Etching & Ion Implantation
Wet vs Dry Etching
Chemical wet etch, isotropic vs anisotropic, and selectivity
Etching Fundamentals
Etching Fundamentals
Etching selectively removes material from the wafer surface using the photoresist or hardmask pattern as a template. Two main approaches:
- Wet etching: Immerse wafers in liquid chemicals that dissolve the target material. Simple, cheap, but usually isotropic (etches equally in all directions).
- Dry etching: Use plasma/reactive gases to remove material. Can be highly anisotropic (etches preferentially downward), enabling vertical sidewalls.
Three critical etch parameters:
- Selectivity: How much faster the target material etches vs. the mask or underlying layer. Higher is better (e.g., 50:1 means 50× faster etch of target).
- Anisotropy: The ratio of vertical to lateral etch rate. 1.0 = perfectly directional.
- Uniformity: Consistent etch rate across the entire wafer.
Analogy: Carving a Sculpture
Wet etching is like dissolving material with acid — it removes material in all directions equally (isotropic). Dry etching is like using a chisel pointing straight down — you can carve vertical walls without undercutting (anisotropic).
Knowledge Check
Knowledge Check
1 / 1What does 'selectivity' mean in etching?