Diffractive X-ray optics, like Fresnel zone plate lenses, are widely employed X-ray optics for collimation and focusing. While they are extremely versatile and easy to use optical elements, they generally suffer from limited efficiency due to limitations in fabrication possibilities. Near-field stacking is an established concept for overcoming fabrication limitations, yet its existing implementations suffer from issues regarding complexity and stability. In this work, an alternative stacking concept is explored, by patterning both the front and back sides of a single membrane. Such double-sided zone plates are shown to exchange conventional zone plate stacks in increasing the efficiency or resolution of conventional zone plate optics. In conventional stacking, they achieve 9.9% focusing efficiency at 9 keV with 30 nm smallest half-pitch and diffraction limited optical performance. Following the blazed stacking scheme, they are shown to provide up to 54.7% diffraction efficiency at 6.2 keV. Finally, using the novel concept of interlaced stacking, they demonstrate the feasibility of large aperture X-ray optics for sub-10 nm X-ray nanofocusing.
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.
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.
Actinic mask defect inspection is an essential process step for the implementation of 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 first goal of the RESCAN project is to develop a tool capable of inspecting an EUV reticle in about 7 hours and detect mask defects down to a size of 10 nm. The lensless imaging concept allows to overcome the resolution limitations due to the numerical aperture (NA) and lens aberrations of conventional actinic 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 actinic aerial image providing us also with extremely valuable information about phase and mask 3D 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.
Actinic mask inspection for EUV lithography with targeted specifications of resolution, sensitivity, and throughput
remains a big hurdle for the successful insertion of EUVL into high volume manufacturing and effective solutions are
needed to address this. We present a method for actinic mask inspection based on scanning coherent scattering
microscopy. In this method, the mask is scanned with an EUV beam of relatively small spot size and the scattered light is
recorded with a pixel detector. Customized algorithms reconstruct the aerial image by iteratively solving the phaseproblem
using over-determined diffraction data gathered by scanning across the specimen with a finite illumination. This
approach provides both phase and amplitude of actinic aerial images of the mask with high resolution without the need to
use high NA (numerical aperture) lenses. Futher, we describe a reflective mode EUV mask scanning lensless imaging
tool (RESCAN), which was installed at the XIL-II beamline and later at the SIM beamline of the Swiss Light Source and
show reconstructed aerial images down to 10 nm (on-wafer) resolution. As a complementary method, the a-priori
knowledge of the sample is employed to identify potential defect sites by analyzing the diffraction patterns. In this
method, the recorded diffraction patterns are compared with the die or database data (i.e. previously measured or
calculated diffraction data from the defect-free mask layout respectively) and their difference is interpreted as the defect
signal. Dynamic software filtering helps to suppress the strong diffraction from defect-free structures and allows
registration of faint defects with high sensitivity. Here, we discuss the basic principles of these Fourier domain
techniques and its potential for actinic mask inspection with high signal-to-noise ratio and high throughput.
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.
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.
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.
KEYWORDS: Zone plates, Diffraction, Binary data, Optical alignment, Phase shifts, X-rays, Lenses, Scanning electron microscopy, Near field optics, Actuators
The focusing efficiency of binary Fresnel zone plate lenses is fundamentally limited and higher efficiency requires a
multi step lens profile. To overcome the manufacturing problems of high resolution and high efficiency multistep zone
plates, we investigate the concept of stacking two different binary zone plates in each other’s optical near-field. We use a
coarse zone plate with π phase shift and a double density fine zone plate with π/2 phase shift to produce an effective 4-
step profile. Using a compact experimental setup with piezo actuators for alignment, we demonstrated 47.1% focusing
efficiency at 6.5 keV using a pair of 500 μm diameter and 200 nm smallest zone width. Furthermore, we present a
spatially resolved characterization method using multiple diffraction orders to identify manufacturing errors, alignment
errors and pattern distortions and their effect on diffraction efficiency.
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