DE102013105400B4 - Method of forming a bump structure - Google Patents

Abstract

A method of forming a bump structure, comprising: forming an under bump metallurgy (UBM) structure (14) on a substrate (12); Forming a copper pillar (16) on the UBM structure (14), wherein the copper pillar (16) is shaped to have a tapered arcuate profile (22), the sidewalls of the copper pillar (16 ) are concave along the entire length of the copper pillar; Forming a metal cap (18) on the copper pillar (16); and forming a solder structure (20) on the metal cap (18); the underside (24) of the copper pillar (16) facing the UBM structure (14) being wider than its top surface (26) facing the metal cap (18); and the metal cap is wider than the top of the copper pillar, where the copper pillar (16) abuts the metal cap (18); wherein forming the copper pillar, the metal cap, and the solder structure further comprises: applying a photoresist to the UBM structure and the substrate, creating a ladder bump profile opening in the photoresist through a photolithography process; Forming the copper pillar and the metal cap in the opening in the photoresist; Forming the solder structure on the metal cap; Removing the photoresist and etching the UBM structure, wherein the etching of the UBM structure also creates an overhang of the metal cap at the top of the copper pillar; the method further comprising: forming a copper oxide film on the sidewalls of the copper pillar (16); Mounting the solder pattern to a second substrate (60) and inserting a underfill (64) between the substrate (12) of the bump structure and the second substrate (60), wherein the copper oxide film adheres to the underfill (64).

Description

background

In a typical Copper (Cu) Pillar Bump process, solder with or without nickel (Ni) below always has a larger critical dimension (CD) than the bottom of the Cu pillar due to Cu overetching. This bump profile with large top and small bottom is critical for fine-pitch mounting yield, especially in bump-on-trace (BOT) assembly. Since an upper Under Bump Metallurgy (UBM) is closer to an adjacent common Cu track, there is a greater risk that the Lot part will undesirably cause a bump to trace bridge.

In addition, a conventional bumping process has an inversion tin (IT) layer along the Cu-pillar sidewall. This inversion tin may undesirably increase the risk of delamination due to poor adhesion to or on a composite material (i.e., underfill material).

The invention provides a method according to claim I before. Embodiments are specified in the subclaims.

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:

1 a cross-sectional view of an embodiment of a ladder-bump structure shows;

2 Figure 12 is a cross-sectional view of the embodiment of the ladder-bump structure providing exemplary dimensions;

3 is a table showing results of simulated stress;

4 the embodiment of the ladder-bump structure of i representing a mechanical bump-on-trace (BOT) connection; and

5 FIG. 10 is a flowchart showing a method of forming the embodiment of the ladder bump structure of FIG 1 represents.

Corresponding numbers and symbols in the various figures generally refer to corresponding parts unless otherwise specified. The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale.

Detailed description of exemplary embodiments

The preparation and use of the presently preferred embodiments are set forth in detail below. It should be understood, however, that the present disclosure provides many applicable inventive concepts that may be embodied in a wide variety of specific contexts. The specific embodiments discussed are for illustration only and do not limit the scope of the disclosure.

The present invention will be described with reference to preferred embodiments in a specific context, ladder-bump structure for bump-on-trace (BOT) assembly. However, the concepts of the disclosure may also apply to other semiconductor structures or circuits.

In i is an embodiment of a ladder-bump structure 10 shown. As shown, the ladder bump structure is included 10 a substrate 12 , an under bump metallurgy (UBM) structure 14 , a copper pillar 16 , a metal cap 18 and a solder structure 20 , The substrate 12 can z. B. be a silicon wafer or a silicon-containing material layer. In some embodiments, an integrated circuit (not shown) is on and / or in the substrate 12 as is known in the art. Numerous layers and structures of the substrate 12 , including transistors, interconnect layers, dielectric layer, passivation layers, post-passivation interconnects, rewiring layers, and the like, have been omitted from the figures for the sake of clarity, as they are not necessary to the understanding of the present disclosure.

Continue to the 1 Referring to, the substrate carries 12 the UBM structure 14 on or in the substrate 12 can be mounted. As shown, the UBM structure carries 14 generally the copper pillar 16 , The copper pillar 16 has a constricting profile 22 (also known as an arched, tapered profile) whose width is from a bottom 24 to a top 26 and the copper pillar 16 , as in I shown, decreases. In other words, the bottom is 24 of the copper pillar 16 wider than the top 26 of the copper pillar 16 , As used herein, the "underside" of the copper pillar 16 the part that is the substrate 12 particularly close as the "top" of the copper pillar 16 the part is that of the substrate 12 farthest away. In one embodiment, the copper pillar 16 side walls 28 on the from the bottom 24 to the top 26 along the entire height 30 (ie, or length) of the sidewalls 28 of the copper pillar 16 are generally concave.

Unchanged to the 1 Referring to, the metal cap 18 from the copper pillar 16 worn or attached to it. In one embodiment, the metal cap 18 made of nickel (Ni). Likewise, other metals can be used for the metal cap 18 suitable to be used. As shown, the metal cap towers over 18 generally the copper pillar 16 where the metal cap is 18 and the copper pillar 16 meet or contiguous. In other words, the metal cap 18 wider than the copper pillar 16 where the copper pillar 16 to the metal cap 18 near the top 16 of the copper pillar 16 followed.

In one embodiment, the process of etching creates or induces the UBM structure 14 the overhang of the metal cap 18 and / or the constricting profile 22 of the copper pillar 16 , A ladder photoresist (photoresist (PR)) is sprayed onto a silicon (Si) wafer coated with a UBM film. A well-controlled photolithography process is used to create a ladder-bump profile having smaller upper and lower critical dimensions (CD). After the ladder bump profile is created, a normal bump process, copper plating and the metal cap in the photoresist opening, removal of surrounding photoresist, and etching of exposed and unwanted UBM film by chemical etching, follow so-called ladder bump existing on the wafer comprises. The overhang of the metal cap 18 provides a larger contact area and has strong adhesion to or on, for example, a molded underfill (MUF) or underfill composite.

Continue to the 1 Referring to, the solder structure becomes 20 on or over the metal cap 18 assembled. In one embodiment, the solder structure 20 a ball, a bump or the like, which are contacted with another electrical component and can be reflow-soldered to electrically connect the two components together.

Now on the 2 In one embodiment, a ladder bump structure is shown 10 the ratio of the width 38 of the copper pillar 16 halfway up 30 (ie length) of the copper pillar 16 to the width 36 the bottom 24 of the copper pillar 16 between about 0.9 to about 0.93. Additionally, in one embodiment, the ratio of the width is 40 of the copper pillar 16 on a quarter of the height 30 of the copper pillar 16 (from the top 26 of the copper pillar 16 measured) to the width 36 the bottom 24 of the copper pillar 16 between about 0.92 to about 0.94.

3 delivers the results 42 Simulation studies that have been carried out to evaluate the possible improvement in load capacity, especially stress, on underlying and / or surrounding layers, such as Extra Low K (ELK) dielectric layers, passivation layers, UBM layers, and polyimide layers, typically one Form part of a packaged integrated circuit structure. Note that the simulation results suggest that the in the 1 and 2 shown structure will lead to a reduced stress in the underlying and / or surrounding layers and structures.

4 illustrates two exemplary ladder-bump structures 10 which are mounted to the substrate 12 (which may include one or more integrated circuit devices) on an underlying substrate 60 to join. In the illustrated embodiment, the substrate is 60 mounted using a Bond on Trace (BOT) solution. Using the illustrated ladder-bump structures 10 a fine pitch assembly can be achieved. It can be an assembly yield, reducing the rate of bridging from solder to substrate web 62 and the void risk of a bump-to-bump mold underfill (MUF) 64 -, be achieved. Further, the exemplary ladder bump process does not require any on the sidewalls 28 of the copper pillar 16 formed inversion tin (Inversion Tin (IT)) coating. This reduces the costs. Furthermore, the lack of IT coating allows for a copper oxide (CuO) film along sidewalls 28 of the copper pillar 16 train. This CuO film has a higher adhesion to or on the composite material 64 and / or the underfill as inversion tin, and also improves durability, such as moisture resistance, as evidenced by performance in Highly Accelerated Stress Testing (HAST).

Now on the 5 Referring to, a method 70 for forming the embodiment of a ladder-bump structure 10 from 1 provided. At block 72 becomes the UBM structure 14 on the Si substrate 12 applied. At block 74 For example, a special photoresist (photoresist (PR)) called Ladder-PR is sprayed on a Si wafer with UBM film applied. At block 76 For example, a well-controlled photolithography process is used to create the ladder bump profile having a smaller upper and lower critical dimension (CD). After that, at block 78 the copper pillar 16 grown in the ladder PR opening. In particular, the copper pillar 16 a tapered arcuate profile (ie, the constricting profile). After that, the metal cap 18 and the lot 20 on the copper pillar 16 generated. Then at block 80 the surrounding PR is removed and etching of exposed and unwanted UBM film is performed by chemical etching. At block 82 becomes the so-called ladder bump on the wafer 12 educated. It goes without saying that additional or intermediate steps to the procedure 70 can be added or included in other embodiments.

From the foregoing, it should be understood that the embodiments of the bump ladder structures 10 provide advantageous features or properties. For example, the bump structure (ie, ladder bump structure) for fine pitch bond-on-trace (BOT) assembly is achieved with yield improvement by bridging solder-to-substrate web (solder to substrate trace (SBT)) and / or voids in the bump-to-bump mold underfill (MUF) are avoided. Further, the exemplary bump structure is constructed of a Ni overhang / Cu pillar crimp profile, the dimensions of which are larger at the bottom than at the top.

The innovative bumping process described herein omits a conventional inversion tin (IT) layer around the copper pillar and the bump surface has some CuO on the Cu sidewall which is responsible for higher adhesion to or on top of the Cu Composite or underfill material provides.

Advantages of some embodiments described include that the solder with Ni (or other metal) has a larger dimension than a top of the Cu pillar. An exemplary UBM etch process induces Ni overhang and Cu-pillar constriction. The Ni overhang provides for a larger contact area and has stronger adhesion to or on a composite material, such as underfill or composite material (molding compound). The exemplary ladder-bump structure has a wider lower than upper dimension of Ni and the Cu-pillar crimp profile can reduce Extremely Low-k Dielectric (ELK), passivation, UBM, and polyimide (PI) stress. In addition, the illustrated embodiments provide a larger contact area for copper pillar / composite adhesion enhancement. Another advantage may be that the Cu pillar does not have a conventional inversion tin (IT) coating and instead uses copper oxide (CuO) on the sidewalls to improve reliability testing reliability.

The following references relate to methods for making bump structures: US 2011/0285 023 A1 . US 2006/0223 313 A1 . US 2012/0049 346 A1 . US 2012/0007 230 A1

Claims (5)

A method of forming a bump structure, comprising: forming an underbump metallurgy (UBM) structure ( 14 ) on a substrate ( 12 ); Forming a copper pillar ( 16 ) on the UBM structure ( 14 ), the copper pillar ( 16 ) is designed so that it has a tapered arcuate profile ( 22 ), wherein the side walls of the copper pillar ( 16 ) are concave along the entire length of the copper pillar; Forming a metal cap ( 18 ) on the copper pillar ( 16 ); and forming a solder structure ( 20 ) on the metal cap ( 18 ); where the bottom ( 24 ) of the copper pillar ( 16 ), the UBM structure ( 14 ) is wider than its top ( 26 ), the metal cap ( 18 facing); and the metal cap is wider than the top of the copper pillar, where the copper pillar ( 16 ) to the metal cap ( 18 ) adjoins; wherein forming the copper pillar, the metal cap, and the solder structure further comprises: applying a photoresist to the UBM structure and the substrate, creating a ladder bump profile opening in the photoresist through a photolithography process; Forming the copper pillar and the metal cap in the opening in the photoresist; Forming the solder structure on the metal cap; Removing the photoresist and etching the UBM structure, wherein the etching of the UBM structure also creates an overhang of the metal cap at the top of the copper pillar; the method further comprising: forming a copper oxide film on the sidewalls of the copper pillar ( 16 ); Mounting the solder structure on a second substrate ( 60 ) and introducing an underfill ( 64 ) between the substrate ( 12 ) of the bump structure and the second substrate ( 60 ), wherein the copper oxide film on the underfill ( 64 ) liable. Method according to claim 1, wherein the ratio of the width ( 38 ) of the copper pillar ( 16 ) at half the height of the copper pillar ( 16 ) to the width ( 36 ) of the underside ( 24 ) of the copper pillar ( 16 ) is between about 0.9 and about 0.93. Method according to claim 1, wherein the ratio of the width ( 40 ) of the copper pillar ( 16 ) at a quarter height of the copper pillar measured from its top ( 26 ), to the width ( 36 ) of the underside ( 24 ) of the copper pillar is between about 0.92 and about 0.94. Method according to one of the preceding claims, wherein the metal cap ( 18 ) is formed of nickel. Method according to one of the preceding claims, wherein side walls of the copper pillar ( 16 ) are free from any tin coating.

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