MicroVision Labs, Inc. - Analytical Microscopy and SEM/EDS Solutions.

Scanning Electron Microscopy (SEM)

Principle of Operation
An SEM is a very powerful microscope (typical magnification range from ~10X to over 100,000X) which uses a thin focused beam of electrons to produce an image by scanning over the surface of the sample. Electrons bouncing off of the sample are collected by a detector, amplified, and scanned across a computer monitor in synch with the beam that is scanning across the sample. The brightness of any pixel on the monitor corresponds to the amount of electrons, or signal, collected from the corresponding point on the sample when the electron beam passes over it. The pattern of the scanning beam, or the raster pattern, thus generates the image we see. By controlling the size of the sample area scanned, typical a square shaped area, we can control the magnification of the image since the size of the image on the monitor remains constant. So if the monitor is 10 inches wide and the electron beam scans an area that is 1 inch wide the magnification is 10X. But if we shorten the width of the area scanned on the sample to say a tenth of an inch, keeping the display on the monitor at 10 inches wide, the magnification becomes 100X.

Depending on the type of information desired from the sample either secondary electrons or backscatter electrons are collected coming off of the sample. Secondary electrons produce better topographical or 3 dimensional images, than backscatter electrons and are by far the most common images recorded from SEMs. In a secondary electron image, high points on a sample will appear brighter while low points, or valley, will appear darker. Backscatter electrons provide excellent elemental density information of the sample but not very good topographical information. In a backscatter electron image, denser areas of the sample will appear brighter than less dense areas, and all things being equal metals with higher atomic numbers will appear brighter than those of lower atomic numbers. This is extremely useful when you are hunting for a needle in a hay stack. More literal examples would be a glass fiber in a dust sample or a mineral grain containing rare earth elements (REE) in a sample of sand.

X-Rays are another type of signal produced when the electron bean hit the sample. By the use of a detector that can collect and differentiate the energy levels of the X-Rays produced, an Energy Dispersive X-Ray Spectroscopy (EDS) detector, one can obtain instantaneous elemental data from the sample. This ability has, of course, an almost unlimited number of applications and makes the SEM the powerful and versatile analytical tools around today.

Advantages:
The SEM can operate at a wide range of magnifications, including very high magnifications, and produce images with incredible resolution and great depth of field. It can outperform any other common type of microscope, even at very low magnifications. The SEM is extremely useful tool for morphological investigations, sizing and particle counting. It also allows for non-destructive analysis of most samples. When equipped with an Ultra Thin Window Light Element EDS detected, as our SEM is, it can also provide elemental information (carbon and up) about the sample as you image it.

Limitations:
Samples must be electrically conductive otherwise they must be able to be coated with a conductive material. We are equipped to coat most samples on site. Maximum sample size is typically less than 3 inches in diameter, so large samples must be cut down to size. Samples must be able to withstand vacuum pressures in the 10-6 torr range as well as a high energy electron beam. The SEM only images surface features of samples and can not image internal features such as a particle imbedded in glass or clear plastic, even though they are visible under a light microscope. Another difference from the light microscope is that the SEM cannot take color images, since color is a function of light waves and not electrons. False color can be added to the digital SEM image later by a number of computer imaging programs should you marketing types need that extra zip.

-John H. Knowles
President
MicroVision Labs, Inc.

The Power of SEM/EDS:

The Scanning Electron Microscope (SEM), combined with Energy Dispersive X-ray Spectroscopy (EDS), is the most versatile and powerful analytical tool available today. SEM images have high resolution, and an equally important large depth of field. Our SEM uses a LaB6 filament, which allows for incredible resolution (5 nm attainable). EDS provides real-time elemental data (from particles as small as 1 micron) during imaging. Additionally, our EDS detector has an ultra thin window, which detects and quantifies elements as light as Carbon. The combination of the morphological and elemental information provided by the SEM/EDS is therefore indispensable for identifying unknowns, analyzing failures, or benchmarking competitive products.

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