Intermittent electroless nickel deposition in a fine trench flip chip bump pad

Article abstract of DOI:10.1149/1.1393575

An intermittent electroless nickel deposition process was examined which involved periodically removing and reinserting the silicon wafer in the deposition bath. This process results in electroless nickel deposits with a smooth surface appearance and uniform thickness within the bump opening. The intermittent deposition process also results in enhancement in deposition speed. The benefits of this process are ascribed to replenishment of the surface concentration and the breakdown of entrapped gas bubble in the trench area of bump pad.

Full text of DOI:10.1149/1.1393575

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Journal of The Electrochemical Society, 147 (7) 2604-2606 (2000)
S0013-4651(99)07-094-9 CCC: $7.00 © The Electrochemical Society, Inc.
Intermittent Electroless Nickel Deposition in a Fine Trench
Flip Chip Bump Pad
z
Kwang-Lung Lin and Chih-Li Chen
Department of Materials Science and Engineering, National Cheng Kung University, Tainan, 701 Taiwan
An intermittent electroless nickel deposition process was examined which involved periodically removing and reinserting the sili-
con wafer in the deposition bath. this process results in electroless nickel deposits with a smooth surface appearance and uniform
thickness within the bump opening. The intermittent deposition process also results in enhancement in deposition speed. The ben-
efits of this process are ascribed to replenishment of the surface concentration and the breakdown of entrapped gas bubble in the
trench area of bump pad.
©
2000 The Electrochemical Society. S0013-4651(99)07-094-9. All rights reserved.
Manuscript submitted July 22, 1999; revised manuscript received April, 2000.
Amorphous Ni-P alloys produced by electroless deposition are
among the under bump metallurgy (UBM) alternatives for solder
respectively. The corresponding standard deviation of the bumps are
1.11, 2.24, and 2.00 ␮m, i.e., 1.54 Ϯ 1.11, 3.22 Ϯ 2.24, 3.17 Ϯ 2.00.
It is quite evident that the electroless nickel bump heights are wide-
ly scattered. The above results suggest that the bump growth almost
stops after 60 min deposition. Nevertheless, the continuous deposi-
tion speed on 100 ϫ 100 ␮m bump pad can be as great as
0.160 ␮m/min, i.e., a bump height of 9.5 ␮m was achieved in 1 h.
Furthermore, as shown in Fig. 2, the electroless nickel bumps ob-
tained on 60 ␮m bump pad were unsatisfactory even after 1 h of
reaction, although this poor result was not observed for 100 ϫ
100 ␮m bump pad. The imperfections include incomplete deposition
and pudding geometry. The rim of certain bumps show thin deposits.
The bump then exhibits pudding shape structure. The bump surface
also exhibits porosity. The above-mentioned results and observations
indicate that electroless nickel deposition in a trench of high aspect
ratio like 10 ␮m (trench wall)/60 ␮m (pad size), in comparing with
normal pad size of 100 ␮m or above, would result in slow deposition
speed, pudding bump, and defected bump structure.
1
,2
bump of flip chip technology. The amorphous structure lacks grain
boundary and other extended defects which aid diffusional process
3
and thus is ideal for diffusion barrier application. Nickel also reacts
4
relatively slowly with Sn. In manufacturing solder bumps electro-
less nickel alloy may be deposited on the patterned openings that are
5
produced lithographically.
A typical flip chip solder bump may have a diameter of less than
1
00 ␮m with a trench depth of 10ϳ30 ␮m. Problems have been
encountered with deposition in such features largely due to the
effects of nonuniform concentration gradients that develop during
6
,7
the deposition process. For example, deposition in a concave
geometry such as a trench, suffers from anisotropic deposition,
which significantly alters shape of the deposit. Surface adsorption of
a stabilizer from the deposition bath may be used to poison the depo-
sition reaction at the edge of the deposit, resulting in a pyramidal
7
geometry. This paper describes a novel method for improving depo-
sition speed, geometry, and surface properties of electroless nickel
alloy deposited in fine patterned concave structures.
Figure 3 is an SEM image of the bump deposited by intermittent
deposition at an on-interval of 5 min for a total deposition period of
9
0 min. In comparison with Fig. 2, it is evident that the bumps
Experimental
obtained with intermittent deposition exhibit smooth surface appear-
ance as well as complete coverage of the pad.
Electroless nickel was deposited on silicon wafer simulating the
UBM of flip chip solder bump. Accordingly, Al and Cu films were
deposited on silicon wafer followed by photolithography to produce
Figure 4 presents the growth of the electroless nickel bump ob-
tained by various on-intervals, including continuous deposition. The
lowest curve shows that continuous deposition without intermission
can only obtain a maximum bump height of around 3 ␮m. On the
other hand, all other curves show that much greater bump height (as
presented by deposit thickness) was obtained by intermittent deposi-
tion especially after 30 min of deposition time. For instance, a bump
height of only 1.54 ␮m is obtained for continuous deposition at
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the bump opening. Photolithography was then applied to produce
desired bump opening of 100 ϫ 100 or 60 ␮m in diam. Electroless
nickel was deposited on the opening by using nickel sulfate as metal
source and nickel hypophosphite as reducing agent at pH 4.6 and
0ЊC. The composition of the bath consists of 87 g/L nickel sulfate,
4 g/L sodium hypophosphite, 30 g/L sodium acetate, 2 g/L citric
acid, 4.1 g/L sodium succinate, and 0.0015 g/L lead acetate. The
wafer was set vertically and clipped to a piece of iron in order to acti-
vate the copper surface for electroless nickel deposition. The inter-
mittent electroless nickel deposition was conducted by periodically
removing the substrate from the deposition bath for a few seconds
during deposition. The periodic nature of this operation results in a
deposition behavior, shown in Fig. 1, analogous to that of pulse elec-
9
2
3
3
5
0 min, while an on-interval of 5 min gives rise to a bump height of
␮m for the same total deposition time.
8
troplating and is thus termed “intermittent electroless nickel depo-
sition.” The duration of the “on-interval” is varied while the “off-
interval” is as short as 1-2 s. The deposit thickness of the electroless
nickel bump was measured with a profilometer. The surface appear-
ance of the bump was investigated with scanning electron micro-
scopy (SEM).
Results and Discussion
For comparison purposes, the electroless nickel deposited with-
out interruption on 60 ␮m diam bump pads gave rise to bump height
of 1.54, 3.22, and 3.17 ␮m when deposited for 30, 60, and 90 min,
z
E-mail: matkllin@mail.ncku.edu.tw
Figure 1. Intermittent electroless nickel deposition process.
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Journal of The Electrochemical Society, 147 (7) 2604-2606 (2000)
2605
S0013-4651(99)07-094-9 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 2. Defected appearance of electroless nickel bump deposited on 60
␮m bump pad without intermittent.
Figure 4. Variation of deposition speed with respect to deposition intervals.
In addition, all intermittent deposition curves show a sigmoid be-
havior. It is evident that an on-interval of 5 min gives rise to the
fastest deposition speed. The inflection point of the sigmoid curve is
a function of the deposition interval. The inflection time is in the
order of continuous deposition > 10 min (interval) > 1 min > 3 min >
thin deposit on the perimeter of the feature. Both cases shown in
Fig. 5a and b would result in the imperfect deposit as seen in Fig. 2.
The hydrogen gas bubble was destroyed when exposed to air dur-
ing off-interval. In such a case, the opening always provides clear
path for deposition reaction. Therefore, the intermittent deposition
process results in elimination of hydrogen gas bubble and thus uni-
form smooth bump is obtained as visualized in Fig. 3. It is expected
that hydrogen bubble phenomenon becomes more prominent for
high aspect ratio opening. Nevertheless, the intermittent deposition
operation eliminates this effect and thus the smooth nickel bump
deposit can be obtained regardless of aspect ratio.
5
min. Note that the deposition behavior of the continuous deposi-
tion follows a parabolic curve, indicating a typical diffusion control
mechanism. However, the intermittent deposition, especially when
the on-interval is 3 or 5 min, would give rise to a more linear corre-
lation rather than a parabolic curve after the inflection point. The lin-
ear correlation corresponds to a much faster deposition speed than
that achievable with continuous deposition. For instance a bump
height of around 12.5 ␮m was obtained at 90 min for on-interval of
Conclusion
5
min, while only 3.17 ␮m was deposited by continuous deposition.
Intermittent electroless nickel deposition provides removal of
trapped gas bubbles and complete replenishment of the metal ion
concentration within the small bump opening, resulting in a higher
The deposition of electroless nickel bump was performed on the
bump opening. Electroless nickel deposition produces hydrogen gas
in addition to nickel deposit. Hydrogen gas is easily entrapped,
attaching to the hydrophobic photoresist, as indicated in Fig. 5. The
hydrogen gas bubble tends to stay and grow, especially when the
opening is as fine as 60 ␮m. A large gas bubble that completely cov-
ers the 60 ␮m opening, Fig. 5a, will definitely stop the deposition
reaction while a small gas bubble attached to the opening wall,
Fig. 5b, affects the growth and thus the geometry of the deposit. Sub-
sequently, the bump deposit become puddinglike structure with a
Figure 5. The entrapment of hydrogen gas bubble by hydrophobic photore-
sist opening wall, (a) complete coverage by gas bubble, (b) partial coverage,
(c) no entrapment due to intermittent process.
Figure 3. Electroless nickel bumps deposited on 60 ␮m bump pad with on-
interval of 5 min.
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606
Journal of The Electrochemical Society, 147 (7) 2604-2606 (2000)
S0013-4651(99)07-094-9 CCC: $7.00 © The Electrochemical Society, Inc.
deposition rate. Intermittent deposition also gives rise to smooth
bump deposit with complete coverage of the bump opening. An on-
interval of 5 min gives the highest deposition speed.
2. M. Inaba, K. Yamakawa, and N. Iwase, IEEE Trans. Compon. Hybrids. Manuf. Tech-
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Acknowledgment
Financial support of this work from the National Science Coun-
cil of Republic of China under NSC87-2216-E-006-027 and NSC88-
5
. K. L. Lin and Y. C. Liu, in Electronic Components and Technology, Proceedings of
the 49th Conference, IEEE (1999).
6. A. M. T. van der Putten and J. W. G. de Bakker, J. Electrochem. Soc., 140, 2221
1993).
(
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216-E-006-025 is gratefully acknowledged.
Dr. Lin assisted in meeting the publication costs of this article.
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