Potential Oscillations in Galvanostatic Cu Electrodeposition: Antagonistic and Synergistic Effects among SPS, Chloride, and Suppressor Additives

Abstract

Polymerizates of imidazole and epichlorohydrin (Imep) serve as one of the benchmarks for today's chemistry development of leveler additives in context of the industrial copper Damascene process. We therefore studied the synergistic and antagonistic interplay of the Imep polymer with other additives, commonly present in copper plating baths used for the state-of-the-art IC manufacturing. Characteristic oscillations in the applied electrode potential appear in galvanostatic copper electrodeposition when Imep is used in combination with SPS (bis­(sodium sulfopropyl) disulfide). We identified the reversible Cu­(I) coordination chemistry of the Imep polymer as a second prospective driving force beyond interfacial anion/cation pairing toward the formation of such suppressor/leveler ensembles at the interface. OH groups of the pristine Imep polymer coordinate with H2O-Cu­(I)-MPS units (primary effect) that appear as side products of the copper electrodeposition in the presence of SPS. The latter transforms during copper deposition into monomeric MPS (mercaptopropanesulfonic acid/sulfonate) as result of the adsorptive SPS dissociation on the copper surface. Electrostatic coupling between the anionic sulfonate of the MPS and the cationic imidazolium group in the formed linear, bidentate Imep-Cu­(I)-MPS complex results into a neutral, hydrophobic species that finally precipitates (secondary effect). The presence of diamagnetic Cu­(I) species in those precipitates is proven by elementary analysis in combination with magnetic SQUID measurements. The observed potential oscillations under galvanostatic conditions are discussed in terms of an alternating precipitation and dissolution of the Imep-Cu­(I)-MPS suppressor ensemble at the copper/electrolyte interface. Linear sweep experiments prove a partially hidden, N-shaped negative differential resistance (HN-NDR) as physical origin for the observed instabilities under galvanostatic conditions. SIMS (secondary ion mass spectroscopy) depth profiling of copper films deposited under such oscillatory conditions reveals periodic modulations in the contamination level parallel to the surface normal. Cross-sectional FIB analysis of the grown copper deposit reveals periodically repeating lines of grain boundaries in the copper deposit.

Towards Structural Analysis of Polymeric Contaminants in Electrodeposited Cu films

“…The physical origin of these temporal instabilities is a so-called N-shaped negative differential resistance (N-NDR) [39,40] that, within a relatively narrow current range, is manifested by potential oscillations during chronopotentiometric measurements. The plot shows seven regular potential oscillations that are attributed to sequential formation of the stable Imep-Cu(I)-MPS suppressor film and its partial dissolution (deactivation) at the interface [29,30]. The intervals at higher overpotentials comprised between sharp potential steps correspond to an active Imep-Cu(I)-MPS diffusional barrier being operative at the interface.…”

“…Recent mechanistic studies proposed an adduct of the Imep polymer with H2O-Cu(I)-MPS complexes as the active suppressor ensemble being operative at the interface during Cu plating in the presence of the two-component Imep/SPS additive package [29,30]. The H2O-Cu(I)-MPS complex is a byproduct of the SPS which undergoes adsorptive dissociation on the Cu surface [29,30].…”

“…The H2O-Cu(I)-MPS complex is a byproduct of the SPS which undergoes adsorptive dissociation on the Cu surface [29,30]. A combination of Cu(I) coordination coupled to inner-salt formation and inter-chain anion/cation pairing has been proven to be the chemical origin of the surfaceconfined precipitation of these suppressor/leveler adducts ( Fig.…”

“…1). To determine to which extent and under which experimental conditions these species get incorporated into the copper film upon electrodeposition, we make use of a particular nonlinear instability of the Imep/SPS additive system that occurs under specific galvanostatic plating conditions [29,30]. The physical origin of these temporal instabilities is a so-called N-shaped negative differential resistance (N-NDR) [39,40] that, within a relatively narrow current range, is manifested by potential oscillations during chronopotentiometric measurements.…”

“…In this context, such more advanced approaches of leveling are not predominantly based on the classical transportation and inclusion concept but rely on sophisticated synergistic and antagonistic interactions with SPS as recently discussed [29][30][31]. Prototypical additives exhibiting this bifunctionality are polymerizates of epichlorohydrin and imidazole (denoted Imep hereinafter, see Fig.…”