EUV Mask Blanks & Pellicles

Every EUV lithography exposure begins with a photomask -- the reflective master template that carries the circuit pattern. The mask is built on a mask blank: a low-thermal-expansion glass substrate coated with a precise multilayer reflective stack. Protecting that mask during production is a pellicle -- an ultra-thin membrane mounted over the mask face that keeps particles out of focus range so contamination prints as a blur rather than a defect. Both components are Tier 1 chokepoints: the blank because only two companies in the world produce EUV-grade blanks at commercial scale, and the pellicle because EUV's extreme operating conditions make pellicle durability a fundamental materials engineering problem that took over a decade to approach at usable transmittance and power levels.

EUV Mask Blank Structure

An EUV mask blank is not a simple coated glass substrate. EUV light at 13.5nm cannot be transmitted through conventional optical materials -- it is absorbed by almost everything. Instead, EUV masks are reflective: the blank is built to reflect EUV photons from its surface rather than transmit them through it. This requires a Bragg reflector: 40 to 50 alternating layers of molybdenum (Mo) and silicon (Si), each layer precisely one-quarter wavelength thick (~3nm per layer), deposited by ion beam sputtering. The Mo/Si stack reflects approximately 65-70% of incident EUV at peak reflectivity -- the theoretical maximum given absorption losses in each layer. On top of the Mo/Si stack sits a ruthenium (Ru) capping layer to prevent oxidation, and a tantalum-based absorber layer that is patterned by the mask shop to define the circuit features.

The substrate beneath all of this must have an ultra-low coefficient of thermal expansion -- ideally near zero at operating temperature -- because any dimensional change from EUV-induced heating distorts the reflected pattern. The substrate material is ultra-low expansion (ULE) glass, a titanium-silicate composition developed specifically for applications where dimensional stability under thermal cycling is critical. The substrate must also be polished to sub-angstrom surface roughness and be essentially defect-free across its area -- a single phase defect in the Mo/Si multilayer that scatters EUV at the wrong phase can print as a killer defect on the wafer below.

Mask Blank Supplier Landscape: The AGC/Hoya Duopoly

Two companies supply essentially all EUV mask blanks in commercial production: AGC (formerly Asahi Glass) and Hoya, both Japanese. Their combined share is approximately 90-93% of the global EUV mask blank market. The duopoly is not a market accident -- it reflects the extraordinary technical barriers to producing defect-free Mo/Si multilayer stacks at the required specification. Phase defect density below 0.1 per cm² at 13.5nm actinic wavelengths is the threshold that separates production-viable blanks from scrap. Achieving and sustaining this requires ion beam deposition tools calibrated to atomic precision, actinic inspection systems (which themselves are scarce -- Lasertec is the primary actinic blank inspection tool supplier), and years of process development to understand and eliminate defect sources at each deposition layer.

The tool-and-material co-development cycle in EUV blank production spans 5-7 years, sequenced with scanner development roadmaps. A well-funded entrant with state-of-the-art deposition equipment could not displace either AGC or Hoya before the next EUV generation was obsolete. Blank qualification at a mask shop is itself a lengthy process -- mask shops qualify blank suppliers on defect density, flatness, reflectivity uniformity, and layer-to-layer reproducibility across batches. Once qualified, the relationship is deeply entrenched.

EUV Pellicles: Function & Physics

A pellicle is a thin membrane stretched across a frame and mounted a few millimeters above the mask surface. Any particle that falls on the pellicle is too far out of focus to print as a sharp defect on the wafer -- it appears as a diffuse blur, low enough in contrast to be below the imaging threshold. Without a pellicle, particles landing directly on the mask surface print as killer defects. In DUV lithography, pellicles are standard practice: a thin polymer film transmits 193nm light efficiently, and the operating conditions are mild enough that pellicles last for millions of exposures.

EUV pellicles face three problems that DUV pellicles do not. First, transmittance: EUV at 13.5nm is absorbed by nearly all materials, so a pellicle film thick enough to be mechanically stable will absorb a significant fraction of the EUV light that should reach the wafer. Second, power: high-volume EUV scanners operate at source powers of 250-500W and above; the pellicle membrane absorbs some of this energy as heat, and must withstand extreme thermal stress without degrading or rupturing. Third, hydrocarbon contamination: the hydrogen atmosphere used in EUV systems to suppress tin debris from the EUV plasma source interacts with organic pellicle materials over time, causing gradual degradation of transmittance.

Pellicle Supplier Landscape & Technology Evolution

Pellicle Durability at High EUV Power

The durability challenge is the central constraint in EUV pellicle deployment. Early polysilicon pellicle membranes developed by ASML achieved 78% transmittance but saw less than half of membranes survive expected EUV source power levels in testing. The progression from 78% to 82% to 85% and finally to over 90% transmittance tracked concurrent improvements in membrane materials and structure. The current Mo/Si multilayer approach from Mitsui achieves over 90% transmittance with demonstrated 400W power durability.

The next challenge is High-NA EUV, which requires higher source powers (above 500W) and introduces a new geometric constraint: the pellicle must function at a higher angle of incidence due to the High-NA scanner's larger acceptance cone. This changes the effective transmittance and the angular distribution of heating in the membrane, requiring a different pellicle design than current low-NA EUV tools use. Carbon nanotube pellicles -- being developed by Canatu in partnership with Imec, and now with Mitsui Chemicals for commercial scale-up -- are the primary candidate for High-NA EUV. CNT films combine high EUV transmittance with thermal conductivity and mechanical properties that Si-based membranes cannot match at the required film thickness.

The practical consequence of pellicle constraints is that some EUV layers in production fabs have historically been run without pellicles, accepting the yield risk from particle contamination in exchange for avoiding throughput loss from pellicle absorption and the operational complexity of pellicle mounting, inspection, and replacement. As pellicle transmittance and durability have improved, pellicle adoption has increased -- but it remains an active tradeoff managed layer-by-layer at each fab.

Actinic Inspection: The Enabling Bottleneck

A chokepoint adjacent to mask blanks is actinic patterned mask inspection (APMI) -- inspection of EUV masks at 13.5nm wavelength rather than at optical or e-beam wavelengths. Only Lasertec (Japan) produces commercial actinic mask inspection tools. Optical inspection cannot detect phase defects in the Mo/Si multilayer stack that are invisible at visible wavelengths but scatter EUV light at the wafer plane. Actinic inspection is the only reliable way to find and characterize these defects before a mask enters production. The scarcity of Lasertec APMI tools -- which carry multi-year lead times similar to ASML EUV scanners -- is a production throughput constraint that affects how quickly mask shops can qualify new blanks and bring new masks into production.

Supply Chain Outlook

EUV mask blanks represent one of the purest Tier 1 chokepoints in advanced semiconductor manufacturing: two Japanese companies, no credible near-term alternatives, qualification cycles that span years, and demand that scales directly with every new EUV scanner that ASML ships. Applied Materials' development efforts represent the only plausible path to a third supplier, but a commercial ramp from Applied would take the better part of a decade from current status. Pellicle supply is less concentrated -- Mitsui, S&S Tech, and internal fab development provide more sources than exist for mask blanks -- but the technology transition to High-NA EUV pellicles reopens the supply question, and CNT commercialization is not yet proven at volume. Both components will remain supply-constrained relative to EUV scanner deployment growth as long as ASML continues shipping tools faster than the materials supply chain can qualify new capacity.

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