143_1.pdf

Preparation and Characterizations of High-/r Gate Dielectric
CaZrO3 Thin Films by Sol-gel Technology
Ting Yu*, Weiguang Zhu*, Changhong Chen*, Xiaofeng Chen*,
R. Gopal Krishnan*
^Microelectronics Center, School of Electrical and Electronic Engineering, Nanyang Technological University,
Singapore 639798
'Singapore Institute of Microelectronics, 11 Science Park Road, Singapore 117685
Abstract. Perovskite CaZrO3 gate dielectric thin films were prepared by the sol-gel wet chemical technology, followed by
post annealing in O2 ambient at different temperatures from 550 to 700 °C. Based on our best knowledge, it is the first time
in the literature to successfully prepare the CaZrO3 thin films using wet chemical deposition methods, including sol-gel and
metallo-organic decomposition (MOD) technique. These thin films were systematically characterized using differential
thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM),
Auger spectra (AES) and electrical and dielectric measurements. Using these techniques, the different reactions in various
processing steps have been clarified. The dielectric constant of crystalline CaZrO3 films is about 20 determined from the C-f
measurement and the thickness. The sol-gel derived CaZrO3 films exhibit stable dielectric properties nearly independent on
the applied electrical field and frequency at room temperature. The leakage current density of the CaZrO3 thin film annealed
at 650 °C for 1 hour is approximately 9.5x10'8 A/cm2 at high applied electrical field 2.6 MV/cm. The high dielectric
constant, low leakage current density and high breakdown strength suggest that the CaZrO3 thin film is a promising
candidate for gate applications.
belong to the class of high-temperature perovskitetype proton conductors. Their appreciable protonic
conductivity makes them promising candidates to
serve as a solid electrolyte in some novel
electrochemical devices, such as solid oxide fuel cells
and gas sensors [6]. But little work has been done on
CaZrO3 perovskite oxides for its possible application
as a gate material. The aim of the present study is to
prepare and characterize the properties of CaZrO3
thin films using the sol-gel method.
1. INTRODUCTION
As the dimensions of the microelectronic devices
are scaled down, the tunneling current of SiO2 gate
dielectric increases exponentially with decreasing the
film thickness. For films below 20A, the leakage
current rises to 1-10 A cm"2 [1], which is becoming
an increasingly critical problem. The material
properties of alternative higher-^ dielectrics have
therefore attracted increasing attention. Metal-oxides
with high dielectric constants have the potential to
extend scaling of transistor gate capacitance beyond
that of SiO2. Many materials have been investigated
to replace silicon dioxide as possible gate dielectrics,
such as Ta2O5 [2], A12O3 [3], ZrO2 [4] and SrTiO3 [5],
etc.
Materials with the perovskite phase are of
fundamental significance for their electrical
properties including ferroelectricity, piezoelectricity
and superconductivity and they also offer interesting
high temperature properties. CaZrO3-based oxides
2. EXPERIMENTAL
Zirconium butoxide solution and calcium acetate
were used to synthesis the precursor. Zirconium
butoxide was dissolved with glacial acetic acid by
stirring and heating at 60 °C. Calcium acetate was
mixed with glacial acetic acid diluted with water.
Both solutions were mixed and stirred to form 0.2 M
CaZrO3 precursor solution. The precursor solution
was first spin-coated onto Pt/Ti/SiO2/Si substrates at
CP683, Characterization and Metrology for VLSI Technology: 2003 International Conference,
edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula
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143
ascribed to the decomposition of the acetate
complexes to carbonates. This decomposition causes
a continuous decrease in sample weight in this
temperature region. At 675 °C, an endothermic peak
is found and attributed to the decomposition of the
carbonates to oxides, accompanied by the loss of CO2.
It can be seen that the crystallization temperature for
the CaZrO3 perovskite phase is relatively high.
The XRD spectra taken after calcining the
precursor at 150 (Ih) (drying), 450 (3h), 800 (3h),
1000 (5h), 1000 °C (lOh) are presented in Fig. 2. All
peaks with # mark observed for the dried precursor
can be attributed to calcium acetate (C4H6CaO4). If
the precursor is calcined at 450 °C, the calcium
acetate complex decomposes to CaCO3, confirming
the experimental results from the TG-DTA
measurements. In the precursor calcined at 800 °C,
two different phases are identified, a CaZrO3
perovskite phase and a CaZr4O9 cubic phase. If the
precipitated precursor is calcined at 1000 °C, then the
solid-state reactions will progress even further
towards formation of the desired CaZrO3 phase, as
shown in Fig. 2 of the XRD pattern obtained after
calcining the precursor for 5h at 1000 °C. It can be
easily seen that the XRD peaks of the CaZrO3
perovskite phase have increased in intensity. The
intensity of the CaZr4O9 phase has decreased
dramatically, indicating a substantial decrease in the
amount of this phase. If the precursor is calcined at
1000 °C for lOh, the conversion to CaZrO3 phase
progresses further and the CaZr4O9 phase nearly can
not be detected.
4000 rpm and then baked at 250 and 450 °C for 5 min
respectively in air to remove most organics in the
film. This process was repeated several times to
obtain the desired film thickness. The dried films
were post-annealed in the furnace at the temperature
ranging from 550 to 700 °C for Ih in O2 atmosphere.
Top electrodes of Au with a diameter of 0.3 mm were
evaporated for electrical measurement.
The decomposition behavior of the dried
precursor was studied using program controlled
thermogravimetry ( TGA ) and differential thermal
analysis ( DTA ) up to a temperature of 1000 °C with
a heating rate of 2 °C/min. For the DTA analysis,
A12O3 powder was used as a reference. XRD analysis
was used to study the different phases in the calcined
powder at temperatures between 150 and 1000 °C.
The depth profile of the film was investigated by
Auger Spectra (AES). Microstructure and
morphology of the films were investigated by X-ray
diffraction and scanning electron microscopy (SEM),
respectively. Capacitance (C) and leakage current
density (J) were measured by a HP 4284A LCR
meter and a HP 4155B semiconductor parameter
analyzer, respectively.
3. RESULTS AND DISCUSSION
The calcination behavior was studied at
temperatures up to 1000 °C using DTA and TGA
technique at a heating rate of 2 °C/min to distinguish
the low-temperature reaction steps. Fig. 1 shows the
typical DTA and TGA curves for the dried 0.2M
CaZrO3 precursor.
= CaCCX
-2.8
1000C/10h
200
400
600
800
1000
Temperature ( C )
FIGURE 2. XRD patterns of the precursor calcined at
different temperatures between 150 and 1000 °C. The
labeled peaks are CaZrO3.
FIGURE 1. DTA and TGA curves of CaZrO3 precursor at
a heating rate of 2 °C/min in air.
Fig. 3 shows the XRD patterns of annealed
CaZrO3 thin films prepared on Pt/Ti/SiO2/Si
It is seen from this figure that with the
endothermic peaks at 56 °C and 180 °C in the DTA
curve are due to the evaporation of adsorbed water
and solvents. The exothermic peak at 378 °C is
substrates and annealed at a series of temperature,
550, 600, 650 and 700 °C for Ih in pure oxygen
atmosphere. The XRD results of the film annealed at
144
The microstructure such as grain size is an
important parameter determining the dielectric and
electrical properties of thin films. Fig. 5 shows the
SEM micrographs of the CaZrO3 films annealed at
different temperatures (550, 600, 650 and 700 °C) for
1 hour in O2 atmosphere. From Fig. 5, it is seen that
the grain size is increased with increasing the
annealing temperature. Films annealed at 650 and
700 °C average consist of tiny crystal grains and they
are dense and smooth, in consistent with the XRD
results. Furthermore, the morphology is changed little
with the increase of temperatures.
550 °C, which is much lower than the crystallization
temperature, indicates that the film has its amorphous
structure. It seems that the CaZrO3 film begins to
crystallize at 600°C. With further increasing the
annealing temperatures, the intensity of perovskite
CaZrO3 phase is increased, indicating that the
crystallization progresses in CaZrO3 thin films further.
There is no tendency of preferential orientation in the
sol-gel-derived CaZrOa films fabricated on
Pt/Ti/SiO2/Si substrates.
20
30
40
50
60
70
2e/degrees
FIGURE 3. XRD patterns of thin films prepared on
Pt/Ti/SiO2/Si substrates annealed at different temperatures
in O2.
The AES depth profile of a flat thin film annealed
at 550 °C is shown in Fig. 4. The buffer layer Ti may
be assumed to infiltrate into the bottom electrode. It
may have an effect on the leakage current.
FIGURE 5. SEM micrographs of thin films post-annealed
at different temperatures in O2.
FIGURE 6. The cross-section image of the thin film
deposited on Pt/Ti/SiO2/Si substrate annealed at 600 °C for
lhinO2.
FIGURE 4. The AES depth file of the thin film postannealed at 550 °C for 1 h in O?.
145
Fig. 6 shows a typical cross-section image of thin
film annealed at 600 °C. The thickness of the film is
about 140nm.
The dielectric characteristics of CaZrOs thin films
annealed at different temperatures of 550, 600, 650
and 700 °C in O2 atmosphere as a function of
frequency measured at room temperature are shown
in Fig. 8. During the measurement the applied
voltage was zero and the oscillation level was 50 mV.
It can be easily seen from the Fig. 8 that the dielectric
constant of these films is nearly independent of
frequency. The dielectric loss is very low for the
samples annealed in Oi-
1E-6
1E-7
1E-8
24
1E-9
T 700°C
• 650°C
• 600°C
1E-10
-
3
-
2
-
1
0
1
.
2
I
3
Applied field (MV/cm)
• 550°C
18
TAV/7II11 IlllV
o
.2
12
FIGURE 7. Leakage current density (J-E) of CaZrO3 thin
films post-annealed at different temperatures for 1 h in O 2 .
0
b
The characteristics of leakage current density
versus
applied
field
for
capacitors
of
Au/Ti/CaZrOa/Pt are shown in Fig. 7. The leakage
current density at room temperature fits a linear
dependence of log J with E1/2. It indicates that the
Schottky emission is the dominant conduction
process in the sol-gel derived CaZrO3 thin films. The
leakage current density of the CaZrO3 thin film
annealed at 650 °C for 1 hour is approximately
9.5x10'8 A/cm2 at high applied electrical field 2.6
MV/cm. For the film post-annealed at 700 °C, the
poor leakage property is attributed to some defects
resulted from the polycrystalline grain growth and
thus open grain boundaries in thin films which are
annealed at relatively high temperature [7].
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Applied field (MV/cm)
FIGURE 9. Dielectric constant dependence on applied
field of the thin films annealed at different temperatures in
02.
The dielectric constant dependence on the
applied field is shown in Fig. 9. During the
measurement the oscillation level was 50 mV and the
frequency was 100 kHz. The flat curves with no any
significant change in dielectric constant indicate that
the dielectric constant of these samples does not
depend on the applied field. Typically at a frequency
of 100 kHz, the dielectric constants of those films
annealed at 550, 600, 650 and 700 °C are 12.8, 14.4,
15.7 and 16.0, and the corresponding values of
dielectric loss are 0.0018, 0.0027, 0.0026 and 0.0027,
respectively. The dielectric constant of film annealed
at 650 °C is nearly same as that of post-annealed at
700°C. It is agreed with the XRD results that the
films are crystallized more completely when the postannealing temperature increased to 650 °C and higher.
700°C
650°C
600°C
550°C
12-0.01
4. CONCLUSION
0.00
1k
10k
100k
Perovskite CaZrO3 dielectric thin films have
prepared by the sol-gel wet chemical technology,
followed by post-annealing in O2 at different
temperatures from 550 to 700 °C. It is found that
CaZrO3 film is in amorphous phase when annealed at
temperature of 550 °C and crystallized into
perovskite structure when annealed at temperature
Frequency (Hz)
FIGURE 8. Dielectric constant and dielectric loss
dependence on frequency of the CaZrO3 films annealed at
different temperatures in O2.
146
above 600 °C on Pt/Ti/SiO2/Si substrate, and the
crystallinity is also supported by SEM surface
morphology observation and dielectric measurements.
The sol-gel derived CaZrO3 films exhibit stable
dielectric properties nearly independent from the
applied electrical field and frequency at room
temperature. The leakage current density of the
CaZrO3 thin film annealed at 650 °C for 1 hour is
approximately 9.5x10"8 A/cm2 at high applied
electrical field 2.6 MV/cm. The high dielectric
constant, low leakage current density and high
breakdown strength suggest that the CaZrO3 thin film
is a promising candidate for gate applications.
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