The properties of Ru films deposited by remote plasma atomic layer deposition on Ar plasma-treated SiO2

Article abstract of DOI:10.1002/pssa.201127280

By using remote plasma atomic layer deposition (ALD), ruthenium thin films were deposited on SiO₂ using bis(ethylcyclopentadienyl)ruthenium [Ru(EtCp)₂] as a Ru precursor and an ammonia plasma as a reactant. Different plasma treatments were applied, and the best results were obtained with the Ar plasma-treated SiO₂ surface. The initial transition region usually observed with Ru deposition before continuous film formation was present, and the number of ALD cycles required to obtain a continuous film was reduced to about 35 cycles on the Ar plasma-treated SiO₂ substrates. The transition region of Ru cluster growth on Ar plasma-treated SiO₂ was investigated with transmission electron microscopy (TEM). Most of the Ru clusters were larger and better crystallized on the Ar plasma-treated SiO ₂ than on untreated SiO₂. Also, Ru films deposited on the treated SiO₂ exhibited a (002) preferred orientated structure with a film resistivity of about 10.26 μω-cm. The growth rates of Ru after passing the transition region were similar on both the treated and untreated SiO₂ at about 1.7 A/cycles. From the Auger electron spectroscopy (AES) spectrum, a very low content of oxygen was observed in the Ru films. About 9% carbon was detected by a rutherford backscattering spectrometer (RBS). Copyright

Full text of DOI:10.1002/pssa.201127280

Phys. Status Solidi A 209, No. 2, 302–305 (2012) / DOI 10.1002/pssa.201127280
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applications and materials science
The properties of Ru films deposited
by remote plasma atomic layer
deposition on Ar plasma-treated SiO2
Taeyong Park, Dongjin Choi, Hagyoung Choi, and Hyeongtag Jeon*
Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea
Received 12 May 2011, revised 31 August 2011, accepted 24 October 2011
Published online 21 November 2011
Keywords remote plasma atomic layer deposition, ruthenium films, TEM
*
Corresponding author: e-mail hjeon@hanyang.ac.kr, Phone: þ82 2 2220 0387, Fax: þ82 2 2292 3523
By using remote plasma atomic layer deposition (ALD), with transmission electron microscopy (TEM). Most of the Ru
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ruthenium thin films were deposited on SiO using bis(ethylcy- clusters were larger and better crystallized on the Ar plasma-
clopentadienyl)ruthenium [Ru(EtCp) ] as a Ru precursor and treated SiO than onuntreated SiO . Also, Ru filmsdeposited on
2
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an ammonia plasma as a reactant. Different plasma treatments the treated SiO
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exhibited a (002) preferred orientated structure
were applied, and the best results were obtained with the Ar with a film resistivity of about 10.26 mV-cm. The growth rates
plasma-treated SiO2 surface. The initial transition region of Ru after passing the transition region were similar on
˚
usually observed with Ru deposition before continuous film both the treated and untreated SiO2 at about 1.7 A/cycles.
formation was present, and the number of ALD cycles required From the Auger electron spectroscopy (AES) spectrum, a
to obtain a continuous film was reduced to about 35 cycles on very low content of oxygen was observed in the Ru films.
the Ar plasma-treated SiO substrates. The transition region of About 9% carbon was detected by a rutherford backscattering
2
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Ru cluster growth on Ar plasma-treated SiO was investigated spectrometer (RBS).
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Introduction In the past several decades, many is a conducting material (ꢀ46 mV cm) and also a high work
transition metals had been employed in various parts of function material [12, 13]. Ru thin films have been studied
electronic devices. Typical uses of these metals are in gate as adhesion layers or seed layers for Cu electrochemical
and capacitor electrodes, and interconnect, diffusion barrier, plating (ECP) in a metallization process because of its good
and adhesion layers [1–4]. Ruthenium (Ru), a novel metal, adhesion properties and immiscibility with Cu [14–16]. Ru
has recently been extensively studied because it exhibits a exhibitsgood processability in contrast toother novel metals,
high work function (4.5–5.3 eV) and low resistivity [5, 6]. Ru because Ru can be etched easily by oxygen plasma or ozone
is a candidate for application as a next generation metal– gas [5]. However, ruthenium has some limitations for
insulator–metal (MIM) capacitor electrode [7], which uses applications in mass produced systems such as the long
an ultra high-k material such a barium-strontium titanium transition time required to form a continuous film [17, 18]
oxide (BST) as Ru has good compatibility with BST [8, 9]. and its high material cost.
Generally, high work function metal electrodes enable an There are two main methods of Ru film deposition. One
increased conduction band offset between the electrode and is thermal atomic layer deposition (ALD) with oxygen gas,
BST [10, 11]. The conduction band offset is an electron and the other is plasma enhanced ALD with NH plasma.
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barrier, and a large band offset prevents leakage current Simply, the major difference between these two methods is
between the top and bottom electrodes. The Ru (002) plane is the presence of plasma. For both methods, it is important to
also well aligned with the BST (110) plane. The dielectric reduce the initial transient region before the continuous film
constants of BST films deposited on (002) preferred oriented deposition. It has been reported that NH3 plasma pre-
Ru films increased [6]. It is also expected that the loss of treatment enhances the nucleation rate of Ru on a SiO2
conductivity and electrical inhomogeneity in the MIM substrate with thermal ALD [18]. In the present study,
junction with high-k oxide are negligible, because Ru oxide several plasma pre-treatments were carried out on SiO2
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Original
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Phys. Status Solidi A 209, No. 2 (2012)
303
substrates and Ru films were deposited by remote plasma
atomic layer deposition (RPALD). The characteristics
of the films deposited on Ar plasma pre-treated SiO2
exhibited the best results among the several plasma treated
substrates.
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RPALD using diethylruthenocene (98.9%, Ru(EtCp) ) as a
Experimental Ru thin films were deposited by
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Ru precursor and NH plasma as a reactant. Ar gas was used
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both as a carrier gas and as a purge gas. All the gases flowed
through a gas purifier to trap oxygen and moisture. Si
substrates with thermally grown 100 nm thick SiO layers
2
were used. In order to compare the plasma effects, the Ru
films were deposited on SiO substrates that were plasma
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Figure 1 (online color at: www.pss-a.com) Evolution of thickness
treated with argon, hydrogen, oxygen, and ammonia at asafunctionofthenumberofcyclesofRufilmsdepositedonplasma-
00 8C, and a SiO substrate without plasma treatment. A treated and untreated SiO substrates.
4
2
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downstream-type RPALD reactor with a 13.56 MHz radio
frequency (rf) power source was used in this study. In order
to prepare the plasma-treated SiO substrates, 400 W of physical etching or changes of the surface bonding state.
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remote plasma was irradiated for 2 min in each gas However, for Ar plasma treatment, the chemical effects
environment. The substrate and canister containing were negligible because Ar is an inert gas. The change of
Ru(EtCp)2 precursor were heated at 400 and 80 8C, surface roughness by Ar plasma treatment was also negligible
respectively. The ALD process consists of the following as the difference of the root mean square (rms) roughness
˚
four steps. First, Ar carrier gas was flowed at 50 sccm measured byatomic forcemicroscopy (AFM) wasunder 0.2 A.
(
standard cubic centimeter per minute) for 5 s for each cycle.
Figure 2 exhibits the impurities of the Ru films obtained
Second, excess precursor was purged by flowing 80 sccm Ar by the AES depth profiles and differentiated AES spectra.
gas for 10 s. Next, 100 W of NH plasma was irradiated for Figures 2(a) and (b) show the AES depth profiles of the Ru
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purged by flowing 80 sccm of Ar gas for 10 s. The plasma untreated SiO substrates, respectively. The profiles of the
5 s. Finally, the remaining gas and by-products were also films deposited for 150 cycles on the Ar plasma-treated and
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was an inductively coupled plasma type (ICP) and was impurities in the Ru films did not change with increasing film
generated at a distance of 15 cm from the substrate to the depth. Figures 2(c) and (d) show differentiated AES spectra
bottom of plasma region. The film thickness was measured which indicate that there are very low contents of oxygen and
by X-ray reflectivity. The film resistivity was calculated by a nitrogen impurities in the Ru films. Carbon detection is not
four point probe (FPP) using the thickness data. A available with AES because carbon KLL exactly matches the
transmission electron microscopy (TEM) was used to ruthenium MNN. Therefore, we utilized Rutherford back-
observe cross-sectional images of 40 nm thick Ru films on scattering spectrometer (RBS) measurements in order to
SiO substrates. X-ray diffraction (XRD) was also used to measure the carbon contents of the Ru films. About 9.8 at%
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investigate the crystal structures of the deposited Ru films.
of carbon was detected in the 50 nm thick Ru film by RBS, as
shown in Fig. 3. The impurity levels could also be estimated
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rates of Ru films deposited on various plasma-treated SiO2 deposited on Ar plasma-treated and untreated SiO were
Results and discussion Figure 1 shows the growth from the film resistivity. The film resistivities of Ru
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substrates. Film thickness was measured by AES depth 10.26 and 15.25 mV cm, respectively. These values are
profiles with XRR calibration. A continuous Ru film was slightly higher than the bulk resistivity of about 7 mV cm due
formed on the Ar plasma-treated SiO substrate at about 30 to carbon contamination and a thickness effect. The impurity
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ALD cycles, and the films on the other plasma-treated SiO2 levels of Ru films deposited on the treated and untreated SiO2
substrates were formed at around 65 cycles. In thermal ALD, were almost the same but the film resistivity varied. The
the NH plasma pre-treatment affected the nucleation rate of grain sizes of the Ru films on the Ar plasma-treated and
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Ru films [18]. However, in the RPALD process, there was no untreated SiO substrate were probably different.
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change of nucleation rate due to the NH plasma pre-
Figure 4(a) and (b) shows plane view TEM images of
treatment because the substrate was exposed to NH plasma Ru clusters on Ar plasma-treated and untreated SiO after 20
3
3
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during each reaction step of the ALD cycles, not during pre- and 30 ALD cycles, respectively. Ru clusters were larger on
treatment. After the continuous Ru film formed, the same the Ar plasma-treated SiO than the clusters on the untreated
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linear growth rate of about 1.7 A/cycle was achieved. In the SiO surface. The number densities of Ru clusters on the
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plasma treatment, we expect both chemical and physical treated and untreated SiO were about 3.33 ꢁ 10
and
2
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effects. The chemical effects are the results of chemical 6.28 ꢁ 10 cm, respectively. As shown in the cross-
reactions between the plasma and substrate, and the physical sectional TEM images in Fig. 4(c) and (d), the Ru films
effects involve physical disorder of the SiO surface such as deposited on the Ar plasma-treated SiO consisted of larger
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a
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T. Park et al.: The properties of Ru films deposited on Ar plasma-treated SiO
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Figure 4 TEM plane view images of Ru clusters deposited for (a)
0 cycles on Ar plasma-treated SiO and (b) 30 cycles on untreated
2
SiO
2
2
. Cross-sectional view images of about 40 nm thick Ru films
deposited on (c) Ar plasma-treated SiO and (d) untreated SiO . The
2
2
scale bars in the images are 20 nm for plane view images and 50 nm
for cross-sectional view images.
Figure 2 (online color at: www.pss-a.com) AES depth profiles of
Ru films deposited for 150 cycles on (a) Ar plasma-treated and
Figure 5 shows XRD patterns of hexagonal close-packed
HCP) Ru films deposited for 50, 150, and 300 ALD cycles
(
on Ar plasma-treated and untreated SiO substrates. The
(
(
b) untreated SiO substrates. And differentiated AES spectra of
2
c) nitrogen and (d) oxygen.
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samples were measured in the 2u range of 25–608. In Fig. 5,
the (002) peak (2u ¼ 44.18) of the Ru film deposited on the Ar
grains than the films deposited on the untreated SiO2 plasma-treated SiO was about six times higher than that on
2
substrate. The different number densities of the Ru clusters untreated SiO2 in the diffraction patterns of the films
may have caused the grain size differences in the thick subjected to 150 cycles. The intensity ratios of the major
continuous Ru films. In addition, the Ru grains grew peaks of fully randomly oriented Ru films are shown at the
vertically and finally formed columnar structured films. bottom of Fig. 5 along with a bar graph for reference. XRD
Therefore, the orientations of the Ru films should be data indicate that the c-axis (002) preferred orientation
measured by XRD.
Figure 5 (online color at: www.pss-a.com) XRD patterns of Ru
films deposited on Ar plasma-treated and untreated SiO . The
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Figure 3 (online color at: www.pss-a.com) RBS data for a 50 nm samples were measured in the 2u range from 258 to 608. The
thick Ru film deposited on an untreated SiO
2
substrate.
(002) peaks were observed at 44.18.
ß 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
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Phys. Status Solidi A 209, No. 2 (2012)
305
occurred significantly on the Ar plasma-treated SiO2 in the transition region and finally, large (002) preferred
substrate. Figure 4(a) also shows that the clusters on the Ar oriented grains were observed in the continuous film. By Ar
plasma-treated SiO were formed with a hexagonal shape. plasma pre-treatment, some surface hydroxyls were replaced
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Generally, additional Ru adatoms could be attached easily on with amines. The presence of too many oxidative adsorption
a loosely-packed plane because the plane is the most similar sites prevents lateral growth of Ru clusters by limiting the
structure with gas phase. Then Ru clusters with a loosely capturing area. Consequently, Ar plasma pre-treatment
packed (100) plane which is parallel to the surface, become enhances adatom mobility by removing oxidative reaction
larger. As a result, the Ru clusters which faced a non-(002) sites which prevents lateral growth of Ru clusters and the
directiongrewslowerthanthosethatfacedthe (002)direction. (002) preferred orientated Ru film with large grains were
Then, the small clusters finally disappeared by coalescence.
The preferred orientation frequently occurred on SiO2. deposition in the RPALD method, which used NH plasma as
deposited. The application of Ar plasma treatment before Ru
3
In the growth of clusters, the maximum density of clusters is a reactant, enabled a reduction of the process time and the
controlled by the capturing area if there is no cluster formation of a well matched Ru film with other perovskite
coalescence. The ideal capturing area is a circular area inside structured films such as BST and STO.
of a radius of the surface diffusion length of an additional
adatom. In the capturingarea, the adatoms are easily attached
to existing clusters through surface diffusion rather than
staying on fixed bonded sites. The surface diffusivity is
mainly related to the deposition temperature. In addition, the
capturing radius of a nucleus increases with increasing
temperature. However, the true mobility of Ru adatoms is
affected not only by surface diffusivity but also by the
surface roughness and chemical state of traps (e.g.,
adsorption sites). When there are too many adsorption sites,
growth is disturbed because the capturing areas overlap each
other. Therefore, the number densities of adsorption sites
Acknowledgements This research was supported by The
National Research Program for Terabit Nonvolatile Memory
Development sponsored by the Korean Ministry of Knowledge
Economy.
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4
Conclusions In the initial stages of deposition, the
characteristics of deposited films were affected mainly by
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treatments were carried out, and among the treatments
chosen in this study, the Ar plasma treatment of the SiO2
[
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On the SiO substrate, relatively large clusters were formed
2
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