Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making electrical device
Reexamination Certificate
1998-09-18
2001-07-31
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making electrical device
C430S312000, C430S318000, C430S319000
Reexamination Certificate
active
06268114
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method for forming solder bumps and devices formed and more particularly, relates to a method for forming fine-pitched solder bumps by incorporating an additional coating layer of leachable metal between an under bump metallurgy and solder ball such that a photoresist layer which has a larger thickness may be used to form solder bumps at a smaller pitch distance and semiconductor devices formed by such method.
BACKGROUND OF THE INVENTION
In the fabrication of modem semiconductor devices, the ever increasing device density and decreasing device dimensions demand more stringent requirements in the packaging or interconnecting techniques of such high density devices. Conventionally, a flip-chip attachment method has been used in packaging of semiconductor chips. In the flip-chip attachment method, instead of attaching a semiconductor die to a lead frame in a package, an array of solder bumps is formed on the surface of the die. The formation of the solder bumps may be carried out in an evaporation method by using a composite material of tin and lead through a mask for producing a desired pattern of solder bumps. The technique of electrodeposition has been more recently developed to produce solder bumps in flip-chip packaging process.
Other techniques that are capable of solder-bumping a variety of substrates to form solder balls have also been proposed. The techniques generally work well in bumping semiconductor substrates that contain solder structures over a minimal size. For instance, one of such widely used techniques is a solder paste screening method which has been used to cover the entire area of an eight inch wafer. However, with recent trend in the miniaturization of device dimensions and the necessary reduction in bump-to-bump spacing (or pitch), the use of the solder paste screening technique has become impractical for several reasons. One of the problems in utilizing solder paste screening technique in bonding modern semiconductor devices is the paste composition itself. A solder paste is formed by a flux material and solder alloy particles. The consistency and uniformity of the solder paste composition become more difficult to control as the solder bump volume decreases. Even though a solution of the problem has been proposed by using solder paste that contain extremely small and uniform solder particles, it can only be achieved at a high cost penalty. A second problem in utilizing the solder paste screening technique in modern high density semiconductor devices is the available space between solder bumps. It is known that a large volume reduction occurs when a solder changes from a paste state to a cured stated, the screen holes for the solder paste must be significantly larger in diameter than the actual solder bumps to be formed. The large volume shrinkage ratio thus makes the solder paste screening technique difficult to carry out in high density devices.
Other techniques for forming solder bumps such as the controlled collapse chip connection (C
4
) technique and the thin film electrodeposition technique have also been used in recent years in the semiconductor fabrication industry. The C
4
technique is generally limited by the resolution achievable by a molybdenum mask which is necessary for the process. Fine-pitched solder bumps are therefore difficult to be fabricated by the C
4
technique. Similarly, the thin film electrodeposition technique which also requires a ball limiting metallurgy layer to be deposited and defined by an etching process which has the same limitations as the C
4
technique. For instance, a conventional thin film electrodeposition process for depositing solder bumps is shown in FIGS.
1
A~
1
H.
A conventional semiconductor structure
10
is shown in FIG.
1
A. The semiconductor structure
10
is built on a silicon substrate
12
with active devices built therein. A bond pad
14
is formed on a top surface
16
of the substrate
12
for making electrical connections to the outside circuits. The bond pad
14
is normally formed of a conductive metal such as aluminum. The bond pad
14
is passivated by a final passivation layer
20
with a window
22
opened by a photolithography process to allow electrical connection to be made to the bond pad
14
. The passivation layer
20
may be formed of any one of various insulating materials such as oxide, nitride or organic materials. The passivation layer
20
is applied on top of the semiconductor device
10
to provide both planarization and physical protection of the circuits formed on the device
10
.
Onto the top surface
24
of the passivation layer
20
and the exposed top surface
18
of the bond pad
14
, is then deposited an under bump metallurgy layer
26
. This is shown in FIG.
1
B. The under bump metallurgy (UBM) layer
26
normally consists of an adhesion diffusion barrier layer
30
and a wetting layer
28
. The adhesion diffusion barrier layer
30
may be formed of Ti, TiN or other metal such as Cr. The wetting layer
28
is normally formed of a Cu layer or a Ni layer. The UMB layer
26
improves bonding between a solder ball to be formed and the top surface
18
of the bond pad
14
.
In the next step of the process, as shown in
FIG. 1C
, a photoresist layer
34
is deposited on top of the UMB layer
26
and then patterned to define a window opening
38
for the solder ball to be subsequently formed. In the following electrodeposition process, a solder ball
40
is electrodeposited into the window opening
38
forming a structure protruded from the top surface
42
of the photoresist layer
34
. The use of the photoresist layer
34
must be carefully controlled such that its thickness is in the range between about 30 &mgr;m and about 40 &mgr;m, preferably at a thickness of about 35 &mgr;m. The reason for the tight control on the thickness of the photoresist layer
34
is that, for achieving a fine-pitched solder bump formation, a photoresist layer of a reasonably small thickness must be used such that a high imaging resolution can be achieved. It is known that, during a photolithography process, the thicker the photoresist layer, the poorer is the imaging process. To maintain a reasonable accuracy in the imaging process on the photoresist layer
34
, a reasonably thin photoresist layer
34
must be used which results in a mushroom configuration of the solder bump
40
deposited therein. The mushroom configuration of the solder bump
40
contributes greatly to the inability of a conventional process in producing fine-pitched solder bumps.
Referring now to
FIG. 1E
, wherein the conventional semiconductor structure
10
is shown with the photoresist layer
34
removed in a wet stripping process. The mushroom-shaped solder bump
40
remains while the under bump metallurgy layer
26
is also intact. In the next step of the process, as shown in
FIG. 1F
, the UBM layer
26
is etched away by using the solder bump
40
as a mask in an wet etching process. The solder bump
40
is then heated in a reflow process to form solder ball
42
. The reflow process is conducted at a temperature that is at least the reflow temperature of the solder material.
FIG. 1G
shows an enlarged, cross-sectional view of the solder bump
40
of FIG.
1
D. The mushroom-shaped solder bump is formed over the photoresist layer
34
. The dimension for the base of the solder bump is X=150 &mgr;m, for the outer diameter of the mushroom is Y=205 &mgr;m, for the height of the base of the solder bump is h=34 &mgr;m, and for the total height of the mushroom configuration is H=62 &mgr;m. In this specific example, it is seen that the total height of the mushroom is almost twice as the height of the base of the solder bump in the window and furthermore, the outside diameter of the mushroom is significantly larger than the outside diameter of the base portion of the solder bump
40
. A comparison in the areas occupied by the base portion
46
and the top portion of the mushroom
48
is shown in
FIG. 1H. A
simple calculation in
Kung Ling-Chen
Lu Szu-Wei
Uang Ruoh-Huey
Wen Ying-Nan
Huff Mark F.
Mohamedulla Saleha R.
Taiwan Semiconductor Manufacturing Company Ltd
Tung & Associates
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