# Patent

## Title
Conductive contacts having varying widths and method of manufacturing same

## Abstract

A bump structure includes a contact element formed on a substrate and a passivation layer overlying the substrate. The passivation layer includes a passivation opening exposing the contact element. The bump structure also includes a polyimide layer overlying the passivation layer and an under bump metallurgy (UBM) feature electrically coupled to the contact element. The polyimide layer has a polyimide opening exposing the contact element, and the under bump metallurgy feature has a UBM width. The bump structure further includes a copper pillar on the under bump metallurgy feature. A distal end of the copper pillar width, and the UBM width is greater than the pillar width.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Application No. 61/707,644, filed on Sep. 28, 2012, entitled “Metal Bump and Method of Manufacturing Same,” of U.S. Provisional Application No. 61/702,624, filed on Sep. 18, 2012, entitled “Ladd Bump Structures and Methods of Making the Same,” of U.S. Provisional Application No. 61/707,609, filed on Sep. 28, 2012, entitled “Interconnection Structure Method of Forming Same,” and of U.S. Provisional Application No. 61/707,442, filed on Sep. 28, 2012, entitled “Bump Structure and Method of Forming Same,” which applications are hereby incorporated herein by reference.
This application relates to the following co-pending and commonly assigned patent applications: U.S. patent application Ser. No. 13/712,722, filed Dec. 12, 2012 entitled “Bump Structure and Method of Forming Same,” U.S. patent application Ser. No. 13/734,811, filed Jan. 04, 2013 entitled “Ladder Bump Structure and Method of Forming Same,” and U.S. patent application Ser. No. 13/744,361, filed Jan. 17, 2013 entitled “Interconnection Structure and Method of Forming Same,” each of which claim priority to U.S. Provisional Application No. 61/707,644, U.S. Provisional Application No. 61/702,624, U.S. Provisional Application No. 61/707,609, and U.S. Provisional Application No. 61/707,442.


BACKGROUND
Generally, in the development of increasingly denser integrated circuit (IC) packaging, as pitch between adjacent connectors (e.g., metal bumps) decreases, the feature size of under bump metallurgy (UBM) also decreases. Concomitantly, the size of the opening in the overlying layers, such as the polyimide (PI) layer decreases as well. This results in higher contact resistance (Rc).
In order to have more bump cell design flexibility, an innovated bump structure is needed, preferably a structure that also provides meet low stress impact on underlying layers, such as extremely low-k (ELK) dielectric, passivation layers, and the like.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a cross sectional view of an embodiment ladder bump structure;
FIG. 2 is a cross sectional view of an embodiment ladder structure electrically coupled to a substrate trace to form a bump on trace (BOT) assembly;
FIG. 3 is a chart illustrating stress modeling for the embodiment ladder bump structures of FIGS. 1-2; and
FIG. 4 is a flow diagram illustrating a method of forming the embodiment ladder structure of FIG. 1.



Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides 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.
The present disclosure will be described with respect to preferred embodiments in a specific context, namely a ladder bump structure for a bump on trace (BOT) assembly. The concepts in the disclosure may also apply, however, to other semiconductor structures or circuits.
Referring now to FIG. 1, an embodiment ladder bump structure 10 is illustrated. As shown, the ladder bump structure 10 includes a substrate 12, an insulating layer 14, a contact element 16, a passivation layer 18, a polyimide layer 20, an under bump metallurgy (UBM) feature 22, a copper pillar 24, and a solder feature 26.
The substrate 12 may be, for example, a silicon wafer or silicon-containing layer of material. In an embodiment, the substrate 12 may be, for instance, a top layer of an integrated circuit device, such as a top metal layer a passivation layer, or the like. In an embodiment, an integrated circuit (not shown) is formed on and/or in the substrate 12, as is known in the art. Various layers and features of the substrate 12, including transistors, interconnect layers, post passivation interconnects, redistribution layers, and the like are omitted from the figures for the sake of clarity, as they are not necessary to an understanding of the present disclosure.
Still referring to FIG. 1, the substrate 12 supports insulating layer 14. In an embodiment, the insulating layer 14 is an extremely low-k (ELK) dielectric. As shown, the insulating layer 14 generally supports the contact element 16. In an embodiment, the contact element 16 is a metal pad (e.g., an aluminum pad). In an embodiment, the contact element 16 comprises another suitable metal or conductive material.
Still referring to FIG. 1, the passivation layer 18 is formed over the insulating layer 14 and the substrate 12. In other words, the passivation layer 18 overlies the insulating layer 14 and the substrate 12. As shown, the passivation layer 18 extends over outer portions of the contact element 16. In an embodiment, the passivation layer 18 abuts or directly engages the contact element 16. The passivation layer 18 also defines a passivation opening 28, which exposes the underlying the contact element 16. In an embodiment, the passivation opening 28 is between about 12.5 μm to about 37.5 μm.
Still referring to FIG. 1, the polyimide layer 20 is formed over the passivation layer 18. In other words, the polyimide layer 20 overlies the passivation layer 18. As shown, the polyimide layer 20 extends over outer portions of the contact element 16. In an embodiment, portions of the passivation layer 18 are interposed between the polyimide layer 20 and the contact element 16. The polyimide layer 20 also defines a polyimide opening 30, which exposes the underlying the contact element 16. In an embodiment, the polyimide opening 30 is between about 25 μm to about 75 μm.
Still referring to FIG. 1, the UBM feature 22 is electrically coupled to the contact element 16. In an embodiment, the UBM feature 22 is formed from titanium (Ti), titanium nitride (TiN) copper nickel (CuNi), aluminum (Al), and the like to a thickness of, perhaps, about 0.1 μm to about 5 μm, depending on the application. In an embodiment, the UBM feature 22 is mounted so as abut or engage with portions of the polyimide layer 20, the passivation layer 18, and the contact element 16. As shown, the UBM feature 22 defines a UBM width 32. The UBM width 32 (a.k.a., the UBM opening) is generally the lateral or horizontal length of the UBM feature 22 as oriented and depicted in FIG. 1.
Still referring to FIG. 1, the copper pillar 24 (a.k.a., bump) is disposed or mounted on the UBM feature 22. In an embodiment, instead of being formed from copper the pillar 24 may be formed from another material such as, for example, tin, aluminum, or another suitable material. In an embodiment, a metal oxide (e.g., cupric oxide, CuO, cuprous oxide, Cu2O, aluminum oxide, Al2O3, etc.) is formed on sidewalls 34 of the copper pillar 24. A distal end 36 of the copper pillar 24, which is the end furthest from the substrate 12, defines a pillar width 38. The pillar width 38 is generally the lateral or horizontal length of the distal end 36 of the copper pillar 24 as oriented and depicted in FIG. 1. In an embodiment, the distal end 36 is between about 30 μm to about 80 μm. The mounted end 40 of the copper pillar 24, which is the ladder-shaped end closest to the substrate 12, defines a mount width 42. In an embodiment, the mount width 42 of the mounted end 40 is between about 40 μm to about 90 μm.
From the foregoing, it should be recognized that the mount width 42 is greater than the pillar width 38. This condition may be satisfied by, for example, making a mounted end 40 of the copper pillar 24 larger relative to the distal end 36. This condition may also be satisfied by, for example, making the distal end 36 of the copper pillar 24 smaller relative to the mounted end 40 as shown in FIG. 2.
One skilled in the art will recognize that it is not desirable to increase the pitch between adjacent bumps. This means that the pillar width 38 of the distal end 36 should not be increased beyond design dimensions. Hence, in order to get the truncated cone structure for the copper pillar 24, the mount width 42 of the mounted end 40 should be increased in order to obtain the advantageous structure. The wider mount width 42 of the mounted end 40 may also serve to lessen the possibility of delamination between the copper pillar 24 and the polyimide layer 20 and may also serve to lessen stress impact on underlying layers such as underlying ELK layers (e.g., insulating layer 14).
The copper pillar 24 generally has a tapering or sloped profile as depicted in FIGS. 1-2. Indeed, the copper pillar 24 generally has the shape of a truncated cone. In an embodiment, the sidewalls 34 of the copper pillar 24 are linear from the distal end 36 to the mounted end 40 along an entire height (i.e., or length) of the sidewalls 34 of the copper pillar 24.
In an embodiment, a photolithography process is used to shape the copper pillar 24 as shown in FIGS. 1-2. Indeed, in the photolithography process a photoresist may be shaped appropriately in order to produce the copper pillar 24 in the form illustrated in FIGS. 1-2. The ladder profile maybe created by suitable techniques such as the use of multiple photoresist layers with different properties or multiple exposures using different masks.
Still referring to FIG. 1, the solder feature 26 is mounted on or over the copper pillar 24. In an embodiment, the solder feature 26 may be a ball, a bump, or the like, that may be contacted to another electrical device and reflowed to electrically bond the two devices together. By way of example, the solder feature 26 may be connected to a trace 44 on another device 46 (i.e., package, integrated circuit, etc.) and reflowed to produce a ball on trace (BOT) assembly 48 as shown in FIG. 2. Notably, the sloped sidewalls 34 of the copper pillar 24 provide a sufficient distance 50 between the adjacent trace 42 to prevent undesirable bridging.
Another advantageous feature of the illustrated embodiment is shown in FIG. 2. This figure illustrates the connection between the BOT assembly 48 and the trace 44 on the underlying device 46 to which the integrated circuit of substrate 12 has been electrically connected. Device 46 could be, e.g., another integrated circuit device, an interposer, a PCB (Printed Circuit Board), or the like. By having the truncated cone shape, the pillar width 38 of the top of copper pillar 24 is less than the mount width 42 of the bottom of copper pillar 24, as discussed above.
This means that, for a given spacing of adjacent bumps (not shown) on substrate 46, the spacing (i.e. the pitch) between adjacent tops of the bumps is greater. FIG. 2 illustrates that traces 44 on underlying device 46 can be placed closer together, for a given pitch of bumps on substrate 46 with a truncated cone shape then would be possible if the mount width 42 of the mounted end 40 at the top of the bump was the same as the pillar width 38 of the distal end 36 at the bottom of the bump. This feature allows for a tighter spacing of traces 44 and for minimizing the possibility of bridging or shorting between adjacent bump/trace joints.
One skilled in the art will recognize that the specific dimensions for the various widths and spacing discussed herein are matters of design choice and are dependent upon the particular technology node, and application employed.
Another advantageous feature of the present disclosure is that the width of the opening 30 in the polyimide layer 20 is wider than the width of the opening 28 in the passivation layer 18 as shown in FIG. 1. The wide opening 30 provides for a low contact resistance (Rc) and may also contribute to low stress impact on the underlying passivation layer 18. As illustrated, the combination of openings 30, 28 gives copper pillar 24 or bump a ladder profile or stair step profile.
In addition, in an embodiment, the pillar width 38 is greater than the polyimide opening 30. Also, in an embodiment the pillar width 38 is greater than the passivation opening 28. In an embodiment, a ratio of the pillar width 38 to the UBM width 32 is between about 0.75 to about 0.97. In an embodiment, a ratio of the passivation opening 28 to the polyimide opening 30 is between about 0.2 to about 0.5. In an embodiment, a ratio of the polyimide opening 30 to the UBM width 32 is between about 0.2 to about 0.7.
Still referring to FIG. 1, in an embodiment, the UBM width 32 is greater than the pillar width 38, the pillar width 38 is greater than the polyimide opening 30, and the polyimide opening 30 is greater than the passivation opening 28. In addition, the contact element 16 is larger than the polyimide opening 30 and the passivation opening 28. Also, in an embodiment the contact element 16 is approximately the same size as the pillar width 38 but smaller than the UBM width 32.
Referring now to FIG. 3, a chart 52 illustrating stress modeling for the embodiment ladder bump structures of FIGS. 1-2 is provided. As represented in FIG. 3, when the ratio of the polyimide opening 30 relative to the UBM width 32 (which is labeled Ratio of PIO(04)/UMB(05)) is between about 0.2 to about 0.7 (see the shaded boxes), the stress on the insulating layer 14 (i.e., the extremely low-k dielectric) and the UBM feature 22 are relatively low. In addition, when the ratio of the passivation opening 28 relative to the polyimide opening 30 (which is labeled Ratio of Passivation(03)/PIO(04)) is between about 0.2 to about 0.5 (see the shaded boxes), the stress on the passivation layer 18 is relatively low.
Referring now to FIG. 4, a method 60 of forming the embodiment ladder bump structure 10 of FIG. 1 is provided. In block 62, a contact element 16 is formed over the substrate 12. In block 64, a passivation layer is formed over the substrate 12. The passivation layer 18 includes the passivation opening 28 to expose the contact element 16. In block 66, the polyimide layer 20 is formed over the passivation layer 18. The polyimide layer 20 includes the polyimide opening 30 to expose the contact element 16.
In block 68, the UBM feature 22 is electrically coupled with the contact element 16. As noted above, the UBM feature 22 defines the UBM width 32. In block 70, the copper pillar 24 is formed on the UBM feature 22. The distal end 36 of the copper pillar 24 defines the pillar width 38. The UBM width 32 is greater than (i.e., larger) than the pillar width 38. In an embodiment, the solder feature is then mounted over the top of the copper pillar 24.
From the foregoing it should be recognized that embodiment bump ladder structures 10 provide advantageous features. For example, the bump structure (i.e., ladder bump structure) is created for fine pitch bump on trace (BOT) assembly 48 without undesirably bridging. In addition, the embodiment bump ladder structures 10 provide more bump cell design flexibility, provide low stress impact on the layer of silicon, extremely low-k dielectric, passivation, and so on. Moreover, the embodiment bump ladder structures 10 provide lower contact resistance (Rc), and a ladder structure for the copper pillar 24. Also, the embodiment bump ladder structures 10 inhibit or prevent delamination of the extremely low-k dielectric and cracking of the passivation layer 18 and the UBM feature 22. Still further, the embodiment bump ladder structures 10 provide a good assembly yield.
The following references are related to subject matter of the present application. Each of these references is incorporated herein by reference in its entirety:

    
    
        U.S. Publication No. 2011/0285023 of Shen, et al. filed on Nov. 24, 2011, entitled “Substrate Interconnections Having Different Sizes.”
    
    


An embodiment bump structure includes a contact element formed over a substrate, a passivation layer overlying the substrate, the passivation layer having a passivation opening exposing the contact element, a polyimide layer overlying the passivation layer, the polyimide layer having a polyimide opening exposing the contact element, an under bump metallurgy (UBM) feature electrically coupled to the contact element, the under bump metallurgy feature having a UBM width, and a copper pillar on the under bump metallurgy feature, a distal end of the copper pillar having a pillar width, the UBM width greater than the pillar width.
An embodiment bump structure includes a contact element formed over a substrate, a passivation layer overlying the substrate, the passivation layer having a passivation opening exposing the contact element, a polyimide layer overlying the passivation layer, the polyimide layer having a polyimide opening exposing the contact element, the polyimide opening greater than the passivation opening, an under bump metallurgy (UBM) feature overlying portions of the polyimide layer and the passivation layer and electrically coupled with the contact element, and a copper pillar on the under bump metallurgy feature.
An embodiment method of forming a bump structure includes forming a contact element over a substrate, forming a passivation layer over the substrate, the passivation layer having a passivation opening exposing the contact element, forming a polyimide layer over the passivation layer, the polyimide layer having a polyimide opening exposing the contact element, electrically coupling an under bump metallurgy (UBM) feature with the contact element, the under bump metallurgy feature having a UBM width, and forming a copper pillar on the under bump metallurgy feature, a distal end of the copper pillar having a pillar width, the UBM width greater than the pillar width.
While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.

## Claims

1. A bump structure, comprising:
a contact element formed over a substrate;
a passivation layer overlying the substrate, the passivation layer having a passivation opening exposing the contact element;
a polyimide layer overlying the passivation layer, the polyimide layer having a polyimide opening exposing the contact element;
an under bump metallurgy (UBM) feature electrically coupled to the contact element, the under bump metallurgy feature having a UBM width; and
a copper pillar on the under bump metallurgy feature, a distal end of the copper pillar having a pillar width, the UBM width greater than the pillar width, wherein a portion of the copper pillar over a topmost surface of the under bump metallurgy feature has a substantially linear profile.

2. The structure of claim 1, wherein the polyimide width of the polyimide opening is greater than a passivation width of the passivation opening.

3. The structure of claim 1, wherein the pillar width is greater than a polyimide width of the polyimide opening.

4. The structure of claim 1, wherein the pillar width is greater than a passivation width of the passivation opening.

5. The structure of claim 1, wherein the portion of the copper pillar over the UBM has a tapering profile.

6. The structure of claim 1, wherein a ratio of the pillar width to the UBM width is between about 0.75 to about 0.97.

7. The structure of claim 1, wherein sidewalls of the copper pillar are coated with a metal oxide.

8. The structure of claim 1, wherein under bump metallurgy feature abuts portions of the polyimide layer and the passivation layer.

9. The structure of claim 1, wherein the contact element is an aluminum pad and the substrate comprises silicon.

10. The structure of claim 1, wherein an extremely low-k dielectric layer is interposed between at least one of the substrate and the passivation layer and the substrate and the contact element.

11. The structure of claim 1, wherein a solder feature is on the distal end of the copper pillar.

12. A bump structure, comprising:
a contact element formed over a substrate;
a passivation layer overlying the substrate, the passivation layer having a passivation opening exposing the contact element;
a polyimide layer overlying the passivation layer, the polyimide layer having a polyimide opening exposing the contact element, wherein the polyimide opening has a polyimide width at a surface of the polyimide layer closest to the substrate, wherein the passivation opening has a passivation width, and wherein the polyimide width is greater than the passivation width;
an under bump metallurgy (UBM) feature overlying portions of the polyimide layer and the passivation layer and electrically coupled with the contact element; and
a copper pillar on the under bump metallurgy feature.

13. The structure of claim 12, wherein a ratio of the passivation width to the polyimide width is between about 0.2 to about 0.5.

14. The structure of claim 12, wherein the under bump metallurgy has a UBM width, a ratio of the polyimide width to the UBM width is between about 0.2 to about 0.7.

15. The structure of claim 12, wherein a distal end of the copper pillar has a pillar width and the under bump metallurgy has a UBM width, the UBM width greater than the passivation opening, the polyimide opening, and the pillar width.

16. The structure of claim 12, wherein the copper pillar has a tapering linear profile.

17. The structure of claim 12, wherein sidewalls of the copper pillar comprise a metal oxide.

18. A method of forming a bump structure, comprising:
forming a contact element over a substrate;
forming a passivation layer over the substrate, the passivation layer having a passivation opening exposing the contact element;
forming a polyimide layer over the passivation layer, the polyimide layer having a polyimide opening exposing the contact element;
electrically coupling an under bump metallurgy (UBM) feature with the contact element, the under bump metallurgy feature having a UBM width; and
forming a copper pillar on the under bump metallurgy feature, a distal end of the copper pillar having a pillar width, the UBM width greater than the pillar width, wherein a portion of the copper pillar over a topmost surface of the under bump metallurgy feature has a substantially linear profile.

19. The method of claim 18, wherein a ratio of the pillar width to the UBM width is between about 0.75 to about 0.97.

20. The method of claim 18, wherein a ratio of a passivation width of the passivation opening to a polyimide width of the polyimide opening is between about 0.2 to about 0.5 and a ratio of the polyimide width to the UBM width is between about 0.2 to about 0.7.