On-Axis Electrode Aberration Correctors for Scanning Electron/Ion Microscopes

A. Khursheed, W. K. Ang
2015 Microscopy and Microanalysis  
This paper presents a method for correcting spherical and chromatic aberrations of scanning electron/ion microscopes through the use of on-axis electrode corrector units. Unlike previous annular systems, which either required insertion of a corrector unit into the objective lens gap [1] , or involved the use of special off-axis lenses [2], the on-axis electrode corrector units proposed here do not require any change to the microscope column design. They, together with an annular aperture,
more » ... e a conventional scanning electron/ion microscope's final hole-aperture unit, as shown in Figure 1 . Apart from this, no other modification to the microscope design is required. Figure 2 shows electric and magnetic designs for a spherical aberration on-axis electrode corrector. The electrostatic corrector is similar to a coaxial cable layout with a zero volt on-axis wire electrode, where the electric field, neglecting the end fringe field effects, is inversely proportional to the radius. In the magnetic case, an on-axis current carrying wire generates a circular magnetic field (Ampere's Law) whose strength is also inversely proportional to the radius. In both cases, the strength of the corrector unit focusing lens action applied to the primary electron/ion beam decreases as a function of radius, opposite to the over-focusing action of the objective lens, whose strength increases with radius. In this way, the spherical aberration effect of an objective lens can be reduced. Obviously, these correctors are more effective with decreasing annular aperture width. Whether the corrector requires a divergent or convergent action on the primary beam depends on the incoming angle of the primary beam and the working distance, however, this is not the key to understanding the way the corrector compensates for the spherical aberration, compensation is achieved by its falling field strength as a function of radius. Consider the simple electrostatic Einzel objective lens column shown in Figure 3 . An annular aperture electrostatic corrector unit is located 5 cm from a 10 kV field emission source, and is a distance of 20 mm from the first electrode of the objective lens. The middle electrode of the objective lens is set to 20 kV (V F ), which forms a focal point 6.53 mm beyond the last objective lens electrode (the working distance). The field emission gun tip is modelled to be 5 µm high and has a radius of 50 nm. Direct ray tracing simulations were carried out by the commercial software, Lorentz 2EM [3]. The program calculates fields having rotational symmetry (two dimensions), and neglects all three dimensional complications, such as end effects in the corrector unit. Any loss of beam current due to the radial supports at the corrector end plates are assumed to be small and neglected. Six rays at a single energy of 0.2 eV for uniformly distributed emission angles from the cathode-tip were plot to go through the annular aperture, and the value of V C in the corrector unit was adjusted so that the RMS value of the final spot size was minimized. Figure 4 shows that the simulated optimized corrected spherical aberration radius as a function of final semi-angle for a 70-80 µm aperture, together with the uncorrected (conventional) spherical aberration for a hole aperture that gives the same current (0 to 38.73 µm). The RMS value of the corrected spherical aberration radius (1.75 nm) is typically around one order of magnitude lower than its conventional counterpart (18.5 nm) and is centered on a semi-angle of 9 mrad. The correctors shown in Figure 2 have obvious applications in situations where spherical aberration is dominant (low diffraction and chromatic aberrations), such as in the Helium Ion Microscope [4]. 106
doi:10.1017/s1431927615013227 fatcat:dm7kuxnlvfbo5nafkxvipxtx7a