passivation

Passivation in plasma etching refers to the deposition of protective polymer films on feature sidewalls that enable anisotropic etching by preventing lateral material removal. Fluorocarbon gases like CHF₃, C₄F₈, or C₄F₆ deposit carbon-rich polymers during etching. Ion bombardment continuously removes passivation from horizontal surfaces while sidewalls remain protected, creating vertical profiles. The balance between passivation deposition and removal determines etch anisotropy and profile shape. Too much passivation causes etch stop or grass formation, while too little results in isotropic etching and undercut. Passivation is critical for high aspect ratio etching of contacts, vias, and trenches. The Bosch process alternates between etch and passivation steps for deep silicon etching. Passivation chemistry must be tuned based on feature size, aspect ratio, and materials being etched. Temperature affects passivation stability—lower temperatures promote polymer formation while higher temperatures increase volatility.