Characterization of the Electron Beam Specimen Interaction in the ESEM with SIM Imaging
Scott A. Wight, John G. Gillen, G B. Saupe
A Secondary Ion Mass Spectrometry (SIMS) study has been undertaken to examine surfaces and films modified in the environmental scanning electron microscope (ESEM). Examination of the modifications induced by ESEM electrons and ions lead to a better understanding of the beam/gas/specimen dynamic. Primary electron beam interactions with specimens in the Electroscan ESEM, model 2020* are complicated by the presence of gas molecules (typically water vapor) in the low vacuum of the specimen chamber [133-2660 Pa(1-20 Torr)]. Electrons are scattered out of the focused electron beam by interaction with water vapor molecules. These scattered electrons strike the specimen in a broad, low intensity skirt. Both primary and scattered electrons can create secondary electrons, backscattered electrons, and x-rays. Secondary electrons that escape the specimen are accelerated towards the gaseous secondary electron detector (GSED) by an applied positive bias. These secondary electrons ionize the water molecules producing positively charged water ions and additional electrons. These electrons are in turn accelerated towards the detector resulting in further ionization events. This ionization cascade amplifies the secondary electron signal and is the basis for a working low vacuum secondary electron detector. The positively charged gas molecules produced by the gas cascade of the ESEM secondary electron detector are accelerated towards the specimen surface where they recombine with excess electrons thereby compensating for surface charging .
Institute Physics Conference
July 9-13, 2000
International Union of Microbeam Analysis Societies