Electrochemical deposition of copper and ruthenium on titanium

Article abstract of DOI:10.1016/j.electacta.2006.02.016

Copper electrochemical deposition on titanium with a ruthenium seed layer was investigated. The chemicals for the acid-bath ruthenium electrochemical deposition were ruthenium(III) chloride hydrate (RuCl₃·3H₂O), hydrochloric acid (HCl), sulfamic acid (NH₂SO₃H), and polyethylene glycol. The chemicals for the acid-bath copper electrochemical depositions were copper(II) sulfate hydrate (CuSO₄·5H₂O), sulfuric acid (H₂SO₄), and polyethylene glycol. Results were analyzed by field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS). Ruthenium thin film of ～30 nm thickness, with equiaxial grains <10 nm, was deposited, on a blanket Ti with a root mean square roughness of 8.3 nm, at 2 V for 90 s. XPS and RBS analyses showed the presence of metallic Ru. The Ti substrate was found stable with respect to ECD of Ru but the Ru/Ti bilayer was not found stable in the Cu acid bath, resulting in the diffusion of Ti into Ru film. The depth profiling studies indicates that Ru film thickness ca. 1.4 nm and deposition time of 10 s can act as a good seed layer.

Full text of DOI:10.1016/j.electacta.2006.02.016

Electrochimica Acta 51 (2006) 5445–5451
Electrochemical deposition of copper and ruthenium on titanium
a
a
a
a
a
Young-Soon Kim , Hyung-Il Kim , Joong-Hee Cho , Hyung-Kee Seo , Gil-Sung Kim ,
a
b
c
a,∗
S.G. Ansari , Gilson Khang , Jay J. Senkevich , Hyung-Shik Shin
Thin Film Technology Laboratory, School of Chemical Engineering, Chonbuk National University, Jeonju 561756, Republic of Korea
a
b
Department of Polymer/Nano Science and Technology, Chonbuk National University, Jeonju 561756, Republic of Korea
c
Brewer Science Inc., Rolla, MO 65401, USA
Received 26 October 2005; received in revised form 3 February 2006; accepted 15 February 2006
Available online 5 April 2006
Abstract
Copper electrochemical deposition on titanium with a ruthenium seed layer was investigated. The chemicals for the acid-bath ruthenium elec-
trochemical deposition were ruthenium(III) chloride hydrate (RuCl ·3H O), hydrochloric acid (HCl), sulfamic acid (NH SO H), and polyethylene
3
2
2
3
glycol. The chemicals for the acid-bath copper electrochemical depositions were copper(II) sulfate hydrate (CuSO O), sulfuric acid (H SO ),
4
·5H
2
2
4
and polyethylene glycol. Results were analyzed by field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray
diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS). Ruthenium thin film of ∼30 nm
thickness, with equiaxial grains <10 nm, was deposited, on a blanket Ti with a root mean square roughness of 8.3 nm, at 2 V for 90 s. XPS and RBS
analyses showed the presence of metallic Ru. The Ti substrate was found stable with respect to ECD of Ru but the Ru/Ti bilayer was not found
stable in the Cu acid bath, resulting in the diffusion of Ti into Ru film. The depth profiling studies indicates that Ru film thickness ca. 1.4 nm and
deposition time of 10 s can act as a good seed layer.
©
2006 Elsevier Ltd. All rights reserved.
Keywords: Ruthenium seed layer; Copper; Electrochemical deposition; Rutherford backscattering spectrometry
1
. Introduction
can expect ECD Cu a suitable candidate for the interconnect
metallization in future technology nodes. ECD Cu requires a
seed layer as a conducting path for the electrons that are needed
for the redox surface reactions [3]. Josell et al. and Chyan et al.
have reported the application of ruthenium for Cu interconnects
as well as a good candidate for the seed layer [4,5]. In addition,
ruthenium could potentially replace the Cu/Ta/TaNX stack; how-
ever, Ru diffuses into Cu during subsequent annealing, which
increases the Cu line resistivity [6,8]. Instead, a thin layer of Ru
can act as a seed layer to minimally impact Cu line resistivity.
Electrochemical deposition of ruthenium is an attractive tech-
nique to deposit a conformal thin seed layer because of its good
step coverage, low tool cost of ownership, and its amenability to
ECD Cu. In our previous studies, we reported growth study of
conformal seed layer using ECD Ru on patterned TiN 130 nm
trenches [9]. This technology would have to address the Cu con-
formal deposition on high aspect ratio (A/R) features with a size
of 22 nm and 33 nm nodes, and three-dimensional devices.
Reid and Blake undertook ECD Ru with a number of aqueous
electrolytes based on simple ruthenium salts and nitrosyl deriva-
tives [10]. Nitrosyl sulfamate emerged as the acid-bath reagent.
In the semiconductor industry, aluminum has been the con-
ductor of choice, even though superseded by copper due to its
lower resistivity and better resistance to electromigration in case
of ultra-large scale integrated (ULSI) devices. In case of Cu met-
allization, the Cu diffuses rapidly into the interlevel dielectrics
(
ILDs). Therefore, refractory metal or metal nitrides such as Ti,
Ta, TiN, and TaN are used as a diffusion barrier. Sputtered cop-
per has limitations for step coverage and cost of ownership for
depositing a seed on the barrier stacks.
On the other hand, electrochemical deposition (ECD) pro-
vides a continuous buildup of metal coating on a substrate in a
suitable aqueous solution. Electrochemically deposited copper
(
ECD Cu) has emerged as the most efficient way to fill nano
features that are based on the dual-damascene technology [1,2].
Because of its high filling capability and low process cost we
∗
Corresponding author. Tel.: +82 63 270 2438; fax: +82 63 270 2306.
E-mail address: hsshin@chonbuk.ac.kr (H.-S. Shin).
0
013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2006.02.016

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Y.-S. Kim et al. / Electrochimica Acta 51 (2006) 5445–5451
Table 1
X-ray photoelectron spectroscopy (XPS, Model—Kratos Ami-
ECD Ru Bath composition and parameters
cus). The surface topology of the film was examined by tapping
mode atomic force microscope (AFM, Multimode + Bioscope,
Digital Instruments/Veeco). A standard X-ray diffractometer
was used with Cu K␣ radiation at 30 kV/20 mA. The thicknesses
of Ti, Ru, and Cu were estimated from Rutherford backscatter-
ing spectrometry (RBS) on the 4.0 MeV Dynamitron accelerator
◦
Temperature
Constant voltage
Polyethylene glycol
NH2SO3H
HC1
RuCl3·3H2O
60 C
1.5–5.0 V
5 ␮l/l
3.84 g/1
1.75 l/l
5.18 g/1
(
Nisshin high voltage corporation, NT 1700 HS accelerator,
4
◦
using 2.0 MeV He ions and a scattering angle of 165 ). Mea-
surements were made with 3.0 MeV alpha particles. The RBS
data was simulated using the ERNIE software. The estimated
areal density from RBS measurement was converted into an
equivalent thickness by dividing with the bulk atomic density
However, Reddy and Taimsalu proposed an electrolyte based
on the N-bridged complex of ruthenium(IV) nitrosyl-chloride
(
RuNC, (NH4)3(RuCl4)N(RuCl4)) for Ru ECD [11]. The reac-
tion for the RuNC is:
22
3
of titanium, ruthenium, and copper viz. 5.71 × 10 atom/cm ,
2
RuCl3·3H2O + 2HCl + 4NH2SO3H
22
3
22
3
7
.27 × 10 atom/cm , and 8.45 × 10 atom/cm , respectively.
2
−
→
(NH4)3{(RuCl4)N(RuCl4)}(H2O)2 + 4SO4
8H + H2↑
+
3. Results
+
(1)
The investigation of the basic growth conditions and bath
The FE-SEM images of the Ti substrate and the electrochem-
composition of Ru electrodeposition in addition to the Cu elec-
trodeposition on the Ti barrier layer is of primary importance.
In this paper, ruthenium was electrochemically deposited onto
blanket Ti substrates followed by electrochemical deposition of
Cu for metallization. The structural and morphological analysis
was carried out using XPS, XRD, AFM, and SEM in addition
to the depth profiling of Ru and ECD Cu on the Ti layer.
ically deposited Ru films as a function of applied voltage are
showninFig. 1. ItcanbeseenthattheTisubstratesurfaceisgran-
ular with grain diameter ranging from 10 nm to 50 nm (Fig. 1(a)).
Fig. 1(b–d) shows the changes in the Ru film surface morphol-
ogy grown on the Ti substrate when deposited at three different
voltages viz. 1.5 V, 2.0 V, and 5.0 V, for 90 s. At 2.0 V, a uni-
form surface coverage of the Ru films was obtained (Fig. 1(c)).
Cleavage marks and agglomerated grain can be seen at lower and
higher applied voltages (Fig. 1(b and d)). The size of particles
in Fig. 1(d) is about 200 nm whereas the surface is inhomoge-
neous, at the applied voltage of 5.0 V, which clearly indicates
that the deposited Ru film is non-continuous and cluster-type
film. Hence it can be clearly seen from these images that the
best morphology results when Ru was deposited at an applied
voltage of 2.0 V. Therefore, 2.0 V was chosen as the best condi-
tion for ECD Ru to compensate for the Ohmic drop under the
experimental conditions employed here [12]. The results agree
with our previous report where we found that the grain growth
occurred when the deposition time was increased from 10 s to
60 s, withanappliedvoltageof2.0 V [9]. Ourresultsalsosupport
the conclusion of Popov et al. who reported that the electrochem-
ical overpotentials and current densities must increase in order
to compensate for the Ohmic drop between two symmetrical
points in a homogeneous field on the anode and cathode.
The X-ray diffraction spectrum of Ru thin film deposited on
Ti for 90 s and 2.0 V is presented in Fig. 2(a). The spectrum
shows only two diffraction peaks corresponding to Ru (1 0 1)
and Ti (2 0 0) (JCPDS card No: 6-663). The Ru (1 0 1) peak
is broad and asymmetrical indicating smaller grains stressed
asymmetrically.
2
. Experimental
An e-beam deposited Ti film of 85 nm was used as sub-
strate. The chemicals for the ruthenium electrochemical bath
were RuCl3·3H2O (ruthenium(III) chloride hydrate, 99% Pure,
Sigma–Aldrich), HCl (35.0–37.0% hydrochloric acid, Samchun
Pure Chemical Co. Ltd., Korea), NH2SO3H (sulfamic acid, 99%
pure, Sigma–Aldrich) and 5 ␮l/l polyethylene glycol was used
as a surfactant. The complex salt was prepared with ruthenium
chloride, which was ultrasonically activated in hydrochloric acid
reacted with sulfamic acid for long periods at reflux. The detailed
experimental information and resonance structure of the RuNC
salt using ECD Ru can be obtained from our previous work
reported elsewhere [9]. The bath solution was maintained at
◦
6
0 C with electrolyte pH value of 1.1 for ECD Ru. The constant
DC voltages from 1.5 V to 5 V for ECD Ru were applied. The
Ru bath composition is detailed in Table 1. Prior to electrode-
position of Ru on Ti, first the substrate was cleaned in acetone
(
99% pure), at room temperature for 10 min, to remove oxide
layer. The substrate was immersed then into the HCl solution
for 5 min to remove possible contaminants, followed by rinsing
with deionized (DI) water and drying with dry nitrogen.
Chemicals for the copper electrochemical bath were 0.08 M
CuSO4·5H2O (copper(II) sulfate hydrate), 0.44 M H2SO4 (sul-
furic acid), and 5 ␮l/l polyethylene glycol. A constant DC volt-
age of 1.5 V for ECD Cu was applied by maintaining the solution
at room temperature.
The 3-D AFM image of the Ru film deposited at 2.0 V for
90 s is shown in Fig. 2(b). Root mean square (RMS) surface
roughness of the film is 8.3 nm, estimated from the AFM image.
In our earlier investigation, we found that increase in the film
roughness with increasing deposition time (10–60 s) was a little
i.e. 8.7–9.5 nm [9]. The difference in the surface roughness value
in the two cases can be correlated to the particle size of the
deposited film [9].
The microstructures of samples were examined by a field
emission scanning electron microscope (FE-SEM, JEOL JSM-
6
330F). The surface chemical bonding was analyzed by using

Y.-S. Kim et al. / Electrochimica Acta 51 (2006) 5445–5451
5447
Fig. 1. FESEM images of (a) the Ti substrate and the electrochemically deposited Ru films for 90 s at (b) 1.5 V, (c) 2.0 V, and (d) 5.0 V.
Fig. 3 shows the XPS spectra of the Ru films deposited at
.0 V for 10 s, 30 s, and 60 s by electrochemical deposition on
2
Ti. The wide scan spectra (Fig. 3(a)) show all the standard photo-
electron lines of Ru (viz. Ru3s, Ru3p, Ru3d, Ru4s), O1s and C1s
[13]. The peak at 531.2 eV, corresponding to O1s can be due to
the water and wet air on the surface while electrochemical depo-
sition of Ru. The photoelectron peaks at 462 eV and 484 eV are
referenced to Ru3p3/2 and Ru3p1/2, respectively [13].
Fig. 3(b) shows the high-resolution spectra of C1s and Ru3d
peaks along with the peak fitting using Shirley-type back-
ground and Lorentzian and Doniac-Sunsic curves convoluted
with Gaussian. The peaks are relatively symmetrical and are
typically the narrowest lines observed in the spectra. Photo-
electron lines of pure metal can, however, exhibit considerable
asymmetry due to core–hole coupling with the conduction elec-
trons [13,14]. Therefore, as can be seen in Fig. 3(b), the XPS
spectra of C1s and Ru3d were further fitted with Doniac-Sunsic
(exponential tail) curve to account for the peaks asymmetry
arising from core–hole coupling. To remove the surface car-
bonaceous species, Chan et al. reported that all samples were
◦
initially heated in flowing O2 at 400 C for 30 min, followed by
◦
reduction in H2 at 300 C to obtain a metallic Ru surface [15].
The best fitting of the C1s (because the samples were not treated
for removal of surface carbonaceous species) and Ru3d spec-
trum of the Ru film from 290 eV to 278 eV was achieved with
five peaks at binding energies of: 285.9 eV, 285.0 eV, 284.3 eV,
281.1 eV, and 280.1 eV. The C1s peak is assigned to a binding
energy of 285.0 eV and which is most likely due to the sur-
face contamination. Since adventitious C1s (285.0 eV) overlaps
with Ru3d3/2 (284.3 eV) therefore the reported literature mainly
relies on Ru3d5/2 (280.1 eV) to differentiate the electronic states
of Ru [16–20]. These two peaks at 284.3 eV and 280.1 eV, sepa-
rated by 4.2 eV corresponds to pure Ru metal. Additionally, the
Fig. 2. (a) XRD pattern and (b) AFM image of the electrochemically deposited
Ru film for 90 s at 2.0 V.

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Y.-S. Kim et al. / Electrochimica Acta 51 (2006) 5445–5451
Fig. 4. (a) RBS spectra of the Ti substrate and the electrochemically deposited
Ru film for 10 s at 2.0 V and (b) simulating the spectrum of the electrochemical
deposited Ru film for 10 s at 2.0 V in (a).
Fig. 3. (a) Wide scan XPS (b) high-resolution narrow scan C1s and Ru3d spectra
of the electrochemically deposited Ru films at 2.0 V for 10 s, 30 s, and 60 s.
peaks at 285.9 eV and 281.1 eV can be due to core–hole coupling
which would yield as asymmetrical broadening of the XPS peak
towards higher binding energy, yielding so-called “shake-up”
peaks for photoelectron lines of pure Ru metal [15].
The RBS spectra of the Ti substrate and Ru film (Ru(10)/Ti)
deposited for 10 s by electrochemical plating on the Ti blanket
film is shown in Fig. 4(a). The Ti peak (1.436 MeV) at the Ru thin
film (Ru(10)/Ti) shifts towards lower energy due to the presence
of the Ru film on the top of it. The 1.710 MeV peak is due
to the Ru layer on the top of Ti. On simulating the spectrum
of the Ti substrate (Fig. 4(a)), the thickness of the Ti substrate
is found to be 86 nm. Fig. 4(b) shows the simulated spectrum
of the Ru(10)/Ti film (Fig. 4(a)), and the thickness of Ti and
Ru films after simulation was found to be 85.8 nm and 1.4 nm,
respectively. This Ti layer is stable with respect to the acid-
bath ruthenium at open and applied potentials. Also, there is no
oxygen layer on the top, which is in consistent with XPS results.
Fig. 5(a) shows RBS spectra of the Ru film deposited for 30 s
by electrochemical plating on the Ti blanket film. Simulating the
spectrum of the Ru film (Fig. 5(b)), shows that the thickness of
Ti and Ru films are 85.8 nm and 12.4 nm, respectively.
Fig. 5. (a) RBS spectrum and (b) simulating the spectrum of the electrochemi-
cally deposited Ru film for 30 s at 2.0 V.

Y.-S. Kim et al. / Electrochimica Acta 51 (2006) 5445–5451
5449
Fig. 7. (a) RBS spectrum and (b) simulating the spectrum of the electrochemi-
cally deposited Cu film for 60 s at 1.5 V on the electrochemically deposited Ru
seed layer for Ru film for 10 s at 2.0 V.
Table 2
Depth profiling of the electrochemically deposited Cu film for 60 s at 1.5 V on
the electrochemically deposited Ru seed layer for Ru film for 10 s at 2.0 V
Sublayers
Thickness (nm)
Composition
1
2
3
4
5
3
88
8
Cu:O = 1:2 (surface oxide)
Cu
Cu:Ru:Ti = 2:1:1
Ti
Si
80
10000
Fig. 6. (a) RBS spectrum, (b) simulating the spectrum, and (c) depth profiling
of the electrochemically deposited Ru film for 60 s at 2.0 V.
electroless depositing solution and do not diffuse into the cop-
◦
per trench at elevated temperature annealing e.g. >400 C [6–8].
Fig. 6(a) shows RBS spectra of the Ru film deposited for 60 s
by electrochemical plating on the Ti blanket film. Simulating the
spectrum of the Ru film (Fig. 6(b)), gives the thickness of Ti and
Ru films as 73.6 nm and 26.1 nm, respectively. The reduction of
Platinum group metals, viz. Ru [4,5,7], Rh [6], Pd [22], Ag, Os,
Ir [23,24], Pt, Au as barrier layers may not diffuse during both Cu
1
2.4 nmfromca. 86 nmto73.6 nmoftheTithicknessrevealsthat
Ti was etched during the deposition for 60 s. The depth profiling
in Fig. 6(c) indicates that the Ru seed layer intermixed with the
Ti layer during Ru electrodeposition for 60 s. This indicates that
Ru film thickness ca. 1.4 nm with deposition time of 10 s can act
as good seed layer. Etching of a barrier layer can be a problem for
electrodepositing solution. Previously, we reported that Group
VIII metals such as Pt, Pd, etc., were stable in the electroless
bath for the deposition of Cu [21,22].
For metallization, copper was electrodeposited for 60 s at
1
.5 V, on the electrochemically deposited Ru film (10 s depo-
sition time). Fig. 7(a) shows RBS spectrum of the Cu film on
Ru(10 s)/Ti. The peak at 1.559 MeV is due to the Cu layer having
the thickness of 94 nm. It is also evident from the simulated data,
shown in Fig. 7(b) as well as the depth profiling in Table 2, that Ti
diffuses into the Ru and Cu film during Cu ECD. Ultrathin noble
metal layers that can activate barrier layers are attractive to elec-
troless deposition processes but only if they are not etched in the
Fig. 8. (a) RBS spectrum and (b) simulating the spectrum of the electrochemi-
cally deposited Cu film for 100 s at 1.5 V on the electrochemically deposited Ru
seed layer for Ru film for 60 s at 2.0 V.

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Y.-S. Kim et al. / Electrochimica Acta 51 (2006) 5445–5451
Table 3
in increase of Cu layer thickness, where as the diffusion profile
shows similar trend.
Depth profiling of the electrochemically deposited Cu film for 100 s at 1.5 V on
the electrochemically deposited Ru seed layer for Ru film for 60 s at 2.0 V
Fig. 9(a) shows the FE-SEM image of the Cu film deposited
for 90 s on the Ru(60 s)/Ti substrate. The Cu film is seen rea-
sonably uniform but exhibits a rough morphology. The struc-
tural analysis of the Cu thin film deposited on Ti for 100 s
using X-ray diffraction is shown in Fig. 9(b). The diffrac-
Sublayers
Thickness (nm)
Composition
1
2
3
4
5
6
7
3
237
25
9
6
Cu:O = 1:2 (surface oxide)
Cu
Cu:Ru = l:4
Cu:Ru:Ti = 1:1:2
Ru:Ti = 1:3
Ti
Si
◦
◦
tion peaks at 2θ = 43.3 , 50.4 corresponds to ꢀ1 1 1ꢁ and
ꢀ
2 0 0ꢁ Cu-planes (JCPDS card No. 4-836). The peak intensity
61
10000
ratio [I(1 1 1)/I(2 0 0)] was found to be 2.17 whereas the full
width at half maximum (FWHM) of peak corresponding to Cu
◦
(
[
1 1 1) peak was 0.25 . Similar results for the intensity ratios
I(1 1 1)/I(2 0 0)] of 2.6 and 2.09 were reported in our earlier
ECD and annealing for ultra-large scale integration. The RBS
depth profiling study indicates that the diffusion of the barrier
layer can be a problem for Cu electrodepositing solution.
Further, copper was electrodeposited for 100 s at 1.5 V, on
the electrochemically deposited Ru film (60 s deposition time).
Fig. 8(a) shows the RBS spectrum of the Cu film on Ru(60 s)/Ti.
The peak at 1.559 MeV is due to the Cu layer having the thick-
ness of 247 nm. It is also evident from the simulated data, shown
in Fig. 7(b) as well as the depth profiling in Table 3, that Ti has
diffused into the Ru and Cu film during Cu ECD. On comparing
Tables 2 and 3, it is clear that increase in deposition time results
reports [21,22].
4. Conclusions
In conclusion it can be said that electrochemical deposi-
tion of ruthenium is an attractive technique and a suitable seed
layer candidate material for electroplating of Cu. Ruthenium
was electrochemically deposited onto Ti followed by the elec-
◦
trochemical deposition of copper, both at 60 C with acid-based
bath chemistries. Samples were characterized by FE-SEM, XPS,
AFM, XRD, and RBS. With increasing applied voltage while
deposition, FE-SEM results indicated an irregular distribution
and inhomogeneous surface consisting of Ru islands on the Ti
film. At an optimal applied voltage of 2.0 V the sample was
homogeneous and smooth with a RMS surface roughness of
8
.3 nm. X-ray diffraction showed preferential ꢀ1 0 1ꢁ texture
◦
but a broad asymmetrical Ru peak at 2θ = 44.0 . XPS analysis
showed the presence of metallic Ru with a thin RuOX layer. The
Ti substrate was found stable with respect to ECD of Ru but the
Ru/Ti bilayer was not found stable in the Cu acid bath, resulting
in the diffusion of Ti into Ru film. The depth profiling studies
indicates that Ru film thickness ca. 1.4 nm with deposition time
of 10 s can act as a good seed layer.
Acknowledgments
We are grateful to Nobuaki Ito for assistance in RBS mea-
surements. This work was supported by the Korea Science
and Engineering Foundation (research grant No. R01-2004-
000-10792-0) and Korea Ministry of Science and Technology
(research grant No. 2004-01352). S.G. Ansari would like to
acknowledge the financial support from Korean Research Foun-
dation and The Korean Federation of Science and Technology
Societies Grant (Brain pool program).
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