Weilun Chao

Lawrence Berkeley National Laboratory, Berkeley, California, United States

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Publications (135)233.51 Total impact

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    ABSTRACT: Coherent extreme ultraviolet beams from tabletop high harmonic generation offer several revolutionary capabilities for observing nanoscale systems on their intrinsic length and time scales. By launching and monitoring hypersonic acoustic waves in such systems, we characterize the mechanical properties of sub-10nm layers and find that the material densities remain close to their bulk values while their elastic properties are significantly modified. Moreover, within the same measurement, by following the heat dissipation dynamics from 30-750nm-wide nanowires, we uncover a new thermal transport regime in which closely-spaced nanoscale heat sources can surprisingly cool more efficiently than widely-spaced heat sources of the same size.
    SPIE Metrology, Inspection, and Process Control for Microlithography XXIX, San Jose, CA; 02/2016
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    ABSTRACT: Understanding thermal transport from nanoscale heat sources is important for a fundamental description of energy flow in materials, as well as for many technological applications including thermal management in nanoelectronics and optoelectronics, thermoelectric devices, nanoenhanced photovoltaics, and nanoparticle-mediated thermal therapies. Thermal transport at the nanoscale is fundamentally different from that at the macroscale and is determined by the distribution of carrier mean free paths and energy dispersion in a material, the length scales of the heat sources, and the distance over which heat is transported. Past work has shown that Fourier’s law for heat conduction dramatically overpredicts the rate of heat dissipation from heat sources with dimensions smaller than the mean free path of the dominant heat-carrying phonons. In this work, we uncover a new regime of nanoscale thermal transport that dominates when the separation between nanoscale heat sources is small comparedwith the dominant phononmean free paths. Surprisingly, the interaction of phonons originating from neighboring heat sources enables more efficient diffusive-like heat dissipation, even from nanoscale heat sources much smaller than the dominant phonon mean free paths. This finding suggests that thermal management in nanoscale systems including integrated circuits might not be as challenging as previously projected. Finally, we demonstrate a unique capability to extract differential conductivity as a function of phonon mean free path in materials, allowing the first (to our knowledge) experimental validation of predictions from the recently developed first-principles calculations.
    Proceedings of the National Academy of Sciences 03/2015; 112(16). DOI:10.1073/pnas.1503449112 · 9.81 Impact Factor
  • IEEE Photonics Journal 02/2015; 7(1):1-1. DOI:10.1109/JPHOT.2015.2389957 · 2.33 Impact Factor
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    ABSTRACT: Analytical probes capable of mapping molecular composition at the nanoscale are of critical importance to materials research, biology and medicine. Mass spectral imaging makes it possible to visualize the spatial organization of multiple molecular components at a sample's surface. However, it is challenging for mass spectral imaging to map molecular composition in three dimensions (3D) with submicron resolution. Here we describe a mass spectral imaging method that exploits the high 3D localization of absorbed extreme ultraviolet laser light and its fundamentally distinct interaction with matter to determine molecular composition from a volume as small as 50 zl in a single laser shot. Molecular imaging with a lateral resolution of 75 nm and a depth resolution of 20 nm is demonstrated. These results open opportunities to visualize chemical composition and chemical changes in 3D at the nanoscale.
    Nature Communications 01/2015; 6:6944. DOI:10.1038/ncomms7944 · 10.74 Impact Factor
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    ABSTRACT: Beamline 2.1 (XM-2) is a transmission soft X-ray microscope in sector 2 of the Advanced Light Source at Lawrence Berkeley National Laboratory. XM-2 was designed, built and is now operated by the National Center for X-ray Tomography as a National Institutes of Health Biomedical Technology Research Resource. XM-2 is equipped with a cryogenic rotation stage to enable tomographic data collection from cryo-preserved cells, including large mammalian cells. During data collection the specimen is illuminated with `water window' X-rays (284–543 eV). Illuminating photons are attenuated an order of magnitude more strongly by biomolecules than by water. Consequently, differences in molecular composition generate quantitative contrast in images of the specimen. Soft X-ray tomography is an information-rich three-dimensional imaging method that can be applied either as a standalone technique or as a component modality in correlative imaging studies.
    Journal of Synchrotron Radiation 11/2014; 21. DOI:10.1107/S1600577514015033 · 3.02 Impact Factor
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    ABSTRACT: X-ray microscopy is powerful in that it can probe large volumes of material at high spatial resolution with exquisite chemical, electronic and bond orientation contrast1, 2, 3, 4, 5. The development of diffraction-based methods such as ptychography has, in principle, removed the resolution limit imposed by the characteristics of the X-ray optics6, 7, 8, 9, 10. Here, using soft X-ray ptychography, we demonstrate the highest-resolution X-ray microscopy ever achieved by imaging 5 nm structures. We quantify the performance of our microscope and apply the method to the study of delithiation in a nanoplate of LiFePO4, a material of broad interest in electrochemical energy storage11, 12. We calculate chemical component distributions using the full complex refractive index and demonstrate enhanced contrast, which elucidates a strong correlation between structural defects and chemical phase propagation. The ability to visualize the coupling of the kinetics of a phase transformation with the mechanical consequences is critical to designing materials with ultimate durability.
    Nature Photonics 09/2014; advance online publication. DOI:10.1038/nphoton.2014.207 · 29.96 Impact Factor
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    ABSTRACT: A self-contained electro-optical module for scanning extreme ultraviolet (EUV) reflection microscopy at 13.5 nm wavelength has been developed. The system has been designed to work with stand-alone commercially available EUV high harmonic generation (HHG) sources through the implementation of narrowband harmonic selecting multilayers and off-axis elliptical short focal length zoneplates. The module has been successfully integrated into an EUV mask scanning microscope achieving diffraction limited imaging performance (84 nm point spread function).
    Optics Express 08/2014; 22(17). DOI:10.1364/OE.22.020144 · 3.53 Impact Factor
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    ABSTRACT: Energy efficient nanomagnetic logic (NML) computing architectures propagate and process binary information by relying on dipolar field coupling to reorient closely-spaced nanoscale magnets. Signal propagation in nanomagnet chains of various sizes, shapes, and magnetic orientations has been previously characterized by static magnetic imaging experiments with low-speed adiabatic operation; however the mechanisms which determine the final state and their reproducibility over millions of cycles in high-speed operation (sub-ns time scale) have yet to be experimentally investigated. Monitoring NML operation at its ultimate intrinsic speed reveals features undetectable by conventional static imaging including individual nanomagnetic switching events and systematic error nucleation during signal propagation. Here, we present a new study of NML operation in a high speed regime at fast repetition rates. We perform direct imaging of digital signal propagation in permalloy nanomagnet chains with varying degrees of shape-engineered biaxial anisotropy using full-field magnetic soft x-ray transmission microscopy after applying single nanosecond magnetic field pulses. Further, we use time-resolved magnetic photo-emission electron microscopy to evaluate the sub-nanosecond dipolar coupling signal propagation dynamics in optimized chains with 100 ps time resolution as they are cycled with nanosecond field pulses at a rate of 3 MHz. An intrinsic switching time of 100 ps per magnet is observed. These experiments, and accompanying macro-spin and micromagnetic simulations, reveal the underlying physics of NML architectures repetitively operated on nanosecond timescales and identify relevant engineering parameters to optimize performance and reliability.
    Nature Communications 08/2014; 6. DOI:10.1038/ncomms7466 · 10.74 Impact Factor
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    ABSTRACT: Gray-scale e-beam lithography has been performed to match the EUV and e-beam aerial image log slope for studying shot noise fundamentals in the two mechanisms through line-edge roughness (LER) measurements for 50 nm lines and spaces patterned on a leading chemically amplified EUV resist. The measured e-beam exposure latitude decreased from 0.4 with binary patterning to 0.28 with gray-scale e-beam exposure designed to match the EUV incident image profile, closely matching the EUV exposure latitude of 0.26. Calculations of absorption statistics with EUV and e-beam suggest that the shot noise with e-beam patterning is expected to be 10% larger than the shot noise with EUV patterning. However, despite the matched image gradients and close to identical absorbed quanta predictions, the e-beam patterned LER is 2.5× larger than the EUV patterned LER.
    SPIE Advanced Lithography; 04/2014
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    ABSTRACT: The SEMATECH High Numerical Aperture Actinic Reticle Review Project (SHARP) is a synchrotron-based extreme ultraviolet (EUV) microscope dedicated to photomask research. SHARP has been operational and serving users since June, 2013, and in eight months, SHARP has recorded over 71,000 high-resolution images. Exposure times are 5 to 8 seconds, and 8 or more through-focus series can be collected per hour at positions spanning the entire mask surface. SHARP’s lossless coherence-control illuminator and variable numerical aperture (NA) enable researchers to emulate the imaging properties of both current and future EUV lithography tools. SHARP’s performance continues to improve over time due to tool learning and upgraded capabilities, described here. Within a centered, 3-μm square image region, we demonstrate an illumination power stability above 99%, and an average uniformity of 98.4%. Demonstrations of through-focus imaging with various illumination coherence settings highlight the capabilities of SHARP.
    SPIE Advanced Lithography; 04/2014
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    ABSTRACT: Nanomagnetic logic is an energy efficient computing architecture that relies on the dipole field coupling of neighboring magnets to transmit and process binary information. In this architecture, nanomagnet chains act as local interconnects. To assess the merits of this technology, the speed and reliability of magnetic signal transmission along these chains must be experimentally determined. In this work, time-resolved pump-probe x-ray photo-emission electron microscopy is used to observe magnetic signal transmission along a chain of nanomagnets. We resolve successive error-free switching events in a single nanomagnet chain at speeds on the order of 100 ps per nanomagnet, consistent with predictions based on micromagnetic modeling. Errors which disrupt transmission are also observed. We discuss the nature of these errors, and approaches for achieving reliable operation.
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    ABSTRACT: A table top nanofabrication system which combines the classic Talbot imaging effect and a compact table top soft-x ray laser is described in this paper. Periodic nanostructures on millimeter square are fabricated using this robust, simple and defect tolerant fabrication method. Talbot lithography allows for a complete coherent extreme ultraviolet lithography process in a compact table top system. Double exposure allowed for the reduction of the feature sizes.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2014; DOI:10.1117/12.2041688 · 0.20 Impact Factor
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    ABSTRACT: We present a single-shot Fourier transform holography setup with ~100nm spatial resolution and 1 ns temporal resolution using a tabletop extreme ultraviolet (EUV) laser. Flash images allowed for the imaging of nano-pillars oscillating at MHz frequencies that will enable the evaluation of mechanical properties of nanoscale mechanical oscillators.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2014; DOI:10.1117/12.2041698 · 0.20 Impact Factor
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    ABSTRACT: We recorded the fast oscillation of sub-micron cantilevers using time-resolved extreme ultraviolet (EUV) Fourier transform holography. A tabletop capillary discharge EUV laser with a wavelength of 46.9 nm provided a large flux of coherent illumination that was split using a Fresnel zone plate to generate the object and the reference beams. The reference wave was produced by the first order focus while a central opening in the zone plate provided a direct illumination of the cantilevers. Single-shot holograms allowed for the composition of a movie featuring the fast oscillation. Three-dimensional displacements of the object were determined as well by numerical back-propagation, or "refocusing" of the electromagnetic fields during the reconstruction of a single hologram.
    Optics Express 02/2014; 22(4):4161-7. DOI:10.1364/OE.22.004161 · 3.53 Impact Factor
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    ABSTRACT: New approach to full-field soft x-ray holographic microscopy is being developed based on the principle of self-interferencce incoherent digital holography, with simple optical configurations and higher efficiencies. Theorectial and preliminary experimental results will be presented.
    Digital Holography and Three-Dimensional Imaging; 01/2014
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    ABSTRACT: Diffractive optical elements such as Fresnel zoneplate lenses have many uses at extreme ultraviolet (EUV), particularly in short focal length, high-resolution applications. However, the diffraction efficiency of a pure absorption zoneplate is limited to about 10%, and it suffers additional loss through the membrane support material. To this end, the authors explored the possibility of silicon nitride (Si3N4) as a EUV phase shifting material. At an etched depth of 244 nm, they measured a diffraction efficiency of 18% in the first order and 18% in the zero order, which compares favorably to an amplitude grating of 10% and 25%, respectively. The measured efficiency as a function of etch depth matches the scalar theory quite well using a measured EUV index of refraction 0.9790 + 0.0066i at the wavelength of 13.5 nm. To further increase the efficiency, zoneplates were made freestanding, with the support membrane completely removed, and a 15% absolute efficiency was obtained. Vector electromagnetic calculations showed that at normal incidence, these optics produce excellent wavefront and efficiency for outer zones of 50 nm or larger. Zoneplates of narrower zones or those illuminated obliquely can suffer larger wavefront errors and low efficiency and would require careful design optimization. In the work, the authors also demonstrated a technique to package zoneplates and associated apertures for high precision insertion and removal from a EUV instrument. This technique has yielded alignment accuracy from a few microns to few 10s microns, depending on the exact design.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 11/2013; 31(6):06F606-06F606-5. DOI:10.1116/1.4826695 · 1.36 Impact Factor
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    ABSTRACT: A compact nanofabrication system that combines Talbot lithography and a table-top extreme ultraviolet laser is presented. The lithographic method based on the Talbot effect provides a robust and simple experimental setup that is capable to print periodic structures over millimeter square areas free of defects. Test structures were printed and transferred into metal layers showing a complete coherent extreme ultraviolet lithographic process in a table-top system. (C) 2013 American Vacuum Society.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 11/2013; 31(6):06F604-06F604-7. DOI:10.1116/1.4826344 · 1.36 Impact Factor
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    ABSTRACT: We demonstrate single-shot Fourier transform holography with the use of a tabletop extreme ultraviolet laser with spatial resolution of 169 nm and temporal resolution of 1ns.
    Frontiers in Optics; 10/2013
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    ABSTRACT: We demonstrate the first general tabletop EUV coherent microscope that can image extended, non-isolated, non-periodic, objects. By implementing keyhole coherent diffractive imaging with curved mirrors and a tabletop high harmonic source, we achieve improved efficiency of the imaging system as well as more uniform illumination at the sample, when compared with what is possible using Fresnel zone plates. Moreover, we show that the unscattered light from a semi-transparent sample can be used as a holographic reference wave, allowing quantitative information about the thickness of the sample to be extracted from the retrieved image. Finally, we show that excellent tabletop image fidelity is achieved by comparing the retrieved images with scanning electron and atomic force microscopy images, and show superior capabilities in some cases.
    Optics Express 09/2013; 21(19):21970-21980. DOI:10.1364/OE.21.021970 · 3.53 Impact Factor
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    ABSTRACT: The authors are extending the capabilities of the SHARP microscope (SEMATECH High-NA Actinic Reticle review Project) by implementing wave front coding as a complementary imaging mode. SHARP, using a single off-axis lens has a tilted focal plane, reducing the instrument’s field of view to a few micrometers. Wave-front coding increases the depth of field of an incoherent imaging system without affecting its resolution and light gathering power, rendering clear, large-field images for navigation and analysis. The resolution of the resulting image is close to the diffraction limit of the unmodified system. The authors have designed and nanofabricated zone plate lenses with a modified pattern that combines focusing power and wave front coding in a single optical element. The study clears the path to further applications of wave front coded zone plates in lab- and synchrotron-based microscopes and metrology tools. The authors have demonstrated wave-front coding in visible-light optical systems using Fresnel zone plate lenses in an offaxis configuration similar to SHARP. Simulations complementing the visible light experiments assess the performance of wave-front coded zone plates in the SHARP microscope.
    SPIE Photomask Technology; 09/2013

Publication Stats

1k Citations
233.51 Total Impact Points

Institutions

  • 2000–2014
    • Lawrence Berkeley National Laboratory
      • Center for X-Ray Optics
      Berkeley, California, United States
    • University of California, Berkeley
      Berkeley, California, United States
  • 2007–2012
    • Colorado State University
      • • Department of Physics
      • • Electrical & Computer Engineering
      • • Extreme Ultraviolet Science and Technology Center
      Fort Collins, CO, United States