Metal-organic chemical vapour deposition of lead scandium tantalate: Chemical issues and precursor selection

Article abstract of DOI:10.1016/S0277-5387(99)00365-4

MOCVD is a promising technique for the deposition of the pyroelectric oxide lead scandium tantalate, Pb(Sc_0.5Ta_0.5)O₃. In order to exploit the full potential of the method, it is important to identify the optimum combination of precursors so that process parameters and film properties are optimised. In this paper, issues involved in the selection of suitable Pb, Sc and Ta precursors are discussed and the molecular design of new Ta and Sc sources is described. It is shown how the use of carefully matched precursors allows the growth of Pb(Sc_0.5Ta_0.5)O₃ in the required perovskite phase at low substrate temperatures. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00365-4

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 351–355
Metal–organic chemical vapour deposition of lead scandium tantalate:
chemical issues and precursor selection
a
b
b
Anthony C. Jones a, , Timothy J. Leedham , Hywel O. Davies , Kirsty A. Fleeting ,
*
Paul O’Brien 1,b, Michael J. Crosbie c, Peter J. Wright c, Dennis J. Williams c, Penelope A. Lane c
a Inorgtech Limited, 25 James Carter Road, Mildenhall, Suffolk IP28 7DE, UK
b Department of Chemistry, Imperial College of Science Technology and Medicine, London SW7 2AZ, UK
c Defence Evaluation and Research Agency, St. Andrews Road, Great Malvern, Worcestershire WR14 3PS, UK
Received 2 May 1999; accepted 16 June 1999
Abstract
MOCVD is a promising technique for the deposition of the pyroelectric oxide lead scandium tantalate, Pb(Sc0.5Ta0.5)O3. In order to exploit
the full potential of the method, it is important to identify the optimum combination of precursors so that process parameters and film properties
are optimised. In this paper, issues involved in the selection of suitable Pb, Sc and Ta precursors are discussed and the molecular design of
new Ta and Sc sources is described. It is shown how the use of carefully matched precursors allows the growth of Pb(Sc0.5Ta0.5)O3 in the
required perovskite phase at low substrate temperatures.
q2000 Elsevier Science Ltd All rights reserved.
Keywords: Metal–organic chemical vapour deposition; Precursors; Lead scandium tantalate; Ferroelectric oxides
1. Introduction
tures in the region of 5008C, to prevent degradation of the
underlying silicon circuitry and metal interconnects.
The most accessible and widely used precursors for fer-
roelectric oxide MOCVD are metal alkoxides and b-diketon-
ates [9], which generally have quite low equilibrium vapour
pressures, typically in the order of 1 Torr at 2008C. In order
to overcome this inherent involatility, liquid injection
MOCVD, in which a solution of the precursor is injected into
a heated evaporator, is now widely used [10,11]. This
approach minimises thermal degradation of precursors, but
leads to the added requirements that the precursor must be
stable for long periods in solution, and must not react with
other precursors in the solution. Precursor solutions should
also evaporate at a common temperature without decom-
position, otherwise blockages will eventually occur in the
evaporator. Ideally, the separate precursors should also
decompose to deposit oxide film in a similar temperature
regime in order to optimise film uniformity. Althoughaccept-
able MOCVD performance and high quality ferroelectric
films can often be obtained using existing precursors, in the
case of more complex oxides it is sometimes necessary to
‘tailor’ the precursor to improve compatibility with other
precursors and to optimise process parameters such as pre-
cursor evaporation temperature and oxide deposition temper-
ature. In this paper we discuss one approach to the selection
Thin films of the ferroelectric oxides Pb(Zr,Ti)O3,
Pb(Mg,Nb)O3 and Pb(Sc0.5Ta0.5)O3 have important appli-
cations in pyroelectric [1], and piezoelectric devices [2].
Various techniques have been used for the deposition of such
oxides, including reactive sputtering [3], sol–gel deposition
[4,5] and metal–organic chemical vapour deposition
(MOCVD) [6–8]. Of these techniques, MOCVD offers the
most flexible approach, having the advantages of large area
deposition capability, good composition control and film uni-
formity, high film densities and deposition rates, and excel-
lent conformal step coverage at device dimensions less than
2 mm, which is particularly important in microelectronics
applications.
An essential requirement for successful MOCVD is the
availability of suitable precursors with the appropriate phys-
ical properties for transport and reactivity for deposition.
There must be an adequate temperature window between
precursor vaporisation and decomposition, and for most elec-
tronics applications oxide deposition is limited to tempera-
* Corresponding author. Fax: q44-163-871-8259; e-mail: tony@
inorgtech.co.uk
1 Also corresponding author.
0277-5387/00/$ - see front matter q2000 Elsevier Science Ltd All rights reserved.
PII S0277-5387(99)00365-4
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A.C. Jones et al. / Polyhedron 19 (2000) 351–355
Fig. 1. Molecular structures of some lead, scandium and tantalum compounds used in this study. (1) Pb(thd)2, (2) Ta(OR)4(thd), (3) Sc(thd)3, (4,5)
Sc(tmod)3. In the solid state Sc(tmod)3 exists as a mixture of meridional and facial isomers (4) and (5). In solution the statistically preferred facial isomer
predominates.
and design of precursors for the deposition of Pb(Sc0.5
-
acceptable PbO growth rates at substrate temperatures above
4008C, with maximum growth rates occurring in the region
450–5008C [11]. However, at evaporator temperatures
greater than 2508C, Pb(thd)2 begins to decompose to deposit
an oxide residue, and at substrate temperatures greater than
5008C desorption of PbO from the substrate occurs. There-
fore, for the growth of Pb(Sc0.5Ta0.5)O3 with good unifor-
mity, it is necessary to identify the Sc and Ta sources which
most closely match the evaporation and deposition charac-
teristics of Pb(thd)2.
Ta0.5)O3 by liquid injection MOCVD. Pb(Sc0.5Ta0.5)O3 is a
sensitive pyroelectric oxide with a high figure of merit which
hasapplicationsin uncooledthermalimaging,includingnight
sight technologies and fire detection [12].
2. Precursor selection
2.1. Pb precursors
2.2. Sc precursors
Although PbO and Pb(Zr,Ti)O3 can be deposited using
lead alkyls or alkoxides such as tetraethyllead, PbEt4, lead
tert-butoxide, Pb(OBut)2, and triethyllead neopentoxide,
Et3Pb(OCH2C(CH)3), their high toxicity and reactivity,
coupled with their high volatility, make them difficult to
handle and extremely hazardous to use in liquid injection
MOCVD.
In contrast, lead b-diketonatesarelesstoxicandhavelower
vapour pressures, which minimises the toxic release hazard.
Therefore the following precursors have all been used in
MOCVD: Pb(thd)2 (thds2,2,6,6-tetramethyl-3,5-heptane-
dionate), Pb(acetylacetonate)2, Pb(hexafluoroacetyl-
acetonate)2, Pb(fod)2 (fods1,1,1,2,2,3,3-heptafluoro-
7,7-dimethyloctane-4,6-dionate) [6]. The fluorine-contain-
ing compounds Pb(hfac)2 and Pb(fod)2 can lead to high
levels of fluorine contamination in the oxide films, decom-
posing above 3008C to form a PbF2 phase [6]. Consequently,
the most volatile of the non-fluorine containing precursors
Pb(thd)2 is at present the precursor ofchoicefortheMOCVD
of lead oxide based ferroelectrics Pb(Zr,Ti)O3 and
Pb(Sc0.5Ta0.5)O3.
The choice of Sc precursor is more limited than that of Pb,
with the available Sc sources beingrestrictedtob-diketonates
such as Sc(thd)3. This molecule is monomeric [14] with
octahedral coordination at the metal atoms, see Fig. 1, and it
deposits oxide films over a conveniently wide temperature
regime of 450 to at least 6008C (see Fig. 2), showing rea-
sonable overlap with the Pb(thd)2 oxide deposition zone
(450–5008C) [15]. However, Sc(thd)3 is significantly less
Pb(thd)2 is a monomeric molecule [13], see Fig. 1, which
gives it a reasonably high vapour pressure for a b-diketonate
compound of 0.1 Torr at 1288C. In liquid injection MOCVD
[11], an evaporator temperature of 2008C is sufficient togive
Fig. 2. Variation in oxide growth rate with substrate temperature for
Sc(thd)3, Sc(tmod)3 and Sc(mhd)3.
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353
volatile than Pb(thd)2, and consequently the evaporator tem-
perature must be approximately 3008C to obtain adequate
Sc2O3 growth rates. This can lead to decomposition of the
Pb(thd)2 and the Ta alkoxide precursors, and so a more
volatile Sc oxide source than Sc(thd)3 would be very
desirable.
Asymmetric b-diketonate ligands R9C(O)CHC(O)R0,
containing two different R groups, such as tmod (2,2,7-tri-
methyloctane-3,5-dionate) and mhd (6-methylheptane-2,4-
dionate) (see Fig. 3) have been shown to influence
significantly the volatility of yttrium b-diketonates[16]. The
lack of symmetry and increased disorder associated with the
tmod and mhd ligands can be expected to disrupt the molec-
ular packing of the metal b-diketonates, thereby increasing
the precursor vapour pressure [16]. We have therefore in-
vestigated the sterically hindered scandium b-diketonates
Sc(tmod)3 and Sc(mhd)3 as alternatives to Sc(thd)3, and a
schematic of the Sc(tmod)3 structure is shown in Fig. 1. TGA
data for the various Sc b-diketonates, shown in Fig. 4, indi-
cate that Sc(tmod)3 and Sc(mhd)3 both evaporate at lower
temperature than Sc(thd)3. The residues of between 5 and
10 wt.% remaining after the evaporation of Sc(tmod)3 and
Sc(mhd)3 are not intrinsic features, and arise from slight
sample decomposition due to the presence of trace oxygen
and moisture during analysis of very small samples (ca. 10
mg).
Fig. 5. Variation in growth rate with substrate temperature for Ta alkoxides:
(a) Ta(OEt)5, (b) Ta(OMe)4(thd), (c) Ta(OEt)4(thd), (d)
Ta(OPri)4(thd).
particular, the new Sc(tmod)3 source looks to be ideally
suited to the MOCVD of Pb(Sc0.5Ta0.5)O3, however the
tmodH ligand is significantly more expensive than thdH
which may somewhat limit the utility of Sc(tmod)3 as an
alternative to Sc(thd)3.
2.3. Ta precursors
The selection of a suitable Ta source presents a major
challenge in the MOCVD of Pb(Sc0.5Ta0.5)O3. There are no
tantalum b-diketonates available, and the simple alkoxide
precursorTa(OEt)5 hasasignificantlylowerthermalstability
than Sc(thd)3, which leads to oxide deposition in a different
temperature regime, and also gives rise to decomposition of
Ta(OEt)5 in the evaporator and reactor inlet pipes at the
temperatures used to evaporate Sc(thd)3.
Both Sc(tmod)3 and Sc(mhd)3 were shown to deposit
oxide over a wide temperature range (Fig. 2), fully compat-
ible with the oxide deposition temperature from Pb(thd)2. In
However, the thermal stability of tantalum alkoxide pre-
cursors can be increased by the insertion of b-diketonate
groups, or by altering the alkoxide group. For instance, the
oxide deposition temperature is higher for Ta(OEt)4(thd)
than for Ta(OEt)5 and increases in the order
Ta(OPri)4(thd)-Ta(OEt)4(thd)-Ta(OMe)4(thd) [17],
as shown by the growth rate data in Fig. 5. These precursors
are monomeric (Fig. 1) and are less air sensitive than the
parent alkoxides. It has also been shown that Ta(OR)4(thd)
compounds undergo significantly less decomposition than
Ta(OEt)5 in the evaporator at temperatures greater than
2008C [17]. Therefore Ta(OEt)4(thd) and Ta(OMe)4(thd)
can be selected as most compatible with Sc(thd)3 and
Pb(thd)2 for Pb(Sc0.5Ta0.5)O3 growth.
Fig. 3. b-diketone ligands used to influence the volatility of Sc(b-diketon-
ate)3 complexes.
3. MOCVD of Pb(Sc0.5Ta0.5)O3
The application of Pb(Sc0.5Ta0.5)O3 in pyroelectric
devices has been restricted by the high temperatures
()10008C) usually required to synthesise the desired per-
ovskite phase and to eliminate the pyrochlore phase, with its
poor dielectric properties, which tends to form at low tem-
perature. Although Pb(Sc0.5Ta0.5)O3 in the perovskite phase
has been grown at low temperature by sol-gel techniques
Fig. 4. Typical TGA data for some tris-b-diketonates of scandium: (a)
[Sc(thd)3], (b) [Sc(mhd)3], (c) [Sc(tmod)3]. Sample weight ca. 10 mg,
Pt cell, argon atmosphere, heating rate 208C miny1
.
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A.C. Jones et al. / Polyhedron 19 (2000) 351–355
[18,19], there have been relatively few reports of successful
MOCVD studies. Early studies [20] reported that a process-
ing temperature of 8008C and a post growth anneal were
required to obtain the perovskite phase. More recent studies
[21] have shown that Pb(Sc0.5Ta0.5)O3 in the perovskite
phase can be grown in one step at temperatures as low as
6008C. However, in both of these studies conventional
MOCVD was employed, in which the Pb, Sc and Ta precur-
sors were held in bubblers at relatively high source temper-
atures, leading to the possibilityofsourcedecompositionwith
time.
In order to avoid source decomposition we have employed
liquid injection MOCVD, and Pb(Sc0.5Ta0.5)O3 films have
been successfully grown at substrate temperaturesofbetween
400 and 6008C using a single solution of Pb(thd)2, Sc(thd)3
and Ta(OEt)4(thd), dissolved in tetrahydrofuran (THF)
[22]. This precursor combination was selected initially as
the best compromise between precursor availability and cost
and MOCVD performance.
Fig. 6. X ray diffraction data for Pb(Sc0.5Ta0.5)O3 grown at 6008C using
Pb(thd)2, Sc(thd)3 and Ta(OEt)4(thd). Pe diffraction peaks from perov-
skite phase, Pt diffraction peaks from platinum substrate.
sublimation. Each compound was fully characterised by 1H
NMR, 13C{1H} NMR, massspectroscopy,andmicroanalysis,
and full details are given elsewhere [14].
At substrate temperatures of 5008C and below the material
was found to be a mixture of the perovskite and pyrochlore
phases. Increasing the substrate temperature led to increasing
amounts of the perovskite phase, and at 6008C the perovskite
phase was found to be predominant (see Fig. 6). These low
growth temperatures are fully compatible with device fabri-
cation requirements, indicating that the use of liquid injection
MOCVD techniques, together with the use of carefully
selected precursors, should lead to increased exploitation of
this important technology.
Ta(OR)4(thd) compounds were prepared from the reac-
tion between Ta(OR)5 and 1 mol equivalent of thdH in n-
hexane solvent. After removal of the solvent and volatiles
in vacuo, the Ta(OR)4(thd) products were purified by vac-
uum distillation. The synthesis and characterisation of
Ta(OR)4(thd) compounds are described more fully else-
where [24].
5.2. Growth of single oxide films
PbO, Sc2O3 and Ta2O5 films were deposited on Si(100)
substrates at 1000mbar, using a liquid injection MOCVD
reactor described elsewhere[11]. The films were grown from
individual solutions of the Pb, Sc and Ta oxide precursors
dissolved in THF. The growth conditions are summarised in
Table 1.
4. Conclusions
Careful selection and molecular design of the Pb, Sc and
Ta precursors have allowed us to match more closely the
physical properties of each precursor, and to achieve oxide
growth in a similar temperature regime. The use of the pre-
cursor combination Pb(thd)2, Sc(thd)3 and Ta(OEt)4(thd)
has allowed the growth of Pb(Sc0.5Ta0.5)O3 in the required
perovskite phase at substrate temperatures as low as 6008C.
5.3. Growth of Pb(Sc0.5Ta0.5)O3 films
Thin films of Pb(Sc0.5Ta0.5)O3 were deposited onto
Pt(111) substrates by the evaporation of a single solution of
precursors in THF using an MOCVD reactor described in
detail elsewhere [11]. An evaporator temperature of 3008C
5. Experimental
Table 1
Growth conditions used for deposition of single metal oxides a
5.1. Precursor synthesis
Precursor
THF solution
concentration
(molar)
Injection
rate
Evaporator
temperature
(8C)
The b-diketones thdH, tmodH and mhdH used in thisstudy
were purchased from Lancaster synthesis. Pb(thd)2 was syn-
thesised by the addition of Na(thd) to PbCl2 in aqueous
methanol [13]. The Sc b-diketonates Sc(thd)3, Sc(tmod)3
and Sc(mhd)3 were synthesised by the reaction of
Sc(NO3)3(H2O)x with the sodium salts of the respective b-
diketonates (3 mol equivalents, prepared in situ by reaction
of NaOH with the b-diketone in aqueous methanol) [23].
After isolation of the solid products by filtration, the Sc b-
diketonates were dried overnightandthenpurifiedbyvacuum
(cm3 hy1
)
Pb(thd)2
Sc(thd)3,
Sc(tmod)3
Sc(mhd)3
Ta(OR)4(thd)
0.16
0.1
0.1
0.1
0.1
1
2
2
2
2
200
300
300
300
225
a Reactor pressure 1000 mbar, substrate Si(100), Ar flow rate 4000 cm3
miny1, oxygen flow rate 1000 cm3 miny1
.
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A.C. Jones et al. / Polyhedron 19 (2000) 351–355
355
Table 2
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(1983) 6601.
Growth conditions used for deposition of Pb(Sc0.5Ta0.5)O3 from a single
[4] K.D. Budd, S.K. Dey, D.A. Payne, Proc. Br. Ceram. Soc. 36 (1985)
107.
precursor solution
[5] B.A. Tuttle, R.W. Schwartz, MRS Bull. 21 (1996) 49.
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heim, 1996, Ch. 1, Section 1.5, and references therein.
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Williams, K.A. Fleeting, D.J. Otway, P. O’Brien, Chem. Vap. Depo-
sition 4 (1998) 46.
Reactor pressure (mbar)
Substrate
THF solution composition (%)
1000
Pt(111)
Pb(thd)2 35.5, Sc(thd)3 32.25,
Ta(OEt)4(thd) 32.25
Solution injection rate (cm3 hy1
)
2
Argon flow rate (cm3 miny1
)
4000
1000
300
450–600
0.1–1
Oxygen flow rate (cm3 miny1
Evaporator temperature (8C)
Substrate temperature (8C)
)
Film growth rate (mm hy1
)
R.W. Whatmore, S.B. Stringfellow, N.M. Shorrocks, Infrared Tech-
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M.A. Malik, P.O’Brien, M. Motevalli, A.C. Jones, Polyhedron 18
(1999) 1641.
[12]
[13]
[14]
[15]
[16]
[17]
was used to prevent condensation of the least volatile pre-
cursor Sc(thd)3, and minimaldecompositionofPb(thd)2 and
Ta(OEt)4(thd) was observed. A full summary of growth
conditions is given in Table 2.
K.A. Fleeting, P.O. Brien, H.O. Davies, A.J.P. White, D.J. Williams,
A.C. Jones, T.J. Leedham, Chem. Vap. Deposition 5 (1999) 261.
A.C. Jones, T.J. Leedham, P.J. Wright, M.J. Crosbie, D.J. Williams,
P.A. Lane, P. O’Brien, Mater. Res. Soc. Symp. Proc. 495 (1998) 11.
H.A. Luten, W.S. Rees, V.L. Goedken, Chem. Vap. Deposition 2
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M.J. Crosbie, P.J. Wright, D.J. Williams, P.A. Lane, J. Jones, A.C.
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EURO CVD 12, Sitges, Spain, September 5–10, 1999, J. Phys. IV
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Acknowledgements
We gratefully acknowledge the support of the Department
of Trade and Industry for a Teaching Company Fellowship
to HOD. POB is the Sumitomo/STS Professor of Materials
Chemistry. KAF thanks the EPSRC and Inorgtechforsupport
via the CASE scheme.
[18]
[19]
[20]
[21]
[22]
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