C. G. H. Walker

The secondary electron emission yield for 24 solid elements excited by primary electrons in the range 250-5000 ev: a theory/experiment comparison.

The secondary electron (SE) yield, delta, was measured from 24 different elements at low primary beam energy (250-5,000 eV). Surface contamination affects the intensity of delta but not its variation with primary electron energy. The experiments suggest that the mean free path of SEs varies across the d bands of transition metals in agreement with theory. Monte Carlo simulations suggest that surface plasmons may need to be included for improved agreement with experiment.

Theory Experiment Comparison of the Electron Backscattering Factor from Solids at Low Electron Energy (250–5,000 eV)

The electron backscattering factor was measured from 24 different elements at low primary beam energy (250-5,000 eV). The results were compared with Monte Carlo simulations from a variety of freely available programs and an in-house developed program. The results suggest that a thin film of oxide can modify the backscattering factor at low primary energy. In addition, a number of problems have been identified with the freely available programs.

Scatter diagrams in energy analysed digital imaging: application to scanning Auger microscopy

In order to improve methods for the systematic characterization of inhomogeneous materials the procedures of multi‐spectral imaging and scatter diagram construction have been deployed. Although the techniques described are relevant to all instruments which detect signals on several different information channels (e.g. wavelength or energy analysed optical, X‐ray or electron imaging), they are illustrated with scanning Auger microscopy (SAM). The construction and use of bivariate correlation diagrams is described by reference to simple samples consisting of patterns of Al film upon Si substrates. The method is then applied to LaNi5 and GaInAs samples each with different signal/noise ratios and chemical characteristics. The software windowing of scatter diagrams by computer combined with presentation of false colour images is demonstrated. The multi‐spectral Auger mapping (MUSLAM) procedure thus evolved is demonstrated to be a powerful, general analytical technique for characterizing the number, abundance and chemistry of each type of region in the surface of an inhomogeneous solid.

Classification of protein crystallization images using Fourier descriptors

The two-dimensional Fourier transform (2D-FT) is well suited to the extraction of features to differentiate image texture, and the classification of images based on information acquired from the frequency domain provides a complementary method to approaches based within the spatial domain. The intensity, I, of the Fourier-transformed images can be modelled by an equation of power law form, I = Arα, where A and α are constants and r is the radial spatial frequency. The power law is fitted over annuli, centred at zero spatial frequency, and the parameters, A and α, determined for each spatial frequency range. The variation of the fitted parameters across wedges of fixed polar angle provides a measure of directionality and the deviation from the fitted model can be exploited for classification. The classification results are combined with an existing method to classify individual objects within the crystallization drop to obtain an improved overall classification rate.

The role of zirconium and sulphur in the adherence of oxides on superalloys

Abstract Two Ni-based superalloys were studied using scanning Auger microscopy (SAM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). One alloy contained 0.3 at% Zr whereas the other did not. It was found after heating to 1050°C in identical conditions that S segregated to the surface of the Zr free alloy whereas Zr segregated to the surface of the zirconiated alloy. The Zr was found to be oxidised. It was also found that the surface topology of the two systems was considerably different after heating although this could not be ascribed to the different oxidation characteristics of the material.

On the role of rare earth additives in the oxidation of superalloys

Abstract The surface segregation of zirconium in a NiCrAlZr(0.5% Zr by weight) is reported for the first time. After in situ annealing of the alloy in a Scanning Auger Microscope (SAM) to above 690 K for 16 h, zirconium has segregated over the two surface phases. An AES estimate of the segregated zirconium shows it to be in a film of about 1 nm thick. The surface of the unheated alloy showed no zirconium using SAM, despite the presence of zirconium-rich precipitates (1–2 μm) observed on the alloy surface using EPMA.

Toward Quantitative Scanning Electron Microscopy

Abstract The scanning electron microscope (SEM) has been a fundamental tool that has underpinned much advancement in research and engineering in various disciplines over many decades. However, it cannot be regarded yet as a true quantitative instrument, particularly with regard to the resulting image contrast and when imaging is carried out at the nanoscale level. Such a limitation is not applied to the SEM’s use as a measuring instrument, in which it performs exceptionally well as a critical dimension tool (CD-SEM). This lack of material quantification manifests itself further when the instrument is operated with low-energy electrons, in what is referred to as low-voltage SEM (LVSEM). This is due to the presence of carbonaceous deposits at the surface and a poor understanding of the emission of secondary electrons from the materials. In this chapter, a short review is given of some of the progress made in the efforts to improve the quantification of the SEM, with the emphasis on research carried out at York. Our results strongly suggest that the currently accepted theory, which explains why there is a correlation between the secondary electron yield and the work function of a metal, is incorrect. In addition, we show that the backscattering coefficient from materials can be strongly influenced by surface layers at low primary electron energy, and that a secondary electron contribution to the backscattering coefficient occurs at low primary beam energy. Finally, we present Auger electron spectra that have been in situ acquired from clean surfaces at high speeds in a high vacuum (10 -7 mbar), and thus represent a new way to determine the composition of nanostructures in an SEM.

Contrast reversal in SAM mapping due to changes in the substrate atomic number

Abstract A new data processing approach for Auger mapping in SAM is proposed. This method avoids elemental contrast reversal in conventional mapping due to the way Auger electron peak heights are estimated. The height of an Auger peak is usually estimated in SAM imaging as the difference in count N 1 - N 2 at two energies E 1 on the peak and E 2 on its high energy side. This method can lead, in some cases, to the difference in count at E 1 and E 2 of the background from another constituent element, to be as large as, or more than, that of the peak height in question. Auger maps of such a system are shown to give a reverse contrast to the actual elemental distribution on the surface as identified by SEM and point AES analysis. In the present approach, the background count N 2 is replaced with another estimated count N b at the same energy as that of the Auger peak and is shown to give the correct chemical contrast.

Simulations and measurements in scanning electron microscopes at low electron energy

The advent of new imaging technologies in Scanning Electron Microscopy (SEM) using low energy (0-2 keV) electrons has brought about new ways to study materials at the nanoscale. It also brings new challenges in terms of understanding electron transport at these energies. In addition, reduction in energy has brought new contrast mechanisms producing images that are sometimes difficult to interpret. This is increasing the push for simulation tools, in particular for low impact energies of electrons. The use of Monte Carlo calculations to simulate the transport of electrons in materials has been undertaken by many authors for several decades. However, inaccuracies associated with the Monte Carlo technique start to grow as the energy is reduced. This is not simply associated with inaccuracies in the knowledge of the scattering cross-sections, but is fundamental to the Monte Carlo technique itself. This is because effects due to the wave nature of the electron and the energy band structure of the target above the vacuum energy level become important and these are properties which are difficult to handle using the Monte Carlo method. In this review we briefly describe the new techniques of scanning low energy electron microscopy and then outline the problems and challenges of trying to understand and quantify the signals that are obtained. The effects of charging and spin polarised measurement are also briefly explored. SCANNING 38:802-818, 2016.

The sensitivity of backscattering coefficients to elastic scattering cross-sections and electron stopping powers.

The sensitivity of Monte Carlo estimates of backscattering coefficients η to the accuracy of their input data is examined by studying the percentage change in η due to changes of 10% and 20% in the differential elastic scattering cross-section dσ/dΩ and corresponding changes in the stopping power S(E) in the primary energy range 200-10,000 eV. To a good approximation equivalent elastic and inelastic scattering changes produce equal and opposite shifts in η, a result consistent with predictions of transport theory. For medium to high atomic numbers an x% error in the specification of either S(E) or dσ/dΩ produces a percentage change in η significantly less than x%, while at low atomic number Δη/η increases approximately linearly with ln E so that Monte Carlo predictions are then more sensitive to parameter precision at high energy.