Scanning electron microscopy (SEM) is a method for high-resolution imaging of surfaces. The SEM uses electrons for imaging, much as a light microscope uses visible light. The advantages of SEM over light microscopy include much higher magnification (>100,000 X) and greater depth of field up to 100 times that of light microscopy. Qualitative and quantitative chemical analysis information is also obtained using an energy dispersive X-Ray spectrometer (EDS) and wavelength dispersive spectrometers (WDS) with the SEM.
The SEM generates a beam of incident electrons in an electron column above the sample chamber. The electrons are produced by a thermal emission source, such as a heated tungsten filament, or by a field emission cathode. The energy of the incident electrons can be as low as 100 eV or as high as 30 keV depending on the evaluation objectives. The electrons are focused into a small beam by a series of electromagnetic lenses in the SEM column. Scanning coils near the end of the column direct and position the focused beam onto the sample surface. The electron beam is scanned in a raster pattern over the surface for imaging. The beam can also be focused at a single point or scanned along a line for X-Ray analysis. The beam can be focused to a final probe diameter as small as about 10 Å.
The incident electrons cause electrons to be emitted from the sample due to elastic and inelastic scattering events within the sample's surface and near-surface material. High-energy electrons that are ejected by an elastic collision of an incident electron, typically with a sample atom's nucleus, are referred to as backscattered electrons. The energy of backscattered
electrons will be comparable to that of the incident electrons. Emitted lower-energy electrons resulting from inelastic scattering are called secondary electrons. Secondary electrons can be formed by collisions with the nucleus
where substantial energy loss occurs or by the ejection of loosely bound electrons from the sample atoms. The energy of secondary electrons is typically 50 eV or less.
To create an SEM image, the incident electron beam is scanned in a raster pattern across the sample's surface. The emitted electrons are detected for each position in the scanned area by an electron detector. The intensity of the emitted electron signal is displayed as brightness on a cathode ray tube (CRT). By sychromizing the CRT scan to that of the scan of the incident electron beam, the CRT display represents the morphology of the sample surface area scanned by the beam. Magnification of the CRT image is the ratio of the image display size to the sample area scanned by the electron beam. Two electron detector types are predominantly used for SEM imaging. Scintillator type detectors (Everhart-Thornley) are used for secondary electron imaging. This detector is charged with a positive voltage to attract electrons to the detector for improved signal to noise ratio. Detectors for backscattered electrons can be scintillator types or a solid-state detector.
The SEM column and sample chamber are at a moderate vacuum to allow the electrons to travel freely from the electron beam source to the sample
and then to the detectors. High-resolution imaging is done with the chamber at higher vacuum, typically from 10
-5 to 10
-7 Torr. Imaging of nonconductive,
volatile, and vacuum-sensitive samples can be performed at higher pressures.
Interaction electron beam-sample.
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Secondary electron image. |
Analytical Information
Secondary Electron Imaging - This mode provides high-resolution imaging of fine surface morphology. Inelastic electron scattering caused by the interaction between the sample's electrons and the incident electrons results in the emission of low-energy electrons from near the sample's surface. The
topography of surface features influences the number of electrons that reach the secondary electron detector from any point on the scanned surface. This
local variation in electron intensity creates the image contrast that reveals the surface morphology. The secondary electron image resolution for an ideal
sample is about 3.5 nm for a tungsten-filament electron source SEM or 1.5 nm for field emission SEM.
Backscatter Electron Imaging - This mode provides image contrast as a function of elemental composition, as well as, surface topography. Backscattered electrons are produced by the elastic interactions between the sample and the incident electron beam. These high-energy electrons can
escape from much deeper than secondary electrons, so surface topography is not as accurately resolved as for secondary electron imaging. The production effeciency for backscattered electrons is proportional to the sample material's mean atomic number, which results in image contrast as a function of composition, i.e., higher atomic number material appears brighter than low atomic number material in a backscattered electron image. The optimum resolution for backscattered electron imaging is about 5.5 nm.
Typical Applications in Rolls
- Microscopic feature measurement (microstructures, cracks, retempered and rehardened areas)
- Fracture characterization (spalling, fatique fractures)
- Microstructure studies (carbides, inclusions)
- Coating evaluations (Cr coatings)
- Surface contamination examination (foreign particles, material welding on roll surface)
Modern Scanning Electron Microscope (SEM).
Image by courtesy of Carl Zeiss company.
