Wet Anisotropic Etching uses orientation-dependent etch rates in crystalline materials to create precisely shaped structures, commonly using KOH or TMAH on silicon.
## What Is Wet Anisotropic Etching?
- Mechanism: Different crystal planes etch at different rates
- Etchants: KOH (potassium hydroxide), TMAH (tetramethylammonium hydroxide)
- Rate Ratio: {100}:{111} can exceed 100:1
- Applications: MEMS cavities, V-grooves, sharp tips, through-wafer vias
## Why Anisotropic Wet Etching Matters
Etching self-terminates on slow-etching {111} planes, creating atomically smooth surfaces and precisely defined angles without expensive plasma equipment.
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Anisotropic Etch in (100) Silicon:
Starting: After KOH etch:
──────────── ────────────
│ Mask │ ╲ ╱
├──────────┤ ╲ ╱
│ │ → ╲╱
│ Silicon │ ╲ ╱ ← 54.7° angle
│ │ ╲ ╱ ({111} planes)
└──────────┘ ╲╱
Self-limiting V-groove (111 planes resist etching)
Etchant Comparison:
| Property | KOH | TMAH |
|----------|-----|------|
| {100}/{111} ratio | ~400 | ~35 |
| CMOS compatible | No (K+ contaminant) | Yes |
| Cost | Low | Higher |
| Surface roughness | Better | Good |