A multi-beam apparatus for multi-beam inspection with an improved source conversion unit providing more beamlets with high electric safety, mechanical availability and mechanical stabilization has been disclosed. The source-conversion unit comprises an image-forming element array having a plurality of image-forming elements, an aberration compensator array having a plurality of micro-compensators, and a pre-bending element array with a plurality of pre-bending micro-deflectors. In each of the arrays, adjacent elements are placed in different layers, and one element may comprise two or more sub-elements placed in different layers. The sub-elements of a micro-compensator may have different functions such as micro-lens and micro-stigmators.