EUV Photomask Imaging — SHARP

The SHARP (SHARP Hyper-NA Actinic Reticle review Project) microscope is a synchrotron-based, full-field EUV mask microscope operating at beamline 11.3.2 of the Advanced Light Source at Lawrence Berkeley National Laboratory. Commissioned in 2013, SHARP provides actinic (at-wavelength, 13.5 nm) imaging of EUV photomasks, enabling mask makers and lithographers to study defect printability, pattern fidelity, and three-dimensional mask effects under conditions that directly replicate the lithographic wavelength. With interchangeable optics, SHARP can emulate the imaging conditions of 0.33 NA, 0.55 High-NA, and 0.75 Hyper-NA scanner generations.

SHARP uses Fresnel zoneplate lenses as imaging objectives. Because zoneplates are diffractive optics fabricated by electron-beam lithography, they can be made diffraction-limited at EUV wavelengths with far lower cost and lead time than reflective mirror systems. New zoneplates can be produced in weeks, allowing SHARP to rapidly configure optics for studying prospective scanner generations long before those scanners exist.

SHARP’s fully programmable lossless Fourier-synthesis illuminator produces arbitrary pupil fill patterns, including the standard illuminators of the various scanner generations and freeform sources. The mask-side angular range of 20 degrees along the plane of incidence by 30 degrees laterally exceeds 0.75 Hyper-NA.

SHARP also supports advanced computational imaging techniques such as quantitative phase imaging and Fourier Ptychographic Microscopy (FPM), enabling wafer-side through-focus emulation and arbitrary pupil fill synthesis from a single dataset.

Scanning-electron microscope image of a Hyper-NA Fresnel zoneplate lens used in the SHARP microscope — SEM of a Hyper-NA zoneplate: (a) full view showing the central stop and silicon membrane, (b) detailed view of the outer zone pattern with gold absorber on silicon. (Benk et al., J. Micro/Nanopattern. Mater. Metrol. 2025)

Instrument Specifications

Wavelength	13.5 nm (EUV, tunable around the central wavelength)
Imaging Optics	Fresnel zoneplate lenses
Imaging NA	Up to 0.85NA mask-side equivalent: 0.33 4xNA, 0.55 high-NA (4x/8x), 0.75 and 0.85 Hyper-NA (4x/8x)
Magnification	900x at 0.33 4xNA; 1250x at high-NA; 1400x at Hyper-NA
Resolution	20 nm half-pitch (mask scale) at Hyper-NA; 5 nm half-pitch (wafer-scale, 1x) demonstrated
Illumination	Programmable lossless Fourier-synthesis illuminator
Anamorphic Support	Elliptical zoneplate apertures, anamorphic imaging mode
Light Source	Synchrotron: ALS bend-magnet beamline 11.3.2

Key Capabilities

Actinic Defect Review

Only actinic imaging tools can capture the printability of a defect on an EUV mask. The SHARP aerial image is true to the wafer print, providing accurate printability assessment and repair verification.

Comparison of mask SEM, wafer SEM, and actinic imaging of a large defect — Comparison of (a) mask SEM, (b) wafer SEM, and (c) actinic imaging of a large defect, revealing that the actinic image captures fine details not seen by mask SEM.

Mask 3D Effect

EUV masks are reflective multilayer structures illuminated at oblique angles. The finite thickness of the multilayer and absorber creates shadowing and phase effects that shift and distort the printed image. These mask 3D effects grow more severe with oblique illumination and are a central concern for high-NA and Hyper-NA lithography. While lithography simulations can model idealized structures, only studies on real photomasks capture the full complexity of fabrication-induced variations. SHARP provides the experimental platform for these studies, enabling researchers to evaluate next-generation absorber materials and mask architectures under realistic imaging conditions.

SHARP through-focus Hyper-NA image data of 10 nm half-pitch vertical lines and spaces — Through-focus Hyper-NA image data of 10-nm hp (1x) vertical lines and spaces, comparing optimized and increased pole-offset dipole illumination. (Benk et al., 2025)

Fourier Ptychographic Microscopy (FPM)

SHARP’s programmable illuminator enables Fourier Ptychographic Microscopy (FPM), a computational imaging technique that reconstructs high-resolution, complex-valued images by stitching together data acquired under multiple illumination angles. FPM recovers both the amplitude and phase of the mask reflectivity while simultaneously reconstructing the pupil function of the imaging system.

Full field-of-view Fourier Ptychography reconstruction of EUV mask data at 13.5 nm, showing raw measurements, computational reconstructions, and pupil phase maps across the field — Full field-of-view Fourier Ptychography reconstruction at 13.5 nm: (a) raw measurement vs. computational reconstruction for three sample regions, (b) pupil wavefront reconstruction across the full FOV. FPM achieves 1.8× resolution enhancement beyond the optical limit. (Gu et al., Proc. SPIE 2025)

High-NA & Hyper-NA Imaging

The industry is moving from today’s 0.33 NA EUV scanners to 0.55 High-NA systems, with Hyper-NA tools expected to follow after 2035. The transition to smaller lithography nodes brings tighter process margins. SHARP has supported research towards 0.55 High-NA EUV lithography since 2015, a decade before the first High-NA scanners became available. In 2024, Hyper-NA zoneplates at 0.75 4x/8xNA and 0.85 4x/8xNA were added to the instrument, providing a platform for research and development towards Hyper-NA EUV lithography, again many years ahead of the introduction of the corresponding lithography nodes into production. SHARP’s Hyper-NA zoneplates have demonstrated imaging of 5 nm hp (wafer-scale) lines and spaces.

SHARP Hyper-NA image data showing 5-nm half-pitch lines and spaces at wafer-equivalent scale — Mask-side Hyper-NA image data of 5-nm hp (1x) lines and spaces: (a) dipole illumination, (b) monopole illumination, (c) frequency-doubled imaging of 40-nm hp mask features. (Benk et al., 2025)

Increasing the mask-side numerical aperture to Hyper-NA poses new challenges. An increased chief ray angle and extended angular range on the photomask amplify mask 3D effects. The multilayer on the photomask has to support an increased angular bandwidth. Smaller pitches have reduced depth of focus. SHARP mask-side Hyper-NA imaging confronts these challenges experimentally, measuring how different mask architectures, absorber materials, and illumination schemes perform under real-world conditions.

Related Research

SHARP supports EUV Lithography Science and Coherent Optics research programs.

SHARP Team

Markus Benk

Staff Scientist