The thickness of gold sputter coating typically ranges from 2 to 20 nm for SEM applications. This ultra-thin coating is applied to non-conducting or poorly conducting specimens to prevent charging and enhance the signal to noise ratio by increasing the emission of secondary electrons.
Detailed Explanation:
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Purpose and Application: Gold sputter coating is primarily used in scanning electron microscopy (SEM) to coat non-conductive or poorly conductive samples. This coating is essential because it prevents the accumulation of static electric fields on the specimen, which could otherwise interfere with the imaging process. Additionally, the metallic coating increases the emission of secondary electrons from the specimen's surface, improving the visibility and clarity of the images captured by the SEM.
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Thickness Range: The reference materials indicate that the typical thickness of sputtered gold films for SEM is between 2 and 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 secondary electron emission.
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Specific Examples and Techniques:
- In one example, a 6" wafer was coated with 3 nm of gold/palladium (Au/Pd) using a SC7640 Sputter Coater. The settings used were 800V and 12mA with argon gas and a vacuum of 0.004 bar. This coating was found to be even across the entire wafer.
- Another example involves the deposition of a 2 nm platinum film on a carbon-coated Formvar film, also using the SC7640 Sputter Coater. The settings were 800V and 10mA with argon gas and a vacuum of 0.004 bar.
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Technical Details and Formulas: The thickness of the Au/Pd coating can be calculated using the formula: [ Th = 7.5 I t ] where ( Th ) is the thickness in angstroms, ( I ) is the current in mA, and ( t ) is the time in minutes. This formula is applicable when the voltage is 2.5KV and the target to specimen distance is 50mm.
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Limitations and Suitability: Gold is not ideal for high-magnification imaging due to its high secondary electron yield, which leads to rapid sputtering and the formation of large islands or grains in the coating. These structures can be visible at high magnifications, potentially obscuring the details of the specimen's surface. Therefore, gold sputtering is better suited for imaging at lower magnifications, typically under 5000×.
In summary, the thickness of gold sputter coating for SEM is carefully controlled within the range of 2 to 20 nm to optimize the balance between conductivity, secondary electron emission, and preservation of specimen detail.
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