A computer study to simulate a magnetic objective lens and analyze the effect of the distance between pole pieces on its focal properties

Abstract

In this research, a magnetic objective lens has been developed and its optical properties have been analyzed using simulation programs based on the finite element method. The optical properties are significantly improved, and low values of electron probe diameter on the sample surface are obtained together with spherical, chromatic, diffraction, and Gaussian aberrations and an optimal aperture angle. A double-pole symmetrical magnetic lens was designed and showed excellent optical properties. The effect of varying distances S between the electrode pieces (S = 2, 4, 6, 8 mm) on the performance of the magnetic lens was studied in order to determine its best design. The results showed that when constant excitation NI = 1 kA‑t and acceleration voltage Vr = 10 kV were applied, the distance S = 2 mm was optimal, where good optical properties were achieved: focal length fo = 4.04 mm, spherical aberration Cs = 3.75 mm, and chromatic aberration Cc = 3.12 mm. The largest axial magnetic flux Bz = 0.18 T was also reached. The effect of the electronic probe diameter as a function of the aperture angle was studied, showing that an aperture angle of 2 mrad significantly improved the optical properties to obtain dp = 12.24 mm, ds = 5.64 mm, dc = 0.0005 mm, dd = 4.06 mm, and dg = 10.06 mm. The lense with S = 2 mm was selected as the best design, and its optical properties were examined in more detail.