Thursday, June 27, 2013

Au coating

Unless one wishes to image at very low beam energies-- 0.5-2.0 kV for most materials-- insulating materials must be coated with a conducting material before imaging. This conducting over-layer allows electrons to be sourced from the grounded sample stage to neutralize positive surface charge due to secondary and back-scattered electron current leaving the sample. In the case of an environmental SEM or low-vacuum SEM this charging can also be dissipated using a quench gas such as water vapor or environmental gasses.  The CMMP JEOL 5900 does not allow this.

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
The above data was taken by coating microscope cover slips, scratching the surface and profiling the scratch with a stylus profilometer.

Saturday, June 1, 2013

Which Beam Energy Should I Use?

A common question is which primary beam energy is most desirable for imaging a particular sample. The question can only be answered by identifying imaging objectives and considering the physics of electron-beam sample interactions. While SEM column specifications are often optimal at higher energies, typically 30-35 kV, it is generally not primary beam diameter that determines image quality unless one is looking at nano-structures that are comparable in size to the probe size.

In this study activated charcoal was imaged at different beam energies at the same magnification, probe current and working distance. The same porous area of the charcoal surface was imaged, though some image rotation is noticeable due to the Lorentz force exerted on the rastered e-bean in the objective lens. Being graphitic, the sample was uncoated and was known to contain ash mineral contamination. To avoid operator "aesthetic bias" all images were ACB'd-- automatic contrast and brightness adjusted-- to highlight systematic changes with beam energy.

The top image was taken at 30keV.  It clearly shows the pore structures in the activated charcoal-- the horizontal ridges on the sample surface and the pores that go into the sample volume. The pores appear quite clearly, and there is some evidence of edge effects-- the edge brightness enhancement on the horizontal margins of some of the larger pores. All in all, the surface of the porous tubes looks fairly smooth, and there is some evidence of little particles on their surface. This isn't very shocking as graphitic materials have a low efficiency for generating secondary electrons. The sample looks "soft" and largely featureless, much like the carbon tape that holds it down.  It would look more so without the particulate material coating the pores.

Dropping the beam energy 10 kV is shown in the second image. It's clearly the same structure-- but it is slightly nuanced. The electron range is significantly less at 10 kV, even in a low density matrix like activated charcoal. This results in the enhancement of edge effects.  In the second image all of the pore edges are brighter due to edge enhancement. Edge enhancement occurs in secondary electron imaging--SEI-- because secondary electrons can be collected not only from the sample surface, but also the edge of the feature and perhaps even hidden surfaces of a feature. Much more surface topography of the pores is also visible at 10 kV. There are clearly ribs and ridges on the pore surface, and smaller secondary pores that emerge from the larger pores.

Most striking is the enhancement of the particulate surface trash. It is very prominent at 10 kV. It could simply be due to an increase in secondary electron emission efficiency at 10 kV. In general, we have to differentiate SE1, secondary electrons produced by the primary beam, and SE2, secondaries produced by back-scattered electrons. The lower electron range at 10 kV in the graphitic matrix increases the probability of generating SE2, and this perhaps is the cause of increased contrast of the surface trash.

In the final image the beam energy is dropped to 1 kV. The efficiency for generating secondary electrons is very low at these low beam energies, and I like to characterize these low-energy images as "chalky". We aren't inventing or losing any morphological features as we change beam energies. The same features-- pores, pore ridges, small side pores, particulate matter-- are all visible, as the such image artifacts as edge enhancement, but the image quality is largely driven by detection efficiency. We get a very clear sense of the relief of the pores actually penetrating the sample.

So, which is correct? They all are. The SEM isn't lying. Which is best? It depends upon the story one wishes to tell with the SEM images. We finish the example with three images taken at much lower magnification, taken at 1 kV, 10 kV and 30 kV. Same general morphology-- different aesthetic quality and different features naturally enhanced.