Freeze Drying

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.

Conventional vs. Freeze Drying:Conventional Drying: Freeze Drying: │ Liquid │ │ Ice │ │ ↓ │ │ ↓ │ ───┤ ✕ ├─── → Collapse ───┤ ├─── → Intact │surface │ │vapor│ │tension │ │ │ Surface tension pulls structures together (stiction)

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)

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