High Resolution Secondary Electron Research Articles

Applications and techniques of thickness measurement for sections and thin films

The thickness of a specimen to be examined in the electron microscope is a significant parameter for many applications. However, due to the lack of ideal methodology for absolute thickness measurement, most quantitative analyses are performed using approaches which are more convenient. For example, theoretical factors, indirect relative thickness or ratio methods are used to eliminate the necessity to obtain the absolute thickness. These alternatives are theoretically valid. However, experimental verification using standards with known thicknesses are still imperative. Since absolute thickness remains indispensable, continuous development and improvement of techniques for thickness measurement are still much needed. The purpose of this study is to critically survey the importance of absolute thickness measurement and assess the available methods for thickness determination.Knowledge of specimen thickness can be applied for both qualitative and quantitative purposes. Specimen thickness can be used to study imaging properties in different electron microscopy modes. Monitoring of film thickness is, for example, essential when coating specimens for high resolution secondary electron imaging, preparing multilayer specimens in material sciences or preparing ultrathin support films.

Evaluation of myocardial pathology, including cardiac homograft rejection, using acoustical microscopy

Controlled formation of UH3 on uranium metal surfaces was induced by a precisely limited uptake of hydrogen at 250 °C and 320 °C with 500 mbar H2 pressure. The hydride (UH3) growth sites on the sample surfaces were studied using focused ion beam (FIB) milling with high resolution secondary electron imaging. Electron backscatter diffraction (EBSD) analysis was also used to examine grain orientation of the metal in the region of the corrosion sites.Results of the analysis indicated that the location of hydride sites was predominantly found to be associated with grain boundaries at the metal surface. In addition, FIB cross-sectioning of multiple hydride sites indicates the observed morphology of very small hydride growths to be consistent with nucleation at, and not beneath, the oxide–metal interface.

The Origins of High Spatial Resolution Secondary Electron Microscopy

AbstractThe secondary electron generation process is studied in an ultra-high vacuum scanning transmission electron microscope using electron coincidence spectroscopy. Production pathways for secondary electrons are determined by analyzing coincidences between secondary electrons and individual excitation events. The ultimate spatial resolution available in scanning electron microscopy is limited by the delocalization of the secondary electron generation process. This delocalization is studied using momentum resolved coincidence electron spectroscopy. The fraction of secondary electrons resulting from localized excitations can explain the high spatial resolution observed in secondary electron microscopy images.

Topographic contrast of monatomic surface steps on Si(100) in secondary electron microscopy

A phenomenonological model of topographic contrast in secondary electron microscopy is presented. This model involves exponential attenuation of isotropically generated secondary electrons. The effect of primary beam diameter and the material dependent secondary electron attenuation length on secondary electron images is demonstrated by explicitly computing linescans of images of straight surface steps. These computed linescans are directly compared with those obtained from monatomic surface steps on Si(100) imaged at normal incidence in an ultrahigh-vacuum scanning transmission electron microscope. An asymmetry in the experimental linescan cannot be fit by any combination of model parameters suggesting that this contrast is not simply due to surface topography. A simple explanation for the contrast reversal observed in secondary electron images of surface steps when the primary beam changes direction from the upstairs to the downstairs direction is presented. The possibility of determining secondary electron emission parameters and extracting surface chemical and electronic information using high spatial resolution secondary and Auger electron imaging is briefly discussed.

High-resolution Auger Electron Microscopy and spectroscopy of supported metal particles

A detailed understanding of catalytic processes requires structural knowledge of the catalyst system. The surface properties of small metal particles which are highly dispersed on insulating supports must play a dominant role in determining this system's catalytic behavior. Therefore characterization of surface topography and composition of heterogeneous catalysts is important. Surface topography of the carrier materials can be obtained by high resolution secondary electron imaging (SEI) in a STEM. The small metal particles can also be located and their topographic relationship with the support morphology determined. However, an elementally specific signal such as the Auger electrons must be used to extract chemical information about the surface species. High resolution Auger electron spectroscopy (AES) and scanning Auger microscopy (SAM) have recently been incorporated into a UHV STEM. In this paper we report some results from the application of this instrumentation to the study of supported metal clusters.

Secondary Electron Imaging of Crystal Surface Steps

High resolution secondary electron (SE) images can be obtained in a scanning transmission electron microscope (STEM) by collecting the type I secondary electrons generated directly by the incident electron beam. The high resolution SE signal is, in general, localized to about 1 nm in the generation process. The escape depth of the secondary electrons, however, varies from about 1 nm for most metals to about 10 nm for some insulating materials. Surface steps can be imaged by collecting high resolution secondary electrons Among other possible contrast mechanisms for SE imaging of surface steps, topographic contrast plays an important role. The topographic contrast of surface steps depends on: (1) the incident electron probe size; (2) step height to SE escape depth ratio; (3) localization of the generated SE signals and (4) the incident beam angle relative to the specimen surface.In our UHV STEM (MIDAS) instrument, both the entrance and exit surfaces of the sample can be imaged by collecting secondary electrons. For thin samples, the resolution is < lnm for SE images of entrance and exit surfaces but for thicker samples the resolution of the SE image obtained from the exit surface will be degraded by the beam broadening effects. For studying steps on thick or bulk crystal surfaces, secondary electrons emitted from the entrance surfaces are, in general, collected to form images with high contrast and high resolution. In this report, all the SE images are obtained, in MIDAS, by collecting secondary electrons emitted from the entrance surfaces of the samples. Figure 1 is a SE image of a MgO smoke crystal revealing the surface growth steps with high contrast. The parallel straight steps, along <100> directions, constitute the edges of unfinished atomic planes on the MgO {100} crystal surfaces. This image indicates that the {100} surfaces of MgO smoke crystals are flat surfaces. Figure 2 shows a SE image of a MoO3 crystal with surface growth steps spiraling from an end-on screw dislocation (the black dots in the SE image represent electron beam induced reduction products). The dark lines (indicated by D) represent up-steps and the bright lines (indicated by B) represent down-steps. It is interesting to note that the two-dimensional crystal growth around the dislocation is anisotropic, forming terraces with long edges parallel to [010] direction.

The oxidation of vicinal copper (100) surfaces

High resolution secondary electron imaging is used to observe the interaction of copper oxide nuclei and steps on nominally (100) copper surfaces at 300 °C and oxygen pressures around 10 −5 Torr. This is a dynamic event with steps moving across the surface until an oxide island nucleates on a step and pins it locally. As the islands grow they influence the surface morphology so that they are observed to be situated on high points of the surface.

High resolution secondary electron imaging and spectroscopy

Secondary electron signals with 1 nm spatial resolution and 1 eV energy resolution obtained in a STEM are presented and discussed. Reflection SE images of oxidised Cu show oxide islands and details of their interaction with surface steps. Stopping power theory identifies inner shell excitations and well localised valence excitations rather than plasmon rather than plasmon excitations as the dominant SE generation mechanisms and accounts for the spatial resolution obtained.

Studies of supported metal catalysts using high resolution secondary electron imaging in a STEM

SUMMARYAn additional technique for use in the characterization of catalysts by electron microscopy is presented. High resolution secondary electron images obtained in a VG HB501 scanning transmission electron microscope have been used to study the surface topography of catalysts consisting of small metal particles on high surface area carbon supports. Surface features down to nanometre dimensions can be seen, allowing the examination of micropores in the support as well as larger pore structures. The results are compared with pore size distributions determined by gas adsorption methods, and are shown to yield valuable additional information. In addition, the method in principle allows examination of the locations of small metal catalyst particles on the support.

High-resolution scanning electron microscopy of surface reactions

High-resolution ( < 10 A ̊ ) and high-quality secondary electron (SE) images can be obtained from specimens in the high-resolution imaging position of a HB5 dedicated STEM instrument, in parallel with STEM or SREM imaging and with microdiffraction and microanalysis using EELS. The additional information obtained by the SEM arises from the possibility of revealing three-dimensional morphology and sensitivity to the chemical composition of surface layers. Examples include evidence of a reaction between Pd and MgO substrate and the formation of Ni by reduction of NiO under electron beam irradiation.

Digital image processingmethods for improvement of quality of high-resolution secondary electron images.

Digital image processingmethods for improvement of quality of high-resolution secondary electron images.

High resolution at low voltage: The SEM philosopher's stone?

A new type of scanning electron microscope (SEM) has been designed to produce nanometre-level high resolution secondary electron images of the surfaces of uncoated non-conducting specimens such as structural ceramics, cement (Fig.1), supported catalyst particles (Fig.2), fine scale metallic features (Fig.3) including radiation damage, and eventually also biological material. Clean low voltage systems also look promising for microanalysis of surface layers e.g. Fig.4.Very useful 5-20nm resolution images of surfaces are obtained with a conventional electron microscope operated at 25-100kV (Figs.2,3) but at these high accelerating voltages the secondary electron yield of most materials is very low, typically &lt; 0.1, and there are therefore severe surface charging problems with non-conducting regions of specimens, the images of which are consequently of poor quality.

High Resolution Scanning Electron Microscopy in Biology: Artefacts Caused By the Nature and Mode of Application of the Coating Material

Results of high resolution secondary electron observations of the haemocyanin molecule from the marine whelk Buccinum undatum are given. The choice of metal used for surface coating of the sample (gold, gold--palladium or carbon/gold--palldium) and the mode of application of this metal (thermal vacuum evaporation or diode sputtering) were both found to be of utmost importance when working at high magnifications in the scanning electron microscope. Sputter coating with gold gives poor results because of a granular and cracked appearance of the surface film. The haemocyanin molecules were difficult to recognize in these preparations. Best results were obtained by a thermal vacuum evaporation of gold--palladium. It is suggested that the inferior results seen after sputter coating may be due to the high temperatures wihch the specimen experiences during the sputtering, even when cooling of the specimen stage is carried out. Other specimens (diatom valves and latex spheres) were also studied at high magnification giving the same results as the haemocyanin molecules.