Highly conformal deposition of pure Co films by MOCVD using Co2 (CO)8 as a precursor

Article abstract of DOI:10.1149/1.2189950

Highly conformal Co thin films were deposited on Si O2 trenches with an aspect ratio of 13 by metallorganic chemical vapor deposition (MOCVD) using Co2 (CO)8 as a precursor in a low-temperature regime of 50-70°C where the growth rate was 3.5-7.0 nmmin. Lowering the pressure of the process reduces the number of collisions in the gas phase and, thus, widens the temperature regime in which the surface reaction controls the growth rate. A processing pressure of 26.7 Pa (0.2 Torr) allows for conformal deposition only at 50°C, whereas deposition at a reduced pressure of 4.0 Pa (0.03 Torr) widens the temperature regime (50-70°C) in which excellent conformality can be obtained. The conformal Co thin film, produced at 50°C and 4.0 Pa, showed a resistivity of 10-12 μ cm and contained 1.0 atom % oxygen and less than 1.0 atom % carbon. After annealing this film at 600°C, its resistivity was reduced to 6 μ cm, which is close to the bulk resistivity (5.7 μ cm) of Co. Therefore, this low-temperature process, which allows for the excellent conformal deposition of pure Co films, can be utilized to produce silicided contacts for advanced devices which require a low contact resistance and good electrical performance.

Full text of DOI:10.1149/1.2189950

Journal of The Electrochemical Society, 153 ͑6͒ G539-G542 ͑2006͒
G539
0013-4651/2006/153͑6͒/G539/4/$20.00 © The Electrochemical Society
Highly Conformal Deposition of Pure Co Films by MOCVD
Using Co2„CO…8 as a Precursor
,z
J. Lee,a, H. J. Yang,a J. H. Lee,a J. Y. Kim,a W. J. Nam,a H. J. Shin,a
*
Y. K. Ko,a J. G. Lee,a, E. G. Lee, and C. S. Kim
b
c
*
aSchool of Advanced Materials Engineering, Kookmin University, Seoul 136-702, Korea
bDepartment of Materials Science and Engineering, Chosun University, Kwangju 501-759, Korea
cKorea Research Institute of Standards and Science, Materials Evaluation Center, Taejon 305-600, Korea
Highly conformal Co thin films were deposited on SiO2 trenches with an aspect ratio of 13 by metallorganic chemical vapor
deposition ͑MOCVD͒ using Co2͑CO͒ as a precursor in a low-temperature regime of 50–70°C where the growth rate was
8
3.5–7.0 nm/min. Lowering the pressure of the process reduces the number of collisions in the gas phase and, thus, widens the
temperature regime in which the surface reaction controls the growth rate. A processing pressure of 26.7 Pa ͑0.2 Torr͒ allows for
conformal deposition only at 50°C, whereas deposition at a reduced pressure of 4.0 Pa ͑0.03 Torr͒ widens the temperature regime
͑50–70°C͒ in which excellent conformality can be obtained. The conformal Co thin film, produced at 50°C and 4.0 Pa, showed
a resistivity of 10–12 ␮⍀ cm and contained 1.0 atom % oxygen and less than 1.0 atom % carbon. After annealing this film at
600°C, its resistivity was reduced to 6 ␮⍀ cm, which is close to the bulk resistivity ͑5.7 ␮⍀ cm͒ of Co. Therefore, this low-
temperature process, which allows for the excellent conformal deposition of pure Co films, can be utilized to produce silicided
contacts for advanced devices which require a low contact resistance and good electrical performance.
© 2006 The Electrochemical Society. ͓DOI: 10.1149/1.2189950͔ All rights reserved.
Manuscript submitted September 19, 2005; revised manuscript received February 13, 2006.
Available electronically April 11, 2006.
Titanium has been widely used to form good ohmic contacts to
heavily doped Si in metal oxide semiconductor ͑MOS͒ devices be-
cause of its ability to reduce the native oxide layer on the Si surface
and the fact that the Schottky barrier height of Ti on n-Si is equal to
approximately one half the silicon bandgap.1 However, since the
scaling down of these devices to dimensions below 100 nm, it has
been found that the Ti layer has the drawback of increasing the
contact resistance, especially in p-type ͑PMOS͒ devices, due to the
rapid depletion of the boron in the p-type junction, which results
from the high reactivity of Ti with boron ͑TiB2, ⌬Gf =
−319.6 kJ/mol͒.2,3 Compared with Ti, Co is less likely to form the
corresponding boride ͑CoB, ⌬Gf = −92.5 kJ/mol͒ and, thus, is less
likely to cause the depletion of boron in the B-doped junction of
PMOS devices. As a result, the use of Co in the P+ contacts of these
devices provides for a much lower contact resistance than that of
Ti.2 Therefore, Co has rapidly replaced Ti in advanced devices.4-6
However, when it is used to deposit Co, the conventional physical
vapor deposition ͑PVD͒ method exhibits a lack of conformal cover-
age and, thus, is not able to produce uniform, conformal thin films in
sub-0.1-␮m devices with complex shapes and structures. Therefore,
most of the current research appears to focus on the use of chemical
carbon contamination and a low resistivity of 5–10 ␮⍀ cm are able
to be produced using Co2͑CO͒ as a Co precursor.9-12However, the
8
facile thermal decomposition of Co2͑CO͒ can produce reactive Co
8
13
carbonyl species such as Co2͑CO͒7 and Co4͑CO͒ , which may be
11
the main contributors to the growth of the Co films. These reactive
intermediate products created in the gas phase tend to degrade the
conformality of the Co films over high-aspect-ratio contact holes
structures.
Therefore, we investigated the effects of various experimental
variables, including the processing pressure, temperature, and addi-
tion of Ar carrier gas on gas-phase collisional activation of the re-
actants, which is known to strongly affect the conformality and
growth rate of Co films. Based on the understanding of the gas-
phase collisional activation on the growth and conformality of the
films, which was obtained from this investigation, the process con-
ditions were optimized in order to obtain uniform, conformal thin
films of Co. Finally, the quality of the conformal Co thin films was
investigated.
Experimental
Cobalt was deposited on either SiO2-coated or trench-patterned
wafers by metallorganic chemical vapor deposition ͑MOCVD͒ using
dicobalt octacarbonyl, Co2͑CO͒8, as a precursor. The trench-
patterned wafers, consisting of 0.2-␮m-wide and 2.6-␮m-deep SiO2
trench structures on Si, were used to investigate the influence of
pressure, temperature, and the addition of Ar carrier gas on the con-
formality of the Co films. The deposition was carried out at a tem-
vapor deposition ͑CVD͒ as
a unique conformal deposition
method.2,7
Kang et al. reported the conformal coating of CVD Co films over
high-aspect-ratio contacts, however, the level of carbon impurity in
the films was found to be quite high, although this was able to be
reduced to about 3 atom % with the addition of H2. The atomic layer
deposition of Co films was also attempted using bis͑N,N -
Ј
perature of 50–200°C and
a
pressure of 4.0–80.0 Pa
diisopropylacetamidinato͒cobalt͑II͒ at temperatures between 260
and 350°C, and was found to result in the excellent conformal depo-
sition of the Co films, which showed a resistivity of 46 ␮⍀ cm for
the 40-nm-thick Co films.8 However, the growth rate of the Co thin
films of less than 1.0 Å/min which is obtained using this method
needs to be increased before it can be used for device fabrication.
The most commonly used precursors for MOCVD Co are cobalt
carbonyls due to the large number of such compounds having suffi-
cient volatility. In addition, the cobalt center in the compound is in
the zero-valent state and, thus, no reductant such as H2 needs to be
added to deposit pure cobalt. As a result, pure cobalt films with low
͑0.03–0.6 Torr͒. The typical flow rate of the Ar ͑99.9999% purity͒
carrier gas was 5 sccm and the pressure was varied by using a
throttle valve in the pumping line. The Co precursor was introduced
into the reaction chamber without Ar carrier gas to obtain a mini-
mum process pressure of 4.0 Pa. The Co bubbler was maintained at
35°C and the delivery gas line was kept at 40°C to prevent any
condensation from occurring inside the lines. The MOCVD system
consisted of a coldwall reactor with a halogen lamp heating system
and was equipped with a mechanical pump. In addition, laser reflec-
tance was used to monitor the variation in the in situ reflectivity
from the substrate surface, which reflects the growth of Co on the
substrate.
The sheet resistance of the Co films was measured using a four-
point probe and the thickness measured by means of a surface pro-
filometer. A ␪-2␪ X-ray diffractometer ͑XRD͒, equipped with a Cu
*
Electrochemical Society Active Member.
z E-mail: lgab@kookmin.ac.kr
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G540
Journal of The Electrochemical Society, 153 ͑6͒ G539-G542 ͑2006͒
Figure 2. Cross-sectional scanning electron microscopy ͑SEM͒ images of
the trenches with an aspect ratio of about 13:1 in which Co films were
deposited using Co2͑CO͒ at 26.7 Pa, ͑a͒ at 50 and ͑b͒ 60°C, respectively.
8
Figure 1. Arrhenius plot for MOCVD Co films as functions of temperature
and process pressure.
arity ͑high Co/CO ratio͒.13 From this mechanistic point of view, the
gas-phase decomposition process can produce a variety of reactive
cobalt carbonyl species with higher sticking coefficient compared
tube, operated at 50 kV and 200 mA, with a monochromatic wave-
length ͑Cu K␣͒ of 0.154 nm, was used to identify the phase and
texture of the films. Auger electron spectroscopy ͑AES͒ was em-
ployed to detect the impurities such as carbon and oxygen in the Co
films. Field-emission scanning electron spectroscopy ͑FESEM͒ and
atomic force microscopy ͑AFM͒ were used to examine the step cov-
erage and surface roughness of the films, respectively.
with Co2͑CO͒ and thus, allow for the increased concentration of
8
the adsorbed precursors on the surface. This results in a much higher
growth rate of Co in temperature regime II. Also the dipole interac-
tion between the electron-deficient cobalt fragments and the SiO2
surface should be an important factor in determining the nature and
extent of the Co–SiO2 surface interaction. In the case of the 4.0 Pa
process, there is a transition temperature of 70–80°C, below which
the deposition of the Co thin films is limited by the surface reaction
and above which it is limited by the gas-phase collisional activation
of the reactants.
Results and Discussion
Figure 1 shows the Arrhenius plot for the deposition rate of the
Co films as functions of the temperature and pressure of the process.
In the temperature range of 50–90°C, the Co growth rate rapidly
increases with increasing temperature and processing pressure, indi-
cating that the reaction is limited by the pressure-related process as
well as by the surface reaction. The deposition of Co at an extremely
low pressure of 4.0 Pa, with no Ar carrier gas, can provide two
distinct temperature regimes. Between 50 and 70°C ͑regime I͒, there
is a moderate increase in the deposition rate of Co with increasing
temperature, while at temperatures above 70°C ͑regime II͒, a sharp
increase in the growth rate of Co with increasing temperature can be
seen. The low-temperature regime, regime I, would be expected to
be surface-reaction-limited. In this temperature regime, the
Ar carrier gas ͑5 sccm͒ was introduced into the reaction chamber
and the pressure was increased to 4.0–80.0 Pa by varying the pump-
ing speed. Increasing the pressure significantly increases the growth
rate, in part due to the increased number of collisions in the gas
phase between the precursor and Ar carrier gas, and in part due to
the increased partial pressure of the reactants. These gas-phase col-
lisions may result in the increased collisional activation of the reac-
tants in the gas phase, thereby leading to the production of reactive
cobalt carbonyl species, which are major contributors to the in-
creased growth rate. Therefore, the transition temperature seems to
disappear as the pressure is increased from 4.0 to 4.0–80.0 Pa.
Further increasing the temperature above 90°C tends to cause the
Co growth rate to level off. Additionally, the growth rate is likely to
decrease as the temperature is further increased above 130°C. This
can be attributed to the homogeneous reaction of the Co precursors,
which results in their loss.9
Co2͑CO͒ precursor can be adsorbed on the surface and then be
8
thermally decomposed to deposit metallic cobalt.13 As the tempera-
ture increases above 70°C ͑regime II͒, the precursor thermally de-
composes to produce intermediate products such as Co2͑CO͒7,
Co4͑CO͒ in the gas phase, and it is this process which is respon-
12
sible for the rapid increase in the growth rate with increasing tem-
perature observed in this regime.
The bottom coverage of the Co films over high-aspect-ratio
trenches was investigated in the temperature range of 50–90°C, in
which the growth rate is competitively controlled by the gas-phase
collision effects and the surface reaction. Figure 2 shows a cross-
sectional FESEM image of the trenches with an aspect ratio of about
The thermal decomposition process of Co2͑CO͒ in the gas
8
phase to produce zero-valent cobalt particles consists of the follow-
ing reactions9
13 in which cobalt films were deposited using Co2͑CO͒ at a pres-
8
2Co2͑CO͒
→ Co4͑CO͒
+ 4CO
͑g͒
͓1͔
͓2͔
8͑g͒
12͑g͒
sure of 26.7 Pa and a temperature of 50–70°C. Co films with a
thickness of approximately 50 nm were uniformly coated on the
walls of the narrow trenches at a temperature of 50°C, revealing a
bottom coverage of 85%, as shown in Fig. 2a. However, the bottom
coverage drastically decreased to 14% ͑Fig. 2b͒ and 7% when the
temperature was increased to 60 and 70°C, respectively.
This abrupt reduction in the conformal coverage of the Co films
at temperatures above 60°C can be attributed to the gas-phase col-
lisional activation of the reactants, possibly resulting from the col-
lisions between the Co precursor and Ar carrier gas. Because these
gas-phase collisions are strongly influenced by the temperature and
Co4͑CO͒
→ 4Co + 12CO
12͑g͒
͑g͒
However, the decomposition of Co2͑CO͒8 to Co4͑CO͒12 and fur-
ther to metallic Co is expected to be a complex process which can
be comprised of several mechanisms. The CO group can be removed
from Co2͑CO͒ and generate an electron-deficient complex,
8
Co2͑CO͒7. This cobalt carbonyl species further reacts with
Co2͑CO͒ to yield an intermediate, Co4͑CO͒12. In addition, this
intermediate can produce a cascade of electron-deficient fragments,
which in turn may react to form cobalt complexes with high nucle-
8
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Journal of The Electrochemical Society, 153 ͑6͒ G539-G542 ͑2006͒
G541
Figure 3. Cross-sectional SEM images of the Co films deposited at a re-
duced pressure of 4.0 Pa and temperatures of ͑a͒ 50, ͑b͒ 60, ͑c͒ 70, and ͑d͒
80°C, respectively.
pressure of the process, the pressure was decreased from
26.7 to 4.0 Pa by introducing Co precursors into the reactor without
Ar carrier gas in order to lower the production of these activated
intermediates.
Figure 5. XRD patterns of about 100-nm-thick Co films deposited on
SiO2-coated Si wafers at 4.0 Pa with various temperatures.
Figure 3 shows the cross-sectional FESEM images of the Co
films deposited on the walls of the trenches at a reduced pressure of
4.0 Pa and temperatures ranging from 50 to 80°C. Excellent confor-
mal deposition of the Co films was obtained up to 70°C, as shown in
͑a–c͒. However, further increasing the temperature to 80°C caused
the bottom coverage to be decreased significantly to about 2–3%,
possibly due to the reaction in the gas phase. This result is consistent
with the existence of two growth temperature regions that are ap-
parent at 4.0 Pa and 50–90°C, as shown in Fig. 1. In the low-
temperature regime ͑50–70°C͒, where the surface reaction controls
the rate, conformal deposition of the Co thin films was obtained. In
the high-temperature regime ͑70–90°C͒, where the gas collision ef-
fect controls the rate, the conformality was significantly degraded. It
can be understood from this that the transition temperature below
which the reaction is surface-reaction-limited needs to be raised in
order to widen the process window for the conformal deposition of
the Co thin films, and this transition temperature is strongly depen-
dent on the process pressure and the nature of the carrier gas.
The film quality of the Co thin films deposited in the temperature
and process regime, which is suitable for the production of highly
conformal coatings, was examined. Figure 4 shows the variation in
resistivity of Co films deposited on Si substrates as functions of
temperature and pressure. The resistivity of the as-deposited Co
films is in the range of 10–15 ␮⍀ cm. In addition, the Co thin films
deposited at 26.7 Pa and 50–60°C show slight higher resistivity than
any of those deposited at 4.0 Pa; however, the difference is within
experimental error.
According to the XRD analysis, the as-deposited Co films at
4.0 Pa have a microcrystalline structure ͑Fig. 5͒. Additionally, AES
analysis reveals very low carbon content in Co film deposited at
4.0 Pa and 50°C, as shown Fig. 6. The low content of carbon ͑below
AES detection limit͒ in the as-deposited Co film deposited at 4.0 Pa
provides evidence for the complete decomposition of the adsorbed
precursor on the surface at the low temperature of 50°C.
About 100-nm-thick Co films were deposited at 4.0 Pa and 50°C
on SiO2-coated Si wafers and then annealed in a vacuum at
100–700°C. Figure 7 shows the variation in the resistivity of the Co
thin films as a function of the annealing temperature. The resistivity
rapidly decreases from 14 to 8 ␮⍀ cm, as the annealing tempera-
ture increases from room temperature to 300°C, possibly due to
grain growth in the Co films. The resistivity continues to decrease as
the temperature is further increased, reaching 6 ␮⍀ cm, almost
equal to the bulk resistivity ͑5.7 ␮⍀ cm͒ of Co, after heating at
Figure 4. Variation in the resistivity of about 100-nm-thick Co films depos-
ited on SiO2-coated Si wafers with temperature and pressure.
Figure 6. AES depth profile of MOCVD Co film deposited on SiO2-coated
Si wafers at 50°C and 4.0 Pa.
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G542
Journal of The Electrochemical Society, 153 ͑6͒ G539-G542 ͑2006͒
be concluded that the extremely low temperature ͑50–70°C͒ and
pressure ͑4.0 Pa͒ regime provides for excellent conformality over
high-aspect-ratio trenches and produces pure microcrystalline Co
thin films which are free of carbon contamination.
Conclusion
We investigated the effects of various experimental variables,
including the pressure and temperature, on the growth rate, confor-
mality, and film quality of Co films. The observed dependence of the
growth rate on the temperature and pressure shows the existence of
a transition temperature below which the deposition process is lim-
ited by the surface reaction and above which it is limited by the
gas-phase collisional activation. Lowering the pressure significantly
increases the transition temperature and thus widens the temperature
window for excellent conformality. The 26.7-Pa process shows the
transition temperature of about 50°C and provides for excellent con-
formality only at 50°C. Lowering the pressure to 4.0 Pa increases
the transition temperature up to 70°C and widens the temperature
window, which allows for the conformal deposition of Co. The wid-
ening of the temperature window for conformal deposition of Co
may be due to the effective suppression of the gas-phase reaction in
the extremely low pressure region. The growth rate of the conformal
Co films was 3.5–7.0 nm/min, depending on the temperature and
pressure. In addition, the as-deposited Co films deposited at 4.0 Pa
and 50°C have a resistivity of 10 ␮⍀ cm and low carbon content in
it, indicating that the low temperature allows for the complete de-
composition of the Co precursor. Therefore, it can be concluded that
this extremely low temperature process, which allows for the con-
formal deposition of high-quality Co films, can be employed to pro-
duce silicided contacts for the advanced devices, which require a
low contact resistance and good electrical performance.
Figure 7. Variation in the resistivity of about 100-nm-thick Co thin films
deposited on SiO2-coated Si wafers as a function of the vacuum-annealing
temperature.
600°C. Figure 8 shows the XRD patterns of the annealed Co films at
various temperatures. The as-deposited Co films consist of micro-
crystalline ␣-Co ͑hexagonal close-packed͒ structure, and the inten-
sities of the ␣-Co phase increase as the annealing temperature is
increased up to 400°C, corresponding to grain growth in the tem-
perature range of 100–400°C. In addition, annealing at 600°C trans-
formed the ␣-Co structure into ␤-Co ͑face centered cubic͒ with a
strong Co͑111͒ texture, indicating that a large amount of grain
growth occurred by surface minimization. From these results, it can
Acknowledgments
This work was supported in part by the ERC program of MOST/
KOSEF ͑R11-2005-048-00000-0͒ and in part through the Center for
Nanostructured Materials Technology by the Korean Ministry of
Science.
Kookmin University assisted in meeting the publication costs of this
article.
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Figure 8. XRD patterns of about 100-nm-thick Co films deposited on
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