Scandium dipivaloyl methanate as a volatile precursor for thin film deposition. Coupling of mass spectrometer to thermobalance

Article abstract of DOI:10.1023/A:1019929521641

The coupling of a quadrupole mass spectrometer (QMS) via a heated capillary to a commercial thermogravimetric analyser is described. The amu and temperature ranges available were up to 1000 amu and 1500°C, respectively. The system was evaluated with test compounds, yielding gaseous species in the m/z range of 17-80, and then used for the study of thermal behaviour of scandium dipivaloyl methanate or Sc(thd)₃ which is discussed in detail. Sc(thd)₂ appears as the major Sc-containing species with m/z=411 in the gas phase at 200-300°C.

Full text of DOI:10.1023/A:1019929521641

Journal of Thermal Analysis and Calorimetry, Vol. 69 (2002) 65–75
SCANDIUM DIPIVALOYL METHANATE AS A
VOLATILE PRECURSOR FOR THIN FILM DEPOSITION
Coupling of mass spectrometer to thermobalance
K. Mészáros-Szécsényi1,2, J. Päiväsaari1, M. Putkonen1, L. Niinistö1* and
G. Pokol3
1Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology,
P.O. Box 6100, FIN-02015 Espoo, Finland
2Permanent address: University of Novi Sad, Faculty of Sciences, Institute of Chemistry,
YU-21000 Novi Sad, Trg. Dositeja Obradovica 3, Yugoslavia
3Institute of General and Analytical Chemistry, Budapest University of Technology and
Economics, H-1521 Budapest, Hungary
Abstract
The coupling of a quadrupole mass spectrometer (QMS) via a heated capillary to a commercial
thermogravimetric analyser is described. The amu and temperature ranges available were up
to 1000 amu and 1500°C, respectively. The system was evaluated with test compounds, yielding
gaseous species in the m/z range of 17–80, and then used for the study of thermal behaviour of
scandium dipivaloyl methanate or Sc(thd)3 which is discussed in detail. Sc(thd)2 appears as the ma-
jor Sc-containing species with m/z=411 in the gas phase at 200–300°C.
Keywords: ALD, CVD, Sc precursor, TG-MS, thin films
Introduction
Combining mass spectrometry simultaneously with thermogravimetry significantly
increases the information content of a thermoanalytical run and enhances the possi-
bilities to correctly interpret the mechanism of thermal decomposition reactions. The
information obtainable from a combined TG-MS run can also be used to evaluate pre-
cursors and processes for the growth of thin films by chemical vapour deposition
(CVD) and atomic layer deposition (ALD) methods. For a better understanding and
control of a CVD or ALD process it is necessary to know, besides the thermal stabil-
ity of the precursor, also the composition of the vapour phase. To solve this kind of
problem, i.e. to carry out an evolved gas analysis (EGA), mass spectrometry has
proven to be a very powerful tool.
Due to its superiority over most other EGA methods, mass spectrometry has es-
tablished itself over the past 30 years or so as the method of choice for monitoring of
gaseous species evolved in a thermoanalytical process (TG or DTA). The coupled
TG-MS methods have been frequently reviewed [1–5] and several TG-MS interface
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MÉSZÁROS SZÉCSÉNYI et al.: SCANDIUM DIPIVALOYL METHANATE
constructions have been described. The interface constructions are most often based
on a heated capillary to reduce the pressure and to transport the gaseous sample [6, 7].
Other interface solutions for a simultaneous EGA, i.e. coupling of a TA instrument to
a gas analyser, have been presented as well. They involve a single- or double-step
coupling to a MS instrument through orifice [8], jet separator [9] or skimmer system
[10]. We have earlier described the coupling of a quadrupole MS (QMS) to a
thermogravimetric analyser via a heated capillary [7]. There are also commercially
available coupled TG-MS instruments. On the other hand, a more versatile solution is
to tailor a connection mode between available MS and TG instruments as we did ear-
lier in the case of Perkin Elmer TGA 7 and Leybold–Heraeus IQ 200 quadrupole
mass spectrometer [7]. In this paper we describe a connection mode between Hiden
QMS and Seiko TG-DTA analyser through a heated capillary interface. The perfor-
mance of the instrument was tested with three test substances, viz. CaC2O4·H2O,
CuSO4·5H2O and Fe(NH4)(SO4)2·12H2O. The system was then used to study the ther-
mal decomposition of scandium dipivaloyl methanate, Sc(thd)3 (dipivaloyl
methanate or thd = 2,2,6,6-tetramethyl-3,5-heptanedionate).
Scandium dipivaloyl methanate, Sc(thd)3 belongs to the metal b-diketonates. These
compounds are often used as precursors for thin film deposition by CVD and ALD meth-
ods because of their volatility and relatively high thermal stability in the vapour phase
[11]. The mass spectra of several rare earth b-diketonates have been reported in literature,
including those of Y(thd)3 [12], Ce(thd)4 [13, 14] and Gd(thd)3 [15].
In this work, we present a TG-MS study of Sc(thd)3, which has been used as a
precursor for atomic layer deposition [16, 17]; the results of the ALD growth of Sc2O3
thin films have been reported elsewhere [18]. Recently, Sc2O3 thin films from a
Sc(thd)3 precursor have also been deposited by CVD [19]. Also the integration of a
QMS instrument into an ALD reactor has been described and used for in situ analysis
of the gas phase during the deposition process [20, 21].
Experimental
Thermoanalytical measurements were carried out using a Seiko simultaneous
TG-DTA 320 instrument while the simultaneous EGA measurements were per-
formed by means of a QMS instrument of Hiden Analytical Ltd, type HAL-1001 with
mass range up to 1000 amu.
Sc(thd)3 (CAS 15492-49-6, M=594.77 g mol–1) was synthesised with the method
presented in literature [22]. As a first step, Sc(NO3)3·xH2O was prepared from 2 g of
Sc2O3, which was dissolved in 6.1 cm3 nitric acid (65 mass%). After the evaporation of
the solution, 10 g of Sc(NO3)3·xH2O crystals were obtained. Sc(NO3)3·xH2O was vac-
uum-dried at 50°C. The number of water molecules, i.e. the value of x, was verified to be
4.6 by means of thermogravimetry. Then 10.3 cm3 of H(thd) was dissolved in 30 cm3 of
96 vol% ethanol (Etax A, Primalco) in a round-bottomed flask. The solution was contin-
uously stirred. Next, 2 g of NaOH pellets dissolved in 50 cm3 of 50 vol% ethanol were
slowly added. Finally, 5.3 g of Sc(NO3)3·4.6 H2O dissolved in 50 cm3 of 50 vol% ethanol
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MÉSZÁROS SZÉCSÉNYI et al.: SCANDIUM DIPIVALOYL METHANATE
67
was added slowly. Solution turned into white as Sc(thd)3 was formed. After about one
hour of stirring the mixture was distilled under vacuum. Sc(thd)3 was separated by vac-
uum filtration, dried in vacuum at 50°C and purified by sublimation at 110°C. Yield after
sublimation was 9.1 g (91%). The purity and volatility of the product were verified by
FTIR and thermogravimetry in vacuum, respectively.
Coupling the mass spectrometer to the thermobalance
In contrast to Perkin Elmer TGA 7 [7], the Seiko TG-DTA 320 instrument is a horizontal
thermobalance with a horizontal gas-flow outlet. This outlet was employed for the cou-
pling of the QMS through a pressure reducing heated silica capillary interface. Sampling
into capillary (Æi=0.32 mm, length=2 m) was from the atmospheric pressure inside the
TA instrument. The temperature of the resistively heated capillary was adjusted to ap-
proximately 150°C by changing the current. The ceramic outlet tube of the thermo-
balance was fitted into the capillary through a connecting metal tube, which was fitting
well the outlet in order to keep the capillary assembly fixed. The tube was connected to
the heated capillary line and it reached the inside of the balance near to the sample holder.
The end of the tube, which was fixed to the sampling line, contained holes for the purge
gas outlet (Fig. 1). Through the metal tube placed near to the sample holder a representa-
tive sample could be obtained for the MS measurement without any time lag between the
decomposition and the detection. Actually, because of the greater sensitivity of the MS
detection compared with the sensitivity of the balance, no time-lag was observed be-
tween the TG (DTG, DTA)- and MS-signals.
Fig. 1 The connection between the coupled thermobalance and QMS through a heated capillary
The Seiko instrument is a thermobalance controlled by a built-in computer,
while the QMS was controlled by PC and MASsoft (Ver. 3.5.3.) software developed
by Hiden Analytical Ltd. Both instruments could send their output signals to a com-
mon plotter and the results could be assessed by overlaying the corresponding time
range curves. It was also possible to transfer data to a data analyser programme like
Origin and evaluate the results after the measurement. The block diagram of the cou-
pled instruments is presented in Fig. 2.
This configuration gave the expected, satisfactory results with the test samples
(CaC2O4·H2O, CuSO4·5H2O and (Fe(NH4)(SO4)2·12 H2O) the decomposition products
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MÉSZÁROS SZÉCSÉNYI et al.: SCANDIUM DIPIVALOYL METHANATE
Fig. 2 A block diagram of the configuration used in the simultaneous TG-MS measurements
of which are gases or highly volatile compounds with molecular masses ranging from
17 to 80 (NH3, H2O, CO, CO2, SO2, SO3). However, for the detection of the decompo-
sition products with low volatility, an outside thermal isolation of the gas-flow outlet,
consisting of a metal tube and unheated part of the capillary, is necessary. In addition,
the metal tube reaching the inside of the furnace improves the heat transfer around the
capillary tip outside of the heated transfer line. With this isolation the thermal stabil-
ity of the fused silica capillary and the limited capillary heating temperature deter-
mine the limits of the MS sampling.
Results and discussion
Thermal decomposition of the scandium dipivaloyl methanate
With the equipment described above, Sc(thd)3 decomposition was studied with heat-
ing rates of 5 and 10°C min–1 both in air and argon atmospheres. The sample mass
ranged from 5 to 25 mg, a typical value being 8 mg. Aluminium oxide crucibles were
employed with aluminium oxide as DTA reference material.
The thermal decomposition curves of the title compound, recorded with 5 and
10°C min–1 heating rates in an argon atmosphere, are presented in Fig. 3. Essentially simi-
lar decomposition curves are obtained in air. DTA curve shows a phase transition at
123°C onset temperature and at 153°C the melting of the sample is observed. According
to the TG curve the evaporation and the thermal decomposition of the compound is a sin-
gle-step process which is completed in a temperature range of approximately
160–320°C, depending on the sample mass. For the typical sample mass of about 8 mg
the decomposition range was 165–265°C. The evolving of gaseous decomposition prod-
ucts during the evaporation was monitored by the QMS assembly described above.
MS fragmentation of the sample was followed in the 50–130, 350–420 and
530–600 amu ranges over a dynamic temperature interval of 20–350°C using 70 eV
ionisation potential and emission current of 1000 mA. The mass spectra of the
Sc(thd)3 at maximum peak intensities are shown in Fig. 4., while the ionic pressure
intensities detected with secondary electron multiplier (SEM) in the mass spectra at
different temperatures are listed in Table 1.
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MÉSZÁROS SZÉCSÉNYI et al.: SCANDIUM DIPIVALOYL METHANATE
69
Fig. 3 Thermal decomposition curves of Sc(thd)3 recorded with heating rates 5 (dashed
line) and 10°C min–1 (solid line) in an Ar-atmosphere
Table 1 Ionic pressure intensities (i) produced by Sc(thd)3 at different temperatures
Temperature/°C
»200
>250
>300
i·1011/torr
m/z fragment
594 Sc(thd)+3
i·1011/torr i/imax/% i·1011/torr
i/imax/%
–
i/imax/%
0.2
–
–
–
–
–
0.1
1.2
4.5
0.3
0.2
0.5
3.5
0.70
537 [Sc(thd)3–C4H9]+
0.5
1.0
0.1
0.05
0.1
1.7
0.7
3.0
3.3
1.0
2.7
411 Sc(thd)+2
0.05
–
3.0
–
30.0
2.0
10.0
0.7
396 [Sc(thd)2–CH3]+
381 [Sc(thd)2–2CH3]+
354 [Sc(thd)2–C4H9]+
127 [(thd)–C4H8]+
126 [(thd)–C4H9]+
57 C4H+9
–
–
1.0
0.4
–
–
2.0
1.1
1.00
0.70
1.50
1.50
67.0
47.0
100.0
100.0
51.0
21.0
91.0
100.0
7.8
1.5
10.0
45.5
22.0
100.0
56 C4H+8
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MÉSZÁROS SZÉCSÉNYI et al.: SCANDIUM DIPIVALOYL METHANATE
As data in Table 1 show, the intensity of all fragments increases with tempera-
ture, indicating an increasing decomposition of the sample at higher temperatures.
The peaks with highest intensity belong to the Sc(thd)+2 , [Sc(thd)3–C4H9]+, Sc(thd)+3
molecular fragments and to the fragments of the ligand. Besides these, fragments
with composition of [Sc(thd)2–CH3]+, [Sc(thd)2–2CH3]+, and [Sc(thd)2–C4H9]+ were
detected.
In the beginning of the decomposition only the fragments of the Sc(thd)+2 and the
ligand peaks were detected with significant intensity [23]. Peaks with m/z ratio of 56
(C4 H+8 ) and 57 amu (C4 H+9 ) appeared with the same intensities. Fragments of C7H10O2+
and C7H11O+2 were also present in the spectra with approximately same intensities. Ionic
current due to the other fragments was negligible at this stage of the decomposition. With
increasing temperature all peak intensities increased but at the same time the intensity ra-
tio also changed: the ionic current of the C4 H+9 fragment decreased while the intensity of
theC4 H+8 fragment increased together with the intensity of theC7H11O+2 fragment. Above
250°C all peaks listed were present with significant intensities.
The highest peak of the spectra at all temperatures belonged to the C4 H+8 frag-
ment. When comparing the intensity of this peak with the intensity ratio of the other
peaks, it was noted that it changed with temperature indicating probably a different
thermal decomposition mechanism. On the basis of these data, we assume that the de-
composition proceeds through the following stages:
1. At lower temperatures mainly dissociation of one ligand takes place:
Sc(thd)3(g) ® Sc(thd)2(g)+thd(g)
(1)
The thd radical formed decomposes immediately, partly through a McLafferty rear-
rangement.
Fig. 4a Mass spectra of Sc(thd)3 at maximum peak intensities in the 50–130 amu range
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MÉSZÁROS SZÉCSÉNYI et al.: SCANDIUM DIPIVALOYL METHANATE
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Fig. 4b Mass spectra of Sc(thd)3 at maximum peak intensities in the 350–420 amu range
Fig. 4c Mass spectra of Sc(thd)3 at maximum peak intensities in the 530–600 amu range
2. With an increasing temperature the decomposition of the C7H11O+2 and
C7H10O+2 fragments also increases which is seen as a sharp rise of the intensities of the
C4 H+8 and C4 H+9 fragments.
In contrast to the fragmentation of the Y(thd)3 complex, where at lower tempera-
tures the peak with highest intensity belongs to the [Y(thd)3–C4H9]+ fragment [12],
the corresponding Sc(thd)3 compound produces the [Sc(thd)3–C4H9]+ peak at higher
temperatures with a significantly lower relative intensity. As the tercbutyl-group is an
electron donor group, it transfers some of its electrons to the chelate ring, weakening
this way the C–C(CH3)3 bond. However, this transfer is less obvious in the Sc(thd)3,
resulting in the cleavage of the terc-butyl-group at higher temperatures.
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Fig. 5 Peak intensities of the fragments m/z=56, 411 and 537 amu as a function of time
Fig. 6 Shape of the MS-peak of the Sc(tdh)+2 (m/z=411 amu) fragment after repeated
measurements in the same capillary showing the signs of retention (tailing)
Thus, the proportion of the [Sc(thd)3–C4H9]+ fragment increases with increasing
temperature while the relative intensities of the other peaks decrease:
Sc(thd)3 ® [Sc(thd)3–C4H9]+C4H9
(2)
At higher temperatures the McLafferty’s rearrangement of the ligand seems to
be preferred over simple fragmentation; above 300°C the C4 H+9 fragment probably
belongs to the broken off terc-butyl-groups. Under dynamic conditions it is likely
that both processes proceed simultaneously.
Besides the different decomposition pattern, the overall thermal stability of the
Sc(thd)3 complex is significantly higher than the stability of the corresponding yttrium
compound. This is a consequence of the greater bond strength between the ligand donor
atoms and scandium ion, mainly due to the higher charge/radius ratio as compared to that
of yttrium.
In principle, all the fragments can be monitored during the decomposition process
on a temperature (time) scale (Fig. 5). The most easily followed fragment is that with the
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MÉSZÁROS SZÉCSÉNYI et al.: SCANDIUM DIPIVALOYL METHANATE
73
Fig. 7 Combined MS and DTG curves recorded for the Sc(tdh)+2 fragment originating
from the Sc(thd)3 sample
highest ionic current intensity which in this case belongs to the fragment with 56 amu.
However, this peak is not specific for the present compound alone, but is typical for all
Ar–n-C5H11, Ar–iso-C5H11, ArO–n-C4H9, ArO–iso-C4H9 compounds, pentyl ketone, etc.
Out of the many compound-characteristic peaks, the Sc(thd)+2 peak (411 amu) appears
suitable for continuous monitoring. However, the retention of the this fragment, being
coordinatively unsaturarated, is sometimes high, especially when the measurement is re-
peated with the same capillary, as can be seen in Fig. 6. On the other hand, there is no sig-
nificant retention of the smaller 56 amu fragment. The combined DTG-MS curves at 5
and 10°C min–1 heating rates are presented in Fig. 7.
Conclusions
The Hiden QMS is a universal MS instrument. In order to make a full-scale use of it,
the connection adapter for every specific purpose should be easily commutable. In
this case it is achieved by unscrewing the metal tube; the QMS is then ready for adap-
tation to other instruments, e.g., to the ALD reactor.
The TG-MS measurements of the test samples with a heated capillary to 150°C
were easily conducted and without condensation. However, difficulties arose during
the measurement of a compound yielding less volatile decomposition products. At
150°C the capillary was clogged by condensation. Therefore an increase of the capil-
lary temperature up to 180°C was applied. Even at this temperature the retention of
the sample was observed. Only with long empty runs and degassing we could get rid
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MÉSZÁROS SZÉCSÉNYI et al.: SCANDIUM DIPIVALOYL METHANATE
of the retained compound. Significant peak broadening was observed with high sam-
ple masses and low purge gas flow rate especially in experiments with low heating
rate. With a clean capillary, a lower sample mass and an optimal purge gas flow, how-
ever, well resolved mass spectra can be obtained which provide very valuable data for
the interpretation of the decomposition mechanism. The sharp MS-peak – time
curves can be easily related to the thermal decomposition (TG-DTG) curves.
Compared with our previous system also based on a TG-QMS coupling [7], the pres-
ent one has a significantly wider mass range and higher sensitivity, meaning smaller
samples. An integration of MS data to a thermogram to produce a practical and infor-
mative plotter output would require some additional programming work, however.
TG-MS data reveal that when Sc(thd)3 is heated, Sc(thd)2 appears as a major
Sc-containing species in the gas phase. Sc(thd)2 being volatile, sufficiently stable and
coordinatively unsaturated is probably responsible for the Sc2O3 thin film growth
from the Sc(thd)3 precursor in the recently developed ALD/ALE process [18]. The
relatively small size of the Sc(thd)2 species makes it an attractive choice for special
applications such as conformal coating of edges and even pores, which is an emerg-
ing application of ALD [24, 25].
* * *
One of us (K.M.Sz.) wishes to acknowledge scholarship for a research stay at HUT from the Finnish
Center for International Mobility (CIMO). The support of CIMO for shorter research visits between
HUT and BUTE for L. N., J. P. and G. P. is acknowledged as well. The authors wish also to thank
Ms. Minna Nieminen (M. Sc.) for the synthesis of the Sc precursor.
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