Acceleration voltage vs. specimen type
In theory, an increase in accelerating voltage will result in a higher signal (and lower noise) in the final image (micrograph). But the situation is not so simple. There are some disadvantages:
- Reduction in structural details of the specimen surface in SE mode
- Increased electron build up in insulating samples, causing charging artefacts
- Increased heating and the possibility of specimen damage
With a higher accelerating voltage the electron beam penetration is greater and the interaction volume is larger. Therefore, the spatial resolution of micrographs created from those signals will be reduced. So there will be a brigher image because the number of backscattered electrons (BSEs) will increase but the resolution will be worse. For secondary electron (SE) imaging at typical voltages (say 15 keV), BSEs can enter the secondary electron detector and degrade resolution because they come from deeper in the sample.
Show the interaction volumes of:
Accelerating voltage (kV or keV) is the voltage difference between the filament and the anode which accelerates the electron beam towards the anode. The accelerating voltage (kV or High Tension) of a typical SEM ranges from 0 to 30kV. In particular, the greater the kV, the greater the power of penetration by the beam into the sample. The disadvantages associated with the use of higher kV are addressed in detail under troubleshooting.
A working guide to the selection of an appropriate accelerating voltage is provided in the table. Experimentation is always necessary to determine the optimum settings for any sample.
Table: A working guide to the selection of an appropriate accelerating voltage.
|1-5kV||delicate or uncoated specimens|
|5-10kV||coated biological samples|
|10-30kV||physical science samples|