THE CHARGING BEHAVIOR AND INTERNAL ELECTRIC-FIELD OF PMMA IRRADIATED BY A KILOELECTRONVOLT ELECTRON-BEAM

A scanning electron microscope (SEM) is employed in the investigation of the charging behaviour of polymethylmethacrylate (PMMA) specimens. Charges trapped in the PMMA specimens are measured using the SEM mirror image method under different conditions of radiation time, accelerating voltages and beam currents. The influence of the surface potential established and the internal electric field arising in the PMMA specimen is studied, and the latter is found to play a more important role. There exists a saturation value for the charge that can be trapped in the specimen under a fixed charging condition. Beyond the saturation value, the trapped charge destabilizes and surface discharge is observed. A Gaussian model of charge distribution is used for the calculation of the electric field. The strength of the electric field initiating surface discharge is estimated.