Both TEM (transmission electron microscopy) and SEM (scanning electron microscopy) use electrons to acquire images of samples and they both use an electron source, a series of electromagnetic and electrostatic lenses which control the trajectory and shape of the electron beam and electron apertures, all housed within a chamber under a high vacuum.
Both TEM and SEM imaging technologies use focused electron beams as a source of illumination, meaning that the images produced are at a significantly better resolution than light-based microscopes. The key difference between SEM imaging and TEM is that SEM produces an image by detecting secondary or backscattered electrons, whereas TEM uses transmitted electrons to form an image.
The Basic Principles of TEM Imaging
TEM imaging is based on a beam of electrons passing through and interacting with an ultra-thin specimen; the transmitted electrons are then recorded with a camera further down the electron column. For this reason, TEM can give vital information regarding the inner structure of samples such as crystal structure, stress state information, morphology at the atomic scale, whereas SEM imaging offers valuable insight into the sample’s 3D surface and composition.
Since the sample must be very thin to allow electron transmission, the number of materials specimens that can be viably imaged is limited adding a difficult and expensive sample preparation step to the imaging workflow.
TEM imaging can achieve excellent spatial resolutions, with resolutions of less than 50pm being reported whereas SEM imaging is limited to ~0.5 nm.
SEM Imaging Basics
SEM imaging is very popular with scientists in the materials and life science research areas as its resolution and depth of field capabilities are a significant improvement on those of traditional optical microscopy. SEM imaging uses deflector coils which alter the path of the electron beam so it scans a sample in a raster pattern. Typically, three detectors are positioned at angles in the sample chamber, these are an X-ray detector, a back-scattered electron detector, and a secondary electron detector. None of these elements are reliant on transmission, meaning sample thickness is not a significant issue.
This does cause a relative drop in resolution; however, SEM imaging offers 3D surface mapping whereas TEM imaging offers 2D internal imagery.
Choosing between TEM and SEM Imaging
The choice of whether to use TEM or SEM imaging comes down to three considerations: 1) required resolution; 2) speed and ease of analysis; 3) ability to adequately prepare the specimen. SEM imaging is often considered the quicker, more versatile, and convenient option as TEM sometimes has issues with material applicability which can only be resolved through a time-consuming thinning process. TEM, however, allows for more resolving power and the ability to visualize and analyze atomic level information.
At JEOL, we create equipment for both TEM and SEM imaging and our experts can advise you on which process will work best for your application, just contact us today for more information.