# Patent

## Title
Method of forming wafer-level molded structure for package assembly

## Abstract

A method of forming an integrated circuit structure is provided. In an embodiment, the method includes bonding top dies onto a bottom wafer and then molding a first molding material onto and in between the top dies and the bottom wafer. The bottom wafer, the top dies, and the first molding material are sawed to form molding units. Each of the molding units includes one of the top dies and a bottom die sawed from the bottom wafer. The molding units are bonded onto a package substrate and a second molding material is molding onto the one of the molding units and the package substrate. Thereafter, the package substrate and the second molding material are sawed to form package-molded units.

## Background
This patent application is a divisional of co-pending U.S. patent application Ser. No. 14/224,921, filed Mar. 25, 2014, entitled “Method of Forming Wafer-Level Molded Structure for Package Assembly,” which is a divisional of co-pending U.S. patent application Ser. No. 12/813,979, filed Jun. 11, 2010, entitled “Wafer-Level Molded Structure for Package Assembly,” which claims the benefit of U.S. Provisional Application No. 61/237,153 filed on Aug. 26, 2009, entitled “Wafer-Level Molded Structure for Package Assembly,” each of which are incorporated herein by reference.


TECHNICAL FIELD
This disclosure relates generally to integrated circuits, and more particularly to methods of stacking dies, and even more particularly to package assemblies including stacked dies and methods of packaging the same.
BACKGROUND
The semiconductor industry has experienced continued rapid growth due to continuous improvements in the integration density of various electronic components (i.e., transistors, diodes, resistors, capacitors, etc.). For the most part, this improvement in integration density has come from repeated reductions in minimum feature size, which allows more components to be integrated into a given area.
These integration improvements are essentially two-dimensional (2D) in nature, in that the volume occupied by the integrated components is essentially on the surface of the semiconductor wafer. Although dramatic improvement in lithography has resulted in considerable improvement in 2D integrated circuit formation, there are physical limits to the density that can be achieved in two dimensions. One of these limits is the minimum size needed to make these components. Also, when more devices are put into one chip, more complex designs are required.
An additional limit comes from the significant increase in the number and length of interconnections between devices as the number of devices increases. When the number and length of interconnections increase, both circuit RC delay and power consumption increase.
Three-dimensional (3D) integrated circuits (ICs) are therefore created to resolve the above-discussed limitations. In a typical formation process of 3D ICs, two wafers, each including an integrated circuit, are formed. The wafers are then bonded with the devices aligned. Deep vias are then formed to interconnect devices in the two wafers.
An alternative scheme for forming 3D ICs is bonding dies. Conventionally, to bond two dies together, each of the dies is bonded onto a package substrate, and then the package substrates are further bonded together to form a package. The known methods include package-in-package (PIP) bonding and package-on-package (POP) bonding. These bonding methods, however, suffer from drawbacks. With the usage of package substrates, which are typically larger than the dies, the size of the final package is increased over any of the dies, which may not be desirable. Further, in conventional packaging schemes, a molding compound is used. However, in some high-performance applications, a significant amount of heat is generated in dies, and the molding compounds, which are often not good thermal conductors, reduce the efficiency in heat dissipation.
SUMMARY
In accordance with one aspect of the embodiment, a method of forming an integrated circuit structure includes bonding top dies onto a bottom wafer, molding a first molding material onto and in between the top dies and the bottom wafer, sawing the bottom wafer, the top dies and the first molding material to form molding units, wherein each of the molding units comprises one of the top dies and a bottom die sawed from the bottom wafer, bonding one of the molding units onto a package substrate, molding a second molding material onto the one of the molding units and the package substrate, and sawing the package substrate and the second molding material to form package-molded units.
Other embodiments are also disclosed.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIGS. 1 through 9 are views of intermediate stages in the manufacturing of a package assembly in accordance with an embodiment; and
FIGS. 10 through 17 are views of intermediate stages in the manufacturing of additional package assemblies, wherein molding compounds are removed from the additional package assemblies.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The making and using of the embodiments are discussed in detail below. It should be appreciated, however, that the embodiments provide many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the package structure.
A novel package structure and the method of forming the same are presented. The intermediate stages of manufacturing an embodiment are illustrated. The variations of the embodiment are then discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.
FIG. 1 illustrates the bonding of top dies 10 onto bottom wafer 12 to form a stacking structure. Before the bonding, bottom wafer 12 may be mounted on carrier 8. Both top dies 10 and bottom wafer 12 may include integrated circuits (not shown), such as complementary metal-oxide-semiconductor (CMOS) transistors, therein. Top dies 10 may be bonded to bottom wafer 12 through flip-chip bonding, although wire bonding may also be used. Top dies 10 may have sizes smaller than that of bottom dies 14 (not shown in FIG. 1, refer to FIG. 6) in bottom wafer 12. An underfill (not shown) may be filled into the space between top dies 10 and bottom wafer 12 to protect the bonds.
Referring to FIG. 2, a wafer-level molding is performed, and molding compound 16 is molded to cover top dies 10 and bottom wafer 12, for example, using spin coating or printing. Molding compound 16 has a flat top surface. In an embodiment, molding compound 16 provides protection of the stacking structure, and is left in the final structure. Accordingly, molding compound 16 may use commonly used molding compound materials such as resins. In alternative embodiments, molding compound 16 is removed in subsequent process steps, and may use reusable materials such as wax, adhesives (glues), and the like. The reusable materials may be collected after the removal and may be used again. Accordingly, molding compound 16 is also referred to as reusable material 16.
In FIG. 2, reusable material 16 provides a temporary flat surface so that a dicing tape may be mounted thereon. FIG. 3 illustrates dicing tape 19 being mounted onto molding compound 16. Dicing tape 19 may include dicing frame 17 therein. After being mounted, dicing tape 19 is adhered to molding compound 16. Next, as shown in FIG. 4, carrier 8 is de-bonded from bottom wafer 12. In an embodiment, carrier 8 is adhered to bottom wafer 12 through an ultra-violet (UV) glue, and hence may be de-bonded by exposing the UV glue to a UV light. In other embodiments, chemicals may be used to remove the adhesive between carrier 8 and bottom wafer 12.
FIG. 5 illustrates the dicing of the stacking structure. A cross-sectional view of a piece of stacked dies (referred to as wafer-level molding unit 18 hereinafter) sawed from the stacking structure is shown in FIG. 6. In the resulting wafer-level molding unit 18, top die 10 is covered by molding compound 16 from the top and sides. However, bottom die 14, which is sawed from bottom wafer 12, shown in FIG. 5, is only covered by molding compound 16 from the top. The edges of molding compound 16 are aligned with edges of bottom die 14. It is observed that since molding compound 16 does not extend onto the edges of bottom die 14, the horizontal size of wafer-level molding unit 18 is equal to the horizontal size of bottom die 14, and is smaller than if molding compound 16 extends to contact the edges of bottom die 14.
Referring to FIG. 7, wafer-level molding units 18 are bonded onto package substrate 20. The bonding may be flip-chip bonding. In which case, bump balls (not shown) may be pre-mounted onto the bottom side of bottom die 14 shown in FIG. 6. Alternatively, the bump balls may be pre-mounted onto the top surface of package substrate 20 before the bonding is performed. In alternative embodiments, wafer-level molding unit 18 may be bonded onto package substrate 20 through wire bonding.
Referring to FIG. 8, a package molding is performed, and molding compound 24 is molded onto wafer-level molding units 18 and package substrate 20. Ball-grid-array (BGA) balls 32, which are solder balls, are also mounted onto package substrate 20. Next, as shown in FIG. 9, a singulation is performed, and package-molded unit 26 is sawed from the structure shown in FIG. 8. The solder balls or wires that are used for bonding top die 10 to bottom die 14, and bottom die 14 to package substrate 30 (a piece sawed from package substrate 20 shown in FIG. 7) are also shown. In an embodiment, molding compounds 16 and 24 comprise different materials, and hence visible interfaces 28 may be observed between molding compounds 16 and 24. In alternative embodiments, molding compounds 16 and 24 are formed of a same material. However, since they are applied at different times, interfaces 28 may still be visible, although they may also sometimes be invisible. Further, package substrate 30 has its edges vertically aligned to edges of molding compound 24.
It is observed that in package-molded unit 26, top die 10 and bottom die 14 do not need to be bonded onto package substrates first before they are bonded together. As a result, fewer process steps and fewer package substrates are needed. The size of the final package is also small.
FIGS. 10 through 12 illustrate an alternative embodiment. The initial steps of this embodiment are essentially the same as shown in FIGS. 1 through 7. It is noted that in the step shown in FIG. 2, molding compound 16 is formed of a reusable material. Accordingly, after the step as shown in FIG. 7 is performed, reusable material 16 in wafer-level molding units 18 is removed, for example, using water or other solvents, wherein the desirable solvents depend on the type of the reusable material. The resulting structure is shown in FIG. 10. As a result of the removal of reusable material 16, top dies 10 and bottom dies 14 are exposed to external environment, such as open air. The removed reusable material may be collected and reused. In the reuse of the reusable material, the process steps as shown in FIGS. 1 and 2 are repeated on other top dies and bottom wafers, and the collected reusable material may be molded again to form another molding compound, which is similar to that shown in FIG. 2.
Next, as shown in FIG. 11, a singulation is performed, and packaged die unit 34 is sawed from the structure shown in FIG. 10. BGA balls 32 are also mounted on package substrate 30, which is a piece sawed from package substrate 20 as shown in FIG. 10. In subsequent process steps, packaged die unit 34 may be bonded to other structures such as a printed circuit board (PCB) 38, as shown in FIG. 12. No molding compound is formed to cover top die 10 and/or bottom die 14. Accordingly, top die 10 and bottom die 14 may have a better heat-dissipating ability. For simplicity, the underfill between top die 10 and bottom die 14 and the underfill between bottom die 14 and package substrate 30 are not illustrated.
FIGS. 13 through 17 illustrate yet another embodiment. The initial steps of this embodiment are essentially the same as shown in FIGS. 1 through 4, and hence are not repeated herein. Again, in the step shown in FIG. 2, molding compound 16 is formed of a reusable material. After the step as shown in FIG. 4 is performed, the step as shown in FIG. 13 is performed, and dicing tape 19 along with dicing frame 17 is de-mounted from reusable material 16. In FIG. 14, dicing tape 40, which may be the same dicing tape 19 as shown in FIG. 13 or a different one, is attached to bottom wafer 12. Therefore, the steps shown in FIGS. 13 and 14 are equivalent to re-mounting a dicing tape from one side to another of the combined structure including bottom wafer 12 and reusable material 16. As a result, reusable material 16 is exposed, and is removed using water or other solvents, for example. The resulting structure is shown in FIG. 15.
Referring to FIG. 16, the stacking structure, including top dies 10 and bottom wafer 12, are sawed, forming stacking dies 44 as shown in FIG. 17. Next, stacking dies 44 are picked up and bonded onto package substrate 20, followed by underfill (not shown) dispensing and curing, wherein the underfill is dispensed into the space between stacking dies 44 and package substrate 20. The details of the bonding are essentially the same as described for FIG. 7, and hence are not repeated herein. After the step as shown in FIG. 17 is performed, the resulting structure is essentially the same as shown in FIG. 10. Subsequently, the process steps as shown in FIGS. 11 and 12 may be performed.
The embodiments have several advantageous features. By bonding top dies directly to bottom wafers without through package substrates, the package size may be reduced, and the process time and the cost are also reduced. The embodiments provide a multi-die stacking solution, in which a temporary flat surface is formed for the processes such as carrier de-bonding, testing, singulation, and the like. Further, with the use of reusable materials, the manufacturing cost is further reduced. The removal of the molding compound also improves the heat-dissipating ability of package assemblies.
An embodiment method of forming an integrated circuit structure includes bonding top dies onto a bottom wafer, molding a first molding material onto and in between the top dies and the bottom wafer, sawing the bottom wafer, the top dies and the first molding material to form molding units, wherein each of the molding units comprises one of the top dies and a bottom die sawed from the bottom wafer, bonding one of the molding units onto a package substrate, molding a second molding material onto the one of the molding units and the package substrate, and sawing the package substrate and the second molding material to form package-molded units.
An embodiment method of forming an integrated circuit structure includes bonding a top die onto a bottom die, molding a first molding material onto and in between the top die and the bottom die, the first molding material in contact with edges of the top die and having edges vertically aligned with respective edges of the bottom die, and bonding a package substrate to the bottom die.
An embodiment method of forming an integrated circuit structure includes bonding a top die onto a bottom die, molding a first molding material onto and in between the top die and the bottom die, the first molding material in contact with a top surface and edges of the top die, the first molding material having edges vertically aligned with respective edges of the bottom die, bonding a package substrate to the bottom die, and molding a second molding material over the first molding material and the package substrate and in between the bottom die and the package substrate, the second molding material engaging sidewalls of the bottom die.
In yet another embodiment, a method of forming an integrated circuit structure is provided. The method includes bonding a top die onto a bottom die and forming a first molding material onto and between the top die and the bottom die, such that the first molding material is in contact with edges of the top die and having edges vertically aligned with respective edges of the bottom die. A package substrate is bonded to the bottom die.
In yet another embodiment, a method of forming an integrated circuit structure is provided. The method includes bonding a top die onto a bottom die and forming a first molding material on the bottom die, the first molding material extending along sidewalls of the top die. The bottom die is bonded to a package substrate, and a second molding material is formed over package substrate, the second molding material extending along sidewalls of the bottom die.
In yet another embodiment, a method of forming an integrated circuit structure is provided. The method includes bonding a top die onto a bottom die of a wafer, forming a first molding material along sidewalls of the top die, and singulating the bottom die from the wafer. The bottom die is bonded onto a packaging substrate, and the packaging substrate is singulated.
Although the embodiments and their advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods, and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps. In addition, each claim constitutes a separate embodiment, and the combination of various claims and embodiments are within the scope of the invention.

## Claims

1. A method of forming an integrated circuit structure, the method comprising:
attaching a bottom die to a carrier using an adhesive;
bonding a top die onto the bottom die such that the bottom die is interposed between the top die and the carrier;
forming a first molding material onto and between the top die and the bottom die;
after forming the first molding material, removing the carrier from the bottom die; and
bonding a package substrate to the bottom die such that the bottom die is interposed between the top die and the package substrate, the bottom die being electrically coupled to the package substrate, the first molding material being in contact with edges of the top die and having edges vertically aligned with respective edges of the bottom die while bonding the package substrate to the bottom die.

2. The method of claim 1, further comprising bonding the package substrate to the bottom die using solder balls.

3. The method of claim 1, further comprising, after bonding the package substrate to the bottom die, forming a second molding material over the first molding material and the bottom die.

4. The method of claim 3, wherein the first molding material and the second molding material are formed of a same material.

5. The method of claim 3, wherein the first molding material and the second molding material are formed of different materials.

6. The method of claim 3, further comprising molding the second molding material directly upon a top surface of the first molding material.

7. The method of claim 1, further comprising, removing the first molding material.

8. The method of claim 7, wherein the removing is performed after bonding the package substrate to the bottom die and prior to singulating the package substrate.

9. The method of claim 1, wherein the bottom die is part of a wafer and further comprising:
after forming the first molding material, singulating the bottom die from the wafer; and
after bonding the package substrate to the bottom die, singulating the package substrate.

10. The method of claim 9, further comprising, after bonding the bottom die onto the package substrate, removing the first molding material.

11. The method of claim 10, wherein the removing the first molding material is performed prior to singulating the package substrate.

12. The method of claim 9, further comprising, prior to bonding the bottom die onto the package substrate, removing the first molding material.

13. The method of claim 9, further comprising, after bonding the bottom die onto the package substrate, forming a second molding material along sidewalls of the bottom die and over a surface of the package substrate.

14. The method of claim 13, wherein the first molding material and the second molding material are formed of a same material.

15. The method of claim 13, wherein the first molding material and the second molding material are formed of different materials.

16. The method of claim 13, further comprising molding the second molding material directly upon a top surface of the first molding material.

17. A method of forming an integrated circuit structure, the method comprising:
attaching a bottom die to a carrier using an adhesive;
bonding a top die onto the bottom die such that the bottom die is interposed between the top die and the carrier;
forming a first molding material onto and between the top die and the bottom die;
after forming the first molding material, removing the carrier from the bottom die;
bonding a package substrate to the bottom die such that the bottom die is interposed between the top die and the package substrate, the bottom die being electrically coupled to the package substrate, the first molding material being in contact with edges of the top die and having edges vertically aligned with respective edges of the bottom die while bonding the package substrate to the bottom die; and
after bonding the package substrate to the bottom die, forming a second molding material over the package substrate, the second molding material extending along sidewalls of the bottom die, sidewalls of the first molding material, and over a top surface of the top die, at least a portion of the first molding material being interposed between the top surface of the top die and the second molding material.

18. The method of claim 17, wherein the first molding material and the second molding material are formed of a same material.

19. The method of claim 17, wherein the first molding material and the second molding material are formed of different materials.

20. The method of claim 17, wherein a width of the top die is less than a width of the bottom die, and the width of the bottom die is less than a width of the package substrate.