A Scanning Electron Microscope (SEM) offers several major advantages over the more common and familial optical microscope. (1) The SEM has higher magnification capabilities than an optical microscope (100,000X compared to 1000X for an optical microscope). (2) The SEM can obtain in-focus images of rough samples which have a large variation in vertical height. In other words, the SEM can focus on both the “peaks” and “valleys” of a rough fracture surface at the same time while an optical microscope can only focus on the “peaks” or “valleys”. This large depth of focus (300 times deeper than an optical microscope) is one of the SEM’s greatest assets. Since it provides 3-D like images of fractures, it allows the analyst to visually identify the fracture type and origin, a critical step in any failure analysis. (3) The illumination source for SEM, a beam of high energy electrons, causes the sample to emit low level x-rays. These x-rays can be used to perform chemical analyses of the sample corresponding to the area viewed on the SEM. By increasing the magnification and thereby illuminating a smaller and smaller area, pinpoint chemical analyses of microscopic features and particles can be performed. This last feature is discussed further under the entry for Energy Dispersive Spectroscopy.
An optical microscope uses light to illuminate a sample for examination. A scanning electron microscope uses a beam of electrons. Sophisticated electronic circuitry is utilized to generate a stable electron beam which is then focused on the sample with electro-magnetic lenses. Additional circuitry transfers the focused image of the sample to a monitor for viewing. Images are then collected and saved in digital format. The beam and sample are under extreme vacuum during the examination process.
The AMRAY 1830i Scanning Electron Microscope is shown at the top of the page (right). The image to the immediate right, while not a typical materials engineering subject, demonstrates the high magnification and depth of focus capabilities of the SEM. The object at the upper left (A) is a human red blood cell. A white blood cell is shown at left center (B). The smaller spherical object at lower left is a bacteria. Magnification of this image is 20,000X.