In situ STM study of Cu electrodeposition on TBPS-modified Au(111) electrodes

Article abstract of DOI:10.1149/2.057202jes

The impact of 3,3′^′-thiobis(1-propanesulfonic acid, sodium salt) (TBPS) on Cu/Au(111) electrodeposition has been investigated by electrochemical methods and scanning tunneling microscopy (STM). Cyclic voltammetry and galvanostatic experiments indicate that Cu growth on Au(111) - which is known to be strongly kinetically hindered in additive-free, aqueous perchloric acid solutions - proceeds significantly faster in the presence of TBPS. The TBPS molecules either float on top of the growing film or become incorporated into the deposit. Complementary in situ STM studies show that Cu underpotential deposition (UPD) proceeds via two distinct mechanisms. One-dimensional growth of Cu stripes was observed between 0.05 and 0.35 V _RHE for TBPS-modified Au(111) electrodes. Each stripe is composed of two or three parallel rows of Cu atoms oriented along the main crystallographic directions of the Au(111) substrate. An increase of the TBPS concentration near the solid/liquid interface restricts the Cu stripe growth to a narrow potential regime between 0.3 and 0.35 V_RHE and two-dimensional Cu island growth becomes the favored growth mechanism. The latter fully dominates in TBPS-containing electrolyte. Cu growth in the overpotential deposition (OPD) regime results in a smooth Cu film with low surface roughness, in contrast to defect-mediated 3D island growth in additive-free electrolytes.

Full text of DOI:10.1149/2.057202jes

D84
Journal of The Electrochemical Society, 159 (2) D84-D90 (2012)
0013-4651/2012/159(2)/D84/7/$28.00 © The Electrochemical Society
In Situ STM Study of Cu Electrodeposition on TBPS-Modified
Au(111) Electrodes
Yung-Fang Liu,a Klaus Krug,a Pin-Chun Lin,a Yong-Da Chiu,b Wei-Ping Dow,b,∗
Shueh-Lin Yau,c and Yuh-Lang Leea,z
aDepartment of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan
bDepartment of Chemical Engineering, National Chung Hsing University, Taichung 40227, Taiwan
cDepartment of Chemistry, National Central University, Jhong-Li 32001, Taiwan
The impact of 3,3ꢀ-thiobis(1-propanesulfonic acid, sodium salt) (TBPS) on Cu/Au(111) electrodeposition has been investigated by
electrochemical methods and scanning tunneling microscopy (STM). Cyclic voltammetry and galvanostatic experiments indicate
that Cu growth on Au(111) - which is known to be strongly kinetically hindered in additive-free, aqueous perchloric acid solutions
- proceeds significantly faster in the presence of TBPS. The TBPS molecules either “float” on top of the growing film or become
incorporated into the deposit. Complementary in situ STM studies show that Cu underpotential deposition (UPD) proceeds via two
distinct mechanisms. One-dimensional growth of Cu stripes was observed between 0.05 and 0.35 VRHE for TBPS-modified Au(111)
electrodes. Each stripe is composed of two or three parallel rows of Cu atoms oriented along the main crystallographic directions of
the Au(111) substrate. An increase of the TBPS concentration near the solid/liquid interface restricts the Cu stripe growth to a narrow
potential regime between 0.3 and 0.35 VRHE and two-dimensional Cu island growth becomes the favored growth mechanism. The
latter fully dominates in TBPS-containing electrolyte. Cu growth in the overpotential deposition (OPD) regime results in a smooth
Cu film with low surface roughness, in contrast to defect-mediated 3D island growth in additive-free electrolytes.
© 2011 The Electrochemical Society. [DOI: 10.1149/2.057202jes] All rights reserved.
Manuscript submitted September 29, 2011; revised manuscript received October 26, 2011. Published December 19, 2011. This was
Paper 1568 presented at the Montreal, QC, Canada, Meeting of the Society, May 1−6, 2011.
Cu damascene plating is a central process in modern microelec-
deposition.13 Cu bulk growth in the overpotential deposition (OPD)
regime proceeds via instantaneous nucleation at Au(111) surface
defects (e.g., at step edges or dislocations) and subsequent forma-
tion of 3D Cu islands in 0.1 M HClO4 solution.10,20 By the use of
specific additives the crystallite growth perpendicular to the surface
can be suppressed with respect to the lateral growth, resulting in a
smooth Cu film on Au(111) substrates.20,21 Adlayers of alkanethiols
with a chain length smaller than 9 carbon atoms are pervious for
Cu2 + ions.22
In the present paper we describe the influence of 3,3 Thiobis(1-
propanesulfonic acid, sodium salt) (TBPS) on Cu/Au(111) growth.
TBPS is a thioether derivative (NaO3S-(CH2)3-S-(CH2)3-SO3Na) with
a monosulfur head group, two comparatively short alkyl chains and
two functional groups of sulfonate. Apart from different headgroups (1
versus 2 sulfur atoms) the molecular structure closely resembles that of
tronic industry to fill nanoscale trenches with large height-to-width
aspect ratios. To achieve defect-free filling the plating bath typically
contains additives which suppress or accelerate Cu growth. A common
“suppressor” additive is polyethylene glycol (PEG) which forms a
growth inhibiting layer in synergy with chloride ions.1–3 “Accelerator”
additives displace the suppressor and increase the local deposition
rate, a key mechanism to realize bottom-up filling.4,5 Growth acceler-
ators are usually sulfur-containing mercapto-sulphonate compounds,
such as bis-3-sodiumsulfopropyl disulfide (SPS) and its monomer
3-mercapto-1-propanesulfonate (MPS). Previous studies indicated
that the sulfur head group tethers the accelerator molecule to the
substrate surface, while the charged sulfonate end group disrupts the
rate-suppressing PEG-Cl film and increases the molecules ability to
float on the surface during rapid metal deposition.6 In general, an
effective accelerator (i) diffuses faster than the suppressor species,
(ii) displaces the suppressor slowly, (iii) is not incorporated into the Cu
deposit, and (iv) does not break down into species which may interfere
with the electrodeposition process (i.e., does not alter chemically).
SPS and MPS partly comply with these requirements and produce
satisfying results. However, continuously shrinking feature sizes
necessitate alternative compounds with even better filling capability.
We recently investigated a dialkyl sulfide molecule called TBPS (see
below) which is a promising accelerator candidate with regard to
molecular structure and adsorption behavior.7 The present, prelimi-
nary study focuses on the influence of TBPS on Cu electrodeposition
using supressor-free electrolytes and single crystal gold substrates.
Heteroepitaxial electrodeposition of Cu on Au(111) is an important
model system which has been extensively studied by electrochemical
and structure-sensitive methods in aqueous, acidic electrolytes.8–17 It
is well known that Cu forms a monolayer on Au(111) positive of
the Nernst potential for Cu bulk deposition, a phenomenon termed
as underpotential deposition (UPD). In perchloric acid solution two
(quasi-) hexagonal UPD adlayers have been reported, a non-primitive,
commensurate (2×2) Cu adatom structure and an incommensurate
“(5×5)” CuCl(111)-like bilayer structure.11,16–19 An atomic force mi-
croscopy study of Manne et al. furthermore evidences an incom-
mensurate, close-packed (1×1)R30◦ Cu-UPD layer prior to Cu bulk
SPS. We have demonstrated previously that TBPS self-assembles into
√
a (6×3 3) adlayer phase on Au(111) if no overpotential is applied,
i.e., at open circuit potential (OCP).7 This phase is stable in TBPS-free
solution at potentials more positive than 0.7 VRHE and disorders toward
more negative potentials, i.e., in the potential regime where Cu de-
position occurs. In TBPS-containing electrolyte a complex, potential-
dependent adsorption behavior has been observed, including two dis-
ordered phases (σ and σa) and two ordered stripe phases (α and β). The
stripe phases form between ≈0.6 and ≈0.8 VRHE and partially overlap
with the potential regime of Cu-UPD (≈0.35−0.6 VRHE). Negative
of 0.35 VRHE, i.e., in the Cu-OPD regime, the TBPS adlayer is pre-
dominantly disordered, although a third ordered phase (γ) has been
observed in a narrow potential regime between 0.23 and 0.3 VRHE
in cathodic potential sweeps.7 At all electrode potentials, the TBPS
molecules do not dissociate and intactly adsorb on the Au(111) sur-
face, predominantly with a “lying down” orientation.
In the studies presented here, the time-dependent evolution of the
Cu deposit morphology in the presence of TBPS has been investi-
gated by electrochemical methods and in-situ scanning tunneling mi-
croscopy (STM) in the UPD and OPD potential regimes. For this, two
common techniques have been employed, that is i) the ex-situ prepa-
ration of self-assembled TBPS-adlayers (in the following termed as
“TBPS-modified Au(111)”) and subsequent Cu growth in TBPS-free
solution and ii) Cu growth in TBPS-containing electrolyte. In both
cases a pronounced influence of TBPS on the growth behavior and on
the resulting deposit morphology was determined.
∗
Electrochemical Society Active Member.
z E-mail: yllee@mail.ncku.edu.tw
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Journal of The Electrochemical Society, 159 (2) D84-D90 (2012)
Experimental
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Cyclic voltammetry and galvanostatic experiments were per-
formed with a CH Instruments potentiostat (model CHI703A) in an
electrochemical cell with three-electrode configuration, using a re-
versible hydrogen reference electrode (RHE) and a platinum counter
electrode. The Au working electrode was produced by melting one end
of a gold wire (99.95%, Guv Team International Co., Ltd.) followed
by mechanical polishing of the bead to obtain a (111)-oriented surface
(5 mm2). Prior to each experiment the Au single crystal was annealed
in a hydrogen gas flame and quenched in hydrogen-saturated, ultra-
pure water (Milli-Q, 18.2 Mꢀ cm) as described in the literature.23–26
Subsequently, self-assembled monolayers were prepared at room tem-
perature by immersing the Au(111) electrode for 2 min in 0.1 M
HClO4 containing 155 μM TBPS. An optional rinsing process (Milli-
Q for 10 s) removed excess amounts of TBPS from the interface.
Contact between as-prepared (rinsed or unrinsed) Au(111) electrodes
and Cu-containing electrolytes was established at OCP via dipping
technique.27 All electrolytes were prepared from Milli-Q water, ul-
trapure perchloric acid (70% HClO4, Merck), TBPS (Sigma Aldrich)
and cupric perchlorate (98% Cu(ClO4)2, Merck) without further pu-
rification. The solutions and the cell compartment were purged with
nitrogen gas (N 5.5) to reduce the oxygen concentration.
Scanning probe experiments were performed using a NanoScope E
scanning tunneling microscope (Veeco Instruments Inc., ECM model)
and an electrochemical cell (made from Teflon) containing two plat-
inum wires as counter and quasi-reference electrode, respectively. For
convenience, all electrode potentials are quoted with respect to RHE in
this paper. TBPS-modified Au(111) electrodes were transfered to the
STM cell (initially filled with 0.1 M HClO4) and potential control was
established close to OCP (ca. 0.75 V). Subsequently, Cu-containing
electrolyte was added from a stock solution at 0.8 V, which is well
positive to the onset of Cu deposition. STM imaging was started at
potentials close to this value. For measurements in TBPS-containing
solution the molecules were added in the potential regime of TBPS
desorption (0.1 V) prior to the addition of cupric perchlorate at 0.8 V.
Tungsten tips were produced by electrochemical etching of 0.25 mm
diameter polycrystalline wires in 3 M KOH and coated with nail polish
in order to minimize faradaic currents at the tip/electrolyte interface.
All STM images were acquired in the constant current mode with
tunneling currents of typically 1 to 3 nA. Data analysis was carried
out with the WSxM software.28
Figure 1. Cyclic voltammograms (50 mV/s) of a) bare Au(111), b) TBPS-
modified Au(111), and c) unrinsed, TBPS-modified Au(111) in 0.1 M HClO4
+ 100 μM Cu2 +. d) Cu-OPD deposit (θ in monolayers of Au(111)) on un-
rinsed, TBPS-modified Au(111) as obtained from the charge of peak A2 in
CVs with different cathodic potential limits.
Results and Discussion
Cyclic voltammetry.— Figure 1 shows cyclic voltammograms
(CV) of a bare, of a TBPS-modified, and of an unrinsed, TBPS-
respectively. In the reverse anodic scan, the Cu bulk deposit is stripped
from the surface as indicated by an increased anodic current density
between 0.25 and 0.57 V (A2). The sharp anodic current peak A1 at
≈ 0.62 V corresponds to the stripping of the UPD layer. A net charge
transfer of ≈285 μC cm−2 is measured from A1, possibly containing
contributions from TBPS adsorption and coadsorbed anions.
Nucleation sites are blocked and have to be activated at sufficiently
cathodic overpotentials, since the current curve forms a loop after re-
versing the scan direction (Figure 1b, ca. −0.2 V). The same loop
was found in previous studies and has been attributed to the inhibitive
properties of the adlayer towards charge transfer.22,30,31 In addition,
the HER proceeds at potentials negative of −0.25 V, i.e., ≈100 mV
more cathodic with respect to Cu growth on bare Au(111). These two
characteristics indicate that (at least in the initial stage of the depo-
sition process on the foreign substrate and for a potential sweep rate
of 50 mV/s) TBPS remains adsorbed on top of the Cu film. Appar-
ently, the deposited Cu monolayer prevents reductive desorption of
TBPS, since the molecules desorb at more negative potentials as com-
pared to TBPS-modified Au(111) in Cu-free electrolytes (desorption
peak at ≈0.06 V).7 A similar effect has been reported for Cu growth
on ethanethiol-modified22 and propanethiol-modified32 Au(111)
electrodes.
modified Au(111) electrode in 0.1 M HClO4 + 100 μM Cu2 +
.
Cu Growth on Bare Au(111).—For a bare Au(111) surface (Figure 1a),
a shallow broad cathodic current peak at ≈0.52 V (C) and a sharp an-
odic current peak at ≈0.54 V (A) are observed, corresponding to Cu
underpotential deposition and to the dissolution of the Cu monolayer,
respectively. Note that no pronounced copper bulk deposition pro-
ceeds between peak C and the hydrogen evolution regime (HER, ca.
−0.15 V) at a sweep rate of 50 mV/s. The present CV is in good
agreement with those in the literature and shows that both UPD and
OPD of Cu are strongly hindered in HClO4 solution.11,14–16,29 The in-
tegrated charge of peak A, 212 μC cm−2, is at least 34% smaller than
the charge required to dissolve a Cu-UPD layer (e.g., ≈284 μC cm−2
for the low coverage (2×2) phase11) due to the coadsorption of per-
chlorate ions.9 An onset potential for Cu bulk deposition (≈0.35 V)
was estimated on the basis of additional CVs recorded with a sweep
rate of 1 mV/s (not shown here).
Cu Growth on TBPS-modified Au(111).—The characteristics of the
CV change for TBPS-modified Au(111) electrodes. The CV in
Figure 1b was obtained in TBPS-free solution after the Au(111)
surface was pre-modified with TBPS. Two cathodic current peaks
C1 and C2 at 0.49 V and −0.2 V belong to Cu-UPD and Cu-OPD,
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Journal of The Electrochemical Society, 159 (2) D84-D90 (2012)
Cu Growth on unrinsed, TBPS-modified Au(111).—Figure 1c shows
a CV of unrinsed, TBPS-modified Au(111), i.e., with an excess of
TBPS near the solid/liquid interface. The characteristics of the I-
V curve resemble those of TBPS-modified electrodes, however, the
current peaks corresponding to Cu-UPD deposition (C1 at 0.47 V),
Cu-OPD deposition (C2 at −0.035 V), and to the dissolution of the
Cu film (A2 at 0.47 V) are more pronounced. This indicates a faster
deposition of the Cu-UPD layer and a less hampered transition from
UPD to OPD growth (see below). On the other hand, a broad UPD
stripping peak (A1 at 0.56 V) suggests a slow dissolution kinetics.
Additional experiments of this system were performed by sweep-
ing the potential to various cathodic limits (EC). The Cu deposit
was obtained from the charge of A2 for each I-V curve. Figure 1d
summarizes the copper coverages θCu (in monolayers of Au(111))
as a function of EC. For EC ≥ 0.05 V, the Cu-OPD deposit is small
(≤1.1 MLAu), while θCu steeply increases for EC < 0.05 V, corre-
sponding to the increase in cathodic current density in the CV (peak
C2). The current maximum C2 can be safely assigned to Cu bulk de-
position. It is noteworthy that about 3.4 MLAu Cu were deposited for
EC = −0.15 V (Figure 1d), while no significant amount is deposited
on the bare Au(111) electrode (no stripping peak in Figure 1a). We
thus conclude that TBPS promotes Cu growth in the initial stage of
Cu deposition on Au(111) substrates.
once the film is dissolved, or that TBPS is consumed and incorporated
into the Cu deposit. Additional deposition experiments carried out
by in-situ Fourier transform infrared (FTIR) spectroscopy on TBPS-
modified Au(111) confirm the presence of TBPS on top of the growing
Cu film, i.e., a good floating ability of TBPS molecules. The FTIR
results will be presented elsewhere.
Scanning tunneling microscopy study.— The time-dependent evo-
lution of the Cu deposit morphology has been monitored by in-situ
STM in the potential regimes of Cu-UPD and Cu-OPD. After adding
100 μM Cu2 + to the electrolyte at 0.8 V, a time span of 15 min was
allowed for the system to reach equilibrium conditions. Subsequently,
the electrode potential was lowered in steps of 20–50 mV (approxi-
mately every 5–10 min) and STM images were taken simultaneously.
Underpotential deposition.— In the presence of TBPS, the Cu-
UPD layer forms via twodistinct growthmechanisms. The first mecha-
nism (“A”) is characterized by nucleation and one-dimensional growth
of Cu stripes. The second mechanism (“B”) is characterized by nucle-
ation and two-dimensional growth of Cu islands. We first describe our
STM observations and then discuss the interfacial conditions under
which both mechanisms occured.
Galvanostatic study.— To further characterize the influence of
TBPS on Cu growth, galvanostatic experiments were performed
in an aqueous solution containing 0.1 M HClO4 and 1 mM Cu2 +
.
Figure 2 shows potential-time transients for various electrodes
and electrolytes. Prior to the measurements (t < 0 s), the samples
were hold at their OCP. For t ≥ 0 s, a constant current density of
j = −20 μA/cm2 was applied, corresponding to a Cu deposition rate
of 2.7 MLAu/min (2.1 MLCu/min, MLAu and MLCu are monolayers of
Au(111) and Cu(111), respectively). With regard to the pseudomor-
phic (1×1) Cu-UPD phase that is formed on bare Au(111) prior to Cu
bulk deposition,11 the Cu coverage (θCu) of thin films is adequately
expressed using MLAu. For thicker films, i.e., homoepitaxial Cu
deposition, MLCu is used to quantify θCu. Coverages determined via
Faradays law are in good agreement with those from linear sweep
voltammetry (anodic Cu bulk stripping peaks).
As shown by the solid lines in Figure 2a and Figure 2b, the potential
of a bare Au(111) electrode steeply decreases from 0.62 V to −0.06 V
within the first 19 s, then slowly increases and approaches an almost
constant potential of ≈0.27 V for t > 130 s (θCu > 6 MLAu). The
potential minimum correlates with the completion of the Cu-UPD
layer (i.e., θCu ≈ 1 MLAu, Figure 2b). Apparently, both the Cu-UPD
and the UPD to OPD transition are kinetically hindered in additive-
free HClO4 solution, in agreement with the literature.11,14,29
In the presence of TBPS (dash-dotted and dashed line), a markedly
different behavior is observed, particularly in the initial period of the
deposition process (Figure 2b). The potential first decreases from
OCP (≈0.76 V) to 0.25 V and then remains constant for t > 90 s
(θCu > 4 MLAu). It is evident that Cu deposition occurred at sig-
nificantly less negative overpotentials on TBPS-modified electrodes
(dash-dotted line), particularly with excess of TBPS at the interface
(dashed line). Kinetic limitations are effectively reduced by TBPS.
Potential plateaus at ≈0.5 V correlate with cathodic current peaks
of UPD deposition in cyclic voltammograms (not shown here, c.f.
Figure 1).
Our data indicates a pronounced influence of TBPS on Cu growth
up to a deposit amount of θCu ≈ 6 MLAu. For θCu > 6 MLAu all
galvanostatic curves exhibit a similar saturation potential. However,
even though the potential-time transients do not reveal differences in
later stages of Cu film growth (OPD), TBPS may still have a pro-
nounced influence on the kinetic growth behavior and on the deposit
morphology. It is interesting to note that even after stripping of thicker
Cu films (≈70 MLCu) from TBPS-modified Au(111) electrodes, the
same characteristic shape is obtained in subsequent galvanostatic
potential-time transients. This implies either that TBPS “floats” on
top of the growing Cu film and readsorbs on the bare Au(111) surface
Figure 2. Galvanostatic potential-time transients of Au(111) in 0.1 M HClO4
+ 1 mM Cu2 +. The curves show data of bare Au(111) (solid lines),
TBPS-modified Au(111) (dash-dotted lines), and Au(111) in 155 μM TBPS-
containing solution (dashed lines). Panel b enlarges the initial time period. For
t ≥ 0 s a fixed current density of j = −20 μA/cm2 was applied.
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Journal of The Electrochemical Society, 159 (2) D84-D90 (2012)
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a
b
d
f
3√3
6
0.85V
0.60V
0.37V
0.15V
c
e
g
0.42V
0.33V
0.05V
Figure 4. a-b) High-resolution STM images (a: 20×20 nm2 and b:
10×10 nm2) of Cu stripes on TBPS-modified Au(111) in 0.1 M HClO4
+ 100 μM Cu2 +. The color scale in panel b has been adjusted to empha-
size the linear array of small spots along the rim of Cu stripes (blue circles).
c) Cross-section profile along one row of Cu atoms (red circles) in the direction
of the stripe. d) Illustration of the (2×2) arrangement of Cu atoms (black balls)
on top of the Au(111) substrate (grey balls).
anisotropically into Cu stripes. Figure 3e illustrates the initial Cu
stripe morphology at 0.33 V with isolated Cu stripes and small Cu
islands. However, the blurry appearance of the deposit and rapid
changes in subsequent STM images (including the vanish of existing
stripes and the formation of new stripes) indicate a pronounced
mobility of Cu adatoms and a low stability of the stripes. The latter
may be either a potential-dependent effect or related to an influence
of the scanning tip on the growth process. A stable Cu stripe phase
is obtained at more negative potentials as demonstrated for 0.15 V
in Figure 3f. In this potential regime, the stripes are well defined
and oriented in angles of 60 or 120◦ with respect to each other,
reflecting the three-fold symmetry of the Au(111) substrate. A cross
section analysis reveals an average stripe height of 0.2 0.03 nm,
stripe thicknesses between 2 and 3 nm, and a maximum stripe length
of approximately 80 nm. Since TBPS adlayers have a height of
0.1 0.03 nm, the stripe height clearly evidences Cu atoms.
The transition from UPD to OPD is slow, even at high cathodic
overpotentials. An STM image at 0.05 V (Figure 3g) shows small
bright spots on top of the Cu stripes, which are attributed to second
layer Cu growth. At first sight, a slow UPD to OPD transition seems
to be at variance with the results of our galvanostatic study, since a
low kinetic hindrance was determined for TBPS-modified Au(111)
electrodes. In order to understand the discrepancy, the 10 times lower
Cu concentration in the STM studies and a possible shielding of Cu
ion transport by the STM tip have to be taken into account. It seems
reasonable that the UPD to OPD transition proceeds more quickly
with increasing Cu2 + concentration. For a comparatively high Cu2 +
concentration (e.g., 1 mM Cu2 +), the kinetic behavior is similar for
TBPS-modified Au(111) and for Au(111) in TBPS-containing elec-
trolyte, respectively, as indicated by the shapes of potential-time tran-
sients (Figure 2). In this case, the presence (but not the concentra-
tion) of TBPS at the interface seems to be critical for the lowering
of kinetic limitations. However, at low Cu2 + concentrations (e.g.,
0.1 mM Cu2 +), the cyclic voltammograms of rinsed (Figure 1b) and
unrinsed (Figure 1c) TBPS-modified Au(111) show prominent differ-
ences, indicating that the amount of TBPS plays an important role
for the transition from UPD to OPD. The slow onset of OPD in the
Figure 3. Mechanism A of UPD Growth: STM images (a-f: 200×200 nm2,
g: 300×300 nm2) of TBPS-modified Au(111) in 0.1 M HClO4 + 100 μM
√
Cu2 +. a) In Cu-free electrolyte (0.1 M HClO4) a (6 × 3 3) TBPS adlayer
phase (right panel, 5 × 5 nm2) is observed at 0.85 V. b-g) In Cu-containing
electrolyte, the Cu-UPD phase forms via one-dimensional growth of Cu stripes
at more negative potentials.
Mechanism A of Cu-UPD growth.—Figure 3 shows STM images of a
TBPS-modified Au(111) electrode. At 0.85 V, prior to the addition of
√
Cu ions (0.1 M HClO4), the Au substrate is covered by a (6 × 3 3)
TBPS adlayer phase (Figure 3a). After injection of the Cu species
(0.1 M HClO4 + 100 μM Cu2 +) and lowering of the electrode
potential to 0.6 V, the TBPS-adlayer is disordered (Figure 3b), in
agreement with our previous studies in Cu-free electrolytes.7 No
evidence for Cu growth has been found at 0.6 V. By further decreasing
the potential to 0.42 V, the Cu deposition process starts at step edges
(Figure 3c). First nucleations in the center of Au terraces are
observed at 0.37 V (bright spots in Figure 3d) and the nuclei grow
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Journal of The Electrochemical Society, 159 (2) D84-D90 (2012)
aforementioned STM study is in agreement with the moderate promo-
tion of Cu growth in the CV of rinsed electrodes (Figure 1b). Cu-UPD
starts at more negative potentials in the STM study (growth at the step
edge at ≈0.42 V, Figure 3c) as compared to the CV study (current peak
at ≈0.5 V, Figure 1b) which is possibly caused by a slower deposition
rate due to tip shielding effects.
The panels a and b of Figure 4 show high-resolution STM images of
the Cu stripes. Most of the stripes are composed by two (seldom three)
parallel rows of large bright spots (Figure 4a) which are attributed to
Cu atoms (red circles in Figure 4b). A cross section analysis along
the Cu rows (Figure 4c) reveals an average interatomic distance of
≈0.58 nm, i.e., twice the nearest distance between Au atoms (aAu
= 2.885 Å). This suggests that Cu stripes are oriented along the
closed-packed crystallographic directions of the Au(111) substrate. In
addition, two neighbored atomic rows are shifted by aAu with respect
to each other in the direction of the stripe, leading to a rather open
(2×2) arrangement of Cu atoms (illustrated in Figure 4d). Figure 4b
highlights the rims of Cu stripes and unveils a linear array of smaller
spots (blue circles) next to the Cu atoms. These features may represent
TBPS molecules. A laterally attached complexing agent would be
in agreement with the anisotropic growth behavior (see below), the
preferred stripe width of two atomic Cu rows, and the fact that no full
coverage UPD layer is formed, even for prolonged holding times at
more cathodic potentials.
Similar ramified or dendritic structures have been often observed
during metal epitaxy in ultra-high vacuum (UHV).33–35 However,
the UHV phases are usually less well-defined and the deposit of-
ten exhibits no long-range correlation with the trigonal substrate
symmetry.34 In addition, the UHV structures are tree-shaped with
a large amount of branches while only a low degree of branching
occurs on TBPS-modified Au(111) electrodes (see Figure 3f). Brune
et al. proposed that dendritic growth is caused by anisotropic perime-
ter diffusion at the rim of close-packed nuclei, resulting in a preferred
growth in favored in-plane directions.35 In contrast, the (2×2) arrange-
ment in the present study exhibits a two times larger distance between
two nearest neighbor Cu atoms. We thus assume that perimeter dif-
fusion plays only a minor role in the present system. As insinuated
above, the anisotropic growth behavior may be explained by a TBPS-
induced blocking at two opposite sides of the unit cell (long sides of
the stripes) while growth along the two remaining sides (both stripe
ends) becomes favorable. Such a mechanism requires the successive
or simultaneous aggregation of Cu adatoms and TBPS molecules (Cu-
TBPS surface complexes, respectively) at the growing stripe ends.
The present stripe phase also differs from stripe phases previously
observed in electrochemical environment. Hagenstro¨m et al. and Cav-
alleri et al. determined ramified Cu deposits on ethanethiol-modified
Au(111) electrodes22 and on hexane-covered Au(111) electrodes,36
respectively, which resemble those in UHV. Tao et al. observed the
growth of Ag nanowires on Au(111) in aqueous sulfuric acid solution
containing sodium dodecylsulfide (SDS).37 Although the morphology
of the Ag nanowires resembles the morphology of Cu stripes on TBPS-
modified Au(111), both the growth conditions and the stability of the
nanostructures differ markedly. While the Cu stripes form within a
disordered TBPS phase and are stable at high cathodic overpotentials,
the Ag nanowires grow into an ordered SDS adlayer (parallel lines of
SDS hemicylinders) and collapse into circular islands towards more
negative potentials. Apparently TBPS plays a significant role in the
Cu stripe formation process and in the stabilization of the Cu stripes.
Figure 5. Mechanism B of UPD Growth: STM images (a: 100×100 nm2,
b-f: 200×200 nm2) of Au(111) in 0.1 M HClO4 + 100 μM Cu2 + + 155 μM
TBPS. The UPD phase forms via two-dimensional growth of Cu islands.
Cu atoms rapidly fill the Au etch pits and two-dimensional Cu islands
form on top of gold terraces. These islands grow laterally with time
and toward more cathodic potentials. At 0.5 V almost all Au etch
pits have been filled and the size of Cu islands markedly increased
(Figure 5d). The island density, the average island size, and the Cu
surface coverage saturate at 0.4 V (Figure 5e). Repeated monitoring
at 0.4 V or at slightly more negative potentials yields similar STM
images (Figure 5e and Figure 5f). Apparently, if the electrolyte con-
tains TBPS, no well-defined UPD layer with large domains can form
Mechanism B of Cu-UPD growth.—Figure 5 shows STM images for
Au(111) in 0.1 M HClO4 + 100 μM Cu2 + containing 155 μM TBPS.
An STM image at 0.8 V (Figure 5a) shows an ordered TBPS stripe
phase (β), pinholes in the adlayer, and monoatomic high vacancy is-
lands (in the following termed as “etch pits” for the sake of brevity)
in the topmost Au layer. The latter have been attributed to an TBPS-
induced etching process at anodic electrode potentials.7 The TBPS
adlayer is disordered at 0.65 V (Figure 5b) and small bright spots with
a height of ≈0.2 nm indicate the onset of Cu-UPD. The surface mor-
phology drastically changes at 0.6 V as demonstrated in Figure 5c.
Figure 6. STM images (138×116 nm2) of Au(111) in 0.1 M HClO4 + 100 μM
Cu2 + + 155 μM TBPS. Panels a and b show the same surface area before
(0.65 V) and after (0.55 V) Cu deposition, respectively. Blue polygons in both
images mark the rim of etch pits observed at 0.65 V.
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Journal of The Electrochemical Society, 159 (2) D84-D90 (2012)
D89
concentration of TBPS near the solid/liquid interface, the Cu depo-
sition rate, and the electrode potential. An important additional pa-
rameter is the holding time at constant electrode potential. Although
several STM experiments have been performed using identical prepa-
ration procedures, no optimal conditions for a pronounced Cu stripe
growth could be determined. By contrast, Mechanism B, i.e., pure 2D
island growth without the occurence of Cu stripes, has been always
observed in TBPS-containing electrolyte. In this case, the Cu-UPD
phase is fully established at 0.4 V, which is significantly more positive
than the typical potential regime for Cu stripe growth (E ≤ 0.35 V).
In addition, a high density of Au etch pits does not provide large, flat
surface areas which are required for the growth of Cu stripes.
Overpotential deposition.— Copper overpotential deposition has
been monitored by in situ STM in TBPS-containing solution. Figure 7
shows the time-dependent evolution of the Cu deposit morphology. At
0.35 V (Figure 7a) the Au(111) substrate is covered by the Cu-UPD
phase, predominantly consisting of monoatomic high Cu islands. The
root mean square (RMS) roughness of this surface, σRMS = 0.1 nm,
is approximately half the height of a Cu(111) layer (0.21 nm) and
corresponds to a perfectly terminated surface covered by ≈0.5 ML
Cu. Subsequent to a potential step from 0.35 V to 0.3 V (t = 0 min), a
three-dimensional growth of homogeneously distributed Cu islands is
observed (Figure 7b–7d). At first sight this growth behavior resembles
3D island growth in additive-free solutions,10,20 except that the growth
is not restricted to step edges or dislocations. However, when the scan
area was increased from 200×200 nm2 to 300×300 nm2 a strong
influence of the scanning tip on the growth behavior became apparent
(Figure 7e). Figure 7g shows a cross-section profile along the blue
line in Figure 7e. From the height difference of ≈2.5 nm between the
lowest point and the constant height in shaded areas, a minimum film
thickness of 12 MLCu is estimated.
Aside from previously scanned regions (e.g., in the 50 nm broad
stripe at the top of Figure 7e), the deposit morphology significantly
differs from the 3D deposit and a low surface roughness indicates
a smooth Cu film, i.e., a “smooth multilayer” or layer-by-layer type
of growth. In order to confirm this, STM images were acquired at
additional lateral positions on the sample surface. The surface mor-
phology in those regions (see Figure 7f) strongly resembles the one in
Figure 7a with predominantly monoatomic high Cu islands and a few
(potentially tip-induced) Cu islands of up to 3 ML height. After the
deposition of more than 12 ML Cu, the surface roughness only mod-
erately increased from σRMS = 0.1 nm (Figure 7a) to σRMS = 0.18 nm
(Figure 7f).
Even though the tip-induced 3D growth does not allow us to follow
the real growth behavior in-situ, the present study shows that Cu
deposition in TBPS-containing solution leads to high quality Cu films
on planar Au(111) surfaces.
Figure 7. STM images (a-d,f: 200×200 nm2, e: 300×300 nm2) of Au(111) in
0.1 M HClO4 + 100 μM Cu2 + + 155 μM TBPS. a) Cu-UPD at 0.35 V. b-e)
Time-dependent evolution of the Cu-OPD film morphology after stepping the
potential from 0.35 V to 0.3 V at t = 0 min. f) Morphology at another lateral
position on the sample surface. g) Cross section profile along the blue line in
panel e.
Conclusions
We have studied Cu electrodeposition on Au(111) electrodes in
aqueous perchloric acid solution and in the presence of TBPS. Elec-
trochemical methods indicate that TBPS accelerates Cu growth in the
UPD regime and in the transition regime from UPD to OPD. TBPS
molecules either “float” on top of the deposit or become incorporated
into the Cu film.
The time- and potential-dependent evolution of the Cu deposit mor-
phology was monitored by in-situ STM. In the regime of Cu-UPD,
a novel Cu stripe phase was observed for TBPS-modified Au(111)
electrodes. Under these conditions, the TBPS concentration at the in-
terface is low and TBPS potentially assists one-dimensional Cu stripe
growth by lateral attachment perpendicular to the stripe direction.
In contrast to ramified Cu deposits reported previously in UHV or
electrochemical environment, the TBPS-induced Cu stripe phase is
well-defined, i.e., it exhibits a low amount of branches and a good
correlation with the substrate symmetry. With increasing concentra-
tion of TBPS at the interface, a crossover from Cu stripe growth
to two-dimensional Cu island growth was observed, which fully
on the Au(111) surface. This behavior is likely related to the large
amount of Au etch pits. Indeed, a comparison of STM images before
Cu deposition (Figure 6a) with those at later stages of Cu growth
(Figure 6b) reveals that Cu islands preferentially grow in etch pit free
surface regions, while almost no Cu islands are observed on top of
Cu-filled etch pits.
Observations of A and B.—The UPD growth mechanism strongly de-
pends on the experimental conditions. Mechanism A, in the form as
discussed above, has been only observed on TBPS-modified Au(111)
electrodes, i.e., in the presence of (ideally) a single monolayer of ad-
sorbed TBPS. However, even for rinsed electrodes a less well-defined
Cu stripe growth in a narrow potential regime between 0.3 and 0.35 V
and 2D island growth have been frequently monitored. Based on the
fact that unrinsed electrodes showed a similar behavior, we are led
to assume that the occurence, the quality, and the stability of the Cu
stripe phase critically depend on several system parameters, e.g., the
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D90
Journal of The Electrochemical Society, 159 (2) D84-D90 (2012)
dominates in TBPS-containing electrolyte. At the presence of ex-
cess TBPS, the formation of large UPD domains is hindered by a high
amount of etch pits in the topmost Au layer. In a TBPS-containing
solution, the deposition in the regime of Cu-OPD results in a smooth
Cu film with a low surface rougness. This behavior differs signifi-
cantly from defect-mediated 3D island growth in additive-free elec-
trolytes and indicates a “smooth multilayer” or “layer-by-layer” type
of growth.
Although several additives have been proven to affect Cu bulk
growth in a similar way, TBPS may be an interesting and promis-
ing molecule with regard to industrial applications. In contrast to
commonly employed dialkyl disulfides (such as SPS), TBPS shows
only intact adsorption and thus allows to investigate the impact on
Cu growth without being influenced by the presence of dissociation
products (such as MPS).
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We gratefully acknowledge financial support by the National Sci-
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