Tuning the electrical properties of zirconium oxide thin films

Article abstract of DOI:10.1063/1.1448667

ZrO₂ films obtained using plasma-enhanced chemical-vapor deposition exhibit various electrical properties tuned by the flow rate ratio of O₂ to precursor-carrying Ar (O₂/Ar), which controls the hydrocarbon incorporation in the films and the interfacial layer formation. As-deposited ZrO₂ films obtained in oxygen-rich plasmas (O ₂/Ar≥1) showed good electrical properties, including an overall dielectric constant of 16, a flatband voltage shift of -24mV, an interfacial trap density of ～10¹¹cm^-2eV^-1, and a leakage current density of 3.3×10^-6A/cm² at an equivalent oxide thickness of 25 A, suitable for metal-oxide-semiconductor device applications.

Full text of DOI:10.1063/1.1448667

Tuning the electrical properties of zirconium oxide thin films
Byeong-Ok Cho, Jianjun Wang, Lin Sha, and Jane P. Chang
Citation: Applied Physics Letters 80, 1052 (2002); doi: 10.1063/1.1448667
View online: http://dx.doi.org/10.1063/1.1448667
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APPLIED PHYSICS LETTERS
VOLUME 80, NUMBER 6
11 FEBRUARY 2002
Tuning the electrical properties of zirconium oxide thin films
Byeong-Ok Cho, Jianjun Wang, Lin Sha, and Jane P. Changa)
Department of Chemical Engineering, University of California, Los Angeles, California 90095
͑Received 11 September 2001; accepted for publication 5 December 2001͒
ZrO2 films obtained using plasma-enhanced chemical-vapor deposition exhibit various electrical
properties tuned by the flow rate ratio of O2 to precursor-carrying Ar (O2 /Ar), which controls the
hydrocarbon incorporation in the films and the interfacial layer formation. As-deposited ZrO2 films
obtained in oxygen-rich plasmas (O2 /Arу1) showed good electrical properties, including an
overall dielectric constant of 16, a flatband voltage shift of Ϫ24 mV, an interfacial trap density of
ϳ1011 cmϪ2 eVϪ1, and a leakage current density of 3.3ϫ10Ϫ6 A/cm2 at an equivalent oxide
thickness of 25 Å, suitable for metal–oxide–semiconductor device applications. © 2002 American
Institute of Physics. ͓DOI: 10.1063/1.1448667͔
High-dielectric-constant materials are being actively re-
searched to enable the continuous downscaling of ultra-
large-scale integrated circuits, including their applications as
a gate dielectric material in metal–oxide–semiconductor
͑MOS͒ transistors and as a storage capacitor in dynamic ran-
dom access memory ͑DRAM͒ devices.1 ZrO2 has attracted
much attention as one of the most promising candidates due
to its high-dielectric constant of ϳ25, wide-band gap of 4.6–
7.8 eV, low-leakage-current level, and superior thermal
stability.2–6 In DRAM applications, the dielectric films
should be deposited over high-aspect-ratio capacitor struc-
tures, and chemical-vapor-deposition ͑CVD͒ processes with
superior step coverage are preferred to physical-vapor-
deposition processes. The plasma-enhanced CVD ͑PECVD͒
process has a distinct advantage over other thermal CVD
methods in that higher deposition rates can be achieved at
much lower substrate temperatures.7 This low-temperature
process can circumvent the diffusion of shallow junctions
and reduce thermal strains leading to the defect generation.8
In this work, we deposited ZrO2 films using zirconium
tetra-tert-butoxide ͓Zr(OC4H9)4, ͑ZTB͔͒ as a metal–organic
precursor, Ar as a carrier of the ZTB vapor, and O2 as an
oxidant in an electron-cyclotron-resonance ͑ECR͒ high-
density-plasma reactor. The temperature of a bubbler con-
taining liquid ZTB was controlled at 50 °C to provide a con-
stant vapor pressure. The 2.45 GHz microwave power, the
currents through upper and lower electromagnets, and the
chamber pressure were set constant at 300 W, 120 A, 150 A,
and 20 mTorr, respectively. We used 4 in. p-type Si͑100͒
wafers with a resistivity of 1–2 ⍀ cm as the substrates,
which were cleaned in a buffered HF solution for 30 s, rinsed
in deionized water for 60 s, and dried in N2 to yield a
hydrogen-terminated surface. Neither heating nor biasing
was applied to substrates during the PECVD experiments.
The substrate temperature is expected to be slightly higher
than room temperature, as Devine and Vallier9 reported that
the temperature of an unbiased and nonheated Si wafer rose
to ϳ60 °C during depositing Ta2O5 using a PECVD process.
No postdeposition annealing was performed in this work.
The key parameter varied in this work was the O2-to-Ar flow
rate ratio (O2 /Ar), as it was determined to dominate the
plasma gas composition and deposited film properties in our
previous work.10 A low O2 /Ar ratio resulted in high hydro-
carbon content in the gas phase via decomposition of the
bulky butoxyl ligands, while a high O2 /Ar ratio yielded an
oxygen-rich plasma.
We measured the film thickness using a Gaertner L116B
ellipsometer with a 632 nm He–Ne laser. All films were
confirmed to be amorphous by the x-ray diffraction measure-
ment. Capacitors were formed by sputter deposition of 300
nm Al on ZrO2 followed by photolithographic patterning to
form the top electrode with area sizes of 2ϫ10Ϫ5 –8
ϫ10Ϫ5 cm2. An Al contact was also made to the backside
surface of the wafers. Capacitance–voltage (C–V) and
current–voltage (I–V) characteristics were obtained using
an HP 4284B LCR meter and an HP 4145B analyzer, respec-
tively.
Figure 1 shows the C–V curves normalized to the accu-
mulation capacitance (CAcc) at the electrode voltage of
Ϫ4 V for capacitors built with thin ZrO2 films deposited at
different O2 /Ar ratios. The film thicknesses were 133 Å at
FIG. 1. Normalized C–V curves of thin ZrO2 films obtained at different
O2 /Ar flow rate ratios. The solid line indicates the calculated curve accord-
ing to the exact charge theory and a frequency of 1 MHz was used for the
measurement.
a͒
Electronic mail: jpchang@ucla.edu
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Appl. Phys. Lett., Vol. 80, No. 6, 11 February 2002
Cho et al.
1053
FIG. 2. Dependence of the dielectric constant and flatband voltage shift as a
function of O2 /Ar flow rate ratios.
FIG. 3. Leakage current density as a function of equivalent oxide thickness.
As-deposited ZrO2 films at (O2 /Ar)у1 showed lower leakage levels than
SiO2 films of equivalent oxide thickness. Inset shows the leakage current
density as a function of applied voltage.
(O2 /Ar)ϭ0.5 and 91 Å at (O2 /Ar)ϭ1, 2, and 4. The theo-
retical C–V curve shown for comparison with the experi-
mental curve at (O2 /Ar)ϭ1 was calculated from the exact
charge theory11 using the known oxide thickness of 91 Å, a
dielectric constant of 16, and a dopant concentration of 8
ϫ1015 cmϪ3 in Si. The work function difference of
Ϫ0.94 eV between the Al electrode and the p-type Si was
also taken into account. Theoretical curves at other O2 /Ar
ratios were almost identical to that at (O2 /Ar)ϭ1 except for
the different levels of C/CAcc in the inversion regime
(CInv /CAcc). At (O2 /Ar)ϭ0.5, where the plasma is hydro-
carbon rich, the experimental and theoretical CInv /CAcc val-
ues differed only 5%. However, at (O2 /Ar)у1, where the
plasma is oxygen rich, the experimental values of CInv /CAcc
were always higher than the theoretical calculations by
ϳ70%. This indicates the presence of a lower-dielectric-
constant interfacial layer between ZrO2 and Si, decreasing
the overall dielectric constant.11,12
To identify this interfacial layer, we analyzed the as-
deposited films by high-resolution transmission electron mi-
croscopy ͑HRTEM͒. We confirmed the presence of an amor-
phous interfacial layer of ϳ2 nm between ZrO2 and Si. This
layer could be either silicon oxide (SiOx) ͑Refs. 13 and 14͒
or zirconium silicate (ZrSixOy) ͑Ref. 4͒ according to ZrO2
deposition studies using similar process conditions in the lit-
erature. In either case, the diffusion of excess oxygen
through ZrO2 is necessary for interfacial layer
formation.4,13,14 We reported in our previous study10 that
oxygen became excessive at O2 /Arу1 and its concentration
increased with increasing O2 /Ar, which could lead to the
formation of the observed interfacial layer. Moreover, taking
into account the physical thicknesses of the interfacial layer
and the ZrO2 layer measured by HRTEM, and ZrO2 dielec-
tric constant ranging from 20 to 25,3,15 we calculated a cor-
responding dielectric constant of the interfacial layer to be
5–7, which would explain the higher CInv /CAcc measure-
ments discussed above.
method.16 The decrease in Dit with increasing (O2 /Ar) is
also likely resulting from the formation of a superior inter-
face of either SiOx or ZrSixOy .3
Figure 2 shows that the dielectric constant of the depos-
ited ZrO2 increases from 13 at (O2 /Ar)ϭ0.5 to 16 at
(O2 /Ar)ϭ1, then decreases gradually to 14 at (O2 /Ar)ϭ4.
The lowest dielectric constant at (O2 /Ar)ϭ0.5 is related to
the highest hydrocarbon content in the film.17 We reported
earlier10 that a significant amount of hydrocarbon fragments
from ZTB ligands can be incorporated into the ZrO2 films
under oxygen-deficient conditions, and a linear correlation
was found between the x-ray photoelectron spectroscopy-
determined carbon composition in ZrO2 and the optical
emission spectroscopy ͑OES͒ intensity ratio of C2 ͑516.52
nm͒ to O ͑777.42 nm͒, which increases with decreasing
(O2 /Ar). The compositional depth profile by Auger electron
spectroscopy ͑AES͒ also showed the highest C/O atomic ra-
tio in the film at (O2 /Ar)ϭ0.5.
Figure 2 also shows the flatband ͑FB͒ voltage shifts,
⌬VFB (ϭVFBϪ␾MS), where VFB is the flatband voltage and
␾
is the Al–Si work-function difference. It is noteworthy
MS
that ⌬VFB scales linearly with increasing (O2 /Ar) at
O2 /Arу1. However, the thicker film deposited at (O2 /Ar)
ϭ0.5 shows a significant deviation (Ϫ520 mV). This indi-
cates that the film deposited at (O2 /Ar)ϭ0.5 is abundant in
positive oxide-trapped charge (Qox) and fixed charge (Qf),
but the net charge decreases and eventually becomes nega-
tive as the (O2 /Ar) ratios increase. The density of fixed
charges, Qf /q, at (O2 /Ar)ϭ2 is estimated to be at most
2.1ϫ1011 cmϪ2, assuming the ⌬VFB contributed solely by
Qf .
The inset in Fig. 3 shows the leakage current density (J)
versus bias voltage (Vb) characteristics from the PECVD
films at different (O2 /Ar) ratios. The leakage current density
at positive gate bias distinctly decreases with the increasing
(O2 /Ar) ratio. The log(J) showed a linear dependence on
V1b/2 at all O2 /Ar ratios, consistent with the Schottky emis-
sion mechanism,18 where the charge conduction is dominated
The slopes of the C–V curves gradually increase with
the increasing (O2 /Ar) ratio, indicating the decrease in the
interfacial trap density, Dit . We estimated the Dit to be
ϳ1012 cmϪ2 eVϪ1 for films deposited at (O2 /Ar)ϭ0.5 and
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ϳ1011 cmϪ2 eVϪ1 those at (O2 /Ar)у1 by Terman’s
by the interface between ZrO2 and Si substrate. As indicated
128.248.155.225 On: Sat, 22 Nov 2014 17:36:04

1054
Appl. Phys. Lett., Vol. 80, No. 6, 11 February 2002
Cho et al.
by the C–V and HRTEM analyses, the interfacial layer for-
mation with increasing (O2 /Ar) ratios could dominate the
decrease in J. The leakage current level under negative bias
was much higher and invariant as (O2 /Ar) varied, indicating
that the Al–ZrO2 interface had a high level of defect density
due to the sputter-deposited Al. Leakage current densities of
the as-deposited PECVD ZrO2 films measured at Vbϭ1 V
are compared with those of SiO2 in Fig. 3.19 Although the
film deposited at (O2 /Ar)ϭ0.5 shows a much higher leak-
age level than SiO2 at the same equivalent oxide thickness
͑EOT͒, the as-deposited films obtained at (O2 /Ar)у1
yielded one to two orders of magnitude lower leakage cur-
rent density than SiO2 . Especially the film at (O2 /Ar)ϭ4
has a low-leakage-current density of Jϭ3.3ϫ10Ϫ6 A/cm2 at
an EOT of 25 Å.
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In conclusion, the electrical properties of a PECVD
ZrO2 film such as its dielectric constant, flatband voltage
shifts, interfacial trap density, and leakage current density
could be tuned by the O2-to-Ar flow rate ratio, which con-
trols the carbon content in the film and the formation of the
interfacial layer.
The authors are grateful to Jae-Ho Min and Dr. Sang H.
Moon at Seoul National University for the AES measure-
ments and to Dr. Igor Levin at NIST for the HRTEM analy-
sis. The authors acknowledge financial support from Na-
tional Science Foundation Career Award No. CTS-9985511,
a UCLA Chancellor’s Faculty Career Development Award,
and the donation of the ECR reactor from Bell Labs, Lucent
Technologies.
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