Unit for Nanocharacterization Equipment and Techniques ESEM
Scanning Electron Microscope Quanta 200
ESEM - Overview
ESEM - Basics and Tutorials
Basics and Tutorials
The scanning electron microscope (SEM) generates a beam of electrons in a vacuum. That beam is collimated and focused by electromagnetic lenses and scanned across the surface of the sample by electromagnetic deflection coils. Interaction of primary electron beam with the material of the sample in SEM causes excitation of secondary, backscattered, Auger electrons, characteristic X-ray radiation and photons of light. The primary imaging method is by collecting electrons that are released by the sample. Depending on their energy, angular distribution and the excitation energy of the primary beam, the electrons emitted by a sample are detected by different electron detectors mounted in the microscope chamber up to a sample surface. Detection of the electron signals is done either through solid state silicon-based detectors, or via photomultiplier-type detector involving double-conversion of a signal through light photons. By correlating the sample scan position with the detected signal, an image of a sample is formed. This image could be strikingly similar to what would be seen through an optical microscope and is, therefore, more or less intuitively understood by a human brain. Generally, each detected signal provides specific type of imaging in SEM. Imaging in secondary electrons (former samples' electrons leaving its surface with up to 250 eV excessive energy) provides mainly topographic information. Imaging in back-scattered electrons uses high energy electrons that emerge nearly 180 degrees from the illuminating beam direction. The backscatter electron yield is a function of the average atomic number of each point on the sample, and thus can give compositional information. Scanning electron microscopes are often coupled with X-ray analysers. The energetic electron beam - sample interactions generate X-rays that are characteristic of the elements presenting in the sample and are used to identify local chemical composition of the samples (energy dispersive X-ray spectroscopy (EDS)).
An electron microscope requires a good vacuum for the generation and propagation of the electron beam, which in the past meant that the specimen under examination had to be placed also in vacuum. Scanning of the non-conductive sample with a beam of negatively charged electrons causes its surface charging, which seriously complicate or exclude at all observation of such specimens without conductive coatings. However, it is now possible to view specimens inside a gaseous environment so that wet samples in a water vapor environment can also be examined. The conductive coating of specimens according to conventional practice is no more necessary because the gaseous layer around the specimen becomes ionized and suppresses charge accumulation. The gas itself can also be used as detection medium giving rise to novel detection and imaging techniques. These innovations have resulted in the Environmental Scanning Electron Microscope (ESEM) .
In our ESEM Quanta 200 we employ water vapors to generate either low vacuum (LV) mode with pressure < 1 Torr, or ESEMTM mode with pressure as high as 20 Torr.
For basic education and principle understanding of Scanning Electron
Microscopy, please, use following web resources:
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