Review of Recent Advances in Gas-Assisted Focused Ion Beam Time-of-Flight Secondary Ion Mass Spectrometry (FIB-TOF-SIMS).

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a powerful chemical characterization technique allowing for the distribution of all material components (including light and heavy elements and molecules) to be analyzed in 3D with nanoscale resolution. Furthermore, the sample's surface can be probed over a wide analytical area range (usually between 1 µm2 and 104 µm2) providing insights into local variations in sample composition, as well as giving a general overview of the sample's structure. Finally, as long as the sample's surface is flat and conductive, no additional sample preparation is needed prior to TOF-SIMS measurements. Despite many advantages, TOF-SIMS analysis can be challenging, especially in the case of weakly ionizing elements. Furthermore, mass interference, different component polarity of complex samples, and matrix effect are the main drawbacks of this technique. This implies a strong need for developing new methods, which could help improve TOF-SIMS signal quality and facilitate data interpretation. In this review, we primarily focus on gas-assisted TOF-SIMS, which has proven to have potential for overcoming most of the aforementioned difficulties. In particular, the recently proposed use of XeF2 during sample bombardment with a Ga+ primary ion beam exhibits outstanding properties, which can lead to significant positive secondary ion yield enhancement, separation of mass interference, and inversion of secondary ion charge polarity from negative to positive. The implementation of the presented experimental protocols can be easily achieved by upgrading commonly used focused ion beam/scanning electron microscopes (FIB/SEM) with a high vacuum (HV)-compatible TOF-SIMS detector and a commercial gas injection system (GIS), making it an attractive solution for both academic centers and the industrial sectors.

3D High-Resolution Chemical Characterization of Sputtered Li-Rich NMC811 Thin Films Using TOF-SIMS.

Massive demand for Li-ion batteries stimulates the research of new materials such as high-capacity cathodes, metal anodes, and solid electrolytes, which should ultimately lead to new generations of batteries such as all-solid-state batteries. Such material discovery often requires knowledge on lithium's content and local distribution in complex Li-containing systems, which is a challenging analytical task. The state-of-the-art time-of-flight secondary-ion mass spectrometry (TOF-SIMS) is one of the few chemical analysis techniques allowing for parallel detection of all sample components and representing their distributions in 3D with nanoscale resolution. In this work, we explore the outstanding potential of TOF-SIMS for comprehensive chemical and nano-/micro-structural characterization of novel Li-rich nickel manganese cobalt oxide thin films, which are potential cathode materials for the future generation batteries. Off-stoichiometric thin films of Li- and Ni-rich layered oxide with the composition of LixNi0.8Mn0.1Co0.1O2 (LR-NMC811, x > 1) were deposited using reactive magnetron sputtering. Such thin films do not contain any conductive additives or binders and therefore serve as model 2D systems to investigate compositional fluctuations, surface and interface phenomena, and their aging. TOF-SIMS revealed the presence of 400 ± 100 nm overlithiated grains and 100 ± 30 nm nanoparticles with an increased 7Li16O+ ion content in the buried part of LR-NMC811. The Li-rich agglomerates could potentially serve as Li reservoirs for compensating Li losses during cathode fabrication and cell operation. Interestingly, these sub-micron structures decomposed in time upon exposure to ambient conditions for 30 days.

In Situ Time-of-Flight Mass Spectrometry of Ionic Fragments Induced by Focused Electron Beam Irradiation: Investigation of Electron Driven Surface Chemistry inside an SEM under High Vacuum.

Recent developments in nanoprinting using focused electron beams have created a need to develop analysis methods for the products of electron-induced fragmentation of different metalorganic compounds. The original approach used here is termed focused-electron-beam-induced mass spectrometry (FEBiMS). FEBiMS enables the investigation of the fragmentation of electron-sensitive materials during irradiation within the typical primary electron beam energy range of a scanning electron microscope (0.5 to 30 keV) and high vacuum range. The method combines a typical scanning electron microscope with an ion-extractor-coupled mass spectrometer setup collecting the charged fragments generated by the focused electron beam when impinging on the substrate material. The FEBiMS of fragments obtained during 10 keV electron irradiation of grains of silver and copper carboxylates and shows that the carboxylate ligand dissociates into many smaller volatile fragments. Furthermore, in situ FEBiMS was performed on carbonyls of ruthenium (solid) and during electron-beam-induced deposition, using tungsten carbonyl (inserted via a gas injection system). Loss of carbonyl ligands was identified as the main channel of dissociation for electron irradiation of these carbonyl compounds. The presented results clearly indicate that FEBiMS analysis can be expanded to organic, inorganic, and metal organic materials used in resist lithography, ice (cryo-)lithography, and focused-electron-beam-induced deposition and becomes, thus, a valuable versatile analysis tool to study both fundamental and process parameters in these nanotechnology fields.

Detection of Au+ Ions During Fluorine Gas-Assisted Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) for the Complete Elemental Characterization of Microbatteries.

Due to excellent electric conductivity and chemical inertness, Au can be used in new microdevices for energy applications, microelectronics, and biomedical solutions. However, the chemical analysis of Au-containing systems using time-of-flight secondary ion mass spectrometry (TOF-SIMS) can be difficult because of the negative ionization of Au, as most metals form positive ions, and therefore cannot be detected from the same analytical volume. In this work, we present the potential of fluorine gas coinjection for altering the polarity, from the negative to positive, of Au secondary ions generated under Ga+ beam bombardment. The importance of detecting Au+ ions and representing their spatial distribution in nanoscale was demonstrated using a novel solid electrolyte for Li-ion solid-state batteries, amorphous Li7La3Zr2O12 (aLLZO). This allowed for assessing the migration of mobile Li+ ions outside the aLLZO layer and alloying the Au layer with Li, which explained the presence of an internal electric field observed during the polarization measurements. Remarkably, during fluorine gas-assisted TOF-SIMS measurements, the trace amount of Au content (5 ppm) was detected in a Pt layer (unattainable under standard vacuum conditions). In conclusion, fluorine gas-assisted TOF-SIMS can help understanding operation mechanisms and potential degradation processes of microdevices and therefore help optimizing their functionality.

Mechanisms of Fluorine-Induced Separation of Mass Interference during TOF-SIMS Analysis.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is one of very few analytical techniques allowing sample chemical structure to be characterized in three-dimensional (3D) with nanometer resolution. Due to the excellent sensitivity in the order of ppm-ppb and capability of detecting all ionized elements and molecules, TOF-SIMS finds many applications for analyzing nanoparticle-containing systems and thin films used in microdevices for new energy applications, microelectronics, and biomedicine. However, one of the main drawbacks of this technique is potential mass interference between ions having the same or similar masses, which can lead to data misinterpretation. In this work, we present that this problem can be easily solved by delivering fluorine gas to a sample surface during TOF-SIMS analysis and we propose mechanisms driving this phenomenon. Our comprehensive studies, conducted on complex thin films made of highly mass-interfering elements, show that fluorine modifies the ionization process, leading to element-specific changes of ion yields (which can vary by several orders of magnitude), and affects the efficiency of metal hydride and oxide formation. In conjunction, these two effects can efficiently induce separation of mass interference, providing more representative TOF-SIMS data with respect to the sample composition and significant enhancement of chemical image resolution. Consequently, this can improve the chemical characterization of complex multilayers in nanoscale.

High Sensitivity of Fluorine Gas-Assisted FIB-TOF-SIMS for Chemical Characterization of Buried Sublayers in Thin Films.

In this work, we present the potential of high vacuum-compatible time-of-flight secondary ion mass spectrometry (TOF-SIMS) detectors, which can be integrated within focused ion beam (FIB) instruments for precise and fast chemical characterization of thin films buried deep under the sample surface. This is demonstrated on complex multilayer systems composed of alternating ceramic and metallic layers with thicknesses varying from several nanometers to hundreds of nanometers. The typical problems of the TOF-SIMS technique, that is, low secondary ion signals and mass interference between ions having similar masses, were solved using a novel approach of co-injecting fluorine gas during the sample surface sputtering. In the most extreme case of the Al/Al2O3/Al/Al2O3/.../Al sample, a <10 nm thick Al2O3 thin film buried under a 0.5 µm material was detected and spatially resolved using only 27Al+ signal distribution. This is an impressive achievement taking into account that Al and Al2O3 layers varied only by a small amount of oxygen content. Due to its high sensitivity, fluorine gas-assisted FIB-TOF-SIMS can be used for quality control of nano- and microdevices as well as for the failure analysis of fabrication processes. Therefore, it is expected to play an important role in the development of microelectronics and thin-film-based devices for energy applications.

Elemental Characterization of Al Nanoparticles Buried under a Cu Thin Film: TOF-SIMS vs STEM/EDX.

In this work, we present a comprehensive comparison of time-of-flight secondary ion mass spectrometry (TOF-SIMS) and scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy (STEM/EDX), which are currently the most powerful elemental characterization techniques in the nano- and microscale. The potential and limitations of these methods are verified using a novel dedicated model sample consisting of Al nanoparticles buried under a 50 nm thick Cu thin film. The sample design based on the low concentration of nanoparticles allowed us to demonstrate the capability of TOF-SIMS to spatially resolve individual tens of nanometer large nanoparticles under ultrahigh vacuum (UHV) as well as high vacuum (HV) conditions. This is a remarkable achievement especially taking into account the very small quantities of the investigated Al content. Moreover, the imposed restriction on the Al nanoparticle location, i.e., only on the sample substrate, enabled us to prove that the measured Al signal represents the real distribution of Al nanoparticles and does not originate from the artifacts induced by the surface topology. The provided comparison of TOF-SIMS and STEM/EDX characteristics delivers guidelines for choosing the most optimal method for efficient characterization of nano-objects.

Fluorine Gas Coinjection as a Solution for Enhancing Spatial Resolution of Time-of-Flight Secondary Ion Mass Spectrometry and Separating Mass Interference.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) detectors have been intensively developed in recent decades due to their unprecedented capability of representing a sample elemental composition in a three-dimensional space from nano- to submilliscale with high spatial resolution and mass resolution. A compact high-vacuum-compatible version of these detectors can be integrated into a focused ion beam (FIB) system which, assembled with scanning electron microscopy (SEM), is the most popular tool used in nanotechnology and material science. This gives a new opportunity for combining TOF-SIMS analysis with other instruments within the same analytical chamber. In this work we present the results of conducting elemental characterization of a dedicated model multilayer sample composed of 100 nm thick thin films of Cu, Zr, and ZrCuAg alloy in a fluorine gas atmosphere provided by an in situ gas injection system (GIS). In general, the secondary ion signals were significantly enhanced by up to 3 orders of magnitude, leading to much higher spatial resolution. The quality of elemental images and depth profiles was improved during a single measurement (which usually cannot be obtained at standard vacuum conditions) at a high beam energy of 20 keV. Moreover, fluorine assistance has enabled a mass interference between 107Ag+ and 91Zr16O+ ions to be separated. This remarkable finding has never been reported before and is expected to play an important role in the future evolution of TOF-SIMS analytical protocols, as currently the mass interference between ions remains one of the main drawbacks of the technique.

3D Imaging of Nanoparticles in an Inorganic Matrix Using TOF-SIMS Validated with STEM and EDX.

Imaging nano-objects in complex systems such as nanocomposites using time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a challenging task. Due to a very small amount of the material and a matrix effect, the number of generated secondary ions can be insufficient to represent a 3D elemental distribution despite being detected in a mass spectrum. Therefore, a model sample consisting of a ZrCuAg matrix with embedded Al nanoparticles is designed. A high mass difference between the light Al and heavy matrix components limits mass interference. The chemical structure measurements using a pulsed 60 keV Bi32+ beam or a continuous 30 keV Ga+ beam reveals distinct Al signal segregation. This can indicate a spatially resolved detection of single 10s of nanometer large particles and/or their agglomerates for the first time. However, TOF-SIMS images of 50 nm or smaller objects do not necessarily represent their exact size and shape but can rather be their convolutions with the primary ion beam shape. Therefore, the size of nanoparticles (25-64 nm) was measured using scanning transmission electron microscopy. Our studies prove the capability of TOF-SIMS to image chemical structure of nanohybrids which is expected to help building new functional materials and optimize their properties.

Application of a Gas-Injection System during the FIB-TOF-SIMS Analysis-Influence of Water Vapor and Fluorine Gas on Secondary Ion Signals and Sputtering Rates.

Combining a Gas-Injection System (GIS) with the Focused Ion Beam (FIB) has a broad scope of applications in sample preparation such as protective layer deposition, increasing material sputtering rates, and reducing FIB-related artifacts. On the other hand, injecting certain specific gases during a Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) analysis can significantly increase element ionization probability and, therefore, improve the quality of 3D representation of a sample elemental structure. In this work, for the first time, the potential of GIS for enhancing secondary ion signals acquired using a TOF detector incorporated into a commercial Ga+ FIB-SEM (Focused Ion Beam combined with Scanning Electron Microscope) instrument is presented. The depth profiles of pure metals (thin films of Cu, Zr, Ag, and W with the thickness in the order of 100 nm) were acquired under ambient vacuum conditions as well as under an exposure to water and fluorine gases. The influence of supplementary gases on the ion yields and sputtering rates was studied. Simulations were performed to assess the local gas pressure at the location of FIB-TOF-SIMS analysis. The highest enhancement of ionization probability was achieved in the case of the Cu thin film (10 times during water vapor coinjection and 510 times when using a fluorine gas). Regarding the sputtering rates, the response of Zr to the effect of the gases was the strongest. Compared to standard background pressure measurements, this thin film was milled around 6 times faster under exposure to water vapor and over 2 times faster when fluorine gas was supplied.

Application of a novel compact Cs evaporator prototype for enhancing negative ion yields during FIB-TOF-SIMS analysis in high vacuum.

The potential of a novel caesium evaporator for enhancing negative secondary ion yields during Focused Ion Beam Time-Of-Flight Secondary Ion Mass Spectrometry (FIB-TOF-SIMS) elemental analysis is presented. The design of the lab prototype Cs evaporator is based on alkali metal dispensers containing a stable Cs salt and therefore it can be safely used for FIB-TOF-SIMS measurements under high vacuum conditions. Continuous in-situ Cs0 deposition on an Au sample surface has allowed the TOF-SIMS signal to be increased by a factor of 260, which confirms the functionality of the technique. A series of tests were conducted in positive and negative ion detection modes to study the response of the system to currents applied on Cs dispensers and generated Joule heating. A stable Cs flux (represented by a Cs+ signal measured with the TOF-SIMS) was achieved over around 1.5 h when using two Cs dispensers simultaneously. Moreover, the presence of Cs0 has enabled the stability of an enhanced Si- signal measured by the TOF-SIMS to be maintained over several hours. This time is much longer than a typical duration of FIB-TOF-SIMS depth profiling, thus the presented method exhibits the potential for studying heterogeneous and multi-layer structures in a three-dimensional space.

Voids and compositional inhomogeneities in Cu(In,Ga)Se2 thin films: evolution during growth and impact on solar cell performance.

Structural defects such as voids and compositional inhomogeneities may affect the performance of Cu(In,Ga)Se2 (CIGS) solar cells. We analyzed the morphology and elemental distributions in co-evaporated CIGS thin films at the different stages of the CIGS growth by energy-dispersive x-ray spectroscopy in a transmission electron microscope. Accumulation of Cu-Se phases was found at crevices and at grain boundaries after the Cu-rich intermediate stage of the CIGS deposition sequence. It was found, that voids are caused by Cu out-diffusion from crevices and GBs during the final deposition stage. The Cu inhomogeneities lead to non-uniform diffusivities of In and Ga, resulting in lateral inhomogeneities of the In and Ga distribution. Two and three-dimensional simulations were used to investigate the impact of the inhomogeneities and voids on the solar cell performance. A significant impact of voids was found, indicating that the unpassivated voids reduce the open-circuit voltage and fill factor due to the introduction of free surfaces with high recombination velocities close to the CIGS/CdS junction. We thus suggest that voids, and possibly inhomogeneities, limit the efficiency of solar cells based on three-stage co-evaporated CIGS thin films. Passivation of the voids' internal surface may reduce their detrimental effects.

A novel PFIB sample preparation protocol for correlative 3D X-ray CNT and FIB-TOF-SIMS tomography.

We present a novel sample preparation method that allows correlative 3D X-ray Computed Nano-Tomography (CNT) and Focused Ion Beam Time-Of-Flight Secondary Ion Mass Spectrometry (FIB-TOF-SIMS) tomography to be performed on the same sample. In addition, our invention ensures that samples stay unmodified structurally and chemically between the subsequent experiments. The main principle is based on modifying the topography of the X-ray CNT experimental setup before FIB-TOF-SIMS measurements by incorporating a square washer around the sample. This affects the distribution of extraction field lines and therefore influences the trajectories of secondary ions that are now guided more efficiently towards the detector. As the result, secondary ion detection is significantly improved and higher, i.e. statistically better, signals are obtained.

State-of-the-Art Three-Dimensional Chemical Characterization of Solid Oxide Fuel Cell Using Focused Ion Beam Time-of-Flight Secondary Ion Mass Spectrometry Tomography.

In this paper the potential of time-of-flight secondary ion mass spectroscopy combined with focused ion beam technology to characterize the composition of a solid oxide fuel cell (SOFC) in three-dimension is demonstrated. The very high sensitivity of this method allows even very small amounts of elements/compounds to be detected and localized. Therefore, interlayer diffusion of elements between porous electrodes and presence of pollutants can be analyzed with a spatial resolution of the order of 100 nm. However, proper element recognition and mass interference still remain important issues. Here, we present a complete elemental analysis of the SOFC as well as techniques that help to validate the reliability of obtained results. A discussion on origins of probable artifacts is provided.