M. M. El Gomati

Monte Carlo calculations of the spatial resolution in a scanning auger electron microscope

Abstract The Monte Carlo methods used in calculations of spatial effects in scanning electron microscopy and X-ray microprobe analysis are applied to scanning Auger electron microscopy (SAM). It is concluded that the spatial resolution limits in SAM are determined almost entirely by the profile of the incident electron beam. Backscattered electrons yield a very low, slowly varying, background 0.2–0.5 microm in extent which should be eliminated with simple discrimination techniques in existing SAM designs. In addition, the backscattering factor degrades the spatial resolution of a step in the composition but only by a factor of the order of 2.

ELECTRONIC EXCITATIONS ON CLEAN AND ADSORBATE COVERED Pd( 111) BY ANGLE RESOLVED ELECTRON ENERGY LOSS SPECTROSCOPY

Abstract Electronic excitations on clean and adsorbate covered Pd(111) have been investigated by angle resolved electron energy loss spectroscopy. Primary energies in the range of 50–1000 eV were chosen for strong specular reflection to emphasize elastic diffraction-before-loss processes. The clean Pd spectra are compared with optical data, and good correspondence is found for the optical limit (q ⋍ 0). The loss features are interpreted in terms of plasmon resonances and interband transitions within the framework of a recent band structure calculation. Virtually no dispersion is observed for the intrinsic Pd losses, but vertical interband transitions decay fast in the dispersive (q ≠ 0) spectra. Two adsorbate systems are investigated in this study: CO in a disordered adsorbate layer and bromine in a well-ordered ( 3× 3 )R30° structure. Adsorbate derived loss features are generally prominent in the nonspecular (q ≠ 0) spectra. While no dispersion is seen for the intramolecular 13.5 eV excitation of adsorbed CO, dispersion up to 1 eV is found for the Br 4p derived loss feature of the ordered overlayer. This is discussed in terms of a two-dimensional adsorbate band structure of bromine.

A fast, parallel acquisition, electron energy analyzer: The hyperbolic field analyzer

This article describes a new kind of electrostatic charged particle analyzer capable of the parallel detection of a large kinetic energy range. The main purpose envisaged is for the simultaneous detection of electrons scattered from surfaces and having energies between a few tens of eV to greater than 2000 eV. A prototype has been constructed that approximates a hyperbolic deflection field for the electrons entering an entrance slit. It exhibits an energy resolution of a few eV and a collection efficiency of 0.05% of 2π sr. Useful Auger spectra can be acquired in a time of less than 2 s. The significant improvement in spectrum acquisition time that this represents offers many possibilities to further Auger and photoelectron spectroscopy.

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 and hotelling transforms: application to surface analytical microscopy

Abstract The collection of electrons from several Auger peaks together with other electrons scattered from a sample in a scanning Auger electron microscope provides a means of analysing many features of an inhomogeneous surface. The use of multi-image sets in the analysis of samples of Al on Si, W on Si, LaNi compounds and superconducting wires are given as examples. The use of scatter diagrams as a means of classifying the number and coverage of different surface phases is illustrated. The Hotelling transform is described and then applied both to data from a real W/Si sample and to simulated images which are chosen to illustrate the ways in which this transform can be useful. The design of a new 23 channel Auger microscope intended to take advantage of the multi-spectral approach to data acquisition is outlined.

The interpretation of the spatial resolution of the scanning Auger electron microscope: A theory/experiment comparison

Abstract Two modes of scanning Auger microscopy for oblique incidence and sharp chemical edges are defined and discussed using a Monte-Carlo method reported earlier. A theory/experiment comparison for a silver edge on a tungsten substrate confirms that the spatial resolution in SAM is controlled by incident beam size. The orientation of the edge with respect to the scan direction is shown to have a strong effect upon the spatial resolution.

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.

Scanning auger electron microscopy with high spatial or high energy resolution

Abstract An all-electrostatic, computer controlled, high spatial and high energy resolution, scanning Auger electron microscope incorporating a RHEED camera is described. The electron column has a 50 nm spatial resolution at 55 mm working distance. The energy analyser allows high energy resolution of up to 630 or constant energy windows of down to 1 eV over the energy range of 20 eV to 1000 eV. A system software allowing flexibility of the computer control is described. 128 × 128 pixel Auger images of Al and SiO 2 , with frame scan times of the order of 8 min, are presented.

Scanning low energy electron loss microscopy (SLEELM)

High resolution scanning low energy electron loss imaging of a well characterized system of W on Si is demonstrated in reflection geometry at a primary electron energy Ep of 740 eV and spatial resolution of 3 μm. Characteristic losses below 60 eV are chosen to examine contrast between two types of regions (tungsten and silicon dominated, respectively). By differencing between a maximum in loss intensity and an adjacent trough, high quality chemical images are obtained with contrast far superior to Auger electron images produced in comparable times. It should be noted that, in contrast to scanning Auger microscopy, the spatial resolution of SLEELM is limited only by the profile of the incident beam. In addition, the technique is readily capable of chemical analysis at the surface of bulk specimens. A further advantage of low electron loss imaging is that the depth resolution may be varied by altering the primary electron energy, with the possibility of bridging the gap between Auger electron microscopy and X‐ray microprobe analyser. Possible disadvantages of the technique lie in the weakness of the elastic backscattering which is essential to observe the losses in reflection geometry, and the possibility of overlapping low energy characteristic loss peaks, an effect which is, however, less bad than for low energy Auger peaks. It is shown that electron probes with characteristics rather different from those currently used in Auger microscopes are required to optimize the technique, but that SLEELM has the potential to be a very useful form of surface microscopy which will complement other surface imaging techniques.

A Digital Scanning Auger Electron Microscope Incorporating a Concentric Hemispherical Analyser

A scanning electron microscope with a field-emitting electron gun, digitally controlled scanning and a concentric hemispherical energy analyser is described and compared with different types of energy dispersive scanning electron microscopes. The advantages of hemispherical analysers accessed by electrostatic lenses are flexible electron optical control of working distances and resolving power. A 4 element lens is described which results in an analyser with a 2 eV window over an energy range of 20 eV to 800 eV. Auger images of calcium and sulphur on titanium are given. Spatial resolutions of 50 nm and frame scan times of a few minutes are possible.