Scanning electron microscopes (SEMs) can be used in many different applications to observe the nanostructural features of a sample without the need for extensive sample prep. For the analysis of some samples the samples must first be sputter coated for the SEM image to be of the required quality.
This article will cover the sputter coating process and which samples require sputter coating.
The Sputter Coating Process
SEMs are capable of imaging many different sample types including semiconductors, metals and alloys, polymers, ceramics and biological samples. Some samples can be more difficult to image and must be sputter coated to ensure a high quality image can be taken.
Sputter coating involves coating the sample with a thin layer (around 10 nm) of a conductive metal such as chromium, platinum, gold or silver.
Sputtering involves bombarding the metal for the coating with heavy particles, which causes it to erode and eject atoms. When this ejection is controlled between two electrodes (using the glow discharge process) it can be used to coat a target material.
The conditions for sputtering are strict and an appropriate ionization gas must be used. Depending on what target metal is bombarded the sample can be coated in palladium, platinum, gold or silver.
Samples that need Sputter Coating
Beam-Sensitive Samples
Beam-sensitive samples must be sputter coated before analysis. The majority of beam-sensitive samples are biological however some polymer-based samples are also beam sensitive.
The electron beam used for SEM imaging is of a high energy, which means it heats samples as they are imaged. If samples are heat or electron sensitive they can be damaged during imaging. Sputter coating the sample protects it from damage by the electron beam.
Non-Conductive Materials
Non-conductive materials can also be sputter coated to improve the SEM images taken of them. Non-conductive materials can behave as an electron trap, meaning electrons gather on their surface through a process known as charging. Charging creates extremely white regions in the SEM image (Figure 1a) that impact the image analysis.
Removal of bright white artifacts is frequently achieved by reducing the vacuum level of the chamber, which encourages positively charged molecules to accumulate at the sample’s surface.
Electrons in the beam interact with the positive molecules and are neutralized meaning charging does not occur. Whilst this method works the image quality is reduced due to the presence of more gaseous molecules within the chamber.
A more effective method is sputter coating the sample to give it a conductive coating. This coating allows the charged electrons to conductively leave the material. Figure 1b shows how sputter coating a sample in gold removes the charging effect.
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Figure 1: a) Charging effect on a non-conductive sample and b) BSD imaging of this sample after 10 nm gold coating.
For some applications sputter coating can be used to improve the resolution and quality of an image. The high conductivity of the metallic coating can result in an improved signal-to-noise ratio, meaning a sharper image is produced.
The Disadvantages of Sputter Coating
Sputter coating is not without its disadvantages. Firstly, it represents an additional step, which requires more time and effort than simply loading the sample in the SEM, and the process is technical and requires the optimal coating parameters to be set.
The greatest disadvantage though is that the surface of the sample is cloaked with the coating, meaning information on the samples atomic number information is lost. In addition, in some cases the topography of the surface can be adjusted, or incorrect atomic information can be collected. For this reason control of sputter parameters is essential to ensure that these issues are avoided.
If researchers familiar with the technique carry out sputter coating it remains a useful method of ensuring high quality SEM images are taken.
Which Coating Should be Used?
The conventional favorite for sputter coating is gold as it has a small grain size, which facilitates high resolution imaging, and has a high conductivity. In cases where EDX analysis is needed carbon is a popular choice of coating as carbons X-ray peak does not interfere with any other elemental signals.
As sputter coating has increased in popularity different metals have been used as the coating material. Chromium, tungsten or iridium are popular choices for ultra-high resolution imaging as they have extremely fine grain sizes. Other popular coatings are palladium and platinum; with silver also being a popular choice because the coating process is reversible.
This information has been sourced, reviewed and adapted from materials provided by Phenom-World BV.
For more information on this source, please visit Phenom-World BV.