Solubility of several short-chain lithium dialkyldithiocarbamates in liquid and supercritical carbon dioxide

Article abstract of DOI:10.1021/je0502952

The solubility of lithium diethyldithiocarbamate, lithium dipropyldithiocarbamate, lithium dibutyldithiocarbamate, and lithium bis(trifluoroethyl)dithiocarbamate was measured in liquid and supercritical carbon dioxide at (298, 308, and 318) K and at pressures between (80 and 250) bar. The solubilites were measured experimentally through traditional cloud point observations using a variable-volume stirred vessel with a sapphire window for visual inspection of the contents. These four short-chain lithium dialkyldithio-carbamates showed an increase in solubility with increasing density (or pressure) at fixed temperature. This increase was fairly independent of the temperature. Solubilites also increased with increasing temperature. The data were accurately modeled using an equation derived from association laws, which had a strong solvent density and temperature dependence.

Full text of DOI:10.1021/je0502952

2088
J. Chem. Eng. Data 2005, 50, 2088-2093
Solubility of Several Short-Chain Lithium Dialkyldithiocarbamates
in Liquid and Supercritical Carbon Dioxide
Randy D. Weinstein,*,† Laurel L. Grotzinger,† Patrick Salemo,† Donna M. Omiatek,‡ and
Carol A. Bessel‡
Department of Chemical Engineering and Department of Chemistry, 800 Lancaster Avenue,
Villanova University, Villanova, Pennsylvania 19085
The solubility of lithium diethyldithiocarbamate, lithium dipropyldithiocarbamate, lithium dibutyldithio-
carbamate, and lithium bis(trifluoroethyl)dithiocarbamate was measured in liquid and supercritical carbon
dioxide at (298, 308, and 318) K and at pressures between (80 and 250) bar. The solubilites were measured
experimentally through traditional cloud point observations using a variable-volume stirred vessel with
a sapphire window for visual inspection of the contents. These four short-chain lithium dialkyldithio-
carbamates showed an increase in solubility with increasing density (or pressure) at fixed temperature.
This increase was fairly independent of the temperature. Solubilites also increased with increasing
temperature. The data were accurately modeled using an equation derived from association laws, which
had a strong solvent density and temperature dependence.
Introduction
that over 32 000 g of wastewater along with 72 g of various
other chemicals are required to produce a 2 g microchip.10
Organic slurries are equally troublesome as they often
include halogenated species such as chloroform or carbon
tetrachloride.
The generalized production of integrated circuits1 is a
multistep process in which the interlayer dielectric is
etched into trenches and vias through lithographic pro-
cesses. A barrier layer is added, and then void-free inter-
connect filling in the vias and trenches is typically accom-
plished using electrochemical deposition. This gives a
nonuniform conductive coating, with superfilling of metal
above the vias.2 To successfully build the required multi-
level metal interconnect structures, the metallic overbur-
den must be removed and the conductor/SiO2 surface
planarized before the next level of interconnect can be
added. Recently, copper has received much attention as an
interconnect for computer chip manufacturing since it has
greater electromigration resistance and higher current
density than traditional interconnects.3-7
Although copper’s properties are outstanding, there exist
several major challenges to its implementation8 including
the fact that conventional dry-etch techniques to remove
the overburden are not adequate.9 Solution-based tech-
niques to remove the overburden are commonly conducted
in an aqueous slurry media; however, the porous low-
dielectric inorganic and organic interlayer dielectric ma-
terials in the circuit are highly incompatible with water.
Water removal is an extremely energy intensive and costly
process. Inefficient water removal can result in higher than
expected dielectric constants and an increase in intercon-
nect defects during the manufacturing process. Water also
has a high surface tension and viscosity that can collapse,
by capillary action, the exquisitely fine (high aspect ratio)
structures of the interconnect trenches and vias. The water
used in interconnect manufacturing must be of ultrahigh
purity as suspended particulates or dissolved ions or
compounds can contaminate the wafer surfaces causing
shorting or unwanted conductive pathways. It is estimated
Instead of using water or traditional organic solvents,
we plan to use liquid and supercritical carbon dioxide as
the solvent for copper overburden removal in a chemical
mechanical planarization (CMP) process. Carbon dioxide
has low viscosity, has high diffusivity, and can easily be
reused in the CMP processes by reducing the pressure to
remove the impurities. Liquid carbon dioxide has a very
low surface tension. It is also inexpensive, environmentally
friendly, and will not oxidize to produce impurities during
the process. We have recently explored the use of various
â-diketones11,12 and bis(acetylacetone)ethylenediimine lig-
ands13 for copper removal in carbon dioxide. We have also
used the lithium dialkyldithiocarbamates studied in this
paper for copper removal14 and have found them to have
removal rates on the order of 250 monolayers/min, which
is comparable to the currently required removal rates of
100 monolayers/min observed in industrial protocols. In
this paper, we are not reporting the results of our CMP
studies, but rather the solubilities of several short-chain
lithium dialkyldithiocarbamates in liquid and supercritical
carbon dioxide. These solubilities are critical for the scale-
up, condition selection, design, and implementation of
carbon dioxide-based CMP processes.
Several other investigators have explored the use of
several other dithiocarbmates in carbon dioxide for metal
removal, separation, and solubilite studies,15-24 and some
have used organic cosolvents to enhance solubilites.25,26
However, organic cosolvents have the potential to contami-
nate copper interconnects, and we wish to avoid their use.
Interestingly, none of the studies found in the literature
explored the solubilites of short-chained lithium dialkyl-
dithiocarbamates or their use in copper CMP removal.
Short-chain dithiocarbamates with small counterions (such
as lithium) are advantageous as they do not easily decom-
* Corresponding author. Phone: (610)519-4954. Fax: (610)519-7354.
E-mail: randy.weinstein@villanova.edu.
† Department of Chemical Engineering.
‡ Department of Chemistry.
10.1021/je0502952 CCC: $30.25 © 2005 American Chemical Society
Published on Web 10/12/2005

Journal of Chemical and Engineering Data, Vol. 50, No. 6, 2005 2089
Table 1. Names and Structures of Lithium Dialkyldithiocarbamates Synthesized
to the PEA vessel. The PEA vessel’s temperature (( 0.2
K) was monitored by a type K thermocouple, and pres-
sure was measured by a Honeywell pressure transducer
(( 0.1 %).
For an experimental solubility measurement, the PEA
vessel was preheated to the desired temperature using a
recirculating bath. The vessel was flushed with low pres-
sure carbon dioxide, and a small amount of solid was
weighed with an analytical balance, placed in the bottom
of the vessel, and sealed. More low pressure carbon dioxide
was purged through the vessel, which was then set to the
smallest volume (5.5 mL) by moving the piston via a
computer interface. The vessel was pressurized to the
desired set point, and stirring was initiated. Once the
desired pressure was obtained, the pressure was held
constant by the syringe pump (within 0.5 bar) while the
vessel volume was increased by raising the piston slowly
until all solid was observed to dissolve. The vessel was then
isolated from the pump via a valve, and the volume of the
vessel was increased slowly until the cloud point was
observed (the point at which material dropped out of
solution). At the cloud point, the vessel volume, tempera-
ture, and pressure were recorded. The solubility was then
calculated using the known amount of solid loaded into the
vessel along with the amount of carbon dioxide present in
the vessel, which was calculated from the known volume,
temperature, and pressure using the NIST31 tables for
carbon dioxide. The vessel was then vented, cleaned with
ethanol and acetone, and thoroughly dried prior to the next
measurement.
Figure 1. Phase equilibrium apparatus.
pose to produce bulky organic byproducts. Decomposition
products have the potential to contaminate integrated
circuits. The lithium dithiocarbamates are preferred as the
larger sodium cations are much less soluble in condensed
carbon dioxide media. It is important to note that the alkali
metal counterion is necessary (i.e., lithium cannot be
replaced with hydrogen) since the protonated forms of the
dithiocarbamates are not stable and quickly decompose.27
Experimental Section
Materials. Research grade 5 carbon dioxide with a
purity of 99.999 % was supplied by BOC Gases. Diethyl-
amine, dipropylamine, dibutylamine, butyllithium (2.5
mol‚L-1 in hexanes), diethyl ether, and carbon disulfide
were supplied by Sigma-Aldrich. Bis(trifluoroethyl)amine
was obtained from Synquest. All chemicals were used as
received.
Lithium Dialkyldithiocarbamate Synthesis. In a
typical synthesis, (0.013 to 0.017) mol of one of the amines
was added to 10 mL of diethyl ether in a 100 mL round-
bottom flask with stir bar held in an acetone-dry ice bath.
The flask was purged with nitrogen, and a molar equiva-
lent of n-butyllithium was added slowly to the contents.
After 1 h, carbon disulfide (molar equivalent) in 10 mL
diethyl ether was slowly added via a dropping funnel, and
the flask was placed in a water-ice bath and then allowed
to warm to room temperature overnight. The solution was
reduced by rotary evaporation to about half the initial
volume, vacuum filtered, and then purified via recrystal-
lization or a silica gel column (methylene chloride eluent).
Details of the synthesis, purification, and characterization
of the lithium diakyldithiocarbamate (Li DDC) products
(shown in Table 1) are presented elsewhere.14
Solubility Measurements. Solubilites of each lithium
dialkyldithiocarbamate were found through cloud point
measurements using a system manufactured by Thar
Design Technologies (PEA-30ML phase equilibrium ana-
lyzer shown in Figure 1). We have successfully used this
system to match the solubilities of ketoprofen found in the
literature,28,29 and we have measured the solubilites of
several anesthetics29 and â-adrenergic blocking agents30 in
liquid and supercritical carbon dioxide previously.
The PEA vessel contained a stirrer, a sapphire window
to observe the cloud point, a heating jacket, and a mov-
able piston that could change the PEA volume between (5.5
and 30) mL. A syringe pump (cooled to 5 °C) was used to
deliver a fixed and controllable pressure of carbon dioxide
Results and Discussion
The solubility mole fractions of the four lithium dialkyl-
dithiocarbamates in liquid and supercritical carbon dioxide
are shown in Tables 2-5 and Figures 2-5 (the dashed lines
in these four figures are just linear fits of the data to assist
the reader in observing trends, and they are not accurate
models of the data). Solubilites were on the order of 10-5
to 10-4 mole fraction and increased with increasing density
(and hence pressure) of carbon dioxide. These relatively low
mole fractions were expected because of the ionic nature
of the Li DDCs. However, even with low solubilites, copper
removal rates with these compounds in carbon dioxide were
faster than current industrial protocols.14
Some unique observations can be made concerning the
solubility trends. First of all, the solubility of Li PDDC
(with an odd number of carbons in the R group) is sig-
nificantly lower than the solubility of both the Li EDDC
and the Li BDDC (both with an even number of carbons
in the R group). Since the alkyl groups are short, no
significant difference in solubility would be expected
based only on their length since longer chain lengths are
generally required to see significant effects on solu-
bility.32 The two R groups attached to the nitrogen atom,
however, do have the ability to twist and form different
orientations when in solution. There might be some ori-
entation with even chain lengths that place the methylene
and methyl groups in a favorable orientation to enhance
solubility. Several others investigators33-36 have also ob-

2090 Journal of Chemical and Engineering Data, Vol. 50, No. 6, 2005
Table 2. Mole Fraction Solubility (y) of Li EDDC in Liquid and Supercritical Carbon Dioxide
T ) 298 K
F/g‚cm-3
T ) 308 K
F/g‚cm-3
T ) 318 K
F/g‚cm-3
P/bar
y × 105
P/bar
y × 105
P/bar
y × 105
85
100
125
150
175
200
225
250
0.79
0.82
0.85
0.88
0.90
0.92
0.93
0.94
0.026
1.02
1.97
2.91
3.65
4.09
4.46
5.84
100
125
150
175
200
225
250
0.72
0.78
0.82
0.84
0.87
0.89
0.90
1.65
3.61
3.92
5.36
5.97
6.96
10.02
125
150
175
187
200
225
0.68
0.74
0.78
0.80
0.81
0.84
2.04
5.60
8.83
11.46
12.70
16.47
Table 3. Mole Fraction Solubility (y) of Li PDDC in Liquid and Supercritical Carbon Dioxide
T ) 298 K
F/g‚cm-3
T ) 308 K
F/g‚cm-3
T ) 318 K
F/g‚cm-3
P/bar
y × 105
P/bar
y × 105
P/bar
y × 105
122
151
175
200
225
237
250
0.85
0.88
0.90
0.92
0.93
0.94
0.94
0.05
0.25
0.55
0.75
1.17
1.37
1.99
100
125
150
175
200
225
237
250
0.71
0.78
0.82
0.84
0.87
0.89
0.89
0.90
0.056
0.23
0.59
0.90
1.87
2.10
2.42
2.60
121
152
175
200
225
237
250
0.66
0.75
0.78
0.81
0.84
0.85
0.86
0.20
1.02
1.97
2.33
2.74
3.15
4.00
Table 4. Mole Fraction Solubility (y) of Li BDDC in Liquid and Supercritical Carbon Dioxide
T ) 298 K
F/g‚cm-3
T ) 308 K
F/g‚cm-3
T ) 318 K
F/g‚cm-3
P/bar
y × 105
P/bar
y × 105
P/bar
y × 105
80
89
0.78
0.80
0.82
0.85
0.88
0.90
0.91
0.93
0.94
2.57
3.15
3.94
4.80
5.74
7.48
8.34
9.16
11.34
85
99
0.61
0.71
0.78
0.81
0.84
0.86
0.88
0.90
3.90
5.22
81
90
0.25
0.34
0.50
0.67
0.74
0.78
0.81
0.84
0.86
9.23
10.29
10.56
12.64
14.33
14.26
14.90
15.24
16.89
99
125
149
175
199
224
249
6.98
100
124
149
174
199
224
249
124
149
174
199
224
248
8.19
10.29
10.84
12.35
14.37
Table 5. Mole Fraction Solubility (y) of Li FDDC in Liquid and Supercritical Carbon Dioxide
T ) 298 K
F/g‚cm-3
T ) 308 K
F/g‚cm-3
T ) 318 K
F/g‚cm-3
P/bar
y × 105
P/bar
y × 105
P/bar
y × 105
99
124
149
173
199
224
248
0.82
0.85
0.88
0.90
0.91
0.93
0.94
2.57
3.41
3.68
5.13
5.13
5.66
6.47
99
99
0.71
0.71
0.77
0.81
0.84
0.86
0.88
0.90
0.90
3.15
3.22
3.96
4.29
5.35
5.69
5.79
6.08
6.18
99
124
149
173
199
224
248
0.48
0.67
0.74
0.78
0.81
0.84
0.86
1.73
3.64
4.45
5.29
5.81
5.94
6.86
124
149
173
199
224
249
249
served some changes in solubilities for odd versus even
chain lengths.
to model the solubilites using a traditional equation of
state-fugacity approach. First of all, equation of state
parameters, while known for pure carbon dioxide, are not
available for the lithium dialkydithiocarbamamtes. The
critical properties, acentric factor, and other properties of
the lithium dialkydithiocarbamates are not available, nor
are they easily measured since the compounds decompose
before they reach a critical temperature. Estimation tech-
niques, such as models based upon group contribution
theory, have not been worked out for these unique ionic
compounds. Furthermore, sublimation pressures of the
solids would be required as a function of temperature to
accurately use the equation of state-fugacity approach to
model and predict solubilites, and again this property for
the pure lithium dialkydithiocarbamates is not available
or easily predicted. Instead of using a rigorous equation of
state-based model, we examined the available literature
for models that showed heavy dependence on both the
density and the temperature of carbon dioxide. The model
also needed to have a density dependence that was able to
The Li FDDC (Figure 5) solubility also showed some
interesting results when compared to its unfluorinated
analogue, Li EDDC (Figure 2). Adding fluorinated func-
tional groups to compounds tends to increase their solubil-
ity in carbon dioxide.37-41 However, to show significant
solubility increases, a large number of carbons usually need
to be fully fluorinated. We were only able to fluorinate one
carbon on the ethyl functional group due to the availability
of starting amines. This low level of fluorination had
minimal effect on the solubility. At low temperatures, it
appeared the Li FDDC had a slightly increased solubility
over Li EDDC. At higher temperatures, the Li FDDC
tended to have a lower solubility.
By examining Figures 2-5, it is clear that both density
and temperature have a strong influence on the solubilities
of the lithium dialkyldithiocarbamates in liquid and su-
percritical carbon dioxide. As both density and temperature
are increased, the solubility increases. It would be difficult

Journal of Chemical and Engineering Data, Vol. 50, No. 6, 2005 2091
Figure 2. Mole fraction solubility of Li EDDC in liquid and
supercritical carbon dioxide as a function of density. [, 298 K; 9,
308 K; 2, 318 K.
Figure 5. Mole fraction solubility of Li FDDC in liquid and
supercritical carbon dioxide as a function of density. [, 298 K; 9,
308 K; 2, 318 K.
Table 6. Fitted Model Parameters
lithium dialkyldithiocarbamate
k
a
b
Li EDDC
Li PDDC
Li BDDC
Li FDDC
9.5
12.7
5.2
-10,699
-9,895
-5,023
-2,409
-31.3
-56.7
-19.8
-22.2
4.3
equilibrium with the system rather than pure solid lithium
dialkyldithiocarbamate in the carbon dioxide rich phase
(see eq 1):
DDC + kCO2 a DDC‚CO2
(1)
k
The association number (k) is not expected to be an integer
as, in many cases, the solvated complexes are not stoichio-
metric.42
It is possible to derive equilibrium expressions from the
mechanism shown in eq 1 (see Chrastil’s original work42
for the details) to derive a final expression shown in eq 2:
Figure 3. Mole fraction solubility of Li PDDC in liquid and
supercritical carbon dioxide as a function of density. [, 298 K; 9,
308 K; 2, 318 K.
a
T/K
c/g‚L-1 ) (F/g‚L-1)k exp
+ b
(2)
(
)
where c is the concentration of Li DDC in carbon dioxide,
F is the density of carbon dioxide, k is the association
number, T is the temperature, and a and b are constants.
The two constants, a and b, have physical significance
shown in eqs 3 and 4, respectively:
-∆H
R
a )
b ) ln(MDDC + MCO ) + q - k ln(MCO
)
2
(4)
2
where ∆H is the enthalpy change of solvation and phase
change combined, R is the universal gas constant, Mi is
the molecular weight of species i, and q is an integration
constant when the Clausius equation was applied and has
some dependence upon the melting points of the solute and
solvent.43 The fitted (by least-squares method) model
parameters for the four lithium dialkyldithiocarbamates
are presented in Table 6. A parity plot for the model is
shown in Figure 6, which reveals a relatively good fit of
the data.
The enthalpy of solvation (expressed by the a constant)
tended to become less negative with increasing alkyl chain
length. It also increased significantly when the alkyl
substituent was partially fluorinated. The association
number (k) tended to decrease with increasing chain length
Figure 4. Mole fraction solubility of Li BDDC in liquid and
supercritical carbon dioxide as a function of density. [, 298 K; 9,
308 K; 2, 318 K.
vary independent of the temperature since the slopes of
straight lines in Figures 2-5 changed with varying tem-
perature. The model derived from the association laws of
Chrastil42 met all of our requirements.
Chrastil’s model is based on the assumption that when
a molecule of lithium dialkyldithiocarbamate dissolves into
dense carbon dioxide a specific number of carbon dioxide
molecules (k) associate closely with the solvated mole-
cule forming a shell that has large interactions with the
solvated molecule.43 It is this solvated complex that is in

2092 Journal of Chemical and Engineering Data, Vol. 50, No. 6, 2005
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(with the exception of the Li PDDC) and fluorination. As
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interact differently with carbon dioxide, possibly forming
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Conclusions
The solubilities of these four short-chain dialkyldithio-
carbamates were high enough to be effective chelating
agents for the removal of copper in liquid and supercritical
carbon dioxide. A simple association model with temper-
ature and solvent density dependence accurately captured
the solubility data and can be used for the design of a
carbon dioxide based CMP process. Solubilities increased
with increasing density and temperature.
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Acknowledgment
The authors appreciate the early work of Joseph Gribbin
of Villanova University on the solubility apparatus.
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Received for review July 26, 2005. Accepted September 6, 2005.
L.L.G. appreciates the support through the Villanova Center for the
Environment. This material is based upon work supported by the
National Science Foundation under Grant 0416040.
JE0502952