Sputter coating for SEM typically involves the application of an ultra-thin, electrically-conducting metal layer with a thickness range of 2–20 nm.
This coating is crucial for non-conductive or poorly conductive specimens to prevent charging and enhance the signal-to-noise ratio in SEM imaging.
4 Key Points Explained
1. Purpose of Sputter Coating
Sputter coating is primarily used to apply a thin layer of conductive metal onto non-conductive or poorly conductive specimens.
This layer helps in preventing the accumulation of static electric fields, which can interfere with the imaging process in SEM.
By doing so, it also enhances the emission of secondary electrons from the specimen's surface, thereby improving the signal-to-noise ratio and the overall quality of the SEM images.
2. Typical Thickness
The thickness of the sputtered films typically ranges from 2 to 20 nm.
This range is chosen to ensure that the coating is thin enough not to obscure the fine details of the specimen but thick enough to provide effective electrical conductivity and prevent charging.
For lower magnification SEM, coatings of 10-20 nm are generally sufficient and do not significantly affect the imaging.
However, for higher magnification SEM, especially those with resolutions less than 5 nm, thinner coatings (as low as 1 nm) are preferred to avoid obscuring the sample details.
3. Materials Used
Common metals used for sputter coating include gold (Au), gold/palladium (Au/Pd), platinum (Pt), silver (Ag), chromium (Cr), and iridium (Ir).
These materials are chosen for their conductivity and ability to improve the imaging conditions in SEM.
In some cases, a carbon coating might be preferred, especially for applications like x-ray spectroscopy and electron backscatter diffraction (EBSD), where it is crucial to avoid mixing information from the coating and the sample.
4. Benefits of Sputter Coating
The benefits of sputter coating for SEM samples include reduced beam damage, increased thermal conduction, reduced sample charging, improved secondary electron emission, reduced beam penetration with improved edge resolution, and protection of beam-sensitive specimens.
These benefits collectively enhance the quality and accuracy of the SEM imaging, making it a critical step in the preparation of certain types of samples for SEM analysis.
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