PROCESSING METHOD OF BONDED WAFER AND PROCESSING APPARATUS

A processing method of a bonded wafer includes forming a plurality of modified layers in a form of rings through positioning focal points of laser beams with a wavelength having transmissibility with respect to a first wafer inside the first wafer, from which a chamfered part is to be removed, from a back surface of the first wafer and executing irradiation, holding a second wafer side on a chuck table, and grinding the back surface of the first wafer to thin the first wafer. In the forming the modified layers, the focal points of the laser beams are set in such a manner as to gradually get closer to a joining layer in a direction from an inner side of the first wafer toward an outer side thereof, so that the plurality of ring-shaped modified layers are formed in a form of descending stairs.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a processing method of a bonded wafer and a processing apparatus.

Description of the Related Art

A wafer having a front surface on which a plurality of devices such as integrated circuits (ICs) and large-scale integration (LSI) circuits are formed in such a manner as to be marked out by a plurality of planned dividing lines that intersect is divided into individual device chips by a dicing apparatus, and the device chips thus obtained are used for pieces of electrical equipment such as mobile phones and personal computers.

Further, in order to improve the degree of integration of devices, in some cases, wafers after formation of a pattern are bonded to each other, and thereafter one of the wafers is ground to be thinned.

However, when a back surface of the one wafer is ground to execute thinning, there is a problem that a chamfered part formed at an outer circumference of the wafer becomes a sharp shape like a knife-edge and that an injury of a worker is induced or cracks develop from the knife-edge to the inside of the wafer and the devices are damaged.

Hence, a technique has been proposed in which a cutting blade or grinding abrasive stones are directly positioned to the outer circumference of a wafer for which the back surface is to be ground, and the chamfered part is removed to suppress occurrence of a knife-edge (for example, refer to Japanese Patent Laid-open No. 2010-225976 and Japanese Patent Laid-open No. 2016-96295).

SUMMARY OF THE INVENTION

In the technique disclosed in Japanese Patent Laid-open No. 2010-225976 and Japanese Patent Laid-open No. 2016-96295, there is a problem that it takes a considerable length of time to remove the chamfered part by the cutting blade or the grinding abrasive stones and the productivity is low. In addition, there is a problem that the other wafer in the bonded wafer is scratched.

Thus, an object of the present invention is to provide a processing method of a bonded wafer and a processing apparatus that can eliminate a problem that it takes long to remove a chamfered part and the productivity is low and a problem that the other wafer in the bonded wafer is scratched.

In accordance with an aspect of the present invention, there is provided a processing method of a bonded wafer formed through bonding, by a joining layer, a front surface of a first wafer and a front surface or a back surface of a second wafer, the first wafer having, on the front surface thereof, a device region in which a plurality of devices are formed and an outer circumferential surplus region that surrounds the device region and that includes a chamfered part formed at an outer circumferential edge. The processing method includes a focal point setting step of causing a laser beam with a wavelength having transmissibility with respect to the first wafer to branch into a plurality of branch laser beams and setting focal points of the respective branch laser beams at different positions, a modified layer forming step of forming a plurality of modified layers in a form of rings through holding a side of the second wafer by a first chuck table, positioning the focal points of the branch laser beams inside the first wafer on an inner side in a radial direction relative to the chamfered part from a back surface of the first wafer, and executing irradiation with the branch laser beams, and a grinding step of holding the side of the second wafer by a second chuck table and grinding the back surface of the first wafer to thin the first wafer after the modified layer forming step is executed. In the modified layer forming step, the focal points of the branch laser beams are formed in a form of descending stairs in such a manner as to gradually get closer to the joining layer in a direction from an inner side toward an outer side of the first wafer, and a surface that links the plurality of modified layers formed by the plurality of focal points forms a side surface of a truncated cone.

Preferably, cracks develop from the plurality of modified layers toward the joining layer, and in the grinding step, the modified layers are removed due to the grinding of the back surface, and the chamfered part is removed from the first wafer due to the cracks. Preferably, the cracks develop toward an outer side of the joining layer in the radial direction.

In accordance with another aspect of the present invention, there is provided a processing apparatus that processes a wafer, the processing apparatus including a chuck table that has a rotational drive mechanism and that holds a wafer, a laser beam irradiation unit that executes irradiation with a laser beam with a wavelength having transmissibility with respect to the wafer held by the chuck table, an X-axis feed mechanism that executes processing feed of the chuck table and the laser beam irradiation unit relative to each other in an X-axis direction, a Y-axis feed mechanism that executes processing feed of the chuck table and the laser beam irradiation unit relative to each other in a Y-axis direction, and a controller. The laser beam irradiation unit includes a focal point forming unit capable of forming a plurality of focal points of the laser beam in a form of descending stairs. The controller includes a coordinate storing section that stores an X-coordinate and a Y-coordinate in the wafer to be processed, and controls the X-axis feed mechanism, the Y-axis feed mechanism, and the rotational drive mechanism of the chuck table on the basis of the coordinates stored in the coordinate storing section.

According to the processing method of a bonded wafer of the present invention, compared with the case of directly removing the chamfered part by use of a cutting blade or grinding abrasive stones, the processing period of time is shortened, and the productivity improves. In addition, the problem that the other wafer is scratched is also eliminated.

According to the processing apparatus of the present invention, compared with the case of directly removing the chamfered part by use of a cutting blade or grinding abrasive stones, the processing period of time is shortened, and the productivity improves. In addition, the problem that the other wafer is scratched can be solved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A processing method of a bonded wafer according to an embodiment of the present invention and a processing apparatus suitable to execute the processing method of a bonded wafer will be described in detail below with reference to the accompanying drawings.

InFIG.1, an overall perspective view of a processing apparatus1of the present embodiment is illustrated. The processing apparatus1is an apparatus that executes laser processing for a bonded wafer W obtained by stacking a first wafer10and a second wafer12like illustrated ones. The processing apparatus1includes a holding unit3including a chuck table35that has an unillustrated rotational drive mechanism and that holds the bonded wafer W, and a laser beam irradiation unit7that executes irradiation with a laser beam with a wavelength having transmissibility with respect to the first wafer10held by the chuck table35. The processing apparatus1includes also an X-axis feed mechanism4athat executes processing feed of the chuck table35and the laser beam irradiation unit7relative to each other in an X-axis direction, a Y-axis feed mechanism4bthat executes processing feed of the chuck table35and the laser beam irradiation unit7relative to each other in a Y-axis direction orthogonal to the X-axis direction, and a controller100that controls the respective operating parts.

The processing apparatus1is disposed on a base2and includes, in addition to the above-described configuration, an imaging unit6that images the bonded wafer W held by the chuck table35and that executes alignment, and a frame body5including a vertical wall part5aerected on a lateral side of the X-axis feed mechanism4aand the Y-axis feed mechanism4bover the base2and a horizontal wall part5bthat extends in a horizontal direction from an upper end part of the vertical wall part5a.

The holding unit3is means that includes the chuck table35having an XY-plane defined based on the X-coordinate and the Y-coordinate as a holding surface and that holds the bonded wafer W. As illustrated inFIG.1, the holding unit3includes a rectangular X-axis direction movable plate31mounted over the base2movably in the X-axis direction, a rectangular Y-axis direction movable plate32mounted over the X-axis direction movable plate31movably in the Y-axis direction, a circular cylindrical support column33fixed to an upper surface of the Y-axis direction movable plate32, and a rectangular cover plate34fixed to an upper end of the support column33. The chuck table35is disposed to pass through a long hole formed in the cover plate34and extend upward, and is configured to be rotatable by the unillustrated rotational drive mechanism that is housed in the support column33. The holding surface of the chuck table35includes a suction adhesion chuck36of a porous material having air permeability and is connected to unillustrated suction means by a flow path that passes through the support column33.

The X-axis feed mechanism4aconverts rotational motion of a motor42to linear motion through a ball screw43and transmits the linear motion to the X-axis direction movable plate31to move the X-axis direction movable plate31in the X-axis direction along a pair of guide rails2A disposed along the X-axis direction on the base2. The Y-axis feed mechanism4bconverts rotational motion of a motor45to linear motion through a ball screw44and transmits the linear motion to the Y-axis direction movable plate32to move the Y-axis direction movable plate32in the Y-axis direction along a pair of guide rails31adisposed along the Y-axis direction on the X-axis direction movable plate31. Due to inclusion of such a configuration, the chuck table35can be moved to any position of the X-coordinate and the Y-coordinate.

The imaging unit6and an optical system that configures the above-described laser beam irradiation unit7are housed inside the horizontal wall part5bof the frame body5. On a lower surface side of a tip part of the horizontal wall part5b, a light collector71that configures part of the laser beam irradiation unit7and that focuses a laser beam to irradiate the bonded wafer W with the laser beam is disposed. The imaging unit6is a camera that images the bonded wafer W held by the holding unit3and that detects the position and orientation of the bonded wafer W, laser processing positions that should be irradiated with the laser beam, and so forth, and is disposed at a position adjacent to the above-described light collector71in the X-axis direction indicated by an arrow X in the diagram.

InFIG.2, a block diagram illustrating an outline of the optical system of the above-described laser beam irradiation unit7is illustrated. The laser beam irradiation unit7includes a laser oscillator72that emits a laser beam LB, an attenuator73that adjusts output power of the laser beam LB emitted by the laser oscillator72, and a focal point forming unit74that causes the laser beam LB having passed through the attenuator73to branch and that forms a plurality of focal points in a form of descending stairs inside the bonded wafer W held by the chuck table35.

For example, as illustrated inFIG.2, the focal point forming unit74of the present embodiment includes a first half wave plate75a, a first beam splitter76a, a second half wave plate75b, a second beam splitter76b, a third half wave plate75c, a third beam splitter76c, a first beam expander77a, a second beam expander77b, a third beam expander77c, a first reflective mirror78a, a second reflective mirror78b, a third reflective mirror78c, a fourth reflective mirror78d, and a fourth beam splitter79.

The laser beam LB that has passed through the above-described attenuator73is introduced to the first beam splitter76athrough the first half wave plate75a, and the rotation angle of the first half wave plate75ais adjusted as appropriate. Due to this, a first branch laser beam LB1(s-polarized light) with the ¼ light amount with respect to the above-described laser beam LB is made to branch from the first beam splitter76aand is introduced to the first beam expander77a. Further, the remaining laser beam (p-polarized light) that is not made to branch by the first beam splitter76ais introduced to the second beam splitter76bthrough the second half wave plate75b, and the rotation angle of the second half wave plate75bis adjusted as appropriate. Due to this, a second branch laser beam LB2(s-polarized light) with the ¼ light amount with respect to the above-described laser beam LB is made to branch from the second beam splitter76band is introduced to the second beam expander77b.

Moreover, the remaining laser beam (p-polarized light) that is not made to branch by the second beam splitter76bis introduced to the third beam splitter76cthrough the third half wave plate75c, and the rotation angle of the third half wave plate75cis adjusted as appropriate. Due to this, a third branch laser beam LB3(s-polarized light) with the ¼ light amount with respect to the above-described laser beam LB is made to branch from the third beam splitter76cand is introduced to the third beam expander77c. The remaining laser beam (p-polarized light) that is not made to branch by the third beam splitter76cbecomes a fourth branch laser beam LB4(p-polarized light) with the ¼ light amount with respect to the above-described laser beam LB and is introduced to the fourth reflective mirror78d. As described above, the first to fourth branch laser beams LB1to LB4are each made to branch with the ¼ light amount with respect to the above-described laser beam LB.

The first branch laser beam LB1is the s-polarized light. Hence, after the beam diameter thereof is adjusted by the first beam expander77a, the first branch laser beam LB1is reflected by the first reflective mirror78a, introduced to the fourth beam splitter79to be reflected, and then introduced to a collecting lens72aof the light collector71. Further, the second branch laser beam LB2is also the s-polarized light. After the beam diameter thereof is adjusted by the second beam expander77b, the second branch laser beam LB2is reflected by the second reflective mirror78b, introduced to the fourth beam splitter79to be reflected, and then introduced to the collecting lens72aof the light collector71. Moreover, the third branch laser beam LB3is also the s-polarized light. After the beam diameter thereof is adjusted by the third beam expander77c, the third branch laser beam LB3is reflected by the third reflective mirror78c, introduced to the fourth beam splitter79to be reflected, and then introduced to the collecting lens72aof the light collector71. In addition, the fourth branch laser beam LB4reflected by the fourth reflective mirror78dis the p-polarized light, and travels straight through the fourth beam splitter79and is introduced to the collecting lens72aof the light collector71. The magnitude of the respective beam diameters is adjusted as appropriate by the first to third beam expanders77ato77cto satisfy a relation of LB1>LB2>LB3>LB4. In addition, the angle of the first to fourth reflective mirrors78ato78dis adjusted. Due to this, as illustrated inFIG.2, focal points P1to P4corresponding to the first to fourth branch laser beams LB1to LB4are formed at different positions in an upward-downward direction and the horizontal direction and are formed in a form of descending stairs toward the left side in the diagram from the focal point P4toward the focal point P1.

For convenience of explanation, the example in which the above-described focal point forming unit74causes the laser beam LB having passed through the attenuator73to branch into the first to fourth branch laser beams LB1to LB4and forms four focal points (the number of branches is 4) has been described. However, the present invention is not limited thereto. It is possible to make the setting to form more branch laser beams by increasing the half wave plate, the beam splitter, the beam expander, the reflective mirror, and so forth, and the focal point forming unit74may form focal points according to the number of branches in a form of descending stairs.

The controller100is configured by a computer and includes a central processing unit (CPU) that executes calculation processing in accordance with a control program, a read-only memory (ROM) that stores the control program and so forth, a readable-writable random access memory (RAM) for temporarily storing a detection value detected, a calculation result, and so forth, an input interface, and an output interface (illustration about details is omitted). A coordinate storing section102that stores the X-coordinate and the Y-coordinate of processing positions in the bonded wafer W is disposed in the controller100. The X-axis feed mechanism4a, the Y-axis feed mechanism4b, and the unillustrated rotational drive mechanism of the chuck table35are connected to the controller100and are controlled based on the information on the X-coordinate and the Y-coordinate stored in the coordinate storing section102.

The processing apparatus1of the present embodiment has the configuration described above substantially, and the processing method of a wafer according to the present embodiment will be described below.

A workpiece of the processing method of a wafer executed in the present embodiment is the bonded wafer W illustrated inFIG.3AandFIG.3B, for example. The bonded wafer W is a wafer with a diameter of 300 mm, for example, and is a bonded wafer obtained by bonding the first wafer10and the second wafer12. The first wafer10is, for example, a silicon on insulator (SOI) wafer in which an oxide film layer is formed inside a silicon substrate, and a plurality of devices D are formed on a front surface10ain such a manner as to be marked out by a plurality of planned dividing lines L that intersect, as illustrated in the diagram. The front surface10aof the first wafer10includes a device region10A that is closer to the center and in which the above-described devices D are formed, and an outer circumferential surplus region10B that surrounds the device region10A. Moreover, at an outer circumferential end part of the outer circumferential surplus region10B, an annular chamfered part10C formed into a curved surface shape is formed as is understood fromFIG.3B. InFIG.3A, a segmentation line16that makes segmentation into the device region10A and the outer circumferential surplus region10B is illustrated. However, the segmentation line16is illustrated for convenience of explanation and is not given to the front surface10aof the actual first wafer10. The second wafer12of the present embodiment has substantially the same configuration as the first wafer10, and detailed description thereof is omitted. As is understood fromFIG.3Bin addition toFIG.3A, the bonded wafer W is formed through inverting the first wafer10to orient the front surface10adownward and joining the front surface10aof the first wafer10and a front surface12aof the second wafer12with the interposition of a joining layer20based on an appropriate adhesive. The bonded wafer W processed by the processing method of a wafer according to the present invention is not limited to the above-described bonded wafer or layer-stacked wafer obtained by joining the front surface10aof the first wafer10and the front surface12aof the second wafer12and may be a bonded wafer obtained by joining the front surface10aof the first wafer10and a back surface12bof the second wafer12.

When the processing method of a wafer according to the present embodiment is executed, the above-described bonded wafer W is conveyed to the processing apparatus1described based onFIG.1and is placed on the chuck table35in such a manner that a back surface10bof the first wafer10that configures the bonded wafer W is oriented upward and the side of the back surface12bof the second wafer12is oriented downward. Then, the above-described suction means is actuated to hold the bonded wafer W under suction. Subsequently, the X-axis feed mechanism4aand the Y-axis feed mechanism4bare actuated to position the bonded wafer W directly under the imaging unit6, and imaging is executed from the side of the upper surface of the bonded wafer W (back surface10bof the first wafer10). Then, as illustrated inFIG.4, the X-coordinate and the Y-coordinate (x0, y0) of a center C are identified from the coordinates of an outer edge of the bonded wafer W, for example. Then, because the X-coordinate and the Y-coordinate of processing positions18in the bonded wafer W that should be processed by a laser beam are stored in the coordinate storing section102of the controller100as described above, the X-coordinate and the Y-coordinate on the chuck table35regarding the processing positions18at which the focal points P1to P4of the above-described branch laser beams LB1to LB4are to be positioned and a modified layer is to be formed are identified due to the identification of the X-coordinate and the Y-coordinate of the center C of the bonded wafer W. The processing positions18are set to exist in the above-described outer circumferential surplus region10B (on an outer side relative to the above-described segmentation line16although illustration is omitted) and on an inner side as viewed in a diameter direction relative to the chamfered part10C having a curved surface. For example, the processing positions18are set at positions of a radius of 149.5 mm from the center C. In the illustration, the processing positions18to which the focal point P4is positioned are illustrated by one annular dashed line for convenience of explanation. However, because the laser beam irradiation unit7of the processing apparatus of the present embodiment forms the plurality of focal points P1to P4in a form of descending stairs as described above, the X-coordinate and the Y-coordinate of the processing positions18may be identified to correspond to the respective focal points P1to P4.

After the processing positions18are identified as described above, the X-axis feed mechanism4aand the Y-axis feed mechanism4bare actuated by the controller100, and the processing position18in the bonded wafer W is positioned directly under the light collector71of the laser beam irradiation unit7as illustrated in FIG.5A. Subsequently, the above-described laser beam irradiation unit7is actuated to execute irradiation with the first to fourth branch laser beams LB1to LB4, and the plurality of focal points P1to P4formed in a form of descending stairs are positioned in such a manner as to gradually get closer to the joining layer20in the direction from the inner side of the first wafer10toward the outer side thereof as illustrated inFIG.5B. The chuck table35is rotated in a direction indicated by an arrow R1(seeFIG.5A). In the present embodiment, the same place is irradiated with the laser beam twice by causing the chuck table35to make two revolutions. By executing the modified layer forming step as above, as illustrated inFIG.5C, modified layers S1to S4are formed to make a truncated cone shape inside the first wafer10that configures the bonded wafer W, along the processing positions18in the outer circumferential surplus region10B of the first wafer10, and cracks11that couple the respective modified layers S1to S4are formed. By the present embodiment, it becomes possible to execute irradiation with the first to fourth branch laser beams LB1to LB4from the inner side with avoidance of the chamfered part10C. Therefore, diffuse reflection at the chamfered part10C having a curved surface is avoided, and the modified layers can be formed with high accuracy. The interval of the respective focal points P1to P4formed by the above-described first to fourth branch laser beams LB1to LB4is set to 10 μm as viewed in the horizontal direction and in a range of 1 to 10 μm as viewed in the upward-downward direction, for example.

Laser processing conditions adopted when the laser processing in the above-described modified layer forming step is executed are set as follows, for example.Wavelength: 1342 nmRepetition frequency: 60 kHzOutput power: 2.4 WNumber of branches: 4Chuck table rotation speed: 107.3 deg/s (circumferential speed 280 mm/s)

After the modified layer forming step is executed as described above, the bonded wafer W is conveyed to a grinding apparatus60illustrated inFIG.6A(only part thereof is illustrated). As illustrated inFIG.6A, the grinding apparatus60includes a grinding unit62for grinding and thinning the bonded wafer W held under suction on a chuck table61. The grinding unit62includes a rotating spindle63rotated by an unillustrated rotational drive mechanism, a wheel mount64mounted on a lower end of the rotating spindle63, and a grinding wheel65attached to a lower surface of the wheel mount64, and a plurality of grinding abrasive stones66are annularly disposed on a lower surface of the grinding wheel65.

After the bonded wafer W for which the above-described modified layer forming step has been executed is conveyed to the grinding apparatus60and the side of the second wafer12is placed on the chuck table61and is held under suction, while the rotating spindle63of the grinding unit62is rotated at, for example, 6000 rpm in a direction indicated by an arrow R2inFIG.6A, the chuck table61is rotated at, for example, 300 rpm in a direction indicated by an arrow R3. Then, while grinding water is supplied onto the back surface10bof the first wafer10of the bonded wafer W by an unillustrated grinding water supply unit, the grinding abrasive stones66are brought into contact with the back surface10bof the first wafer10, and grinding feed of the grinding wheel65is executed in a direction indicated by an arrow R4at a grinding feed rate of 1 μm/second, for example. At this time, the grinding can be advanced while the thickness of the first wafer10is measured by an unillustrated measuring gauge of a contact type. As illustrated inFIG.6B, by grinding the back surface10bof the first wafer10by a predetermined amount, the above-described modified layers S1to S4are removed, and the chamfered part10C of the first wafer10is scattered and removed due to the cracks11. After the chamfered part10C is removed, the grinding unit62is stopped, and the grinding step of grinding the bonded wafer W is completed through cleaning, drying, and other steps regarding which explanation is omitted, so that the processing method of a wafer according to the present embodiment is completed.

As described above, by executing the modified layer forming step with use of the processing apparatus1of the present embodiment, the plurality of focal points P1to P4are set in a form of descending stairs, and the modified layers S1to S4are formed on a side surface of a truncated cone inside the first wafer10that configures the bonded wafer W. Further, the cracks11develop in such a manner as to connect the modified layers S1to S4, so that the cracks11develop to an outer side of the above-described joining layer20while extending toward the joining layer20. By executing the above-described grinding step for such a bonded wafer W, the chamfered part10C is removed due to the cracks11. Therefore, compared with removal of the chamfered part by use of a cutting blade or grinding abrasive stones, the processing period of time is shortened, and the productivity improves. In addition, the problem that the other wafer, that is, the second wafer, is scratched is also eliminated.

In the above-described embodiment, the focal point forming unit74that configures the laser beam irradiation unit7is implemented by combining the plurality of half wave plates, the plurality of beam splitters, the plurality of beam expanders, the plurality of reflective mirrors, and so forth. However, the present invention is not limited thereto. For example, the following configuration may be employed. A spatial light modulator (liquid crystal on silicon (LCOS)) is disposed instead of the focal point forming unit74illustrated inFIG.2, the laser beam LB emitted from the laser oscillator72is made to be incident on the spatial light modulator, and the laser beam LB is made to branch into a plurality of branch laser beams. Further, a plurality of focal points of the respective branch laser beams are formed in a form of descending stairs in such a manner as to gradually get closer to the joining layer in the direction from the inner side of a wafer toward the outer side thereof, and modified layers are formed to make a truncated cone shape corresponding to the plurality of focal points.

Moreover, in the above-described embodiment, the bonded wafer W is conveyed to the grinding apparatus60in the state in which the chamfered part10C of the first wafer10is left, the grinding step is executed, and the chamfered part10C is then removed by a crushing force at the time of grinding with the cracks11formed among the modified layers S1to S4being the point of origin. However, the present invention is not limited thereto, and the chamfered part10C may be removed by giving an external force before the bonded wafer W is carried in to the grinding apparatus60.