Broadening the applications of the atom probe technique by ultraviolet femtosecond laser

Laser assisted field evaporation using ultraviolet (UV) wavelength gives rise to better mass resolution and signal-to-noise ratio in atom probe mass spectra of metals, semiconductors and insulators compared to infrared and green lasers. Combined with the site specific specimen preparation techniques using the lift-out and annular Ga ion milling in a focused ion beam machine, a wide variety of materials including insulating oxides can be quantitatively analyzed by the three-dimensional atom probe using UV laser assisted field evaporation. After discussing laser irradiation conditions for optimized atom probe analyses, recent atom probe tomography results on oxides, semiconductor devices and grain boundaries of sintered magnets are presented.     

Microscopy and microanalysis of complex nanosized strengthening precipitates in new generation commercial Al–Cu–Li alloys

Precipitates (ppts) in new generation aluminum–lithium alloys (AA2099 and AA2199) were characterised using scanning and transmission electron microscopy and atom probe tomography. Results obtained on the following ppts are reported: Guinier–Preston zones, T₁ (Al₂CuLi), β’ (Al₃Zr) and δ’ (Al₃Li). The focus was placed on their composition and the presence of minor elements. X‐ray energy‐dispersive spectrometry in the electron microscopes and mass spectrometry in the atom probe microscope showed that T₁ ppts were enriched in zinc (Zn) and magnesium up to about 1.9 and 3.5 at.%, respectively. A concentration of 2.5 at.% Zn in the δ’ ppts was also measured. Unlike Li and copper, Zn in the T₁ ppts could not be detected using electron energy‐loss spectroscopy in the transmission electron microscope because of its too low concentration and the small sizes of these ppts. Indeed, Monte Carlo simulations of EEL spectra for the Zn L_2,3 edge showed that the signal‐to‐noise ratio was not high enough and that the detection limit was at least 2.5 at.%, depending on the probe current. Also, the simulation of X‐ray spectra confirmed that the detection limit was exceeded for the Zn K_α X‐ray line because the signal‐to‐noise ratio was high enough in that case, which is in agreement with our observations.     

Some aspects of the field evaporation behaviour of GaSb

In-depth analysis of pulsed laser atom probe tomography (APT) data on the field evaporation of the III-V semiconductor material GaSb reveals strong variations in charge states, relative abundances of different cluster ions, multiplicity of detector events and spatial correlation of evaporation events, as a function of the effective electric field at the specimen surface. These variations are discussed in comparison with the behaviour of two different metallic specimen materials, an Al-6XXX series alloy and pure W, studied under closely related experimental conditions in the same atom probe instrument. It is proposed that the complex behaviour of GaSb originates from a combination of spatially correlated evaporation events and the subsequent field induced dissociation of cluster ions, the latter contributing to inaccuracies in the overall atom probe composition determination for this material.     

Automated focusing in bright-field microscopy for tuberculosis detection

Pulsed‐laser atom‐probe tomography is used to compare the field‐evaporation mass spectrum and spatial distribution of molecular fragments from various poly(3‐alkylthiophene) films deposited on sharpened aluminium specimen carriers using two different deposition methods. Films deposited via a modified solution‐cast methodology yield small fragments with a uniform structural morphology whereas films deposited via an electrospray ionization methodology yield a wide range of fragments with a very non‐uniform structural morphology. The main field‐evaporated chemical species identified for both deposition types were, in order of typical relative abundance, C₂H₅⁺, CH₃⁺, C₂H₄⁺, followed by C₃H_7,8⁺/SC⁺ and SCH⁺. Thick electrospray depositions allowed investigation of the influence of laser‐pulse energy on the analysis. Evidence is presented supporting the presence of a critical laser‐pulse energy whereby changes in film morphology are signalled by the appearance of a new mass fragment at 190 Da.     

Towards the imaging of Weibel–Palade body biogenesis by serial block face‐scanning electron microscopy

Electron microscopy is used in biological research to study the ultrastructure at high resolution to obtain information on specific cellular processes. Serial block face‐scanning electron microscopy is a relatively novel electron microscopy imaging technique that allows three‐dimensional characterization of the ultrastructure in both tissues and cells by measuring volumes of thousands of cubic micrometres yet at nanometre‐scale resolution. In the scanning electron microscope, repeatedly an image is acquired followed by the removal of a thin layer resin embedded biological material by either a microtome or a focused ion beam. In this way, each recorded image contains novel structural information which can be used for three‐dimensional analysis. Here, we explore focused ion beam facilitated serial block face‐scanning electron microscopy to study the endothelial cell–specific storage organelles, the Weibel–Palade bodies, during their biogenesis at the Golgi apparatus. Weibel–Palade bodies predominantly contain the coagulation protein Von Willebrand factor which is secreted by the cell upon vascular damage. Using focused ion beam facilitated serial block face‐scanning electron microscopy we show that the technique has the sensitivity to clearly reveal subcellular details like mitochondrial cristae and small vesicles with a diameter of about 50 nm. Also, we reveal numerous associations between Weibel–Palade bodies and Golgi stacks which became conceivable in large‐scale three‐dimensional data. We demonstrate that serial block face‐scanning electron microscopy is a promising tool that offers an alternative for electron tomography to study subcellular organelle interactions in the context of a complete cell.     

Laser‐induced reversion of precipitates in an Al‐Li alloy: Study on temperature rise in pulsed laser atom probe

The influence of tuning the laser pulse energy during the analyses on the resulting microstructure in a specimen utilizing an ultra‐fast laser assisted atom probe was demonstrated by a case study of a binary Al‐Li alloy. The decomposition parameters, such as the size, number density, volume fraction, and composition of precipitates, were carefully monitored after each analysis. A simple model was employed to estimate the corresponding specimen temperature for each value of the laser energy. The results indicated that the corresponding temperatures for the laser pulse energy in the range of 10 to 80 pJ are located inside the miscibility gap of the binary Al‐Li phase diagram and fall into the metastable equilibrium field. In addition, the corresponding temperature for a laser pulse energy of 100 pJ was in fairly good agreement with reported range of solvus temperature, suggesting a result of reversion upon heating due to laser pulsing. Microsc. Res. Tech. 79:727–737, 2016. © 2016 Wiley Periodicals, Inc.     

Optimization of pulsed laser atom probe (PLAP) for the analysis of nanocomposite Ti–Si–N films

Laser-assisted atom probe tomography was used to investigate the nanostructure and composition of high-performance, ultra-hard Ti–Si–N nanocomposite films. However, the quality of data is heavily dependent on analysis conditions. In order to obtain reliable data from these, and other ‘less conducting’ specimens, the analysis parameter space was thoroughly investigated to optimize the mass resolution and hit multiplicity obtained in atom probe tomography. Geometric factors including tip radius and shank angle were found to play a significant role in mass resolution but had no apparent effect on the number of multiple hits observed. Increased laser energy led to a gradual increase in the number of single hits, but a modest improvement in mass resolution. The influence of other instrumental factors including detection rate and base temperature was investigated separately. Preliminary PLAP results are presented, and correlated with TEM analysis of the microstructure of the film.     

Impact of laser pulsing on the reconstruction in an atom probe tomography

The implementation of fast pulsed laser has significantly improved the performance of the atom probe technique by enabling near-atomic-scale three-dimensional analysis of poorly conducting materials. This has broadened the range of applications for the atom probe, addressing a major limitation of the technique. Despite this, the implications of lasing on the tomographic reconstruction of atom probe data have yet to be fully characterised. Here, we demonstrate how changes in the shape of the specimen surface, induced by laser pulsing, affect the ion trajectories, and hence the projection parameters used to build the three-dimensional map.     

Pulsed laser atom probe analysis of semiconductor materials

As the size of semiconductor devices is reduced the active volumes of material in each device is also decreased. Under these circumstances it becomes more important to understand the microchemistry of semiconducting materials, as small fluctuations in composition can dramatically affect both the operation of the devices, and of the contacts to semiconductors. Atom probe microanalysis has been shown to be able to analyse the microchemistry of metallic materials with plane-by-plane resolution, and by using a pulsed laser to replace the more conventional voltage pulses the analysis of semiconducting and insulating materials becomes possible. The pulsed laser atom probe has been shown to give very accurate chemical analysis of the stoichiometry of extremely small volumes of III-V compound semiconductors, and the composition of the interfacial layer between silicon dioxide and silicon has been identified as SiO of thickness about 0.3 nm. It has been shown to be possible to prepare specimens for analysis from thin films of semiconductors, thus allowing the microanalysis of a wide range of materials that are deposited in thin film form.     

Materials analysis with a position-sensitive atom probe

A position-sensitive detector has been combined with time-of-flight mass spectrometry in the atom probe field-ion microscope to yield a system in which both chemical identity and spatial information are obtained for individual ions field-evaporated from the specimen surface. This allows the variations in composition originally present in the sample to be reconstructed in 3-D with sub-nanometre resolution. The prototype position-sensitive atom probe is being used to study phase chemistry in a number of metallurgical alloys, including accurate composition determination of 1–2 nm Cu-rich precipitates formed in Fe–1.3% Cu–1.4%Ni aged to peak hardness. Other applications of the position-sensitive atom probe (POSAP) include the analysis of surface layers on superconductors and atom probe studies of semiconductor multiple quantum wells. These initial applications of the instrument are reported, and the limitations and intended improvements to the instrument are discussed.     

Time‐of‐flight mass spectrometry: Introduction to the basics

The intention of this tutorial is to introduce into the basic concepts of time‐of‐flight mass spectrometry, beginning with the most simple single‐stage ion source with linear field‐free drift region and continuing with two‐stage ion sources combined with field‐free drift regions and ion reflectors—the so‐called reflectrons. Basic formulas are presented and discussed with the focus on understanding the physical relations of geometric and electric parameters, initial distribution of ionic parameters, ion flight times, and ion flight time incertitude. This tutorial is aimed to help the applicant to identify sources of flight time broadening which limit good mass resolution and sources of ion losses which limit sensitivity; it is aimed to stimulate creativity for new experimental approaches by discussing a choice of instrumental options and to encourage those who toy with the idea to build an own time‐of‐flight mass spectrometer. Large parts of mathematics are shifted into a separate chapter in order not to overburden the text with too many mathematical deviations. Rather, thumb‐rule formulas are supplied for first estimations of geometry and potentials when designing a home‐built instrument, planning experiments, or searching for sources of flight time broadening. © 2016 Wiley Periodicals, Inc. Mass Spec Rev 36:86–109, 2017.     

Investigations of field-evaporated end forms in voltage- and laser-pulsed atom probe tomography

Atom Probe Tomography (APT) consists of analyzing a needle-shaped specimen on an atom-by-atom basis. In recent years, instruments have become commercially available, enabling the sequential analysis of the same specimen in both laser- and voltage-pulsed modes. In this contribution, a comparison of field evaporated end-forms as a function of the voltage and laser power is presented for silicon. Electron microscopy is utilized for visual inspection of the final tip end-forms. The field of evaporation for silicon is calculated based on these radius measurements for voltage and laser pulsing. Electron microscopy and analysis of the atom probe data show that the specimen end-forms for both pulsing modes can be different. We have observed two effects on the shape of a field-ion emitter when irradiated by a focused laser beam. One is a change in the 3-dimensional topology of the emitter due to different crystallographic orientations. Secondly, exposure to focused laser beam from one side may lead to a non-hemispherical tip shape especially when reasonably high laser energy is utilized. For comparison purposes to the laser mode, the voltage pulse evaporated tip end form is also analyzed for different specimen temperatures. Consequently, evaporation fields are calculated for different temperatures and laser conditions for silicon.     

Ion mobility spectrometry‐mass spectrometry (IMS‐MS) of small molecules: Separating and assigning structures to ions

The phenomenon of ion mobility (IM), the movement/transport of charged particles under the influence of an electric field, was first observed in the early 20th Century and harnessed later in ion mobility spectrometry (IMS). There have been rapid advances in instrumental design, experimental methods, and theory together with contributions from computational chemistry and gas‐phase ion chemistry, which have diversified the range of potential applications of contemporary IMS techniques. Whilst IMS‐mass spectrometry (IMS‐MS) has recently been recognized for having significant research/applied industrial potential and encompasses multi‐/cross‐disciplinary areas of science, the applications and impact from decades of research are only now beginning to be utilized for “small molecule” species. This review focuses on the application of IMS‐MS to “small molecule” species typically used in drug discovery (100–500 Da) including an assessment of the limitations and possibilities of the technique. Potential future developments in instrumental design, experimental methods, and applications are addressed. The typical application of IMS‐MS in relation to small molecules has been to separate species in fairly uniform molecular classes such as mixture analysis, including metabolites. Separation of similar species has historically been challenging using IMS as the resolving power, R, has been low (3–100) and the differences in collision cross‐sections that could be measured have been relatively small, so instrument and method development has often focused on increasing resolving power. However, IMS‐MS has a range of other potential applications that are examined in this review where it displays unique advantages, including: determination of small molecule structure from drift time, “small molecule” separation in achiral and chiral mixtures, improvement in selectivity, identification of carbohydrate isomers, metabonomics, and for understanding the size and shape of small molecules. This review provides a broad but selective overview of current literature, concentrating on IMS‐MS, not solely IMS, and small molecule applications. © 2012 Wiley Periodicals, Inc., Mass Spec Rev 32:43–71, 2013     

太阳能级晶体硅中杂质的质谱检测方法

太阳能级晶体硅材料中杂质的种类和含量直接影响太阳能电池的发电效率,因此硅材料中杂质含量和分布的检测至关重要。当今社会光伏产业的迅猛发展,推进了晶体硅检测技术的更新和发展。本文总结了近年来在硅材料杂质检测中所使用的方法以及这些方法的不足;比较了电感耦合等离子体质谱（ICP-MS）、辉光放电质谱（GDMS）、二次离子质谱（SIMS）和激光电离质谱（LIMS）四种可用于太阳能级晶体硅检测的原理和优缺点。     

Ion spectrometric detection technologies for ultra‐traces of explosives: A review

In recent years, explosive materials have been widely employed for various military applications and civilian conflicts; their use for hostile purposes has increased considerably. The detection of different kind of explosive agents has become crucially important for protection of human lives, infrastructures, and properties. Moreover, both the environmental aspects such as the risk of soil and water contamination and health risks related to the release of explosive particles need to be taken into account. For these reasons, there is a growing need to develop analyzing methods which are faster and more sensitive for detecting explosives. The detection techniques of the explosive materials should ideally serve fast real‐time analysis in high accuracy and resolution from a minimal quantity of explosive without involving complicated sample preparation. The performance of the in‐field analysis of extremely hazardous material has to be user‐friendly and safe for operators. The two closely related ion spectrometric methods used in explosive analyses include mass spectrometry (MS) and ion mobility spectrometry (IMS). The four requirements—speed, selectivity, sensitivity, and sampling—are fulfilled with both of these methods. © 2011 Wiley Periodicals, Inc., Mass Spec Rev 30:940–973, 2011     

Investigation of the analysis parameters and background subtraction for high-k materials with atom probe tomography

In this paper we present depth profiles of a high-k layer consisting of HfO₂ with an embedded sub-nm thick ZrO₂ layer obtained with atom probe tomography (APT). In order to determine suitable measurement parameters for reliable, reproducible, and quantitative analysis, we have investigated the influence of the laser energy and the specimen temperature on the resulting elemental composition. In addition we devise a procedure for local background subtraction both for the composition and the depth scale that is crucial for gaining reproducible results. We find that the composition of the high-k material remains unaffected even for extreme laser energies and base temperatures, while higher laser energies lead to an accumulation of silicon at the upper interface of the high-k layer. Furthermore we show that APT is capable of providing sub-nm depth resolution for high-k materials with high reproducibility, good compositional accuracy, and high measurement yield.     

Quantitative APT analysis of Ti(C,N)

A specially produced Ti(C,N) standard material, with a known nominal composition, was investigated with laser assisted atom probe tomography. The occurrence of molecular ions and single/multiple events was found to be influenced by the laser pulse energy, and especially C related events were affected. Primarily two issues were considered when the composition of Ti(C,N) was determined. The first one is connected to detector efficiency, due to the detector dead-time. The second one is connected to peak overlap in the mass spectrum. A method is proposed for quantification of the C content in order to establish the C/N ratio. A correction was made to the major C peaks, C at 6 and 12 Da, with the ¹³C isotopes, at 6.5 and 13 Da, according to the known natural abundance. In addition, a correction of the peak at 24 Da, where C and Ti overlap, is proposed based on the occurrence of single/multiple events for respective element. The results were compared to the results from other techniques such as electron energy loss spectroscopy, chemical analysis and X-ray diffraction. After applying the corrections, atom probe tomography results were satisfactory. Furthermore, the content of dissolved O in Ti(C,N) was successfully quantified.     

Laser assisted atom probe analysis of thin film on insulating substrate

We demonstrate that the atom probe analyses of metallic thin films on insulating substrates are possible using laser assisted field evaporation. The tips with metallic thin film and insulating substrate (0.6-3 μm in thickness) were prepared by the lift-out and annular ion beam milling techniques on tungsten supports. In spite of the existence of thick insulating layer between the metallic film and the tungsten support, atom probe tomography with practical mass resolution, signal-to-noise ratio and spatial resolution was found to be possible using laser assisted field evaporation.     

Effect of laser power and specimen temperature on atom probe analyses of magnesium alloys

The influence of laser power, wave length, and specimen temperature on laser assisted atom probe analyses for Mg alloys was investigated. Higher laser power and lower specimen temperature led to improved mass and spatial resolutions. Background noise and mass resolutions were degraded with lower laser power and higher specimen temperature. By adjusting the conditions for laser assisted atom probe analyses, atom probe results with atomic layer resolutions were obtained from all the Mg alloys so far investigated. Laser assisted atom probe investigations revealed detailed chemical information on Guinier-Preston zones in Mg alloys.