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 greater magnification (up to 100,000X) and much greater depth of field.

An incident electron beam is raster-scanned across the sample's surface, and the resulting electrons emitted from the sample are collected to form an image of the surface. Imaging is typically obtained using secondary electrons for the best resolution of fine surface topographical features. Alternatively, imaging with backscattered electrons gives contrast based on atomic number to resolve microscopic composition variations, as well as, topographical information. Qualitative and quantitative chemical analysis information is also obtained using an energy dispersive x-ray spectrometer with the SEM (see separate web page for EDS analysis).

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Secondary electron imaging - provides high-resolution imaging of fine surface morphology. Inelastic electron scattering caused by the interaction between the sample's electrons and the incident electron beam results in the emission of low-energy electrons from near the sample's surface. The orientation of surface features influences the number of electrons that reach the secondary electron detector, which creates variations in image contrast that represent the sample's surface topography. The secondary electron image resolution for an ideal sample is about 3.5 nm.

Backscattered electron imaging - provides elemental composition variation, 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. The efficiency of production of backscattered electrons is proportional to the sample material's mean atomic number, which results in image contrast as a function of composition - higher atomic number material appears brighter than low atomic number material. The optimum image resolution for backscattered electron imaging is about 5.5 nm.

Quantification - Microscope image magnification is calibrated against a reference standard. Lateral features dimension can be readily quantified to an accuracy of less than 0.1µm. Secondary computer analysis of images can quantify area/volume fractions and particles shapes and sizes.

Data formats - Images can be recorded on Polaroid instant film, low-cost video prints, videotape, or as bitmap (BMP), tagged-image (TIFF), or other computer file formats.

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• Microscopic feature measurement
• Fracture characterization
• Microstructure studies
• Thin coating evaluations
• Surface contamination examination
• Failure analysis

MEE has a JEOL JSM-5800LV digital SEM. This SEM operates using conventional high-vacuum SEM techniques, as well as, using a variable-pressure mode to image and analyze nonconductive and wet samples.

Samples up to 8 in. (200 mm) in diameter can be readily accommodated. Larger samples, up to about 12 in. (300mm) across can be loaded with limited stage movement. Sample height is limited to 2 in. (50 mm). Backscattered electron imaging can be performed on conductive or nonconductive samples. For secondary electron imaging, samples must be electrically conductive. Nonconductive materials can be evaporatively coated with carbon or gold to obtain conductivity without affecting observed surface morphology.

Samples must be compatible with moderate vacuum. For secondary imaging, the sample environment is at a pressure of about 1x10-5 Torr. The pressure for backscattered electron imaging can be adjusted up to 2 Torr for vacuum sensitive samples.

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