Understanding the Role of Concentrated Phosphoric Acid Solutions as High-Temperature Silicon Nitride Etchants

Abstract

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).