Glossary of EM & Surface Analysis Terms
BSEM Backscattered Scanning Electron Microscopy
The technique is similar to SEM in that it employs a focussed beam of electrons that is rastered across the surface of the sample. However, in this case a solid-state detector is employed that is sensitive to the electrons that are backscattered from the sample. The signal is a function of the atomic number of the sample and so this technique is used to distinguish phases in the scanning electron microscope. Backscatter detectors are often constructed in segments and the signals from different segments are mixed together to optimize the compositional or topographical information in the image.
(C)TEM (Conventional) Transmission Electron Microscopy
A term applied to 'normal' TEM imaging. The electron beam is passed through a thin film sample (typically ~1-200 nm thick). Bright field diffraction contrast images are formed with the direct (undiffracted) beam. Dark field images are formed with a selected diffracted beam. CTEM imaging is used in the general observation of samples and careful selection of the diffracting conditions of the sample will allow the analysis of defect structures within the sample.
CBED Convergent Beam Electron Diffraction
The technique of convergent beam electron diffraction employs a tightly focussed electron probe to obtain crystallographic information from small regions of the specimen. The patterns frequently contain a wealth of detail which may yield information on the crystal symmetry and atomic and electronic structure of the sample.
HREM High Resolution Electron Microscopy
High Resolution Electron Microscopy is phase contrast microscopy of the atomic structure of materials. In most crystalline inorganic materials and a number of polymeric materials HREM allows the imaging of individual atomic columns. The images can frequently be interpreted in terms of the projected crystal potential, although it is often necessary to match the experimental images with those calculated from multislice algorithms.
(P)EELS (Parallel) Electron Energy Loss Spectroscopy
Electron energy loss spectroscopy analyzes the inelastically scattered electrons present in the beam after it has been transmitted through the sample. An EEL spectrum typically consists of a monoatomically decreasing background on which are superimposed a number of peaks. Each peak is characteristic of the scattering process that has occured in the sample. The peaks can be used to obtain information about the chemical composition and electronic structure of the sample. EEL spectra are acquired typically in a magnetic sector spectrometer located under the camera chamber of hte AEM/TEM. Spatial resolution is typically limited by the minimum probe diameter of the microscope. EELS tends to be complimentary to XEDS in that it can be used to analyze very thin samples of low Z materials.
SAED Selected Area Electron Diffraction
In this diffraction mode an aperture is used to define the area from which a diffraction pattern is to be recorded from a thin sample. This aperture is typically located in an image plane below the sample. SAED patterns are simple spot patterns and are of use in phase determination (lattice spacing mesaurement) and defect analysis (sample orientation).
SEM Scanning Electron Microscopy
Scanning electron microscopy is performed by scanning a focussed probe across the surface of the sample to be studied. Secondary electrons emitted from the sample are typically detected by a photomultiplier system, the output of which is used to modulate the brightness of a TV monitor that is rastered in synchronization with the with the electron beam scan. The more electrons a particular region emits, the brighter the image at that point. SEM images typically contain a good deal of topographical detail.
SFM Scanning Force Microscopy
Initially called Atomic Force Microscopy (AFM), this technique is now more typically termed Scanning Force Microscopy (SFM) or Scanning Probe Microscopy (SPM). This instrument is essentially an extremely high resolution profilometer. A sharp tip, typically fabricated from silicon nitride, is scanned across the surface of a sample at a constant force by three piezoelectric ceramics. The piezoelectric ceramics are computer controlled via a feedback loop which monitors the position of the tip by means of an optical lever (a laser is focussed on the top of the tip support and the beam reflected into a position sensitive detector). The changes in height of the tip are used to form an image as the tip is scanned across the sample.
STEM Scanning Transmission Electron Microscopy
n STEM, the electron beam is rastered across the surface of a sample in a similar manner to SEM, however, the sample is a thin TEM section and the diffraction contrast image is collected on a solid-state detector.
WDS Wavelength Dispersive Spectroscopy
WDS is a microanalytical technique that is based on the characteristic X-ray peaks that are generated when the high energy beam of the electron microscope interacts with the specimen. Each element yields a characteristic spectral fingerprint that may be used to identify the presence of that element within the sample. The relative intensities of the spectral peaks may be used to determine the relative concentrations of each element in the specimen. The X-ray signal is detected by one of a series of diffraction spectrometers.
XEDS X-ray Energy Dispersive Spectroscopy
XEDS is a microanalytical technique that is based on the characteristic X-ray peaks that are generated when the high energy beam of the electron microscope interacts with the specimen. Each element yields a characteristic spectral fingerprint that may be used to identify the presence of that element within the sample. The relative intensities of the spectral peaks may be used to determine the relative concentrations of each element in the specimen. The X-ray signal is detected by a solid-state silicon-lithium detector and the construction and efficiency of this detector sets a lower limit on the atomic number that may be detected. Generally elements heavier than carbon are detectable.