Formation of CoSi2 on various polycrystalline silicon structures and its effects on thermal stability

Article abstract of DOI:10.1149/1.1393392

We have investigated formation of CoSi₂ on various grain sizes of polycrystalline Si (poly-Si) with emphasis on its thermal stability. As the grain size of poly-Si decreases, CoSi₂ phase is formed at lower temperature because of the diffusion of Co atoms along grain boundaries of poly-Si during the rapid thermal annealing process. The enhanced reaction of cobalt with silicon on small-grain-sized poly-Si creates a rough CoSi₂/poly-Si interface, which becomes thermally unstable. CoSi₂ formed on amorphous Si showed less thermal stability than that found on medium and large grain sized poly-Si.

Full text of DOI:10.1149/1.1393392

Journal of The Electrochemical Society, 147 (4) 1551-1554 (2000)
1551
S0013-4651(99)07-029-9 CCC: $7.00 © The Electrochemical Society, Inc.
Formation of CoSi on Various Polycrystalline Silicon Structures and Its
2
Effects on Thermal Stability
z
Jong-Uk Bae, Dong Kyun Sohn,* Ji-Soo Park, Chang Hee Han, and Jin Won Park
LG Semiconductor Company, Limited, Research and Development Division, Cheongju-si 361-480, Korea
We have investigated formation of CoSi on various grain sizes of polycrystalline Si (poly-Si) with emphasis on its thermal sta-
2
bility. As the grain size of poly-Si decreases, CoSi phase is formed at lower temperature because of the diffusion of Co atoms
2
along grain boundaries of poly-Si during the rapid thermal annealing process. The enhanced reaction of cobalt with silicon on
small-grain-sized poly-Si creates a rough CoSi /poly-Si interface, which becomes thermally unstable. CoSi formed on amorphous
2
2
Si showed less thermal stability than that found on medium and large grain sized poly-Si.
©
2000 The Electrochemical Society. S0013-4651(99)07-029-9. All rights reserved.
Manuscript submitted July 7, 1999; revised manuscript received December 20, 1999.
For sub-quarter-micrometer complementary metal oxide semicon-
ductors (CMOS), formation of dual-doped gate structure becomes
dominant over the single-doped gate process because of the surface
operation mode for both n- and p-channel MOS transistors, which
helps to improve the short-channel effect immunity. Recently, gate
electrodes using small-grain-sized polycrystalline Si (poly-Si) have
been fabricated to simultaneously dope the gate and source/drain re-
The process conditions are summarized in Table I. After a standard
cleaning process, the samples were dipped in a diluted HF solution
prior to Co film deposition. We sputtered a thick Co film with Ti cap-
10
ping layer without breaking the vacuum. Subsequent RTA was car-
ried out at 450-700ЊC for 60 s in an N ambient (first RTA). Unreact-
2
ed metals were then removed in an NH OH:H O :H O followed by
4
2
2
2
an HCl:H O :H O solution. Additional annealing was performed at
2
2
2
1
gion. This technology provides simple dual-gate fabrication by sav-
750ЊC for 30 s to form low resistivity CoSi (second RTA). Thermal
2
ing two photolithography steps. Since dopants mainly diffuse through
grain boundaries in the poly-Si gate, the grain size must be kept to a
minimum for suppression of gate depletion due to insufficient diffu-
sion of dopant during rapid thermal annealing (RTA).
stability of CoSi /poly-Si structure was evaluated after annealing at
2
various temperatures ranging from 800 to 1000ЊC for 30 s. Sheet
resistance of Co silicide was measured by four-point probe method.
Phase changes were analyzed by X-ray diffraction (XRD), and film
Cobalt silicide has been widely used as low resistance gate elec-
trodes and local interconnections. One problem with using silicides
morphology of CoSi was observed using Philips CM200-FEG trans-
mission electron microscopy (TEM).
2
2
in integrated circuits is that the morphology of the silicide can change
during the postannealing process. That is, a rough and discontinuous
3
Results and Discussion
CoSi film can cause an increase of sheet resistance and junction
Sheet resistance of partially reacted film as a function of first
RTA temperatures for various types of poly-Si is shown in Fig. 1.
The sheet resistance of Co film on large grain sized poly-Si is almost
constant up to 600ЊC and decreases abruptly after annealing at
650ЊC for 60 s. Above this temperature, Co film is transformed to the
2
leakage current. The degradation of silicides on poly-Si is more
severe than that on single crystalline Si because grain boundaries pro-
vide an additional driving force for morphological changes. Nygren
and Johnson reported that a CoSi /poly-Si layered structure becomes
2
morphologically metastable because of the grain growth of poly-Si
low resistivity CoSi film. It is worth noting that the decrease of
2
4
during post annealing. Additionally, it has been known that the
sheet resistance of Co film is shifted to lower temperature as the
grain size of poly-Si decreases. That is, the sheet resistance of Co
film on fine-grain-sized poly-Si starts to decrease after annealing at
500ЊC, which is 100ЊC lower than that on large-grain-sized poly-Si.
The changes in sheet resistance can be directly correlated to the
phase transformation of Co silicide. Figure 2 depicts XRD spectra of
films on poly-Si after RTA at 600ЊC for 60 s. We can observe that the
Co film on a-Si or fine-grain-sized poly-Si is transformed to low re-
CoSi /poly-Si structure becomes unstable when the initial grain size
2
of the poly-Si decreases.
Previous reports focused mainly on the stability and agglomera-
3
-9
tion of CoSi films. However, the influence of poly-Si grain size
2
on cobalt silicide formation or agglomeration has not been exten-
sively studied. In this work, we have investigated the formation and
thermal stability of cobalt silicide on various grain sizes of poly-Si
using the Ti-capped Co system and its effect on thermal stability. It
is further shown that the initial morphology of the as-formed CoSi2
film has a relation to agglomeration during post-heat-treatment.
sistivity CoSi , while the Co film on medium or large-grain-sized
2
poly-Si remains high in resistivity CoSi phase. Formation of the low
resistivity CoSi phase from the reaction between a deposited Co
2
layer and a-Si substrate proceeds sequentially by Co/Si r Co Si r
2
Experimental
CoSi r CoSi at 350, 450ЊC, and higher temperatures, respective-
2
The substrate used in this study were p-type Si (100) wafers with
a resistivity of 5-9 ⍀ cm. After standard RCA cleaning, thermal oxide
(4.5 nm thick) was grown and 200 nm thick poly-Si was deposited by
low-pressure chemical vapor deposition (LPCVD). The grain size of
poly-Si film was varied by controlling deposition temperature and
postanneal conditions. Large grain sized poly-Si was formed by solid
phase crystallization of amorphous Si (a-Si) at 850ЊC for 30 min, in
which a-Si was deposited at 580ЊC. Medium and fine-grain-sized
poly-Si was made by deposition at 660 and 680ЊC, respectively. The
final grain size of fine, medium, and large poly-Si were 50, 150, and
Table I. Deposition conditions for poly-Si, its grain size, and
grain boundary energy (E ϭ ␥/r, ␥ ϭ 0.3 J/cm ).
2
Deposition
Mean grain
Poly-Si structures
-Si
Fine grain size poly-Si
Medium grain
temperature (ЊC) size (nm) 2␥/r (kJ/mol)
␣
580
680
660
—
65
<150
10.6
0.11
0.0452
200 nm, respectively. To compare with crystalline Si, a-Si film was
directly deposited on fine grain sized poly-Si (a-Si/poly-Si bilayer).
size poly-Si
Large grain size poly-Si
580
>200
0.0424
*
Electrochemical Society Active Member.
E-mail: jongukB@netsgo.com
z
CoSi r CoSi2 ⌬H ϭ Ϫ2.5 kJ/mol
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1
552
Journal of The Electrochemical Society, 147 (4) 1551-1554 (2000)
S0013-4651(99)07-029-9 CCC: $7.00 © The Electrochemical Society, Inc.
Therefore, the enhancement of CoSi formation on fine-grain-sized
2
poly-Si cannot be explained by the grain boundary energy of poly-
Si films. Instead, we can speculate on the effect of poly-Si grain
boundaries on CoSi formation. CoSi phase is formed following
2
2
1
4
CoSi after the second RTA. Pramanick et al. suggested that CoSi2
was mainly formed by the following reaction: 2CoSi r CoSi ϩ Co.
2
The reaction generates Co interstitial atoms which can produce the
CoSi phase by reaction: Co ϩ 2Si r CoSi via grain boundary. Park
2
2
1
5
et al. also reported that CoSi was formed by the fast diffusion of
2
Co atoms along defects in Si such as dislocations. Because grain
boundaries of poly-Si act as a diffusion path, we postulated that the
progression for CoSi growth was influenced by the total area of
2
grain boundaries of poly-Si. This tendency can be confirmed by
measuring sheet resistance after the second RTA (750ЊC).
Figure 3 shows the sheet resistance of Co silicide film as a func-
tion of the first RTA temperature with various grain sizes of poly-Si
structures after the second RTA at 750ЊC for 30 s. It is important to
note that sheet resistance of Co silicide film has a higher value as the
grain size of poly-Si decreases. Considering that a 750ЊC anneal is at
a sufficient temperature for the transformation of CoSi , the differ-
2
Figure 1. Sheet resistance of Co film as a function of first RTA temperature
for various grain sizes of poly-Si: a-Si/fine grain, fine grain, medium grain,
and large grain sized poly-Si.
ence of sheet resistance can only be ascribed to a difference of final
16
thickness of the CoSi film or to an irregular interface morphology.
2
Cross-sectional TEM observation after the second RTA (750ЊC,
3
0 s) indicates that the sheet resistance difference is caused by the
ly.11 Therefore, we can conclude that the Co-Si reaction is enhanced
as grain size of poly-Si decreases.
A rough estimate of grain boundary energy of poly-Si is E ϭ ␥/r,
where ␥ is grain boundary energy per unit volume and r is the grain
size of poly-Si. The relationship between grain size of poly-Si and its
grain boundary energy is summarized in Table I. Because a-Si has a
roughness of CoSi /poly-Si interface. In Fig. 4a, CoSi film on a-
Si/fine grain-sized poly-Si shows a rough interface and grooves at the
CoSi /poly-Si. Because CoSi formed on a-Si/fine-grain-sized poly-Si
has a smaller grain size than that on single crystalline or large-grain-
sized poly-Si, Co atoms fast diffuse along the grain boundary, and
CoSi is rapidly formed. Xiao et al. demonstrated that the phase
transformation to CoSi on amorphized Si was considerably acceler-
ated, and the silicide grain size was smaller. The CoSi formed on fine
grain sized poly-Si [Fig. 4b] also shows a rough morphology of the
CoSi /poly-Si interface. On the other hand, the CoSi formed on
large-grain-size poly-Si [Fig. 4c] shows a smooth interface. This
result suggests that the grain boundary can be a rapid diffusion path
of Co atoms. The formation of CoSi at the grain boundary results in
a rough interface. Thus, we conclude that the difference of sheet resis-
tance is ascribed to an irregular interface morphology [Fig. 3]. The
difference of morphology can affect the thermal stability of CoSi2
film, which was measured via sheet resistance as shown in Fig. 5.
2
2
2
6
2
high internal energy for crystallization, it can reduce the CoSi for-
2
2
mation temperature. Although the poly-Si has a higher energy state
than single crystalline Si due to the existence of grain boundaries, the
difference is small compared with the heat of formation of transfor-
2
2
2
mation from Co monosilicide to Co disilicides (CoSi r CoSi : ⌬H ϭ
2
Ϫ2.5 kJ/mol). For transformation from CoSi to CoSi through the
2
reaction CoSi ϩ Si ϭ CoSi on single crystalline Si, the difference in
2
2
enthalpy is 2.5 kJ/mol between CoSi (HCoSi ϭ Ϫ100.4 kJ/mol) and
CoSi (H
ϭ Ϫ102.9 kJ/mol). Because of this small chemical dri-
2
CoSi2
ving force, it is known that nucleation becomes the limiting factor in
12
the CoSi formation. This theoretical prediction is in good agree-
2
13
ment with a previous report. Colgan et al. reported the thermody-
namic driving force for silicide formation on poly-Si should be very
similar to that on single crystalline Si.
The sheet resistance of CoSi film on a-Si and fine-grain-sized
2
poly-Si starts to increase after annealing at 800ЊC, and shows an
abrupt increase at 875ЊC. However, as grain size increases, the in-
crease of sheet resistance is greatly reduced. For CoSi film on large-
grain-sized poly-Si, the sheet resistance remains constant up to 950ЊC,
which is almost 150ЊC higher than that on fine-grain-sized poly-Si.
From the view point of thermodynamic driving force, the struc-
ture of poly-Si does not affect the silicide formation temperature.
2
Figure 2. XRD spectra of Co film on Si substrate with various poly-Si struc-
Figure 3. Sheet resistance of CoSi as a function of first RTA temperature
2
tures after first RTA (600ЊC in an N for 60 s).
after second RTA (750ЊC in N for 60 s) for various grain sizes of poly-Si.
2
2
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Journal of The Electrochemical Society, 147 (4) 1551-1554 (2000)
1553
S0013-4651(99)07-029-9 CCC: $7.00 © The Electrochemical Society, Inc.
Since the increase of sheet resistance with postannealing results from
agglomeration, we can conclude the fast increase of sheet resistance
of CoSi2 film on a-Si/fine-grain-sized poly-Si or fine-grain-sized
poly-Si is caused by more rapid agglomeration. Thus, the poly-Si
structures and initial morphology CoSi /poly-Si determines the ther-
2
mal stability of CoSi film.
2
To investigate the degradation of CoSi film, the CoSi /poly-Si
2
2
interface was observed by cross-sectional TEM microview after an-
nealing at 850ЊC for 30 s as shown in Fig. 6. The microstructure
changes observed correlation of the grain size of poly-Si. The CoSi2
films formed on a-Si/fine-grain-sized poly-Si [Fig. 6a] and fine grain
sized poly-Si [Fig. 6b] grow into the poly-Si due to silicide dissoci-
1
7
ation. Metal diffusion along grain boundaries and formation of
CoSi result in more rough interface. At the surface of the poly-Si,
2
Si grains grow through the silicide to the top surface. In medium-
grain-sized poly-Si, CoSi films show a discontinuous morphology
2
Figure 5. Sheet resistance of CoSi film as a function of RTA temperature for
2
3
0 s with various grain sizes of poly-Si. The thermal stability of CoSi film
2
is greatly deteriorated as grain size decreases.
[
Fig. 6c]. Meanwhile, The CoSi on large-grain-sized poly-Si shows
2
a smooth and continuous interface [Fig. 6d]. The TEM results show
that the morphology of CoSi /poly-Si interface affects CoSi ag-
2
2
glomeration during postannealing, and the CoSi agglomeration is
2
accelerated by the number of grain boundaries of poly-Si.
Conclusions
As the grain size of poly-Si decreases, the formation of CoSi is
2
enhanced after RTA. We conclude that Co atoms rapidly diffuse along
the grain boundary by the reaction of 2CoSi r CoSi ϩ Co. That is,
2
Co atom diffusion along grain boundaries results in an intense reac-
tion of Co with Si, especially on fine-grain-sized poly-Si. Therefore,
the morphology of the CoSi on fine-grain-sized poly-Si is much
2
rougher than that on large-grain-sized poly-Si. From the results of
Figure 6. Cross-sectional TEM microview of CoSi /poly-Si after annealing
2
at 850ЊC for 30 s. CoSi on (a) a-Si and (b) fine grain sized poly-Si is formed
2
Figure 4. Cross-sectional TEM images of CoSi formed on (a) a-Si/fine grain
into poly-Si along grain boundary. (c) CoSi /medium grain sized poly-Si
2
2
sized poly-Si, (b) fine grain sized poly-Si, and (c) large grain sized poly-Si
shows a discontinuous morphology. Meanwhile, (d) CoSi /large grain sized
2
formed for 30 s at 750ЊC.
poly-Si shows a smooth and continuous interface.
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1
554
Journal of The Electrochemical Society, 147 (4) 1551-1554 (2000)
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TEM analysis, we find it likely that the rough interface promotes the
agglomeration process, thereby thermal stability is also degraded.
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