Coating with a conducting material also improves image resolution as it enhances secondary electron contrast in SEI imaging modes. Since SEI images are essentially maps of secondary electron current versus lateral surface position, metal coating can greatly improve spatial resolution by increasing the secondary electron contrast between small structural features. Even if a sample is conducting or semi-conducting enough to allow SEM imaging, it is often beneficial to perform metal coating.
Generally AuPd is used to coat samples as the metal grains are on the order of 20-25 nm. Given that the JEOL 5900 has a resolution on the order of ~ 20 nm at 30 kV and a 6 mm working distance, such grains would never been seen, though they would certainly be seen in a field emission SEM.
Recently the AuPd sputter target failed, and having several Au targets in stock we have migrated to Au coating for the time being. Au coating is a little less desirable as the metal grains are on the order of ~ 50 nm, potentially just visible at the highest magnifications in the JEOL 5900. For routine work at several 10kX and below, it's a non-issue. Like AuPd, Au is also a very good secondary electron contrast coating.
One important difference between the AuPd and Au sputter targets is the sputtering yield. Au is softer than the AuPd allow and produces more sputtered material per unit time per unit of plasma current. While the AuPd target with a 25 mm target distance and 30 mA plasma current produced a ~ 15 nm film, the Au target produces a ~ 45 nm film under the same conditions.
The figure above shows the fit of Au film thickness versus sputtering time for a 25 mm target distance and a 30 mA plasma current. The fit is: thickness [nm] = 0.629 [nm/s] * sputter time [s] + 1.389 [nm].
The target thickness is really dependent upon the surface morphology and the method of coating. A very smooth surface with very little morphological relief can be coated with as little as 5 nm of a small-grain metal coating like AuPd or Ir-- just thick enough to produce a continuous film. Samples with very large surface relief require thicker coatings or coating with the planetary motion attachment to assist coating the peaks and valleys in the sample topography. Given that the resolution of the JEOL 5900 is ~ 20 nm, we've been aiming at ~ 15 nm coatings. Very rough topographies might warrant 30 nm coatings-- even thicker or coating using the planetary motion attachment.
For a 25 mm target distance and a 30 mA plasma current the following guidelines can be used:
- 10 nm: 14 seconds
- 15 nm: 22 seconds
- 20 nm: 30 seconds
- 25 nm: 38 seconds
- 30 nm: 46 seconds