Full text of DOI:10.1063/1.114931

Influence of hydrogen on chemical vapor deposition of tungsten on sputterdeposited
TiN layers
S.L. Zhang, R. Palmans, J. Keinonen, C. S. Petersson, and K. Maex
Citation: Applied Physics Letters 67, 2998 (1995); doi: 10.1063/1.114931
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Influence of hydrogen on chemical vapor deposition of tungsten
on sputter-deposited TiN layers
S.-L. Zhanga)
Royal Institute of Technology, Department of Electronics, Box Electrum 229, S-164 40 Kista, Sweden
R. Palmans
IMEC, Kapeldreef 75, 3001 Leuven, Belgium
J. Keinonen
University of Helsinki, Accelerator Laboratory, H a¨ meentie 100, SF-00550 Helsinki, Finland
C. S. Petersson
Royal Institute of Technology, Department of Electronics, Box Electrum 229, S-164 40 Kista, Sweden
K. Maex
IMEC, Kapeldreef 75, 3001 Leuven, Belgium
͑
Received 5 June 1995; accepted for publication 7 September 1995͒
Tungsten ͑W͒ films are deposited on sputter-deposited TiN adhesion layers in a cold-wall chemical
vapor deposition reactor, initiated with the deposition of a W nucleation layer by SiH reduction of
4
WF . H is also introduced in the reactor for some depositions. The electrical resistivity and
6
2
mechanical stress of the W films are found to be dependent on the underlying TiN layers as well as
on the presence of H during W nucleation layer deposition. A higher resistivity is obtained when
2
the W is deposited on the TiN prepared at 250 °C than on the TiN prepared at 450 °C. For the W
deposited on the low-temperature TiN, the resistivity is reduced by adding H to the reactants during
2
W nucleation layer deposition; while for the W deposited on the high-temperature TiN, the
resistivity is almost insensitive to the H addition. More oxygen and fluorine are found at the
2
W–TiN interface for the W deposited on the low-temperature TiN than on the high-temperature TiN.
Introduction of H to the reactants during W nucleation layer deposition reduces the concentrations
2
of interfacial fluorine and oxygen, in agreement with thermodynamic predictions. A lower film stress
is obtained for the W deposited on the high-temperature TiN layers and/or with H addition. The W
2
films become less textured when H is introduced to the reactants during W nucleation layer
2
deposition. © 1995 American Institute of Physics.
Chemical vapor deposition ͑CVD͒ of tungsten ͑W͒ using
by several groups.11,12 Structural and mechanical properties
1
3
tungsten hexafluoride (WF ) is attractive for contact or via
have been of major concern. Our recent results show that
both the electrical resistivity and mechanical stress of W
films are strongly dependent on the structure of the underly-
ing TiN. In the present work, emphasis will mainly be placed
6
filling in very-large-scale integration ͑VLSI͒ technology, ow-
1,2
ing to its low film resistivity of about 10 ␮⍀ cm and low
deposition temperature typically at 250–450 °C.1 W depo-
–6
3
–9
on the impact of H addition to the reactants during W nucle-
sition from WF is sensitive to the surface conditions.
2
6
ation layer deposition on the electrical and mechanical prop-
erties of the W films. It will be shown that the amounts of
interfacial oxygen and fluorine are reduced and that the films
This makes selective W deposition difficult if the surface of
concern contains oxide and other contaminants even in trace
amounts. Selective deposition of W into contact windows of
various depths surrounded by dielectric materials further suf-
fers from W creeping and lateral growth on top of the surface
of the dielectric materials around shallow windows. There-
fore, blanket deposition of W is preferred because contact
windows of various depths can be filled up simultaneously.
This concept relies on the deposition of an adhesion layer
over the entire wafer surface prior to W deposition. Among
many alternatives, titanium nitride ͑TiN͒ is a fairly good con-
ductor with a resistivity of about 50 ␮⍀ cm, and it is ther-
mally stable which makes it a good diffusion barrier.10 TiN
adheres well on various surfaces and permits W deposition to
occur on its own surface using a nucleation step involving
become less textured by introducing a large amount of H to
2
the reactants during W nucleation layer deposition, leading to
a decrease in both film resistivity and film stress.
Silicon substrates used in this study were 125 mm diam
Si wafers with ͗100͘ orientation, covered with a 250 nm
thick plasma enhanced CVD silicon dioxide film deposited at
4
00 °C. Ti and TiN layers were sequentially sputter depos-
ited on the PECVD oxide, in the same sputter chamber with-
out breaking the vacuum between the two deposition steps.13
Two different TiN/Ti adhesion bilayer systems were pre-
pared. The low-temperature-type bilayer ͑denoted as low-T
TiN͒ consisted of 100 nm thick TiN on top of 20 nm thick Ti,
both deposited at 250 °C ͑wafer susceptor temperature͒,
whereas the high-temperature-type bilayer ͑denoted as
high-T TiN͒ was composed of 80 nm TiN on top of 30 nm Ti,
both deposited at 450 °C susceptor temperature. These thick-
nesses were all nominal values. The low-T TiN was found
silane (SiH ) reduction of WF6
4
Deposition of W on top of TiN has recently been studied
a͒Electronic mail: shili@ele.kth.se
2
998
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Appl. Phys. Lett. 67 (20), 13 November 1995
0003-6951/95/67(20)/2998/3/$6.00
© 1995 American Institute of Physics
1

TABLE I. Summary of measurement results of the 40 nm thick W nucleation layers and of about 200 nm thick
bulk W layers on top of the nucleation layers.
TiN/Ti
bilayer
Deposition of W Total thickness of W Resistivity of W Stress of W Integrated intensity
nucleation layer
͑nm͒
͑␮⍀ cm͒
͑GPa͒
of W-͑110͒, XRD
High-T
Low-T
High-T
Low-T
High-T
Low-T
High-T
Low-T
WF , SiH , H
39
41
41
19.6
22.0
19.3
23.4
12.7
13.1
12.7
13.4
1.52
3.83
1.55
4.17
1.13
1.62
1.14
1.72
317
2307
419
2654
1126
10007
1056
14725
6
4
2
2
WF , SiH , H
6
4
WF , SiH
6
4
WF , SiH
39
6
4
WF , SiH , H
244
242
253
241
6
4
2
WF , SiH , H
6
4
2
WF , SiH
6
4
WF , SiH
6
4
slightly ͑about 5 at. %͒ Ti deficient.13 W deposition was per-
formed in a cold-wall CVD reactor at 475 °C wafer suscep-
tor temperature, which amounts to about 434–438 °C wafer
terfacial F is proportional to the amount of interfacial O, and
that the concentrations of interfacial O and F at the W–TiN
interface are higher for the W deposited on the low-T TiN
than on the high-T TiN. As expected, the O and F concentra-
tions in the W bulk are independent of the type of adhesion
bilayer or of the W nucleation layer, because the same H2
1
3
temperature. The nucleation step was performed at a cham-
ber pressure of 4.5 Torr with SiH reduction of WF . H was
4
6
2
also introduced in the reactor during the nucleation step for
some depositions. The bulk W deposition was always per-
formed at 80 Torr with H reduction of WF . After W depo-
reduction of WF process was used for deposition of all bulk
6
W films.
2
6
sition, no heat treatment was performed.
Figure 2 shows the quantitative F concentration in
atomic percentages versus depth ͑counting from the W sur-
face͒ for the W nucleation layers of about 40 nm thickness,
determined by nuclear resonance broadening ͑NRB͒.14 The F
concentrations are higher for the W deposited on the low-T
TiN than on the high-T TiN, in agreement with the SIMS
results. For the W films deposited on the low-T TiN, the F
Table I summarizes the electrical and mechanical mea-
surement results for W films of two different thicknesses,
i.e., about 40 nm thick nucleation layers and ϳ200 nm thick
bulk W on top of the 40 nm nucleation layers. Clearly, the
resistivity and stress of the W films are strongly influenced
by the underlying TiN/Ti bilayers as well as by H addition
2
to the reactants during the deposition of W nucleation layers.
As the resistivity values given are averaged ones calculated
over the entire thickness of the W films, the higher resistivity
of the W films deposited on the low-T TiN is thus attributed
to the higher concentrations of interfacial oxygen ͑O͒ and
fluorine ͑F͒ piling up at the W–TiN interface. The depth
profiles of O and F, obtained by secondary ion mass spec-
trometry ͑SIMS͒, are shown in Fig. 1 for the W films of
about 240 nm thickness on the TiN/Ti bilayers. The thicker
W films were chosen so as to also reveal the O and F con-
centrations in the W bulk. It is clear that the amount of in-
concentration is higher for the W deposition without H ad-
2
dition to the reactants. The introduction of H to the reactants
2
does not affect the amount of interfacial F for the W depos-
ited on the high-T TiN. Furthermore, the F peaks are all
extending into the underlying TiN layer, and the degree of
this extension strongly depends on the type of TiN. Fluorine
penetrates through the entire low-T TiN layer of 100 nm
thickness and the concentration of F in TiN is somewhat
reduced by H addition to the reactants. In contrast, it does
2
not penetrate the high-T TiN layer of 80 nm thickness, and
the concentration of F in this type of TiN is substantially
reduced and insensitive to H addition. In all cases, the F
2
FIG. 1. SIMS results showing the O and F depth distributions in the W/TiN/
Ti/SiO structure for the W films of about 240 nm thickness for two different
wafers. The broken vertical line indicates the approximate position of the
W–TiN interface.
FIG. 2. NRB results of the F depth distributions, in atomic percentages,
showing F piling up at the W–TiN interface and tailing considerably into the
TiN layers for the W nucleation layers of 40 nm thickness.
2
Appl. Phys. Lett., Vol. 67, No. 20, 13 November 1995
Zhang et al.
2999
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1

WF –SiH mixture, the impact of H is almost negligible
6
4
2
and the main solid products will always be TiF and W. All
3
these seem to be supported by the results of Figs. 1 and 2.
Apparently, more oxidation takes place on the TiN surface of
the low-T TiN, because this TiN has substantially more grain
boundaries per unit surface area as compared to the high-T
TiN.13 The grains of the low-T TiN extend right through the
whole TiN layer down to the underlying Ti layer. If sufficient
F reaches the Ti underlayer, delamination can occur due to
the interaction of F with Ti. The nonstoichiometric TiN layer
also seems to promote the interaction between WF and TiN.
6
Therefore, to reduce the impurity level at the W–TiN inter-
face, a stoichiometric TiN layer is preferred and a large
amount of H should be introduced to the WF –SiH gas
2
6
4
mixture during the deposition of W nucleation layers. The
slight W penetration into the underlying TiN is ascribed to
FIG. 3. RBS spectra for the W nucleation layers of 40 nm thickness depict-
ing a small difference in the low-energy edge of the W peaks among the
three wafers presented.
the inhomogeneous interaction between WF and the par-
6
tially oxidized TiN surface. The latter is formed because of
nonuniform surface oxidation of TiN in air at room tempera-
ture.
concentration decays rapidly from the peak values of 1.0–2.5
at. % at the W–TiN interface to about 0.1 at. % at the W
surface ͑depthϭ0 nm͒ only about 40 nm apart. Presumably,
most of the O impurities are also found in the similar depth
range.
Tailing of F into the TiN layer is closely associated with
the W penetration into the underlying TiN. The Rutherford
backscattering spectrometry ͑RBS͒ results of Fig. 3 depict
that the degree of extension of the low-energy edge of the W
peaks towards lower energies follows the same trend as the
degree of F tailing shown in Fig. 2. Since both the W surface
and the W–TiN interface for the W nucleation layer depos-
ited on the low-T TiN are smoother than those of the W
nucleation layer deposited on the high-T TiN,13 the W pen-
etration into the underlying TiN is concluded to be the main
cause for the extension of the low-energy edge of the W
peaks observed in Fig. 3. Thus, depositing W on the high-T
The W films are all under tensile stress, and the ampli-
tude of the stress is not only strongly influenced by the type
of adhesion TiN/Ti bilayers,13 but also affected by H addi-
2
tion to the reactants during W nucleation layer deposition
͑see Table I͒. According to x-ray diffraction ͑XRD͒ analysis,
the W films deposited are all preferentially oriented to the W
͗110͘, and the strong impact of the underlying TiN on the
film texture of W has been reported elsewhere.13 The inte-
grated intensities of the W-͑110͒ peak given in Table I are
normalized values. The W films are less textured for the W
deposited with H addition to the reactants during the depo-
2
sition of W nucleation layers. Thus, the difference in stress is
attributed to the difference in the texture of the W films and
possibly to the interfacial impurities as well.
The authors wish to thank J. Cardenas for SIMS and N.
Lundberg for RBS. This work was financially supported by
the Swedish Board for Technical Development ͑Nutek͒ and
by the Flemish Institute for the Advancement of Scientific-
Technological Research in Industry ͑IWT͒.
TiN or adding H to the reactants during W deposition leads
2
to reduced W penetration. It should, however, be noted that
the depth of W extending into the underlying TiN is much
shallower than the depth of F tailing into the same TiN. This
is interpreted as a consequence of the much more rapid F
penetration along the grain boundaries of TiN, as compared
to W into TiN.
1
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͑
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because of the presence of O there. Large amounts of F
atoms are incorporated at the W–TiN interface also due to
the formation of nonvolatile TiF , a product of the chemical
3
8
9
0
interaction between WF and TiN under the deposition con-
6
ditions used. Thermodynamic calculations predict that when
a TiO surface is exposed to the WF –SiH mixture with a
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2
6
4
1
¨
H. Norstr o¨ m, S. Nygren, P. Wiklund, M. Ostling, R. Buchta, and C. S.
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4
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main solid products will be WO , TiF , and W. H addition to
11
2
3
2
the same WF –SiH mixture reduces the amount of solid
6
4
12
oxide phase͑s͒ by enhancing the formation of gaseous H O.
2
The formation of TiF will also be considerably reduced by
3
13
14
S.-L. Zhang, R. Palmans, S. Petersson, and K. Maex ͑unpublished͒.
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2
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Appl. Phys. Lett., Vol. 67, No. 20, 13 November 1995
Zhang et al.
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