Scanning electron microscopes have emerged as a very valuable characterization method in recent years, following the major technological developments and the continuous shrinking of material dimensions. SEMs are versatile tools that allow users to perform many different types of analyses on a wide range of materials and to achieve the best results, users should carefully select the main microscope settings. One of those settings is the spot size, i.e. the diameter of the probe at the sample. In this blog, I explain how to adjust the spot size in a SEM — and how to achieve the right balance between high-resolution imaging and a high beam current to get the results you’re looking for.
As mentioned above, SEMs can image almost all kind of samples; ceramics, metals and alloys, semiconductors, polymers, biological samples and many more. However, certain types of samples are more challenging and require an extra step in sample preparation to enable the user to gather high-quality information from a SEM. This extra step involves coating your sample with an additional thin layer (~10 nm) of a conductive material, such as gold, silver, platinum or chromium etc.
When a metallic target material is bombarded with heavy particles, the erosion of this material begins. Sputtering occurs when the erosion process takes place in conditions of glow discharge between an anode and a cathode. In this way, and by careful selection of the ionization gas and the target material, an additional thin layer (¬ 10nm) of a conductive material, such as gold, silver, platinum or palladium will coat your sample.
In some cases, the sputter coating sample preparation technique can be used to improve image quality and resolution. Due to their high conductivity, coating materials can increase the signal-to-noise ratio during SEM imaging and therefore produce better quality images.
As can be easily understood, there are a few concerns when it comes to using sputter coating for SEM imaging. Initially, it requires additional time and effort by the user to define the optimal coating parameters.
However, there is an even more important downside of sputter coating; the surface of the sample does not contain the original material but the sputter-coated one, and therefore the atomic number-contrast is lost.
In some extreme cases, it may lead to altered surface topography or false elemental information about the sample. Nevertheless, in most cases, the parameters of the sputter coating procedure are carefully selected and these issues do not appear and therefore the user is able to acquire high-quality images that carry the type of information that is required.
Historically, the most used sputter coating material has been gold, due to its high conductivity and its relatively small grain size that enables high-resolution imaging. Also, if EDX analysis is required, SEM users typically coat their samples with carbon because carbon’s X-ray peak does not conflict with the peak of any other element.
Nowadays, people are also using other coating materials with even finer grain sizes such as tungsten, iridium or chromium when ultra-high resolution imaging is required. Other coating materials include, platinum, palladium and silver, with the latter having the advantage of reversibility.
It goes without saying that certain type of samples need some extra steps of sample preparation to achieve the best possible result in the SEM. If you’d like to know more about how to prepare your samples for SEM imaging, you can download our detailed sample preparation e-guide.