# Patent

## Title
Forming low stress joints using thermal compress bonding

## Abstract

A method of forming a bump structure includes providing a first work piece including a dielectric layer having a top surface; placing a second work piece facing the first work piece; placing a heating tool contacting the second work piece; and heating the second work piece using the heating tool to perform a reflow process. A first solder bump between the first and the second work pieces is melted to form a second solder bump. Before the second solder bump solidifies, pulling the second work piece away from the first work piece, until an angle formed between a tangent line of the second solder bump and the top surface of the dielectric layer is greater than about 50 degrees, wherein the tangent line is drawn at a point where the second solder bump joins the dielectric layer.

## Background
TECHNICAL FIELD
This disclosure relates generally to integrated circuit manufacturing processes, and more particularly to forming low-stress joints using thermal compress bonding.
BACKGROUND
Integrated circuits are formed on semiconductor wafers, which are then sawed into semiconductor chips. The semiconductor chips are then bonded onto package substrates. FIGS. 1 through 3 illustrate cross-sectional views of intermediate stages in a conventional bonding process. Referring to FIG. 1, package substrate 100 is provided, and bond pads 108 are at the surface of package substrate 100. Chip 102 is picked up and flipped over, with solder bumps 104 at a surface of chip 102 facing down. Flux 106 is also applied on solder bumps 104.
Next, as shown in FIG. 2, chip 102 is placed on package substrate 100, with solder bumps 104 being placed against bond pads 108. Package substrate 100 and chip 102 then go through a reflow process, during which package substrate 100 and chip 102, and solder bumps 104 are heated. The resulting bonded structure is shown in FIG. 3. Due to the weight of chip 102 and solder bumps 104, solder bumps 104, when melted, are crushed, and width W1 of solder bumps 104 increases.
One of the problems found in the conventional bond structures is that cracking often occurs in solder bumps 104 after the bonding process, particularly near wherein solder bumps 104 join solder resists 112 and 114. In addition, due to the increase in width W1 of solder bumps 104, the spacing between neighboring solder bumps 104 need to be increased to prevent solder bumps 104 from shorting with each other.
SUMMARY
In accordance with one aspect, a method of forming a bump structure includes providing a first work piece including a dielectric layer having a top surface; placing a second work piece facing the first work piece; placing a heating tool contacting the second work piece; and heating the second work piece using the heating tool to perform a reflow process. A first solder bump between the first and the second work pieces is melted to form a second solder bump. Before the second solder bump solidifies, pulling the second work piece away from the first work piece, until an angle formed between a tangent line of the second solder bump and the top surface of the dielectric layer is greater than about 50 degrees, wherein the tangent line is drawn at a point where the second solder bump joins the dielectric layer.
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:
FIGS. 1 through 3 illustrate cross-sectional views of intermediate stages in a conventional bonding process;
FIGS. 4 through 8B are cross-sectional views of intermediate stages in the manufacturing of a bond structure in accordance with various embodiments; and
FIG. 9 illustrates spacing S (FIG. 8) between neighboring solder bumps as a function of pull up heights, and heights H of solder bumps as a function of the pull up heights.



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, and do not limit the scope of the disclosure.
A novel thermal compress bonding (TCB) process is provided. The intermediate stages of manufacturing various embodiments are illustrated. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements.
FIG. 4 illustrates a cross-sectional view of work piece 26. In an embodiment, work piece 26 is a package substrate or an interposer, which does not have active devices such as transistors therein. In alternative embodiments, work piece 26 is a device die having active devices such as transistors (not shown) therein. Metal pads 28 may be formed in work piece 26, and exposed through an opening in solder resist 30, which is formed of a dielectric material. Solder bumps 32 are formed on the top surface of, and are electrically coupled to, metal pads 28. Solder bumps 32 may include portions over dielectric layer 30, and portions extending into the openings in dielectric layer 30. In alternative embodiments, bumps 32 are non-reflowable metal bumps such as copper pillar bumps.
FIGS. 5A and 5B illustrate work pieces 40, which may be device dies (chips) comprising active devices (not shown) therein, wherein the active devices may be transistors. Alternatively, work pieces 40 may be interposers, package substrates, or the like. Metal pads 42 may be formed in work pieces 40. Dielectric layer 44 is formed over metal pads 42, and may be formed of polyimide, silicon oxide, silicon nitride, or the like. In an embodiment, as shown in FIG. 5A, solder bumps 46 are formed on the top surfaces of, and are electrically coupled to, metal pads 42. In alternative embodiments, as shown in FIG. 5B, under-bump metallurgies (UBMs) 47 are formed, with each of UBMs 47 including a portion extending into the respective opening in dielectric layer 44, and a portion over dielectric layer 44. Solder bumps 46 are formed on the top surfaces of the respective UBMs 47. UBMs 47 may comprise a titanium layer and a copper layer on the titanium layer, for example, although different metallic materials may be used. In an embodiment, bumps 46 are solder bumps, although they can be other type of bumps such as copper pillar bumps. However, at least one, and may be both, of metal bumps 32 (FIG. 4) and 46 are solder bumps. In the following illustrated exemplary embodiments, metal bumps 32 and 46 are all solder bumps.
Referring to FIG. 6, work piece 40 is picked up, flipped over, and placed onto work piece 26. Metal bumps 32 are in contact with solder bumps 46. Next, as shown in FIG. 7, a bonding is performed. Heating tool 60 is placed over and contacting work piece 40, and may apply a downward force (symbolized by arrow 62) to work piece 40, so that metal bumps 32 and 46 are pressed against each other to prevent work piece 40 from slipping. Heating tool 60 heats work piece 40, and the heat is conducted to, and cause the reflow of, solder bumps 32 and 46. The resulting solder bumps formed of the combined solder bumps 32 and 46 are referred to as solder bumps 66. During the melting of solder bumps 32 and 46, downward force 62 may be, or may not be, applied.
Referring to FIGS. 8A and 8B, after the reflow to form solder bumps 66 and before solder bumps 66 solidify, heating tool 60 applies an upward force, as symbolized by arrow 68, so that the height H of solder bumps 66 is increased, and horizontal size W2 of solder bumps 66 is reduced. The application of the upward force may be achieved, for example, by forming a vacuum in air duct 67 in heating tool 60 to suck work piece 40 up. The pulling up of work piece 40 is continued until angles α1 and α2 are higher than certain values (such as 50 degrees), as will be discussed in detail in subsequent paragraphs. The temperature of heating tool 60 is then reduced until solder bumps 66 solidify. The bonding process is then finished. After the bonding, underfill 70 is filled into the gap between work pieces 26 and 40.
In the resulting structure as shown in FIGS. 8A and 8B, at the joints where solder bumps 66 join dielectric layer 30, solder bumps 66 have tangent lines 72, which form angle α1 with horizontal surface 30a (which is in a horizontal plane) of dielectric layer 30. Each of tangent lines 72 may have one end meeting a joint of solder bumps 66 and dielectric layer 30, wherein the joint may be the corner of dielectric layer 30, which corner is where horizontal surface 30a intercepts a sidewall of dielectric layer 30. Similarly, at the joints where solder bumps 66 join dielectric layer 44, solder bumps 66 have tangent lines 76, which form angles α2 with horizontal surface 44a (which is in a horizontal plane) of dielectric layer 44. In the embodiment wherein UBMs 47 are formed in work piece 40, as shown in FIG. 8B, angles α2 may be considered as between tangent lines 76 and horizontal surface 47a of UBMs 47. In an embodiment, angles α1 and α2 are greater than about 50 degrees, greater than about 60 degrees, or even greater than about 75 degrees. It is known that increased angles α1 may result in the decrease in the stress at the corners where solder bumps 66 join dielectric layer 30, where the stress in solder bumps 66 is often the highest near the corners. Accordingly, the cracking in solder bumps 66 caused by the stress is reduced. Similarly, the increased angles α2 may result the decrease in the stress at the corners where solder bumps 66 join dielectric layer 44 or UBMs 47, and hence the reduction in the cracking in solder bumps 66. It is observed that when height H of solder bumps 66 is increased, angles α1 and α2 also increase. Accordingly, by pulling up work piece 40 in the reflow process, angles α1 and α2 may increase, and the cracking in solder bumps 66 may be reduced.
The increase in height H also results in spacing S between neighboring solder bumps 66 to be increased. Assuming solder bumps 66 in some sample bond structures are pulled up from an original height H equal to 64 μm, and a pull up height is used to measure the increase in height H from the original height of 64 μm, then in FIG. 9, line 84 may be used to illustrate spacing S (FIGS. 8A and 8B) as a function of the pull up heights, and line 82 may be used to illustrate height H of solder bumps 66 as a function of the pull up height. It is observed that both spacing S and height H increase substantially linearly with the increase in the pull up height. In other words, by pulling up work piece 40 in the reflow process, spacing S may be increased linearly with the increase in solder bump height H. Accordingly, the risk of shorting is reduced when solder bump height H is increased, and solder bump density may be increased. The possibility of cracking in solder bumps 66 is also reduced.
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 method comprising:
providing a first work piece comprising a dielectric layer having a top surface;
placing a second work piece facing the first work piece;
placing a heating tool contacting the second work piece;
heating the second work piece using the heating tool to perform a reflow process, wherein a first solder bump between the first and the second work pieces is melted to form a second solder bump; and
before the second solder bump solidifies, pulling the second work piece away from the first work piece, until an angle formed between a tangent line of the second solder bump and the top surface of the dielectric layer is greater than about 50 degrees, wherein the tangent line is drawn at a point where the second solder bump joins the dielectric layer.

2. The method of claim 1, wherein the angle is greater than about 60 degrees.

3. The method of claim 2, wherein the angle is greater than about 75 degrees.

4. The method of claim 1, wherein the first work piece is a package substrate.

5. The method of claim 1, wherein the first work piece is a device die.

6. The method of claim 1, wherein before the step of placing the second work piece facing the first work piece, the first solder bump is attached to the first work piece, and a third solder bump is attached to the second work piece, and wherein the first and the third solder bumps are melted to form the second solder bump.

7. The method of claim 1, wherein at a joint where the second solder bump joins the second work piece, an additional tangent line of the second solder bump forms an additional angle greater than about 50 degrees with a top surface of an additional dielectric layer at a surface of the second work piece, and wherein the additional tangent line is drawn at a joint of the second bump and the additional dielectric layer.