pellicle (euv)

**An EUV pellicle** is an ultra-thin transparent membrane mounted a few millimeters above the **EUV reticle (mask)** surface to protect it from particle contamination during exposure. Any particle landing on the reticle would print as a defect on every wafer — the pellicle prevents this by keeping particles out of the focus plane. **Why Pellicles Are Critical** - In optical lithography (DUV), pellicles have been standard for decades — a transparent polymer film keeps particles away from the mask surface. - At EUV wavelengths (**13.5 nm**), the challenge is extreme: virtually all materials **absorb** EUV light, making a transparent pellicle extraordinarily difficult to create. - Without a pellicle, masks must be inspected and cleaned frequently, adding cost and risk of damage. **EUV Pellicle Requirements** - **High Transmission**: Must transmit >90% of EUV light (the beam passes through the pellicle twice — going to and reflecting from the mask). - **Ultra-Thin**: Thickness typically **40–60 nm** to minimize EUV absorption. For comparison, this is only ~100 atoms thick. - **Large Area**: Must span the full mask field — approximately **110 × 140 mm** — without support structures in the beam path. - **Mechanical Strength**: Must survive the vacuum, thermal loads, and electrostatic forces inside the scanner. - **Thermal Resistance**: Must withstand heating from absorbed EUV light (temperatures can reach 500°C+). **Pellicle Materials** - **Polysilicon (p-Si)**: ASML's current pellicle solution. A free-standing polysilicon membrane ~50 nm thick with a capping layer to improve durability. Transmission ~85–88%. - **Carbon Nanotube (CNT)**: Membranes of aligned carbon nanotubes offer high transmission and thermal conductivity. Under development. - **SiN and SiC**: Silicon nitride and silicon carbide membranes explored for their combination of EUV transparency and mechanical robustness. - **Graphene**: Explored for its extreme thinness and strength, but achieving continuous large-area films is challenging. **Challenges** - **Transmission Loss**: Even 10% absorption means significant light loss in an already photon-starved EUV system, directly reducing scanner throughput. - **Thermal Damage**: At high-NA EUV power levels, pellicles absorb enough energy to risk rupture or degradation. - **Flatness**: Any wrinkle or sag creates imaging errors (phase distortion). EUV pellicle development is one of the **most challenging materials engineering problems** in semiconductor manufacturing — creating a membrane thin enough to transmit EUV light yet strong enough to survive the harsh scanner environment.