Advancing the Hexapole Cs-Corrector for the Transmission Electron Microscope

Abstract

* presenting author: linck@ceos-gmbh.de TL11 TL12 TL22 TL21 ADL HP1 HP2 OL original CETCOR DCOR technology BCOR technology +QP +QP +QP +QP +QP,HP +QP,HP Corrector outline Since 1998, the correction of image aberrations in TEM [1] has been a huge success story. The basic design of the first commercial version of the TEM corrector (CETCOR) has hardly changed over the years. However, recent instrumental developments such as improved mechanical stability, cold FEGs and monochromators and new applications such as low-kV TEM imaging have pushed the requirements on aberration correctors to a new level. High-resolution phase contrast imaging up to the information limit of state-of-the-art TEMs requires correction for the dominating six-fold astigmatism A5 and full fourth-order axial aberration correction; this is out of reach with the present CETCOR design. Also, intrinsic off-axial aberrations strongly limit the aberration-free field of view for high-resolution TEM imaging. A careful redesign of the CETCOR with optimized hexapole geometries and additional stigmators can remove these limitations for aberration-corrected TEM. Our results from a prototype of the Advanced TEM CORrector (ATCOR) operated in a JEOL JEM-ARM200F NEOARM electron microscope equipped with high-resolution pole piece and cold field emission gun show several substantial improvements for TEM techniques and indicate that the corrector is well-prepared for ultrahigh-resolution TEM using monochromated illumination. The advanced TEM corrector inherits the DCOR/ASCOR-type length-optimized hexapoles for minimization of six-fold astigmatism A5 and full fourth-order axial aberration correction that had been introduced in 2006 during the face-lift of the hexapole corrector for STEM [2]. The BCOR, which had been designed for correction of the unavoidable azimuthal off-axial coma [3], introduced several additional degrees of freedom to correct for low-order parasitic off-axial aberrations. This knowledge has substantially influenced the redesign of the "old" CETCOR in that the BCOR-type middle transfer optics and additional stigmators have been adapted for the advanced TEM corrector. The new corrector has 45 channels to correct for all axial aberrations up to fourth order including the fifth-order six-fold astigmatism and to control lower-order parasitic off-axial aberrations such as off-axial twofold astigmatism and twofold generalized off-axial coma.