Diffusion barrier properties of tungsten nitride films grown by atomic layer deposition from bis(tert-butylimido)bis(dimethylamido)tungsten and ammonia

Article abstract of DOI:10.1063/1.1565699

The synthesis of the highly uniform, smooth and conformal coatings of tungsten nitride (WN) using atomic layer deposition (ALD) from vapors of bis(tert-butylimido)bis(dimethylamido)tungsten and ammonia was discussed. The diffusion barrier properties of th

Full text of DOI:10.1063/1.1565699

Diffusion barrier properties of tungsten nitride films grown by atomic layer deposition
from bis(tert-butylimido)bis(dimethylamido)tungsten and ammonia
Jill S. Becker and Roy G. Gordon
Citation: Applied Physics Letters 82, 2239 (2003); doi: 10.1063/1.1565699
View online: http://dx.doi.org/10.1063/1.1565699
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APPLIED PHYSICS LETTERS
VOLUME 82, NUMBER 14
7 APRIL 2003
Diffusion barrier properties of tungsten nitride films grown by atomic
layer deposition from bis„tert-butylimido…bis„dimethylamido…tungsten
and ammonia
Jill S. Becker and Roy G. Gordona)
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138
͑Received 27 November 2002; accepted 14 February 2003͒
Highly uniform, smooth, and conformal coatings of tungsten nitride ͑WN͒ were synthesized by
atomic layer deposition ͑ALD͒ from vapors of bis͑tert-butylimido͒bis͑dimethylamido͒tungsten and
ammonia. The films are shiny, silver colored, and electrically conducting. The films were amorphous
as deposited. 100% step coverage was obtained inside holes with aspect ratios greater than 40:1.
WN films as thin as 1.5 nm proved to be good barriers to diffusion of copper for temperatures up
to 600 °C. Annealing for 30 min at temperatures above 725 °C converted the WN to pure,
polycrystalline tungsten metal. ALD of copper onto the surface of the WN produced strongly
adherent copper films that could be used as ‘‘seed’’ layers for chemical vapor deposition or
electrodeposition of thicker copper coatings. © 2003 American Institute of Physics.
͓DOI: 10.1063/1.1565699͔
Tungsten nitride ͑WN͒ is considered to be a good barrier
against diffusion of copper in microelectronic circuits.1 WN
can also be used in electrodes for thin-film capacitors and
field-effect transistors.2 WN has been made by reactive sput-
tering, but the uniformity of film thickness inside narrow
features ͑‘‘step coverage’’͒ is not expected to be adequate for
use in future microelectronic devices having narrow features
with high aspect ratios.3
Atomic layer deposition ͑ALD͒ ͑also known as atomic
layer epitaxy͒ is a process for depositing thin layers of solid
materials from two or more vapor precursors.4 The surface of
a substrate onto which film is to be deposited is exposed to a
dose of vapor from one precursor. Then any excess unreacted
vapor from that precursor is pumped away. Next, a vapor
dose of the second precursor is brought to the surface and
allowed to react. This cycle of steps can be repeated to build
up thicker films. One particularly important aspect of this
process is that the ALD reactions are self-limiting, in that
only a certain maximum thickness can form in each cycle,
after which no further deposition occurs during that cycle,
even if excess reactant is available. Because of this self-
limiting character, ALD reactions produce coatings with
highly uniform thicknesses. Uniformity of ALD film thick-
nesses extends not only over flat substrate surfaces, but also
into narrow holes and trenches. This ability of ALD to make
conformal films is called ‘‘good step coverage.’’
In the present work, WN barrier films were made by
ALD using bis͑tert-butylimido͒bis͑dimethylamido͒tungsten,
(tBuN)2(Me2N)2W, and ammonia at low substrate tempera-
tures (250–350 °C). The basic bulk properties of these films
were investigated, as well as their performance as a barrier to
the diffusion of copper.
The flow reactor shown in Fig. 1 was used to deposit
the tungsten nitride coatings by ALD. Bis͑tert-butyl-
imido͒bis͑dimethylamido͒tungsten͑VI͒,6 with a vapor pres-
sure of 0.037 Torr at 30 °C, was placed in a stainless steel
container ͑W͒. A detailed description of the ALD system is
given elsewhere.6
The thickness per cycle values for ALD WN films, as
determined by scanning electron microscopy ͑SEM͒, was
0.05 nm at 300 °C, and 0.1 nm at 350 °C.7 Both growth rates
are smaller than an ideal monolayer ͑ML͒/cycle. ͑With the
atomic density of tungsten nitride dϭ5.68ϫ1022 cmϪ3, 1
ML can be defined as dϪ1/3ϭ0.26 nm by assuming simple
Coatings of WN made by ALD from WF6 and NH3 have
good step coverage.5 A disadvantage of this process is that
WF6 and/or its reaction byproduct, HF, attacks substrates
made of Si or SiO2 . Also, this process can leave the WNx
surface with a fluorine residue that may impede adhesion of
copper to the surface. In particular, adhesion of Cu deposited
by chemical vapor deposition ͑CVD͒ is often considered to
be poor in part because of fluorine contamination at the in-
terface between the tungsten nitride and the copper.
FIG. 1. Schematic diagram of the deposition system showing the location of
the nitrogen mass flow controllers and the nitrogen source (N2), the ammo-
nia vapor reservoir (NH3), the two-position, three-way gas-chromatography
valve, the three on-off diaphragm valves, the nitrogen assist dose volume
(V), the tungsten precursor ͑W͒, the pressure measurement (P), and the
rotary vane pump ͑pump͒ with respect to the precursor T(precursors) and
deposition T(deposition) isothermal heating zones.
͓ ͔
͓
͔
a͒
Electronic mail: gordon@chemistry.harvard.edu
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0003-6951/2003/82(14)/2239/3/$20.00 2239 © 2003 American Institute of Physics
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2240
Appl. Phys. Lett., Vol. 82, No. 14, 7 April 2003
J. S. Becker and R. G. Gordon
FIG. 3. A cross-sectional scanning electron micrograph of holes 0.18 ␮m in
diameter and 7.3 ␮m deep in a silicon wafer uniformly coated with a 33-
nm-thick tungsten nitride film as deposited at 300 °C ͑600 cycles͒. An ex-
posure of approximately 2.1ϫ104 langmuirs/cycle was used for the tungsten
precursor and 2.9ϫ107 langmuirs/cycle for ammonia.
FIG. 2. ͑a͒ HRTEM image of a 20-nm-thick tungsten nitride film as depos-
ited at 300 °C ͑400 cycles͒. The reference bar is 20 nm long. ͑b͒ HRTEM
image of a 14-nm-thick tungsten film. Note that the film has fully crystal-
lized and that it has shrunk from 20 nm ͓see Fig. 2͑a͔͒ to 14 nm ͑about
30%͒. The reference bar is 20 nm long.
A SEM picture was taken of a wafer with narrow holes
͑0.18 ␮m in diameter and 7.3 ␮m deep͒ having aspect ratio
greater than 40:1 coated with WN and then cleaved to show
a cross section of the coated holes. The SEM in Fig. 3 shows
that the walls of the narrow hole are covered with a perfectly
conformal coating. This result demonstrates the excellent
step coverage achieved by ALD utilizing this new tungsten
precursor.
cubic packing.͒ This is probably due to the steric bulk of the
(tBuN)2W(NMe)2 limiting the surface adsorption density.
The films are pure tungsten nitride. Rutherford back-
scattering spectroscopy8 ͑RBS͒ showed that there was no
appreciable oxygen in the film. Carbon was less than the
detectable limit, Ͻ 12 at. %. X-ray photoelectron spectro-
scopy9 ͑XPS͒ confirmed that no carbon was detectable in the
film ͑detection limit Ͻ 14 at. %͒. The films were silver and
shiny. The density of the material was found to be about
12 g/cm3 by combining RBS and SEM data. RBS deter-
The resistivity15 of typical ALD tungsten nitride coating
is about 1.5ϫ10Ϫ3 –4.0ϫ10Ϫ3 ⍀ cm. The resistivity was re-
duced to 4.8ϫ10Ϫ4 –9.6ϫ10Ϫ4 ⍀ cm by annealing in form-
ing gas at 700 °C for 30 min to form the polycrystalline WN
phase. The resistivities were even further reduced to 1.2
ϫ10Ϫ4 –5.0ϫ10Ϫ4 ⍀ cm when the films were annealed at
temperatures greater than 725 °C for 30 min. The annealed
film, at temperatures greater than 725 °C, lost its nitrogen to
become pure tungsten metal and crystallized to the cubic
tungsten structure. RBS and XPS confirmed the loss of ni-
trogen. XRD too confirmed that once the as-deposited amor-
phous films were annealed at temperatures greater than
725 °C, they contained the diffraction peaks corresponding
to the known cubic, crystalline phase of W.12,16 A decrease in
thickness ͑30%͒ occurred as the films turned from WN into
the more dense W films. This decrease in thickness can be
seen by comparing the HRTEM images of the same sample
before annealing, when the film is still just WN ͓Fig. 2͑a͔͒,
and after annealing, when the film has turned to pure W
metal ͓Fig. 2͑b͔͒. The polycrystalline W films remained
smooth as was seen by AFM ͑rms roughnessϭ0.34 nm). The
annealed films remained adherent to the substrates.
mined the chemical composition of the film to be WN1.1Ϯ0.1
.
X-ray diffraction10 ͑XRD͒ confirmed that once the as-
deposited amorphous films were annealed they contained the
diffraction peaks corresponding to the known cubic, crystal-
line phase of WN.11,12 The x-ray diffraction peaks were still
very broad for the crystallized material, indicating that the
film is composed of nanograins smaller than 20 Å.
The WN films possessed very smooth surface character-
istics. Atomic force microscopy13 ͑AFM͒ confirmed that the
surface roughness of the deposited layer ͑rms roughness
ϭ0.33 nm) was very similar, if not equal to that of the sub-
strate ͑rms roughnessϭ0.2–0.3 nm) on which it was depos-
ited. The RMS roughness was independent of film thickness.
SEM also showed that the films were smooth and featureless.
XRD showed that the as-deposited layer was amorphous.
This structural information was confirmed by high-resolution
transmission electron microscopy ͑HRTEM͒,6,14 which
When the tungsten nitride film was deposited directly on
silicon, without an oxide interlayer produced by the UV-
ozone treatment, annealing the WN to a temperature of
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showed the film to be amorphous ͓Fig. 2͑a͔͒.
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Appl. Phys. Lett., Vol. 82, No. 14, 7 April 2003
J. S. Becker and R. G. Gordon
2241
by heating the sample to 500 °C in a hydrogen atmosphere
for 30 min. The resulting shiny copper layer adhered strongly
to the tungsten nitride, and could not be removed by adhe-
sive tape applied to the copper. This thin ALD copper could
be used as a ‘‘seed’’ layer to initiate CVD or electrodeposi-
tion of copper.
In conclusion, tungsten nitride films were synthesized
from
vapors
of
tungsten
bis͑tert-butylimide͒
bis͑dimethylamide͒ and ammonia gas supplied in alternate
doses to surfaces heated to 250–350 °C. This process pro-
duced coatings of tungsten nitride having very uniform
thickness and excellent step coverage in holes with aspect
ratios up to at least 40:1. The films are metallic electrical
conductors. Suitable applications in microelectronics include
barriers to the diffusion of copper and electrodes for capaci-
tors. Films as thin as 1.5 nm proved to be good barriers up to
600 °C.
FIG. 4. A sample annealed at 650 °C showed numerous crystals of copper
silicide due to the complete breakdown of the barrier.
The authors would like to thank Martin Gutsche of Infi-
neon Technologies for providing the etched wafer and the
SEMs of the coated holes shown in Fig. 3. Supported in part
by the National Science Foundation.
1000 °C produced an interfacial layer of tungsten silicide,
identified by RBS.12
The ALD tungsten nitride films were shown to be good
barriers to the diffusion of copper by the following tests. 100
nm of copper was sputtered on top of various films of tung-
sten nitride ranging in thickness from 1.5 to 100 nm on sili-
con substrates. Samples of these Si/WN/Cu structures were
annealed in forming gas for 30 min at various temperatures.
The copper on the surface was dissolved in nitric acid solu-
tion, and then the tungsten nitride was dissolved in ammonia/
hydrogen peroxide solution. Examination of the silicon by
SEM and RBS showed no change in samples annealed at
450, 500, 550, or 600 °C. A sample annealed at 600 °C
showed a few bright crystals of copper silicide due to iso-
lated breakdown of the tungsten nitride barrier at a few de-
fect sites. ͑None of these experiments were done in a clean-
room.͒ A sample annealed at 650 °C showed numerous
crystals of copper silicide due to complete breakdown of the
barrier ͑Fig. 4͒. These results are conventionally interpreted
to mean that the tungsten nitride is stable to 600 °C and is an
excellent barrier to the diffusion of copper.
Copper oxide was deposited on tungsten nitride films by
ALD from 1000 cycles of alternating exposure to copper͑II͒
bis͑sec-butylacetoacetate͒ vapor and an ozone/oxygen gas
mixture at a substrate temperature of 300 °C using an appa-
ratus described by Fig. 1 with the copper precursor in the
‘‘ALD bubbler’’ and the ozone/oxygen mixture from an
ozone generator passing through the two-position valve.
Copper oxide ͑CuO͒ was deposited at a rate of about 0.05
nm per cycle. The copper oxide was reduced to copper metal
1 M. Takeyama and A. Noya, Jpn. J. Appl. Phys., Part 1 36, 2261 ͑1997͒.
2 B. Park, M. Lee, K. Moon, H. Lee, and H. Kang, IEEE International
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3 Semiconductor Industry Association International. International Technol-
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4
¨
M. Ritala and M. Leskela, Deposition and Processing, Handbook of Thin
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5 J. W. Klaus, S. J. Ferro, and S. M. George, J. Electrochem. Soc. 147, 1175
͑2000͒.
6 J. S. Becker, S. Suh, and R. G. Gordon, Chem. Mater. ͑to be published͒.
7 Thickness measurements and step coverage ͑on cleaved samples͒ were
made by SEM ͑Leo 982; resolution of Ϯ3 nm).
8 Composition and number of atoms per unit area were determined by RBS
͑General Ionics model 4117, 1.7 MeV Tandetron͒ of samples grown on
glassy carbon substrates.
9 Composition was also verified by XPS ͑Surface Science Lab SSX-100͒
and energy dispersive x-ray spectroscopy ͑JEOL 2010F equipped with an
energy dispersive x-ray spectrometer͒.
10 XRD ͑Scintag model XDS2000͒ and electron diffraction by transmission
electron microscopy ͑JEOL 2010F͒ determined that the films are amor-
phous.
11 V. I. Khitrova and Z. G. Pinsker, Kristallografiya 4, 545 ͑1959͒.
12 J. S. Becker, Ph.D. thesis, Harvard University, Cambridge, MA, 2003.
13 Roughness measurements were made by AFM ͑Nanoscope III and IV,
Digital Instruments͒. Surface morphology was also determined by SEM.
14 High-resolution transmission electron microscopy were measured on a
JEOL 2010F.
15 Electrical resistivities were obtained on glass substrates using a four-point
probe ͑Veeco model No. FPP-100͒.
16 Swanson, Tatge, Natl. Bur. Stand. ͑US͒. Circ. 539, I, 28 ͑1953͒.
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