# Patent

## Title
Post-passivation interconnect structure and method of forming the same

## Abstract

A semiconductor device includes a conductive layer formed on the surface of a post-passivation interconnect (PPI) structure by an immersion tin process. A polymer layer is formed on the conductive layer and patterned with an opening to expose a portion of the conductive layer. A solder bump is then formed in the opening of the polymer layer to electrically connect to the PPI structure.

## Background
TECHNICAL FIELD
This disclosure relates to the fabrication of semiconductor devices and, more particularly, to the fabrication of a post-passivation interconnect (PPI) structure.
BACKGROUND
Modern integrated circuits are made up of literally millions of active devices such as transistors and capacitors. These devices are initially isolated from each other, but are later interconnected together to form functional circuits. Typical interconnect structures include lateral interconnections, such as metal lines (wirings), and vertical interconnections, such as vias and contacts. Interconnections are increasingly determining the limits of performance and the density of modern integrated circuits. On top of the interconnect structures, bond pads are formed and exposed on the surface of the respective chip. Electrical connections are made through bond pads to connect the chip to a package substrate or another die. Bond pads can be used for wire bonding or flip-chip bonding. Flip-chip packaging utilizes bumps to establish electrical contact between a chip's input/output (I/O) pads and the substrate or lead frame of the package. Structurally, a bump actually contains the bump itself and an “under bump metallurgy” (UBM) located between the bump and an I/O pad.
Wafer level chip scale packaging (WLCSP) is currently widely used for its low cost and relatively simple processes. In a typical WLCSP, post-passivation interconnect (PPI) lines such as redistribution lines (RDLs) are formed on passivation layers, followed by the formation of polymer films and bumps. The interface between the bump and the polymer layer, however, has poor adhesion and suffers moisture attack, which may induce delamination in polymer layers.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-5 are cross-sectional views of illustrating various intermediate stages of a method of forming a semiconductor device having a post-passivation interconnect (PPI) structure in accordance with exemplary embodiments;
FIG. 6 is a cross-sectional view of a PPI structure with an intermetallic compound (IMC) layer in accordance with an exemplary embodiment; and
FIG. 7 is a cross-sectional view of a packaging assembly in accordance with an exemplary embodiment.



DETAILED DESCRIPTION
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 of specific ways to make and use the embodiments, and do not limit the scope of the disclosure. Embodiments described herein relate to the use of bump structures for use with semiconductor devices. As will be discussed below, embodiments are disclosed that utilize a bump structure for the purpose of attaching one substrate to another substrate, wherein each substrate may be a die, wafer, interposer substrate, printed circuit board, packaging substrate, or the like, thereby allowing for die-to-die, wafer-to-die, wafer-to-wafer, die or wafer to interposer substrate or printed circuit board or packaging substrate, or the like. Throughout the various views and illustrative embodiments, like reference numerals are used to designate like elements.
Reference will now be made in detail to exemplary embodiments illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. In the drawings, the shape and thickness may be exaggerated for clarity and convenience. This description will be directed in particular to elements forming part of, or cooperating more directly with, an apparatus in accordance with the present disclosure. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. Further, when a layer is referred to as being on another layer or “on” a substrate, it may be directly on the other layer or on the substrate, or intervening layers may also be present. Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. It should be appreciated that the following figures are not drawn to scale; rather, these figures are merely intended for illustration.
FIGS. 1-5 illustrate various intermediate stages of a method of forming a semiconductor device in accordance with an embodiment. Referring first to FIG. 1, a portion of a substrate 10 having electrical circuitry formed thereon is shown, in accordance with an embodiment. The substrate 10 may comprise, for example, bulk silicon, doped or undoped, or an active layer of a semiconductor-on-insulator (SOI) substrate. The substrate 10 may be provided as a wafer level scale or a chip level scale. Other substrates, such as a multi-layered or gradient substrate may also be used.
Electrical circuitry 12 formed on the substrate 10 may be any type of circuitry suitable for a particular application. In an embodiment, the electrical circuitry 12 includes electrical devices formed on the substrate 10 with one or more dielectric layers overlying the electrical devices. Metal layers may be formed between dielectric layers to route electrical signals between the electrical devices. Electrical devices may also be formed in one or more dielectric layers. For example, the electrical circuitry 12 may include various N-type metal-oxide semiconductor (NMOS) and/or P-type metal-oxide semiconductor (PMOS) devices, such as transistors, capacitors, resistors, diodes, photo-diodes, fuses, and the like, interconnected to perform one or more functions. The functions may include memory structures, processing structures, sensors, amplifiers, power distribution, input/output circuitry, or the like. One of ordinary skill in the art will appreciate that the above examples are provided for illustrative purposes only to further explain applications of some illustrative embodiments and are not meant to limit the disclosure in any manner. Other circuitry may be used as appropriate for a given application.
Also shown in FIG. 1 is an inter-layer dielectric (ILD) layer 14. The ILD layer 14 may be formed, for example, of a low-K dielectric material, such as phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), fluorinated silicate glass (FSG), SiOxCy, Spin-On-Glass, Spin-On-Polymers, silicon carbon material, compounds thereof, composites thereof, combinations thereof, or the like, by any suitable method, such as spinning, chemical vapor deposition (CVD), and/or plasma-enhanced CVD (PECVD). In some embodiments, the ILD layer 14 may comprise a plurality of dielectric layers. Contacts (not shown) may be formed through the ILD layer 14 to provide an electrical contact to the electrical circuitry 12. The contacts may be formed of, for example, one or more layers of TaN, Ta, TiN, Ti, CoW, copper, tungsten, aluminum, silver, or the like, or combinations thereof.
One or more inter-metal dielectric (IMD) layers 16 and the associated metallization layers 18 are formed over the ILD layer 14. Generally, the one or more IMD layers 16 and the associated metallization layers (such as metal lines 18 and vias 19) are used to interconnect the electrical circuitry 12 to each other and to provide an external electrical connection. The IMD layers 16 may be formed of a low-K dielectric material, such as FSG formed by PECVD techniques or high-density plasma CVD (HDPCVD), or the like, and may include intermediate etch stop layers. In some embodiments, one or more etch stop layers (not shown) may be positioned between adjacent ones of the dielectric layers, e.g., the ILD layer 14 and the IMD layers 16. Generally, the etch stop layers provide a mechanism to stop an etching process when forming vias and/or contacts. The etch stop layers are formed of a dielectric material having a different etch selectivity from adjacent layers, e.g., the underlying semiconductor substrate 10, the overlying ILD layer 14, and the overlying IMD layers 16. In an embodiment, etch stop layers may be formed of SiN, SiCN, SiCO, CN, combinations thereof, or the like, deposited by CVD or PECVD techniques.
In some embodiments, the metallization layers may be formed of copper or copper alloys, or of other metals. One skilled in the art will realize the formation details of the metallization layers. Further, the metallization layers include a top metal layer 20 formed and patterned in or on the uppermost IMD layer to provide external electrical connections and to protect the underlying layers from various environmental contaminants. In some embodiments, the uppermost IMD layer may be formed of a dielectric material, such as silicon nitride, silicon oxide, undoped silicon glass, and the like. In subsequent drawings, semiconductor substrate 10, electrical circuitry 12, ILD layer 14, and metallization layers 18 and 19 are not illustrated. In some embodiments, the top metal layer 20 is formed as a part of the top metallization layer on the uppermost IMD layer.
Thereafter, a conductive pad 22 is formed and patterned to contact the top metal layer 20, or alternatively, electrically coupled to top metal layer 20 through a via. In some embodiments, the conductive pad 22 may be formed of aluminum, aluminum copper, aluminum alloys, copper, copper alloys, or the like.
With reference to FIG. 1, one or more passivation layers, such as passivation layer 24, are formed and patterned over the conductive pads 22. In some embodiments, the passivation layer 24 may be formed of a dielectric material, such as undoped silicate glass (USG), silicon nitride, silicon oxide, silicon oxynitride or a non-porous material by any suitable method, such as CVD, PVD, or the like. The passivation layer 24 is formed to cover the peripheral portion of the conductive pad 22, and to expose the central portion of conductive pad 22 through the opening in passivation layer 24. The passivation layer 24 may be a single layer or a laminated layer. One of ordinary skill in the art will appreciate that a single layer of conductive pad and a passivation layer are shown for illustrative purposes only. As such, other embodiments may include any number of conductive layers and/or passivation layers.
Next, a first protective layer 26 is formed and patterned over the passivation layer 24. In some embodiments, the first protective layer 26 may be, for example, a polymer layer, which is patterned to form an opening 27, through which the conductive pad 22 is exposed. In some embodiments, the polymer layer may be formed of a polymer material such as an epoxy, polyimide, benzocyclobutene (BCB), polybenzoxazole (PBO), and the like, although other relatively soft, often organic, dielectric materials can also be used. The formation methods include spin coating or other methods.
Referring to FIG. 2, at least one metallization layer is formed on the first protective layer 26 and fills the opening 27 and then patterned as an interconnect layer 28, which is electrically connected to the conductive pad 22 and may expose a portion of the first protective layer 26. In at least an embodiment, the interconnect layer 28 is a post-passivation interconnect (PPI) structure 28, which may also function as power lines, re-distribution lines (RDL), inductors, capacitors or any passive components. The PPI structure 28 includes an interconnect line region 281 and a landing pad region 28P. In some embodiments, the interconnect line region 281 and the landing pad region 28P may be formed simultaneously, and may be formed of a same conductive material. A bump feature will be formed over and electrically connected to the landing pad region 28P in subsequent processes. In some embodiments, the PPI structure 28 may include copper, aluminum, copper alloy, or other mobile conductive materials using plating, electroless plating, sputtering, chemical vapor deposition methods, and the like. In one embodiment, the PPI structure 28 includes a copper layer or a copper alloy layer. In the embodiment of FIG. 2, the landing region 28P is not directly over the conductive pad 22. In other embodiments, through the routing of PPI structure 28, the landing pad region 28P is directly over the conductive pad 22.
With reference to FIG. 3, a conductive layer 34 is formed on the PPI structure 28. In an embodiment, the conductive layer 34 is a metallic layer comprising tin. In some embodiments, the conductive layer 34 comprises at least one tin layer or at least one tin alloy layer. In some embodiments, the conductive layer 34 may protect the surface of the PPI structure 28 to prevent copper in the PPI structure 28 from diffusing into a bonding material. In some embodiments, the conductive layer 34 may also function as an anti-oxidation layer to prevent the copper surface of the PPI structure 28 from oxidation during subsequent processing. In some embodiments, the conductive layer 34 may further function as an adhesion layer, which improves the interface adhesion between the PPI structure 28 and a subsequently formed polymer layer. Therefore the conductive layer 34 can increase the reliability and bonding strength of a package. In some embodiments, the conductive layer 34 is less than about 3 μm thick, for example, about 0.1 μm to about 3 μm thick.
In some embodiments, the formation method of the conductive layer 34 includes an immersion process or an electroless plating process, in which the conductive layer 34 is formed on the surface of the PPI structure 28 in a self-alignment manner. In one embodiment, the conductive layer 34 is a single-layer structure including an immersion Sn layer. In one embodiment, the conductive layer 34 is a triple-layer structure including an electroless Ni layer, an electroless Pd layer, and an immersion Au layer, which is also known as an ENEPIG structure. For example, the ENEPIG structure may have the electroless Ni layer with a thickness of at least 0.5 μm, the electroless Pd layer with a thickness of at least 0.02 μm and the immersion Au layer with a thickness of at least 0.01 μm. In one embodiment, the conductive layer 34 is a dual-layer structure including an electroless Ni layer and an electroless Pd layer, named an ENEP structure. In one embodiment, the barrier layer 34 is a dual-layer structure including an electroless Ni layer and an immersion Au layer, which is also known as an ENIG structure.
With reference to FIG. 4, a second protective layer 30 is then formed on the substrate 10 to cover the conductive layer 34. In some embodiments, the second protective layer 30 extends to cover the exposed portions of the first protective layer 26. Using photolithography and/or etching processes, the second protective layer 30 is further patterned to form an opening 32 exposing at least a portion of the conductive layer 34 in the landing pad region 28P of the PPI structure 28. The formation methods of the opening 32 may include lithography, wet or dry etching, laser drill, and/or the like. In some embodiments, the second protective layer 30 is formed of a polymer layer, such as an epoxy, polyimide, benzocyclobutene (BCB), polybenzoxazole (PBO), and the like, although other relatively soft, often organic, dielectric materials may also be used. In some embodiments, the second protective layer 30 is formed of a non-organic material selected from un-doped silicate glass (USG), silicon nitride, silicon oxynitride, silicon oxide, and combinations thereof.
As shown in FIG. 5, a solder bump 36 is formed on the exposed potion of the conductive layer 34 so as to electrically connect to the PPI structure 28. In one embodiment, the solder bump 36 is formed by attaching a solder ball in the opening 32 and then thermally reflowing the solder material. In some embodiments, the solder bump 36 may include a lead-free pre-solder layer, SnAg, or a solder material including alloys of tin, lead, silver, copper, nickel, bismuth, or combinations thereof. In an embodiment, the solder bump 36 has a thickness greater than 30 μm. In some embodiments, the s solder bump 36 has a thickness about 40 μm to about 70 μm, although the thickness may be greater or smaller. In some embodiments, the solder bump may be formed by plating a solder layer with photolithography technologies followed by reflowing processes. In some embodiments, the solder bump 36 has a diameter of about 200 μm to about 300 μm. In other embodiments, the solder bump 36 has a diameter of about 100 μm to about 200 μm. In still other embodiments, the solder bump 36 has a diameter of about 50 μm to about 100 μm. In further embodiments, the solder bump 36 has a diameter of about 10 μm to about 50 μm. In some embodiments, the solder bump 36 includes so-called “micro-bumps”.
In some embodiments, during the thermally reflowing process, the tin (Sn) in the conductive layer 34 tends to react with copper (Cu) in the PPI structure 28 to form an intermetallic compound (IMC) layer therebetween. In one embodiment, the conductive layer 34 is fully consumed during the IMC formation, resulting in a Cu—Sn IMC layer 34a between the PPI structure 28 and the second protective layer 30. In some embodiments, the tin (Sn) in the conductive layer 34 tends to react with tin (Sn) in the solder bump 36 and copper (Cu) in the PPI structure 28 to form another intermetallic compound (IMC) layer therebetween. In one embodiment, the conductive layer 34 is fully consumed during the IMC formation, resulting in a Cu—Sn IMC layer 34b between the solder bump 36 and the landing pad regions 28P of the PPI structure 28. In an embodiment, the Cu—Sn IMC layer 34b is thicker than the Cu—Sn IMC layer 34a. Accordingly, a semiconductor device 100 with the PPI structure 28 and the solder bmp 36 is completed.
The presented embodiments provide the conductive layer 34 as an anti-oxidation film on the PPI structure 28 to avoid copper oxidation in processing. The conductive layer 34 also serves as an adhesion film between the PPI structure 28 and the second protective layer 30, which can increase the interface adhesion between the copper layer and the polymer layer and protect the copper layer from moisture attack, and the delamination issue between the polymer layers or the delamination issue between the solder bump and the polymer layer are therefore eliminated. The conductive layer 34 further serves as a protection film between the solder bump 36 and the landing pad region 28P to prevent copper in the PPI structure 28 from diffusing into the solder material. Accordingly, in packaging assembly processes, joint reliability can be increased and bump fatigue can be reduced.
After the bump formation, for example, an encapsulant may be formed, a singulation process may be performed to singulate individual dies, and wafer-level or die-level stacking or the like may be performed. It should be noted, however, that embodiments may be used in many different situations. For example, embodiments may be used in a die-to-die bonding configuration, a die-to-wafer bonding configuration, a wafer-to-wafer bonding configuration, die-level packaging, wafer-level packaging, or the like.
FIG. 7 is a cross-sectional diagram depicting an exemplary embodiment of a flip-chip assembly. The device 100 shown in FIG. 6 is flipped upside down and attached to another substrate 200 at the bottom of FIG. 6. In some embodiments, the substrate 200 may be a package substrate, board (e.g., a printed circuit board (PCB)), a wafer, a die, an interposer substrate, or other suitable substrate. The bump structure is coupled to the substrate 200 through various conductive attachment points. For example, a conductive region 202 is formed and patterned on the substrate 100. The conductive region 202 is a contact pad or a portion of a conductive trace, which is presented by a mask layer 204. In one embodiment, the mask layer 204 is a solder resist layer formed and patterned on the substrate 200 to expose the conductive region 202. The mask layer 204 has a mask opening, which provides a window for solder joint formation. For example, a solder layer including alloys of tin, lead, silver, copper, nickel, bismuth, or combinations thereof may be provided on the conductive region 202. In some embodiments, the device 100 can be coupled to the substrate 200 to form a joint solder structure 206 between the conductive layer 34 and the conductive region 202. An exemplary coupling process includes a flux application, chip placement, reflowing of melting solder joints, and/or cleaning of flux residue. The integrated circuit device 100, the joint solder structure 206, and the other substrate 200 may be referred to as a packaging assembly 300, or in the present embodiment, a flip-chip packaging assembly.
In according with one aspect of the exemplary embodiment, a semiconductor device includes a passivation layer overlying the semiconductor substrate, an interconnect layer overlying the passivation layer and patterned with a line region and a landing pad region, a conductive layer including tin (Sn) formed on the surface of the interconnect layer, a protective layer formed on the conductive layer and having an opening exposing a portion of the conductive layer on the landing pad region, and a solder bump formed on the opening of the protective layer and electrically connected to the conductive layer. In an embodiment, the conductive layer includes an intermetallic compound (IMC) layer. In an embodiment, the IMC layer includes copper and tin. In an embodiment, the conductive layer includes a first IMC layer on the line region of the interconnect layer, and a second IMC layer on the landing pad region. In an embodiment, the second IMC layer is thicker than the first IMC layer. In some embodiments, the protective layer includes a polymer layer, the interconnect layer includes a copper layer or a copper alloy layer. In other embodiments, another protective layer is formed between the interconnect layer and the passivation layer, and the protective layer includes a polymer layer.
In accordance with another aspect of the exemplary embodiment, a packaging assembly includes a semiconductor device electrically coupled to a substrate through a solder structure. The semiconductor device includes a post-passivation interconnect (PPI) structure including a line region and a landing pad region, an intermetallic compound (IMC) layer on the surface of the line region and the landing pad region of the PPI structure, and a protective layer on the IMC layer and exposing a portion of the IMC layer on the landing pad region of the PPI structure. The IMC layer includes tin and copper, and the solder structure is formed on the exposed portion of the IMC layer. In an embodiment, the substrate includes a conductive trace, and the solder structure is formed between the IMC layer and the conductive trace. In an embodiment, the IMC layer on the landing pad region of the PPI structure is thicker than the IMC layer on the line region of the PPI structure. In some embodiments, the protective layer includes a polymer layer, and the PPI structure includes a copper layer or a copper alloy layer.
In accordance with the other aspect of the exemplary embodiment, a method includes providing a semiconductor substrate; forming a passivation layer overlying the semiconductor substrate; forming an interconnect layer overlying the passivation layer, including a line region and a landing pad region; forming a metallic layer including tin on the surface of the interconnect layer using an immersion process; forming a protective layer on the metallic layer; and forming an opening in the protective layer to expose a portion of the metallic layer on the landing pad region of the interconnect layer. In an embodiment, the method further includes forming a solder bump on the opening of the protective layer, and performing a thermally reflowing process on the solder bump. In an embodiment, the method further includes forming an intermetallic compound layer including tin and copper between the interconnect layer and the protective layer. In an embodiment, the method further includes forming an intermetallic compound layer including tin and copper between the interconnect layer and the solder bump.
In the preceding detailed description, the disclosure is described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications, structures, processes, and changes may be made thereto without departing from the broader spirit and scope of the disclosure. The specification and drawings are, accordingly, to be regarded as illustrative and not restrictive. It is understood that the disclosure is capable of using various other combinations and environments and is capable of changes or modifications within the scope of inventive concepts as expressed herein.

## Claims

1. A semiconductor device, comprising:
a semiconductor substrate;
a passivation layer overlying the semiconductor substrate;
an interconnect layer overlying the passivation layer and comprising a conductive line region and a conductive landing pad region;
a conductive layer formed on a surface of the interconnect layer, a sidewall surface of the conductive line region of the interconnect layer and a sidewall surface of the conductive landing pad region, wherein the conductive layer comprises tin (Sn);
a protective layer formed on the conductive layer and comprising an opening exposing a portion of the conductive layer on the conductive landing pad region; and
a solder bump formed in the opening of the protective layer and configured to be electrically connected to the conductive layer.

2. The semiconductor device of claim 1, wherein the conductive layer comprises an intermetallic compound (IMC) layer.

3. The semiconductor device of claim 2, wherein the IMC layer comprises copper and tin.

4. The semiconductor device of claim 2, wherein the conductive layer comprises a first IMC layer on the conductive line region of the interconnect layer, and a second IMC layer on the conductive landing pad region.

5. The semiconductor device of claim 4, wherein the second IMC layer is thicker than the first IMC layer.

6. The semiconductor device of claim 1, wherein the protective layer comprises a polymer layer.

7. The semiconductor device of claim 1, wherein the interconnect layer comprises a copper layer or a copper alloy layer.

8. The semiconductor device of claim 1, further comprising another protective layer between the interconnect layer and the passivation layer.

9. The semiconductor device of claim 8, wherein the another protective layer comprises a polymer layer.

10. A packaging assembly, comprising a semiconductor device electrically coupled to a substrate through a solder structure, the semiconductor device comprising:
a post-passivation interconnect (PPI) structure comprising a conductive line region and a conductive landing pad region;
an intermetallic compound (IMC) layer on a surface of the conductive line region of the PPI structure, a surface of the conductive landing pad region of the PPI structure, a sidewall surface of the conductive line region and a sidewall surface of the conductive landing pad region opposite the sidewall surface of the conductive line region, wherein the IMC layer comprises tin and copper; and
a protective layer on the IMC layer and exposing a portion of the IMC layer on the conductive landing pad region of the PPI structure,
wherein the solder structure is formed on the exposed portion of the IMC layer.

11. The packaging assembly of claim 10, wherein the substrate comprises a conductive trace.

12. The packaging assembly of claim 11, wherein the solder structure is formed between the IMC layer and the conductive trace.

13. The packaging assembly of claim 10, wherein the IMC layer on the conductive landing pad region of the PPI structure is thicker than the IMC layer on the conductive line region of the PPI structure.

14. The packaging assembly of claim 10, wherein the protective layer comprises a polymer layer.

15. The packaging assembly of claim 10, wherein the PPI structure comprises a copper layer or a copper alloy layer.

16. A semiconductor device, comprising:
a semiconductor substrate;
a passivation layer overlying the semiconductor substrate;
an interconnect layer overlying the passivation layer and comprising a conductive line region and a conductive landing pad region;
an intermetallic compound (IMC) layer on a surface of the interconnect layer, a sidewall surface of the conductive line region of the interconnect layer and a sidewall surface of the conductive landing pad region, wherein the IMC layer comprises copper (Cu) and tin (Sn);
a protective layer formed on the IMC layer and comprising an opening exposing a portion of the IMC layer on the conductive landing pad region; and
a solder bump formed in the opening of the protective layer and configured to be electrically connected to the IMC layer.

17. The semiconductor device of claim 16, wherein the IMC layer covers each of the sidewalls of the interconnect layer.

18. The semiconductor device of claim 16, further comprising a top metal layer in the semiconductor substrate, wherein the interconnect layer is electrically connected to the top metal layer.

19. The semiconductor device of claim 18, further comprising a conductive pad between the top metal layer and the interconnect layer.

20. The semiconductor device of claim 16, wherein the IMC layer comprises:
a first IMC portion on the conductive line region of the interconnect layer, and
a second IMC portion on the conductive landing pad region, wherein the second IMC portion is thicker than the first IMC portion.