| 摘要 |
The invention relates to an electron-optical corrector for eliminating third-order aberrations, such as spherical aberrations, field curvature and off-axis astigmatism; said corrector being devoid of third-order off-axis coma, third-order distortion and first-order chromatic aberration of the first degree. The corrector has a construction which is symmetrical about the central plane in the direction of the linear optical axis. A hexapole S1 of length l1 is first positioned in the direction of the beam path, followed by a circular lens R1, a hexapole S2 of length l2 and subsequently a circular lens R2 which is followed by a third hexapole S3 with the same strength with the same strength of the hexapole S1 and double the length of the latter l3=211. The separation of the two circular lenses R1, R2 and the distance from the circular lens to the first hexapole S1 is chosen in such a way that the internal plane of S1 comes to rest in the front principal focus of the circular lens that is positioned downstream and the center of the hexapoles S2 and S3 is located on the focal plane. Additional elements of the corrector also follow in sequence, said elements being symmetrical about the central plane Zm of the hexapole S3.
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