Correction of sample tilt in FIB instruments

Many particle optical instruments use ion beams focused onto a tilted sample and the rotationally symmetrical beam creates an elliptical spot. Furthermore, an additional electrostatic field may be present near the titled sample, an extracting field of a detector or a spectrometer or a retarding field on the sample, for example. As a result, the dipole and quadrupole components of the field near the axis of the column cause an additional astigmatic focusing of the beam, which leads to even more elongated shape of the spot. A method of determining a correction has been developed, which allows to compensate for both the ellipticity due to the tilt and the additional astigmatism and to achieve a circular spot on the titled sample. The compensation is provided by a suitable setting of two correcting quadruples and by refocusing the objective lens. The required settings of the quadrupole excitations and objective lens are obtained by a paraxial optimization. The results are checked by computing an intensity distribution and determining the beam shape.