JP2024152504A - Wafer processing method - Google Patents

Abstract

[Problem] To provide a wafer processing method capable of suppressing misalignment of a modified layer formed to suppress device damage during grinding. [Solution] The wafer processing method processes a wafer having a chamfer formed along its outer periphery, and includes an annular modified layer forming step 2 in which a focal point of a laser beam having a wavelength that is transparent to the wafer is positioned a predetermined distance inward from the outer periphery of the wafer, and the laser beam is irradiated from the back side of the wafer along the outer periphery to form an annular modified layer inside the wafer, and a thinning step 3 in which, after the annular modified layer forming step 2, the back side of the wafer is ground to thin it down to a predetermined thickness, and in the annular modified layer forming step 2, the laser beam is irradiated in a direction inclined at a predetermined angle with respect to the thickness direction of the wafer so that the laser beam enters the wafer radially inward from the chamfer and heads radially outward. [Selected Figure] Figure 3

Description

The present invention relates to a wafer processing method.

In recent years, the development of 3D stacked semiconductor wafers has progressed in line with the trend toward lower profile and higher integration of device chips. For example, TSV (Through-Silicon Via) wafers enable the connection of the electrodes of two chips by bonding them together using through electrodes.

These wafers are ground and thinned while bonded to a supporting wafer (silicon, glass, ceramics, etc.) that serves as the base. Normally, the outer edge of the wafer is chamfered, so when the wafer is ground to an extremely thin thickness, the outer edge becomes a knife edge, and chipping of the edge occurs easily during grinding. This can cause the chip to extend to the device, potentially resulting in damage to the device.

As a countermeasure against the knife edge, an edge trimming method has been devised in which after bonding the wafers together, a laser beam is irradiated along the outer periphery of the device to form an annular modified layer, thereby preventing chipping of the wafer edge that occurs during grinding from extending into the device (see Patent Document 1).

JP 2020-057709 A

In the method of Patent Document 1, since an annular modified layer is formed near the outer periphery of the wafer, the area to which the laser beam is irradiated may be set to a chamfered portion formed on the outer periphery of the wafer. In this case, the position to which the laser beam is irradiated is an inclined surface, which causes problems such as the processing position of the modified layer being shifted.

The present invention was made in consideration of these problems, and its purpose is to provide a wafer processing method that can suppress misalignment of the processing position of the modified layer formed to suppress device damage during grinding.

In order to solve the above problems and achieve the object, the wafer processing method of the present invention is a wafer processing method for processing a wafer having a chamfered portion formed along its outer periphery, and includes an annular modified layer forming step in which a focal point of a laser beam having a wavelength that is transparent to the wafer is positioned a predetermined distance inward from the outer periphery of the wafer, and the laser beam is irradiated from the back side of the wafer along the outer periphery to form an annular modified layer inside the wafer, and a thinning step in which, after the annular modified layer forming step, the back side of the wafer is ground to thin it to a predetermined thickness, and in the annular modified layer forming step, the laser beam is irradiated in a direction inclined at a predetermined angle with respect to the thickness direction of the wafer so that the laser beam enters the wafer radially inward from the chamfered portion and heads radially outward.

The wafer processing method of the present invention may further include a bonding step of bonding the front side of the wafer to a support wafer different from the wafer before carrying out the thinning step.

The present invention can reduce misalignment of the modified layer formed to prevent damage to the device during grinding.

FIG. 1 is a perspective view showing an example of a wafer to be processed by a wafer processing method according to an embodiment. FIG. 2 is a cross-sectional view taken along the line II-II shown in FIG. FIG. 3 is a flowchart showing a flow of the wafer processing method according to the embodiment. FIG. 4 is a perspective view showing one state of the bonding step shown in FIG. FIG. 5 is an enlarged cross-sectional view of a portion of the wafer after the bonding step of FIG. FIG. 6 is a side view, partly in section, showing one state of the annular modified layer forming step shown in FIG. FIG. 7 is an enlarged cross-sectional view showing a part of the wafer in the annular modified layer forming step of FIG. FIG. 8 is an enlarged cross-sectional view showing a part of a wafer in the annular modified layer forming step of a comparative example. FIG. 9 is a side view, partly in section, showing one state of the thinning step shown in FIG. FIG. 10 is an enlarged cross-sectional view of a portion of the wafer after the thinning step of FIG.

The following describes in detail the form (embodiment) for carrying out the present invention with reference to the drawings. The present invention is not limited to the contents described in the following embodiment. The components described below include those that a person skilled in the art can easily imagine and those that are substantially the same. Furthermore, the configurations described below can be combined as appropriate. Various omissions, substitutions, or modifications of the configuration can be made without departing from the spirit of the present invention.

[Embodiment]
A method for processing a wafer 10 according to an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a perspective view showing an example of a wafer 10 to be processed by the method for processing a wafer 10 according to the embodiment. Fig. 2 is a cross-sectional view taken along line II-II shown in Fig. 1.

1 and 2 is a wafer such as a disk-shaped semiconductor wafer or an optical device wafer, and is a silicon wafer in the embodiment, having a substrate 11 made of silicon (Si), sapphire (Al ₂ O ₃ ), gallium arsenide (GaAs), silicon carbide (SiC), or the like. The wafer 10 has a chamfered portion formed along an outer circumferential edge 12 as shown in FIG.

After the front surface 13 of the wafer 10, which is the device surface, is bonded to, for example, a support wafer 30 (see FIG. 4, etc.) to form a bonded wafer, the back surface 14 is ground to thin it. In the wafer 10 of the embodiment, the chamfer on the back surface 14 is wider and gentler than the chamfer on the front surface 13. The back surface 14 of the wafer 10 includes an inclined surface 15 of the chamfer and a flat surface 16.

As shown in FIG. 1, the wafer 10 includes a device region 17 on the surface 13 side of the substrate 11, and an outer peripheral region 18 surrounding the device region 17. The device region 17 has a plurality of planned division lines 19 set in a grid pattern on the surface 13 of the substrate 11, and devices 20 formed in each region partitioned by the planned division lines 19. The outer peripheral region 18 is a region that surrounds the device region 17 all around, and in which no devices 20 are formed.

In the embodiment, the device 20 constitutes a 3D NAND flash memory and includes an electrode pad and a through electrode connected to the electrode pad. The through electrode penetrates the back surface 14 side of the substrate 11 when the substrate 11 is thinned and the devices 20 are individually separated from the wafer 10. That is, the wafer 10 of the embodiment is a so-called TSV wafer in which the individually separated devices 20 have through electrodes. Note that the wafer 10 of the present invention is not limited to a TSV wafer having through electrodes as in the embodiment, and may be a device wafer without through electrodes.

Figure 3 is a flow chart showing the flow of the processing method of the wafer 10 according to the embodiment. As shown in Figure 3, the processing method of the wafer 10 according to the embodiment includes a bonding step 1, an annular modified layer forming step 2, and a thinning step 3. The processing method of the wafer 10 is a processing method in which an annular modified layer 21 is formed along the outer peripheral edge 12 of the wafer 10, which has a chamfered portion formed along the outer peripheral edge 12, at a predetermined distance inside the wafer 10, and then the back surface 14 side is ground to thin the wafer 10 to a predetermined thickness 23 and the outer peripheral region 18 is removed.

(Lamination step 1)
Fig. 4 is a perspective view showing one state of the bonding step 1 shown in Fig. 3. Fig. 5 is an enlarged cross-sectional view showing a part of the wafer 10 after the bonding step 1 of Fig. 4. The bonding step 1 is a step of bonding the front surface 13 side of the wafer 10 to a support wafer 30 different from the wafer 10.

The support wafer 30 shown in FIG. 4 is a disk-shaped substrate wafer having a substrate 31 made of silicon, glass, ceramics, or the like and the same diameter as the wafer 10, but in the present invention, it may be a TSV wafer similar to the wafer 10. The support wafer 30 has a chamfered portion formed along the outer periphery 32, similar to the wafer 10. Also, in the embodiment of the support wafer 30, the chamfered portion on the other surface 34 is wider and more gentle than the chamfered portion on one surface 33, which is the surface to be bonded to the wafer 10.

In the bonding step 1, first, as shown in FIG. 4, the front surface 13 of the wafer 10 and one surface 33 of the support wafer 30 are opposed to each other with a gap therebetween. Next, the front surface 13 of the wafer 10 and one surface 33 of the support wafer 30 are bonded together. This forms a bonded wafer.

In this case, if a bonding layer is provided between the wafer 10 and the support wafer 30, the bonding layer is laminated on one of the front surface 13 of the wafer 10 and one surface 33 of the support wafer 30, and then the front surface 13 of the wafer 10 and one surface 33 of the support wafer 30 are bonded together via the bonding layer. The bonding layer may be a double-sided tape with an adhesive layer laminated on the front and back surfaces of a base layer, an oxide film, or one formed by applying an adhesive containing a resin or the like. The wafer 10 and the support wafer 30 may also be directly bonded together without using a bonding layer.

As shown in the flowchart in FIG. 3, in the embodiment, the lamination step 1 is performed before the annular modified layer formation step 2, but it may also be performed after the annular modified layer formation step 2 and before the thinning step 3.

In the embodiment of the method for processing the wafer 10, each processing of the wafer 10 is performed in a state in which the support wafer 30 is bonded to the front surface 13 side of the wafer 10. However, the wafer 10 processed in the present invention is not necessarily limited to the wafer 10 constituting a bonded wafer, and a single wafer 10 may be processed. In other words, the bonding step 1 does not have to be performed.

(Annular modified layer formation step 2)
6 is a side view showing one state of the annular modified layer forming step 2 shown in FIG. 3 in a partial cross section. FIG. 7 is a cross-sectional view showing an enlarged portion of the wafer 10 in the annular modified layer forming step 2 of FIG. 6. The annular modified layer forming step 2 is a step of forming an annular modified layer 21 inside the wafer 10 by positioning a focal point 44 of a laser beam 43 having a wavelength that is transparent to the wafer 10 at a predetermined distance inside from the outer peripheral edge 12 of the wafer 10 and irradiating the laser beam 43 from the back surface 14 side of the wafer 10 along the outer peripheral edge 12. The annular modified layer forming step 2 of the embodiment is performed after the bonding step 1 is performed.

Here, the modified layer 21 refers to a region in which the density, refractive index, mechanical strength or other physical properties are different from those of the surrounding area as a result of irradiation with the laser beam 43. The modified layer 21 is, for example, a melting process region, a crack region, an insulation breakdown region, a refractive index change region, or a region in which these regions are mixed. The modified layer 21 has a lower mechanical strength, etc. than other parts of the wafer 10.

In the annular modified layer formation step 2 of the embodiment, an annular modified layer 21 is formed inside the wafer 10 by stealth dicing using a laser processing device 40. The laser processing device 40 includes a holding table 41, a laser beam irradiation unit 42, a moving unit (not shown) that moves the holding table 41 and the laser beam irradiation unit 42 relative to each other, and an imaging unit (not shown) that images the wafer 10 held on the holding table 41.

The holding table 41 holds the wafer 10 on the holding surface (upper surface) via the support wafer 30 and can rotate around a vertical axis. The laser beam irradiation unit 42 irradiates the wafer 10 held on the holding table 41 via the support wafer 30 with a laser beam 43. The laser beam irradiation unit 42 has, for example, a laser oscillator, a condenser that condenses and irradiates the laser beam 43 emitted from the laser oscillator onto the wafer 10 held on the holding table 41 via the support wafer 30, and various optical components that propagate the laser beam 43 from the laser oscillator to the condenser. In the laser beam irradiation unit 42, the optical axis of the laser beam 43 passing through the condenser is inclined at a predetermined angle with respect to the thickness direction of the wafer 10 held on the holding table 41 via the support wafer 30.

In the annular modified layer formation step 2, a laser beam 43 is irradiated from the back surface 14 side of the wafer 10. At this time, the laser beam 43 is irradiated in a direction inclined at a predetermined angle with respect to the thickness direction of the wafer 10 so that the laser beam 43 enters the wafer 10 at the flat surface 16, which is radially inward from the chamfered portion, and heads radially outward. The laser beam 43 is a laser beam with a wavelength that is transparent to the wafer 10, such as infrared rays (IR).

In the annular modified layer formation step 2, first, the other surface 34 of the support wafer 30 is sucked and held on the holding surface (upper surface) of the holding table 41. Next, the wafer 10 is aligned with the collector of the laser beam irradiation unit 42. Specifically, the holding table 41 is moved to the processing area below the laser beam irradiation unit 42 by a moving unit (not shown).

Next, the wafer 10 is photographed and aligned using an imaging unit (not shown), and the irradiation portion of the laser beam irradiation unit 42 is positioned so that the direction in which the laser beam 43 is irradiated faces the radially outward direction of the wafer 10 in a plan view. Furthermore, the laser beam 43 is aligned so that it is incident on the flat surface 16, which is radially inward from the chamfered portion, on the back surface 14 of the wafer 10, and faces radially outward, so that the focal point 44 is positioned at a position a predetermined distance inward from the outer circumferential edge 12 of the wafer 10.

In the annular modified layer formation step 2, the holding table 41 is then rotated around a vertical axis while the laser beam 43 is irradiated from the laser beam irradiation unit 42 onto the flat surface 16 of the back surface 14 of the wafer 10. In this manner, the laser beam 43 is irradiated in a ring shape along the outer peripheral edge 12 with the focal point 44 of the laser beam 43 positioned a predetermined distance inward from the outer peripheral edge 12 of the wafer 10, thereby forming an annular modified layer 21 inside the wafer 10.

The crack 22 extends from the modified layer 21 formed by irradiation with the laser beam 43, and a splitting starting point is formed along the side of a truncated cone having a predetermined angle of inclination from the inside of the wafer 10 toward the surface 13. In the annular modified layer formation step 2, it is preferable to irradiate the laser beam 43 so that the crack 22 extending from the modified layer 21 appears on the surface 13. Note that "along the side of a truncated cone having a predetermined angle of inclination" means that the inclination of an approximate plane that approximates the entire extended crack 22 to a plane is within ±5 degrees, preferably within ±2 degrees, with respect to a virtual plane of a body of revolution formed by rotating the optical axis of the laser beam 43 around the axis of the holding table 41.

In the annular modified layer formation step 2, the height of the focal point 44 of the laser beam 43 is changed in the optical axis direction of the laser beam 43 and the laser beam 43 is irradiated multiple times, that is, after the laser beam 43 having one focal point 44 is irradiated once along an area a predetermined distance inside from the outer peripheral edge 12 of the wafer 10, the position of the focal point 44 is changed in the optical axis direction of the laser beam 43 and the laser beam 43 is irradiated multiple times in the same manner, thereby forming multiple modified layers 21 along the side of the truncated cone having a predetermined angle of inclination. Alternatively, multiple modified layers 21 along the side of the truncated cone having a predetermined angle of inclination may be formed by irradiating a laser beam 43 having multiple focal points 44 spaced apart in the optical axis direction of the laser beam 43.

Here, a comparative example of the annular modified layer formation step 2 will be described. FIG. 8 is an enlarged cross-sectional view showing a portion of the wafer 10 in the annular modified layer formation step 2 of the comparative example. In the annular modified layer formation step 2 of the comparative example, the laser beam 43 is irradiated in a direction perpendicular to the front surface 13 of the wafer 10, i.e., in the thickness direction of the wafer 10. In this case, in order to form the modified layer 21 at the boundary position between the device region 17 and the outer peripheral region 18, it is necessary to irradiate the laser beam 43 from the inclined surface 15 on the back surface 14 side of the wafer 10, as shown in FIG. 8.

The laser beam 43 focused by the condenser is refracted when it enters the wafer 10, so the focal point 44 is shifted upward compared to when it travels straight outside the wafer 10. When positioning the focal point 44 inside the wafer 10 to form the modified layer 21, the position of the focal point 44 is set taking the above into consideration. However, when the laser beam 43 is made incident from the inclined surface 15, the height position of incidence is more likely to shift compared to when it is made incident from the flat surface 16, and when it is made incident from a curved surface, it becomes difficult to control the focusing of the laser beam 43, so there is a possibility that the position where the modified layer 21 is formed may shift unintentionally.

(Thinning Step 3)
Fig. 9 is a side view, partially in cross section, showing one state of the thinning step 3 shown in Fig. 3. Fig. 10 is an enlarged cross-sectional view showing a part of the wafer 10 after the thinning step 3 of Fig. 9. The thinning step 3 is performed after the annular modified layer forming step 2 is performed. The thinning step 3 is a step of grinding the back surface 14 of the wafer 10 to thin it down to a predetermined thickness 23.

In the thinning step 3 of the embodiment, the back surface 14 of the wafer 10 is ground by a grinding device 50 to thin the wafer 10 to a predetermined thickness 23. The grinding device 50 includes a holding table 51, a grinding unit 52, and a moving unit (not shown) that moves the holding table 51 and the grinding unit 52 relative to one another.

The holding table 51 holds the wafer 10 by suction on the holding surface (upper surface) via the support wafer 30, and rotates around a vertical axis. The grinding unit 52 includes a spindle 53 which is a rotating shaft member, a grinding wheel 54 attached to the lower end of the spindle 53, a grinding stone 55 attached to the underside of the grinding wheel 54, and a grinding water supply unit (not shown). The spindle 53 and the grinding wheel 54 rotate on a rotation axis parallel to the axis of the holding table 51. The diameter of the grinding wheel 54 is at least greater than the radius of the wafer 10.

In the thinning step 3, first, the other surface 34 of the support wafer 30 is suction-held on the holding surface (upper surface) of the holding table 51. Next, the wafer 10 held on the holding table 51 via the support wafer 30 is aligned with the grinding wheel 54. Specifically, the holding table 51 is moved to the processing area below the grinding wheel 54 by a moving unit (not shown), and the grinding wheel 55 is brought into opposition to the back surface 14 of the wafer 10.

Next, while the holding table 51 is rotated about its axis, the grinding wheel 54 is rotated about its axis. Grinding water is supplied to the processing point by a grinding water supply unit (not shown), and the grinding wheel 55 of the grinding wheel 54 is brought closer to the holding table 51 at a predetermined feed speed, so that the back surface 14 side of the wafer 10 is ground by the grinding wheel 55, thinning the wafer 10 to a predetermined thickness 23 shown in FIG. 10. At this time, the area (peripheral area 18) on the outer periphery 12 side of the modified layer 21 of the wafer 10 is removed by the grinding load.

As described above, the embodiment of the method for processing the wafer 10 forms an annular modified layer 21 inside the wafer 10, which serves as a starting point for dividing the outer peripheral region 18. Here, the irradiation area onto which the laser beam 43 for forming the modified layer 21 is irradiated is near the outer peripheral edge 12, and therefore may be set to the chamfered portion of the outer peripheral edge 12, i.e., the inclined surface 15.

When the inclined surface 15 is irradiated with the laser beam 43, the thickness of the wafer 10 in the irradiated area is not constant, so the height position of the modified layer 21 that is formed may shift unintentionally. In addition, when the laser beam 43 is incident on the inclined surface 15, which is a curved surface, it is diffusely reflected, and the modified layer 21 may not be formed stably.

In contrast, in the embodiment, in the annular modified layer formation step 2, a laser beam 43 inclined at a predetermined angle is irradiated from the back surface 14 side of the wafer 10 so as to move radially outward from the flat surface 16 radially inward from the chamfered portion. This makes it possible to avoid problems such as diffuse reflection of the laser beam 43 by the chamfered portion, and has the effect of enabling a stable formation of the modified layer 21 along the outer circumferential edge 12.

The present invention is not limited to the above-described embodiment and modified examples. In other words, various modifications can be made without departing from the gist of the present invention. For example, in the annular modified layer formation step 2, the method is not limited to tilting the optical axis of the laser beam 43 in the embodiment, but the holding table 41 may be tilted, or a spatial light modulator such as LCOS (Liquid-Crystal on Silicon) may be used to control the wavefront to form a predetermined angle.

In addition, to facilitate removal of the peripheral region 18 in the thinning step 3, laser processing to divide the peripheral region 18 into multiple regions in advance may be performed prior to the thinning step 3. In this case, for example, the peripheral region 18 may be divided radially by forming a radial modified layer in the peripheral region 18 by stealth dicing.

Alternatively, the outer peripheral region 18 may be removed by applying an external force. The external force may be, for example, a mechanical external force such as a shearing force caused by pressing the outer peripheral region 18 from above or lifting the outer peripheral region 18, or crushing by a roller, or may be an ultrasonic vibration, a collision force between a liquid and/or a solid when the liquid and/or a solid are sprayed onto the outer peripheral region 18, or a radial external force caused by expanding an expandable tape attached to the grinding finish surface of the wafer 10.

REFERENCE SIGNS LIST 10 wafer 11 substrate 12 outer periphery 13 front surface 14 back surface 15 inclined surface 16 flat surface 17 device region 18 outer periphery region 19 planned division line 20 device 21 modified layer 22 crack 23 thickness 30 support wafer 43 laser beam 44 focal point

Claims (2)

A wafer processing method for processing a wafer having a chamfered portion formed along an outer periphery, comprising:
an annular modified layer forming step of positioning a focal point of a laser beam having a wavelength that is transparent to the wafer at a predetermined distance inside from the outer periphery of the wafer, and irradiating the laser beam from the back side of the wafer along the outer periphery to form an annular modified layer inside the wafer;
After the annular modified layer forming step is performed, a thinning step of grinding the back surface of the wafer to a predetermined thickness,
In the annular modified layer forming step,
a laser beam is applied to the wafer in a direction inclined at a predetermined angle with respect to a thickness direction of the wafer so that the laser beam is incident on the wafer radially inward from the chamfered portion and directed radially outward. Prior to carrying out the thinning step,
The wafer processing method according to claim 1 , further comprising a bonding step of bonding the front surface side of the wafer to a support wafer different from the wafer.

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