Ru cyclooctatetraene precursors for MOCVD

Article abstract of DOI:10.1039/c1dt11454a

A series of Ru⁰ cyclooctatetraene complexes are presented with optimal properties for MOCVD (metal organic chemical vapour deposition) applications, including combinations of the two lowest melting points and lowest decomposition temperatures yet reported for such materials. The compounds are easy to handle and lead to highly conformal thin films of Ru on SiO₂ features; even within holes with aspect ratios of 40:1. SEM, AFM and XPS studies confirm the near ideal nature of the resulting conformal thin film.

Full text of DOI:10.1039/c1dt11454a

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An international journal of inorganic chemistry
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Volume 41 | Number 6 | 14 February 2012 | Pages 1637–1904
ISSN 1477-9226
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Ogo et al.
Ru cyclooctatetraene precursors for MOCVD
1477-9226(2012)41:6;1-E

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PAPER
Ru cyclooctatetraene precursors for MOCVD†
Tatsuya Ando,a Naoki Nakata,a,b Kazuharu Suzuki,b Takahiro Matsumotoa,c and Seiji Ogo*a,c,d
Received 2nd August 2011, Accepted 5th September 2011
DOI: 10.1039/c1dt11454a
A series of Ru0 cyclooctatetraene complexes are presented with optimal properties for MOCVD (metal
organic chemical vapour deposition) applications, including combinations of the two lowest melting
points and lowest decomposition temperatures yet reported for such materials. The compounds are
easy to handle and lead to highly conformal thin films of Ru on SiO2 features; even within holes with
aspect ratios of 40 : 1. SEM, AFM and XPS studies confirm the near ideal nature of the resulting
conformal thin film.
Several MOCVD precursors have been developed.5–16 The ideal
MOCVD precursor compound would have a complete set of
complementary physical properties. Its melting point would be
low, so as to enable easy transition to the liquid state; as a liquid,
it would have a high vapour pressure to enable easy transport; and
it would have a low decomposition temperature to enable easy
removal of the organic ligands. With respect to these properties,
Fig. 1 shows some of the previously-reported candidates A–H of
precursors for MOCVD.5a–c,7a,7b,8b,12–16 The highest aspect ratio of
6.4 : 1 was published by using B,16 however, a Ru precursor with a
higher aspect ratio has yet to be reported.
Introduction
In order for next-generation devices to become more efficient
and complex, engineers must be able to reliably coat thin films
onto ever smaller and more closely-packed features. That such
films can be made efficiently from Ru is becoming increasingly
apparent as a result of its low resistivity and good deposition
characteristics. Currently, it is employed as an electrode in dynamic
RAM devices1 and as a diffusion barrier for Cu interconnects in
integrated circuits.2
Although there are several methods for preparing layers of Ru,
MOCVD (metal organic chemical vapour deposition) and ALD
(atomic layer deposition) are suitable for forming Ru thin films.
Gordon and co-workers achieved the deposition of a Ru film
with an aspect ratio of 40 : 1 using a RuII amidinate complex by
ALD.3 However, the ALD requires a number of reactive cycles to
generate thin films and a slow deposition rate.4 MOCVD provides
highly conformal and uniform films, even on challenging three-
dimensional surfaces such as narrow holes with high aspect ratios.
Furthermore, the growth rate of the films is fast enough to support
modern and future manufacturing for microelectronics. For these
reasons, developing Ru thin film precursor compounds has become
an important target for technology industries.
aDepartment of Chemistry and Biochemistry, Graduate School of Engi-
neering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka, 819-0395,
Japan. E-mail: ogotcm@mail.cstm.kyushu-u.ac.jp; Fax: +81-92-802-2823;
Tel: +81-92-802-2818
bSpecialty Chemicals Section, Technology Development Department, Tech-
nical Division, Tanaka Kikinzoku Kogyo K. K., 22 Wadai, Tsukuba, Ibaraki,
300-4247, Japan
cInternational Institute for Carbon-Neutral Energy Research (I2CNER),
744 Moto-oka, Nishi-ku, Fukuoka, 819-0395, Japan
dCore Research for Evolutional Science and Technology (CREST), Japan
Science and Technology Agency (JST), Kawaguchi Center Building, 4-1-8
Honcho, Kawaguchi-shi, Saitama, 332-0012, Japan
† Electronic supplementary information (ESI) available: Experimental
details, Table S1 and Fig. S1–S16. CCDC reference number 837556. For
ESI and crystallographic data in CIF or other electronic format, see DOI:
10.1039/c1dt11454a
Fig. 1 Some of the previously-reported candidates A–H of precursors for
MOCVD.
1678 | Dalton Trans., 2012, 41, 1678–1682
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We report herein Ru precursors for MOCVD with the highest
aspect ratio of 40 : 1 by tuning the nature of Ru0 COT complexes,
with Cu-Ka radiation generated at 40 kV and 40 mA {scan speed:
20◦ min-1 (2q = 25–55◦), 1◦ min-1 (2q = 37–45◦)}.
4
[Ru0(h -COT-R)(CO)3] (R = H: 1, R = Me: 2, R = Et: 3, COT-
[Ru0(g4-COT-Me)(CO)3] (2)
R = R-cycloocta-1,3,5,7-tetraene, Fig. 2).17 Introduction of alkyl
groups such as a methyl and ethyl group into COT makes the
nature of a Ru0 COT complex close to the ideal properties for
MOCVD. The resulting Ru thin films have good properties as
revealed by scanning electron microscopy (SEM), atomic force
microscopy (AFM) and X-ray photoelectron spectroscopy (XPS).
The films were deposited on a SiO2 substrate and were conformal
even within a hole with an aspect ratio of 40 : 1. We believe these
compounds will be of great benefit in the continuing drive towards
greater miniaturisation.
A suspension of [Ru0 (CO)12] (0.750 g, 1.17 mmol) in COT-Me
3
(3.00 g, 25.4 mmol) and cyclohexane (25 mL) was stirred for
72 h with irradiation by UV light. Some unreacted [Ru0 (CO)12]
3
was removed by filtration, and the solvent wa◦s removed in vacuo.
The residue was purified by sublimation (23 C, 40 Pa) followed
by recrystallisation from a hexane solution at -40 ◦C. 2 was
obtained as orange crystals (yield: 46% based on the metal atoms in
[Ru0 (CO)12]). 1H NMR (300 MHz, in CDCl3, reference to TMS):
3
d 5.85 (t, 2H, C8H7CH3), 5.55 (t, 1H, C8H7CH3), 5.12 (t, 2H,
C8H7CH3), 4.70 (d, 2H, C8H7CH3), 1.99 (s, 3H, C8H7CH3). FT-
IR (cm-1, KBr disk): 2065 (CO), 1997 (CO). Anal. Calcd for 2:
C12H10O3Ru: C, 47.52; H, 3.32. Found: C, 47.38; H, 3.22.
[Ru0(g4-COT-Et)(CO)3] (3)
Complex 3 was synthesised by the same method as 2 using
[Ru0 (CO)12] (0.750 g, 1.17 mmol) and COT-Et (3.36 g, 25.4
3
mmol). After removal of the solvent, distillation (23 ◦C, 39 Pa)
of the residue afforded 3 as an orange liquid (yield: 21% based
on the metal atoms in [Ru0 (CO)12]). 1H NMR (300 MHz, in
3
CDCl3, reference to TMS): d 5.90 (t, 2H, C8H7CH2CH3), 5.61
(t, 1H, C8H7CH2CH3), 5.13 (t, 2H, C8H7CH2CH3), 4.62 (d,
2H, C8H7CH2CH3), 2.12 (q, 2H, C8H7CH2CH3), 1.07 (t, 3H,
C8H7CH2CH3). FT-IR (cm-1, KBr disk): 2062 (CO), 1987 (CO).
Anal. Calcd for 3: C13H12O3Ru: C, 49.21; H, 3.81. Found: C,
48.98; H, 3.65.
Fig. 2 Structures and melting points of 1–3.
Experimental
Materials and methods
All reactions for the synthesis of 1–3 and COT-R were carried out
under an N2 atmosphere using standard vacuum line techniques
or handled in an N2-glovebox. Complex 1, COT-Me and COT-
Et were prepared using the previously reported methods.18–20
X-Ray crystallographic analysis
An X-ray quality crystal of 2 was prepared by recrystallisation
of a hexane solution at -40 ◦C for a few days. Measurements
were made on a Rigaku/MSC Saturn CCD diffractometer with
confocal monochromated Mo-Ka radiation (l = 0.7107 A). Data
were collected and processed using the teXsan crystallographic
[Ru0 (CO)12] was supplied by Tanaka kikinzoku kogyo KK and
3
˚
other reagents were purchased from Aldrich, Kanto Chemical
Co., Inc. and Wako Pure Chemical Industries. All reagents were
used as received without further purification.
software package of Molecular Structure Corporation.
1H NMR spectra were recorded on a JEOL JNM-AL300 at
25 ◦C. IR spectra were recorded on a Thermo Nicolet NEXUS 870
FT-IR instrument usi◦ng 2 cm-1 standard resolution in a range of
4000 to 400 cm-1 at 25 C. Thermal properties of 1–3 were analysed
under an N2 atmosphere by thermogravimetry-differential thermal
analysis (TG-DTA) using a Seiko TG-DTA320 instrument. N2
flow rate is 100 mL min-1. Precursors 1–3 (5 mg) were loaded in
open type aluminium crucibles. The measurements were done with
a heating temperature rate of 5 ◦C min-1.
Ru thin films were analysed by scanning electron microscopy
(SEM) using Hitachi S-5000. The film purities were determined by
X-ray photoelectron spectroscopy (XPS) using a Nissan Arc PHI
5800 system with an Al/Mg dual anode X-ray source. Binding
energies were calibrated by the Si 2p peak of SiO2 substrates
at 103.3 eV.21 Atomic force microscopy (AFM) was carried out
to observe the surface morphology and estimate the root mean
square (RMS) roughness value. Electrical resistivity of Ru thin
films was measured by a four-point probe method at 25 ◦C, using
a Mitsubishi Chemical MCP-T610. The crystal orientation of the
deposited Ru films was evaluated by X-ray diffraction (XRD)
patterns using an X-ray diffractometer (Rigaku Ultramax IV)
Deposition of ruthenium thin films
Depositions of Ru thin films were conducted using a cold wall
MOCVD reactor, with the substrates maintained at 155–175 ◦C.
For each◦MOCVD experiment, precursors 1–3 were vaporized
at 70–75 C under a pressure of 330 Pa under a flow of N2 {10
standard cc min-1 (sccm)} and H2 (1 sccm). N2 and H2 are the
carrier gas and the reactant gas, respectively. The deposition time
was set to a period of 60–70 min. Deposited Ru films were annealed
at 450 ◦C for 30 s under a flow of H2 (10 sccm).
The substrates had an 80 nm layer of SiO2 on Si and were
used as received from the supplier (Ites). Patterned SiO2 substrates
(Tei solutions) containing holes with a diameter of 200 nm and
an aspect ratio of 40 : 1 were used to determine the conformal
deposition of Ru thin films.
Results and discussion
Synthesis and characterisation of zero-valent Ru precursors
Ru0 precursors 1–3 were prepared by the photochemical reaction
of [Ru0 (CO)12] with COT-R (R = H, Me and Et) in cyclohexane,
3
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Table 1 Physical properties of the Ru precursors for MOCVD
Precursor
Melting point (◦C)
Decomposition temperature (◦C)
Vapour pressure (Pa/◦C)
Reference
1
78a
97a
20/23b
this work
this work
this work
12a–c
13
14
5a,15
2
24a
95a
40/23b
3
<-20a
20–25
17
71.5
150
100a
110–150
270
160–200
150
39/23b
27/56
5.3/60
0.036–20/45
0.4/40
A
B
C
D
a The melting points and decomposition temperatures were determined by TG-DTA (Fig. S5–S7 in ESI†). b The values of the vapour pressures were
determined by sublimation of 1 and 2 or distillation of 3.
3 typical of n(CO) (Fig. S1 and S2†). The carbonyl stretches are
red-shifted with respect to the free CO vibration (2143 cm-1) due
to the back-donation of electron density from the dp orbital of
Ru0 to the p* anti-bonding orbital of the CO ligands.
1
Fig. S3 and S4† show H NMR spectra of 2 and 3 in CDCl3,
respectively. The signal at 1.99 ppm observed in 2 is assigned
to the methyl protons and the signals at 2.12 and 1.07 ppm
in 3 are responsible for the ethyl protons. The protons of the
Fig. 3 Synthesis of 1–3.
cyclooctatetraene rings in 2 and 3 are observed at 4.6–6.0 ppm.
respectively (Fig. 3). Molecular structures of 2 and 3 were
characterised by FT-IR (Fig. S1 and S2 in ESI†) and H NMR
Physical properties of zero-valent Ru complexes 1–3
1
spectroscopy (Fig. S3 and S4 in ESI†), but also the structure of 2
was determined by X-ray analysis.
The physical properties of 1–3 and the previously-reported precur-
sors A–D are summarised in Table 1 and a three-dimensional plot
of the physical properties is shown in Fig. 5. Thermal properties of
1–3 were analysed by TG-DTA under a flow of N2 gas (Fig. S5–S7
in ESI†). The DTA curves of 1 and 2 show endothermic peaks at
78 and 24 ◦C, respectively, without weight loss in the TG analysis,
indicative of melting without decomposition. Complex 3 solidified
below -20 ◦C, indicating the melting point is <-20 ◦C. The melting
point depends on the bulkiness of the ligand substituent. The order
of the melting points is 3 < 2 < 1. This result is probably attributed
to the lowering of the symmetry of the molecular structures of Ru0
complexes 2 and 3 by introduction of the methyl and ethyl groups
into COT.
Recrystallisation of 2 from a hexane solution at -40 ◦C yielded
orange crystals of 2, which were available for X-ray analysis (Fig.
4). Fig. 4 reveals that the Ru atom is surrounded by one COT-
Me and three CO ligands. Two C C bonds of COT-Me are
4
coordinated to the Ru centre in an h -fashion. The bond distances
between Ru and the C atoms of CO ligands {1.927(2), 1.936(2),
˚
and 1.933(2) A for Ru1–C1, Ru1–C2 and Ru1–C3, respectively}
are much shorter than those between Ru and the C atoms of
˚
COT-Me {2.205(2), 2.189(2), 2.262(2) and 2.238(2) A for Ru1–
C4, Ru1–C11, Ru1–C5 and Ru1–C10, respectively}.
Fig. 5 A three-dimensional plot of physical properties of the Ru
precursors for MOCVD. These data are obtained from Table 1.
Fig. 4 An ORTEP drawing of 2 with ellipsoids at 50% probability.
◦
˚
Selected interatomic distances (l/A) and angles (f/ ): Ru1–C1 = 1.927(2),
Ru1–C2 = 1.936(2), Ru1–C3 = 1.933(2), Ru1–C4 = 2.205(2), Ru1–C5
= 2.262(2), Ru1–C10 = 2.238(2), Ru1–C11 = 2.189(2), Ru1–C1–O1 =
175.3(2), Ru1–C2–O2 = 178.0(2), Ru1–C3–O3 = 179.1(2).
At 95–100 ◦C, weight losses were observed in the TG analyses
of 1–3 with endothermic peaks in the DT analyses (Fig. S5–S7 in
ESI†), indicative of decompositions (Table 1).
IR spectra of 2 and 3 in the solid state indicate strong vibration
bands at 1997 and 2065 cm-1 for 2 and 1987 and 2062 cm-1 for
Sufficient volatility is a very important requirement for
MOCVD precursors and quantitatively estimated by determining
1680 | Dalton Trans., 2012, 41, 1678–1682
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the vapour pressure. The vapour pressures of 1–3 were determined
as 20, 40 and 39 Pa, respectively, by sublimation of 1 and 2 or
distillation of 3 at 23 ◦C without decomposition (Table 1).
As mentioned above, an ideal precursor has a high vapour
pressure, low melting point and low decomposition point, which
corresponds to the top of the front-left corner of the diagram in
Fig. 5. Fig. 5 demonstrates that 2 and 3 are almost exactly in this
sweet spot, relative to the best previously-reported candidates A–D
of precursors for MOCVD.
Preparation of Ru films by MOCVD
The deposition of Ru thin films from 1–3 was carried out by using a
cold wall type CVD apparatus. The deposited films were analysed
by SEM (Fig. 6 and Fig. S8 and S9 in ESI†), AFM (Fig. 7 and
Fig. S10 and S11 in ESI†), XRD (Fig. S12 in ESI†) and XPS
(Fig. 8 and Fig. S13 and S14 in ESI†). The deposition conditions
◦
were as follows; dep◦osition temperature: 155–175 C, Ru source
temperature: 70–75 C, a flow rate of carrier N2 gas: 10 sccm, a
flow rate of reactant H2 gas: 1 sccm, pressure: 330 Pa, deposition
time: 60–70 min. Deposited Ru films were annealed at 450 ◦C for
30 s under a flow of H2 (10 sccm). Fig. 6 shows a SEM image of a
Ru film deposited on a SiO2 substrate from 2 and its film thickness
deposited at 155 ◦C was about 12 nm. The SEM image reveals that
Fig. 8 (a) XPS spectrum of a Ru film deposited from 2 at 155 ◦C under a
flow of N2 (10 sccm) and H2 (1 sccm). Peaks for O 1s and Si 2p originate
from a SiO2 substrate. (b) Magnification of Ru 3d3/2 and 3d5/2 peaks in (a).
Fig. 6 SEM image of a Ru film deposited from 2 at 155 ◦C under a flow
of N2 (10 sccm) and H2 (1 sccm).
a continuous Ru film is deposited and the film is planar. The same
is true for 1 and 3 (Fig. S8 and S9 in ESI†).
AFM was employed to study the surface roughness of the
deposited Ru thin films from 1–3 (Fig. 7 and Fig. S10 and S11
in ESI†). RMS roughness values are 1.1 nm for a 13 nm thick
film deposited from 1 at 165 ◦C, 1.1 nm for a 12 nm thick film
deposited from 2 at 155 ◦C and 1.5 nm for a 49 nm thick film
deposited from 3 at 175 ◦C, which correspond to fairly smooth
films. XRD analysis shows the diffraction peaks typical of a Ru
crystalline film (Fig. S12 in ESI†).
Ru films from 1–3 were analysed by XPS after being cleaned
by sputtering with Ar ions (Fig. 8 and Fig. S13 and S14 in ESI†).
These experiments indicate that the binding energies of Ru 3d3/2
and 3d5/2 correspond to Ru0.21 No peaks derived from Ru oxide
or ligands were observed,8a meaning the Ru0 centres in 1–3 were
not oxidised and the supporting ligands were completely removed
during the deposition process.
For Ru films deposited from 1–3, resistivity measurements gave
values of 93, 152 and 125 mX cm, respectively. Although these
values are higher than that of bulk Ru (7 mX cm),5a they are
comparable to previously-reported values for Ru thin films.22,23
Fig. 7 AFM image of a 12 nm Ru film deposited from 2 at 155 ◦C under
a flow of N2 (10 sccm) and H2 (1 sccm). (a) Two- and (b) three-dimensional
views.
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The excellent properties of 1–3 allowed even coverage of Ru
films within features with very high aspect ratios. Ru films were
deposited on the surface of a high aspect ratio hole (40 : 1).
SEM images of the cross section of Ru films show that very
conformal Ru films with about 70% step coverage were formed
on the substrates using precursors 1–3 (Fig. 9 and Fig. S15 and
S16 in ESI†). Although the values of step coverage were a little
lower than that of the RuII amidinate complex (80%),3 it shows that
suitable tuning of the nature of the precursors enables us to deposit
Ru films with a very high aspect ratio which is comparable to
ALD.
Global COE Program, “Science for Future Molecular Systems”
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), Japan and the Basic Research Programs
CREST Type, “Development of the Foundation for Nano-
Interface Technology” from JST, Japan.
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We conclude that zero-valent Ru complexes 1–3 are outstanding
MOCVD precursors because they have both a high volatility
and a low decomposition point. Pure Ru films with small RMS
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films on holes with aspect ratios of 40 : 1. We believe these
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for miniaturised devices.
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Acknowledgements
This work was supported by the World Premier International Re-
search Center Initiative (WPI Program), grants-in-aid: 18065017
(Chemistry of Concerto Catalysis), 19205009 and 23655053, the
1682 | Dalton Trans., 2012, 41, 1678–1682
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The Royal Society of Chemistry 2012
©