Growth of hafnium aluminate thin films by direct liquid injection metallorganic CVD using Hf [N(C2H5)2] 4 and Al(OiC3H7)3

Article abstract of DOI:10.1149/1.1851058

A mixtured solution of Hf[N(C₂H₅)₂] ₄ (tetrakis-diethylamido-hafnium, TDEAH) and Al(OⁱC ₃H₇)₃ (aluminum iso-propoxide) dissolved in n-octane was used in the direct liq

Full text of DOI:10.1149/1.1851058

C108
Journal of The Electrochemical Society, 152 ͑2͒ C108-C112 ͑2005͒
0013-4651/2005/152͑2͒/C108/5/$7.00 © The Electrochemical Society, Inc.
Growth of Hafnium Aluminate Thin Films by Direct Liquid
Injection Metallorganic CVD Using Hf†N„C2H5…2‡4
and Al„OiC3H7…3
,z
*
Moon-Kyun Song, Sang-Woo Kang, and Shi-Woo Rhee
Laboratory for Advanced Materials Processing, Department of Chemical Engineering, Pohang University
of Science and Technology, Pohang 790-784, Korea
A mixtured solution of Hf͓N͑C2H5)2]4 ͑tetrakis-diethylamido-hafnium, TDEAH͒ and Al͑OiC3H7)3 ͑aluminum iso-propoxide͒
dissolved in n-octane was used in the direct liquid injection metallorganic chemical vapor deposition ͑DLI-MOCVD͒ for the
deposition of hafnium aluminate (HfAlxOy) thin films. The effect of the deposition temperature and the concentration of the
precursors in the solution on the deposition rate and the composition of the hafnium aluminate film were studied in the deposition
temperature range of 200-475°C. The deposition rate was increased up to the deposition temperature of 350°C with the activation
energy of about 3.6 kcal/mol and then decreased due to the gas-phase dissociation of the precursor. The incorporation of aluminum
into the film depends almost linearly on the concentration of the aluminum precursor in the solution. As the Al incorporation in the
film was increased, crystallization temperature of the film was increased, but the grain size measured by scanning electron
microscope was not significantly changed with the increase of the annealing temperature. For 100 nm thin film, the dielectric
constant and the leakage current density of Hf aluminate film ͑about 65% aluminum phase͒ annealed at 800°C was 11.1 and
2.3 ϫ 10Ϫ7 A/cm2 at 5 V, respectively.
© 2005 The Electrochemical Society. ͓DOI: 10.1149/1.1851058͔ All rights reserved.
Manuscript submitted November 21, 2003; revised manuscript received August 9, 2004. Available electronically January 14, 2005.
10
According to the scaling rules of silicon metal-oxide-
been prepared by methods such as pulsed laser deposition ͑PLD͒
semiconductor field effect transistor ͑MOSFET͒ device, high k ͑di-
electric constant͒ gate dielectrics with an equivalent oxide thickness
͑EOT͒ below 1-1.5 nm will soon be required for sub-0.1 ␮m de-
vices. Unfortunately, the interface reaction between the dielectric
and Si may limit the effective EOT value of some high k dielectrics.
Therefore, the search for thermodynamically stable high k dielectric
on Si is essential to achieve small EOT. HfO2 is among the candi-
date materials for high-k applications because of its high dielectric
constant, thermodynamic compatibility of the interface with Si, gate
poly-Si compatibility, high chemical stability, and a relatively large
bandgap ͑5.68 eV͒.1 Pure HfO2 , however, becomes crystalline at
temperatures lower than 500°C.2 For high-k gate dielectrics, an
amorphous structure is always preferred because polycrystalline film
can introduce high leakage paths along grain boundaries with non-
uniformities in k value and increased surface roughness.3 Although
the low-temperature deposition may be used in an attempt to prevent
crystallization during deposition, the post-thermal cycle in the con-
ventional complimentary metal-oxide-semiconductor ͑CMOS͒ pro-
cess flow might cause crystallization of the HfO2 film. More impor-
tantly, HfO2 is not a good barrier to oxygen diffusion4,5 and
annealing in oxygen-rich ambient will lead to the fast diffusion of
oxygen through HfO2 , causing the growth of uncontrolled low-k
interfacial layers.6
Hf silicate has been proposed as an alternative high k material, as
it is a good barrier against oxygen diffusion, and it is expected to
remain amorphous at relatively high temperatures. But due to the
low k value ͑ϳ3.9͒ of SiO2 , a relatively low k value is expected for
Si-rich films. Al2O3 is a well-known good oxygen diffusion barrier
that may protect the Si surface from oxidation,7,8 and Al2O3 is ther-
modynamically stable in contact with Si. Similar to SiO2 , Al2O3 is
also a good former of glass and, if alloyed with HfO2 , their amor-
phous structure can be stabilized during the high-temperature an-
nealing. In addition, Al2O3 has a large bandgap ͑ϳ8.8 eV͒ and large
band offset with Si. To take advantage of both HfO2 and Al2O3 , it is
desirable to make multicomponent HfAlxOy films with high dielec-
tric constants that are thermodynamically stable in contact with Si.
Some thermal stability studies on Hf aluminate films have been
reported recently.9 Thin films of metal aluminate have previously
molecular beam deposition ͑MBD͒,11 atomic laser deposition
12,13
͑ALD͒
and chemical vapor deposition ͑CVD͒.14 The CVD
method has the ability to control the microstructure and texture of
the films, along with good step coverage and uniformity over a large
area.
The direct liquid injection ͑DLI͒-MOCVD employed in this
study is an attractive method to feed the precursor into the chamber,
especially for multicomponent films. This method opens up the
CVD process to a wider range of precursors and allows the deposi-
tion of materials that are very difficult to deposit by conventional
vapor transport CVD. DLI-MOCVD has been successfully applied
to deposit oxides such as (Ba, Sr͒TiO3 ,15 Pb͑Zr, Ti͒O3 ,16 Ru,17
ZrO2 ,18 and La2O3 .19
We used the DLI-MOCVD process for the deposition of Hf alu-
minate thin films. The precursors used were tetrakisdiethylami-
dohafnium ͑TDEAH͒ and aluminum isopropoxide Al͑OiC H ) ].
͓
3
7 3
TDEAH is a useful precursor for the deposition of Hf oxide20 be-
cause it is liquid at room temperature and has a moderate vapor
pressure ͑1 Torr at 75°C͒.21 Aluminum isopropoxide is a white solid
at room temperature and has a low vapor pressure ͑1 Torr at 360°C͒,
but this precursor has a high solubility in a solvent.25
In this paper, we report the effect of the deposition temperature
and concentration of precursors in the solution on the deposition rate
and the incorporation of Hf and Al elements in the film. Crystalli-
zation temperature and the electrical properties of the film were also
studied.
Experimental
A schematic diagram of the experimental setup was shown
elsewhere18 and operating conditions for the deposition are summa-
rized in Table I. 0.05 M solution of each precursor was made with
n-octane, and the Al͑OiPr͒ •TDEAH͔
ratio in the solution
͓
3
solution
was varied between 1:1 and 9:1 to study the effect of the precursor
ratio by taking different amounts of each solution to make a mixture.
In other cases, a 1:1 ratio was used. A precise amount of the liquid
precursor ͑0.1 mL/min͒ was introduced into the flash evaporator
with a syringe pump. Vaporizer temperature was held at 210°C to
prevent the condensation of the Hf and Al precursor and vaporizer
pressure was maintained at 5 Torr. The reactor pressure was fixed at
1.2 Torr by using the throttle valve between the pump and the reac-
tion chamber. The O2 flow rate was 350 sccm and Ar carrier gas
flow rate was 150 sccm. The wafer used was ͑100͒ oriented p-type
* Electrochemical Society Active Member.
z E-mail: srhee@postech.ac.kr
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Journal of The Electrochemical Society, 152 ͑2͒ C108-C112 ͑2005͒
C109
Table I. Deposition parameters in the direct injection MOCVD.
Parameter
Condition
Hf aluminate
Hf single
oxide
Al single
oxide
Deposition process
DLI-MOCVD
Substrate temperature
Precursor
250-450°C
TDEAH and
Al͑OiPr)3
300-475°C
TDEAH
200-450°C
Al͑OiPr)3
Cocktail solution ratio
0.05 mol/L
1:1 mixture
0.05 mol/L
0.05 mol/L
Injection speed
0.1 mL/min
210°C
O2 , 1 h
150 sccm
350 sccm
1.2 Torr
n-octane
Vaporizer temperature
Annealing condition
Carrier gas ͑Ar͒
Oxidizing gas ͑Ar͒
Deposition pressure
Solvent
Figure 2. The Arrhenius plot of the deposition rate and fraction of alumi-
num component in the Hf aluminate film as a function of the deposition
temperature using a mixture solution of TDEAH and Al͑OiPr)3 ͑1:1͒.
Si wafer with a resistivity of 8-12 ⍀ cm and a modified RCA
method was used for the predeposition cleaning. The thickness of Hf
aluminate thin film for characterization was fixed at 100 nm. After
the deposition of Hf aluminate film, it was annealed at temperatures
ranging from 500-1000°C.
Results and Discussion
Figure 1 shows the deposition rate of Hf aluminate thin films
deposited at the substrate temperature of 350°C as a function of the
vaporizer temperature. The best efficiency of the evaporator gives
the maximum deposition rate and the range of optimum vaporizer
temperature of Hf and Al precursor was 200 to about 220°C. Below
this temperature range, the injected precursor was not fully vapor-
ized because of the insufficient supply of the thermal energy for
vaporization. Above this temperature range, the growth rate of the
Hf aluminate film was decreased because of the decomposition of
the injected precursor. The amount of residue left in the vaporizer
was lowest at the optimum temperature range and the optimum va-
porizer temperature was fixed at 210°C.
The Arrhenius plot of the deposition rate of the film along with
the composition is shown in Fig. 2. Also the Arrhenius plot of the
deposition rate of Al single oxide, and Hf single oxide films along
with Hf aluminate is shown in Fig. 3. The deposition rate was mea-
sured as a function of the substrate temperature, and the concentra-
tion ratio of Hf and Al precursor in the solution was 1:1. On a p-type
Si surface, the deposition rate of Hf aluminate film was determined
by the mass-transfer rate with the apparent activation energy about
3.6 kcal/mol. The shape of the deposition rate curve of the Hf alu-
minate film and that of the Al oxide film were similar to those
shown in Fig. 3. The deposition rate of the Hf aluminate film and
also the aluminum content were dependent on the decomposition
characteristics of the Al precursor. The Al content was increased
with the deposition temperature below the deposition temperature of
350°C and decreased with the deposition temperature above 350°C.
Above the substrate temperature of 350°C, it appears that the Al
precursor was dissociated in the gas phase that led to the decrease of
the deposition rate and Al incorporation. Figure 4 shows the alumi-
num incorporation into the Hf aluminate film as a function of the
composition ratio of the cocktail solution. The aluminum content in
the film increased almost linearly as the aluminum precursor ratio
was increased in the solution.
The thickness of the Hf aluminate film was measured with an
ellipsometer ͑MX75/100T͒, and some of the measurements were
compared with cross-sectional scanning electron microscopy ͑SEM,
JEOL JSM-840A͒. The crystallinity was examined using X-ray dif-
fraction ͑XRD, Mac-Science M18XHF͒ and the surface morphology
was analyzed by scanning electron microscopy ͑SEM͒. The compo-
sition of the film was determined by inductively coupled plasma-
atomic emission spectrometry ͑ICP-AES, Jobin-Yvon, JY38plus͒.
For electrical characterization, metal-insulator-semiconductor ͑MIS͒
capacitors were fabricated with deposited oxides. The aluminum dot
electrode, 6.34 ϫ 10Ϫ4 cm2 in area, was deposited by thermal
evaporation through a metallic mask. The capacitance-voltage ͑C-V͒
characteristics were analyzed at high frequency ͑1 MHz͒ using
HP4275 multifrequency LCR meter with a sweep voltage of Ϫ8 to
ϩ8 V. The current-voltage (I-V) characteristics were obtained with
HP 4155 semiconductor parameter analyzer to investigate the break-
down strength and the leakage current through the oxide film.
The plane-view SEM photographs were shown in Fig. 5 for the
Hf aluminate thin films ͑65% Al phase͒ deposited at the substrate
temperature of 350°C and annealed at 600, 800, and 1000°C for 60
min in O2 . The grain size of the as-deposited film was increased
during annealing, but the annealing temperature did not have a sig-
nificant influence on the grain size. Figure 6a, b, c, and d are XRD
peaks of the hafnium aluminate film as functions of the film com-
position obtained by varying the concentration ratio of Al͑OiPr)3
and TDEAH, 9:1, 7:3, 3:7, and 1:9. ͑The film composition can be
found for each case in Fig. 4, and it was 94.1, 79.6, 48.2, 18.5% Al
Figure 1. Deposition rate of Hf aluminate thin film deposited at a substrate
temperature of 350°C as a function of the vaporizer temperature using a
mixture solution of TDEAH and Al͑OiPr)3 ͑1:1͒.
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Journal of The Electrochemical Society, 152 ͑2͒ C108-C112 ͑2005͒
Figure 3. The Arrhenius plot of the deposition rate of the Hf aluminate, Al
single-oxide, and Hf single-oxide films.
phase, respectively͒. This shows the XRD patterns of the as-
deposited films and films annealed at 600, 800, and 1000°C for 60
min in O2 . Regardless of the chemical composition, the Hf alumi-
nate thin films deposited at 350°C were amorphous. In Fig. 6a and b,
it can be seen that when the aluminum phase was higher than about
80%, the film remained amorphous up to the annealing temperature
of 1000°C. In the case of the film with 65% Al phase, the film also
remained amorphous up to the annealing temperature of 1000°C
with XRD patterns similar to Fig. 6b. In Fig. 6c and d, the peak
intensity of the XRD pattern was increased with the increase of the
annealing temperature. The XRD pattern of the Hf-rich film was
similar to the peaks observed in the monoclinic phase of Hf single-
oxide film (HfO2) with a small decrease in 2␪ value. Accordingly,
the d value ͑the lattice spacing͒ for the Hf aluminate was increased
compared to the Hf oxide. The increase in the amount of Al element
and annealing temperature leads to a larger d value. For example,
the d value of the monoclinic ͑111͒ of Hf oxide was 2.8234. How-
ever, the d value of Hf aluminate with Al/͑Al ϩ Hf) ϭ 0.185 an-
Figure 5. SEM micrographs of the Hf aluminate thin films ͑a͒ as-deposited
͑350°C͒ and at the annealing temperature of ͑b͒ 600, ͑c͒ 800, and ͑d͒ 1000°C
͑65% Al phase͒.
nealed at 1000°C was 2.9285 and with Al/͑Al ϩ Hf) ϭ 0.482 an-
nealed at 1000°C, it was 2.9341. The shift in XRD peaks indicates
the change in the crystal structure due to the aluminum incorpora-
Figure 4. Fraction of aluminum component in the Hf aluminate thin film
as a function of the feed ratio of Al and Hf precursors ͑deposition tempera-
ture, 350°C͒.
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Journal of The Electrochemical Society, 152 ͑2͒ C108-C112 ͑2005͒
C111
Figure 6. XRD pattern of Hf alumi-
nate thin films deposited at 350°C as a
function of the composition: ͑a͒ 94.1,
͑b͒ 79.6, ͑c͒ 48.2, and ͑d͒ 18.5% alu-
mina phase.
tion into the lattice of Hf oxide. The film with an aluminum phase
below about 20% was crystallized above the annealing temperature
of 600°C.
in Hf aluminate on the physical and electrical properties.26 They
showed that the crystallization temperature increased monotonically
as a function of Al content and the dielectric constant of the film
decreased linearly with aluminum content. It was also pointed out
that the effective dielectric constant was decreased with the increase
of the annealing temperature due to the growth of an interfacial
layer, such as silicon dioxide or hafnium silicate at the Hf
aluminate/Si interface.11 The flatband voltage, VFB , of the oxide
film was measured from the C-V curve. The ideal flatband voltage of
Al/Hf aluminate/p-type Si is about Ϫ0.9 V, which is from the dif-
ference in work function between Al and Si. Compared with the
ideal case, the flatband voltage of the Hf aluminate film shifted to
the positive direction as shown in Fig. 7. Generally, the flatband
Figure 7 shows the C-V plot of the Al/Hf aluminate/p-type
Si͑100͒ ͑metal-insulator-semiconductor ͑MIS͒ structure measured at
the high frequency of 1 MHz. The film used for the capacitance-
voltage ͑C-V͒ measurement was deposited at 350°C using the con-
centration ratio of Al͑OiPr)3 . TDEAH ϭ 1.1 and Al/͑Hf ϩ Al) of
the film was about 0.65. From the accumulated capacitance of the
MIS structure, the effective dielectric constant of the Hf aluminate
thin film deposited at 350°C is 12.6, and those of the films annealed
at 600, 800, and 1000°C are 12.2, 11.1, and 9.9, respectively. Zhu
et al. recently investigated the effect of the aluminum composition
Figure 7. Capacitance-voltage characteristics of the Al/Hf aluminate/p-type
Si͑100͒ structure with Hf aluminate films ͑65% alumina phase͒ deposited at
͑a͒ 350°C, and annealed at ͑b͒ 600, ͑c͒ 800, and ͑d͒ 1000°C.
Figure 8. I-V characteristic of the Al/Hf aluminate/p-type Si͑100͒ structure
with Hf aluminate films ͑65% alumina phase͒ deposited at ͑a͒ 350°C, and
annealed at ͑b͒ 600, ͑c͒ 800, and ͑d͒ 1000°C.
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C112
Journal of The Electrochemical Society, 152 ͑2͒ C108-C112 ͑2005͒
voltage of the aluminum oxide film was shifted to the positive
side.23,24 In our case, flatband voltage of the Hf aluminate film
showed the same tendency as the aluminum single oxide. The posi-
tive shift in flatband voltage is attributed to a negative charge at the
Si interface or in the bulk oxide.22 The flatband voltage shift could
be reduced by annealing at high temperature.
Figure 8 shows the I-V characteristics of the MIS structure
and with the Hf aluminate thin film deposited at 350°C, a leakage
current of 4.3 ϫ 10Ϫ4 A/cm2 at 5 V was obtained and the break-
down was not observed up to 20 V. After annealing at higher tem-
peratures, the leakage of the Hf aluminate thin film was significantly
reduced. This is probably due to the increase of the interfacial oxide
layer thickness as in C-V measurement results.11 The leakage cur-
rent of the Hf aluminate thin film annealed at 800 and 1000°C was
2.3 ϫ 10Ϫ7 and 1.6 ϫ 10Ϫ8 A/cm2 at 5 V, respectively.
References
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Conclusions
Hf aluminate film was deposited with Hf͓N͑C2H5)2]4 and
Al͑OiC3H7)3 mixture solution using DLI-MOCVD. The deposition
rate was increased up to the deposition temperature of 350°C with
the activation energy of about 3.6 kcal/mol and then decreased due
to the gas-phase dissociation of the precursor. The incorporation of
aluminum into the film depends almost linearly on the concentration
of the aluminum precursor in the solution. For the film with an
aluminum phase above 80%, crystallization was not observed up to
the annealing temperature of 1000°C. For the 100 nm thin film, the
dielectric constant and the leakage current density of Hf aluminate
film ͑about 65% aluminum phase͒ annealed at 800°C was 11.1 and
2.3 ϫ 10Ϫ7 A/cm2 at 5 V, respectively. It appears that the suppres-
sion of crystallization is more effective with the incorporation of the
alumina phase than the silica phase.
Acknowledgment
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Financial support of this work by the System IC 2010 project is
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Pohang University of Science and Technology assisted in meeting the
publication costs of this article.
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