Inorganic Replication in Electron Microscopy

I H E basic purpose of micrography of surfaces is to exhibit structural topography. Present day electron microscopes are transmission-type Instruments. Practical limitations of experimental technique establish a voltage of the order of 50 kv as the most useful accelerating potential for the electrons used for illumination. In bright field transmission microscopy, the image consists of a field with local variations of intensity produced because the object has partially absorbed, or scattered, the incident radiation. In electron imaging scattering is the predominant factor, limiting direct examination to objects whose mass thickness does not exceed about 50 /xg/cm2.* Thicker specimens can be examined only in profile. Optical microscopy of surfaces is concerned with their appearance as seen by reflected light, the counterpart of which is not practicable 1 with electrons. The electron microscopist has therefore devised means of transferring surface structural details to thin films called replicas.2 These films must present to the electron beam locally varying thickness corresponding to the surface details. A simple type is the plastic replica 3 consisting of an appropriately thin plastic film stripped from the surface. A second type * Some microscopes provide a range of accelerating potentials, up to 100 kv or more, permitting direct examination of thicker objects. 1 Zworykin et al. "Electron Optics and the Electron Microscope," pp. 98-106. 2 J. Roy. Micro. Soc., 70, 1950, "The Practise of Electron Microscopy," ed.