Design of an objective lens pole piece for a transmission electron microscope with a resolution less than 0.1 nm at 200 kV

Abstract

The limit of the chromatic aberration coefficient C c of a 200 kV transmission electron microscope has been calculated numerically. The limit is 0.34 mm, if no limitation exists for the specimen insertion and machining of the pole piece. The corresponding image side maximum of the axial flux density is 7.8 T. The computed C c is nearly equal to the theoretical minimum of 0. 33 mm obtained from Schiske's equation (P Schiske Optik 13 (1956) 502). Under practical conditions, C c=0.63 mm and the corresponding spherical aberration coefficient C s=0.55 mm are obtained. The dimensions of the pole piece are the gap length 2 S=1.2 mm, the bore radius b=0.2 and the top face radius D=0.3 mm. Axial magnetic field distributions of such a pole piece have two peaks in the gap, because of the small bore and the top face area of the pole piece compared with the gap length. The resolution of the information limit is estimated to be 0.07 nm, if the stability of the high voltage is below 1×10 −7 and a field emission gun is used.