Mononuclear precursor for MOCVD of HfO2 thin films

Article abstract of DOI:10.1039/b405015k

We report the precursor characteristics of a novel mononuclear mixed alkoxide compound [Hf(OⁱPr)₂(tbaoac)₂] and its application towards MOCVD of HfO₂ thin films in a production tool CVD reactor.

Full text of DOI:10.1039/b405015k

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Mononuclear precursor for MOCVD of HfO2 thin films†
Arne Baunemann,a Reji Thomas,b Ralf Becker,a Manuela Winter,a Roland A. Fischer,a Peter
Ehrhart,b Rainer Waserb and Anjana Devi*a
a Inorganic Materials Chemistry Group, Lehrstuhl für Anorganische Chemie II, Ruhr-University Bochum,
Universitätsstr. 150, D-44780 Bochum, Germany. E-mail: anjana.devi@ruhr-uni-bochum.de; Fax: +49
234 3224174; Tel: +49 234 3224150
b Institut für Festkörperforschung/EKM, Forschungszentrum Jülich gmbH, D-52425 Jülich, Germany
Received (in Cambridge, UK) 6th April 2004, Accepted 18th May 2004
First published as an Advance Article on the web 17th June 2004
We report the precursor characteristics of a novel mononuclear
mixed alkoxide compound [Hf(OiPr)2(tbaoac)2] and its applica-
tion towards MOCVD of HfO2 thin films in a production tool
CVD reactor.
center is expected to be enhanced and the system should be more
stabilized. In this communication, we report the synthesis‡ and
structural characterisation§ of the mononuclear mixed-alkoxide
compound [Hf(OiPr)2(tbaoac)2](1). The precursor exhibits a high
degree of solubility in organic solvents and possesses promising
thermal properties for CVD applications. Thin films of HfO2 were
grown in a production tool liquid injection MOCVD reactor and the
observed electrical properties of the films seem promising for gate
oxide applications.
Hafnium dioxide is one of the promising candidates to replace SiO2
as the gate oxide material in the submicron generation of
complementary metal oxide semiconductor (CMOS) devices
because of its relatively high dielectric constant and stability.1
There are several reports where thin films of HfO2 have been
produced by laser ablation, sputtering, sol–gel, ALD and CVD
processes.2 MOCVD is one of the appealing techniques for the
deposition of thin films of various materials as it possesses some
inherent advantages. The metalorganic compounds used as pre-
cursors for this process play a pivotal role in the resulting properties
of the films obtained.
Compound 1 is a white crystalline solid with a low melting point
of 44 °C and the synthesis can be easily scaled up with high
yields.
From single crystal X-ray diffraction studies, the complex was
found to be monomeric and crystallizes in the monoclinic space
group C2/c. The ligands are oriented in cis-geometry due to the
stronger trans effect of the alkoxy groups (Fig. 1). The methyl
groups at the tbaoac ligand are stronger donors than the tert-butoxy
groups. Therefore the tert-butyl-moieties of the b-ketoesters are
trans to the isopropoxy groups (strong electron donators). The Hf–
O bonds trans to the isopropoxy groups are lengthened (Hf1–O5 =
2.200(5) Å, Hf1–O1 = 2.081(6) Å). The difference in the Hf1–O5
and the Hf1–O1 bond lengths causes the two chelating ligands to
form an angle of 83.82(19)° for the O1–Hf1–O5A bond and even
79.28(18)° for the O5–Hf1–O5A bond. To neutralise the stress
caused by this compression of the b-ketoester rings, the two
isopropoxy groups form angles greater than 90° (O6–Hf1–O6A =
102.6(3)°). These data correspond to the data reported for the
analogous Zr complex [Zr(tbaoac)2(OiPr)2].7
Thermal analysis (simultaneous TG/DTA) was carried out to test
the suitability of the precursor for CVD applications. The
compound sublimes at relatively low temperatures (70–90 °C, from
isothermal studies using TG) and the decomposition temperature is
around 225 °C and hence suitable for MOCVD applications and
low temperature deposition.
Crystalline HfO2 films with monoclinic phase were obtained in
the susceptor temperature range 500–750 °C as can be seen in Fig.
2 and films were amorphous below 500 °C.
Several precursors have been tried for the CVD of HfO2 films
and among them alkoxides and b-diketonates of Hf have been
extensively used. However, there are some limitations associated
with these precursors. The homoleptic hafnium ethoxide and
isopropoxide compounds tend to oligomerize and are therefore less
suitable for MOCVD applications. Hafnium tert-butoxide exhibits
a much higher volatility due to its monomeric structure (sterical
demand of the tert-butyl groups). Hence, it is utilized in MOCVD.3
The hafnium center is highly unsaturated because it is only fourfold
substituted and hafnium favours the coordination number eight.
This results in a high sensitivity towards moisture and air. The low
coordination number also reduces the shelf life of hafnium
alkoxides in solution and limits the usage for liquid injection CVD.
To increase the stability of the compounds, bidentate ligands,
mainly b-diketonate ligands could be introduced. The eight
coordinated complexes [Hf(acac)4] and [Hf(thd)4] (acac = acet-
ylacetonate, thd = 2,2,6,6-tetramethyl-3,5-heptadionate) are less
sensitive towards moisture but at the same time they display a low
volatility. It is possible to increase the volatility of these homoleptic
compounds using fluorinated ligands. An example is [Hf(tfac)4]
(tfac = trifluoroacetylacetonate). The drawback of this precursor is
the incorporation of fluorine into the gate oxide layer that cannot be
avoided.4 Amido precursors of the type [Hf(NRRA)4] (R, RA =
alkyl) have also been established not only for the MOCVD of HfO2
thin films but also for atomic layer deposition (ALD) processes.5 In
general a compromise has to be made between the stability and the
volatility and a coordination number of eight seems to be
favourable. Examples of hafnium complexes with the coordination
number six or with fluxional ligand systems are [Hf(mmp)4],
[Hf(OtBu)2(mmp)2] and [Hf(OiPr)2(thd)2] (mmp = 1-methoxy-
2-methyl-2-propanolate).6
Capacitace–voltage (C–V) and current–voltage (I–V) character-
istics of the Pt/HfO2/SiOx/p–Si(100) MIS structures shown in Fig.
3 were evaluated for the gate oxide application. The C–V curve
shows small hysteresis (DVfb = 13 mV) with a loop in the
Our approach to the synthesis of volatile precursors for the
MOCVD of HfO2 is the introduction of specific changes in the
ligand sphere of already established Hf-key structures (b-diketo-
nates like acac and thd). By using the b-diketoester, Htbaoac
(tbaoac = tert-butylacetoacetate) the Lewis acidity of the metal
† Electronic supplementary information (ESI) available: TG/DTA, and
isothermal studies, RBS and AFM data of HfO2 film. See http://
www.rsc.org/suppdata/cc/b4/b405015k/
Fig. 1 Molecular structure of [Hf(OiPr)2(tbaoac)2] in the solid state.
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C h e m . C o m m u n . , 2 0 0 4 , 1 6 1 0 – 1 6 1 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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C6D6): d 1.37 (18H, s, C(CH3)3 tbaoac), 1.41 (12H, d, CH(CH3)2 OiPr, 1J
= 6,28 Hz), 1.75 (6H, s, CH3 tbaoac), 4.71 (2H, sept., CH(CH3)2 OiPr), 5.06
(2H, s, CH tbaoac). 13C{H}NMR: (RT, 62.5 MHz, C6D6): d 25.68 (CH3
tbaoac), 27.39 (CH3 OiPr), 28.47 (C(CH3)3 tbaoac), 71.18 (CH OiPr), 81.24
(OC(CH3)3 tbaoac), 90.45 (CH tbaoac); 173.92 (CO tbaoac). 186.15
(OCCH3 tbaoac). EI-mass spectrum (70 eV): m/z 612 [M]+, 553 [M 2
OiPr]+, 497 [M 2 OiPr, 2 isobutene (C4H8)]+, 455 [M 2 tbaoac]+, 441 [M
2 OiPr, 2 2 C4H8]+, 397 (calcd. 396) [M 2 OiPr, 2 tbaoac]+, 355 (calcd.
i
353) [M 2 OiPr, 2 tbaoac, 2 Pr]+.
§
Single crystal X-ray diffraction: data collection for [Hf(OiPr)2(t-
baoac)2] was performed on a Bruker-AXS-SMART (CCD 1000) dif-
fractometer, equipped with a cryogenic nitrogen cold stream to prevent loss
of solvent and using graphite monochromated Mo–Ka radiation (0.71073
Å). The structures were solved by direct methods and refined anisotrop-
ically with SHELXL-97 program suite. CCDC 236724. See http://
www.rsc.org/suppdata/cc/b4/b405015k/ for crystallographic data in .cif or
other electronic format. Crystallographic data for [Hf(OiPr)2(tbaoac)2] (red.
0.48 3 0.42 3 0.38 mm), C22H40HfO8, Mr = 611.02, monoclinic, a =
Fig. 2 XRD patterns of HfO2 films on Si substrates deposited at various
susceptor temperatures in a MOCVD production tool.
9.872(3), b
= 15.312(4), c = 18.599(4) Å, b = 97.548(7)°, U =
2787.1(12) Å3, T = 213(2) K, space group C2/c, Z = 4, 7901 reflections
collected, 2471 unique (Rint. = 0.127) which were used in all calculations.
The final wR(F2) was 0.0953 (all data). Simultaneous thermogravimetric
and differential thermal analysis (TG/DTA) was carried out using a Seiko
TG/DTA 6300S11 in an argon atmosphere (300 mL min21, sample size ~
10 mg, ambient pressure, RT–500 °C, heating rate of 5 °C min21).
Film depositions were performed in an AIXTRON 2600G3 planetary
reactor equipped with a TRIJET liquid injection system, which allows
deposition on 5 3 6 in. wafers simultaneously.8 The precursor was
dissolved in n-butylacetate (0.05 mol). The deposition conditions: substrate
temperature 350–750 °C, vaporisation temperature 170–240 °C, reactor
pressure 1.0–1.5 mbar, O2 flow of 200 sccm, period 0.32 s and opening time
0.8 ms. Films were deposited on p-type silicon wafers without removing the
native oxide layer. Film thickness was calculated by measuring the areal
mass of the film’s Hf atoms from the X-ray fluorescence (RIGAKU ZSX-
100e). The crystal structure of the films was characterized using an X-ray
diffractometer (Philips Analytical) employing grazing incidence and Cu–
Ka radiation. Surface morphology of the films was studied with AFM (SIS
Picostation) and the RMS roughness was around 0.1 nm. The composition
of the films deposited were analysed by Rutherford back scattering and X-
ray photoelectron spectroscopy. Electrical properties of the films were
studied after sputter deposition and patterning of Pt top electrodes.
Electrode annealing was done at 400 °C in N2 atmosphere and the area used
for the electrical characterization was 0.0491 mm2. C–V and I–V
characteristics of the films in the MIS configuration were obtained, using a
HP8284 LCR meter and a 617 Keithley programmable electrometer,
respectively.
Fig. 3 C–V and I–V characteristics of HfO2/SiOx/Si deposited at 700 °C in
a MOCVD production tool.
anticlockwise direction, indicating oxide trapping of electrons
injected into the oxide at positive voltage and subsequent ejection
of trapped electrons at the negative bias. This effect corresponds to
a density of rechargeable oxide traps ~ 1 3 1011 cm22. Equivalent
oxide thickness (EOT) calculated from the accumulation capaci-
tance was about 2.6 nm for the 3 nm thickness HfO2 films. Average
flatband voltage was about 0.32 V. Interface trapped charges, vital
in the degradation of the MOS device, were calculated from the
slope of C–V curve at the flat band voltage, and was found to be 7.0
3 1011 eV21 cm22, comparable with the SiO2 based MOS devices
without the forming gas anneal. Leakage current density at 21 V
was about 4.6 3 1026 A cm22, less than the corresponding SiO2
based structures.
In summary, the novel mononuclear mixed alkoxide of Hf has
been synthesised and structurally characterised. Application of this
compound as a precursor in a production tool MOCVD reactor
resulted in HfO2 thin films exhibiting electrical properties which
are promising for device applications.
The authors gratefully acknowledge the financial support from
the DFG (CVD-SPP-1119, DE 790/3-2 and WA 908/13-2). Thanks
to Dr. H. Parala for critically reviewing the manuscript.
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Notes and references
‡ [Hf(OiPr)2(tbaoac)2] was synthesized by dissolving [Hf(OiPr)4] (10
mmol, 4.14 g) in 80 ml hexane and 1 ml (13 mmol) isopropanol. During
refluxing of the mixture (T = 80 °C), tert-butylacetoacetate (20 mmol, 3.32
ml) diluted in 5 ml hexane was added slowly. After a refluxing period of 2
h, the solvent was removed in vacuo yielding a slightly yellow, viscous
product. Short-path distillation of the product at 85 °C/0.07 Torr resulted in
a pure white compound. Yield: 5.1 g (8.3 mmol, 80% based on [Hf(OiPr)4]);
mp (uncorrected): 44 °C; anal.: calcd. for C22H40HfO8, C: 43.24; H: 6.60;
1
found: C: 42.64 H: 6.63%; H NMR: (room temperature (RT), 250 MHz,
C h e m . C o m m u n . , 2 0 0 4 , 1 6 1 0 – 1 6 1 1
1611