I need to decide which SEM (with EDX) model to buy. I am told that in going from 100 to 200 nA, there is not much to be gained. I just needed to cross check this claim and educate myself about specific cases where it is advisable to buy one vs another.
100 to 200 nA? Are you sure? It is huge even for a FIB (typically max ion current is about 100 nA). The spot size will be terrible (micrometer?!). Most common parameters for EDX - current from 0.1 to 1 nA, HT from 10 to 30 kV, almost all normal SEMs meet these conditions.
No worries. Be careful, sometimes manufacturers highlight some non-important parameters or useless features and hide really important things.
Good luck!
A factor 2 increase in beam current means that your measurements will take half the time to get the same quality or, for the same acquisition time will have signal-to-noise ratio worse by a factor of 1/roo(2)~0.7. This is not much. The condenser aperture choice and the X-ray detector collection aperture set-up will be much more relevant!
Thomas Walther, yes, but it is necessary to remember about the spot size - it will be increased too, spoiling the spatial resolution.
In this case (100 and 200 nA) definitely something is wrong (misreading, misunderstanding, etc.), because it is really huge current. I don't believe that someone could use it for EDX. For some GIS system? Perhaps. But it is pretty exotic. Nevertheless, the difference between 100 and 200 nA would be not dramatic.
Electron probes (EPMA) may have currents up to several microamper. So, setup with higher current could be useful for WDS, but mostly useless for EDS.
Pushing instrument to the limit of one of the parameters usually degrades overall performance. To help with making informed decision about usefulness of 200nA vs. 100nA beam current, ask OEM(s) for beam profile (Point Spread Function) data at both current settings. In highly unlikely event of receiving such data, upon comparison you would most likely see that 200nA spot size is so much larger (current density is lower) that loss of spatial resolution could far outweigh limited benefits of increase in intensity and S/N ratio of EDS signal. If your main application is EDS and speed is of the essence, then good EDS detector with wide solid angle is of much more value then electron beam current.
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