For improving spatial resolution in electron microscopy, as is well known, the spherical aberration (Cs) has to be compensated. Currently, the Cs-correction devices consisted of multi-pole lenses have successfully realized sub-angstrom resolution in the scanning / transmission electron microscopes (S/TEMs) [1-3]. These correctors, however, require complex control of multiple optical components with high accuracy and stability. In addition, the microscope columns should be reconfigurated to insert additionally rather large corrector components, resulting in huge cost. In order to solve these problems, one of the coauthor Ikuta had newly proposed the very simple and compact Cs-corrector with axially-symmetric electrostatic-filed formed between annular and circular electrodes [4], as schematically shown in Fig. 1(a). We called it “ACE corrector”, meaning the Cs-corrector using Annular and Circular Electrodes. In the present paper, we report preliminary results of the ACE corrector installed in 200kV-STEM apparatus.

It can be simply explained how the ACE corrector compensates the Cs, as follows. In the electrostatic field formed around the circular electrode, the electrons going through the field are a little focused. In contrast, around the annular electrode, the electron trajectories are spread. They indicate that the field between the electrodes provides the compound lens effect of the convex and concave lenses arising from the circular and annular electrodes, respectively. Totally, as schematically shown in Fig. 1(b), the ACE corrector has the negative Cs value, while the effective area is restricted to be in the off-axis by the annular slit.

Fig. 2(a) is a cross-sectional illustration of the electrodes with typical sizes. The circular electrode can be easily obtained by the photolithography as well as the conventional apertures for the electron microscopes. Since the annular electrodes contain complicated structures, we have employed the focused ion beam (FIB) technique for their fabrication. Fig. 2(b) shows a SEM image of the annular slit corresponding to that in Fig. 2(a). This structure was processed at the center of the base tantalum plate having the size of 3mm in diameter and 10m in thickness. Two electrodes were assembled in the small device, as shown in Fig. 2(c), by sandwiching the insulator film between them. This device was installed in the STEM (Hitachi HD-2300S; 200kV) by attaching to the tip of the conventional aperture holder instrument, which were connected to the voltage supply. The constant negative voltage was applied to the circular electrode, and the annular electrode was grounded, via two lines attached to the device as in Fig. 2(c).

Figs. 3(a) show annular dark-field (ADF) images of CeO2 particles taken at different Cs conditions, i.e. the voltage applied to the ACE corrector varied from 0 V to 15 V. They indicate that the image obtained at 10 V show most clear contrast, which is consistent with the appropriate value predicted in advance by the simulation. In high-resolution condition, as shown in Figs. 3(b), a Cs corrected image taken at 10V can clearly exhibit atomic columns. These results demonstrate that our developed electrostatic device can effectively correct the intrinsic spherical-aberration of the objective lens.

References
[1] H. Rose, Optik 85 (1990) 19
[2] M. Haider, et al., Optik 99 (1995) 167    
[3] O. L. Krivanek, et al., Inst. Phys. Conf., 153 (1997) 35
[4] T. Kawasaki, et al., Proc. ALC, 27p-P-58, (2015)

Figures:

Figure 1. (a) Schematic illustrations of the ACE corrector, and (b) its equivalent lens system.

Figure 2. (a) Cross-sectional drawing of the ACE corrector with scales, (b) SEM image of an annular electrode fabricated by FIB, and (c) developed device to attach the electrodes.

Figure 3. (a) ADF STEM images of CeO2 particles observed by using the ACE corrector with 200kV-STEM (HD2300S; Hitachi); taken by changing the applied voltage from 0 to 15V. (b) High-resolution ADF STEM images of a CeO2 particle taken at 0V and 10V.

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EMC Abstracts - https://emc-proceedings.com/abstract/development-of-a-new-electrostatic-cs-corrector-consisted-of-annular-and-circular-electrodes/