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.
I agree with Elisabeth Müller
, but you did not address the difference in clarity and accuracy in the picture, in your opinion, which is better?
Dear Mohammed Hassnawy , what is "accuracy in the picture"?
Anyway, for modern microscopes at magnifications below 20k and accelerating voltages above 5 kV there is no difference in picture quality.
Compared with convention scanning electron microscopy (SEM), field emission SEM (FESEM) produces clearer, less electrostatically distorted images with spatial resolution down to 1 1/2 nanometers – three to six times better.
Compared with convention scanning electron microscopy (SEM), field emission SEM (FESEM) produces clearer, less electrostatically distorted images with spatial resolution down to 1 1/2 nanometers – three to six times better.
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.
Elisabeth described well the probe size issue. But there is also another benefit of FE-SEM. Modern, more expensive microscopes has more advanced detection systems like in-lens or semi-in lens detectors which can collect only SE type I . This also will give You better resolution than regular SE detector. If You try to compare images from the microscope for 600K versus 2M Euro this difference in price is not only in the source type.
The difference between a FESEM and a SEM lies in the electron generation system. As a source of electrons, the FESEM uses a field emission gun that provides extremely focused high and low-energy electron beams, which greatly improves spatial resolution and enables work to be carried out at very low potentials .This helps to minimise the charging effect on non-conductive specimens and to avoid damage to electron beam sensitive samples.
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