Prospects of three-dimensional high resolution electron microscopy of non-periodic structures.
In the first part of the paper the general features of three-dimensional electron microscopy of non-periodic structures at high resolution are discussed. Such an analysis requires series of 20 or more exposures of mechanically moved specimens. It is fundamentally important that the integral radiation dose, however, need not be increased--three-dimensional analysis yields extra information without additional radiation loading. It is demonstrated that the constraints characteristic for atomic resolution can in future facilitate many difficulties like restricted tilting range (conical tilting), inaccuracies in the refinement of the origin determination etc. Data collection up to atomic resolution is possible with existing image reconstruction methods if the chromatic spread of the cathodes can be partially corrected. The relatively great radiation induced changes of "stable" specimens will require additional averaging in the experiment (quasi-simultaneous registration). The advantage of a time dependent analysis of radiation sensitive structures--especially the possibility of using redundancies in the analysis (trace structure analysis)--is discussed. The experimental part (studies of radiation damage and three-dimensional work on carbon foils) presents examples of such analyses. The main experimental difficulty consists in the collection of many exposures with no extra load of the specimen in the intervals between exposures.