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.
How does sputter coating work?
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.
Challenging samples that require sputter coating
- Beam-sensitive samples
The first type of samples that are usually sputter-coated prior to loading in the SEM are the beam-sensitive samples. These are mainly biological samples, but they can also be other types, such as materials made from plastics. The electron beam in a SEM is highly energetic and, during its interaction with the sample, it carries part of its energy to the sample mainly in the form of heat. If the sample consists of a material that is sensitive to the electron beam, this interaction can damage part or their entire structure. In this case, sputter-coating with a material that is not beam-sensitive can act as a protective layer against such kind of damage.
- Non-conductive materials
Another class of materials that is frequently subjected to sputter coating is non-conductive materials. Due to their non-conductive nature, their surface acts as an electron trap. This accumulation of electrons on the surface is called “charging” and creates the extra-white regions on the sample that can be seen below, which can influence the image information.
In order to remove this artefact, a common approach is to lower the vacuum level inside the chamber. This introduces positively-charged molecules near the surface of the sample. These interact with the charging electrons and neutralize them, thereby removing this charging effect. This has proven to be an effective approach, however the air molecules that are introduced in the vacuum chamber interact with the primary electrons reducing the quality of the image.
For this reason, if a high-quality electron image is required, the use of sputter coater is recommended; the conductive coating material acts as a channel that allows the charging electrons to be removed from the material. Here you can see how the charging effect has been removed with the application of a gold coating.
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.
The drawbacks of sputter coating for SEM
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.
Which materials should you use to sputter-coat your sample?
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.
Download the free Sample Preparation E-guide
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.