A) is a perspective view showing the upper surface side of a polishing tool 48 having a plurality of polishing layers 70, and FIG. 5(B) is a perspective view showing the bottom surface side of the polishing tool 48 having a plurality of polishing layers 70.

[0047] As shown in FIGS. 5(A) and 5(B), a plurality of polishing layers 70 may be fixed to the base 60. For example, four polishing layers 70 formed in a teardrop shape (petal shape) are arranged at substantially equal intervals along the circumferential direction of the base 60. The lower surfaces of the polishing layers 70 each constitute a flat polishing surface 70a that contacts the wafer 11 and polishes the wafer 11. Note that the configuration of the polishing layer 70 is the same as that of the polishing layer 62 (see FIG. 4).

[0048] Next, a specific example of a method for polishing the wafer 11 using the polishing tool 48 will be described. FIG. 6 is a cross-sectional view showing a polishing apparatus 2 for polishing the wafer 11.

[0049] When polishing the wafer 11 with the polishing tool 48, the polishing tool 48 is mounted on the polishing unit 40 of the polishing apparatus 2. Also, the wafer 11 is held by the chuck table 24. Specifically, the wafer 11 is placed on the chuck table 24 such that the surface 11a side (the protective member 17 side) faces the holding surface 24a and the back surface 11b is exposed upward. When the suction force (negative pressure) of the suction source is applied to the holding surface 24a in this state, the wafer 11 is suction-held by the chuck table 24 via the protective member 17.

[0050] The chuck table 24 holding the wafer 11 is positioned below the polishing unit 40 by the moving mechanism 18 (see FIG. 1). At this time, the wafer 11 is arranged such that the entire back surface 11b (the surface to be polished) overlaps with the polishing surface 62a of the polishing layer 62.

[0051] Next, while rotating the chuck table 24 and the spindle 44, the polishing unit 40 is lowered by the moving mechanism 28 (see FIG. 1). As a result, the rotating polishing layer 62 is pressed against the back surface 11b side of the wafer 11, and the back surface 11b side of the wafer 11 is polished by the polishing surface 62a. For example, the wafer 11 is processed by dry polishing in which no polishing liquid is supplied to the wafer 11 and the polishing tool 48 during polishing.

[0052] When the polishing unit 40 descends to a predetermined position, the polishing amount of the wafer 11 (the difference in the thickness of the wafer 11 before and after polishing) reaches a predetermined value, and the polishing process of the wafer 11 is completed. As a result, the back surface 11b side of the wafer 11 is planarized, and the grinding marks remaining on the back surface 11b side of the wafer 11 are removed.

[0053] In addition, when the wafer 11 is polished with the polishing tool 48, static electricity may be generated between the wafer 11 and the polishing layer 62 that are in contact with each other, and the surface to be polished of the wafer 11 (the back surface 11b side) may be charged. The charging of the wafer 11 may cause damage or malfunction of the device 15 (see FIG. 2) formed on the wafer 11.

[0054] Here, the polishing tool 48 according to the present embodiment includes a polishing layer 62 (see FIG. 4) in which a conductive material 68 is dispersed. When the wafer 11 is polished with the polishing tool 48, the conductive material 68 exposed on the lower surface 64b (polishing surface 62a) of the binder 64 contacts the wafer 11. As a result, the wafer 11 is connected to a ground terminal (not shown) via the conductive material 68 dispersed in the polishing layer 62, the base 60 made of a conductive metal, the mount 46, and the spindle 44. Thereby, a discharge path for static electricity generated between the wafer 11 and the polishing layer 62 is formed, and the static electricity is removed from the wafer 11.

[0055] The conductive material 68 is generally uniformly dispersed over the entire area of the polishing layer 62, and the wear amount of the polishing layer 62 (the decrease amount of the thickness of the polishing layer 62) when the wafer 11 is polished with the polishing tool 48 is generally uniform over the entire area of the polishing layer 62. That is, the polishing surface 62a of the polishing layer 62 is maintained flat. Thereby, the state in which the conductive material 68 exposed on the polishing surface 62a of the polishing layer 62 contacts the wafer 11 is maintained, and the static electricity removal effect continues.

[0056] As described above, the polishing tool 48 according to the present embodiment includes a polishing layer 62 in which a conductive material 68 is dispersed. Thus, when the wafer 11 is polished with the polishing tool 48, the state in which the conductive material 68 is in contact with the wafer 11 is maintained, and the static electricity generated between the wafer 11 and the polishing layer 62 is surely removed.

[0057] In the above embodiment, the case where the wafer 11 is polished by dry polishing has been described, but the wafer 11 can also be polished by wet polishing. In this case, when the wafer 11 is polished by the polishing tool 48, a polishing liquid is supplied to the wafer 11 and the polishing tool 48 through the through holes 60b and 62b from a polishing liquid supply path 72 (see FIG. 6) formed inside the polishing unit 40. For example, as the polishing liquid, an alkaline solution containing sodium hydroxide, potassium hydroxide, etc., an acidic liquid containing permanganate, etc., pure water, etc. are used.

[0058] In addition, the structure, method, etc. according to the above embodiment can be appropriately changed and implemented as long as they do not deviate from the scope of the object of the present invention.

[0059] (Example 1) Next, the results of evaluating the characteristics of the polishing tool according to the present invention will be described. In this example, a substrate corresponding to the polishing layer 62 (see FIG. 4) of the polishing tool 48 was prepared, and the resistance value of the substrate was measured.

[0060] FIG. 7(A) is a perspective view showing a substrate 21 for evaluation. The substrate 21 was formed in the same manner as the polishing layer 62 (see FIG. 4). Specifically, a disk-shaped substrate 21 was formed by dispersing abrasive grains and a conductive material in a binder (rubber particles). As the abrasive grains, silica having an average particle size of 5 μm was used, and as the conductive material, carbon fibers having an average fiber length of 10 μm and an average fiber diameter of 0.2 μm were used. The diameter of the substrate 21 was 150 mm, and the thickness of the substrate 21 was 10 mm.

[0061] In this embodiment, nine substrates 21 with different carbon fiber contents were prepared. The carbon fiber content in each substrate 21 was adjusted to be 0 wt%, 1.0 wt%, 2.0 wt%, 3.0 wt%, 3.5 wt%, 4.0 wt%, 4.5 wt%, 5.0 wt%, and 15.0 wt%. This content corresponds to the ratio of the mass of carbon fiber to the mass of the substrate 21 containing abrasive grains (the sum of the mass of the binder, the mass of the abrasive grains, and the mass of the carbon fiber).

[0062] Then, the resistance value in the thickness direction of the substrate 21 was measured. The resistance value was measured by applying the probes of a resistance meter (tester) 80 to the front surface 21a and the back surface 21b of the substrate 21.

[0063] FIG. 7(B) is a graph showing the relationship between the carbon fiber content and the resistance value of the evaluation substrate 21. The resistance values of the substrates 21 with carbon fiber contents of 0 wt%, 1.0 wt%, and 2.0 wt% were 3000 kΩ or more, which is the measurement upper limit value of the resistance meter 80. On the other hand, when the carbon fiber content reached 3.0 wt%, the resistance value of the substrate 21 decreased rapidly to 236 kΩ. This is presumably because the carbon fibers contained in the substrate 21 increased, making it easier to form a conductive path from the front surface 21a to the back surface 21b of the substrate 21. Thereby, it was confirmed that the carbon fiber content contained in the polishing layer 62 (see FIG. 4) is preferably 3.0 wt% or more.

[0064] Furthermore, every time the carbon fiber content reached 3.5 wt%, 4.0 wt%, 4.5 wt%, and 5.0 wt%, the resistance value of the substrate 21 decreased to 94 kΩ, 24 kΩ, 11 kΩ, and 8 kΩ. Thereby, it was confirmed that the carbon fiber content contained in the polishing layer 62 (see FIG. 4) is preferably 3.5 wt% or more, or 4.0 wt% or more, or 4.5 wt% or more, or 5.0 wt% or more. And when the carbon fiber content was 5.0 wt% and 15.0 wt%, the resistance value of the substrate 21 became the minimum value of 8 kΩ.

[0065] However, when the carbon fiber content exceeds 15.0 wt%, although the resistance value of the substrate 21 is maintained low, it was confirmed that the substrate 21 becomes brittle and the mechanical strength of the substrate 21 decreases. Therefore, in order to polish the wafer 11 while forming the polishing layer 62 (see FIG. 4) into a desired shape and maintaining the shape of the polishing layer 62, the carbon fiber content is preferably 15.0 wt% or less.

[0066] From the above results, it was confirmed that by including carbon fibers in the polishing layer 62 (see FIG. 4) of the polishing tool 48, it is possible to lower the resistance value in the thickness direction of the polishing layer 62 and exhibit conductivity effective for removing static electricity.

[0067] (Example 2) Next, the results of polishing a wafer with the polishing tool according to the present invention will be described. In this example, the charging of the wafer during the polishing process was monitored by measuring the voltage on the surface of the wafer while polishing the wafer with the polishing tool 48.

[0068] FIG. 8(A) is a bottom view showing the polishing tool 48 used for polishing the wafer. The polishing tool 48 shown in FIG. 8(A) has the same configuration as the polishing tool 48 shown in FIGS. 5(A) and 5(B), except that the number of polishing layers 70 is five.

[0069] The diameter of the base 60 was 450 mm, and the thickness of the five teardrop-shaped (petal-shaped) polishing layers 70 was 10 mm. The polishing layer 70 was formed by dispersing abrasive grains and a conductive material in a binder (rubber particles). As the abrasive grains, silica with an average particle size of 5 μm was used, and as the conductive material, carbon fibers with an average fiber length of 10 μm and an average fiber diameter of 0.2 μm were used. The carbon fiber content was adjusted to 5 wt%.

[0070] Then, the above polishing tool 48 was attached to the polishing unit 40 (see FIG. 1) of the polishing apparatus 2, and the wafer was polished with the polishing tool 48. FIG. 8(B) is a partial cross-sectional front view showing the polishing tool 48 used for polishing the wafer 23.

[0071] As the wafer 23, a silicon wafer with a diameter of 300 mm and a thickness of 100 μm was used. Then, the surface 23a side of the wafer 23 was held by a chuck table 24 (see FIG. 6), and the back surface 23b side of the wafer 23 was polished by a polishing layer 70. The rotation speed of the chuck table 24 (see FIG. 6) was 100 rpm, the rotation speed of the spindle 44 (see FIG. 6) was 1000 rpm, and the lowering speed of the polishing tool 48 was adjusted so that a load of 200 N was applied to the wafer 23.

[0072] Under the above polishing conditions, 48 wafers 23 were each polished by dry polishing for 220 seconds. During the polishing of the wafer 23, the voltage of the back surface 23b of the wafer 23 was measured using a non-contact voltage measuring device 82. The voltage measuring device 82 was installed at the center of the base 60 of the polishing tool 48, and the voltage of the region positioned directly below the voltage measuring device 82 on the back surface 23b of the wafer 23 was measured.

[0073] The measured voltage was in the range of -50 V or more and 50 V or less during the polishing of all 48 wafers 23, and was kept substantially constant. That is, no increase or decrease in voltage due to the charging of the wafer 23 was confirmed. This is presumably because the static electricity generated between the wafer 23 and the polishing layer 70 during polishing was removed by the carbon fibers contained in the polishing layer 70.

[0074] From the above results, it was confirmed that by incorporating carbon fibers in the polishing layer 70 of the polishing tool 48, charging of the wafer 23 was effectively prevented.

Explanation of Reference Numerals

[0075] 11 Wafer 11a Surface (First Surface) 11b Back Surface (Second Surface) 13 Street (Division Planned Line) 15 Device 17 Protection Member 21 Substrate 21a Surface 21b Back Surface 23 Wafer 23a surface 23b back surface 2 grinding device 4 base 4a, 4b openings 6a, 6b cassette placement area (cassette placement table) 8a, 8b cassettes 10 first transfer mechanism 12 operation panel 14 position adjustment mechanism 16 second transfer mechanism (loading arm) 18 moving mechanism 20 moving table 22 dust and drip proof cover 24 chuck table (holding table) 24a holding surface 24b suction path 26 support structure 28 moving mechanism 30 guide rail 32 moving plate 34 ball screw 36 pulse motor 38 support member 40 grinding unit 42 housing 44 spindle 46 mount 48 grinding tool (grinding pad) 50 bolt 52 third transfer mechanism (unloading arm) 54 cleaning mechanism 60 base 60a screw hole 60b through hole 62 grinding layer 62a grinding surface 62b through hole 64 bonding material (base material) 64a upper surface 64b lower surface 66 abrasive grains (fixed abrasive grains) 68 conductive material 70 grinding layer 70a grinding surface 72 Abrasive liquid supply path 80 Resistance meter (tester) 82 Voltage measuring device

Claims

**Claim 1** A polishing tool for polishing a wafer, comprising: a base and a polishing layer fixed to the base, the polishing layer including a binder in which a conductive material for removing static electricity generated when the polishing layer contacts the wafer is dispersed, the conductive material being carbon fiber, the content of the carbon fiber being 3 wt% or more and 15 wt% or less, the carbon fiber forming a conductive path from the upper surface to the lower surface of the binder. A polishing tool characterized by this. **Claim 2** The polishing tool according to claim 1, wherein the content of the carbon fiber is 5 wt% or more.

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