Monomeric malonate precursors for the MOCVD of HfO2 and ZrO 2 thin films

Article abstract of DOI:10.1039/b810528f

New Hf and Zr malonate complexes have been synthesized by the reaction of metal amides with different malonate ligands (L = dimethyl malonate (Hdmml), diethyl malonate (Hdeml), di-tert-butyl malonate (Hdbml) and bis(trimethylsilyl) malonate (Hbsml)). Homo

Full text of DOI:10.1039/b810528f

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PAPER
www.rsc.org/dalton | Dalton Transactions
Monomeric malonate precursors for the MOCVD of HfO2 and
ZrO2 thin films†
Ramasamy Pothiraja, Andrian Milanov, Harish Parala, Manuela Winter, Roland A. Fischer and
Anjana Devi*
Received 24th June 2008, Accepted 29th September 2008
First published as an Advance Article on the web 21st November 2008
DOI: 10.1039/b810528f
New Hf and Zr malonate complexes have been synthesized by the reaction of metal amides with
different malonate ligands (L = dimethyl malonate (Hdmml), diethyl malonate (Hdeml), di-tert-butyl
malonate (Hdbml) and bis(trimethylsilyl) malonate (Hbsml)). Homoleptic eight-coordinated
monomeric compounds of the type ML4 were obtained for Hf with all the malonate ligands employed.
In contrast, for Zr only Hdmml and Hdeml yielded the eight-coordinated monomeric compounds of
the type ML4, while using the bulky Hdbml and Hbsml ligands resulted into mixed alkoxo-malonato
six-coordinated compounds of the type [ML2(OR)2]. Single crystal X-ray diffraction studies of all the
compounds are presented and discussed, and they are found to be monomeric. The complexes are solids
and in solution, they retain their monomeric nature as evidenced by NMR measurements. Compared to
the classical b-diketonate complexes, [M(acac)4] and [M(thd)4] (M = Hf, Zr; acac: acetylacetonate; thd:
tetramethylheptadione), the new malonate compounds are more volatile, decompose at lower
temperatures and have lower melting points. In particular, the homoleptic diethyl malonate complexes
of Hf and Zr melt at temperatures as low as 62 ◦C. In addition, the compounds are very stable in air
and can be sublimed quantitatively. The promising thermal properties makes these compounds
interesting for metal-organic chemical vapor deposition (MOCVD). This was demonstrated by
depositing HfO2 and ZrO2 thin films successfully with two representative Hf and Zr complexes.
stability during the evaporation process and lower decomposition
temperature are some of the features desired.
Introduction
Silicon dioxide (SiO2) has conventionally been used as the
dielectric in metal oxide semiconductor field effect transistors
(MOSFETs).1 However, further downscaling of the dielectric
thickness will lead to direct electron tunneling and high leakage
currents which can degrade the device performance1 High permit-
tivity (high-k) insulators which give higher gate capacitances with
physically thicker dielectric films are needed to address this issue.1,2
Research studies in the last few years identified Hf and Zr based
dielectrics (e.g. oxides, silicates, aluminates) as the most promising
high-k materials to replace SiO2 in CMOS applications.1–3 Metal-
organic chemical vapor deposition (MOCVD) has been estab-
lished as a very useful fabrication process for the deposition of
high quality thin films. It has several advantages such as growth of
uniform layers on large substrates, high growth rates, good com-
position control, conformal coverage on substrates with complex
geometries etc. The success of MOCVD process relies strongly
on the precursors employed. The availability of volatile metal-
organic precursors possessing sufficient vapor pressure, thermal
Metal-organic compounds such as metal halides, classical
or functional alkoxides, b-diketonates, alkylamides and metal
nitrates are some of the commonly used precursors for MOCVD
of Hf and Zr oxide thin films.4,5 There are certain drawbacks
associated with some of these precursors such as lower volatility,
higher evaporation and decomposition temperatures, carbon and
halogen incorporation in the films and high air and moisture
sensitivity. Our earlier approach of modifying the existing class
of precursors, for example, usage of different chelating ligands
in combination with Hf and Zr alkoxides, showed promise in
terms of improved thermal properties, moisture sensitivity etc.6 In
a continuation of our efforts to develop precursors with improved
properties, in terms of precursor volatility, ease of handling, ther-
mal stability, reduction in decomposition temperature in order to
achieve film growth at reduced substrate temperatures, we pursued
our work further. In this work, our main objective was to synthesise
complexes of Hf and Zr with malonates as alternative chelating
ligands with a goal to obtain precursors, which are more volatile
and have lower decomposition temperature than the classical b-
diketonates like [M(acac)4] or [M(thd)4], and easier to handle
than the parent alkoxides or amides. The structural and thermal
properties of malonate based Hf and Zr compounds are discussed
in detail. As a representative example, compounds [Hf(dmml)4]
and [Zr(dbml)2(OBut)2] (Hdmml = dimethyl malonate, Hdbml =
di-tert-butyl malonate) are tested for the MOCVD growth of HfO2
and ZrO2 respectively and the thin film properties are investigated.
Inorganic Materials Chemistry, Lehrstuhl fu¨r Anorganische Chemie II,
Universita¨tsstr. 150, Ruhr-Universita¨t Bochums D-44780 Bochum,
Germany. E-mail: anjana.devi@rub.de; Fax: +49 2343214174; Tel: +49
2343224150
† Electronic supplementary information (ESI) available: Molecular struc-
tures, 1H NMR spectra, mass spectra, TG curves and crystallographic data.
CCDC reference numbers 692629–692636. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b810528f
654 | Dalton Trans., 2009, 654–663
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(5) and [Zr(deml)4] (6) (Scheme 1). However, when bulkier di-
tert-butyl and bis(trimethylsilyl) malonates were used as ligands,
only six-coordinated mixed alkoxide-malonate of Zr compounds
[Zr(dbml)2(OBut)2] (7) and [Zr(bsml)2(OSiMe3)2] (8) were ob-
tained (Scheme 2). The alkoxide/siloxide ions on Zr were gen-
erated during thermal activation of malonic diester (malonate).8,9
The products were purified by crystallization in toluene or
toluene/acetonitrile mixture. Except for 3, 4 and 7, all the other
compounds crystallize with toluene in the crystal lattice. However,
toluene can be removed from the crystal lattice by prolonged
evacuation at reduced pressure. Crystal and data collection
parameters for all compounds are given in Table S1 in the ESI.†
Compound 1 was crystallized in toluene at 5 ◦C and it crystallizes
in the monoclinic P21/c space group with a toluene molecule in
the crystal lattice. The coordination geometry around the Hf ion
is square antiprism (Fig. 1).10 Eight oxygen atoms of the four
chelating malonate ions are coordinated to the central Hf ion with
Results and discussion
As a part of our research to develop improved precursors for
HfO2 and ZrO2, we used malonates as chelating ligands, since
these ligands saturate the metal centre and can also lead to
the formation of monomeric compounds, which is important
in terms of volatility. The other advantage is that, the presence
of the ester groups in the molecule can act as cleavage points,
which can trigger decomposition at low temperatures enabling
film growth at reduced substrate temperatures. In this study,
the bulkiness of the malonates was also varied (by changing
the substituent on malonate) to study its effect on product
formation, crystallization, volatility and decomposition. We have
thus synthesized a series of Hf and Zr malonate complexes through
the amide route by substituting amide groups on metal with
chelating malonate ligands. Though in principle it is possible to
synthesize all the complexes through alkoxide or salt metathesis
routes, the amide route was chosen for the following reasons.
In alkoxide route, the alcohol, formed during the reaction, can
facilitate some other transformation (or product decomposition).7
The salt metathesis route includes the heterogeneous reaction of
metal salt suspension and the metal salt formed as a byproduct
could possibly contaminate the precursor. In the amide route, the
amines formed as side products can easily be removed by applying
vacuum since their boiling points are lower than alcohols.
˚
the average Hf–O bond length of 2.169 A, which is shorter than
11
˚
average Hf–O bond length (2.178 A) in [Hf(acac)4]. The bite
angles of Hf and malonates are wider in 1 (av. 75.58◦) compared
to the same in [Hf(acac)4] (av. 75.12◦).
Compound 2 was crystallized in toluene at -30 ◦C. Similar to 1,
compound 2 crystallizes in a monoclinic P21/c space group with
toluene molecule in the crystal lattice (Fig. S1†). The coordination
polyhedron of Hf ion is identical to that of 1. The average Hf–O
˚
bond length (2.172 A) is comparable to the same in 1 and shorter
than the same in [Hf(acac)4] (av. 2.178 A). However, the bite angles
The reaction of Hf amide with four equivalents of different mal-
onates resulted in the formation of eight-coordinated monomeric
Hf complexes [Hf(dmml)4] (1), [Hf(deml)4] (2), [Hf(dbml)4] (3)
and [Hf(bsml)4] (4) (Hdmml = dimethyl malonate, Hdeml =
diethyl malonate, Hdbml =di-tert-butyl malonate, Hbsml =
bis(trimethylsilyl) malonate) (Scheme 1).
˚
(av. 75.37◦) are narrow compared to the same in 1 (av. 75.58◦).
As the size of the alkyl group on malonate was increased from
ethyl to tert-butyl, it was not possible to crystallize the product
[Hf(dbml)4] (3) in toluene. However, X-ray quality crystals were
obtained for this compound in toluene/acetonitrile mixture at
room temperature. Compound 3 crystallizes in tetragonal space
In the case of Zr, only Hdmml and Hdeml yielded ho-
moleptic eight-coordinated monomeric complexes [Zr(dmml)4]
Scheme 1 Synthesis of eight-coordinated Hf and Zr malonates.
Scheme 2 Synthesis of six-coordinated mixed alkoxide-malonates of Zr.
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Fig. 1 Molecular structure (left) and coordination polyhedron (right) of [Hf(dmml)4] (1). (Hydrogen atoms and toluene molecule are removed for
◦
˚
clarity.) Selected bond lengths (A) and angles ( ); Hf(1)-O(11) 2.146(2), Hf(1)-O(25) 2.153(2), Hf(1)-O(45) 2.155(2), Hf(1)-O(31) 2.158(2), Hf(1)-O(41)
2.174(2), Hf(1)-O(21) 2.186(2), Hf(1)-O(15) 2.190(2), Hf(1)-O(35) 2.192(2), O(45)-Hf(1)-O(41) 75.35(8), O(25)-Hf(1)-O(21) 75.78(7), O(11)-Hf(1)-O(15)
75.74(8), O(31)-Hf(1)-O(35) 75.44(8).
group of P4/ncc with no solvent molecule in it (Fig. S2†). The
coordination polyhedron of Hf is a square antiprism.10 The
molecule has C4 symmetry and the Hf–O bond lengths (av.
˚
a consequence, C(2)-O(1) bond length (1.282(4) A) is longer than
˚
C(4)-O(5) bond length (1.250(4) A).
˚
This resulted in the shortening of C(2)-C(3) (1.376(4) A) bond
length compared to C(3)-C(4) (1.400(4) A) bond length. The
˚
2.176 A) are close to the same in [Hf(acac)4] and are comparable
˚
to that of 1 and 2.
bonding here can be best explained as intermediate position
between a and b shown in Scheme 3, as formation of position
b is important for McLafferty rearrangement.
However, it is longer than Hf–O distances observed in [Hf(thd)4]
(av. 2.146 A). Hf-malonate bite angle (79.02(8)◦) is wider
7
˚
than the same in other Hf malonate compounds reported here.
Though it is an eight-coordinated compound, its bite angle is
comparable to the same in six-coordinated [Hf(NEt2)2(dbml)2]
and [Hf(N(MeEt)2(dbml)2].12,13 Solvent free [Hf(bsml)4] (4) was
crystallized in tetragonal I41/a space group in toluene (Fig. 2).
There are two types of Hf–O bonds, which differ in their lengths
˚
(Hf(1)-O(1) 2.105(2) and Hf(1)-O(5) 2.262(2) A). It indicates
that Hf is strongly coordinated to O(1) compared to O(5). As
Scheme 3 View of bonding situation in [Hf(bsml)4] (4).
Very similar to 1 and 2, [Zr(dmml)4] (5) and [Zr(deml)4] (6) were
crystallized in toluene. Compound 5 (Fig. S3†) was crystallized
at 5 ◦C, but compound 6 (Fig. 3) was crystallized at -30 ◦C.
Both crystallize in a monoclinic P21/c space group with toluene
molecule in the crystal lattice. Also in both cases, eight oxygen
atoms of the four chelating malonate ions are coordinated to the
˚
central Zr ion with the average Zr–O bond length of 2.18 A, which
are comparable to Zr–O bond lengths in [Zr(acac)4] and other
eight coordinated Zr compounds.14,16
However, the coordination geometry around Zr ion in 6 is very
close to square antiprism (Fig. 3) than the geometry in 5. The bite
angles of Zr and malonates are wider in 5 (av. 75.36◦) compared
to the same in 6 (av. 75.13◦). Overall Zr malonate bite angles in 5
and 6 are narrower than Hf malonates 1 and 2. All other structural
features of 5 and 6 are very similar to their Hf analogues 1 and 2.
¯
Compound 7 crystallizes in triclinic space group P1 (Fig. 4).
There are two types of Zr in the crystal lattice, which differ
slightly in their coordination geometry. In each molecule, two di-
tert-butyl malonates and two tert-butoxy ligands are octahedrally
coordinated to the central Zr ion. Two tert-butoxy groups on each
Fig. 2 Molecular structure of [Hf(bsml)4] (4) (hydrogen atoms are
removed for clarity). Selected bond lengths (A) and angles ( ); Hf(1)-O(1)
2.105(2), Hf(1)-O(5) 2.262(2), C(2)-O(1) 1.282(4), C(4)-O(5) 1.250(4),
C(2)-C(3) 1.376(4), C(3)-C(4) 1.400(4), O(1)-Hf(1)-O(5) 76.85(8).
◦
˚
656 | Dalton Trans., 2009, 654–663
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the widening of O–Zr–O angle of the tert-butoxy groups. All these
structural features are very similar to the related six-coordinated
mixed alkoxide-ketoester and mixed amide-malonate compounds
reported by our group earlier.6
Compound 8 crystallizes with toluene in crystal lattice in the
monoclinic space group C2/c forming a distorted octahedral
environment at the Zr centre (Fig. S4†). Since the toluene molecule
was highly disordered, it was squeezed out with the program
Platon 1.13.17 The Zr molecule has C2 symmetry. Zr–O bond
˚
length of siloxy groups (1.949(3) A) is longer than Zr–O bond
˚
˚
length of tert-butoxy groups (1.917–1.928 A with av. 1.922 A) in
7. It shows that Zr–O bond of siloxide in 8 is weak compared to
Zr–O bond of tert-butoxy group in 7. It results in the reduced trans
influence on Zr–O bonds of malonate which are trans to OSiMe3,
Fig. 3 Molecular structure of [Zr(deml)4] (6) (Hydrogen atoms and
˚
toluene molecule are removed for clarity). Selected bond lengths (A)
and angles (◦); Zr(1)-O(45) 2.159(2), Zr(1)-O(15) 2.162(2), Zr(1)-O(31)
2.182(2), Zr(1)-O(35) 2.184(2), Zr(1)-O(41) 2.185(2), Zr(1)-O(25) 2.188(2),
Zr(1)-O(21) 2.188(2), Zr(1)-O(11) 2.214(2), O(31)-Zr(1)-O(35) 75.13(8),
O(45)-Zr(1)-O(41) 75.18(8), O(25)-Zr(1)-O(21) 75.07(8), O(15)-Zr(1)-
O(11) 75.15(7).
˚
and hence its bond length (2.175 A) is shorter than the Zr–O bond
˚
˚
lengths of malonate (2.185–2.218 A with av. 2.198 A) which are
trans to OBut in 7. All other structural features are similar to
compound 7.
Apart from being monomer in solid state, all these compounds
(1–8) retain their mononuclearity in solution too. This was
confirmed by solution state 1H and 13C NMR spectroscopic studies
in C6D6. 1H NMR spectra of dimethyl malonates of Hf (1) and Zr
(5) show only two singlets with the intensity ratio of 6 : 1 (d 3.40,
4.95 ppm respectively for 1 and d 3.45, 4.99 ppm respectively for 5)
corresponding to CH3 and CH of dimethyl malonates respectively
(Fig. S5†). It clearly shows that only one type of dmml is present
in the solution. This fact is also confirmed in 13C NMR spectra, as
they exhibit only three peaks at d 51.31, 68.17 and 175.52 ppm for
1; at d 51.29, 68.39 and 175.28 ppm for 5 corresponding to OCH3,
1
=
CH and C O respectively. H NMR spectra of diethyl malonates
of Hf (2) and Zr (6) exhibit one triplet (at d 1.08 ppm for 2 and at
d 1.07 for 6) for CH3, one quartet (at d 4.16 ppm for 2 and at d
4.11 ppm for 6) for CH2 and one singlet (at d 5.02 ppm for 2 and
at d 4.99 for 6) for CH. 13C NMR spectra show four peaks at d
14.89, 60.33, 68.54 and 175.08 ppm for 2; at d 14.90, 60.30, 68.73
Fig. 4 Molecular structure of [Zr(dbml)2(OBut)2] (7) (Hydrogen atoms
◦
˚
are removed for clarity). Selected bond lengths (A) and angles ( );
Zr(2)-O(51) 1.917(2), Zr(2)-O(61) 1.928(2), Zr(2)-O(21) 2.119(2), Zr(2)-
O(29) 2.122(2), Zr(2)-O(25) 2.185(2), Zr(2)-O(210) 2.218(2), O(51)-Zr(2)-
O(61) 100.9(1), O(51)-Zr(2)-O(21) 97.7(1), O(61)-Zr(2)-O(21) 94.2(1),
O(51)-Zr(2)-O(29) 94.0(1), O(61)-Zr(2)-O(29) 99.3(1), O(21)-Zr(2)-O(29)
160.21(9), O(51)-Zr(2)-O(25) 166.3(1), O(61)-Zr(2)-O(25) 91.6(1), O(21)-
Zr(2)-O(25) 86.91(9), O(29)-Zr(2)-O(25) 78.35(9), O(51)-Zr(2)-O(210)
88.48(9), O(61)-Zr(2)-O(210) 168.96(9), O(21)-Zr(2)-O(210) 78.60(9),
O(29)-Zr(2)-O(210) 85.77(9), O(25)-Zr(2)-O(210) 79.76(9).
=
and 174.83 ppm for 6 corresponding to CH3, CH2, CH and C O
respectively.
As expected for a mononuclear compound, the 1H NMR
spectrum of compound 3 exhibits one singlet at d 1.54 ppm for
CH3 of tert-butyl group and one singlet at d 4.88 ppm for CH
of malonate. The 13C NMR spectrum also confirms this fact as it
shows only four peaks at d 29.66, 73.73, 79.46 and 174.70 ppm
=
corresponding to CH3, OC, CH and C O respectively. There are
two singlets for 1H NMR spectrum of compound 4 (at d 0.37 ppm
Zr ion are cis to each other. Both tert-butyl groups on Zr(1)-O
are disordered. However, there is no such disorder observed in
the Zr(2) based molecules. Zr–O bond lengths for the tert-butoxy
for SiMe3 and at d 4.86 ppm for CH). The 13C NMR spectrum
shows three peaks at d 0.92, 74.80 and 173.44 ppm corresponding
1
=
to SiMe3, CH and C O respectively. The H NMR spectrum of
˚
˚
groups are in the range of 1.917–1.928 A (av. 1.922 A), which are
shorter than the Zr–O bond lengths of malonate ligands (2.119–
compound 7 exhibits three singlets of equal intensity at d 1.42, 1.46
and 1.53 ppm for tert-butyl groups. It indicates that the two tert-
butyl groups on each malonate are different due to the trans effect
of tBuO on Zr and hence gives two singlets. This phenomenon also
supports the trans effect observed in X-ray crystal structure. The
third singlet is due to the tert-butoxy group on Zr. The singlet at
d 4.86 ppm is due to the presence of CH of malonates. This trans
effect is also seen in 13C NMR spectrum, as it shows two peaks at
d 28.96 and 29.05 ppm for CH3 of tert-butyl groups of malonate,
two peaks at d 79.83 and 79.96 ppm for CCH3 of tert-butyl groups
˚
˚
2.218 A (av. 2.162 A)). Among the two sets of Zr–O bond lengths
˚
of malonates on each Zr, one (av. 2.198 A) which is trans to Zr–
t
˚
OBu is longer than other set of Zr–O bond lengths (av. 2.126 A)
due to the trans effect of alkoxides on the metal. All the O–Zr–O
bond angles strongly deviate from the ideal octahedral angle which
varies from 78.4 to 100.9◦ for cis and from 159.1 to 169.0◦ for trans.
Among the cis angles, tBuO-Zr-OBut angles (100.4 and 100.9◦ for
Zr(1) a◦nd Zr (2) respectively) are wider than other cis angles (78.4
to 99.6 ) which could be due to the steric effect of tBu group. Also
the small bite angles of two malonate esters (78.4–79.0◦) facilitate
=
of malonate, two peaks at d 175.66 and 175.85 ppm for C O of
malonates, and one peak at d 71.42 ppm for CH of malonates.13
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Table 1 Melting points of Hf and Zr compounds
Table 2 Overview of fragments of malonate based complexes of Hf and
Zr detected by mass spectrometry
Compound
Mp/◦C
Compound
Mp/◦C
Mass, m/z (relative intensity, %)
[Hf(acac)4]
[Hf(thd)4]
19315,18
[Zr(acac)4]
[Zr(thd)4]
19415,18
>32019
>2606,b
139–140
62
>32019
Fragments
1
2
5
6
[Hf(tbaoac)4]
[Hf(dmml)4] (1)
[Hf(deml)4] (2)
[Hf(dbml)4] (3)
[Hf(bsml)4] (4)
180–21020,a [Zr(tbaoac)4]
139–140
62
194
147–152
M+ - ROC(O)CHC(O)
M+ - L
604 (<1) 702 (1)
573 (2)
514 (1)
657 (43) 483 (8)
612 (1)
567 (97)
[Zr(dmml)4] (5)
[Zr(deml)4] (6)
M+ - L - ROC(O)CHC(O) 473 (2)
M+ - L - 2ROC(O)CHC(O) 373 (2)
M+ - L - 3ROC(O)CHC(O) 273 (1)
543 (100) 383 (26) 453 (100)
429 (69) 283 (31) 339 (82)
[Zr(dbml)2(OBut)2] (7)
197
[Zr(bsml)2(OSiMe3)2] (8) >270
315 (5)
160 (4)
183 (20) 225 (12)
132 (4) 160 (4)
L+
132 (4)
a Sublimes at 180–210 ◦C, 2.5 ¥ 10-4 Torr. b Decomposes without melting
ROC(O)CHC(O)+
101 (100) 115 (90) 101 (100) 115 (93)
at 260 ◦C.
L = dmml and deml.
1
The H NMR spectrum of compound 8 exhibits one broad
M+ - L, M+ - L - ROC(O)CHC(O), M+ - L - 2ROC(O)CHC(O),
M+ - L - 3ROC(O)CHC(O) and L+ROC(O)CHC(O)+ were
observed. It clearly indicates that malonate ligands on metal
decompose into (metal) alkoxide and ROC(O)CHC(O). It also
explains the abundance of ROC(O)CHC(O) ions in the spec-
tra.Table 2 gives an overview of the fragments observed in the
spectra for dimethyl and diethyl malonates of Hf and Zr. This
shows that the decomposition mechanism here is different from
thd-based Hf and Zr compounds. During fragmentation in thd
based Hf and Zr compounds, diketone part (RC(O)CHC(O)) of
RC(O)CHC(O)R is intact with the metal.7,23 In malonate based
Hf and Zr compounds, the diketone part (ROC(O)CHC(O)) of
ROC(O)CHC(O)OR is not intact with metal, but only the “OR”
part of ROC(O)CHC(O)OR is intact with metal.
The EI mass spectrum of compound 7 clearly shows the
decomposition of each tert-butyl group into isobutene gas. A
schematic view of proposed mechanism of fragmentation of
[Zr(dbml)2(OBut)2] is shown in the Scheme 4. It shows a molecular
ion peak at m/z 666. This is followed by stepwise loss of two
isobutene molecules as there are two consecutive peaks at m/z 610
(a) and 554 (b).24 These two isobutene molecules are most probably
from two tert-butoxy groups on the metal as the following step
is the elimination of water.25 It is more likely (or more feasible)
that two hydroxyl groups on the metal, which are cis to each
other, condense to eliminate water than two hydroxyl groups on
malonates.7
signal at d 0.31 ppm corresponding to the Me3Si on malonates,
followed by one sharp singlet of half of its intensity due to
the presence of Me3SiO on Zr (Fig. S6†). As expected, CH of
malonates shows one singlet at 5.00 ppm. The 13C NMR spectrum
shows two peaks at d 0.33 and 2.61 ppm for SiMe3, one peak at
=
d 73.91 ppm for CH and one peak at d 175.66 ppm for C O. These
data indicate the presence of only one type of malonate ligands and
also supports the reduced trans influence of OSiMe3 on malonates
as discussed in the crystal structure description. The above data
confirm the mononuclearity of compounds 7 and 8 in solution.
The melting points of all the compounds were measured in
sealed capillaries (listed in Table 1) and compared with melting
points reported for the classical b-diketonates of Hf and Zr. It
was observed that the melting points of the new compounds are
relatively lower compared to the related analogues known in the
literature, especially with the methyl and ethyl malonate ligands.
For example, the difference between [Zr(acac)4] and [Zr(dmml)4]
is only the substitution on carbonyl group. In [Zr(acac)4], methyl
group is bonded to carbonyl group, whereas in [Zr(dmml)4],
methoxy group is bonded to carbonyl group. However, the melting
point of [Zr(dmml)4] (139–140 ◦C) is much lower compared to the
same for [Zr(acac)4] (194 ◦C). The same trend also exists between
[Hf(acac)4] and [Hf(dmml)4], and [Hf(thd)4] and [Hf(dbml)4].
Apart from compound 8, all the other compounds melted at
temperatures below 200 ◦C. A striking observation is that the
melting point values of the [Hf(deml)] (2) and [Zr(deml)] (6) are
as low as 62 ◦C. This shows that a slight change in the substituent
on the malonates results in a drastic change in the melting point.
This could be due to the difference in packing of the molecule in
the crystal lattice.
EI mass spectra were recorded for the compounds 1, 2, 5, 6
and 7. These data provide information about the fragmentation
characteristics of the complexes under gas phase ionization
conditions. The utilization of mass spectral data to predict the
possible mechanism for CVD processes has been previously
established.21 As has been discussed in the literature, care must
be taken in extrapolating any fragmentation information from
mass spectral data to the CVD process since the latter is thermally
induced.22 Nevertheless, mass spectral data have provided insights
into probable fragmentation patterns of the Hf and Zr malonate
precursors.
Hence, the peak at m/z 537 could be assigned to [Zr(O)(dbml)2]+
(c). The following four steps (at m/z 480, 424, 368 and
313) could be due to the elimination of four isobutene
molecules (through McLafferty rearrangement) from four tert-
butyl groups on malonates.12,13,20,26 This leads to the formation of
[Zr(O){HO2CCHCO2H}2] (at m/z 313) (d). The highest intensity
peak (base peak) corresponds to isobutene molecular ion at m/z
57. The overview of fragments of compound 7 is shown in Table 3.
Thermal characterization
In order to assess the suitability of the Hf and Zr malonate
complexes for MOCVD, the thermal characteristics were studied
by thermogravimetric (TG) analysis and the results are shown
in Fig. 5. The homoleptic Hf methyl (1) and ethyl malonates
(2) volatalise in the temperature range of 50–150 ◦C. This is
followed by two shoulders at 187 ◦C (75 weight%) and 334 ◦C
Dimethyl and diethyl malonates of Hf and Zr (1, 2, 5 and
6) exhibit similar fragmentation pattern. None of these four
compounds shows a molecular ion peak. However, peaks corre-
sponding to the subsequent fragments viz. M+ - ROC(O)CHC(O),
◦
(50 weight%) for compound 1, and at 225 C (65 weight%) and
◦
277 C (50 weight%) for compound 2. A final residue of around
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Scheme 4 Proposed mechanism of fragmentation of [Zr(dbml)2(OtBu)2] (7).
Table 3 Overview of fragments of compound 7 detected by mass
reduced temperatures in these Hf and Zr complexes compared
to the classical b-diketonates. Detailed mechanistic studies are
necessary to verify these claims on precursor decomposition
mechanism.
Isothermal thermogravimetric measurements at atmospheric
pressure were carried out for compounds 1 and 5. From the
isothermal curves (Fig. 6), it was observed that [Hf(dmml)4] (1)
and [Zr(dmml)]4 (5) sublime at a constant and appreciable rate
(2.27 mg min-1 for compound 1 and 5.06 mg min-1 for compound
5) for a long period (400 min) at 100 ◦C. From the thermal analysis,
it can be inferred that the compounds are volatile. The linearity in
the isothermal TG curves indicates mass transport behaviour of
these compounds which is desirable for the MOCVD process.
spectrometry
Fragments
Mass, m/z (relative intensity, %)
M+
666 (3)
610 (6)
554 (3)
537 (8)
480 (26)
424 (34)
368 (44)
313 (9)
57 (100)
M+ - isobutene
M+ - 2isobutene
M+ - 2isobutene - H2O
M+ - 3isobutene - H2O
M+ - 4isobutene - H2O
M+ - 5isobutene - H2O
M+ - 6isobutene - H2O
Isobutene
40% for compound 1 and 35% for compound 2 is left behind.
For compound 3, the volatilisation begins at higher temperature
(>150 ◦C), but the residue left behind is much lower (25%).
Similar trends were observed in the evaporation behaviour of Zr
compounds 5–7 (Fig. 5). Higher residues were obtained for the
Zr methyl (5) and ethyl (6) malonates compared to the mixed
alkoxide-butyl malonates (7).
A precise analysis of the residues hasn’t been carried out,
however, the steps in the TG curves could be due to the stepwise
loss of the different terminal alkyl groups present in the malonates.
As discussed earlier, the cleavage points in the ligand structure
of malonates could facilitate decomposition reactions at much
MOCVD and characterisation of HfO2 and ZrO2 thin films
The new Hf and Zr complexes have suitable thermal properties
as discussed above (low melting point, low decomposition tem-
perature, can be sublimed quantitatively, sufficient temperature
window between volatilisation and decomposition). Therefore, it
was interesting to test these compounds for thin film deposition
by MOCVD and, as a representative example, the homoleptic
[Hf(dmml)4] (1) was tested for MOCVD of HfO2. In the case
of Zr, the mixed alkoxide-malonate, [Zr(dbml)2(OBut)2] (7) was
used for MOCVD of ZrO2. The HfO2 and ZrO2 films grown
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Fig. 5 Thermogravimetric analysis curves for the Hf complexes {[Hf(dmml)4] (1), [Hf(deml)4] (2) and [Hf(dbml)4] (3)} and Zr complexes {[Zr(dmml)4]
(5), [Zr(deml)4] (6) and [Zr(dbml)2(OBut)2] (7)}.
Fig. 7 XRD patterns of HfO2 (a) and ZrO2 (b) films deposited from
[Hf(dmml)4] (1) and [Zr(dbml)2(OBut)2] (7) respectively at 600 ◦C.
Fig. 6 Isothermal curves for [Hf(dmml)4] (1) and [Zr(dmml)4] (5) at
100 ◦C.
at temperatures below 500 ◦C were amorphous. XRD analysis
revealed that the crystallinity of the films was increased with
the increase in substrate temperature. The crystalline films of
HfO2 and ZrO2 stabilised in the monoclinic phase (JCPDS card
00–043–1017 for HfO2 and 00–037–1484 for ZrO2) as shown in
Fig. 7. Similar observations of these materials crystallising in
the monoclinic phase have been reported using other Hf and Zr
precursors in the same temperature range.6,27
The surface morphology of the as-deposited films was analysed
by SEM (Fig. 8). There were no distinct surface features on the
films grown at temperatures lower than 500 ◦C confirming the
amorphous nature of the films obtained.
The SEM micrograph of an HfO2 film grown at 700 ◦C (Fig. 8a)
shows the formation of fine grains and Fig. 8b shows the cross
Fig. 8 SEM images of HfO2 (a and b) and ZrO2 (c and d) films deposited
section of the film (thickness = 60 nm), which reveals the densely
packed structure of the film tending towards columnar growth.
Similar features were observed in the case of ZrO2 (Fig. 8c and d),
however at the same temperature longitudinal ZrO2 grains and the
columnar growth was much more clearly evident (film thickness =
360 nm). A detailed study on the deposition and characterization
from [Hf(dmml)4] (1) and [Zr(dbml)2(OBut)2] (7) respectively at 700 ◦C.
of HfO2 and ZrO2 films grown varying different CVD process
conditions is currently underway. It should be noted that all the
compounds reported here are stable in solution and show a high
degree of solubility in commonly used organic solvents. Therefore
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it will be interesting to test them for liquid injection MOCVD
applications.
sensitive detector (PSD). All the films were analysed in the q–2q
geometry. The morphology of the film was studied using a LEO
Gemini SEM 1530 electron microscope.
Summary and conclusions
Synthesis of [Hf(dmml)4] (1)
A series of new Hf and Zr precursors with different chelating
malonate ligands (compounds 1–8) were synthesised in high
yields. The Hf and Zr malonate compounds are monomeric. The
thermal properties of the precursors are suitable for MOCVD
applications. Preliminary MOCVD experiments are carried out
to deposit thin films of HfO2 using [Hf(dmml)4] (1) and ZrO2
films using [Zr(dbml)2(OBut)2] (7). The HfO2 and ZrO2 films
grown at temperatures above 500 ◦C crystallize in the monoclinic
phase and exhibit columnar grain growth. We have demonstrated
that the simple and straightforward synthetic route resulted in
significant improvement in the MOCVD precursor development
for HfO2 and ZrO2. The new Hf and Zr compounds have improved
thermal properties compared to the classical b-diketonate class of
compounds like [M(acac)4] and [M(thd)4]. A detailed MOCVD
studies using these compounds are currently in progress.
To the solution of [Hf(NEt2)4] (0.467 g, 1 mmol) in 20 ml of diethyl
ether, Hdmml (0.528 g, 4 mmol) in 20 ml of diethyl ether was added
drop wise over a period of 1 h at room temperature. After stirring
for 24 h at ambient conditions, the solvent was removed under
reduced pressure yielding a white solid. The resulting solid was
dissolved in toluene and kept at 5 ◦C for 24 h to afford colorless
crystals suitable for single crystal X-ray analysis. Yield: 0.633 g
(90%). Mp: 139–140 ◦C. Elemental analysis: calc. for C20H28O16Hf:
C 34.17%, H 4.01%; found: C 33.92%, H 4.06%. 1H NMR (room
temp., 200 MHz, C6D6): dH 3.40 (24H, s, CH3), 4.95 (4H, s,
CH). 13C NMR (room temp., 200 MHz, C6D6): dC 51.31 (OCH3),
68.17 (CH), 175.52 (OCO). EI-MS (70 eV): 604, <1% (M+
-
MeOC(O)CHC(O)); 573, 2% (M+ - dmml); 473, 2% (M+ - dmml -
MeOC(O)CHC(O)); 373, 2% (M+ - dmml -2MeOC(O)CHC(O));
273, 1% (M+ - dmml - 3MeOC(O)CHC(O)); 132, 4% (Hdmml);
101, 100% (MeOC(O)CHC(O)).
Experimental
All reactions were performed using a conventional vacuum/argon
line with standard Schlenck techniques. Samples for all analyses
were prepared in argon filled glove boxes. Elemental analyses were
performed by the analytical service center at faculty of Chemistry
at Ruhr-Universita¨t Bochum (CHNSO Vario EL 1998). Electronic
ionization (EI) mass spectra were recorded using a Varian MAT
Synthesis of [Hf(deml)4] (2)
[Hf(NEt2)4] (0.467 g, 1 mmol) was dissolved in 20 ml of diethyl
ether. To this solution, four equivalents of Hdeml (0.641 g, 4 mmol)
in 20 ml of diethyl ether was added drop wise over a period of 1 h
at room temperature. The resultant solution was stirred for 24 h
at ambient conditions and then the solvent was removed under
vacuum. The remaining sticky solid was dissolved in toluene and
kept at -30 ◦C for 24 h to afford colorless crystals suitable for single
crystal X-ray analysis. Yield: 0.720 g (88%). Mp: 62 ◦C. Elemental
analysis: calc. for C28H44O16Hf: C 41.26%, H 5.44%; found: C
41.39%, H 4.98%. 1H NMR (room temp., 200 MHz, C6D6): dH 1.08
(24H, t, CH3), 4.16 (16H, q, CH2), 5.02 (4H, s, CH). 13C NMR
(room temp., 200 MHz, C6D6): dC 14.89 (CH3), 60.33 (OCH2),
1
spectrometer supplied with an ionizing energy of 70 eV. H and
13C NMR spectra were recorded on the Bruker Advance DRX-
200. Thermal analysis was carried out using a Seiko TG/DTA
6300S11 at ambient pressure (Sample weight ~10 mg, heating
◦
rate 5 C min-1). ZrCl4 (Alfa Aesar), HfCl4 (Alfa Aesar), n-
BuLi (Merck), HNEt2 (Fluka), dimethyl malonate (Acros), diethyl
malonate (Janssen Chimica), di-tert-butyl malonate (Fluka) and
bis(trimethylsilyl) malonate (Fluka) were used as received. The
Zr and Hf amide compounds [Zr(NEt2)4] and [Hf(NEt2)4] were
synthesized according to a previously reported procedure.28 Crys-
tallographic data were collected on a Xcalibur 2 Oxford using
68.54 (CH), 175.08 (OCO). EI–MS (70 eV): 702, 1% (M+
-
EtOC(O)CHC(O)); 657, 43% (M+ - deml); 543, 100% (M+ - deml -
EtOC(O)CHC(O)); 429, 69% (M+ - deml -2EtOC(O)CHC(O));
315, 5% (M+ - deml - 3EtOC(O)CHC(O)); 160, 4% (Hdeml); 115,
90% (EtOC(O)CHC(O)).
˚
graphite monochromated Mo-Ka radiation (l = 0.71073 A). The
structure was solved using the SHELXL-97 software package and
refined by full matrix least-squares methods based on F2 with all
observed reflections.
Synthesis of [Hf(dbml)4] (3)
MOCVD of HfO2 and ZrO2
A solution of Hdbml (0.865 g, 4 mmol) in 20 ml of toluene was
added drop wise to a stirred solution of [Hf(NEt2)4] (0.467 g,
1 mmol) in 20 ml of toluene over a period of 1 h at room
temperature. After stirring for 5 h at 110 ◦C, the solvent was
removed under reduced pressure yielding a sticky solid. The
obtained solid was dissolved in toluene/acetonitrile mixture and
kept at room temperature for 7 days to afford colorless crystals
suitable for single crystal X-ray analysis. Yield: 0.865 g (83%). Mp:
The screening of the metal-organic precursors was carried out
using a self-built horizontal cold wall CVD reactor operating at
reduced pressure.29 Nitrogen (purity 6.0, flow rate: 50 sccm) and
oxygen (purity 4.8, flow rate: 50 sccm) were used as carrier gas and
oxidant respectively. The flow rates were controlledusing mass flow
controllers (MKS). The films were grown on Si(100) substrates,
which were cleaned prior to deposition. The substrate temperature
was varied between 400–700 ◦C and the temperature of the
precursor bubbler was maintained at 100 ◦C. The reactor pressure
was set at 1.00 mbar. The as-grown films were characterised
by X-ray diffraction (XRD) using a Bruker D8 Advance AXS
◦
194 C. Elemental analysis: calc. for C44H76O16Hf: C 50.84%, H
1
7.37%; found: C 50.36%, H 7.32%. H NMR (room temp., 200
MHz, C6D6): dH 1.54 (72H, s, CH3), 4.88 (4H, s, CH). 13C NMR
(room temp., 200 MHz, C6D6): dC 29.66 (CH3), 73.73 (C–O), 79.46
(CH), 174.70 (OCO).
˚
diffractometer with Cu-Ka radiation (1.5481 A) with a position
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Synthesis of [Hf(bsml)4] (4)
stirring for 5 h at 110 ◦C, the solvent was removed under reduced
pressure yielding a white solid. The resulting solid was dissolved in
toluene/acetonitrile mixture and kept at 5 ◦C for 7 days to afford
colorless crystals suitable for single crystal X-ray analysis. Yield:
0.5 g (75% based on [Zr(NEt2)4]). Mp: 197 ◦C. Elemental analysis:
calc. for C30H56O10Zr: C 53.94%, H 8.45%; found: C 53.32%, H
To the diethyl ether (20 ml) solution of [Hf(NEt2)4] (0.467 g,
1 mmol), four equivalents of Hbsml (0.994 g, 4 mmol) in 20 ml
of diethyl ether was added drop wise over a period of 1 h at
room temperature. The resultant solution was stirred for 24 h at
room temperature. Removal of the solvent under reduced pressure
yielded a slightly sticky solid which was dissolved in toluene and
kept at -30 ◦C for 24 h to afford colorless crystals suitable for
single crystal X-ray analysis. Yield: 0.92 g (79%). Mp: 147–152 ◦C.
Elemental analysis: calc. for C36H76O16Si8Hf: C 37.01%, H 6.56%;
1
9.98%. H NMR (room temp., 200 MHz, C6D6): dH 1.42 (18H,
s, CH3), 1.46 (18H, s, CH3) 1.53 (18H, s, CH3) 4.86 (2H, s, CH).
13C NMR (room temp., 200 MHz, C6D6): dC 28.96 (CH3 of mal),
29.05 (CH3 of mal), 32.91(CH3 of tBuO), 71.42 (CH of mal), 75.77
(C–CH3 of tBuO), 79.83 (C–CH3 of mal), 79.96 (C–CH3 of mal)
175.66 (OCO), 175.85 (OCO). EI-MS (70 eV): 666, 3% (M+); 610,
6% (M+ - isobutene); 554, 3% (M+ - 2isobutene); 537, 8% (M+ -
2isobutene - H2O); 480, 26% (M+ - 3isobutene - H2O); 424, 34%
(M+ - 4isobutene - H2O); 368, 44% (M+ - 5isobutene - H2O);
313, 9% (M+ - 6isobutene - H2O); 57, 100% (isobutene).
1
found: C 36.39%, H 6.10%. H NMR (room temp., 200 MHz,
C6D6): dH 0.37 (72H, s, CH3), 4.86 (4H, s, CH). 13C-NMR (room
temp., 200 MHz, C6D6): dC 0.92 (CH3), 74.80 (CH), 173.44 (OCO).
Synthesis of [Zr(dmml)4] (5)
A solution of Hdmml (0.528 g, 4 mmol) in diethyl ether (20 ml)
was added drop wise to a stirred solution of [Zr(NEt2)4] (0.380 g,
1 mmol) in 20 ml of diethyl ether over a period of 1 h at room
temperature. After stirring for 24 h at ambient conditions, the
solvent was removed under reduced pressure yielding a white
solid. The solid obtained was dissolved in toluene and kept at
5 ◦C for 24 h to afford colorless crystals suitable for single crystal
X-ray analysis. Yield: 0.585 (95%). Mp: 139–140 ◦C. Elemental
analysis: calc. for C20H28O16Zr: C 39.02%, H 4.58%; found: C
Synthesis of [Zr(bsml)2(OSiMe3)2] (8)
Four equivalents of Hbsml (0.994 g, 4 mmol) in 20 ml of toluene
was added drop wise over a period of 1 h to a solution of
[Zr(NEt2)4] (0.380 g, 1 mmol) in 20 ml of toluene at room
temperature. After stirring for 5 h at 110 ◦C, the solvent was
removed under reduced pressure yielding a◦pale yellow solid which
was dissolved in toluene and kept at -30 C for 7 days to afford
colorless crystals suitable for single crystal X-ray analysis. Yield:
0.48 g (63% based on [Zr(NEt2)4]). Mp: >270 ◦C. Elemental
analysis: calc. for C24H56O10Si6Zr: C 37.71%, H 7.38%; found: C
1
38.96%, H 4.56%. H NMR (room temp., 200 MHz, C6D6): dH
3.45 (24H, s, CH3), 4.99 (4H, s, CH). 13C NMR (room temp.,
200 MHz, C6D6): dC 51.29 (OCH3), 68.39 (CH), 175.28 (OCO).
EI–MS (70 eV): 514, 1% (M+ - MeOC(O)CHC(O)); 483, 8%
(M+ - dmml); 383, 26% (M+ - dmml - MeOC(O)CHC(O));
283, 31% (M+ - dmml - 2MeOC(O)CHC(O)); 183, 20% (M+ -
dmml - 3MeOC(O)CHC(O)); 132, 4% (Hdmml); 101, 100%
(MeOC(O)CHC(O)).
1
30.38%, H 6.50%. H NMR (room temp., 200 MHz, C6D6): dH
0.31 (36H, s(br), CH3), 0.34 (18H, s, CH3) 5.00 (2H, s, CH). 13C
NMR (room temp., 200 MHz, C6D6): dC 0.33 (CH3), 2.61 (CH3),
73.91 (CH), 175.66 (OCO).
Acknowledgements
Synthesis of [Zr(deml)4] (6)
The authors are grateful to the German Science Foundation
for funding this work (DFG-CVD-SPP-1119-DE-390–3). One of
the authors (RP) thanks the Alexander von Humboldt (AvH)
foundation for the fellowship.
Hdeml (0.641 g, 4 mmol) in 20 ml of diethyl ether was added
dropwise to a solution of [Zr(NEt2)4] (0.380 g, 1 mmol) in 20 ml
of diethyl ether over a period of 1 h at room temperature. After
stirring for 24 h at ambient conditions, the solvent was removed
under reduced pressure yielding a white solid. The resulting
solid was dissolved in toluene and kept at -30 ◦C for 24 h,
which yielded colorless crystals suitable for single crystal X-ray
analysis. Yield: 0.68 g (93%). Mp: 62 ◦C. Elemental analysis:
calc. for C28H44O16Zr: C 46.20%, H 6.09%; found: C 46.28%,
H 5.37%. 1H NMR (room temp., 200 MHz, C6D6): dH 1.07
(24H, t, CH3), 4.11 (16H, q, CH2) 4.99 (4H, s, CH). 13C NMR
(room temp., 200 MHz, C6D6): dC 14.90 (CH3), 60.30 (OCH2),
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