# Patent

## Title
Package structures and methods for forming the same

## Abstract

A device includes a redistribution line, and a polymer region molded over the redistribution line. The polymer region includes a first flat top surface. A solder region is disposed in the polymer region and electrically coupled to the redistribution line. The solder region includes a second flat top surface not higher than the first flat top surface.

## Background
BACKGROUND
The fabrication of modern integrated circuits typically involves several steps. Integrated circuits are first fabricated on a semiconductor wafer, which contains multiple duplicated semiconductor chips, each comprising integrated circuits therein. The semiconductor chips are then sawed from the wafer and packaged. The packaging processes have two main purposes: to protect delicate semiconductor chips, and to connect interior integrated circuits to exterior pins.
In conventional packaging processes, a semiconductor chip may be mounted on a package component using flip-chip bonding. An Underfill is dispensed into the gap between the semiconductor chip and the package component to prevent cracks from being formed in solder bumps or solder balls, wherein cracks are typically caused by thermal stresses. The package component may be an interposer that includes metal connections for routing electrical signals between opposite sides. The chip may be bonded to the interposer through direct metal bonding, solder bonding, or the like.
With the increasing demand for more functions, package-on-package (PoP) technology is used to further expand the integration ability of the packages. When the PoP technology is used, packages are stacked. There are various designs regarding how the PoP structures can be formed. By using the PoP technology, the package design becomes more flexible and less complex. Time-to-market is also reduced for product upgrades. With a high degree of integration, the electrical performance of the resulting package is also improved due to the shortened connecting paths between the package components.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIGS. 1 through 10 are cross-sectional views of intermediate stages in the formation of a package in accordance with various embodiments.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The making and using of the embodiments of the disclosure 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, and do not limit the scope of the disclosure.
A package structure and the method of forming the same are provided in accordance with various embodiments. The intermediate stages of forming the package structure are illustrated. The variations of the embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.
FIGS. 1 through 10 illustrate the cross-sectional views of intermediate stages in the manufacturing of a package in accordance with embodiments. FIG. 1 illustrates carrier 20 and release layer 22 formed on carrier 20. Carrier 20 may be a glass carrier, a ceramic carrier, or the like. Release layer 22 may be formed of a polymer-based material, which is capable of being removed later, so that the structures formed over release layer 22 may be demounted from carrier 20. Release layer 22 may be formed of a thermal-release, chemical-release, UV-release, or laser-release material. In some embodiments, release layer 22 is dispensed as a liquid, and is then cured. The top surface of release layer 22 is leveled to have a high degree of co-planarity. In alternative embodiments, release layer 22 is a laminate film, and is laminated onto carrier 20.
Referring to FIG. 2, redistribution layers 24 are formed on release layer 22. Redistribution layers 24 include metal lines 26 and vias 28 interconnecting metal lines 26, wherein metal lines 26 and vias 28 are formed in inter-layer dielectrics (ILDs) 30. Metal lines 26 and vias 28 are referred to as redistribution lines (RDLs) 26/28 hereinafter. The bottom layer of redistribution layers 24 may be in contact with the top surface of release layer 22. In an embodiment, RDLs 26/28 may be formed of metals or metal alloys, such as copper, aluminum, aluminum copper, nickel, or the like. ILDs 30 may be formed of a photo-sensitive material such as polyimide or polybenzoxazole (PBO). Alternatively, ILDs 30 are formed of a nitride such as silicon nitride. In yet other embodiments, ILDs 30 may be formed of an oxide such as silicon oxide, phosphosilicate glass (PSG), borosilicate glass (BSG), boron-doped phosphosilicate glass (BPSG), or the like. In accordance with some exemplary embodiments, the formation of RDLs 26/28 and ILDs 30 may include forming and patterning one of ILDs 30, depositing a metal layer, and then patterning the metal layer. Alternatively, a damascene process may be used, which includes forming ILD 30 as a blank layer, forming openings in the respective ILDs 30, filling a metallic material in the respective ILDs 30, and performing a chemical mechanical polish (CMP).
In an embodiment, metal finish 32 may be formed optionally as a portion of the top layer of RDLs 26/28. In some embodiments, metal finish 32 may be a nickel layer. In other embodiments, metal finish 32 may be formed of materials and methods, including, but not limited to, electro-less nickel immersion gold (ENIG), electro-less nickel electro-less gold (ENEG), electro-less nickel electro-less palladium immersion gold (ENEPIG), direct immersion gold (DIG), immersion tin, or the like.
In alternative embodiments, instead of forming redistribution layers 24 starting from release layer 22, redistribution layers 24 may be pre-formed and attached onto release layer 22. In some embodiments, a package substrate may be used as redistribution layers 24.
Referring to FIG. 3, package component 36 is bonded to metal layers 24. In some embodiments, package component 36 is a device die including active devices such as transistors (not shown) therein. In Alternative embodiments, package component 36 is a package that comprises a device die bonded to an interposer, a package substrate, or the like. The bonding of package component 36 to metal layers 24 may be performed through connectors 38, which may include solder regions, metal pillar, metal pads, and/or the like. The bonding may be solder bonding, metal pillar to metal pillar bonding, Burn-on-Trace (BOT) bonding, or the like.
FIG. 4 illustrates the formation of Z-interconnectors 40, which are named due to their function of connecting features in the direction (Z-direction), which is perpendicular to the major top surface of redistribution layers 24. Z-interconnectors 40 are alternatively referred to as connectors 40 throughout the description. In the resulting structure, at least top portions, and possibly entireties, of Z-interconnectors 40 comprise solder regions that have round top surfaces. In some embodiments, Z-interconnectors 40 are solder balls, and are placed on metal finish 32/RDLs 26/28, and then reflowed. Accordingly, the formation of the respective Z-interconnectors 40 may include placing and reflowing Z-interconnectors 40. In alternative embodiments, Z-interconnectors 40 may have other structures comprising, for example, metal pillars and solder caps on the metal pillars. Accordingly, the formation of the respective Z-interconnectors 40 may include forming a mask layer (not shown), forming openings in the mask layer, plating the metal pillars and the solder caps, and removing the mask layer. The solder caps may then be reflowed. As shown in FIG. 4, one of top RDLs 26 includes a first end portion overlapped by, and bonded to, package component 36, a second end having a recess, with Z-interconnector 40 in the recess, and an intermediate portion connecting the first end portion to the second end portion. Metal finish 32 continuous extends through the first end portion, the second end portion, and the intermediate portion.
FIG. 5 illustrates the molding of polymer region 42, which may comprise a molding compound, a molding underfill, an epoxy, or the like. Polymer region 42 covers package component 36 and Z-interconnectors 40, with top surface 42A of polymer region 42 higher than top surface 36A of package component 36 and top surface 40A of Z-interconnectors 40, so that package component 36 and Z-interconnectors 40 are embedded in polymer region 42.
Referring to FIG. 6, a planarization, such as a grinding process, is performed to remove a top layer of polymer region 42. A top portion of each of Z-interconnectors 40 is also removed during the planarization. As a result, top surfaces 40A of Z-interconnectors 40 are flat, and are level with top surface 42A of polymer region 42. The sidewalls of Z-interconnectors 40, on the other hand, are in contact with polymer region 42, and are rounded. After the planarization, top surface 36A of package component 36 may be level with, or lower than, top surfaces 40A and 42A.
FIG. 7 illustrates the optional recessing of Z-interconnectors 40, wherein after the step as shown in FIG. 6 is formed, an etching step is performed using an etchant that attacks Z-interconnectors 40, and does not attack polymer region 42. Accordingly, recesses 41 are formed, and top surfaces 40A of Z-interconnectors 40 are lower than top surface 42A of polymer region 42. Top surfaces 40A remain to be substantially flat after the recessing of Z-interconnectors 40. In an exemplary embodiment, recessing depth D1, which is the height difference between top surfaces 40A and 42A, is greater than about 10 μm, for example, although different values may also be used.
FIG. 8 illustrates the releasing of carrier 20 from redistribution layers 24 and the formation of connectors 48. In an embodiment, the releasing may be achieved by performing a step (such as a UV light exposure) to cause release layer 22 to lose adhesion, so that carrier 20 may be removed from release layer 22. Release layer 22 may then be removed, resulting in the structure shown in FIG. 8.
FIG. 8 also illustrates the formation of connectors 48 on the bottom layer of redistribution layers 24, wherein connectors 48 and Z-interconnectors 40 are on opposite sides of redistribution layers 24. In some embodiments, connectors 48 are solder balls, and are placed on the bottom layer of redistribution layers 24 and reflowed. A singulation may then be performed along scribe lines 52, so that a plurality of packages is formed, wherein each of the packages may be identical to the illustrated package 50.
Next, as shown in FIGS. 9 and 10, package component 54 is bonded to package 50. As shown in FIG. 9, package component 54 is first placed on package 50, with connectors 56 of package component 54 residing on Z-interconnectors 40. In some embodiments, package component 54 is a package that includes device die 55 bonded to interposer (or package substrate) 57, and hence the resulting structure in FIG. 10 is a Package-on-Package (PoP) structure. In alternative embodiments, package component 54 may be another type of components such as a device die. Connectors 56 that are used for bonding may be solder balls, which have round surfaces, although connectors 56 may have other structures such as metal pillars and solder caps (balls) on the metal pillars. It is observed that since the top surfaces of Z-interconnectors 40 are flat, connectors 56 are unlikely to shift on Z-interconnectors 40. Furthermore, in the embodiments wherein the top surfaces of Z-interconnectors 40 are recessed from top surface 42A of polymer region 42, connectors 56 are better confined in recesses 41.
After the surface of connectors 56 are in contact with top surfaces 40A of Z-interconnectors 40, a reflow is performed to form connectors 58, which join package component 54 to package 50. The resulting structure is shown in FIG. 10.
It is observed that in the resulting structure in FIG. 10, recesses 41 as in FIG. 7 may further confine the molten solder during the reflow, so that the assembly yield for the packaging process is increased.
In accordance with embodiments, a device includes a redistribution line, and a polymer region molded over the redistribution line. The polymer region includes a first flat top surface. A solder region is disposed in the polymer region and electrically coupled to the redistribution line. The solder region includes a second flat top surface not higher than the first flat top surface.
In accordance with other embodiments, a device includes a redistribution line, and a molding compound molded over the redistribution line, wherein the molding compound comprises a first flat top surface. A solder ball is disposed in the molding compound, wherein the solder ball is over and bonded to the redistribution line. The solder ball includes round sidewalls and a second flat top surface not higher than the first flat top surface. A die is disposed over and bonded to the redistribution line, and is located in the molding compound. A top surface of the die is not higher than the first flat top surface.
In accordance with yet other embodiments, a method includes forming a connector over and electrically coupled to a redistribution line, wherein the connector include a solder region having a round top surface. A polymer region is molded to cover the connector and the redistribution line. The polymer region and the solder region are then grinded, until the solder region forms a first top surface level with a second top surface of the polymer region.
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 embodiments 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, 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 disclosure. 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 disclosure.

## Claims

1. A device comprising:
a bottom package comprising:
a redistribution line forming a first recess and a second recess, with an entirety of the redistribution line formed of a non-solder conductive material, wherein the redistribution line comprises:
a first end portion comprising a portion underlying the first recess, and additional portions forming sidewalls of the first recess;
a second end portion higher than the first end portion; and

a polymer region molded over the redistribution line, wherein the polymer region comprises a flat top surface, and the polymer region extends into the second recess;
a device die molded in the polymer region, wherein the device die overlaps, and is bonded to, the second end portion of the redistribution line; and
a solder region comprising a portion in the polymer region, wherein the solder region extends into the first recess and is in contact with a bottom and sidewalls of the first end portion; and

a top package bonded to the bottom package through the solder region.

2. The device of claim 1, wherein the top package comprises:
a package substrate; and
a device die bonded to the package substrate.

3. The device of claim 1, wherein a top surface of the device die is lower than the flat top surface.

4. The device of claim 1, wherein a back surface of the device die is coplanar with the flat top surface of the polymer region.

5. The device of claim 1, wherein the redistribution line is in a top redistribution layer of a plurality of redistribution layers, and wherein the device further comprises a connector attached to a bottom layer of the plurality of redistribution layers.

6. The device of claim 1, wherein the redistribution line further comprises:
an intermediate portion having a first end physically connected to the first end portion, and a second end physically connected to the second end portion, wherein an entire top surface of the intermediate portion of the redistribution line that continuously connects the first end portion to the second end portion of the redistribution line is in physical contact with the polymer region.

7. The device of claim 1, wherein the polymer region comprises:
a first flat top surface;
a recess extending from the first flat top surface to a second flat top surface of the solder region; and
a curved sidewall surface in the recess, wherein the curved sidewall surface of the polymer region has a first end connected to the first flat top surface, and a second end connected to the second flat top surface.

8. A device comprising:
a bottom package comprising:
a redistribution line forming a first recess and a second recess, wherein the redistribution line comprises a first portion underlying the first recess, and second portions forming sidewalls of the first recess;
a molding compound molded over the redistribution line, wherein the molding compound comprises a flat top surface, and the molding compound extends into the second recess;
a solder region comprising:
a first portion in the molding compound, wherein the solder region extends into the first recess and is in contact with a top surface of the first portion and sidewall surfaces of the second portions of the first recess; and
a second portion over the flat top surface, wherein the first portion and the second portion of the solder region are continuously connected to each other, and the first portion of the solder region is in physical contact with the second portion of the solder region;

a die over and bonded to the redistribution line through flip-chip bonding, wherein the die is located in the molding compound, wherein a top surface of the die is not higher than the flat top surface; and

a top package bonded to the bottom package through the solder region.

9. The device of claim 8, wherein the top surface of the die is lower than the flat top surface.

10. The device of claim 8, wherein the top surface of the die is level with the flat top surface.

11. The device of claim 8, wherein no non-solder region separates the first portion and the second portion of the solder region from each other.

12. The device of claim 8, wherein the redistribution line comprises:
a first end portion overlapped by, and bonded to, the die;
a second end portion, with the first recess in the second end portion; and
an intermediate portion having a first end physically connected to the first end portion, and a second end physically connected to the second end portion, wherein an entire top surface of the intermediate portion of the redistribution line that continuously connects the first end portion to the second end portion of the redistribution line is in physical contact with the molding compound.

13. A device comprising:
a bottom package comprising:
a redistribution line forming a first recess and a second recess, with an entirety of the redistribution line formed of a non-solder conductive material, wherein the redistribution line comprises:
a first end portion comprising a portion underlying the first recess, and additional portions forming sidewalls of the first recess;
a second end portion higher than the first end portion; and

a polymer region molded over and in physical contact with the redistribution line, wherein the polymer region comprises a flat top surface, and the polymer region extends into the second recess;
a device die molded in the polymer region, wherein the device die overlaps, and is bonded to, the second end portion of the redistribution line, wherein the flat top surface of the polymer regions extends to opposite sides of the device die, and forms a continuous top surface with a top surface of the device die; and
a solder region comprising a portion in the polymer region, wherein the solder region extends into the first recess and is in contact with a bottom and sidewalls of the first end portion; and

a top package bonded to the bottom package through the solder region.

14. The device of claim 13, wherein the top package comprises:
a package substrate; and
a device die bonded to the package substrate.

15. The device of claim 13 further comprising an additional solder region bonding the top package to the bottom package, wherein the polymer region is further in contact with the additional solder region.

16. The device of claim 13, wherein the redistribution line is in a top redistribution layer of a plurality of redistribution layers, and wherein the device further comprises a connector attached to a bottom layer of the plurality of redistribution layers.

17. The device of claim 13, wherein the redistribution line further comprises:
an intermediate portion having a first end physically connected to the first end portion, and a second end physically connected to the second end portion, wherein an entire top surface of the intermediate portion of the redistribution line that continuously connects the first end portion to the second end portion of the redistribution line is in physical contact with the polymer region.

18. The device of claim 13, wherein the polymer region comprises:
a first flat top surface;
a recess extending from the first flat top surface to a second flat top surface of the solder region; and
a curved sidewall surface in the recess, wherein the curved sidewall surface of the polymer region has a first end connected to the first flat top surface, and a second end connected to the second flat top surface.

19. The device of claim 13 further comprising an additional solder region at a bottom of the bottom package, wherein the additional solder region is electrically coupled to the solder region.