Sputter coating in SEM involves applying an ultra-thin layer of electrically-conducting metal onto non-conducting or poorly conducting specimens.
This process is crucial for preventing specimen charging and enhancing the signal-to-noise ratio in SEM imaging.
The coating, typically 2–20 nm thick, is applied using a technique that involves generating a metal plasma and depositing it onto the sample.
5 Key Points to Understand Sputter Coating in SEM
1. Purpose of Sputter Coating
Sputter coating is primarily used to address the issue of specimen charging in SEM.
Non-conductive materials can accumulate static electric fields when exposed to the electron beam, which distorts the image and can damage the sample.
By applying a conductive layer, such as gold, platinum, or their alloys, the charge is dissipated, ensuring a clear and undistorted image.
2. Technique and Process
The sputter coating process involves creating a metal plasma through glow discharge, where ion bombardment of a cathode erodes the material.
The sputtered atoms then deposit onto the sample, forming a thin, conductive film.
This process is carefully controlled to ensure uniform and consistent coating, often using automated equipment to maintain high precision and quality.
3. Benefits for SEM Imaging
Besides preventing charging, sputter coating also enhances the emission of secondary electrons from the sample's surface.
This increase in secondary electron yield improves the signal-to-noise ratio, leading to clearer and more detailed images.
Additionally, the conductive coating can help reduce thermal damage to the sample by conducting away heat generated by the electron beam.
4. Types of Metals Used
Common metals used for sputter coating include gold (Au), gold/palladium (Au/Pd), platinum (Pt), silver (Ag), chromium (Cr), and iridium (Ir).
The choice of metal depends on factors such as the sample's properties and the specific requirements of the SEM analysis.
5. Thickness of the Coating
The thickness of the sputtered film is critical and typically ranges from 2 to 20 nm.
A film that is too thin might not adequately prevent charging, while a film that is too thick can obscure details of the sample's surface.
Therefore, achieving the right balance is essential for optimal SEM imaging.
In summary, sputter coating is a vital preparatory step in SEM for non-conductive or poorly conductive samples, enhancing their imaging quality by preventing charging and improving the signal-to-noise ratio.
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