| 摘要 |
PURPOSE:To obtain the captioned lens having a low aberration characteristic by setting a center electrode, asymmetrically with respect to the center of a lens, especially selecting this matter out of various elements of configuration of an electrode. CONSTITUTION:The spot diameter d0 of a beam probe is given by the equation I, where Cc and Cs are color aberration coefficient and spherical aberration coefficient respectively, theta is the semivertical angle of the convergent beam, PHI is drift potential, DELTAE is the half-width of the energy distribution of charged particles, m is the mangnifying power of the lens, and dg is the spot diameter of the beam. In order to reduce do on the basis of the euqationI, each aberration coefficient should be reduced. Cc and Cs are given by the equationsIand II, if the potential on axis is shown by PHI(Z). r=r(Z) is the paraxial orbit of the charged particle and given as the solution of the equation III. Therefore, if the potential on axis PHI(R) is obtained from the euqationsI-IV, each aberration coefficient can be calculated. Each aberration coefficient decreases as a center electrode 2 is brought close to the image side from the object surface side with respect to the center of the lens. |
