JP2018170313A - Wafer, wafer thinning method, and wafer thinning apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To eliminate wafer damage after performing wafer shape processing and damage of a wafer end face in a step of pressing a polishing tape against an edge portion of the wafer end face.SOLUTION: In a wafer W thinned to a desired thickness by back grinding, an edge portion of a wafer periphery is processed as a chamfered portion W-1 and an upper portion of the chamfered portion W- 1 is horizontally removed from an outer periphery to an inner periphery of the wafer W to a target thickness (T) thinned from an upper surface of the wafer W. When denoting a removal length removed from the outer periphery as L, denoting an angle of the chamfered portion W-1 with respect to a horizon as φ, and denoting a margin as α, the wafer W is processed into a shape so as to satisfy L=T/tan φ + α.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a method for manufacturing a semiconductor substrate, and more particularly to a wafer, a wafer thinning method, and a wafer thinning apparatus that can improve the yield when a semiconductor wafer is thinned.

  Semiconductor manufacturing technology includes machining technology, lithography technology, thin film formation technology, etching technology, ion implantation technology, and cleaning technology. Semiconductor machining technologies include cutting (slicing), grinding, lapping, polishing, CMP (chemical mechanical polishing), and backgrinding in the wafer manufacturing process, wafer processing process, chip forming process, and assembly process. And dicing.

  Recently, the demand for applications such as thin cards and wireless tags is expanding, and the demand for high-density mounting is increasing. In order to meet these demands, it is necessary to make the wafer extremely thin, and the demand for ultra-thinning technology is increasing. In order to make the wafer extremely thin, various devices have been devised for grinding the wafer. In addition, wafer diameters are increasing in order to improve productivity and reduce costs. To achieve ultra-thinness in large-diameter wafers, polishing / grinding methods, finishing methods, processing equipment, and transport methods Many ideas are needed.

  Semiconductor wafer thinning is performed by backgrinding the semiconductor wafer from the back surface to the desired thickness. When the semiconductor wafer is simply ground from the back surface, the mechanical strength of the semiconductor wafer decreases. Easily break. Further, the semiconductor wafer is likely to warp, and as a result, when the layer is thinned, the defect rate of the semiconductor wafer due to cracking or warping increases rapidly.

  In order to prevent the defect rate of the semiconductor wafer from increasing due to cracking and warping caused by backgrinding, the inner surface of the outer edge of the semiconductor wafer is thinned by grinding the back surface of the semiconductor wafer while leaving a few mm of the outer edge. Stratify. That is, it is known to form a recess inside the outer edge, and is described in, for example, Patent Document 1.

  In addition, an annular step is formed at the peripheral edge on the surface side of the chamfered wafer before thinning to prevent a knife edge, and these steps are circular corresponding to the diameter of the wafer finally obtained. Patent Document 2 describes that the depth is a depth corresponding to the target thickness of the wafer after thinning.

JP 2007-335659 A JP 2007-152906 A

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

  The object of the present invention is to solve the above-mentioned problems of the prior art and eliminate damage to the wafer end face in a process of pressing the polishing tape against the edge of the wafer end face by handling the wafer after processing the wafer shape. To improve productivity and profitability.

  In order to achieve the above object, according to the present invention, in a wafer thinned to a desired thickness by back grinding, an edge portion of the outer periphery of the wafer is processed as a chamfered portion, and an upper portion of the chamfered portion is an upper surface of the wafer. From the outer periphery to the inner periphery of the wafer from the outer periphery to the target thickness (T) to be thinned, L is the machining allowance length removed from the outer periphery, the angle φ relative to the horizontal of the chamfered portion, the margin allowance α, Is processed into a shape of L = T / tanφ + α.

  In the above wafer, the margin [alpha] is preferably 0.1 to 0.5 mm.

  Further, in the above wafer, it is desirable that the angle φ is determined to be equal to an angle of a portion of the inner surface of the wafer transfer cassette that contacts 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 a contact angle with a polishing tape for polishing the outer peripheral portion of the wafer.

  Further, the present invention is a wafer thinning method for processing a wafer to a desired thickness by backgrinding, wherein the edge portion on the outer periphery of the wafer is processed as a chamfered portion, and the 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 periphery toward the inner periphery. , L = T / tanφ + α is processed into a shape.

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

  Furthermore, in the above, after processing the wafer into the shape, it is preferable that the outermost peripheral portion of the wafer is mirror-finished with a polishing tape.

  Furthermore, in the above, it is desirable that the angle φ is determined to be equal to the contact angle with the polishing tape.

  Furthermore, in the above, it is desirable that the angle φ is determined so as to be equal to the angle of the inner surface of the wafer transfer cassette that contacts the outer peripheral portion of the wafer.

  The present invention also provides a wafer thinning apparatus for processing a wafer to a desired thickness by backgrinding, a chamfering apparatus for processing the edge portion of the outer periphery of the wafer as a chamfered portion, and before the backgrinding processing. A grindstone that horizontally cuts and removes the upper part of the chamfered portion that has been chamfered in advance from the outer periphery to the inner periphery of the wafer to a target thickness (T), and the removal allowance length is L, The margin is α, 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, and is processed into a shape of L = T / tan φ + α.

  According to the present invention, the upper portion of the chamfered portion on the outer periphery of the wafer is horizontally removed from the outer periphery toward the inner periphery until the target thickness (T) to be thinned, the chamfering length is L, and the angle φ relative to the horizontal of the chamfered portion. Since the margin is α, L = T / tanφ + α is processed so that the wafer is damaged by handling after the wafer is processed, and the wafer is pressed against the edge of the wafer end surface. Backgrinding can be realized without damage to the end face. Therefore, the productivity and profitability of the thinned wafer can be improved.

The front view which shows the principal part concerning one Embodiment of this invention The top view which shows the principal part concerning one Embodiment of this invention Sectional drawing which shows the shape of the wafer W at the time of chamfering concerning one Embodiment of this invention Sectional drawing which shows incision by the trimming grindstone concerning one Embodiment of this invention Sectional drawing which shows the detailed shape of the outer peripheral part of the wafer W concerning one Embodiment of this invention The side view which showed the state of the contact of the wafer W and polishing tape concerning one Embodiment of this invention The figure which shows the structure of the grinding | polishing apparatus concerning one Embodiment of this invention. Sectional drawing which shows the shape of the back grinding process and wafer W concerning one Embodiment of this invention

  Embodiments of the present invention will be described below in detail with reference to the drawings. Semiconductor wafer thinning is performed by backgrinding the semiconductor wafer from the backside to the desired thickness, and in the subsequent manufacturing process, the periphery of the wafer is chamfered before the wafer is thinned. In addition, processing is performed to make it difficult to cause damage such as cracks and chips at the periphery during handling.

  In addition, when the chamfered wafer is thinned, the cross-section of the periphery is sharpened so as to become thinner toward the outer periphery, and changes to a knife edge, so that the periphery is a surface along the thickness direction (surface direction) In order to prevent the knife edge from being formed before the thinning, a cutting process is performed in advance.

  First, a chamfering device will be described. FIG. 1 is a front view showing a main part of a chamfering apparatus 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 feeding unit 50A and a grinding unit 50B. A pair of Y-axis guide rails 52, 52 are laid at a predetermined interval on a base plate 51 disposed horizontally in the wafer feeding unit 50A. A Y-axis table 56 is slidably supported on the pair of Y-axis guide rails 52 and 52 via Y-axis linear guides 54 and 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 to the nut member 58. Both ends of the Y-axis ball screw 60 are rotatably supported by bearing members 62 and 62 disposed on the base plate 51 between the pair of Y-axis guide rails 52 and 52. A 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 and 66 are laid on the Y-axis table 56 so as to be orthogonal to the pair of Y-axis guide rails 52 and 52. An X-axis table 70 is slidably supported on the pair of X-axis guide rails 66, 66 via 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 to the nut member 72. Both ends of the X-axis ball screw 74 are rotatably supported by bearing members 76 and 76 disposed on the X-axis table 70 between the pair of X-axis guide rails 66 and 66. An output shaft of an X-axis motor (not shown) is connected to the end.

  Accordingly, 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 and 66 via the nut member 72. To do.

A Z-axis base 80 is erected vertically on the X-axis table 70, and a pair of Z-axis guide rails 82 and 82 are laid on the Z-axis base 80 with a predetermined interval. A Z-axis table 86 is slidably supported by the pair of Z-axis guide rails 82 and 82 via Z-axis linear guides 84 and 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 to the nut member 88. Both ends of the Z-axis ball screw 90 are rotatably supported by bearing members 92 and 92 disposed on the Z-axis base 80 between the pair of Z-axis guide rails 82 and 82, and the lower end thereof. The output shaft of the Z-axis motor 94 is connected to the motor.

  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 (rotation axis θ) parallel to the Z-axis as a center is connected to the output shaft of the θ-axis motor 96, and a suction table (holding table) is attached 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 feeding 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. The Z-axis motor 94 is driven to move vertically in the Z-axis direction in the figure, and the θ-axis motor 96 is driven to rotate around the θ-axis.

  A mount 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 whose axis is a rotation axis CH parallel to the Z axis is connected to the output shaft of the outer peripheral motor 104. A grindstone wheel 108 that chamfers the outer peripheral portion of the wafer W is detachably mounted on the spindle 106, and rotates by driving the outer peripheral motor 104. A nozzle 121 that discharges coolant (grinding fluid) toward a processing point where the wafer W comes into contact with the grinding wheel 108 is provided.

  In the chamfering apparatus 50 configured as described above, the wafer W is chamfered as follows. As a preparatory step before chamfering, 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 grinding wheel 108 and the suction table 10 at high speed in the same direction. For example, the rotational speed of the grinding wheel 108 is set to 3000 rpm, and the rotational speed of the suction table 10 is set to a speed at which the outer peripheral speed of the wafer W is 5 mm / sec.

  Further, the height of the suction table 10 is adjusted by driving the Z-axis motor 94 so that the height of the wafer W corresponds to the height of the grinding wheel 108. Further, the X-axis motor is driven so that the θ-axis (rotation axis θ) serving as the rotation axis of the wafer W matches the position in the X-axis direction with the rotation axis CH of the grinding wheel 108.

  Next, as a chamfering process, the Y-axis motor 64 is driven to send the wafer W toward the grinding wheel 108. And it decelerates just before the outer peripheral part of the wafer W contact | abuts to the grindstone wheel 108, and sends the wafer W toward the grindstone wheel 108 at low speed after that.

  As a result, the outer peripheral portion of the wafer W comes into sliding contact with the grindstone wheel 108 and is ground and chamfered by a minute amount as described below. In addition, coolant is discharged from the nozzle 121 toward the processing point, and the grinding point and abrasive powder (crushed and dropped abrasive grains) are discharged along with cooling of the processing point.

  Then, when a predetermined time elapses after the outer peripheral portion of the wafer W reaches the deepest portion of the grinding groove, the chamfering process is finished, and the wafer W is moved away from the grinding wheel 108 and moved to a position where the wafer W is recovered. Let In the 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 chamfering 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 slightly rounded shape.

  The wafer W is placed on the suction table 10, and the back grinding is performed by thinning the back side with the back grinding grindstone 21 while holding the wafer W as shown by the dotted line in FIG. 8. If this is left, the edge portion W-10 has a sharp shape due to the chamfer angle. And mechanical strength becomes very weak and edge chipping arises. In addition, the edge portion flutters due to the influence of grinding water and processing conditions, and chipping occurs, which causes wafer damage.

  In the back grinding, as for the grinding conditions, it is preferable to reduce the edge chipping by reducing the feeding speed of the back grinding wheel 21 in the vicinity of the final finish thickness for both rough grinding and finish grinding. Further, edge trimming in which grooving is performed in advance on the outer peripheral edge portion of the wafer W before backgrinding is effective.

  However, simply by grooving at a depth corresponding to the target thickness T of the thinned wafer W, chipping near the final finished thickness, scratches on the outer shape when the wafer W is stored in the cassette, etc. It is difficult to reduce conveyance damage and damage when polishing an edge using an abrasive tape.

  In the edge trimming, the hatched portion in FIG. 3 is processed by the trimming grindstone 20 before the back grinding, and the edge surface of the wafer W is edge trimmed to remove the damage. The trimming grindstone 20 is suitably a vitrified grindstone, No. 3000 or higher resin grindstone. After edge trimming, fine irregularities on the ground wafer W are polished, and mirror finish is performed so as not to affect the surface performance.

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

  Thereby, workability is improved and the cavitation effect is promoted, and clogging at the processing point due to the difficulty of entering the coolant and the difficulty of discharging the sludge can be prevented. Therefore, the cleaning of the grinding surface and the discharge of grinding scraps are promoted, and the protruding amount of the abrasive grains is stably ensured to improve the processing quality.

  FIG. 5 is a cross-sectional view showing a detailed shape of the outer peripheral portion of the wafer W. As shown in FIG. L is a machining allowance length necessary for avoiding conveyance damage and damage due to the polishing tape, and is removed from the outer periphery of the wafer W toward the inner periphery. If the value of L is not appropriate, it is difficult to improve quality by reducing damage in the process after chamfering.

  The inner surface of a transfer cassette (not shown) is normally in contact with the outer periphery of the wafer W at a predetermined angle. φ is an angle chamfered so as to be substantially equal to a contact angle with a holder such as a transport cassette or a polishing tape in a subsequent polishing step. φ is necessary to prevent the corner W-3 of the edge of the wafer W from making point contact with the inner surface of a holder such as a transfer 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. If the machining allowance length L to be processed is equal to or less than T / tanφ, the corner W-3 of the wafer W to be used collides with the inner surface of the transfer cassette and the polishing tape 31, and the corner W- The risk of chipping in 3 increases. The polishing tape 31 is guided and fed out from the feed bobbin 34 by 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 the polishing tape 31, and a slight unevenness is finished to a slightly rounded shape by polishing. Therefore, α is a margin (preferably 0.1 to 0.5 mm),
Processing into a shape of L = T / tanφ + α.
T is the target thickness of the wafer W to be used, φ is a chamfering angle substantially equal to the contact angle with a holder such as a transfer cassette or a polishing tape, and processed before backgrinding. The damage that can occur is avoided.

  FIG. 7 is a diagram showing the configuration of the 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 a parallel direction, an edge polishing unit 5 as a polishing tape unit for polishing an edge portion of the wafer W, A polishing moving mechanism 6 for moving the edge polishing unit 5 relative to the edge portion of the wafer W held on the wafer stage 3 is provided.

  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 supports the wafer stage 3 so that the wafer stage 3 can be moved by the base 13. And a table 14. The movable part 12 is movable in the direction indicated by the arrow Y. The cylindrical member 30 is configured to be rotatable about the β axis as a rotation axis.

  The edge polishing unit 5 includes a polishing head portion 32 as an edge contact portion that contacts and presses the polishing tape (band-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. And a feed mechanism 33.

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

  The polishing moving mechanism 6 includes a shaft base 41 having a cylindrical member 40 that holds the edge polishing unit 5 including the polishing head unit 32 and the polishing tape feed mechanism 33, a movable unit 42 that can move integrally with the shaft base 41, A shaft base 43 that supports the portion 42 and a movable portion 44 that can move integrally with the shaft base 43 are provided.

  As described above, before processing the backgrinding, the outer peripheral edge portion of the wafer W is not simply grooved at a depth corresponding to the target thickness T in advance, but the outer portion when the wafer W is stored in the cassette. Optimum edge trimming is performed in consideration of transport damage such as scratches or damage when polishing an edge using a polishing tape. This effectively prevents damage during backgrinding and chipping near the final finished thickness.

  When edge trimming is performed, 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 vibration acceleration motion of the abrasive grains are added to improve the grinding performance. . Thereby, it can be set as the low damage process to a Z direction. Further, cavitation of abrasive grains is promoted on the side that is parallel to the cutting direction, and no damage occurs on the surface of the rising portion W-2 that becomes the vertical surface. In addition, since the lubrication failure and processing load on the abrasive grains are dispersed, the shape maintaining performance of the grindstone can be improved, the workpiece quality can be stabilized, and the grindstone life can be improved.

  W: Wafer, W-1: Chamfered part, W-2 ... Rising part, W-3 ... Corner part, W-4 ... Outermost peripheral part, W-10 ... Edge part, φ ... Chamfering angle, L ... Chamfering 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 ... backgrinding grindstone, 30 ... cylindrical member, 31 ... polishing tape, 32 ... polishing head part, 33 ... Abrasive tape feed mechanism, 34 ... feed bobbin, 35 ... winding 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 ... Peripheral motor, 106 ... Spindle, 108 ... Grinding wheel, 121 …nozzle

Claims (10)

A wafer thinning method for processing a wafer to a desired thickness by backgrinding,
Processing the edge of the wafer outer periphery as a chamfer,
The upper part of the chamfered portion is removed horizontally from the outer periphery to the inner periphery of the wafer up to a target thickness (T) for thinning from the upper surface of the wafer,
A method of thinning a wafer, characterized in that a machining allowance length to be removed from the outer periphery is L, an angle φ with respect to the horizontal of the chamfered portion, and a margin allowance α, and processing into a shape of L = T / tan φ + α.   2. The wafer thinning method according to claim 1, wherein the margin [alpha] is 0.1 to 0.5 mm.   The wafer thinning method according to claim 1, wherein after processing the wafer into the shape, the outermost peripheral portion of the wafer is mirror-finished with a polishing tape.   4. The wafer thinning method according to claim 1, wherein the angle φ is determined to be equal to a contact angle with the polishing tape. 5.   5. The wafer according to claim 1, 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 is in contact with the outer peripheral portion of the wafer. Thinning method. In a wafer thinning device that processes a wafer to a desired thickness by backgrinding,
A chamfering device for processing the edge portion of the outer periphery of the wafer as a chamfered portion;
A grindstone that horizontally cuts and removes the upper part of the chamfered portion that has been chamfered in advance before processing the back grinding to a target thickness (T) from the outer periphery to the inner periphery of the wafer;
The length of the machining allowance to be removed is L, the margin is α, and the angle of the inner surface of the transfer cassette of the wafer that contacts the outer peripheral portion of the wafer is φ, and processed into a shape of L = T / tanφ + α A wafer thinning apparatus characterized by: In wafers that are thinned to the desired thickness by backgrinding,
The edge part of the outer periphery of the wafer is processed as a chamfered part,
The upper portion of the chamfered portion is horizontally removed from the outer periphery to the inner periphery of the wafer from the upper surface of the wafer to a target thickness (T) to be thinned.
A wafer that is processed into a shape of L = T / tan φ + α, where L is a machining allowance length to be removed from the outer periphery, an angle φ with respect to the horizontal of the chamfered portion, and a margin α.   The wafer according to claim 7, wherein the margin α is set to 0.1 to 0.5 mm.   9. The wafer according to claim 7, 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 contacts the outer peripheral portion of the wafer.   10. The wafer according to claim 7, wherein the angle φ is determined to be equal to a contact angle with a polishing tape for polishing an outer peripheral portion of the wafer.