Ion damage to metal films inside an electron microscope

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Abstract
The origin of small spot and loop features observed in electron micrographs of thin evaporated single-crystal gold films has been investigated. It is found that the features are introduced into the specimens whilst they are being examined in the electron microscope. Conclusive evidence is given to show that they arise from bombardment by energetic negative ions emitted from the tungsten filament of the electron gun, although the ions have not been identified. The rate of ion damage can be considerably increased by coating the tungsten filament with a standard oxide emitter. The annealing characteristics of the damage have been studied. Above about 300–350°c the majority of the spot features anneal out, and ion damage above about 350° consists of the formation of small tetrahedra of stacking fault. The ion damage is assumed to be very similar in character to that produced by primary knock-ons resulting from irradiation by other particles (e.g. neutrons).