EUV mask

**Extreme Ultraviolet (EUV) Mask Infrastructure and Blank Defectivity** is **the ecosystem of materials, inspection tools, and defect management strategies required to produce defect-free reflective photomasks for 13.5 nm EUV lithography** — because EUV masks operate in reflection rather than transmission, their fabrication and qualification are fundamentally more complex than those of conventional optical masks. - **Multilayer Reflector**: An EUV mask blank consists of approximately 40 alternating pairs of Mo/Si layers deposited by ion-beam sputtering on an ultra-low-thermal-expansion (ULE) glass substrate. Peak reflectivity reaches about 67% at 13.5 nm wavelength and is extremely sensitive to layer thickness uniformity. - **Blank Defectivity**: Even a single particle or pit on the blank substrate propagates through all 40 bilayers, creating a printable phase defect. Blank suppliers target fewer than 0.003 defects per cm² at 30 nm detection sensitivity. Achieving this requires ultra-clean deposition chambers and extensive blank inspection. - **Absorber and Capping Layers**: A TaBN absorber (or next-generation low-n absorber) pattern defines the circuit features, while a thin Ru capping layer protects the Mo/Si multilayer from oxidation during mask processing and use. - **Actinic Inspection**: Defect inspection at the 13.5 nm operating wavelength (actinic inspection) is necessary because some defects visible at DUV wavelengths are not printable at EUV and vice versa. Actinic patterned-mask inspection tools are being deployed to catch buried multilayer defects. - **Pellicle Challenges**: EUV pellicles must be ultra-thin (< 50 nm) to maintain transmission at 13.5 nm, yet survive high thermal loads from absorbed EUV and infrared radiation. Polysilicon, SiN, carbon nanotube, and metal-capped membranes are under development with transmission targets above 90%. - **Mask Lifetime and Cleaning**: Repeated EUV exposures degrade the capping layer; hydrogen plasma cleaning removes surface contamination without damaging the multilayer. Mask lifetime management tracks exposure dose and cleaning cycles. - **Phase and Amplitude Defect Repair**: Focused ion beam and electron-beam-induced deposition can repair absorber defects; compensating buried multilayer phase defects remains a research challenge. - **Cost and Supply**: A single EUV mask blank costs significantly more than a DUV blank, and only a handful of global suppliers can produce them at the required defect density. EUV mask infrastructure remains the single most critical and expensive element of the EUV lithography ecosystem, with blank defect density directly determining the yield of every advanced-node wafer printed.