JP6896476B2 - Wafer and wafer thinning method and wafer thinning equipment - Google Patents

Description

The present invention relates to a method for manufacturing a semiconductor substrate, and more particularly to a wafer and a wafer thinning method capable of improving a yield when thinning a semiconductor wafer, and a wafer thinning device.

Semiconductor manufacturing technology includes machining technology, lithography technology, thin film formation technology, etching technology, ion implantation technology, cleaning technology, and the like. Semiconductor machining technology includes cutting (slicing), grinding, lapping, polishing, CMP (chemical mechanical polishing), and back grinding in each process of wafer manufacturing process, wafer processing process, chipping process, and assembly process. , Dicing, etc.

Recently, the demand for applications such as thin cards and wireless tags is increasing, and the demand for high-density mounting is also increasing. In order to meet these demands, it is necessary to make the wafer ultra-thin, and the demand for ultra-thin technology is increasing. In order to make the wafer extremely thin, various devices and methods have been devised for grinding the wafer. In addition, wafers are becoming larger in diameter in order to improve productivity and reduce costs. However, in order to realize ultra-thin wafers in large-diameter wafers, polishing / grinding methods, finishing methods, processing equipment, and transport methods A lot of ingenuity is required for such things.

The thinning of the semiconductor wafer is performed by back grinding in which the semiconductor wafer is ground from the back surface to obtain a desired thickness, and when the semiconductor wafer is simply ground from the back surface, the mechanical strength of the semiconductor wafer is lowered. It becomes easy to break. Further, the semiconductor wafer tends to warp, and as a result, when the layer is thinned, the defect rate of the semiconductor wafer due to cracking or warping increases sharply.

In order to prevent the defective rate of the semiconductor wafer from increasing due to cracking and warpage due to back grinding, the back surface of the semiconductor wafer is ground leaving a few mm of the outer edge portion, and the inside of the outer edge portion of the semiconductor wafer is thinned. Layer. That is, it is known that a recess is formed inside the outer edge portion, and is described in, for example, Patent Document 1.

In addition, an annular step is formed on the peripheral edge of the chamfered unthinned wafer on the surface side to prevent knife edges, and these steps have a circular shape corresponding to the diameter of the finally obtained wafer. It is described in Patent Document 2 that the depth corresponds to the target thickness of the thinned wafer.

Japanese Unexamined Patent Publication No. 2007-335569 JP-A-2007-152906

The above-mentioned prior art is effective in reducing the transfer risk of thin wafers and reducing warpage, and can reduce edge chipping. However, it has been difficult to improve the quality by mechanically polishing the wafer in order to prevent the wafer from being damaged by the microcrack (fine scratches and cracks) layer caused by back grinding.

An object of the present invention is to solve the above-mentioned problems of the prior art and eliminate damage to the wafer end surface due to handling work after shape processing of the wafer, damage to the wafer end surface in a process of pressing a polishing tape against an edge portion of the wafer end surface, and the like. The purpose is to improve productivity and profitability.

In order to achieve the above object, in the present invention, in a wafer thinned to a desired thickness by back grinding, the edge portion of the outer circumference of the wafer is processed as a chamfered portion, and the upper portion of the chamfered portion is the upper surface of the wafer. The length of the chamfer that is horizontally removed from the outer circumference to the inner circumference of the wafer from the outer circumference to the target thickness (T) to be thinned is L, the angle φ of the chamfered portion with respect to the horizontal, the margin α, As a result, it is processed into a shape such that L = T / tanφ + α.

Further, in the above wafer, it is desirable that the margin α is 0.1 to 0.5 mm.

Further, in the above wafer, it is desirable that the angle φ is determined to be equal to the angle of the portion of the inner surface of the transfer cassette of the wafer that comes into contact with the outer peripheral portion of the wafer.

Further, in the above wafer, it is desirable that the angle φ is determined to be equal to the contact angle with the polishing tape for polishing the outer peripheral portion of the wafer.

Further, the present invention is a method for thinning a wafer in which a wafer is processed to a desired thickness by back grinding, in which an edge portion on the outer periphery of the wafer is processed as a chamfered portion and an upper portion of the chamfered portion is formed on the wafer. From the upper surface to the target thickness (T) to be thinned, the wafer is horizontally removed from the outer circumference toward the inner circumference, and the removal allowance length removed from the outer circumference is L, the angle φ of the chamfered portion with respect to the horizontal, and the margin α. As a result, it is processed into a shape such that L = T / tanφ + α.

Further, in the above, it is desirable that the margin α is 0.1 to 0.5 mm.

Further, in the above, it is desirable that after the wafer is processed into the shape, the outermost peripheral portion of the wafer is mirror-finished with a polishing tape.

Further, in the above, it is desirable to determine the angle φ to be equal to the contact angle with the polishing tape.

Further, in the above, it is desirable to determine the angle φ to be equal to the angle of the portion of the inner surface of the transfer cassette of the wafer that comes into contact with the outer peripheral portion of the wafer.

Further, according to the present invention, in a wafer thinning device for processing a wafer to a desired thickness by back grinding, a chamfering device for processing the edge portion of the outer periphery of the wafer as a chamfer portion and a chamfering device before processing the back grinding. The upper portion of the chamfered portion that has been chamfered in advance is provided with a grindstone that horizontally cuts and removes the upper portion of the chamfered portion up to the target thickness (T) from the outer circumference to the inner circumference of the wafer. It is processed into a shape such that L = T / tan φ + α, where φ is the margin α and the angle of the portion of the inner surface of the transfer cassette of the wafer that comes into contact with the outer peripheral portion of the wafer.

According to the present invention, the upper part of the chamfered portion on the outer periphery of the wafer is horizontally removed from the outer periphery toward the inner circumference up to the target thickness (T) to be thinned, the chamfer length is L, and the angle φ with respect to the horizontal of the chamfered portion Since the shape is processed so that L = T / tanφ + α as the margin α, the wafer is damaged due to the handling work after the shape of the wafer is processed, and the wafer is pressed against the edge of the end face of the wafer. Back grinding can be achieved by eliminating damage to the end face. Therefore, the productivity and profitability of the thinned wafer can be improved.

Front view showing a main part according to an embodiment of the present invention. Top view showing the main part which concerns on one Embodiment of this invention Cross-sectional view showing the shape of the wafer W at the time of chamfering according to the embodiment of the present invention. Cross-sectional view showing a cut by a trimming grindstone according to an embodiment of the present invention. Sectional drawing which shows the detailed shape of the outer peripheral part of the wafer W which concerns on one Embodiment of this invention. A side view showing a state of contact between the wafer W and the polishing tape according to the embodiment of the present invention. The figure which shows the structure of the polishing apparatus which concerns on one Embodiment of this invention. A cross-sectional view showing a back grinding process and a shape of a wafer W according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The thinning of the semiconductor wafer is performed by back grinding in which the semiconductor wafer is ground from the back surface to obtain a desired thickness, and in the subsequent manufacturing process, the peripheral edge of the wafer is chamfered before the wafer is thinned. However, the peripheral edge is processed to prevent damage such as cracks and chips during handling.

Further, when the chamfered wafer is thinned, the cross section of the peripheral edge is sharpened so as to become thinner toward the outer peripheral side and changes to a knife edge shape, so that the peripheral edge is a surface along the thickness direction (plane direction). Before thinning, the surface is cut so that it does not become an acute angle.

First, the chamfering device will be described. FIG. 1 is a front view showing a main part of a chamfering device according to an embodiment of the present invention, and FIG. 2 is a plan view.

As shown in the figure, the chamfering device 50 includes a wafer feed unit 50A and a grinding unit 50B. A pair of Y-axis guide rails 52, 52 are laid at predetermined intervals on the horizontally arranged base plate 51 of the wafer feed unit 50A. A Y-axis table 56 is slidably supported on the pair of Y-axis guide rails 52, 52 via Y-axis linear guides 54, 54.

A nut member 58 is fixed to the lower surface of the Y-axis table 56, and a Y-axis ball screw 60 is screwed into the nut member 58. The Y-axis ball screw 60 is rotatably supported at both ends of the pair of Y-axis guide rails 52, 52 by bearing members 62, 62 arranged on the base plate 51, and Y-axis ball screw 60 is rotatably supported at one end thereof. The shaft motor 64 is connected.

By driving the Y-axis motor 64, the Y-axis ball screw 60 rotates, and the Y-axis table 56 slides in the horizontal direction (Y-axis direction) along the Y-axis guide rails 52 and 52 via the nut member 58. ..

A pair of X-axis guide rails 66, 66 are laid on the Y-axis table 56 so as to be orthogonal to the pair of Y-axis guide rails 52, 52. The X-axis table 70 is slidably supported on the pair of X-axis guide rails 66, 66 via the X-axis linear guides 68, 68, ....

A nut member 72 is fixed to the lower surface of the X-axis table 70, and an X-axis ball screw 74 is screwed onto the nut member 72. The X-axis ball screw 74 is rotatably supported between a pair of X-axis guide rails 66, 66 by bearing members 76, 76 having both ends arranged on the X-axis table 70, one of which is rotatably supported. The output shaft of an X-axis motor (not shown) is connected to the end.

Therefore, by driving the X-axis motor, the X-axis ball screw 74 rotates, and the X-axis table 70 slides in the horizontal direction (X-axis direction) along the X-axis guide rails 66, 66 via the nut member 72. To do.

A Z-axis base 80 is vertically erected on the X-axis table 70, and a pair of Z-axis guide rails 82, 82 are laid on the Z-axis base 80 at predetermined intervals. A Z-axis table 86 is slidably supported on the pair of Z-axis guide rails 82, 82 via Z-axis linear guides 84, 84.

A nut member 88 is fixed to the side surface of the Z-axis table 86, and a Z-axis ball screw 90 is screwed into the nut member 88. The Z-axis ball screw 90 is rotatably supported between a pair of Z-axis guide rails 82, 82 by bearing members 92, 92 whose both ends are arranged on the Z-axis base 80, and the lower end portions thereof. The output shaft of the Z-axis motor 94 is connected to the Z-axis motor 94.

By driving the Z-axis motor 94, the Z-axis ball screw 90 rotates, and the Z-axis table 86 slides in the vertical direction (Z-axis direction) along the Z-axis guide rails 82 and 82 via the nut member 88. .. A θ-axis motor 96 is installed on the Z-axis table 86.

A θ-axis shaft 98 having a θ-axis (rotational axis θ) parallel to the Z-axis as an axis is connected to the output shaft of the θ-axis motor 96, and a suction table (holding table) is connected to the upper end of the θ-axis shaft 98. ) 10 are connected horizontally. The wafer W is placed on the suction table 10 and held by vacuum suction.

In the wafer feed unit 50A, the suction table 10 moves horizontally in the Y-axis direction in the figure by driving the Y-axis motor 64, and horizontally moves in the X-axis direction in the figure by driving the X-axis motor. Then, by driving the Z-axis motor 94, it moves vertically in the Z-axis direction in the drawing, and by driving the θ-axis motor 96, it rotates around the θ-axis.

A gantry 102 is installed on the base plate 51 of the grinding unit 50B. An outer peripheral motor 104 is installed on the gantry 102, and a spindle 106 having a rotation axis CH parallel to the Z axis as an axis is connected to the output shaft of the outer peripheral motor 104. The grindstone wheel 108 for chamfering the outer peripheral portion of the wafer W is detachably attached to the spindle 106 and rotates by driving the outer peripheral motor 104. The grindstone wheel 108 is provided with a nozzle 121 that discharges coolant (grinding liquid) toward a processing point where the wafer W contacts.

In the chamfering device 50 configured as described above, the wafer W is chamfered as follows. As a preparatory step before the chamfering is performed, the wafer W to be chamfered is placed on the suction table 10 and sucked and held. Then, the outer peripheral motor 104 and the θ-axis motor 96 are driven to rotate both the grindstone wheel 108 and the suction table 10 at high speed in the same direction. For example, the rotation speed of the grindstone wheel 108 is 3000 rpm, and the rotation speed of the suction table 10 is such that the outer peripheral speed of the wafer W is 5 mm / sec.

Further, the Z-axis motor 94 is driven to adjust the height of the suction table 10 so that the height of the wafer W corresponds to the height of the grindstone wheel 108. Further, the X-axis motor is driven to match the positions of the θ-axis (rotational axis θ), which is the rotation axis of the wafer W, with the rotation axis CH of the grindstone wheel 108 in the X-axis direction.

Next, as an execution step of chamfering, the Y-axis motor 64 is driven to feed the wafer W toward the grindstone wheel 108. Then, the speed is reduced immediately before the outer peripheral portion of the wafer W comes into contact with the grindstone wheel 108, and then the wafer W is sent toward the grindstone wheel 108 at a low speed.

As a result, the outer peripheral portion of the wafer W is in sliding contact with the grindstone wheel 108, and is ground and chamfered in small amounts as described below. Further, the coolant is discharged from the nozzle 121 toward the machining point, and the grinding debris and the abrasion powder of the grindstone (crushed / fallen abrasive grains) are discharged together with the cooling of the machining point.

Then, when the outer peripheral portion of the wafer W reaches the deepest portion of the grinding groove and a predetermined time elapses, the chamfering process is completed, the wafer W is moved in a direction away from the grindstone wheel 108, and the wafer W is moved to a position where the wafer W is collected. Let me. For chamfering and shape processing, the outer peripheral portion of the wafer is processed with a CBN or SD diamond grindstone.

FIG. 3 is a cross-sectional view showing the shape of the wafer W when the chamfering process is completed. FIG. 8 is a cross-sectional view showing the back grinding process and the shape of the wafer W. The cross-sectional shape of the edge portion of the wafer W is chamfered, and the chamfered portion W-1 has a shape with a slight radius.

The wafer W is placed on the suction table 10, and the back grinding is performed by grinding the back surface side with a back grinding wheel 21 as shown by the dotted line in FIG. 8 while holding the wafer W to thin the wafer W. If this is left as it is, the edge portion W-10 will have a sharp shape depending on the chamfer angle. Then, the mechanical strength becomes very weak and edge chipping occurs. In addition, the edge portion flutters due to the influence of grinding water and processing conditions, causing chipping and causing wafer damage.

In back grinding, regarding the grinding conditions, it is good for reducing edge chipping to perform low load grinding by reducing the feeding speed of the back grinding grindstone 21 near the final finish thickness in both rough grinding and finish grinding. Further, it is effective to perform edge trimming in which the outer peripheral edge portion of the wafer W is grooved in advance before the back grinding process.

However, simply grooving at a depth corresponding to the target thickness T of the thinned wafer W may cause chipping near the final finish thickness, scratches on the outer shape when the wafer W is stored in the cassette, and the like. It is difficult to reduce the transfer damage and the damage when polishing the edge using the polishing tape.

In edge trimming, the shaded portion of FIG. 3 is processed by the trimming grindstone 20 before back grinding to trim the edge of the wafer W end face to remove damage. As the trimming grindstone 20, a vitrified grindstone, a resin grindstone of No. 3000 or higher, or the like is suitable. After edge trimming, minute irregularities on the ground wafer W are polished and mirror-finished so as not to affect the surface performance.

FIG. 4 is a cross-sectional view showing a cut by the trimming grindstone 20. The upper portion of the chamfered portion W-1, which is the shaded portion in FIG. 3, is removed from the upper surface of the wafer W by 50 to 150 μm corresponding to the target thickness T, and is smoothed horizontally in the Y direction. Next, the rising portion W-2, which swings in the Z direction and is a plane perpendicular to the cutting direction, is mirror-finished to prevent cracks from entering.

As a result, it is possible to improve the workability and promote the cavitation effect, and prevent clogging at the processing point due to the difficulty in entering coolant and the difficulty in discharging sludge. Therefore, cleaning of the ground surface and discharge of grinding debris are promoted, the amount of protrusion of the grinding abrasive grains is stably secured, and the processing quality is improved.

FIG. 5 is a cross-sectional view showing a detailed shape of the outer peripheral portion of the wafer W. L is a take-up length required to avoid transport damage and damage due to the polishing tape, and is removed from the outer circumference to the inner circumference of the wafer W. Unless the value of L is adjusted appropriately, it becomes difficult to reduce damage in the process after chamfering and improve quality.

The inner surface of the transport cassette (not shown) usually comes into contact with the outer peripheral portion of the wafer W at a predetermined angle. φ is an angle chamfered so as to be substantially equal to the contact angle with the holder such as a transport cassette or the polishing tape in the subsequent polishing process. φ is necessary so that the corner portion W-3 of the edge of the wafer W does not come into point contact with the inner surface of a holder such as a transport cassette or the polishing tape.

FIG. 6 is a side view showing a state of contact between the wafer W and the polishing tape 31. When the take-off length L to be processed is T / tanφ or less, the corner portion W-3 of the wafer W to be used collides with the inner surface of the transport cassette and the polishing tape 31 in point contact, and the corner portion W- The risk of chipping in 3 increases. The polishing tape 31 is guided and sent out from the feed bobbin 34 by the guide rollers 38 and 39. Then, the polishing tape 31 comes into contact with the outer peripheral portion of the wafer W and is wound around the winding bobbin 35 via the guide rollers 36 and 37.

The outermost peripheral portion W-4 of the wafer W needs to be sufficiently mirror-finished with a polishing tape 31, and fine irregularities are polished to a slightly rounded shape. Therefore, α is set as a margin (preferably 0.1 to 0.5 mm).
Process into a shape such that L = T / tanφ + α.
T is the target thickness of the wafer W to be used, φ is a chamfer angle substantially equal to the contact angle with a holder such as a transport cassette or polishing tape, and the end face of the wafer W is processed before back grinding. You can avoid the damage that can occur in.

FIG. 7 is a diagram showing a configuration of a polishing apparatus. The polishing apparatus includes a wafer stage 3 for holding the wafer W, a wafer stage moving mechanism 4 for moving the wafer stage 3 in parallel directions, an edge polishing unit 5 as a polishing tape portion for polishing the edge portion of the wafer W, and the like. It is provided with a polishing movement mechanism 6 for moving the edge polishing unit 5 with respect to the edge portion of the wafer W held by the wafer stage 3.

The wafer stage moving mechanism 4 includes a shaft base 11 that rotatably supports the cylindrical member 30, a movable portion 12 that can move integrally with the shaft base 11, and a support that movably supports the wafer stage 3 on the base 13. It is equipped with a stand 14. The movable portion 12 can be moved in the direction indicated by the arrow Y. Further, the cylindrical member 30 is configured to be rotatable around the β axis as a rotation axis.

The edge polishing unit 5 includes a polishing head portion 32 as an edge contact portion that abuts and presses the polishing tape (belt-shaped polishing member) 31 against the edge portion of the wafer W, and a polishing tape that sends the polishing tape 31 to the polishing head portion 32. It is provided with a feed mechanism 33.

The polishing tape feeding mechanism 33 has a feeding bobbin 34 as a tape feeding section for feeding the polishing tape 31 to the polishing head section 32, and a winding bobbin as a tape winding section for winding the polishing tape 31 fed to the polishing head section 32. 35 and a rotation mechanism (not shown) for rotating the take-up bobbin 35 are provided. As the polishing tape 31, a polishing tape in which abrasive particles of polishing particles are applied to a base film is used on one side of the polishing surface.

The polishing moving mechanism 6 is movable with a shaft base 41 having a cylindrical member 40 for holding an edge polishing unit 5 including a polishing head portion 32 and a polishing tape feeding mechanism 33, and a movable portion 42 that can be moved integrally with the shaft base 41. It includes a shaft base 43 that supports the portion 42, and a movable portion 44 that can move integrally with the shaft base 43.

As described above, not only the outer peripheral edge portion of the wafer W is preliminarily grooved to a depth corresponding to the target thickness T before the back grinding process, but also the outer shape portion when the wafer W is stored in the cassette. Optimal edge trimming is performed in consideration of transport damage such as scratches and damage when polishing the edge using a polishing tape. This makes it possible to effectively prevent damage during back grinding and chipping near the final finish thickness.

When edge trimming, the trimming grindstone 20 is swung in the Z direction to vibrate the abrasive grains in the Z direction, and the original cutting ability and the vibration acceleration motion of the abrasive grains are added to improve the grinding performance. .. As a result, low damage processing can be performed in the Z direction. Further, the cavitation of the abrasive grains is promoted on the side parallel to the cutting direction, and the surface of the rising portion W-2, which is the vertical surface, is not damaged. Then, poor lubrication to the abrasive grains and dispersion of the machining load improve the shape maintenance performance of the grindstone, stabilize the work quality, and improve the life of the grindstone.

W ... Wafer, W-1 ... Chamfered part, W-2 ... Rising part, W-3 ... Corner part, W-4 ... Outermost part, W-10 ... Edge part, φ ... Chamfering angle, L ... Chamfering allowance length T ... Target thickness, α ... Margin, 3 ... Wafer stage, 4 ... Wafer stage moving mechanism, 5 ... Edge polishing unit, 6 ... Polishing moving mechanism, 10 ... Suction table, 11, 41, 43 ... Shaft base , 12, 42, 44 ... Movable part, 13 ... Base, 14 ... Support base, 20 ... Trimming grindstone, 21 ... Back grinding grindstone, 30 ... Cylindrical member, 31 ... Polishing tape, 32 ... Polishing head part, 33 ... Polishing tape feed mechanism, 34 ... feed bobbin, 35 ... take-up bobbin, 36, 37, 38, 39 ... guide roller, 40 ... cylindrical member, 50 ... chamfering device, 50A ... wafer feed unit, 50B ... grinding unit, 51 ... Base plate, 52 ... Y-axis guide rail, 54 ... Y-axis linear guide, 56 ... Y-axis table, 58, 72, 88 ... nut member, 60 ... Y-axis ball screw, 62 ... bearing member, 64 ... Y-axis motor, 66 ... X-axis guide rail, 68 ... X-axis linear guide, 70 ... X-axis table, 74 ... X-axis ball screw, 76, 92 ... bearing member, 80 ... Z-axis base, 82 ... Z-axis guide rail, 84 ... Z-axis linear guide , 86 ... Z-axis table, 90 ... Z-axis ball screw, 94 ... Z-axis motor, 96 ... θ-axis motor, 98 ... θ-axis shaft, 102 ... mount, 104 ... outer circumference motor, 106 ... spindle, 108 ... grind wheel, 121 …nozzle

Claims (8)

A method for thinning a wafer, in which the wafer is processed to a desired thickness by back grinding.
The edge portion on the outer circumference of the wafer is processed as a chamfered portion.
The upper part of the chamfered portion is horizontally removed from the upper surface of the wafer to the target thickness (T) to be thinned from the outer circumference to the inner circumference of the wafer.
The up Dainaga is to be removed from the outer peripheral L, the angle φ with respect to the horizontal of the chamfered portion, allowance alpha, as was processed into a shape to be L = T / tanφ + α, the margin alpha is 0.1 to 0 A method for thinning a wafer, which is characterized by having a thickness of .5 mm. The method for thinning a wafer according to claim 1 , wherein after the wafer is processed into the shape, the outermost peripheral portion of the wafer is mirror-finished with a polishing tape. The method for thinning a wafer according to claim 2 , wherein the angle φ is determined to be equal to the contact angle with the polishing tape. The method for thinning a wafer according to claim 1 or 2 , wherein the angle φ is determined to be equal to the angle of a portion of the inner surface of the transfer cassette of the wafer that comes into contact with the outer peripheral portion of the wafer. In a wafer thinning device that processes a wafer to a desired thickness by back grinding,
A chamfering device that processes the edge portion of the outer periphery of the wafer as a chamfered portion,
A grindstone that removes the upper part of the chamfered portion, which has been chamfered in advance before the back grinding process, by cutting horizontally from the outer circumference to the inner circumference of the wafer to the target thickness (T).
The shape is processed so that L = T / tanφ + α, where L is the removal allowance length, the allowance α is, and the angle of the portion of the inner surface of the wafer transfer cassette that contacts the outer peripheral portion of the wafer is φ. However, the wafer thinning device is characterized in that the margin α is 0.1 to 0.5 mm. In wafers that are thinned to the desired thickness by back grinding
The edge portion on the outer circumference of the wafer is processed as a chamfered portion.
The upper part of the chamfered portion is horizontally removed from the upper surface of the wafer to the target thickness (T) to be thinned from the outer circumference to the inner circumference of the wafer.
The up Dainaga is to be removed from the outer peripheral L, the angle φ with respect to the horizontal of the chamfered portion, allowance alpha, as are processed into a shape to be L = T / tanφ + α, the margin alpha is 0.1 to 0 A wafer characterized by being 5.5 mm. The wafer according to claim 6 , wherein the angle φ is determined to be equal to an angle of a portion of the inner surface of the transfer cassette of the wafer that comes into contact with the outer peripheral portion of the wafer. The wafer according to claim 6 , wherein the angle φ is determined to be equal to a contact angle with a polishing tape for polishing the outer peripheral portion of the wafer.

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