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.

<svg viewBox="0 0 460 245" xmlns="http://www.w3.org/2000/svg" style="max-width:100%;height:auto" role="img"><rect x="0" y="0" width="460" height="245" rx="12" fill="#0d1117"/><g font-family="ui-monospace,SFMono-Regular,Menlo,Consolas,&quot;Liberation Mono&quot;,monospace" font-size="14"><text xml:space="preserve" x="20" y="31.7"><tspan fill="#c9d1d9">Anisotropic Etch in (100) Silicon:</tspan></text><text xml:space="preserve" x="20" y="50.7"><tspan fill="#c9d1d9">Starting:              After KOH etch:</tspan></text><text xml:space="preserve" x="20" y="69.7"><tspan fill="#c9d1d9"> </tspan><tspan fill="#6e7681">────────────</tspan><tspan fill="#c9d1d9">           </tspan><tspan fill="#6e7681">────────────</tspan></text><text xml:space="preserve" x="20" y="88.7"><tspan fill="#c9d1d9"> </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">  Mask    </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">              ╲    ╱</tspan></text><text xml:space="preserve" x="20" y="107.7"><tspan fill="#c9d1d9"> </tspan><tspan fill="#6e7681">├──────────┤</tspan><tspan fill="#c9d1d9">               ╲  ╱</tspan></text><text xml:space="preserve" x="20" y="126.7"><tspan fill="#c9d1d9"> </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">          </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">     </tspan><tspan fill="#6e7681">→</tspan><tspan fill="#c9d1d9">          ╲╱</tspan></text><text xml:space="preserve" x="20" y="145.7"><tspan fill="#c9d1d9"> </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">  Silicon </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">           ╲    ╱  </tspan><tspan fill="#6e7681">←</tspan><tspan fill="#c9d1d9"> 54.7° angle</tspan></text><text xml:space="preserve" x="20" y="164.7"><tspan fill="#c9d1d9"> </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">          </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">            ╲  ╱     ({111} planes)</tspan></text><text xml:space="preserve" x="20" y="183.7"><tspan fill="#c9d1d9"> </tspan><tspan fill="#6e7681">└──────────┘</tspan><tspan fill="#c9d1d9">             ╲╱</tspan></text><text xml:space="preserve" x="20" y="202.7"><tspan fill="#c9d1d9"> </tspan></text><text xml:space="preserve" x="20" y="221.7"><tspan fill="#c9d1d9">Self-limiting V-groove (111 planes resist etching)</tspan></text></g></svg>

Etchant Comparison: