Sample preparation for space charge characterization

Charge trapping occurs at defects or insulator interfaces which have different permittivities and different conductivities. Independent of the charge injection technique (electron beam, corona discharge, poling, etc.) it is first necessary to remove the surface contamination before characterizing the bulk insulator intrinsic charging properties. Surface contamination may be due to handling, dust, adsorbed organic layers, condensed water, hydroxide and carbonate groups, surface diffusion and surface segregation. Dust can be removed by a gas jet directed at the specimen surface, but this often causes the insulator to build up static electricity and attract even more dust. Removal of the other contaminants requires cleaning of not only the whole sample (front, back and sides), but the fixture holding the specimen as well. In this work a frozen CO₂ cleaning process has been tested using Auger Electron Spectroscopy on a single crystal of MgO. The purpose was to compare the cleaning efficiency and the effect of the cleaning technique on the intrinsic crystal charging properties. It has been observed that MgO fired in air does not accumulate charge whereas it does when subject to atmospheric contaminants. All contaminated surfaces seem to contribute to charging. Testing the intrinsic property of MgO then requires removal of this contamination over the whole specimen and specimen holder surface (due to surface diffusion). Cleaning the single crystal MgO and specimen holder using a snow-jet CO ₂ spray preceding electron beam exposure results in the crystal being able to fully dissipate the incident electron exposure