The interfacial reaction of Ni on (100) Si1-xGex
(x=0, 0.25) and (111) Ge
L.J. Jin,1 K.L. Pey,1, 2 W.K. Choi,1,2 E.A. Fitzgerald,1,3 D.A. Antoniadis,1,3 A.J. Pitera,3 M.L. Lee3 and
D.Z. Chi4
1. Singapore-MIT Alliance, 4 Engineering Drive 3, Singapore 117576
2. Department of Electrical & Computer Engineering, National University of Singapore, 4 Engineering
Drive 3, Singapore 117576
3. Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-66307
4. Institute of Materials Science and Engineering, 3 Research Link, Singapore 117602
Abstract—The interfacial reaction of Ni with Si, Si0.75Ge0.25, and
Ge at 400 ºC has been investigated. A uniform epitaxial NiSi film
was obtained at 400oC for Ni-Silicidation on Si using rapid thermal
annealing method. Similarly, uniform film of NiGe was formed at
400ºC for Ni reaction with Ge. Whereas using in situ annealing at
400ºC, Ni3Ge2 and NiGe were observed. For the interfacial
reaction of Ni with relaxed Si0.75Ge0.25 films rapid thermal
annealed at 400ºC, a mixed layer consisting of Ni3(Si1-xGex)2,
Ni(Si1-yGey), and Si1-zGez (z>y>x) was formed; whereas only
Ni3(Si1-xGex)2 and Ni(Si1-yGey) were observed by in situ annealing.
Index Terms—Ni-germanosilicide, in situ annealing
I.
O
INTRODUCTION
species (avoids lateral encroachment of silicide over the oxide
spacer that will occur when dominant diffusing species is Si).
Studies of the effect of Ge concentration in the relaxed SixGe1-x
on Ni silicidation process for different annealing temperatures5-7
have revealed that ternary compounds always form, and at the
same time, Ge segregation has been observed when annealing
temperature is above 400ºC. As Ge segregation will degrade the
contact performance, the control of the Ge segregation is
critical. Some reports showed that the onset temperature for
obtaining uniform low resistivity Ni-SiGe is a strong function of
the Ge concentration.8 In addition, some reports further showed
that the current Rapid Thermal Annealing (RTA) technology
has very narrow process window for various Ge concentrations
and may not a viable manufacturable technology for fabricating
uniform Ni-SiGe.
Developing robust technology for forming good Ni-SiGe on
relaxed SiGe S/D regions and ultimately developing a Ni
silicidation process for application in future SiGe-based
MOSFET devices with high performance is of technological
interest. The silicide technology must be thermally stable and
form a uniform low sheet resistance silicide film.
In this article, in order to use Ni silicide in ultra shallow
junction, 10nm thickness Ni was investigated. In situ annealing
method at 400ºC was experimented to prevent Ge segregation
and avoid oxide contamination.5
The interfacial reactions of Ni with relaxed Si0.75Ge0.25 films
at 400ºC by RTA and in situ annealing methods were studied.
The interfacial reactions of Ni with pure (100) Si and pure (111)
Ge at 400ºC were also investigated.
ne of the problems limiting the performance of Silicon
MOSFET is the speed that carriers can move from source
to drain. Si/SiGe heterostructures on Si wafers are currently
explored as channel materials for high performance MOSFETs.
To further reduce the RC delay, increase driving current, and
enhance the performance of deep submicron CMOS devices,
self-aligned silicide is formed at the source and drain regions.
Formation of silicides can cause a reduction in the sheet
resistance of the source and drain areas. Many metals such as Ti,
Co, Pt, Pd, Cu, and Ni have been studied for silicidation. 1-3
Ni-silicide has been identified as the next potential candidate for
advanced CMOS Si technology because Ni consumes less
silicon (this is important for device applications on ultra-thin Si
layers), 4 and has less risk of spiking in ultra shallow junctions.
In addition, NiSi can be formed by one step annealing at low
temperature (400-600ºC) without any agglomeration as Ni
II. EXPERIMENT
monosilicide has lower resistivity and junction leakage. Other
In our study, The SiGe growth is carried out by ultrahigh
advantages are good resistance to bridging between gate and
vacuum
chemical vacuum deposition (UHVCVD) technology
source or drain (S/D) regions, since Ni is the dominant diffusing
using SiH4 and GeH4 as the source gases. The starting wafer is a
4” Si wafer, and a compositional graded layer with a gradually
L. J. Jin is with the Singapore-MIT Alliance, 4 Engineering Drive 3,
increasing Ge composition was grown subsequently at 900ºC
Singapore 117576 (email: smap1004@nus.edu.sg).
and 25mTorr on the Si wafer until it reached the desired Ge
composition. In order to get a Si0.75Ge0.25 buffer wafer, ten
100
Sheet resistance(ohm /sq)
90
(As-deposited 10nm Ni )
0
(10nm Ni sputter depositon by RTA at 400 C)
(10nm Ni sputter deposition by in-situ annealing
0
at 400 C)
80
70
60
50
40
30
20
10
0.00
0.25
0.50
0.75
1.00
Ge percent(at%)
graded layers of 2000Å at 2.5% Ge steps were deposited.
Hence, the graded region was 2µm thick, followed by 1.5µm
thick relaxed Si0.75Ge0.25. The details of the growth conditions
and the characterization of the relaxed SiGe films can be found
elsewhere. 9-11 In order to compare different Ge compositions,
p-type (100) Si wafer with a resistivity of 4~8Ω/sq and pure
(111) Ge wafers were also used.
The relaxed SiGe and Ge wafers were cleaned by piranha,
while the Si wafer was cleaned by the standard RCA solution.
Prior to Ni deposition, all the samples were cleaned by dilute HF
to remove oxide and then immediately loaded into a sputtering
chamber. About 100 Å thick Ni film was deposited onto the
wafers at room temperature by sputtering method at a deposition
rate of 2 Å/s. The base pressure was below 5x10-7 Torr, and the
deposition pressure was about 3x10-3 Torr. Two annealing
methods were used. One was rapid thermal annealing at 400ºC
for 60s in N2 ambient, while the other was in situ annealing at
400ºC in the sputtering chamber without breaking the vacuum
after the deposition. The sheet resistance of the films was
measured by the four-point probe method. The silicide films
were characterized by micro-Raman technique. Cross-sectional
transmission electron microscopy (XTEM) was adopted to
study the surface morphology and the interfacial structure of
these films.
III.
RESULTS AND DISCUSSION
Figure 1 shows a typical Atomic Force Microscopy (AFM)
image of a relaxed Si0.75Ge0.25 substrate after surface cleaning.
The relaxed SiGe films were very smooth after the cleaning.
The characteristic cross-hatch pattern of the relaxed Si0.75Ge0.25
layer is absent due to CMP technology employed during the
SiGe growth.12
Figure 2 shows the sheet resistances of a 10nm Ni-silicided
films on the relaxed Si0.75Ge0.25, (100) Si and (111) Ge using
rapid thermal annealing and in situ annealing at an annealing
Fig. 2. Sheet resistance of 10nm Ni-silicided different substrates
using different annealing methods at 400ºC. As-deposited
sample is included.
temperature of 400ºC. The as-deposited samples are included
for comparison. For the relaxed Si0.75Ge0.25 samples, the in situ
annealed sample gave a lower sheet resistance, 8.78Ω/sq,
compared to the sample rapid thermal annealed method
(9.87Ω/sq). For the (100) Si sample, the sheet resistances are
8.84 and 15.1Ω/sq for RTA and the in situ annealing,
respectively. For the (111) Ge samples, the sheet resistances are
7.64 and 8.56 Ω/ sq for the in situ annealing and RTA,
respectively.
Figures 3, 4 and 5 show the results of micro-Raman
experiments at room temperature for the Si, Si0.75Ge0.25, and Ge
samples annealed at 400ºC by RTA and the in situ annealing
methods. In Fig. 3, the strongest Raman peak corresponding to
NiSi is at ~ 213-217cm-1.13 The less prominent NiSi peak is at
199cm-1. Other NiSi peaks occur at 258, 296, 367cm-1. 14 From
Fig. 4, the Raman peak of Ni/Si0.75Ge0.25 system appears ~ 199
and 213cm-1 for the in situ annealing, and around 213cm-1 for
RTA. For the Ni/Ge system, two peaks exist at ~140 and
200cm-1 in Fig. 5 regardless of the annealing technology. There
5000
Ni-Si
Intensity(A.U.)
Fig. 1. AFM image of relaxed Si0.75Ge0.25 substrate after
surface cleaning (surface roughness Ra~0.236nm).
Ni-Si
RTA
Ni-Si
Ni-Si
Ni-Si
In-situ annealing
0
100
200
300
400
500
-1
Raman shift(cm )
Fig. 3. Raman spectra of Ni on Si at 4000C by RTA and in situ
annealing ( λ L=632.8nm).
5000
Ge-Ge
Si-Si(SiGe sub)
60000
4000
50000
Localized Si-Si(Ge)
Ni-Si(Ge)
30000
Intensity(A.U.)
Intensity(A.U.)
40000
Ge-Ge
Si-Ge
20000
Ni3Ge2
3000
2000
NiGe
in-situ annealing
10000
0
1000
RTA
RTA
In-situ annealing
As-deposited
0
100
200
300
400
500
600
-1
Raman shift(cm )
Fig. 4. Raman spectra of Ni on Si0.75Ge0.25 annealed at 400ºC
by RTA and in situ annealing ( λ L=632.8nm). The
as-deposited sample is included for comparison.
are no reports of the Raman peaks for NixGey in the literature.
According to our TEM results, the two peaks correspond to
NiGe. XRD experiments are currently being carried out to
confirm the presence of Ni3Ge2 phase.
Figures 6(a) and 6(b) show the XTEM images of the
Ni-silicided on Si substrate annealed at 400ºC by RTA and in
situ annealing methods, respectively. Energy dispersive x-ray
analysis(EDX) was used to determine the elemental
composition. With RTA, a uniform layer of NiSi with a
thickness of about 15.3nm was found. The top layer was a
residual metal layer. The monosilicide layer was an epitaxial
layer, as shown by the inserted diffraction patterns. This is the
first time that an epitaxial Ni-silicided layer has been formed on
(100) Si. The formation of epitaxial NiSi layer is probably
related to the Ni thickness. On the other hand, epitaxial layer
NiSi could not be detected with the in situ experiments. 15
Instead, the Ni3Si2 and NiSi phases were found. The interface
between Ni silicide and Si is not very uniform compared to the
sample using the RTA method. When thin film of Ni reacts with
thick Si substrates, it is generally agreed that Ni first transforms
into Ni2Si and then into NiSi phase. Ni3Si2 is generally not
observed in the thin film reaction of Ni with crystalline silicon
due to complicated orthogonal structure. 16-18
Figure 7 reveals the cross-sectional TEM micrographs of the
Ni-silicided Si0.75Ge0.25 films annealed at 400ºC for 60s by the
in situ annealing. The interface between Ni germanosilicide
and relaxed Si0.75Ge0.25 substrate is very rough. The EDX
results showed that the silicided films were Ni(Si1-xGex)
(x=0.17) and Ni3(Si1-yGey)2 (y=0.16) phase. The interface is
worse by the RTA method (not shown), besides Ni(Si1-xGex)
(x=0.13) and Ni3(Si1-yGey)2 (y=0.16) phase, Ge-rich Si1-zGez
(z>0.25) existed. In all the samples, Ni3(Si1-yGey)2 phase was
observed possibly due to the Ge effect. The lattice constant of
Ge is larger than that of Si; the Ge in the Si-Ge system has a
lower mobility than Si leading to a reduction of the activation
energy for the formation of Ni3(Si1-yGey)2. So it is easy to form
Ni3(Si1-yGey)2 from reaction kinetics view. 8 The cross-
100
150
200
250
300
350
400
-1
Raman shift(cm )
Fig. 5. Raman spectra of Ni on Ge annealed at 400ºC by
RTA and the in situ annealing ( λ L=632.8nm).
(a)
(b)
Fig. 6. Cross-sectional TEM micrographs of the 10nm
Ni-silicided Si annealed at 400ºC by (a) rapid thermal
annealing (b) in situ annealing.
sectional TEM micrographs of the Ni/Ge annealed at 400ºC by
RTA and the in situ annealing methods are shown in Figs. 8 (a)
and 8 (b). Some previous studies showed that epitaxial growth
of Ni2Ge and NiGe by interfacial reactions between nickel thin
films and Ge (111) was possible at 160ºC and 250ºC.19 Another
study showed that orthorhombic NiGe nucleates and grows
between 300 and 400ºC and the formation of NiGe is through a
thermally activated process with an activation energy of 1.3ev.
20
According to the EDX analysis in Fig. 8(b), two phases exist
as NiGe and Ni3Ge2 by the in situ annealing, whereas, only
uniform NiGe phase exists using the RTA method (Fig. 8 (a)).
Formation of Ni3Ge2 phase in thin film reaction is reported to be
impossible,16-18 so the exact mechanism for the formation of the
phase in the thin film reaction of Ni with Ge substrate is not very
clear.
Based on our TEM analysis, Ni3Si2, Ni3(Si1-yGey)2, and
Ni3Ge2 phase can be observed when Ni reacts with different Ge
atomic percent substrates. So it is postulated that ramp rate in
the annealing process plays a key role in forming those phases.
However, the exact mechanism is still under investigation.
IV.
NiSiGe=52:40:8
NiSiGe=60:32:8
SiGe=72:28
Fig. 7. Cross-sectional TEM micrographs of the 10nm
Ni-silicided Si0.75Ge0.25 annealed at 400ºC by in situ annealing.
CONCLUSION
Uniform, epitaxial and low sheet resistance NiSi can be
formed on p (100)-Si substrate at 400ºC using RTA method. It is
postulated that this is due to the very thin Ni used. In the
Ni/SiGe system, a mixed layer consisting of Ni3(Si1-xGex)2,
Ni(Si1-yGey), and Si1-zGez (z>y>x) were detected using the RTA
method; whereas only Ni3(Si1-xGex)2 and Ni(Si1-yGey) were
observed in the in situ annealing. In the Ni/Ge system, uniform
NiGe phase can be obtained by RTA at 400ºC; however, a
mixing layer of NiGe and Ni3Ge2 was obtained by the in situ
annealing at 400ºC.
NiGe=53:47
(111) Ge
ACKNOWLEDGMENT
The authors would like to acknowledge the Singapore-MIT
Alliance (SMA) for providing all the necessary resources. In
addition, the authors would like to acknowledge Lee Tek Po
Rinus of IMRE and Dr. Tung Chih Hang of IME for their help
in the sputter deposition and TEM analysis, respectively, as
well as the Physics Department of the National University of
Singapore (NUS) in performing the Raman spectra. Lastly, the
authors would like to acknowledge SMA for providing her
research scholarship.
(a)
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