Mechanistic Investigations of Ruthenium Polishing Enabled by Heterogeneous Catalysis with Titania Based Slurries
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Mechanistic Investigations of Ruthenium Polishing Enabled by Heterogeneous Catalysis with Titania Based Slurries Daniela White, John Parker, R. Nagarajan CABOT MICROELECTRONICS, 870 N. Commons Drive, Aurora, IL 60504 ABSTRACT Difficulties and challenges have been widely encountered in the chemical mechanical planarization (CMP) of noble metals used as diffusion barrier films due to hardness, chemical inertness and toxic oxidation products, in particular for Ru polishing. A commercial CMC Ru polishing slurry that successfully addressed the safety concerns was based on a high pH (8.4) polymer-treated α-alumina-based multi-additive slurry containing complexing reagents (carboxylic acids and salts), corrosion inhibitors, surfactants, and hydrogen peroxide as a mild, non-aggressive oxidizer. Recently, our efforts had been redirected towards the development of a new generation of low pH barrier slurries based on an entirely new concept employing unique Lewis acid type abrasives. This class of materials is well known to be highly reactive in dielectrics polishing (ceria, zirconia, titania), but far less is known about their interactions with Ru, Cu or Ta. In this presentation, we will elaborate the unexpected discovery that hard, chemically inert, noble metals such as ruthenium can be polished easily by virtually abrasive-free and/or oxidizer-free TiO2 based slurries. Of particular importance is the finding that the polymorphic phase of TiO2 (anatase or rutile), its surface chemistry (functional groups, Lewis acid/base character), pH induced surface modification and reactivity, can all mediate the abrasive particle’s adsorption properties, catalytic activity and ultimately, its slurry polishing performance. INTRODUCTION Interconnect schemes for high performance ICs like microprocessors use Cu interconnects imbedded in dielectric materials with a low dielectric constant separated by diffusion barrier films based on Ta/TaN or more recently, Ru thin layers which prevent Cu ion migration into the interlayer dielectric (ILD) films. For Ru polishing, current commercial slurries include hard abrasives (α-alumina), very strong oxidizers ((NH4)2S2O8, KMnO4, (NH4)2Ce(NO3)6 used at high concentrations between 0.1 – 5 M), and are only effective at low pH’s (1-3) after pH adjustment with excess nitric acid, HNO3 (2-6%), [1,2]. Ru(0) metal is rapidly dissolved by these slurries as a result of direct oxidation to its highest oxidation state (VIII). This reaction is accompanied by the formation of the volatile, highly toxic compound ruthenium tetroxide (RuO4) which is very unstable in aqueous solutions and ultimately reduces via perruthenates (RuO4-) and ruthenates (RuO42-) to a black precipitate of hydrated RuO2x2H2O. An approach used by CMC in addressing this safety issue and enabling Ru, Cu and Ta polishing has been the formulation of barrier slurries, based on a high pH, high solids, α-alumina-based multi-additive slurry with hydrogen peroxide as the oxidizer of choice. Another key CMP performance criterion demanded by cu
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