Lensless imaging with short-wavelength light is a promising method for achieving high-resolution, chemically sensitive images of a wide variety of samples. The use of 13 nm illumination is of particular interest for materials science and the imaging of next-generation nanofabricated devices. Prior to this work, there was an unmet need for a microscope that can image general samples with extreme ultraviolet light, which requires a reflection geometry. Here, we fulfill this need by performing lensless imaging using a 13 nm high-harmonic beam at grazing incidence, where most materials are reflective. Furthermore, we demonstrate to our knowledge the first 13 nm reflection-mode lensless microscope on a tabletop by using a compact high-harmonic generation source. Additionally, we present an analytic formalism that predicts when general lensless imaging geometries will yield Nyquist sampled data. Our grazing-incidence ptychographic approach, which we call GLIDER, provides the first route for achieving wide field-of-view, high-resolution, lensless images of general samples with extreme ultraviolet and soft x-ray light.

General-purpose, wide field-of-view reflection imaging with a tabletop 13 nm light source

Actinic mask defect inspection is an essential process step for the implementation of extreme ultraviolet (EUV) lithography in high-volume manufacturing. The main challenges for any mask defect inspection platform are resolution, sensitivity, and throughput. The reflective-mode EUV mask scanning lensless imaging microscope (RESCAN) is being developed to provide actinic patterned mask inspection capabilities for defects and patterns with high resolution and high throughput for node 7 and beyond. Namely, the goal of the RESCAN project is to develop a tool capable of inspecting an EUV reticle in 7 h and detect mask defects down to a size of 10  nm×10  nm. The lensless imaging concept allows overcoming the resolution limitations due to the numerical aperture and lens aberrations of conventional mask imaging systems. With the increasing availability of computational power and the refinement of iterative phase reconstruction algorithms, lensless imaging became a powerful tool to synthesize the complex amplitude of the reticle image providing us also with extremely valuable information about phase and mask three-dimensional effects. Here, we present a brief description of the current prototype of the RESCAN platform and illustrate a few experimental examples of programmed defect detection.

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RESCAN: an actinic lensless microscope for defect inspection of EUV reticles

For the successful implementation of extreme ultraviolet (EUV) lithography in the upcoming technology nodes, a major challenge to overcome is the stable and reliable detection and characterization of mask defects. We have recently presented a reflective mode EUV mask scanning lensless imaging tool (RESCAN) which was installed at the XIL-II beamline of the swiss light source and showed reconstructed aerial images of test patterns on EUV masks. RESCAN uses scanning coherent diffractive imaging (SCDI) methods to obtain actinic aerial images of EUV photomasks and was designed for 80 nm onmask resolution. Our SCDI algorithm reconstructs the measured sample by iteratively solving the phase problem using overdetermined diffraction data gathered by scanning across the specimen with a finite illumination. It provides the phase and amplitude aerial images of EUV photomasks with high resolution without the need to use high numerical aperture (NA) lenses. Contrary to scanning microscopy and full-field microscopy, where the resolution is limited by the spot size or NA of the lens, the achievable resolution with our method depends on the detector noise and NA of the detector. To increase the resolution of our tool, we upgraded RESCAN with a detector and algorithms. Here, we present the results obtained with the tool that is capable of up to 40-nm onmask resolution. We believe that the realization of our prototype marks a significant step toward overcoming the limitations imposed by methods relying on imaging optics and shows a viable solution for actinic mask metrology.

https://www.spiedigitallibrary.org/journals/Journal-of-MicroNanolithography-MEMS-and-MOEMS/volume-15/issue-3/034006/Scanning-coherent-diffractive-imaging-methods-for-actinic-extreme-ultraviolet-mask/10.1117/1.JMM.15.3.034006.pdf

Scanning coherent diffractive imaging methods for actinic extreme ultraviolet mask metrology

The re ective-mode EUV mask scanning lensless imaging microscope (RESCAN) is being developed to provide actinic mask inspection capabilities for defects and patterns with high resolution and high throughput, for 7 nm node and beyond. Here we, will report on our progress and present the results on programmed defect detection on random, logic-like patterns. The defects we investigated range from 200 nm to 50 nm size on the mask. We demonstrated the ability of RESCAN to detect these defects in die-to-die and die-to-database mode with a high signal to noise ratio. We also describe future plans for the upgrades to increase the resolution, the sensitivity, and the inspection speed of the demo tool.

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Actinic inspection of EUV reticles with arbitrary pattern design

With extreme ultraviolet (EUV) lithography getting ready to enter high volume manufacturing, there is an imminent need to address EUV mask metrology infrastructure. Actinic defect inspection of patterned EUV photomasks has been identified as an essential step for mask qualification, but there is no commercial tool available right now. We address this gap with the RESCAN tool, a defect inspection platform being built at Paul Scherrer Institut (PSI), co-developed in collaboration with Nuflare Inc, Japan. RESCAN uses Scanning Scattering Contrast Microscopy (SSCM) and Scanning Coherent Diffraction Imaging (SCDI) for fast defect detection and fine defect localization. The development of a stand-alone tool based on these techniques relies on the availability of (1) a bright coherent EUV source with a small footprint and (2) a high frame-rate pixel detector with extended dynamic range and high quantum efficiency for EUV. We present two in-house projects at PSI addressing the development of these components: COSAMI and JUNGFRAU. COSAMI (COmpact Source for Actinic Mask Inspection), is a high-brightness EUV source optimized for EUV photons with a relatively small footprint. JUNGFRAU (adJUstiNg Gain detector FoR the Aramis User station) is a silicon-based hybrid pixel detector, developed in house at PSI and prototyped for EUV. With a high frame rate and dynamic range at 13.5 nm, this sensor solution is an ideal candidate for the RESCAN platform. We believe that these ongoing source and sensor programs will pave the way towards a comprehensive solution for actinic patterned mask inspection bridging the gap of actinic defect detection and identification on EUV reticles.

/pdf/towards-a-stand-alone-high-throughput-euv-actinic-photomask-5317srl0mf.pdf

Towards a stand-alone high-throughput EUV actinic photomask inspection tool: RESCAN

Actinic mask inspection for EUV lithography with targeted specification of sensitivity and throughput is a big challenge and effective solutions are needed. We present a novel method for actinic mask inspection, i.e. scanning scattering contrast microscopy. In this method the EUV mask is scanned with a beam of relatively small spot size and the scattered light is recorded with a pixel detector. Since the mask layout is known, the scattering profile of a defect-free mask at the detector can be calculated. The signal between the measured and calculated signal provides the deviation between the real mask and its ideal counterpart and a signal above a certain threshold indicates the existence of a defect within the illumination area. Dynamic software filtering helps to suppress strong diffraction from defect free structures and allows registration of faint defects with high sensitivity. With the continuous scan of the whole mask area, a defect map can be obtained with high throughput. Therefore, we believe that this method has the potential of providing an effective solution for actinic mask inspection. Here we discuss the basic principles of the method, present proof-of-principle experiments, describe the basic components of a feasible stand-alone tool and present early results of the performance estimations of such a tool.

/pdf/scanning-scattering-contrast-microscopy-for-actinic-euv-mask-44jnolb1yh.pdf

Scanning scattering contrast microscopy for actinic EUV mask inspection

For the successful implementation of extreme ultraviolet (EUV) lithography in the upcoming technology nodes, a major challenge to overcome is the stable and reliable detection and characterization of mask defects. We have recently presented a reflective mode EUV mask scanning lensless imaging tool (RESCAN) which is installed at the XIL-II beamline of the Swiss Light Source and showed reconstructed aerial images of test patterns on EUV masks. RESCAN uses scanning coherent diffractive imaging (SCDI) methods to obtain actinic aerial images of EUV photomasks and was designed for 20 nm on-wafer resolution. Our SCDI algorithm reconstructs the measured sample by iteratively solving the phase-problem using over-determined diffraction data gathered by scanning across the specimen with a finite illumination. It provides phase and amplitude aerial images of EUV photomasks with high resolution without the need to use high NA (numerical aperture) lenses. Contrary to scanning microscopy and full-field microscopy, where the resolution is limited by the spot size or NA of the lens, the achievable resolution with our method depends on the detector noise and NA of the detector. To increase the resolution of our tool, we upgraded RESCAN with a new detector and algorithms. Here we present the results obtained with the new tool that is capable of up to 10 nm on-wafer resolution. We believe that the realization of our prototype marks a significant step towards overcoming the limitations imposed by methods relying on imaging optics and shows a viable solution for actinic mask metrology.

Scanning coherent diffractive imaging methods for actinic EUV mask metrology

Background: The purpose of EUV pellicles is to protect the surface of EUV lithography masks from particle contamination. It is important to ensure that the optical characteristics of the pellicle membrane do not critically affect the reticle image quality. Aim: We want to verify the possibility to integrate pellicle inspection and characterization capabilities in reflective-mode EUV mask scanning microscope (RESCAN), our actinic mask inspection platform based on coherent diffraction imaging. Approach: We studied the impact of a few selected EUV pellicle prototypes on the quality and the contrast of the reticle image obtained with RESCAN. Results: We measured the scattering distribution of the pellicles, and we correlated it with the mask image contrast and fidelity. We also detected the presence of a 6.5-μm-diameter fiber on the pellicle surface. Conclusions: We demonstrated that RESCAN is suitable for through-pellicle actinic mask inspection and can be also used to characterize and monitor the pellicle quality.

Experimental evaluation of the impact of carbon nanotube EUV pellicles on reticle imaging

Background: One of the challenges for extreme ultraviolet (EUV) lithography is the mitigation of mask three-dimensional effects arising from the oblique incident angle and the mask topography. As the scanners’ numerical aperture and the pattern aspect ratio increase, these effects become more prominent. A potential solution to reduce them consists in replacing the current TaBN absorber for a higher-k material. Aim: We demonstrate the potential of a mask inspection platform to evaluate the impact of different absorber materials on actinic defect inspection. Approach: We evaluate the performance of a reflective-mode EUV mask scanning microscope (RESCAN), our actinic lensless inspection tool, with three different absorber materials (hydrogen silsesquioxane, TaBN, and Ni). We study the effect of these materials on the image formation and compare the defect maps. Results: The Ni absorber mask exhibits a better contrast compared to the TaBN one, even though the thickness of the layers differs only by 10 nm. Programmed defects are localized and detected with a high signal-to-noise ratio (SNR). Conclusions: The gain in contrast for the Ni absorber being significant, the SNR is higher for a smaller defect in a TaBN absorber photomask. RESCAN allows the evaluation of the performance of absorber materials in defectivity and image formation on small samples.

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Rajeev Rajendran

Papers

Towards a stand-alone high-throughput EUV actinic photomask inspection tool: RESCAN

Scanning scattering contrast microscopy for actinic EUV mask inspection

Scanning coherent diffractive imaging methods for actinic EUV mask metrology

Experimental evaluation of the impact of carbon nanotube EUV pellicles on reticle imaging

Comparative study of extreme ultraviolet absorber materials using lensless actinic imaging