Direct liquid injection metallorganic chemical vapor deposition of ZrO2 thin films using Zr(dmae)4 as a novel precursor

Article abstract of DOI:10.1149/1.1421605

A novel liquid Zr precursor with a donor-functionalized alkoxy ligand [Zr(OCH₂CH₂NMe₂)₄, Zr(dmae)₄] (dmae = dimethylaminoethoxide) has been characterized by thermogravimetry (TG) and differential scanning calorimetry (DSC) analysis, nuclear magnetic resonance, and mass spectrometry. Zr(dmae)₄ vaporizes near 320°C and reacts with oxygen at around 310°C under our TG/DSC measurement conditions. The bond strength of Zr-dmae was found to be similar to that of Zr-OⁱPr. The ZrO₂ thin films deposited at 300-480°C by a direct liquid injection metallorganic chemical vapor deposition process had a dense and smooth morphology. The film had a weak monoclinic phase in an amorphous background without any other metastable phase such as tetragonal or cubic. The high-frequency (1 MHz) capacitance-voltage curves showed that the flatband voltage (V_FB) of the ZrO₂ thin films deposited at 400°C was close to the theoretical value of -0.9 V. The interface trap density near the midgap was found to be less than 1 × 10¹¹ cm^-2 eV^-1, which was calculated by the Terman method.

Full text of DOI:10.1149/1.1421605

Journal of The Electrochemical Society, 149 ͑1͒ C23-C27 ͑2002͒
C23
0013-4651/2001/149͑1͒/C23/5/$7.00 © The Electrochemical Society, Inc.
Direct Liquid Injection Metallorganic Chemical Vapor
Deposition of ZrO2 Thin Films Using Zr„dmae…4
as a Novel Precursor
,z
*
Jeong Seok Na, Dae-Hwan Kim, Kijung Yong, and Shi-Woo Rhee
Laboratory for Advanced Materials Processing, Department of Chemical Engineering, Pohang University
of Science and Technology, Pohang 790-784, Korea
A
novel liquid Zr precursor with a donor-functionalized alkoxy ligand Zr͑OCH CH NMe ͒ , Zr͑dmae͒4] (dmae
͓
2 2 2 4
ϭ dimethylaminoethoxide) has been characterized by thermogravimetry ͑TG͒ and differential scanning calorimetry ͑DSC͒ analy-
sis, nuclear magnetic resonance, and mass spectrometry. Zr͑dmae͒4 vaporizes near 320°C and reacts with oxygen at around 310°C
under our TG/DSC measurement conditions. The bond strength of Zr-dmae was found to be similar to that of Zr-OiPr. The ZrO2
thin films deposited at 300-480°C by a direct liquid injection metallorganic chemical vapor deposition process had a dense and
smooth morphology. The film had a weak monoclinic phase in an amorphous background without any other metastable phase such
as tetragonal or cubic. The high-frequency ͑1 MHz͒ capacitance-voltage curves showed that the flatband voltage (VFB) of the ZrO2
thin films deposited at 400°C was close to the theoretical value of Ϫ0.9 V. The interface trap density near the midgap was found
to be less than 1 ϫ 1011 cmϪ2 eVϪ1, which was calculated by the Terman method.
© 2001 The Electrochemical Society. ͓DOI: 10.1149/1.1421605͔ All rights reserved.
Manuscript submitted February 28, 2001; revised manuscript received August 17, 2001. Available electronically November 27,
2001.
Zirconia, ZrO2, has attracted much interest because of its high
dielectric constant ͑ϳ25͒, wide energy bandgap ͑ϳ5 eV͒, high index
of refraction ͑Ͼ2͒, and good mechanical and chemical stability.1
The ZrO2 film can be potentially used as an alternative dielectric
material for the storage capacitor in the dynamic random access
memory ͑DRAM͒.2 In addition, the related ferroelectric oxide
Pb͑Zr, Ti͒O3 has a potential application in nonvolatile memories.
Recently, ZrO2 is also investigated as a possible replacement for
SiO2 gate dielectrics. More importantly, it has been reported that
ZrO2 is stable on the Si surfaces.3,4 The ZrO2 thin films have been
deposited by high-temperature chloride chemical vapor deposition
͑CVD͒,5 low-temperature metallorganic chemical vapor deposition
͑MOCVD͒,6-10 reactive dc magnetron sputtering,11 and electron-
beam evaporation.12 In particular, MOCVD is a suitable process for
the preparation of good quality ZrO2 thin films by the thermal de-
composition of appropriate metallorganic precursors at a relatively
low temperature, while the chloride CVD requires a high substrate
temperature.
So far the ZrO2 thin films deposited by MOCVD have been
prepared using zirconium oxygenated derivatives such as zirconium
alkoxide,6,7 ␤-diketonates,8 fluorinated ␤-diketonates,9 and anhy-
drous nitrates.10 In particular, zirconium tetrakis͑t-butoxide͒
͓Zr͑OtBu͒4, ZTTB͔ is the most frequently investigated alkoxide pre-
cursor, mainly because it has a high vapor pressure ͑0.1 Torr at
31°C͒ and forms films in the temperature range 350-500°C.9 Unfor-
tunately, this compound is very sensitive to hydrolysis. Zirconium
MOCVD process. We characterized Zr͑dmae͒ with thermogravim-
4
etry ͑TG͒ and differential scanning calorimetry ͑DSC͒ analysis,
nuclear magnetic resonance ͑NMR͒, and mass spectrometry. Also,
we investigated the deposition characteristics of the ZrO2 thin films
with a DLI MOCVD using Zr͑dmae͒ in an n-butyl acetate solvent.
4
Experimental
Preparation and characterization of Zr(dmae)4.—Zirconium tet-
ra͑diethylamine͒ ͓Zr͑NEt2͒4, 24.68 g, 65 mmol͔ was added to 150
mL of dry toluene and N,N-dimethylaminoethanol ͑26 mL, 260
mmol͒ was then added slowly to that solution. The mixture was
refluxed for 20 h and then cooled. The solvent was removed under a
reduced pressure to obtain Zr͑dmae͒ as a yellow viscous liquid
4
1
͑yield Ͼ90%͒. H NMR ͑C6D6, 300 MHz͒: ␦ 4.35 ͑s, CH2, 8H͒,
2.61 ͑s, CH2, 8H͒, 2.30 ͑s, CH3, 24H͒.
To investigate the thermal properties of the sample, TG and DSC
analysis ͑Rheometric͒ were used in N2 and O2 atmospheres at a
heating rate of 10°C/min. The cracking patterns of a sample were
obtained with a mass spectrometer ͑JEOL, JMS 700͒. The sample
was ionized by the electron impact ͑EI͒ method and the scanned
mass range was from 1 to 2000 m/z.
Deposition and characterization of the ZrO2 films.— The ZrO2
thin films were deposited using Zr͑dmae͒ as a source material by a
4
DLI MOCVD process described in a previous work.15 Vaporizer
temperature was held at 230°C, and the feed line after the vaporizer
was held at 240°C to prevent the condensation of the precursor. The
susceptor was heated by the tungsten halogen lamps. The reactor
pressure was fixed at 1.6 Torr by using the throttle valve between the
pump and the reaction chamber. Table I shows the typical deposition
conditions. The wafers used were a ͑100͒-oriented p-type Si with the
resistivity of 8-12 ⍀ cm. The modified RCA method, ͑i͒ dipping in
a H2SO4 :H2O2 3:1 solution for 10 min and rinsing with deionized
͑DI͒ water, ͑ii͒ dipping in a HF:H2O 1:7 solution for 30 s and rinsing
with DI water, and ͑iii͒ blowing with N2 gas, was used for the
predeposition cleaning.
tetrakis͑2,2,6,6,-tetramethyl-3,5-heptanedionate͒ Zr͑tmhd͒ , a ho-
͓
͔
4
moleptic zirconium ␤-diketonate, has a high thermal stability and
allows the optimized growth of ZrO2 at the substrate temperature
greater than 600°C, which is not suitable for the low-temperature
CVD process required in the majority of microelectronics applica-
tions.
To overcome these problems, a new approach was pursued by
using a donor-functionalized alkoxy ligand such as dimethylamino-
ethoxide ͑dmae͒.13,14 Dmae provides an additional Lewis base site
which is able to form the chelate rings. It increases the coordinative
saturation relative to an alkoxide precursor such as ZTTB, which
contains an unsaturated ZrIV center.
The deposition rate was measured by an ␣-step 500 surface pro-
filer ͑Tencor͒, and the surface morphology was investigated using a
scanning electron microscope ͑SEM, Hitachi S-4200͒. The crystal
structure was analyzed by an X-ray diffractometer ͑XRD, Mac-
Science M18XHF͒ with Cu K␣ radiation operating at the 30 kV and
40 mA. The depth profile and film composition were investigated by
X-ray photoelectron spectroscopy ͑XPS, PHI 5400 ESCA͒ with a Al
K␣ radiation. For electrical characterizations, the metal-insulator-
In the present work, we introduced Zr͑dmae͒ 14 as an alternative
4
zirconium precursor in a direct liquid injection ͑DLI͒ for the
* Electrochemical Society Active Member.
z E-mail: srhee@postech.ac.kr
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C24
Journal of The Electrochemical Society, 149 ͑1͒ C23-C27 ͑2002͒
Table I. Typical deposition conditions for MOCVD of ZrO2 thin
films.
Substrate
p-type Si͑100͒
300-480°C
1.6 Torr
Substrate temperature
Deposition pressure
Deposition time
Gas flow rate
15 min
R ϭO /͑Ar ϩ O ͒ ϭ 0 ͑Ar 400 sccm͒
͓
͔
2
2
R ϭ 0.3 ͑Ar 280 sccm, O2 120 sccm͒
R ϭ 0.5 ͑Ar 200 sccm, O2 200 sccm͒
0.1 mL/min with 0.1 M precursor solution
n-butyl acetate
Solution injection rate
Solvent
Vaporizer pressure
Vaporizer temperature
8 Torr
230°C
semiconductor ͑MIS͒ capacitors were fabricated with the deposited
oxides. The aluminum dot electrode, 1.963 ϫ 10Ϫ3 cm2 in area,
was deposited by the thermal evaporation through a metallic mask.
The capacitance-voltage ͑C-V͒ characteristics were analyzed at high
frequency ͑1 MHz͒ using an HP 4275 multifrequency LCR meter
with a sweep voltage range of Ϫ4 to ϩ4 V. The interface trap
density at the ZrO2 /Si interface was obtained from the C-V data
using the Terman method.16
Results and Discussion
Characterization of the Zr precursors.—Figure 1 shows a sche-
matic representation of the structure of Zr͑dmae͒4. Bharara et al.
synthesized Zr͑dmae͒ with zirconium isopropoxide Zr͑OiPr͒ and
͓
͔
4
4
dimethylaminoethanol in 1977 for the first time. They revealed that
Zr͑dmae͒ is a yellow viscous liquid and exhibits monomeric
4
behavior.14 In this study, we synthesized Zr͑dmae͒ with Zr͑NEt͒
4
4
and dimethylaminoethanol to obtain a high-purity precursor having
no mono-, di-, and tris-dimethylaminoethanol derivatives of zirco-
nium, i.e., Zr͑OiPr)x(dmae)4Ϫx (x ϭ 1-3).
To investigate the thermal properties of Zr͑dmae͒4, thermogravi-
metric analysis ͑TGA͒ and DSC were performed at a heating rate of
10°C/min. In N2 atmosphere, an endothermic peak and a rapid
weight loss were detected at around 320°C, as shown in Fig. 2a,
indicating that the vaporization occurred at around 320°C. Figure 2b
shows DSC curves in an oxidizing ambient, which compares the
Figure 2. ͑a͒ TGA/DSC curve of Zr͑dmae͒ in Ar atmosphere. ͑b͒ DSC
4
curves of Zr͑dmae͒4, Zr2͑tmhd͒ ͑OiPr͒6, and Zr͑OtBu͒ in the oxygen atmo-
2
4
sphere. ͑760 Torr, dT/dt ϭ 10°C/min.͒
chemical behavior of Zr͑dmae͒ with that of Zr͑OtBu)4 , a most
frequently used alkoxide precursor, and that of Zr2͑tmhd͒ ͑OiPr͒6.
4
2
Zr͑OtBu͒4 shows an endothermic peak at 170°C, indicating the high
volatility of the precursor and then an exothermic peak at about
280°C. Zr͑dmae͒ exhibited an exothermic peak at about 310°C,
4
while Zr2͑tmhd͒ ͑OiPr͒ showed two exothermic peaks at 315 and
2
6
392°C, as shown in Fig. 2b. Based on the report of Lee et al.,17 it is
believed that the exothermic peak at about 310°C for both
Zr͑dmae͒ and Zr2͑tmhd͒ ͑OiPr͒ represents the dissociation of the
4
2
6
precursor by the oxidation of Zr-dmae and Zr-OiPr bond, respec-
tively. On the other hand, the exothermic peak at 280°C for
Zr͑OtBu͒ and 392°C for Zr2͑tmhd͒ ͑OiPr͒ represents the oxidation
4
2
6
of Zr-OtBu and Zr-tmhd bond, respectively. Therefore, it can be
expected that the bond strength of Zr-dmae is similar to that of
Zr-OiPr, slightly stronger than that of Zr-OtBu and lower than that
of Zr-tmhd.
Mass spectrometry was used to investigate the vaporized species
of Zr͑dmae͒4. The typical mass spectra of Zr͑dmae͒ heated at
4
165°C is shown in Fig. 3. Peaks mainly appeared at positions lower
than the molecular weight of Zr͑dmae͒ (m/z ϭ 444), and aggre-
4
gated species of Zr͑dmae͒ such as dimer were not detected notice-
4
ably. These results might indicate that Zr͑dmae͒ exists mainly as
4
a monomer in the gas phase as well as in the liquid state as
proposed by Bharara et al.14
MOCVD of the ZrO2 thin films.—The ZrO2 thin films were de-
posited using flash-vaporized metallorganic precursor solutions with
Zr͑dmae͒4 dissolved in an n-butyl acetate solvent. Because the solu-
Figure 1. A schematic representation of the structure of Zr͑dmae͒4.
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Journal of The Electrochemical Society, 149 ͑1͒ C23-C27 ͑2002͒
C25
Figure 3. Mass cracking patterns of Zr͑dmae͒ at a pressure of 10Ϫ5 Torr
4
after heating at 165°C.
bility of Zr͑dmae͒ is high enough, various solvents can be used.
4
Among various solvents, we selected n-butyl acetate that has a high
boiling point (bp ϭ 126°C). Also, there is no precipitation in the
precursor solution for a month.
Figure 4 shows the Arrhenius plot of the deposition rate as a
function of the substrate temperature in the presence of O2 and the
absence of O2 using Zr͑dmae͒ as a new precursor. The variation of
4
the deposition rate with a substrate temperature shows a sharp in-
crease reaching a maximum value at about 360 and 420°C, respec-
tively, in the presence of O2 and the absence of O2. The decrease of
the growth rate at high temperatures is probably due to the prereac-
tion of precursor molecules with oxygen at high temperatures.18 In
Figure 5. Plan-view ͑top͒ and cross-sectional ͑bottom͒ SEM micrographs of
the ZrO2 film at a deposition temperature of 400°C with R ϭ 0.5.
fact, the growth rate of Zr͑dmae͒ with O2 is lower than that of
4
Zr͑dmae͒4 without O2 at temperatures above 380°C. Since we used a
cold-wall CVD reactor, the precursor reaction with the reactor wall,
which can occur at the high wall temperature, is negligible at the
high substrate temperatures. However, more investigations of the
decrease of growth rate at high temperatures are required. In the
low-temperature region less than 380°C, oxygen has an apparent
positive effect on the rate of film growth, but in the higher tempera-
ture region above 380°C this effect is negative. The apparent acti-
vation energy of the surface chemical reaction calculated from the
slope of Fig. 4 is about 34 kJ/mol for both cases. Compared with the
reports using other precursors, the rate of ZrO2 growth has an inter-
mediate value between the value using Zr͑OtBu)49 and that using
10
4
Zr͑tmhd͒
.
Figure 5 shows the SEM images of the ZrO2 thin film as-
deposited at 400°C. The cross-sectional view reveals a uniform film
with a fine microstructure, and the plan-view micrograph reveals a
fine-grained and dense film surface with an average grain size of
300-400 Å. The grain size was not significantly changed in the sub-
strate temperature range of 340-450°C.
Figure 6 shows XRD patterns of the ZrO2 thin films. ZrO2 can
adopt three different polymorphs; monoclinic, tetragonal, and cubic
phase. Generally, the monoclinic phase is stable below 1170°C. The
stability of polymorphs can be affected by the crystallite size or the
Figure 4. Arrhenius plot of deposition rate as a function of the substrate
temperature in ͑a͒ the absence of oxygen ͑Ar 400 sccm͒ and ͑b͒ presence of
oxygen with R ϭO /͑Ar ϩ O ͒ ϭ 0.5.
͓
͔
2
2
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Journal of The Electrochemical Society, 149 ͑1͒ C23-C27 ͑2002͒
Figure 8. C-V characteristics of ZrO2 as deposited in an Al/ZrO2 /p-type
Si͑100͒ structure at ͑a͒ 340°C, R ϭ 0.3; ͑b͒ 340°C, R ϭ 0.5; ͑c͒ 400°C,
R ϭ 0.3; and ͑d͒ 400°C, R ϭ 0.5.
Figure 6. XRD patterns of the ZrO2 film at various deposition temperatures
with R ϭ 0.5: ͑a͒ 320, ͑b͒ 360, and ͑c͒ 400°C.
mechanical strain present in thin films in the form of the residual
stress.19 The films deposited in the temperature range 320-400°C
exhibit a weak ͑020͒ peak of the monoclinic ZrO2 in an amorphous
background without any other metastable phases in most conditions.
When annealed at 700°C for 30 min in air, the phase transformation
of ZrO2 was not observed and only peak intensities of monoclinic
phase were slightly increased.
Figure 7 shows the typical XPS depth profile of the as-deposited
ZrO2 film and the variation of the XPS spectra of C 1s for an as-
deposited film and a 500°C annealed film in air. The as-deposited
ZrO2 film was found to have an O/Zr ratio of about 1.62 with no
significant change after annealing. The nitrogen content was not
detected in the films ͑detection limit ϳ1 atom %͒. XPS depth profile
shows a uniform concentration distribution of Zr, O, and even car-
bon impurities through the deposited film. Annealing temperature,
500°C, was selected according to the report of Ma et al.20 The film
as-deposited at 400°C has an average carbon content of around 7
atom %, which drops to about 2 atom % by annealing. It is believed
that carbon exists as a carbidic state of Zr-C ͓average binding energy
͑BE͒ found: 282.4 eV͔ in the film. The carbon content of the ZrO2
film as-deposited at 400°C using Zr͑dmae͒ is comparable to about
4
2
10 atom % using Zr͑tfacac͒ ͑at 450°C͒ and 16 atom % using
4
Zr͑acac͒ ͑hfip͒ ͑at 400°C͒.21
2
2
Figure 7. ͑a͒ Typical XPS depth profiles of the as-deposited ZrO2 film with
Ar ion sputtering time and ͑b͒ XPS spectra of C 1s. ͑As deposited at 400°C,
R ϭ 0.5 and annealed at 500°C for 30 min in air.͒
Figure 9. C-V characteristics of ͑a͒ as-deposited ZrO2 film at 400°C,
R ϭ 0.3, and ͑b͒ annealed film at 500°C for 30 min in air.
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Journal of The Electrochemical Society, 149 ͑1͒ C23-C27 ͑2002͒
C27
with oxygen at around 310°C under our TG/DSC measurement con-
ditions. The bond strength of Zr-dmae is similar to that of Zr-OiPr
and stronger than that of Zr-OtBu. The ZrO2 films deposited at 300-
480°C by a DLI MOCVD process showed a dense and smooth mor-
phology. The film had a weak monoclinic phase in an amorphous
background without any other metastable phases. The flatband volt-
age of the ZrO2 thin films deposited at 400°C was close to the
theoretical value of Ϫ0.9 V. The interface trap density near the mid-
gap was found to be less than 1 ϫ 1011 cmϪ2 eVϪ1. It is thought
that Zr͑dmae͒ is a useful candidate among the Zr sources for the
4
deposition of the ZrO2 thin films at the temperature range used here.
Acknowledgment
This work is partly financed by the Consortium of Semiconduc-
tor Advanced Research ͑project no. 00-B6-C0-00-09-00-01͒.
Pohang University of Science and Technology assisted in meeting the
publication costs of this article.
Figure 10. Interface trap density (Dit) calculated from 1 MHz C-V curve:
͑a͒ T ϭ 400°C, R ϭ 0.5 and ͑b͒ ideal Dit on Si͗100͘.
References
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Figure 8 shows the typical C-V plot of the MIS device of the
Al/ZrO2 /p-type Si͑100͒ structure measured at a high frequency of 1
MHz. From the accumulation capacitance of the MIS structure, the
relative dielectric constant of the as-deposited ZrO2 thin films is
estimated to be in the range 15-20. As shown in Fig. 8, the flatband
voltage VFB of the ZrO2 films increases with the increasing deposi-
tion temperature from 340 to 400°C, implying an increase of the
negative charges in the Zr thin films.22 The ideal flatband voltage of
the Al/ZrO2 /p-type Si is about Ϫ0.9 V, which is ␾ms , the difference
in work function between Al and Si. The flatband voltage (VFB) of
the ZrO2 thin films deposited at 400°C is found to be close to this
value.
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Figure 9 shows the C-V characteristics of the as-deposited ZrO2
film and the annealed film at 500°C for 30 min in air. It is shown
that the accumulation capacitance of the ZrO2 film decreases by
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ZrO2 /Si interface and the crystallization of ZrO2 film. This behavior
was also observed in other literatures.23,24
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An interface trap density Dit can be measured by a high-
frequency method developed by Terman.16 Figure 10 shows both the
experimental and the ideal C-V curves for the as-deposited ZrO2
thin film at 400°C, O2 /͑Ar ϩ O2) ϭ 0.5. An interface trap density
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near the midgap was found to be less than 1 ϫ 1011 cmϪ2 eVϪ1
,
which is close to the ideal interface trap density on Si͗100͘. The
results from Fig. 8-10 demonstrate that good electrical properties in
both the bulk and interface regions of the ZrO2 /Si system are ob-
tained for the films grown at 400°C by MOCVD using Zr͑dmae͒4 as
a newly developed precursor.
Conclusions
A liquid Zr precursor Zr͑OCH CH NMe ͒ , Zr͑dmae͒ ] has
been characterized. The precursor vaporizes near 320°C and reacts
͓
23. H. Fukumoto, M. Morita, and Y. Osaka, J. Appl. Phys., 65, 5210 ͑1989͒.
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2
2
2 4
4
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