Tungsten thin-film deposition on a silicon wafer: The formation of silicides at W-Si interface

Article abstract of DOI:10.1134/S002016850902006X

The interphase boundary formed in the process of tungsten thin-film deposition on a silicon wafer is investigated. These films are produced via (1) a CVD technique relying on hydrogen reduction of tungsten hexafluoride, (2) the same technique supplemented

Full text of DOI:10.1134/S002016850902006X

ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 2, pp. 140–144. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © S.V. Plyushcheva, G.M. Mikhailov, L.G. Shabel’nikov, S.Yu. Shapoval, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 2, pp. 176–180.
Tungsten Thin-Film Deposition on a Silicon Wafer:
The Formation of Silicides at W–Si Interface
S. V. Plyushcheva, G. M. Mikhailov, L. G. Shabel’nikov, and S. Yu. Shapoval
Institute of Technological Problems of Microelectronics and High Purity Materials, Russian Academy of Sciences,
Chernogolovka, Moscow oblast, 142432 Russia
e-mail: plush@iptm.ru
Received December 5, 2007
Abstract—The interphase boundary formed in the process of tungsten thin-film deposition on a silicon wafer
is investigated. These films are produced via (1) a CVD technique relying on hydrogen reduction of tungsten
hexafluoride, (2) the same technique supplemented with plasmochemical action, and (3) magnetron deposition
used for comparison purposes. It is shown that a nanometer tungsten silicide W5Si3 layer is formed at the tung-
sten–silicon interface only under gas-phase deposition. The effect of annealing on the specimen composition
and surface resistance is investigated. It is shown that the formation and growth of a silicide WSi2 layer com-
mences at 700°C for CVD films and at above 750°C for films obtained with plasmochemical deposition; this results
in a drastic increase in their electrical resistance. Under optimal conditions, tungsten films of 8 × 10–6 Ω cm resistiv-
ity are produced.
DOI: 10.1134/S002016850902006X
INTRODUCTION
EXPERIMENTAL
The employment of tungsten as a current-carrying
material for metallization of ICs is of growing interest
owing to its unique properties; as electronic devices
become smaller, this material continues to withstand
progressively greater current densities. Despite the fact
that a tungsten layer has higher electrical resistance
than an aluminum layer, in the production of semicon-
ductor ICs, the former offers a number of advantages to
the process of metallization [1–4]. Tungsten films are to
a smaller degree prone to electromigration, and they
differ but little in their thermal-expansion coefficient
from silicon and silica, which are most often used in
technology.
Tungsten films on silicon were obtained via the
CVD technique by the hydrogen reduction of tungsten
hexafluoride (in a thermal process with the activation
energy of 70.32 kJ/mol) and gas-phase deposition with
the plasmochemical stimulation of hydrogen. In the lat-
ter case, the activation energy is 15.91 kJ/mol. The speci-
mens produced with magnetron deposition were taken for
comparison. High-resistance 〈100〉-oriented silicon
wafers were taken as substrates. These wafers were pre-
pared by the conventional procedure [5].
The isothermal annealing of all the specimens was
performed in hydrogen in the range from room temper-
ature to 900°C in 50°C steps in the course of 30 min in
the interior of an SUOL tube-furnace equipped with a
precision temperature regulator. Hydrogen was cleaned
by filtering with the use of a Palladii 0.5 installation
equipped with palladium filters.
A metal film is deposited directly on silicon; there-
fore, in some cases, when the occasion requires, in the
process of manufacturing devices, a silicide layer is
produced under a later thermal treatment or the metal
layer is left unchanged. Knowledge of the basic laws
that govern film–substrate interface interactions in the
specific structures and feasibility of control over these
processes with the use of various growth methods and
different film growth conditions serve as a physical
foundation in the manufacture of electronic devices.
Moreover, the conditions at the interlayer boundary, by
and large, dictate the useful life of the devices.
The film thickness was measured with a Talystep
profilometer. The steps were formed both by masking a
substrate in the process of metal film deposition and
also by partially stripping the film out of the substrate
surface. The electrical resistance of the films was mea-
sured according to the four-point probe method.
The chemical interaction of silicon substrate surface
The aim of this paper is to investigate the chemical layers and a tungsten film was investigated with Ruth-
interactions occurring at the tungsten–silicon interface erford backscattering spectrometry (RBS) and thin-film
in the process of tungsten thin-film deposition onto a x-ray diffractometry (TFXRD) [6–8]. The special fea-
silicon wafer with the use of various techniques of tures of the TFXRD technique are the specimen posi-
obtaining these films with a low electrical resistance.
tion with a low-angle primary x-ray beam and the
140

TUNGSTEN THIN-FILM DEPOSITION ON A SILICON WAFER
141
(a)
(a)
110
110
200
211
200
W Si
3
211
310
W Si
W Si
220
5
3
5
5
3
40
50
60
70
(b)
(b)
110
110
211
200
200
211
220
310
220
310
40
60
80
100 2θ, deg
40
60
80
100 2θ, deg
Fig. 2. Diffraction patterns of tungsten films obtained on sil-
icon with the use of (a) plasmochemical deposition of tung-
sten and (b) magnetron deposition of tungsten at 100–
200°ë.
Fig. 1. Diffraction patterns of tungsten films deposited on
silicon; the diffraction patterns are obtained with the use of
(a) conventional and (b) TFXRD techniques.
detection of diffracted radiation as an x-ray detector gen show that these layers are not uncombined tungsten
rotates.
[9, 10]. The well-discernible tails of the silicon distri-
bution are almost universally present in the spectra of
these layers in the vicinity of the interface. This con-
firms that silicon penetrates to a large measure into a
tungsten layer. In [9, 10], the details on characteristics
of the observed tails were omitted.
The diffraction patterns show that, inside the tung-
sten films, a WF6 layer localized at the film–substrate
interface is formed; this holds for the tungsten films
obtained by gas-phase deposition from a WF6–H2 mix-
ture both in the case of true thermal deposition and also
Figure 1 presents the diffraction patterns of tungsten
films deposited on silicon; these patterns are obtained
with the conventional recording procedure with the use
of the TFXRD scheme. As is seen from the figure, the
application of the TFXRD scheme ensures a substantial
increase in the diffraction line intensity for the film
specimens.
RESULTS AND DISCUSSION
The tungsten layer deposition begins with interac- for the plasmochemical technique (Fig. 2). There is no
tion in accordance with the reaction [9]
silicide layer in the films produced with magnetron
deposition at temperatures between 100 and 300°C
(Fig. 2b) or with gas-phase deposition of tungsten onto
silica substrates; all this provides the basis to assume
that silicide arises by the interaction between tungsten
hexafluoride and silicon according to the reaction
2WF6 + 3Si = 2W + 3SiF4.
(1)
Then, the tungsten layer growth occurs owing to hydro-
gen reduction according to the reaction
WF6 + 3H2 = W + 6HF.
(2)
10WF6 + 21Si = 2W5Si3 + 15SiF4.
(3)
It is thought that no silicide layer can occur under
conventional temperatures of 250 to 400°C. However,
the Auger and RBS spectra for the tungsten layers the possibility of reaction (3). The Helmholtz energy
obtained under the interaction between WF6 and hydro- (∆G) of tungsten silicide W5Si3 formation is close to
The thermodynamic calculations performed support
INORGANIC MATERIALS Vol. 45 No. 2 2009

142
PLYUSHCHEVA et al.
The Helmholtz energy of reaction products formed at W–Si
interface
ilicide WSi2 of tetragonal modification show up in the
specimens annealed at 800°C.
Comparing the diffraction patterns produced with
the different techniques, we found that, in the case of
plasmochemically deposited films, the silicide layer
thickness (<10 nm) was much less than the ones
obtained by conventional thermal gas-phase deposition
(~20 nm). Conceivably, this might be due to activated
hydrogen in the reduction reaction of tungsten hexaflu-
oride. Being favorable to the course of reaction (2),
activated hydrogen makes silicide formation difficult,
because tungsten silicide WF6 is divided among reac-
tions (2) and (3). The part played by active hydrogen
was mentioned in (9) as well. Conceivably, the activa-
tion of a gaseous mixture with the use of an rf discharge
results in weakening the interaction between WF6 and
silicon; this may happen owing to competition between
deposition reactions (1) and (3) [5].
–∆G, kJ/mol
Reaction
298 K 373 K 473 K 573 K
WF6 + 3H2
W + 6HF 0.92 25.95 58.73 91.25
2WF6 + 3Si
3SiF4
2W +
657.20 664.74 671.85 678.55
10WF6 + 21Si
2W5Si3 + 15SiF4
687.34 692.36 698.64 704.08
this value of tungsten formation according to reaction
(1) (see table). Moreover, as is known, the heteroge-
neous interaction between tungsten and silicon escapes
detection up to 600°C.
The XRF analysis of annealed specimens does not
show a change in their composition up to 700°C. The
diffraction patterns for pre- and postannealed tungsten
films are shown in Fig. 3. The growth of silicide phase
700°C is observed at annealing temperatures up to
700°C; this is confirmed by an increase in its diffraction
peak intensity with a rise in temperature. Irrespective of
the film growth technique, strong peaks of tungsten dis-
Internal strains in the tungsten films were measured
according to the TFXRD technique (Fig. 4). In the ini-
tial stage, a CVD specimen shows the linear relation-
ship ε(sin2ψ); this points to a uniform state of stresses. The
film deformation found by extrapolation to sin2ψ = 0 cor-
responds to compression. After the film was annealed,
a decrease in stresses and change in their sign were
revealed; in accordance with Fig. 4, the stresses corre-
spond to expansion. In films obtained with magnetron
deposition, a nonuniformly stressed state is observed;
after annealing, this state becomes uniform and the
internal stresses show a substantial rise in value.
(a)
110
The grain-boundary angles in the texture were mea-
sured with the use of 110 reflection in the diffraction
pattern shown in Fig. 5. The half-width of the curve
I110(ω) serves as the measure of misorientation for
200
211
W Si
W Si
W Si
220
310
5
3
5
3
5
3
3
ε × 10
10
2
40
60
80
100
5
110
(b)
3
WSi
2
1
0
WSi
2
211
200
WSi
–5
4
WSi
2
2
0
0.1
0.2
0.3
sin2ψ
20
40
60
2θ, deg
80
100
∆d
----------------
2
Fig. 4. The deformation given by ε =
vs. sin ψ,
d(211)
where(ψ = Θ – ω): 1, 2) magnetron deposition at 750°C
before and after annealing, respectively; (3, 4) CVD depo-
sition at 600°ë before and after annealing, respectively.
Fig. 3. Diffraction patterns of CVD tungsten films obtained
on silicon: (a) before and (b) after annealing.
INORGANIC MATERIALS Vol. 45 No. 2 2009

TUNGSTEN THIN-FILM DEPOSITION ON A SILICON WAFER
143
sure internal strains in the tungsten films and grain-
boundary angles between 〈110〉 axes of grains in the
texture and the normal to the substrate surface; it was
revealed that, after annealing, the half-width of the
curves narrows; this effect is attributable to a decline of
microstresses in film grains or to a growth in their size.
16
12
3
2
In CVD films and in films obtained with the CVD
technique supplemented with rf activation of hydrogen,
the effect of annealing on the film composition and
electrical resistance was investigated. As a rule, the
films produced at 350–400°C and deposited with no
stimulation show a lower electrical resistance as com-
pared with films obtained at a temperature of about
~100°C under plasma-stimulated deposition. Upon the
thermal treatment of films in hydrogen, the surface
resistance shows a certain decrease with increasing
temperature; then, at a temperature above 700°C, a
jump in the electrical resistance occurs.
8
4
1
0
10
20
30
40
Incidence angle ω, deg
The electrical resistance versus annealing tempera-
ture is presented in Fig. 6 for various specimens. The
specimens obtained under plasmochemical stimulation
of hydrogen show a growth in the electrical resistance
at a higher temperature (800°C). One can suppose that
the shift into a higher temperature range is caused by
the kinetics of formation of silicides and related to a
lower thickness of the self-restricted film obtained
under plasmochemical deposition. This layer with the
active film–substrate interaction is thought to be the
major contributor of nucleation centers of silicide
growth under annealing. The rise in annealing temper-
ature up to 700°C results in a slight reduction of the
electrical resistance; this phenomenon may be due to a
decrease in the number of defects in the film. Using the
results of the x-ray investigations, we can ascribe a
drastic increase in the electrical resistance to the
appearance tungsten disilicide of the tetragonal phase.
Fig. 5. Intensity of 110 diffraction reflection vs. x-ray inci-
dence beam angle: (1) initial specimen, (2) specimen
annealed at 600°C, and (3) specimen annealed at 800°C.
grains in a textured film. The films obtained with mag-
netron deposition show a more pronounced texture as
compared to the films produced by gas-phase deposi-
tion. The TFXRD technique made it possible to mea-
R /R
s
o
1
4
3
2
The performed investigations made it possible to
optimize the growth processes and to produce thin
tungsten layers with a resistivity of 8 × 10–6 Ωcm,
which is close to the resistivity of the bulk metal.
2
CONCLUSIONS
Tungsten films were obtained on a silicon wafer
with the use of various techniques: the W–Si interfaces
were investigated.
1
The gas-phase deposition of tungsten layers was
performed from a mixture of tungsten hexafluoride and
hydrogen; it was shown that tungsten silicide W5Si3 is
formed at the interface in accordance with the reaction
WF6 + Si
W5Si3 + SiF4.
0
400
500
600
t, °C
700
800
In the case of plasmochemical deposition of tung-
sten, the activation of hydrogen makes the reaction of
silicide formation difficult; as a consequence, these
specimens have a thinner W5Si3 film (below 10 nm).
Annealing in hydrogen at a temperature up to 650°C
leaves the tungsten film composition unaltered. The
Fig. 6. Relative electrical resistance of tungsten layers vs.
annealing temperature; the films were produced using
(1) CVD technique and (2) CVD technique supplemented
with activation of hydrogen.
INORGANIC MATERIALS Vol. 45 No. 2 2009

144
PLYUSHCHEVA et al.
Substrate Temperature, Appl. Surf. Sci., 1990, vol. 45,
reduction of the electrical resistance is correlated to a
decrease in the defect density, internal strains, and
microstresses in the film; this is supported by x-ray dif-
fraction results.
The formation and growth of silicide WSi2 com-
mences at 700°C in the films obtained with gas-phase
deposition; in the case of the films obtained with plas-
mochemical deposition, this is true at a temperature
above 750°C; the formation of WSi2 is accompanied by
a drastic growth in the electrical resistance of these
films.
pp. 257–261.
3. Malikov, I.V., Plyuscheva, S.V., and Shapoval, S.Yu.,
Gas-Phase Deposition of Tungsten Thin Films with the
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5. Luft, B.D., Fiziko-khimicheskie metody obrabotki polu-
provodnikov (Physicochemical Methods of Semicon-
ductor Processing), Moscow: Radio i Svyaz’, 1982.
The obtained thin tungsten films have a resistivity of
8 × 10–6 Ωcm, which is close to the resistivity of the
bulk metal.
6. Shabel’nikov, L.G. and Zuev, A.P., Thin Films x-Ray
Diffractometry Using SR Beams, Phys. Res. Nucl.
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INORGANIC MATERIALS Vol. 45 No. 2 2009