Next-generation nano- and quantum devices have increasingly complex 3D structure. As the dimensions of these devices shrink to the nanoscale, their performance is often governed by interface quality or precise chemical or dopant composition. Here, we present the first phase-sensitive extreme ultraviolet imaging reflectometer. It combines the excellent phase stability of coherent high-harmonic sources, the unique chemical sensitivity of extreme ultraviolet reflectometry, and state-of-the-art ptychography imaging algorithms. This tabletop microscope can nondestructively probe surface topography, layer thicknesses, and interface quality, as well as dopant concentrations and profiles. High-fidelity imaging was achieved by implementing variable-angle ptychographic imaging, by using total variation regularization to mitigate noise and artifacts in the reconstructed image, and by using a high-brightness, high-harmonic source with excellent intensity and wavefront stability. We validate our measurements through multiscale, multimodal imaging to show that this technique has unique advantages compared with other techniques based on electron and scanning probe microscopies.

https://advances.sciencemag.org/content/advances/7/5/eabd9667.full.pdf

Nondestructive, high-resolution, chemically specific 3D nanostructure characterization using phase-sensitive EUV imaging reflectometry

Extreme ultraviolet microscopy and wavefront sensing are key elements for next-generation ultrafast applications, such as chemically-resolved imaging, focal spot diagnostics in pump-and-probe experiments, and actinic metrology for the state-of-the-art lithography node at 13.5 nm wavelength. Ptychography offers a robust solution to the aforementioned challenges. Originally adapted by the electron and synchrotron communities, advances in the stability and brightness of high-harmonic tabletop sources have enabled the transfer of ptychography to the laboratory. This review covers the state of the art in tabletop ptychography with high harmonic generation sources. We consider hardware options such as illumination optics and detector concepts as well as algorithmic aspects in the analysis of multispectral ptychography data. Finally, we review technological application cases such as multispectral wavefront sensing, attosecond pulse characterization, and depth-resolved imaging. 

Advances in laboratory-scale ptychography using high harmonic sources [Invited]

Background: Reliable photomask metrology is required to reduce the risk of yield loss in the semiconductor manufacturing process as well as for the research on absorber materials. Actinic pattern inspection (API) of EUV reticles is a challenging problem to tackle with a conventional approach. For this reason, we developed RESCAN, an API platform based on coherent diffraction imaging. Aim: We want to verify the sensitivity of our platform to absorber and phase defects. Approach: We designed and manufactured two EUV mask samples with absorber and phase defects, and we inspected them with RESCAN in die-to-database mode. Results: We reconstructed an image of an array of programmed absorber defects, and we created a defect map of our sample. We inspected two programmed phase defect samples with buried structures of 3.5 and 7.8 nm height. Conclusions: We verified that RESCAN, in its current configuration, can detect absorber defects in random patterns and buried (phase) defects down to 50  ×  50  nm2.

/pdf/quantitative-characterization-of-absorber-and-phase-defects-5d53icrqb0.pdf

Quantitative characterization of absorber and phase defects on EUV reticles using coherent diffraction imaging

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.

/pdf/illumination-control-in-lensless-imaging-for-euv-mask-1hbzkkk6k8.pdf

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

Reliable photomask metrology is required to reduce the risk of yield loss in the semiconductor manufacturing process. Actinic pattern inspection (API) of EUV reticles is a challenging problem to tackle with a conventional approach. For this reason we developed an API platform based on coherent diffraction imaging. Aim: We want to verify the sensitivity of our platform to absorber and phase defects. Approach: We designed and manufactured two EUV mask samples with absorber and phase defects and we inspected them with RESCAN in die-to-database mode. Results: We reconstructed an image of an array of programmed absorber defects and we created a defect map of our sample. We inspected two programmed phase defect samples with buried structures of 3.5 nm and 7.8 nm height. Conclusions: We verified that RESCAN in its current configuration can detect absorber defects in random patterns and buried (phase) defects down to 50 × 50 nm2.

/pdf/absorber-and-phase-defect-inspection-on-euv-reticles-using-4hmhufrl6h.pdf

Absorber and phase defect inspection on EUV reticles using RESCAN

The production of modern semiconductor devices is based on photolithography, a process through which a pattern engraved on a mask is projected on a silicon wafer coated with a photosensitive material. In the past few decades, continuous technological progress in this field allowed the industry to follow Moore’s law by reducing the size of the printed features. This was achieved by progressively increasing the numerical aperture of the projection system and reducing the wavelength. The latest lithography platforms for semiconductor manufacturing employ Extreme Ultra Violet (EUV) light at a wavelength of 13.5 nm. The metrology for the optics and the components of such platforms is not fully mature yet. Specifically, the inspection of the EUV photomask is still an open issue as no commercial solutions are currently available. Here we describe a lensless approach to this problem, based on coherent diffraction imaging at EUV that overcomes the main technological issues linked to the conventional mask inspection approach.

/pdf/lensless-metrology-for-semiconductor-lithography-at-euv-1iumtcr469.pdf

Atoosa Dejkameh

Papers

Absorber and phase defect inspection on EUV reticles using RESCAN

Quantitative characterization of absorber and phase defects on EUV reticles using coherent diffraction imaging

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 metrology for semiconductor lithography at EUV