D178
Journal of The Electrochemical Society, 158 ͑3͒ D178-D186 ͑2011͒
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013-4651/2011/158͑3͒/D178/9/$28.00 © The Electrochemical Society
Characterization and Purification of Commercial SPS and MPS
by Ion Chromatography and Mass Spectrometry
a
a
b
Ryan G. Brennan, Melissa M. Phillips, Liang-Yueh Ou Yang, and
b, ,z
Thomas P. Moffat *
a
b
Analytical Chemistry Division, and Metallurgy Division, Material Measurement Laboratory, National
Institute of Standards and Technology, Gaithersburg, Maryland 20899-8551, USA
SPS ͑bis-͑3-sulfopropyl͒ disulfide͒ is an essential electrolyte additive used in the fabrication of copper interconnects by elec-
trodeposition. In electroplating baths, the disulfide component of SPS may be cleaved to form the thiol analog, MPS ͑3-
mercaptopropyl sulfonate͒, by either homogenous interactions with the Cu͑I͒ reaction intermediate or by dissociative adsorption
onto the copper surface. However, mechanistic studies into the role of these additives in copper electrodeposition are presently
constrained by limited knowledge of the purity of commercially available SPS and MPS. This report details the use of ion
chromatography ͑IC͒ and electrospray ionization mass spectrometry to characterize aqueous solutions of commercial SPS and
MPS source materials. Sulfate ͑2.0%͒ and propane disulfonic acid ͑0.9%͒ ͑PDS͒ were determined to be the principal impurities in
SPS ͑96.3% estimated purity, mass fraction͒. IC fractionation was used to purify and isolate SPS for surface and electroanalytical
studies. Stability of SPS, MPS, and PDS in the presence of O and Cu͑II͒ was also examined. No degradation of SPS or PDS in
2
aqueous solution was observed over a 3-month period. Solutions of MPS were metastable to O2 saturation, but the addition of
Cu͑II͒ resulted in formation of SPS by dimerization as well as parasitic PDS generation.
©
2011 The Electrochemical Society. ͓DOI: 10.1149/1.3537819͔ All rights reserved.
Manuscript submitted November 15, 2010; revised manuscript received December 20, 2010. Published January 21, 2011.
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State-of-the-art Cu wiring for microelectronic circuitry is fabri-
that generated immediately adjacent to the electrode interface.
1,2
cated by electrochemical deposition. The electroplating process
requires the use of a specific combination of additives in an acidic
Cu͑II͒ plating bath to enable void-free filling of recessed surface
features such as trenches and vias. Commercial additive packages
The concentration of Cu͑I͒ is an exponential function of potential,
with the prospect for homogenous Cu͑I͒/SPS chemistry being most
significant during metal deposition at small overpotentials. In a free
standing electrolyte, MPS is thought to be unstable to oxidative
dimerization by Cu͑II͒ to form SPS and Cu͑I͒, among other
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comprised at least three species: Cl , an accelerator such as bis-͑3-
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sulfopropyl͒ disulfide ͑SPS͒, and a polyether-based suppressor such
as polyethylene glycol ͑PEG͒ or a related block or branched
possiblities.
For plating cells open to the atmosphere, Cu͑I͒
species are also constantly being scavenged by O2 to regenerate
Cu͑II͒. Adding to the above complexity are results from recent vol-
tammetric and STM experiments in our laboratory indicate that in-
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copolymer. Chloride is a required coadsorbate for the formation of
the inhibiting PEG layer as well as the subsequent formation of the
SPS-derived accelerating surface phase. Feature filling involves a
−
dicate the rate constant for MPS adsorption on a Cl saturated sur-
−
competition between SPS and the polyether for Cl -saturated Cu
face may exceed that of SPS by a factor of 100 or more. Such a
significant difference opens the possibility that the low levels of
MPS might dominate studies formally aimed at quantifying the ki-
netics of SPS adsorption. Unambiguous knowledge of the purity of
the commercially available starting materials is required to rigor-
ously evaluate the relative contributions of the respective disulfide
and thiol species to the formation and function of the accelerator
surface phase. Such knowledge will also support the development of
a more quantitative understanding the Cu͑I͒/Cu͑II͒ disulfide/thiol
redox chemistry and its interaction with O2.
Beyond the mechanistic concerns detailed above, industrial users
have spent significant effort in development of tools for online mea-
surements of the mass fractions and stability of SPS and related
decomposition products that arise during electrolysis. Cyclic volta-
mmetric stripping has been widely used for additive control,
whereby the influence of the additives on the rate of metal deposi-
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surface sites. As the local surface area decreases, such as within a
filling trench, the more tightly bound SPS-derived adsorbates remain
on the surface while the polyether suppressor is displaced into the
electrolyte. This displacement results in an accelerated rate of Cu
deposition on the SPS-enriched concave surface segments, leading
to bottom–up superconformal filling. Several quantitative descrip-
tions of feature filling based on the curvature enhanced accelerator
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coverage ͑CEAC͒ mechanism are available. Nevertheless, much
remains unknown about the physical and chemical nature of the
SPS-derived accelerator surface phase.
A recent scanning tunneling microscope ͑STM͒ study of SPS
−
adsorption on a Cl saturated Cu͑100͒ surface revealed a plurality of
−
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lattice gas species diffusing on top of, or within, the Cl adlayer. In
addition to the dimer-like SPS species, smaller molecules suggestive
of the related thiol monomer known as MPS ͑3-mercaptopropyl sul-
fonate͒ were also evident. MPS could be formed by a dissociative
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tion is monitored via integrated charge.
Some potential-
−
reductive adsorption of SPS dimers at defects within the Cl covered
dependent information is lost by integration; therefore, a more de-
tailed analysis of the voltammetry or chronoamperometry represents
an interesting opportunity for further refinement. In a related fash-
ion, the application of impedance methods is also being explored.
Similarly, a variety of other analytical methods ͑e.g., chromatogra-
phy͒ have been developed for direct speciation of the additives and
Cu surface. Alternatively, Cu͑I͒, a well-established reaction interme-
diate in both the Cu deposition and dissolution reactions, may react
homogenously with SPS to produce MPS that subsequently may
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5
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adsorb to form thiolate species within the Cl adlayer. Such metal
ion-mediated thiol/disulfide redox chemistry is widely known in
synthetic organic chemistry and biology. In the case of electroplat-
ing, Cu͑I͒ generation at Cu anodes has been a source of significant
bath aging, causing difficulties both in printed circuit board
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their breakdown products.
Online quantitative mass spectrom-
etry has also been considered to track the accumulation of SPS and
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PEG breakdown products in practical plating baths.
These
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fabrication and more recently in Damascene wafer plating. Subse-
quent use of proton-selective membranes to separate the anode and
cathode compartments has significantly reduced Cu͑I͒ generation,
such that the only source of Cu͑I͒ in the work piece compartment is
studies verified that SPS breakdown was oxidative with the most
rapid deterioration being associated with reactions occurring at the
anode. A variety of oxidation products were identified, but no ac-
count of the purity or speciation of the initial source materials was
provided.
In order to develop a more detailed view of the structure, com-
position, and dynamics of the SPS/MPS derived accelerator surface
phase, this work utilizes ion chromatography ͑IC͒ and mass spec-
*
Electrochemical Society Fellow.
E-mail: thomas.moffat@nist.gov
z