Sputter coating for SEM typically involves applying an ultra-thin layer of metal, such as gold, gold/palladium, platinum, silver, chromium, or iridium, onto non-conductive or poorly conductive specimens. The purpose of this coating is to prevent specimen charging and enhance the signal-to-noise ratio by increasing the emission of secondary electrons. The thickness of sputtered films generally ranges from 2 to 20 nm.
Detailed Explanation:
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Thickness Range: The standard thickness for sputter coatings used in scanning electron microscopy (SEM) is between 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 adequate electrical conductivity and prevent charging.
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Specific Examples:
- A 6" wafer was coated with 3 nm of gold/palladium using the SC7640 Sputter Coater, demonstrating that even thinner coatings (down to 3 nm) can be achieved with precision equipment.
- A TEM image showed a 2 nm sputtered platinum film, indicating the capability to produce very thin coatings suitable for high-resolution imaging.
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Calculation of Thickness: Experiments using interferometric techniques have provided a formula to calculate the thickness of Au/Pd coatings: [ Th = 7.5 I t \text{ (angstroms)} ] where ( Th ) is the thickness in angstroms, ( I ) is the current in mA, and ( t ) is the time in minutes. This formula is applicable under specific conditions (V = 2.5KV, target to specimen distance = 50mm).
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Coating Uniformity and Precision: High-end sputter coaters equipped with features like high vacuum, inert gas environments, and film thickness monitors can deposit coatings as thin as 1 nm. These precision tools are crucial for applications requiring high resolution, such as EBSD analysis, where even the smallest details matter.
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Impact of Coating Thickness on Imaging: For SEM with high-resolution capabilities (<5 nm), coating thicknesses of 10-20 nm can start to obscure finer details of the sample. Therefore, thinner coatings are preferred to maintain the integrity of the sample's surface features.
In summary, the thickness of sputter coatings for SEM typically ranges from 2 to 20 nm, with advanced equipment capable of achieving even thinner coatings down to 1 nm. The choice of coating thickness depends on the specific requirements of the imaging or analysis being performed, with thinner coatings generally preferred for high-resolution applications.
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