Auger Images Research Articles

Removing photoemission features from Auger-yield NEXAFS spectra

Two-dimensional spectral images, as opposed to one-dimensional spectra, are crucial for both recognizing the presence and verifying the removal of photoemission features in Auger-yield NEXAFS. We present a procedure for removing such photoemission features, which relies on describing the measured Auger-yield NEXAFS image as three simple, one-dimensional spectra: NEXAFS, Auger, and XPS. This allows for a very successful and fast removal of even extreme photoemission features, as illustrated by four examples. The procedure requires no additional reference measurements other than photon flux, and provides a significant amount of additional information, such as NEXAFS, Auger and XPS spectra and images, which can be used to verify the validity of the underlying assumptions and success of the cleanup process.

Improved Accuracy of Root Length Density Measurements for a Wheat and Cotton Intercropping System or Monocultures by Combining Root Auger Sampling and Image Analysis Methods

Improved Accuracy of Root Length Density Measurements for a Wheat and Cotton Intercropping System or Monocultures by Combining Root Auger Sampling and Image Analysis Methods

Effects of precipitates in Cu upon impact fracture: an ultra-high-vacuum study with local probe Scanning Auger/Electron Microscopy

In situ fracture under ultra-high vacuum (UHV) conditions of copper-alloys containing copper sulfide precipitates exhibits areas in the form of pits. The wide variety of morphologies depends significantly on the size of the existing precipitate. For large precipitates, the fractured surface reveals loops surrounded by step-terrace like structures with sulfur remaining preferentially on straight ledges as nanometer scale Auger images reveal in comparison with scanning electron images. For smaller precipitates (<3–4 μm), only well defined facets are formed having an almost uniform distribution of sulfur. Qualitatively the loop and facet formation could be explained due to accommodation of the Cu sulfide to the Cu matrix, leading for small precipitates to facet formation, and for large ones to step-terrace structures. Asymmetry in loop structures could be attributed to asymmetry of precipitate shape, while in some areas plasticity effects are also present. Finally, a comparison with distinctly different behavior in Cu–Bi alloys is made.

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. © John Wiley & Sons, Ltd.

Development of high spatial resolution Auger microscope as applied to semiconductor analysis

An Auger microscope equipped with aZrO/WSchottky field emitter was developed. For Auger analysis, spatial resolution under a probe current of more than 1 nA is important. Under such conditions, the aberration by the condenser lens is not negligible. To reduce it, the distance from the emitter to the principal plane of the first condenser lens is reduced by forming its magnetic field overlapping with the accelerating field of the electron. As a result, a spherical aberration coefficient of 10 mm (accelerating voltage 30 kV) was achieved and the spatial resolution was about 10 nm with a probe current of 10 nA and an accelerating voltage of 25 kV. The newly developed Auger microscope combined with the probe tracking system has enabled an Auger image to be obtained, in which a 40 nm gate oxide layer in a cross-sectioned integrated circuit device is clearly observed.

Noise removal from Auger images by using adaptive binomial filter

The application of the new smoothing routine based on adaptive binomial filter for Scanning Auger Microscopy has been investigated. The main advantage of the adaptive binomial filter is the possibility to reduce the noise without noticeable signal broadening. Experimental Auger images of the various samples (Au films on alumina and porous silicon films) were processed by this routine. It was demonstrated that the employment of the new smoothing routine improves the visual quality of chemical images and elemental scattering diagrams. The detailed surface chemical composition of the samples investigated was finally revealed by using elemental scattering diagram analysis.

Quantitative surface chemical mapping with Auger and backscattered electron signals

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.

Scanning Auger microscopy and x-ray photoelectron spectroscopy studies of Roman bronzes

Scanning Auger microscopy and x-ray photoelectron spectroscopy (XPS) techniques are used to study the surface and interface microchemical composition of a series of Roman leaded bronzes of the third century B.C. The materials’ surface composition, as obtained from large-area XPS analysis, compares fairly well with that of the bulk, except for a significant surface lead segregation observed in one case. We find that, in the sample with the highest tin content, tin plays a sacrificial role in preventing the corrosion of copper. The tin is present mainly in the oxidized form, even at a depth of about 100 Å, whereas the copper is virtually all in the metallic form. Evidence is given for the presence of ‘‘white lead’’ [2PbCO3⋅Pb (OH)2] at the surface of the sample with the highest lead content. Auger point analysis evidences significant lateral inhomogeneities in the microchemistry of the materials, as well as sulfur segregation in the original patina. Auger images bring to light tin and lead lateral segregations with spatial resolutions of about 0.5 and 0.1 μm, respectively.

Al2Cu precipitate distribution in AlCu interconnects after electromigration stressing: A study by Auger microscopy and ion beam bevelling

The York multi-spectral Auger microscope has been used to study unpassivated AlCu interconnects on silicon, which have undergone electromigration stressing. Auger, energy dispersive x-ray, and backscattered electron images have been obtained simultaneously from a sample which has been bevelled using Xe+ ions. The exposed surface on the bevel makes an angle of about 10−3 rad with the virgin surface so exposing laterally the depth distribution of the elements in the material. Cu-rich precipitates are observed in the tracks. The combination of Auger, energy dispersive x-ray, and backscattered electron imaging allows the determination of the location of both surface and subsurface AlCu precipitates. The faceted character of the particles can lead to an apparent excess of oxygen in their surfaces. The backscattered electron images are used to correct the Auger images for these topographical artifacts.

Three-dimensional scatter diagrams: application to surface analytical microscopy

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.

Spatial resolution limitation in Auger microanalysis of compounds

The width of the resolved gap between the features of Auger images was calculated by Rayleigh criterion. When the threshold contrast is provided, this width is almost independent of the examined specimen nature and is defined only by probing electron beam diameter. The relation between the irradiation dose, contrast and spatial resolution is deduced. The flexible dependence allowing to evaluate both the real and maximum attainable spatial resolution of Auger images is obtained.

Polymer analysis by Auger electron spectroscopy using sectioning and cryogenic cooling

Efficient polymer analyses have been performed by Auger electron spectroscopy in combination with sectioning and cryogenic cooling, to prevent surface charging and polymer damage during electron beam exposure. Polymer samples of sub-micron thickness have been found to be free of surface-charge effects, and cryogenic cooling (which was applied to the polymer AES analysis for the first time) has made it possible to measure volatile components (components other than carbon). A high resolution Cl Auger image has been obtained for acrylonitrile/vinylidene chloride copolymer. The distribution of oxygen and nitrogen in the cross-section of oxidized polyacrylonitrile (PAN) fibers has been measured by this method, with results which show it to be effective for the microanalysis of polymer samples.

Auger imaging from rough, chemically inhomogenous, materials

AbstractSurface analysis of practical materials often presents a challenging situation in Auger spectroscopy and imaging because the yield of Auger electrons depends not only on the composition of the surface but also upon the local surface inclination (the topography) and the composition of the material below the surface. The use of simultaneous imaging of the same area of a sample with an electron spectrometer and four quadrants of a backscattered electron (BSE) detector has been demonstrated previously to be very useful for the separate correction of either topographical contrast or sub‐surface composition effects. In this work the possibility is tested of using BSE detector images to correct simultaneously acquired Auger images for the presence of both topographical and sub‐surface composition effects in the same sample. Measurements on model samples with known topography and sub‐surface composition are used. The samples are formed from anisotropically etched Si(100) coated in some places with a thick Au film and then overcoated everywhere with either a thin film of CuSn alloy or partially coated with a thin film of Au. The correction process is successful in reducing the artefacts everywhere and the extent of this correction is discussed.

Nanometer-resolution surface analysis with Auger electrons

High spatial resolution Auger electron spectra and images have been obtained in a UHV scanning transmission electron microscope. An edge resolution of ≈ 1 nm has been achieved in Auger electron images of thin specimens. The attenuation lengths of the collected Auger electrons and the escape depth of the high energy background secondary electrons have been estimated from these nanometer-resolution images. Surface steps are clearly visible in Auger electron images with high contrast and high resolution. These techniques have been applied to study a model catalyst system. A method has been developed to estimate the number of atoms in metal particles which are smaller than the probe size. The most recent results show that Ag clusters containing less than 10 atoms can be readily detected when supported on thin substrates.

Automatic removal of substrate backscattering effects in Auger imaging and spectroscopy

AbstractThe York multi‐spectral scanning Auger microscope has been used to investigate the correlations between Auger and backscattered electron images collected simultaneously from a electron spectrometer and the quadrants of a Sip‐n junction backscattered electron detector. Digital signal processing of the four backscattered electron signals allows the calculation of the Auger backscattering factor at each pixel position, the division of which into the Auger image allows the removal of contrast due to subsurface composition variations. The method, which allows the quantification of complicated heterogeneous samples, requires no fitting parameters or prior knowledge regarding the sample.

Testing the success of analytical image correction routines: Surface images

AbstractMultispectral techniques in Auger microscopy involve the acquisition of data as several spatially registered images or signals from the Auger, specimen absorption current, characteristic x‐ray, backscattered and secondary electron signals. Each signal contains information from different interactions and volumes within the sample. Various combinations of these signals can be used along with simple physical models to remove those contrast variations in Auger images that are not due to compositional variations of the sample within the Auger escape depth. Such contrast may, for example, be due to beam current fluctuations during data acquisition, substrate backscattering or topographical effects. To measure the success of these corrections, two tests have been devised: the calculation of the number of resolvable levels in the Auger image before and after correction; and the measurement of the modulus of correlation |CAR| between the corrected Auger image and the correction factor image. The tests may be used to evaluate any new image correction method. They have been applied to simulated and experimental data for a sample with varying subsurface composition. The results show that both tests are useful for evaluating the accuracy of a correction, and that measurement of |CAR| is the more sensitive test.

Observation of a titanium surface by desorbed ion images with a scanning electron microscope

When metallic titanium is annealed above 900°C, grain growth and segregation of sulfur to the surface occur. Using an electron scanning microscope, we have observed the secondary electron image, the sulfur Auger image and the electron stimulated desorbed ion image for a titanium specimen annealed above 900°C and exposed to hydrogen. It is found that the intrinsic titanium surface adsorbs hydrogen which diffuses from the surface into the bulk, and that segregated sulfur blocks such a hydrogen transfer. We also found that sulfur does not segregate at the grain boundaries but over the whole surface. However, the sulfur Auger image suggests that the segregation occurs easily in some crystalline surfaces but not in others.

Above-surface neutralization of highly charged ions: The formation of hollow atoms

We investigate the interaction of multiply charged ions with metallic surfaces. Simulations are presented displaying the formation of hollow atoms when slow highly charged ions approach the surface. It is shown that above surface neutralization proceeds via hollow-atom formation. Relaxation of the multiply excited states to the ground state occurs only subsequent to the close encounter with the topmost atomic layer. Recent experimental data on K Auger emission and image acceleration confirm this scenario.

Segmentation and scatter diagram analysis of scanning Auger images ? A critical comparison of results

In surface science, Scanning Auger Microscopy (SAM) is an important method for investigating the chemical composition of surfaces and obtaining information about the spatial distribution of chemical elements. Images obtained by SAM give a qualitative impression of the concentration of the selected elements on the surface. For the systematic characterization of inhomogeneous materials the evaluation of multispectral SAM-images can be facilitated by image processing techniques. Two methods, classification and segmentation, are applied to SAM images and the results are compared. Scatter diagrams have been used to classify the number and coverage of different surface phases. In SAM-literature (e.g. [1]) it is demonstrated that classification is a valuable and easy to use tool to interpret the content of multispectral images. Segmentation decomposes the images into homogeneous connected regions of similar surface composition, based on the information contained in the elemental maps. Segmentation makes it possible to extract statistical and topological features of single objects, whereas scatter diagram analysis gives information only about different surface phases.

Noise suppression in scanning Auger Images ? comparison of various digital filters

Due to the small number of electrons collected for each pixel of the image, images from Scanning Auger Microscopy (SAM) are typically noisy. Therefore it is necessary to apply noise suppression techniques both to enhance the optical impression of the picture and to support further image processing methods, such as the analysis of scatter diagrams or segmentation, which are the basis for qualitative and quantitative investigations. A variety of techniques have been proposed to reduce noise in digital images. These can be judged according to their capability of smoothing noise inside homogeneous regions, preservation of edges, sensitivity to outlier contamination, ease of implementation and performance. During the segregation of sulphur in polycrystalline high purity iron, the SAM-images of sulphur show regions of different, homogeneous sulphur coverage [1], which correspond to grains and are sharply separated by grain boundaries. Several noise suppression techniques were applied to these images and the results were judged with respect to the above mentioned characteristics.