Provided is an assembly for inspecting the surface of a sample. The assembly includes two or more multi-beam electron column units. Each unit has: a single thermal field emitter for emitting a diverging electron beam towards a beam splitter; wherein the beam splitter includes a first multi-aperture plate having multiple apertures for creating multiple primary electron beams; a collimator lens for collimating the diverging electron beam from the emitter; an objective lens unit for focusing said multiple primary electron beams on said sample; and a multi-sensor detector system for separately detecting the intensity of secondary electron beams created by each one of said focused primary electron beams on said sample. The two or more multi-beam electron column units are arranged adjacent to each other for inspecting different parts of the surface of the sample at the same time.