Wafer processing method

A method of processing a wafer having a device area where a plurality of devices are formed and a peripheral marginal area surrounding the device area on the front side of the wafer is disclosed. The devices are formed in regions defined by division lines. Each device has a plurality of bump electrodes on the front side. A first laser beam is applied through dicing tape from the back side along the boundary between the device area and the peripheral marginal area, with the focal point of the first laser beam set inside the wafer, thereby forming an annular modified layer inside the wafer. A second laser beam is applied through the dicing tape from the back side along each division line with the focal point of the second laser beam set inside the wafer, thereby forming a modified layer inside the wafer along each division line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wafer processing method of processing a wafer having a front side on which a plurality of crossing division lines are formed to define a plurality of separate regions where a plurality of devices are respectively formed.

2. Description of the Related Art

In a semiconductor device fabrication process, a plurality of crossing division lines are formed on the front side of a substantially disk-shaped semiconductor wafer to thereby define a plurality of separate regions where a plurality of devices such as ICs and LSIs are respectively formed. The semiconductor wafer is cut along the division lines to thereby divide the regions where the devices are formed from each other, thus obtaining individual semiconductor chips.

As a method of dividing a wafer such as a semiconductor wafer along the division lines, there has been tried a laser processing method using a pulsed laser beam having a transmission wavelength to the wafer, wherein the pulsed laser beam is applied to the wafer in the condition where the focal point of the pulsed laser beam is set inside the wafer in an area to be divided. A wafer dividing method using this laser processing method includes the steps of applying a pulsed laser beam having a transmission wavelength to the wafer from one side thereof in the condition where the focal point of the pulsed laser beam is set inside the wafer along each division line, thereby continuously forming a modified layer as a break start point inside the wafer along each division line, and next applying an external force to the wafer along each division line where the strength has been reduced by the formation of the modified layer, thereby dividing the wafer. This technique is expected to produce an effect that the width of each division line can be minimized (see Japanese Patent No. 3408805, for example).

In the above method including the steps of forming a modified layer as a break start point inside the wafer along each division line and then dividing the wafer along each division line where the modified layer is formed, the modified layer is formed inside the wafer along each division line having a small width and it is therefore desirable to apply the laser beam to the wafer from the back side thereof where the devices are not formed. Further, in the step of picking up each device (device chip) obtained by dividing the wafer along each division line, it is desirable to expose the front side of the wafer where the devices are formed. In view of these circumstances, a laser beam having a transmission wavelength to the wafer is applied to the wafer from the back side thereof along each division line to thereby form a modified layer inside the wafer along each division line. Thereafter, a dicing tape is attached to the back side of the wafer in which the modified layer is formed, and the dicing tape is supported at its peripheral portion to an annular frame. Thereafter, an external force is applied to the wafer to thereby divide the wafer into the individual devices (see Japanese Patent Laid-open No. 2006-54246, for example).

However, in the above method including the steps of forming the modified layer inside the wafer along each division line and next attaching the dicing tape to the back side of the wafer in which the modified layer is formed, there is a possibility that the wafer may be broken in attaching the dicing tape. To cope with this problem, there has been proposed a method including the steps of attaching the front side of the dicing tape to the back side of the wafer, supporting the peripheral portion of the dicing tape to the annular frame, and next applying a laser beam through the dicing tape from the back side thereof along each division line in the condition where the focal point of the laser beam is set inside the wafer, thereby forming a modified layer inside the wafer along each division line (see Japanese Patent Laid-open No. 2012-84618, for example).

SUMMARY OF THE INVENTION

However, in the case that the wafer is of such a type that the electrodes formed on each device are projecting bump electrodes, the following problem arises. That is, when the front side of the wafer whose back side is attached to the dicing tape is held on a chuck table of a laser processing apparatus and the annular frame supporting the dicing tape is fixed by frame fixing means mounted on the chuck table, the peripheral portion of the wafer is depressed by the dicing tape. Accordingly, in applying a laser beam along each division line to form a modified layer inside the wafer along each division line, the devices adjacent to the peripheral portion of the wafer may be damaged by a stress generated by the depression force of the dicing tape.

It is therefore an object of the present invention to provide a wafer processing method of processing a wafer of such a type that the electrodes formed on each device are projecting bump electrodes which method can form a desired modified layer inside the wafer along each division line without damage to each device, by applying a laser beam through the dicing tape from the back side thereof along each division line in the condition where the focal point of the laser beam is set inside the wafer.

In accordance with an aspect of the present invention, there is provided a wafer processing method of processing a wafer having a device area where a plurality of devices are formed and a peripheral marginal area surrounding the device area on the front side of the wafer, the devices being respectively formed in a plurality of separate regions defined by a plurality of division lines, each device having a plurality of bump electrodes on the front side, the wafer processing method including a wafer supporting step of attaching the front side of a dicing tape to the back side of the wafer and supporting the dicing tape at its peripheral portion to an annular frame; a wafer holding step of holding the front side of the wafer on the upper surface of a chuck table as a holding surface in the condition where the back side of the dicing tape is oriented upward and fixing the annular frame by means of clamps; an annular modified layer forming step of applying a first laser beam having a transmission wavelength to the dicing tape and the wafer through the dicing tape from the back side thereof along the boundary between the device area and the peripheral marginal area in the condition where the focal point of the first laser beam is set inside the wafer, thereby forming an annular modified layer inside the wafer along the boundary between the device area and the peripheral marginal area; and a modified layer forming step of applying a second laser beam having a transmission wavelength to the dicing tape and the wafer through the dicing tape from the back side thereof along each division line in the condition where the focal point of the second laser beam is set inside the wafer after performing the annular modified layer forming step, thereby forming a modified layer inside the wafer along each division line.

According to the wafer processing method of the present invention, the annular modified layer forming step is performed before performing the modified layer forming step, thereby forming the annular modified layer inside the wafer along the boundary between the device area and the peripheral marginal area. Accordingly, the device area is isolated from the peripheral marginal area by the annular modified layer, so that the stress generated in the peripheral marginal area by the depression force of the dicing tape can be prevented from being transmitted from the peripheral marginal area to the device area in the modified layer forming step. Accordingly, it is possible to solve the problem that the devices adjacent to the peripheral marginal area maybe broken in forming the modified layer inside the wafer along each division line in the modified layer forming step.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the wafer processing method according to the present invention will now be described in detail with reference to the attached drawings. Referring toFIG. 1, there is shown a perspective view of a semiconductor wafer2to be processed by the wafer processing method according to the present invention. The semiconductor wafer2shown inFIG. 1is formed from a silicon wafer having a thickness of 300 μm and a diameter of 200 mm. The semiconductor wafer2has a front side2aand a back side2b. A plurality of crossing division lines21are formed on the front side2aof the semiconductor wafer2, thereby defining a plurality of rectangular separate regions where a plurality of devices22such as ICs and LSIs are respectively formed. The semiconductor wafer2includes a device area23where the devices22are formed and a peripheral marginal area24surrounding the device area23. Further, a plurality of bump electrodes221are formed on the front side of each device22. Each bump electrode221has a height of about 50 to 200 μm.

In forming a modified layer inside the semiconductor wafer2along each division line21, a wafer supporting step is first performed in such a manner that the front side of a dicing tape is attached to the back side2bof the semiconductor wafer2and the dicing tape is supported at its peripheral portion to an annular frame. More specifically, as shown inFIG. 2, the peripheral portion of a dicing tape30is preliminarily mounted to an annular frame3so as to close the inside opening of the annular frame3, and the back side2bof the semiconductor wafer2is attached to the front side (adhesive side)30aof the dicing tape30. For example, the dicing tape30is formed from a polyvinyl chloride (PVC) sheet.

Referring toFIG. 3, there is shown a perspective view of a laser processing apparatus4for performing laser processing in forming the modified layer inside the semiconductor wafer2along each division line21. The laser processing apparatus4shown inFIG. 3includes a stationary base40, a chuck table mechanism5for holding a workpiece, the chuck table mechanism5being provided on the stationary base40so as to be movable in a feeding direction (X direction) shown by an arrow X, and a laser beam applying unit6provided on the stationary base40, the laser beam applying unit6having laser beam applying means to be hereinafter described.

The chuck table mechanism5includes a pair of guide rails51provided on the stationary base40so as to extend parallel to each other in the X direction, a first slide block52provided on the guide rails51so as to be movable in the X direction, a second slide block53provided on the first slide block52so as to be movable in an indexing direction (Y direction) shown by an arrow Y perpendicular to the X direction, a cover table55supported by a cylindrical member54standing on the second slide block53, and a chuck table56as wafer holding means. The chuck table56has a vacuum chuck561formed of a porous material. The semiconductor wafer2as a workpiece is adapted to be held under suction on the upper surface of the vacuum chuck561as a holding surface by operating suction means (not shown). The chuck table56is rotatable by a pulse motor (not shown) provided in the cylindrical member54. The chuck table56is provided with clamps562as frame holding means for fixing the annular frame3supporting the semiconductor wafer2as a workpiece through the dicing tape30. The clamps562as the frame holding means are so configured as to hold the annular frame3at a vertical position lower than that of the upper surface of the chuck table56as the holding surface.

The lower surface of the first slide block52is formed with a pair of guided grooves521for slidably engaging the pair of guide rails51mentioned above. A pair of guide rails522are provided on the upper surface of the first slide block52so as to extend parallel to each other in the Y direction. Accordingly, the first slide block52is movable in the X direction along the guide rails51by the slidable engagement of the guided grooves521with the guide rails51. The chuck table mechanism5further includes feeding means57for moving the first slide block52in the X direction along the guide rails51. The feeding means57includes an externally threaded rod571extending parallel to the guide rails51so as to be interposed therebetween and a pulse motor572as a drive source for rotationally driving the externally threaded rod571. The externally threaded rod571is rotatably supported at one end thereof to a bearing block573fixed to the stationary base40and is connected at the other end to the output shaft of the pulse motor572so as to receive the torque thereof. The externally threaded rod571is engaged with a tapped through hole formed in an internally threaded block (not shown) projecting from the lower surface of the first slide block52at a central portion thereof. Accordingly, the first slide block52is moved in the X direction along the guide rails51by operating the pulse motor572to normally or reversely rotate the externally threaded rod571.

The lower surface of the second slide block53is formed with a pair of guided grooves531for slidably engaging the pair of guide rails522provided on the upper surface of the first slide block52as mentioned above. Accordingly, the second slide block53is movable in the Y direction along the guide rails522by the slidable engagement of the guided grooves531with the guide rails522. The chuck table mechanism5further includes indexing means58for moving the second slide block53in the Y direction along the guide rails522. The indexing means58includes an externally threaded rod581extending parallel to the guide rails522so as to be interposed therebetween and a pulse motor582as a drive source for rotationally driving the externally threaded rod581. The externally threaded rod581is rotatably supported at one end thereof to a bearing block583fixed to the upper surface of the first slide block52and is connected at the other end to the output shaft of the pulse motor582so as to receive the torque thereof. The externally threaded rod581is engaged with a tapped through hole formed in an internally threaded block (not shown) projecting from the lower surface of the second slide block53at a central portion thereof. Accordingly, the second slide block53is moved in the Y direction along the guide rails522by operating the pulse motor582to normally or reversely rotate the externally threaded rod581.

The laser beam applying unit6includes a support member61provided on the stationary base40, a casing62supported by the support member61so as to extend in a substantially horizontal direction, laser beam applying means7provided on the casing62, and imaging means8provided on the casing62at a front end portion thereof for detecting a subject area to be laser-processed. The laser beam applying means7includes pulsed laser beam oscillating means (not shown) provided in the casing62and focusing means71provided at the front end portion of the casing62for focusing a pulsed laser beam oscillated from the pulsed laser beam oscillating means. Although not shown, the pulsed laser beam oscillating means includes a pulsed laser beam oscillator and repetition frequency setting means. The imaging means8includes an ordinary imaging device (CCD) for imaging the workpiece by using visible light, infrared light applying means for applying infrared light to the workpiece, an optical system for capturing the infrared light applied to the workpiece by the infrared light applying means, and an imaging device (infrared CCD) for outputting an electrical signal corresponding to the infrared light captured by the optical system. An image signal output from the imaging means8is transmitted to control means (not shown).

In forming a modified layer inside the semiconductor wafer2along each division line21by using the laser processing apparatus4after performing the wafer supporting step mentioned above, a wafer holding step is first performed in such a manner that the front side2aof the semiconductor wafer2is held on the upper surface of the chuck table56as the holding surface and the annular frame3is held by the clamps562as the frame holding means. More specifically, as shown inFIG. 4, the semiconductor wafer2supported through the dicing tape30to the annular frame3is placed on the upper surface of the chuck table56as the holding surface in the condition where the front side2aof the semiconductor wafer2is oriented downward. At this time, a porous sheet31is preferably interposed between the front side2aof the semiconductor wafer2and the upper surface of the chuck table56, so as to protect the devices22including the bump electrodes221. Thereafter, the suction means (not shown) is operated to hold the semiconductor wafer2through the porous sheet31on the upper surface of the chuck table56under suction. Thereafter, the annular frame3is fixed by the clamps562. Accordingly, the semiconductor wafer2is held on the chuck table56in the condition where the back side (nonadhesive side)30bof the dicing tape30attached to the semiconductor wafer2is oriented upward. When the annular frame3supporting the peripheral portion of the dicing tape30is fixed by the clamps562, the peripheral marginal area24of the semiconductor wafer2is depressed by the dicing tape30with the bump electrodes221functioning as a fulcrum because the bump electrodes221are higher in position than the clamps562, causing the generation of stress in the peripheral marginal area24.

After performing the wafer holding step mentioned above, the feeding means57is operated to move the chuck table56holding the semiconductor wafer2under suction to a position directly below the imaging means8. When the chuck table56is positioned directly below the imaging means8, a first alignment step is performed by the imaging means8and the control means (not shown) to detect the boundary between the device area23and the peripheral marginal area24of the semiconductor wafer2. More specifically, the imaging means8and the control means (not shown) detect the position radially inside from the outer circumference of the semiconductor wafer2by an amount of 5 mm, for example, and then store the coordinate values of this position into a memory constituting the control means. Although the dicing tape30is present on the upper side of the semiconductor wafer2, the outer circumference of the semiconductor wafer2can be imaged from the back side30bof the dicing tape30because the imaging means8includes the infrared light applying means for applying infrared light, the optical system for capturing the infrared light, and the imaging device (infrared CCD) for outputting an electrical signal corresponding to the infrared light.

After performing the first alignment step mentioned above, an annular modified layer forming step is performed in such a manner that a laser beam having a transmission wavelength to the dicing tape30and the semiconductor wafer2is applied through the dicing tape30from the back side30bthereof along the boundary between the device area23and the peripheral marginal area24in the condition where the focal point of the laser beam is set inside the semiconductor wafer2, thereby forming an annular modified layer inside the semiconductor wafer2along the boundary between the device area23and the peripheral marginal area24. More specifically, in performing this annular modified layer forming step, the feeding means57is operated to move the chuck table56holding the semiconductor wafer2from the condition where the first alignment step has been performed to a laser beam applying area where the focusing means71is located. That is, the position radially inside from the outer circumference of the semiconductor wafer2by an amount of 5 mm, for example, is set directly below the focusing means71as shown inFIG. 5. Thereafter, the focal point P of the pulsed laser beam LB to be applied from the focusing means71is set near the intermediate position in the direction of the thickness of the semiconductor wafer2. Thereafter, the pulsed laser beam LB having a transmission wavelength to the dicing tape30and the semiconductor wafer2(silicon wafer) is applied from the focusing means71. At the same time, the chuck table56holding the semiconductor wafer2under suction is rotated 360 degrees in the direction shown by an arrow56ainFIG. 5. As a result, an annular modified layer25is formed inside the semiconductor wafer2along the boundary between the device area23and the peripheral marginal area24as shown inFIG. 5.

For example, the annular modified layer forming step mentioned above may be performed under the following processing conditions.

Light source: YAG pulsed laser

After performing the annular modified layer forming step mentioned above, a modified layer forming step is performed in such a manner that a laser beam having a transmission wavelength to the dicing tape30and the semiconductor wafer2is applied through the dicing tape30from the back side30bthereof along each division line21in the condition where the focal point of the laser beam is set inside the semiconductor wafer2, thereby forming a modified layer inside the semiconductor wafer2along each division line21. More specifically, in performing this modified layer forming step, the feeding means57is operated to move the chuck table56holding the semiconductor wafer2under suction to the position directly below the imaging means8. In this condition, a second alignment step is performed to detect a subject area of the semiconductor wafer2to be laser-processed. More specifically, the imaging means8and the control means (not shown) are operated to make the alignment between the division lines21extending in a first direction on the semiconductor wafer2and the focusing means71for applying the laser beam along the division lines21. Similarly, this alignment is made between the focusing means71and the remaining division lines21extending in a second direction perpendicular to the first direction on the semiconductor wafer2. Although the front side2aon which the division lines21of the semiconductor wafer2are formed is oriented downward, the division lines21can be imaged from the back side30bof the dicing tape30and the back side2bof the semiconductor wafer2because the imaging means8includes the infrared light applying means for applying infrared light, the optical system for capturing the infrared light, and the imaging device (infrared CCD) for outputting an electrical signal corresponding to the infrared light.

After performing the second alignment step mentioned above, the chuck table56is moved to the laser beam applying area where the focusing means71of the laser beam applying means7is located, and a predetermined one of the division lines21extending in the first direction on the semiconductor wafer2held on the chuck table56is positioned directly below the focusing means71as shown inFIG. 6A. At this time, one end (left end as viewed inFIG. 6A) of the predetermined division line21is positioned directly below the focusing means71as shown inFIG. 6A. Thereafter, the focal point P of the pulsed laser beam LB to be applied from the focusing means71is set near the intermediate position in the direction of the thickness of the semiconductor wafer2. Thereafter, the pulsed laser beam LB having a transmission wavelength to the dicing tape30and the semiconductor wafer2(silicon wafer) is applied from the focusing means71. At the same time, the chuck table56is moved in the direction shown by an arrow X1inFIG. 6Aat a predetermined feed speed (modified layer forming step).

When the other end (right end as viewed inFIG. 6B) of the predetermined division line21reaches the position directly below the focusing means71as shown inFIG. 6B, the application of the pulsed laser beam LB is stopped and the movement of the chuck table56is also stopped. As a result, a modified layer26is formed inside the semiconductor wafer2along the predetermined division line21as shown inFIG. 6B.

For example, the modified layer forming step mentioned above may be performed under the following processing conditions.

Light source: YAG pulsed laser

After performing the modified layer forming step along the predetermined division line21as mentioned above, the chuck table56is moved in the Y direction by an amount corresponding to the pitch of the division lines21(indexing step), and the modified layer forming step is performed similarly along the next division line21extending in the first direction. After performing the modified layer forming step along all of the division lines21extending in the first direction, the chuck table56is rotated 90 degrees to similarly perform the modified layer forming step along all of the remaining division lines21extending in the second direction perpendicular to the first direction.

As described above, the annular modified layer forming step is performed before performing the modified layer forming step, thereby forming the annular modified layer25inside the semiconductor wafer2along the boundary between the device area23and the peripheral marginal area24. Accordingly, the device area23is isolated from the peripheral marginal area24by the annular modified layer25, so that the stress generated in the peripheral marginal area24by the depression force of the dicing tape30can be prevented from being transmitted from the peripheral marginal area24to the device area23in the modified layer forming step. Accordingly, it is possible to solve the problem that the devices adjacent to the peripheral marginal area24may be broken in forming the modified layer26inside the semiconductor wafer2along each division line21in the modified layer forming step.