# Patent

## Title
Improving the strength of micro-bump joints

## Abstract

A device includes a work piece including a metal bump; and a dielectric layer having a portion directly over the metal bump. The metal bump and a surface of the portion of the dielectric layer form an interface. A metal finish is formed over and contacting the metal bump. The metal finish extends from over the dielectric layer to below the interface.

## Background
This application is a continuation of U.S. patent application Ser. No. 14/521,233, entitled “Strength of Micro-Bump Joints” filed on Oct. 22, 2014, which is a continuation of U.S. patent application Ser. No. 12/789,696 (now U.S. Pat. No. 8,901,736, Issued Dec. 2, 2014), entitled “Strength of Micro-Bump Joints” filed on May 28, 2010, which applications are hereby incorporated herein by reference.


TECHNICAL FIELD
This disclosure relates generally to integrated circuits, and more particularly to micro-bump joints with improved strength and method of forming the same.
BACKGROUND
In the manufacturing of wafers, integrated circuit devices such as transistors are first formed at the surfaces of semiconductor substrates. Interconnect structures are then formed over the integrated circuit devices. Bumps are formed on the surfaces of the semiconductor wafers, and are electrically coupled to integrated circuit devices. The semiconductor wafers are sawed into semiconductor chips, also commonly known as dies.
In the packaging of the semiconductor chips, the semiconductor chips are often bonded with other chips or package substrates using flip-chip bonding. Solders are used to join the bumps in the semiconductor chips, or join the bumps in the semiconductor chips to the bond pads in the package substrates. When two semiconductor chips (or one semiconductor chip and a package substrate) are bonded, a solder bump may be pre-formed on one of the bumps or bond pads. A re-flow is then performed so that the solder bump joins the semiconductor chips. Conventional bumps were typically large, and hence micro-bumps were developed. Micro-bump flip-chip interconnections allow for high bonding densities.
FIG. 1 illustrates an exemplary micro-bump on chip 200, with micro-bump 210 formed at a surface of chip 200. Micro-bump 210 includes nickel layer 212, and copper pad 214 on nickel layer 214. Silicon nitride layer 216 covers the edge portion of micro-bump 210, with a center portion of the top surface of copper pad 214 not covered. Electroless nickel electroless palladium immersion gold (ENEPIG) finish 220 is formed to cover copper pad 214, which is exposed through the opening in silicon nitride layer 216. Bump 210 as shown in FIG. 1 will be joined with bump 230 of chip 232 by re-flowing solder cap 234, so that chips 200 and 232 are bonded together.
SUMMARY
In accordance with one aspect of the embodiment, a device includes a work piece including a metal bump; and a dielectric layer having a portion directly over the metal bump. The metal bump and a surface of the portion of the dielectric layer form an interface. A metal finish is formed over and contacting the metal bump. The metal finish extends from over the dielectric layer to below the interface.
Other embodiments are also disclosed.



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:
FIG. 1 illustrates a cross-sectional view of a conventional micro-bump structure;
FIGS. 2 through 6 are cross-sectional views of intermediate stages in the formation of bond structures in accordance with various embodiments; and
FIGS. 7 through 9 are cross-sectional views and top views of intermediate stages in the formation of bond structures in accordance with various alternative 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 of specific ways to make and use the embodiments, and do not limit the scope of the disclosure.
A novel bond structure is provided in accordance with an embodiment. The variations of the embodiment are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.
Referring to FIG. 2, work piece 2, which includes substrate 10, is provided. Work piece 2 may be a device die that includes active devices such as transistors therein, although it may also be a package substrate or an interposer that does not have active devices therein. In an embodiment wherein work piece 2 is a device die, substrate 10 may be a semiconductor substrate such as a silicon substrate, although it may include other semiconductor materials. Semiconductor devices 14 such as transistors may be formed at a surface of substrate 10. Interconnect structure 12, which includes metal lines and vias (not shown) formed therein and connected to semiconductor devices 14, is formed over substrate 10. The metal lines and vias may be formed of copper or copper alloys, and may be formed using the well-known damascene processes. Interconnect structure 12 may include a commonly known inter-layer dielectric (ILD) and inter-metal dielectrics (IMDs).
Metal pad 28 is formed over interconnect structure 12. Metal pad 28 may comprise aluminum (Al), copper (Cu), silver (Ag), gold (Au), nickel (Ni), tungsten (W), alloys thereof, and/or multi-layers thereof. Metal pad 28 may be electrically coupled to semiconductor devices 14, for example, through the underlying interconnection structure 12. Passivation layer 30 may be formed to cover edge portions of metal pad 28. In an exemplary embodiment, passivation layer 30 is formed of polyimide or other known dielectric materials.
Under bump metallurgy (UBM) 32 is formed on, and electrically connected to, metal pad 28. UBM 32 may include a copper layer and a titanium layer (not shown). Copper bump 34 is formed on UBM 32. In an embodiment, copper bump 34 is formed by plating. An exemplary plating process includes forming a blanket UBM layer (not shown, wherein UBM 32 is a part of the UBM layer), forming a mask (not shown) on the UBM layer, patterning the mask to form an opening, plating copper bump 34 into the opening, and removing the mask and the portion of the UBM layer previously covered by the mask. Copper bump 34 may be formed of substantially pure copper or copper alloys.
Metal finish 36 may be formed on copper bump 34, for example, by plating. Metal finish 36 may comprise different materials and layers, and may be used to prevent the oxidation and the diffusion of copper bump 34 to/from solder cap 40. In an embodiment, metal finish 36 is formed of nickel, although other metals may be added. Alternatively, metal finish 36 may be formed of electroless nickel electroless palladium immersion gold (ENEPIG), which includes a nickel layer, a palladium layer on the nickel layer, and a gold layer on the palladium layer. Metal finish 36 may be limited in the region directly over copper bump 34, and is not formed on sidewalls of copper bump 34. Alternatively, metal finish 36 is also formed on the sidewalls of copper bump 34. In subsequent discussion, UBM 32, copper bump 34, and metal finish 36 in combination are referred to as metal bump 38. Solder cap 40 may be formed on metal bump 38, and may comprise a lead-free solder material containing, for example, SnAg, SnAgCu, and the like, although solder cap 40 may also be formed of an eutectic solder material containing, for example, lead (Pb) and tin (Sn).
FIG. 3 illustrates work piece 100, which may be a semiconductor chip, although it may also be a package substrate. Further, the structure shown in FIG. 3 may be a backside or a front side of a silicon chip/die/wafer. Work piece 100 may include metal bump 110. Through-substrate vias (TSVs, not shown) may be formed in work piece 100 for forming three-dimensional integrated circuits (3DICs). Electrical connections such as redistribution lines (not shown) may be formed in work piece 100. Through the electrical connections, metal bump 110 may be electrically coupled to the integrated circuit devices 108 in work piece 100 and/or the TSVs therein.
Metal bump 110 comprises metal layer 112, which may include nickel layer 112A and copper seed layer 112B, for example. Metal layer 112 may act as an UBM and a barrier layer for the formation of metal layer 114. In an exemplary embodiment, the thickness of nickel layer 112A may be between about 200 Å to about 1000 Å, and the thickness of copper seed layer 112B may be between about 1 kÅ to about 5 kÅ. One skilled in the art will realize, however, that the dimensions recited throughout the description are merely examples, and will change if different formation technologies are used. Metal layer 114 is formed over metal layer 112, for example, by electro or electroless plating. Metal layer 114 may be formed of copper (for example, pure or substantially pure copper), aluminum, silver, and alloys thereof. Thickness T of metal layer 114 may be between about 1 μm to about 10 μm. Throughout the description, metal layer 112 and metal layer 114 in combination are referred to micro-bump 110. In a top view, micro-bump 110 may appear to have a rectangular, square, or a circular shape. A horizontal dimension W1 (which may be a length, a width, or a diameter, depending on the shape of micro-bump 110) may be between about 5 μm and about 30 μm, although different dimensional may be used.
After the formation of micro-bump 110, dielectric layer 118 is blanket formed to cover the surface of work piece 100. The formation methods of dielectric layer 118 may include chemical vapor deposition (CVD) methods such as plasma enhanced CVD (PECVD) or other applicable methods. The thickness of dielectric layer 118 may be between about 0.1 μm and about 1 μm. Dielectric layer 118 may be a conformal layer, wherein the thickness of sidewall portions on the sidewalls of micro-bump 110 is close to the thickness of horizontal portions. Further, dielectric layer 118 comprises a first portion 118A directly over micro-bump 110, a sidewall portion 118B, and a second portion 118C not directly over micro-bump 110, wherein portions 118A and 118C are connected to opposite ends of sidewall portion 118B. A patterning is then performed to form opening 120 in dielectric layer 118, with micro-bump 110 being exposed through opening 120. Dielectric layer 118 may be formed of silicon nitride, although other dielectric materials such as silicon oxide, silicon oxynitride, or the like, may be used. After the formation of opening 120, the top surface of micro-bump 110 includes two portions, center portion 110A that is exposed through opening 120, and edge portion 110B that is covered by dielectric layer 118, wherein portion 110A is substantially level with portion 110B. Edge portion 110B of the top surface of micro-bump 110 is also the interface between the bottom surface of dielectric portion 118A and the top surface of micro-bump 110. Accordingly, the interface is also denoted as 110B.
Referring to FIG. 4, an etch is performed using an etchant attacking micro-bump 110/metal layer 114, and not attacking dielectric layer 118. The etch may be isotropic, although an anisotropic effect may be combined with the isotropic effect. Accordingly, a wet etch may be used, for example, using H2SO4 as an etchant. Metal layer 114 is thus recessed, with recess 124 being formed. In an exemplary embodiment, depth D1 of recess 124 is greater than about 1 μm, or even greater than about 2 μm. Depth D1 may also be between about 1,000 Å and about 2 μm. Due to the nature of isotropic etching, recess 124 may include a substantially flat bottom 126, which is also the top surface of the recessed portion of metal layer 114. Further, corners 128 of recess 124 may be rounded. Recess 124 may extend to directly underlying dielectric layer 118 to form undercut 124′, wherein width W2 of undercut 124′ may be greater than about 0.5 μm, or even greater than about 2 μm.
Next, as shown in FIG. 5, metal finish 132 is formed. In an embodiment, metal finish 132 may be formed of nickel, although other metals may be added. Alternatively, metal finish 132 may be formed of electroless nickel electroless palladium immersion gold (ENEPIG), which includes a nickel layer, a palladium layer on the nickel layer, and a gold layer on the palladium layer. The gold layer may be formed using immersion plating. In other embodiments, metal finish 132 may be formed of other known finish materials and methods, including, but not limited to, electroless nickel immersion gold (ENIG), direct immersion gold (DIG), or the like. Depending on the type of metal finish 132, methods including electroless plating, immersion, and the like, may be used to selectively form metal finish 132 starting from the exposed portion of metal layer 114. Accordingly, recess 124, including undercut 124′, is filled. The resulting metal finish 132 includes portion 132A over portion 118A of dielectric layer 118, portion 132B that is level with portion 118A of dielectric layer 118, and portion 132C that is lower than portion 118A of dielectric layer 118 and extending into metal layer 114. Furthermore, portion 132D, which is a part of 132C, is in undercut 124′, and is directly under dielectric layer 118. The top surface of metal finish 132 may be higher than the top surface of dielectric layer 118. Metal finish 132 may extend to directly over portions of dielectric layer 118. Further, the top surface of metal finish 132 may be rounded.
Work piece 2 and work piece 100 may be bonded through flip-chip bonding, as shown in FIG. 6. A reflow process is performed to melt solder cap 40 (FIG. 2). Solder cap 40 thus bonds work piece 2 and work piece 100. In the resulting structure, the interface between metal finish 132 and metal (copper) layer 114 is below the interface 110B, which were conventionally the weak points that prone to cracking and breaking. Instead, the internal bond of metal finish 132, which is much stronger than the interface between metal finish 132 and metal layer 114, is located at the same level as interface 110B. Further, metal finish 132 not only forms a large interface area with metal layer 114, it may also extend directly below dielectric layer 118. Therefore, the bonding between metal finish 132 and metal layer 114 is also strong. The reliability of the entire bond structure is improved.
In the embodiments shown in FIGS. 2 through 6, recess 124 is formed using dielectric layer 118 as a self-aligned mask. In alternative embodiments, an additional mask may be used to form recesses in metal layer 114. FIGS. 7 through 9 illustrate variations of embodiments. Unless specified otherwise, the reference numerals in these embodiments represent like elements in the embodiments illustrated in FIGS. 2 through 6. The initial steps of this embodiment are essentially the same as shown in FIGS. 2 and 3. Next, as shown in FIG. 7, mask 140 is formed on the surface of work piece 100, wherein mask 140 may be a photo resist. Mask 140 is patterned so that some portions of top surface 110A of metal layer 114 are exposed, while some other portions are covered. An etch is then performed to etch exposed portions of metal layer 114 to form recesses 124, as shown in FIGS. 8A through 8C. The etch may be an isotropic etch, an anisotropic etch, or the combinations thereof. In the resulting structure, depth D2 of recesses 124 may be greater than about 1 μm, or even greater than about 2 μm. Depth D2 may also be between about 1,000 Å and about 3 μm.
After the etching, mask 140 is removed. The resulting structure is shown in FIGS. 8A through 8C, wherein FIG. 8A is a cross-sectional view, while FIGS. 8B and 8C are top views in accordance with different embodiments. In the top views, it is observed that recesses 124 may have various different patterns. For example, as shown in FIG. 8B, recesses are isolated holes arranged in a periodic pattern such as an array. In FIG. 8C, recesses 124 are parallel trenches, in which the strips of metal finished 132 are formed. Further, recesses 124 may extend directly under dielectric layer 118, as shown in FIG. 8C using dotted lines.
Referring to FIG. 9, metal finish 132 is formed, the resulting work piece 100 is then bonded to work piece 2. The formation process and the materials of metal finish 132 may be essentially the same as in the embodiments shown in FIG. 5, and hence are not discussed herein. In the resulting structure, it is observed that metal finish 132 may form vias extending into metal layer 114. As a result, the interface area between metal layer 114 and metal finish 132 is increased, resulting in the improvement in the strength of the resulting bond structure. Further, metal finish 132 also extends directly under dielectric layer 118, so that the strength of the bond structure is further improved.
In the embodiments, by extending metal finish 132 below the interface between dielectric layer 118 and micro-bump 110 (metal layer 114), the strength of the resulting bonding is significantly improved. Experiments have been performed to study conventional bond structures in which the interfaces between the metal finishes and the micro-bumps are level with the interface between the dielectric layers and the micro-bumps. In the experiments, two chips bonded through the conventional micro-bumps were pulled away from each other. The experiment results revealed that 80 percent of the bond broke at the interfaces between the metal finishes and the micro-bumps. Accordingly, with the embodiments, the conventional weak points are strengthened, and the reliability of the bond structures is improved.
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 bump structure comprising:
a metal pad comprising a second copper layer overlying a first copper layer, the second copper layer having a recess extending partially into the second copper layer with a portion of the second copper layer being below a lowermost surface of the recess;
a passivation layer over the metal pad, the passivation layer having an opening exposing a portion of the metal pad; and
a metal bump over the metal pad, the metal bump having a lip extending laterally beneath a lowermost surface of the passivation layer, the lip anchoring the metal bump to the passivation layer.

2. The bump structure of claim 1, wherein the lip has a recess depth measured from a sidewall of the passivation layer to a distal end of the lip, the recess depth greater than zero micrometers.

3. The bump structure of claim 1, wherein the passivation layer comprises silicon nitride.

4. The bump structure of claim 1, wherein the metal bump comprises one of nickel, Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG), Electroless Nickel Immersion Gold (ENIG), and Direct Immersion Gold (DIG).

5. The bump structure of claim 1, wherein the metal pad comprises copper, aluminum, silver, and alloys thereof.

6. The bump structure of claim 1, wherein the bump structure is supported by a bottom die, the bottom die configured to be joined with a top die to form a bump joint.

7. The bump structure of claim 1, wherein the metal pad has a recess, the recess extending below the passivation layer, wherein the metal bump is positioned in the recess.

8. A structure comprising:
a top die including a first bump structure; and
a bottom die mounted to the top die, the bottom die including a second bump structure, the second bump structure comprising:
a metal pad, the metal pad comprising:
a nickel layer;
a copper seed layer over and contacting the nickel layer; and
a metal layer over and contacting the copper seed layer, the metal layer having a recess extending partially into the metal layer with a portion of the metal layer being below a lowermost surface of the recess; and

a metal bump over and contacting the metal layer in the recess with a lip extending laterally beneath a lowermost surface of a passivation layer, the lip anchoring the metal bump to the passivation layer, the recess extending under the passivation layer.

9. The structure of claim 8 further comprising:
a solder joint joining the first bump structure to the metal bump.

10. The structure of claim 8, wherein the metal layer comprises copper, aluminum, silver, and alloys thereof.

11. The structure of claim 8, wherein the passivation layer comprises silicon nitride.

12. The structure of claim 9, wherein the metal bump comprises one of nickel, Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG), Electroless Nickel Immersion Gold (ENIG), and Direct Immersion Gold (DIG).

13. A method comprising:
forming an opening in a passivation layer to expose a metal pad on an integrated circuit die;
etching the metal pad to form an undercut beneath the passivation layer; and
forming a bump on the metal pad, the bump having a lip projecting into the undercut, the lip of the bump physically contacting a lower surface of the passivation layer in the undercut for a first width, the first width being greater than 0.5 μm, the bump being configured to be directly connected to an external device.

14. The method of claim 13, wherein the etching comprises a wet etch process.

15. The method of claim 13, wherein the metal pad has a concave top surface after the etching.

16. The method of claim 13 further comprising:
attaching the bump on the integrated circuit die to a bump structure on the external device using a solder joint.

17. The method of claim 13, wherein the step of etching the metal pad to form an undercut beneath the passivation layer is performed using the passivation layer as a self-aligned mask, with no additional mask formed over the passivation layer.

18. The method of claim 13, wherein etching the metal pad to form an undercut beneath the passivation layer further forms a recess in the metal pad, wherein the recess comprises a plurality of isolated portions isolated from each other by un-etched portions of the metal pad.

19. The bump structure of claim 1, wherein the lip of the metal bump contacts the lowermost surface of the passivation layer for a first width, the first width being greater than 0.5 μm.