A scanning electron microscope (SEM) scans a focused electron beam over a surface to produce an image. The electrons in the beam interact with the sample, creating differing signals that can be employed to gain information about the surface topography and composition.
Why is There a Need for SEM Imaging?
With enough light, a healthy human eye can distinguish points 0.2mm apart without the help of additional lenses. A lens or a microscope (using a group of lenses) can be employed to magnify this distance and allow the eye to see points closer together than 0.2mm.
Modern light microscopes can magnify up to 1000x, limited by the number and quality of the lenses and also the wavelength of the light used for illumination. The electron microscope was created when wavelength became the limiting factor in light microscopes. Electrons have significantly shorter wavelengths, resulting in much better resolution. Materials and devices are shrinking, meaning many structures cannot be characterized by light microscopy.
How Do SEM Microscopes Work?
The main components of an SEM microscope are a source of electrons, a column for the electrons to travel down with electromagnetic lenses, electron detector, sample chamber, and a computer with a display to see the images.
Electrons are produced at the top of the column, moved down through a group of lenses and apertures to create a focused beam of electrons which hits the surface of the sample. The sample is mounted on a stage in the chamber area and both the column and chamber are evacuated by a combination of pumps. The level of vacuum will be respective to the design of the microscope.
Scan coils above the objective lens control the position of the electron beam on the sample. The coils mean the beam can be scanned across the surface of the sample. This enables information about a specific area of the sample to be obtained. The electron-sample interaction produces a number of signals which are then detected.
SEM imaging occurs by scanning the sample with a high-energy beam of electrons. When these electrons interact with the sample they create secondary electrons, characteristic x-rays, and backscattered electrons. One or more detectors collect these signals and form images that can be seen on a computer screen.
When the electron beam impacts the surface, it pierces the sample to the depth of a few microns, depending on the accelerating voltage and density of the sample. A lot of signals such as secondary electrons and X-rays are created because of the interaction within the sample.
Applications of SEM Imaging
SEM Imaging is used in many different areas to learn more about the composition and topography of both naturally occurring and man made materials. SEM imaging has aided biologists understanding of microscopic organisms such as bacteria and viruses in addition to aiding geologists learn more about crystalline structures.
Manufacturing businesses can use SEM imaging to learn more about the composition and topography of products and components. Stainless steel is one such example as it must be evenly coated with particular chemicals for its best performance. SEM imaging can help locate cracks and contaminants on the surface.
Industries such as cosmetics that work with tiny particles can employ SEM imaging to understand more about the shape and size of small particles they work with. Particles which are the wrong size may have an impact on the consistency of the product.