Freeze Drying (Lyophilization) in semiconductor processing uses sublimation to dry delicate structures, avoiding surface tension damage from liquid evaporation.

What Is Freeze Drying?

• Process: Freeze liquid → Sublimate ice directly to vapor
• Advantage: Eliminates liquid-gas interface that causes stiction
• Applications: MEMS release, porous materials, delicate structures
• Equipment: Vacuum chamber with cold trap

Why Freeze Drying Matters

Surface tension during conventional drying collapses fine structures like MEMS cantilevers. Sublimation bypasses the liquid phase entirely.

<svg viewBox="0 0 502 207" xmlns="http://www.w3.org/2000/svg" style="max-width:100%;height:auto" role="img"><rect x="0" y="0" width="502" height="207" 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">Conventional vs. Freeze Drying:</tspan></text><text xml:space="preserve" x="20" y="50.7"><tspan fill="#c9d1d9">Conventional Drying:          Freeze Drying:</tspan></text><text xml:space="preserve" x="20" y="69.7"><tspan fill="#c9d1d9">     </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9"> Liquid </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">                   </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9"> Ice </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">   </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><tspan fill="#6e7681">↓</tspan><tspan fill="#c9d1d9">  </tspan><tspan fill="#6e7681">│</tspan></text><text xml:space="preserve" x="20" y="107.7"><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">  Collapse   </tspan><tspan fill="#6e7681">───┤</tspan><tspan fill="#c9d1d9">     </tspan><tspan fill="#6e7681">├───</tspan><tspan fill="#c9d1d9">  </tspan><tspan fill="#6e7681">→</tspan><tspan fill="#c9d1d9">  Intact</tspan></text><text xml:space="preserve" x="20" y="126.7"><tspan fill="#c9d1d9">     </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">surface </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">                   </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">vapor</tspan><tspan fill="#6e7681">│</tspan></text><text xml:space="preserve" x="20" y="145.7"><tspan fill="#c9d1d9">     </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">tension </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">                   </tspan><tspan fill="#6e7681">│</tspan><tspan fill="#c9d1d9">     </tspan><tspan fill="#6e7681">│</tspan></text><text xml:space="preserve" x="20" y="164.7"><tspan fill="#c9d1d9">     </tspan></text><text xml:space="preserve" x="20" y="183.7"><tspan fill="#c9d1d9">Surface tension pulls structures together (stiction)</tspan></text></g></svg>

Freeze Drying Process: 1. Rinse with water or t-butanol 2. Freeze below solvent melting point (-40°C typical) 3. Apply vacuum (<1 mbar) 4. Sublimate ice over hours 5. Warm to room temperature under vacuum

Alternative: Supercritical CO₂ drying (faster, no freezing damage)