Wet Anisotropic Etch

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.

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 |

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