Superconformal electrodeposition of silver from a KAg(CN)2-KCN-KSeCN electrolyte

Article abstract of DOI:10.1149/1.1531195

Electrodeposition of silver from a KAg(CN)₂-KCN electrolyte was investigated. The addition of potassium selenocyanate (KSeCN) results in a hysteretic current-voltage response, specular films, and superconformal growth in submicrometer vias. These observations are well described by the recently proposed curvature enhanced accelerator coverage model of film growth.

Full text of DOI:10.1149/1.1531195

Journal of The Electrochemical Society, 150 ͑2͒ C61-C66 ͑2003͒
C61
0013-4651/2002/150͑2͒/C61/6/$7.00 © The Electrochemical Society, Inc.
Superconformal Electrodeposition of Silver
from a KAg„CN…2-KCN-KSeCN Electrolyte
b
b
a
a,z
B. C. Baker,a, M. Freeman, B. Melnick, D. Wheeler, D. Josell,
*
and T. P. Moffata,
*
aNational Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
bMotorola, Advanced Product Research and Development Laboratory, Austin, Texas 78721, USA
Electrodeposition of silver from a KAg͑CN)2-KCN electrolyte was investigated. The addition of potassium selenocyanate
͑KSeCN͒ results in a hysteretic current-voltage response, specular films, and superconformal growth in submicrometer vias. These
observations are well described by the recently proposed curvature enhanced accelerator coverage model of film growth.
© 2002 The Electrochemical Society. ͓DOI: 10.1149/1.1531195͔ All rights reserved.
Manuscript submitted May 28, 2002; revised manuscript received July 19, 2002. Available electronically December 23, 2002.
The success of copper electrodeposition for interconnect metal-
lization has spurred interest in the potential application of silver
which has an even higher electrical conductivity and thus the prom-
ise of a lower resistance-capacitance circuit time constant.1 To be a
viable alternative, the silver films must also exhibit comparable or
superior resistance to electromigration2,3 and the deposition process
must be capable of void or seam-free filling of submicrometer fea-
tures. Recently, a commercial silver cyanide plating solution was
shown to be capable of filling submicrometer trenches4 and vias.5
The results were described by the curvature-enhanced accelerator
coverage ͑CEAC͒ model which successfully explained superconfor-
mal electrodeposition of copper6-8 and silver4,5 and chemical vapor
deposition ͑CVD͒ of copper.9 The essence of the CEAC model is
that ͑i͒ the local deposition rate is determined by the coverage of a
catalytic surface species and (ii) the local coverage changes in re-
sponse to the changing area ͑therefore curvature͒ of the growth
front. Thus, the diminishing area associated with growth at the bot-
tom of a trench or via results in an increase in catalyst coverage and
an accelerated metal deposition rate which gives rise to ‘‘bottom-
up’’ or superconformal deposition. In the case of silver plating, se-
lenium was identified as the catalytic surface species because its
coverage scaled with rising current-time ͑chronoamperometric͒ tran-
sients. A significant limitation of the early silver work was incom-
plete knowledge of the electrolyte constituents, i.e., the nature of the
catalyst precursor and the influence of additional surfactants. The
unknown composition ͑constituents͒ of the electrolyte prevented
deeper understanding of the silver deposition process, hindering fur-
ther development for superconformal filling processes.
into which aliquots of KSeCN solution were added. A sweep rate of
1 mV/s was used when cycling voltage.
Studies of feature filling in the silver cyanide electrolytes took
place in the same setup used for the studies with planar substrates. A
silver plated clip was used to contact scribed wafer pieces. Wafers,
patterned with vias of a wide range of spacing and diameter, with a
100 nm copper seed, were used. Samples were plated potentiostati-
cally vs. the Ag reference electrode. The stability of the electrolytes
was tested by ensuring that current-voltage i-␩ curves in the solution
at the beginning and end of each set of experiments were reproduc-
ible. After plating, the specimens were covered in epoxy and a glass
coverslip and cross sectioned. Polishing was accomplished using
diamond lapping films down to 0.1 ␮m grit followed by Arϩ ion
polishing at 13° from the plane of the specimen.
Results
Electrochemical.—The i-␩ curve for the KAg͑CN)2-KCN solu-
tion is shown in Fig. 1a. Slight hysteresis is apparent, with smaller
current on the returning sweep. In sharp contrast, additions of
KSeCN result in marked hysteresis of the i-␩ curves whereby the
current is substantially increased on the return sweeps ͑Fig. 1a and
b͒. The hysteresis is significant even at a concentration of only 0.2
␮mol/L. The resulting deposits were also visibly brighter than those
deposited in the absence of the catalyst. The hysteretic behavior
results from cumulative adsorption of SeCNϪ catalyst on the metal
surface occurring on the same time scale as the voltage sweep. The
minimal hysteresis between the negative and reverse potential
sweeps for the 19.4 ␮mol/L electrolyte indicates that saturation of
the catalyst coverage occurs rapidly at this higher concentration in
the electrolyte. The deposits at this concentration are not optically
bright, unlike specimens deposited at the lower concentrations. This
occurs despite the significantly shorter duration exposure to the de-
stabilizing silver concentration gradient ͑i.e., swept to only Ϫ0.4 V
compared to Ϫ0.6 V for 4.9 ␮mol/L i-␩ curve͒. This is an expected
consequence of the CEAC mechanism for a saturated surface. Ac-
celeration of deposition rate in chronoamperometric the experiments
due to the gradual adsorption of the catalyst, is also as expected
͑Fig. 2͒.
In this paper, a silver cyanide electrolyte is fully disclosed which
enables a more precise assessment of the CEAC model as it relates
to this new class of ‘‘superfilling’’ electrolytes relevant to new in-
dustrial silver processes.10
Experimental
Silver cyanide solutions consisted of 0.34 mol/L KAg͑CN)2 and
2.3 mol/L KCN. Selenium was added as KSeCN and diluted from a
200 ␮mol/L KSeCN solution in the matrix shown above to concen-
trations between 0.02 and 20 ␮mol/L.
Electrochemical data was obtained on a planar silver electrode in
solutions with no agitation at room temperature ͑ϳ23°C͒. The silver
cathode was polished with 320 SiC paper, rinsed with distilled wa-
ter, and dried with N2 prior to each deposition run. The masked area
for deposition was 0.97 cm2. A silver ͑99.99%͒ rod, polished and
rinsed between experiments, was used as a reference electrode,
while a Pt anode was situated coplanar with the working electrode.
Experiments were done in a cell containing 100 mL of electrolyte
Simulations.—The CEAC mechanism was used to describe the
electrochemical data presented above. A summary of the relevant
equations is shown below
CiAg
␣ ␪͒F
1 Ϫ ␣ ␪͒ F
͑
͓
͑
͔
i ␪, ␩͒ ϭ i ␪͒
exp Ϫ
␩
Ϫ exp
␩
͑
͑
ͫ
ͩ
ͪ
ͩ
ͪ
ͬ
o
CAg
RT
RT
* Electrochemical Society Active Member.
z E-mail: daniel.josell@nist.gov
͓1͔
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Journal of The Electrochemical Society, 150 ͑2͒ C61-C66 ͑2003͒
Figure 2. Current transient plots for 0.34 mol/L KAg͑CN)2 , 2.3 mol/L
KCN, and 1 ␮mol/L KSeCN for three overpotential values.
kinetics ͑for concentration CSi e at the interface͒. Note that the time
rate of change of local area equals the normal velocity
v
ϭ i⍀Ag /F, with ⍀Ag ϭ 10.3 cm3/mol and Faraday’s constant, F
ϭ 96,485 C/mol, multiplied by the local curvature ␬. For planar
substrates, such as were used in acquiring the i-␩ data, the curvature
␬ ϭ 0.
For planar substrates, mass balance, assuming a psuedo-steady-
state approximation, yields Eq. 3, which matches the flux of catalyst
adsorbing on the surface with that diffusing across a boundary layer
of thickness ␦, for diffusion coefficient DAg . Likewise, Eq. 4
matches the rate of silver ion incorporation into the deposit with the
flux of silver ions across the boundary layer.
The i-␩ data were fit as follows. First, the time-dependent cov-
erage of selenium, ␪(t,k), is obtained for a particular electrolyte
concentration CSe and rate constant k, by numerically integrating
Eq. 2 for a Ϫ1 mV/s sweep rate. The time-dependent current is then
obtained using Eq. 1, for particular functions ␣͑␪͒ and io(␪). The
functions ␣͑␪͒ and io(␪), both presumed to be linear in ␪, and the
rate constant k were obtained by optimizing the fit to the experimen-
tal i-␩ curves obtained for the eleven different concentrations of
KSeCN catalyst. Values used for ⌫, the maximum possible coverage
of adsorbed catalyst, and DAg in Eq. 1-3 were the same as those used
in a previous publication4: 7.6 ϫ 10Ϫ10 mol/cm2 and 7.6 ϫ 10Ϫ6
cm2/s, respectively. The diffusion coefficient DSe for the selenium
catalyst was set equal to DAg . The bulk concentration of the catalyst
SeCNϪ was assumed to be the known concentration of KSeCN; the
bulk concentration of silver is assumed to be the same as the con-
centration of KAg͑CN)2 . A voltage independent k ϭ 1.05 ϫ 106
cm3/mol s, with F/RT ϭ 38.92 VϪ1 at T ϭ 293 K and overpoten-
tial ␩ in volts, was found to fit the data well. The resulting fits for
the kinetic parameters are io ϭ 0.18 ϩ 5.4␪ mA/cm2 and ␣ ϭ 0.23
ϩ0.12␪. The boundary layer thickness ␦ was also used as a fitting
parameter; a value of 95 ␮m optimized the fit. For these parameters,
the limiting current associated with transport of the metal ions, equal
to DAgCAgF/␦, has a value of 26.2 mA/cm2. The fits for the eleven
curves, each one a hysteretic cycle containing inflection points on
both the outgoing and return sweeps, have an average root mean
square deviation of less than 0.8 mA/cm2 using only seven fitting
parameters. Several of the i-␩ data curves and the associated simu-
lations are shown in Fig. 3.
Figure 1. Effect of KSeCN concentration on the i-␩ response in electrolyte
containing 0.34 mol/L KAg͑CN)2 , 2.3 mol/L KCN, and ͑a͒ 0 to 1 ␮M
KSeCN and ͑b͒ 1 to 19.4 ␮mol/L KSeCN.
d␪
i⍀Ag
ϭ
␬␪ ϩ kCiSe 1 Ϫ ␪͒
͓2͔
͓3͔
͓4͔
͑
dt
F
C
Ϫ Ci
͒
Se
͑
DSe
Se
kCiSe 1 Ϫ ␪͒ ϭ
͑
⌫
␦
C
Ag Ϫ CiAg
͒
͑
i
ϭ DAg
F
␦
Equations 1 and 2 are the mathematical expressions of the curvature
enhanced accelerator coverage model.4-9 Equation 1 details how the
local silver deposition rate increases with the coverage of adsorbed
catalyst, while Eq. 2 describes how the local coverage of adsorbed
catalyst changes during metal deposition. More specifically, the
Butler-Volmer equation, Eq. 1, expresses the local metal deposition
rate as a function of the overpotential ␩, catalyst coverage ␪, and the
silver ion concentration at the interface CiAg ͑for bulk concentration
C
Ag). Evaluation of the coverage of the adsorbed catalyst is de-
These kinetic parameters can now be utilized in the CEAC model
to predict superfilling behavior in vias. The same equations are used
for modeling feature filling as were used for modeling the i-␩ re-
sults. The only differences are that ͑i͒ all kinetic parameters are now
scribed by Eq. 2 where the first term describes local change of the
catalyst coverage through local area change and the second term
accounts for adsorption from the electrolyte according to Langmuir
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Journal of The Electrochemical Society, 150 ͑2͒ C61-C66 ͑2003͒
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Figure 3. Comparison of experimental i-␩ curves ͑solid line͒ obtained on planar substrates with those obtained for the optimized kinetic parameters. Each plot
differs only predicted ͑dashed line͒ in the concentration of KSeCN catalyst in the electrolyte. With the exception of the experimental results for the additive-free
case, the outgoing sweeps are the curves with the lower current at a given voltage for both the experimental and modeling results. Both the bare surface
͑additive-free͒ and saturated surface ͑for 19.4 ␮mol/L KSeCN͒ behaviors are captured as is the time-dependent accumulation of the catalyst as manifested in the
hysteresis loops at intermediate concentrations.
Concentrations were assumed to be independent of position within
the vias. All vias studied were 1.5 ␮m apart, nearly five times the
largest via dimension, making additional metal ion or catalyst deple-
tion due to deposition on the extra surface area ͑i.e., the sidewalls͒
of the vias small enough to ignore. Time-dependent concentrations
equal to those over a planar substrate for deposition under equiva-
lent conditions ͑as per Eq. 3 and 4͒ were therefore used. Results are
shown in Fig. 4 for electrolytes, deposition potentials and via aspect
ratios chosen to match the experimental results. Aspect ratios were
specified and (ii) the curvature ␬ of the surface is generally
nonzero.
Modeling of Via Filling
A front tracking code, analogous to that previously used for mod-
eling fill of trenches,6 was used to model via filling. The code was
appropriately modified to account for the cylindrical geometry of the
via. Silver ion and catalyst depletion were approximated as that
obtained for deposition on a planar substrate as per Eq. 3 and 4.
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Journal of The Electrochemical Society, 150 ͑2͒ C61-C66 ͑2003͒
Figure 4. Filling predicted by the
CEAC model. Simulations are shown for
silver deposition in electrolytes contain-
ing 0.34 mol/L KAg͑CN)2 , 2.3 mol/L
KCN with different KSeCN concentra-
tions and via aspect ratios indicated. Re-
sults for overpotential Ϫ0.2 V are shown
on top and for Ϫ0.3 V on the bottom.
Superconformal filling is predicted in all
cases, as indicated by the bottom-to-top
filling.
chosen to match those obtained using the experimental via heights
divided by their diameters at midheight. Predictions based on the
kinetics obtained from the i-␩ studies on planar substrates and the
evolution Eq. 1 through 3 are shown in Fig. 4.
Deposition on patterned substrates.—Filling of vias with an as-
pect ratio of 1.3 ͑defined as above͒ in electrolytes at a deposition
potential of Ϫ0.2 V and varying concentration of catalyst is shown
in Fig. 5. The absence of voids in the via filled in 0.2 ␮mol/L
KSeCN is consistent with superconformal filling ͑Fig. 5, top͒. The
time sequence for deposition in 1 ␮mol/L KSeCN shows the
bottom-to-top filling that is an unambiguous sign of superconformal
deposition ͑Fig. 5, middle͒. A further increase of the catalyst con-
centration to 5 ␮mol/L KSeCN yields improved superconformal fill-
ing ͑based on the reduced thickness of the deposits on the sidewalls͒
at an aspect ratio ͑AR͒ of 2.0 ͑Fig. 5, bottom͒.
Filling of vias at a deposition potential of Ϫ0.3 V and varying
concentrations of catalyst is shown in Fig. 6. In contrast to the
results obtained at Ϫ0.2 V ͑Fig. 5͒, a void in the feature, and a cusp
over it, both clear signs of conformal filling, are evident in the via
filled in the electrolyte with 0.2 ␮mol/L KSeCN ͑Fig. 6, top͒. A time
sequence at 1 ␮mol/L KSeCN shows the evolution of such failure
͑Fig. 6, middle͒. However, for a concentration of 2 ␮mol/L KSeCN,
filling is clearly consistent with bottom-to-top superconformal filling
͑Fig. 6, bottom͒. The off-center void most likely formed as the up-
ward moving bottom surface went across that location on the side-
wall deposit.
Consistent with the experimental results at Ϫ0.2 V ͑Fig. 5͒, su-
perconformal filling was predicted for the via of AR 1.3 via in 0.2
␮mol/L KSeCN, AR 1.6 via in 1 ␮mol/L KSeCN and the AR 2.0 via
in 5 ␮mol/L KSeCN ͑Fig. 4, top row͒. The sidewall thickness and
height of the bottom surface are also generally consistent with the
predicted times. In contrast, while the experimental results for filling
of AR 1.3 vias at Ϫ0.3 V showed superconformal filling only for a
catalyst concentrations of 2 ␮mol/L ͑Fig. 6, bottom͒, superconfor-
mal filling was predicted for 0.2, 1, and 2 ␮mol/L concentrations
͑Fig. 4, bottom row͒.
Figure 5. Silver filling of vias at an overpotential of Ϫ0.2 V in electrolytes
containing 0.34 mol/L KAg͑CN)2 , 2.3 mol/L KCN, and ͑top͒ AR 1.3 for 0.2
␮mol/L KSeCN, ͑middle͒ AR 1.6 for 1 ␮mol/L KSeCN, and ͑bottom͒ AR
2.0 for 5 ␮mol/L KSeCN. The absence of a void for 0.2 ␮mol/L KSeCN is
consistent with the evident superconformal filling for the 1 ␮mol/L KSeCN
and the 5 ␮mol/L KSeCN.
In light of the rapid motion of the bottom surface during the
period of superconformal filling, agreement between predictions and
experiment is excellent for filling in 5 ␮mol/L at Ϫ0.2 V and in 2
␮mol/L at Ϫ0.3 V. The source of the disagreement between model
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Journal of The Electrochemical Society, 150 ͑2͒ C61-C66 ͑2003͒
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Figure 8. Silver fill of vias in electrolytes containing 0.34 mol/L
KAg͑CN)2 , 2.3 mol/L KCN, and 2 ␮mol/L KSeCN at ͑top͒ Ϫ0.2 V and
͑bottom͒ Ϫ0.3 V. The sidewall thicknesses on the vias that have nearly
finished bottom-to-top filling permit the maximum aspect ratios for super-
conformal filling to be estimated.
Figure 6. Silver filling of vias at an overpotential of Ϫ0.3 V in electrolytes
containing 0.34 mol/L KAg͑CN)2 , 2.3 mol/L KCN, and ͑top͒ 0.2 ␮mol/L
KSeCN, ͑middle͒ 1 ␮mol/L KSeCN, and ͑bottom͒ 2 ␮mol/L KSeCN. The
central void and cusped surface for 0.2 ␮mol/L KSeCN are consistent with
conformal filling as shown for the 1 ␮mol/L KSeCN. Superconformal
bottom-to-top filling is evident for the 2 ␮mol/L KSeCN in spite of the
off-center void.
top and middle͒. Surface roughness has previously been noted to
lead to void formation where it is not otherwise expected.4
Once bottom-to-top superconformal filling of the feature is fin-
ished, a bump develops over the feature ͑Fig. 7͒. This is in agree-
ment with model predictions shown in Fig. 4, and with previous
experimental and modeling predictions for superconformal copper
deposition.5-9 As per the model predictions, the rapid upward motion
of the bottom surface is due to the increased catalyst coverage there
induced through area decrease during deposition on the concave
surface. The resulting increased catalyst coverage causes enhanced
deposition on this surface even after it leaves the via, creating the
bump visible above the fully filled feature. The overfill bump is thus
a direct result of the same CEAC mechanism that is responsible for
the superconformal bottom-to-top filling of the feature itself. Note
that, due to the opposite sign curvature of the surface, which be-
comes convex after leaving the via, deposition on the bump deac-
celerates toward that on the planar surface with further deposition.
and experiment for the 0.2 and 1 ␮mol/L catalyst concentrations at
Ϫ0.3 V is unclear, although one possibility is surface roughness.
Note particularly the roughness of the sidewall deposits for the ex-
perimental results that the model failed to predict accurately ͑Fig. 6,
Estimating the limits.—It is possible to estimate the maximum
aspect ratio feature that can be filled using superconformal filling
results on a lower aspect ratio feature. This maximum value is the
feature height divided by the twice the sidewall thickness just prior
to completion of filling. While this construct is strictly accurate for
filling of trenches, it is only approximate for filling of vias because
the fractional decrease of sidewall area for a given thickness of
deposit scales inverselt with the radius. This makes estimates of the
maximum fillable aspect ratio based on larger diameter vias liberal.
As an example, a sidewall thickness of Ϸ0.1 ␮m just prior to filling
of the two larger diameter vias in 2 ␮mol/L KSeCN at Ϫ0.2 V ͑Fig.
8, top͒ yields a minimum via diameter of Ϸ0.2 ␮m for superconfor-
mal filling. Using the via height of 0.46 ␮m, this indicates that AR
2.3 is the highest that can be filled for these conditions. This is
reasonably consistent with the failure of the AR 2 via for these
conditions. Similarly, the sidewall thickness of Ϸ0.125 ␮m just
prior to via filling of the larger diameter via in 2 ␮mol/L KSeCN at
Ϫ0.3 V
Figure 7. An overfill bump is visible above the fully filled via. Silver depo-
sition was accomplished in an electrolyte containing 0.34 mol/L KAg͑CN)2 ,
2.3 mol/L KCN, and 1 ␮mol/L KSeCN at an overpotential of Ϫ0.2 V for
85 s.
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Journal of The Electrochemical Society, 150 ͑2͒ C61-C66 ͑2003͒
͑Fig. 8, bottom͒ yields a minimum diameter of Ϸ0.25 ␮m and a
maximum AR 1.8 for the same electrolyte at Ϫ0.3 V. Again, this is
reasonably consistent with the marginal failure observed in the 1.7
AR feature for these conditions. Based on the Ϸ0.05 ␮m thick side-
walls for the via filled for 25 s in 5 ␮mol/L KSeCN at Ϫ0.2 V ͑see
Fig. 5, bottom͒, a minimum diameter of Ϸ0.1 ␮m and a maximum
AR 4.6 are obtained.
References
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Conclusion
This work has presented an electrolyte containing a single Se-
based additive that yields superconformal silver electrodeposition in
fine features. The curvature enhanced accelerator coverage ͑CEAC͒
model, using kinetics obtained from deposition behavior on planar
substrates, was used to quantitatively predict the filling behavior.
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͑2001͒.
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The National Institute of Standards and Technology assisted in meeting
the publication costs of this article.
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