Thin Solid Films 497 (2006) 121 – 129
www.elsevier.com/locate/tsf
Cu electrodeposition on Ru(0001): Perchlorate dissociation and its effects on
Cu deposition
J. Lei, S. Rudenja, N. Magtoto *, J.A. Kelber
Department of Chemistry, University of North Texas, UNT Box 305070, Denton, TX 76203, USA
Received 12 January 2005; received in revised form 15 July 2005; accepted 18 October 2005
Available online 13 December 2005
Abstract
The electrodeposition of Cu on Ru(0001) and polycrystalline Ru was studied in a combined ultrahigh vacuum/electrochemistry system (UHVEC) in 0.1 M HClO4. The Cu(ClO4)2 concentrations were varied from 0.005 M to 0.0005 M for deposition on Ru(0001), while a solution of
0.05M H2SO4/0.005 M CuSO4/0.001 M NaCl was used for deposition on Ru(poly). Cyclic voltammograms show well-defined Cu underpotential
(UPD) and overpotential deposition (OPD) peaks. X-ray photoelectron spectroscopy (XPS) was used to determine surface composition.
Photoelectron spectra of Ru electrodes emersed from perchloric acid solution at cathodic potentials show that ClO4 dissociates into adsorbed Cl
and ClOx species. The deposited Cu film consists of a bottom layer of Cu(I) and a top layer of insoluble Cu(II). In contrast, only Cu(0) is obtained
when Cu is deposited on polycrystalline Ru in a solution consisting of H2SO4, CuSO4 and NaCl, indicating that the formation of Cu(II) and Cu(I)
involves both Cl and ClO4 interactions with the deposited Cu. A pre-adsorbed layer of iodine on the Ru(0001) surface inhibits perchlorate
dissociation and promotes the deposition of Cu(0) in the perchlorate bath. XPS depth profile analysis demonstrates that the iodine monolayer
‘‘floats’’ on top of the deposited film, in agreement with previous results, effectively protecting the Cu film from air oxidation.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Copper; X-ray photoelectron spectroscopy; Electrodeposition
1. Introduction
The continued shrinkage of interconnect geometries in
integrated circuits has increased interest in the direct electrodeposition of Cu on barrier electrodes without the use of a Cu
‘‘seed’’ layer applied ex-situ to the plating bath [1 –3]. Ru is of
interest as either a stand-alone diffusion barrier [4], or,
possibly, for surface modification to enhance the plateability
of conventional Ta/TaN barriers.
The electrodeposition of Cu on Ru(0001) has been
previously explored [5,6] with a focus on the comparison of
voltammetric stripping data and thermally programmed desorption data for Cu monolayers bound directly to Ru. The
focus of this report is on how solvent – electrode interactions
affect both the electronic state of the deposited Cu and the
electrochemical behavior of the Ru electrode, which have not
* Corresponding author. Tel.: +1 940 565 3265.
E-mail addresses: [email protected] (J.A. Kelber),
[email protected] (N. Magtoto).
0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2005.10.057
been explored previously [7,8] but obviously pertinent to
current technological applications.
HClO4 is commonly used for fundamental electrochemical
studies because perchlorate anions generally interact less
strongly with electrode surfaces than anions such as (bi)sulfate
anions [8– 10]. However, the potential for perchlorate dissociation at Ru electrodes, which has been previously reported
[7], raises the issue of the interaction of deposited Cu with
adsorbed Cl and/or partially dissociated perchlorate species at
the Ru electrode surface. Since Cl anions are common
additives in plating baths [11 – 14], Cu deposition on
Ru(0001) in perchlorate electrolytes was compared to the
following: (1) deposition on Ru(poly) in sulfuric acid solution
containing 1 mM NaCl and (2) deposition in perchloric acid on
Ru(0001) electrodes modified by a partial monolayer of preadsorbed iodine. The results indicate that the presence of both
Cl and perchlorate at the Ru electrode surface results in
formation of an insoluble Cu(II) species with Cu(I) beneath the
Cu(II) layer. In the absence of perchlorate, only Cu(0) is
observed even if Cl is added in the bath (i.e., 0.05 M H2SO4/
0.005 M CuSO4/0.001 M NaCl). Similarly, only Cu(0) is
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J. Lei et al. / Thin Solid Films 497 (2006) 121 – 129
formed in perchloric acid solution on an I-modified Ru
electrode, which inhibits perchlorate dissociation and the
formation of adsorbed Cl. The results indicate that both
perchlorate and chloride species are involved in the formation
of the insoluble Cu(II)/Cu(I) layer.
2. Experimental details
The experiments were performed in a UHV-EC system that
consisted of a main chamber (base pressure < 6 10 8 Pa), a
hemispherical analyzer and dual anode X-ray source for XPS, an
Ar ion sputter gun, and an electrochemical chamber (base
pressure <2.7 10 4 Pa) with the ability to carry out electrochemical measurements under an inert atmosphere. Each
chamber was pumped by a turbomolecular pump to permit high
gas loadings. A detailed description of this system and
methodology has been published previously [15]. The surface
was characterized by XPS under UHV condition after cleaning it
in the main chamber by repeated cycles of Ar ion sputtering
followed by annealing. The sample was then transferred to the
electrochemical chamber via a gate valve. The electrochemical
chamber was brought to > 1 atmospheric pressures by backfilling
it with dry N2. The electrochemical cell filled with electrolyte
was then introduced into the electrochemical chamber through a
gate valve in proximity to the sample allowing the meniscus to
wet the electrode surface. In the solution vapor exposure
experiment, the electrode surface was positioned as close as
possible to the electrolyte meniscus (< 5 10 3 m) without
touching the solution for an exposure period of 2 min.
XPS spectra were acquired with a hemispherical electron
energy analyzer at a constant pass energy of 23.5 eV using an
unmonochromatized Mg Ka X-ray source operated at 15 kV
and 300 W. The calculated surface coverage and chemical
composition were based upon the integrated intensities of core
level transitions calibrated by atomic sensitivity factors
appropriate to the analyzer (PHI Model 10 –360) [16]. The
peak fitting software and procedure have been described
thoroughly in earlier publications [17,18]. A brief account of
the important steps in the fitting procedure is reported here. The
Shirley background subtraction method was applied in this
study since this approach proved to be effective for fitting the
short energy range found in typical core XPS spectra [19].
Spectral components with full width at half maximum
(FWHM) value of 1.7 eV were used to fit experimental
O(1s) spectra. The assignment of this FWHM is based on a
300
recent study in our laboratory, which involved a systematic
examination of oxide film growth on Ru(0001) and Ru(poly)
[20]. The binding energy and FWHM of the spectral
components remained fixed during the fitting procedure.
The Ru(0001) single crystal electrode (MaTecK, Germany)
had a diameter of 1 cm and a thickness of 0.5 mm, a purity of
99.99%, roughness < 0.3 Am and was oriented to within 0.4-.
The sample was mounted onto two tantalum leads that were
spot-welded directly onto the backside of the disc to support
the sample and provided electrical conductivity for resistive
heating. The sample temperature was monitored by a K-type
(chromel – alumel) thermocouple spot-welded directly onto the
back of the sample disc.
The Ru(0001) sample was cleaned in the UHV chamber by
repeated cycles of Ar ion bombardment (3.5 kV) at 800 K for 3
h followed by annealing to 950 K for 30 min. Before
electrochemical experiments, the sample was annealed again
to 850 K for 2 h. Due to the absence of low energy electron
diffraction capability on this system, the degree of long-range
order could not be directly determined. Surface cleanliness was
verified by XPS. The binding energy overlap between the
C(1s) and Ru(3d 3/2) transitions complicates analysis of this
portion of the XPS spectrum. Surface cleanliness was
calibrated by measuring the Ru(3d 5/2)/Ru(3d 3/2) intensity ratio
as a measurement of surface cleanliness [16], a clean surface
yielding an intensity ratio close to 1.4 [16,21]. This cleaning
procedure yielded reproducible cyclic voltammogram (CV)
data, as shown in Fig. 1. The electrochemical cell consisted of
the classic three-electrode configuration with a coiled platinum
wire as the counter electrode. A copper or a platinum wire
served as reference electrode and was positioned close to the
electrode surface in order to minimize the Ohmic drop. Before
electrochemical measurement, the reference electrode was
calibrated with Ag/AgCl. The electrode potentials in this report
are referenced to Ag/AgCl unless stated otherwise. N2 was
used to de-aerate the electrolyte for about 2 h before each
electrochemical experiment and to maintain an air-free
atmosphere over the solution during the experiment. After
each electrochemical measurement, the sample was rinsed with
Millipore deionized water (resistivity >18.2 MV) to remove
the residual electrolyte. All the electrochemical measurements
were acquired using a commercially available potentiostat
(EG&G 263) and software.
Iodine-modified Ru(0001) electrode surfaces were prepared
by polarizing the UHV-cleaned Ru(0001) electrode at 0.2 V
(a) 0.0005 M Cu(ClO4)2/0.1 M HClO4
1500
0.005M Cu(ClO4)2 /0.1 M HClO4
1000
100
I/µA
I/µA
200
(b)
0
500
0
-100
Scan rate = 10 mV/s
-400
-200
0
200
400
E/mV (Ag/AgCl)
600
-500
Scan rate = 10 mV/s
-400
-200
0
200
400
E/mV (Ag/AgCl)
Fig. 1. Cyclic voltammograms (CVs) for Ru(0001) in (a) 0.0005 M Cu(ClO4)2/0.1 M HClO4 and (b) 0.005 M Cu(ClO4)2/0.1 M HClO4.
J. Lei et al. / Thin Solid Films 497 (2006) 121 – 129
vs. Ag/AgCl for 40 min in 0.005 M KI/0.1 M HClO4 solution
[10].
The average thickness of an overlayer on the Ru electrode
was estimated by measuring the attenuation of the Ru(3d 5/2)
XPS intensity according to [22]:
I ¼ I0 expð d=kÞ
ð1Þ
In Eq. (1), I 0 and I are the Ru(3d 5/2) XPS core level signal
intensities of the clean Ru(0001) and Ru(0001) after experiment, k is the calculated [23] electron inelastic mean-free path
(IMFP) for an Ru(3d 5/2)electron in Ru metal (14.1 Å), and d is
the estimated thickness of the overlayer.
The thickness of the Cu overlayer after pulse deposition was
estimated according to [22]:
NA
1 exp½ d=kA ðEA Þ
¼
exp½ d=kB ðEB Þ
NB
ð2Þ
In Eq. (2), N A and N B are the atomic concentration of the Cu
overlayer and Ru substrate, respectively. k A and k B are the
respective calculated IMFP values for a Cu(2p3/2) electron (13
Å) and for an Ru(3d 5/2) electron (14.1 Å), and d is the
estimated thickness of the Cu overlayer.
The fractional monolayer coverage of iodine overlayer on
Ru(0001) after iodine adsorption experiment was estimated
according to [22]:
hA f1 exp½ aA =kA ðEA Þg
NA
¼
1 hf1 exp½ aA =kB ðEB Þg
NB
ð3Þ
where N A and N B are the atomic concentrations of the iodine
overlayer and Ru substrate, respectively, and a A is the covalent
diameter of iodine atom (2.66 Å). k A and k B are the IMFP
values for an I(3d 5/2) electron (16.3 Å) and Ru(3d 5/2) electron
(14.1 Å), respectively. h A is the estimated fractional monolayer
coverage of iodine overlayer.
3. Results
123
charge from the UPD region (Fig. 1a) indicates the reduction of
¨ 1.3 1015 Cu ions on the electrode area (assuming 100% of
the current was due to Cu+ 2 reduction, electrode reaction area
¨ 1 cm2), or the deposition of (at a maximum) ¨ 1 monolayer
of Cu metal [24]. The hydrogen evolution peak begins at
0.35 V and is well separated from the Cu OPD peak at 0.11
V. By comparison, CV data acquired under identical conditions, but with a 5 10 3 M Cu(ClO4)2 solution (Fig. 1b)
show evidence of distortion. The main features and peak
positions are, however, comparable to previously reported
results [1].
After stripping (polarization to + 0.6 V) and relaxation to
OCP (+ 0.36 V) followed by rinsing in deionized H2O, the
sample was emersed from the electrolyte then transferred into
the main UHV chamber for XPS analysis. XPS data acquired
before and after the CV in 5 10 4 M Cu(ClO4)2/0.1 M
HClO4 are displayed in Fig. 2a– d, which show an electrode
surface free of detectable Cu and negligible Cl signal. Since
electrode exposure to the vapor above the electrochemical cell
after emersion is an unavoidable aspect of this process, XPS
data for the electrode emersed from the electrolyte after CV are
compared to the data for a clean Ru(0001) electrode exposed
only to vapor above the electrochemical cell (Fig. 2). The O(1s)
core level signals after the vapor exposure and after CV are
well fit with a FWHM of 1.7 eV at binding energies of 531.2
eV and 532.4 eV, consistent with chemisorbed oxygen
[20,25,26] and hydroxide [20,27] respectively. The thickness
of the chemisorbed oxygen and hydroxide overlayer estimated
by Eq. (1) is 2 Å for exposure to the vapor above the
electrolyte, and 4 Å after CV and emersion at + 0.36 V. The
absence of an O(1s) component with binding energy at ¨ 530
eV – expected for RuO2 [20,25,26,28] – indicates the absence
of a true oxide phase upon either exposure to vapor or anodic
polarization and emersion. The corresponding binding energy
at 280.1 eV and FWHM of 1 eV for Ru(3d 5/2) remained
unchanged (Fig. 2) after solution vapor exposure and anodic
polarization, which also indicates lack of metal oxide
formation.
3.1. Ru(0001) after cyclic voltammetry in 0.1 M HClO4/0.5 mM
Cu(ClO4)2
3.2. Reaction of Cl with Ru(0001) electrodes
CV data for freshly prepared samples immersed in 0.0005
M and in 0.005 M Cu(ClO4)2 solutions in 0.1 M HClO4 are
compared in Fig. 1. In each case, the scan was started at open
circuit potential (OCP) (+ 0.36 V in 0.0005 Cu(ClO4)2 with 0.1
M HClO4 and + 0.38 V in 0.005 M Cu(ClO4)2 with 0.1 M
HClO4) and extended in the cathodic direction, with a scan
speed of 10 mV/s. The scan direction was reversed at 0.4 V
then scanned to + 0.6 V (0.0005 M Cu(ClO4)2) or to + 0.4 V
(0.005 M Cu(ClO4)2) and then back to OCP prior to sample
emersion. The potentials for the Cu deposition and stripping
peaks are in good agreement with those previously reported on
polycrystalline [1] and single crystal [5] Ru electrodes.
Although a UPD feature appears to be present for deposition
in the 0.0005 M solution (Fig. 1a), no corresponding stripping
peak for the UPD feature is observed, due to the kinetically
determined width of the bulk-stripping feature. The integrated
In order to examine the effects of perchlorate ions on the Cu
electrodeposition process, the Ru(0001) electrode potential was
stepped from open circuit potential to cathodic potential (from
0.15 V to 0.35 V vs. Ag/AgCl) in 0.005 M Cu(ClO4)2/0.1
M HClO4, for 5 s. The current-time transient obtained from this
experiment is shown in Fig. 3. The absence of a broad
maximum in the deposition current (shown as a negative
current in Fig. 3) is notable and similar to that observed in
sulfate solutions at comparable [Cu+ 2] concentrations in our
laboratory. Integration of the area under the curve (current-time
transient at 0.25 V; Fig. 3) indicated (assuming 100%
efficiency in Cu+ 2 reduction) the deposition of 5.8 1015 Cu
atoms (1.8 10 3 Coulomb), or ¨ 3.6 ML (according to an
areal density of 1.58 1015 Cu atoms cm 2 [6]).
In order to further investigate chemical reactions during the
current-time transient process at cathodic potentials on
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J. Lei et al. / Thin Solid Films 497 (2006) 121 – 129
Ru (3d5/2)
Ru 3d
50000 cps
(b)
OHads
Oads
O1s
After CV
Solution vapor
exposed
3000 cps
XPS intensity
(a)
Clean
Ru(0001)
290
285
280
275
540
Binding Energy (eV)
(c)
536
532
528
524
Binding Energy (eV)
(d)
Cl2p
Cu2p
Solution vapor
exposed
4500 cps
400 cps
XPS intensity
After CV
Clean
Ru(0001)
215
210
205
200
195
Binding Energy (eV)
960
940
920
Binding Energy (eV)
Fig. 2. Ru(0001) XPS spectrum: clean, solution vapor exposed and emersion at +0.36 V (open circuit) after CV in 0.0005 M Cu(ClO4)2/0.1 M HClO4.
Ru(0001) substrate, the electrode was pulsed from OCP to
0.25 V (well short of the hydrogen evolution region) for 5 s,
emersed at that potential, rinsed several times with deionized
water, and then examined by XPS. The photoelectron spectra
for the clean Ru(0001) electrode are shown in Fig. 4. Due to
the presence of O, Cl and Cu, a determination of the average
overlayer thickness is complicated. An estimation of the total
overlayer average thickness from the attenuation of the
absolute Ru(3d 5/2) intensity (Eq. (1)) yields a value of 10 Å.
The XPS-derived Cu overlayer thickness (Eq. (2)) is only 1.5
Å. This is in contrast to the total amount of reduced Cu
estimated from integration of the current-time transient (Fig. 3)
of 3.6 ML, or about 10 Å, assuming an average Cu covalent
0.0
a
d
e
Curent (mA)
-0.5
c
b
-1.0
a:
b:
c:
d:
e:
-1.5
-2.0
0
1
2
3
-0.15 V
-0.20 V
-0.25 V
-0.30 V
-0.35 V
4
5
time (s)
Fig. 3. Current vs. time transients at different potentials (vs. Ag/AgCl) in 0.005
M Cu(ClO4)2/0.1 M HClO4 for 5 s.
diameter of 2.7 Å. A disagreement between the chronoamperometry (Fig. 3) and XPS estimates of the amount of Cu
deposited is not surprising in view of the presence of other
species within the adsorbate layer. The discrepancy indicates
that either the chronoamperometry is affected by side reactions,
and/or that Cu is not being deposited uniformly on the surface.
The Ru(3d 5/2)/Ru(3d 3/2) peak intensity ratio (Fig. 4a) is 1.4
(T 0.1) for clean Ru(0001), and 1.3 (T0.1) for Ru(0001) after
pulse experiments, indicating a negligible carbon contamination during and after the measurement. Three components with
FWHM of 1.7 eV in O(1s) transition for the clean Ru(0001)
(upper level, Fig. 4b) with binding energy at 530.2 eV, 532.3
eV, and 533.6 eV can be assigned to metal oxide [20,29],
adsorbed hydroxide overlapped with adsorbed perchloric
species [20,30], and adsorbed water [16,30], respectively.
The Cl(2p) spectra in Fig. 4c indicate the presence of two
Cl(2p3/2,1/2) doublets at 207.5 eV and 198.5 eV. Each doublet is
well fit by a 2p3/2:2p1/2 intensity ratio of 2:1, with a 1.6 eV
separation [16,31] (The binding energy values reported here
refer to the Cl(2p3/2) photoelectron line). The Cl(2p) doublet
features at 207.5 eV and 198.5 eV are well fit with FWHM of
1.7 eV for each component. Based on the binding energy
position of Cl(2p3/2) and O(1s) for the clean Ru(0001) after
current-time transient experiment, the Cl(2p) components are
attributed to adsorbed perchlorate (207.5 eV) [16,30] and
adsorbed chloride (198.5 eV) [16,31], both of which were
strongly adsorbed on the electrode surface after the final rinsing
procedure. The appearance of the chloride peak implies the
dissociation of perchloric species either in solution or upon the
Ru electrode surface. The Cu(2p) spectrum given in Fig. 4d
(upper level) shows a satellite features at ¨ 945 eV and ¨ 965
eV, which are of higher binding energy than parent photoelec-
J. Lei et al. / Thin Solid Films 497 (2006) 121 – 129
(b)
Ru 3d
30000 cps
288
282
276
Cleane
Ru(0001)
540
Binding Energy (eV)
(c)
metal oxide
H2Oads
OHads/ClO4
535
530
525
Binding Energy (eV)
(d)
Cl2p
Cu2p
2p1/2
2p3/2
5000 cps
Emersed at – 0.25V
3000 cps
XPS intensity
O1s
Emersed at – 0.25V
50000 cps
XPS intensity
(a)
125
215
Cleane
Ru(0001)
210
205
200
195
Binding Energy (eV)
970
960
950
940
930
920
Binding Energy (eV)
Fig. 4. Ru, Cl, O and Cu XPS for clean Ru(0001) (bottom trace), and after emersion at 0.25 V vs. Ag/AgCl (top trace) in a solution of 0.005 M Cu(ClO4)2/0.1 M
HClO4.
tron line (932.7 eV for 2p3/2 and 952.4 eV for 2p1/2), indicative
of adsorbed Cu(II) layer formation [32 –34]. A 2.1 eV FWHM
at 932.7 eV indicates the presence of multiple Cu electronic
states, since clean metallic Cu yields a Cu(2p3/2) transition with
a FWHM of only 1.5 eV under these experimental conditions.
The O(1s) feature at 530.2 eV (Fig. 4b, upper trace) is
attributable to CuO [29], which is consistent with the Cu(II)related features observed in the corresponding Cu(2p) spectrum
(Fig. 4d, upper trace). The intensity ratio of O(1s) (532.3 eV)/
Cl(2p) (207.5 eV) feature is about 7, larger than the expected
value of 4 for adsorbed ClO4. While this result is consistent
with perchlorate ion dissociation, the comparison is complicated by the fact that the expected O(1s) binding energy for
adsorbed perchlorate is 532.3 eV [30], very close to the value
532.4 eV for a surface hydroxyl species [20,27]. Although the
presence of adsorbed H2O species at 533.6 eV is uncommon in
UHV environment at room temperature [35], the H2O
associated with a counter ionic species may be stabilized
under UHV conditions [30].
Similar results were obtained on Ru(poly) as on Ru(0001).
In addition, scanning the Ru(poly) electrode from OCP to
0.45 V prior to emersion produced Cu deposits with XPS
spectra identical to those obtained by pulsing the Ru(0001) to
0.25 V for 5 s and emersed at that potential. Typical results
are displayed in Fig. 5 for an Ru(poly) electrode in 0.005 M
Cu(ClO4)2/0.1 M HClO4. The electrode was held at OCP
initially and then scanned to 0.45 V vs. Ag/AgCl (scan
speed = 20 mV/s) followed by emersion at that potential. The
Cu(2p) spectra of the Ru(poly) emersed at 0.45 V are
displayed in Fig. 5. The spectra were acquired at both normal
take-off angle (Fig. 5, top) and grazing take-off angle of 60with respect to the surface normal (Fig. 5, bottom). The data in
Fig. 5 indicate that the lower binding energy component of the
Cu(2p3/2) feature displays greater relative intensity at normal
incidence. In addition, the relative intensities of the satellite
features to the main peaks are less at normal compared to
grazing take-off angle. Since reducing the photoelectron takeoff angle reduces the average depth from which the XPS signal
is obtained, the data in Fig. 5 indicate that Cu(II) is located
primarily at the outer surface of the film. The Cu(2p3/2) core
level signals can be fit with two components (each with 1.5 eV
FWHM), at 932.6 eV and 934.7 eV, consistent with Cu(0) or
Cu(I) [32], and Cu(II) [36] respectively. Since the XPS Cu(2p3/
2) binding energy difference between Cu(0) and Cu(I) is < 0.2
eV [16], the X-ray excited Cu(L3VV) Auger spectrum was
used to distinguish between Cu(0) and Cu(I) [29]. Both normal
take-off angle (Fig. 5c) and grazing take-off angle measurements (Fig. 5d) yielded a Cu(L3VV) Auger lineshape with a
single peak at 916.8 eV kinetic energy, indicating adsorbed
Cu(I) [29,32,33,37], but providing no evidence for the presence
of metallic Cu.
3.3. Iodine inhibition of Cl dissociation and Cu(II) formation
In order to assess the effects of pre-adsorbed I on Cl/Cu
interactions at the Ru electrode surface, a clean Ru(0001)
electrode was polarized to 0.2 V vs. Ag/AgCl in 0.1 M
HClO4 containing 5 mM KI for 40 min, followed by emersion
at this potential and rinsing in deionized water. The I(3d 3/2) and
Ru(3d) core level spectra before and after this experiment are
shown in Fig. 6. The I coverage after emersion was calculated
(Eq. (3)) to be 0.25 monolayers (ML), slightly less than the
0.32 ML coverage reported previously [20] for I 2 vapor
reaction with an Ru(0001) surface under UHV conditions.
Corresponding XPS data are shown in Fig. 6b, e. The Imodified Ru(0001) electrode was then immersed in 0.005 M
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J. Lei et al. / Thin Solid Films 497 (2006) 121 – 129
(a)
CuL3VV
2000 cps
2000 cpsc
2p1/2
Auger intensity
XPS intensity
(c)
Cu 2p
2p3/2
2000 cps
(b)
Cu(I)
(d)
2p3/2
2p1/2
975
960
945
930
Binding Energy (eV)
906
912
918
924
930
Kinetic Energy(eV)
Fig. 5. Take-off angle resolved spectra for Cu(2p) on clean Ru(poly) after emersion at 0.45 V vs. Ag/AgCl from a 0.005 M Cu(ClO4)2/0.1 M HClO4 solution.
(Bottom) XPS core level and Auger spectra acquired at grazing take-off angle. (Top) Spectra acquired at normal take-off angle.
Cu(ClO4)2/0.1 M HClO4 solution. After CV was acquired, the
electrode was pulsed for 5 s at 0.25 V vs. Ag/AgCl and
emersed at that potential as a final step. Corresponding Ru(3d)
and I(3d 5/2)XPS spectra are shown in Fig. 6c, f. With iodine
modification, the CV (not shown) of I-modified Ru(0001)
indicated energetically more favorable anodic process and
more distinct copper deposition peak compared to the CV of
clean Ru(0001) [20]. The absolute intensity of the iodine peak
is not attenuated by the deposited Cu film after Cu deposition
for 5 s at 0.25 V in the iodine-free 0.005 M Cu(ClO4)2/0.1 M
(c)
XPS intensity
(f)
Ru 3d
Ru3d5/2
100000 cps
10000 cps
I 3d5/2
(b)
(d)
(a)
(e)
625
620
615
Binding Energy (eV)
610
290
285
280
275
Binding Energy (eV)
Fig. 6. I and Ru XPS data for I/Ru(0001) after iodine adsorption and after
emersion at 0.25 V vs. Ag/AgCl from a 0.005 M Cu(ClO4)2/0.1 M HClO4;
I(3d 5/2) spectra obtained before immersion in KI solution, after immersion in
KI solution, and after Cu electrodeposition, respectively (a – c); corresponding
Ru(3d) spectra (d – f).
HClO4 (Fig. 6b, c). The I(3d 5/2) XPS spectrum obtained after
pulse deposition (Fig. 6c) is well fit by a single peak with
FWHM of 1.6 eV at 619.7 eV binding energy with a small
shoulder at higher binding energy [16,31], in agreement with
published values [38,39]. In those reports, the binding energy
of molecular iodine and chemisorbed iodine on Ag were
assigned to 619.9 eV and 619.2 eV correspondingly. The
binding energy at 619.7 eV for chemisorbed iodine in our study
demonstrates partially charge transfer between adsorbed iodine
atoms and substrate. On the contrary, the I(3d 5/2) core-level
spectrum after polarizing in iodide containing solution (Fig. 6b)
is broader and of less intensity than that after subsequent CV
and current transient experiment in iodide free 0.005 M
Cu(ClO4)2/0.1 M HClO4 solution (Fig. 6c). This result
suggests that a variety of bonding states of iodine to
Ru(0001) exist after polarization in and emersion from
iodide-containing solution. The increase of the absolute
I(3d 5/2) peak intensity after Cu pulse deposition (Fig. 6b and
c) is <8%, which indicates that the total amount of iodine
remained constant before and after pulse deposition (chemisorbed iodine remains at the surface of the growing Cu film)
and is consistent with previously reported results [20]. Lack of
K+ XPS signal (not shown) also indicates that iodide anions in
solution oxidized to chemisorbed iodine and partially bonded
to Ru(0001) surface.
Cu deposition under potentiostatic conditions results in a
17% decrease in Ru(3d 5/2) intensity (Fig. 6e, f). The total
amount of Cu reduced, determined by integrating the charge in
the current-time transient experiment (not shown) and assuming 100% efficiency in Cu+ 2 reduction, is 7.4 1015 Cu atoms
(2.3 10 3 Coulomb), or ¨ 4.6 ML. This corresponds to an
average Cu overlayer thickness of 12 Å, assuming a Cu
covalent diameter of 2.7 Å. Assuming the attenuation of
Ru(3d 5/2) after current-time transient experiment is due to Cu
deposition, an estimation of the Cu average thickness from the
J. Lei et al. / Thin Solid Films 497 (2006) 121 – 129
XPS data (Eq. (1)) yields a value of only 4 Å. A disagreement
between the chronoamperometry and XPS-derived estimates of
the amount of Cu deposited suggests that either the chronoamperometry is affected by side reactions, and/or that the Cu
is not being deposited uniformly on the surface.
The effects of pre-adsorbed I on Cu electrodeposition are
displayed in Fig. 7, in which Ru(3d), O(1s), Cl(2p), and
Cu(2p) core level spectra are compared for clean and Imodified Ru(0001) after 5 s polarization and emersion at
0.25 V in 0.1 M HClO4/0.005 M Cu(ClO4)2 solution (the
core level spectra for the unmodified electrode are the same as
in Fig. 4). The total average overlayer thickness – derived
from attenuation of the Ru(3d 5/2) intensity (Fig. 7a) – is
estimated (Eq. (1)) to be 7 Å for I –Ru(0001) compared to 10
Å for clean Ru(0001). The total O(1s) and Cl(2p) intensities
are greatly reduced on I –Ru(0001) compared to the unmodified electrode (Fig. 7b, c). The O(1s) spectra for both clean
and I pre-adsorbed Ru(0001) can be well fit with three
components with FWHM of 1.7 eV at binding energies of
530.2 eV, 532.3 eV, and 533.6 eV, consistent with metal oxide
[20,29], adsorbed hydroxide overlapped or adsorbed perchloric species [20,30], and adsorbed water [16,30], respectively.
The O(1s) component at 530.2 eV is clearly present after
deposition on clean Ru(0001) surface but is of much less
intensity following deposition on I – Ru(0001). This is
consistent with a previous report [20] indicating that an I
adlayer hinders oxide formation on Ru(0001) and Ru(poly),
but not the formation of surface OH or O species. Assuming
the quantity of surface OH species is the same on both
electrodes, after subtracting the intensity due to OH from
O(1s) intensity at 532.3 eV for clean Ru(0001) electrode, the
O(1s) (532.3 eV)/Cl(2p) (207.5 eV) on clean Ru(0001) ratio is
4. The greatly reduced intensity of the central O(1s)
component at 532.3 eV on the I-modified electrode (Fig. 7b)
is also consistent with the greatly reduced levels of adsorbed
perchlorate and complete absence of observable Cl species
(Fig. 7c) on the I-modified surface. The data in Fig. 7b, c
indicate that the presence of pre-adsorbed I inhibits adsorption
and dissociation of perchlorate species on the Ru electrode
surface. Comparison of the Cu(2p) spectra for the two
surfaces (Fig. 7d) indicates the absence of observable amounts
of Cu(II) species on the I – Ru(0001) electrode surface.
Inspection of the corresponding Cu(L3VV) Auger spectra
(Fig. 8) indicates that Cu is present on the I –Ru(0001) surface
as Cu(0). The data in Figs. 7 and 8 indicate that the presence
of pre-adsorbed I inhibits adsorption and dissociation of
perchlorate species on the Ru surface, and also enhances the
deposition of metallic Cu.
In order to test whether the formation of Cu(I) or Cu(II)
species on the unmodified Ru surface was specifically related
to the presence of adsorbed Cl (Cla), deposition experiments
were carried out with an unmodified Ru(poly) electrode in a
sulfuric acid solution with the same cupric ion concentration
(0.005 M CuSO4/0.05 M H2SO4). One solution contained
0.001 M NaCl, the other had no other additives. In both cases,
the UHV cleaned Ru(poly) were held at OCP initially, and then
scanned to 0.45 V vs. Ag/AgCl (scan speed = 20 mV/s),
followed by emersion at that potential.
The XPS core level and Cu(LVV) Auger spectra acquired
after emersion at 0.45 V vs. Ag/AgCl from 0.005 M CuSO4
containing 0.05 M H2SO4 with and without NaCl are
compared in Fig. 9. The presence of adsorbed Cl is clearly
indicated (Fig. 9c) for the sample emersed from the Clcontaining solution. The Cu(L3VV) Auger lineshape displayed
in Fig. 9f shows the kinetic energies at 918.4 eV for the
electrodes emersed from Cl containing and Cl free solution,
(b)
Ru3d
-0.25 V 5s
clean Ru(0001)
metal oxide
H2Oads
-0.25 V 5s
Ru(0001)/Iodine
288
276
282
540
Binding Energy (eV)
OHads/ClO4
534
528
522
Binding Energy (eV)
(d)
Cl2p
Cu2p
13000 cps
-0.25 V 5s
clean Ru(0001)
3000 cps
XPS intensity
(c)
O1s
25000 cps
60000 cps
XPS intensity
(a)
127
-0.25 V 5s
Ru(0001)/Iodine
216
210
204
198
Binding Energy (eV)
975
960
945
930
Binding Energy (eV)
Fig. 7. Ru, Cl, O and Cu XPS for clean, and iodine modified Ru(0001) after emersion at 0.25 V in 0.005 M Cu(ClO4)2/0.1 M HClO4.
128
J. Lei et al. / Thin Solid Films 497 (2006) 121 – 129
solution or Cla at the electrode surface is not sufficient to
induce the formation of oxidized Cu.
CuL3VV
(a)
Cu(I)
4. Discussion
(b)
Cu(0)
3000 cps
Auger intensity
Clean Ru(0001)
Iodine-modified
Ru(0001)
906
912
918
924
930
Kinetic Energy (eV)
Fig. 8. X-ray excited Cu(L3VV) Auger spectra for clean (top trace), and iodinemodified Ru(0001) (bottom trace) after polarization at 0.25 V for 5 s
followed by emersion in 0.005 M Cu(ClO4)2/0.1 M HClO4.
which indicates the formation of Cu(0) [40] on the Ru(poly) in
the presence of Cl. Subsequent XPS depth profile analysis
(not shown) demonstrated that impurities of Cl, O, and S
decreased dramatically for the Ru(poly) emersed from NaCl
containing electrolyte after brief Ar ion sputtering (3.5 kV
beam voltage, 25 mA emission current) indicating that the
impurities were located at the surface of the Cu film. The data
in Fig. 9, therefore, indicate that the presence of Cl in
The dissociation and adsorption of perchlorate anions at the
Ru electrode surface has been previously reported [7]. XPS
data (Fig. 2) demonstrate that the potential cycling of the Ru
electrode in Cu+ 2-containing perchloric acid solution and
emersion at 0.36V (OCP) results in no observable Cu and
insignificant amounts of perchlorate species remaining on the
surface. At more cathodic potentials, however, perchlorate
adsorption and dissociation are observed (Fig. 4) together with
the formation of a Cu(II) surface species (Fig. 5) and Cu(I)
formed underneath. The presence of pre-adsorbed I blocks
perchlorate adsorption/dissociation, and enhances the formation of metallic Cu during electrodeposition (Fig. 7). In the
absence of surface I, the presence of Cl in sulfuric acid
electrolyte does not lead to the formation of an oxidized Cu
deposit (Fig. 9). These data therefore indicate that (a) preadsorbed I inhibits perchlorate dissociation on the Ru electrode
surface, and (b) both perchlorate and chloride species are
required to form an insoluble Cu(II) surface film during Cu
electrodeposition.
An interesting question is the origin of the insoluble Cu(II)
film observed for deposition on clean Ru in perchloric acid,
and the fact that the substrate is largely or completely Cu(I) in
nature (Figs. 4, 5). The fact that Cu(II) formation is only
Ru 3d
(a)
30000
20000
10000
0
288
282
100000
XPS intensity (cps)
XPS intensity (cps)
40000
80000
(b)
60000
2p3/2
2p1/2
40000
20000
0
970
276
Binding Energy (eV)
1200
Cl2p
(c)
900
600
300
0
30000
-300
215
210
205
200
(e)
S2p
3000
2000
1000
0
174
168
162
Binding Energy (eV)
930
O1s
10000
0
535
530
525
Binding Energy (eV)
4000
-1000
940
(d)
-1000
540
195
156
Auger intensity (cps)
XPS intensity (cps)
5000
950
20000
Binding Energy (eV)
6000
960
Binding Energy (eV)
XPS intensity (cps)
XPS intensity (cps)
1500
Cu2p
40000
(f)
CuL3VV
32000
24000
16000
8000
0
906
912
918
924
930
Kinetic Energy (eV)
Fig. 9. XPS for Ru(poly) emersed at 0.45 V from 0.005 M CuSO4/0.05 M H2SO4 solution with 0.001 M NaCl (solid line) and without NaCl (dotted line).
J. Lei et al. / Thin Solid Films 497 (2006) 121 – 129
observed in the presence of both perchlorate and chloride
species suggests that Cu ions reduced at the Ru electrode
abstract an adsorbed chloride species, forming Cu(I). Reaction
with perchlorate species, either adsorbed or from solution,
could then yield a Cu(II)(Cl)(ClO4) complex. The XPS data
and the fact that Cu continues to be deposited indicate that Cu
deposition occurs underneath the Cu(II) film. One possible way
this might happen would be for Cu deposition to occur in a
spatially non-uniform manner, allowing continued abstraction
of adsorbed Cl from the Ru substrate.
The data here do indicate that the Cu electrodeposition on
Ru in perchloric electrolytes poses complications, and that
sulfuric acid is a better choice for the deposition of metallic Cu.
The data also indicate that a pre-adsorbed layer of iodine will
protect the Ru electrode surface from perchlorate adsorption
and dissociation to form adsorbed Cl.
5. Conclusion
In summary, the dissociation of perchlorate species at
cathodic potential on Ru(0001) and Ru(poly) leads to Cu(II)
and Cu(I) formation. Iodine can be chemisorbed on Ru(0001)
by polarizing the electrode in acidic iodide containing solution,
and the subsequent Cu deposition on I-modified Ru at cathodic
potential in perchlorate bath yields Cu(0) while inhibiting
perchlorate reaction and dissociation at the electrode surface.
Chemisorbed iodine remains at the surface of the growing Cu
film after pulse deposition in iodide free 0.005 M Cu(ClO4)2/
0.1 M HClO4 electrolyte. Cu(II) formation occurs only in the
presence of both Cl and perchlorate. The presence of Cl
alone (e.g., as an additive in a sulfuric acid bath) results in the
deposition of metallic Cu.
Acknowledgements
This work was supported by a Novellus/Semiconductor
Research Corporation Customized Research project, and by the
Robert Welch Foundation under Grant no. B-1356.
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