etching basics

**Etching** — selectively removing material from a wafer surface to define circuit patterns, using either chemical solutions (wet) or reactive plasmas (dry). **Wet Etching** - Immerse wafer in chemical solution - Isotropic (etches equally in all directions) — undercuts the mask - Simple, cheap, high selectivity - Used for cleaning, stripping, and non-critical features **Dry (Plasma) Etching** - Reactive gases ionized into plasma bombard the wafer - Anisotropic (directional) — etches mainly downward, preserving sidewall profiles - Essential for sub-micron features - Types: RIE (Reactive Ion Etching), ICP (Inductively Coupled Plasma), ALE (Atomic Layer Etching) **Key Parameters** - **Selectivity**: Ratio of etch rates between target material and mask/underlayer. Higher = better - **Anisotropy**: Vertical vs lateral etch. 1.0 = perfectly vertical - **Uniformity**: Consistent etch rate across the wafer - **Etch rate**: nm per minute **Modern Challenges** - Atomic-scale precision needed at 3nm and below - High aspect ratio etching (memory trenches >100:1) - ALE provides single-atomic-layer removal control **Etching** defines every physical feature on a chip — without precise etch, no pattern transfer is possible.