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.     

Evolution of tip shape during field evaporation of complex multilayer structures

Atom-probe tomography analysis of complex multilayer structures is a promising avenue for studying interfacial properties. However, significant artefacts in the three-dimensional reconstructed data arise due to the field evaporation process. To clarify the origin and impact of these artefacts for a FeCoB/FeCo/MgO/FeCo/IrMn multilayer, tip shapes were observed by transmission electron microscopy and compared to those obtained by finite difference modelling of electric fields and evaporation processes. It was found that the emitter shape is not spherical and its surface morphology evolves during successive evaporation of the different layers. This evolving morphology contributes to the artefacts generally observed in the reconstructed atom-probe data for multilayer structures because algorithms for three-dimensional reconstruction are based on the assumption that the shape of the emitter during field evaporation is spherical. Some proposed improvements to data reconstruction are proposed.     

Effect of laser pulsing on the composition measurement of an Al–Mg–Si–Cu alloy using three-dimensional atom probe

The effect of laser pulse energy on the composition measurement of an Al–Mg–Si–Cu alloy (AA6111) specimen has been investigated over a base temperature range of 20–80 K and a voltage range of 2.5–5 kV. Laser pulse energy must be sufficiently higher to achieve pulse-controlled field evaporation, which is at least 0.9 nJ with a beam spot size of about 5 μm, providing an equivalent voltage pulse fraction, ∼14% at 80 K for the alloy specimen. In contrast to the cluster composition, the measured specimen composition is sensitive to base temperature and laser energy changes. The exchange charge state under the influence of laser pulsing makes the detection of Si better at low base temperature, but detection of Cr and Mn is better at a higher temperature and using higher laser energy. No such effect occurs for detection of Mg and Cu under laser pulsing, although Mg concentration is sensitive to the analysis temperature under voltage pulsing. Mass resolution at full-width half-maximum is sensitive to local taper angle near the apex, but has little effect on composition measurement.     

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.     

Overcoming challenges in the study of nitrided microalloyed steels using atom probe

Nitrided steels are widely used in the engineering field due to their superior hardness and other attractive properties. Atom probe tomography (APT) was employed to study two Nb-microalloyed CASTRIP steels with different N contents. A major challenge of using APT to study this group of materials is the presence of tails after Fe peaks in the mass spectra, which overestimates the composition for alloying elements such as Nb and Cu in the steels. One important factor that contributes to the tails is believed to be delayed field evaporation from Fe²⁺. This artefact of the mass spectrum was observed to be the most severe when voltage pulsing was used. The application of laser pulses with energy ranging from 0.2 to 1.2 nJ successfully reduced the tails and lead to better compositional measurement accuracy. Spatial resolution in the z-direction (along the tip direction) was observed to be less affected by changing laser energy but deteriorates in x-y direction with increasing laser energy. This investigation suggests that pulsed-laser atom probe with ∼0.4 nJ laser energy can be used to study this group of materials with improved mass resolution while still maintaining high spatial resolution.     

Resolution enhancing using cantilevered tip-on-aperture silicon probe in scanning near-field optical microscopy

Scanning near-field optical microscopy (SNOM) achieves a resolution beyond the diffraction limit of conventional optical microscopy systems by utilizing subwavelength aperture probe scanning. A problem associated with SNOM is that the light throughput decreases markedly as the aperture diameter decreases. Apertureless scanning near-field optical microscopes obtain a much better resolution by concentrating the light field near the tip apex. However, a far-field illumination by a focused laser beam generates a large background scattering signal. Both disadvantages are overcome using the tip-on-aperture (TOA) approach, as presented in previous works. In this study, a finite difference time domain analysis of the degree of electromagnetic field enhancement is performed to verify the efficiency of TOA probes. For plasmon enhancement, silver is deposited on commercially available cantilevered SNOM tips with 20nm thicknesses. To form the aperture and TOA in the probes, electron beam-induced deposition and focused ion beam machining were applied at the end of the sharpened tip. The results show that cantilevered TOA probes were highly efficient for improvements of the resolution of optical and topological measurement of nanostructures.     

Effects of laser pulsing on analysis of steels by atom probe tomography

When performing atom probe tomography analysis, laser pulsing was found very helpful for some types of steels, which are prone to premature sample failure under voltage pulsing. Rather accurate chemical compositions for both matrix and precipitates were obtained by voltage- and laser-pulsed mode. Some special issues on the effects of laser are discussed. A simple correction based on the ¹³Cⁿ⁺ and ¹⁰Bⁿ⁺ peak was used to quantify C in the carbide (M₂₃C₆, M=Fe, Cr, Mo) and B in the boride (M₃B₂, M=Mo, Fe, Cr, V). The mass resolution in laser mode is sufficient to resolve ⁵⁶Fe²⁺ and ¹⁴N₂¹⁺ at 28 Da. Small peak shifts were found in the spectrum due to reflectron aberrations.     

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.     

Even illumination in total internal reflection fluorescence microscopy using laser light

In modern fluorescence microscopy, lasers are a widely used source of light, both for imaging in total internal reflection and epi-illumination modes. In wide-field imaging, scattering of highly coherent laser light due to imperfections in the light path typically leads to nonuniform illumination of the specimen, compromising image analysis. We report the design and construction of an objective-launch total internal reflection fluorescence microscopy system with excellent evenness of specimen illumination achieved by azimuthal rotation of the incoming illuminating laser beam. The system allows quick and precise changes of the incidence angle of the laser beam and thus can also be used in an epifluorescence mode.     

Enhanced angular current intensity from Schottky emitters

Even though the Schottky emitter is a high‐brightness source of choice for electron beam systems, its angular current intensity is substantially lower than that of thermionic cathodes, rendering the emitter impractical for applications that require high beam current. In this study, two strategies were attempted to enhance its angular intensity, and their experimental results are reported. The first scheme is to employ a higher extraction field for increasing the brightness. However, the tip shape transformation was found to induce undesirably elevated emission from the facet edges at high fields. The second scheme exploits the fact that the angular intensity is proportional to the square of the electron gun focal length [ Fujita, S. & Shimoyama, H. (2005) Theory of cathode trajectory characterization by canonical mapping transformation. J. Electron Microsc. 54 , 331–343], which can be increased by scaling‐up the emitter tip radius. A high angular current intensity (J_Ω∼ 1.5 mA sr⁻¹) was obtained from a scaled‐up emitter. Preliminary performance tests were conducted on an electron probe‐forming column by substituting the new emitter for the original tungsten filament gun. The beam current up to a few microamperes was achieved with submicron spatial resolution.     

A reproducible method for damage-free site-specific preparation of atom probe tips from interfaces

Atom probe tomography (APT) is a mass spectrometry method with atomic-scale spatial resolution that can be used for the investigation of a wide range of materials. The main limiting factor with respect to the type of problems that can be addressed is the small volume investigated and the randomness of common sample preparation methods. With existing site-specific specimen preparation methods it is still challenging to rapidly and reproducibly produce large numbers of successful samples from specifically selected grain boundaries or interfaces for systematic studies. A new method utilizing both focused ion beam (FIB) and transmission electron microscopy (TEM) is presented that can be used to reproducibly produce damage-free atom probe samples with features of interest at any desired orientation with an accuracy of better than 50 nm from samples that require very little prior preparation.     

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.     

The study on the atomic force microscopy base nanoscale electrical discharge machining

This study proposes an innovative atomic force microscopy (AFM) based nanoscale electrical discharge machining (AFM-based nanoEDM) system which combines an AFM with a self-produced metallic probe and a high-voltage generator to create an atmospheric environment AFM-based nanoEDM system and a deionized water (DI water) environment AFM-based nanoEDM system. This study combines wire-cut processing and electrochemical tip sharpening techniques on a 40-μm thick stainless steel sheet to produce a high conductive AFM probes, the production can withstand high voltage and large current. The tip radius of these probes is approximately 40 nm. A probe test was executed on the AFM using probes to obtain nanoscales morphology of Si wafer surface. The silicon wafer was as a specimen to carry out AFM-base nanoEDM process in atmospheric and DI water environments by AFM-based nanoEDM system. After experiments, the results show that the atmospheric and DI water environment AFM-based nanoEDM systems operate smoothly. From experimental results, it can be found that the electric discharge depth of the silicon wafer at atmospheric environments is a mere 14.54 nm. In a DI water environment, the depth of electric discharge of the silicon wafer can reach 25.4 nm. This indicates that the EDM ability of DI water environment AFM-based nanoEDM system is higher than that of atmospheric environment AFM-based nanoEDM system. After multiple nanoEDM process, the tips become blunt. After applying electrochemical tip sharpening techniques, the tip radius can return to approximately 40 nm. Therefore, AFM probes produced in this study can be reused.     

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-dependent abundances of hydride molecular ions in atom probe tomography of III-N semiconductors

We investigate the microscopic behaviour of hydrogen-containing species formed on the surface of III-N semiconductor samples by the residual hydrogen in the analysis chamber in laser-assisted atom probe tomography (APT). We analysed AlGaN/GaN heterostructures containing alternate layers with a thickness of about 20 nm. The formation of H-containing species occurs at field strengths between 22 and 26 V/nm and is independent of the analysed samples. The 3D APT reconstruction makes it possible to map the evolution of the surface behaviour of these species issued by chemical reactions. The results highlight the strong dependence of the relative abundances of hydrides on the surface field during evaporation. The relative abundances of the hydrides decrease when the surface field increases due to the evolution of the tip shape or the different evaporation behaviour of the different layers.     

Atom probe trajectory mapping using experimental tip shape measurements

Atom probe tomography is an accurate analytical and imaging technique which can reconstruct the complex structure and composition of a specimen in three dimensions. Despite providing locally high spatial resolution, atom probe tomography suffers from global distortions due to a complex projection function between the specimen and detector which is different for each experiment and can change during a single run. To aid characterization of this projection function, this work demonstrates a method for the reverse projection of ions from an arbitrary projection surface in 3D space back to an atom probe tomography specimen surface. Experimental data from transmission electron microscopy tilt tomography are combined with point cloud surface reconstruction algorithms and finite element modelling to generate a mapping back to the original tip surface in a physically and experimentally motivated manner. As a case study, aluminium tips are imaged using transmission electron microscopy before and after atom probe tomography, and the specimen profiles used as input in surface reconstruction methods. This reconstruction method is a general procedure that can be used to generate mappings between a selected surface and a known tip shape using numerical solutions to the electrostatic equation, with quantitative solutions to the projection problem readily achievable in tens of minutes on a contemporary workstation.     

A study of phase separated Ni66Nb17Y17 metallic glass using atom probe tomography

Microstructural characterization of Ni₆₆Nb₁₇Y₁₇ as spun metallic glass ribbon was carried out using atom probe tomography. A comparison of different experimental conditions for pulsed laser and pulsed voltage field evaporation reveal that the laser pulsing can be optimized to avoid preferential evaporation of yttrium. Atom probe tomography measurements illustrate that the sample undergoes phase separation resulting in two interconnected phases during the process of vitrification. The yttrium-enriched phase was depleted in niobium and yttrium-depleted phase was enriched in niobium. Moreover, detailed analyses of the roller-contact and non-contact sides of the melt-spun ribbon show different wavelength of phase separated regions revealing that the degree of phase separation is directly associated with the cooling rate.     

Quantitative atom probe analyses of rare-earth-doped ceria by femtosecond pulsed laser

We have investigated the irradiation conditions of femtosecond laser pulses for quantitative atom probe analyses of rare-earth (RE) doped ceria. The influence of laser wavelength, power, pulse frequency, as well as specimen temperature on mass resolution and background noise of atom probe mass spectra were investigated. Furthermore, quantitative atom probe analysis of yttrium distribution in Y-doped ceria was carried out with the optimized evaporation conditions. The distribution of yttrium was found to be uniform within the grains, but they were confirmed to be segregated at grain boundaries.     

The shape of field emitters and the ion trajectories in three-dimensional atom probes

The lateral resolution of three-dimensional atom probes is mainly controlled by the aberrations of the ion trajectories near the specimen surface. For the first time, a simulation program has been developed to reconstruct the ion trajectories near a sharp hemispherical electrode defined at the atomic scale. Surface atoms submitted to the highest field were removed one by one. The consecutive gradual change of the surface topology was taken into account in the calculation of ion trajectories. As the tip was 'field evaporated', the initial spherical shape of the emitter was observed to transform gradually into a polygonal shape. When the tip reached its equilibrium shape, the field distribution at the tip surface was found to be much more uniform compared to the initial distribution. The calculated distribution of ion impacts on the detector exhibits the presence of depleted zones both at the centre of low index poles and along <001> zone axes. These predictions are in excellent agreement with experiments.     

‘Collapsing rings’ on Schottky electron emitters

The electron beam in systems that use a Schottky emitter as the electron source can display periodic fluctuations when the emitter is operated at an extraction voltage that gives a relatively low field strength at the tip. In the past, these fluctuations have been associated with the so-called “collapsing rings” without much further information. In this paper, the tip’s geometry changes associated with these beam instabilities are investigated in more detail by recording the evolution of the emission pattern of a Schottky emitter showing ‘collapsing rings’ for different operating conditions. Scanning electron microscope (SEM) images of different Schottky emitters have been used to support the interpretation.