JP2013232449A - Wafer dividing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wafer dividing method capable of dividing a wafer into individual devices without breaking the wafer.SOLUTION: A method for dividing a wafer 2 having a device region and an outer periphery excess region includes: a protective tape adhering step of adhering a protective tape 3 having an annular adhesive layer 30 on its surface to a surface only at an outer periphery corresponding to the outer periphery excess region 24 of the wafer 2; a rear face grinding step of forming thickness of the device region 23 into predetermined finish thickness by grinding a region corresponding to the device region 23, and forming an annular reinforcing section by leaving a region corresponding to the outer periphery excess region 24; a modification layer formation step of forming a modification layer along a street 21 inside by radiating permeable laser light beam to the wafer 2; and a wafer division step of providing an external force to the wafer 2, breaking the wafer 2 along the street 21, and dividing it into individual devices 22.

Description

  The present invention relates to a wafer comprising a device region in which devices are formed in a plurality of regions partitioned by a plurality of streets formed in a lattice pattern on the surface, and an outer peripheral surplus region surrounding the device region. This invention relates to a method of dividing a wafer along the lines.

  In the semiconductor device manufacturing process, a plurality of regions are defined by dividing lines called streets arranged in a lattice pattern on the surface of a semiconductor wafer having a substantially disk shape, and ICs, LSIs, and microelectronics are defined in the partitioned regions. Form devices such as mechanical systems (MEMS). Then, the semiconductor wafer is cut along the streets to divide the region in which the device is formed to manufacture individual devices.

  As a method of dividing the wafer along the street, a laser processing method has been attempted in which a pulsed laser beam having transparency to the wafer is used, and the focused laser beam is aligned within the region to be divided and irradiated with the pulsed laser beam. . The dividing method using this laser processing method is to irradiate the inside of the wafer with a pulsed laser beam having a wavelength that is transparent to the wafer by aligning the condensing point from one side of the wafer to the inside. A modified layer is continuously formed along the street, and an external force is applied along the street whose strength is reduced by the formation of the modified layer, thereby dividing the wafer into individual devices. (For example, refer to Patent Document 1.)

  In general, a wafer is formed to have a predetermined thickness by grinding the back surface before dividing into individual devices. When grinding the back surface of the wafer, a protective tape is attached to the surface of the wafer in order to protect the device formed on the surface of the wafer. However, when the protective tape is peeled off from the wafer surface after grinding, a part of the adhesive layer remains on the surface of the device, contaminating the device bumps and bonding pads, leading to disconnection, and imaging such as CCDs. There is a problem that the imaging accuracy of the element is lowered. In particular, when the device is a micro electro mechanical system (MEMS), there is a problem that the micro electro mechanical system (MEMS) is destroyed when the protective tape is peeled off.

In order to solve such problems, a protective tape that has an adhesive layer only in the region corresponding to the outer peripheral surplus region of the wafer and does not have an adhesive layer in the region corresponding to the device region is attached to the surface of the wafer. A processing method for grinding the back surface of the wafer has been proposed.
(For example, see Patent Document 2.)

Japanese Patent No. 3408805 Japanese Patent Publication No. 2001-196404

  Thus, after the back surface of the wafer is ground and formed to a predetermined thickness, a condensing point of a laser beam having a wavelength that is transmissive to the wafer is positioned inside and irradiated along the street to enter the inside of the wafer. When the modified layer is formed along the street, there is a problem that when the wafer is unloaded from the laser processing apparatus and conveyed to the wafer dividing step, the wafer is damaged during unloading or during conveyance.

  The present invention has been made in view of the above-mentioned facts, and the main technical problem thereof is when the protective tape attached to the surface of the wafer is peeled off or after the modified layer is formed along the street inside the wafer. It is an object of the present invention to provide a wafer dividing method capable of dividing the wafer into individual devices without damaging the wafer.

In order to solve the above main technical problem, according to the present invention, a plurality of regions are partitioned by a plurality of streets formed in a lattice shape on the surface, and a device region in which a device is formed in the partitioned region, and the device region A wafer processing method for dividing a wafer having a peripheral area surrounding a device area along a plurality of streets,
A protective tape attaching step of attaching a protective tape having a diameter corresponding to the diameter of the wafer and having an annular adhesive layer on the surface only on the outer peripheral portion corresponding to the outer peripheral surplus area of the wafer;
The area corresponding to the device area on the back surface of the wafer on which the protective tape attaching process has been performed is ground to form the thickness of the device area to a predetermined finished thickness, and the area corresponding to the outer peripheral surplus area on the back surface of the wafer A back surface grinding step for forming an annular reinforcing portion by leaving
Modification that forms a modified layer along the street inside the wafer by irradiating along the street with a condensing point of a laser beam having a wavelength that is transparent to the wafer subjected to the back grinding process A layer forming step;
A frame support step of attaching the back surface of the wafer on which the modified layer forming step has been performed to the surface of a dicing tape that is attached to an annular frame and has an adhesive layer on the surface;
A protective tape peeling step for peeling the protective tape from the surface of the wafer on which the frame supporting step has been performed;
A wafer dividing step of breaking the wafer along the streets whose strength has been reduced by applying an external force to the wafer subjected to the protective tape peeling step and forming a modified layer to divide the wafer into individual devices. Including,
A method for processing a wafer is provided.

  In the modified layer forming step, a pulse laser beam having a wavelength that is transparent to the wafer is also applied to the outer peripheral area on the street extension line to assist in forming a modified layer in the annular reinforcing portion. It is desirable to include a modified layer forming step.

In the wafer dividing method according to the present invention, a protective tape having a diameter corresponding to the diameter of the wafer and having an annular adhesive layer on the surface only on the outer peripheral portion corresponding to the outer peripheral surplus region of the wafer is attached to the surface of the wafer. Since the backside grinding process is performed, when the protective tape is peeled after the backside grinding process is performed, the protective tape has only a ring-shaped adhesive layer attached to the outer peripheral area of the wafer. Even an electromechanical system (MEMS) is not damaged. If the device is an image sensor such as an IC, LSI, or CCD, a part of the adhesive layer remains on the surface of the device, contaminating the device bumps, bonding pads, etc. The problem of lowering the imaging accuracy can be prevented beforehand.
Further, in the back grinding process, the region corresponding to the outer peripheral surplus region is left to form the annular reinforcing portion, so that the wafer on which the modified layer forming step has been carried out is carried out from the laser processing apparatus and carried out to the next step. At this time, since the wafer is reinforced by the annular reinforcing portion, the problem that the wafer breaks during unloading or during transfer is solved.

The perspective view of the wafer divided | segmented by the division | segmentation method of the wafer by this invention. Explanatory drawing of the masking tape sticking process in the division | segmentation method of the wafer by this invention. The principal part perspective view of the grinding device for implementing the back surface grinding process in the division | segmentation method of the wafer by this invention. Explanatory drawing of the back surface grinding process in the division | segmentation method of the wafer by this invention. The expanded sectional view of the wafer in which the back surface grinding process shown in FIG. 4 was implemented. The principal part perspective view of the laser processing apparatus for implementing the modified layer formation process in the division | segmentation method of the wafer by this invention. Explanatory drawing of the modified layer formation process in the division | segmentation method of the wafer by this invention. Explanatory drawing of the frame support process in the division | segmentation method of the wafer by this invention. Explanatory drawing of the masking tape peeling process in the division | segmentation method of the wafer by this invention. The perspective view of the tape expansion apparatus for implementing the wafer division | segmentation process in the division | segmentation method of the wafer by this invention. Explanatory drawing of the wafer division | segmentation process in the division | segmentation method of the wafer by this invention.

  Preferred embodiments of a wafer dividing method according to the present invention will be described below in detail with reference to the accompanying drawings.

  FIG. 1 shows a perspective view of a wafer processed by dividing the wafer according to the present invention. The wafer 2 shown in FIG. 1 is made of a silicon wafer having a thickness of, for example, 600 μm, and a plurality of regions are defined by a plurality of streets 21 formed in a lattice shape on the surface 2a. As a result, a micro electro mechanical system (MEMS) is formed. The wafer 2 configured as described above includes a device region 23 in which the device 22 is formed, and an outer peripheral surplus region 24 surrounding the device region 23.

  In order to divide the wafer 2 into individual devices along the street 21, first, the surface has a diameter corresponding to the diameter of the wafer and an annular adhesive layer only on the outer peripheral portion corresponding to the outer peripheral surplus region of the wafer. A protective tape attaching process for attaching the protective tape to the surface of the wafer is performed. That is, as shown in FIGS. 2A and 2B, an annular adhesive layer 30 having a diameter corresponding to the diameter of the wafer 2 and only on the outer peripheral portion corresponding to the outer peripheral surplus region 24 of the wafer 2 is formed on the surface 3a. The provided protective tape 3 is attached to the surface 2 a of the wafer 2. Therefore, the protective tape 3 has the annular adhesive layer 30 attached to the outer peripheral surplus region 24 of the wafer 2.

  If the above-described protective tape attaching step is performed, the region corresponding to the device region 23 on the back surface of the wafer 2 is ground to form the thickness of the device region 23 to a predetermined finished thickness, and the back surface of the wafer 2 is also formed. A back grinding process is performed in which a region corresponding to the outer peripheral surplus region 24 is left to form an annular reinforcing portion. This back grinding process is performed by a grinding apparatus shown in FIG.

  The grinding apparatus 4 shown in FIG. 3 includes a chuck table 41 that holds a wafer as a workpiece, and a grinding means 42 that grinds the processed surface of the wafer held on the chuck table 41. The chuck table 41 sucks and holds the wafer on its upper surface and is rotated in the direction indicated by the arrow 41a in FIG. The grinding means 42 includes a spindle housing 421, a rotating spindle 422 that is rotatably supported by the spindle housing 421 and rotated by a rotation driving mechanism (not shown), a mounter 423 attached to the lower end of the rotating spindle 422, and the mounter And a grinding wheel 424 attached to the lower surface of 423. The grinding wheel 424 includes a disk-shaped base 425 and a grinding wheel 426 that is annularly mounted on the lower surface of the base 425, and the base 425 is attached to the lower surface of the mounter 423.

  In order to perform the back surface grinding process using the grinding apparatus 4 described above, the protective member 3 side of the semiconductor wafer 2 conveyed by the wafer carry-in means (not shown) is placed on the upper surface (holding surface) of the chuck table 41, The semiconductor wafer 2 is sucked and held on the chuck table 41. Here, the relationship between the semiconductor wafer 2 held on the chuck table 41 and the annular grinding wheel 426 constituting the grinding wheel 424 will be described with reference to FIG. The rotation center P 1 of the chuck table 41 and the rotation center P 2 of the annular grinding wheel 426 are eccentric, and the outer diameter of the annular grinding wheel 426 is the boundary line between the device region 23 and the outer peripheral surplus region 24 of the semiconductor wafer 2. The diameter is set to be smaller than the diameter of 25 and larger than the radius of the boundary line 25, and the annular grinding wheel 426 passes through the rotation center P 1 (the center of the semiconductor wafer 2) of the chuck table 41.

  Next, as shown in FIGS. 3 and 4, while rotating the chuck table 41 in the direction indicated by arrow 41a at 300 rpm, the grinding wheel 424 is rotated in the direction indicated by arrow 424a at 6000 rpm, and the grinding wheel 424 is moved downward. The grinding wheel 426 is moved to contact the back surface of the semiconductor wafer 2. Then, the grinding wheel 424 is ground and fed downward by a predetermined amount at a predetermined grinding feed speed. As a result, on the back surface of the semiconductor wafer 2, a region corresponding to the device region 23 is ground and removed to form a circular recess 23b having a predetermined thickness (for example, 60 μm) as shown in FIG. In the illustrated embodiment, a region corresponding to the region 24 remains in a thickness of 540 μm and is formed in the annular reinforcing portion 24b (back surface grinding step).

  If the above-described back grinding process is performed, a condensing point of a laser beam having a wavelength that is transmissive to the wafer 2 is positioned inside and irradiated along the street, and the inside of the wafer 2 is modified along the street. A modified layer forming step of forming a layer is performed. This modified layer forming step is performed using a laser processing apparatus 5 shown in FIG. A laser processing apparatus 5 shown in FIG. 6 has a chuck table 51 that holds a workpiece, a laser beam irradiation means 52 that irradiates a workpiece held on the chuck table 51 with a laser beam, and a chuck table 51 that holds the workpiece. An image pickup means 53 for picking up an image of the processed workpiece is provided. The chuck table 51 is configured to suck and hold a workpiece. The chuck table 51 is processed and fed in a direction indicated by an arrow X in FIG. 6 by a processing feed unit (not shown) and is also shown in FIG. 6 by an index feed unit (not shown). Indexing and feeding are performed in the direction indicated by Y.

  The laser beam irradiation means 52 irradiates a pulsed laser beam from a condenser 522 attached to the tip of a cylindrical casing 521 arranged substantially horizontally. The imaging means 53 attached to the tip of the casing 521 constituting the laser beam irradiation means 52 is an infrared illumination that irradiates the workpiece with infrared rays in addition to a normal imaging device (CCD) that captures an image with visible light. And an image pickup device (infrared CCD) that outputs an electric signal corresponding to the infrared rays captured by the optical system, and the like. A signal is sent to control means (not shown).

The modified layer forming process performed using the laser processing apparatus 5 described above will be described with reference to FIGS.
In order to carry out this modified layer forming step, the protective tape 3 side adhered to the surface 2a of the wafer 2 on which the above-described back grinding step has been performed on the chuck table 51 of the laser processing apparatus 5 shown in FIG. The wafer 2 is placed and operated by suction means (not shown) to hold the wafer 2 on the chuck table 51 by suction. Therefore, the back surface of the wafer 2 held on the chuck table 51 is the upper side. The chuck table 51 that sucks and holds the wafer 2 in this way is positioned directly below the image pickup means 53 by a processing feed means (not shown).

  When the chuck table 51 is positioned immediately below the image pickup means 53, an alignment operation for detecting a processing region to be laser processed along the street 21 formed on the surface 2a of the wafer 2 is executed by the image pickup means 53 and a control means (not shown). To do. That is, the imaging unit 53 and the control unit (not shown) align the street 21 formed in a predetermined direction of the wafer 2 with the condenser 522 of the laser beam irradiation unit 52 that irradiates the laser beam along the street 21. Image processing such as pattern matching is performed to perform alignment of the laser beam irradiation position. In addition, the alignment of the laser beam irradiation position is similarly performed on a plurality of streets 21 formed in a direction orthogonal to the streets 21 formed in a predetermined direction on the wafer 2. At this time, the surface of the wafer 2 on which the streets 21 are formed is located on the lower side, but the imaging means 53 is an infrared illumination means for irradiating infrared rays and an optical system for capturing the infrared rays emitted by the infrared illumination means. And an image pickup device (infrared CCD) that outputs an electrical signal corresponding to the infrared rays captured by the optical system, etc., so that the street 21 can be imaged through the back surface (upper surface) side of the wafer 2. it can.

  If the street 21 formed on the surface 2a of the wafer 2 held on the chuck table 51 is detected as described above and the laser beam irradiation position is aligned, as shown in FIG. The chuck table 51 is moved to the laser beam irradiation area where the condenser 522 of the laser beam irradiation means 52 is located, and one end (the left end in FIG. 7A) of the outer peripheral surplus area 24 on the extension line of the predetermined street 21 is irradiated with the laser beam. Positioned just below the light collector 522 of the means 52. Then, the condensing point P of the pulse laser beam irradiated from the condenser 522 is adjusted to a position of, for example, 30 μm from the front surface 2a (lower surface) of the wafer 2. Next, the chuck table 51 is moved at a predetermined processing feed rate in the direction indicated by the arrow X1 in FIG. 7A while irradiating a pulsed laser beam having a wavelength that is transmissive to the wafer 2 from the condenser 522. . Then, as shown in FIG. 7B, the other end (the right end in FIG. 7B) of the outer peripheral surplus region 24 on the extension line of the street 21 reaches the irradiation position of the condenser 522 of the laser beam irradiation means 52. Then, the irradiation of the pulse laser beam is stopped and the movement of the chuck table 51 is stopped. As a result, on the wafer 2, the modified layer 200 is formed along the street 21 in the middle portion in the thickness direction of the device region 23 and the outer peripheral surplus region 24 (annular reinforcing portion 24 b). In the modified layer forming step, the modified layer may be formed only in the device region 23 without irradiating the outer peripheral surplus region 24 (annular reinforcing portion 24b) with the pulse laser beam.

The processing conditions in the modified layer forming step are set as follows, for example.
Light source: Yb laser: Ytterbium-doped fiber laser Wavelength: 1045 nm pulse laser Repetition frequency: 100 kHz
Average output: 0.3W
Condensing spot diameter: φ1 ~ 1.5μm
Processing feed rate: 100 mm / sec

Under the above processing conditions, the thickness of the modified layer 200 formed on the wafer 2 is about 40 μm.
As described above, if the modified layer forming step is performed along all the streets 21 formed in a predetermined direction of the wafer 2, the chuck table 51 holding the wafer 2 is positioned at a position rotated 90 degrees. . Then, the modified layer forming step is performed along all the streets 21 formed in the direction orthogonal to the predetermined direction of the wafer 2.

  In the modified layer forming step described above, the outer peripheral surplus region 24 on the extended line of the street 21 is also irradiated with a pulse laser beam having a wavelength that is transmissive to the wafer, and the modified portion 24b is also modified inside. Since the layer is formed (auxiliary modified layer forming step), breakage along the wafer street can be assisted in the dividing step described later.

  When the modified layer forming step described above is performed, a frame supporting step is performed in which the back surface of the wafer 2 is attached to the surface of a dicing tape that is attached to an annular frame and has an adhesive layer on the surface. That is, as shown in FIGS. 8 (a) and 8 (b), the surface of the dicing tape 7 in which the rear surface 2b of the wafer 2 subjected to the modified layer forming step is mounted on the annular frame 6 and has an adhesive layer on the surface. Adhere to (frame support process). When the wafer 2 on which the modified layer forming process has been performed is carried out from the chuck table 51 of the laser processing apparatus 5 and carried out to the frame support process which is the next process, the wafer 2 is an area corresponding to the outer peripheral surplus area 24. Since it is reinforced by the annular reinforcing portion 24b formed in the above, the problem that the wafer 2 breaks during unloading or during transfer is solved.

  When the frame supporting process described above is performed, the protective tape 3 is peeled off from the surface 2a of the wafer 2 as shown in FIG. 9 (protective tape peeling process). At this time, since only the annular adhesive layer 30 of the protective tape 3 is attached to the outer peripheral surplus region 24 of the wafer 2, the device 22 made of a micro electro mechanical system (MEMS) is not damaged when it is peeled off. . If the device is an image sensor such as an IC, LSI, or CCD, a part of the adhesive layer remains on the surface of the device, contaminating the device bumps, bonding pads, etc. The problem of lowering the imaging accuracy can be prevented beforehand.

  If the above-described protective tape peeling step is performed, the wafer 2 is broken along the streets 21 whose strength has been reduced by applying an external force to the wafer 2 and forming a modified layer, thereby forming individual devices 22. A wafer dividing step for dividing is performed. This wafer dividing step is performed using a tape expansion device 8 shown in FIG. 10 includes a frame holding means 81 for holding the annular frame 6 and a tape extending means for expanding the dicing tape 7 attached to the annular frame 6 held by the frame holding means 81. 82 and a pickup collet 83. The frame holding means 81 includes an annular frame holding member 811 and a plurality of clamps 812 as fixing means disposed on the outer periphery of the frame holding member 811. An upper surface of the frame holding member 811 forms a mounting surface 811a on which the annular frame 6 is mounted, and the annular frame 6 is mounted on the mounting surface 811a. The annular frame 6 placed on the placement surface 811 a is fixed to the frame holding member 811 by a clamp 812. The frame holding means 81 configured as described above is supported by the tape expanding means 82 so as to be able to advance and retreat in the vertical direction.

  The tape expansion means 82 includes an expansion drum 821 disposed inside the annular frame holding member 811. The expansion drum 821 has an inner diameter and an outer diameter that are smaller than the inner diameter of the annular frame 6 and larger than the outer diameter of the wafer 2 attached to the dicing tape 7 attached to the annular frame 6. The expansion drum 821 includes a support flange 822 at the lower end. The tape expansion means 82 in the illustrated embodiment includes support means 823 that can advance and retract the annular frame holding member 811 in the vertical direction. The support means 823 includes a plurality of air cylinders 823 a arranged on the support flange 822, and the piston rod 823 b is connected to the lower surface of the annular frame holding member 811. In this way, the support means 823 including the plurality of air cylinders 823a is configured such that the annular frame holding member 811 and the mounting surface 811a are substantially at the same height as the upper end of the expansion drum 821, as shown in FIG. Then, as shown in FIG. 11 (b), it is moved in the vertical direction between the extended positions below the upper end of the expansion drum 821 by a predetermined amount.

  A wafer dividing process performed using the tape expansion device 8 configured as described above will be described with reference to FIG. That is, the annular frame 6 on which the dicing tape 7 to which the wafer 2 is attached is attached to the mounting surface 811a of the frame holding member 811 constituting the frame holding means 81 as shown in FIG. And fixed to the frame holding member 811 by the clamp 812 (frame holding step). At this time, the frame holding member 811 is positioned at the reference position shown in FIG.

  When the above-described frame holding step is performed, the plurality of air cylinders 823a as the supporting means 823 constituting the tape expanding means 82 are operated as shown in FIG. Lower to the extended position. Accordingly, the annular frame 6 fixed on the mounting surface 811a of the frame holding member 811 is also lowered, so that the dicing tape 7 mounted on the annular frame 6 is an expansion drum as shown in FIG. Expansion is performed in contact with the upper edge of 821 (tape expansion process). As a result, a tensile force acts radially on the wafer 2 adhered to the dicing tape 7. When a tensile force acts radially on the wafer 2 in this manner, the strength of the modified layer 200 formed along the street 21 is reduced, and thus the modified layer 200 in which the strength of the wafer 2 is reduced. Is broken along the street 21 as a starting point of breakage and divided into individual devices 22. At this time, in the above-described modified layer forming step, the outer peripheral surplus region 24 on the extension line of the street 21 is also irradiated with a pulse laser beam having a wavelength that is transmissive to the wafer, so that the inside of the annular reinforcing portion 24b. In addition, by forming the modified layer (auxiliary modified layer forming step), breakage along the street 21 of the wafer 2 is facilitated.

  If the wafer 2 is broken along the street 21 on which the modified layer 200 is formed and divided into individual devices 22 by performing the above-described wafer dividing step, a pickup as shown in FIG. The collet 83 is operated to adsorb the device 22, peeled off from the dicing tape 7 and picked up. In the pickup process, since the gap S between the individual devices 22 is widened, the pickup can be easily performed without contact with the adjacent devices 22.

2: Wafer 21: Street 22: Device 23: Device region 24: Peripheral surplus region 3: Protection tape 30: Annular adhesive layer 4: Grinding device 41: Chuck table 42: Grinding means 424: Grinding wheel 5: Laser processing device 51 : Chuck table 52: laser beam irradiation means 522: light collector 6: annular frame 7: dicing tape 8: tape expansion device 81: frame holding means 82: tape expansion means

Claims (2)

A wafer having a plurality of regions partitioned by a plurality of streets formed in a lattice pattern on the surface and a device region in which a device is formed in the partitioned region and an outer peripheral surplus region surrounding the device region, A method of dividing a wafer along multiple streets,
A protective tape attaching step of attaching a protective tape having a diameter corresponding to the diameter of the wafer and having an annular adhesive layer on the surface only on the outer peripheral portion corresponding to the outer peripheral surplus area of the wafer;
The area corresponding to the device area on the back surface of the wafer on which the protective tape attaching process has been performed is ground to form the thickness of the device area to a predetermined finished thickness, and the area corresponding to the outer peripheral surplus area on the back surface of the wafer A back surface grinding step for forming an annular reinforcing portion by leaving
Modification that forms a modified layer along the street inside the wafer by irradiating along the street with a condensing point of a laser beam having a wavelength that is transparent to the wafer subjected to the back grinding process A layer forming step;
A frame support step of attaching the back surface of the wafer on which the modified layer forming step has been performed to the surface of a dicing tape that is attached to an annular frame and has an adhesive layer on the surface;
A protective tape peeling step for peeling the protective tape from the surface of the wafer on which the frame supporting step has been performed;
A wafer dividing step of breaking the wafer along the streets whose strength has been reduced by applying an external force to the wafer subjected to the protective tape peeling step and forming a modified layer to divide the wafer into individual devices. Including,
A wafer dividing method characterized by the above.   The modified layer forming step irradiates a pulse laser beam having a wavelength that is permeable to the wafer to the outer peripheral surplus area on the extended line of the street to assist the formation of the modified layer in the annular reinforcing portion. 2. The wafer dividing method according to claim 1, further comprising a modified layer forming step.

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