What is an SEM?


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White Blood Cell

Scanning Electron Microscope (SEM) image of a White Blood Cell

SEM stands for Scanning Electron Microscope. The term "SEM" is used to describe a microscope used to create high magnification, black and white images of devices with dimensions from a few tenths of a micron to several millimeters in diameter. The term SEM is also used to describe the photographs taken by the Scanning Electron Microscope. Hence, SEM refers both to the microscope, and the photograph created by the microscope.

Traditional optical microscopes have a very shallow depth of field. This means that images created with optical microscopes have only a small area that is in focus. Optical microscopes can create quality images of things that are flat, but can not create good images of three dimensional structures because of the focus issues. Scanning Electron Microscopes, on the other hand, have a very large depth of field. This means that they can image complex three dimensional structures, with all parts of the device being imaged being in focus. This large depth of field results in stunning, sharp photographs of very small things.

Adaptive Optics

SEM photograph of a Microscopic Motor Connected to a Positional Mirror. Note the Clarity of the photograph, and the sharp focus of all elements of the micorstructure.

Optical Microscope Photograph of a Microscopic Motor and Positional Mirror. Note the poor focus on the spring structure, in comparison to SEM at left.

SEMs are only able to create Black and White images. When SEM images are used in publications or books, they are sometimes colored by a graphic artist for dramatic impact. It should be understood, however, that the colors were added by an artist, and in no way reflect the actual structure being depicted. Optical microscopes, on the other hand, can create true color images. In comparing the two photographs above, it can be seen that the SEM creates much sharper images than what can be achieved with an optical microscope. The two structures pictured above are MEMS (MicroElectroMechanical System) devices. These devices are relatively flat. The advantage of the SEM becomes more pronounced as devices become more three dimensional.

A scanning electron microscope operates by using a beam of electrons to image extremely small structures. The electron beam is created by a cathode typically made of tungsten. An electric field is used to accelerate the electron beam towards the cathode, where the structure to be imaged is located. Coils or deflector plates are used to scan the beam across the sample to be imaged. The resolution that can be achieved depends on the spot size of the electron beam. The smaller the beam, the better the resolution that can be achieved.

In order to achieve proper creation, control, and detection of the electron beam, the imaging must be done in a vacuum. In addition, in order to get good contrast and resolution, the surface of the device to be imaged must be very conductive. These requirements of conductive surfaces, and operation in a vacuum requires that a sample be properly prepared in order to achieve good images. Biological samples can be damaged when placed in a vacuum, as the water in the sample will vaporizes, which can cause the sample to explode or dehydrate.

A typical sample preparation for non-conductive solids, and biological samples is to coat the sample with gold prior to imaging. A very thin coating of gold will provide the conductivity needed, and will offer protection for fragile biological material.  The images presented on this site were all created by first gold coating the samples.

More recently, a new type of SEM has been developed which does not require high levels of vacuum. These environmental SEMS allow imaging of devices which can not be coated with gold, or prepared with other techniques. Environmental SEMS offer the ability to image structures which previously were not compatible with SEM imaging. The resolution and contrast of images in an environmental SEM are typically not as good as a traditional SEM.

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