Coherence control and flexible pupil fill play a key role in the imaging of EUV reticles. This is also true for lensless metrology applications based on coherent diffraction imaging. We describe the concept and the key components of a Fourier synthesis illuminator designed to provide the RESCAN microscope with flexible illumination capabilities and to improve its resolution limit. In particular, we discuss the characteristics of the three mirrors of the new illuminator and the requirements for their multilayer coating.

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Illumination control in lensless imaging for EUV mask inspection and review

Actinic EUV mask metrology is essentially needed for EUV lithography in the semiconductor device manufacturing process. At PSI, we are developing RESCAN, a coherent diffractive imaging (CDI)-based platform that can meet current and future mask inspection resolution requirements. In CDI, the diffraction patterns obtained by illuminating the sample with coherent light are recorded by a pixel detector, and these are used to reconstruct the complex-amplitude image of an object through an iterative phase retrieval algorithm. While in a conventional optical system, aberrations can compromise the final image's resolution, the CDI approach is inherently aberration-free. Nevertheless, a careful preprocessing of the diffraction signal is necessary to avoid artifacts in the reconstructed image. In particular, since our system works in reflection mode with an angle of incidence of 6° and uses a flat detector, it is necessary to correct the recorded diffraction patterns that are conically distorted due to the non-telecentricity. This paper discusses the impact of the diffraction data preprocessing on the reconstructed image quality and demonstrates the defect sensitivity improvement by applying an optimized data preprocessing pipeline in the RESCAN microscope. As a result, we achieve defect sensitivity down to 20 nm on the photomask and uniform image quality in a large field-of-view.

/pdf/high-resolution-and-uniform-image-reconstruction-in-a-large-1r7j3137kx.pdf

High resolution and uniform image reconstruction in a large field-of-view for EUV actinic mask review

Anamorphic mask design is necessary to overcome the geometrical limitation imposed by the reflective geometry used in EUV lithography projection systems and will be used in high-NA EUVL scanners. In this study, we demonstrate the capability of anamorphic imaging using CDI. We created anamorphic EUV masks with programmed defects and inspected it with RESCAN, a lensless microscope dedicated to EUV mask inspection.

/pdf/lensless-euv-mask-inspection-for-anamorphic-patterns-1t7bawyad0.pdf

Lensless EUV mask inspection for anamorphic patterns

Recent breakthroughs in high harmonic generation have extended the reach of bright tabletop coherent light sources
from a previous limit of ≈100 eV in the extreme ultraviolet (EUV) all the way beyond 1 keV in the soft X-ray region.
Due to its intrinsically short pulse duration and spatial coherence, this light source can be used to probe the fastest
physical processes at the femtosecond timescale, with nanometer-scale spatial resolution using a technique called
coherent diffractive imaging (CDI). CDI is an aberration-free technique that replaces image-forming optics with a
computer phase retrieval algorithm, which recovers the phase of a measured diffraction amplitude. This technique
typically requires the sample of interest to be isolated; however, it is possible to loosen this constraint by imposing
isolation on the illumination. Here we extend previous tabletop results, in which we demonstrated the ability to image a
test object with 22 nm resolution using 13 nm light [3], to imaging of more complex samples using the keyhole CDI
technique adapted to our source. We have recently demonstrated the ability to image extended objects in a transmission
geometry with ≈100 nm resolution. Finally, we have taken preliminary CDI measurements of extended nanosystems in
reflection geometry. We expect that this capability will soon allow us to image dynamic processes in nanosystems at the
femtosecond and nanometer scale.

Tabletop coherent diffractive imaging of extended objects in transmission and reflection geometry

RESCAN is a coherent diffraction imaging based APMI microscope prototype. A complex image of the EUV reticle is reconstructed from diffraction patterns collected on a CCD detector. With the next upgrade of the tool, the resolution will be enhanced from the current 34 nm down to 20 nm on mask. Also the illumination NA value will change from the current range of 0.002 to 0.02 to a value of 0.035. Here, we study how a change of the illumination NA affects the EUV mask inspection in simulation. We observe a better image quality, lower object error and higher defect sensitivity with increasing illumination NA.

/pdf/effects-of-the-illumination-na-on-euv-mask-inspection-with-421uktrujz.pdf

Hyun-Su Kim

Papers

Illumination control in lensless imaging for EUV mask inspection and review

High resolution and uniform image reconstruction in a large field-of-view for EUV actinic mask review

Lensless EUV mask inspection for anamorphic patterns

Effects of the illumination NA on EUV mask inspection with coherent diffraction imaging