Ultramicroscopy 3 (1979) 381-386 Q North-Holland Publishing Company RADIATION DAMAGE IN ELECTRON MICROSCOPY OF INORGANIC SOLIDS L.W. HOBBS Department of Metallurgy and Materials Science, Case Western Reserve University, Cleveland, OH 44106, USA 1. Introduction The electron beamof an electron optical instru- ment is a probe of prodigiousenergy density, and its “inelastic” interaction with a solid specimen results in energy transfer to the specimen under examination at ratesbetween 10sand 1Or2W/mm3. Most of this energy eventually appears as heat and can result in deleteriouselevation of specimen temperature. The remainderof this energy appears largely asinforma- tive electromagnetic radiation (cathodoluminescence, X-ray emission) but occasionally as atom rearrange- ments, which are the subject to this contribution. Such atom rearrangements, known collectively as radiation damage, limit the analytical information obtainable from a sensitive specimen either because (i) accumulation of damage masks important image or diffraction features arisingfrom otherwise unaf- fected material, or (ii) the specimen itself deteriorates to such an extent that it becomes unrepresentative. Minimizing the energy channeled into atom rearrange- ments means first minimizing the overall electron dose - which reduces as well the number of inter- acting electrons that contribute to the imageor ana- lytical information, degrading imageor information statistics.However, knowledge of the operative mech- anisms of damage can in many cases suggest additio- nal measures which may be beneficially applied. 2. Electron - solid interactions A fast electron interacts electrostatically with both atomic electrons and atom nuclei in the solid specimen. Owing to the enormous disparity between electron and nuclear masses, significant electron- electron interactions are typically -lo4 times more numerous than electron-nucleus interactions, and so a fast electron loses most of its energy in electron- electron interactions. In order to effect ‘permanent’ alterations to the specimen, a minimum energy trans- fer exists because all atomic electronsand nuclei are bound locally to some extent. For the electron- electron interactions, this is the exciton energy to promote valence electrons to bound electron-hole pair states (excitons) (fig. 1) or to promote nominally- free conduction electrons(or most weakly-bound valence electrons) into localized collective oscillations (plasmons). Theseenergy transfer thresholds lie typ- ically in the 5-30 eV range,and the lifetime or per- manency of suchexcited electronic states can be as short as lo-*’ s or aslong asmany seconds. For electron-nucleus interactions, the threshold is the Fig. 1. EELS of NaCl and KC1 showing exciton peaks and broad plasmon resonance. 381