Scan rate and signal to noise ratios
It is conventional to reduce the scan rate when collecting an image for later use or publication. The slower scan rate allows more electrons to be collected at each point along the line of the beam scan. This produces a better quality image.
The image quality of the SEM is limited by the spot size and the ratio of the signal (S) produced by the electron beam to the noise (N) imparted by the electronics of the instrument in the displaying of this signal (S/N). Noise pulses are derived from such sources as beam brightness, condenser lens settings (spot size), and SE detector ‘gain’, and may impart a salt-and-pepper (grainy) appearance to the image. When the SEM is set up for high resolution imaging it will often have a low S/N ratio and appear grainy. This may be unavoidable.
The image quality in an SEM and hence its S/N ratio is improved as the total number of electrons recorded per picture point is increased. Tungsten (W) filaments characteristically have low yields of electrons resulting in a low brightness image. Thus, at condenser lens settings for high resolution (small spot sizes), the quantity of electrons reaching the specimens to interact with the specimen is low. Therefore SE production is low and a high current must be used in the SEM electronics (e.g. the photomultiplier tube) in order to produce an image. This results in an unfavourable S/N ratio.
To overcome the limitation of W filaments, and to improve the S/N ratio, bright sources such as the field emission gun (FEG) have been developed.