Challenging samples for SEM
Scanning electron microscopes (SEMs) are versatile tools that can provide nanoscale-level information about a wide range of samples with little or no sample preparation. In some cases, however, sputter coating the samples before working with SEMs is necessary to obtain a good-quality SEM image.
SEMs can image all kinds of samples: ceramics, metals and alloys, semiconductors, polymers, biological samples, and much more. However, certain types of samples are more challenging and require an extra step as the sample is prepared to gather high-quality information. This extra step involves coating your sample with an additional thin layer of about 10 nanometers (nm) of a conductive material such as gold, silver, platinum, or chromium.
When sputter coating is needed
Due to their high conductivity, coating materials can increase the signal-to-noise ratio during SEM imaging and therefore produce better quality images. Among the challenging samples that require sputter coating are beam-sensitive and non-conductive materials.
- Beam-sensitive samples: Beam-sensitive samples are mainly biological samples, but they can also be other types, such as materials made from plastics. The electron beam in an 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’s sensitive to the electron beam, this interaction can damage part or all of the structure. In this case, sputter coating with a material that’s not beam-sensitive can act as a protective layer against this kind of damage.
- Non-conductive materials: Due to their non-conductive nature, the surface of non-conductive materials acts as an electron trap. The resulting accumulation of electrons on the surface is called “charging” and creates extra-white regions on the sample, which can influence the image information. When sputter coating is used, the conductive coating material acts as a channel that allows the charging electrons to be removed from the material.
The drawbacks of sputter coating for SEM
Sputter coating SEM samples has its downsides. For one, additional time and effort is required to define the optimal coating parameters. Even more importantly, 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, the technique may lead to altered surface topography or false elemental information about the sample.
In most cases, however, the parameters of the sputter coating procedure are carefully selected and these issues do not arise, allowing the user to obtain the accurate, high-quality images they need.
Materials used to sputter coat your sample
Historically, the most frequently used sputter coating material has been gold because of its high conductivity and relatively small grain size, which makes it ideal for high-resolution imaging. If energy-dispersive X-ray (EDX) analysis is required, SEM users typically coat their samples with carbon due to the fact that 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, and chromium when ultra-high-resolution imaging is required. Other coating materials include platinum, palladium and silver, with the latter having the advantage of reversibility.
To achieve the best possible SEM images, certain type of samples require extra sample preparation. Sputter coating can be an effective technique for obtaining high-quality SEM images when working with challenging samples such as beam-sensitive and non-conductive materials.
To learn more about sputter coating as an SEM sample preparation technique, fill out this form to speak with an expert.
Erik Luyk is a marketing communications specialist at Thermo Fisher Scientific.