Extreme ultraviolet radiation for coherent diffractive imaging with high spatial resolution

EUV lithography is promising for addressing upcoming, <10nm nodes for the semiconductor industry, but with this promise comes the need for reliable metrology techniques. In particular, there is a need for actinic mask inspection in which the imaging wavelength matches that of the intended lithography process, so that the most relevant defects are detected. Here, we demonstrate tabletop, ptychographic, coherent diffraction imaging (CDI) in reflection- and transmission-modes of extended samples, using a 13 nm high harmonic generation (HHG) source. We achieve the first sub-wavelength resolution EUV image (0.9λ) in transmission, the highest spatial resolution using any 13.5 nm source to date. We also present the first reflection-mode image obtained on a tabletop using 12.7 nm light. This work represents the first 12.7 nm reflection-mode image using any source of a general sample.

This report is based on a BES Workshop on Compact Light Sources, held May 11-12, 2010, to evaluated the advantages and disadvantages of compact light source approaches and compared their performance to the third generation storage rings and free-electron lasers. The workshop examined the state of the technology for compact light sources and their expected progress. The workshop evaluated the cost efficiency, user access, availability, and reliability of such sources. Working groups evaluated the advantages and disadvantages of Compact Light Source (CLS) approaches, and compared their performance to the third-generation storage rings and free-electron lasers (FELs). The primary aspects of comparison were 1) cost effectiveness, 2) technical availability v. time frame, and 3) machine reliability and availability for user access. Five categories of potential sources were analyzed: 1) inverse Compton scattering (ICS) sources, 2) mini storage rings, 3) plasma sources, 4) sources using plasma-based accelerators, and 5) laser high harmonic generation (HHG) sources. Compact light sources are not a substitute for large synchrotron and FEL light sources that typically also incorporate extensive user support facilities. Rather they offer attractive, complementary capabilities at a small fraction of the cost and size of large national user facilities. In the far term they may offer the potential for a new paradigm of future national user facility. In the course of the workshop, we identified overarching R&D topics over the next five years that would enhance the performance potential of both compact and large-scale sources: Development of infrared (IR) laser systems delivering kW-class average power with femtosecond pulses at kHz repetition rates. These have application to ICS sources, plasma sources, and HHG sources. Development of laser storage cavities for storage of 10-mJ picosecond and femtosecond pulses focused to micron beam sizes. Development of high-brightness, high-repetition-rate electron sources. Development of continuous wave (cw) superconducting rf linacs operating at 4 K, while not essential, would reduce capital and operating cost.

Through generation of coherent extreme ultraviolet (XUV) radiation, high-harmonic generation (HHG) has opened up a wealth of possibilities for table-top research, ranging from attosecond dynamics to high resolution coherent imaging. The ability to perform interferometry with HHG sources would enable powerful coherent optical techniques such as Fourier transform spectroscopy in the XUV spectral range. However, interferometry and Fourier transform spectroscopy (FTS) remain challenging, due to the need for a pulse pair with a controllable, yet stable, attosecond delay. To overcome this challenge we have developed a method to produce such ultrastable XUV pulse pairs, using a birefringence-based common path interferometer. This interferometer is suitable for high-intensity laser pulses, which enables the generation of fully coherent XUV pulse pairs through HHG. The resulting pulse pairs have sub-attosecond timing variation, a tunable time delay and a clean Gaussian spatial mode profile, as shown in Figure 1 [1].

Widely used MRI methods show brain morphology both in vivo and ex vivo at very high resolution. Many of these methods (e.g., T2*-weighted imaging, phase-sensitive imaging, or susceptibility-weighted imaging) are sensitive to local magnetic susceptibility gradients produced by subtle variations in tissue composition. However, the spectral resolution of commonly used methods is limited to maintain reasonable run-time combined with very high spatial resolution. Here, the authors report on data acquisition at increased spectral resolution, with 3-dimensional high spectral and spatial resolution MRI, in order to analyze subtle variations in water proton resonance frequency and lineshape that reflect local anatomy. The resulting information compliments previous studies based on T2* and resonance frequency. The proton free induction decay was sampled at high resolution and Fourier transformed to produce a high-resolution water spectrum for each image voxel in a 3D volume. Data were acquired using a multigradient echo pulse sequence (i.e., echo-planar spectroscopic imaging) with a spatial resolution of 50 × 50 × 70 μm(3) and spectral resolution of 3.5 Hz. Data were analyzed in the spectral domain, and images were produced from the various Fourier components of the water resonance. This allowed precise measurement of local variations in water resonance frequency and lineshape, at the expense of significantly increased run time (16-24 h). High contrast T2*-weighted images were produced from the peak of the water resonance (peak height image), revealing a high degree of anatomical detail, specifically in the hippocampus and cerebellum. In images produced from Fourier components of the water resonance at -7.0 Hz from the peak, the contrast between deep white matter tracts and the surrounding tissue is the reverse of the contrast in water peak height images. This indicates the presence of a shoulder in the water resonance that is not present at +7.0 Hz and may be specific to white matter anatomy. Moreover, a frequency shift of 6.76 ± 0.55 Hz was measured between the molecular and granular layers of the cerebellum. This shift is demonstrated in corresponding spectra; water peaks from voxels in the molecular and granular layers are consistently 2 bins apart (7.0 Hz, as dictated by the spectral resolution) from one another. High spectral and spatial resolution MR imaging has the potential to accurately measure the changes in the water resonance in small voxels. This information can guide optimization and interpretation of more commonly used, more rapid imaging methods that depend on image contrast produced by local susceptibility gradients. In addition, with improved sampling methods, high spectral and spatial resolution data could be acquired in reasonable run times, and used for in vivo scans to increase sensitivity to variations in local susceptibility.

In this paper, we apply a tabletop, ultrafast, high-harmonic generation (HHG) source to measure the element-specific ferromagnetic resonance (FMR) in ultrathin magnetic alloys and multilayers on an opaque $\mathrm{Si}$ substrate. We demonstrate a continuous-wave bandwidth up to 62 GHz, with a promise to extend it to 100 GHz or higher. This laboratory-scale instrument detects the FMR using ultrafast, extreme-ultraviolet (EUV) light, with photon energies spanning the M-edges of the most relevant magnetic elements. A radiofrequency frequency comb generator is used to produce a microwave excitation that is intrinsically synchronized to the EUV pulses with a timing jitter of 1.1 ps or better. We apply this system to measure the dynamics in a multilayer system as well as $\mathrm{Ni}$-$\mathrm{Fe}$ and $\mathrm{Co}$-$\mathrm{Fe}$ alloys. Since this instrument operates in reflection mode, it is a milestone toward measuring and imaging the dynamics of the magnetic state and spin transport of active devices on arbitrary substrates on a tabletop. The higher bandwidth also enables measurements of materials with high magnetic anisotropy, as well as ferrimagnets, antiferromagnets, and short-wavelength (high-wavevector) spin waves in nanostructures or nanodevices. Furthermore, the coherence and short wavelength of the EUV will enable extending these studies using dynamic nanoscale lensless imaging techniques such as coherent diffractive imaging, ptychography, and holography.

In this paper, we present our latest results on development of a high harmonic generation (HHG) based EUV source at KMLabs. We have successfully demonstrated the first commercially available HHG source that meets critical specifications for metrology and imaging applications. We attribute this performance to our patented hollow core fiber technology and the modularized beamline developed to reliably characterize this EUV source.

The challenge of nanometric imaging drives intense efforts in applied sciences and fundamental research. Following the physical law of diffraction, the utilization of extreme ultraviolet (EUV) or X-ray radiation for imaging extends the ultimate resolution limit given by the illumination wavelength down to nanoscale dimensions. However, despite the great potential, the resolution is governed by imperfections inherent to optical elements employed. In this cumulative thesis, a table-top source of EUV radiation based on high harmonic generation (HHG), is developed and applied for lensless coherent diffractive imaging (CDI). Real-space images of microscopic objects are obtained solely on the basis of their far-field diffraction patterns, eliminating the need of any optical element and therefore any aberrations they may have. Benefiting from the exceptional coherence of the source developed, a diffraction-limited imaging is demonstrated with a spatial resolution of illuminating wavelength down to 30 nm. The microscopic images obtained via CDI allow for a detailed study of electric field distributions on the exit surface of the investigated structures. The experiments reveal that electric fields emerging from metallic structures are strongly affected by the polarization of the illuminating probe as well as propagation effects within the sample, indicating the limitations of the projection approximation. The above observations are attributed to waveguiding phenomena, thus, demonstrating extreme ultraviolet waveguiding in nanoscale slab structures. By direct imaging of the spatial distribution of the underlying electromagnetic modes exiting the sample, a strong polarization contrast of metallic waveguides with high aspect-ratio is experimentally identified. The guiding and polarization-sensitiveness are explored through an analytical approach based on the expansion in eigenmodes as well as numerical simulations to corroborate the experimental findings. Additionally, a novel approach to analyze and control the polarization state of EUV radiation by means of single image acquisition is developed. Such an analyzer, employing a set of nanoscale slab waveguides provides for an advanced characterization scheme for the polarization state of high harmonics. These results on CDI beyond the projection approximation using high harmonics demonstrate the potential of such a source for imaging, and pave the way towards applications in material science, for example, the spatial investigation of chiral phenomena, such as magnetism, enabling nanoscale magnetic imaging with a compact tabletop setup. Furthermore, HHG enables to date shortest possible pulse durations offering the opportunity to study ultrafast phenomena with unprecedented temporal resolution.

Hyperspectral images (HSI) have high spectral and low spatial resolutions. However, multispectral images (MSI) usually have low spectral and high spatial resolutions. In various applications HSI with high spectral and spatial resolutions are required. In this paper, a new method for spatial resolution enhancement of HSI using high resolution MSI based on sparse coding and linear spectral unmixing (SCLSU) is introduced. In the proposed method (SCLSU), high spectral resolution features of HSI and high spatial resolution features of MSI are fused. In this case, the sparse representation of some high resolution MSI and linear spectral unmixing (LSU) model of HSI and MSI is simultaneously used in order to construct high resolution HSI (HRHSI). The fusion process of HSI and MSI is formulated as an ill-posed inverse problem. It is solved by the Split Augmented Lagrangian Shrinkage Algorithm (SALSA) and an orthogonal matching pursuit (OMP) algorithm. Finally, the proposed algorithm is applied to the Hyperion and ALI datasets. Compared with the other state-of-the-art algorithms such as Coupled Nonnegative Matrix Factorization (CNMF) and local spectral unmixing, the SCLSU has significantly increased the spatial resolution and in addition the spectral content of HSI is well maintained.

Coherent diffraction imaging (CDI) is a powerful method for visualizing the structure of an object with a high spatial resolution that exceeds the performance limits of the lens. Single-frame CDI in the X-ray region has potential use for probing dynamic phenomena with a high spatiotemporal resolution. Here, we experimentally demonstrate a general method for single-frame X-ray CDI using a triangular aperture and a Fresnel zone plate. Using 5 keV synchrotron radiation X-rays, we reconstructed the object image of the locally illuminated area with a spatial resolution of higher than 50 nm and an exposure time of more than 0.1 s without prior information about the sample. After a 10 s exposure, a resolution of 17 nm was achieved. The present method opens new frontiers in the study of dynamics at the nanoscale by using next-generation synchrotron radiation X-rays/free-electron lasers as light sources.

Improved efficiency of high resolution thermal and cold neutron imaging

The combination of high-harmonic generation (HHG) illumination with coherent diffractive imaging (CDI) marks the beginning of a new era in nanoscale imaging. From early experiments relying on large-scale synchrotron facilities to today’s compact tabletop HHG sources, CDI is evolving from a proof-of-concept technique into a practical imaging tool. HHG-CDI systems offer exceptional flexibility, low cost, compact footprint, and simple architecture, while moving closer to meeting the demands of next-generation nanoscale imaging: high resolution, aberration-free imaging, non-destructive and non-contact operation, high efficiency, 3D reconstruction, and dynamic imaging. Furthermore, these systems enable chemical composition and concentration mapping of samples. As a result, HHG-CDI techniques are finding real-world applications in both scientific research and industrial settings, particularly in semiconductor inspection. In this thesis, we have explored and advanced the field of coherent EUV imaging through the development and application of tabletop HHG sources, ptychographic techniques, and Fourier transform holographic methodologies. Our work provides important steps towards high-resolution, material-sensitive, and spectrally resolved imaging for applications in materials science, nanotechnology, and semiconductor metrology.

Most modern satellites can provide two types of images: 1) panchromatic (PAN) image and 2) multispectral (MS) image. The former has high spatial resolution and low spectral resolution, while the latter has high spectral resolution and low spatial resolution. To obtain images with both high spectral and spatial resolution, pansharpening has emerged to fuse the spatial information of the PAN image and the spectral information of the MS image. However, most pansharpening methods fail to preserve spatial and spectral information simultaneously. In this article, we propose a framelet-based convolutional neural network (CNN) for pansharpening which makes it possible to pursue both high spectral and high spatial resolution. Our network consists of three subnetworks: 1) feature embedding net; 2) feature fusion net; and 3) framelet prediction net. Different from conventional CNN methods directly inferring high-resolution MS images, our approach learns to predict their framelet coefficients from available PAN and MS images. The introduction of multilevel feature aggregation and hybrid residual connection makes full use of spatial information of PAN image and spectral information of MS image. Quantitative and qualitative experiments at reduced- and full-resolution demonstrate that the proposed method achieves more appealing results than other state-of-the-art pansharpening methods. The source code and trained models are available at https://github.com/TingMAC/FrMLNet.

We present here a two-level model that not only reproduces experimentally observed plateau, but also allows us to evaluate its characteristics in explicit analytic form and predict new dramatic effects. Rare gases and rare-gas like ions in which high order odd harmonic generation (HHG) has been observed so far, may not be inconsistent with two-level description as far as HHG is concerned. >

Coastal ocean areas are very dynamic regions subject to strong anthropogenic pressure (e.g. industry, tourism, renewable energies, population). Satellite data constitute a unique tool to study and monitor these complex areas at high spatial and temporal resolutions. While &amp;#8220;traditional&amp;#8221; ocean colour satellites like Sentinel-3 provide daily temporal resolution, their sensors do not measure at the spatial resolution needed to correctly resolve complex coastal dynamics. On the other hand, while high spatial resolution sensors, like the MSI onboard Sentinel-2 (10m - 60m resolution), are able to resolve these small scales, their revisit time is far from optimal (2-5 days for the Sentinel-2 A &amp; B tandem). Additionally, both high spatial resolution datasets and traditional ones are hindered by the presence of clouds, resulting in a large amount of missing data.&amp;#160;Given the high complementarity of these two measurement strategies (Sentinel-3 and Sentinel-2), we present a methodology to derive gap-filled multi resolution ocean colour products from the synergistic use of Sentinel-2 and Sentinel-3 data. Applying DINEOF (Data Interpolating Empirical Orthogonal Functions), we exploit and combine both the high-resolution spatial variability information contained in Sentinel-2 products as well as the high temporal information of Sentinel-3. Both Sentinel-2 and Sentinel-3 products are generated using the Copernicus Marine High-Resolution processor, which uses automated switching algorithms adapted to the local water conditions (at the pixel level) to retrieve optimal remote sensing spectra and water quality variables. With this approach, we address the high variability of different water types with small scale changes. The combined Sentinel-2 and Sentinel-3 products consist of Chlorophyll-a concentration and turbidity retrieved through a multi-algorithm approach with optimized quality flagging.&amp;#160;An analysis of turbidity and chlorophyll daily data at 100m resolution in the northern Adriatic Sea and the Belgian coastal zone will be presented, alongside an analysis of the spatial scales resolved by the original and merged DINEOF datasets.

Hyperspectral (HS) images obtained from space are useful for monitoring different natural phenomena on regional to global scales. The Environmental Mapping and Analysis Program (EnMAP) is a satellite recently launched by Germany to monitor the environment and explore the capabilities of hyperspectral sensors in the 420 and 2450 nm range of the spectrum. However, the data captured by the EnMAP mission have a ground sampling distance (GSD) of 30 m. This limits the use of the data for some applications that require higher spatial resolution (&lt;10 m). This study examines the potential for improving the resolution of hyperspectral data using high resolution multispectral (MS) data obtained by Cubesats. Specifically, this work uses the data captured by the PlanetScope constellation, which has more than 150 CubeSats in low Earth orbit, with a high spatial and temporal resolution. The approach adopted leverages (1) the spectral capability of the hyperspectral EnMAP sensor, with a bandwidth of 6.5 nm in the visible and near infrared (VNIR) range (420&amp;#8211;1000 nm) and 10 nm in the SWIR range (900&amp;#8211;2450 nm), and (2) the spatial capability of the multispectral PlanetScope data, with a GSD of 3 meters, to enable significant spatial improvements due to its high spatial resolution. The main components of this work include: (i) area of interest clipping (ii) data co-registration, (iii) HS-MS data fusion, and (iv) quality assessments using the Jointly Spectral and Spatial Quality Index (QNR). In this study, a 2 km x 2 km area of interest was selected in the Malaucene region of France, where six state-of-the-art HS-MS fusion methods were evaluated: (1) fast multi-band image fusion algorithm (FUSE), (2) coupled nonnegative matrix factorization (CNMF), (3) smoothing filtered-based intensity modulation (SFIMHS), (4) maximum a posteriori stochastic mixing model (MAPSMM), (5) Hyperspectral Superresolution (HySure), and (6) generalized laplacian pyramid hypersharpening (GLPHS). Quality assessments of the enhanced data showed that high spectral and spatial fidelity are maintained, with the best performing fusion method being FUSE with a QNR of 0.625 followed by the MAPSMM method with a QNR of 0.604. Overall, this study advocates the benefits associated with the fusion of hyperspectral and multispectral data to obtain enhanced EnMAP data at 3 m GSD.&amp;#160;