Extreme Ultraviolet Light Research Articles

Diagnostics of tin droplet-based laser produced plasma by triple Langmuir probe

Laser produced plasma extreme ultraviolet light source, as the exposure light source for next-generation lithography, faces urgent challenges due to its low conversion efficiency and significant debris contamination. Using a triple Langmuir probe, we conducted a study on the parameters and kinetic characteristics of tin droplet-based laser produced plasma that serves as extreme ultraviolet light source. In this paper, we report a design of triple Langmuir probe circuit based on the BWL (Bulk Wirewound Low-value) sampling resistor, which ensures that high-frequency signals remain undistorted during acquisition. Utilizing the “shadowgraph” method, we calculated the average deflection angles of droplet debris jets under different alignment conditions. Combining the diagnostic results from the probe, we found that the alignment accuracy between the laser and droplet directly influences the parameters of plasma. The parameters and kinetic characteristics of plasma were also diagnosed and analyzed at different angles and distances by moving the probe. Employing the triple Langmuir probe, measurements and analyses of laser produced plasma parameters and kinetic characteristics were achieved, providing a convenient method for diagnosing extreme ultraviolet lithography light sources. This work also offers an effective basis for optimizing the light sources.

Sun-as-a-star Analysis of the X1.6 Flare on 2023 August 5: Dynamics of Postflare Loops in Spatially Integrated Observational Data

Postflare loops are loop-like plasmas observed during the decay phase of solar flares, and they are expected to exist for stellar flares. However, it is unclear how postflare loops are observed in stellar flares’ cases. To clarify behaviors of postflare loops in spatially integrated data, we performed the Sun-as-a-star analysis of the X1.6 flare that occurred on 2023 August 5, using GOES X-ray flux (∼107 K), extreme ultraviolet (EUV) images taken by Atmospheric Imaging Assembly on board the Solar Dynamic Observatory (≥104.9 K), and Hα data taken by Solar Dynamics Doppler Imager on board the Solar Magnetic Activity Research Telescope at Hida Observatory, Kyoto University (∼104 K). As a result, we found that this flare showed signatures corresponding to the important dynamics of the postflare loops even in the spatially integrated data: (1) The Hα light curve showed two distinct peaks corresponding to the flare ribbons and the postflare loops. The plasma cooling in the postflare loops generated different peak times in soft X-rays, EUV, and Hα light curves. (2) Downflows were confirmed as simultaneous redshifted/blueshifted absorptions in the Hα spectra. (3) The apparent rise of postflare loops was recognized as a slowing of the decay for the Hα light curve. These results are the key to investigating stellar postflare loops with spatially integrated data. We also discuss the dependence of our results on flare locations and their possible applications to stellar observations.

The Sun from space: Discoveries from space missions over the past fifty years

Abstract The Sun is a magnetically active star that displays many exciting phenomena when observed from space in X-rays, extreme ultraviolet wavelengths and visible light: coronal holes, active regions, magnetic loops, flares, huge mass eruptions and the solar wind. Observations from space have revealed incredible mysteries about the Sun’s atmosphere and its extended corona, and even the interior of the Sun. This paper reviews and illustrates the progress and achievements over the last fifty years, presenting selected instruments and observations from fifteen solar missions, starting from Skylab in 1973 up to the latest Solar Orbiter mission, giving unprecedented insight into how the Sun works.

Spectral and spatial resolution of an extreme ultraviolet broadband imaging spectrometer based on dispersion-matched zone plates

We present a combined 1D imaging and broadband spectroscopy tool for analyzing laser-produced plasma sources of extreme ultraviolet light using a tapered zone plate that is dispersion-matched to a transmission grating. Specifically, we follow up on prior work [Mostafa et al. Opt. Lett. 48, 4316 (2023)] to obtain the actual spectral and spatial resolution of the imaging spectrometer and compare it to the design values. The imaging spectrometer is shown to have a spectral resolution of 1.2 nm at 13.5 nm, close to its design value, by assessing spectra obtained from carbon laser-produced plasma in a 5–180 nm wavelength band. The spatial resolution was obtained by placing slits near the object plane and back-illuminating the slit with a tin laser-produced plasma and found to be 17(5) µm, somewhat larger than the design specifications but still well within design limits for use for diagnosing plasma.

Optimization of efficiency of short-pulsed fast flow CO2 laser amplifier with dual-band and multispectral lines

In this paper, we optimize the amplification efficiency of a nanosecond pulse CO2 laser in a fast flow amplifier using dual-band and multispectral lines techniques. Utilizing a six-temperature model and a random rotational relaxation model, we simulate the time-domain amplification process of dual-band and multispectral lines short-pulse seeds in a fast flow CO2 laser amplifier, analyzing the effects of input pulse fluence, pulse width, and spectral line composition on amplification efficiency. Compared to single-line 10P(20) amplification, the extraction efficiency of 10.6-µm and 9.6-µm dual-band two-line 15-ns pulses is improved by approximately 70%, surpassing that of 10.6-µm single-band four-line amplification. This study is of great significance for the efficient amplification of short-pulse CO2 lasers and the generation of extreme ultraviolet light from laser produced plasma.

Optical Coherence Tomography of Van Der Waals Heterostructures Using Extreme Ultraviolet Light

AbstractNew experimental methods with high out‐of‐plane spatial sensitivity combined with ultrafast temporal resolution can revolutionize the understanding of charge‐ and heat‐transfer dynamics occurring at interfaces. In this work, a step forward is taken in this direction by applying coherence tomography with extreme ultraviolet (EUV) light to different van der Waals heterostructures, which enables a 3D sample reconstruction with nanoscopic axial resolution. Furthermore, the measurements and, more in general, the approach is confirmed by ab initio calculations of the refractive index of layered materials that we compare to existing databases of empirical data. The EUV coherence tomography contrast is estimated in a broad spectral range (photon energy 65 –100 eV). This work sets the basis for the development of a new spectroscopy tool that, thanks to the temporal profile of EUV light sources and the high axial resolution of coherence tomography, can become the ideal probe of ultrafast processes occurring in van der Waals heterostructures and buried nanoscale opto‐electronic devices.

Theoretical Investigation of Laser Ablation Propulsion Using Micro-Scale Fluid in Atmosphere

Laser ablation propulsion based on liquid propellants is a type of propulsion technology with a high specific impulse and good controllability that can be applied to space thrusters, gas metal arc welding, and extreme ultraviolet light. However, its basic mechanisms, such as flow evolution and thrust formation, have not yet been described in detail. In this study, the laser ablation of micro-scale fluid in the atmosphere was investigated. Flow evolution with different laser energy and fluid mass was observed using a schlieren system. According to the characteristic of flow evolution, a theoretical model of laser ablation propulsion in the atmosphere was established. For the first time, a theoretical hypothesis was proposed that the laser energy is divided into two parts, which act on fluid and air respectively. The model indicates that the impulses generated by fluids and air follow power laws with the laser energy, while the exponentials are 0.5 and 1, respectively. In the atmosphere, due to the shielding effect of a laser-maintained detonation wave on laser, the energy absorbed by the fluid is basically unchanged, while only the energy absorbed by the air changes. Significantly, the theoretical model is consistent with the impulse experiment and current studies.

First-principles simulations of high-order harmonics generation in thin films of wide bandgap materials [Invited

High-order harmonics generation (HHG) is the only process that enables tabletop-sized sources of extreme ultraviolet (XUV) light. The HHG process typically involves light interactions with gases or plasma––material phases that hinder wider adoption of such sources. This motivates the research in HHG from nanostructured solids. Here, we employ the time-dependent density function theory (TDDFT) to investigate material platforms for HHG at the nanoscale using first-principles supercomputer simulations. We reveal that wide bandgap semiconductors, aluminum nitride (AlN) and silicon nitride (Si3N4), are highly promising for XUV light generation when compared to silicon, one of the most common nonlinear nanophotonic materials. In our calculations, we assume excitation with a 100 fs pulse duration, 1×1013W/cm2 peak power, and 800 nm central wavelength. We demonstrate that in AlN material the interplay between the crystal symmetry and the incident light direction and polarization can enable the generation of both even and odd harmonics. Our results should advance the development of high-harmonics generation of XUV light from nanostructured solids.

Impact of MoS2 Monolayers on the Thermoelastic Response of Silicon Heterostructures.

Understanding the thermoelastic response of a nanostructure is crucial for the choice of materials and interfaces in electronic devices with improved and tailored transport properties at the nanoscale. Here, we show how the deposition of a MoS2 monolayer can strongly modify the nanoscale thermoelastic dynamics of silicon substrates close to their interface. We demonstrate this by creating a transient grating with extreme ultraviolet light, using ultrashort free-electron laser pulses, whose ≈84 nm period is comparable to the size of elements typically used in nanodevices, such as electric contacts and nanowires. The thermoelastic response, featuring coherent acoustic waves and incoherent relaxation, is tangibly modified by the presence of monolayer MoS2. Namely, we observed a major reduction of the amplitude of the surface mode, which is almost suppressed, while the longitudinal mode is basically unperturbed, aside from a faster decay of the acoustic modulations. We interpret this behavior as a selective modification of the surface elasticity, and we discuss the conditions to observe such effect, which may be of immediate relevance for the design of Si-based nanoscale devices.

A new framework for soft x-ray transient gratings

The capability to use extreme ultraviolet (EUV) light for generating transient gratings (TGs) has enabled the study of thermoelastic and magnetic dynamics at the nanoscale, in thin solid samples and surfaces, without the need of specially modifying them. However, the current mirror-based setup for generating EUV TG limits both its extension to the soft x-ray photon energy range and the attainment of few femtosecond time-resolution. Here we propose to overcome these limitations with an alternative experimental scheme based on diffractive optical elements that has become feasible with the current technology. In addition, some aspects of the discussed setup may facilitate the implementation of the EUV TG approach at table-top high-harmonic generation sources.

High-Resolution Electron Ionization Mass Spectrometry of Stannane: Deconvolution of Superimposed Fragmentation Patterns.

In a pulsed laser plasma driven extreme ultraviolet (EUV) light, tin droplets undergo evaporation, eventually depositing on different surfaces. The removal of surface bound tin is commonly achieved with a hydrogen plasma, resulting in the formation of stannane (SnH4). The mechanisms leading to the formation and decomposition of stannane remain incompletely understood. To analyze these mechanisms mass spectrometrically, a reference is crucial, necessitating a high-resolution and thoroughly characterized mass spectrum of stannane. In this paper, a high-resolution 70 eV electron ionization (EI) mass spectrum of stannane is presented. The mass spectrum comprises all ten natural isotopes of the stannane fragments generated through EI. Utilizing the custom analysis program RASP, the relative distribution of fragments is calculated from the isotopically superimposed mass signals, offering crucial insights into the occurring processes. Furthermore, the dependence of fragment formation on ion source pressure and temperature was determined.

State-selective dissociative double ionization of CH3I and CH2I2 via I 4d core-hole states studied by multi-electron-ion coincidence spectroscopy.

The dissociative double ionization of CH3I and CH2I2 irradiated with extreme ultraviolet light at hv = 100eV is investigated by multi-electron-ion coincidence spectroscopy using a magnetic bottle type electron spectrometer. The spin-orbit state-resolved Auger electron spectra for the I 4d core-hole states, (I 4d3/2)-1 and (I 4d5/2)-1, provide clear identifications of electronic states of CH3I2+ and CH2I22+. The dominant ion species produced after the double ionization correlate with the Auger electron energy, showing that different fragmentation pathways are open depending on the electronic states populated by the Auger decay. Theoretical calculations are performed to understand the fragmentation from the doubly charged states and the observed spin-orbit specificity in the Auger decay.

On the role of target mass in extreme ultraviolet light generation from CO2-driven tin plasmas for nanolithography

Target conditioning is a crucial ingredient of high-power extreme ultraviolet (EUV) source operation in state-of-the-art nanolithography. It involves deforming tin microdroplets into tens of nanometer-thin sheets, sheets which are subsequently irradiated by intense CO2 laser radiation to form a hot, EUV-emitting plasma. Recent experiments have found that a substantial fraction of the initial droplet mass is lost in the deformation phase through fragmentation. The goal of the present study is to investigate, using radiation-hydrodynamic modeling, how variations in the sheet mass affect EUV source power and the laser-to-in-band conversion efficiency (CE). It is found that high-mass sheets can “feed” the plasma with sufficient mass to sustain the production of in-band-emitting charge states over the course of laser irradiation. Low-mass sheets, on the contrary, cannot supply enough mass to sustain this production over the pulse, thus leading to a reduction in in-band power and CE. The dependence of CE on laser energy and target thickness is quantified, and a rather weak reduction of CE with increasing laser energy for high-mass sheets is identified.

Producing focused extreme ultraviolet vortex with Fermat-spiral photon sieves

Extreme ultraviolet (EUV) light is difficult to focus due to strong absorption of most materials. Photon sieves (PS), rather than Fresnel zone plates (FZP), can focus EUV to smaller spot and suppress the higher orders of secondary maxima by several orders of magnitude. The number of pinholes used in PS is far more than that of transparent rings used in FZP, providing a great flexibility to manipulate structured focusing in EUV. In this work we investigate the Fermat-spiral PS to produce focused vortices with different topological charges. Experiment at the wavelength of 46.9 nm is carried out and multi-planar coherent diffractive imaging is used to retrieve the phase map of the focused EUV vortices. These results show the enormous potential of PS for manipulating EUV light. This study not only provides a compact, affordable substitute to focusing vortices where transmissive optics materials are unavailable, but also provides a route of converting various complex light manipulation ranging from visible light to EUV and soft x-ray.

Generation of extreme-ultraviolet and x-ray light from a propagating nanometer electron layer in few-cycle laser interaction with solid targets

Generation of extreme-ultraviolet and x-ray light from a propagating nanometer electron layer in few-cycle laser interaction with solid targets

Optical design of reflective Bragg mirrors for EUV photolithography

In this article. we investigate the optical characteristics of reflective extreme-ultraviolet (EUV) mirrors for the application of lithography for 7 nm node and below. By using the conventional quarter-wavelength stacked Bragg reflective configuration of Si-based metal multilayers centered at soft x-ray 13.5 nm, the maximum reflection is numerically computed to be approximately 0.7 at nearly normal incidence, which is consistent to the typical values reported by Advanced Semiconductor Materials Lithography (ASML) for Mo/Si stacked multilayers. The full width half maximum (FWHM) bandwidth is 0.65∼0.78 nm. In our numerical simulation, moreover, we find Nb/Si reflective mirrors can be potentially utilized to replace Mo/Si stacked multilayers in order to harvest more EUV light inside the scanners.

Azimuthally extreme-ultraviolet focal splitter by modified spiral photon sieves

Extreme Ultraviolet (EUV) radiation is a short-wavelength light source that has important applications in many fields, such as optical communication, particle manipulation, and ultrahigh resolution imaging. However, the highly absorptive nature of EUV light makes it challenging to design suitable focusing optics, such as focal splitters, to properly manipulate the energetic light. Here, we propose modified spiral photon sieves to transform EUV laser light into azimuthally splitting focusing. A genetic algorithm was used to design and optimize the azimuthally focal splitters. A capillary discharge EUV laser at 46.9 nm was used to verify the effectiveness of our proposed method, and PMMA targets were used to record the focused laser spot. The profile of the recorded patterns measured by atomic force microscopy shows that the focal spots in the experiment are diffraction-limited and agreed with the theoretical analysis. The proposed technique provides a new way for manipulating EUV light and further extends the applications ranging from EUV to soft x rays.

Design and simulation of an extreme ultraviolet metalens based on the Pancharatnam-Berry phase.

Extreme ultraviolet (EUV) radiation plays a key role in the fields of material science, attosecond metrology, and lithography. However, the reflective optical components typically used in EUV systems contribute to their bulky size, weight, and increased costs for fabrication. In this paper, we theoretically investigate transmissive metalens designs capable of focusing the EUV light based on the Pancharatnam-Berry phase. The designed metalens is composed of nanoscale elliptical holes, which can guide and manipulate EUV light due to the higher refractive index of the vacuum holes compared to that of the surrounding material. We designed an EUV metalens with a diameter of 10µm, which supports a focal length of 24µm and a numerical aperture of up to 0.2. It can focus 55-nm EUV incident light to a diffraction-limited spot, and the focusing efficiency is calculated to be as high as about 7% over a broad EUV frequency range (50-65nm). This study reveals the possibility of applying a dielectric metalens in the EUV region without a transmissive optical material.

Effect of multiply excited states to the EUV emission from yttrium-like tin

The spectral emission rate of yttrium-like tin (Sn11+) is investigated as the typical ion, which has a strong emission of extreme ultra-violet (EUV) light near the wavelength of 13.5 nm. The energy level structure of the tin ion is investigated using the non-relativesitic configuration average model to calculate the population in the local thermodynamic equilibrium, and the spectral structure of the unresolved transition array that has EUV emission is investigated based on the calculated atomic data using the HULLAC code. It is shown that the broad main peak is accompanied by the tail structure for shorter and longer wavelengths, which arises from multiply and inner-shell excited levels. The emission channels that significantly contribute to the spectrum are determined from a convergence analysis. The excited states below the ionization limit with a large population are shown to have a significant contribution to the emission.

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.