Silicidation of Co/Si Core Shell Nanowires
Han-Bo-Ram Leea, Gil Ho Gub, Chan Gyung Parkb, and Hyungjun Kima*
a
School of Electrical and Electronic Engineering, Yonsei University, 262 Seongsanno,
Seodaemun-gu, Seoul, Korea (120-749)
b
Department of Materials Science and Engineering, POSTECH, San-31, Nam-gu, Pohang,
Korea (790-784)
ABSTRACT
The formation of CoSi2 nanowires (NWs) via the annealing of Si NWs conformally
coated with atomic layer deposited (ALD) Co layers was investigated. Co ALD was carried
out using Co(iPr-AMD)2 and NH3 as a precursor and a reactant, respectively. Rapid thermal
annealing (RTA) of Co/Si core-shell NWs produced Co oxide instead of CoSi2 due to oxygen
contamination during annealing. To prevent oxygen contamination, a highly conformal ALD
Ru layer was used as a capping layer. X-ray diffraction showed the formation of CoSi2 when
the Co/Si core-shell NWs were annealed at over 800 °C. To investigate the deposition and
silicidation process on a nanometer scale, high resolution Cs-corrected scanning transmission
electron microscopy was utilized with electron energy loss spectroscopy and energy
dispersion spectroscopy. In contrast to previous reports on silicidation of NWs, the dominant
diffusion species was found to be Si instead of Co based on a nucleation-controlled reaction.
Corresponding email: hyugnjun@yonsei.ac.kr
2
1. Introduction
Silicides are important materials for contacts in modern Si devices due to their low
contact resistance and good compatibility. Silicide is usually formed by metal deposition on a
Si layer followed by annealing at the appropriate temperature through a solid-state reaction.1
Recently, silicide nanowires (NWs) have attracted great attention since they have the high
conductivity and compatibility with metal required for contacts in nanowire devices. Silicide
NWs are usually synthesized by chemical vapor deposition (CVD). For instance, three kinds
of cobalt silicide NWs, Co2Si, Co3Si, and CoSi, were synthesized by the reaction of CoCl4
vapor and Si substrate.2 Also, SiH4 gas was reacted with Ni thin film to form NiSi NW.3, 4
Similarly, several silicide NWs have been reported, such as TiSi25, Ti5Si36, TaSi27, Fe5Si38,
FeSi9, and Ni2Si10. Another approach of synthesizing silicide NWs is via solid-state reaction
between metal and Si NWs. This method has a benefit of enabling contact formation for NW
based device fabrication. However, there have been few studies on the silicide NW synthesis
using solid-state reaction between Si NWs and metal films.
Recently, the formation of Co and Ni silicide NWs by in situ annealing of point
contact between Si NWs and metal in a UHV-TEM system was reported, focusing on the
silicide formation kinetics.11-13 The epitaxial layer-by-layer growth of CoSi2 along the Si NW
axis was observed, which was explained by the limited supply of Co atoms due to point
contact.12 In that report, it was found that having a lower interface energy between the native
oxide and Si than between native oxide and silicide prevented heterogeneous nucleation,
resulting in homogeneous nucleation.13 Meanwhile, silicidation of a single Si NW partially
covered with Pt deposited by evaporation was reported focusing on the effects of annealing
environment.14 However, due to the limited coverage of evaporation technique, uniform
silicide NWs could not be synthesized.
3
Atomic layer deposition (ALD) is a promising deposition technique for nanoscale
3D device fabrication due to its excellent conformality and thickness controllability.15
Recently, we reported high quality Co ALD using bis(N,N’-diisopropyl-acetamidinato)cobalt(II) [Co(iPr-AMD)2] and NH3 as a precursor and a reactant, respectively.16 In the
present study, we utilized Co ALD to conformally coat Si NWs and investigated the Si NWs
silicidation via annealing through solid-state reaction between Si NW and ALD Co layer. The
microstructure change was analyzed by X-ray diffraction (XRD) and scanning transmission
electron spectroscopy (STEM). To prevent oxygen contamination during annealing, we
employed an ALD Ru film as a capping layer.
2. Experimental Details
In this study, a commercial ALD chamber (Quros Plus 150TM) with a loadlock
chamber was used. Two metal-organic precursors for Co and Ru ALD processes were in
separate bubblers. For Co ALD, Co(iPr-AMD)2 and NH3 were used as the precursor and
reactant, respectively. One ALD cycle was composed of four steps consisting of precursor
exposure (ts), purging (tp), reactant exposure (tr), and purging, where the flow of Ar purging
gas was 50 sccm. The ts, tr, and tp were 3 s, 6 s, and 1 s, respectively, and typical deposition
temperature was 300 °C. The precursor for Ru ALD was DER (Ru(DMPD)(EtCp)) with O2
counter reactant. The DER precursor diluted with ethylcyclohexane was delivered into the
chamber with Ar carrier gas through liquid injection system and vaporizer. More information
and detailed experimental setup for Co and Ru ALD can be found in our previous reports.16, 17
The Si(001) substrates were routinely used for silicidation experiments on planar
substrate. The substrates were cleaned by dipping in buffered oxide etchant (6 : 1) for 10 s to
remove native oxide, and followed by deionized (DI) water rinsing and N2 blowing resulting
in H-terminated Si. After cleaning, Si(001) substrates were immediately loaded into the
4
chamber to prevent the formation of a native oxide layer. Si NWs were fabricated by CVD
with Au catalysis and SiH4 gas exposure. The Au catalyst was formed by agglomeration of
very thin Au film deposited on SiO2 substrate by thermal evaporation. When the Co film was
deposited on Si NWs, the same cleaning step was repeated to remove the native oxide formed
around Si NWs. For silicidation experiment, the Ru capping layer was deposited in situ after
Co ALD without breaking vacuum. The specimens were annealed by rapid thermal annealing
(RTA) at various annealing temperatures (Ta) from 400 °C to 900 °C for 30 s with N2
ambient. To remove residual O2 inside RTA chamber, N2 flushing was carried out for 5 min
after specimen loading. Most RTA experiments were performed at atmosphere but several
silicidation was carried out under vacuum to prevent O2 contamination.
Field emission scanning electron microscopy (FE-SEM) was used to analyze the
morphology. The resistivity of the Co films was measured by 4-point probe. The change of
microstructure to silicide phases was analyzed by XRD with a synchrotron source (Pohang
acceleration laboratory, 3C2 beam line) as well as Cu k1. For the nanoscale structural and
chemical composition analysis, high resolution Cs-corrected STEM (JEOL JEM-2100F) with
electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy (EDS) was
employed. The STEM was operated at 200 kV and its probe size was reduced to 0.1 nm for
precise EELS analysis.
3. Results and Discussions
Figures 1a and 1b show SEM images obtained before and after Co ALD on Si NWs,
respectively. The diameters of individual Si NWs are approximately 20 nm as shown in
Figure 1a before Co deposition. 833 growth cycles were carried out to deposit 20 nm-thick
Co film on the Si NWs, based on the growth rate of 0.24 Å/cycle. The Co ALD produced Si
NWs uniformly coated with a Co layer as shown in Figure 1b. The final diameter of Si NWs
5
was approximately 60 nm, indicating that Co layer with 20 nm of thickness was conformally
deposited all around Si NWs. Although the Si NWs are each over a few micrometers long
and entangled with one another, it was confirmed that the diameter of Co coated Si NW is
almost constant due to the excellent conformality of the current ALD Co process.16 In this
case, the conformality was defined by the ratio of Co/Si NW diameters at upper part and
lower part of NWs. Higher magnification SEM image can be found in supplementary
materials (Figure S1a).18 Because ALD Co has relatively rough surface morphology on Si
surface as shown in Figure S1b, the Si NWs covered with ALD Co look like rugged wire.18
The core-shell Co/Si NWs were annealed using RTA under N2 ambient. Typically, in
the annealing of Co films on Si substrates, phase transformation occurs by the following
sequence: metal-rich silicide (Co2Si), monosilicide (CoSi), and disilicide (CoSi2).1
Sometimes, the formation of Co2Si is not observed. The formation temperature of CoSi is
between Ta = 400 C and 600 C, and CoSi2 begins to form at Ta > 700 C. Also, in our
previous study on silicidation of Si substrates with PE-ALD Co, CoSi2 formation was
observed at over Ta = 700 C.19 Thus, the specimen was annealed at Ta = 900 C which is
known to be a sufficiently high temperature to form CoSi2. Figure 2 presents the XRD
spectrum of the annealed sample showing two XRD peaks at 31.2  and 36.8 , corresponding
to Co3O4(220) and Co3O4(331), respectively.
The Co/Si core-shell NWs annealed at Ta = 900 C were further analyzed by STEMEELS. The TEM image in Figure 3a shows that the inner core seems to remain intact with a
20 nm diameter. STEM-EELS was carried out along the black line perpendicular to the NW
shown in Figure 3a. The STEM-EELS shows that the Si signal was only observed at the
center of NW within the 20 nm core region, as seen in Figure 3b. Co was detected along the
whole scan line and the relative intensity was higher in the outer layer region than in the core
region. A similar trend was observed for O, indicating that Co and O existed in the outer
6
layer due to the circular shape of the sample cross section. Thus, we infer that Si and Co did
not react under this condition and Co reacted with O to form a Co oxide shell, which is
corroborated by the XRD result. We tried to remove the oxygen contamination by annealing
under vacuum. However, similar results were observed.
It is well known that the oxygen contamination disturbs the formation of Co silicides.
Thus, it is a common practice that a Ti capping layer, prepared by sputtering or evaporation,
is used for preventing O2 contamination and scavenging residual oxygen in the film. However,
conventional Ti deposition process cannot be used as a capping layer for annealing core-shell
NWs due to the limited conformality. It should be noted that Ti ALD processes have rarely
been reported. Thus, an alternate capping material, which can be deposited by ALD, should
be employed for the current process scheme. Ru is a stable material which can be easily
prepared by the ALD process.17 Although Ru has little tendency to scavenge oxygen like Ti,
Ru has high resistance to oxygen at high temperatures. Thus, ALD Ru was chosen as an
alternative capping layer to Ti. Separate experiments for the feasibility of Ru capping layer
were carried out for ALD Co film on Si substrate, confirming the silicide formation at Ta >
800 C, which will be published elsewhere.
Ru/Co/Si core-shell NWs were prepared by carrying out an additional 500 cycles of
ALD Ru following the Co ALD without vacuum break. Figure 4a shows the SEM image of
Si NWs sequentially coated with ALD Co and ALD Ru. The STEM bright field image in
Figure 4b shows a cross-section view of a core-shell NW composed of two polycrystalline
layers. The STEM-EDS mapping on this NW shown in Figure 4c clearly shows that the inner
layer is Co and outer layer is Ru. The thickness of each layer is 20 nm, and total diameter of
this NW including two layers is measured to be around 100 nm. The outer most Ru shell has
a thickness of 20 nm, which agrees with the expected value calculated from the growth rate
of Ru ALD, 0.4 Å/cycle.
7
Ru/Co/Si core-shell NWs were annealed using RTA at various temperatures from Ta
= 400 ºC to 900 ºC and synchrotron radiation XRD analysis was carried out as shown in
Figure 5. In contrast to annealed NWs without Ru capping, two strong diffraction peaks were
observed at 28.1 º and 35.2 º, corresponding to CoSi2(111) and (200), respectively, at Ta ≥
800 ºC.20, 21 The three additional peaks at 38.4 º, 42.2 º, and 44.1 º were indexed as Ru(100),
(002), and (101), respectively. Figure 6a shows SEM image of Ru/Co/Si core-shell NW after
RTA at Ta = 900 ºC. The surface morphology of the annealed NW is more textured and
rougher compared to the as-prepared NW in Figure 4. In Figure 5, Ru XRD peak becomes
stronger and narrower with increasing Ta. Thus, the change of morphology can be understood
by the recrystallization of Ru capping layer during annealing. The diameter change of
Ru/Co/Si NWs after RTA was plotted with annealing temperature in Figure 6b. The
diameters of core-shell NWs were measured from SEM images, and the average diameter and
deviation were calculated by measurement on different NWs in the same SEM image. The
average diameter and deviation of as-prepared Ru/Co/Si NWs was 102.4 ± 5.6 nm, and are
denoted by the dashed line and shading in Figure 6b. Diameters of core-shell NWs remained
almost same up to Ta = 700 ºC. At Ta = 800 ºC and 900 ºC, however, diameters increased
abruptly reaching 130 nm. Together with XRD data in Figure 5, it can be concluded that
phase transformation into CoSi2 leads to a volume expansion of Ru/Co/Si NWs along the
radial direction.
The Ru/Co/Si core-shell NWs annealed at Ta = 900 ºC were further analyzed by
STEM with EDS mapping. Figure 7a shows a cross sectional STEM bright field image of the
annealed core-shell NWs. The image shows that the cross section is composed of a dark ringshape region between bright core and shell regions. The diameter of NWs is larger than the
pre-annealed NWs, agreeing with the SEM results. Figure 7b shows a dark field STEM image
and Figure 7c displays the EDS mapping result. Two hollow regions are clearly observed in
8
the center of cross-section area in the DF image, which are denoted by the black arrows. In
addition, the other NW in the bottom of the image in Figure 7a shows a hollow region as
well. The EDS mapping indicates that the ring-like shape is composed of Ru and the core and
shell are composed of Co and Si. Figure 7d is an HR-TEM image of the region denoted by a
square in Figure 7b. Various lattice fringes from crystal orientations were observed,
indicating that the annealed NWs have a polycrystalline microstructure. The spacing of lattice
fringes in Figure 7d is 3.1 Å, corresponding to the d-spacing of CoSi2(111) plane, which is
consistent with the XRD results shown in Figure 5.
The presence of both a shell and a core region of CoSi2 indicates that there may be
interdiffusion between Ru coating and Co/Si core during silicidation. The detailed
mechanisms on the inward-diffusion of Ru and outward-diffusion of Co/Si are still not clear.
The formation enthalpy of CoSi2 (-98.7 kJ/mol) is larger than that of Ru silicides (RuSi (-58.1
kJ/mol), and Ru2Si3 (-60.7 kJ/mol)) in the reported thermodynamic data.22, 23 In addition, a
solid solution exists in the phase diagram of Ru and Co form.24 Thus, it is plausible that the
interdiffusion occurs during the annealing, which results in the formation of CoSi2 in core
and shell.
Kinetically, silicidation occurs through two different reaction modes: diffusioncontrolled and nucleation-controlled reactions. Laterally uniform growth is typically observed
in diffusion-controlled reactions in which reaction rate at the interface between the metal (or
silicon) and the silicide is much higher than the diffusion rate. For instance, epitaxial CoSi2
fabricated by utilizing a diffusion-controlled reaction shows atomically sharp interface.25, 26
On the contrary, non-uniform nucleation occurs in nucleation-controlled reactions. In the
previous reports on silicidation of NWs through point contact under UHV system, the
reaction occurs through a diffusion-controlled reaction due to the limited contact area
between Si and metal, resulting in the formation of epitaxial CoSi2 with atomically sharp
9
edge.13, 27 Since the current core-shell NW system has large contact area between Si NW and
Co layer, however, there is little barrier for diffusion. So, the silicidation is thought to be
governed by a nucleation-controlled reaction resulting in polycrystalline CoSi2 formation.
Kirkendall voids are formed when there is large difference in diffusion flux.28 Thus,
when the outward-diffusion from the core part of core-shell NW is faster than the inwarddiffusion from the outer layer, voids can be formed inside of the inner shell. The formation of
nanocavities when annealing oxide coreshell NWs has previously been reported.29, 30 Also, in
the previous reports on silicidation, voids were observed in nanoparticles as well as planar
system.28, 31, 32 Thus, the formation of voids in the coreshell can be explained by the faster Si
outward-diffusion than Co inward-diffusion, resulting in the Kirkendall void formation. If Co
atoms diffuse out to Ru, voids can be generated at the interface between Co and Ru.
However, since the voids were mainly observed in the CoSi2 region, it is likely that the voids
are formed by Si outward-diffusion. Until now, researchers have intensively investigated
what material is the dominant diffusion species (DDS) during the formation of silicides in a
thin film system. Usually, marker experiments have been employed to determine the DDS.3335
While it is generally accepted that the DDS for CoSi is Si, there are contradicted reports on
DDS for CoSi2 and both Co36,
37
or Si38-40 have been reported. DDS can be changed
depending on various factors, such as diffusion-controlled or nucleation-controlled reaction
mechanisms and strain caused by volume mismatch.41 In this system, it appears that Co and
Si interdiffuse into each sides and that Si is DDS in our system.
Also, we need to consider the volume change during silicidation. Because lattice
constant of Si (5.43 Å) is almost same as that of CoSi2 (5.36 Å), the volume change in the Si
core caused by Co atoms is not significant. However, the volume change during the
transformation from Co (3.51 Å) to CoSi2 in Co layer by Si diffusion is significant. If Si
atoms in a NW diffuse into Co shell side and the space occupied by Si NW is preserved in
10
core region despite of Si outward-diffusion, it can be calculated with assuming single
crystalline microstructure that a 1 nm-thick Co shell deposited around a 20 nm diameter Si
core is changed to 3.2 nm-thick CoSi2. This volume change is more significant in
polycrystalline microstructure system. This volume expansion at the Co outer layer may
induce tensile stress to the outermost Ru layer, resulting in breaking of Ru layer as shown in
Figure 7. In addition, a recrystallization and grain growth of Ru during annealing can
contribute the total volume change. Because our system is 1-D NW system, the growth of
grains to the tangential direction is limited leading to volume expansion to the radial direction,
resulting in large volume change.
Therefore, the CoSi2 NWs were formed by dominant diffusion of Si atom into Co
shell side, resulting in the formation of void in core region. In addition, the diameter of coreshell NW is changed after annealing because the silicidation mainly takes place in the Co
shell region by diffusing Si atoms.
4. Summary
The formation of CoSi2 NWs was investigated by direct silicidation of Ru/Co/Si core
shell NW prepared by ALD followed by RTA. The ALD Co film was deposited from Co(iPrAMD)2 and NH3 as a precursors and reactant, respectively. After RTA of Co/Si core-shell
NW, only Co oxide was observed instead of CoSi2 due to oxygen contamination. To prevent
oxygen contamination, a highly conformal ALD Ru film was used for capping layer. CoSi2
diffraction peaks were observed in XRD results, and confirmed from STEM images. In the
contrast to previous NW silicidation results, the dominant diffusion species is Si instead of
Co atom.
5. ACKNOWLEDGEMENTS
11
This work was supported by the Technology Innovation Program Industrial Strategic
Technology Development Program, 10035430, Development of reliable fine-pitch
metallization technologies funded by the Ministry of Knowledge Economy MKE, Korea.
12
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15
FIGURE LEGENDS
FIG. 1 FE-SEM images of the Si NWs (a) before and (b) after Co ALD.
FIG. 2 XRD result of Co/Si core-shell NWs after RTA at Ta = 900 °C
FIG. 3 (a) STEM image and (b) EELS line profile of annealed Co/Si core-shell NW used in
Figure 2. EELS line profile was carried out along the black line in Figure 3a.
FIG. 4 (a) FE-SEM, (b) cross-sectional HR-TEM image, and (c) EDS mapping image of
Ru/Co/Si core-shell NW. In Figure 4c, green, blue, and red dots correspond to Ru, Co, and Si
signals, respectively.
FIG. 5 XRD results of ALD Ru/ALD Co/Si NWs core-shell structure after RTA at from Ta =
400 ºC to 900 ºC. The Ru (100), (002), and (101) diffraction peaks are denoted by the closed
circle (●), closed square (■), and closed triangle (▲), respectively.
FIG. 6 (a) FE-SEM image of the Ru/Co/Si core-shell NW annealed at Ta = 900 ºC. (b) The
change of Ru/Co/Si core-shell NW diameter with increasing annealing temperature.
FIG. 7 (a) Cross-sectional STEM bright field image, (b) STEM dark field image, (c) EDS
mapping image, and (d) HR-TEM image of Ru/Co/Si core-shell NW annealed at Ta = 900 ºC.
Figure 7d is magnified image from the square of Figure 7b. The arrows in Figure 7a and 7b,
and guidelines in Figure 7d indicate hollows and d-spacing, respectively. In Figure 7c, green,
blue, and red dots correspond to Ru, Co, and Si signals, respectively.