Hartel P, Gerheim V, Linck M, Müller H, Uhlemann S, Zach Z, Haider M
Corrected Electron Optical Systems GmbH, Englerstr. 28, Heidelberg 69126, Germany.
Corrected Electron Optical Systems GmbH, Englerstr. 28, Heidelberg 69126, Germany.
Ultramicroscopy. 2019 Nov;206:112821. doi: 10.1016/j.ultramic.2019.112821. Epub 2019 Aug 3.
After the introduction of the hexapole C-correctors for scanning transmission electron microscopes (STEM), the next big step forward was the strong reduction of the six-fold astigmatism A by means of an advanced hexapole design (DCOR/ASCOR). As a result all axial aberrations up to fifth order are sufficiently small to allow for large semi-aperture angles beyond 40 mrad for electron energies in the range of 30 to 300 kV without deterioration of the STEM resolution. In this paper we derive simple expressions for the optimum hexapole strength for minimum A and the size of the residual A. Both quantities are intrinsic properties of the hexapoles and the transfer lens doublet in between. The optimum hexapole strength scales with the inverse of the electron wavelength, while the residual A does not depend on the electron energy directly, but on the spherical aberration C of the pole piece. With the given properties of the DCOR/ASCOR and typical values of C in the range of 0.5 to 2.7 mm, at all acceleration voltages A remains in the range from 0.03 to 0.4 mm, the latter even for a large-gap pole piece.
在为扫描透射电子显微镜（STEM）引入六极C校正器之后，下一步的重大进展是通过先进的六极设计（DCOR/ASCOR）大幅降低六重像散A。结果，对于30至300 kV范围内的电子能量，所有直至五阶的轴向像差都足够小，从而允许在半孔径角超过40 mrad的情况下，而不会降低STEM的分辨率。在本文中，我们推导了使A最小化的最佳六极强度以及残余A大小的简单表达式。这两个量都是六极和其间的传输透镜双合透镜的固有属性。最佳六极强度与电子波长的倒数成比例，而残余A并不直接取决于电子能量，而是取决于极靴的球差C。根据DCOR/ASCOR的给定属性以及C在0.5至2.7 mm范围内的典型值，在所有加速电压下，A都保持在0.03至0.4 mm的范围内，对于大间隙极靴，后者也是如此。
