The resolution of a SEM is basically given by the minimal spot size which can be formed and then rastered over your sample. This probe is formed by demagnifying the image of your gun. A field emission gun emits the electrons from a much smaller area then a therionic gun. Therefore the probe will principally be smaller for a FEG-instrument compared to a W- or a LaB6 instrument. Additionally the coherency is much higher, the energy spread smaller. This again allows to more perfectly focus the beam.
The resolution of a SEM is basically given by the minimal spot size which can be formed and then rastered over your sample. This probe is formed by demagnifying the image of your gun. A field emission gun emits the electrons from a much smaller area then a therionic gun. Therefore the probe will principally be smaller for a FEG-instrument compared to a W- or a LaB6 instrument. Additionally the coherency is much higher, the energy spread smaller. This again allows to more perfectly focus the beam.
Field Emission source(Gun) was used in the FESEM.
FE Gun having the following advantages over the others(W or LAB6).
Cross over diameter is less.
Spot diameter also less compared to thermionic emission.
more coherent and energy spred will very low.
The brightness will be very high compared to others thermionic source.
All these will make FESEM to give better resolution than SEM.
Crossover – the point at which the electrons are focused when leaving electron gun; virtual electron source. For SEM – smaller crossover gives gives smaller final electron spot, i.e. higher resolution.
@Vladimir Dusevich What is cross over free beam path, and what is the advantage of it?
thanks for your comment.
Crossover-free beam path – way of forming beam without crossover, implemented by some manufacturers. Theoretically eliminates some aberrations caused by electron-electron interactions in crossover spot. Since it is not supported by all manufacturers, we can assume that other ways of increasing resolution are at least as good as that one.
can anyone help me with interpretation of FESEM images of shale sample.?
There are two classes of emission source: thermionic emitter and field emitter. I believe that the emitter type is the main difference between the SEM and the FESEM. thermionic emitter and field emitter.
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