EUV mask blank

**EUV Mask Blank Technology** encompasses the **fabrication of the specialized mask substrates used in extreme ultraviolet lithography — consisting of an ultra-flat, low-thermal-expansion glass substrate coated with a 40-layer Mo/Si Bragg reflector mirror stack, a capping layer, and a patterned absorber** — where defect requirements are among the most stringent in all of materials science. Unlike DUV masks that transmit light through a transparent quartz substrate, EUV masks operate in **reflection**: 13.5nm EUV light reflects off the multilayer (ML) mirror at near-normal incidence. The ML mirror consists of 40 alternating pairs of molybdenum (Mo, ~2.8nm) and silicon (Si, ~4.1nm) layers, with a bilayer period of ~6.9nm — exactly half the EUV wavelength for constructive interference at ~6° off-normal incidence. The theoretical peak reflectivity is ~74%, and production blanks achieve 66-68% (losses from interface roughness, intermixing, and absorption). Each ML bilayer must maintain thickness uniformity to <0.01nm across the 152×152mm mask area. The mask blank fabrication sequence: start with an ultra-low thermal expansion material (ULETM — Corning, or Clearceram — AGC) substrate, polished to <0.15nm RMS roughness over all spatial frequency ranges. Any substrate defect — particle, pit, or bump >1nm in height — will print as a phase defect in the reflected EUV wavefront. The ML is deposited by **ion beam deposition (IBD)** — the most controlled thin-film process available — in cleanroom conditions targeting zero printable defects on the entire mask blank. Finally, a Ru capping layer (~2.5nm) protects the ML from oxidation. The **absorber layer** (historically TaN-based, ~60-70nm thick) is deposited on top of the ML/capping stack. When patterned by e-beam writing and dry etch, the absorber blocks EUV reflection in dark regions. Next-generation absorbers include **high-k materials** (Ru-based, Ni-based, or Cr-based) that are thinner (~30-40nm) to reduce mask 3D shadowing effects (where the thick absorber casts shadows due to the angled EUV illumination), improving pattern fidelity at tight pitches. Defect management is the critical challenge: **ML defects** (embedded particles, pits, or thickness non-uniformities) cannot be repaired after ML deposition and are the primary yield limiter for EUV mask blanks. Zero-defect blanks are the target — even a single 20nm defect can print as a CD error on every exposed wafer. Blank inspection uses **actinic (at-wavelength, 13.5nm) inspection tools** for the most sensitive defect detection, complemented by DUV and e-beam inspection. The global supply of EUV mask blanks is concentrated in a few suppliers (AGC, Hoya, Schott for substrates; industry-internal or specialized ML deposition), making this a critical supply chain bottleneck. **EUV mask blank technology embodies the extreme end of precision manufacturing — a multi-billion-dollar lithography ecosystem depends on glass substrates polished to atomic smoothness and coated with 80 alternating nanolayers deposited to sub-angstrom precision, all without a single printable defect.**