Ultramicroscopy最新文献

Microlens array (MLA), through which all the sub-beams are focused, is widely used in multi-electron-beam systems. In this work, based on the differential algebraic (DA) method, we propose an approach in calculating the high-order aberrations for both axial and off-axial microlenses, considering the multipole fields that are introduced by the neighborhood structures in MLA, as well as the rotationally symmetric field. To perform the DA calculation, the electric fields of the microlenses are analyzed by using the azimuthal Fourier analysis and the Fourier-Bessel series Expansion. The resulting field components, including both rotationally symmetric field and the multipole fields, are transferred into DA arguments and operated as per DA methodology. Then, by developing and employing the DA theory and algorithm, the primary and high-order aberrations are calculated and obtained simultaneously for both the axial and off-axial microlenses by tracing only one reference ray. Finally, we calculate, analyze and discuss the primary and high-order aberrations of two example MLAs, for both axial and off-axial microlenses. The effects of the dodecapole fields on the aberrations are also analyzed.

By working out the Bethe sum rule, a boundary condition that takes the form of a linear equality is derived for the fine structure observed in ionization edges present in electron energy-loss spectra. This condition is subsequently used as a constraint in the estimation process of the elemental abundances, demonstrating starkly improved precision and accuracy and reduced sensitivity to the number of model parameters. Furthermore, the fine structure is reliably extracted from the spectra in an automated way, thus providing critical information on the sample's electronic properties that is hard or impossible to obtain otherwise. Since this approach allows dispensing with the need for user-provided input, a potential source of bias is prevented.

Ultrasonic atomic force microscopy (UAFM) is a powerful nondestructive subsurface imaging tool that is widely used to inspect material defects and analyze biological cells. The contrast in UAFM images, which is crucial for subsurface imaging quality, is directly influenced by the contact force between the probe and material. This contact force affects the subsurface contrast by influencing the propagation of the stress field from the vibrating probe into the material. Therefore, optimizing the contact force is essential for achieving superior subsurface contrast with better resolution and greater detectable depth. This paper proposes a model for determining the optimal contact force for high-contrast, high-resolution subsurface imaging. The model was designed to improve UAFM imaging across samples with a wide range of Young's moduli, from tens to hundreds of GPa. The use of this model resulted in significant improvements to imaging quality, with a detectable depth exceeding 337.7 nm and lateral resolution below 56.9 nm. Hence, this model demonstrates better results than experiments conducted under arbitrary contact forces. This study provides a pathway for optimizing subsurface imaging and delivering enhanced contrast, higher resolution, and greater detectable depth. Consequently, the results of this study contribute to the advancement of the capabilities of subsurface imaging techniques.

The rich information of electron energy-loss spectroscopy (EELS) comes from the complex inelastic scattering process whereby fast electrons transfer energy and momentum to atoms, exciting bound electrons from their ground states to higher unoccupied states. To quantify EELS, the common practice is to compare the cross-sections integrated within an energy window or fit the observed spectrum with theoretical differential cross-sections calculated from a generalized oscillator strength (GOS) database with experimental parameters. The previous Hartree-Fock-based and DFT-based GOS are calculated from Schrödinger's solution of atomic orbitals, which does not include the full relativistic effects. Here, we attempt to go beyond the limitations of the Schrödinger solution in the GOS tabulation by including the full relativistic effects using the Dirac equation within the local density approximation, which is particularly important for core-shell electrons of heavy elements with strong spin-orbit coupling. This has been done for all elements in the periodic table (up to Z = 118) for all possible excitation edges using modern computing capabilities and parallelization algorithms. The relativistic effects of fast incoming electrons were included to calculate cross-sections that are specific to the acceleration voltage. We make these tabulated GOS available under an open-source license to the benefit of both academic users and to allow integration into commercial solutions.

Acquiring multiple high magnification, high resolution images with scanning electron microscopes (SEMs) for quantitative analysis is a time consuming and repetitive task for microscopists. We propose a workflow to automate SEM image acquisition and demonstrate its use in the context of nanoparticle (NP) analysis. Acquiring multiple images of this type of specimen is necessary to obtain a complete and proper characterization of the NP population and obtain statistically representative results. Indeed, a single high magnification image only scans a small area of sample, containing only few NPs. The proposed workflow is successfully applied to obtain size distributions from image montages at three different magnifications (20,000x, 60,000x and 200,000x) on the same area of the sample using a Python based script. The automated workflow consists of sequential repositioning of the electron beam, stitching of adjacent images, feature segmentation, and NP size computation. Results show that NPs are best characterized at higher magnifications, since lower magnifications are limited by their pixel size. Increased accuracy of feature characterization at high magnification highlights the importance of automation: many high-magnification acquisitions are required to cover a similar area of the sample at low magnification. Therefore, we also present feature tracking with smart beam positioning as an alternative to blind acquisition of very large image arrays. Feature tracking is achieved by integrating microscope tasks with image processing tasks, and only areas of interest will be imaged at high resolution, reducing total acquisition duration.

The impact of the laser wavelength on accuracy in elemental composition analysis in atom probe tomography (APT) was investigated. Three different commercial atom probe systems - LEAP 3000X HR, LEAP 5000 XR, and LEAP 6000 XR - were systematically compared for a TiN model coating studying the effect of shorter laser wavelengths, especially in the deep ultraviolet (DUV) range, on the evaporation behavior. The findings demonstrate that the use of shorter wavelengths enhances the accuracy in elemental composition, while maintaining similar electric field strengths. Thus, thermal effects are reduced, which in turn improves mass resolving power. An important aspect of this research includes the estimation of energy density ratios of the different instruments. The reduction in wavelength is accompanied by increasing energy densities due to smaller laser spot sizes. Furthermore, advancements in the detector technology were studied. Finally, the detector dead-times were determined and dead-zones were evaluated to investigate the ion pile-up behavior in APT measurements of nitrides with the LEAP 6000 XR.

To fully evaluate the atomic structure, and associated properties of materials using transmission electron microscopy, examination of samples from three non-collinear orientations is needed. This is particularly challenging for thin films and nanoscale devices built on substrates due to limitations with plan-view sample preparation. In this work, a new method for preparation of high-quality, site-specific, plan-view TEM samples from thin-films grown on substrates, is presented and discussed. It is based on using a dual-beam focused ion beam scanning electron microscope (FIB-SEM) system. To demonstrate the method, the samples were prepared from thin films of perovskite oxide BaSnO₃ grown on a SrTiO₃ substrate and metal oxide IrO₂ on a TiO₂ substrate, ranging from 20-80 nm in thicknesses using molecular beam epitaxy. While the method is optimized for the thin films, it can be extended to other site-specific plan-view samples and devices build on wafers. Aberration-corrected STEM was used to evaluate the quality of the samples and their applicability for atomic-resolution imaging and analysis.

Scanning transmission electron microscopy (STEM) provides high-resolution visualization of atomic structures as well as various functional imaging modes utilizing phase contrasts. In this study we introduce a semicircular aperture in STEM bright field imaging, which gives a phase contrast transfer function that becomes complex and includes both lower and higher spatial frequency contrast transfer. This approach offers significant advantages over conventional phase plate methods, having no charge accumulation, degradation, or unwanted background noise, which are all problematic in the phase plate material. Also compared to the differential phase contrast or ptychography equipment, this semicircular aperture is far less costly. We apply this approach to visualization of polymer, biological and magnetic samples.

Electron energy-loss spectroscopy (EELS) performed in a scanning transmission electron microscope (STEM) is susceptible to noise, just like every other measurement. EELS measurements are also affected by signal blurring, related to the energy distribution of the electron beam and the detector point spread function (PSF). Moreover, the signal blurring caused by the detector introduces correlation effects, which smooth the noise. A general understanding of the noise is essential for evaluating the quality of measurements or for designing more effective post-processing techniques such as deconvolution, which especially in the context of EELS is a common practice to enhance signals. Therefore, we offer theoretical insight into the noise smoothing by convolution and characterize the resulting noise correlations by Pearson coefficients. Additional effects play a role in EELS mapping, where multiple spectra are acquired sequentially at various specimen positions. These three-dimensional datasets are affected by energy drifts of the electron beam, causing spectra to shift relative to each other, and by beam current deviations, which alter their relative proportion. We investigate several energy alignment techniques to correct energy drifts on a sub-channel level and describe the intensity normalization necessary to correct for beam current deviations. Both procedures affect noises and uncertainties of the measurement to various degrees. In this paper, we mathematically derive an applied noise model for EELS measurements, which is straightforward to use. Therefore, we provide the necessary methods to determine the most important noise parameters of the EELS detector enabling users to adapt the model. In summary, we aim to provide a comprehensive understanding of the noises faced in EELS and offer the necessary tools to apply this knowledge in practice.

Advances in analytical scanning transmission electron microscopy (STEM) and in microelectronic mechanical systems (MEMS) based microheaters have enabled in-situ materials' characterization at the nanometer scale at elevated temperature. In addition to resolving the structural information at elevated temperatures, detailed knowledge of the local temperature distribution inside the sample is essential to reveal thermally induced phenomena and processes. Here, we investigate the accuracy of plasmon energy expansion thermometry (PEET) as a method to map the local temperature in a tungsten (W) lamella in a range between room temperature and 700 °C. In particular, we address the influence of sample thickness in the range of a typical electron-transparent TEM sample (from 30 nm to 70 nm) on the temperature-dependent plasmon energy. The shift in plasmon energy, used to determine the local sample temperature, is not only temperature-dependent, but in case of W also seems thickness-dependent in sample thicknesses below approximately 60 nm. It is believed that the underlying reason is the high susceptibility of the regions with thinner sample thickness to strain from residual load induced during FIB deposition, together with increased thermal expansion in these areas due to their higher surface-to-volume ratio. The results highlight the importance of considering sample thickness (and especially thickness variations) when analyzing the local bulk plasmon energy for temperature measurement using PEET. However, in case of W, an increasing beam broadening (FWHM) of the bulk plasmon peak with decreasing sample thickness can be used to improve the accuracy of PEET in TEM lamellae with varying sample thickness.