Which SEM Microscope is Right for Which Application?
There are many types of SEMs, ranging from the more common type, which use a tungsten filament as an electron source, to the more specialized type which, with a field emission (FE) electron gun mounted, attains higher resolution and magnification. Choosing the ideal SEM instrument is dependent on many factors.
Below, we summarize some of the main considerations when selecting an SEM.
1. Microscope Magnification
Since electron wavelengths are up to 100,000 times smaller than the wavelengths of visible light, SEMs resolve details hundreds of thousands of times smaller than optical microscopes.
The field of view (FOV) in a microscope defines the size of the feature to be imaged. This value can range between millimeters, microns and nanometers. To define the FOV required to image samples, first the end goal must be decided. If the number of particles in a sample is what is of interest, having multiple particles per image is not an issue, so an SEM that provides a FOV of 100 times greater is enough. However, if a particle's structure is of interest, a closer FOV is needed to see the required detail. This is shown in Figures 3-5, which compare tabletop, tungsten, multipurpose FE and ultra-high resolution (UHR) FE SEM instruments.
Tabletop scanning electron microscopy can be very efficient for basic applications requiring magnification ranges up to 100,000X and some selectable settings. The relaxed vacuum requirements and small evacuated volume enable fast image production without extensive sample preparation.
Additionally, operating a tabletop SEM is simple enough to do it by the individual who requires the information instead of a dedicated scanning electron microscopist. As well as obtaining answers quickly, it is also beneficial to be able to carry out analysis straight away and for the user to be able to manage it in real-time response to observations.
For instances where higher magnification is needed, but space is also a limiting factor, conventional tungsten SEMs are an option to simplify specimen navigation, and advanced automation delivers crisp secondary and backscatter images in seconds. If a specimen is challenging to analyze, FE SEMs and UHR FE SEMs provide topographical and elemental information at magnifications of 10X up to 1,000,000X.
Figure 3: Materials SEM comparisons.
Figure 4: Biological SEM Comparisons.
Figure 5. SEM comparisons for imaging Titanium alloy cross section.