Metallurgical Coatings 1988. Proceedings of the 15th International Conference on Metallurgical Coatings, San Diego, CA, U.S.A., April 11–15, 1988

390 Pages·1988·63.49 MB·English

by  R. Krutenat (Eds.)

METALLURGICAL
COATINGS
1988
Proceedings of the 15th International Conference
on Metallurgical Coatings, San Diego, CA, U.S.A.,
April 11-15,1988
Volume III
Reprinted from the journal
Thin Solid Films
Vol. 166, Nos. 1-2
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International Conference on Metallurgical
Coatings (15th: 1988: San Diego, Calif.)
1. Metallic
I. Title II. Krutenat, R. C. III. Surface
and coatings technology. IV. Thin solid
/
films.
667.9
ISBN 1-85166-983-3 (V.l)
ISBN 1-85166-984-1 (V.2)
ISBN l-85166-985-X(V.3)
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V
PUBLICATION SCHEDULE
The Sessions of the 1988 International Conference on Metallurgical Coatings
are distributed between Volumes I—III as follows:
VOLUME I
Corrosion resistant coatings and materials development
High temperature wear and clearance control
Thermal barrier coatings
Fundamentals of hard coatings
Hard coating processes
Applications of hard coatings
Superhard coatings
Reduction of friction through coatings and/or surface modification
Erosion corrosion resistance through coatings and/or surface modification
Protective coatings and/or surface modifications
Adhesion micromechanical properties and testing of coatings
VOLUME II
Characterization of hard coatings
Tribological and mechanical properties of hard coatings
Coating materials and applications
Production equipment
Ancillary equipment process controls and surface modifications
Author and subject indexes for VOLUMES I and II
VOLUME III
Thin film barrier layers
Metallization for VLSI circuits
Thin films used in packaging technology
New materials and emerging technologies
Synthesis and microstructure of high T c superconductors
Surface and thin film analysis techniques
Microstructural characterization techniques
Mechanical properties of films and coatings
Non-destructive characterization techniques
Protective coatings for magnetic and optical thin film media
Thick and thin film sensors
Author and subject indexes for VOLUME III
Thin Solid Films, 166 (1988) 1-14
1
REACTION KINETICS IN TUNGSTEN/BARRIER METAL/SILICON
SYSTEMS*
K. SUGURO, Y. NAKASAKI, T. INOUE,f S. SHIMA AND M. KASHIWAGI
VLSI Research Centre, Toshiba Corporation, 1 Komukai Toshiba-cho, Saiwai-ku, Kawasaki 210 (Japan)
(Received March 21, 1988)
The reaction kinetics and electrical properties of the W/TiN/TiSi 2/Si and
W/TiN/Si systems are described. By using the TiN/TiSi 2 and TiN diffusion barrier
layer, the tungsten silicidation rate is successfully lowered by 2 orders and 4.5 orders
of magnitude respectively, compared with the silicidation rate of the W/Si system.
The tungsten resistivity is 8 |iQ cm after 950 °C annealing in N 2. By considering the
activation energy of WSi 2 formation, the reaction rate for the W/TiN/TiSi 2 system is
found to be controlled by the silicon supply at the bottom of the tungsten layer, and
the rate-limiting process is considered to be silicon bond breaking at the TiSi 2—Si
interface. The driving force for silicon diffusion is considered to be the free energy
reduction caused by tungsten silicidation at the W-TiN interface. 100 A TiN films
are thick enough to maintain the thermal stability, i.e. to suppress reactions and to
maintain adhesion even after 6 annea2ling at 950 °C for 5 h. A good oh+mic contact
resistivity of the order of 10 ~ Q cm could be obtained for 1t9he T-iN/3p -Si contact by
increasing the interfacial hole density up to (8-10) x 10 cm . Some application
examples are proposed with W/barrier layer/Si systems.
1. INTRODUCTION
A continuous effort has been put into miniaturization to satisfy the need for
higher speed, lower power and lower cost per function in ultralarge scale integrated
(ULSI) circuits. Higher performance and reliability requirements for enormous
numbers of lower submicron contacts in a ULSI chip will become more and more
stringent than ever. To improve the thermal stability of the contacts, diffusion
barrier layers will be necessary in metal-oxide-semiconductor (MOS) ULSI circuits
as well as bipolar devices. There are two important interfaces where thermal stability
is required. They are Al-Si and refractory metal-Si interfaces. In the Al/Si system,
nitrides, borides and carbides of the 4A group metals such as titanium, zirconium
•Paper presented at the 15th International Conference on Metallurgical Coatings, San Diego, CA,
U.S.A., April 11-15,1988.
f Present address: Iwaki Meisei University, Fukushima, Japan.
0040-6090/88/S3.50 © Elsevier Sequoia/Printed in The Netherlands
2 K. SUGURO et al
1
and hafnium are considered to be promising barrier metals . Deep-hole2-fi-llin6g
tungsten, formed by selective or blanket chemical vapour deposition (CVD) , will
also be expected to be useful as an Al/Si barrier metal as long as the processing
temperature does not exceed 500 °C. Low pressure CVD (LP CVD) tungsten is also
one of the most promising materials among a number of candidates for use as an
interconnecting material. Tungsten has a resistivity (bulk resistivity, 5.5 |iQ cm)
twice as large as that of aluminium; however, tungsten has the highest melting point
(3410 °C) of all the elements and is very attractive for its high activation energy (4 eV) 7
for grain boundary diffusion which is c8losely related to the eleetromigration .
Refractory metal local interconnections are considered to be more reliable than
aluminium multilevel interconnections, the performance of which can be strongly
affected by stress and electric fields. 9
However, tungsten is known to react with silicon to form WSi 2 at tempera­
tures above 650 °C. A recent article has shown that tungsten silicidation occurs at 1 0
575 °C in sequentially deposited tungsten and silicon multilayered systems .
Therefore, if a tungsten plug is made in a contact hole opened on single-crystal
silicon or polycrystalline silicon (poly-Si), very rapid tungsten silicidation would
occur after heat treatment at higher temperatures. As a result, the plugging material
surface level would go down as a r3esult of volume shrinkage during silic3idation,
because 1 mol of tungste3n (9.5 cm ) reacts with 2 mol of silicon (24.1 cm ) to form
1 mol of WSi 2 (24.6 cm ). This means that almost all the plugged tungsten can be
embedded in the silicon layer in the form of silicide during tungsten silicidation.
When tungsten is used for interconnections, tungsten silicidation should cause
a resistivity increase. In order to suppress rapid silicidation, a barrier layer is
necessary. Ox1yg1en 1as 3an impurity is known to hinder the reaction between tungsten
and silicon . However, a barrier metal which can supress the reaction between
tungsten and silicon while suppressing the contact resistance increase has not yet
been reported. An ideal diffusion barrier for a contact metallization is defined as a
chemically and mechanically stable and elec1trically conductive layer that inhibits
interdiffusion between its bordering media . TiN has been reported as a diffus1ion 4 - 1 6
barrier material which acts as an impermeable barrier to silicon atoms .
Furthermore, the TiN/TiSi 2/Si structure has been reported to be stable for
700-1000 °C annealing, since TiN contacts more stably with 1Ti7Si 2 than Si 3N4 does,
as is obvious from the Ti-Si-N ternary phase diagram . However, the thermal
stability of the W/TiN system or the W/TiN/TiSi 2/Si system has not yet been
reported.
In our study, TiN 1or 8Ti-N/2Ti1Si 2 layers were selected as a barrier metal between
tungsten and silicon . A similar structure of a diffusion barrier between tungsten
and silicon consisting of TiN/TiSi 2, which 2is 2formed by thermal nitridation during
titanium silicidation, was also reported . TiN was employed in the present
structure on the basis of thermodynamic considerations as follows. The change AH f
in heaxt of formation by nitridation is more negat1ively for titanium (TiN, — 80 kcal
mol" ) than for tungsten (W 2N, -17 kcal mol' ) at 25 °C (ref. 23). Because of the
lack of entropy data for W 2N, an exact value for the change in the Gibbs free energy
caused by nitridation is n o3t 2available. However, from the empirical formulation for
the free energy of Mo 2N (since molybdenum shows chemical properties similar to
REACTION KINETICS IN W/BARRIER METAL/Si SYSTEMS 3
those of tungsten), the Gibbs free energy change for the nitridation of tungsten can
be roughly estimated. The Gibbs 1free energy changes at 900 °C 1for TiN and W 2N are
estimated to be — 55 kcal mol" (ref. 23) and — 3 kcal mol" respectively. Therefore
the W-TiN interface is considered to be stable. If a strong barrier metal is interposed
at the W-Si interface, tungsten interconnections can be realized even when high
temperature processes such as phosphorus gettering, reflow of borophosphosilicate
glass, rapid thermal annealing, upper level gate oxidation, and laser or electron
beam annealing are used after tungsten metallization.
24 The thermal stability of the W/TiN structure is supported by other results. So et
ai showed that nitrogen atoms in the W-N layer redistribute into the titanium
layer after annealing of a Ti/W 7N030 structure at temperatures above 500 °C. The
results can be explained by the thermodynamic considerations mentioned above.
From a thermodynamic point of view, the idea of W/TiN local inter8connections
formed by interaction between W-N and titanium is very interesting .
Contradictory results exist on 2t5he feasibility of 2L6PCVD tungsten film
deposition on TiN. Broadbent et al. and Chen et ai reported that the TiN
formed by rapid thermal annealing in nitrogen ambien1t8 co1mp2letely prohib2it1s the
growth of selective tungsten. In contrast, our group and Rana et al have
reported tungsten growth on reactively sputtered TiN. Surface morphology,
composition (oxygen content), and tungsten CVD conditions are key parameters in
CVD tungsten deposition.
The purpose of the present paper is to clarify the reaction kinetics in W/TiN/Si
and W/TiN/TiSi2/Si structures in order to utilize tungsten for interconnections and
for plugging contact holes, even when the tungsten undergoes high temperature
processing. Application examples of these structures are also described in this paper.
2. EXPERIMENTAL PROCEDURES
0.3-1 jam Si0 2 films were formed by thermal oxidation of n- or p-type
1-10 D cm Si(100) substrates or by deposition with atmospheric pressure CVD (AP
CVD). In some samples, contact holes with sizes ranging from 0.75 to 3.5 (im were
opened in oxides by using conventional photolithography and reactive ion etching
(RIE) with a gas mixture of CHF 3 and H 2. Fluorocarbon contamination during RIE
was removed by 0 2 plasma treatment followed by boiling in a mixture of H 20 2 and
H2S04. After dipping in a diluted HF solution and deionized water, the silicon wafer
was spin dried with N 2 blowing. In some samples, 500 A of TiSi 2 film as deposited by
d.c. sputtering in a magnetron sputtering system with a composite target (composed
of 99.99% purity titanium and silicon) in argon without substrate 5heating. Before
sputtering, the chamber was evacuated to a base pressure of 10" Pa and the argon
pressure during sputtering was 0.5-0.6 Pa. Various thicknesses of golden-appearing
TiN films were deposited by reactive sputtering a 99.99% purity titanium target in a
gas mixture of 50% N 2 and 50% Ar without bias. The substrate was heated to 200 °C
during deposition. The base pressure and total sputtering pressure were the same as
for the TiSi2 deposition. In the case of the bilayer structure, TiN and TiSi 2 layers
were sequentially d9eposited w2ithout breaking vacuum. The film stress of the TiN
was about 5xl0 dyncm" in the compressive direction. Tungsten films of
4 K. SUGURO et al.
1
0.1-1.2 jam thickne-s1s were deposited in a gas mixture of 0.3-0.45 standard 1 min~ of
WF36 and -1llmin of H2 by cold-wall-type LPCVD. In some cases 90 standard
cm min of SiH4 was added to the gas mixture in order to increase deposition
rates of tungsten. In this case silicon was incorporated in the tungsten films up to
1.8 at.% W. Deposition pressure and temperature were 30-35 Pa and 400-450 °C
respectively. -Th1ese samples were annealed in a 1 atm N 2 ambient. The N 2 flow rate
was 31 min . Before st4arting the N 2 flow, the IR furnace was evacuated to a
pressure below 1.3 x 10~ Pa to avoid tungsten and TiN oxidation. Reaction rates of
tungsten with silicon we+re measured by Talystep measurement, X-ray diffraction
(XRD), and 2 MeV He Rutherford backscattering spectrometry (RBS). Morph­
ological changes at interfaces between individual layers and surfaces were observed
by scanning electron microscopy (SEM).
3. RESULTS AND DISCUSSION
3.1. Silicidation rates in IV/Si, W/TiN/TiSi2/Si and W/TiN/Si systems
In order to clarify the product phases after annealing, XRD analysis was carried
out. Figures 1(a) and 1(b) show XRD patterns for the structure with and without a
barrier layer. The (400) reflection of silicon appears at 29 = 69.2° and a X/2 reflection
with the same spacing occurs at 20 = 32.95°. Here, 9 and X are the Bragg angle and
X-ray wavelength respectively. With the TiN(1000 A)/TiSi 2(500 A) barrier layer, the
(110) peak intensity of W(1000 A) did not change during annealing at 900 °C for
30 min. Tungsten, TiN and TiSi 2 were identified as b.c.c. oc-W, cubic TiN and
orthorhombic C54-type TiSi 2 phases respectively. The reacted tungsten volume was
negligible. Very slow silicidation occurred when prolonged annealing was carried
out. The TiN grain shape was pillar like and the grain size was ca2lcu8lated from SEM
observation results and XRD by using Scherrer's equation . The grain size was
250-350 A for as-deposited TiN and increased to 500-700 A after annealing at
900 °C for 30 min in N 2. In contrast, the tungsten film completely reacted with
silicon in the W(1000A)/Si system. Only diffractions from tetragonal WSi 2 and
Si(400) existed. Secondary ion mass spectrometry analysis has been carried out in
order to ascerta1in 9the X-ray results and to clarify the depth profiles of atomic
concentrations . The silicidation rate was calculated by measuring the reduction
rate of the W(110) peak intensity. The thickness sensitivity was a few tens of
angstroms for the average thickness of reacted tungsten. In contrast, WSi 2(103),
WSi2(110) and WSi 2(101) peaks appeared after a few hundred angstroms of WSi 2
was formed, because X-ray analysis has a low sensitivity to small amounts of
crystalline material. Precautions were taken to avoid tungsten oxidation which
caused a reduction in W(l 10) peak height.
Tungsten silicidation proceeded in the following fashion. First, the silicide grew
linearly with annealing time because of the reaction-rate-limited growth. As the
silicide thickness increased, the silicide grew linearly with the square root of
annealing time because of the diffusion-limited growth. Silicon diffusion-li2m9ited
growth has been reported for thick (above 1 (im) tungsten silicide films . The
linear-parabolic transition region cannot be exactly determined; however, the
silicide growth mode changed from linear growth to parabolic growth for the
REACTION KINETICS IN W/BARRIER METAL/Si SYSTEMS 5
WSi2 900°C 30min
W(110) (002X101)
-^-(400)Si
900°C 30min
-ob 1T(i0S6i02) o W,Si22 n )
I TiN(200) CO wsi2 ,
(200)(202)l
z>
o o
o (J
20 30 40 50 60 70 20 30 40 50 60 70
(a) 2d C° ) (b) 26 [° )
Fig. 1. X-ray diffraction patterns after annealing at 900 °C for 30 min in N2: (a) W(1000 A)/TiN(1000 A)/
TiSi2(500 A)/Si(100); (b) W(1000 A)/Si(100).
thickness range 1000-2000 A at temperatures from 800 to 950 °C. Lajzerowicz et al.
have shown that the growth is parabolic in the annealing of evap3or0ated W/Si(100),
where the evaporation took place at a 500 °C heated substrate . In the LPCVD
W/Si system linear growth was observed in thin film region. Oxygen depth profiles
were measured by Auger analysis; however, the oxygen concentration was under the
detection limit (< 0.1 at.%). In the linear growth region, the silicidation rate can be
expressed as a reacted tungsten thickness per minute, which was measured by X-ray
analysis and Talystep step height measurement. The result is shown in an Arrhenius
plot in Fig. 2. CVD tungsten films were deposited by H 2 reduction on Si(100)
substrates. 1Em9pirical formulations for linear rate constants were clearly found to be
as follows : 1 5 1
dTw/dt= 1.5xl0 exp(-2.9eV/^T)Amin- (1)
for the W/Si system and
1 0 1
dTw/dr = 2.5xl0 exp(-2.3eV//cT)Amin- (2)
for the W/TiN/TiSi2/Si system. Here, TW(A), t (min), k and T(K) are the reacted W
TEMPERATURE C°C]
950 900
1 i
W/Si
^ Ea-2.9eV
W/TiN/TiSi2/Si
Ea«2.29eV
1 I 4 I 11 i
8.0 85
10 /T [ K" ]
Fig. 2. Rate of WSi2 formation as a function of the reciprocal of the temperature. The growth is linear
with the annealing duration in both cases.
6 K. SUGURO et al.
thickness, annealing time, Boltzmann constant and temperature respectively. The
reduction rate of tungsten in the system with a TiN/TiSi 2 layer is 2-2.5 orders of
magnitude lower than that for the W/Si system. The XRD patterns after annealing at
950 °C for 30 min show a clear contrast between with and without a TiSi 2 layer,
which can act as a "catalyser" as shown in Fig. 3. When the TiSi 2 layer did not exist,
no tungsten silicidation occurred after annealing at 950 °C for 30 min. Even after
prolonged annealing up to 6 h was carried out, very little of the tungsten changed
into WSi2.
W/TiN/TiSi2/Si W010)
W/TiN/Si
TiN (200)
ZD ZD I
oo iiujir t . \ ji O
o
20 30 40 50 60 70 20 30 40 50 60 70
26 26
(a) (b)
Fig. 3. XRD patterns obtained after annealing at 950 °C for 30 min in N2: (a) W/TiN/TiSi2/Si; (b)
W/TiN/Si.
The silicidation rate for the W/TiN/Si system was measured by RBS analysis.
Figure 4 show an RBS spectrum of a W(1000 A)/TiN(1000 A)/Si system after 950 °C
for 6 h. The spectrum shows that 2on0ly a small amount of silicon diffused from the
substra2te 1to the tungsten layer . A detailed discussion will be submitted else­
where . By calculating the integral of the silicon spectrum in the tungsten layer, the
reaction product WSi 2 thickness was estimated to be about 60 A. The t1ungsten
silicidation rate at 950 °C in this system was calculated to be 0.07 A min ~~ expressed
as a reaction rate of tungsten. This rate is 1/130 and 1/25 000 of that in the
W/TiN/TiSi2/Si system and the W/Si system respectively. The above results are
summarized in Fig. 5, which shows schematic illustrations of W/Si, W/TiN/Si and
W/TiN/TiSi2/Si systems as deposited (Fig. 5(a)), after annealing at 900 °C for 30 min
(Fig. 5(b)) and after annealing at 950 °C for 30 min (Fig. 5(c)).
3.2. Driving force for silicidation and diffusing species in WI barrier metalj Si systems
The activation energy values in eqns. (1) and (2) were 2.9 eV and 2.3 eV
respectively. The activation energy value for the linear rate constant (derived from
the thin WSi 2 growth region) in the W/Si system a1g2rees with previously repor3ted1
values of 2.7-3.1 eV for a sputtered W/Si system and other W/Si systems . In
contrast, the activation energy for the parabolic rate constant is reported to be
2.2 eV and 3.4 eV for annealing at temperatures from 675 to 760 °C and for laser
annealing above 1137°C r3esp2ectively. Silicon is known as the dominant diffusing
species in WSi2 formation . The activation energy for the W/TiN/TiSi 2/Si system is
0.6 eV lower than that for the W/Si system. This result means that the rate-limiting
process is not a W-Si reaction. Silicon diffusion is not the rate-limiting process
either because the silicon diffusion in thin WSi 2 is considered to be very fast. The
REACTION KINETICS IN W/BARRIER METAL/Si SYSTEMS 7
0.1/i.m O.l^tm X30.0
+
2.0 MeV
Si TilSB W -He
25°
-as depo
-950°C 6h. in N2
Si
Ti
0.0 0.5 1.0 1.5 2.0
ENERGY (MeV)
Fig. 4. RBS spectra for a W(1000 A)/TiN(1000 A)/Si system before and after annealing at 950 °C for 6 h in
N2. After annealing, silicon diffusion to the tungsten layer is observed in the region where the scattering
yield is enlarged by factor of 30.
Si
/ si
(a) TiSi2
V
6
WSi2 TiN^
Si 'Si
(b) TiSi2
SB
WSi2 \TiN
Si Si
(c) • » I 1 1/ TiSi2
Fig. 5. Schematic illustrations of cross-sections for W/Si, W/TiN/Si and W/TiN/TiSi2/Si structures: (a)
as deposited; (b) after annealing at 900 °C for 30 min in N2; (c) after annealing at 950 °C for 30 min in N2.
rate-limiting process is considered to be the silicon supply at the bottom of the
tungsten layer.2 The silicon can be supplied by diffusion through localized defective
spots (0.6 \im~) in the TiN/TiSi 2 layer because of heterogeneous nucleation of a
WSi2 phase which was observed by SEM. The pre-exponential factor in eqn. (2) is 4.5
orders of magnitude smaller than that in eqn. (1). This is considered to be a reason
for the very small amount of silicon supplied in this system. In order to determine where
silicon atoms come from, a W/TiN/TiSi 2/Si02 structure was analysed by XRD with
the result shown in Fig. 6. The result shows that tungsten silicidation which resulted
from decomposition of TiSi 2 did not occur. This result shows that the silicon source
is the silicon substrate. Further evidence of silicon diffusion from the substrate is the