Characteristics of ZrO2 films with Al and Pt gate electrodes

Article abstract of DOI:10.1149/1.1624843

We investigated interfacial stabilities of ZrO₂ films with Al and Pt electrodes formed by magnetron sputtering upon annealing and consequent changes of their metal-oxide-semiconductor capacitor characteristics. The as-deposited ZrO₂ films deposited using a sputtering power of 300 W were amorphous, while after annealing in N₂ at 600°C for 5 min the films became polycrystalline with a mixture of monoclinic and tetragonal phases. After the deposition of electrodes, we found that the amorphous interlayer which is presumed to be Al₂O₃ was formed at the ZrO₂/Al interface, while platinum (Pt) electrodes showed no interlayer at the interface with ZrO₂ films. The value of the capacitance equivalent thickness for the ZrO₂ film with the Al electrode was larger than that of the case with the Pt electrode by about 12 A, which is due to the presence of the additional Al₂O₃ interlayer at the Al/ZrO₂ interface. The capacitance-voltage measurement showed that the difference in flatband voltage (V_FB) between the ZrO₂ films and the two different electrodes is about 1.2 V, which is due to the work function difference between the two electrode materials.

Full text of DOI:10.1149/1.1624843

Journal of The Electrochemical Society, 150 ͑12͒ G849-G853 ͑2003͒
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0013-4651/2003/150͑12͒/G849/5/$7.00 © The Electrochemical Society, Inc.
Characteristics of ZrO2 Films with Al and Pt Gate Electrodes
Seok-Woo Nam,a Jung-Ho Yoo,a Suheun Nam,a Dae-Hong Ko,a,z
Cheol-Woong Yang,b and Ja-Hum Kuc
aDepartment of Ceramic Engineering, Yonsei University, Seodaemoon-ku, Seoul 120-749, Korea
bSchool of Metallurgical and Materials Engineering, Sungkyunkwan University, Jangan-gu,
Suwon 440-746, Korea
cSamsung Electronics Company, Limited, Semiconductor Research and Development Division,
Kyunggi-do, Korea
We investigated interfacial stabilities of ZrO2 films with Al and Pt electrodes formed by magnetron sputtering upon annealing and
consequent changes of their metal-oxide-semiconductor capacitor characteristics. The as-deposited ZrO2 films deposited using a
sputtering power of 300 W were amorphous, while after annealing in N2 at 600°C for 5 min the films became polycrystalline with
a mixture of monoclinic and tetragonal phases. After the deposition of electrodes, we found that the amorphous interlayer which
is presumed to be Al2O3 was formed at the ZrO2 /Al interface, while platinum ͑Pt͒ electrodes showed no interlayer at the interface
with ZrO2 films. The value of the capacitance equivalent thickness for the ZrO2 film with the Al electrode was larger than that of
the case with the Pt electrode by about 12 Å, which is due to the presence of the additional Al2O3 interlayer at the Al/ZrO2
interface. The capacitance-voltage measurement showed that the difference in flatband voltage (VFB) between the ZrO2 films and
the two different electrodes is about 1.2 V, which is due to the work function difference between the two electrode materials.
© 2003 The Electrochemical Society. ͓DOI: 10.1149/1.1624843͔ All rights reserved.
Manuscript submitted December 2, 2002; revised manuscript received June 13, 2003. Available electronically November 5, 2003.
Silicon dioxide has been used as the gate dielectric material in
metal oxide semiconductor field effect transistor ͑MOSFET͒ devices
for over 40 years. The conventional SiO2 dielectrics, however,
showed the issue of direct tunneling of carriers in the sub-2.0 nm
thickness regime, which results in significant leakage current flow-
ing through the SiO2 layer.1-3 Therefore, the fabrication of the sub-
0.1 ␮m MOSFET devices requires the development of new gate
dielectric materials with lower leakage current levels than SiO2 at
the same equivalent oxide thickness values. Alternative dielectric
rotation for the uniformity of deposited films. Postdeposition anneal-
ing ͑PDA͒ by furnace in a N2 ambient was performed at 600°C for
5 min. Postannealing by furnace in either an O2 or N2 ambients was
performed at 700-800°C for 10 min. Al and Pt were deposited as the
top electrode materials. The top electrodes of Al and Pt films were
prepared using the dc magnetron sputtering method from Al and Pt
metal targets with 99.9% purity in the Ar gas ambient. Sputtering
was performed at room temperature with 100 W power.
The thickness of the deposited films was confirmed by ellipsom-
etry or X-ray reflectometry. Microstructures of both the films and the
interfaces between the electrodes and the films were characterized
and compared by a cross-sectional transmission electron microscopy
͑TEM͒ and an X-ray photoelectron spectroscopy ͑XPS͒. For XPS
data acquisition, Al K␣ radiation ͑1486.6 eV͒ was used as photon
source and the analysis chamber was maintained at 5 ϫ 10Ϫ9 Torr.
All experimental scans were performed at a constant pass energy of
23.5 eV and a constant takeoff angle of 45°. In addition, the XPS
spectra were all obtained at room temperature before and after Ar
ion sputtering. For the Ar ion sputtering we used an ion energy of 3
keV and a beam current density of 1.5 ϫ 10Ϫ5 A/cm2.
materials, such as Si3N4 , Si3N4 /SiO2 stack,4-8 Y2O3 ,9 Al2O3 ,10
17,18
TiO2 ,11,12 Ta2O5 ,13-16 SrTiO3 , and (BaxSr1Ϫx)TiO3
have been
suggested as candidate materials. These materials, however, have
demonstrated issues of low dielectric constant values or poor ther-
mal stability with Si substrates.11-18 Recently, ZrO2 ,19-21 HfO2 ,22,23
and their silicates24,25 have been considered as promising alternative
gate dielectric materials due to their high dielectric constant values
and good thermal stabilities with Si substrates.26
Among the key issues related to the high-k gate stack such as
gate poly depletion effects or boron penetration,27 is interfacial in-
stability in direct contact with gate electrode materials as well as
with Si substrates. In particular, the interfacial reactions between
electrodes and high-k oxides affect the electrical properties of
complementary metal-oxide semiconductor ͑CMOS͒ devices such as
decrease of the effective capacitance values. Therefore, good ther-
mal stability with gate electrodes must be acquired in order to be
employed as gate dielectrics. In this paper we report on the stability
of sputtered ZrO2 films in contact with two different gate electrode
materials of Al and Pt, which are generally used for the evaluation
of metal-oxide semiconductor ͑MOS͒ capacitor structures.
Capacitance-voltage ͑C-V͒ characteristics of MOS structures
with ZrO2 films with Al or Pt electrodes were measured using an
HP4284A. The area of the MOS (Al/ZrO2 /Si or Pt/ZrO2 /Si) ca-
pacitors was about 1.1 ϫ 10Ϫ4 cm2. The capacitance equivalent
thickness ͑CET͒ was calculated from the accumulation capacitance
at 1 MHz, and quantum mechanical effect was not considered. The
gate bias sweeping was from ϩ2 to Ϫ4 V for the C-V measurement.
Results and Discussion
The thicknesses of the as-deposited and annealed ZrO2 films
were measured by high-resolution TEM ͑HRTEM͒. Figure 1 shows
the cross-sectional HRTEM micrographs of the ZrO2 films on Si
substrates after deposition and after annealing at 600°C for 5 min in
an N2 ambient. The physical thicknesses of the ZrO2 films were
measured to be about 9-10 nm for both films. The as-deposited film
is amorphous ͑see Fig. 1a͒ and changes to its crystalline phase after
annealing ͑see Fig. 1b͒. The annealed ZrO2 films show the crystal-
line lattice fringes having spacing of about 2.94 Å that corresponds
to the ͑101͒ plane of the tetragonal ZrO2 phase. In order to identify
the crystalline structures of annealed films, we examined the
samples by plan-view diffraction pattern analysis. The plan-view
diffraction pattern in Fig. 1c demonstrated that the film consists of
monoclinic and tetragonal ZrO2 phases. Notably, an interfacial
Experimental
p-Type silicon͑100͒ wafers with a resistivity of 8-15 ⍀ cm were
cleaned by the RCA cleaning method and then the native oxide was
removed with a 1% HF solution. The ZrO2 films were prepared via
the reactive dc magnetron sputtering method from a Zr metal target
with a 99.9% purity in the Ar ϩ O2 gas ambients. The sputtering
was performed at room temperature with a power of 300 W. The
system was initially pumped down to 2 ϫ 10Ϫ7 Torr and the pres-
sure during sputtering maintained at 6 ϫ 10Ϫ3 Torr with an O2 flow
rate of 5.4 sccm (Ar/O2 ϭ 5). The substrate underwent a 4 rpm
z E-mail: dhko@yonsei.ac.kr

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Journal of The Electrochemical Society, 150 ͑12͒ G849-G853 ͑2003͒
Figure 2. Al 2p signal in XPS spectra for ZrO2 film with Al top electrode.
layer was measured to be about 8 Å in the as-deposited sample and
it increased to about 15 Å after annealing. As reported in our previ-
ous publications, the growth of the oxide layer is due to the oxida-
tion of the substrate by the diffusion of oxidizing species from the
ambient through the ZrO2 film.28 Similar results were observed in
the CeO2 films29 and the Y2O3 films30 deposited on Si substrates.
Al electrodes and Pt electrodes were deposited on ZrO2 films by
dc magnetron sputtering. The microstructures of the interfaces be-
tween the electrodes and the oxide films were analyzed by XPS and
HRTEM. In order to investigate the interfacial reactions in the
Al/ZrO2 /Si and the Pt/ZrO2 /Si systems, the chemical binding states
at the Al/ZrO2 and Pt/ZrO2 interface were analyzed by XPS. For the
XPS analysis, 10 nm thick Al/ZrO2 /Si and 10 nm thick Pt/ZrO2 /Si
gate stacks were fabricated and annealed under the same annealing
conditions. Initially, 10 nm thick Al and Pt films were etched to
Al/ZrO2 and Pt/ZrO2 interface by Ar sputtering in an XPS chamber,
after which spectra for Al 2p, Pt 4f, Zr 3d, and O 1s were obtained
at each etching step from the Al/ZrO2 and Pt/ZrO2 interface to the
Si substrate. Figure 2 illustrates the changes of the Al 2p signal in
XPS spectra as the Al electrode was etched ͑level 1 meaning
Al/ZrO2 and level 6 meaning ZrO2). The peaks with binding energy
corresponding to photoelectrons in metallic Al ͑72.9 eV͒ as well as
with higher binding energies were observed at the Al/ZrO2 inter-
faces. Detailed XPS spectra at stage 4 are shown in Fig. 3a. The Al
2p binding energy for the metallic Al and the binding energy of 75.5
eV are clearly shown in the spectra, which demonstrates the pres-
Figure 1. Cross-sectional HRTEM micrographs of ͑a͒ as-deposited ZrO2
film and ͑b͒ postannealed ZrO2 film at 600°C in N2 for 5 min. ͑c͒ Plan-view
diffraction pattern image of postannealed ZrO2 film at 600°C in N2 for 5
min.
amorphous layer is clearly observed between the ZrO2 films and the
Si substrates in the HRTEM image. The thickness of the interface

Journal of The Electrochemical Society, 150 ͑12͒ G849-G853 ͑2003͒
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Figure 3. XPS spectra of ͑a͒ Al 2p and ͑b͒ Zr 3d for ZrO2 film with Al top
electrode at level 4 of the electrode etching process.
Figure 4. ͑a͒ Low-magnification and ͑b͒ high-resolution cross-sectional
TEM micrographs of ZrO2 film with Al top electrode.
ence of metallic Al and Al2O3 phases.31 At the same etching stage,
the binding energy of Zr 3d5/2 shows three different peak positions
͑see Fig. 3b͒ of 178.9, 181.0, and 183.0 eV, indicating the binding
energy of photoelectron characteristics of metallic Zr and ZrO2Ϫx
phases at the Al/ZrO2 interfaces.31
͑level 1 meaning Pt/ZrO2 and level 6 meaning ZrO2). In the spectra
only peaks with binding energy corresponding to photoelectrons in
metallic Pt 4f are observed in all stages of etching. Figure 6a shows
the XPS spectra at stage 4, which demonstrated the presence of the
Pt 4f7/2 peak at 71.2 eV. In the same stage, the binding energy of Zr
3d5/2 is observed only at 183.0 eV, corresponding to that of the
stoichiometric ZrO2 ͑see Fig. 6b͒. Such data demonstrate that there
is no interaction between the Pt electrode and the ZrO2 films. Figure
7 shows the cross-sectional TEM micrograph of ZrO2 film with Pt
top electrode in which no interfacial layer is observed between the
Pt electrode and ZrO2 film. Because the free energies for formation
of platinum oxides show positive values from thermodynamic
consideration,32 it is more likely that no interfacial layer is formed at
the Pt/ZrO2 interface. In addition, Pt has commonly been used as an
electrode because it allows the deposition of dielectric films at high
temperature in an oxidizing atmosphere without causing any oxida-
tion of the electrode.
Because the chemical reaction on the interface between Al and
ZrO2
4Al ϩ 3ZrO2 → 2Al2O3 ϩ 3Zr
has the negative free energy (⌬Gr ϭ Ϫ45.37 kJ/mol at 298 K͒,32 Al
reduces the top layer of ZrO2 film into either metallic zirconium or
oxygen-deficient zirconia (ZrO2Ϫx) during the deposition of Al.
Consequently, the interlayer of the Al-O compound which is pre-
sumed to be Al2O3 is formed at the Al/ZrO2 interface. The forma-
tion of this interlayer is clearly displayed in the low-magnification
TEM micrograph of ZrO2 film with an Al electrode ͑Fig. 4a͒. The
HRTEM image in Fig. 4b clearly shows the presence of a 20 Å thick
amorphous interlayer, which is presumed to be an Al2O3 layer.
In contrast to the Al/ZrO2 system, the structure with the Pt elec-
trode demonstrated different results. Figure 5 illustrates the evalua-
tion of the Pt 4f signal in XPS spectra as the electrode was etched
Figure 8 shows the high-frequency C-V characteristics of the
ZrO2 films after annealing in a tube furnace at 600°C for 5 min in an
N2 gas ambient. Al and Pt are deposited as electrodes after annealing

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Journal of The Electrochemical Society, 150 ͑12͒ G849-G853 ͑2003͒
Figure 5. Pt 4f signal in XPS spectra for ZrO2 film with Pt top electrode.
of ZrO2 films, and no additional postannealing was performed after
the deposition of the electrodes. Although the TEM image indicates
the physical thickness of the ZrO2 film at 9-10 nm in both cases, the
measured accumulation capacitance values strongly depend on top
electrode materials. CET values extracted from an accumulation ca-
pacitance without considering the quantum mechanical effect for the
annealed ZrO2 films with Pt electrode are measured to be about 34.8
Å, which is smaller than 47.2 Å for the one with an Al electrode.
This is explained by the presence of the additional amorphous inter-
facial layer at the Al/ZrO2 interface. The dielectric constant of the
interfacial layer ki is defined as
Figure 6. XPS spectra of ͑a͒ Pt 4f and ͑b͒ Zr 3d for ZrO2 film with Pt top
electrode at level 4 of the electrode etching process.
Conclusion
We investigated the interfacial stabilities of ZrO2 films with Al
and Pt electrodes formed by a magnetron sputtering process upon
annealing and consequent changes of their MOS capacitor charac-
teristics. The polycrystalline with both monoclinic and tetragonal
ZrO2 phases was observed by HR-TEM images and plan-view dif-
fraction after postannealing at 600°C. In the case of the ZrO2 film
with an Al electrode, we observed the formation of the interfacial
Al2O3 layer through XPS and HRTEM analyses. In contrast, the
structure with Pt electrode displayed no interfacial layer. The CET
value for the ZrO2 films with Pt electrode was calculated to 34.8 Å,
while the value for the films with Al electrode was 47.2 Å. This
difference is related to the presence of the interfacial layer in the
k ϭ k T /T ͒
͑
s
i
i
eq
where Teq is the electrical equivalent thickness for the insulating
layer, ks is the dielectric constant of SiO2 , and Ti is the physical
thickness of interfacial amorphous layer. Because the difference in
CET values between the two cases is about 12.4 Å and Ti is 20 Å,
the calculated ks value is approximately 6.3, which is close to the
dielectric constant value of the interfacial Al2O3 layer. In addition,
the difference of flatband voltage between films having two kinds of
electrodes was observed in Fig. 8. The difference is about 1.2 V,
resulting from the work function difference between the two elec-
trodes of Al ͑4.08 eV͒ and Pt ͑5.32 eV͒.

Journal of The Electrochemical Society, 150 ͑12͒ G849-G853 ͑2003͒
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Figure 8. The high-frequency C-V characteristics of Al/ZrO2 /p-Si(100)
and Pt/ZrO2 /p-Si(100) diodes.
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Figure 7. ͑a͒ Low-magnification and ͑b͒ high-resolution cross-sectional
TEM micrographs of ZrO2 film with Pt top electrode.
Al/ZrO2 structures. The difference of the flatband voltage (VFB
)
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between Pt/ZrO2 /Si and Al/ZrO2 /Si was about 1.2 V because of
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Acknowledgment
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This work was partially supported by a scholarship grant from
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Yonsei University assisted in meeting the publication costs of this article.
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