Electrochemically deposited tin-silver-copper ternary solder alloys

Article abstract of DOI:10.1149/1.1530153

A study on film properties of electrochemically deposited tin-silver-copper (SnAgCu) alloys was performed with an alkaline bath. This research focused on the bath and process development for dendrite-free, near-eutectic SnAgCu alloy deposition through the investigation of cathodic polarization, morphological transition, and film composition. Effects of process parameters on surface morphology, film composition, and diffusion-limited current density (LCD) were also examined. Ternary alloys were obtained only when the current density was driven beyond the mass-transfer limitation of noble metals (silver and copper), which seemed to cause the transition of surface morphology and film composition with increasing current density. The morphological transition occurred through four stages (dendrites, suppression of dendrites, nodules, and columns/dendrites), and the content of noble metals in the film tended to drop with increasing current density. With increasing the concentration of noble metals, bath temperature, and agitation, the morphology at low current densities became increasingly dendrite-dominated and the content of noble metals in the film was enhanced at a fixed current density. The morphology of stage four was influenced by the ratio of the applied current density to the LCD, where the LCD was significantly influenced by the metal concentration, bath temperature, and agitation.

Full text of DOI:10.1149/1.1530153

Journal of The Electrochemical Society, 150 ͑2͒ C53-C60 ͑2003͒
C53
0013-4651/2002/150͑2͒/C53/8/$7.00 © The Electrochemical Society, Inc.
Electrochemically Deposited Tin-Silver-Copper Ternary Solder
Alloys
,z
*
*
Bioh Kim and Tom Ritzdorf
Semitool, Incorporated, ECD Division, Kalispell, Montana, 59901, USA
A study on film properties of electrochemically deposited tin-silver-copper ͑SnAgCu͒ alloys was performed with an alkaline bath.
This research focused on the bath and process development for dendrite-free, near-eutectic SnAgCu alloy deposition through the
investigation of cathodic polarization, morphological transition, and film composition. Effects of process parameters on surface
morphology, film composition, and diffusion-limited current density ͑LCD͒ were also examined. Ternary alloys were obtained
only when the current density was driven beyond the mass-transfer limitation of noble metals ͑silver and copper͒, which seemed
to cause the transition of surface morphology and film composition with increasing current density. The morphological transition
occurred through four stages ͑dendrites, suppression of dendrites, nodules, and columns/dendrites͒, and the content of noble metals
in the film tended to drop with increasing current density. With increasing the concentration of noble metals, bath temperature, and
agitation, the morphology at low current densities became increasingly dendrite-dominated and the content of noble metals in the
film was enhanced at a fixed current density. The morphology of stage four was influenced by the ratio of the applied current
density to the LCD, where the LCD was significantly influenced by the metal concentration, bath temperature, and agitation.
© 2002 The Electrochemical Society. ͓DOI: 10.1149/1.1530153͔ All rights reserved.
Manuscript submitted January 9, 2002; revised manuscript received July 23 2002. Available electronically December 23, 2002.
Mounting is a critical step in semiconductor-related device
manufacture. With the electronic devices miniaturized and the cir-
cuitry complicated, the mounting of a microelectronic substrate to
another substrate becomes more and more difficult. Therefore, inter-
connection techniques with high density, performance, and reliabil-
ity are desirable for microelectronic substrates. Flip chip technology,
which involves turning over a chip and then bonding the terminals
directly to a wiring substrate, is used instead of conventional mount-
ing technologies such as wire bonding or tape automated bonding.
The advantages of this technology include high-density bonding,
gang bonding, improved electrical performance, self-alignment, re-
liability and manufacturability.1,2
The most widely used solders for flip chip applications are based
on lead-tin alloys. However, there is currently substantial interest in
looking for lead-free solder alloys because of legislation due to the
toxicity of lead.3 Another concern for upcoming technology genera-
tions is soft error generation caused by lead in flip-chip solder con-
nections. This is due to alpha particle emission from the lead.4 Tin-
silver and tin-silver-based alloys are being developed to use as lead-
free solder bumps.5-10
Near-eutectic SnAgCu alloys are receiving a growing interest
due to such merits as good melting, solderability, ability to cope
with lead contamination, and reliability.11,12 To use these alloys for
general purpose solders, the contents of silver and copper in alloys
are limited to near-eutectic compositions ͑3.4-4.1 wt % Ag and 0.45-
0.9 wt % Cu͒ on account of the limited level of melting points for
bumping applications.12 SnAgCu alloys are being investigated as
alternatives to lead-bearing alloys for solder bumps, but the main-
stream method of formation has been limited to screen printing tech-
nology, because this is a low-cost method for producing relatively
large geometry bumps.13-19 However, this method has serious limi-
tation as the size/pitch of solder bumps decreases. As the density of
patterns and the complexity of circuitry increase, electrodeposition
technology becomes preferred as an alternative method to screen
printing.20
among polarization, surface morphology, and film composition was
discussed. Effects of process parameters on surface morphology,
film composition, and diffusion-limited current density ͑LCD͒ were
also examined.
Experimental
Wafers.—Wafers for testing cathodic polarization had the struc-
ture of Si͑substrate͒/SiO2 /TiN ͑40 nm͒/Pt ͑50 nm͒. Platinum seed
layer was sputter-deposited. Wafers for characterizing surface mor-
phology, film composition, and LCD had the structure of Si
͑substrate͒/SiO2 /Ta ͑25 nm͒/Cu ͑100 nm͒/Cu ͑5 ␮m͒. A thin copper
layer was sputter-deposited, whereas the thick copper layer was
electrochemically deposited.
Cathodic polarization.—In order to mount the wafers in a con-
ventional rotating disk electrode ͑RDE͒ with as little hydrodynamic
disturbance as possible, a device was manufactured which is re-
ferred to as the modified rotating disk electrode ͑MRDE͒ and is
illustrated in Fig. 1.21 To use this MRDE, the wafer was sectioned
into 1.5 ϫ 1.5 cm pieces and the wafer piece was then mounted to
the MRDE. The result was that no conducting part except for the
exposed wafer with a 1.2 cm diam came into contact with the plat-
ing solution. Additional characterization of the bath was performed
using an EG&G platinum RDE with a 4 mm diam. The MRDE and
The purpose of this study was to develop a bath and process that
could produce dendrite-free, near-eutectic SnAgCu alloys for lead-
free bumping applications. Within this study, several process condi-
tions using an alkaline bath were tested and their influences were
measured. For example, the cathodic polarization tests of a new bath
were conducted, surface morphology and film composition were in-
vestigated with varying current densities, and the relationship
* Electrochemical Society Active Member.
z E-mail: bkim@semitool.com
Figure 1. Schematic drawing of MRDE.
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Journal of The Electrochemical Society, 150 ͑2͒ C53-C60 ͑2003͒
Table I. Summary of process conditions used for experiments.
Parameters
Conditions
Bath concentration
Bath type
Sn
Ag
Cu
͑mol/L͒
1
2
3
4
5
6
7
0.500
0.500
0.500
0.500
0.750
0.400
0.400
0.008
0.012
0.012
0.024
0.012
0.010
0.005
0.004
0.004
0.006
0.012
0.006
0.010
0.005
Temperature ͑°C͒
Agitation ͑rpm͒
pH
20, 30, 40, and 50 (Ϯ0.5)
0, 100, and 200 (Ϯ10)
7.5, 8.5, and 9.5 (Ϯ0.2)
RDE were rotated at the desired rpm using an EG&G model 616
RDE motor. The potentiostat used for linear scans was the EG&G
model 263A with EG&G 270 electrochemical software. The scan
rate was 20 mV/s. An Ag/AgClreference electrode was used and the
measured potential was converted to standard hydrogen electrode
͑SHE͒.
Surface morphology, film composition, and LCD.—To evaluate
film properties, wafer segments with an area of approximately
3.0 cm2 were used as a cathode and a large area of platinized-
titanium sheet was used as an anode. Experiments were performed
in a 1.5 L beaker containing 1 L of plating solution. The power
supply used for these experiments was an HP model E3617E DC.
The deposition time for each current density was adjusted to meet
the target thickness ͑10 ␮m͒. For example, it took 25 min to deposit
a 10 ␮m thick film with a current density of 10 mA/cm2 ͑at room
temperature and no agitation͒. This rate was used as a reference
value to set up the deposition time for different current densities.
Tested process conditions including bath concentrations are listed in
Table I. Each bath was composed of tin ions, silver ions, copper
ions, and two complexing agents. Complexing agents were used to
shift the reduction potential of noble metal ions ͑silver and copper͒
to more cathodic values by forming ion complexes. A scanning elec-
tron microscope ͑SEM, AMRAY model 3600 FE SEM͒ and an
energy-dispersive spectrometer ͑EDS, Noran Voyager͒ were used to
observe surface morphology and examine film composition, respec-
tively. Film thickness was measured with a Veeco surface metrology
tool ͑Dektak 300-Si͒. Chronopotentiometric data were obtained with
the same potentiostat in order to evaluate the LCD of ternary alloy
deposition.
Figure 2. ͑a͒ Cathodic polarization curve of SnAgCu bath showing two
plateaus, one by the mass-transfer limitation of noble metals and the other by
that of tin and ͑b͒ initial parts showing first plateau and deposition potential
‘‘a’’ used for potentiostatic deposition. 20 mV/s scan rate, bath 6 ͑0.4 mol/L
Sn, 0.01 mol/L Ag, and 0.01 mol/L Cu͒, room temperature, 100 rpm, and pH
9.5.
Results and Discussion
Cathodic polarization, morphological transition, and film
composition.—A cyanide-free, alkaline bath was developed for elec-
trochemically depositing ternary SnAgCu solder alloys. As the con-
tents of silver and copper in these alloys for bumping applications
Figure 3. ͑a͒ EDS spectrum ͑average composition in wt %: 56 Ag-44 Cu͒ and ͑b͒ surface morphology of deposit obtained at potential ‘‘a’’. Bath 6 ͑0.4 mol/L
Sn, 0.01 mol/L Ag, and 0.01 mol/L Cu͒, room temperature, 100 rpm, and pH 9.5.
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Journal of The Electrochemical Society, 150 ͑2͒ C53-C60 ͑2003͒
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Figure 4. Morphological transition with increasing current density. Electric charge density of 18 C/cm2 at each current density, bath 5 ͑0.75 mol/L Sn, 0.012
mol/L Ag, and 0.006 mol/L Cu͒, 40°C, 200 rpm, and pH 9.5.
are very low ͑generally, Ͻ4.1 wt %Ag and Ͻ0.9 wt %Cu),12 the
EDS spectrum. It was found from more studies that the composition
of binary alloys varied with changing potential due to the difference
of deposition potential between silver and copper in the bath. The
deposit was very dendritic as seen in Fig. 3b, and there was no
change of induced current density in a wide range of potential as
seen in Fig. 2b. It can be deduced from these results that the first
plateau comes from the mass-transfer limitation of noble metal ions
and that ternary alloys are obtained only when the current density is
driven beyond the mass-transfer limitation of noble metals. Driving
current density beyond this point causes the voltage to increase to a
point where tin is codeposited, leading to ternary alloy deposition.
At a potential of approximately Ϫ0.7 V, the curve begins to rise
sharply until it reaches the potential where the mass-transfer limita-
tion of tin ions occurs. This rise is related to the inclusion of the tin
in the deposit, as mentioned previously. Therefore, ternary alloys
bath also contains small amounts of silver and copper. Baths used
for these experiments are listed in Table I. These baths did not show
any precipitation or color change for more than 6 months without
applying current.
Polarization tests were conducted with an MRDE to characterize
the surface morphology and film composition of deposits obtained at
each potential. Experiments were performed with bath 6 at room
temperature, 100 rpm, and pH 9.5. The result is illustrated in Fig. 2a.
The initial part that was gently sloping and then forming a plateau,
shown in Fig. 2b, corresponds to the potentials at which primarily
the binary silver-copper alloys are deposited. As shown in Fig. 3a,
the deposit obtained potentiostatically from the deposition potential
a in Fig. 2b was a binary alloy composed of 56 wt % Ag and 44 wt
% Cu. No tin was observed in this alloy, as can be seen from the
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Journal of The Electrochemical Society, 150 ͑2͒ C53-C60 ͑2003͒
Figure 5. Approximated relationship between polarization curve and surface
morphology.
can be obtained from this potential. Another rise at about Ϫ1.5 V
after the second plateau results from proton discharge ͑hydrogen gas
evolution͒. After the polarization tests were completed, electrodepo-
sition runs were conducted at various current densities in order to
investigate the transition of surface morphology and film composi-
tion with varying current densities.
The morphological transition of ternary alloys with increasing
current density was examined and is shown in Fig. 4. The morpho-
logical transition of deposits occurred through four stages with in-
creasing current density; dendritic growth ͑stage 1͒, suppression of
dendrites ͑stage 2͒, nodular growth ͑stage 3͒, and columnar/dendritic
growth ͑stage 4͒. The following can be considered as the driving
force for the morphological transition with increasing current den-
sity. Stage 1 seems to result from the mass-transfer limitation of
silver and copper as seen from Fig. 3, where the ternary alloy depo-
sition occurs at a current density higher than the formation of the
dendritic, binary silver-copper alloys. Up to a certain potential after
the first plateau, the initial, ternary alloys containing high amounts
of noble metals can also be under the influence of the mass-transfer
limitation of noble metals. In order words, the low concentration of
noble metals in the bath can be the source of dendrites. Because of
the higher ͑less cathodic͒ reduction potential of silver and copper
relative to tin, these ions tend to be preferentially deposited at low
potentials. However, the supply of these ions to the surface is very
limited, resulting in dendrites at low potentials. Tin is deposited
preferentially with an increase in current density, suppressing den-
Figure 7. Effects of process parameters on the noble metal LCD ͑first pla-
teau͒. ͑a͒ Effect of the concentration of noble metals. 20 mV/s scan rate, bath
6 ͑0.4 mol/L Sn, 0.01 mol/L Ag, and 0.01 mol/L Cu͒ and bath 7 ͑0.4 mol/L
Sn, 0.005 mol/L Ag, and 0.005 mol/L Cu͒, 1000 rpm, room temperature, and
pH 9.5. ͑b͒ Effect of bath temperature: 20 mV/s scan rate, bath 7 ͑0.4 mol/L
Sn, 0.005 mol/L Ag, and 0.005 mol/L Cu͒, 1000 rpm, and pH 9.5. ͑c͒ Effect
of agitation. 20 mV/s scan rate, bath 6 ͑0.4 mol/L Sn, 0.01 mol/L Ag, and
0.01 mol/L Cu͒, room temperature, and pH 9.5.
drites induced by the mass-transfer limitation of noble metals ͑stage
2͒. In this case, the transition seems to occur between 10 and
20 mA/cm2. Fully suppressed, smooth surfaces were obtained be-
tween 60 and 70 mA/cm2. Deposits were transformed to stage 3
͑development of nodules͒ with further increasing current density,
where the mechanism of deposition is thought to change from
charge-transfer control to mixed control, resulting in secondary,
nodular growth. The topography of stage 3 morphology was en-
hanced with increasing current density. Under mass-transfer control
Figure 6. The change of film composition with increasing current density.
Bath 5 ͑0.75 mol/L Sn, 0.012 mol/L Ag, and 0.006 mol/L Cu͒, 40°C, 200
rpm, and pH 9.5.
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Journal of The Electrochemical Society, 150 ͑2͒ C53-C60 ͑2003͒
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Figure 8. Effects of process parameters on the morphology of deposits obtained at low current densities. ͑a͒ Effect of the concentration of noble metals:
2.5 mA/cm2 , bath 1 ͑0.5 mol/L Sn, 0.008 mol/L Ag, and 0.004 mol/L Cu͒, bath 3 ͑0.5 mol/L Sn, 0.012 mol/L Ag, and 0.006 mol/L Cu͒ and bath 4 ͑0.5 mol/L
Sn, 0.024 mol/L Ag, and 0.012 mol/L Cu͒, 30°C, 0 rpm, and pH 9.5. ͑b͒ Effect of bath temperature: 5.0 mA/cm2 , bath 3 ͑0.5 mol/L Sn, 0.012 mol/L Ag, and
0.006 mol/L Cu͒, 0 rpm, and pH 9.5. ͑c͒ Effect of agitation. 2.5 and 5.0 mA/cm2 , bath 3 ͑0.5 mol/L Sn, 0.012 mol/L Ag, and 0.006 mol/L Cu͒, room
temperature, and pH 9.5. ͑d͒ Effect of pH. 2.5 mA/cm2 , bath 3 ͑0.5 mol/L Sn, 0.012 mol/L Ag, and 0.006 mol/L Cu͒, room temperature, and 0 rpm.
͑stage 4͒, the morphology changed to columnar dendrites at
110 mA/cm2 and eventually fine dendrites at 120 mA/cm2 with se-
vere hydrogen gas evolution. Figure 5 shows the approximated re-
lationship between the polarization curve and surface morphology.
The exact boundary between stages was not defined clearly due to
the gradual change of morphology with increasing current density.
More study will be devoted to defining the exact boundary between
stages, especially between stage 1 and stage 2, in the future.
The content of silver and copper, whose deposition potential is
higher ͑less cathodic͒ than that of tin, tended to drop with increasing
current density and is illustrated in Fig. 6. The abrupt drop of the
content of noble metals between 10 and 20 mA/cm2 is accompanied
by the significant change of morphology between two current den-
sities, as shown in Fig. 4. In the case of alloy deposition, the content
of each metal in the deposit is thought to be proportional to the
partial current ͑density͒ induced by each discharge reaction. The
absolute partial current density by the discharge of noble metals is
constant after the first plateau, but the relative partial current density
decreases with increasing current density due to the increase of par-
tial current density by tin discharge. This result indicates that
Figure 9. Extension of stage 1 with increasing agitation: 20 mV/s scan rate,
bath 6 ͑0.4 mol/L Sn, 0.01 mol/L Ag, and 0.01 mol/L Cu͒, room temperature,
and pH 9.5.
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Journal of The Electrochemical Society, 150 ͑2͒ C53-C60 ͑2003͒
Figure 13. Effect of agitation ͑0, 100, and 200 rpm͒ on film composition.
Bath 3 ͑0.5 mol/L Sn, 0.012 mol/L Ag, and 0.006 mol/L Cu͒, room tempera-
ture, and pH 9.5.
Figure 10. Effect of the ratio of the applied current density (Ja) to the LCD
Ͻ
(JL) on the morphology: ͑a͒ Ja
JL , stage 3, ͑b͒ Ja Ϸ JL , stage 4, ͑c͒ Ja
Ͼ JL , stage 4, and ͑d͒ Ja ӷ JL , stage 4. Bath 1 ͑0.5 mol/L Sn, 0.008 mol/L
Ag, and 0.004 mol/L Cu͒, room temperature, 0 rpm, and pH 9.5.
Figure 11. Effect of concentration ͑bath 1 and 3͒ of noble metals on film
composition. Bath 1 ͑0.5 mol/L Sn, 0.008 mol/L Ag, and 0.004 mol/L Cu͒
and bath 3 ͑0.5 mol/L Sn, 0.012 mol/L Ag, and 0.006 mol/L Cu͒, room
temperature, no agitation, and pH 9.5.
Figure 14. Effect of pH ͑7.5, 8.5, and 9.5͒ on film composition. Bath 3 ͑0.5
mol/L Sn, 0.012 mol/L Ag, and 0.006 mol/L Cu͒, room temperature, and no
agitation.
Table II. Effects of process parameters on the LCD of ternary
alloy deposition „second plateau…; conditions in bold increased
the LCD.
Conditions
LCD
Bath Temperature Agitation
Parameters
͑°C͒
͑rpm͒
pH
(mA/cm2)
Base line
3
5
3
3
3
5
20
20
50
20
20
40
0
0
0
200
0
200
9.5
9.5
9.5
20 (Ϯ2.5)
25 (Ϯ2.5)
30 (Ϯ2.5)
60 (Ϯ5.0)
20 (Ϯ2.5)
100 (Ϯ10)
Concentration
Temperature
Agitation
pH
9.5
7.5 and 8.5
9.5
Figure 12. Effect of bath temperature ͑20, 40, and 50°C͒ on film composi-
tion. Bath 3 ͑0.5 mol/L Sn, 0.012 mol/L Ag, and 0.006 mol/L Cu͒, no agita-
tion, and pH 9.5.
Combination
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Journal of The Electrochemical Society, 150 ͑2͒ C53-C60 ͑2003͒
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Figure 15. Comparison between chro-
nopotentiometry and surface morphol-
ogy. Bath 1 ͑0.5 mol/L Sn, 0.008 mol/L
Ag, and 0.004 mol/L Cu͒, room tempera-
ture, no agitation, and pH 9.5.
SnAgCu alloy deposition with this type of bath is of a normal
codeposition type.
A chronopotentiometric method was used to define the LCD of
ternary alloy deposition ͑second plateau͒ and a comparison was
made with the surface morphology. As can be seen from Fig. 15,
voltage fluctuation as well as gas evolution was detected along with
the columnar growth. More severe fluctuation and gas evolution
were associated with finer dendrites. The codeposition of protons is
thought to be the source of potential fluctuation. In the case of
mass-transfer limitation of metals, the potential drops quickly to
proton reduction potential in order to keep the current constant. The
LCD of each condition is summarized in Table II. Similar to the
results in Fig. 7, the concentration of metal ions ͑tin in this case͒,
bath temperature, and agitation had a large influence on the LCD.
By properly combining those factors, five times higher LCD than
that of the baseline was obtained. As expected, the effect of pH was
negligible.
Effects of process conditions on surface morphology, film
composition, and LCD.—Effects of process parameters on the sur-
face morphology of stage 1 and stage 4 were investigated. Most
efforts were devoted to evaluating the influence of the concentration
of noble metals, bath temperature, and agitation on the morphology
of stage 1, based on the assumption that process parameters influ-
encing mass transfer might affect the morphology of stage 1, as
these dendrites were caused by the mass-transfer limitation of noble
metals. The results were in good agreement with the assumption.
Figure 7 shows the experimental results with a conventional RDE,
illustrating the change of noble metal LCD with varying process
conditions. The LCD of noble metals ͑first plateau͒ was increased
with increasing the concentration of noble metals, bath temperature,
and agitation. It is shown in Fig. 8 that the surface morphology
became increasingly dendrite-dominated at a fixed current density
with increasing the concentration of noble metals, bath temperature,
and agitation. The extension of stage 1 with increasing mass transfer
is due to the increase of the noble metal LCD, extending the range at
which stage 1 growth was observed. The extension of stage 1 to
higher current density with increasing agitation, as an example, is
shown in Fig. 9. Accordingly, the shift of stage due to the variation
of noble metal LCD can be considered to cause the morphology
change at a fixed current density. As can be seen from Fig. 8d, the
effect of pH was negligible due to the negligible effect of pH on the
noble metal LCD. The morphology of stage 4 was dependent on the
ratio of the current density to the LCD and is illustrated in Fig. 10.
With increasing the ratio of the applied current density to the LCD,
the tip radius of dendrites decreased and the corresponding morphol-
ogy varied from columnar to finer dendrites due to the increased
growth rate. Baths 1-5 all showed the same trend.
Conclusions
A bath and process were developed for dendrite-free, near-
eutectic SnAgCu alloy deposition through the investigation of ca-
thodic polarization, morphological transition, and film composition.
Ternary alloys were obtained only when the current density was
driven beyond the mass-transfer limitation of noble metals. With
increasing current density, the morphological transition occurred
through four stages; dendrites, suppression of dendrites, nodules,
and columns/dendrites. The content of silver and copper in the film
tended to drop with increasing current density.
With increasing the concentration of noble metals, bath tempera-
ture, and agitation, the transition current density from stage 1 to
stage 2 increased. With increasing the concentration of noble metals,
bath temperature, and agitation, the content of noble metals in the
film increased at a fixed current density due to an increase in noble
metal LCD. The morphology of stage 4 was dependent on the ratio
of the applied current density to the LCD, where the LCD of ternary
alloy deposition was mainly affected by the concentration of tin
ions, bath temperature, and agitation.
Figures 11-14 represent the effect of process conditions on film
composition according to the applied current density. The corre-
sponding maximum current density depicted in each figure is close
to the corresponding LCD. With increasing the concentration of
noble metals ͑Fig. 11͒, bath temperature ͑Fig. 12͒, and agitation
͑Fig. 13͒, the content of noble metals in the film increased. How-
ever, the effect of pH was negligible, as can be seen from Fig. 14.
That means the deposition of noble metals is under mass-transfer
control. An increase in mass transfer increased the LCD of noble
metals ͑first plateau͒, resulting in the increase of noble metal content
in the film at a fixed current density.
Semitool, Incorporated, assisted in meeting the publication costs of this
artcle.
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