The ongoing demands for increased storage capacity flash memory in 2D-NAND structures resulted in their replacement by more advanced 3D-NAND structures, with the memory cells made of multiple, vertically stacked silicon oxide/silicon nitride layers. A critical step is selectively etching the silicon nitride films involving a wet etch technique using concentrated phosphoric acid at high temperatures. Concentrated phosphoric acid solutions demonstrate unique behaviors and have particularly high electrical conductivity, but the etching mechanism remains poorly understood. This study investigates the fundamental role of phosphoric acid in the silicon nitride etching and proposes complex active species for the silicon nitride surface protonation and hydroxylation. Characterization methods include 31P-NMR, XPS, FTIR, conductometry, viscometry and ellipsometry. We conclude that the unique performance of concentrated phosphoric acid as silicon nitride etchant results from an anomalously fast proton transport via the Grotthuss diffusion mechanism based on an intramolecular proton transfer driven by easily polarizable, hydrogen bond rearrangements between dissociated molecules as dimers, trimers and triple ions. By contrast, dilute phosphoric acid solutions and other strong protic acids (methanesulfonic acid, sulfuric acid, nitric acid), at both high and low concentrations exhibit protonic conductivity based on molecular diffusion of the H3O+/H2O/anions as separate entities (classical vehicle mechanism).

We discuss several mechanistic approaches and experimental data for improving post-CMP cleaning of W plugs with TiN as barrier liner, and dielectric substrates SiO2 and Si3N4 for use at the 10 nm technology node (metal pitch of 40 nm). Particle charge in the low pH, W CMP slurries are usually positive, and the W surface is always negatively charged at pH >3. Therefore, a strong electrostatic attraction is expected to occur between the W surface and the residual particles during post-CMP cleaning. Two main approaches were chosen to break down the strong particles-W surface post-CMP electrostatic interactions, as well as particles dispersion and prevention of redeposition: (1) using cleaning additives able to adsorb at the W surface and reverse the W surface charge; (2) using organic additives to reverse the particle charge. The latter approach results in two strongly negative charged surfaces, which are able to repulse each other, and leads to the best cleaning.

Nanoparticles provide multiple functionalities to the performance of CMP slurries. These include mechanical surface abrasion, mass transport of slurry chemistries between the pad and wafer surface, or increased chemical reactivity of some key additives by in-situ interactions with the particles surfaces. Since most of the inorganic nanosized oxides used as common abrasives in CMP slurries (silica, alumina, ceria) have chemically reactive hydrophilic surface functionalities in a large pH range (2 – 12), we can assume that significant interactions between the inorganic particles surface and some of the slurry additives (organic surfactants, oxidizers, film-forming ligands, removal rates promoters, etc.) could have a meaningful impact on slurry ultimate performances (removal rates, planarity, defectivity, etc.)In this presentation we will highlight the significance of slurry abrasives as surface modified nanoparticles/chemical carriers, able to directly participate and control the metal oxidation/removal mechanism, removal rates and other polishing characteristics. We will discuss two relevant examples, both involving fumed silica as the carrier particle in two different low pH slurries of variable complexities in terms of design (with/without particles surface modifiers) and performance requirements for tungsten CMP. We will provide a complex variety of analytical evidence (TEM, SEM, FT-IR, GPC, cyclic voltammetry, MS-TOF) in order to support the proposed mechanism of “chemically activated fumed silica”, in its natural (no interactions with organic additives in the slurry) and surface modified form (in-situ interactions with organic additives), as a carrier of selective slurry components with enhanced chemical activity, that ultimately controls the tungsten CMP mechanism and the ability of the slurry to efficiently and predictably remove the oxidized tungsten film formed at the wafer surface.

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