Atomic-layer deposition of ZrO2 with a Si nitride barrier layer

Article abstract of DOI:10.1063/1.1510584

ZrO₂ thin films for gate dielectrics have been formed at low temperatures (200°C) by an atomic-layer deposition (ALD) technique using Zr(t-OC₄H₉)₄ and H₂O source gases. An ultrathin (physical thickness T_phy of ～0.5 nm) Si nitride layer was deposited on a Si substrate by ALD before the deposition of ZrO ₂. Transmission electron microscopy showed that the Si nitride barrier layer successfully suppressed the formation of a SiO₂ interfacial layer. Because of the extremely uniform thickness control capability in the ultrathin region and the low thermal budget of the ALD process, the ALD process for the ZrO₂/Si nitride stack structure is a promising candidate for fabricating the ultrathin gate dielectrics for sub-0.1-μm complementary metal-oxide-semiconductor transistors. 2002 American Institute of Physics.

Full text of DOI:10.1063/1.1510584

Atomic-layer deposition of ZrO 2 with a Si nitride barrier layer
Anri Nakajima, Toshirou Kidera, Hiroyuki Ishii, and Shin Yokoyama
Citation: Applied Physics Letters 81, 2824 (2002); doi: 10.1063/1.1510584
View online: http://dx.doi.org/10.1063/1.1510584
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 15
7 OCTOBER 2002
Atomic-layer deposition of ZrO2 with a Si nitride barrier layer
Anri Nakajima,a) Toshirou Kidera, Hiroyuki Ishii, and Shin Yokoyama
Research Center for Nanodevices and Systems, Hiroshima University 1-4-2 Kagamiyama, Higashi-
Hiroshima, Hiroshima 739-8527, Japan
͑Received 9 April 2002; accepted 5 August 2002͒
ZrO2 thin films for gate dielectrics have been formed at low temperatures ͑200 °C͒ by an
atomic-layer deposition ͑ALD͒ technique using Zr(t-OC4H9)4 and H2O source gases. An ultrathin
͑physical thickness Tphy of ϳ0.5 nm͒ Si nitride layer was deposited on a Si substrate by ALD before
the deposition of ZrO2 . Transmission electron microscopy showed that the Si nitride barrier layer
successfully suppressed the formation of a SiO2 interfacial layer. Because of the extremely uniform
thickness control capability in the ultrathin region and the low thermal budget of the ALD process,
the ALD process for the ZrO2 /Si nitride stack structure is a promising candidate for fabricating the
ultrathin gate dielectrics for sub-0.1-␮m complementary metal–oxide–semiconductor transistors.
© 2002 American Institute of Physics. ͓DOI: 10.1063/1.1510584͔
Recently, the substitution of conventional SiO2 with a
high-dielectric-constant thin film as the gate dielectrics for
sub-0.1-␮m metal–oxide–semiconductor field-effect transis-
tors ͑MOSFETs͒ has received extensive attention.1–9 The
major reason is that the scaling down of the SiO2 thickness is
reaching its limit from the viewpoint of gate leakage current.
One of the most promising candidates for the replacement of
SiO2 is ZrO2 , due to its high relative dielectric constant
value (␧rϭ20–25),4 its thermodynamic stability in contact
with Si,5 its large energy band gap ͑5.2 eV͒,6 and small lat-
tice mismatch of 2.1% with Si ͑100͒.4
Various methods have been proposed for the formation
of ZrO2 gate dielectrics such as sputtering7,8 and chemical
vapor deposition methods.6,9 Recently, in view of film uni-
formity, thickness control capability in the small thickness
region, and low thermal budget, the application of self-
limiting atomic-layer deposition ͑ALD͒ is accelerating with
apparently significant benefits in the fabrication of various
gate dielectrics.10–14 For the ALD of ZrO2 gate dielectrics,
the alternating exposure of ZrCl4 and H2O gases has most
commonly been applied to date.15,16 However, in the ALD
using these source gases, ZrO2 shows island-like growth
when deposited directly on Si.15 There is also a risk of Cl
contamination and particle adhesion to the substrate surface
because ZrCl4 is in a solid state at room temperature. Zirco-
thin ZrO2 layer by ALD using ZTB and H2O as source gases,
and we report its structural characterization. We have also
formed an ultrathin Si nitride layer by ALD between ZrO2
and the Si substrate and found that it acts as an effective
barrier against oxygen indiffusion.
The ALD of ZrO2 layers was carried out by alternately
supplying ZTB and H2O gases on p-type Si ͑001͒ wafers
͑ϳ10 ⍀ cm͒. The wafers were cleaned with an
NH4OH:H2O2 :H2Oϭ0.15:3:7 solution at 80 °C for 10 min
and were terminated with hydrogen in a 0.5% HF solution, in
order to suppress native oxidation before the ALD. ZTB ex-
posure followed by H2O exposure was cyclically repeated
2–15 times at the substrate temperature (Tsub) of 200 °C.
The H2O exposure time was 60 s. The vapor pressures of
ZTB and H2O during the deposition were controlled to 0.04
and 0.13–1.05 kPa, respectively. Just after the ALD, in-situ
N2 annealing was carried out for 5 min at 400 °C. In the
ALD–ZrO2/ALD–Si–nitride stack structure, about 0.5-nm-
thick Si nitride was deposited by the ALD process using
SiCl4 and NH3 gases.13,14 The microstructure and thickness
of the deposited film were investigated using transmission
electron microscopy ͑TEM͒, employing a Hitachi HF-2100
field emission TEM operating at 200 keV.
Figure 1 shows the self-limiting properties of the film
growth with ZTB exposure time. Vapor pressure of H2O was
0.70 kPa. A ZrO2 film thickness with five deposition cycles
tends to saturate at ZTB exposure times longer than 60 s. The
film thickness was measured by ellipsometry, under the as-
sumption that the refractive index of ZrO2 was 2.05.4
nium tertiary-butoxide Zr(t-OC H ) ,(ZTB) is one of the
͓
͔
4
9 4
alternative Zr precursors with the highest vapor pressure, al-
lowing evaporation at low temperatures. However, only a
few reports17 have been published regarding the ALD of
ZrO2 using ZTB as a gas source. In particular, the growth in
the ultrathin region has not yet been examined.
On the other hand, it is well known that a chemical
reaction between ZrO2 and the Si substrate occurs during
film deposition in oxygen ambient or oxygen-containing
source gas ambient.9,15 The growth of the interfacial oxide
layer increases equivalent oxide thickness ͑EOT͒. To prevent
this growth, it is efficient to form a thin barrier layer for
oxygen indiffusion. In this study, we have formed an ultra-
FIG. 1. Dependence of the ALD ZrO2 film thickness on the ZTB exposure
times after five deposition cycles. Vapor pressure of H2O was 0.70 kPa.
a͒
Electronic mail: nakajima@sxsys.hiroshima-u.ac.jp
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Appl. Phys. Lett., Vol. 81, No. 15, 7 October 2002
Nakajima et al.
2825
FIG. 2. Dependence of the ALD ZrO2 film thickness on the H2O vapor
pressure after five deposition cycles. ZTB exposure time was 60 s.
Figure 2 also shows the self-limiting properties of the
film growth with vapor pressure of H2O. A saturated film
thickness of about 2.5 nm was achieved with five deposition
cycles from around 0.1 to 1.05 kPa, which is consistent with
the result for ZTB exposure time of 60 s shown in Fig. 1.
Figure 3 shows the thickness of the ALD ZrO2 as a
function of the number of deposition cycles. The deposited
thickness is in linear relation with the number of deposition
cycles although some offset thickness occurred. This offset
thickness is about 1.5 nm and is considered to be due to the
presence of the interfacial oxidized Si layer. The interfacial
layer is also observed by TEM measurements ͓Fig. 5͑a͔͒.
From the slope of the linear line in the figure, the growth rate
is estimated to be about 0.2 nm/cycle. One monolayer of
amorphous ZrO2 is estimated to be ϳ0.2 nm thick since the
Zr–O distance is obtained to be 0.22 nm from the ionic
radius.18,19 Therefore, it is indicated that the layer-by-layer
growth of ZrO2 takes place in our experiment.
Figure 4͑a͒ shows the x-ray photoelectron spectroscopy
͑XPS͒ spectra of the Zr3d core level for ALD ZrO2 deposited
under 0.70 kPa H2O vapor pressure. It shows a strong signa-
ture of typical ZrO2 bonding, namely, the shifted Zr3d oxide
doublet at 182.3 and 184.7 eV.9,20 Figure 4͑b͒ shows the Si2p
signal of the film. The Si2p peak at 102.4 eV indicates the
existence of Si–O bonds in the interfacial layer. The separa-
tion between oxidized and unoxidized Si signals is 2.9 eV,
which is lower than the ϳ4 eV measured for SiO2 on Si.21
This indicates that the interfacial Si oxide is substoichiomet-
ric.
FIG. 4. XPS spectra of the ͑a͒ Zr3d and ͑b͒ Si2p core levels for the ALD
ZrO2 . Number of deposition cycles was 5. ZTB exposure time was 60 s.
H2O vapor pressure was 0.70 kPa. Take-off angle was 90°.
thickness in Fig. 3. Figure 5͑b͒ shows the
ALD–ZrO2 /ALD–Si-nitride stack structure. In this sample,
annealing at 850 °C for 3 min in N2 ambient was added to
400 °C annealing. For the 850 °C-annealed sample, crystal-
lized ZrO2 is observed. With this crystallization, the surface
of the ZrO2 becomes rough. A noteworthy feature is that a
smooth interface was observed between the ZrO2 and Si ni-
tride layers. The growth of the interfacial Si oxide layer is
observed to be suppressed. This is understood from the fact
that the thickness of the interfacial amorphous layer ͑ϳ0.5
nm͒ coincides with that of the initially deposited ALD Si
nitride.
Figure 6 shows the capacitance–voltage (C–V) curve of
Figure 5 shows a high-resolution cross-sectional TEM
micrograph of the formed films. It is shown in Fig. 5͑a͒ that
ALD ZrO2 has an amorphous structure even after annealing
at 400 °C. Uniform thickness of ALD ZrO2 is observed. The
thickness of the interfacial layer is observed to be ϳ1.2 nm.
Considering the ambiguity of the boundary between the in-
terfacial layer and ZrO2 in the TEM micrograph, the ob-
served interfacial layer thickness is consistent with the offset
FIG. 5. High-resolution cross-sectional TEM micrograph of ͑a͒ ALD ZrO2
and ͑b͒ ALD–ZrO2 /ALD–Si-nitride stack films. ZTB exposure time was 60
s. H2O vapor pressure was 0.70 kPa. For the stack film, 850 °C annealing
was added for 3 min after the ALD and the annealing ͑400 °C for 5 min͒ of
ZrO . Number of deposition cycles of underlying ALD Si nitride was 2
FIG. 3. Thickness of ALD ZrO2 versus number of deposition cycles. The
thickness of ZrO was measured by ellipsometry. ZTB exposure time was
60 s. H2O vapor pressure was 0.70 kPa.
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(Tphyϭϳ0.5 nm).
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2826
Appl. Phys. Lett., Vol. 81, No. 15, 7 October 2002
Nakajima et al.
pressed the formation of the Si oxide interfacial layer. Be-
cause of an extremely uniform thickness control capability in
the ultrathin region and the low thermal budget of the ALD
process, the ZrO2 /Si–nitride stack structure formed by the
ALD process is a promising candidate for the ultrathin gate
dielectrics of sub-0.1-␮m complementary metal–oxide–
semiconductor transistors.
Part of this work has been supported by a Grant-in-Aid
for Scientific Research ͑B͒ from the Ministry of Education,
Culture, Sports, Science and Technology, Japanese Govern-
ment ͑No. 13450129͒, and the Semiconductor Technology
Academic Research Center ͑STARC͒.
FIG. 6. C–V characteristics at 20 kHz for ALD–ZrO2 /ALD–Si–nitride
capacitor ͑solid lines͒. Number of deposition cycles was 15 and 2 for ZrO2
and Si nitride, respectively. ZTB exposure time was 60 s. H2O vapor pres-
sure was 0.70 kPa. The calculated C–V curve ͑broken line͒ is also shown
using the doping level of the Si substrate and the work function of the Al
gate.
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an Al/ALD–ZrO2 /ALD–Si–nitride capacitor measured at
20 kHz. The number of deposition cycles of ZrO2 was 15.
The observed hysteresis (⌬VFBϭ50 mV) is considered to be
due to charge trapping in ZrO2 and/or the ZrO2 /Si–nitride
interface. The damage that occurred during Al sputtering for
electrode formation is also a possible reason because we did
not carry out postmetallization annealing after sputtering.
The EOT value of the stack dielectrics is obtained to be 1.6
nm from the comparison with the calculated capacitance
͑broken line͒ for SiO2 dielectrics. The physical thickness
(Tphy) is observed to be 4.7 nm from TEM for the stack film.
Tphy consists of the ZrO2 layer (Tphyϭ4.2 nm) and the un-
derlying Si nitride layer (Tphyϭ0.5 nm). Taking these Tphy
values into account, the ⑀r value of the ALD ZrO2 layer is
obtained to be 12. In comparison with the calculated curve
͑broken curve͒, the interface trap density Dit was estimated
to be about 5ϳ4ϫ1012 cmϪ2 eVϪ1 between 0.1 and 0.3 eV
above the valence band edge. Also, the density of the posi-
tive fixed charge was estimated to be 5ϫ1012 cmϪ2 from the
flatband voltage shift. Details of the electrical properties in-
cluding leakage current will be reported elsewhere.
In summary, ultrathin ZrO2 films were successfully
formed by alternately supplying ZTB and H2O gases. Self-
limiting properties of film growth with ZTB exposure time
and H2O vapor pressure were achieved at a growth tempera-
ture of 200 °C. The deposited ZrO2 was amorphous when
annealed at 400 °C in the N2 ambient. Good smoothness at
the surface of ALD ZrO2 was obtained. XPS spectra showed
that the formed film consists of ZrO2 . TEM observation
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