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MICROSCOPY RESEARCH AND TECHNIQUE 40:136–151 (1998)
High Resolution Electron Microscopy of Amorphous
Interlayers Between Metal Thin Films and Silicon
L.J. CHEN,* J.H. LIN, T.L. LEE, C.H. LUO, W.Y. HSIEH, J.M. LIANG, AND M.H. WANG
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China
KEY WORDS
solid-state diffusion; growth kinetics; structure; stability; auto-correlation function analysis
ABSTRACT
High-resolution electron microscopy of amorphous interlayers (a-interlayer) formed
by solid-state diffusion between metal thin films and silicon is reviewed. In this paper, an overview of
the development is presented. Pertinent data obtained on the growth kinetics and structure of
a-interlayers in polycrystalline metal thin films on single-crystal silicon are reported.
For the Ti/Si, Zr/Si, Hf/Si, V/Si, Nb/Si and Ta/Si systems, the growth of a-interlayer was found to
follow a linear law in the initial stage. Si atoms were found to be the dominant diffusing species in
the solid phase amorphization in the Ti/Si, Zr/Si, and Hf/Si systems. For the Y/Si system, the
stability of amorphous interlayer depends critically on the composition of the amorphous films.
Auto-correlation function analysis was utilized to determine the structure of the amorphous
interlayers. HRTEM in conjunction with the fast Fourier transform were applied to determine the
first nucleated crystalline phase. Simultaneous presence of multiphases was observed to occur in a
number of refractory metal/Si systems. Microsc. Res. Tech. 40:136–151, 1998. r 1998 Wiley-Liss, Inc.
INTRODUCTION
The study of amorphous metallic alloys dates back to
investigations during the 1950s. The discovery of the
formation of amorphous interlayer (a-interlayer) by
solid-state diffusion in diffusion couples is relatively
recent ( Johnson, 1988). For metal thin films on silicon,
although the presence of an amorphous membrane at
the metal-silicon interface was proposed to predict the
first nucleated phase of the system in 1976, direct
confirmation of solid phase amorphization was not
realized until much later (Walser and Bene, 1976).
Interfacial reactions of metal thin films on silicon have
been a subject of continuing interest for many years
(Nicolet and Lau, 1983; Tu and Mayer, 1978). The
formation of a-interlayer is assumed to be driven by the
large negative heat of mixing of the two components
and can proceed in a broad concentration range as long
as the temperature is high enough to allow sufficiently
rapid interdiffusion of the atoms, but low enough to
avoid any nucleation of the crystalline compound during the reaction time ( Johnson, 1988). The free energies of mixing between the undercooled liquid metals
and silicon were estimated with the method proposed
by Miedema et al. (1976). Negative free energy of
mixing, which provides the driving force for the formation of a-interlayer, was obtained for all refractory
metal/Si systems, with the exception of the W/Si system, and all rare-earth metal/Si systems investigated
so far (Chen et al., 1994). As a rule, the crystalline
compound phase appearing in the equilibrium phase
diagram is more stable than the amorphous alloy of the
same composition. An example is shown in Figure 1.
For metal-silicon systems, the formation of amorphous metal-Si alloys between metal thin films and
amorphous silicon during low-temperature annealing
was first discovered in the Rh/a-Si system (Herd et al.,
1983). Subsequently, it has been found to occur in all
r 1998 WILEY-LISS, INC.
refractory metal/Si and a number of rare-earth metal
and platinum group metal and crystalline silicon systems (Abelson et al., 1988; Cheng and Chen, 1990, 1991;
Hsieh et al., 1993; Lee and Chen, 1993; Liang and Chen,
1994; Lin and Chen, 1995; Lin et al., 1996; Lur and Chen,
1989; Wang and Chen, 1992). Both conventional and
high-resolution transmission electron microscopy (HRTEM), reflection high-energy electron diffraction (RHEED),
low-energy electron diffraction (LEED), Auger electron
spectroscopy (AES), X-ray diffraction, differential scanning
calorimetry, and electrical measurements have been applied to characterize the structures and properties of the
systems. HRTEM has played a critical role in clarifying
many key issues in the formation of amorphous layers in
metal-silicon systems. In this paper, we present an overview of the development and report the results obtained on
the growth kinetics and structure of a-interlayer in polycrystalline metal thin films on single-crystal silicon. Reactions
in metal/amorphous silicon multilayers were reviewed
recently (Sinclair and Konno, 1993). The scope of the
present review is mostly confined to the reactions
between metal thin films and single-crystal silicon
unless it is deemed appropriate to use some of the
results obtained for metal/a-Si multilayer reactions.
MATERIALS AND METHODS
For growth kinetics study, single crystal, 3–8 V-cm, 2
or 3 inches in diameter, phosphorus doped (001), (111),
and (011) oriented silicon wafers were used. The wafers
were cleaned chemically by a standard procedure. Thin
metal films, 10–60 nm in thickness, were then deposContract grant sponsor: Republic of China National Science Council; Contract
grant number: NSC84-2215-E007-011.
*Correspondence to: L.J. Chen, Department of Materials Science and Engineering National Tsing Hua University, Hsinchu, Taiwan, Republic of China.
E-mail: ljchen@mse.nthu.edu.tw
Received 16 March 1996; accepted in revised form 21 March 1996.
HREM OF AMORPHOUS INTERLAYERS
Fig. 1.
137
Metastable free energy diagram at 500°C for the Ti/Si system.
ited onto the wafers at room temperature in a UHV
system (with a base pressure better than 1 3 10210
Torr) or in a non-UHV system. An a-Si layer, 10–20 nm
in thickness, was deposited onto the metal layers to
protect the metal thin films from oxidation during
subsequent annealings. The vacua during depositions
were maintained to be better than 1 3 1029 and 5 3
1027 Torr in UHV and non-UHV systems, respectively.
To determine the effects of composition on the stability
of amorphous interlayer, multilayer metal/Si samples
were also prepared under UHV conditions.
The deposited samples were annealed isothermally
in an oil-free vacuum furnace or in a nitrogen gasflowing diffusion furnace at temperatures ranging from
200 to 625°C. HRTEM imaging was carried out in a
JEOL (Peabody, MA) 4000EX electron microscope operating at 400 keV with a point-to-point resolution of 0.18
nm. Most of the cross-sectional TEM (XTEM) micrographs were taken under symmetric [110] diffraction
condition. In situ RHEED, LEED, XRD, and AES
analyses of the thin film systems were also carried out.
For electrical characterization, sheet resistance, Schottky barrier height, contact resistance, and leakage
current were measured (Liauh et al., 1993).
GROWTH LAW AND ACTIVATION ENERGY
For UHV deposited Ti/(111)Si samples, RHEED analysis revealed that after the first 0.2-nm-thick Ti layer
Fig. 2. HRTEM image of an as-deposited sample, cross-sectional
view (c-s).
was deposited, the diffraction peaks corresponding to
(7 3 7) Si surface reconstruction disappeared. An intermixed amorphous layer was formed during the deposition of the first 1.7-nm-thick Ti layer. The characteristic
diffraction pattern of Ti showed up following 2-nmthick Ti deposition. XTEM examination of as-deposited
samples revealed that a 2-nm-thick amorphous inter-
138
L.J. CHEN ET AL.
Fig. 3. HRTEM images of V/(001)Si samples (a) as-deposited and
annealed at 440°C for (b) 15, (c) 45, (d) 75, (e) 105 minutes, showing
the growth of the amorphous interlayer.
layer was indeed formed. The Ti thin film was also
found to exhibit an (0, 1, 21, 0) texture. An example is
shown in Figure 2 (Wang and Chen, 1992).
For Ti/Si, Zr/Si, Hf/Si, V/Si, Nb/Si, and Ta/Si samples, amorphous interlayers were observed to form
in as-deposited samples. The thickness of the ainterlayer was found to increase with annealing temperature and time (Chen et al., 1994; Cheng and Chen,
1990, 1991; Lur and Chen, 1989). Examples are shown
in Figure 3. In the initial stage the growth follows a
linear law. The growth then slows down and comes to a
complete stop as a critical thickness is reached. The
activation energies of the growth of a-interlayer are
listed in Table 1.
A 1-nm-thick a-interlayer was found to form in asdeposited Mo/Si samples. The maximum thickness was
measured to be 4 nm, which is considerably thinner than
that found in non-UHV samples. For as-deposited W/Si
samples prepared under UHV environment, no continuous
a-interlayer was detected to form. In samples annealed at
500°C for 1 hour, a 3-nm-thick disordered layer was detected at the W/c-Si interface. In samples annealed at a
temperature as high as 700 or 750°C for 20 seconds, no
a-interlayer was found to form (Chen et al., 1994).
139
HREM OF AMORPHOUS INTERLAYERS
TABLE 1. Correlations Among Physical Parameters and Kinetic Data in the Formation and Growth of Amorphous Interlayers in Refractory
Metal/Si and Y/Si Systems
Metal
Atomic radius (nm)
Atomic size difference between metal and Si (%)
Atomic weight (g)
Electronegativity
Chemical potentials for electrons (V)
DG (kJ/g atom)
Maximum thickness (nm)
Activation energy (ev)
Dominant diffusing species
Ti
Zr
Hf
Nb
Ta
V
Y
Si
0.147
11.3
47.9
1.5
3.65
239
10
1.6
—
0.16
21.2
91.2
1.4
3.4
253
17
1.4
Si
0.167
26.5
178.5
1.3
3.55
258
27
1.2
Si
0.146
10.6
92.9
1.6
4.25
223
8
0.8
—
0.149
12.8
181
1.5
4.00
221
7.5
0.9
—
0.134
1.5
50.9
1.6
4.05
215
4.5
1.1
—
0.178
34.9
88.9
1.3
3.20
239
.10
—
—
0.132
—
28.1
1.8
4.70
—
—
—
DOMINANT DIFFUSING SPECIES
For the Zr/Si and Hf/Si systems, Kirkendall voids
were found to be present beneath the a-interlayer/c-Si
interfaces (Cheng and Chen, 1990). Similarly, the voids
were seen to form at the a-interlayer/a-Si interface in
Ti/a-Si multilayer system (Holloway and Sinclair, 1987).
The observation indicated that silicon is the dominant
diffusing species during the reactions in these systems.
AVERAGE COMPOSITION OF THE
AMORPHOUS INTERLAYERS
A scheme using an ultrathin (,1 nm) oxide layer and
capping Mo layer to define the reference planes for
interdiffusion has been utilized to determine the average compositions of amorphous interlayer in ultrahigh
vacuum (UHV) deposited V/c-Si samples. The buried
oxide layer is present at the epitaxial Si/c-Si interface
in UHV deposited samples. A thin Mo layer was deposited on the V layer to serve as an inert cap and define
the upper surface of the metal layer. The ratios of
participating Si and V atoms in the reactions were
measured to be 5:6 and 5:9 in samples annealed at
465°C for 10 and 30 min, respectively. An example is
shown in Figure 4 (Lin and Chen, 1996). A similar
scheme using an ultrathin (,1 nm) oxide layer and
capping W layer to define the reference planes for
interdiffusion has been utilized to determine the average composition of amorphous interlayer in Zr/c-Si
samples. The ratio of participating Si and Zr atoms in
the reactions in samples annealed at 420°C for 50
minutes was measured to be 3:5.
RARE-EARTH METAL/Si SYSTEMS
For 10-nm-thick yttrium thin film on silicon, an
11-nm-thick amorphous interlayer located between the
amorphous Si capping layer and Si substrate was
observed. An example is shown in Figure 5. In samples
annealed at 200°C, crystalline Y5Si3 and Si were always
observed to nucleate within the amorphous interlayer.
In addition, about 1.5-nm-thick epitaxial YSi2-x layer
was found to form at the amorphous interlayer/c-Si
interface in samples annealed for 30 minutes. An
example is shown in Figure 6. In samples annealed at
temperatures higher than 250°C, only epitaxial YSi2-x
was found to form at the amorphous interlayer/c-Si
interface. The amorphous interlayer was found to completely transform to epitaxial YSi2-x in a sample annealed at 300°C in 5 minutes. An example is shown in
Figure 7 (Lee and Chen, 1993).
Fig. 4. XTEM images of V/(111)Si samples (a) as-deposited and (b)
annealed at 465°C for 10 minutes, showing the movement of the
interfaces. c, d: Schematic diagrams of a and b, respectively.
140
L.J. CHEN ET AL.
Fig. 5. HRTEM image of an as-deposited a 5 Si(20 nm)/Y(10 nm)/(111)Si sample viewed along the
[112 ]Si direction.
Fig. 6. HRTEM image of an a-Si(20 nm)/Y(10 nm)/Si sample annealed at 200°C for 30 minutes viewed
along the [110]Si direction.
HREM OF AMORPHOUS INTERLAYERS
Fig. 7.
HRTEM image of an a-Si(20 nm)/Y(10 nm)/Si sample annealed at 300°C for 5 minutes, c-s view.
Fig. 8. HRTEM image of an as-deposited a-Si(20 nm)/Y(30 nm)/Si sample viewed along the [110]Si
direction.
141
142
L.J. CHEN ET AL.
Fig. 9. Schematic diagrams illustrating the phase
formation in as-deposited samples. a: Initial configuration following deposition of a thin layer of Y. b:
Growth of a-interlayer as the thickness of the deposited layer increases. c: Crystalline phases are formed
when the critical thickness of the a-interlayer is
exceeded. d: Same sample as that of c but annealed
at 200°C.
It is apparent that the deposited 10-nm-thick yttrium
thin film was completely intermixed with atoms from
the Si substrate to form an amorphous layer with a high
Si concentration before the amorphous Si capping layer
was deposited. As a result, the further intermixing of Si
atoms from the amorphous capping layer becomes
rather difficult. To check this conjecture, a-Si(20 nm)/
Y(20 nm)/Si, a-Si(20 nm)/Y(30 nm)/Si, and a-Si(20
nm)/Y(60 nm)/Si samples were also prepared. Crystalline phases were found to form in all these samples in
as-deposited forms by both XRD and TEM. The XRD
spectra revealed the phases formed in as-deposited samples
free from any artifacts induced by ion milling. Considerable
intermixing between the amorphous Si capping layer and
yttrium thin films was found in these samples from AES
concentration depth profiles. TEM observation and diffraction analysis revealed the simultaneous presence of polycrystalline Y, Y5Si3, YSi, Si, and amorphous interlayer.
In as-deposited 20- to 60-nm-thick yttrium thin films
on silicon samples, a 2.5-nm-thick amorphous interlayer was also found to form at the Y/c-Si interface.
Y5Si3 was also found to be present. An example is shown
in Figure 8. The thickness of the amorphous interlayer
was found to increase with annealing time in samples
annealed at 250°C up to 60 minutes. Compared to the
results obtained for a-Si(20 nm)/Y(10 nm)/Si samples, it
is apparent that as the critical thickness of the amorphous layer was exceeded, further intermixing of Y and
Si was overtaken by the formation of crystalline phases
at room temperature. Schematic diagrams of the interfacial reactions in the Y/Si system at low temperatures
are shown in Figure 9.
Amorphous interlayers were also found to form in
Er/Si, Tb/Si, Dy/Si, and Gd/Si systems. For both Er/Si
and Tb/Si samples, a 2.5-nm-thick amorphous interlayer was observed in the as-deposited samples. The
growth followed a linear growth law in samples annealed at 190–250°C. The maximum thicknesses of the
a-interlayers were measured to be 9 and 10 nm for the
Er/Si and Tb/Si systems, respectively. Examples are
shown in Figure 10 (Luo et al., unpublished data).
PLATINUM GROUP METAL/Si SYSTEMS
Amorphous interlayers were found to form in a
Rh/a-Si diffusion couple as early as 1983 (Herd et al.,
1983). More recently, solid state amorphization was
also found to occur in the Rh/c-Si system (Tu et al.,
1991). For metal/Si systems, Pt/Si was the first system
HREM OF AMORPHOUS INTERLAYERS
143
Fig. 10. HRTEM images of Er/(001)Si samples annealed at 225°C for (a) 10, (b) 30, (c) 60, and (d) 90
minutes, showing the growth of the amorphous interlayer.
in which amorphization by solid-state diffusion was
discovered. However, the layer was rather thin and
discontinuous (Abelson et al., 1988). A recent study
showed that the amorphous interlayer is also present in
as-deposited Ir thin film on silicon (Chen et al., 1994).
EFFECTS OF ATOMIC COMPOSITION ON THE
THERMAL STABILITY OF AMORPHOUS
INTERLAYERS IN Y-Si MULTILAYER SYSTEMS
Three sets of multilayer films with overall atomic
composition ratios 1Y:2Si, 1Y:1Si, and 5Y:3Si were
prepared. The films consisted of a total of 13 alternating
Si and Y layers, with the starting and final layers being
Si. The thickness of the Si layer was fixed at 5 nm,
whereas the thicknesses of the Y layers varied from 5 to
17.5 nm to yield the desired average atomic concentrations. The top a-Si layer also served as a capping layer
to protect the thin film assembly from oxidation during
subsequent heat treatments. The configurations of the
three sets of samples are depicted in Figure 11.
A 65-nm-thick amorphous Y-Si intermixing layer was
found to form in multilayer films with an overall
composition of YSi2 at room temperature. An XTEM
image and AES depth profile are shown in Figure 12.
Homogenization of atomic compositions was found to
proceed in samples annealed at 250–350°C. Examples
are shown in Figure 13. The multilayer remained
amorphous in this temperature range. In samples
annealed at 400°C, the amorphous layer was found to
convert to crystalline YSi2.
For multilayer films with an overall composition of
YSi, crystalline phases were found to be present between amorphous layers in as-deposited samples. Figure 14 shows an HRTEM image of an as-deposited
sample. About 6-nm-thick amorphous interlayers were
found to form between crystalline phases. From TEM
diffraction analysis, crystalline Y5Si3, YSi, and Y are
present. The average grain size was measured to be 20
nm. The results are consistent with those obtained from
XRD spectrum. From the AES profile, significant interdiffusion was found to occur in as-deposited samples. In
samples annealed at 400°C for 5 min, Y5Si3 and YSi
were found to form at the expense of the amorphous
layer. The average grain size was increased to 25 nm. In
samples annealed at 500°C for 10 minutes, both Y5Si3
and Y2O3 were found to be present. In samples annealed at and higher than 600°C, Y2O3 was the only
compound phase that remained.
The results obtained for multilayer films with an
overall composition of Y5Si3 are similar to those of the
1Y:1Si films. XRD spectrum of an as-deposited sample
revealed that crystalline Y5Si3, YSi, and Y phases were
144
L.J. CHEN ET AL.
Fig. 11.
A schematic diagram showing the configurations of as-deposited multilayer samples.
found to be present. About 5.5-nm-thick amorphous
interlayers were found to form between crystalline
phases. From TEM diffraction analysis, crystalline
Y5Si3, YSi, and Y are present. The average grain size
was measured to be 15 nm. Figure 15 shows an HRTEM
image. Y5Si3 was found to be the only silicide phase
formed in samples annealed at 400°C. The average
grain sizes of Y5Si3 were found to increase from 20 nm
to 1 µm as the annealing temperature was increased
from 300 to 400°C.
The formation of crystalline Y5Si3 and YSi was found
in as-deposited samples prepared with excess Y so that
the composition of the top intermixing layer exceeds a
critical value. The Y overlayer was found to react with
the amorphous layer to form polycrystalline Y5Si3, YSi,
and Si in samples annealed at temperatures lower than
400°C. In 5Y:3Si films, Y5Si3 was found to be the only
silicide phase formed in samples annealed at 400°C.
The results indicated that the nucleation of the crystalline phase depends on the composition. The thickness of
the amorphous layer was found to decrease with annealing temperature and/or annealing time. The height of
nucleation barriers to form silicides in the Y-Si system
was seen to depend critically on the atomic composition
of the layer, which in turn significantly influences the
phase formation sequence of the system.
Metastable free energy diagrams provide a basis for
predicting whether the solid-state amorphization can
proceed. Based on a growth control model, it was
concluded that the formation of the amorphous layer at
room temperature was controlled by nucleation (Lee
and Chen, 1994).
CORRELATIONS WITH PHYSICAL
PARAMETERS
Miedema has expressed the energy in terms of differences in chemical potential and electron density at the
Wigner-Seitz cell of the two elements (Miedema et al.,
1976). A large difference in chemical potential is required for the significant intermixing of two elements.
Previous studies revealed the existence of good correlations among differences in atomic size between metal
and Si atoms, the calculated free energy difference in
forming amorphous phase, and critical and maximum
amorphous interlayer thickness for many refractory
metals and silicon (Ti/Si, Zr/Si, Hf/Si, V/Si, Nb/Si, and
Ta/Si) and Y/Si systems (Chen et al., 1994). Pertinent
data are listed in Table 1. It is seen that the differences
in both size and electronegativity between Y and Si
atoms are considerably larger than those between
refractory metal and Si atoms. In addition, the free
energy of mixing for the Y/Si system is large and
negative. As a result, a-interlayer is expected to be
more prone to form in the Y/Si system. Indeed, the Y/Si
system is the only system found up to date among all
metal/Si systems in which the a-interlayer can be
grown to a thickness of 10 nm during deposition at room
temperature.
STRUCTURE OF AMORPHOUS INTERLAYERS
HRTEM in conjunction with optical diffractometry
has been used to analyze amorphous interlayer and
identify the first nucleated phase in metal thin films on
silicon systems. However, crystallites less than 1 nm in
HREM OF AMORPHOUS INTERLAYERS
145
Fig. 12. (a) HRTEM image and (b) AES depth profile of the
alternating a-Si and Y multilayer films with an atomic composition
ratio close to 1Y:2Si.
size may be embedded in the amorphous interlayer and
undetectable with direct imaging technique in HRTEM. On the other hand, a correlation technique is a
statistical analysis of images in real space. The autocorrelation function (ACF) is equivalent to a twodimensional form of Patterson function, well known in
X-ray crystallography in that it represents a map of
interatomic vectors (Fan and Cowley, 1985). The correlation functions were introduced into electron microscopy when it became clear from optical diffraction
experiments that different electron micrographs of the
same specimen can be aligned with high accuracy
despite their apparent differences due to contamination, radiation damage, electron and photographic noise,
and change in defocus (Fan and Cowley, 1985; Frank,
1980).
ACF analysis has been applied to the high-resolution
transmission electron microscope images of amorphous
interlayers formed in the interfacial reactions of ultrahigh vacuum deposited V, Zr, and Mo thin films on
(111)Si. The results demonstrate the usefulness of the
ACF analysis of atomic images in amorphous interlayers formed in the initial stage of reactions in metal thin
films on silicon.
146
L.J. CHEN ET AL.
Fig. 13. (a) HRTEM image and (b) AES depth profile of an 1Y:2Si
multilayer film annealed at 300°C for 30 minutes.
In V/Si samples, an amorphous interlayer was found
to form. Figure 16 shows an example of a sample
annealed at 430°C for 120 minutes. The presence of a
crystalline phase and/or a short-range order in the
amorphous region is not evident. ACF analysis was
carried out on templates of 1.2 3 1.2 nm2 with each
pixel corresponding to 0.019 3 0.019 nm2 cut along the
interlayer in the HRTEM micrograph. The templates
were patched on double in size square regions with a
homogeneous background that is set to be equal to the
average value of the original intensity so that the
ACF-processed images are free from wrap-around arti-
fact. Periodic structures were observed in some of the
ACF images that revealed the short-range ordering in
the amorphous interlayers. Typical examples of ACFprocessed images are shown in Figure 17. The crosshatch pattern was found to correspond to V3Si and
V5Si3 from both interplanar spacing and angular measurements. It was found that the ACF-processed image
and the simulated image following Patterson function
treatment match rather well. Examples are shown in
Figure 18. The calculated Patterson function images
indicated that the thickness of the region with a
short-range order is less than 2 nm. Similar analysis
HREM OF AMORPHOUS INTERLAYERS
Fig. 14.
HRTEM image of an as-deposited 1Y:1Si multilayer film.
Fig. 15.
HRTEM image of an as-deposited 5Y:3Si multilayer film.
.
147
148
L.J. CHEN ET AL.
Fig. 16.
minutes.
HRTEM image of an a-Si (20 nm)/V(30 nm)/(001)Si sample annealed at 430°C for 120
Fig. 17.
ACF-processed images of the outlined regions shown in Figure 16.
was carried out for the amorphous interlayer formed in
the Zr/Si system. Zr2Si, ZrSi, and ZrSi2 were found to
form (Chen et al., 1994). For Mo/Si system, Mo3Si was
identified to be the first nucleated crystalline phase
that is correlated to the stable structure of the amorphous Mo-Si alloy. Both Mo3Si and Mo5Si3 were found
to form simultaneously under certain annealing conditions (Liang and Chen, 1994).
FORMATION OF CRYSTALLINE PHASES
The first phase nucleation is one of the long-standing
problems in the interfacial reactions of metal thin films
HREM OF AMORPHOUS INTERLAYERS
Fig. 18.
V5Si3.
149
(a) HRTEM image, (b) ACF-processed image, (c) pattern function, and (d) simulated image of
Fig. 19. XTEM micrograph, HRTEM image of a sample annealed at 450°C for 1 hour. Inset is the
optical diffraction pattern corresponding to [ 110]Ti5Si3.
on silicon. Theoretically, difficulties were encountered
in predicting the reaction product of a basically nonequilibrium process from equilibrium thermodynamics.
Experimentally, conventional techniques, such as X-ray
diffraction and electron diffraction, are more appropriate for the detection of the growth phase rather than for
the unambiguous identification of the first nucleated
phase owing to their limited sensitivity and resolution
in the determination of the phase. For the Ti/Si system,
as many as four different crystalline phases (Ti5Si3,
TiSi, C49-TiSi2, and C54-TiSi2 ) were reported to form in
thin film reactions. The close proximity and/or overlap-
150
L.J. CHEN ET AL.
Fig. 20. Schematic diagram of formation of Ti-silicides in samples
annealed at 475°C for 30–60 minutes or at 500°C for 10–20 minutes.
ping of diffraction rings of these silicides rendered the
unambiguous identification of phases during the initial
stages of reactions rather difficult. As a result, although
the final stable phase is known to be C54-TiSi2, the
phase formation sequence is still unclarified. Ti5Si3,
TiSi, and C49-TiSi2 were variously reported to be the
first nucleated phase. HRTEM in conjunction with
optical diffractometry (or fast Fourier transform technique) were used to identify the first nucleated phase in
silicide formation. The combined techniques are capable of unambiguous identification of a phase as small
as 1 nm in size. The sensitivity compared favorably
with conventional selected area electron diffraction and
microdiffraction, which correlate diffraction patterns
with areas about 500 and 100 nm in size in the
specimens, respectively. Since Ti is a strong oxygen
getter, to minimize the influence of oxygen on the
interfacial reactions in the Ti/Si system, UHV deposition of the thin films is considered to be most appropriate.
In the Ti/Si system, Ti5Si3, located at the
Ti/a-interlayer interface, was identified to be the first
nucleated phase. Ti5Si3, Ti5Si4, TiSi, and C49-TiSi2
along with the amorphous interlayer were observed to
be present simultaneously in samples annealed at
higher temperatures. Fundamental issues in silicide
formation need to be addressed in light of the discovery
of the formation of the amorphous interlayer and as
many as four different silicide phases in the initial
stages of interfacial reactions of UHV deposited Ti thin
films on silicon. An example and a schematic diagram
are shown in Figures 19 and 20, respectively (Wang and
Chen, 1992). Simultaneous occurrence of multiphases
was also observed in the interfacial reactions of the
Zr/Si, Hf/Si, V/Si, Cr/Si, Mo/Si, and Y/Si systems (Chen
et al., 1994; Hsieh et al., 1993; Lee and Chen, 1993;
Liang and Chen, 1994; Lin et al., 1996).
CONCLUSIONS
HRTEM has been fruitfully applied to investigate the
formation of the amorphous interlayers in metal/Si
systems. Amorphous interlayers were found to form in
all refractory metal and a number of rare-earth and
platinum group metal/Si systems. For Ti/Si, Zr/Si,
Hf/Si, V/Si, Nb/Si, and Ta/Si systems, the growth of
a-interlayer follows a linear law in the initial stage. The
maximum thickness of amorphous interlayers and activation energy of the growth are correlated with the free
energy of mixing, in particular to the difference in
electronegativity between elemental metals and silicon
for the metals in the same group in the periodic table
and silicon systems. Si atoms were found to be the
dominant diffusing species in the solid phase amorphization in Ti/Si, Zr/Si, and Hf/Si systems. For Y/Si system,
the stability of amorphous interlayer depends critically
on the composition of the amorphous films.
Auto-correlation function analysis was utilized to
determine the structure of the amorphous interlayers.
Taking advantage of the distinct contrast between
amorphous interlayer and crystalline phase, HRTEM,
in conjunction with the optical diffractometry (or fast
Fourier transform technique), as applied to determine
the first nucleated crystalline phase. The simultaneous
presence of multiphases was observed to occur in a
number of refractory metal/Si systems.
ACKNOWLEDGMENTS
The research was supported by the Republic of China
National Science Council through grant NSC84-2215E007-011.
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