Three-dimensional atom probe analysis of heavily drawn steel wires by probing perpendicular to the pearlitic lamellae

Three-dimensional atom probe analysis of heavily drawn pearlitic steel wires was conducted by probing in the direction perpendicular to the pearlitic lamellae. A needle tip perpendicularly intersecting the lamellar structure was prepared using focused ion beam (FIB) milling combined with the lift-out method. The specimen preparation technique enabled the analysis of many lamellae and their interfaces with higher depth spatial resolution, which was suitable for drawn pearlitic steel wires having inhomogeneous and fine lamellar structure. Carbon concentration peaks at the cementite lamellae appeared higher and narrower than those obtained by probing parallel to the lamellae, which implies that the conventional analysis overestimated the extent of cementite decomposition in drawn pearlitic steel wires.     

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.     

On the many advantages of local-electrode atom probes

Local extraction electrodes offer several crucial advantages for operation of atom probes. Because of the proximity of the local extraction electrode to the specimen, the electric field produced at the specimen apex by a given voltage is enhanced and the voltage required for field evaporation is reduced. In a voltage-pulsed atom probe, the absolute magnitude of the energy uncertainty is correspondingly reduced. High mass resolution (m/deltam > 1000) may therefore be obtained by accelerating the evaporated ions to a greater total potential after the local extraction electrode. The low extraction voltage may also be pulsed rapidly (100 ps rise time) and at high repetition rates (up to 10(5) pulses per second) using currently available solid-state pulsers. Furthermore, a local electrode and intermediate electrodes may be used as optical elements to control the image magnification. All of these benefits may be applied to any type of atom probe. Local-electrode atom probes (LEAP) should be especially advantageous for developing three-dimensional atom probes with high mass resolution and a large field of view. A sample has been developed that consists of many microtips formed on a planar sample using ion beam mask etching. Microtip samples are especially suited to LEAP. Analysis of electrically insulating samples may also be possible with microtip samples in a LEAP. This combination of features suggests flexible, high speed, high mass resolution atom probes that can work with either conventional needle-shaped specimens or the new style of planar microtip specimens.     

A study of the anomalous field evaporation of Sm(Co0.68 Fe0.20 Cu0.10 Zr0.02)7.5 alloy by 3D atom probe

3D atom probe analysis of the composition of a Sm(Co0.68 Fe0.20 Cu0.10 Zr0.02)7.5 alloy was conducted by varying the probing temperature from 10.6 to 65 K and pulse fraction from 10% to 20%. It was found that the preferential evaporation of Sm occurred at 65 K, due to the very low evaporation field of Sm, 15.2 V/nm calculated by using the charge exchange model. With decreasing the specimen temperature, preferential evaporation of Sm was alleviated. The optimum analysis conditions which give reasonably good measurement of the composition were: the specimen temperature of 20 K and a pulse fraction 15%. The effects of the specimen temperature and pulse fraction on the measured composition of the alloy are discussed, based on the charge exchange model.     

Shaping the lens of the atom probe: fabrication of site specific, oriented specimens and application to grain boundary analysis

The random sampling provided by classical atom probe sample preparation methods is one of the major factors limiting the types of problems that can be addressed using this powerful technique. A focused ion beam enables not only site-specific preparation, but can also be used to give the specimen, which acts as the lens in an atom probe experiment, a specific shape. In this paper we present a technique that uses low accelerating voltages (10 and 5 kV) in the focused ion beam (FIB) to reproducibly produce specimens with selected grain boundaries <100 nm from the tip at any desired orientation. These tips have a high rate of successfully running in the atom probe and no Ga contamination within the region of interest.This technique is applied to the analysis of grain boundaries in a high purity iron wire and a strip-cast steel. Lattice resolution is achieved around the boundary in certain areas. Reconstruction of these datasets reveals the distribution of light and heavy elements around the boundary. Issues surrounding the uneven distribution of certain solute elements as a result of field-induced diffusion are discussed.     

Characterization of dilute species within CVD‐grown silicon nanowires doped using trimethylboron: protected lift‐out specimen preparation for atom probe tomography

Three‐dimensional quantitative compositional analysis of nanowires is a challenge for standard techniques such as secondary ion mass spectrometry because of specimen size and geometry considerations; however, it is precisely the size and geometry of nanowires that makes them attractive candidates for analysis via atom probe tomography. The resulting boron composition of various trimethylboron vapour–liquid–solid grown silicon nanowires were measured both with time‐of‐flight secondary ion mass spectrometry and pulsed‐laser atom probe tomography. Both characterization techniques yielded similar results for relative composition. Specialized specimen preparation for pulsed‐laser atom probe tomography was utilized and is described in detail whereby individual silicon nanowires are first protected, then lifted out, trimmed, and finally wet etched to remove the protective layer for subsequent three‐dimensional analysis.     

Field evaporation behaviour in the gamma phase in Ti-Al during analysis in the tomographic atom probe

A Ti-48 at% Al alloy has been successfully investigated, using atom probe field ion microscopy and transmission electron microscopy. After a specific heat treatment, this alloy has a (alpha2 + gamma) lamellar microstructure. Using the tomographic atom probe (TAP), it has been possible to image the stacking of superlattice planes of gamma and to identify titanium as the highest evaporation field element. In addition, the influence of analysis site on atom probe measurements has been estimated for this phase. A TAP analysis has also made it possible to observe an extremely thin step along a gamma/gamma interface at a near atomic scale.     

The study of quantitativeness in atom probe analysis of alloying elements in steel

The quantitativeness in atom probe analysis of dilute solute alloying elements in steel was systematically investigated. The samples of binary Fe–Si, Fe–Ti, Fe–Cr, Fe–Cu, Fe–Mn and Fe–Mo alloys were prepared for present study. The apparent compositions of alloying elements were examined by three-dimensional atom probe (3DAP) under various experimental conditions. The temperature dependence of the apparent compositions varied largely with the alloys, which indicated that the degree of preferential evaporation or retention varied with the alloying elements. Furthermore, the analysis direction dependence of the apparent Mn composition was examined in the Fe–Mn alloy. The experimental results indicated that the order of the field evaporation rate of elements in steel was Cu>Cr>Mn∼Mo>Fe>Ti∼Si. The field evaporability of alloying elements in steel was discussed in terms of the solution enthalpy of the alloying elements and the phase types of the binary Fe alloys.     

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.     

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.     

Applications of scanning optical microscopy in materials science to detect bulk microdefects in semiconductors

A three-dimensional atom probe permits the elemental reconstruction of a small volume of a specimen by determining the x , y and z positions and mass-to-charge ratio of the atoms in that volume. The historical development of this new type of atom probe is described. Several variants of these instruments including the position-sensitive atom probe, the optical atom probe and the tomographic atom probe are reviewed. The various methods of data visualization and analysis are summarized. The performance of the three-dimensional atom probe is compared with the energy-compensated atom probe.     

Atom probe specimen fabrication methods using a dual FIB/SEM

A dual FIB/SEM provides solutions to many challenges in atom probe specimen preparation. When combined with an in situ lift-out capability, the versatility of this tool allows almost any region of interest, in almost any geometry, to be placed at the apex of a specimen tip. Several preparation techniques have been developed in response to specific application requirements; for example, in cases where materials are not suitable for electropolishing, or where site-specific analysis is required. Two general techniques, with wide-ranging potential applications, are described in detail here. The first is a 'cut-out' technique that provides a relatively quick means of micro-tip specimen preparation from bulk material samples. The second method is a 'lift-out' technique that can be used in an in situ or ex situ mode and does not require the preparation of pre-sharpened mounting points.     

New tomographic atom probe at University of Muenster, Germany

Currently atom probe tomography provides the highest spatial resolution compared to all other volume analysis techniques. Owing to its single atom sensitivity, it is specially suited to study nano-structured materials. Therefore, a new atom probe was installed at the Institute for Material Physics at University of Muenster, Germany, to study thin film reactions. Since the available budget was rather limited, a cost-effective non-commercial atom probe was constructed. The instrument is based on a 2D delay line detector system of 120 mm diameter. To achieve a large collecting angle and thus large volumes of analysis, a straight flight tube without a reflectron is used. This way, the flight distance may be reduced down to 160 mm. However, the variable chamber layout allows using a reflectron as an alternative. Furthermore, a laser system is implemented that delivers pulses in the 500 ps range to make possible laser-assisted evaporation of atoms. The article describes instrumental details and presents first characteristic data.     

Some aspects of the silicon behaviour under femtosecond pulsed laser field evaporation

Three dimension atom probe analysis of semiconductor materials requires the ability to bring high electric field at the specimen apex to remove atoms. It is shown that, if voltage pulses are used to evaporate doped silicon, the resistivity of the material has to be lower than about 10(2) Omega cm. To overcome this problem, voltage pulses have been replaced by femtosecond laser pulses. The laser pulses give rise to field evaporation by two processes. Both thermal and optical field evaporation have been observed. Thermal evaporation takes place at high laser intensities and with short wavelengths while the evaporation is assisted by the rectification of the optical field for lower intensities and in the infrared domain. Using the optical field evaporation, reproducible and good analyses in term of spatial and mass resolutions could be conducted.     

Analysis conditions of an industrial Al-Mg-Si alloy by conventional and 3D atom probes.

Industrial 6016 Al-Mg-Si(Cu) alloys are presently regarded as attractive candidates for heat treatable sheet materials. Their mechanical properties can be adjusted for a given application by age hardening of the alloys. The resulting microstructural evolution takes place at the nanometer scale, making the atom probe a well suited instrument to study it. Accuracy of atom probe analysis of these aluminium alloys is a key point for the understanding of the fine scale microstructural evolution. It is known to be strongly dependent on the analysis conditions (such as specimen temperature and pulse fraction) which have been widely studied for ID atom probes. The development of the 3D instruments, as well as the increase of the evaporation pulse repetition rate have led to different analysis conditions, in particular evaporation and detection rates. The influence of various experimental parameters on the accuracy of atom probe data, in particular with regard to hydride formation sensitivity, has been reinvestigated. It is shown that hydrogen contamination is strongly dependent on the electric field at the specimen surface, and that high evaporation rates are beneficial. Conversely, detection rate must be limited to smaller than 0.02 atoms/pulse in order to prevent drastic pile-up effect.     

Fabrication and characterization of APT specimens from high dose heavy ion irradiated materials

The next generations of advanced energy systems will require materials that can withstand high doses of irradiation at elevated temperatures. Therefore, a methodology has been developed for the fabrication of high-dose ion-irradiated atom probe tomography specimens at a specific dose with the use of a focused ion beam milling system. The method also enables the precise ion dose of the atom probe tomography specimen to be estimated from the local concentration of the implanted ions. The method has been successfully applied to the characterization of the distribution of nanoclusters in a radiation-tolerant 14YWT nanostructured ferritic steel under ion irradiation to doses up to 400 displacements per atom.     

Determining the composition of small features in atom probe: bcc Cu-rich precipitates in an Fe-rich matrix

Aberrations in the ion trajectories near the specimen surface are an important factor in the spatial resolution of the atom probe technique. Near the boundary between two phases with dissimilar evaporation fields, ion trajectory overlaps may occur, leading to a biased measurement of composition in the vicinity of this interface. In the case of very small second-phase precipitates, the region affected by trajectory overlaps may extend to the centre of the precipitate prohibiting a direct measurement of composition. A method of quantifying the aberrant matrix contribution and thus estimating the underlying composition is presented. This method is applied to the Fe–Cu-alloy system, where the precipitation of low-nanometre size Cu-rich precipitates is of considerable technical importance in a number of materials applications. It is shown definitively that there is a non-zero underlying level of Fe within precipitates formed upon thermal ageing, which is augmented and masked by trajectory overlaps. The concentration of Fe in the precipitate phase is shown to be a function of ageing temperature. An estimate of the underlying Fe level is made, which is at lower levels than commonly reported by atom probe investigations.     

Atom probe analysis of titanium hydride precipitates

It is expected that the three-dimensional atom probe (3DAP) will be used as a tool to visualize the atomic scale of hydrogen atoms in steel is expected, due to its high spatial resolution and very low detection limit. In this paper, the first 3DAP analysis of titanium hydride precipitates in metal titanium is reported in terms of the quantitative detection of hydrogen. FIB fabrication techniques using the lift-out method have enabled the production of needle tips of hydride precipitates, of several tens of microns in size, within a titanium matrix. The hydrogen concentration estimated from 3DAP analysis was slightly smaller than that of the hydride phase predicted from the phase diagram. We discuss the origin of the difference between the experimental and predicted values and the performance of 3DAP for the quantitative detection of hydrogen.     

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.     

APT analysis of WC-Co based cemented carbides

A method for quickly producing sharp and site-specific atom probe specimens from WC-Co based cemented carbides was developed using a combination of electropolishing, controlled back-polishing and FIB milling. Also, a method for measuring the amount of segregated atoms to an interface between two phases with a big difference in field needed for field evaporation was developed. Using atom probe tomography, the interface chemistry of WC/WC grain boundaries, WC/(M,W)C phase boundaries and WC/binder phase boundaries was analysed. In addition, the transition metal solubility in WC was determined.