We report on the use of multilayer Laue lenses to focus the intense X-ray Free Electron Laser (XFEL) beam at the European XFEL to a spot size of a few tens of nanometers. We present the procedure to align and characterize these lenses and discuss challenges working with the pulse trains from this unique x-ray source.
Optical trapping of light-absorbing particles in a gas environment is usually dominated by laser-induced thermal or photophoretic forces, which can be orders of magnitude higher than the force due to radiation pressure. Particle guiding with photophoretic forces over large distances in open air was recently realised by an optical pipeline, formed by a vortex laser beam of doughnut-like intensity profile, with a high-intensity ring of light that surrounds a dark core. We are adapting the optical pipeline concept for the purpose of guiding aerosolized particles into the intense focus of a x-ray free-electron laser (XFEL), in order to enable high-efficiency femtosecond x-ray coherent diffractive imaging (CDI). XFEL-based CDI allows single-shot nanometer-resolution imaging, and multi-shot Angstrom-resolution tomography in the case of reproducible nanoparticles, at a time resolution better than 10 femtoseconds. Remarkably, by imaging at timescales shorter than atomic motion, the crucial resolution-limiting effects of radiation damage may be overcome for radiation-sensitive targets such as viruses and biomolecules. Following on our previous work, we are developing an optical first-order Bessel-like beam with a variable-diameter hollow core and an axial-to-lateral aspect ratio up to ~2000, that can be used to guide particles with a spatial precision of less than a few µm over centimetre-long distances. We present the ways to control the beam divergence aiming to focus the stream of particles by thermal forces and forces of radiation pressure, analyse the forces acting on the particle in the beam, and uncover the beam structure and intensity to apply for a real-time experiment with XFEL.
Joe P. J. Chen, Richard Kirian, Kenneth Beyerlein, Richard Bean, Andrew Morgan, Oleksandr Yefanov, Romain Arnal, David Wojtas, Phil Bones, Henry Chapman, John C. H. Spence, Rick Millane
Serial femtosecond nanocrystallography (SFX) is a form of x-ray coherent diffraction imaging that utilises a stream of tiny nanocrystals of the biological assembly under study, in contrast to the larger crystals used in conventional x-ray crystallography using conventional x-ray synchrotron x-ray sources. Nanocrystallography utilises the extremely brief and intense x-ray pulses that are obtained from an x-ray free-electron laser (XFEL). A key advantage is that some biological macromolecules, such as membrane proteins for example, do not easily form large crystals, but spontaneously form nanocrystals. There is therefore an opportunity for structure determination for biological molecules that are inaccessible using conventional x-ray crystallography. Nanocrystallography introduces a number of interesting image reconstruction problems. Weak diffraction patterns are recorded from hundreds of thousands of nancocrystals in unknown orientations, and these data have to be assembled and merged into a 3D intensity dataset. The diffracted intensities can also be affected by the surface structure of the crystals that can contain incomplete unit cells. Furthermore, the small crystal size means that there is potentially access to diffraction information between the crystalline Bragg peaks. With this information, phase retrieval is possible without resorting to the collection of additional experimental data as is necessary in conventional protein crystallography. We report recent work on the diffraction characteristics of nanocrystals and the resulting reconstruction algorithms.
Optical trapping of light-absorbing particles in a gaseous environment is governed by a laser-induced photophoretic force, which can be orders of magnitude stronger than the force of radiation pressure induced by the same light intensity. In spite of many experimental studies, the exact theoretical background underlying the photophoretic force and the prediction of its influence on the particle motion is still in its infancy. Here, we report the results of a quantitative analysis of the photophoretic force and the stiffness of trapping achieved by levitating graphite and carbon-coated glass shells of calibrated sizes in an upright diverging hollow-core vortex beam, which we refer to as an ‘optical funnel’. The measurements of forces were conducted in air at various gas pressures in the range from 5 mbar to 2 bar. The results of these measurements lay the foundation for developing a touch-free optical system for precisely positioning sub-micrometer bioparticles at the focal spot of an x-ray free electron laser, which would significantly enhance the efficiency of studying nanoscale morphology of proteins and biomolecules in femtosecond coherent diffractive imaging experiments.
The photon density profile of an X-ray free-electron laser (XFEL) beam at the focal position is a critical parameter for serial femtosecond crystallography (SFX), but is difficult to measure because of the destructive power of the beam. A novel high intensity radiation induced phasing method (HIRIP) has been proposed as a general experimental approach for protein structure determination, but has proved to be sensitive to variations of the X-ray intensity, with uniform incident fluence desired for best performance. Here we show that experimental SFX data collected at the nano-focus chamber of the Coherent X-ray Imaging end-station at the Linac Coherent Light Source using crystals with a limited size distribution suggests an average profile of the X-ray beam that has a large variation of intensity. We propose a new method to improve the quality of high fluence data for HI-RIP, by identifying and removing diffraction patterns from crystals exposed to the low intensity region of the beam. The method requires crystals of average size comparable to the width of the focal spot.
We report on the fabrication of novel diffraction gratings for soft x-ray and extreme ultra-violet (EUV) photon energies based on asymmetric-cut multilayer structures. Asymmetric-cut multilayers are highly dispersive and highly efficient gratings obtained by slicing a thick multilayer coating. Multilayer deposition techniques enable sub-ångström precision in layer thickness control, which leads to close to perfect blazed gratings. However, the final grating size is limited by the maximum multilayer thickness for which one can still control the layer thickness, stress and roughness. Here, we present a new approach in which we substantially extend the grating size by combining specially prepared substrates, thick multilayer deposition and final polishing. Gratings prepared by this method, like asymmetric multilayers deposited on plane substrates, are highly dispersive and efficient. Their extended size make them ideal for use in monochromators, spectrometers and pulse compressors.
We have developed a high-aspect ratio optical pipeline aiming to produce a highly collimated stream of micron-size
particles in either gaseous or vacuum environments. A hollow, first-order quasi-Bessel beam with variable divergence
was generated with a phase-only spatial light modulator (SLM), by superimposing the quadratic phase of a lens and an
axicon with a 0.5° base angle. The beam was further re-imaged to form a centimetre-long funnel beam with ~5μm
diameter and up to 2000 length-to-diameter aspect ratio. The divergence of the central core of the Bessel beam was
controlled by varying the effective lens in the hologram. The SLM-based optical beam was compared to a similar
beam composed using a physical axicon. The experimental tests were conducted with 2-μm size polystyrene spherical
particles to evaluate the optical force. We present estimated optical forces exerted on the particles in the transverse
plane, both depending on the particle size, laser power, and background-gas pressure.
Adam F. Leontowich, Andrew Aquila, Francesco Stellato, Richard Bean, Holger Fleckenstein, Mauro Prasciolu, Mengning Liang, Daniel DePonte, Anton Barty, Fenglin Wang, Jakob Andreasson, Janos Hajdu, Henry Chapman, Saša Bajt
A super-polished substrate with an off-axis parabola figure was coated with a Sc/B4C/Cr multilayer. This optic was used to focus pulses of 4.3 nm photons from the Free-electron LASer in Hamburg (FLASH) at normal incidence. Beam imprints were made in poly(methyl methacrylate) to align the optic and to measure the beam profile at the focal plane. The intense interaction resulted in imprints with raised perimeters, surrounded by ablated material extending out several micrometres. These features interfere with the beam profile measurement. The effect of a post-exposure development step on the beam imprints was investigated.
N. Loh, Dmitri Starodub, Lukas Lomb, Christina Hampton, Andrew Martin, Raymond Sierra, Anton Barty, Andrew Aquila, Joachim Schulz, Jan Steinbrener, Robert Shoeman, Stephan Kassemeyer, Christoph Bostedt, John Bozek, Sascha Epp, Benjamin Erk, Robert Hartmann, Daniel Rolles, Artem Rudenko, Benedikt Rudek, Lutz Foucar, Nils Kimmel, Georg Weidenspointner, Günther Hauser, Peter Holl, Emanuele Pedersoli, MengNing Liang, Mark Hunter, Lars Gumprecht, Nicola Coppola, Cornelia Wunderer, Heinz Graafsman, Filipe R. N. Maia, Tomas Ekeberg, Max Hantke, Holger Fleckenstein, Helmut Hirsemann, Karol Nass, Thomas White, Herbert Tobias, George Farquar, W. Henry Benner, Stefan Hau-Riege, Christian Reich, Andreas Hartmann, Heike Soltau, Stefano Marchesini, Sasa Bajt, Miriam Barthelmess, Lothar Strueder, Joachim Ullrich, Philip Bucksbaum, Keith Hodgson, Mathias Frank, Ilme Schlichting, Henry Chapman, Michael Bogan
Profiling structured beams produced by X-ray free-electron lasers (FELs) is crucial to both maximizing signal intensity for weakly scattering targets and interpreting their scattering patterns. Earlier ablative imprint studies describe how to infer the X-ray beam profile from the damage that an attenuated beam inflicts on a substrate. However, the beams in-situ profile is not directly accessible with imprint studies because the damage profile could be different from the actual beam profile. On the other hand, although a Shack-Hartmann sensor is capable of in-situ profiling, its lenses may be quickly damaged at the intense focus of hard X-ray FEL beams. We describe a new approach that probes the in-situ morphology of the intense FEL focus. By studying the translations in diffraction patterns from an ensemble of randomly injected sub-micron latex spheres, we were able to determine the non-Gaussian nature of the intense FEL beam at the Linac Coherent Light Source (SLAC National Laboratory) near the FEL focus. We discuss an experimental application of such a beam-profiling technique, and the limitations we need to overcome before it can be widely applied.
X-ray free-electron lasers enable high-resolution imaging of biological materials by using short enough pulses to outrun many of the effects of radiation damage. Experiments conducted at the LCLS have obtained diffraction data from single particles and protein nanocrystals at doses to the sample over 3 GGy. The details of the interaction of the X-ray FEL pulse with the sample determine the limits of this new paradigm for imaging. Recent studies suggest that in the case of crystalline samples, such as protein nanocrystals, the atomic displacements and loss of bound electrons in the crystal (due to the high X- ray intensity) has the effect of gating the diffraction signal, and hence making the experiment less radiation sensitive. Only the incident photon intensity in the first part of the pulse, before the Bragg diffraction has died out, is relevant to acquiring signal and the rest of the pulse will mainly contribute to a diffuse background. In this work we use a plasma based non-local thermodynamic equilibrium code to explore the displacement and the ionization of a protein nanocrystal at various X-ray wavelengths and intensities.
Andrew Martin, Jakob Andreasson, Andrew Aquila, Saša Bajt, Thomas R. Barends, Miriam Barthelmess, Anton Barty, W. Henry Benner, Christoph Bostedt, John Bozek, Phillip Bucksbaum, Carl Caleman, Nicola Coppola, Daniel DePonte, Tomas Ekeberg, Sascha Epp, Benjamin Erk, George Farquar, Holger Fleckenstein, Lutz Foucar, Matthias Frank, Lars Gumprecht, Christina Hampton, Max Hantke, Andreas Hartmann, Elisabeth Hartmann, Robert Hartmann, Stephan Hau-Riege, Günther Hauser, Peter Holl, André Hoemke, Olof Jönsson, Stephan Kassemeyer, Nils Kimmel, Maya Kiskinova, Faton Krasniqi, Jacek Krzywinski, Mengning Liang, Ne-Te Duane Loh, Lukas Lomb, Filipe R. N. Maia, Stefano Marchesini, Marc Messerschmidt, Karol Nass, Duško Odic, Emanuele Pedersoli, Christian Reich, Daniel Rolles, Benedikt Rudek, Artem Rudenko, Carlo Schmidt, Joachim Schultz, M. Marvin Seibert, Robert Shoeman, Raymond Sierra, Heike Soltau, Dmitri Starodub, Jan Steinbrener, Francesco Stellato, Lothar Strüder, Martin Svenda, Herbert Tobias, Joachim Ullrich, Georg Weidenspointner, Daniel Westphal, Thomas White, Garth Williams, Janos Hajdu, Ilme Schlichting, Michael Bogan, Henry Chapman
Results of coherent diffractive imaging experiments performed with soft X-rays (1-2 keV) at the Linac Coherent
Light Source are presented. Both organic and inorganic nano-sized objects were injected into the XFEL beam
as an aerosol focused with an aerodynamic lens. The high intensity and femtosecond duration of X-ray pulses
produced by the Linac Coherent Light Source allow structural information to be recorded by X-ray diffraction
before the particle is destroyed. Images were formed by using iterative methods to phase single shot diffraction
patterns. Strategies for improving the reconstruction methods have been developed. This technique opens
up exciting opportunities for biological imaging, allowing structure determination without freezing, staining or
crystallization.
The high intensity of free-electron lasers now allows for the possibility of obtaining measurable diffraction from
biological samples with a single X-ray pulse. An important consequence of diffract-before-destroy imaging is that the
sample is destroyed and therefore must be replaced preferably at the repetition rate of the FEL. This presents an
interesting challenge; the sample must be rapidly replaced within the X-ray focus at the proper particle density and
degree of hydration without damaging or denaturing the sample. If particle number density is too high, for example due
to clustering or evaporation, the diffraction pattern resulting from coherent illumination of multiple particles may be
discarded when sorting for 3D reconstruction. If number density is too low the hit rate, percentage of pulses with
measurable scattered intensity, may also be too low to collect a complete data set. Evaporation will also leave behind less
volatile material and this change of concentration may be damaging to the sample. On the other hand the similarity in
electron density for water and biological material provides poor contrast for fully hydrated material. It is often also
necessary to consider sample consumption. While high, near unity, hit rate can be obtained using liquid jets, a liquid
flow rate greater then 1 microliter per minute must be maintained. Several sample injection possibilities, drop on
demand, aerosols, liquid jets, aerodymamic lenses, have been explored and a review of these results is presented.
The recent commissioning of a X-ray free-electron laser triggered an extensive research in the area of X-ray ablation of
high-Z, high-density materials. Such compounds should be used to shorten an effective attenuation length for obtaining
clean ablation imprints required for the focused beam analysis. Compounds of lead (Z=82) represent the materials of first
choice. In this contribution, single-shot ablation thresholds are reported for PbWO4 and PbI2 exposed to ultra-short
pulses of extreme ultraviolet radiation and X-rays at FLASH and LCLS facilities, respectively. Interestingly, the
threshold reaches only 0.11 mJ/cm2 at 1.55 nm in lead tungstate although a value of 0.4 J/cm2 is expected according to
the wavelength dependence of an attenuation length and the threshold value determined in the XUV spectral region, i.e.,
79 mJ/cm2 at a FEL wavelength of 13.5 nm. Mechanisms of ablation processes are discussed to explain this discrepancy.
Lead iodide shows at 1.55 nm significantly lower ablation threshold than tungstate although an attenuation length of the
radiation is in both materials quite the same. Lower thermal and radiation stability of PbI2 is responsible for this finding.
We report on the x-ray absorption of Warm Dense Matter experiment at the FLASH Free Electron Laser (FEL) facility at DESY. The FEL beam is used to produce Warm Dense Matter with soft x-ray absorption as the probe of electronic structure. A multilayer-coated parabolic mirror focuses the FEL radiation, to spot sizes as small as 0.3μm in a ~15fs pulse of containing >1012 photons at 13.5 nm wavelength, onto a thin sample. Silicon photodiodes measure the transmitted and reflected beams, while spectroscopy provides detailed measurement of the temperature of the sample. The goal is to measure over a range of intensities approaching 1018 W/cm2. Experimental results will be presented along with theoretical calculations. A brief report on future FEL efforts will be given.
The beam of Free-Electron Laser in Hamburg (FLASH) tuned at either 32.5 nm or 13.7 nm was focused by a grazing
incidence elliptical mirror and an off-axis parabolic mirror coated by Si/Mo multilayer on 20-micron and 1-micron spot,
respectively. The grazing incidence and normal incidence focusing of ~10-fs pulses carrying an energy of 10 μJ lead at
the surface of various solids (Si, Al, Ti, Ta, Si3N4, BN, a-C/Si, Ni/Si, Cr/Si, Rh/Si, Ce:YAG, poly(methyl methacrylate)
- PMMA, stainless steel, etc.) to an irradiance of 1013 W/cm2 and 1016 W/cm2, respectively. The optical emission of the
plasmas produced under these conditions was registered by grating (1200 lines/mm and/or 150 lines/mm) spectrometer
MS257 (Oriel) equipped with iCCD head (iStar 720, Andor). Surprisingly, only lines belonging to the neutral atoms
were observed at intensities around 1013 W/cm2. No lines of atomic ions have been identified in UV-vis spectra emitted
from the plasmas formed by the FLASH beam focused in a 20-micron spot. At intensities around 1016 W/cm2, the OE
spectra are again dominated by the atomic lines. However, a weak emission of Al+ and Al2+ was registered as well. The
abundance ratio of Al/Al+ should be at least 100. The plasma is really cold, an excitation temperature equivalent to 0.8 eV was found by a computer simulation of the aluminum plasma OE spectrum. A broadband emission was also
registered, both from the plasmas (typical is for carbon; there were no spectral lines) and the scintillators (on Ce:YAG
crystal, both the luminescence bands and the line plasma emission were recorded by the spectrometer).
A multilayer-coated 27-cm focal length parabola, optimized to reflect 13.5 nm wavelength at normal incidence,
was used in multiple FLASH experiments and focused the beam to a sub-micron beam size. The intensity of the beam
was measured indirectly from the depths of craters left by the FLASH beam on PMMA-coated substrates. Comparing
simulated and experimental shapes of the craters we found the best match for a wavefront error of 0.45 nm, or λ/30. We
further estimated that the FWHM of the focal spot was 350 nm and that the intensity in the focus was 1018 W/cm2. The
sub-micron FLASH beam provided extreme intensity conditions essential for warm dense matter experiments. The same
optic was used in multiple experiments and survived the beam. However, after the first measurements, which took place
over several days, the optical surface was contaminated. This contamination reduced the mirror reflectivity, which was
partially recovered by oxygen plasma cleaning. However, even the partially cleaned multilayer-coated optic is still
diffraction limited and can focus the beam in future experiments to a sub-micron beam size.
Short-pulse ultraviolet and x-ray free electron lasers of unprecedented peak brightness are in the process of
revolutionizing physics, chemistry, and biology. Optical components for these new light sources have to be able to
withstand exposure to the extremely high-fluence photon pulses. Whereas most optics have been designed to stay intact
for many pulses, it has also been suggested that single-pulse optics that function during the pulse but disintegrate on a
longer timescale, may be useful at higher fluences than multiple-pulse optics. In this paper we will review damage-resistant
single-pulse optics that recently have been demonstrated at the FLASH soft-x-ray laser facility at DESY,
including mirrors, apertures, and nanolenses. It was found that these objects stay intact for the duration of the 25-fs
FLASH pulse, even when exposed to fluences that exceed the melt damage threshold by fifty times or more. We present
a computational model for the FLASH laser-material interaction to analyze the extent to which the optics still function
during the pulse. Comparison to experimental results obtained at FLASH shows good quantitative agreement.
Multilayers are artificially layered structures that can be used to create optics and optical elements for a broad
range of x-ray wavelengths, or can be optimized for other applications. The development of next generation x-ray
sources (high brightness synchrotrons and x-ray free electron lasers) requires advances in x-ray optics. Newly
developed multilayer-based mirrors and optical elements enabled efficient band-pass filtering, focusing and time
resolved measurements in recent FLASH (Free Electron LASer in Hamburg) experiments. These experiments are
providing invaluable feedback on the response of the multilayer structures to high intensity, short pulsed x-ray sources.
This information is crucial to design optics for future x-ray free electron lasers and to benchmark computer codes that
simulate damage processes.
We have designed and commissioned an apparatus for serial crystallography of hydrated proteins at the Advanced Light
Source. Serial crystallography is a recently proposed method of imaging uncrystallized proteins at a third generation
synchrotron source. This paper describes the design of the apparatus and results from the first experiment, which
recorded x-ray diffraction patterns from 8 micron droplets containing photosystem 1 protein molecules.
Since 1993, research in the fabrication of extreme ultraviolet (EUV) optical imaging systems, conducted at Lawrence Berkeley National Laboratory (LBNL) and Lawrence Livermore National Laboratory (LLNL), has produced the highest resolution optical systems ever made. We have pioneered the development of ultra-high-accuracy optical testing and alignment methods, working at extreme ultraviolet wavelengths, and pushing wavefront-measuring interferometry into the 2-20-nm wavelength range (60-600 eV). These coherent measurement techniques, including lateral shearing interferometry and phase-shifting point-diffraction interferometry (PS/PDI) have achieved RMS wavefront measurement accuracies of 0.5-1-Å and better for primary aberration terms, enabling the creation of diffraction-limited EUV optics. The measurement accuracy is established using careful null-testing procedures, and has been verified repeatedly through high-resolution imaging. We believe these methods are broadly applicable to the advancement of short-wavelength optical systems including space telescopes, microscope objectives, projection lenses, synchrotron beamline optics, diffractive and holographic optics, and more. Measurements have been performed on a tunable undulator beamline at LBNL's Advanced Light Source (ALS), optimized for high coherent flux; although many of these techniques should be adaptable to alternative ultraviolet, EUV, and soft x-ray light sources. To date, we have measured nine prototype all-reflective EUV optical systems with NA values between 0.08 and 0.30 (f/6.25 to f/1.67). These projection-imaging lenses were created for the semiconductor industry's advanced research in EUV photolithography, a technology slated for introduction in 2009-13. This paper reviews the methods used and our program's accomplishments to date.
Zone plates with depth to zone-width ratios as large as 100 are needed for focusing of hard x-rays. Such high aspect ratios are challenging to produce by lithography. We are investigating the fabrication of high-aspect-ratio linear zone plates by multilayer deposition followed by sectioning. As an initial step in this work, we present a synchrotron x-ray study of constant-period multilayers diffracting in Laue (transmission) geometry. Data are presented from two samples: a 200 period W/Si multilayer with d-spacing of 29 nm, and a 2020 period Mo/Si multilayer with d-spacing of 7 nm. By cutting and polishing we have successfully produced thin cross sections with section depths ranging from 2 to 12 μm. Transverse scattering profiles (rocking curves) across the Bragg reflection exhibit well-defined interference fringes originating from the depth of the sample, in agreement with dynamical diffraction theory for a multilayer in Laue geometry.
We demonstrate direct flare measurements of 4-mirror projection optics in the Engineering Test Stand (ETS) using a conventional resist clearing method (the Kirk method). Two extreme UV lithographic projection optics, one with higher flare than the other, have been characterized and the results compared. The measured results have also been compared to analytical calculations based on measured mirror roughness and the extended point spread function. Full-field flare across the 24 mm field width has been measured, and we have verified that flare is constant across the field for EUV lithography as predicted. Horizontal (H) and vertical (V) flare bias has been observed and the cause of the H-V flare bias has been investigated. The main cause has been identified to be anisotropic mirror polishing. Simulations with the 2D Power Spectral density function have confirmed the experimental results.
Static and scanned images of 100nm dense features for a developmental set of l/14 optics (projection optics box # 1, POB 1) in the Engineering Test Stand (ETS) were successfully obtained with various LPP source powers last year. The ETS with POB1 has been used to understand initial system performance and lithographic learning. Since then, numerous system upgrades have been made to improve ETS lithographic performance to meet or exceed the original design objectives. The most important upgrade is the replacement of POB 1 with an improved projection optics system, POB2, having lower figure error (l/20 rms wavefront error) and lower flare. Both projection optics boxes are a four-mirror design with a 0.1 numerical aperture. Scanned 70-nm dense features have been successfully printed using POB2. Aerial image contrast measurements have been made using the resist clearing method. The results are in good agreement to previous POB2 aerial image contrast measurements at the subfield exposure station (SES) at Lawrence Berkeley National Laboratory. For small features the results deviate from the modeling predictions due to the inherent resolution limit of the resist. The intrinsic flare of POB2 was also characterized. The experimental results were in excellent agreement with modeling predictions. As predicted, the flare in POB2 is less than 20% for 2μm features, which is two times lower than the flare in POB1. EUV flare is much easier to compensate for than its DUV counterpart due to its greater degree of uniformity and predictability. The lithographic learning obtained from the ETS will be used in the development of EUV High Volume Manufacturing tools. This paper describes the ETS tool ETS tool setup, both static and scanned, that was required after the installation of POB2. The paper will also describe the lithographic characterization of POB2 in the ETS and cmpare those results to the lithographic results obtained last year with POB1.
The development of defect-free reticle blanks is an important challenge facing the commercialization of extreme ultraviolet lithography (EUVL). The basis of an EUVL reticle are mask blanks consisting of a substrate and a reflective Mo/Si multilayer. Defects on the substrate or defects introduced during multilayer deposition can result in critical phase and amplitude defects. Amplitude- or phase-defect repair techniques are being developed with the goal to repair many of these defects. In this paper we discuss the selection of a capping layer for amplitude-defect repair, and report on experimental results of the reflectance variation over the amplitude-defect repair zone for different capping layers. Our results suggest that carbon and silicon carbide are the leading candidates for capping layer materials. We further performed a quantitative assessment of the yield improvement due to defect repair. We found that amplitude- and phase-defect repair have the potential to significantly improve mask blank yield, and that yield can be maximized by increasing the number of Mo/Si bilayers.
Full-field imaging with a developmental projection optic box (POB 1) was successfully demonstrated in the alpha tool Engineering Test Stand (ETS) last year. Since then, numerous improvements, including laser power for the laser-produced plasma (LPP) source, stages, sensors, and control system have been made. The LPP has been upgraded from the 40 W LPP cluster jet source used for initial demonstration of full-field imaging to a high-power (1500 W) LPP source with a liquid Xe spray jet. Scanned lithography at various laser drive powers of >500 W has been demonstrated with virtually identical lithographic performance.
The reflectance stability of multilayer coatings for extreme ultraviolet lithography (EUVL) in a commercial tool environment is of utmost importance to ensure continuous exposures with minimum maintenance cost. We have made substantial progress in designing the protective capping layer coatings, understanding their performance and estimating their lifetimes based on accelerated electron beam and EUV exposure studies. Our current capping layer coatings have about 40 times longer lifetimes than Si-capped multilayer optics. Nevertheless, the lifetime of current Ru-capped multilayers is too short to satisfy commercial tool requirements and further improvements are essential.
As the quality of optical systems for extreme ultraviolet lithography improves, high-accuracy wavefront metrology for alignment and qualification becomes ever more important. To enable the development of diffraction-limited EUV projection optics, visible-light and EUV interferometries must work in close collaboration. We present a detailed comparison of EUV and visible-light wavefront measurements performed across the field of view of a lithographic-quality EUV projection optical system designed for use in the Engineering Test Stand developed by the Virtual National Laboratory and the EUV Limited Liability Company. The comparisons reveal that the present level of RMS agreement lies in the 0.3-0.4-nm range. Astigmatism is the most significant aberration component for the alignment of this optical system; it is also the dominant term in the discrepancy, and the aberration with the highest measurement uncertainty. With EUV optical systems requiring total wavefront quality in the (lambda) EUV/50 range, and even higher surface-figure quality for the individual mirror elements, improved accuracy through future comparisons, and additional studies, are required.
While interferometry is routinely used for the characterization and alignment of lithographic optics, the ultimate performance metric for these optics is printing in photoresist. The comparison of lithographic imaging with that predicted from wavefront performance is also useful for verifying and improving the predictive power of wavefront metrology. To address these issues, static, small-field printing capabilities have been added to the EUV phase- shifting point diffraction interferometry implemented at the Advanced Light Source at Lawrence Berkeley National Laboratory. The combined system remains extremely flexible in that switching between interferometry and imaging modes can be accomplished in approximately two weeks.
EUV mask blanks are fabricated by depositing a reflective Mo/Si multilayer film onto super-polished substrates. Small defects in this thin film coating can significantly alter the reflected field and introduce defects in the printed image. Ideally one would want to produce defect-free mask blanks; however, this may be very difficult to achieve in practice. One practical way to increase the yield of mask blanks is to effectively repair multilayer defects, and to this effect we present two complementary defect repair strategies for use on multilayer-coated EUVL mask blanks. A defect is any area on the mask which causes unwanted variations in EUV dose in the aerial image obtained in a printing tool, and defect repair is correspondingly defined as any strategy that renders a defect unprintable during exposure. The term defect mitigation can be adopted to describe any strategy which renders a critical defect non-critical when printed, and in this regard a non-critical defect is one that does not adversely affect device function. Defects in the patterned absorber layer consist of regions where metal, typically chrome, is unintentionally added or removed from the pattern leading to errors in the reflected field. There currently exists a mature technology based on ion beam milling and ion beam assisted deposition for repairing defects in the absorber layer of transmission lithography masks, and it is reasonable to expect that these this technology will be extended to the repair of absorber defects in EUVL masks . However, techniques designed for the repair of absorber layers can not be directly applied to the repair of defects in the mask blank, and in particular the multilayer film. In this paper we present for the first time a new technique for the repair of amplitude defects as well as recent results on the repair of phase defects.
Static and scanned images of 100 nm dense features were successfully obtained with a developmental set of projection optics and a 500W drive laser laser-produced-plasma (LPP) source in the Engineering Test Stand (ETS). The ETS, configured with POB1, has been used to understand system performance and acquire lithographic learning which will be used in the development of EUV high volume manufacturing tools. The printed static images for dense features below 100 nm with the improved LPP source are comparable to those obtained with the low power LPP source, while the exposure time was decreased by more than 30x. Image quality comparisons between the static and scanned images with the improved LPP source are also presented. Lithographic evaluation of the ETS includes flare and contrast measurements. By using a resist clearing method, the flare and aerial image contrast of POB1 have been measured, and the results have been compared to analytical calculations and computer simulations.
The EUV Engineering Test Stand (ETS) has demonstrated the printing of 100-nm-resolution scanned images. This milestone was first achieved while the ETS operated in an initial configuration using a low power laser and a developmental projection system, PO Box 1. The drive laser has ben upgraded to a single chain of the three-chain Nd:YAG laser developed by TRW. The result in exposure time is approximately 4 seconds for static exposures. One hundred nanometer dense features have been printed in step-and-scan operation with the same image quality obtained in static printing. These experiments are the first steps toward achieving operation using all three laser chains for a total drive laser power of 1500 watts. In a second major upgrade the developmental wafer stage platen, used to demonstrate initial full-field imaging, has been replaced with the final low-expansion platen made of Zerodur. Additional improvements in the hardware and control software have demonstrated combined x and jitter from 2 to 4 nm RMS Over most of the wafer stage travel range, while scanning at the design scan speed of 10 mm/s at the wafer. This value, less than half of the originally specified jitter, provides sufficient stability to support printing of 70 nm features as planned, when the upgraded projection system is installed. The third major upgrade will replace PO Box 1 with an improved projection system, PO Box 2, having lower figure error and lower flare. In addition to these upgrades, dose sensors at the reticle and wafer planes and an EUV- sensitive aerial image monitor have been integrated into the ETS. This paper reports on ETS system upgrades and the impact on system performance.
The Engineering Test Stand (ETS) is an EUV lithography tool designed to demonstrate full-field EUV imaging and provide data required to accelerate production-tool development. Early lithographic results and progress on continuing functional upgrades are presented and discussed. In the ETS a source of 13.4 nm radiation is provided by a laser plasma source in which a Nd:YAG laser beam is focused onto a xenon- cluster target. A condenser system, comprised of multilayer-coated and grazing incidence mirrors, collects the EUV radiation and directs it onto a reflecting reticle. The resulting EUV illumination at the reticle and pupil has been measured and meets requirements for acquisition of first images. Tool setup experiments have been completed using a developmental projection system with (lambda) /14 wavefront error (WFE), while the assembly and alignment of the final projection system with (lambda) /24 WFE progresses in parallel. These experiments included identification of best focus at the central field point and characterization of imaging performance in static imaging mode. A small amount of astigmatism was observed and corrected in situ, as is routinely done in advanced optical lithographic tools. Pitch and roll corrections were made to achieve focus throughout the arc-shaped field of view. Scan parameters were identified by printing dense features with varying amounts of magnification and skew correction. Through-focus scanned imaging results, showing 100 nm isolated and dense features, will be presented. Phase 2 implementation goals for the ETS will also be discussed.
One of the most critical tasks in the development of extreme ultraviolet lithography (EUVL) is the accurate deposition of reflective multilayer coatings for the mirrors comprising the EUVL tool. The second set (Set 2) of four imaging optics for an alpha-class EUVL system has been coated successfully. All four mirrors (M1, M2, M3, M4) were Mo/Si- coated during a single-deposition run with a production- scale DC-magnetron sputtering system. Ideally, the multilayer coatings should not degrade the residual wavefront error of the imaging system design. For the present EUVL camera, this requirement is equivalent to depositing multilayer coatings that would add a figure error of less than 0.11 nm rms. In addition, all mirrors should be matched in centroid wavelength, in order to insure maximum throughput of the EUVL tool. In order to meet these constraints, the multilayer deposition process needs to be controlled to atomic precision. EUV measurements of the coated mirrors determined that the added figure errors due to the multilayer coatings are 0.032 nm rms (M1), 0.037 nm rms (M2), 0.040 nm rms (M3) and 0.015 nm rms (M4), well within the aforementioned requirement of 0.11 nm rms. The average wavelength among the four projection mirrors is 13.352 nm, with an optic-to-optic matching of 1(sigma) =0.010 nm. This outstanding level of wavelength matching produces 99.3% of the throughput of an ideally matched four-mirror system. Peak reflectances are 63.8% (M1), 65.2% (M2), 63.8% (M3) and 66.7% (M4). The variation in reflectance values between the four optics is consistent with their high frequency substrate roughness. It is predicted that the multilayer coatings will not introduce any aberrations in the lithographic system performance, for both static and scanned images of 70 nm - dense features.
The Engineering Test Stand (ETS) is a developmental lithography tool designed to demonstrate full-field EUV imaging and provide data for commercial-tool development. In the first phase of integration, currently in progress, the ETS is configured using a developmental projection system, while fabrication of an improved projection system proceeds in parallel. The optics in the second projection system have been fabricated to tighter specifications for improved resolution and reduced flare. The projection system is a 4-mirror, 4x-reduction, ring-field design having a numeral aperture of 0.1, which supports 70 nm resolution at a k1 of 0.52. The illuminator produces 13.4 nm radiation from a laser-produced plasma, directs the radiation onto an arc-shaped field of view, and provides an effective fill factor at the pupil plane of 0.7. The ETS is designed for full-field images in step-and-scan mode using vacuum-compatible, magnetically levitated, scanning stages. This paper describes system performance observed during the first phase of integration, including static resist images of 100 nm isolated and dense features.
The Engineering Test Stand (ETS) is an EUV laboratory lithography tool. The purpose of the ETS is to demonstrate EUV full-field imaging and provide data required to support production-tool development. The ETS is configured to separate the imaging system and stages from the illumination system. Environmental conditions can be controlled independently in the two modules to maximize EUV throughput and environmental control. A source of 13.4 nm radiation is provided by a laser plasma source in which a YAG laser beam is focused onto a xenon-cluster target. A condenser system, comprised of multilayer-coated mirrors and grazing-incidence mirrors, collects the EUV radiation and directs it onto a reflecting reticle. A four-mirror, ring-field optical system, having a numerical aperture of 0.1, projects a 4x-reduction image onto the wafer plane. This design corresponds to a resolution of 70 nm at a k1 of 0.52. The ETS is designed to produce full- field images in step-and-scan mode using vacuum-compatible, one-dimension-long-travel magnetically levitated stages for both reticle and wafer. Reticle protection is incorporated into the ETS design. This paper provides a system overview of the ETS design and specifications.
At-wavelength, extreme ultraviolet interferometric measurements of a new, four-mirror, ring-field projection optical system have been made. Designed for operation at 13.4- nm wavelength with a 0.1 numerical aperture and a 26 mm field of view at the wafer, the nearly diffraction-limited wavefront quality of the system has been verified interferometrically. After assembly and alignment with visible-light interferometry, the optic was transported to Lawrence Berkeley National Laboratory where the at-wavelength testing with a phase-shifting point diffraction interferometer was performed. Measurement of the system wavefront at a number of points across the field of view reveals the optical performance of the system over its large, ring-field imaging area.
A condenser for a ring-field extreme ultra-violet (EUV) projection lithography camera is presented. The condenser consists of a gently undulating mirror, that we refer to as a ripple plate, and which is illuminated by a collimated beam at grazing incidence. The light is incident along the ripples rather than across them, so that the incident beam is reflected onto a cone and subsequently focused onto the arc of the ring field. A quasi-stationary illumination is achieved, since any one field point receives light from points on the ripples, which are distributed throughout the condenser pupil. The design concept can easily be applied to illuminate projection cameras with various ring-field and numerical aperture specifications. Ray-tracing results are presented of a condenser for a 0.25 NA EUV projection camera.
The capabilities of the EUV 10x microstepper have been substantially improved over the past year. The key enhancement was the development of a new projection optics system with reduced wavefront error, reduced flare, and increased numerical aperture. These optics and concomitant developments in EUV reticles and photoresists have enabled dramatic improvements in EUV imaging, illustrated by resolution of 70 nm dense lines and spaces (L/S). CD linearity has been demonstrated for dense L/S over the range 100 nm to 80 nm, both for the imaging layer and for subsequent pattern transfer. For a +/- 10 percent CD specification, we have demonstrated a process latitude of +/- micrometers depth of focus and 10 percent dose range for dense 100 nm L/S.
Recent experiments with four 10x EUV imaging systems provide the first direct comparisons of visible-light and at- wavelength EUV interferometers performed using the state-of- the-art measurement tools that will be used to assemble and align the next generation of EUV imaging systems. Measurements from four individual multilayer-coated Schwarzschild objectives are discussed. Favorable agreement has been achieved between EUV and visible-light system wavefront measurements in all four optical systems. Measurements made in the presence of surface contamination and multilayer thickness variation, however, do show expected localized differences between the two measurements.
The assembly of an optical system requires the correction of aberrations in the entire imaging field by making selected rigid-body motions of the optical elements. We present a rigorous method for determining which adjustment motions, called compensators, to use for alignment. These compensators are found by employing techniques from linear algebra that choose the most independent vectors from a set which are interdependent. The method finds the applied to a four-mirror scanning ring-field EUV lithography system. It is shown that out of 32 degrees of freedom in the configuration of the optical elements, only eight compensators are required on the optics. By adjusting these compensators a misaligned configuration giving 30(lambda) wavefront error can be assembled to (lambda) /50 in the absence of measurement noise.
The imaging specifications for extreme ultraviolet lithography (EUVL) projection optics parallel those of other optical lithographies. Specifications are scaled to reflect the 100 nm critical dimension for the first generation EUVL systems. The design being fabricated for the Engineering Test Stand, an EUVL alpha tool, consists of a condenser with six channels to provide an effective partial coherence factor of 0.7. The camera contains four mirrors; three of the mirrors are aspheres and the fourth is spherical. The design of the optical package has been constrained so that the angles of incidence and the variations in the angle of incidence of all rays allow for uniform multilayer coatings. The multilayers introduce a slight shift in image position and magnification. We have shown that a system aligned with visible light is also aligned at 13.4 nm. Each mirror must be fabricated with an RMS figure error of less than 0.25 nm and better than 0.2 nm RMS roughness. Optical surfaces that exceed each of these specifications individually have been fabricated. The success of EUVL requires that these specifications be met simultaneously. Keywords: EUV projection lithography, optical design, multilayer coatings, aspheric optics
Jorg Maser, Henry Chapman, Chris Jacobsen, Alex Kalinovsky, Janos Kirz, Angelika Osanna, Steve Spector, Steve Wang, Barry Winn, Sue Wirick, Xiaodong Zhang
The Stony Brook scanning transmission x-ray microscope (STXM) has been operating at the X1A beamline at the NSLS since 1989. A large number of users have used it to study biological and material science samples. We report on changes that have been performed in the past year, and present recent results. To stabilize the position of the micro probe when doing spectral scans at high spatial resolution, we have constructed a piezo-driven flexure stage which carries out the focusing motion of the zone plate needed when changing the wavelength. To overcome our detector limitation set by saturation of our gas-flow counter at count rates around 1 MHz, we are installing an avalanche photo diode with an active quenching circuit which we expect to respond linearly to count rates in excess of 10 MHz. We have improved the enclosure for STXM to improve the stability of the Helium atmosphere while taking data. This reduces fluctuations of beam absorption and, therefore, noise in the image. A fast shutter has been installed in the beam line. We are also developing a cryo- STXM which is designed for imaging frozen hydrated samples at temperatures below 120 K. At low temperatures, radiation sensitive samples can tolerate a considerably higher radiation dose than at room temperature. This should improve the resolution obtainable from biological samples and should make recording of multiple images of the same sample area possible while minimizing the effects of radiation damage. This should enable us to perform elemental and chemical mapping at high resolution, and to record the large number of views needed for 3D reconstruction of the object.
X-ray focusing using square channel capillary arrays is reviewed. We present some experimental results obtained using a variety of array configurations and we deduce that channel misalignment and surface roughness are the prime factors limiting the performance of these devices. We present results obtained using a stacked array of commercial precision bore square tubing and deduce that the reflectivity from these unetched surfaces is superior to that from etched micro-channel plate blanks. The improved surface quality implied by this reflectivity result is confirmed using atomic force microscopy. We also present results of a new drawing technique that we have developed.
We present an analysis of the geometric optics of spherically curved arrays of reflective surfaces. In particular, we consider optical devices in which reflective surfaces are arranged on a spherical interface and every ray reflects once from a reflector. The orientation of the reflective surfaces is not necessarily related in any way to the orientation of the interface. The analysis can be applied to any radiation that may specularly reflect from the reflectors. This may be reflection from stacks of mirrors or diffraction from the atomic planes. In this paper the principles are applied to x-ray optical systems such as capillary arrays and curved crystals. The calculations are used to find optimum configurations of reflective arrays for applications such as x-ray condensers and telescopes, to find the tolerances to which reflective arrays must be constructed, and to find the conditions where primary aberrations are eliminated.
X-ray optical devices based on arrays of capillaries and single tapered capillaries can focus and concentrate x rays by the reflection of near-grazing-incidence rays at the interior walls of the channels. Capillary arrays are true imaging devices and can be used as focusing, condensing,and collimating optics, suitable for x-ray microcopy and astronomy. Rays which enter channels and are reflected once or twice, via total external reflection, may be redirected towards the image. Theoretical calculations of the focusing performance of arrays consisting of capillaries of square and circular cross-section are given. Experimental investigations have been made using microchannel plate (MCP) detector blanks and various x-ray sources.
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