APPLIED PHYSICS LETTERS
VOLUME 80, NUMBER 18
6 MAY 2002
Excellent thermal stability of Al2O3 ÕZrO2 ÕAl2O3 stack structure
for metal–oxide–semiconductor gate dielectrics application
Hyo Sik Chang, Sanghun Jeon, and Hyunsang Hwanga)
Department of Materials Science and Engineering, Kwangju Institute of Science and Technology,
Gwangju 500-712, Korea
Dae Won Moon
Nano Surface Group, Korea Research Institute of Standards and Science, Daejeon 305-600, Korea
͑Received 29 November 2001; accepted for publication 9 March 2002͒
The thermal stability of a nanolaminate (Al2O3 /ZrO2 /Al2O3) gate stack prepared by atomic layer
chemical vapor deposition was investigated using medium-energy ion scattering spectroscopy, and
x-ray photoelectron spectroscopy. We observed that the structure was stable up to 1000 °C under
ultrahigh vacuum conditions. However, annealing in a nitrogen or oxygen ambient at 1 atm yielded
the formation of an interfacial Zr–Al silicate layer at much lower temperatures. The growth of the
interfacial silicate layer could be significantly reduced during furnace annealing via the use of
plasma nitridation. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1477266͔
As the gate oxide thickness of a metal–oxide–
semiconductor field-effect transistor device was scaled down
to below 1.5 nm, conventional thermal SiO2 is no longer
applicable because of the excess direct tunneling leakage
current. Conventional high-k candidates, such as SrTiO3 ,
Ta2O5 , TiO2 , Y2O3 , ZrO2 , HfO2 , Zr silicate, and Hf sili-
cate, have been investigated as alternative gate dielectric
materials.1 However, considering the thermal stability and
dielectric constant, ZrO2 , HfO2 and their silicates are the
most promising.1–3
Based on phase diagrams and thermodynamic predic-
tions, an excellent thermal stability of ZrO2 on silicon can be
predicted. However, the formation of an interfacial oxide
layer and silicide at the ZrO2 /Si interface has been
reported.2–4 In addition, ZrO2 films which are directly depos-
ited on a HF-last Si͑001͒ surface caused an island-type
nucleation which degrades film roughness and leakage
current.4,5 Although a single-ZrO2 layer structure is preferred
for the process simplicity, the degradation of thermal stabil-
ity was observed for ultrahigh vacuum ͑UHV͒ annealing at
1000 °C.4 Therefore, the introduction of an Al2O3 layer has
several advantages such as the formation of an atomically
flat film and robustness against the silicide formation at
1000 °C.5–7
In this letter, we report the investigation of the effect of
high-temperature annealing ambients such that UHV and 1
atm nitrogen on the thermal stability of a nanolaminate
(Al2O3 /ZrO2 /Al2O3) structure. In addition, to improve the
thermal stability, we evaluated the effect of plasma nitrida-
tion which provides the sufficient stability at a high-
temperature annealing.
A uniform ultrathin Al2O3 /ZrO2 /Al2O3 layered struc-
ture was deposited on a 4 in. Si͑001͒ wafer by atomic layer
chemical vapor deposition. After standard cleaning, an ultra-
thin ͑less than 1 nm thick͒ amorphous Al2O3 buffer layer
was deposited using trimethylaluminum and H2O vapor as
source gases. Then, a 4 nm-thick ZrO2 layer was prepared
using ZrCl4 and H2O vapor as source gases. Finally, a 0.5
nm-thick Al2O3 buffer layer was deposited on top of the
ZrO2 . The substrate temperature and the process pressure
were 300 °C and 1 Torr, respectively. After the deposition of
a 150 nm-thick layer of Pt, metal–oxide-semiconductor
͑MOS͒ devices with a gate area of 9ϫ10Ϫ6 cm2 were de-
fined by photolithography and etching. The electrical prop-
erties of the nanolaminate film were characterized by
capacitance–voltage (C–V), and current density–voltage
(J–V) measurements. For the thermal stability measure-
ments, samples were transferred to the UHV-medium-energy
ion scattering ͑MEIS͒ system with sample heating capabili-
ties.
MEIS analysis was accomplished with a 100 keV proton
beam in the double alignment in order to reduce contribu-
tions from the crystalline Si substrates, allowing the decon-
volution of spectra into contributions from the bottom
Al2O3 , the underlying SiO2 , and the interfacial Si signals.8
The incident ions were along ͓111͔ in the ͑011͒ plane and the
¯
scattered ions were along 001 with a scattering angle of
͓
͔
125°. The energy resolution of electrostatic energy analyzer
of MEIS is almost single atomic layer depth resolution in the
near surface.9 During MEIS experiments, the specimen was
moved and the Hϩ dose was maintained below
2
ϫ1015 cmϪ2 in order to prevent damage. In addition to
MEIS, other powerful analytical techniques, such as x-ray
photoelectron spectroscopy ͑XPS͒, high-resolution transmis-
sion electron microscopy ͑HRTEM͒, and electrical measure-
ments, were used in this study.
Figure 1 shows a proton backscattering energy spectrum
from the as-deposited and vacuum-annealed nanolaminate
film. Using a simulation program for scattering analysis, the
best-fit result for the compositional depth profile was ob-
tained as shown in the inset of Fig. 1.10,11 For the bottom
Al2O3 film deposited on the Si substrate, a 1.4Ϯ0.3 nm thick
interfacial silicate layer was found, which is composed of Zr,
Al, Si, and O. Based on the contrast difference in HRTEM, a
0.6 nm-thick interfacial layer was identified as being related
a͒
Electronic mail: hwanghs@kjist.ac.kr
0003-6951/2002/80(18)/3385/3/$19.00 3385 © 2002 American Institute of Physics
130.18.123.11 On: Sun, 21 Dec 2014 02:50:32
Chang, Hyo Sik
