How to estimate the resolution of a focused beam?
An easy way to estimate the resolution of a focused beam is by scanning over a sharp edge. The integral of the transmitted intensity of the Gaussian beam becomes a smooth step function (edge response). The resolution can be defined in terms of a height fraction (rise-distance) of the step function. Naturally, the choice of this fraction will significantly influence the value of the resolution.
What is a max positive bias voltage to be applied to a sample (in the beam deceleration mode)?
Max negative voltage to be applied to a sample: From 5 to 3 kV. If the accelerating voltage is 30kV, sample bias is 5kV and landing voltage (energy) is 25kV. That is the disposable maximum. For landing energy down to 1kV the sample bias is still 5kV. For the landing voltage below 1kV, that means down to 50V, the stage bias is linearly decreased to 3 kV. Only the experienced users are allowed to change sample bias down to 2.5 kV (such biasing can be used for cases when charging needs to be compensated).
Why FESEM gives better resolution than SEM?
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.