Patent Number: 043895719
Section: claims

1. In an electron beam device having a source of electrons, the improvement in the device for correcting higher order aberration, comprising: first and second sextupoles, each for focusing a beam in an image plane, and each having a second-order astigmatism distribution, an intermediate lens positioned between said first and said second sextupoles for focusing the beam to a point in the center of said second, sextupole, a first lens for focusing the beam to a point in the center of said first, sextupole, and a second lens positioned in said image plane of said second, sextu-pole for focusing the beam to a point in a final image plane. 2. The device of claim 1 wherein said first and said second focusing means comprise first and second sextupoles, respectively. 3. The device of claim 2, wherein said second sextupole is located downstream of said first sextupole and has between 40% and 80% of the strength of said first sextupole. 4. The device of claim 2, further including steering means for aligning said beam along the optical axis of the device. 5. The device of claim 4 wherein said steering means includes two pairs of steering coils with one pair of coils positioned upstream of said first sextupole and the second pair of coils positioned downstream of said second sextupole. 6. The device of claim 3, wherein said second lens has a focal length of 1 mm, a value of spherical aberration coefficient of 0.4 mm, and the source produces 75,000 volt electrons.