Gold sputter coating is a crucial process in scanning electron microscopy (SEM). It helps to prevent charging and improve the quality of images. The thickness of this coating typically ranges from 2 to 20 nanometers. This ultra-thin layer is applied to non-conducting or poorly conducting specimens. It enhances the signal-to-noise ratio by increasing the emission of secondary electrons.
5 Key Points Explained
1. Purpose and Application
Gold sputter coating is primarily used in 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. This could otherwise interfere with the imaging process. Additionally, the metallic coating increases the emission of secondary electrons from the specimen's surface. This improves the visibility and clarity of the images captured by the SEM.
2. Thickness Range
The typical thickness of sputtered gold films for SEM is between 2 and 20 nanometers. This range is chosen to ensure that the coating is thin enough not to obscure the fine details of the specimen. It is also thick enough to provide adequate electrical conductivity and secondary electron emission.
3. Specific Examples and Techniques
In one example, a 6" wafer was coated with 3 nanometers 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 nanometer 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.
4. Technical Details and Formulas
The thickness of the Au/Pd coating can be calculated using the formula: [ Th = 7.5 I t ]. Here, ( 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.
5. Limitations and Suitability
Gold is not ideal for high-magnification imaging due to its high secondary electron yield. This 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×.
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