Surface nucleation of Ti silicides at elevated temperatures

Article abstract of DOI:10.1063/1.115630

The nucleation of Ti silicide at the surfaces of Si was studied. Deposition of Ti and codeposition of TiSix at elevated temperatures on single crystal and amorphous Si led to the direct growth of silicides. The temperature and composition of the deposition and the crystallinity of the substrate were found to have a strong effect on the phases(s) of the silicide layer. A remarkably low nucleation temperature, ～500°C, for the low-resistivity C54-TiSi2 phase was observed on amorphous Si. Stoichiometric and uniform TiSi2 layers were grown with the depositions of pure Ti. On crystalline Si, uniform TiSi2 layers were also grown at ～500°C with the deposition of essentially Ti. The significant difference between silicide formation in the present scheme and that under conventional silicide processing was discussed in terms of a possible circumvention of precursor amorphous salicide phases during surface nucleation.

Full text of DOI:10.1063/1.115630

Surface nucleation of Ti silicides at elevated temperatures
R. T. Tung
Citation: Applied Physics Letters 68, 1933 (1996); doi: 10.1063/1.115630
View online: http://dx.doi.org/10.1063/1.115630
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Surface nucleation of Ti silicides at elevated temperatures
R. T. Tunga)
ATT Bell Laboratories, Lucent Technologies, Murray Hill, New Jersey 07974
͑Received 23 October 1995; accepted for publication 28 January 1996͒
The nucleation of Ti silicide at the surfaces of Si was studied. Deposition of Ti and codeposition of
TiSix at elevated temperatures on single crystal and amorphous Si led to the direct growth of
silicides. The temperature and composition of the deposition and the crystallinity of the substrate
were found to have a strong effect on the phases͑s͒ of the silicide layer. A remarkably low
nucleation temperature, ϳ500 °C, for the low-resistivity C54-TiSi2 phase was observed on
amorphous Si. Stoichiometric and uniform TiSi2 layers were grown with the depositions of pure Ti.
On crystalline Si, uniform TiSi2 layers were also grown at ϳ500 °C with the deposition of
essentially Ti. The significant difference between silicide formation in the present scheme and that
under conventional silicide processing was discussed in terms of a possible circumvention of
precursor amorphous salicide phases during surface nucleation. © 1996 American Institute of
Physics. ͓S0003-6951͑96͒00314-1͔
The performance of Si-based ultra-large-scale integra-
tion ͑ULSI͒ devices can improve significantly with the incor-
poration of thin film silicides.1 TiSi2 is among the best can-
didates for self-aligned silicide ͑salicide͒ in the deep
submicron ͑Ͻ0.5 ␮m͒ design rules regime, because of the
low resistivity of the C54-TiSi2 phase and the maturity of the
Ti salicide process. However, the difficulty with the poly-
morphic transformation, C49-TiSi2→C54-TiSi2, in narrow
lines2 and thin layers3 is well known. This and the agglom-
eration problem of thin and narrow TiSi2 layers still present
engineers and scientists with a significant challenge in push-
ing this process beyond the 0.25 ␮m generation. Parameters
governing the nucleation of the C54 phase need to be estab-
lished to meet this challenge.
In a conventional salicide process, Ti is deposited, usu-
ally by sputtering, onto a patterned Si substrate held at close
to room temperature. The deposition is followed by anneal-
ing, usually in nitrogen, which leads first to a precursor in-
terfacial amorphous silicide layer with a graded composition,
before any crystalline silicide phases are formed.4,5 The
C49-TiSi2 phase is believed to nucleate at the interface be-
tween Si and the precursor amorphous silicide layer.5,6 The
formation of the C54-TiSi2 phase is always preceded by the
C49-TiSi2 phase,7 and it is the transformation between these
two polymorphs which is presently the biggest issue in Ti
silicide. Recently, a preamorphization step was found to be
crucial to the reduction of the C54-TiSi2 transformation tem-
perature on narrow lines,8 suggesting that the crystallinity of
the Si is an important factor in the formation of the
C54-TiSi2 phase. The silicide reaction still goes through the
amorphous TiSix stage on amorphous Si.
This work attempts to avoid the ␣-TiSix phases by using
͑co-͒depositions at temperatures exceeding their crystalliza-
tion temperatures.9
Blanket Si͑100͒ wafers, oxidized Si wafers with a 200
nm thick plasma enhanced chemical vapor deposition
͑PECVD͒ grown amorphous-Si layer, and preamorphized
͑PAM͒ Si wafers were used in this study. The PAM wafers
were prepared with a 50 keV 3E15 As implant, an activating
anneal, and a 50 keV 3E14 As amorphizing implant. Sub-
strates were cleaned chemically and loaded in an ultrahigh
vacuum ͑UHV͒ system. Amorphous Si and PAM wafers
were first given a dilute HF dip. The thin oxide layer on
single crystal substrate surfaces was removed in situ using
the Si beam method. Ti and Si were deposited using e-beam
sources while the Si substrate was held at a specific tempera-
ture. Ti deposition rates of ϳ0.03–0.5 nm/s were used.
*
Throughout this letter, a ‘‘1 nm TiSix’’ is defined as a code-
posited TiSix layer which contains the equivalent of 1 nm Ti.
Shown in Fig. 1 are transmission electron microscopy
͑TEM͒ images and diffraction patterns ͑TED͒ of silicide lay-
ers grown by deposition of 3 nm Ti, at ϳ0.1 nm/s, on single
crystal Si͑100͒. After Ti-rich depositions ͑Ti or TiSix, x
р1) at ϳ400–465 °C, amorphous thin films (TiSix) are
found at the surface. At ϳ490 °C, crystalline TiSi phase is
identified along with the presence of amorphous silicide ͓Fig.
1͑b͔͒. There is no evidence for the growth of TiSi2 phase͑s͒
at temperatures below 500 °C. However, when Si-rich
codepositions (TiSix , xϾ1.3) are used, C49-TiSi2 is grown
directly on Si͑100͒ at temperatures as low as ϳ350 °C. This
apparent dependence of the silicide phase on the deposition
composition all but vanishes when the deposition tempera-
ture is raised to above ϳ500 °C. Crystalline C49-TiSi2 phase
is grown after Ti-rich depositions at temperatures of
Ͼ500 °C. In the temperature range 500–630 °C, a significant
fraction of the C49-TiSi2 islands are found to occupy epitax-
ial orientations.10 The TED pattern ͓Fig. 1͑c͔͒ shows long
streaks, possibly from Si surface or interface microstructures,
stabilized by Ti. No evidence for the C54-TiSi2 phase is
observed in layers grown at р600 °C. At deposition tem-
peratures of у630 °C the orientations of the C-49TiSi2
grains become randomized and the presence of the
Since the C54 phase is thermodynamically more stable
than C49-TiSi2, the predominance of the C49 phase in the
initial stage of reaction is speculated to arise from a kinetic
advantage. One notes that the vast majority of existing work
on Ti silicide formation involves processing conditions
which make the amorphous silicide precursor phase inevi-
table. It is possible that the nucleation advantage enjoyed by
the C49-TiSi2 phase is related to the precursor ␣-TiSix layer.
a͒
Electronic mail: rtt@clockwise.att.com
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ϩ
*
FIG. 3. Glancing angle, 2.0 MeV He , RBS spectra from 15 nm TiSix
layers deposited at 470 °C on ␣-Si with the following deposition stoichiom-
etries: Ti, TiSi0.5, and TiSi2.0
.
FIG. 1. Plan-view, bright field, TEM images of 3 nm Ti deposited on
Si͑100͒ at the following temperatures: ͑a͒ 465 °C, ͑b͒ 490 °C, ͑c͒ 560 °C,
and ͑d͒ 730 °C.
amount of crystalline Si with temperature is revealed by
TED, possibly the result of silicide-induced crystallization of
amorphous Si.9 A plan-view TEM image and a TED pattern
of surface nuclei grown by deposition of 3 nm Ti at 550 °C
are shown in Fig. 2.
C54-TiSi2 phase can be clearly detected ͓Fig. 1͑d͔͒. While
these results support the general notion that the C54-TiSi2
phase requires a higher nucleation temperature than C49,
they also suggest that the concept of a ‘‘threshold
thickness’’10 is not applicable in this system. With increasing
deposition temperature, the fraction of C54 islands increases.
However, even at a temperature of 780 °C, the C49-TiSi2
phase still occupies a detectable fraction of the islands. In
general, layers grown at lower deposition temperatures are
more uniform than those grown at higher temperatures. Also,
codepositions lead to more uniform layers than pure Ti depo-
sitions.
*
Thicker ͑ϳ15 nm Ti͒ silicide layers have also been
grown at elevated deposition temperatures. Shown in Fig. 3
are Rutherford backscattering spectra ͑RBS͒ of silicide lay-
ers grown at 470 °C on amorphous Si. A TiSi layer is grown
after pure Ti or TiSi0.5 depositions, while a TiSi2 layer is
grown after a TiSi1.8 deposition. The amorphous silicide
grown at this temperature ͑last paragraph͒ is limited in its
thickness, after which is passed, pure Ti accumulates and
reacts with the ␣-TiSix into TiSi, much like the sequence
seen in conventional Ti silicide reaction.6 Ti depositions and
codepositions at Ͼ500 °C lead to the growth of TiSi2 on
amorphous Si. An increasing proportion of the C54-TiSi2
phase is observed with temperature. A somewhat surprising
result is shown in Fig. 4, where the surface morphology of
silicide layers grown from deposited 15 nm Ti seems to be
smooth at 470 and 560 °C, and yet it appears rough at the
intermediate temperature, 500 °C. The layer smoothness is
explained by a high nucleation density, respectively, for the
TiSi2 phase͑s͒ at 560 °C and for the TiSi phase at 470 °C,
while a low nucleation density of the TiSi2 phase͑s͒ at
500 °C likely caused the observed layer roughness at this
temperature. On amorphous Si, the growth of stoichiometric
silicide from deposition of pure Ti ͑15 nm thick͒ appears not
The nucleation of C54-TiSi2 is much easier on amor-
phous Si than on crystalline Si. At a temperature of ϳ400–
*
480 °C, 3 nm Ti depositions and 3 nm Ti-rich codepositions
͑xϽ1͒ lead to no observable crystalline phase. At the same
temperatures, Si-rich codepositions ͑xϳ2͒ lead to the growth
of the C49-TiSi2 phase. TED spots from the C54-TiSi2 phase
*
are clearly identified on amorphous Si, after 3 nm Ti-rich
depositions at temperatures of 500 °C or higher. At 550 °C
and higher, the diffraction patterns are in agreement with that
of pure C54-TiSi2 phase, even though the presence of a small
amount of C49-TiSi2 cannot be ruled out. An increasing
FIG. 2. Plan-view, bright field, TEM image, and TED pattern of 3 nm Ti
deposited at 550 °C on amorphous Si.
FIG. 4. Nomarsky images of the silicide surface after deposition of 15 nm
Ti on ␣-Si at ͑a͒ 560 °C, ͑b͒ 500 °C, and ͑c͒ 465 °C.
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1934 Appl. Phys. Lett., Vol. 68, No. 14, 1 April 1996 R. T. Tung
130.216.129.208 On: Sat, 06 Dec 2014 09:37:21

TABLE I. Phase identification and electrical sheet resistance of silicide
*
layers grown by deposition of 15 nm TiSix on ␣-Si.
Deposition
composition
T
depϭ470 °C
T
depϭ520 °C
T
depϭ550 °C
TiSi
52⍀/ᮀ
C54ϩC49
8.9⍀/ᮀ
C54ϾC49
6.6 ⍀/ᮀ
Ti
TiSi
78 ⍀/ᮀ
C54ϾC49
6.4 ⍀/ᮀ
C54ϾC49
7.4 ⍀/ᮀ
TiSi0.5
TiSi1.8
C54ϩC49
14 ⍀/ᮀ
C54ϩC49
13 ⍀/ᮀ
C54ϾC49
a
16 ⍀/ᮀ
aDiscontinuous layer.
FIG. 6. Plan-view TEM images of Ti silicide layers grown on c-Si. Layer
͑a͒ was grown by the deposition of 15 nm at 480 °C. Layer ͑b͒ was grown
to be limited by diffusivity at Ͼ500 °C, at least for deposi-
tion rates up to 0.2 nm/s. Table I summarizes the dominant
phase͑s͒, from TED studies, and sheet resistance, from four-
point probe measurements, of thicker silicide layers grown
on amorphous Si.
*
by the codeposition of 1.5 nm TiSi2.2 at room temperature, annealing at
500 °C for 1 min, and the deposition of 13.5 nm Ti at 480 °C.
Silicide phase formation is largely dependent on the
nucleation process. The initial reaction in a continuous
growth process can have a decisive effect on the final result.
If a thin Ti layer ͑1.5 nm͒ is deposited on ␣-Si at room
temperature before the deposition of Ti at elevated tempera-
tures begins, the growth of stoichiometric TiSi2 is no longer
observed ͑Fig. 5͒. The 1.5 nm Ti layer prevents the surface
nucleation of TiSi2 phase͑s͒, leading to the growth of the
TiSi phase, as determined by TED, even after subsequent
deposition at 540 °C. On the other hand, a thin TiSix ͑xϳ2͒,
layer deposited on c-Si at low temperature and annealed at
ϳ500 °C forms an essentially continuous, randomly ori-
ented, C49-TiSi2 layer which serves as a template layer for
subsequent growth. Deposition of Ti at as low as 480 °C on
such a template leads to the growth of uniform TiSi2 layers,
predominantly C49, on c-Si, as shown in Fig. 6͑b͒. Without
a template, deposition of 15 nm Ti at 480 °C leads to the
growth of TiSi ͓Fig. 6͑a͔͒.
Si at ϳ500 °C. Furthermore, there is no apparent minimum
thickness to the formation of the C54-TiSi2 phase. These
unusual silicide reaction characteristics demonstrated in the
present study are likely the result of direct Ti-Si reaction
without involving the ␣-TiSix phase. The fact that nucleation
occurs at or near a free surface is also speculated to favor the
C54-TiSi2 phase. At a surface, the nucleation process is not
as limited by kinetic constraints as bulk or interface nucle-
ation would be, thus removing some of the kinetic advan-
tages with the growth of the C49-TiSi2 phase over the
growth of the C54-TiSi2 phase. In addition, the clean UHV
conditions and the use of ␣-Si may also tend to favor the
nucleation of the C54-TiSi2 phase.
In summary, the nucleation and growth of Ti silicide at
elevated deposition temperatures has been studied. A low
nucleation temperature, ϳ500 °C, for the low-resistivity
C54-TiSi2 phase was observed on ␣-Si. This low C54 forma-
tion temperature was speculated to arise from the avoidance
of amorphous silicide formation. Stoichiometric and uniform
TiSi2 layers were grown on amorphous Si at ϳ500 °C with
the depositions of pure Ti. On c-Si, uniform TiSi2 layers
were also grown at 500 °C, but with the use of a thin tem-
plate.
In a conventional salicide process, the C54-TiSi2 phase
is observed to grow only at Ͼ650 °C. Presently, the nucle-
ation of C54-TiSi2 is observed on the surface of amorphous
The author thanks F. Schrey, G. R. Weber, and K. Fujii
for their assistance
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