Extreme Ultraviolet Photons Research Articles

Optical design of the time-resolved ARPES beamline of the new material spectroscopy experimental station for the update of CAEP THz-FEL facility

Abstract The Chinese Academy of Engineering Physics Terahertz Free Electron Laser Facility (CAEP THz FEL, CTFEL) is the only high-average power free electron laser terahertz source based on superconducting accelerators in China. The update of the CTFEL is now undergoing and will expand the frequency range from 0.1-4.2 THz to 0.1-125 THz. Two experimental stations for material spectroscopy and biomedicine will be built. A high harmonic generation (HHG) lightsource based beamline at the material spectroscopy experimental station for time-resolved angle-resolved photoemission spectroscopy (ARPES) research will be constructed and the optical design is presented. The HHG lightsource covers the extreme ultraviolet (XUV) photon energy range of 20-50 eV. A Czerny-Turner monochromator with two plane gratings worked in conical diffraction configuration is employed for maintaining the transmission efficiency and preserving the pulse time duration. The calculated beamline transmission efficiency is better than 5% in the whole photon energy range. To our knowledge, this is the first time in China to combine THz-infrared FEL with HHG light source, and this experimental station will be a powerful and effective instrument and give new research opportunities in the future for users doing research on dynamic evolution of excited electron band structure of material surface.

Novel low-temperature and high-flux hydrogen plasma source for extreme-ultraviolet lithography applications

Inside extreme-ultraviolet (EUV) lithography machines, a hydrogen plasma is generated by ionization of the background gas by EUV photons. This plasma is essential for preventing carbon build-up on the optics, but it might affect functional performance and the lifetime of other elements inside the machine. The interaction of scanner materials and components with hydrogen plasma is investigated in controlled experiments using laboratory (off-line) setups, where the properties of EUV-generated plasmas are mimicked. Here, we present a novel experimental setup at TNO, where a low-temperature hydrogen plasma is generated by means of electron-impact ionization using a high-current, high-pressure electron beam (e-beam) gun. We show that the produced ion flux, peak ion energies, and radical-to-ion ratio are similar to that of the EUV-generated plasma. Since the e-gun has the option of operating the e-gun in the pulsed mode, it is possible to reproduce the time-dependent behavior of the scanner plasma as well. Moreover, as shown by Luo et al. [RSC Adv. 10, 8385 (2020)], electrons that impinge on surfaces mimic EUV photons in the generation of secondary electrons, which often drive radiation-induced processes (e.g., surface oxidation, reduction, and growth of carbon). We conclude that e-beam generated hydrogen plasma is a very promising technology for cost-effective lifetime testing of materials and optics, as compared to setups with EUV sources.

Attosecond delays in X-ray molecular ionization.

The photoelectric effect is not truly instantaneous but exhibits attosecond delays that can reveal complex molecular dynamics1-7. Sub-femtosecond-duration light pulses provide the requisite tools to resolve the dynamics of photoionization8-12. Accordingly, the past decade has produced a large volume of work on photoionization delays following single-photon absorption of an extreme ultraviolet photon. However, the measurement of time-resolved core-level photoionization remained out of reach. The required X-ray photon energies needed for core-level photoionization were not available with attosecond tabletop sources. Here we report measurements of the X-ray photoemission delay of core-level electrons, with unexpectedly large delays, ranging up to 700 as in NO near the oxygen K-shell threshold. These measurements exploit attosecond soft X-ray pulses from a free-electron laser to scan across the entire region near the K-shell threshold. Furthermore, we find that the delay spectrum is richly modulated, suggesting several contributions, including transient trapping of the photoelectron owing to shape resonances, collisions with the Auger-Meitner electron that is emitted in the rapid non-radiative relaxation of the molecule and multi-electron scattering effects. The results demonstrate how X-ray attosecond experiments, supported by comprehensive theoretical modelling, can unravel the complex correlated dynamics of core-level photoionization.

Quantifying External Energy Inputs for Giant Planet Magnetospheres

AbstractThe long‐standing “energy crisis” at the giant planets refers to the anomalous heating of planetary thermospheres compared to the available energy from solar irradiance. The coupling between planetary magnetospheres and their upper atmospheres is thought to address these crises, though the sources and pathways of energy transport have not been fully explored at each system. In particular, the total available energy from the upstream solar wind at each planet has not been comprehensively quantified. Here we apply recently developed models of energy conversion by magnetic reconnection and the Kelvin‐Helmholtz instability to each of the Giant Planets, providing estimates of the average external energy inputs for each system between 1985 and 2020. We find that external energy associated with solar‐wind‐magnetospheric coupling significantly exceeds that from solar extreme ultraviolet photons. While internal energy sources are known to dominate at Jupiter and Saturn, external sources may be significant at Uranus and Neptune.

Science challenges and research opportunities for plasma applications in microelectronics

Low-temperature plasmas (LTPs) are essential to manufacturing devices in the semiconductor industry, from creating extreme ultraviolet photons used in the most advanced lithography to thin film etching, deposition, and surface modifications. It is estimated that 40%–45% of all process steps needed to manufacture semiconductor devices use LTPs in one form or another. LTPs have been an enabling technology in the multidecade progression of the shrinking of device dimensions, often referred to as Moore’s law. New challenges in circuit and device design, novel materials, and increasing demands to achieve environmentally benign processing technologies require advances in plasma technology beyond the current state-of-the-art. The Department of Energy Office of Science Fusion Energy Sciences held a workshop titled Plasma Science for Microelectronics Nanofabrication in August 2022 to discuss the plasma science challenges and technical barriers that need to be overcome to continue to develop the innovative plasma technologies required to support and advance the semiconductor industry. One of the key outcomes of the workshop was identifying a set of priority research opportunities (PROs) to focus attention on the most strategic plasma science challenges to address to benefit the semiconductor industry. For each PRO, scientific challenges and recommended strategies to address those challenges were identified. This article summarizes the PROs identified by the workshop participants.

Comparison of wavefront sensing and ablation imprinting for FEL focus diagnostics at FLASH2

Extreme ultraviolet (EUV) photon beam characterization techniques, Hartmann wavefront sensing and single shot ablation imprinting, were compared along the caustic of a tightly focused free-electron laser (FEL) beam at beamline FL24 of FLASH2, the Free-electron LASer in Hamburg at DESY. The transverse coherence of the EUV FEL was determined by a Young’s double pinhole experiment and used in a back-propagation algorithm which includes partial coherence to calculate the beam intensity profiles along the caustic from the wavefront measurements. A very good agreement of the profile structure and size is observed for different wavelengths between the back-propagated profiles, an indirect technique, and ablation imprints. As a result, the Hartmann wavefront sensor including its software MrBeam is a very useful, single shot pulse resolved and fast tool for non-invasive determination of focal spot size and shape and also for beam profiles along the caustic.

Modeling the structure of the dayside Venusian ionosphere: Impacts of protonation and Coulomb interaction

The CO$_2$-dominated thick atmosphere of Venus coexists with an ionosphere that is mainly formed, on the dayside, via the ionization of atmospheric neutrals by solar extreme ultraviolet and soft X-ray photons. Despite extensive modeling efforts that have reproduced the electron distribution reasonably well, we note two main shortcomings with respect to prior studies. The effects of protonation and Coulomb interaction are crucial to unveiling the structure and composition of the Venusian ionosphere. We evaluate the role of protonated species on the structure of the dayside Venusian ionosphere for the first time. We also evaluate the role of ion-ion Coulomb collisions, which are neglected in many existing models. Focusing on the solar minimum condition for which the effect of protonation is expected to be more prominent, we constructed a detailed one-dimensional photochemical model for the dayside Venusian ionosphere, incorporating more than 50 ion and neutral species (of which 17 are protonated species), along with the most thorough chemical network to date. We included both ion-neutral and ion-ion Coulomb collisions. Photoelectron impact processes were implemented with a two-stream kinetic model. Our model reproduces the observed electron distribution reasonably well. The model indicates that protonation tends to diverge the ionization flow into more channels via a series of proton transfer reactions along the direction of low to high proton affinities for parent neutrals. In addition, the distribution of O$_2^+$ is enhanced by protonation by a factor of nearly 2 at high altitudes, where it is efficiently produced via the reaction between O and OH$^+$. We find that Coulomb collisions influence the topside Venusian ionosphere not only directly by suppressing ion diffusion, but also indirectly by modifying ion chemistry. Two ion groups can be distinguished in terms of the effects of Coulomb collisions: one group preferentially produced at high altitudes and accumulated in the topside ionosphere, which is to be compared with another group that is preferentially produced at low altitudes and, instead, depleted in the topside ionosphere. Both protonation and Coulomb collisions have appreciable impacts on the topside Venusian ionosphere, which account for many of the significant differences in the model ion distribution between this study and early calculations.

A high-efficiency programmable modulator for extreme ultraviolet light with nanometre feature size based on an electronic phase transition

AbstractThe absence of efficient light modulators for extreme ultraviolet (EUV) and X-ray photons considerably limits their real-life application, particularly when even slight complexity of the beam patterns is required. Here we report on a novel approach to reversible imprinting of a holographic mask in an electronic Wigner crystal material with a sub-90-nm feature size. The structure is imprinted on a sub-picosecond timescale using EUV laser pulses, and acts as a high-efficiency diffraction grating that deflects EUV or soft X-ray light. The imprinted nanostructure is stable after the removal of the exciting beams at low temperatures, but can be easily erased by a single heating beam. Modelling shows that the efficiency of the device can exceed 1%, approaching state-of-the-art etched gratings, but with the benefit of being programmable and tunable over a large range of wavelengths. The observed effect is based on the rapid change of lattice constant upon transition between metastable electronically ordered phases in a layered transition metal dichalcogenide. The proposed approach is potentially useful for creating tunable light modulators in the EUV and soft X-ray spectral ranges.

Two-body fragmentation of methane induced by extreme ultraviolet and high charge ions

CH<sub>4</sub> is abundant in planetary atmospheres, and the study of CH<sub>4</sub> dissociation dynamics is of great importance and can help to understand the atmospheric evolution process in the universe. At present, the CH<sub>4</sub><sup>2+</sup>→ CH<sub>3</sub><sup>+</sup>+H<sup>+</sup> channel has been extensively studied, but the explanation of the dissociation mechanism for this channel is controversial. In this work, the double-photoionization experiment of CH<sub>4</sub> by extreme ultraviolet photon (XUV) in the energy range of 25-44 eV and the collision experiment between 1 MeV Ne<sup>8+</sup> and CH<sub>4</sub> were carried out on the reaction microscope. The 3D momenta of CH<sub>3</sub><sup>+</sup> and H<sup>+</sup> ions were measured in coincidence, the corresponding kinetic energy release (KER) was reconstructed, and fragmentation dynamics from the parent ion CH<sub>4</sub><sup>2+</sup> to the CH<sub>3</sub><sup>+</sup>+H<sup>+</sup> ion pair were investigated. In the photoionization experiment, we observed two peaks in the KER spectrum, one locates around 4.75 eV, and the other one lies at 6.09 eV. Benefiting from the conclusions of previous experiments and the theoretical calculations of Williams [19] et al, we discussed the corresponding mechanism of each KER peak. For the 6.09 eV peak, we attributed it to the CH<sub>4</sub><sup>2+</sup> dissociation mediated by the Jahn-Teller effect, as this value is consistent with the energy difference between the CH<sub>4</sub><sup>2+</sup> <sup>1</sup>E initial state and the CH<sub>3</sub><sup>+</sup> /H+ final state involving the Jahn-Teller effect. For the 4.75 eV peak, we proposed that it may come from the direct dissociation of CH<sub>4</sub><sup>2+</sup> without the Jahn-Teller effect. In more detail, Williams<sup>[19]</sup> et al presented the potential energy curve for one C-H bond stretching to 8 a.u., while other C-H bonds are fixed at the initial geometry of the CH<sub>4</sub> molecule. Based on the reflection approximation, we deduced the additional energy release from the internuclear distance of 8 a.u. to infinity. We found the sum KER is 4.7 eV, this is consistent with the experimental observation and suggests that the KER peak at 4.75 eV may arise from the direct dissociation of CH<sub>4</sub><sup>2+</sup> without the Jahn-Teller effect. In addition, in the 1 MeV Ne<sup>8+</sup> ion collision experiment, we observed three KER peaks with the mean kinetic energy release values of around 4.65, 5.75, and 7.94 eV. By comparing the branching ratio of each peak with the previous experimental results, it is suggested that the velocity effect is not significant in KER spectra.

Detection of carbon contamination in EUV multilayer mirrors based on secondary electrons

The issue of carbon contamination on the surface of extreme ultraviolet (EUV) optical elements needs to be addressed urgently. In the process of removing carbon contamination, it is necessary to detect the thickness of the carbon contaminated layer in order to avoid damage to EUV multilayer mirrors (MLMs) by overcleaning. The yield of secondary electrons (SEY) generated by the excitation of EUV photons and primary electrons has been proved to be correlated with the thickness of the carbon layer, and thus the cutoff point for carbon layer cleaning can be found by detecting the SEY. In this study, the spatial distribution and motion state of secondary electrons (SEs) in the vacuum cavity of the EUV system during the cleaning process are described, utilizing the Particle-in-cell model and Monte Carlo method, and the variation relationship between SEY and the thickness of the carbon layer is obtained. The results show that under the set conditions, the SEY generated by EUV photons is very low, but it will generate primary electrons that are 1.05 times as numerous as itself, of which 24% bombarded to the carbon layer, resulting in SEs which amount to 16% of primary electrons generation. Moreover, as the carbon layer thickness decreases to 2 nm or below, SEY increases significantly, ranging from 0.55 to 0.71. The present work is of great significance in realizing the accurate detection of the carbon layer thickness of EUV multilayer film mirrors and the recovery of the reflectivity of MLMs.

Production of 13.5 nm light with 5% conversion efficiency from 2 μ m laser-driven tin microdroplet plasma

We demonstrate the efficient generation of extreme ultraviolet (EUV) light from laser-produced plasma (LPP) driven by 2 μm wavelength laser light as an alternative for 10 μm CO2 gas LPP currently employed in EUV lithography machines for high-volume manufacturing of semiconductor devices. High conversion efficiencies of laser light into “in-band” EUV photons up to 5.0% are achieved by homogeneously heating the plasma that is laser-generated from preshaped tin microdroplet targets. Scaling the laser pulse duration, spot size, and intensity yields a high in-band EUV energy output of up to 12.5 mJ. The EUV emission source size is studied under a similar parameter range and is shown to match typical etendues of EUV optic columns. Our findings make 2 μm LPP a particularly promising candidate to power future EUV nanolithography.

Photoevaporation and Close Encounters: How the Environment around Cygnus OB2 Affects the Evolution of Protoplanetary Disks

In our Galaxy, star formation occurs in a variety of environments, with a large fraction of stars formed in clusters hosting massive stars. OB stars have an important feedback on the evolution of protoplanetary disks orbiting around nearby young stars and likely on the process of planet formation occurring in them. The nearby massive association Cyg OB2 is an outstanding laboratory to study this feedback. It is the closest massive association to our Sun and hosts hundreds of massive stars and thousands of low-mass members, both with and without disks. In this paper, we analyze the spatial variation of the disk fraction (i.e., the fraction of cluster members bearing a disk) in Cyg OB2 and study its correlation with the local values of far-ultraviolet (FUV) and extreme-ultraviolet (EUV) radiation fields and the local stellar surface density. We present definitive evidence that disks are more rapidly dissipated in the regions of the association characterized by intense local UV fields and large stellar density. In particular, the FUV radiation dominates disk dissipation timescales in the proximity (i.e., within 0.5 pc) of the O stars. In the rest of the association, EUV photons potentially induce a significant mass loss from the irradiated disks across the entire association, but the efficiency of this process is reduced at increasing distances from the massive stars owing to absorption by the intervening intracluster material. We find that disk dissipation due to close stellar encounters is negligible in Cyg OB2 and likely to have affected 1% or fewer of the stellar population. Disk dissipation is instead dominated by photoevaporation. We also compare our results to what has been found in other young clusters with different massive populations, concluding that massive associations like Cyg OB2 are potentially hostile to protoplanetary disks but that the environments where disks can safely evolve in planetary systems are likely quite common in our Galaxy.

Influence of nuclear dynamics on molecular attosecond photoelectron interferometry.

In extreme ultraviolet spectroscopy, the photoionization process occurring in a molecule due to the absorption of a single photon can trigger an ultrafast nuclear motion in the cation. Taking advantage of attosecond photoelectron interferometry, where the absorption of the extreme ultraviolet photon is accompanied by the exchange of an additional infrared quantum of light, one can investigate the influence of nuclear dynamics by monitoring the characteristics of the photoelectron spectra generated by the two-color field. Here, we show that attosecond photoelectron interferometry is sensitive to the nuclear response by measuring the two-color photoionization spectra in a mixture of methane (CH4) and deuteromethane (CD4). The effect of the different nuclear evolution in the two isotopologues manifests itself in the modification of the amplitude and contrast of the oscillations of the photoelectron peaks. Our work indicates that nuclear dynamics can affect the coherence properties of the electronic wave packet emitted by photoionization on a time scale as short as a few femtoseconds.

Direct tracking of ultrafast proton transfer in water dimers.

Upon ionization, water forms a highly acidic radical cation H2O+· that undergoes ultrafast proton transfer (PT)-a pivotal step in water radiation chemistry, initiating the production of reactive H3O+, OH[Formula: see text] radicals, and a (hydrated) electron. Until recently, the time scales, mechanisms, and state-dependent reactivity of ultrafast PT could not be directly traced. Here, we investigate PT in water dimers using time-resolved ion coincidence spectroscopy applying a free-electron laser. An extreme ultraviolet (XUV) pump photon initiates PT, and only dimers that have undergone PT at the instance of the ionizing XUV probe photon result in distinct H3O+ + OH+ pairs. By tracking the delay-dependent yield and kinetic energy release of these ion pairs, we measure a PT time of (55 ± 20) femtoseconds and image the geometrical rearrangement of the dimer cations during and after PT. Our direct measurement shows good agreement with nonadiabatic dynamics simulations for the initial PT and allows us to benchmark nonadiabatic theory.

Efficient Indirect Interatomic Coulombic Decay Induced by Photoelectron Impact Excitation in Large Pure Helium Nanodroplets.

Ionization of matter by energetic radiation generally causes complex secondary reactions that are hard to decipher. Using large helium nanodroplets irradiated by extreme ultraviolet (XUV) photons, we show that the full chain of processes ensuing primary photoionization can be tracked in detail by means of high-resolution electron spectroscopy. We find that elastic and inelastic scattering of photoelectrons efficiently induces interatomic Coulombic decay (ICD) in the droplets. This type of indirect ICD even becomes the dominant process of electron emission in nearly the entire XUV range in large droplets with radius ≳40 nm. Indirect ICD processes induced by electron scattering likely play an important role in other condensed-phase systems exposed to ionizing radiation as well, including biological matter.

X-FAST: A versatile, high-throughput, and user-friendly XUV femtosecond absorption spectroscopy tabletop instrument.

We present the X-FAST (XUV Femtosecond Absorption Spectroscopy Tabletop) instrument at the University of Wisconsin-Madison. The instrument produces femtosecond extreme ultraviolet photon pulses via high-harmonic generation in the range of 40-72eV, as well as optical pump pulses for transient-absorption experiments. The system implements a gas-cooled sample cell that enables studying the dynamics of thermally sensitive thin-film samples. This paper provides potential users with specifications of the optical, vacuum, data acquisition, and sample cooling systems of the X-FAST instrument, along with performance metrics and data of an ultrafast laser-induced phase transition in a Ni2MnGa Heusler thin film.

Probability of ionization of the noble gas atoms and ions for high harmonic generation

Radiation produced in high-order harmonic generation (HHG) process can be utilized to generate attosecond pulses. According to the semi-classical theory of HHG, the maximum emitted photon energy is [Formula: see text]. The ionization potential ([Formula: see text] and ponderomotive energy ([Formula: see text], play important roles in the generated photon energy; therefore, choosing the material and field for HHG is important. Here, investigating the probability of ionization of material in laser and plasmonic field for HHG is important. The ionization probability of noble gases atoms and ions was investigated. Using Ammosov–Delone–Krainov (ADK) model formulas, the parameters for ions and atoms were determined. The results show that helium is the most suitable noble gas atom for HHG in both fields. After ionizing the atoms, the ionization potential increases significantly, and the ionization probability also decreases. Then, ions are better than their corresponding atoms for HHG. Among noble gas atoms and ions, the helium ion is the most suitable for HHG. In addition, the plasmonic field increases the ponderomotive energy and decreases the probability of ionization for all atoms, as shown by the simulation. Then, extreme ultraviolet (XUV) high-energy photon can be efficiently generated using noble gas ions.

Resistless EUV lithography: Photon-induced oxide patterning on silicon

In this work, we show the feasibility of extreme ultraviolet (EUV) patterning on an HF-treated silicon (100) surface in the absence of a photoresist. EUV lithography is the leading lithography technique in semiconductor manufacturing due to its high resolution and throughput, but future progress in resolution can be hampered because of the inherent limitations of the resists. We show that EUV photons can induce surface reactions on a partially hydrogen-terminated silicon surface and assist the growth of an oxide layer, which serves as an etch mask. This mechanism is different from the hydrogen desorption in scanning tunneling microscopy-based lithography. We achieve silicon dioxide/silicon gratings with 75-nanometer half-pitch and 31-nanometer height, demonstrating the efficacy of the method and the feasibility of patterning with EUV lithography without the use of a photoresist. Further development of the resistless EUV lithography method can be a viable approach to nanometer-scale lithography by overcoming the inherent resolution and roughness limitations of photoresist materials.

Shape resonances in angle-resolved photoionization of aligned CF3I molecules

The photoionization of CF3I molecules in the photon energy range between 10 eV and 40 eV is investigated using angle-resolved photoelectron spectroscopy in the molecular frame. Using a femtosecond laser pulse, the molecules are impulsively aligned prior to their ionization by a series of extreme ultra-violet photon energies obtained by high-harmonic generation. The manifestation of molecular alignment in the photoelectron angular distributions (PADs) is well reproduced by non-relativistic quantum-mechanical photoionization calculations carried out with the ePolyScat software package, taking into account all open ionization channels. Our analysis points to two prominent shape resonances that dominate the photoelectron scattering dynamics and that therefore largely explain the observed changes in the PADs as a function of molecular alignment.

Simulations of plasmas driven by laser wavelengths in the 1.064—10.6 μ m range for their characterization as future extreme ultraviolet light sources

We characterize the properties of extreme ultraviolet (EUV) light source plasmas driven by laser wavelengths in the λ laser = 1.064 − 10.6 μ m range and laser intensities of I laser = 0.5 − 5 × 10 11 W cm−2 for λ laser = 1.064 μ m. Detailed numerical simulations of laser-irradiated spherical tin microdroplet targets reveal a strong laser-wavelength dependence on laser absorptivity and the conversion efficiency of generating in-band EUV radiation. For λ laser = 1.064 μ m irradiation, the increase in in-band radiation with increasing laser intensity is offset by only a minor reduction in conversion efficiency. Radiative losses are found to dominate the power balance for all laser wavelengths and intensities, and a clear shift from kinetic to in-band radiative losses with increasing laser wavelength is identified. Yet, with increasing laser intensity, such a shift is absent. We find that the existence of a maximum conversion efficiency, near λ laser = 4 μ m, originates from the interplay between the optical depths of the laser light and the in-band EUV photons for this specific droplet-target geometry.