Growth of Ti and TiSi2 films on Si(111) by low energy Ti+ beam deposition

Article abstract of DOI:10.1016/S0039-6028(00)00339-3

The deposition of titanium and titanium disilicide thin films on Si(111) by low-energy (10-500 eV) Ti⁺ beams in the temperature range 25-700°C has been investigated by in-situ Auger electron spectroscopy (AES) and reflection high-energy electron diffraction (RHEED) as well as ex-situ depth profile X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and transmission electron diffraction (TED). Small XPS chemical shifts in the Si_2p and Ti_2p peaks and changes in the AES line shapes originating from elemental silicon and titanium and the titanium disilicide compound have been resolved. The XPS shifts were used for resolution of the silicide layer in the depth profiles. This interfacial silicide layer is formed even at 25°C, and its characteristics are dependent on both Ti⁺ energy and Si temperature. The RHEED and TED results confirm the growth of a polycrystalline Ti film for T≤500°C, the C49-TiSi₂ phase for T～600°C, the C54-TiSi₂ phase for T～700°C or above, and the existence of synergistic effects between the substrate temperature and Ti⁺ kinetic energy on the crystalline quality of the films. The AFM images exhibit a broad range of morphologies as a function of growth conditions. The effects of energetic ions and temperature in Ti and TiSi₂ film deposition are discussed accordingly.

Full text of DOI:10.1016/S0039-6028(00)00339-3

Surface Science 453 (2000) 159–170
www.elsevier.nl/locate/susc
Growth of Ti and TiSi films on Si(111) by low energy Ti+
2
beam deposition
S.M. Lee, E.T. Ada, H. Lee, J. Kulik, J.W. Rabalais *
Department of Chemistry, University of Houston, Houston, TX 77204-5641, USA
Received 11 October 1999; accepted for publication 27 January 2000
Abstract
The deposition of titanium and titanium disilicide thin films on Si(111) by low-energy (10–500 eV) Ti+ beams in
the temperature range 25–700°C has been investigated by in-situ Auger electron spectroscopy (AES) and reflection
high-energy electron diffraction (RHEED) as well as ex-situ depth profile X-ray photoelectron spectroscopy ( XPS),
atomic force microscopy (AFM), and transmission electron diffraction (TED). Small XPS chemical shifts in the
Si and Ti peaks and changes in the AES line shapes originating from elemental silicon and titanium and
2
p
2p
the titanium disilicide compound have been resolved. The XPS shifts were used for resolution of the silicide layer in
the depth profiles. This interfacial silicide layer is formed even at 25°C, and its characteristics are dependent on both
Ti+ energy and Si temperature. The RHEED and TED results confirm the growth of a polycrystalline Ti film for
T≤500°C, the C49–TiSi phase for T~600°C, the C54–TiSi phase for T~700°C or above, and the existence of
2
2
synergistic effects between the substrate temperature and Ti+ kinetic energy on the crystalline quality of the films.
The AFM images exhibit a broad range of morphologies as a function of growth conditions. The effects of energetic
ions and temperature in Ti and TiSi film deposition are discussed accordingly. © 2000 Elsevier Science B.V. All
2
rights reserved.
Keywords: Ion–solid interactions; Silicon; Surface structure, morphology, roughness, and topography; Titanium
1
. Introduction
grown by physical vapor deposition or sputter
deposition onto silicon surfaces followed by high-
temperature treatment. Four different crystalline
phases (Ti Si , TiSi, C49–TiSi , and C54–TiSi )
The growth of titanium disilicide has been
extensively studied due to interest in its material
properties, such as low resistivity, relatively good
thermal stability to high temperatures, and com-
patibility with current processing steps. It is consid-
ered to be an optimum choice for applications as
contacts and interconnects on Si metal oxide semi-
conductors (MOS) [1–5]. In processing and most
of the studies reported to date, silicide films are
5
3
2
2
and an amorphous phase have been reported to
form in these thin films. C54–TiSi is reported to
2
be the thermodynamically favorable phase at tem-
peratures above 700°C. C49–TiSi is the kinetically
2
favorable metastable structure that forms at the
lower temperatures of 500–600°C in this thickness
region. While C54–TiSi is the most stable state
2
with the lowest resistance, it has several drawbacks,
such as rough surface morphology, incomplete
surface coverage, and high fabrication temperature
*
Corresponding author. Fax: +1-713-743-2709.
E-mail address: rabalais@uh.edu (J.W. Rabalais)
0
039-6028/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0039-6028 ( 00 ) 00339-3

1
60
S.M. Lee et al. / Surface Science 453 (2000) 159–170
that are incompatible with the fabrication of shal-
low junctions. Various film fabrication methods,
such as codeposition of Si and Ti and Ti deposition
onto amorphous Si substrates, have been
attempted in order to overcome these
shortcomings.
It has been observed that the Schottky barrier
in titanium disilicide is established during the early
stages of interface formation and is relatively inde-
pendent of postannealing conditions [6]. As a
result, in order to understand the growth mecha-
nism and influencing factors and to reduce the
processing temperature, many studies have concen-
trated on understanding the interfacial reactions
between Ti and Si in the initial phase of silicide
formation and the roll of impurities such as oxygen
during film growth [7]. Several techniques have
been used to study the silicide interface by depth
profiling, including Auger electron spectroscopy
beam epitaxy, six distinct high-quality single crys-
tal orientations of the hcp Ti surface structure
were reported [18]. On MgO, Ti is known to grow
as a face-centered tetragonal structure [19].
However, to the best of our knowledge, no ordered
structure of Ti on Si has been reported. At submo-
nolayer coverages, Ti does not form any ordered
surface phase on Si at both low and elevated
temperatures [20]. When Ti is deposited to form
˚
a relatively thick (50~500 A) film, only a diffuse
textured reflection high-energy electron diffraction
(RHEED) pattern is observed, which represents
an amorphorized surface structure [2,21].
In this paper, we present a study of Ti and
TiSi film growth by low-energy Ti+ beam depos-
2
ition on clean Si(111) surfaces. By comparing films
grown from Ti+ beams of different kinetic energies
(10–500 eV) and temperatures (25–700°), we show
how the energy of the impinging Ti+ affects the
structure and composition of the film surface, the
silicide phase, and the film–substrate interface.
Synergism between ion-beam-induced effects and
substrate temperature has been observed, suggest-
ing the possibility of film processing at low temper-
atures [22,23]. Ti film deposition is superseded by
direct formation of titanium silicide at temper-
atures between 500 and 600°C [24]. Hence, the
results reported herein cover both the Ti metal
(
AES), X-ray photoelectron spectroscopy ( XPS),
secondary ion mass spectrometry (SIMS), and
Rutherford backscattering spectrometry (RBS)
[
8–10]. The analyses produced depth profiles that
only show the changes in the elemental ratio of
the total amount of Ti and Si along the profiles.
These studies have not defined the silicide com-
pound in the depth profiles [7,11]. Since the elec-
tronegativities of Ti and Si are similar (Si=1.8
and Ti=1.5 in Pauling scale) [12], it has been
difficult to observe a chemical shift between pure
Si and silicide in the XPS spectra. Accordingly,
only a small change in peak shape has been
observed in AES [13]. SIMS and RBS do not
provide direct information regarding the chemical
bonding state of the elements in the film. The
effect of Si substrate crystallinity on the silicide
formation process has been studied [14], but many
points remain uncertain in this area also.
film and the TiSi film. The crystallinity of these
2
films was monitored by transmission electron
diffraction (TED) and in-situ RHEED, the surface
morphology was studied by atomic force micro-
scopy (AFM), the ratio of Ti to Si was monitored
by in-situ AES, and the interfacial silicide layer
was resolved by depth-profiling XPS.
2. Experimental
The role of the Ti film crystallinity in silicide
formation has rarely been studied, even though Ti
film growth is one of the major steps in the actual
formation of titanium disilicide. The high reactivity
of Ti with residual oxygen and carbon and segre-
gation of sulfur from the bulk during the high-
temperature cleaning process have impeded such
studies [15,16]. A low-energy electron diffraction
The ion-beam deposition system consists of a
Freeman ion source, laminated magnets, and a
retarding lens [25]. The ions were accelerated to
10 keV for transport and mass selection and
retarded to the specified ion energy on the
grounded target. An energy range of 5 eV–10 keV
is available. The energy profile of the beam has a
Gaussian shape with a FWHM of ± 3 eV. The
unscanned beam has a ribbon-like shape, flat over
10–15 mm along the vertical axis with a Gaussian-
(
LEED) study of the Ti(0001) surface is consistent
with two different terminations [17]. When Ti was
grown on the bcc metals Ta and Mo by molecular

S.M. Lee et al. / Surface Science 453 (2000) 159–170
161
like shape of FWHM of 3–5 mm along the hori-
zontal axis. This FWHM was increased to 8–
0 mm, and uniformity was improved by scanning
1
the beam horizontally. The current density of the
scanned 100 eV 48Ti+ beam was ~2 mA/cm2. The
base and beam-on-target pressures of the depos-
ition chamber were 2×10−8 and 1× 10−7 Pa,
respectively. The sample temperature was moni-
tored by using an infra-red pyrometer and cal-
ibrated with a thermocouple. The instrument is
equipped with in-situ AES, XPS, and RHEED.
The surface structure of the titanium film was
examined in situ by using RHEED at 20 kV accel-
eration voltage with an incident angle of 1.5° to
the sample surface. Ex-situ XPS spectra and depth
profiles using 5 keV Ar+ sputtering were collected
on a Physical Electronics Model 5700 instrument
using standard AlKa X-rays. TED patterns were
obtained with a JEOL-JEM2000FX transmission
electron microscope (TEM) operated at 200 kV.
Si(111) samples (CZ, p-type, B-doped, 0.3 Vcm)
measuring ~7×7 mm were ultrasonically cleaned
in methanol prior to loading into the UHV cham-
ber. Before deposition, the samples were annealed
at 1200°C for 1 min, and the surface cleanness and
crystallinity were checked by in-situ AES and
RHEED. Ti+ beams of four different kinetic ener-
gies (10, 50, 100, and 500 eV) were used for
deposition, while the substrate was maintained at
temperatures of 25, 200, 400, 500, 600 and 700°C.
The Ti+ ion current to the sample was
Fig. 1. AES spectra showing the Si LMM (a) and Ti LMM (b
and c) peaks after 10 eV Ti+ deposition of 1×1017 Ti/cm2 on
Si at 400 and 500°C. At 400°C, the surface is completely covered
with Ti, and no Si AES peak is observed.
Fig. 1. The dose of 1×1017 ions/cm2 is large
enough to cover the surface with a metallic Ti film
(
Fig. 1b) when the temperature is below 400°C;
no Si peak is observed at these low temperatures.
As the attenuation length of Si-LMM electrons
below a Ti film is estimated to be ~2.1 ML [26],
the thickness of Ti film on the Si(111) substrate
should be at least 4.5 nm. When the temperature
is 500°C or higher, elemental Si (Fig. 1a) is
observed, and the Ti spectrum changes to that
characteristic of titanium silicide (Fig. 1c). The
ratio is [Si]/[Ti]~20, as determined from the AES
sensitivity factors (Fig. 1a and c). This indicates
that the onset for diffusion of Si into Ti is between
400 and 500°C. It has already been demonstrated
that Si is the major diffusant in the Ti/Si system
[27]. The surface free energy of Si is 1.107 J m−2,
while that of Ti is 2.570 J m−2 [28]. At higher
temperatures where the Si is mobile, surface free
energy is reduced by diffusion of Si through the
Ti film and onto its surface.
1
8
5 nA/mm2, and the deposition spot size was
×10 mm2. Depositions were carried out for vary-
ing time periods up to a maximum dose of
×1017 ions/cm2, which is equivalent to ~40 nm
thickness of Ti assuming unit sticking probability
9]. After deposition, AES and RHEED were used
1
[
to study the film composition and structure. The
samples were then removed from the deposition
chamber for AFM, XPS, and TED studies.
3
3
3
. Results
.1. Titanium film
.1.1. Auger electron spectra (AES)
The Ti-LMM and Si-LMM AES spectra after
Ti+ deposition at 400 and 500°C are shown in

1
62
S.M. Lee et al. / Surface Science 453 (2000) 159–170
3.1.2. Transmission electron diffraction (TED)
A TED image from a plan view sample depos-
ited at 200°C is shown in Fig. 2. It exhibits a ring
pattern corresponding to a polycrystalline film that
agrees well with the crystal structure of Ti. The
measured d-spacings of the diffraction rings corre-
spond to those of metallic Ti within 1% error
range. A dark-field image recorded using a portion
of the first (lowest angle) diffraction ring reveals
Ti grains of ~10–20 nm in size. For deposition at
500°C, the TED image exhibited the same poly-
crystalline Ti phase, but its surface contained some
elemental silicon that had diffused from the
substrate.
3.1.3. Reflection high-energy electron diffraction
(
RHEED)
Fig. 3. RHEED patterns of clean Si and the surface after Ti+
deposition at 10 and 100 eV at 25°C.
RHEED patterns for the Ti films grown under
different conditions are shown in Figs. 3–5. The
25°C images of Fig. 3 show the clean Si(111)
substrate and 10 and 100 eV Ti deposition images.
The clean Si surface exhibits a 7×7 pattern as
expected. The Ti film exhibits a textured 2×1
pattern at 10 eV that blurred at 100 eV. The blur-
ring indicates imperfections in the single crystalline
grains and their 3D features, which contributes to
formation of the cigar shaped patterns. For depos-
ition at 200°C in Fig. 4, energies of 50 eV or lower
yield sharp 1×1 streaky patterns, indicating that
the Ti film is crystalline. At higher energies, this
streaky pattern changed to a blurred 2×1 textured
pattern as observed at 25°C. For deposition at
400°C in Fig. 5, the images from all four energies
exhibit long streaky patterns that are sharper than
those from samples at lower temperatures. This is
due to the enhanced mobility of the atoms, that
eventually contributes in reducing the 3D features
and defect density in the surface region. At the
higher energies used in Fig. 5, the patterns lose
their intensity rather than becoming blurred spotty
patterns, as observed at lower temperatures. The
images of all the Ti films exhibit no azimuthal
angle dependency to the incident electron beam.
Such behavior has been observed previously for
Sb film growth on GaAs and is attributed to
Volmer–Weber growth, where each single crystal-
line island has the same contact plane with respect
to the substrate, but with random azimuthal orien-
tation. Therefore, our observations indicate that
the Ti films consist of single crystal islands with
random azimuthal orientations whose crystalline
quality depends on the growth conditions, i.e.
kinetic energy and temperature.
In order to compare deposition at 25°C followed
by annealing with the above results, 10 eV Ti+
was deposited on Si(111) at 25°C and post-
Fig. 2. TED image from a plan view sample of a Ti film depos-
ited on Si(111) from a 50 eV Ti+ beam at 200°C.

S.M. Lee et al. / Surface Science 453 (2000) 159–170
163
10 eV, 400°C and 50 eV, 200°C exhibit the best
crystalline order with comparable qualities.
It has been reported that the size distribution
of islands during film growth was reflected in the
RHEED images for In films on GaAs [29]. For
the Ti/Si system, our RHEED images are similar
for deposition conditions that produce different
surface morphologies, e.g. 50 eV, 200°C and 10 eV,
400°C (Figs. 4 and 5). This indicates that the
quality of the RHEED images of Ti/Si is deter-
mined by atomic structures of a scale <1 mm that
do not necessarily reflect the surface morphology.
3.1.4. Atomic force microscopy (AFM)
AFM images of the surfaces of clean Si and Si
after deposition of Ti+ at different conditions are
shown in Fig. 6. The clean Si surface consists of
ridges with an average height and width of 13 and
Fig. 4. RHEED patterns after Ti+ deposition at 10, 50, 100,
and 500 eV at 200°C.
˚
00 A, respectively. The surface morphology of
5
the Ti film exhibits a strong dependence on the
growth conditions. The 10 eV, 25°C sample still
maintains the ridge structure of the substrate, but
Fig. 5. RHEED patterns after Ti+ deposition at 10, 50, 100,
and 500 eV at 400°C.
annealed while monitoring the RHEED pattern as
a function of temperature. The textured streaky
pattern became vague as the temperature
increased. This implies that the observation of
sharp streaky patterns from Ti films deposited
from Ti+ beams is a result of a combined synergis-
tic effect of temperature and kinetic energy. For
example, the films formed under the conditions
Fig. 6. AFM images of the surfaces of clean Si and after depos-
ition of Ti+ at different energies and temperatures. (a) and (j)
have 0.5×0.5 mm2 scales, and all the others have
1.0×1.0 mm2 scales.

1
64
S.M. Lee et al. / Surface Science 453 (2000) 159–170
2p line is at 99.6 eV for Si and at 99.3 eV for the
interfacial silicide region of the Ti deposited
sample. This shift of 0.3 eV to a lower binding
energy for the deposited sample reflects the more
negative electronic environment of the interfacial
silicide compared to elemental Si, as expected for
titanium silicide. The Ti 2p
lines after Ti
1/2,3/2
deposition are shown in Fig. 8b. These lines are
broad and asymmetrical, but their maxima are
clearly shifted by ~0.1–0.2 eV. This small shift is
typical of chemical shifts observed between met-
allic Ti and silicide. Therefore, the XPS spectra of
this sample deposited at room temperature indicate
the formation of a metallic Ti layer with a thin
interfacial silicide layer between the metal and
elemental Si.
Fig. 7. RMS roughness values for the clean Si and the Ti depos-
ited films in Fig. 6 as a function of ion energy.
An example of an XPS depth profile of a
it is covered with small grains that reflect the 3D
growth mode of the Ti film. As the ion energy
increases, these small grains of ~200 A dominate,
TiSi film that was deposited from a 50 eV Ti+
2
beam at 600°C is shown in Fig. 9. The small silicide
˚
chemical shift (~0.3 eV) was used to separate the
elemental Si and Si in the silicide film. Fig. 9 shows
a clearly resolved titanium silicide film layer in
which elemental Si is below the detection level.
Note that the relative atomic concentrations of Si
and Ti in the silicide film have an almost perfect
and the ridge structure becomes less obvious.
When the temperature is increased to 200°C, the
surface exhibits different ridge structures, depend-
ing on the ion energy. For 10 eV, 200°C, the ridge
morphology is conserved, while it develops larger
features for 500 eV, 200°C whose average height
stoichiometric composition of TiSi . These indivi-
2
˚
and width are 150 and 2000 A, respectively. For
dual elemental concentrations are relatively flat
1
0 eV, 400°C, large sharp protrusions are observed
over a long sputtering time, indicating that the
composition is uniform over the width of the film.
This stoichiometry persists for Ti+ deposition ener-
gies of less than 100 eV. The d-XPS plot shows
that there are three regions of interest in the
profile: (1) the outermost surface layers where
concentrations are changing rapidly due to intro-
duction of Ti into the Si; (2) the silicide film region
where the concentrations are constant as a function
of sputtering time; (3) the interface region between
the silicide film and the silicon substrate where
concentrations again change rapidly.
that evolve into randomly disordered features for
higher ion energies. The RMS values for clean Si
and the Ti deposited films are plotted in Fig. 7 as
a function of ion energy. The RMS value for the
1
00 eV, 25°C sample is very near that of clean Si,
while the 10 and 500 eV points are slightly higher.
The 200°C sample exhibits a fourfold increase in
RMS over the energy range, while the 400°C
sample exhibits little energy dependence.
3.2. Titanium silicide film
3.2.1. X-ray photoelectron spectroscopy (XPS)
3.2.2. Transmission electron diffraction (TED)
depth profiles
The phase of the resulting TiSi films was
determined by TED. Diffraction patterns from
2
XPS spectra of the Si 2p
lines from clean
1/2,3/2
elemental Si and Si after Ti+ deposition (50 eV,
plan view samples were recorded. The camera
length was calibrated using the reflections from
the Si substrate. The ring pattern from the sample
deposited at the lower temperatures, i.e. 25–600°C,
was readily indexed using the cell parameters for
25°C, 1 × 1017 ions/cm2) are shown in Fig. 8a. A
chemical shift between the two samples is observed,
indicating that a chemical reaction has occurred
between the Ti and Si. The peak position of the

S.M. Lee et al. / Surface Science 453 (2000) 159–170
165
Fig. 8. XPS spectra of the (a) Si 2p
line from elemental and silicide Si and the (b) Ti 2p
lines after Ti+ deposition (50 eV,
1/2,3/2
1
/2,3/2
2
5°C, 1 × 1017 ions/cm2) on Si. The dashed lines in (a) correspond to the fitted Si 2p and 2p components.
1/2
3/2
Fig. 9. Ex-situ XPS depth profile of a TiSi film grown by means of a 50 eV Ti+ beam on a Si(111) substrate at 600°C. The depth
2
profile used 5 keV Ar+ for sputtering. Clear resolution of the TiSi film from the substrate Si and a sharp interface region are
2
observed. The inset shows the chemical shift between the 2p peaks of the silicide Si and the elemental Si in the bulk.
the C49–TiSi phase. The higher temperature
depositions, i.e. 700°C, were readily indexed using
example of the ring pattern observed from the C54
phase is shown in Fig. 10. This pattern exhibits
rather spotty rings because of the relatively large
2
the cell parameters of the C54–TiSi phase. An
2

1
66
S.M. Lee et al. / Surface Science 453 (2000) 159–170
Fig. 10. TED image from a plan view sample of a C54–TiSi film deposited on Si(111) from a 50 eV Ti+ beam at 500°C.
2
grains — on the order of 0.5 mm in size. Three
3.3. Titanium silicide–silicon substrate interface
conventional bright-field images that show the
grain structure of the C54–TiSi phase are shown
in Fig. 11. In each image, one grain is seen in
strong contrast because of dynamical diffraction
conditions, while the surrounding grains, although
still visible, exhibit a weaker contrast because of
kinematic or near-kinematic diffraction conditions.
3.3.1. X-ray photoelectron spectra (XPS)
2
Fig. 12 shows depth profiling analyses for Ti
films grown with Ti+ energies of 50 and 500 eV at
temperatures of 25 and 400°C. The thickness of
the metallic Ti overlayer film decreases by about
50% as the deposition energy increases from 50 to
Table 1
Details of the interfacial silicide layer derived from the XPS data
Deposition conditions
50 eV, 25°C
500 eV, 25°C
50 eV, 400°C
500 eV, 400°C
Atomic percentage of silicide (from XPS data of Fig. 12)
FWHM of interfacial silicide (nm)
28
18.5
40
13.5
26
23.4
38
18.0

S.M. Lee et al. / Surface Science 453 (2000) 159–170
167
500 eV as a result of the higher self-sputtering
yield and lower Ti sticking probability at a higher
energy. For the silicide profiles, the maxima
increase as the deposition energy increases, and
the FWHM increases as the temperature increases.
The profiles clearly demonstrate the existence of
an interfacial silicide layer and its dependence on
the Ti+ energy and the Si temperature, as shown
by the data of Table 1.
3.3.2. Classical ion trajectory simulations
Classical ion trajectory simulations were per-
formed using the TRIM program [30] for Ti+
impinging on a Ti film at normal incidence as a
function of kinetic energy in order to determine
the number of vacancies (V) produced and the
sputtering yield (Y) per incident Ti+. The vacancy
results are shown in Fig. 13a. The number of
vacancies (V) increases linearly with increasing ion
energy. These vacancies accumulate in the subsur-
face region as the film grows under a continuous
deposition flux, and their densities are higher for
higher Ti+ energies. With this constant generation
of defects in the film during deposition, there is a
higher probability of void formation in the Ti film.
Such void formation inhibits the diffusion of Si
into Ti for those films formed at the higher Ti+
energies. This is most evident for ion energies
above 500 eV [24,31].
The self-sputtering yield of Ti (Y ) and the
Ti
sputtering yield of Ti on Si (Y ) were calculated
Si
for Ti+ at 50 and 500 eV impinging on a film
composed of homogeneously mixed Ti and Si
atoms as a function of the [Ti]/([Ti]+[Si]) atomic
ratio (Fig. 13b). Such a film simulates the initial
stages of Ti deposition on Si. The Y values at
50 eV are <0.03, as expected. At 500 eV, these Y
values become significant; the maxima are
Y ~0.1 at [Ti]/([Ti]+[Si])=0.5 and Y ~0.5 at
Si
Ti
[
5
Ti]/([Ti]+[Si])=1.0. Therefore, for the case of
0 eV ions, the Ti film grows quickly as a result
of the near unity sticking probability, i.e.
S ~(1−Y )~1, of the impinging ions. For
Ti
Ti
5
00 eV ions, the sticking probability drops,
S ~(1−Y )~0.5, after the surface becomes cov-
Ti
Ti
ered with Ti. The rate of formation of the inter-
facial silicide layer formed at both energies should
be similar until Ti/Si>0.5, at which point, the
growth rate for the 500 eV ions would be slower.
Fig. 11. Bright field images showing the grain structure of the
C54–TiSi phase corresponding to the TED image of Fig. 10.
2

1
68
S.M. Lee et al. / Surface Science 453 (2000) 159–170
Fig. 12. XPS depth profiles of Ti films deposited at substrate temperatures of 25 and 400°C using 50 and 500 eV Ti+ beams.
4
. Discussion
island formation mode. From high-resolution
TEM and XRD studies, they found that the Ti
film bulk consists of single crystal domains that
are mainly oriented vertical to the Si(111) surface
with about a deviation of 10°. Our AFM morphol-
ogy study for the 10 eV, 25° films clearly shows
many 3D grains along the ridges and valleys of
the Si(111) substrate (Fig. 6). The measurement
of the lattice constant from the spacing between
streaks on the RHEED images of the Ti film
(Figs. 3–5) matches the reported value of the
closely packed Ti(0001) plane within 0.7%.
Therefore, it is reasonable to conclude that Ti
films grown by low-energy Ti deposition also grow
in the Volmer–Weber mode, where each island has
a single crystalline structure mainly oriented so
that the Ti(0001) plane is parallel to the Si(111)
plane. RHEED streaks from islands that have
random azimuthal orientation relative to the sub-
strate are known to lie on inward curving lines
[32]. We did not observe this curvature (Figs. 3–
5) due to the limited size of our RHEED screen.
From the TEM measurement, the grain size of
each single crystalline domain can be estimated to
be ~10–20 nm.
How do the Si temperature and Ti+ kinetic
energy affect the surface crystallinity? As the tem-
perature increases, thermal vibrations, atomic
mobility, and the ability to anneal Frenkel pairs
are enhanced, resulting in enhanced crystallinity.
Kinetic energy can have both beneficial and delete-
rious effects as a result of changes in the dynamical
processes that dominate film growth as a function
of ion energy. As the kinetic energy transfer from
the impinging ion increases above the level of bond
energies, up to the atomic displacement threshold,
and to more energetic levels, processes conducive
to crystalline growth such as subsurface penetra-
tion, local atomic rearrangements, Frenkel pair
creation, and enhanced mobility are replaced by
processes that are deleterious to crystal growth
such as permanent defect formation, lattice
damage, sputtering, and atomic mixing. The syner-
getic effect of simultaneous low-temperature
annealing and low-energy ion irradiation has been
shown to result in crystal growth and epitaxy at
temperatures considerably lower than those
required for standard annealing treatments [22,23].
Choi et al. [21] studied Ti film growth on
Si(111) by thermal evaporation and reported that
the Ti film was polycrystalline and grew in the
Huth and Flynn [19] used AFM to study the
morphology of Ti film growth on Al O by molecu-
2
3
lar beam epitaxy (MBE). Even though their Ti

S.M. Lee et al. / Surface Science 453 (2000) 159–170
169
atures as low as 700°C using 10 eV Ti+. The
relatively large grain sizes of ~0.5 mm indicate
that this technique has potential for development
as a useful growth technique for C54–TiSi . While
2
the TED images show the C49–TiSi phase at
6
2
00°C, they exhibit a Ti-film phase at 500°C.
Considering that the surface of the film formed at
500°C has elemental Si, it is obvious that the
diffusion coefficient of Si into Ti is large at this
temperature, while the phase transition to the C49
titanium silicide phase is not yet activated.
The observation of small chemical shifts (Fig. 8)
in both the Ti and Si peaks from their elemental
2p
2p
form indicates the formation of an interfacial
titanium silicide compound. As these chemical
shifts are small, there have been several reports
that such shifts were not observed from titanium
silicide [7,11]. It has been reported that the phase
of the interfacial silicide layer is amorphous and
that it develops from Ti Si to TiSi as more silicon
5
3
2
becomes available for reaction at higher temper-
atures [2,10,33]. Our XPS (Fig. 12) measurements
demonstrate the existence of the interfacial silicide
layer that is buried beneath the Ti film for temper-
atures below 400°C, but we do not have the ability
to analyze its crystalline state. Therefore, it can be
inferred that the compositional ratio of the inter-
facial silicide should be Ti-rich. Questions have
been raised as to whether there is an interfacial
silicide layer between the Ti film and Si substrate
when the film is grown by physical vapor depos-
ition at room temperature [2,10,33]. This study
demonstrates that interfacial silicide formation
does occur during ion-beam deposition as low
as 25°C.
The thickness of the interfacial layer depends
on the deposition conditions. At 400°C, the layer
is broadened by 30%, and its atomic concentration
stays constant relative to that at 25°C. This temper-
ature-induced increase in the relative amount of
interfacial silicide is due to diffusion. At 500 eV,
the relative ratio of silicide/metal increases by
~44% relative to that at 50 eV. This ion-energy-
induced increase in the relative amount of inter-
facial silicide is due to the enhanced chemical
reactions between energetic Ti and substrate Si.
The kinetic energy transferred to the surface is
dissipated through atomic collisions that result in
Fig. 13. TRIM simulations showing (a) the number of vacancies
formed per incident Ti+ as a function of ion energy and (b)
the sputtering yields of Si (Y ) and Ti (Y ) as a function of the
fraction of Ti atoms in the surface for 50 and 500 eV Ti+.
Si
Ti
films on Al O show various features according to
2
3
the growth temperature, the RMS values were in
˚
the range of 2–5 A. This is considerably smaller
than that of the Ti film grown on Si(111) in this
study (Fig. 7). TEM results show that the Ti–Si
interface is relatively rough due to columnar struc-
ture and silicide formation [7]. The XPS depth
profiles (Fig. 12) show that the Ti film begins to
form a TiSi layer at 25°C, which develops as a
x
function of the temperature and kinetic energy.
Therefore, the rough surface morphology (Figs. 6
and 7) of the Ti film on Si(111) observed herein
reflects the development of the rough interfacial
silicide layer.
The TED images (Figs. 10 and 11) demonstrate
that the C54–TiSi phase can be formed at temper-
2

1
70
S.M. Lee et al. / Surface Science 453 (2000) 159–170
[
[
[
[
[
3] C.A. Sukow, R.J. Nemanich, J. Mater. Res. 9 (1994) 1214.
energetic, reactive atoms, defects (Fig. 13) such as
interstitials and vacancies, and broken chemical
bonds. Since the energetic bombarding particle
4] R.T. Tung, Appl. Phys. Lett. 68 (1996) 1933.
5] K. Maex, Mat. Sci. Eng. Rep. R11 (1993) 53.
6] A. Franciosi, J.H. Weaver, Physica B 118 (1983) 846.
7] R. Butz, G.W. Rubloff, P.S. Ho, J. Vac. Sci. Technol. A 1
(1986) 149.
(
Ti) is one of the reactants, the chemical reaction
is directly enhanced. Such irradiation-enhanced
reactions occur even with inert gas ions [23,34].
[
[
8] A.T.S. Wee, A.C.H. Huan, T. Osipowicz, K.K. Lee, W.H.
Thian, K.L. Tan, R. Hogan, Thin Solid Films 283
(
1996) 130.
9] S.B. Herner, V. Krishnamoorthy, A. Naman, K.S. Jones,
H.J. Gossmann, R.T. Tung, Thin Solid Films 302 (1997)
1
5. Conclusions
27.
10] R. Butz, G.W. Rubloff, T.Y. Tan, P.S. Ho, Phys. Rev. B
0 (1984) 5421.
11] J.M.M. De Nijs, Appl. Surf. Sci. 40 (1990) 349.
12] L. Pauling, The Nature of the Chemical Bond, Cornell
University Press, Ithaca, NY, 1960.
[13] X. Wallart, J.P. Nys, H.S. Zeng, G. Dalmai, I. Lefebvre,
M. Lannoo, Phys. Rev. B 41 (1990) 3087.
14] W. Lur, L.J. Chen, Appl. Phys. Lett. 54 (1989) 1217.
15] I.H. Kahn, Surf. Sci. 48 (1975) 537.
16] J. Mischenko III,, P.R. Watson, Surf. Sci. 209 (1989) L105.
[17] H.D. Snih, F. Jona, D.W. Jepsen, P.M. Marcus, J. Phys.
C: Solid State Phys. 9 (1976) 1405.
18] J.C.A. Huang, R.R. Du, C.P. Flynn, Phys. Rev. Lett. 66
1991) 341.
19] M. Huth, C.P. Flynn, Appl. Phys. Lett. 71 (1997) 2466.
20] V.G. Lifshitz, A.A. Saranin, A.V. Zotov (Eds.), Surface
Phases on Silicon Preparation Structures and Properties,
Wiley, New York, 1994.
[
Titanium and titanium disilicide films have been
3
successfully grown by low-energy (50–500 eV)
Ti+ deposition on Si(111) in the temperature
range of 25–600°C. The titanium films are poly-
crystalline with single crystalline grain sizes of 10–
[
[
20 nm. A broad range of surface morphologies is
[
[
[
observed as a function of growth conditions. The
Ti film completely covers the Si substrate in the
temperature range 25–400°C, while Si diffuses
through the film to the surface at higher temper-
atures. Synergetic effects between the substrate
temperature and Ti+ kinetic energy on the crystal-
line quality of the Ti film were observed. The
[
(
[
[
titanium silicide films are the C49–TiSi phase for
2
T~600°C and the C54–TiSi phase for T≥700°C
2
[21] C. Choi, H. Park, J.Y. Lee, K. Cho, M. Paek, O. Kwon,
K. Kim, S. Yang, J. Cryst. Growth 115 (1991) 579.
22] J.W. Rabalais, A..H.. Al-Bayati, K.J. Boyd, D. Marton,
J. Kulik, Z. Zhang, W.K. Chu, Phys. Rev. B 53 (1996)
10781.
with grain sizes of ~0.5 mm. XPS chemical shifts
in the Si and Ti peaks between the elemental
[
2p
2p
and silicide phases allow resolution of the inter-
facial silicide layer between the Ti film and Si
substrate. The interfacial silicide is formed even at
[
[
[
23] S.M. Lee, C.J. Fell, D. Marton, J.W. Rabalais, J. Appl.
Phys. 83 (1998) 5217.
25°C, and its characteristics are dependent on both
24] S.M. Lee, E.T. Ada, H. Lee, D. Marton, J.W. Rabalais,
Nucl. Instrum. Meth. Phys. Res. B 157 (1999) 220.
25] A.H. Al-Bayati, D. Marton, S.S. Todorov, K.J. Boyd, J.W.
Rabalais, D.G. Armour, J.S. Gordon, G. Duller, Rev. Sci.
Instrum. 65 (1994) 2680.
26] D. Briggs, M.P. Seah, Auger and X-ray Photoelectron
Spectroscopy, Practical Surface Analysis Vol. 1 Wiley,
New York, 1990.
the Ti+ energy and the Si temperature.
Acknowledgements
[
This work was supported in part under the
MRSEC Program of the National Science
Foundation under Award Number DMR-9632667
and the National Science Foundation Grant No.
DMR-9616440. The assistance of M.S. Lim and
S.S. Perry in obtaining AFM images is appreciated.
[27] S.P. Murarka, J. Vac. Sci. Technol. 17 (1980) 775.
28] L.Z. Mezey, J. Giber, Jpn. J. Appl. Phys. 21 (1982) 1569.
29] D.E. Savage, M.G. Lagally, J. Vac. Sci. Technol. B 4
[
[
(
1986) 943.
[
[
[
[
30] J.F. Ziegler, J.P. Biersack, The Stopping and Range of
Ions in Solids, Pergamon Press, New York, 1985.
31] S.M. Lee, E.T. Ada, H. Lee, D. Marton, J.W. Rabalais,
Nucl. Instrum. Meth. B 157 (1999) 220.
32] D.E. Savage, M.G. Lagally, Appl. Phys. Lett. 50 (1987)
References
1
719.
33] J. Vahakangas, Y.U. Idzerda, E.D. Williams, R.L. Park,
Phys. Rev. B 33 (1986) 8716.
[34] Y.H. Xie, K.L. Wang, Y.C. Kao, J. Vac. Sci. Technol. A
3 (1985) 1035.
[
[
1] K. Maex, L. van Den Hove, R.F. Keersmaecker, This Solid
Films 140 (1986) 149.
2] M.H. Wang, L.J. Chen, J. Appl. Phys. 66 (1992) 5918.