X-ray microscopy has proven its advantages for resolving nanoscale objects. High Harmonic Generation (HHG) sources allow performing nanoimaging experiments at the lab scale and their femtosecond pulse duration and synchrony to an optical laser renders them useful for studying dynamic processes. HHG sources regularly provide high average photon flux but relatively low single-shot flux limiting time-resolved applications to adiabatic processes. Here, we show that soft X-ray lasers (SXRL) in turn provide high flux due to an X-ray lasing transition, but the coherence of an SXRL operating in the amplified-spontaneous-emission scheme is limited. The coherence properties of an SXRL seeded by an HHG source can be significantly improved allowing single-shot nanoscale imaging. In combination with ptychography, source properties are measured with high fidelity. This is applied to study the plasma dynamics of SXRL amplification in unprecedented quality.
We evaluated the capabilities of an intense ultrafast high-harmonic seeded soft X-ray laser at 32.8 nm wavelength regarding single-shot lensless imaging and ptychography. Additionally the wave front at the exit of the laser plasma amplifier is monitored in amplitude and phase using high resolution ptychography and backpropagation techniques.Characterizing the laser plasma amplifier performance depending on the arrival time of the seed pulse with respect to pump pulses provides insight into the light plasma interaction in the soft X-ray range.
We use three dimensional finite-difference-time-domain simulations to study the dynamics of extraordinary optical
transmission through arrays of nanoholes in 200 nm-thick Au films on silicon nitride substrates. By diving the light
source into two identical 5 femtosecond pulses and tuning the relative delay between them, we are able to modulate both
the intensity and spectra of the transmitted light on ultrashort time scales. Simulations demonstrate that the intensity and
distribution of the electric fields on the surface of the film and within the nanoholes are altered by changing the pulse
delay.
Recent advances in the development of attosecond soft X-ray sources ranging into the ‘water window’ spectral range,
between the carbon 1s and oxygen 1s states (284 eV - 543 eV), are also driving the development of suited broadband
multilayer optics for attosecond beam steering and dispersion management. The relatively low intensity of current High
Harmonic Generation (HHG) soft X-ray sources calls for an efficient use of photons, thus the development of low-loss
multilayer optics is of uttermost importance. Here, we report about the realization of atomically smooth interfaces in
broadband CrSc multilayer mirrors by an optimized ion beam deposition and assisted interface polishing process.
Chirped broadband multilayer mirrors are key components to shape attosecond pulses in the XUV range. Compressing
high harmonic pulses to their Fourier limit is the major goal for attosecond physics utilizing short pulse pump-probe
experiments. Here, we report about the first implementation of multilayers fulfilling these requirements in the “waterwindow”
spectral range.
The excitation of surface plasmons in metallic nanostructures by resonant ultrashort femtosecond light pulses produces
interesting phenomena such as optical field nanolocalization, nanoscale electric field enhancement and ultrafast sub-femtosecond beating of the plasmon eigenmodes. Nonlinear two-photon photoemission electron microscopy has proven
to be a powerful tool for spatiotemporal characterization of such effects on the nanoscale below the optical diffraction
limit. As a step toward using intense, few-cycle 4 femtosecond laser pulses to excite and control surface plasmons, we
report on the multiphoton-photoemission electron microscopy experiments on lithographically-fabricated gold
nanostructures excited by these few-cycle laser pulses. In addition, the effects of the shape and size of silver plasmonic
structures, as well as the polarization of the excitation source are examined in the two-photon photoemission induced by
picosecond laser pulses. Potential approaches toward spatiotemporal control of lightfield nanolocalization are described.
A new actinic mask inspection technology to probe nano-scaled defects buried underneath a
Mo/Si multilayer reflection coating of an Extreme Ultraviolet Lithography mask blank has
been implemented using EUV Photoemission Electron Microscopy (EUV-PEEM). EUV
PEEM images of programmed defect structures of various lateral and vertical sizes recorded
at around 13 nm wavelength show that 35 nm wide and 4 nm high buried line defects are
clearly detectable. The imaging technique proves to be sensitive to small phase jumps
enhancing the visibility of the edges of the phase defects which is explained in terms of a
standing wave enhanced image contrast at resonant EUV illumination.
A new method for the actinic at-wavelength inspection of defects inside and ontop of Extreme Ultraviolet Lithography (EUVL) multilayer-coated mask blanks is presented. The experimental technique is based on PhotoElectron Emission Microscopy (PEEM) supported by the generation of a standing wave field inside and above the multilayer mask blank when illuminated near the resonance Bragg wavelength at around 13.5 nm wavelength. Experimental results on programmed defect samples based on e-beam lithographic structures or PSL equivalent silica balls overcoated with an EUV multilayer show that buried defects scaling down to 50 nm in lateral size are detectable with further scalability down to 30 nm and smaller due to the PEEM's instrumental performance. Furthermore, phase structures as shallow as 6 nm in height on a programmed phase grating sample has been detected by this technique. The visibility of the phase defect structures has been shown to be strongly dependent on and controlled by the phase of the standing wave field at the mask blank surface and thus can be optimized by tuning the illumination wavelength between 12.5 nm and 13.8 nm.
Bufferlayer and caplayer engineering strategies are getting progressively important for improving crucial properties of EUVL multilayer optics and EUV reflection masks. While bufferlayers modifying the contact between the reflecting interference multilayer and the superpolished substrate aim for a partial smoothing of the residual substrate roughness or for a mitigation of local substrate defects as well as for multilayer film stress relaxation, surface caplayers are capable to enhance the multilayer reflectivity of EUV mirrors and masks. We present experimental results on various bufferlayer systems (singlelayer and multilayer) applied to different substrate materials (ULE, Zerodur, silicon). The bufferlayers have been deposited by e-beam evaporation and ion-polishing techniques at UHV conditions and substrates with and without bufferlayer have been coated with standard Mo/Si multilayers (50 doublelayers, d-spacing 6.8 nm) in the same deposition run. The samples have been analyzed exsitu by means of AFM, TEM, X-ray scattering and reflection and normal-incidence EUV reflectance measurements. We have found significant improvement (+ 0.7 %) of the Mo/Si multilayer EUV reflectivity for some bufferlayer systems applied to substrates with 0.2 - 0.3 nm r.m.s. high spatial frequency roughness (HSFR), while no effect was found on superpolished substrates exhibiting 0.1 nm r.m.s HSFR. The effect of caplayer modification applied to Mo/Si multilayers has been examined regarding EUV reflectivity. The native siliconoxide layer on top of the Mo/Si multilayer coatings has been replaced by an ultrathin (2 nm) chemically inert caplayer. We have found a 1 % - 1.5 % improvement in EUV peak reflectance for one cap material applied to several Mo/Si multilayer in comparison to a native siliconoxide cap.
Extreme Ultraviolet Lithography (EUVL) is a candidate for future application by the semiconductor industry in the production of sub-100 nm feature sizes in integrated circuits. Using multilayer reflective coatings optimized at wavelengths ranging from 11 to 14 nm, EUVL represents a potential successor to currently existing optical lithography techniques. In order to assess lifetimes of the multilayer coatings under realistic conditions, a series of radiation stability tests has been performed. In each run a dose of EUV radiation equivalent to several months of lithographic operation was applied to Mo/Si and Mo/Be multilayer coatings within a few days. Depending on the residual gas concentration in the vacuum environment, surface deposition of carbon during the exposure lead to losses in the multilayer reflectivity. However, in none of the experimental runs was structural damage within the bulk of the multilayers observed. Mo/Si multilayer coatings recovered their full original reflectivity after removal of the carbon layer by an ozone cleaning method. Auger depth profiling on Mo/Be multilayers indicate that carbon penetrated into the Be top layer during illumination with high doses of EUV radiation. Subsequent ozone cleaning fully removed the carbon, but revealed enhanced oxidation of the area illuminated, which led to an irreversible loss in reflectance on the order of 1%.
Ulf Kleineberg, Hans-Juergen Stock, D. Menke, O. Wehmeyer, Ulrich Heinzmann, Detlef Fuchs, Peter Bulicke, Marco Wedowski, Gerhard Ulm, Klaus Heidemann, K. Osterried
We report about the diffraction efficiencies of two new types of
multilayer diffraction gratings, one of them being proposed for high
spectral dispersion ability due to a high line density in the photon
energy range around 90 eV, while the other is being proposed as a
multilayer grating working in the water window spectral range around
470 eV at near normal incidence angles. For a 6600 L/mm sinusoidal interference grating coated with 18 Mo/Si doublelayers of d = 8.2 nm absolute diffraction efficiencies up to 11 % at a wavelength .\ = 13.2 nm were measured in each first diffraction order. A 2400 L/mm laminar grating was coated with 60 Ti/C bilayers of d = 2.2 nm for high efficiency in the water window spectral range near the Ti-L edge (.\ = 2.77 nm). For a similar Ti/C multilayer mirror refiectivities of 11 % were measured at a wavelength ) = 2.7 nm and an angle of incidence a = 59 deg. First order diffraction efficiencies of
about 0.7 % were measured for the Ti/C multilayer grating.
Ulf Kleineberg, Hans-Juergen Stock, D. Menke, K. Osterried, Bernt Schmiedeskamp, Ulrich Heinzmann, Detlef Fuchs, Peter Mueller, Frank Scholze, Klaus Heidemann, Bruno Nelles, Juergen Thieme
Here we report on the fabrication and analysis of two different types of laterally structured multilayers, which act as highly efficient x-ray optical elements for focusing or spectral dispersion of soft x-rays at (near) normal incidence. The microfocusing of soft x-rays at (near) normal incidence with plane reflection optics can be obtained by multilayer Bragg-Fresnel zone plates. A circular condensor zone plate structure (2.9 mm diameter, 417 zones, inner zone radius 50 micrometers , outer zone width 0.9 micrometers ) has been recorded by holographic lithography followed by ion beam etching into a Mo-Si multilayer mirror of 24 periods and a doublelayer thickness of 7.2 nm. The focusing properties analyzed by soft x-ray reflectometry at (lambda) equals 13.8 nm show a focal spot size smaller than 40 micrometers measured 160 mm behind the multilayer zone plate. Multilayer blazed gratings offer the opportunity for highly resolved spectral dispersion of soft x-rays due to the possible combination of high efficiency and high diffraction orders with almost the whole intensity diffracted in one diffraction order. The sawtooth profile of a blazed grating structure (1221 l/mm, blaze angle 1.5 degree(s)) has been ruled into a 200 nm thick Au-film which has been deposited onto a plane glass substrate. A Mo/Si multilayer of 15 periods and a doublelayer thickness of 7.2 nm has been deposited onto the grating substrate. In order to smooth the rough Au-surface and to prevent interdiffusion of the Au-film with the upper Mo-Si multilayer a carbon film has been evaporated onto the Au-grating surface before the deposition of the soft x-ray coating. This procedure results in a significant increase of diffraction efficiency. The multilayer grating has been matched working on blaze in the third diffraction order, where an absolute diffraction efficiency of 3.4% at (lambda) equals 14 nm has been measured, while only 1.1% has been achieved for a similar grating without a carbon interlayer.
Bernt Schmiedeskamp, Andreas Kloidt, Hans-Juergen Stock, Ulf Kleineberg, Thorsten Doehring, Michael Proepper, Steffen Rahn, Kerstin Hilgers, Bernhard Heidemann, Thorsten Tappe, Ulrich Heinzmann, Michael Krumrey, Peter Mueller, Frank Scholze, Klaus Heidemann
For the wavelength region above the Si-L edge normal incidence, soft x-ray mirrors are produced with peak reflectivities close to 60%. The multilayer systems consist of molybdenum and silicon and are fabricated by electron beam evaporation in ultrahigh vacuum. A smoothing of the boundaries, and thereby a drastic enhancement of the reflectivity, is obtained by thermal treatment of the multilayer systems during growth. The thermal stability of the multilayer stacks could be improved considerably up to 850° C by mixing Mo and Si in the absorber layers and producing thus MoxSiy/Si multilayers with x and y denoting the amounts of Mo and Si in the absorber layer, respectively. First attempts are reported to produce mirrors with a bilayer thickness of 2.6 nm. An improvement in the quality of these interfaces can be obtained by bombardment with Ar+ ions. We report on normal incidence reflectivity measurements of the mirrors with synchrotron radiation and finally on the normal incidence diffraction efficiencies of a Mo/Si multilayer coated grating, for which values of 5.5% are achieved for the + 1'st and - 1'st diffraction orders.
Andreas Kloidt, Hans-Juergen Stock, Ulf Kleineberg, Thorsten Doehring, Michael Proepper, Kerstin Nolting, Bernhard Heidemann, Thorsten Tappe, Bernt Schmiedeskamp, Ulrich Heinzmann, Michael Krumrey, Peter Mueller, Frank Scholze, Steffen Rahn, F. Hormes, Klaus Heidemann
For the wavelength region above the Si-L edge normal incidence soft X-ray mirrors are produced with peak reflectivities around 55 percent. The Mo/Si multilayer systems are fabricated by electron beam evaporation in ultrahigh vacuum. Analysis of the quality of the stack is made by using an in situ monitoring system measuring the reflection of the C-K line and ex situ grazing X-ray reflection of the Cu-K-alpha line. A smoothing of the boundaries and thereby a drastic enhancement of the reflectivity can be obtained by thermal treatment of the multilayer system during growth. The microstructure of the multilayer systems is investigated by means of Rutherford Backscattering spectroscopy and Sputter/AES technique. Baking the final stack after deposition up to 900 C is applied to study the thermal stability of the soft X-ray mirror. Near normal incidence mirrors even for short wavelengths, e.g., the water window (2.4 - 4.4 nm), are produced with a Mo/Si bilayer thickness of 2.6 nm. An improvement in the quality of the interfaces for such ultrathin multilayer systems can be obtained by bombardment of the deposited layers with Ar(+) ions as well as by thermal treatment of the multilayer system and mixing of Mo and Si in the absorber layer during the deposition run. We report on reflectivity measurements of the mirrors and their behavior as polarizers and analyzers and on the diffraction efficiencies of laterally structured multilayer systems as gratings.
SXPL (soft X-ray projection lithography) is one of the most promising applications of X-ray reflecting optics using multilayer mirrors. Within our collaboration, such multilayer mirrors were fabricated, characterized, laterally structured and then used as reflection masks in a projecting lithography procedure. Mo/Si-multilayer mirrors were produced by electron beam evaporation in UHV under thermal treatment with an in-situ X-ray controlled thickness in the region of 2d equals 14 nm. The reflectivities measured at normal incidence reached up to 54%. Various surface analysis techniques have been applied in order to characterize and optimize the X-ray mirrors. The multilayers were patterned by reactive ion etching (RIE) with CF4, using a photoresist as the etch mask, thus producing X-ray reflection masks. The masks were tested in the synchrotron radiation laboratory of the electron accelerator ELSA at the Physikalisches Institut of Bonn University. A double crystal X-ray monochromator was modified so as to allow about 0.5 cm2 of the reflection mask to be illuminated by white synchrotron radiation. The reflected patterns were projected (with an energy of 100 eV) onto the resist (Hoechst AZ PF 514), which was mounted at an average distance of about 7 mm. In the first test-experiments, structure sizes down to 8 micrometers were nicely reproduced over the whole of the exposed area. Smaller structures were distorted by Fresnel-diffraction. The theoretically calculated diffraction images agree very well with the observed images.
The fabrication, by electron beam evaporation, of Mo/Si and Ta/Si mutilayers designed as soft-X-ray mirrors is described. The mirrors were characterized using surface analytical methods (RBS and sputtering in combination with AES), Cu-k(alpha) reflection, and soft-X-ray optical methods, and their soft-X-ray optical properties were correlated with microstructural characteristics. A comparison of in situ C-k reflectivity curves with calculations disclosed the existence of roughnesses at the interfaces, which can not be completely described by multiplying the reflected amplitude at each interface by a Debye-Waller factor. It was found that heating of Ta/Si samples induces a considerable change (up to and above 10 percent) in the d-spacing of multilayers, while the reflected amplitude is only reduced to two thirds of its original value.
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