Background information - What is scanning electron microscopy?
A Scanning Electron Microscope (SEM) is a tool for seeing otherwise invisible worlds of microspace (1 micron = 10-6m) and nanospace (1 nanometer = 10-9m). By using a focussed beam of electrons, the SEM reveals levels of detail and complexity inaccessible by light microscopy.
SEMs can magnify an object from about 10 times up to 300,000 times. A scale bar is often provided on an SEM image. From this the actual size of structures in the image can be calculated.
Essentially, the way the scanning electron microscope "looks" at the surface can be compared to a person alone in a dark room using a fine beamed torch to scan for objects on a wall. By scanning the torch systematically side-to-side and gradually moving down the wall, the person can build up an image of the objects in their memory. The SEM uses an electron beam instead of a torch, an electron detector instead of eyes, and a viewing screen and camera as memory.
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Electrons are negatively charged particles within the atom. Unlike light photons, electrons cannot be focussed by glass lenses, but electromagnets are capable of guiding and concentrating electrons just as glass bends light.
The scanning electron microscope (SEM) provides the competent user with an advantage over the light microscope (LM) in three key areas:
- Resolution at high magnification. Resolution can be defined as the least distance between two closely opposed points, at which they may be recognized as two separate entities. The best resolution possible in a LM is about 200 nm whereas a typical SEM has a resolution of better than 10 nm (typically 5 nm).
- Depth of field i.e. the height of a specimen that appears in focus in an image - more than 300 times the depth of field compared to the LM. This means that great topographical detail can be obtained. For many users, the three dimensional (3D) appearance of the specimen image, is the most valuable feature of the SEM. This is because such images, even at low magnifications, can provide much more information about a specimen than is available using the LM. The use of "stereo pair" SEM images can give even greater information about the sample.
- Microanalysis i.e. the analysis of sample composition including information about chemical composition, as well as crystallographic, magnetic and electrical characteristics.
One drawback to the SEM
One drawback to the use of the SEM is that it operates under vacuum and in many SEMs the samples must be rendered conductive to be viewed. This is often achieved by coating with a very thin layer of metal or carbon. However, there are a number of different types of SEMs which all have specific purposes, often associated with additional pieces of equipment like specialised stages or collectors. Some of these do not require dry or conductive samples. They include the following:
- Low vacuum scanning electron microscopy (LVSEM)
- Using cryo on a scanning electron microscope (Cryo-SEM)
- Environmental scanning electron microscope (ESEM)
- Focused ion beam (FIB) technology
- E-beam lithography (EBL)
Fundamentally and functionally, electron microscopes are in many ways analogous to their optical counterparts (light microscopes: LM). This is somewhat surprising at first glance, given the contrast between the simple technology of the LM and the complex electronics, vacuum equipment, voltage supplies and electron optics system of electron microscopes. A comparison of these features is used frequently in textbooks as a starting point for any discussion of SEM.