Wafer processing method

Abstract

A wafer processing method of processing a wafer (11) having a device surface (17) in which a plurality of devices (15) are formed, and a peripheral marginal surface (19) surrounding the device surface, wherein a front side (11a) of the wafer is partitioned by a plurality of intersecting dividing lines (13) to define a plurality of separate regions where the devices are respectively formed, the wafer processing method comprising: a holding step of holding the front side of the wafer (11) using a holding means (20) having a recess (26) designed to be opposite to the device surface of the wafer (17) and a peripheral part (24) surrounding the recess (26) so as to be raised from the outer periphery of the recess (26), the peripheral part (24) having a holding surface (24a) for holding the peripheral marginal surface (19) of the wafer (11) in contact therewith; and a modified layer forming step of applying the laser beam (30) to the wafer (11) held on the holding means (20) from a back side (11b) of the wafer (11) along the dividing lines (13) under a condition in which a focal point (32) of the laser beam (30) is set within the wafer (11), thereby forming a modified layer within the wafer along each dividing line (13), wherein the modified layer (34) is not formed in the peripheral marginal area (19) of the wafer (11).

Description

BACKGROUND OF THE INVENTIONField of the invention

[0001] The present invention relates to a wafer processing method for applying a laser beam to a wafer so as to focus the laser beam within the wafer, thereby forming a modified layer as a division starting point within the wafer. Description of related prior art

[0002] A wafer processing method has been proposed in which a laser beam is applied to a wafer, the laser beam being focused within the wafer, thereby forming a modified layer of a division starting point within the wafer to divide the wafer into a plurality of chips (see, for example, Japanese Patent No. 3408805). The modified layer is formed along each division line formed on the wafer. Thereafter, by applying a tension to the wafer, the wafer can be divided along each division line.

[0003] A plurality of devices, such as integrated circuits, are formed on a front surface of the wafer as a workpiece. In addition to these devices, a plurality of elements for testing, called TEGs (test element groups) or films, are formed on the front surface of the wafer. The devices are formed only in a plurality of regions partitioned by the dividing lines and not in a different area corresponding to each dividing line. However, the TEG or film is also formed in such an area corresponding to each dividing line.

[0004] In the above-mentioned wafer processing method, the laser beam is applied to the wafer from its front side. Accordingly, in the case where the TEG or film is formed in the area corresponding to each dividing line, a reduction in the light intensity of the laser beam occurs, reaching a position where the modified layer is to be formed. As a result, there is a likelihood that a good modified layer suitable for dividing the wafer can be formed.

[0005] A wafer processing method has also been considered in which a tape is applied to the back of a wafer and a laser beam is applied to the wafer from the back of the wafer through the tape (see, for example, Japanese Patent Application Laid-Open No. 2006-148175 A). Using this wafer processing method, the above problem can be solved because the laser beam is applied from the back of the wafer.

[0006] Laid-open publication JP 2012 - 178 523 A describes a method using two deteriorated layer forming steps, whereby a deteriorated layer is formed along a street extending along an entire wafer. SUMMARY OF THE INVENTION

[0007] However, in the case where the laser beam is applied to the wafer through the belt from the back of the belt, as mentioned above, the applied laser beam is partially absorbed by the belt, resulting in a reduction in the utilization efficiency of the laser beam. Furthermore, there is a possibility that the belt may be worn down by the application of the laser beam.

[0008] To overcome this problem, it is considered that no tape is attached to the back of the wafer and the laser beam is applied to the wafer from the back of the wafer. However, since the wafer is not firmly held to the tape in this method, the area for forming the modified layer expands due to the application of the laser beam, causing wafer warping. If wafer warping occurs, the focal position of the laser beam may deviate, resulting in variations in the formation position of the modified layer in a direction along the wafer thickness.

[0009] It is also worth considering that a tape is adhered to the front side of the wafer to prevent wafer warping. However, when performing a pick-up step of picking up the chips obtained by dividing the wafer with the tape applied to the front side, it is necessary to attach another tape to the back of the wafer and remove the tape previously applied to the front side of the wafer, thereby exposing the front side of the wafer. Accordingly, the pick-up step becomes complicated. Furthermore, attaching and detaching these tapes causes an increase in the cost of the tapes.

[0010] Furthermore, this processing method is not suitable for processing a wafer with MEMS (Micro-Electro-Mechanical Systems) devices or imaging devices formed on the front side. This means that since each MEMS device is a weak structure, there is a risk of breakage due to the attachment and detachment of tapes. Furthermore, if foreign material, such as adhesive from the tape attached to the front side of the wafer, adheres to the imaging devices, the expected imaging characteristics may not be obtained.

[0011] It is therefore an object of the present invention to provide a wafer processing method that can form a good modified layer within a wafer.

[0012] According to one aspect of the present invention, there is provided a wafer processing method of processing a wafer having a device surface in which a plurality of devices are formed and a peripheral marginal surface surrounding the device surface, wherein the front side of the wafer is partitioned by a plurality of intersecting dividing lines to define a plurality of separate regions where the devices are respectively formed, the wafer processing method including a holding step of holding the front side of the wafer using holding means having a recess configured to face the device surface of the wafer and a peripheral part surrounding the recess so as to be raised from the outer periphery of the recess,wherein the peripheral part has a holding surface for holding the peripheral marginal surface of the wafer in contact therewith; and a modified layer forming step of applying the laser beam to the wafer held on the holding means from the back side of the wafer along the dividing lines under a condition where a focal point of the laser beam is set within the laser, thereby forming a modified layer within the wafer along each dividing line, the modified layer being formed only in the device surface except for the peripheral marginal surface of the wafer in the modified layer forming step.

[0013] According to the present invention, the front surface of the wafer is held on the holding means including the recess designed to face the device surface, so that the device surface does not come into contact with the holding means. Accordingly, it is unnecessary to attach a tape to the front surface of the wafer so as to protect the devices. Further, the modified layer is formed only in the device surface. In other words, the modified layer is not formed in the peripheral marginal surface. Accordingly, the device surface is firmly supported to the peripheral marginal surface, so that the warping of the wafer during processing can be reduced without using any tape for firmly supporting the front or back surface of the wafer.This means that the forming position of the modified layer can be made constant in the direction along the thickness of the wafer without attaching a tape to the front or back of the wafer.

[0014] Since no tape is attached to the front side of the wafer, the modified layer can be satisfactorily formed in the wafer without causing failure in a weak structure such as a MEMS device or in the imaging device, whose performance would be reduced by the deposition of foreign matter. Furthermore, since no tape is attached to the back side of the wafer, there is no possibility that the laser beam applied to the wafer from the back side of the wafer could be partially absorbed by the tape, which would lead to a reduction in the utilization efficiency of the laser beam. Furthermore, there is no possibility that the tape would be worn away by the application of the laser beam.

[0015] Conventionally, a picking-up step of picking up chips obtained by dividing a wafer is performed under the condition where the back side of each chip is attached to an expansion tape. Accordingly, in the case where a protective tape is attached to the front side of the wafer, the expansion tape must be attached to the back side of the wafer before performing the picking-up step, and the protective tape must be removed from the front side of the wafer. In contrast, according to the present invention, it is unnecessary to perform the attachment and detachment of tapes even when performing the picking-up step after dividing the wafer. Accordingly, it is possible to suppress the complication of steps and reduce the cost of tapes.Additionally, since the laser beam is applied to the wafer from the back side, the width of the area to be processed can be reduced compared to the case where the laser beam is applied to the wafer from the front side. As a result, a reduction in the number of streets can be achieved, thereby increasing the number of chips that can be obtained.

[0016] Thus, the present invention can provide a wafer processing method that forms a good modified layer within a wafer.

[0017] The above and other objects, features and advantages of the present invention and the manner of carrying them out will become more apparent, and the invention itself will be better understood, from a study of the following description and appended claims with reference to the accompanying drawings which show certain preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view showing a configuration of a wafer as a workpiece to be processed by a wafer processing method according to a preferred embodiment of the present invention; Fig. 2 is a partially sectional side view schematically showing a holding step; Fig. 3A is a partially cutaway side view schematically showing a modified layer forming step; Fig. 3B is a supervision that indicates a state of the Fig. 3A shows a wafer processed in the modified layer formation step; Fig. 4A is a partial sectional side view schematically showing a boundary-modified layer formation step; Fig. 4B is a plan view schematically showing a state of the Fig. 4A shows a wafer processed in the boundary-modified layer formation step; Fig. 5 is a perspective view schematically showing a tape attaching step; Fig. 6A and Fig. 6B are partially sectioned side views schematically showing an expansion step; Fig. 7A is a partially sectional side view schematically showing a boundary-modified layer forming step according to a modification of the second preferred embodiment; and Fig. Fig. 7B is a plan view schematically showing the state of the Fig. 7A shows the boundary-modified layer formation step-processed wafer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018] A preferred embodiment of the present invention will now be described with reference to the accompanying drawings. In this preferred embodiment, a wafer having devices formed on the front side is used as a workpiece to be processed by the wafer processing method according to the present invention. However, in the present invention, the workpiece is not limited to such a wafer.

[0019] The wafer processing method in this preferred embodiment includes a holding step (see Fig. 2), a modified layer formation step (see Fig. 3A and Fig. 3B), a boundary-modified layer formation step (see Fig. 4A and Fig. 4B), a tape application step (see Fig. 5) and an expanding step (see Fig. 6A and Fig. 6B).

[0020] In the holding step, a wafer 11 is held on a holding means (holding table) 20 such that a device surface 17 of the wafer 11 is opposite a recess 26 of the holding means 20 (see Fig. 2). In the modified layer forming step, a laser beam 30 is applied to the wafer 11 along dividing lines (streets) 13 to thereby form a modified layer 34 as a division starting point within the wafer 11 along each dividing line 13 (see Fig. 3A and Fig. 3B). In this modified layer formation step, the modified layer 34 is formed along each dividing line 13 only in one area (device area 17), except for a peripheral marginal area 19 of the wafer 11. In the boundary-modified layer formation step, the laser beam 30 is applied to the wafer 11 along the boundary between the device area 17 and the peripheral marginal area 19, thereby forming a boundary-modified layer 36 as a division starting point within the wafer 11 along this boundary (see Fig. 4A and Fig. 4B). In the tape attachment step, the wafer 11 is attached to an expansion tape 40 (see Fig. 5). In the expanding step, the expansion band 40 is expanded to thereby divide the wafer 11 along the modified layer 34 and the boundary-modified layer 36 as the division starting points (see Fig. 6A and Fig. 6B). These steps of the wafer processing method according to this preferred embodiment will now be described in more detail.

[0021] Fig. 1 is a perspective view showing the configuration of the wafer 11 as a workpiece. As shown in Fig. 1, the wafer 11 as a workpiece is a semiconductor wafer having a circular outer shape. The wafer 11 has a front side 11a and a back side 11b. The front side 11a of the wafer 11 consists of the device surface 17 as a central surface and the peripheral marginal surface 19 surrounding the device surface 17. The device surface 17 is partitioned into a plurality of regions by the plurality of intersecting dividing lines 13, and the device 15, such as a MEMS device, is formed in each region. The wafer 11 has an outer periphery 11e chamfered to have an arcuate cross-section (see Fig. 2).

[0022] In the wafer processing method according to this preferred embodiment, the holding step is first performed to hold the wafer 11 on the holding means 20. As shown in Fig. 2, the holding means 20 is a circular member having an outer diameter equal to that of the wafer 11. The holding means 20 consists of a central portion 22 corresponding to the device surface 17 of the wafer 11 and a peripheral portion 24 surrounding the central portion 22. The central portion 22 is of a lower level than the peripheral portion 24 to form the recess 26 corresponding to the device surface 17 of the wafer 11. The peripheral portion 24 has a holding surface 24a for holding the peripheral marginal surface 19 of the wafer 11 under suction. A vacuum created by a vacuum source (not shown) acts on the holding surface 24a via a passage (not shown) formed in the holding means 20, thereby generating a holding force for holding the wafer 11 under suction.

[0023] In the holding step, the wafer 11 is first positioned over the holding means 20 such that the device surface 17 of the wafer 11 is opposite the recess 26 of the holding means 20. Thereafter, the peripheral marginal surface 19 of the wafer 11 is brought into contact with the holding surface 24a of the peripheral part 24 and then held on the peripheral part 24 under suction. Accordingly, the wafer 11 is held on the holding means 20. As described above, the recess 26 is formed on the holding means 20. Accordingly, when the wafer 11 is held on the holding means 20, the device surface 17 of the wafer 11 does not come into contact with an upper surface 20a of the central part 22. Accordingly, it is unnecessary to attach a tape for protecting the devices 15 to the front side 11a of the wafer 11.

[0024] After performing the holding step, the modified layer forming step is performed to form the modified layer 34 as a division starting point along each division line 13 in the device surface 17. Fig. 3A is a partial sectional side view schematically showing the modified layer forming step, and Fig. 3B is a plan view schematically showing a state of the wafer 11 processed by the modified film forming step.

[0025] In the modified layer forming step, a laser processing head 28 is positioned over the device surface 17 to apply the laser beam 30 to the wafer 11, while the holding means 20 and the laser processing head 28 are moved relative to each other, as shown in Fig. 3A. The laser processing head 28 uses, for example, YAG or YVO4 as a laser medium, and the laser beam 30 is oscillated from the laser medium. The laser beam 30 is applied to the wafer 11 from the back surface 11b thereof. The laser beam 30 is applied along each dividing line 13 by the relative movement of the holding means 20 and the laser processing head 28. A focal point 32 of the laser beam 30 is adjusted within the wafer 11. In the case of using a silicon wafer as the wafer 11, a laser beam having a wavelength (for example, 1065 nm) in the infrared region is used as the laser beam 30. By using the laser beam 30 having this wavelength, the modified layer 34 can be satisfactorily formed within the wafer 11.

[0026] As in the Fig. 3A and Fig. As shown in Fig. 3B, the modified layer 34 is formed along each dividing line 13 only in the device surface 17 of the wafer 11. In other words, the modified layer 34 is not formed in the peripheral marginal surface 19 of the wafer 11. Accordingly, the device surface 17 is firmly supported on the peripheral marginal surface 19, so that even if the area for forming the modified layer 34 expands due to the application of the laser beam 30, the warping of the wafer 11 can be suppressed. Accordingly, the formation position of the modified layer 34 can be made constant in the direction along the thickness of the wafer 11 without attaching a tape to the front surface 11a or the back surface 11b of the wafer 11.

[0027] Since no tape is attached to the front surface 11a of the wafer 11, the modified layer 34 can be satisfactorily formed in the wafer 11 without causing failure in a weak structure such as a MEMS device or an imaging device whose performance is reduced by the deposition of foreign matter. Furthermore, since no tape is attached to the back surface 11b of the wafer 11, there is no possibility that the laser beam 30 applied to the wafer 11 from the back surface 11b thereof can be partially absorbed by the tape to cause a reduction in the utilization efficiency of the laser beam. Furthermore, there is also no possibility that the tape is worn by the application of the laser beam 30.

[0028] Conventionally, a picking-up step of picking up chips obtained by dividing a wafer is performed under the condition where the back side of each chip is attached to an expansion tape. Accordingly, in the case where a protective tape is applied to the front side of the wafer, the expansion tape must be attached to the back side of the wafer before performing the picking-up step, and the protective tape must be removed from the front side of the wafer. In contrast, according to the present invention, it is unnecessary to perform the attachment and detachment of tapes even when performing the picking-up step after dividing the wafer 11. Accordingly, it is possible to reduce the complication of steps and lower the cost of tapes.In addition, since the laser beam 30 is applied to the wafer 11 from the back side 11b thereof, the width of an area to be processed can be reduced compared to the case where the laser beam 30 is applied to the wafer 11 from the front side 11a thereof. As a result, a street reduction can be realized, thereby increasing the number of chips that can be obtained.

[0029] After performing the modified layer forming step, the boundary-modified layer forming step is performed to form the boundary-modified layer 36 as a division starting point along the boundary between device surface 17 and the peripheral marginal surface 19. Fig. 4A is a partial sectional side view schematically showing the boundary-modified layer formation step, and Fig. 4B is a plan view schematically showing a condition of the Fig. 4A shows a wafer 11 processed in the boundary-modified layer formation step.

[0030] In the boundary-modified layer forming step, the laser processing head 28 is positioned directly above the layer between the device surface 17 and the peripheral marginal layer 19 of the wafer 11, and the laser beam 30 is directed onto the wafer 11 while rotating the processing means 20, as shown in Fig. 4A. The laser beam 30 is applied to the wafer 11 from the back side 11b thereof along the boundary between the device surface 17 and the peripheral marginal surface 19. As a result, the boundary-modified layer 36 is formed within the wafer 11 along the boundary between the device surface 17 and the peripheral marginal surface 19, as shown in Fig. 4A and Fig. 4B. The other processing conditions in the boundary-modified layer formation step may be the same as those in the modified layer formation step.

[0031] After performing the boundary-modified layer forming step, the tape attaching step is performed to attach the expansion tape 40 to the wafer 11. Fig. Fig. 5 is a perspective view schematically showing the tape attachment step. In the tape attachment step, the expansion tape 40, temporarily held on a ring frame 38, is attached to the back surface 11b of the wafer 11, as shown in Fig. 5 shown.

[0032] After performing the tape attaching step, the expanding step is performed to expand the expansion tape 40, thereby dividing the wafer 11 along the modified layer 34 and the boundary-modified layer 36 as division starting points. Fig. 6A and Fig. 6B are partial cross-sectional side views schematically showing the expanding step.

[0033] As in Fig. As shown in Fig. 6A, the frame 38 supporting the wafer 11 by the expansion belt 40 is placed on a ring table 42 and fixed to the ring frame 42 by a plurality of clamps 44 provided on the outer periphery of the ring table 42. Under this condition, an upper end of an expanding drum 46 is positioned between the outer periphery of the wafer 11 and the inner periphery of the frame 38 to come into contact with the expansion belt 40.

[0034] Thereafter, the ring table 42 is lowered by a vertical movement mechanism 48 so that the expanding drum 46 is raised relatively with respect to the ring table 42, as shown in Fig. 6B. Accordingly, the expansion belt 40 is pushed up and expanded by the expanding drum 46. As a result, tension in a direction to expand the expansion belt 40 is applied to the modified layer 34 and the boundary-modified layer 36. Accordingly, the device surface 17 of the wafer 11 is divided (broken) into a plurality of chips along the modified layer 34 and the boundary-modified layer 36 as the division starting points. At the same time, the peripheral marginal surface 19 is also broken.

[0035] As a modification, an additional step of forming a peripheral marginal surface fracture starting point may be performed as a fracture starting point for the peripheral marginal surface 19 before performing the expanding step. In this case, the peripheral marginal surface fracture starting point may be provided by forming radial grooves or the like on the peripheral marginal surface 19 with a laser beam or a cutting blade.

[0036] In the above-mentioned wafer processing method, the order of the boundary-modified layer formation step, the tape attachment step, and the expanding step can be changed. For example, this order can be changed to the order of the tape attachment step, the boundary-modified layer formation step, and the expanding step. In the wafer processing method according to this second modification, the tape attachment step of attaching the expansion tape 40 supported by the ring frame 38 to the back surface 11b of the wafer 11 is performed after performing the modified layer formation step (see Fig. 5).

[0037] After performing the tape attachment step, the boundary-modified layer formation step is performed as shown in Fig. 7A and Fig. 7B shown. Fig. 7A is a partial sectional side view schematically showing the boundary-modified layer forming step in the wafer processing method according to the second modification, and Fig. Fig. 7B is a plan view schematically showing a state of the Fig. 7A shows a wafer 11 processed in the boundary-modified layer formation step.

[0038] In Fig. In the boundary-modified layer formation step shown in Fig. 7A, the wafer 11 supported by the expansion belt 40 on the ring frame 38 is held on a chuck table 50 under suction in a state where the expansion belt 40 attached to the back surface 11b of the wafer 11 is in contact with the chuck table 50. In this state, the laser processing head 28 is positioned directly above the boundary between the device surface 17 and the peripheral marginal surface 19 of the wafer 11, and the laser beam 30 is applied to the wafer 11 while rotating the chuck table 50, as shown in Fig. 7A. In this modification, the expansion tape 40 is attached to the back side 11b of the wafer 11. Accordingly, the laser beam 30 is applied to the wafer 11 from the front side 11a thereof along the boundary between the device surface 17 and the peripheral marginal surface 19. By performing this boundary-modified layer formation step, the boundary-modified layer 36 is formed within the wafer 11 along the boundary between the device surface 17 and the peripheral marginal surface 19, as shown in Fig. 7B shown.

[0039] After performing the boundary-modified layer forming step, the expanding step is performed to expand the expansion band 40, thereby dividing the wafer 11 along the modified layer 34 and the boundary-modified layer 36 as the division starting points (see Fig. 6A and Fig.6B). Accordingly, the wafer 11 is divided (broken) into a plurality of chips along the modified layer 34 and the boundary-modified layer 36 as the division starting points.

[0040] The present invention is not limited to the above-mentioned preferred embodiment, but various modifications may be made. For example, while the boundary-modified layer forming step of forming the boundary-modified layer 36 is performed in the above preferred embodiment, the boundary-modified layer forming step may be replaced by another step. For example, a cutting step of cutting the wafer 11 with a cutting blade or an ablation step of performing ablation with a laser beam having an absorption wavelength (e.g., 355 nm) on the wafer 11 may be performed to separate the device surface 17 and the peripheral marginal surface 19 from each other.In this case, the boundary region between the device surface 17 and the peripheral marginal surface 19 may be fully cut over the entire thickness of the wafer 11 (full cutting) or may be half cut over half the thickness of the wafer 11 (half cutting).

[0041] In the case of full cutting, it is preferable to remove the peripheral marginal surface 19 before performing the expanding step. For example, by fully cutting the boundary region between the device surface 17 and the peripheral marginal surface 19 to receive the peripheral marginal surface 19 and then performing the tape attachment step, the peripheral marginal surface 19 can be removed before performing the expanding step. In this case, tension can be easily applied to the modified layer 34 in the expanding step, so that the wafer 11 can be reliably divided.

Claims

[1] A wafer processing method of processing a wafer (11) having a device surface (17) in which a plurality of devices (15) are formed and a peripheral marginal surface (19) surrounding the device surface, wherein a front side (11a) of the wafer is partitioned by a plurality of intersecting dividing lines (13) to define a plurality of separate regions where the devices are respectively formed, the wafer processing method comprising: a holding step of holding the front side of the wafer (11) using a holding means (20) having a recess (26) designed to be opposite to the device surface of the wafer (17) and a peripheral part (24) surrounding the recess (26) so as to be raised from the outer periphery of the recess (26), the peripheral part (24) having a holding surface (24a) for holding the peripheral marginal surface (19) of the wafer (11) in contact therewith; and a modified layer forming step of applying the laser beam (30) to the wafer (11) held on the holding means (20) from a back side (11b) of the wafer (11) along the dividing lines (13) under a condition in which a focal point (32) of the laser beam (30) is set within the wafer (11), thereby forming a modified layer within the wafer along each dividing line (13), wherein the modified layer (34) is not formed in the peripheral marginal area (19) of the wafer (11).

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