euv mask
**EUV Mask (Reticle)** is the **reflective photomask used in extreme ultraviolet lithography at 13.5nm wavelength** — fundamentally different from transmissive DUV masks, using multilayer mirrors and absorber patterns etched onto ultra-flat quartz substrates.
**EUV Mask Architecture**
- **Substrate**: Ultra-low thermal expansion (ULE) quartz, 152x152x6.35mm.
- **Multilayer (ML) Stack**: 40 bilayers of Mo/Si, each 7nm thick, total ~280nm.
- Mo/Si multilayer acts as Bragg reflector at 13.5nm wavelength.
- Peak reflectivity: ~67% — lossy (33% absorbed even in best case).
- **Capping Layer**: Ruthenium (Ru) ~2.5nm — protects ML from oxidation and cleaning.
- **Absorber**: TaN ~60nm — patterns the image, absorbs EUV photons where no exposure desired.
**EUV vs. DUV Mask**
| Feature | DUV (ArF) | EUV |
|---------|-----------|-----|
| Wavelength | 193nm | 13.5nm |
| Mask type | Transmissive | Reflective |
| Substrate | Fused silica | LTEM quartz |
| Pattern | Cr absorber | TaN absorber |
| Flatness requirement | 100nm | 50nm |
| Defect printability | ~1x | ~1x |
**EUV Mask Blank Defects**
- Multilayer defects (phase defects): Buried bumps or pits in ML stack print as CD variation.
- Defect density target: < 0.01 defects/cm² printable defects.
- Defect mitigation: Absorber pattern shifted to avoid defect positions (mask-to-defect matching).
- HOYA, AGC, S&S (Shin-Etsu) produce mask blanks — high barrier to entry.
**EUV Pellicle**
- Thin membrane (PolyS, SiN, CNT) ~50nm thick spanning the mask.
- Keeps particles off mask surface during scanner use.
- EUV transmission > 85% target — early pellicles only 83%.
- Currently: Optional, Intel and TSMC using pellicles at HVM (2024).
EUV masks are **the most critical, expensive, and difficult-to-manufacture consumables in chipmaking** — each EUV reticle set costs $500K–$1M and requires perfect defect control to enable sub-7nm patterning.