Modification of the high-voltage circuitry of the thermionic electron
gun of a commercial scanning electron microscope (SEM) to a fully comp
uter-controlled, cathode-stabilized system is described. The modificat
ion permits automatic filament saturation and monitoring. Using this s
ystem, variations and drifts in the emission current over time can be
adjusted and compensated automatically with a feedback loop in the com
puter-controlled system. This change also enables the accurate determi
nation and setting of the primary electron beam energy. Therefore, the
accelerating voltage is known for comparison to Monte Carlo computer
modeling of the electron beam specimen interaction.