Gold sputtering typically results in a film with a thickness range of 2–20 nm.
This range is particularly relevant for applications in scanning electron microscopy (SEM).
In SEM, the coating serves to prevent specimen charging and enhance the signal-to-noise ratio by increasing the emission of secondary electrons.
How Thick is Gold Sputtering? 4 Key Points to Understand
1. Purpose of Gold Sputtering in SEM
In SEM, non-conductive or poorly conductive specimens can accumulate static electric fields, which interfere with imaging.
To mitigate this, a thin layer of conductive material like gold is applied through sputtering.
This process involves depositing a metal onto a surface by bombarding it with energetic particles, typically in a high-vacuum environment.
The applied metal layer helps conduct the electric charge away from the specimen, preventing distortion in the SEM images.
2. Thickness of Gold Sputtering
The reference provided indicates that sputtered films for SEM applications generally have a thickness between 2 and 20 nm.
This range is chosen to balance the need for conductivity with the requirement to avoid obscuring the specimen's surface details.
Thicker coatings might introduce artifacts or alter the specimen's surface properties, while thinner coatings might not provide adequate conductivity.
3. Specific Examples and Techniques
Gold/Palladium Coating: An example given describes a 6" wafer coated with 3 nm of gold/palladium using specific settings (800V, 12mA, argon gas, and a vacuum of 0.004 bar).
This example demonstrates the precision achievable in sputtering, with the coating being even across the entire wafer.
Calculation of Coating Thickness: Another method mentioned uses interferometric techniques to calculate the thickness of Au/Pd coatings at 2.5KV.
The formula provided (Th = 7.5 I t) allows for the estimation of the coating thickness (in angstroms) based on the current (I in mA) and time (t in minutes).
This method suggests that typical coating times might range from 2 to 3 minutes with a current of 20 mA.
4. Limitations and Suitability of Gold Sputtering
While gold sputtering is effective for many applications, it is noted that gold is not ideal for high-magnification imaging due to its high secondary electron yield and the formation of large grains in the coating.
These characteristics can interfere with the visibility of fine specimen details at high magnifications.
Therefore, gold sputtering is more suitable for lower magnification imaging, typically below 5000×.
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