In this master’s thesis, a mechanical design of an in-column secondary electrons (SE) detector is presented. It is an ultravacuum compatible fibre-scintillation detector designed for use in an ultrahigh vacuum scanning electron microscope (UHV SEM). The designed in-column SE detector was manufactured and tested upon overcoming R&D challenges. The first section of this thesis deals with theoretical basis needed for understanding of functional principles of UHV SEM system and means of SE’s detection. In the second section, mechanical design of the in-column SE detector is described. The last section describes functionality of the designed detector. Effectiveness of light shielding of the detector was tested and the detective quantum efficiency was measured. Finally, images created by the designed in-column detector and an in-chamber SE detector were evaluated and compared.