From emsuite wiki

Probe Current or Spot Size in a SEM

A.    The size of the probe in the SEM has an affect in a number of areas –

1. The ultimate imaging resolution.
2. The level of imaging signal from a sample.
3. The level of x-ray signal from the sample.
4. The relative levels of secondary and backscattered electrons emitted from the specimen; larger spots result in more backscattered electron influence in the image.

B.    The actual probe size on the surface of the specimen relates to a number of other settings within the instrument

1. The position of the filament in the cathode (shorter distances give a higher emission level allowing a higher bias to be applied reducing the source size whilst retaining a usable emission level).
2. The applied bias level (as above).
3. The accelerating voltage (the extraction of electrons from the cathode is less efficient as the kV is lowered due to the anode to cathode distance being retained at that desired for the highest instrument kV.  The distance should be 1mm for every 2 kV for maximum gun efficiency, therefore in theory the anode should be raised as the kV is lowered.)
4. The setting of the condenser lenses (A high strength first condenser provides a smaller spot.)
5. The focal length (WD) of the final lens. (A longer focal length requires a weaker lens to focus and as a result a higher level of spherical and chromatic aberration.  A longer focal length also results in more outside interference from electrical fields in the laboratory.)

C.   The ultimate resolution of an instrument depends upon the settings mentioned in “B” above when a test specimen is the subject.  This specimen should consist of an artificial structure containing heavy particles, usually gold, on a light element support, usually carbon or latex, this configuration of “something on nothing” exaggerates performance but may provide a resolution standard for most instruments.  In reality the resolution attained with a day to day specimen depends upon that specimen’s ability to emit electrons, for example carbon based materials provide a weaker signal.
D.   The size of the beam spot, should it require measurement, may be determined using the edge affect method.
A disk aperture is placed in the microscope sitting over a hole in a stub.

1. The magnification is increased to maximum and the edge of the aperture brought to focus at the desired probe size.
2. A line scan is run across the edge of the aperture.
3. A wave form is produced from the line scan where a measurement is taken from 10%* horizontally into the slope up to 90%* horizontally into the slope, this figure is related to the magnification and is said to be the actual probe size. (check these figures *)

E.    The performance of the microscope is better checked through the use of a sample as described in “C”.  Periodic tests where the image quality is related to the previous test are sufficient for most laboratories, but point to point measurements on the structures may also be made.  If measurements are taken a magnification calibration will also be required.  Great care should be taken to ensure that the magnification calibration is performed under identical conditions, including spot size, to that of the resolution test.  Changing the spot size on many older instruments changes the magnification due to a change in final lens focal length.