Experimental and computational studies of the homogeneous thermal decomposition of the tungsten dimethylhydrazido complexes Cl4(RCN) W(NNMe2)

Article abstract of DOI:10.1149/2.002206jes

The thermal decomposition pathways of the tungsten dimethylhydrazido complexes Cl₄(RCN)W(NNMe₂) (1a: R=CH₃; 1b: R=Ph), precursors for single source deposition of WN_xC_y, were investigated using a combination of experiments and calculations. Raman scattering studies were performed in an impinging-jet, up-flow, aerosol-assisted CVD reactor to identify reaction intermediates. Density Functional Theory calculations (B3LYP/LanL2DZ) were used to estimate Raman active frequencies and explore the reaction surface. Dimethylamine and methylmethyleneimine, products from N-N cleavage of the hydrazido ligand, were observed under deposition conditions and identified by comparison with previously reported Raman shifts and calculated frequencies.

Full text of DOI:10.1149/2.002206jes

Journal of The Electrochemical Society, 159 (5) H545-H553 (2012)
H545
0013-4651/2012/159(5)/H545/9/$28.00 © The Electrochemical Society
Experimental and Computational Studies of the Homogeneous
Thermal Decomposition of the Tungsten Dimethylhydrazido
Complexes Cl4(RCN)W(NNMe2)
Jooyoung Lee,a Dojun Kim,a Oh Hyun Kim,a Tim Anderson,a,∗,z Ju¨rgen Koller,b
Dan R. Denomme,b Sophia Z. Habibi,b Mohammad Ehsan,b
John R. Eyler,b and Lisa McElwee-Whiteb,∗
aDepartment of Chemical Engineering and bDepartment of Chemistry, University of Florida, Gainesville,
Florida 32611-7200, USA
The thermal decomposition pathways of the tungsten dimethylhydrazido complexes Cl4(RCN)W(NNMe2) (1a: R=CH3; 1b:
R=Ph), precursors for single source deposition of WNxCy, were investigated using a combination of experiments and calcula-
tions. Raman scattering studies were performed in an impinging-jet, up-flow, aerosol-assisted CVD reactor to identify reaction
intermediates. Density Functional Theory calculations (B3LYP/LanL2DZ) were used to estimate Raman active frequencies and
explore the reaction surface. Dimethylamine and methylmethyleneimine, products from N–N cleavage of the hydrazido ligand, were
observed under deposition conditions and identified by comparison with previously reported Raman shifts and calculated frequencies.
© 2012 The Electrochemical Society. [DOI: 10.1149/2.002206jes] All rights reserved.
Manuscript submitted December 30, 2011; revised manuscript received February 23, 2012. Published March 21, 2012.
Continual reduction of the average feature size found in integrated
circuits and the concomitant changes in via and line resistivity, cur-
rent density, and RC time delays have driven the gradual replacement
of Al-based metallization to that based on Cu given its lower elec-
trical resistivity and higher resistance to electromigration. Use of Cu
metallization requires prevention of Cu migration into the underly-
ing Si and SiO2, which can produce an increase in contact resis-
tance, leaky pn junctions, variations in the barrier height, and con-
tact layer embrittlement.1,2 To avoid Cu-Si interaction an amorphous
diffusion barrier layer at a thickness well below the via dimension
is conformally deposited at low temperature. As these requirements
become more demanding, a chemical deposition method will likely
be required. WNxCy films grown by both chemical vapor deposition
(CVD)3,4 and atomic layer deposition5 have shown promise as a suit-
able diffusion barrier material. Tungsten-based barriers have demon-
strated improved adhesion to Cu and ease of chemical mechanical
planarization as compared to the commonly used TaNx.6,7 We re-
cently reported the preparation of the diorganohydrazido(2-) tungsten
complexes Cl4(CH3CN)W(NNR2) (R2 = Me2, Ph2, and −(CH2)5−)
and Cl4(pyridine)W(NNPh2)8 as precursors for the CVD of WNxCy
films for use as Cu diffusion barriers.9,10 Other than these, researchers
also have reported tungsten carbonitride film deposition using single
source precursors.11–13
This study reports the mechanism of depositing tungsten carboni-
tride from the hydrazido complex Cl4(CH3CN)W(NNMe2) (1a). The
synthesized precursor 1a was dissolved in benzonitrile to allow trans-
port by nebulization. The precursor rapidly coverts to its benzonitrile
derivative Cl4(PhCN)W(NNMe2) (1b) upon dissolution in benzoni-
trile. The gas-phase decomposition kinetics of 1b were investigated
using in situ Raman spectroscopy in a vertical up-flow, cold-wall CVD
reactor. Additional experiments involved NMR kinetics and observa-
tion of the fragmentation of 1b during electrospray ionization (ESI)
in a Fourier transform ion cyclotron resonance (FTICR) mass spec-
trometer. Comparison of the experimental results with literature data
and DFT calculations was used for assignment of the observed Raman
bands and evaluation of likely decomposition pathways.
ure 1).14 Since under deposition conditions (1a in benzoni-
trile solvent and delivered as aerosol to substrate at temperature
> 250◦C), the equilibrium between 1a and 1b is dominated by 1b (vide
infra), these mixtures will subsequently be referred to as “1b.” The
reactor is interfaced with an in situ Raman spectrometer (Ramanor
U-1000, Jobin Yvon), which uses the 532.08 nm line of Nd:YAG
solid-state laser as the light source and includes a double additive
monochromator fitted with a diffraction grating of 1800 grooves/mm.
The Raman system was calibrated against the 546.07 nm emission
line of mercury. As described elsewhere,14 this up-flow, impinging-
jet CVD reactor was designed to produce a stable 2-D flow pattern
while isolating the reactants from the reactor walls to prevent parasitic
reactions. Since the entire CVD reactor chamber assembly could be
translated in the x-y-z directions, it was possible to measure gas-phase
composition and temperature profiles inside the reactor to quantita-
tively study the gas phase decomposition kinetics of complex 1b.
The three concentric inlet tubes shown in Figure 1 are packed with
3 mm glass beads to establish an equal velocity and uniform-flow in-
let boundary condition. Aerosol-assisted CVD (AACVD) was carried
out for the Raman experiment with 1b because this complex has low
volatility.15 The precursor 1b was prepared by dissolving solid 1a in
benzonitrile (PhCN, 0.0174 mol/L). The solution is then pumped into
a nebulizer from a syringe. A piezoelectric material in the nebulizer
vibrates at a frequency of 1.44 MHz, which generates an aerosol of
precursor 1b and benzonitrile. The aerosol is introduced into the reac-
tor with N2 (99.999%, Airgas) carrier at a flow rate of 2.5 cm/s, which
transports the mixture of 1b and solvent that was injected at a rate of
1.0 mL/h. The susceptor is heated by a resistive ceramic heater placed
on the inside of the quartz flat-one-side tube and the experiments
were conducted at a set point of 850◦C. For the concentric annular
and sweep flows, pure N2 gas was delivered with the same flow ve-
locity (2.5 cm/s) and sufficient gas flow time was allowed for the
reactor to reach steady state before measurements were made. Based
on a previous study16 that validated a steady-state, two-dimensional
mass transport model and included CH4 tracer experiments, disrup-
tive recirculation flow patterns in the reactor are not anticipated. A
3W Nd:YAG solid-state laser line (532.08 nm) was used to excite 1b
and benzonitrile, while the N2 vibrational Raman excitation lines were
recorded along the centerline to estimate the temperature profile.
To assist in the assignment of these additional Raman bands as
well as to assess possible reaction pathways, DFT calculations were
performed using the Gaussian 03 program suite.17 For all bond dis-
sociation, recombination, activation energy, thermodynamic proper-
ties, and frequency calculations, the spin polarized hybrid density
functional B3LYP was combined with the LanL2DZ basis set for all
Experimental and Theoretical Methods
The homogeneous thermal decomposition of mixtures of the
tungsten dimethylhydrazido complex 1a and its benzonitrile deriva-
tive 1b was studied in a custom designed CVD reactor (Fig-
∗
Electrochemical Society Active Member.
z E-mail: tim@ufl.edu
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Journal of The Electrochemical Society, 159 (5) H545-H553 (2012)
Vent
Vent
Ceramic cap
z
r
Heating
element
Quartz wall
Center flow
Annular flow
Sweep flow
Stainless screen
Quartz balls
LASER
(532.08 nm)
Sweep inlet
Figure 2. Plot of ln(k/T) vs. 1/T(K) for acetonitrile exchange in complex 1a.
M.O. source
(1a/benzonitrile)
(N2)
sidered as the predominant precursor species during deposition of
WNxCy from solutions made from 1a. Because the rates for dissocia-
tion of acetonitrile and benzonitrile from transition metal complexes
are similar,23 these experiments also establish that the coordinately
unsaturated intermediate 2 will be readily accessible from either 1a
or 1b under deposition conditions.
Annular inlet
(N2)
Carrier gas
(N2)
Nebulizer
Figure 1. Schematic of the nebulizer-assisted experimental reactor for in situ
Raman spectroscopic measurements.
Ion cyclotron resonance experiments.— We have previously
demonstrated that mass spectrometry can provide insight into the gas
phase dissociation chemistry of the related tungsten imido complexes
elements. Crystallographic structure data8 of complex 1a collected at
173 K were used to obtain the starting geometry for calculations.
NMR kinetic studies of the dissociation of acetonitrile from 1a
were conducted on a Varian Inova at 500 MHz. Compound 1a along
with a molar equivalent of acetonitrile was dissolved in toluene-d8.
The 1H spectrum displayed signals for 1a at 0.53 ppm and acetonitrile
at 0.83 ppm with a ratio of 1:2.09.
For ESI-FTICR mass spectrometric analyzes solutions of 1b were
prepared by dissolving 1a at a concentration of 100 μM in dry ben-
zonitrile (purchased from Sigma-Aldrich as anhydrous grade in 99.9%
purity and degassed by purging with Ar) in a dry-box under N2. The
complex was ionized (negative ionization mode) using a commercial
electrospray ionization (ESI) source (Analytica of Branford, Branford;
CT, USA) with a user-modified heated metal capillary18–20 modified
with a conical capillary inlet21 set at a temperature of 125◦C. The ESI
source was confined in a purge box maintained at a positive pressure of
dry nitrogen gas. The complex was introduced in benzonitrile solution
with no additional solvents (e.g., water, methanol). Mass spectra of
the WCl4N− (nominal m/z 340) ions were obtained with a Bruker 47e
FTICR mass spectrometer (Bruker Daltonics; Billerica, MA, USA)
equipped with a 4.7 T superconducting magnet (Magnex Scientific
Ltd.; Abington, UK) and Infinity cell.22
i
Cl4(CH3CN)W(NR) where R = Pr, Ph and allyl3,24,25 as well as the
series of tungsten hydrazido complexes Cl4(CH3CN)W(NNR2).8 The
chemical ionization mass spectra of 1a, however, were anomalous in
that the tungsten nitrido fragment Cl4WN+, which is a marker for the
ability of the imido complexes to deposit high quality films at low
temperatures, could not be detected unambiguously in the spectra of
1a, despite the fact that 1a is an excellent CVD precursor for WNxCy.
To further investigate the gas phase ion dissociation pathways of 1a
and their relevance to the deposition conditions, a mass spectrometric
study with negative electrospray ionization and FTICR ion trapping
and detection was carried out.
Figure 3 shows the ions detected (negative ion mode) follow-
ing electrospray ionization of 1b prepared by dissolving 1a in
Results and Discussion
Kinetics of acetonitrile dissociation from 1a.— To estimate the
rate of loss of acetonitrile from 1a to form Cl4W(NNMe2) (2) in the
CVD reactor, the kinetics of acetonitrile exchange were determined
via variable temperature NMR spectroscopy. The exchange rate k
was determined by line shape analysis in the temperature interval 50
to 84◦C. A plot of ln(k/T) vs. 1/T (Figure 2) afforded an activation
enthalpy of 23.0
0.2 kcal/mol and an activation entropy of 28.8
0.6 cal/mol · K, consistent with dissociative exchange. The cor-
responding Gibbs energy of activation is 14.4 kcal/mol, indicating
that dissociation of acetonitrile from 1a should be facile under CVD
conditions.
Based on these results it can be assumed that benzonitrile solutions
of 1a have been converted almost completely to 1b by dissociative
exchange of the acetonitrile ligand with solvent before the solution
is introduced into the CVD reactor. Thus complex 1b can be con-
Figure 3. Mass spectrum of Cl4(PhCN)W(NNMe2) (1b). Negative ions were
generated through electrospray ionization (ESI) technique and detected using
a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. The
most abundant peak in the array was a combination of the ions WCl4N− and
WCl4O−, in a ratio of 21:79. Inset: comparison of experimental intensities to
calculated intensities using the 21:79 ratio determined through least squares
analysis.
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Journal of The Electrochemical Society, 159 (5) H545-H553 (2012)
H547
Figure 4. Raman spectra of gas-phase
dimethylhydrazido complex 1b in benzoni-
trile as a function of distance below the heated
susceptor along the reactor centerline. Spec-
tra in panel E (nitrogen vibrational mode) are
marked with reduced scale for concentration
reference. Raman shift values for the primary
modes are specified at the top of each panel.
Raman spectra profiles are marked as a func-
tion of position from the heated susceptor
surface as (a) 1 mm, (b) 3 mm, (c) 4 mm,
(d) 5 mm, (e) 7 mm, (f) 9 mm and (g) 11.5 mm.
Panels D and F show only three high resolu-
tion scans acquired in a single run.
benzonitrile. The envelope of peaks centered about m/z 340 corre-
sponds to WCl4N−, and a second, lower intensity, envelope of peaks
centered about m/z 323 corresponds to WO2Cl3−. Observing the lat-
ter oxidation products suggested that the major peaks in the mass
spectrum might represent overlapping WCl4N− and WCl4O− distri-
butions, which are displaced by 2 amu. Comparison of the observed
mass spectrum with simulations based on mixtures of the calculated
distributions of the nitrido- and oxo- species revealed that the major
envelope of peaks in Figure 3 is actually composed of 21% WCl4N−
and 79% WCl4O− (Figure 3, inset). Although the configuration of
the sample inlet resulted in formation of the oxo anion WCl4O−due
to unavoidable brief exposure of the samples to the atmosphere, the
observation of the nitrido cleavage product provides evidence of N–N
bond cleavage of the hydrazido ligand of 1b in the gas phase under
mass spectrometry conditions.
In situ Raman experiments.— To investigate the thermal de-
composition of hydrazido complex 1b, in situ Raman experiments
were performed using a susceptor set point temperature of 850◦C.
Figure 4 shows the observed Raman bands along the centerline at
seven distances below the heated susceptor (labeled a-g in Figure 4).
To minimize the influence of blackbody radiation, Raman signals of
the blank reactor were recorded and subtracted for each measurement
position. For the results shown in panels A and D, each spectrum was
deconvolved using the Gaussian-Lorentzian peak shape routine in the
commercial software package PeakFIT (v4.12). An example deconvo-
lution of spectrum (g) from Panel A (Figure 4) is shown in Figure 5A.
Measurements on neat benzonitrile provided frequency values for the
C–H in plane bending coupled with C–CN stretching (1181 cm−1) and
the C–H in plane bending (1197 cm−1) modes in this region. Other
modes detected in this region include C–CN stretching for complex
1b (1178 cm−1), W≡N1 stretching with methyl scissoring for com-
plex 2 Cl4W(NNMe2) (1189 cm−1), and C-Ph stretching, W–N1 and
W–N3 stretching with C–N–C torsion for complex 1b (1193 cm−1
)
with the assignments based on DFT calculations. Estimation of the
local gas phase temperature along the reactor centerline was made
by analysis of the N2 rotational state distribution with accuracy 20 to
30◦C in the temperature range of interest.26 The measured tempera-
ture is plotted in Figures 5B and 5C as a function of reactor position
and the position associated with each spectrum listed in Figure 4. The
concentration profiles were obtained as shown in Figures 5B and 5C.
In Figure 5B, detected species concentrations are shown in relative
concentration because the Raman cross section of 1b is not available.
Since the inlet concentration of benzonitrile is known the Raman
scattering cross-section was estimated and the concentration profiles
indicated in Figure 5C estimated values.
Figure 5. A) Peak deconvolution of spectrum at probe position (g) 11.5 mm,
panel A, Figure 4; B) Relative species concentration and gas phase temperature
profile (x) along the reactor centerline for 1b and 2; C) Concentration for
benzonitrile (BN) and gas phase temperature profile (x).
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H548
Journal of The Electrochemical Society, 159 (5) H545-H553 (2012)
The results shown in panel D (Figure 4) support the presence
of two C≡N vibrational modes. It is noted, however, that since this
region also includes signal from the periodic vibration of excited N2,
these two vibrations can be extracted from the spectra in panel D
by comparison with the gas-phase data from authentic benzonitrile.
Peak deconvolution of spectrum g in Panel D (Figure 4) reveals six
components: four peaks associated with the N2 vibrational mode and
two with C≡N stretching (Figure 6A). Examination of the spectra
(a) from additional N2 only experiment with (b), (c) and (d) reveals
the four periodic N2 vibrational peaks (spectrum a) at 2221, 2229,
2237, and 2245 cm−1. Two additional C≡N stretching frequencies
are detected at 2234 and 2241 cm−1 as shown in Figure 6B (spectra
b, c, and d). These last two frequencies lie in the reported ranges for
νCN of benzonitrile as observed in gas-phase Raman experiments27–29
and the origin of the second band under our experimental conditions
is not yet understood.
To probe for additional Raman active species that were not de-
tected in the gas-phase experiments, supplementary liquid phase Ra-
man experiments were performed for 1b generated by dissolving 1a in
benzonitrile. Using a standard liquid chamber, the 532.08 nm line of a
Nd:YAG solid-state laser at 0.1 W was used to excite neat benzonitrile
and the vibrational Raman excitation lines were recorded in the range
100∼4000 cm−1. Solution phase Raman spectra of 1b (the solutions
are prepared as 0.0174 mol/L 1a in benzonitrile) and pure benzonitrile
were measured and spectra for three selected wave number ranges are
shown in Figure 7. It is evident that the higher liquid phase density
affords strong scattering intensity to produce very good signal to noise
(S/N) ratio. It is also evident that spectra for the liquid samples show
features not detected in the gas-phase experiments. Specifically, the
three spectra show peaks at 361 cm−1, associated with the W–Cl4
vibrational mode, 1133 cm−1, assigned to the C–N–C asymmetric vi-
brational mode, and 1396 cm−1 assigned to the symmetrical umbrella
mode of two terminal CH3 groups. These Raman shifts were not de-
tected in the gas-phase experiments and more detailed experiments
are in progress.
DFT calculations.— DFT calculations were performed to better
interpret the experimental results and evaluate possible mechanistic
steps in the thermal decomposition of 1b. We have reported further
comparison of DFT calculations to experimental results for aryl- and
alkylimido precursors.30 The initial calculations were a geometry op-
timization (B3LYP/LanL2DZ) using experimentally determined bond
lengths and angles for 1a as a test of the computational method (Fig-
ure 8). Although the results of the NMR study indicate that 1a will
Figure 6. Peak deconvolution results: (A) spectrum g in panel D of
Figure 4 and (B) N2 vibrational bands from additional experiment (spectrum
a) and peaks associated with N2 vibrational modes and with C≡N stretching
frequencies (spectra b, c, and d are the same as spectra g, d, and a of panel D
in Figure 4, respectively).
Figure 7. Liquid Raman spectra of 1b in benzonitrile solution (solid line) compared to pure liquid benzonitrile (dashed line). Note that the red spectra are intensity
shifted for clarity.
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Journal of The Electrochemical Society, 159 (5) H545-H553 (2012)
H549
Table I. Experimental and Computationally Optimized Bond
Lengths (Å) and Bond Angles (◦) for Complex 1a.
Experimental8
Split Basis Set31,a
This Workb
W1-N1
W1-N3
W1-Cl
N1-N2
1.769(5)
2.224(7)
2.347(16)
1.271(8)
1.438(7)
95.9(4)
1.749
2.268
2.388
1.289
1.467
96.8
1.757
2.220
2.429
1.283
1.478
96.2
N2-C1
N1-W1-Cl
N3-W1-Cl
N2-N1-W1
84.07(4)
180.0(0)
83.24
178.0
83.75
179.6
a B3LYP/LanL2DZ for W, 6-311G for other elements,
b B3LYP/LanL2DZ for all elements
Table II. Calculated Bond Dissociation Enthalpy (ꢀH◦) and Gibbs
Energy Change (ꢀG◦) for the W–N1 and N1–N2 Bonds.
BDE(W–N1) (kcal/mol)a
BDE(N1–N2) (kcal/mol)a
Figure 8. Computationally optimized geometry of 1a.
ꢀHo (298 K)
ꢀHo (900 K)
ꢀGo (298 K)
ꢀGo (900 K)
113.1
111.4
101.0
77.8
88.0
86.9
73.2
43.7
have been converted to 1b before it is injected into the reactor, only
the substituent of the nitrile ligand is different and the critical bond
lengths and bond angles of 1a should carry over to 1b. The computa-
tionally optimized geometry of 1a (Figure 8) is summarized in Table I
along with experimental data8 and previous calculations using a split
valence basis set.31 For most of the calculated values in Table I, the
B3LYP/LanL2DZ geometry optimization more closely approximates
the experimental solid state structure.
a B3LYP/LanL2DZ
Since the dissociation of acetonitrile from 1a was observed to
be facile near room temperature, calculations assumed that loss of
the nitrile was a rapid first step in decomposition of 1a in the reac-
tor. Further calculations on possible intermediates thus began with
Cl4W(NNMe2) (2), the product of acetonitrile loss from 1a (or loss
of benzonitrile from 1b). The experimental observation of dimethy-
lamine and methylmethyleneimine among the decomposition prod-
ucts suggest two possible dissociative reactions of 2 as depicted in
Scheme 1. We have considered both dissociation reactions since
AACVD from 1b involves high deposition temperature. Values for
the enthalpy and Gibbs energy of W–N1 and N1–N2 dissociation (Ta-
ble II) were obtained using statistical thermodynamics. Homolysis of
W–N1 affords open shell products and the spin states were set accord-
ingly during calculations. In the case of N1–N2 homolysis, the prod-
ucts are doublets. The calculated W–N1 bond dissociation enthalpy is
significantly higher than the N1–N2 dissociation energy over the 298
to 900 K temperature range. These bond strengths can be viewed in
terms of the limiting resonance structures A and B (Figure 9). Crys-
tallographic data are consistent with representation B being the major
Figure 9. Limiting resonance structures of complexes 1a and 1b.
contributor, which is in accord with calculated N1–N2 BDE values that
are higher than the experimental value for free 1,1-dimethylhydrazine
(49.6 kcal/mol).32
The dissociation of benzonitrile to form Cl4W(NNMe2) (2)
(Scheme 2) is postulated to be the first step in thermal decompo-
sition of 1b on the basis of the nitrile dissociation rates for conversion
of 1a to 1b. The assumed parallel between the reactivity of 1a and
N1
H3C
CH3
N1-N2 cleavage
H3C
H3C
CH3
N2
N1
W
Cl
Cl
Cl
Cl
N2
W
3
4
Cl
Cl
Cl
Cl
CH3
N2
N1
Cl
Cl
Cl
Cl
W
W-N1 cleavage
2
5
6
Scheme 1. Products of cleavage of the W–N1 and N1–N2 bonds of complex 2. Gibbs energy values (ꢀG◦ ) are in kcal/mol.
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H550
Journal of The Electrochemical Society, 159 (5) H545-H553 (2012)
Scheme 2. Gibbs energy change (ꢀG◦) for initial ligand substitution and dissociation reactions of complex 1. All energy values are in kcal/mol at 298 K.
the imido complex Cl4(CH3CN)W(NiPr) is consistent with compu-
tational assessment of the bond strength between W and N3 using
the Wiberg bond index in the natural bond orbital (NBO) analysis of
complex 1a. As shown in Figure 10, the coordinate covalent bond of
the acetonitrile ligand with W has the weakest bond order (0.3310).
Dimethylaminyl radical (3)33–35 and 1,1-dimethyldiazene (5) have
been previously generated by other methods and their reactions have
been reported. Dimerization of 1,1-alkyldiazenes is known to form
tetrazene derivatives, which in the case of 5 would afford tetramethyl-
2-tetrazene (7). Since decomposition of 7 affords two equivalents of
dimethylaminyl radical 3 (Scheme 3), the critical reactive intermediate
from cleavage of either the W–N1 or N1–N2 bond of 2 (Scheme 1)
will be 3.
Known reactions of dimethylaminyl radical (disproportiona-
tion, dimerization, dissociation and rearrangement) are summarized
in Scheme 4. Two dimethylaminyl radicals can yield methyl-
methyleneimine (8) and dimethylamine (9)36–38 or tetramethylhy-
drazine (7) by disproportionation or recombination, respectively34,39
(Scheme 4, path a). Since bimolecular reactions of radicals have low
activation energies, products of recombination and disproportion are
expected to be formed in similar quantities.40 In addition to these re-
actions, dimethylaminyl radical undergoes dissociation into methyl-
nitrene and methyl radical (Scheme 4, path b). Subsequent hydro-
gen shift in methylnitrene affords methyleneimine41–43 (Scheme 4,
path c).
Comparison of experimental results with DFT calculations.— The
calculated vibrational frequencies can be correlated to products pos-
tulated in the proposed decomposition mechanisms (Schemes 1 and
4a). Calculated and corrected Raman active stretching frequencies
with experimental values for all compounds including the gas-phase
authentic sample of benzonitrile are listed in Table III.
Assignments for 9 of the observed bands attributed to benzonitrile
(462, 762, 1003, 1181, 1197, 2234, 3077, 3128, 3175 cm−1) were
derived from gas phase benzonitrile Raman spectra and deconvolu-
tion data as well as DFT calculations. The reported Raman bands of
1240 and 3377 cm−1 of gas-phase dimethylamine44 and DFT results
were used for assigning 1247 and 3377 cm−1 to the CH3 rocking and
N–H stretching motions in dimethylamine, respectively. In addition,
1638 cm−1 was assigned to C=N stretching of methylmethyleneimine
(8). This assignment is supported by IR data39 and the Raman charac-
teristic group frequency.45 Given that dimethylamine (9) and imine 8
would ultimately result from homolysis of the N1–N2 bond followed
by disproportionation of radical 3, observation of these products is
consistent with N1–N2 cleavage. The possibility of W–N1 dissocia-
tion, however, cannot be excluded since this reaction can produce the
same radical intermediate (3) through tetramethyltetrazene formation
(Scheme 4).
CH3
H3C
H3C
CH3
0.9321
N2
0.9320
0.9321
0.9322
N2
N1
W
1.2990
1.2965
N1
1.8954
1.7417
1.0526
Cl
1.1096
0.8380
Cl
The peak at 3426 cm−1 is tentatively assigned to the N–H stretch-
ing mode of HNWCl4 (14) based on the predicted gas phase reaction
chemistry and the experimentally observed N–H vibrational frequen-
cies of W(VI) model compounds with imido (NH) and amido (NH2)
ligands.46 The N–N cleavage reaction (Scheme 1) would produce the
nitrogen-centered radical · NWCl4 (4) as the inorganic product. Sub-
sequent abstraction of a hydrogen atom by 4 would afford the W(VI)
parent imido complex 14. The Nsp-H bond of 14 would be expected
to have a high bond dissociation energy, making hydrogen abstraction
from several species in the reactor energetically favorable.47
Cl
Cl
1.0554
W
0.8390
0.7806
1.1108
Cl
Cl
Cl
Cl
0.5161
0.7849
N3
0.3310
N3
2.6326
2.694
C
C
1.1164
1.1278
CH3
Estimate of aerosol evaporation time.— The analysis of this study
assumes that the decomposition reactions that occur are for inlet
species 1b, benzonitrile, and acetonitrile, and that they occur in the gas
phase along the probed centerline of the reactor. The aerosol assisted
delivery of the low volatility precursor complicates the analysis of the
experimental results since the precursor is initially in a liquid solution
but quickly evolves to a gas mixture due to the high vapor pressure of
the solvent. It is of interest therefore to estimate the position at which
1a
1b
Figure 10. Wiberg bond indexes for 1a and 1b.
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Journal of The Electrochemical Society, 159 (5) H545-H553 (2012)
H551
Scheme 3. Production of dimethylaminyl radical (3) after W–N1 cleavage to form 5. Gibbs energy values are in kcal/mol.
H3C
CH3
CH2
H3C
H3C
Δ
Δ
G
G
ο = −44.23
ο = −21.89
N
N
H
8
9
H3C
H3C
CH3
CH3
(a)
N
CH3
N
N
3
H3C
H3C
CH3
10
CH3
(b)
(c)
Δ
Δ
G
G
ο = 51.06
N
N
12
11
3
‡
Δ
G‡ = 49.42
> 400 oC
H
H
H
N
H3C
ο = −88.83
H2C NH
C
N
11
13
11TS
Scheme 4. Known reactions of dimethylaminyl radicals and their calculated (B3LYP/LanL2DZ) reaction energies at 298 K. Gibbs energy values are in kcal/mol.
Table III. Calculated and Corrected Raman Active Stretching Frequencies.
Molecule
Experimental
Calculated
Corrected
Assignment
BN
BN
BN
1b
BN
2
1b
BN
9
8
–
BN
–
–
–
–
–
BN
BN
BN
9
462
762
466
769
461
761
Ph ring stretching
Ph ring stretching
Ph ring breathing
C–H in plane bend
C–H in plane bend+ C–CN vibration
CH3 scissoring + W≡N stretching
C–Ph stretching + N3-W-N1 stretching + C–N–C torsion
C–H in plane bend
1003
1177
1181
1189
1193
1197
1247
1638
1708
2234
2241
2516
2547
2580
2834
3077
3128
3175
3377
3426
1025
1226
1236
1246
1243
1248
1279
1696
1005
1177
1187
1196
1193
1198
1228
1638
CH3 rocking
C=N stretching
2335
2237
C≡N stretching
C≡N stretching in BN?
CH3 symmetric stretching (-N(CH3)2; aromatics)
C–H asymmetric stretching
C–H asymmetric stretching
3142
3192
3238
3079
3128
3173
NH stretching
NH stretching
14
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H552
Journal of The Electrochemical Society, 159 (5) H545-H553 (2012)
the aerosol can be assumed to be sufficiently evaporated and thus not
affect the measurement.
To investigate the homogeneity of the introduced precursor, a
model for droplet formation and evaporation was used to estimate
the droplet distribution and time for evaporation. The results were
then compared to experiment. The initial droplet size produced by the
nebulizer was first estimated using Lang’s correlation.48 This model
assumes each droplet generated by the ultrasonic nebulizer is a homo-
geneous sphere. The droplet diameter estimated for the system used
in this study is ca. 2.7 μm. Next, the rate of evaporation was estimated
by modeling the evaporation process for a droplet of pure benzonitrile
in a laminar carrier gas. Briefly, the energy balance equation for a
benzonitrile droplet gives:
hc AꢀT
Nc =
[1]
λ
where Nc, hc, A, ꢀT and λ denote the solvent evaporation rate, heat
transfer coefficient across the liquid-gas, total droplet surface area
introduced to the reactor (6.18 × 10−4 m2), temperature difference
between droplet surface and ambient, and latent heat of vaporization
(3.67 × 102 J/g), respectively. To estimate the value of the heat transfer
coefficient, the Nusselt number, Nu, is calculated for a liquid droplet
in a flowing gas, using the empirical relation:
985
995
1005
1015
Raman shift [cm-1]
Figure 11. Detected Raman spectra along the center line for injected ben-
zonitrile. The observed wavenumber 1003 cm−1was assigned to the phenyl
breathing mode of benzonitrile. Raman spectra are shifted for clarity. Black
bottom signal was recorded at the gas inlet (cold region) and top signal just
below the heated susceptor.
Nu = 2.0 + 0.6Re0.5 Pr0.33
[2]
where Re is the Reynolds number and Pr is the Prandtl number. Using
this equation, the heat transfer coefficient, hc is estimated as:
the lack of a measurable peak shift in the phenyl ring breathing mode,
the assumption of homogeneous gas-phase reaction in the whole CVD
reactor is substantiated.
ꢀ
ꢁ
ꢀ
ꢁ
0.5
0.33
hc D
Dνρ
Cpμ
= 2.0 + 0.6
[3]
κd
μ
κd
whereD and κd are the droplet diameter and liquid thermal conductiv-
ity (0.1317 W/mK at 366.5 K). In addition, ν, ρ, μ, and Cp denote mean
fluid velocity (2.5 cm/s), density (1.03 g/cm3), viscosity (1.25 × 10−3
Pa · s), and specific heat capacity at constant pressure (0.4369 J/kg K),
respectively. With the property values mentioned above, Re and Pr
have the value of 5.45 × 10−5 and 3.81, respectively. From Eq. 3, the
heat transfer coefficient is calculated as 9.79 × 104 J/sm2K.
The value of the temperature difference between the injected
droplet and the gas ambient was assumed to be 10 K since the drop size
is relatively small. Eq. 1 gives the value of the solvent evaporation rate
as 1.65 g/s and applying this model to the reaction conditions gives a
drop evaporation time of only 17 ms for a droplet of mean diameter
2.7 μm. Using the inlet velocity of 2.5 cm/s velocity, the distance
traveled by this droplet for full evaporation is 0.43 mm, which is well
before the measurement zone.
Conclusions
The decomposition pathways of the dimethylhydrazido tungsten
complexes (RCN)Cl4W(NNMe2) (1a: R = CH3; 1b: R = Ph) were
investigated using Raman spectroscopy, NMR kinetics, ion cyclotron
resonance and DFT calculations at the B3LYP/LanL2DZ level of
theory. The rate of loss of acetonitrile from 1a was determined via
variable temperature NMR spectroscopy to yield an activation en-
thalpy of 23.0 0.2 kcal/mol and an activation entropy of 28.8 0.6
cal/mol · K in the temperature range 50 to 84◦C. These results indi-
cate that acetonitrile dissociation is rapid and the precursor injected
from benzonitrile solutions of 1a is thus predominantly 1b, which
rapidly dissociates to 2. Raman scattering experiments in the up-flow,
cold-wall aerosol-assisted CVD reactor identified peaks consistent
with methylmethyleneimine (8), dimethylamine (9), and HNWCl4
(14). These frequencies were assigned using gas-phase DFT calcu-
lations and literature data. The bond cleavages of both W–N1 and
N1–N2 are possible, although the calculated bond strength of W–N1
is higher than that of N1–N2. Detection of methylmethyleneimine (8)
and dimethylamine (9) is consistent with homolysis of the N1–N2 bond
followed by disproportionation of radical 3, however, this radical can
also be produced by W–N1 dissociation. DFT calculations suggested
that homolysis of the N1–N2 bond can lead to HNWCl4 (14) after
disproportionation of the resulting radicals. Tentative assignment of
the peak at 3426 cm−1 to the N–H stretching mode of HNWCl4 (14)
evidences N1–N2 bond homolysis, consistent with experimental evi-
dence on the deposition of films from the related tungsten imido com-
The droplet diameter is predicted to show a normal distribution
with standard deviation, σ, as follows:49
ρ0.1
0.41τ0.50μ0.18
σ = 4.6 × 10−5
[4]
f
where f is the nebulizer frequency (1.44 MHz), τ is the surface tension
(0.041 N/m) and μ is the dynamic viscosity (1.25 × 10−3 Pa · s). Using
these values the standard deviation of drop diameter, σ, is 5.0 μm. This
implies that for a droplet at 2σ diameter (17.7 μm) only 1.8 particles
are present in the reactor volume and will be evaporated at 2.0 mm
position in the reactor.
There is experimental evidence to support this conclusion. Frolov
et al.49 reported experimental results supported by calculations that
show 70 μm n-tetradecane droplets vaporize in 70 ms at a liquid sur-
face temperature of 293.15 K and a gas temperature of 573.15 K. It is
noted that n-tetradecane has a higher boiling temperature (526 K) than
benzonitrile (464 K) and their droplet diameter was approximately 4
times larger than that in this study. As further evidence, the spectral
positions of the strongest phenyl ring breathing mode of benzonitrile
were carefully measured. No displacement of peak positions, as would
be expected between liquid and gas phase samples, was detected along
the reactor centerline as shown in Figure 11. Based on the modeling
results, comparison to experiment using a less volatile solvent, and
i
plexes Cl4(CH3CN)W(NR) where R = Pr, Ph and allyl.3,24,25 Finally,
homogeneity of introduced precursor was confirmed by conducting
droplet evaporation analysis based on the analytical and spectroscopic
evidences.
Acknowledgments
We thank the National Science Foundation for support under
NSF grant CHE-0911640. S.Z.H thanks the Arnold and Mabel Beck-
man Foundation for support of her undergraduate research through a
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Journal of The Electrochemical Society, 159 (5) H545-H553 (2012)
H553
Beckman Scholar Award. M.E. thanks the UF HHMI Science for Life
program for support of his undergraduate research.
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