METHOD OF GRINDING WAFER

A method of grinding a wafer includes a grinding step of grinding, by a grinding wheel, a central region of a wafer held by a holding table, to form a circular recessed part and form a ring-shaped reinforcing part 18 on the outer circumferential side of the circular recessed part. The wafer has such a thickness that the deformation of the wafer in which the circular recessed part 17 is formed is able to be suppressed by the ring-shaped reinforcing part.

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Description
BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method of grinding a wafer.

Description of the Related Art

It is known that the rigidity of a wafer thinned by grinding lowers and handling of the wafer in the post-process becomes difficult as a result. Techniques relating to such a problem are described in Japanese Patent Laid-open No. 2007-019461 and Japanese Patent Laid-open No. 2022-044315, for example.

Japanese Patent Laid-open No. 2007-019461 has proposed TAIKO (registered trademark) grinding, in which only a region on the center side in a wafer on which devices are disposed is ground and a region on the outer circumferential side is left as it is to keep at a certain degree the rigidity of the wafer that has been subjected to the grinding.

Even when the TAIKO grinding has been executed, the rigidity of the wafer may not be sufficiently ensured, and the deformation of the wafer, such as warpage or bending, may occur in the post-process. In view of this, Japanese Patent Laid-open No. 2022-044315 has proposed a technique in which the TAIKO grinding is executed after a support substrate is stuck to a wafer.

SUMMARY OF THE INVENTION

As described above, the problem relating to the rigidity of a thinned wafer is significantly suppressed by the TAIKO grinding. However, for dealing with the problem more surely, troublesome work such as sticking the support substrate to the wafer is caused.

Thus, an object of the present invention is to provide a grinding method that can give high rigidity to a wafer that has been subjected to grinding, without sticking a support substrate to the wafer.

In accordance with an aspect of the present invention, there is provided a method of grinding a wafer. The grinding method includes a grinding step of grinding, by a grinding wheel, a central region of the wafer held by a holding table, to form a circular recessed part and form a ring-shaped reinforcing part on an outer circumferential side of the circular recessed part. The wafer has such a thickness that deformation of the wafer in which the circular recessed part is formed is able to be suppressed by the ring-shaped reinforcing part.

According to the present invention, it is possible to provide a grinding method that can give high rigidity to a wafer that has been subjected to grinding, without sticking a support substrate to the wafer.

The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating one example of a front surface of a wafer;

FIG. 2 is a perspective view illustrating one example of a grinding step;

FIG. 3 is a perspective view illustrating one example of a back surface of the wafer that has been subjected to the grinding step;

FIG. 4 is a sectional view illustrating one example of a metal film forming step;

FIG. 5 is a sectional view illustrating one example of a support member fixing step;

FIG. 6 is a perspective view illustrating one example of a ring-shaped reinforcing part removal step;

FIG. 7 is a sectional view illustrating the example of the ring-shaped reinforcing part removal step;

FIG. 8 is a perspective view illustrating one example of a dividing step;

FIG. 9 is a diagram illustrating a relation between the diameter and the thickness of the wafer in the SEMI standard;

FIG. 10 is a diagram illustrating a relation among the diameter, the thickness, and the ratio between the diameter and the thickness regarding the wafer; and

FIG. 11 is a diagram illustrating thresholds of a coefficient that is the ratio between the diameter and the thickness of the wafer.

 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A method of grinding a wafer according to the present embodiment will be described with reference to the drawings. The wafer as a grinding target is not particularly limited to any kind. The wafer is not limited to a semiconductor wafer or an optical device wafer, and it suffices for the wafer to be a plate-shaped workpiece that is a grinding target.

The method of grinding a wafer according to the present embodiment includes a grinding step in which TAIKO (registered trademark of Disco Corporation) grinding is executed. The TAIKO grinding refers to grinding processing to form a ring-shaped projecting part by thinning a wafer in such a manner that an outer circumferential region of the wafer is left. By the TAIKO grinding, the thinned wafer is reinforced by the ring-shaped projecting part, and accordingly, the deformation of the wafer is suppressed compared with a case in which the projecting part is not formed. Hereinafter, the ring-shaped projecting part formed in the outer circumferential region of a wafer will be referred to as a ring-shaped reinforcing part.

However, in the grinding method of the related art, the effect of suppression of the deformation of a wafer obtained by the ring-shaped reinforcing part is not necessarily complete. In particular, when a large-diameter wafer with a diameter as large as approximately 300 mm (12 inches) or larger is thinned, the deformation of the wafer is likely to occur.

As the result of strenuous studies regarding the above-described problem, the inventor of the present application has found out a novel grinding method that can sufficiently suppress the deformation of a wafer even with a large-diameter wafer without using such a support substrate as described in Japanese Patent Laid-open No. 2022-044315.

First, with reference to FIGS. 1 to 8, a wafer processing procedure including a grinding step in which the TAIKO grinding is executed will be described together with the necessity for suppression of the deformation of the wafer.

FIG. 1 is a perspective view illustrating one example of a front surface 11 of a wafer 10. A device region 12 in which a plurality of devices 14 are formed is disposed in the front surface 11 of the wafer 10 as a grinding target. The device region 12 is segmented by a plurality of planned dividing lines 15 that intersect. Further, an outer circumferential surplus region 13 in which the device 14 is not formed is disposed on the outer circumferential side of the device region 12.

That is, the front surface 11 of the wafer 10 includes the device region 12 and the outer circumferential surplus region 13 that surrounds the device region 12.

FIG. 2 is a perspective view illustrating one example of the grinding step. The grinding step is executed by a grinding apparatus illustrated in FIG. 2, for example. The grinding apparatus illustrated in FIG. 2 includes a grinding mechanism 100 and a holding table 110. The holding table 110 is a chuck table that holds the front surface 11 of the wafer 10 under suction. The wafer 10 is held by the holding table 110 in a state in which a back surface 16 thereof is oriented toward the grinding mechanism 100.

A mount 102 is disposed at the lower end of a spindle 101 of the grinding mechanism 100. A small-size grinding wheel 103 for the TAIKO grinding on which a large number of grinding abrasive stones 104 are annularly disposed is mounted on the lower surface of the mount 102. The annular grinding abrasive stones 104 are formed through binding diamond abrasive grains by a bond such as a metal bond or a resin bond and have, for example, an outer diameter that is larger than the radius of the device region 12 but smaller than the radius of the wafer 10.

In the grinding step, the holding table 110 that holds the wafer 10 is rotated at a low speed by a motor or the like that is not illustrated. Moreover, the grinding mechanism 100 lowers while the grinding wheel 103 on which the annular grinding abrasive stones 104 are disposed rotates at a high speed in the same direction as the holding table 110. At this time, the annular grinding abrasive stones 104 are controlled to be constantly in contact with the rotation center of the back surface 16 of the wafer 10 that is rotated in association with the rotation of the holding table 110. This executes the TAIKO grinding by which the wafer 10 is thinned with the outer circumferential region of the wafer 10 left.

FIG. 3 is a perspective view illustrating one example of the back surface 16 of the wafer 10 that has been subjected to the grinding step. In the TAIKO grinding, a central region of the wafer 10 held by the holding table 110 is ground and thinned by the grinding wheel 103 on which the grinding abrasive stones 104 are disposed. Therefore, a circular recessed part 17 is formed in the back surface 16 of the wafer 10 as illustrated in FIGS. 2 and 3. Further, in conjunction with the formation of the circular recessed part 17, a ring-shaped reinforcing part 18 that keeps the thickness before the grinding is formed on the outer circumferential side of the circular recessed part 17 (outer circumferential region of the wafer 10).

The ring-shaped reinforcing part 18 is formed in a region in the back surface 16 corresponding to the outer circumferential surplus region 13 in the front surface 11. In the grinding step according to the present embodiment, such a positional relation between the ring-shaped reinforcing part 18 and the outer circumferential surplus region 13 is implemented by constantly bringing the annular grinding abrasive stones 104 having the outer diameter that is larger than the radius of the device region 12 but smaller than the radius of the wafer 10 into contact with the rotation center of the wafer 10.

The lowering of the rigidity of the wafer 10 thinned by the grinding is suppressed by the presence of the ring-shaped reinforcing part 18. However, the rigidity of the thinned wafer 10 lowers compared with the wafer 10 that has not yet been subjected to the grinding. When the rigidity of the thinned wafer 10 that has been subjected to the grinding is not sufficient, deformation such as warpage occurs in the wafer 10.

Such deformation of the wafer 10 possibly becomes a cause of various kinds of errors and the breakage of the wafer 10 in a subsequent step. To avoid the occurrence of these undesired events, the rigidity of the wafer 10 that has been subjected to the grinding needs to be sufficiently ensured to suppress the deformation of the wafer 10.

FIG. 4 is a sectional view illustrating one example of a metal film forming step. In the metal film forming step executed after the grinding step, a metal film 19 of gold, silver, titanium, or the like is formed on the circular recessed part 17 in the back surface 16 of the wafer 10. The metal film 19 formed on the back surface 16 (circular recessed part 17) is disposed in order to execute an electrical test for each device 14 formed on the front surface 11 (device region 12).

The film deposition method for forming the metal film 19 on the wafer 10 is not particularly limited to any kind. A desired film deposition method such as a sputtering method, an evaporation method, or a chemical vapor deposition (CVD) method can be employed.

The metal film 19 formed on the back surface 16 of the wafer 10 has a thermal shrinkage rate different from that of the wafer 10. Therefore, the wafer 10 formed with the metal film 19 is liable to generate warpage or the like due to the difference in the thermal shrinkage rate, compared with the wafer 10 that has not yet been formed with the metal film 19. Thus, deformation such as warpage occurs in the wafer 10 when the rigidity of the thinned wafer 10 that has been subjected to the grinding is insufficient. Further, the magnitude of the deformation that occurs also possibly becomes higher than that before the metal film formation.

The significant deformation of the wafer 10 is likely to cause the above-described errors and the breakage of the wafer 10. Thus, in the case in which a metal film is formed in the processing process of the wafer 10, it is extremely important to sufficiently ensure the rigidity of the wafer 10 that has been subjected to grinding and suppress the deformation of the wafer 10. After the metal film forming step, electrical characteristics of each device 14 are tested.

FIG. 5 is a sectional view illustrating one example of a support member fixing step. In the support member fixing step executed after the test of the devices 14, a frame unit 22 in which the wafer 10 and a ring frame 21 that is a support member are integrated by a dicing tape 20 is formed.

The method of forming the frame unit 22 is not particularly limited to any kind. For example, the frame unit 22 may be formed through sticking the dicing tape 20 to the ring frame 21 and the wafer 10 by rotating a sticking roller 202 illustrated in FIG. 5 while pressing, by the sticking roller 202, the dicing tape 20 against the ring frame 21 and the wafer 10, the dicing tape 20 being drawn out from a roll tape that is not illustrated.

The sticking of the dicing tape 20 is executed in a state in which the wafer 10 is held under suction by a table 201. However, when deformation such as warpage has occurred in the wafer 10 due to insufficiency of the rigidity of the wafer 10 that has been subjected to grinding, a gap is generated between the holding surface of the table 201 and the front surface 11 of the wafer 10, and therefore, it becomes difficult to firmly hold the wafer 10 under suction.

When it is impossible to firmly hold the wafer 10 under suction, the sticking of the dicing tape 20 also becomes difficult. Therefore, the occurrence of an error increases in the support member fixing step. To avoid the occurrence of such an error, the rigidity of the wafer 10 that has been subjected to grinding needs to be sufficiently ensured to suppress the deformation of the wafer 10.

FIG. 6 is a perspective view illustrating one example of a ring-shaped reinforcing part removal step. FIG. 7 is a sectional view illustrating the example of the ring-shaped reinforcing part removal step. After being formed, the frame unit 22 is disposed in a cutting apparatus including a cutting mechanism 300 (see FIGS. 6 and 7).

In the cutting apparatus, the back surface 16 of the wafer 10 is held by a holding table 310 illustrated in FIG. 7, and the ring frame 21 is held by clamps that are not illustrated. This causes the frame unit 22 to be held by the cutting apparatus in a state in which the front surface 11 is oriented toward the cutting mechanism 300.

The holding table 310 is a chuck table including a porous plate 312 embedded in a circular recessed part of a frame body 311. The wafer 10 is held under suction by the holding table 310 by use of a negative pressure generated at the surface of the porous plate 312 by suction operation of a suction source that is not illustrated.

After the frame unit 22 is held by the holding table 310, the cutting apparatus executes circle cutting for the wafer 10. Specifically, as illustrated in FIGS. 6 and 7, the cutting mechanism 300 lowers toward the wafer 10 while rotating a spindle 302, to press a cutting blade 301 against the boundary between the device region 12 and the outer circumferential surplus region 13 in the front surface 11 of the wafer 10 and cut the wafer 10. At this time, an unillustrated spindle that is for the holding table 310 and has a rotation axis in a direction orthogonal to the spindle 302 of the cutting mechanism 300 rotates. Owing to this, the frame unit 22 also rotates together with the holding table 310.

As a result, the cutting blade 301 of the cutting mechanism 300 cuts the wafer 10 in a circular shape along the boundary between the device region 12 and the outer circumferential surplus region 13 of the wafer 10 to separate the device region 12 and the outer circumferential surplus region 13. This removes, from the wafer 10 (device region 12), the ring-shaped reinforcing part 18 formed in the region corresponding to the outer circumferential surplus region 13.

FIG. 8 is a perspective view illustrating one example of a dividing step. In the dividing step executed after the circle cutting, the cutting mechanism 300 cuts the wafer 10 along the planned dividing lines 15 to execute dicing. This divides the wafer 10 into chips on which the individual devices 14 are formed.

Specifically, the cutting mechanism 300 lowers toward the wafer 10 while rotating the spindle 302, to press the cutting blade 301 against the planned dividing line 15 and cut the wafer 10. At this time, the wafer 10 is divided along the planned dividing line 15 by moving a base of the holding table 310 in a direction parallel to the planned dividing line 15. Such treatment is executed for each planned dividing line 15. This divides the wafer 10 into the chips on which the individual devices 14 are formed.

Next, with reference to FIGS. 9 to 11, detailed description will be made regarding the wafer grinding method that suppress the lowering of the rigidity of the thinned wafer 10 that has been subjected to grinding.

As the result of strenuous studies, the inventor of the present application has concluded that the cause of the higher likelihood of the deformation of a wafer when a large-diameter wafer is thinned is incapability of sufficiently supporting, by the ring-shaped reinforcing part, the thinned wafer that has been subjected to the grinding, that is, insufficiency of support by the ring-shaped reinforcing part. Further, the inventor of the present application has found out that it is possible to lower the likelihood of the deformation of a wafer even when a large-diameter wafer is thinned, by forming the ring-shaped reinforcing part that sufficiently supports the thinned wafer that has been subjected to the grinding, in the method of grinding a wafer.

In light of the fact that the thickness of the ring-shaped reinforcing part formed in the TAIKO grinding is equivalent to the original thickness of the wafer, it suffices for the wafer to have such a thickness (original thickness) that the deformation of the wafer in which the circular recessed part is formed can be suppressed by the ring-shaped reinforcing part in the method of grinding a wafer. In the grinding method according to the present embodiment, a wafer satisfying such a condition is used.

In the following, description will be made regarding an example in which a wafer has such an original thickness that the deformation of the wafer can be suppressed by the ring-shaped reinforcing part in the grinding method using a silicon wafer on which devices are formed.

FIG. 9 is an explanatory diagram illustrating a relation between the diameter and the thickness of the wafer in the SEMI standard. The SEMI standard refers to a standard defined by the Semiconductor Equipment and Materials International (SEMI), which is an international industry group of manufacturing equipment manufacturers, material manufacturers, and so forth of the semiconductor.

In the SEMI standard, the relation between the diameter and the thickness is settled as illustrated in FIG. 9 as a standard specification of dimensions of the silicon wafer. Thus, the relation between the diameter and the thickness in many silicon wafers distributed in the market will also satisfy the relation illustrated in FIG. 9.

FIG. 10 is a diagram illustrating a relation among the diameter, the thickness, and the ratio between the diameter and the thickness regarding the wafer. Parts with a white background indicate the specification of the wafer with a low likelihood of causing deformation after the TAIKO grinding. Parts with a black background indicate the specification of the wafer that is likely to cause deformation after the TAIKO grinding or the specification of the wafer that is not suitable for practical use.

In FIG. 10, the black background is used for the specifications in the range of the SEMI standard of the 12-inch wafer because, in the case in which a large-diameter wafer with a diameter equal to or larger than approximately 300 mm (12 inches) is thinned, the deformation of a wafer is likely to occur even when the TAIKO grinding is executed. Moreover, also for the specification of the wafer whose thickness is too large (diameter: 300 mm, thickness: 10 mm), the black background is used on the basis of a determination that the practicality is low in terms of the grinding cost.

The inventor of the present application has paid attention to the point that, in a case in which a 12-inch wafer is thinned, the deformation of a wafer is likely to occur even when the TAIKO grinding is executed, and has checked 12-inch wafers for which the TAIKO grinding has been executed. As a result, the inventor of the present application has found a phenomenon in which the ring-shaped reinforcing part has been deformed together with the circular recessed part, and has concluded that insufficiency of the thickness (original thickness) of the 12-inch wafer satisfying the standard specification causes insufficiency of support of the wafer by the ring-shaped reinforcing part.

According to the specification of the 12-inch wafer in the SEMI standard, it is supposed that 12-inch wafers having a thickness of at most 795 μm are distributed in the market, and a wafer having a thickness equal to or larger than 800 μm, which exceeds the thickness (original thickness) of the 12-inch wafer satisfying the standard specification, is not distributed in the market. By use of such a wafer having a thickness equal to or larger than 800 μm, which is not typically used, the deformation of the wafer that has been subjected to grinding can be suppressed compared with the related art because the thickness of the ring-shaped reinforcing part also becomes larger than that of the related art.

Therefore, it is desirable that the thickness of a wafer be equal to or larger than 800 μm in the method of grinding a wafer. By use of such a wafer that is out of the SEMI standard, which is not typically used, high rigidity can be given to the wafer that has been subjected to grinding, without a support substrate being stuck to the wafer, and accordingly, the effect of suppression of the deformation of the wafer can be obtained.

Further, as the result of studies on the relation illustrated in FIG. 10, the inventor of the present application has paid attention to the fact that the ratio between the original thickness and the diameter of the wafer with the standard specification defined in the SEMI standard illustrated in FIG. 9 becomes lower, as the diameter of the wafer becomes larger as indicated below, and has concluded that the lowering of the ratio between the thickness and the diameter has an influence as the cause of the higher likelihood of the deformation of a wafer when a large-diameter wafer is thinned.

    • 4-inch wafer: 0.0052
    • 5-inch wafer: 0.005
    • 6-inch wafer: 0.0045
    • 8-inch wafer: 0.00363
    • 12-inch wafer: 0.00258

Moreover, a wafer has a low likelihood of deformation after the TAIKO grinding in the case of wafers with a diameter equal to or smaller than 8 inches, whereas the deformation of a wafer is comparatively more likely to occur after the TAIKO grinding in a 12-inch wafer. Thus, the inventor of the present application has concluded that the deformation of a wafer can be suppressed after the TAIKO grinding irrespective of the size of the wafer as long as the ratio between the thickness and the diameter of the 8-inch wafer is kept.

Therefore, it is desirable that a wafer satisfy the following expression (1) in the method of grinding a wafer.


the thickness of the wafer≥the diameter of the wafer×0.725/200  (1)

By use of a wafer satisfying expression (1), irrespective of the size (inch, diameter) of the wafer, high rigidity can be given to the wafer that has been subjected to grinding, without a support substrate being stuck to the wafer, and accordingly, the deformation of the wafer can be suppressed after the TAIKO grinding. In particular, it is desirable to satisfy expression (1) when a wafer with a diameter equal to or larger than 300 mm is used. When expression (1) is satisfied, the deformation of a wafer that has been subjected to grinding, which is likely to occur when a 12-inch wafer satisfying the standard specification of the SEMI standard is used, can be suppressed.

FIG. 11 is a diagram illustrating thresholds of a coefficient that is the ratio between the diameter and the thickness of the wafer. In FIG. 11, the ratios (coefficients) between the diameter and the thickness of the wafer in the above-described two specific examples are indicated as the thresholds. A threshold THa is the ratio (coefficient) between the thickness and the diameter of a 12-inch wafer having a thickness of 800 μm, and is 0.00267. A threshold THb is the ratio (coefficient) between the thickness and the diameter of an 8-inch wafer satisfying the standard specification of the SEMI standard, and is 0.00363.

By use of a wafer having a ratio (coefficient) equal to or higher than the threshold THa in the method of grinding a wafer, particularly in a 12-inch wafer, the rigidity of the wafer that has been subjected to the TAIKO grinding can be made higher than that in the related art, and accordingly, the deformation of the wafer can be suppressed. Moreover, by use of a wafer having a ratio (coefficient) equal to or higher than the threshold THb, irrespective of the size of the wafer, the rigidity of the wafer that has been subjected to the TAIKO grinding can be sufficiently ensured, and accordingly, the deformation of the wafer can be suppressed more surely.

Therefore, using a wafer having at least a ratio equal to or higher than the threshold THa can suppress the occurrence of various kinds of errors including the breakage of the wafer in a step after the TAIKO grinding. Further, using a wafer having a ratio equal to or higher than the threshold THb can further suppress the occurrence of various kinds of errors in a step after the TAIKO grinding.

The various kinds of errors that occur in a step after the TAIKO grinding due to the deformation of the wafer include a conveyance error in a step subsequent to the grinding step, an error of bending and breakage of the wafer when the wafer is housed in a cassette after execution of the grinding step, and an error of failure in holding the wafer under suction on the holding table in the support member fixing step.

Embodiments of the present invention are not limited to the above-described embodiment and may be changed, replaced, or modified in various ways without departing from the gist of technical ideas of the present invention. Moreover, if a technical idea of the present invention can be implemented in another way on the basis of advancement of a technique or another technique that is derivative, the technical idea may be carried out by using such a method. Therefore, the scope of claims covers all embodiments that can be included in the range of technical ideas of the present invention.

As described above, according to a method of grinding a wafer in accordance with the present invention, high rigidity can be given to a wafer that has been subjected to grinding, without a support substrate being stuck to the wafer. Thus, the method of grinding a wafer is very useful particularly in grinding of a large-diameter wafer in which the rigidity is likely to be insufficient after the grinding.

The present invention is not limited to the details of the above described preferred embodiment. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.

Claims

1. A method of grinding a wafer, the method comprising:

a grinding step of grinding, by a grinding wheel, a central region of the wafer held by a holding table, to form a circular recessed part and form a ring-shaped reinforcing part on an outer circumferential side of the circular recessed part,
wherein the wafer has such a thickness that deformation of the wafer in which the circular recessed part is formed is able to be suppressed by the ring-shaped reinforcing part.

2. The method of grinding a wafer according to claim 1,

wherein the wafer has, in a front surface of the wafer, a device region in which a plurality of devices are formed and an outer circumferential surplus region that surrounds the device region, and
the ring-shaped reinforcing part is formed in a region corresponding to the outer circumferential surplus region.

3. The method of grinding a wafer according to claim 1,

wherein the thickness of the wafer is equal to or larger than 800 μm.

4. The method of grinding a wafer according to claim 1,

wherein the wafer satisfies an expression of the thickness of the wafer≥a diameter of the wafer×0.725/200.

5. The method of grinding a wafer according to claim 1,

wherein a diameter of the wafer is equal to or larger than 300 mm.
Patent History
Publication number: 20240383090
Type: Application
Filed: May 2, 2024
Publication Date: Nov 21, 2024
Inventor: Kazuma SEKIYA (Tokyo)
Application Number: 18/653,465
Classifications
International Classification: B24B 7/22 (20060101);