Ruthenium electrodeposition on silicon from a room-temperature ionic liquid

Article abstract of DOI:10.1016/j.electacta.2009.01.012

Electrochemical deposition of ruthenium on n-type silicon from an ionic liquid is reported for the first time. The study was performed by dissolving ruthenium(III) chloride in a 1-butyl-3-methyl imidazolium hexafluorophosphate (BMIPF₆) room-tem

Full text of DOI:10.1016/j.electacta.2009.01.012

Electrochimica Acta 54 (2009) 6042–6045
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Ruthenium electrodeposition on silicon from a room-temperature ionic liquid
a
a
b
b
a,∗
Ofer Raz , Gil Cohn , Werner Freyland , Olivier Mann , Yair Ein-Eli
a
Department of Materials Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel
Institute of Physical Chemistry, Karlsruhe Institute of Technology (KIT), Kaiserstrasse 12, 76128 Karlsruhe, Germany
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 October 2008
Received in revised form 24 December 2008
Accepted 7 January 2009
Available online 15 January 2009
Electrochemical deposition of ruthenium on n-type silicon from an ionic liquid is reported for the first
time. The study was performed by dissolving ruthenium(III) chloride in a 1-butyl-3-methyl imidazolium
hexafluorophosphate (BMIPF6) room-temperature ionic liquid (RTIL). Cyclic voltammetry (CV) studies
demonstrate reduction and stripping peaks at −2.1 and 0.2 V vs. Pt quasi-reference, corresponding to
the deposition and dissolution of ruthenium, respectively. Metallic Ru films of ∼100 nm thickness have
been deposited and were analyzed using scanning electron microscopy (SEM) and X-ray photoelectron
spectroscopy (XPS).
Keywords:
Ionic liquids
Electrodeposition
Ruthenium
©
2009 Elsevier Ltd. All rights reserved.
1
. Introduction
[14,16]. Thus, Ru is recognized as a candidate material for many
electronic devices, in particular as adhesion promoter and a dif-
fusion barrier between silicon and copper in ultra large system
integrated (ULSI) [17]. Ruthenium thin films are also being con-
sidered in fabrication of dynamic random access memories (DRAM)
[18] and ferroelectric random access memories (FRAM) [19]. Ruthe-
nium ECD is currently considered as a mean to achieve a diffusion
barrier between metal layers, as capacitor electrode in memories
and slip-ring electric contacts [15].
Room-temperature ionic liquids (RTILs) have attracted much
attention as a replacement for classical solvents in many applica-
tions due to their remarkable chemical and physical properties [1,2]
which include high chemical and thermal stability, high ionic con-
ductivity, low vapor pressure and good solubility of many organic
and inorganic compounds. Since RTILs are composed of ions in
which both cation and anion can be varied, a large number of
anion–cation pairs can be combined, each one with the desirable
unique properties [3].
In order to achieve the above mentioned applications, Ru thin
films must be deposited onto silicon. Some methods for Ru ECD
Most RTILs are highly conductive with a wide electrochemical
window which makes them attractive for use in electrochemical
applications, such as electrochemical deposition (ECD). This has
been studied for a variety of transition metals and semiconduc-
tors [4–8]. Such type of deposition is highly useful in preparation
of new materials having specific characteristics, such as thin films
and nanostructures. Comparing it with other deposition methods
like Chemical Vapor Deposition (CVD) [9], Electroless deposition
were already reported; hydrous ruthenium oxide (RuOx·nH O) with
2
high pseudocapacitance has been electroplated from an aqueous
solution containing RuCl3 [20]. Also, Ru deposition in solutions
containing Ru(NO)Cl3 dissolved in H N–SO –OH [21] leading to
2
2
uniform coating of 1–2 ␮m of ruthenium has been reported. Since
ruthenium tends to oxidize in aqueous solutions it is highly diffi-
cult to obtain pure metallic ruthenium films. Utilization of a molten
◦
salt solution (KCN and NaCN) at a temperature of 560 C yielding
(
ELD) [10] or Atomic Layer Deposition (ALD) [11–13], ECD can be
smooth 100 ␮m thick Ru layers was also reported [14]. Furthermore,
ruthenium films have been deposited at a temperature of 90 C by
dissolving RuCl3 in non aqueous solution of dimethylformamide
(DMF), which is highly volatile and toxic [14].
◦
in many cases more effective, least expensive and simple to adapt.
Consequently, studying the electrochemical deposition of metals
and semiconductors in RTILs is of great importance.
One of the most interesting elements to be deposited in RTILs is
ruthenium. Ru is a noble metal (being inexpensive compared with
other noble metals such as Pt, Rh and Pd [14]), with high hardness
having a high melting temperature of 2130 C [15], good corrosion
resistance, and may serve as an effective oxygen diffusion barrier
Even though many attempts have been made to find the opti-
mum conditions for Ru deposition, still there are problems as
for composition instability, low cathodic current densities and
film morphology. Recently, we reported on Ru electrodeposition
from 1-butyl-3-methyl dicyanamide (BMIDCA) on Au substrates via
sequential transformation of Ru ion valence state 3+ to 4+ and a
final reduction to metallic Ru [22]. Herein, we report on a direct
electrodeposition of metallic ruthenium on silicon from 1-butyl-
3-methyl imidazolium hexafluorophosphate (BMIPF6) solution
◦
∗ Corresponding author.
E-mail address: eineli@tx.technion.ac.il (Y. Ein-Eli).
0
013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.01.012

O. Raz et al. / Electrochimica Acta 54 (2009) 6042–6045
6043
Fig. 1. Cyclic voltammograms obtained from polarizing n-Si (1 0 0) at a scan rate
◦
of 50 mV/s in BMIPF6 at room temperature and 70 C, and in a solution containing
◦
4
mM RuCl3 dissolved in BMIPF6 at 70 C.
containing Ru3+ ions. We also discuss the morphology and spec-
troscopic characteristics of the uniform Ru film electrodeposited
on blank n-type silicon.
2. Experimental
Ruthenium (III) chloride (RuCl , 99.99%, Leico Industries Inc.),
3
and BMIPF6 (electrochemical grade >99.5%, Covalent) were used.
◦
−5
BMIPF was dried overnight at 80 C under vacuum (10
1
to
mbar) in order to further reduce the water content. Solu-
tion preparations as well as electrochemical measurements were
6
−
0
6
performed in an Ar-filled glove box (MBraun Unilab, H O and O2
2
below 3 ppm). Electrochemical studies were carried out with an
EG&G 273A potentiostat/galvanostat (Princeton Applied Research).
A three electrode electrochemical configuration was employed
Fig. 2. (a) Potentiostatic current transients curves obtained from polarizing n-Si
(1 0 0) in solutions containing 4 mM RuCl dissolved in BMIPF6 at a temperature of
3
◦
in RTIL media, where n-type silicon wafers (100 oriented, with
70 C at different cathodic potentials. Starting potential was at −0.5 V vs. Pt/Pt(II).
2
(b) nondimensional plots for instantaneous and progressive nucleation model in
comparison with experimental transients.
1
–10 ꢀ cm and an effective working electrode area of 0.1 cm ) were
used as a working electrode. Silicon wafers were initially immersed
in H O :H SO (1:1) solution for 10 min in order to remove organic
2
2
2
4
contamination, followed by 10 s immersion in HF:H O (1:50) solu-
50 mV/s until a current density of about 100 ␮A/cm2 was achieved.
2
tion to remove surface oxides. Finally, Si substrates were washed
and rinsed in deionized water and dried under a nitrogen stream.
The Si wafer was contacted from the back side by mechanically
squeezing of a stainless steel plate. A Pt foil was used as a counter
electrode while a Pt wire served as a quasi-reference electrode.
The Pt electrodes were flame annealed with butane-flame prior to
each experiment. A polyetheretherketone (PEEK) round bottom cell
was used as the electrochemical compartment adapted for small
volumes of solution (0.5 mL).
Within these current limits the electrochemical potential window
of the Ru-free RTIL at room temperature is found to be nearly 4 V, in
agreement with an other report [1]. The electrochemical potential
◦
window is reduced by ∼0.5 V at 70 C. Attempts to electrodeposit Ru
on n-Si substrate were conducted in BMIPF containing 4 mM RuCl .
6
3
◦
The CV recorded at 70 C with a scan rate of 50 mV/s is shown in
Fig. 1, as well. Measurements were initiated at a potential of −0.4 V,
1
00 mV above the open circuit potential (−0.5 V), swept negatively,
and back-scanned at a potential of −2.7 V. A shoulder is visible in the
Scanning electron microscopy (FEI Quanta 200) was utilized
in surface morphology studies of the deposited layers, and XPS
analysis was performed on a Sigma probe X-ray photoelectron
spectrometer (Thermo V.G. Scientific) using a monochromatized
aluminum anode X-ray source (Al K␣ 1486.68 eV). Sample sputter-
cathodic sweep around −1.4 V, followed by a clear reduction peak at
−
2.1 V. We assign the latter to Ru deposition reaction; Whether the
reduction of Ru(III) to Ru(0) occurs in one step or passes through an
intermediate oxidation state, which may be indicated by the redox
reaction corresponding to the shoulder at −1.4 V, is an open ques-
tion. A stripping peak is observed at 0.2 V in the anodic scan, well
below the anodic limit of BMIPF6 RTIL.
+
ing was performed by a 2 kV Ar ion beam. Etching rate calibration
was carried out with a thick Ta O5 standard.
2
Typical potentiostatic current transients recorded at different
cathodic potentials (from −1.4 to −2 V) during Si polarization in
3. Results and discussion
◦
a BMIPF6 solution containing 4 mM of RuCl3 at 70 C are shown
Fig. 1 presents cyclic voltammograms (CV) obtained from polar-
in Fig. 2a. The potential was stepped for all current transients
presented in Fig. 2 from OCP (near −0.5 V) to the respective over-
potentials (between −1.4 V and −2.0 V). A contribution of the
double layer charging in ionic liquids is expected well below 0.1 s
[23], has been subtracted from the transients shown in Fig. 2a. All
◦
izing n-type silicon in pure BMIPF6 at room temperature and 70 C.
Polarization was initiated 100 mV above the measured stable open
circuit potential (OCP, recorded as −0.5 V vs. Pt/Pt(II)) and was
progressed by shifting the potential cathodically at a scan rate of

6
044
O. Raz et al. / Electrochimica Acta 54 (2009) 6042–6045
Fig. 4. XPS spectrum of a Ru film electrodeposited for 1 h at −2 V vs. Pt/Pt(II). from
a solution of BMIPF6 containing 4 mM RuCl3. recorded on a film of 50 nm thickness
after argon sputtering.
sional plots [23,24], presented in Fig. 2b. Assuming 3D nucleation
followed by hemispherical diffusion, the following limiting cases
are of interest: instantaneous nucleation on a fixed number of active
sites (Eq. (1)) and progressive nucleation on an infinite number of
active sites (Eq. (2)). These are given by the following equations
[
23,24]:
ꢀ
ꢁ
2
ꢀ
ꢀ
ꢁꢀ
ꢂ
ꢀ
ꢀ
ꢁꢃꢁ
2
I
tm
t
t
=
=
1.9542
1.2254
1 − exp −1.2564
(1)
(2)
Im
tm
ꢄ
ꢅ
ꢆꢇ
ꢀ
ꢁ
2
ꢁ
ꢁ
2
2
I
tm
t
t
1 − exp −2.2367
Im
tm
Here Im is the maximum current and tm is the maximum current
time. A comparison of the time evolution of the experimental tran-
sients with these model plots is presented in Fig. 2b. It is observed
that the experimental data follow the model curve of instanta-
neous nucleation at short times (t < tm). At longer times there is no
agreement with either model curves which must be due to decom-
position currents of the ionic liquid, mentioned above.
The morphology of several Ru films on Si substrates was ana-
lyzed by scanning electron microscopy (SEM) subsequent to a
potentiostatic process in BMIPF /RuCl3 solution. Ru deposits onto
6
silicon substrates were rinsed and washed in acetonitrile subse-
quent to the deposition process. Surface morphologies of Ru films,
deposited at a potential of −2 V vs. Pt/Pt(II) for different time peri-
ods of 10 min, 1 and 2 h, are shown in Fig. 3. The surface obtained
within 10 min (Fig. 3a) exhibits spread grains morphology with a
grain diameter of ∼1 ␮m. After 1 h the obtained morphology turns
into a compact dense grainy film with similar grain sizes of 1 ␮m.
Within an extended time period (2 h), the structure becomes gran-
ulated and coarser (Fig. 3c).
In order to obtain an independent proof of Ru deposition and
to further analyze the composition of the deposited film, we per-
formed XPS measurements. A typical XPS of the film shown in
Fig. 3b spectrum is shown in Fig. 4.The thickness of this film was
estimated to be ∼100 nm using SIMS measurements. Before record-
ing the XPS spectra the thickness of the film was reduced to ∼50 nm
by ion sputtering inside the XPS vacuum chamber. The two main
peaks at a binding energy of 280 and 284 eV quantitatively agree
with the 3d5/2 and 3d3/2 excitations of pure Ru metal, respectively.
Fitting these peaks with single Gaussians having a slight asymme-
try towards higher binding energy becomes visible. This is most
probably due to core–hole coupling which yield “shake up” peaks
of Ru expected near 281 and 285 eV [25]. In addition, a contribution
Fig. 3. SEM micrographs of Ru films on Si (1 0 0) after electrochemical deposition
from a solution of BMIPF6 containing 4 mM RuCl3, at a potential of −2 V vs. Pt/Pt(II).
At different time periods: (a) 10 min, (b) 1 h and (c) 2 h.
the transients exhibit a clear maximum around 100 ms and than
decay towards longer times. They do not merge at extended times;
instead, they approach different constant currents which we assign
to a cathodic decomposition reaction of the RTIL, as indicated in the
◦
CV of the pure RTIL at 70 C (shown in Fig. 1). In order to obtain a first
insight into the nucleation and growth mechanism of Ru on Si (1 0 0)
we discuss the current transient in the approximation of nondimen-

O. Raz et al. / Electrochimica Acta 54 (2009) 6042–6045
6045
to the asymmetry of the 3d3/2 peak could be due to the presence of
a small amount of C impurities [22].
[4] W. Freyland, C.A. Zell, S. Zein El Abedin, F. Endres, Electrochim. Acta 48 (2003)
053.
3
[5] J. Dogel, W. Freyland, Phys. Chem. Chem. Phys. 5 (2003) 2484.
6] C.C. Tai, F.Y. Su, I.W. Sun, Electrochim. Acta 50 (2005) 5504.
[
4
. Conclusion
[7] I. Mukhopadhyay, W. Freyland, Chem. Phys. Lett. 377 (2003) 223.
[
[
8] F. Endres, D. McFarlene, A. Abbott (Eds.), Electrodeposition from Ionic Liquids,
Wiley–VCH, Weinheim, 2008.
9] M.L. Green, M.E. Gross, L.E. Papa, K.J. Schomes, D. Barsen, J. Electrochem. Soc.
182 (1985) 2677.
Ruthenium is a prime candidate to serve as a diffusion bar-
rier in various semiconductor applications which necessitates the
development of an inexpensive and reproducible method to deposit
ultrathin Ru films. For this aim we have studied the electrode-
position of Ru on n-Si wafers from an imidazolium based ionic
liquid, BMIPF . Electrodeposition of Ru confirmed by XPS study was
performed by directly depositing Ru from a dissolved RuCl3 salt
[
[
10] A. Crown, A. Wieckowski, Phys. Chem. Chem. Phys. 3 (2001) 3290.
11] T. Aatonen, A. Rahti, M. Ritala, M. Leskela, Electrochem. Solid State Lett. 6 (9)
2003) C130.
(
[12] T. Aatonen, P. Alen, M. Ritala, M. Leskela, Chem. Vap. Dep. 9 (1) (2003) 45.
[13] T. Aatonen, M. Ritala, K. Arstilsa, J. Keinonen, M. Leskela, Chem. Vap. Dep. 10 (4)
6
(
2004) 215.
[
14] T. Jones (Ed.), Electrodeposition & Electroless Deposition of the Precious Met-
als: Rhodium, Ruthenium, Iridium, Osmium, Rhenium, Finishing Publications
Limited, 2003.
15] T. Jones, Metal Finishing 99 (6) (2001) 121.
16] Y. Zhang, L. Huang, T.N. Arunagiri, O. Ojeda, S. Flores, O. Chyan, R.M. Wallace,
Electrochem. Solid-State Lett. 7 (9) (2004) C107.
[17] S.P. Murarka, in: D. Gupta (Ed.), Diffusion Processes in Advanced Technolog-
ical Materials, Springer-Verlag Berlin and Heidelberg GmbH & Co. K, 2005,
p. 239.
18] C.S. Hwang, Mater. Sci. Eng. B56 (1998) 178.
[19] J. Bondaru, T. Sands, L. Tsakalakos, J. Appl. Phys. 84 (2) (1998) 1121.
20] C.C. Hu, Y.H. Huang, J. Electrochem. Soc. 146 (7) (1999) 2465.
21] E. Molenda, M. Mielcarski, J. Radioanal. Nucl. Chem. 238 (1998) 159.
◦
in BMIPF6 at 70 C. CV studies indicate RedOx peaks correspond-
ing to Ru deposition and stripping at potentials of −2.1 and 0.2 V
vs. Pt/Pt(II), respectively. It should be stated that Ru deposition is
accompanied by RTIL reduction (from a potential of −1.4 to −2 V)
Nevertheless, Potentiostatic studies indicate a 3D instantaneous
nucleation and growth mechanism. Further characterizations of the
electrodeposited Ru films by SEM show the growth of homogenous
films within 2 h.
[
[
[
[
[
[
[
References
22] O. Mann, W. Freyland, O. Raz, Y. Ein-Eli, Chem. Phys. Lett. 460 (2008) 178.
23] O. Mann, W. Freyland, J. Phys. Chem. C 111 (2007) 9832.
[1] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley–VCH, 2003.
[2] H. Ohno, Electrochemical Aspects of Ionic Liquids, Wiley-Interscience, 2005.
[3] R. Hagiwara, Y. Ito, J. Fluorine Chem. 105 (2000) 221.
[24] G. Gunawardena, G. Hills, I. Montenegro, B. Scharifker, J. Electroanal. Chem. 138
(1982) 225.
[25] H.Y.H. Chan, C.G. Takoudis, M.J. Weaver, J. Catal. 172 (1997) 336.