Thermal stability of polycrystalline silicon electrodes on ZrO2 gate dielectrics

Article abstract of DOI:10.1063/1.1499513

Thermal stability of gate stack structures composed of ZrO₂ gate dielectrics and silicon electrodes was investigated. The ZrO₂ films were deposited by atomic layer deposition, while the polycrystalline silicon electrodes were deposited using different variants of chemical (CVD) and physical vapor deposition (PVD). Zirconium silicide formation was noted in all CVD-electroded samples after subsequent annealing treatments at temperatures above 750°C, but not in the room temperature PVD-electroded samples, even after gate annealing at 1050°C. The dependence of zirconium silicide formation on the Si deposition process was explained using thermodynamic arguments which explicitly include the effects of oxygen deficiency of the metal oxide films.

Full text of DOI:10.1063/1.1499513

Thermal stability of polycrystalline silicon electrodes on ZrO 2 gate dielectrics
Charles M. Perkins, Baylor B. Triplett, Paul C. McIntyre, Krishna C. Saraswat, and Eric Shero
Citation: Applied Physics Letters 81, 1417 (2002); doi: 10.1063/1.1499513
View online: http://dx.doi.org/10.1063/1.1499513
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 8
19 AUGUST 2002
Thermal stability of polycrystalline silicon electrodes on ZrO2
gate dielectrics
Charles M. Perkins, Baylor B. Triplett, and Paul C. McIntyrea)
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305
Krishna C. Saraswat
Department of Electrical Engineering, Stanford University, Stanford, California 94305
Eric Shero
ASM America, 3440 East University Drive, Phoenix, Arizona 85034
͑
Received 10 December 2001; accepted for publication 15 June 2002͒
Thermal stability of gate stack structures composed of ZrO gate dielectrics and silicon electrodes
2
was investigated. The ZrO2 films were deposited by atomic layer deposition, while the
polycrystalline silicon electrodes were deposited using different variants of chemical ͑CVD͒ and
physical vapor deposition ͑PVD͒. Zirconium silicide formation was noted in all CVD-electroded
samples after subsequent annealing treatments at temperatures above 750 °C, but not in the room
temperature PVD-electroded samples, even after gate annealing at 1050 °C. The dependence of
zirconium silicide formation on the Si deposition process was explained using thermodynamic
arguments which explicitly include the effects of oxygen deficiency of the metal oxide films.
©
2002 American Institute of Physics. ͓DOI: 10.1063/1.1499513͔
High-␬ metal oxides have been extensively studied as
were 60 to 90 min. Samples from the third test group were
capped by 800 Å poly-Si films deposited by room tempera-
ture physical vapor deposition ͑PVD͒. The sputtering process
utilized a lightly boron-doped silicon target and a base pres-
sure of approximately 10 Torr. Deposition times were sev-
eral minutes with a 3–4 h pump down period.
alternative gate dielectric materials to SiO in metal–oxide–
2
1
semiconductor devices beyond the 100 nm technology node.
Because dual metal gate electrode replacements impose ad-
ditional processing complexities and limitations on operating
Ϫ7
2
voltages, conventional polycrystalline silicon ͑poly-Si͒ re-
mains an interesting gate electrode material. Recently, HfO2
has emerged as a promising gate dielectric replacement due
to its high dielectric constant and superior thermal stability in
After depositing the poly-Si films, the atmospheric CVD
samples were annealed in a horizontal furnace using reagent
grade N gas. Temperatures ranged from 700 to 1000 °C
2
3
contact with Si-based electrodes. Similar stability has been
while dwell times were kept constant at 1 h. Annealing of the
low-pressure CVD and PVD samples utilized a rapid thermal
reported for ZrO in certain special cases; for example, with
2
4
a diffusion barrier between the ZrO and silicon electrode.
annealer ͑RTA͒ and reagent grade N gas. Temperatures were
2
2
In this letter, we demonstrate the thermal stability of the
Si/SiO /ZrO /Si system under conventional dopant activa-
tion conditions without a barrier layer and show how stabil-
varied between 700 and 1050 °C with the dwell times of 5 s
to 3 min. Cross sections of the gate stacks were examined by
high-resolution transmission electron microscopy ͑TEM͒ us-
ing a Philips CM20 microscope operating at 200 kV. Crys-
talline phases within the gate stack were also monitored and
indexed by selected area electron diffraction ͑SAD͒ of plan
view samples.
2
2
ity of the poly-Si/ZrO interface depends on the temperature
2
and oxygen activity of the poly-Si growth process.
Uniform ultrathin ͑ϳ50 Å͒ zirconium oxide films were
ϩ
deposited on 200 mm p-epi/p silicon test wafers by atomic
layer deposition ͑ALD͒ at ASM America Inc., using alternat-
Figure 1 shows high-resolution cross-sectional TEM im-
ages of a Si/SiO /ZrO /Si gate stack with a top silicon elec-
ing surface-saturating reactions of ZrCl and H O at 300 °C.
4
2
x
2
The ZrO films in the first two of three test groups were
2
trode deposited by atmospheric CVD using SiH at 620 °C.
4
deposited directly onto the original chemical silicon oxide of
the as-received wafers. Films in the third test group were
Fast Fourier transforms were performed on both cross-
sectional micrographs to determine the interplanar spacings
of the different polycrystalline regions. The majority of the
deposited onto an ultrathin ͑ϳ15 Å͒ SiO film grown by
2
rapid thermal oxidation. The first test group utilized atmo-
spheric chemical vapor deposition ͑CVD͒ to grow 1500 Å
silicon films on the ZrO films using SiH and H carrier gas
ZrO film was indexed as tetragonal while a small percent-
age of the film exhibited diffraction features characteristic of
the monoclinic phase. The silicide grains in both images
2
2
4
2
͑Ͼ10 SLM͒ at 620 °C. Deposition times were approximately
were indexed as the orthorhombic ZrSi phase. To analyze a
2
1
–2 min. Low-pressure CVD was performed in a vertical
more statistically significant sample area, SAD patterns were
recorded using a SAD aperture that included a diffracting
furnace for samples in the second test group to deposit 500 Å
silicon films using pure SiH or Si H at 510 and 440 °C,
2
4
2
6
area of ϳ2 ␮m . A representative electron diffraction ring
respectively, at a pressure of a few Torr. Deposition times
pattern of the as-deposited sample is shown in Fig. 2͑a͒. The
only rings not indexed to tetragonal ZrO interplanar spac-
2
a͒Electronic mail: pcm1@leland.stanford.edu
ings were indexed to the ZrSi phase. The most intense ͑131͒
2
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003-6951/2002/81(8)/1417/3/$19.00 1417 © 2002 American Institute of Physics
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0
1

1418
Appl. Phys. Lett., Vol. 81, No. 8, 19 August 2002
Perkins et al.
FIG. 3. Cross-sectional TEM micrograph illustrates a ZrSi2 grain in a
5
10 °C SiH CVD sample after a 1050 °C N anneal.
4
2
icide particles or detectable thinning of the SiO layer. Upon
2
subsequent annealing, the Si H samples exhibited the best
2
6
thermal stability, as silicide diffraction signatures were not
observed below 800 °C. As illustrated in Fig. 3 for a low-
temperature SiH sample annealed at 1050 °C, higher tem-
4
perature post-deposition anneals were again found to signifi-
cantly thin the interfacial layer and cause silicon to diffuse to
the base of the silicide grains.
FIG. 1. Cross-sectional TEM images of a Si/SiO /ZrO /Si gate stack uti-
lizing a 620 °C SiH4 CVD process ͑a͒ as-deposited and ͑b͒ after a subse-
The extent of silicide growth depended upon the post-
electrode annealing temperature. Recent x-ray photoelectron
spectroscopy studies reported thermal stability up to 900 °C
x
2
quent 1 h N furnace anneal at 850 °C.
2
5
,6
for the Si/SiO /ZrO /Si system. Because the initial onset
2
2
ZrSi reflection was used to determine the occurrence of sil-
icide formation in all samples in this study. The cross-
sectional micrograph of the as-deposited gate stack also
of silicidation is a sparse and localized phenomenon, this
technique may not be sensitive enough to detect the initial
precipitates until their volume fraction in the gate stack
reaches a few atomic percent. As a result, the onset of ther-
mal instability in these previous studies5 actually may have
occurred at lower temperatures, similar to those reported in
this paper.
A third set of samples was fabricated using a room tem-
perature sputtering process to deposit the poly-Si gate elec-
trode ͑Fig. 4͒. Unlike the samples made using CVD methods,
2
shows a local thinning of the SiO -based interfacial layer
2
,6
beneath the silicide grain. The degree of thinning increased
and was observed to spread to regions that were not in the
immediate vicinity of a silicide particle during the subse-
quent high-temperature anneal. Epitaxial growth of silicon at
the Si ͑100͒ wafer interface is also evident in the annealed
sample around the sides of the silicide grain in Fig. 1͑b͒,
suggesting diffusion of silicon from the decomposing SiO2
into the Si wafer during the anneal. It should be noted that
the PVD material was stable with respect to ZrSi formation
2
at temperatures up to those of conventional dopant activation
anneals. Thinning of the interfacial layer was observed after
the anneal.
high temperature N annealing of bare ͑unelectroded͒ ZrO2
2
films did not result in detectable ZrSi formation.
2
A second set of CVD poly-Si deposition experiments
explored the effects of lower thermal budgets and different
Several groups have suggested that the observed silicid-
ation during vacuum annealing of uncapped ZrO films re-
2
silicon precursors on ZrSi formation. Figure 2͑b͒ is a rep-
sults from SiO evaporation which decomposes the SiO layer
2
2
and removes oxygen from the ZrO2 .5 Oxygen loss by SiO
,7
resentative SAD pattern of the as-deposited Si H sample.
2
6
As with the as-deposited low temperature SiH samples, no
silicide diffraction signatures were observed; all detected
evaporation is well established for thin SiO films for tem-
4
2
peratures between 700 and 1000 °C.8 Reduction of the zir-
conia films to form oxygen-deficient ZrO2Ϫx by loss of mo-
lecular oxygen during the early stages of poly-Si deposition
,9
rings were indexed to tetragonal ZrO . Cross-sectional TEM
2
images corroborated these results, showing no zirconium sil-
FIG. 2. SAD patterns of as-deposited samples show ͑a͒ ZrSi2 diffraction
FIG. 4. Cross-sectional TEM micrographs of a Si/SiO /ZrO /Si gate stack
x
2
signatures with the 620 °C SiH CVD process and ͑b͒ thermal stability of
utilizing a room temperature PVD process in both the ͑a͒ as-deposited con-
4
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the 510 °C SiH CVD sample.
dition and ͑b͒ after a 10 s N RTA step at 1050 °C.
4
2
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Appl. Phys. Lett., Vol. 81, No. 8, 19 August 2002
Perkins et al.
1419
change of the combined process is also large and negative
ϳϪ190 kJ/mol͒,1
0–12
indicating a large driving force for
͑
zirconium silicide formation from oxygen deficient ZrO via
2
this solid state reaction.
Although the predicted driving forces for SiO decom-
2
position and ZrSi2 formation at the ZrO2Ϫx /SiO2Ϫx and
Si/ZrO2Ϫx interfaces, respectively, are not dissimilar, our an-
nealing experiments on PVD poly-Si electroded samples
showed a complete absence of silicidation but a detectable
SiO thinning. This suggests that the ALD ZrO films were
2
2
somewhat oxygen deficient as-deposited and that the PVD
poly-Si deposition process did not add to this initial oxygen
nonstoichiometry. Furthermore, one expects a substantial ki-
netic barrier to nucleation of the silicide particles associated
with formation of Si/ZrSi and ZrSi /ZrO interfaces. No
FIG. 5. The proposed process of silicidation involves ͑1͒ decomposition of
ZrO , ͑2͒ Zr diffusion and reaction with Si to form ZrSi , and ͑3͒ O diffu-
sion and filling of oxygen vacancies in ZrO2Ϫx regions. Note concurrent O
2
2
diffusion from the SiO layer.
2
2
2
2
new interfaces are needed for SiO thinning to take place.
2
The elevated temperature of the CVD processes may pro-
is also possible. The poly-Si CVD precursor ambients ͑e.g.,
H /SiH ͒ are expected to have extremely low oxygen activ-
mote formation of ZrSi nuclei at the poly-Si/ZrO interface.
2
2
2
4
These nuclei would then grow during post-electrode anneals,
consistent with the observed particle formation during an-
nealing of the CVD samples.
In summary, we have demonstrated a correlation be-
tween poly-Si gate electrode deposition conditions and the
ity. As a result, it is reasonable that oxygen loss should occur
perhaps involving SiO͒ during the initial phases of silicon
deposition given the tendency of ZrO to become oxygen
deficient. In this study, thinning of the interfacial layer was
not observed in the lower temperature ͑440–510 °C͒ CVD
and the PVD samples until after post-electrode annealing.
͑
2
1
2
silicidation of ZrO gate dielectrics, whereby high deposition
2
temperatures and reducing CVD ambients encourage forma-
Once a thick Si layer caps the ZrO film, exchange of oxy-
2
tion of ZrSi particles. We have also shown that ZrSi for-
gen between the sample and the ambient is kinetically inhib-
2
2
ited. Therefore, continual SiO evaporation5 cannot account
,7
mation occurs during postelectrode annealing of ZrO2
samples that are capped by a thick, continuous poly-Si elec-
for the observed silicidation and SiO thinning during post-
2
trode layer, when the electrode deposition occurs under typi-
cal CVD conditions. These results are consistent with ther-
modynamically favored processes in which oxygen
vacancies present in the ZrO2 dielectric are annihilated
electrode anneals.
The elevated temperatures and low oxygen activity of
conventional CVD silicon deposition may create oxygen va-
cancies in the ZrO , in addition to oxygen vacancies present
2
through SiO decomposition and ZrSi formation.
in the as-deposited metal oxide films. Reported thermody-
namic data suggest that there is a driving force for oxygen
exchange between oxygen deficient ZrO and SiO via the
2
2
This work was supported in part by the NSF/SRC Engi-
neering Research Center for Environmentally Benign Semi-
conductor Manufacturing and Intel Corporation.
2
2
1
2
following reaction ͑using Kr o¨ ger-Vink notation͒
Ϫ
x
O
4
e ϩ2V¨ ϩSiO ͑s͒→Si͑s͒ϩ2O ,
O
2
1
where the individual processes can be thought off as: ͑1͒
decomposition of SiO into its elemental constituents and ͑2͒
filling of oxygen vacancies in ZrO . The enthalpy change of
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3
͑
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2
7
8
9
0
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x
O
ZrO ϩ4e ϩ2V¨ ϩ2Si͑s͒→ZrSi ͑s͒ϩ2O ,
2
O
2
which can be thought of as to the summation of three pro-
1
cesses: ͑1͒ decomposition of ZrO into its elemental con-
2
stituents, ͑2͒ reaction between the resulting metallic Zr and
11
Si to form ZrSi , and ͑3͒ filling of two oxygen vacancies in
2
12
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1