Xe Plasma Research Articles

Demonstration of a kHz-repetition-rate extreme ultraviolet laser at 41.8 nm.

We demonstrate the operation of a plasma-based extreme ultraviolet (XUV) laser at a 1 kHz repetition rate driven by infrared pump pulses of less than 20 mJ. The 41.8 nm laser pulses were generated in a Xe plasma created by optical-field ionization by the L1 Allegra laser at ELI Beamlines. The output power of the XUV laser lies in the few microwatt range, and the energy efficiency of this pumping scheme opens the way for further scaling in repetition rate and average power.

Artifact-free sample preparation of metal thin films using Xe plasma-focused ion beam milling for atomic resolution and in situ biasing analyses

The focused ion beam (FIB) technique using Ga ion beams is generally employed for transmission electron microscopy (TEM) sampling owing to its location-specific beam controllability on materials. However, Ga ion beam bombardment-induced damage hinders reliable structural analysis. Furthermore, Ga ions implanted in the damaged surface layer substantially lower the chemical stability of the samples, such as metal thin films. Here, aiming for atomic-level interface structural analysis, we propose a useful approach for the TEM sample preparation of heteroepitaxial Ag/Cu metal films without physical damage or chemical instabilities using the Xe plasma FIB (PFIB) system. Xe plasma-assisted sampling ensured that the interface structure of the Ag/Cu metal film remained intact; in contrast, the sample prepared using the focused Ga ion beam was damaged by elemental mixing. Furthermore, the sample prepared by Xe plasma milling exhibited robust structural stability over time after exposure to air in contrast to the sample prepared by Ga-ion milling, which induced structural decomposition owing to oxidation. In addition to the static structural analysis, the Cu thin film sample prepared by Xe PFIB exclusively exhibited pristine structural properties under in situ electrical biasing experiments, thus confirming the feasibility of conducting a fundamental study of Cu electromigration without artifacts.

Laser-excited Xe plasma as a radiation source for lithography at wavelengths near 11 nm

Subject of study. The study focuses on the plasma excited by a laser on a gas-jet Xe target. Aim of study. The aim was to achieve such a high emissivity of the Xe plasma at a wavelength of 11.2 nm that would meet the requirements of industrial lithography. In addition to the applied aspect, the work also has a diagnostic component, the purpose of which is to study the internal structure of the laser plasma and determine its internal parameters, such as temperature, concentration, and mean ion charge. Method. The paper contains a review of works devoted to the comprehensive investigation of the laser-produced plasma with a Xe gas-jet target. In the course of this research, several methods of measurements and plasma diagnostics were developed and tested, including multimirror Bragg spectrometry, probing the plasma with the same infrared laser radiation that creates it, and determination of mean ion charge of this nonequilibrium and short-lived laser-produced plasma using the measured absorption of the laser energy in the plasma and a database on cross-sections of ionization by electron impact. Main results. A multimirror Bragg spectrometry technique was developed and tested. This technique was then used to obtain quantitatively calibrated spectra of the laser-produced Xe plasma. An effective mode of irradiating the target with a wide, defocused laser beam has been found, which made it possible to achieve the coefficient of conversion of laser pulse energy into the energy of the narrow-band plasma radiation around λ=11.2nm, approximately equal to 4%. This value is currently a world record for plasmas of such a type. Additionally, an analytical method for determining the temperature and ion charge of the plasma based on experimental measurements was developed. Practical significance. The achieved conversion efficiency of the radiation source under consideration is sufficient for its application in high production industrial lithography. The use of the simple and “clean” radiation source with Xe plasma could eliminate a number of problems encountered in contemporary lithography with a wavelength of 13.5 nm and a source with targets of metallic tin.

Optical emission spectroscopy of fiber laser sustained Xe plasma

Laser-sustained plasma (LSP), a unique method of obtaining high brightness broadband light sources, has been used for rapid optical inspection of wafer defects and has thus attracted great interest. Then Xe plasma sustained under high pressure by a continuous fiber laser was investigated using optical emission spectroscopy (OES). Emission spectra of spatial integration diagnosis have shown that with the increase of laser power and the decrease of Gaussian beam focusing number F, the electron temperature fitted by Boltzmann plot method increased from 0.45 eV to 0.85 eV, the corresponding electron density was roughly an order of magnitude of 1016-1017 cm-3, and the calculated absorption coefficient of plasma was distributed in 0.9–1.5 cm-1. To further analyze the spatial distribution of plasma parameters, the spatial resolved OES of LSP was acquired, and the two-dimensional electron temperature and electron density were determined. The non-uniform distribution of the plasma parameters was analyzed due to the existence of thermal gravity convection and forced convection. The serious self-reversal of emission lines of 881.94 nm and 823.16 nm was observed and analyzed.

Bridging macro to micro-scale fatigue crack growth by advanced fracture mechanical testing on the meso-scale

Fatigue and fracture mechanics testing is strictly regulated by standards that specify a minimum specimen size. However, when component sizes or material volume decrease below this limit, test methods need to be adapted for small scales. In fatigue crack growth studies, experiments become complex and challenging. Thus, there is lack of systematic scale-bridging studies from meso- to micro-scales on ductile materials. This study outlines a method to bridge this scale gap by performing meso-scale studies. Cantilver bending specimens with a cross section of 50x50µm2 and a lever arm length of 200µm of nanocrystalline nickel, as a model material, are milled by Xe plasma focused ion beam technique and tested following ASTM E647 standard. The gathered da/dN-curves are highly reproducible. The threshold stress intensity factor ΔKth is obtained by step-wise load reduction and discussed in context of crack closure. So, not only the gap between macro and micro-scaled testing was bridged, but also a pre-cracking step was shown to be essential for obtaining reliable data, and a new method to determine the intrinsic threshold stress intensity factor was introduced.

ToF-SIMS on a Xe Plasma FIB: Dos and Don'ts.

ToF-SIMS on a Xe Plasma FIB: Dos and Don'ts.

Study on the ionization and acceleration of a microwave discharge cusped field thruster

The microwave discharge cusped field thruster is a novel concept electric micro propulsion device, also a candidate thruster for the gravitational detection mission. A coaxial transmission line resonator is utilized to feed the microwave into the thruster to generate Xe plasma steadily with a mass flow rate as low as 0.1 sccm. Due to the separation of ionization and acceleration, the thruster performs high operation mode stability over a wide range of voltage in low mass flow conditions. Experimental and simulation methods are carried out to study the ionization and ion acceleration of the thruster. The results show that in operating conditions with a mass flow rate of 0.1 sccm, an anode voltage of 0 V to 1000 V, and a microwave power of 2 W, the right-hand circularly polarized wave (R wave) and the ordinary wave (O wave) play the most important role in the ionization process. The ion acceleration region locates around the exit magnetic separatrix, and the acceleration region tends to converge toward the separatrix as the anode voltage increases, resulting in an increased focus of the thruster plume and concentration of the ion energy distribution. Due to the separation of the ionization and acceleration regions, the thruster performs a divergence efficiency of 0.5–0.8, and an acceleration efficiency of 0.9.

An open source framework for advanced Multi-physics and multiscale modelling of solid oxide fuel cells

Solid oxide fuel cells are high-efficiency renewable energy devices and considered one of the most promising net-zero carbon energy technologies. Numerical modelling is a powerful tool for the virtual design and optimisation of the next-generation solid oxide fuel cells but needs to tackle issues for incorporating the multi-scale character of the cell and further improving the accuracy and computational efficiency. While most of solid oxide fuel cell models were developed based on closed source platforms which limit the freedom of customisation in numerical discretization schemes and community participation. Here, an open source multi-physics and multiscale platform for advanced SOFC simulations consisting of both cell- and pore-scale performance models was developed using OpenFOAM. The modelling aspects are elucidated in detail, involving the coupling of various physical equations and the implementation of the pore-scale electrode in the performance model. The entire platform was carefully validated against experimental data and the other numerical models which were implemented in commercial software ANSYS Fluent and based on the lattice Boltzmann method. The cell-scale model is subsequently employed to study the effects of different fuels, i.e., pure hydrogen and different ratios of pre-reformed methane gas under various operating temperatures. It is found that the cell-scale model reasonably predicts the effects of these parameters on the cell performance, aligning well with the Fluent model. This study further identified the size of representative element volume with respect to the current density for the anode via the pore-scale model where the realistic microstructures reconstructed by a Xe plasma focused ion beam–scanning electron microscopy are employed as computational domains. It is found that a volume element size of 1243 voxels is sufficient to yield the representative current density of the whole. All these numerical investigations show that OpenFOAM is a potential multi-physics and multi-scale computational platform that is capable of accurately predicting both cell-scale and pore-scale performance and spatial information of solid oxide fuel cells. The developed models are also made public in GitHub to inspire community to further develop around it.

Improved Methodology of Cross-Sectional SEM Analysis of Thin-Film Multilayers Prepared by Magnetron Sputtering

In this work, an improved methodology of cross-sectional scanning electron microscopy (SEM) analysis of thin-film Ti/V/Ti multilayers was described. Multilayers with various thicknesses of the vanadium middle layer were prepared by magnetron sputtering. The differences in cross sections made by standard fracture, focused ion beam (FIB)/Ga, and plasma focused ion beam (PFIB)/Xe have been compared. For microscopic characterization, the Helios NanoLab 600i microscope and the Helios G4 CXe with the Quanta XFlash 630 energy dispersive spectroscopy detector from Bruker were used. The innovative multi-threaded approach to the SEM preparation itself, which allows us to retain information about the actual microstructure and ensure high material contrast even for elements with similar atomic numbers was proposed. The fracture technique was the most noninvasive for microstructure, whereas FIB/PFIB results in better material contrast (even than EDS). There were only subtle differences in cross sections made by FIB-Ga and PFIB-Xe, but the decrease in local amorphization or slightly better contrast was in favor of Xe plasma. It was found that reliable information about the properties of modern nanomaterials, especially multilayers, can be obtained by analyzing a two-part SEM image, where the first one is a fracture, while the second is a PFIB cross section.

Methods to expose subsurface objects of interest identified from 3D imaging: The intermediate sample preparation stage in the correlative microscopy workflow.

The correlative imaging workflow is a method of combining information and data across modes (e.g. SEM, X-ray CT, FIB-SEM), scales (cm to nm) and dimensions (2D-3D-4D), providing a more holistic interpretation of the research question. Often, subsurface objects of interest (e.g. inclusions, pores, cracks, defects in multilayered samples) are identified from initial exploratory nondestructive 3D tomographic imaging (e.g. X-ray CT, XRM), and those objects need to be studied using additional techniques to obtain, for example, 2D chemical or crystallographic data. Consequently, an intermediate sample preparation step needs to be completed, where a targeted amount of sample surface material is removed, exposing and revealing the object of interest. At present, there is not one singular technique for removing varied thicknesses at high resolution and on a range of scales from cm to nm. Here, we review the manual and automated options currently available for targeted sample material removal, with a focus on those methods which are readily accessible in most laboratories. We summarise the approaches for manual grinding and polishing, automated grinding and polishing, microtome/ultramicrotome, and broad-beam ion milling (BBIM), with further review of other more specialist techniques including serial block face electron microscopy (SBF-SEM), and ion milling and laser approaches such as FIB-SEM, Xe plasma FIB-SEM, and femtosecond laser/LaserFIB. We also address factors which may influence the decision on a particular technique, including the composition, shape and size of the samples, sample mounting limitations, the amount of surface material to be removed, the accuracy and/or resolution of peripheral parts, the accuracy and/or resolution of the technique/instrumentation, and other more general factors such as accessibility to instrumentation, costs, and the time taken for experimentation. It is hoped that this study will provide researchers with a range of options for removal of specific amounts of sample surface material to reach subsurface objects of interest in both correlative and non-correlative workflows.

Three-dimensional high-resolution crystallographic observation of the entire volume of microstructurally small fatigue cracks in Ni-Co based superalloy

The crystallographic features of microstructurally small fatigue cracks (MSFCs) in a Ni-Co-based superalloy were analyzed by using a Xe plasma focused ion beam scanning electron microscope (PFIB-SEM) system in conjunction with electron backscatter diffraction (EBSD) analysis. The crystallographic orientation of the three-dimensional (3D) fatigue crack growth path was successfully observed using a large-volume, high-resolution 3D image. The major parts of the crack surface were close to the {111} slip plane. Our results indicated that the Mode II fatigue crack growth mechanism is locally more predominant than the general blunting-resharpening Mode I fatigue crack growth mechanism, not only in the Stage I region but also in the larger part of the Stage II region of MSFC growth process.

Large Volume 3D Electron Backscatter Diffraction Characterization of Lath Martensite in 13%Cr-4%Ni Stainless Steel by Xe Plasma FIB

Large Volume 3D Electron Backscatter Diffraction Characterization of Lath Martensite in 13%Cr-4%Ni Stainless Steel by Xe Plasma FIB

Modification of Porous Ultralow-k Film by Vacuum Ultraviolet Emission

Modification of spin-on-deposited porous PMO (periodic mesoporous organosilica) ultralow-k (ULK) SiCOH films (k = 2.33) containing both methyl terminal and methylene bridging groups by vacuum ultraviolet (VUV) emission from Xe plasma is studied. The temporal evolution of chemical composition, internal defects, and morphological properties (pore structure transformation) is studied by using Fourier transform infrared spectroscopy, in situ laser ellipsometry, spectroscopic ellipsometry, ellipsometric porosimetry (EP), positron-annihilation lifetime spectroscopy (PALS), and Doppler broadening positron-annihilation spectroscopy. Application of the different advanced diagnostics allows making conclusions on the dynamics of the chemical composition and pore structure. The time frame of the VUV exposure in the current investigation can be divided into two phases. During the first short phase, film loses almost all of its surface methyl and matrix bridging groups. An increase of material porosity due to removal of methyl groups with simultaneous matrix shrinkage is found by in situ ellipsometry. The removal of bridging bonds leads to an increase of matrix intrinsic porosity. Nevertheless, when the treated material is exposed to the ambient air, the sizes of micro- and mesopores and pores interconnectivity decrease with the VUV exposure time according to PAS and EP data. The last is the result of the additional film shrinkage caused by atmosphere exposure. During the second phase the increase of mesopore size is detected by both EP and PAS. The increase of mesopore size goes all the time as it is expected from in situ ellipsometry, but it is masked by the air exposure.

Research of continuous fiber laser sustained Xe plasma

High-speed optical inspection of wafers requires a high-brightness and broadband light source. The brightness of traditional arc discharge lamps is difficult to increase, which is limited by the cathode current density. Laser sustained plasma (LSP) is a promising method to overcome the limitations of existing light source technologies. Then the characteristics of fiber laser sustained plasma in a xenon bulb were investigated, the absorption coefficient was measured, and the spectrum was analyzed. The results show that with the increase of laser power, the volume of plasma gradually increases and moves towards the direction of laser incident. With the increase of Gaussian beam focusing number F, the threshold laser power required to maintain the stable existence of plasma decreases gradually. There are upper and lower limits for laser power at which the LSP could be sustained. When the laser incident at the optimal incident angle, the laser propagation in the plasma is the optimal path for laser energy deposition, and the plasma has higher absorption coefficient, temperature and brightness. When F ≥ 10, the Xe plasma will show spatial instability due to refraction effect. Compared with the traditional short-arc xenon lamp, the LSP spectrum has a higher proportion in short wavelength.

Three-dimensional modeling of intense unipolar THz pulses formation during their amplification in nonequilibrium extended Xe plasma channel

We develop a three-dimensional (3D) fully self-consistent model for analysis of an ultrashort THz pulse propagation and amplification in a nonequilibrium plasma channel formed in xenon by a femtosecond UV laser pulse. The model is based on the self-consistent solution of a second order wave equation in the cylindrical geometry and the kinetic Boltzmann equation for the electron velocity distribution function (EVDF) at different points of the spatially inhomogeneous nonequilibrium plasma channel. We analyze the wide range of plasma and seed pulse parameters and reveal the optimal regimes for producing high intensity outgoing THz fields as well as highly unipolar THz pulses within the proposed mechanism. It is demonstrated that the process of EVDF relaxation in plasma limits the amplification of THz pulses at the level of ∼10^{7}W/cm^{2}. Both focusing features of nonequilibrium plasma and the possibility of producing THz pulses with a high degree of unipolarity are confirmed for the case of 3D geometry.

Using Xe Plasma FIB for High-Quality TEM Sample Preparation.

A direct comparison between electron transparent transmission electron microscope (TEM) samples prepared with gallium (Ga) and xenon (Xe) focused ion beams (FIBs) is performed to determine if equivalent quality samples can be prepared with both ion species. We prepared samples using Ga FIB and Xe plasma focused ion beam (PFIB) while altering a variety of different deposition and milling parameters. The samples’ final thicknesses were evaluated using STEM-EELS t/λ data. Using the Ga FIB sample as a standard, we compared the Xe PFIB samples to the standard and to each other. We show that although the Xe PFIB sample preparation technique is quite different from the Ga FIB technique, it is possible to produce high-quality, large area TEM samples with Xe PFIB. We also describe best practices for a Xe PFIB TEM sample preparation workflow to enable consistent success for any thoughtful FIB operator. For Xe PFIB, we show that a decision must be made between the ultimate sample thickness and the size of the electron transparent region.

Effect of Xe plasma processing on characteristics of carbon nanotube-based field emission cathodes

We developed an experimental setup and procedure to investigate improving the electron emission stability in the long-term operation of carbon nanotube (CNT)-based field emission cathodes (FECs) by using Xe plasma processing aging method. To find the optimum plasma processing conditions, we varied the FEC's exposure time, plasma environment, and bias voltage. Plasma processing increased the gate voltage required for a certain number of electron emissions, whereas we confirmed that the emission stability improved and the aging effect of plasma processing. A bias voltage of approximately −15 V and a shorter exposure time were most suitable for plasma processing. Because the number of ion and electron collisions and the collision energies cause a moderate change in the CNT structure with minimal damage to the CNTs, we expect that the proposed plasma processing will improve the lifetime of CNT-based FECs, which is essential if they are to be used as electron emission sources for space propulsion systems.

Etching resistance and etching behavior of h-BN textured ceramics under Xe plasma condition

The h-BN ceramic with two-dimensional laminar structure is an important constituent material of Hall thruster channel, and the differences in its chemical bonding structures lead to anisotropy of BN grains in different orientations. In this paper, h-BN matrix textured ceramics are prepared by hot-press sintering. Xe plasma is used to etch the textured ceramics with different preparation processes and orientations. The effects of microstructure and h-BN grain orientation on plasma etching resistance of the materials are investigated. The results show that the intergranular phase has superior plasma etching resistance compared to the h-BN material. During the etching, the sample surface maintains dynamic equilibrium on the macroscopic scale, while preferential etching exists on the microscopic scale. The h-BN grains exhibit point etching, line etching and composite etching behavior during the etching process. The etching resistance of textured ceramics is related to the intrinsic plasma etching resistance of the matrix material, but less correlation to the preparation process, microstructure, and orientation of the BN grains of the matrix material, moreover, there is no anisotropic etching behavior.

Dynamic thermal behavior of polycrystalline LaB6 hollow cathodes

Lanthanum hexaboride (LaB6) hollow cathodes have demonstrated a capability for long life operation, which is critical to many space exploration missions. Thermal characterization of LaB6 hollow cathodes has revealed lower than expected electron emitter temperatures when the cathode reaches a steady state. This phenomenon is observed at discharge currents ranging from 5 to 35 A and xenon mass flow rates of 5–25 SCCM in cathodes with three different orifice diameters. Thus, the currently accepted value of the work function for polycrystalline LaB6, 2.67 eV, does not describe well the emission characteristics of LaB6 hollow cathodes operating with internal gas discharges at a steady state. We use empirically measured temperatures combined with a model of the hollow cathode emitter and xenon discharge to estimate the value of the work function, yielding a value ranging from 2.1 to 2.44 eV. This lower work function value implies that LaB6 hollow cathodes are expected to have even longer lifetimes than previously anticipated, further establishing them as a more suited alternative to other conventional cathode technologies for the task of long duration travel. Direct measurements of the work function as a function of depth on a hollow cathode emitter using x-ray photoelectron spectroscopy and ion beam milling indicate that the work function decreases with depth. We postulate several mechanisms that could explain the observed work function enhancement. Altogether, our results have important implications to the design, study approach, and operation of LaB6 cathodes and potentially other cathodes with hollow configuration. Finally, our work opens the question of why the work function is reduced upon interaction with Xe plasma.

Expanding the Capability of Xe Plasma Focused Ion Beam Sample Preparation for Transmission Electron Microscopy

Expanding the Capability of Xe Plasma Focused Ion Beam Sample Preparation for Transmission Electron Microscopy