I. R. Barkshire

Multi-spectral scanning Auger microscopy of tribological surfaces

Abstract The novel surface analysis technique of multi-spectral Auger microscopy (MULSAM) has been used to study simultaneously and at high resolution the chemical and topographic variations across wear scars generated with model oil formulations. This is the first time this technique has been applied to anything other than well-characterised samples from the semiconductor industry. The MULSAM image manipulations prove sufficiently robust to allow meaningful work on these extremely non-ideal specimens. The work leads to new insights into the composition and spatial distribution of anti-wear films. In particular, the results show significant correlations between the surface topography and the local chemical nature of the anti-wear film.

Three-dimensional scatter diagrams : application to surface analytical microscopy

Abstract Multivariate statistical techniques, such as principal component analysis and two-dimensional scatter diagrams, are increasingly being applied to multispectral images in analytical electron and X-ray microscopy. Combining data from different sources can often reveal more information than would be obtained if each image was processed separately. Three-dimensional data manipulation and visualisation tools are gaining popularity in many scientific fields. However, three-dimensional scatter diagrams are not yet being exploited much in analytical microscopy. This paper demonstrates the advantages of three-dimensional scatter diagrams and interactive correlation partitioning. An introduction to the methodology is included. Examples using multi-element Auger and topographic images from the York multispectral analytical electron microscope (MULSAM) are presented. Implementation issues are discussed briefly to help other workers who may wish to investigate the techniques. Some areas of future work are suggested.

Correction of topographical artefacts in Auger images using four backscattered electron detectors

Abstract The York multi-spectral scanning Auger microscope has been used to examine the correlations between Auger and backscattered electron images collected simultaneously from an electron spectrometer and the quadrants of an Si p-n junction backscattered electron detector. Digital signal processing of the four backscattered electron images allows the complete characterisation of the surface topography and also gives the effective atomic number at each pixel of the image. The topographical images thus calculated can then be used to correct the topographical artefacts in the Auger images. This method, which allows quantitative surface analysis of samples with complicated surface topographies with no prior knowledge of the sample, is tested using a carbon-coated anisotropically etched Si sample.

A miniature, all-electrostatic, field emission electron column for surface analytical microscopy

The design, construction and application of a miniature, electrostatic, two-lens field emission column are presented and demonstrated. The column can provide a finely focused beam of electrons with energies ranging from 100 eV to 15 keV, with corresponding beam currents of 5 - 20 nA, onto a sample at a working distance of 10 mm. Edge resolution measurements show that the beam diameter at the sample is (at worst) at 200 eV falling to 120 nm at 5 keV.

Multi-imaging and multivariate statistics used for 3D characterization at surfaces

Several microanalytical imaging techniques—energy dispersive X-ray detection, parallel electron energy loss spectroscopy, secondary ion mass spectroscopy, photoelectron spectroscopy and XPS) and scanning Auger microscopy—have reached the stage where they are capable of producing images of a surface with a section of a spectrum in each pixel. The resulting image-spectrum is a complex data structure which requires the use of special methodologies if the data are to be interpreted effectively. Appropriate methods have been developed for Earth satellite image processing and are directly applicable to surface microanalysis. The use of scatter diagrams, interactive correlation partitioning, factor and target factor analysis and principal component analysis are outlined in this paper and their application to semiconducting, catalytic and magnetic structures is illustrated. This field of endeavour can be thought of as being the beginning of an area of study which may be called surface chemometrics.

Quantitative surface chemical mapping with Auger and backscattered electron signals

Abstract The objective of surface analytical imaging methods is to map the quantitative distribution of the elements in the surface of a solid. Auger spectroscopy and imaging are often used for this purpose but the contrast in scanning Auger microscope (SAM) images of inhomogeneous, rough, surfaces show some features which are not due to the spatial variations of the surface chemical composition. Various schemes devised to reduce the effects on the image contrast of roughness and sub-surface inhomogeneity are reviewed in this paper. A model sample with a range of known surface inclinations and with known surface and sub-surface composition variations is used to demonstrate the effects of two general classes of correction schemes intended to reduce the contrast of such artefacts. The first class is based upon various arithmetic combinations of the number of electrons detected by a spectrometer set at the energy of an Auger peak and then at an energy (or a number of energies) on the spectral background above the peak. The extent to which these different combinations are successful in reducing topographical contrast and revealing otherwise hidden chemical contrast is summarised. It will be shown how a simple estimate of the Auger peak height leaves large topographical effects in the image contrast and that ratios of measurements at two energies provide a good first approximation for the removal of topographical artefacts. However, even those ratios which provide the best topographic compensation can overcorrect for variations in the sub-surface composition and its effects upon the Auger backscattering factor. In addition, they can introduce new problems for quantitative surface microanalysis. A second class of corrections involves the use of the signals from a set of four backscattered electron (BSE) detectors which collect BSE imaes at the same time as the energy-analysed images. The sum and difference signals from these detectors can be used, together with appropriate calibration, to compute three images in which the pixel intensities give the local surface inclinations and effective atomic numbers of the sub-surface material. These images can be used, in turn, to derive correction images for the Auger maps so as to yield a more accurate map of the surface chemical composition. The combination of the correlated Auger and BSE signals for each place on the surface requires the use of an instrument which collects these signals simultaneously from several detectors. Various methods of using such correlations are discussed. Surfaces with large variations in the contrast in the “raw” Auger images can be analysed with a precision of approximately ±5 at% after exploitation of such correlations.

Automatic and interactive correlation partitioning compared : Application to TiN/Ti/SiO2

A new method has been developed for the partitioning of sets of images with the objective of automatically identifying the number and locations of different regions in a material. The method is called automatic correlation partitioning and it involves the identification of clusters in the n-dimensional intensity histogram of a set of n images that are spatially registered. The method uses the peaks located in the simple intensity histograms of each image in the set to produce a list of all possible clusters in the entire data set. This list is then searched in order to find the actual clusters. The method is tested using data from a multi-imaging Auger electron microscope, which yields sets of Auger images characteristic of the spatial distributions of selected kinds of atoms in the surface of a solid. The first tests involve the use of a model sample consisting of a W overlay pattern on a Si substrate. The second tests are done on a TiN/Ti/SiO2 planar layer structure that has been ion beam bevelled to reveal a cross-section of the composition depth profile. The first set contains two images and the second set contains five images. The results of the new automatic method are compared with those obtained by the analyst working interactively with the data set to identify the clusters subjectively. Cluster analysis of the second sample reveals details of the interfacial layer chemistry not revealed by the interactive method and is consistent with published XPS depth profiling experiments reporting a titanium silicide layer at the Ti/SiO2 interface.

Quantitative surface microanalysis of samples with extreme topography utilising image interpretation by scatter diagrams and principal component analysis

Abstract Quantitative surface analysis (by Auger imaging, SIMS, EPMA, etc.) of many samples of technological interest is not possible due to the image artefacts arising from surface topography or other features of the sample. A methodology is presented for the identification and removal of regions within an image where artefacts dominate the contrast. This enables meaningful quantification of the regions not dominated by artefacts. The method employs multi-variate statistical techniques including 3D scatter diagrams and principal component analysis (PCA). PCA proves to be a powerful method for measuring the extent of any remnant artefact within image sets. The methodology is applied to the characterisation of a Pt Rh catalyst using the multi-spectral scanning Auger microscope at York.

The application of a low energy loss electron detector in conjunction with scanning Auger microscopy: an aid to quantitative surface microscopy

The application of a low loss detector is demonstrated as a quantification aid in scanning Auger microscopy. The low loss detector collects only those electrons that have lost less than a few hundred electron volts during their passage within, and escape from the specimen. The signal thus generated originates from within a comparable information depth to that of the Auger signal and the contrast is predominantly due to the average atomic number within this information depth. Calibration of the detector using elemental standards enables rapid identification of the position of, and the local atomic number of the various phases present at the surface of the specimen. This allows rapid identification of the regions for subsequent Auger spectroscopy or imaging. The low loss detector is deemed superior to conventional secondary or backscattered electron detectors for this purpose. The methodology is demonstrated on a W/TiN/Ti/Si system of trenches and contact holes that had been back polished in order to generate a bevelled cross-section through the structure.