photoresist

**Photoresist** — a light-sensitive polymer material used as a temporary mask during lithography, enabling selective patterning of underlying layers. **Types** - **Positive Resist**: Exposed areas become soluble in developer. Unexposed resist remains. Most common for IC fabrication (better resolution) - **Negative Resist**: Exposed areas cross-link and become insoluble. Unexposed resist washes away. Used for some MEMS and packaging **Chemically Amplified Resist (CAR)** - Standard for DUV (248nm, 193nm) and EUV lithography - Photon absorption generates acid catalyst - Post-exposure bake: Acid catalyzes deprotection reaction (positive) or cross-linking (negative) - One photon catalyzes many reactions — amplifies the exposure effect **EUV Resist Challenges** - Fewer photons per feature area → shot noise (stochastic defects) - Trade-off triangle: Resolution vs. Sensitivity vs. Line Edge Roughness (RLS trade-off) - Metal-containing resists being developed for higher EUV absorption **Process Steps** 1. Spin coat (1-2um thick) 2. Soft bake (drive off solvent) 3. Expose (DUV or EUV) 4. Post-exposure bake 5. Develop (TMAH developer) 6. Hard bake (optional, for etch resistance) **Photoresist** is the temporary "stencil" that makes every pattern on a chip possible.