It can be a sample that we deliberately prepared with depositing layers with nm thicknesses or an unknown sample we suppose contains different layers - what beam energy, what detectors we can use to visualize this sample in SEM?
If we use high energy beam and detect BSE electrons we are going to get superposition of contents from layers that are in interaction volume or the image will mainly be representative of the layer that is in deepest part of interaction volume?
Depending on your deposition and expected marerial contrast i would try an angled polish with bse detector and 20 keV. Also an in-lens detector can give good material contrast. I one can see something plausible one can reduce the energy to find a good compromise of lateral resolution and mat. contrast.
If possible a FIB cross section can be milled alternatively, which can show sharp multilayer pictures. Then inspecting the shaft by in-lens- or se-detector, depending on the contrast. I hope it can help a bit.
I would attempt some sort of fracturing, either at room temperature, or cryo fracture, then do lateral imaging. FIB may be a further option if you have that available. With mechanical polishing, I would be concerned about sample alteration due to heat generation during polishing, particularly at nm scale. Given that you mention BSE detector, you expect different elemental composition of samples. if your sample is flakey and breaks with larger horizontal pieces of each nm layer, you could also try EDAX and element mapping, with µm horizontal resolution, but nm vertical resolution. EDAX mapping may give you cleaner compositional info than BSE. Re kV, follow general EDS/EDAX approaches.
All depends on specifics. With new samples, I often try a number of approaches and see what works best. I have been surprised by what works best in the past.
" layers with nm thicknesses" is too uncertain a definition. A few nm? Tens or hundreds of nm? If it is really a few nm, then FIB and TEM are your best bet. Otherwise you may want to try polished cross sections (difficult, but doable, especially with embedding) or cross fractures. Use lower kV for SE, higher for BSE. Exact voltage depends on your SEM and on thickness of layers on your specimen, so you need to play a bit with your SEM to find the best imaging conditions.
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