| 摘要 |
An objective lens arrangement includes a first (123), second (125) and third (163) pole pieces, each being substantially rotationally symmetric. The first, second and third pole pieces are disposed on a same side of an object plane (101). An end of the first pole piece is separated from an end of the second pole piece to form a first gap, and an end of the third pole piece is separated from an end of the second pole piece to form a second gap. A first excitation coil (129) generates a focusing magnetic field in the first gap, and a second excitation coil (167) generates a compensating magnetic field in the second gap. First and second power supplies (141,169) supply current to the first and second excitation coils, respectively. A magnetic flux (142) generated in the second pole piece is oriented in a same direction as a magnetic flux generated in the second pole piece. |
