A multi-beam inspection apparatus including an improved source conversion unit is disclosed. The improved source conversion unit may comprise a micro-structure deflector array including a plurality of multipole structures. The micro-deflector deflector array may comprise a first multipole structure having a first radial shift from a central axis of the array and a second multipole structure having a second radial shift from the central axis of the array. The first radial shift is larger than the second radial shift, and the first multipole structure comprises a greater number of pole electrodes than the second multipole structure to reduce deflection aberrations when the plurality of multipole structures deflects a plurality of charged particle beams.