Solderless interconnection structure and method of forming same

Abstract

An embodiment bump on trace (BOT) structure includes a contact element supported by an integrated circuit, an under bump metallurgy (UBM) feature electrically coupled to the contact element, a metal ladder bump mounted on the under bump metallurgy feature, the metal ladder bump having a first tapering profile, and a substrate trace mounted on a substrate, the substrate trace having a second tapering profile and coupled to the metal ladder bump through direct metal-to-metal bonding. An embodiment chip-to-chip structure may be fabricated in a similar fashion.

Description

BACKGROUND

Generally, the conventional flip chip bumps have vertical or nearly vertical sidewalls and are connected to an underlying trace (such as on a substrate, a printed circuit board, an interposer, another chip, or the like) using a solder reflow process.

The solder joint method forms intermetallic compounds (IMCs) between the metal-solder interface. The IMCs may cause higher electrical resistivity (contact resistance). The higher electrical resistivity leads to increased electromigration, which further increases the contact resistance. In addition, with a small area under bump metallurgy (UBM), the solder/metal electromigration issue may be of greater concern.

As device packaging dimensions shrink, the smaller distance between the bump and an adjacent trace may lead to undesirable bridging during reflow. In addition, as device packaging dimensions shrink interconnect bump sizes also shrink. The reduction in bump size has led to an increase in interconnect resistance and capacitance (RC) that is the cause of signal transmission delay (RC delay). Smaller bump sizes also increases the risk of extremely low-k (ELK) dielectric delamination.

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 bump on trace (BOT) structure;

FIG. 2 is a cross sectional view of an embodiment chip-to-chip structure;

FIG. 3 is a cross section of a metal bump from the BOT structure of FIG. 1 or the chip-to-chip structure of FIG. 2 illustrating a tapering profile and a metal oxide formed on sidewalls;

FIG. 4 is a plan view of the metal bump from the BOT structure of FIG. 1 or the chip-to-chip structure of FIG. 2 illustrating various periphery shapes; and

FIG. 5 is a method of forming the BOT 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

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 or a flip-chip chip scale package (FCCSP). The concepts in the disclosure may also apply, however, to other semiconductor structures or circuits.

Referring now to FIG. 1, an embodiment bump on trace (BOT) structure 10 is illustrated. As shown, the BOT structure 10 includes a contact element 12, an under bump metallurgy (UBM) feature 14, a metal ladder bump 16, a substrate trace 18, and a substrate (SBT) 20. As shown, the contact element 12 is generally supported by the integrated circuit 22 (i.e., chip). In an embodiment, an insulating layer 24 is disposed between the contact element 12 and the integrated circuit 22. In an embodiment, the contact element 12 is an aluminum pad. In an embodiment, the insulating layer 24 comprises an extremely low-k (ELK) dielectric.

In an embodiment, a passivation layer 26 overlies the integrated circuit 22 (and/or the insulating layer 24). As shown in FIG. 1, the passivation layer 26 may have a passivation opening exposing the contact element 12. In an embodiment, a polyimide layer 28 overlies the passivation layer 26. The polyimide layer 28 may have a polyimide opening exposing the contact element 12.

Various layers and features of the integrated circuit 22, 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 UBM feature 14 is electrically coupled to the contact element 12. In an embodiment, the UBM feature 14 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. As shown, various layers including, for example, a passivation layer and a polyimide layer, may be disposed between portions of the UBM feature 14 and the contact element 12.

Still referring to FIG. 1, the metal ladder bump 16 is mounted on the UBM feature 14. In an embodiment, the metal ladder bump 16 has a tapering profile. In an embodiment, the metal ladder bump 16 has a linear tapering profile. Indeed, the metal ladder bump 16 generally has the shape of a truncated cone. In an embodiment, sidewalls 30 of the metal ladder bump 16 are linear from a distal end 32 to a mounted end 34 of the metal ladder bump 16 along an entire height (i.e., or length) of the sidewalls 30 of the metal ladder bump 16.

In an embodiment, the metal ladder bump 16 is formed from a suitable material such as, for example, copper (Cu), nickel (Ni), gold (Au), palladium (Pd), titanium (Ti), and so on, or alloys thereof. The mounted end 34 of the metal ladder bump 16, which is the end closest to the integrated circuit 22, has a greater width than the distal end 32 of the metal ladder bump 16, which is the end furthest from the integrated circuit 22. In an embodiment, the distal end 32 has a width of between about 10 μm to about 80 μm. In an embodiment, the mounted end 34 has a width of between about 20 μm to about 90 μm.

From the foregoing, it should be recognized that the mounted end 34 is wider or larger than the distal end 32. This condition may be satisfied by, for example, making the mounted end 34 of the metal ladder bump 16 larger relative to the distal end 32. This condition may also be satisfied by, for example, making the distal end 32 of the metal ladder bump 16 smaller relative to the mounted end 34.

One skilled in the art will recognize that it is not desirable to increase the pitch between adjacent bumps. This means that the width of the distal end 32 should not be increased beyond design dimensions. Hence, in order to get the truncated cone structure for the metal ladder bump 16, the width of the mounted end 34 should be increased in order to obtain the advantageous structure. The wider width of the mount end 34 may also serve to lessen the possibility of delamination between the metal ladder bump 16 and adjacent layers and may also serve to lessen stress impact on underlying layers such as underlying ELK layers (e.g., insulating layer 24). As shown in FIG. 1, by forming the distal end 32 of the metal ladder bump 16 smaller than the mounted end 34, the distance, d, between the adjacent trace 18 and the bonded substrate trace 18/metal bump 16 is greater to prevent bridging.

In an embodiment, a photolithography process is used to shape the metal ladder bump 16 as shown in FIG. 1. Indeed, in the photolithography process a photoresist may be shaped appropriately in order to produce the metal ladder bump 16 in the form illustrated in FIG. 1. In an embodiment, the metal ladder bump 16 and/or the substrate trace 18 may be formed using an electrolytic plating process.

Still referring to FIG. 1, the substrate trace 18 is generally mounted on the substrate 20. In an embodiment, the substrate trace 18 is formed from copper (Cu), nickel (Ni), gold (Au), aluminum (Al), silver (Ag), and so on, or alloys thereof. As shown, the substrate trace 18 also has a tapering profile. Indeed, a mounted end 36 of the substrate trace 18, which is the end mounted to the substrate 20, has a greater width than a distal end 38 of the substrate trace 18, which is the end furthest from the substrate 20.

In addition to the above, the substrate trace 18 is structurally and electrically coupled to the metal ladder bump 16 through direct metal-to-metal bonding. Indeed, ends of the metal ladder bump 16 and the substrate trace 18 are each free of solder. Because direct metal-to-metal bonding is used instead of solder, the metal ladder bump 16 is operably coupled to the substrate trace without forming any undesirably intermetallic compounds at or proximate the bonded joint. In addition, the absence of solder reduces the potential for undesirably bridging of the substrate trance 18 and/or the metal ladder bump 16 with an adjacent substrate trace 18.

In an embodiment, the direct metal-to-metal bonding process includes several steps. For example, the top portions or surfaces of the metal ladder bump 16 and/or substrate trace 18 are appropriately cleaned to remove debris or contaminants that may detrimentally affect bonding or bonding strength. Thereafter, the metal ladder bump 16 and the substrate trace 18 are aligned with each other. Once aligned, a permanent bonding process such as, for example, a thermo-compression bonding is performed to bond the metal ladder bump 16 to the substrate trace 18. In an embodiment, an annealing step may be performed to increase the bond strength. For example, the metal ladder bump 16 and the substrate trace 18 may be subjected to a temperature of about 100° C. to about 400° C. for about 1 hour to about 2 hours.

Referring now to FIG. 2, an embodiment chip-to-chip structure 40 is illustrated. The chip-to-chip structure 40 is similar to the BOT structure 10 of FIG. 1. However, the chip-to-chip structure 40 of FIG. 2 includes a second metal ladder bump 42 mounted on a second UBM feature 44 of a second integrated circuit 46 instead of the substrate trace 18 on the substrate 20. As shown, the second metal ladder bump 42 also has a tapering profile and is structurally and electrically coupled to the first metal ladder bump 16 through direct metal-to-metal bonding.

In an embodiment, the second integrated circuit 46 includes a second passivation layer 48, a second insulating layer 50 (e.g., ELK dielectric), and a second contact element 52 (e.g., aluminum pad). Various layers and features of the second integrated circuit 46, 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. In addition, the second metal ladder bump 42 may be formed in similar fashion and with similar dimensions relative to the metal ladder bump 16 of FIG. 1.

As shown in FIG. 3, in an embodiment a metal oxide 54 (e.g., cupric oxide, CuO, cuprous oxide, Cu₂O, aluminum oxide, Al₂O₃, etc.) is formed on the sidewalls 30 of the metal ladder bump 16, substrate trace 18, or second metal ladder bump 42. In an embodiment, a ratio of the width (W_T) of the distal end 32 of the metal ladder bump 16 to the width (W_B) of the mounted end 34 of the metal ladder bump 16 is between about 0.75 to about 0.97. In an embodiment, a ratio of the width (W_T) of the distal end 38 of the substrate trace 18 to the width (W_B) of the mounted end 36 of the substrate trace 18 is between about 0.75 to about 0.97. In an embodiment, a ratio of the width (W_T) of the distal end 56 of the second metal ladder bump 42 to the width (W_B) of the mounted end 58 of the second metal ladder bump 42 is between about 0.75 to about 0.97.

As shown in FIG. 4, a periphery of the metal ladder bump 16 (or the second metal ladder bump 42) may take or resemble a variety of different shapes when viewed from above. In an embodiment, the metal ladder bump 16 (or the second metal ladder bump 42) is in the form of a circle, a rectangle, an ellipse, an obround, a hexagon, an octagon, a trapezoid, a diamond, a capsule, and combinations thereof when viewed from the mounted end 34, 58. In FIG. 4, the periphery of the metal ladder bump 16 (or the second metal ladder bump 42) is shown relative to the underlying metal substrate trace 18 (FIG. 1).

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.

Referring now to FIG. 5, an embodiment method 60 of forming the BOT structure 10 of FIG. 1 is provided. In block 62, the contact element 12 is formed on the integrated circuit 22. In block 64, the UBM feature is electrically coupled to the contact element 12. Then, in block 66, the metal ladder bump 16 with the tapering profile is mounted on the UBM feature 14. In block 68, the substrate trace 18 with the tapering profile is mounted on the substrate 20. Thereafter, in block 70, the metal ladder bump 16 and the substrate trace 18 are coupled together through direct metal-to-metal bonding as described herein. Those skilled in the art will recognize that the chip-to-chip structure of FIG. 2 may be formed in similar fashion. Therefore, a detailed description of such a method has not been repeated herein for the sake of brevity.

From the foregoing it should be recognized that embodiment BOT structure 10 and chip-to-chip structure 40 provide advantageous features. For example, without having to rely on solder bonding, the BOT structure 10 and chip-to-chip structure 40 are free of any undesirably intermetallic compounds (IMCs). In addition, the BOT structure 10 and chip-to-chip structure 40 provide lower electrical resistivity, lower risk of electromigration failure, and a significantly reduced interconnect RC delay relative to conventional devices. Moreover, the structures 10, 40 inhibit or prevent delamination of the insulating layer 24, 46 (the ELK dielectric). In addition, the smaller top surface area of the metal ladder bump 16, substrate trace 18, and/or second metal ladder bump 42 provide for easier bonding. Still further, the bonding time and the interfacial seam voids may be reduced using the structures 10, 40 and methods disclosed herein.

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 on trace (BOT) structure includes a contact element supported by an integrated circuit, an under bump metallurgy (UBM) feature electrically coupled to the contact element, a metal ladder bump mounted on the under bump metallurgy feature, the metal ladder bump having a first tapering profile, and a substrate trace mounted on a substrate, the substrate trace having a second tapering profile and coupled to the metal ladder bump through direct metal-to-metal bonding.

An embodiment chip-to-chip structure includes a first contact element supported by a first integrated circuit, a first under bump metallurgy (UBM) feature electrically coupled to the first contact element, a first metal ladder bump mounted on the first under bump metallurgy feature, the first metal ladder bump having a first tapering profile, and a second metal ladder bump mounted on a second under bump metallurgy feature of a second integrated circuit, the second metal ladder bump having a second tapering profile and coupled to the second metal ladder bump through direct metal-to-metal bonding.

An embodiment method of forming a bump on trace (BOT) structure includes forming a contact element on an integrated circuit, electrically coupling an under bump metallurgy (UBM) feature to the contact element, mounting a metal ladder bump on the under bump metallurgy feature, the metal ladder bump having a first tapering profile, mounting a substrate trace on a substrate, the substrate trace having a second tapering profile, and coupling the metal ladder bump and the substrate trace together through direct metal-to-metal bonding.

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 device comprising: a trace extending along a substrate by a first length, the trace having a first end mounted to the substrate and having a second end distal to the first end, the first end having a first width, the second end having a second width, the first length greater than the first width and the second width; anda chip comprising: an integrated circuit;an insulating layer on the integrated circuit;a contact pad in the insulating layer;a passivation layer on the contact pad and the insulating layer;a polyimide layer on the passivation layer and in physical contact with the contact pad;an under bump metallurgy extending through the polyimide layer and the passivation layer, the under bump metallurgy coupled to the contact pad;a bump having a third end mounted to the under bump metallurgy and having a fourth end distal to the third end, the third end having a third width, the fourth end having a fourth width, the fourth width less than the third width, wherein the second end of the trace is directly bonded to the fourth end of the bump, the second end of the trace and the fourth end of the bump being free of solder and of intermetallic compounds; andan oxide layer disposed on an entirety of a first sidewall of the bump, the first sidewall extending from the third end to the fourth end.

2. The device of claim 1, wherein the second width is less than the first width.

3. The device of claim 1, wherein the bump has a second length, the first length greater than the second length.

4. The device of claim 1, wherein sidewalls of the bump taper linearly from the third end to the fourth end.

5. The device of claim 1, wherein the insulating layer comprises an extremely low-k dielectric.

6. The device of claim 1, wherein the trace has a first periphery shape, the bump has a second periphery shape, and the second periphery shape is different from the first periphery shape.

7. The device of claim 1, wherein the oxide layer comprises a metal oxide.

8. A device comprising: a metal trace extending along a major surface of a first substrate, the metal trace having a first end mounted to the first substrate and having a second end distal to the first end, the first end having a greater width than the second end; anda metal bump extending from a second substrate, the metal bump having a third end mounted to the second substrate and having a fourth end distal to the third end, the third end having a greater width than the fourth end, wherein the metal bump and the metal trace are physically and electrically coupled together through direct metal-to-metal bonds, an interface between the metal bump and the metal trace being free of solder, and wherein the metal trace has a greater length than the metal bump in a direction parallel to the major surface of the first substrate; anda first oxide layer disposed on an entirety of a sidewall of the metal bump, wherein the sidewall extends from the third end to the fourth end.

9. The device of claim 8, wherein a first periphery of the second end of the metal trace has a first shape, and a second periphery of the fourth end of the metal bump has a second shape, the second shape different from the first shape.

10. The device of claim 8, wherein the interface between the metal bump and the metal trace is free of intermetallic compounds.

11. The device of claim 8, wherein sidewalls of the metal bump taper linearly from the third end to the fourth end.

12. The device of claim 11 further comprising: a second oxide layer on the sidewalls of the metal trace.

13. The device of claim 8 further comprising: an insulating layer on the second substrate;a contact pad in the insulating layer;a passivation layer on the contact pad and the insulating layer; andan under bump metallurgy extending through the passivation layer, the under bump metallurgy coupled to the contact pad,wherein the metal bump is disposed on the under bump metallurgy.

14. The device of claim 13, wherein the insulating layer comprises an extremely low-k dielectric.

15. The device of claim 8, wherein the fourth end of the metal bump has a greater width than the second end of the metal trace.

16. A device comprising: a metal trace mounted on a first substrate, the metal trace having a first width, the metal trace extending along a surface of the first substrate by a first length, the first length greater than the first width; anda chip comprising: a contact pad over a second substrate;an insulating layer surrounding the contact pad;a polyimide layer over the contact pad and the insulating layer;an under bump metallurgy over the polyimide layer, the contact pad, and the insulating layer; anda metal bump mounted over the second substrate and on the under bump metallurgy, the metal bump having a second width, the second width decreasing along a second direction extending from the second substrate to the first substrate, wherein the under bump metallurgy isolates the metal bump from the polyimide layer, and wherein the metal bump and the metal trace are physically and electrically coupled together without solder.

17. The device of claim 16, wherein the first width decreases along a first direction extending from the first substrate to the second substrate.

18. The device of claim 16, wherein the first width is less than the second width at an interface of the metal bump and the metal trace.

19. The device of claim 16, wherein the metal bump has a second length, the second length greater than the second width.

20. The device of claim 19, wherein the first length is greater than the second length.