Spatial Resolution Of X-ray Microanalysis Research Articles

The Spatial Resolution of X-Ray Microanalysis with EDS in the Transmission Electron Microscope

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

X-Ray Microanalysis in the Environmental SEM Using Mapping and Fourier Deconvolution Techniques

X-ray microanalysis of any type of specimen in its natural state without the use of conventional SEM specimen preparation techniques has immense potential in a wide range of scientific and industrial applications. This capability would be particularly useful in microanalysis applications where it is highly desirable to preserve the integrity of the specimen, for example in semiconductor failure analysis and forensic investigations. in principle, this X-ray microanalysis goal can be achieved in an environmental or variable pressure scanning electron microscope (VPSEM) because specimen charging and vacuum stability problems are negated by the presence of a gas in the specimen chamber. However, the accuracy and spatial resolution of X-ray microanalysis in the VPSEM is significantly degraded by the chamber gas as it scatters primary beam electrons, generating spurious X-rays far from the analysis point. to date, two different X-ray measurement strategies have been developed to facilitate X-ray microanalysis at high chamber pressure in the VPSEM.

Quantitative Composition and Thickness Mapping With High Spatial Resolution By XEDS In a 300kv FEG-AEM

Abstract Since the first demonstration by Cosslett and Duncumb, X-ray mapping by an electron probe microanalyzer (EPMA) has become a most popular approach in microanalysis because elemental distributions of constituents in a bulk sample can be displayed visually. The major disadvantage of EPMA mapping is poor spatial resolution (∼ 1 μm). The spatial resolution of X-ray microanalysis can be improved to a few nanometers using electron transparent thin-specimens in the analytical electron microscope (AEM). However, X-ray count rates from thin specimens are strictly limited because of the improved spatial resolution (i.e. smaller interaction volume) and the poor collection efficiency of X-ray. To obtain reasonable counts for accurate quantification in the AEM, extraordinarily long mapping times are required. Therefore, quantitative X-ray mapping is rarely attempted in the AEM. However, these limitations can be overcome by use of intermediate-volt age instruments combined with field-emission guns to increase the beam current-density, careful stage design to maximize the X-ray collection efficiency and the peak-to-background ratio, and ultrahigh vacuum system to reduce contamination.

Low-voltage EDS of magnesium ferrite Dendrites in a FEG-SEM

Energy-dispersive X-ray spectrometry (EDS) performed at low (≤ 5 kV) accelerating voltages in the SEM has the potential for providing quantitative microanalytical information with a spatial resolution of ∼100 nm. In the present work, EDS analyses were performed on magnesium ferrite spinel [(MgxFe1−x)Fe2O4] dendrites embedded in a MgO matrix, as shown in Fig. 1. spatial resolution of X-ray microanalysis at conventional accelerating voltages is insufficient for the quantitative analysis of these dendrites, which have widths of the order of a few hundred nanometers, without deconvolution of contributions from the MgO matrix. However, Monte Carlo simulations indicate that the interaction volume for MgFe2O4 is ∼150 nm at 3 kV accelerating voltage and therefore sufficient to analyze the dendrites without matrix contributions.Single-crystal {001}-oriented MgO was reacted with hematite (Fe2O3) powder for 6 h at 1450°C in air and furnace cooled. The specimen was then cleaved to expose a clean cross-section suitable for microanalysis.

Definition of the spatial resolution of X-ray microanalysis in thin foils

The spatial resolution of X-ray microanalysis in thin foils is defined in terms of the incident electron beam diameter and the average beam broadening. The beam diameter is defined as the full width tenth maximum of a Gaussian intensity distribution. The spatial resolution is calculated by a convolution of the beam diameter and the average beam broadening. This definiyion of the spatial resolution can be related simply to experimental measurements of composition profiles across interphase interfaces. Monte Carlo calculations using a high-speed parallel supercomputer show good agreement with this definition of the spatial resolution and calculations based on this definition. The agreement is good over a range of specimen thicknesses and atomic number, but is poor when excessive beam tailing distorts the assumed Gaussian electron intensity distributions. Beam tailing occurs in low-Z materials because of fast secondary electrons and in high-Z materials because of plural scattering.

The influence of X-ray absorption on the spatial resolution of X-ray microanalysis of thin foils

The spatial resolution of X-ray microanalysis of thin foils is determined by the initial diameter of the electron probe and by subsequent beam broadening in the specimen due to electron scattering. The influence of X-ray absorption on resolution is investigated and found to be negligible up to a critical thickness depending on the composition and tilt conditions of the specimen.

A simple spot-size versus pixel-size criterion for X-ray microanalysis of thin foils

Spatial resolution of X-ray microanalysis in the (S)TEM is limited by the initial electron probe size and by subsequent beam broadening in the foil. A finite probe size will thus inevitably deteriorate spatial resolution when compared to the resolution figure of a hypothetical point source. It can, however, be argued that this induced alteration is acceptible if the relative deterioration in resolution is either small or in accordance with the system resolution, which is determined by the microscope and energy-dispersive system settings (magnification, number of pixels). When acquiring an X-ray map or a line scan, the probe FWHM or spot size and the sampling interval on the specimen or unmagnified pixel size should be tuned to each other in order to optimize the acquisition with respect to time. This tuning criterion is translated into a set of coupled equations for the spot size and pixel size as function of a measure (to be defined) for the alteration of resolution due to the use of a finite source. These simple analytic equations, which obviously depend on parameters such as accelerating voltage, specimen thickness, density, atomic weight and number, can easily be implemented in the software of any X-ray microanalysis system.

The spatial resolution of x-ray microanalysis in thin foils

The spatial resolution of x-ray microanalysis in a thin foil is determined by the size of the beam-specimen interaction volume. This volume is a combination of the incident electron beam diameter (d) and the beam broadening (b) due to elastic scatter within the specimen. Definitions of spatial resolution have already been proposed on this basis but all present a worst case value for the resolution based on the dimensions of the beam emerging from the exit face of the foil.

Digital x-ray mapping of nanometer-size supported bimetallic catalysts

Understanding the role of metal cluster composition in determining catalytic selectivity and activity is of major interest in heterogeneous catalysis. The electron microscope is well established as a powerful tool for ultrastructural and compositional characterization of support and catalyst. Because the spatial resolution of x-ray microanalysis is defined by the smallest beam diameter into which the required number of electrons can be focused, the dedicated STEM with FEG is the instrument of choice. The main sources of errors in energy dispersive x-ray analysis (EDS) are: (1) beam-induced changes in specimen composition, (2) specimen drift, (3) instrumental factors which produce background radiation, and (4) basic statistical limitations which result in the detection of a finite number of x-ray photons. Digital beam techniques have been described for supported single-element metal clusters with spatial resolutions of about 10 nm. However, the detection of spurious characteristic x-rays away from catalyst particles produced images requiring several image processing steps.

THEORETICAL STUDY OF THE SPATIAL RESOLUTION OF X-RAY MICROANALYSIS IN ANALYTICAL ELECTRON MICROSCOPY

The spatial resolution of X-ray microanalysis has been investigated by Monte Carlo calculations, including knock-on secondary electron production. The results for AuMα in a l000A thin Au film at 100 keV showed an appreciable difference from the ones without knock-out events. Also calculations have been done with the Mott cross-section instead of the screened Rutherford cross-section for elastic scattering. A significant difference was found in the results among the three models.

A comment on ‘the spatial resolution of x-ray microanalysis in the scannning transmission electron microscope’

A comment on ‘the spatial resolution of x-ray microanalysis in the scannning transmission electron microscope’

Comment on ‘The spatial resolution of X-ray microanalysis in the scanning transmission electron microscope’

Abstract The recent treatment of electron beam spreading by Doig, Lonsdale and Flewitt (1980) predicts an incorrect relationship between beam spreading and foil thickness, Its use may lead to substantial errors. Theoretical approaches appropriate to Various experimental situations are recommended.

The spatial resolution of X-ray microanalysis in the scanning transmission electron microscope

Abstract The spatial resolution of X-ray microanalysis within thin foils in the scanning transmission electron microscope (STEM), interfaced with an energy dispersive X-ray spectrometer, has been analysed. The electron beam divergence within the foil has been described analytically and the resulting X-ray intensity profiles have been numerically evaluated for a range of simple elemental composition distributions within the foil. The effects of incident electron probe diameter, electron accelerating voltage and foil thickness on the measured X-ray intensity ratio profiles are considered and discussed.

Applications of high spatial resolution hicroanalysis to materials problems using dedicated STEM

The scanning transmission electron microscope has afforded a dramatic improvement in the spatial resolution of X-ray microanalysis of thin specimens, allowing the investigation of extremely localized compositional variations in materials systems. In this paper, the results of high resolution composition profile analysis in several materials are presented. The materials were analyzed in a 100 kV field emission STEM manufactured by VG Microscopes, Ltd., and fitted with an energy dispersive X-ray spectrometer. The specimens were held in a double-tilt graphite cartridge which allowed X-ray detection in the tilt range 0°-20° about each axis. The vacuum in the specimen chamber was ∿ 2 x 10-9 torr during analysis. Electron probe spot sizes of 5-10 Å were used, corresponding to probe currents in the range of 10-10-10-9 amps.For a given specimen composition, the spatial resolution of X-ray microanalysis in thin specimens is a function of probe size, accelerating voltage, specimen atomic number, and thickness.

Spatial resolution in x-ray microanalysis of thin foils in stem

The influence of electron scattering in the spatial resolution of X-ray microanalysis of thin foils of silicon has been measured as a function of specimen thickness and accelerating voltage. The experimental observations show that high angle elastic scattering controls the spatial resolution and that this is not influenced by the diffraction conditions. Monte Carlo calculations have also been carried out and the general form of the theoretical and experimental results are in good agreement. For foil thicknesses normally used in defect analysis in silicon, ~5000 A, the spatial resolution as measured in this experiment (taken as the region within which 90% of the X-ray signal is generated) is about 700 A at 100 kV and 1800 A at 40 kV.

Spatial resolution of X-ray microanalysis in thin foils

There is increasing interest in X-ray microanalysis of thin specimens and the present paper attempts to define some of the factors which govern the spatial resolution of this type of microanalysis. One of these factors is the spreading of the electron probe as it is transmitted through the specimen. There will always be some beam-spreading with small electron probes, because of the inevitable beam divergence associated with small, high current probes; a lower limit to the spatial resolution is thus 2αst where 2αs is the beam divergence and t the specimen thickness.In addition there will of course be beam spreading caused by elastic and inelastic interaction between the electron beam and the specimen. The angle through which electrons are scattered by the various scattering processes can vary from zero to 180° and it is clearly a very complex calculation to determine the effective size of the beam as it propagates through the specimen.