Vapor deposition of ruthenium from an amidinate precursor

Article abstract of DOI:10.1149/1.2789294

Atomic layer deposition (ALD) and pulsed chemical vapor deposition (CVD) were used to make ruthenium (Ru) thin films from a volatile Ru amidinate precursor, bis (N, N′ -di-tert-butylacetamidinato)ruthenium(II) dicarbonyl. The CVD films were grown without any coreactant, while the ALD films used ammonia as a coreactant. The films are fine-grained polycrystalline ruthenium with high purity (<0.2% impurities). Ru grew as a continuous, electrically conductive, pinhole-free film on tungsten nitride (WN) films even for films as thin as 2 nm. The resistivities of the films match those of pure sputtered ruthenium of the same thickness. Roughness is <2% of the film thickness. The films are very conformal, with 80% step coverage over holes with high aspect ratios (40:1). This thermal process does not use any oxidant or plasma as a second reagent, thereby avoiding damage to sensitive substrates. The ALD growth rate can reach 1.5 Acycle at a substrate temperature of 300°C.

Full text of DOI:10.1149/1.2789294

D642
Journal of The Electrochemical Society, 154 ͑12͒ D642-D647 ͑2007͒
0013-4651/2007/154͑12͒/D642/6/$20.00 © The Electrochemical Society
Vapor Deposition of Ruthenium from an Amidinate Precursor
,z
Huazhi Li,a Damon B. Farmer,b Roy G. Gordon,a, Youbo Lin,
and
*
b,**
Joost Vlassakb
aDepartment of Chemistry and Chemical Biology, and bSchool of Engineering and Applied Sciences,
Harvard University, Cambridge, Massachusetts 02138, USA
Atomic layer deposition ͑ALD͒ and pulsed chemical vapor deposition ͑CVD͒ were used to make ruthenium ͑Ru͒ thin films from
a volatile Ru amidinate precursor, bis͑N,N -di-tert-butylacetamidinato͒ruthenium͑II͒ dicarbonyl. The CVD films were grown
Ј
without any coreactant, while the ALD films used ammonia as a coreactant. The films are fine-grained polycrystalline ruthenium
with high purity ͑Ͻ0.2% impurities͒. Ru grew as a continuous, electrically conductive, pinhole-free film on tungsten nitride ͑WN͒
films even for films as thin as 2 nm. The resistivities of the films match those of pure sputtered ruthenium of the same thickness.
Roughness is Ͻ2% of the film thickness. The films are very conformal, with 80% step coverage over holes with high aspect ratios
͑40:1͒. This thermal process does not use any oxidant or plasma as a second reagent, thereby avoiding damage to sensitive
substrates. The ALD growth rate can reach 1.5 Å/cycle at a substrate temperature of 300°C.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2789294͔ All rights reserved.
Manuscript submitted May 9, 2007; revised manuscript received August 9, 2007. Available electronically October 11, 2007.
Thin films of ruthenium have many current and potential appli-
reagent to avoid the formation of interfacial metal oxides, but a
plasma may not achieve high step coverage because of rapid recom-
bination of plasma-generated radicals on the surfaces of features.15
Also, plasma reactors are more complicated than thermal reactors,
and plasmas can damage substrates.
cations. They can be used as electrodes for capacitors in which their
high work function leads to low leakage currents.1 Furthermore, the
conductive nature of RuO2 interlayers with oxide insulators pro-
motes high capacitance density. As gate electrodes, the high work
function of Ru provides low threshold voltages for PMOS
transistors.2,3 Because Ru has a low solid solubility in copper and
strong adhesion to copper, it could be used as a seed and/or adhesion
layer for copper interconnects.4 Multilayer magnetoresistance struc-
tures, such as those found in read/write heads in hard drives and in
magnetic memory ͑MRAM͒, can use Ru as a nonmagnetic spacer
between magnetic layers.5 Some plasma displays incorporate Ru
films.6 Heterogeneous catalysts can be made from Ru films depos-
ited on a support.7
Physical vapor deposition ͑PVD͒ methods, such as sputtering,
can be used to deposit ruthenium films. Because of the directional
nature of the deposition flux and the high sticking probability, PVD
technology cannot deposit ruthenium inside narrow holes and com-
plex structures. Ru films with better conformity are sometimes
achieved by using chemical vapor deposition ͑CVD͒. Atomic layer
deposition ͑ALD͒ can make completely conformal coating even in
structures with high aspect ratios.
Here we present a vapor deposition process for ruthenium
using bis͑N,N -di-tert-butylacetamidinato͒ruthenium͑II͒ dicarbonyl,
Ј
Ru͑tBu-Me-amd͒ ͑CO͒ 1, as shown in Fig. 1. This precursor can
2
2
be used alone in a pulsed CVD mode or with reducing agents, such
as molecular hydrogen or ammonia in a mixed CVD-ALD mode.
The processes are thermally activated and do not require the use of
oxygen, ozone, or plasmas.
Experimental
Pulsed CVD of ruthenium was carried out by thermal decompo-
sition of bis͑N,N -di-tert-butylacetamidinato͒ruthenium͑II͒ dicarbo-
Ј
nyl, Ru͑tBu-Me-amd͒ ͑CO͒2 without any other reactant or reducing
2
agent. The detailed synthesis and characterization of the precur-
sor will be published separately.16 The compound is monomeric,
with a distorted octahedral structure. The vapor pressure of
Ru͑tBu-Me-amd͒ ͑CO͒ is ϳ0.05 Torr at 130°C, based on subli-
2
2
Many CVD Ru processes have been reported. Precursors based
mation data. The Ru precursor ͑10 g͒ was placed in a stainless steel
container with a volume of 130 cm3 and heated ͑typically, to a tem-
perature of 130°C͒ in an oven. Nitrogen carrier gas ͑2.5
ϫ 10−4 mol͒ was admitted to the headspace volume above the pre-
cursor, resulting in a gas mixture of 1 mol % Ru precursor and
99 mol % N2 at a total pressure of ϳ6.4 mbar. Meanwhile, the
deposition chamber ͑a stainless steel tube with volume of 386 cm3͒
in a tube furnace was pumped down to a base pressure of
ϳ0.05 mbar and then an air-actuated valve between the deposition
chamber and the pump was closed, admitting a dose of 1.8
ϫ 10−6 mol of Ru precursor to the deposition chamber. The coated
area ͑including the substrates, the substrate holder, and the heated
chamber walls͒ was 686 cm2; thus, the dose was 0.26 nanomol/cm2.
The valve between the precursor source and the deposition chamber
was next opened for 2 s and then closed. After an additional period
of 2 s, the valve between the deposition chamber and the pump was
opened for 25 s, and 5.6 ϫ 10−4 mol of nitrogen purging gas was
admitted to help sweep out by-products and any unreacted precursor
vapor. The exposure of the Ru precursor to the surfaces was
0.086 mbar s. This CVD dose was then repeated a specified number
of times.
on the cyclopentadienyl ͑Cp͒ ligands, such as RuCp2 and Ru͑EtCp͒
2
͑Et = ethyl͒, have been investigated.8 Ruthenium tris-␤-diketonates
were also widely studied in CVD ruthenium and ruthenium
oxide films.9 These include Ru͑acac͒ ͑acac = 2,4-pentanedionate͒,
3
Ru͑thd͒ ͑thd = 2,2,6,6-tetramethyl-3,5-heptanedionate͒, Ru͑tfa͒
3
3
͑tfa = 1,1,1-trifluoro-2,4-pentanedionate͒, and Ru͑OD͒
͑OD
3
= 2,4-octanedionate͒. Ru3͑CO͒12, Ru͑CO͒ ͑hfb͒ ͑hfb = hexafluoro-
4
2-butyne͒,
Ru͑CO͒ ͑hfac͒
͑hfac = 1,1,1,5,5,5-hexafluoro-
2
2
2,4-pentanedionate͒, and Ru͑CO͒ ͑thd͒2 were also tried as CVD Ru
2
precursors.10 However, these precursors either lack thermal stability,
do not produce pure films, and/or fail to provide high step coverage.
Film growth in ALD is based on alternating, self-limited surface
reactions. When precursors show this self-limiting growth mecha-
nism, the films deposited by ALD have excellent conformality and
superior uniformity. ALD Ru based on precursors RuCp2,11
14
Ru͑EtCp͒2,12 Ru͑thd͒,13 and Ru͑OD͒
have been the subject of
3
recent studies. However, all these processes require using O2 or
ozone as a second reagent, which can potentially form interfacial
metal oxide films that can cause interconnect failure or increase
contact resistance. NH3 plasma has also been tried as the second
ALD of ruthenium was carried out with the same equipment as
was used for the pulsed CVD of Ru. The doses of Ru precursor
vapor were alternated with doses of ammonia gas. The ammonia
dose from a measured volume ͑22 cm3 at a pressure of 1.24 bar͒
flowed into the evacuated deposition region, where it was also con-
*
Electrochemical Society Active Member.
Electrochemical Society Student Member.
**
z E-mail: gordon@chemistry.harvard.edu
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Journal of The Electrochemical Society, 154 ͑12͒ D642-D647 ͑2007͒
D643
Figure 2. Ru growth rate with NH3 ͑ALD͒ or without NH3 ͑pulsed CVD͒ at
different temperatures.
cycle was ϳ5 ␮m. Prior to spin-coating the epoxy, 40 nm of Ti and
400 nm of Cu were sputter deposited onto the Ru to increase the
distance between the film stack and the epoxy. All four-point bend
tests were performed in dry N2 at 25°C at a crosshead speed of
0.3 ␮m/s. All experimental loading curves displayed well-
developed load plateaus during crack extension allowing the deter-
mination of the adhesion energy.18
Results and Discussion
Film growth.— Pulsed CVD consisted of ͑a͒ supplying
a
dose of Ru precursor vapor ͑bis͑N,N -di-tert-butylaceta-
1
Ј
midinato͒ruthenium͑II͒ dicarbonyl͒ mixed with N2 carrier gas to a
uniformly heated chamber containing substrates; ͑b͒ purging the
deposition chamber with a vacuum pump and the assistance of a
dose of pure N2 gas. Figure 2 shows the pulsed thermal CVD
growth of Ru from precursor 1 on thermal SiO2 substrates. Above
200°C there was substantial deposition, while below 200°C there
was little or no deposition. At 300°C, the observed CVD rate was
ϳ0.4 Å/pulse. Figure 3 shows the Arrhenius representation of the
Figure 1. ͑Color online͒ ͑a͒ Chemical formula of Ru precursor
bis͑N,N -di-tert-butylacetamidinato͒ruthenium͑II͒ dicarbonyl,
Ј
Ru͑Bu-Me-amd͒ ͑CO͒2. and ͑b͒ its molecular structure by X-ray crystallog-
2
raphy ͑hydrogen atoms not shown͒.
fined for a time period ͑typically 3 s͒ before purging. The dose of
ammonia was 1.06 ␮mol cm−2, and the exposure was 200 mbar s.
The substrates used had 300 nm of thermal oxide on Si͑100͒.
Before deposition, the substrates were treated for 4 min with UV/
ozone to remove organic contaminants. ALD WN as a diffusion
barrier and adhesion layer was deposited on the SiO2 substrates at
380°C using ALD with bis͑tert-butylimido͒bis͑dimethylamido͒-
tungsten͑VI͒ and ammonia gas as precursors.17 Typically, 10 cycles
were used to make a WN layer of ϳ2.5 nm thick.
Some Ru samples were annealed in a rapid thermal annealing
͑RTA͒ furnace at 500 and 750°C for 1 min in a vacuum ͑14.7 Torr͒.
The crystal orientation of the as-deposited and annealed Ru films
was evaluated by using a ␪-2␪ X-ray diffractometer ͓͑XRD͒, Scin-
tag. Inc. ͑now Thermo Optek Corp.͒, Cupertino, CA, USA͔ with Cu
K␣ radiation generated at 40 kV and 30 mA. Film resistivity was
calculated from the sheet resistance measured with a four-point
probe and the film thickness. The film thickness was measured using
2 MeV He+ Rutherford backscattering spectroscopy ͑RBS͒. TEM
pictures were taken with Ru/WN stacks on amorphous SiO2 or
Si3N4 membrane grids supplied by SPI Supplies.
Adhesion measurements were performed using the four-point
bend technique. Test specimens were prepared by bonding Si wafers
to the substrates with the Ru/WN/SiO2 film stack using a spin-on
epoxy ͑EPOTEK353ND͒. The epoxy was cured at 90°C for 60 min
under a pressure of 8 kPa. The epoxy thickness after the curing
Figure 3. ͑Color online͒ Pulsed CVD growth rate of Ru without using NH3
͑᭿͒ and ALD growth rate ͑b͒ as a function of temperature.
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D644
Journal of The Electrochemical Society, 154 ͑12͒ D642-D647 ͑2007͒
CVD rate as a function of the substrate temperature for the CVD
process. Over a wide range of temperatures ͑200–300°C͒, the curve
is linear, which means that the CVD rate is kinetically controlled in
that temperature range. From these data, the activation energy of the
CVD process was calculated to be 81 kJ/mol.
NH3 is an optional coreactant, but using NH3 can boost the
growth rate of Ru films as shown in Fig. 2 and 3. A typical ALD
cycle was composed of four separate steps: ͑a͒ one dose of 1 using
N2 injected into the headspace of the bubbler to assist the transport
of the precursor vapor into the reactor; ͑b͒ N2 purge; ͑c͒ one dose of
NH3; ͑d͒ N2 purge. Figure 2 demonstrates that in the ALD process
with NH3 as a second reagent, the deposition rate is somewhat
higher than with pulsed CVD ͑self-decomposition͒ of the precursor
without any coreactant. The ALD growth per cycle was further in-
creased to 1.5 Å/cycle by increasing the size of the vapor doses, by
using a bubbler with a larger head-space and containing more pre-
cursor. Under these optimized conditions, the vapor concentration in
the reactor increases and, thus, the pulsed CVD pathway provides
more growth. In other words, the ALD growth is not self-limiting
because it contains a CVD pathway as well as a self-limiting ALD
reaction.
Because the decomposition of the precursor on the surface is
slow, the film thickness can still be uniform and conformal if the
temperature of the substrate is uniform, even though the growth is
not self-limiting. The thickness nonuniformity of films over a 25 cm
long deposition zone was Ͻ5% at 300°C. Above 300°C, the thick-
ness nonuniformity was Ͼ5% ͑ϳ15% at 325°C͒. The less uniform
thickness at higher temperatures may be due to the surface reactions
becoming so fast that the growth is limited by transport in the vapor,
rather than by surface reactions. As a result, 300°C was used as the
optimized growth temperature.
Figure 4. ͑Color online͒ SIMS depth profiles of a ruthenium film 20 nm
thick deposited on thermal SiO2 at 300°C; the observed carbon and oxygen
counts/second have been converted to concentrations; the ruthenium curve
represents the signal in counts per second because a sensitivity factor is not
known for Ru.
A proposed mechanism of the thermal decomposition is shown in
Eq. 1. Here, the ruthenium precursor decomposes by releasing car-
bon monoxide and the ligand dimer 2, which joins at the N atoms
while giving off two electrons to reduce the adjacent Ru͑II͒ to
Ru͑0͒. This ligand dimer was isolated and the structure 2 illustrated
AFM image of the film morphology. The root-mean-square ͑rms͒
roughness is 1.5 nm for an 87 nm thick film deposited at 300°C,
which corresponds to a fairly smooth film.
Examination of thicker films by XRD showed that they are poly-
crystalline ruthenium films. Figure 6 shows X-ray diffraction spectra
measured from a 180 nm thick film deposited at 300°C. These films
were randomly oriented. Annealing these films at 500 and 750°C in
N2 at 15 Torr for 1 min did not cause the XRD spectrum to change
significantly, indicating that Ru recrystallization is negligible at
these temperatures.
1
in Eq. 1 was confirmed by H NMR as well as high-resolution mass
spectroscopy and elemental analysis. Additional evidence support-
ing this mechanism was acquired by using carbon monoxide as the
carrier gas during deposition. When this was done, the Ru deposi-
tion rate was decreased by almost half, which means that the leaving
of carbon monoxide is reversible, and is relevant to the kinetics of
the deposition reaction
The TEM image of a 30 nm thick film on a Si3N4 membrane
shows that the grain size is about 5–20 nm ͑Fig. 7͒. Because the film
was deposited on both sides of the membrane, some Moire patterns
appear due to interference between grains in the two films. These
investigations lead to the conclusions that the Ru films are smooth,
nanocrystalline, and structurally stable.
͓1͔
A test of film continuity was made by depositing a 2 nm thick Ru
film on a 3 nm thick freshly deposited tungsten nitride film on a
30 nm Si3N4 membrane. The TEM of this structure in Fig. 8 shows
that the Ru grains on WN are much smaller and more densely
spaced than on Si3N4 ͑Fig. 7͒. This sample was then submerged in a
WN etching solution ͓H2O2:NH3: H2O ͑1:1:5͔͒ for 10 min. In a
separate experiment, this solution was found not to etch Ru films.
Energy dispersive X-ray spectroscopy ͑EDX͒ analysis revealed the
presence of Ru and W both before and after submersion in the
etchant. In contrast, a WN film that was not capped with a 2 nm Ru
layer was completely etched in the same solution within seconds.
These results show that there are no gaps or pinholes in this very
thin Ru film. TEM analysis of the Ru-capped film before and after
etching showed no difference in terms of the size or density of
nuclei.
The mechanism by which the NH3 enhances the ALD deposition
rate is not known.
Film properties.— RBS analyses of films on glassy carbon sub-
strates show no evidence of carbon, nitrogen, or oxygen in either the
CVD or ALD films, which indicates that NH3 molecules are not
incorporated in the films ͑data not shown here͒. The lack of nitrogen
incorporation is not surprising because ruthenium does not have any
stable nitride phase. Secondary ion mass spectra ͑SIMS͒ in Fig. 4
show that the impurity content of the ruthenium films grown at
300°C is very low. The carbon content in the films is ϳ0.05% while
the oxygen content is ϳ0.2%. These results verify that the surface
reactions are fully complete under the conditions used and that ru-
thenium is not oxidized during the deposition process or in the am-
bient environment.
To extend on these results, the layers were intentionally pierced
with holes and then left in the etching solution for ϳ12 h to ensure
complete dissolution of the WN by undercutting from the edges of
Atomic force microscopy ͑AFM͒ was employed to study the sur-
face roughness of the as-deposited Ru thin films. Figure 5 shows an
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Journal of The Electrochemical Society, 154 ͑12͒ D642-D647 ͑2007͒
D645
Figure 6. XRD patterns obtained from a Ru film as deposited and after
annealing.
films do not have any significant effect on their electrical resistance.
One of the challenges in multilevel metallization of the sub-
90 nm generation devices is the void-free filling of high aspect ratio
contact holes. Nonconformal Ru films can cause shoulders near the
top of the holes, leading to void formation when the holes are filled
by Cu or W. The CVD and ALD Ru films were highly conformal
inside holes in Si with aspect ratios of 40:1. Figure 11 shows a
scanning electron microscopy ͑SEM͒ image of cleaved cross sec-
tions of ALD Ru films inside UV/ozone treated Si holes. The film
thickness at the top of the holes is found to be 50 nm, while the
thickness at the middle and bottom of the holes is 40 nm. This gives
a step coverage of 80%. The deviation away from ideal 100% step
coverage may be due to the partial self-decomposition of the pre-
cursor or to insufficient exposure to the precursor.20
Figure 5. ͑Color online͒ AFM image of a 87 nm ruthenium film grown at
300°C: ͑a͒ two dimensional view and ͑b͒ three dimensional view.
Ru adhesion energies are listed in Table I. The Ru/SiO2 interface
can be separated by Scotch tape. Four-point bend tests for Ru/SiO2
the holes. After etching, some fragments of the Ru film settled back
onto the substrate. Figure 9 shows a TEM image of one of these
pieces of Ru film that had been released from the substrate by dis-
solution of the WN. The Ru film is clearly continuous, free of pin-
holes, and self-supporting. The existence of these intact fragments is
further proof of the continuity and uniformity of the 2 nm thick Ru
films.
The in-plane resistivities of the Ru films on WN were measured
with a four-point probe. A small correction was made for the parallel
conduction through the WN. The Ru resistivity increases with de-
creasing film thickness owing to several effects: grain boundary
scattering, electron-surface scattering, surface-roughness-induced
scattering, and impurity scattering. The curve in Fig. 10 is calculated
from a simple model that includes only grain-boundary scattering19
␳/␳ = 1 + 1.5͕R/͑1 − R͖͒͑␭/g͒
͓2͔
0
where ␳0 is the bulk resistance of 7.1 ␮⍀ cm, ␭ is the electron mean
free path ͑estimated to be 10 nm͒, g is the average size of the grains
͑approximated as the film thickness͒, and Rϳ 0.58 is the grain-
boundary-scattering coefficient ͑obtained by fitting the data in Fig.
10͒. It is remarkable that the data for Ru films produced by ALD
follow this model down to thicknesses of 2 nm. This agreement
between measured conductivity and the model suggests that the ru-
thenium film must be continuous and nearly uniformly covering the
WN surface. The agreement between the resistivity of the ALD Ru
films and pure sputtered Ru films shows that impurities in the ALD
Figure 7. TEM images of a Ru film ͑30 nm͒ deposited over Si3N4 mem-
brane ͑30 nm͒.
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D646
Journal of The Electrochemical Society, 154 ͑12͒ D642-D647 ͑2007͒
Figure 8. TEM of a Ru film grown on freshly deposited tungsten nitride on
a Si3N4 TEM membrane.
Figure 10. The resistivity of Ru thin films as a function of Ru film thickness
on WN substrates. ᭿ are data from this study, b are data from sputtered Ru.
21
give an adhesion energy of Ͻ3 J m−2
.
In contrast, the
Acknowledgments
Ru/WN/SiO2 stack passes the tape test and exhibits strong adhe-
sion. Four-point bend tests indicate that the WN/SiO2 interface has
an adhesion energy of 17.6 3.8 J/m2, where the location of the
adhesive failure was confirmed by X-ray photoelectron spectro-
scopic analysis of both interfaces exposed by the crack. It was not
possible to break the Ru/WN interface in the film stack under the
same testing conditions.
We appreciate the help with the AFM imaging from Dr. Zheng-
wen Li and Qiaobing Xu. Y.L. acknowledges support from SRC,
contract no. 2005-KC-1292.010. A portion of the work presented
here was carried out at the Center for Nanoscale Systems at Harvard
University. This work was supported in part by the National Science
Foundation.
Harvard University assisted in meeting the publication costs of this
article.
Conclusion
We conclude that CVD and ALD of Ru on WN can provide
highly conformal, continuous, and strongly adherent layers that are
suitable for many applications. The process does not require oxidiz-
ing conditions or plasmas, which can damage substrates. The Ru
films nucleate densely on freshly deposited WN and are continuous
and pinhole-free at thicknesses down to 2 nm. The polycrystalline
films are substantially free of impurities and have resistivities
matching those of pure sputtered Ru films.
Figure 9. TEM image of a fragment of a Ru film of after etching of WN in
a pierced film of Ru͑2 nm͒/WN͑3 nm͒/SiO2 deposited on Si3N4 membrane
͑30 nm͒. The darker areas are regions where the film folded over on itself.
Figure 11. Ru films deposited inside the high aspect ratio ͑40:1͒ trench
structure showing excellent conformality of pulsed CVD process. The white
bar on the bottom of each image represents 200 nm.
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Journal of The Electrochemical Society, 154 ͑12͒ D642-D647 ͑2007͒
D647
8. M. L. Green, M. E. Gross, L. E. Papa, K. J. Schnoes, and D. Brasen, J. Electro-
chem. Soc., 132, 2677 ͑1985͒; T. Aoyama and K. Eguchi, Jpn. J. Appl. Phys., Part
2, 38, L1134 ͑1999͒.
Table I. Adhesion measurements for ALD and CVD multilayer
structures. Adhesion energy was evaluated by four-point bend
tests.
9. J. C. Viguié and J. Spitz, J. Electrochem. Soc., 122, 585 ͑1975͒; J. Vetrone, C. M.
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Adhesion energy
Structure
Scotch tape test
͑J/m2͒
Ru/SiO2
WN/SiO2
Ru/WN/SiO2
Failed
Passed
Passed
Ͻ3
17.6 3.8 J/m2
Ͼ17.6 J/m2
11. T. Aaltonen, P. Alén, M. Ritala, and M. Leskelä, Chem. Vap. Deposition, 9, 45
͑2003͒.
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G109 ͑2004͒.
13. T. Aaltonen, M. Ritala, K. Arstila, J. Keinonen, and M. Leskelä, Chem. Vap. Depo-
sition, 10, 215 ͑2004͒.
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