FIELD KMISSION MICROSCOPY 



the more refractory metals. At a sufficiently 

 high field atoms of all metals will be "field- 

 evaporated" in the form of positive ions, 

 even at a temperature close to absolute zero. 

 In order to observe a steady surface the 

 evaporation field of the tip material must 

 be above the field necessary for the ioniza- 

 tion of the image producing gas. 



Helium, requiring 450 MV/cm for ioniza- 

 tion and gi\'ing the best images, can be used 

 for tips of C, W, Re, Ta, Mo, Nb, Ir, Pt, 

 Rhj and with restrictions also for tips of Zr. 

 Pd, Ni, Fe, Co, Si, and of alloys of these 

 metals. Gases with lower ionization poten- 

 tials require less field strength, and fairly 



good images have been obtained with neon, 

 hydrogen, nitrogen, oxygen, and argon, al- 

 though the resolution is inferior and diffi- 

 culties are encountered with adsorption and 

 possibly field induced chemical reactions of 

 these gases. 



The luiique feature of operating the field 

 ion microscope with heUum is that, once the 

 tip surface has been cleaned from all con- 

 taminations by field evaporating a few sur- 

 face layers of the metal, not one contamina- 

 tion atom can reach the tip surface as long as 

 the high field is maintained. All gases have a 

 lower ionization potential than helium, so 

 that all contamination molecules will be ion- 



FiG. 4. Ion microscope picture of a platinum crystal with nearly perfect lattice, with 001 in the cen- 

 ter, and the four 111 planes in the four corners. Many of the about 1000 high index net planes are re- 

 solved in individual atoms. 



330 



