The effects of an iodine surface layer on ru reactivity in air and during Cu electrodeposition

Article abstract of DOI:10.1149/1.1842072

X-ray photoelectron spectroscopy (XPS), low energy electron diffraction, and cyclic voltammetry have been used to study the adsorption of iodine on the Ru(0001), air, and water exposure to both clean and iodine covered Ru(0001) surfaces and the stability of the iodine adlayer during Cu overpotential electrodeposition. A Ru(0001)-(√3 × 3)R30°-I structure was observed after I₂ vapor exposure of the Ru(0001) surface at room temperature. The Ru(0001)-(√3 ×3)R30°-I structure was found to be stable toward ambient air and water exposure. The I ad-layer passivates the Ru(0001) surface against significant hydroxide, chemisorbed oxygen, or oxide formation during exposure to air. Immersion of I-Ru(0001) results in greater hydroxide and chemisorbed oxygen formation than air exposure. A saturation coverage of I on a Ru(poly) electrode similarly passivated the Ru surface against oxidation upon exposure to water vapor over an electrochemical cell in an ultrahigh vacuum electrochemistry transfer system. Studies with combined electrochemical and XPS techniques show that iodine surface adlayer remained on top of the surface after cycles of overpotential electrodeposition/dissolution of copper on Ru(0001). These results indicate the potential bifunctionality of the iodine layer to both passivate the Ru surface in the microelectronic processing and to act as a surfactant for copper electrodeposition.

Full text of DOI:10.1149/1.1842072

Journal of The Electrochemical Society, 152 ͑2͒ G115-G121 ͑2005͒
G115
0013-4651/2004/152͑2͒/G115/7/$7.00 © The Electrochemical Society, Inc.
The Effects of an Iodine Surface Layer on Ru Reactivity
in Air and during Cu Electrodeposition
,z
*
J. Liu, J. Lei, N. Magtoto, S. Rudenja, M. Garza, and J. A. Kelber
Department of Chemistry, University of North Texas, Denton, Texas 76203, USA
X-ray photoelectron spectroscopy ͑XPS͒, low energy electron diffraction, and cyclic voltammetry have been used to study the
adsorption of iodine on the Ru͑0001͒, air, and water exposure to both clean and iodine covered Ru͑0001͒ surfaces and the stability
of the iodine adlayer during Cu overpotential electrodeposition. A Ru(0001)-(ͱ3 ϫ
ͱ
3)R30°-I structure was observed after I2
vapor exposure of the Ru͑0001͒ surface at room temperature. The Ru(0001)-(ͱ3 ϫ ͱ3)R30°-I structure was found to be stable
toward ambient air and water exposure. The I ad-layer passivates the Ru͑0001͒ surface against significant hydroxide, chemisorbed
oxygen, or oxide formation during exposure to air. Immersion of I-Ru͑0001͒ results in greater hydroxide and chemisorbed oxygen
formation than air exposure. A saturation coverage of I on a Ru͑poly͒ electrode similarly passivated the Ru surface against
oxidation upon exposure to water vapor over an electrochemical cell in an ultrahigh vacuum electrochemistry transfer system.
Studies with combined electrochemical and XPS techniques show that iodine surface adlayer remained on top of the surface after
cycles of overpotential electrodeposition/dissolution of copper on Ru͑0001͒. These results indicate the potential bifunctionality of
the iodine layer to both passivate the Ru surface in the microelectronic processing and to act as a surfactant for copper
electrodeposition.
© 2004 The Electrochemical Society. ͓DOI: 10.1149/1.1842072͔ All rights reserved.
Manuscript submitted November 14, 2003; revised manuscript received July 15, 2004. Available electronically December 17,
2004.
The ability to passivate metallic surfaces under non-ultrahigh
vacuum ͑nonUHV͒ conditions is not only of fundamental interest,
but also of growing practical importance in catalysis and microelec-
tronics. The catalytic properties of Ru have led to intensive study of
the reactivity of Ru surfaces in vapor1,2 and solution environments.3
In addition, there is growing interest in this material as a barrier to
Cu diffusion during the fabrication of Cu interconnects for micro-
electronics applications.4 As the feature size of semiconductor de-
vices keeps on shrinking, currently used diffusion barriers, such as
Ta or TaN, may become too thin to operate properly because of the
consequent high resistivity.5 Ruthenium is a promising diffusion bar-
rier candidate for Cu interconnect fabrication in microdevices be-
cause of its high melting point ͑2310°C͒, higher conductivity ͑7.6
␮⍀ cm͒ and better resistance to oxidation in ambient than Ta. The
increasing interest in direct electrodeposition of Cu on the barrier
without a vacuum-deposited seed layer6,7 enhances the advantages
of a barrier material which will not form an insulating interfacial
oxide under conditions relevant to Cu plating. The potential use of
Ru as a Cu diffusion barrier for microelectronics applications en-
hances the importance of a method to passivate the surface against
exposure to ambient during processing.
An iodine adlayer on some seminoble single-crystal surfaces has
been demonstrated to protect these surfaces from oxidation and con-
tamination under ambient conditions.8-11 In addition to surface pas-
sivation, iodine adlayers may have an additional benefit with respect
to Cu electrodeposition, acting as a surfactant to enhance conformal
Cu growth. A monolayer of adsorbed iodine has been demon-
strated12 to inhibit roughness evolution of metallorganic chemical
vapor deposition ͑MOCVD͒ Cu films, and may therefore also serve
as a surfactant during Cu electrodeposition. Such an application,
obviously, demands that iodine ad-atoms originally adsorbed at the
electrode surface not be buried by the deposited Cu film but remain
on the surface of the Cu film during deposition ͑or etching͒. While
this tendency has been demonstrated for Cu underpotential deposi-
tion on various electrodes,13-15 the stability of I adlayers during Cu
overpotential deposition has not been demonstrated.
against oxide formation. Similar XPS results were obtained for an
I-Ru͑poly͒ surface exposed to the vapor above an electrochemical
cell in a UHV-electrochemistry ͑UHV-EC͒ transfer system. XPS and
LEED show that iodine passivation is less pronounced upon Ru
immersion in deionized water. Electrochemical and XPS measure-
ments under UHV-EC conditions indicate that Cu electrodeposition
on and dissolution from an I-modified Ru͑poly͒ electrode in an
I-free electrolyte does not result in a decrease of I surface coverage.
The results indicate the potential for I adlayers to both passivate Ru
barrier surfaces during microelectronics processing and to act as a
surfactant for Cu electrodeposition.
Experimental
The system used for Ru͑0001͒ single-crystal study consisted of
an UHV chamber with a hemispherical electron energy analyzer
͑VSW, Cheshire, U.K.͒ for XPS and 3-grid LEED optics ͑Omicron,
Germany͒, an unmonochromated X-ray source ͑Physical Electron-
ics, Eden Prairie, MN͒ and an ion gun ͑Specs, Germany͒ for sample
cleaning. The UHV chamber was connected to an antechamber ca-
pable for sample loading, and the two chambers were separated by
means of a gate valve. The pressures of the UHV chamber and the
antechamber were maintained at Ͻ5 ϫ 10Ϫ10 and Ͻ1 ϫ 10Ϫ7 Torr,
respectively, by two different turbo molecular pumps. Pressures
were monitored by nude ion gauges out of line of site with the
sample, and calibrated for N2 . The temperature of the sample in the
main chamber could be varied between 120 and 1300 K by a com-
bination of resistive heating and liquid nitrogen cooling. Sample
temperature was monitored by a type K thermocouple spot-welded
to the sample holder. LEED spectra were acquired with a beam
energy of 71.4 eV. XPS spectra were acquired in constant pass en-
ergy mode ͑22 eV͒ using an Mg K␣ X-ray source operated at 300
W. Spectra were acquired with the analyzer axis parallel to the
sample normal. XPS spectral analysis procedures were as previously
reported.16 Briefly, peak fitting of the XPS spectra was accom-
plished using commercially available software, electron spectros-
copy for chemical analysis ͑ESCA͒ tools, which were utilized as a
toolbox in the MATLAB environment. Shirley background subtrac-
tion was used in this study due to its effectiveness for fitting the
short energy range found in typical core XPS spectra.17 The experi-
mental O ͑1s͒ spectra was fitted with components of a full width at
half maximum ͑fwhm͒ of 1.7 eV. This fwhm was used to fit the O
͑1s͒ spectra since pure SiO2 results in a O ͑1s͒ spectra of a single
peak with fwhm of 1.7 eV under the same analysis conditions. The
X-ray photoelectron spectroscopy ͑XPS͒ and low energy electron
diffraction ͑LEED͒ results reported here indicate that exposure
of a Ru͑0001͒ surface to I2 vapor results in the formation of a
ͱ3 ϫ ͱ3 iodine (I-( 3 ϫ ͱ3)) adlayer that remains stable during
ͱ
exposure to air at 300 K, and effectively passivates the surface
* Electrochemical Society Active Member.
z E-mail: kelber@unt.edu
Downloaded on 2015-05-02 to IP 152.14.136.77 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

G116
Journal of The Electrochemical Society, 152 ͑2͒ G115-G121 ͑2005͒
binding energies of synthesized O ͑1s͒ spectral components were
allowed to vary by ϽϮ0.1 eV throughout the fitting procedure.
The exposure studies on Ru͑poly͒ and electrochemical studies on
Ru͑0001͒ were carried out in a system designed for transfer between
UHV and electrochemical environments without sample exposure
to atmosphere. A detailed description of this system and the method
for performing electrochemical experiments is reported else-
where.16,18,19 Briefly, however, the main chamber was equipped with
a hemispherical analyzer and unmonochromated X-ray source, and
with facilities for ion bombardment and sample heating/cooling, but
with the absence of the LEED capability. XPS acquisition and
analysis procedures were similar to those used in the other chamber.
The Ru͑0001͒ surface was cleaned by cycles of argon ion sput-
tering ͑3 keV, 10 ␮A͒ at 1000 K for 4.5 h followed by another 30
min annealing at 1100 K. The cleanness of the surface was checked
by LEED20 and by XPS. XPS analysis of Ru is complicated by the
overlap between the C ͑1s͒ and Ru (3d3/2) transitions. Therefore the
Ru (3d5/2)/Ru (3d3/2) intensity ratio was compared to literature
values21 as a measure of surface cleanliness, a method previously
used for the analysis of Ru thin films.22 The XPS spectra were
referenced to Ru (3d5/2) binding energy of 280.1 eV for a clean Ru
surface.21 The cleaning of Ru͑poly͒ was performed under a similar
procedure as for Ru͑0001͒ and the cleanness of the surface was also
similarly monitored by XPS.
Figure 1. Iodine uptake curve on clean Ru͑0001͒ at room temperature. A
bright Ru(0001)-(ͱ3 ϫ ͱ3)R30°-I structure was formed after 10.4 L io-
dine dosing (␪ ϭ 0.34).
The thickness of the oxide overlayer ͑d͒ formed after exposure
was estimated by the following equation24,26
1 Ϫ exp Ϫd/␭ E ͒
͔
͖
A
NA
NB
͓
͑
A
͕
ϭ
͓2͔
A surface iodine layer on Ru͑0001͒ for electrochemical studies
was prepared by immersion of a clean Ru͑0001͒ sample into a 0.1 M
HClO4 solution with 5 mM KI and polarization at a potential of
Ϫ0.6 V ͑vs. RHE͒ for 40 min.23 In the studies of adsorption behav-
ior of iodine on Ru͑0001͒ and in the exposure experiments, a surface
iodine layer was prepared by sample exposure to I2 vapor. The
molecular iodine was produced by sublimation from solid io-
dine ͑99.999% in purity, Strem Chemicals͒ and introduced into the
UHV main chamber as vapor through a retractable, stainless steel
tubular doser. The iodine vapor pressure in the UHV chamber was
maintained at 5 ϫ 10Ϫ8 Torr during dosing through a manual leak
valve. I2 exposures are reported here in terms of Langmuir ͑L͒
(1L ϭ 10Ϫ6 Torr s͒ and have not been corrected for ion gauge
sensitivity or flux to the sample. The iodine coverage was monitored
by XPS and calculated from the intensity ratio of I (3d5/2)/Ru
(3d5/2) peaks by the following equation for fractional monolayer
exp Ϫd/␭ E ͒
͔
B
͓
͑
B
where NA and NB are the atomic concentrations of the oxygen over-
layer and Ru substrate, respectively, and EA and EB represent the
XPS electron energy for O ͑1s͒ and Ru (3d5/2) electrons, ␭A and ␭
B
are the IMFP for oxygen and ruthenium, respectively. The
calculated25 IMFP of Ru (3d5/2) electrons in element Ru is
␭
B ϭ 14.1 Å. In order to estimating the IMFP of O ͑1s͒ electrons in
the oxygen overlayer, the overlayer composition is assumed to be
RuO2 and the calculated25 IMFP is 15.1 Å.
Results
Iodine adsorption on Ru(0001).—Figure 1 shows the variation of
XPS-derived I surface coverage vs. I2 exposure for adsorption on
clean Ru͑0001͒ at 300 K. The iodine displayed a rapid uptake on the
surface at low coverages, and approached saturation at higher cov-
erages. A saturation coverage of 0.44 ML was observed after expo-
sure of 39 L I2 . During the exposure process, LEED was employed
to monitor the ordering of iodine on the surface. A faint
24
coverage, ␪
␪
1 Ϫ exp Ϫa /␭ E ͒
͔
͖
A
NA
NB
͓
͑
A
͕
A
A
ϭ
͓1͔
1 Ϫ ␪ 1 Ϫ exp Ϫa /␭ E ͒
͔
͖
B
͓
͑
B
͕
A
A
(ͱ3 ϫ
ͱ
3)R30°-I superlattice pattern developed with increasing I2
where ␪ is the iodine coverage and aA is the covalent diameter of
the iodine atom ͑ϳ2.66 Å͒, NA and NB are the atomic concentra-
A
exposure. A bright, but not very sharp, LEED pattern was observed
after 10.4 L I2 exposure ͑Fig. 1͒. The surface coverage at this point
was estimated ͑Eq. 1͒ to be about 0.34 ML, close to the theoretical
surface coverage of 1/3 ML of iodine for (ͱ3 ϫ ͱ3)R30° structure
formation. Further incorporation of iodine during the buildup of the
adsorbate layer introduced disorder into this superstructure and fi-
nally blurred the pattern from the Ru͑0001͒ substrate after 39 L
iodine exposure.
tions of the iodine overlayer and Ru substrate, respectively, ␭ and
A
␭
are the inelastic mean-free paths ͑IMFPs͒ for iodine and ruthe-
B
nium, and EA and EB represent the XPS electron energy for I (3d5/2
)
and Ru (3d5/2) electrons, respectively. The calculated25 IMFP of Ru
(3d5/2) electrons in elemental Ru is ␭B ϭ 14.1 Å and IMFP of I
(3d5/2) in element I is ␭A ϭ 16.3 Å.
A saturated iodine layer was formed on both Ru͑0001͒ and
Ru͑poly͒ to prepare an iodine-covered surface for the exposure ex-
periments. In the clean Ru͑0001͒ and I-Ru͑0001͒ air exposure ex-
periments, the prepared and characterized sample was transferred
from UHV into the antechamber, which was then isolated via the
gate valve. The antechamber was then opened to atmosphere for 30
min. For the water exposure, the Ru͑0001͒ or I-Ru͑0001͒ sample
was transferred out of the UHV system and drops of deionized water
were applied onto the surface for 5 min. The sample was then dried
by nitrogen gas and transferred back into the UHV system for analy-
sis. The Ru͑poly͒ experiments were carried out in the UHV-EC
system. The Ru͑poly͒ or I-Ru͑poly͒ sample surface was exposed to
the water vapor from the electrochemical cell, 2 cm away, for
30 min for the water vapor experiments, or contacted to the water
meniscus of the electrochemical cell for 5 min for the water immer-
sion studies.
ͱ3 ϫ ͱ3R30° patterns were observed upon
Sharp and bright
annealing the iodine saturated Ru͑0001͒ single crystal for 5 min. at
400 or 500 K in UHV. The superlattice structure disappeared com-
pletely after annealing at 700 K for 5 min.
Effect of surface iodine layer on Ru(0001) reactivity toward air
and liquid water.—Figure 2a,b shows the comparison of O ͑1s͒ core
level spectra between clean and I-Ru͑0001͒ surfaces after exposure
to air for 30 min. Both experimental O ͑1s͒ spectra are well fit with
three components, each with a fwhm of 1.7 eV at binding energies
of 530.0, ϳ531.1, and 531.6 eV. The components at 530.0, 531.1,
and 532.5 eV are assigned, respectively, to RuO2 ,27-29 chemisorbed
oxygen,27,28 and hydroxide30 species by comparison with results in
the literature. The hydroxide and chemisorbed oxygen features are
the major components of the O ͑1s͒ spectrum on I-Ru͑0001͒. In
contrast, oxide is the dominant component for the air-exposed clean
Downloaded on 2015-05-02 to IP 152.14.136.77 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 152 ͑2͒ G115-G121 ͑2005͒
G117
Figure 2. XPS O ͑1s͒ spectra after air exposure to ͑a͒
iodine-covered Ru͑0001͒ and ͑b͒ clean Ru͑0001͒ and
after further liquid water exposure to ͑c͒ iodine-covered
Ru͑0001͒ and ͑d͒ clean Ru͑0001͒.
Ru͑0001͒ O ͑1s͒ spectrum. The thickness of the oxide layer was
estimated ͑Eq. 2͒ to be 1.8 Å for air-exposed Ru͑0001͒, and 0.07 Å
for I-Ru͑0001͒, respectively. The intensity ratios of O ͑1s͒/Ru
(3d5/2) for components in the experimental O ͑1s͒ spectra and the
estimated oxide thickness are summarized in Table I. The pro-
nounced difference in the oxide layer thickness after air exposure
indicates that the iodine adlayer passivates the substrate surface and
inhibits the formation of oxide on it when exposed to ambient.
The formed Ru(0001)-(ͱ3 ϫ ͱ3)R30°-I structure was stable
not only toward exposure to air but also toward immersion in water
͑Fig. 3͒. The stability of adsorbed iodine toward water exposure, as
demonstrated by LEED ͑right panel of Fig. 3͒, would lead to an
expectation of the same strong inhibition of oxygen adlayer forma-
tion in water exposure as in air exposure. However, when such air-
exposed surfaces were further exposed to deionized water, the dif-
ference in the resulting O ͑1s͒ spectra of two surfaces is less
pronounced, as shown in Fig. 2c,d. These O ͑1s͒ spectra are fit
with three components, each with a fwhm of 1.7 eV at binding
energies of 530.1, ϳ531.1, and 532.5 eV, which was also assigned to
oxide, chemisorbed oxygen, and hydroxide, respectively. The esti-
mated thickness of oxide overlayers are 0.3 Å for I-Ru͑0001͒ sur-
face and 2.2 Å for uncovered Ru͑0001͒ surface, indicating a less
pronounced iodine passivation for the surface toward interaction
with liquid water.
ments ͑Fig. 2͒. In each case the spectrum can also be well fit with
three components with a fwhm of 1.7 eV at binding energies of
530.0, 531.1, and 532.5 eV which are similarly assigned to the ox-
ide, chemisorbed oxygen, and Ru hydroxide bonds, respectively
͑Table I͒. The pronounced difference in the intensities of peaks at
530.0 eV clearly demonstrates the inhibition effect of a surface io-
dine adlayer toward the oxide formation when exposed to water
vapor. The estimated thicknesses of the oxide layers are 0.1 and 3.4
Å for I-Ru͑poly͒ and clean Ru͑poly͒, respectively. Figure 4c,d
shows the XPS O ͑1s͒ spectra acquired for the above water-vapor
exposed samples after subsequent exposure to liquid water. The O
͑1s͒ spectra in both I-Ru͑poly͒ and clean Ru͑poly͒ are similarly fit
as that for water-vapor exposed samples ͑Table I͒. The components
at binding energies of 530.0, ϳ531.1, and ϳ532.5 eV are also as-
signed to the RuO2 , chemisorbed oxygen, and Ru hydroxide bonds,
respectively. The I-Ru͑poly͒ surface formed a thicker oxide layer
when exposed to liquid water than when exposed only to water
vapor, but the thickness is still smaller than that observed for the
clean Ru͑poly͒ surface exposed to liquid water. The estimated thick-
nesses of the oxide overlayers are 0.6 and 4.4 Å for I-Ru͑poly͒ and
clean Ru͑poly͒, respectively. The results show an effective but lim-
ited passivation effect of iodine adlayer on the Ru͑poly͒ surface
toward liquid water exposure.
A similar passivation effect of a surface iodine layer was also
observed on Ru͑poly͒. Figure 4a,b shows the XPS O ͑1s͒ spectra
after exposing the Ru͑poly͒ surface and the I-Ru͑poly͒ surface to
water vapor above an electrochemical cell. Both O ͑1s͒ core level
spectra are similar to those from the Ru͑0001͒ air-exposure experi-
Surfactant effect of iodine adlayer during Cu electrodeposition
on Ru(0001).—After the cleaning of Ru͑0001͒ in the UHV-EC
chamber, another 2 h annealing at 850 K was conducted on the
Ru͑0001͒ sample in order to achieve a well-ordered surface before
the electrochemical experiments.
Table I. Intensity ratios of O „1s…ÕRu „3d5Õ2… for synthesized components „fwhm Ä 1.7 eV… of experimental O „1s… spectra and the estimated
oxide thickness on Ru„0001… and Ru„poly… under different exposure conditions.
Estimated
thickness
of oxide
overlayer
͑Å͒
Intensity ratios of O ͑1s͒/Ru (3d5/2) for components
in experimental O ͑1s͒ spectra
Oxide
Chemisorbed
Hydroxide
Samples and exposure conditions
͑ϳ530 eV͒
Oxygen ͑ϳ531.1 eV͒
͑ϳ532.5 eV͒
Ru͑0001͒
Air exposure
Ru͑0001͒ ϩ air
I-Ru͑0001͒ ϩ air
Ru͑0001͒ ϩ air ϩ water
I-Ru͑0001͒ ϩ air ϩ water
Ru͑poly͒ ϩ vapor
I-Ru͑poly͒ ϩ vapor
Ru͑poly͒ ϩ vapor ϩ water
I-Ru͑poly͒ ϩ vapor ϩ water
0.030
0.001
0.049
0.006
0.065
0.001
0.093
0.009
0.012
0.005
0.019
0.021
0.044
0.009
0.056
0.048
0.007
0.007
0.012
0.008
0.025
0.009
0.030
0.028
1.8
0.07
2.2
0.3
3.4
0.1
4.4
0.6
Liquid
Water exposure
Water
Vapor exposure
Liquid
Ru͑poly͒
Water exposure
Downloaded on 2015-05-02 to IP 152.14.136.77 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

G118
Journal of The Electrochemical Society, 152 ͑2͒ G115-G121 ͑2005͒
Figure 3. Ru(0001)-(ͱ3 ϫ ͱ3)R30°-I stability toward exposure to air and
water. Ru(0001)-(ͱ3 ϫ ͱ3)R30°-I pattern was taken ͑a͒ after air exposure,
͑b͒ after air exposure followed by water͑liquid͒ exposure. The beam energy
was 71.4 eV.
The surface iodine layer was deposited on well-annealed
Ru͑0001͒ electrochemically before the copper deposition process,
with an estimated coverage of 0.25ML from XPS ͑Eq. 1͒. Since the
UHV-EC chamber in which the electrochemical/XPS studies were
performed does not possess LEED capability, we can come to no
firm conclusions on the structure of the 0.25 ML I adlayer on
Ru͑0001͒. However, the 0.25 ML coverage calculated from XPS is
in agreement with a ͱ3 ϫ ͱ3 structure.
Figure 5a shows the CV observed for clean and I-Ru͑0001͒ elec-
trodes immersed in a 5 mM Cu͑ClO4)2 solution with a 0.1 M HClO4
supporting electrolyte. The iodine-modified sample was scanned
from open-circuit potential ͑OCP͒ to Ϫ0.5 V vs. RHE and then
reversed to anodic sweep and back to OCP. A similar scan procedure
was applied to the unmodified Ru͑0001͒ sample, but the negative
potential limit was extended to Ϫ0.6 V vs. RHE. As shown in Fig. 5,
the iodine modification has a pronounced effect on the Cu elec-
trodeposition process. With iodine modification, the anodic peak po-
tential shifts about 0.07 V more negative, indicating an energetically
more favorable anodic process. In the cathodic sweep on the un-
modified Ru͑0001͒, the deposition take places over a wide potential
range without distinct peak formation, since H2 evolution overlaps
the Cu deposition region. In contrast, a wave at Ϫ0.42 V is notice-
able on I/Ru͑0001͒.
Figure 5. Cyclic voltammograms for Cu electrodeposition on ͑a͒ I/Ru͑0001͒
or ͑b͒ clean Ru͑0001͒ in 0.1 M HClO4 solution with 5 mM Cu͑ClO4)2 . Scan
rate, 10 mV/s.
Identified with the Cu Auger parameter, the deposited Cu is Cu
metal ͑Fig. 6e͒. The sulfur contamination is negligible both before
and after the deposition process as evidenced by the very weak XPS
S ͑2p͒ spectra signals ͑Fig. 6c͒. The Ru spectra ͑Fig. 6a͒ intensity
was attenuated by the deposited Cu layer after the electrodeposition
process. In contrast, the intensity of I (3d5/2) spectra was not attenu-
ated ͑the slight increase indicates inhomogeneous coverage of the
sample͒ ͑Fig. 6b͒. This result demonstrates that surface iodine is not
covered by the deposited Cu, but floats on top of the surface during
the deposition process. This conclusion is verified by the depth pro-
file of the Cu electrodeposited I/Ru͑0001͒ sample ͑Fig. 7͒. After
about 250 min of argon sputtering, the XPS intensity ratio of I
(3d5/2)/Ru (3d5/2) become constant and close to null as shown in
Fig. 7. The intensity ratio of Cu (2p)/Ru (3d5/2) is, however, de-
creased by only about 30%. This result clearly demonstrates that
iodine definitely floats on top of the Cu layer surface as a surfactant
during the electrodeposition.
XPS and X-ray excited Cu͑LVV͒ Auger spectra of the
I-Ru͑0001͒ surface before and after Cu electrodeposition are shown
in Fig. 6. The XPS and Auger measurements were carried out after
four CV cycles and one subsequent pulse experiment on the
I-Ru͑0001͒ sample, which was emmersed at potential of Ϫ0.5 V.
Discussion
LEED results ͑Fig. 1͒ indicate that a (ͱ3 ϫ ͱ3)R30°-I over-
layer is formed by Ru͑0001͒ exposure to I2 at 300 K, and that this
Figure 4. XPS O ͑1s͒ spectra after water vapor expo-
sure to ͑a͒ iodine covered Ru͑poly͒ and ͑b͒ clean
Ru͑poly͒ and after further liquid water exposure to ͑c͒
iodine-covered Ru͑poly͒ and ͑d͒ clean Ru͑poly͒.
Downloaded on 2015-05-02 to IP 152.14.136.77 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 152 ͑2͒ G115-G121 ͑2005͒
G119
Figure 6. XPS spectra after iodine adsorption ͑solid line͒ on Ru͑0001͒ and after subsequent Cu electrodeposition ͑dotted line͒ on I/Ru͑0001͒.
ad-layer becomes disordered at higher I2 exposures. Similar disor-
dering of a halogen adlayer upon saturation exposure has been re-
ported for Br2 /Pt(111)31 and for I2 /Fe-Cr-Ni(111).11 In the experi-
ment reported here, mild annealing in vacuum restores the ordered
ad-layer. XPS data ͑Fig. 2a,b; Table I͒ and LEED ͑Fig. 3͒ demon-
strate that the ordered I ad-layer inhibits oxide and adsorbed oxygen
formation on the Ru͑0001͒ surface upon exposure to ambient at
room temperature. A similar effect is observed for an I-modified
polycrystalline electrode exposed to water vapor above an electro-
chemical cell ͑Fig. 4͒. The ordered I ad-layer on the single-crystal
substrate will also survive immersion in water under open circuit
conditions. While the surface is less completely passivated under
such conditions, an inhibition of surface oxide formation relative
that of the unmodified Ru surface is still observed ͑Fig. 2, 4; Table
I͒. These data indicate the practical robustness of I-modification of
Ru surfaces for passivation and electrochemical processing under
industrial conditions.
The kinetic inhibition of RuO2 formation on Ru͑0001͒ upon ex-
Figure 7. Depth profile of Cu electrodeposited I/Ru͑0001͒ sample.
posure to O2 at 300 K is well known32 and accounts for the limited
Downloaded on 2015-05-02 to IP 152.14.136.77 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

G120
Journal of The Electrochemical Society, 152 ͑2͒ G115-G121 ͑2005͒
oxide layer thickness observed for clean Ru͑0001͒ exposed to room
air, or air and liquid water ͑Table I͒. This kinetic barrier is less
apparent for polycrystalline surfaces, however, as demonstrated by
the substantially thicker oxide formed on the polycrystalline as com-
pared to a single-crystal surface ͑Table I͒. While a greater reactivity
of the polycrystalline as compared to the close-packed single-crystal
surface is not surprising, there are obvious implications for the pro-
cessing of Ru diffusion barriers during microelectronics fabrication,
since such barrier surfaces are more likely to resemble the polycrys-
talline rather than the single-crystal surface.
A detailed analysis of the interaction of the I-adlayer with liquid
water compared to combined H2O/O2 vapor ͑ambient air͒ is com-
plicated due to some uncertainty in the assignment of O ͑1s͒ features
at Ru surfaces. There is general consensus27-29 that an O ͑1s͒ bind-
ing energy near 530.0 eV on Ru is indicative of oxide formation,
RuO2 . An O ͑1s͒ binding energy of ϳ532 eV ͑Fig. 2 and 4͒ has
been attributed to a surface hydroxyl species,30,33 as well as to a
chemisorbed O species.28 The fact that exposure to H2O vapor re-
sults in features at both 532.5 and 531 eV ͑Fig. 4͒, motivates the
assignment of the 532.5 eV feature to a surface OH species. An
assignment of this feature to molecularly adsorbed water is improb-
able due to the stability of this feature at 300 K in UHV on both
clean and I-modified surfaces, and to the fact that molecularly ad-
sorbed H2O dissociates on Ru well below room temperature.34 The
assignment of the 532.5 eV feature to chemisorbed OH is further
supported by the fact dissociative chemisorption and OH formation
have been reported for both clean Ru͑0001͒1,35 and Ru͑0001͒ modi-
fied by chemisorbed oxygen.36 Assignment of the 532.5 eV feature
to OH then indicates the most logical assignment of the 531 eV
feature is to chemisorbed oxygen.
Figure 8. Possible adsorption sites for oxygen and water molecules on the
Ru͑0001͒ surface with a Ru(0001)-(ͱ3 ϫ ͱ3)R30°-I superstructure.
vapor-phase exposure ͑Fig. 2a,b; Fig. 4a,b͒, interactions at the
liquid-solid interface may be complicated by the presence of a
double layer and charged species. In addition, the further exposure
of the surface to vapor upon emersion from the liquid must be con-
sidered. The specific I inhibition of oxide formation, however, is
explainable if the formation of a complete Oa (1 ϫ 1)ad-layer is
presumed to be a necessary precursor to the penetration of subsur-
face oxygen,39,40 which leads to the formation of RuO2 ,41 as indi-
cated by UHV studies and theoretical calculations. In such a case,
the high activity of H2O at the liquid/solid interface could result in
high occupancy of available OHa and Oa sites, as apparent in Fig.
2c,d and Fig. 4c,d. As long as a significant number of 3-fold sites
remain occupied by Ia, the formation of the Oa(1 ϫ 1) ad-layer will
be prevented, and the formation of RuO2 therefore inhibited.
Given the O ͑1s͒ assignments in Table I, inspection of the data
for H2O vapor exposure to clean and I-modified Ru͑poly͒ ͑Fig. 4a,b;
Table I͒ indicates that the I overlayer inhibits complete H2O disso-
ciation to form either chemisorbed O or RuO2 , although OH forma-
tion occurs. These results are consistent with those observed for
I-Ru͑0001͒ ͑Fig. 2a,b; Table I͒ and suggest that although the partial
dissociation of H2O occurs on both clean and I-modified surfaces
H2O → OHa ϩ Ha
͓3͔
͑a)
The further dissociation of OH is inhibited by the I-adlayer
OHa → Oa ϩ Ha ͑inhibited by Ia)
͓4͔
Results of exposure to either H2O vapor or ambient air differ
significantly from the results observed for immersion in water solu-
tion ͑Fig. 2c,d; Fig. 4c,d; Table I͒. Immersion in the liquid phase
results in substantial OHa and Oa formation on both single-crystal
and polycrystalline surfaces. In both cases, it is the formation of the
oxide which is inhibited by the presence of an I adlayer. This result
is somewhat similar to that observed for S-modified Cu18 and
S-modified Fe polycrystalline19 electrodes exposed to aqueous elec-
trolytes under controlled potential conditions. In those cases, the
presence of S resulted in relatively greater hydroxide vs. oxide for-
mation, and this was attributed to S inhibition of the complete dis-
sociation of H2Oa, as in Fig. 4. The results observed here ͑Fig. 2,
Fig. 4͒ suggest that Ia /Ru blocks OHa dissociation under vapor con-
ditions, but blocks conversion of Oa to RuO2 in liquid.
The ability of Ia to block OH dissociation is readily accounted for
by a simple steric model in which I occupies 3-fold sites on the
Ru͑0001͒ surface ͑Fig. 8͒. Halogen occupancy of high coordination
sites is generally observed for seminoble surfaces.37 Some of the
3-fold sites are blocked by I occupancy, but on-top and some 3-fold
sites are accessible. Oa on Ru also occupies 3-fold sites,38 and there-
fore the formation of an Oa surface layer is partly inhibited by I. In
contrast, H2O adsorption occurs on on-top sites.1 Therefore, while
the initial chemisorption of H2O is not inhibited by the I adlayer,
further dissociation to Oa is limited by access to surface 3-fold sites
͑Fig. 8͒. While this model accounts for the results of ambient air or
The ability of Ia to float on top of a depositing ͑or etching͒ Cu
film is apparent from the data presented in Fig. 6 and 7. The floating
ability of I has been demonstrated for repeated cycles of underpo-
tential deposition ͑UPD͒ on I-modified Pt14,15 and other surfaces.13
As pointed out by Kolb and co-workers,42 however, overpotential
deposition ͑OPD͒ is an electronically and physically distinct process
from UPD, resulting in complete discharge of the Cu ion. Further,
even a small increase in potential from the UPD to OPD region can
result in a qualitative change in nucleation kinetics, as observed for
Pd OPD on Au͑111͒.43 Therefore, the ability of adsorbed I to remain
on top of a film deposited under OPD conditions can in no way be
assumed on the basis of UPD studies. The data in Fig. 6 and 7
therefore demonstrate that Ia on Ru has the necessary stability to
survive cycling of the electrode over a reasonably broad potential
range, and the ability to be continually displaced to surface sites of
the Cu film during OPD. The apparent increase in I intensity upon
Cu deposition ͑Fig. 6͒ is attributable to the fact that the immersion
method, which involves the electrode surface being placed on top of
the electrochemical cell so that the meniscus wets the surface16,18
can result in inhomogeneous concentrations and/or deposition across
the ϳ1 cm diam electrode surface. This was verified by further
immersion/polarization studies on I-modified polycrystalline elec-
trodes, which showed marked variation in the XPS intensities of Cu,
I, and adsorbed anion species as the sample source area was system-
atically varied across the surface. The data in Fig. 7 demonstrate that
Downloaded on 2015-05-02 to IP 152.14.136.77 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 152 ͑2͒ G115-G121 ͑2005͒
G121
University of North Texas assisted in meeting the publication costs of
this article.
Cu deposition results in no significant I penetration into the bulk of
either the growing Cu film or the Ru substrate.
Although concentration inhomogeneties across the electrode sur-
face obviate any detailed kinetic analysis of I effects on the elec-
trodeposition process, the data in Fig. 5 demonstrate that Ia shifts the
onset of H2 evolution to higher potentials, and yields a distinct Cu
deposition wave. Although the apparent Cu deposition current is
larger for the I-modified electrode ͑Fig. 5͒, the possible effects of a
larger effective electrode area due to slight changes in the position
of the surface relative to the meniscus cannot be rigorously ex-
cluded. In addition, it is not yet understood whether the shift of the
stripping peak to lower potentials on the I-modified surface is due
specifically to I, or to some other effect related to the excursion to
larger negative potentials on the I-modified electrode.
References
1. M. Nakamura and M. Ito, Chem. Phys. Lett., 325, 293 ͑2000͒.
2. H. Shi and K. Jacobi, Surf. Sci., 317, 45 ͑1994͒.
3. H. Kim, I. R. Moraes, G. Tremiliosi-Filho, R. Haasch, and A. Wieckowski, Surf.
Sci., 474, L203 ͑2001͒.
4. O. Chyan, T. N. Arunagiri, and T. Ponnuswany, J. Electrochem. Soc., 150, C347
͑2003͒.
5. K. M. Takahashi, J. Electrochem. Soc., 147, 1414 ͑2000͒.
6. C. Wang, J. Lei, S. Rudenja, N. Magtoto, and J. Kelber, Electrochem. Solid-State
Lett., 5, C82 ͑2002͒.
7. G. Oskam, P. M. Vereecken, and P. C. Searson, J. Electrochem. Soc., 146, 1436
͑1999͒.
8. A. Wieckowski, S. D. Rosasco, B. C. Schardt, J. L. Stickney, and A. T. Hubbard,
Inorg. Chem., 23, 565 ͑1984͒.
Conclusions
9. F. Lu, G. N. Salaita, H. Baltruschat, and A. T. Hubbard, J. Electroanal. Chem., 222,
305 ͑1987͒.
XPS and LEED measurements have been carried out on
Ru͑0001͒ electrodes exposed to I2 vapor at 300 K and on I-modified
and clean Ru͑0001͒ and Ru͑poly͒ electrodes exposed to ambient
vapor and liquid H2O environments at 300 K. The results show that
10. A. T. Hubbard, Chem. Rev. (Washington, D.C.), 88, 633 ͑1988͒.
11. G. A. Garwood, Jr. and A. T. Hubbard, Surf. Sci., 121, L524 ͑1982͒.
12. E. S. Hwang and J. Lee, Electrochem. Solid-State Lett., 3, 138 ͑2000͒.
13. A. Martinez-Ruiz, J. Valenzuela-Benavides, L. M. D. L. Garza, and N. Batina, Surf.
Sci., 476, 139 ͑2001͒.
an ordered (ͱ3 ϫ
ͱ
3)R30°-I adlayer is formed after 10.4 L I2
exposure at 300 K, but becomes disordered upon higher I2 expo-
sures at this temperature. Annealing to 400 K restores the ordered
adlayer, while annealing to 700 K in UHV removes I from the
surface.
14. J. L. Stickney, S. D. Rosasco, and A. T. Hubbard, J. Electrochem. Soc., 131, 260
͑1984͒.
15. S. Chia-Haw and Y. Shueh-Lin, J. Phys. Chem. B, 105, 5489 ͑2001͒.
16. T. C. Lin, G. Seshadri, and J. A. Kelber, Langmuir, 14, 3673 ͑1998͒.
17. P. M. A. Sherwood, J. Vac. Sci. Technol. A, 14, 1424 ͑1996͒.
18. G. Seshadri, H. C. Xu, and J. A. Kelber, J. Electrochem. Soc., 146, 1762 ͑1999͒.
19. G. Seshadri, T. C. Lin, and J. A. Kelber, Corros. Sci., 39, 987 ͑1997͒.
20. J. T. Grant and T. W. Haas, Surf. Sci., 21, 76 ͑1970͒.
21. J. R. Moulder, W. F. Stickle, P. E. Sobol, K. D. Bomben, J. Chastain, and R. C.
King, Handbook of X-Ray Photoelectron Spectroscopy, Physical Electronics, Eden
Prairie, MN ͑1995͒.
(ͱ3 ϫ ͱ3)R30°-I
The
Ru͑0001͒ sample to ambient air or to air and immersion in water
under open-circuit conditions. The ad-layer passivates the
adlayer survives exposure of the
I
Ru͑0001͒ surface against significant hydroxide, chemisorbed oxy-
gen, or oxide formation during exposure to air. Immersion of
I-Ru͑0001͒ results in greater hydroxide and chemisorbed oxygen
formation than air exposure. The I adlayer, however, still greatly
inhibits oxide formation. Similar results are observed for I-modified
Ru͑poly͒.
XPS studies have also been carried out on clean and I-modified
Ru͑poly͒ electrodes Cu-containing, I-free electrolyte, and polarized
at potentials between Ϫ0.6 and 0.09 V vs. RHE, and then transferred
to UHV without atmospheric exposure. Cyclic voltammetric mea-
surements indicate that Ia shifts H2 evolution to more negative po-
tentials, permitting the observation of a distinct Cu deposition wave
in the voltammogram. XPS measurements show that I is not buried
by Cu deposition under OPD conditions, but floats on top of the
growing Cu film.
The above data indicate that I bonding to Ru is sufficiently ro-
bust as to survive air exposure and immersion in water, while pas-
sivating the surface against oxide formation. In addition, adsorbed I
is not buried during Cu OPD on Ru, but floats to the surface of the
growing Cu film. These findings have practical implications for the
use of I ad-layers for passivation and as a surfactant for the elec-
trodeposition of Cu onto Ru diffusion barriers for microelectronics
fabrication.
22. K. C. Smith, Y.-M. Sun, N. R. Mettlach, R. L. Hance, and J. M. White, Thin Solid
Films, 376, 73 ͑2000͒.
23. P.-C. Lu, C.-H. Yang, S.-L. Yau, and M.-S. Zei, Langmuir, 18, 754 ͑2002͒.
24. M. P. Seah, Practical Surface Analysis, Wiley, New York ͑1990͒.
25. S. Tanuma, C. J. Powell, and D. R. Penn, Surf. Interface Anal., 21, 165 ͑1994͒.
26. P. J. Cumpson and M. P. Seah, Surf. Interface Anal., 25, 430 ͑1997͒.
27. S. Bhaskar, P. S. Dobal, S. B. Majumder, and R. S. Katiyar, J. Appl. Phys., 89, 2987
͑2001͒.
28. H. Y. H. Chan, C. G. Takoudis, and M. J. Weaver, J. Catal., 172, 336 ͑1997͒.
29. J. Y. Shen, A. Adnot, and S. Kaliaguine, Appl. Surf. Sci., 51, 47 ͑1991͒.
30. H. Madhavaram, H. Idriss, S. Wendt, Y. D. Kim, M. Knapp, J. Over, J. Abamann,
E. Loffler, and M. Muhler, J. Catal., 202, 296 ͑2001͒.
31. E. Bertel, K. Schwaha, and F. P. Netzer, Surf. Sci., 83, 439 ͑1979͒.
32. M. Tanaka and M. Ami, J. Am. Ceram. Soc., 81, 1969 ͑1998͒.
33. R. Koetz, H. J. Lewerenz, and S. Stucki, J. Electrochem. Soc., 130, 825 ͑1983͒.
34. G. Pirug, C. Ritke, and H. P. Bonzel, Surf. Sci., 241, 289 ͑1991͒.
35. P. J. Feibelman, Science, 295, 99 ͑2002͒.
36. M. Kiskinova, G. Pirug, and H. P. Bonzel, Surf. Sci., 150, 319 ͑1985͒.
37. S. B. Dicenzo, G. K. Wertheim, and D. N. E. Buchanan, Surf. Sci., 121, 411 ͑1982͒.
38. T. E. Madey, H. A. Engelhardt, and D. Menzel, Surf. Sci., 48, 304 ͑1975͒.
39. A. Bottcher and H. Niehus, J. Chem. Phys., 110, 3186 ͑1999͒.
40. C. Stampfl, S. Schwegmann, J. Over, M. Scheffler, and G. Ertl, Phys. Rev. Lett., 77,
3371 ͑1996͒.
Acknowledgments
41. K. Reuter, C. Stampfl, M. V. Ganduglia-Pirovano, and M. Scheffler, Chem. Phys.
Lett., 352, 311 ͑2002͒.
42. L. A. Kibler, M. Kleinert, R. Randler, and D. M. Kolb, Surf. Sci., 443, 19 ͑1999͒.
43. M. T. Quayum, S. Ye, and K. Uosaki, J. Electroanal. Chem., 520, 126 ͑2002͒.
This work was supported by the Semiconductor Research Cor-
poration under Research Task 1000.001, and by the Robert Welch
Foundation under grant no. B-1356.
Downloaded on 2015-05-02 to IP 152.14.136.77 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).