PROCESSING METHOD FOR STACKED WAFERS

Abstract

Method for processing a stacked wafer (W) comprising a first wafer (10) and a second wafer (20) stacked at a front face (10a) of the first wafer (10), wherein the first wafer (10) has several components (12) formed in corresponding areas on the front face (10a) delimited by several intersecting provided division lines (14), the method comprising: a formation step for a modified layer, a positioning of a focused spot of a laser beam (LB) with a wavelength transmissible through the second wafer (20), within the second wafer (20) within two sides (14a) defining one of the provided division lines (14), and a deposition of the laser beam (LB) from an upper surface (20b) of the second wafer (20) onto the second wafer (20).wherein at least two strips of modified layers (100) are formed within the second wafer (20) along the designated parting lines (14); a band attachment step of attaching a band (T2) to the upper surface (20b) of the second wafer (20); and an exposure step for designated parting lines (14), of peeling the band (T2) from the upper surface (20b) of the second wafer (20) to remove residues (22) of the second wafer (20) corresponding to the designated parting lines (14) in which the modified layers (100) are formed along the designated parting lines (14) of the second wafer (20), thereby exposing the designated parting lines (14) formed on the front face (10a) of the first wafer (10).

Description

BACKGROUND OF THE INVENTIONField of the InventionThe present invention relates to a method of processing a stacked wafer including a first wafer and a second wafer stacked on a front side of the first wafer, the first wafer having a plurality of devices formed in respective regions on the front side delimited by a plurality of crossing provided division lines.Description of Related ArtWafers having a plurality of devices such as integrated circuits (ICs) and large scale integrations (LSIs) formed in respective regions defined on their front sides by a plurality of division lines provided crossing each other are divided into individual device chips by a dividing device or a laser processing device. The fabricated device chips are used in electrical equipment such as mobile phones and personal computers.Stacked wafers such as silicon-on-insulator (SOI) wafers including a first wafer and a second wafer stacked on the front side of the first wafer, the first wafer including a plurality of devices formed in respective regions on the front side delimited by a plurality of crossing provided division lines, are also divided into individual two-layered device chips along the provided division lines (see, for example, JP 2015-230971 A).Further prior art helpful for understanding the present invention can be found in the following documents:DE 10 2010 030 339 A1 relates to a processing method for a semiconductor wafer having a plurality of semiconductor circuits.JP 2019-71 475 A relates to a method for cutting a wafer, in which a modified region is formed with laser light.SUMMARY OF THE INVENTIONIn processing such a stacked wafer as described above, a processing step in which portions of the second wafer corresponding to the respective provided division lines are removed and the provided division lines are exposed on the front side of the first wafer may be performed. If the regions of the second wafer corresponding to the respective intended division lines are removed with a cutting blade, then the cutting blade may possibly contact the first wafer, which tends to degrade the quality of the first wafer. One solution would be to adjust the depth at which the cutting blade cuts into the second wafer to prevent the cutting blade from contacting the first wafer as the cutting blade removes the areas of the second wafer. In practice, however, it is extremely difficult to perform the task of adjusting the cutting blade. Another problem is that when the second wafer is cut by the blade, cutting water, i.e., water supplied to the cutting blade during the cutting step, tends to reach the first wafer through the cutting grooves in the second wafer, thereby reducing the quality of the devices on the first wafer.It is therefore an object of the present invention to provide a method of processing a stacked wafer including a first wafer and a second wafer stacked on the front side of the first wafer without degrading the quality of the first wafer when the regions of the second wafer corresponding to respective provided division lines are removed and the provided division lines are exposed on the front side of the first wafer.According to an aspect of the present invention, there is provided a method of processing a stacked wafer having a first wafer and a second wafer stacked on a front side of the first wafer, the first wafer having a plurality of devices formed in respective regions on the front side delimited by a plurality of crossing provided division lines, the method comprising: a modified layer forming step of positioning a focused spot of a laser beam having a wavelength transmissive through the second wafer within the second wafer within two sides defining a respective one of the provided division lines, and applying the laser beam from an upper surface of the second wafer to the second wafer, thereby forming at least two strips of modified layers within the second wafer along the provided division lines, a tape fixing step of fixing a tape to the upper surface of the second wafer, and a provided dicing line exposing step of peeling the tape from the upper surface of the second wafer to remove residues of the second wafer corresponding to the provided dicing lines and in which the modified layers are formed along the provided dicing lines from the second wafer, thereby exposing the provided dicing lines formed on the front side of the first wafer.Preferably, the tape attaching step includes an ultraviolet curable tape attaching step of attaching an ultraviolet curable tape whose adhesive force is lowered upon exposure to ultraviolet radiation to the upper surface of the second wafer, and an ultraviolet radiation applying step of applying ultraviolet radiation to regions of the ultraviolet curable tape other than regions thereof corresponding to the intended division lines, thereby lowering an adhesive force of the ultraviolet curable tape in the other regions. Preferably, the tape used in the tape fixing step comprises a thermoplastically compressing bonding tape containing polyolefin, and the tape fixing step comprises a step of fixing the the thermoplastically compressing bonding tape to the upper surface of the second wafer by heating and pressing the the thermoplastically compressing bonding tape put on the upper surface of the second wafer.A thinning step of thinning the second wafer by grinding or polishing the upper surface of the second wafer may be performed before the modified layer forming step or after the modified layer forming step.In the stacked wafer processing method of the present invention, the regions corresponding to the intended division lines of the first wafer can be removed from the second wafer without using a cutting blade. This prevents the first wafer from undergoing a degradation caused by contact of a cutting blade with the first wafer. Moreover, insofar as cutting water is not used to remove the regions of the second wafer corresponding to the intended division lines of the first wafer, the first wafer is prevented from suffering a degradation caused by the cutting water.The above and other objects, features and advantages of the present invention and its mode of operation will be best understood by studying the following specification and appended claims with reference to the accompanying drawings which show some preferred embodiments of the invention, and the invention is best understood thereby.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is an exploded perspective view of a stacked wafer as a workpiece to be processed by a processing method according to an embodiment of the present invention; FIG. 2 is a perspective view illustrating a thinning step of the machining method; FIG. 3 is a perspective view of a laser processing apparatus; FIG. 4A is a perspective view illustrating a modified layer forming step of the processing method; FIG. 4B is an enlarged fragmentary plan view of a first wafer; FIG. 4C is an enlarged fragmentary cross-sectional view of the stacked wafer in the modified layer forming step; FIG. 5 is a perspective view illustrating an alternative thinning step of the machining method; FIG. 6 is a perspective view illustrating a fixing step of fixing an ultraviolet curable tape in a tape fixing method of the processing method; FIG. 7 is a perspective view illustrating an ultraviolet ray applying step in the tape fixing method; and FIG. 8 is an enlarged fragmentary cross-sectional view illustrating an intended division line exposing step of the machining method.DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTSA processing method for a stacked wafer, also referred to as "processing method", according to preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.FIG. 1 illustrates, in an exploded perspective, a stacked wafer W as a workpiece processed by a processing method according to an embodiment of the present invention. As illustrated in an upper portion of FIG. 1, the stacked wafer W includes a first wafer 10 and a second wafer 20 stacked on the first wafer 10. The first wafer 10 comprises, for example, a wafer made of silicon and has a front side 10 athat faces upwards and has a plurality of devices 12 that are formed in corresponding regions delimited on the front side 10 aby a plurality of crossing provided dividing lines 14. Similar to the first wafer 10, the second wafer 20 comprises, for example, a wafer of silicon and has a downwardly facing front side 20 acomprising a plurality of devices, not shown, formed in corresponding regions delimited on the front side 20 aby a plurality of crossing provided dividing lines (not shown). The second wafer 20 is stacked on the first wafer 10 such that the front side 20 aof the second wafer 20 faces the front side 10 aof the first wafer 10, and the first and second wafers 10 and 20 are integrally fixed to each other, thereby forming the stacked wafer W, as provided in a lower portion of FIG. 1.The provided division lines 14 on the front side 10 aof the first wafer 10 and the provided division lines on the front side 20 aof the second wafer 20 are formed at respective positions, i.e., they are aligned with each other across the stacked wafer W. In order to integrally connect the first wafer 10 and the second wafer 20 to each other, their orientations are adjusted by aligning notches defined therein, respectively, and the first wafer 10 and the second wafer 20 are fixed to each other (see the lower portion of FIG. 1 ). Therefore, the first wafer 10 and the second wafer 20 are stacked on each other and integrally connected to each other, so that the provided dividing lines 14 on the first wafer 10 and the provided dividing lines on the second wafer 20 are held in complete alignment with each other. An oxide film or the like is disposed between the first wafer 10 and the second wafer 20, thereby integrally bonding the first wafer 10 and the second wafer 20 to each other. As described above, when the stacked wafer W to be processed according to the present embodiment has been prepared, the processing method to be described below is performed on the stacked wafer W.In order to perform the processing method on the stacked wafer W according to the present embodiment, a thinning step may be performed in advance to grind or polish the upper surface of the second wafer 20, i.e., a back surface 20 bof the second wafer 20, thereby thinning the second wafer 20. The thinning step will be described below with reference to FIG. 2.FIG. 2 illustrates a grinding apparatus 30 used to perform the thinning step, wherein the grinding apparatus 30 is only partially illustrated. The grinding apparatus 30 includes a holding unit 31 for holding the stacked wafer W and a grinding unit 32 for grinding the stacked wafer W held on the holding unit 31.The holding unit 31 has a holding surface, not shown, which is air-permeable and is connected, for example, to a suction source, also not shown. The holding surface may hold the stacked wafer W thereon under suction forces transmitted from the suction source. The grinding unit 32 includes a rotary spindle 34 rotatable about its central axis by a rotary drive mechanism, not shown, a disk mount 35 attached to a lower end of the rotary spindle 34, and a grinding disk 36 attached to a lower surface of the disk mount 35. A plurality of grinding stones 37 are arranged in an annular array on a lower surface of the grinding wheel 36.The grinding apparatus 30 performs the thinning step as follows: A stacked wafer W to be ground is supplied to the grinding apparatus 30, and the stacked wafer W with the first wafer 10 facing downward is held on the holding unit 31 under suction. Then, as illustrated in the figure, the stacked wafer W is positioned under the grinding unit 32. The grinding unit 32 rotates the rotary spindle 34 around its central axis at a speed of 6000 U / min, for example, in the direction indicated by an arrow R 1. At the same time, the holding unit 31 is rotated at a speed of, for example, 300 U / min about its central axis in a direction indicated by an arrow R2. While grinding water is supplied from a not-illustrated grinding water supply means to the back surface 20 bof the second wafer 20, the grinding wheel 36 is grinding-supplied downward at a grinding supply speed of, for example, 1 μm / second, and the back surface 20 bof the second wafer 20 is continuously ground in such a state that the grinding stones 37 are in abrasive contact with the back surface 20 bof the second wafer 20. At this time, an unillustrated contact measurement device continues to measure the thickness of the stacked wafer W. When the grinding stones 37 have ground the back surface 20 bof the second wafer 20 of the stacked wafer W by a predetermined depth, the grinding unit 32 stops grinding of the stacked wafer W. If necessary, the stacked wafer W that has been ground is cleaned and dried, whereupon the thinning step is completed.In a case where the amount of thinning the back surface 20 bof the second wafer 20 is small or the back surface 20 bof the second wafer 20 needs to be polished to mirror finish in the above-described thinning step, the grinding unit 32 may be replaced or combined with a polishing apparatus for polishing and thinning the back surface 20 bof the second wafer 20 with a polishing pad or the like, not illustrated. Alternatively, in a case where the second wafer 20 of the stacked wafer W has already a desired thickness at a time when the stacked wafer W is manufactured, the above-described thinning step may be omitted.The stacked wafer W thus produced from the thinning step or without the thinning step is then processed in a modified layer forming step described below. To execute the modified layer forming step, the stacked wafer W is held on an annular frame F with an adhesive tape T 1 as illustrated in FIG. 3 and supplied to a laser processing apparatus 1.FIG. 3 perspectively illustrates the laser processing apparatus 1 in its entirety suitable for carrying out the processing method according to the present embodiment. The laser processing apparatus 1 includes a holding unit 2, a base 3, a moving mechanism 4, a laser beam applying unit 6, an image pickup unit 7, and a control unit not illustrated.The holding unit 2 includes a rectangular X-axis movable plate 2 athat is movably attached to the base 3 for movement along X-axis directions, a rectangular Y-axis movable plate 2 cthat is movably attached to the X-axis movable plate 2 afor movement in Y-axis directions along a pair of guide rails 2 bon the X-axis movable plate 2 a, a hollow cylindrical column 2 dfixed to an upper surface of the Y-axis movable plate 2 c, and a rectangular cover plate 2 gfixed to an upper end of the column 2 d. The holding unit 2 also includes a chuck table 2 ecomprising a circular member attached to the cover plate 2 gand extending upward through an elongated hole defined in the cover plate 2 g. The chuck table 2e is rotatable about its central axis by rotary drive means not shown. The chuck table 2e has a holding surface 2f made of an air-permeable porous material and lying in a plane defined by the X-axis directions and the Y-axis directions. The support surface 2f is connected to suction means (not shown) via a fluid channel defined in and extending through the column 2d. In FIG. 3, the X-axis directions are represented by the directions indicated by an arrow X, and the Y-axis directions are represented by the directions indicated by an arrow Y and perpendicular to the X-axis directions. The plane defined by the X-axis directions and the Y-axis directions is substantially horizontal.The moving mechanism 4 includes an X-axis moving mechanism 41 for moving the chuck table 2 eof the holding unit 2 and the laser beam applying unit 6 and the image pickup unit 7 relative to each other along the X-axis directions, and a Y-axis moving mechanism 42 for moving the chuck table 2 eof the holding unit 2 and the image pickup unit 7 relative to each other along the Y-axis directions. The X-axis moving mechanism 41 includes a ball screw 44 extending across the base 3 in the X-axis directions, and an electric motor 43 coupled to one end of the ball screw 44. The ball screw 44 is screwed by an unillustrated nut fixed to a lower surface of the X-axis movable plate 2 a. The X-axis moving mechanism 41 converts the rotational motion of the electric motor 43 into a linear motion via the ball screw 44 and the nut combined therewith, and transmits the linear motion to the X-axis movable plate 2 a, the X-axis movable plate 2 abeing reciprocated in the X-axis directions along a pair of guide rails 3 amounted on the base 3. The Y-axis moving mechanism 42 includes a ball screw 46 extending in the Y-axis directions across the X-axis movable plate 2 a, and an electric motor 45 coupled to one end of the ball screw 46. The ball screw 46 is screwed by an unillustrated nut fixed to a lower surface of the Y-axis movable plate 2 c. The Y-axis moving mechanism 42 converts the rotational motion of the electric motor 45 into a linear motion via the ball screw 46 and the nut combined therewith, and transmits the linear motion to the Y-axis movable plate 2 c, thereby reciprocating the Y-axis movable plate 2 cin the X-axis directions along the guide rails 2 bmounted on the X-axis movable plate 2 a.A frame 5 having a vertical wall 5a extending upward from an upper surface of the base 3 and a horizontal arm 5b extending horizontally from the vertical wall 5a in overhanging relation to the holding unit 2 is erected from the base 3 behind the holding unit 2. The laser beam applying unit 6 and the image pickup unit 7 each have an optical system accommodated in the horizontal arm 5b. The laser beam application unit 6 includes a beam condenser 6 adisposed on a lower surface of a distal end portion of the horizontal arm 5 b. The image pickup unit 7 includes a lens assembly disposed on the lower surface of the distal end portion of the horizontal arm 5 bat a position spaced apart from the beam condenser 6 ain the X-axis directions. The image pickup unit 7 also has a visible light emitting means and a usual visible light receiving means, and an infrared radiation emitting means and an infrared image pickup element for receiving infrared radiation. The laser beam application unit 6, the moving mechanism 4, the image pickup unit 7, and the like are electrically connected to the not-illustrated control unit, and perform laser processing on a workpiece on the chuck table 2 eaccording to the command signals supplied from the control device.The stacked wafer W supplied to the laser processing apparatus 1 is placed on the chuck table 2 eof the holding unit 2 with the back surface 20 bof the second wafer 20 facing upward, and is held on the holding surface 2 fwith the adhesive tape T 1 interposed therebetween under suction. The stacked wafer W on the chuck table 2 eis moved by the moving mechanism 4 and positioned directly below the image pickup unit 7 to be processed in an alignment step. In the aligning step, the image pickup unit 7 acquires an image of the first wafer 10 of the stacked wafer W from the back surface 20 bof the second wafer 20 and detects the positions of the provided division lines 14 on the front surface 10 aof the first wafer 10. As described above, the first wafer 10 and the second wafer 20 are stacked on each other such that the provided division lines 14 on the front side 10 aof the first wafer 10 and the not-illustrated provided division lines on the front side 20 aof the second wafer 20 are aligned with each other across the stacked wafer W. In the aligning step, therefore, the provided division lines of the second wafer 20 are aligned with the X-axis directions at the same time as the provided division lines 14 of the first wafer 10 are aligned with the X-axis directions. The position information of the provided division lines 14 detected in the aligning step is stored in the unillustrated control unit.Based on the position information applied in the above-described aligning step, the beam condenser 6 aof the laser beam application unit 6 is positioned above a machining start position corresponding to a provision of the division lines 14. Then, as shown in FIG. 4A, the modified layer forming step is performed on the stacked wafer W to apply a laser beam LB having a wavelength transmissive through the second wafer 20 to the second wafer 20 to form modified layers 100 in the second wafer 20. The modified layer forming step will be described in detail below with reference to Figs. 4A to 4C.In the modified layer forming step according to the present embodiment, as indicated in FIG. 4B, the focused spot of the laser beam LB inside the second wafer 20 is positioned in a region sandwiched between two sides 14 aof two adjacent devices 12 defining one of the provided division lines 14 on the first wafer 10 in a plan view. The chuck table 2e and thus the stacked wafer W are processed in one of the X-axis directions, i.e., in the direction indicated by X in FIG. 4A. The laser beam LB now forms modified layers 100 within the second wafer 20 along the intended dividing line 14 of the stacked wafer W. Moreover, according to the present embodiment, the laser beam LB whose focused spot is positioned inside the second wafer 20 in the region sandwiched between two sides 14 aconstitutes at least two stripes of modified layers 102 and 104 along the two sides 14 a. These modified layers 102 and 104 together form the modified layers 100. According to the present embodiment, as illustrated in FIG. 4C, the laser beam LB forms the modified layers 102 and 104 at multiple positions at different depths of the second wafer 20.After the laser beam LB forms the modified layers 100 including the two strips of modified layers 102 and 104 within the second wafer 20 in the region corresponding to the intended division line 14 along the X-axis directions, the above-described moving mechanism 4 is operated to index the stacked wafer W in one of the Y-axis directions to position a region of the second wafer 20 corresponding to an unprocessed intended division line 14 adjacent to the already processed intended division line 14 directly below the beam condenser 6 a. Then, the focused spot of the laser beam LB inside the second wafer 20 is positioned in the region corresponding to the unprocessed intended division line 14, and the stacked wafer W is process-fed in one of the X-axis directions, thereby forming modified layers 100 having two stripes of modified layers 102 and 104, in the same manner as described above. Thereafter, the stacked wafer W is processed in one of the X-axis directions and index-fed in one of the Y-axis directions, and the laser beam LB is applied to the stacked wafer W until modified layers 100 are formed inside the second wafer 20 along all the provided division lines 14 of the first group along the X-axis directions. Then, the stacked wafer W is rotated 90° about its central axis together with the chuck table 2 eto align a second group of provided division lines 14 aligned in a second direction perpendicular to the first direction in which the modified layers 100 described above have already been formed with the X-axis directions. Then, the focused spot of the laser beam LB of the second wafer 20 is positioned in the region corresponding to each of the unprocessed provided division lines 14 of the second group, and the stacked wafer W is process-fed in one of the X-axis directions, thereby forming modified layers 100 having two stripes of modified layers 102 and 104, in the same manner as described above. Thereafter, the stacked wafer W is processed in one of the X-axis directions and index-fed in one of the Y-axis directions, and the laser beam LB is applied to the stacked wafer W until modified layers 100 are formed inside the second wafer 20 along all the provided division lines 14 of the second group along the X-axis directions. The modified layer forming step is now complete.Laser processing conditions for the modified layer forming step described above are formed as follows, for example:Wavelength: 1064 nmAverage output power: 1.0 WRepetition frequency: 100 kHzMachining feed speed: 100 mm / secondIn the modified layer forming step according to the present embodiment described above, the focal spot of the laser beam LB inside the second wafer 20 is positioned inside the two sides 14 athat define each of the provided division lines 14 of the first wafer 10, and is deposited on the second wafer 20 from the upper surface, i.e., the back surface 20 b, of the second wafer 20, forming the modified layers 100 including the two strips of modified layers 102 and 104 in the second wafer 20 along the provided division line 14. The present invention is not limited to the formation of such modified layers. For example, modified layers comprising three or more stripes of modified layers could be formed in the second wafer 20.According to the present embodiment, the thinning step of thinning the back surface 20 bof the second wafer 20 is performed before the modified layer forming step is performed. However, the present invention is not limited to such details. The thinning step may be performed after the above-described modified layer forming step is performed. For example, as illustrated in FIG. 5, the stacked wafer W on which the modified layer forming step has been performed is supplied to the grinding apparatus 30, which grinds and / or polish the back surface 20 bof the second wafer 20 until the second wafer 20 is thinned to a desired thickness by the same method as the above-described thinning step. When the thinning step is performed after the modified layer forming step is performed, external forces are applied to the regions where the modified layers 100 are formed, thereby dividing the second wafer 20 along the modified layers 100, i.e., the modified layers 102 and 104, to thereby form fractured regions 110 in the second wafer 20. Since the areas divided along the modified layers 100 by the thinning step are extremely narrow, the grinding water supplied to the back surface 20 bof the second wafer 20 in the grinding does not find the way to the first wafer 10.When the modified layer forming step described above has been performed, a tape fixing step is performed to fix a tape having an adhesive force to the upper surface, i.e., the back surface 20 b, of the second wafer 20. To perform the tape mounting step, a tape T 2 (see FIG. 6 ) having substantially the same dimensions as the stacked wafer W in plan view is prepared. The tape T 2 to be attached to the second wafer 20 is not limited to specific tapes. According to the present embodiment, as the tape T 2, an ultraviolet curable tape that reduces its adhesive force and cures upon exposure to ultraviolet radiation is selected. For example, the ultraviolet curable tape may comprise a polyvinyl chloride (PVC) base. When the tape T 2 has been prepared, an ultraviolet curable tape attaching step is performed to attach the tape T 2 to the back surface 20 bof the second wafer 20 of the stacked wafer W in which the modified layers 100 have been formed to achieve an integral shape, as illustrated in FIG. 6.In a case where an ultraviolet curable tape is used as the above-described tape T 2, it is preferable to perform an ultraviolet radiation applying step as illustrated in FIG. 7. The ultraviolet curable tape attaching step and the ultraviolet radiation applying step are included in the tape attaching step.To perform the ultraviolet ray application step, as illustrated in FIG. 7, a mask 120 is placed over the tape T 2 on the stacked wafer W held by the adhesive tape T 1. The mask 120 includes an annular frame 122 defining an opening 122a complementary in shape to the stacked wafer W, and a grid 124 masking the portions of the tape T2 mounted on the stacked wafer W corresponding to the respective provided dividing lines 14. The mesh 124 is attached to the annular frame 122 and positioned in the opening 122 a. When the mask 120 is placed over the tape T 2 on the stacked wafer W held by the adhesive tape T 1, the regions of the tape T 2 corresponding to the respective provided division lines 14 are hidden by the mesh 124 in a plan view. When the mask 120 is placed on the adhesive tape T 1 over the tape T 2, an ultraviolet radiation application device 50 is positioned over the stacked wafer W and applies an ultraviolet radiation P from an ultraviolet radiation application surface 50 ato at least the mask 120 in its entirety. The ultraviolet ray P is applied to regions of the tape T2 other than the regions corresponding to the intended division lines 14, thereby reducing the adhesive force of the regions of the tape T2 other than the regions corresponding to the intended division lines 14.After the tape fixing step including the ultraviolet curable tape fixing step and the ultraviolet radiation applying step is performed, a provided division line exposing step is performed as described below.As illustrated in FIG. 8, the intended division line exposing step is performed by pulling the tape T 2 attached to the back surface 20 bof the second wafer 20 of the stacked wafer W upward in a direction indicated by an arrow R 3. At this time, regions T 2 aof the tape T 2 exposed to the ultraviolet radiation in the ultraviolet radiation application step have a reduced adhesive force and can be easily peeled from the back surface 20 bof the second wafer 20. Conversely, in the regions T 2 bof the tape T 2 masked by the mesh 124 of the mask 120, in the above-described ultraviolet ray application step, the adhesive force is not attenuated but is maintained, and in addition, the regions of the second wafer 20 corresponding to the intended division lines 14 of the first wafer 10 respectively have the two stripes of the modified layers 102 and 104 as fracture initiation points. Therefore, regions of the second wafer 20 where the modified layers 102 and 104 are formed to correspond to the intended division lines 14 of the first wafer 10 remain adhered to the tape T 2 and separated from the second wafer 20 as residues 22 when the tape T 2 is peeled from the second wafer 20. As a result, the residues 22 in which the modified layers 102 and 104 are formed to correspond to the provided division lines 14 are removed from the second wafer 20, thereby exposing the provided division lines 14 on the front surface 10 aof the first wafer 10.According to the present embodiment described above, in the stacked wafer processing method, the regions of the second wafer 20 corresponding to the intended division lines 14 of the first wafer 10 are separated from the second wafer 20 as the debris 22 when the tape T 2 is peeled off from the second wafer 20. Since the debris 22 is removed from the second wafer 20 without using a cutting blade, the first wafer 10 is prevented from suffering a degradation caused by a cutting blade contacting the first wafer 10. In addition, insofar as cutting water is not used to remove the regions of the second wafer 20 corresponding to the intended division lines 14 of the first wafer 10, the first wafer 10 is also prevented from undergoing a degradation caused by the cutting water.According to the present embodiment described above, an ultraviolet curable tape is used as the tape T 2 that is attached to the second wafer 20 in the tape attaching step, and the ultraviolet curable tape attaching step and the ultraviolet radiation applying step are performed in the tape attaching step. However, the present invention is not limited to such details. Rather, a polyolefin-containing thermoplastically joining tape could be used as the tape T2 which is laid on the upper surface of the second wafer 20 in the tape-attaching step and can be attached, for example, by heating and pressing against the upper surface of the second wafer 20. In this case, the tape attaching step is followed by peeling the tape T 2 from the upper surface, i.e., the back surface 20 b, of the second wafer 20 to remove the debris 22, where the modified layers 102 and 104 are formed to correspond to the provided division lines 14 of the second wafer 20, thereby exposing the provided division lines 14 on the front surface 10 aof the first wafer 10.

Claims

A method for processing a stacked wafer (W) having a first wafer (10) and a second wafer (20) stacked on a front side (10a) of the first wafer (10), the first wafer (10) having a plurality of devices (12) formed in respective regions on the front side (10a) delimited by a plurality of crossing provided dividing lines (14), the method comprising: a modified layer forming step of positioning a focused spot of a laser beam (LB) having a wavelength transmissive by the second wafer (20) within the second wafer (20) within two sides (14a) defining a respective one of the provided dividing lines (14), and applying the laser beam (LB) from an upper surface (20b) of the second wafer (20) to the second wafer (20), thereby forming at least two stripes of modified layers (100) within the second wafer (20) along the provided division lines (14); a tape fixing step of fixing a tape (T2) to the upper surface (20b) of the second wafer (20); and a provided dividing line exposing step (14), peeling the tape (T2) from the upper surface (20b) of the second wafer (20) to remove residues (22) of the second wafer (20) corresponding to the provided dividing lines (14) and in which the modified layers (100) are formed along the provided dividing lines (14) from the second wafer (20), thereby exposing the provided dividing lines (14) formed on the front side (10a) of the first wafer (10).The method for processing a stacked wafer (W) according to claim 1, wherein the tape attaching step includes an ultraviolet curable tape attaching step of attaching an ultraviolet curable tape whose adhesive force is lowered upon exposure to an ultraviolet ray to the upper surface (20b) of the second wafer (20), and an ultraviolet ray applying step of applying an ultraviolet ray to regions of the ultraviolet curable tape other than regions thereof corresponding to the intended division lines (14), thereby lowering an adhesive force of the ultraviolet curable tape in the other regions.The method for processing a stacked wafer (W) according to claim 1, wherein the tape used in the tape attaching step has a thermoplastically compressing bonding tape containing polyolefin, and the tape attaching step includes a step of attaching the the thermoplastically compressing bonding tape to the upper surface (20b) of the second wafer (20) by heating and pressing the the thermoplastically compressing bonding tape put on the upper surface (20b) of the second wafer (20).The method for processing a stacked wafer (W) according to any one of the preceding claims, further comprising: a thinning step of thinning the second wafer (20).The method for processing a stacked wafer (W) according to any one of the preceding claims, further comprising: after the modified layer forming step, a thinning step of thinning the second wafer (20) by grinding or polishing the upper surface (20b) of the second wafer (20).

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