FIELD EMISSION MICHOSCOPY 



ized way above the surface, and carried away 

 by the field. This makes it possible to design 

 a field ion microscope with verj^ modest vac- 

 uum requirements, providing, for instance, 

 greased joints for easy tip specimen replace- 

 ment. In spite of the resulting poor vacuum 

 conditions (10~^ mm residual pressure), a 

 specimen surface stays atomically clean for 

 any desired length of time, that is without 

 even the adsorption of onlj^ one contaminat- 

 ing atom. The best results are obtained when 

 the microscope is cooled with liquid h\'dro- 

 gen or Hquid hehum, although quite useful 

 images can be made with liquid nitrogen 

 cooling. 



The most promising application of the 

 field ion microscope is the direct obsen^ation 

 of individual imperfections in metal crystals. 

 The regular arrangement of the atoms in the 

 faultless crystal can more accurately be de- 

 termined by the classical x-ray diffraction 

 methods. The field ion microscope, however, 

 shows directl^y the individual imperfections 

 such as single vacancies, interstitials and 

 dislocations, the structure of lattice steps 

 and of grain bomidaries. Although only the 

 surface is shown in the image, controlled 

 field e\-aporation can be used to remove one 

 surface la3^er of the specimen after another, 

 so that with this "sectioning technique" the 

 interior of the tip crystal becomes acces- 

 sible. The high field exerts a large electro- 

 static stress FVStt at the surface, which 

 amounts to approximately 1000 kg/mm^ 

 when helium ions are used. Nevertheless, 

 all the metals mentioned above can stand 

 this stress in spite of their much lower bulk 

 strength. By pulsing the field the fluctuating 

 stress can be used for fatigue experiments in 

 situ. As no heating is necessary for removing 

 surface contaminations by field evaporation, 

 it is possible to study metal structures that 

 are subject to changes at elevated tempera- 

 tures, such as cold work effects, quenched-in 



Fig. 5. Ion image of a section of a platinum 

 crystal with many defects. Each fine bright dot 

 represents an individual atom. 



defects, precipitation in alloy's, or damage by 

 irradiation with high energy particles. The 

 impact of one indi\idual a particle can be 

 seen directlV; and the disarrangement of the 

 metal atoms in the displacement spike can 

 be investigated in detail. It can be assumed 

 that in the future the field ion microscope 

 will increasingly contribute to our knowledge 

 of the atomic structure of solids. 



REFERENCES 



1. Good, R. H., Jr., and Miller, E. W., "En- 



cyclopedia of Physics" (2nd ed.), Springer 

 Verlag, Vol. 21, p. 17t>-231 (195C). 



2. GoMER, R. "Advances in Catalysis,'" Academic 



Press, Vol. 7, p. 93-134 (1955). 



3. Becker, J. A., "Solid State Physics, "Academic 



Press, Vol. 7, p. 379-424 (1958). 



4. Dyke, W. P. .\nd Dolan, W. W., "Advances in 



Electronics," Academic Press, Vol. 8, p. 

 90^185 (1956). 



5. ^It'LLER, E. W., "Fourth International Con- 



gress on Electron Microscopj," Berlin, 1958, 

 Springer Verlag, p. 820-835 (1960). 



6. MtJLLER, E. W., "Advances in Electronics," 



Academic Press. Vol. 13. p. 83-179 (1960). 



Erwix W. jNIuller 



331 



