coma-free axis

"Coma-free axis" is an axis where the axial parasitic coma aberration of the objective lens becomes 0 (zero). Alignment of the coma-free axis means to align the incidence direction of the electron beam with the coma-free axis. The axial coma aberration is a parasitic aberration that is originated from mechanical disagreement between the axes of lenses. It should be noted that this aberration is different from the (off-axial) coma aberration among five Seidel aberrations, which is intrinsic to the lens. In the case of an electron microscope without a Cs corrector, the axial coma aberration due to the disagreement of the axis of the objective lens is corrected by the alignment coils above the specimen chamber. The coma-free axis is not so different from the accelerating voltage center. On the other hand, in the case of an electron microscope equipped with a Cs corrector, the axial coma aberration due to the disagreement of the axis of the Cs corrector is the matter of concern. A double-deflection coil system is provided for the alignment of the coma-free axis other than the alignment coil system for accelerating voltage center. The coefficient of the axial coma can be corrected to a value of a few nm. In actual alignment, using an amorphous carbon (C) thin foil or an amorphous germanium (Ge) thin film, the incident beam is tilted to more than some 10 mrads and 4 to 16 micrographs of the thin foil are taken at 0°to 360°in the azimuth directions to form a diffractogram tableau. Then, the tilt of the electron beam to the Cs corrector placed after the objective lens is adjusted so that a set of the tableau becomes centro-symmetric with respect to the center of the tableau (position at a tilt angle of 0°). This alignment is important when taking high-resolution images.

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