Improvements in growth behavior of CVD Ru films on film substrates for memory capacitor integration

Article abstract of DOI:10.1149/1.1827595

Ru thin films were grown by metallorganic chemical vapor deposition using a cyclopentadienylpropylcyclopentadienlylruthenium(II) on the Ta ₂O₅, TiN, Si₃N₄, SiO₂, TiO₂ thin-film substrates and their nucleation and growth behaviors were investigated. It was observed that the bonding type between atoms of the substrate thin films has a profound effect on the nucleation behaviors. The more ionic the bonding, the smaller the nucleation barrier and smoother Ru films were obtained. The poor nucleation property of Ru films on TiN, which has a covalent bonding nature, was successfully improved by the Ar-plasma treatment on TiN substrate prior to the deposition of Ru film. It was found that the Ar plasma treatment selectively removes N ions from the surface and made the TiN surface more metallic or ionic (due to the residual Ti-O bonding) and reduced the nucleation barrier. Furthermore, oxidation resistance of Ru/TiN layers was improved by H₂ annealing due to the densification of the Ru films. 2004 The Electrochemical Society. All rights reserved.

Full text of DOI:10.1149/1.1827595

Journal of The Electrochemical Society, 152 ͑1͒ C15-C19 ͑2005͒
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0013-4651/2004/152͑1͒/C15/5/$7.00 © The Electrochemical Society, Inc.
Improvements in Growth Behavior of CVD Ru Films on Film
Substrates for Memory Capacitor Integration
,z
*
*
Sang Yeol Kang, Cheol Seong Hwang, and Hyeong Joon Kim
School of Materials Science and Engineering and Interuniversity Semiconductor Research Center,
Seoul National University, Kwanak-ku, Seoul 151-742, Korea
Ru thin films were grown by metallorganic chemical vapor deposition using
a
cyclopentadienylpropylcyclo-
pentadienlylruthenium͑II͒ on the Ta2O5 , TiN, Si3N4 , SiO2 , TiO2 thin-film substrates and their nucleation and growth behaviors
were investigated. It was observed that the bonding type between atoms of the substrate thin films has a profound effect on the
nucleation behaviors. The more ionic the bonding, the smaller the nucleation barrier and smoother Ru films were obtained. The
poor nucleation property of Ru films on TiN, which has a covalent bonding nature, was successfully improved by the Ar-plasma
treatment on TiN substrate prior to the deposition of Ru film. It was found that the Ar plasma treatment selectively removes N ions
from the surface and made the TiN surface more metallic or ionic ͑due to the residual Ti-O bonding͒ and reduced the nucleation
barrier. Furthermore, oxidation resistance of Ru/TiN layers was improved by H2 annealing due to the densification of the Ru films.
© 2004 The Electrochemical Society. ͓DOI: 10.1149/1.1827595͔ All rights reserved.
Manuscript submitted March 25, 2004; revised manuscript received June 15, 2004. Available electronically November 22, 2004.
Ruthenium ͑Ru͒ thin films have been extensively studied as the
electrodes for dynamic random access memory ͑DRAM͒ capacitors
having a design rule Ͻ90 nm when high dielectric films, such as
Ta2O5 or (Ba, Sr͒TiO3 ͑BST͒, are adopted as the capacitor dielectric
due to its low resistivity and good etching property.1-3 In addition,
the metallorganic chemical vapor deposition ͑MOCVD͒ process for
the Ru thin growth has been greatly improved in the semiconductor
industry during the past few years. The conformal deposition prop-
erty of low-temperature ͑Ͻ350°C͒ Ru-MOCVD is one of the key
parameters that the capacitor electrode films should have for gigabit-
scale DRAM applications.4 However, there are several requirements
for successful integration of Ru electrodes into DRAM capacitors in
addition to the conformal deposition property of the electrodes. Fig-
ure 1 shows the schematic diagram of the Ru/insulator/Ru ͑RIR͒
concave capacitor structure of the gigabit-scale DRAMs,5 where the
Ru bottom electrodes should be deposited not only on Ta2O5 adhe-
sion layers but also on TiN layers that work as the diffusion and
reaction barrier between the bottom electrode material and W or
poly-Si plug. The plug electrically connects the capacitor to the
source region of the select transistor. The Ta2O5 adhesion layer was
adopted in order to improve the adhesion between the SiO2 layer
and Ru electrode and to make the film smooth by the enhanced
nucleation of Ru on Ta2O5 .
poor nucleation behavior of the Ru film on the TiN surface was
improved by an Ar plasma treatment of the TiN film surface, and the
reason for the improvement was investigated. Oxidation resistance
of the Ru/TiN layers was also studied.
Experimental
The MOCVD apparatus consists of a vertical warm wall reactor,
a resistive substrate heater which can handle wafers with 6 in. diam,
and the bubbler source supply system. Details of the MOCVD sys-
tem have been reported earlier.6,7 Experimental conditions for the
Ru film deposition are summarized in Table I. Ar gas was used as a
carrier gas as well as a diluent gas, and O2 gas was introduced into
the reactor in order to eliminate carbon incorporation into the film
and also to enhance the decomposition of the metallorganic precur-
sors. Ru films were deposited on SiO2 , Si3N4 , TiN, TiO2 , and
Ta2O5 substrate layers at temperatures ranging from 300 to 350°C.
The SiO2 , Si3N4 , TiN, TiO2 , and Ta2O5 films were prepared by
thermal oxidation, low-pressure CVD, reactive sputtering ͑TiN and
TiO2), and MOCVD, respectively. Some of the Ru films were an-
nealed in an atmosphere-controlled furnace under N2 or H2 /Ar at-
mosphere. Some of the TiN films were Ar plasma treated under a
pressure of 5 mTorr and input plasma power of 100 W.
Phase identification and resistivity measurements of Ru thin
films were performed by an X-ray diffraction ͑XRD͒ and a four-
point probe, respectively. The depth profiles of elements in the films
were obtained by Auger electron spectroscopy ͑AES͒. The surface
morphology, cross-sectional image, and surface roughness of the
films were observed by scanning electron microscopy ͑SEM͒, trans-
mission electron microscopy ͑TEM͒, and atomic force microscopy
͑AFM͒, respectively. Changes in surface states of the TiN films due
to the Ar plasma treatment were investigated by X-ray photoelectron
spectroscopy ͑XPS͒.
However, the MOCVD Ru films usually suffer from poor nucle-
ation and rough surface morphology on Si, SiO2 , and TiN film
surfaces, although the MOCVD Ru films on Ta2O5 film shows good
morphologies.6,7 This makes the fabrication of capacitors and inte-
gration processes difficult using the MOCVD Ru electrodes. There-
fore, it is important to understand the origin of the different deposi-
tion behaviors of Ru on various substrates and to find the solutions
to enhance the growth behaviors of Ru films on the TiN films that
have high nucleation barriers.
In addition, during the deposition and annealing process of the
high-dielectric thin films, Ru/TiN electrode stacks are exposed to
oxidizing atmosphere. With the decreasing feature sizes of the
memory devices, the Ru electrode thickness should also be de-
creased to Ͻ20 nm, which results in oxidation of the TiN surface
due to the oxidant penetration through the thin Ru electrode. This
results in a serious increase in the contact resistance of the inte-
grated capacitors.
Results and Discussion
Figure 2a and b shows the SEM surface morphologies of Ru thin
films deposited on Ta2O5 and TiN substrates, respectively, with a
low magnification. Ru film deposited on Ta2O5 has a smooth surface
and uniform grains as shown in Fig. 2c. However, the Ru film de-
posited on TiN film has many nondeposited regions, shown as dark
spots in Fig. 2b and even the deposited regions show rough mor-
phology and grains with sharp edges ͑Fig. 2d͒. The root-mean-
square ͑rms͒ roughnesses of the Ru films on Ta2O5 and TiN were
3.48 and 4.37 nm, respectively. It seems that the TiN surface has a
higher nucleation barrier for deposition of Ru films compared to
Ta2O5 film. In order to understand the nucleation behavior depend-
ing on the types of the substrate films, Ru film deposition behaviors
on various substrates, such as Si3N4 , SiO2 , Ta2O5 , TiN, and TiO2 ,
were investigated. For these experiments, the Ru films were depos-
Therefore, in this study, Ru thin films were prepared by MOCVD
using
cyclopentadienyl-propylcyclopentadienlylruthenium͑II͒
͓RuCp͑i-PrCp͔͒ and the nucleation behaviors of Ru films on Ta2O5 ,
TiN, Si3N4 , SiO2 , TiO2 thin-film substrates were investigated. The
* Electrochemical Society Active Member.
z E-mail: cheolsh@plaza.snu.ac.kr
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Journal of The Electrochemical Society, 152 ͑1͒ C15-C19 ͑2005͒
Figure 2. Surface morphologies of Ru films deposited on ͑a,c͒ Ta2O5 films
and ͑b,d͒ TiN films. ͑d͒ The SEM image of film deposited region ͑b͒ in.
͑BST͒ and TiN films were due to the different bonding type of the
substrate atoms.9 At the initial stage of the Ru film growth, the Ru
precursors chemically adsorbed on the substrate and thermally de-
composed with the aid of oxygen. It has been reported that the
chemisorbed Ru ions form dimer and these Ru dimers form strong
surface complex by the interaction with the surface atoms of the
substrate.9 The surface complex works as the nuclei for the Ru film
growth. When the surface complex formation is not favorable, the
adsorbed Ru dimers evaporate back to the vapor and no nucleation
occurs. It is believed that the surface complex formation between
the Ru dimer and substrate atoms is favorable when the substrate
atoms are of the metallic or ionic nature, considering the metallic
property of the Ru dimer. When the substrate atoms have the metal-
lic nature, electron clouds are shared between the substrate atom and
Ru dimers, forming strong surface complexes. When the substrate
atoms have ionic bonding nature, the negative charges of anions
attract the Ru dimers which also results in strong surface complex
formation. In addition, chemical interaction between the anions, es-
Figure 1. Structure of RIR concave-type capacitor.
ited with an oxygen flow rate of 15 sccm at 300°C, which is an
unfavorable condition ͑too small oxygen flow rate͒ for the deposi-
tion of Ru films.8 Under this oxygen flow rate, the Ru precursor
decomposition is relatively inactive, but this condition was adopted
in order to clearly show the substrate-dependent growth behavior.
Figure 3a-e shows the surface morphologies of Ru films deposited
on the Si3N4 , SiO2 , Ta2O5 , TiN, and TiO2 film substrate, respec-
tively. While the films deposited on Ta2O5 and TiO2 films have
continuous and smooth surface morphologies, the films on Si3N4 ,
SiO2 , and TiN have island-like and discontinuous surface mor-
phologies. Therefore, it is believed that the Ru nucleation barrier
energies on Ta2O5 and TiO2 , and Si3N4 , SiO2 , and TiN substrates
are low and high, respectively.
It was found that the different nucleation behaviors on the vari-
ous substrates depend on the bonding types between the atoms in
substrate materials. Table II shows the differences in the electrone-
gativities (⌬n) of the constituent atoms of the various substrate
films. It can be understood that when ⌬n is Ͼ1.9, i.e., the bonding
is largely ionic, the Ru film grows in a uniform manner, and when
the ⌬n is Ͻ1.6, i.e., the bonding is less ionic or close to covalent,
the Ru film grows in a discrete island form. A similar report has
been made by Smith et al.9 They reported that different adhesion
and deposition behaviors of Ru films on barium strontium titanate
Table I. Experimental conditions for deposition of Ru films.
Precursor
RuCp͑i-PrCp͒
Source temperature
Carrier Ar flow rate
Substrate temperature
Oxygen flow rate
Total gas flow rate
Pressure
85°C
100 sccm
300-350°C
100-200 sccm
1000 sccm
3 Torr
Figure 3. Surface morphologies of Ru films deposited on ͑a͒ Si3N4 , ͑b͒
SiO2 , ͑c͒ Ta2O5 , ͑d͒ TiN, and ͑e͒ TiO2 at 300°C with 15 sccm of O2 flow.
Film thickness
300-400 Å
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Journal of The Electrochemical Society, 152 ͑1͒ C15-C19 ͑2005͒
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Table II. Differences in electronegativities between elements of
the substrate films.
Difference in electronegativities
Si-N
Ti-N
Si-O
Ti-O
Ta-O
1.14 ͑covalent bonding͒
1.50
1.54
1.90
1.94 ͑ionic bonding͒
pecially oxygen ions, and Ru metal ions contribute to the surface
complex formation. Therefore, the Ru film growth on the Ta2O5 and
TiO2 bears low nucleation barrier and shows smooth surface mor-
phologies. When the MOCVD Ru films were grown on sputtered Ru
film, the nucleation and growth was also favorable and excellent
MOCVD Ru films were obtained.
However, when the bonding between substrate atoms is covalent,
such as SiO2 , Si3N4 , and TiN cases, little electron density is avail-
able for bonding with the Ru dimer and no strong interfacial com-
plex is formed. Therefore, the nucleation barrier becomes high and
only island-like films were obtained because the Ru atoms are ad-
sorbed only on the previously formed Ru nuclei.
Figure 5. SEM images of Ru films deposited for 1 min on ͑a͒ bare TiN and
͑b͒ TiN treated by Ar plasma at 100 W for 5 min.
The formation of RuO2 phase can be assumed to occur in two
steps: the deposition of Ru nuclei ͑Eq. 1͒ and its oxidation ͑Eq. 2͒
Ru deposition step: RuCp͑i-PrCp) ϩ O2 ϭ Ru ϩ by product↑
͓1͔
Ru oxidation step: Ru ϩ O2 ϭ RuO2
͓2͔
The difficulty in the nucleation of the metal Ru film on the co-
valent bonded substrate influenced the Ru or RuO2 formation be-
havior under a deposition condition where high oxygen flow rate
was used. Figure 4a and b shows the XRD patterns of the films
deposited on Ta2O5 and TiN, respectively, at an Ar carrier flow rate
of 50 sccm and an oxygen flow rate of 450 sccm, which corresponds
to a thermodynamic condition where the RuO2 is more stable than
the metallic Ru.7,8 This is an experimental condition where the dif-
ference in phase formation ͑Ru or RuO2) on the different substrates
is clearly shown. It can be observed that the Ta2O5 film has a higher
Ru XRD peak intensity than that of the RuO2 even under this con-
dition, whereas the film on the TiN surface is an almost pure RuO2 .
This can be understood from the following discussion.
On the Ta2O5 surface with a low nucleation barrier for Ru deposi-
tion, the Ru deposition rate is higher than the oxidation rate. In this
case, the rate-determining step of the film growth is Ru oxidation.
Therefore, the formation of Ru phases is more favorable than that of
RuO2 phases. The deposition rate of Ru thin films on a TiN surface
is lower than the oxidation rate due to its high nucleation barrier.
Once the Ta2O5 and TiN surfaces are covered with the Ru and
RuO2 nucleation layers, respectively, the main growth material
should have been the RuO2 because RuO2 is thermodynamically
favorable under the given deposition conditions. However, the major
phase on the Ta2O5 is the Ru, as shown by the XRD in Fig. 4a. This
can be understood from the following; first, the difference in the
volume free energies of the Ru and RuO2 under the given conditions
is not large.7,8 Second, the growth of RuO2 on the Ru nuclei pro-
duces interfaces which increase the total free energy of the system.
This interface energy increase might be larger than that of the bulk
free energy difference between the Ru and RuO2 . Therefore, Ru
film dominantly formed on top of the Ta2O5 surface.
As discussed previously, Ru has to be deposited on the TiN bar-
rier as well as the Ta2O5 adhesion layer in the integrated capacitor
structure. Actually, the uniform and dense deposition of Ru film on
the TiN barrier is crucial in order not to oxidize the TiN barrier
surface and obtain good electrical contact between the capacitor and
transistor. Because the reason for the poor nucleation behavior of the
Ru film on the TiN surface was the covalent bonding nature of the
TiN, the TiN surface was modified by plasma treatment. It was
expected that the Ar plasma treatment removes some of the nitrogen
atoms by knocking out the mechanism and metallic Ti atoms remain
on the TiN surface.10 It was also noted that only a few Ti atomic
layers were sufficient to enhance the Ru nucleation. Further removal
of the nitrogen degrades the diffusion barrier property of the TiN.
There has been a report on the effects of plasma treatment on the
RuO2 nucleation enhancement. Park et al.11 reported that the en-
hancement of the nucleation density of RuO2 films was obtained by
using plasma treatment prior to the deposition of the film by CVD.
They concluded that active oxygen chemical species which were
adsorbed on the substrate surface by O2 plasma treatment help the
dissociation of the Ru precursors. They also reported that the en-
hancement of the nucleation density by Ar or H2 plasma is mainly
due to generation of the nucleation site by ion bombardment. In the
present study, Ar plasma was used because O2 plasma oxidizes the
TiN barrier surface and increases the contact resistance.8
Figure 5 shows the surface morphologies of Ru films deposited
for 1 min on ͑a͒ untreated and ͑b͒ Ar-plasma-treated TiN film. The
Figure 4. XRD patterns of Ru-RuO2 films deposited on ͑a͒ Ta2O5 and ͑b͒
TiN layers.
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Journal of The Electrochemical Society, 152 ͑1͒ C15-C19 ͑2005͒
Figure 6. XPS results of ͑a͒ N 1s and ͑b͒ O 1s spectra from nontreated and
Ar-plasma-treated TiN films.
plasma treatment was performed for 5 min. It can be observed that
the Ru film grows in an island manner and shows a discontinuous
surface on the untreated TiN, while the film has a continuous and
smooth surface on the plasma-treated TiN.
Figure 7. The deconvoluted Ti 2p spectra of the ͑a͒ untreated and ͑b͒
plasma-treated sample.
Surface roughnesses of the untreated and plasma-treated TiN
films measured by the AFM were 1.01 and 0.92 nm, respectively,
suggesting that the physical damage on the TiN surface by Arϩ ion
bombardment11 is not the origin of the nucleation enhancement.
In order to investigate the reasons for the improvement in the
nucleation behavior of the Ru film on the plasma-treated TiN, un-
treated TiN and Ar-plasma treated TiN were investigated by XPS.
Figure 6 shows the ͑a͒ N 1s and ͑b͒ O 1s XPS spectra of the
untreated and plasma-treated TiN samples. The deconvoluted Ti 2p
spectra of the untreated and plasma-treated TiN samples are shown
in Fig. 7a and b, respectively. The relative N concentration of the
plasma-treated sample is much smaller than that of the untreated
sample, whereas the decrease in the oxygen concentration was much
smaller. The oxygen on the TiN film surface originates from the air
exposure of the TiN film before the Ru deposition. It has been re-
ported that the air exposure of the TiN film results in 15-20% sur-
face oxygen concentration.12 Therefore, the Ar plasma treatment se-
lectively removes only N ions from the TiN film surface. The almost
invariant O 1s peak intensity is due to the air exposure of the sample
after the Ar plasma treatment. This can also be confirmed from the
Ti 2p spectra change as shown in Fig. 7. Here, the peak deconvolu-
tion and peak assignment were done based on Refs. 10 and 13. The
separation between Ti 2p3/2 and Ti 2p5/2 peaks was 5.54 eV. The
peaks at 458.4, 456.7, and 455.3 eV are assigned to be Ti 2p3/2 peaks
of TiO2 , TiNxOy , and TiN, respectively.10 It can be seen that the Ti
2p peak intensity related to the TiN and TiNxOy largely decrease but
that of the TiO2 is almost invariant. Therefore, the Ar plasma treat-
ment selectively removes some of the N ions by knocking out the
mechanism so that the surface bonding nature of TiN becomes more
metallic, due to remaining Ti ions. This greatly improved the Ru
nucleation behavior.
not only the electrode but also the oxygen permeation barrier to the
underlying TiN barrier. As previously mentioned, during the depo-
sition and annealing process of high-dielectric thin films, Ru/TiN
stacks should be exposed to oxidizing atmosphere. Figure 8 shows
the AES depth profiles of ͑a͒ as-deposited, ͑b͒ postannealed at
600°C under oxygen ambient using rapid thermal processing ͑RTP͒
for 30 s, and ͑c͒ postannealed Ru/TiN stack under H2 atmosphere at
400°C using a furnace for 30 min followed by O2 atmosphere at
600°C using RTP for 30 s. The as-deposited Ru film contains a
negligible amount of oxygen ͑Ͻ1%͒. It seems that the relatively
high oxygen concentration on the TiN surface is due to the exposure
of the TiN film to air prior to Ru film growth, as previously men-
tioned. Figure 8b shows that RTP under the oxygen ambient oxi-
dized the surface of the Ru film and oxygen diffused into the TiN
layer. However, in case of the H2-annealed Ru/TiN ͑Fig. 8c͒, oxygen
did not diffuse into the TiN and only the Ru surface was oxidized
during annealing under the oxygen ambient.
Figure 9 shows the cross-sectional TEM images of ͑a͒ as-
deposited and ͑b͒ postannealed under H2 ambient Ru/TiN stack
samples. The film microstructures did not change largely after the
H2 annealing. Therefore, it is not easy to understand the reason for
the improved oxygen-blocking property of the H2-annealed Ru
films. The reason might be related to some change in the grain
boundary structure or density of the Ru films which cannot be
clearly observed in the present TEM analysis.
Conclusions
Ru thin films were prepared by MOCVD using RuCp͑i-PrCp͒
and O2 as the precursor and reaction aids, respectively, and the
nucleation behaviors on Si3N4 , SiO2 , Ta2O5 , TiN, and TiO2 thin-
film substrates were investigated. It was found that the difference in
In the integrated capacitor structure, the Ru layer should work as
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Journal of The Electrochemical Society, 152 ͑1͒ C15-C19 ͑2005͒
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Figure 9. The cross-sectional TEM images of ͑a͒ as-deposited samples and
͑b͒ samples postannealed under H2 ambient.
erty of TiN was improved by Ar plasma treatment, which selectively
removes N ions from the TiN surface and produced more metallic or
ionic surface. In addition, the diffusion of oxygen atoms to TiN
through Ru film was largely suppressed by the H2 annealing of the
Ru/TiN stacked films.
Acknowledgments
This work was supported by the Korean Ministry of Science and
Technology through the National Research Laboratories program
and National R&D Project for Nano Science and Technology
͑M10214000097-02B1500-01500͒ and the System IC 2010 project
of the Korean government. The authors also acknowledge Tanaka
Kikinzoku Co., Japan, for their supply of RuCp͑i-PrCp͒ and finan-
cial support of Samsung Electronics.
Seoul National University assisted in meeting the publication costs of this
article.
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nucleation behavior on various substrates contributes to the different
bonding types between atoms in substrate materials; the ionic bond-
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smooth and continuous Ru films, whereas the covalent bonding
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the high nucleation barrier. The covalent nature of TiN results in a
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Ru film is grown on the Ta2O5 substrate. The poor nucleation prop-
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