EI.ECTRON MICROSCOPY 



PLASTICS. Srr GENERAL MICROSCOPY, p. 390. 



PULP AND PAPER. See GENERAL MICROS- 

 COPY, p. 394. 



REFLECTION I 



In the early days of electron microscopy 

 prior to the de\elopment of replica methods, 

 most specimens were so thick as to be elec- 

 tron-opaque and only their silhouettes could 

 be examined. To enable direct surface studies 

 to be made, Ruska (1) in 1933 attempted to 

 image electrons scattered from a metal sur- 

 face. The specimen was irradiated with an 

 electron beam which was at 90° to the axis 

 of the imaging system, but the resolution 

 was limited in this first experiment to about 

 20 to 30 microns by the chromatic aberra- 

 tion of the lens and the very large bandwidth 

 of the energy spectrum of the scattered elec- 

 trons. Further work by Ruska and Miiller, 

 (2) using glancing angles of incidence, and 

 viewing at 90°, yielded a resolution of 5,000 

 A. Von Borries (3) in 1940 suggested the use 

 of glancing angles of both illumination and 

 viewing, thus reducing the energy loss in the 

 scattered beam and giving a resolution of 



o 



about 250 A. Such low angles (the total 

 deviation of the beam being about 8°) make 

 for extremely difficult interpretation of the 



Fig. 1. Schematic diagram of reflection elec- 

 tron microscope. 



image and moreover only the smoothest 

 surfaces can be examined. It was for these 

 reasons that with the advent of repUca meth- 

 ods, interest in reflection electron microscopy 

 diminished. 



There was a revival of interest in the years 

 1951 to 1953 when it was realized that in 

 certain circumstances the reflection method 

 of Von Borries could have distinct advan- 

 tages over the now well established replica 

 method and this revival took place almost 

 simultaneously in several schools. Kushnir, 

 Biberman and Levkin (4), Cosslett and 

 Jones (5), and Fert and Saporte (6) worked 

 with microscopes designed and built for 

 reflection work while Menter (7), and Haine 

 and Hirst (8) made adaptations for the 

 Metropolitan Vickers microscope. 



The characteristics of the image in this 

 type of work are very different from those 

 pertaining to other forms of microscopy and 

 can be considered under the following head- 

 ings: 



Viewpoint. The highly oblique viewpoint 

 is a most unusual one; Von Borries stated 

 that the image has the appearance of a road 

 illuminated by the headlights of an ap- 

 proaching car. The most important feature 

 of this obhque viewpoint is the marked fore- 

 shortening effect in the image. Distances 

 along the line of sight (direction y, Fig. 1) 

 are foreshortened with a viewing angle of 

 for example 5° by a factor of 12:1. Thus a 

 circle in the plane of the specimen appears 

 as an ellipse of this eccentricity; this led 

 Emerton (9) to suggest that the scale on 

 reflection electron micrographs should be 

 indicated by ellipses, the diameter of which 

 could be quoted. It also follows that linear 

 features on the specimen appear to be ori- 

 ented more nearly perpendicular to the line 

 of sight (direction x). A line at an angle of 

 45° to the line of sight appears in a micro- 

 graph (foreshortened 12: 1) to be at an angle 

 of 85°. 



This viewpoint does nevertheless have 

 certain advantages. The form of the surface 



220 



