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).
Wet Etch Chemistries and Where They Still Win
Wet Etch Chemistries and Where They Still Win
Even in the dry-etch era, wet processes survive wherever isotropy, selectivity, or simplicity are decisive. Classic recipes:
| Material | Etchant | Notes |
|---|---|---|
| SiO₂ | Buffered HF (BHF / BOE) | NH₄F buffer keeps etch rate stable; ~1000 Å/min |
| Si₃N₄ | Hot H₃PO₄ (~160 °C) | Very selective to SiO₂; used to strip nitride spacers |
| Si (anisotropic) | KOH or TMAH | Etches {100} much faster than {111} → V-grooves; used in MEMS |
| Al | PAN (H₃PO₄ + HNO₃ + acetic + H₂O) | Smooth profiles for bond pads |
| Cu | FeCl₃ or H₂SO₄/H₂O₂ | Mostly limited to PCB / packaging; in-fab Cu uses CMP not wet etch |
Wet etching still dominates for:
- Stripping sacrificial layers (pad oxide removal, nitride spacer pull) — selectivities of 100:1+ are easy
- Surface preparation — HF dip removes native oxide and leaves an H-terminated Si surface ready for epi
- MEMS bulk micromachining — KOH/TMAH carve cavities through the wafer along crystal planes
- Cost-sensitive back-end — wet stations are an order of magnitude cheaper per wafer than plasma tools
Key Concept: Why Wet Etch Lost Patterning
At sub-micron CDs, isotropic etch undercuts the mask by roughly the film thickness — a 500 nm film undercuts ~500 nm, larger than the line itself. Dry etching's anisotropy is the only way to print vertical sidewalls below ~1 µm.
Knowledge Check
Knowledge Check
1 / 2What does 'selectivity' mean in etching?